Osteocytic Sclerostin Expression as an Indicator of Altered Bone Turnover

Abstract

Renal osteodystrophy (ROD) is a complex and serious complication of chronic kidney disease (CKD), a major global health problem caused by loss of renal function. Currently, the gold standard to accurately diagnose ROD is based on quantitative histomorphometric analysis of trabecular bone. Although this analysis encompasses the evaluation of osteoblast and osteoclast number/activity, tfigurehe interest in osteocytes remains almost nihil. Nevertheless, this cell type is evidenced to perform a key role in bone turnover, particularly through its production of various bone proteins, such as sclerostin. In this study, we aim to investigate, in the context of ROD, to which extent an association exists between bone turnover and the abundance of osteocytes and osteocytic sclerostin expression in both the trabecular and cortical bone compartments. Additionally, the effect of parathyroid hormone (PTH) on bone sclerostin expression was examined in parathyroidectomized rats. Our results indicate that PTH exerts a direct inhibitory function on sclerostin, which in turn negatively affects bone turnover and mineralization. Moreover, this study emphasizes the functional differences between cortical and trabecular bone, as the number of (sclerostin-positive) osteocytes is dependent on the respective bone compartment. Finally, we evaluated the potential of sclerostin as a marker for CKD and found that the diagnostic performance of circulating sclerostin is limited and that changes in skeletal sclerostin expression occur more rapidly and more pronounced. The inclusion of osteocytic sclerostin expression and cortical bone analysis could be relevant when performing bone histomorphometric analysis for diagnostic purposes and to unravel pathological mechanisms of bone disease.

Keywords: bone turnover; chronic kidney disease (CKD); osteocyte; parathyroid hormone (PTH); parathyroidectomy (PTX); renal osteodystrophy (ROD); sclerostin.

Figure 1

Serum iPTH and sclerostin levels. Serum iPTH and sclerostin levels were measured in PTX rats (n = 10) and non-PTX rats (n = 9) using specific enzyme-linked immunosorbent assays (ELISA). (A) Total exposure of the bone to iPTH during the 12 weeks of the study (measured as area under the curve (AUC), pg/mL * weeks); (B) Serum iPTH levels at 12 weeks (shown as median ± IQR); (C) Total exposure of the bone to sclerosin during the 12 weeks of the study (measured as AUC, pg/mL * weeks). ** p ≤ 0.01.

Figure 2

Cell counts in (non-)PTX RATS. Different cell counts (i.e., osteocytes, total number of lacunae and sclerostin-positive osteocytes) were determined in the cortical and trabecular bone of PTX rats (n = 10) and non-PTX rats (n = 9). Data shown as mean ± SD. *** p ≤ 0.001.

Figure 3

Cell counts in the patient cohort. Different cell counts (i.e., osteocytes, total number of lacunae and sclerostin-positive osteocytes) were determined in the cortical and trabecular bone of CKD5/transplant patients with different degrees of bone turnover (n = 10/group). Data shown as mean ± SD. * p ≤ 0.05, ** p ≤ 0.01, *** p ≤ 0.001.

Figure 4

PTH/Sclerostin interaction. PTH exerts a direct effect on cAMP signaling, resulting in reduced SOST mRNA expression due to impairment of the MEF2-stimulated SOST promotor. Lower sclerostin levels cause increased WNT signaling and, eventually, bone formation. Abbreviation: PTHR, parathyroid hormone receptor.