Mechanisms ensuring robust repression of the Drosophila female germline stem cell maintenance factor Nanos via posttranscriptional regulation

An RNA-binding regulatory cascade controls the switch from proliferation to differentiation in the Drosophila male germ cell lineage

The switch from precursor cell proliferation to onset of differentiation in adult stem cell lineages must be carefully regulated to produce sufficient progeny to maintain and repair tissues, yet prevent overproliferation that may enable oncogenesis. In the Drosophila male germ cell lineage, spermatogonia produced by germ line stem cells undergo a limited number of transit amplifying mitotic divisions before switching to the spermatocyte program that sets up meiosis and eventual spermatid differentiation. The number of transit amplifying divisions is set by accumulation of the bag-of-marbles (Bam) protein to a critical threshold. In bam mutants, spermatogonia proliferate through several extra rounds of mitosis and then die without becoming spermatocytes. Here, we show that a key role of Bam for the mitosis to differentiation switch is repressing expression of Held Out Wings (how), homolog of mammalian Quaking. Knockdown of how in germ cells was sufficient to allow spermatogonia mutant for bam or its partner benign gonial cell neoplasm to differentiate, while forced expression of nuclear-targeted How protein in spermatogonia wild-type for bam resulted in continued proliferation at the expense of differentiation. Our findings suggest that Bam targets how RNA for degradation by acting as an adapter to recruit the CCR4-NOT deadenylation complex via binding its subunit, Caf40. As How is itself an RNA-binding protein with roles in RNA processing, our findings reveal that the switch from proliferation to meiosis and differentiation in the Drosophila male germ line adult stem cell lineage is regulated by a cascade of RNA-binding proteins.

Comparative Analysis of Drosophila Bam and Bgcn Sequences and Predicted Protein Structural Evolution

The protein encoded by the Drosophila melanogaster gene bag of marbles (bam) plays an essential role in early gametogenesis by complexing with the gene product of benign gonial cell neoplasm (bgcn) to promote germline stem cell daughter differentiation in males and females. Here, we compared the AlphaFold2 and AlphaFold Multimer predicted structures of Bam protein and the Bam:Bgcn protein complex between D. melanogaster, D. simulans, and D. yakuba, where bam is necessary in gametogenesis to that in D. teissieri, where it is not. Despite significant sequence divergence, we find very little evidence of significant structural differences in high confidence regions of the structures across the four species. This suggests that Bam structure is unlikely to be a direct cause of its functional differences between species and that Bam may simply not be integrated in an essential manner for GSC differentiation in D. teissieri. Patterns of positive selection and significant amino acid diversification across species is consistent with the Selection, Pleiotropy, and Compensation (SPC) model, where detected selection at bam is consistent with adaptive change in one major trait followed by positively selected compensatory changes for pleiotropic effects (in this case perhaps preserving structure). In the case of bam, we suggest that the major trait could be genetic interaction with the endosymbiotic bacteria Wolbachia pipientis. Following up on detected signals of positive selection and comparative structural analysis could provide insight into the distribution of a primary adaptive change versus compensatory changes following a primary change.

The TRIM-NHL RNA-binding protein MEI-P26 modulates the size of Drosophila Type I neuroblast lineages

The Drosophila TRIM-NHL RNA-binding protein (RBP), MEI-P26, has previously been shown to suppress tumor formation in the germline. Here we show that, in the Drosophila larval central brain, cell-type-specific expression of MEI-P26 plays a vital role in regulating neural development. MEI-P26 and another TRIM-NHL RBP, Brain tumor (BRAT), have distinct expression patterns in Type I neuroblast (NB) lineages: While both proteins are expressed in NBs, BRAT is expressed in ganglion mother cells (GMCs) but not neurons, whereas MEI-P26 is expressed in neurons but not GMCs. Knockdown of MEI-P26 leads to re-expression of the stem cell marker Deadpan (DPN) and over-production of neurons. In contrast, ectopically expressed MEI-P26 reduces NB lineage size by repressing division of GMCs, resulting in reduced neuron production. We show that MEI-P26 positively regulates expression of Prospero (PROS), a transcription factor that is known to repress cell cycle-related genes. Ectopic expression of PROS phenocopies ectopic expression of MEI-P26. In both cases, Cyclin B (CYCB) expression is downregulated. Importantly, knockdown of PROS in the context of ectopic MEI-P26 rescues the neural lineage. Based on these results, we conclude that MEI-P26 functions to prevent over-production of neurons by promoting production of PROS which, in turn, downregulates cell division.

An RNA binding regulatory cascade controls the switch from proliferation to differentiation in the Drosophila male germ cell lineage

The switch from precursor cell proliferation to onset of differentiation in adult stem cell lineages must be carefully regulated to produce sufficient progeny to maintain and repair tissues, yet prevent overproliferation that may enable oncogenesis. In the Drosophila male germ cell lineage, spermatogonia produced by germ line stem cells undergo a limited number of transit amplifying mitotic divisions before switching to the spermatocyte program that sets up meiosis and eventual spermatid differentiation. The number of transit amplifying divisions is set by accumulation of the bag-of-marbles (Bam) protein to a critical threshold. In bam mutants, spermatogonia proliferate through several extra rounds of mitosis then die without becoming spermatocytes. Here we show that a key role of Bam for the mitosis to differentiation switch is repressing expression of Held Out Wings (how), homolog of mammalian Quaking. Knockdown of how in germ cells was sufficient to allow spermatogonia mutant for bam or its partner benign gonial cell neoplasm (bgcn) to differentiate, while forced expression of nuclear-targeted How protein in spermatogonia wild-type for bam resulted in continued proliferation at the expense of differentiation. Our findings suggest that Bam targets how RNA for degradation by acting as an adapter to recruit the CCR4-NOT deadenylation complex via binding its subunit, Caf40. As How is itself an RNA binding protein with roles in RNA processing, our findings reveal that the switch from proliferation to meiosis and differentiation in the Drosophila male germ line adult stem cell lineage is regulated by a cascade of RNA-binding proteins.

Comparative Analysis of Drosophila Bam and Bgcn Sequences and Predicted Protein Structural Evolution

The protein encoded by the Drosophila melanogaster gene bag of marbles (bam) plays an essential role in early gametogenesis by complexing with the gene product of benign gonial cell neoplasm (bgcn) to promote germline stem cell daughter differentiation in males and females. Here, we compared the AlphaFold2 and AlphaFold Multimer predicted structures of Bam protein and the Bam:Bgcn protein complex between D. melanogaster, D. simulans, and D. yakuba, where bam is necessary in gametogenesis to that in D. teissieri, where it is not. Despite significant sequence divergence, we find very little evidence of significant structural differences in high confidence regions of the structures across the four species. This suggests that Bam structure is unlikely to be a direct cause of its functional differences between species and that Bam may simply not be integrated in an essential manner for GSC differentiation in D. teissieri. Patterns of positive selection and significant amino acid diversification across species is consistent with the Selection, Pleiotropy, and Compensation (SPC) model, where detected selection at bam is consistent with adaptive change in one major trait followed by positively selected compensatory changes for pleiotropic effects (in this case perhaps preserving structure). In the case of bam, we suggest that the major trait could be genetic interaction with the endosymbiotic bacteria Wolbachia pipientis. Following up on detected signals of positive selection and comparative structural analysis could provide insight into the distribution of a primary adaptive change versus compensatory changes following a primary change.

New insights into ginsenoside Rg1 regulating the niche to inhibit age-induced germline stem cells depletion through targeting ECR/BMP signaling pathway in Drosophila

PURPOSE

The age-induced imbalance in ecological niches leads to the loss of GSCs, which is the main reason for ovarian germline senescence. Ginsenoside Rg1 can delay ovarian senescence. Here, we shed light on new insights of ginsenoside Rg1 in regulating the niche to maintain GSCs self-renewal and discussing related molecular mechanisms.

METHODS

The differences among GSC number, reproductive capacity of naturally aging female Drosophila after ginsenoside Rg1 feeding were analyzed by immunofluorescence and behavior monitoring. The expressions of the active factors in the niche and the BMP signaling were analyzed through Western blot and RT-qPCR. The target effect was verified in the ECR mutant and combined with the molecular docking.

RESULTS

Ginsenoside Rg1 inhibited the age-induced reduction of the GSCs number and restored offspring production and development. Ginsenoside Rg1 promoted the expression of anchor proteins E-cadherin, stemness maintenance factor Nos and differentiation promoting factor Bam, thereby GSCs niche homeostasis was regulated. In addition, ginsenoside Rg1 was bound to the LBD region of the hormone receptor ECR. Ginsenoside Rg1 promotes the regeneration of GSCs by targeting the ECR to increase pSmad1/5/8 expression and thereby activating the BMP signaling pathway. In addition, ginsenoside Rg1 maintenance of niche homeostasis to promote GSCs regeneration is dependent on ECR as demonstrated in ECR mutants.

CONCLUSIONS

Ginsenoside Rg1 regulated the ecological niche homeostasis of GSCs and promoted the regeneration of GSCs by targeting the ECR/BMP signaling pathway in hormone-deficient states in aging ovaries. It is of great significance for prolonging fertility potential and delaying ovarian senescence.

Wolbachia infection at least partially rescues the fertility and ovary defects of several new Drosophila melanogaster bag of marbles protein-coding mutants

The D. melanogaster protein coding gene bag of marbles (bam) plays a key role in early male and female reproduction by forming complexes with partner proteins to promote differentiation in gametogenesis. Like another germline gene, Sex lethal, bam genetically interacts with the endosymbiont Wolbachia, as Wolbachia rescues the reduced fertility of a bam hypomorphic mutant. Here, we explored the specificity of the bam-Wolbachia interaction by generating 22 new bam mutants, with ten mutants displaying fertility defects. Nine of these mutants trend towards rescue by the wMel Wolbachia variant, with eight statistically significant at the fertility and/or cytological level. In some cases, fertility was increased a striking 20-fold. There is no specificity between the rescue and the known binding regions of bam, suggesting wMel does not interact with one singular bam partner to rescue the reproductive phenotype. We further tested if wMel interacts with bam in a non-specific way, by increasing bam transcript levels or acting upstream in germline stem cells. A fertility assessment of a bam RNAi knockdown mutant reveals that wMel rescue is specific to functionally mutant bam alleles and we find no obvious evidence of wMel interaction with germline stem cells in bam mutants.

Wolbachia infection at least partially rescues the fertility and ovary defects of several new Drosophila melanogaster bag of marbles protein-coding mutants

The D. melanogaster protein coding gene bag of marbles ( bam ) plays a key role in early male and female reproduction by forming complexes with partner proteins to promote differentiation in gametogenesis. Like another germline gene, Sex lethal , bam genetically interacts with the endosymbiont Wolbachia , as Wolbachia rescues the reduced fertility of a bam hypomorphic mutant. Here, we explored the specificity of the bam-Wolbachia interaction by generating 22 new bam mutants, with ten mutants displaying fertility defects. Nine of these mutants trend towards rescue by the w Mel Wolbachia variant, with eight statistically significant at the fertility and/or cytological level. In some cases, fertility was increased a striking 20-fold. There is no specificity between the rescue and the known binding regions of bam , suggesting w Mel does not interact with one singular bam partner to rescue the reproductive phenotype. We further tested if w Mel interacts with bam in a non-specific way, by increasing bam transcript levels or acting upstream in germline stem cells. A fertility assessment of a bam RNAi knockdown mutant reveals that w Mel rescue is specific to functionally mutant bam alleles and we find no obvious evidence of w Mel interaction with germline stem cells in bam mutants.

Author Summary

Reproduction in the Drosophila melanogaster fruit fly is dependent on the bag of marbles ( bam ) gene, which acts early in the process of generating eggs and sperm. Mutations to this gene negatively impact the fertility of the fly, causing it to be sterile or have fewer progeny. Interestingly, we find that the bacteria Wolbachia , which resides within reproductive cells across a wide range of insects, partially restores the fertility and ovary phenotype of several bam mutants of which the resultant Bam protein is altered from wildtype. The protein function of Bam is further suggested to be important by the lack of rescue for a fly that has a fertility defect due to low expression of a non-mutated bam gene. Previous work makes similar conclusions about Wolbachia with another reproductive gene, Sex lethal ( Sxl ), highlighting the potential for rescue of fertility mutants to occur in a similar way across different genes. An understanding of the ways in which Wolbachia can affect host reproduction provides us with context with which to frame Wolbachia 's impact on host genes, such as bam and Sxl, and consider the evolutionary implications of Wolbachia 's infection in D. melanogaster fruit flies.

The Regulation of Germline Sex Determination in Drosophila by Sex lethal

BACKGROUND

The establishment of male or female identity (sex determination) is essential for creating the anatomical, physiological, and behavioral differences between 2 sexes of the same species (sexual dimorphism). In many organisms, including mammals and Drosophila, sex is determined by inheritance of sex chromosomes, while in other animals, sex is determined by environmental factors. Arguably the most important consequence of sex determination is the production of healthy gametes necessary for reproduction: female oocytes and male spermatids.

SUMMARY

The generation of sperm and oocytes requires cooperation between 2 different cell types within the gonad: germ cells and somatic cells. Defects in sex determination in either the somatic gonad or germline lead to disorders of sexual development and infertility. In Drosophila, the gene Sex lethal (Sxl) is the key determinant of sex in both the soma and the germline. However, how Sxl controls sex determination is much more well understood in the soma than the germline. Key Mesage: This review will focus on Sxl in the germline, how it is activated specifically in female germ cells, and how it regulates germline sex determination and sexual development.