Osteogenesis imperfecta (OI) is a heterogeneous heritable connective tissue disorder associated with reduced bone mineral density and skeletal fragility. Bone is inherently mechanosensitive, with bone strength being proportional to muscle mass and strength. Physically active healthy children accrue more bone than inactive children. Children with type I OI exhibit decreased exercise capacity and muscle strength compared with healthy peers. It is unknown whether this muscle weakness reflects decreased physical activity or a muscle pathology. In this study, we used heterozygous G610C OI model mice (þ/G610C), which model both the genotype and phenotype of a large Amish OI kindred, to evaluate hindlimb muscle function and physical activity levels before evaluating the ability of þ/G610C mice to undergo a treadmill exercise regimen. We found þ/G610C mice hindlimb muscles do not exhibit compromised muscle function, and their activity levels were not reduced relative to wild-type mice. The þ/G610C mice were also able to complete an 8-week treadmill regimen. Biomechanical integrity of control and exercised wild-type and þ/G610C femora were analyzed by torsional loading to failure. The greatest skeletal gains in response to exercise were observed in stiffness and the shear modulus of elasticity with alterations in collagen content. Analysis of tibial cortical bone by Raman spectroscopy demonstrated similar crystallinity and mineral/matrix ratios regardless of sex, exercise, and genotype. Together, these findings demonstrate þ/G610C OI mice have equivalent muscle function, activity levels, and ability to complete a weight-bearing exercise regimen as wildtype mice. The þ/G610C mice exhibited increased femoral stiffness and decreased hydroxyproline with exercise, whereas other biomechanical parameters remain unaffected, suggesting a more rigorous exercise regimen or another exercise modality may be required to improve bone quality of OI mice.

During fetal development, the uterine environment can have effects on offspring bone architecture and integrity that persist into adulthood; however, the biochemical and molecular mechanisms remain unknown. Myostatin is a negative regulator of muscle mass. Parental myostatin deficiency (Mstn tm1Sjl/+ ) increases muscle mass in wild-type offspring, suggesting an intrauterine programming effect. Here, we hypothesized that Mstn tm1Sjl/+ dams would also confer increased bone strength. In wild-type offspring, maternal myostatin deficiency altered fetal growth and calvarial collagen content of newborn mice and conferred a lasting impact on bone geometry and biomechanical integrity of offspring at 4 mo of age, the age of peak bone mass. Second, we sought to apply maternal myostatin deficiency to a mouse model with osteogenesis imperfecta (Col1a2 oim ), a heritable connective tissue disorder caused by abnormalities in the structure and/or synthesis of type I collagen. Femora of male Col1a2 oim/+ offspring from natural mating of Mstn tm1Sjl/+ dams to Col1a2 oim/+ sires had a 15% increase in torsional ultimate strength, a 29% increase in tensile strength, and a 24% increase in energy to failure compared with age, sex, and genotype-matched offspring from natural mating of Col1a2 oim/+ dams to Col1a2 oim/+ sires. Finally, increased bone biomechanical strength of Col1a2 oim/+ offspring that had been transferred into Mstn tm1Sjl/+ dams as blastocysts demonstrated that the effects of maternal myostatin deficiency were conferred by the postimplantation environment. Thus, targeting the gestational environment, and specifically prenatal myostatin pathways, provides a potential therapeutic window and an approach for treating osteogenesis imperfecta.osteogenesis imperfecta | developmental origins of health and disease | fetal programming | myostatin | bone health O steogenesis imperfecta (OI) is a genetically and clinically heterogeneous heritable connective tissue disorder characterized by anomalies in type I collagen-containing tissues, particularly bone. Clinical manifestations include osteopenia/osteoporosis, fractures, skeletal deformities, short stature, and skeletal muscle weakness. Current therapies entail use of antiresorptive drugs and surgical interventions but have limited success (1). New approaches that decrease fracture risk while minimizing long-term damage to bone integrity are greatly needed. Myostatin, or growth and differentiation factor 8 (GDF8), is a TGF-β family member and negative regulator of muscle growth and development. It is highly conserved across species, primarily expressed in muscle cells, and participates in paracrine and endocrine signaling (2). Recently, we and others have shown that inhibiting myostatin, either genetically or pharmacologically, increases muscle mass and improves bone microarchitecture (3) and mechanical strength (4) in the OI murine (Col1a2 oim ) model. In addition to regulating muscle growth postnatally, myostatin deficiency in the maternal environment increases muscle mass in ...