Interactions Between HR-pQCT Bone Density and D3 Cr Muscle Mass (or HR-pQCT Bone Structure and HR-pQCT Muscle Density) in Predicting Fractures: The Osteoporotic Fractures in Men Study

We examined if an interaction exists between bone and muscle in predicting fractures in older men. Prospective data from the Osteoporotic Fractures in Men study was used to build Cox proportional hazards models. Predictors included HR-pQCT total volumetric BMD (Tt.BMD), trabecular BMD (Tb.BMD), cortical BMD (Ct.BMD) and cortical area (Ct.Ar) at distal radius/tibia, HR-pQCT muscle volume and density (diaphyseal tibia), D3 -creatine dilution (D3 Cr) muscle mass, and grip strength and leg force, analyzed as continuous variables and as quartiles. Incident fractures were self-reported every 4 months via questionnaires and centrally adjudicated by physician review of radiology reports. Potential confounders (demographics, comorbidities, lifestyle factors, etc.) were considered. A total of 1353 men (mean age 84.2 ± 4.0 years, 92.7% white) were followed for 6.03 ± 2.11 years. In the unadjusted (continuous) model, there were no interactions (p > 0.05) between any muscle variable (D3 Cr muscle mass, muscle volume, muscle density, grip strength or leg force) and Tt.BMD at distal radius/tibia for fractures (all: n = 182-302; nonvertebral: n = 149-254; vertebral: n = 27-45). No consistent interactions were observed when interchanging Tt.BMD for Tb.BMD/Ct.BMD or for Ct.Ar (bone structure) at the distal radius/tibia in the unadjusted (continuous) models. Compared with men in quartiles (Q) 2-4 of D3 Cr muscle mass and Q2-4 of distal tibia Tt.BMD, men in Q1 of both had increased risk for all fractures (hazard ratio (HR) = 2.00; 95% confidence interval [CI] 1.24-3.23, p = 0.005) and nonvertebral fractures (HR = 2.10; 95% CI 1.25-3.52, p < 0.001) in the multivariable-adjusted model. Confidence intervals overlapped (p > 0.05) when visually inspecting other quartile groups in the multivariable-adjusted model. In this prospective cohort study of older men, there was no consistent interactions between bone and muscle variables on fracture risk. Larger sample sizes and longer follow-up may be needed to clarify if there is an interaction between bone and muscle on fracture risk in men. © 2023 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Short-Term Carbohydrate Restriction Impairs Bone Formation at Rest and During Prolonged Exercise to a Greater Degree than Low Energy Availability

Bone stress injuries are common in athletes, resulting in time lost from training and competition. Diets that are low in energy availability have been associated with increased circulating bone resorption and reduced bone formation markers, particularly in response to prolonged exercise. However, studies have not separated the effects of low energy availability per se from the associated reduction in carbohydrate availability. The current study aimed to compare the effects of these two restricted states directly. In a parallel group design, 28 elite racewalkers completed two 6-day phases. In the Baseline phase, all athletes adhered to a high carbohydrate/high energy availability diet (CON). During the Adaptation phase, athletes were allocated to one of three dietary groups: CON, low carbohydrate/high fat with high energy availability (LCHF), or low energy availability (LEA). At the end of each phase, a 25-km racewalk was completed, with venous blood taken fasted, pre-exercise, and 0, 1, 3 hours postexercise to measure carboxyterminal telopeptide (CTX), procollagen-1 N-terminal peptide (P1NP), and osteocalcin (carboxylated, gla-OC; undercarboxylated, glu-OC). Following Adaptation, LCHF showed decreased fasted P1NP (~26%; p < 0.0001, d = 3.6), gla-OC (~22%; p = 0.01, d = 1.8), and glu-OC (~41%; p = 0.004, d = 2.1), which were all significantly different from CON (p < 0.01), whereas LEA demonstrated significant, but smaller, reductions in fasted P1NP (~14%; p = 0.02, d = 1.7) and glu-OC (~24%; p = 0.049, d = 1.4). Both LCHF (p = 0.008, d = 1.9) and LEA (p = 0.01, d = 1.7) had significantly higher CTX pre-exercise to 3 hours post-exercise but only LCHF showed lower P1NP concentrations (p < 0.0001, d = 3.2). All markers remained unchanged from Baseline in CON. Short-term carbohydrate restriction appears to result in reduced bone formation markers at rest and during exercise with further exercise-related increases in a marker of bone resorption. Bone formation markers during exercise seem to be maintained with LEA although resorption increased. In contrast, nutritional support with adequate energy and carbohydrate appears to reduce unfavorable bone turnover responses to exercise in elite endurance athletes. © 2022 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

A Neuroskeletal Atlas: Spatial Mapping and Contextualization of Axon Subtypes Innervating the Long Bones of C3H and B6 Mice

Nerves in bone play well-established roles in pain and vasoregulation and have been associated with progression of skeletal disorders, including osteoporosis, fracture, arthritis, and tumor metastasis. However, isolation of the region-specific mechanisms underlying these relationships is limited by our lack of quantitative methods for neuroskeletal analysis and precise maps of skeletal innervation. To overcome these limitations, we developed an optimized workflow for imaging and quantitative analysis of axons in and around the bone, including validation of Baf53b-Cre in concert with R26R-tdTomato (Ai9) as a robust pan-neuronal reporter system for use in musculoskeletal tissues. In addition, we created comprehensive maps of sympathetic adrenergic and sensory peptidergic axons within and around the full length of the femur and tibia in two strains of mice (B6 and C3H). In the periosteum, these maps were related to the surrounding musculature, including entheses and myotendinous attachments to bone. Three distinct patterns of periosteal innervation (termed type I, II, III) were defined at sites that are important for bone pain, bone repair, and skeletal homeostasis. For the first time, our results establish a gradient of bone marrow axon density that increases from proximal to distal along the length of the tibia and define key regions of interest for neuroskeletal studies. Lastly, this information was related to major nerve branches and local maps of specialized mechanoreceptors. This detailed mapping and contextualization of the axonal subtypes innervating the skeleton is intended to serve as a guide during the design, implementation, and interpretation of future neuroskeletal studies and was compiled as a resource for the field as part of the NIH SPARC consortium. © 2021 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR)..

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).

Higher Undercarboxylated to Total Osteocalcin Ratio Is Associated With Reduced Physical Function and Increased 15-Year Falls-Related Hospitalizations: The Perth Longitudinal Study of Aging Women

Evidence from animal models suggests that undercarboxylated osteocalcin (ucOC) is involved in muscle mass maintenance and strength. In humans, the ucOC to total (t)OC ratio may be related to muscle strength and perhaps physical function and falls risk, but data are limited. We tested the hypothesis that ucOC and ucOC/tOC ratio are associated with muscle function (muscle strength and physical function) in older women and 15-year falls-related hospitalizations. Serum tOC and ucOC were assessed in 1261 older women (mean age 75.2 ± 2.7 years) forming the Perth Longitudinal Study of Aging Women (1998 to 2013). Timed-up-and-go (TUG) and grip strength were assessed at baseline and at 5 years. Falls-related hospitalizations (14.5-year follow-up) were captured by the Hospital Morbidity Data Collection, via the Western Australian Data Linkage System. At baseline, women with higher ucOC/tOC ratio (quartile 4) had slower TUG performance compared with quartile 1 (~0.68 seconds, p < .01). Grip strength and 5-year change of TUG and grip were not different (p > .05) between quartiles. Fear of falling limiting house, outdoor, and combined activities was significantly different across quartiles (p < .05). Higher ucOC/tOC was significantly associated with poorer TUG performance at baseline and 5-year change in performance, increased walking aid use, and fear of falling (all p < .05). Higher ucOC was related to lower grip strength at baseline (p < .05) but not 5-year change in strength. Those with the highest ucOC/tOC had greater falls-related hospitalizations (unadjusted log rank, p = .004) remaining significant after adjusting for key variables (hazard ratio [HR] = 1.31, 95% confidence interval [CI] 1.09-1.57, p = .004). We identified a large proportion of older women with high ucOC/tOC ratio who had reduced physical function, including its long-term decline and increased risk of falls-related hospitalizations. Early identification of women at higher risk can enable prevention and intervention strategies to occur, reducing risk for injurious falls. © 2020 American Society for Bone and Mineral Research (ASBMR)..

Irisin Correlates Positively With BMD in a Cohort of Older Adult Patients and Downregulates the Senescent Marker p21 in Osteoblasts

Irisin is a myokine produced by skeletal muscle during exercise in both mice and humans. We previously showed that irisin treatment ameliorates immobility-induced osteoporosis and muscular atrophy in mice. Data in humans showed a positive association between irisin and bone mineral density (BMD) in athletes and a population of healthy children. However, the role of this myokine regarding the state of muscle and bone in the same population remained to be determined. For this purpose, 62 patients (age 68.71 ± 12.31 years) undergoing total hip or knee replacement were recruited. Our results showed that irisin serum levels negatively correlated with age (R = -0.515; p = .000018) and positively correlated with femoral BMD (R = 0.619; p = .001) and vertebral BMD (R = 0.201; p = .0001). Irisin was also positively associated with Fndc5 mRNA in muscle biopsies (R = 0.248; p = .016), as well as with Osteocalcin (Ocn) mRNA in bone biopsies (R = 0.708; p = .006). In skeletal muscle, FNDC5 positive fibers positively correlate with BMD of total femur (R = 0.765; p = .0014) and BMD of femoral neck (R = 0.575; p = .031), Interestingly, by analyzing patients divided by their T-score, we found lower irisin levels (p = .0011) in patients with osteopenia/osteoporosis (OP) compared to healthy controls matched for age and sex. By analyzing the senescence marker p21, we found a significant increase of its mRNA expression in the bone biopsies of OP patients compared to control ones. Therefore, we investigated in vitro whether rec-irisin had a direct effect on this senescence marker, showing that p21 mRNA expression was significantly downregulated in osteoblasts by the treatment with irisin. Overall, these results indicate that higher irisin levels are associated with a lower rate of age-related osteoporosis and that irisin could be effective in delaying the osteoblast aging process, suggesting a potential senolytic action of this myokine. © 2020 American Society for Bone and Mineral Research (ASBMR).

Bone Microarchitecture Assessed by Trabecular Bone Score Is Independent of Mobility Level or Height in Pediatric Patients with Cerebral Palsy

Bone strength and fracture risk do not only depend on bone density, but also on bone structure. The trabecular bone score (TBS) evaluates homogeneity of bone microarchitecture indirectly by measuring gray-level variations of two-dimensional (2D) DXA images. Although TBS is well-established for adults, there have been only few publications in pediatrics. In this monocentric retrospective analysis, we investigated TBS in children and adolescents with cerebral palsy (CP), a patient group vulnerable to low bone mineral mass due to impaired mobility. The influence of different parameters on TBS and areal BMD (aBMD) were evaluated, as well as the relationship between TBS and aBMD. We compared TBS values of our study population to a reference population. A total of 472 lumbar spine-dual-energy X-ray absorptiometry (LS-DXA) scans of children and adolescents with CP (205 female), aged between 4 and 18 years, were analyzed. The DXA-scans were part of the routine examination. The children had no records of fractures or specific bone diseases. Our study population with CP had similar TBS as the reference population. TBS did not increase with age until an inflection point at 10 years in females, and 12 years in males. Girls had significantly higher TBS than boys (p = .049) and pubertal girls aged 8 to 13 years had significantly higher TBS than prepubertal girls (p = .009). TBS standard deviation score for age (SDS-TBS) and aBMD Z-scores correlated weakly (p < .001; R = 0.276 [males], R = 0.284 [females]). Other than for aBMD Z-scores, SDS-TBS was not influenced by age-adjusted height Z-scores and there was no significant difference in SDS-TBS when grouped by mobility levels, using the Gross Motor Function Classification System (GMFCS). Our results indicate that children with CP have a similar homogeneous distribution of trabecular microarchitecture as controls. Puberty initiation appears to be essential for increase of TBS with age and for sex differences. TBS seems less influenced by body composition, height, and mobility than aBMD. © 2020 The Authors. Journal of Bone and Mineral Research published by American Society for Bone and Mineral Research.

Irisin Prevents Disuse-Induced Osteocyte Apoptosis

Previous results showed that intermittently administered irisin improves bone mass in normal mice and prevents the development of disuse-induced osteoporosis and muscular atrophy in hindlimb-suspended mice, a murine model able to mimic the absence of mechanical loading. A recent study showed that irisin increases survival of osteocytes acting through integrin αV/β5 receptors. To better understand the action of irisin on these cells, we investigated the downstream signaling cascades in osteocyte-like cells (MLO-Y4) treated with recombinant irisin (rec-irisin) in vitro and we analyzed survival of osteocytes and caspase activation in cortical bone of osteoporotic mice treated with rec-irisin in vivo. Our results revealed that rec-irisin activated the MAP kinases Erk1 and Erk2 and increased the expression of the transcription factor Atf4 (2.5-fold, p < .05) through an Erk-dependent pathway in osteocytes. Some key genes expressed by MLO-Y4 cells were modulated by long-term irisin treatment, either continuously administered or given with intermittent short pulses. Interestingly, Sost mRNA was severely downregulated only upon intermittent irisin administration (10-fold, p < .001). Furthermore, rec-irisin upregulated Tfam mRNA (fourfold, p < .05) and Bcl2/Bax ratio (twofold, p < .05) in MLO-Y4 cells. By detecting caspase-9 and caspase-3, we also found that rec-irisin inhibited apoptosis induced by hydrogen peroxide and dexamethasone, respectively. In cortical bone of unloading C57BL6 mice treated with vehicle (unload-veh), irisin prevented disuse-induced reduction of viable osteocytes (+30% versus unload-veh, p < .05) and increase of empty lacunae (+110% versus unload-veh, p < .05), as well as caspase-9 (threefold, p < .05) and caspase-3 (twofold, p < .05) activations. Our findings revealed underlying mechanisms of irisin action on osteocytes, which increases their functions and exerts anti-apoptotic effects, confirming that mechanosensor cells of bone are sensitive to the exercise-mimetic myokine irisin. © 2019 American Society for Bone and Mineral Research.

Age at Onset of Walking in Infancy Is Associated With Hip Shape in Early Old Age

Bones' shapes and structures adapt to the muscle and reaction forces they experience during everyday movements. Onset of independent walking, at approximately 12 months, represents the first postnatal exposure of the lower limbs to the large forces associated with bipedal movements; accordingly, earlier walking is associated with greater bone strength. However, associations between early life loading and joint shape have not been explored. We therefore examined associations between walking age and hip shape at age 60 to 64 years in 1423 individuals (740 women) from the MRC National Survey of Health and Development, a nationally representative British birth cohort. Walking age in months was obtained from maternal interview at age 2 years. Ten modes of variation in hip shape (HM1 to HM10), described by statistical shape models, were ascertained from DXA images. In sex-adjusted analyses, earlier walking age was associated with higher HM1 and HM7 scores; these associations were maintained after further adjustment for height, body composition, and socioeconomic position. Earlier walking was also associated with lower HM2 scores in women only, and lower HM4 scores in men only. Taken together, this suggests that earlier walkers have proportionately larger (HM4) and flatter (HM1, HM4) femoral heads, wider (HM1, HM4, HM7) and flatter (HM1, HM7) femoral necks, a smaller neck-shaft angle (HM1, HM4), anteversion (HM2, HM7), and early development of osteophytes (HM1). These results suggest that age at onset of walking in infancy is associated with variations in hip shape in older age. Early walkers have a larger femoral head and neck and smaller neck-shaft angle; these features are associated with reduced hip fracture risk, but also represent an osteoarthritic-like phenotype. Unlike results of previous studies of walking age and bone mass, associations in this study were not affected by adjustment for lean mass, suggesting that associations may relate directly to skeletal loading in early life when joint shape changes rapidly. © 2018 American Society for Bone and Mineral Research.

Associations Between Lean Mass, Muscle Strength and Power, and Skeletal Size, Density and Strength in Older Men

Studies examining the relationship between muscle parameters and bone strength have not included multiple muscle measurements and/or both central and peripheral skeletal parameters. The purpose of this study was to explore the relationship between lean mass, muscle strength and power, and skeletal size, bone density, and bone strength. We studied the association between appendicular lean mass (ALM), grip strength, and leg power, and central quantitative computed tomography (QCT) parameters in 2857 men aged 65 years or older; peripheral QCT was available on a subset (n = 786). ALM, grip strength, and leg power were measured by dual-energy X-ray absorptiometry (DXA), Jamar dynamometer, and the Nottingham Power Rig, respectively. Multivariable models adjusting for potential confounders including age, race, study site, BMI, and muscle measurements were developed and least squares means were generated from linear regression models. For the multivariable model, percent differences of bone parameters between lowest (Q1) and highest quartiles (Q4) of ALM, grip strength, and leg power were reported. ALM was significantly associated with central and peripheral QCT parameters: percent higher values (Q4 versus Q1) ranging from 3.3% (cortical volumetric bone mineral density [vBMD] of the femoral neck) to 31% (vertebral strength index of the spine). Grip strength was only significantly associated with radial parameters: percent higher values (Q4 versus Q1) ranging from 2.5% (periosteal circumference) to 7.5% (33% axial strength index [SSIx]). Leg power was associated with vertebral strength and lower cross-sectional area with percent lower values (Q4 versus Q1) of -11.9% and -2.7%, respectively. In older men, stronger associations were observed for ALM compared to muscle strength and power. Longitudinal studies are needed to examine the relationship between independent changes in muscle measurements and skeletal size, density and strength. © 2018 American Society for Bone and Mineral Research.

Bone Turnover in Young Adult Men: Cross-Sectional Determinants and Associations With Prospectively Assessed Bone Loss

Biochemical markers of bone turnover are higher in young adult men than in middle-aged men or young adult women. Nonetheless, little is known about the determinants and clinical significance hereof. The present study examined determinants of serum bone turnover markers in men around peak bone mass age, and explored whether bone turnover at this age predicts subsequent changes in bone mass. We used cross-sectional and longitudinal data from 973 and 428 healthy men, respectively, aged 25 to 45 years at baseline, including baseline procollagen type I amino-terminal propeptide (P1NP), osteocalcin, and C-terminal telopeptide of type I collagen (CTX) from fasting serum samples, baseline questionnaire-assessed physical activity levels, and baseline and follow-up dual-energy X-ray absorptiometry-derived areal bone mineral density (aBMD) and body composition. Mean follow-up time was 12.4 ± 0.4 years. At baseline, all bone turnover markers were inversely associated with total body fat mass (β ≤ -0.20, p < 0.001), and positively with physical activity during sports activities (β ≥ 0.09, p ≤ 0.003), and, albeit not independently from fat mass, total body lean mass (β ≥ 0.20, p ≤ 0.003). Mean annual aBMD changes in the longitudinal cohort were -0.19% ± 0.24% at the total body, -0.14% ± 0.42% at the spine, -0.49% ± 0.47% at the femoral neck, and -0.25% ± 0.37% at the total hip (all p < 0.001). Higher bone turnover markers at baseline were associated with larger decreases in aBMD at all measurement sites (β ≤ -0.08, p ≤ 0.081 for P1NP; β ≤ -0.16, p ≤ 0.002 for osteocalcin; and β ≤ -0.21, p < 0.001 for CTX). In conclusion, our findings show that sports activities and body composition, primarily fat mass, are the main identified determinants of bone turnover in men around peak bone mass age. Further, bone turnover at this age is an important determinant of subsequent changes in bone mass, with higher levels of bone turnover markers being associated with greater decreases in aBMD. © 2017 American Society for Bone and Mineral Research.

Effect of Two-Year Caloric Restriction on Bone Metabolism and Bone Mineral Density in Non-Obese Younger Adults: A Randomized Clinical Trial

Although caloric restriction (CR) could delay biologic aging in humans, it is unclear if this would occur at the cost of significant bone loss. We evaluated the effect of prolonged CR on bone metabolism and bone mineral density (BMD) in healthy younger adults. Two-hundred eighteen non-obese (body mass index [BMI] 25.1 ± 1.7 kg/m(2) ), younger (age 37.9 ± 7.2 years) adults were randomly assigned to 25% CR (CR group, n = 143) or ad libitum (AL group, n = 75) for 2 years. Main outcomes were BMD and markers of bone turnover. Other outcomes included body composition, bone-active hormones, nutrient intake, and physical activity. Body weight (-7.5 ± 0.4 versus 0.1 ± 0.5 kg), fat mass (-5.3 ± 0.3 versus 0.4 ± 0.4 kg), and fat-free mass (-2.2 ± 0.2 versus -0.2 ± 0.2 kg) decreased in the CR group compared with AL (all between group p < 0.001). Compared with AL, the CR group had greater changes in BMD at 24 months: lumbar spine (-0.013 ± 0.003 versus 0.007 ± 0.004 g/cm(2) ; p < 0.001), total hip (-0.017 ± 0.002 versus 0.001 ± 0.003 g/cm(2) ; p < 0.001), and femoral neck (-0.015 ± 0.003 versus -0.005 ± 0.004 g/cm(2) ; p = 0.03). Changes in bone markers were greater at 12 months for C-telopeptide (0.098 ± 0.012 versus 0.025 ± 0.015 μg/L; p < 0.001), tartrate-resistant acid phosphatase (0.4 ± 0.1 versus 0.2 ± 0.1 U/L; p = 0.004), and bone-specific alkaline phosphatase (BSAP) (-1.4 ± 0.4 versus -0.3 ± 0.5 U/L; p = 0.047) but not procollagen type 1 N-propeptide; at 24 months, only BSAP differed between groups (-1.5 ± 0.4 versus 0.9 ± 0.6 U/L; p = 0.001). The CR group had larger increases in 25-hydroxyvitamin D, cortisol, and adiponectin and decreases in leptin and insulin compared with AL. However, parathyroid hormone and IGF-1 levels did not differ between groups. The CR group also had lower levels of physical activity. Multiple regression analyses revealed that body composition, hormones, nutrients, and physical activity changes explained ∼31% of the variance in BMD and bone marker changes in the CR group. Therefore, bone loss at clinically important sites of osteoporotic fractures represents a potential limitation of prolonged CR for extending life span. Further long-term studies are needed to determine if CR-induced bone loss in healthy adults contributes to fracture risk and if bone loss can be prevented with exercise.

Deletion of connexin43 in osteoblasts/osteocytes leads to impaired muscle formation in mice

It is well-established that muscle forces are necessary for bone development as well as proper bone modeling and remodeling. Recent work has also suggested that bone acts as an endocrine organ that can influence the development of other organs. Connexin43 (Cx43), a gap junction protein that transduces mechanical signals, is an important determinant of cortical bone modeling. Using an osteoblast/osteocyte-specific ablation of the Cx43 gene (Gja1) driven by the 2.3-kb Col1 α1 promoter (cKO) in the mouse, in this study we confirmed reduced cortical bone thickness and density with expanded bone marrow cavity in the cKO humerus. Surprisingly, Gja1 deletion in bone cells also affected skeletal muscle development, resulting in lower fast muscle weight, grip strength, and maximum absolute and specific tetanic forces (60% to 80%, 85%, and 50%, respectively, of WT mice). The normally fast twitch extensor digitorum longus (EDL) muscle exhibited increased slow twitch fibers in cKO mice. These muscle defects were accompanied by a 40% to 60% reduction in mRNA abundance for genes encoding osteocalcin in the humerus, relative to WT mice. Accordingly, both carboxylated and undercarboxylated isoforms of osteocalcin were reduced by over 30% in the circulation of cKO mice. Moreover, the active, undercarboxylated isoform of osteocalcin (glu-OC) promoted myotube formation in C2C12 myoblast cultures, and glu-OC injections to cKO mice rescued EDL muscle cross-sectional area and grip strength in vivo. These findings demonstrate that Cx43 in osteoblasts/osteocytes indirectly modulates skeletal muscle growth and function, potentially via an endocrine effect of glu-OC.

METTL21C is a potential pleiotropic gene for osteoporosis and sarcopenia acting through the modulation of the NF-κB signaling pathway

Sarcopenia and osteoporosis are important public health problems that occur concurrently. A bivariate genome-wide association study (GWAS) identified METTL21c as a suggestive pleiotropic gene for both bone and muscle. The METTL21 family of proteins methylates chaperones involved in the etiology of both myopathy and inclusion body myositis with Paget's disease. To validate these GWAS results, Mettl21c mRNA expression was reduced with siRNA in a mouse myogenic C2C12 cell line and the mouse osteocyte-like cell line MLO-Y4. At day 3, as C2C12 myoblasts start to differentiate into myotubes, a significant reduction in the number of myocytes aligning/organizing for fusion was observed in the siRNA-treated cells. At day 5, both fewer and smaller myotubes were observed in the siRNA-treated cells as confirmed by histomorphometric analyses and immunostaining with myosin heavy chain (MHC) antibody, which only stains myocytes/myotubes but not myoblasts. Intracellular calcium (Ca(2+)) measurements of the siRNA-treated myotubes showed a decrease in maximal amplitude peak response to caffeine, suggesting that less Ca(2+) is available for release due to the partial silencing of Mettl21c, correlating with impaired myogenesis. In siRNA-treated MLO-Y4 cells, 48 hours after treatment with dexamethasone there was a significant increase in cell death, suggesting a role of Mettl21c in osteocyte survival. To investigate the molecular signaling machinery induced by the partial silencing of Mettl21c, we used a real-time PCR gene array to monitor the activity of 10 signaling pathways. We discovered that Mettl21c knockdown modulated only the NF-κB signaling pathway (ie, Birc3, Ccl5, and Tnf). These results suggest that Mettl21c might exert its bone-muscle pleiotropic function via the regulation of the NF-κB signaling pathway, which is critical for bone and muscle homeostasis. These studies also provide rationale for cellular and molecular validation of GWAS, and warrant additional in vitro and in vivo studies to advance our understanding of role of METTL21C in musculoskeletal biology.