Impaired Osteoblast and Osteocyte Maturation in the Pathogenesis of Renal Osteodystrophy

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ABSTRACT Bone disease in chronic kidney disease (CKD) has been traditionally defined by changes in bone turnover stemming from altered circulating parathyroid hormone (PTH) concentrations. However, skeletal mineralization defects and increased numbers of fibroblast growth factor 23 (FGF23)-expressing osteocytes are present in children with very early CKD; these abnormalities are unrelated to circulating calcium, phosphorus, and PTH concentrations and contribute to bone deformities, fractures, poor growth, and cardiovascular disease that are not treated by current therapies. Our preliminary data suggest that osteoblasts and osteocytes from CKD patients have impaired maturation. Our data demonstrate that locally secreted signals regulate this maturation failure in primary osteoblast in vitro, likely via the Wingless/Integrin 1 (Wnt) signaling pathway. Local deficiencies in extracellular matrix concentrations dentin matrix protein 1 (DMP1), with subsequent alterations in Wnt signaling, may contribute to impaired osteocyte maturation, excess FGF23 expression, and skeletal mineralization defects in vivo. To test the hypothesis that CKD-mediated skeletal mineralization defects occur in response to abnormal secretion of local factors critical to osteoblast and osteocyte maturation, we will first establish that osteocyte maturation defects co-occur with or precede skeletal mineralization defects in vivo. We will evaluate markers of osteocyte maturity by immunohistochemistry and immunofluorescence in intact human iliac-crest sections from patients with early and late CKD. We will correlate expression of these markers with histomorphometric and quantitative backscatter electron imaging (qBEI) measures of bone mineralization. We will evaluate expression and location of DMP1 and Wnt signaling proteins in whole bone and in extracellular matrix by immunohistochemistry, immunofluorescence, and Western blot analysis. Next, we will characterize CKD osteoblast maturation and determine which secreted factor(s) modulate osteoblast maturation and Wnt signaling in vitro. We will characterize markers of osteoblast maturity in control and CKD osteoblasts during proliferative, matrix-generating, and mineralizing phases of maturation. We will rescue the CKD phenotype in the presence of healthy extracellular matrix and conditioned media and we will characterize signaling pathways affected by phenotype rescue. We will identify specific factors missing from CKD extracellular matrix that rescue the CKD phenotype and normalize Wnt signaling in vitro. We will attempt to recreate the CKD phenotype by siRNA knockdown of these same factors in control cells. At the completion of this study, we anticipate insight into the secreted factors and mechanistic pathways that contribute to osteoblast and osteocyte maturation failure in CKD. We anticipate that these findings will propel further research into understanding the mechanisms of defective skeletal mineralization in CKD and in defining therapeutic targets for improving bone health in the CKD population.