Astrocyte regulation of neuronal AMPA glutamate receptors

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ABSTRACT This project addresses the role of astrocytes in regulating neuronal synapses, specifically by regulating levels of AMPA glutamate receptors (AMPARs) at postsynaptic sites. Astrocyte-secreted glypican 4 and 6 (Gpc4&6) are sufficient to induce nascent synapses by clustering calcium-permeable GluA1 AMPA receptors on the postsynaptic dendrite. Further, the mechanism Gpc4 employs is by signaling through presynaptic protein tyrosine phosphatase receptor delta, to induce release of the AMPAR clustering factor neuronal pentraxin 1. In this proposal the mouse visual cortex (VC) will be used to study the relative contribution of astrocyte-expressed glypicans to synapse formation, maturation and plasticity. Preliminary studies showed Gpc4&6 are most highly expressed by astrocytes at the time of synapse formation, fitting with their role inducing nascent synapses, while another astrocyte enriched glypican, Gpc5, is upregulated with synapse maturation and remains high in the adult, suggesting it may regulate synapse maturation or plasticity. This proposal addresses 3 questions related to the role of astrocyte-enriched glypicans in the formation and maturation of excitatory synapses in the VC. 1) What is the role of astrocyte-expressed Gpc5 in synapse maturation and plasticity? Mice lacking Gpc5 in astrocytes have decreased presynaptic size of thalamo-cortical synapses, and altered recruitment of postsynaptic AMPARs. Based on this it is hypothesized that Gpc5 regulates presynaptic maturation, which will be investigated using electrophysiology, immunohistochemistry and electron microscopy. Further, as Gpc4&6 regulate GluA1 AMPARs in development, the hypothesis that Gpc5 is required to increase synaptic levels of GluA1 during plasticity in the adult brain will be tested. 2) What is the role of Gpc4 in excitatory synapse formation onto inhibitory interneurons? Whether astrocytes regulate excitatory synapses onto inhibitory interneurons (INs) is an important unanswered question. As synapses onto INs contain high levels of GluA1 AMPARs, which Gpc4 regulates on principal neurons, it is hypothesized that Gpc4 induces synapses onto INs. Preliminary experiments found a deficit in GluA1 on IN dendrites in the Gpc4 KO. This study will investigate the mechanism underlying this deficit using electrophysiology and immunohistochemistry; ask if the same synaptogenic pathway is employed as at principal neuron synapses; and if lack of Gpc4 affects plasticity at IN synapses. 3) What is the relative contribution of astrocyte-expressed Gpc4&6 to synaptic function? Gpc4&6 are redundant in vitro in inducing synaptic activity. Preliminary studies found Gpc4&6 have different spatial and temporal expression in the developing VC, leading to the hypothesis they have stage and synapse-specific roles in synaptogenesis. This will be analyzed using astrocyte-specific Gpc4 and Gpc6 single KO mice, and astrocyte-specific Gpc4/Gpc6 double KO mice, and synapse formation and function across cortical layers and timepoints examined using electrophysiology and immunohistochemistry. These experiments will reveal region, time and synapse-specific roles of multiple astrocyte glypicans in sculpting neuronal circuit development.