Proplatelet formation is selectively inhibited by collagen type I through Syk-independent GPVI signaling.

Collagen receptors GPVI (also known as GP6) and integrin α2ß1 are highly expressed on blood platelets and megakaryocytes, their immediate precursors. After vessel injury, subendothelial collagen becomes exposed and induces platelet activation to prevent blood loss. Collagen types I and IV are thought to have opposite effects on platelet biogenesis, directing proplatelet formation (PPF) towards the blood vessels to prevent premature release within the marrow cavity. We used megakaryocytes lacking collagen receptors or treated megakaryocytes with blocking antibodies, and could demonstrate that collagen-I-mediated inhibition of PPF is specifically controlled by GPVI. Other collagen types competed for binding and diminished the inhibitory signal, which was entirely dependent on receptor-proximal Src family kinases, whereas Syk and LAT were dispensable. Adhesion assays indicate that megakaryocyte binding to collagens is mediated by α2ß1, and that collagen IV at the vascular niche might displace collagen I from megakaryocytes and thus contribute to prevention of premature platelet release into the marrow cavity and thereby directionally promote PPF at the vasculature.

Identification of fibronectin as a major factor in human serum to recruit subchondral mesenchymal progenitor cells.

Human serum has the potential for mesenchymal progenitor cell recruitment in repair of articular cartilage lesions. It is unclear which factor(s) in serum mediate this migratory effect. Our goal was to identify cell recruiting factors in human serum fractions obtained by ion exchange chromatography. The recruiting activity of serum fractions on human subchondral mesenchymal progenitor cells was analyzed using 96-well chemotaxis assays. Protein composition of recruiting serum fractions were analyzed by mass spectrometry and showed 58 potential candidates. Fibronectin, gelsolin, lumican, thrombospondin-1 and WNT-9a were identified as key candidates for progenitor cell recruitment. Only human plasma derived and recombinant fibronectin showed significant recruiting activity on progenitors reaching 50-90% of the recruiting activity of normal human serum. Presence of fibronectin in all human serum fractions with recruiting activity was verified by Western blot analysis. This study shows that fibronectin is a key factor in human serum to recruit mesenchymal progenitor cells and might be involved in subchondral mesenchymal progenitor cell migration into cartilage defects after microfracture.