InteractiveFly: GeneBrief

Blimp-1 : Biological Overview | References

BIOLOGICAL OVERVIEW

The specification of tissue identity during embryonic development requires precise spatio-temporal coordination of gene expression. Many transcription factors required for the development of organs have been identified and their expression patterns are known; however, the mechanisms through which they coordinate gene expression in time remain poorly understood. This study shows that hormone-induced transcription factor Blimp-1 participates in the temporal coordination of tubulogenesis in Drosophila melanogaster by regulating the expression of many genes involved in tube maturation. In particular, it was demonstrated that Blimp-1 regulates the expression of genes involved in chitin deposition and F-actin organization. Blimp-1 is involved in the temporal control of lumen maturation by regulating the beginning of chitin deposition. Blimp-1 also represses a variety of genes involved in tracheal maturation. Finally, the kinase Btk29A serves as a link between Blimp-1 transcriptional repression and apical extra-cellular matrix organization (Ozturk-Colak, 2018).

Specialized cellular functions and cell lineage fates are usually regulated by only a few key instructive transcription factors required to activate or repress specific patterns of gene expression. During the development of multicellular organisms, these events must be precisely timed. As a result, organogenesis requires an accurate spatio-temporal regulation of gene expression over extended periods. While many transcription factors required for the development of organs have been identified and their expression pinpointed spatially, the mechanisms through which they coordinate downstream gene expression in time remain poorly understood. This study addressed part of this question during the development of the Drosophila melanogaster tracheal system -- a model used to study epithelial organ development (Ozturk-Colak, 2018).

The tracheal system of Drosophila is formed by a network of epithelial tubes that requires tight temporal regulation of gene expression. Tracheal tube maturation involves the timely and spatially regulated deposition of a chitinous apical extracellular matrix (aECM), a process governed by downstream effectors of the midembryonic ecdysone hormone pulse. One of these ecdysone response genes is the D. melanogaster B-lymphocyte inducing maturation protein-1 (Blimp-1) (Ng, 2006; Chavoshi, 2010). Blimp-1 is the homolog of human Prdm1 (positive regulatory domain containing 1). Blimp-1/PRDM1 is a zinc finger transcriptional repressor that belongs to the Prdm gene family, and it was originally identified as a silencer of β-interferon gene expression. Prdm family members contain a conserved N-terminal domain, known as a positive regulatory (PR) domain. This domain has been associated with the SET methyltransferase domain, which is important for the regulation of chromatin-mediated gene expression. In addition, Prdm family proteins contain multiple zinc fingers that mediate sequence-specific DNA binding and protein--protein interactions. Prdm family members modulate key cellular processes, including cell fate, and the aberrant function of some members may lead to malignant transformation. During embryonic development, Blimp-1 controls a plethora of cell-fate decisions in many organisms. In D. melanogaster, Blimp-1 serves as an ecdysone-inducible gene that regulates ftz-f1 in pupal stages (Agawa, 2007). By acting as a transcriptional repressor, Blimp-1 prevents the premature expression of ftz-f1, thereby influencing the temporal regulation of events that are crucial for insect development. The expression level and stability of Blimp-1 is critical for the precise timing of pupariation (Akagi, 2016; Ozturk-Colak, 2018 and references therein).

Blimp-1 exerts a function in tracheal system morphogenesis during embryonic development (Ng, 2006; Ozturk-Colak, 2016). However, the question remains as to how this transcription factor regulates tube maturation events downstream of the hormone ecdysone. The role of Blimp-1 in the transcriptional regulation of the regulation of tracheal tube maturation in D. melanogaster was examined in this study. Blimp-1 was found to be a transcriptional repressor of many genes involved in tracheal development and its levels are critical for the precise timing of luminal maturation and the final stages of tubulogenesis in the embryo. The results indicate that Blimp-1, working downstream of ecdysone, acts as a link of hormone action during tube maturation in organogenesis (Ozturk-Colak, 2018).

This study found that Blimp-1 regulates multiple tracheal targets, thus acting as a key gene in tracheal development. Blimp-1 is an ecdysone response gene (Beckstead, 2005; Chavoshi, 2010) and, therefore, a link between the hormonal signal and the timing of tracheal tube maturation in both embryos and larvae. Blimp-1 was found to regulate the expression of many genes required for tube maturation. Interestingly, in silico, four Blimp-1 binding sites were detected in Blimp-1 regulatory sequences using the parameters described, which suggests that Blimp-1 may regulate its own expression. This is in agreement with recent data showing that Blimp-1/PRDM1 is also able to regulate its own expression in mammals (Mitani, 2017). Self-regulation of expression is consistent with the feedback loops in which Blimp-1/PRDM1 participates, and also with its role in regulating many developmental processes. Blimp-1 binding sites were found in the region of Tramtrack (Ttk), another transcription factor involved in many features of tube maturation (Ozturk-Colak, 2018).

Furthermore, Blimp-1 was observed to modulate the timing of the expression of reb and exp, two genes involved in the genetic program triggering timely chitin deposition. Untimely chitin deposition was shown to disturb tube maturation, thereby demonstrating that this process has to be tightly regulated during tracheal development. Tracheal overexpression of reb leads to earlier chitin deposition in all branches from stage 13 and sometimes chitin appearance at stage 12. Accordingly, the results show that Reb is expressed earlier in Blimp-1 embryos. This agrees with the early chitin deposition phenotypes observed in Blimp-1 mutants. Furthermore, Blimp-1 also modulated knk expression during tube maturation stages. Knk is involved in directing chitin assembly in the trachea and correct amounts of Knk at specific times during metamorphosis are important for correct wing cuticle differentiation and function. Taken together, these in silico and in vivo results indicate that Blimp-1 is a transcription factor that acts downstream of ecdysone, and that it is involved in the correct timing of chitin synthesis and deposition during embryonic development (Ozturk-Colak, 2018).

Blimp-1 binding sites were also found in the aPKC coding region. aPKC is involved in the junction anisotropies that orient both actin rings and taenidial ridges in the lumen of tracheal tubes. In Blimp-1 mutants, both actin rings and taenidial ridges are either undetectable or misoriented (Ozturk-Colak, 2016) --observations that are consistent with changes in junction anisotropy (Ozturk-Colak, 2018).

Previous work has shown that Blimp-1 regulates chitin deposition levels and architecture, and that, subsequently, the chitin aECM feeds back on the cellular architecture by stabilizing F-actin bundling and cell shape via the modulation of Src42A phosphorylation levels. However, this report provided no link between the chitinous aECM and Src42A. Btk29A mutant larvae have an aECM phenotype, which may be the result of their actin bundle phenotype. This study found that Btk29A removal can partially rescue the Blimp-1 taenidial orientation and expansion phenotype. In view of these results, it is proposed that the contribution of Btk29A can be added to the feedback model for the generation of supracellular taenidia put forward in Ozturk-Colak (2016). This study now adds to a previous model by hypothesizing that Blimp-1 acts as a link between the aECM and cells by regulating the levels of Btk29A. Btk29A and Src42A, together with the formin DAAM, have been shown to regulate the actin cytoskeleton. In agreement with the current results, it is speculated that Btk29A might phosphorylate Src42A, and that this phosphorylation event could be modulated by Blimp-1 and DAAM (Ozturk-Colak, 2018).

To conclude, the results indicate that Blimp-1 is a key player in the regulation of tracheal tube maturation, and, consequently, in the feedback mechanism involved in the generation of supracellular taenidia (Ozturk-Colak, 2018).

Proteasome activity determines pupation timing through the degradation speed of timer molecule Blimp-1

The transcriptional repressor Blimp-1 is a labile protein. This characteristic is key for determining pupation timing because the timing of the disappearance of Blimp-1 affects pupation timing by regulating the expression of its target betaftz-f1. However, the molecular mechanisms that regulate the protein turnover of Blimp-1 are still unclear. This study demonstrates that Blimp-1 is regulated by the ubiquitin proteasome system. Blimp-1 degradation is inhibited by proteasome inhibitor MG132. Pupation timing was delayed in mutants of 26S proteasome subunits as well as FBXO11, which recruits target proteins to the 26S proteasome as a component of the SCF ubiquitin ligase complex by slowing down the degradation speed of Blimp-1. Delay in pupation timing in the FBXO11 mutant was suppressed by the induction of betaFTZ-F1. Furthermore, fat-body-specific knockdown of proteasomal activity was sufficient to induce a delay in pupation timing. These results suggest that Blimp-1 is degraded by the 26S proteasome and is recruited by FBXO11 in the fat body, which is important for determining pupation timing (Aly, 2018).

This study showed that Drosophila Blimp-1 is degraded by the 26S proteasome system and is recruited by FBXO11 as the substrate-recognition component of the SCF complex. Furthermore, this study showed the importance of proteasome activity in the fat body to determine pupation timing. The results are correlated with previously described results that the biological timer system for pupation is located in the fat body (Akagi, 2016; Aly, 2018 and references therein).

A delay was observed in pupation timing in all of the examined heterozygous mutants of 26S proteasome components. These results suggest gene dosage effects due to loss-of-function mutations of these 26S proteasome components. In addition, a heterozygous mutant of recruiter FBXO11 also exhibited the same level of delay in pupation timing. These results indicate that the expression level of these components is an important factor to determine pupation timing; therefore, pupation timing can be controlled by the expression level of these components. Thus, it is assumed that the UPS contributes to determine pupation timing as one of the components in the biological timer during the early prepupal period. Of note, a sudden increase in the concentration of the 26S proteasome at 0 to 4 hr APF has been reported, suggesting the importance of protein degradation in developmental control. Furthermore, RNA-Seq data in the modENCODE developmental transcriptome of D. melanogaster showed that the expression of the FBXO11 increases gradually from the 3rd instar larval stage (L3) to a moderately high level at pupation and then starts to decrease again 24 hr later. These developmental changes may allow control of the degradation speed of specific targets, including Blimp-1, among many UPS target proteins that must be degraded at appropriate time points (Aly, 2018).

This study has shown that both the Blimp-1 and βftz-f1 are induced by 20E and are temporally expressed in almost all organs (Akagi, 2016), but the identified target genes are still limited in number. βFTZ-F1 has multiple functions in each organ during the mid to late prepupal period. For instance, βFTZ-F1 regulates two pupal cuticle genes that are expressed in slightly different parts of the epidermis, and it also regulates a protease that is expressed in the fat body and contributes to its morphological change. Furthermore, the expression of βFTZ-F1 in the inka cells is essential for releasing the ecdysis-triggering hormone ETH, which induces pupation in the late prepupal period, and also βFTZ-F1 expression in muscles is necessary to determine the timing of muscle apoptosis during metamorphosis. Moreover, βFTZ-F1 is a master regulator of late prepupal gene expression, which is essential for histolysis of the salivary gland cells during the early pupal period. In addition, the expression timing of βFTZ-F1 is not completely the same among different organs. In a large transcriptional profiling platform, involving 29 dissected tissues from larval, pupal, and adult stages of Drosophila, FBXO11 appeared to be expressed in many tissues and/or during development with specific upregulation in the fat body from L3 up to pupation. It is deduced that the expression levels of the 26S proteasome and FBXO11 may differ depending on tissue and contribute to the determination of timing of tissue-specific developmental events through control of the degradation speed of Blimp-1 (Aly, 2018).

In C. elegans, Blmp-1 was previously identified using RNAi-based suppressor screening to suppress dre-1 heterochronic phenotypes (Horn, 2014). A dre-1 mutant showed retarded migration of the gonad, whereas a Blmp-1 mutant showed precocious gonadal migration during L2 to L3 larva and was able to suppress the retarded phenotype of dre-1. In addition, precocious fusion and differentiation of epidermal stem cells, called seam cells, was partially suppressed by the Blmp-1 mutant in C. elegans. Moreover, similar genetic interactions were observed between DRE-1 and Blmp-1 for dauer formation. These observations suggest a conserved role of Blimp-1 degradation for the determination of developmental timing across taxa (Aly, 2018).

The Blimp-1 transcription factor acts in non-neuronal cells to regulate terminal differentiation of the Drosophila eye

The formation of a functional organ such as the eye requires specification of the correct cell types and their terminal differentiation into cells with the appropriate morphologies and functions. This study shows that the zinc-finger transcription factor Blimp-1 acts in secondary and tertiary pigment cells in the Drosophila retina to promote the formation of a bi-convex corneal lens with normal refractive power, and in cone cells to enable complete extension of the photoreceptor rhabdomeres. Blimp-1 expression depends on the hormone ecdysone, and loss of ecdysone signaling causes similar differentiation defects. Timely termination of Blimp-1 expression is also important, as its overexpression in the eye has deleterious effects. Transcriptomic analysis revealed that Blimp-1 regulates the expression of many structural and secreted proteins in the retina. Blimp-1 may function in part by repressing another transcription factor; Slow border cells is highly upregulated in the absence of Blimp-1, and its overexpression reproduces many of the effects of removing Blimp-1. This work provides insight into the transcriptional networks and cellular interactions that produce the structures necessary for visual function (Wang, 2022).

The Blimp-1 transcription factor has been shown to play important regulatory roles in the development of many cell types and organs in both flies and mammals. The only reported function for Blimp-1 in the mouse retina is to stabilize the photoreceptor cell fate; however, this study found that Drosophila Blimp-1 acts at a later stage of retinal development to promote the terminal differentiation of several cell types. It has no visible function in photoreceptors, but acts in the cone cells to enable normal photoreceptor rhabdomere extension and in the higher order pigment cells to promote the formation of a biconvex corneal lens. Blimp-1 controls the terminal differentiation of these cells by regulating a battery of genes that encode likely structural components of the eye as well as Slbo and other transcription factors. Upregulation of Blimp-1 expression in the retina requires ecdysone signaling, illustrating a role for steroid hormones in regulating sensory organ morphogenesis (Wang, 2022).

Although humans and Drosophila have shared common ancestors only in the very distant past and their lenses have distinct molecular compositions, both species evolved bi-convex lens shapes that enable them to focus light effectively onto the retina. The fly corneal lens cannot change its curvature through muscle contractions as the human lens does; to compensate for its fixed parameters, the fly retina is able to detect light throughout its depth. However, the thickness of the retina is insufficient to compensate for the massive reduction in the refractive power of the corneal lens resulting from the plano-convex shape caused by Blimp-1 loss-of-function, especially as loss of Blimp-1 also reduces retinal thickness. Loss of curvature on the external surface of Blimp-1 mutant corneal lenses is more detrimental to their refractive power than flattening of the internal surface facing the pseudocone would be, as the refractive indices of the corneal lens and pseudocone are similar to each other, but quite different from that of air (Stavenga, 2003) (Wang, 2022).

Although major corneal lens proteins such as Crystallin and Retinin are produced by the cone and primary pigment cells, Blimp-1 acts in the more peripheral secondary and tertiary pigment cells to generate the external curvature of the corneal lens. Blimp-1 has been shown to regulate chitin deposition during embryonic cuticle formation and tracheal maturation (Ozturk-Colak, 2018), and chitin is a major component of the corneal lens that may determine its shape. Blimp-1 also controls the expression of numerous cuticle proteins that could be secreted from higher-order pigment cells to give the periphery of the corneal lens a composition distinct from the center. Besides regulating the expression of direct structural components of the corneal lens, Blimp-1 could contribute to the bi-convex shape by regulating the forces that pigment cells exert on the periphery of the corneal lens through its effects on the expression of cell surface and cytoskeletal proteins (Wang, 2022).

In addition to its effect on corneal lens shape, Blimp-1 loss-of-function causes a defect in photoreceptor morphology. Instead of extending throughout the depth of the retina, rhabdomeres are absent from the proximal retina, where the photoreceptors form stalk-like structures lacking rhabdomere markers. Photoreceptor nuclei are also displaced to the proximal ends of the rhabdomeres. During development, photoreceptors interact with cone cells as they undergo a 90 ° rotation of their apical membrane surface. Microvilli projecting from this apical membrane subsequently form rhabdomeres that extend proximally down the retina, growing to ~90 &mi;m in length. As cone cells are known to structurally and physiologically support photoreceptors during development, loss of Blimp-1 in these cells may affect their ability to contribute to photoreceptor development. Genes encoding cell surface, cytoskeletal and secreted proteins that this study found to be regulated by Blimp-1, including trol, could promote rhabdomere extension or attachment to an ECM produced by the cone cell feet (Wang, 2022).

Loss of Blimp-1 leads to a dramatic upregulation of the transcription factor Slbo in non-neuronal retinal cells. Overexpressing slbo in the eye results in a plano-convex lens and shortened rhabdomeres, similar to what is observed in Blimp-1 mutants. slbo overexpression could thus contribute to the Blimp-1 loss-of-function phenotypes. However, removing slbo is not sufficient to restore normal retinal development in the absence of Blimp-1, indicating that at least one crucial target gene requires direct input from Blimp-1 in addition to the absence of Slbo. Two distinct mechanisms, one regulated by Slbo and one by loss of Blimp-1 independently of Slbo, can thus lead to this constellation of phenotypes (Wang, 2022).

Most previous studies of slbo have focused on its requirement for normal border cell migration in the ovary and have shown that its overexpression is detrimental to most other tissues. As the eye is quite distinct from the ovary in its structure and morphogenesis, it is surprising that only a single transcription factor prevents slbo from being expressed there. It is more common to find a factor that is poised for expression when a single repressor is removed in tissues that share a common origin or developmental pathway. For example, knot is required for wing patterning but is repressed by Ultrabithorax in the haltere disc, which is a structure homologous to the wing disc. It is not known what activators drive slbo expression in the absence of repression by Blimp-1. In the ovary, slbo is activated by JAK/STAT signaling, which acts early in eye development to promote growth of the larval eye disc, but is not known to function in the pupal retina. The mouse slbo homologue Cebpa is activated by Blimp-1 in uterine tissues , suggesting possible conservation of the regulation of C/EBP genes by Blimp-1, but not of the direction of this effect (Wang, 2022).

Given the presence of Blimp-1 at the periphery of the developing mouse cornea , these results raise the intriguing possibility that its role in regulating the curvature of refractive surfaces is conserved. Further investigation of this hypothesis may provide insight into the genesis of the refractive disorders that afflict a growing number of people (Wang, 2022).

Blimp-1/PRDM1 and Hr3/RORbeta specify the blue-sensitive photoreceptor subtype in Drosophila by repressing the hippo pathway

During terminal differentiation of the mammalian retina, transcription factors control binary cell fate decisions that generate functionally distinct subtypes of photoreceptor neurons. For instance, Otx2 and RORbeta activate the expression of the transcriptional repressor Blimp-1/PRDM1 that represses bipolar interneuron fate and promotes rod photoreceptor fate. Moreover, Otx2 and Crx promote expression of the nuclear receptor Nrl that promotes rod photoreceptor fate and represses cone photoreceptor fate. Mutations in these four transcription factors cause severe eye diseases such as retinitis pigmentosa. This study shows that a post-mitotic binary fate decision in Drosophila color photoreceptor subtype specification requires ecdysone signaling and involves orthologs of these transcription factors: Drosophila Blimp-1/PRDM1 and Hr3/RORbeta promote blue-sensitive (Rh5) photoreceptor fate and repress green-sensitive (Rh6) photoreceptor fate through the transcriptional repression of warts/LATS, the nexus of the phylogenetically conserved Hippo tumor suppressor pathway. Moreover, this study identified a novel interaction between Blimp-1 and warts, whereby Blimp-1 represses a warts intronic enhancer in blue-sensitive photoreceptors and thereby gives rise to specific expression of warts in green-sensitive photoreceptors. Together, these results reveal that conserved transcriptional regulators play key roles in terminal cell fate decisions in both the Drosophila and the mammalian retina, and the mechanistic insights further deepen understanding of how Hippo pathway signaling is repurposed to control photoreceptor fates for Drosophila color vision (Bunker, 2023).

Juvenile hormone and 20-hydroxyecdysone coordinately control the developmental timing of matrix metalloproteinase-induced fat body cell dissociation

Tissue remodeling is a crucial process in animal development and disease progression. Coordinately controlled by the two main insect hormones, juvenile hormone (JH) and 20-hydroxyecdysone (20E), tissues are remodeled context-specifically during insect metamorphosis. Previous work has discovered that two matrix metalloproteinases (Mmps) cooperatively induce fat body cell dissociation in Drosophila. However, the molecular events involved in this Mmps-mediated dissociation are unclear. This study reports that JH and 20E coordinately and precisely control the developmental timing of Mmps-induced fat body cell dissociation. During the larval-prepupal transition, the anti-metamorphic factor Kr-h1 was found to transduce JH signaling, which directly inhibited Mmps expression and activated expression of tissue inhibitor of metalloproteinases (timp), and thereby suppressed Mmps-induced fat body cell dissociation. It is also noted that upon a decline in the JH titer, a prepupal peak of 20E suppresses Mmps-induced fat body cell dissociation through the 20E primary-response genes, E75 and Blimp-1, which inhibited expression of the nuclear receptor and competence factor βftz-F1. Moreover, upon a decline in the 20E titer, βftz-F1 expression was induced by the 20E early-late response gene DHR3, and then βftz-F1 directly activated Mmps expression and inhibited timp expression, causing Mmps-induced fat body cell dissociation during 6-12 hrs after puparium formation. In conclusion, coordinated signaling via JH and 20E finely tunes the developmental timing of Mmps-induced fat body cell dissociation. These findings shed critical light on hormonal regulation of insect metamorphosis (Jia, 2017).

MMPs and tissue inhibitor of metalloproteinases (TIMPs) play crucial roles in regulating tissue remodeling in both vertebrates and Drosophila. Previous work has demonstrated the collaborative functions of Mmp1 and Mmp2 in inducing fat body cell dissociation in Drosophila (Jia, 2014). timp mutant adults show autolyzed tissue in the abdominal cavity and inflated wings, a phenotype consistent with the role of timp in BM integrity and remodeling. The current study clarified the role of timp in inhibiting the enzymatic activity of Mmps and thus, Mmp-induced fat body cell dissociation. In mammals, Mmps activity in vivo is controlled at different levels, including the regulation by gene expression, the zymogens activation, and the inhibition of active enzymes by TIMPs. These studies unify the important inhibitory roles of timp/TIMP in regulating tissue remodeling in both Drosophila and mammals. In addition to regulating Mmp expression, JH and 20E signals differentially regulate timp expression, with the stimulatory role of Kr-h1 and the inhibitory role of βftz-F1. Because timp inhibits the enzymatic activity of Mmps in the Drosophila fat body, it is concluded that JH and 20E coordinately control Mmps activity at both the mRNA and enzymatic levels (Jia, 2017).

Previously work has show the requirement of both JH and its receptors to inhibit fat body cell dissociation in Drosophila. This study demonstrated the ability of Kr-h1 to transduce JH signaling to decrease Mmp expression and to induce timp expression during larval-prepupal transition. Moreover, a Kr-h1-binding sites (KBS) was identified in the Mmp1 promoter, indicating that Kr-h1 directly represses Mmp1 expression. Interestingly, Kr-h1 expression gradually increases from initiation of wandering (IW) to 3 h APF when induced by JH and 20E in an overlapping manner, thus inhibiting the enzymatic activity of Mmps and Mmp-induced fat body cell dissociation during the larval-prepupal transition. Moreover, Kr-h1 acts as an anti-metamorphic factor by inhibiting 20E signaling. It is proposed, in addition to directly affecting the expression of Mmps and timp, that Kr-h1 might also indirectly regulate their expression by inhibiting 20E signaling (Jia, 2017).

Two consecutive 20E pulses control timely metamorphosis in Drosophila. Together with previous findings, the current results show that the conserved 20E transcriptional cascade precisely controls the timing of Mmp-induced fat body cell dissociation. In general, the first 20E signal pulse plays an inhibitory role during the larval-prepupal transition; however, it is a prerequisite for the expression of βftz-F1, which induces the second 20E signal pulse during the prepupal-pupal transition and the expression of Mmps. Because of the requirement for the first 20E signal pulse, blockade of the 20E receptor prevents fat body cell dissociation. When JH titer declines, the prepupal peak of 20E activates expression of two 20E primary-response genes, E75 and Blimp-1, to inhibit fat body cell dissociation: E75 represses DHR3 transactivation of βftz-F1 expression, and Blimp-1 directly represses βftz-F1 expression. During the prepupal-pupal transition, DHR3 directly induces βftz-F1 expression from 6 h APF to 12 APF. Before pupation, βftz-F1 induces Mmp expression and represses timp expression. Moreover, an FBS was identified in the Mmp2 promoter, demonstrating that βftz-F1 directly induces Mmp2 expression. Finally, within 6 h before pupation, Mmp1 and Mmp2 cooperatively induce fat body cell dissociation, with each assuming a distinct role (Jia, 2017).

Insect metamorphosis is coordinately controlled by JH and 20E, whereas the hormonal control of tissue remodeling is strictly context-specific. Different larval tissues and adult organs might have distinct, yet precise, developmental fates and timing. Knowledge regarding this question is poor. Based on previous preliminary information, this study clarified the detailed molecular mechanisms by which JH and 20E precisely control the developmental timing of Mmp-induced fat body cell dissociation at both mRNA and enzymatic levels in Drosophila, and a working model is provided of hormonal control of tissue remodeling in animals (see Model showing developmental timing of Mmp-induced fat body cell dissociation is coordinately and precisely controlled by JH and 20E in Drosophila). In summary, at first, Kr-h1 transduces JH signaling to inhibit Mmp-induced fat body cell dissociation during larval-prepupal transition. Then when JH titer declines, the prepupal peak of 20E suppresses Mmp-induced fat body cell dissociation through E75 and Blimp-1, which inhibit βftz-F1 expression. Finally, until 20E titer declines, DHR3 induces βftz-F1 expression, and βftz-F1 covers the 20E-triggered transcriptional cascade to activate Mmp-induced fat body cell dissociation within 6 h before pupation. This study provides an excellent sample for better understanding the hormonal regulation of insect metamorphosis (Jia, 2017).

A biological timer in the fat body comprising Blimp-1, betaFtz-f1 and Shade regulates pupation timing in Drosophila melanogaster

During the development of multicellular organisms, many events occur with precise timing. In Drosophila, pupation occurs about 12 hours after puparium formation, and its timing is believed to be determined by the release of a steroid hormone, ecdysone (E), from the prothoracic gland. This study demonstrates that the ecdysone-20-monooxygenase, Shade, determines the pupation timing by converting E to 20-hydroxyecdysone (20E) in the fat body, which is the organ that senses nutritional status. The timing of shade expression is determined by its transcriptional activator βFTZ-F1. The βFTZ-F1 gene is activated after a decline in the expression of its transcriptional repressor Blimp-1, which is temporally expressed around puparium formation in response to a high titer of 20E. The expression level and stability of Blimp-1 is critical for the precise timing of pupation. Thus, it is proposed that Blimp-1 molecules function as sands in an hourglass for this precise developmental timer system. Furthermore, the data suggest a biological advantage results from both the use of a transcriptional repressor for the time determination, and association of developmental timing with nutritional status of the organism (Akagi, 2016).

This study shows that the gene regulatory pathway consisting of Blimp-1, βftz-f1 and shade works as a biological timer to measure a specific period during the prepupal period. The results suggest that a biological advantage results from use of a transcriptional repressor for the precise timer system. The timing of gene expression could be determined by either induction of its transcriptional activator or depletion of its transcriptional repressor. Accuracy of the induction timing of a gene depends on the rate of accumulation and disappearance of its transcriptional activator and repressor, respectively. It is possible to increase the amounts of repressor molecules produced to elongate the time without changing their rate of degradation. Thus, if the affinity of DNA binding by transcription factors is at the same level, the repressor (e.g., Blimp-1) can maintain high accuracy to determine the timing of expression of the target gene after receiving the upstream induction signal. This study further demonstrated the importance of the expression level and stability of the transcriptional repressor Blimp-1 for the time measurement system. Accordingly, it is proposed that Blimp-1 molecules work like sand in an accurate hourglass to determine pupation timing (Akagi, 2016).

Blimp-1 is a conserved factor among Metazoa and contributes to cell fate decision in many organs during development (Bikoff, 2009). A recent report showed that degradation of Blimp-1 by DRE-1/FBXO11 coordinates developmental timing in Caenorhabditis elegans and this protein interaction is conserved in mammals (Horn, 2014), suggesting the conserved importance of the degradation of Blimp-1 for the timing decision during the development (Akagi, 2016).

This study found that the pupation timer is composed of Blimp-1, βftz-f1 and shade; however, the delays in pupation by induction of Blimp-1 or knockdown of either βftz-f1 or shade were different. The difference may be caused by the expression levels of each transgene, including the RNAi efficiency and the minimum requirement of each protein for timing determination. Other organs may also contribute to this pathway, because relatively low levels of shade expression are detected in the midgut and the Malpighian tubules in addition to the fat body. This multi-organ contribution could be one of the reasons that fat body-specific knockdown of shade did not cause lethality (Akagi, 2016).

Although this study found that the identified timer system is crucial to determine pupation timing, several observations reveal that pupation timing is restricted only to a specific period, suggesting that other factor(s) provide competence for pupation. In addition to these results, it was not possible to rescue the pupation deficiency by injecting 20E in the ftz-f1 mutant prepupae, suggesting that βFtz-f1 controls not only 20E production through Shade but also expression of other factors necessary for pupation. Ecdysis triggering hormone (ETH), a peptide hormone that regulates the pupation behavior, is potentially one of these factors, as βFtz-f1 is essential for release of ETH from the inka cells. The timing of E production, which is triggered by prothoracicotropic hormone (PTTH), is an important factor for pupation; it was observed that pupation timing was delayed for several hours in prepupae where PTTH-producing neurons are ablated. However, neither ectopic Blimp-1 induction nor βftz-f1 knockdown affected the expression of either ptth or eth transcripts. It has been reported that the chromatin remodeling protein INO80 has an effect on the pupation timing by regulating the regression of ecdysone-regulated genes including βftz-f1. These results suggest the presence of other mechanisms, acting independently of the identified timer system, to restrict pupation timing (Akagi, 2016).

This study identified the fat body as an essential tissue necessary to drive this developmental timer system. Several reports have shown a link between nutrient status and developmental timing, and it has been suggested that the fat body is the central tissue for coordinating this link. Thus, it is expected that the fat body incorporates the nutritional status of the animal and sends a cue for the final decision of pupation independent of the timer system (Akagi, 2016).

A recent publication proposed that shade gene expression could be finely tuned by acetylation of Ftz-f1. This result supports the idea that βFtz-f1 directly regulates the shade gene. On the other hand, βFtz-f1 is expressed after decline of 20E level in almost all organs at the late embryonic stage and each larval and pupal stage during development, but the upregulation of shade is limited around the high ecdysteroid period for puparium formation. These results suggest that the activation of the shade gene by βFtz-f1 is restricted only in the prepupal period. Further studies are needed to understand the regulation mechanism of the shade gene, including epigenetic regulation of the time-measuring mechanism (Akagi, 2016).

In this transcriptional cascade, the mechanism for initiating the timer is crucial to drive the system. The regulatory mechanism required to determine the period of ecdysone pulse, which induces puparium formation, has been unveiled recently at the molecular level. This system potentially works as a switch for the Blimp-1 timer to determine the specific period after the decline of the 20E level (Akagi, 2016).

A feedback mechanism converts individual cell features into a supracellular ECM structure in Drosophila trachea

The extracellular matrix (ECM), a structure contributed to and commonly shared by many cells in an organism, plays an active role during morphogenesis. This study used the Drosophila tracheal system to study the complex relationship between the ECM and epithelial cells during development. There is an active feedback mechanism between the apical ECM (aECM) and the apical F-actin in tracheal cells. Furthermore, cell-cell junctions are key players in this aECM patterning and organisation, and individual cells contribute autonomously to their aECM. Strikingly, changes in the aECM influence the levels of phosphorylated Src42A (pSrc) at cell junctions. Therefore, it is proposed that Src42A phosphorylation levels provide a link for the ECM environment to ensure proper cytoskeletal organisation (Ozturk-Colak, 2016).

This study examined the apical ECM (aECM) of Drosophila melanogaster trachea, the insect respiratory system. Once the different branches of the tracheal system have been established to cover the overall embryonic body, tracheal cells begin to secrete the components of a chitin-rich aECM that lines up the lumen of the tracheal tubes and can be visualised by the incorporation of chitin-binding probes. A distinctive feature of this aECM are taenidial folds, a series of cuticle ridges that compose a helical structure running perpendicular to the tube length along the entire lumen. Taenidia are believed to confer mechanical strength to the tubes and have been compared to a coiled spring within a rubber tube or to the corrugated hose of a vacuum cleaner. From the very first descriptions, it was noticed that taenidia are unaffected by the presence of cell boundaries, thereby indicating that they are a supracellular structure and suggesting a substantial degree of intercellular coordination. It has been reported that taenidial organisation correlates with that of the apical F-actin bundles in underlying cells—the formation of these bundles preceding the appearance of taenidia. However, the relationship between these bundles and taenidia is still poorly understood. In addition, physical modelling has recently revealed that the interaction of the apical cellular membrane and the aECM determines the stability of biological tubes, thus generating more questions about how this interaction occurs (Ozturk-Colak, 2016).

This study reports that there is a dynamic relationship between sub-apical F-actin and taenidial folds during tracheal lumen formation. Cell-cell junctions participate in organising F-actin bundles and the taenidial fold supracellular aECM and this chitinous aECM contributes to regulating F-actin organisation in a two-way regulatory mechanism (Ozturk-Colak, 2016).

The contribution of chitin deposition to the organisation of taenidia was examined. As when studying the contribution of tracheal actin rings to this process, mutants were chosen that do not completely inhibit chitin deposition, as these mutants would probably heavily impair tracheal development, thus hindering specific analysis of the morphogenesis of taenidia. Thus, this investigation turned to Blimp-1, an ecdysone response gene (Chavoshi, 2010; Beckstead, 2005) that encodes the Drosophila homolog of the transcriptional factor B-lymphocyte-inducing maturation protein gene and whose mutants have been reported to have misshapen trachea almost completely devoid of taenidia (Ozturk-Colak, 2016).

Indeed, Blimp-1 mutant embryos were grossly inflated compared to the wild-type, a phenotype associated with weaker embryonic cuticles caused by mutations impairing the deposition or organisation of chitin. Consistent with this observation, Blimp-1 mutants showed a pale ectodermal cuticle with smaller denticles, although their phenotype is weaker than that of the kkv chitin synthase mutants. This observation suggests that, while chitin deposition is severely impaired, some still accumulated in the cuticle of Blimp-1 mutant embryos. In support of this hypothesis, lower levels of fluostain signal were detected in the trachea of Blimp-1 mutants compared to the wild-type. Thus, this study expected to find similarly less conspicuous taenidia, which was indeed the case. However, the most obvious abnormal feature of taenidia was their pattern, as they were not organised in folds perpendicular to the tube axis but instead ran parallel to it. Given the close correlation between taenidia and actin bundle organisation, actin arrangement was examined in Blimp-1 mutants, and it was found to be severely impaired. In most Blimp-1 mutants examined, no tracheal actin rings were observed. However, in the mutant embryos in whichd apical actin bundles were detected, these were oriented in parallel to the tube length like the chitin structures. Thus, as is the case for the other mutant genotypes examined so far, in Blimp-1 embryos the lack of a proper arrangement of taenidial folds correlates with either the absence or abnormal pattern of actin rings (Ozturk-Colak, 2016).

Detailed ultrastructural analysis by TEM confirmed the close interplay between actin and chitin in both tal/pri and Blimp-1 mutants. In wild-type embryos, each taenidium is formed by a plasma membrane protrusion and the taenidia have a regular shape. Arrangement of plasma membrane protrusions in tal/pri and Blimp-1 mutant tracheal cells is irregular. At the end of embryogenesis, whereas the breadth of these taenidia is very constant in wt animals, it is highly variable in tal/pri and Blimp-1 mutants. This result is in line with the finding that proper F-actin ring organisation and chitin deposition are necessary for taenidial morphogenesis (Ozturk-Colak, 2016).

The observation of an effect of a mutation in a gene required for proper chitin arrangement on actin bundling was unexpected. To assess whether the effect of Blimp-1 mutations on actin organisation was indeed a consequence of abnormal chitin deposition in the tracheal cuticle rather than the result of a direct and yet unknown role of Blimp-1 in F-actin bundling, tracheal actin organisation was examined in mutants for kkv, a gene required for chitin morphogenesis only. Surprisingly, kkv mutants also lacked actin rings, thereby indicating a feedback role of proper chitin-mediated tracheal cuticle in F-actin organisation. In addition, F-actin bundles formed normally and thereafter collapsed in kkv mutants. This finding indicates that a proper cuticle is not required for the establishment of the F-actin rings but instead for their maintenance. This implies that proper chitin deposition/organisation contributes to ensure the proper organisation and stability of the apical F-actin rings (Ozturk-Colak, 2016).

How could the apical chitin in the ECM influence actin bundling? It was observed that both kkv and Blimp-1 mutations had an effect on tracheal cell shape. In the wild-type trachea, the cells of the DT were organised such that the longest axis of their apical shape is parallel to the tube axis. However, in both Blimp-1 and kkv mutant trachea, the anteroposterior elongation of the cells of the DT was lost, causing cells to be more square shaped. Thus, it was hypothesised that the change in taenidial orientation in kkv and Blimp-1 mutants could be attributed to the alteration in the overall orientation or shape of the tracheal cells. Interestingly, a modification of cell shape/orientation also occurs in embryos mutant for the Src-family kinase Src42A. However, and as previously reported for F-actin, this study found taenidia to follow the same organisation in Src42A mutant embryos as the wild-type indicating that proper organisation of taenidia can be uncoupled from correct tracheal cell shape/orientation and thus that the former is not merely a consequence of the latter (Ozturk-Colak, 2016).

Having identified and characterised genes that specifically affect taenidial patterning, the individual cell contributions to this supracellular organisation was examined by impairing genetic functions in mosaics. It was not possible to generate mosaics by mitotic recombination since there are no cell divisions after tracheal invagination and RNAi-mediated knockdown often does not work in Drosophila embryogenesis; this was indeed the case upon expression of UAS-RNAi constructs for either tal/pri or Blimp-1 in the embryonic tracheal cells. Thus, alternative approaches were used to produce tracheal cellular chimeras (Ozturk-Colak, 2016).

First, advantage was taken of the effect of Blimp-1 overexpression on taenidial formation. To generate tracheal DTs with distinct cellular composition, an AbdB-Gal4 line was used that drives expression only in the posterior part of the embryo. This approach served as an internal control within the same embryo. Upon expression of UASBlimp-1 under these conditions, lower levels of chitin were detected in the posterior metameres. Thus, chitin deposition seems to be highly dependent on the levels of Blimp-1 activity as both loss-of-function mutations and overexpression of Blimp-1 induce low levels of chitin. It is also noted that overexpression of Blimp-1 gives rise to tracheal cells with a less elongated apical side, like that of Blimp-1 and kkv mutants. Then the trachea at the border of the AbdB-Gal4 domain were examined, finding a perfect correlation between the different physical appearance of taenidia and cells and their genotype, with either wild-type or increased levels of Blimp-1. Flip-out clones expressing Blimp-1 were generated in a wild-type background, and similar results were obtained in these clones. Thus, it is concluded that Blimp-1 regulates chitin accumulation in a cell-autonomous manner and that each cell contributes independently to the chitin deposition of their corresponding segments of the taenidial folds (Ozturk-Colak, 2016).

As a second approach to mosaic analysis, he same AbdBGal4 line was used to drive expression of tal/pri and Blimp-1 in tal/pri and in Blimp-1 loss-of-function mutant backgrounds, respectively. For both mutants, a rescuing effect was seen in the posterior tracheal metameres as taenidial folds became organised perpendicularly to the tube length. Using this approach, it was possible to generate borders of cells with and without tal/pri and Blimp-1 function and taenidia were analyzed in these conditions. In the case of the tal/pri rescue experiment, a difference was detected between the cells expressing the wild-type tal/pri gene and those with a wild-type phenotype, an observation consistent with the non-cell autonomous function of the Tal/Pri peptides. However, in the case of the Blimp-1 rescue experiment, taenidia tended to follow the orientation dictated by the genotype of their respective cells. Moreover, and due to the expression domain of the AbdBGAL4 driver not being completely continuous, single cells of one of the genotypes were observed surrounded by cells of the other and either mutant cells could be detected with a longitudinal arrangement of the taenidia or 'rescued' cells with a perpendicular arrangement; in this case, there was a correlation between the physical appearance of taenidia and the corresponding cell genotype. Interestingly, intermediate orientations between the prototypical longitudinal taenidia were also detected in the mutant domain and the perpendicular ones in the rescued domain. These results suggest that cells 'adapt' the orientation of 'their' segments of the taenidia to the global orientation of the segments of the taenidia contributed by neighbouring cells (Ozturk-Colak, 2016).

These results show that tracheal taenidia can form proper rings even when the neighbouring cells do not. This indicates that, to a certain degree, segments of taenidia can organise properly even in the absence of proper subjacent actin rings provided that the segments of taenidia contributed by the neighbouring cells are properly organised (Ozturk-Colak, 2016).

The role for the apical chitin ECM in tracheal actin organisation indicates a feedback mechanism to generate the supracellular taenidial structures. In the light of the above and previously published results, the following model is proposed for the formation of the taenidial folds that expand the overall diameter of the tracheal tube. On the one hand, actin polymerises in rings at the apical side of the tracheal cells in a tal/pri-dependent process; these actin rings are then required for the particular accumulation of the kkv chitin synthase and for the appearance of folds in the plasma membrane. In turn, kkv accumulation leads to a localised increased production and deposition of chitin along specific enriched stripes above the actin rings in a Blimp-1-mediated process. On the other hand, the cellular AJs are instrumental in ensuring that apical F-actin bundles from each cell follow a supracellular organ arrangement. It has to be noted that each cell appears to independently organise or maintain, to a certain degree, the proper orientation of their actin bundles, as determined by Blimp-1 clonal analysis and the disruption of cell adhesion by downregulation of α-Cat and, consequently, DE-Cad. These results further suggest cell polarity along the circumferential axis of the tracheal tube. Nevertheless, this is not an absolute value as cells also have the capacity to modify the orientation of their sections of the taenidia to keep the continuity of these structures along the tube. In this regard, cell adhesion is central to ensure the continuity of the intracellular actin bundles as a patterning element for the overall tube. Subsequently, the chitin aECM feeds back on to the cellular architecture by stabilising F-actin bundling and cell shape via the modulation of Src42A phosphorylation levels. The combination of all these phenomena explain just how it is that the cells of the tracheal epithelium can cooperate unconsciously so as to form a helicoid [chitinous] thickening continuous from one end of the trachea to another (Ozturk-Colak, 2016).

Blimp1 (Prdm1) prevents re-specification of photoreceptors into retinal bipolar cells by restricting competence

During retinal development, photoreceptors and bipolar cells express the transcription factor Otx2 (Drosophila homolog: Orthodenticle). Blimp1 is transiently expressed in Otx2+ cells. Blimp1 deletion results in excess bipolar cell formation at the expense of photoreceptors. In principle, Blimp1 could be expressed only in Otx2+ cells that are committed to photoreceptor fate. Alternatively, Blimp1 could be expressed broadly in Otx2+ cells and silenced to allow bipolar cell development. To distinguish between these alternatives, the fate of Blimp1 expressing cells was followed using Blimp1-Cre mice and Lox-Stop-Lox reporter strains. Blimp1+ cells were observed to give rise to all photoreceptors, but also to one third of bipolar cells, consistent with the latter alternative: that Blimp1 inhibits bipolar competence in Otx2+ cells and must be silenced to allow bipolar cell generation. To further test this hypothesis, transitioning rod photoreceptors were looked for in Blimp1 conditional knock-out (CKO) mice carrying the NRL-GFP transgene, which specifically labels rods. Control animals lacked NRL-GFP+ bipolar cells. In contrast, about half of the precociously generated bipolar cells in Blimp1 CKO mice co-expressed GFP, suggesting that rods become re-specified as bipolar cells. Birthdating analyses in control and Blimp1 CKO mice showed that bipolar cells were birthdated as early as E13.5 in Blimp1 CKO mice, five days before this cell type was generated in the wild-type retina. Taken together, these data suggest that early Otx2+ cells upregulate photoreceptor and bipolar genes, existing in a bistable state. Blimp1 likely forms a cross-repressive network with pro-bipolar factors such that the winner of this interaction stabilizes the photoreceptor or bipolar state, respectively (Brzezinski, 2013).

Regulatory mechanisms of ecdysone-inducible Blimp-1 encoding a transcriptional repressor that is important for the prepupal development in Drosophila

Blimp-1 is an ecdysone-inducible transcription factor that is expressed in the early stage of the prepupal period. The timing of its disappearance determines expression timing of the FTZ-F1 gene, whose temporally restricted expression is essential for the prepupal development. To elucidate the termination mechanism of Blimp-1 gene expression, this study examined the regulation of the Blimp-1 gene using an organ culture system. The results showed that the Blimp-1 gene is transcribed in cultured organs taken from a low ecdysteroid period even after extended exposure to 20-hydroxyecdysone, while well-known early genes such as E75A are repressed under the same conditions. Similar selective transcription was observed in the cultured organs obtained from a high ecdysteroid period. This study further showed that Blimp-1 transcripts quickly disappeared in the presence of actinomycin D. From these results, it is concluded that the Blimp-1 gene is transcribed when the ecdysteroid titer is high, but the expressed mRNA degrades rapidly; these unique regulations limit its expression to the high ecdysteroid stage (Akagi, 2011).

Drosophila Blimp-1 is a transient transcriptional repressor that controls timing of the ecdysone-induced developmental pathway

Regulatory mechanisms controlling the timing of developmental events are crucial for proper development to occur. ftz-f1 is expressed in a temporally regulated manner following pulses of ecdysteroid and this precise expression is necessary for the development of Drosophila melanogaster. To understand how insect hormone ecdysteroids regulate the timing of FTZ-F1 expression, a DNA binding regulator of Ftz-f1 was purified. Mass spectroscopy analysis revealed this protein to be a fly homolog of mammalian B lymphocyte-induced maturation protein 1 (Blimp-1). Drosophila Blimp-1 (dBlimp-1) is induced directly by 20-hydroxyecdysone, and its product exists during high-ecdysteroid periods and turns over rapidly. Forced expression of dBlimp-1 and RNA interference analysis indicate that dBlimp-1 acts as a repressor and controls the timing of FTZ-F1 expression. Furthermore, its prolonged expression results in delay of pupation timing. These results suggest that the transient transcriptional repressor dBlimp-1 is important for determining developmental timing in the ecdysone-induced pathway (Agawa, 2007).

A homologue of the vertebrate SET domain and zinc finger protein Blimp-1 regulates terminal differentiation of the tracheal system in the Drosophila embryo

The B-lymphocyte-inducing maturation protein (Blimp-1) gene encodes a zinc finger and SET/PR domain-containing transcriptional factor. A number of functional studies in a variety of vertebrate species have demonstrated that Blimp-1 is a master regulator of cell fate determination and cell differentiation in a wide diversity of developmental contexts. Despite all of this significance, the role, if any, of a homologue of Blimp-1 in directing morphogenetic events during embryonic development of invertebrates has so far remained completely unexplored. This report describes the identification of a Drosophila homologue of Blimp-1 and shows that the gene is expressed in diverse cell types during the course of embryogenesis. Further, using genetic analysis, it was demonstrated that its wild-type activity is critically required for the maturation of the tracheal system into properly differentiated tubes (Ng, 2006).

Sex-specific adipose tissue imprinting of regulatory T cells

Adipose tissue is an energy store and a dynamic endocrine organ. In particular, visceral adipose tissue (VAT) is critical for the regulation of systemic metabolism. Impaired VAT function-for example, in obesity-is associated with insulin resistance and type 2 diabetes. Regulatory T (Treg) cells that express the transcription factor FOXP3 are critical for limiting immune responses and suppressing tissue inflammation, including in the VAT. This study uncovered pronounced sexual dimorphism in Treg cells in the VAT. Male VAT was enriched for Treg cells compared with female VAT, and Treg cells from male VAT were markedly different from their female counterparts in phenotype, transcriptional landscape and chromatin accessibility. Heightened inflammation in the male VAT facilitated the recruitment of Treg cells via the CCL2-CCR2 axis. Androgen regulated the differentiation of a unique IL-33-producing stromal cell population specific to the male VAT, which paralleled the local expansion of Treg cells. Sex hormones also regulated VAT inflammation, which shaped the transcriptional landscape of VAT-resident Treg cells in a BLIMP1 transcription factor-dependent manner. Overall, this study found that sex-specific differences in Treg cells from VAT are determined by the tissue niche in a sex-hormone-dependent manner to limit adipose tissue inflammation (Vasanthakumar, 2020).

A critical role of PRDM14 in human primordial germ cell fate revealed by inducible degrons

PRDM14 is a crucial regulator of mouse primordial germ cells (mPGCs), epigenetic reprogramming and pluripotency, but its role in the evolutionarily divergent regulatory network of human PGCs (hPGCs) remains unclear. Besides, a previous knockdown study indicated that PRDM14 might be dispensable for human germ cell fate. This study used inducible degrons for a more rapid and comprehensive PRDM14 depletion. PRDM14 loss results in significantly reduced specification efficiency and an aberrant transcriptome of hPGC-like cells (hPGCLCs) obtained in vitro from human embryonic stem cells (hESCs). Chromatin immunoprecipitation and transcriptomic analyses suggest that PRDM14 cooperates with TFAP2C and BLIMP1 to upregulate germ cell and pluripotency genes, while repressing WNT signalling and somatic markers. Notably, PRDM14 targets are not conserved between mouse and human, emphasising the divergent molecular mechanisms of PGC specification. The effectiveness of degrons for acute protein depletion is widely applicable in various developmental contexts (Sybirna, 2020).

Control of germinal center localization and lineage stability of follicular regulatory T cells by the Blimp1 transcription factor

Follicular regulatory T (TFR) cells are a specialized suppressive subset that controls the germinal center (GC) response and maintains humoral self-tolerance. The mechanisms that maintain TFR lineage identity and suppressive activity remain largely unknown. This study shows that expression of Blimp1 by FoxP3(+) TFR cells is essential for TFR lineage stability, entry into the GC, and expression of regulatory activity. Deletion of Blimp1 in TFR cells reduced FoxP3 and CTLA-4 expression and increased pro-inflammatory cytokines and spontaneous production of autoantibodies, including elevated IgE. Maintenance of TFR stability reflected Blimp1-dependent repression of the IL-23R-STAT3 axis and activation of the CD25-STAT5 pathway, while silenced IL-23R-STAT3 or increased STAT5 activation rescued the Blimp1-deficient TFR phenotype. Blimp1-dependent control of CXCR5/CCR7 expression also regulated TFR homing into the GC. These findings uncover a Blimp1-dependent TFR checkpoint that enforces suppressive activity and acts as a gatekeeper of GC entry (Shen, 2019).

Principles for the regulation of multiple developmental pathways by a versatile transcriptional factor, BLIMP1

Single transcription factors (TFs) regulate multiple developmental pathways, but the underlying mechanisms remain unclear. This study quantitatively characterized the genome-wide occupancy profiles of BLIMP1, a key transcriptional regulator for diverse developmental processes, during the development of three germ-layer derivatives (photoreceptor precursors, embryonic intestinal epithelium and plasmablasts) and the germ cell lineage (primordial germ cells). BLIMP1-binding sites were found to be shared among multiple developmental processes, and such sites were highly occupied by BLIMP1 with a stringent recognition motif and were located predominantly in promoter proximities. A subset of bindings common to all the lineages exhibited a new, strong recognition sequence, a GGGAAA repeat. Paradoxically, however, the shared/common bindings had only a slight impact on the associated gene expression. In contrast, BLIMP1 occupied more distal sites in a cell type-specific manner; despite lower occupancy and flexible sequence recognitions, such bindings contributed effectively to the repression of the associated genes. Recognition motifs of other key TFs in BLIMP1-binding sites had little impact on the expression-level changes. These findings suggest that the shared/common sites might serve as potential reservoirs of BLIMP1 that functions at the specific sites, providing the foundation for a unified understanding of the genome regulation by BLIMP1, and, possibly, TFs in general (Mitani, 2017).

Dusky-like is required for epidermal pigmentation and metamorphosis in Tribolium castaneum

Dusky-like (Dyl) is associated with the morphogenesis of embryonic denticle, adult sensory bristle and wing hair in Drosophila melanogaster. And whether Dyl involved in insect post-embryonic development and its signal transduction are poorly understood. In this study, phylogenetic analysis revealed that dyl displayed one-to-one orthologous relationship among insects. In Tribolium castaneum, dyl is abundantly expressed at the late embryonic stage. Tissue-specific expression analysis at the late adult stage illustrated high expression of dyl in the fat body and ovary. Knockdown of dyl resulted in the defects in larval epidermal pigmentation and completely blocked the transitions from larval to pupal and pupal to adult stages of T. castaneum. It was further discovered that dyl RNAi phenotypes were phenocopied by blimp-1 or shavenbaby (svb) silencing, and dyl was positively regulated by blimp-1 through svb in T. castaneum. These results suggest that Dyl functions downstream of Blimp-1 through Svb for larval epidermal pigmentation and metamorphosis. Moreover, ftz-f1 was down-regulated after RNA interference (RNAi) suppressing any of those three genes, indicating that Ftz-f1 works downstream of Dyl to mediate the effects of Blimp-1, Svb and Dyl on metamorphosis in T. castaneum. This study provides valuable insights into functions and signaling pathway of insect Dyl (Li, 2016).

The transcriptional repressor Blimp-1 acts downstream of BMP signaling to generate primordial germ cells in the cricket Gryllus bimaculatus

Segregation of the germ line from the soma is an essential event for transmission of genetic information across generations in all sexually reproducing animals. Although some well-studied systems such as Drosophila and Xenopus use maternally inherited germ determinants to specify germ cells, most animals, including mice, appear to utilize zygotic inductive cell signals to specify germ cells during later embryogenesis. Such inductive germ cell specification is thought to be an ancestral trait of Bilateria, but major questions remain as to the nature of an ancestral mechanism to induce germ cells, and how that mechanism evolved. It was previously reported that BMP signaling-based germ cell induction is conserved in both the mouse Mus musculus and the cricket Gryllus bimaculatus, which is an emerging model organism for functional studies of induction-based germ cell formation. In order to gain further insight into the functional evolution of germ cell specification, this study examined the Gryllus ortholog of the transcription factor Blimp-1 (also known as Prdm1), which is a widely conserved bilaterian gene known to play a crucial role in the specification of germ cells in mice. Functional analyses of the Gryllus Blimp-1 ortholog revealed that it is essential for Gryllus primordial germ cell development, and is regulated by upstream input from the BMP signaling pathway. This functional conservation of the epistatic relationship between BMP signaling and Blimp-1 in inductive germ cell specification between mouse and cricket supports the hypothesis that this molecular mechanism regulated primordial germ cell specification in a last common bilaterian ancestor (Nakamura, 2016).

DRE-1/FBXO11-dependent degradation of BLMP-1/BLIMP-1 governs C. elegans developmental timing and maturation

Developmental timing genes catalyze stem cell progression and animal maturation programs across taxa. Caenorhabditis elegans DRE-1/FBXO11 functions in an SCF E3-ubiquitin ligase complex to regulate the transition to adult programs, but its cognate proteolytic substrates are unknown. This study identified the conserved transcription factor BLMP-1 as a substrate of the SCF(DRE-1/FBXO11) complex. blmp-1 deletion suppressed dre-1 mutant phenotypes and exhibited developmental timing defects opposite to dre-1. blmp-1 also opposed dre-1 for other life history traits, including entry into the dauer diapause and longevity. BLMP-1 protein was strikingly elevated upon dre-1 depletion and dysregulated in a stage- and tissue-specific manner. The role of DRE-1 in regulating BLMP-1 stability is evolutionary conserved, as direct protein interaction and degradation function were observed for worm and human counterparts. Taken together, posttranslational regulation of BLMP-1/BLIMP-1 by DRE-1/FBXO11 coordinates C. elegans developmental timing and other life history traits, suggesting that this two-protein module mediates metazoan maturation processes (Horn, 2014).

Blimp1 associates with Prmt5 and directs histone arginine methylation in mouse germ cells

Blimp1, a transcriptional repressor, has a crucial role in the specification of primordial germ cells (PGCs) in mice at embryonic day 7.5 (E7.5). This SET-PR domain protein can form complexes with various chromatin modifiers in a context-dependent manner. Blimp1 has a novel interaction with Prmt5, an arginine-specific histone methyltransferase that mediates symmetrical dimethylation of arginine 3 on histone H2A and/or H4 tails (H2A/H4R3me2s). Prmt5 has been shown to associate with Tudor, a component of germ plasm in Drosophila melanogaster. Blimp1-Prmt5 colocalization results in high levels of H2A/H4 R3 methylation in PGCs at E8.5. However, at E11.5, Blimp1-Prmt5 translocates from the nucleus to the cytoplasm, resulting in the loss of H2A/H4 R3 methylation at the time of extensive epigenetic reprogramming of germ cells. Subsequently, Dhx38, a putative target of the Blimp1-Prmt5 complex, is upregulated. Interestingly, expression of Dhx38 is also seen in pluripotent embryonic germ cells that are derived from PGCs when Blimp1 expression is lost. This study demonstrates that Blimp1 is involved in a novel transcriptional regulatory complex in the mouse germ-cell lineage (Ancelin, 2006).

Characterization of the B lymphocyte-induced maturation protein-1 (Blimp-1) gene, mRNA isoforms and basal promoter

Blimp-1 is a transcriptional repressor that is both required and sufficient to trigger terminal differentiation of B lymphocytes and monocyte/macrophages. This study reports the organization of the mouse Blimp-1 gene, an analysis of Blimp-1 homologs in different species, the characterization of Blimp-1 mRNA isoforms and initial studies on the transcription of Blimp-1. The murine Blimp-1 gene covers approximately 23 kb and contains eight exons. There are Blimp-1 homologs in species evolutionarily distant from mouse (Caenorhabditis elegans and Drosophila melanogaster) but no homolog was found in the unicellular yeast Saccharomyces cerevisiae. The three major Blimp-1 mRNA isoforms result from the use of different polyadenylation sites and do not encode different proteins. Run-on transcription analyses were used to show that the developmentally regulated expression of Blimp-1 mRNA in B cells is determined by transcription initiation. Multiple Blimp-1 transcription initiates sites were mapped near an initiator element and a region conferring basal promoter activity has been identified (Tunyaplin, 2000).

PRDI-BF1/Blimp-1 repression is mediated by corepressors of the groucho family of proteins

The PRDI-BF1/Blimp-1 protein is a transcriptional repressor required for normal B-cell differentiation, and it has been implicated in the repression of beta-interferon (IFN-beta) and c-myc gene expression. PRDI-BF1 represses transcription of the IFN-beta promoter and of an artificial promoter through an active repression mechanism. A minimal repression domain has been identified in PRDI-BF1 that is sufficient for transcriptional repression when tethered to DNA as a Gal4 fusion protein. Remarkably, this repression domain interacts specifically with hGrg, TLE1, and TLE2 proteins, all of which are members of the Groucho family of transcriptional corepressors. In addition, the hGrg protein itself can function as a potent repressor when tethered to DNA through the Gal4 DNA-binding domain. The amino-terminal glutamine-rich domains of hGrg and TLE1 are found to be sufficient to mediate dimerization of the two Groucho family proteins. Proteins containing only this domain can function as a dominant-negative inhibitor of PRDI-BF1 repression, and can significantly increase the IFN-beta promoter activity after virus induction. It is concluded that PRDI-BF1/Blimp-1 represses transcription by recruiting a complex of Groucho family proteins to DNA, and it is suggested that such corepressor complexes are required for the postinduction repression of the IFN-beta promoter (Ren, 1999).

Search PubMed for articles about Drosophila Blimp-1

Agawa, Y., Sarhan, M., Kageyama, Y., Akagi, K., Takai, M., Hashiyama, K., Wada, T., Handa, H., Iwamatsu, A., Hirose, S. and Ueda, H. (2007). Drosophila Blimp-1 is a transient transcriptional repressor that controls timing of the ecdysone-induced developmental pathway. Mol Cell Biol 27(24): 8739-8747. PubMed ID: 17923694

Akagi, K. and Ueda, H. (2011). Regulatory mechanisms of ecdysone-inducible Blimp-1 encoding a transcriptional repressor that is important for the prepupal development in Drosophila. Dev Growth Differ 53(5): 697-703. PubMed ID: 21671917

Akagi, K., Sarhan, M., Sultan, A. R., Nishida, H., Koie, A., Nakayama, T. and Ueda, H. (2016). A biological timer in the fat body comprising Blimp-1, betaFtz-f1 and Shade regulates pupation timing in Drosophila melanogaster. Development 143(13): 2410-2416. PubMed ID: 27226323

Aly, H., Akagi, K. and Ueda, H. (2018). Proteasome activity determines pupation timing through the degradation speed of timer molecule Blimp-1. Dev Growth Differ 60(8): 502-508. PubMed ID: 30368781

Ancelin, K., Lange, U. C., Hajkova, P., Schneider, R., Bannister, A. J., Kouzarides, T. and Surani, M. A. (2006). Blimp1 associates with Prmt5 and directs histone arginine methylation in mouse germ cells. Nat. Cell Biol. 8: 623-630. Medline abstract: 16699504

Beckstead, R. B., Lam, G. and Thummel, C. S. (2005). The genomic response to 20-hydroxyecdysone at the onset of Drosophila metamorphosis. Genome Biol 6(12): R99. PubMed ID: 16356271

Bikoff, E. K., Morgan, M. A. and Robertson, E. J. (2009). An expanding job description for Blimp-1/PRDM1. Curr Opin Genet Dev 19(4): 379-385. PubMed ID: 19592232

Brzezinski, J. A. t., Uoon Park, K. and Reh, T. A. (2013). Blimp1 (Prdm1) prevents re-specification of photoreceptors into retinal bipolar cells by restricting competence. Dev Biol 384: 194-204. PubMed ID: 24125957

Bunker, J., Bashir, M., Bailey, S., Boodram, P., Perry, A., Delaney, R., Tsachaki, M., Sprecher, S. G., Nelson, E., Call, G. B. and Rister, J. (2023). Blimp-1/PRDM1 and Hr3/RORbeta specify the blue-sensitive photoreceptor subtype in Drosophila by repressing the hippo pathway. Front Cell Dev Biol 11: 1058961. PubMed ID: 36960411

Chavoshi, T. M., Moussian, B. and Uv, A. (2010). Tissue-autonomous EcR functions are required for concurrent organ morphogenesis in the Drosophila embryo. Mech Dev 127(5-6): 308-319. PubMed ID: 20093179

Horn, M., Geisen, C., Cermak, L., Becker, B., Nakamura, S., Klein, C., Pagano, M. and Antebi, A. (2014). DRE-1/FBXO11-dependent degradation of BLMP-1/BLIMP-1 governs C. elegans developmental timing and maturation. Dev Cell 28(6): 697-710. PubMed ID: 24613396

Jia, Q., Liu, Y., Liu, H. and Li, S. (2014). Mmp1 and Mmp2 cooperatively induce Drosophila fat body cell dissociation with distinct roles. Sci Rep 4: 7535. PubMed ID: 25520167

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date revised: 25 August 2023

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