Mechanotransduction in human bone: in vitro cellular physiology that underpins bone changes with exercise

Mechanobiological optimization of scaffolds for bone tissue engineering

Synthetic bone graft scaffolds aim to generate new bone tissue and alleviate the limitations of autografts and allografts. To meet that aim, it is essential to have a design approach able to generate scaffold architectures that will promote bone formation. Here, we present a topology-varying design optimization method, the "mixed-topology" approach, that generates new designs from a set of starting structures. This approach was used with objective functions focusing on improving the scaffold's local mechanical microenvironments to mechanobiologically promote bone formation within the scaffold and constraints to ensure manufacturability and achieve desired macroscale properties. The results demonstrate that this approach can successfully generate scaffold designs with improved microenvironments, taking into account different combinations of relevant stimuli and constraints.

Physiologic Osseous Remodeling of the Anterior Wall of the Spinal Canal after Anterior Cervical Corpectomy and Fusion: A Retrospective Observational Study

BACKGROUND

 Anterior cervical corpectomy and fusion (ACCF) has been widely used in the treatment of cervical spondylotic myelopathy (CSM), ossification of posterior longitudinal ligament (OPLL), cervical trauma, and other cervical diseases, but few studies have reported the osseous and physiologic remodeling of the anterior wall of the spinal canal following ACCF. In this study, we analyze that remodeling process and its influence on titanium mesh cage (TMC) subsidence.

METHODS

 We performed a clinical and radiologic analysis of consecutive patients treated with ACCF. Growth rates (GRs) reflecting the extent of remodeling of the remnants of the resected vertebral bodies were measured. We compared the computed tomography (CT) scans taken immediately and at least 1 year after surgery, and a literature review was conducted.

RESULTS

 In all, 48 patients underwent ACCF at a mean age of 61.5 ± 12.0 years. The median follow-up was 36 months, and 159 CT images were analyzed. The GR values of the remnants of the vertebral bodies on CT images immediately and 1 year after surgery were 0.505 ± 0.077 and 0.650 ± 0.022 (p < 0.001), respectively, and the GR value at ≥4 years was 1. Axial CT scans showed that remodeling starts from the lateral remnants of the resected vertebral bodies, finally reaching the center. When fusion of the vertebral bodies and the titanium cage was complete during the first year after ACCF, osteogenesis and remodeling were initiated in the osseous anterior wall of the spinal canal. The remodeling of the osseous anterior wall of the spinal canal was completed at the fourth year after surgery, without recompressing the spinal cord, as seen on both axial and lateral CT scans. According to the literature review, there was no TMC subsidence at more than 4 years after surgery.

CONCLUSION

 The anterior wall of the spinal canal undergoes osseous remodeling after ACCF. The process is complete in the fourth year after surgery and prevents TMC subsidence.

Time of exercise differentially impacts bone growth in mice

Although physical training has been shown to improve bone mass, the time of day to exercise for optimal bone growth remains uncertain. Here we show that engaging in physical activity during the early active phase, as opposed to the subsequent active or rest phase, results in a more substantial increase in bone length of male and female mice. Transcriptomic and metabolomic methodologies identify that exercise during the early active phase significantly upregulates genes associated with bone development and metabolism. Notably, oxidative phosphorylation-related genes show a rhythmic expression in the chondrification centre, with a peak at the early active phase, when more rhythmic genes in bone metabolism are expressed and bone growth is synergistically promoted by affecting oxidative phosphorylation, which is confirmed by subsequent pharmacological investigations. Finally, we construct a signalling network to predict the impact of exercise on bone growth. Collectively, our research sheds light on the intricacies of human exercise physiology, offering valuable implications for interventions.

FLRT3 and TGF-β/SMAD4 signalling: Impacts on apoptosis, autophagy and ion channels in supraventricular tachycardia

To explore the underlying molecular mechanisms of supraventricular tachycardia (SVT), this study aimed to analyse the complex relationship between FLRT3 and TGF-β/SMAD4 signalling pathway, which affects Na+ and K+ channels in cardiomyocytes. Bioinformatics analysis was performed on 85 SVT samples and 15 healthy controls to screen overlapping genes from the key module and differentially expressed genes (DEGs). Expression profiling of overlapping genes, coupled with Receiver Operating Characteristic (ROC) curve analyses, identified FLRT3 as a hub gene. In vitro studies utilizing Ang II-stimulated H9C2 cardiomyocytes were undertaken to elucidate the consequences of FLRT3 silencing on cardiomyocyte apoptosis and autophagic processes. Utilizing a combination of techniques such as quantitative reverse-transcription polymerase chain reaction (qRT-PCR), western blotting (WB), flow cytometry, dual-luciferase reporter assays and chromatin immunoprecipitation polymerase chain reaction (ChIP-PCR) assays were conducted to decipher the intricate interactions between FLRT3, the TGF-β/SMAD4 signalling cascade and ion channel gene expression. Six genes (AADAC, DSC3, FLRT3, SYT4, PRR9 and SERTM1) demonstrated reduced expression in SVT samples, each possessing significant clinical diagnostic potential. In H9C2 cardiomyocytes, FLRT3 silencing mitigated Ang II-induced apoptosis and modulated autophagy. With increasing TGF-β concentration, there was a dose-responsive decline in FLRT3 and SCN5A expression, while both KCNIP2 and KCND2 expressions were augmented. Moreover, a direct interaction between FLRT3 and SMAD4 was observed, and inhibition of SMAD4 expression resulted in increased FLRT3 expression. Our results demonstrated that the TGF-β/SMAD4 signalling pathway plays a critical role by regulating FLRT3 expression, with potential implications for ion channel function in SVT.

[Role of Piezo mechanosensitive ion channels in the osteoarticular system]

Objective

To summarize the role of Piezo mechanosensitive ion channels in the osteoarticular system, in order to provide reference for subsequent research.

Methods

Extensive literature review was conducted to summarize the structural characteristics, gating mechanisms, activators and blockers of Piezo ion channels, as well as their roles in the osteoarticular systems.

Results

The osteoarticular system is the main load-bearing and motor tissue of the body, and its ability to perceive and respond to mechanical stimuli is one of the guarantees for maintaining normal physiological functions of bones and joints. The occurrence and development of many osteoarticular diseases are closely related to abnormal mechanical loads. At present, research shows that Piezo mechanosensitive ion channels differentiate towards osteogenesis by responding to stretching stimuli and regulating cellular Ca 2+ influx signals; and it affects the proliferation and migration of osteoblasts, maintaining bone homeostasis through cellular communication between osteoblasts-osteoclasts. Meanwhile, Piezo1 protein can indirectly participate in regulating the formation and activity of osteoclasts through its host cells, thereby regulating the process of bone remodeling. During mechanical stimulation, the Piezo1 ion channel maintains bone homeostasis by regulating the expressions of Akt and Wnt1 signaling pathways. The sensitivity of Piezo1/2 ion channels to high strain mechanical signals, as well as the increased sensitivity of Piezo1 ion channels to mechanical transduction mediated by Ca 2+ influx and inflammatory signals in chondrocytes, is expected to become a new entry point for targeted prevention and treatment of osteoarthritis. But the specific way mechanical stimuli regulate the physiological/pathological processes of bones and joints still needs to be clarified.

Conclusion

Piezo mechanosensitive ion channels give the osteoarticular system with important abilities to perceive and respond to mechanical stress, playing a crucial mechanical sensing role in its cellular fate, bone development, and maintenance of bone and cartilage homeostasis.

A Comprehensive Mechanical Characterization of Subject-Specific 3D Printed Scaffolds Mimicking Trabecular Bone Architecture Biomechanics

This study presents a polymeric scaffold designed and manufactured to mimic the structure and mechanical compressive characteristics of trabecular bone. The morphological parameters and mechanical behavior of the scaffold were studied and compared with trabecular bone from bovine iliac crest. Its mechanical properties, such as modulus of elasticity and yield strength, were studied under a three-step monotonic compressive test. Results showed that the elastic modulus of the scaffold was 329 MPa, and the one for trabecular bone reached 336 MPa. A stepwise dynamic compressive test was used to assess the behavior of samples under various loading regimes. With microcomputed tomography (µCT), a three-dimensional reconstruction of the samples was obtained, and their porosity was estimated as 80% for the polymeric scaffold and 88% for trabecular bone. The full-field strain distribution of the samples was measured using in situ µCT mechanics and digital volume correlation (DVC). This provided information on the local microdeformation mechanism of the scaffolds when compared to that of the tissue. The comprehensive results illustrate the potential of the fabricated scaffolds as biomechanical templates for in vitro studies. Furthermore, there is potential for extending this structure and fabrication methodology to incorporate suitable biocompatible materials for both in vitro and in vivo clinical applications.

Harnessing mechanical cues in the cellular microenvironment for bone regeneration

At the macroscale, bones experience a variety of compressive and tensile loads, and these loads cause deformations of the cortical and trabecular microstructure. These deformations produce a variety of stimuli in the cellular microenvironment that can influence the differentiation of marrow stromal cells (MSCs) and the activity of cells of the MSC lineage, including osteoblasts, osteocytes, and chondrocytes. Mechanotransduction, or conversion of mechanical stimuli to biochemical and biological signals, is thus part of a multiscale mechanobiological process that drives bone modeling, remodeling, fracture healing, and implant osseointegration. Despite strong evidence of the influence of a variety of mechanical cues, and multiple paradigms proposed to explain the influence of these cues on tissue growth and differentiation, even a working understanding of how skeletal cells respond to the complex combinations of stimuli in their microenvironments remains elusive. This review covers the current understanding of what types of microenvironmental mechanical cues MSCs respond to and what is known about how they respond in the presence of multiple such cues. We argue that in order to realize the vast potential for harnessing the cellular microenvironment for the enhancement of bone regeneration, additional investigations of how combinations of mechanical cues influence bone regeneration are needed.

Effect of stress on the dissolution/crystallization of apatite in aqueous solution: a thermochemical equilibrium study

Bone mineralization is critical to maintaining tissue mechanical function. The application of mechanical stress via exercise promotes bone mineralization via cellular mechanotransduction and increased fluid transport through the collagen matrix. However, due to its complex composition and ability to exchange ions with the surrounding body fluids, bone mineral composition and crystallization is also expected to respond to stress. Here, a combination of data from materials simulations, namely density functional theory and molecular dynamics, and experimental studies were input into an equilibrium thermodynamic model of bone apatite under stress in an aqueous solution based on the theory of thermochemical equilibrium of stressed solids. The model indicated that increasing uniaxial stress induced mineral crystallization. This was accompanied by a decrease in calcium and carbonate integration into the apatite solid. These results suggest that weight-bearing exercises can increase tissue mineralization via interactions between bone mineral and body fluid independent of cell and matrix behaviours, thus providing another mechanism by which exercise can improve bone health. This article is part of a discussion meeting issue 'Supercomputing simulations of advanced materials'.

Cyclic Stretch-Induced Mechanical Stress Applied at 1 Hz Frequency Can Alter the Metastatic Potential Properties of SAOS-2 Osteosarcoma Cells

Recently, there has been an increasing focus on cellular morphology and mechanical behavior in order to gain a better understanding of the modulation of cell malignancy. This study used uniaxial-stretching technology to select a mechanical regimen able to elevate SAOS-2 cell migration, which is crucial in osteosarcoma cell pathology. Using confocal and atomic force microscopy, we demonstrated that a 24 h 0.5% cyclic elongation applied at 1 Hz induces morphological changes in cells. Following mechanical stimulation, the cell area enlarged, developing a more elongated shape, which disrupted the initial nuclear-to-cytoplasm ratio. The peripheral cell surface also increased its roughness. Cell-based biochemical assays and real-time PCR quantification showed that these morphologically induced changes are unrelated to the osteoblastic differentiative grade. Interestingly, two essential cell-motility properties in the modulation of the metastatic process changed following the 24 h 1 Hz mechanical stimulation. These were cell adhesion and cell migration, which, in fact, were dampened and enhanced, respectively. Notably, our results showed that the stretch-induced up-regulation of cell motility occurs through a mechanism that does not depend on matrix metalloproteinase (MMP) activity, while the inhibition of ion-stretch channels could counteract it. Overall, our results suggest that further research on mechanobiology could represent an alternative approach for the identification of novel molecular targets of osteosarcoma cell malignancy.

The Impact of Plasma Membrane Ion Channels on Bone Remodeling in Response to Mechanical Stress, Oxidative Imbalance, and Acidosis

The extracellular milieu is a rich source of different stimuli and stressors. Some of them depend on the chemical–physical features of the matrix, while others may come from the ‘outer’ environment, as in the case of mechanical loading applied on the bones. In addition to these forces, a plethora of chemical signals drives cell physiology and fate, possibly leading to dysfunctions when the homeostasis is disrupted. This variety of stimuli triggers different responses among the tissues: bones represent a particular milieu in which a fragile balance between mechanical and metabolic demands should be tuned and maintained by the concerted activity of cell biomolecules located at the interface between external and internal environments. Plasma membrane ion channels can be viewed as multifunctional protein machines that act as rapid and selective dual-nature hubs, sensors, and transducers. Here we focus on some multisensory ion channels (belonging to Piezo, TRP, ASIC/EnaC, P2XR, Connexin, and Pannexin families) actually or potentially playing a significant role in bone adaptation to three main stressors, mechanical forces, oxidative stress, and acidosis, through their effects on bone cells including mesenchymal stem cells, osteoblasts, osteoclasts, and osteocytes. Ion channel-mediated bone remodeling occurs in physiological processes, aging, and human diseases such as osteoporosis, cancer, and traumatic events.

Homo sapiens—A Species Not Designed for Space Flight: Health Risks in Low Earth Orbit and Beyond, Including Potential Risks When Traveling beyond the Geomagnetic Field of Earth

Homo sapiens and their predecessors evolved in the context of the boundary conditions of Earth, including a 1 g gravity and a geomagnetic field (GMF). These variables, plus others, led to complex organisms that evolved under a defined set of conditions and define how humans will respond to space flight, a circumstance that could not have been anticipated by evolution. Over the past ~60 years, space flight and living in low Earth orbit (LEO) have revealed that astronauts are impacted to varying degrees by such new environments. In addition, it has been noted that astronauts are quite heterogeneous in their response patterns, indicating that such variation is either silent if one remained on Earth, or the heterogeneity unknowingly contributes to disease development during aging or in response to insults. With the planned mission to deep space, humans will now be exposed to further risks from radiation when traveling beyond the influence of the GMF, as well as other potential risks that are associated with the actual loss of the GMF on the astronauts, their microbiomes, and growing food sources. Experimental studies with model systems have revealed that hypogravity conditions can influence a variety biological and physiological systems, and thus the loss of the GMF may have unanticipated consequences to astronauts’ systems, such as those that are electrical in nature (i.e., the cardiovascular system and central neural systems). As astronauts have been shown to be heterogeneous in their responses to LEO, they may require personalized countermeasures, while others may not be good candidates for deep-space missions if effective countermeasures cannot be developed for long-duration missions. This review will discuss several of the physiological and neural systems that are affected and how the emerging variables may influence astronaut health and functioning.

Effect of tension and compression on dynamic alveolar histomorphometry

Here, we tested the hypothesis that tensile and compressive stresses generated in the alveolar bone proper regulate site-specific cellular and functional changes in osteoclasts and osteoblasts. Thirty-two 13-week-old male mice were randomly divided into four groups: two experimental groups with vertical loading obliquely from the palatal side to the buccal side of the maxillary molar (0.9 N) 30 min per day for 8 or 15 days and unloaded controls (n = 8). Calcein and alizarin were administered 8 and 2 days before euthanization, respectively, to detect the time of bone formation. Undecalcified sections parallel to the occlusal plane were prepared on the palatal root and the surrounding alveolar bone in the middle of the root length. The alveolar perimeter was divided into 12 equal regions for site analysis, and the bone histomorphometric parameters were obtained for each region. Data from in vivo microfocus computed tomography were used to construct animal-specific finite element models. 2D stress distribution images were overlain on histology images obtained from the same location. Significant differences in the total perimeter between groups and between loading and unloading in each region were statistically analyzed (α = 0.05). Osteoclast counts and the alizarin label ratio were significantly higher in the loaded group than in the unloaded group in regions where the maximum von Mises and principal tensile stresses were the highest along the perimeter. The label ratio of calcein was significantly lower in the 8-day loaded group than in the unloaded group, indicating that the calcein-labeled surface was resorbed by osteoclasts that appeared during the loading period. The effect of loading was mitigated by an increase in the maximum principal compressive stress. We conclude that bone resorption and bone formation are functions of site-specific tension and compression in the alveolar bone proper, confirming our hypothesis. This finding is critical for the advancement of diagnosis and treatment planning in clinical dentistry.

Liquid flow in scaffold derived from natural source: experimental observations and biological outcome

This study investigates the biological effects on a 3D scaffold based on hydroxyapatite cultured with MC3T3 osteoblasts in response to flow-induced shear stress (FSS). The scaffold adopted here (B-HA) derives from the biomorphic transformation of natural wood and its peculiar channel geometry mimics the porous structure of the bone. From the point of view of fluid dynamics, B-HA can be considered a network of micro-channels, intrinsically offering the advantages of a microfluidic system. This work, for the first time, offers a description of the fluid dynamic properties of the B-HA scaffold, which are strongly connected to its morphology. These features are necessary to determine the FSS ranges to be applied during in vitro studies to get physiologically relevant conditions. The selected ranges of FSS promoted the elongation of the attached cells along the flow direction and early osteogenic cell differentiation. These data confirmed the ability of B-HA to promote the differentiation process along osteogenic lineage. Hence, such a bioactive and naturally derived scaffold can be considered as a promising tool for bone regeneration applications.

Modelling Changes in Bone and Body Composition Over a Season in Elite Male Footballers

This study investigated the change in bone and body composition characteristics of elite football players and recreationally active control participants across the course of a season. Fortysix participants (20 footballers and 26 recreationally active controls) were assessed by dual-energy x-ray absorptiometry and peripheral Quantitative Computed Tomography for a range of bone and body composition characteristics at four points over the course of a competitive season. Multilevel modelling was used to examine changes. Footballers had higher characteristics than controls for 24 out of 29 dual-energy x-ray absorptiometry and peripheral Quantitative Computed Tomography variables (all p<0.05). However, there was also significant random inter-individual variation in baseline values for all variables, for both footballers and controls (p < 0.05). Wholebody bone mineral density, leg and whole-body bone mineral content, tibial bone mass and area (38%) increased across the season in footballers (p < 0.05), and there was significant random inter-individual variation in the rate of increase of leg and whole-body bone mineral content (p<0.05). Whole-body bone mineral density, leg and whole-body bone mineral content, tibial bone mass and area (38%) increased over the course of the season in elite football players. The modelling information on expected changes in bone characteristics provides practitioners with a method of identifying those with abnormal bone response to football training and match-play.

The mechanosensory and mechanotransductive processes mediated by ion channels and the impact on bone metabolism: A systematic review

Mechanical environments were associated with alterations in bone metabolism. Ion channels present on bone cells are indispensable for bone metabolism and can be directly or indirectly activated by mechanical stimulation. This review aimed to discuss the literature reporting the mechanical regulatory effects of ion channels on bone cells and bone tissue. An electronic search was conducted in PubMed, Embase and Web of Science. Studies about mechanically induced alteration of bone cells and bone tissue by ion channels were included. Ion channels including TRP family channels, Ca²⁺ release-activated Ca²⁺ channels (CRACs), Piezo1/2 channels, purinergic receptors, NMDA receptors, voltage-sensitive calcium channels (VSCCs), TREK2 potassium channels, calcium- and voltage-dependent big conductance potassium (BK_Ca) channels, small conductance, calcium-activated potassium (SK_Ca) channels and epithelial sodium channels (ENaCs) present on bone cells and bone tissue participate in the mechanical regulation of bone development in addition to contributing to direct or indirect mechanotransduction such as altered membrane potential and ionic flux. Physiological (beneficial) mechanical stimulation could induce the anabolism of bone cells and bone tissue through ion channels, but abnormal (harmful) mechanical stimulation could also induce the catabolism of bone cells and bone tissue through ion channels. Functional expression of ion channels is vital for the mechanotransduction of bone cells. Mechanical activation (opening) of ion channels triggers ion influx and induces the activation of intracellular modulators that can influence bone metabolism. Therefore, mechanosensitive ion channels provide new insights into therapeutic targets for the treatment of bone-related diseases such as osteopenia and aseptic implant loosening.

At the nuclear envelope of bone mechanobiology

The nuclear envelope and nucleoskeleton are emerging as signaling centers that regulate how physical information from the extracellular matrix is biochemically transduced into the nucleus, affecting chromatin and controlling cell function. Bone is a mechanically driven tissue that relies on physical information to maintain its physiological function and structure. Disorder that present with musculoskeletal and cardiac symptoms, such as Emery-Dreifuss muscular dystrophies and progeria, correlate with mutations in nuclear envelope proteins including Linker of Nucleoskeleton and Cytoskeleton (LINC) complex, Lamin A/C, and emerin. However, the role of nuclear envelope mechanobiology on bone function remains underexplored. The mesenchymal stem cell (MSC) model is perhaps the most studied relationship between bone regulation and nuclear envelope function. MSCs maintain the musculoskeletal system by differentiating into multiple cell types including osteocytes and adipocytes, thus supporting the bone's ability to respond to mechanical challenge. In this review, we will focus on how MSC function is regulated by mechanical challenges both in vitro and in vivo within the context of bone function specifically focusing on integrin, β-catenin and YAP/TAZ signaling. The importance of the nuclear envelope will be explored within the context of musculoskeletal diseases related to nuclear envelope protein mutations and nuclear envelope regulation of signaling pathways relevant to bone mechanobiology in vitro and in vivo.

Bone Quality in Chronic Kidney Disease Patients: Current Concepts and Future Directions - Part II

Background

Patients with chronic kidney disease (CKD) have an increased risk of osteoporotic fractures, which is due not only to low bone volume and mass but also poor microarchitecture and tissue quality. The pharmacological and nonpharmacological interventions detailed, herein, are potential approaches to improve bone health in CKD patients. Various medications build up bone mass but also affect bone tissue quality. Antiresorptive therapies strikingly reduce bone turnover; however, they can impair bone mineralization and negatively affect the ability to repair bone microdamage and cause an increase in bone brittleness. On the other hand, some osteoporosis therapies may cause a redistribution of bone structure that may improve bone strength without noticeable effect on BMD. This may explain why some drugs can affect fracture risk disproportionately to changes in BMD.

Summary

An accurate detection of the underlying bone abnormalities in CKD patients, including bone quantity and quality abnormalities, helps in institution of appropriate management strategies. Here in this part II, we are focusing on advancements in bone therapeutics that are anticipated to improve bone health and decrease mortality in CKD patients.

Key Messages

Therapeutic interventions to improve bone health can potentially advance life span. Emphasis should be given to the impact of various therapeutic interventions on bone quality.

Piezo Channels: Awesome Mechanosensitive Structures in Cellular Mechanotransduction and Their Role in Bone

Piezo channels are mechanosensitive ion channels located in the cell membrane and function as key cellular mechanotransducers for converting mechanical stimuli into electrochemical signals. Emerged as key molecular detectors of mechanical forces, Piezo channels' functions in bone have attracted more and more attention. Here, we summarize the current knowledge of Piezo channels and review the research advances of Piezo channels' function in bone by highlighting Piezo1's role in bone cells, including osteocyte, bone marrow mesenchymal stem cell (BM-MSC), osteoblast, osteoclast, and chondrocyte. Moreover, the role of Piezo channels in bone diseases is summarized.

High-protein supplementation facilitates weight training-induced bone mineralization in baseball players

OBJECTIVES

The aim of this study was to determine whether weight training combined with high-protein intake enhances total and regional bone mineral density (BMD) in athletes.

METHODS

BMD of 27 Division 1 collegiate baseball players 18 to 22 y of age (N = 13, 2 dropouts) received either 14% protein or isocaloric 44% protein supplements and were assessed by dual-energy x-ray absorptiometry before and after a 12-wk weight training program (challenging upper and lower body).

RESULTS

Baseline data showed unequivocally greater humerus BMD in the dominant arm than their contralateral non-dominant arm (∼20 %) among all baseball players. Humerus BMD of the non-dominant arm was enhanced by 2.7% after weight training for both low- and high-protein groups (main effect, P = 0.008), concurrent with an unexpected small decrease in total body BMD (main effect, P = 0.014). Humerus BMD of the dominant arm with greater baseline value than the non-dominant arm was not increased unless high protein was supplemented (+2.7 %; P < 0.05).

CONCLUSION

Bones with relatively higher BMD show blunt adaptation against training, which can be relieved by high-protein supplementation. Total BMD of athletes cannot be further elevated by weight training.

Optimal Human Functioning Requires Exercise Across the Lifespan: Mobility in a 1g Environment Is Intrinsic to the Integrity of Multiple Biological Systems

It is widely acknowledged that achieving and maintaining a healthier lifestyle can be enhanced through regular participation in sport and physical activity. Coevally, a growing number of health professionals regard exercise as a legitimate intervention strategy for those who have lost their health. Exercise has been shown to be effective for overweight or obese individuals, who are at risk to lose their health due to development of type II diabetes, cardiovascular disease, as well as, infiltration of muscles, bone and other organs with fat, so it can be considered medicine. However, exercise and associated mobility likely also have a strong prevention component that can effectively contribute to the maintenance of the integrity of multiple biological systems for those who do not have overt risk factors or ongoing disease. While prevention is preferred over intervention in the context of disease, it is clear that exercise and associated mobility, generally, can be an effective influence, although overtraining and excessive loading can be deleterious to health. The basis for the generally positive influence of exercise likely lies in the fact that many of our physiological systems are designed to function in the mechanically dynamic and active 1g environment of Earth (e.g., muscles, cartilage, ligaments, tendons, bones, and cardiovascular system, and neuro-cognitive function), and nearly all these systems subscribe to the "use it or lose it" paradigm. This conclusion is supported by the changes observed over the more than 50 years of space flight and exposure to microgravity conditions. Therefore, the premise advanced is: "exercise is preventative for loss of health due to age-related decline in the integrity of several physiological systems via constant reinforcement of those systems, and thus, optimal levels of exercise and physical activity are endemic to, essential for, and intrinsic to optimal health and wellbeing."

Impact of changes in fat mass and lean soft tissue on bone mineral density accrual in adolescents engaged in different sports: ABCD Growth Study

Body composition can have a significant impact on bone mineral density (BMD) in pediatric patients and may cause bone disease. This study demonstrated that lean soft tissue (LST) seems to have a greater impact on BMD gain in the lower limbs of adolescents.

PURPOSE

To analyze the impact of changes in lean soft tissue (LST) and fat mass on areal bone mineral density (BMD) accrual in the lower limbs among adolescents engaged in sports with different weight-bearing levels.

METHODS

Longitudinal (12 months: measurements at two time points). Adolescents of both sexes (n = 191; 62 girls [32.5%] and 129 boys [67.5%]) were divided into three groups: control group (n = 65), swimming group (n = 25), and weight-bearing sports group (n = 100). Absolute changes in LST (kg) and fat mass (kg) were the independent variables, while BMD accrual (left and right legs) was the dependent variable. Linear regression was used to assess the relationship between dependent and independent variables in a multivariate model adjusted by sex, body weight, somatic maturation, serum osteocalcin, and baseline values of BMD of the lower limbs.

RESULTS

For the left leg, LST was positively related to areal BMD accrual in the control (β = 0.021 [95%CI: 0.001 to 0.042]) and weight-bearing sport groups (β = 0.051 [95%CI: 0.037 to 0.065]), but not among swimmers (β = 0.029 [95%CI: - 0.004 to 0.062]). For the right leg, LST was positively related to areal BMD accrual in the swimming group (β = 0.065 [95%CI: 0.031 to 0.100]) and weight-bearing sport groups (β = 0.048 [95%CI: 0.034 to 0.062]), but not in the control group (β = 0.014 [95%CI: - 0.002 to 0.030]). Fat mass was not significantly related to areal BMD in either leg.

CONCLUSIONS

Changes in LST were the most relevant determinant of BMD accrual in the lower limbs, mainly among adolescents engaged in sports.

Maximal strength training improves musculoskeletal health in amphetamine users in clinical treatment

Amphetamine use leads to impaired skeletal health and elevated risk of osteoporosis. In the current study, we document that maximal strength training (MST), as a part of clinical treatment, works as a countermeasure, improving muscle force generating capacity, body composition, and skeletal health at sites particularly prone to osteoporotic fractures.

INTRODUCTION

Amphetamine users have attenuated musculoskeletal health. MST with heavy loads, few repetitions, and emphasis on maximal mobilization in the concentric phase may increase muscle force generating capacity and skeletal health. This study investigated if MST-induced improvements in force generating capacity improved bone mineral density (BMD), trabecular bone score, and body composition in amphetamine users participating in 3-months clinical treatment.

METHODS

Of 40 randomized patients, 23 completed the study: 11 in the supervised training group (TG; 8 men, 3 women, 34 ± 10 years) and 12 in the control group (CG; 9 men, 3 women, 32 ± 8 years). The TG performed hack-squat MST three times a week for 12 weeks with an intensity of ~90% of one repetition maximum (1RM). Both groups attended conventional clinical treatment. Pre-training and post-training, we assessed hack-squat 1RM and rate of force development (RFD), BMD, body composition and trabecular bone score by dual X-ray absorptiometry, and serum bone metabolism markers.

RESULTS

MST induced increases in 1RM (70%) and RFD (86%), and resulted in BMD improvements at lumbar spine (3.6%) and total hip (2.4%); all improvements were different from CG (p < 0.05). Both the 1RM and RFD increases were associated with BMD improvements (lumbar spine: r = 0.73 (1RM), r = 0.60 (RFD); total hip: r = 0.61 (1RM); all p < 0.05). No differences were observed in trabecular bone score or bone metabolism markers.

CONCLUSIONS

MST improved force generating capacity and skeletal health at sites prone to bone loss in amphetamine users, and advocate that MST should be implemented as a clinical strategy to restore the patients' musculoskeletal health.

Intensity‑dependent effect of treadmill running on differentiation of rat bone marrow stromal cells

The effect of running on bone mass depends on its intensity. However, the underlying molecular mechanism that associates running intensity with bone mass is unclear. The current study examined the effects of treadmill running at different intensities on bone mass and osteogenic differentiation of bone marrow stromal cells (BMSCs) in a rat model. A total of 24 male Wistar rats were randomly divided into groups and subjected to no running (Con group), low‑intensity running (LIR group), moderate‑intensity running (MIR group), and high‑intensity running (HIR group). Histological, immunohistochemistry and micro‑CT examinations were performed on the femora harvested after 8 weeks of treadmill running. The study demonstrated that treadmill running affected trabecular bone mass in an intensity‑dependent manner. In addition, such an intensity‑dependent effect was also demonstrated on the osteogenic and adipogenic differentiation and proliferation of BMSCs. Furthermore, the Wnt/β‑catenin signaling pathway may be involved in the running‑induced increase in bone mass in rats in the MIR group. There appears to be a biomechanical 'window', in which running‑induced strain signals can increase the number of BMSCs and progenitor cells (specific to the osteoblast lineage) causing upregulation of osteogenesis and downregulation of adipogenesis of BMSCs. This finding may provide insight into the molecular and cellular mechanisms responsible for bone homeostasis.

Translation of remote control regenerative technologies for bone repair

The role of biomechanical stimuli, or mechanotransduction, in normal bone homeostasis and repair is understood to facilitate effective osteogenesis of mesenchymal stem cells (MSCs) in vitro. Mechanotransduction has been integrated into a multitude of in vitro bone tissue engineering strategies and provides an effective means of controlling cell behaviour towards therapeutic outcomes. However, the delivery of mechanical stimuli to exogenous MSC populations, post implantation, poses a significant translational hurdle. Here, we describe an innovative bio-magnetic strategy, MICA, where magnetic nanoparticles (MNPs) are used to remotely deliver mechanical stimuli to the mechano-receptor, TREK-1, resulting in activation and downstream signalling via an external magnetic array. In these studies, we have translated MICA to a pre-clinical ovine model of bone injury to evaluate functional bone repair. We describe the development of a magnetic array capable of in vivo MNP manipulation and subsequent osteogenesis at equivalent field strengths in vitro. We further demonstrate that the viability of MICA-activated MSCs in vivo is unaffected 48 h post implantation. We present evidence to support early accelerated repair and preliminary enhanced bone growth in MICA-activated defects within individuals compared to internal controls. The variability in donor responses to MICA-activation was evaluated in vitro revealing that donors with poor osteogenic potential were most improved by MICA-activation. Our results demonstrate a clear relationship between responders to MICA in vitro and in vivo. These unique experiments offer exciting clinical applications for cell-based therapies as a practical in vivo source of dynamic loading, in real-time, in the absence of pharmacological agents.

Lab-on-a-chip platforms for quantification of multicellular interactions in bone remodeling

Researchers have been using lab-on-a-chip systems to isolate factors for study, simulate laboratory analysis and model cellular, tissue and organ level processes. The technology is increasing rapidly, but the bone field has been slow to keep pace. Novel models are needed that have the power and flexibility to investigate the elegant and synchronous multicellular interactions that occur in normal bone turnover and in disease states in which remodeling is implicated. By removing temporal and spatial limitations and enabling quantification of functional outcomes, the platforms should provide unique environments that are more biomimetic than single cell type systems while minimizing complex systemic effects of in vivo models. This manuscript details the development and characterization of lab-on-a-chip platforms for stimulating osteocytes and quantifying bone remodeling. Our platforms provide the foundation for a model that can be used to investigate remodeling interactions as a whole or as a standard mechanotransduction tool by which isolated activity can be quantified as a function of load.

Obesity, Metabolic Syndrome, and Musculoskeletal Disease: Common Inflammatory Pathways Suggest a Central Role for Loss of Muscle Integrity

Inflammation can arise in response to a variety of stimuli, including infectious agents, tissue injury, autoimmune diseases, and obesity. Some of these responses are acute and resolve, while others become chronic and exert a sustained impact on the host, systemically, or locally. Obesity is now recognized as a chronic low-grade, systemic inflammatory state that predisposes to other chronic conditions including metabolic syndrome (MetS). Although obesity has received considerable attention regarding its pathophysiological link to chronic cardiovascular conditions and type 2 diabetes, the musculoskeletal (MSK) complications (i.e., muscle, bone, tendon, and joints) that result from obesity-associated metabolic disturbances are less frequently interrogated. As musculoskeletal diseases can lead to the worsening of MetS, this underscores the imminent need to understand the cause and effect relations between the two, and the convergence between inflammatory pathways that contribute to MSK damage. Muscle mass is a key predictor of longevity in older adults, and obesity-induced sarcopenia is a significant risk factor for adverse health outcomes. Muscle is highly plastic, undergoes regular remodeling, and is responsible for the majority of total body glucose utilization, which when impaired leads to insulin resistance. Furthermore, impaired muscle integrity, defined as persistent muscle loss, intramuscular lipid accumulation, or connective tissue deposition, is a hallmark of metabolic dysfunction. In fact, many common inflammatory pathways have been implicated in the pathogenesis of the interrelated tissues of the musculoskeletal system (e.g., tendinopathy, osteoporosis, and osteoarthritis). Despite these similarities, these diseases are rarely evaluated in a comprehensive manner. The aim of this review is to summarize the common pathways that lead to musculoskeletal damage and disease that result from and contribute to MetS. We propose the overarching hypothesis that there is a central role for muscle damage with chronic exposure to an obesity-inducing diet. The inflammatory consequence of diet and muscle dysregulation can result in dysregulated tissue repair and an imbalance toward negative adaptation, resulting in regulatory failure and other musculoskeletal tissue damage. The commonalities support the conclusion that musculoskeletal pathology with MetS should be evaluated in a comprehensive and integrated manner to understand risk for other MSK-related conditions. Implications for conservative management strategies to regulate MetS are discussed, as are future research opportunities.

HOW DO BONE CELLS SENSE MECHANICAL LOADING?

Influenced by gravidity, bone tissue experiences stronger or lighter deformation according to the strength of the activities of daily life. Activities resulting in impact are particularly known to stimulate osteogenesis, thus reducing bone mass loss. Knowing how bone cells recognize the mechanical deformation imposed to the bone and trigger a series of biochemical chain reactions is of crucial importance for the development of therapeutic and preventive practices in orthopaedic activity. There is still a long way to run until we can understand the whole process, but current knowledge has shown a strong progression, with researches being conducted focused on therapies. For a mechanical sign to be transformed into a biological one (mechanotransduction), it must be amplified at cell level by the histological structure of bone tissue, producing tensions in cell membrane proteins (integrins) and changing their spatial structure. Such change activates bindings between these and the cytoskeleton, producing focal adhesions, where cytoplasmatic proteins are recruited to enable easier biochemical reactions. Focal adhesion kinase (FAK) is the most important one being self-activated when its structure is changed by integrins. Activated FAK triggers a cascade of reactions, resulting in the activation of ERK-1/2 and Akt, which are proteins that, together with FAK, regulate the production of bone mass. Osteocytes are believed to be the mechanosensor cells of the bone and to transmit the mechanical deformation to osteoblasts and osteoclasts. Ionic channels and gap junctions are considered as intercellular communication means for biochemical transmission of a mechanical stimulus. These events occur continuously on bone tissue and regulate bone remodeling.

Overcoming translational challenges - The delivery of mechanical stimuli in vivo

Despite major medical advances, non-union bone fractures and skeletal defects continue to place significant burden on the patient, the clinicians and the healthcare system as a whole. Current bone substitute approaches are still limited in effectiveness and to date no adequate bone substitute material has been developed for routine clinical application. Tissue engineering presents a novel approach to tackling this clinical burden and developing an acceptable solution for the treatment of skeletal defects. Over the past three decades the field has evolved to appreciate the key biological, material and physical parameters influencing the development of a cell-based tissue engineered therapy and to create associated technologies to exploit such parameters. In recent years a number of therapies have started progressing along the pre-clinical pipeline to build a case for regulatory approval and ultimately clinical adoption. However, little emphasis has been given to the translational challenges faced when moving from "bench-to-bedside". One particular challenge lies in the delivery of functional mechanical stimuli to implanted cell populations to activate and promote osteogenic activities. This review introduces novel bio-magnetic approaches to overcoming this challenge.

microRNA-103a functions as a mechanosensitive microRNA to inhibit bone formation through targeting Runx2

Emerging evidence indicates that microRNAs (miRNAs) play essential roles in regulating osteoblastogenesis and bone formation. However, the role of miRNA in osteoblast mechanotransduction remains to be defined. In this study, we aimed to investigate whether miRNAs regulate mechanical stimulation-triggered osteoblast differentiation and bone formation through modulation of Runx2, the master transcription factor for osteogenesis. We first investigated the role of mechanical loading both in a mouse model and in an osteoblast culture system and the outcomes clearly demonstrated that mechanical stimuli can regulate osteogenesis and bone formation both in vivo and in vitro. Using bioinformatic analyses and subsequent confirmation by quantitative real-time PCR (qRT-PCR), we found that multiple miRNAs that potentially target Runx2 were responding to in vitro mechanical stimulation, among which miR-103a was fully characterized. miR-103a and its host gene PANK3 were both downregulated during cyclic mechanical stretch (CMS)-induced osteoblast differentiation, whereas Runx2 protein expression was upregulated. Overexpression of miR-103a significantly decreased and inhibition of miR-103a increased Runx2 protein level, suggesting that miR-103a acts as an endogenous attenuator of Runx2 in osteoblasts. Mutation of putative miR-103a binding sites in Runx2 mRNA abolishes miR-103a-mediated repression of the Runx2 3'-untranslated region (3'UTR) luciferase reporter activity, suggesting that miR-103a binds to Runx2 3'UTR. Osteoblast marker gene profiling and osteogenic phenotype assays demonstrated that miR-103a negatively correlates with CMS-induced osteogenesis. Further, the perturbation of miR-103a also has a significant effect on osteoblast activity and matrix mineralization. More importantly, we found an inhibitory role of miR-103a in regulating bone formation in hindlimb unloading mice, and pretreatment with antagomir-103a partly rescued the osteoporosis caused by mechanical unloading. Taken together, our data suggest that miR-103a is the first identified mechanosensitive miRNA that regulates osteoblast differentiation by directly targeting Runx2, and therapeutic inhibition of miR-103a may be an efficient anabolic strategy for skeletal disorders caused by pathological mechanical loading.

Simulated microgravity inhibits L-type calcium channel currents partially by the up-regulation of miR-103 in MC3T3-E1 osteoblasts

L-type voltage-sensitive calcium channels (LTCCs), particularly Cav1.2 LTCCs, play fundamental roles in cellular responses to mechanical stimuli in osteoblasts. Numerous studies have shown that mechanical loading promotes bone formation, whereas the removal of this stimulus under microgravity conditions results in a reduction in bone mass. However, whether microgravity exerts an influence on LTCCs in osteoblasts and whether this influence is a possible mechanism underlying the observed bone loss remain unclear. In the present study, we demonstrated that simulated microgravity substantially inhibited LTCC currents and suppressed Cav1.2 at the protein level in MC3T3-E1 osteoblast-like cells. In addition, reduced Cav1.2 protein levels decreased LTCC currents in MC3T3-E1 cells. Moreover, simulated microgravity increased miR-103 expression. Cav1.2 expression and LTCC current densities both significantly increased in cells that were transfected with a miR-103 inhibitor under mechanical unloading conditions. These results suggest that simulated microgravity substantially inhibits LTCC currents in osteoblasts by suppressing Cav1.2 expression. Furthermore, the down-regulation of Cav1.2 expression and the inhibition of LTCCs caused by mechanical unloading in osteoblasts are partially due to miR-103 up-regulation. Our study provides a novel mechanism for microgravity-induced detrimental effects on osteoblasts, offering a new avenue to further investigate the bone loss induced by microgravity.

Function and regulation of primary cilia and intraflagellar transport proteins in the skeleton

Primary cilia are microtubule-based organelles that project from the cell surface to enable transduction of various developmental signaling pathways. The process of intraflagellar transport (IFT) is crucial for the building and maintenance of primary cilia. Ciliary dysfunction has been found in a range of disorders called ciliopathies, some of which display severe skeletal dysplasias. In recent years, interest has grown in uncovering the function of primary cilia/IFT proteins in bone development, mechanotransduction, and cellular regulation. We summarize recent advances in understanding the function of cilia and IFT proteins in the regulation of cell differentiation in osteoblasts, osteocytes, chondrocytes, and mesenchymal stem cells (MSCs). We also discuss the mechanosensory function of cilia and IFT proteins in bone cells, cilia orientation, and other functions of cilia in chondrocytes.

The roles of P2Y2 purinergic receptors in osteoblasts and mechanotransduction

We previously demonstrated, using osteoblastic MC3T3-E1 cells, that P2Y₂ purinergic receptors are involved in osteoblast mechanotransduction. In this study, our objective was to further investigate, using a knockout mouse model, the roles of P2Y₂ receptors in bone mechanobiology. We first examined bone structure with micro-CT and measured bone mechanical properties with three point bending experiments in both wild type mice and P2Y₂ knockout mice. We found that bones from P2Y₂ knockout mice have significantly decreased bone volume, bone thickness, bone stiffness and bone ultimate breaking force at 17 week old age. In order to elucidate the mechanisms by which P2Y₂ receptors contribute to bone biology, we examined differentiation and mineralization of bone marrow cells from wild type and P2Y₂ knockout mice. We found that P2Y₂ receptor deficiency reduces the differentiation and mineralization of bone marrow cells. Next, we compared the response of primary osteoblasts, from both wild type and P2Y₂ knockout mice, to ATP and mechanical stimulation (oscillatory fluid flow), and found that osteoblasts from wild type mice have a stronger response, in terms of ERK1/2 phosphorylation, to both ATP and fluid flow, relative to P2Y₂ knockout mice. However, we did not detect any difference in ATP release in response to fluid flow between wild type and P2Y₂ knock out osteoblasts. Our findings suggest that P2Y₂ receptors play important roles in bone marrow cell differentiation and mineralization as well as in bone cell mechanotransduction, leading to an osteopenic phenotype in P2Y₂ knockout mice.

Nox family NADPH oxidases in mechano-transduction: mechanisms and consequences

SIGNIFICANCE

The majority of cells in a multi-cellular organism are continuously exposed to ever-changing physical forces. Mechano-transduction links these events to appropriate reactions of the cells involving stimulation of signaling cascades, reorganization of the cytoskeleton and alteration of gene expression.

RECENT ADVANCES

Mechano-transduction alters the cellular redox balance and the formation of reactive oxygen species (ROS). Nicotine amide adenine dinucleotide reduced form (NADPH) oxidases of the Nox family are prominent ROS generators and thus, contribute to this stress-induced ROS formation.

CRITICAL ISSUES

Different types and patterns of mechano-stress lead to Nox-dependent ROS formation and Nox-mediated ROS formation contributes to cellular responses and adaptation to physical forces. Thereby, Nox enzymes can mediate vascular protection during physiological mechano-stress. Despite this, over-activation and induction of Nox enzymes and a subsequent substantial increase in ROS formation also promotes oxidative stress in pathological situations like disturbed blood flow or extensive stretch.

FUTURE DIRECTIONS

Individual protein targets of Nox-mediated redox-signaling will be identified to better understand the specificity of Nox-dependent ROS signaling in mechano-transduction. Nox-inhibitors will be tested to reduce cellular activation in response to mechano-stimuli.

Mechanobiological modulation of cytoskeleton and calcium influx in osteoblastic cells by short-term focused acoustic radiation force

Mechanotransduction has demonstrated potential for regulating tissue adaptation in vivo and cellular activities in vitro. It is well documented that ultrasound can produce a wide variety of biological effects in biological systems. For example, pulsed ultrasound can be used to noninvasively accelerate the rate of bone fracture healing. Although a wide range of studies has been performed, mechanism for this therapeutic effect on bone healing is currently unknown. To elucidate the mechanism of cellular response to mechanical stimuli induced by pulsed ultrasound radiation, we developed a method to apply focused acoustic radiation force (ARF) (duration, one minute) on osteoblastic MC3T3-E1 cells and observed cellular responses to ARF using a spinning disk confocal microscope. This study demonstrates that the focused ARF induced F-actin cytoskeletal rearrangement in MC3T3-E1 cells. In addition, these cells showed an increase in intracellular calcium concentration following the application of focused ARF. Furthermore, passive bending movement was noted in primary cilium that were treated with focused ARF. Cell viability was not affected. Application of pulsed ultrasound radiation generated only a minimal temperature rise of 0.1°C, and induced a streaming resulting fluid shear stress of 0.186 dyne/cm(2), suggesting that hyperthermia and acoustic streaming might not be the main causes of the observed cell responses. In conclusion, these data provide more insight in the interactions between acoustic mechanical stress and osteoblastic cells. This experimental system could serve as basis for further exploration of the mechanosensing mechanism of osteoblasts triggered by ultrasound.

The impact of different types of physical activity on total and regional bone mineral density in young Brazilian athletes

Bone turnover is affected by exercise throughout the lifespan, especially during childhood and adolescence. The objective of this study was to investigate the impact of different sports on total and regional bone mineral density in male Brazilian adolescent athletes. Forty-six adolescents aged 10-18 years participated in the study: 12 swimmers, 10 tennis players, 10 soccer players, and 14 sedentary individuals. The athletes had engaged in physical activities for more than 10 h per week in the previous 6 months. Bone mineral density of the lumbar spine (L1-L4), left proximal femur region, and whole body was evaluated by dual-energy X-ray absorptiometry. Results showed higher mean values in the proximal femur region of tennis and soccer players (1.02 ± 0.18; 0.96 ± 0.16, respectively) than swimmers and controls (0.91 ± 0.14 and 0.87 ± 0.06, respectively) (P < 0.05). In relation to the impact of sporting activities based on bone age determination, we observed significant differences in bone mineral density at all evaluated sites at the end of puberty (16-18 years) compared with 10-12 years, with increases of 78% in the lumbar spine, 47% in the proximal femur, and 38% in the whole body.

The effects of vibration loading on adipose stem cell number, viability and differentiation towards bone-forming cells

Mechanical stimulation is an essential factor affecting the metabolism of bone cells and their precursors. We hypothesized that vibration loading would stimulate differentiation of human adipose stem cells (hASCs) towards bone-forming cells and simultaneously inhibit differentiation towards fat tissue. We developed a vibration-loading device that produces 3g peak acceleration at frequencies of 50 and 100 Hz to cells cultured on well plates. hASCs were cultured using either basal medium (BM), osteogenic medium (OM) or adipogenic medium (AM), and subjected to vibration loading for 3 h d(-1) for 1, 7 and 14 day. Osteogenesis, i.e. differentiation of hASCs towards bone-forming cells, was analysed using markers such as alkaline phosphatase (ALP) activity, collagen production and mineralization. Both 50 and 100 Hz vibration frequencies induced significantly increased ALP activity and collagen production of hASCs compared with the static control at 14 day in OM. A similar trend was detected for mineralization, but the increase was not statistically significant. Furthermore, vibration loading inhibited adipocyte differentiation of hASCs. Vibration did not affect cell number or viability. These findings suggest that osteogenic culture conditions amplify the stimulatory effect of vibration loading on differentiation of hASCs towards bone-forming cells.

A pilot investigation of load-carrying on the head and bone mineral density in premenopausal, black African women

Although the influence of weight-bearing activity on bone mass has been widely investigated in white women, few studies have been conducted in black, African populations. We investigated bone mineral density (BMD) in black South African women, with and without a history of load-carrying on the head. We also investigated whether load carrying may offer protection against low BMD in users of injectable progestin contraception (IPC). Participants were 32 black, South African women (22.4 +/- 3.2 years). Load carrying history was determined by questionnaire and interview; participants were grouped as load carriers (LC; n = 18) or non-load carriers (NLC; n = 14). Ten women were using IPC and 6 were load-carriers. Total body (TB), lumbar spine (LS) and total hip (H) BMD were measured by dual energy X-ray absorptiometry. There were no differences in BMD between LC and NLC, and after controlling for age and BMI using two-tailed partial correlations. IPC users had lower BMD at all sites compared to non-IPC users (p < 0.05) and there were no associations between load carrying and BMD in this group. When IPC users were excluded from analysis, LC had higher LS BMD than NLC (p < 0.005). Correlations were found between the weight of load carried and LS BMD (r = 0.743, p < 0.005), and between years of load carrying and LS and TB BMD (r = 0.563, r = 0.538, respectively; both p < 0.05). Load carrying on the head may offer osteogenic benefits to the spine but these benefits did not appear in women using IPC.

Bone rigidity to neuromuscular performance ratio in young and elderly men

Given the adaptation of bone to prevalent loading, bone loss should follow, but lag behind, the decline in physical performance during aging. Furthermore, bone responsiveness to load-induced strains is believed to decrease with aging. However, the relationship between bone and lean body ( approximately muscle) mass appears to remain rather constant throughout adulthood. The purpose of this study was to examine the association between age and bone to neuromuscular performance ratio. Young (N=20, age 24 SD+/-2 years, body mass 77+/-11 kg, height 178+/-6 cm) and elderly (N=25, 72+/-4 years, 75+/-9 kg, 172+/-5 cm) men served as subjects. Bone structural traits were measured at the right distal tibia and tibial mid-shaft with peripheral quantitative computed tomography (pQCT). Maximal section modulus (Z(max50)), total area (ToA(d)), cortical area (CoA(50)), total density (ToD(d)) and cortical density (CoD(50)) were determined from the pQCT images. Neuromuscular performance was measured by recording vertical ground reaction force (GRF) in maximal bilateral hopping. Load-induced strains were estimated by calculating appropriate indices for compressive and tensile loading that took into account both the bone structure and apparent biomechanics of the given bone site. Young subjects had significantly higher maximal GRF compared to older men (4260+/-800 N vs. 3080+/-600 N, P<0.001). They also had smaller ToA(d) (1100+/-170 mm(2) vs. 1200+/-100 mm(2), P=0.028) while their ToD(d) was higher (370+/-46 g/cm(3) vs. 330+/-22 g/cm(3), P=0.002). The Z(max50) did not differ significantly between young (1660+/-320 mm(3)) and elderly men (1750+/-320 mm(3)) (P=0.224). Compressive (0.484+/-0.102 vs. 0.399+/-0.078, P=0.016) and tensile (0.107+/-0.016 vs. 0.071+/-0.018, P<0.001) strain indices were significantly higher in the younger group. In conclusion, the difference in bone to loading ratio at the tibial mid-shaft is bigger than expected from the delay in bone adaptation alone. Potential candidates to explain this phenomenon include a decrease in mechanosensitivity with aging, inability of maximal physical performance to adequately represent the bone loading environment, or the need to maintain constant safety factors to functional strains.

Functional alterations in mechanical loading of condylar cartilage induces changes in the bony subcondylar region

Bone remodeling is orchestrated by cells of the osteoblast lineage and involves an intricate network of cell-cell and cell-matrix interactions. This dynamic process engages systemic hormones, locally produced cytokines and growth factors, as well as the mechanical environment of the cells. In growing subjects, the mandibular condyle consists of both articular and growth components and the presence of progenitor cells is verified by their anabolic responses to growth hormones. The pathways of chondrocyte and osteoblast differentiation during endochondral bone formation are interconnected and controlled by key transcription factors. The present study was undertaken to explore the possibility and the extent by which the mechano-transduction events in chondrocytes are 'sensed' in the subchondral bony area under altered functional loading. To this end, the involvement of the JNK/ERK-AP-1/Runx2 signaling axe was investigated by immunohistochemistry in temporomandibular joints of young rats subjected to different functional mastication loads. Our results showed that mechanical load triggers differentiation phenomena through the induction of master tissue regulators, namely the expression and/or activation of the JNK-c-Jun signaling pathway components and c-Fos in subchondral osteoblasts, as well as the activation of ERK/MAPK and the cellular expression of the transcription factor Runx2 in subchondral osteoblasts.

Cholesterol-regulated stress fiber formation

Dynamic interactions between cellular membranes and the cytoskeleton are known to play major roles in many cellular responses to environmental cues. External signals resulting in proliferation, differentiation, polarization, and motility must be translated from chemical signals into changes of state, often involving the cytoskeleton-dependent altering of cell shape and redistribution of molecules. Cholesterol, a critical component of eukaryotic cell membranes, performs vital roles in regulating membrane dynamics and function. Here we demonstrate, using mesenchymal and epithelial cell lines, that depletion of membrane cholesterol results in Src kinase-mediated Rho activation and caveolin phosphorylation, which together collaborate to form stress fibers. These results demonstrate that cholesterol is a critical regulator of membrane-cytoskeletal dynamics and suggest that altered cholesterol concentrations may result in dramatic changes in cellular responses mediated by the cytoskeleton.