Platelet aggregation is initiated by the release of mediators as adenosine diphosphate (ADP) stored in platelet granules. Possible candidates for transport proteins mediating accumulation of these mediators in granules include multidrug resistance protein 4 (MRP4, ABCC4), a transport pump for cyclic nucleotides and nucleotide analogs. We investigated the expression of MRP4 in human platelets by immunoblotting, detecting a strong signal at 170 kDa. Immunofluorescence microscopy using 2 MRP4-specific antibodies revealed staining mainly in intracellular structures, which largely colocalized with the accumulation of mepacrine as marker for delta-granules and to a lower extent at the plasma membrane. IntroductionThe critical role played by platelets in hemostasis and thrombosis is related to their function as exocytotic cells that secrete effector molecules at the side of vascular injury. Platelets contain at least 3 types of intracellular granules, in which these mediators are stored and concentrated, known as alpha, dense, and lysosomal granules. 1 While alpha granules contain mainly polypeptides, as fibrinogen, von Willebrand factor, growth factors, and protease inhibitors, dense granules contain small molecules, specifically adenosine diphosphate (ADP), adenosine triphosphate (ATP), serotonin, and calcium. 2 Humans with defective dense granule exocytosis suffer from delta storage pool disease associated with a moderate bleeding tendency. The most severe delta storage pool disease is observed in Hermansky-Pudlak syndrome (HPS), a rare autosomal recessive disorder in which oculocutaneous albinism, bleeding, and lysosomal ceroid storage result from defects of melanosomes, platelet-dense granules, and lysosomes. [3][4][5] Little is known, however, about transport proteins mediating accumulation of the effector molecules in granules or their transport across the plasma membrane. Possible candidates include the multidrug resistance protein 4 (MRP4/ABCC4) and MRP5 (ABCC5). These belong to the C-branch of the human ATP-binding cassette (ABC) transporter superfamily, which consists of 12 members, 9 of which comprise the group of multidrug resistance proteins (MRP1-9; ABCC1-6 and ABCC10-12). 6,7 MRPs are integral membrane glycoproteins that mediate the primary active unidirectional export of organic anions from cells. Conjugates of lipophilic compounds with glutathione, glucuronate, and sulfate are preferred substrates of MRP1-3, [8][9][10] while cyclic purine nucleotides and nucleotide analogs have been identified as substrates for MRP4 and MRP5. 11-15 MRP5 mRNA has been detected in many tissues, 16,17 and the MRP5 protein could be localized in erythrocytes, 12 in smooth muscle cells of the genitourinary tract, 18 and in human heart cardiomyocytes, vascular endothelial, and smooth muscle cells. 19 MRP4 mRNA was detected in prostate, liver, testis, ovary, brain, kidney, and adrenal gland. 16,[20][21][22] Studies in membrane vesicles containing recombinant MRP4 indicate that it represents a transporter with a relatively br...

MYH9-related hereditary macrothrombocytopenia does not necessarily protect against postoperative venous thromboembolism, and affected patients who do not evince bleeding diathesis should be considered for routine postoperative pharmacologic thromboprophylaxis.

We previously showed that the MRP4 (ABCC4) transporter is expressed in human platelet delta-granules and may be involved in ADP transport. We now demonstrate by immunoblotting and immunofluorescence microscopy that platelet MRP4 is absent in two patients with a platelet delta-storage pool deficiency (delta-SPD)-like phenotype with reduced platelet adenine nucleotide (AN) but normal serotonin levels, whereas their other membrane marker proteins of platelet granules were normally expressed and localized. In these patients, MRP4 was present in lymphocytes, and the coding region of their MRP4/ABCC4 gene did not show any mutation that explained the lack of expression. In platelets with "classic" delta-SPD (low AN and serotonin levels), MRP4 was quantitatively (immunoblot) normal, but, like other delta-granules membrane marker proteins (eg, LAMP2), was mostly displaced from delta-granules to patches at the plasma membrane, suggesting that platelets with classic delta-SPD have an abnormality that impairs the assembly of normal delta-granules. Thus, defective expression of platelet MRP4 is associated with selective defect in AN storage. The genetic basis of the new delta-SPD phenotype remains to be elucidated.