Reduced amounts of cartilage collagen fibrils and growth plate anomalies in transgenic mice harboring a glycine-to-cysteine mutation in the mouse type II procollagen alpha 1-chain gene.

We have generated transgenic mice harboring a glycine-to-cysteine mutation in residue 85 of the triple helical domain of mouse type II collagen. The offspring of different founders displayed a phenotype of severe chondrodysplasia characterized by short limbs and trunk, cranio-facial deformities, and cleft palate. The affected pups died of acute respiratory distress caused by an inability to inflate lungs at birth. Staining of the skeleton showed a severe retardation of growth for practically all bones. Light microscopic examination indicated a decrease in cartilage matrix density, a severe disorganization of growth plate architecture, and the presence of streaks of fibrillar material in the cartilage matrix. Electron microscopic analysis showed a pronounced decrease in the number of typical thin cartilage collagen fibrils, distension of the rough endoplasmic reticulum of chondrocytes, and the presence of abnormally large banded collagen fibril bundles. The level of expression of the mutant type II procollagen alpha 1 chain transgene in cartilage tissues was approximately equal to that of the endogenous gene in two of the strains. We propose that the principal consequence of the mutation is a considerable reduction in density of the typical thin cartilage collagen fibrils and that this phenomenon causes the severe disorganization of the growth plate. We also postulate that the abnormal thick collagen fibrils are probably related to a defect in crosslinking between the collagen molecules. The cartilage anomalies displayed by these transgenic mice are remarkably similar to those of certain human chondrodysplasias.     

Expression profiles of mRNAs for osteoblast and osteoclast proteins as indicators of bone loss in mouse immobilization osteopenia model.

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.