Adynamic Bone Decreases Bone Toughness During Aging by Affecting Mineral and Matrix

Rejuvenating Aged Bone Repair through Multihierarchy Reactive Oxygen Species-Regulated Hydrogel

Aging exacerbates the dysfunction of tissue regeneration at multiple levels and gradually diminishes individual's capacity to withstand stress, damage, and disease. The excessive accumulation of reactive oxygen species (ROS) is considered a hallmark feature of senescent stem cells, which causes oxidative stress, deteriorates the host microenvironment, and eventually becomes a critical obstacle for aged bone defect repair. Till now, the strategies cannot synchronously and thoroughly regulate intracellular and extracellular ROS in senescent cells. Herein, a multihierarchy ROS scavenging system for aged bone regeneration is developed by fabricating an injectable PEGylated poly(glycerol sebacate) (PEGS-NH2 )/poly(γ-glutamic acid) (γ-PGA) hydrogel containing rapamycin-loaded poly(diselenide-carbonate) nanomicelles (PSeR). This PSeR hydrogel exhibits highly sensitive ROS responsiveness to the local aged microenvironment and dynamically releases drug-loaded nanomicelles to scavenge the intracellular ROS accumulated in senescent bone mesenchymal stem cells. The PSeR hydrogel effectively tunes the antioxidant function and delays senescence of bone mesenchymal stem cells by safeguarding DNA replication in an oxidative environment, thereby promoting the self-renewal ability and enhancing the osteogenic capacity for aged bone repair in vitro and in vivo. Thus, this multihierarchy ROS-regulated hydrogel provides a new strategy for treating degenerative diseases.

Effect of gamma irradiation and supercritical carbon dioxide sterilization with Novakill™ or ethanol on the fracture toughness of cortical bone

Sterilization of structural bone allografts is a critical process prior to their clinical use in large cortical bone defects. Gamma irradiation protocols are known to affect tissue integrity in a dose dependent manner. Alternative sterilization treatments, such as supercritical carbon dioxide (SCCO2 ), are gaining popularity due to advantages such as minimal exposure to denaturants, the lack of toxic residues, superior tissue penetration, and minor impacts on mechanical properties including strength and stiffness. The impact of SCCO2 on the fracture toughness of bone tissue, however, remains unknown. Here, we evaluate crack initiation and growth toughness after 2, 6, and 24 h SCCO2 -treatment using Novakill™ and ethanol as additives on ~11 samples per group obtained from a pair of femur diaphyses of a canine. All mechanical testing was performed at ambient air after 24 h soaking in Hanks' balanced salt solution (HBSS). Results show no statistically significant difference in the failure characteristics of the Novakill™-treated groups whereas crack growth toughness after 6 and 24 h of treatment with ethanol significantly increases by 37% (p = .010) and 34% (p = .038), respectively, compared to an untreated control group. In contrast, standard 25 kGy gamma irradiation causes significantly reduced crack growth resistance by 40% (p = .007) compared to untreated bone. FTIR vibrational spectroscopy, conducted after testing, reveals a consistent trend of statistically significant differences (p < .001) with fracture toughness. These trends align with variations in the ratios of enzymatic mature to immature crosslinks in the collagen structure, suggesting a potential association with fracture toughness. Additional Raman spectroscopy after testing shows a similar trend with statistically significant differences (p < .005), which further supports that collagen structural changes occur in the SCF-treated groups with ethanol after 6 and 24 h. Our work reveals the benefits of SCCO2 sterilization compared to gamma irradiation.

Bone Material Properties in Bone Diseases Affecting Children

PURPOSE OF REVIEW

Metabolic and genetic bone disorders affect not only bone mass but often also the bone material, including degree of mineralization, matrix organization, and lacunar porosity. The quality of juvenile bone is moreover highly influenced by skeletal growth. This review aims to provide a compact summary of the present knowledge on the complex interplay between bone modeling and remodeling during skeletal growth and to alert the reader to the complexity of bone tissue characteristics in children with bone disorders.

RECENT FINDINGS

We describe cellular events together with the characteristics of the different tissues and organic matrix organization (cartilage, woven and lamellar bone) occurring during linear growth. Subsequently, we present typical alterations thereof in disorders leading to over-mineralized bone matrix compared to those associated with low or normal mineral content based on bone biopsy studies. Growth spurts or growth retardation might amplify or mask disease-related alterations in bone material, which makes the interpretation of bone tissue findings in children complex and challenging.

Physicochemical understanding of biomineralization by molecular vibrational spectroscopy: From mechanism to nature

The process and mechanism of biomineralization and relevant physicochemical properties of mineral crystals are remarkably sophisticated multidisciplinary fields that include biology, chemistry, physics, and materials science. The components of the organic matter, structural construction of minerals, and related mechanical interaction, etc., could help to reveal the unique nature of the special mineralization process. Herein, the paper provides an overview of the biomineralization process from the perspective of molecular vibrational spectroscopy, including the physicochemical properties of biomineralized tissues, from physiological to applied mineralization. These physicochemical characteristics closely to the hierarchical mineralization process include biological crystal defects, chemical bonding, atomic doping, structural changes, and content changes in organic matter, along with the interface between biocrystals and organic matter as well as the specific mechanical effects for hardness and toughness. Based on those observations, the special physiological properties of mineralization for enamel and bone, as well as the possible mechanism of pathological mineralization and calcification such as atherosclerosis, tumor micro mineralization, and urolithiasis are also reviewed and discussed. Indeed, the clearly defined physicochemical properties of mineral crystals could pave the way for studies on the mechanisms and applications.

Nuclear Factor-Kappa B Regulation of Osteoclastogenesis and Osteoblastogenesis

Maintenance of skeletal integrity requires the coordinated activity of multinucleated bone-resorbing osteoclasts and bone-forming osteoblasts. Osteoclasts form resorption lacunae on bone surfaces in response to cytokines by fusion of precursor cells. Osteoblasts are derived from mesenchymal precursors and lay down new bone in resorption lacunae during bone remodeling. Nuclear factorkappa B (NF-κB) signaling regulates osteoclast and osteoblast formation and is activated in osteoclast precursors in response to the essential osteoclastogenic cytokine, receptor activator of NF-κB ligand (RANKL), which can also control osteoblast formation through RANK-RANKL reverse signaling in osteoblast precursors. RANKL and some pro-inflammatory cytokines, including tumor necrosis factor (TNF), activate NF-κB signaling to positively regulate osteoclast formation and functions. However, these cytokines also limit osteoclast and osteoblast formation through NF-κB signaling molecules, including TNF receptor-associated factors (TRAFs). TRAF6 mediates RANKL-induced osteoclast formation through canonical NF-κB signaling. In contrast, TRAF3 limits RANKL- and TNF-induced osteoclast formation, and it restricts transforming growth factor β (TGFβ)-induced inhibition of osteoblast formation in young and adult mice. During aging, neutrophils expressing TGFβ and C-C chemokine receptor type 5 (CCR5) increase in bone marrow of mice in response to increased NF-κB-induced CC motif chemokine ligand 5 (CCL5) expression by mesenchymal progenitor cells and injection of these neutrophils into young mice decreased bone mass. TGFβ causes degradation of TRAF3, resulting in decreased glycogen synthase kinase-3β/β-catenin-mediated osteoblast formation and age-related osteoporosis in mice. The CCR5 inhibitor, maraviroc, prevented accumulation of TGFβ+/CCR5+ neutrophils in bone marrow and increased bone mass by inhibiting bone resorption and increasing bone formation in aged mice. This paper updates current understanding of how NF-κB signaling is involved in the positive and negative regulation of cytokine-mediated osteoclast and osteoblast formation and activation with a focus on the role of TRAF3 signaling, which can be targeted therapeutically to enhance bone mass.

Pathophysiology of bone disease in chronic kidney disease: from basics to renal osteodystrophy and osteoporosis

Chronic kidney disease (CKD) is a highly prevalent disease that has become a public health problem. Progression of CKD is associated with serious complications, including the systemic CKD-mineral and bone disorder (CKD-MBD). Laboratory, bone and vascular abnormalities define this condition, and all have been independently related to cardiovascular disease and high mortality rates. The "old" cross-talk between kidney and bone (classically known as "renal osteodystrophies") has been recently expanded to the cardiovascular system, emphasizing the importance of the bone component of CKD-MBD. Moreover, a recently recognized higher susceptibility of patients with CKD to falls and bone fractures led to important paradigm changes in the new CKD-MBD guidelines. Evaluation of bone mineral density and the diagnosis of "osteoporosis" emerges in nephrology as a new possibility "if results will impact clinical decisions". Obviously, it is still reasonable to perform a bone biopsy if knowledge of the type of renal osteodystrophy will be clinically useful (low versus high turnover-bone disease). However, it is now considered that the inability to perform a bone biopsy may not justify withholding antiresorptive therapies to patients with high risk of fracture. This view adds to the effects of parathyroid hormone in CKD patients and the classical treatment of secondary hyperparathyroidism. The availability of new antiosteoporotic treatments bring the opportunity to come back to the basics, and the knowledge of new pathophysiological pathways [OPG/RANKL (LGR4); Wnt-ß-catenin pathway], also affected in CKD, offers great opportunities to further unravel the complex physiopathology of CKD-MBD and to improve outcomes.

TGFβ1+CCR5+ neutrophil subset increases in bone marrow and causes age-related osteoporosis in male mice

TGFβ1 induces age-related bone loss by promoting degradation of TNF receptor-associated factor 3 (TRAF3), levels of which decrease in murine and human bone during aging. We report that a subset of neutrophils (TGFβ1+CCR5+) is the major source of TGFβ1 in murine bone. Their numbers are increased in bone marrow (BM) of aged wild-type mice and adult mice with TRAF3 conditionally deleted in mesenchymal progenitor cells (MPCs), associated with increased expression in BM of the chemokine, CCL5, suggesting that TRAF3 in MPCs limits TGFβ1+CCR5+ neutrophil numbers in BM of young mice. During aging, TGFβ1-induced TRAF3 degradation in MPCs promotes NF-κB-mediated expression of CCL5 by MPCs, associated with higher TGFβ1+CCR5+ neutrophil numbers in BM where they induce bone loss. TGFβ1+CCR5+ neutrophils decreased bone mass in male mice. The FDA-approved CCR5 antagonist, maraviroc, reduced TGFβ1+CCR5+ neutrophil numbers in BM and increased bone mass in aged mice. 15-mon-old mice with TGFβRII specifically deleted in MPCs had lower numbers of TGFβ1+CCR5+ neutrophils in BM and higher bone volume than wild-type littermates. We propose that pharmacologic reduction of TGFβ1+CCR5+ neutrophil numbers in BM could treat or prevent age-related osteoporosis.

Fluid shear stress-induced down-regulation of miR-146a-5p inhibits osteoblast apoptosis via targeting SMAD4

Fluid shear stress (FSS) plays an important role in osteoblast apoptosis. However, the role of miRNA in osteoblast apoptosis under FSS and possible molecular mechanisms remain unknown. Our aim of the study was to explore whether miR-146a-5p regulates osteoblast apoptosis under FSS and its molecular mechanisms. FSS could down-regulate the expression of miR-146a-5p in MC3T3-E1 cells. We confirm that up-regulation of miR-146a-5p promotes osteoblasts apoptosis and down-regulation of miR-146a-5p inhibits osteoblasts apoptosis. We further demonstrated that FSS inhibits osteoblast apoptosis by down-regulated miR-146a-5p. Dual-luciferase reporter assay validated that SMAD4 is a direct target gene of miR-146a-5p. In addition, mimic-146a-5p suppressed FSS-induced up-regulation of SMAD4 protein levels, which suggests that FSS elevated SMAD4 protein expression levels via regulation miR-146a-5p. Further investigations showed that SMAD4 could inhibit osteoblast apoptosis. We demonstrated that miR-146a-5p regulates osteoblast apoptosis via targeting SMAD4. Taken together, our present study showed that FSS-induced down-regulation miR-146a-5p inhibits osteoblast apoptosis via target SMAD4. These findings may provide novel mechanisms for FSS to inhibit osteoblast apoptosis, and also may provide a potential therapeutic target for osteoporosis.

Is Adynamic Bone Always a Disease? Lessons from Patients with Chronic Kidney Disease

Renal osteodystrophy (ROD) is a common complication of end-stage kidney disease that often starts early with loss of kidney function, and it is considered an integral part in management of patients with chronic kidney disease (CKD). Adynamic bone (ADB) is characterized by suppressed bone formation, low cellularity, and thin osteoid seams. There is accumulating evidence supporting increasing prevalence of ADB, particularly in early CKD. Contemporarily, it is not very clear whether it represents a true disease, an adaptive mechanism to prevent bone resorption, or just a transitional stage. Several co-players are incriminated in its pathogenesis, such as age, diabetes mellitus, malnutrition, uremic milieu, and iatrogenic factors. In the present review, we will discuss the up-to-date knowledge of the ADB and focus on its impact on bone health, fracture risk, vascular calcification, and long-term survival. Moreover, we will emphasize the proper preventive and management strategies of ADB that are pivotal issues in managing patients with CKD. It is still unclear whether ADB is always a pathologic condition or whether it can represent an adaptive process to suppress bone resorption and further bone loss. In this article, we tried to discuss this hard topic based on the available limited information in patients with CKD. More studies are needed to be able to clearly address this frequent ROD finding.

Compositional assessment of bone by Raman spectroscopy

Raman spectroscopy (RS) is used to analyze the physiochemical properties of bone because it is non-destructive and requires minimal sample preparation. With over two decades of research involving measurements of mineral-to-matrix ratio, type-B carbonate substitution, crystallinity, and other compositional characteristics of the bone matrix by RS, there are multiple methods to acquire Raman signals from bone, to process those signals, and to determine peak ratios including sub-peak ratios as well as the full-width at half maximum of the most prominent Raman peak, which is nu1 phosphate (ν1PO4). Selecting which methods to use is not always clear. Herein, we describe the components of RS instruments and how they influence the quality of Raman spectra acquired from bone because signal-to-noise of the acquisition and the accompanying background fluorescence dictate the pre-processing of the Raman spectra. We also describe common methods and challenges in preparing acquired spectra for the determination of matrix properties of bone. This article also serves to provide guidance for the analysis of bone by RS with examples of how methods for pre-processing the Raman signals and for determining properties of bone composition affect RS sensitivity to potential differences between experimental groups. Attention is also given to deconvolution methods that are used to ascertain sub-peak ratios of the amide I band as a way to assess characteristics of collagen type I. We provide suggestions and recommendations on the application of RS to bone with the goal of improving reproducibility across studies and solidify RS as a valuable technique in the field of bone research.

Patterns of renal osteodystrophy one year after kidney transplantation

BACKGROUND

Renal osteodystrophy is considered common, but is not well characterized, in contemporary kidney transplant recipients. This study reports extensively on bone phenotype by bone histomorphometry, bone densitometry, and novel bone biomarkers 1 year after kidney transplantation.

METHODS

A transiliac bone biopsy and dual energy x-ray absorptiometry were performed in 141 unselected kidney transplant recipients in this observational cohort study. Blood and 24 hr urine samples were collected simultaneously.

RESULTS

Median age was 57 ± 11 years, 71% were men, and all were of Caucasian ethnicity. Bone turnover was normal in 71% of patients, low in 26%, and high in just four cases (3%). Hyperparathyroidism with hypercalcemia was present in 13% of patients, of which one had high bone turnover. Delayed bone mineralization was detected in 16% of patients, who were characterized by hyperparathyroidism (137 vs. 53 ρg/mL), a higher fractional excretion of phosphate (40 vs. 32%), and lower levels of phosphate (2.68 vs 3.18 mg/dL) and calcidiol (29 vs. 37 ng/mL) compared to patients with normal bone mineralization. Osteoporosis was present in 15-46% of patients, with the highest prevalence at the distal skeleton. The proportion of osteoporotic patients was comparable across categories of bone turnover and mineralization.

CONCLUSION

The majority of kidney transplant recipients, including patients with osteoporosis, have a normal bone turnover at 1-year post-transplant. Low bone turnover is seen in a substantial subset, while high bone turnover is rare. Vitamin D deficiency and hypophosphatemia represent potential interventional targets to improve bone health post-transplant.

Differentiating the causes of adynamic bone in advanced chronic kidney disease informs osteoporosis treatment

Patients with chronic kidney disease (CKD) have an increased fracture risk because of impaired bone quality and quantity. Low bone mineral density predicts fracture risk in all CKD stages, including advanced CKD (CKD G4-5D). Pharmacological therapy improves bone mineral density and reduces fracture risk in moderate CKD. Its efficacy in advanced CKD remains to be determined, although pilot studies suggest a positive effect on bone mineral density. Currently, antiresorptive agents are the most commonly prescribed drugs for the prevention and therapy of osteoporosis. Their use in advanced CKD has been limited by the lack of large clinical trials and fear of causing kidney dysfunction and adynamic bone disease. In recent decades, adynamic bone disease has evolved as the most predominant form of renal osteodystrophy, commonly associated with poor outcomes, including premature mortality and progression of vascular calcification. Evolving evidence indicates that reduction of bone turnover by parathyroidectomy or pharmacological therapies, such as calcimimetics and antiresorptive agents, are not associated with premature mortality or accelerated vascular calcification in CKD. In contrast, chronic inflammation, oxidative stress, malnutrition, and diabetes can induce low bone turnover and associate with poor prognosis. Thus, the conditions causing suppression of bone turnover rather than the low bone turnover per se may account for the perceived association with outcomes. Anabolic treatment, in contrast, has been suggested to improve turnover and bone mass in patients with advanced CKD and low bone turnover; however, uncertainty about safety even exceeds that of antiresorptive agents. Here, we critically review the pathophysiological concept of adynamic bone disease and discuss the effect of low bone turnover on the safety and efficacy of anti-osteoporosis pharmacotherapy in advanced CKD.

Traditional and Non-traditional Risk Factors for Osteoporosis in CKD

Osteoporosis is a state of bone fragility with reduced skeletal resistance to trauma, and consequently increased risk of fracture. A wide range of conditions, including traditional risk factors, lifestyle choices, diseases and their treatments may contribute to bone fragility. It is therefore not surprising that the multi-morbid patient with chronic kidney disease (CKD) is at a particularly high risk. CKD is associated with reduced bone quantity, as well as impaired bone quality. Bone fragility in CKD is a composite of primary osteoporosis, accumulation of traditional and uremia-related risk factors, assaults brought on by systemic disease, and detrimental effects of drugs. Some risk factors are modifiable and represent potential targets for intervention. This review provides an overview of the heterogeneity of bone fragility in CKD.

No Role of Osteocytic Osteolysis in the Development and Recovery of the Bone Phenotype Induced by Severe Secondary Hyperparathyroidism in Vitamin D Receptor Deficient Mice

Osteocytic osteolysis/perilacunar remodeling is thought to contribute to the maintenance of mineral homeostasis. Here, we utilized a reversible, adult-onset model of secondary hyperparathyroidism to study femoral bone mineralization density distribution (BMDD) and osteocyte lacunae sections (OLS) based on quantitative backscattered electron imaging. Male mice with a non-functioning vitamin D receptor (VDRΔ/Δ) or wild-type mice were exposed to a rescue diet (RD) (baseline) and subsequently to a low calcium challenge diet (CD). Thereafter, VDRΔ/Δ mice received either the CD, a normal diet (ND), or the RD. At baseline, BMDD and OLS characteristics were similar in VDRΔ/Δ and wild-type mice. The CD induced large cortical pores, osteomalacia, and a reduced epiphyseal average degree of mineralization in the VDRΔ/Δ mice relative to the baseline (-9.5%, p < 0.05 after two months and -10.3%, p < 0.01 after five months of the CD). Switching VDRΔ/Δ mice on the CD back to the RD fully restored BMDD to baseline values. However, OLS remained unchanged in all groups of mice, independent of diet. We conclude that adult VDRΔ/Δ animals on an RD lack any skeletal abnormalities, suggesting that VDR signaling is dispensable for normal bone mineralization as long as mineral homeostasis is normal. Our findings also indicate that VDRΔ/Δ mice attempt to correct a calcium challenge by enhanced osteoclastic resorption rather than by osteocytic osteolysis.

Routine serum biomarkers, but not dual-energy X-ray absorptiometry, correlate with cortical bone mineral density in children and young adults with chronic kidney disease

Background. Biomarkers and dual-energy X-ray absorptiometry (DXA) are thought to be poor predictors of bone mineral density (BMD). The Kidney Disease: Improving Global Outcomes guidelines suggest using DXA if the results will affect patient management, but this has not been studied in children or young adults in whom bone mineral accretion continues to 30 years of age. We studied the clinical utility of DXA and serum biomarkers against tibial cortical BMD (CortBMD) measured by peripheral quantitative computed tomography, expressed as Z-score CortBMD, which predicts fracture risk.

Methods. This was a cross-sectional multicentre study in 26 patients with CKD4 and 5 and 77 on dialysis.

Results. Significant bone pain that hindered activities of daily living was present in 58%, and 10% had at least one low-trauma fracture. CortBMD and cortical mineral content Z-scores were lower in dialysis compared with CKD patients (P = 0.004 and P = 0.02). DXA BMD hip and lumbar spine Z-scores did not correlate with CortBMD or biomarkers. CortBMD was negatively associated with parathyroid hormone (PTH; r = −0.44, P &lt; 0.0001) and alkaline phosphatase (ALP; r = −0.22, P = 0.03) and positively with calcium (Ca; r = 0.33, P = 0.001). At PTH &lt;3 times upper limit of normal, none of the patients had a CortBMD below −2 SD (odds ratio 95% confidence interval 7.331 to infinity). On multivariable linear regression PTH (β = −0.43 , P &lt; 0.0001), ALP (β = −0.36, P &lt; 0.0001) and Ca (β = 0.21, P = 0.005) together predicted 57% of variability in CortBMD. DXA measures did not improve this model.

Conclusions. Taken together, routinely used biomarkers, PTH, ALP and Ca, but not DXA, are moderate predictors of cortical BMD. DXA is not clinically useful and should not be routinely performed in children and young adults with CKD 4–5D.

Mechanosensitivity of Cells and Its Role in the Regulation of Physiological Functions and the Implementation of Physiotherapeutic Effects (Review)

Regulatory signals in the body are not limited to chemical and electrical ones. There is another type of important signals for cells: those are mechanical signals (coming from the environment or arising from within the body), which have been less known in the literature. The review summarizes new information on the mechanosensitivity of various cells of connective tissue and nervous system. Participation of mechanical stimuli in the regulation of growth, development, differentiation, and functioning of tissues is described. The data focus on bone remodeling, wound healing, neurite growth, and the formation of neural networks. Mechanotransduction, cellular organelles, and mechanosensitive molecules involved in these processes are discussed as well as the role of the extracellular matrix. The importance of mechanical characteristics of cells in the pathogenesis of diseases is highlighted. Finally, the possible role of mechanosensitivity in mediating the physiotherapeutic effects is addressed.

Impact of Weekly Teriparatide on the Bone and Mineral Metabolism in Hemodialysis Patients With Relatively Low Serum Parathyroid Hormone: A Pilot Study

Adynamic bone disease in HD patients is characterized by skeletal resistance to parathyroid hormone (PTH) or suppression of PTH release, leading to a downregulated bone turnover and bone fracture. Hence, we examined the efficacy of weekly teriparatide for HD patients with low PTH indicating adynamic bone disease without a history of parathyroidectomy. Fifteen HD patients with low PTH were recruited in this prospective observational study. Of them, 10 received teriparatide for 12 months and five nontreated patients were enrolled as control. Primary outcomes were defined as the changes in bone mineral density and bone turnover markers. Bone mineral density at the lumbar spine increased by 3.7% and 2.5% at 6 and 12 months, respectively, and bone formation markers increased, while bone resorption markers did not change in the teriparatide group. At 12 months after teriparatide administration, endogenous PTH was secreted followed by the recovery of low bone turnover. 40% of patients in the teriparatide group dropped out due to adverse events and the most common adverse event was transient hypotension. This study suggests that weekly teriparatide for HD patients with low PTH in the absence of parathyroidectomy accelerates bone formation and bone turnover, leading to increased trabecular bone mass and secretion of endogenous PTH.

Renal insufficiency and bone loss

PURPOSE OF REVIEW

Patients with chronic kidney disease have a high risk of fractures and no established treatments that have been shown to prevent the bone disease. The physiology of renal osteodystrophy is complex and recently more factors have been found that complicate the mineral metabolism. The recognition that vascular calcifications are related to bone disease has made treatment even more challenging.

RECENT FINDINGS

The most exciting new findings relate to the signaling pathways that are seen in kidney disease and how they cause abnormalities in bone physiology. In particular, wnt and activin signaling pathways are seen early in the course of renal disease. The bones react by increasing FGF-23, which targets both renal phosphate secretion and a variety of other systemic effects. Secreted klotho is another newly described hormone with effects on several systems.Clinical studies have focused on treatments for hyperparathyroidism and phosphate, and frustrating limitations of the treatments used for ordinary osteoporosis.

SUMMARY

Treatment of bone disease in patients with chronic kidney disease is challenging, and understanding the physiological pathways could lead to novel therapies.

The Role of Dickkopf-1 in Thyroid Hormone-Induced Changes of Bone Remodeling in Male Mice

Thyroid hormones regulate bone homeostasis, and exogenously induced hyperthyroidism and hypothyroidism in mice was recently found to be associated with an altered expression of the Wnt inhibitor Dickkopf-1 (Dkk1), a determinant of bone mass. Here, we assessed the role of Dkk1 in thyroid hormone-induced changes in bone using conditional Dkk1 knockout mice. Male mice with a global (Dkk1fl/fl;Rosa26-CreERT2) or osteocyte-specific (Dkk1fl/fl;Dmp1:Cre) deletion of Dkk1 were pharmacologically rendered hypothyroid or hyperthyroid. The bone phenotype was analyzed using micro-CT analysis, dynamic histomorphometry, and serum concentrations of bone turnover markers. Hypothyroid and hyperthyroid Cre-negative mice of either Cre line revealed the expected changes in bone volume with hypothyroid mice displaying a 40% to 60% increase in vertebral trabecular bone volume, while hyperthyroid mice lost 45% to 60% of bone volume. Similar changes were observed at the spine. Interestingly, Cre-positive mice of both lines did not gain or lose as much bone at the femur when rendered hypothyroid or hyperthyroid. While Cre-negative hypothyroid mice gained 80% to 100% bone volume, Cre-positive hypothyroid mice only increased their bone volume by 55% to 90%. Similarly, Cre-negative hyperthyroid mice lost 74% to 79% bone, while Cre-positive hyperthyroid mice merely lost 40% to 54%. Despite these site-specific differences, both global and osteocyte-specific Dkk1 knockout mice displayed similar changes in bone turnover as their Cre-negative controls in the hypothyroid and hyperthyroid states. While osteoblast and osteoclast parameters were increased in hyperthyroidism, hypothyroidism potently suppressed bone cell activities. Loss of Dkk1 is not sufficient to fully reverse thyroid hormone-induced changes in bone mass and bone turnover.

Transient peak-strain matching partially recovers the age-impaired mechanoadaptive cortical bone response

Mechanoadaptation maintains bone mass and architecture; its failure underlies age-related decline in bone strength. It is unclear whether this is due to failure of osteocytes to sense strain, osteoblasts to form bone or insufficient mechanical stimulus. Mechanoadaptation can be restored to aged bone by surgical neurectomy, suggesting that changes in loading history can rescue mechanoadaptation. We use non-biased, whole-bone tibial analyses, along with characterisation of surface strains and ensuing mechanoadaptive responses in mice at a range of ages, to explore whether sufficient load magnitude can activate mechanoadaptation in aged bone. We find that younger mice adapt when imposed strains are lower than in mature and aged bone. Intriguingly, imposition of short-term, high magnitude loading effectively primes cortical but not trabecular bone of aged mice to respond. This response was regionally-matched to highest strains measured by digital image correlation and to osteocytic mechanoactivation. These data indicate that aged bone’s loading response can be partially recovered, non-invasively by transient, focal high strain regions. Our results indicate that old murine bone does respond to load when the loading is of sufficient magnitude, and bones’ age-related adaptation failure may be due to insufficient mechanical stimulus to trigger mechanoadaptation.

Bone and mineral disorders in chronic kidney disease: implications for cardiovascular health and ageing in the general population

The patient with chronic kidney disease (CKD) represents an extreme model for arteriosclerosis, vascular calcification, and bone disorders, all of which are also associated with ageing in the general population. These pathological features are also relevant to other common chronic health disorders such as diabetes, and chronic inflammatory and cardiovascular diseases. Although management and interventions for these major risk factors are now incorporated into most public health guidelines (eg, smoking cessation and control of bodyweight and blood pressure, as well as glucose and cholesterol concentrations), some residual cardiovascular risk is not reduced by implementation of these interventions. CKD should be regarded as an atypical disease in which both traditional and novel cardiovascular risk factors have effects on outcomes. But CKD can also be viewed conceptually as an accelerator of traditional cardiovascular risk factors. Findings from research into mineral bone disorder associated with CKD (CKD-MBD) could help the medical community to better understand the vascular actions of certain molecules, such as phosphates, fibroblast growth factor 23, parathyroid hormone, sclerostin, or vitamin D and their relevance to the management of different pathologies in the general population. Importantly, these components, which are recognised in nephrology, could help to explain residual risk of cardiovascular events in the general population. Thus, achieving a better understanding of CKD-MBDs could provide substantial insight into future treatments for arteriosclerosis and osteoporosis, which are strongly associated with ageing and morbidity in the general population.

Trabecular health of vertebrae based on anisotropy in trabecular architecture and collagen/apatite micro-arrangement after implantation of intervertebral fusion cages in the sheep spine

Healthy trabecular bone shows highly anisotropic trabecular architecture and the preferential orientation of collagen and apatite inside a trabecula, both of which are predominantly directed along the cephalocaudal axis. This makes trabecular bone stiff in the principally loaded direction (cephalocaudal axis). However, changes in these anisotropic trabecular characteristics after the insertion of implant devices remain unclear. We defined the trabecular architectural anisotropy and the preferential orientation of collagen and apatite as parameters of trabecular bone health. In the present study, we analyzed these parameters after the implantation of two types of intervertebral fusion cages, open and closed box-type cages, into sheep spines for 2 and 4months. Alteration and evolution of trabecular health around and inside the cages depended on the cage type and implantation duration. At the boundary region, the values of trabecular architectural anisotropy and apatite orientation for the closed-type cages were similar to those for isotropic conditions. In contrast, significantly larger anisotropy was found for open-type cages, indicating that the open-type cage tended to maintain trabecular anisotropy. Inside the open-type cage, trabecular architectural anisotropy and apatite orientation significantly increased with time after implantation. Assessing trabecular anisotropy might be useful for the evaluation of trabecular health and the validation and refinement of implant designs.

Mechanosensitive miRNAs and Bone Formation

Mechanical stimuli are required for the maintenance of skeletal integrity and bone mass. An increasing amount of evidence indicates that multiple regulators (e.g., hormone, cytoskeleton proteins and signaling pathways) are involved in the mechanical stimuli modulating the activities of osteogenic cells and the process of bone formation. Significantly, recent studies have showed that several microRNAs (miRNAs) were sensitive to various mechanical stimuli and played a crucial role in osteogenic differentiation and bone formation. However, the functional roles and further mechanisms of mechanosensitive miRNAs in bone formation are not yet completely understood. This review highlights the roles of mechanosensitive miRNAs in osteogenic differentiation and bone formation and underlines their potential therapeutic application for bone loss induced by the altering of mechanical stimuli.

Molecular Abnormalities Underlying Bone Fragility in Chronic Kidney Disease

Prevention of bone fractures is one goal of therapy for patients with chronic kidney disease-mineral and bone disorder (CKD-MBD), as indicated by the Kidney Disease: Improving Global Outcomes guidelines. CKD patients, including those on hemodialysis, are at higher risk for fractures and fracture-related death compared to people with normal kidney function. However, few clinicians focus on this issue as it is very difficult to estimate bone fragility. Additionally, uremia-related bone fragility has a more complicated pathological process compared to osteoporosis. There are many uremia-associated factors that contribute to bone fragility, including severe secondary hyperparathyroidism, skeletal resistance to parathyroid hormone, and bone mineralization disorders. Uremia also aggravates bone volume loss, disarranges microarchitecture, and increases the deterioration of material properties of bone through abnormal bone cells or excess oxidative stress. In this review, we outline the prevalence of fractures, the interaction of CKD-MBD with osteoporosis in CKD patients, and discuss possible factors that exacerbate the mechanical properties of bone.

Management of adynamic bone disease in chronic kidney disease: A brief review

The Kidney Disease: Improving Global Outcomes (KDIGO) work group released recommendations in 2006 to define the bone-related pathology associated with chronic kidney disease as renal osteodystrophy. In 2009, KDIGO released revised clinical practice guidelines which redefined systemic disorders of bone and mineral metabolism due to chronic kidney disease as chronic kidney disease-mineral and bone disorders. Conditions under this overarching term include osteitis fibrosa cystica, osteomalacia, and adynamic bone disease. We aim to provide a brief review of the histopathology, pathophysiology, epidemiology, and diagnostic features of adynamic bone disease, focusing on current trends in the management of this complex bone disorder.