Loss of synaptopodin impairs structural and functional mGluR-LTD at hippocampal 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 brain 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 mature and more stable dendritic spines containing synaptopodin (SP), an actin-associated protein involved in local protein synthesis and calcium dynamics, are critical for structural 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 employ a combination of electrophysiological and biochemical methods and 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.