TRANSLATIONAL INHIBITION OF CYCLIN B1 AND APPEARANCE OF CYCLIN D1 VERY EARLY IN THE DIFFERENTIATION OF MOUSE TROPHOBLAST GIANT-CELLS

Investigation of the basis of uncoupling of replication of the genome from mitosis in the mouse trophoblast has so far been neglected despit e its significance for understanding both placental development and ce ll cycle control. In order to obtain clues about the molecular basis o f the switch from proliferation to endoreduplication, we have investig ated changes in the expression of cyclins and cyclin-dependent kinases in diploid versus giant trophoblast cells. Interestingly, while cycli n B1 transcripts were found in both diploid and giant cells, the prote in was found exclusively in diploid cells. This could be explained by either inhibition of translation or by constitutive degradation of the protein. The latter was ruled out by examining blastocysts which had been cultured in the presence of the proteasome inhibitor N-acetyl-leu -leu-norleucinal followed by immunostaining for cyclin B1. In these ex periments cyclin B1 protein accumulated in diploid but not in giant ce lls. Fusion of trophoblast giant cells with secondary oocytes, which a re rich in maturation promoting factor (MPF) activity, revealed that a n exogenous source of active MPF could cause chromosome condensation a nd nuclear envelope breakdown in endocycling cells; therefore endoredu plication via polyteny evidently requires the suppression of MPF activ ity. In addition, cyclin D1 transcripts were found only in giant cells and, interestingly, the beginning of its expression was evident prior to that of placental lactogen I, an early marker of trophoblast diffe rentiation. The results suggest that supression of MPF activity, by in hibition of translation of cyclin B1, is a key mechanism for the estab lishment of the endocycle in the mouse trophoblast.