Abstract
Immature glutamatergic synapses in cultured neurons contain high-release probability (Pr) presynaptic sites coupled to postsynaptic sites bearing GluN2B-containing NMDA receptors (NMDARs), which mature into low-Pr, GluN2B-deficient synapses. Whether this coordinated maturation of high-Pr, GluN2B(+) synapses to low-Pr, GluN2B-deficient synapses actually occurs in vivo, and if so, what factors regulate it and what role it might play in long-term synapse function and plasticity are unknown. We report that loss of the integrin-regulated Abl2/Arg kinase in vivo yields a subpopulation of "immature" high-Pr, GluN2B(+) hippocampal synapses that are maintained throughout late postnatal development and early adulthood. These high-Pr, GluN2B(+) synapses are evident in arg(-/-) animals as early as postnatal day 21 (P21), a time that precedes any observable defects in synapse or dendritic spine number or structure in arg(-/-) mice. Using focal glutamate uncaging at individual synapses, we find only a subpopulation of arg(-/-) spines exhibits increased GluN2B-mediated responses at P21. As arg(-/-) mice age, these synapses increase in proportion, and their associated spines enlarge. These changes coincide with an overall loss of spines and synapses in the Arg-deficient mice. We also demonstrate that, although LTP and LTD are normal in P21 arg(-/-) slices, both forms of plasticity are significantly altered by P42. These data demonstrate that the integrin-regulated Arg kinase coordinates the maturation of presynaptic and postsynaptic compartments in a subset of hippocampal synapses in vivo, and this coordination is critical for NMDAR-dependent long-term synaptic stability and plasticity. SIGNIFICANCE STATEMENT: Synapses mature in vitro from high-release probability (Pr) GluN2B(+) to low-Pr, GluN2B(-), but it is unknown why this happens or whether it occurs in vivo High-Pr, GluN2B(+) synapses persist into early adulthood in Arg-deficient mice in vivo and have elevated NMDA receptor currents and increased structural plasticity. The persistence of these high-Pr, GluN2B(+) synapses is associated with a net synapse loss and significant disruption of normal synaptic plasticity by early adulthood. Together, these observations suggest that the maturation of high-Pr, GluN2B(+) synapses to predominantly low-Pr, GluN2B(-) synapses may be essential to preserving a larger dynamic range for plasticity while ensuring that connectivity is distributed among a greater number of synapses for optimal circuit function.