Superoxide dismutase activity and glutathione concentration during the calcium-induced differentiation of Physarum polycephalum microplasmodia.

Microplasmodia of Physarum polycephalum differentiate into spherules when the CaCI2 concentration of their nutrient medium is increased to 54mM (high- calcium). The salts starvation medium routinely used to induce differentiation contains 8mM CaCI2. This medium will not induce spherulation in the ab- sence of a calcium salt; no other metal is essential. High-calcium also induces the spherulation of a strain of Physarum that had not been previously ob- served to spherulate. The striking increase in superoxide dismutase activity (SOD) and the decrease in glutathione concentration (GSH) that are charac- teristic of salts-induced spherulation do not occur in salts media containing high-calcium. In the absence of calcium, no significant change in SOD is observed and very little change in GSH occurs. The immediate effect of the oxidative stress associated with spherulation may be the release of calcium stores into the cytosol. The parameters modulating this stress are, in turn, sensitive to exogenous calcium concentrations. Cellular differentiation is ultimately due to qualita- tive and quantitative alterations in gene expression. At present the nature of the mechanisms governing devel- opmental changes in gene expression remains obscure. Cytoplasmic fields, gradients, and morphogenetic deter- minants are widely believed to modulate the transition of embryonic tissues into differentiated cell types (Na- menwirth, 1974; Summerbell, 1979; Summerbell et al., 1973; Wolpert, 1978). Among the factors which appear to play a role in development are the metabolic and chemical gradients observed in developing organisms; these arise from regional variations in surfaceholume ratio and from differential vascularization of the embry- onic tissues (Boveri, 1901; Browder, 1980; Caplan and Ordahl, 1978; Child, 1915). The effects of metabolic gra- dients on development may be partly due to differential nutrient supply; however, there is also evidence that variations in molecular oxygen concentration play a role in the regulation of developmental processes (Jargiello and Caplan, 1983; Caplan and Koutroupas, 1973). The mechanism by which oxygen and its metabolites affect gene expression is presently unknown.