Contribution of endoplasmic reticulum to Ca²⁺signals inDictyosteliumdepends on extracellular Ca²⁺

Abstract

We recently reported the first molecular genetic evidence that Dictyostelium Ca(2+) responses to chemoattractants include a contribution from the endoplasmic reticulum (ER) - responses are enhanced in mutants lacking calreticulin or calnexin, two major Ca(2+)-binding proteins in the ER, even though the influx of Ca(2+) into the mutants is reduced. Compared with wild-type cells, the ER in the mutants contributes at least 30-70 nM additional Ca(2+) to the responses. Here we report that this additional ER contribution to the cytosolic Ca(2+) signal depends upon extracellular Ca(2+)- it does not occur in the absence of extracellular Ca(2+), increases to a maximum as the extracellular Ca(2+) levels rise to 10 microM and then remains constant at extracellular Ca(2+) concentrations up to at least 250 microM. These results suggest that Ca(2+) influx causes the intracellular release, in the simplest scenario by a mechanism involving Ca(2+)-induced Ca(2+) release from the ER. By way of contrast, we show that Ca(2+) responses to mechanical stimulation are reduced, but still occur in the absence of extracellular Ca(2+). Unlike the responses to chemoattractants, mechanoresponses thus include contributions from the ER that are independent of extracellular Ca(2+).

Membrane Targeting of C2GAP1 Enables Dictyostelium discoideum to Sense Chemoattractant Gradient at a Higher Concentration Range

“…In contrast to several other C2 domains, which require calcium binding for their membrane translocation (Damer and Creutz, 1994;Ilacqua et al, 2018), we found a decreased membrane localization of C2GAP1 in the presence of high [Ca 2+ ] (Figure 1A). Consistent with the above result, iplA − cells without [Ca 2+ ] increase in response to cAMP stimulation, displayed an increased, prolonged membrane translocation of C2GAP1-YFP, while cbpG − cells with higher basal [Ca 2+ ] and stronger, quicker peaking [Ca 2+ ] upon cAMP stimulation exhibited little membrane translocation of C2GAP1, instead, more further withdrawal of C2GAP1 from the plasma membrane to the cytoplasm in response to cAMP stimulation (Figures 1C-E; Wilczynska et al, 2005;Fisher and Wilczynska, 2006). An intracellular calcium gradient that was higher at the trailing edge of chemotaxing cells has been previously reported (Yumura et al, 1996).…”

A study of wound repair in Dictyostelium cells by using novel laserporation

“…The above results in the presence of EGTA indicate that only the wound without Ca 2+ influx does not induce the elevation of Ca i 2+ . However, the influx of Ca 2+ may induce Ca 2+ release from intracellular Ca 2+ stores via a calcium-induced calcium release (CICR) mechanism 38 , 39 , and this release may also contribute to the wound repair. To examine this possibility, iplA null cells expressing Dd-GCaMP6s were wounded.…”

The effects of extracellular calcium on motility, pseudopod and uropod formation, chemotaxis, and the cortical localization of myosin II in Dictyostelium discoideum

“…Extracellular Ca ++ has been shown to affect the level of free cytosolic Ca ++ in a variety of cell types, including D. discoideum (Clapham et al , 2001, 2007; Berridge et al , 2003; Malchow et al , 1996; Berridge, 2005; Clark and Petty, 2008; Fisher and Wilczynska, 2006; Taylor, 2002; Wheeler and Brownlee, 2008). We therefore considered the hypothesis that the effects of extracellular Ca ++ on cell behavior and on the cortical localization of myosin II were mediated through changes in the concentration of free cytosolic calcium.…”

“…First, it can do so by regulating the intracellular concentration of free cytosolic Ca ++ , the source of which can be bound Ca ++ stores (Berridge, 2005; Berridge et al , 2003; Oh-Hora et al , 2008) and/or extracellular Ca ++ (Berridge, 2005; Berridge et al , 2003; Taylor, 2002; Wheeler and Brownlee, 2008; Fisher and Wilczynska, 2006; Lombardi et al , 2008). Second, extracellular Ca ++ can affect behavior through Ca ++ receptors (Clark et al , 2008; Hofer and Brown, 2003; Sharan et al , 2008; Martinac et al , 2008) that activate signaling pathways.…”