Ca 2+-dependent contractility of isolated and demembranated macronuclei in the hypotrichous ciliate Euplotes aediculatus

Abstract

The hypotrichous ciliated protozoan Euplotes aediculatus possesses a characteristic C-shaped somatic nucleus (macronucleus) within the cytoplasm, which shows dynamic shape change during the cell cycle. It is shown that isolated macronuclei possess Ca 2+-dependent contractility. Macronuclei were isolated, stuck fast on the glass surface, and subjected to different concentrations of Ca 2+ in a Ca 2+-EGTA buffer. The nuclei became expanded at [Ca 2+]<10 −7 M, and they contracted on subsequent addition of higher concentrations of Ca 2+. Cycles of expansion and contraction of the nucleus could be repeated many times by alternate addition of EGTA and Ca 2+, indicating that the size of isolated nuclei can be regulated by [Ca 2+] alone. The nuclear contraction was observed in all phases of the cell cycle, but contractility was less evident around replication bands in the S phase. In addition to the hypotrichous ciliate Euplotes, similar Ca 2+-dependent nuclear contractility was found to exist in other cell types, including protozoans of different taxa (a heliozoon Actinophrys sol and a peniculine ciliate Paramecium bursaria), and also mammalian culture cells (HeLa cells). Our findings suggest a possibility that Ca 2+-dependent nuclear contractility may be shared among diverse eukaryotic organisms.