Sister chromatid separation and the metaphase-anaphase transition in mouse oocytes

The paper reports on the effect of experimentally inhibiting chromatid separation on meiotic progression and maturation-promoting factor (MPF) activity in both metaphase I (Experiment 1) and metaphase II mouse oocytes (Experiment 2) subjected to combinations of inhibitors of (a) protein synthesis, (b) topoisomerase II, and (c) cytokinesis-cytoskeleton integrity. The results from Experiment 1 showed that the inhibition of protein synthesis invariably results in the extrusion of the first polar body and the formation of an interphase nucleus. Furthermore, this inhibition induces a rapid decline in MPF activity. Similarly, in Experiment 2 the exposure of metaphase II oocytes to cycloheximide initiated a rapid fall in MPF activity, progression to anaphase, the extrusion of the second polar body, and the formation of a pronucleus. While the inhibition of protein synthesis hastened progression through the meiotic cycle, the opposite effect was observed when chromatid separation was prevented by etoposide or colcemid treatment. The results in Experiment 1 demonstrated that the inhibition of chromatid separation totally blocked meiotic progression by preventing the metaphase I to anaphase I transition. These oocytes were characterized by the persistence of high MPF activity for extended periods of time (> 20 hr). This activity declined slowly in oocytes exposed both to inhibitors of chromatin separation and protein synthesis. In Experiment 2 the results showed that the prevention of chromatin separation induced changes which paralleled those observed with MI oocytes. The prevention of chromatid separation with either etoposide or colcemid converted the oocytes from being sensitive to activation stimuli to being entirely resistant to standard activation. In addition, MPF activity remained persistently elevated and declined only when protein synthesis was inhibited. The decline in intracellular MPF activity reached basal levels 6 to 10 hr after the addition of cycloheximide and was accompanied by the slow and gradual decondensation of chromatin. Our results are in accord with those from recent experiments in yeast, insects, and amphibia which suggest that chromatid separation provides an essential signal for cell cycle progression beyond M-phase. We postulate first that exist from both metaphase I and metaphase II, and the characteristic reduction in MPF activity at anaphase in mouse oocytes, are initiated by chromosome (chromatid) separation. Second, we suggest that chemically induced chromosome (chromatid) separation block prevents the anaphase to telophase transition by inhibiting MPF degradation. Third, we postulate that the slow escape from metaphase arrest in oocytes treated with both etoposide and cycloheximide reflects a gradual decrease of MPF activity due to normal protein turnover without new synthesis.