Acidification of α-granules in megakaryocytes by vacuolar-type adenosine triphosphatase is essential for organelle biogenesis

Background: Platelets coordinate blood coagulation at sites of vascular injury and play fundamental roles in a wide variety of (patho)physiological processes. Key to many platelet functions is the transport and secretion of proteins packaged within x-granules, organelles produced by platelet precursor megakaryocytes. Prominent among x-granule cargo are fibrinogen endocytosed from plasma and endogenously synthesized von Willebrand factor. These and other proteins are known to require acidic pH for stable packaging. Luminal acidity has been confirmed for mature x-granules isolated from platelets, but direct measurement of megakaryocyte granule acidity has not been reported. Objectives: To determine the luminal pH of x-granules and their precursors in megakaryocytes and assess the requirement of vacuolar-type adenosine triphosphatase (VATPase) activity to establish and maintain the luminal acidity and integrity of these organelles. Methods: Cresyl violet staining was used to detect acidic granules in megakaryocytes. Endocytosis of fibrinogen tagged with the pH-sensitive fluorescent dye fluorescein isothiocyanate was used to load a subset of these organelles. Ratiometric fluorescence analysis was used to determine their luminal pH. Results: We show that most of the acidic granules detected in megakaryocytes appear to be x-granules/precursors, for which we established a median luminal pH of 5.2 (IQR, 5.0-5.5). Inhibition of megakaryocyte V-ATPase activity led to enlargement of cargo- containing compartments detected by fluorescence microscopy and electron microscopy. Conclusion: These observations reveal that V-ATPase activity is required to establish and maintain a luminal acidic pH in megakaryocyte x-granules/precursors, confirming its importance for stable packaging of cargo proteins such as von Willebrand factor.