Distinct changes in intranuclear lamin A/C organization during myoblast differentiation

Intranuclear lamin foci or speckles have been observed in various cell type s. In order to explore the possibility of changes in internal lamin organiz ation during muscle differentiation, we have examined the appearance of A-t ype lamin speckles that associate with RNA splicing factor speckles in C2C1 2 myoblasts and myotubes. Lamin speckles were observed in dividing myoblast s but disappeared early during the course of differentiation in postmitotic myocytes, and were absent in myotubes and muscle fibers. However, no chang es were seen in the typical peripheral organization of lamins A/C or B1 or in RNA splicing factor speckles. Lamin speckles were also absent in quiesce nt myoblasts but reappeared as cells were reactivated to enter the cell cyc le. These changes were not observed in other quiescent cell types. Immunobl ot analysis indicated that the abundance and migration of lamins A and C wa s not altered in differentiated myoblasts. When myotube or quiescent myobla st nuclei were extracted with nucleases and detergent, a uniformly stained internal lamina was revealed, indicating that lamins A/C were antigenically masked in these cells, probably owing to structural reorganization of the lamina during differentiation or quiescence. Our results suggest that muscl e cell differentiation is accompanied by regulated rearrangements in the or ganization of the A-type lamins.