InsP₃-mediated intracellular calcium signalling is altered by expression of synaptojanin-1

Phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P₂] plays an important physiological role as a precursor for the InsP₃-mediated intracellular calcium (Ca²⁺) signalling cascade. It also regulates membrane trafficking, actin function and transmembrane proteins. SJ-1 (synaptojanin-1), a phosphoinositide phosphatase, regulates the turnover of a PtdIns(4,5)P₂ pool involved in clathrin and actin dynamics at the cell surface. We tested the interrelationship of this pool with PtdIns(4,5)P₂ pools involved in Ca²⁺ signalling by expressing in Chinese-hamster ovary cells full-length SJ-1 or its 5-Pase (inositol 5-phosphatase) domain. SJ-1 significantly attenuated the generation of Ca²⁺ oscillations induced by ATP and the 5-Pase domain mimicked this effect. These changes correlated with increased PtdIns(4,5)P₂ phosphatase activity of cellular extracts. Overexpression of the endoplasmic reticulum-anchored PtdIns(4)P phosphatase Sac1 did not affect Ca²⁺ oscillations, although it increased the Ca²⁺ efflux rate from intracellular stores. The ability of SJ-1 to alter intracellular Ca²⁺ signalling indicates a close functional interrelationship between plasma membrane PtdIns(4,5)P₂ pools that control actin and endocytosis and those involved in the regulation of specific spatio-temporal Ca²⁺ signals.