Cation homeostasis in megakaryocytes and platelets
Agonist-induced elevation of the intracellular Ca2+ concentration ([Ca2+]i) is a central event during platelet activation and aggregation that occurs through Ca2+ release from intracellular stores and/or Ca2+ influx through plasma membrane channels, most notably receptor operated Ca2+ channels (ROC) and store operated Ca2+ channels (SOC).
We have identified the Ca2+ sensor stromal interaction molecule 1 (STIM1) and the four transmembrane domain protein Orai1 as the principal mediators of store-operated calcium entry (SOCE) in platelets, thus establishing this mechanism as a potential target for antithrombotic therapy. Our findings furthermore revealed that TRPC6 is the major and only DAG-mediated ROC channel in platelets. The pathophysiological imbalance in Ca2+ homeostasis is usually associated with the abnormal function of Ca2+ channels, Ca2+ pumps or Na+/Ca2+
Although it has been known for more than 50 years that Mg2+ is a natural antagonist of Ca2 and hypomagnesaemia is related to elevated [Ca2+]i in mammalian cells, the molecular mechanisms underlying these observations are still not resolved. We speculate that abnormal function of Mg2+ channels may contribute to the development of a Ca2+-overload phenotype in platelets, thereby inducing abnormal platelet activation and a pro-thrombotic phenotype in vivo. Although Mg2+ entry has been described in megakaryocytes and platelets, the involved Mg2+ channels and their signaling function have not been identified. We are therefore using knock-out and knock-in mouse models for the Mg2+ channels magnesium transporter 1 (MAGT1) and transient receptor potential melastatin-like 7 (TRPM7) to identify the major Mg2+ entry mechanism in murine megakaryocytes and platelets and study Mg2+ receptor signaling under normal and pathophysiological conditions. Altered Mg2+ homeostasis is described in many cardiovascular diseases, especially in patients with coronary heart disease, arrhythmia, acute myocardial infarction, hypertension and stroke. Under specific clinical conditions, Mg2+ therapy is essential to prevent thrombosis, myocardial infarction or stroke. Therefore, we analyze the physiological consequence of impaired Mg2+ entry mechanism in our mouse models in experimental models of cardiovascular diseases, including thrombosis and ischemic stroke.
Moreover, our recent findings indicate that also the zinc (Zn2+) homeostasis is critically involved in hemostatic and thrombo-inflammatory processes. By utilizing platelets from patients with Zn2+-related platelet disorders and transgenic mouse models we plan to identify the intracellular Zn2+ stores and the molecular components that mediate Zn2+ transport in platelets, and to investigate their role in platelet signaling.