Astrocyte regulation of neuronal AMPA glutamate receptors

描述(由申请人提供)：发育过程中神经元突触的正确形成和成熟对神经元终生回路功能至关重要。一种能力 神经元形成突触并不完全是固有的，因为突触的形成和功能可以通过与大脑中其他类型的细胞相互作用来调节，包括星形胶质细胞。星形胶质细胞分泌调节谷氨酸能突触形成的因子，包括增加突触上AMPA谷氨酸受体(AMPAR)数量的因子。Glypicans 4和6(GPC4和6)是星形胶质细胞分泌的因子，是增加突触AMPAR水平和突触强度的必要条件和充分条件。全球缺乏GPC4的小鼠海马神经元中的谷氨酸能突触较弱，这表明在体内星形胶质细胞和GPC4在突触发育中发挥着重要作用。突触的AMPAR水平决定突触反应的大小，而突触位置AMPAR的调节增加和移除是学习和记忆的分子机制。因此，确定GPC4用来调节突触AMPAR的机制对于理解突触强度在发育过程中如何正常调节，在学习和记忆过程中如何改变，以及在自闭症和精神分裂症等神经疾病中如何被错误调节具有重要意义。这项建议解决了有关GPC4作用机制的主要悬而未决的问题，GPC4将增加对发育中大脑中星形胶质细胞-神经元相互作用的了解。1)GPC4在向突触募集AMPAR方面的作用相对较慢，在目标1中，我们研究了GPC4通过调节神经元中促突触生成因子的转录来发挥作用的假说。2)星形胶质细胞在生后发育早期高水平表达GPC4，在突触形成旺盛时期，随着突触数量趋于稳定，GPC4在星形胶质细胞中的表达随发育成熟而降低。目的2研究激活神经元信号下调星形胶质细胞GPC4表达，从而限制神经元兴奋性的假说。3)GPC4基因缺陷小鼠海马区突触结构存在缺陷，但目前尚不清楚是否所有突触都受到同样的影响，或者在特定的突触亚群中存在缺陷。目标3将确定GPC4是否平等地调节神经元上所有突触的强度，例如通过转录控制(目标1)，或者可以局部调节特定突触亚群的强度。这将区分星形胶质细胞和GPC4在突触发育中的全球许可和局部指导作用。4)在成人脑兴奋性神经元亚群中，当星形胶质细胞GPC4的表达降低时，GPC4的表达上调。目标4研究了星形胶质细胞和神经元GPC4在发育和可塑性中对神经元突触功能的相对贡献。这些实验将对星形胶质细胞和神经元在整个生命过程中双向相互作用的分子机制提供重要的见解，从而导致神经回路的正确形成和功能。

DESCRIPTION (provided by applicant): The correct formation and maturation of neuronal synapses during development is essential for neuronal circuit function throughout life. The ability of neurons to form synapses is not entirely intrinsic, as synapse formation and function can be regulated by interactions with other cell types in the brain, including astrocytes. Astrocytes secrete factors that regulate the formation of glutamatergic synapses, including factors that increase the number of AMPA glutamate receptors (AMPARs) at synapses. Glypicans 4 and 6 (Gpc4 and 6) are astrocyte secreted factors that are necessary and sufficient to increase synaptic levels of AMPARs and synaptic strength. Mice globally lacking Gpc4 have weaker glutamatergic synapses in hippocampal neurons, demonstrating an important in vivo role for astrocytes and Gpc4 in synaptic development. The levels of AMPARs at a synapse determine the size of the synaptic response, and the regulated addition and removal of AMPARs at synaptic sites is the molecular mechanism underlying learning and memory. Hence, identifying the mechanisms that Gpc4 uses to regulate synaptic AMPARs has important implications for understanding how synaptic strength is normally regulated during development, how it is altered during learning and memory, and how it can be misregulated in neurological disorders such as autism and schizophrenia. This proposal addresses major unanswered questions regarding the mechanism of action of Gpc4 which will increase knowledge of astrocyte-neuron interactions in the developing brain. 1) The action of Gpc4 in recruiting AMPARs to synapses is relatively slow, and in Aim 1 the hypothesis that Gpc4 acts by regulating the transcription of pro-synaptogenic factors in neurons is investigated. 2) Astrocytes express high levels of Gpc4 in early postnatal development during periods of intense synapse formation, and Gpc4 expression in astrocytes decreases with maturation when synapse numbers have stabilized. Aim 2 investigates the hypothesis that active neurons signal to downregulate the expression of Gpc4 in astrocytes, thus limiting neuronal excitability. 3) Gpc4 deficient mice have defects in hippocampal synapse formation, but it is not known whether all synapses are affected equally, or there are defects in specific subsets of synapses. Aim 3 will determine if Gpc4 regulates the strength of all synapses on a neuron equally e.g. by transcriptional control (Aim 1), or can locally regulate the strength o specific subsets of synapses. This will differentiate between a globally permissive and locally instructive role for astrocytes and Gpc4 in synaptic development. 4) In the adult brain subsets of excitatory neurons upregulate expression of Gpc4, at a time when astrocyte expression of Gpc4 has decreased. The relative contribution of astrocytic and neuronal Gpc4 to neuronal synaptic function in development and plasticity are investigated in Aim 4. These experiments will give important insight into the molecular mechanisms astrocytes and neurons use to bidirectionally interact throughout life, leading to the correct formation and function of neural circuits.