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Serotonin (5-HT) Transport in Human Platelets is Modulated by Src-Catalysed Tyr-Phosphorylation of the Plasma Membrane Transporter SERT

Background/Aim: platelets possess tightly regulated systems for serotonin (5-HT) transport. This study analysed whether the 5-HT transport mediated by the plasma-membrane transporter SERT is regulated by its Tyr-phosphorylation. Methods: 5-HT transport was determined by filtration techniques, while immunoblotting procedures were adopted for detecting the Tyr-phosphorylation of SERT in human platelet fractions. Results: 5-HT accumulation in platelets pre-treated with reserpine, which prevents the neurotransmitter transport into the dense granules, decreased upon cellular exposure to PP2 and SU6656, two structurally unrelated inhibitors of Src-kinases. By contrast, the protein Tyr-phosphatase inhibitor pervanadate increased the 5-HT accumulation. Anti-SERT immunostaining of the platelet fractions showed a major band displaying an apparent molecular mass of 50 κDa, indicating that, during the analytical procedure, SERT underwent proteolysis, which was counteracted by addition of 4 M urea in the cellular disrupting medium. The Tyr-phosphorylation degree of SERT immunoprecipitated from membrane extracts decreased by platelet treatment with SU6656 or PP2, and enhanced upon pervanadate treatment. The anti-SERT immunoprecipitates displayed anti-Src immunostaining and in vitro kinase activity towards a Src-specific peptide-substrate. Platelet treatment with PP2 or SU6656 also caused a decrease in the imipramine binding to platelets. It was concluded that the Src-mediated SERT Tyr-phosphorylation regulates the 5-HT transport by affecting the neurotransmitter binding sites.