Astrocyte-neuron circuits underlying cortical mechanisms of learned behavior

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Astrocytes are the major non-neuronal cell type in the cortex and are increasingly recognized as key contributors to the development, plasticity and function of neuronal circuits. Yet, how they participate with neurons in learned behavior and dynamically shape the underlying cortical circuits is poorly understood. The primary motor cortex is required for learning and executing voluntary movements: the acquisition of a cued, stereotyped, movement in mice is accompanied by synaptic remodeling of motor cortex neurons and the emergence of coordinated movement-related ensemble neuronal activity. Here, we propose to examine functional astrocyte mechanisms in motor cortex that mediate synaptic plasticity and neuronal dynamics during motor learning. Astrocytes have highly ramified fine processes that contact nearly all synapses in the cortex, where they modulate synaptic transmission and plasticity by mechanisms that include uptake of glutamate and GABA, primarily via the transporters GLT1 and GAT3 respectively. Astrocytes also respond to, as well as modulate, synaptic activity with spatiotemporally heterogeneous calcium transients in their processes, termed microdomains. We will examine the role of astrocytes in shaping motor cortex circuits as mice learn a forelimb lever push movement, including cued response onset and reliable movement trajectory, using a range of cutting-edge approaches: simultaneous high-resolution imaging of astrocytes and neurons in vivo, computational encoding-decoding models of astrocyte and neuronal activity, astrocyte-specific gene expression analyses, and novel astrocyte optogenetic and CRISPR tools alongside established chemogenetic and viral knockdown methods. Building on our preliminary data, which demonstrate parallel learning-related changes in astrocyte microdomain responses and neuronal responses, along with gene expression changes in astrocyte GLT1 and GAT3, in Aim 1 we will determine functional astrocyte calcium signatures in motor cortex during learning and their relationship to neuronal activity and behavior. We hypothesize that astrocytes shape neuronal plasticity during task learning with corresponding plasticity in their microdomain calcium responses, which we will specify computationally. In Aim 2, we will determine the effect of astrocyte calcium signaling on motor learning and neuronal responses. We hypothesize that disruption of calcium transients alters the emergence of neuronal ensembles and expert behavior, potentially by altering astrocyte gene expression of transporter mechanisms. In Aim 3, we will determine the role of astrocyte neurotransmitter transporter function in motor cortex circuits and learning. We hypothesize that disrupting astrocytic modulation of excitatory transmission via GLT1, and inhibitory neurotransmission via GAT3, disrupts astrocytic calcium responses together with neuronal circuit plasticity and behavior. Together, these studies will provide a mechanistic, computational view of astrocyte involvement in the function and plasticity of cortical circuits, reveal their task-specific contributions to neuronal responses and learned behavior, and provide the basis for understanding their role in a range of brain disorders and diseases.