Abstract
The distinct features of neonatal megakaryocytes, high proliferation and inefficient platelet production, have clinical repercussions. A diminished capacity for stress thrombopoiesis, the response to acute drops in platelet counts, contributes to the high prevalence of thrombocytopenia in premature infants and to impaired platelet recovery after umbilical cord blood stem cell transplantation. High proliferation also promotes leukemogenesis in babies with Down Syndrome (DS). The transcriptional coactivator Mkl1/MrtfA participates in programming the ontogenic shift from fetal/neonatal to adult-type megakaryopoiesis; in this activity it is opposed by the DS-associated kinase Dyrk1a. In a screen for downstream ontogenic effectors in human progenitors, we identified the kinesin Kifc3 as a factor selectively decreased in adult megakaryocytes and whose knockdown in neonatal megakaryocytes induced adult-type morphogenesis with augmented platelet release. Kifc3 acts as a minus-end directed motor for centrosomal delivery of various cargos. Centrosomal release of Cep192 has recently been found induce cellular process extensions through actin remodeling, reminiscent of megakaryocyte platelet release. In our studies, Cep192 showed striking upregulation and dispersion in adult vs neonatal megakaryocytes, and Kifc3 knockdown recapitulated this effect in neonatal megakaryocytes. A role for Cep192 in promoting megakaryocyte morphogenesis, distinct from its role in centrosome biogenesis, was demonstrated in vitro and in vivo. In silico screening for Kifc3 inhibitors identified a small molecule that affected neonatal megakaryocytes similarly to Kifc3 knockdown, indicating feasibility for therapeutic targeting of the Kifc3-Cep192 pathway in clinical conditions associated with fetal-type megakaryopoiesis.