P₀-Related Protein Accelerates the Migration of Human Mesenchymal Stem Cells and Their Progeny by Modulating VLA-5 Interactions with Fibronectin.

Human mesenchymal stem cells (hMSCs) are capable of expansion, self renewal and generation of several tissues such as bone, fat and cartilage. They also generate microenviromental niche cells that regulate angiogenesis and hematopoiesis. During development and in the adult, hMSCs and their progeny migrate to specific niches where they provide support for hematopoietic cells or they play an important role in tissue regeneration, inflammatory responses and cancer surveillance. There is particular interest in understanding the molecular basis of hMSCs migration which involves numerous transmembrane receptors, cell adhesion and signaling molecules. Here, we demonstrate, for the first time, that human P₀ - related protein, hPZR, has the ability to accelerate VLA-5-mediated migration of cultured hMSCs on the extracellular matrix (ECM) protein, fibronectin. This is mediated via the immunotyrosine-inhibitory motif (ITIM) sequences within the hPZR cytoplasmic domain, and the ECM ligand, since it cannot be recapitulated with the hPZR ITIM-less isoform, hPZRb, nor with other ECM components such as laminin or Type IV collagen. It is mediated by interactions with the docking phosphatase, SHP-2. The specificity of these results was confirmed by knocking-down hPZR and hPZRb using siRNA technology. We further demonstrated that hPZR clusters with VLA-5 at the leading edge of the cell, using high-resolution confocal microscopy together with immunoprecipitation and immunoblotting technologies. We propose that hPZR plays a key role in negatively regulating VLA-5 adhesion to fibronectin, a process critical for cell detachment from fibronectin and for the migration of hMSCs and their progeny.