REMODELING OF THE NUCLEAR PERIPHERY DURING MUSCLE-CELL DIFFERENTIATION IN-VITRO

We have examined the composition and ultrastructure of the nuclear per iphery during in vitro myogenesis of the rat myoblast cell line, L6E9. Immunofluorescence labelling and immunoblotting showed that lamins A/ C and B were all present in undifferentiated cells, but that they incr eased significantly before extensive cell fusion had occurred, with la mins A/C increasing proportionately more. Electron microscopic observa tions were consistent with these results, showing an increase in the p rominence of the lamina during differentiation. On the other hand, imm unofluorescence labelling suggested that the P1 antigen began to disap pear from the nuclear periphery as the cells were fusing, after the in crease in lamin quantity, and was no longer detectable in multinucleat ed cells. Unexpectedly, however, P1 was readily detected in isolated n uclei, whether prepared from myoblast or differentiated cultures, as w ell as in both myoblast and myotube nuclear matrices. It appears proba ble, therefore, that the fading of P1 labelling is due to masking of t he epitope by a soluble factor recruited to the nuclear periphery as c ells differentiate. These data, together with evidence that the genome is substantially rearranged during L6E9 myogenesis [Chaly and Munro, 1996], suggest that L6E9 cells are a useful model system in which to s tudy the interrelationship of nuclear envelope organization, chromatin spatial order, and nuclear function. (C) 1996 Wiley-Liss, Inc.