The adult Drosophila gastric and stomach organs are maintained by a multipotent stem cell pool at the foregut/midgut junction in the cardia (proventriculus)

Leptin- and cytokine-like unpaired signaling in Drosophila

Animals have evolved a multitude of signaling pathways that enable them to orchestrate diverse physiological processes to tightly regulate systemic homeostasis. This signaling is mediated by various families of peptide hormones and cytokines that are conserved across the animal kingdom. In this review, we primarily focus on the unpaired (Upd) family of proteins in Drosophila which are evolutionarily related to mammalian leptin and the cytokine interleukin 6. We summarize expression patterns of Upd in Drosophila and discuss the parallels in structure, signaling pathway, and functions between Upd and their mammalian counterparts. In particular, we focus on the roles of Upd in governing metabolic homeostasis, growth and development, and immune responses. We aim to stimulate future studies on leptin-like signaling in other phyla which can help bridge the evolutionary gap between insect Upd and vertebrate leptin and cytokines like interleukin 6.

Understanding Developmental Cell Death Using Drosophila as a Model System

Cell death plays an essential function in organismal development, wellbeing, and ageing. Many types of cell deaths have been described in the past 30 years. Among these, apoptosis remains the most conserved type of cell death in metazoans and the most common mechanism for deleting unwanted cells. Other types of cell deaths that often play roles in specific contexts or upon pathological insults can be classed under variant forms of cell death and programmed necrosis. Studies in Drosophila have contributed significantly to the understanding and regulation of apoptosis pathways. In addition to this, Drosophila has also served as an essential model to study the genetic basis of autophagy-dependent cell death (ADCD) and other relatively rare types of context-dependent cell deaths. Here, we summarise what is known about apoptosis, ADCD, and other context-specific variant cell death pathways in Drosophila, with a focus on developmental cell death.

Recent trends in insect gut immunity

The gut is a crucial organ in insect defense against various pathogens and harmful substances in their environment and diet. Distinct insect gut compartments possess unique functionalities contributing to their physiological processes, including immunity. The insect gut's cellular composition is vital for cellular and humoral immunity. The peritrophic membrane, mucus layer, lumen, microvilli, and various gut cells provide essential support for activating and regulating immune defense mechanisms. These components also secrete molecules and enzymes that are imperative in physiological activities. Additionally, the gut microbiota initiates various signaling pathways and produces vitamins and minerals that help maintain gut homeostasis. Distinct immune signaling pathways are activated within the gut when insects ingest pathogens or hazardous materials. The pathway induced depends on the infection or pathogen type; include immune deficiency (imd), Toll, JAK/STAT, Duox-ROS, and JNK/FOXO regulatory pathways. These pathways produce different antimicrobial peptides (AMPs) and maintain gut homeostasis. Furthermore, various signaling mechanisms within gut cells regulate insect gut recovery following infection. Although some questions regarding insect gut immunity in different species require additional study, this review provides insights into the insect gut's structure and composition, commensal microorganism roles in Drosophila melanogaster and Tenebrio molitor life cycles, different signaling pathways involved in gut immune systems, and the insect gut post-infection recovery through various signaling mechanisms.

Compromised junctional integrity phenocopies age-dependent renal dysfunction in Drosophila Snakeskin mutants

Transporting epithelia provide a protective barrier against pathogenic insults while allowing the controlled exchange of ions, solutes and water with the external environment. In invertebrates, these functions depend on formation and maintenance of 'tight' septate junctions (SJs). However, the mechanism by which SJs affect transport competence and tissue homeostasis, and how these are modulated by ageing, remain incompletely understood. Here, we demonstrate that the Drosophila renal (Malpighian) tubules undergo an age-dependent decline in secretory capacity, which correlates with mislocalisation of SJ proteins and progressive degeneration in cellular morphology and tissue homeostasis. Acute loss of the SJ protein Snakeskin in adult tubules induced progressive changes in cellular and tissue architecture, including altered expression and localisation of junctional proteins with concomitant loss of cell polarity and barrier integrity, demonstrating that compromised junctional integrity is sufficient to replicate these ageing-related phenotypes. Taken together, our work demonstrates a crucial link between epithelial barrier integrity, tubule transport competence, renal homeostasis and organismal viability, as well as providing novel insights into the mechanisms underpinning ageing and renal disease.

Insect Gut Regeneration

In adult insects, as in vertebrates, the gut epithelium is a highly regenerative tissue that can renew itself rapidly in response to changing inputs from nutrition, the gut microbiota, ingested toxins, and signals from other organs. Because of its cellular and genetic similarities to the mammalian intestine, and its relevance as a target for the control of insect pests and disease vectors, many researchers have used insect intestines to address fundamental questions about stem cell functions during tissue maintenance and regeneration. In Drosophila, where most of the experimental work has been performed, not only are intestinal cell types and behaviors well characterized, but numerous cell signaling interactions have been detailed that mediate gut epithelial regeneration. A prevailing model for regenerative responses in the insect gut invokes stress sensing by damaged enterocytes (ECs) as a principal source for signaling that activates the division of intestinal stem cells (ISCs) and the growth and differentiation of their progeny. However, extant data also reveal alternative mechanisms for regeneration that involve ISC-intrinsic functions, active culling of healthy epithelial cells, enhanced EC growth, and even cytoplasmic shedding by infected ECs. This article reviews current knowledge of the molecular mechanisms involved in gut regeneration in several insect models (Drosophila and Aedes of the order Diptera, and several Lepidoptera).

No Evidence that Wnt Ligands Are Required for Planar Cell Polarity in Drosophila

The frizzled (fz) and dishevelled (dsh) genes are highly conserved members of both the planar cell polarity (PCP) pathway and the Wnt signaling pathway. Given these dual functions, several studies have examined whether Wnt ligands provide a tissue-scale orientation cue for PCP establishment during development, and these studies have reached differing conclusions. Here, we re-examine this issue in the Drosophila melanogaster wing and notum using split-Gal4 co-expression analysis, multiplex somatic CRISPR, and double RNAi experiments. Pairwise loss-of-function experiments targeting wg together with other Wnt genes, via somatic CRISPR or RNAi, do not produce PCP defects in the wing or notum. In addition, somatic CRISPR against evi (aka wntless), which is required for the secretion of Wnt ligands, did not produce detectable PCP phenotypes. Altogether, our results do not support the hypothesis that Wnt ligands contribute to PCP signaling in the Drosophila wing or notum.

Previous studies have come to differing conclusions on whether Wnt ligands provide a tissue-level orientation cue for the planar cell polarity pathway. Ewen-Campen et al. re-examine this question in Drosophila using multiplex in vivo CRISPR and double RNAi against Wnt ligands and find no evidence that Wnts are required for PCP patterning.

Identification and characterization of GAL4 drivers that mark distinct cell types and regions in the Drosophila adult gut

The gastrointestinal tract in the adult Drosophila serves as a model system for exploring the mechanisms underlying digestion, absorption and excretion, stem cell plasticity, and inter-organ communication, particularly through the gut-brain axis. It is also useful for studying the cellular and adaptive responses to dietary changes, alterations in microbiota and immunity, and systematic and endocrine signals. Despite the various cell types and distinct regions in the gastrointestinal tract, few tools are available to target and manipulate the activity of each cell type and region, and their gene expression. Here, we report 353 GAL4 lines and several split-GAL4 lines that are expressed in enteric neurons (ENs), progenitors (ISCs and EBs), enterocytes (ECs), enteroendocrine cells (EEs), or/and other cell types that are yet to be identified in distinct regions of the gut. We had initially collected approximately 600 GAL4 lines that may be expressed in the gut based on RNA sequencing data, and then crossed them to UAS-GFP to perform immunohistochemistry to identify those that are expressed selectively in the gut. The cell types and regional expression patterns that are associated with the entire set of GAL4 drivers and split-GAL4 combinations are annotated online at http://kdrc.kr/index.php (K-Gut Project). This GAL4 resource can be used to target specific populations of distinct cell types in the fly gut, and therefore, should permit a more precise investigation of gut cells that regulate important biological processes.

Heterogeneity of midgut cells and their differential responses to blood meal ingestion by the mosquito, Aedes aegypti

Mosquitoes are the most notorious hematophagous insects and due to their blood feeding behavior and genetic compatibility, numerous mosquito species are highly efficient vectors for certain human pathogenic parasites and viruses. The mosquito midgut is the principal organ of blood meal digestion and nutrient absorption. It is also the initial site of infection with blood meal acquired parasites and viruses. We conducted an analysis based on single-nucleus RNA sequencing (snRNA-Seq) to assess the cellular diversity of the midgut and how individual cells respond to blood meal ingestion to facilitate its digestion. Our study revealed the presence of 20 distinguishable cell-type clusters in the female midgut of Aedes aegypti. The identified cell types included intestinal stem cells (ISC), enteroblasts (EB), differentiating EB (dEB), enteroendocrine cells (EE), enterocytes (EC), EC-like cells, cardia cells, and visceral muscle (VM) cells. Blood meal ingestion dramatically changed the overall midgut cell type composition, profoundly increasing the proportions of ISC and three EC/EC-like clusters. In addition, transcriptional profiles of all cell types were strongly affected while genes involved in various metabolic processes were significantly upregulated. Our study provides a basis for further physiological and molecular studies on blood digestion, nutrient absorption, and cellular homeostasis in the mosquito midgut.

Fly foregut and transmission of microbes

Two areas of research that have greatly increased in attention are: dipterans as vectors and the microbes they are capable of vectoring. Because it is the front-end of the fly that first encounters these microbes, this review focuses on the legs, mouthparts, and foregut, which includes the crop as major structures involved in dipteran vectoring ability. The legs and mouthparts are generally involved in mechanical transmission of microbes. However, the crop is involved in more than just mechanical transmission, for it is within the lumen of the crop that microbes are taken up with the meal of the fly, stored, and it is within the lumen that horizontal transmission of bacterial resistance has been demonstrated. In addition to storage of microbes, the crop is also involved in depositing the microbes via a process known as regurgitation. Various aspects of crop regulation are discussed and specific examples of crop involvement with microorganisms are discussed. The importance of biofilm and biofilm formation are presented, as well as, some physical parameters of the crop that might either facilitate or inhibit biofilm formation. Finally, there is a brief discussion of dipteran model systems for studying crop microbe interactions.

Essential long-range action of Wingless/Wnt in adult intestinal compartmentalization

Signal transduction activated by Wingless/Wnt ligands directs cell proliferation and fate specification in metazoans, and its overactivation underlies the development of the vast majority of colorectal cancers. In the conventional model, the secretion and movement of Wingless to cells distant from its source of synthesis are essential for long-range signaling in tissue patterning. However, this model was upended recently by an unanticipated finding: replacement of wild-type Drosophila Wingless with a membrane-tethered form produced viable adults with largely normal external morphology, which suggested that Wingless secretion and movement are dispensable for tissue patterning. Herein, we tested this foundational principle in the adult intestine, where Wingless signaling gradients coincide with all major boundaries between compartments. We find that the critical roles of Wingless during adult intestinal development, which include regulation of target gene activation, boundary formation, stem cell proliferation, epithelial cell fate specification, muscle differentiation, gut folding, and signaling crosstalk with the Decapentaplegic pathway, are all disrupted by Wingless tethering. These findings provide new evidence that supports the requirement for the direct, long-range action of Wingless in tissue patterning, with relevance for animal development, tissue homeostasis and Wnt-driven disease.

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.

Temporospatial induction of homeodomain gene cut dictates natural lineage reprogramming

Understanding how cellular identity naturally interconverts with high efficiency and temporospatial precision is crucial for regenerative medicine. Here, we revealed a natural midgut-to-renal lineage conversion event during Drosophila metamorphosis and identified the evolutionarily-conserved homeodomain protein Cut as a master switch in this process. A steep Wnt/Wingless morphogen gradient intersects with a pulse of steroid hormone ecdysone to induce cut expression in a subset of midgut progenitors and reprogram them into renal progenitors. Molecularly, ecdysone-induced temporal factor Broad physically interacts with cut enhancer-bound Wnt pathway effector TCF/β-catenin and likely bridges the distant enhancer and promoter region of cut through its self-association. Such long-range enhancer-promoter looping could subsequently trigger timely cut transcription. Our results therefore led us to propose an unexpected poising-and-bridging mechanism whereby spatial and temporal cues intersect, likely via chromatin looping, to turn on a master transcription factor and dictate efficient and precise lineage reprogramming.

As an embryo develops, an organism transforms from a single cell into an organized collection of different cells, tissues and organs. Regulated by genes and messenger molecules, non-specialized cells known as precursor cells, move, divide and adapt to produce the different cells in the adult body.

However, sometimes already-specialized adult cells can acquire a new role in a process known as lineage reprogramming. Finding ways to artificially induce and control lineage reprogramming could be useful in regenerative medicine. This would allow cells to be reprogrammed to replace those that are lost or damaged. So far, scientists have been unable to develop a clear view of how lineage reprogramming happens naturally.

Here, Xu et al. identified a cell-conversion event in the developing fruit fly. As the fly larva develops into an adult, a group of cells in the midgut reprogramme to become renal cells – the equivalent to human kidney cells. The experiments revealed that a combination of signals from a cell messenger system important for cell specialization (called Wnt) and the hormone that controls molting in insects, activate a gene called cut, which controls the midgut-to-renal lineage reprogramming.

Together, Wnt and the hormone ensure that cut is activated only in a small, specific group of midgut precursor cells at a precise time. The reprogrammed cells then move into the excretory organs, the renal tubes, where they give rise to renal cells. Midgut precursor cells in which cut had been experimentally removed, still traveled into the renal tubes. However, they failed to switch their identity and gave rise to midgut cells instead.

Further examination revealed that both Wnt and the ecdysone hormone are needed to activate the cut gene. This is probably achieved by creating loops in the DNA to bring together the two distantly located key regulatory elements of cut gene expression. If this mechanism can be seen in other contexts it may be possible to adapt it for medical purposes. The ability to reprogramme groups of cells with high specificity could transform medicine. It would make it easier for our bodies to regenerate and repair.

Wingless/Wnt Signaling in Intestinal Development, Homeostasis, Regeneration and Tumorigenesis: A Drosophila Perspective

In mammals, the Wnt/β-catenin signal transduction pathway regulates intestinal stem cell maintenance and proliferation, whereas Wnt pathway hyperactivation, resulting primarily from the inactivation of the tumor suppressor Adenomatous polyposis coli (APC), triggers the development of the vast majority of colorectal cancers. The Drosophila adult gut has recently emerged as a powerful model to elucidate the mechanisms by which Wingless/Wnt signaling regulates intestinal development, homeostasis, regeneration, and tumorigenesis. Herein, we review recent insights on the roles of Wnt signaling in Drosophila intestinal physiology and pathology.

Application of the Mesolens for subcellular resolution imaging of intact larval and whole adult Drosophila

In a previous paper, we showed a new giant lens called the Mesolens and presented performance data and images from whole fixed and intact fluorescently‐stained 12.5‐day old mouse embryos. Here, we show that using the Mesolens we can image an entire Drosophila larva or adult fly in confocal epifluorescence and show subcellular detail in all tissues. By taking several hundreds of optical sections through the entire volume of the specimen, we show cells and nuclear details within the gut, brain, salivary glands and reproductive system that normally require dissection for study. Organs are imaged in situ in correct 3D arrangement. Imaginal discs are imaged in mature larvae and it proved possible to image pachytene chromosomes in cells within ovarian follicles in intact female flies. Methods for fixing, staining and clearing are given.

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.

Metamorphosis of the Drosophila visceral musculature and its role in intestinal morphogenesis and stem cell formation

The visceral musculature of the Drosophila intestine plays important roles in digestion as well as development. Detailed studies investigating the embryonic development of the visceral muscle exist; comparatively little is known about postembryonic development and metamorphosis of this tissue. In this study we have combined the use of specific markers with electron microscopy to follow the formation of the adult visceral musculature and its involvement in gut development during metamorphosis. Unlike the adult somatic musculature, which is derived from a pool of undifferentiated myoblasts, the visceral musculature of the adult is a direct descendant of the larval fibers, as shown by activating a lineage tracing construct in the larval muscle and obtaining labeled visceral fibers in the adult. However, visceral muscles undergo a phase of remodeling that coincides with the metamorphosis of the intestinal epithelium. During the first day following puparium formation, both circular and longitudinal syncytial fibers dedifferentiate, losing their myofibrils and extracellular matrix, and dissociating into mononuclear cells ("secondary myoblasts"). Towards the end of the second day, this process is reversed, and between 48 and 72h after puparium formation, a structurally fully differentiated adult muscle layer has formed. We could not obtain evidence that cells apart from the dedifferentiated larval visceral muscle contributed to the adult muscle, nor does it appear that the number of adult fibers (or nuclei per fiber) is increased over that of the larva by proliferation. In contrast to the musculature, the intestinal epithelium is completely renewed during metamorphosis. The adult midgut epithelium rapidly expands over the larval layer during the first few hours after puparium formation; in case of the hindgut, replacement takes longer, and proceeds by the gradual caudad extension of a proliferating growth zone, the hindgut proliferation zone (HPZ). The subsequent elongation of the hindgut and midgut, as well as the establishment of a population of intestinal stem cells active in the adult midgut and hindgut, requires the presence of the visceral muscle layer, based on the finding that ablation of this layer causes a severe disruption of both processes.

A Drosophila LexA Enhancer-Trap Resource for Developmental Biology and Neuroendocrine Research

Novel binary gene expression tools like the LexA-LexAop system could powerfully enhance studies of metabolism, development, and neurobiology in Drosophila However, specific LexA drivers for neuroendocrine cells and many other developmentally relevant systems remain limited. In a unique high school biology course, we generated a LexA-based enhancer trap collection by transposon mobilization. The initial collection provides a source of novel LexA-based elements that permit targeted gene expression in the corpora cardiaca, cells central for metabolic homeostasis, and other neuroendocrine cell types. The collection further contains specific LexA drivers for stem cells and other enteric cells in the gut, and other developmentally relevant tissue types. We provide detailed analysis of nearly 100 new LexA lines, including molecular mapping of insertions, description of enhancer-driven reporter expression in larval tissues, and adult neuroendocrine cells, comparison with established enhancer trap collections and tissue specific RNAseq. Generation of this open-resource LexA collection facilitates neuroendocrine and developmental biology investigations, and shows how empowering secondary school science can achieve research and educational goals.

The lipolysis pathway sustains normal and transformed stem cells in adult Drosophila

Cancer stem cells (CSCs) may be responsible for tumour dormancy, relapse and the eventual death of most cancer patients. In addition, these cells are usually resistant to cytotoxic conditions. However, very little is known about the biology behind this resistance to therapeutics. Here we investigated stem-cell death in the digestive system of adult Drosophila melanogaster. We found that knockdown of the coat protein complex I (COPI)-Arf79F (also known as Arf1) complex selectively killed normal and transformed stem cells through necrosis, by attenuating the lipolysis pathway, but spared differentiated cells. The dying stem cells were engulfed by neighbouring differentiated cells through a draper-myoblast city-Rac1-basket (also known as JNK)-dependent autophagy pathway. Furthermore, Arf1 inhibitors reduced CSCs in human cancer cell lines. Thus, normal or cancer stem cells may rely primarily on lipid reserves for energy, in such a way that blocking lipolysis starves them to death. This finding may lead to new therapies that could help to eliminate CSCs in human cancers.

Maintenance of the adult Drosophila intestine: all roads lead to homeostasis

Maintenance of tissue homeostasis is critical in tissues with high turnover such as the intestinal epithelium. The intestinal epithelium is under constant cellular assault due to its digestive functions and its function as a barrier to chemical and bacterial insults. The resulting high rate of cellular turnover necessitates highly controlled mechanisms of regeneration to maintain the integrity of the tissue over the lifetime of the organism. Transient increase in stem cell proliferation is a commonly used and elaborate mechanism to ensure fast and efficient repair of the gut. However, tissue repair is not limited to regulating ISC proliferation, as emerging evidence demonstrates that the Drosophila intestine uses multiple strategies to ensure proper tissue homeostasis that may also extend to other tissues.

Keap1-Independent Regulation of Nrf2 Activity by Protein Acetylation and a BET Bromodomain Protein

Mammalian BET proteins comprise a family of bromodomain-containing epigenetic regulators with complex functions in chromatin organization and gene regulation. We identified the sole member of the BET protein family in Drosophila, Fs(1)h, as an inhibitor of the stress responsive transcription factor CncC, the fly ortholog of Nrf2. Fs(1)h physically interacts with CncC in a manner that requires the function of its bromodomains and the acetylation of CncC. Treatment of cultured Drosophila cells or adult flies with fs(1)h RNAi or with the BET protein inhibitor JQ1 de-represses CncC transcriptional activity and engages protective gene expression programs. The mechanism by which Fs(1)h inhibits CncC function is distinct from the canonical mechanism that stimulates Nrf2 function by abrogating Keap1-dependent proteasomal degradation. Consistent with the independent modes of CncC regulation by Keap1 and Fs(1)h, combinations of drugs that can specifically target these pathways cause a strong synergistic and specific activation of protective CncC- dependent gene expression and boosts oxidative stress resistance. This synergism might be exploitable for the design of combinatorial therapies to target diseases associated with oxidative stress or inflammation.

Nrf2-related transcription factors regulate gene expression programs that protect organisms against chemical or oxidative stress. Nrf2-activating drugs hold promise for the treatment of diseases that are connected to oxidative stress or inflammation. We identified Fs(1)h, a bromodomain-containing BET protein, as a negative regulator of Nrf2 function in Drosophila. BET proteins are involved in transcription regulation and chromatin organization and have been implicated in several diseases, including cancer. Fs(1)h interacts with acetylated lysines on CncC, the homolog of Nrf2 in Drosophila, and thereby prevents target gene activation. Nrf2 can be released from this inhibitory effect by small molecules that specifically interfere with the binding of BET proteins to acetylated targets. Fs(1)h regulates Nrf2 independently of Keap1, a well-studied Nrf2 regulator. Consequently, chemical inhibitors of Keap1 and of Fs(1)h can be combined to achieve synergistic activation of Nrf2 target genes and strongly boost oxidative stress tolerance in Drosophila. The Keap1-independent mechanism of Nrf2 regulation is conserved in mammals. We suggest that the synergistic effect of combinatorial Nrf2 targeting drugs may be effective for the treatment of different oxidative stress and inflammation-related diseases.

Regulation of Stem Cell Proliferation and Cell Fate Specification by Wingless/Wnt Signaling Gradients Enriched at Adult Intestinal Compartment Boundaries

Intestinal stem cell (ISC) self-renewal and proliferation are directed by Wnt/β-catenin signaling in mammals, whereas aberrant Wnt pathway activation in ISCs triggers the development of human colorectal carcinoma. Herein, we have utilized the Drosophila midgut, a powerful model for ISC regulation, to elucidate the mechanisms by which Wingless (Wg)/Wnt regulates intestinal homeostasis and development. We provide evidence that the Wg signaling pathway, activation of which peaks at each of the major compartment boundaries of the adult intestine, has essential functions. Wg pathway activation in the intestinal epithelium is required not only to specify cell fate near compartment boundaries during development, but also to control ISC proliferation within compartments during homeostasis. Further, in contrast with the previous focus on Wg pathway activation within ISCs, we demonstrate that the primary mechanism by which Wg signaling regulates ISC proliferation during homeostasis is non-autonomous. Activation of the Wg pathway in absorptive enterocytes is required to suppress JAK-STAT signaling in neighboring ISCs, and thereby their proliferation. We conclude that Wg signaling gradients have essential roles during homeostasis and development of the adult intestine, non-autonomously controlling stem cell proliferation inside compartments, and autonomously specifying cell fate near compartment boundaries.

The highly conserved Wingless/Wnt signal transduction pathway directs many cellular processes in metazoans and its deregulation underlies numerous human congenital diseases and cancers. Most notably, more than 80% of colon cancers arise from aberrant activation of the Wnt pathway. A better understanding of how Wnt signaling functions in the intestinal stem cells (ISCs) during homeostasis and in disease states is thus critical. The Drosophila digestive tract provides a powerful genetic model and an entry point to study these questions. Here, we find that the Wg ligand and pathway activation are enriched at Drosophila intestinal compartment boundaries and are essential for development and homeostasis of the adult gut. During homeostasis, Wg signaling in enterocytes is required to prevent the overproliferation of ISCs non-autonomously. In addition, during development, Wg signaling ensures proper cell fate specification near compartment boundaries. These findings provide insight into the mechanisms underlying the Wg-dependent regulation of adult intestinal function.

All for one and one for all: Regionalization of the Drosophila intestine

Physiological responses are the ultimate outcomes of the functional interactions and proper organization of the different cell types that make up an organ. The digestive tract represents a good example where such structure/function correlation is manifested. To date, the molecular mechanisms that establish and/or maintain gut segmentation and functional specialization remain poorly understood. Recently, the use of model systems such as Drosophila has enriched our knowledge about the gut organization and physiology. Here, we review recent studies deciphering the morphological and functional properties of the Drosophila adult midgut compartments. Intestinal compartments are established through the differentiation of regionalized stem cell populations in concert with the joint activity of patterned transcription factors and locally produced morphogens. The maintenance of a compartmentalized gut structure is vital to the organism, allowing sequentially the ingestion and digestion of food, absorption of nutrients, and excretion of waste products in addition to the compartmentalization of immune and homeostatic functions. Further characterization of the gene regulatory networks underlying gut compartmentalization will pave the way for a better understanding of gastrointestinal function in insects and mammals, in both health and disease conditions.

Regional Cell-Specific Transcriptome Mapping Reveals Regulatory Complexity in the Adult Drosophila Midgut

Deciphering contributions of specific cell types to organ function is experimentally challenging. The Drosophila midgut is a dynamic organ with five morphologically and functionally distinct regions (R1-R5), each composed of multipotent intestinal stem cells (ISCs), progenitor enteroblasts (EBs), enteroendocrine cells (EEs), enterocytes (ECs), and visceral muscle (VM). To characterize cellular specialization and regional function in this organ, we generated RNA-sequencing transcriptomes of all five cell types isolated by FACS from each of the five regions, R1-R5. In doing so, we identify transcriptional diversities among cell types and document regional differences within each cell type that define further specialization. We validate cell-specific and regional Gal4 drivers; demonstrate roles for transporter Smvt and transcription factors GATAe, Sna, and Ptx1 in global and regional ISC regulation, and study the transcriptional response of midgut cells upon infection. The resulting transcriptome database (http://flygutseq.buchonlab.com) will foster studies of regionalization, homeostasis, immunity, and cell-cell interactions.

Enteroendocrine cells are generated from stem cells through a distinct progenitor in the adult Drosophila posterior midgut

Functional mature cells are continually replenished by stem cells to maintain tissue homoeostasis. In the adult Drosophila posterior midgut, both terminally differentiated enterocyte (EC) and enteroendocrine (EE) cells are generated from an intestinal stem cell (ISC). However, it is not clear how the two differentiated cells are generated from the ISC. In this study, we found that only ECs are generated through the Su(H)GBE(+) immature progenitor enteroblasts (EBs), whereas EEs are generated from ISCs through a distinct progenitor pre-EE by a novel lineage-tracing system. EEs can be generated from ISCs in three ways: an ISC becoming an EE, an ISC becoming a new ISC and an EE through asymmetric division, or an ISC becoming two EEs through symmetric division. We further identified that the transcriptional factor Prospero (Pros) regulates ISC commitment to EEs. Our data provide direct evidence that different differentiated cells are generated by different modes of stem cell lineage specification within the same tissues.

Bacterial uracil modulates Drosophila DUOX-dependent gut immunity via Hedgehog-induced signaling endosomes

Genetic studies in Drosophila have demonstrated that generation of microbicidal reactive oxygen species (ROS) through the NADPH dual oxidase (DUOX) is a first line of defense in the gut epithelia. Bacterial uracil acts as DUOX-activating ligand through poorly understood mechanisms. Here, we show that the Hedgehog (Hh) signaling pathway modulates uracil-induced DUOX activation. Uracil-induced Hh signaling is required for intestinal expression of the calcium-dependent cell adhesion molecule Cadherin 99C (Cad99C) and subsequent Cad99C-dependent formation of endosomes. These endosomes play essential roles in uracil-induced ROS production by acting as signaling platforms for PLCβ/PKC/Ca2+-dependent DUOX activation. Animals with impaired Hh signaling exhibit abolished Cad99C-dependent endosome formation and reduced DUOX activity, resulting in high mortality during enteric infection. Importantly, endosome formation, DUOX activation, and normal host survival are restored by genetic reintroduction of Cad99C into enterocytes, demonstrating the important role for Hh signaling in host resistance to enteric infection.

The digestive tract of Drosophila melanogaster

The digestive tract plays a central role in the digestion and absorption of nutrients. Far from being a passive tube, it provides the first line of defense against pathogens and maintains energy homeostasis by exchanging neuronal and endocrine signals with other organs. Historically neglected, the gut of the fruit fly Drosophila melanogaster has recently come to the forefront of Drosophila research. Areas as diverse as stem cell biology, neurobiology, metabolism, and immunity are benefitting from the ability to study the genetics of development, growth regulation, and physiology in the same organ. In this review, we summarize our knowledge of the Drosophila digestive tract, with an emphasis on the adult midgut and its functional underpinnings.

EGFR and Notch signaling respectively regulate proliferative activity and multiple cell lineage differentiation of Drosophila gastric stem cells

Quiescent, multipotent gastric stem cells (GSSCs) in the copper cell region of adult Drosophila midgut can produce all epithelial cell lineages found in the region, including acid-secreting copper cells, interstitial cells and enteroendocrine cells, but mechanisms controlling their quiescence and the ternary lineage differentiation are unknown. By using cell ablation or damage-induced regeneration assays combined with cell lineage tracing and genetic analysis, here we demonstrate that Delta (Dl)-expressing cells in the copper cell region are the authentic GSSCs that can self-renew and continuously regenerate the gastric epithelium after a sustained damage. Lineage tracing analysis reveals that the committed GSSC daughter with activated Notch will invariably differentiate into either a copper cell or an interstitial cell, but not the enteroendocrine cell lineage, and loss-of-function and gain-of-function studies revealed that Notch signaling is both necessary and sufficient for copper cell/interstitial cell differentiation. We also demonstrate that elevated epidermal growth factor receptor (EGFR) signaling, which is achieved by the activation of ligand Vein from the surrounding muscle cells and ligand Spitz from progenitor cells, mediates the regenerative proliferation of GSSCs following damage. Taken together, we demonstrate that Dl is a specific marker for Drosophila GSSCs, whose cell cycle status is dependent on the levels of EGFR signaling activity, and the Notch signaling has a central role in controlling cell lineage differentiation from GSSCs by separating copper/interstitial cell lineage from enteroendocrine cell lineage.

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.

Migration of Drosophila intestinal stem cells across organ boundaries

All components of the Drosophila intestinal tract, including the endodermal midgut and ectodermal hindgut/Malpighian tubules, maintain populations of dividing stem cells. In the midgut and hindgut, these stem cells originate from within larger populations of intestinal progenitors that proliferate during the larval stage and form the adult intestine during metamorphosis. The origin of stem cells found in the excretory Malpighian tubules ('renal stem cells') has not been established. In this paper, we investigate the migration patterns of intestinal progenitors that take place during metamorphosis. Our data demonstrate that a subset of adult midgut progenitors (AMPs) move posteriorly to form the adult ureters and, consecutively, the renal stem cells. Inhibiting cell migration by AMP-directed expression of a dominant-negative form of Rac1 protein results in the absence of stem cells in the Malpighian tubules. As the majority of the hindgut progenitor cells migrate posteriorly and differentiate into hindgut enterocytes, a group of the progenitor cells, unexpectedly, invades anteriorly into the midgut territory. Consequently, these progenitor cells differentiate into midgut enterocytes. The midgut determinant GATAe is required for the differentiation of midgut enterocytes derived from hindgut progenitors. Wingless signaling acts to balance the proportion of hindgut progenitors that differentiate as midgut versus hindgut enterocytes. Our findings indicate that a stable boundary between midgut and hindgut/Malpighian tubules is not established during early embryonic development; instead, pluripotent progenitor populations cross in between these organs in both directions, and are able to adopt the fate of the organ in which they come to reside.

Morphological and molecular characterization of adult midgut compartmentalization in Drosophila

Although the gut is a central organ of Eumetazoans and is essential for organismal health, our understanding of its morphological and molecular determinants remains rudimentary. Here, we provide a comprehensive atlas of Drosophila adult midgut. Specifically, we uncover a fine-grained regional organization consisting of 14 subregions with distinct morphological, histological, and genetic properties. We also show that Drosophila intestinal regionalization is defined after adult emergence, remains stable throughout life, and reestablishes following acute tissue damage. Additionally, we show that this midgut compartmentalization is achieved through the interplay between pan-midgut and regionalized transcription factors, in concert with spatial activities of morphogens. Interestingly, disruption of the midgut compartmentalization leads to a loss of intestinal homeostasis characterized by an increase in stem cell proliferation and aberrant immune responses. Our integrative analysis of Drosophila midgut compartmentalization provides insights into the conserved mechanisms underlying intestinal regionalization in metazoans.

APC loss-induced intestinal tumorigenesis in Drosophila: Roles of Ras in Wnt signaling activation and tumor progression

Adenomatous polyposis coli (APC) and K-ras are the two most frequently mutated genes found in human colorectal cancers. In human colorectal cancers, Wnt signaling activation after the loss of APC is hypothesized to be the key event for adenoma initiation, whereas additional mutations such as Ras activation are required for the progression from adenoma to carcinoma. However, accumulating data have led to conflicting views regarding the precise role of Ras in APC loss-induced tumorigenesis. Here, using Drosophila midgut as a model system, we show that in the absence of Ras, APC mutant epithelial cells cannot initiate hyperplasia, suggesting that Ras plays an essential role in tumor initiation. Conversely, activating Ras by expressing oncogenic Ras or Raf in APC-deficient cells led to a blockage of cell differentiation and to preinvasive tumor outgrowth, characteristics that are shared by advanced colorectal carcinoma in humans. Mechanistically, we find that Ras is not required for Wnt signaling activation after APC loss, although Ras hyperactivation is able to potentiate Wnt signaling by increasing the cytoplasmic and nuclear accumulation of Armadillo/β-catenin via mechanisms independent of JNK/Rac1 or PI3K-Akt signaling, partly owing to the downregulation of DE-cadherin. Together with the data from gene expression analyses, our results indicate that both parallel and cooperative mechanisms of Wnt and Ras signaling are responsible for the initiation and progression of intestinal tumorigenesis after APC loss.

Stem cells in the Drosophila digestive system

Adult stem cells maintain tissue homeostasis by continuously replenishing damaged, aged and dead cells in any organism. Five types of region and organ-specific multipotent adult stem cells have been identified in the Drosophila digestive system: intestinal stem cells (ISCs) in the posterior midgut; hindgut intestinal stem cells (HISCs) at the midgut/hindgut junction; renal and nephric stem cells (RNSCs) in the Malpighian Tubules; type I gastric stem cells (GaSCs) at foregut/midgut junction; and type II gastric stem cells (GSSCs) at the middle of the midgut. Despite the fact that each type of stem cell is unique to a particular organ, they share common molecular markers and some regulatory signaling pathways. Due to the simpler tissue structure, ease of performing genetic analysis, and availability of abundant mutants, Drosophila serves as an elegant and powerful model system to study complex stem cell biology. The recent discoveries, particularly in the Drosophila ISC system, have greatly advanced our understanding of stem cell self-renewal, differentiation, and the role of stem cells play in tissue homeostasis/regeneration and adaptive tissue growth.

Genetic control of intestinal stem cell specification and development: a comparative view

Stem cells of the adult vertebrate intestine (ISCs) are responsible for the continuous replacement of intestinal cells, but also serve as site of origin of intestinal neoplasms. The interaction between multiple signaling pathways, including Wnt/Wg, Shh/Hh, BMP, and Notch, orchestrate mitosis, motility, and differentiation of ISCs. Many fundamental questions of how these pathways carry out their function remain unanswered. One approach to gain more insight is to look at the development of stem cells, to analyze the "programming" process which these cells undergo as they emerge from the large populations of embryonic progenitors. This review intends to summarize pertinent data on vertebrate intestinal stem cell biology, to then take a closer look at recent studies of intestinal stem cell development in Drosophila. Here, stem cell pools and their niche environment consist of relatively small numbers of cells, and questions concerning the pattern of cell division, niche-stem cell contacts, or differentiation can be addressed at the single cell level. Likewise, it is possible to analyze the emergence of stem cells during development more easily than in vertebrate systems: where in the embryo do stem cells arise, what structures in their environment do they interact with, and what signaling pathways are active sequentially as a result of these interactions. Given the high degree of conservation among genetic mechanisms controlling stem cell behavior in all animals, findings in Drosophila will provide answers that inform research in the vertebrate stem cell field.

The Drosophila larva as a tool to study gut-associated macrophages: PI3K regulates a discrete hemocyte population at the proventriculus

Immune cells not only patrol the body in the circulation but also importantly, associate with specific tissues, such as the intestinal epithelium. The complex interactions between immune cells and their target tissues are difficult to study and simple, genetically tractable models are lacking. Here, we present the first thorough characterization of gut-associated macrophages in Drosophila larvae. We analyze their gene expression, morphology, development and lineage and importantly, demonstrate that they are functional (phagocytic) macrophages. We test their regulation by phosphoinositide 3-kinase (PI3K) and show evidence that this pathway regulates the population size of gut hemocytes and their phagocytic activity, reminiscent of recent findings in mammalian colitis models. Our data suggest that PI3K signaling modifies the adhesive properties of hemocytes, a possible mechanism for gut-hemocyte regulation. These results demonstrate the potential of the Drosophila larva as a simple tool to uncover mechanisms regulating recruitment and maintenance of innate immune cells at their target tissues.

Drosophila models of epithelial stem cells and their niches

Epithelial stem cells are regulated through a complex interplay of signals from diffusible ligands, cellular interactions, and attachment to the extracellular matrix. The development of Drosophila models of epithelial stem cells and their associated niche has made it possible to dissect the contribution of each of these factors in vivo, during both basal homeostasis and in response to acute damage such as infection. Studies of Drosophila epithelial stem cells have also provided insight into the mechanisms by which a healthy population of stem cells are maintained throughout adulthood by demonstrating, for example, that stem cells have a finite lifespan and may be displaced by replacement cells competing for niche occupancy. Here, we summarize the literature on each of the known Drosophila epithelial stem cells, with a focus on the two most well-characterized types, the follicle stem cells (FSCs) in the ovary and the intestinal stem cells (ISCs) in the posterior midgut. Several themes have emerged from these studies, which suggest that there may be a common set of features among niches in a variety of epithelia. For example, unlike the simpler Drosophila germline stem cell niches, both the FSC and ISC niches produce multiple, partially redundant, niche signals, some of which activate pathways such as Wnt/Wingless, Hedgehog, and epidermal growth factor (EGF) that also regulate mammalian epithelial tissue renewal. Further study into these relatively new stem cell models will be of use in understanding both the specifics of epithelial regeneration and the diversity of mechanisms that regulate adult stem cells in general.

Generation and staining of intestinal stem cell lineage in adult midgut

Stem cell-mediated tissue repair is a promising approach in regenerative medicine. Intestinal epithelium is the most rapidly self-renewing tissue in adult mammals. Recently, using lineage tracing and molecular marker labeling, intestinal stem cells (ISCs) have been identified in Drosophila adult midgut. ISCs reside at the basement membrane and are multipotent as they produce both enterocytes and enteroendocrine cells. The adult Drosophila midgut provides an excellent in vivo model organ to study ISC behavior during aging, stress, regeneration, and infection. It has been demonstrated that Notch, Janus kinase/signal transducer and activator of transcription, epidermal growth factor receptor/mitogen-activated protein kinase, Hippo, and wingless signaling pathways regulate ISCs proliferation and differentiation. There are plenty of genetic tools and markers developed in recent years in Drosophila stem cell studies. These tools and markers are essential in the precise identification of stem cells as well as manipulation of genes in stem cell regulation. Here, we describe the details of genetic tools, markers, and immunolabeling techniques used in identification and characterization of adult midgut stem cells in Drosophila.

Intestinal stem cells in the adult Drosophila midgut

Drosophila has long been an excellent model organism for studying stem cell biology. Notably, studies of Drosophila's germline stem cells have been instrumental in developing the stem cell niche concept. The recent discovery of somatic stem cells in adult Drosophila, particularly the intestinal stem cells (ISCs) of the midgut, has established Drosophila as an exciting model to study stem cell-mediated adult tissue homeostasis and regeneration. Here, we review the major signaling pathways that regulate the self-renewal, proliferation and differentiation of Drosophila ISCs, discussing how this regulation maintains midgut homeostasis and mediates regeneration of the intestinal epithelium after injury.

Quiescent gastric stem cells maintain the adult Drosophila stomach

The adult Drosophila copper cell region or "stomach" is a highly acidic compartment of the midgut with pH < 3. In this region, a specialized group of acid-secreting cells similar to mammalian gastric parietal cells has been identified by a unique ultrastructure and by copper-metallothionein fluorescence. However, the homeostatic mechanism maintaining the acid-secreting "copper cells" of the adult midgut has not been examined. Here, we combine cell lineage tracing and genetic analysis to investigate the mechanism by which the gastric epithelium is maintained. Our investigation shows that a molecularly identifiable population of multipotent, self-renewing gastric stem cells (GSSCs) produces the acid-secreting copper cells, interstitial cells, and enteroendocrine cells of the stomach. Our assays demonstrate that GSSCs are largely quiescent but can be induced to regenerate the gastric epithelium in response to environmental challenge. Finally, genetic analysis reveals that adult GSSC maintenance depends on Wnt signaling. Characterization of the GSSC lineage in Drosophila, with striking similarities to mammals, will advance the study of both homeostatic and pathogenic processes in the stomach.

Drosophila as a model of wound healing and tissue regeneration in vertebrates

Understanding the molecular basis of wound healing and regeneration in vertebrates is one of the main challenges in biology and medicine. This understanding will lead to medical advances allowing accelerated tissue repair after wounding, rebuilding new tissues/organs and restoring homeostasis. Drosophila has emerged as a valuable model for studying these processes because the genetic networks and cytoskeletal machinery involved in epithelial movements occurring during embryonic dorsal closure, larval imaginal disc fusion/regeneration, and epithelial repair are similar to those acting during wound healing and regeneration in vertebrates. Recent studies have also focused on the use of Drosophila adult stem cells to maintain tissue homeostasis. Here, we review how Drosophila has contributed to our understanding of these processes, primarily through live-imaging and genetic tools that are impractical in mammals. Furthermore, we highlight future research areas where this insect may provide novel insights and potential therapeutic strategies for wound healing and regeneration.

Comparative endocrinology of aging and longevity regulation

Hormones regulate growth, development, metabolism, and other complex processes in multicellular animals. For many years it has been suggested that hormones may also influence the rate of the aging process. Aging is a multifactorial process that causes biological systems to break down and cease to function in adult organisms as time passes, eventually leading to death. The exact underlying causes of the aging process remain a topic for debate, and clues that may shed light on these causes are eagerly sought after. In the last two decades, gene mutations that result in delayed aging and extended longevity have been discovered, and many of the affected genes have been components of endocrine signaling pathways. In this review we summarize the current knowledge on the roles of endocrine signaling in the regulation of aging and longevity in various animals. We begin by discussing the notion that conserved systems, including endocrine signaling pathways, "regulate" the aging process. Findings from the major model organisms: worms, flies, and rodents, are then outlined. Unique lessons from studies of non-traditional models: bees, salmon, and naked mole rats, are also discussed. Finally, we summarize the endocrinology of aging in humans, including changes in hormone levels with age, and the involvement of hormones in aging-related diseases. The most well studied and widely conserved endocrine pathway that affects aging is the insulin/insulin-like growth factor system. Mutations in genes of this pathway increase the lifespan of worms, flies, and mice. Population genetic evidence also suggests this pathway's involvement in human aging. Other hormones including steroids have been linked to aging only in a subset of the models studied. Because of the value of comparative studies, it is suggested that the aging field could benefit from adoption of additional model organisms.