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重要过程。突触强度的变化可能是短暂的，也可能持续很长一段时间。突触可塑性的持久形式在发育过程中以及学习和记忆中神经元连接的细化中起着至关重要的作用。在哺乳动物中，突触传递的NMDA受体依赖性长时程增强（LTP）和长时程抑制（LTD）是突触可塑性的两种主要形式。 AMPA受体在突触内外的运动可能是LTP和LTD中突触效能变化的细胞机制。 半胱天冬酶是参与程序性细胞死亡或细胞凋亡的关键蛋白水解酶，并且被分组为作用于细胞凋亡的外在（配体结合）和内在（线粒体）途径的启动子或效应子。活性半胱天冬酶的存在被认为导致不可逆的细胞死亡，因此是广泛使用的凋亡细胞的标记物。然而，活性半胱天冬酶也可以在注定不会死亡的细胞中检测到，并且现在广泛接受的是，半胱天冬酶可以在各种发育和生理环境中发挥非凋亡作用。 2012年，我们综述了半胱天冬酶在生理和病理条件下改变突触传递的功能及其与认知的相关性。在过去的几年里，来自几个小组的研究共同指出，半胱天冬酶在突触可塑性中的重要功能，独立于神经元细胞死亡。 两个启动器半胱天冬酶，1和9，和效应器半胱天冬酶-3调节海马神经元中的持久突触可塑性。我们的小组提供了令人信服的证据表明，NMDA受体依赖性LTD的诱导严重依赖于caspase-3的激活，活性caspase-3足以诱导突触抑制，BAD和BAX诱导的线粒体释放细胞色素c在LTD中激活caspase-3中起着至关重要的作用。 突触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，据报道半胱天冬酶-3和-1有助于学习和记忆，如在斑胸草雀中所见，其中半胱天冬酶-3是鸟鸣学习期间记忆巩固所必需的。此外，在老年大鼠中长期脑灌注半胱天冬酶-1抑制剂改善海马依赖性上下文记忆。也有新的证据表明，半胱天冬酶在阿尔茨海默病的早期阶段是活跃的，并且可以在细胞死亡和神经变性出现之前介导突触功能障碍和丧失。这些新的见解突出了正常和病理条件下，半胱天冬酶在突触可塑性中的调节作用。 尽管半胱天冬酶在突触可塑性中的重要性，但半胱天冬酶控制突触传递的机制仍不清楚。2012年，我们与Sanford Markey博士（NIMH）合作，筛选可能参与LTD的caspase底物。发现LTD诱导后神经元中的几种蛋白质被切割。通过半胱天冬酶切割试验证实了这些推定的半胱天冬酶底物在LTD过程中的蛋白水解，并通过诱变确定切割位点。我们已经产生了抗半胱天冬酶的突变体，并开始测试它们对突触传递的影响。 除了caspase途径，我们还研究了多巴胺D2受体（D2 R）在突触发育中的作用。D2 R主要通过调节突触传递在脑功能中起关键作用。精神分裂症中的D2 R功能障碍是众所周知的。所有的抗精神病药物都能拮抗D2 R，但对认知障碍影响甚微，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。精神分裂症中神经元连接障碍的潜在机制仍然难以捉摸。在2012年，通过使用D2 R活性的药理学和遗传操作，我们发现D2 R调节树突棘形态发生，并且D2 R过度激活对棘的影响可以通过阻断D2 R的抗精神病药物来减轻。这些发现提供了证据表明，精神分裂症中的D2 R功能障碍有助于神经元连接障碍和随后的认知障碍。

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.