Marc Claret

Ribosomal Protein S6 Kinase 1 Signaling Regulates Mammalian Life Span

Mimicking Caloric Restriction The extended life span and resistance to age-related diseases in animals exposed to caloric restriction has focused attention on the biochemical mechanisms that produce these effects. Selman et al. (p. 140; see the Perspective by Kaeberlein and Kapahi) explored the role of the mammalian ribosomal protein S6 kinase 1 (S6K1), which regulates protein translation and cellular energy metabolism. Female knockout mice lacking expression of S6K1 showed characteristics of animals exposed to caloric restriction, including improved health and increased longevity. The beneficial effects included reduced fat mass in spite of increased food intake. Thus, inhibition of signaling pathways activated by S6K1 might prove beneficial in protecting against age-related disease. A signaling pathway in mice mediates the effects of caloric restriction that protect against age-related diseases. Caloric restriction (CR) protects against aging and disease, but the mechanisms by which this affects mammalian life span are unclear. We show in mice that deletion of ribosomal S6 protein kinase 1 (S6K1), a component of the nutrient-responsive mTOR (mammalian target of rapamycin) signaling pathway, led to increased life span and resistance to age-related pathologies, such as bone, immune, and motor dysfunction and loss of insulin sensitivity. Deletion of S6K1 induced gene expression patterns similar to those seen in CR or with pharmacological activation of adenosine monophosphate (AMP)–activated protein kinase (AMPK), a conserved regulator of the metabolic response to CR. Our results demonstrate that S6K1 influences healthy mammalian life-span and suggest that therapeutic manipulation of S6K1 and AMPK might mimic CR and could provide broad protection against diseases of aging.

Evidence for lifespan extension and delayed age-related biomarkers in insulin receptor substrate 1 null mice

Recent evidence suggests that alterations in insulin/insulin–like growth factor 1 (IGF1) signaling (IIS) can increase mammalian life span. For example, in several mouse mutants, impairment of the growth hormone (GH)/IGF1 axis increases life span and also insulin sensitivity. However, the intracellular signaling route to altered mammalian aging remains unclear. We therefore measured the life span of mice lacking either insulin receptor substrate (IRS) 1 or 2, the major intracellular effectors of the IIS receptors. Our provisional results indicate that female Irs1–/– mice are long–lived. Furthermore, they displayed resistance to a range of age–sensitive markers of aging including skin, bone, immune, and motor dysfunction. These improvements in health were seen despite mild, lifelong insulin resistance. Thus, enhanced insulin sensitivity is not a prerequisite for IIS mutant longevity. Irs1–/– female mice also displayed normal anterior pituitary function, distinguishing them from long–lived somatotrophic axis mutants. In contrast, Irs2–/– mice were short–lived, whereas Irs1–/– and Irs2+/– mice of both sexes showed normal life spans. Our results therefore suggest that IRS1 signaling is an evolutionarily conserved pathway regulating mammalian life span and may be a point of intervention for therapies with the potential to delay age–related processes.—Selman, C., Lingard, S., Choudhury, A. I., Batterham, A. L., Claret, M., Clements, M., Ramadani, F., Okkenhaug, K., Schuster, E., Blanc, E., Piper, M. D., Al‐Qassab, H., Speakman, J. R., Carmignac, D., Robinson, I. C. A., Thornton, J. M., Gems, D., Partridge, L., Withers, D. J. Evidence for lifespan extension and delayed age‐related biomarkers in insulin receptor substrate 1 null mice. FASEB J. 22, 807–818 (2008)

AMPK is essential for energy homeostasis regulation and glucose sensing by POMC and AgRP neurons

Hypothalamic AMP-activated protein kinase (AMPK) has been suggested to act as a key sensing mechanism, responding to hormones and nutrients in the regulation of energy homeostasis. However, the precise neuronal populations and cellular mechanisms involved are unclear. The effects of long-term manipulation of hypothalamic AMPK on energy balance are also unknown. To directly address such issues, we generated POMC alpha 2KO and AgRP alpha 2KO mice lacking AMPK alpha2 in proopiomelanocortin- (POMC-) and agouti-related protein-expressing (AgRP-expressing) neurons, key regulators of energy homeostasis. POMC alpha 2KO mice developed obesity due to reduced energy expenditure and dysregulated food intake but remained sensitive to leptin. In contrast, AgRP alpha 2KO mice developed an age-dependent lean phenotype with increased sensitivity to a melanocortin agonist. Electrophysiological studies in AMPK alpha2-deficient POMC or AgRP neurons revealed normal leptin or insulin action but absent responses to alterations in extracellular glucose levels, showing that glucose-sensing signaling mechanisms in these neurons are distinct from those pathways utilized by leptin or insulin. Taken together with the divergent phenotypes of POMC alpha 2KO and AgRP alpha 2KO mice, our findings suggest that while AMPK plays a key role in hypothalamic function, it does not act as a general sensor and integrator of energy homeostasis in the mediobasal hypothalamus.

Critical role for the p110α phosphoinositide-3-OH kinase in growth and metabolic regulation

The eight catalytic subunits of the mammalian phosphoinositide-3-OH kinase (PI(3)K) family form the backbone of an evolutionarily conserved signalling pathway; however, the roles of most PI(3)K isoforms in organismal physiology and disease are unknown. To delineate the role of p110α, a ubiquitously expressed PI(3)K involved in tyrosine kinase and Ras signalling, here we generated mice carrying a knockin mutation (D933A) that abrogates p110α kinase activity. Homozygosity for this kinase-dead p110α led to embryonic lethality. Mice heterozygous for this mutation were viable and fertile, but displayed severely blunted signalling via insulin-receptor substrate (IRS) proteins, key mediators of insulin, insulin-like growth factor-1 and leptin action. Defective responsiveness to these hormones led to reduced somatic growth, hyperinsulinaemia, glucose intolerance, hyperphagia and increased adiposity in mice heterozygous for the D933A mutation. This signalling function of p110α derives from its highly selective recruitment and activation to IRS signalling complexes compared to p110β, the other broadly expressed PI(3)K isoform, which did not contribute to IRS-associated PI(3)K activity. p110α was the principal IRS-associated PI(3)K in cancer cell lines. These findings demonstrate a critical role for p110α in growth factor and metabolic signalling and also suggest an explanation for selective mutation or overexpression of p110α in a variety of cancers.

The role of insulin receptor substrate 2 in hypothalamic and β cell function

Insulin receptor substrate 2 (Irs2) plays complex roles in energy homeostasis. We generated mice lacking Irs2 in β cells and a population of hypothalamic neurons (RIPCreIrs2KO), in all neurons (NesCreIrs2KO), and in proopiomelanocortin neurons (POMCCreIrs2KO) to determine the role of Irs2 in the CNS and β cell. RIPCreIrs2KO mice displayed impaired glucose tolerance and reduced β cell mass. Overt diabetes did not ensue, because β cells escaping Cre-mediated recombination progressively populated islets. RIPCreIrs2KO and NesCreIrs2KO mice displayed hyperphagia, obesity, and increased body length, which suggests altered melanocortin action. POMCCreIrs2KO mice did not display this phenotype. RIPCreIrs2KO and NesCreIrs2KO mice retained leptin sensitivity, which suggests that CNS Irs2 pathways are not required for leptin action. NesCreIrs2KO and POMCCreIrs2KO mice did not display reduced β cell mass, but NesCreIrs2KO mice displayed mild abnormalities of glucose homeostasis. RIPCre neurons did not express POMC or neuropeptide Y. Insulin and a melanocortin agonist depolarized RIPCre neurons, whereas leptin was ineffective. Insulin hyperpolarized and leptin depolarized POMC neurons. Our findings demonstrate a critical role for IRS2 in β cell and hypothalamic function and provide insights into the role of RIPCre neurons, a distinct hypothalamic neuronal population, in growth and energy homeostasis.

Akkermansia muciniphila inversely correlates with the onset of inflammation, altered adipose tissue metabolism and metabolic disorders during obesity in mice

Recent evidence indicates that the gut microbiota plays a key role in the pathophysiology of obesity. Indeed, diet-induced obesity (DIO) has been associated to substantial changes in gut microbiota composition in rodent models. In the context of obesity, enhanced adiposity is accompanied by low-grade inflammation of this tissue but the exact link with gut microbial community remains unknown. In this report, we studied the consequences of high-fat diet (HFD) administration on metabolic parameters and gut microbiota composition over different periods of time. We found that Akkermansia muciniphila abundance was strongly and negatively affected by age and HFD feeding and to a lower extend Bilophila wadsworthia was the only taxa following an opposite trend. Different approaches, including multifactorial analysis, showed that these changes in Akkermansia muciniphila were robustly correlated with the expression of lipid metabolism and inflammation markers in adipose tissue, as well as several circulating parameters (i.e., glucose, insulin, triglycerides, leptin) from DIO mice. Thus, our data shows the existence of a link between gut Akkermansia muciniphila abundance and adipose tissue homeostasis on the onset of obesity, thus reinforcing the beneficial role of this bacterium on metabolism.

Hypothalamic and brainstem neuronal circuits controlling homeostatic energy balance.

Alterations in adequate energy balance maintenance result in serious metabolic disturbances such as obesity. In mammals, this complex process is orchestrated by multiple and distributed neuronal circuits. Hypothalamic and brainstem neuronal circuits are critically involved in the sensing of circulating and local factors conveying information about the energy status of the organism. The integration of these signals culminates in the generation of specific and coordinated physiological responses aimed at regulating energy balance through the modulation of appetite and energy expenditure. In this article, we review current knowledge on the homeostatic regulation of energy balance, emphasizing recent advances in mouse genetics, electrophysiology, and optogenetic techniques that have greatly contributed to improving our understanding of this central process.

Dominant role of the p110β isoform of PI3K over p110α in energy homeostasis regulation by POMC and AgRP neurons

Summary PI3K signaling is thought to mediate leptin and insulin action in hypothalamic pro-opiomelanocortin (POMC) and agouti-related protein (AgRP) neurons, key regulators of energy homeostasis, through largely unknown mechanisms. We inactivated either p110α or p110β PI3K catalytic subunits in these neurons and demonstrate a dominant role for the latter in energy homeostasis regulation. In POMC neurons, p110β inactivation prevented insulin- and leptin-stimulated electrophysiological responses. POMCp110β null mice exhibited central leptin resistance, increased adiposity, and diet-induced obesity. In contrast, the response to leptin was not blocked in p110α-deficient POMC neurons. Accordingly, POMCp110α null mice displayed minimal energy homeostasis abnormalities. Similarly, in AgRP neurons, p110β had a more important role than p110α. AgRPp110α null mice displayed normal energy homeostasis regulation, whereas AgRPp110β null mice were lean, with increased leptin sensitivity and resistance to diet-induced obesity. These results demonstrate distinct metabolic roles for the p110α and p110β isoforms of PI3K in hypothalamic energy regulation.

Deletion of the von Hippel–Lindau gene in pancreatic β cells impairs glucose homeostasis in mice

Defective insulin secretion in response to glucose is an important component of the beta cell dysfunction seen in type 2 diabetes. As mitochondrial oxidative phosphorylation plays a key role in glucose-stimulated insulin secretion (GSIS), oxygen-sensing pathways may modulate insulin release. The von Hippel-Lindau (VHL) protein controls the degradation of hypoxia-inducible factor (HIF) to coordinate cellular and organismal responses to altered oxygenation. To determine the role of this pathway in controlling glucose-stimulated insulin release from pancreatic beta cells, we generated mice lacking Vhl in pancreatic beta cells (betaVhlKO mice) and mice lacking Vhl in the pancreas (PVhlKO mice). Both mouse strains developed glucose intolerance with impaired insulin secretion. Furthermore, deletion of Vhl in beta cells or the pancreas altered expression of genes involved in beta cell function, including those involved in glucose transport and glycolysis, and isolated betaVhlKO and PVhlKO islets displayed impaired glucose uptake and defective glucose metabolism. The abnormal glucose homeostasis was dependent on upregulation of Hif-1alpha expression, and deletion of Hif1a in Vhl-deficient beta cells restored GSIS. Consistent with this, expression of activated Hif-1alpha in a mouse beta cell line impaired GSIS. These data suggest that VHL/HIF oxygen-sensing mechanisms play a critical role in glucose homeostasis and that activation of this pathway in response to decreased islet oxygenation may contribute to beta cell dysfunction.

Stable and functional regeneration of pancreatic beta-cell population in nSTZ-rats treated with tungstate.

Aims/hypothesisSodium tungstate has recently emerged as an effective oral treatment for diabetes. We examined the effects of tungstate administration in the beta-cell mass of the pancreas as well as its therapeutic potential.MethodsSodium tungstate was administered via drinking water to healthy and neonatal streptozotocin (nSTZ)-diabetic rats for one month. The pancreas from each rat was removed and morphometric and immunocytochemical studies were carried out. The molecular mechanism of tungstate’s action was also studied.ResultsIn nSTZ rats administration of this compound normalised glycaemia, and increased insulinaemia and islet insulin content. Blood glucose concentrations were normalised as early as on day 4 of treatment, and tungstate treatment produced a partial recovery of beta-cell mass. The rats remained normoglycaemic after tungstate withdrawal. Morphometric studies showed that the increase in beta-cell mass was not due to beta-cell hypertrophy but to hyperplasia, with an increase in islet density in treated diabetic rats. Tungstate treatment increased extra-islet beta-cell replication without modifying intra-islet beta-cell replication rates. Moreover, the treatment induced increases in insulin-positive cells located close to ducts; and in PDX-1 positive cells scattered in the exocrine tissue, suggesting active neogenesis. In islets from treated diabetic rats, tungstate is able to increase the phosphorylation state of PDX-1 through the activation of p38.Conclusion/interpretationThese observations indicate that tungstate treatment is able to regenerate a stable, functional pancreatic beta-cell population which leads to and maintains normoglycaemia.