Nucleated blood platelets are produced by their bone marrow resident precursors, the megakaryocytes, in a unique process in mammalian physiology. Terminally differentiated, polyploid megakaryocytes are the largest cells in the bone marrow evolving from hematopoietic stem cells. Megakaryocytes are localised in close proximity to sinusoidal blood vessels and convert their cytoplasmic, membranous network, the demarcation membrane system, into long, cytoplasmic protrusions (proplatelets)1, which extend into the lumen of bone marrow sinusoids and finally platelets are sequentially released from their ends 2. Thus, the terminal stage of platelet production occurs in the bloodstream, where platelets attain their final shape and size.
At sites of vascular injury, circulating platelets come into contact with exposed sub endothelial components (e.g. collagens), and form a plug to prevent excessive blood loss (haemostasis) 3. 1011 platelets are produced daily in the human body to fulfil the normal hemostatic function 4. The individual’s platelet count is maintained in a range of approximately 150.000-400.000/μL, requiring a constant balance of platelet production and clearance. Conditions that cause insufficient platelet production or accelerated platelet clearance pose a risk for death from bleeding. This is of major clinical significance when patients acquire a low platelet count (thrombocytopenia) resulting from radiation exposure, chemotherapy, transplants, surgery, or other causes as well as for patients suffering from inherited thrombocytopenias due to mutations in specific genes that play an important role in megakaryocyte or platelet development. Platelet transfusion is a very important option for treating thrombocytopenia, however, an ever-expanding demand for platelets and an inadequate response of patients to transfused platelets requires alternative strategies, such as donor-independent sources, to treat patients suffering from thrombocytopenia associated with bleeding complications 5.
On the other hand, however, if platelet aggregation occurs in an uncontrolled manner it may also lead to thrombotic events causing life-threatening disease states such as myocardial infarction or ischemic stroke, which are the leading causes of disability and mortality in the Western world. Therefore, there is still a strong demand for the development and production of selective, powerful, yet safe antithrombotic drugs.
Despite recent advances, a more detailed understanding of the complex process of platelet production and function is crucial to better treat patients with bleeding complications without increasing the risk of thrombotic events.
Consequently, in order to better understand the underlying mechanisms, our group capitalises on genetically-modified mouse lines to identify key molecules involved in these processes. Mice are the most frequently used species in thrombosis research because of their small size, high fertility, exceptional reproductive capacity and the similarity to humans in anatomy, physiology, and genetics.
How animal models can serve to study the complex world of human diseases will be shown as follows: we were recently able to show 6 that a hitherto unsuspected protein, the cytoskeleton-regulatory protein Profilin 1, is involved in the development of the Wiskott-Aldrich syndrome, a rare and severe hereditary disease, caused by mutations in the Wiskott-Aldrich syndrome protein (WASp). This disease is characterised by the triad macrothrombocytopenia (small and few platelets) with increased risk of bleeding, immune deficiency and eczema 7. We revealed6 that mice with a megakaryocyte/platelet-specific Profilin 1 deficiency also displayed a macrothrombocytopenia, thereby reproducing a central hallmark of the Wiskott-Aldrich syndrome in humans. Profilin 1-deficient mouse platelets contained misarranged and hyperstable microtubules (a component of the cell skeleton) that were causative for the smaller platelet size, a defect that we also found in platelets from Wiskott-Aldrich syndrome patients. Based on our findings, we speculate that WASp acts as a modulator of Profilin 1 function in megakaryocytes and that this process is disturbed in Wiskott-Aldrich syndrome patients. These results point to a previously unrecognised mechanism underlying the platelet formation defect in Wiskott-Aldrich syndrome patients.
Moreover, identification of misarranged/ hyperstable microtubules in human platelets by immunostaining can now be used as a quick diagnostic marker to identify humans suffering from this disease.
Taken together, our group seeks to understand (I) which proteins are key regulators in platelet formation, (II) how a mutation in a gene leads to impaired platelet production, (III) how we can manipulate this process to ultimately increase the platelet count in thrombocytopenic patients, and finally (IV) what kind of new strategies can be developed to avoid platelet transfusion shortages. Similarly, we have spent much effort in identifying key molecules involved in hemostasis and pathological thrombus formation. Therefore, mice deficient in various proteins have been subjected to bleeding time assays and different experimental in vivo arterial thrombosis models. A major goal is to find new suitable, powerful antithrombotic targets that have no or only a minor impact on sealing a wound.
Interestingly, an emerging body of evidence over the last few years shows that blood platelets are not only important as the “band-aids” of the bloodstream but also play various roles beyond hemostasis and thrombosis. Platelets have been increasingly recognised to be involved in processes such as inflammation, tumour metastasis and lymphatic blood vessel separation.
Thus, intensive platelet research is necessary to better diagnose and treat platelet-related diseases affecting the lives of many human beings.
References
1 Bender M, Thon J, Ehrlicher A, et al. Microtubule sliding drives proplatelet elongation and is dependent on cytoplasmic dynein. Blood. 2014.
2 Hartwig J, Italiano J. The birth of the platelet. Journal of thrombosis and haemostasis: JTH. 2003;1(7):1580-1586.
3 Nieswandt B, Pleines I, Bender M. Platelet adhesion and activation mechanisms in arterial thrombosis and ischaemic stroke. Journal of thrombosis and haemostasis: JTH. 2011;9 Suppl 1:92-104.
4 Branehög I, Ridell B, Swolin B, Weinfeld A. Megakaryocyte quantifications in relation to thrombokinetics in primary thrombocythaemia and allied diseases. Scandinavian journal of haematology. 1975;15(5):321-332.
5 Leavitt A. Are there more tricks in the bag for treating thrombocytopenia? The Journal of clinical investigation. 2010;120(11):3807-3810.
6 Bender M, Stritt S, Nurden P, et al. Megakaryocyte-specific Profilin1-deficiency alters microtubule stability and causes a Wiskott-Aldrich syndrome-like platelet defect. Nature communications. 2014;5:4746.
7 Thrasher A, Burns S. WASP: a key immunological multitasker. Nature reviews Immunology. 2010;10(3):182-192.
Dr Markus Bender
Group leader – Emmy-Noether Group of the DFG
Department of Experimental Biomedicine
University Hospital Wuerzburg and Rudolf Virchow Center
Tel: +49 931 31 80362
Bender_M1@ukw.de
