Loss of synaptopodin impairs mGluR5 and protein synthesis dependent mGluR-LTD at CA3-CA1 synapses

Metabotropic glutamate receptor-dependent long-term depression (mGluR-LTD) is an important form of synaptic plasticity that occurs in many regions of the CNS and is the underlying mechanism for several learning paradigms. In the hippocampus, mGluR-LTD is manifested by the weakening of synaptic transmission and elimination of dendritic spines. Interestingly, not all spines respond or undergo plasticity equally in response to mGluR-LTD. A subset of dendritic spines containing synaptopodin (SP), an actin-associated protein, are critical for mGluR-LTD and protect spines from elimination through mGluR1 activity. The precise cellular function of SP is still enigmatic and it is still unclear how SP contributes to the functional aspect of mGluR-LTD despite of its modulation on the structural plasticity. In the present study, we show that the lack of SP impairs mGluR-LTD by negatively affecting the mGluR5-dependent activity. Such impairment of mGluR5 activity is accompanied by a significant decrease of surface mGluR5 level in SP knockout (SPKO) mice. Intriguingly, the remaining mGluR-LTD becomes a protein synthesis-independent process in the SPKO and is mediated instead by endocannabinoid signaling. These data show for the first time that the postsynaptic protein SP can regulate the locus of expression of mGluR-LTD and provide insight to our understanding of spine/synapse-specific plasticity. Significance statementHippocampal group I metabotropic glutamate receptor dependent long-term depression (mGluR-LTD), a form of learning and memory, is misregulated in many murine models of neurodevelopmental disorders. Despite extensive studies there is a paucity of information on the molecular mechanism underlying mGluR-LTD. Previously, we reported that loss of synaptopodin, an actin-associated protein found in a subset of mature dendritic spines, impairs mGluR-LTD. In the current study, we uncover the molecular and cellular deficits involved. We find that synaptopodin is required for the mGluR5-Homer interaction and uncover synaptopodin as a molecular switch for mGluR-LTD expression, as mGluR-LTD becomes protein synthesis-independent and relies on endocannabinoid signaling in synaptopodin knock-out. This work provides insight into synaptopodin as a gatekeeper to regulate mGluR-LTD at hippocampal synapses.