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

Study protocol for the ROLEX-DUO randomised placebo-controlled trial: ROmosozumab Loaded with EXercise - DUal effects on bone and muscle in postmenopausal Osteoporosis and Osteopenia

INTRODUCTION

Novel strategies are needed to address the rising burden of osteoporosis and fragility fractures. High-intensity resistance and impact (HiRIT) exercise has shown benefit in improving bone density in postmenopausal women with osteoporosis/osteopenia. Whether HiRIT can enhance the therapeutic effects of osteoporosis pharmacotherapy has not been established. ROLEX-DUO is a randomised controlled trial designed to assess the efficacy of romosozumab on various bone and muscle outcomes in combination with different exercise interventions in women with postmenopausal osteoporosis/osteopenia.

METHODS AND ANALYSIS

ROLEX-DUO is an 8-month randomised placebo-controlled trial conducted at two tertiary referral centres for patients with osteoporosis/osteopenia in Sydney, New South Wales, Australia. The study is implementing the combination of romosozumab or placebo with different forms of exercise in postmenopausal women with osteoporosis/osteopenia without recent fragility fracture (n=102). Eligible women will be randomised 1:1:1 into one of three groups: (1) romosozumab with supervised HiRIT, (2) romosozumab with unsupervised low-intensity exercise or (3) placebo with unsupervised low-intensity exercise. Co-primary outcomes are the mean percentage change in lumbar spine bone mineral density (BMD), and mean change in five times sit-to-stand test performance (seconds) at 8 months. Secondary/exploratory outcomes include BMD changes at the femoral neck, total hip and distal radius, three-dimensional dual-energy X-ray absorptiometry (DXA) hip outcomes, DXA-derived lean and fat mass, serum markers of bone turnover (procollagen type 1 peptide, C-telopeptide of type 1 collagen) and bone biomarkers (dickkopf-1), serum extracellular vesicle analyses, 36-Item Short Form Survey (SF-36) quality-of-life scores, Menopause-Specific Quality Of Life (MENQOL) Questionnaire menopause symptom burden scores, number of falls and fractures. Mixed-effects models will be performed to compare longitudinal outcome results between groups using intention-to-treat analysis.

ETHICS AND DISSEMINATION

The trial was approved by the Northern Sydney Local Health District Human Research Ethics Committee (2022/ETH01794, protocol V.8, dated 03 July 2024). Participants will provide written informed consent prior to inclusion. Findings will be disseminated via peer-reviewed journals, scientific conferences and summary reports to funding bodies.

TRIAL REGISTRATION NUMBER

ACTRN12623000867695.

Transcriptomics reveals transient and dynamic muscle fibrosis and atrophy differences following spinal cord injury in rats

BACKGROUND

The rate and magnitude of skeletal muscle wasting after severe spinal cord injury (SCI) exceeds most other disuse conditions. Assessing the time course of molecular changes can provide insight into the progression of muscle wasting post-SCI. The goals of this study were (1) to identify potential targets that may prevent the pathologic features of SCI in soleus muscles and (2) to establish therapeutic windows for treating these pathologic changes.

METHODS

Four-month-old Sprague-Dawley male rats received T9 laminectomy (SHAM surgery) or severe contusion SCI. Hindlimb locomotor function was assessed weekly, with soleus muscles obtained 1 week, 2 weeks, 1 month and 3 months post-surgery (n = 6-7 per group per timepoint). RNA was extracted from muscles for bulk RNA-sequencing analysis (n = 3-5 per group per timepoint). Differentially expressed genes (DEGs) were evaluated between age-matched SHAM and SCI animals. Myofiber size, muscle fibre type and fibrosis were assessed on contralateral muscles.

RESULTS

SCI produced immediate and persistent hindlimb paralysis, with Basso-Beattie-Bresnahan locomotor scores remaining below 7 throughout the study, contributing to a progressive 25-50% lower soleus mass and myofiber atrophy versus SHAM (P < 0.05 at all timepoints). Transcriptional comparisons of SCI versus SHAM resulted in 184 DEGs (1 week), 436 DEGs (2 weeks), 133 DEGs (1 month) and 1200 DEGs (3 months). Upregulated atrophy-related genes included those associated with cell senescence, nuclear factor kappa B, ubiquitin proteasome and unfolded protein response pathways, along with upregulated genes that negatively influence muscle growth through the transforming growth factor beta pathway and inhibition of insulin-like growth factor-I/Akt/mechanistic target of rapamycin and p38/mitogen-activated protein kinase signalling. Genes associated with extracellular matrix (ECM), including collagens, collagen crosslinkers, proteoglycans and those regulating ECM integrity, were enriched within upregulated DEGs at 1 week but subsequently downregulated at 2 weeks and 3 months and were accompanied by >50% higher ECM areas and hydroxyproline levels in SCI muscles (P < 0.05). Myofiber remodelling genes were enriched in upregulated DEGs at 2 weeks and 1 month and were downregulated at 3 months. Genes that regulate neuromuscular junction remodelling were evident in muscles post-SCI, along with slow-to-fast fibre-type shifts: 1 week and 2 weeks SCI muscles were composed of 90% myosin heavy chain (MHC) type I fibres, which decreased to only 16% at 3 months and were accompanied by 50% fibres containing MHC IIX (P < 0.05). Metabolism genes were enriched in upregulated DEGs at 1 month and were further enriched at 3 months.

CONCLUSIONS

Our results substantiate many known pathologic features of SCI-induced wasting in rat skeletal muscle and identify a progressive and dynamic transcriptional landscape within the post-SCI soleus. Future studies are warranted to consider these therapeutic treatment windows when countering SCI muscle pathology.

Musculoskeletal crosstalk in chronic obstructive pulmonary disease and comorbidities: Emerging roles and therapeutic potentials

Chronic Obstructive Pulmonary Disease (COPD) is a multifaceted respiratory disorder characterized by progressive airflow limitation and systemic implications. It has become increasingly apparent that COPD exerts its influence far beyond the respiratory system, extending its impact to various organ systems. Among these, the musculoskeletal system emerges as a central player in both the pathogenesis and management of COPD and its associated comorbidities. Muscle dysfunction and osteoporosis are prevalent musculoskeletal disorders in COPD patients, leading to a substantial decline in exercise capacity and overall health. These manifestations are influenced by systemic inflammation, oxidative stress, and hormonal imbalances, all hallmarks of COPD. Recent research has uncovered an intricate interplay between COPD and musculoskeletal comorbidities, suggesting that muscle and bone tissues may cross-communicate through the release of signalling molecules, known as "myokines" and "osteokines". We explored this dynamic relationship, with a particular focus on the role of the immune system in mediating the cross-communication between muscle and bone in COPD. Moreover, we delved into existing and emerging therapeutic strategies for managing musculoskeletal disorders in COPD. It underscores the development of personalized treatment approaches that target both the respiratory and musculoskeletal aspects of COPD, offering the promise of improved well-being and quality of life for individuals grappling with this complex condition. This comprehensive review underscores the significance of recognizing the profound impact of COPD on the musculoskeletal system and its comorbidities. By unravelling the intricate connections between these systems and exploring innovative treatment avenues, we can aspire to enhance the overall care and outcomes for COPD patients, ultimately offering hope for improved health and well-being.

Effects of sclerostin injection on soleus and extensor digitorum longus muscle tissue in male mice

Sclerostin, a potent inhibitor of the Wnt signaling pathway, plays a critical role in bone homeostasis. Evidence suggests that sclerostin may also be involved in crosstalk between other tissues, including muscle. This pilot study attempted to examine the effects of sclerostin on soleus and extensor digitorum longus (EDL) muscle tissue from male mice that were given continuous recombinant sclerostin injections for 4 weeks. A total of 48 10-week-old male C57BL/6J mice were assigned to be sedentary or perform 1 h treadmill running per day for 4 weeks and administered subcutaneous injections of either saline or recombinant sclerostin 5 days/week. Sclerostin injection led to a reduction in the soleus myosin heavy chain (MHC) I, MHC I/IIA, MHC IIA/X, and MHC IIB cross-sectional area (p < 0.05) with no exercise effects on these reductions. In contrast, there were no effects of sclerostin injections or exercise on the fast-twitch EDL muscle in terms of size, MHC protein, or markers of Wnt signaling. These findings provide preliminary evidence of sclerostin's endocrine role in muscle via decreases in myofiber cross-sectional area, which seems to be independent of fiber type but muscle type-specific. More studies, however, are needed to confirm these preliminary results.

Τhe story of sclerostin inhibition: the past, the present, and the future

Sclerostin inhibits osteoblast activity by hampering activation of the canonical Wnt signaling pathway and simultaneously stimulates osteoclastogenesis through upregulation of the receptor activator of NFκB ligand (RANKL). Thus, antibodies against sclerostin (Scl-Abs), besides promoting bone formation, suppress bone resorption and dissociate bone formation from resorption. This dual action results in remarkable increases of bone mineral density which are of a greater magnitude compared to the other antiosteoporotic treatments and are accompanied by decreases of fracture risk at all skeletal sites. The anabolic effect subsides after the first few months of treatment and a predominantly antiresorptive effect remains after this period, limiting its use to 12 months. Furthermore, these effects are largely reversible upon discontinuation; therefore, subsequent treatment with antiresorptives is indicated to maintain or further increase the bone gains achieved. Romosozumab is currently the only Scl-Ab approved for the treatment of severe postmenopausal osteoporosis. Indications for use in other populations, such as males, premenopausal women, and patients with glucocorticoid-induced osteoporosis, are pending. Additionally, the efficacy of Scl-Abs in other bone diseases, such as osteogenesis imperfecta, hypophosphatasia, X-linked hypophosphatemia, and bone loss associated with malignancies, is under thorough investigation. Cardiovascular safety concerns currently exclude patients at high cardiovascular risk from this treatment.

Sclerostin: clinical insights in muscle-bone crosstalk

Sclerostin, a protein encoded by the sclerostin (SOST) gene, is mostly expressed in osteocytes. First described in the pathogenesis of three disorders, sclerosteosis, van Buchem's disease, and craniodiaphyseal dysplasia, sclerostin has been identified as an important regulator of bone homeostasis, controlling bone formation by osteoblasts through inhibition of the canonical Wnt signaling pathway. Recent studies have highlighted a hypothetical role of sclerostin in myogenesis, thus modulating the interaction between bone and muscle. This narrative review provides an overview of the clinical implications of sclerostin modulation on skeletal muscle mass and function, and bone metabolism. Improving knowledge about muscle-bone crosstalk may represent a turning point in the development of therapeutic strategies for musculoskeletal disorders, particularly osteosarcopenia.

Passive Cycle Training Promotes Bone Recovery after Spinal Cord Injury without Altering Resting-State Bone Perfusion

INTRODUCTION

Spinal cord injury (SCI) produces diminished bone perfusion and bone loss in the paralyzed limbs. Activity-based physical therapy (ABPT) modalities that mobilize and/or reload the paralyzed limbs (e.g., bodyweight-supported treadmill training (BWSTT) and passive-isokinetic bicycle training) transiently promote lower-extremity blood flow (BF). However, it remains unknown whether ABPT alter resting-state bone BF or improve skeletal integrity after SCI.

METHODS

Four-month-old male Sprague-Dawley rats received T 9 laminectomy alone (SHAM; n = 13) or T 9 laminectomy with severe contusion SCI ( n = 48). On postsurgery day 7, SCI rats were stratified to undergo 3 wk of no ABPT, quadrupedal (q)BWSTT, or passive-isokinetic hindlimb bicycle training. Both ABPT regimens involved two 20-min bouts per day, performed 5 d·wk -1 . We assessed locomotor recovery, bone turnover with serum assays and histomorphometry, distal femur bone microstructure using in vivo microcomputed tomography, and femur and tibia resting-state bone BF after in vivo microsphere infusion.

RESULTS

All SCI animals displayed immediate hindlimb paralysis. SCI without ABPT exhibited uncoupled bone turnover and progressive cancellous and cortical bone loss. qBWSTT did not prevent these deficits. In comparison, hindlimb bicycle training suppressed surface-level bone resorption indices without suppressing bone formation indices and produced robust cancellous and cortical bone recovery at the distal femur. No bone BF deficits existed 4 wk after SCI, and neither qBWSTT nor bicycle altered resting-state bone perfusion or locomotor recovery. However, proximal tibia BF correlated with several histomorphometry-derived bone formation and resorption indices at this skeletal site across SCI groups.

CONCLUSIONS

These data indicate that passive-isokinetic bicycle training reversed cancellous and cortical bone loss after severe SCI through antiresorptive and/or bone anabolic actions, independent of locomotor recovery or changes in resting-state bone perfusion.

Role of the Osteocyte in Musculoskeletal Disease

PURPOSE OF THE REVIEW

The purpose of this review is to summarize the role of the osteocyte in muscle atrophy in cancer patients, sarcopenia, spinal cord injury, Duchenne's muscular dystrophy, and other conditions associated with muscle deterioration.

RECENT FINDINGS

One type of bone cell, the osteocyte, appears to play a major role in muscle and bone crosstalk, whether physiological or pathological. Osteocytes are cells living within the bone-mineralized matrix. These cells are connected to each other by means of dendrites to create an intricately connected network. The osteocyte network has been shown to respond to different types of stimuli such as mechanical unloading, immobilization, aging, and cancer by producing osteocytes-derived factors. It is now becoming clear that some of these factors including sclerostin, RANKL, TGF-β, and TNF-α have detrimental effects on skeletal muscle. Bone and muscle not only communicate mechanically but also biochemically. Osteocyte-derived factors appear to contribute to the pathogenesis of muscle disease and could be used as a cellular target for new therapeutic approaches.

Fingolimod Nanoemulsions at Different Particle Sizes Define the Fate of Spinal Cord Injury Recovery

Spinal cord injury (SCI) is a debilitating condition for which no definitive treatment has yet been identified. Notably, it influences other tissues through inflammatory reactions and metabolic disturbances. Therefore, fingolimod (FTY-720), as an FDA-approved inflammatory modulator, would be promising. In the present study, nanocarriers with two distinct monodisperse particle sizes of 60 (nF60) and 190 (nF190) nm were prepared via low-(stirring) and high-energy (probe ultrasound) emulsion oil in water (O/W) methods. Larger nanocarriers showed higher EE% and sustained-release profile than smaller nanocarriers. Neural stem cell (NSC) viability and lactate dehydrogenase (LDH) release were studied in the presence of nanocarriers and free FTY-720. The results indicated that nanocarriers and free FTY-720 enhanced NSC viability compared with the control group. However, nF190 induced significantly less cell membrane damage than nF60. Nanocarriers and free FTY-720 enhanced motor neuron recovery in SCI rats, while body weight and return to bladder reflux by nF190 were significantly higher than those in the nF60 group. Return to bladder reflux might be due to the role of FTY-720 in the regulation of detrusor muscle tone and preservation of the integrity of vessels by acting on endothelial cells. Moreover, nF190 gained higher soleus muscle weight than the free drugs; probably decreasing proinflammatory cytokines in the soleus diminishes muscular atrophy in SCI rats. In summary, it might be said that larger nanocarriers with sustained-release profile and less cell membrane damage seem to be more efficient than smaller ones to manage SCI and enhance bladder reflux. These data will help pharmaceutical companies select the correct particle size for nanodrugs and develop more efficient drug formulations to treat SCI.

Decreased Serum Level of Sclerostin in Older Adults with Sarcopenia

BACKGRUOUND

Although muscles and bones interact with each other through various secretory factors, the role of sclerostin, an osteocyte-secreted factor, on muscle metabolism has not been well studied. We investigated the levels of serum sclerostin in Korean older adults with sarcopenia.

METHODS

Blood samples were collected from 129 participants who underwent evaluation of muscle mass and function in an outpatient geriatric clinic of a teaching hospital. Sarcopenia and related parameters were determined using cutoff values for the Asian population. Serum sclerostin levels were measured using an enzyme-linked immunosorbent assay.

RESULTS

The mean age of the participants was 69.6 years, and 20 participants (15.5%) were classified as having sarcopenia. After adjusting for age, sex, and body mass index, serum sclerostin levels were significantly lower in participants with sarcopenia, low muscle mass, or weak muscle strength (P=0.003 to 0.045). Serum sclerostin levels were positively associated with skeletal muscle index and grip strength after adjusting for confounders (P=0.001 and P=0.003), whereas sarcopenic phenotype score showed a negative association (P=0.006). These increases in muscle mass and strength were also dose dependent as serum sclerostin levels increased (P for trends=0.003 and P for trends=0.015). Higher serum sclerostin levels were associated with lower odds ratio (ORs) for sarcopenia, low muscle mass, and weak muscle strength after adjusting for confounders (OR, 0.27 to 0.50; P<0.001 to 0.025).

CONCLUSION

Higher serum sclerostin levels were associated with a lower risk of sarcopenia, low muscle mass, and weak muscle strength in Korean older adults.

Bone loss after severe spinal cord injury coincides with reduced bone formation and precedes bone blood flow deficits

Diminished bone perfusion develops in response to disuse and has been proposed as a mechanism underlying bone loss. Bone blood flow (BF) has not been investigated within the unique context of severe contusion spinal cord injury (SCI), a condition that produces neurogenic bone loss that is precipitated by disuse and other physiological consequences of central nervous system injury. Herein, 4-mo-old male Sprague-Dawley rats received T₉ laminectomy (SHAM) or laminectomy with severe contusion SCI (n = 20/group). Time course assessments of hindlimb bone microstructure and bone perfusion were performed in vivo at 1- and 2-wk postsurgery via microcomputed tomography (microCT) and intracardiac microsphere infusion, respectively, and bone turnover indices were determined via histomorphometry. Both groups exhibited cancellous bone loss beginning in the initial postsurgical week, with cancellous and cortical bone deficits progressing only in SCI thereafter. Trabecular bone deterioration coincided with uncoupled bone turnover after SCI, as indicated by signs of ongoing osteoclast-mediated bone resorption and a near-complete absence of osteoblasts and cancellous bone formation. Bone BF was not different between groups at 1 wk, when both groups displayed bone loss. In comparison, femur and tibia perfusion was 30%-40% lower in SCI versus SHAM at 2 wk, with the most pronounced regional BF deficits occurring at the distal femur. Significant associations existed between distal femur BF and cancellous and cortical bone loss indices. Our data provide the first direct evidence indicating that bone BF deficits develop in response to SCI and temporally coincide with suppressed bone formation and with cancellous and cortical bone deterioration.NEW & NOTEWORTHY We provide the first direct evidence indicating femur and tibia blood flow (BF) deficits exist in conscious (awake) rats after severe contusion spinal cord injury (SCI), with the distal femur displaying the largest BF deficits. Reduced bone perfusion temporally coincided with unopposed bone resorption, as indicated by ongoing osteoclast-mediated bone resorption and a near absence of surface-level bone formation indices, which resulted in severe cancellous and cortical microstructural deterioration after SCI.

Pharmacologic approaches to prevent skeletal muscle atrophy after spinal cord injury

Skeletal muscle atrophy is a hallmark of severe spinal cord injury (SCI) that is precipitated by the neural insult and paralysis. Additionally, other factors may influence muscle loss, including systemic inflammation, low testosterone, low insulin-like growth factor (IGF)-1, and high-dose glucocorticoid treatment. The signaling cascades that drive SCI-induced muscle loss are common among most forms of disuse atrophy and include ubiquitin-proteasome signaling and others. However, differing magnitudes and patterns of atrophic signals exist after SCI versus other disuse conditions and are accompanied by endogenous inhibition of IGF-1/PI3K/Akt signaling, which combine to produce exceedingly rapid atrophy. Several well-established anabolic agents, including androgens and myostatin inhibitors, display diminished ability to prevent SCI-induced atrophy, while ursolic acid and β2-agonists more effectively attenuate muscle loss. Strategies combining physical rehabilitation regimens to reload the paralyzed limbs with drugs targeting the underlying molecular pathways hold the greatest potential to improve muscle recovery after severe SCI.

Osteoporosis Treatment with Anti-Sclerostin Antibodies-Mechanisms of Action and Clinical Application

Osteoporosis is characterized by reduced bone mass and disruption of bone architecture, resulting in increased risk of fragility fractures and significant long-term disability. Although both anti-resorptive treatments and osteoanabolic drugs, such as parathyroid hormone analogues, are effective in fracture prevention, limitations exist due to lack of compliance or contraindications to these drugs. Thus, there is a need for novel potent therapies, especially for patients at high fracture risk. Romosozumab is a monoclonal antibody against sclerostin with a dual mode of action. It enhances bone formation and simultaneously suppresses bone resorption, resulting in a large anabolic window. In this opinion-based narrative review, we highlight the role of sclerostin as a critical regulator of bone mass and present human diseases of sclerostin deficiency as well as preclinical models of genetically modified sclerostin expression, which led to the development of anti-sclerostin antibodies. We review clinical studies of romosozumab in terms of bone mass accrual and anti-fracture activity in the setting of postmenopausal and male osteoporosis, present sequential treatment regimens, and discuss its safety profile and possible limitations in its use. Moreover, an outlook comprising future translational applications of anti-sclerostin antibodies in diseases other than osteoporosis is given, highlighting the clinical significance and future scopes of Wnt signaling in these settings.

Bone and Muscle Crosstalk in Aging

Osteoporosis and sarcopenia are two age-related diseases that affect the quality of life in the elderly. Initially, they were thought to be two independent diseases; however, recently, increasing basic and clinical data suggest that skeletal muscle and bone are both spatially and metabolically connected. The term "osteosarcopenia" is used to define a condition of synergy of low bone mineral density with muscle atrophy and hypofunction. Bone and muscle cells secrete several factors, such as cytokines, myokines, and osteokines, into the circulation to influence the biological and pathological activities in local and distant organs and cells. Recent studies reveal that extracellular vesicles containing microRNAs derived from senescent skeletal muscle and bone cells can also be transported and aid in regulating bone-muscle crosstalk. In this review, we summarize the age-related changes in the secretome and extracellular vesicle-microRNAs secreted by the muscle and bone, and discuss their interactions between muscle and bone cells during aging.

Muscle, Bone, and Fat Crosstalk: the Biological Role of Myokines, Osteokines, and Adipokines

PURPOSE OF REVIEW

Skeletal muscle and bone are connected anatomically and physiologically, and play a crucial role in human locomotion and metabolism. Historically, the coupling between muscle and bone has been viewed in light of mechanotransduction, which dictates that the mechanical forces applied to muscle are transmitted to the skeleton to initiate bone formation. However, these organs also communicate through the endocrine system, orchestrated by a family of cytokines namely myokines (derived from myocytes) and osteokines (derived from bone cells). A third player in this biochemical crosstalk is adipose tissue and the secretion of adipokines (derived from adipocytes). In this review, we discuss the bidirectional effects of myokines and osteokines on muscle and bone metabolism, and the impact of adipokines on both of these secretory organs.

RECENT FINDINGS

Several myokines, notably, IL6, irisin, IGF-1, BDNF, myostatin, and FGF2 exert anabolic/catabolic effects on bone, while the osteokines osteocalcin and sclerostin have shown to induce muscle anabolism and catabolism, respectively. Adipokines, such as leptin, resistin, adiponectin, and TNFα (released from adipose tissue), can also modulate muscle and bone metabolism. Contrarily, exercise-mediated release of lipolytic myokines (IL6, irisin, and LIF) stimulates thermogenesis by promoting the browning of adipocytes. Myokines, osteokines, and adipokines exert autocrine/paracrine effects locally as well as through the endocrine system, to regulate muscle, bone, and fat metabolism. Reductions in physical activity and increases in energy intake, both linked with aging, leads to adipocyte hypertrophy and the recruitment of immunological cells (macrophages). In turn, this releases pro-inflammatory adipokines which induces chronic low-grade inflammation (LGI), a key player in the pathology of several diseases. However, exercise-induced stimulation of bioactive cytokines, through muscle-bone-fat crosstalk, increases muscle anabolism, bone formation, mitochondrial biogenesis, glucose utilization, and fatty acid oxidation, and attenuates chronic LGI.

Characterization of Skeletal Muscle Endocrine Control in an In Vitro Model of Myogenesis

Skeletal muscle has remarkable regenerative abilities regulated by a highly orchestrated process involving the activation of cellular and molecular responses, which are dependent on satellite cells. These cells maintain the stem cell population and provide numerous myogenic cells that proliferate, differentiate, fuse and lead to new myofiber formation for a functional contractile tissue. We have isolated and characterized satellite cells obtained from human biopsies and established an in vitro model of myogenesis, evaluating muscle regeneration, monitoring the dynamic increases of the specific myogenic regulatory factors and the final formation of multinucleated myofibers. As the skeletal muscle is an endocrine tissue able of producing many substances that can act on distant organs, and it can be physiologically modulated by a variety of hormones, we embarked in a project of characterization of muscle cell endocrinology machinery. The expression of a large array of hormone receptors was quantified during the process of myogenesis. The results obtained showed a significant and generalized increase of all the tested hormone receptors along the process of differentiation of human cultured cells from myoblasts to myocytes. Interestingly, also the production of the myokine irisin increased in a parallel manner. These findings point to the human cultured myoblasts as an ideal model to characterize the skeletal muscle endocrine machinery and its hormonal regulation.

Contusion spinal cord injury upregulates p53 protein expression in rat soleus muscle at multiple timepoints but not key senescence cytokines

To determine whether muscle disuse after a spinal cord injury (SCI) produces elevated markers of cellular senescence and induces markers of the senescence-associated secretory phenotypes (SASPs) in paralyzed skeletal muscle. Four-month-old male Sprague-Dawley rats received a moderate-severe (250 kiloDyne) T-9 contusion SCI or Sham surgery and were monitored over 2 weeks, and 1-, 2-, or 3 months. Animals were sacrificed via isoflurane overdose and terminal exsanguination and the soleus was carefully excised and snap frozen. Protein expression of senescence markers p53, p27, and p16 was determined from whole soleus lysates using Western immunoblotting and RT-qPCR was used to determine the soleus gene expression of IL-1α, IL-1β, IL-6, CXCL1, and TNFα. SCI soleus muscle displayed 2- to 3-fold higher total p53 protein expression at 2 weeks, and at 1 and 2 months when compared with Sham. p27 expression was stable across all groups and timepoints. p16 protein expression was lower at 3 months in SCI versus Sham, but not earlier timepoints. Gene expression was relatively stable between groups at 2 weeks. There were Surgery x Time interaction effects for IL-6 and TNFα mRNA expression but not for IL-1α, IL-1β, or CXCL1. There were no main effects for time or surgery for IL-1α, IL-1β, or CXCL1, but targeted t tests showed reductions in IL-1α and CXCL1 in SCI animals compared to Sham at 3 months and IL-1β was reduced in SCI animals compared to Sham animals at the 2-month timepoint. The elevation in p53 does not appear consistent with the induction of SASP because mRNA expression of cytokines associated with senescence was not uniformly upregulated and, in some instances, was downregulated in the early chronic phase of SCI.

Locomotor training with adjuvant testosterone preserves cancellous bone and promotes muscle plasticity in male rats after severe spinal cord injury

Loading and testosterone may influence musculoskeletal recovery after spinal cord injury (SCI). Our objectives were to determine (a) the acute effects of bodyweight-supported treadmill training (TM) on hindlimb cancellous bone microstructure and muscle mass in adult rats after severe contusion SCI and (b) whether longer-term TM with adjuvant testosterone enanthate (TE) delivers musculoskeletal benefit. In Study 1, TM (40 min/day, 5 days/week, beginning 1 week postsurgery) did not prevent SCI-induced hindlimb cancellous bone loss after 3 weeks. In Study 2, TM did not attenuate SCI-induced plantar flexor muscles atrophy nor improve locomotor recovery after 4 weeks. In our main study, SCI produced extensive distal femur and proximal tibia cancellous bone deficits, a deleterious slow-to-fast fiber-type transition in soleus, lower muscle fiber cross-sectional area (fCSA), impaired muscle force production, and levator ani/bulbocavernosus (LABC) muscle atrophy after 8 weeks. TE alone (7.0 mg/week) suppressed bone resorption, attenuated cancellous bone loss, constrained the soleus fiber-type transition, and prevented LABC atrophy. In comparison, TE+TM concomitantly suppressed bone resorption and stimulated bone formation after SCI, produced near-complete cancellous bone preservation, prevented the soleus fiber-type transition, attenuated soleus fCSA atrophy, maintained soleus force production, and increased LABC mass. 75% of SCI+TE+TM animals recovered voluntary over-ground hindlimb stepping, while no SCI and only 20% of SCI+TE animals regained stepping ability. Positive associations between testosterone and locomotor function suggest that TE influenced locomotor recovery. In conclusion, short-term TM alone did not improve bone, muscle, or locomotor recovery in adult rats after severe SCI, while longer-term TE+TM provided more comprehensive musculoskeletal benefit than TE alone.

Longitudinal Examination of Bone Loss in Male Rats After Moderate-Severe Contusion Spinal Cord Injury

To elucidate mechanisms of bone loss after spinal cord injury (SCI), we evaluated the time-course of cancellous and cortical bone microarchitectural deterioration via microcomputed tomography, measured histomorphometric and circulating bone turnover indices, and characterized the development of whole bone mechanical deficits in a clinically relevant experimental SCI model. 16-weeks-old male Sprague-Dawley rats received T₉ laminectomy (SHAM, n = 50) or moderate-severe contusion SCI (n = 52). Outcomes were assessed at 2-weeks, 1-month, 2-months, and 3-months post-surgery. SCI produced immediate sublesional paralysis and persistent hindlimb locomotor impairment. Higher circulating tartrate-resistant acid phosphatase 5b (bone resorption marker) and lower osteoblast bone surface and histomorphometric cancellous bone formation indices were present in SCI animals at 2-weeks post-surgery, suggesting uncoupled cancellous bone turnover. Distal femoral and proximal tibial cancellous bone volume, trabecular thickness, and trabecular number were markedly lower after SCI, with the residual cancellous network exhibiting less trabecular connectivity. Periosteal bone formation indices were lower at 2-weeks and 1-month post-SCI, preceding femoral cortical bone loss and the development of bone mechanical deficits at the distal femur and femoral diaphysis. SCI animals also exhibited lower serum testosterone than SHAM, until 2-months post-surgery, and lower serum leptin throughout. Our moderate-severe contusion SCI model displayed rapid cancellous bone deterioration and more gradual cortical bone loss and development of whole bone mechanical deficits, which likely resulted from a temporal uncoupling of bone turnover, similar to the sequalae observed in the motor-complete SCI population. Low testosterone and/or leptin may contribute to the molecular mechanisms underlying bone deterioration after SCI.

Targeting the Muscle-Bone Unit: Filling Two Needs with One Deed in the Treatment of Duchenne Muscular Dystrophy

PURPOSE OF REVIEW

In Duchenne muscular dystrophy (DMD), the progressive skeletal and cardiac muscle dysfunction and degeneration is accompanied by low bone mineral density and bone fragility. Glucocorticoids, which remain the standard of care for patients with DMD, increase the risk of developing osteoporosis. The scope of this review emphasizes the mutual cohesion and common signaling pathways between bone and skeletal muscle in DMD.

RECENT FINDINGS

The muscle-bone interactions involve bone-derived osteokines, muscle-derived myokines, and dual-origin cytokines that trigger common signaling pathways leading to fibrosis, inflammation, or protein synthesis/degradation. In particular, the triad RANK/RANKL/OPG including receptor activator of NF-kB (RANK), its ligand (RANKL), along with osteoprotegerin (OPG), regulates bone matrix modeling and remodeling pathways and contributes to muscle pathophysiology in DMD. This review discusses the importance of the muscle-bone unit in DMD and covers recent research aimed at determining the muscle-bone interactions that may eventually lead to the development of multifunctional and effective drugs for treating muscle and bone disorders regardless of the underlying genetic mutations in DMD.

Activity-Based Physical Rehabilitation with Adjuvant Testosterone to Promote Neuromuscular Recovery after Spinal Cord Injury

Neuromuscular impairment and reduced musculoskeletal integrity are hallmarks of spinal cord injury (SCI) that hinder locomotor recovery. These impairments are precipitated by the neurological insult and resulting disuse, which has stimulated interest in activity-based physical rehabilitation therapies (ABTs) that promote neuromuscular plasticity after SCI. However, ABT efficacy declines as SCI severity increases. Additionally, many men with SCI exhibit low testosterone, which may exacerbate neuromusculoskeletal impairment. Incorporating testosterone adjuvant to ABTs may improve musculoskeletal recovery and neuroplasticity because androgens attenuate muscle loss and the slow-to-fast muscle fiber-type transition after SCI, in a manner independent from mechanical strain, and promote motoneuron survival. These neuromusculoskeletal benefits are promising, although testosterone alone produces only limited functional improvement in rodent SCI models. In this review, we discuss the (1) molecular deficits underlying muscle loss after SCI; (2) independent influences of testosterone and locomotor training on neuromuscular function and musculoskeletal integrity post-SCI; (3) hormonal and molecular mechanisms underlying the therapeutic efficacy of these strategies; and (4) evidence supporting a multimodal strategy involving ABT with adjuvant testosterone, as a potential means to promote more comprehensive neuromusculoskeletal recovery than either strategy alone.