Mitochondrial ROS promotes susceptibility to infection via gasdermin D-mediated necroptosis.

Although mutations in mitochondrial-associated genes are linked to inflammation and susceptibility to infection, their mechanistic contributions to immune outcomes remain ill-defined. We discovered that the disease-associated gain-of-function allele Lrrk2G2019S (leucine-rich repeat kinase 2) perturbs mitochondrial homeostasis and reprograms cell death pathways in macrophages. When the inflammasome is activated in Lrrk2G2019S macrophages, elevated mitochondrial ROS (mtROS) directs association of the pore-forming protein gasdermin D (GSDMD) to mitochondrial membranes. Mitochondrial GSDMD pore formation then releases mtROS, promoting a switch to RIPK1/RIPK3/MLKL-dependent necroptosis. Consistent with enhanced necroptosis, infection of Lrrk2G2019S mice with Mycobacterium tuberculosis elicits hyperinflammation and severe immunopathology. Our findings suggest a pivotal role for GSDMD as an executer of multiple cell death pathways and demonstrate that mitochondrial dysfunction can direct immune outcomes via cell death modality switching. This work provides insights into how LRRK2 mutations manifest or exacerbate human diseases and identifies GSDMD-dependent necroptosis as a potential target to limit Lrrk2G2019S-mediated immunopathology.

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.

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.

Integrin-ECM interactions and membrane-associated Catalase cooperate to promote resilience of the Drosophila intestinal epithelium.

Balancing cellular demise and survival constitutes a key feature of resilience mechanisms that underlie the control of epithelial tissue damage. These resilience mechanisms often limit the burden of adaptive cellular stress responses to internal or external threats. We recently identified Diedel, a secreted protein/cytokine, as a potent antagonist of apoptosis-induced regulated cell death in the Drosophila intestinal midgut epithelium during aging. Here, we show that Diedel is a ligand for RGD-binding Integrins and is thus required for maintaining midgut epithelial cell attachment to the extracellular matrix (ECM)-derived basement membrane. Exploiting this function of Diedel, we uncovered a resilience mechanism of epithelial tissues, mediated by Integrin-ECM interactions, which shapes cell death spreading through the regulation of cell detachment and thus cell survival. Moreover, we found that resilient epithelial cells, enriched for Diedel-Integrin-ECM interactions, are characterized by membrane association of Catalase, thus preserving extracellular reactive oxygen species (ROS) balance to maintain epithelial integrity. Intracellular Catalase can relocalize to the extracellular membrane to limit cell death spreading and repair Integrin-ECM interactions induced by the amplification of extracellular ROS, which is a critical adaptive stress response. Membrane-associated Catalase, synergized with Integrin-ECM interactions, likely constitutes a resilience mechanism that helps balance cellular demise and survival within epithelial tissues.

Membrane therapy using DHA suppresses epidermal growth factor receptor signaling by disrupting nanocluster formation.

Epidermal growth factor receptor (EGFR) signaling drives the formation of many types of cancer, including colon cancer. Docosahexaenoic acid (DHA, 22∶6Δ4,7,10,13,16,19), a chemoprotective long-chain n-3 polyunsaturated fatty acid suppresses EGFR signaling. However, the mechanism underlying this phenotype remains unclear. Therefore, we used super-resolution microscopy techniques to investigate the mechanistic link between EGFR function and DHA-induced alterations to plasma membrane nanodomains. Using isogenic in vitro (YAMC and IMCE mouse colonic cell lines) and in vivo (Drosophila, wild type and Fat-1 mice) models, cellular DHA enrichment via therapeutic nanoparticle delivery, endogenous synthesis, or dietary supplementation reduced EGFR-mediated cell proliferation and downstream Ras/ERK signaling. Phospholipid incorporation of DHA reduced membrane rigidity and the size of EGFR nanoclusters. Similarly, pharmacological reduction of plasma membrane phosphatidic acid (PA), phosphatidylinositol-4,5-bisphosphate (PIP2) or cholesterol was associated with a decrease in EGFR nanocluster size. Furthermore, in DHA-treated cells only the addition of cholesterol, unlike PA or PIP2, restored EGFR nanoscale clustering. These findings reveal that DHA reduces EGFR signaling in part by reshaping EGFR proteolipid nanodomains, supporting the feasibility of using membrane therapy, i.e., dietary/drug-related strategies to target plasma membrane organization, to reduce EGFR signaling and cancer risk.

A virus-acquired host cytokine controls systemic aging by antagonizing apoptosis.

Aging is characterized by degeneration of unique tissues. However, dissecting the interconnectedness of tissue aging remains a challenge. Here, we employ a muscle-specific DNA damage model in Drosophila to reveal secreted factors that influence systemic aging in distal tissues. Utilizing this model, we uncovered a cytokine-Diedel-that, when secreted from muscle or adipose, can attenuate age-related intestinal tissue degeneration by promoting proliferative homeostasis of stem cells. Diedel is both necessary and sufficient to limit tissue degeneration and regulate lifespan. Secreted homologs of Diedel are also found in viruses, having been acquired from host genomes. Focusing on potential mechanistic overlap between cellular aging and viral-host cell interactions, we found that Diedel is an inhibitor of apoptosis and can act as a systemic rheostat to modulate cell death during aging. These results highlight a key role for secreted antagonists of apoptosis in the systemic coordination of tissue aging.

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.

Promoting longevity by maintaining metabolic and proliferative homeostasis.

Aging is characterized by a widespread loss of homeostasis in biological systems. An important part of this decline is caused by age-related deregulation of regulatory processes that coordinate cellular responses to changing environmental conditions, maintaining cell and tissue function. Studies in genetically accessible model organisms have made significant progress in elucidating the function of such regulatory processes and the consequences of their deregulation for tissue function and longevity. Here, we review such studies, focusing on the characterization of processes that maintain metabolic and proliferative homeostasis in the fruitfly Drosophila melanogaster. The primary regulatory axis addressed in these studies is the interaction between signaling pathways that govern the response to oxidative stress, and signaling pathways that regulate cellular metabolism and growth. The interaction between these pathways has important consequences for animal physiology, and its deregulation in the aging organism is a major cause for increased mortality. Importantly, protocols to tune such interactions genetically to improve homeostasis and extend lifespan have been established by work in flies. This includes modulation of signaling pathway activity in specific tissues, including adipose tissue and insulin-producing tissues, as well as in specific cell types, such as stem cells of the fly intestine.

Lifespan extension by preserving proliferative homeostasis in Drosophila.

Regenerative processes are critical to maintain tissue homeostasis in high-turnover tissues. At the same time, proliferation of stem and progenitor cells has to be carefully controlled to prevent hyper-proliferative diseases. Mechanisms that ensure this balance, thus promoting proliferative homeostasis, are expected to be critical for longevity in metazoans. The intestinal epithelium of Drosophila provides an accessible model in which to test this prediction. In aging flies, the intestinal epithelium degenerates due to over-proliferation of intestinal stem cells (ISCs) and mis-differentiation of ISC daughter cells, resulting in intestinal dysplasia. Here we show that conditions that impair tissue renewal lead to lifespan shortening, whereas genetic manipulations that improve proliferative homeostasis extend lifespan. These include reduced Insulin/IGF or Jun-N-terminal Kinase (JNK) signaling activities, as well as over-expression of stress-protective genes in somatic stem cell lineages. Interestingly, proliferative activity in aging intestinal epithelia correlates with longevity over a range of genotypes, with maximal lifespan when intestinal proliferation is reduced but not completely inhibited. Our results highlight the importance of the balance between regenerative processes and strategies to prevent hyperproliferative disorders and demonstrate that promoting proliferative homeostasis in aging metazoans is a viable strategy to extend lifespan.

A distinct Acyl-CoA binding protein (ACBP6) shapes tissue plasticity during nutrient adaptation in Drosophila.

Nutrient availability is a major selective force in the evolution of metazoa, and thus plasticity in tissue function and morphology is shaped by adaptive responses to nutrient changes. Utilizing Drosophila, we reveal that distinct calibration of acyl-CoA metabolism, mediated by Acbp6 (Acyl-CoA binding-protein 6), is critical for nutrient-dependent tissue plasticity. Drosophila Acbp6, which arose by evolutionary duplication and binds acyl-CoA to tune acetyl-CoA metabolism, is required for intestinal resizing after nutrient deprivation through activating intestinal stem cell proliferation from quiescence. Disruption of acyl-CoA metabolism by Acbp6 attenuation drives aberrant 'switching' of metabolic networks in intestinal enterocytes during nutrient adaptation, impairing acetyl-CoA metabolism and acetylation amid intestinal resizing. We also identified STAT92e, whose function is influenced by acetyl-CoA levels, as a key regulator of acyl-CoA and nutrient-dependent changes in stem cell activation. These findings define a regulatory mechanism, shaped by acyl-CoA metabolism, that adjusts proliferative homeostasis to coordinately regulate tissue plasticity during nutrient adaptation.

Mutant APC reshapes Wnt signaling plasma membrane nanodomains by altering cholesterol levels via oncogenic ß-catenin.

Although the role of the Wnt pathway in colon carcinogenesis has been described previously, it has been recently demonstrated that Wnt signaling originates from highly dynamic nano-assemblies at the plasma membrane. However, little is known regarding the role of oncogenic APC in reshaping Wnt nanodomains. This is noteworthy, because oncogenic APC does not act autonomously and requires activation of Wnt effectors upstream of APC to drive aberrant Wnt signaling. Here, we demonstrate the role of oncogenic APC in increasing plasma membrane free cholesterol and rigidity, thereby modulating Wnt signaling hubs. This results in an overactivation of Wnt signaling in the colon. Finally, using the Drosophila sterol auxotroph model, we demonstrate the unique ability of exogenous free cholesterol to disrupt plasma membrane homeostasis and drive Wnt signaling in a wildtype APC background. Collectively, these findings provide a link between oncogenic APC, loss of plasma membrane homeostasis and CRC development.

Glutamate metabolism directs energetic trade-offs to shape host-pathogen susceptibility in Drosophila.

Individual hosts within populations often show inter-individual variation in their susceptibility to bacterial pathogen-related diseases. Utilizing Drosophila, we highlight that phenotypic variation in host-pathogen susceptibility within populations is driven by energetic trade-offs, facilitated by infection-mediated changes in glutamate metabolism. Furthermore, host-pathogen susceptibility is conditioned by life history, which adjusts immunometabolic sensing in muscles to direct vitamin-dependent reallocation of host energy substrates from the adipose tissue (i.e., a muscle-adipose tissue axis). Life history conditions inter-individual variation in the activation strength of intra-muscular NF-κB signaling. Limited intra-muscular NF-κB signaling activity allows for enhanced infection-mediated mitochondrial biogenesis and function, which stimulates glutamate dehydrogenase-dependent synthesis of glutamate. Muscle-derived glutamate acts as a systemic metabolite to promote lipid mobilization through modulating vitamin B enzymatic cofactor transport and function in the adipose tissue. This energy substrate reallocation improves pathogen clearance and boosts host survival. Finally, life history events that adjust energetic trade-offs can shape inter-individual variation in host-pathogen susceptibility after infection.

Insulin and JNK: optimizing metabolic homeostasis and lifespan.

Metabolic adaptation to environmental changes is crucial for the long-term survival of an organism. Signaling mechanisms that govern this adaptation thus influence lifespan. One such mechanism is the insulin/insulin-like growth factor signaling (IIS) pathway, a central regulator of metabolism in metazoans. Recent studies have identified the stress-responsive Jun-N-terminal kinase (JNK) pathway as a regulator of IIS signaling, providing a link between environmental challenges and metabolic regulation. JNK inhibits IIS activity and, thus, promotes lifespan extension and stress tolerance. Interestingly, this interaction is also at the center of age-related metabolic diseases. Here, we review recent advances illuminating the mechanisms of the JNK-IIS interaction and its implications for metabolic diseases and lifespan in metazoans.

The Drosophila midgut and the systemic coordination of lipid-dependent energy homeostasis.

The evolution of complex organ systems in metazoans has dictated that the maintenance of energy homeostasis requires coordinating local and systemic energy demands between organs with specialized functions. The gastrointestinal tract is one of many organs that is indispensable for the systemic coordination of energy substrate uptake, storage, and usage, and the spatial organization of this organ (i.e. proximity to other metabolic organs) within a complex body plan underlies its role in organ crosstalk. Studies of various arthropod intestines, and in particular insects, have shed light on the evolution and function of the gastrointestinal tract in the maintenance of energy homeostasis. This brief review focuses on studies and theories derived from the insect intestine (particularly the midgut) of adult Drosophila melanogaster to inform on the how, what, and why of the gastrointestinal tract in the systemic regulation of lipids, the most common form of stored energy in insects.

Diet-MEF2 interactions shape lipid droplet diversification in muscle to influence Drosophila lifespan.

The number, size, and composition of lipid droplets can be influenced by dietary changes that shift energy substrate availability. This diversification of lipid droplets can promote metabolic flexibility and shape cellular stress responses in unique tissues with distinctive metabolic roles. Using Drosophila, we uncovered a role for myocyte enhancer factor 2 (MEF2) in modulating diet-dependent lipid droplet diversification within adult striated muscle, impacting mortality rates. Muscle-specific attenuation of MEF2, whose chronic activation maintains glucose and mitochondrial homeostasis, leads to the accumulation of large, cholesterol ester-enriched intramuscular lipid droplets in response to high calorie, carbohydrate-sufficient diets. The diet-dependent accumulation of these lipid droplets also correlates with both enhanced stress protection in muscle and increases in organismal lifespan. Furthermore, MEF2 attenuation releases an antagonistic regulation of cell cycle gene expression programs, and up-regulation of Cyclin E is required for diet- and MEF2-dependent diversification of intramuscular lipid droplets. The integration of MEF2-regulated gene expression networks with dietary responses thus plays a critical role in shaping muscle metabolism and function, further influencing organismal lifespan. Together, these results highlight a potential protective role for intramuscular lipid droplets during dietary adaptation.

Dietary Adaptation of Microbiota in Drosophila Requires NF-κB-Dependent Control of the Translational Regulator 4E-BP.

Dietary nutrients shape complex interactions between hosts and their commensal gut bacteria, further promoting flexibility in host-microbiota associations that can drive nutritional symbiosis. However, it remains less clear if diet-dependent host signaling mechanisms also influence these associations. Using Drosophila, we show here that nuclear factor κB (NF-κB)/Relish, an innate immune transcription factor emerging as a signaling node linking nutrient-immune-metabolic interactions, is vital to adapt gut microbiota species composition to host diet macronutrient composition. We find that Relish is required within midgut enterocytes to amplify host-Lactobacillus associations, an important bacterial mediator of nutritional symbiosis, and thus modulate microbiota composition in response to dietary adaptation. Relish limits diet-dependent transcriptional inducibility of the cap-dependent translation inhibitor 4E-BP/Thor to control microbiota composition. Furthermore, maintaining cap-dependent translation in response to dietary adaptation is critical to amplify host-Lactobacillus associations. These results highlight that NF-κB-dependent host signaling mechanisms, in coordination with host translation control, shape diet-microbiota interactions.

NF-κB Shapes Metabolic Adaptation by Attenuating Foxo-Mediated Lipolysis in Drosophila.

Metabolic and innate immune signaling pathways have co-evolved to elicit coordinated responses. However, dissecting the integration of these ancient signaling mechanisms remains a challenge. Using Drosophila, we uncovered a role for the innate immune transcription factor nuclear factor κB (NF-κB)/Relish in governing lipid metabolism during metabolic adaptation to fasting. We found that Relish is required to restrain fasting-induced lipolysis, and thus conserve cellular triglyceride levels during metabolic adaptation, through specific repression of ATGL/Brummer lipase gene expression in adipose (fat body). Fasting-induced changes in Brummer expression and, consequently, triglyceride metabolism are adjusted by Relish-dependent attenuation of Foxo transcriptional activation function, a critical metabolic transcription factor. Relish limits Foxo function by influencing fasting-dependent histone deacetylation and subsequent chromatin modifications within the Bmm locus. These results highlight that the antagonism of Relish and Foxo functions are crucial in the regulation of lipid metabolism during metabolic adaptation, which may further influence the coordination of innate immune-metabolic responses.

Long-Chain n-3 Fatty Acids Attenuate Oncogenic KRas-Driven Proliferation by Altering Plasma Membrane Nanoscale Proteolipid Composition.

Ras signaling originates from transient nanoscale compartmentalized regions of the plasma membrane composed of specific proteins and lipids. The highly specific lipid composition of these nanodomains, termed nanoclusters, facilitates effector recruitment and therefore influences signal transduction. This suggests that Ras nanocluster proteolipid composition could represent a novel target for future chemoprevention interventions. There is evidence that consumption of fish oil containing long-chain n-3 polyunsaturated fatty acids (n-3 PUFA) such as eicosapentaenoic acid (EPA, 20:5Δ5,8,11,14,17) and docosahexaenoic acid (DHA, 22:6Δ4,7,10,13,16,19) may reduce colon cancer risk in humans, yet the mechanism underlying this effect is unknown. Here, we demonstrate that dietary n-3 PUFA reduce the lateral segregation of cholesterol-dependent and -independent nanoclusters, suppressing phosphatidic acid-dependent oncogenic KRas effector interactions, via their physical incorporation into plasma membrane phospholipids. This results in attenuation of oncogenic Ras-driven colonic hyperproliferation in both Drosophila and murine models. These findings demonstrate the unique properties of dietary n-3 PUFA in the shaping of Ras nanoscale proteolipid complexes and support the emerging role of plasma membrane-targeted therapies.Significance: The influence of dietary long chain n-3 polyunsaturated fatty acids on plasma membrane protein nanoscale organization and KRas signaling supports development of plasma membrane-targeted therapies in colon cancer.Graphical Abstract: http://cancerres.aacrjournals.org/content/canres/78/14/3899/F1.large.jpg Cancer Res; 78(14); 3899-912. ©2018 AACR.