Cadmium modification of nucleolar ultrastructure and RNA synthesis in Physarum polycephalum

摘要

Exposure of the acellular slime mold Physarum polycephalum to cadmium resulted in distortion of nucleolar structure and in inhibition of RNA synthesis. The extent of these lesions was dependent on concentration of cadmium, as well as duration and timing of exposure in the cell cycle. A 30-min exposure to 5 x 10-4 m CdSO4, initiated just as DNA synthesis began, inhibited RNA synthesis by 50% and caused subtle nucleolar changes: eccentric placement of nucleoli in nuclei, and the appearance of multiple nucleolar bodies in low incidence. Exposure to either 5 x 10-4 m or 1.5 x 10-3 m CdSO4 for 4 hr depressed RNA synthesis to 20-25% of control values and caused ring-shaped nucleoli. In electron micrographs of cadmium-treated cells, nucleoli appeared as electron dense rings of nucleolar material enclosing less intensely staining central zones, in contrast to control nucleoli which appeared as uniformly dense, nearly spherical bodies. Three-dimensional reconstructions showed that the "ring" was, in actuality, a sphere of nucleolar material completely surrounding a central cavity. A 4-hr exposure to 5 x 10-4 or 1.5 x 10-3 m CdSO4 after postmitotic reconstruction was complete and DNA synthesis had been underway for 2 hr or more inhibited RNA synthesis by 50%, but nucleolar rings were not formed. These observations identify the nucleus as a target for cadmium, or for effectors which mediate cadmium toxicity, and they suggest that disruption of nucleolar function (i.e., synthesis of RNA) and/or of nucleolar structure may be underlying mechanisms of cadmium toxicity.