A two-state model for Ca2+/CaM-dependent protein kinase II (αCaMKII) in response to persistent Ca2+ stimulation in hippocampal neurons

SummaryPersistent elevation of the intracellular free Ca2+ concentration [Ca2+]i is neurotoxic and therefore it is important to understand how it affects downstream components of the Ca2+ signaling pathway. The response of calmodulin (CaM) and αCa2+/CaM-dependent protein kinase II (αCaMKII), to intracellular Ca2+ overload in hippocampal neurons is studied by confocal imaging of fluorescently tagged proteins. Transient and persistent redistribution of CaM and αCaMKII together is seen from the cytosol to dendritic and somatic punctae. Typical persistent redistribution occurs following a lag of 138 ± (S.E.M.) 12 s and is complete at 460 ± (S.E.M.) 34 s (n = 18), lack of Thr286-autophosphorylation of αCaMKII however promotes the formation of early transient punctae (peak at 40 s). In contrast, the T286D-mimick of phospho-Thr286-αCaMKII forms punctae with a delay >10 min, indicating that Thr286-autophosphorylation is antagonistic to CaMKII clustering. A two-state model is proposed in which phospho-Thr286-αCaMKII, formed immediately upon Ca2+ stimulation, is primarily responsible for target interactions and memory functions of αCaMKII. However, a distinct clustering form denoted αCaMKIIc, generated upon persistent intracellular free Ca2+ elevation, is deposited in the punctae which are made of self-interacting CaM/CaMKII complexes. Punctate deposition disables both the interactions and the activity of CaMKII.