Biological oxidation and the mobilization of mitochondrial calcium during the differentiation of Physarum polycephalum

We have previously reported that calcium is required for the starvation-induced differentiation of the slime mold, Physarum polycephalum. With the exception of calcium, each component of the complex starvation medium may be withheld and the organism will still differentiate into spherules. The results of the present study reveal that spherulation will proceed normally when the microplasmoidal cells are transferred from nutrient medium to a citrate buffer containing only 8 mM Ca CI2. Electron microscopy and X-ray microprobe analysis reveal that there is an initial increase in the population of calcium-containing mitochondrial granules when the microplasmodia are induced to differentiate. However, as differentiation proceeds, these granules decrease in number and are virtually absent from the mitochondria of mature spherules. The accumulation and depletion of calcium-containing granules is not observed in a nondifferentiating strain of Physarum cultured under standard conditions, but is observed when this strain is first treated with a calcium-enriched nutrient medium that conditions it for spherulation. Changes in the cellular concentrations of NADH and lipid peroxides, and in the activity of superoxide dismutase, correspond temporally to the pattern of increase and depletion of the calcium-containing inclusions. The oxidative stress associated with starvation-induced spherulation may be a consequence of the active accumulation of calcium; the mobilization of this calcium may then be the event that initiates differentiation.