Thrombopoiesis

Approximately 750 billion platelets circulate in human blood, and 10¹¹ platelets are produced daily in the human body to fulfill their physiological functions. The individual’s platelet count is maintained in a range of approximately 150.000-400.000 platelets per μL blood, ensured by a constant balance of platelet production and clearance.

Platelets are derived from megakaryocytes (MKs), which differentiate from hematopoietic stem cells (HSC). The process of platelet biogenesis in the bone marrow is unique in mammalian physiology. During their maturation, MKs become polyploid, markedly increase their size and form complex membrane invaginations termed demarcation membrane system (DMS), that serves as a membrane reservoir for the newly formed platelets. Mature MKs reside at vascular sinusoids where they form unidirectional long cytoplasmic extensions called proplatelets, which pass the endothelial barrier and are shed into the circulation, where the final sizing into hundreds of virtually identical platelets from a single MK takes place.

Defects in platelet biogenesis may affect the number of platelets (thrombocytopenia: low platelet count; thrombocytosis: high platelet count). Thrombocytopenia represents a frequent clinical problem as it poses an increased risk of bleeding. The demand for platelet transfusions for the treatment of thrombocytopenia is steadily increasing. However, a major problem in transfusion medicine is the gap of demand and supply of platelet concentrates, and a short shelf life of platelet bags due to the quick decline of platelet viability under storage conditions. Platelet production for transfusion purposes under in vitro conditions has been very challenging in the past decades and there is still a remarkable gap of knowledge with regard to the mechanisms that regulate proplatelet formation in vivo. Thus, a more detailed understanding of this complex biological process is required.

Our central research questions are:

• Which proteins are key regulators of platelet formation in vivo and in vitro?
• Which are the molecular triggers of transendothelial proplatelet formation?
• How does an MK ‘sense’ its localization in the bone marrow compartment?
• How is the platelet mass equilibrium altered in different disease settings?

We address these questions by investigating the role of MK receptors and their signaling pathways, cations (e.g. Mg²⁺ and Zn²⁺) and the actin and tubulin cytoskeleton in platelet formation in vitro and in vivo using a multitude of transgenic mouse lines.