Cellular Biology Location Matters Clarifying the Concept of Nuclear and Cytosolic CaMKII Subtypes

Rationale: Differential effects of B and C subtypes of Ca /calmodulin-dependent protein kinase (CaMKII) on cardiomyocyte Ca handling and survival have been suggested to result from their respective nuclear versus cytosolic localizations. CaMKII subtype localization and its relationship to enzyme activation and target phosphorylation have not, however, been systematically evaluated. Objective: To determine whether CaMKII subtypes are restricted to a particular subcellular location and assess the relationship of localization to enzyme activation and function. Methods and Results: CaMKII is highly expressed in mouse heart and cardiomyocytes and concentrated in sarcoplasmic reticulum (SR)/membrane and nuclear fractions. CaMKII B and C subtypes differ by a nuclear localization sequence, but both are present in nuclear and SR/membrane fractions. Nonselective subtype distribution is also seen in mice overexpressing CaMKII B or C, even in a CaMKII null background. Fluorescently tagged CaMKII B expressed in cardiomyocytes concentrates in nuclei whereas C concentrates in cytosol, but neither localization is exclusive. Mouse hearts exposed to phenylephrine show selective CaMKII activation in the nuclear (versus SR) compartment, whereas caffeine selectively activates CaMKII in SR (versus nuclei), independent of subtype. Compartmentalized activation extends to functional differences in target phosphorylation at CaMKII sites: phenylephrine increases histone deacetylase 5 phosphorylation (Ser498) but not phospholamban (Thr17), whereas the converse holds for caffeine. Conclusions: These studies demonstrate that CaMKII B and C are not exclusively restricted to the nucleus and cytosol and that spatial and functional specificity in CaMKII activation is elicited by mobilization of different Ca stores rather than by compartmentalized subtype localization. (Circ Res. 2011;109:1354-1362.)