Changes in bone characteristics precede serum mineral deterioration in early stage chronic kidney disease

Calcium metabolism is tightly regulated and involves hormonal communication between intestine, kidney, and bone. Renal failure leads to impaired calcium homeostasis, but how each of the calcium-handling tissues adapts during the initial phases of the disease, remains to be explored. To this end, we used Col4a3-/- mice in which chronic kidney disease (CKD) develops progressively, as shown by the gradually decreased glomerular filtration rate and increased uremia. Mineral homeostasis was disturbed with increased serum levels of phosphaturic hormones fibroblast growth factor 23 and parathyroid hormone and decreased 1,25-dihydroxyvitamin D3 levels. These hormonal adaptations in Col4a3-/- mice preserved normal serum calcium levels and maintained intestinal calcium absorption and renal fractional clearance of calcium. However, bone mass was already affected starting with a decrease in trabecular bone mass and later evolving to additional cortical bone loss. Histomorphometric analysis of Col4a3-/- mice revealed reduced osteoblast number and activity, whereas osteoclast numbers were not different. Interestingly, the decrease in bone formation was associated with an increase in bone marrow adiposity. Taken together, we show that the renal impairment in Col4a3-/- mice leads to the typical hormonal alterations of CKD that preserve normal serum calcium levels at the early stage, but the renal dysfunction already negatively affects bone formation and increases bone marrow adiposity.