Enhancement of osteoclastic bone resorption and suppression of osteoblastic bone formation in response to reduced mechanical stress do not occur in the absence of osteopontin - PubMed (original) (raw)

Enhancement of osteoclastic bone resorption and suppression of osteoblastic bone formation in response to reduced mechanical stress do not occur in the absence of osteopontin

M Ishijima et al. J Exp Med. 2001.

Abstract

Reduced mechanical stress to bone in bedridden patients and astronauts leads to bone loss and increase in fracture risk which is one of the major medical and health issues in modern aging society and space medicine. However, no molecule involved in the mechanisms underlying this phenomenon has been identified to date. Osteopontin (OPN) is one of the major noncollagenous proteins in bone matrix, but its function in mediating physical-force effects on bone in vivo has not been known. To investigate the possible requirement for OPN in the transduction of mechanical signaling in bone metabolism in vivo, we examined the effect of unloading on the bones of OPN(-/-) mice using a tail suspension model. In contrast to the tail suspension-induced bone loss in wild-type mice, OPN(-/-) mice did not lose bone. Elevation of urinary deoxypyridinoline levels due to unloading was observed in wild-type but not in OPN(-/-) mice. Analysis of the mechanisms of OPN deficiency-dependent reduction in bone on the cellular basis resulted in two unexpected findings. First, osteoclasts, which were increased by unloading in wild-type mice, were not increased by tail suspension in OPN(-/-) mice. Second, measures of osteoblastic bone formation, which were decreased in wild-type mice by unloading, were not altered in OPN(-/-) mice. These observations indicate that the presence of OPN is a prerequisite for the activation of osteoclastic bone resorption and for the reduction in osteoblastic bone formation in unloaded mice. Thus, OPN is a molecule required for the bone loss induced by mechanical stress that regulates the functions of osteoblasts and osteoclasts.

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Figures

Figure 1

μ-CT tomographs and trabecular BV/TV of the tibiae of loaded and unloaded mice. (A) μ-CT pictures of the midsagittal planes of the proximal regions of the tibiae after 4 wk of tail suspension (Susp) or loading (Load) in wild-type or OPN−/− mice. μ-CT analyses were conducted as described in Materials and Methods. (B) Fractional trabecular BV/TV was quantified based on the image analysis of μ-CT pictures of the tibiae after 4 wk of either tail suspension (Susp) or loading (Load) in wild-type or OPN−/− mice shown in A. Analyses were conducted in the rectangular 280 × 1,400 μm area of 340–620 μm distal to the growth plate of the proximal ends of the tibiae. Each of the four groups consisted of six mice. Data are expressed as means and standard errors. *Statistically significant difference from respective control (P < 0.05).

Figure 4

An unloading-induced reduction in osteoblastic activity in vivo does not occur in OPN−/− mice. (A and B) In the undecalcified sections of the proximal ends of the tibiae, (A) BFR and (B) MAR at 350–600 μm distal to the growth plate in the metaphyseal region was measured as described in Materials and Methods. The mice were injected intraperitoneally with calcein at 4 mg/kg 4 and 2 d before killing at 2 wk. Data are expressed as means and standard errors for six bones from each of the wild-type and OPN−/− mice groups. *Statistically significant difference from respective control (P < 0.05). (C) Calcein double-labeled surfaces of the bones at the ends of the tibiae after tail suspension (Susp) or loading (Load) in wild-type or OPN−/− mice. Arrows indicate the lines of calcein labeling (light green) used to obtain data shown in A and B.

Figure 2

Urinary Dpyd levels of loaded and unloaded mice. Urine of either tail suspension (Susp) or loading (Load) from both wild-type or OPN−/− mice was collected during the last 24 h (on day 14). Urine from two mice was combined and three independent samples per group were analyzed by ELISA. Data are expressed as means and standard errors. *Statistically significant difference from respective control (P < 0.05).

Figure 3

An unloading-induced increase in the N.Oc/BS does not occur in OPN−/− mice. (A and B) In the decalcified sections at the ends of the tibiae, the N.Oc/BS (A) and the Oc.S/BS (B) were measured within an area of 350–600 μm distal to the growth plate. The N.Oc/BS was calculated as the number of osteoclasts per bone surface, and the Oc.S/BS was calculated as the percentage of bone surface covered by osteoclast per total bone surface. Data are expressed as means and standard errors for six bones from each of the wild-type and OPN−/− mice. *Statistically significant difference from respective control (P < 0.05). (C) Osteoclasts on the cancellous bones in the decalcified 5-μm-thick midsagittal sections of the ends of the tibiae after tail suspension (Susp) or loading (Load) in wild-type or OPN−/− mice. TRAP-positive multinucleated cells (red cells) attached to cancerous bone were counted as osteoclasts to obtain data shown in A and B.

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Enhancement of osteoclastic bone resorption and suppression of osteoblastic bone formation in response to reduced mechanical stress do not occur in the absence of osteopontin - PubMed (original) (raw)