Erythropoiesis

 

FetalLiverEnucleation

Red blood cell (RBC) biogenesis (erythropoiesis) is a highly orchestrated process of regulated gene expression with a series of cell divisions leading to cell cycle exit, coupled to dramatic changes in cell and nuclear morphology, and culminating in nuclear expulsion (enucleation). Cell and nuclear size and transcriptional activity decreases, the nucleus moves to one side of the cell and is extruded, and the future RBC membrane proteins are sorted to the membrane of the nascent reticulocyte, which is then remodeled to a mature biconcave RBC lacking internal organelles or cytoskeleton.

While the enucleation process has been described morphologically, the exact mechanisms that drive the process remain unclear. A key question is to what extent and how actin filament (F-actin) assembly and dynamics, or non-muscle myosin (NMII) contractility, provides forces to polarize and then extrude the nucleus. We evaluate erythroblast differentiation and enucleation in hematopoietic organs (bone marrow, spleen, fetal liver), and the molecular architecture of the actin cytoskeleton in erythroblasts from transgenic mice with mutations in tropomodulins (Tmods), tropomyosins (TMs) or NMIIs, as well as in human CD34+ cells differentiated to erythroblasts in culture. We also use time-lapse microscopy of living erythroblasts to examine dynamics of fluorescent-tagged F-actin, NMII, membranes and nuclei to model the forces required for nuclear polarization and expulsion. The results from these studies will elucidate molecular mechanisms and pathogenesis of human congenital anemias due to defects in RBC biogenesis and enucleation, and also provide new strategies for optimizing RBC production in vitro for future applications in transfusion medicine.