Myostatin deficiency partially rescues the bone phenotype of osteogenesis imperfecta model mice

Abstract

Mice with osteogenesis imperfecta (+/oim), a disorder of bone fragility, were bred to mice with muscle over growth to test whether increasing muscle mass genetically would improve bone quality and strength. The results demonstrate that femora from mice carrying both mutations have greater mechanical integrity than their +/oim littermates.

Introduction: Osteogenesis imperfecta is a heritable connective tissue disorder due primarily to mutations in the type I collagen genes resulting in skeletal deformity and fragility. Currently, there is no cure, and therapeutic strategies encompass the use of antiresorptive pharmaceuticals and surgical bracing, with limited success and significant potential for adverse effects. Bone, a mechanosensing organ, can respond to high mechanical loads by increasing new bone formation and altering bone geometry to withstand increased forces. Skeletal muscle is a major source of physiological loading on bone, and bone strength is proportional to muscle mass.

Methods: To test the hypothesis that congenic increases in muscle mass in the osteogenesis imperfecta murine model mouse (oim) will improve their compromised bone quality and strength, heterozygous (+/oim) mice were bred to mice deficient in myostatin (+/mstn), a negative regulator of muscle growth. The resulting adult offspring were evaluated for hindlimb muscle mass, and bone microarchitecture, physiochemistry, and biomechanical integrity.

Results: +/oim mice deficient in myostatin (+/mstn +/oim) were generated and demonstrated that myostatin deficiency increased body weight, muscle mass, and biomechanical strength in +/mstn +/oim mice as compared to +/oim mice. Additionally, myostatin deficiency altered the physiochemical properties of the +/oim bone but did not alter bone remodeling.

Conclusions: Myostatin deficiency partially improved the reduced femoral bone biomechanical strength of adult +/oim mice by increasing muscle mass with concomitant improvements in bone microarchitecture and physiochemical properties.

Keywords: Biomechanics; Bone; Myostatin; Osteogenesis imperfecta; Raman spectroscopy; Skeletal muscle.

Fig. 1

A Schematic of the +/mstn (dam) X +/oim (sire) mice breeding scheme with the resultant genotypes of the offspring indicated. B Representative graphs showing the torque applied on the femur of Wt, +/mstn, +/mstn +/oim, and +/oim offspring per angular displacement. B–G Myostatin deficiency partially rescues the bone phenotype of +/oim mice as seen in 4-month-old +/mstn +/oim femora. C Torsional ultimate strength (T_max), D tensile strength (Su), E energy to failure (U), F torsional stiffness (Ks), G shear modulus of elastcity. The genotype main effect was evaluated. Values are the LSmeans±SE of the combined genotype values regardless of sex. Values with the same superscript letter are not significantly different. Differences were considered significant at p≤0.05. Wt (solid black), n=21; +/mstn (black and white), n=25–26; +/mstn, +/oim (black and gray), n=18; +/oim (gray), n=18

Fig. 2

Myostatin deficiency in 4-month-old +/mstn +/oim tibiae increases the PO₄/amide I (mineral to collagen) ratio as compared to +/oim tibiae. Cortical bone A PO₄/CO₃ ratio (indication of carbonate substitution of phosphate in the crystal lattice), B PO₄/CH₂ ratio (relative amount of mineral phosphate to matrix protein), C PO₄/amide I ratio (relative amount of mineral to collagen), and D femoral hydroxyproline content (μg/g bone). The genotype main effect was evaluated. Values are the LSmeans±SE of the combined genotype values regardless of sex. Values with the same superscript letter are not significantly different. Differences were considered significant at p≤0.05. Wt (solid black), n=13–14; +/mstn (black and white), n=11–17; +/mstn, +/oim (black and gray), n=12–17; +/oim (gray), n=13–14

Fig. 3

Evaluation of serum bone turnover markers, N-terminal propeptide of type I procollagen [PINP (A)] and C-terminal cross-linked telopeptide of type I collagen [CTX (B)] demonstrated an increase in serum CTX associated with +/oim and +/mstn +/oim mice. The genotype main effect was evaluated. Values are the LSmeans±SE of the combined genotype values regardless of sex. Values with the same superscript letter are not significantly different. Differences were considered significant at p≤0.05. Wt (solid black), n=13; +/mstn (black and white), n=16–19; +/mstn, +/oim (black and gray), n=17–18; +/oim (gray), n=15–16