InteractiveFly: GeneBrief

Otefin: | References

BIOLOGICAL OVERVIEW

Nuclear envelope proteins play important roles in chromatin organization, gene regulation, and signal transduction; however, the physiological role of these proteins remains elusive. This study found that otefin (ote), which encodes a nuclear lamin, is essential for germline stem cell (GSC) maintenance. Ote, as an intrinsic factor, is both necessary and sufficient to regulate GSC fate. Furthermore, ote is required for the Dpp/BMP signaling pathway to silence bam transcription. By structure-function analysis, it was demonstrated that the nuclear membrane localization of Ote is essential for its role in GSC maintenance. Ote physically interacts with Medea/Smad4 at the bam silencer element to regulate GSC fate. Thus, this study demonstrates that specific nuclear membrane components mediate signal-dependent transcriptional effects to control stem cell behavior (Jiang, 2008).

In adult tissues, stem cells are characterized by their unique capacity to produce daughter stem cells for self-renewal as well as differentiated daughter cells for maintaining homeostasis. Understanding how the self-renewal and differentiation processes of stem cells are controlled will not only reveal the fundamental biological mechanisms that govern the formation and maintenance of tissues, but may also influence future stem cell-based therapies for regenerative medicine (Jiang, 2008).

The Drosophila ovarian germline stem cells (GSCs) within the germarium region provide an attractive system to study the regulatory mechanisms that determine stem cell fate. A typical Drosophila ovary is composed of 16-20 ovarioles, each consisting of an anterior functional unit called a germarium and a linear string of differentiated egg chambers posterior to the germarium. In the tip of the germarium, GSCs normally divide asymmetrically to ensure that one daughter remains attached to the stromal somatic cap cells (or niche cells) for self-renewal. The remaining daughter cell is displaced from the niche and becomes a cystoblast (CB), which initiates differentiation and sustains oogenesis. During this process, one gene, bag-of-marbles (bam), has been shown to act autonomously in the germline to play an instructive role in CB differentiation. In contrast, gene products, such as Piwi and Dpp, a homolog of BMP2/4 in mammals, are produced from niche cells; however, they function as maintenance factors for GSC self-renewal. It has been shown that Dpp signaling from stromal cells activates Smad signaling in GSCs, directly silences bam transcription, and blocks the formation of Bam:Bgcn complexes that would otherwise antagonize translational repression. However, the issue of how Dpp/Smad signaling is transduced in the nucleus and acts especially at the bam silencer element to repress bam transcription remains poorly understood (Jiang, 2008).

The nuclear envelope separates the nucleoplasm from cytoplasm and is composed of outer and inner membranes that are separated by the perinuclear space and joined at nuclear pore complexes. The nuclear lamina is a network of lamin polymers and lamin-associated proteins that are embedded in the inner membrane (Gruenbaum, 2005). Increasing evidence indicates that these nuclear membrane proteins play important roles in chromatin organization, gene regulation, and signal transduction at the cellular level. However, the physiological roles of these proteins remain elusive. Otefin (Ote) is one member of the 'LEM' family, which represents an important group of nuclear membrane-associated proteins that share a conserved LEM domain (Wagner, 2007). Previous studies have shown that Ote physically interacted with lamin B and YA proteins and localized at the nuclear envelope (Ashery-Padan, 1997a; Ashery-Padan, 1997b; Goldberg, 1998). Although inhibition of lamin activity by anti-lamin antibody prevented nuclear assembly in vitro, RNAi experiments demonstrated that knockdown of Ote exhibited no effect on Drosophila Kc167 cells, which suggests that Ote might not be a limiting component for the maintenance of the nuclear architecture (Wagner, 2004). Thus, the function and physiological role of Ote remain elusive (Jiang, 2008).

This study shows that otefin (ote), which encodes a nuclear lamin, is essential for GSC maintenance. Ote, as an intrinsic factor, is both necessary and sufficient for GSC maintenance by silencing bam transcription via interaction with Dpp signaling. Furthermore, nuclear membrane localization of Ote is critical for its function in the maintenance of GSC. Finally, biochemical evidence is presented to support that Ote physically interacts with Medea, a Drosophila Smad4, at the bam silencer element to regulate GSC fate. Thus, these data indicate that an integral membrane protein, the nuclear lamin Ote, functions at target gene loci to mediate BMP signal-dependent repression (Jiang, 2008).

This study has identified and characterized Otefin (Ote) as a protein the plays an important role in the regulation of GSC fate via BMP/Dpp signaling. The data support the notion that specific nuclear membrane components mediate signal-dependent transcriptional effects to control stem cell behavior (Jiang, 2008).

Observation of the abnormality and loss of germ cells in ote mutant ovaries prompted an exploration of whether ote is involved in the regulation of GSC fate. Using germline clonal analysis and rescue tests, it was demonstrated that ote plays an intrinsic role in GSC self-renewal. In addition, it was also observed that ectopic expression of ote increased the number of GSC-like cells, most likely through repression of GSC/CB differentiation. Thus, the results suggest that, like Dpp signaling, Ote is also both necessary and sufficient to regulate GSC fate. A previous study has demonstrated that knockdown of Ote by RNAi interference exhibited no effect on Drosophila Kc167 cells, suggesting that Ote might not be a limiting factor for the maintenance of the nuclear architecture in cultured cells (Wagner, 2004). Consistently, clonal data showed that ote mutant GSCs could develop into normal cysts and egg chambers rather than undergo apoptosis, suggesting that Ote plays a specific role in maintaining GSC self-renewal but not germ cell viability. As supportive evidence, it was also shown that loss of function of ote did not affect the nuclear architecture and the normal expression of other nuclear lamin components in ovaries. In addition, it was found, except in germ cells, neither overexpression nor loss of function of ote exhibited obvious defects in other developmental processes. Together, these data suggest that Ote may play a role in the maintenance of GSC and germ cell development rather than performing a general cell biological function (Jiang, 2008).

Previous studies have revealed two major signaling mechanisms, dpp-dependent bam transcriptional silencing and bam-independent translational repression, that function cooperatively in the repression of GSC differentiation. In GSCs, Pum/Nos-mediated and microRNA-mediated translational control have been proposed to repress translation of the mRNA pool that promotes GSC/CB differentiation; in contrast, Dpp signaling from the niche cells is responsible for silencing bam transcription in GSCs by activating Smad complexes that physically bind the bam silencer element. Thus, the question becomes how Ote integrates into this signal network. Several lines of genetic evidence strongly suggest that Ote acts through the Dpp signaling pathway rather than through a parallel (Dpp-independent) pathway. (1) The removal of Ote activity not only results in the loss of GSCs, but also replicates the mad or med mutant phenotypes. (2) ote suppressed the TKVca-overexpression phenotype, suggesting that the function of Dpp signaling required Ote activity in order to repress germ cell differentiation. (3) Genetic analysis showed that the ote and dpp pathway are functionally dependent on each other. Thus, these results strongly suggest that Ote serves as a positive component in the Dpp signaling pathway rather than acting through a parallel (Dpp-independent) pathway to regulate GSC fate (Jiang, 2008).

The loss of ote results in a female sterile phenotype but does not affect Dpp signaling in other developmental stages, implying that Ote regulates Dpp signaling only in the ovary. It is possible that ote plays a specific role in regulation of the Dpp pathway in ovary, but is dispensable for dpp pathway regulation in other tissues. A similar example is brinker (brk), which also functions in a tissue-specific manner. It has been shown that brk acts as a negative regulator of the dpp pathway in wing growth control; however, it is dispensable for the dpp pathway in the regulation of GSC fate. Another possibility is that ote could have a redundant function with other nuclear membrane protein(s) in the regulation of the dpp pathway in other tissues (Jiang, 2008).

Structure-function analysis revealed that nuclear membrane localization is essential for Ote function in the regulation GSC fate, the co-IP and FRET assays showed a direct interaction between Ote and Med at the nuclear membrane, and the ChIP assay verified that Ote associated with the bam silencer element in a Med-dependent manner, indicating that Ote/Med interaction might be important for recruiting the bam locus to the nuclear envelope. Combined with the data that Ote is necessary and sufficient for bam silencing in vivo, the results further suggest that Ote/Med-mediated relocalization of the bam locus to the nuclear periphery might be important for bam silencing in the regulation of GSC fate. It has been proposed that subnuclear environments at the nuclear periphery promote gene silencing and activation. Silenced regions of the genome, such as centromeres and telomeres, are statically tethered to the nuclear envelope (Ahmed, 2007). Thus, Ote/Med interaction recruiting the bam locus to the nuclear periphery that results in bam silencing may provide an interesting example to support the role of the nuclear periphery in target-gene silencing at the transcriptional level to maintain the identity of the specific type cells (Jiang, 2008).

It has been shown that Schnurri (Shn), a negatively acting Mad cofactor (Affolter, 2007), is genetically required for GSC maintenance. The biochemical evidence showed that the bam silencer element could also form a ShnCT-containing protein-DNA complex with high affinity when Dpp signaling was activated (Pyrowolakis, 2004). Thus, studies have proposed that Shn probably serves as a component in the bam silencing complexes/Smad complexes required for bam silencing, and germline stem cells are maintained by Shn recruitment to the bam silencer element. However, so far, the direct experimental evidence that loss of shn results in derepression of bam in GSCs is still lacking. Since Shn, like Ote, has tissue-specific functions mediated by its ability to confer repressive activity on Smad complexes, it will be interesting to test whether Shn acts together with Ote at the bam silencer element in GSCs (Jiang, 2008).

The LEM family represents an important group of nuclear membrane-associated proteins that share a conserved LEM domain. A number of studies have focused on the potential biochemical properties of these proteins and their relationship with nuclear assembly and cell division at the cellular level (Ashery-Padan, 1997a; Goldberg, 1998; Gruenbaum, 2005; Mattout-Drubezki, 2003). Recently, several studies revealed that certain nuclear envelope components are involved in signal transductions, such as MAN1, a nuclear membrane protein that binds Smad2 and Smad3 and antagonizes TGF-β signaling in vertebrates. These findings are in contrast with the current results indicating that Ote functions positively to regulate Dpp signaling transduction in the regulation of GSC. It has been reported that a Drosophila LEM domain protein encoded by the annotated gene CG3167, named dman1, is the putative ortholog to vertebrate MAN1 (Wagner, 2006). Similar to Ote, downregulation of dMAN1 by RNAi has no obvious effect on Kc167 cells, suggesting that the dMAN1 protein is also not a limiting component of the nuclear architecture either (Wagner, 2004; Wagner, 2006). Since ote and dman1 possess opposite roles in the regulation of TGF-β/BMP signaling, and dMan1 potentially interacts with Mad in yeast two-hybrid assays and co-IP assays in S2 cells, it would be interesting to determine whether Ote and dMan1 collaborate together to balance the self-renewal and differentiation of GSCs by controlling the proper induction of Dpp pathway activity. There is no known counterpart to Ote in mammals; however, Emerin has a domain arrangement similar to Ote, since it also contains a LEM at its N terminus and a single TM at its C terminus. It has been reported that mutations in emerin cause Emery-Dreifuss muscular dystrophy in humans; however, the molecular mechanism of these mutations and their phenotypes remain poorly understood. This study has characterized a new role in the regulation of stem cells for the nuclear lamin Otefin. It will also be interesting to determine whether nuclear lamina components in mammals, including humans, are also involved in fate determination of stem cells, as well as in mediating signal-dependent gene silencing related to human diseases (Jiang, 2008).

Survival of Drosophila germline stem cells requires the chromatin binding protein Barrier-to-autointegration factor

The nuclear lamina (NL) is an extensive protein network that underlies the inner nuclear envelope. This network includes LAP2-emerin-MAN1-domain (LEM-D) proteins that associate with the chromatin and DNA binding protein Barrier-to-autointegration factor (BAF). this study investigated the partnership between three NL Drosophila LEM-D proteins and BAF. In most tissues, only D-emerin/Otefin is required for NL enrichment of BAF, revealing an unexpected dependence on a single LEM-D protein. Prompted by these observations, BAF contributions were studied in the ovary, a tissue where D-emerin/Otefin function is essential. Germ cell-specific BAF knockdown causes phenotypes that mirror d-emerin/otefin mutants. Loss of BAF disrupts NL structure, blocks differentiation and promotes germ cell loss, phenotypes that are partially rescued by inactivation of the ATR and Chk2 kinases. These data suggest that similar to d-emerin/otefin mutants, BAF depletion activates the NL checkpoint that causes germ cell loss. Taken together, these findings provide evidence for a prominent NL partnership between the LEM-D protein D-emerin/Otefin and BAF, revealing that BAF functions with this partner in the maintenance of an adult stem cell population (Duan, 2020).

The nuclear lamina (NL) is an extensive protein network that underlies the inner nuclear membrane. Comprising lamins and hundreds of associated proteins, the NL builds contacts with the genome to regulate transcription, replication and DNA repair. The NL also connects the nucleus with the cytoskeleton, facilitating transduction of regulatory information between cellular compartments. The composition of the NL is cell-type specific, providing a diverse platform for the integration of developmental regulatory signals. Changes in NL structure occur during physiological aging and disease, suggesting that maintenance of NL function is crucial for cellular health and longevity (Duan, 2020).

One prominent family of NL proteins are LEM domain (LEM-D) proteins, named after the founding human members: LAP2, emerin and MAN1. The defining feature of this conserved family is the LEM domain (LEM-D), an ∼40 amino acid domain that directly interacts with the metazoan chromatin-binding protein Barrier-to-autointegration factor (BAF, sometimes referred to as BANF1). Purified human BAF directly binds double-stranded DNA, the A-type lamin and histones in vitro, suggesting that BAF also promotes chromatin-NL connections using non-LEM-D-dependent mechanisms. In dividing metazoan cells, regulated formation of complexes between LEM-D proteins, BAF and lamin controls mitotic spindle assembly and positioning, as well as the reformation of the nucleus. In non-dividing metazoan cells, LEM-D proteins and BAF cooperate to tether the genome to the nuclear periphery and form repressed chromatin. These properties highlight central connections between LEM-D proteins and BAF in NL function (Duan, 2020).

Studies in Drosophila melanogaster have begun to define the role of LEM-D proteins and BAF in development. Drosophila has three NL LEM-D proteins that bind BAF, including two emerin orthologues (Emerin/Otefin and Emerin2/Bocksbeutel) and MAN1. Each LEM-D protein is globally expressed during development. Even so, loss of individual NL LEM-D proteins causes different, non-overlapping defects in the several tissues, including the ovaries, testes, wings and the nervous system. These restricted mutant phenotypes reflect functional redundancy among the Drosophila LEM-D proteins, as loss of any two proteins is lethal. Strikingly, phenotypes of the emerin double mutants (otefin-/-; bocksbeutel-/-) phenocopy baf null mutants (Furukawa, 2003). Both baf and the emerin double mutants die before pupation, resulting from decreased mitosis and increased apoptosis of imaginal discs (Barton, 2014; Furukawa, 2003). In contrast, emerin/otefin; MAN1 or emerin2/bocksbeutel; MAN1 die during pupal development, without associated defects in mitosis or apoptosis (Barton, 2014). Together, genetic studies indicate that the Drosophila emerin orthologues and BAF are important partners (Duan, 2020).

This study extend investigations of the Drosophila NL LEM-D and BAF protein partnership. Using a CRISPR generated gfp-baf allele, this study confirmed that BAF is a globally expressed nuclear protein that shows strong enrichment at the NL in diploid cells. Strikingly, this NL enrichment largely depends upon one LEM-D protein, Emerin/Otefin. Prompted by these observations, BAF contributions were studied in the ovary, a tissue where Emerin/Otefin function is essential. In germline stem cells (GSCs), loss of Emerin/Otefin causes a thickening of the NL and reorganization of heterochromatin. These structural nuclear defects are linked to activation of two kinases of the DNA damage response pathway: Ataxia Telangiectasia and Rad3-related (ATR) and Checkpoint kinase 2 (Chk2). Although oogenesis in emerin/otefin mutants is rescued by loss of these DDR kinases, canonical triggers are not responsible for pathway activation. Instead, ATR and Chk2 activation is linked to defects in NL structure itself (Barton, 2018). Given the roles of BAF in mitotic nuclear envelope formation and repair (Halfmann, 2019; Samwer, 2017; Mehsen, 2018), it was reasoned that checkpoint activation in emerin/otefin mutants might result from altered BAF function. This prediction was tested using germ cell-specific RNA interference (RNAi) to knockdown BAF. This study shows that BAF depletion disrupts NL structure, blocks differentiation and promotes GSC loss, mutant phenotypes that mirror Emerin/Otefin loss. Additionally, mutation of atr or chk2 partially restores germ cell differentiation in the baf mutant background, supporting the possibility that BAF depletion activates the NL checkpoint. Taken together, these findings suggest that Emerin/Otefin plays a dominant role in the enrichment of BAF to the NL and provide evidence that BAF functions with this prominent partner in the maintenance of an adult stem cell population (Duan, 2020).

This study extended in vivo studies of the BAF and LEM-D partnership. Capitalizing on a newly generated gfp-baf allele, this study shows that NL localization of BAF largely depends upon a single LEM-D protein, Emerin/Otefin. Loss of Emerin/Otefin is sufficient to disperse BAF in cells that express the A- and B-type lamins, Emerin2/Bocksbeutel and MAN1 in the NL. These data establish the in vivo existence of a prominent NL partnership between one LEM-D protein and BAF (Duan, 2020).

The basis for the unexpected reliance on Emerin/Otefin is unknown. One possibility is that LEM-Ds have different affinities for BAF. Pairwise alignment of amino acid residues within LEM-Ds shows the highest conservation between Drosophila emerin orthologues (70% similarity; Barton., 2014). Nonetheless, all LEM-Ds are strongly conserved in BAF-binding residues (42% identical, 67% similar). A second possibility is that the interaction of LEM-D proteins with BAF depends upon how a given LEM-D protein assembles into the NL network. Self-association of emerin influences both BAF and lamin binding. Finally, post-translational modifications (PTMs) of LEM-D proteins might impact BAF partnerships. As an example, O-GlcNAcylation modification of emerin affects BAF association, representing a regulated PTM that has the potential to alter NL function in response to nutrient availability. However, such signal-dependent PTMs are likely to be tissue specific, predicting a tissue-restricted, not global, effect on the NL enrichment of BAF. Further studies are needed to resolve the basis for the strong partnership between Emerin/Otefin and BAF (Duan, 2020).

BAF is essential for viability, with dying baf null larvae exhibiting a typical mitotic mutant phenotype that is associated with high levels of apoptosis (Furukawa, 2007). Several observations suggest that loss of NL BAF is not equivalent to complete loss of BAF. First, emerin/otefin null animals are viable, even though there is a global loss of NL BAF. Second, emerin/otefin null animals have lower levels of apoptosis in larval tissues than baf animals, without effects on the development of adult structures. Third, emerin/otefin mutant imaginal disc cells display an unchanged nuclear shape and chromatin architecture (Barton, 2018), whereas these cells are affected in baf mutants (Furukawa, 2003). Based on these data, it is suggested that BAF function at the NL during interphase is not essential. It is predicted that the essential BAF function relates to its contributions in mitosis and depends upon both Drosophila emerin orthologues, as these double mutant animals die with a mitotic mutant phenotype (Duan, 2020).

Effects of mislocalized BAF share features resulting from BAF overexpression in other systems. In emerin/otefin mutant germ cells, BAF dispersal contributes to the aggregation of heterochromatin. Defects in HP1a distribution have also been found in human cells overexpressing BAF or expressing a BAF mutant defective in interacting with NL components. Furthermore, several diseases affecting expression and processing of lamin A alter the distribution of BAF and resemble a BAF overexpression phenotype. Together, these findings support a model in which BAF contributes to the deleterious effects resulting from lamin or LEM-D mutations (Duan, 2020).

BAF is required for maintenance of Drosophila GSCs. Germ cell-specific BAF knockdown caused GSC loss, with remaining GSCs displaying a thickened and irregular NL structure, a phenotype shared with emerin/otefin mutants. These data support a model in which Emerin/Otefin and BAF function together to build NL structure in this cell type. Such a dependence on Emerin/Otefin for NL structure is consistent with limiting levels of the second Drosophila Emerin ortholog, Emerin2/Bocksbeutel (Barton, 2014). It is predicted that, in GSCs, the Emerin/Otefin and BAF might have a shared function in nuclear reformation at the end of mitosis (Duan, 2020).

Activation of the NL checkpoint is linked to NL deformation (Barton, 2018). Strikingly, baf mutant phenotypes are partially suppressed in atr/chk2; nos>bafRNAi animals, with double mutant ovaries showing increased germ cell survival and differentiation. Yet cell death remained in the double mutant backgrounds. Based on these observations, it is predicted that BAF loss in germ cells has multiple consequences. First, NL structure is affected. Second, loss of nuclear BAF might affect transcriptional networks required for GSC maintenance, suggested from studies showing BAF is an epigenetic regulator (Montes de Oca, 2011). Notably, the maintenance of mammalian stem cells also depends on BAF. Knockdown of BAF in either mouse or human embryonic stem cells promoted premature differentiation and reduced survival, phenotypes associated with an altered cell cycle. It remains possible that loss of Drosophila BAF in GSCs perturbs mitosis, which might induce apoptosis. Additional studies are needed to elucidate cell cycle contributions of BAF in GSCs (Duan, 2020).

These studies emphasize the important role of BAF within the NL network. Evidence is presented for consequences of BAF dispersal and loss during development, showing BAF dysfunction causes cell-type specific responses. Further definition of the developmental contributions of BAF will advance understanding of laminopathies, including the Nestor-Guillermo syndrome: a rare hereditary progeroid disorder caused by a missense mutation in BAF/BANF1 (Duan, 2020).

Recruitment of BAF to the nuclear envelope couples the LINC complex to endoreplication

DNA endoreplication has been implicated as a cell strategy to grow in size and in tissue injury. This study demonstrates that barrier to autointegration factor (BAF), represses endoreplication in Drosophila myofibers. This study shows that BAF localization at the nuclear envelope was eliminated either in mutants of the Linker of Nucleoskeleton and Cytoskeleton (LINC) complex, in which the LEM-domain protein Otefin was similarly excluded, or after disruption of the nucleus-sarcomere connections. Furthermore, BAF localization at the nuclear envelope required the activity of the BAF kinase VRK1/Ball, and consistently non-phosphorytable BAF-GFP was excluded from the nuclear envelope. Importantly, removal of BAF from the nuclear envelope correlated with increased DNA content in the myonuclei. E2F1, a key regulator of endoreplication was found to overlap BAF localization at the myonuclear envelope, and BAF removal from the nuclear envelope resulted with increased E2F1 levels in the nucleoplasm, and subsequent elevated DNA content. It is suggested that LINC-dependent, and phospho-sensitive attachment of BAF to the nuclear envelope, through its binding to Otefin, tethers E2F1 to the nuclear envelope thus inhibiting its accumulation at the nucleoplasm (Unnikannan, 2020).

Endoreplication emerges as an important strategy of differentiated cells, enabling them to grow in size or rescue tissue integrity following injury, in a wide range of non-dividing cell types. Recent experimental studies have proposed a functional link between mechanical inputs and endoreplication events in various cell types. Moreover, mechanical signals transmitted across the nuclear membrane have been implicated in the regulation of cell cycle, epigenetic events and gene transcription. As part of the mechanism linking cell cycle events with mechanical inputs, the translocation of specific essential factors into the nucleus has been proposed. However, the molecular link between nuclear translocation of such factors, mechanical inputs on the nuclear envelope and endoreplication is still elusive (Unnikannan, 2020).

The linker of nucleoskeleton and cytoskeleton (LINC) complex has been suggested to mediate mechanically induced nuclear entry of essential factors (Driscoll, 2015; Horn, 2014; Osmanagic-Myers, 2015). It physically connects the cytoskeleton and the nucleoskeleton at the interface of the nuclear envelope and has been associated with various human myopathies. The LINC complex is composed of Nesprin protein family members, which associate at their cytoplasmic N-terminal end with distinct cytoskeletal components, and on their nuclear C-terminal end with SUN domain proteins at the perinuclear space. SUN domain proteins bind to various nuclear lamina components, resulting in a physical link between the cytoskeleton and the nucleoskeleton. Recent results indicate that, in Drosophila larval muscles, the LINC complex is essential for arresting endoreplication in the muscle nuclei (myonuclei) and that LINC mutants exhibit additional rounds of DNA replication, resulting in elevated polyploidy. The molecular nature of this process is currently elusive (Unnikannan, 2020).

In an attempt to reveal the components downstream of the LINC-dependent arrest of DNA endoreplication, a screen was performed for genes whose transcription changes in Drosophila Nesprin/klar mutant muscles. One of the identified genes was barrier-to-autointegration factor (baf), shown to be significantly reduced at the transcription level. BAF is a small protein of 89 amino acids that binds dsDNA as well as the nuclear envelope, and in addition forms homodimers. Furthermore, BAF binds to the inner components of the nuclear membrane, including the Lap-2, Emerin, MAN1 (LEM) domain proteins, as well as to lamins A/C and B. Thus, BAF dimers might bridge between dsDNA and the nuclear envelope. Proteomic analysis of BAF partners indicate its potential association with additional proteins, including transcription factors, damage-specific DNA binding proteins and histones. Furthermore, the binding of BAF to its potential partners might be regulated by its phosphorylation state. For example, phosphorylated BAF associates with LEM-domain proteins, whereas de-phosphorylated BAF favors binding to dsDNA. One kinase that has been implicated in BAF phosphorylation is the threonine-serine VRK1 kinase, whose homolog in Drosophila is Ballchen (Ball, also known as NHK-1) (Unnikannan, 2020).

BAF has a crucial role in the condensation and assembly of post-mitotic DNA. Its interaction with both dsDNA and the nuclear lamina enables DNA compaction through cross-bridges between chromosomes and the nuclear envelope, a process essential for the assembly of DNA within a single nucleus following mitosis. Likewise, BAF is recruited to the sites of ruptured nuclear membrane, where it is essential for resealing the ruptured nuclear membrane. Interestingly, in humans a single amino acid substitution of BAF causes Nestor-Guillermo progeria syndrome (NGPS); however, the molecular basis for the disease awaits further investigation (Unnikannan, 2020).

Previous studies demonstrated that in Drosophila, muscle-specific knockdown of BAF increases the levels of DNA endoreplication, phenocopying the LINC mutant outcome. This led to the hypothesis that BAF acts downstream of the LINC complex-dependent mechanotransduction in promoting the arrest of DNA endoreplication in muscle. This study demonstrates that BAF localization at the nuclear envelope is crucial for that process, and that it is downstream of the LINC complex, depends on nucleus-sarcomere connections, and is phosphosensitive. Importantly, elimination of BAF from the nuclear envelope correlates with increased DNA content in the myonuclei and a concomitant increase in E2F1 levels in the nucleoplasm. Taken together, these findings suggest a model in which a LINC-dependent localization of BAF at the nuclear envelope promotes E2F1 tethering to the nuclear envelope to inhibit its accumulation in the nucleoplasm (Unnikannan, 2020).

This study demonstrates the contribution of a novel mechanosensitive component, BAF, in controlling the nuclear accumulation of E2F1, a crucial transcription factor required for the regulation of endoreplication. Whereas previous reports implicated BAF in promoting the condensation and assembly of post-mitotic dsDNA into single nuclei, this study demonstrates that BAF is also essential for the arrest of DNA endoreplication in fully differentiated muscle fibers. Importantly, only BAF that localizes to the nuclear envelope appears to be relevant for this function in post-mitotic differentiated cells. The contribution of BAF to larval muscle functionality is unclear, as baf mutants did not survive up to third instar stage and BAF knockdown in muscles by using RNAi did not eliminate BAF very efficiently (Unnikannan, 2020).

In Drosophila muscle fibers, it was found that BAF was detected in various subcellular sites, including the cytoplasm, nuclear envelope, nucleoplasm and at the nucleolus borders. Yet, only the portion of BAF localized at the nuclear envelope was found to change following elimination of a functional LINC complex. It is well accepted that the LINC complex transmits cytoplasmic mechanical inputs from the cytoskeleton to the nucleoskeleton in various cell types. Moreover, nuclear deformations (from oval into spheroid shape) observed both in larval muscles of LINC complex mutants and in conditions where nuclei detach from the sarcomeres or following Sls knockdown are indicative of changes in the mechanical inputs applied on the nuclear envelope. Because BAF localization at the nuclear envelope was specifically impaired in both conditions, it is proposed that maintenance of BAF at the nuclear envelope is mechanically sensitive (Unnikannan, 2020).

In control myofibers, BAF exhibited a relatively broad distribution along the outlines of the nuclear envelope, often extending beyond the Lamin C expression domain towards the cytoplasm, overlapping with the nucleus-associated microtubules. This suggested that, in addition to its association with the inner aspects of the nuclear membrane through binding to LEM-domain proteins and Lamin A/C, BAF associates with the outer aspects of the nuclear membrane. Previous experiments indicate that despite its small size BAF does not diffuse passively from the cytoplasm to the nucleus. Furthermore, photobleaching experiments with GFP-BAF indicate that BAF-dependent repair of nuclear ruptures occurs when cytoplasmic BAF, but not nuclear BAF, rapidly associates with the ruptured sites and further recruits LEM-domain proteins to establish membrane sealing. The authors suggest that their findings are consistent with a dynamic exchange of BAF between cytoplasmic and nuclear pools, where BAF in the cytoplasm primarily responds to mechanical signals. Because the current experiments indicate that BAF phosphorylation is crucial for its maintenance at the nuclear membrane, it is possible that the exchange of BAF localization between the cytoplasm and the nucleus is stabilized by its phosphorylation. The contribution of the LINC complex to BAF association with the nuclear envelope could be either direct (e.g. by binding to components of the LINC complex) or indirect (e.g. through an effect of the LINC complex on the distribution of LEM proteins at the nuclear envelope). The results support the latter model, in which the LINC complex maintains the localization of the LEM protein Otefin at the nuclear envelope to mediate BAF association with the nuclear envelope. Hence, a model is suggested in which the contribution of the LINC complex to BAF localization at the nuclear envelope is through an effect on Otefin localization at the nuclear envelope (Unnikannan, 2020).

Endoreplication has been implicated in a wide variety of differentiated cells in a broad range of species, including human tissues. A link between mechanical tension and endoreplication has been recently suggested. However, the molecular mechanism coupling mechanical tension with the endoreplication process is still elusive. This study found that a key regulator of endoreplication, E2F1, exhibits a specific distribution at the nuclear envelope in fully differentiated myofibers, where it probably resides non-actively. Changes in the mechanical environment of the nuclear envelope correlate with the localization of E2F1 and promote its accumulation within the nucleoplasm, where it is expected to promote DNA synthesis. It will be of interest to find which proteins associate directly with E2F1 at the nuclear envelope. Attempts to co-immunoprecipitate BAF with Msp300 or E2F1 failed to show a specific protein interaction between these proteins. From a physiological point of view, no detectable changes in muscle size or movement were observed in the baf knockdown muscles, and the larvae developed up to adult stage. The baf homozygous mutant did not develop up to the third instar larval stage, so the full physiological contribution of BAF to muscle growth awaits experiments in which a more efficient reduction in BAF levels is induced in muscle tissue (Unnikannan, 2020).

In summary, these results reveal a novel insight into the role of the LINC complex in coupling endoreplication with changes in the nuclear envelope composition in mature muscle fibers. In particular, the mechanosensitive component, BAF, whose localization at the nuclear envelope is tightly regulated by the LINC complex, is shown to negatively control the nuclear accumulation of the cell cycle regulator E2F1 at the level of the nuclear envelope. The localization of Otefin in the nuclear envelope and BAF phosphorylation by Ball kinase are both crucial in this context. This process might be part of a mechanosensitive pathway that regulates polyploidy in a wide variety of differentiated cells (Unnikannan, 2020).

Nuclear lamina dysfunction triggers a germline stem cell checkpoint

LEM domain (LEM-D) proteins are conserved components of the nuclear lamina (NL) that contribute to stem cell maintenance through poorly understood mechanisms. The Drosophila emerin homolog Otefin (Ote) is required for maintenance of germline stem cells (GSCs) and gametogenesis. This study shows that ote mutants carry germ cell-specific changes in nuclear architecture that are linked to GSC loss. Strikingly, both GSC death and gametogenesis are rescued by inactivation of the DNA damage response (DDR) kinases, ATR and Chk2. Whereas the germline checkpoint draws from components of the DDR pathway, genetic and cytological features of the GSC checkpoint differ from the canonical pathway. Instead, structural deformation of the NL correlates with checkpoint activation. Despite remarkably normal oogenesis, rescued oocytes do not support embryogenesis. Taken together, these data suggest that NL dysfunction caused by Otefin loss triggers a GSC-specific checkpoint that contributes to maintenance of gamete quality (Barton, 2018).

The Drosophila emerin homolog Ote has an essential requirement for GSC survival and germ cell differentiation. This study shows that Ote loss causes GSC-specific nuclear defects that include a thickened and irregular NL and aggregation of heterochromatin. Strikingly, inactivation of two DDR kinases, ATR, and Chk2, rescues oogenesis in ote-/- females, a rescue that is cell-type specific. Genetic and cytological features of the checkpoint pathway present in ote mutant GSCs differ from those found in canonical DDR pathways. In addition, although heterochromatin coalesce is present, such defects by themselves do not trigger the checkpoint. Instead, the data correlate Chk2 activity with defects in NL structure, indicating that NL dysfunction is responsible for the activation of a checkpoint pathway in GSCs. Despite remarkably normal oogenesis, rescued oocytes do not support embryogenesis. It is suggested that this NL checkpoint pathway functions in GSCs to ensure that only healthy gametes are passed on to the next generation (Barton, 2018).

These studies identify ATR as the critical responder kinase and Chk2 as the critical transducer kinase in the NL checkpoint. This signaling axis differs from the canonical ATM-to-Chk2 or ATR-to-Chk1 axes. Several factors might contribute to the choice of responder and transducer kinase. First, species-specific constraints might exist. ATR, but not ATM is essential in mammals, whereas ATM, but not ATR, is essential in Drosophila. Second, cell-type specific distinctions are apparent. In both the fly and mouse germline, persistent meiotic double-strand breaks activate ATR and Chk2, implying that the ATR-Chk2 axis might be dominant in germ cells. Third, the nature of the trigger might influence which proteins are involved in signaling. For example in Drosophila, ATR and Chk2 are both required for the patterning defects caused by a failure to repair meiotic double-strand breaks. However, in DNA-damaged GSCs, ATR protects against GSC death, whereas Chk2 promotes it. These data suggest that in the case of the NL checkpoint, both ATR and Chk2 promote germ cell death. These studies add to growing evidence that the DDR pathway is modular, with selective use of pathway components in response to various cellular stresses (Barton, 2018).

Activated Chk2 is commonly associated with phosphorylation and activation of p53. Canonically, p53 activation leads to cell cycle arrest and apoptosis. These studies demonstrate that GSC loss persists in ote-/-; p53-/- females, suggesting that classical apoptosis is not responsible for GSC death. These findings are consistent with the absence of classic markers of apoptosis in ote mutants and observations that the p53 regulatory network differs in GSCs. Recently, an alternative cell death pathway was identified in spermatogonia of Drosophila testes. This pathway is responsible for spontaneous elimination of spermatogonia, using activated lysosomal and mitochondrial-associated factors. Additional studies are needed to determine whether ATR/Chk2-dependent GSC loss in ote mutants targets a similar pathway (Barton, 2018).

The data suggest that NL dysfunction is the primary cause of the ATR/Chk2 checkpoint in GSCs. Notably, NL defects are found only in affected cells and persist in rescued chk2-/-, ote-/- double mutants. Multiple mechanisms might connect nuclear architecture changes to ATR/Chk2 activation. First, altered NL structure might change genomic contacts needed for appropriate transcriptional regulation, with resulting gene expression changes prompting activation of the checkpoint. While global transcriptional changes during oogenesis were not observed, identification of transcriptional changes specific to GSCs or early germ cells would have been masked in these studies. Second, disruptions in the NL might affect trafficking of products between the nucleus and cytoplasm. Notably, a recent study identified large ribonucleoparticles (megaRNPs) that exit the nucleus by egress or budding through the inner and outer nuclear membranes, a process disrupted by defects in the NL. As such, it remains possible that the thickened NL in ote-/- GSCs disrupts large ribonucleoprotein (megaRNP) egress, leading to cellular stress and ATR/Chk2 activation. Third, defects in the NL structure might alter scaffolding of components of the DDR pathway, leading to checkpoint activation. Indeed, proteomic studies from Drosophila cultured somatic cells found that Ote interacted with proteins involved in DNA replication and repair, implying that Ote might assemble responder and transducer kinases complexes at the NL. However, observations that the ATR/Chk2-dependent checkpoint is GSC-specific, coupled with findings that meiotic double-strand breaks are repaired appropriately in chk2-/-, ote-/- germaria, argue against this model. Fourth, structural alteration in the nuclear envelope itself might trigger ATR/Chk2 activation. Indeed, emerging evidence implicates ATR as a general sensor of the structural integrity of cellular components. Further studies are needed to identify how NL dysfunction triggers the GSC-specific checkpoint (Barton, 2018).

Mutations in NL LEM-D proteins cause dystrophic diseases. Much evidence suggests that these diseases result from compromised stem cell populations that underlie the defects in tissue homeostasis. Indeed, a wealth of evidence links NL defects to increased DNA damage. The data are consistent with these reports, as it was shown that elevated accumulation of the commonly used DNA damage marker. However, this study found that phosphorylation of the H2A variant occurs downstream of Chk2, suggesting that accumulation of DNA damage in cells with a dysfunctional NL might be a consequence of cells dying, not the primary cause. These unexpected results suggest that caution is needed in linking causation of γH2Av/H2X accumulation to DNA damage and a failure in DNA repair. Indeed, recent studies of progerin-expressing cells indicated that the cellular defect in Hutchinson-Gilford progeria cells does not lie in defective DNA repair and DNA damage, even though these cells accumulate phosphorylated H2AX. These findings establish a new context for consideration of mechanisms of laminopathic diseases, suggesting that detrimental effects of NL dysfunction are primary events that are linked to checkpoint activation and stem cell loss (Barton, 2018).

Interactions among Drosophila nuclear envelope proteins lamin, otefin, and YA

Lamin, Otefin, and YA are the three Drosophila nuclear envelope proteins that have been characterized in early embryos. The yeast two-hybrid system was used to explore the interactions between pairs of these proteins. The ubiquitous major lamina protein, Lamin Dm, interacts with both Otefin, a peripheral protein of the inner nuclear membrane, and YA, an essential, developmentally regulated protein of the nuclear lamina (see A model for the organization of lamin, otefin, and YA in the nuclear envelope.). In agreement with this interaction, Lamin and Otefin can be coimmunoprecipitated from the vesicle fraction of Drosophila embryos and will colocalize in nuclear envelopes of Drosophila larval salivary gland nuclei. The two-hybrid system was further used to map the domains of interaction among Lamin, Otefin, and YA. Lamin's rod domain interacts with the complete otefin protein, with otefin's hydrophilic NH2-terminal domain, and with two different fragments derived from this domain. Analogous probing of the interaction between Lamin and YA shows that the lamin rod and tail plus part of its head domain are needed for interaction with full-length YA in the two-hybrid system. YA's COOH-terminal region is necessary and sufficient for interaction with lamin. These results suggest that interactions with lamin might mediate or stabilize the localization of Otefin and YA in the nuclear lamina. They also suggest that the need for both Otefin and Lamin in mediating association of vesicles with chromatin might reflect the function of a protein complex that includes these two proteins. Since the hydrophobic COOH terminus of Otefin is required for targeting to the inner nuclear membrane, Otefin may connect with the inner nuclear membrane through its COOH terminus and with the nuclear lamina through other regions of otefin. Interaction between Otefin and Lamin may stabilize the localization of Otefin. This could be similar to the case of the lamin B receptor (LBR) in vertebrates, which has a hydrophilic NH2 terminus and a hydrophobic COOH terminus that is capable of targeting the LBR to the inner nuclear membrane. Since the NH2 terminus of the LBR alone targets a cytosolic protein to the nucleus but a type II integral protein to the inner nuclear membrane, this suggests that targeting a protein to the inner nuclear membrane requires a special domain, such as one mediating interaction with other nuclear envelope proteins (Goldberg, 1998).

Previous studies provided indirect evidence for an in vivo interaction between lamin and otefin. This evidence included the peripheral nucleoplasmic localization of both otefin and lamin, the similar levels of resistance of otefin and of lamin to extraction with Triton X-100, the finding that in early embryos both proteins remain associated with the spindle envelope during mitosis, and the finding that both lamin and otefin are required during nuclear assembly for the attachment of membrane vesicles to chromatin. In addition, in the maternal pool, otefin is associated with the same membrane vesicle fractions as lamin Dmmit. The present study has provided in vitro and in vivo evidence for such an interaction and has shown that otefin and lamin colocalize in salivary gland cell nuclei (Goldberg, 1998).

The two-hybrid experiments in yeast cells revealed that otefin and lamin can interact with one another in the absence of any other Drosophila proteins. Interaction domains delineated in the yeast two-hybrid assay suggest that otefin interacts with the lamin rod domain through otefin's hydrophilic NH2 terminus, including otefin's aa 35 to 172, which have been shown previously to stabilize otefin's localization to the nuclear envelope. Since the hydrophobic COOH terminus of otefin is required for targeting to the inner nuclear membrane, otefin may connect with the inner nuclear membrane through its COOH terminus and with the nuclear lamina through other regions of otefin. Interaction between otefin and lamin may stabilize the localization of otefin. This could be similar to the case of the lamin B receptor (LBR) in vertebrates, which has a hydrophilic NH2 terminus and a hydrophobic COOH terminus that is capable of targeting LBR to the inner nuclear membrane. Since the NH2 terminus of LBR alone targets a cytosolic protein to the nucleus but a type II integral protein to the inner nuclear membrane, this suggests that targeting a protein to the inner nuclear membrane requires a special domain, such as one mediating interaction with other nuclear envelope proteins (Goldberg, 1998).

In vitro studies have shown that the rod domain of lamin has several biological activities. The heptad repeats in the rod domain are involved in coiled-coil interactions, and sequences at both ends are involved in the head-to-tail organization of lamin filaments. The rod domain contains a chromatin binding site, and it can bind M/SAR sequences with high affinity. The current results show that the rod domain can also serve as a binding domain for otefin (Goldberg, 1998).

Distinct regions specify the targeting of Otefin to the nucleoplasmic side of the nuclear envelope

Otefin is a 45-kDa nuclear envelope protein with no apparent homology to other known proteins. It includes a large hydrophilic domain, a single carboxyl-terminal hydrophobic sequence of 17 amino acids, and a high content of serine and threonine residues. Cytological labeling located otefin on the nucleoplasmic side of the nuclear envelope. Chemical extraction of nuclei from Drosophila embryos reveals that Otefin is a peripheral protein whose association with the nuclear envelope is stronger than that of lamin. Deletion mutants of otefin were expressed in order to identify regions that direct Otefin to the nuclear envelope. These experiments revealed that the hydrophobic sequence at the carboxyl terminus is essential for correct targeting to the nuclear envelope, whereas additional regions in the hydrophilic domain of Otefin are required for its efficient targeting and stabilization in the nuclear envelope (Ashery-Padan, 1997a).

Localization and posttranslational modifications of otefin, a protein required for vesicle attachment to chromatin, during Drosophila melanogaster development

Otefin is a peripheral protein of the inner nuclear membrane in Drosophila melanogaster. During nuclear assembly in vitro, it is required for the attachment of membrane vesicles to chromatin. With the exception of sperm cells, otefin colocalizes with lamin Dm0 derivatives in situ and presumably in vivo and is present in all somatic cells examined during the different stages of Drosophila development. In the egg chamber, otefin accumulates in the cytoplasm, in the nuclear periphery, and within the nucleoplasm of the oocyte, in a pattern similar to that of lamin Dm0 derivatives. There is a relatively large nonnuclear pool of otefin present from stages 6 to 7 of egg chamber maturation through 6 to 8 h of embryonic development at 25°C. In this pool, otefin is peripherally associated with a fraction containing the membrane vesicles. This association is biochemically different from the association of otefin with the nuclear envelope. Otefin is a phosphoprotein in vivo and is a substrate for in vitro phosphorylation by cdc2 kinase and cyclic AMP-dependent protein kinase. A major site for cdc2 kinase phosphorylation in vitro was mapped to serine 36 of otefin. Together, these data suggest an essential role for otefin in the assembly of the Drosophila nuclear envelope (Ashery-Padan, 1997b).

Isolation and characterization of the Drosophila nuclear envelope otefin cDNA

The primary translation otefin gene product has a calculated mass of 45 kDa, contains many serine and threonine residues, and is mostly hydrophilic. However, in the carboxyl terminus, there is a hydrophobic region which may serve as a membrane anchoring domain. RNA blot analysis indicated that the otefin gene codes for a single poly(A+) transcript of 1.6 kilobases and that relatively large amounts of this transcript are present during developmental stages in which many nuclear divisions occur. Polyclonal antibodies raised against the cDNA translation product react with a 58-kDa mammalian nuclear envelope protein, demonstrating evolutionary conservation (Padan, 1990).

Persistence of major nuclear envelope antigens in an envelope-like structure during mitosis in Drosophila melanogaster embryos

Using monoclonal antibodies, the fate of three different nuclear envelope proteins during mitosis in Drosophila early embryos was followed by indirect immunofluorescence microscopy. Two of these proteins, lamin and otefin, a newly characterized nuclear envelope polypeptide with an apparent Mr of 53,000, are apparently present in an envelope-like structure that is present throughout mitosis. Immunoelectron microscopy of interphase nuclei indicates that otefin, like lamin, is not a component of nuclear pore complexes. In contrast with lamin and otefin, gp188, a putative pore complex component, was completely redistributed through the surrounding cytoplasm during prophase in comparable early embryo specimens and was present in an envelope only in interphase. Together with previous morphological studies, these data suggest that the entire mitotic apparatus including condensed chromosomes and spindle is enclosed by an envelope throughout mitosis during early embryogenesis in Drosophila. This 'spindle envelope' contains both lamin and otefin but probably not pore complex proteins (Harel, 1989).

Search PubMed for articles about Drosophila Otefin

Affolter, M. and Basler, K. (2007) The Decapentaplegic morphogen gradient: from pattern formation to growth regulation, Nat. Rev. Genet. 8 (2007): 663-674. PubMed ID: 17703237

Ahmed, S. and Brickner, J. H. (2007). Regulation and epigenetic control of transcription at the nuclear periphery. Trends Genet. 23: 396-402. PubMed ID: 17566592

Ashery-Padan, R., Weiss, A. M., Feinstein, N. and Gruenbaum, Y. (1997a). Distinct regions specify the targeting of otefin to the nucleoplasmic side of the nuclear envelope. J. Biol. Chem. 272(4): 2493-9. PubMed ID: 8999964

Ashery-Padan, R., et al. (1997b). Localization and posttranslational modifications of otefin, a protein required for vesicle attachment to chromatin, during Drosophila melanogaster development. Mol. Cell Biol. 17(7): 4114-23. PubMed ID: 9199347

Barton, L. J., Wilmington, S. R., Martin, M. J., Skopec, H. M., Lovander, K. E., Pinto, B. S. and Geyer, P. K. (2014). Unique and shared functions of nuclear lamina LEM domain proteins in Drosophila. Genetics 197(2): 653-665. PubMed ID: 24700158

Barton, L. J., Duan, T., Ke, W., Luttinger, A., Lovander, K. E., Soshnev, A. A. and Geyer, P. K. (2018). Nuclear lamina dysfunction triggers a germline stem cell checkpoint. Nat Commun 9(1): 3960. PubMed ID: 30262885

Duan, T., Kitzman, S. C. and Geyer, P. K. (2020). Survival of Drosophila germline stem cells requires the chromatin binding protein Barrier-to-autointegration factor. Development. PubMed ID: 32345742

Furukawa, K., Sugiyama, S., Osouda, S., Goto, H., Inagaki, M., Horigome, T., Omata, S., McConnell, M., Fisher, P. A. and Nishida, Y. (2003). Barrier-to-autointegration factor plays crucial roles in cell cycle progression and nuclear organization in Drosophila. J Cell Sci 116(Pt 18): 3811-3823. PubMed ID: 12902403

Furukawa, K., Aida, T., Nonaka, Y., Osoda, S., Juarez, C., Horigome, T. and Sugiyama, S. (2007). BAF as a caspase-dependent mediator of nuclear apoptosis in Drosophila. J Struct Biol 160(2): 125-134. PubMed ID: 17904382

Goldberg, M., et al. (1998). Interactions among Drosophila nuclear envelope proteins lamin, otefin, and YA. Mol. Cell Biol. 18(7): 4315-23. PubMed ID: 9632815

Gruenbaum, Y., et al. (2005). The nuclear lamina comes of age. Nat. Rev. Mol. Cell Biol. 6: 21-31. PubMed ID: 15688064

Halfmann, C. T., Sears, R. M., Katiyar, A., Busselman, B. W., Aman, L. K., Zhang, Q., O'Bryan, C. S., Angelini, T. E., Lele, T. P. and Roux, K. J. (2019). Repair of nuclear ruptures requires barrier-to-autointegration factor. J Cell Biol 218(7): 2136-2149. PubMed ID: 31147383

Harel, A., et al. (1989). Persistence of major nuclear envelope antigens in an envelope-like structure during mitosis in Drosophila melanogaster embryos. J. Cell Sci. 94: 463-70. PubMed ID: 2517292

Jiang, X., et al. (2008). Otefin, a nuclear membrane protein, determines the fate of germline stem cells in Drosophila via interaction with Smad complexes. Dev. Cell 14(4): 494-506. PubMed ID: 18410727

Mattout-Drubezki, A. and Gruenbaum, Y. (2003). Dynamic interactions of nuclear lamina proteins with chromatin and transcriptional machinery. Cell. Mol. Life Sci. 60: 2053-2063. PubMed ID: 14618255

Mehsen, H., Boudreau, V., Garrido, D., Bourouh, M., Larouche, M., Maddox, P. S., Swan, A. and Archambault, V. (2018). PP2A-B55 promotes nuclear envelope reformation after mitosis in Drosophila. J Cell Biol 217(12): 4106-4123. PubMed ID: 30309980

Montes de Oca, R., Andreassen, P. R. and Wilson, K. L. (2011). Barrier-to-Autointegration Factor influences specific histone modifications. Nucleus 2(6): 580-590. PubMed ID: 22127260

Padan, R., Nainudel-Epszteyn, S., Goitein, R., Fainsod, A. and Gruenbaum, Y. (1990). Isolation and characterization of the Drosophila nuclear envelope otefin cDNA. J. Biol. Chem. 265(14): 7808-13. PubMed ID: 2186029

Pyrowolakis, G., Hartmann, B., Müller, B., Basler, K. and Affolter, M. (2004). A simple molecular complex mediates widespread BMP-induced repression during Drosophila development. Dev. Cell 7(2): 229-40. PubMed ID: 15296719

Samwer, M., Schneider, M. W. G., Hoefler, R., Schmalhorst, P. S., Jude, J. G., Zuber, J. and Gerlich, D. W. (2017). DNA cross-bridging shapes a single nucleus from a set of mitotic chromosomes. Cell 170(5): 956-972 e923. PubMed ID: 28841419

Unnikannan, C. P., Reuveny, A., Grunberg, D. and Volk, T. (2020). Recruitment of BAF to the nuclear envelope couples the LINC complex to endoreplication. Development. PubMed ID: 33168584

Wagner, N., Schmitt, J. and Krohne G. (2004). Two novel LEM-domain proteins are splice products of the annotated Drosophila melanogaster gene CG9424 (Bocksbeutel). Eur. J. Cell Biol. 82(12): 605-16. PubMed ID: 15035436

Wagner, N., Kagermeier, B., Loserth, S. and Krohne, G. (2006). The Drosophila melanogaster LEM-domain protein MAN1. Eur. J. Cell Biol. 85(2): 91-105. PubMed ID: 16439308

Wagner, N. and Krohne, G. (2007). LEM-Domain proteins: new insights into lamin-interacting proteins. Int. Rev. Cytol. 261:1-46. PubMed ID: 17560279

date revised: 5 November 2023

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