Regulation of Drosophila germline stem cells

Escargot a Snail superfamily member and its multiple roles in Drosophila melanogaster development

The Snail superfamily of transcription factors plays a crucial role in metazoan development; one of the most important vertebrate members of this family is Snai1 which is orthologous to the Drosophila melanogaster esg gene. This review offers a comprehensive examination of the roles of the esg gene in Drosophila development, covering its expression pattern and downstream targets, and draws parallels between the vertebrate Snai1 family proteins on controlling the epithelial-to-mesenchymal transition and esg. This gene regulates stemness, ploidy, and pluripontency. esg is expressed in various tissues during development, including the gut, imaginal discs, and neuroblasts. The functions of the esg include the suppression of differentiation in intestinal stem cells and the preservation of diploidy in imaginal cells. In the nervous system development, esg expression also inhibits neuroblast differentiation, thus regulating the number of neurons and the moment in development of neuronal differentiation. Loss of esg function results in diverse developmental defects, including defects in intestinal stem cell maintenance and differentiation, and alters imaginal disc and nervous system development. Expression levels of esg also play a role in regulating longevity and metabolism in adult stages. This review provides an overview of the current understanding of esg's developmental role, emphasizing cellular and tissue effects that arise from its loss of function. The insights gained may contribute to a better understanding of evolutionary conserved developmental mechanisms and certain metabolic diseases.

Communication between the stem cell niche and an adjacent differentiation niche through miRNA and EGFR signaling orchestrates exit from the stem cell state in the Drosophila ovary

The signaling environment, or niche, often governs the initial difference in behavior of an adult stem cell and a derivative that initiates a path towards differentiation. The transition between an instructive stem cell niche and differentiation niche must generally have single-cell resolution, suggesting that multiple mechanisms might be necessary to sharpen the transition. Here, we examined the Drosophila ovary and found that Cap cells, which are key constituents of the germline stem cell (GSC) niche, express a conserved microRNA (miR-124). Surprisingly, loss of miR-124 activity in Cap cells leads to a defect in differentiation of GSC derivatives. We present evidence that the direct functional target of miR-124 in Cap cells is the epidermal growth factor receptor (EGFR) and that failure to limit EGFR expression leads to the ectopic expression of a key anti-differentiation BMP signal in neighboring somatic escort cells (ECs), which constitute a differentiation niche. We further found that Notch signaling connects EFGR activity in Cap cells to BMP expression in ECs. We deduce that the stem cell niche communicates with the differentiation niche through a mechanism that begins with the selective expression of a specific microRNA and culminates in the suppression of the major anti-differentiation signal in neighboring cells, with the functionally important overall role of sharpening the spatial distinction between self-renewal and differentiation environments.

Asymmetric division of stem cells and its cancer relevance

Asymmetric division is a fundamental process for generating cell diversity and maintaining the stem cell population. During asymmetric division, proteins, organelles, and even RNA are distributed unequally between the two daughter cells, determining their distinct cell fates. The mechanisms orchestrating this process are extremely complex. Dysregulation of asymmetric division can potentially trigger cancer progression. Cancer stem cells, in particular, undergo asymmetric division, leading to intra-tumoral heterogeneity, which contributes to treatment refractoriness. In this review, we delve into the cellular and molecular mechanisms that govern asymmetric division and explore its relevance to tumorigenesis.

The Drosophila histone methyl-transferase SET1 coordinates multiple signaling pathways in regulating male germline stem cell maintenance and differentiation

Many cell types come from tissue-specific adult stem cells that maintain the balance between proliferation and differentiation. Here, we study how the H3K4me3 methyltransferase, Set1, regulates early-stage male germ cell proliferation and differentiation in Drosophila. Early-stage germline-specific knockdown of set1 results in a temporally progressed defects, arising as germ cell loss and developing to overpopulated early-stage germ cells. These germline defects also impact the niche architecture and cyst stem cell lineage in a non-cell-autonomous manner. Additionally, wild-type Set1, but not the catalytically inactive Set1, could rescue the set1 knockdown phenotypes, highlighting the functional importance of the methyl-transferase activity of the Set1 enzyme. Further, RNA-seq experiments reveal key signaling pathway components, such as the JAK-STAT pathway gene stat92E and the BMP pathway gene mad, that are upregulated upon set1 knockdown. Genetic interaction assays support the functional relationships between set1 and JAK-STAT or BMP pathways, as mutations of both the stat92E and mad genes suppress the set1 knockdown phenotypes. These findings enhance our understanding of the balance between proliferation and differentiation in an adult stem cell lineage. The germ cell loss followed by over-proliferation phenotypes when inhibiting a histone methyl-transferase raise concerns about using their inhibitors in cancer therapy.

Single-cell RNA sequencing identifies eggplant as a regulator of germ cell development in Drosophila

Drosophila ovarian germline stem cells (GSCs) are a powerful model for stem cell research. In this study, we use single-cell RNA sequencing (scRNA-seq), an RNAi screen and bioinformatic analysis, to identify genes involved in germ cell differentiation, including 34 genes with upregulated expression during early germ cell development and 19 genes that may regulate germ cell differentiation. Among these, a gene we have named eggplant (eggpl) is highly expressed in GSCs and downregulated in early daughter cells. RNAi knockdown of eggpl causes germ cell proliferation and differentiation defects. In flies fed a rich yeast diet, the expression of eggpl is significantly lower and knockdown or knockout of eggpl phenocopies a rich diet. In addition, eggpl knockdown suppresses the reduction in germ cell proliferation caused by inhibition of the insulin effector PI3K. These findings suggest that downregulation of eggpl links nutritional status to germ cell proliferation and differentiation. Collectively, this study provides new insights into the signaling networks that regulate early germ cell development and identifies eggpl as a key player in this process.

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.

Targeting the stem cell niche micro-environment as therapeutic strategies in aging

Adult stem cells (ASCs) reside throughout the body and support various tissue. Owing to their self-renewal capacity and differentiation potential, ASCs have the potential to be used in regenerative medicine. Their survival, quiescence, and activation are influenced by specific signals within their microenvironment or niche. In better words, the stem cell function is significantly influenced by various extrinsic signals derived from the niche. The stem cell niche is a complex and dynamic network surrounding stem cells that plays a crucial role in maintaining stemness. Studies on stem cell niche have suggested that aged niche contributes to the decline in stem cell function. Notably, functional loss of stem cells is highly associated with aging and age-related disorders. The stem cell niche is comprised of complex interactions between multiple cell types. Over the years, essential aspects of the stem cell niche have been revealed, including cell-cell contact, extracellular matrix interaction, soluble signaling factors, and biochemical and biophysical signals. Any alteration in the stem cell niche causes cell damage and affects the regenerative properties of the stem cells. A pristine stem cell niche might be essential for the proper functioning of stem cells and the maintenance of tissue homeostasis. In this regard, niche-targeted interventions may alleviate problems associated with aging in stem cell behavior. The purpose of this perspective is to discuss recent findings in the field of stem cell aging, heterogeneity of stem cell niches, and impact of age-related changes on stem cell behavior. We further focused on how the niche affects stem cells in homeostasis, aging, and the progression of malignant diseases. Finally, we detail the therapeutic strategies for tissue repair, with a particular emphasis on aging.

A single N-terminal amino acid determines the distinct roles of histones H3 and H3.3 in the Drosophila male germline stem cell lineage

Adult stem cells undergo asymmetric cell divisions to produce 2 daughter cells with distinct cell fates: one capable of self-renewal and the other committed for differentiation. Misregulation of this delicate balance can lead to cancer and tissue degeneration. During asymmetric division of Drosophila male germline stem cells (GSCs), preexisting (old) and newly synthesized histone H3 are differentially segregated, whereas old and new histone variant H3.3 are more equally inherited. However, what underlies these distinct inheritance patterns remains unknown. Here, we report that the N-terminal tails of H3 and H3.3 are critical for their inheritance patterns, as well as GSC maintenance and proper differentiation. H3 and H3.3 differ at the 31st position in their N-termini with Alanine for H3 and Serine for H3.3. By swapping these 2 amino acids, we generated 2 mutant histones (i.e., H3A31S and H3.3S31A). Upon expressing them in the early-stage germline, we identified opposing phenotypes: overpopulation of early-stage germ cells in the H3A31S-expressing testes and significant germ cell loss in testes expressing the H3.3S31A. Asymmetric H3 inheritance is disrupted in the H3A31S-expressing GSCs, due to misincorporation of old histones between sister chromatids during DNA replication. Furthermore, H3.3S31A mutation accelerates old histone turnover in the GSCs. Finally, using a modified Chromatin Immunocleavage assay on early-stage germ cells, we found that H3A31S has enhanced occupancy at promoters and transcription starting sites compared with H3, while H3.3S31A is more enriched at transcriptionally silent intergenic regions compared to H3.3. Overall, these results suggest that the 31st amino acids for both H3 and H3.3 are critical for their proper genomic occupancy and function. Together, our findings indicate a critical role for the different amino acid composition of the N-terminal tails between H3 and H3.3 in an endogenous stem cell lineage and provide insights into the importance of proper histone inheritance in specifying cell fates and regulating cellular differentiation.

The Vast Utility of Drosophila Oogenesis

In this chapter, we highlight examples of the diverse array of developmental, cellular, and biochemical insights that can be gained by using Drosophila melanogaster oogenesis as a model tissue. We begin with an overview of ovary development and adult oogenesis. Then we summarize how the adult Drosophila ovary continues to advance our understanding of stem cells, cell cycle, cell migration, cytoplasmic streaming, nurse cell dumping, and cell death. We also review emerging areas of study, including the roles of lipid droplets, ribosomes, and nuclear actin in egg development. Finally, we conclude by discussing the growing conservation of processes and signaling pathways that regulate oogenesis and female reproduction from flies to humans.

Niche formation and function in developing tissue: studies from the Drosophila ovary

Adult stem cells have a unique ability to self-renew and to generate differentiated daughter cells that are required in the body tissues. The identity of adult stem cells is maintained by extrinsic signals from other cell types, known as niche cells. Thus, the niche is required for appropriate tissue homeostasis. Niche is formed and recruits stem cells during tissue development; therefore, it is essential to establish niche cells and stem cells in proper numbers during development. A small niche may recruit too few stem cells and cause tissue degeneration, while a large niche may maintain too many stem cells and lead to tumorigenesis. Given that vertebrate tissues are not suitable for large-scale forward genetics studies, the Drosophila ovary stands out as an excellent model for studying how multiple niche cell types and germ cells (GCs) are coordinately regulated in vivo. Recent studies are beginning to reveal how various signaling molecules regulate niche formation and how niche cells non-autonomously influence GC number. In this review, we summarize the ovarian niche structure, the key signaling pathways for niche formation, and how niche cells generate extrinsic factors to control GC proliferation during ovarian development. Video abstract.

Recent insights into the microRNA and long non-coding RNA-mediated regulation of stem cell populations

Stem cells are undifferentiated cells that have multi-lineage differentiation. The transition from self-renewal to differentiation requires rapid and extensive gene expression alterations. Since different stem cells exhibit diverse non-coding RNAs (ncRNAs) expression profiles, the critical roles of ncRNAs in stem cell reprogramming, pluripotency maintenance, and differentiation have been widely investigated over the past few years. Hence, in this current review, the two main categories of ncRNAs, microRNAs (miRNAs) and long non-coding RNAs (lncRNAs), are discussed. While the primary way by which miRNAs restrict mRNA transcription is through miRNA-mRNA interaction, lncRNAs have a wide range of effects on mRNA functioning, including interactions with miRNAs. Both of these ncRNAs participate in the post-transcriptional regulation of crucial biological mechanisms, such as cell cycle regulation, apoptosis, aging, and cell fate decisions. These findings shed light on a previously unknown aspect of gene regulation in stem cell fate determination and behavior. Overall, we summarized the key roles of miRNAs (including exosomal miRNAs) and lncRNAs in the regulation of stem cell populations, such as cardiac, hematopoietic, mesenchymal, neural, and spermatogonial, as well ncRNAs' influence on malignancy through modulating cancer stem cells, which might significantly contribute to clinical stem cell therapy and in regenerative medicine.

Interchromosomal interaction of homologous Stat92E alleles regulates transcriptional switch during stem-cell differentiation

Pairing of homologous chromosomes in somatic cells provides the opportunity of interchromosomal interaction between homologous gene regions. In the Drosophila male germline, the Stat92E gene is highly expressed in a germline stem cell (GSC) and gradually downregulated during the differentiation. Here we show that the pairing of Stat92E is always tight in GSCs and immediately loosened in differentiating daughter cells, gonialblasts (GBs). Disturbance of Stat92E pairing by relocation of one locus to another chromosome or by knockdown of global pairing/anti-pairing factors both result in a failure of Stat92E downregulation, suggesting that the pairing is required for the decline in transcription. Furthermore, the Stat92E enhancer, but not its transcription, is required for the change in pairing state, indicating that pairing is not a consequence of transcriptional changes. Finally, we show that the change in Stat92E pairing is dependent on asymmetric histone inheritance during the asymmetric division of GSCs. Taken together, we propose that the changes in Stat92E pairing status is an intrinsically programmed mechanism for enabling prompt cell fate switch during the differentiation of stem cells.

Asymmetric inheritance of organelles, proteins and RNAs occurs during stem cell division. Here the authors show the strength of pairing of homologous Stat92E loci, a stem cell-specific gene, changes immediately after the asymmetric division due to asymmetric inheritance of new histones to one of the daughter cells and is important for turning off gene expression in this cell as it differentiates.

Functional Divergence of the bag-of-marbles Gene in the Drosophila melanogaster Species Group

In Drosophila melanogaster, a key germline stem cell (GSC) differentiation factor, bag of marbles (bam) shows rapid bursts of amino acid fixations between sibling species D. melanogaster and Drosophila simulans, but not in the outgroup species Drosophila ananassae. Here, we test the null hypothesis that bam's differentiation function is conserved between D. melanogaster and four additional Drosophila species in the melanogaster species group spanning approximately 30 million years of divergence. Surprisingly, we demonstrate that bam is not necessary for oogenesis or spermatogenesis in Drosophila teissieri nor is bam necessary for spermatogenesis in D. ananassae. Remarkably bam function may change on a relatively short time scale. We further report tests of neutral sequence evolution at bam in additional species of Drosophila and find a positive, but not perfect, correlation between evidence for positive selection at bam and its essential role in GSC regulation and fertility for both males and females. Further characterization of bam function in more divergent lineages will be necessary to distinguish between bam's critical gametogenesis role being newly derived in D. melanogaster, D. simulans, Drosophila yakuba, and D. ananassae females or it being basal to the genus and subsequently lost in numerous lineages.

Adult stem cells and niche cells segregate gradually from common precursors that build the adult Drosophila ovary during pupal development

Production of proliferative follicle cells (FCs) and quiescent escort cells (ECs) by follicle stem cells (FSCs) in adult Drosophila ovaries is regulated by niche signals from anterior (cap cells, ECs) and posterior (polar FCs) sources. Here we show that ECs, FSCs, and FCs develop from common pupal precursors, with different fates acquired by progressive separation of cells along the AP axis and a graded decline in anterior cell proliferation. ECs, FSCs, and most FCs derive from intermingled cell (IC) precursors interspersed with germline cells. Precursors also accumulate posterior to ICs before engulfing a naked germline cyst projected out of the germarium to form the first egg chamber and posterior polar FC signaling center. Thus, stem and niche cells develop in appropriate numbers and spatial organization through regulated proliferative expansion together with progressive establishment of spatial signaling cues that guide adult cell behavior, rather than through rigid early specification events.

Mitochondrial Control of Stem Cell State and Fate: Lessons From Drosophila

Over the years, Drosophila has served as a wonderful genetically tractable model system to unravel various facets of tissue-resident stem cells in their microenvironment. Studies in different stem and progenitor cell types of Drosophila have led to the discovery of cell-intrinsic and extrinsic factors crucial for stem cell state and fate. Though initially touted as the ATP generating machines for carrying various cellular processes, it is now increasingly becoming clear that mitochondrial processes alone can override the cellular program of stem cells. The last few years have witnessed a surge in our understanding of mitochondria's contribution to governing different stem cell properties in their subtissular niches in Drosophila. Through this review, we intend to sum up and highlight the outcome of these in vivo studies that implicate mitochondria as a central regulator of stem cell fate decisions; to find the commonalities and uniqueness associated with these regulatory mechanisms.

Epigenetic regulation of drosophila germline stem cell maintenance and differentiation

Gametogenesis is one of the most extreme cellular differentiation processes that takes place in Drosophila male and female germlines. This process begins at the germline stem cell, which undergoes asymmetric cell division (ACD) to produce a self-renewed daughter that preserves its stemness and a differentiating daughter cell that undergoes epigenetic and genomic changes to eventually produce haploid gametes. Research in molecular genetics and cellular biology are beginning to take advantage of the continually advancing genomic tools to understand: (1) how germ cells are able to maintain their identity throughout the adult reproductive lifetime, and (2) undergo differentiation in a balanced manner. In this review, we focus on the epigenetic mechanisms that address these two questions through their regulation of germline-soma communication to ensure germline stem cell identity and activity.

Nuclear receptors linking physiology and germline stem cells in Drosophila

Maternal nutrition and physiology are intimately associated with reproductive success in diverse organisms. Despite decades of study, the molecular mechanisms linking maternal diet to the production and quality of oocytes remain poorly defined. Nuclear receptors (NRs) link nutritional signals to cellular responses and are essential for oocyte development. The fruit fly, Drosophila melanogaster, is an excellent genetically tractable model to study the relationship between NR signaling and oocyte production. In this review, we explore how NRs in Drosophila regulate the earliest stages of oocyte development. Long-recognized as an essential mediator of developmental transitions, we focus on the intrinsic roles of the Ecdysone Receptor and its ligand, ecdysone, in oogenesis. We also review recent studies suggesting broader roles for NRs as regulators of maternal physiology and their impact specifically on oocyte production. We propose that NRs form the molecular basis of a broad physiological surveillance network linking maternal diet with oocyte production. Given the functional conservation between Drosophila and humans, continued experimental investigation into the molecular mechanisms by which NRs promote oogenesis will likely aid our understanding of human fertility.

Loss of Drosophila E3 Ubiquitin Ligase Hyd Promotes Extra Mitosis in Germline Cysts and Massive Cell Death During Oogenesis

The Drosophila hyperplastic disc (hyd) gene is the ortholog of mammalian tumor suppressor EDD, which is implicated in a wide variety of cellular processes, and its regulation is impaired in various tumors. It is a member of the highly conserved HECT family of E3 ubiquitin ligases, which directly attach ubiquitin to targeted substrates. In early works, it was shown that Drosophila Hyd may be a tumor suppressor because it is involved in the control of imaginal-disc cell proliferation and growth. In this study, we demonstrated that Hyd is also important for the regulation of female germ cell proliferation and that its depletion leads to additional germline cell mitoses. Furthermore, we revealed a previously unknown Hyd function associated with the maintenance of germ cells' viability. A reduction in hyd expression by either mutations or RNA interference resulted in large-scale germ cell death at different stages of oogenesis. Thus, the analysis of phenotypes arising from the hyd deficiency points to Hyd's role in the regulation of germline metabolic processes during oogenesis.

The apical cell - An enigmatic somatic cell in leech ovaries - Structure and putative functions

Although somatic cells play an integral role in animal gametogenesis, their organization and function are usually poorly characterized, especially in non-model systems. One such example is a peculiar cell found in leech ovaries - the apical cell (AC). A single AC can be found at the apical tip of each ovary cord, the functional unit of leech ovaries, where it is surrounded by other somatic and germline cells. The AC is easily distinguished due to its enormous size and its numerous long cytoplasmic projections that penetrate the space between neighboring cells. It is also characterized by a prominent accumulation of mitochondria, Golgi complexes and electron-dense vesicles. ACs are also enriched in cytoskeleton, mainly in form of intermediate filaments. Additionally, the AC is connected to neighboring cells via junctions that structurally resemble hemidesmosomes. In spite of numerous descriptive data about the AC, its functions remain poorly understood. Its suggested functions include a role in forming skeleton for the germline cells, and a role in defining a niche for germline stem cells. The latter is more speculative, since germline stem cells have not been identified in leech ovaries. Somatic cells with similar morphological properties to those of the AC have been found in gonads of nematodes - the distal tip cell - and in insects - Verson's cell, hub cells and cap cells. In the present article we summarize information about the AC structure and its putative functions. AC is compared with other well-described somatic cells with potentially similar roles in gametogenesis.

Drosophila SERTAD domain protein Taranis is required in somatic cells for maintenance of male germline stem cells

BACKGROUND

Polycomb proteins are essential for maintaining stem cell identity across different stem cell niches. However, how they function to maintain stem cell niches is not fully understood.

RESULTS

Here we show that the SERTAD protein Taranis (Tara), which is a Polycomb-trithorax group protein, is expressed in the adult testis niche and plays a role in its maintenance in Drosophila. We found that tara is expressed in early cyst cells, likely including somatic cyst stem cells (CySCs) of Drosophila male testis tip region, which houses both germline and somatic cyst stem cells along with the hub cells, forming the stem cell niche. Consistent with its expression, we found that, while loss of tara in germline cells only had minimal effects, tara knockdown in all cells or only in somatic cells of the niche reduced the number of not only somatic cells, but also germline stem cells (GSCs). We further found that Tara might antagonize Notch signaling in CySCs to maintain the stem cell niche.

CONCLUSIONS

Our studies suggest that Tara might function in somatic CySCs for GSC maintenance in the Drosophila testis.

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.

Nuclear architecture as an intrinsic regulator of Drosophila female germline stem cell maintenance

Homeostasis of Drosophila germline stem cells (GSC) depends upon the integration of intrinsic and extrinsic signals. This review highlights emerging data that support nuclear architecture as an intrinsic regulator of GSC maintenance and germ cell differentiation. Here, we focus on the nuclear lamina (NL) and the nucleolus, two compartments that undergo alterations in composition upon germ cell differentiation. Loss of NL or nucleolar components leads to GSC loss, resulting from activation of GSC quality control checkpoint pathways. We suggest that the NL and nucleolus integrate signals needed for the switch between GSC maintenance and germ cell differentiation, and propose regulation of nuclear actin pools as one mechanism that connects these compartments.

Regulation of adult female germline stem cells by nutrient-responsive signaling

Insect oogenesis is greatly affected by nutrient availability. When nutrients are abundant, oocytes are rapidly generated, but the process is slowed to conserve energy under nutrient-deficient conditions. To properly allocate limited resources toward oogenesis, systemic factors coordinate the behavioral response of ovarian germline stem cells (GSCs) to nutritional inputs by acting on the GSC itself, GSC supporting cells (the niche), or the adipose tissue surrounding the ovary. In this review, we describe current knowledge of the Drosophila ovarian GSC-niche-adipocyte system and major nutrient sensing pathways (insulin/IGF signaling, TOR signaling, and GCN2-dependent amino acid sensing) that intrinsically or extrinsically regulate GSC responses to nutrient signals.