Altered surface mGluR5 dynamics provoke synaptic NMDAR dysfunction and cognitive defects in Fmr1 knockout mice

Abstract

Metabotropic glutamate receptor subtype 5 (mGluR5) is crucially implicated in the pathophysiology of Fragile X Syndrome (FXS); however, its dysfunction at the sub-cellular level, and related synaptic and cognitive phenotypes are unexplored. Here, we probed the consequences of mGluR5/Homer scaffold disruption for mGluR5 cell-surface mobility, synaptic N-methyl-D-aspartate receptor (NMDAR) function, and behavioral phenotypes in the second-generation Fmr1 knockout (KO) mouse. Using single-molecule tracking, we found that mGluR5 was significantly more mobile at synapses in hippocampal Fmr1 KO neurons, causing an increased synaptic surface co-clustering of mGluR5 and NMDAR. This correlated with a reduced amplitude of synaptic NMDAR currents, a lack of their mGluR5-activated long-term depression, and NMDAR/hippocampus dependent cognitive deficits. These synaptic and behavioral phenomena were reversed by knocking down Homer1a in Fmr1 KO mice. Our study provides a mechanistic link between changes of mGluR5 dynamics and pathological phenotypes of FXS, unveiling novel targets for mGluR5-based therapeutics.

Regulation and Function of Activity-Dependent Homer in Synaptic Plasticity

“…As a result, Fmr1 KO mice displayed a decrease in NMDA current amplitude, and mGluR-mediated LTD of postsynaptic NMDA currents was absent in the CA1 of Fmr1 KO mice. Deficits in hippocampal-dependent memory and NMDA receptor function/plasticity were rescued by specific knockdown of Homer1a in the CA1 of Fmr1 KO mice [152]. Together, these studies suggest that a higher ratio of short/long Homer proteins interacting with mGluR5 is responsible for disrupting mGluR5-mediated signaling in fragile X syndrome, contributing to the disorder.…”

“…In more recent work, the consequences of disrupted mGluR5–long Homer interactions were assessed in a new, second-generation Fmr1 KO mouse model with total absence of detectable Fmr1 transcripts [152, 153]. In this genetic model, mGluR5 was found to be more mobile at hippocampal synapses, resulting in increased clustering of mGluR5 with NMDA receptors.…”

“…In Fmr1 KO mice modeling Fragile X syndrome, the deletion of murine Fmr1 alters the balance between long and short Homer1, although the precise mechanism is unknown [149, 150]. This results in a bias toward short Homers interacting with mGluR5, which deleteriously enhances mGluR5 processes of protein synthesis and mGluR-LTD; additional phenotypes observed in a newer, 2nd generation of Fmr1 KO mice are in italics [152]. Similar imbalances between HOMER1 isoforms have been recorded in schizophrenia patients [136, 161]; a disorder that that can relate to FMR1 dysregulation directly or indirectly via targets of CYFIP1 and FMRP.…”

“…Similar imbalances between HOMER1 isoforms have been recorded in schizophrenia patients [136, 161]; a disorder that that can relate to FMR1 dysregulation directly or indirectly via targets of CYFIP1 and FMRP. Intriguingly, in a preclinical model of Fmr1 and Homer1a haploinsufficiency, the removal of Homer1a shifts the balance back to mGluR5-long Homer interactions, resulting in rescue of many Fragile X Syndrome-relevant phenotypes [152]. mGluR5, metabotropic glutamate receptor 5; NMDAR, N-methyl-D-aspartate receptor; FMRP, Fragile X mental retardation protein; LTD, long-term depression.…”

Activity-dependent changes in synaptic protein complex composition are consistent in different detergents despite differential solubility

“…We initially examined the effect of lysis buffer reagents on Homer_mGluR5, a well characterized synaptic protein interaction known to play an important role in glutamatergic signaling (19,26,29,35). Significantly more Homer_mGluR5 co-association was detected in NP-40 and Triton compared to DOC ( Figure 3B, C).…”