Background: The secondary hyperparathyroidism of chronic kidney disease (CKD) produces a high turnover osteodystrophy that is associated with peritrabecular fibrosis. The nature of the cells involved in the development of peritrabecular fibrosis may represent osteoprogenitors expressing a fibroblastic phenotype that are retarded from progressing through osteoblast differentiation.
Methods: To test the hypothesis that osteoblast differentiation is retarded in secondary hyperparathyroidism due to CKD producing bone marrow fibrosis, we administered bone morphogenetic protein 7 (BMP-7), a physiologic regulator of osteoblast regulation, to C57BL6 mice that had CKD produced by electrocautery of one kidney followed by contralateral nephrectomy two weeks later. Following the second surgical procedure, a subgroup of mice received daily intraperitoneal injections of BMP-7 (10 microg/kg). Three to six weeks later, the animals were sacrificed, blood was obtained for measurements of blood urea nitrogen (BUN) and parathyroid hormone (PTH) levels, and the femora and tibiae were processed for histomorphometric analysis.
Results: The animals had significant renal insufficiency with BUN values of 77.79 +/- 22.68 mg/dL, and the level of renal impairment between the CKD untreated mice and the CKD mice treated with BMP-7 was the same in the two groups. PTH levels averaged 81.13 +/- 51.36 and 75.4 +/- 43.61 pg/mL in the CKD and BMP-7 treated groups, respectively. The animals with CKD developed significant peritrabecular fibrosis. In addition, there was an increase in osteoblast surface and osteoid accumulation as well as increased activation frequency and increased osteoclast surface consistent with high turnover renal osteodystrophy. Treatment with BMP-7 eliminated peritrabecular fibrosis, increased osteoblast number, osteoblast surface, mineralizing surface and single labeled surface. There was also a significant decrease in the eroded surface induced by treatment with BMP-7.
Conclusions: These findings indicate that BMP-7 treatment in the setting of high turnover renal osteodystrophy prevents the development of peritrabecular fibrosis, affects the osteoblast phenotype and mineralizing surfaces, and decreases bone resorption. This is compatible with a role of osteoblast differentiation in the pathophysiology of osteitis fibrosa.