Regulation of excitatory synapses and fear memory by the synaptic adhesion molecule SALM4

Abstract

PART I Synaptic adhesion molecules regulate various aspects of synapse development, function, and plasticity. These functions mainly involve trans-synaptic interactions and positive regulations, whereas cis interactions and negative regulations are less understood. Here we report that SALM4, a member of the SALM/Lrfn family of synaptic adhesion molecules, suppresses excitatory synapse development through a cis inhibition of SALM3, another SALM family protein with synaptogenic activity. Salm4-mutant (Salm4$^{-/-}$) mice show increased excitatory synapse numbers in the hippocampus. SALM4 cis-interacts with SALM3, inhibits trans-synaptic SALM3 interaction with presynaptic LAR family receptor tyrosine phosphatases, and suppresses SALM3-dependent presynaptic differentiation. Importantly, additional deletion of Salm3 in Salm4$^{-/-}$mice (Salm3$^{-/-}$

Salm4$^{-/-}$) normalizes the increased excitatory synapse number. These results suggest that SALM4 negatively regulates excitatory synapses via cis inhibition of the trans-synaptic SALM3-LAR adhesion.

PART II I analyzed Salm4-mutant (Salm4$^{-/-}$) mice using behavioral, electrophysiological, and biochemical approaches. Salm4$^{-/-}$ mice exhibit increased contextual fear memory and impaired fear extinction that is alleviated by fluoxetine, a medication for PTSD. Salm4$^{-/-}$ mice at 3-4 postnatal weeks display reduced long-term potentiation (LTP), increased NMDA/AMPA ratio, and increased levels of the GluN2A subunit of NMDA receptors in hippocampus. These results suggest that SALM4 regulates excitatory synaptic transmission and plasticity, and fear conditioning behavior. In addition, given that reduced fear conditioning is associated posttraumatic stress disorder (PTSD), a psychiatric disorder characterized by repetitive flashbacks, avoidance of the reminder, and continuous hyperarousal, these results may help us understand the mechanisms underlying the PTSD.