Formation of Mast Cell Granules in Cell Cycle Mutants of an Undifferentiated Mastocytoma Line: Evidence for Two Different States of Reversible Proliferative Quiescence ARTHUR ZIMMERMANN, JEAN-CLAUDE SCHAER, DOMINIQUE

A heat-sensitive (hs, arrested at 39.5°C, multiplying at 33°C) and a cold-sensitive (cs, arrested at 33°C, multiplying at 39.5°C) cell cycle variant were isolated from an undifferentiated P-815 murine mastocytoma line. At the respective nonpermissive temperature, both the hs and the cs variant cells were reversibly arrested with a DNA content, typical of GI phase. The cells of two cs variant subclones, when exposed to the nonpermissive temperature of 33°C, formed metachromaticaIly staining granules with an ultrastructure resembling that of mature mast cells. In addition, the cellular 5-hydroxytryptamine content underwent a marked increase, and the cells responded to compound 48/80 by degranulation as described for normal mast cells. On the other hand, in cells of two hs variant subclones, essentially no mast cell granules were detectable at either 33 or 39.5°C. As previously reported, the cs cell cycle variant phenotype is expressed dominantly in heterokaryons obtained by fusing cs with wild-type cells, whereas hs cell cycle variant cells, similar to other hs mutants, were found to behave recessively under these conditions. Thus the state of proliferative quiescence induced in the cs cells at 33°C is qualitatively different from the state of cell cycle arrest observed in hs cells at 39.5°C and may represent a model for proliferative quiescence of differentiated cells in the intact organism. Condit ional cell cycle mutants o f animal cells represent promising tools for analyzing mechanisms involved in the transition o f a cell from proliferative activity into a state o f reversible proliferative quiescence (commonly referred to as Go phase) and back into active cell proliferation. Whereas heat-sensitive (hs, arrested at ~39.5°C, multiplying at ~33°C) cell cycle mutants are presently available in relatively large numbers, only a few cold-sensitive (c.s, arrested at 33°C, multiplying at 39.5°C) cell cycle mutants have been described. We have recently reported that a cs cell cycle mutant of the murine P815 line exhibits dominant behavior in heterokaryons obtained by fusion with wild-type cells (1), whereas hs mutants generally behave recessively (1, 2). Since the cells o f the P-815 line and the P-815-derived as and ]as cell cycle mutants grow in suspension without attaching to solid substrates, cell-to-cell interactions such as contact inhibition may be minimized in studying the effects of various culture conditions. This communicat ion shows that as cell cycle mutants also differ from hs cell cycle mutants with respect to their capacity to undergo cellular differentiation when brought to the nonpermissive tempera-