Early mechanisms mediating the organization of the axon initial segment impact the formation of axo-axonic synapses - PROJECT SUMMARY Brain development is a complex process that requires the production and differentiation of numerous cells and cell types, with the additional challenge of spatial and temporal precision in the neural circuitry that connects these cells. Improper formation of neural circuitry leads to impaired control over brain activity patterns and is broadly thought to contribute to a number of childhood neurological disorders. Multiple lines of research suggest that inhibitory GABAergic circuitry in particular contributes to the pathophysiology of neurodevelopmental disor- ders, yet these disorders remain poorly treated. This project is focused on the GABAergic synapse formed be- tween a chandelier cell and a specialized neuronal compartment called the axon initial segment (AIS), forming an axo-axonic synapse. Chandelier cells exert powerful control over neural activity patterns exerting shunting inhibition at the AIS of large groups of principal cells, which impacts their probability of generating an electrical signal. Axo-axonic synapses are characterized by GABAA receptors containing the α2 subunit and collybistin, a specific interacting partner we have recently identified. We have developed and characterized a mouse model with a substitution mutation in the GABAA receptor α2 subunit (Gabra2-1) that diminishes interaction with colly- bistin, causing reduced numbers of axo-axonic synapses and spontaneous seizures during development. On this premise, we hypothesize that the α2 subunit contributes to the organization of the AIS in order to facilitate the formation of axo-axonic synapses. In this proposal we will examine axo-axonic synapse development and AIS organization using immunohistochemistry and expansion microscopy, as well as circuit dysfunction using cortical field recordings. We will examine how these changes correlate with symptom onset in the Gabra2-1 mouse and also evaluate novel methods to replace lost control by using a novel light activated cation channel targeted to the AIS. Relevance to human health: The proposed project is expected to yield detailed information about how the AIS organizes during development, as well as the formation of axo-axonic synapses in both typical and pathological conditions, providing both basic knowledge about brain development and translational insights for neurodevelopmental disorders. This proposal also offers a novel means of actively manipulating the AIS, a site known for its potential to control cortical activity patterns. Outcomes: This work will be conducted entirely by undergraduate and graduate students at UNLV, which is one of the nation’s most diverse campuses. Thus, this project will provide instrumental training opportunities for the diversification of the biomedical workforce, as well as enhance the research environment of our institution.
