A fast Ca 2+-induced Ca 2+-release mechanism in Dictyostelium discoideum

Abstract

In vertebrate cells calcium- induced calcium release (CICR) is thought to be responsible for rapid cytosolic Ca 2+ elevations despite the occurrence of strong Ca 2+ buffering within the cytosol. In Dictyostelium, a CICR mechanism has not been reported. While analyzing Ca 2+ regulation in a vesicular fraction of Dictyostelium rich in Ca 2+-flux activity, containing contractile vacuoles (CV) as the main component of acidic Ca 2+ stores and ER, we detected a rapid Ca 2+ change upon addition of Ca 2+ (CIC). CIC was three times larger in active stores accumulating Ca 2+ than before Ca 2+ uptake and in inactivated stores. Ca 2+ release was demonstrated with the calmodulin antagonist W7 that inhibits the V-type H +ATPase activity and Ca 2+ uptake of acidic Ca 2+ stores. W7 caused a rapid and large increase of extravesicular Ca 2+ ([Ca 2+] e), much faster and larger than thapsigargin (Tg), a Ca 2+-uptake inhibitor of the ER. W7 treatment blocked CIC indicating that a large part of CIC is due to Ca 2+ release. The height of CIC depended on the filling state of the Ca 2+ stores. CIC was virtually unchanged in the iplA − strain that lacks a putative IP 3 or ryanodine receptor thought to be located at the endoplasmic reticulum. By contrast, CIC was reduced in two mutants, HGR8 and lvsA −, that are impaired in acidic Ca 2+-store function. Purified Ca 2+ stores enriched in CV still displayed CIC, indicating that CV are a source of Ca 2+-induced Ca 2+ release. CIC-defective mutants were altered in their oscillatory properties. The irregularity of the HGR8 oscillation suggests that the principal oscillator is affected in this mutant.