Spatiotemporal maps of CaMKII in dendritic spines

The calcium calmodulin dependent kinase (CaMKII) is important for long-term potentiation at dendritic spines. Photo-activatable GFP (PaGFP) - CaMKII fusions were used to map CaMKII movements between and within spines in dissociated hippocampal neurons. Photo-activated PaGFP (GFP*) generated in the shaft spread uniformly, but was retained for about 1 s in spines. The differential localization of GFP*-CaMKII isoforms was visualized with hundred nanometer precision frame to frame using de-noising algorithms. GFP*-CaMKII alpha localized to the tips of mushroom spines. The spatiotemporal profiles of native and kinase defective GFP*-CaMKII beta, differed markedly from GFP*-CaMKII alpha and mutant GFP*-CaMKII beta lacking the association domain. CaMKII beta bound to cortical actin in the dendrite and the stable actin network in spine bodies. Glutamate produced a transiently localized GFP*-CaMKII alpha fraction and a soluble GFP*-CaMKII beta fraction in spine bodies. Single molecule simulations of the interplay between diffusion and biochemistry of GFP* species were guided by the spatiotemporal maps and set limits on binding parameters. They highlighted the role of spine morphology in modulating bound CaMKII lifetimes. The long residence times of GFP*-CaMKII beta relative to GFP*-CaMKII alpha followed as consequence of more binding sites on the actin cytoskeleton than the post-synaptic density. These factors combined to retain CaMKII for tens of seconds, sufficient to outlast the calcium transients triggered by glutamate, without invoking complex biochemistry.