Changes in cytoskeletal proteins and their mRNAs during maturation of human erythroid progenitor cells.

We have used highly purified human early erythroid progenitors to study changes in cytoskeletal proteins during their maturation and terminal differentiation. When erythroid progenitors at the burst-forming unit-erythroid (BFU-E) stage of development are grown in the presence of erythropoietin, the cells mature and terminally differentiate into reticulocytes during a 14-15-day culture period. We have shown by immunofluorescence that spectrin is present in day 3 BFU-E, at which time proteins band 3, ankyrin, and band 4.1 cannot be detected. Ankyrin and band 4.1 were detected in the majority of the cells by day 7 of culture, at the colony-forming unit (CFU)-E stage, whereas only 15% of the cells were positive for band 3 protein on day 7 of culture. The mRNA level for spectrin was already at its maximum on day 8 whereas the mRNAs for band 3, ankyrin, and band 4.1 were just beginning to accumulate. After enucleation, spectrin, band 3, ankyrin, and band 4.1 fluorescence were all associated with the reticulocytes. Actin was localized at the constriction between the extruding nucleus and the incipient reticulocyte in enucleating erythroblasts suggesting a key role for actin in the enucleation of human erythroblasts. Our investigations have also shown that purified human erythroid progenitors cultured in serum-free suspension media are capable of enucleating without the requirement of an extracellular matrix. These results demonstrate that the synthesis and expression of major cytoskeletal proteins in the human erythrocyte membrane occur in an asynchronous manner and that the remodeling of the membrane skeleton begins at a very early stage during erythrocyte development.