Overexpression of secreted frizzled-related protein 1 inhibits bone formation and attenuates parathyroid hormone bone anabolic effects

Wnt/β-catenin signaling components and mechanisms in bone formation, homeostasis, and disease

Wnts are secreted, lipid-modified proteins that bind to different receptors on the cell surface to activate canonical or non-canonical Wnt signaling pathways, which control various biological processes throughout embryonic development and adult life. Aberrant Wnt signaling pathway underlies a wide range of human disease pathogeneses. In this review, we provide an update of Wnt/β-catenin signaling components and mechanisms in bone formation, homeostasis, and diseases. The Wnt proteins, receptors, activators, inhibitors, and the crosstalk of Wnt signaling pathways with other signaling pathways are summarized and discussed. We mainly review Wnt signaling functions in bone formation, homeostasis, and related diseases, and summarize mouse models carrying genetic modifications of Wnt signaling components. Moreover, the therapeutic strategies for treating bone diseases by targeting Wnt signaling, including the extracellular molecules, cytosol components, and nuclear components of Wnt signaling are reviewed. In summary, this paper reviews our current understanding of the mechanisms by which Wnt signaling regulates bone formation, homeostasis, and the efforts targeting Wnt signaling for treating bone diseases. Finally, the paper evaluates the important questions in Wnt signaling to be further explored based on the progress of new biological analytical technologies.

Research Progress on the Mechanism of the SFRP-Mediated Wnt Signalling Pathway Involved in Bone Metabolism in Osteoporosis

Osteoporosis (OP) is a metabolic bone disease linked to an elevated fracture risk, primarily stemming from disruptions in bone metabolism. Present clinical treatments for OP merely alleviate symptoms. Hence, there exists a pressing need to identify novel targets for the clinical treatment of OP. Research indicates that the Wnt signalling pathway is modulated by serum-secreted frizzled-related protein 5 (SFRP5), potentially serving as a pivotal regulator in bone metabolism disorders. Moreover, studies confirm elevated SFRP5 expression in OP, with SFRP5 overexpression leading to the downregulation of Wnt and β-catenin proteins in the Wnt signalling pathway, as well as the expression of osteogenesis-related marker molecules such as RUNX2, ALP, and OPN. Conversely, the opposite has been reported when SFRP5 is knocked out, suggesting that SFRP5 may be a key factor involved in the regulation of bone metabolism via the Wnt signalling axis. However, the molecular mechanisms underlying the action of SFRP5-induced OP have yet to be comprehensively elucidated. This review focusses on the molecular structure and function of SFRP5 and the potential molecular mechanisms of the SFRP5-mediated Wnt signalling pathway involved in bone metabolism in OP, providing reasonable evidence for the targeted therapy of SFRP5 for the prevention and treatment of OP.

Therapeutic approaches to activate the canonical Wnt pathway for bone regeneration

Activation of the canonical Wingless-related integration site (Wnt) pathway has been shown to increase bone formation and therefore has therapeutic potential for use in orthopedic conditions. However, attempts at developing an effective strategy to achieve Wnt activation has been met with several challenges. The inherent hydrophobicity of Wnt ligands makes isolating and purifying the protein difficult. To circumvent these challenges, many have sought to target extracellular inhibitors of the Wnt pathway, such as Wnt signaling pathway inhibitors Sclerostin and Dickkopf-1, or to use small molecules, ions and proteins to increase target Wnt genes. Here, we review systemic and localized bioactive approaches to enhance bone formation or improve bone repair through antibody-based therapeutics, synthetic Wnt surrogates and scaffold doping to target canonical Wnt. We conclude with a brief review of emerging technologies, such as mRNA therapy and Clustered Regularly Interspaced Short Palindromic Repeats technology, which serve as promising approaches for future clinical translation.

Inflammation and Bone Metabolism in Rheumatoid Arthritis: Molecular Mechanisms of Joint Destruction and Pharmacological Treatments

Rheumatoid arthritis (RA) is an inflammatory disease characterized by a variety of symptoms and pathologies often presenting with polyarthritis. The primary symptom in the initial stage is joint swelling due to synovitis. With disease progression, cartilage and bone are affected to cause joint deformities. Advanced osteoarticular destruction and deformation can cause irreversible physical disabilities. Physical disabilities not only deteriorate patients' quality of life but also have substantial medical economic effects on society. Therefore, prevention of the progression of osteoarticular destruction and deformation is an important task. Recent studies have progressively improved our understanding of the molecular mechanism by which synovitis caused by immune disorders results in activation of osteoclasts; activated osteoclasts in turn cause bone destruction and para-articular osteoporosis. In this paper, we review the mechanisms of bone metabolism under physiological and RA conditions, and we describe the effects of therapeutic intervention against RA on bone.

Wnt Pathway Extracellular Components and Their Essential Roles in Bone Homeostasis

The Wnt pathway is involved in several processes essential for bone development and homeostasis. For proper functioning, the Wnt pathway is tightly regulated by numerous extracellular elements that act by both activating and inhibiting the pathway at different moments. This review aims to describe, summarize and update the findings regarding the extracellular modulators of the Wnt pathway, including co-receptors, ligands and inhibitors, in relation to bone homeostasis, with an emphasis on the animal models generated, the diseases associated with each gene and the bone processes in which each member is involved. The precise knowledge of all these elements will help us to identify possible targets that can be used as a therapeutic target for the treatment of bone diseases such as osteoporosis.

A Na+/K+ ATPase Pump Regulates Chondrocyte Differentiation and Bone Length Variation in Mice

The genetic and developmental mechanisms involved in limb formation are relatively well documented, but how these mechanisms are modulated by changes in chondrocyte physiology to produce differences in limb bone length remains unclear. Here, we used high throughput RNA sequencing (RNAseq) to probe the developmental genetic basis of variation in limb bone length in Longshanks, a mouse model of experimental evolution. We find that increased tibia length in Longshanks is associated with altered expression of a few key endochondral ossification genes such as Npr3, Dlk1, Sox9, and Sfrp1, as well reduced expression of Fxyd2, a facultative subunit of the cell membrane-bound Na⁺/K⁺ ATPase pump (NKA). Next, using murine tibia and cell cultures, we show a dynamic role for NKA in chondrocyte differentiation and in bone length regulation. Specifically, we show that pharmacological inhibition of NKA disrupts chondrocyte differentiation, by upregulating expression of mesenchymal stem cell markers (Prrx1, Serpina3n), downregulation of chondrogenesis marker Sox9, and altered expression of extracellular matrix genes (e.g., collagens) associated with proliferative and hypertrophic chondrocytes. Together, Longshanks and in vitro data suggest a broader developmental and evolutionary role of NKA in regulating limb length diversity.

Wnt3a and ASCs are capable of restoring mineralization in staph aureus-infected primary murine osteoblasts

INTRODUCTION

Bone infections are one of the main reasons for impaired bone regeneration and non-union formation. In previous experimental animal studies we could already demonstrate that bone defects due to prior infections showed a markedly reduced healing capacity, which could effectively be enhanced via application of Wnt3a and Adipose-derived stromal cells (ASCs). For a more in-depth analysis, we investigated proliferation and mineralization of cultured osteoblasts infected with staph aureus and sought to investigate effects of Wnt3a and ASCs on infected osteoblasts.

MATERIALS AND METHODS

Primary murine osteoblasts were isolated from calvariae and infected with staph aureus. Infected osteoblasts received treatment via application of recombinant Wnt3a, ASC conditioned medium and were furthermore cocultured with ASCs. Osteoblasts were evaluated by Alamar blue assay for metabolic activity, TUNEL-assay for apoptosis, ALP and Alizarin Red staining for mineralization. In addition, immunoflourescent staining (IF) and qRT-PCR analyses were performed.

RESULTS

Infected osteoblasts showed a markedly reduced ability for mineralization and increased apoptosis, which could be restored to physiological levels by Wnt3a and ASC treatment. Interestingly, metabolic activity of osteoblasts seemed to be unaffected by staph aureus infection. Additional analyses of Wnt-pathway activity revealed effective enhancement of canonical Wnt-pathway activity in Wnt3a-treated osteoblasts.

CONCLUSIONS

In summary, we gained further osteoblast-related insights into pathomechanisms of reduced bone healing capacity upon infections.

The SFRP1 Inhibitor WAY-316606 Attenuates Osteoclastogenesis Through Dual Modulation of Canonical Wnt Signaling

Osteoporosis, a noteworthy age-related disease induced by imbalanced osteogenesis and osteoclastogenesis, is a serious economic burden on both individuals and society. Small molecule drugs with dual effects on both bone resorption and mineralization are pressingly needed. Secreted frizzled-related protein 1 (SFRP1), a well-known extracellular repressor of canonical Wnt signaling, has been reported to regulate osteogenesis. Global SFRP1 knockout mice show significantly elevated bone mass. Although osteoclasts (OCs) express and secrete SFRP1, the role of SFRP1 produced by OCs in osteoclastogenesis and osteoporosis remains unclear. In this work, the levels of SFRP1 were found to be increased in patients with osteoporosis compared with healthy controls. Pharmacological inhibition of SFRP1 by WAY-316606 (WAY)- attenuated osteoclastogenesis and bone resorption in vitro. The expressions of OC-specific genes were suppressed by the SFRP1 inhibitor, WAY. Mechanistically, both extracellular and intracellular SFRP1 could block activation of the canonical Wnt signaling pathway, and WAY reverse the silent status of canonical Wnt through dual effects, leading to osteoclastogenesis inhibition and osteogenesis promotion. Severe osteopenia was observed in the ovariectomized (OVX) mouse model, and WAY treatment effectively improved the OVX-induced osteoporosis. In summary, this work found that SFRP1 supports OC differentiation and function, which could be attenuated by WAY through dual modulation of canonical Wnt signaling, suggesting its therapeutic potential. © 2021 American Society for Bone and Mineral Research (ASBMR).

Anabolic actions of PTH in murine models: two decades of insights

Parathyroid hormone (PTH) is produced by the parathyroid glands in response to low serum calcium concentrations where it targets bones, kidneys, and indirectly, intestines. The N-terminus of PTH has been investigated for decades for its ability to stimulate bone formation when administered intermittently (iPTH) and is used clinically as an effective anabolic agent for the treatment of osteoporosis. Despite great interest in iPTH and its clinical use, the mechanisms of PTH action remain complicated and not fully defined. More than 70 gene targets in more than 90 murine models have been utilized to better understand PTH anabolic actions. Because murine studies utilized wild-type mice as positive controls, a variety of variables were analyzed to better understand the optimal conditions under which iPTH functions. The greatest responses to iPTH were in male mice, with treatment starting later than 12 weeks of age, a treatment duration lasting 5-6 weeks, and a PTH dose of 30-60 μg/kg/day. This comprehensive study also evaluated these genetic models relative to the bone formative actions with a primary focus on the trabecular compartment revealing trends in critical genes and gene families relevant for PTH anabolic actions. The summation of these data revealed the gene deletions with the greatest increase in trabecular bone volume in response to iPTH. These included PTH and 1-α-hydroxylase (Pth;1α(OH)ase, 62-fold), amphiregulin (Areg, 15.8-fold), and PTH related protein (Pthrp, 10.2-fold). The deletions with the greatest inhibition of the anabolic response include deletions of: proteoglycan 4 (Prg4, -9.7-fold), low-density lipoprotein receptor-related protein 6 (Lrp6, 1.3-fold), and low-density lipoprotein receptor-related protein 5 (Lrp5, -1.0-fold). Anabolic actions of iPTH were broadly affected via multiple and diverse genes. This data provides critical insight for future research and development, as well as application to human therapeutics. © 2021 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Effective Osteogenic Priming of Mesenchymal Stem Cells through LNA-ASOs-Mediated Sfrp1 Gene Silencing

Mesenchymal stem cell (MSC) transplantation has emerged as a promising approach for bone regeneration. Importantly, the beneficial effects of MSCs can be improved by modulating the expression levels of specific genes to stimulate MSC osteogenic differentiation. We have previously shown that Smurf1 silencing by using Locked Nucleic Acid-Antisense Oligonucleotides, in combination with a scaffold that sustainably releases low doses of BMP-2, was able to increase the osteogenic potential of MSCs in the presence of BMP-2 doses significantly smaller than those currently used in the clinic. This would potentially allow an important reduction in this protein in MSs-based treatments, and thus of the side effects linked to its administration. We have further improved this system by specifically targeting the Wnt pathway modulator Sfrp1. This approach not only increases MSC bone regeneration efficiency, but is also able to induce osteogenic differentiation in osteoporotic human MSCs, bypassing the need for BMP-2 induction, underscoring the regenerative potential of this system. Achieving successful osteogenesis with the sole use of LNA-ASOs, without the need of administering pro-osteogenic factors such as BMP-2, would not only reduce the cost of treatments, but would also open the possibility of targeting these LNA-ASOs specifically to MSCs in the bone marrow, allowing us to treat systemic bone loss such as that associated with osteoporosis.

Regulation of pathophysiological and tissue regenerative functions of MSCs mediated via the WNT signaling pathway (Review)

Tissues have remarkable natural capabilities to regenerate for the purpose of physiological turnover and repair of damage. Adult mesenchymal stem cells (MSCs) are well known for their unique self-renewal ability, pluripotency, homing potential, paracrine effects and immunomodulation. Advanced research of the unique properties of MSCs have opened up new horizons for tissue regenerative therapies. However, certain drawbacks of the application of MSCs, such as the low survival rate of transplanted MSCs, unsatisfactory efficiency and even failure to regenerate under an unbalanced microenvironment, are concerning with regards to their wider therapeutic applications. The activity of stem cells is mainly regulated by the anatomical niche; where they are placed during their clinical and therapeutic applications. Crosstalk between various niche signals maintains MSCs in homeostasis, in which the WNT signaling pathway plays vital roles. Several external or internal stimuli have been reported to interrupt the normal bioactivity of stem cells. The irreversible tissue loss that occurs during infection at the site of tissue grafting suggests an inhibitory effect mediated by microbial infections within MSC niches. In addition, MSC-seeded tissue engineering success is difficult in various tissues, when sites of injury are under the effects of a severe infection despite the immunomodulatory properties of MSCs. In the present review, the current understanding of the way in which WNT signaling regulates MSC activity modification under physiological and pathological conditions was summarized. An effort was also made to illustrate parts of the underlying mechanism, including the inflammatory factors and their interactions with the regulatory WNT signaling pathway, aiming to promote the clinical translation of MSC-based therapy.

Microarray analysis identification of key pathways and interaction network of differential gene expressions during osteogenic differentiation

BACKGROUND

Adult bone marrow-derived mesenchymal stem cells (BM-MSCs) are multipotent stem cells that can differentiate into three lineages. They are suitable sources for cell-based therapy and regenerative medicine applications. This study aims to evaluate the hub genes and key pathways of differentially expressed genes (DEGs) related to osteogenesis by bioinformatics analysis in three different days. The DEGs were derived from the three different days compared with day 0.

RESULTS

Gene expression profiles of GSE37558 were obtained from the Gene Expression Omnibus (GEO) database. A total of 4076 DEGs were acquired on days 8, 12, and 25. Gene ontology (GO) enrichment analysis showed that the non-canonical Wnt signaling pathway and lipopolysaccharide (LPS)-mediated signaling pathway were commonly upregulated DEGs for all 3 days. KEGG pathway analysis indicated that the PI3K-Akt and focal adhesion were also commonly upregulated DEGs for all 3 days. Ten hub genes were identified by CytoHubba on days 8, 12, and 25. Then, we focused on the association of these hub genes with the Wnt pathways that had been enriched from the protein-protein interaction (PPI) by the Cytoscape plugin MCODE.

CONCLUSIONS

These findings suggested further insights into the roles of the PI3K/AKT and Wnt pathways and their association with osteogenesis. In addition, the stem cell microenvironment via growth factors, extracellular matrix (ECM), IGF1, IGF2, LPS, and Wnt most likely affect osteogenesis by PI3K/AKT.

Identification of osteogenic progenitor cell-targeted peptides that augment bone formation

Activation and migration of endogenous mesenchymal stromal cells (MSCs) are critical for bone regeneration. Here, we report a combinational peptide screening strategy for rapid discovery of ligands that not only bind strongly to osteogenic progenitor cells (OPCs) but also stimulate osteogenic cell Akt signaling in those OPCs. Two lead compounds are discovered, YLL3 and YLL8, both of which increase osteoprogenitor osteogenic differentiation in vitro. When given to normal or osteopenic mice, the compounds increase mineral apposition rate, bone formation, bone mass, and bone strength, as well as expedite fracture repair through stimulated endogenous osteogenesis. When covalently conjugated to alendronate, YLLs acquire an additional function resulting in a “tri-functional” compound that: (i) binds to OPCs, (ii) targets bone, and (iii) induces “pro-survival” signal. These bone-targeted, osteogenic peptides are well suited for current tissue-specific therapeutic paradigms to augment the endogenous osteogenic cells for bone regeneration and the treatment of bone loss.

Activation of osteogenic cells is essential for bone regeneration. Here, the authors screen a peptide library and identify 2 compounds that promote osteogenic progenitor cell differentiation in vitro, and show that they increase bone formation and fracture repair in mice.

MiR-1-3p regulates the differentiation of mesenchymal stem cells to prevent osteoporosis by targeting secreted frizzled-related protein 1

Osteoporosis (OP) is a systemic skeletal disorder with the characteristics of bone mass reduction and microarchitecture deterioration, resulting in bone fragility and increased fracture risk. A reduction in the osteoblast-differentiation of bone marrow mesenchymal stem cells (BMSCs) is considered as a basic pathogenesis of osteoporosis. miRNAs play a substantial role in the development and differentiation of BMSCs. In the present study, we found that miR-1-3p was significantly downregulated in the bones of Chinese osteoporotic patients (n = 29). Secreted frizzled-related protein 1 (SFRP1) was predicted as a target gene of miR-1-3p via the TargetScan and PicTar softwares and validated by dual-luciferase reporter assays. The findings revealed that the expression of SFRP1 was inversely correlated with miR-1-3p in osteoporotic patients. We induced mouse MSCs (mMSCs) to osteogenesis or adipogenesis and found that miR-1-3p was upregulated during osteogenesis but downregulated during adipogenesis. The overexpression of miR-1-3p stimulated osteogenesis and inhibited adipogenesis of mMSCs. In addition, ovariectomized (OVX) mice were tested and the function of miR-1-3p in vivo was explored. Immunohistochemistry and histomorphometric assays showed that in vivo inhibition of miR-1-3p increased the expression level of SFRP1 and reduced bone formation and bone mass. Furthermore, tartrate-resistant acid phosphatase (TRAP) staining indicated that the in vivo suppression of miR-1-3p promoted osteoclast activity, suggesting that miR-1-3p may influence bone mass by regulating bone resorption. It can be concluded that miR-1-3p plays a pivotal role in the pathogenesis of osteoporosis via targeting SFRP1 and may be a potential therapeutic target for osteoporosis.

Subclinical inflammation in the preclinical phase of rheumatoid arthritis might contribute to articular joint damage

The first degree relatives of rheumatoid arthritis (RA) patients have a higher risk of developing RA, which is related to the expression of autoantibodies against citrullinated proteins (ACPA). Remarkably, prior to the onset of RA, cartilage damage is already initiated, whereas ACPA autoantibodies are already expressed. Here we show that both TNF-α and IL-6 are also increased prior to the onset of RA. Furthermore, when the levels of DKK1 and Sclerostin were evaluated in first degree relatives of RA patients, we found that the serum levels of TNF- α correlate with the expression levels of both DKK1 and Sclerostin. Interestingly, when the disease is already established, the correlation of TNF- α with DKK1 is lost in RA patients, whereas the correlation of Sclerostin with both TNF- α and IL-6 is further increased. Our data suggest a subclinical inflammation in patients at high risk of developing RA, which might lead to an increase in the levels of both DKK1 and Sclerostin, contributing to joint damage in the preclinical phase of the disease linked to the expression of ACPA autoantibodies.

Glucocorticoids Influencing Wnt/β-Catenin Pathway; Multiple Sites, Heterogeneous Effects

Glucocorticoid hormones are vital; their accurate operation is a necessity at all ages and in all life situations. Glucocorticoids regulate diverse physiological processes and they use many signaling pathways to fulfill their effect. As the operation of these hormones affects many organs, the excess of glucocorticoids is actually detrimental to the whole human body. The endogenous glucocorticoid excess is a relatively rare condition, but a significant proportion of adult people uses glucocorticoid medication for the treatment of chronic illnesses, therefore they are exposed to the side effects of long-term glucocorticoid treatment. Our review summarizes the adverse effects of glucocorticoid excess affecting bones, adipose tissue, brain and skin, focusing on those effects which involve the Wnt/β-catenin pathway.

Adjuvant drug-assisted bone healing: Part III - Further strategies for local and systemic modulation

In this third in a series of reviews on adjuvant drug-assisted bone healing, further approaches aiming at influencing the healing process are discussed. Local and systemic modulation of bone metabolism is pursued with use of a number of drugs with completely different indications, which are characterized by a pleiotropic spectrum of action. These include drugs used to treat lipid disorders (HMG-CoA reductase inhibitors), hypertension (ACE inhibitors), osteoporosis (bisphosphonates), cancer (proteasome inhibitors) and others. Potential applications to enhance bone healing are discussed.

Selective inhibition of progesterone receptor in osteochondral progenitor cells, but not in mature chondrocytes, modulated subchondral bone structures

OBJECTIVE

The presence or relative proportion of progesterone nuclear receptors (PR) in different tissues may contribute to sexual dimorphism in these tissues. PR is expressed in chondrocytes, but its function is mostly unknown. We hypothesized that the PR may regulate chondrocyte metabolism and affect subchondral bone structure.

METHODS

We utilized genetic fate mapping and immunohistochemistry to elucidate PR expression in and effect on cartilage. To define sex-dependent and chondrocyte-specific effects of the PR on subchondral bone, we selectively deleted PR in osteochondrogenic progenitor cells marked by Prx1 (Prx1; PRcKO) and Collagen 2 (Col2; PRcKO), or in matured chondrocytes marked by aggrecan (Acan; PRcKO) and evaluated subchondral bone structure at 4 months of age. Chondrocyte aging was monitored by anti-senescence marker p16INK4a, and MMP13, one of the Senescence-Associated Secretary Phenotype (SASP) components.

RESULTS

Compared to wild-type (WT) mice, the female Prx1; PRcKO and the Col2; PRcKO mice had greater total subchondral bone volume and greater subchondral cortical bone thickness, with increased estimated subchondral bone stiffness and failure load in both female and male Col2; PRcKO mice. Moreover, Col2; PRcKO mice from both sexes had greater bone formation and bone strength at the femurs. In contrast, we did not observe any subchondral bone changes in Acan; PRcKO mice other than higher work-to-failure observed in the male Acan; PRcKO mice. Despite no detected difference in articular cartilage between the WT and the PR; chondrocyte conditional deletion mice, there were greater numbers of senescent chondrocytes and increased MMP13 expression, especially in the male mutant mice.

CONCLUSION

These findings suggest that selective inhibition of PR in osteoprogenitor cells, but not in terminally differentiated chondrocytes, induced an increased subchondral bone phenotype and high estimated subchondral bone strength, which might be associated with the development of osteoarthritis in older age.

Emerging therapeutic targets for osteoporosis

Introduction: Osteoporosis is a chronic, skeletal disorder characterized by compromised bone strength and increased fracture risk; it affects 50% of women and 20% of men. In the past two decades, there have been substantial improvements in the pharmacotherapy of osteoporosis which have yielded potent inhibitors of bone resorption or stimulators of bone formation.Areas covered: This review discusses newly identified targets and pathways and conceptual approaches to the prevention of multiple age-related disorders. Furthermore, it summarizes existing therapeutic strategies for osteoporosis.Expert opinion: Our enhanced understanding of bone biology and the reciprocal interactions between bone and other tissues have allowed the identification of new targets that may facilitate the development of novel drugs. These drugs will hopefully achieve the uncoupling of bone formation from resorption and possibly exert a dual anabolic and antiresorptive effect on bone. Alas, limitations regarding adherence, efficacy on nonvertebral fracture prevention and the long-term adverse events still exist for currently available therapeutics. Moreover, the efficacy of most agents is limited by the tight coupling of osteoblasts and osteoclasts; hence the reduction of bone resorption invariably reduces bone formation, and vice versa. This field is very much 'a work in progress.'

Targeting fundamental aging mechanisms to treat osteoporosis

Introduction: Osteoporotic fractures represent a growing burden of mortality, morbidity and socioeconomic cost to health-care systems worldwide. Osteoporosis is a disease uniquely associated with aging, therefore, an understanding of the physiological mechanisms underpinning its development as we age may open new avenues for therapeutic exploitation. Novel treatments, as well as refinement of the current approaches, are vital in the effort to sustain healthy, independent patients across the lifespan.Areas covered: This review covers the anabolic and catabolic pathways seen in bone maintenance, highlighting how they are changed with age, leading to osteoporosis. It will also discuss how these changes may be targeted therapeutically, in the development of new therapies, and the refinement of those already in use.Expert opinion: New effective and safe treatments for osteoporosis are still needed. Bone anabolics seem to be the most appropriate therapeutic approach to osteoporosis in older persons. Considering that bone and muscle mass synchronically decline with aging thus predisposing older persons to falls and fractures, combined therapeutic approaches to osteosarcopenia with a dual anabolic effect on muscle and bone will be a major advance in the treatment of these devastating conditions in the future.

The genomic basis for colonizing the freezing Southern Ocean revealed by Antarctic toothfish and Patagonian robalo genomes

Background

The Southern Ocean is the coldest ocean on Earth but a hot spot of evolution. The bottom-dwelling Eocene ancestor of Antarctic notothenioid fishes survived polar marine glaciation and underwent adaptive radiation, forming >120 species that fill all water column niches today. Genome-wide changes enabling physiological adaptations and the rapid expansion of the Antarctic notothenioids remain poorly understood.

Results

We sequenced and compared 2 notothenioid genomes—the cold-adapted and neutrally buoyant Antarctic toothfish Dissostichus mawsoni and the basal Patagonian robalo Eleginops maclovinus, representing the temperate ancestor. We detected >200 protein gene families that had expanded and thousands of genes that had evolved faster in the toothfish, with diverse cold-relevant functions including stress response, lipid metabolism, protein homeostasis, and freeze resistance. Besides antifreeze glycoprotein, an eggshell protein had functionally diversified to aid in cellular freezing resistance. Genomic and transcriptomic comparisons revealed proliferation of selcys–transfer RNA genes and broad transcriptional upregulation across anti-oxidative selenoproteins, signifying their prominent role in mitigating oxidative stress in the oxygen-rich Southern Ocean. We found expansion of transposable elements, temporally correlated to Antarctic notothenioid diversification. Additionally, the toothfish exhibited remarkable shifts in genetic programs towards enhanced fat cell differentiation and lipid storage, and promotion of chondrogenesis while inhibiting osteogenesis in bone development, collectively contributing to the achievement of neutral buoyancy and pelagicism.

Conclusions

Our study revealed a comprehensive landscape of evolutionary changes essential for Antarctic notothenioid cold adaptation and ecological expansion. The 2 genomes are valuable resources for further exploration of mechanisms underlying the spectacular notothenioid radiation in the coldest marine environment.

Low Serum Levels of DKK2 Predict Incident Low-Impact Fracture in Older Women

There are currently no robust noninvasive markers of fragility fractures. Secreted frizzled related protein‐1 (sFRP‐1), dickkopf‐related protein 1 (DKK1) and DKK2, and sclerostin (SOST) inhibit Wnt signaling and interfere with osteoblast‐mediated bone formation. We evaluated associations of serum levels of sFRP‐1, DKK1, DKK2, and SOST with incident low‐impact fracture and BMD in 828 women aged ≥65 years from EpiDoC, a longitudinal population‐based cohort. A structured questionnaire during a baseline clinical appointment assessed prevalent fragility fractures and clinical risk factors (CRFs) for fracture. Blood was collected to measure serum levels of bone turnover markers and Wnt regulators. Lumbar spine and hip BMD were determined by DXA scanning. Follow‐up assessment was performed through a phone interview; incident fragility fracture was defined by any new self‐reported low‐impact fracture. Multivariate Cox proportional hazard models were used to analyze fracture risk adjusted for CRFs and BMD. During a mean follow‐up of 2.3 ± 1.0 years, 62 low‐impact fractures were sustained in 58 women. A low serum DKK2 level (per 1 SD decrease) was associated with a 1.5‐fold increase in fracture risk independently of BMD and CRFs. Women in the two lowest DKK2 quartiles had a fracture incidence rate of 32 per 1000 person‐years, whereas women in the two highest quartiles had 14 fragility fractures per 1000 person‐years. A high serum sFRP1 level was associated with a 1.6‐fold increase in fracture risk adjusted for CRFs, but not independently of BMD. Serum levels of SOST (r = 0.191; p = 0.0025) and DKK1(r = −0.1725; p = 0.011) were correlated with hip BMD, but not with incident fragility fracture. These results indicate that serum DKK2 and sFRP1 may predict low‐impact fracture. The low number of incident fractures recorded is a limitation and serum levels of Wnt regulators should be further studied in other populations as potential noninvasive markers of fragility fractures. © 2019 The Authors. JBMR Plus published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research.

The Wnt Antagonist SFRP1: A Key Regulator of Periodontal Mineral Homeostasis

The function of mammalian periodontal tissues depends on the presence of a nonmineralized periodontal ligament (PDL) juxtaposed in between mineralized tooth anchorage tissues alveolar bone (AB) and root cementum. In the present study we have hypothesized that the Wnt antagonist secreted frizzled related protein 1 (SFRP1) is an essential regulator of periodontal tissue mineral homeostasis. Our immunoreactions and western blot data demonstrated that SFRP1 was substantially expressed higher in PDL fibroblasts than in surrounding AB progenitors and cementoblasts. SFRP1 was also detected at higher levels in PDL fibroblasts than in dental follicle (DF) cells, but the difference was less pronounced. Preferential H3K4me3 active histone mark enrichment on the SFRP1 promoter and a lack of H3K27me3 repression were most dramatic in PDL progenitors, to a lesser degree in DF cells, and not detected in AB progenitors and cementoblasts. Selective inhibition of SFRP1 using a small molecule inhibitor WAY-316606 demonstrated that SFRP1 block increased PDL cell mineralization and mineralization gene expression such as β-catenin, alkaline phosphatase, osteocalcin, collagen I, and RUNX2. The effect of SFRP1 inhibition on PDL cell mineral homeostasis was confirmed by RNA silencing. These studies also demonstrated that SFRP1 knockdown promotes PDL differentiation through histone H3K4me3-mediated activation of RUNX2 and SP7. Finally, when SFRP1 inhibition and silencing studies were performed using AB progenitors instead of PDL progenitors, there was little effect on mineralized state control and gene expression, with the exception of osteocalcin, which was dramatically upregulated upon SFRP1 silencing. Together, the results of our study document the highly specific role of the Wnt inhibitor SFRP1 in maintaining the nonmineralized state of PDL progenitors.

Identification of differentially expressed microRNAs in the bone marrow of osteoporosis patients

This study aimed to identify specific microRNAs (miRNAs) related to postmenopausal osteoporosis (OP) in human. A total of 67 conserved miRNAs, including 50 miRNAs significantly up-regulated and 17 miRNAs significantly downregulated, showed differential expression between OP group and control group. 180 hairpin structures were predicted and 199 potential novel miRNA candidates with 18 to 25 nt in length, which will greatly enrich the human miRBase. 4 miRNAs (miR-518b, miR-582-3p, miR-148a-3p and miRNA-223-3p) had upregulated expression and 4 (miR-7d-5p, miR-210-3p, miR-324-5p and miR-654-3p) showed down-regulated expression. Target genes of these miRNAs were involved in bone development, cell proliferation in bone marrow, osteoblast development, negative regulation of osteoblast differentiation, and negative regulation of osteoclast development, as well as several osteogenesis related pathways. Canonical Wnt signaling pathway was selected for verification and function analysis. The expression of Wnt1, FZD10, LRP5, DVL2 and LEF1 was down-regulated significantly, while that of SFRP1, DKK1, and CHD8 was up-regulated markedly. In conclusion, these genes play important roles in OP, which improves our understanding of pathogenesis of OP.

Cancer driver G-protein coupled receptor (GPCR) induced β-catenin nuclear localization: the transcriptional junction

G protein-coupled receptors (GPCRs) comprise the main signal-transmitting components in the cell membrane. Over the past several years, biochemical and structural analyses have immensely enhanced our knowledge of GPCR involvement in health and disease states. The present review focuses on GPCRs that are cancer drivers, involved in tumor growth and development. Our aim is to highlight the involvement of stabilized β-catenin molecular machinery with a specific array of GPCRs. We discuss recent advances in understanding the molecular path leading to β-catenin nuclear localization and transcriptional activity and their implications for future cancer therapy research.

Interplay between microRNAs and Wnt, transforming growth factor-β, and bone morphogenic protein signaling pathways promote osteoblastic differentiation of mesenchymal stem cells

Osteoblasts are terminally differentiated cells with mesenchymal origins, known to possess pivotal roles in sustaining bone microstructure and homeostasis. These cells are implicated in the pathophysiology of various bone disorders, especially osteoporosis. Over the last few decades, strategies to impede bone resorption, principally by bisphosphonates, have been mainstay of treatment of osteoporosis; however, in recent years more attention has been drawn on bone-forming approaches for managing osteoporosis. MicroRNAs (miRNAs) are a broad category of noncoding short sequence RNA fragments that posttranscriptionally regulate the expression of diverse functional and structural genes in a negative manner. An accumulating body of evidence signifies that miRNAs direct mesenchymal stem cells toward osteoblast differentiation and bone formation through bone morphogenic protein, transforming growth factor-β, and Wnt signaling pathways. MiRNAs are regarded as excellent future therapeutic candidates because of their small size and ease of delivery into the cells. Considering their novel therapeutic significance, this review discusses the main miRNAs contributing to the anabolic aspects of bone formation and illustrates their interactions with corresponding signaling pathways involved in osteoblastic differentiation.

Dissecting the Genetics of Osteoporosis using Systems Approaches

Osteoporosis is a condition characterized by low bone mineral density (BMD) and an increased risk of fracture. Traits contributing to osteoporotic fracture are highly heritable, indicating that a comprehensive understanding of bone requires a thorough understanding of the genetic basis of bone traits. Towards this goal, genome-wide association studies (GWASs) have identified over 500 loci associated with bone traits. However, few of the responsible genes have been identified, and little is known of how these genes work together to influence systems-level bone function. In this review, we describe how systems genetics approaches can be used to fill these knowledge gaps.

cGMP-dependent protein kinase-2 regulates bone mass and prevents diabetic bone loss

NO/cGMP signaling is important for bone remodeling in response to mechanical and hormonal stimuli, but the downstream mediator(s) regulating skeletal homeostasis are incompletely defined. We generated transgenic mice expressing a partly-activated, mutant cGMP-dependent protein kinase type 2 (PKG2R242Q) under control of the osteoblast-specific Col1a1 promoter to characterize the role of PKG2 in post-natal bone formation. Primary osteoblasts from these mice showed a two- to three-fold increase in basal and total PKG2 activity; they proliferated faster and were resistant to apoptosis compared to cells from WT mice. Male Col1a1-Prkg2R242Q transgenic mice had increased osteoblast numbers, bone formation rates and Wnt/β-catenin-related gene expression in bone and a higher trabecular bone mass compared to their WT littermates. Streptozotocin-induced type 1 diabetes suppressed bone formation and caused rapid bone loss in WT mice, but male transgenic mice were protected from these effects. Surprisingly, we found no significant difference in bone micro-architecture or Wnt/β-catenin-related gene expression between female WT and transgenic mice; female mice of both genotypes showed higher systemic and osteoblastic NO/cGMP generation compared to their male counterparts, and a higher level of endogenous PKG2 activity may be responsible for masking effects of the PKG2R242Q transgene in females. Our data support sexual dimorphism in Wnt/β-catenin signaling and PKG2 regulation of this crucial pathway in bone homeostasis. This work establishes PKG2 as a key regulator of osteoblast proliferation and post-natal bone formation.

Postnatal deletion of β-catenin in osterix-expressing cells is necessary for bone growth and intermittent PTH-induced bone gain

wnt/β-catenin signaling has been shown to influence bone homeostasis and is important for parathyroid hormone (PTH)-induced bone gain. To further understand the role of β-catenin in the early stages of osteoblastic lineage cells for postnatal bone homeostasis and the anabolic actions of PTH on bone, we examined mice with postnatal disruption of β-catenin in osterix-expressing cells (β-catenin KO mice) by mating floxed β-catenin mice with transgenic mice expressing cre under the control of the osterix promoter suppressible by doxycycline. After withdrawal of doxycycline, β-catenin KO mice developed progressive bone loss, ectopic cartilage formation, accumulation of mesenchymal stromal cells, and bone marrow adiposity. The β-catenin-defective osteoblasts sorted by flow cytometry from β-catenin KO mice exhibited decreased EdU incorporation, increased annexin V activity, and profound alterations in gene expression including wnt target genes, osteoclast regulators, and osteoblast markers. A dramatic increase in osteoclasts was observed in both neonatal and postnatal β-catenin KO mice. Intermittent administration of PTH for 4 weeks significantly increased bone mass in control mice; however, this anabolic effect of PTH was substantially blunted in β-catenin KO mice. Our data indicate that β-catenin in osterix-expressing cells is required for postnatal osteoblast differentiation, osteoblast proliferation, and bone resorption, and is essential for the anabolic actions of PTH in bone.

A specific haplotype in potential miRNAs binding sites of secreted frizzled-related protein 1 (SFRP1) is associated with BMD variation in osteoporosis

PURPOSE

Osteoporosis is an important multifactorial disease which is largely influenced by Wnt signaling pathway. Considering regulatory single nucleotide polymorphisms in Wnt signaling pathway may pave the road of understanding the genetic basis of predisposition to osteoporosis. The aim of this study was to determine the possible association between variants of SFRP1 and WNT5b, and osteoporosis incidence risk.

METHODS

The study population comprised 186 osteoporotic patients and 118 normal subjects from Amirkola Health and Ageing Project. rs1127379 (c.1406A>G) and rs3242 (c.3132C>T) variants in 3'UTR of SFRP1 gene, and rs3803164 (c.236C>T) in 3'UTR and rs735890 (c.622-536A>G) in intron 4 of WNT5b gene were genotyped using polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) method. Regression analyses were used to calculate the association of genotype frequencies with bone mineral density (BMD) and bone mineral content (BMC) values of participants. Bioinformatics algorithms were used to detect the effect of each SNP on the secondary structure of mRNA, and predict putative 3'UTR microRNA target sites and splicing sites changes by related SNPs.

RESULTS

WNT5b rs735890 was associated with lumbar spine BMD, BMC, and femoral neck BMC (P = 0.035, P = 0.007, and P = 0.038, respectively). WNT5b rs3803164, and SFRP1 rs3242 were significantly associated with lumbar spine BMD (P = 0.028 and P = 0.030, respectively). SFRP1 rs1127379 was associated with lumbar spine BMD in the male gender. Haplotype analysis showed a significant association of SFRP1 c.[1406A; 3132C] haplotype with lumbar spine BMD, and BMC (P = 0.019 and P = 0.030, respectively), and SFRP1 c.[1406G; 3132C] haplotype with lumbar spine BMC (P = 0.045). In silico analyses revealed that the G allele of SFRP1 rs1127379, and WNT5b rs3803164 appear as more possible target sites for many miRNAs.

CONCLUSIONS

This study is the first evidence of the association of WNT5b rs735890, and c.[1406A; 3132C] and c.[1406G; 3132C] haplotypes of SFRP1 with BMD variation in osteoporosis, probably by altering microRNA target sites, in elderly persons.

Sex-Dependent, Osteoblast Stage-Specific Effects of Progesterone Receptor on Bone Acquisition

The role of the progesterone receptor (PR) in the regulation of sexual dimorphism in bone has yet to be determined. Here we utilized genetic fate mapping and Western blotting to demonstrate age-dependent PR expression in the mouse femoral metaphysis and diaphysis. To define sex-dependent and osteoblast stage-specific effects of PR on bone acquisition, we selectively deleted PR at different stages of osteoblast differentiation. We found that when Prx1-Cre mice were crossed with PR floxed mice to generate a mesenchymal stem cell (MSC) conditional KO model (Prx1; PRcKO), the mutant mice developed greater trabecular bone volume with higher mineral apposition rate and bone formation. This may be explained by increased number of MSCs and greater osteogenic potential, particularly in males. Age-related trabecular bone loss was similar between the Prx1; PRcKO mice and their WT littermates in both sexes. Hormone deficiency during the period of rapid bone growth induced rapid trabecular bone loss in both the WT and the Prx1; PRcKO mice in both sexes. No differences in trabecular bone mass was observed when PR was deleted in mature osteoblasts using osteocalcin-Cre (Bglap-Cre). Also, there were no differences in cortical bone mass in all three PRcKO mice. In conclusion, PR inactivation in early osteoprogenitor cells but not in mature osteoblasts influenced trabecular bone accrual in a sex-dependent manner. PR deletion in osteoblast lineage cells did not affect cortical bone mass. © 2017 American Society for Bone and Mineral Research.

Acceleration of Fracture Healing by Overexpression of Basic Fibroblast Growth Factor in the Mesenchymal Stromal Cells

In this study, we engineered mesenchymal stem cells (MSCs) to over‐express basic fibroblast growth factor (bFGF) and evaluated its effects on fracture healing. Adipose‐derived mouse MSCs were transduced to express bFGF and green fluorescence protein (ADSC^bFGF‐GFP). Closed‐femoral fractures were performed with osterix‐mCherry reporter mice of both sexes. The mice received 3 × 10⁵ ADSCs transfected with control vector or bFGF via intramuscular injection within or around the fracture sites. Mice were euthanized at days 7, 14, and 35 to monitor MSC engraftment, osteogenic differentiation, callus formation, and bone strength. Compared to ADSC culture alone, ADSC^bFGF increased bFGF expression and higher levels of bFGF and vascular endothelial growth factor (VEGF) in the culture supernatant for up to 14 days. ADSC^bFGF treatment increased GFP‐labeled MSCs at the fracture gaps and these cells were incorporated into the newly formed callus. quantitative reverse transcription polymerase chain reaction (qRT‐PCR) from the callus revealed a 2‐ to 12‐fold increase in the expression of genes associated with nervous system regeneration, angiogenesis, and matrix formation. Compared to the control, ADSC^bFGF treatment increased VEGF expression at the periosteal region of the callus, remodeling of collagen into mineralized callus and bone strength. In summary, MSC^bFGF accelerated fracture healing by increasing the production of growth factors that stimulated angiogenesis and differentiation of MSCs to osteoblasts that formed new bone and accelerated fracture repair. This novel treatment may reduce the time required for fracture healing. Stem Cells Translational Medicine2017;6:1880–1893

Bone as a Target Organ in Rheumatic Disease: Impact on Osteoclasts and Osteoblasts

Dysregulated bone remodeling occurs when there is an imbalance between bone resorption and bone formation. In rheumatic diseases, including rheumatoid arthritis (RA) and seronegative spondyloarthritis, systemic and local factors disrupt the process of physiologic bone remodeling. Depending upon the local microenvironment, cell types, and local mechanical forces, inflammation results in very different effects on bone, promoting bone loss in the joints and in periarticular and systemic bone in RA and driving bone formation at enthesial and periosteal sites in diseases such as ankylosing spondylitis (AS), included within the classification of axial spondyloarthritis. There has been a great deal of interest in the role of osteoclasts in these processes and much has been learned over the past decade about osteoclast differentiation and function. It is now appreciated that osteoblast-mediated bone formation is also inhibited in the RA joint, limiting the repair of erosions. In contrast, osteoblasts function to produce new bone in AS. The Wnt and BMP signaling pathways have emerged as critical in the regulation of osteoblast function and the outcome for bone in rheumatic diseases, and these pathways have been implicated in both bone loss in RA and bone formation in AS. These pathways provide potential novel approaches for therapeutic intervention in diseases in which inflammation impacts bone.

Improved Mobilization of Exogenous Mesenchymal Stem Cells to Bone for Fracture Healing and Sex Difference

Mesenchymal stem cell (MSC) transplantation has been tested in animal and clinical fracture studies. We have developed a bone-seeking compound, LLP2A-Alendronate (LLP2A-Ale) that augments MSC homing to bone. The purpose of this study was to determine whether treatment with LLP2A-Ale or a combination of LLP2A-Ale and MSCs would accelerate bone healing in a mouse closed fracture model and if the effects are sex dependent. A right mid-femur fracture was induced in two-month-old osterix-mCherry (Osx-mCherry) male and female reporter mice. The mice were subsequently treated with placebo, LLP2A-Ale (500 μg/kg, IV), MSCs derived from wild-type female Osx-mCherry adipose tissue (ADSC, 3 x 10⁵ , IV) or ADSC + LLP2A-Ale. In phosphate buffered saline-treated mice, females had higher systemic and surface-based bone formation than males. However, male mice formed a larger callus and had higher volumetric bone mineral density and bone strength than females. LLP2A-Ale treatment increased exogenous MSC homing to the fracture gaps, enhanced incorporation of these cells into callus formation, and stimulated endochondral bone formation. Additionally, higher engraftment of exogenous MSCs in fracture gaps seemed to contribute to overall fracture healing and improved bone strength. These effects were sex-independent. There was a sex-difference in the rate of fracture healing. ADSC and LLP2A-Ale combination treatment was superior to on callus formation, which was independent of sex. Increased mobilization of exogenous MSCs to fracture sites accelerated endochondral bone formation and enhanced bone tissue regeneration. Stem Cells 2016;34:2587-2600.

The Relationship between Metabolic Syndrome and Osteoporosis: A Review

Metabolic syndrome (MetS) and osteoporosis are two major healthcare problems worldwide. Metabolic syndrome is a constellation of medical conditions consisting of central obesity, hyperglycemia, hypertension, and dyslipidemia, in which each acts on bone tissue in different ways. The growing prevalence of MetS and osteoporosis in the population along with the controversial findings on the relationship between both conditions suggest the importance for further investigation and discussion on this topic. This review aims to assess the available evidence on the effects of each component of MetS on bone metabolism from the conventional to the contemporary. Previous studies suggested that the two conditions shared some common underlying pathways, which include regulation of calcium homeostasis, receptor activator of NF-κB ligand (RANKL)/receptor activator of the NF-κB (RANK)/osteoprotegerin (OPG) and Wnt-β-catenin signaling pathways. In conclusion, we suggest that MetS may have a potential role in developing osteoporosis and more studies are necessary to further prove this hypothesis.

Osteogenesis induced by frizzled-related protein (FRZB) is linked to the netrin-like domain

Abnormal Wnt signaling is associated with bone mass disorders. Frizzled-related protein (FRZB, also known as secreted frizzled-related protein-3 (SFRP3)) is a Wnt modulator that contains an amino-terminal cysteine-rich domain (CRD) and a carboxy-terminal Netrin-like (NTN) motif. Frzb(-/-) mice show increased cortical thickness. However, the direct effect of FRZB on osteogenic differentiation and the involvement of the structural domains herein are not fully understood. In this study, we observed that stable overexpression of Frzb in MC3T3-E1 cells increased calcium deposition and osteoblast markers compared with control. Western blot analysis showed that the increased osteogenesis was associated with reduced canonical, but increased non-canonical Wnt signaling. On the contrary, loss of Frzb induced the opposite effects on osteogenesis and Wnt signaling. To translationally validate the positive effects of FRZB on primary human cells, we treated human periosteal and human bone marrow stromal cells with conditioned medium from MC3T3-E1 cells overexpressing Frzb and observed an increase in Alizarin red staining. We further studied the effect of the domains. FrzbNTN overexpression induced similar effects on osteogenesis as full-length Frzb, whereas FrzbCRD overexpressing cells mimicked loss of Frzb experiments. The CRD is considered as the Wnt binding domain, but the NTN domain also has important effects on bone biology. FRZB and other SFRPs or their specific domains may hold surprising potential as therapeutics for bone and joint disorders considering that excess of SFRPs has effects that are not expected under physiological, endogenous expression conditions.

Mediators of inflammation and bone remodeling in rheumatic disease

Remodeling of bone is a continuous process that occurs throughout life. Under normal physiologic conditions, bone-resorbing osteoclasts and bone-forming osteoblasts are tightly coupled and regulated to ensure proper balance, such that there is no net change in bone mass. However, inflammation perturbs normal bone homeostasis. The impact of inflammation on bone is dependent upon the anatomic site affected, cell types, factors and cytokines present in the local microenvironment, and local mechanical forces. Cytokines are central to the pathogenesis of inflammation-induced bone loss and contribute to the uncoupling of osteoclast-mediated bone resorption and osteoblast-mediated bone formation, thereby disrupting normal remodeling. In this review, we will discuss the effects of cytokines on bone in two settings, rheumatoid arthritis and spondyloarthritis, a disease category that includes ankylosing spondylitis, psoriatic arthritis, reactive arthritis, inflammatory bowel disease, and juvenile onset spondyloarthropathy. The outcome for bone in these disease settings is quite different, and an understanding of the pathogenic mechanisms leading to the net impact on bone has been essential in developing new therapeutic approaches to bone health in these diseases.

Transcription Factors in Craniofacial Development: From Receptor Signaling to Transcriptional and Epigenetic Regulation

Craniofacial morphogenesis is driven by spatial-temporal terrains of gene expression, which give rise to stereotypical pattern formation. Transcription factors are key cellular components that control these gene expressions. They are information hubs that integrate inputs from extracellular factors and environmental cues, direct epigenetic modifications, and define transcriptional status. These activities allow transcription factors to confer specificity and potency to transcription regulation during development.

Multi-lineage differentiation of mesenchymal stem cells - To Wnt, or not Wnt

Mesenchymal stem cells (MSCs) are multipotent precursor cells originating from several adult connective tissues. MSCs possess the ability to self-renew and differentiate into several lineages, and are recognized by the expression of unique cell surface markers. Several lines of evidence suggest that various signal transduction pathways and their interplay regulate MSC differentiation. To that end, a critical player in regulating MSC differentiation is a group of proteins encoded by the Wnt gene family, which was previously known for influencing various stages of embryonic development and cell fate determination. As MSCs have gained significant clinical attention for their potential applications in regenerative medicine, it is imperative to unravel the mechanisms by which molecular regulators control differentiation of MSCs for designing cell-based therapeutics. It is rather coincidental that the functional outcome(s) of Wnt-induced signals share similarities with cellular redox-mediated networks from the standpoint of MSC biology. Furthermore, there is evidence for a crosstalk between Wnt and redox signalling, which begs the question whether Wnt-mediated differentiation signals involve the intermediary role of reactive oxygen species. In this review, we summarize the impact of Wnt signalling on multi-lineage differentiation of MSCs, and attempt to unravel the intricate interplay between Wnt and redox signals.

β-Ecdysone Augments Peak Bone Mass in Mice of Both Sexes

BACKGROUND

One of the strongest predictors for osteoporosis is peak bone mass. Interventions to augment peak bone mass have yet to be developed. β-Ecdysone (βEcd), a natural steroid-like compound produced by arthropods to initiate metamorphosis, is believed to have androgenic effects and so may be used to augment bone mass.

QUESTIONS/PURPOSES

The purpose of this study was to use both male and female (1) gonadal-sufficient; and (2) -insufficient mice to investigate sex differences in terms of bone development and structure after βEcd administration.

METHODS

Two-month-old male and female Swiss-Webster mice were randomized to receive either vehicle or βEcd (0.5 mg/kg) for 3 weeks. In a separate experiment to evaluate the effects of βEcd on sex hormone-deficient mice, gonadectomy was performed in male (orchiectomy [ORX]) and female mice (ovariectomy [OVX]). Sham-operated and the ORX/OVX mice were then treated for 3 weeks with βEcd. Primary endpoints for the study were trabecular bone structure and bone strength.

RESULTS

In male mice, the trabecular bone volume was 0.18±0.02 in the placebo-treated (PL) and 0.23±0.02 in the βEcd-treated group (p<0.05 versus PL); and 0.09±0.01 in the ORX group (p<0.05 versus PL) and 0.12±0.01 in the ORX+βEcd group. Vertebral bone strength (maximum load) was 43±2 in PL and 51±1 in the βEcd-treated group (p<0.05 versus PL); and 30±4 in the ORX group (p<0.05 versus PL) and 37±3 in the ORX+βEcd group. In female mice, trabecular bone volume was 0.23±0.02 in PL and 0.26±0.02 in the βEcd-treated group (p<0.05 versus PL); and 0.15±0.01 in the OVX group (p<0.05 versus PL) and 0.14±0.01 in the OVX+βEcd group. Maximum load of the vertebrae was 45±2 in PL and 48±4 in the βEcd-treated group; and 39±4 in the OVX group (p<0.05 versus PL) and 44±4 in the OVX+βEcd group.

CONCLUSIONS

These findings suggest the potential use of βEcd in the augmentation of bone mass in growing male and female mice. It may also partially prevent the detrimental effects of gonadectomy on trabecular bone.

CLINICAL RELEVANCE

Our results support the potential use of βEcd or nature products that are rich in βEcd to augment peak bone mass. βEcd may differ from the other anabolic hormone treatments that may have severe side effects such as serious cardiac complications. However, its effects on humans remain to be determined.

Prevention of glucocorticoid induced bone changes with beta-ecdysone

Beta-ecdysone (βEcd) is a phytoecdysteroid found in the dry roots and seeds of the asteraceae and achyranthes plants, and is reported to increase osteogenesis in vitro. Since glucocorticoid (GC) excess is associated with a decrease in bone formation, the purpose of this study was to determine if treatment with βEcd could prevent GC-induced osteoporosis. Two-month-old male Swiss-Webster mice (n=8-10/group) were randomized to either placebo or slow release prednisolone pellets (3.3mg/kg/day) and treated with vehicle control or βEcd (0.5mg/kg/day) for 21days. GC treatment inhibited age-dependent trabecular gain and cortical bone expansion and this was accompanied by a 30-50% lower bone formation rate (BFR) at both the endosteal and periosteal surfaces. Mice treated with only βEcd significantly increased bone formation on the endosteal and periosteal bone surfaces, and increased cortical bone mass were their controls to compare to GC alone. Concurrent treatment of βEcd and GC completely prevented the GC-induced reduction in BFR, trabecular bone volume and partially prevented cortical bone loss. In vitro studies determined that βEcd prevented the GC increase in autophagy of the bone marrow stromal cells as well as in whole bone. In summary, βEcd prevented GC induced changes in bone formation, bone cell viability and bone mass. Additional studies are warranted of βEcd for the treatment of GC induced bone loss.

Lipid metabolism disorders and bone dysfunction--interrelated and mutually regulated (review)

The association between lipid and bone metabolism has become an increasing focus of interest in recent years, and accumulating evidence has shown that atherosclerosis (AS) and osteoporosis (OP), a disorder of bone metabolism, frequently co-exist. Fat and bone are known to share a common progenitor cell: Multipotent mesenchymal stem cells (MSC) in the bone marrow (BM), which are able to differentiate into various cell phenotypes, including osteoblasts, adipocytes and chondrocytes. Laboratory-based and clinical trials have shown that increasing adipocytes are accompanied by a decrease in bone mineral density (BMD) and bone mass. Statins, lipid-lowering drugs used to treat hyperlipidemia, also provide benefit in the treatment of OP. There is thus evidence that the metabolism of lipids is correlated with that of bone, and that the two are mutually regulated. The present review primarily focuses on the potential association between lipid metabolism disturbance and OP, based on biological metabolism, pathophysiological processes, results from clinical and experimental animal studies, processes involved in the differentiation of adipocytes and osteoblasts, as well as pharmacological treatments of these diseases.

Inhibited Wnt signaling causes age-dependent abnormalities in the bone matrix mineralization in the Apert syndrome FGFR2(S252W/+) mice

Apert syndrome (AS) is a type of autosomal dominant disease characterized by premature fusion of the cranial sutures, severe syndactyly, and other abnormalities in internal organs. Approximately 70% of AS cases are caused by a single mutation, S252W, in fibroblast growth factor receptor 2 (FGFR2). Two groups have generated FGFR2 knock-in mice Fgfr2S252W/+ that exhibit features of AS. During the present study of AS using the Fgfr2S252W/+ mouse model, an age-related phenotype of bone homeostasis was discovered. The long bone mass was lower in 2 month old mutant mice than in age-matched controls but higher in 5 month old mutant mice. This unusual phenotype suggested that bone marrow-derived mesenchymal stem cells (BMSCs), which are vital to maintain bone homeostasis, might be involved. BMSCs were isolated from Fgfr2S252W/+ mice and found that S252W mutation could impair osteogenic differentiation BMSCs but enhance mineralization of more mature osteoblasts. A microarray analysis revealed that Wnt pathway inhibitors SRFP1/2/4 were up-regulated in mutant BMSCs. This work provides evidence to show that the Wnt/β-catenin pathway is inhibited in both mutant BMSCs and osteoblasts, and differentiation defects of these cells can be ameliorated by Wnt3a treatment. The present study suggested that the bone abnormalities caused by deregulation of Wnt pathway may underlie the symptoms of AS.

Sex steroid actions in male bone

Sex steroids are chief regulators of gender differences in the skeleton, and male gender is one of the strongest protective factors against osteoporotic fractures. This advantage in bone strength relies mainly on greater cortical bone expansion during pubertal peak bone mass acquisition and superior skeletal maintenance during aging. During both these phases, estrogens acting via estrogen receptor-α in osteoblast lineage cells are crucial for male cortical and trabecular bone, as evident from conditional genetic mouse models, epidemiological studies, rare genetic conditions, genome-wide meta-analyses, and recent interventional trials. Genetic mouse models have also demonstrated a direct role for androgens independent of aromatization on trabecular bone via the androgen receptor in osteoblasts and osteocytes, although the target cell for their key effects on periosteal bone formation remains elusive. Low serum estradiol predicts incident fractures, but the highest risk occurs in men with additionally low T and high SHBG. Still, the possible clinical utility of serum sex steroids for fracture prediction is unknown. It is likely that sex steroid actions on male bone metabolism rely also on extraskeletal mechanisms and cross talk with other signaling pathways. We propose that estrogens influence fracture risk in aging men via direct effects on bone, whereas androgens exert an additional antifracture effect mainly via extraskeletal parameters such as muscle mass and propensity to fall. Given the demographic trends of increased longevity and consequent rise of osteoporosis, an increased understanding of how sex steroids influence male bone health remains a high research priority.

High-throughput screening of mouse gene knockouts identifies established and novel skeletal phenotypes

Screening gene function in vivo is a powerful approach to discover novel drug targets. We present high-throughput screening (HTS) data for 3 762 distinct global gene knockout (KO) mouse lines with viable adult homozygous mice generated using either gene-trap or homologous recombination technologies. Bone mass was determined from DEXA scans of male and female mice at 14 weeks of age and by microCT analyses of bones from male mice at 16 weeks of age. Wild-type (WT) cagemates/littermates were examined for each gene KO. Lethality was observed in an additional 850 KO lines. Since primary HTS are susceptible to false positive findings, additional cohorts of mice from KO lines with intriguing HTS bone data were examined. Aging, ovariectomy, histomorphometry and bone strength studies were performed and possible non-skeletal phenotypes were explored. Together, these screens identified multiple genes affecting bone mass: 23 previously reported genes (Calcr, Cebpb, Crtap, Dcstamp, Dkk1, Duoxa2, Enpp1, Fgf23, Kiss1/Kiss1r, Kl (Klotho), Lrp5, Mstn, Neo1, Npr2, Ostm1, Postn, Sfrp4, Slc30a5, Slc39a13, Sost, Sumf1, Src, Wnt10b), five novel genes extensively characterized (Cldn18, Fam20c, Lrrk1, Sgpl1, Wnt16), five novel genes with preliminary characterization (Agpat2, Rassf5, Slc10a7, Slc26a7, Slc30a10) and three novel undisclosed genes coding for potential osteoporosis drug targets.

Low-dose PTH increases osteoblast activity via decreased Mef2c/Sost in senescent osteopenic mice

Intermittent administration of parathyroid hormone (PTH) 1-34 at a standard dose has been shown to induce anabolic effects in bone. However, whether low-dose PTH promotes bone formation during senescence is unknown. To address this issue, we determined the effects of low-dose PTH and analysed the underlying mechanisms in prematurely senescent mice that display osteopenia. Treatment of 9-week-old Samp6 mice for 6 weeks with PTH at a standard dose (100 μg/kg per day) increased vertebral and femoral bone mass and improved bone microarchitecture as a result of increased bone-forming surfaces and mineral apposition rate (MAR). At a tenfold lower dose (10 μg/kg per day), PTH increased axial bone volume and trabecular thickness, as detected by bone histomorphometry but not by micro-computed tomography analysis. This anabolic effect resulted from increased osteoblast activity, as reflected by increased serum N-terminal propeptide of type 1 procollagen (P1NP) levels and MAR, with unchanged bone-forming surface or osteoblast surface. Mechanistically, low-dose PTH increased the expression of osteoblast markers in bone marrow stromal cells and mature osteoblasts, which was associated with increased expression of the Wnt effector Wisp1. Moreover, low-dose PTH decreased the expression of the Mef2c transcription factor, resulting in decreased Sost expression in osteoblasts/osteocytes. These results indicate that PTH at a low dose is effective at promoting bone formation and increased bone volume in senescent osteopenic mice through increased osteoblast activity and modulation of specific Wnt effectors, which raises the potential therapeutic use of intermittent PTH at low dose to increase bone forming activity and bone mass in skeletal senescence.

Improved trabecular bone structure of 20-month-old male spontaneously hypertensive rats

A few clinical studies have reported that elderly male participants with hypertensive disease frequently have higher bone mineral density (BMD) than the normotensive participants at several skeletal sites. The detailed mechanism is still unknown; therefore, a study of bone structure and density using the hypertensive animal models could be informative. We used micro-computed tomography to quantitatively evaluate the tibial and 3rd lumbar vertebral bones in the 20-month-old male spontaneous hypertensive rat (SHR). The BMD, volume fraction, and the microarchitecture changes of the SHR were compared to those of same-age normotensive controls (Wistar-Kyoto rat, WKY). We found that in the very old (20 month) male rats, the trabecular bone fraction and microstructure were higher than those in the same-age normotensive controls. The observation of the association of hypertension with BMD and bone strength in hypertensive rats warrants further investigations of bone mass and strength in elderly males with hypertension.

Reversing bone loss by directing mesenchymal stem cells to bone

Bone regeneration by systemic transplantation of mesenchymal stem cells (MSCs) is problematic due to the inability to control the MSCs' commitment, growth, and differentiation into functional osteoblasts on the bone surface. Our research group has developed a method to direct the MSCs to the bone surface by conjugating a synthetic peptidomimetic ligand (LLP2A) that has high affinity for activated α4β1 integrin on the MSC surface, with a bisphosphonates (alendronate) that has high affinity for bone (LLP2A-Ale), to direct the transplanted MSCs to bone. Our in vitro experiments demonstrated that mobilization of LLP2A-Ale to hydroxyapatite accelerated MSC migration that was associated with an increase in the phosphorylation of Akt kinase and osteoblastogenesis. LLP2A-Ale increased the homing of the transplanted MSCs to bone as well as the osteoblast surface, significantly increased the rate of bone formation and restored both trabecular and cortical bone loss induced by estrogen deficiency or advanced age in mice. These results support LLP2A-Ale as a novel therapeutic option to direct the transplanted MSCs to bone for the treatment of established bone loss related to hormone deficiency and aging.