Regional and trans-regional heterogeneity of astrocyte morphology as a functional determinant of synaptic astrocyte-neuron interactions in the hippocampus

神经元和星形胶质细胞在突触水平上的快速相互信号交换调节突触传递及其可塑性。这种星形胶质细胞-神经元信号传导通常涉及可扩散的信号。因此，其效率可能取决于神经元结构和突触周围星形胶质细胞过程之间的空间接近度，从而取决于星形胶质细胞形态。因此，星形胶质细胞形态的异质性对突触传递具有重要意义。事实上，我们可以证明，星形胶质细胞在啮齿动物CA 1层辐射的结构异质性有重要的后果突触前释放在CA 3-CA 1突触，在细胞神经生理学的标准模型系统。使用双光子激发荧光显微镜和新的电生理方法的组合，我们可以揭示，星形胶质细胞的结构复杂性的变化决定突触前短期可塑性通过星形胶质细胞谷氨酸受体，可能通过星形胶质细胞过程的突触的差异覆盖。在这个项目中，我们现在将建立在这一发现的基础上，首先揭示星形胶质细胞形态的区域细胞间异质性如何决定神经元CA 3-CA 1突触和GABA能连接的局部覆盖。为此，我们建立了扩展显微镜。ExM使我们能够超分辨突触和突触周围星形胶质细胞结构之间的结构关系，这是衍射限制显微镜无法完全可视化的。其次，我们将扩大我们的调查，以相邻层的CA 1区，星形胶质细胞显示不同的形态，并涵盖其他树突状域相同的CA 1锥体细胞神经元和他们的传入兴奋性连接。通过将成像和电生理技术与ExM相结合，我们将揭示星形胶质细胞形态的跨区域异质性如何决定单个神经元细胞类型所接收的兴奋性突触的局部特性。第三，我们将建立星形胶质细胞的形态和它的细胞间和亚细胞的专业化决定了星形胶质细胞突触活动的反应。在这里，我们将集中在星形胶质细胞的过程，以mammatergic突触的接近程度决定的可能性，突触刺激的结果在当地的星形胶质细胞钙瞬变，以及它如何影响其幅度和潜在的信号级联。这些功能实验将再次得到使用ExM的结构分析的支持。第四，我们将探索人类组织样本中星形胶质细胞结构、其异质性和突触星形胶质细胞-神经元相互作用之间的关系。总之，我们将建立如何区域和跨区域的结构异质性和专业化的星形胶质细胞决定当地的突触覆盖星形胶质细胞的过程和功能的相关性星形胶质细胞-神经元的相互作用。

Synaptic transmission and its plasticity are modulated by fast reciprocal signal exchange between neurons and astrocytes on the synaptic level. This astrocyte-neuron signalling often involves diffusible signals. Therefore, its efficiency is likely to depend on the spatial proximity between neuronal structures and perisynaptic astrocyte processes and thus on astrocyte morphology. As a consequence, the heterogeneity of astrocyte morphology should have important implications for synaptic transmission. Indeed, we could demonstrate that the structural heterogeneity of astrocytes in the rodent CA1 stratum radiatum has important consequences for presynaptic release at the CA3-CA1 synapse, a standard model system in cellular neurophysiology. Using combinations of two-photon excitation fluorescence microscopy and novel electrophysiological approaches we could reveal that variations of the structural complexity of astrocytes determines presynaptic short-term plasticity via astrocytic glutamate receptors, likely through differential coverage of synapses by astrocyte processes. In this project, we will now build on this finding by first uncovering how regional intercellular heterogeneity of astrocyte morphology determines the local coverage of glutamatergic CA3-CA1 synapses and also GABAergic connections. To this end, we have established expansion microscopy. ExM enables us to super-resolve the structural relationship between synapses and perisynaptic astrocyte structures, which cannot be fully visualized by diffraction-limited microscopy. Second, we will extend our investigation to adjacent layers of the CA1 region, where astrocytes display different morphologies and cover other dendritic domains of the same CA1 pyramidal cell neurons and their incoming excitatory connections. By combining imaging and electrophysiological techniques with ExM we will reveal how trans-regional heterogeneity of astrocyte morphology determines the local properties of excitatory synapses received by a single neuronal cell type. Third, we will establish how astrocyte morphology and its intercellular and subcellular specialisation determines the responsiveness of astrocyte to synaptic activity. Here we will focus on how the proximity of astrocyte processes to glutamatergic synapses determines the likelihood that a synaptic stimulus results in a local astrocytic Ca2+ transients and how it affects their magnitude and underlying signalling cascades. These functional experiments will again be supported by structural analysis using ExM. Fourth, we will explore the relationship between astrocyte structure, its heterogeneity and synaptic astrocyte-neuron interactions in human tissue samples. In summary, we will establish how regional and trans-regional structural heterogeneity and specialisation of astrocytes determine local synaptic coverage by astrocyte processes and the functional relevance for astrocyte-neuron interactions.