Activity-dependent translation and release of BDNF

描述（由申请人提供）：脑源性神经营养因子（BDNF）是神经营养因子家族的一员，在神经元存活、分化、回路形成/重建和突触可塑性中起着重要作用。BDNF功能的缺乏与精神疾病、抑郁症和学习记忆缺陷有关。BDNF对神经元回路的作用是回路和突触选择性作用，暗示可能参与了BDNF蛋白的局部表达/释放。初级BDNF转录物在两个可选的多聚腺苷化位点进行加工，产生两个BDNF mrna库，它们分别具有0.35 kb和2.85 kb的3'短utr和长utr，但编码相同的BDNF蛋白。长3'UTR包含额外的不同序列实体，提供特定的BDNF转录后调控。携带短3'UTR的BDNF mRNA在细胞体中受到限制，而携带长3'UTR的BDNF mRNA则存在于树突区域。重要的是，长3'UTR负责BDNF mRNA的树突靶向，这可以通过神经元活动增强。此外，长3'UTR被发现抑制静止神经元中的BDNF翻译，但在树突中进行活性依赖的BDNF翻译。BDNF长3'UTR缺失导致树突棘发育和可塑性缺陷，表明BDNF通过长转录本在树突表达中起重要作用。我们假设长3'UTR的BDNF mRNA可能在响应局部突触活动的选择性树突区域介导BDNF的翻译，导致BDNF释放在时空上受到限制，从而对突触进行特异性修饰。本研究将评估这一令人兴奋的假设，并解决有关长3'UTR转录本产生BDNF的几个关键问题。最重要的是BDNF蛋白是否通过突触活性在局部由长3'UTR转录本产生，包装成囊泡，并从受刺激的突触释放。了解从短转录本和长转录本翻译的BDNF蛋白在其分布、运输和释放方面是否对局部突触活动有不同的反应也很重要。高级活细胞成像将用于解决三个具体问题：(1)长3'UTR BDNF转录物是否介导BDNF活性依赖的局部翻译；(2)由长3'UTR转录本合成的BDNF蛋白是否被包装在囊泡中，并能定位到突触活动部位；(3)由长3' utr转录本合成的BDNF蛋白是否真的能在突触活动的反应中释放。这些问题的答案将为BDNF产生、运输和分泌的新机制提供支持，该机制可能在突触修饰中发挥重要作用。这项研究的结果可能会对我们理解正常和疾病大脑中BDNF功能的分子和细胞机制产生重大影响。

DESCRIPTION (provided by applicant): Brain derived neurotrophic factor (BDNF), a member of the neurotrophin family, plays fundamental roles in neuronal survival, differentiation, circuitr formation/reconstruction, and synaptic plasticity. Deficiency in BDNF functions has been implicated in psychiatric disorders, depression, and learning-memory deficits. BDNF actions on neuronal circuitry are circuitry- and synapse-selective actions, implicating a potential involvement of localized expression/release of BDNF proteins. The primary BDNF transcript is processed at two alternative polyadenylation sites, giving rise to two pools of BDNF mRNAs harboring a short and a long 3'UTR of 0.35 kb and 2.85 kb but encoding the same BDNF protein. The long 3'UTR contains additional distinct sequence entities, which offer specific posttranscriptional regulation of BDNF. BDNF mRNA bearing the short 3'UTR is restricted in the cell body whereas that bearing long 3'UTR is present in the dendritic region. Importantly, the long 3'UTR is responsible for dendritic targeting of BDNF mRNA, which is enhanced by neuronal activity. Furthermore, the long 3'UTR was found to repress BDNF translation in resting neurons but undergoes activity-dependent BDNF translation in dendrites. Loss of the BDNF long 3'UTR results in defects in dendritic spine development and plasticity, indicating an important role for dendritic expression of BDNF through the long transcripts. We hypothesize that the long 3'UTR BDNF mRNA may mediate translation of BDNF in selective dendritic regions in response to local synaptic activities, leading to spatiotemporally restricted BDNF release for synapse-specific modification of synapses. This proposed study will evaluate this exciting hypothesis and address several crucial issues concerning BDNF generated from the long 3'UTR transcripts. Of most importance is whether BDNF protein is locally produced from long 3'UTR transcripts by synaptic activity, packed into vesicles, and released from stimulated synapses. It is also important to know if BDNF proteins translated from the short and long transcripts behave differentially in terms of their distribution, trafficking, and release in respose to local synaptic activity. Advanced live cell imaging will be performed to address three specific questions: (1) whether the long 3'UTR BDNF transcripts mediate activity-dependent localized translation of BDNF; (2) whether BDNF proteins synthesized from the long 3'UTR transcripts are packed in vesicles, and can localize to the site of synaptic activities; (3) whether BDNF proteins synthesized from the long 3'UTR transcripts can actually be released in response to synaptic activities. Answers to these questions will provide the support to a novel mechanism concerning BDNF production, trafficking, and secretion that may play an important role in synaptic modifications. Results from this study could have a major impact on our understanding of the molecular and cellular mechanisms underlying BDNF functions in normal and disease brains. PUBLIC HEALTH RELEVANCE: Brain derived neurotrophic factor (BDNF) plays a crucial role in brain development and functions but is produced from two pools of BDNF mRNAs harboring a short and a long 3'UTR. This study will evaluate if the long 3'UTR BDNF mRNA mediate local translation and release of BDNF for synaptic modification. Results from this study could have a major impact on our understanding of BDNF functions in normal and disease brains, thus having direct relevance to public health.