The musculoskeletal effect of exercise and soluble activin receptor type 2b in mouse models of osteogenesis imperfecta
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Osteogenesis Imperfecta (OI) is a heritable connective tissue disorder primarily due to mutations in the type I collagen containing tissues, such as bone, skin and blood vessels. The clinical manifestations of OI include skeletal deformity and fragility, scoliosis, growth retardation, and hearing loss. There is no cure for OI and current treatment is limited with anti-resorptive drugs, the bisphosphonates, and/or surgical intervention, which comes with adverse side-effects and high risk of device failure, respectively. Thus identifying an alternative strategy to strengthen the skeletal properties of OI is still critically needed. Bone and muscle are intimate tissues in a relation to their proximate locations and biochemical cross-talks. Bone responds and adapts to external stimuli, mainly the muscle mass and contractile strength, to alter its shape and mass due to its mechanosensing characteristic, and this makes muscle and bone mass to be positively correlated in normal condition. As a potential therapeutic option, we sought to enhance the muscle mass and function via physical exercise to indirectly improve the bone properties in OI. We have investigated the effects of the threadmill exercise in G610C OI mouse model, which has a glycine to cysteine substitution at position 610 of proα2 collagen chain and mimics the phenotype of mild type I/IV OI individuals. Treadmill exercised heterozygous G610C (+/G610C) mice exhibited similar exercise capacity as wildtype littermates and had increased femoral stiffness without altering bone biomechanical strength. Muscle mass can be regulated by myostatin, a negative regulator of muscle growth, and deficiency of myostatin in mice lead to abnormal muscle fiber growth. As an alternative approach, we have investigated the effects of pharmacological myostatin inhibition by using a soluble fusion protein activin receptor type IIB-mFc (sActRIIB-mFc). Myostatin signals through activin receptor type IIB (ActRIIB) on cell surface to regulate downstream signaling pathways and the sActRIIB-mFc act as “ligand trap” to bind any circulating myostatin proteins and prevent them from binding to their endogenous cellular receptors. As first part of this study, we investigated the effects of sActRIIB-mFc on muscle properties of two molecularly distinct OI mouse models, G610C and oim. Unlike G610C mouse model, homozygous oim (oim/oim) has a mutation in col1α2 genes thus synthesizing nonfunctional proα2(I) collagen chain and leading to synthesis of homotrimeric α1(I)3, instead of normal heterotrimeric α1(I)2α2(I). oim/oim also exhibit muscle atrophy with compromised muscle contractile strength. 8 weeks of bi-weekly sActRIIB-mFc (10mg/kg) treatment in +/G610C and oim/oim mice was able to induce the increase in body weight and skeletal muscle mass. In addition, oim/oim mice exhibited increase in absolute contractile strength without altering relative and specific muscle function, suggesting a potential therapeutic option for muscle weakness in oim/oim mice. As second part of this study, we investigated the effects of sActRIIB-mFc on skeletal properties of these two OI mouse models. sActRIIB-mFc treated +/G610C and oim/oim mice exhibited increase in trabecular bone microarchitecture, and +/G610C mice had further increase in cortical bone geometry and biomechanical strength. Overall, my current study demonstrated that sActRIIB-mFc treatment was effective in both G610C and oim mouse models to enhance their muscle and bone properties, although they exhibited different responses in such that G610C mice did not show a statistically significant increase in muscle contractile function while the oim mice did not show increase in cortical bone geometry and biomechanical strength. I postulate that this was potentially due to the differences in molecular mutation and severity of the phenotype, thus more thorough investigation in molecular and cellular mechanisms of sActRIIB-mFc in these two different OI mouse models will hold promise in developing more targeted therapeutic option for OI.