RC3/neurogranin, a postsynaptic calpacitin for setting the response threshold to calcium influxes

In this review, we attempt to cover the descriptive, biochemical and molecular biological work that has contributed to our current knowledge about RC3/neurogranin function and its role in dendritic spine development, long-term potentiation, long-term depression, learning, and memory. Based on the data reviewed here, we propose that RC3, GAP-43, and the small cerebellum-enriched peptide, PEP-19, belong to a protein family that we have named the calpacitins. Membership in this family is based on sequence homology and, we believe, a common biochemical function. We propose a model wherein RC3 and GAP-43 regulate calmodulin availability in dendritic spines and axons, respectively, and calmodulin regulates their ability to amplify the mobilization of Ca2+ in response to metabotropic glutamate receptor stimulation. PEP-19 may serve a similar function in the cerebellum, although biochemical characterization of this molecule has lagged behind that of RC3 and GAP-43. We suggest that these molecules release CaM rapidly in response to large influxes of Ca2+ and slowly in response to small increases. This nonlinear response is analogous to the behavior of a capacitor, hence the name calpacitin. Since CaM regulates the ability of RC3 to amplify the effects of metabotropic glutamate receptor agonists, this activity must, necessarily, exhibit nonlinear kinetics as well. The capacitance of the system is regulated by phosphorylation by protein kinase C, which abrogates interactions between calmodulin and RC3 or GAP-43. We further propose that the ratio of phosphorylated to unphosphorylated RC3 determines the sliding LTP/LTD threshold in concert with Ca2+/ calmodulin-dependent kinase II. Finally, we suggest that the close association between RC3 and a subset of mitochondria serves to couple energy production with the synthetic events that accompany dendritic spine development and remodeling.