Latrophilin Function in Synapse Formation

About This Grant

Neural circuits are constructed by synapses that connect neurons into vast networks. Although many neural circuits have been characterized, the molecular mechanisms that construct their synaptic architecture remain largely unknown. Synapse formation in neural circuits is thought to be controlled by bi-directional signaling via trans-synaptic adhesion molecules. Strikingly, genetic changes in trans-synaptic adhesion molecules often predispose to neuropsychiatric disorders such as autism and schizophrenia, suggesting that dysfunction of the synaptic architecture of neural circuits contributes to neuropsychiatric disorders, although the nature of the dysfunction is poorly understood. We previously showed that in hippocampal circuits, postsynaptic latrophilins (Lphns) perform a central activity-dependent role in organizing synapses by forming large and diverse trans- synaptic complexes. Surprisingly, Lphn-based complexes include not only presynaptic neurexins, teneurins and FLRTs but also postsynaptic Unc5s, proteins that were previously only known as repulsive axon-guidance cues and not as synapse organizers. Moreover, our previous findings demonstrate that Lphns act as adhesion-GPCRs that organize synapses via two parallel pathways, local cAMP signaling and recruitment of postsynaptic scaffold proteins as phase-separated condensates. However, how Lphn complexes mediate synapse formation in an activity-dependent manner, what the role of Unc5s is in these complexes, and how significant the function of different Lphn-based complexes is for circuit assembly remains largely unknown. To address this knowledge gap, the present application proposes to test the following overall hypotheses in four Specific Aims. Aim 1 will test the hypothesis that Lphns organize synapses by engaging in large trans-synaptic protein complexes that include teneurins, FLRTs, neurexins and Unc5s and that can be reconstituted from purified proteins with associated pre- and postsynaptic specializations; Aim 2 will test the hypothesis that the components of Lphn- based complexes perform distinct essential functions in synapse formation; Aim 3 will test the hypothesis that postsynaptic Lphns and Unc5s utilize diverse signaling pathways to collaboratively organize synapse assembly; and Aim 4 will test the hypothesis that the differential expression and activity-dependent alternative splicing of Lphn-based complex components regulate synapse diversity and circuit plasticity, enabling circuit adaptations critical for learning and memory. Although the Aims of this application are independent of each other, they together form a logical sequence that will advance our understanding of synapse assembly and, given the close genetic association of the pre- and postsynaptic components of Lphn-based complexes with autism and schizophrenia, provide insight into the pathogenesis of neuropsychiatric disorders. In pursuit of these Aims, the application proposes to employ mice as a primary model system and to use an interdisciplinary methodology ranging from reductionist biochemical experiments in order to reconstitute synapses from purified proteins to synaptic function assays in cultured neurons and in vivo to analyses of neural circuits and behavior.