Regulation of adult stem cell behavior by nutrient signaling

Spermatogonial Dedifferentiation into Germline Stem Cells in Drosophila Testes

Stem cell pools are dynamic and capable of reacting to insults like injury and starvation. Recent work has highlighted the key role of dedifferentiation as a conserved mechanism for replenishing stem cell pools after their loss, thereby maintaining tissue homeostasis. The testis of the fruit fly Drosophila melanogaster offers a simple but powerful system to study dedifferentiation, the process by which differentiating spermatogonia can revert their fate to become fully functional germline stem cells (GSCs). Dedifferentiated GSCs show interesting characteristics, such as being more proliferative than their wild-type sibling GSCs. To facilitate the study of the cellular and molecular mechanisms underlying the process of germline dedifferentiation in the Drosophila testis, here we describe techniques for inducing high rates of dedifferentiation and for unambiguously labeling dedifferentiated GSCs.

Influence of Nutrition, Food Supplements and Lifestyle in Hair Disorders

Genetic, androgenic, epigenetic, environmental, lifestyle, diet, and nutritional factors have influence over promoting or regulating inflammation, immunity, and genetic expression predisposing to hair loss. Oxidative stress is a major mediator for various mechanisms of hair loss, including the release of transforming growth factor-β ( TGF-β). Nutrients counter oxidative stress, repair cellular damage, support cellular functions, and restore hair growth. Nutrients can be synergistic or antagonistic. Covert subclinical nutritional deficiencies are common. Higher dose of nutrients does not mean higher efficiency but may reverse the benefits converting an antioxidant to become prooxidant. Nutrients do not work alone but are supported by accessory micronutrients which ensure biological utilization. Providing proper nutritional environment can neutralize free radicals and perpetuate active hair growth cycles.

Cellular mechanisms underlying adult tissue plasticity in Drosophila

Adult tissues in Metazoa dynamically remodel their structures in response to environmental challenges including sudden injury, pathogen infection, and nutritional fluctuation, while maintaining quiescence under homoeostatic conditions. This characteristic, hereafter referred to as adult tissue plasticity, can prevent tissue dysfunction and improve the fitness of organisms in continuous and/or severe change of environments. With its relatively simple tissue structures and genetic tools, studies using the fruit fly Drosophila melanogaster have provided insights into molecular mechanisms that control cellular responses, particularly during regeneration and nutrient adaptation. In this review, we present the current understanding of cellular mechanisms, stem cell proliferation, polyploidization, and cell fate plasticity, all of which enable adult tissue plasticity in various Drosophila adult organs including the midgut, the brain, and the gonad, and discuss the organismal strategy in response to environmental changes and future directions of the research.

Investigating Adult Stem Cells Through Lineage analyses

A simple, universal and fundamental definition of adult stem cell communities is proposed. Key principles of cell lineage methods for defining adult stem cell numbers, locations and behaviors are critically evaluated, emphasizing the imperatives of capturing the full spectrum of individual stem cell behaviors, examining a variety of experimental time periods and avoiding unwarranted assumptions. The focus is first on defining fundamentals and then addresses stem cell heterogeneity, potential hierarchies and how individual cells serve the function of a stem cell community.

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.

Male reproductive aging arises via multifaceted mating-dependent sperm and seminal proteome declines, but is postponable in Drosophila

Declining ejaculate performance with male age is taxonomically widespread and has broad fitness consequences. Ejaculate success requires fully functional germline (sperm) and soma (seminal fluid) components. However, some aging theories predict that resources should be preferentially diverted to the germline at the expense of the soma, suggesting differential impacts of aging on sperm and seminal fluid and trade-offs between them or, more broadly, between reproduction and lifespan. While harmful effects of male age on sperm are well known, we do not know how much seminal fluid deteriorates in comparison. Moreover, given the predicted trade-offs, it remains unclear whether systemic lifespan-extending interventions could ameliorate the declining performance of the ejaculate as a whole. Here, we address these problems using Drosophila melanogaster. We demonstrate that seminal fluid deterioration contributes to male reproductive decline via mating-dependent mechanisms that include posttranslational modifications to seminal proteins and altered seminal proteome composition and transfer. Additionally, we find that sperm production declines chronologically with age, invariant to mating activity such that older multiply mated males become infertile principally via reduced sperm transfer and viability. Our data, therefore, support the idea that both germline and soma components of the ejaculate contribute to male reproductive aging but reveal a mismatch in their aging patterns. Our data do not generally support the idea that the germline is prioritized over soma, at least, within the ejaculate. Moreover, we find that lifespan-extending systemic down-regulation of insulin signaling results in improved late-life ejaculate performance, indicating simultaneous amelioration of both somatic and reproductive aging.

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.

A study of the transit amplification divisions during spermatogenesis in Oncopetus fasciatus to assess plasticity in sperm numbers or sperm viability under different diets

Oncopeltus fasciatus males fed the ancestral diet of milkweed seeds prioritize reproduction over lifespan as evidenced by higher rates of fertility and shorter lifespans than males from the same population fed the adapted diet of sunflower seeds. We examined the proximate mechanisms by which milkweed-fed males maintained late-life fertility. We tested the hypothesis that older milkweed-fed males maintained fertility by producing more, higher quality sperm. Our results, that older males have more sperm, but their sperm do not have higher viability, are in general agreement with other recent studies on how nutrition affects male fertility in insects. We further examined the mechanisms by which sperm are produced by examining the progression of spermatogonial cells through the cell cycle during the transit amplification divisions. We demonstrated that diet affects the likelihood of a spermatocyst being in the S-phase or M-phase of the cell cycle. Given work in model systems, these results have implications for subtle effects on sperm quality either through replication stress or epigenetic markers. Thus, viability may not be the best marker for sperm quality and more work is called for on the mechanisms by which the germline and the production of sperm mediate the cost of reproduction.

Spen limits intestinal stem cell self-renewal

Precise regulation of stem cell self-renewal and differentiation properties is essential for tissue homeostasis. Using the adult Drosophila intestine to study molecular mechanisms controlling stem cell properties, we identify the gene split-ends (spen) in a genetic screen as a novel regulator of intestinal stem cell fate (ISC). Spen family genes encode conserved RNA recognition motif-containing proteins that are reported to have roles in RNA splicing and transcriptional regulation. We demonstrate that spen acts at multiple points in the ISC lineage with an ISC-intrinsic function in controlling early commitment events of the stem cells and functions in terminally differentiated cells to further limit the proliferation of ISCs. Using two-color cell sorting of stem cells and their daughters, we characterize spen-dependent changes in RNA abundance and exon usage and find potential key regulators downstream of spen. Our work identifies spen as an important regulator of adult stem cells in the Drosophila intestine, provides new insight to Spen-family protein functions, and may also shed light on Spen's mode of action in other developmental contexts.

Pink1 and Parkin regulate Drosophila intestinal stem cell proliferation during stress and aging

Intestinal stem cells (ISCs) maintain the midgut epithelium in Drosophila melanogaster Proper cellular turnover and tissue function rely on tightly regulated rates of ISC division and appropriate differentiation of daughter cells. However, aging and epithelial injury cause elevated ISC proliferation and decreased capacity for terminal differentiation of daughter enteroblasts (EBs). The mechanisms causing functional decline of stem cells with age remain elusive; however, recent findings suggest that stem cell metabolism plays an important role in the regulation of stem cell activity. Here, we investigate how alterations in mitochondrial homeostasis modulate stem cell behavior in vivo via RNA interference-mediated knockdown of factors involved in mitochondrial dynamics. ISC/EB-specific knockdown of the mitophagy-related genes Pink1 or Parkin suppresses the age-related loss of tissue homeostasis, despite dramatic changes in mitochondrial ultrastructure and mitochondrial damage in ISCs/EBs. Maintenance of tissue homeostasis upon reduction of Pink1 or Parkin appears to result from reduction of age- and stress-induced ISC proliferation, in part, through induction of ISC senescence. Our results indicate an uncoupling of cellular, tissue, and organismal aging through inhibition of ISC proliferation and provide insight into strategies used by stem cells to maintain tissue homeostasis despite severe damage to organelles.

Effect of essential amino acids on enteroids: Methionine deprivation suppresses proliferation and affects differentiation in enteroid stem cells

We investigated the effects of essential amino acids on intestinal stem cell proliferation and differentiation using murine small intestinal organoids (enteroids) from the jejunum. By selectively removing individual essential amino acids from culture medium, we found that 24 h of methionine (Met) deprivation markedly suppressed cell proliferation in enteroids. This effect was rescued when enteroids cultured in Met deprivation media for 12 h were transferred to complete medium, suggesting that Met plays an important role in enteroid cell proliferation. In addition, mRNA levels of the stem cell marker leucine-rich repeat-containing G protein-coupled receptor 5 (Lgr5) decreased in enteroids grown in Met deprivation conditions. Consistent with this observation, Met deprivation also attenuated Lgr5-EGFP fluorescence intensity in enteroids. In contrast, Met deprivation enhanced mRNA levels of the enteroendocrine cell marker chromogranin A (ChgA) and markers of K cells, enterochromaffin cells, goblet cells, and Paneth cells. Immunofluorescence experiments demonstrated that Met deprivation led to an increase in the number of ChgA-positive cells. These results suggest that Met deprivation suppresses stem cell proliferation, thereby promoting differentiation. In conclusion, Met is an important nutrient in the maintenance of intestinal stem cells and Met deprivation potentially affects cell differentiation.

Amitosis of Polyploid Cells Regenerates Functional Stem Cells in the Drosophila Intestine

Organ fitness depends on appropriate maintenance of stem cell populations, and aberrations in functional stem cell numbers are associated with malignancies and aging. Symmetrical division is the best characterized mechanism of stem cell replacement, but other mechanisms could also be deployed, particularly in situations of high stress. Here, we show that after severe depletion, intestinal stem cells (ISCs) in the Drosophila midgut are replaced by spindle-independent ploidy reduction of cells in the enterocyte lineage through a process known as amitosis. Amitosis is also induced by the functional loss of ISCs coupled with tissue demand and in aging flies, underscoring the generality of this mechanism. However, we also found that random homologous chromosome segregation during ploidy reduction can expose deleterious mutations through loss of heterozygosity. Together, our results highlight amitosis as an unappreciated mechanism for restoring stem cell homeostasis, but one with some associated risk in animals carrying mutations.

Somatic stem cell differentiation is regulated by PI3K/Tor signaling in response to local cues

Stem cells reside in niches that provide signals to maintain self-renewal, and differentiation is viewed as a passive process that depends on loss of access to these signals. Here, we demonstrate that the differentiation of somatic cyst stem cells (CySCs) in the Drosophila testis is actively promoted by PI3K/Tor signaling, as CySCs lacking PI3K/Tor activity cannot differentiate properly. We find that an insulin peptide produced by somatic cells immediately outside of the stem cell niche acts locally to promote somatic differentiation through Insulin-like receptor (InR) activation. These results indicate that there is a local 'differentiation' niche that upregulates PI3K/Tor signaling in the early daughters of CySCs. Finally, we demonstrate that CySCs secrete the Dilp-binding protein ImpL2, the Drosophila homolog of IGFBP7, into the stem cell niche, which blocks InR activation in CySCs. Thus, we show that somatic cell differentiation is controlled by PI3K/Tor signaling downstream of InR and that the local production of positive and negative InR signals regulates the differentiation niche. These results support a model in which leaving the stem cell niche and initiating differentiation are actively induced by signaling.

Phosphoinositide signaling in sperm development

Phosphatidylinositol phosphates (PIPs)¹ are membrane lipids with crucial roles during cell morphogenesis, including the establishment of cytoskeletal organization, membrane trafficking, cell polarity, cell-cycle control and signaling. Recent studies in mice (Mus musculus), fruit flies (Drosophila melanogaster) and other organisms have defined germ cell intrinsic requirements for these lipids and their regulatory enzymes in multiple aspects of sperm development. In particular, PIP levels are crucial in germline stem cell maintenance, spermatogonial proliferation and survival, spermatocyte cytokinesis, spermatid polarization, sperm tail formation, nuclear shaping, and production of mature, motile sperm. Here, we briefly review the stages of spermatogenesis and discuss the roles of PIPs and their regulatory enzymes in male germ cell development.

Simultaneous control of stemness and differentiation by the transcription factor Escargot in adult stem cells: How can we tease them apart?

The homeostatic turnover of adult organs and their regenerative capacity following injury depend on a careful balance between stem cell self-renewal (to maintain or enlarge the stem cell pool) and differentiation (to replace lost tissue). We have recently characterized the role of the Drosophila Snail family transcription factor escargot (esg) in testis cyst stem cells (CySCs) (1,2) and intestinal stem cells (ISCs). (3,4) CySCs mutant for esg are not maintained as stem cells, but they remain capable of differentiating normally along the cyst cell lineage. In contrast, esg mutant CySCs that give rise to a closely related lineage, the apical hub cells, cannot maintain hub cell identity. Similarly, Esg maintains stemness of ISCs while regulating the terminal differentiation of progenitor cells into absorptive enterocytes or secretory enteroendocrine cells. Therefore, our findings suggest that Esg may play a conserved and pivotal regulatory role in adult stem cells, controlling both their maintenance and terminal differentiation. Here we propose that this dual regulatory role is due to simultaneous control by Esg of overlapping genetic programs and discuss the exciting challenges and opportunities that lie ahead to explore the underlying mechanisms experimentally.

The regulated elimination of transit-amplifying cells preserves tissue homeostasis during protein starvation in Drosophila testis

How tissues adapt to varying nutrient conditions is of fundamental importance for robust tissue homeostasis throughout the life of an organism, but the underlying mechanisms are poorly understood. Here, we show that Drosophila testis responds to protein starvation by eliminating transit-amplifying spermatogonia (SG) while maintaining a reduced pool of actively proliferating germline stem cells (GSCs). During protein starvation, SG die in a manner that is mediated by the apoptosis of somatic cyst cells (CCs) that encapsulate SG and regulate their development. Strikingly, GSCs cannot be maintained during protein starvation when CC-mediated SG death is inhibited, leading to an irreversible collapse of tissue homeostasis. We propose that the regulated elimination of transit-amplifying cells is essential to preserve stem cell function and tissue homeostasis during protein starvation.

Stem cell autotomy and niche interaction in different systems

The best known cases of cell autotomy are the formation of erythrocytes and thrombocytes (platelets) from progenitor cells that reside in special niches. Recently, autotomy of stem cells and its enigmatic interaction with the niche has been reported from male germline stem cells (GSCs) in several insect species. First described in lepidopterans, the silkmoth, followed by the gipsy moth and consecutively in hemipterans, foremost the milkweed bug. In both, moths and the milkweed bug, GSCs form finger-like projections toward the niche, the apical cells (homologs of the hub cells in Drosophila). Whereas in the milkweed bug the projection terminals remain at the surface of the niche cells, in the gipsy moth they protrude deeply into the singular niche cell. In both cases, the projections undergo serial retrograde fragmentation with progressing signs of autophagy. In the gipsy moth, the autotomized vesicles are phagocytized and digested by the niche cell. In the milkweed bug the autotomized vesicles accumulate at the niche surface and disintegrate. Autotomy and sprouting of new projections appears to occur continuously. The significance of the GSC-niche interactions, however, remains enigmatic. Our concept on the signaling relationship between stem cell-niche in general and GSC and niche (hub cells and cyst stem cells) in particular has been greatly shaped by Drosophila melanogaster. In comparing the interactions of GSCs with their niche in Drosophila with those in species exhibiting GSC autotomy it is obvious that additional or alternative modes of stem cell-niche communication exist. Thus, essential signaling pathways, including niche-stem cell adhesion (E-cadherin) and the direction of asymmetrical GSC division - as they were found in Drosophila - can hardly be translated into the systems where GSC autotomy was reported. It is shown here that the serial autotomy of GSC projections shows remarkable similarities with Wallerian axonal destruction, developmental axon pruning and dying-back degeneration in neurodegenerative diseases. Especially the hypothesis of an existing evolutionary conserved “autodestruction program” in axons that might also be active in GSC projections appears attractive. Investigations on the underlying signaling pathways have to be carried out. There are two other well known cases of programmed cell autotomy: the enucleation of erythroblasts in the process of erythrocyte maturation and the segregation of thousands of thrombocytes (platelets) from one megakaryocyte. Both progenitor cell types - erythroblasts and megakaryocytes - are associated with a niche in the bone marrow, erythroblasts with a macrophage, which they surround, and the megakaryocytes with the endothelial cells of sinusoids and their extracellular matrix. Although the regulatory mechanisms may be specific in each case, there is one aspect that connects all described processes of programmed cell autotomy and neuronal autodestruction: apoptotic pathways play always a prominent role. Studies on the role of male GSC autotomy in stem cell-niche interaction have just started but are expected to reveal hitherto unknown ways of signal exchange. Spermatogenesis in mammals advance our understanding of insect spermatogenesis. Mammal and insect spermatogenesis share some broad principles, but a comparison of the signaling pathways is difficult. We have intimate knowledge from Drosophila, but of almost no other insect, and we have only limited knowledge from mammals. The discovery of stem cell autotomy as part of the interaction with the niche promises new general insights into the complicated stem cell-niche interdependence.

Transgenerational inheritance of diet-induced genome rearrangements in Drosophila

Ribosomal RNA gene (rDNA) copy number variation modulates heterochromatin formation and influences the expression of a large fraction of the Drosophila genome. This discovery, along with the link between rDNA, aging, and disease, high-lights the importance of understanding how natural rDNA copy number variation arises. Pursuing the relationship between rDNA expression and stability, we have discovered that increased dietary yeast concentration, emulating periods of dietary excess during life, results in somatic rDNA instability and copy number reduction. Modulation of Insulin/TOR signaling produces similar results, indicating a role for known nutrient sensing signaling pathways in this process. Furthermore, adults fed elevated dietary yeast concentrations produce offspring with fewer rDNA copies demonstrating that these effects also occur in the germline, and are transgenerationally heritable. This finding explains one source of natural rDNA copy number variation revealing a clear long-term consequence of diet.

A somatic permeability barrier around the germline is essential for Drosophila spermatogenesis

Interactions between the soma and germline are essential for gametogenesis. In the Drosophila testis, differentiating germ cells are encapsulated by two somatic cells that surround the germline throughout spermatogenesis. chickadee (chic), the fly ortholog of Profilin, mediates soma-germline interactions. Knockdown of Chic in the soma results in sterility and severely disrupted spermatogenesis due to defective encapsulation. To study this defect further, we developed a permeability assay to analyze whether the germline is isolated from the surrounding environment by the soma. We find that germline encapsulation by the soma is, by itself, insufficient for the formation of a permeability barrier, but that such a barrier gradually develops during early spermatogenesis. Thus, germline stem cells, gonialblasts and early spermatogonia are not isolated from the outside environment. By late spermatocyte stages, however, a permeability barrier is formed by the soma. Furthermore, we find that, concomitant with formation of the permeability barrier, septate junction markers are expressed in the soma and localize to junctional sites connecting the two somatic cells that surround the germline. Importantly, knockdown of septate junction components also disrupts the permeability barrier. Finally, we show that germline differentiation is delayed when the permeability barrier is compromised. We propose that the permeability barrier around the germline serves an important regulatory function during spermatogenesis by shaping the signaling events that take place between the soma and the germline.

The Drosophila cyst stem cell lineage: Partners behind the scenes?

In all animals, germline cells differentiate in intimate contact with somatic cells and interactions between germline and soma are particularly important for germline development and function. In the male gonad of Drosophila melanogaster, the developing germline cells are enclosed by somatic cyst cells. The cyst cells are derived from cyst stem cells (CySCs) of somatic origin and codifferentiate with the germline cells. The fast generation cycle and the genetic tractability of Drosophila has made the Drosophila testis an excellent model for studying both the roles of somatic cells in guiding germline development and the interdependence of two separate stem cell lineages. This review focuses on our current understanding of CySC specification, CySC self-renewing divisions, cyst cell differentiation, and soma-germline interactions. Many of the mechanisms guiding these processes in Drosophila testes are similarly essential for the development and function of tissues in other organisms, most importantly for gametogenesis in mammals.

Steroid signaling promotes stem cell maintenance in the Drosophila testis

Stem cell regulation by local signals is intensely studied, but less is known about the effects of hormonal signals on stem cells. In Drosophila, the primary steroid twenty-hydroxyecdysone (20E) regulates ovarian germline stem cells (GSCs) but was considered dispensable for testis GSC maintenance. Male GSCs reside in a microenvironment (niche) generated by somatic hub cells and adjacent cyst stem cells (CySCs). Here, we show that depletion of 20E from adult males by overexpressing a dominant negative form of the Ecdysone receptor (EcR) or its heterodimeric partner ultraspiracle (usp) causes GSC and CySC loss that is rescued by 20E feeding, uncovering a requirement for 20E in stem cell maintenance. EcR and USP are expressed, activated and autonomously required in the CySC lineage to promote CySC maintenance, as are downstream genes ftz-f1 and E75. In contrast, GSCs non-autonomously require ecdysone signaling. Global inactivation of EcR increases cell death in the testis that is rescued by expression of EcR-B2 in the CySC lineage, indicating that ecdysone signaling supports stem cell viability primarily through a specific receptor isoform. Finally, EcR genetically interacts with the NURF chromatin-remodeling complex, which we previously showed maintains CySCs. Thus, although 20E levels are lower in males than females, ecdysone signaling acts through distinct cell types and effectors to ensure both ovarian and testis stem cell maintenance.

Nutritional regulation of stem and progenitor cells in Drosophila

Stem cells and their progenitors are maintained within a microenvironment, termed the niche, through local cell-cell communication. Systemic signals originating outside the niche also affect stem cell and progenitor behavior. This review summarizes studies that pertain to nutritional effects on stem and progenitor cell maintenance and proliferation in Drosophila. Multiple tissue types are discussed that utilize the insulin-related signaling pathway to convey nutritional information either directly to these progenitors or via other cell types within the niche. The concept of systemic control of these cell types is not limited to Drosophila and may be functional in vertebrate systems, including mammals.

Caloric restriction and aging stem cells: the stick and the carrot?

Adult tissue stem cells have the ability to adjust to environmental changes and affect also the proliferation of neighboring cells, with important consequences on tissue maintenance and regeneration. Stem cell renewal and proliferation is strongly regulated during aging of the organism. Caloric restriction is the most powerful anti-aging strategy conserved throughout evolution in the animal kingdom. Recent studies relate the properties of caloric restriction to its ability in reprogramming stem-like cell states and in prolonging the capacity of stem cells to self-renew, proliferate, differentiate, and replace cells in several adult tissues. However this general paradigm presents with exceptions. The scope of this review is to highlight how caloric restriction impacts on diverse stem cell compartments and, by doing so, might differentially delay aging in the tissues of lower and higher organisms.

Eat to reproduce: a key role for the insulin signaling pathway in adult insects

Insects, like all heterotrophic organisms, acquire from their food the nutrients that are essential for anabolic processes that lead to growth (larval stages) or reproduction (adult stage). In adult females, this nutritional input is processed and results in a very specific output, i.e., the production of fully developed eggs ready for fertilization and deposition. An important role in this input-output transition is attributed to the insulin signaling pathway (ISP). The ISP is considered to act as a sensor of the organism's nutritional status and to stimulate the progression of anabolic events when the status is positive. In several insect species belonging to different orders, the ISP has been demonstrated to positively control vitellogenesis and oocyte growth. Whether or not ISP acts herein via a mediator action of lipophilic insect hormones (ecdysteroids and juvenile hormone) remains debatable and might be differently controlled in different insect orders. Most likely, insulin-related peptides, ecdysteroids and juvenile hormone are involved in a complex regulatory network, in which they mutually influence each other and in which the insect's nutritional status is a crucial determinant of the network's output. The current review will present an overview of the regulatory role of the ISP in female insect reproduction and its interaction with other pathways involving nutrients, lipophilic hormones and neuropeptides.

Mutations in the IMD pathway and mustard counter Vibrio cholerae suppression of intestinal stem cell division in Drosophila

Vibrio cholerae is an estuarine bacterium and an intestinal pathogen of humans that causes severe epidemic diarrhea. In the absence of adequate mammalian models in which to study the interaction of V. cholerae with the host intestinal innate immune system, we have implemented Drosophila melanogaster as a surrogate host. We previously showed that immune deficiency pathway loss-of-function and mustard gain-of-function mutants are less susceptible to V. cholerae infection. We find that although the overall burden of intestinal bacteria is not significantly different from that of control flies, intestinal stem cell (ISC) division is increased in these mutants. This led us to examine the effect of V. cholerae on ISC division. We report that V. cholerae infection and cholera toxin decrease ISC division. Because IMD pathway and Mustard mutants, which are resistant to V. cholerae, maintain higher levels of ISC division during V. cholerae infection, we hypothesize that suppression of ISC division is a virulence strategy of V. cholerae and that accelerated epithelial regeneration protects the host against V. cholerae. Extension of these findings to mammals awaits the development of an adequate experimental model.

[The Drosophila midgut as a model to study adult stem cells]

Constant renewal of cells occurs in most tissues throughout the adult lifetime and is insured by the activity of resident stem cells. Recent work has demonstrated the presence of adult stem cells in the Drosophila intestine and consequently, the Drosophila intestine has become a powerful model to understand adult stem cells in vivo. In this review, we summarize our current understanding of the mechanisms controlling cell fate decisions of the intestinal stem cells with a particular focus on the role of the Notch pathway in this process. We also summarize what is known about proliferation control of the intestinal stem cells, which is crucial to maintain tissue homeostasis during normal and environmentally stressful conditions.

Recent advances in Drosophila male germline stem cell biology

The ability of stem cells to divide asymmetrically to produce both self-renewing and differentiating daughter cells sustains many adult tissues, but germline stem cells (GSCs) are unique among stem cells as they perpetuate the genome of the species. The cellular and molecular mechanisms regulating most mammalian stem cells in their endogenous local microenvironments, or niches, are quite challenging to study. However, studies of stem cell niches such as those found in the Drosophila gonads have proven very useful. In these tissues, GSCs are housed in a readily identifiable niche, and the ability to genetically manipulate these cells and their neighbors has uncovered several fundamental mechanisms that are relevant to stem cells more generally. Here, we summarize recent work on the regulation of GSCs in the Drosophila testis niche by intercellular signals, and on the intracellular mechanisms that cooperate with these signals to ensure the survival of the germline. This review focuses on GSCs within the adult Drosophila testis; somatic stem cells in this tissue are reviewed by Zoller and Schulz in this issue.(1) For a review of the testis niche as a whole, see de Cuevas and Matunis,(2) and for more comprehensive reviews of the Drosophila testis, refer to Fuller(3) and Davies and Fuller.(4).

Drosophila midgut homeostasis involves neutral competition between symmetrically dividing intestinal stem cells

The Drosophila adult posterior midgut has been identified as a powerful system in which to study mechanisms that control intestinal maintenance, in normal conditions as well as during injury or infection. Early work on this system has established a model of tissue turnover based on the asymmetric division of intestinal stem cells. From the quantitative analysis of clonal fate data, we show that tissue turnover involves the neutral competition of symmetrically dividing stem cells. This competition leads to stem-cell loss and replacement, resulting in neutral drift dynamics of the clonal population. As well as providing new insight into the mechanisms regulating tissue self-renewal, these findings establish intriguing parallels with the mammalian system, and confirm Drosophila as a useful model for studying adult intestinal maintenance.

Control of adult stem cells in vivo by a dynamic physiological environment: diet-dependent systemic factors in Drosophila and beyond

Adult stem cells are inextricably linked to whole-body physiology and nutrient availability through complex systemic signaling networks. A full understanding of how stem cells sense and respond to dietary fluctuations will require identifying key systemic mediators, as well as elucidating how they are regulated and integrated with local and intrinsic factors across multiple tissues. Studies focused on the Drosophila germline have generated valuable insights into how stem cells are controlled by diet-dependent pathways, and increasing evidence suggests that diverse adult stem cell populations respond to nutrients through similar mechanisms. Systemic signals, including nutrients themselves and diet-regulated hormones such as Insulin/Insulin-like growth factor or steroid hormones, can directly or indirectly affect stem cell behavior by modifying local cell-cell communication or intrinsic factors. The physiological regulation of stem cells in response to nutritional status not only is a fascinating biological problem, but also has clinical implications, as research in this field holds the key to noninvasive approaches for manipulating stem cells in vivo. In addition, given the known associations between diet, stem cells, and cancer risk, this research may inspire novel anticancer therapies.

Efficiency of spermatogonial dedifferentiation during aging

Background

Adult stem cells are critical for tissue homeostasis; therefore, the mechanisms utilized to maintain an adequate stem cell pool are important for the survival of an individual. In Drosophila, one mechanism utilized to replace lost germline stem cells (GSCs) is dedifferentiation of early progenitor cells. However, the average number of male GSCs decreases with age, suggesting that stem cell replacement may become compromised in older flies.

Methodology/Principal Findings

Using a temperature sensitive allelic combination of Stat92E to control dedifferentiation, we found that germline dedifferentiation is remarkably efficient in older males; somatic cells are also effectively replaced. Surprisingly, although the number of somatic cyst cells also declines with age, the proliferation rate of early somatic cells, including cyst stem cells (CySCs) increases.

Conclusions

These data indicate that defects in spermatogonial dedifferentiation are not likely to contribute significantly to an aging-related decline in GSCs. In addition, our findings highlight differences in the ways GSCs and CySCs age. Strategies to initiate or enhance the ability of endogenous, differentiating progenitor cells to replace lost stem cells could provide a powerful and novel strategy for maintaining tissue homeostasis and an alternative to tissue replacement therapy in older individuals.