Coumarin derivatives such as warfarin represent the therapy of choice for the long-term treatment and prevention of thromboembolic events. Coumarins target blood coagulation by inhibiting the vitamin K epoxide reductase multiprotein complex (VKOR). This complex recycles vitamin K 2,3-epoxide to vitamin K hydroquinone, a cofactor that is essential for the post-translational gamma-carboxylation of several blood coagulation factors. Despite extensive efforts, the components of the VKOR complex have not been identified. The complex has been proposed to be involved in two heritable human diseases: combined deficiency of vitamin-K-dependent clotting factors type 2 (VKCFD2; Online Mendelian Inheritance in Man (OMIM) 607473), and resistance to coumarin-type anticoagulant drugs (warfarin resistance, WR; OMIM 122700). Here we identify, by using linkage information from three species, the gene vitamin K epoxide reductase complex subunit 1 (VKORC1), which encodes a small transmembrane protein of the endoplasmic reticulum. VKORC1 contains missense mutations in both human disorders and in a warfarin-resistant rat strain. Overexpression of wild-type VKORC1, but not VKORC1 carrying the VKCFD2 mutation, leads to a marked increase in VKOR activity, which is sensitive to warfarin inhibition.

To explore the initial steps by which transplanted mesenchymal stem cells (MSCs) interact with the vessel wall in the course of extravasation, we studied binding of human MSCs to endothelial cells (ECs). In a parallel plate flow chamber, MSCs bound to human umbilical vein ECs (HUVECs) similar to peripheral-blood mononuclear cells (PBMCs) or CD34 ؉ hematopoietic progenitors at shear stresses of up to 2 dynes/cm 2 . This involved rapid extension of podia, rolling, and subsequent firm adhesion that was increased when ECs were prestimulated with TNF-␣. MSC binding was suppressed when ECs were pretreated with function-blocking anti-P-selectin antibody, and rolling of MSCs was induced on immobilized P-selectin, indicating that P-selectin was involved in this process. IntroductionIn recent years, mesenchymal stem cells (MSCs) have been characterized as adherent-cell populations originating from bone marrow, capable of expanding in vitro as undifferentiated cells or differentiating into osteocytes, chondrocytes, tenocytes, adipocytes, or smooth muscle cells. [1][2][3] MSCs have been used in a number of preclinical models to mediate the regeneration of muscle, endothelial, neuronal, skin, or renal epithelial tissue. [4][5][6][7] In vitro differentiation studies have demonstrated the potential of MSCs to also form alveolar and airway epithelial cells or cardiac pacemaker cells. 8,9 Moreover, MSCs have been transplanted intravenously and shown to distribute to spleen, bone, lung, and cartilage in several rodent models. [10][11][12] Intravenously injected MSCs have already been used in patients to accelerate hematopoietic reconstitution after hematopoietic stem cell transplantation, to overcome the molecular defect in children with osteogenesis imperfecta, or to alleviate the outcome after myocardial infarction. [13][14][15][16][17] Transplantation experiments in mice and primates have shown that intravenously applied MSCs distribute to several tissues and may accumulate in the lungs. 10,12,18,19 However, currently it is poorly understood to what degree MSCs use specific adhesion mechanisms for egress from the bloodstream and whether they home in a tissue-specific manner. To leave the bloodstream, mature leukocytes and hematopoietic progenitor cells (HPCs) have been shown to undergo a coordinated sequence of adhesion steps, initiated by tethering events, which are mainly mediated by selectins and their ligands. 20,21 Subsequently, the captured cells roll and encounter chemokines, which eventually activate integrins, resulting in firm arrest and subsequent transendothelial migration.To elucidate the potential of MSCs to undergo coordinated steps of interaction with endothelial cells (ECs), we investigated human MSCs under shear flow using a parallel plate flow chamber and by intravital microscopy in mice. We show here that human MSCs home to different tissues and display coordinated rolling and adhesion behavior on ECs. Although P-selectin glycoprotein ligand 1 (PSGL-1) is not expressed by MSCs, MSCs bind to ECs in a P-sele...

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