Expression Profiles of mRNAs for Osteoblast and Osteoclast Proteins as Indicators of Bone Loss in Mouse Immobilization Osteopenia Model

Abstract

An experimental mouse model for disuse osteopenia was developed using unilateral cast immobilization. Analysis of the distal femurs and proximal tibias by quantitative histomorphometry revealed significant osteopenia within 10-21 days of immobilization. At 3 weeks, bone loss was also demonstrated with peripheral quantitative computed tomography as diminished bone mineral content and as concomitant reduction in the cross-sectional moment of inertia. These structural and geometrical alterations resulted in decreased strength of the distal femurs tested by cantilever bending. Analysis of the underlying cellular and molecular mechanisms of bone loss revealed a rapid increase in bone resorption within 3 days of immobilization. The mRNA levels for cathepsin K, matrix metalloproteinase-9, and tartrate resistant acid phosphatase were all significantly increased during the 21-day immobilization period, but with different expression profiles. These increases were paralleled by an increased number of osteoclasts as measured by histomorphometry. By day 6 of immobilization, the balance of bone turnover was further shifted toward net bone loss as the mRNA levels for major bone components (type I collagen and osteocalcin) were decreased. In histomorphometric analysis this was observed as reduced rates of mineral apposition and bone formation after 10 days of immobilization. The results of this study demonstrate that immobilization has a dual negative effect on bone turnover involving both depressed bone formation and enhanced bone resorption.

Histomorphometric and Densitometric Changes in the Femora of Spinal Cord Transected Mice

“…However, preliminary data from SCI patients suggest that both a decrease of osteoblastic activity and an increase of osteoclastic activity contribute to bone loss after trauma (Roberts et al, 1998). This finding closely reflects some results obtained in a mouse model of disuse (hindlimb immobilization with a cast) showing a rapid decrease of osteocalcin and a sharp increase of acid phosphatase (i.e., markers of osteoblastic and osteoclastic activities respectively) within a few days to a few weeks postimmobilization (Rantakokko et al, 1999). It will be of interest in future experiments to determine whether comparable changes in biomarker levels can be found in SCI mice.…”

“…For instance, a 10-30% (up to 50% after 18 months) decrease of cancellous bone was found within a few weeks in the ipsilateral femur of rats that had their hindlimbs immobilized with a cast or an elastic bandage (Li et al, 1990;Chen et al, 1992;Maeda et al, 1993). Some of the changes associated with disuse are believed to be mediated by both an increase of osteoclastic bone resorption and a decrease of osteoblastic bone formation (Rantakokko et al, 1999;Kingery et al, 2003).…”

Alcohol and Adaptation to Mechanical Usage

“…We have also shown that chronic consumption of alcohol can suppress bone formation and cause bone loss without a continuous reduction in mRNA levels for bone matrix proteins (Turner, unpublished data). Taken together, these findings suggest, but do not prove, that alcoholinduced inhibition of bone formation may be due in part to post-transcriptional alterations.Alternatively, the mRNA levels for bone matrix proteins may depend upon the interval between feeding and sacrifice.PTH resulted in histomorphometric changes consistent with those reported in published studies, including increases in mineralizing perimeter and bone formation rate (29,33,34). The large increases in mRNA levels for bone matrix proteins following PTH-treatment have also been reported (29).…”

Early adaptive changes in chronic paraplegic mice: a model to study rapid health degradation after spinal cord injury

“…In fact, a murine model of disuse (hindlimb immobilization with a cast) provided evidence suggesting that bone loss occurring within a few days to a few weeks post-immobilization involves both a sharp decrease of osteoblastic activity and a rapid increase of osteoclastic activity (that is, low osteocalcin and high acid phosphatase levels). 19 Further studies in mice (for example, SCI, cast-immobilization, tail-suspension) are likely to provide new insights into the molecular mechanisms of rapid bone loss after paralysis or immobilization.…”