Megakaryocytopoiesis

Project leader: Daphne Thijssen-Timmer PhD

Platelet assembly and release are the final events of a multi-step process involving commitment, proliferation and differentiation of hematopoietic stem cells (HSC) to megakaryocytes (MK). Multipotent HSC have self-renewal capacity but during commitment decisions they loose the ability to self-renew as well as their multipotency. When MK differentiate from a bipotent megakaryocyte-erythroblast progenitor (MEP) they follow a distinct pathway. The so-called megakaryocytopoiesis starts with an increase in cytoplasm and endomitosis resulting in large polyploid cells, followed by proplatelet formation and finally shedding of platelets.

Within Sanquin a robust human in vitro culture system for the generation of mature MK from hematopoietic stem (CD34+) cells has been developed over the past 10 years. However, a robust system for in vitro generation of human platelets from cultured megakaryocytes does not exist. Efforts are now made to optimize and extend our protocol towards in vitro generation of platelets and to this end we need a better understanding of the (transcription) factors that are specific for megakaryocytopoiesis and platelet release. We performed comparative transcriptome analysis on all mature blood cells, cultured erythroblasts and megakaryocytes, and found that the transcription factor MEIS1 was specifically expressed in MK. We are currently studying the role of MEIS1 in megakaryocytopoiesis by performing overexpression and knockdown studies of MEIS1 using lentiviral RNAi technology in CD34+ cells. Silencing MEIS1 in a MK cell line has already provided us with a list of potential MEIS1 regulated genes which may play a role in the late stages of megakaryocytopoiesis and platelet formation. Together these studies will elucidate the role of MEIS1 in megakaryocytopoiesis and shed more light on factors important for platelet formation.

Megakaryocyte forming proplatelets and platelets,
like 'beads on a string'. Stained are the MK marker
CD42a (red) actin (green) and the nucleus (pink).