Identifying early pathogenic events during vascular calcification in uremic rats

Vascular calcification in chronic kidney disease is a very complex process traditionally explained in multifactorial terms. Here we sought to clarify relevance of the diverse agents acting on vascular calcification in uremic rats and distinguish between initiating and complicating factors. After 5/6 nephrectomy, rats were fed a 1.2% phosphorus diet and analyzed at different time points. The earliest changes observed in the aortic wall were noticed 11 weeks after nephrectomy: increased Wnt inhibitor Dkk1 mRNA expression and tissue non-specific alkaline phosphatase (TNAP) expression and activity. First deposits of aortic calcium were observed after 12 weeks in areas of TNAP expression. Increased mRNA expressions of Runx2, BMP2, Pit1, Pit2, HOXA10, PHOSPHO1, Fetuin-A, ANKH, OPN, Klotho, cathepsin S, MMP2, and ENPP1 were also found after TNAP changes. Increased plasma concentrations of activin A and FGF23 were observed already at 11 weeks post-nephrectomy, while plasma PTH and phosphorus only increased after 20 weeks. Plasma pyrophosphate decreased after 20 weeks, but aortic pyrophosphate was not modified, nor was the aortic expression of MGP, Msx2, several carbonic anhydrases, osteoprotegerin, parathyroid hormone receptor-1, annexins II and V, and CD39. Thus, increased TNAP and Dkk1 expression in the aorta precedes initial calcium deposition, and this increase is only preceded by elevations in circulating FGF23 and activin A. The expression of other agents involved in vascular calcification only changes at later stages of chronic kidney disease, in a complex branching pattern that requires further clarification.