Activation phosphorylation and dephosphorylation dynamics of Ca2+/Calmodulin-dependent protein kinase Iα in HeLa cells

Ca²⁺/calmodulin-dependent protein kinase I (CaMKI), a multifunctional CaM-activated protein kinase, is involved in various Ca²⁺ signaling pathways including neuronal development. Here, we characterize the phosphorylation at Thr177 (an activation Thr residue) and subsequent dephosphorylation dynamics of CaMKIα in HeLa cells upon physiological stimulation that elevates intracellular Ca²⁺. ATP induced CaMKIα phosphorylation within 5–10 min; phosphorylation was then blocked by CaMKK inhibitor TIM-063, or by the depletion of extracellular Ca²⁺. This was followed by gradual dephosphorylation to basal levels within 30–60 min. Histamine induced CaMKIα phosphorylation, peaking within 3–4 min; this process was abolished by treatment with either TIM-063 or intracellular Ca²⁺ chelation using BAPTA-AM and thapsigargin; however, not by extracellular Ca²⁺ depletion. CaMKIα was then rapidly dephosphorylated to basal levels within 10 min. Consistently, histamine-induced (but not ATP-induced) CaMKIα phosphorylation was absent in triple IP₃ receptor-knockout HeLa cells. Dephosphorylation of CaMKIα after ATP-induced phosphorylation was unaffected by okadaic acid or CaMK phosphatase (CaMKP, known as PPM1F) inhibitors (ANS and ANDS). We found that HeLa cell extracts contained Mg2+/Mn2+-dependent CaMKIα dephosphorylation activity that was insensitive to ANS and ANDS. Furthermore, co-expression of PP2Cα fully abolished ATP-, histamine-, or ionomycin-stimulated CaMKIα phosphorylation, which is consistent with in vitro dephosphorylation of CaMKIα at Thr177 by recombinant PP2Cα. Taken together, these results reveal that agonist-induced Ca²⁺ influx from the extracellular space or release from intracellular stores transiently activates CaMKK-CaMKIα signaling in HeLa cells, which is shut off by dephosphorylation catalyzed by PP2Cα as a promising candidate for CaMKIα phosphatase.