Regional Roles of Central Trkb Receptors in Energy Balance and Regulation by Ptp1b

Protein tyrosine phosphatase 1B (PTP1B) is a ubiquitously expressed phosphatase implicated in energy balance regulation. CNS-specific PTP1B-deficiency results in a lean phenotype with resistance to diet-induced obesity. PTP1B antagonizes actions of leptin, which regulates central energy balance by suppressing food intake and elevating energy expenditure. Although the metabolic effects of PTP1B-deficiency have been largely attributed to improved leptin sensitivity, mice lacking both leptin and PTP1B weigh less compared to the mice lacking leptin only, suggesting leptin-independent metabolic effects of PTP1B-deficiency. Biochemical studies have identified tropomyosin receptor kinase B (TrkB) as a potential substrate for PTP1B. Since TrkB ligand brain-derived neurotrophic factor (BDNF) is a key player in energy balance, this dissertation tests the hypothesis that PTP1B is a physiological regulator of central BDNF/TrkB signaling and further examines the metabolic role of endogenous hypothalamic and hindbrain BDNF/TrkB signaling. To assess whether PTP1B is a physiological regulator of central BDNF/TrkB signaling, an immortalized human neuronal SH-SY5Y-TrkB cell line was utilized in biochemical studies in vitro, and a mouse model of global PTP1B-deficiency (Ptpn1-/-) was used to test the metabolic response to exogenous central BDNF delivery in vivo. In SH-SY5Y-TrkB cells, PTP1B overexpression and PTP1B inhibition impairs and augments TrkB signaling, respectively. Furthermore, PTP1B interacts with the BDNF-activated TrkB receptor. Ptpn1-/- mice exhibit enhanced hypothalamic TrkB phosphorylation, and are hypersensitive to central BDNF-induced increase in core temperature. Whether Ptpn1-/- mice show increased hypothalamic neurogenesis was explored through BrdU studies. To further elucidate the role of endogenous BDNF/TrkB signaling in central metabolic control, hypothalamus (Nkx2.1-Ntrk2-/-) or hindbrain (Phox2b-Ntrk2+/-) specific TrkB-deficient mice were generated and their metabolic phenotype was analyzed in comparison to wild type controls. Nkx2.1-Ntrk2-/- mice display increased body weight and adiposity due to alterations in food intake and energy expenditure, and have glucose homeostasis impairments. Interestingly, female mice lacking TrkB in the hypothalamus have a more robust metabolic phenotype. Phox2b-Ntrk2+/- mice exhibit pronounced hyperphagia despite the absence of a body weight phenotype. In summary, these data clearly establish PTP1B as a novel, physiological regulator of central BDNF/TrkB signaling, and that endogenous hypothalamic and hindbrain TrkB signaling are essential to central metabolic control

Ablation of intact hypothalamic and/or hindbrain TrkB signaling leads to perturbations in energy balance

Objective: Brain-derived neurotrophic factor (BDNF) and its receptor, tropomyosin receptor kinase B (TrkB), play a paramount role in the central regulation of energy balance. Despite the substantial body of genetic evidence implicating BDNF- or TrkB-deficiency in human obesity, the critical brain region(s) contributing to the endogenous role of BDNF/TrkB signaling in metabolic control remain unknown. Methods: We assessed the importance of intact hypothalamic or hindbrain TrkB signaling in central regulation of energy balance by generating Nkx2.1-Ntrk2−/− and Phox2b-Ntrk2+/− mice, respectively, and comparing metabolic parameters (body weight, adiposity, food intake, energy expenditure and glucose homeostasis) under high-fat diet or chow fed conditions. Results: Our data show that when fed a high-fat diet, male and female Nkx2.1-Ntrk2−/− mice have significantly increased body weight and adiposity that is likely driven by reduced locomotor activity and core body temperature. When maintained on a chow diet, female Nkx2.1-Ntrk2−/− mice exhibit an increased body weight and adiposity phenotype more robust than in males, which is accompanied by hyperphagia that precedes the onset of a body weight difference. In addition, under both diet conditions, Nkx2.1-Ntrk2−/− mice show increased blood glucose, serum insulin and leptin levels. Mice with complete hindbrain TrkB-deficiency (Phox2b-Ntrk2−/−) are perinatal lethal, potentially indicating a vital role for TrkB in visceral motor neurons that control cardiovascular, respiratory, and digestive functions during development. Phox2b-Ntrk2+/− heterozygous mice are similar in body weight, adiposity and glucose homeostasis parameters compared to wild type littermate controls when maintained on a high-fat or chow diet. Interestingly, despite the absence of a body weight difference, Phox2b-Ntrk2+/− heterozygous mice exhibit pronounced hyperphagia. Conclusion: Taken together, our findings suggest that the hypothalamus is a key brain region involved in endogenous BDNF/TrkB signaling and central metabolic control and that endogenous hindbrain TrkB likely plays a role in modulating food intake and survival of mice. Our findings also show that female mice lacking TrkB in the hypothalamus have a more robust metabolic phenotype

Single-cell transcriptomic profiling of the aging mouse brain

The mammalian brain is complex, with multiple cell types performing a variety of diverse functions, but exactly how each cell type is affected in aging remains largely unknown. Here we performed a single-cell transcriptomic analysis of young and old mouse brains. We provide comprehensive datasets of aging-related genes, pathways and ligand–receptor interactions in nearly all brain cell types. Our analysis identified gene signatures that vary in a coordinated manner across cell types and gene sets that are regulated in a cell-type specific manner, even at times in opposite directions. These data reveal that aging, rather than inducing a universal program, drives a distinct transcriptional course in each cell population, and they highlight key molecular processes, including ribosome biogenesis, underlying brain aging. Overall, these large-scale datasets (accessible online at https://portals.broadinstitute.org/single_cell/study/aging-mouse-brain) provide a resource for the neuroscience community that will facilitate additional discoveries directed towards understanding and modifying the aging process