Schlatterer, Christina
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cAMP controls cytosolic Ca2+ levels in Dictyostelium discoideum
Differentiating Dictyostelium discoideum amoebae respond upon cAMP-stimulation with an increase in the cytosolic free Ca2+ concentration ([Ca2+]i) that is composed of liberation of stored Ca2+ and extracellular Ca2+-influx. In this study we investigated whether intracellular cAMP is involved in the control of [Ca2+]i.
Dissection of the cAMP induced cytosolic calcium response in Dictyostelium discoideum : the role of cAMP receptor subtypes and G protein subunits
The cAMP signaling cascade leading to changes in [Ca2+]i in Dictyostelium discoideum was analyzed using cell lines overexpressing single cAMP receptor subtypes (cAR1–cAR3) or lacking the Gα2 or Gβ subunit of the G protein. Imaging of fura2‐dextran‐loaded amoebae revealed cAMP‐induced [Ca2+]i changes characteristic for each receptor subtype activated. Cells expressing distinct subtypes sort to defined zones during multicellular development suggesting involvement of the specific [Ca2+]i transients in patterning processes. Whereas generation of the [Ca2+]i increase was Gα2‐independent, only few cells devoid of Gβ displayed a [Ca2+]i change after stimulation indicating its participation in the regulation of the calcium homeostasis.
The contractile vacuole in Ca2+-regulation in Dictyostelium : its essential function for cAMP-induced Ca2+-influx
cAMP-induced Ca2+-influx in Dictyostelium is controlled by at least two non-mitochondrial Ca2+-stores: acidic stores and the endoplasmic reticulum (ER). The acidic stores may comprise the contractile vacuole network (CV), the endosomal compartment and acidocalcisomes. Here the role of CV in respect to function as a potential Ca2+-store was investigated.
A link of Ca2+ to cAMP oscillations in Dictyostelium : the calmodulin antagonist W-7 potentiates cAMP relay and transiently inhibits the acidic Ca2+-store
During early differentiation of Dictyostelium the attractant cAMP is released periodically to induce aggregation of the cells. Here we pursue the question whether pulsatile cAMP signaling is coupled to a basic Ca2+-oscillation.
Ca2+ regulation in the absence of the iplA gene product in Dictyostelium discoideum
Stimulation of Dictyostelium discoideum with cAMP evokes an elevation of the cytosolic free Ca2+ concentration ([Ca2+]i). The [Ca2+]i-change is composed of liberation of stored Ca2+ and extracellular Ca2+-entry. The significance of the [Ca2+]i-transient for chemotaxis is under debate. Abolition of chemotactic orientation and migration by Ca2+-buffers in the cytosol indicates that a [Ca2+]i-increase is required for chemotaxis. Yet, the iplA- mutant disrupted in a gene bearing similarity to IP3-receptors of higher eukaryotes aggregates despite the absence of a cAMP-induced [Ca2+]i-transient which favours the view that [Ca2+]i-changes are insignificant for chemotaxis.
Calcium stores in differentiated Dictyostelium discoideum : prespore cells sequester calcium more efficiently than prestalk cells
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 prespore
cells (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.