Energy imbalance due to excess of calories is considered to be a major player in the current worldwide obesity pandemic and could be accompanied by systemic and central inflammation and mitochondrial dysfunctions. This hypothesis was tested by comparing the wild-derived diet-induced obesity- (DIO-) resistant mouse strain WSB/EiJ to the obesity-prone C57BL/6J strain. We analysed circulating and hypothalamic markers of inflammatory status and hypothalamic mitochondrial activity in both strains exposed to high-fat diet (HFD). We further analysed the regulations of hypothalamic genes involved in inflammation and mitochondrial pathways by high throughput microfluidic qPCR on RNA extracted from laser micro-dissected arcuate (ARC) and paraventricular (PVN) hypothalamic nuclei. HFD induced increased body weight gain, circulating levels of leptin, cholesterol, HDL and LDL in C57BL/6J whereas WSB/EiJ mice displayed a lower inflammatory status, both peripherally (lower levels of circulating cytokines) and centrally (less activated microglia in the hypothalamus) as well as more reactive mitochondria in the hypothalamus. The gene expression data analysis allowed identifying strain-specific hypothalamic metabolic pathways involved in the respective responses to HFD. Our results point to the involvement of hypothalamic inflammatory and mitochondrial pathways as key factors in the control of energy homeostasis and the resistance to DIO.

Adult-onset hypothyroidism is associated with learning and cognitive dysfunctions, which may be related to alterations in synaptic plasticity. Local reduced levels of thyroid hormones (THs) may impair glia morphology and activity, and promote the increase of pro-inflammatory cytokine levels mainly in the hippocampus. Given that neuroinflammation induces memory impairments, hypothyroidism-related glia dysfunction may participate in brain disorders. Thus, we investigated the mechanisms linking hypothyroidism and neuroinflammation, from a protective perspective. We induced hypothyroidism in adult C57BL/6J and wild-derived WSB/EiJ male mice by a seven-week propylthiouracil (PTU) treatment. We previously showed that WSB/EiJ mice were resistant to high-fat diet (HFD)-induced obesity, showing no neuroinflammatory response through adaptive abilities, unlike C57BL/6J. As PTU and HFD treatments are known to induce comparable inflammatory responses, we hypothesized that WSB/EiJ mice might also be protected against hypothyroidism-induced neuroinflammation. We showed that hypothyroid WSB/EiJ mice depicted no hippocampal neuroinflammatory response and were able to maintain their hippocampal thyroid signalling despite low circulatisng TH levels. In contrast, C57BL/6J mice exhibited disturbed hippocampal TH signalling, accompanied by neuroinflammation and memory impairment. Our results reinforce the preponderance of the hippocampal TH regulatory system over TH circulating levels in the hippocampal glial reactivity.

In mammals, the maintenance of energy homeostasis relies on complex mechanisms requiring tight synchronization between peripheral organs and the brain. Thyroid hormones (TH), among their pleiotropic actions, play a central role in these regulations. Hypothyroidism, which is characterized by low circulating TH levels, slows down the metabolism, leading to a reduction in energy expenditure, as well as in lipid and glucose metabolism, and to insulin resistance. Our objective was to evaluate whether metabolic deregulations induced by hypothyroidism could be avoid by regulatory mechanisms involved in metabolic flexibility. To this aim, we compared the response to hypothyroidism in two mouse strains, the wild-derived WSB/EiJ mouse strain characterized by a diet-induced obesity (DIO) resistance due to its high metabolic flexibility phenotype and the C57BL/6J mice, prone to DIO. Adult mice were fed with a low-iodine diet supplemented with 6-n-propyl-2-thiouracyl (PTU) for 7 weeks to induce hypothyroidism. Our results show that hypothyroidism, characterized by a decrease in serum T4 levels, led to metabolic deregulations, as an alteration of lipid metabolism in the liver of both strains. However, the decrease in hepatic lipid synthesis was compensated in WSB/EiJ mice by a mobilization of lipid reserves from white adipose tissue, but not in the C57BL/6J mice. No peripheral or hypothalamic inflammatory response to hypothyroidism was observed in both strains. Moreover, gene expression analysis showed that hypothyroidism stimulates the hypothalamic orexigenic circuit in both strains, but unchanged Mc4r and LepR expression in hypothyroid WSB/EiJ mice strain, which reflect their adaptability to maintain their body weight, contrary to C57BL/6J mice. Our results show that WSB/EiJ mice displayed a phenotype of resistance to metabolic dysregulations induced by hypothyroidism, by compensatory mechanisms. This response as well as their resistance to HFD-induced obesity highlights their adaptive capacities to maintain metabolic homeostasis, namely, their high metabolic flexibility, despite serum hypothyroidism. This model sheds light on the importance of local thyroid homeostasis to maintain lipid metabolism and metabolic homeostasis.

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Adult-onset hypothyroidism is associated with learning and cognitive dysfunctions, which may be related to alterations in synaptic plasticity. Local reduced levels of thyroid hormones (THs) may impair glia morphology and activity, and promote the increase of proinflammatory cytokine levels mainly in the hippocampus. Given that neuroinflammation induces memory impairments, hypothyroidism-related glia dysfunction may participate in brain disorders. Thus, we investigated the mechanisms linking hypothyroidism, neuroinflammation and spatial memory, from a protective prospective. We induced hypothyroidism in adult C57BL/6J and wild-derived WSB/EiJ male mice by a seven-week propylthiouracil (PTU) treatment. We previously showed that WSB/EiJ mice were resistant to high-fat diet (HFD)-induced obesity, showing no neuroinflammatory response through adaptive abilities, unlike C57BL/6J. As PTU and HFD treatments are known to induce comparable inflammatory responses, we hypothesized WSB/EiJ mice might also be protected against hypothyroidism induced neuroinflammation. We showed that hypothyroid WSB/EiJ mice depicted no hippocampal neuroinflammatory response and were able to maintain their hippocampal thyroid signalling despite low circulating TH levels. In contrast, C57BL/6J mice exhibited disturbed hippocampal TH signalling, accompanied by neuroinflammation and memory impairment. Our results reinforce the preponderance of local TH levels over circulating levels and the major role of hippocampal glia reactivity in the establishment of memory deficits.