神经突触是神经元与其靶细胞之间进行信息交流的特化结构。突触生长过程的精确调控对于神经环路的形成和可塑性至关重要，突触发育和功能的异常导致多种神经精神疾病包括智力低下、自闭症、精神分裂症和神经变性病等。因此，寻找和鉴定突触发育和功能调控基因一直是神经生物学家的重要研究内容之一。
果蝇脑肿瘤基因brat（brain tumor）是一个进化上非常保守的基因，该基因突变后导致果蝇大脑产生肿瘤，造成大脑半球明显增大。中国科学院遗传与发育生物学研究所张永清实验室通过多学科的实验手段的研究发现，brat突变体的神经肌肉突触过度生长，其显著特征是突触卫星扣结数大大增加。同时，电镜实验结果表明，与野生型相比，brat突变体突触扣结负责神经递质传递的活性区的大小增大。电生理测试结果表明，brat突变导致自发性的微兴奋性突触后电位升高，突触传递效率下降。并且，FM1-43荧光染料内吞实验显示brat突变体的突触内吞功能受损，表明Brat参与调控突触内吞功能。遗传学互作及生化实验表明，发现brat通过抑制BMP （Bone Morphogenic Protein）信号途径中转录因子Mad（Mothers against decapentaplegic）的表达，从而调控了神经肌肉突触的形态、结构和功能。该研究结果揭示，脑肿瘤蛋白Brat除了在神经系统早期分化发育过程起重要作用外，在已成熟的神经元的突触生长过程中也起着关键作用。该研究将有助于进一步认识突触发育的分子调控机制以及神经系统相关疾病的发病机制。
该研究结果于7月24日正式发表在国际学术期刊Journal of Neuroscience。张永清课题组的博士生石文文和陈严为该论文的共同第一作者。合作实验室包括中科院遗传发育所的许执恒研究员实验室和东南大学的谢维教授实验室。
该研究得到了国家自然科学基金委和中科院的资助。（生物谷 Bioon.com）
生物谷推荐的英文摘要
The Journal of Neuroscience doi: 10.1523/?JNEUROSCI.0386-13.2013
Brain Tumor Regulates Neuromuscular Synapse Growth and Endocytosis in Drosophila by Suppressing Mad Expression
Wenwen Shi1,*, Yan Chen1,*, Guangming Gan2, Dan Wang1, Jinqi Ren1, Qifu Wang1, Zhiheng Xu1, Wei Xie2, and Yong Q. Zhang1
The precise regulation of synaptic growth is critical for the proper formation and plasticity of functional neural circuits. Identification and characterization of factors that regulate synaptic growth and function have been under intensive investigation. Here we report that brain tumor (brat), which was identified as a translational repressor in multiple biological processes, plays a crucial role at Drosophila neuromuscular junction (NMJ) synapses. Immunohistochemical analysis demonstrated that brat mutants exhibited synaptic overgrowth characterized by excess satellite boutons at NMJ terminals, whereas electron microscopy revealed increased synaptic vesicle size but reduced density at active zones compared with wild-types. Spontaneous miniature excitatory junctional potential amplitudes were larger and evoked quantal content was lower at brat mutant NMJs. In agreement with the morphological and physiological phenotypes, loss of Brat resulted in reduced FM1-43 uptake at the NMJ terminals, indicating that brat regulates synaptic endocytosis. Genetic analysis revealed that the actions of Brat at synapses are mediated through mothers against decapentaplegic (Mad), the signal transduction effector of the bone morphogenetic protein (BMP) signaling pathway. Furthermore, biochemical analyses showed upregulated levels of Mad protein but normal mRNA levels in the larval brains of brat mutants, suggesting that Brat suppresses Mad translation. Consistently, knockdown of brat by RNA interference in Drosophila S2 cells also increased Mad protein level. These results together reveal an important and previously unidentified role for Brat in synaptic development and endocytosis mediated by suppression of BMP signaling.
