Reduced Bone Density and Cortical Bone Indices in Female Adiponectin-Knockout Mice

Aerobic exercise training-induced bone and vascular adaptations in mice lacking adiponectin

Adiponectin regulates lipid and glucose metabolism, and insulin sensitivity in various target organs; however, the effects of adiponectin on bone health remain controversial. Exercise training can enhance bone density, bone microarchitecture, and blood flow. This study aimed to elucidate the role of adiponectin in adaptations of bone microarchitecture and bone vasculature in response to aerobic exercise training. Adult male C57BL/6 wild-type (WT) and homozygous adiponectin knockout (AdipoKO) mice were either treadmill exercise trained or remained sedentary for 8-10 weeks. The trabecular structures of the distal femoral metaphysis, a weight-bearing bone, and the mandible, a non-weight-bearing bone, were examined using microcomputed tomography. The femoral principal nutrient arteries were isolated to assess vasoreactivity (vasodilation and vasoconstriction) and structural remodeling. At the femoral metaphysis, impaired trabecular bone structures, including reduced connectivity density and increased trabecular spacing, were observed in AdipoKO mice compared to WT mice. In addition, nitric oxide-mediated, endothelium-dependent vasodilation was substantially reduced, and wall-to-lumen ratio was significantly increased in the femoral principal nutrient artery of AdipoKO mice. Interestingly, although exercise training-induced enhancements in trabecular connectivity density were observed at the femoral metaphysis of both WT and AdipoKO, increased vasoconstrictor responses were only observed in the femoral principal nutrient artery of WT mice, not in the AdipoKO mice. In mandibular trabecular bone, exercise training increased trabecular bone volume fraction (BV/TV, %) and intersection surface in the mandible of both WT and AdipoKO mice. These findings indicate that adiponectin is crucial for maintaining normal bone microarchitecture and vasculature. Although the absence of adiponectin compromises bone vascular adaptation to exercise training in mice, some exercise training-induced alterations in bone microarchitecture occurred in the absence of adiponectin, suggesting contribution of compensatory mechanisms during exercise training.

Preptin Deficiency Does Not Protect against High-Fat Diet-Induced Metabolic Dysfunction or Bone Loss in Mice

Preptin is derived from the cleavage of the E-peptide of pro-insulin-like growth factor (IGF)-II and is an insulin secretagogue. Observational studies have linked elevated circulating preptin to metabolic dysfunction in humans; however, a causal role for preptin in metabolic dysfunction has not been established. Additionally, preptin can promote osteoblast proliferation and differentiation, suggesting a link with skeletal health. We previously described a global preptin knockout (KO) model. In this study, we sought to uncover the impact of preptin KO in mice on the response to a moderately high-fat diet (HFD) and low-fat diet (LFD). HFD groups had higher weight and fat mass gain, lower trabecular and cortical bone volume and fracture load, and higher liver triglycerides. In males, preptin deficiency led to lower blood glucose than wild-type (WT) mice under LFD conditions. This was accompanied by differences in bone microarchitecture, including lower trabecular bone volume fraction, trabecular number, and lower cortical thickness. These differences were absent in female mice, although KO females had a HFD-driven increase in fat mass and liver triglycerides that was absent in WT mice. Female WT mice had increased glucose-stimulated insulin secretion under HFD conditions that was absent in female KO mice. Overall, preptin may have a detrimental impact on metabolism and a positive impact on bone health in male mice and may protect against liver fat storage in females while enabling islet compensation under HFD conditions. When we consider that serum preptin levels are elevated in humans of both sexes in pathological states in which insulin levels are elevated, the impact of preptin on comorbidity risk needs to be better understood. © 2023 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

The multiple regulatory effects of white adipose tissue on bone homeostasis

White adipose tissue (WAT) is not only an energy storage reservoir that is critical in energy homeostasis but is also a highly metabolically active endocrine organ. WAT can secrete a variety of adipocytokines, including leptin (LEP), adiponectin (APN), resistin, visfatin, tumor necrosis factor-α (TNF-α), interleukin-6 (IL-6), and osteopontin (OPN). It can also synthesize and secrete exosomes, which enhance intercellular communication and participate in various physiological processes in the body. It can also synthesize and secrete exosomes to enhance intercellular communication and participate in a variety of physiological processes in the body. The skeleton is an important organ for protecting internal organs. It forms the scaffolding of the body and gives the body its basic form. It drives muscle contraction to produce movement under the regulation of the nervous system. It is also an important hematopoietic organ; and it is regulated by the cytokines secreted by WAT. As research related to the release of adipocytokines from WAT to affect the skeleton continues to progress, an inextricable link between bone lipid regulation has been identified. In this paper, we review the literature to summarize the structure, function and metabolism of WAT, elaborate the specific molecular mechanisms by which WAT-secreted hormones, cytokines and exosomes regulate skeletal cells, provide a theoretical basis for the in-depth study of WAT cross-organ regulation of bone, and provide new ideas for finding new adipose-secreted targeting factors for the treatment of skeletal diseases.

Dietary supplementation of clinically utilized PI3K p110α inhibitor extends the lifespan of male and female mice

Diminished insulin and insulin-like growth factor-1 signaling extends the lifespan of invertebrates^1-4; however, whether it is a feasible longevity target in mammals is less clear^5-12. Clinically utilized therapeutics that target this pathway, such as small-molecule inhibitors of phosphoinositide 3-kinase p110α (PI3Ki), provide a translatable approach to studying the impact of these pathways on aging. Here, we provide evidence that dietary supplementation with the PI3Ki alpelisib from middle age extends the median and maximal lifespan of mice, an effect that was more pronounced in females. While long-term PI3Ki treatment was well tolerated and led to greater strength and balance, negative impacts on common human aging markers, including reductions in bone mass and mild hyperglycemia, were also evident. These results suggest that while pharmacological suppression of insulin receptor (IR)/insulin-like growth factor receptor (IGFR) targets could represent a promising approach to delaying some aspects of aging, caution should be taken in translation to humans.

Exercise Promotes Bone Marrow Microenvironment by Inhibiting Adipsin in Diet-Induced Male Obese Mice

Obesity is a growing global epidemic linked to many diseases, including diabetes, cardiovascular diseases, and musculoskeletal disorders. Exercise can improve bone density and decrease excess bone marrow adipose tissue (BMAT) in obese individuals. However, the mechanism of exercise regulating bone marrow microenvironment remains unclear. This study examines how exercise induces bone marrow remodeling in diet-induced obesity. We employed unbiased RNA-Seq to investigate the effect of exercise on the bone marrow of diet-induced obese male mice. Bone mesenchymal stem cells (BMSCs) were isolated to explore the regulatory effects of exercise in vitro. Our data demonstrated that exercise could slow down the progression of obesity and improve trabecular bone density. RNA-seq data revealed that exercise inhibited secreted phosphoprotein 1 (Spp1), which was shown to mediate bone resorption through mechanosensing mechanisms. Interactome analysis of Spp1 using the HINT database showed that Spp1 interacted with the adipokine adipsin. Moreover, exercise decreased BMAT, which induced osteoclast differentiation and promoted bone loss. Our study reveals that exercise improves the bone marrow microenvironment by at least partially inhibiting the adipsin-Spp1 signaling pathway so as to inhibit the alternative complement system from activating osteoclasts in diet-induced obese mice.

Impairment of APPL1/Myoferlin facilitates adipogenic differentiation of mesenchymal stem cells by blocking autophagy flux in osteoporosis

An imbalance of human mesenchymal stem cells (hMSCs) adipogenic and osteogenic differentiation is crucial in the pathogenesis of osteoporosis, and elucidation of the underlying mechanism is urgently needed. APPL1, an adaptor protein of the adiponectin receptor, was recently shown to be closely related to bone mass. However, the role of APPL1 in the imbalance of hMSC differentiation in osteoporosis is unclear. Therefore, we aimed to explore the mechanisms by which APPL1 alters hMSCs adipogenic differentiation in osteoporosis. Here, we found that APPL1 expression was downregulated in elderly patients with osteoporosis and in mouse osteoporosis model. APPL1 negatively regulated hMSC adipogenic differentiation in vivo and in vitro. Mechanistically, by enhancing ubiquitination-mediated Myoferlin degradation, downregulated APPL1 expression increased the risk of lysosome dysfunction during hMSCs adipogenic differentiation. Lysosomal dysfunction inhibited autophagy flux by suppressing autophagosome degradation and promoted hMSC differentiation towards the adipocyte lineage. Our findings suggest that APPL1/Myoferlin downregulation promoted hMSCs adipogenic differentiation by inhibiting autophagy flux, further impairing the balance of hMSCs adipogenic and osteogenic differentiation in osteoporosis; the APPL1/ Myoferlin axis may be a promising diagnostic and therapeutic target for osteoporosis.

Lipolysis of bone marrow adipocytes is required to fuel bone and the marrow niche during energy deficits

To investigate roles for bone marrow adipocyte (BMAd) lipolysis in bone homeostasis, we created a BMAd-specific Cre mouse model in which we knocked out adipose triglyceride lipase (ATGL, Pnpla2 gene). BMAd-Pnpla2-/- mice have impaired BMAd lipolysis, and increased size and number of BMAds at baseline. Although energy from BMAd lipid stores is largely dispensable when mice are fed ad libitum, BMAd lipolysis is necessary to maintain myelopoiesis and bone mass under caloric restriction. BMAd-specific Pnpla2 deficiency compounds the effects of caloric restriction on loss of trabecular bone in male mice, likely due to impaired osteoblast expression of collagen genes and reduced osteoid synthesis. RNA sequencing analysis of bone marrow adipose tissue reveals that caloric restriction induces dramatic elevations in extracellular matrix organization and skeletal development genes, and energy from BMAd is required for these adaptations. BMAd-derived energy supply is also required for bone regeneration upon injury, and maintenance of bone mass with cold exposure.

Cystinosin deficient rats recapitulate the phenotype of nephropathic cystinosis

The lysosomal storage disease cystinosis is caused by mutations in CTNS, encoding a cystine transporter, and in its severest form leads to proximal tubule dysfunction followed by kidney failure. Patients receive the drug-based therapy cysteamine from diagnosis. However, despite long-term treatment, cysteamine only slows the progression of end-stage renal disease. Pre-clinical testing in cystinotic rodents is required to evaluate new therapies; however, the current models are sub-optimal. To solve this problem we generated a new cystinotic rat model using CRISPR/Cas9-mediated gene editing to disrupt exon 3 of Ctns and measured various parameters over a 12-month time-course. Ctns^-/- rats display hallmarks of cystinosis by 3-6 months of age as seen by a failure to thrive, excessive thirst and urination, cystine accumulation in tissues, corneal cystine crystals, a loss of Lrp2 in proximal tubules and immune cell infiltration. High levels of glucose, calcium, albumin and protein are excreted at 6-months of age, consistent with the onset of Fanconi syndrome, with a progressive diminution of urine urea and creatinine from 9-months of age, indicative of chronic kidney disease. The kidney histology and immunohistochemistry showed proximal tubule atrophy and glomerular damage as well as classic 'swan neck' lesions. Overall, Ctns^-/- rats show a disease progression that more faithfully recapitulates nephropathic cystinosis than existing rodent models. The Ctns^-/- rat provides an excellent new rodent model of nephropathic cystinosis that is ideally suited for conducting pre-clinical drug testing and a powerful tool to advance cystinosis research.

A longitudinal analysis of serum adiponectin levels and bone mineral density in postmenopausal women in Taiwan

Since bone and fat mass are derived from mesenchyme in early development, adipokines secreted by adipose tissue may have an effect on bone metabolism. The relationship between adiponectin and bone mineral density (BMD) has been inconsistent in previous reports, with results being dependent on age, gender, menopausal status and bone sites. We investigated the relationship between serum adiponectin levels and the BMD of proximal femur and vertebrae bones in a 96-week longitudinal study of post-menopausal women with repeated measures of both. Linear regression models were used to determine the relation between adiponectin and BMD at each time point cross-sectionally, and a generalized estimating equation (GEE) model was used to investigate the longitudinal trends. Among 431 subjects, 376 (87%) provided baseline adiponectin measurements and 373 provided more than two measurements for longitudinal analysis. The means of serum adiponectin and BMD decreased with time. In linear regression models, adiponectin at baseline, the 48th week and the 96th week appeared to be inversely associated with BMD of proximal femur bone, but not lumbar spine after adjusting for age and various confounders. However, they all turn insignificant with further adjustment of body mass index. The inverse association between adiponectin and BMD of proximal femur is substantiated by all generalized equation models. Before adding the BMI in the model, the increase of 1 mg/dL of adiponectin can accelerate the decrease of proximal femur BMD by 0.001 (SE = 0.0004, p = 0.008). With BMI in the model, the drop rate was 0.0008 (SE = 0.0004, p = 0.026) and remained similar with further adjustment of two bone turnover markers. In this longitudinal analysis with both adiponectin and BMD measured at three time points, we demonstrate that with the increase of adiponectin level, the decline of proximal femur BMD in postmenopausal women accelerated during a period of 96 weeks.

Genetic deletion of menin in mouse mesenchymal stem cells: an experimental and computational analysis

Loss‐of‐function mutations in the MEN1 tumor‐suppressor gene cause the multiple endocrine neoplasia type 1 syndrome. Menin, the MEN1 gene product, is expressed in many tissues, including bone, where its function remains elusive. We conditionally inactivated menin in mesenchymal stem cells (MSCs) using paired‐related homeobox 1 (Prx1)‐Cre and compared resultant skeletal phenotypes of Prx1‐Cre;Men1^f/f menin‐knockout mice (KO) and wild‐type controls using in vivo and in vitro experimental approaches and mechanics simulation. Dual‐energy X‐ray absorptiometry demonstrated significantly reduced bone mineral density, and 3‐dimensional micro‐CT imaging revealed a decrease in trabecular bone volume, altered trabecular structure, and an increase in trabecular separation in KO mice at 6 and 9 months of age. Numbers of osteoblasts were unaltered, and dynamic histomorphometry demonstrated unaltered bone formation; however, osteoclast number and activity and receptor activator of NF‐κB ligand/osteoprotegerin (RANKL/OPG) mRNA profiles were increased, supporting increased osteoclastogenesis and bone resorption. In vitro, proliferative capabilities of bone marrow stem cells and differentiation of osteoblasts and mineralization were unaltered; however, osteoclast generation was increased. Gross femur geometrical alterations observed included significant reductions in length and in mid‐metaphyseal cross‐sectional area. Atomic force microscopy demonstrated significant decreases in elasticity of both cortical and trabecular bone at the nanoscale, whereas three‐point bending tests demonstrated a 30% reduction in bone stiffness; finite element analysis showed morphological changes of the femur microgeometry and a significantly diminished femur flexural rigidity. The biomechanical results demonstrated the detrimental outcome of the accelerated osteoclastic bone resorption. Our studies have a twofold implication; first, MEN1 deletion from MSCs can negatively regulate bone mass and bone biomechanics, and second, the experimental and computational biomechanical analyses employed in the present study should be applicable for improved phenotypic characterization of murine bone. Furthermore, our findings of critical menin function in bone may underpin the more severe skeletal phenotype found in hyperparathyroidism associated with loss‐of‐function of the MEN1 gene. © 2022 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Leptin to adiponectin ratio in puberty is associated with bone mineral density in 18-year-old males

Introduction

Inconsistent associations of leptin and adiponectin with bone mineral characteristics in puberty and adolescence have been reported. We aimed to examine the associations between leptin to adiponectin ratio (LAR) in puberty and bone mineral characteristics at the age of 18 years in healthy males.

Materials and methods

88 white Caucasian boys were investigated at T1 (mean age 12.1 years), T2 (14.0 years) and T3 (18.0 years). Serum leptin and adiponectin were measured and LAR was calculated at T1, T2 and T3, bone mineral density (BMD) and bone mineral apparent density (BMAD) for total body and lumbar spine (LS) at T1 and T3. Spearman correlation coefficient and partial correlation analyses were used to describe the associations between mean pubertal LAR and BMD at T3.

Results

Mean pubertal LAR was negatively correlated with both LS BMD (r = −0.23; P < 0.05) and LS BMAD at T3 (r = −0.33; P < 0.05). These associations remained significant also in partial correlation analysis after controlling for total body fat percentage, total testosterone, HOMA-IR and physical activity at T1 (r = −0.31; P < 0.05 and r = −0.41; P < 0.05 respectively).

Conclusion

LAR in puberty is negatively associated with lumbar spine BMD and lumbar spine BMAD at the age of 18 years.

The Effect of Caloric Restriction with and without n-3 PUFA Supplementation on Bone Turnover Markers in Blood of Subjects with Abdominal Obesity: A Randomized Placebo-Controlled Trial

Weight loss contributes to an increased risk of hip fracture, especially in postmenopausal women. Omega-3 polyunsaturated fatty acid (n-3 PUFA) supplementation could diminish the adverse effect of weight loss on bone health. The aim of this randomized, placebo-controlled, double-blind parallel trial was to investigate the effect of caloric restriction and n-3 PUFA supplement intake on osteogenic markers (carboxylated osteocalcin (Gla-OC); procollagen I N-terminal propeptide (PINP)), as well as a bone resorption marker (C-terminal telopeptide of type I collagen (CTX-I)) in a serum of 64 middle aged individuals (BMI 25-40 kg/m²) with abdominal obesity. Bone remodeling, metabolic and inflammatory parameters and adipokines were determined before and after 3 months of an isocaloric diet (2300-2400 kcal/day) or a low-calorie diet (1200 kcal/day for women and 1500 kcal/day for men) along with n-3 PUFA (1.8 g/day) or placebo capsules. CTX-I and adiponectin concentrations were increased following 7% weight loss independently of supplement use. Changes in CTX-I were positively associated with changes in adiponectin level (rho = 0.25, p = 0.043). Thus, an increase in serum adiponectin caused by body weight loss could adversely affect bone health. N-3 PUFAs were without effect.

Identical summary statistics were uncommon in randomized trials and cohort studies

OBJECTIVE

To examine the proposition that identical summary statistics (mean and/or SD) in different randomized controlled trials (RCT) or clinical cohorts can be explained by common or homogeneous source populations.

STUDY DESIGN

We estimated the probability of identical summary data in studies with high proportions of identical summary statistics, in simulations, and in control datasets.

RESULTS

The probability of both an identical mean and an identical SD for a variable in separate RCT is low (<~3%), unless the variable is rounded to 1 significant figure. In two RCT with identical summary statistics for 16 of 39 shared variables, simulations indicated the probability of the observed matches was <1 in 100,000. In 34 clinical cohorts with publication integrity concerns, the proportion of summary statistics from variables reported in ≥10 studies that were identical in ≥2 cohorts were high (42% for means, 52% for SD, and 29% for both), and improbable based on simulations and comparisons to control datasets.

CONCLUSIONS

The likelihood of multiple identical summary statistics within an individual RCT or across a body of RCT or cohort studies by the same research group is low, especially when both the mean, and the SD are identical, unless the variables are rounded to 1 significant figure.

Lack of Adiponectin Drives Hyperosteoclastogenesis in Lipoatrophic Mice

Long bones from mammals host blood cell formation and contain multiple cell types, including adipocytes. Physiological functions of bone marrow adipocytes are poorly documented. Herein, we used adipocyte-deficient PPARγ-whole body null mice to investigate the consequence of total adipocyte deficiency on bone homeostasis in mice. We first highlighted the dual bone phenotype of PPARγ null mice: one the one hand, the increased bone formation and subsequent trabecularization extending in the long bone diaphysis, due to the well-known impact of PPARγ deficiency on osteoblasts formation and activity; on the other hand, an increased osteoclastogenesis in the cortical bone. We then further explored the cause of this unexpected increased osteoclastogenesis using two independent models of lipoatrophy, which recapitulated this phenotype. This demonstrates that hyperosteoclastogenesis is not intrinsically linked to PPARγ deficiency, but is a consequence of the total lipodystrophy. We further showed that adiponectin, a cytokine produced by adipocytes and mesenchymal stromal cells is a potent inhibitor of osteoclastogenesis in vitro and in vivo. Moreover, pharmacological activation of adiponectin receptors by the synthetic agonist AdipoRon inhibited mature osteoclast activity both in mouse and human cells by blocking podosome formation through AMPK activation. Finally, we demonstrated that AdipoRon treatment blocks bone erosion in vivo in a murine model of inflammatory bone loss, providing potential new approaches to treat osteoporosis.

Obesity and Bone Health: A Complex Link

So far, the connections between obesity and skeleton have been extensively explored, but the results are inconsistent. Obesity is thought to affect bone health through a variety of mechanisms, including body weight, fat volume, bone formation/resorption, proinflammatory cytokines together with bone marrow microenvironment. In this review, we will mainly describe the effects of adipokines secreted by white adipose tissue on bone cells, as well as the interaction between brown adipose tissue, bone marrow adipose tissue, and bone metabolism. Meanwhile, this review also reviews the evidence for the effects of adipose tissue and its distribution on bone mass and bone-related diseases, along with the correlation between different populations with obesity and bone health. And we describe changes in bone metabolism in patients with anorexia nervosa or type 2 diabetes. In summary, all of these findings show that the response of skeleton to obesity is complex and depends on diversified factors, such as mechanical loading, obesity type, the location of adipose tissue, gender, age, bone sites, and secreted cytokines, and that these factors may exert a primary function in bone health.

Mechano-adaptive Responses of Alveolar Bone to Implant Hyper-loading in a pre-clinical in vivo model

OBJECTIVES

Oral implants transmit biting forces to peri-implant bone. In turn, those forces subject peri-implant bone to mechanical stresses and strains. Here, our objective was to understand how peri-implant bone responded to conditions of normal versus hyper-loading in a mouse model.

MATERIAL AND METHODS

Sixty-six mice were randomly assigned to 2 groups; both groups underwent bilateral maxillary first molar extraction followed by complete healing. Titanium alloy implants were placed in healed sites and positioned below the occlusal plane. After osseointegration, a composite crown was affixed to the implant so masticatory loading would ensue. In controls, the remaining dentition was left intact but in the hyper-loaded (test) group, the remaining molars were extracted. 3D finite element analysis (FEA) calculated peri-implant strains resulting from normal and hyper-loading. Peri-implant tissues were analyzed at multiple time points using micro-computed tomography (µCT) imaging, histology, enzymatic assays of bone remodeling, and vital dye labeling to evaluate bone accrual.

RESULTS

Compared to controls, hyper-loaded implants experienced a 3.6-fold increase in occlusal force, producing higher peri-implant strains. Bone formation and resorption were both significantly elevated around hyper-loaded implants, eventually culminating in a significant increase in peri-implant bone volume/total volume (BV/TV). In our mouse model, masticatory hyper-loading of an osseointegrated implant was associated with increased peri-implant strain, increased peri-implant bone remodeling, and a net gain in bone deposition.

CONCLUSION

Hyper-loading results in bone strain with catabolic and anabolic bone responses, leading to a net gain in bone deposition.

APPL1 negatively regulates bone mass, possibly by controlling the fate of bone marrow mesenchymal progenitor cells

Adiponectin is an adipokine that can exert a regulatory function on bone metabolism. However, there are many contradictions between clinical and pre-clinical studies on adiponectin. APPL1 is an adaptor protein that can interact with adiponectin receptors. In the current study, we found that knockout of the Appl1 gene in male mice was associated with higher bone volume and numbers of trabeculae than in females or controls. The trabecular thickness, cortical thickness, ratio of bone volume/trabecular volume, cross-sectional bone area, and mean polar moment of inertia increased in Appl1 KO mice compared with wild-type mice. The number of osteoblasts increased but the number of adipocytes decreased in Appl1 KO mice. Knockdown of Appl1 impaired adipogenesis in bone marrow-derived mesenchymal stem cells. Mineralization was increased by knockdown of Appl1 during osteoblast differentiation. Data from differentiation-related genes showed results consistent with the in vivo effects. In summary, this study provides further clarification of the effect of the adiponectin signaling pathway on bone metabolism.

Marrow Fat-Secreted Factors as Biomarkers for Osteoporosis

PURPOSE OF REVIEW

The age-related accumulation of bone marrow adipose tissue (BMAT) negatively impacts bone metabolism and hematopoiesis. This review provides an overview about BMAT-secreted factors as biomarkers for BMAT accumulation and osteoporosis risk.

RECENT FINDINGS

The adipokines leptin and adiponectin are regulators of BMAT. It remains to be clarified if locally produced adipokines substantially contribute to their peripheral serum levels and if they influence bone metabolism beyond that of extraosseous adipokine production. Existing data also suggests that BMAT disturbs bone metabolism primarily through palmitate-mediated toxic effects on osteoblasts and osteocytes, including dysregulated autophagy and apoptosis. BMAT-secreted factors are important modulators of bone metabolism. However, the majority of our understanding about MAT-secreted factors and their paracrine and endocrine effects is derived from in vitro studies and animal experiments. Therefore, more research is needed before BMAT-secreted biomarkers can be applied in medical practice.

Effect of adiponectin secreted from adipose-derived stem cells on bone-fat balance and bone defect healing

The efficacy of adiponectin (APN) in regulating bone metabolism remains controversial. This study aimed to investigate the role of APN secreted from adipose-derived stem cells on adipogenesis and osteogenesis. Human APN gene was transfected via recombinant adenovirus into adipose derived stem cells (ASCs) in vitro and were cocultured with bone marrow mesenchymal stem cells (BMSCs) in using a transwell chamber. Adipogenesis was inhibited in APN-transfected ASCs; in BMSCs, adipogenesis was inhibited, but osteogenesis was promoted in coculture with APN-transfected ASCs. Next, the same adenovirus construct was transfected into the abdominal adipose tissue of a Sprague Dawley rat in vivo, and then a tibia defect was established in the same rat. We confirmed there was higher gene and protein expression of APN in ASCs and the abdominal adipose tissue of these rat models. Development of adipocytes in abdominal adipose tissue was suppressed, and less new bone was formed in the bone defect area. In conclusion, APN secreted from ASCs could directly inhibit adipogenesis in ASCs and BMSCs and promote osteogenesis in the latter. However, APN overexpression in adipose tissue was inversely associated with bone formation in tibia defects potentially due to decreased levels of circulating bone-activating hormones.

Low bone mass is associated with carotid calcification plaque in Chinese postmenopausal women: the Chongqing osteoporosis study

Background: The aim of this study was to examine the relationship between low bone mass and the risk of carotid calcification plaques in Chinese postmenopausal women.Methods: We conducted a 5 years prospective study. Bone mineral density (BMD) was measured by dual-energy X-ray absorptiometry (DXA) scanning. Carotid computed tomography angiography (CTA) was conducted using a 64-multidetector row scanner to assess carotid arterial plaque at baseline and during follow-up. Cox proportional hazards analysis was used to evaluate the association of BMD and risk of carotid calcification plaques.Results: Four hundred and eighty-eight women sustained prospective carotid plaques during the follow-up. Women with carotid calcification plaques had low BMD than those with carotid non-calcification plaques. After adjustment for potential confounders, BMD, age, years since menopause, levels of plasma osteoprotegerin and adiponectin, hypertension, diabetes mellitus and hyperlipidemia were independently associated with increased risk of carotid calcification plaques. For carotid calcification plaques, a significant inverse correlation was indicated between BMD and the plaques, and a significant positive correlation was indicated between bone loss and plaques.Conclusions: This study suggested that lower BMD and increased loss rate of BMD were associated with a higher risk of carotid calcification plaques in Chinese postmenopausal women.

The Impact of Childhood Obesity on Skeletal Health and Development

Increased risk of fracture identified in obese children has led to a focus on the relationship between fat, bone, and the impact of obesity during skeletal development. Early studies have suggested that despite increased fracture risk, obese children have a higher bone mass. However, body size corrections applied to account for wide variations in size between children led to the finding that obese children have a lower total body and regional bone mass relative to their body size. Advances in skeletal imaging have shifted the focus from quantity of bone in obese children to evaluating the changes in bone microarchitecture that result in a change in bone quality and strength. The findings suggest that bone strength in the appendicular skeleton does not appropriately adapt to an increase in body size which results in a mismatch between bone strength and force from falls. Recent evidence points to differing influences of fat compartments on skeletal development-visceral fat may have a negative impact on bone which may be related to the associated adverse metabolic environment, while marrow adipose tissue may have an independent effect on trabecular bone development in obese children. The role of brown fat has received recent attention, demonstrating differences in the influence on bone mass between white and brown adipose tissues. Obesity results in a shift in growth and pubertal hormones as well as influences bone development through the altered release of adipokines. The change in the hormonal milieu provides an important insight into the skeletal changes observed in childhood obesity.

Adiponectin Deficiency Triggers Bone Loss by Up-Regulation of Osteoclastogenesis and Down-Regulation of Osteoblastogenesis

Osteoporosis and bone disorders related to the metabolic syndrome are often associated with adipokines secreted by adipocytes in bone. Adiponectin, a type of adipokine, is a regulator of immune responses and metabolic processes, but its role in bone biology remains uncertain. We investigated the role of adiponectin in bone metabolism using adiponectin-deficient mice in vivo and in vitro. Adiponectin-deficient mice exhibited reduced bone mass and increased adiposity. Adiponectin-deficient calvarial cells were prone to differentiate into adipocytes rather than osteoblasts. Although bone marrow macrophages (BMMs) from adiponectin-deficient mice had low osteoclastogenic potential as osteoclast precursors with increasing interferon regulatory factor 5 expression, under co-culture conditions of calvarial cells and BMMs, the enhanced receptor activator of nuclear factor κB ligand/osteoprotegerin (RANKL/OPG) ratio of adiponectin-deficient mesenchymal progenitor cells facilitated osteoclast differentiation. In addition, increased RANKL/OPG ratio was observed in the bone marrow extracellular fluid of adiponectin-deficient mice compared to that of wild-type mice. Notably, recombinant adiponectin treatment enhanced RANKL-induced osteoclast differentiation from BMMs but up-regulated OPG production in recombinant adiponectin-exposed calvarial cells, which inhibited osteoclast differentiation. Taken together, these results suggest that adiponectin plays an inhibitory role in bone metabolism through cross talk between precursor cells of both osteoclasts and osteoblasts by regulating RANKL/OPG ratio in the bone marrow microenvironment.

The Unique Metabolic Characteristics of Bone Marrow Adipose Tissue

Bone marrow adipose tissue (MAT) is distinct from white adipose tissue (WAT) or brown adipose tissue (BAT) for its location, feature and function. As a largely ignored adipose depot, it is situated in bone marrow space and resided with bone tissue side-by-side. MAT is considered not only as a regulator of bone metabolism through paracrine, but also as a functionally particular adipose tissue that may contribute to global metabolism. Adipokines, inflammatory factors and other molecules derived from bone marrow adipocytes may exert systematic effects. In this review, we summary the evidence from several aspects including development, distribution, histological features and phenotype to elaborate the basic characteristics of MAT. We discuss the association between bone metabolism and MAT, and highlight our current understanding of this special adipose tissue. We further demonstrate the probable relationship between MAT and energy metabolism, as well as glucose metabolism. On the basis of preliminary results from animal model and clinical studies, we propose that MAT has its unique secretory and metabolic function, although there is no in-depth study at present.

Adiponectin signaling and its role in bone metabolism

Adiponectin, the most prevalent adipo-cytokine in plasma plays critical metabolic and anti-inflammatory roles is fast emerging as an important molecular target for the treatment of metabolic disorders. Adiponectin action is critical in multiple organs including cardio-vascular system, muscle, liver, adipose tissue, brain and bone. Adiponectin signaling in bone has been a topic of active investigation lately. Human association studies and multiple mice models of gene deletion/modification failed to define a clear cause and effect of adiponectin signaling in bone. The most plausible reason could be the multimeric forms of adiponectin that display differential binding to receptors (adipoR1 and adipoR2) with cell-specific receptor variants in bone. Discovery of small molecule agonist of adipoR1 suggested a salutary role of this receptor in bone metabolism. The downstream signaling of adipoR1 in osteoblasts involves stimulation of oxidative phosphorylation leading to increased differentiation via the likely suppression of wnt inhibitor, sclerostin. On the other hand, the inflammation modulatory effect of adiponectin signaling suppresses the RANKL (receptor activator of nuclear factor κ-B ligand) - to - OPG (osteprotegerin) ratio in osteoblasts leading to the suppression of osteoclastogenic response. This review will discuss the adiponectin signaling and its role in skeletal homeostasis and critically assess whether adipoR1 could be a therapeutic target for the treatment of metabolic bone diseases.

Fat and bone in children - where are we now?

The risk of fracture secondary to low-impact trauma is greater in obese children, suggesting obese children are at risk of skeletal fragility. However, despite this finding, there is a lack of agreement about the impact of excessive adiposity on skeletal development. The combination of poor diet, sedentary lifestyle, greater force generated on impact through falls, and greater propensity to falls may in part explain the increased risk of fracture in obese children. To date, evidence suggests that in early childhood years, obesity confers a structural advantage to the developing skeleton. However, in time, this relationship attenuates and then reverses, such that there is a critical period during skeletal development when obesity has a detrimental effect on skeletal structure and strength. Fat mass may be important to the developing cortical and trabecular bone compartments, provided that gains in fat mass are not excessive. However, when fat accumulation reaches excessive levels, unfavorable metabolic changes may impede skeletal development. Evidence from studies examining bone microstructure suggests skeletal adaption to excessive load fails, and bone strength is relatively diminished in relation to body size in obese children. Mechanisms that may explain these changes include changes in the hormonal environment, particularly in relation to alterations in adipokines and fat distribution. Given the concomitant rise in the prevalence of childhood obesity and fractures, as well as adult osteoporosis, further work is required to understand the relationship between obesity and skeletal development.

Role of Marrow Adipocytes in Regulation of Energy Metabolism and Bone Homeostasis

PURPOSE OF REVIEW

The goal of this review is to gain a better understanding of marrow adipocyte development, its regulation of energy, and its characterization responsible for bone homeostasis.

RECENT FINDINGS

Despite major advances in uncovering the complex association of bone-fat in the marrow, the underlying basic biological process of adipose tissue development, as well as its interaction with bone homeostasis in pathophysiological conditions, is still not well understood. This review identifies many pro- and anti-osteogenic factors secreted by adipocytes to play a role in the manipulating the fate of mesenchymal stem cells as well as the osteoblastic activity during bone remodeling. It also addresses the function of adipose tissue capable of negative regulation of the hematopoietic microenvironment to influence the bone quantity and the nature of bone homeostasis.

Adiponectin Reduces Bone Stiffness: Verified in a Three-Dimensional Artificial Human Bone Model In Vitro

Primary human osteoblasts and osteoclasts incubated in a rotating coculture system without any scaffolding material, form bone-like tissue that may be used to evaluate effects of various compounds on mechanical strength. Circulating adiponectin has been found to be negatively associated with BMD and strength and was therefore assessed in this system. Osteospheres of human osteoblasts and osteoclasts were generated with and without adiponectin. The osteospheres were scanned using micro-computed tomography, the mechanical properties were tested by flat punch compression using nanoindentation equipment, and the cellular morphology characterized by microscopy. The association between autologously produced adiponectin and biomechanical properties was further evaluated by quantitation of adiponectin levels using quantitative polymerase chain reaction (qPCR) and immunoassays, and identification of stiffness by bending test of rat femurs. The molecular mechanisms were examined in vitro using human bone cells. Mechanical testing revealed that adiponectin induced a more compliant osteosphere compared with control. The osteospheres had a round, lobulated appearance with morphologically different areas; inner regions containing few cells embedded in a bone-like material surrounded by an external area with a higher cell quantity. The expression of adiponectin was found to correlate positively to ultimate bending moment and ultimate energy absorption and deflection, on the other hand, it correlated negatively to bending stiffness, indicating autocrine and/or paracrine effects of adiponectin in bone. Adiponectin enhanced proliferation and expression of collagen, leptin, and tumor necrosis factor-alpha in osteoblasts and stimulated proliferation, but not the functional activity of osteoclasts. Our results indicate that both administration of adiponectin during osteosphere production and in situ elevated levels of adiponectin in rat femurs, reduced stiffness of the bone tissues. An increase in undifferentiated cells and extracellular matrix proteins, such as collagen, may explain the reduced bone stiffness seen in the osteospheres treated with adiponectin.

Globular adiponectin reverses osteo-sarcopenia and altered body composition in ovariectomized rats

Adiponectin regulates various metabolic processes including glucose flux, lipid breakdown and insulin response. We recently reported that adiponectin receptor1 (adipoR1) activation by a small molecule reverses osteopenia in leptin receptor deficient db/db (diabetic) mice. However, the role of adiponectin in bone metabolism under the setting of post-menopausal (estrogen-deficiency) osteopenia and associated metabolic derangements has not been studied. Here, we studied the therapeutic effect of the globular form of adiponectin (gAd), which is predominantly an adipoR1 agonist, in aged ovariectomized (OVX) rats and compared it with standard-of-care anti-osteoporosis drugs. In OVX rats with established osteopenia, gAd completely restored BMD and load bearing capacity and improved bone quality. Skeletal effects of gAd were comparable to PTH (osteoanabolic) but better than alendronate (anti-catabolic). Both osteoanabolic and anti-catabolic mechanisms led to the anti-osteoporosis effect of gAd. In cultured osteoblasts and bones, gAd increased a) adipoR1 and peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC1α) expression to promote mitochondrial respiration, which likely fueled osteoblast differentiation, b) suppressed sclerostin (a wnt antagonist) in a sirtuin1-dependent manner and c) decreased receptor-activator of nuclear factor κB ligand (RANKL) to achieve its anti-catabolic effect. The OVX-induced sarcopenia and insulin resistance were also improved by gAd. We conclude that gAd has therapeutic efficacy in estrogen deficiency-induced osteoporosis, sarcopenia and insulin resistance and hold metabolic disease modifying potential in postmenopausal women.

The Activity of Adiponectin in Bone

The adipokine adiponectin affects multiple target tissues and plays important roles in glucose metabolism and whole-body energy homeostasis. Circulating adiponectin levels in obese people are lower than in non-obese, and increased serum adiponectin is associated with weight loss. Numerous clinical studies have established that fat mass is positively related to bone mass, a relationship that is maintained by communication between the two tissues through hormones and cytokines. Since adiponectin levels inversely correspond to fat mass, its bone effects and its potential contribution to the relationship between fat and bone have been investigated. In clinical observational studies, adiponectin was found to be negatively associated with bone mineral density, suggesting it might be a negative regulator of bone metabolism. In order to identify the mechanisms that underlie the activity of adiponectin in bone, a large number of laboratory studies in vitro and in animal models of mice over-expressing or deficient of adiponectin have been carried out. Results of these studies are not entirely congruent, partly due to variation among experimental systems and partly due to the complex nature of adiponectin signaling, which involves a combination of multiple direct and indirect mechanisms.

Myocardin-related transcription factor A (MRTFA) regulates the fate of bone marrow mesenchymal stem cells and its absence in mice leads to osteopenia

Objective

Arising from common progenitors in the bone marrow, adipogenesis and osteogenesis are closely associated yet mutually exclusive during bone marrow mesenchymal stem cell (BMSC) development. Previous studies have shown that morphological changes can affect the early commitment of pluripotent BMSCs to the adipose versus osteoblastic lineage via modulation of RhoA activity. The RhoA pathway regulates actin polymerization to promote the incorporation of globular actin (G-actin) into filamentous actin (F-actin). In doing so, myocardin-related transcription factors (MRTFs) dissociate from bound G-actin and enter the nucleus to co-activate serum response factor (SRF) target gene expression. In this study, we investigated whether MRTFA/SRF is acting downstream of the RhoA pathway to regulate BMSC commitment in mice.

Methods

The effects of knocking out MRTFA on skeletal homeostasis was studied in MRTFA KO mice using micro-CT, QPCR and western blot assays. To determine how MRTFA affects the mechanisms regulating BMSC fate decisions, primary bone marrow stromal cells from WT and MRTFA KO mice as well as C3H10T1/2 cell lines were analyzed in vitro.

Results

Global MRTFA KO mice have lower whole body weight, shorter femoral and tibial lengths as well as significantly decreased bone mass in their femurs. BMSCs isolated from the KO mice show increased adipogenesis and reduced osteogenesis when compared to WT littermates. KO mice, particularly females, develop osteopenia with age, and this was enhanced by a high fat diet. Over-expression of MRTFA or SRF enhances osteogenesis in CH310T1/2 cell lines. Sca1⁺, CD45⁻ cells from KO marrow express lower amounts of smooth muscle actin (SMA) and TAZ/YAP target genes compared to WT counterparts.

Conclusion

This study identified MRTFA as a novel regulator of skeletal homeostasis by regulating the balance between adipogenic and osteogenic differentiation of BMSCs. We propose that MRTFA promotes the osteogenic activity of TAZ/YAP by maintaining SMA production in BMSCs.