RGS14 restricts plasticity in hippocampal CA2 by limiting postsynaptic calcium signaling
Pyramidal neurons in hippocampal area CA2 are distinct from neighboring CA1 in that they resist synaptic long-term potentiation (LTP) at CA3 Schaffer Collateral synapses. Regulator of G Protein Signaling 14 (RGS14) is a complex scaffolding protein enriched in CA2 dendritic spines that naturally blocks CA2 synaptic plasticity and hippocampus-dependent learning, but the cellular mechanisms by which RGS14 gates LTP are largely unexplored. A previous study has attributed the lack of plasticity to higher rates of calcium (Ca2+) buffering and extrusion in CA2 spines. Additionally, a recent proteomics study revealed that RGS14 interacts with two key Ca2+-activated proteins in CA2 neurons: calcium/calmodulin, and CaMKII. Here, we investigate whether RGS14 regulates Ca2+ signaling in its host CA2 neurons. We find the nascent LTP of CA2 synapses due to genetic knockout (KO) of RGS14 in mice requires Ca2+-dependent postsynaptic signaling through NMDA receptors, CaMK, and PKA, revealing similar mechanisms to those in CA1. We report RGS14 negatively regulates the long-term structural plasticity of dendritic spines of CA2 neurons. We further show that wild-type (WT) CA2 neurons display significantly attenuated spine Ca2+ transients during structural plasticity induction compared with the Ca2+ transients from CA2 spines of RGS14 KO mice and CA1 controls. Finally, we demonstrate that acute overexpression of RGS14 is sufficient to block spine plasticity, and elevating extracellular Ca2+ levels restores plasticity to RGS14-expressing neurons. Together, these results demonstrate for the first time that RGS14 regulates plasticity in hippocampal area CA2 by restricting Ca2+ elevations in CA2 spines and downstream signaling pathways.\n\nSignificance StatementRecent studies of hippocampal area CA2 have provided strong evidence in support of a clear role for this apparently plasticity-resistant subregion of the hippocampus in social, spatial, and temporal aspects of memory. Regulator of G Protein Signaling 14 (RGS14) is a critical factor that inhibits synaptic plasticity in CA2, but the molecular mechanisms by which RGS14 limits LTP remained unknown. Here we provide new evidence that RGS14 restricts spine calcium (Ca2+) in CA2 neurons and that key downstream Ca2+-activated signaling pathways are required for CA2 plasticity in mice lacking RGS14. These results define a previously unrecognized role for RGS14 as a natural inhibitor of postsynaptic Ca2+ signaling in hippocampal area CA2.