βν integrin inhibits chronic and high level activation of JNK to repress senescence phenotypes in Drosophila adult midgut

Apical-basal polarity in the gut

Apical-Basal polarity is a fundamental property of all epithelial cells that underlies both their form and function. The gut is made up of a single layer of intestinal epithelial cells, with distinct apical, lateral and basal domains. Occluding junctions at the apical side of the lateral domains create a barrier between the gut lumen and the body, which is crucial for tissue homeostasis, protection against gastrointestinal pathogens and for the maintenance of the immune response. Apical-basal polarity in most epithelia is established by conserved polarity factors, but recent evidence suggests that the gut epithelium in at least some organisms polarises by novel mechanisms. In this review, we discuss the recent advances in understanding polarity factors by focussing on work in C. elegans, Drosophila, Zebrafish and Mouse.

Vinculin recruitment to α-catenin halts the differentiation and maturation of enterocyte progenitors to maintain homeostasis of the Drosophila intestine

Mechanisms communicating changes in tissue stiffness and size are particularly relevant in the intestine because it is subject to constant mechanical stresses caused by peristalsis of its variable content. Using the Drosophila intestinal epithelium, we investigate the role of vinculin, one of the best characterised mechanoeffectors, which functions in both cadherin and integrin adhesion complexes. We discovered that vinculin regulates cell fate decisions, by preventing precocious activation and differentiation of intestinal progenitors into absorptive cells. It achieves this in concert with α-catenin at sites of cadherin adhesion, rather than as part of integrin function. Following asymmetric division of the stem cell into a stem cell and an enteroblast (EB), the two cells initially remain connected by adherens junctions, where vinculin is required, only on the EB side, to maintain the EB in a quiescent state and inhibit further divisions of the stem cell. By manipulating cell tension, we show that vinculin recruitment to adherens junction regulates EB activation and numbers. Consequently, removing vinculin results in an enlarged gut with improved resistance to starvation. Thus, mechanical regulation at the contact between stem cells and their progeny is used to control tissue cell number.

Epithelial Cell Polarity During Drosophila Midgut Development

The adult Drosophila midgut epithelium is derived from a group of stem cells called adult midgut precursors (AMPs) that are specified during the migration of the endoderm in early embryogenesis. AMPs are maintained and expanded in AMP nests that lie on the basal side of the larval midgut throughout the larval development. During metamorphosis, the larval midgut undergoes histolysis and programmed cell death, while the central cells in the AMP nests form the future adult midgut and the peripheral cells form the transient pupal midgut. Here we review what is known about how cells polarise in the embryonic, larval, pupal and adult midgut, and discuss the open questions about the mechanisms that control the changes in cell arrangements, cell shape and cell polarity during midgut development.

The Emerging Roles of JNK Signaling in Drosophila Stem Cell Homeostasis

The Jun N-terminal kinase (JNK) pathway is an evolutionary conserved kinase cascade best known for its roles during stress-induced apoptosis and tumor progression. Recent findings, however, have identified new roles for this pleiotropic pathway in stem cells during regenerative responses and in cellular plasticity. Here, we provide an overview of recent findings about the new roles of JNK signaling in stem cell biology using two well-established Drosophila models: the testis and the intestine. We highlight the pathway’s roles in processes such as proliferation, death, self-renewal and reprogramming, and discuss the known parallels between flies and mammals.

Differential effects of commensal bacteria on progenitor cell adhesion, division symmetry and tumorigenesis in the Drosophila intestine

Microbial factors influence homeostatic and oncogenic growth in the intestinal epithelium. However, we know little about immediate effects of commensal bacteria on stem cell division programs. In this study, we examined the effects of commensal Lactobacillus species on homeostatic and tumorigenic stem cell proliferation in the female Drosophila intestine. We identified Lactobacillus brevis as a potent stimulator of stem cell divisions. In a wild-type midgut, L.brevis activates growth regulatory pathways that drive stem cell divisions. In a Notch-deficient background, L.brevis-mediated proliferation causes rapid expansion of mutant progenitors, leading to accumulation of large, multi-layered tumors throughout the midgut. Mechanistically, we showed that L.brevis disrupts expression and subcellular distribution of progenitor cell integrins, supporting symmetric divisions that expand intestinal stem cell populations. Collectively, our data emphasize the impact of commensal microbes on division and maintenance of the intestinal progenitor compartment.

Tissue Adaptation to Environmental Cues by Symmetric and Asymmetric Division Modes of Intestinal Stem Cells

Tissues must adapt to the different external stimuli so that organisms can survive in their environments. The intestine is a vital organ involved in food processing and absorption, as well as in innate immune response. Its adaptation to environmental cues such as diet and biotic/abiotic stress involves regulation of the proliferative rate and a switch of division mode (asymmetric versus symmetric) of intestinal stem cells (ISC). In this review, we outline the current comprehension of the physiological and molecular mechanisms implicated in stem cell division modes in the adult Drosophila midgut. We present the signaling pathways and polarity cues that control the mitotic spindle orientation, which is the terminal determinant ensuring execution of the division mode. We review these events during gut homeostasis, as well as during its response to nutrient availability, bacterial infection, chemical damage, and aging. JNK signaling acts as a central player, being involved in each of these conditions as a direct regulator of spindle orientation. The studies of the mechanisms regulating ISC divisions allow a better understanding of how adult stem cells integrate different signals to control tissue plasticity, and of how various diseases, notably cancers, arise from their alterations.

Drosophila Peptide Hormones Allatostatin A and Diuretic Hormone 31 Exhibiting Complementary Gradient Distribution in Posterior Midgut Antagonistically Regulate Midgut Senescence and Adult Lifespan

Enteroendocrine cells (EEs) are evolutionarily conserved gastrointestinal secretory cells that show scattered distribution in the intestinal epithelium. These cells classified into several subtypes based on the hormones they produce in both mammals and insects. In the fruit fly Drosophila, it has been suggested that nearly equal numbers of two subtypes of EEs (Allatostatin A: AstA and Diuretic hormone 31 : Dh31) are alternately produced from the intestinal stem cells in the posterior midgut. However, we found that these two subtypes are not always present in this manner, but are rather distributed in a complementary frequency gradient along the posterior midgut. We show that midgut-preferential RNA knockdown of the peptide hormones AstA or Dh31 respectively results in decreased or increased adult lifespan. This effect on longevity is apparently correlated with the midgut senescence phenotypes as a result of direct hormone action through both hormone receptors expressed in the enteroblasts or other midgut cell types. However, gut senescence does not appear to be the direct cause for longevity regulation, as knockdown of both hormone receptors did not affect adult lifespan. Furthermore, these senescence phenotypes appear to be independent of insulin signaling and manifest in an organ-specific manner. These results indicate that the two intestinal secretory peptides antagonistically regulate adult lifespan and intestinal senescence through multiple pathways, irrespective of insulin, which implicates a complementary gradient distribution of each of the hormone-producing EEs, consistent with local requirements for cell activity along the posterior midgut.

Organelle aging: Lessons from model organisms

Most cellular processes descend into failure during aging. While a large collection of longevity pathways has been identified in the past decades, the mechanism for age-related decline of cellular homeostasis and organelle function remains largely unsolved. It is known that many organelles undergo structural and functional changes during normal aging, which significantly contributes to the decline of tissue function at old ages. Since recent studies have revealed an emerging role of organelles as regulatory hubs in maintaining cellular homeostasis, understanding of organelle aging will provide important insights into the cellular basis of organismal aging. Here we review current progress on the characterization of age-dependent structural and functional alterations in the more well-studied organelles, as well as the known mechanisms governing organelle aging in model organisms, with a special focus on the fruit fly Drosophila melanogaster.

Designing a Green Fluorogenic Protease Reporter by Flipping a Beta Strand of GFP for Imaging Apoptosis in Animals

A family of proteases called caspases mediate apoptosis signaling in animals. We report a GFP-based fluorogenic protease reporter, dubbed "FlipGFP", by flipping a beta strand of the GFP. Upon protease activation and cleavage, the beta strand is restored, leading to reconstitution of the GFP and fluorescence. FlipGFP-based TEV protease reporter achieves 100-fold fluorescence change. A FlipGFP-based executioner caspase reporter visualized apoptosis in live zebrafish embryos with spatiotemporal resolution. FlipGFP also visualized apoptotic cells in the midgut of Drosophila. Thus, the FlipGFP-based caspase reporter will be useful for monitoring apoptosis during animal development and for designing reporters of proteases beyond caspases. The design strategy can be further applied to a red fluorescent protein for engineering a red fluorogenic protease reporter.

Adult Intestine Aging Model

The Drosophila adult has an intestine composed of a series of differentiated cells and tissue stem cells, all of which are similar to the mammalian intestinal cells. The aged adult intestine shows apparent characteristics such as multilayering of absorptive cells, misexpression of cell type-specific genes, and hyperproliferation of stem cells. Recent studies have revealed various gene networks responsible for progression of these aged phenotypes. The molecular mechanism for senescence of the Drosophila adult midgut and its relation with the corresponding mechanism in mammals are overviewed. In addition, a basic method for observing aged phenotypes of the midgut is described.

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.

Activity-Induced Synaptic Structural Modifications by an Activator of Integrin Signaling at the Drosophila Neuromuscular Junction

Activity-induced synaptic structural modification is crucial for neural development and synaptic plasticity, but the molecular players involved in this process are not well defined. Here, we report that a protein named Shriveled (Shv) regulates synaptic growth and activity-dependent synaptic remodeling at the Drosophila neuromuscular junction. Depletion of Shv causes synaptic overgrowth and an accumulation of immature boutons. We find that Shv physically and genetically interacts with βPS integrin. Furthermore, Shv is secreted during intense, but not mild, neuronal activity to acutely activate integrin signaling, induce synaptic bouton enlargement, and increase postsynaptic glutamate receptor abundance. Consequently, loss of Shv prevents activity-induced synapse maturation and abolishes post-tetanic potentiation, a form of synaptic plasticity. Our data identify Shv as a novel trans-synaptic signal secreted upon intense neuronal activity to promote synapse remodeling through integrin receptor signaling.SIGNIFICANCE STATEMENT The ability of neurons to rapidly modify synaptic structure in response to neuronal activity, a process called activity-induced structural remodeling, is crucial for neuronal development and complex brain functions. The molecular players that are important for this fundamental biological process are not well understood. Here we show that the Shriveled (Shv) protein is required during development to maintain normal synaptic growth. We further demonstrate that Shv is selectively released during intense neuronal activity, but not mild neuronal activity, to acutely activate integrin signaling and trigger structural modifications at the Drosophila neuromuscular junction. This work identifies Shv as a key modulator of activity-induced structural remodeling and suggests that neurons use distinct molecular cues to differentially modulate synaptic growth and remodeling to meet synaptic demand.

miR-263a Regulates ENaC to Maintain Osmotic and Intestinal Stem Cell Homeostasis in Drosophila

Proper regulation of osmotic balance and response to tissue damage is crucial in maintaining intestinal stem cell (ISC) homeostasis. We found that Drosophila miR-263a downregulates the expression of epithelial sodium channel (ENaC) subunits in enterocytes (ECs) to maintain osmotic and ISC homeostasis. In the absence of miR-263a, the intraluminal surface of the intestine displays dehydration-like phenotypes, Na⁺ levels are increased in ECs, stress pathways are activated in ECs, and ISCs overproliferate. Furthermore, miR-263a mutants have increased bacterial load and expression of antimicrobial peptides. Strikingly, these phenotypes are reminiscent of the pathophysiology of cystic fibrosis (CF) in which loss-of-function mutations in the chloride channel CF transmembrane conductance regulator can elevate the activity of ENaC, suggesting that Drosophila could be used as a model for CF. Finally, we provide evidence that overexpression of miR-183, the human ortholog of miR-263a, can also directly target the expressions of all three subunits of human ENaC.

GATAe regulates intestinal stem cell maintenance and differentiation in Drosophila adult midgut

Adult intestinal tissues, exposed to the external environment, play important roles including barrier and nutrient-absorption functions. These functions are ensured by adequately controlled rapid-cell metabolism. GATA transcription factors play essential roles in the development and maintenance of adult intestinal tissues both in vertebrates and invertebrates. We investigated the roles of GATAe, the Drosophila intestinal GATA factor, in adult midgut homeostasis with its first-generated knock-out mutant as well as cell type-specific RNAi and overexpression experiments. Our results indicate that GATAe is essential for proliferation and maintenance of intestinal stem cells (ISCs). Also, GATAe is involved in the differentiation of enterocyte (EC) and enteroendocrine (ee) cells in both Notch (N)-dependent and -independent manner. The results also indicate that GATAe has pivotal roles in maintaining normal epithelial homeostasis of the Drosophila adult midgut through interaction of N signaling. Since recent reports showed that mammalian GATA-6 regulates normal and cancer stem cells in the adult intestinal tract, our data also provide information on the evolutionally conserved roles of GATA factors in stem-cell regulation.

Niche appropriation by Drosophila intestinal stem cell tumours

Mutations that inhibit differentiation in stem cell lineages are a common early step in cancer development, but precisely how a loss of differentiation initiates tumorigenesis is unclear. We investigated Drosophila intestinal stem cell (ISC) tumours generated by suppressing Notch (N) signalling, which blocks differentiation. Notch-defective ISCs require stress-induced divisions for tumour initiation and an autocrine EGFR ligand, Spitz, during early tumour growth. On achieving a critical mass these tumours displace surrounding enterocytes, competing with them for basement membrane space and causing their detachment, extrusion and apoptosis. This loss of epithelial integrity induces JNK and Yki/YAP activity in enterocytes and, consequently, their expression of stress-dependent cytokines (Upd2, Upd3). These paracrine signals, normally used within the stem cell niche to trigger regeneration, propel tumour growth without the need for secondary mutations in growth signalling pathways. The appropriation of niche signalling by differentiation-defective stem cells may be a common mechanism of early tumorigenesis.