Muscle and bone, two interconnected tissues

The Importance of Biophysical and Biochemical Stimuli in Dynamic Skeletal Muscle Models

Classical approaches to engineer skeletal muscle tissue based on current regenerative and surgical procedures still do not meet the desired outcome for patient applications. Besides the evident need to create functional skeletal muscle tissue for the repair of volumetric muscle defects, there is also growing demand for platforms to study muscle-related diseases, such as muscular dystrophies or sarcopenia. Currently, numerous studies exist that have employed a variety of biomaterials, cell types and strategies for maturation of skeletal muscle tissue in 2D and 3D environments. However, researchers are just at the beginning of understanding the impact of different culture settings and their biochemical (growth factors and chemical changes) and biophysical cues (mechanical properties) on myogenesis. With this review we intend to emphasize the need for new in vitro skeletal muscle (disease) models to better recapitulate important structural and functional aspects of muscle development. We highlight the importance of choosing appropriate system components, e.g., cell and biomaterial type, structural and mechanical matrix properties or culture format, and how understanding their interplay will enable researchers to create optimized platforms to investigate myogenesis in healthy and diseased tissue. Thus, we aim to deliver guidelines for experimental designs to allow estimation of the potential influence of the selected skeletal muscle tissue engineering setup on the myogenic outcome prior to their implementation. Moreover, we offer a workflow to facilitate identifying and selecting different analytical tools to demonstrate the successful creation of functional skeletal muscle tissue. Ultimately, a refinement of existing strategies will lead to further progression in understanding important aspects of muscle diseases, muscle aging and muscle regeneration to improve quality of life of patients and enable the establishment of new treatment options.

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.

Current Status of Sarcopenia in Korea: A Focus on Korean Geripausal Women

Sarcopenia is defined as an age-associated decline in muscle mass and function caused by several etiologies and mechanisms. Muscle mass and function do not decrease concurrently, and a loss of muscle function may be more highly associated with adverse health outcomes. Despite the clinical significance of sarcopenia, no universally operational definition of sarcopenia or standardized intervention programs are currently available. Sarcopenia, osteoporosis, and obesity share several pathophysiological mechanisms, and a combination of these entities may lead to an increased risk of musculoskeletal, cardio-metabolic, and psychological morbidities especially in geripause populations. Treatment for sarcopenia is mainly nonpharmacological, however, various drugs are currently being developed. It is conceivable that sarcopenia is the next immediate clinical target in musculoskeletal science.

Prolonged high force high repetition pulling induces osteocyte apoptosis and trabecular bone loss in distal radius, while low force high repetition pulling induces bone anabolism

We have an operant rat model of upper extremity reaching and grasping in which we examined the impact of performing a high force high repetition (High-ForceHR) versus a low force low repetition (Low-ForceHR) task for 18weeks on the radius and ulna, compared to age-matched controls. High-ForceHR rats performed at 4 reaches/min and 50% of their maximum voluntary pulling force for 2h/day, 3days/week. Low-ForceHR rats performed at 6% maximum voluntary pulling force. High-ForceHR rats showed decreased trabecular bone volume in the distal metaphyseal radius, decreased anabolic indices in this same bone region (e.g., decreased osteoblasts and bone formation rate), and increased catabolic indices (e.g., microcracks, increased osteocyte apoptosis, secreted sclerostin, RANKL, and osteoclast numbers), compared to controls. Distal metaphyseal trabeculae in the ulna of High-ForceHR rats showed a non-significant decrease in bone volume, some catabolic indices (e.g., decreased trabecular numbers) yet also some anabolic indices (e.g., increased osteoblasts and trabecular thickness). In contrast, the mid-diaphyseal region of High-ForceHR rats' radial and ulnar bones showed few to no microarchitecture differences and no changes in apoptosis, sclerostin or RANKL levels, compared to controls. In further contrast, Low-ForceHR rats showed increased trabecular bone volume in the radius in the distal metaphysis and increased cortical bone area its mid-diaphysis. These changes were accompanied by increased anabolic indices, no microcracks or osteocyte apoptosis, and decreased RANKL in each region, compared to controls. Ulnar bones of Low-ForceHR rats also showed increased anabolic indices, although fewer than in the adjacent radius. Thus, prolonged performance of an upper extremity reaching and grasping task is loading-, region-, and bone-dependent, with high force loads at high repetition rates inducing region-specific increases in bone degradative changes that were most prominent in distal radial trabeculae, while low force task loads at high repetition rates induced adaptive bone responses.

Bone health assessment in older people with or without muscle health impairment

This study investigated the relationship between muscle and bone status in elderly individuals. Our results suggested links between sarcopenia and osteoporosis; impairment in muscle status (i.e., muscle mass, muscle strength, and physical performance) is associated with deterioration in bone mass and texture subsequently leading to an increased risk of fracture.

INTRODUCTION

Accumulating evidence has shown associations between sarcopenia and osteoporosis, but existing studies face inconsistencies in the clinical definition of both conditions. Thus, we sought to investigate bone health among older individuals with or without muscle health impairment.

METHODS

We conducted an analysis of cross-sectional data available from the Sarcopenia and Physical Impairment with Advancing Age (SarcoPhAge) study. Sarcopenia was diagnosed according to the European Working Group on Sarcopenia in Older People (EWGSOP) (i.e., a low muscle mass plus either low muscle strength or low physical performance). Muscle mass and areal bone mineral density (aBMD) were determined using dual-energy X-ray absorptiometry (DEXA). Muscle strength was assessed using a hand dynamometer, and physical performance was assessed with the Short Physical Performance Battery test. Using the cutoff limits proposed by the EWGSOP, we have classified women in the "low SMI group" when its value was < 5.50 kg/m², in the "low muscle strength group" when strength was < 20 kg, and in the "low physical performance group" when SPPB < 8 points. The thresholds of < 7.26 kg/m² (for SMI), < 30 kg (for muscle strength), and SPPB < 8 points were used for men. The 10-year fracture risk was obtained using the FRAX® tool. Moreover, bone texture was determined using the trabecular bone score (TBS) method.

RESULTS

The study sample consisted of 288 older subjects aged 74.7 ± 5.7 years, and 59.0% of the subjects were women. Sarcopenia was diagnosed in 43 individuals (14.9%), and osteoporosis was diagnosed in 36 subjects (12.5%). Moreover, aBMD values were, most of the time, lower in older men and women with muscle impairment (i.e., low muscle mass, low muscle strength, and low physical performance). For these subjects, we also noted a higher probability of fracture. When comparing bone quality, there were no significant differences in the TBS values between sarcopenic and non-sarcopenic older men and women or between those with low and high muscle mass. However, when controlling for confounders (i.e., age, BMI, number of co-morbidities, smoking status, and nutritional status), TBS values were lower in older women with low muscle strength (p = 0.04) and in older men with low physical performance (p = 0.01).

CONCLUSIONS

Our study showed interrelationships between components of sarcopenia and osteoporosis, with older subjects with muscle impairment having poorer bone health.

FAM210A is a novel determinant of bone and muscle structure and strength

Osteoporosis and sarcopenia are common comorbid diseases, yet their shared mechanisms are largely unknown. We found that genetic variation near FAM210A was associated, through large genome-wide association studies, with fracture, bone mineral density (BMD), and appendicular and whole body lean mass, in humans. In mice, Fam210a was expressed in muscle mitochondria and cytoplasm, as well as in heart and brain, but not in bone. Grip strength and limb lean mass were reduced in tamoxifen-inducible Fam210a homozygous global knockout mice (TFam210a^-/- ), and in tamoxifen-inducible Fam210 skeletal muscle cell-specific knockout mice (TFam210aMus^-/- ). Decreased BMD, bone biomechanical strength, and bone formation, and elevated osteoclast activity with microarchitectural deterioration of trabecular and cortical bones, were observed in TFam210a^-/- mice. BMD of male TFam210aMus^-/- mice was also reduced, and osteoclast numbers and surface in TFam210aMus^-/- mice increased. Microarray analysis of muscle cells from TFam210aMus^-/- mice identified candidate musculoskeletal modulators. FAM210A, a novel gene, therefore has a crucial role in regulating bone structure and function, and may impact osteoporosis through a biological pathway involving muscle as well as through other mechanisms.

Steroid-associated osteonecrosis animal model in rats

OBJECTIVE

Established preclinical disease models are essential for not only studying aetiology and/or pathophysiology of the relevant diseases but more importantly also for testing prevention and/or treatment concept(s). The present study proposed and established a detailed induction and assessment protocol for a unique and cost-effective preclinical steroid-associated osteonecrosis (SAON) in rats with pulsed injections of lipopolysaccharide (LPS) and methylprednisolone (MPS).

METHODS

Sixteen 24-week-old male Sprague-Dawley rats were used to induce SAON by one intravenous injection of LPS (0.2 mg/kg) and three intraperitoneal injections of MPS (100 mg/kg) with a time interval of 24 hour, and then, MPS (40 mg/kg) was intraperitoneally injected three times a week from week 2 until sacrifice. Additional 12 rats were used as normal controls. Two and six weeks after induction, animals were scanned by metabolic dual energy X-ray absorptiometry for evaluation of tissue composition; serum was collected for bone turnover markers, Microfil perfusion was performed for angiography, the liver was collected for histopathology and bilateral femora and bilateral tibiae were collected for histological examination.

RESULTS

Three rats died after LPS injection, i.e., with 15.8% (3/19) mortality. Histological evaluation showed 100% incidence of SAON at week 2. Dual energy X-ray absorptiometry showed significantly higher fat percent and lower lean mass in SAON group at week 6. Micro-computed tomography (Micro-CT) showed significant bone degradation at proximal tibia 6 weeks after SAON induction. Angiography illustrated significantly less blood vessels in the proximal tibia and significantly more leakage particles in the distal tibia 2 weeks after SAON induction. Serum amino-terminal propeptide of type I collagen and osteocalcin were significantly lower at both 2 and 6 weeks after SAON induction, and serum carboxy-terminal telopeptide was significantly lower at 6 weeks after SAON induction. Histomorphometry revealed significantly lower osteoblast surface and higher marrow fat fraction and oedema area in SAON group. Hepatic oedema appeared 2 weeks after SAON induction, and lipid accumulation appeared in the liver of SAON rats 6 weeks after SAON induction.

CONCLUSION

The present study successfully induced SAON in rats with pulsed injection of LPS and MPS, which was well simulating the clinical feature and pathology. Apart from available large animal models, such as bipedal emus or quadrupedal rabbits, our current SAON small model in rats could be a cost-effective preclinical experimental model to study body metabolism, molecular mechanism of SAON and potential drugs developed for prevention or treatment of SAON.

THE TRANSLATIONAL POTENTIAL OF THIS ARTICLE

The present study successfully induced SAON in a small animal model in rats with pulsed injection of LPS and MPS. The evaluation protocols with typical histopathologic ON features and advanced evaluation approaches to identify the metabolic disorders of SAON could be used in future rat SAON studies. The SAON rat model is a suitable and cost-effective animal model to study molecular mechanism of SAON and potential drugs developed for prevention and treatment of SAON.

Sarcopenic osteoarthritis: a new entity in geriatric medicine?

PURPOSE

Osteoarthritis, a disease characterized by cartilage degradation, abnormal subchondral bone remodeling and some grade of inflammation, and sarcopenia, a condition of low muscle mass associated with reduced strength and function, are prevalent disorders in older adults. In this review, we examine what is known about the relationship between osteoarthritis and sarcopenia, with particular focus on the older population. We also discuss how osteoarthritis and sarcopenia may interact and affect each other in clinical progression and the potential benefits from developing treatments that address such muscular-skeletal interaction.

METHODS

We searched in Pubmed and Scopus through a combination of search and MESH terms, for osteoarthritis and sarcopenia.

RESULTS

Even if more literature is needed, there is increasing evidence that decline in lower limb muscle strength is associated with knee or hip osteoarthritis in a pathological network of pain, altered joint stability, maladapted postures and defective neuromuscular communication. At the cellular levels, chondrocytes and myoblasts share common pathways, and the close anatomical location of both cell types also suggest the possibility of paracrine communication.

CONCLUSIONS

Sarcopenia and osteoarthritis are significantly intercorrelated and in the near future should be considered as an only entity, as we have recently proposed for sarcopenia and osteoporosis. The treatment of both sarcopenia and osteoarthritis is based on physical exercise and nutritional interventions with the aim of improving cartilage, bone and muscle health. Future studies are needed, particularly to know the exact prevalence of sarcopenia in people with osteoarthritis, its peculiar consequences and the most appropriate treatments.

Effects of pharmacologic sclerostin inhibition or testosterone administration on soleus muscle atrophy in rodents after spinal cord injury

Sclerostin is a circulating osteocyte-derived glycoprotein that negatively regulates Wnt-signaling after binding the LRP5/LRP6 co-receptors. Pharmacologic sclerostin inhibition produces bone anabolic effects after spinal cord injury (SCI), however, the effects of sclerostin-antibody (Scl-Ab) on muscle morphology remain unknown. In comparison, androgen administration produces bone antiresorptive effects after SCI and some, but not all, studies have reported that testosterone treatment ameliorates skeletal muscle atrophy in this context. Our purposes were to determine whether Scl-Ab prevents hindlimb muscle loss after SCI and compare the effects of Scl-Ab to testosterone enanthate (TE), an agent with known myotrophic effects. Male Sprague-Dawley rats aged 5 months received: (A) SHAM surgery (T8 laminectomy), (B) moderate-severe contusion SCI, (C) SCI+TE (7.0 mg/wk, im), or (D) SCI+Scl-Ab (25 mg/kg, twice weekly, sc). Twenty-one days post-injury, SCI animals exhibited a 31% lower soleus mass in comparison to SHAM, accompanied by >50% lower soleus muscle fiber cross-sectional area (fCSA) (p<0.01 for all fiber types). Scl-Ab did not prevent soleus atrophy, consistent with the relatively low circulating sclerostin concentrations and with the 91-99% lower LRP5/LRP6 gene expressions in soleus versus tibia (p<0.001), a tissue with known anabolic responsiveness to Scl-Ab. In comparison, TE partially prevented soleus atrophy and increased levator ani/bulbocavernosus (LABC) mass by 30-40% (p<0.001 vs all groups). The differing myotrophic responsiveness coincided with a 3-fold higher androgen receptor gene expression in LABC versus soleus (p<0.01). This study provides the first direct evidence that Scl-Ab does not prevent soleus muscle atrophy in rodents after SCI and suggests that variable myotrophic responses in rodent muscles after androgen administration are influenced by androgen receptor expression.

Comparison of muscle/lean mass measurement methods: correlation with functional and biochemical testing

DXA-measured lean mass is often used to assess muscle mass but has limitations. Thus, we compared DXA lean mass with two novel methods-bioelectric impedance spectroscopy and creatine (methyl-d3) dilution. The examined methodologies did not measure lean mass similarly and the correlation with muscle biomarkers/function varied.

INTRODUCTION

Muscle function tests predict adverse health outcomes better than lean mass measurement. This may reflect limitations of current mass measurement methods. Newer approaches, e.g., bioelectric impedance spectroscopy (BIS) and creatine (methyl-d3) dilution (D3-C), may more accurately assess muscle mass. We hypothesized that BIS and D3-C measured muscle mass would better correlate with function and bone/muscle biomarkers than DXA measured lean mass.

METHODS

Evaluations of muscle/lean mass, function, and serum biomarkers were obtained in older community-dwelling adults. Mass was assessed by DXA, BIS, and orally administered D3-C. Grip strength, timed up and go, and jump power were examined. Potential muscle/bone serum biomarkers were measured. Mass measurements were compared with functional and serum data using regression analyses; differences between techniques were determined by paired t tests.

RESULTS

Mean (SD) age of the 112 (89F/23M) participants was 80.6 (6.0) years. The lean/muscle mass assessments were correlated (.57-.88) but differed (p < 0.0001) from one another with DXA total body less head being highest at 37.8 (7.3) kg, D3-C muscle mass at 21.1 (4.6) kg, and BIS total body intracellular water at 17.4 (3.5) kg. All mass assessment methods correlated with grip strength and jump power (R = 0.35-0.63, p < 0.0002), but not with gait speed or repeat chair rise. Lean mass measures were unrelated to the serum biomarkers measured.

CONCLUSIONS

These three methodologies do not similarly measure muscle/lean mass and should not be viewed as being equivalent. Functional tests assessing maximal muscle strength/power (grip strength and jump power) correlated with all mass measures whereas gait speed was not. None of the selected serum measures correlated with mass. Efforts to optimize muscle mass assessment and identify their relationships with health outcomes are needed.

Quantitative imaging techniques for the assessment of osteoporosis and sarcopenia

Bone and muscle are two deeply interconnected organs and a strong relationship between them exists in their development and maintenance. The peak of both bone and muscle mass is achieved in early adulthood, followed by a progressive decline after the age of 40. The increase in life expectancy in developed countries resulted in an increase of degenerative diseases affecting the musculoskeletal system. Osteoporosis and sarcopenia represent a major cause of morbidity and mortality in the elderly population and are associated with a significant increase in healthcare costs. Several imaging techniques are currently available for the non-invasive investigation of bone and muscle mass and quality. Conventional radiology, dual energy X-ray absorptiometry (DXA), computed tomography (CT), magnetic resonance imaging (MRI) and ultrasound often play a complementary role in the study of osteoporosis and sarcopenia, depicting different aspects of the same pathology. This paper presents the different imaging modalities currently used for the investigation of bone and muscle mass and quality in osteoporosis and sarcopenia with special emphasis on the clinical applications and limitations of each technique and with the intent to provide interesting insights into recent advances in the field of conventional imaging, novel high-resolution techniques and fracture risk.

The Expanding Life and Functions of Osteogenic Cells: From Simple Bone-Making Cells to Multifunctional Cells and Beyond

During the last three decades, important progress in bone cell biology and in human and mouse genetics led to major advances in our understanding of the life and functions of cells of the osteoblast lineage. Previously unrecognized sources of osteogenic cells have been identified. Novel cellular and molecular mechanisms controlling osteoblast differentiation and senescence have been determined. New mechanisms of communications between osteogenic cells, osteocytes, osteoclasts, and chondrocytes, as well as novel links between osteogenic cells and blood vessels have been identified. Additionally, cells of the osteoblast lineage were shown to be important components of the hematopoietic niche and to be implicated in hematologic dysfunctions and malignancy. Lastly, unexpected interactions were found between osteogenic cells and several soft tissues, including the central nervous system, gut, muscle, fat, and testis through the release of paracrine factors, making osteogenic cells multifunctional regulatory cells, in addition to their bone-making function. These discoveries considerably enlarged our vision of the life and functions of osteogenic cells, which may lead to the development of novel therapeutics with immediate applications in bone disorders. © 2017 American Society for Bone and Mineral Research.

Relationship between the Mediterranean dietary pattern and musculoskeletal health in children, adolescents, and adults: systematic review and evidence map

Context: An understanding of the modifiable effects of diet on bone and skeletal muscle mass and strength over the life course will help inform strategies to reduce age-related fracture risk. The Mediterranean diet is rich in nutrients that may be important for optimal musculoskeletal health. Objective: The aim of this systematic review was to investigate the relationship between a Mediterranean diet and musculoskeletal outcomes (fracture, bone density, osteoporosis, sarcopenia) in any age group. Data Sources: Ten electronic databases were searched. Study Selection: Randomized controlled trials and prospective cohort studies that investigated a traditional Mediterranean diet, published in any language, were eligible. Studies using other designs or other definitions of the Mediterranean diet were collated separately in an evidence map. Data Extraction: Details on study design, methods, population, dietary intervention or exposure, length of follow-up, and effect on or association with musculoskeletal outcomes were extracted. Results: The search yielded 1738 references. Data from eligible randomized controlled trials (n = 0) and prospective cohort studies (n = 3) were synthesized narratively by outcome for the systematic review. Two of these studies reported on hip fracture incidence, but results were contradictory. A third study found no association between the Mediterranean diet and sarcopenia incidence. Conclusions: Overall, the systematic review and evidence map demonstrate a lack of research to understand the relationship between the Mediterranean diet and musculoskeletal health in all ages. Systematic Review Registration: PROSPERO registration number IDCRD42016037038.

Impact of Futsal and Swimming Participation on Bone Health in Young Athletes

Physical activity plays a crucial role in bone mass acquisition during childhood and adolescence, with weightbearing and high-impact sport activities being more beneficial. This study sought to evaluate the impact of different sports activities on bone mineral density and content in male Portuguese athletes. Seventy adolescent boys (aged 12-15 years) including 28 futsal players (FG), 20 swimmers (SG) and 22 non-athletic adolescents used as control subjects (CG), participated in the current study. Areal bone mineral density (aBMD) and areal bone mineral content (aBMC) were measured by dual energy x-ray absorptiometry (DEXA). Futsal players had significantly higher aBMD (lumbar spine - FG: 0.95 ± 0.18, SG: 0.80 ± 0.13, CG: 0.79 ± 0.13 g/cm2, p = 0.001; pelvis - FG: 1.17 ± 0.21, SG: 0.91 ± 0.12, CG: 0.98 ± 0.10 g/cm2, p < 0.001; lower limbs - FG: 1.21 ± 0.19, SG: 0.97 ± 0.10, CG: 0.99 ± 0.09 g/cm2, p < 0.001) and aBMC (lumbar spine - FG: 51.07 ± 16.53, SG: 40.19 ± 12.47, CG: 40.50 ± 10.53 g, p = 0.013; pelvis - FG: 299.5 ± 110.61, SG: 170.02 ± 55.82, CG: 183.11 ± 46.78 g, p < 0.001; lower limbs - FG: 427.21 ± 117.11, SG: 300.13 ± 76.42, CG: 312.26 ± 61.86 g/cm2, p < 0.001) than swimmers and control subjects. Data suggest that futsal, as a weightbearing and high or odd-impact sport, may improve bone mass during childhood and adolescence.

Physical Activity, Physical Fitness, Body Composition, and Nutrition Are Associated with Bone Status in University Students

Understanding the modifiable factors that improve and maximize peak bone mass at an early age is necessary to design more effective intervention programs to prevent osteoporosis. To identify these modifiable factors, we analyzed the relationship of physical activity (PA), physical fitness, body composition, and dietary intake with bone stiffness index (SI), measured by quantitative ultrasonometry in young university students (18–21 years). Moderate-to-vigorous PA (MVPA) was the strongest predictor of SI (β = 0.184; p = 0.035). SI was most closely related with very vigorous PA in males (β = 0.288; p = 0.040) and with the number of steps/day in females (β = 0.319; p = 0.002). An association between thigh muscle and SI was consistent in both sexes (β = 0.328; p < 0.001). Additionally, extension maximal force was a bone SI predictor factor in females (β = 0.263; p = 0.016) independent of thigh muscle perimeter. Calcium intake was the only nutrition parameter that had a positive relationship with SI (R = 0.217; p = 0.022). However, it was not included as a predictor for SI in our regression models. This study identifies predictors of bone status in each sex and indicates that muscle and bone interrelate with PA and fitness in young adults.

The risks of sarcopenia, falls and fractures in patients with type 2 diabetes mellitus

Fracture risk in patients with type 2 diabetes mellitus (T2DM) is increased, and the mechanism is multifactorial. Recent research on T2DM-induced bone fragility shows that bone mineral density (BMD) is often normal or even slightly elevated. However, bone turnover may be decreased and bone material and microstructural properties are altered, especially when microvascular complications are present. Besides bone fragility, extra-skeletal factors leading to an increased propensity to experience falls may also contribute to the increased fracture risk in T2DM, such as peripheral neuropathy, retinopathy and diabetes medication (e.g. insulin use). One of the probable additional contributing factors to the increased fall and fracture risks in T2DM is sarcopenia, the age-related decline in skeletal muscle mass, quality and function. Although the association between sarcopenia, fall risk, and fracture risk has been studied in the general population, few studies have examined the association between T2DM and muscle tissue and the risks of falls and fractures. This narrative review provides an overview of the literature regarding the multifactorial mechanisms leading to increased fracture risk in patients with T2DM, with a focus on sarcopenia and falls.

A review of sarcopenia: Enhancing awareness of an increasingly prevalent disease

Sarcopenia is defined as an age associated decline in skeletal muscle mass. The pathophysiology of sarcopenia is multifactorial, with decreased caloric intake, muscle fiber denervation, intracellular oxidative stress, hormonal decline, and enhanced myostatin signaling all thought to contribute. Prevalence rates are as high as 29% and 33% in elderly community dwelling and long-term care populations, respectively, with advanced age, low body mass index, and low physical activity as significant risk factors. Sarcopenia shares many characteristics with other disease states typically associated with risk of fall and fracture, including osteoporosis, frailty, and obesity. There is no current universally accepted definition of sarcopenia. Diagnosing sarcopenia with contemporary operational definitions requires assessments of muscle mass, muscle strength, and physical performance. Screening is recommended for both elderly patients and those with conditions that noticeably reduce physical function. Sarcopenia is highly prevalent in orthopedic patient populations and correlates with higher hospital costs and rates of falling, fracture, and mortality. As no muscle building agents are currently approved in the United States, resistance training and nutritional supplementation are the primary methods for treating sarcopenia. Trials with various agents, including selective androgen receptor modulators and myostatin inhibitors, show promise as future treatment options. Increased awareness of sarcopenia is of great importance to begin reaching consensus on diagnosis and to contribute to finding a cure for this condition.

Association between low lean mass and low bone mineral density in 653 women with hip fracture: does the definition of low lean mass matter?

BACKGROUND AND AIMS

Loss of both muscle and bone mass results in fragility fractures with increased risk of disability, poor quality of life, and death. Our aim was to assess the association between low appendicular lean mass (aLM) defined according to different criteria and low bone mineral density (BMD) in hip-fracture women.

METHODS

Six hundred fifty-three women admitted to our rehabilitation hospital underwent dual energy X-ray absorptiometry 19.1 ± 4.1 (mean ± SD) days after hip-fracture occurrence. Low aLM was identified according to either Baumgartner's definition (aLM/height² less than two standard deviations below the mean of the young reference group) or FNIH criteria: aLM <15.02 kg, or aLM adjusted for body mass index (BMI) <0.512. Low BMD was diagnosed with a T-score <-2.5 at the unfractured femoral neck.

RESULTS

Using Baumgartner's definition, the association between low aLM/height² and low BMD was significant: χ ²(1, n = 653) = 8.52 (p = 0.004), but it was erased by adjustments for age and fat mass. Using the FNIH definition the association between low aLM and low BMD was significant: χ ²(1, n = 653) = 42.5 (p < 0.001), and it was confirmed after adjustment for age and fat mass (p < 0.001). With the FNIH definition based on aLM/BMI ratio the association between low aLM/BMI ratio and low BMD was nonsignificant: χ ²(1, n = 653) = 0.003 (p = 0.957).

CONCLUSIONS

The association between low aLM and low BMD in women with hip fracture dramatically depends on the adopted definition of low aLM. FNIH threshold for aLM (<15.02 kg) emerges as a useful tool to capture women with damage of the muscle-bone unit.

Low bone mineral density in middle-aged women: a red flag for sarcopenia

OBJECTIVE

This study evaluated whether low bone density, a condition related to aging, is associated with low muscle mass, a surrogate for sarcopenia, and whether it could be used as a marker of the condition.

METHODS

We studied 483 women aged 35 to 69 years old who appeared healthy and attended a preventive gynecological examination. Dual-energy X-ray absorptiometry was used to measure bone mineral density (BMD) and regional body composition. BMD was assessed using the T-score. Low appendicular lean mass (aLM) adjusted by height (aLM index) was defined according to Baumgartner et al (<5.45 kg/m). The association of low aLM index with bone mass was evaluated with a binary logistic regression using a cutoff point on the receiver operating characteristic curves for the T-score of -1.5.

RESULTS

The participants had a mean age of 54.7 ± 9.1 years, body mass index of 24.6 ± 3.6 kg/m, aLM index of 5.9 ± 0.6 kg/m (22.6% showed sarcopenia), abdominal fat percentage of 44.0 ± 9.1%, and T-score of -0.48 ± 0.97. In the logistic regression model, we found that low BMD implied a significant risk for sarcopenia (odds ratio [OR] 1.77; 95% CI, 1.02-3.06). In contrast, excess body weight was a protective factor (OR 0.12; 95% CI, 0.06-0.25). Neither age nor abdominal fat percentage, however, influenced the likelihood of sarcopenia in these women.

CONCLUSIONS

A BMD T-score below -1.5 suggests low muscle mass in middle-aged women, which is a central element in the diagnosis of sarcopenia. Early diagnosis provides the opportunity to introduce preventive and therapeutic options.

Vitamin D and osteosarcopenia: an update from epidemiological studies

PURPOSE OF REVIEW

The review summarizes recent epidemiological studies that examined the relationship between osteoporosis and sarcopenia to assess the impact of vitamin D status or supplementation on health outcomes related to these two medical conditions.

RECENT FINDINGS

Osteoporosis and sarcopenia are major public health problems, but whether these two diseases should be considered alone or combined into a single condition is not clear. No consensual definition of osteosarcopenia is largely accepted. Most observational studies demonstrate some relationship between muscle and bone health. Vitamin D status is generally lower in study participants with bone or muscle wasting. Studies on the effects of vitamin D supplementation on muscle or bone health have provided conflicting results, likely because of the heterogeneity between studies. However, the most positive results were observed in study participants with low vitamin D status and in studies that avoided massive boluses of vitamin D.

SUMMARY

More observational and interventional studies are needed to confirm the exact role of vitamin D in the pathophysiology and treatment of osteosarcopenia.

Dietary Magnesium May Be Protective for Aging of Bone and Skeletal Muscle in Middle and Younger Older Age Men and Women: Cross-Sectional Findings from the UK Biobank Cohort

Although fragility fractures, osteoporosis, sarcopenia, and frailty are becoming more prevalent in our aging society the treatment options are limited and preventative strategies are needed. Despite magnesium being integral to bone and muscle physiology, the relationship between dietary magnesium and skeletal muscle and bone health has not been investigated concurrently to date. We analysed cross-sectional associations between dietary magnesium and skeletal muscle mass (as fat free mass-FFM), grip strength, and bone density (BMD) in 156,575 men and women aged 39-72 years from the UK Biobank cohort. FFM was measured with bioelectrical impedance and was expressed as the percentage of body weight (FFM%) or as divided by body mass index (FFMBMI). Adjusted mean grip strength, FFM%, FFMBMI, and BMD were calculated according to quintiles of dietary magnesium, while correcting for covariates. Significant inter-quintile differences across intakes of magnesium existed in men and women, respectively, of 1.1% and 2.4% for grip strength, 3.0% and 3.6% for FFM%, 5.1% and 5.5% for FFMBMI, and 2.9% and 0.9% for BMD. These associations are as great or greater than annual measured losses of these musculoskeletal outcomes, indicating potential clinical significance. Our study suggests that dietary magnesium may play a role in musculoskeletal health and has relevance for population prevention strategies for sarcopenia, osteoporosis, and fractures.

Osteosarcopenia: where bone, muscle, and fat collide

As the world's population ages, the prevalence of chronic diseases increases. Sarcopenia and osteoporosis are two conditions that are associated with aging, with similar risk factors that include genetics, endocrine function, and mechanical factors. Additionally, bone and muscle closely interact with each other not only anatomically, but also chemically and metabolically. Fat infiltration, a phenomenon observed in age-related bone and muscle loss, is highly prevalent and more severe in sarcopenic and osteoporotic subjects. Clinically, when individuals suffer a combination of both disorders, negative outcomes such as falls, fractures, loss of function, frailty, and mortality increase, thus generating significant personal and socio-economic costs. Therefore, it is suggested that when bone mineral density loss is synchronic with decreased muscle mass, strength, and function, it should be interpreted as a single diagnosis of osteosarcopenia, which may be preventable and treatable. Simple interventions such as resistance training, adequate protein and calcium dietary intake, associated with maintenance of appropriate levels of vitamin D, have a dual positive effect on bone and muscle, reducing falls, fractures, and, consequently, disability. It is essential that fracture prevention approaches-including postfracture management-involve assessment and treatment of both osteoporosis and sarcopenia. This is of particular importance as in older persons the combination of osteopenia/osteoporosis and sarcopenia has been proposed as a subset of frailer individuals at higher risk of institutionalization, falls, and fractures. This review summarizes osteosarcopenia epidemiology, pathophysiology, diagnosis, outcomes, and management strategies.

Dysmobility Syndrome and Risk of Mortality for Community-Dwelling Middle-Aged and Older Adults: The Nexus of Aging and Body Composition

Dysmobility syndrome is a newly proposed concept to comprehensively consider bone-muscle-adiposity as a whole to associate with mortality and other adverse outcomes in the older adults. Little was known in Asian populations since the body composition was highly related to ethnicity. The study aimed to evaluate the association between dysmobility syndrome and mortality and to explore the most optimal operational definition for dysmobility syndrome. The prevalence of dysmobility syndrome was 3.9–10.1% based on different operational definitions of adiposity and skeletal muscle index. Subjects with dysmobility syndrome were older, more often to be women, having higher adiposity, lower lean body mass and bone mineral density. Multivariate Cox proportional hazard model showed that dysmobility and pre-dysmobility syndrome had higher risk of mortality than the robust group (Hazard ratio (HR): 11.3, 95% confidence interval (CI): 1.2–109.1; and HR 8.7, 95% CI 1.1-67.3, respectively). Overall, the modified operational definition of dysmobility syndrome in Asian populations using FNIH-adjusted skeletal muscle mass and waist circumference-defined adiposity may be the most optimal model for mortality prediction. Taking the nexus of body composition as a whole to evaluate the mortality risk of older adults is an important improvement beyond sarcopenia and osteoporosis.

Osteoporosis in Frail Patients: A Consensus Paper of the Belgian Bone Club

In this consensus paper, the Belgian Bone Club aims to provide a state of the art on the epidemiology, diagnosis, and management of osteoporosis in frail individuals, including patients with anorexia nervosa, patients on dialysis, cancer patients, persons with sarcopenia, and the oldest old. All these conditions may indeed induce bone loss that is superimposed on physiological bone loss and often remains under-recognized and under-treated. This is of particular concern because of the major burden of osteoporotic fractures in terms of morbidity, mortality, and economic cost. Therefore, there is an urgent need to appreciate bone loss associated with these conditions, as this may improve diagnosis and management of bone loss and fracture risk in clinical practice.

Measuring myokines with cardiovascular functions: pre-analytical variables affecting the analytical output

In the last few years, a growing number of molecules have been associated to an endocrine function of the skeletal muscle. Circulating myokine levels, in turn, have been associated with several pathophysiological conditions including the cardiovascular ones. However, data from different studies are often not completely comparable or even discordant. This would be due, at least in part, to the whole set of situations related to the preparation of the patient prior to blood sampling, blood sampling procedure, processing and/or store. This entire process constitutes the pre-analytical phase. The importance of the pre-analytical phase is often not considered. However, in routine diagnostics, the 70% of the errors are in this phase. Moreover, errors during the pre-analytical phase are carried over in the analytical phase and affects the final output. In research, for example, when samples are collected over a long time and by different laboratories, a standardized procedure for sample collecting and the correct procedure for sample storage are acknowledged. In this review, we discuss the pre-analytical variables potentially affecting the measurement of myokines with cardiovascular functions.

Irisin and musculoskeletal health

Irisin is a hormone-like myokine produced in abundance by skeletal muscle in response to exercise, both in mice and humans. Once released into the circulation, irisin acts on white adipocytes to induce the browning response and subsequently activates nonshivering thermogenesis. We have examined the premise that irisin produced during exercise may subserve further functions in the musculoskeletal system. We review evidence for its possible skeletal effects, including the central role that irisin plays in the control of bone mass, with positive effects on cortical mineral density and geometry in mice. We also review the autocrine effects of irisin in skeletal muscle, in which it upregulates the expression of its precursor (FNDC5). Since loss of bone and muscle mass occurs with aging, immobility, and several metabolic diseases, future studies exploring the efficacy of irisin in restoring bone and reversing muscle wasting could be important to establishing irisin as a molecule that combines beneficial effects for treating osteoporosis and muscular atrophy. If the results from mice were confirmed in human studies, an irisin-based therapy could be developed for physically disabled or bedridden patients.

Sexual dimorphism of frailty and cognitive impairment: Potential underlying mechanisms (Review)

The aim of the present study was to assess systematically gender differences in susceptibility to frailty and cognitive performance decline, and the underlying mechanisms. A systematic assessment was performed of the identified reviews of cohort, mechanistic and epidemiological studies. The selection criteria of the present study included: i) Sexual dimorphism of frailty, ii) sexual dimorphism of subjective memory decline (impairment) and atrophy of hippocampus during early life, iii) sexual dimorphism of late‑onset Alzheimer's disease and iv) sexual dimorphism mechanisms underlying frailty and cognitive impairment. Males exhibit a susceptibility to poor memory performance and a severe atrophy of the hippocampus during early life and females demonstrate a higher prevalence for frailty and late‑life dementia. The different alterations within the hypothalamic‑pituitary‑gonadal/adrenal axis, particularly with regard to gonadal hormones, cortisol and dehydroepiandrosterone/sulfate‑bound dehydroepiandrosterone prior to and following andropause in males and menopause in females, serve important roles in sexual dimorphism of frailty and cognitive impairment. These endocrine changes may accelerate immunosenescence, weaken neuroprotective and neurotrophic effects, and promote muscle catabolism. The present study suggested that these age‑associated endocrine alterations interact with gender‑specific genetic and epigenetic factors, together with immunosenescence and iron accumulation. Environment factors, including psychological factors, are additional potential causes of the sexual dimorphism of frailty and cognitive impairment.

Galectin-1 promotes an M2 macrophage response to polydioxanone scaffolds

Regulating soft tissue repair to prevent fibrosis and promote regeneration is central to creating a microenvironment conducive to soft tissue development. Macrophages play an important role in this process. The macrophage response can be modulated using biomaterials, altering cytokine and growth factor secretion to promote regeneration. Electrospun polydioxanone (PDO) fiber scaffolds promoted an M2 phenotype when macrophages were cultured on large diameter, highly porous scaffolds, but an M1 phenotype on smaller diameter fibers. In this study, we investigated whether incorporation of galectin-1, an immunosuppressive protein that enhances muscle regeneration, could promote the M2 response. Galectin-1 was incorporated into large and small fiber PDO scaffolds during electrospinning. Galectin-1 incorporation increased arginase-1 and reduced iNOS and IL-6 production in mouse bone-marrow derived macrophages compared with PDO alone for both scaffold types. Inhibition of ERK mitogen-activated protein kinase did not alter galectin-1 effects on arginase-1 and iNOS expression, but reversed IL-6 suppression, indicating that IL-6 is mediated by a different mechanism. Our results suggest that galectin-1 can be used to modulate macrophage commitment to a pro-regenerative M2 phenotype, which may positively impact tissue regeneration when using small diameter PDO scaffolds. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 105A: 2562-2571, 2017.

Network architecture associated with the highly specialized hindlimb of frogs

Network analyses have been increasingly used in the context of comparative vertebrate morphology. The structural units of the vertebrate body are treated as discrete elements (nodes) of a network, whose interactions at their physical contacts (links) determine the phenotypic modules. Here, we use the network approach to study the organization of the locomotor system underlying the hindlimb of frogs. Nodes correspond to fibrous knots, skeletal and muscular units. Edges encode the ligamentous and monoaxial tendinous connections in addition to joints. Our main hypotheses are that: (1) the higher centrality scores (measured as betweenness) are recorded for fibrous elements belonging to the connective system, (2) the organization of the musculoskeletal network belongs to a non-trivial modular architecture and (3) the modules in the hindlimb reflect functional and/or developmental constraints. We confirm all our hypotheses except for the first one, since bones overpass the fibrous knots in terms of centrality. Functionally, there is a correlation between the proximal-to-distal succession of modules and the progressive recruitment of elements involved with the motion of joints during jumping. From a developmental perspective, there is a correspondence between the order of the betweenness scores and the ontogenetic chronology of hindlimbs in tetrapods. Modular architecture seems to be a successful organization, providing of the building blocks on which evolution forges the many different functional specializations that organisms exploit.

Fat Mass Is Positively Associated with Estimated Hip Bone Strength among Chinese Men Aged 50 Years and above with Low Levels of Lean Mass

This study investigated the relationships of fat mass (FM) and lean mass (LM) with estimated hip bone strength in Chinese men aged 50-80 years (median value: 62.0 years). A cross-sectional study including 889 men was conducted in Guangzhou, China. Body composition and hip bone parameters were generated by dual-energy X-ray absorptiometry (DXA). The relationships of the LM index (LMI) and the FM index (FMI) with bone phenotypes were detected by generalised additive models and multiple linear regression. The associations between the FMI and the bone variables in LMI tertiles were further analysed. The FMI possessed a linear relationship with greater estimated hip bone strength after adjustment for the potential confounders (p < 0.05). Linear relationships were also observed for the LMI with most bone phenotypes, except for the cross-sectional area (p < 0.05). The contribution of the LMI (4.0%-12.8%) was greater than that of the FMI (2.0%-5.7%). The associations between the FMI and bone phenotypes became weaker after controlling for LMI. Further analyses showed that estimated bone strength ascended with FMI in the lowest LMI tertile (p < 0.05), but not in the subgroups with a higher LMI. This study suggested that LM played a critical role in bone health in middle-aged and elderly Chinese men, and that the maintenance of adequate FM could help to promote bone acquisition in relatively thin men.

Muscle strength rather than muscle mass is associated with osteoporosis in older Chinese adults

BACKGROUND

Assessment of the association of muscle strength and muscle mass with osteoporosis (OP) is of special interest as muscles are a potential target for interventions (i.e., strength training).

METHODS

A cross-sectional descriptive study encompassing people aged ≥ 60 years (average age: 66.9 ± 6.2 years; men, n = 516; women, n = 652) in the Hangu area of Tianjin, China. The study populations were invited to participate in a comprehensive geriatric assessment. OS was identified by measuring the calcaneal using a quantitative ultrasound and a T score of less than -2.5. Muscle characteristics included grip strength and appendicular skeletal muscle mass (ASM).

RESULTS

The prevalence of OS in this study was 61.6% (male 52.1%, female 69.1%). Grip strength was negatively related to OS and after adjusting for all other variables, higher grip strength was found to be associated with a lower OS risk (p = 0.023). ASM/height² was not associated with OS (p = 0.205).

CONCLUSION

Based on our study, muscle strength rather than muscle mass is negatively associated with OS in older people; thus, we should pay more attention to muscle strength training in the early stage of the OS.

Low serum concentrations of Irisin are associated with increased risk of hip fracture in Chinese older women

OBJECTIVE

Irisin derived from muscle in response to exercise may be the molecular entity responsible for muscle wasting-osteoporosis connectivity in the elderly. The objective of the study was to determine whether serum Irisin (sIrisin) provides information on hip fracture prediction which were independent of bone mineral density (BMD) and the fracture risk assessment tool (FRAX) algorithm.

METHODS

This study enrolled 160 older women (ages, 70-90y) with minimal trauma hip fractures (MTHFs) and 160 age-matched women without fracture serving as controls. Clinical features, BMD and bone turnover markers including sIrisin levels were measured after fracture within 2 days as baseline.

RESULTS

sIrisin levels were significantly lower (361.5±140.0ng/mL vs 478.5±159.6ng/mL, P<0.001) in cases than controls. After multivariate analysis, sIrisin remained as an independent variable of BMD, which explained 17.8% of femoral neck BMD and 22.5% of lumbar spine BMD, respectively. The odds ratio (OR) of MTHFs comparing the lowest (<320.1ng/mL) to highest (>524.5ng/mL) quartiles was 1.95 (95% CI 1.23-3.79, P<0.05) for sIrisin. Adjustment for age, body mass index, time since menopause and exercise ≥30min/day yielded similar results, and BMD of femoral neck also did not change these associations. Taking FRAX score into account attenuated the association somewhat: OR of hip fracture was 1.81 (95% CI 1.26-3.49, P<0.05) in first versus fourth quartile of sIrisin. There was a negative gradient of risk by decreasing quartile in sIrisin.

CONCLUSIONS

Low concentrations of sIrisin in older women were independently associated with increased risk of hip fractures when adjusted for BMD or FRAX score.

Serum sclerostin decreases following 12months of resistance- or jump-training in men with low bone mass

PURPOSE

We previously reported that 12months of resistance training (RT, 2×/wk, N=19) or jump training (JUMP, 3×/wk, N=19) increased whole body and lumbar spine BMD and increased serum bone formation markers relative to resorption in physically active (≥4h/wk) men (mean age: 44±2y; median: 44y) with osteopenia of the hip or spine. The purpose of this secondary analysis was to examine the effects of the RT or JUMP intervention on potential endocrine mediators of the exercise effects on bone, specifically IGF-I, PTH and sclerostin.

METHODS

Fasting blood samples were collected after a 24-h period of no exercise at baseline and after 12months of RT or JUMP. IGF-I, PTH and sclerostin were measured in serum by ELISA. The effects of RT or JUMP on IGF-I, PTH and sclerostin were evaluated using 2×2 repeated measures ANOVA (time, group). This study was conducted in accordance with the Declaration of Helsinki and was approved by the University of Missouri IRB.

RESULTS

Sclerostin concentrations in serum significantly decreased and IGF-I significantly increased after 12months of RT or JUMP; while PTH remained unchanged.

CONCLUSION

The beneficial effects of long-term, progressive-intensity RT or JUMP on BMD in moderately active men with low bone mass are associated with decreased sclerostin and increased IGF-I.

Modified step aerobics training and neuromuscular function in osteoporotic patients: a randomized controlled pilot study

BACKGROUND

Training programs directed to improve neuromuscular and musculoskeletal function of the legs are scarce with respect to older osteoporotic patients. We hypothesized that a modified step aerobics training program might be suitable for this purpose and performed a randomized controlled pilot study to assess the feasibility of conducting a large study. Here we report on the training-related effects on neuromuscular function of the plantar flexors.

PATIENTS AND METHODS

Twenty-seven patients with an age of at least 65 years were enrolled and randomized into control and intervention group. The latter received supervised modified step aerobics training (twice weekly, 1 h per session) over a period of 6 months. At baseline, and after 3 and 6 months neuromuscular function of the plantar flexors, i.e., isometric maximum voluntary torque, rate of torque development and twitch torque parameters were determined in detail in all patients of both groups.

RESULTS

Twenty-seven patients (median age 75 years; range 66-84 years) were randomized (control group n = 14; intervention group n = 13). After 3 and 6 months of training, maximum voluntary contraction strength in the intervention group was significantly higher by 7.7 Nm (9.1%; 95% CI 3.3-12.2 Nm, P < 0.01) and 12.4 Nm (14.8%; 95% CI 6.4-18.5 Nm, P < 0.01) compared to controls. These changes were most probably due to neural and muscular adaptations.

CONCLUSION

It is worthwhile to investigate efficacy of this training program in a large randomized trial. However, a detailed neuromuscular assessment appears feasible only in a subset of participants.

Effect of the Lipoxygenase Inhibitor Baicalein on Muscles in Ovariectomized Rats

Sarcopenia, a loss of muscle mass accompanying osteoporosis, leads to falls and fall-related injuries. Baicalein, as a phytochemical agent, has an antioxidative and anti-inflammatory effect in muscle. In this study, sixty-one female Sprague Dawley rats were divided into five groups: four groups were ovariectomized (OVX) and one control group was nonovariectomized (NON-OVX). Eight weeks after ovariectomy, three disparate concentrations (1 mg/kg body weight (BW), 10 mg/kg BW, and 100 mg/kg BW) of baicalein were applied subcutaneously daily in three OVX groups. Mm. soleus, gastrocnemius, and longissimus were extracted; their diameter, area, relation to body, and muscle weights as well as number of capillaries per fibre were recorded. In Mm. soleus and gastrocnemius, the baicalein effect (increasing number of capillaries per fibre) was proportional to the dose applied. The fibre diameters and area under baicalein treatment were significantly greater compared to OVX and NON-OVX groups. In M. longissimus, we observed a shift to type IIa fibres. Serum creatine kinase levels were significantly lower in highest baicalein concentration group. We conclude that baicalein can stimulate angiogenesis, though not fibre type-specific, in skeletal muscle and reduce the estrogen-related loss of fibre diameter and area in the skeletal muscle in rats. Therefore, a protective effect of baicalein on muscle cells can be assumed.

Implications of exercise-induced adipo-myokines in bone metabolism

Physical inactivity has been recognized, by the World Health Organization as the fourth cause of death (5.5 % worldwide). On the contrary, physical activity (PA) has been associated with improved quality of life and decreased risk of several diseases (i.e., stroke, hypertension, myocardial infarction, obesity, malignancies). Bone turnover is profoundly affected from PA both directly (load degree is the key determinant for BMD) and indirectly through the activation of several endocrine axes. Several molecules, secreted by muscle (myokines) and adipose tissues (adipokines) in response to exercise, are involved in the fine regulation of bone metabolism in response to the energy availability. Furthermore, bone regulates energy metabolism by communicating its energetic needs thanks to osteocalcin which acts on pancreatic β-cells and adipocytes. The beneficial effects of exercise on bone metabolism depends on the intermittent exposure to myokines (i.e., irisin, IL-6, LIF, IGF-I) which, instead, act as inflammatory/pro-resorptive mediators when chronically elevated; on the other hand, the reduction in the circulating levels of adipokines (i.e., leptin, visfatin, adiponectin, resistin) sustains these effects as well as improves the whole-body metabolic status. The aim of this review is to highlight the newest findings about the exercise-dependent regulation of these molecules and their role in the fine regulation of bone metabolism.

Bone and Muscle Endocrine Functions: Unexpected Paradigms of Inter-organ Communication

Most physiological functions originate with the communication between organs. Mouse genetics has revived this holistic view of physiology through the identification of inter-organ communications that are unanticipated, functionally important, and would have been difficult to uncover otherwise. This Review highlights this point by showing how two tissues usually not seen as endocrine ones, bone and striated muscles, influence several physiological processes in a significant manner.

The Relationship of Physical Activity and Anthropometric and Physiological Characteristics to Bone Mineral Density in Postmenopausal Women

The aim of this study is to investigate the relationship of physical activity and anthropometric and physiological characteristics to bone mineral density (BMD) in postmenopausal women. Ninety-seven postmenopausal women with an average age of 50.71 ± 6.86 yr were selected to participate in this study. After completing consent forms and the questionnaire on physical activity, the amounts of calcium and 25-hydroxyvitamin D levels in participants' blood were measured by blood tests. The BMDs of the subjects in the lumbar spine (L2-L4) and hip were measured by dual-energy X-ray absorptiometry device and the results were recorded. Also, anthropometric characteristics including height, weight, body fat percentage, body mass index, waist-to-hip ratio (WHR), digit ratio (2D:4D), skeletal muscle mass index, hand and calf circumferences and physiological parameters, including handgrip strength, quadriceps isotonic extension strength and balance of the subjects, were measured. The results showed that the 2D:4D ratio and skeletal muscle mass index had a significantly positive relationship with BMD of the lumbar spine (p ≤ 0.05) and the hip (p ≤ 0.05). Also, there was a negative relationship between the BMD of lumbar spine and hip and WHR (p ≤ 0.05). Moreover, there was a positive relationship between the calf circumferences and lumbar spine BMD (p ≤ 0.05). Contrary to this, there was no significant relationship between the calf circumference and the hip BMD, and between hand circumference with lumbar spine and hip BMD (p > 0.05). Results of physiological indices showed a significant positive relationship between physical activity, handgrip strength, quadriceps isotonic extension strength, standing on 1 foot with the lumbar spine and hip BMD (p ≤ 0.05). But the relationship was not observed between BMD and the ability to squat down on the floor (p > 0.05). Based on these results, it seemed that we can use some physiological and anthropometric indices that are important determinants of BMD and risk of osteoporosis in postmenopausal women.

Quantitative in vivo micro-computed tomography for assessment of age-dependent changes in murine whole-body composition

Micro-computed tomography (micro-CT) is used routinely to quantify skeletal tissue mass in small animal models. Our goal was to evaluate repeated in vivo micro-CT imaging for monitoring whole-body composition in studies of growth and aging in mice. Male mice from 2 to 52 weeks of age were anesthetized and imaged using an eXplore Locus Ultra and/or eXplore speCZT scanner. Images were reconstructed into 3D volumes, signal-intensity thresholds were used to classify each voxel as adipose, lean or skeletal tissue, and tissue masses were calculated from known density values. Images revealed specific changes in tissue distribution with growth and aging. Quantification showed biphasic increases in total CT-derived body mass, lean and skeletal tissue masses, consisting of rapid increases to 8 weeks of age, followed by slow linear increases to 52 weeks. In contrast, bone mineral density increased rapidly to a stable plateau at ~ 14 weeks of age. On the other hand, adipose tissue mass increased continuously with age. A micro-CT-derived total mass was calculated for each mouse and compared with gravimetrically measured mass, which differed on average by < 3%. Parameters were highly reproducible for mice of the same age, but variability increased slightly with age. There was also good agreement in parameters for the same group of mice scanned on the eXplore Locus Ultra and eXplore speCZT systems. This study provides reference values for normative comparisons; as well, it demonstrates the usefulness of in vivo single-energy micro-CT scans to quantify whole-body composition in high-throughput studies of growth and aging in mice.

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Novel use of micro-CT to monitor body composition during growth and aging of mice.

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Skeletal, adipose and lean tissue masses were quantified using in vivo imaging.

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Marked differences in composition occur during rapid growth from 2 to 5 weeks of age.

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Reference data for other investigators using mice as disease models.

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Micro-CT is an accurate tool for quantification of body composition in mice.

The genetics of bone mass and susceptibility to bone diseases

Osteoporosis is characterized by low bone mass and an increased risk of fracture. Genetic factors, environmental factors and gene-environment interactions all contribute to a person's lifetime risk of developing an osteoporotic fracture. This Review summarizes key advances in understanding of the genetics of bone traits and their role in osteoporosis. Candidate-gene approaches dominated this field 20 years ago, but clinical and preclinical genetic studies published in the past 5 years generally utilize more-sophisticated and better-powered genome-wide association studies (GWAS). High-throughput DNA sequencing, large genomic databases and improved methods of data analysis have greatly accelerated the gene-discovery process. Linkage analyses of single-gene traits that segregate in families with extreme phenotypes have led to the elucidation of critical pathways controlling bone mass. For example, components of the Wnt-β-catenin signalling pathway have been validated (in both GWAS and functional studies) as contributing to various bone phenotypes. These notable advances in gene discovery suggest that the next decade will witness cataloguing of the hundreds of genes that influence bone mass and osteoporosis, which in turn will provide a roadmap for the development of new drugs that target diseases of low bone mass, including osteoporosis.

COPD and osteoporosis: links, risks, and treatment challenges

Chronic obstructive pulmonary disease (COPD) is a chronic inflammatory airway disease associated with various systemic comorbidities including osteoporosis. Osteoporosis and its related fractures are common and have significant impacts on quality of life and even respiratory function in patients with COPD. COPD-associated osteoporosis is however extremely undertreated. Recent studies have suggested that both decreased bone mineral density (BMD) and impaired bone quality contribute to bone fragility, causing fractures in COPD patients. Various clinical risk factors of osteoporosis in COPD patients, including older age, emaciation, physical inactivity, and vitamin D deficiency, have also been described. It is critically important for pulmonologists to be aware of the high prevalence of osteoporosis in COPD patients and evaluate them for such fracture risks. Routine screening for osteoporosis will enable physicians to diagnose COPD patients with comorbid osteoporosis at an early stage and give them appropriate treatment to prevent fracture, which may lead to improved quality of life as well as better long-term prognosis.

Long-duration bed rest as an analog to microgravity

Long-duration bed rest is widely employed to simulate the effects of microgravity on various physiological systems, especially for studies of bone, muscle, and the cardiovascular system. This microgravity analog is also extensively used to develop and test countermeasures to microgravity-altered adaptations to Earth gravity. Initial investigations of bone loss used horizontal bed rest with the view that this model represented the closest approximation to inactivity and minimization of hydrostatic effects, but all Earth-based analogs must contend with the constant force of gravity by adjustment of the G vector. Later concerns about the lack of similarity between headward fluid shifts in space and those with horizontal bed rest encouraged the use of 6 degree head-down tilt (HDT) bed rest as pioneered by Russian investigators. Headward fluid shifts in space may redistribute bone from the legs to the head. At present, HDT bed rest with normal volunteers is the most common analog for microgravity simulation and to test countermeasures for bone loss, muscle and cardiac atrophy, orthostatic intolerance, and reduced muscle strength/exercise capacity. Also, current physiologic countermeasures are focused on long-duration missions such as Mars, so in this review we emphasize HDT bed rest studies with durations of 30 days and longer. However, recent results suggest that the HDT bed rest analog is less representative as an analog for other important physiological problems of long-duration space flight such as fluid shifts, spinal dysfunction and radiation hazards.

Irisin, an exercise-induced myokine as a metabolic regulator: an updated narrative review

Irisin, as a new hormone-like myokine, is discovered in the presence of exercise-induced peroxisome proliferator-activated receptor gamma coactivator-1-alpha (PGC-1α). Which substance plays an important role in energy metabolism in each organ in the body and the regulation of metabolic diseases such as obesity and diabetes. The finding of irisin can contribute to the exploration of the novel and effective therapeutic targets or therapeutic strategies of these metabolic diseases or metabolism-associated health issues. To date, little is known regarding the functions and regulatory mechanisms of irisin with respect to metabolic diseases or metabolism-associated health issues. In this narrative review article, we systematically introduce its structural characteristics, production and distribution in tissues and organs, and the regulation and corresponding mechanisms for metabolic diseases or metabolism-associated health issues of irisin. Meanwhile, its future prospects and the development of irisin-related products for the promotion of human health have also been proposed, which will benefit future research and application of irisin. Copyright © 2015 John Wiley & Sons, Ltd.