Calcium stores in differentiated Dictyostelium discoideum : prespore cells sequester calcium more efficiently than prestalk cells

Abstract

Dictyostelium discoideum pseudoplasmodia exhibit a gradient of the cytosolic free Ca2]-concentration ([Ca2]]i) along their anterior posterior axis involved in cell-type specific differentiation. [Ca2]]i is high in prestalk and low in prespore cells.We determined the content and localization of calcium and other elements in cryosectioned cells of pseudoplasmodia and fruiting bodies by X-ray microanalysis. Granular stores rich in Ca, Mg and P were identified. Average Ca was higher in prespore than prestalk granules (225 vs 111 mmol/kg dry weight). Total Ca stored in granules was also higher in prespore than prestalk cells. The amount of P and S in granules differed between the two cell types indicating different store composition. In spores mean granular Ca was 120 mmol/kg dry weight. Stalk cells had smaller granules with 360 mmol Ca/kg dry weight. Complementary to microanalysis, vesicular Ca2]-fluxes were studied in fractionated cell homogenates. The rate of Ca2]-uptake was higher in pellet fractions of prespore than prestalk amoebae (4.7 vs 3.4 nmol/minemg). Ca2]-release was greater in supernatant fractions from prestalk than presporecells (16.5 vs 7.7 nmol/108 cells). In summary, prestalk and prespore cells possess qualitatively different, high-capacity stores containing distinct amounts of Ca and probably being involved in regulation of the anterior posterior [Ca2]]i-gradient.