PGRP-SC2 promotes gut immune homeostasis to limit commensal dysbiosis and extend lifespan.

Interactions between commensals and the host impact the metabolic and immune status of metazoans. Their deregulation is associated with age-related pathologies like chronic inflammation and cancer, especially in barrier epithelia. Maintaining a healthy commensal population by preserving innate immune homeostasis in such epithelia thus promises to promote health and longevity. Here, we show that, in the aging intestine of Drosophila, chronic activation of the transcription factor Foxo reduces expression of peptidoglycan recognition protein SC2 (PGRP-SC2), a negative regulator of IMD/Relish innate immune signaling, and homolog of the anti-inflammatory molecules PGLYRP1-4. This repression causes deregulation of Rel/NFkB activity, resulting in commensal dysbiosis, stem cell hyperproliferation, and epithelial dysplasia. Restoring PGRP-SC2 expression in enterocytes of the intestinal epithelium, in turn, prevents dysbiosis, promotes tissue homeostasis, and extends lifespan. Our results highlight the importance of commensal control for lifespan of metazoans and identify SC-class PGRPs as longevity-promoting factors.

Aging: seeking mitonuclear balance.

Metabolic imbalances accompany the aging process in many organisms, and signaling mechanisms that allay or prevent these imbalances can extend lifespan. Two recent studies by Auwerx and colleagues, including one in this issue, identify a conserved signaling network centered on mitochondrial stress responses that promotes longevity in response to changes in mitochondrial translation and NAD(+) metabolism.

Gut cytokines modulate olfaction through metabolic reprogramming of glia.

Infection-induced aversion against enteropathogens is a conserved sickness behaviour that can promote host survival1,2. The aetiology of this behaviour remains poorly understood, but studies in Drosophila have linked olfactory and gustatory perception to avoidance behaviours against toxic microorganisms3-5. Whether and how enteric infections directly influence sensory perception to induce or modulate such behaviours remains unknown. Here we show that enteropathogen infection in Drosophila can modulate olfaction through metabolic reprogramming of ensheathing glia of the antennal lobe. Infection-induced unpaired cytokine expression in the intestine activates JAK-STAT signalling in ensheathing glia, inducing the expression of glial monocarboxylate transporters and the apolipoprotein glial lazarillo (GLaz), and affecting metabolic coupling of glia and neurons at the antennal lobe. This modulates olfactory discrimination, promotes the avoidance of bacteria-laced food and increases fly survival. Although transient in young flies, gut-induced metabolic reprogramming of ensheathing glia becomes constitutive in old flies owing to age-related intestinal inflammation, which contributes to an age-related decline in olfactory discrimination. Our findings identify adaptive glial metabolic reprogramming by gut-derived cytokines as a mechanism that causes lasting changes in a sensory system in ageing flies.

Metabolic homeostasis: HDACs take center stage.

Hormonal regulation of glucose and lipid metabolism is pivotal for metabolic homeostasis and energy balance. Two studies in this issue of Cell (Mihaylova et al., 2011 and Wang et al., 2011) introduce a new conserved signaling mechanism controlling catabolic gene expression: class IIa histone deacetylases (HDACs) regulate Foxo activity in Drosophila and mice.

Host autophagy mediates organ wasting and nutrient mobilization for tumor growth.

During tumor growth-when nutrient and anabolic demands are high-autophagy supports tumor metabolism and growth through lysosomal organelle turnover and nutrient recycling. Ras-driven tumors additionally invoke non-autonomous autophagy in the microenvironment to support tumor growth, in part through transfer of amino acids. Here we uncover a third critical role of autophagy in mediating systemic organ wasting and nutrient mobilization for tumor growth using a well-characterized malignant tumor model in Drosophila melanogaster. Micro-computed X-ray tomography and metabolic profiling reveal that RasV12 ; scrib-/- tumors grow 10-fold in volume, while systemic organ wasting unfolds with progressive muscle atrophy, loss of body mass, -motility, -feeding, and eventually death. Tissue wasting is found to be mediated by autophagy and results in host mobilization of amino acids and sugars into circulation. Natural abundance Carbon 13 tracing demonstrates that tumor biomass is increasingly derived from host tissues as a nutrient source as wasting progresses. We conclude that host autophagy mediates organ wasting and nutrient mobilization that is utilized for tumor growth.

Intestinal Stem Cell Aging: Origins and Interventions.

Regenerative processes that maintain the function of the gastrointestinal (GI) epithelium are critical for health and survival of multicellular organisms. In insects and vertebrates, intestinal stem cells (ISCs) regenerate the GI epithelium. ISC function is regulated by intrinsic, local, and systemic stimuli to adjust regeneration to tissue demands. These control mechanisms decline with age, resulting in significant perturbation of intestinal homeostasis. Processes that lead to this decline have been explored intensively in Drosophila melanogaster in recent years and are now starting to be characterized in mammalian models. This review presents a model for age-related regenerative decline in the fly intestine and discusses recent findings that start to establish molecular mechanisms of age-related decline of mammalian ISC function.

SKNy worms and long life.

Interactions between insulin signaling and stress-response pathways can markedly impact life span. In this issue, Tullet et al. (2008) demonstrate that the worm homolog of Nrf2, called SKN-1, a transcription factor that switches on expression of antioxidant genes, is an important component of such signaling interactions.

Signal integration by Ca(2+) regulates intestinal stem-cell activity.

Somatic stem cells maintain tissue homeostasis by dynamically adjusting proliferation and differentiation in response to stress and metabolic cues. Here we identify Ca(2+) signalling as a central regulator of intestinal stem cell (ISC) activity in Drosophila. We show that dietary L-glutamate stimulates ISC division and gut growth. The metabotropic glutamate receptor (mGluR) is required in ISCs for this response, and for an associated modulation of cytosolic Ca(2+) oscillations that results in sustained high cytosolic Ca(2+) concentrations. High cytosolic Ca(2+) concentrations induce ISC proliferation by regulating Calcineurin and CREB-regulated transcriptional co-activator (Crtc). In response to a wide range of dietary and stress stimuli, ISCs reversibly transition between Ca(2+) oscillation states that represent poised or activated modes of proliferation, respectively. We propose that the dynamic regulation of intracellular Ca(2+) levels allows effective integration of diverse mitogenic signals in ISCs to adapt their proliferative activity to the needs of the tissue.

Mutations of mitochondrial DNA are not major contributors to aging of fruit flies.

Mammals develop age-associated clonal expansion of somatic mtDNA mutations resulting in severe respiratory chain deficiency in a subset of cells in a variety of tissues. Both mathematical modeling based on descriptive data from humans and experimental data from mtDNA mutator mice suggest that the somatic mutations are formed early in life and then undergo mitotic segregation during adult life to reach very high levels in certain cells. To address whether mtDNA mutations have a universal effect on aging metazoans, we investigated their role in physiology and aging of fruit flies. To this end, we utilized genetically engineered flies expressing mutant versions of the catalytic subunit of mitochondrial DNA polymerase (DmPOLγA) as a means to introduce mtDNA mutations. We report here that lifespan and health in fruit flies are remarkably tolerant to mtDNA mutations. Our results show that the short lifespan and wide genetic bottleneck of fruit flies are limiting the extent of clonal expansion of mtDNA mutations both in individuals and between generations. However, an increase of mtDNA mutations to very high levels caused sensitivity to mechanical and starvation stress, intestinal stem cell dysfunction, and reduced lifespan under standard conditions. In addition, the effects of dietary restriction, widely considered beneficial for organismal health, were attenuated in flies with very high levels of mtDNA mutations.

Dietary restriction improves intestinal cellular fitness to enhance gut barrier function and lifespan in D. melanogaster.

Loss of gut integrity is linked to various human diseases including inflammatory bowel disease. However, the mechanisms that lead to loss of barrier function remain poorly understood. Using D. melanogaster, we demonstrate that dietary restriction (DR) slows the age-related decline in intestinal integrity by enhancing enterocyte cellular fitness through up-regulation of dMyc in the intestinal epithelium. Reduction of dMyc in enterocytes induced cell death, which leads to increased gut permeability and reduced lifespan upon DR. Genetic mosaic and epistasis analyses suggest that cell competition, whereby neighboring cells eliminate unfit cells by apoptosis, mediates cell death in enterocytes with reduced levels of dMyc. We observed that enterocyte apoptosis was necessary for the increased gut permeability and shortened lifespan upon loss of dMyc. Furthermore, moderate activation of dMyc in the post-mitotic enteroblasts and enterocytes was sufficient to extend health-span on rich nutrient diets. We propose that dMyc acts as a barometer of enterocyte cell fitness impacting intestinal barrier function in response to changes in diet and age.

Tis11 mediated mRNA decay promotes the reacquisition of Drosophila intestinal stem cell quiescence.

Adult stem cell proliferation rates are precisely regulated to maintain long-term tissue homeostasis. Defects in the mechanisms controlling stem cell proliferation result in impaired regeneration and hyperproliferative diseases. Many stem cell populations increase proliferation in response to tissue damage and reacquire basal proliferation rates after tissue repair is completed. Although proliferative signals have been extensively studied, much less is known about the molecular mechanisms that restore stem cell quiescence. Here we show that Tis11, an Adenine-uridine Rich Element (ARE) binding protein that promotes mRNA degradation, is required to re-establish basal proliferation rates of adult Drosophila intestinal stem cells (ISC) after a regenerative episode. We find that Tis11 limits ISC proliferation specifically after proliferation has been stimulated in response to heat stress or infection, and show that Tis11 expression and activity are increased in ISCs during tissue repair. Based on stem cell transcriptome analysis and RNA immunoprecipitation, we propose that Tis11 activation represents an integral part of a negative feedback mechanism that limits the expression of key components of several signaling pathways that control ISC function and proliferation. Our results identify Tis11 mediated mRNA decay as an evolutionarily conserved mechanism of re-establishing basal proliferation rates of stem cells in regenerating tissues.

PERK Limits Drosophila Lifespan by Promoting Intestinal Stem Cell Proliferation in Response to ER Stress.

Intestinal homeostasis requires precise control of intestinal stem cell (ISC) proliferation. In Drosophila, this control declines with age largely due to chronic activation of stress signaling and associated chronic inflammatory conditions. An important contributor to this condition is the age-associated increase in endoplasmic reticulum (ER) stress. Here we show that the PKR-like ER kinase (PERK) integrates both cell-autonomous and non-autonomous ER stress stimuli to induce ISC proliferation. In addition to responding to cell-intrinsic ER stress, PERK is also specifically activated in ISCs by JAK/Stat signaling in response to ER stress in neighboring cells. The activation of PERK is required for homeostatic regeneration, as well as for acute regenerative responses, yet the chronic engagement of this response becomes deleterious in aging flies. Accordingly, knocking down PERK in ISCs is sufficient to promote intestinal homeostasis and extend lifespan. Our studies highlight the significance of the PERK branch of the unfolded protein response of the ER (UPRER) in intestinal homeostasis and provide a viable strategy to improve organismal health- and lifespan.

Ubx dynamically regulates Dpp signaling by repressing Dad expression during copper cell regeneration in the adult Drosophila midgut.

The gastrointestinal (GI) tract of metazoans is lined by a series of regionally distinct epithelia. To maintain structure and function of the GI tract, regionally diversified differentiation of somatic stem cell (SC) lineages is critical. The adult Drosophila midgut provides an accessible model to study SC regulation and specification in a regionally defined manner. SCs of the posterior midgut (PM) have been studied extensively, but the control of SCs in the middle midgut (MM) is less well understood. The MM contains a stomach-like copper cell region (CCR) that is regenerated by gastric stem cells (GSSCs) and contains acid-secreting copper cells (CCs). Bmp-like Decapentaplegic (Dpp) signaling determines the identity of GSSCs, and is required for CC regeneration, yet the precise control of Dpp signaling activity in this lineage remains to be fully established. Here, we show that Dad, a negative feedback regulator of Dpp signaling, is dynamically regulated in the GSSC lineage to allow CC differentiation. Dad is highly expressed in GSSCs and their first daughter cells, the gastroblasts (GBs), but has to be repressed in differentiating CCs to allow Dpp-mediated differentiation into CCs. We find that the Hox gene ultrabithorax (Ubx) is required for this regulation. Loss of Ubx prevents Dad repression in the CCR, resulting in defective CC regeneration. Our study highlights the need for dynamic control of Dpp signaling activity in the differentiation of the GSSC lineage and identifies Ubx as a critical regulator of this process.

Control of apoptosis by Drosophila DCAF12.

Regulated Apoptosis (Programmed Cell Death, PCD) maintains tissue homeostasis in adults, and ensures proper growth and morphogenesis of tissues during development of metazoans. Accordingly, defects in cellular processes triggering or executing apoptotic programs have been implicated in a variety of degenerative and neoplastic diseases. Here, we report the identification of DCAF12, an evolutionary conserved member of the WD40-motif repeat family of proteins, as a new regulator of apoptosis in Drosophila. We find that DCAF12 is required for Diap1 cleavage in response to pro-apoptotic signals, and is thus necessary and sufficient for RHG (Reaper, Hid, and Grim)-mediated apoptosis. Loss of DCAF12 perturbs the elimination of supernumerary or proliferation-impaired cells during development, and enhances tumor growth induced by loss of neoplastic tumor suppressors, highlighting the wide requirement for DCAF12 in PCD.

Integration of UPRER and oxidative stress signaling in the control of intestinal stem cell proliferation.

The Unfolded Protein Response of the endoplasmic reticulum (UPRER) controls proteostasis by adjusting the protein folding capacity of the ER to environmental and cell-intrinsic conditions. In metazoans, loss of proteostasis results in degenerative and proliferative diseases and cancers. The cellular and molecular mechanisms causing these phenotypes remain poorly understood. Here we show that the UPRER is a critical regulator of intestinal stem cell (ISC) quiescence in Drosophila melanogaster. We find that ISCs require activation of the UPRER for regenerative responses, but that a tissue-wide increase in ER stress triggers ISC hyperproliferation and epithelial dysplasia in aging animals. These effects are mediated by ISC-specific redox signaling through Jun-N-terminal Kinase (JNK) and the transcription factor CncC. Our results identify a signaling network of proteostatic and oxidative stress responses that regulates ISC function and regenerative homeostasis in the intestinal epithelium.

Control of metabolic adaptation to fasting by dILP6-induced insulin signaling in Drosophila oenocytes.

Metabolic adaptation to changing dietary conditions is critical to maintain homeostasis of the internal milieu. In metazoans, this adaptation is achieved by a combination of tissue-autonomous metabolic adjustments and endocrine signals that coordinate the mobilization, turnover, and storage of nutrients across tissues. To understand metabolic adaptation comprehensively, detailed insight into these tissue interactions is necessary. Here we characterize the tissue-specific response to fasting in adult flies and identify an endocrine interaction between the fat body and liver-like oenocytes that regulates the mobilization of lipid stores. Using tissue-specific expression profiling, we confirm that oenocytes in adult flies play a central role in the metabolic adaptation to fasting. Furthermore, we find that fat body-derived Drosophila insulin-like peptide 6 (dILP6) induces lipid uptake in oenocytes, promoting lipid turnover during fasting and increasing starvation tolerance of the animal. Selective activation of insulin/IGF signaling in oenocytes by a fat body-derived peptide represents a previously unidentified regulatory principle in the control of metabolic adaptation and starvation tolerance.

Studying aging in Drosophila.

Drosophila melanogaster represents one of the most important genetically accessible model organisms for aging research. Studies in flies have identified single gene mutations that influence lifespan and have characterized endocrine signaling interactions that control homeostasis systemically. Recent studies have focused on the effects of aging on specific tissues and physiological processes, providing a comprehensive picture of age-related tissue dysfunction and the loss of systemic homeostasis. Here we review methodological aspects of this work and highlight technical considerations when using Drosophila to study aging and age-related diseases.

Notch-mediated suppression of TSC2 expression regulates cell differentiation in the Drosophila intestinal stem cell lineage.

Epithelial homeostasis in the posterior midgut of Drosophila is maintained by multipotent intestinal stem cells (ISCs). ISCs self-renew and produce enteroblasts (EBs) that differentiate into either enterocytes (ECs) or enteroendocrine cells (EEs) in response to differential Notch (N) activation. Various environmental and growth signals dynamically regulate ISC activity, but their integration with differentiation cues in the ISC lineage remains unclear. Here we identify Notch-mediated repression of Tuberous Sclerosis Complex 2 (TSC2) in EBs as a required step in the commitment of EBs into the EC fate. The TSC1/2 complex inhibits TOR signaling, acting as a tumor suppressor in vertebrates and regulating cell growth. We find that TSC2 is expressed highly in ISCs, where it maintains stem cell identity, and that N-mediated repression of TSC2 in EBs is required and sufficient to promote EC differentiation. Regulation of TSC/TOR activity by N signaling thus emerges as critical for maintenance and differentiation in somatic stem cell lineages.

Exploring the physiology and pathology of aging in the intestine of Drosophila melanogaster.

The gastrointestinal tract, due to its role as a digestive organ and as a barrier between the exterior and interior milieus, is critically impacted by dietary, environmental, and inflammatory conditions that influence health and lifespan. Work in flies is now uncovering the multifaceted molecular mechanisms that control homeostasis in this tissue, and establishing its central role in health and lifespan of metazoans. The Drosophila intestine has thus emerged as a productive, genetically accessible model to study various aspects of the pathophysiology of aging. Studies in flies have characterized the maintenance of regenerative homeostasis, the development of immune senescence, the loss of epithelial barrier function, the decline in metabolic homeostasis, as well as the maintenance of epithelial diversity in this tissue. Due to its fundamental similarity to vertebrate intestines, it can be anticipated that findings obtained in this system will have important implications for our understanding of age-related changes in the human intestine. Here, I review recent studies exploring age-related changes in the fly intestine, and their insight into the regulation of health and lifespan of the animal.

Schnurri regulates hemocyte function to promote tissue recovery after DNA damage.

Tissue recovery after injury requires coordinated regulation of cell repair and apoptosis, removal of dead cells and regeneration. A critical step in this process is the recruitment of blood cells that mediate local inflammatory and immune responses, promoting tissue recovery. Here we identify a new role for the transcriptional regulator Schnurri (Shn) in the recovery of UV-damaged Drosophila retina. Using an experimental paradigm that allows precise quantification of tissue recovery after a defined dose of UV, we find that Shn activity in the retina is required to limit tissue damage. This function of Shn relies on its transcriptional induction of the PDGF-related growth factor Pvf1, which signals to tissue-associated hemocytes. We show that the Pvf1 receptor PVR acts in hemocytes to induce a macrophage-like morphology and that this is required to limit tissue loss after irradiation. Our results identify a new Shn-regulated paracrine signaling interaction between damaged retinal cells and hemocytes that ensures recovery and homeostasis of the challenged tissue.