Selective Prevention of MDGA-Neuroligin-2 Interactions Increases Inhibitory Synaptic Transmission, Fear Memory, and Anxiety

Abstract

Within the CNS, synapse organizer proteins serve as potent modulators of synapse development and function. Among the known synapse organizers, neuroligins (NLs) have well-established roles in promoting synapse development. In contrast, MAM domain containing glycosylphosphatidylinositol anchor (MDGA) proteins repress synapse formation. Human gene linkage analysis has implicated NLs and MDGAs in brain dysfunction. For example, mutations in NL2 or MDGAs contribute to major neurodevelopmental disorders, including autism. However, how NL2/MDGA interactions specifically regulate synapse properties and behavioral outputs remains unclear. NL2 is an inhibitory synapse-specific synapse organizer, whereas MDGAs suppress synapses through blocking the interaction of NL2 and presynaptic NRXNs. To examine the roles of NL2-MDGA interactions in regulating excitatory (glutamatergic) and inhibitory (GABAergic) balance (E/I balance) in synaptic connectivity, we designed a novel and specific NL2ΔSite II transgenic mouse model with the NL2 site II mutated which selectively disrupts NL2/MDGA binding, without affecting other protein interactions. Using a combination of immunohistochemistry, diverse behavior paradigms, and electrophysiology techniques, we sought to determine how synapse development, neural transmission and behaviors are altered in male NL2ΔSite II transgenic mice. We found that NL2ΔSite II mice showed increased anxiety, elevated fear memory and impaired social memory. These mice also exhibited increased expression of inhibitory synapse proteins and upregulated GABAergic transmission whereas excitatory synapse proteins and synaptic transmission appeared normal. Overall, our results suggest that NL2ΔSite II specifically modulates inhibitory synapses and leads to behavioral abnormalities, which could have clinical implications for treatment of neurodevelopmental disorders including autism, schizophrenia and bipolar disorders. A second major project focused on addressing the critical question of how loss of both MDGAs (Mdga1/2 double knockout; DKO) in adult mice affects synapse development in vivo. Preliminary results revealed that MDGA1/2 DKO in hippocampal CA1 pyramidal neurons elevates intrinsic excitability of CA1 pyramidal neurons, increases excitatory synaptic transmission but decreases inhibitory synaptic transmission. These findings suggest dual loss of MDGAs elevates the excitation/inhibition ratio if favor of glutamatergic transmission which may have implications for understanding increased susceptibility to neurodevelopmental disorders following MDGA loss of function.