InteractiveFly: GeneBrief

Lamin C: Biological Overview | References

BIOLOGICAL OVERVIEW

The levels of nuclear protein Lamin A/C (Drosophila Lamin C) are crucial for nuclear mechanotransduction. Lamin A/C levels are known to scale with tissue stiffness and extracellular matrix levels in mesenchymal tissues. But in epithelial tissues, where cells lack a strong interaction with the extracellular matrix, it is unclear how Lamin A/C is regulated. This study shows in epithelial tissues that Lamin A/C levels scale with apico-basal cell compression, independent of tissue stiffness. Using genetic perturbations in Drosophila epithelial tissues, it was shown that apico-basal cell compression regulates the levels of Lamin A/C by deforming the nucleus. Further, in mammalian epithelial cells, this study show that nuclear deformation regulates Lamin A/C levels by modulating the levels of phosphorylation of Lamin A/C at Serine 22, a target for Lamin A/C degradation. Taken together, these results reveal a mechanism of Lamin A/C regulation which could provide key insights for understanding nuclear mechanotransduction in epithelial tissues (Iyer, 2021).

Mechanotransduction is a process through which mechanical forces are converted to biochemical signaling or gene expression and is crucial for physiology. Impaired mechanotransduction is at the heart of various diseases. Mechanotransduction could be activated by either extrinsic forces like fluid shear flow in blood vessels or intrinsic forces through actomyosin contractility. Recent experiments have highlighted the importance of the transmission of forces to the nucleus through the cytoskeleton. In this process, the nuclear scaffold plays an important role in activating mechanotransduction in the nucleus (Aureille, 2017; Iyer, 2021).

Nuclear Lamins are the primary component of the nuclear scaffold. Lamins are type V intermediate filaments, comprising of A-type and B-type Lamins. In vertebrates, A-type Lamins are composed of two splice isoforms of the Lamin A gene, Lamin A, and Lamin C, whereas B-Type Lamins include Lamin B1 and Lamin B2. In contrast, in invertebrates like Drosophila, Lamin C is the only form of A-type Lamin, and Lamin DM0 is the only type of B-type Lamin. Lamins are known to influence the mechanical properties of the nucleus. Lamin A/C contributes to the stiffness and viscosity of the nucleus whereas Lamin B is responsible for the elasticity of the nucleus. Recent experiments have shown that levels of Lamin A/C are crucial for mechanotransduction in the nucleus (Lammerding, 2004). Not only does Lamin influence force-induced stiffening of the nucleus (Guilluy, 2014; Dahl, 2008; Osmanagic-Myers, 2015), it also promotes, nuclear translocation of MKL, a co-factor of Serum Response factor (SRF). Interestingly, Lamin B is expressed uniformly in all tissues during development, but Lamin A/C is developmentally regulated and has tissue-specific expression profiles (Ho, 2013; Iyer, 2021).

Over the last decade, studies have focused on identifying how Lamin A/C is regulated in different tissues. Experiments in mesenchymal stem cells (MSC), mesenchymal tissues and some non-mesenchymal tissues have shown that Lamin A/C scales with extracellular matrix (ECM) stiffness in these tissues. Epithelial tissues are one of the most abundant adult tissues. ECM in these tissues is scant, but the cells adhere to each other through a plethora of cell-cell junctions along the apico-basal axis. These cell-cell junctions bear most of the mechanical stress in the tissue. In the absence of strong interactions with the ECM, it is still unclear how Lamin A/C levels are regulated in these tissues. Studying this would be key to understand the role of Lamin A/C in mechanotransduction and the interplay between tissue mechanics and nuclear mechanotransduction in epithelial tissues (Iyer, 2021).

In this work, using Drosophila epithelial tissues and mammalian Madin Darby Canine Kidney (MDCK) cells as model systems, strong evidence is provided that apico-basal cell compression is an ECM-independent mechanism for regulation of Lamin A/C in epithelial tissues. Nuclear deformation in response to apico-basal cell compression modulates Lamin A/C levels. By combining genetic perturbations in vivo, and altered cell packing, in cultured mammalian cells, this study has shown that apico-basal cell compression-based regulation of Lamin A/C is evolutionarily conserved in epithelial tissues (Iyer, 2021).

Lamin A/C is an important element of the cellular mechanotransduction pathway which bridges mechanical forces to the cell nucleus. In mesenchymal and non-epithelial tissues Lamin A/C is known to be regulated by ECM stiffness (Swift, 2013). How Lamin A/C is regulated in epithelial tissues was not known. This work has shown that Lamin A/C levels in epithelial tissues depend on apico-basal cell compression. In many different epithelial tissues and under different genetic perturbations Lamin A/C levels can be reliably predicted solely based on apico-basal cell compression, suggesting that cell shape regulates Lamin A/C levels. It was also shown that the nucleus flattens in response to apical-basal cell compression. It has been shown in recent studies on mesenchymal and non-epithelial tissues that the levels of Lamin A/C are higher when nuclei flatten on a stiff ECM. Recent studies in mesenchymal tissues have shown that tissue stiffness and cytoskeletal tension that lead to increased forces on the nucleus deform the nucleus. In contrast, epithelial tissues alter cell shape by modulating cortical tension and properties of cell-cell junctions. The current findings suggest that in epithelial tissues, Lamin A/C is regulated independent of tissue stiffness and ECM levels and that Lamin A/C levels depend on the flattening of the nucleus. Thus, nuclear deformation is a common feature of Lamin A/C regulation in both mesenchymal (Buxboim, 2017) and epithelial tissues, suggesting the at the scale of the nucleus similar mechanisms regulate the levels of Lamin A/C in both types of tissues. While this study showed that LamC is regulated by apico-basal cell compression at a protein level, a recent study has shown that specific transcription factors could regulate the transcription of LamC52 (Schulze, 2005; Iyer, 2021 and references therein).

In response to apico-basal cell compression, the nuclei of MDCK cells flatten and the nuclear surface area increases, resulting in stretching of the Lamin network. This stretching of Lamin network strongly correlates with dephosphorylation of Lamin A/C at serine 22, which could inhibit the degradation of Lamin A/C as observed previously in mesenchymal cells in culture. This could result in higher levels of Lamin A/C in apico-basally compressed cells. Interestingly Lamin A/C levels depend non-linearly on both apico-basal compression of the nucleus and on Lamin network stretching. This study found that nuclear compression or Lamin network stretching had to exceed a threshold before Lamin A/C levels increased. Recent studies in cell lines and in Zebrafish embryos show a similar non-linear response of myosin II activity to a reduction of nuclear height. An increase in myosin II activity is observed only when the nucleus is compressed below a critical height, at which cytosolic phospholipase A2 (cPLA2) was activated. This phenomenon is called cellular proprioception, whereby the nucleus could sense and respond to changes in shape and size of a cell. In the current work, pSer22 could act like a sensor of nuclear stretch to inhibit or activate degradation of Lamin A/C. Thus, cellular proprioception could be the mechanism through which nuclei in epithelial tissues sense apico-basal cell compression and modulate the levels of Lamin A/C (Iyer, 2021).

Interestingly, this study also found that while cell morphology regulates Lamin A/C levels, the reverse is not true: changes in Lamin A/C levels do not influence the morphology of cells in Drosophila epithelial tissues. Even in Lamin C null mutants the morphology of the cells in the wing disc is not significantly different from wild-type, suggesting that altering Lamin A/C levels does not affect cytoskeletal organization in these cells. However, as previously described, the Lamin C null larvae do not survive beyond the late third instar stage of development. This finding is in accordance with the role of Lamin A/C in protecting the genome against DNA damage and in mediating nuclear mechanotransduction. Thus, in LamC null larvae, hindered translocation of mechanotransducers like YAP57 and MKL58 or accumulation of DNA damage during larval stages could lead to lethality at later development (Iyer, 2021).

Taken together, these results suggest that regulation of Lamin A/C could be the underlying mechanism coupling cell mechanics and nuclear mechanotransduction. The nature of this mechanism and how nuclear mechanotransduction could influence developmental process are major questions that this study raises for future investigations (Iyer, 2021).

Live imaging of chromatin distribution reveals novel principles of nuclear architecture and chromatin compartmentalization

The three-dimensional organization of chromatin contributes to transcriptional control, but information about native chromatin distribution is limited. Imaging chromatin in live Drosophila larvae, with preserved nuclear volume, revealed that active and repressed chromatin separates from the nuclear interior and forms a peripheral layer underneath the nuclear lamina. This is in contrast to the current view that chromatin distributes throughout the nucleus. Furthermore, peripheral chromatin organization was observed in distinct Drosophila tissues, as well as in live human effector T lymphocytes and neutrophils. Lamin A/C up-regulation resulted in chromatin collapse toward the nuclear center and correlated with a significant reduction in the levels of active chromatin. Physical modeling suggests that binding of lamina-associated domains combined with chromatin self-attractive interactions recapitulate the experimental chromatin distribution profiles. Together, these findings reveal a novel mode of mesoscale organization of peripheral chromatin sensitive to lamina composition, which is evolutionary conserved (Amiad-Pavlov, 2021).

Robust chromatin organization within the nucleus has been linked to the control of gene transcription, yet information about the 3D chromatin distribution under physiological conditions in a living organism is limited. This study has revealed the 3D distribution of chromatin in muscle nuclei of live Drosophila larvae at high resolution. This analysis demonstrates a novel mode of chromatin organization at the nuclear mesoscale of fully differentiated cells, wherein a high density of chromatin, including both active and repressed regions, is distributed at the nuclear periphery. Chromatin density decreases sharply as a function of distance to the center of nucleus, giving rise to a chromatin-devoid region at the center of the nucleus. This novel mode of nuclear-scale chromatin architecture was detectable only in unfixed tissue within a live organism and preservation of nuclear volume was critical for that. The peripheral chromatin organization was sensitive to lamin A/C levels and contributed to maintaining adequate levels of the active epigenetic mark H3K9ac. Model simulations show that a peripheral chromatin organization was obtained when lamina-associated domains (LADs) and chromatin attractive interactions dominate the entropic tendency to distribute the chromatin uniformly throughout the nucleus. Peripheral chromatin organization was observed not only in larval muscles but also in other live Drosophila larval tissues, as well as in live human effector T lymphocytes and neutrophils, suggesting an evolutionary conserved mode of chromatin organization at the nuclear periphery (Amiad-Pavlov, 2021).

A key experimental feature that allowed viewing nuclear partitioning was preservation of nuclear volume. Previous global chromatin organization studies were performed primarily on fixed preparation or on live flat cultured cells, which might obscure the native spatial chromatin distribution. Specifically, prevailing fixation methods may cause cell and nuclear volume changes, often at the Z axis, while the dimensions in the imaging X-Y plane are less affected, possibly due to robust nuclear-cytoskeletal interactions in this plane. Furthermore, cells grown on rigid surfaces are relatively spread and flat, with 40% to 50% less water content relative to cells grown on soft matrices. As nuclear and cytoplasmic volumes were shown to be interconnected and adapt to external signals, through tight control of their mutual water content (23, 25, 55), preservation of nuclear volume within the tissue (as in the current experimental system) appears to be critical. This was also deduced from model simulations where nuclear volume was found to be directly linked to chromatin mesoscale organization. Significantly, changes in nuclear volume and shape were shown to have critical functional effects such as altered chromatin organization, gene expression, and DNA synthesis (Amiad-Pavlov, 2021).

Although a few previous studies already reported specific cases of peripheral chromatin distribution, it was considered as a transient stage that correlated with the differentiation state of the cell, for example, in the transition from 8- to 20-cell stages of in vitro fertilized bovine embryos, T cell differentiation, or during myeloid cell differentiation. The reported alternations in chromatin organization were accompanied by volume changes; however, the link between these two parameters was not explored. the current data suggest that chromatin peripheral organization is common to a wide range of differentiated tissues. Furthermore, chromatin to nucleus volume fraction is a major regulator of chromatin mesoscale organization, as indicated by experimental and model results. Drosophila myonuclei are polyploid, where DNA copy number was shown to correlate with nuclear size. This study demonstrate a constant chromatin/nucleus volume ratio (0.31) for a wide range of muscle nuclei volumes suggesting that it is a tightly regulated feature. A constant chromatin volume fraction in polyploid nuclei is not unique to larval myonuclei and was also found in Drosophila embryos as well as in plants and fish (Amiad-Pavlov, 2021).

A recent study challenged the prevailing view that repressed and active chromatin compartments partition radially within the nuclear volume, i.e., repressed chromatin predominates at the nuclear periphery and active chromatin prevails near the nuclear center. In agreement with this report, this study did not observe a clear radial separation between active and repressed chromatin distributions and also detected active chromatin at the nuclear peripheral compartment (Amiad-Pavlov, 2021).

The nuclear lamina controls radial chromatin organization by its dynamic tethering of chromatin to the nuclear periphery through LADs. Because the nucleus is thought to be a mechanosensitive organelle in which the close proximity of the chromatin to the nuclear envelope might sensitize the chromatin to both mechanical inputs and changes in the nuclear lamina composition, a peripheral chromatin organization might couple between mechanical inputs and transcriptional regulation. This is especially relevant for fully differentiated nondividing cells, in which the program of gene transcription has been well established; however, because this analysis did not include undifferentiated cells, it remains unclear whether chromatin peripheral organization also takes place in all cell types. In addition, the global chromatin detachment from the periphery and its condensation toward the center that was observed upon elevated lamin C levels agrees with previous reports on reduced chromatin-lamina tethering following up- or down-regulation of lamin C, as well as with observations in cells with laminopathy-associated mutations. These chromatin structural changes were accompanied by chromatin decondensation and were linked to transcriptional regulation. The current data support a link between 3D chromatin distribution and transcriptional regulation as a significant reduction was observed in the active, H3K9ac levels in the nuclei overexpressing lamin C-GFP. The centrally located, condensed chromatin might prompt reduced availability to enzymes (such as acetyltransferase), leading to a net decrease in acetylated H3K9, predicted to induce reduction in gene transcription. Chromatin regions that are already densely packed (such as repressed chromatin) might be less affected from the collapse of chromatin to the center as observed with the preserved H3K9me3 levels in nuclei overexpressing lamin C-GFP (Amiad-Pavlov, 2021).

The simulations of chromatin distribution dynamics involved only a small number of parameters extracted from the literature, including the percentage of LADs per chromosome, the tendency of chromatin to effectively self-attract and the extent of nuclear confinement. The similarity between the simulation and the experimental results suggests that these are among the most critical factors in determination of the partitioning of chromatin on the nuclear scale. It further suggests that a balance between chromatin-lamina interactions through LADs, entropy, and chromatin self-association may represent the major driving forces for the nuclear partitioning observed experimentally. The physical model suggests that the experimentally observed mesoscale chromatin collapse from the periphery to the nucleus interior upon lamin C OE might be driven by reduced lamina-LAD interactions. It has been shown that both lamin A/C and B types compete with one another on similar LAD-binding sites. Although lamin B contacts with LAD exclusively at the nuclear periphery because its localization is restricted to the nuclear membrane due to its farnesylation, lamin A/C was shown to interact with LADs at the nuclear periphery and at the nucleoplasm. This study demonstrated that in contrast to chromatin collapse following lamin C OE, lamin Dm0 (lamin B homolog) OE did not result in chromatin detachment from the lamina and the peripheral chromatin organization was preserved. This might be due to the nucleoplasmic localization of lamin C or due to distinct association of each of the lamin types with other nuclear membrane components affecting the recruitment and preservation of chromatin-lamina interactions at the periphery (Amiad-Pavlov, 2021).

In summary, this study reveals a novel mode of nuclear mesoscale chromatin organization in fully differentiated cells in which chromatin density is high at the nuclear periphery and undetectable in the nuclear center, creating an effectively central chromatin-devoid region. Simulation of chromatin organization based on LAD binding and chromatin self-attraction recapitulated the experimental observations, supported that preservation of nuclear volume is critical and predicted that changing each of these parameters may disrupt peripheral chromatin density profile. Experimental OE of lamin C disrupted the peripheral chromatin organization, supporting the basic assumptions of the model, and had functional consequence of reduced active epigenetic mark (Amiad-Pavlov, 2021).

Suppression of myopathic lamin mutations by muscle-specific activation of AMPK and modulation of downstream signaling

Laminopathies are diseases caused by dominant mutations in the human LMNA gene encoding A-type lamins. Lamins are intermediate filaments that line the inner nuclear membrane, provide structural support for the nucleus, and regulate gene expression. Human disease-causing LMNA mutations were modeled in Drosophila Lamin C (LamC) and expressed in indirect flight muscle (IFM). IFM-specific expression of mutant, but not wild-type LamC, caused held-up wings indicative of myofibrillar defects. Analyses of the muscles revealed cytoplasmic aggregates of nuclear envelope (NE) proteins, nuclear and mitochondrial dysmorphology, myofibrillar disorganization, and up-regulation of the autophagy cargo receptor p62. It was hypothesized that the cytoplasmic aggregates of NE proteins trigger signaling pathways that alter cellular homeostasis, causing muscle dysfunction. In support of this hypothesis, transcriptomics data from human muscle biopsy tissue revealed misregulation of the AMPK/4E-BP1/autophagy/proteostatic pathways. S6K mRNA levels were increased and AMPKalpha and mRNAs encoding downstream targets were decreased in muscles expressing mutant LMNA relative controls. The Drosophila laminopathy models were used to determine if altering the levels of these factors modulated muscle pathology. Muscle-specific over-expression of AMPKalpha and down-stream targets 4E-BP, Foxo and PGC1alpha, as well as inhibition of S6K, suppressed the held-up wing phenotype, myofibrillar defects, and LamC aggregation. These findings provide novel insights on mutant LMNA-based disease mechanisms and identify potential targets for drug therapy (Chandran, 2018).

Laminopathies are a collection of diseases caused by dominant mutations in the human LMNA gene encoding A-type lamins. Lamins are intermediate filaments that line the inner nuclear membrane where they provide structural support for the nucleus and regulate gene expression. Laminopathies include autosomal dominant Emery-Dreifuss muscular dystrophy (EDMD2, OMIM #181350), Limb-Girdle muscular dystrophy type 1B (LGMD1B, 159001), congenital muscular dystrophy (MDC, OMIM #613205), dilated cardiomyopathy type 1A (CMD1A, OMIM #115200), familial partial lipodystrophy type 2 (FPLD2, OMIM #151660) and early on-set aging syndromes such as Hutchinson-Gilford progeria syndrome (HGPS; OMIM #176670). It is unclear how LMNA mutations result in tissue-specific defects when mutant lamins are expressed in nearly all tissue. The pathogenic mechanisms of laminopathies are not well defined; hence, a greater understanding is needed to support the development of therapeutic interventions (Chandran, 2018).

Over 400 distinct mutations have been identified in the LMNA gene, among the highest number of mutations discovered in a single human gene. The majority of these are point mutations throughout the gene that give rise to single amino acid substitutions in lamins A and C, two isoforms derived from alternatively spliced LMNA messenger RNA (mRNA). Amino acid substitutions that give rise to skeletal muscular dystrophy are often accompanied by congenital muscular dystrophy (CMD). EDMD2 in particular is characterized by progressive muscle weakness, joint contractures and CMD with conduction defects. While much is known about the functions of lamins in the nucleus where they play a role in maintaining nuclear envelope (NE) integrity and organizing the genome, their functions in signaling pathways are becoming equally important with respect to disease mechanisms. For example, mutant lamins cause perturbations of the mammalian target of rapamycin (mTOR) pathway, which can be partially reversed with mTOR inhibitors such as rapamycin and temsiromilus. Genetic ablation of S6K1 (encoding ribosomal protein S6 protein kinase 1), a downstream substrate of mTOR, improved muscle function and extended lifespan of Lmna-/- mice. mTOR activity inversely correlates with the rate of autophagy, which plays a role in regulating nuclear-to-cytoplasmic transport and degradation of Lamin B1. Consistent with these findings, activation of autophagy suppressed cardiac laminopathy in a Drosophila model. Thus, regulation of the mTOR pathway is critical for muscle health in the context of laminopathies, however, which factors upstream and downstream of mTOR play key roles needed further investigation (Chandran, 2018).

To evaluate the role of TOR signaling and autophagy in lamin-associated muscle disease, Drosophila melanogaster (fruit fly) models of laminopathies were established. Drosophila models have proved to be powerful in defining the mechanistic basis of human disease, including muscle disorders associated with cytoskeletal defects. In addition, Drosophila models have been used to identify potential therapeutic targets for human aging disorders. Relevant for this study, Drosophila indirect flight muscle (IFM) models have been successfully used to define the molecular basis for muscle organization and disorganization. Importantly, expression of dominant negative (DN) mutants and knock-down (KD) of IFM-specific genes does not cause lethality in flies, allowing evaluation of pathophysiological aspects of progressive muscle degeneration without effects on the remainder of the organism. A dominant flightless phenotype with abnormal wing position provides powerful visual markers of defective IFM function. D. melanogaster, with its high degree of genome conservation to humans and manipulability through versatile genetic techniques, is an excellent model for understanding the molecular mechanisms of mutant lamin-induced skeletal muscle defects (Chandran, 2018).

The expression of D. melanogaster Lamin C (LamC) gene is developmentally regulated and nearly ubiquitously expressed, similar to the human LMNA gene. LamC shares amino acid sequence identity with human lamins A and C. Lamins have a conserved protein domain structure with a globular head, coiled-coil rod and a tail domain possessing an immunoglobulin-fold (Ig-fold). In addition, LamC localizes to the NE in all Drosophila tissues investigated including cardiac and larval body wall muscle tissue, supporting Drosophila as a useful model. Furthermore, the pathogenic genes and pathways described in this study are highly conserved between Drosophila and humans, offering the possibilities for the identification of conserved drug targets. The genetic and pharmacological manipulation of these pathways will provide mechanistic tests for potential skeletal muscle laminopathy therapies (Chandran, 2018).

To address the molecular basis of skeletal muscle laminopathies, mutations were made in Drosophila LamC analogous to those that cause muscle disease in humans. Muscle-specific expression of mutant LamC resulted in muscle functional defects that were accompanied by a plethora of cellular abnormalities including cytoplasmic aggregation of NE proteins. It was hypothesized that these cytoplasmic aggregates trigger signaling pathways and alter cellular and metabolic homeostasis, which results in muscle dysfunction. In support of this hypothesis and to reveal relevance to human pathology, transcriptomics data obtained from human muscle biopsy tissue showed misregulation of genes in the AMP-activated protein kinase (AMPK)/TOR/autophagy signaling pathways. Genetic manipulation of these pathways in Drosophila IFM suppressed the muscle defects, suggesting that misregulation of these pathways was causal to the muscle pathology. Overall, this analysis identified potential new therapeutic targets for lamin-associated skeletal myopathies and possibly other laminopathies (Chandran, 2018).

Although several hundred mutations in the LMNA gene have been identified and many studies have been performed on lamins, the pathogenic mechanisms of laminopathies remain not well understood. Greater insights are needed for therapeutic interventions. To address the molecular pathology of laminopathies and to understand the functions of lamins, Drosophila models of skeletal myopathies were developed. Mutations in Drosophila LamC were generated that are analogous to human LMNA mutations and expressed exclusively in the IFM, a muscle that produces a readily visible held-up wing phenotype upon muscle dysfunction (Chandran, 2018).

Three of the four LamC mutants examined in this study (R205W, G489V and V528P) caused severe muscle defects upon expression with Act88F and Fln Gal4 drivers (expressed before sarcomere assembly/maturation). In contrast, A177P caused only moderate functional defects when expressed with the same drivers, despite similar levels of LamC protein. These data demonstrate that the severity of the abnormal phenotypes is mutation-specific. This is similar to the human disease condition in which individuals with different LMNA mutations exhibit a wide range of disease severity depending on the location of the amino acid substitution. Expression of the mutant lamins after sarcomere assembly/maturation via the DJ-694 Gal4 driver resulted in only moderate functional defects. Taken together, these data suggested that mutant LamC interfered with sarcomere assembly/maturation. This idea was supported by TEM images showing disruption of sarcomere organization when using the Act88F and Fln Gal4 drivers. During sarcomere assembly, several proteins are produced de novo and presence of cytoplasmic LamC aggregates might interfere with the formation of multi-protein complexes that are associated with the contractile apparatus. It is also possible that cytoplasmic LamC aggregates interfere with proteostasis by sequestering chaperone proteins that facilitate protein folding following de novo synthesis. Loss of sarcomere structure and mitochondrial defects are known to cause the held-up wing phenotype and loss of flight. Both of these phenotypes are useful phenotypes for drug screens. The fact that this study identified mutation-specific variation in muscle disease severity further suggest that the Drosophila models will be useful for identifying modifier genes, which provide another level of complexity with regard to the range of disease severity observed in individuals, including family members with the same LMNA mutation (Chandran, 2018).

To better understand the molecular and cellular basis of the muscle pathology, an in-depth analysis of the Drosophila models was performed. Cytological analysis revealed cytoplasmic aggregates of LamC and nuclear pore proteins, nuclear blebbing, disruption of the cytoskeletal organization and mitochondrial morphology. During the natural aging process, accumulation of aggregates often results from defective proteostasis. Interestingly, protein aggregation in Huntington disease leads to amyloids that cause sarcomeric assembly defects due to loss of proteostasis. It is proposed that the abnormal accumulation of cytoplasmic NE protein aggregates leads to an impairment of proteostasis, causing loss of muscle function. These findings are consistent with cytoplasmic aggregation of NE proteins in muscle biopsies from individuals with skeletal muscle laminopathy. Thus, the results show that the IFM defects in the Drosophila models share characteristics with the human diseased muscle (Chandran, 2018).

To further define the pathological mechanisms of mutant LamC in skeletal muscle, the effects of mutant LamC on autophagy and metabolic signaling was examined. Ref(2)P, the Drosophila homologue of mammalian polyubiquitin binding protein p62, is up-regulated in IFM expressing mutant LamC relative to controls. Misregulation of nuclear factor (erythroid-derived 2)-like 2 (Nrf2)/Keap1 redox signaling mediated by p62 has also been associated with muscle atrophy and cardiomyopathy, and this pathway is predicted to influence autophagy. p62 is an adaptor protein that binds protein aggregates and targets them for autophagy and proteasome-based destruction. However, it is unknown how p62 influences laminopathy-mediated autophagic defects. Studies in mice show that loss of A-type lamins leads to cardiac and muscle defects due to alterations in mTOR signaling, which influences the rate of authophagy. Autophagy is responsible for the regulation of lamin B1 levels. Thus, it is possible that autophagy flux is impaired by NE protein aggregates. This might lead to the persistence of defective mitochondria, resulting in the up-regulation of the TOR pathway, which in turn contributes to the down-regulation of autophagy (Chandran, 2018).

Transcriptomic data from human laminopathy muscle allowed (1) obtaining unbiased insights into the gene expression profile of human diseased muscle, (2) comparing the data obtained with Drosophila to human diseased muscle to validate the use of the current models and (3) establishing the translational potential of the Drosophila models. Based upon the knowledge gained from the RNA sequencing of human muscle biopsy tissue, this study identified pathogenic pathways and then modulated those pathways using the rapid genetics offered by Drosophila. Through these genetic manipulations, it was possible to reduced (and eliminated in some cases) NE protein aggregation and alter intracellular signaling to ameliorate muscle dysfunction. Through modulation of AMPK, PGC1α, Foxo, S6K and 4E-BP, key players were identified that regulate autophagy in suppressing laminopathy-induced skeletal myopathy and mitochondrial dysmorphology. The pathway components identified might serve as valuable disease markers and provide new targets for the development of rational therapeutic strategies (Chandran, 2018).

Based on human muscle transcriptomics and genetic manipulations in Drosophila, this study has shown that activation of AMPK suppressed muscle laminopathy. AMPK is a sensor of cellular energy and metabolism that is linked to regulating autophagy, proteostasis and mitochondrial function. AMPK has conserved functions in many species, including Drosophila, and occurs universally as heterotrimeric complexes containing catalytic α-subunits and regulatory β-and γ-subunits. Increased expression of AMPK prevented age-related phenotypes in old mice, such as weight gain and decline of mitochondrial function. Activation of the AMPK pathway improved lamin-induced myopathy by removing abnormal aggregates, achieving autophagic and mitochondrial homeostasis. Consistent with these findings, the AMPK activator metformin lowered progerin (a specific mutant form of lamin A/C) levels and suppressed defects in the HGPS-induced pluripotent stem cell model (Chandran, 2018).

The data extend these findings by showing that the positive effects of AMPK activation are mainly through PGC1α, with contributions from Foxo, both of which maintain metabolic and cellular homeostasis. Previously, it was shown that Foxo/4E-BP signaling regulates age-induced proteostasis, including suppression of age-associated aggregation in skeletal muscle. As observed with rapamycin treatment in mouse models, activation of 4E-BP, a key downstream effector of the mTOR complex, is thought to reduce TOR activity. Muscle-specific expression of 4E-BP suppressed age-related protein aggregates and metabolic defects in Drosophila and mouse models. However, whole-body OE of 4E-BP1 shortened the lifespan of Lmna^-/- mice possibly by enhancing lipolysis. In the Drosophila IFM models, activation of S6K enhanced muscle deterioration and a DN version of S6K suppressed muscle dysfunction, presumably by activating autophagy as evidenced by the reduction of cytoplasmic aggregation of NE proteins. Overall, this study identified specific downstream targets of AMPK that suppress muscle laminopathy (Chandran, 2018).

Based upon these findings and those in the literature, a model is proposed that describes how cytoplasmic aggregates of NE proteins impact autophagy and signaling pathways and contribute to muscle pathology. According to this model, cytoplasmic aggregation of NE proteins lead to increased levels of Ref(2)P/p62, which bind to the protein aggregates. Accumulation of p62 causes up-regulation of the TOR pathway that leads to inhibition of autophagy in the skeletal muscle. Accumulation of Ref(2)P/p62 also causes up-regulation of the regulatory associated protein of MTOR complex 1 (RPTOR), which binds mTOR and inhibits autophagy. Thus, autophagy is down-regulated by two mechanisms, causing a disruption in proteostasis. Moreover, up-regulation of the mTOR pathway causes increased S6K activity, which leads to imbalance in energy homeostasis. Consistent with this model, the transcriptomic data from the laminopathy muscle biopsy tissue showed up-regulation of RPTOR and S6K, implying that autophagy is down-regulated. Also, in support of this model, KD of S6K in Drosophila IFM suppressed the muscle defects. Inhibition of autophagy is predicted to cause a reduction in AMPK activity. Consistent with this idea, all three AMPKα transcripts were down-regulated in the laminopathy muscle biopsy tissue. OE of AMPKα in Drosophila IFM suppressed the muscle defects. AMPK inactivation leads to the activation of PI3K/AKT/mTOR pathway, which was also up-regulated in the human muscle biopsy. Another important function of AMPK is to control the expression of genes involved in energy metabolism and aging by enhancing the activity of sirtuin 1 (SIRT1). SIRT1 controls the activity of downstream targets such as PGC-1α, the master regulator of mitochondrial biogenesis, and Foxo, which is involved in delaying the aging process, by reducing protein aggregation through controlling its target 4E-BP. The transcriptomic data showed that SIRT1 and downstream targets, PGC-1α, Foxo and 4E-BP, were down-regulated, which would cause an imbalance in cellular energy metabolism leading to cellular stress and compromising skeletal muscle function. In agreement, OE of dPGC-1, Foxo and 4E-BP in the Drosophila models suppressed the abnormal muscle phenotypes. Several kinases were up-regulated in the human muscle biopsy tissue. Up-regulation of these kinases has been observed in lamin-associated cardiomyopathy. Genetic modulation of these kinases is needed to test their effectiveness in suppressing muscle laminopathy and other lamin-based disorders (Chandran, 2018).

Overall, the data provide new insights on potential targets for small molecular screens. As proof-of-principle, dietary supplementation of rapamycin (TOR inhibitor) or 5-Aminoimidazole-4-carboxamide ribonucleotide (AICAR), activator of AMPK, in Drosophila media suppressed the mutant LamC-induced muscle defects. Drosophila allows evaluation of the effects of these compounds on functional and cellular defects caused by mutant LamC in the context of a whole organism. Promising compounds can then be tested in pre-clinical mouse laminopathy models. Given that lamin-associated muscular dystrophies have pathophysiological features shared with other laminopathies and diseases such as diabetes, these findings have the potential for broad impact (Chandran, 2018).

Increasing autophagy and blocking Nrf2 suppress laminopathy-induced age-dependent cardiac dysfunction and shortened lifespan

Mutations in the human LMNA gene cause a collection of diseases known as laminopathies. These include myocardial diseases that exhibit age-dependent penetrance of dysrhythmias and heart failure. The LMNA gene encodes A-type lamins, intermediate filaments that support nuclear structure and organize the genome. Mechanisms by which mutant lamins cause age-dependent heart defects are not well understood. This study modeled human disease-causing mutations in the Drosophila Lamin C gene and expressed mutant Lamin C exclusively in the heart. This resulted in progressive cardiac dysfunction, loss of adipose tissue homeostasis, and a shortened adult lifespan. Within cardiac cells, mutant Lamin C aggregated in the cytoplasm, the CncC(Nrf2)/Keap1 redox sensing pathway was activated, mitochondria exhibited abnormal morphology, and the autophagy cargo receptor Ref2(P)/p62 was upregulated. Simultaneous over-expression of the autophagy kinase Atg1 gene and an RNAi against CncC eliminated the cytoplasmic protein aggregates, restored cardiac function, and lengthened lifespan. These data suggest that simultaneously increasing rates of autophagy and blocking the Nrf2/Keap1 pathway are a potential therapeutic strategy for cardiac laminopathies (Bhide, 2018).

Mutations in the human LMNA gene are associated with a collection of diseases called laminopathies in which the most common manifestation is progressive cardiac disease. This study has generated Drosophila melanogaster models of age-dependent cardiac dysfunction. In these models, mutations synonymous with those causing disease in humans were introduced into Drosophila LamC. Cardiac-specific expression of mutant LamC resulted in (1) cardiac contractility, conduction, and physiological defects, (2) abnormal nuclear envelope morphology, (3) cytoplasmic LamC aggregation, (4) nuclear enrichment of the redox transcriptional regulator CncC (mammalian Nrf2), (5) and upregulation of autophagy cargo receptor Ref(2)P (mammalian p62). These cardiac defects were enhanced with age and accompanied by increased adipose tissue in the adult fat bodies and a shortened lifespan (Bhide, 2018).

To understand the mechanistic basis of cardiolaminopathy and identify genetic suppressors, advantage was taken of powerful genetic tools available in Drosophila. The presence of cytoplasmic LamC aggregates prompted a determination of whether increasing autophagy would suppress the cardiac defects. Cardiac-specific upregulation of autophagy (Atg1 OE) suppressed G489V-induced cardiac defects. Consistent with this, decreased autophagy due to expression of Atg1 DN resulted in enhanced deterioration of G489V-induced cardiac dysfunction. Interestingly, cardiac-specific Atg5 OE and Atg8a OE, two factors that also promote autophagy, showed little to no suppression of G489V-induced heart dysfunction, suggesting that Atg1 might be rate limiting in this context. These findings are consistent with studies in mouse laminopathy models in which rapamycin and temsirolimus had beneficial effects on heart and skeletal muscle through inhibition of AKT/mTOR signaling. These findings are depicted in a model (see Model for the interactions between the autophagy and CncC/Keap1 signaling pathway in mutant lamin-induced cardiac disease) in which cytoplasmic aggregation of mutant LamC results in upregulation of p62, which in turn inhibits autophagy via activation of TOR and inactivation of AMPK. AMPK inactivation leads to the activation of PI3K/Akt/mTOR pathway and inhibition of autophagy Atg1 OE promoted clearance of the LamC aggregates and restored proteostasis in these Drosophila models. Thus, the data suggest that mutant LamC reduces autophagy, resulting in impairment of cellular proteostasis that leads to cardiac dysfunction (Bhide, 2018).

Cardiac-specific expression of mutant LamC altered CncC subcellular localization. Previously, Drosophila larval body wall muscles expressing G489V were shown to experience reductive stress, an atypical redox state characterized by high levels of reduced glutathione and NADPH, and upregulation CncC target genes (Dialynas, 2015). Cardiac-specific CncC RNAi in the wild-type LamC background did not produce major cardiac defects. Consistent with this, Nrf2 deficiency in mice does not compromise cardiac and skeletal muscle performance. Cardiac-specific CncC RNAi suppressed G489V-induced cardiac dysfunction and reduced cytoplasmic LamC aggregation, but not R205W-induced defects. However, cardiac-specific RNAi against CncC did not affect G489V-induced adipose tissue accumulation and lifespan shortening. Similar to the nuclear enrichment of CncC in hearts expressing G489V, human muscle biopsy tissue from an individual with a point mutation in the LMNA gene that results in G449V (analogous to Drosophila G489V) showed nuclear enrichment of Nrf2 (Dialynas, 2015). Disruption of Nrf2/Keap1 signaling has also been reported for Hutchinson-Gilford progeria, an early-onset aging disease caused by mutations in LMNA. In this case, however, the thickened nuclear lamina traps Nrf2 at the nuclear envelope that results in a failure to activate Nrf2 target genes, leading to oxidative stress. In these studies, CncC nuclear enrichment was observed; however, a redox imbalance was not readily observed at the three-time points investigated. This might indicate that there is a window of time in disease progression in which redox imbalance occurs and that mechanisms are in place to re-establish homeostasis (Bhide, 2018).

It has been postulated that there is cross-talk between autophagy and Nrf2/Keap1 signaling. This was tested by manipulating autophagy and CncC (Nrf2) alone and in combination. CncC RNAi suppressed the cardiac defects caused by G489V, but not the lipid accumulation and lifespan shortening, suggesting the latter two phenotypes are not specifically due to loss of cardiac function. In contrast, Atg1 OE suppressed the cardiac and adipose tissue defects and lengthened the lifespan. The double treatment (simultaneous Atg1 OE and RNAi knockdown of CncC) gave the most robust suppression of the mutant phenotypes and completely restored the lifespan. Interestingly, Atg1 DN and RNAi knockdown of CncC simultaneously did not further deteriorate or improve the mutant phenotypes. Taken together, these data suggest that autophagy plays a key role in suppression of the G498V-induced phenotypes and that knockdown on CncC enhances this suppression (Bhide, 2018).

These findings support a model whereby autophagy and Nrf2 signaling are central to cardiac health. It is proposed that cytoplasmic aggregation of LamC increases levels of Ref(2)P (p62), which competitively binds to Keap1, resulting in CncC (Nrf2) translocation to the nucleus. Inside the nucleus, Nrf2 regulates genes involved in detoxification. Continued expression of antioxidant genes results in the disruption of redox homeostasis, defective mitochondria, and dysregulation of energy homeostasis/energy sensor such as AMPK and its downstream targets. Simultaneously, upregulation of Ref(2)P (p62) causes inhibition of autophagy via activation of TOR, which leads to the inactivation of AMPK. AMPK inactivation in combination with activation of the TOR pathway causes cellular and metabolic stress that leads to cardiomyopathy. In support of this model, transcriptomics data from muscle tissue of an individual with muscular dystrophy expressing Lamin A/C G449V (analogous to Drosophila G489V) showed (1) upregulation of transcripts from Nrf2 target genes, (2) upregulation of genes encoding subunits of the mTOR complex, and (3) downregulation of AMPK, further demonstrating relevance of the Drosophila model for providing insights on human pathology (Bhide, 2018).

Atf3 links loss of epithelial polarity to defects in cell differentiation and cytoarchitecture

Interplay between apicobasal cell polarity modules and the cytoskeleton is critical for differentiation and integrity of epithelia. However, this coordination is poorly understood at the level of gene regulation by transcription factors. This study establish the Drosophila activating transcription factor 3 (atf3) as a cell polarity response gene acting downstream of the membrane-associated Scribble polarity complex. Loss of the tumor suppressors Scribble or Dlg1 induces atf3 expression via aPKC but independent of Jun-N-terminal kinase (JNK) signaling. Strikingly, removal of Atf3 from Dlg1 deficient cells restores polarized cytoarchitecture, levels and distribution of endosomal trafficking machinery, and differentiation. Conversely, excess Atf3 alters microtubule network, vesicular trafficking and the partition of polarity proteins along the apicobasal axis. Genomic and genetic approaches implicate Atf3 as a regulator of cytoskeleton organization and function, and identify Lamin C as one of its bona fide target genes. By affecting structural features and cell morphology, Atf3 functions in a manner distinct from other transcription factors operating downstream of disrupted cell polarity (Donohoe, 2018).

High expression of A-type lamin in the leading front is required for Drosophila thorax closure

Tissue closure involves the coordinated unidirectional movement of a group of cells without loss of cell-cell contact. However, the molecular mechanisms controlling the tissue closure are not fully understood. This study demonstrates that Lamin C, the sole A-type lamin in Drosophila, contributes to the process of thorax closure in pupa. High expression of Lamin C was observed at the leading front of the migrating wing imaginal discs. Live imaging analysis revealed that knockdown of Lamin C in the thorax region affected the coordinated movement of the leading front, resulting in incomplete tissue fusion required for formation of the adult thorax. The closure defect due to knockdown of Lamin C correlated with insufficient accumulation of F-actin at the front. This study indicates a link between A-type lamin and the cell migration behavior during tissue closure (Kosakamoto, 2018).

Overexpression of the lamina proteins Lamin and Kugelkern induces specific ultrastructural alterations in the morphology of the nuclear envelope of intestinal stem cells and enterocytes

The nuclear envelope has a stereotypic morphology consisting of a flat double layer of the inner and outer nuclear membrane, with interspersed nuclear pores. Underlying and tightly linked to the inner nuclear membrane is the nuclear lamina, a proteinous layer of intermediate filament proteins and associated proteins. Physiological, experimental or pathological alterations in the constitution of the lamina lead to changes in nuclear morphology, such as blebs and lobulations. It has so far remained unclear whether the morphological changes depend on the differentiation state and the specific lamina protein. This study analysed the ultrastructural morphology of the nuclear envelope in intestinal stem cells and differentiated enterocytes in adult Drosophila flies, in which the proteins Lam, Kugelkern or a farnesylated variant of LamC were overexpressed. Surprisingly, distinct morphological features specific for the respective protein were detected. Lam induced envelopes with multiple layers of membrane and lamina, surrounding the whole nucleus whereas farnesylated LamC induced the formation of a thick fibrillary lamina. In contrast, Kugelkern induced single-layered and double-layered intranuclear membrane structures, which are likely be derived from infoldings of the inner nuclear membrane or of the double layer of the envelope (Petrovsky, 2018).

Coordinated increase of nuclear tension and lamin-A with matrix stiffness outcompetes lamin-B receptor that favors soft tissue phenotypes

Matrix stiffness that is sensed by a cell or measured by a purely physical probe reflects the intrinsic elasticity of the matrix and also how thick or thin the matrix is. In this study, mesenchymal stem cells (MSCs) and their nuclei spread in response to thickness-corrected matrix microelasticity, with increases in nuclear tension and nuclear stiffness resulting from increases in myosin-II and lamin-A,C. Linearity between the widely varying projected area of a cell and its nucleus across many matrices, timescales, and myosin-II activity levels indicates a constant ratio of nucleus-to-cell volume, despite MSCs' lineage plasticity. Nuclear envelope fluctuations are suppressed on the stiffest matrices, and fluctuation spectra reveal a high nuclear tension that matches trends from traction force microscopy and from increased lamin-A,C. Transcriptomes of many diverse tissues and MSCs further show that lamin-A,C's increase with tissue or matrix stiffness anti-correlates with lamin-B receptor (LBR), which contributes to lipid/sterol biosynthesis. Adipogenesis (a soft lineage) indeed increases LBR:lamin-A,C protein stoichiometry in MSCs versus osteogenesis (stiff). The two factors compete for lamin-B in response to matrix elasticity, knockdown, myosin-II inhibition, and even constricted migration that disrupts and segregates lamins in situ. Matrix stiffness-driven contractility thus tenses the nucleus to favor lamin-A,C accumulation and suppress soft tissue phenotypes (Buxboim, 2017).

Lamin A/C sustains PcG protein architecture, maintaining transcriptional repression at target genes

Beyond its role in providing structure to the nuclear envelope, lamin A/C is involved in transcriptional regulation. However, its cross talk with epigenetic factors--and how this cross talk influences physiological processes--is still unexplored. Key epigenetic regulators of development and differentiation are the Polycomb group (PcG) of proteins, organized in the nucleus as microscopically visible foci. This study shows that lamin A/C, examined both in Drosophila and vertebrates, is evolutionarily required for correct PcG protein nuclear compartmentalization. Confocal microscopy supported by new algorithms for image analysis reveals that lamin A/C knock-down leads to PcG protein foci disassembly and PcG protein dispersion. This causes detachment from chromatin and defects in PcG protein-mediated higher-order structures, thereby leading to impaired PcG protein repressive functions. Using myogenic differentiation as a model, it was found that reduced levels of lamin A/C at the onset of differentiation led to an anticipation of the myogenic program because of an alteration of PcG protein-mediated transcriptional repression. Collectively, these results indicate that lamin A/C can modulate transcription through the regulation of PcG protein epigenetic factors (Cesarubum 2015).

Myopathic lamin mutations cause reductive stress and activate the nrf2/keap-1 pathway

Mutations in the human LMNA gene cause muscular dystrophy by mechanisms that are incompletely understood. The LMNA gene encodes A-type lamins, intermediate filaments that form a network underlying the inner nuclear membrane, providing structural support for the nucleus and organizing the genome. To better understand the pathogenesis caused by mutant lamins, this study performed a structural and functional analysis on LMNA missense mutations identified in muscular dystrophy patients. These mutations perturb the tertiary structure of the conserved A-type lamin Ig-fold domain. To identify the effects of these structural perturbations on lamin function, these mutations were modeled in Drosophila Lamin C and the mutant lamins were expressed in muscle. It was found that the structural perturbations have minimal dominant effects on nuclear stiffness, suggesting that the muscle pathology is not accompanied by major structural disruption of the peripheral nuclear lamina. However, subtle alterations in the lamina network and subnuclear reorganization of lamins remain possible. Affected muscles have cytoplasmic aggregation of lamins and additional nuclear envelope proteins. Transcription profiling revealed upregulation of many Nrf2 target genes. Nrf2 is normally sequestered in the cytoplasm by Keap-1. Under oxidative stress Nrf2 dissociates from Keap-1, translocates into the nucleus, and activates gene expression. Unexpectedly, biochemical analyses revealed high levels of reducing agents, indicative of reductive stress. The accumulation of cytoplasmic lamin aggregates correlates with elevated levels of the autophagy adaptor p62/SQSTM1, which also binds Keap-1, abrogating Nrf2 cytoplasmic sequestration, allowing Nrf2 nuclear translocation and target gene activation. Elevated p62/SQSTM1 and nuclear enrichment of Nrf2 were identified in muscle biopsies from the corresponding muscular dystrophy patients, validating the disease relevance of the Drosophila model. Thus, novel connections were made between mutant lamins and the Nrf2 signaling pathway, suggesting new avenues of therapeutic intervention that include regulation of protein folding and metabolism, as well as maintenance of redox homoeostasis (Dialynas, 2015).

LMNA variants cause cytoplasmic distribution of nuclear pore proteins in Drosophila and human muscle

Mutations in the human LMNA gene, encoding A-type lamins, give rise to laminopathies, which include several types of muscular dystrophy. Heterozygous sequence variants in LMNA, which result in single amino-acid substitutions, were identified in patients exhibiting muscle weakness. To assess whether the substitutions altered lamin function, in vivo analyses was performed using a Drosophila model. Stocks were generated that expressed mutant forms of the Drosophila A-type lamin modeled after each variant. Larvae were used for motility assays and histochemical staining of the body-wall muscle. In parallel, immunohistochemical analyses were performed on human muscle biopsy samples from the patients. In control flies, muscle-specific expression of the wild-type A-type lamin had no apparent affect. In contrast, expression of the mutant A-type lamins caused dominant larval muscle defects and semi-lethality at the pupal stage. Histochemical staining of larval body wall muscle revealed that the mutant A-type lamin, B-type lamins, the Sad1p, UNC-84 domain protein Klaroid and nuclear pore complex proteins were mislocalized to the cytoplasm. In addition, cytoplasmic actin filaments were disorganized, suggesting links between the nuclear lamina and the cytoskeleton were disrupted. Muscle biopsies from the patients showed dystrophic histopathology and architectural abnormalities similar to the Drosophila larvae, including cytoplasmic distribution of nuclear envelope proteins. These data provide evidence that the Drosophila model can be used to assess the function of novel LMNA mutations and support the idea that loss of cellular compartmentalization of nuclear proteins contributes to muscle disease pathogenesis (Dialynas, 2012).

The role of Drosophila Lamin C in muscle function and gene expression

The inner side of the nuclear envelope (NE) is lined with lamins, a meshwork of intermediate filaments that provides structural support for the nucleus and plays roles in many nuclear processes. Lamins, classified as A- or B-types on the basis of biochemical properties, have a conserved globular head, central rod and C-terminal domain that includes an Ig-fold structural motif. In humans, mutations in A-type lamins give rise to diseases that exhibit tissue-specific defects, such as Emery-Dreifuss muscular dystrophy. Drosophila is being used as a model to determine tissue-specific functions of A-type lamins in development, with implications for understanding human disease mechanisms. The GAL4-UAS system was used to express wild-type and mutant forms of Lamin C (the presumed Drosophila A-type lamin), in an otherwise wild-type background. Larval muscle-specific expression of wild type Drosophila Lamin C caused no overt phenotype. By contrast, larval muscle-specific expression of a truncated form of Lamin C lacking the N-terminal head (Lamin C DeltaN) caused muscle defects and semi-lethality, with adult 'escapers' possessing malformed legs. The leg defects were due to a lack of larval muscle function and alterations in hormone-regulated gene expression. The consequences of Lamin C association at a gene were tested directly by targeting a Lamin C DNA-binding domain fusion protein upstream of a reporter gene. Association of Lamin C correlated with localization of the reporter gene at the nuclear periphery and gene repression. These data demonstrate connections among the Drosophila A-type lamin, hormone-induced gene expression and muscle function (Dialynas, 2010).

Loss of Drosophila A-type lamin C initially causes tendon abnormality including disintegration of cytoskeleton and nuclear lamina in muscular defects

Lamins are the major components of nuclear envelope architecture, being required for both the structural and informational roles of the nuclei. Mutations of lamins cause a spectrum of diseases in humans, including muscular dystrophy. This study reports that the loss of the A-type lamin gene, lamin C in Drosophila resulted in pupal metamorphic lethality caused by tendon defects, matching the characteristics of human A-type lamin revealed by Emery-Dreifuss muscular dystrophy (EDMD). In tendon cells lacking lamin C activity, overall cell morphology was affected and organization of the spectraplakin family cytoskeletal protein Shortstop which is prominently expressed in tendon cells gradually disintegrated, notably around the nucleus and in a manner correlating well with the degradation of musculature. Furthermore, lamin C null mutants were efficiently rescued by restoring lamin C expression to shortstop-expressing cells, which include tendon cells but exclude skeletal muscle cells. Thus the critical function of A-type lamin C proteins in Drosophila musculature is to maintain proper function and morphology of tendon cells (Uchino, 2013).

The different function of single phosphorylation sites of Drosophila melanogaster lamin Dm and lamin C

Lamin functions are regulated by phosphorylation at specific sites but understanding of the role of such modifications is practically limited to the function of cdc 2 (cdk1) kinase sites in depolymerization of the nuclear lamina during mitosis. This study used Drosophila lamin Dm (B-type) to examine the function of particular phosphorylation sites using pseudophosphorylated mutants mimicking single phosphorylation at experimentally confirmed in vivo phosphosites (S(25)E, S(45)E, T(435)E, S(595)E). Lamin C (A-type) was also examined and its mutant S(37)E representing the N-terminal cdc2 (mitotic) site as well as lamin Dm R(64)H mutant as a control, non-polymerizing lamin. In the polymerization assay different effects of N-terminal cdc2 site pseudophosphorylation were observed on A- and B-type lamins: lamin Dm S(45)E mutant was insoluble, in contrast to lamin C S(37)E. Lamin Dm T(435)E (C-terminal cdc2 site) and R(64)H were soluble in vitro. It was also confirmed that none of the single phosphorylation site modifications affected the chromatin binding of lamin Dm, in contrast to the lamin C N-terminal cdc2 site. In vivo, all lamin Dm mutants were incorporated efficiently into the nuclear lamina in transfected Drosophila S2 and HeLa cells, although significant amounts of S(45)E and T(435)E were also located in cytoplasm. When farnesylation incompetent mutants were expressed in HeLa cells, lamin Dm T(435)E was cytoplasmic and showed higher mobility in FRAP assay (Zaremba-Czogalla, 2012).

Lamin C and chromatin organization in Drosophila

Drosophila lamin C (LamC) is a developmentally regulated component of the nuclear lamina. The lamC gene is situated in the fifth intron of the essential gene tout velu (ttv). This study carried out genetic analysis of lamC during development. Phenotypic analyses of RNAi-mediated downregulation of lamC expression as well as targeted misexpression of lamin C suggest a role for lamC in cell survival. Of particular interest in the context of laminopathies is the caspase-dependent apoptosis induced by the overexpression of lamin C. Interestingly, misexpression of lamin C in the central nervous system, where it is not normally expressed, did not affect organization of the nuclear lamina. lamC mutant alleles suppressed position effect variegation normally displayed at near-centromeric and telomeric regions. Further, both downregulation and misexpression of lamin C affected the distribution of heterochromatin protein 1. These results suggest that Drosophila lamC has a tissue-specific role during development and is required for chromatin organization (Gurudata, 2010).

A-type and B-type lamins initiate layer assembly at distinct areas of the nuclear envelope in living cells

To investigate nuclear lamina re-assembly in vivo, Drosophila A-type and B-type lamins were artificially expressed in Drosophila lamin Dm(0) null mutant brain cells. Both exogenous lamin C (A-type) and Dm(0) (B-type) formed sub-layers at the nuclear periphery, and efficiently reverted the abnormal clustering of the NPC. Lamin C initially appeared where NPCs were clustered, and subsequently extended along the nuclear periphery accompanied by the recovery of the regular distribution of NPCs. In contrast, lamin Dm(0) did not show association with the clustered NPCs during lamina formation and NPC spacing recovered only after completion of a closed lamin Dm(0) layer. Further, when lamin Dm(0) and C were both expressed, they did not co-polymerize, initiating layer formation in separate regions. Thus, A and B-type lamins reveal differing properties during lamina assembly, with A-type having the primary role in organizing NPC distribution. This previously unknown complexity in the assembly of the nuclear lamina could be the basis for intricate nuclear envelope functions (Furukawa, 2009).

A comparative study of Drosophila and human A-type lamins

Nuclear intermediate filament proteins, called lamins, form a meshwork that lines the inner surface of the nuclear envelope. Lamins contain three domains: an N-terminal head, a central rod and a C-terminal tail domain possessing an Ig-fold structural motif. Lamins are classified as either A- or B-type based on structure and expression pattern. The Drosophila genome possesses two genes encoding lamins, Lamin C and lamin Dm(0), which have been designated A- and B-type, respectively, based on their expression profile and structural features. In humans, mutations in the gene encoding A-type lamins are associated with a spectrum of predominantly tissue-specific diseases known as laminopathies. Linking the disease phenotypes to cellular functions of lamins has been a major challenge. Drosophila is being used as a model system to identify the roles of lamins in development. Towards this end, a comparative study of Drosophila and human A-type lamins was performed. Analysis of transgenic flies showed that human lamins localize predictably within the Drosophila nucleus. Consistent with this finding, yeast two-hybrid data demonstrated conservation of partner-protein interactions. Drosophila lacking A-type lamin show nuclear envelope defects similar to those observed with human laminopathies. Expression of mutant forms of the A-type Drosophila lamin modeled after human disease-causing amino acid substitutions revealed an essential role for the N-terminal head and the Ig-fold in larval muscle tissue. This tissue-restricted sensitivity suggests a conserved role for lamins in muscle biology. In conclusion, this study has shown that (1) localization of A-type lamins and protein-partner interactions are conserved between Drosophila and humans, (2) loss of the Drosophila A-type lamin causes nuclear defects and (3) muscle tissue is sensitive to the expression of mutant forms of A-type lamin modeled after those causing disease in humans. These studies provide new insights on the role of lamins in nuclear biology and support Drosophila as a model for studies of human laminopathies involving muscle dysfunction (Schulze, 2009).

Molecular genetic analysis of the nested Drosophila melanogaster lamin C gene

Lamins are intermediate filaments that line the inner surface of the nuclear envelope, providing structural support and making contacts with chromatin. There are two types of lamins, A- and B-types, which differ in structure and expression. Drosophila possesses both lamin types, encoded by the LamC (A-type) and lamin Dm0 (B-type) genes. LamC is nested within an intron of the essential gene ttv. This study demonstrates that null mutations in LamC are lethal, and expression of a wild-type LamC transgene rescues lethality of LamC but not ttv mutants. Mutations in the human A-type lamin gene lead to diseases called laminopathies. To determine if Drosophila might serve as a useful model to study lamin biology and disease mechanisms, transgenic flies were generated expressing mutant LamC proteins modeled after human disease-causing lamins. These transgenic animals display a nuclear lamin aggregation phenotype remarkably similar to that observed when human mutant A-type lamins are expressed in mammalian cells. LamC aggregates also cause disorganization of lamin Dm0, indicating interdependence of both lamin types for proper lamina assembly. Taken together, these data provide the first detailed genetic analysis of the LamC gene and support using Drosophila as a model to study the role of lamins in disease (Schulze, 2005).

Ectopic overexpression of Drosophila lamin C is stage-specific lethal

To gain insight into the function of the developmentally regulated A-type lamins Drosophila was transformed with a construct containing the hsp70 promoter followed by the Drosophila lamin C (an analog of vertebrate A-type lamins) cDNA. Lamin C is expressed ectopically after heat shock of embryos and localizes to the nucleus. In embryos that normally do not contain lamin C, no phenotypic change is observed after lamin C expression. However, ectopic expression of lamin C during most larval (but not pupal) stages stalls growth, inhibits ecdysteroid signaling (in particular during the larval-prepupal transition), results in the development of melanotic tumors, and ultimately causes death. During pupation in control animals, when massive apoptosis of larval tissues takes place, lamin C is proteolyzed into a fragment with a size similar to that predicted by caspase cleavage. The ectopically expressed lamin C is identically cleaved, resulting in a large increase of the steady-state level of the lamin C fragment. A null mutation of the dcp-1 gene, one of the two known Drosophila caspase genes, also results in development of melanotic tumors and larval death, suggesting that the ectopically expressed lamin C inhibits apoptosis through competitive inhibition of caspase activity (Stuurman, 1999).

Expression of Drosophila lamin C is developmentally regulated: analogies with vertebrate A-type lamins

Invertebrates have long been thought to contain only a single lamin, which in Drosophila is the well-characterized Lamin. However, recently a Drosophila cDNA clone (pG-IF) has been identified that codes for an intermediate filament protein that harbors a nuclear localization signal but lacks a carboxy-terminal CaaX motif. Based on these data the putative protein encoded by pG-IF was tentatively called Drosophila Lamin C. To address whether the pG-IF encoded protein is expressed and whether it encodes a cytoplasmic intermediate filament protein or a nuclear lamin, antibodies were raised against the recombinant pG-IF protein. The antibodies decorate the nuclear envelope in Drosophila Kc tissue culture cells as well as in salivary and accessory glands, demonstrating that pG-IF encodes a nuclear lamin (Lamin C). Antibody decoration, in situ hybridization, western and northern blotting studies show that Lamin C is acquired late in embryogenesis. In contrast, Lamin is constitutively expressed. Lamin C is first detected in late stage 12 embryos in oenocytes, hindgut and posterior spiracles and subsequently also in other differentiated tissues. In third instar larvae Lamin C and Lamin are coexpressed in all tissues tested. Thus, Drosophila has two lamins: Lamin, containing a CaaX motif, is expressed throughout, while Lamin C, lacking a CaaX motif, is expressed only later in development. Expression of Drosophila Lamin C is similar to that of vertebrate lamin A, which loses its CaaX motif during incorporation into the lamina (Riemer, 1995).

A cDNA from Drosophila melanogaster encodes a lamin C-like intermediate filament protein

A novel intermediate filament cDNA, pG-IF, has been isolated from a Drosophila melanogaster embryonic expression library screened with a polyclonal antiserum produced against a 46 kDa cytoskeletal protein isolated from Kc cells. This 46 kDa protein is known to be immunologically related to vertebrate intermediate filament proteins. The screen resulted in the isolation of four different cDNA groups. Of these, one has been identified as the previously characterized Drosophila nuclear lamin cDNA, Dm0, and a second, pG-IF, demonstrates homology to Dm0 by cross hybridization on Southern blots. DNA sequence analysis reveals that pG-IF encodes a newly identified intermediate filament protein in Drosophila. Its nucleotide sequence is highly homologous to nuclear lamins with lower homology to cytoplasmic intermediate filament proteins. pG-IF predicts a protein of 621 amino acids with a predicted molecular mass of 69,855 daltons. In vitro transcription and translation of pG-IF yields a protein with a SDS-PAGE estimated molecular weight of approximately 70 kDa. It contains sequence principles characteristic of class V intermediate filament proteins. Its near neutral pI (6.83) and the lack of a terminal CaaX motif suggests that it may represent a lamin C subtype in Drosophila. In situ hybridization to polytene chromosomes detects one band of hybridization on the right arm of chromosome 2 at or near 51A. This in conjunction with Southern blot analysis of various genomic digests suggests one or more closely placed genes while Northern blot analysis detects two messages in Kc cells (Bossie, 1993).

Search PubMed for articles about Drosophila Lamin C

Amiad-Pavlov, D., Lorber, D., Bajpai, G., Reuveny, A., Roncato, F., Alon, R., Safran, S. and Volk, T. (2021). Live imaging of chromatin distribution reveals novel principles of nuclear architecture and chromatin compartmentalization. Sci Adv 7(23). PubMed ID: 34078602

Aureille, J., Belaadi, N. and Guilluy, C. (2017). Mechanotransduction via the nuclear envelope: a distant reflection of the cell surface. Curr Opin Cell Biol 44: 59-67. PubMed ID: 27876470

Bhide, S., Trujillo, A. S., O'Connor, M. T., Young, G. H., Cryderman, D. E., Chandran, S., Nikravesh, M., Wallrath, L. L. and Melkani, G. C. (2018). Increasing autophagy and blocking Nrf2 suppress laminopathy-induced age-dependent cardiac dysfunction and shortened lifespan. Aging Cell 17(3): e12747. PubMed ID: 29575479

Bossie, C. A. and Sanders, M. M. (1993). A cDNA from Drosophila melanogaster encodes a lamin C-like intermediate filament protein. J. Cell Sci. 104 ( Pt 4): 1263-72. PubMed Citation:

Buxboim, A., Irianto, J., Swift, J., Athirasala, A., Shin, J. W., Rehfeldt, F. and Discher, D. E. (2017). Coordinated increase of nuclear tension and lamin-A with matrix stiffness outcompetes lamin-B receptor that favors soft tissue phenotypes. Mol Biol Cell 28(23): 3333-3348. PubMed ID: 28931598

Chandran, S., Suggs, J. A., Wang, B. J., Han, A., Bhide, S., Cryderman, D. E., Moore, S. A., Bernstein, S. I., Wallrath, L. L. and Melkani, G. C. (2018). Suppression of myopathic lamin mutations by muscle-specific activation of AMPK and modulation of downstream signaling. Hum Mol Genet. PubMed ID: 30239736

Cesarini, E., Mozzetta, C., Marullo, F., Gregoretti, F., Gargiulo, A., Columbaro, M., Cortesi, A., Antonelli, L., Di Pelino, S., Squarzoni, S., Palacios, D., Zippo, A., Bodega, B., Oliva, G. and Lanzuolo, C. (2015). Lamin A/C sustains PcG protein architecture, maintaining transcriptional repression at target genes. J Cell Biol 211(3): 533-551. PubMed ID: 26553927

Dahl, K. N., Ribeiro, A. J. and Lammerding, J. (2008). Nuclear shape, mechanics, and mechanotransduction. Circ Res 102(11): 1307-1318. PubMed ID: 18535268

Dialynas, G., Speese, S., Budnik, V., Geyer, P. K. and Wallrath, L. L. (2010). The role of Drosophila Lamin C in muscle function and gene expression. Development 137(18): 3067-3077. PubMed ID: 20702563

Dialynas, G., Flannery, K. M., Zirbel, L. N., Nagy, P. L., Mathews, K. D., Moore, S. A. and Wallrath, L. L. (2012). LMNA variants cause cytoplasmic distribution of nuclear pore proteins in Drosophila and human muscle. Hum Mol Genet 21(7): 1544-1556. PubMed ID: 22186027

Dialynas, G., Shrestha, O.K., Ponce, J.M., Zwerger, M., Thiemann, D.A., Young, G.H., Moore, S.A., Yu, L., Lammerding, J. and Wallrath, L.L. (2015). Myopathic lamin mutations cause reductive stress and activate the nrf2/keap-1 pathway. PLoS Genet 11: e1005231. PubMed ID: 25996830

Donohoe, C. D., Csordas, G., Correia, A., Jindra, M., Klein, C., Habermann, B. and Uhlirova, M. (2018). Atf3 links loss of epithelial polarity to defects in cell differentiation and cytoarchitecture. PLoS Genet 14(3): e1007241. PubMed ID: 29494583

Furukawa, K., Ishida, K., Tsunoyama, T. A., Toda, S., Osoda, S., Horigome, T., Fisher, P. A. and Sugiyama, S. (2009). A-type and B-type lamins initiate layer assembly at distinct areas of the nuclear envelope in living cells. Exp Cell Res 315(7): 1181-1189. PubMed ID: 19210986

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date revised: 22 February 2022

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