Molecular mechanisms of synapse development and plasticity

突触可塑性是神经系统对先前经验做出反应并适应环境变化的一个重要过程。突触强度的变化可以是暂时性的，也可以是长期的。这种持久的突触可塑性在发育过程中神经元连接的完善以及学习和记忆中发挥着至关重要的作用。在哺乳动物中，NMDA受体依赖的长时程增强(LTP)和突触传递的长时程抑制(LTD)是长时程突触可塑性的两种主要形式。AMPA受体在突触内和突触外的运动似乎是LTP和LTD期间突触效能改变的细胞机制。 半胱氨酸天冬氨酸氨基转移酶是细胞程序性死亡或凋亡过程中的关键蛋白水解酶，在细胞凋亡的外在途径(配体结合)和内在途径(线粒体)中起作用。Caspase活性的存在被认为是不可逆转地导致细胞死亡，因此是被广泛使用的细胞凋亡的标志。然而，在不是注定要死亡的细胞中也可以检测到活性的caspase，现在人们普遍认为caspase可以在各种发育和生理环境中发挥非凋亡性作用。 2012年，我们对caspase在生理和病理条件下改变突触传递的功能及其与认知的关系进行了综述。在过去的几年里，来自几个研究小组的研究共同指出了caspase在突触可塑性中的一个基本功能，它独立于神经细胞死亡。两个启动子caspase 1和9以及效应子caspase-3被证明调节海马神经元的长时程突触可塑性。我们的研究小组提供了有力的证据表明，NMDA受体依赖性LTD的诱导严重依赖于caspase-3的激活，caspase-3的激活足以诱导突触抑制，BAD和Bax诱导的线粒体细胞色素c的释放在激活Caspase-3的LTD中起着至关重要的作用。 突触的可塑性对学习和记忆至关重要。与它们在LTD和LTP中的功能一致，caspase-3和-1已被报道有助于学习和记忆，就像斑马雀所看到的那样，其中caspase-3是在鸟鸣学习过程中记忆巩固所必需的。此外，在老年大鼠脑中长期灌输caspase-1抑制剂可以改善海马区依赖的上下文记忆。也有新的证据表明，caspase在阿尔茨海默病的早期阶段是活跃的，并可能在细胞死亡和神经退化到来之前介导突触功能障碍和丢失。这些新的见解强调了caspase在正常和病理条件下对突触可塑性的调节作用。 尽管caspase在突触可塑性中很重要，但caspase控制突触传递的机制仍然不清楚。2012年，我们与Sanford Markey博士(NIMH)合作，筛选可能与LTD有关的caspase底物。在LTD诱导下，发现有几种蛋白质在神经元中被切割。半胱氨酸天冬氨酸酶的裂解实验证实了这些假定的半胱氨酸酶底物在LTD过程中发生了蛋白降解，并通过突变确定了裂解位点。我们已经产生了抗半胱氨酸氨基转移酶的突变体，并开始测试它们对突触传递的影响。 除了线粒体-caspase途径外，我们还研究了多巴胺D2受体(D2R)在突触发育中的作用。D2R主要通过调节突触传递在脑功能中发挥关键作用。精神分裂症的D2R功能障碍是众所周知的。所有抗精神病药物都能拮抗D2R，但它们对认知障碍几乎没有影响，认知障碍是精神分裂症的核心症状，与神经元间连接受损有关。精神分裂症患者神经元连接障碍的机制仍然难以捉摸。2012年，通过对D2R活性的药理学和遗传操作，我们发现D2R调节树突棘的形态发生，并且D2R过度激活对脊柱的影响可以通过抗精神病药物阻断D2R来缓解。这些发现提供了证据，精神分裂症的D2R功能障碍导致神经元连接障碍和随之而来的认知障碍。

Synaptic plasticity is an important process through which the nervous system responds to prior experience and adapts to environmental changes. The change in synaptic strength can be transient or last for long periods of time. The long-lasting form of synaptic plasticity plays a crucial role in the refinement of neuronal connections during development and in learning and memory. In mammals, NMDA receptor-dependent long-term potentiation (LTP) and long-term depression (LTD) of synaptic transmission are two major forms of long-lasting synaptic plasticity. AMPA receptor movement, both in and out of the synapse appears to be the cellular mechanism subserving the change of synaptic efficacy during LTP and LTD. Caspases are key proteolytic enzymes involved in programmed cell death or apoptosis and are grouped as either initiators or effectors that act on extrinsic (ligand binding) and intrinsic (mitochondria) pathways of apoptosis. The presence of active caspases was believed to lead irreversibly to cell death, and thus was a widely used marker of apoptotic cells. However, active caspases can be also detected in cells that are not destined to die, and it is now widely accepted that caspases can play non-apoptotic roles in various developmental and physiological contexts. In 2012, we reviewed the functions of caspases in altering synaptic transmission under both physiological and pathological conditions, and its relevance to cognition. In the past few years, studies from several groups collectively point to an essential function of caspases in synaptic plasticity, independent of neuronal cell death. Two initiator caspases, 1 and 9, and the effector caspase-3 are shown to regulate long lasting synaptic plasticity in hippocampal neurons. Our group provided compelling evidence that the induction of NMDA receptor-dependent LTD is critically dependent on caspase-3 activation, active caspase-3 is sufficient to induce synaptic depression, and that BAD and BAX induced mitochondrial release of cytochrome c plays a crucial role in activating caspase-3 in LTD. Synaptic plasticity is crucial for learning and memory. Consistent with their functions in LTD and LTP, caspase-3 and -1 have been reported to contribute to learning and memory as seen in zebra finch where caspase-3 is necessary for memory consolidation during birdsong learning. In addition, chronic brain infusion of a caspase-1 inhibitor in aged rats improves hippocampus dependent contextual memory. There are also emerging evidence that caspases are active in early stages of Alzheimers disease, and could mediate synapse dysfunction and loss before the advent of cell death and neurodegeneration. These new insights highlight the regulatory role of caspases in synaptic plasticity under both normal and pathological conditions. Despite the importance of caspases in synaptic plasticity, the mechanism by which caspases controls synaptic transmission is still unclear. In 2012, we collaborated with Dr. Sanford Markey (NIMH) to screen for caspase substrates potentially involved in LTD. Several proteins were found to be cleaved in neurons upon LTD induction. Proteolysis of these putative caspase substrates during LTD were confirmed by caspase cleavage assay, and the cleavage sites were determined by mutagenesis. We have generated caspase-resistant mutants, and started to test their effects on synaptic transmission. In addition to the mitochondria-caspase pathway, we have also investigated the role of dopamine D2 receptors (D2R) in synapse development. D2R plays a pivotal role in brain functions mainly by modulating synaptic transmission. D2R dysfunction in schizophrenia is well known. All antipsychotics antagonize D2R, but they have little effect on cognitive impairment, a core symptom of schizophrenia related to impaired interneuronal connections. The mechanisms underlying neuronal dysconnection in schizophrenia remain elusive. In 2012, by using both pharmacological and genetic manipulations of D2R activity, we find that D2R regulates dendritic spine morphogenesis, and that the effect of D2R overactivation on spines can be alleviated by antipsychotics blocking D2R. These findings provide evidence that D2R dysfunction in schizophrenia contributes to neuronal dysconnectivity and consequent cognitive impairment.