Skeletal muscle weakness in osteogenesis imperfecta mice

Extra-Skeletal Manifestations in Osteogenesis Imperfecta Mouse Models

Osteogenesis imperfecta (OI) is a rare heritable connective tissue disorder of skeletal fragility with an incidence of roughly 1:15,000. Approximately 85% of the pathogenic variants responsible for OI are in the type I collagen genes, COL1A1 and COL1A2, with the remaining pathogenic OI variants spanning at least 20 additional genetic loci that often involve type I collagen post-translational modification, folding, and intracellular transport as well as matrix incorporation and mineralization. In addition to being the most abundant collagen in the body, type I collagen is an important structural and extracellular matrix signaling molecule in multiple organ systems and tissues. Thus, OI disease-causing variants result not only in skeletal fragility, decreased bone mineral density (BMD), kyphoscoliosis, and short stature, but can also result in hearing loss, dentinogenesis imperfecta, blue gray sclera, cardiopulmonary abnormalities, and muscle weakness. The extensive genetic and clinical heterogeneity in OI has necessitated the generation of multiple mouse models, the growing awareness of non-skeletal organ and tissue involvement, and OI being more broadly recognized as a type I collagenopathy.This has driven the investigation of mutation-specific skeletal and extra-skeletal manifestations and broadened the search of potential mechanistic therapeutic strategies. The purpose of this review is to outline several of the extra-skeletal manifestations that have recently been characterized through the use of genetically and phenotypically heterogeneous mouse models of osteogenesis imperfecta, demonstrating the significant potential impact of OI disease-causing variants as a collagenopathy (affecting multiple organ systems and tissues), and its implications to overall health.

Neurocranial growth in the OIM mouse model of osteogenesis imperfecta

Osteogenesis imperfecta (OI) is a disorder of type I collagen characterized by abnormal bone formation. The OI craniofacial phenotype includes midfacial underdevelopment, as well as neurocranial changes (e.g., macrocephaly and platybasia) that may also affect underlying nervous tissues. This study aims to better understand how OI affects the integrated development of the neurocranium and the brain. Juvenile and adult mice with OI (OIM) and unaffected wild type (WT) littermates were imaged using in vivo micro-computed tomography (microCT). Virtual endocast models were used to measure brain volume, and 3D landmarks were collected from the cranium and brain endocasts. Geometric morphometric analyses were used to compare brain shape and integration between the genotypes. OIM mice had increased brain volumes (relative to cranial centroid size) only at the juvenile stage. No significant difference was seen in cranial base angle (CBA) between OIM and WT mice. However, CBA was higher in juvenile than in adult OIM mice. Brain shape was significantly different between OIM and WT mice at both stages, with OIM mice having more globular brains than WT mice. Neurocranial and brain morphology were strongly integrated within both genotypes, while adult OIM mice tended to have lower levels of skull-brain integration than WT mice. These results suggest that neurocranial dysmorphologies in OI may be more severe at earlier stages of postnatal development. Decreased skull-brain integration in adult mice suggests that compensatory mechanisms may exist during postnatal growth to maintain neurological function despite significant changes in neurocranial morphology.

Morphological variability in the inner ear of mice with osteogenesis imperfecta

Osteogenesis imperfecta (OI) is known to cause hearing loss in ~60% of the affected human population. While OI-related pathologies have been studied in the middle ear, the development of cochlear pathologies is less well understood. In this study, we examine OI-related pathologies of the cochlea in a mouse model of OI to (1) document variation between OI and unaffected mice, and (2) assess the intrusion of the otic capsule onto the cochlea by analyzing differences in duct volumes. Juvenile and adult OIM C57BL/6mice were compared to unaffected wildtype (WT) mice using three-dimensional models of the cochlea generated from high resolution micro-CT scans. Two-tailed Mann-Whitney U tests were then used to investigate duct volume differences both within and between the OI and WT samples. Areas of higher ossification were observed at the cochlear base in the OI sample. OI mice also had significant intraindividual differences in duct volume between right and left ears (4%-15%), an effect not observed in WT mice. WT and OI duct volumes showed a large degree of overlap, although the OIM volumes were more variable. Our findings indicate that OIM mice are likely to exhibit more asymmetry and variation in cochlear volume despite minor differences in sample cochlear volumes, possibly due to bony capsule intrusion. This suggests a potential mechanism of hearing loss, and a high potential for cochlear and otic capsule alteration in OIM mice.

Compromised Muscle Properties in a Severe Hypophosphatasia Murine Model

Hypophosphatasia (HPP) is a rare metabolic bone disorder characterized by low levels of tissue non-specific alkaline phosphatase (TNAP) that causes under-mineralization of the bone, leading to bone deformity and fractures. In addition, patients often present with chronic muscle pain, reduced muscle strength, and an altered gait. In this work, we explored dynamic muscle function in a homozygous TNAP knockout mouse model of severe juvenile onset HPP. We found a reduction in skeletal muscle size and impairment in a range of isolated muscle contractile properties. Using histological methods, we found that the structure of HPP muscles was similar to healthy muscles in fiber size, actin and myosin structures, as well as the α-tubulin and mitochondria networks. However, HPP mice had significantly fewer embryonic and type I fibers than wild type mice, and fewer metabolically active NADH+ muscle fibers. We then used oxygen respirometry to evaluate mitochondrial function and found that complex I and complex II leak respiration were reduced in HPP mice, but that there was no disruption in efficiency of electron transport in complex I or complex II. In summary, the severe HPP mouse model recapitulates the muscle strength impairment phenotypes observed in human patients. Further exploration of the role of alkaline phosphatase in skeletal muscle could provide insight into mechanisms of muscle weakness in HPP.

Whole-Body Metabolism and the Musculoskeletal Impacts of Targeting Activin A and Myostatin in Severe Osteogenesis Imperfecta

Mutations in the COL1A1 and COL1A2 genes, which encode type I collagen, are present in around 85%-90% of osteogenesis imperfecta (OI) patients. Because type I collagen is the principal protein composition of bones, any changes in its gene sequences or synthesis can severely affect bone structure. As a result, skeletal deformity and bone frailty are defining characteristics of OI. Homozygous oim/oim mice are utilized as models of severe progressive type III OI. Bone adapts to external forces by altering its mass and architecture. Previous attempts to leverage the relationship between muscle and bone involved using a soluble activin receptor type IIB-mFc (sActRIIB-mFc) fusion protein to lower circulating concentrations of activin A and myostatin. These two proteins are part of the TGF-β superfamily that regulate muscle and bone function. While this approach resulted in increased muscle masses and enhanced bone properties, adverse effects emerged due to ligand promiscuity, limiting clinical efficacy and obscuring the precise contributions of myostatin and activin A. In this study, we investigated the musculoskeletal and whole-body metabolism effect of treating 5-week-old wildtype (Wt) and oim/oim mice for 11 weeks with either control antibody (Ctrl-Ab) or monoclonal anti-activin A antibody (ActA-Ab), anti-myostatin antibody (Mstn-Ab), or a combination of ActA-Ab and Mstn-Ab (Combo). We demonstrated that ActA-Ab treatment minimally impacts muscle mass in oim/oim mice, whereas Mstn-Ab and Combo treatments substantially increased muscle mass and overall lean mass regardless of genotype and sex. Further, while no improvements in cortical bone microarchitecture were observed with all treatments, minimal improvements in trabecular bone microarchitecture were observed with the Combo treatment in oim/oim mice. Our findings suggest that individual or combinatorial inhibition of myostatin and activin A alone is insufficient to robustly improve femoral biomechanical and microarchitectural properties in severely affected OI mice. © 2023 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Zoledronic acid improves bone quality and muscle function in a high bone turnover state

SUMMARY

Zoledronic acid (ZA) prevents muscle weakness in mice with bone metastases; however, its role in muscle weakness in non-tumor-associated metabolic bone diseases and as an effective treatment modality for the prevention of muscle weakness associated with bone disorders, is unknown. We demonstrate the role of ZA-treatment on bone and muscle using a mouse model of accelerated bone remodeling, which represents the clinical manifestation of non-tumor associated metabolic bone disease. ZA increased bone mass and strength and rescued osteocyte lacunocanalicular organization. Short-term ZA treatment increased muscle mass, whereas prolonged, preventive treatment improved muscle mass and function. In these mice, muscle fiber-type shifted from oxidative to glycolytic and ZA restored normal muscle fiber distribution. By blocking TGFβ release from bone, ZA improved muscle function, promoted myoblast differentiation and stabilized Ryanodine Receptor-1 calcium channel. These data demonstrate the beneficial effects of ZA in maintaining bone health and preserving muscle mass and function in a model of metabolic bone disease.

Context and significance

TGFβ is a bone regulatory molecule which is stored in bone matrix, released during bone remodeling, and must be maintained at an optimal level for the good health of the bone. Excess TGFβ causes several bone disorders and skeletal muscle weakness. Reducing excess TGFβ release from bone using zoledronic acid in mice not only improved bone volume and strength but also increased muscle mass, and muscle function. Progressive muscle weakness coexists with bone disorders, decreasing quality of life and increasing morbidity and mortality. Currently, there is a critical need for treatments improving muscle mass and function in patients with debilitating weakness. Zoledronic acid's benefit extends beyond bone and could also be useful in treating muscle weakness associated with bone disorders.

Murine Animal Models in Osteogenesis Imperfecta: The Quest for Improving the Quality of Life

Osteogenesis imperfecta is a rare genetic disorder characterized by bone fragility, due to alterations in the type I collagen molecule. It is a very heterogeneous disease, both genetically and phenotypically, with a high variability of clinical phenotypes, ranging from mild to severe forms, the most extreme cases being perinatal lethal. There is no curative treatment for OI, and so great efforts are being made in order to develop effective therapies. In these attempts, the in vivo preclinical studies are of paramount importance; therefore, serious analysis is required to choose the right murine OI model able to emulate as closely as possible the disease of the target OI population. In this review, we summarize the features of OI murine models that have been used for preclinical studies until today, together with recently developed new murine models. The bone parameters that are usually evaluated in order to determine the relevance of new developing therapies are exposed, and finally, current and innovative therapeutic strategies attempts considered in murine OI models, along with their mechanism of action, are reviewed. This review aims to summarize the in vivo studies developed in murine models available in the field of OI to date, in order to help the scientific community choose the most accurate OI murine model when developing new therapeutic strategies capable of improving the quality of life.

Skeletal muscle mitochondrial function and whole-body metabolic energetics in the +/G610C mouse model of osteogenesis imperfecta

Osteogenesis imperfecta (OI) is rare heritable connective tissue disorder that most often arises from mutations in the type I collagen genes, COL1A1 and COL1A2, displaying a range of symptoms including skeletal fragility, short stature, blue-gray sclera, and muscle weakness. Recent investigations into the intrinsic muscle weakness have demonstrated reduced contractile generating force in some murine models consistent with patient population studies, as well as alterations in whole body bioenergetics. Muscle weakness is found in approximately 80% of patients and has been equivocal in OI mouse models. Understanding the mechanism responsible for OI muscle weakness is crucial in building our knowledge of muscle bone cross-talk via mechanotransduction and biochemical signaling, and for potential novel therapeutic approaches. In this study we evaluated skeletal muscle mitochondrial function and whole-body bioenergetics in the heterozygous +/G610C (Amish) mouse modeling mild/moderate human type I/VI OI and minimal skeletal muscle weakness. Our analyses revealed several changes in the +/G610C mouse relative to their wildtype littermates including reduced state 3 mitochondrial respiration, increased mitochondrial citrate synthase activity, increased Parkin and p62 protein content, and an increased respiratory quotient. These changes may represent the ability of the +/G610C mouse to compensate for mitochondrial and metabolic changes that may arise due to type I collagen mutations and may also account for the lack of muscle weakness observed in the +/G610C model relative to the more severe OI models.

Is COL1A1 Gene rs1107946 Polymorphism Associated with Sport Climbing Status and Flexibility?

The purpose of this study was to compare the frequency of COL1A1 rs1107946 polymorphism between sport climbers and controls from three ethnic groups (Japanese, Polish, and Russian) and investigate the effect of the COL1A1 rs1107946 polymorphism on the age-related decrease in flexibility in the general population. Study I consisted of 1929 healthy people (controls) and 218 climbers, including Japanese, Polish, and Russian participants. The results of the meta-analysis showed that the frequency of the AC genotype was higher in climbers than in the controls (p = 0.03). Study II involved 1093 healthy Japanese individuals (435 men and 658 women). Flexibility was assessed using a sit-and-reach test. There was a tendency towards association between sit-and-reach and the COL1A1 rs1107946 polymorphism (genotype: p = 0.034; dominant: p = 0.435; recessive: p = 0.035; over-dominant: p = 0.026). In addition, there was a higher negative correlation between sit-and-reach and age in the AA + CC genotype than in the AC genotype (AA + CC: r = −0.216, p < 0.001; AC: r = −0.089, p = 0.04; interaction p = 0.037). However, none of these results survived correction for multiple testing. Further studies are warranted to investigate the association between the COL1A1 gene variation and exercise-related phenotypes.

Osteogenesis Imperfecta: The Impact of Genotype and Clinical Phenotype on Adiposity and Resting Energy Expenditure

CONTEXT

Mutations in type I collagen or collagen-related proteins cause osteogenesis imperfecta (OI). Energy expenditure and body composition in OI could reflect reduced mobility or intrinsic defects in osteoblast differentiation increasing adipocyte development.

OBJECTIVE

This study compares adiposity and resting energy expenditure (REE) in OI and healthy controls (HC), for OI genotype- and Type-associated differences.

METHODS

We studied 90 participants, 30 with OI (11 COL1A1 Gly, 8 COL1A2 Gly, 4 COL1A1 non-Gly, 1 COL1A2 non-Gly, 6 non-COL; 8 Type III, 16 Type IV, 4 Type VI, 1 Type VII, 1 Type XIV) and 60 HC with sociodemographic characteristics/BMI/BMIz similar to the OI group. Participants underwent dual-energy x-ray absorptiometry to determine lean mass and fat mass percentage (FM%) and REE. FM% and REE were compared, adjusting for covariates, to examine the relationship of OI genotypes and phenotypic Types.

RESULTS

FM% did not differ significantly in all patients with OI vs HC (OI: 36.6% ± 1.9%; HC: 32.7% ± 1.2%; P = 0.088). FM% was, however, greater than HC for those with non-COL variants (P = 0.016). FM% did not differ from HC among OI Types (P values > 0.05).Overall, covariate-adjusted REE did not differ significantly between OI and HC (OI: 1376.5 ± 44.7 kcal/d; HC: 1377.0 ± 96 kcal/d; P = 0.345). However, those with non-COL variants (P = 0.016) and Type VI OI (P = 0.04) had significantly lower REE than HC.

CONCLUSION

Overall, patients with OI did not significantly differ in either extra-marrow adiposity or REE from BMI-similar HC. However, reduced REE among those with non-COL variants may contribute to greater adiposity.

Female Athletes Genetically Susceptible to Fatigue Fracture Are Resistant to Muscle Injury: Potential Role of COL1A1 Variant

PURPOSE

We aimed to investigate the hypothesis that type I collagen plays a role in increasing bone mineral density (BMD) and muscle stiffness, leading to low and high risks of fatigue fracture and muscle injury, respectively, in athletes. As a potential mechanism, we focused on the effect of the type I collagen alpha 1 chain gene (COL1A1) variant associated with transcriptional activity on bone and skeletal muscle properties.

METHODS

The association between COL1A1 rs1107946 and fatigue fracture/muscle injury was evaluated in Japanese athletes. Effects of the polymorphism on tissue properties (BMD and muscle stiffness) and type I collagen α1/α2 chain ratios in muscles were examined in Japanese nonathletes.

RESULTS

The C-allele carrier frequency was greater in female athletes with fatigue fracture than in those without (odds ratio = 2.44, 95% confidence interval [CI] = 1.17-5.77) and lower in female athletes with muscle injury than in those without (odds ratio = 0.46, 95% CI = 0.24-0.91). Prospective validation analysis confirmed that in female athletes, muscle injury was less frequent in C-allele carriers than in AA genotype carriers (multivariable-adjusted hazard ratio = 0.27, 95% CI = 0.08-0.96). Among female nonathletes, the C-allele of rs1107946 was associated with lower BMD and lower muscle stiffness. Muscle biopsy revealed that C-allele carriers tended to have a larger type I collagen α1/α2 chain ratio than AA genotype carriers (2.24 vs 2.05, P = 0.056), suggesting a higher proportion of type I collagen α1 homotrimers.

CONCLUSION

The COL1A1 rs1107946 polymorphism exerts antagonistic effects on fatigue fracture and muscle injury among female athletes by altering the properties of these tissues, potentially owing to increased levels of type I collagen α1 chain homotrimers.

Gender-Related Impact of Sclerostin Antibody on Bone in the Osteogenesis Imperfecta Mouse

Osteogenesis imperfecta (OI), which is most often due to a collagen type 1 gene mutation, is characterized by low bone density and bone fragility. In OI patients, gender-related differences were reported, but data in the literature are not convergent. We previously observed that sclerostin antibody (Scl-Ab), which stimulates osteoblast Wnt pathway via sclerostin inactivation, improved spine and long-bone parameters and biomechanical strength in female oim/oim mice, a validated model of human type 3 OI. Here, we wanted to highlight the effect of Scl-Ab on male oim/oim bones in order to identify a possible distinct therapeutic effect from that observed in females. According to the same protocol as our previous study with female mice, male wild-type (Wt) and oim/oim mice received vehicle or Scl-Ab from 5 to 14 weeks of age. Clinimetric and quantitative bone parameters were studied using X-rays, peripheral quantitative computed tomography, microradiography, and dynamic histomorphometry and compared to those of females. Contrary to Wt mice, male oim/oim had significantly lower weight, snout-sacrum length, and bone mineral content than females at 5 weeks. No significant difference in these clinimetric parameters was observed at 14 weeks, whereas male oim showed significantly more long-bone fractures than females. Scl-Ab improved bone mineral density and bone volume/total volume ratio (BV/TV) of vertebral body in Wt and oim/oim, without significant difference between male and female at 14 weeks. Male vehicle oim/oim had a significantly lower cortical thickness (Ct.Th) and BV/TV of tibial diaphysis than female and showed a higher number of fractures at 14 weeks. Scl-Ab increased midshaft periosteal apposition rate in such a way that tibial Ct.Th of male oim/oim was not significantly different from the female one at 14 weeks. The number of fractures was lower in male than female oim/oim after 14 weeks of Scl-Ab treatment, but this difference was not significant. Nevertheless, Scl-Ab-treated oim/oim male and female mice remained smaller than the Wt ones. In conclusion, our results highlighted differences between male and female oim/oim at 4 and 14 weeks of age, as well as some male-specific response of cortical bone to Scl-Ab. These gender-related particularities of oim/oim should be considered when testing experimental treatments.

Muscle transcriptome in mouse models of osteogenesis imperfecta

Osteogenesis imperfecta (OI) is a heritable connective tissue disorder that is most often caused by mutations in collagen type I encoding genes. Even though bone fragility is the most conspicuous finding in OI, the muscle system is also affected. In the present study we explored the muscle phenotype related to collagen type I mutations on the transcriptome level. RNA sequencing was performed in gastrocnemius muscles of homozygous oim mice and of heterozygous Jrt mice, two models of severe OI. We found that oim and Jrt mice shared 27 differentially expressed genes, of which 11 were concordantly upregulated and 15 concordantly downregulated. Gene Set Enrichment Analysis revealed that in both oim and Jrt mice, genes involved in 'metabolism of lipids' were significantly enriched among upregulated genes. In addition, several genes coding for extracellular matrix components were upregulated in both oim and Jrt mice. Among downregulated genes, genes involved in 'muscle contraction' were enriched in both OI mouse models. These 'muscle contraction' genes coded for slow-twitch type I muscle fiber components. Another shared downregulated gene was Mss51, a metabolic stress-inducible factor that is found in mitochondria. These data show that two mouse models of severe OI share abnormalities in the expression of genes that code for extracellular matrix proteins, lipid and energy metabolism and structural proteins of type I muscle fibers. The muscle disturbances resulting from the collagen type I mutations in these mouse models could be viewed as a mild form of muscle dystrophy.

Impact of Intrinsic Muscle Weakness on Muscle-Bone Crosstalk in Osteogenesis Imperfecta

Bone and muscle are highly synergistic tissues that communicate extensively via mechanotransduction and biochemical signaling. Osteogenesis imperfecta (OI) is a heritable connective tissue disorder of severe bone fragility and recently recognized skeletal muscle weakness. The presence of impaired bone and muscle in OI leads to a continuous cycle of altered muscle-bone crosstalk with weak muscles further compromising bone and vice versa. Currently, there is no cure for OI and understanding the pathogenesis of the skeletal muscle weakness in relation to the bone pathogenesis of OI in light of the critical role of muscle-bone crosstalk is essential to developing and identifying novel therapeutic targets and strategies for OI. This review will highlight how impaired skeletal muscle function contributes to the pathophysiology of OI and how this phenomenon further perpetuates bone fragility.

Role of myokines and osteokines in cancer cachexia

Cancer-induced muscle wasting, i.e. cachexia, is associated with different types of cancer such as pancreatic, colorectal, lung, liver, gastric and esophageal. Cachexia affects prognosis and survival in cancer, and it is estimated that it will be the ultimate cause of death for up to 30% of cancer patients. Musculoskeletal alterations are known hallmarks of cancer cachexia, with skeletal muscle atrophy and weakness as the most studied. Recent evidence has shed light on the presence of bone loss in cachectic patients, even in the absence of bone-metastatic disease. In particular, we and others have shown that muscle and bone communicate by exchanging paracrine and endocrine factors, known as myokines and osteokines. This review will focus on describing the role of the most studied myokines, such as myostatin, irisin, the muscle metabolite β-aminoisobutyric acid, BAIBA, and IL-6, and osteokines, including TGF-β, osteocalcin, sclerostin, RANKL, PTHrP, FGF23, and the lipid mediator, PGE₂ during cancer-induced cachexia. The interplay of muscle and bone factors, together with tumor-derived soluble factors, characterizes a complex clinical scenario in which musculoskeletal alterations are amongst the most debilitating features. Understanding and targeting the "secretome" of cachectic patients will likely represent a promising strategy to preserve bone and muscle during cancer cachexia thereby enhancing recovery.

Skeletal muscle specific mitochondrial dysfunction and altered energy metabolism in a murine model (oim/oim) of severe osteogenesis imperfecta

Osteogenesis imperfecta (OI) is a heritable connective tissue disorder with patients exhibiting bone fragility and muscle weakness. The synergistic biochemical and biomechanical relationship between bone and muscle is a critical potential therapeutic target, such that muscle weakness should not be ignored. Previous studies demonstrated mitochondrial dysfunction in the skeletal muscle of oim/oim mice, which model a severe human type III OI. Here, we further characterize this mitochondrial dysfunction and evaluate several parameters of whole body and skeletal muscle metabolism. We demonstrate reduced mitochondrial respiration in female gastrocnemius muscle, but not in liver or heart mitochondria, suggesting that mitochondrial dysfunction is not global in the oim/oim mouse. Myosin heavy chain fiber type distributions were altered in the oim/oim soleus muscle with a decrease (-33 to 50%) in type I myofibers and an increase (+31%) in type IIa myofibers relative to their wildtype (WT) littermates. Additionally, altered body composition and increased energy expenditure were observed oim/oim mice relative to WT littermates. These results suggest that skeletal muscle mitochondrial dysfunction is linked to whole body metabolic alterations and to skeletal muscle weakness in the oim/oim mouse.

Impact of Genetic and Pharmacologic Inhibition of Myostatin in a Murine Model of Osteogenesis Imperfecta

Osteogenesis imperfecta (OI) is a genetic connective tissue disorder characterized by compromised skeletal integrity, altered microarchitecture, and bone fragility. Current OI treatment strategies focus on bone antiresorptives and surgical intervention with limited effectiveness, and thus identifying alternative therapeutic options remains critical. Muscle is an important stimulus for bone formation. Myostatin, a TGF-β superfamily myokine, acts through ActRIIB to negatively regulate muscle growth. Recent studies demonstrated the potential benefit of myostatin inhibition with the soluble ActRIIB fusion protein on skeletal properties, although various OI mouse models exhibited variable skeletal responses. The genetic and clinical heterogeneity associated with OI, the lack of specificity of the ActRIIB decoy molecule for myostatin alone, and adverse events in human clinical trials further the need to clarify myostatin's therapeutic potential and role in skeletal integrity. In this study, we determined musculoskeletal outcomes of genetic myostatin deficiency and postnatal pharmacological myostatin inhibition by a monoclonal anti-myostatin antibody (Regn647) in the G610C mouse, a model of mild-moderate type I/IV human OI. In the postnatal study, 5-week-old wild-type and +/G610C male and female littermates were treated with Regn647 or a control antibody for 11 weeks or for 7 weeks followed by a 4-week treatment holiday. Inhibition of myostatin, whether genetically or pharmacologically, increased muscle mass regardless of OI genotype, although to varying degrees. Genetic myostatin deficiency increased hindlimb muscle weights by 6.9% to 34.4%, whereas pharmacological inhibition increased them by 13.5% to 29.6%. Female +/mstn +/G610C (Dbl.Het) mice tended to have similar trabecular and cortical bone parameters as Wt showing reversal of +/G610C characteristics but with minimal effect of +/mstn occurring in male mice. Pharmacologic myostatin inhibition failed to improve skeletal bone properties of male or female +/G610C mice, although skeletal microarchitectural and biomechanical improvements were observed in male wild-type mice. Four-week treatment holiday did not alter skeletal outcomes. © 2020 American Society for Bone and Mineral Research (ASBMR).

Muscle-bone properties after prolonged voluntary wheel running in a mouse model of dominant severe osteogenesis imperfecta

OBJECTIVE

The objective of the current study is to assess the effect of a seven-week voluntary wheel running intervention on muscles and bones properties in a mouse model mimicking dominant severe osteogenesis imperfecta (OI).

METHODS

Female wild-type (WT) and OI (Col1a1^Jrt/+) mice either performed voluntarily wheel-running exercise for 7-weeks or remained sedentary. Running distance and speed, forelimb grip strength, isolated muscle force and fatigability as well as bone morphology and mechanical properties were assessed.

RESULTS

We demonstrate that female WT and OI mice voluntarily performed exercise, although OI mice exercised less than WT littermates. The exercise regimen increased soleus muscle masses in WT and OI but increased relative grip strength in WT mice only. Specific muscle force and fatigability were similar between WT and OI mice and did not improve with exercise. Furthermore, the exercise regimen did not improve the femoral architectural and biomechanical properties in OI mice.

CONCLUSION

Our study suggests that voluntary wheel running is not appropriate to assess the effects of exercise in a mouse model of OI. Findings from exercising OI mice model studies may not necessarily be transferable to humans.

Osteogenesis imperfecta: an update on clinical features and therapies

Osteogenesis imperfecta (OI) is an inherited skeletal dysplasia characterized by bone fragility and skeletal deformities. While the majority of cases are associated with pathogenic variants in COL1A1 and COL1A2, the genes encoding type I collagen, up to 25% of cases are associated with other genes that function within the collagen biosynthesis pathway or are involved in osteoblast differentiation and bone mineralization. Clinically, OI is heterogeneous in features and variable in severity. In addition to the skeletal findings, it can affect multiple systems including dental and craniofacial abnormalities, muscle weakness, hearing loss, respiratory and cardiovascular complications. A multi-disciplinary approach to care is recommended to address not only the fractures, reduced mobility, growth and bone pain but also other extra-skeletal manifestations. While bisphosphonates remain the mainstay of treatment in OI, new strategies are being explored, such as sclerostin inhibitory antibodies and TGF beta inhibition, to address not only the low bone mineral density but also the inherent bone fragility. Studies in animal models have expanded the understanding of pathomechanisms of OI and, along with ongoing clinical trials, will allow to develop better therapeutic approaches for these patients.

Craniofacial allometry in the OIM-/- mouse model of osteogenesis imperfecta

Osteogenesis imperfecta (OI) is a skeletal disorder characterized by the impaired synthesis of type I collagen (Col1). This study tests the hypothesis that the craniofacial phenotype of severe OI is linked to an overall reduction in body size. 3D landmark data were collected from µCT scans of adult OIM^-/- and wild-type (WT) mice and used to calculate centroid sizes (CS) and interlandmark distances (ILDs). To remove the effect of body size, ILDs were scaled against craniomandibular lengths and CS. Mann-Whitney U tests were used to compare CS and absolute/scaled ILDs between genotypes. OIM^-/- mice are smaller than their WT littermates in body mass, craniomandibular CS, and absolute ILDs including skull, basicranial, palatal, mandibular, and toothrow lengths. When linear distances are scaled to CS, OIM^-/- mice have a relatively short midface, short nasal bones, tall mandibular corpora, and long mandibular toothrows. Results underscore the importance of size and scaling in morphometric analyses. The deleterious effect of Col1 mutations on global skeletal dimensions combined with localized morphometric changes may underlie the facial phenotype seen in human patients with severe OI. Attempts to identify these localized changes should first account for the restricted growth and small body sizes present in individuals with OI.

Muscle contraction induces osteogenic levels of cortical bone strain despite muscle weakness in a mouse model of Osteogenesis Imperfecta

Mechanical interactions between muscle and bone have long been recognized as integral to bone integrity. However, few studies have directly measured these interactions within the context of musculoskeletal disease. In this study, the osteogenesis imperfecta murine model (oim/oim) was utilized because it has both reduced bone and muscle properties, allowing direct assessment of whether weakened muscle is able to engender strain on weakened bone. To do so, a strain gauge was attached to the tibia of healthy and oim/oim mice, muscles within the posterior quadrant of the lower hind limb were stimulated, and bone strain during muscle contraction was measured. Results indicated that the relationship between maximum muscle torque and maximum engendered strain is altered in oim/oim bone, with less torque required to engender strain compare to wild-type and heterozygous mice. Maximum muscle torque at 150 Hz stimulation frequency was able to engender ~1500 μɛ in oim/oim animals. However, even though the strain engendered in the oim/oim mice was high relative to historical bone formation thresholds, the maximum strain values were still significantly lower than that of the wild-type mice. These results are promising in that they suggest that muscle stimulation may be a viable means of inducing bone formation in oim/oim and potentially other disease models where muscle weakness/atrophy exist.

Compromised Exercise Capacity and Mitochondrial Dysfunction in the Osteogenesis Imperfecta Murine (oim) Mouse Model

Osteogenesis imperfecta (OI) is a heritable connective tissue disorder that most often arises from type I collagen-COL1A1 and COL1A2-gene defects leading to skeletal fragility, short stature, blue-gray sclera, and muscle weakness. Relative to the skeletal fragility, muscle weakness is much less understood. Recent investigations into OI muscle weakness in both patients and mouse models have revealed the presence of an inherent muscle pathology. Understanding the mechanisms responsible for OI muscle weakness is critical, particularly in light of the extensive cross-talk between muscle and bone via mechanotransduction and biochemical signaling. In the following study we initially subjected WT and oim/oim mice, modeling severe human OI type III, to either weight-bearing (voluntary wheel-running) or non-weight-bearing (swimming) exercise regimens as a modality to improve muscle strength and ultimately bone strength. The oim/oim mice ran only 35% to 42% of the distance run by age- and sex-matched WT mice and exhibited little improvement with either exercise regimen. Upon further investigation, we determined that oim/oim gastrocnemius muscle exhibited severe mitochondrial dysfunction as characterized by a 52% to 65% decrease in mitochondrial respiration rates, alterations in markers of mitochondrial biogenesis, mitophagy, and the electron transport chain components, as well as decreased mitochondrial citrate synthase activity, relative to age- and sex-matched WT gastrocnemius muscle. Thus, mitochondrial dysfunction in the oim/oim mouse likely contributes to compromised muscle function and reduced physical activity levels. © 2019 American Society for Bone and Mineral Research.

Scoliosis and Cardiopulmonary Outcomes in Osteogenesis Imperfecta Patients

STUDY DESIGN

Retrospective clinical study of individuals with osteogenesis imperfecta (OI).

OBJECTIVE

To assess the relationship between severity of scoliosis and pulmonary function, and to assess the relationship between restrictive lung disease and self-reported quality of life in individuals with OI.

SUMMARY OF BACKGROUND DATA

OI is a heritable connective tissue disorder characterized by osteopenia and a predisposition to fracture. Respiratory insufficiency is a leading cause of mortality. Literature on pulmonary function in this population has shown a negative correlation between percent-predicted vital capacity and severity of scoliosis. However, it has been suggested that decreased pulmonary function in OI may be due to intrinsic pulmonary disease, in addition to the impact of vertebral compression fractures and scoliosis.

METHODS

Anterior-posterior spine radiographs and pulmonary function tests from 30 individuals with OI were reviewed. Radiographs were evaluated for scoliosis, defined as a curve ≥ 10°. If more than one curve was present, the largest curve was used. Pulmonary function was defined as the forced expiratory volume in 1 second (FEV1)/forced vital capacity (FVC) ratio. Restrictive pulmonary disease was defined as FEV1/FVC > 80%, while obstructive disease was defined as FEV1/FVC < 70%. Bivariate correlation analysis was performed, using Spearman rho correlation coefficient (P < 0.05). Quality of life was assessed by SF-36.

RESULTS

The mean age was 27.6 years (range: 12-42 yrs). 57.6% were female. OI type IV was the most common (46.7%), followed by OI type III (33.3%), OI type I (10%), OI type IX (6.67% each), and OI type VIII (3.33%). Pulmonary comorbidity was present in 40% of individuals, while 6.67% had a cardiac comorbidity. The correlation between scoliosis and pulmonary function was weak and not significant (R = -0.059, P = 0.747).

CONCLUSION

Pulmonary function is not significantly correlated with scoliosis, supporting the hypothesis that decreased pulmonary function is intrinsic to OI and/or chest wall deformities, rather than secondary to scoliosis.

LEVEL OF EVIDENCE

4.

Sclerostin antibody reduces long bone fractures in the oim/oim model of osteogenesis imperfecta

Osteogenesis imperfecta type III (OI) is a serious genetic condition with poor bone quality and a high fracture rate in children. In a previous study, it was shown that a monoclonal antibody neutralizing sclerostin (Scl-Ab) increases strength and vertebral bone mass while reducing the number of axial fractures in oim/oim, a mouse model of OI type III. Here, we analyze the impact of Scl-Ab on long bones in OI mice. After 9 weeks of treatment, Scl-Ab significantly reduced long bone fractures (3.6 ± 0.3 versus 2.1 ± 0.8 per mouse, p < 0.001). In addition, the cortical thickness of the tibial midshaft was increased (+42%, p < 0.001), as well as BMD (+28%, p < 0.001), ultimate load (+86%, p < 0.05), plastic energy (+184%; p < 0.05) and stiffness (+172%; p < 0.01) in OI Scl-Ab mice compared to OI vehicle controls. Similar effects of Scl-Ab were observed in Wild type (Wt) mice. The plastic energy, which reflects the fragility of the tissue, was lower in the OI than in the Wt and significantly improved with the Scl-Ab treatment. At the tissue level by nanoindentation, Scl-Ab slightly increased the elastic modulus in bones of both OI and Wt, while moderately increasing tissue hardness (+13% compared to the vehicle; p < 0.05) in Wt bones, but not in OI bones. Although it did not change the properties of the OI bone matrix material, Scl-Ab reduced the fracture rate of the long bones by improving its bone mass, density, geometry, and biomechanical strength. These results suggest that Scl-Ab can reduce long-bone fractures in patients with OI.

Genetics of Bone and Muscle Interactions in Humans

PURPOSE OF REVIEW

To summarize the evidence from recent studies on the shared genetics between bone and muscle in humans.

RECENT FINDINGS

Genome-wide association studies (GWAS) have successfully identified a multitude of loci influencing the variability of different bone or muscle parameters, with multiple loci overlapping between the traits. In addition, joint analyses of multiple correlated musculoskeletal traits (i.e., multivariate GWAS) have underscored several genes with possible pleiotropic effects on both bone and muscle including MEF2C and SREBF1. Notably, several of the proposed pleiotropic genes have been validated using human cells or animal models. It is clear that the study of pleiotropy may provide novel insights into disease pathophysiology potentially leading to the identification of new treatment strategies that simultaneously prevent or treat both osteoporosis and sarcopenia. However, the role of muscle factors (myokines) that stimulate bone metabolism, as well as osteokines that affect muscles, is in its earliest stage of understanding.

Skeletal Response to Soluble Activin Receptor Type IIB in Mouse Models of Osteogenesis Imperfecta

Osteogenesis imperfecta (OI) is a heritable connective tissue disorder primarily due to mutations in the type I collagen genes (COL1A1 and COL1A2), leading to compromised biomechanical integrity in type I collagen-containing tissues such as bone. Bone is inherently mechanosensitive and thus responds and adapts to external stimuli, such as muscle mass and contractile strength, to alter its mass and shape. Myostatin, a member of the TGF-β superfamily, signals through activin receptor type IIB to negatively regulate muscle fiber growth. Because of the positive impact of myostatin deficiency on bone mass, we utilized a soluble activin receptor type IIB-mFc (sActRIIB-mFc) fusion protein in two molecularly distinct OI mouse models (G610C and oim) and evaluated their bone properties. Wild-type (WT), +/G610C, and oim/oim mice were treated from 2 to 4 months of age with either vehicle (Tris-buffered saline) or sActRIIB-mFc (10 mg/kg). Femurs of sActRIIB-mFc-treated mice exhibited increased trabecular bone volume regardless of genotype, whereas the cortical bone microarchitecture and biomechanical strength were only improved in WT and +/G610C mice. Dynamic histomorphometric analyses suggest the improved cortical bone geometry and biomechanical integrity reflect an anabolic effect due to increased mineral apposition and bone formation rates, whereas static histomorphometric analyses supported sActRIIB-mFc treatment also having an anti-catabolic impact with decreased osteoclast number per bone surface on trabecular bone regardless of sex and genotype. Together, our data suggest that sActRIIB-mFc may provide a new therapeutic direction to improve both bone and muscle properties in OI. © 2018 American Society for Bone and Mineral Research.

Osteogenesis Imperfecta: Muscle-Bone Interactions when Bi-directionally Compromised

PURPOSE OF REVIEW

Osteogenesis imperfecta (OI) is a hereditary connective tissue disorder of skeletal fragility and more recently muscle weakness. This review highlights our current knowledge of the impact of compromised OI muscle function on muscle-bone interactions and skeletal strength in OI.

RECENT FINDINGS

The ramifications of inherent muscle weakness in OI muscle-bone interactions are just beginning to be elucidated. Studies in patients and in OI mouse models implicate altered mechanosensing, energy metabolism, mitochondrial dysfunction, and paracrine/endocrine crosstalk in the pathogenesis of OI. Compromised muscle-bone unit impacts mechanosensing and the ability of OI muscle and bone to respond to physiotherapeutic and pharmacologic treatment strategies. Muscle and bone are both compromised in OI, making it essential to understand the mechanisms responsible for both impaired muscle and bone functions and their interdependence, as this will expand and drive new physiotherapeutic and pharmacological approaches to treat OI and other musculoskeletal disorders.

Soluble activin receptor type IIB decoy receptor differentially impacts murine osteogenesis imperfecta muscle function

INTRODUCTION

Osteogenesis imperfecta (OI) is characterized by skeletal fragility and muscle weakness. In this study we investigated the effects of soluble activin type IIB receptor (sActRIIB-mFc) on muscle mass and function in 2 distinct mouse models of OI: osteogenesis imperfecta murine (oim) and +/G610C.

METHODS

Wild-type (WT), +/G610C, and oim/oim mice were treated from 2 to 4 months of age with Tris-buffered saline (vehicle) or sActRIIB-mFc and their hindlimb muscles evaluated for mass, morphology, and contractile function.

RESULTS

sActRIIB-mFc-treated WT, +/G610C, and oim/oim mice had increased hindlimb muscle weights and myofiber cross-sectional area compared with vehicle-treated counterparts. sActRIIB-mFc-treated oim/oim mice also exhibited increased contractile function relative to vehicle-treated counterparts.

DISCUSSION

Blocking endogenous ActRIIB was effective at increasing muscle size in mouse models of OI, and increasing contractile function in oim/oim mice. ActRIIB inhibitors may provide a potential mutation-specific therapeutic option for compromised muscle function in OI. Muscle Nerve 57: 294-304, 2018.

Fatigue and disturbances of sleep in patients with osteogenesis imperfecta - a cross-sectional questionnaire study

Background

Persisting fatigue has been reported to be a common complaint by individuals with connective tissue disorders, including Osteogenesis imperfecta (OI). This controlled study evaluated in an adult OI population the subjective experience of fatigue, affecting daily life. Sleep disturbances and chronic pain were examined as hypothesized underlying factors.

Methods

This cross-sectional study analyzed the answers of 56 OI patients and 56 matched healthy controls to a questionnaire, designed to evaluate levels of experienced fatigue and bodily pain, as well as the presence or absence of symptoms related to sleep disturbances or sleep apnea. The relationships between fatigue, pain, and sleep disturbances were evaluated with correlation analysis and regression analysis.

Results

Fatigue was reported by 96%, and daily pain by 87% of the individuals with OI. Notably, the level of fatigue was similarly experienced by patient respondents and controls. In total, 95% of the patients and 77% of the controls reported one to several sleep disturbance symptoms. These symptoms as well as previously diagnosed sleep apnea were statistically significantly more prevalent in the patient group than in the controls (p < 0.05). Likewise, the experienced bodily pain was statistically highly significantly more severe among the respondents with OI (p < 0.001), and correlated with the reported fatigue.

Conclusions

In comparison with age-matched controls, adults with OI do not differ in experienced fatigue, unlike hypothesized. Therefore, sleep disturbances, which based on the frequency of reported related symptoms and previous sleep apnea diagnoses appear to be common in OI patients, may remain undiagnosed.

Electronic supplementary material

The online version of this article (10.1186/s12891-017-1922-5) contains supplementary material, which is available to authorized users.

Severe hypotonia and postnatal growth impairment in a girl with a missense mutation in COL1A1: Implication of expanded phenotypic spectrum of type I collagenopathy

BACKGROUND

It is known that type I collagenopathy has a broad-spectrum phenotypic variability. Here, we report a case of a Korean girl with a heterozygous COL1A1 mutation who had an atypical presentation.

CASE PRESENTATION

A 26-month-old girl presented with delayed motor development and failure to thrive. She had severe growth retardation. She exhibited right-sided plagiocephaly, blue sclerae, and facial dysmorphism, including a small pointed chin, frontal bossing, and a triangular face, but had microcephaly. Whole-exome sequencing revealed a novel de novo heterozygous sequence variant in COL1A1 (p.Gly1127Asp), which was validated by Sanger sequencing. Radiological finding showed generalized osteoporosis with progressive scoliosis of the spine without evidence of platyspondyly related to fractures and bowing of the long bones, and markedly delayed carpal bone age. Muscle pathology showed a marked size variation of myofibers and selective type 1 atrophy.

CONCLUSIONS

This study expanded the clinical and genetic spectrum of type I collagenopathy with a COL1A1 variant. Therefore, we suggest that type I collagenopathy should be considered in the patients who have some features of osteogenesis imperfecta simultaneously with atypical features such as facial dysmorphism.

The Effect of Whole Body Vibration Training on Bone and Muscle Function in Children With Osteogenesis Imperfecta

CONTEXT

Osteogenesis imperfecta (OI) is associated with reduced muscle size, dynamic muscle function, and mobility.

OBJECTIVE

To assess the effect of whole body vibration (WBV) on bone density and geometry, muscle size and function, mobility, and balance in children with OI.

DESIGN

Randomized controlled pilot trial.

SETTING

Tertiary pediatric research center.

PARTICIPANTS

Twenty-four children (5 to 16 years) with OI types 1, 4, and limited mobility [Child Health Assessment Questionnaire (CHAQ) score ≥ 0.13] recruited in sex- and pubertal stage-matched pairs. Incident fractures in two boys (WBV arm) led to exclusion of two prepubertal pairs.

INTERVENTION

Five months of WBV training (3 × 3 minutes twice daily) or regular care.

MAIN OUTCOME MEASURES

Bone and muscle variables measured by dual-energy X-ray absorptiometry (spine, hip, total body) and peripheral quantitative computed tomography (tibia). Mobility assessed by 6-minute walk tests and CHAQ; dynamic muscle function by mechanography.

RESULTS

All participants had reduced walking distances and muscle function (P < 0.001). Body mass index z score was associated with higher CHAQ scores (ρ + 0.552; P = 0.005) and lower walking and two-leg jumping performance (ρ - 0.405 to -0.654, P < 0.05). The WBV and control groups did not differ in the 5-month changes in bone. Total lean mass increased more in the WBV group [+1119 g (+224 to +1744)] compared with controls [+635 g (-951 to +1006)], P = 0.01, without improving mobility, muscle function, or balance.

CONCLUSIONS

The increase in lean mass without changes in muscle function or bone mass suggests reduced biomechanical responsiveness of the muscle-bone unit in children with OI.

Muscle abnormalities in osteogenesis imperfecta

Osteogenesis imperfecta (OI) is mainly characterized by bone fragility but muscle abnormalities have been reported both in OI mouse models and in children with OI. Muscle mass is decreased in OI, even when short stature is taken into account. Dynamic muscle tests aiming at maximal eccentric force production reveal functional deficits that can not be explained by low muscle mass alone. However, it appears that diaphyseal bone mass is normally adapted to muscle force. At present the determinants of muscle mass and function in OI have not been clearly defined. Physiotherapy interventions and bisphosphonate treatment appear to have some effect on muscle function in OI. Interventions targeting muscle mass have shown encouraging results in OI animal models and are an interesting area for further research.

Decreasing maternal myostatin programs adult offspring bone strength in a mouse model of osteogenesis imperfecta

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.

Body Composition in Children and Adolescents with Osteogenesis Imperfecta

OBJECTIVE

To use peripheral quantitative computed tomography to determine the cross-sectional area (CSA) of subcutaneous fat and muscle (fat CSA, muscle CSA) in transverse forearm scans in patients with osteogenesis imperfecta (OI).

STUDY DESIGN

Fat and muscle CSA were quantified in 266 individuals (142 female) aged 5-20 years who had a diagnosis of OI type I, III, or IV and who had mutations in COL1A1 or COL1A2. Results were compared with those of 255 healthy controls.

RESULTS

In a subgroup of 39 patients with OI type I, % fat CSA correlated closely with total body percentage fat mass as determined by dual-energy x-ray absorptiometry (R(2) = 0.69; P < .001). In the entire study cohort, muscle CSA adjusted for age, sex, and forearm length was lower in OI type I and III than in controls (P < .05 each), but fat CSA was similar between OI types and controls. No relationship between the type of disease-causing mutation in the COL1A1 or COL1A2 genes and fat CSA or muscle CSA was found.

CONCLUSIONS

Children and adolescents with OI have low muscle size but a normal amount of subcutaneous fat at the forearm.

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

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.

Bone and muscle: Interactions beyond mechanical

The musculoskeletal system is significantly more complex than portrayed by traditional reductionist approaches that have focused on and studied the components of this system separately. While bone and skeletal muscle are the two largest tissues within this system, this system also includes tendons, ligaments, cartilage, joints and other connective tissues along with vascular and nervous tissues. Because the main function of this system is locomotion, the mechanical interaction among the major players of this system is essential for the many shapes and forms observed in vertebrates and even in invertebrates. Thus, it is logical that the mechanical coupling theories of musculoskeletal development exert a dominant influence on our understanding of the biology of the musculoskeletal system, because these relationships are relatively easy to observe, measure, and perturb. Certainly much less recognized is the molecular and biochemical interaction among the individual players of the musculoskeletal system. In this brief review article, we first introduce some of the key reasons why the mechanical coupling theory has dominated our view of bone-muscle interactions followed by summarizing evidence for the secretory nature of bones and muscles. Finally, a number of highly physiological questions that cannot be answered by the mechanical theories alone will be raised along with different lines of evidence that support both a genetic and a biochemical communication between bones and muscles. It is hoped that these discussions will stimulate new insights into this fertile and promising new way of defining the relationships between these closely related tissues. Understanding the cellular and molecular mechanisms responsible for biochemical communication between bone and muscle is important not only from a basic research perspective but also as a means to identify potential new therapies for bone and muscle diseases, especially for when they co-exist. This article is part of a Special Issue entitled "Muscle Bone Interactions".

Hindlimb Skeletal Muscle Function and Skeletal Quality and Strength in +/G610C Mice With and Without Weight-Bearing Exercise

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 (+/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 +/G610C mice to undergo a treadmill exercise regimen. We found +/G610C mice hindlimb muscles do not exhibit compromised muscle function, and their activity levels were not reduced relative to wild-type mice. The +/G610C mice were also able to complete an 8-week treadmill regimen. Biomechanical integrity of control and exercised wild-type and +/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 +/G610C OI mice have equivalent muscle function, activity levels, and ability to complete a weight-bearing exercise regimen as wild-type mice. The +/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.

Are Changes in Composition in Response to Treatment of a Mouse Model of Osteogenesis Imperfecta Sex-dependent?

BACKGROUND

Osteogenesis imperfecta (OI) is a genetic disease characterized by skeletal fragility and deformity. There is extensive debate regarding treatment options in adults with OI. Antiresorptive treatment reduces the number of fractures in growing oim/oim mice, an animal model that reproducibly mimics the moderate-to-severe form of OI in humans. Effects of long-term treatments with antiresorptive agents, considered for treatment of older patients with OI with similar presentation (moderate-to-severe OI) are, to date, unknown.

QUESTIONS/PURPOSES

Fourier transform infrared (FTIR) imaging, which produces a map of the spatial variation in chemical composition in thin sections of bone, was used to address the following questions: (1) do oim/oim mice show a sex dependence in compositional properties at 6.5 months of age; (2) is there a sex-dependent response to treatment with antiresorptive agents used in the treatment of OI in humans; and (3) are any compositional parameters in oim/oim mice corrected to wild-type (WT) values after treatment?

METHODS

FTIR imaging data were collected from femurs from four to five mice per sex per genotype per treatment. Treatments were 24 weeks of saline, alendronate, or RANK-Fc; and 12 weeks of saline+12 weeks RANK-Fc and 12 weeks of alendronate+RANK-Fc. FTIR imaging compositional parameters measured in cortical and cancellous bones were mineral-to-matrix ratio, carbonate-to-mineral ratio, crystal size/perfection, acid phosphate substitution, collagen maturity, and their respective distributions (heterogeneities). Because of the small sample size, nonparametric statistics (Mann-Whitney U- and Kruskal-Wallis tests with Bonferroni correction) were used to compare saline-treated male and female mice of different genotypes and treatment effects by sex and genotype, respectively. Statistical significance was defined as p<0.05.

RESULTS

At 6.5 months, saline-treated male cortical oim/oim bone had increased mineral-to-matrix ratio (p=0.016), increased acid phosphate substitution (p=0.032), and decreased carbonate-to-mineral ratio (p=0.016) relative to WT. Cancellous bone in male oim/oim also had increased mineral-to-matrix ratio (p=0.016) relative to male WT. Female oim/oim mouse bone composition for all cortical and cancellous bone parameters was comparable to WT (p>0.05). Only the female WT mice showed a response of mean compositional properties to treatment, increasing mineral-to-matrix after RANK-Fc treatment in cancellous bone (p=0.036) compared with saline-treated mice. Male oim/oim increased mineral-to-matrix cortical and cancellous bone heterogeneity in response to all long-term treatments except for saline+RANK-Fc (p<0.04); female oim/oim cortical mineral-to-matrix bone heterogeneity increased with ALN+RANK-Fc and all treatments increased cancellous female oim/oim bone acid phosphate substitution heterogeneity (p<0.04).

CONCLUSIONS

Both oim/oim and WT mice, which demonstrate sex-dependent differences in composition with saline treatment, showed few responses to long-term treatment with antiresorptive agents. Female WT mice appeared to be more responsive; male oim/oim mice showed more changes in compositional heterogeneity. Changes in bone composition caused by these agents may contribute to improved bone quality in oim/oim mice, because the treatments are known to reduce fracture incidence.

CLINICAL RELEVANCE

The optimal drug therapy for long-term treatment of patients with moderate-to-severe OI is unknown. Based on bone compositional changes in mice, antiresorptive treatments are useful for continued treatment in OI. There is a reported sexual dimorphism in fracture incidence in adults with OI, but to date, no one has reported differences in response to pharmaceutical intervention. This study suggests that such an investigation is warranted.

Administration of soluble activin receptor 2B increases bone and muscle mass in a mouse model of osteogenesis imperfecta

Osteogenesis imperfecta (OI) comprises a group of heritable connective tissue disorders generally defined by recurrent fractures, low bone mass, short stature and skeletal fragility. Beyond the skeletal complications of OI, many patients also report intolerance to physical activity, fatigue and muscle weakness. Indeed, recent studies have demonstrated that skeletal muscle is also negatively affected by OI, both directly and indirectly. Given the well-established interdependence of bone and skeletal muscle in both physiology and pathophysiology and the observations of skeletal muscle pathology in patients with OI, we investigated the therapeutic potential of simultaneous anabolic targeting of both bone and skeletal muscle using a soluble activin receptor 2B (ACVR2B) in a mouse model of type III OI (oim). Treatment of 12-week-old oim mice with ACVR2B for 4 weeks resulted in significant increases in both bone and muscle that were similar to those observed in healthy, wild-type littermates. This proof of concept study provides encouraging evidence for a holistic approach to treating the deleterious consequences of OI in the musculoskeletal system.

Mutational and structural characteristics of four novel heterozygous C-propeptide mutations in the proα1(I) collagen gene in Chinese osteogenesis imperfecta patients

OBJECTIVE

Osteogenesis imperfecta (OI) with C-propeptide mutations in proα1(I) collagen gene are rarely reported. We report four novel C-propeptide mutations in COL1A1 gene from Chinese OI patients.

METHODS

Clinical characteristics and radiographic findings were described for four OI patients with C-propeptide mutations in proα1(I) collagen gene. Mutations were identified by traditional DNA sequencing based on PCR. The locations of mutations were mapped, and in silico prediction was conducted to analyse their effects on protein structure. Histology studies of skin, bone and muscle tissues were performed.

RESULTS

All four C-propeptide heterozygous mutations identified were in the COL1A1 gene. Heterozygous mutation of c.4021C>T (p.Q1341X) disrupted the chain recognition sequences and was found in patients with type IV OI. Mutations of c.3893C>A (p.T1298N) and c.3897C>A (p.C1299X) impeded the formation of disulphide bonds and were associated with type IV OI phenotype. Missense mutation of c.3835A>C (p.N1279H) disrupted Ca(2+) binding and led to a severe type III OI phenotype. In silico programs predicted damaging effects for the patients with type III OI and the creation of an exonic splicing enhancer hexamer sequence for the type IV patients. Expansion of the bone marrow cavity and disorganization of osteocyte alignment was evident in bone specimens; and muscle atrophy and enlargement of intramuscular connective tissue were found in muscle specimens.

CONCLUSIONS

Four novel C-propeptide mutations in proα1(I) collagen gene were identified in Chinese OI patients, and their clinical severity ranged from moderate type IV to severe type III. In silico prediction of the mutation effect and histological characteristics of tissue specimens was in accordance with the OI phenotypes.

Muscle anatomy and dynamic muscle function in osteogenesis imperfecta type I

CONTEXT

Results of previous studies suggested that children and adolescents with osteogenesis imperfecta (OI) type I have a muscle force deficit. However, muscle function has only been assessed by static isometric force tests and not in more natural conditions such as dynamic force and power tests.

OBJECTIVE

The purpose of this study was to assess lower extremity dynamic muscle function and muscle anatomy in OI type I.

SETTING

The study was performed in the outpatient department of a pediatric orthopedic hospital.

PATIENTS AND OTHER PARTICIPANTS

A total of 54 individuals with OI type I (6-21 years; 20 male) and 54 age- and sex-matched controls took part in this study.

MAIN OUTCOME MEASURES

Calf muscle cross-sectional area and density were measured by peripheral quantitative computed tomography. Lower extremity muscle function (peak force per body weight and peak power per body mass) was measured by jumping mechanography through 5 tests: multiple two-legged hopping, multiple one-legged hopping, single two-legged jump, chair-rise test, and heel-rise test.

RESULTS

Compared with age- and sex-matched controls, patients with OI type I had smaller muscle size (P = .04) but normal muscle density (P = .21). They also had lower average peak force and lower specific force (peak force/muscle cross-sectional area; all P < .008). Average peak power was lower in patients with OI type I but not significantly so (all P > .054).

CONCLUSIONS

Children and adolescents with OI type I have, on average, a significant force deficit in the lower limb as measured by dynamic force tests. Nonetheless, these data also show that OI type I is compatible with normal muscle performance in some individuals.

Gender-dependence of bone structure and properties in adult osteogenesis imperfecta murine model

Osteogenesis imperfecta (OI) is a dominant skeletal disorder characterized by bone fragility and deformities. Though the oim mouse model has been the most widely studied of the OI models, it has only recently been suggested to exhibit gender-dependent differences in bone mineralization. To characterize the impact of gender on the morphometry/ultra-structure, mechanical properties, and biochemical composition of oim bone on the congenic C57BL/J6 background, 4-month-old oim/oim, +/oim, and wild-type (wt) female and male tibiae were evaluated using micro-computed tomography, three-point bending, and Raman spectroscopy. Dramatic gender differences were evident in both cortical and trabecular bone morphological and geometric parameters. Male mice had inherently more bone and increased moment of inertia than genotype-matched female counterparts with corresponding increases in bone biomechanical strength. The primary influence of gender was structure/geometry in bone growth and mechanical properties, whereas the mineral/matrix composition and hydroxyproline content of bone were influenced primarily by the oim collagen mutation. This study provides evidence of the importance of gender in the evaluation and interpretation of potential therapeutic strategies when using mouse models of OI.