SNX6在神经活性依赖的AMPAR内体-质膜转运中的功能研究

The excitatory synapse is composed of axonal boutons of presynaptic neurons and dendritic spines, membrane protrusions emanating from the plasma membrane of postsynaptic dendrites. Synaptic plasticity is modulation of synaptic strength in response to changes in neural activity. The induction and expression of synaptic plasticity relies on translocation of synaptic proteins between their site of synthesis and synaptic sites. In the central nervous system, dendritic spines are major sites for excitatory inputs. Although the structural and functional plasticity of spines rely heavily on vesicular transport and vesicle-plasma membrane interactions in dendrites, the mechanisms underlying neural activity-dependent membrane trafficking remain to be explored. Previously we found that SNX6 mediates dynein-dynactin-driven vesicular transport of the postsynaptic scaffold protein Homer1b/c, which is critical for the structural and functional integrity of dendritic spines. Most recently we found that SNX6 is also required for long-term potentiation (LTP), a classic form of synaptic plasticity. We also found that SNX6 interacts with both the glutamate receptor AMPAR, the key molecule that determines synaptic strength, and CaMKII, the master regulator of LTP. In order to investigate the molecular mechanisms for neural activity-dependent membrane trafficking in dendrites, we propose to employ biochemical, cell biological and neurobiological approaches to determine the role of SNX6-CaMKII interaction in regulation of translocation of the neurotransmitter receptor AMPAR to postsynaptic sites.

兴奋性突触由突触前神经元的轴突扣结与突触后神经元树突上的膜突起树突棘构成。突触可塑性是神经元细胞响应神经活性变化而调节其突触强度的特性。突触可塑性的产生依赖于突触蛋白如递质受体的转运。作为兴奋性突触的主要接收位点，树突棘的结构和功能可塑性依赖于树突中的囊泡运输及囊泡-质膜相互作用。然而树突中神经活性对膜运输的调控机制尚未阐明。先前我们发现，动力蛋白dynein-dynactin的介导因子SNX6通过介导突触支架蛋白Homer1b/c的囊泡运输参与维持树突棘的结构和功能。最近我们发现，突触可塑性的经典形式长时程突触增强需要SNX6活性，而且SNX6与递质受体AMPAR及可塑性核心调控蛋白CaMKII相互作用，提示SNX6介导AMPAR转运并受到神经活性调控。我们拟运用生化、细胞和神经生物学手段，探讨SNX6-CaMKII互作对于AMPAR靶向运输的调控作用，解析神经活性调控膜运输的分子机制。

神经活性调控递质受体运输是突触可塑性产生的重要机制。我们发现运输调节因子SNX6介导兴奋性递质受体AMPAR亚基GluA2分选进入树突运输途径，在SNX6基因敲除的海马神经元中GluA2被错误分选进入降解途径，导致AMPAR受体复合物组成改变，神经活性诱导的AMPAR质膜转运受到抑制，AMPAR介导的突触传递以及长时程突触增强（LTP）受损。这些结果表明AMPAR受体各亚基的运输及组装机制各不相同。同时，我们还发现突触增强信号诱导神经元树突干胞质内钙升高，使激酶PI4KIII转位到质膜促进膜脂PI4P合成，而活性诱导的AMPAR囊泡胞吐、LTP表达及海马区依赖的长期记忆依赖于质膜PI4P。我们继而发现神经元树突中内质网和质膜的互作受到神经活性调控，内质网-质膜膜接触位点的数量和面积在长时程突触诱导和表达过程中发生动态变化，膜栓系因子E-Syt1相应胞质钙离子浓度升高促进内质网-质膜膜接触位点形成及扩展，并调控神经活性诱导的AMPAR质膜运输。这些发现增进了我们对于树突运输、脂代谢和内质网-质膜互作在突触可塑性中的作用机制及调控作用的了解。