Astrocyte morphogenesis is dependent on BDNF signaling via astrocytic TrkB.T1

Astrocytic morphogenesis and maturation are critical steps in CNS development. The time window of astrocyte morphological development is well defined, but the molecular underpinnings are still unclear. BDNF is a critical growth factor involved in the development of the CNS, including synapse refinement. Here we demonstrate the BDNF receptor at Ntrk2 is enriched in astrocytes relative to all CNS cell populations. RNA sequencing indicates Ntrk2 falls in the top 0.001% of all gene transcripts expressed in juvenile astrocytes, almost exclusively due to truncated TrkB.T1. Astrocyte complexity is increased in the presence of BNDF in vitro, which is dependent upon the presence of TrkB.T1. Furthermore, deletion of TrkB.T1 in vivo revealed astrocytes with significantly reduced volume and branching complexities. Indicating a role for functional astrocyte maturation via BDNF/TrkB.T1 signaling, TrkB.T1 KO astrocytes do not support normal excitatory synaptogenesis. Together, these data suggest a significant role for BDNF/TrkB.T1 signaling in astrocyte morphogenesis and indicate this signaling may contribute to astrocyte regulation of neuronal synapse development. Wild-type (WT) males were bred with heterozygous Mecp2tm1.1Jae/+ (Jaenisch) female mice. Astrocytes were acutely isolated from whole cortex as previously described (Holt and Olsen, 2016)) from WT males ranging from postnatal days 25-31. An n of 5 biological replicates were used. For RNA-Sequencing, 2 technical replicates were run per biological replicate.星形胶质细胞形态发生与成熟是中枢神经系统（CNS）发育的关键环节。星形胶质细胞形态发育的时间窗口已被明确界定，但其背后的分子机制仍未阐明。脑源性神经营养因子（BDNF）是参与中枢神经系统发育（包括突触重塑）的关键生长因子。本研究证实，相较于中枢神经系统所有其他细胞群，BDNF受体Ntrk2在星形胶质细胞中富集。RNA测序（RNA Sequencing）结果显示，Ntrk2的转录本在幼年星形胶质细胞的全部基因转录本中位列前0.001%，这一富集现象几乎完全源于截短型TrkB.T1受体的表达。体外实验中，BDNF可提升星形胶质细胞的形态复杂度，且该效应依赖于TrkB.T1的存在。进一步的体内敲除实验表明，敲除TrkB.T1后，星形胶质细胞的体积与分支复杂度均显著降低。此外，TrkB.T1敲除（KO）的星形胶质细胞无法支持正常的兴奋性突触发生，这提示BDNF/TrkB.T1信号通路可介导星形胶质细胞的功能性成熟。综上，本研究数据表明BDNF/TrkB.T1信号通路在星形胶质细胞形态发生中发挥重要作用，且该信号通路可能参与星形胶质细胞对神经元突触发育的调控。本研究将野生型（WT）雄性小鼠与杂合子Mecp2tm1.1Jae/+（Jaenisch）雌性小鼠繁育。按照此前报道的方法（Holt与Olsen，2016），从出生后第25至31天的野生型雄性小鼠全皮层中急性分离星形胶质细胞。本实验设置5个生物学重复，每个生物学重复对应2个技术重复以进行RNA测序。