Cyclin E controls Drosophila female germline stem cell maintenance independently of its role in proliferation by modulating responsiveness to niche signals

SR Protein Kinase is expressed in Drosophila ovarian germline stem cells but is not essential for their self-renewal

Germline stem cells (GSCs) are necessary for oocyte production in Drosophila . GSC maintenance is regulated by intrinsic factors that promote their self-renewal. One such factor, the beta-importin, Transportin-Serine/Arginine rich , mediates nuclear import of serine/arginine-rich (SR) proteins, which are phosphorylated by SR protein kinases. Here, we investigate whether the kinase encoded by SR protein kinase ( SRPK ) is essential for GSC self-renewal. We find that SRPK is expressed in GSCs and their mitotically-dividing daughters, but is not necessary for GSC establishment or maintenance. We conclude that SRPK is dispensable for GSC self-renewal, and postulate that other protein kinases can compensate for its absence.

Fusome morphogenesis is sufficient to promote female germline stem cell self-renewal in Drosophila

Many tissue-resident stem cells are retained through asymmetric cell division, a process that ensures stem cell self-renewal through each mitotic cell cycle. Asymmetric organelle distribution has been proposed as a mechanism by which stem cells are marked for long-term retention; however, it is not clear whether biased organelle localization is a cause or an effect of asymmetric division. In Drosophila females, an endoplasmic reticulum-like organelle called the fusome is continually regenerated in germline stem cells (GSCs) and associated with GSC division. Here, we report that the β-importin Tnpo-SR is essential for fusome regeneration. Depletion of Tnpo-SR disrupts cytoskeletal organization during interphase and nuclear membrane remodeling during mitosis. Tnpo-SR does not localize to microtubules, centrosomes, or the fusome, suggesting that its role in maintaining these processes is indirect. Despite this, we find that restoring fusome morphogenesis in Tnpo-SR-depleted GSCs is sufficient to rescue GSC maintenance and cell cycle progression. We conclude that Tnpo-SR functionally fusome regeneration to cell cycle progression, supporting the model that asymmetric rebuilding of fusome promotes maintenance of GSC identity and niche retention.

Lethal Giant Disc is a target of Cdk1 and regulates ESCRT-III localization during germline stem cell abscission

Abscission is the final step of cytokinesis that allows the physical separation of sister cells through the scission of the cellular membrane. This deformation is driven by ESCRT-III proteins, which can bind membranes and form dynamic helices. A crucial step in abscission is the recruitment of ESCRT-III proteins at the right time and place. Alix is one of the best characterized proteins that recruits ESCRT-III proteins from yeast to mammals. However, recent studies in vivo have revealed that pathways acting independently or redundantly with Alix are also required at abscission sites in different cellular contexts. Here, we show that Lgd acts redundantly with Alix to properly localize ESCRT-III to the abscission site in germline stem cells (GSCs) during Drosophila oogenesis. We further demonstrate that Lgd is phosphorylated at multiple sites by the CycB/Cdk1 kinase. We found that these phosphorylation events potentiate the activity of Shrub, a Drosophila ESCRT-III, during abscission of GSCs. Our study reveals that redundancy between Lgd and Alix, and coordination with the cell cycle kinase Cdk1, confers robust and timely abscission of Drosophila germline stem cells.

Chromatin and gene expression changes during female Drosophila germline stem cell development illuminate the biology of highly potent stem cells

Highly potent animal stem cells either self renew or launch complex differentiation programs, using mechanisms that are only partly understood. Drosophila female germline stem cells (GSCs) perpetuate without change over evolutionary time and generate cystoblast daughters that develop into nurse cells and oocytes. Cystoblasts initiate differentiation by generating a transient syncytial state, the germline cyst, and by increasing pericentromeric H3K9me3 modification, actions likely to suppress transposable element activity. Relatively open GSC chromatin is further restricted by Polycomb repression of testis or somatic cell-expressed genes briefly active in early female germ cells. Subsequently, Neijre/CBP and Myc help upregulate growth and reprogram GSC metabolism by altering mitochondrial transmembrane transport, gluconeogenesis, and other processes. In all these respects GSC differentiation resembles development of the totipotent zygote. We propose that the totipotent stem cell state was shaped by the need to resist transposon activity over evolutionary timescales.

Genetic and Epigenetic Regulation of Drosophila Oocyte Determination

Primary oocyte determination occurs in many organisms within a germ line cyst, a multicellular structure composed of interconnected germ cells. However, the structure of the cyst is itself highly diverse, which raises intriguing questions about the benefits of this stereotypical multicellular environment for female gametogenesis. Drosophila melanogaster is a well-studied model for female gametogenesis, and numerous genes and pathways critical for the determination and differentiation of a viable female gamete have been identified. This review provides an up-to-date overview of Drosophila oocyte determination, with a particular emphasis on the mechanisms that regulate germ line gene expression.

β-importin Tnpo-SR promotes germline stem cell maintenance and oocyte differentiation in female Drosophila

Germ cell development requires interplay between factors that balance cell fate and division. Early in their development, germ cells in many organisms divide mitotically with incomplete cytokinesis. Key regulatory events then lead to the specification of mature gametes, marked by the switch to a meiotic cell cycle program. Though the regulation of germ cell proliferation and meiosis are well understood, how these events are coordinated during development remains incompletely described. Originally characterized in their role as nucleo-cytoplasmic shuttling proteins, β-importins exhibit diverse functions during male and female gametogenesis. Here, we describe novel, distinct roles for the β-importin, Transportin-Serine/Arginine rich (Tnpo-SR), as a regulator of the mitosis to meiosis transition in the Drosophila ovary. We find that Tnpo-SR is necessary for germline stem cell (GSC) establishment and self-renewal, likely by controlling the response of GSCs to bone morphogenetic proteins. Depletion of Tnpo-SR results in germ cell counting defects and loss of oocyte identity. We show that in the absence of Tnpo-SR, proteins typically suppressed in germ cells when they exit mitosis fail to be down-regulated, and oocyte-specific factors fail to accumulate. Together, these findings provide new insight into the balance between germ cell division and differentiation and identify novel roles for β-importins in germ cell development.

Camptothecin Effectively Regulates Germline Differentiation through Bam-Cyclin A Axis in Drosophila melanogaster

Camptothecin (CPT), first isolated from Chinese tree Camptotheca acuminate, produces rapid and prolonged inhibition of DNA synthesis and induction of DNA damage by targeting topoisomerase I (top1), which is highly activated in cancer cells. CPT thus exhibits remarkable anticancer activities in various cancer types, and is a promising therapeutic agent for the treatment of cancers. However, it remains to be uncovered underlying its cytotoxicity toward germ cells. In this study we found that CPT, a cell cycle-specific anticancer agent, reduced fecundity and exhibited significant cytotoxicity toward GSCs and two-cell cysts. We showed that CPT induced GSC loss and retarded two-cell cysts differentiation in a niche- or apoptosis-independent manner. Instead, CPT induced ectopic expression of a differentiation factor, bag of marbles (Bam), and regulated the expression of cyclin A, which contributed to GSC loss. In addition, CPT compromised two-cell cysts differentiation by decreasing the expression of Bam and inducing cell arrest at G1/S phase via cyclin A, eventually resulting in two-cell accumulation. Collectively, this study demonstrates, for the first time in vivo, that the Bam-cyclin A axis is involved in CPT-mediated germline stem cell loss and two-cell cysts differentiation defects via inducing cell cycle arrest, which could provide information underlying toxicological effects of CPT in the productive system, and feature its potential to develop as a pharmacology-based germline stem cell regulation agent.

Visualizing Fusome Morphology via Tubulin Immunofluorescence in Drosophila Ovarian Germ Cells

In many species, oocytes are initially formed by the mitotic divisions of germline stem cells and their differentiating daughters. These progenitor cells are frequently interconnected in structures called cysts, which may function to safeguard oocyte quality. In Drosophila, an essential germline-specific organelle called the fusome helps maintain and coordinate the mitotic divisions of both germline stem cells and cyst cells. The fusome also serves as a useful experimental marker to identify germ cells during their mitotic divisions. Fusomes are cytoplasmic organelles composed of microtubules, endoplasmic reticulum-derived vesicles, and a meshwork of membrane skeleton proteins. The fusome branches as mitotic divisions progress, traversing the intercellular bridges of germline stem cell/cystoblast pairs and cysts. Here, we provide a protocol to visualize fusome morphology in fixed tissue by stabilizing microtubules and immunostaining for α-Tubulin and other protein constituents of the fusome. We identify a variety of fluorophore-tagged proteins that are useful for visualizing the fusome and describe how these might be combined experimentally. Taken together, these tools provide a valuable resource to interrogate the genetic control of germline stem cell function, oocyte selection, and asymmetric division.

Methods to Analyze Nutritional and Inter-Organ Control of Drosophila Ovarian Germline Stem Cells

Physiological status, particularly dietary input, has major impacts on the Drosophila melanogaster ovarian germline stem cell lineage. Moreover, several studies have shed light on the role that inter-organ communication plays in coordinating whole-organism responses to changes in physiology. For example, nutrient-sensing signaling pathways function within the fat body to regulate germline stem cells and their progeny in the ovary. Together with its incredible genetic and cell biological toolkits, Drosophila serves as an amenable model organism to use for uncovering molecular mechanisms that underlie physiological control of adult stem cells. In this methods chapter, we describe a general dietary manipulation paradigm, genetic manipulation of adult adipocytes, and whole-mount ovary immunofluorescence to investigate physiological control of germline stem cells.

Drosophila MESR4 Gene Ensures Germline Stem Cell Differentiation by Promoting the Transcription of bag of marbles

Ovarian germline stem cells (GSCs) of Drosophila melanogaster provide a valuable in vivo model to investigate how the adult stem cell identity is maintained and the differentiation of the daughter cells is regulated. GSCs are embedded into a specialized cellular microenvironment, the so-called stem cell niche. Besides the complex signaling interactions between the germ cells and the niche cells, the germ cell intrinsic mechanisms, such as chromatin regulation and transcriptional control, are also crucial in the decision about self-renewal and differentiation. The key differentiation regulator gene is the bag of marbles (bam), which is transcriptionally repressed in the GSCs and de-repressed in the differentiating daughter cell. Here, we show that the transcription factor MESR4 functions in the germline to promote GSC daughter differentiation. We find that the loss of MESR4 results in the accumulation of GSC daughter cells which fail to transit from the pre-cystoblast (pre-CB) to the differentiated cystoblast (CB) stage. The forced expression of bam can rescue this differentiation defect. By a series of epistasis experiments and a transcriptional analysis, we demonstrate that MESR4 positively regulates the transcription of bam. Our results suggest that lack of repression alone is not sufficient, but MESR4-mediated transcriptional activation is also required for bam expression.

Cell-cycle exit and stem cell differentiation are coupled through regulation of mitochondrial activity in the Drosophila testis

Whereas stem and progenitor cells proliferate to maintain tissue homeostasis, fully differentiated cells exit the cell cycle. How cell identity and cell-cycle state are coordinated during differentiation is still poorly understood. The Drosophila testis niche supports germline stem cells and somatic cyst stem cells (CySCs). CySCs give rise to post-mitotic cyst cells, providing a tractable model to study the links between stem cell identity and proliferation. We show that, while cell-cycle progression is required for CySC self-renewal, the E2f1/Dp transcription factor is dispensable for self-renewal but instead must be silenced by the Drosophila retinoblastoma homolog, Rbf, to permit differentiation. Continued E2f1/Dp activity inhibits the expression of genes important for mitochondrial activity. Furthermore, promoting mitochondrial biogenesis rescues the differentiation of CySCs with ectopic E2f1/Dp activity but not their cell-cycle exit. In sum, E2f1/Dp coordinates cell-cycle progression with stem cell identity by regulating the metabolic state of CySCs.

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CycE is critical for CySC self-renewal

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E2f/Dp does not act in self-renewal but must be silenced for differentiation

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E2f/Dp inhibits increases in oxidative metabolism involved in normal differentiation

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Increased mitochondrial biogenesis rescues differentiation of E2f/Dp-active cells

Cyromazine Effects the Reproduction of Drosophila by Decreasing the Number of Germ Cells in the Female Adult Ovary

Simple Summary

Cyromazine, an insect growth regulator, is used to control the Dipteran pest population. Previous findings observed that treatment with cyromazine increased the larval mortality, by interfering with the ecdysone signaling. In addition, the application of exogenous 20E significantly reduced the mortality caused by cyromazine. Many studies have also supported the role of ecdysone signaling in the maintenance of germline stem cells (GSCs), where mutations in ecdysone signaling-related genes significantly decreased the number of GSCs. However, to date, no study has reported the effect of cyromazine on the GSCs of Drosophila melanogaster. In the present study, we observed that cyromazine significantly reduced the number of both GSCs and cystoblasts (CBs) in the ovary of adult female. To further understand the effect of cyromazine on germ cells, we selected some key genes related to the ecdysone signaling pathway and evaluated their expression through RT-qPCR. Additionally, we measured the ecdysone titer from the cyromazine-treated ovaries. Our results indicated a significant decrease in the expression of ecdysone signaling-related genes and also in the ecdysone titer. These results further supported our findings that cyromazine reduced the number of germ cells by interfering with the ecdysone signaling pathway.

Abstract

In the present study, we observed a 58% decrease in the fecundity of Drosophila melanogaster, after treatment with the cyromazine. To further elucidate the effects of cyromazine on reproduction, we counted the number of both germline stem cells (GSCs) and cystoblasts (CBs) in the ovary of a 3-day-old adult female. The results showed a significant decrease in the number of GSCs and CBs as compared to the control group. The mode of action of cyromazine is believed to be through the ecdysone signaling pathway. To further support this postulate, we observed the expression of key genes involved in the ecdysone signaling pathway and also determined the ecdysone titer from cyromazine-treated ovaries. Results indicated a significant decrease in the expression of ecdysone signaling-related genes as compared to the control group. Furthermore, the titer of the ecdysone hormone was also markedly reduced (90%) in cyromazine-treated adult ovaries, suggesting that ecdysone signaling was directly related to the decrease in the number of GSCs and CBs. However, further studies are required to understand the mechanism by which cyromazine affects the GSCs and CBs in female adult ovaries.

Canonical Wnt Signaling Promotes Formation of Somatic Permeability Barrier for Proper Germ Cell Differentiation

Morphogen-mediated signaling is critical for proper organ development and stem cell function, and well-characterized mechanisms spatiotemporally limit the expression of ligands, receptors, and ligand-binding cell-surface glypicans. Here, we show that in the developing Drosophila ovary, canonical Wnt signaling promotes the formation of somatic escort cells (ECs) and their protrusions, which establish a physical permeability barrier to define morphogen territories for proper germ cell differentiation. The protrusions shield germ cells from Dpp and Wingless morphogens produced by the germline stem cell (GSC) niche and normally only received by GSCs. Genetic disruption of EC protrusions allows GSC progeny to also receive Dpp and Wingless, which subsequently disrupt germ cell differentiation. Our results reveal a role for canonical Wnt signaling in specifying the ovarian somatic cells necessary for germ cell differentiation. Additionally, we demonstrate the morphogen-limiting function of this physical permeability barrier, which may be a common mechanism in other organs across species.

Active or Autoclaved Akkermansia muciniphila Relieves TNF-α-Induced Inflammation in Intestinal Epithelial Cells Through Distinct Pathways

Intestinal inflammation is a major threat to the health and growth of young animals such as piglets. As a next-generation probiotics, limited studies have shown that Akkermansia muciniphila could alleviate inflammation of intestinal epithelial cells (IECs). In this study, a TNF-α-induced inflammatory model of IPEC-J2 cells, the intestinal porcine enterocytes, was built to evaluate the effects of active or inactive A. muciniphila on the inflammation of IECs. The viability of IPEC-J2 cells was the highest when treated with active (10⁸ copies/mL) or inactive (10⁹ copies/mL) A. muciniphila for 7.5 h (P < 0.01). Treated with 20 ng/mL of TNF-α and followed by a treatment of A. muciniphila, the mRNA level of proinflammatory cytokines (IL-8, IL-1β, IL-6 and TNF-α) was remarkably reduced (P < 0.05) along with the increased mRNA level of tight junction proteins (ZO-1 and Occludin, P < 0.05). Flow cytometry analysis showed that active or inactive A. muciniphila significantly suppressed the rate of the early and total apoptotic of the inflammatory IPEC-J2 cells (P < 0.05). According to results of transcriptome sequencing, active and inactive A. muciniphila may decline cell apoptosis by down-regulating the expression of key genes in calcium signaling pathway, or up-regulating the expression of key genes in cell cycle signaling pathway. And the bacterium may alleviate the inflammation of IECs by down-regulating the expression of PI3K upstream receptor genes. Our results indicate that A. muciniphila may be a promising NGP targeting intestinal inflammation.

Live imaging of the Drosophila ovarian niche shows spectrosome and centrosome dynamics during asymmetric germline stem cell division

Drosophila female germline stem cells (GSCs) are found inside the cellular niche at the tip of the ovary. They undergo asymmetric divisions to renew the stem cell lineage and to produce sibling cystoblasts that will in turn enter differentiation. GSCs and cystoblasts contain spectrosomes, membranous structures essential for orientation of the mitotic spindle and that, particularly in GSCs, change shape depending on the cell cycle phase. Using live imaging and a fusion protein of GFP and the spectrosome component Par-1, we follow the complete spectrosome cycle throughout GSC division and quantify the relative duration of the different spectrosome shapes. We also determine that the Par-1 kinase shuttles between the spectrosome and the cytoplasm during mitosis and observe the continuous addition of new material to the GSC and cystoblast spectrosomes. Next, we use the Fly-FUCCI tool to define, in live and fixed tissues, that GSCs have a shorter G1 compared with the G2 phase. The observation of centrosomes in dividing GSCs allowed us to determine that centrosomes separate very early in G1, before centriole duplication. Furthermore, we show that the anterior centrosome associates with the spectrosome only during mitosis and that, upon mitotic spindle assembly, it translocates to the cell cortex, where it remains anchored until centrosome separation. Finally, we demonstrate that the asymmetric division of GSCs is not an intrinsic property of these cells, as the spectrosome of GSC-like cells located outside of the niche can divide symmetrically. Thus, GSCs display unique properties during division, a behaviour influenced by the surrounding niche.

Summary: Imaging of live Drosophila germline stem cells in the ovarian niche reveals their asymmetric division and centrosome behaviour, whereas tumorous stem cells divide symmetrically.

Germline Stem and Progenitor Cell Aging in C. elegans

Like many animals and humans, reproduction in the nematode C. elegans declines with age. This decline is the cumulative result of age-related changes in several steps of germline function, many of which are highly accessible for experimental investigation in this short-lived model organism. Here we review recent work showing that a very early and major contributing step to reproductive decline is the depletion of the germline stem and progenitor cell pool. Since many cellular and molecular aspects of stem cell biology and aging are conserved across animals, understanding mechanisms of age-related decline of germline stem and progenitor cells in C. elegans has broad implications for aging stem cells, germline stem cells, and reproductive aging.

Orphan nuclear receptor ftz-f1 (NR5A3) promotes egg chamber survival in the Drosophila ovary

Gamete production in mammals and insects is controlled by cell signaling pathways that facilitate communication between germ cells and somatic cells. Nuclear receptor signaling is a key mediator of many aspects of reproduction, including gametogenesis. For example, the NR5A subfamily of nuclear receptors is essential for gonad development and sex steroid production in mammals. Despite the original identification of the NR5A subfamily in the model insect Drosophila melanogaster, it has been unclear whether Drosophila NR5A receptors directly control oocyte production. Ftz-f1 is expressed throughout the ovary, including in germline stem cells, germline cysts, and several populations of somatic cells. We show that ftz-f1 is required in follicle cells prior to stage 10 to promote egg chamber survival at the mid-oogenesis checkpoint. Our data suggest that egg chamber death in the absence of ftz-f1 is due, at least in part, to failure of follicle cells to exit the mitotic cell cycle or failure to accumulate oocyte-specific factors in the germline. Taken together, these results show that, as in mammals, the NR5A subfamily promotes maximal reproductive output in Drosophila. Our data underscore the importance of nuclear receptors in the control of reproduction and highlight the utility of Drosophila oogenesis as a key model for unraveling the complexity of nuclear receptor signaling in gametogenesis.

Cyclin E in normal physiology and disease states

E-type cyclins, collectively called cyclin E, represent key components of the core cell cycle machinery. In mammalian cells, two E-type cyclins, E1 and E2, activate cyclin-dependent kinase 2 (CDK2) and drive cell cycle progression by phosphorylating several cellular proteins. Abnormally elevated activity of cyclin E-CDK2 has been documented in many human tumor types. Moreover, cyclin E overexpression mediates resistance of tumor cells to various therapeutic agents. Recent work has revealed that the role of cyclin E extends well beyond cell proliferation and tumorigenesis, and it may regulate a diverse array of physiological and pathological processes. In this review, we discuss these various cyclin E functions and the potential for therapeutic targeting of cyclin E and cyclin E-CDK2 kinase.

The Involvement of Ubiquitination Machinery in Cell Cycle Regulation and Cancer Progression

The cell cycle is a collection of events by which cellular components such as genetic materials and cytoplasmic components are accurately divided into two daughter cells. The cell cycle transition is primarily driven by the activation of cyclin-dependent kinases (CDKs), which activities are regulated by the ubiquitin-mediated proteolysis of key regulators such as cyclins, CDK inhibitors (CKIs), other kinases and phosphatases. Thus, the ubiquitin-proteasome system (UPS) plays a pivotal role in the regulation of the cell cycle progression via recognition, interaction, and ubiquitination or deubiquitination of key proteins. The illegitimate degradation of tumor suppressor or abnormally high accumulation of oncoproteins often results in deregulation of cell proliferation, genomic instability, and cancer occurrence. In this review, we demonstrate the diversity and complexity of the regulation of UPS machinery of the cell cycle. A profound understanding of the ubiquitination machinery will provide new insights into the regulation of the cell cycle transition, cancer treatment, and the development of anti-cancer drugs.

Characterization of histone inheritance patterns in the Drosophila female germline

Stem cells have the unique ability to undergo asymmetric division which produces two daughter cells that are genetically identical, but commit to different cell fates. The loss of this balanced asymmetric outcome can lead to many diseases, including cancer and tissue dystrophy. Understanding this tightly regulated process is crucial in developing methods to treat these abnormalities. Here, we report that during a Drosophila female germline stem cell asymmetric division, the two daughter cells differentially inherit histones at key genes related to either maintaining the stem cell state or promoting differentiation, but not at constitutively active or silenced genes. We combine histone labeling with DNA Oligopaints to distinguish old versus new histones and visualize their inheritance patterns at a single‐gene resolution in asymmetrically dividing cells in vivo. This strategy can be applied to other biological systems involving cell fate change during development or tissue homeostasis in multicellular organisms.

α-Tubulin Regulates the Fate of Germline Stem Cells in Drosophila Testis

The Drosophila testis provides an exemplary model for analyzing the extrinsic and intrinsic factors that regulate the fate of stem cell in vivo. Using this model, we show that the Drosophila αTub67C gene (full name αTubulin at 67C), which encodes α4-Tubulin (a type of α-Tubulin), plays a new role in controlling the fate of male germline stem cells (GSC). In this study, we have found that Drosophila α4-Tubulin is required intrinsically and extrinsically for GSCs maintenance. Results from green fluorescent protein (GFP)-transgene reporter assays show that the gene αTub67C is not required for Dpp/Gbb signaling silencing of bam expression, suggesting that αTub67C functions downstream of or parallel to bam, and is independent of Gbb/Dpp-bam signaling pathway. Furthermore, overexpression of αTub67C fails to obviously increase the number of GSC/Gonialblast (GB). Given that the α-tubulin genes are evolutionarily conserved from yeast to human, which triggers us to study the more roles of the gene α-tubulin in other animals in the future.

CENP-C functions in centromere assembly, the maintenance of CENP-A asymmetry and epigenetic age in Drosophila germline stem cells

Germline stem cells divide asymmetrically to produce one new daughter stem cell and one daughter cell that will subsequently undergo meiosis and differentiate to generate the mature gamete. The silent sister hypothesis proposes that in asymmetric divisions, the selective inheritance of sister chromatids carrying specific epigenetic marks between stem and daughter cells impacts cell fate. To facilitate this selective inheritance, the hypothesis specifically proposes that the centromeric region of each sister chromatid is distinct. In Drosophila germ line stem cells (GSCs), it has recently been shown that the centromeric histone CENP-A (called CID in flies)—the epigenetic determinant of centromere identity—is asymmetrically distributed between sister chromatids. In these cells, CID deposition occurs in G₂ phase such that sister chromatids destined to end up in the stem cell harbour more CENP-A, assemble more kinetochore proteins and capture more spindle microtubules. These results suggest a potential mechanism of ‘mitotic drive’ that might bias chromosome segregation. Here we report that the inner kinetochore protein CENP-C, is required for the assembly of CID in G₂ phase in GSCs. Moreover, CENP-C is required to maintain a normal asymmetric distribution of CID between stem and daughter cells. In addition, we find that CID is lost from centromeres in aged GSCs and that a reduction in CENP-C accelerates this loss. Finally, we show that CENP-C depletion in GSCs disrupts the balance of stem and daughter cells in the ovary, shifting GSCs toward a self-renewal tendency. Ultimately, we provide evidence that centromere assembly and maintenance via CENP-C is required to sustain asymmetric divisions in female Drosophila GSCs.

Stem cells can divide in an asymmetric fashion giving rise to two daughter cells with different fates. One daughter remains a stem cell, while the other can differentiate and adopt a new cell fate. Germline stem cells in the testes and ovaries give rise to differentiating daughter cells that eventually form the gametes, eggs and sperm. Here we investigate mechanisms controlling germline stem cell divisions occurring in the ovary of the fruit fly Drosophila melanogaster. Centromeres are epigenetically specified loci on chromosomes that make essential connections to the cell division machinery. Our study is focused on the centromere component CENP-C. We show that CENP-C is critical for the correct assembly of centromeres that occurs prior to cell division in germline stem cells. In addition, we find that CENP-C is asymmetrically distributed between stem and daughter cells, with more CENP-C at stem cell centromeres. Finally, we show that CENP-C depletion in germline stem cells disrupts the balance of stem and daughter cells in the developing ovary, impacting on cell fate. Taken together, we propose that CENP-C level and function at centromeres plays an important role in determining cell fate upon asymmetric division occurring in stem cells.

Genome Architecture Facilitates Phenotypic Plasticity in the Honeybee (Apis mellifera)

Phenotypic plasticity, the ability of an organism to alter its phenotype in response to an environmental cue, facilitates rapid adaptation to changing environments. Plastic changes in morphology and behavior are underpinned by widespread gene expression changes. However, it is unknown if, or how, genomes are structured to ensure these robust responses. Here, we use repression of honeybee worker ovaries as a model of plasticity. We show that the honeybee genome is structured with respect to plasticity; genes that respond to an environmental trigger are colocated in the honeybee genome in a series of gene clusters, many of which have been assembled in the last 80 My during the evolution of the Apidae. These clusters are marked by histone modifications that prefigure the gene expression changes that occur as the ovary activates, suggesting that these genomic regions are poised to respond plastically. That the linear sequence of the honeybee genome is organized to coordinate widespread gene expression changes in response to environmental influences and that the chromatin organization in these regions is prefigured to respond to these influences is perhaps unexpected and has implications for other examples of plasticity in physiology, evolution, and human disease.

Mad dephosphorylation at the nuclear pore is essential for asymmetric stem cell division

Stem cells divide asymmetrically to generate a stem cell and a differentiating daughter cell. Yet, it remains poorly understood how a stem cell and a differentiating daughter cell can receive distinct levels of niche signal and thus acquire different cell fates (self-renewal versus differentiation), despite being adjacent to each other and thus seemingly exposed to similar levels of niche signaling. In the Drosophila ovary, germline stem cells (GSCs) are maintained by short range bone morphogenetic protein (BMP) signaling; the BMP ligands activate a receptor that phosphorylates the downstream molecule mothers against decapentaplegic (Mad). Phosphorylated Mad (pMad) accumulates in the GSC nucleus and activates the stem cell transcription program. Here, we demonstrate that pMad is highly concentrated in the nucleus of the GSC, while it quickly decreases in the nucleus of the differentiating daughter cell, the precystoblast (preCB), before the completion of cytokinesis. We show that a known Mad phosphatase, Dullard (Dd), is required for the asymmetric partitioning of pMad. Our mathematical modeling recapitulates the high sensitivity of the ratio of pMad levels to the Mad phosphatase activity and explains how the asymmetry arises in a shared cytoplasm. Together, these studies reveal a mechanism for breaking the symmetry of daughter cells during asymmetric stem cell division.

Asymmetric assembly of centromeres epigenetically regulates stem cell fate

Centromeres are epigenetically defined by CENP-A–containing chromatin and are essential for cell division. Previous studies suggest asymmetric inheritance of centromeric proteins upon stem cell division; however, the mechanism and implications of selective chromosome segregation remain unexplored. We show that Drosophila female germline stem cells (GSCs) and neuroblasts assemble centromeres after replication and before segregation. Specifically, CENP-A deposition is promoted by CYCLIN A, while excessive CENP-A deposition is prevented by CYCLIN B, through the HASPIN kinase. Furthermore, chromosomes inherited by GSCs incorporate more CENP-A, making stronger kinetochores that capture more spindle microtubules and bias segregation. Importantly, symmetric incorporation of CENP-A on sister chromatids via HASPIN knockdown or overexpression of CENP-A, either alone or together with its assembly factor CAL1, drives stem cell self-renewal. Finally, continued CENP-A assembly in differentiated cells is nonessential for egg development. Our work shows that centromere assembly epigenetically drives GSC maintenance and occurs before oocyte meiosis.

Functions of cyclins and CDKs in mammalian gametogenesis†

Cyclins and cyclin-dependent kinases (CDKs) are key regulators of the cell cycle. Most of our understanding of their functions has been obtained from studies in single-cell organisms and mitotically proliferating cultured cells. In mammals, there are more than 20 cyclins and 20 CDKs. Although genetic ablation studies in mice have shown that most of these factors are dispensable for viability and fertility, uncovering their functional redundancy, CCNA2, CCNB1, and CDK1 are essential for embryonic development. Cyclin/CDK complexes are known to regulate both mitotic and meiotic cell cycles. While some mechanisms are common to both types of cell divisions, meiosis has unique characteristics and requirements. During meiosis, DNA replication is followed by two successive rounds of cell division. In addition, mammalian germ cells experience a prolonged prophase I in males or a long period of arrest in prophase I in females. Therefore, cyclins and CDKs may have functions in meiosis distinct from their mitotic functions and indeed, meiosis-specific cyclins, CCNA1 and CCNB3, have been identified. Here, we describe recent advances in the field of cyclins and CDKs with a focus on meiosis and early embryogenesis.

Cyclin E and Cdk1 regulate the termination of germline transit-amplification process in Drosophila testis

An extension of the G1 is correlated with stem cell differentiation. The role of cell cycle regulation during the subsequent transit amplification (TA) divisions is, however, unclear. Here, we report for the first time that in the Drosophila male germline lineage, the transit amplification divisions accelerate after the second TA division. The cell cycle phases, marked by Cyclin E and Cyclin B, are progressively altered during the TA. Antagonistic functions of the bag-of-marbles and the Transforming-Growth-Factor-β signaling regulate the cell division rates after the second TA division and the extent of the Cyclin E phase during the fourth TA division. Furthermore, loss of Cyclin E during the fourth TA cycle retards the cell division and induces premature meiosis in some cases. A similar reduction of Cdk1 activity during this stage arrests the penultimate division and subsequent differentiation, whereas enhancement of the Cdk1 activity prolongs the TA by one extra round. Altogether, the results suggest that modification of the cell cycle structure and the rates of cell division after the second TA division determine the extent of amplification. Also, the regulation of the Cyclin E and CDK1 functions during the penultimate TA division determines the induction of meiosis and subsequent differentiation.

Emerging roles of metazoan cell cycle regulators as coordinators of the cell cycle and differentiation

In multicellular organisms, cell proliferation must be tightly coordinated with other developmental processes to form functional tissues and organs. Despite significant advances in our understanding of how the cell cycle is controlled by conserved cell-cycle regulators (CCRs), how the cell cycle is coordinated with cell differentiation in metazoan organisms and how CCRs contribute to this process remain poorly understood. Here, we review the emerging roles of metazoan CCRs as intracellular proliferation-differentiation coordinators in multicellular organisms. We illustrate how major CCRs regulate cellular events that are required for cell fate acquisition and subsequent differentiation. To this end, CCRs employ diverse mechanisms, some of which are separable from those underpinning the conventional cell-cycle-regulatory functions of CCRs. By controlling cell-type-specific specification/differentiation processes alongside the progression of the cell cycle, CCRs enable spatiotemporal coupling between differentiation and cell proliferation in various developmental contexts in vivo. We discuss the significance and implications of this underappreciated role of metazoan CCRs for development, disease and evolution.

Coordinating Proliferation, Polarity, and Cell Fate in the Drosophila Female Germline

Gametes are highly specialized cell types produced by a complex differentiation process. Production of viable oocytes requires a series of precise and coordinated molecular events. Early in their development, germ cells are an interconnected group of mitotically dividing cells. Key regulatory events lead to the specification of mature oocytes and initiate a switch to the meiotic cell cycle program. Though the chromosomal events of meiosis have been extensively studied, it is unclear how other aspects of oocyte specification are temporally coordinated. The fruit fly, Drosophila melanogaster, has long been at the forefront as a model system for genetics and cell biology research. The adult Drosophila ovary continuously produces germ cells throughout the organism’s lifetime, and many of the cellular processes that occur to establish oocyte fate are conserved with mammalian gamete development. Here, we review recent discoveries from Drosophila that advance our understanding of how early germ cells balance mitotic exit with meiotic initiation. We discuss cell cycle control and establishment of cell polarity as major themes in oocyte specification. We also highlight a germline-specific organelle, the fusome, as integral to the coordination of cell division, cell polarity, and cell fate in ovarian germ cells. Finally, we discuss how the molecular controls of the cell cycle might be integrated with cell polarity and cell fate to maintain oocyte production.

Biology of the Caenorhabditis elegans Germline Stem Cell System

Stem cell systems regulate tissue development and maintenance. The germline stem cell system is essential for animal reproduction, controlling both the timing and number of progeny through its influence on gamete production. In this review, we first draw general comparisons to stem cell systems in other organisms, and then present our current understanding of the germline stem cell system in Caenorhabditis elegans In contrast to stereotypic somatic development and cell number stasis of adult somatic cells in C. elegans, the germline stem cell system has a variable division pattern, and the system differs between larval development, early adult peak reproduction and age-related decline. We discuss the cell and developmental biology of the stem cell system and the Notch regulated genetic network that controls the key decision between the stem cell fate and meiotic development, as it occurs under optimal laboratory conditions in adult and larval stages. We then discuss alterations of the stem cell system in response to environmental perturbations and aging. A recurring distinction is between processes that control stem cell fate and those that control cell cycle regulation. C. elegans is a powerful model for understanding germline stem cells and stem cell biology.

Local and Physiological Control of Germline Stem Cell Lineages in Drosophila melanogaster

The long-term survival of any multicellular species depends on the success of its germline in producing high-quality gametes and maximizing survival of the offspring. Studies in Drosophila melanogaster have led our growing understanding of how germline stem cell (GSC) lineages maintain their function and adjust their behavior according to varying environmental and/or physiological conditions. This review compares and contrasts the local regulation of GSCs by their specialized microenvironments, or niches; discusses how diet and diet-dependent factors, mating, and microorganisms modulate GSCs and their developing progeny; and briefly describes the tie between physiology and development during the larval phase of the germline cycle. Finally, it concludes with broad comparisons with other organisms and some future directions for further investigation.

Regulation of Drosophila germline stem cells

The asymmetric division of adult stem cells into one self-renewing stem cell and one differentiating cell is critical for maintaining homeostasis in many tissues. One paradigmatic model of this division is the Drosophila male and female germline stem cell, which provides two model systems not only sharing common features but also having distinct characteristics for studying asymmetric stem cell division in vivo. This asymmetric division is controlled by a combination of extrinsic signaling molecules and intrinsic factors that are either asymmetrically segregated or regulated differentially following division. In this review, we will discuss recent advances in understanding the molecular and cellular mechanisms guiding this asymmetric outcome, including extrinsic cues, intrinsic factors governing cell fate specification, and cell cycle control.

Maintenance of Proper Germline Stem Cell Number Requires Adipocyte Collagen in Adult Drosophila Females

Stem cells reside in specialized niches and are regulated by a variety of physiological inputs. Adipocytes influence whole-body physiology and stem cell lineages; however, the molecular mechanisms linking adipocytes to stem cells are poorly understood. Here, we report that collagen IV produced in adipocytes is transported to the ovary to maintain proper germline stem cell (GSC) number in adult Drosophila females. Adipocyte-derived collagen IV acts through β-integrin signaling to maintain normal levels of E-cadherin at the niche, thereby ensuring proper adhesion to GSCs. These findings demonstrate that extracellular matrix components produced in adipocytes can be transported to and incorporated into an established adult tissue to influence stem cell number.

Subunits of the DNA polymerase alpha-primase complex promote Notch-mediated proliferation with discrete and shared functions in C. elegans germline

Notch receptor signaling is a highly conserved cell communication system in most multicellular organisms and plays a critical role at several junctures in animal development. In Caenorhabditis elegans,GLP-1/Notch signaling is essential for both germline stem cell maintenance and germ cell proliferation during gonad development. Here, we show that subunits (POLA-1, DIV-1, PRI-1, and PRI-2) of the DNA polymerase alpha-primase complex are required for germ cell proliferation in response to GLP-1/Notch signaling in different tissues at different developmental stages. Specifically, genetic and functional analyses demonstrated that (a) maternally contributed DIV-1 (regulatory subunit) is indispensable non-cell autonomously for GLP-1/Notch-mediated germ cell proliferation during early larval development, whereas POLA-1 (catalytic subunit) and two primase subunits, PRI-1 and PRI-2, do not appear to be essential; (b) germline POLA-1, PRI-1, and PRI-2 play a crucial role in GLP-1/Notch-mediated maintenance of proliferative cell fate during adulthood, while DIV-1 is dispensable; and (c) germline POLA-1, DIV-1, PRI-1, and PRI-2 function in tandem with PUF (Pumilio/FBF) RNA-binding proteins to maintain germline stem cells in the adult gonad. These findings suggest that the subunits of the DNA polymerase alpha-primase complex exhibit both discrete and shared functions in GLP-1/Notch or PUF-mediated germ cell dynamics in C. elegans. These findings link the biological functions of DNA replication machineries to signals that maintain a stem cell population, and may have further implications for Notch-dependent tumors.

Cyclin B3 Deficiency Impairs Germline Stem Cell Maintenance and Its Overexpression Delays Cystoblast Differentiation in Drosophila Ovary

It is well known that cyclinB3 (cycB3) plays a key role in the control of cell cycle progression. However, whether cycB3 is involved in stem cell fate determination remains unknown. The Drosophila ovary provides an exclusive model for studying the intrinsic and extrinsic factors that modulate the fate of germline stem cells (GSCs). Here, using this model, we show that DrosophilacycB3 plays a new role in controlling the fate of germline stem cells (GSC). Results from cycB3 genetic analyses demonstrate that cycB3 is intrinsically required for GSC maintenance. Results from green fluorescent protein (GFP)-transgene reporter assays show that cycB3 is not involved in Dad-mediated regulation of Bmp signaling, or required for dpp-induced bam transcriptional silencing. Double mutants of bam and cycB3 phenocopied bam single mutants, suggesting that cycB3 functions in a bam-dependent manner in GSCs. Deficiency of cycB3 fails to cause apoptosis in GSCs or influence cystoblast (CB) differentiation into oocytes. Furthermore, overexpression of cycB3 dramatically increases the CB number in Drosophila ovaries, suggesting that an excess of cycB3 function delays CB differentiation. Given that the cycB3 gene is evolutionarily conserved, from insects to humans, cycB3 may also be involved in controlling the fate of GSCs in humans.

Cell lineage and cell cycling analyses of the 4d micromere using live imaging in the marine annelid Platynereis dumerilii

Cell lineage, cell cycle, and cell fate are tightly associated in developmental processes, but in vivo studies at single-cell resolution showing the intricacies of these associations are rare due to technical limitations. In this study on the marine annelid Platynereis dumerilii, we investigated the lineage of the 4d micromere, using high-resolution long-term live imaging complemented with a live-cell cycle reporter. 4d is the origin of mesodermal lineages and the germline in many spiralians. We traced lineages at single-cell resolution within 4d and demonstrate that embryonic segmental mesoderm forms via teloblastic divisions, as in clitellate annelids. We also identified the precise cellular origins of the larval mesodermal posterior growth zone. We found that differentially-fated progeny of 4d (germline, segmental mesoderm, growth zone) display significantly different cell cycling. This work has evolutionary implications, sets up the foundation for functional studies in annelid stem cells, and presents newly established techniques for live imaging marine embryos.

Transient transcriptional silencing alters the cell cycle to promote germline stem cell differentiation in Drosophila

Transcriptional silencing is a conserved process used by embryonic germ cells to repress somatic fate and maintain totipotency and immortality. In Drosophila, this transcriptional silencing is mediated by polar granule component (pgc). Here, we show that in the adult ovary, pgc is required for timely germline stem cell (GSC) differentiation. Pgc is expressed transiently in the immediate GSC daughter (pre-cystoblast), where it mediates a pulse of transcriptional silencing. This transcriptional silencing mediated by pgc indirectly promotes the accumulation of Cyclin B (CycB) and cell cycle progression into late-G2 phase, when the differentiation factor bag of marbles (bam) is expressed. Pgc mediated accumulation of CycB is also required for heterochromatin deposition, which protects the germ line genome against selfish DNA elements. Our results suggest that transient transcriptional silencing in the pre-cystoblast “re-programs” it away from self-renewal and toward the gamete differentiation program.

Spaghetti, a homolog of human RPAP3 (RNA polymerase II-associated protein 3), determines the fate of germline stem cells in Drosophila ovary

The Drosophila ovary provides an attractive model for studying the extrinsic or intrinsic factors that regulate the fate of germline stem cells (GSCs). Using this model, we identified a new role for Drosophila spaghetti (spag), encoding a homolog of human RNA polymerase II-associated protein 3 (RPAP3), in regulating the fate of ovarian GSCs. Results from spag knockdown and genetic mosaic studies suggest that spag functions as an intrinsic factor for GSCs maintenance. Loss of Spag by, either spag RNAi or null mutation failed to trigger apoptosis in ovarian GSCs. Overexpression of spag led to negligible increases in the number of GSC/Cystoblast (CB) cells, suggesting that an excess of Spag is not sufficient to accelerate the proliferation of GSCs or delay CBs' differentiation. Our study provides evidence supporting that spag is involved in adult stem cells maintenance. In addition, the RNAi screen results showed that knockdown of Hsp90, one of known Spag interacting partners, led to loss of ovarian GSCs in Drosophila. Heterozygous mutations in hsp90 (hsp90/+) dramatically accelerated the GSC loss in spag RNAi ovaries, suggesting that the Spag-contained complex possibly plays an essential role in controlling the GSCs fate.

Temporal remodeling of the cell cycle accompanies differentiation in the Drosophila germline

Development of multicellular organisms relies upon the coordinated regulation of cellular differentiation and proliferation. Growing evidence suggests that some molecular regulatory pathways associated with the cell cycle machinery also dictate cell fate; however, it remains largely unclear how the cell cycle is remodeled in concert with cell differentiation. During Drosophila oogenesis, mature oocytes are created through a series of precisely controlled division and differentiation steps, originating from a single tissue-specific stem cell. Further, germline stem cells (GSCs) and their differentiating progeny remain in a predominantly linear arrangement as oogenesis proceeds. The ability to visualize the stepwise events of differentiation within the context of a single tissue make the Drosophila ovary an exceptional model for study of cell cycle remodeling. To describe how the cell cycle is remodeled in germ cells as they differentiate in situ, we used the Drosophila Fluorescence Ubiquitin-based Cell Cycle Indicator (Fly-FUCCI) system, in which degradable versions of GFP::E2f1 and RFP::CycB fluorescently label cells in each phase of the cell cycle. We found that the lengths of the G1, S, and G2 phases of the cell cycle change dramatically over the course of differentiation, and identified the 4/8-cell cyst as a key developmental transition state in which cells prepare for specialized cell cycles. Our data suggest that the transcriptional activator E2f1, which controls the transition from G1 to S phase, is a key regulator of mitotic divisions in the early germline. Our data support the model that E2f1 is necessary for proper GSC proliferation, self-renewal, and daughter cell development. In contrast, while E2f1 degradation by the Cullin 4 (Cul4)-containing ubiquitin E3 ligase (CRL4) is essential for developmental transitions in the early germline, our data do not support a role for E2f1 degradation as a mechanism to limit GSC proliferation or self-renewal. Taken together, these findings provide further insight into the regulation of cell proliferation and the acquisition of differentiated cell fate, with broad implications across developing tissues.

Bam-dependent deubiquitinase complex can disrupt germ-line stem cell maintenance by targeting cyclin A

Drosophila germ-line stem cells (GSCs) provide an excellent model to study the regulatory mechanisms of stem cells in vivo. Bag of marbles (bam) has been demonstrated to be necessary and sufficient to promote GSC and cystoblast differentiation. Despite extensive investigation of its regulation and genetic functions, the biochemical nature of the Bam protein has been unknown. Here, we report that Bam is an ubiquitin-associated protein and controls the turnover of cyclin A (CycA). Mechanistically, we found that Bam associated with Otu to form a deubiquitinase complex that stabilized CycA by deubiquitination, thus providing a mechanism to explain how ectopic expression of Bam in GSCs promotes differentiation. Collectively, our findings not only identify a biochemical function of Bam, which contributes to GSC fate determination, but also emphasizes the critical role of proper expression of cyclin proteins mediated by both ubiquitination and deubiquitination pathways in balancing stem cell self-renewal and differentiation.

AMP-activated protein kinase has diet-dependent and -independent roles in Drosophila oogenesis

Multiple aspects of organismal physiology influence the number and activity of stem cells and their progeny, including nutritional status. Previous studies demonstrated that Drosophila germline stem cells (GSCs), follicle stem cells (FSCs), and their progeny sense and respond to diet via complex mechanisms involving many systemic and local signals. AMP-activated protein kinase, or AMPK, is a highly conserved regulator of energy homeostasis known to be activated under low cellular energy conditions; however, its role in the ovarian response to diet has not been investigated. Here, we describe nutrient-dependent and -independent requirements for AMPK in Drosophila oogenesis. We found that AMPK is cell autonomously required for the slow down in GSC and follicle cell proliferation that occurs on a poor diet. Similarly, AMPK activity is necessary in the germline for the degeneration of vitellogenic stages in response to nutrient deprivation. In contrast, AMPK activity is not required within the germline to modulate its growth. Instead, AMPK acts in follicle cells to negatively regulate their growth and proliferation, thereby indirectly limiting the size of the underlying germline cyst within developing follicles. Paradoxically, AMPK is required for GSC maintenance in well-fed flies (when AMPK activity is presumably at its lowest), suggesting potentially important roles for basal AMPK activity in specific cell types. Finally, we identified a nutrient-independent, developmental role for AMPK in cyst encapsulation by follicle cells. These results uncover specific AMPK requirements in multiple cell types in the ovary and suggest that AMPK can function outside of its canonical nutrient-sensing role in specific developmental contexts.

A Genetic Mosaic Screen Reveals Ecdysone-Responsive Genes Regulating Drosophila Oogenesis

Multiple aspects of Drosophila oogenesis, including germline stem cell activity, germ cell differentiation, and follicle survival, are regulated by the steroid hormone ecdysone. While the transcriptional targets of ecdysone signaling during development have been studied extensively, targets in the ovary remain largely unknown. Early studies of salivary gland polytene chromosomes led to a model in which ecdysone stimulates a hierarchical transcriptional cascade, wherein a core group of ecdysone-sensitive transcription factors induce tissue-specific responses by activating secondary branches of transcriptional targets. More recently, genome-wide approaches have identified hundreds of putative ecdysone-responsive targets. Determining whether these putative targets represent bona fide targets in vivo, however, requires that they be tested via traditional mutant analysis in a cell-type specific fashion. To investigate the molecular mechanisms whereby ecdysone signaling regulates oogenesis, we used genetic mosaic analysis to screen putative ecdysone-responsive genes for novel roles in the control of the earliest steps of oogenesis. We identified a cohort of genes required for stem cell maintenance, stem and progenitor cell proliferation, and follicle encapsulation, growth, and survival. These genes encode transcription factors, chromatin modulators, and factors required for RNA transport, stability, and ribosome biogenesis, suggesting that ecdysone might control a wide range of molecular processes during oogenesis. Our results suggest that, although ecdysone target genes are known to have cell type-specific roles, many ecdysone response genes that control larval or pupal cell types at developmental transitions are used reiteratively in the adult ovary. These results provide novel insights into the molecular mechanisms by which ecdysone signaling controls oogenesis, laying new ground for future studies.

Monitoring complete and incomplete abscission in the germ line stem cell lineage of Drosophila ovaries

In most species, cytokinesis is blocked in germ cells during at least some stage of their development. Abscission is difficult to assess directly in germ cells which are located in internal organs. Here, we described several indirect and direct methods to monitor the completion of abscission in Drosophila germ line cells. These methods are based on the observation that cells still connected by some cytoplasm share some degree of synchronization of their cell cycle. This synchrony can be detected on fixed tissue (Section 1.1), including using EdU incorporation to label S-phase (Section 1.2). Mitotic synchrony can also be observed using short-term live imaging (Section 1.3). Finally, we describe how the completion of abscission can be monitored using photoactivatable markers diffusing or not between two cells (Section 1.4).

RNA helicase Belle (DDX3) is essential for male germline stem cell maintenance and division in Drosophila

The present study showed that RNA helicase Belle (DDX3) was required intrinsically for mitotic progression and survival of germline stem cells (GSCs) and spermatogonial cells in the Drosophila melanogaster testes. We found that deficiency of Belle in the male germline resulted in a strong germ cell loss phenotype. Early germ cells are lost through cell death, whereas somatic hub and cyst cell populations are maintained. The observed phenotype is related to that of the human Sertoli Cell-Only Syndrome caused by the loss of DBY (DDX3) expression in the human testes and results in a complete lack of germ cells with preservation of somatic Sertoli cells. We found the hallmarks of mitotic G2 delay in early germ cells of the larval testes of bel mutants. Both mitotic cyclins, A and B, are markedly reduced in the gonads of bel mutants. Transcription levels of cycB and cycA decrease significantly in the testes of hypomorph bel mutants. Overexpression of Cyclin B in the germline partially rescues germ cell survival, mitotic progression and fertility in the bel-RNAi knockdown testes. Taken together, these results suggest that a role of Belle in GSC maintenance and regulation of early germ cell divisions is associated with the expression control of mitotic cyclins.

Antibody Staining in Drosophila Germaria

Drosophila oogenesis is a powerful model for studying a wide spectrum of cellular and developmental processes in vivo. Oogenesis starts in a specialized structure called the germarium, which harbors the stem cells for both germ and somatic cells. The germarium produces egg chambers, each of which will develop into an egg. Active areas of research in Drosophila germaria include stem cell self-renewal, division, and maintenance, cell cycle control and differentiation, oocyte specification, intercellular communication, and signaling, among others. The solid knowledge base, the genetic tractability of the Drosophila model, as well as the availability and fast development of tools and imaging techniques for oogenesis research ensure that studies in this model will keep being instrumental for novel discoveries within cell and developmental biology also in the future. This chapter focuses on antibody staining in Drosophila germaria and provides a protocol for immunostaining as well as an overview of commonly used antibodies for visualization of different cell types and cellular structures. The protocol is well-suited for subsequent confocal microscopy analyses, and in addition we present key adaptations of the protocol that are useful when performing structured illumination microscopy (SIM) super-resolution imaging.

Co-expressed Cyclin D variants cooperate to regulate proliferation of germline nuclei in a syncytium

The role of the G1-phase Cyclin D-CDK 4/6 regulatory module in linking germline stem cell (GSC) proliferation to nutrition is evolutionarily variable. In invertebrate Drosophila and C. elegans GSC models, G1 is nearly absent and Cyclin E is expressed throughout the cell cycle, whereas vertebrate spermatogonial stem cells have a distinct G1 and Cyclin D1 plays an important role in GSC renewal. In the invertebrate, chordate, Oikopleura, where germline nuclei proliferate asynchronously in a syncytium, we show a distinct G1-phase in which 2 Cyclin D variants are co-expressed. Cyclin Dd, present in both somatic endocycling cells and the germline, localized to germline nuclei during G1 before declining at G1/S. Cyclin Db, restricted to the germline, remained cytoplasmic, co-localizing in foci with the Cyclin-dependent Kinase Inhibitor, CKIa. These foci showed a preferential spatial distribution adjacent to syncytial germline nuclei at G1/S. During nutrient-restricted growth arrest, upregulated CKIa accumulated in arrested somatic endoreduplicative nuclei but did not do so in germline nuclei. In the latter context, Cyclin Dd levels gradually decreased. In contrast, the Cyclin Dbβ splice variant, lacking the Rb-interaction domain and phosphodegron, was specifically upregulated and the number of cytoplasmic foci containing this variant increased. This upregulation was dependent on stress response MAPK p38 signaling. We conclude that under favorable conditions, Cyclin Dbβ-CDK6 sequesters CKIa in the cytoplasm to cooperate with Cyclin Dd-CDK6 in promoting germline nuclear proliferation. Under nutrient-restriction, this sequestration function is enhanced to permit continued, though reduced, cycling of the germline during somatic growth arrest.

Translational control in germline stem cell development

Stem cells give rise to tissues and organs during development and maintain their integrity during adulthood. They have the potential to self-renew or differentiate at each division. To ensure proper organ growth and homeostasis, self-renewal versus differentiation decisions need to be tightly controlled. Systematic genetic studies in Drosophila melanogaster are revealing extensive regulatory networks that control the switch between stem cell self-renewal and differentiation in the germline. These networks, which are based primarily on mutual translational repression, act via interlocked feedback loops to provide robustness to this important fate decision.

Abscission is regulated by the ESCRT-III protein shrub in Drosophila germline stem cells

Abscission is the final event of cytokinesis that leads to the physical separation of the two daughter cells. Recent technical advances have allowed a better understanding of the cellular and molecular events leading to abscission in isolated yeast or mammalian cells. However, how abscission is regulated in different cell types or in a developing organism remains poorly understood. Here, we characterized the function of the ESCRT-III protein Shrub during cytokinesis in germ cells undergoing a series of complete and incomplete divisions. We found that Shrub is required for complete abscission, and that levels of Shrub are critical for proper timing of abscission. Loss or gain of Shrub delays abscission in germline stem cells (GSCs), and leads to the formation of stem-cysts, where daughter cells share the same cytoplasm as the mother stem cell and cannot differentiate. In addition, our results indicate a negative regulation of Shrub by the Aurora B kinase during GSC abscission. Finally, we found that Lethal giant discs (lgd), known to be required for Shrub function in the endosomal pathway, also regulates the duration of abscission in GSCs.

Abscission is the final step of cytokinesis which allows the physical separation of sister cells through the scission of a thin cytoplasmic bridge that links them at the end of mitosis. The duration of abscission varies depending on cell types, indicating that the event is developmentally regulated. Recently, we have identified two kinases, Aurora B and CycB/Cdk-1, which regulate the timing of abscission in germ cells and in mammalian cells. However, these kinases are upstream regulators and do not perform abscission per se. Here, we show that Shrub, a potential target of Aurora B and one of the most downstream effectors of abscission, is required for complete abscission in germline stem cells. In the absence of Shrub, the mother stem cell remains linked to its daughter cells, which then share the same cytoplasm and cannot differentiate. Loss of Shrub and Aurora B have opposite effects on abscission duration suggesting that Aurora B regulates negatively Shrub. We further show that Shrub acts together with its interactor Lethal giant disc to ensure proper abscission timing.