Ablation of ATP2A, SERCA2+/-, increases sensitivity of Ca2+ influx and Ca2+ release from ER in mouse parotid gland acinar cells

Objectives:The importance of understanding plasticity and adaptation in Ca²⁺ signaling and Ca²⁺-dependent cell functions in peripheral tissues in vivo is highlighted by the recent demonstration that Darier's disease (DD) is due to mutations in the ATP2A2 gene, which codes for the SERCA2 Ca²⁺ pump. Previously we reported plasticity and adaptation of Ca²⁺ signaling and Ca²⁺-dependent exocytosis in pancreatic acini from SERCA2^+/- mice (EMBO J; 20; 2680-2689, 2001). In the present work, we investigated the effect of modulation of [Ca²⁺]i  on the mechanism of salivary secretion in SERCA2^+/-. Methods:Intracellular Ca²⁺ concentration was measured using spectrofluorometer and amylase release was measured with standard technique. Results: Ca²⁺ release and Ca²⁺ entry in response to maximal concentration of carbachol, muscarinic agonist, in parotid acinar cells from SERCA2^+/- and wild type mice showed similar pattern and the amount of Ca²⁺ in endoplasmic reticulum (ER) was identical. Complete depletion of ER with prolonged incubation with cyclopiazonic acid, ER-Ca²⁺ ATPase inhibitor, induces more Ca²⁺ entry in SERCA2^+/-, suggesting increases in sensitivity of store-operated Ca²⁺ channel. Short stimulations with various concentration of carbachol generate [Ca²⁺]i  increases in a dose-dependent manner in WT. However, at low concentration of agonist SERCA2^+/- shows more [Ca²⁺]i increases compared to WT, suggesting increases in IP₃ production or sensitivity of IP₃ receptor. Increases in [cAMP]i by isoproteronol induce amylase secretion in a dose-dependent manner and no difference between WT and SERCA2^+/-. [Ca²⁺]i increases induce amylase secretion in a dose-dependent manner, but the sensitivity of amylase release in SERCA2+/- shifts to left, indicating that SERCA2+/- has a high sensitivity of Ca²⁺ signaling mechanism compared to WT. Conclusion: Parotid acinar cells from SERCA2^+/- shows high sensitivity of store-operated Ca²⁺ channel and increases in IP₃ production or sensitivity of IP₃ receptor. This study was supported by a grant of the Korea Health 21 R&D Project, Ministry of Health & Welfare, Republic of Korea. (02-PJ1-PG3- 20706-0001). Keyword:SERCA2, Ca²⁺ Signaling, Signaling Protein, Salivary Secretion