神經(jīng)突觸是神經(jīng)元與其靶細胞之間進行信息交流的特化結(jié)構(gòu)。突觸生長過程的精確調(diào)控對于神經(jīng)環(huán)路的形成和可塑性至關重要,，突觸發(fā)育和功能的異常導致多種神經(jīng)精神疾病包括智力低下,、自閉癥,、精神分裂癥和神經(jīng)變性病等。因此,，尋找和鑒定突觸發(fā)育和功能調(diào)控基因一直是神經(jīng)生物學家的重要研究內(nèi)容之一,。
果蠅腦腫瘤基因brat（brain tumor）是一個進化上非常保守的基因，該基因突變后導致果蠅大腦產(chǎn)生腫瘤,，造成大腦半球明顯增大,。中國科學院遺傳與發(fā)育生物學研究所張永清實驗室通過多學科的實驗手段的研究發(fā)現(xiàn)，brat突變體的神經(jīng)肌肉突觸過度生長,，其顯著特征是突觸衛(wèi)星扣結(jié)數(shù)大大增加,。同時，電鏡實驗結(jié)果表明,，與野生型相比,，brat突變體突觸扣結(jié)負責神經(jīng)遞質(zhì)傳遞的活性區(qū)的大小增大。電生理測試結(jié)果表明,，brat突變導致自發(fā)性的微興奮性突觸后電位升高,，突觸傳遞效率下降。并且,，F(xiàn)M1-43熒光染料內(nèi)吞實驗顯示brat突變體的突觸內(nèi)吞功能受損,，表明Brat參與調(diào)控突觸內(nèi)吞功能,。遺傳學互作及生化實驗表明，發(fā)現(xiàn)brat通過抑制BMP （Bone Morphogenic Protein）信號途徑中轉(zhuǎn)錄因子Mad（Mothers against decapentaplegic）的表達,，從而調(diào)控了神經(jīng)肌肉突觸的形態(tài),、結(jié)構(gòu)和功能。該研究結(jié)果揭示,，腦腫瘤蛋白Brat除了在神經(jīng)系統(tǒng)早期分化發(fā)育過程起重要作用外,，在已成熟的神經(jīng)元的突觸生長過程中也起著關鍵作用。該研究將有助于進一步認識突觸發(fā)育的分子調(diào)控機制以及神經(jīng)系統(tǒng)相關疾病的發(fā)病機制,。
該研究結(jié)果于7月24日正式發(fā)表在國際學術期刊Journal of Neuroscience,。張永清課題組的博士生石文文和陳嚴為該論文的共同第一作者。合作實驗室包括中科院遺傳發(fā)育所的許執(zhí)恒研究員實驗室和東南大學的謝維教授實驗室,。
該研究得到了國家自然科學基金委和中科院的資助,。（生物谷Bioon.com）
doi:10.1523/JNEUROSCI.0386-13.2013
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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.
