JAK-STAT signal inhibition regulates competition in the Drosophila testis stem cell niche

Signaling pathways in Drosophila testis niche: Local signals that regulate stem cell fate

Stem cells are located in a well-structured and specialized microenvironment called the niche. The niche provides signaling molecules to control the survival, self-renewal, and differentiation of stem cells. As tissues generally contain different types of stem cells, it is important to understand how these stem cells are coordinately regulated by various signaling pathways. The Drosophila testis niche serves as an excellent model for studying such processes, because it harbors 2 types of stem cells, germline stem cells and somatic cyst stem cells. In this review, we summarize the roles of key signaling pathways in stem cell maintenance and differentiation in the Drosophila testis.

Signals from the niche promote distinct modes of translation initiation to control stem cell differentiation and renewal in the Drosophila testis

Stem cells have the unique ability among adult cells to give rise to cells of different identities. To do so, they must change gene expression in response to environmental signals. Much work has focused on how transcription is regulated to achieve these changes; however, in many cell types, transcripts and proteins correlate poorly, indicating that post-transcriptional regulation is important. To assess how translational control can influence stem cell fate, we use the Drosophila testis as a model. The testis niche secretes a ligand to activate the Janus kinase (JAK)/signal transducer and activator of transcription (STAT) pathway in two stem cell populations, germline stem cells (GSCs) and somatic cyst stem cells (CySCs). We find that global translation rates are high in CySCs and decrease during differentiation, and that JAK/STAT signaling regulates translation. To determine how translation was regulated, we knocked down translation initiation factors and found that the cap binding complex, eIF4F, is dispensable in differentiating cells, but is specifically required in CySCs for self-renewal, acting downstream of JAK/STAT activity. Moreover, we identify eIF3d1 as a key regulator of CySC fate, and show that two eIF3d1 residues subject to regulation by phosphorylation are critical to maintain CySC self-renewal. We further show that Casein Kinase II (CkII), which controls eIF3d1 phosphorylation, influences the binding of eIF3d and eIF4F in mammalian cells, and that CkII expression is sufficient to restore CySC function in the absence of JAK/STAT. We propose a model in which niche signals regulate a specific translation programme in which only some mRNAs are translated. The mechanism we identify allows stem cells to switch between modes of translation, adding a layer of regulation on top of transcription and providing cells with the ability to rapidly change gene expression upon receiving external stimuli.

The Batten disease gene Cln3 is required for the activation of intestinal stem cell during regeneration via JAK/STAT signaling in Drosophila

CLN3 mutation causes Juvenile neuronal ceroid lipofuscinosis (JNCL, also known as Batten disease), an early onset neurodegenerative disorder. Patients who suffer from Batten disease often die at an early age. However, the mechanisms underlying how CLN3 loss develops Batten disease remain largely unclear. Here, using Drosophila midgut system, we demonstrate that Drosophila Cln3 has no effect on midgut homeostasis maintaince, including cellular component, intestinal stem cells (ISCs) proliferation and differentiation, but is necessary for ISC activation upon tissue damage. Cell type-specific Gal4 screening reveals that the failure of ISC activation during regeneration caused by Cln3 loss is ISC-autonomous. Through genetic analyses, we elucidate that JAK/STAT signaling in ISCs is not activated with Cln3 depletion upon tissue damage, and functions downstream of Cln3. Our study provides a potential mechanism underlying the development of CLN3-mediated Batten disease at cellular level.

ZnT35C Maintains Zinc Homeostasis to Regulate Spermatogenesis in Drosophila Testis

Zinc homeostasis contributes significantly to numerous physiological processes. Drosophila ZnT35C protein, a zinc transporter encoded by CG3994, is chiefly located on the cell membrane and facilitates the transport of zinc from the cytoplasm to the extracellular space to sustain zinc homeostasis within the organism. Previous studies about ZnT35C have involved diverse structures such as the Malpighian tubules, adult brain, and sensory nervous system. Nonetheless, the role of ZnT35C in Drosophila spermatogenesis remained unclear. In our study, we discovered that ZnT35C plays a pivotal role in Drosophila spermatogenesis. Its knockdown resulted in sperm loss and male infertility. When ZnT35C was knocked down in cyst cells, zinc was concentrated within cyst cells, inhibiting the proper development of germ cells and thereby causing the incapacity of flies to generate mature sperms. Zinc supplementation can effectively rescue this failure of spermatogenesis. Our research outcomes suggest that ZnT35C, through modulating the zinc environment within the testes, impacts the male fertility of Drosophila, occupying a crucial position in the spermatogenesis process.

Licensing and niche competition in spermatogenesis: mathematical models suggest complementary regulation of tissue maintenance

To maintain and regenerate adult tissues after injury, division and differentiation of tissue-resident stem cells must be precisely regulated. It remains elusive which regulatory strategies prevent exhaustion or overgrowth of the stem cell pool, whether there is coordination between multiple mechanisms, and how to detect them from snapshots. In Drosophila testes, somatic stem cells transition to a state that licenses them to differentiate, but remain capable of returning to the niche and resuming cell division. Here, we build stochastic mathematical models for the somatic stem cell population to investigate how licensing contributes to homeostasis. We find that licensing, in combination with differentiation occurring in pairs, is sufficient to maintain homeostasis and prevent stem cell extinction from stochastic fluctuations. Experimental data have shown that stem cells are competing for niche access, and our mathematical models demonstrate that this contributes to the reduction in the variability of stem cell numbers but does not prevent extinction. Hence, a combination of both regulation strategies, licensing with pairwise differentiation and competition for niche access, may be needed to reduce variability and prevent extinction simultaneously.

Maintenance of niche architecture requires actomyosin and enables proper stem cell signaling and oriented division in the Drosophila testis

Stem cells are essential to repair and regenerate tissues, and often reside in a niche that controls their behavior. Here, we use the Drosophila testis niche, a paradigm for niche-stem cell interactions, to address the cell biological features that maintain niche structure and function during its steady-state operation. We report enrichment of Myosin II (MyoII) and a key regulator of actomyosin contractility (AMC), Rho Kinase (ROK), within the niche cell cortex at the interface with germline stem cells (GSCs). Compromising MyoII and ROK disrupts niche architecture, suggesting that AMC in niche cells is important to maintain its reproducible structure. Furthermore, defects in niche architecture disrupt GSC function. Our data suggest that the niche signals less robustly to adjacent germ cells yet permits increased numbers of cells to respond to the signal. Finally, compromising MyoII in niche cells leads to increased misorientation of centrosomes in GSCs as well as defects in the centrosome orientation checkpoint. Ultimately, this work identifies a crucial role for AMC-dependent maintenance of niche structure to ensure a proper complement of stem cells that correctly execute divisions.

Pyriproxyfen enhances germline stem cell proliferation and reduces reproduction in Drosophila by up-regulating juvenile hormone signaling

BACKGROUND

Pyriproxyfen is an insect growth regulator (IGR) that is effective against various types of insect pests. However, the molecular mechanism underlying pyriproxyfen effects on insect reproduction remains unclear. Thus, in this study, we attempted to uncover the mechanisms underlying the impact of pyriproxyfen on the reproductive system of the model organism Drosophila melanogaster.

RESULTS

A significant decrease in Drosophila reproduction was observed after pyriproxyfen treatment. The juvenile hormone (JH) titer was significantly increased (120.4%) in the ovary samples of pyriproxyfen-treated flies. Likewise, the concentrations of key enzymes and the expression of key genes related to the JH signaling pathway were also increased in the pyriproxyfen-treated group compared with the control group. Furthermore, pyriproxyfen treatment significantly increased (15.6%) the number of germline stem cells (GSCs) and significantly decreased (17%) the number of cystoblasts (CBs). However, no significant differences were observed in the number of somatic cells. We performed RNA interference (RNAi) on five key genes (Met, Tai, gce, ftz-f1, and hairy) related to the JH signaling pathway in germ cells using the germ cell-specific Gal4 driver. Interestingly, RNAi of the selected genes significantly decreased the number of both GSCs and CBs in pyriproxyfen-treated transgenic flies. These results further validate that pyriproxyfen enhances GSC proliferation by up-regulating JH signaling.

CONCLUSION

Our results indicate that pyriproxyfen significantly decreases reproduction by affecting germ cells in female adult ovaries. The effect of pyriproxyfen on germ cell proliferation and differentiation is mediated by an increase in JH signaling. This study has significant implications for optimizing pest control strategies, developing sustainable agriculture practices, and understanding the mechanism of insecticide action. © 2024 Society of Chemical Industry.

A regulatory loop of JAK/STAT signalling and its downstream targets represses cell fate conversion and maintains male germline stem cell niche homeostasis

A specialised microenvironment, termed niche, provides extrinsic signals for the maintenance of residential stem cells. However, how residential stem cells maintain niche homeostasis and whether stromal niche cells could convert their fate into stem cells to replenish lost stem cells upon systemic stem cell loss remain largely unknown. Here, through systemic identification of JAK/STAT downstream targets in adult Drosophila testis, we show that Escargot (Esg), a member of the Snail family of transcriptional factors, is a putative JAK/STAT downstream target. esg is intrinsically required in cyst stem cells (CySCs) but not in germline stem cells (GSCs). esg depletion in CySCs results in CySC loss due to differentiation and non-cell autonomous GSC loss. Interestingly, hub cells are gradually lost by delaminating from the hub and converting into CySCs in esg-defective testes. Mechanistically, esg directly represses the expression of socs36E, the well-known downstream target and negative regulator of JAK/STAT signalling. Finally, further depletion of socs36E completely rescues the defects observed in esg-defective testes. Collectively, JAK/STAT target Esg suppresses SOCS36E to maintain CySC fate and repress niche cell conversion. Thus, our work uncovers a regulatory loop between JAK/STAT signalling and its downstream targets in controlling testicular niche homeostasis under physiological conditions.

Mechanisms of Germline Stem Cell Competition across Species

In this review, we introduce the concept of cell competition, which occurs between heterogeneous neighboring cell populations. Cells with higher relative fitness become "winners" that outcompete cells of lower relative fitness ("losers"). We discuss the idea of super-competitors, mutant cells that expand at the expense of wild-type cells. Work on adult stem cells (ASCs) has revealed principles of neutral competition, wherein ASCs can be stochastically lost and replaced, and of biased competition, in which a winning ASC with a competitive advantage replaces its neighbors. Germline stem cells (GSCs) are ASCs that are uniquely endowed with the ability to produce gametes and, therefore, impact the next generation. Mechanisms of GSC competition have been elucidated by studies in Drosophila gonads, tunicates, and the mammalian testis. Competition between ASCs is thought to underlie various forms of cancer, including spermatocytic tumors in the human testis. Paternal age effect (PAE) disorders are caused by de novo mutations in human GSCs that increase their competitive ability and make them more likely to be inherited, leading to skeletal and craniofacial abnormalities in offspring. Given its widespread effects on human health, it is important to study GSC competition to elucidate how cells can become winners or losers.

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

Many tissue-specific adult stem cell lineages maintain a balance between proliferation and differentiation. Here, we study how the H3K4me3 methyltransferase Set1 regulates early-stage male germ cells in Drosophila. Early-stage germline-specific knockdown of Set1 results in temporally progressive defects, arising as germ cell loss and developing into overpopulated early-stage germ cells. These germline defects also impact the niche architecture and cyst stem cell lineage non-cell-autonomously. Additionally, wild-type Set1, but not the catalytically inactive Set1, rescues the Set1 knockdown phenotypes, highlighting the functional importance of the methyltransferase activity of Set1. Further, RNA-sequencing experiments reveal key signaling pathway components, such as the JAK-STAT pathway gene Stat92E and the BMP pathway gene Mad, which are upregulated upon Set1 knockdown. Genetic interaction assays support the functional relationships between Set1 and JAK-STAT or BMP pathways, as both Stat92E and Mad mutations suppress the Set1 knockdown phenotypes. These findings enhance our understanding of the balance between proliferation and differentiation in an adult stem cell lineage. The phenotype of germ cell loss followed by over-proliferation when inhibiting a histone methyltransferase also raises concerns about using their inhibitors in cancer therapy.

Computational simulation of JAK/STAT signaling in somatic versus germline stem cells

BACKGROUND

The Janus kinase/signal transducer and activator of transcription (JAK/STAT) signaling pathway regulates a variety of cellular processes. A major activation event in this pathway involves the phosphorylation of a tyrosine of STAT, converting unphosphorylated STAT (uSTAT) to phosphorylated STAT (pSTAT), an active transcription factor. In a noncanonical role, uSTAT contributes to the maintenance of heterochromatin stability. As such, an increase in pSTAT concurrently reduces uSTAT, resulting in heterochromatin loss, as observed in Drosophila somatic tissues. Paradoxically, an opposing phenomenon occurs in Drosophila male germline stem cells (GSCs), where the JAK/STAT pathway remains persistently active due to a continuous supply of ligands. Here, computational simulations were employed to dissect JAK/STAT pathway activation under different cellular contexts, mimicking somatic and germline cells. In these simulations, ordinary differential equations were leveraged to replicate the chemical reactions governing JAK/STAT signaling under different conditions.

RESULTS

The outcomes indicate that transient ligand stimulation, typical in somatic tissues, led to a momentary reduction in uSTAT levels. Conversely, sustained ligand stimulation, a characteristic feature of the GSC niche, resulted in elevated uSTAT levels at equilibrium.

CONCLUSION

The simulation suggests that the duration of ligand exposure could explain the observed opposite effects of JAK/STAT activation on heterochromatin in somatic versus GSCs.

Integrin restriction by miR-34 protects germline progenitors from cell death during aging

During aging, regenerative tissues must dynamically balance the two opposing processes of proliferation and cell death. While many microRNAs are differentially expressed during aging, their roles as dynamic regulators of tissue regeneration have yet to be described. We show that in the highly regenerative Drosophila testis, miR-34 levels are significantly elevated during aging. miR-34 modulates germ cell death and protects the progenitor germ cells from accelerated aging. However, miR-34 is not expressed in the progenitors themselves but rather in neighboring cyst cells that kill the progenitors. Transcriptomics followed by functional analysis revealed that during aging, miR-34 modifies integrin signaling by limiting the levels of the heterodimeric integrin receptor αPS2 and βPS subunits. In addition, we found that in cyst cells, this heterodimer is essential for inducing phagoptosis and degradation of the progenitor germ cells. Together, these data suggest that the miR-34-integrin signaling axis acts as a sensor of progenitor germ cell death to extend progenitor functionality during aging.

Rel governs loser elimination during stem cell competition in the Drosophila testis

In the Drosophila testis, a group of stromal cells termed hub provides multiple niche signals for the surrounding germline and somatic stem cells. Stem cells of both populations compete for physical retention in the niche, and clones unable to transduce any one niche signal are rapidly eliminated from the stem cell pool by differentiation. We have mapped the transcriptomes of isolated somatic cyst stem cells and differentiated cyst cells, and found that the stem cells but not their differentiated progeny exhibit the signature of an innate immune response including the NF-κB transcription factor Relish (Rel). Related signalling pathways had previously implicated in cell competition in larval epithelia, prompting the question of whether NF-κB signalling was, despite the clear differences between the two competition scenarios, also involved in stem cell competition in the testis. Here we show i) that in the testis Rel is dispensable for stemness, ii) that loss of Rel or the upstream receptor Toll suppresses loser elimination following a variety of different triggers used to induce loser fate, and iii) that clonal Rel activation is sufficient for the displacement of neutral or winner cells from the niche, even if these cells otherwise retain stem cell properties.

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.

Signaling Pathways in Drosophila gonadal Stem Cells

The stem cells' ability to divide asymmetrically to produce differentiating and self-renewing daughter cells is crucial to maintain tissue homeostasis and development. Stem cell maintenance and differentiation rely on their regulatory microenvironment termed 'niches'. The mechanisms of the signal transduction pathways initiated from the niche, regulation of stem cell maintenance and differentiation were quite challenging to study. The knowledge gained from the study of Drosophila melanogaster testis and ovary helped develop our understanding of stem cell/niche interactions and signal pathways related to the regulatory mechanisms in maintaining homeostasis of adult tissue. In this review, we discuss the role of signaling pathways in Drosophila gonadal stem cell regeneration, competition, differentiation, dedifferentiation, proliferation, and fate determination. Furthermore, we present the current knowledge on how these signaling pathways are implicated in cancer, and how they contribute as potential candidates for effective cancer treatment.

A feedforward loop between JAK/STAT downstream target p115 and STAT in germline stem cells

The maintenance of germline stem cells (GSCs) is essential for tissue homeostasis. JAK/STAT signaling maintains GSC fate in Drosophila testis. However, how JAK/STAT signaling maintains male GSC fate through its downstream targets remains poorly understood. Here, we identify p115, a tER/cis-Golgi golgin protein, as a putative downstream target of JAK/STAT signaling. p115 maintains GSC fate independent of GM130 and GRASP65. p115 localizes in cytosol, the ER and Golgi apparatus in germline cells and is required for the morphology of the ER and Golgi apparatus. Furthermore, depletion of p115 in GSCs results in aberrant spindle orientation. Mechanistically, p115 associates with and stabilizes STAT. Finally, ectopic expression of STAT completely restores GSC loss caused by p115 depletion. Collectively, JAK/STAT signaling and p115 form a feedforward loop to maintain male GSC fate. Our work provides new insights into the regulatory mechanism of how stem cell maintenance is properly controlled by JAK/STAT signaling.

A review on immunological aspects in male reproduction: An immune cells and cytokines

The male reproductive system, particularly the male gamete, offers a unique barrier to the immune system. The growing germ cells in the testis need to be shielded from autoimmune damage. Hence the testis has to establish and sustain an immune-privileged milieu. Sertoli cells create this safe space, protected by the blood-testis barrier. Cytokines are a type of immune reaction that can positively and negatively affect male reproductive health. Inflammation, disease, and obesity are just a few physiological conditions for which cytokines mediate signals. They interact with steroidogenesis, shaping the adrenals and testes to produce the hormones needed for survival. In particular pathological condition, including autoimmune disorders, contains high levels of the same cytokines in semen that play an essential role in the immunomodulation of the male gonad. This review focuses on understanding the immunological role of cytokines in the control and development of male reproduction. Also, in maintaining male reproductive health and diseases linked with their aberrant function in the testis.

Canonical and non-canonical functions of STAT in germline stem cell maintenance

BACKGROUND

Maintenance of the Drosophila male germline stem cells (GSCs) requires activation of the Janus kinase/signal transducer and activators of transcription (JAK/STAT) pathway by niche signals. The precise role of JAK/STAT signaling in GSC maintenance, however, remains incompletely understood.

RESULTS

Here, we show that, GSC maintenance requires both canonical and non-canonical JAK/STAT signaling, in which unphosphorylated STAT (uSTAT) maintains heterochromatin stability by binding to heterochromatin protein 1 (HP1). We found that GSC-specific overexpressing STAT, or even the transcriptionally inactive mutant STAT, increases GSC number and partially rescues the GSC-loss mutant phenotype due to reduced JAK activity. Furthermore, we found that both HP1 and STAT are transcriptional targets of the canonical JAK/STAT pathway in GSCs, and that GSCs exhibit higher heterochromatin content.

CONCLUSIONS

These results suggest that persistent JAK/STAT activation by niche signals leads to the accumulation of HP1 and uSTAT in GSCs, which promote heterochromatin formation important for maintaining GSC identity. Thus, the maintenance of Drosophila GSCs requires both canonical and non-canonical STAT functions within GSCs for heterochromatin regulation.

Pvr regulates cyst stem cell division in the Drosophila testis niche, and has functions distinct from Egfr

Regulation of the rate of stem cell division is one of the key determinants of the abundance of differentiating progeny in stem cell-supported tissues, and mis-regulation can lead to tumorigenesis. The well-studied Drosophila testis niche is an excellent model system to study the regulation of stem cell division in vivo. This niche supports two stem cell populations-the germline stem cells (GSCs) and cyst stem cells (CySCs), which cluster around a group of cells called the hub. The differentiating cells of these two stem cell populations cooperate together to produce sperm. Signal transduction initiated by the epidermal growth factor receptor (Egfr) is a key regulatory pathway in the cyst lineage, and much of the study of this stem cell population has centered around understanding the complexities of the requirements for Egfr signaling. We examined another receptor tyrosine kinase, Pvr, the sole Drosophila PDGF/VEGF homolog, and found that it accumulates in the cyst lineage cells of the testis, while its ligand Pvf1 accumulates in the hub. Pvr inhibition caused a reduction in both CySC numbers and the proportion of CySCs in S phase, similar to Egfr inhibition. However, testes with Pvr inhibition exhibited a low-penetrance non-autonomous germ cell differentiation defect distinct from that observed with Egfr inhibition. Cyst cells with constitutively activated Pvr failed to support germ cell differentiation, as observed with constitutively activated Egfr. However, constitutively activated Pvr promoted tumorous accumulation of cyst cells outside of the niche, a phenotype not observed with constitutively activated Egfr. Thus, Egfr and Pvr have some receptor-specific functions and some shared functions in the cyst lineage cells of the testis.

Comparative transcriptional analysis uncovers molecular processes in early and mature somatic cyst cells of Drosophila testes

The tight interaction between somatic and germline cells is conserved in animal spermatogenesis. The testes of Drosophila melanogaster are the model of choice to identify processes responsible for mature gamete production. However, processes of differentiation and soma-germline interactions occurring in somatic cyst cells are currently understudied. Here we focused on the comparison of transcriptome expression patterns of early and mature somatic cyst cells to find out the developmental changes taking place in them. We employed a FACS-based approach for the isolation of early and mature somatic cyst cells from fly testes, subsequent preparation of RNA-Seq libraries, and analysis of gene differential expression in the sorted cells. We found increased expression of genes involved in cell cycle-related processes in early cyst cells, which is necessary for the proliferation and self-renewal of a crucial population of early cyst cells, cyst stem cells. Genes proposedly required for lamellipodium-like projection organization for proper cyst formation were also detected among the upregulated ones in early cyst cells. Gene Ontology and interactome analyses of upregulated genes in mature cyst cells revealed a striking over-representation of gene categories responsible for metabolic and catabolic cellular processes, as well as genes supporting the energetic state of the cells provided by oxidative phosphorylation that is carried out in mitochondria. Our comparative analyses of differentially expressed genes revealed major peculiarities in early and mature cyst cells and provide novel insight into their regulation, which is important for male fertility.

Reduced adhesion of aged intestinal stem cells contributes to an accelerated clonal drift

Analysis of clonal dynamics of intestinal stem cells supports an accelerated clonal drift upon aging, likely because of reduced adhesion of aged ISCs because of reduced canonical Wnt signaling.

Upon aging, the function of the intestinal epithelium declines with a concomitant increase in aging-related diseases. ISCs play an important role in this process. It is known that ISC clonal dynamics follow a neutral drift model. However, it is not clear whether the drift model is still valid in aged ISCs. Tracking of clonal dynamics by clonal tracing revealed that aged crypts drift into monoclonality substantially faster than young ones. However, ISC tracing experiments, in vivo and ex vivo, implied a similar clonal expansion ability of both young and aged ISCs. Single-cell RNA sequencing for 1,920 high Lgr5 ISCs from young and aged mice revealed increased heterogeneity among subgroups of aged ISCs. Genes associated with cell adhesion were down-regulated in aged ISCs. ISCs of aged mice indeed show weaker adhesion to the matrix. Simulations applying a single cell–based model of the small intestinal crypt demonstrated an accelerated clonal drift at reduced adhesion strength, implying a central role for reduced adhesion for affecting clonal dynamics upon aging.

Injury-induced inflammatory signaling and hematopoiesis in Drosophila

Inflammatory response in Drosophila to sterile (axenic) injury in embryos and adults has received some attention in recent years, and most concentrate on the events at the injury site. Here we focus on the effect sterile injury has on the hematopoietic organ, the lymph gland, and the circulating blood cells in the larva, the developmental stage at which major events of hematopoiesis are evident. In mammals, injury activates Toll-like receptor/NF-κB signaling in macrophages, which then express and secrete secondary, proinflammatory cytokines. In Drosophila larvae, distal puncture injury of the body wall epidermis causes a rapid activation of Toll and Jun kinase (JNK) signaling throughout the hematopoietic system and the differentiation of a unique blood cell type, the lamellocyte. Furthermore, we find that Toll and JNK signaling are coupled in their activation. Secondary to this Toll/JNK response, a cytokine, Upd3, is induced as a Toll pathway transcriptional target, which then promotes JAK/STAT signaling within the blood cells. Toll and JAK/STAT signaling are required for the emergence of the injury-induced lamellocytes. This is akin to the derivation of specialized macrophages in mammalian systems. Upstream, at the injury site, a Duox- and peroxide-dependent signal causes the activation of the proteases Grass and SPE, needed for the activation of the Toll-ligand Spz, but microbial sensors or the proteases most closely associated with them during septic injury are not involved in the axenic inflammatory response.

chinmo-mutant spermatogonial stem cells cause mitotic drive by evicting non-mutant neighbors from the niche

Niches maintain a finite pool of stem cells via restricted space and short-range signals. Stem cells compete for limited niche resources, but the mechanisms regulating competition are poorly understood. Using the Drosophila testis model, we show that germline stem cells (GSCs) lacking the transcription factor Chinmo gain a competitive advantage for niche access. Surprisingly, chinmo-/- GSCs rely on a new mechanism of competition in which they secrete the extracellular matrix protein Perlecan to selectively evict non-mutant GSCs and then upregulate Perlecan-binding proteins to remain in the altered niche. Over time, the GSC pool can be entirely replaced with chinmo-/- cells. As a consequence, the mutant chinmo allele acts as a gene drive element; the majority of offspring inherit the allele despite the heterozygous genotype of the parent. Our results suggest that the influence of GSC competition may extend beyond individual stem cell niche dynamics to population-level allelic drift and evolution.

Merlin and expanded integrate cell signaling that regulates cyst stem cell proliferation in the Drosophila testis niche

The Drosophila testis is a model organism stem cell niche in which two stem cell populations coordinate together to produce sperm; thus, these stem cells must be balanced in the niche. Merlin, a tumor-suppressor and human disease gene required for contact inhibition of proliferation, is known to limit the proliferation of the somatic cyst stem cells in the testis niche. Expanded encodes a protein that is structurally similar to Merlin in Drosophila, and is semi-redundant with Merlin in multiple tissues. We found that expanded depletion caused similar cyst lineage cell over-proliferation as observed with Merlin, and double mutants showed more severe phenotypes than either gene individually. Thus, these genes have partially redundant functions in the cyst lineage cells of this niche. We also expressed non-phosphorylatable constitutively "tumor suppressing" alleles of Merlin in cyst lineage cells, and surprisingly, we observed a similar cyst lineage over-proliferation phenotype. Merlin is known to impact multiple different signaling pathways to exert its effect on proliferation. We found that the Merlin loss of function phenotype was associated with an increase in MAPK/ERK signaling, consistent with Merlin's established role in transmembrane receptor inhibition. Constitutive Merlin displayed a reduction in both MAPK/ERK signaling and PI3K/Tor signaling. PI3K/Tor signaling is required for cyst cell differentiation, and inhibition of this pathway by Merlin activation phenocopied the Tor cyst lineage loss of function phenotype. Thus, Merlin impacts and integrates the activity of multiple signaling pathways in the testis niche. The ability of Merlin to dynamically change its activity via phosphorylation in response to local contact cues provides an intriguing mechanism whereby the signaling pathways that control these stem cells might be dynamically regulated in response to the division of a neighboring germ cell.

The Drosophila gonads: models for stem cell proliferation, self-renewal, and differentiation

The male and female gonads of Drosophila melanogaster have developed into powerful model systems for both the study of stem cell behaviours, and for understanding how stem cell misregulation can lead to cancers. Using these systems, one is able to observe and manipulate the resident stem cell populations in vivo with a great deal of licence. The tractability of the testis and ovary also allow researchers to explore a range of cellular mechanisms, such as proliferation and polarity, as well as the influence exerted by the local environment through a host of highly-conserved signalling pathways. Importantly, many of the cellular behaviours and processes studied in the Drosophila testis and ovary are known to be disrupted, or otherwise misregulated, in human tumourigenic cells. Here, we review the mechanisms relating to stem cell behaviour, though we acknowledge there are many other fascinating aspects of gametogenesis, including the invasive behaviour of migratory border cells in the Drosophila ovary that, though relevant to the study of tumourigenesis, will unfortunately not be covered.

Transcriptomic analysis of feminizing somatic stem cells in the Drosophila testis reveals putative downstream effectors of the transcription factor Chinmo

One of the best examples of sexual dimorphism is the development and function of the gonads, ovaries and testes, which produce sex-specific gametes, oocytes, and spermatids, respectively. The development of these specialized germ cells requires sex-matched somatic support cells. The sexual identity of somatic gonadal cells is specified during development and must be actively maintained during adulthood. We previously showed that the transcription factor Chinmo is required to ensure the male sexual identity of somatic support cells in the Drosophila melanogaster testis. Loss of chinmo from male somatic gonadal cells results in feminization: they transform from squamous to epithelial-like cells that resemble somatic cells in the female gonad but fail to properly ensheath the male germline, causing infertility. To identify potential target genes of Chinmo, we purified somatic cells deficient for chinmo from the adult Drosophila testis and performed next-generation sequencing to compare their transcriptome to that of control somatic cells. Bioinformatics revealed 304 and 1549 differentially upregulated and downregulated genes, respectively, upon loss of chinmo in early somatic cells. Using a combination of methods, we validated several differentially expressed genes. These data sets will be useful resources to the community.

Knockdown of RpL36 in testes impairs spermatogenesis in Drosophila melanogaster

Many ribosomal proteins (RPs) not only play essential roles in ribosome biogenesis, but also have "extraribosomal" functions in various cellular processes. RpL36 encodes ribosomal protein L36, a component of the 60S subunit of ribosomes in Drosophila melanogaster. We report here that RpL36 is required for spermatogenesis in D. melanogaster. After showing the evolutionary conservation of RpL36 sequences in animals, we revealed that the RpL36 expression level in fly testes was significantly higher than in ovaries. Knockdown RpL36 in fly testes resulted in a significantly decreased egg hatch rate when these males mated with wild-type females. Furthermore, 76.67% of the RpL36 knockdown fly testes were much smaller in comparison to controls. Immunofluorescence staining exhibited that in the RpL36 knockdown testis hub cell cluster was enlarged, while the number of germ cells, including germ stem cells, was reduced. Knockdown of RpL36 in fly testis caused much fewer or no mature sperms in seminal vesicles. The terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) signal was stronger in RpL36 knockdown fly testes than in the control testes, but the TUNEL-positive cells could not be stained by Vasa antibody, indicating that apoptotic cells are not germ cells. The percentage of pH3-positive cells among the Vasa-positive cells was significantly reduced. The expression of genes involved in cell death, cell cycle progression, and JAK/STAT signaling pathway was significantly changed by RpL36 knockdown in fly testes. These results suggest that RpL36 plays an important role in spermatogenesis, likely through JAK/STAT pathway, thus resulting in defects in cell-cycle progression and cell death in D. melanogaster testes.

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.

Wnt6 regulates the homeostasis of the stem cell niche via Rac1-and Cdc42-mediated noncanonical Wnt signalling pathways in Drosophila testis

The homeostasis of the stem cell niche is regulated by both intrinsic and extrinsic factors, and the complex and ordered molecular and cellular regulatory mechanisms need to be further explored. In Drosophila testis, germline stem cells (GSCs) rely on hub cells for self-renewal and physical attachment. GSCs are also in contact with somatic cyst stem cells (CySCs). Utilizing genetic manipulation in Drosophila, we investigated the role of Wnt6 in vivo and in vitro. In Drosophila testis, we found that Wnt6 is required for GSC differentiation and CySC self-renewal. In Schneider 2 (S2) cells, we found that Wnt6 regulates cell proliferation and apoptosis. Mechanistically, we demonstrated that Wnt6 can downregulate the expression levels of Arm, Rac1 and Cdc42 in S2 cells. Notably, Rac1 and Cdc42, which act downstream of the noncanonical Wnt signalling pathway, imitated the phenotypes of Wnt6 in Drosophila testis. Thus, the newly discovered Wnt6-Rac1/Cdc42 signal axis is required for the homeostasis of the stem cell niche in the Drosophila testis.

Centromere assembly and non-random sister chromatid segregation in stem cells

Asymmetric cell division (ACD) produces daughter cells with separate distinct cell fates and is critical for the development and regulation of multicellular organisms. Epigenetic mechanisms are key players in cell fate determination. Centromeres, epigenetically specified loci defined by the presence of the histone H3-variant, centromere protein A (CENP-A), are essential for chromosome segregation at cell division. ACDs in stem cells and in oocyte meiosis have been proposed to be reliant on centromere integrity for the regulation of the non-random segregation of chromosomes. It has recently been shown that CENP-A is asymmetrically distributed between the centromeres of sister chromatids in male and female Drosophila germline stem cells (GSCs), with more CENP-A on sister chromatids to be segregated to the GSC. This imbalance in centromere strength correlates with the temporal and asymmetric assembly of the mitotic spindle and potentially orientates the cell to allow for biased sister chromatid retention in stem cells. In this essay, we discuss the recent evidence for asymmetric sister centromeres in stem cells. Thereafter, we discuss mechanistic avenues to establish this sister centromere asymmetry and how it ultimately might influence cell fate.

Stem Cell DNA Damage and Genome Mutation in the Context of Aging and Cancer Initiation

Adult stem cells fuel tissue homeostasis and regeneration through their unique ability to self-renew and differentiate into specialized cells. Thus, their DNA provides instructions that impact the tissue as a whole. Since DNA is not an inert molecule, but rather dynamic, interacting with a myriad of chemical and physical factors, it encounters damage from both endogenous and exogenous sources. Damage to DNA introduces deviations from its normal intact structure and, if left unrepaired, may result in a genetic mutation. In turn, mutant genomes of stem and progenitor cells are inherited in cells of the lineage, thus eroding the genetic information that maintains homeostasis of the somatic cell population. Errors arising in stem and progenitor cells will have a substantially larger impact on the tissue in which they reside than errors occurring in postmitotic differentiated cells. Therefore, maintaining the integrity of genomic DNA within our stem cells is essential to protect the instructions necessary for rebuilding healthy tissues during homeostatic renewal. In this review, we will first discuss DNA damage arising in stem cells and cell- and tissue-intrinsic mechanisms that protect against harmful effects of this damage. Secondly, we will examine how erroneous DNA repair and persistent DNA damage in stem and progenitor cells impact stem cells and tissues in the context of cancer initiation and aging. Finally, we will discuss the use of invertebrate and vertebrate model systems to address unanswered questions on the role that DNA damage and mutation may play in aging and precancerous conditions.

RpS13 controls the homeostasis of germline stem cell niche through Rho1-mediated signals in the Drosophila testis

Objectives

Stem cell niche regulated the renewal and differentiation of germline stem cells (GSCs) in Drosophila. Previously, we and others identified a series of genes encoding ribosomal proteins that may contribute to the self‐renewal and differentiation of GSCs. However, the mechanisms that maintain and differentiate GSCs in their niches were not well understood.

Materials and Methods

Flies were used to generate tissue‐specific gene knockdown. Small interfering RNAs were used to knockdown genes in S2 cells. qRT‐PCR was used to examine the relative mRNA expression level. TUNEL staining or flow cytometry assays were used to detect cell survival. Immunofluorescence was used to determine protein localization and expression pattern.

Results

Herein, using a genetic manipulation approach, we investigated the role of ribosomal protein S13 (RpS13) in testes and S2 cells. We reported that RpS13 was required for the self‐renewal and differentiation of GSCs. We also demonstrated that RpS13 regulated cell proliferation and apoptosis. Mechanistically, we showed that RpS13 regulated the expression of ribosome subunits and could moderate the expression of the Rho1, DE‐cad and Arm proteins. Notably, Rho1 imitated the phenotype of RpS13 in both Drosophila testes and S2 cells, and recruited cell adhesions, which was mediated by the DE‐cad and Arm proteins.

Conclusion

These findings uncover a novel mechanism of RpS13 that mediates Rho1 signals in the stem cell niche of the Drosophila testis.

C1EIP Functions as an Activator of ENO1 to Promote Chicken PGCs Formation via Inhibition of the Notch Signaling Pathway

The production of germ cells, especially primordial germ cells (PGCs), is important for avian stem cells and reproduction biology. However, key factors involved in the regulation of PGCs remain unknown. Here, we report a PGC-related marker gene: C1EIP (Chromosome 1 Expression in PGCs), whose activation and expression are regulated by the transcription factor STAT3 (signal transducer and activator of transcription 3), histone acetylation, and promoter methylation. C1EIP regulates PGCs formation by mediating the expression of PGC-associated genes, such as CVH (Chicken Vasa Homologous) and CKIT (Chicken KIT proto-oncogene). C1EIP knockdown during embryonic development reduces PGC generation efficiency both in vitro and in ovo. Conversely, C1EIP overexpression increases the formation efficiency of PGCs. C1EIP encodes a cytoplasmic protein that interacts with ENO1 (Enolase 1) in the cytoplasm, inhibits the Notch signaling pathway, and positively regulates PGC generation. Collectively, our findings demonstrate C1EIP as a novel gene involved in PGC formation, which regulates genes involved in embryonic stem cell differentiation through interaction with ENO1 and subsequent inhibition of the Notch signaling pathway by the impression of Myc (MYC proto-oncogene).

Single-cyst transcriptome analysis of Drosophila male germline stem cell lineage

The Drosophila male germline stem cell (GSC) lineage provides a great model to understand stem cell maintenance, proliferation, differentiation and dedifferentiation. Here, we use the Drosophila GSC lineage to systematically analyze the transcriptome of discrete but continuously differentiating germline cysts. We first isolated single cysts at each recognizable stage from wild-type testes, which were subsequently applied for RNA-seq analyses. Our data delineate a high-resolution transcriptome atlas in the entire male GSC lineage: the most dramatic switch occurs from early to late spermatocyte, followed by the change from the mitotic spermatogonia to early meiotic spermatocyte. By contrast, the transit-amplifying spermatogonia cysts display similar transcriptomes, suggesting common molecular features among these stages, which may underlie their similar behavior during both differentiation and dedifferentiation processes. Finally, distinct differentiating germ cell cyst samples do not exhibit obvious dosage compensation of X-chromosomal genes, even considering the paucity of X-chromosomal gene expression during meiosis, which is different from somatic cells. Together, our single cyst-resolution, genome-wide transcriptional profile analyses provide an unprecedented resource to understand many questions in both germ cell biology and stem cell biology fields.

The Drosophila RNA Helicase Belle (DDX3) Non-Autonomously Suppresses Germline Tumorigenesis Via Regulation of a Specific mRNA Set

DDX3 subfamily DEAD-box RNA helicases are essential developmental regulators of RNA metabolism in eukaryotes. belle, the single DDX3 ortholog in Drosophila, is required for fly viability, fertility, and germline stem cell maintenance. Belle is involved both in translational activation and repression of target mRNAs in different tissues; however, direct targets of Belle in the testes are essentially unknown. Here we showed that belle RNAi knockdown in testis cyst cells caused a disruption of adhesion between germ and cyst cells and generation of tumor-like clusters of stem-like germ cells. Ectopic expression of β-integrin in cyst cells rescued early stages of spermatogenesis in belle knockdown testes, indicating that integrin adhesion complexes are required for the interaction between somatic and germ cells in a cyst. To address Belle functions in spermatogenesis in detail we performed cross-linking immunoprecipitation and sequencing (CLIP-seq) analysis and identified multiple mRNAs that interacted with Belle in the testes. The set of Belle targets includes transcripts of proteins that are essential for preventing the tumor-like clusters of germ cells and for sustaining spermatogenesis. By our hypothesis, failures in the translation of a number of mRNA targets additively contribute to developmental defects observed in the testes with belle knockdowns both in cyst cells and in the germline.

Signal transduction in the early Drosophila follicle stem cell lineage

The follicle stem cell (FSC) lineage in the Drosophila ovary is a highly informative model of in vivo epithelial stem cell biology. Studies over the past 30 years have identified roles for every major signaling pathway in the early FSC lineage. These pathways regulate a wide variety of cell behaviors, including self-renewal, proliferation, survival and differentiation. Studies of cell signaling in the follicle epithelium have provided new insights into how these cell behaviors are coordinated within an epithelial stem cell lineage and how signaling pathways interact with each other in the native, in vivo context of a living tissue. Here, we review these studies, with a particular focus on how these pathways specify differences between the FSCs and their daughter cells. We also describe common themes that have emerged from these studies, and highlight new research directions that have been made possible by the detailed understanding of the follicle epithelium.

Ectopic Dpp signaling promotes stem cell competition through EGFR signaling in the Drosophila testis

Stem cell competition could select the fittest stem cells and potentially control tumorigenesis. However, little is known about the underlying molecular mechanisms. Here, we find that ectopic Decapentaplegic (Dpp) signal activation by expressing a constitutively active form of Thickveins (Tkv^CA) in cyst stem cells (CySCs) leads to competition between CySCs and germline stem cells (GSCs) for niche occupancy and GSC loss. GSCs are displaced from the niche and undergo differentiation. Interestingly, we find that induction of Tkv^CA results in elevated expression of vein, which further activates Epidermal Growth Factor Receptor (EGFR) signaling in CySCs to promote their proliferation and compete GSCs out of the niche. Our findings elucidate the important role of Dpp signaling in regulating stem cell competition and tumorigenesis, which could be shed light on tumorigenesis and cancer treatment in mammals.

Heterochromatin Protein 1 (HP1) inhibits stem cell proliferation induced by ectopic activation of the Jak/STAT pathway in the Drosophila testis

Stem cells can divide asymmetrically with respect to cell fate, producing a copy of themselves (self-renewal), while giving rise to progeny that will differentiate along a specific lineage. Mechanisms that bias the balance towards self-renewal or extend the proliferative capacity of the differentiating progeny can result in tissue overgrowth and, eventually, the formation of tumors. Recent work has explored the role of heterochromatin and heterochromatin-associated proteins in the regulation of stem cell behavior under homeostatic conditions, but less is known about their possible roles in potentiating or suppressing stem cell overproliferation. Here we used ectopic activation of the Jak/STAT pathway in germline and somatic stem cells of the D. melanogaster testis as an in vivo model to probe the function of Heterochromatin Protein 1 (HP1) in stem cell overproliferation. Forced expression of HP1 in either early germ or somatic cells suppressed the overgrowth of testes in response to ectopic Jak/STAT activation. Interestingly, HP1 expression led to distinct phenotypes, depending on whether it was overexpressed in somatic or germ cells, possibly reflecting different cell-autonomous and non-autonomous effects in each cell type. Our results provide a new framework for further in vivo studies aimed at understanding the interactions between heterochromatin and uncontrolled stem cell proliferation, as well as the complex cross-regulatory interactions between the somatic and germline lineages in the Drosophila testis.

JAK/STAT signaling in stem cells and regeneration: from Drosophila to vertebrates

The JAK/STAT pathway is a conserved metazoan signaling system that transduces cues from extracellular cytokines into transcriptional changes in the nucleus. JAK/STAT signaling is best known for its roles in immunity. However, recent work has demonstrated that it also regulates critical homeostatic processes in germline and somatic stem cells, as well as regenerative processes in several tissues, including the gonad, intestine and appendages. Here, we provide an overview of JAK/STAT signaling in stem cells and regeneration, focusing on Drosophila and highlighting JAK/STAT pathway functions in proliferation, survival and cell competition that are conserved between Drosophila and vertebrates.

Drosophila Jak/STAT Signaling: Regulation and Relevance in Human Cancer and Metastasis

Over the past three-decades, Janus kinase (Jak) and signal transducer and activator of transcription (STAT) signaling has emerged as a paradigm to understand the involvement of signal transduction in development and disease pathology. At the molecular level, cytokines and interleukins steer Jak/STAT signaling to transcriptional regulation of target genes, which are involved in cell differentiation, migration, and proliferation. Jak/STAT signaling is involved in various types of blood cell disorders and cancers in humans, and its activation is associated with carcinomas that are more invasive or likely to become metastatic. Despite immense information regarding Jak/STAT regulation, the signaling network has numerous missing links, which is slowing the progress towards developing drug therapies. In mammals, many components act in this cascade, with substantial cross-talk with other signaling pathways. In Drosophila, there are fewer pathway components, which has enabled significant discoveries regarding well-conserved regulatory mechanisms. Work across species illustrates the relevance of these regulators in humans. In this review, we showcase fundamental Jak/STAT regulation mechanisms in blood cells, stem cells, and cell motility. We examine the functional relevance of key conserved regulators from Drosophila to human cancer stem cells and metastasis. Finally, we spotlight less characterized regulators of Drosophila Jak/STAT signaling, which stand as promising candidates to be investigated in cancer biology. These comparisons illustrate the value of using Drosophila as a model for uncovering the roles of Jak/STAT signaling and the molecular means by which the pathway is controlled.

Protecting and Diversifying the Germline

Gametogenesis represents the most dramatic cellular differentiation pathways in both female and male flies. At the genome level, meiosis ensures that diploid germ cells become haploid gametes. At the epigenome level, extensive changes are required to turn on and shut off gene expression in a precise spatiotemporally controlled manner. Research applying conventional molecular genetics and cell biology, in combination with rapidly advancing genomic tools have helped us to investigate (1) how germ cells maintain lineage specificity throughout their adult reproductive lifetime; (2) what molecular mechanisms ensure proper oogenesis and spermatogenesis, as well as protect genome integrity of the germline; (3) how signaling pathways contribute to germline-soma communication; and (4) if such communication is important. In this chapter, we highlight recent discoveries that have improved our understanding of these questions. On the other hand, restarting a new life cycle upon fertilization is a unique challenge faced by gametes, raising questions that involve intergenerational and transgenerational epigenetic inheritance. Therefore, we also discuss new developments that link changes during gametogenesis to early embryonic development-a rapidly growing field that promises to bring more understanding to some fundamental questions regarding metazoan development.

Direct control of somatic stem cell proliferation factors by the Drosophila testis stem cell niche

Niches have traditionally been characterised as signalling microenvironments that allow stem cells to maintain their fate. This definition implicitly assumes that the various niche signals are integrated towards a binary fate decision between stemness and differentiation. However, observations in multiple systems have demonstrated that stem cell properties, such as proliferation and self-renewal, can be uncoupled at the level of niche signalling input, which is incompatible with this simplified view. We have studied the role of the transcriptional regulator Zfh1, a shared target of the Hedgehog and Jak/Stat niche signalling pathways, in the somatic stem cells of the Drosophila testis. We found that Zfh1 binds and downregulates salvador and kibra, two tumour suppressor genes of the Hippo/Wts/Yki pathway, thereby restricting Yki activation and proliferation to the Zfh1⁺ stem cells. These observations provide an unbroken link from niche signal input to an individual aspect of stem cell behaviour that does not, at any step, involve a fate decision. We discuss the relevance of these findings for an overall concept of stemness and niche function.

JAK/STAT3 regulated global gene expression dynamics during late-stage reprogramming process

BACKGROUND

The generation of induced pluripotent stem cells (iPSCs) has underdefined mechanisms. In addition, leukemia inhibitory factor (LIF) activated Janus kinase/signal transducer and activator of transcription 3 (JAK/STAT3) pathway is the master regulator for naïve-state pluripotency achievement and maintenance. However, the regulatory process to attain naïve pluripotent iPSCs is not well understood.

RESULTS

We performed transcriptome analysis to dissect the genomic expression during mouse iPSC induction, with or without blocking the JAK/STAT3 activity. We describe JAK/STAT3 signaling-specific biological events such as gametogenesis, meiotic/mitotic cell cycle, and DNA repair, and JAK/STAT3-dependent expression of key transcription factors such as the naïve pluripotency-specific genes, developmental pluripotency associated (Dppa) family, along with histone modifiers and non-coding RNAs in reprogramming. We discover that JAK/STAT3 activity does not affect early phase mesenchymal to epithelial transition (MET) but is necessary for proper imprinting of the Dlk1-Dio3 region, an essential event for pluripotency achievement at late-reprogramming stage. This correlates with the JAK/STAT3-dependent stimulation of Dppa3 and Polycomb repressive complex 2 (PRC2) genes. We further demonstrate that JAK/STAT3 activity is essential for DNA demethylation of pluripotent loci including Oct4, Nanog, and the Dlk1-Dio3 regions. These findings correlate well with the previously identified STAT3 direct targets. We further propose a model of pluripotency achievement regulated by JAK/STAT3 signaling during the reprogramming process.

CONCLUSIONS

Our study illustrates novel insights for JAK/STAT3 promoted pluripotency establishment, which are valuable for further improving the naïve-pluripotent iPSC generation across different species including humans.

Cell fate decisions: emerging roles for metabolic signals and cell morphology

Understanding how cell fate decisions are regulated is a fundamental goal of developmental and stem cell biology. Most studies on the control of cell fate decisions address the contributions of changes in transcriptional programming, epigenetic modifications, and biochemical differentiation cues. However, recent studies have found that other aspects of cell biology also make important contributions to regulating cell fate decisions. These cues can have a permissive or instructive role and are integrated into the larger network of signaling, functioning both upstream and downstream of developmental signaling pathways. Here, we summarize recent insights into how cell fate decisions are influenced by four aspects of cell biology: metabolism, reactive oxygen species (ROS), intracellular pH (pHi), and cell morphology. For each topic, we discuss how these cell biological cues interact with each other and with protein-based mechanisms for changing gene transcription. In addition, we highlight several questions that remain unanswered in these exciting and relatively new areas of the field.

DIP1 modulates stem cell homeostasis in Drosophila through regulation of sisR-1

Stable intronic sequence RNAs (sisRNAs) are by-products of splicing and regulate gene expression. How sisRNAs are regulated is unclear. Here we report that a double-stranded RNA binding protein, Disco-interacting protein 1 (DIP1) regulates sisRNAs in Drosophila. DIP1 negatively regulates the abundance of sisR-1 and INE-1 sisRNAs. Fine-tuning of sisR-1 by DIP1 is important to maintain female germline stem cell homeostasis by modulating germline stem cell differentiation and niche adhesion. Drosophila DIP1 localizes to a nuclear body (satellite body) and associates with the fourth chromosome, which contains a very high density of INE-1 transposable element sequences that are processed into sisRNAs. DIP1 presumably acts outside the satellite bodies to regulate sisR-1, which is not on the fourth chromosome. Thus, our study identifies DIP1 as a sisRNA regulatory protein that controls germline stem cell self-renewal in Drosophila.

Stable intronic sequence RNAs (sisRNAs) are by-products of splicing from introns with roles in embryonic development in Drosophila. Here, the authors show that the RNA binding protein DIP1 regulates sisRNAs in Drosophila, which is necessary for germline stem cell homeostasis.

Repression of Abd-B by Polycomb is critical for cell identity maintenance in adult Drosophila testis

Hox genes play a fundamental role in regulating animal development. However, less is known about their functions on homeostasis maintenance in adult stem cells. Here, we report that the repression of an important axial Hox gene, Abdominal-B (Abd-B), in cyst stem cells (CySCs) is essential for the homeostasis and cell identity maintenance in the adult Drosophila testis. Derepression of Abd-B in CySCs disrupts the proper self-renewal of both germline stem cells (GSCs) and CySCs, and leads to an excessive expansion of early stage somatic cells, which originate from both lineages. We further demonstrate that canonical Polycomb (Pc) and functional pathway of Polycomb group (PcG) proteins are responsible for maintaining the germline cell identity non-autonomously via repressing Abd-B in CySCs in the adult Drosophila testis.

Neutral Competition for Drosophila Follicle and Cyst Stem Cell Niches Requires Vesicle Trafficking Genes

The process of selecting for cellular fitness through competition plays a critical role in both development and disease. The germarium, a structure at the tip of the ovariole of a Drosophila ovary, contains two follicle stem cells (FSCs) that undergo neutral competition for the stem cell niche. Using the FSCs as a model, we performed a genetic screen through a collection of 126 mutants in essential genes on the X chromosome to identify candidates that increase or decrease competition for the FSC niche. We identified ∼55 and 6% of the mutations screened as putative FSC hypo- or hyper-competitors, respectively. We found that a large majority of mutations in vesicle trafficking genes (11 out of the 13 in the collection of mutants) are candidate hypo-competition alleles, and we confirmed the hypo-competition phenotype for four of these alleles. We also show that Sec16 and another COPII vesicle trafficking component, Sar1, are required for follicle cell differentiation. Lastly, we demonstrate that, although some components of vesicle trafficking are also required for neutral competition in the cyst stem cells of the testis, there are important tissue-specific differences. Our results demonstrate a critical role for vesicle trafficking in stem cell niche competition and differentiation, and we identify a number of putative candidates for further exploration.

Merlin is required for coordinating proliferation of two stem cell lineages in the Drosophila testis

Although the mechanisms that balance self-renewal and differentiation of a stem cell lineage have been extensively studied, it remains poorly understood how tissues that contain multiple stem cell lineages maintain balanced proliferation among distinct lineages: when stem cells of a particular lineage proliferate, how do the other lineages respond to maintain the correct ratio of cells among linages? Here, we show that Merlin (Mer), a homolog of the human tumor suppressor neurofibromatosis 2, is required to coordinate proliferation of germline stem cells (GSCs) and somatic cyst stem cells (CySCs) in the Drosophila testis. Mer mutant CySCs fail to coordinate their proliferation with that of GSCs in multiple settings, and can be triggered to undergo tumorous overproliferation. Mer executes its function by stabilizing adherens junctions. Given the known role of Mer in contact-dependent inhibition of proliferation, we propose that the proliferation of CySCs are regulated by crowdedness, or confluency, of cells in their lineage with respect to that of germline, thereby coordinating the proliferation of two lineages.