The degree of coupling of nuclear rotation in binucleate 3T3 cells

In order to test the existence of mechanical coupling between the rotational movements of two adjacent nuclei, we prepared binucleate 3T3 cells and observed their nuclear movements by near infrared microscopy and recorded them with time-lapse video techniques. We found that 49 out of 110 (44%) of the selected binucleate cells expressed nuclear rotation. Rotation could occur in just one of the nuclei while the second nucleus remained stationary (31/110) or in both nuclei simultaneously (18/110). In almost all cases where both nuclei rotated simultaneously (15/110) they did so at different speeds and in opposite directions. The nuclei were observed to rotate in the same direction in only three of the examples. The results are consistent with a weak mechanical interaction between a rotating nucleus and its neighbor. Consistent with our previous observations in mononucleate cells, we did not find a characteristic position of the centrosphere or a special distribution of the microtubules or the intermediate filaments in binucleate cells with rotating nuclei. There was an absence of long, well-formed microfilament bundles beneath the nuclei during rotation, even in the local region beneath the rotating nucleus in those cells with one rotating and one stationary nucleus. Also consistent with observations of mononucleate cells, nuclear rotation was inhibited by treatment with colcemid, although the ability of the nuclei to rotate was eventually restored when the colcemid-containing medium was replaced with normal medium.