Molecular Mechanisms of Non-ionotropic NMDA Receptor Signaling in Dendritic Spine Shrinkage

Abstract

Structural plasticity of dendritic spines is a key component of the refinement of synaptic connections during learning. Recent studies highlight a novel role for the NMDA receptor (NMDAR), independent of ion flow, in driving spine shrinkage and LTD. Yet little is known about the molecular mechanisms that link conformational changes in the NMDAR to changes in spine size and synaptic strength. Here, using two-photon glutamate uncaging to induce plasticity at individual dendritic spines on hippocampal CA1 neurons from mice and rats of both sexes, we demonstrate that p38 MAPK is generally required downstream of non-ionotropic NMDAR signaling to drive both spine shrinkage and LTD. In a series of pharmacological and molecular genetic experiments, we identify key components of the non-ionotropic NMDAR signaling pathway driving dendritic spine shrinkage, including the interaction between NOS1AP (nitric oxide synthase 1 adaptor protein) and neuronal nitric oxide synthase (nNOS), nNOS enzymatic activity, activation of MK2 (MAPK-activated protein kinase 2) and cofilin, and signaling through CaMKII. Our results represent a large step forward in delineating the molecular mechanisms of non-ionotropic NMDAR signaling that can drive shrinkage and elimination of dendritic spines during synaptic plasticity.

Non-ionotropic NMDA receptor signaling gates bidirectional structural plasticity of dendritic spines

“…Ion flow-independent NMDAR signaling has been implicated by many independent studies in spine shrinkage and synaptic weakening (Nabavi et al, 2013;Aow et al, 2015;Birnbaum et al, 2015;Stein et al, 2015;Carter and Jahr, 2016;Wong and Gray, 2018;Stein et al, 2020;Thomazeau et al, 2020). Here, we made the unexpected discovery that this non-ionotropic NMDAR signaling pathway is also required for spine growth during synaptic strengthening.…”

“…Here, we made the unexpected discovery that this non-ionotropic NMDAR signaling pathway is also required for spine growth during synaptic strengthening. It may appear contradictory that the same signaling pathway could support both spine shrinkage and spine growth; however, it is notable that cofilin activation, which has been implicated in spine shrinkage during LTD (Zhou et al, 2004;Hayama et al, 2013;Stein et al, 2020) is also important for long-term spine growth during LTP (Bosch et al, 2014). We propose a model for spine structural plasticity (Fig.…”

“…The requirement of p38 MAPK for spine growth, but not for LTP, and the previously implicated role of p38 MAPK in non-ionotropic signaling during LTD and spine shrinkage led us to the unexpected hypothesis that non-ionotropic NMDAR signaling might be required for bidirectional spine structural plasticity. We have previously shown that, upstream of p38 MAPK, nonionotropic NMDAR-dependent spine shrinkage is dependent on the interaction between NOS1AP and nNOS, and that remodeling of the actin cytoskeleton downstream of p38 MAPK is mediated by MK2 and cofilin (Stein et al, 2020). We therefore set out to test whether these molecules are also required for LTP-induced spine growth.…”

“…Glutamate binding to NMDARs initiates a signaling cascade that drives synaptic weakening (LTD) and the shrinkage of dendritic spines (sLTD), even when ion flow is blocked pharmacologically (Nabavi et al, 2013;Stein et al, 2015;Carter and Jahr, 2016;Wong and Gray, 2018). Furthermore, in the absence of ion flux, patterns of glutamatergic stimulation that would normally drive LTP and spine growth, instead drive LTD and spine shrinkage (Nabavi et al, 2013;Stein et al, 2015;Stein et al, 2020). Because the non-ionotropic NMDAR signaling pathway should also be activated with glutamate binding when ion flow through the receptor is not blocked, we wondered whether it plays a role in bidirectional synaptic plasticity.…”

“…Indeed, imaging studies using FRET reporters have shown that NMDA or glutamate binding triggers conformational changes in the NMDAR intracellular domains, and changes its interaction with calcium/calmodulin-dependent protein kinase II (CaMKII) and protein phosphatase 1 (PP1) (Aow et al, 2015;Dore et al, 2015;Ferreira et al, 2017). p38 MAPK has been identified as a key component of the molecular pathway downstream of NMDAR conformational signaling (Nabavi et al, 2013;Stein et al, 2015), and a recent study further identified nNOS, nNOS-NOS1AP interactions, MAPKactivated protein kinase 2 (MK2) and cofilin as part of this signaling pathway (Stein et al, 2020).…”