Tiny Drosophila intestinal stem cells, big power

The signaling pathways are highly conserved between Drosophila and mammals concerning intestinal development, regeneration, and disease. The powerful genetic tools of Drosophila make it a valuable and convenient alternative to answer basic biological questions that can not be addressed using mammalian models. In this review, we discuss recent advances in how we use fly midgut to answer the following key questions: (1) How intestine stem cell niches are established; (2) which factors control asymmetric division of stem cells; (3) how intestinal cells interact with environmental factors, such as tissue damage, microbiota, and diet; (4) how to screen aging/cancer-related factors or drugs by fly intestine stem cells.

mRNA Translation Is Dynamically Regulated to Instruct Stem Cell Fate

Stem cells preserve tissue homeostasis by replacing the cells lost through damage or natural turnover. Thus, stem cells and their daughters can adopt two identities, characterized by different programs of gene expression and metabolic activity. The composition and regulation of these programs have been extensively studied, particularly by identifying transcription factor networks that define cellular identity and the epigenetic changes that underlie the progressive restriction in gene expression potential. However, there is increasing evidence that post-transcriptional mechanisms influence gene expression in stem cells and their progeny, in particular through the control of mRNA translation. Here, we review the described roles of translational regulation in controlling all aspects of stem cell biology, from the decision to enter or exit quiescence to maintaining self-renewal and promoting differentiation. We focus on mechanisms controlling global translation rates in cells, mTOR signaling, eIF2ɑ phosphorylation, and ribosome biogenesis and how they allow stem cells to rapidly change their gene expression in response to tissue needs or environmental changes. These studies emphasize that translation acts as an additional layer of control in regulating gene expression in stem cells and that understanding this regulation is critical to gaining a full understanding of the mechanisms that underlie fate decisions in stem cells.

Roles of Src family kinase, Ras, and mTOR signaling in intestinal epithelial homeostasis and tumorigenesis

The turnover of intestinal epithelial cells (IECs) is relatively rapid (3-5 days in mouse and human), and this short existence and other aspects of the homeostasis of IECs are tightly regulated by various signaling pathways including Wnt-β-catenin signaling. Dysregulation of IEC homeostasis likely contributes to the development of intestinal inflammation and intestinal cancer. The roles of receptor protein tyrosine kinases and their downstream signaling molecules such as Src family kinases, Ras, and mTOR in homeostatic regulation of IEC turnover have recently been evaluated. These signaling pathways have been found to promote not only the proliferation of IECs but also the differentiation of progenitor cells into secretory cell types such as goblet cells. Of note, signaling by Src family kinases, Ras, and mTOR has been shown to oppose the Wnt-β-catenin signaling pathway and thereby to limit the number of Lgr5⁺ intestinal stem cells or of Paneth cells. Such cross-talk of signaling pathways is important not only for proper regulation of IEC homeostasis but for the development of intestinal tumors and potentially for anticancer therapy.

Myogenin is an essential regulator of adult myofibre growth and muscle stem cell homeostasis

Growth and maintenance of skeletal muscle fibres depend on coordinated activation and return to quiescence of resident muscle stem cells (MuSCs). The transcription factor Myogenin (Myog) regulates myocyte fusion during development, but its role in adult myogenesis remains unclear. In contrast to mice, myog-/-zebrafish are viable, but have hypotrophic muscles. By isolating adult myofibres with associated MuSCs, we found that myog-/- myofibres have severely reduced nuclear number, but increased myonuclear domain size. Expression of fusogenic genes is decreased, Pax7 upregulated, MuSCs are fivefold more numerous and mis-positioned throughout the length of myog-/-myofibres instead of localising at myofibre ends as in wild-type. Loss of Myog dysregulates mTORC1 signalling, resulting in an 'alerted' state of MuSCs, which display precocious activation and faster cell cycle entry ex vivo, concomitant with myod upregulation. Thus, beyond controlling myocyte fusion, Myog influences the MuSC:niche relationship, demonstrating a multi-level contribution to muscle homeostasis throughout life.

Regulation of colonic epithelial cell homeostasis by mTORC1

Cell signaling important for homeostatic regulation of colonic epithelial cells (CECs) remains poorly understood. Mammalian target of rapamycin complex 1 (mTORC1), a protein complex that contains the serine-threonine kinase mTOR, mediates signaling that underlies the control of cellular functions such as proliferation and autophagy by various external stimuli. We here show that ablation of tuberous sclerosis complex 2 (Tsc2), a negative regulator of mTORC1, specifically in intestinal epithelial cells of mice resulted in increased activity of mTORC1 of, as well as increased proliferative activity of, CECs. Such Tsc2 ablation also reduced the population of Lgr5-positive colonic stem cells and the expression of Wnt target genes in CECs. The stimulatory phosphorylation of the kinase Akt and inhibitory phosphorylation of glycogen synthase kinase 3β were both markedly decreased in the colon of the Tsc2 conditional knockout (CKO) mice. Development of colonic organoids with cryptlike structures was enhanced for Tsc2 CKO mice compared with control mice. Finally, Tsc2 CKO mice manifested increased susceptibility to dextran sulfate sodium-induced colitis. Our results thus suggest that mTORC1 activity promotes the proliferation of, as well as the expression of Wnt target genes in, CECs and thereby contributes to colonic organogenesis and homeostasis.

The TORC1 inhibitor Nprl2 protects age-related digestive function in Drosophila

Aging and age-related diseases occur in almost all organisms. Recently, it was discovered that the inhibition of target of rapamycin complex 1 (TORC1), a conserved complex that mediates nutrient status and cell metabolism, can extend an individual’s lifespan and inhibit age-related diseases in many model organisms. However, the mechanism whereby TORC1 affects aging remains elusive. Here, we use a loss-of-function mutation in nprl2, a component of GATOR1 that mediates amino acid levels and inhibits TORC1 activity, to investigate the effect of increased TORC1 activity on the occurrence of age-related digestive dysfunction in Drosophila. We found that the nprl2 mutation decreased Drosophila lifespan. Furthermore, the nprl2 mutant had a distended crop, with food accumulation at an early age. Interestingly, the inappropriate food distribution and digestion along with decreased crop contraction in nprl2 mutant can be rescued by decreasing TORC1 activity. In addition, nprl2-mutant flies exhibited age-related phenotypes in the midgut, including short gut length, a high rate of intestinal stem cell proliferation, and metabolic dysfunction, which could be rescued by inhibiting TORC1 activity. Our findings showed that the gastrointestinal tract aging process is accelerated in nprl2-mutant flies, owing to high TORC1 activity, which suggested that TORC1 promotes digestive tract senescence.

Drosophila Gut-A Nexus Between Dietary Restriction and Lifespan

Aging is often defined as the accumulation of damage at the molecular and cellular levels which, over time, results in marked physiological impairments throughout the organism. Dietary restriction (DR) has been recognized as one of the strongest lifespan extending therapies observed in a wide array of organisms. Recent studies aimed at elucidating how DR promotes healthy aging have demonstrated a vital role of the digestive tract in mediating the beneficial effects of DR. Here, we review how dietary restriction influences gut metabolic homeostasis and immune function. Our discussion is focused on studies of the Drosophila digestive tract, where we describe in detail the potential mechanisms in which DR enhances maintenance of the intestinal epithelial barrier, up-regulates lipid metabolic processes, and improves the ability of the gut to deal with damage or stress. We also examine evidence of a tissue-tissue crosstalk between gut and neighboring organs including brain and fat body. Taken together, we argue that the Drosophila gut plays a critical role in DR-mediated lifespan extension.

Anatomy and Physiology of the Digestive Tract of Drosophila melanogaster

The gastrointestinal tract has recently come to the forefront of multiple research fields. It is now recognized as a major source of signals modulating food intake, insulin secretion and energy balance. It is also a key player in immunity and, through its interaction with microbiota, can shape our physiology and behavior in complex and sometimes unexpected ways. The insect intestine had remained, by comparison, relatively unexplored until the identification of adult somatic stem cells in the Drosophila intestine over a decade ago. Since then, a growing scientific community has exploited the genetic amenability of this insect organ in powerful and creative ways. By doing so, we have shed light on a broad range of biological questions revolving around stem cells and their niches, interorgan signaling and immunity. Despite their relatively recent discovery, some of the mechanisms active in the intestine of flies have already been shown to be more widely applicable to other gastrointestinal systems, and may therefore become relevant in the context of human pathologies such as gastrointestinal cancers, aging, or obesity. This review summarizes our current knowledge of both the formation and function of the Drosophila melanogaster digestive tract, with a major focus on its main digestive/absorptive portion: the strikingly adaptable adult midgut.

Stem Cell Intrinsic Hexosamine Metabolism Regulates Intestinal Adaptation to Nutrient Content

The intestine is an organ with an exceptionally high rate of cell turnover, and perturbations in this process can lead to severe diseases such as cancer or intestinal atrophy. Nutrition has a profound impact on intestinal volume and cellular architecture. However, how intestinal homeostasis is maintained in fluctuating dietary conditions remains insufficiently understood. By utilizing the Drosophila midgut model, we reveal a novel stem cell intrinsic mechanism coupling cellular metabolism with stem cell extrinsic growth signal. Our results show that intestinal stem cells (ISCs) employ the hexosamine biosynthesis pathway (HBP) to monitor nutritional status. Elevated activity of HBP promotes Warburg effect-like metabolic reprogramming required for adjusting the ISC division rate according to nutrient content. Furthermore, HBP activity is an essential facilitator for insulin signaling-induced ISC proliferation. In conclusion, ISC intrinsic hexosamine synthesis regulates metabolic pathway activities and defines the stem cell responsiveness to niche-derived growth signals.

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HBP is a mediator of Drosophila midgut adaptation to nutrient content

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ISC intrinsic HBP is a necessary and sufficient driver of stem cell divisions

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HBP activity regulates a Warburg-like metabolic reprogramming of the intestine

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HBP activity determines the output of InR signaling of the ISCs

mTORC1 Activation during Repeated Regeneration Impairs Somatic Stem Cell Maintenance

The balance between self-renewal and differentiation ensures long-term maintenance of stem cell (SC) pools in regenerating epithelial tissues. This balance is challenged during periods of high regenerative pressure and is often compromised in aged animals. Here, we show that target of rapamycin (TOR) signaling is a key regulator of SC loss during repeated regenerative episodes. In response to regenerative stimuli, SCs in the intestinal epithelium of the fly and in the tracheal epithelium of mice exhibit transient activation of TOR signaling. Although this activation is required for SCs to rapidly proliferate in response to damage, repeated rounds of damage lead to SC loss. Consistently, age-related SC loss in the mouse trachea and in muscle can be prevented by pharmacologic or genetic inhibition, respectively, of mammalian target of rapamycin complex 1 (mTORC1) signaling. These findings highlight an evolutionarily conserved role of TOR signaling in SC function and identify repeated rounds of mTORC1 activation as a driver of age-related SC decline.

Nutritional Control of Stem Cell Division through S-Adenosylmethionine in Drosophila Intestine

The intestine has direct contact with nutritional information. The mechanisms by which particular dietary molecules affect intestinal homeostasis are not fully understood. In this study, we identified S-adenosylmethionine (SAM), a universal methyl donor synthesized from dietary methionine, as a critical molecule that regulates stem cell division in Drosophila midgut. Depletion of either dietary methionine or SAM synthesis reduces division rate of intestinal stem cells. Genetic screening for putative SAM-dependent methyltransferases has identified protein synthesis as a regulator of the stem cells, partially through a unique diphthamide modification on eukaryotic elongation factor 2. In contrast, SAM in nutrient-absorptive enterocytes controls the interleukin-6-like protein Unpaired 3, which is required for rapid division of the stem cells after refeeding. Our study sheds light upon a link between diet and intestinal homeostasis and highlights the key metabolite SAM as a mediator of cell-type-specific starvation response.

Longevity and stress resistance are affected by activation of TOR/Myc in progenitor cells of Drosophila gut

Diverse physiological pathways have been shown to regulate longevity, stress resistance, fecundity and feeding rates, and metabolism in Drosophila. Here we tesed physiological traits in flies with Rheb and Myc- Rheb overexpressed in gut progenitor cells, known as enteroblasts (EBs). We found that activation of TOR signaling by overexpression of Rheb in EBs decreases survival and stress resistance. Additionall, we showed that Myc co-expression in EBs reduces fly fecundity and feeding rate. Rheb overexpression enhanced the level of whole body glucose. Higher relative expression of the metabolic genes dilps, akh, tobi and pepck was, however, observed. The role of TOR/Myc in the regulation of genes involved in lipid metabolism and protein synthesis was established. We showed a significant role of TOR/Myc in EBs in the regulation of the JAK/STAT, EGFR and insulin signaling pathways in Drosophila gut. These results highlight the importance of the balance between all different types of cells and confirm previous studies demonstrating that promotion of homeostasis in the intestine of Drosophila may function as a mechanism for the extension of organismal lifespan. Overall, the results demonstrate a role of TOR signaling and its downstream target Myc in EB cells in the regulation of Drosophila physiological processes.

A Phyllopod-Mediated Feedback Loop Promotes Intestinal Stem Cell Enteroendocrine Commitment in Drosophila

The intestinal epithelium in the Drosophila midgut is maintained by intestinal stem cells (ISCs), which are capable of generating both enterocytes and enteroendocrine cells (EEs) via alternative cell fate specification. Activation of Delta-Notch signaling directs ISCs for enterocyte generation, but how EEs are generated from ISCs remains poorly understood. Here, we identified Phyllopod (Phyl) as a key regulator that drives EE generation from ISCs. Phyl, which is normally suppressed by Notch, functions as an adaptor protein that bridges Tramtrack 69 (Ttk69) and E3 ubiquitin ligase Sina for degradation. Degradation of Ttk69 allows the activation of the Achaete-Scute Complex (AS-C)-Pros regulatory axis, which promotes EE specification. Interestingly, expression of AS-C genes in turn further induces Phyl expression, thereby establishing a positive feedback loop for continuous EE fate specification and commitment. This positive feedback circuit-driven regulatory mechanism could represent a common strategy for reliable and irreversible cell fate determination from progenitor cells.

Tissue-resident stem cell activity: a view from the adult Drosophila gastrointestinal tract

The gastrointestinal tract serves as a fast-renewing model for unraveling the multifaceted molecular mechanisms underlying remarkably rapid cell renewal, which is exclusively fueled by a small number of long-lived stem cells and their progeny. Stem cell activity is the best-characterized aspect of mucosal homeostasis in mitotically active tissues, and the dysregulation of regenerative capacity is a hallmark of epithelial immune defects. This dysregulation is frequently associated with pathologies ranging from chronic enteritis to malignancies in humans. Application of the adult Drosophila gastrointestinal tract model in current and future studies to analyze the immuno-physiological aspects of epithelial defense strategies, including stem cell behavior and re-epithelialization, will be necessary to improve our general understanding of stem cell participation in epithelial turnover. In this review, which describes exciting observations obtained from the adult Drosophila gastrointestinal tract, we summarize a remarkable series of recent findings in the literature to decipher the molecular mechanisms through which stem cells respond to nonsterile environments.

EGFR-dependent TOR-independent endocycles support Drosophila gut epithelial regeneration

Following gut epithelial damage, epidermal growth factor receptor/mitogen-activated protein kinase (EGFR/MAPK) signalling triggers Drosophila intestinal stem cells to produce enteroblasts (EBs) and enterocytes (ECs) that regenerate the gut. As EBs differentiate into ECs, they become postmitotic, but undergo extensive growth and DNA endoreplication. Here we report that EGFR/RAS/MAPK signalling is required and sufficient to drive damage-induced EB/EC growth. Endoreplication occurs exclusively in EBs and newborn ECs that inherit EGFR and active MAPK from fast-dividing progenitors. Mature ECs lack EGF receptors and are refractory to growth signalling. Genetic tests indicated that stress-dependent EGFR/MAPK promotes gut regeneration via a novel mechanism that operates independently of Insulin/Pi3K/TOR signalling, which is nevertheless required in nonstressed conditions. The E2f1 transcription factor is required for and sufficient to drive EC endoreplication, and Ras/Raf signalling upregulates E2f1 levels posttranscriptionally. We illustrate how distinct signalling mechanisms direct stress-dependent versus homeostatic regeneration, and highlight the importance of postmitotic cell growth in gut epithelial repair.

In response to gut epithelial damage, Drosophila stem cells proliferate to produce large polyploid enterocytes (EC), which comprise the bulk of the epithelium. Here, the authors show that stress-dependent EGFR/MAP kinase signalling drives both endoreplication and cell growth in newborn ECs.

Rapamycin preserves gut homeostasis during Drosophila aging

Gut homeostasis plays an important role in maintaining the overall body health during aging. Rapamycin, a specific inhibitor of mTOR, exerts prolongevity effects in evolutionarily diverse species. However, its impact on the intestinal homeostasis remains poorly understood. Here, we demonstrate that rapamycin can slow down the proliferation rate of intestinal stem cells (ISCs) in the aging guts and induce autophagy in the intestinal epithelium in Drosophila. Rapamycin can also significantly affect the FOXO associated genes in intestine and up-regulate the negative regulators of IMD/Rel pathway, consequently delaying the microbial expansion in the aging guts. Collectively, these findings reveal that rapamycin can delay the intestinal aging by inhibiting mTOR and thus keeping stem cell proliferation in check. These results will further explain the mechanism of healthspan and lifespan extension by rapamycin in Drosophila.

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.

Bunched and Madm Function Downstream of Tuberous Sclerosis Complex to Regulate the Growth of Intestinal Stem Cells in Drosophila

The Drosophila adult midgut contains intestinal stem cells that support homeostasis and repair. We show here that the leucine zipper protein Bunched and the adaptor protein Madm are novel regulators of intestinal stem cells. MARCM mutant clonal analysis and cell type specific RNAi revealed that Bunched and Madm were required within intestinal stem cells for proliferation. Transgenic expression of a tagged Bunched showed a cytoplasmic localization in midgut precursors, and the addition of a nuclear localization signal to Bunched reduced its function to cooperate with Madm to increase intestinal stem cell proliferation. Furthermore, the elevated cell growth and 4EBP phosphorylation phenotypes induced by loss of Tuberous Sclerosis Complex or overexpression of Rheb were suppressed by the loss of Bunched or Madm. Therefore, while the mammalian homolog of Bunched, TSC-22, is able to regulate transcription and suppress cancer cell proliferation, our data suggest the model that Bunched and Madm functionally interact with the TOR pathway in the cytoplasm to regulate the growth and subsequent division of intestinal stem cells.

Metabolic regulation of stem cell function in tissue homeostasis and organismal ageing

Many tissues and organ systems in metazoans have the intrinsic capacity to regenerate, which is driven and maintained largely by tissue-resident somatic stem cell populations. Ageing is accompanied by a deregulation of stem cell function and a decline in regenerative capacity, often resulting in degenerative diseases. The identification of strategies to maintain stem cell function and regulation is therefore a promising avenue to allay a wide range of age-related diseases. Studies in various organisms have revealed a central role for metabolic pathways in the regulation of stem cell function. Ageing is associated with extensive metabolic changes, and interventions that influence cellular metabolism have long been recognized as robust lifespan-extending measures. In this Review, we discuss recent advances in our understanding of the metabolic control of stem cell function, and how stem cell metabolism relates to homeostasis and ageing.

Inability to activate Rac1-dependent forgetting contributes to behavioral inflexibility in mutants of multiple autism-risk genes

The etiology of autism is so complicated because it involves the effects of variants of several hundred risk genes along with the contribution of environmental factors. Therefore, it has been challenging to identify the causal paths that lead to the core autistic symptoms such as social deficit, repetitive behaviors, and behavioral inflexibility. As an alternative approach, extensive efforts have been devoted to identifying the convergence of the targets and functions of the autism-risk genes to facilitate mapping out causal paths. In this study, we used a reversal-learning task to measure behavioral flexibility in Drosophila and determined the effects of loss-of-function mutations in multiple autism-risk gene homologs in flies. Mutations of five autism-risk genes with diversified molecular functions all led to a similar phenotype of behavioral inflexibility indicated by impaired reversal-learning. These reversal-learning defects resulted from the inability to forget or rather, specifically, to activate Rac1 (Ras-related C3 botulinum toxin substrate 1)-dependent forgetting. Thus, behavior-evoked activation of Rac1-dependent forgetting has a converging function for autism-risk genes.

Gastrointestinal stem cells in health and disease: from flies to humans

The gastrointestinal tract of complex metazoans is highly compartmentalized. It is lined by a series of specialized epithelia that are regenerated by specific populations of stem cells. To maintain tissue homeostasis, the proliferative activity of stem and/or progenitor cells has to be carefully controlled and coordinated with regionally distinct programs of differentiation. Metaplasias and dysplasias, precancerous lesions that commonly occur in the human gastrointestinal tract, are often associated with the aberrant proliferation and differentiation of stem and/or progenitor cells. The increasingly sophisticated characterization of stem cells in the gastrointestinal tract of mammals and of the fruit fly Drosophila has provided important new insights into these processes and into the mechanisms that drive epithelial dysfunction. In this Review, we discuss recent advances in our understanding of the establishment, maintenance and regulation of diverse intestinal stem cell lineages in the gastrointestinal tract of Drosophila and mice. We also discuss the field's current understanding of the pathogenesis of epithelial dysfunctions.

Keeping intestinal stem cell differentiation on the Tramtrack

Intestinal epithelium in adult Drosophila midgut undergoes regular turnover and renewal. This process is fueled by intestinal stem cells (ISCs), which can self-renew as well as produce both absorptive enterocytes (ECs) and secretory enteroendocrine (EE) cells. Notch signaling plays a decisive role in EC differentiation. However, the mechanisms controlling EE specification is much less understood. Recently we identified a BTB-domain containing transcriptional repressor Ttk69 as an intrinsic factor in repressing EE cell specification. Loss of Ttk69 caused all progenitor cells to adopt EE cell specification, regardless the status of Notch activity. Mechanistically, Ttk69 represses EE specification via a Ttk69-acheate-scute complex (AS-C) genes-Prospero (Pros) regulatory axis. Interestingly, depletion of ttk69 is able to bypass the requirements of many known signaling pathways, such as JAK/STAT signaling and Tuberous Sclerosis Complex (Tsc), in EE cell specification. These observations suggest that Ttk69 acts as a master repressor of EE cell fate. Here, we further tested the effect of Ttk69 in mature hormone-producing EE cells. We found that cell-autonomous overexpression of Ttk69 in differentiated EE cells was sufficient to disrupt their hormone-producing activity, further supporting the notion that Ttk69 is a master repressor of EE cell fate. In this Extra View, we also provide a brief discussion of recent progress and remaining questions concerning EE cell specification in adult Drosophila midgut.

Ttk69 acts as a master repressor of enteroendocrine cell specification in Drosophila intestinal stem cell lineages

In adult Drosophila midgut, intestinal stem cells (ISCs) periodically produce progenitor cells that undergo a binary fate choice determined primarily by the levels of Notch activity that they receive, before terminally differentiating into enterocytes (ECs) or enteroendocrine (EE) cells. Here we identified Ttk69, a BTB domain-containing transcriptional repressor, as a master repressor of EE cell specification in the ISC lineages. Depletion of ttk69 in progenitor cells induced ISC proliferation and caused all committed progenitor cells to adopt EE fate, leading to the production of supernumerary EE cells in the intestinal epithelium. Conversely, forced expression of Ttk69 in progenitor cells was sufficient to prevent EE cell specification. The expression of Ttk69 was not regulated by Notch signaling, and forced activation of Notch, which is sufficient to induce EC specification of normal progenitor cells, failed to prevent EE cell specification of Ttk69-depleted progenitors. Loss of Ttk69 led to derepression of the acheate-scute complex (AS-C) genes scute and asense, which then induced prospero expression to promote EE cell specification. These studies suggest that Ttk69 functions in parallel with Notch signaling and acts as a master repressor of EE cell specification in Drosophila ISC lineages primarily by suppressing AS-C genes.

Interaction of polyamines and mTOR signaling in the synthesis of antizyme (AZ)

Tissue polyamine levels are largely determined by the activity of ornithine decarboxylase (ODC, EC 4.1.17), which catalyzes the conversion of ornithine to the diamine putrescine. The activity of the enzyme is primarily regulated by a negative feedback mechanism involving ODC antizyme (AZ). Our previous studies demonstrated that AZ synthesis is stimulated by the absence of amino acids, the levels of which are sensed by the mTOR complex containing TORC1, which is stimulated by amino acids and inhibited by their absence, and TORC2 the function of which is not well defined. Polyamines, which cause a +1 ribosomal frameshift during the translation of AZ mRNA are required to increase AZ synthesis in both the presence and absence of amino acids. Amino acid starvation increases TORC2 activity. We have demonstrated that mTORC2 activity is necessary for AZ synthesis in the absence of amino acids. Tuberous sclerosis protein (TSC), a negative regulator of mTOR function regulates the activities of both the TORC1 and TORC2. TSC2 knockdown increased mTORC1 activity with concomitant inhibition of mTORC2 activity eliminating AZ induction in the absence of amino acids as well as that induced by spermidine. Thus, these results clearly demonstrate that in addition to polyamines, mTORC2 activity is necessary for AZ synthesis. Moreover, our results support a role for mTORC2 in the synthesis of a specific protein, AZ, which regulates growth of intestinal epithelial cells.

A deregulated intestinal cell cycle program disrupts tissue homeostasis without affecting longevity in Drosophila

Recent studies illuminate a complex relationship between the control of stem cell division and intestinal tissue organization in the model system Drosophila melanogaster. Host and microbial signals drive intestinal proliferation to maintain an effective epithelial barrier. Although it is widely assumed that proliferation induces dysplasia and shortens the life span of the host, the phenotypic consequences of deregulated intestinal proliferation for an otherwise healthy host remain unexplored. To address this question, we genetically isolated and manipulated the cell cycle programs of adult stem cells and enterocytes. Our studies revealed that cell cycle alterations led to extensive cell death and morphological disruptions. Despite the extensive tissue damage, we did not observe an impact on longevity, suggesting a remarkable degree of plasticity in intestinal function.

Intestinal inflammation and stem cell homeostasis in aging Drosophila melanogaster

As a barrier epithelium, the intestinal epithelium has to coordinate physiological functions like digestion and nutrient resorption with the control of commensal bacteria and the prevention of pathogenic infections. It can therefore mount powerful innate immune and inflammatory responses, while, at the same time, maintaining tissue homeostasis through regenerative processes. How these different functions are coordinated remains unclear, and further insight is required to understand the age-related loss of homeostasis in this system, as well as the etiology of inflammatory and proliferative diseases of the gut. Recent work in Drosophila melanogaster has provided important new insight into the regulation of regenerative activity, innate immune homeostasis, commensal control, as well as age-related dysfunction in the intestine. Interestingly, many of the identified processes and mechanisms mirror similar homeostatic processes in the vertebrate intestine. This review summarized the current understanding of how innate immune responses, changes in commensal bacteria, and other challenges influence regenerative activity in the aging intestinal epithelium of flies and draws parallels to similar processes in mammals.