CaMKII inactivation by extracellular Ca2+ depletion in dorsal root ganglion neurons

A mechanism by which Ca2+/CaM-dependent protein kinase (CaMKII) is autophosphorylated by changes in extracellular calcium in the absence of detectable changes in cytoplasmic [Ca2+] has been identified. We find that when the external Ca2+ concentration ([Ca2+]O) is lowered, Ca2+ is released from intracellular stores to maintain a constant cytoplasmic Ca2+ level, gradually depleting the endoplasmic Ca2+ stores. Accompanying the store-depletion is a rapid decrease in CaMKII activity. Approximately 25% of the measured CaMKII autophosphorylation in DRG neurons in culture can be regulated by Ca2+ flux from intracellular stores caused by manipulating [Ca2+]O, as shown by blocking refilling of store-operated Ca2+-channels with SK&F 96365, Ruthenium Red, and a partial block with Ni2+. Blocking voltage-gated Ca2+-channels with either isradipine or SR 33805, had no effect on CaMKII autophosphorylation induced by restoring Ca2+O to normal after depleting the intracellular Ca2+ stores. These results show that removal of Ca2+O has profound effects on intracellular Ca2+ signaling and CaMKII autophosphorylation, in the absence of measurable changes in intracellular Ca2+. These findings have wide-ranging significance, because [Ca2+]O is manipulated in many experimental studies. Moreover, this explanation for the paradoxical changes in CaMKII phosphorylation in response to manipulating [Ca2+]O provides a possible mechanism linking activity-dependent depletion of Ca2+ from the synaptic cleft to a protein kinase regulating many neuronal properties.