Final Report Summary - NMDAR-SYNPRUN (The roles of juvenile NMDA receptors in synapse maturation and elimination and their association with cognition) Nerve cells transmit information across structures called synapses. During childhood, brain is subject to postnatal development which is characterized by an overproduction of synapses. Neuronal activity later refines this basic circuitry by strengthening and stabilizing certain synapses but eliminating (or “pruning”) others. This is a fundamental process because it defines mature neural circuits from initially redundant connections, which is critical for memory encoding. A key event that determines whether individual synapses will be selected for stabilization versus pruning is the replacement of immature by mature forms of a type of receptor named NMDA (NMDARs) as a function of synapse use. Yet the factors that control the precise developmental timing and synapse specificity of this process, and how they impart (or limit) synapse stability remain poorly understood. Functional NMDAR are formed by combinations of GluN1 subunit, GluN2 (A-D) and GluN3 (A, B) subunits. GluN2 and GluN3 subunits confer distinct properties to the receptor, and influence their signalling and localization on the neuronal surface. In the adult brain, although there is not an absolute segregation of GluN2A and GluN2B subunits to synaptic and extrasynaptic sites, it has been accepted than GluN2A–containing NMDARs (GluN2A-NMDARs) concentrate in postsynaptic densities (PSDs) whereas GluN2B-NMDARs can be found at both synaptic and extrasynaptic locations. The expression levels of GluN2A subunits increase after birth promoting the substitution of GluN2B subunits, which are expressed from birth, by GluN2A, which become expressed in virtually every CNS region by adulthood. Incorporation of GluN2A subunits into NMDAR complexes enhances their attachment to the PSD and shortens the duration of NMDAR-mediated currents, changing the temporal integration of synaptic inputs, and ultimately affects the ability of synapses to be modified by experience.To date, most studies have focused on the roles of the developmental exchange of GluN2B by GluN2A subunits on the stabilization and maturation of excitatory synapses and the emergence of mature neural circuits. As part of the synaptic stabilization process, NMDARs exchange by lateral diffusion between extrasynaptic and synaptic plasma membranes. In this context, the rate of mobility of the receptor within the plasma membrane and the receptor affinity for transmembrane and intracellular synaptic scaffolds will determine their concentration and distribution between synaptic and extrasynaptic sites. Importantly, both parameters vary depending on the type of GluN2 subunit present in the receptor complex. Indeed, GluN2A–NMDARs are less mobile in the neuronal surface and more retained within synapses than GluN2B–NMDARs. Overexpression of GluN2A subunits decreases the surface diffusion of GluN2B-NMDARs, suggesting a dominant role for GluN2A in the surface dynamics of NMDARs containing this subunits. Yet to date there is no information regarding the mobility of GluN3A-NMDARs. Here we imaged surface GluN3A-NMDARs in live hippocampal neurons and evaluated the impact of GluN3A subunits on the dynamics of synaptic NMDARs. Using a combination of high-resolution single nanoparticle imaging, electrophysiological and biochemical approaches, we show that the diffusion dynamics and synaptic accumulation of GluN2A-, but not GluN2B-NMDARs, are controlled by GluN3A subunits. Our data demonstrate that GluN3A expression increases the lateral mobility of surface GluN2A, and this prevents the stabilization and reduces the synaptic content of GluN2A-NMDARs in immature neurons. We propose that, at early after birth, high natural GluN3A levels contribute to keep mature GluN2A subunits away from synapses and sustain the synaptic expression of GluN2B-NMDARs. Later GluN3A down-regulation permits the incorporation of GluN2A-NMDARs to synapses and drives the maturation and consolidation of adult neuronal networks. In addition, GluN3A subunits binds G protein-coupled receptor kinase-interacting protein (GIT1), a postsynaptic scaffold that assembles actin regulatory complexes, including a factor called βPIX, that promote the activation of a kinase actin signalling pathway named Rac1/p21 in spines. Binding to GluN3A limits the synaptic localization of GIT1 and its ability to form complexes with βPIX, leading to decreased Rac1 activation and reduced spine density and size in neurons. As part of a collaboration work with Dr. Maria Fiuza, our results identify inhibition of Rac1/p21-activated kinase actin signaling pathways as an activity-dependent mechanism mediating the inhibitory effects of GluN3A on spine shape.Disturbances in the balance between synapse maturation and elimination have been implicated in a number of disorders of cognition, including mental retardation, autism, and schizophrenia, that exhibit reduced densities of spines and synapses or a predominance of immature spine morphologies. Our study reveals a mechanism that could explain the inhibitory effects of GluN3A subunits on spine maturation and consolidation under physiological and pathological conditions.TARGET GROUPS. Huntington disease patients, individual with mental disorders.
