Neuronal loss of TRPM8 leads to obesity and glucose intolerance in male mice.

OBJECTIVE: Mice with global deletion of the transient receptor potential channel melastatin family member 8 (TRPM8) are obese, and treatment of diet-induced obese (DIO) mice with TRPM8 agonists decrease body weight. Whether TRPM8 signaling regulates energy metabolism via central or peripheral effects is unknow. Here we assessed the metabolic phenotype of mice with either Nestin Cre-mediated neuronal loss of TRPM8, or with deletion of TRPM8 in Advillin Cre positive sensory neurons of the peripheral nervous system (PNS). METHODS: Nestin Cre- and Advillin Cre-Trpm8 knock-out (KO) mice were metabolically phenotyped under chronic exposure to either chow or high-fat diet (HFD), followed by assessment of energy and glucose metabolism. RESULTS: At room temperature, chow-fed neuronal Trpm8 KO are obese and show decreased energy expenditure when acutely treated with the TRPM8 selective agonist icilin. But body weight of neuronal Trpm8 KO mice is indistinguishable from wildtype controls at thermoneutrality, or when mice are chronically exposed to HFD-feeding. In contrast to previous studies, we show that the TRPM8 agonist icilin has no direct effect on brown adipocytes, but that icilin stimulates energy expenditure, at least in part, via neuronal TRPM8 signaling. We further show that lack of TRPM8 in sensory neurons of the PNS does not lead to a metabolically relevant phenotype. CONCLUSIONS: Our data indicate that obesity in TRPM8-deficient mice is centrally mediated and likely originates from alterations in energy expenditure and/or thermal conductance, but does not depend on TRPM8 signaling in brown adipocytes or sensory neurons of the PVN.

The metabolic response to postnatal leptin in rats varies with age and may be litter dependent.

Hyperleptinemia during postnatal life induces long-term effects on metabolism. However, these effects are controversial as both increased and decreased propensity towards obesity has been reported. To further analyze the effects of chronic neonatal hyperleptinemia on the subsequent metabolic profile, male Wistar rats proceeding from 18 different litters (8 pups/litter) received a daily subcutaneous injection of either saline (10 ml/kg, n=36) or leptin (3 µg/g, n=36) from postnatal day (PND) 2 to PND9. Rats were sacrificed at 10, 40, or 150 days of age. At 10 days of age, leptin treated rats had decreased body weight (p<0.001) and body fat (p<0.05). Leptin levels and glycemia were increased (p<0.01), whereas insulin, total lipids, triglycerides and glycerol levels were decreased (p<0.05). At PND40 rats receiving leptin had increased glycemia (p<0.01) and plasma HDL and LDL levels, but decreased total lipids (p<0.05). At PND150 neonatal leptin treatment induced different effects in rats raised in different litters. Rats from litter 1 had increased body weight (p<0.05), body fat (p<0.01), and plasma leptin (p<0.001), cholesterol (p<0.001), triglyceride (p<0.001), total lipid (p<0.001), LDL (p<0.05), and glycerol (p<0.001) levels. In rats from litter 2 these parameters did not differ from controls. Rats from litter 3 had decreased body weight (p<0.05), visceral fat (p<0.01) and plasma leptin (p<0.001), cholesterol (p<0.001), triglyceride (p<0.001), glycerol (p<0.001), and HDL (p<0.001) levels. In conclusion, the metabolic response to postnatal leptin varies with age, with the response in adulthood being variable and most likely influenced by other factors, including the genetic make-up.

Estrogen, astrocytes and the neuroendocrine control of metabolism.

Obesity, and its associated comorbidities such as type 2 diabetes, cardiovascular diseases, and certain cancers, represent major health challenges. Importantly, there is a sexual dimorphism with respect to the prevalence of obesity and its associated metabolic diseases, implicating a role for gonadal hormones. Specifically, estrogens have been demonstrated to regulate metabolism perhaps by acting as a leptin mimetic in the central nervous system (CNS). CNS estrogen receptors (ERs) include ER alpha (ERα) and ER beta (ERß), which are found in nuclear, cytoplasmic and membrane sites throughout the brain. Additionally, estrogens can bind to and activate a G protein-coupled estrogen receptor (GPER), which is a membrane-associated ER. ERs are expressed on neurons as well as glia, which are known to play a major role in providing nutrient supply for neurons and have recently received increasing attention for their potentially important involvement in the CNS regulation of systemic metabolism and energy balance. This brief overview summarizes data focusing on the potential role of astrocytic estrogen action as a key component of estrogenic modulation responsible for mediating the sexual dimorphism in body weight regulation and obesity.

Activation of microglia in specific hypothalamic nuclei and the cerebellum of adult rats exposed to neonatal overnutrition.

Much attention has been drawn to the possible involvement of hypothalamic inflammation in the pathogenesis of metabolic disorders, especially in response to a high-fat diet. Microglia, the macrophages of the central nervous system, can be activated by proinflammatory signals resulting in the local production of specific interleukins and cytokines, which in turn could exacerbate the pathogenic process. Because obesity itself is considered to be a state of chronic inflammation, we evaluated whether being overweight results in microglial activation in the hypothalamus of rats on a normal diet. Accordingly, we used a model of neonatal overnutrition that entailed adjustment of litter size at birth (small litters: four pups/dam versus normal litters: 12 pups/dam) and resulted in a 15% increase in bodyweight and increased circulating leptin levels at postnatal day 60. Rats that were overnourished during neonatal life had an increased number of activated microglia in specific hypothalamic areas such as the ventromedial hypothalamus, which is an important site for metabolic control. However, this effect was not confined to the hypothalamus because significant microglial activation was also observed in the cerebellar white matter. There was no change in circulating tumour necrosis factor (TNF) α levels or TNFα mRNA levels in either the hypothalamus or cerebellum. Interleukin (IL)6 protein levels were higher in both the hypothalamus and cerebellum, with no change in IL6 mRNA levels. Because circulating IL6 levels were elevated, this rise in central IL6 could be a result of increased uptake. Thus, activation of microglia occurs in adult rats exposed to neonatal overnutrition and a moderate increase in weight gain on a normal diet, possibly representing a secondary response to systemic inflammation. Moreover, this activation could result in local changes in specific hypothalamic nuclei that in turn further deregulate metabolic homeostasis.

Ghrelin treatment protects lactotrophs from apoptosis in the pituitary of diabetic rats.

Poorly controlled diabetes is associated with hormonal imbalances, including decreased prolactin production partially due to increased lactotroph apoptosis. In addition to its metabolic actions, ghrelin inhibits apoptosis in several cell types. Thus, we analyzed ghrelin's effects on diabetes-induced pituitary cell death and hormonal changes. Six weeks after onset of diabetes in male Wistar rats (streptozotocin 70 mg/kg), minipumps infusing saline or 24 nmol ghrelin/day were implanted (jugular). Rats were killed two weeks later. Ghrelin did not modify body weight or serum glucose, leptin or adiponectin, but increased total ghrelin (P<0.05), IGF-I (P<0.01) and prolactin (P<0.01) levels. Ghrelin decreased cell death, iNOS and active caspase-8 (P<0.05) and increased prolactin (P<0.05), Bcl-2 (P<0.01) and Hsp70 (P<0.05) content in the pituitary. In conclusion, ghrelin prevents diabetes-induced death of lactotrophs, decreasing caspase-8 activation and iNOS content and increasing anti-apoptotic pathways such as pituitary Bcl-2 and Hsp70 and serum IGF-I concentrations.

Death of hypothalamic astrocytes in poorly controlled diabetic rats is associated with nuclear translocation of apoptosis inducing factor.

Astrocytes in the hypothalamus of poorly controlled diabetic rats are reduced in number, due to increased apoptosis and decreased proliferation, and undergo morphological changes, including a decrease in projections. These changes are associated with modifications in synaptic proteins and most likely affect neuroendocrine signalling and function. The present study aimed to determine the intracellular mechanisms underlying this increase in hypothalamic cell death. Adult male Wistar rats were injected with streptozotocin (70 mg/kg, i.p) and controls received vehicle. Rats were killed at 1, 4, 6 and 8 weeks after diabetes onset (glycaemia > 300 mg/dl). Cell death, as detected by enzyme-linked immunosorbent assay, increased at 4 weeks of diabetes. Immunohistochemistry and terminal dUTP nick-end labelling (TUNEL) assays indicated that these cells corresponded to glial fibrillary acidic protein (GFAP) positive cells. No significant change in fragmentation of caspases 2, 3, 6, 7, 8, 9, or 12 was observed with western blot analysis. However, enzymatic assays indicated that caspase 3 activity increased significantly after 1 week of diabetes and decreased below control levels thereafter. In the hypothalamus, cell bodies lining the third ventricle, fibres radiating from the third ventricle and GFAP positive cells expressed fragmented caspase 3, with this labelling increasing at 1 week of diabetes. However, because no nuclear labelling was observed and this increase in activity did not correlate temporally with the increased cell death, this caspase may not be involved in astrocyte death. By contrast, nuclear translocation of apoptosis inducing factor (AIF) increased significantly in astrocytes in parallel with the increase in death and AIF was found in TUNEL positive cells. Thus, nuclear translocation of AIF could underlie the increased death, whereas fragmentation of caspase 3 could be associated with the morphological changes found in hypothalamic astrocytes of diabetic rats.