Lrfn2-Mutant Mice Display Suppressed Synaptic Plasticity and Inhibitory Synapse Development and Abnormal Social Communication and Startle Response

Abstract

SALM1 (SALM (synaptic adhesion-like molecule), also known as LRFN2 (leucine rich repeat and fibronectin type III domain containing), is a postsynaptic density (PSD)-95-interacting synaptic adhesion molecule implicated in the regulation of NMDA receptor (NMDAR) clustering largely based on data, although its functions remain unclear. Here, we found that mice lacking SALM1/LRFN2 ( mice) show a normal density of excitatory synapses but altered excitatory synaptic function, including enhanced NMDAR-dependent synaptic transmission but suppressed NMDAR-dependent synaptic plasticity in the hippocampal CA1 region. Unexpectedly, SALM1 expression was detected in both glutamatergic and GABAergic neurons and CA1 pyramidal neurons showed decreases in the density of inhibitory synapses and the frequency of spontaneous inhibitory synaptic transmission. Behaviorally, ultrasonic vocalization was suppressed in pups separated from their mothers and acoustic startle was enhanced, but locomotion, anxiety-like behavior, social interaction, repetitive behaviors, and learning and memory were largely normal in adult male mice. These results suggest that SALM1/LRFN2 regulates excitatory synapse function, inhibitory synapse development, and social communication and startle behaviors in mice. Synaptic adhesion molecules regulate synapse development and function, which govern neural circuit and brain functions. The SALM/LRFN (synaptic adhesion-like molecule/leucine rich repeat and fibronectin type III domain containing) family of synaptic adhesion proteins consists of five known members for which the functions are largely unknown. Here, we characterized mice lacking SALM1/LRFN2 (SALM1 KO) known to associate with NMDA receptors (NMDARs) and found that these mice showed altered NMDAR-dependent synaptic transmission and plasticity, as expected, but unexpectedly also exhibited suppressed inhibitory synapse development and synaptic transmission. Behaviorally, SALM1 KO pups showed suppressed ultrasonic vocalization upon separation from their mothers and SALM1 KO adults showed enhanced responses to loud acoustic stimuli. These results suggest that SALM1/LRFN2 regulates excitatory synapse function, inhibitory synapse development, social communication, and acoustic startle behavior.

Sorting nexin-27 regulates AMPA receptor trafficking through the synaptic adhesion protein LRFN2

“…Importantly, we have also shown that LRFN2 depletion caused a loss of AMPA receptor-mediated synaptic activity, consistent with a role for LRFN2 in SNX27-dependent AMPA receptor trafficking. We also observed an attenuated activity-dependent hippocampal LTP upon LRFN2 suppression, which supports the study of Li and colleagues who reported a reduction of LTP in an LRFN2 knockout mouse model (59). In contrast, Morimura and colleagues reported an increase in silent synapses and an enhancement of LTP in another LRFN2 knockout mouse model (58).…”

“…While the LRFNs have been associated with inhibitory synapses (59) and at the pre-synapse (60), the function of this protein family has principally been linked with organising excitatory synapses (37-39, 41, 61, 62). The PDZ binding motif present in LRFN1, LRFN2 and LRFN4 can associate with PSD-95 (37-39, 63) and, for LRFN2, its extracellular and transmembrane domains associate with the obligatory GluN1 subunit of NMDA receptors and serves to cluster these receptors at the synaptic surface (39,40).…”

SALM 1 controls synapse development by promoting F‐actin/PIP2‐dependent Neurexin clustering

“…Constitutive KO models often lead to milder phenotypes compared to acute CAM depletion using RNAi‐mediated knockdown or conditional KO models (Chih et al , ; Chubykin et al , ; Etherton et al , ; de Wit et al , ). Consistently, two independent studies have recently reported milder phenotypes for SALM1 constitutive KO mice compared to our acute, RNAi‐mediated SALM1 knockdown method (Morimura et al , ; Li et al , ). These studies found little to no effect on hippocampal excitatory synaptic density in SALM1 constitutive KO models.…”

High-Frequency Neuronal Oscillatory Abnormalities in the Phospholipase C-β1 Knockout Mouse Model of Schizophrenia

“…A weak-moderate prepulse stimulus of 8 dB above background was chosen, which in our experience reliably elicits PPI in rodents (Hudson et al, 2016) while minimizing the potential for any startle responses itself. Previous studies in mice have shown negligible startle responses with auditory stimulus <20 dB above background (Weber et al, 2015; Li et al, 2018).…”

“…The most apparent electrophysiological finding from this study was that PLC-β1 −/− mice exhibited a reduction in the power and phase-locking (i.e., inter-trial coherence) of evoked beta and gamma oscillations elicited by auditory stimuli (specifically, the prepulse) during PPI. Importantly, the prepulse is of sufficiently high volume to elicit an electrophysiological response, but 8 dB above background, which is too low to elicit any behavioral (startle) responses itself (Weber et al, 2015; Li et al, 2018), so these measures of evoked oscillations are unlikely to be influenced by movement. While measures of evoked power and coherence both provide information regarding the oscillatory dynamics associated with a particular stimulus, they are independent—a reduction in evoked power indicates a reduction in the amplitude of an event-related oscillatory response, whereas a reduction in inter-trial coherence is indicative of a discrepancy in time-locking of the oscillatory event to the stimulus of interest.…”