Platelet activation is a highly regulated process, and cyclic nucleotide mediated signaling pathways are crucial to effective platelet activation. Vascular injury produces, exposed collagen which binds circulating platelets through the platelet's “collagen” receptor, GPVI, resulting in the activation of guanyly/adenlyl cyclases. These interactions result in the rapid alterations in the cyclic nucleotide concentration inside the platelets leading to activation of protein kinase A and G signaling pathways to modulate platelet function.
While, ABCC4 functions as a plasma membrane transporter for cyclic nucleotides its contribution to platelet activation has been obscured because it was reportedly as primarily intracellular in the platelets dense granules. This original report (Jedlitschky, Tirschmann et al. 2004) evaluated ABCC4 localization by immune-fluorescence of platelets attached to collagen coated coverslips. However, attachment via collagen produces platelet activation leading to mobilization and fusion of alpha and dense granules to the plasma membrane, thus under these conditions distinguishing between plasma membrane and dense granules is not possible. We resolved this problem by labeling quiescent platelets with a cell impermeable biotinylating agent (EZ-Link Sulfo-NHS-LC-LC Biotin). Isolation of membrane and internal fraction demonstrated that of over ninety percent of Abcc4 localizes to the plasma membrane. Furthermore, confocal microscopy of platelets stained with specific antibodies against Abcc4 confirmed Abcc4 localization to the plasma membrane.
We extended these studies to the Abcc4- knockout (KO) mouse model. The Abcc4- KO mouse does not have any change in the number of platelet or dense granules compared to the wild type mouse. Platelet activation in vivo can be initiated by interaction with collagen through the GPVI receptor that is expressed at the plasma membrane of the platelets. At the molecular level, the initiation of platelet activation by collagen results in an increase in the cyclic nucleotide concentration leading to activation of signaling cascade through protein kinase A or G. Expose of Abcc4-KO platelets to collagen and revealed impaired activation in response to collagen. However, Abcc4-KO platelets activated by either thrombin or ADP (which activate either G-coupled PAR receptors or P2Y12 receptor respectively) shows an aggregation profile almost identical to wildtype platelets, thus indicating the defect in Abcc4 -KO platelet aggregation is specific to the collagen pathway. To understand the basis for the impaired collagen aggregation of Abcc4-KO platelets, we investigated the collagen receptor (GPVI) signaling pathway in Abcc4-KO platelets. Interestingly, in the Abcc4-KO platelets after the platelet activation with collagen, cyclic nucleotide dependent phosphorylation of VASP through protein kinase A or G at Ser-157 or Ser-239 respectively is reduced compared to the wildtype. Notably, Abcc4-KO platelets had reduced GPVI surface expression that correlated with the reduced phosphorylation of VASP after collagen stimulation. The similar, protein levels of Syk and Plcg2, (downstream signaling molecules of GPVI signaling pathway), in the Abcc4 wildtype and KO platelets implies that GPVI expression is the primary defect in Abcc4 deficiency. These results suggest that Abcc4 plays a crucial role in regulating cyclic nucleotides in response to GPVI activation by collagen. These findings suggest ABCC4/Mrp4 loss of function or inhibition (by drugs) may disrupt platelet aggregation under conditions of vascular injury. As, many antiplatelet drugs are potent inhibitors of Abcc4 (e.g., Dipyridamole and Sildenafil) these conclusions have strong implications for not just the development of antiplatelet drugs, but also for further exploring the role of Abcc4 in regulating intracellular nucleotide levels and platelet biology.
No relevant conflicts of interest to declare.
This work was supported by NIH and by the ALSAC
Asterisk with author names denotes non-ASH members.