Synapsin 2a/Synaptogyrin-3 interaction regulates fear extinction in mice

The mechanisms behind a lack of efficient fear extinction in some individuals are unclear. Here, by employing a principal components analysis (PCA)-based approach, we differentiated the mice into extinction-resistant and susceptible groups. We identified that elevated synapsin 2a (Syn2a) in the infralimbic cortex (IL) to basolateral amygdala (BLA) circuit disrupted presynaptic orchestration, leading to an excitatory/inhibitory imbalance in the BLA region and causing extinction resistance. Overexpression or silencing of Syn2a levels in IL neurons replicated or alleviated behavioral, electrophysiological, and biochemical phenotypes in resistant mice. We further identified the proline-rich domain H in the C-terminal of Syn2a was indispensable for the interaction with synaptogyrin-3 (Syngr3) and demonstrated that disrupting this interaction restored extinction impairments. Molecular docking revealed ritonavir, an FDA-approved HIV drug, could disrupt Syn2a-Syngr3 binding and rescue fear extinction behavior in Syn2a-elevated mice. In summary, aberrant Syn2a elevation and its interaction with Syngr3 at the presynaptic site were crucial in fear extinction resistance, suggesting a potential therapeutic avenue for related disorders.