Exposure to Uremic Serum Induces a Procalcific Phenotype in Human Mesenchymal Stem Cells

OBJECTIVE: Medial artery calcification in patients with chronic kidney disease proceeds through intramembranous ossification resulting from osteoblast-induced calcification of the collagen extracellular matrix. The current study is based on the hypothesis that mesenchymal stem cells (MSC) constitute critical cells for procalcific extracellular matrix remodeling in patients with chronic kidney disease. METHODS AND RESULTS: Human MSC were cultured in media supplemented with pooled sera from either healthy or uremic patients (20%). Exposure to uremic serum enhanced the proliferation of MSC (cell counting, BrdU incorporation) whereas apoptosis and necrosis were not affected (annexin V and 7-amino-actinomycin staining). Uremic serum-exposed MSC recapitulated osteogenesis by matrix calcification and expression of bone-related genes (bone morphogenetic protein BMP-2 receptor, alkaline phosphatase, osteopontin, and Runx2) in 35 days. The uremic serum-induced osteogenesis was completely blocked by a BMP-2/4 neutralizing antibody or the natural antagonist NOGGIN. Calcification and matrix remodeling were further analyzed in a collagen-embedded osteogenesis model recapitulating the vascular collagen I/III environment. The uremic serum-induced calcification was shown to occur along collagen fibers as shown by scanning electron microscopy, energy-dispersive X-ray spectroscopy, and von Kossa staining and was accompanied by extensive matrix remodeling. CONCLUSIONS: Uremic serum induced in a BMP-2/4-dependent manner an osteoblast-like phenotype in MSC accompanied by matrix remodeling and calcification.