A review of osteocyte function and the emerging importance of sclerostin

T-Type voltage-sensitive calcium channels mediate mechanically-induced intracellular calcium oscillations in osteocytes by regulating endoplasmic reticulum calcium dynamics

One of the earliest responses of bone cells to mechanical stimuli is a rise in intracellular calcium (Ca(2+)), and osteocytes in particular exhibit robust oscillations in Ca(2+) when subjected to loading. Previous studies implicate roles for both the endoplasmic reticulum (ER) and T-Type voltage-sensitive calcium channels (VSCC) in these responses, but their interactions or relative contributions have not been studied. By observing Ca(2+) dynamics in the cytosol (Ca(2+)cyt) and the ER (Ca(2+)ER), the focus of this study was to explore the role of the ER and T-Type channels in Ca(2+) signaling in bone cells. We demonstrate that inhibition of T-Type VSCC in osteocytes significantly reduces the number of Ca(2+)cyt responses and affects Ca(2+)ER depletion dynamics. Simultaneous observation of Ca(2+) exchange among these spaces revealed high synchrony between rises in Ca(2+)cyt and depressions in Ca(2+)ER, and this synchrony was significantly reduced by challenging T-Type VSCC. We further confirmed that this effect was mediated directly through the ER and not through store-operated Ca(2+) entry (SOCE) pathways. Taken together, our data suggests that T-Type VSCC facilitate the recovery of Ca(2+)ER in osteocytes to sustain mechanically-induced Ca(2+) oscillations, uncovering a new mechanism underlying the behavior of osteocytes as mechanosensors.

Direct effects of physical training on markers of bone metabolism and serum sclerostin concentrations in older adults with low bone mass

Background

Both gravitational loading and the forces generated by muscle contraction have direct effects on serum markers of bone metabolism. The object of this study was to examine the direct effects of a single session of resistance exercise or walking on biochemical markers of bone metabolism in participants with low bone mass.

Methods

A total of 150 otherwise healthy female subjects (mean age = 59.1 ± 7.1 years) diagnosed with osteoporosis or osteopenia were randomly allocated to either a resistance exercise group (RG; n = 50), walking group (WG; n = 50), or control group (CG; n = 50). Changes in bone-specific alkaline phosphatase (BALP), carboxy-terminal cross-linked telopeptide of type I collagen (CTX), and serum sclerostin concentrations were measured before and immediately after a single exercise intervention.

Results

There was no significant change in BALP values in any of the groups. Sclerostin levels increased in the RG and WG, and there was significant difference between the WG and CG after the exercise intervention (P < 0.01). In contrast, the changes in CTX concentrations from baseline were significant in the RG (P < 0.01) but not in the WG (P = 0.11), and there was a significant difference between resistance exercise and walking (P < 0.01).

Conclusions

In participants with low bone mass, resistance exercise influenced the serum concentrations of CTX, a marker of bone resorption, but walking did not.

Trial registration

Current Controlled Trials ISRCTN16329455; retrospectively registered on 05/05/2016.

Chronic Psychosocial Stress Impairs Bone Homeostasis: A Study in the Social Isolation Reared Rat

Chronic psychosocial stress is a key player in the onset and aggravation of mental diseases, including psychosis. Although a strong association between this psychiatric condition and other medical co-morbidities has been recently demonstrated, few data on the link between psychosis and bone homeostasis are actually available. The aim of this study was to investigate whether chronic psychosocial stress induced by 4 or 7 weeks of social isolation in drug-naïve male Wistar rats could alter bone homeostasis in terms of bone thickness, mineral density and content, as well as markers of bone formation and resorption (sclerostin, cathepsin K, and CTX-I). We found that bone mineral density was increased in rats exposed to 7 weeks of social isolation, while no differences were detected in bone mineral content and area. Moreover, 7 weeks of social isolation lead to increase of femur thickness with respect to controls, suggesting the development of a hyperostosis condition. Isolated rats showed no changes in sclerostin levels, a marker of bone formation, compared to grouped animals. Conversely, bone resorption markers were significantly altered after 7 weeks of social isolation in terms of decrease in cathepsin K and increase of CTX-I. No alterations were found after 4 weeks of isolation rearing. Our observations suggest that chronic psychosocial stress might affect bone homeostasis, more likely independently from drug treatment. Thus, the social isolation model might help to identify possible new therapeutic targets to treat the burden of chronic psychosocial stress and to attempt alternative therapy choices.

Bone grafts and their substitutes

The continual cycle of bone formation and resorption is carried out by osteoblasts, osteocytes, and osteoclasts under the direction of the bone-signaling pathway. In certain situations the host cycle of bone repair is insufficient and requires the assistance of bone grafts and their substitutes. The fundamental properties of a bone graft are osteoconduction, osteoinduction, osteogenesis, and structural support. Options for bone grafting include autogenous and allograft bone and the various isolated or combined substitutes of calcium sulphate, calcium phosphate, tricalcium phosphate, and coralline hydroxyapatite. Not all bone grafts will have the same properties. As a result, understanding the requirements of the clinical situation and specific properties of the various types of bone grafts is necessary to identify the ideal graft. We present a review of the bone repair process and properties of bone grafts and their substitutes to help guide the clinician in the decision making process.

Relaxin and insulin-like peptide 3 in the musculoskeletal system: from bench to bedside

Skeletal muscles and bones form a joined functional unit sharing a complex mechanical, biochemical and hormonal crosstalk. A number of factors, including sex hormones, physiologically regulate the musculoskeletal system. Striking gender differences in muscle and bone mass, and function are mainly caused by distinct actions exerted by oestrogens and androgens. However, relaxin and relaxin-related peptides, such as insulin-like peptide 3 (INSL3), might contribute to these sex-associated differences in physiological and pathological conditions (such as osteoporosis and sarcopenia). Relaxin is a 'pregnancy' hormone, but it is also produced from the prostate gland, and has recently attracted attention as a potential drug for cardiovascular disorders and fibrosis. In contrast, INSL3 is a male-specific hormone produced by the Leydig cells of the testis with a fundamental role in testicular descent during fetal life. Recent evidence suggests that both hormones have interesting roles in the musculoskeletal system. Relaxin and INSL3, by finely tuning bone formation and resorption, are involved in bone remodelling processes, and relaxin contributes to the healing of injured ligaments and promotes skeletal muscle regeneration. Here, we review the most recent findings on the effects of relaxin and INSL3 on skeletal muscle and the cell components of bone. In the light of the experimental evidence available and animal models, their clinical implications are also discussed.

LINKED ARTICLES

This article is part of a themed section on Recent Progress in the Understanding of Relaxin Family Peptides and their Receptors. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v174.10/issuetoc.

Sclerostin, TNF-alpha and Interleukin-18 Correlate and are Together with Klotho Related to Other Growth Factors and Cytokines in Haemodialysis Patients

Patients with chronic renal failure are known to have renal osteodystrophy (bone disease) and increased calcification of vessels. A new marker of bone disease, sclerostin, the two pro-inflammatory cytokines tumour necrosis factor-alpha (TNF-alpha) and interleukin-18 (IL-18), and the fibroblast growth factor-23 (FGF-23) receptor-associated marker Klotho were tested in 84 haemodialysis (HD) patients and in healthy controls. The patients had significantly higher levels of the three former markers than of the controls while Klotho was significantly higher in the controls. Low level, but significant, correlations were observed in the patient group when the levels of these four markers were compared to each other and to those of 5 cytokines and growth factors tested earlier; high-sensitive CRP (hsCRP), interleukin-6 (IL-6), hepatocyte growth factor (HGF), fibroblast growth factor-23 (FGF-23) and soluble urokinase plasminogen activator (suPAR). Ln sclerostin correlated positively to Ln hsTNF-alpha, Ln HGF and Ln suPAR. Ln hsTNF-alpha correlated positively to Ln sclerostin, Ln hsCRP, Ln IL-6, Ln FGF-23, Ln suPAR and Ln IL-18. Ln IL-18 correlated positively to Ln suPAR and Ln TNF-alpha. Ln Klotho correlated negatively to Ln hsCRP but did not correlate to Ln FGF-23. The markers studied here may be involved in the calcification of vessels seen in HD patients due to a combination of inflammation and bone disease. The mechanisms are still not fully known but may be of importance for future therapeutic possibilities in this group of patients.

Osteoporosis and Bone Mass Disorders: From Gene Pathways to Treatments

Genomic technologies have evolved rapidly contributing to the understanding of diseases. Genome-wide association studies (GWAS) and whole-exome sequencing have aided the identification of the genetic determinants of monogenic and complex conditions including osteoporosis and bone mass disorders. Overlap exists between the genes implicated in monogenic and complex forms of bone mass disorders, largely explained by the clustering of genes encoding factors in signaling pathways crucial for mesenchymal cell differentiation, skeletal development, and bone remodeling and metabolism. Numerous of the remaining discovered genes merit functional follow-up studies to elucidate their role in bone biology. The insight provided by genetic studies is serving the identification of biomarkers predictive of disease, redefining disease, response to treatment, and discovery of novel drug targets for skeletal disorders.

Effectors of mTOR-autophagy pathway: targeting cancer, affecting the skeleton

Although some modulators of autophagy are emerging as drugs or supplements for anti-cancer therapy, the effects of these compounds on normal tissues must be examined carefully. Here, I review the role of autophagy in skeletal tissues in this context. First, I briefly review preclinical studies indicating the role of autophagy in cancer, as well as related on-going clinical trials. Thereafter, the role of autophagy in the physiology of skeletal tissues is discussed, with a focus on recent genetic preclinical studies. Specifically, I discuss the mTOR-autophagy pathway in relationship to epiphyseal chondrocytes, articular chondrocytes, osteoblasts, osteocytes and osteoclasts and potential side effects of targeting either mTOR pathway or autophagy in general in connection with anti-cancer therapy. Current preclinical findings indicate that inhibiting autophagy will not seriously reduce bone mass and enhance osteoporosis. However, inhibition of autophagy might damage articular cartilage and cause osteoarthritis, whereas treatment with rapalogs might result in relatively beneficial effects on articular cartilage. Modulation of the mTOR pathway or autophagy during childhood may have an undesirable influence on adult height, as well as acquisition of bone mass.

Lower uncarboxylated osteocalcin and higher sclerostin levels are significantly associated with coronary artery disease

Systemic roles for bone-derived proteins have emerged from recent studies. In particular, the serum concentration of osteocalcin (OCN) or sclerostin was found to be associated with altered glucose metabolism or atherosclerosis. The aims of this study were to evaluate OCN and sclerostin levels in subjects who underwent coronary artery bypass graft (CABG) surgery compared with those in normal controls and to analyze their relationships with atherosclerosis. This was an age- and sex-matched case-control study that included 61 male subjects who underwent CABG and 61 controls. Forty-six subjects (37.7%) with diabetes and 62 hypertensive subjects (50.8%) were included. Serum sclerostin, uncarboxylated OCN (ucOCN) and carboxylated OCN (cOCN) were measured. Coronary artery calcium (CAC) score was calculated according to Agatston's method, using a 64-slice multi-detector computed tomography scanner. The levels of serum ucOCN were significantly lower and sclerostin concentrations were higher in the CABG group than in the controls (p<0.05 for both), and these significances were maintained after adjusting for atherosclerotic risk factors in both diabetic and nondiabetic patients (p<0.05 in both groups). However, there was no difference in cOCN levels between CABG patients and controls. The group with abnormal CAC scores (CAC scores ≥100) had significantly higher levels of serum sclerostin (p<0.05). In multiple logistic regression analysis, both lower ucOCN and higher sclerostin levels were independently associated with CABG (odds ratio [OR] 0.43, 95% CI 0.22-0.84, p<0.05 for log(ucOCN); and OR 2.09, 95% CI 1.08-4.05, p<0.05 for log(sclerostin)). In subjects with CAD who underwent CABG, the serum ucOCN level was decreased and the sclerostin level was increased compared with those in the controls, regardless of diabetic status. Longitudinal studies are warranted to establish the precise roles of ucOCN and sclerostin in the pathogenesis of atherosclerosis.

The best of both worlds - managing the cancer, saving the bone

In the context of breast cancer, the importance of the skeleton in the regulation of primary tumour development and as a site for subsequent metastasis is well characterized. Our understanding of the contributions made by the host bone and bone marrow cells increasingly demonstrates the extent of the interaction between tumour cells and normal host cells. As a result, the need to develop and utilize therapies that can impede the growth and/or function of tumour cells while sparing normal host bone and bone marrow cells is immense and expanding. The need for these new treatments is, however, superimposed on the orthopaedic management of patients' quality of life, where pain control and continued locomotion are paramount. Indeed, the majority of the anticancer therapies used to date often result in direct or indirect damage to bone. Thus, although the bone microenvironment regulates tumour cell growth in bone, cells within the bone marrow niche also mediate many of the orthopaedic consequences of tumour progression as well as resistance to the antitumour effects of existing therapies. In this Review, we highlight the effects of existing cancer treatments on bone and the bone marrow microenvironment as well as the mechanisms mediating these effects and the current utility of modern orthopaedic interventions.

Regulation of Sclerostin Production in Human Male Osteocytes by Androgens: Experimental and Clinical Evidence

In this study we aimed to elucidate a possible role of T in the regulation of sclerostin, a glycoprotein secreted by osteocytes known to regulate bone mass. To this end, we evaluated the effect of T stimulation on sclerostin production and gene expression in human cultured osteocytes. In addition, we evaluated serum sclerostin levels in a cohort of 20 hypogonadal male patients, compared with 20 age-matched eugonadal controls. Stimulation with DHT decreased sclerostin expression in cultured osteocytes in a time- and dose-dependent manner. Confirming a direct androgen receptor-mediated effect on sclerostin production, flutamide coincubation and silencing of androgen receptor gene in osteocytes abolished the DHT effects. In addition, hypogonadal patients showed higher serum sclerostin levels with respect to controls (145.87 ± 50.83 pg/mL vs 84.02 ± 32.15 pg/mL; P < .001) and in both probands and controls, serum T levels were negatively correlated with sclerostin (R = -0.664, P = 0.007, and R = -0.447, P = .045, respectively). Finally, multiple stepwise regression analysis showed that T represented the only independent predictor of sclerostin levels. In conclusion, by showing a direct correlation between T and sclerostin, both in vivo and in vitro, this study adds further support to the emerging clinical and experimental studies focusing on sclerostin as a therapeutic target for osteoporosis treatment.

Navigating the bone marrow niche: translational insights and cancer-driven dysfunction

The bone marrow niche consists of stem and progenitor cells destined to become mature cells such as haematopoietic elements, osteoblasts or adipocytes. Marrow cells, influenced by endocrine, paracrine and autocrine factors, ultimately function as a unit to regulate bone remodelling and haematopoiesis. Current evidence highlights that the bone marrow niche is not merely an anatomic compartment; rather, it integrates the physiology of two distinct organ systems, the skeleton and the marrow. The niche has a hypoxic microenvironment that maintains quiescent haematopoietic stem cells (HSCs) and supports glycolytic metabolism. In response to biochemical cues and under the influence of neural, hormonal, and biochemical factors, marrow stromal elements, such as mesenchymal stromal cells (MSCs), differentiate into mature, functioning cells. However, disruption of the niche can affect cellular differentiation, resulting in disorders ranging from osteoporosis to malignancy. In this Review, we propose that the niche reflects the vitality of two tissues - bone and blood - by providing a unique environment for stem and stromal cells to flourish while simultaneously preventing disproportionate proliferation, malignant transformation or loss of the multipotent progenitors required for healing, functional immunity and growth throughout an organism's lifetime. Through a fuller understanding of the complexity of the niche in physiologic and pathologic states, the successful development of more-effective therapeutic approaches to target the niche and its cellular components for the treatment of rheumatic, endocrine, neoplastic and metabolic diseases becomes achievable.

A threshold of mechanical strain intensity for the direct activation of osteoblast function exists in a murine maxilla loading model

The response to the mechanical loading of bone tissue has been extensively investigated; however, precisely how much strain intensity is necessary to promote bone formation remains unclear. Combination studies utilizing histomorphometric and numerical analyses were performed using the established murine maxilla loading model to clarify the threshold of mechanical strain needed to accelerate bone formation activity. For 7 days, 191 kPa loading stimulation for 30 min/day was applied to C57BL/6J mice. Two regions of interest, the AWAY region (away from the loading site) and the NEAR region (near the loading site), were determined. The inflammatory score increased in the NEAR region, but not in the AWAY region. A strain intensity map obtained from [Formula: see text] images was superimposed onto the images of the bone formation inhibitor, sclerostin-positive cell localization. The number of sclerostin-positive cells significantly decreased after mechanical loading of more than [Formula: see text] in the AWAY region, but not in the NEAR region. The mineral apposition rate, which shows the bone formation ability of osteoblasts, was accelerated at the site of surface strain intensity, namely around [Formula: see text], but not at the site of lower surface strain intensity, which was around [Formula: see text] in the AWAY region, thus suggesting the existence of a strain intensity threshold for promoting bone formation. Taken together, our data suggest that a threshold of mechanical strain intensity for the direct activation of osteoblast function and the reduction of sclerostin exists in a murine maxilla loading model in the non-inflammatory region.

The past 10 years-new hormones, new functions, new endocrine organs

Since the publication of the first issue of this journal in November 2005, our understanding of the endocrine system has evolved, with the identification of novel hormones and novel endocrine roles for previously identified molecules. Here, we have asked six of our Advisory Board Members to comment on how these insights have led to the recognition that many organs and tissues that were not widely considered part of the classic endocrine system in the past have important endocrine functions.

Targeting oxidant-dependent mechanisms for the treatment of COPD and its comorbidities

Chronic obstructive pulmonary disease (COPD) is an incurable global health burden and is characterised by progressive airflow limitation and loss of lung function. In addition to the pulmonary impact of the disease, COPD patients often develop comorbid diseases such as cardiovascular disease, skeletal muscle wasting, lung cancer and osteoporosis. One key feature of COPD, yet often underappreciated, is the contribution of oxidative stress in the onset and development of the disease. Patients experience an increased burden of oxidative stress due to the combined effects of excess reactive oxygen species (ROS) and nitrogen species (RNS) generation, antioxidant depletion and reduced antioxidant enzyme activity. Currently, there is a lack of effective treatments for COPD, and an even greater lack of research regarding interventions that treat both COPD and its comorbidities. Due to the involvement of oxidative stress in the pathogenesis of COPD and many of its comorbidities, a unique therapeutic opportunity arises where the treatment of a multitude of diseases may be possible with only one therapeutic target. In this review, oxidative stress and the roles of ROS/RNS in the context of COPD and comorbid cardiovascular disease, skeletal muscle wasting, lung cancer, and osteoporosis are discussed and the potential for therapeutic benefit of anti-oxidative treatment in these conditions is outlined. Because of the unique interplay between oxidative stress and these diseases, oxidative stress represents a novel target for the treatment of COPD and its comorbidities.

Dynamic interplay between bone and multiple myeloma: emerging roles of the osteoblast

Multiple myeloma is a B-cell malignancy characterized by the unrelenting proliferation of plasma cells. Multiple myeloma causes osteolytic lesions and fractures that do not heal due to decreased osteoblastic and increased osteoclastic activity. However, the exact relationship between osteoblasts and myeloma cells remains elusive. Understanding the interactions between these dynamic bone-forming cells and myeloma cells is crucial to understanding how osteolytic lesions form and persist and how tumors grow within the bone marrow. This review provides a comprehensive overview of basic and translational research focused on the role of osteoblasts in multiple myeloma progression and their relationship to osteolytic lesions. Importantly, current challenges for in vitro studies exploring direct osteoblastic effects on myeloma cells, and gaps in understanding the role of the osteoblast in myeloma progression are delineated. Finally, successes and challenges in myeloma treatment with osteoanabolic therapy (i.e., any treatment that induces increased osteoblastic number or activity) are enumerated. Our goal is to illuminate novel mechanisms by which osteoblasts may contribute to multiple myeloma disease progression and osteolysis to better direct research efforts. Ultimately, we hope this may provide a roadmap for new approaches to the pathogenesis and treatment of multiple myeloma with a particular focus on the osteoblast.

Embryonic stem cell-derived osteocytes are capable of responding to mechanical oscillatory hydrostatic pressure

Osteoblasts can be derived from embryonic stem cells (ESCs) by a 30 day differentiation process, whereupon cells spontaneously differentiate upon removal of LIF and respond to exogenously added 1,25α(OH)2 vitamin D3 with enhanced matrix mineralization. However, bone is a load-bearing tissue that has to perform under dynamic pressure changes during daily movement, a capacity that is executed by osteocytes. At present, it is unclear whether ESC-derived osteogenic cultures contain osteocytes and whether these are capable of responding to a relevant cyclic hydrostatic compression stimulus. Here, we show that ESC-osteoblastogenesis is followed by the generation of osteocytes and then mechanically load ESC-derived osteogenic cultures in a compression chamber using a cyclic loading protocol. Following mechanical loading of the cells, iNOS mRNA was upregulated 31-fold, which was consistent with a role for iNOS as an immediate early mechanoresponsive gene. Further analysis of matrix and bone-specific genes suggested a cellular response in favor of matrix remodeling. Immediate iNOS upregulation also correlated with a concomitant increase in Ctnnb1 and Tcf7l2 mRNAs along with increased nuclear TCF transcriptional activity, while the mRNA for the repressive Tcf7l1 was downregulated, providing a possible mechanistic explanation for the noted matrix remodeling. We conclude that ESC-derived osteocytes are capable of responding to relevant mechanical cues, at least such that mimic oscillatory compression stress, which not only provides new basic understanding, but also information that likely will be important for their use in cell-based regenerative therapies.

Lipoprotein receptor-related protein 6 is required for parathyroid hormone-induced Sost suppression

Parathyroid hormone (PTH) suppresses the expression of the bone formation inhibitor sclerostin (Sost) in osteocytes by inducing nuclear accumulation of histone deacetylases (HDACs) to inhibit the myocyte enhancer factor 2 (MEF2)-dependent Sost bone enhancer. Previous studies revealed that lipoprotein receptor-related protein 6 (LRP6) mediates the intracellular signaling activation and the anabolic bone effect of PTH. Here, we investigated whether LRP6 mediates the inhibitory effect of PTH on Sost using an osteoblast-specific Lrp6-knockout (LRP6-KO) mouse model. An increased level of Sost mRNA expression was detected in femur tissue from LRP6-KO mice, compared to wild-type littermates. The number of osteocytes expressing sclerostin protein was also increased in bone tissue of LRP6-KO littermates, indicating a negative regulatory role of LRP6 on Sost/sclerostin. In wild-type littermates, intermittent PTH treatment significantly suppressed Sost mRNA expression in bone and the number of sclerostin(+) osteocytes, while the effect of PTH was much less significant in LRP6-KO mice. Additionally, PTH-induced downregulation of MEF2C and 2D, as well as HDAC changes in osteocytes, were abrogated in LRP6-KO mice. These data indicate that LRP6 is required for PTH suppression of Sost expression.