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

Zebrafish Models for Skeletal and Extraskeletal Osteogenesis Imperfecta Features: Unveiling Pathophysiology and Paving the Way for Drug Discovery

In the last decades, the easy genetic manipulation, the external fertilization, the high percentage of homology with human genes and the reduced husbandry costs compared to rodents, made zebrafish a valid model for studying human diseases and for developing new therapeutical strategies. Since zebrafish shares with mammals the same bone cells and ossification types, it became widely used to dissect mechanisms and possible new therapeutic approaches in the field of common and rare bone diseases, such as osteoporosis and osteogenesis imperfecta (OI), respectively. OI is a heritable skeletal disorder caused by defects in gene encoding collagen I or proteins/enzymes necessary for collagen I synthesis and secretion. Nevertheless, OI patients can be also characterized by extraskeletal manifestations such as dentinogenesis imperfecta, muscle weakness, cardiac valve and pulmonary abnormalities and skin laxity. In this review, we provide an overview of the available zebrafish models for both dominant and recessive forms of OI. An updated description of all the main similarities and differences between zebrafish and mammal skeleton, muscle, heart and skin, will be also discussed. Finally, a list of high- and low-throughput techniques available to exploit both larvae and adult OI zebrafish models as unique tools for the discovery of new therapeutic approaches will be presented.

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.

Update on the Genetics of Osteogenesis Imperfecta

Osteogenesis imperfecta (OI) is a heterogeneous heritable skeletal dysplasia characterized by bone fragility and deformity, growth deficiency, and other secondary connective tissue defects. OI is now understood as a collagen-related disorder caused by defects of genes whose protein products interact with collagen for folding, post-translational modification, processing and trafficking, affecting bone mineralization and osteoblast differentiation. This review provides the latest updates on genetics of OI, including new developments in both dominant and rare OI forms, as well as the signaling pathways involved in OI pathophysiology. There is a special emphasis on discoveries of recessive mutations in TENT5A, MESD, KDELR2 and CCDC134 whose causality of OI types XIX, XX, XXI and XXI, respectively, is now established and expends the complexity of mechanisms underlying OI to overlap LRP5/6 and MAPK/ERK pathways. We also review in detail new discoveries connecting the known OI types to each other, which may underlie an eventual understanding of a final common pathway in OI cellular and bone biology.

Combining anabolic loading and raloxifene improves bone quantity and some quality measures in a mouse model of osteogenesis imperfecta

Osteogenesis imperfecta (OI) increases fracture risk due to changes in bone quantity and quality caused by mutations in collagen and its processing proteins. Current therapeutics improve bone quantity, but do not treat the underlying quality deficiencies. Male and female G610C+/- mice, a murine model of OI, were treated with a combination of raloxifene and in vivo axial tibial compressive loading starting at 10 weeks of age and continuing for 6 weeks to improve bone quantity and quality. Bone geometry and mechanical properties were measured to determine whole bone and tissue-level material properties. A colocalized Raman/nanoindentation system was used to measure chemical composition and nanomechanical properties in newly formed bone compared to old bone to determine if bone formed during the treatment regimen differed in quality compared to bone formed prior to treatment. Lastly, lacunar geometry and osteocyte apoptosis were assessed. OI mice were able to build bone in response to the loading, but this response was less robust than in control mice. Raloxifene improved some bone material properties in female but not male OI mice. Raloxifene did not alter nanomechanical properties, but loading did. Lacunar geometry was largely unchanged with raloxifene and loading. However, osteocyte apoptosis was increased with loading in raloxifene treated female mice. Overall, combination treatment with raloxifene and loading resulted in positive but subtle changes to bone quality.

Emerging Landscape of Osteogenesis Imperfecta Pathogenesis and Therapeutic Approaches

Osteogenesis imperfecta (OI) is an uncommon genetic disorder characterized by shortness of stature, hearing loss, poor bone mass, recurrent fractures, and skeletal abnormalities. Pathogenic variations have been found in over 20 distinct genes that are involved in the pathophysiology of OI, contributing to the disorder's clinical and genetic variability. Although medications, surgical procedures, and other interventions can partially alleviate certain symptoms, there is still no known cure for OI. In this Review, we provide a comprehensive overview of genetic pathogenesis, existing treatment modalities, and new developments in biotechnologies such as gene editing, stem cell reprogramming, functional differentiation, and transplantation for potential future OI therapy.

Absence of TRIC-B from type XIV Osteogenesis Imperfecta osteoblasts alters cell adhesion and mitochondrial function - A multi-omics study

Osteogenesis Imperfecta (OI) is a heritable collagen-related bone dysplasia characterized by bone fractures, growth deficiency and skeletal deformity. Type XIV OI is a recessive OI form caused by null mutations in TMEM38B, which encodes the ER membrane intracellular cation channel TRIC-B. Previously, we showed that absence of TMEM38B alters calcium flux in the ER of OI patient osteoblasts and fibroblasts, which further disrupts collagen synthesis and secretion. How the absence of TMEM38B affects osteoblast function is still poorly understood. Here we further investigated the role of TMEM38B in human osteoblast differentiation and mineralization. TMEM38B-null osteoblasts showed altered expression of osteoblast marker genes and decreased mineralization. RNA-Seq analysis revealed that cell-cell adhesion was one of the most downregulated pathways in TMEM38B-null osteoblasts, with further validation by real-time PCR and Western blot. Gap and tight junction proteins were also decreased by TRIC-B absence, both in patient osteoblasts and in calvarial osteoblasts of Tmem38b-null mice. Disrupted cell adhesion decreased mutant cell proliferation and cell cycle progression. An important novel finding was that TMEM38B-null osteoblasts had elongated mitochondria with altered fusion and fission markers, MFN2 and DRP1. In addition, TMEM38B-null osteoblasts exhibited a significant increase in superoxide production in mitochondria, further supporting mitochondrial dysfunction. Together these results emphasize the novel role of TMEM38B/TRIC-B in osteoblast differentiation, affecting cell-cell adhesion processes, gap and tight junction, proliferation, cell cycle, and mitochondrial function.

Delivering a Home-Based Exercise Program to Youth With Osteogenesis Imperfecta: Protocol for a Comparative-Approach Study

BACKGROUND

Osteogenesis imperfecta (OI) is a rare bone fragility disorder associated with muscle weakness. Individuals with OI may therefore benefit from exercise interventions aiming to improve muscle and bone strength. Given the rarity of OI, many patients do not have access to exercise specialists who are familiar with the disorder. As such, telemedicine, the provision of health care through technology to provide care at a distance, may be well suited for this population.

OBJECTIVE

The main objectives are (1) to investigate the feasibility and cost-effectiveness of 2 telemedicine approaches for the delivery of an exercise intervention for youth with OI and (2) to assess the impact of the exercise intervention on muscle function and cardiopulmonary fitness in youth with OI.

METHODS

Patients with OI type I (the mildest form of OI; n=12, aged 12-16 years) from a pediatric orthopedic tertiary hospital will be randomized to receive a 12-week remote exercise intervention in either (1) a supervised group (n=6), monitored every session, or (2) a follow-up group (n=6), receiving monthly progress update appointments. Participants will undergo the following pre- and postintervention evaluations: sit-to-stand test, push-up test, sit-up test, single-legged balance test, and a heel-rise test. Both groups will be given the same 12-week exercise regimen, which includes cardiovascular, resistance, and flexibility training. For each exercise training session involving the supervised group, a kinesiologist will provide instructions to participants through live video sessions using a teleconferencing application. On the other hand, the follow-up group will discuss their progress with the kinesiologist every 4 weeks over a teleconferencing video call. Feasibility will be assessed by recruitment, adherence, and completion rates. A cost-effectiveness analysis of both approaches will be computed. Changes in muscle function and cardiopulmonary fitness will be examined between the 2 groups, pre- and postintervention.

RESULTS

It is anticipated that the supervised group will have higher adherence and completion rates compared to the follow-up group, which may be associated with greater physiological benefits; however, it may not be as cost-effective compared to the follow-up approach.

CONCLUSIONS

By determining the most feasible telemedicine approach, this study may serve as a basis for providing increased access to specialized adjunct therapies for individuals with rare disorders.

INTERNATIONAL REGISTERED REPORT IDENTIFIER (IRRID)

PRR1-10.2196/40262.

Collagen (I) homotrimer potentiates the osteogenesis imperfecta (oim) mutant allele and reduces survival in male mice

The osteogenesis imperfecta murine (oim) model with solely homotrimeric (α1)3 type I collagen, owing to a dysfunctional α2(I) collagen chain, has a brittle bone phenotype, implying that the (α1)2(α2)1 heterotrimer is required for physiological bone function. Here, we comprehensively show, for the first time, that mice lacking the α2(I) chain do not have impaired bone biomechanical or structural properties, unlike oim homozygous mice. However, Mendelian inheritance was affected in male mice of both lines, and male mice null for the α2(I) chain exhibited age-related loss of condition. Compound heterozygotes were generated to test whether gene dosage was responsible for the less-severe phenotype of oim heterozygotes, after allelic discrimination showed that the oim mutant allele was not downregulated in heterozygotes. Compound heterozygotes had impaired bone structural properties compared to those of oim heterozygotes, albeit to a lesser extent than those of oim homozygotes. Hence, the presence of heterotrimeric type I collagen in oim heterozygotes alleviates the effect of the oim mutant allele, but a genetic interaction between homotrimeric type I collagen and the oim mutant allele leads to bone fragility.

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.

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.

Transcranial photobiomodulation therapy ameliorates perioperative neurocognitive disorder through modulation of mitochondrial function in aged mice

Perioperative neurocognitive disorder (PND) is a serious nervous system complication characterized by progressive cognitive impairment, especially in geriatric population. However, the neuropathogenesis of PND is complex, and there are no approved disease-modifying therapeutic options. Mitochondrial dysfunction has been demonstrated to contribute to the occurrence and development of PND. Transcranial near-infrared (tNIR) light treatment helps to improve mitochondrial dysfunction and enhance cognition, but its effect on PND remains unclear. Here, we evaluated the effect of tNIR light treatment on PND caused by anesthesia and surgery in aged mice. We built the PND models with 18-month C57BL/6 male mice by exploratory laparotomy under isoflurane inhalation anesthesia, and treated by tNIR light with wavelength 810 nm for 2 weeks. The short-term and long-term changes in cognitive function were analyzed by behavioral tests. We further explored the effects of tNIR light on mitochondria, synapses, neurons, and signaling pathways through different experimental methods. The results demonstrated that the cognitive impairment and mitochondrial dysfunction in PND mice were ameliorated after tNIR light treatment. Further experiments demonstrated that photobiomodulation therapy (PBMT) increased synapse-related protein expression, neuronal survival, and protected synapse from depletion. Moreover, downregulated sirtuin 1 (SIRT1) and peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α) were increased after tNIR light treatment. Our results suggested that tNIR light was an effective treatment of PND through PBMT effect, accompanied by synaptic and neuronal improvement. The improvement of mitochondrial dysfunction mediated by SIRT1/PGC-1α signaling pathway might participate in this process. Those findings might provide a novel and noninvasive therapeutic target for PND.

Linking Oxidative Stress and DNA Damage to Changes in the Expression of Extracellular Matrix Components

Cells are subjected to endogenous [e.g., reactive oxygen species (ROS), replication stress] and exogenous insults (e.g., UV light, ionizing radiation, and certain chemicals), which can affect the synthesis and/or stability of different macromolecules required for cell and tissue function. Oxidative stress, caused by excess ROS, and DNA damage, triggered in response to different sources, are countered and resolved by specific mechanisms, allowing the normal physiological equilibrium of cells and tissues to be restored. One process that is affected by oxidative stress and DNA damage is extracellular matrix (ECM) remodeling, which is a continuous and highly controlled mechanism that allows tissues to readjust in reaction to different challenges. The crosstalk between oxidative stress/DNA damage and ECM remodeling is not unidirectional. Quite on the contrary, mutations in ECM genes have a strong impact on tissue homeostasis and are characterized by increased oxidative stress and potentially also accumulation of DNA damage. In this review, we will discuss how oxidative stress and DNA damage affect the expression and deposition of ECM molecules and conversely how mutations in genes encoding ECM components trigger accumulation of oxidative stress and DNA damage. Both situations hamper the reestablishment of cell and tissue homeostasis, with negative impacts on tissue and organ function, which can be a driver for severe pathological conditions.

Probiotics Stimulate Bone Formation in Obese Mice via Histone Methylations

Rationale: Manipulation of the gut microbiome can prevent pathologic bone loss. However, the effects of probiotics on mitochondrial epigenetic remodeling and skeletal homeostasis in the high-fat diet (HFD)-linked obesity remains to be explored. Here, we examined the impact of probiotics supplementation on mitochondrial biogenesis and bone homeostasis through the histone methylation mechanism in HFD fed obese mice.

Methods: 16S rRNA gene sequencing was performed to study the microbiota composition in the gut and microbial dysbiosis in obese mouse model. High resolution (microPET/CT) imaging was performed to demonstrate the obese associated colonic inflammation. Obese-associated upregulation of target miRNA in osteoblast was investigated using a microRNA qPCR array. Osteoblastic mitochondrial mass was evaluated using confocal imaging. Overexpression of mitochondrial transcription factor (Tfam) was used to investigate the glycolysis and mitochondrial bioenergetic metabolism using Tfam-transgenic (Tg) mice fed on HFD. The bone formation and mechanical strength was evaluated by microCT analysis and three-point bending analysis.

Results: High-resolution imaging (µ-CT) and mechanical testing revealed that probiotics induced a significant increase of trabecular bone volume and bone mechanical strength respectively in obese mice. Probiotics or Indole-3-propionic acid (IPA) treatment directly to obese mice, prevents gut inflammation, and improved osteoblast mineralization. Mechanistically, probiotics treatment increases mitochondrial transcription factor A (Tfam) expression in osteoblasts by promoting Kdm6b/Jmjd3 histone demethylase, which inhibits H3K27me3 epigenetic methylation at the Tfam promoter. Furthermore, Tfam-transgenic (Tg) mice, fed with HFD, did not experience obesity-linked reduction of glucose uptake, mitochondrial biogenesis and mineralization in osteoblasts.

Conclusions: These results suggest that the probiotics mediated changes in the gut microbiome and its derived metabolite, IPA are potentially be a novel agent for regulating bone anabolism via the gut-bone axis.

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.

Male but not female mice with severe osteogenesis imperfecta are partially protected from high-fat diet-induced obesity

Previously we have shown that young mice with a dominant severe form of osteogenesis imperfecta (OI), caused by mutated collagen type I, exhibit an altered glucose/insulin metabolism and energy expenditure along with elevated levels of osteocalcin, a bone-derived hormone involved in the regulation of whole-body metabolism. This study aimed to examine the long-term effects of a western diet in these OI mice. Male and female OI mice and wild type littermates (WT) were fed a high-fat diet (HFD) or a matched low-fat diet (LFD) for 26 weeks. HFD-induced obesity was observed in male and female WT and female OI mice, but not in male OI mice. HFD-fed WT and OI mice of both sexes developed hyperglycemia and glucose intolerance, but the degree of glucose intolerance was significantly lower in male and female OI mice compared to sex- and diet-matched WT mice. Indirect calorimetry revealed increased movement of male OI mice on HFD compared to LFD and, while HFD lowered energy expenditure in WT mice, energy expenditure was not changed in OI mice. Further, HFD-fed male OI mice demonstrated a diet-induced increased expression of the thermogenesis genes, Ucp1 and Pgc1α, in brown adipose tissue. On LFD, total and Gla-13 osteocalcin levels were similar in 30-week-old WT and OI mice, but on HFD, both were significantly higher in OI mice than WT. Thus, male OI mice respond to HFD with increased movement, energy expenditure, brown adipose tissue thermogenesis, and higher levels of osteocalcin, resulting in partial protection against HFD-induced obesity.

Tendon and motor phenotypes in the Crtap-/- mouse model of recessive osteogenesis imperfecta

Osteogenesis imperfecta (OI) is characterized by short stature, skeletal deformities, low bone mass, and motor deficits. A subset of OI patients also present with joint hypermobility; however, the role of tendon dysfunction in OI pathogenesis is largely unknown. Using the Crtap-/- mouse model of severe, recessive OI, we found that mutant Achilles and patellar tendons were thinner and weaker with increased collagen cross-links and reduced collagen fibril size at 1- and 4-months compared to wildtype. Patellar tendons from Crtap-/- mice also had altered numbers of CD146+CD200+ and CD146-CD200+ progenitor-like cells at skeletal maturity. RNA-seq analysis of Achilles and patellar tendons from 1-month Crtap-/- mice revealed dysregulation in matrix and tendon marker gene expression concomitant with predicted alterations in TGF-β, inflammatory, and metabolic signaling. At 4-months, Crtap-/- mice showed increased αSMA, MMP2, and phospho-NFκB staining in the patellar tendon consistent with excess matrix remodeling and tissue inflammation. Finally, a series of behavioral tests showed severe motor impairments and reduced grip strength in 4-month Crtap-/- mice - a phenotype that correlates with the tendon pathology.

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.

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.

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.

Vitamin K2 Can Rescue the Dexamethasone-Induced Downregulation of Osteoblast Autophagy and Mitophagy Thereby Restoring Osteoblast Function In Vitro and In Vivo

Chronic long-term glucocorticoids (GC) use is associated with glucocorticoid-induced osteoporosis (GIOP) by inhibiting the survival and impairing the functions of osteoblasts. Autophagy and mitophagy play key roles in osteoblast differentiation, mineralization and survival, and mounting evidence have implicated osteoblast autophagy and mitophagy as a novel mechanism in the pathogenesis of GIOP. Vitamin K2 (VK2) is an essential nutrient supplement that have been shown to exert protective effects against osteoporotic bone loss including GIOP. In this study, we showed that the glucocorticoid dexamethasone (Dex) deregulated osteoblast autophagy and mitophagy by downregulating the expression of autophagic and mitophagic markers LC3-II, PINK1, Parkin. This consequently led to inhibition of osteoblast differentiation and mineralization function in vitro. Interestingly, co-treatment with VK2 significantly attenuated the Dex-induced downregulation of LC3-II, PINK1, Parkin, thereby restoring autophagic and mitophagic processes and normal osteoblastic activity. In addition, using an established rat model of GIOP, we showed that VK2 administration can protect rats against the deleterious effects of Dex on bone by reinstating autophagic and mitophagic activities in bone tissues. Collectively, our results provide new insights into the role of osteoblast autophagy and mitophagy in GIOP. Additionally, the use of VK2 supplementation to augment osteoblast autophagy/mitophagy may significantly improve clinical outcomes of GIOP patients.