Background Connexins are a widespread family of membrane proteins that assemble into hexameric hemichannels, also known as connexons. Connexons regulate membrane permeability in individual cells or couple between adjacent cells to form gap junctions and thereby provide a pathway for regulated intercellular communication. We have now examined the role of connexins in platelets, blood cells that circulate in isolation, but upon tissue injury adhere to each other and the vessel wall to prevent blood loss and facilitate wound repair. Methods and Results We report the presence of connexins in platelets, notably connexin37, and that the formation of gap junctions within platelet thrombi is required for the control of clot retraction. Inhibition of connexin function modulated a range of platelet functional responses prior to platelet-platelet contact, and reduced laser induced thrombosis in vivo in mice. Deletion of the Cx37 gene (Gja4) in transgenic mice reduced platelet aggregation, fibrinogen binding, granule secretion and clot retraction indicating an important role for Cx37 hemichannels and gap junctions in platelet thrombus function. Conclusions Together, these data demonstrate that platelet gap junctions and hemichannels underpin the control of haemostasis and thrombosis and represent potential therapeutic targets.

No abstract

Objective Thiol isomerases facilitate protein folding in the endoplasmic reticulum, and several of these enzymes, including PDI and ERp57, are mobilized to the surface of activated platelets, where they influence platelet aggregation, blood coagulation and thrombus formation. In this study we examined for the first time the synthesis and trafficking of thiol isomerases in megakaryocytes, determined their subcellular localization in platelets and identified the cellular events responsible for their movement to the platelet surface upon activation. Approach and Results Immunofluorescence microscopy (IFM) imaging was used to localize PDI and ERp57 in murine and human megakaryocytes at various developmental stages. IFM and subcellular fractionation analysis were used to localize these proteins in platelets to a compartment distinct from known secretory vesicles that overlaps with an inner cell surface membrane region defined by the endoplasmic/sarcoplasmic reticulum proteins calnexin and SERCA3. IFM and flow cytometry were used to monitor thiol isomerase mobilization in activated platelets in the presence and absence of actin polymerization (inhibited by latrunculin), and in the presence or absence of membrane fusion mediated by Munc 13-4 (absent in platelets from Unc13dJinx mice). Conclusions Platelet-borne thiol isomerases are trafficked independently of secretory granule contents in megakaryocytes, and become concentrated in a subcellular compartment near the inner surface of the platelet outer membrane corresponding to the sarco/endoplasmic reticulum of these cells. Thiol isomerases are mobilized to the surface of activated platelets via a process that requires actin polymerization but not SNARE/Munc 13-4-dependent vesicular-plasma membrane fusion.