Translational studies provide insights for the etiology and treatment of cortical bone osteoporosis

The potential benefits and mechanisms of protein nutritional intervention on bone health improvement

Osteoporosis commonly occurs in the older people and severe patients, with the main reason of the imbalance of bone metabolism (the rate of bone resorption exceeding the rate of bone formation), resulting in a decrease in bone mineral density and destruction of bone microstructure and further leading to the increased risk of fragility fracture. Recent studies indicate that protein nutritional support is beneficial for attenuating osteoporosis and improving bone health. This review summarized the classical mechanisms of protein intervention for alleviating osteoporosis on both suppressing bone resorption and regulating bone formation related pathways (promoting osteoblasts generation and proliferation, enhancing calcium absorption, and increasing collagen and mineral deposition), as well as the potential novel mechanisms via activating autophagy of osteoblasts, altering bone related miRNA profiles, regulating muscle-bone axis, and modulating gut microbiota abundance. Protein nutritional intervention is expected to provide novel approaches for the prevention and adjuvant therapy of osteoporosis.

Causal Effect of Selenium Levels on Osteoporosis: A Mendelian Randomization Study

Prior research has demonstrated equivocal associations between selenium (Se) concentrations and osteoporosis (OP), yielding inconclusive findings. The purpose of the current study was to examine the potential correlation between Se levels and the risk of OP by using the Mendelian randomization (MR) study design. The genetic variants related to Se levels were obtained from a meta-analysis of a Genome-Wide Association Study (GWAS) conducted on toenail Se levels (n = 4162) and blood Se levels (n = 5477). The data summary for OP and bone mineral density (BMD) was obtained by utilizing the GWAS database. To examine the association between Se levels and BMD and OP, we employed three statistical methods: inverse variance weighted, weighted median, and MR-Egger. The reliability of the analysis was verified by sensitivity testing. All three methods of MR analysis revealed that Se levels had no effect on OP risk. In addition, the sensitivity analysis revealed no heterogeneity or pleiotropy, and the significance of the overall effect remained unaffected by single-nucleotide polymorphisms (SNPs), as determined by the leave-one-out analysis, indicating that our findings are relatively reliable. The results of our study indicate that there is no causal association between Se levels and the risk of OP. However, additional investigation is necessary to ascertain whether there is a potential association between these variables.

Two-sample Mendelian randomization analysis evaluates causal associations between inflammatory bowel disease and osteoporosis

Introduction

Over the past few years, multiple observational studies have speculated a potential association between inflammatory bowel disease (IBD), which includes ulcerative colitis (UC) and Crohn's disease (CD), and osteoporosis. However, no consensus has been reached regarding their interdependence and pathogenesis. Herein, we sought to further explore the causal associations between them.

Methods

We validated the association between IBD and reduced bone mineral density in humans based on genome-wide association studies (GWAS) data. To investigate the causal relationship between IBD and osteoporosis, we performed a two-sample Mendelian randomization study using training and validation sets. Genetic variation data for IBD, CD, UC, and osteoporosis were derived from published genome-wide association studies in individuals of European ancestry. After a series of robust quality control steps, we included eligible instrumental variables (SNPs) significantly associated with exposure (IBD/CD/UC). We adopted five algorithms, including MR Egger, Weighted median, Inverse variance weighted, Simple mode, and Weighted mode, to infer the causal association between IBD and osteoporosis. In addition, we evaluated the robustness of Mendelian randomization analysis by heterogeneity test, pleiotropy test, leave-one-out sensitivity test, and multivariate Mendelian randomization.

Results

Genetically predicted CD was positively associated with osteoporosis risk, with ORs of 1.060 (95% CIs 1.016, 1.106; p = 0.007) and 1.044 (95% CIs 1.002, 1.088; p = 0.039) for CD in the training and validation sets, respectively. However, Mendelian randomization analysis did not reveal a significant causal relationship between UC and osteoporosis (p > 0.05). Furthermore, we found that overall IBD was associated with osteoporosis prediction, with ORs of 1.050 (95% CIs 0.999, 1.103; p = 0.055) and 1.063 (95% CIs 1.019, 1.109; p = 0.005) in the training and validation sets, respectively.

Conclusion

We demonstrated the causal association between CD and osteoporosis, complementing the framework for genetic variants that predispose to autoimmune disease.

Structural characterization and anti-osteoporosis effects of polysaccharide purified from Eucommia ulmoides Oliver cortex based on its modulation on bone metabolism

EuOCP3, with a molecular weight of 38.1 kDa, is an acidic polysaccharide purified from Eucommia ulmoides Oliver cortex. Herein, we determined that the main backbone of EuOCP3 was predominantly composed of →4)-α-GalpA-(1 → 4)-α-GalpA-(1→, →4)-α-GalpA-(1 → 5)-α-Araf-(1→, →4)-α-GalpA-(1 → 2)-α-Rhap-(1→, and →4)-α-GalpA-(1 → 5)-α-Araf-(1 → 2)-α-Rhap-(1 → repeating blocks, which were connected by →2,3,5)-α-Araf-(1→. The side chains, substituted at C-2 and C-5 of →2,3,5)-α-Araf-(1→, contained T-β-Araf→ and T-β-Araf → 4)-α-GalpA-(1 → residues. In dexamethasone (Dex)-induced osteoporosis (OP) mice, EuOCP3 treatment restored cortical bone thickness, increased mineralized bone area, enhanced the number of osteoblasts, and decreased the number of osteoclasts on the surface of cortical bone. Combining analysis of gut microflora, serum metabolite profiles, and biological detection results, we demonstrated that EuOCP3 regulated the abundance of specific species within the gut microflora, such as g_Dorea and g_Prevotella, and ameliorated oxidative stress. In turn, enhancement of osteogenic function and restoration of bone metabolism via the extracellular signal-regulated kinase (ERK)/c-Jun N-terminal kinase (JNK)/nuclear factor erythroid-2 related factor 2 (Nrf2) signaling pathway was indicated. The current findings contribute to understanding the potential of EuOCP3 in anti-OP treatment.

Direct Assessment of Rabbit Cortical Bone Basic Multicellular Unit Longitudinal Erosion Rate: A 4D Synchrotron-Based Approach

Cortical bone remodeling is carried out by basic multicellular units (BMUs), which couple resorption to formation. Although fluorochrome labeling has facilitated study of BMU formative parameters since the 1960s, some resorptive parameters, including the longitudinal erosion rate (LER), have remained beyond reach of direct measurement. Indeed, our only insights into this spatiotemporal parameter of BMU behavior come from classical studies that indirectly inferred LER. Here, we demonstrate a 4D in vivo method to directly measure LER through in-line phase contrast synchrotron imaging. The tibias of rabbits (n = 15) dosed daily with parathyroid hormone were first imaged in vivo (synchrotron micro-CT; day 15) and then ex vivo 14 days later (conventional micro-CT; day 29). Mean LER assessed by landmarking the co-registered scans was 23.69 ± 1.73 μm/d. This novel approach holds great promise for the direct study of the spatiotemporal coordination of bone remodeling, its role in diseases such as osteoporosis, as well as related treatments. © 2022 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Biological Deciphering of the “Kidney Governing Bones” Theory in Traditional Chinese Medicine

The description of the “kidney” was entirely different from modern medicine. In traditional Chinese medicine (TCM), the kidney was a functional concept regulating water metabolism, which was closely related to the urinary system, reproductive system, nervous system, endocrine, skeleton, hearing, metabolism, immunity, etc. In particular, the kidney in TCM plays an important regulatory role in the processes of growth, development, prime, aging, and reproduction. Hence, “Kidney Governing Bone” (KGB) was a classical theory in TCM, which hypothesized that the function of the kidney was responsible for bone health. However, the related modern physiological mechanisms of this TCM theory are unclear. This present paper proposed a new understanding and explored the biological basis of the KGB theory. After searching through plenty of reported literature, we discovered that the functions of the kidney in TCM were closely associated with the hypothalamic-pituitary-gonadal (HPG) axis in modern science. The physiological mechanism of the KGB was regulated by sex hormones and their receptors. This review deciphered the connotation of the KGB theory in modern medicine and further verified the scientificity of the basic TCM theory.

Periostin: An Emerging Molecule With a Potential Role in Spinal Degenerative Diseases

Periostin, an extracellular matrix protein, is widely expressed in a variety of tissues and cells. It has many biological functions and is related to many diseases: for example, it promotes cell proliferation and differentiation in osteoblasts, which are closely related to osteoporosis, and mediates cell senescence and apoptosis in chondrocytes, which are involved in osteoarthritis. Furthermore, it also plays an important role in mediating inflammation and reconstruction during bronchial asthma, as well as in promoting bone development, reconstruction, repair, and strength. Therefore, periostin has been explored as a potential biomarker for various diseases. Recently, periostin has also been found to be expressed in intervertebral disc cells as a component of the intervertebral extracellular matrix, and to play a crucial role in the maintenance and degeneration of intervertebral discs. This article reviews the biological role of periostin in bone marrow-derived mesenchymal stem cells, osteoblasts, osteoclasts, chondrocytes, and annulus fibrosus and nucleus pulposus cells, which are closely related to spinal degenerative diseases. The study of its pathophysiological effects is of great significance for the diagnosis and treatment of spinal degeneration, although additional studies are needed.

Osteocyte- and late osteoblast-derived NOTUM reduces cortical bone mass in mice

Osteoporosis is a common skeletal disease, with increased risk of fractures. Currently available osteoporosis treatments reduce the risk of vertebral fractures, mainly dependent on trabecular bone, whereas the effect on nonvertebral fractures, mainly dependent on cortical bone, is less pronounced. WNT signaling is a crucial regulator of bone homeostasis, and the activity of WNTs is inhibited by NOTUM, a secreted WNT lipase. We previously demonstrated that conditional inactivation of NOTUM in all osteoblast lineage cells increases the cortical but not the trabecular bone mass. The aim of the present study was to determine if NOTUM increasing cortical bone is derived from osteoblast precursors/early osteoblasts or from osteocytes/late osteoblasts. First, we demonstrated Notum mRNA expression in Dmp1-expressing osteocytes and late osteoblasts in cortical bone using in situ hybridization. We then developed a mouse model with inactivation of NOTUM in Dmp1-expressing osteocytes and late osteoblasts (Dmp1-creNotum^flox/flox mice). We observed that the Dmp1-creNotum^flox/flox mice displayed a substantial reduction of Notum mRNA in cortical bone, resulting in increased cortical bone mass and decreased cortical porosity in femur but no change in trabecular bone volume fraction in femur or in the lumbar vertebrae L5 in Dmp1-creNotum^flox/flox mice as compared with control mice. In conclusion, osteocytes and late osteoblasts are the principal source of NOTUM in cortical bone, and NOTUM derived from osteocytes/late osteoblasts reduces cortical bone mass. These findings demonstrate that inhibition of osteocyte/late osteoblast-derived NOTUM might be an interesting pharmacological target to increase cortical bone mass and reduce nonvertebral fracture risk.NEW & NOTEWORTHY NOTUM produced by osteoblasts is known to regulate cortical bone mass. Our new findings show that NOTUM specifically derived by DMP1-expressing osteocytes and late osteoblasts regulates cortical bone mass and not trabecular bone mass.

Runt-related transcription factor 1 is required for murine osteoblast differentiation and bone formation

Despite years of research investigating osteoblast differentiation, the mechanisms by which transcription factors regulate osteoblast maturation, bone formation, and bone homeostasis is still unclear. It has been reported that runt-related transcription factor 1 (Runx1) is expressed in osteoblast progenitors, pre-osteoblasts, and mature osteoblasts; yet, surprisingly, the exact function of RUNX1 in osteoblast maturation and bone formation remains unknown. Here, we generated and characterized a pre-osteoblast and differentiating chondrocyte-specific Runx1 conditional knockout mouse model to study RUNX1's function in bone formation. Runx1 ablation in osteoblast precursors and differentiating chondrocytes via osterix-Cre (Osx-Cre) resulted in an osteoporotic phenotype and decreased bone density in the long bones and skulls of Runx1^f/fOsx-Cre mice compared with Runx1^f/f and Osx-Cre mice. RUNX1 deficiency reduced the expression of SRY-box transcription factor 9 (SOX9), Indian hedgehog signaling molecule (IHH), Patched (PTC), and cyclin D1 in the growth plate, and also reduced the expression of osteocalcin (OCN), OSX, activating transcription factor 4 (ATF4), and RUNX2 in osteoblasts. ChIP assays and promoter activity mapping revealed that RUNX1 directly associates with the Runx2 gene promoter and up-regulates Runx2 expression. Furthermore, the ChIP data also showed that RUNX1 associates with the Ocn promoter. In conclusion, RUNX1 up-regulates the expression of Runx2 and multiple bone-specific genes, and plays an indispensable role in bone formation and homeostasis in both trabecular and cortical bone. We propose that stimulating Runx1 activity may be useful in therapeutic approaches for managing some bone diseases such as osteoporosis.

The effects of estradiol are modulated in a tissue-specific manner in mice with inducible inactivation of ERα after sexual maturation

Mouse models with lifelong inactivation of estrogen receptor-α (ERα) show that ERα is the main mediator of estrogenic effects in bone, thymus, uterus, and fat. However, ERα inactivation early in life may cause developmental effects that confound the adult phenotypes. To address the specific role of adult ERα expression for estrogenic effects in bone and other nonskeletal tissues, we established a tamoxifen-inducible ERα-inactivated model by crossing CAGG-Cre-ER and ERα^flox/flox mice. Tamoxifen-induced ERα inactivation after sexual maturation substantially reduced ERα mRNA levels in cortical bone, trabecular bone, thymus, uterus, gonadal fat, and hypothalamus, in CAGG-Cre-ERα^flox/flox (inducible ERα^KO) compared with ERα^flox/flox (control) mice. 17β-estradiol (E2) treatment increased trabecular bone volume fraction (BV/TV), cortical bone area, and uterine weight, while it reduced thymus weight and fat mass in ovariectomized control mice. The estrogenic responses were substantially reduced in inducible ERα^KO mice compared with control mice on BV/TV (-67%), uterine weight (-94%), thymus weight (-70%), and gonadal fat mass (-94%). In contrast, the estrogenic response on cortical bone area was unaffected in inducible ERα^KO compared with control mice. In conclusion, using an inducible ERα^KO model, not confounded by lack of ERα during development, we demonstrate that ERα expression in sexually mature female mice is required for normal E2 responses in most, but not all, tissues. The finding that cortical, but not trabecular bone, responds normally to E2 treatment in inducible ERα^KO mice strengthens the idea of cortical and trabecular bone being regulated by estrogen via different mechanisms.

New Targets and Emergent Therapies for Osteoporosis

The 11 existing FDA-approved osteoporosis drug treatments include hormone replacement therapy, 2 SERMs (raloxifene and bazedoxifene), 5 inhibitors of bone-resorbing osteoclasts (4 bisphosphonates and anti-RANKL denosumab), 2 parathyroid hormone analogues (teriparatide and abaloparatide), and 1 WNT signaling enhancer (romosozumab). These therapies are effective and provide multiple options for patients and physicians. As the genomic revolution continues, potential novel targets for future drug development are identified. This review takes a wide perspective to describe potentially rewarding topics to explore, including knowledge of genes and pathways involved in bone cell metabolism, the utility of animal models, targeting drugs to bone, and ongoing advances in drug design and delivery.

Mice lacking plastin-3 display a specific defect of cortical bone acquisition

Although inactivating mutations of PLS3, encoding the actin-bundling protein plastin-3, have been identified to cause X-linked osteoporosis, the cellular and molecular influence of PLS3 on bone remodeling is poorly defined. Moreover, although a previous study has demonstrated moderate osteopenia in 12 week-old Pls3-deficient mice based on μCT scanning, there is no reported analysis of such a model on the basis of undecalcified histology and bone-specific histomorphometry. To fill this knowledge gap we applied a deep phenotyping approach and studied Pls3-deficient mice at different ages. Surprisingly, we did not detect significant differences between wildtype and Pls3-deficient littermates with respect to trabecular bone mass, and the same was the case for all histomorphometric parameters determined at 12 weeks of age. Remarkably however, the cortical thickness in both, tibia and femur, was significantly reduced in Pls3-deficient mice in all age groups. We additionally studied the ex vivo behavior of Pls3-deficient primary osteoblasts, which displayed moderately impaired mineralization capacity. Of note, while most osteoblastogenesis markers were not differentially expressed between wildtype and Pls3-deficient cultures, the expression of Sfrp4 was significantly reduced in the latter, a potentially relevant finding, since Sfrp4 inactivation, in mice and humans, specifically causes cortical thinning. We finally addressed the question, if Pls3-deficiency would impair the osteoanabolic influence of parathyroid hormone (PTH). For this purpose we applied daily injection of PTH into wildtype and Pls3-deficient mice and found a similar response regardless of the genotype. Taken together, our data reveal that Pls3-deficiency in mice only recapitulates the cortical bone phenotype of individuals with X-linked osteoporosis by negatively affecting the early stage of cortical bone acquisition.

Probiotic treatment using a mix of three Lactobacillus strains for lumbar spine bone loss in postmenopausal women: a randomised, double-blind, placebo-controlled, multicentre trial

BACKGROUND

Postmenopausal bone loss in the spine is associated with an increased risk of vertebral fractures. Certain probiotic treatment protects rodents from ovariectomy-induced bone loss. The aim of the present study was to determine if treatment with a combination of three bacterial strains protects against the rapid spine bone loss occurring in healthy early postmenopausal women.

METHODS

This randomised, double-blind, placebo-controlled, multicentre trial was done at four study centres in Sweden. Early postmenopausal women were randomly assigned in a 1:1 ratio to receive probiotic treatment consisting of three Lactobacillus strains (Lactobacillus paracasei DSM 13434, Lactobacillus plantarum DSM 15312, and Lactobacillus plantarum DSM 15313; 1 x 1010 colony-forming units per capsule) or placebo once daily for 12 months. The primary outcome was the percentage change from baseline in lumbar spine bone mineral density (LS-BMD) at 12 months. The primary analysis was done in all participants with BMD measurements available both at baseline and at 12 months. Analyses of adverse events and safety included all participants who had taken at least one capsule of placebo or Lactobacillus. This trial is registered with ClinicalTrials.gov, NCT02722980, and is completed.

FINDINGS

Between April 18 and Nov 11, 2016, 249 participants were randomly assigned to receive probiotic product or placebo, and 234 (94%) completed the analyses required for the primary outcome. Lactobacillus treatment reduced the LS-BMD loss compared with placebo (mean difference 0·71%, 95% CI 0·06 to 1·35). The LS-BMD loss was significant in the placebo group (-0·72%, -1·22 to -0·22), whereas no bone loss was observed in the Lactobacillus-treated group (-0·01%, -0·50 to 0·48). The adverse events were similar between the two groups.

INTERPRETATION

Probiotic treatment using a mix of three Lactobacillus strains protects against lumbar spine bone loss in healthy postmenopausal women.

FUNDING

Probi.

NOTUM inhibition increases endocortical bone formation and bone strength

The disability, mortality and costs caused by non-vertebral osteoporotic fractures are enormous. Existing osteoporosis therapies are highly effective at reducing vertebral but not non-vertebral fractures. Cortical bone is a major determinant of non-vertebral bone strength. To identify novel osteoporosis drug targets, we phenotyped cortical bone of 3 366 viable mouse strains with global knockouts of druggable genes. Cortical bone thickness was substantially elevated in Notum^−/− mice. NOTUM is a secreted WNT lipase and we observed high NOTUM expression in cortical bone and osteoblasts but not osteoclasts. Three orally active small molecules and a neutralizing antibody inhibiting NOTUM lipase activity were developed. They increased cortical bone thickness and strength at multiple skeletal sites in both gonadal intact and ovariectomized rodents by stimulating endocortical bone formation. Thus, inhibition of NOTUM activity is a potential novel anabolic therapy for strengthening cortical bone and preventing non-vertebral fractures.

NOTUM is an enzyme that inactivates WNT proteins (which play a key role in early tissue development), and inhibiting NOTUM has been found to increase the formation of endocortical bone (within the cortex, the hard exterior of bone) and enhance bone strength. Existing therapies for osteoporosis (condition causing bone to become weak and brittle) are effective in reducing vertebral, but not non-vertebral, fractures. A team headed by Robert Brommage at Lexicon Pharmaceuticals, Texas aimed to identify novel osteoporosis drug targets in mice. Following inhibition of NOTUM activity, the authors observed increased cortical bone thickness and strength at multiple skeletal sites through stimulation of endocortical bone formation. The team concluded that inhibiting NOTUM activity has good potential as a new therapeutic strategy and could be beneficial in preventing non-vertebral osteoporotic fractures.

The androgen receptor is required for maintenance of bone mass in adult male mice

Previous studies evaluating the role of the androgen receptor (AR) for bone mass have used mouse models with global or tissue-specific lifelong inactivation of the AR. However, these mouse models have the AR inactivated already early in life and the relative roles of the AR during development, sexual maturation and in adult mice cannot be evaluated separately. The aim of the present study was to determine the specific roles of the AR in bone during sexual maturation and in adult mice. The AR was conditionally ablated at four (pre-pubertal) or ten (post-pubertal) weeks of age in male mice using tamoxifen-inducible Cre-mediated recombination. Both the pre-pubertal and the post-pubertal AR inactivation were efficient demonstrated by substantially lower AR mRNA levels in seminal vesicle, bone and white adipose tissue as well as markedly reduced weights of reproductive tissues when comparing inducible ARKO mice and control mice at 14 weeks of age. Total body BMD, as analyzed by DXA, as well as tibia diaphyseal cortical bone thickness and proximal metaphyseal trabecular bone volume fraction, as analyzed by μCT, were significantly reduced by both pre-pubertal and post-pubertal AR inactivation. These bone effects were associated with an increased bone turnover, indicating a high bone turnover osteoporosis. Pre-pubertal but not post-pubertal AR inactivation resulted in substantially increased fat mass. In conclusion, the AR is required for maintenance of both trabecular and cortical bone in adult male mice while AR expression during puberty is crucial for normal fat mass homeostasis in adult male mice.