Sclerostin Immunoreactivity Increases in Cortical Bone Osteocytes and Decreases in Articular Cartilage Chondrocytes in Aging Mice

Abstract

Sclerostin is a 24-kDa secreted glycoprotein that has been identified as a negative modulator of new bone formation and may play a major role in age-related decline in skeletal function. Although serum levels of sclerostin markedly increase with age, relatively little is known about whether cells in the skeleton change their expression of sclerostin with aging. Using immunohistochemistry and confocal microscopy, we explored sclerostin immunoreactivity (sclerostin-IR) in the femurs of 4-, 9-, and 24-month-old adult C3H/HeJ male mice. In the femur, the only two cell types that expressed detectable levels of sclerostin-IR were bone osteocytes and articular cartilage chondrocytes. At three different sites along the diaphysis of the femur, only a subset of osteocytes expressed sclerostin-IR and the percentage of osteocytes that expressed sclerostin-IR increased from approximately 36% to 48% in 4- vs. 24-month-old mice. In marked contrast, in the same femurs, there were ~40% fewer hypertrophic chondrocytes of articular cartilage that expressed sclerostin-IR when comparing 24- vs. 4-month-old mice. Understanding the mechanism(s) that drive these divergent changes in sclerostin-IR may provide insight into understanding and treating the age-related decline of the skeleton.

Old age causes de novo intracortical bone remodeling and porosity in mice

“…The decrease in Sost gene expression and sclerostin protein in marrow plasma reported herein constitutes additional evidence that aging alters the secretome of cortical osteocytes and is consistent with previous studies (20,64). Decreased sclerostin levels should increase pro-osteoblastogenic Wnt signaling (65), but we have shown previously that expression of Wnt target genes declines in murine cortical bone with age (66,67).…”

Sclerostin does not play a major role in the pathogenesis of skeletal complications in type 2 diabetes mellitus

“…Thus, it could be that sclerostin levels in serum correlate better with the degree of glucose control remaining or the age of the rats, rather than the presence or absence of diabetes. Sclerostin levels greatly increase with age in mice and humans [38, 39], thus it seems unlikely that age contributes to the decrease in sclerostin levels observed in our study at 14 weeks of age. However, we cannot exclude that the lower bone mass observed in ZDF rats at 14 weeks may imply the presence of fewer osteocytes and consequently reduced sclerostin release into the serum of diabetic rats.…”

“…Significantly, Roforth et al [41] suggested that circulating sclerostin is derived from several other non-skeletal sources, as well as from articular cartilage chondrocytes [39], and all this information combined implies that measurement of serum sclerostin may not be useful as a predictive biomarker for impaired bone formation. Serum sclerostin levels have been measured in many clinical conditions and consistent changes were not always observed [42], suggesting the need for a better understanding of the factors that control sclerostin production by osteocytes and possibly more consistent serum sclerostin assays.…”

Immunohistochemical localization of nerve growth factor, tropomyosin receptor kinase A, and p75 in the bone and articular cartilage of the mouse femur

“…The adult and aging mice were deeply anesthetized with ketamine/xylazine (0.01 ml/g, 100 mg/10 kg, s.c.) and perfused intracardially as described previously. 33 – 35 Briefly, mice were perfused intracardially with a fixative solution of 4% formaldehyde/12.5% picric acid solution in 0.1 M PBS (pH 6.9 at 4℃). Following perfusion, hindlimbs were removed and postfixed in the same fixative solution overnight.…”