Excess body weight is frequently associated with low-grade inflammation. Evidence indicates a relationship between obesity and cancer, as well as with other diseases, such as diabetes and non-alcoholic fatty liver disease, in which inflammation and the actions of various adipokines play a role in the pathological mechanisms involved in these disorders. Leptin is mainly produced by adipose tissue in proportion to fat stores, but it is also synthesized in other organs, where leptin receptors are expressed. This hormone performs numerous actions in the brain, mainly related to the control of energy homeostasis. It is also involved in neurogenesis and neuroprotection, and central leptin resistance is related to some neurological disorders, e.g., Parkinson’s and Alzheimer’s diseases. In peripheral tissues, leptin is implicated in the regulation of metabolism, as well as of bone density and muscle mass. All these actions can be affected by changes in leptin levels and the mechanisms associated with resistance to this hormone. This review will present recent advances in the molecular mechanisms of leptin action and their underlying roles in pathological situations, which may be of interest for revealing new approaches for the treatment of diseases where the actions of this adipokine might be compromised.

Dietary intervention is a common tactic employed to curtail the current obesity epidemic. Changes in nutritional status alter metabolic hormones such as insulin or leptin, as well as the insulin-like growth factor (IGF) system, but little is known about restoration of these parameters after weight loss in obese subjects and if this differs between the sexes, especially regarding the IGF system. Here male and female mice received a high fat diet (HFD) or chow for 8 weeks, then half of the HFD mice were changed to chow (HFDCH) for 4 weeks. Both sexes gained weight (p < 0.001) and increased their energy intake (p < 0.001) and basal glycemia (p < 0.5) on the HFD, with these parameters normalizing after switching to chow but at different rates in males and females. In both sexes HFD decreased hypothalamic NPY and AgRP (p < 0.001) and increased POMC (p < 0.001) mRNA levels, with all normalizing in HFDCH mice, whereas the HFD-induced decrease in ObR did not normalize (p < 0.05). All HFD mice had abnormal glucose tolerance tests (p < 0.001), with males clearly more affected, that normalized when returned to chow. HFD increased insulin levels and HOMA index (p < 0.01) in both sexes, but only HFDCH males normalized this parameter. Returning to chow normalized the HFD-induced increase in circulating leptin (p < 0.001), total IGF1 (p < 0.001), IGF2 (p < 0.001, only in females) and IGFBP3 (p < 0.001), whereas free IGF1 levels remained elevated (p < 0.01). In males IGFBP2 decreased with HFD and normalized with chow (p < 0.001), with no changes in females. Although returning to a healthy diet improved of most metabolic parameters analyzed, fIGF1 levels remained elevated and hypothalamic ObR decreased in both sexes. Moreover, there was sex differences in both the response to HFD and the switch to chow including circulating levels of IGF2 and IGFBP2, factors previously reported to be involved in glucose metabolism. Indeed, glucose metabolism was also differentially modified in males and females, suggesting that these observations could be related.

It is now clear that hypothalamic astrocytes participate in maintaining metabolic homeostasis. Both nutrients and metabolic hormones can directly impact on these glial cells to modify their release of gliotransmitters, metabolic factors, growth factors, etc, as well as their physical interaction with neighboring neurons. Another mechanism by which astrocytes could communicate with neurons is through their release of exosomes. We have previously shown (by RNAseq analysis) that exposure to palmitic acid (PA) dramatically modifies the miRNA content of exosomes released by hypothalamic astrocytes. Here our objectives were: 1) To determine if the miRNA changes in hypothalamic astrocyte-derived exosomes in response to oleic acid (OA) differ from those seen in response to PA and 2) Analyze the response of POMC neurons to exosomes derived from astrocytes exposed to either PA or OA. Primary hypothalamic astrocyte cultures were treated with PA (0.5 mM), OA (0.5 mM) or vehicle (V) for 24 hours. Exosomes were purified from the media and used for miRNA analysis and to treat a POMC neuronal cell line (mHypoA-POMC/GFP-1). Both OA and PA modified miRNA levels in exosomes compared to those detected in V exosomes, but these modifications differed between the two fatty acids. Furthermore, the response of POMC neurons to exosomes from vehicle (E-V), OA (E-OA) and PA (E-PA) treated astrocytes for 24 hours differed significantly. The expression of POMC mRNA was significantly decreased in response to E-V and increased in response to both E-OA and E-PA, although the increase in POMC mRNA was significantly greater in E-PA treated neurons. These results suggest that hypothalamic astrocytes can directly communicate with neurons involved in metabolic control through exosomes and that the messages contained within these exosomes are modulated by the nutrient environment.

Astrocytes influence neighboring neurons through the release of a variety of signals, including exosomes, micro-vesicles that contain a vast heterogeneity of molecules such as cytokines, growth factors, RNAs and micro-RNAs (mi-RNAs) that modify target cells. We hypothesized that hypothalamic astrocytes communicate the metabolic status via exosomes to neighboring POMC neurons to modify their functions in the promotion of satiety and energy expenditure. To this end, primary hypothalamic astrocyte cultures were treated with palmitic acid (PA; 0.5 mM), oleic acid (OA; 0.5 mM) or vehicle for 24 hours and exosomes were isolated from the media and applied (1.25 or 2.50 µg/mL) to a POMC neuronal cell line for 24 hours. Exosomes released in response to PA (E-PA) or OA (E-OA) increased POMC expression (p < 0. 05) with no effect on the expression of markers of ER stress (CHOP) and inflammation [interleukin (IL)-6] compared to exosomes released from vehicle treated astrocytes (E-V) or with no exosomes (control). Seahorse Cell Mito Stress test was performed to determinate modifications in metabolism in the POMC neurons in response to these treatments. The mitochondrial spare respiratory capacity of neurons was increased (p < 0. 0001) in response to both doses of E-PA and E-OA, with the maximal respiration (p < 0. 0001) increasing with E-PA (both doses) or 2.50 µg/mL of E-OA compared to E-V or control. Next-generation miRNA sequencing analysis established the modifications of miRNAs contained in exosomes released by hypothalamic astrocytes in response to PA, with miR-199a-3p and miR-145-5p content being higher in E-PA compared to E-V. Transfection of POMC neurons with a mimetic of miR-199a-3p (1.5 pmol) increased POMC expression and insulin-like growth factor 1 receptor (IGF1r) protein levels (p<0. 05). Moreover, levels of mTOR as well as p70S6k, reported targets of miR-199a-3p, were decreased (both p<0. 05). Mimetic overexpression of miR-145-5p reduced POMC expression (p < 0. 001) and protein levels of insulin receptor substrate 1 (IRS1; p < 0. 001), which is a known target of this miRNA. These results suggest that astrocytes communicate with neurons via exosomes, with the exosomes content being modulated in response to the nutritional environment. The messages contained in astrocytic exosomes can directly alter the neuropeptide expression in targeted neurons as well as of the levels of receptors and factors involved in cell protection, metabolism, and nutrient sensing, with specific miRNAs participating in this process. Furthermore, cellular respiration of POMC neurons treated with fatty acid-modified astrocytic exosomes is modified in a manner that suggests they are preparing for a possible respiratory stress by increasing their spare respiratory capacity and maximal respiration. Presentation: No date and time listed