Novel implications of lost serotonin transporter function on platelet biology

Abstract

Reduced platelet aggregation and a mild bleeding phenotype has been observed in patients chronically taking selective serotonin reuptake inhibitors (SSRIs) that block the serotonin transporter (SERT). Here, I explore this relationship between platelet activation, specifically αIIbβ3 activation, and the loss of SERT function. First, using a pharmacological approach, we investigate the role of acute loss of SERT function on platelet αIIbβ3 activation. We find that acute treatment with SSRIs does not alter αIIbβ3 activation but reduced αIIbβ3-mediated platelet spreading. Next, two models of sustained loss of SERT function were used to investigate platelet αIIbβ3 activation; the SERT knockout mouse (SERT-/-) mice and mice treated with citalopram for 6-days. Both models replicate the mild bleeding phenotypes noted in human patients taking SSRIs. Following transfusion of wild-type platelets into SERT-/- mice, SERT-/- bleeding times were reduced to WT levels suggesting defects in platelet function. We examined αIIbβ3 activation (JON/A binding) and granule exocytosis with P-selectin surface expression (CD62p binding) following ADP stimulation and found reduced ADP-mediated αIIbβ3 activation in SERT-/- platelets. Acute treatment of platelets with SSRIs (paroxetine and citalopram) to prevent serotonin uptake during activation did not alter ADP-mediated αIIbβ3 activation. However, 5HT2AR antagonists significantly reduced ADP-mediated αIIbβ3 activation. These findings suggest that serotonin synergizes with ADP through 5HT2AR activation, but not acute SERT uptake. Furthermore, SERT-/- platelets displayed reduced serotonin enhanced ADP-mediated αIIbβ3 activation, likely due to reduced 5HT2AR cell surface levels. This again suggests acute function of SERT does not alter ADP-mediated αIIbβ3 activation but that sustained loss of SERT function alters surface expression of 5HT2AR and ultimately reduces ADP-mediated αIIbβ3 activation. Lastly, I discuss the hypercoaguability phenotype of the β3 integrin mutation, which models the human PIA2 polymorphism. This mutation leads to a primed αIIbβ3 integrin characterized by increased baseline Src-signaling. The hypercoaguability phenotype can be rescued with the Src inhibitor SKI606 but, interestingly, is also hyperserotonergic. We find that blockage of the SERT transporter with 6-day citalopram treatment is also able to rescue the KI thrombin clotting time. These findings further establish a connection between serotonergic regulation and αIIbβ3 activation in platelet function. This body of work expands the current knowledge of serotonin in platelet biology and supports a more physiologically based understanding of the effects of SSRIs on platelet function.