Astrocytic regulation of energy balance on high-fat diet

摘要 脑源性神经营养因子(BDNF)与全长原肌球蛋白受体激酶(TrkB.FL)结合，诱导其 二聚化和活化。这激活了许多共同促进神经元存活的信号级联。 并调节许多脑区的神经元发育和突触传递。这个信号通路是 对能量平衡的控制也很重要，因为TrkB.FL激酶结构域或BDNF的突变导致 老鼠和人类的过度吞噬和严重肥胖。除了TrkB.FL，NTRK2基因还会产生两个 截短的受体，主要的TrkB.T1和次要的TrkB.T2(我们使用TrkB.T来指代这两种异构体)。TrkB.T 有一个较短的细胞内序列，并且缺乏酪氨酸激酶结构域。神经元主要表达TrkB.FL， 而星形胶质细胞只表达TrkB.T.，而BDNF与TrkB.T1结合可诱导钙信号并激活 培养的星形胶质细胞中的Rho GTP酶，星形胶质细胞TrkB.T是否在能量控制中发挥作用尚不清楚 平衡。我们建立了星形胶质细胞特异性NTRK2条件性基因敲除(ANtrk2 CKO)小鼠，其中NTRK2缺失 开始于5周龄，并取消星形胶质细胞中TrkB.T的表达。我们发现NTRK2基因在 成熟星形胶质细胞阻断了下丘脑弓状核(ARH)的星形胶质细胞反应，并给小鼠 通过减少能量摄入和增加能量消耗，全面抵抗饮食诱导的肥胖(DIO)。在……里面 除了对神经元的营养和营养支持外，星形胶质细胞还通过它们的过程接触突触 通过从突触间隙摄取神经递质并释放来调节突触传递 神经胶质递质进入突触裂隙。因此，我们假设TrkB.T介导的钙信号促进 星形胶质细胞对高脂饮食(HFD)喂养的反应，这反过来又扰乱了星形胶质细胞对神经元的支持 以及星形胶质细胞对突触的调节，将能量稳态转变为能量盈余。我们的研究将集中在 在ARH的星形胶质细胞、AgRP/NPY神经元和POMC神经元上。我们建议研究NTRK2的影响 ARH中星形胶质细胞基因缺失(AIM 1)，以确定星形胶质细胞NTRK2缺失对神经元的影响 和突触传递(目标2)，以确定星形胶质细胞TrkB.T消融阻断饮食诱导的位置 肥胖(目标3)，并确定减弱星形细胞钙信号是否可以预防高脂饮食喂养的小鼠肥胖(目标3 4)。总而言之，这个研究项目将测试几个新概念，包括TrkB.T的关键作用- 介导的钙信号诱导星形胶质细胞反应，TrkB.T在能量调节中的积极作用 在高脂饲料喂养的小鼠中，平衡和改变了星形细胞对突触传递的调节。我们的研究很可能会证明 减弱TrkB.T介导的星形胶质细胞内钙信号可能是一种新的有效的治疗策略 人类最常见的肥胖形式--DIO的干预。

Summary Brain-derived neurotrophic factor (BDNF) binds to full-length tropomyosin receptor kinase (TrkB.FL), inducing its dimerization and activation. This activates many signaling cascades that cooperatively promote neuronal survival and regulate neuronal development and synaptic transmission in many brain regions. This signaling pathway is also critical for the control of energy balance, as mutations in the TrkB.FL kinase domain or BDNF lead to hyperphagia and severe obesity in mice and humans. In addition to TrkB.FL, the Ntrk2 gene produces two truncated receptors, a predominant TrkB.T1 and a minor TrkB.T2 (we use TrkB.T to refer both isoforms). TrkB.T has a short intracellular sequence and lacks the tyrosine kinase domain. Neurons mainly express TrkB.FL, whereas astrocytes only express TrkB.T. While binding of BDNF to TrkB.T1 induces Ca2+ signals and activates Rho GTPase in cultured astrocytes, it remains unclear if astrocytic TrkB.T plays a role in the control of energy balance. We generated astrocyte specific Ntrk2 conditional knockout (aNtrk2 cKO) mice where Ntrk2 deletion starts at 5 weeks of age and abolishes TrkB.T expression in astrocytes. We found that the Ntrk2 deletion in mature astrocytes blocked astrocytic reactivity in the arcuate nucleus of the hypothalamus (ARH) and gave mice total resistance to diet-induced obesity (DIO) by reducing energy intake and increasing energy expenditure. In addition to nutritional and trophic support to neurons, astrocytes contact synapses through their processes to regulate synaptic transmission by taking up neurotransmitters from the synaptic cleft and releasing gliotransmitters into the synaptic cleft. Thus, we hypothesize that TrkB.T-mediated Ca2+ signals promote astrocytic reactivity in response to high-fat diet (HFD) feeding, which in turn disrupts astrocytic support to neurons and astrocytic regulation of synapses and shifts energy homeostasis to energy surplus. Our studies will focus on astrocytes, AgRP/NPY neurons, and POMC neurons in the ARH. We propose to examine the impact of Ntrk2 gene deletion on astrocytes in the ARH (Aim 1), to determine the impact of astrocytic Ntrk2 deletion on neurons and synaptic transmission (Aim 2), to identify the site where astrocytic TrkB.T ablation blocks diet-induced obesity (Aim 3), and to determine if attenuating astrocytic Ca2+ signals can prevent obesity in HFD-fed mice (Aim 4). In conclusion, this research project will test several novel concepts, including a crucial role for TrkB.T- mediated Ca2+ signals in induction of astrocytic reactivity, an active role for TrkB.T in the regulation of energy balance, and altered astrocytic regulation of synaptic transmission in HFD-fed mice. Our studies will likely show that attenuating TrkB.T-mediated Ca2+ signals in astrocytes can be a novel and effective strategy for therapeutic interventions of DIO, the most common form of obesity in humans.