Transmembrane signaling through G protein-coupled receptors (GPCRs) controls a diverse array of cellular processes including metabolism, growth, motility, adhesion, neuronal signaling and blood coagulation. The numerous GPCRs and their key roles in both normal physiology and disease have made them the target for more than 50% of all prescribed drugs. GPCR agonists and antagonists act on the extracellular side of the receptors, whereas the intracellular surface has not yet been exploited for development of new therapeutic agents. Here, we demonstrate the utility of novel cell-penetrating peptides, termed 'pepducins', that act as intracellular inhibitors of signal transference from receptors to G proteins. Attachment of a palmitate lipid to peptides based on the third intracellular loop of protease-activated receptor 1 (PAR1) or PAR4 (refs. 3-5) yielded potent inhibitors of thrombin-mediated aggregation of human platelets. Infusion of the anti-PAR4 pepducin into mice extended bleeding time and protected against systemic platelet activation, consistent with the phenotype of PAR4-deficient mice. We show that pepducins might be used to ascertain the physiological roles of GPCRs and rapidly determine the potential therapeutic value of blockade of a particular signaling pathway.

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HHT is an autosomal dominant vascular dysplasia, in which abnormalities of endothelial cells cause patients to develop mucocutaneous telangiectasias, and AVMs of the pulmonary, hepatic, and cerebral circulations. Pulmonary AVMs occur in more than 20% of patients with HHT, and presentation varies from simple diffuse telangiectasias to large complex structures. Surgical management is usually indicated for large pulmonary AVMs. Treatment options for pulmonary AVMs include therapeutic embolization, segmental or lobar resection of the affected lung, and less commonly lung transplantation. Here we report the first successful case of a bilateral lung transplant for a four-yr-old girl with HHT.