The role of WNT10B in physiology and disease: A 10-year update

Regulation of osteoblastogenesis and bone mass by Wnt10b

Wnts comprise a family of secreted signaling proteins that regulate diverse developmental processes. Activation of Wnt signaling by Wnt10b inhibits differentiation of preadipocytes and blocks adipose tissue development; however, the effect of Wnt10b on other mesenchymal lineages has not been defined. To explore the physiological role of Wnt signaling in bone development, we analyzed FABP4-Wnt10b mice, which express the Wnt10b transgene in marrow. Femurs from FABP4-Wnt10b mice have almost four times as much bone in the distal metaphyses and are mechanically stronger. These mice maintain elevated bone mass at least through 23 months of age. In addition, FABP4-Wnt10b mice are protected from the bone loss characteristic of estrogen deficiency. We used pharmacological and genetic approaches to demonstrate that canonical Wnt signaling stimulates osteoblastogenesis and inhibits adipogenesis of bipotential mesenchymal precursors. Wnt10b shifts cell fate toward the osteoblast lineage by induction of the osteoblastogenic transcription factors Runx2, Dlx5, and osterix and suppression of the adipogenic transcription factors C/EBPα and PPARγ. One mechanism whereby Wnt10b promotes osteoblastogenesis is suppression of PPARγ expression. Finally, Wnt10b-/- mice have decreased trabecular bone and serum osteocalcin, confirming that Wnt10b is an endogenous regulator of bone formation.

Wnt10b Increases Postnatal Bone Formation by Enhancing Osteoblast Differentiation

Abstract Overexpression of Wnt10b from the osteocalcin promoter in transgenic mice increases postnatal bone mass. Increases in osteoblast perimeter, mineralizing surface, and bone formation rate without detectable changes in pre-osteoblast proliferation, osteoblast apoptosis, or osteoclast number and activity suggest that, in this animal model, Wnt10b primarily increases bone mass by stimulating osteoblastogenesis. Introduction: Wnt signaling regulates many aspects of development including postnatal accrual of bone. Potential mechanisms for how Wnt signaling increases bone mass include regulation of osteoblast and/or osteoclast number and activity. To help differentiate between these possibilities, we studied mice in which Wnt10b is expressed specifically in osteoblast lineage cells or in mice devoid of Wnt10b. Materials and Methods: Transgenic mice, in which mouse Wnt10b is expressed from the human osteocalcin promoter (Oc-Wnt10b), were generated in C57BL/6 mice. Transgene expression was evaluated by RNase protection assay. Quantitative assessment of bone variables was done by radiography, μCT, and static and dynamic histomorphometry. Mechanisms of bone homeostasis were evaluated with assays for BrdU, TUNEL, and TRACP5b activity, as well as serum levels of C-terminal telopeptide of type I collagen (CTX). The endogenous role of Wnt10b in bone was assessed by dynamic histomorphometry in Wnt10b−/− mice. Results: Oc-Wnt10b mice have increased mandibular bone and impaired eruption of incisors during postnatal development. Analyses of femoral distal metaphyses show significantly higher BMD, bone volume fraction, and trabecular number. Increased bone formation is caused by increases in number of osteoblasts per bone surface, rate of mineral apposition, and percent mineralizing surface. Although number of osteoclasts per bone surface is not altered, Oc-Wnt10b mice have increased total osteoclast activity because of higher bone mass. In Wnt10b−/− mice, changes in mineralizing variables and osteoblast perimeter in femoral distal metaphyses were not observed; however, bone formation rate is reduced because of decreased total bone volume and trabecular number. Conclusions: High bone mass in Oc-Wnt10b mice is primarily caused by increased osteoblastogenesis, with a minor contribution from elevated mineralizing activity of osteoblasts.

Wnt Pathway in Bone Repair and Regeneration – What Do We Know So Far

Wnt signaling plays a central regulatory role across a remarkably diverse range of functions during embryonic development, including those involved in the formation of bone and cartilage. Wnt signaling continues to play a critical role in adult osteogenic differentiation of mesenchymal stem cells. Disruptions in this highly-conserved and complex system leads to various pathological conditions, including impaired bone healing, autoimmune diseases and malignant degeneration. For reconstructive surgeons, critically sized skeletal defects represent a major challenge. These are frequently associated with significant morbidity in both the recipient and donor sites. The Wnt pathway is an attractive therapeutic target with the potential to directly modulate stem cells responsible for skeletal tissue regeneration and promote bone growth, suggesting that Wnt factors could be used to stimulate bone healing after trauma. This review summarizes our current understanding of the essential role of the Wnt pathway in bone regeneration and repair.

Osteocyte Wnt/β-Catenin Signaling Is Required for Normal Bone Homeostasis

β-Catenin-dependent canonical Wnt signaling plays an important role in bone metabolism by controlling differentiation of bone-forming osteoblasts and bone-resorbing osteoclasts. To investigate its ...

The Regulation of Bone Metabolism and Disorders by Wnt Signaling

Wnt, a secreted glycoprotein, has an approximate molecular weight of 40 kDa, and it is a cytokine involved in various biological phenomena including ontogeny, morphogenesis, carcinogenesis, and maintenance of stem cells. The Wnt signaling pathway can be classified into two main pathways: canonical and non-canonical. Of these, the canonical Wnt signaling pathway promotes osteogenesis. Sclerostin produced by osteocytes is an inhibitor of this pathway, thereby inhibiting osteogenesis. Recently, osteoporosis treatment using an anti-sclerostin therapy has been introduced. In this review, the basics of Wnt signaling, its role in bone metabolism and its involvement in skeletal disorders have been covered. Furthermore, the clinical significance and future scopes of Wnt signaling in osteoporosis, osteoarthritis, rheumatoid arthritis and neoplasia are discussed.

Molecular Signaling Pathways and Essential Metabolic Elements in Bone Remodeling: An Implication of Therapeutic Targets for Bone Diseases

Bone is one of the dynamic tissues in the human body that undergoes continuous remodelling through subsequent actions of bone cells, osteoclasts, and osteoblasts. Several signal transduction pathways are involved in the transition of mesenchymal stem cells into osteoblasts. These primarily include Runx2, ATF4, Wnt signaling and sympathetic signalling. The differentiation of osteoclasts is controlled by M-CSF, RANKL, and costimulatory signalling. It is well known that bone remodelling is regulated through receptor activator of nuclear factor-kappa B ligand followed by the binding to RANK, which eventually induces the differentiation of osteoclasts. The resorbing osteoclasts secrete TRAP, cathepsin K, MMP-9 and gelatinase to digest the proteinaceous matrix of type I collagen and form a saucer-shaped lacuna along with resorption tunnels in the trabecular bone. Osteoblasts secrete a soluble decoy receptor, osteoprotegerin that prevents the binding of RANK/RANKL and thus moderating osteoclastogenesis. Moreover, bone homeostasis is also regulated by several growth factors, cytokines, calciotropic hormones, parathyroid hormone and sex steroids. The current review presents a correlation of the probable molecular targets underlying the regulation of bone mass and the role of essential metabolic elements in bone remodelling. Targeting these signaling pathways may help design newer therapies for treating bone diseases.

The Microbial Metabolite Butyrate Stimulates Bone Formation via T Regulatory Cell-Mediated Regulation of WNT10B Expression

Tyagi et al. show that oral supplementation with the widely used probiotic, Lactobacillus rhamnosus GG (LGG), increases bone mass in mice by increasing the serum levels of the short chain fatty acid butyrate. LGG or butyrate increase the frequency of regulatory T (Treg) cells in the intestine and in the bone marrow. Treg cells stimulate CD8+ T cells to secrete the Wnt ligand Wnt10b, which stimulates bone formation by activating Wnt signaling in osteoblasts. Therefore, LGG and butyrate may represent new interventions for the prevention and treatment of osteoporosis.

Loss of Bone and Wnt10b Expression in Male Type 1 Diabetic Mice Is Blocked by the Probiotic Lactobacillus reuteri

Type 1 diabetes (T1D)–induced osteoporosis is characterized by a predominant suppression of osteoblast number and activity, as well as increased bone marrow adiposity but no change in osteoclast activity. The fundamental mechanisms and alternative anabolic treatments (with few side effects) for T1D bone loss remain undetermined. Recent studies by our laboratory and others indicate that probiotics can benefit bone health. Here, we demonstrate that Lactobacillus reuteri, a probiotic with anti-inflammatory and bone health properties, prevents T1D-induced bone loss and marrow adiposity in mice. We further found that L. reuteri treatment prevented the suppression of Wnt10b in T1D bone. Consistent with a role for attenuated bone Wnt10b expression in T1D osteoporosis, we observed that bone-specific Wnt10b transgenic mice are protected from T1D bone loss. To examine the mechanisms of this protection, we focused on TNF-α, a cytokine up-regulated in T1D that causes suppression of osteoblast Wnt10b expression in vitro. Addition of L. reuteri prevented TNF-α–mediated suppression of Wnt10b and osteoblast maturation markers. Taken together, our findings reveal a mechanism by which T1D causes bone loss and open new avenues for use of probiotics to benefit the bone.

TGF-β and BMP signaling in osteoblast, skeletal development, and bone formation, homeostasis and disease

Transforming growth factor-beta (TGF-β) and bone morphogenic protein (BMP) signaling has fundamental roles in both embryonic skeletal development and postnatal bone homeostasis. TGF-βs and BMPs, acting on a tetrameric receptor complex, transduce signals to both the canonical Smad-dependent signaling pathway (that is, TGF-β/BMP ligands, receptors, and Smads) and the non-canonical-Smad-independent signaling pathway (that is, p38 mitogen-activated protein kinase/p38 MAPK) to regulate mesenchymal stem cell differentiation during skeletal development, bone formation and bone homeostasis. Both the Smad and p38 MAPK signaling pathways converge at transcription factors, for example, Runx2 to promote osteoblast differentiation and chondrocyte differentiation from mesenchymal precursor cells. TGF-β and BMP signaling is controlled by multiple factors, including the ubiquitin–proteasome system, epigenetic factors, and microRNA. Dysregulated TGF-β and BMP signaling result in a number of bone disorders in humans. Knockout or mutation of TGF-β and BMP signaling-related genes in mice leads to bone abnormalities of varying severity, which enable a better understanding of TGF-β/BMP signaling in bone and the signaling networks underlying osteoblast differentiation and bone formation. There is also crosstalk between TGF-β/BMP signaling and several critical cytokines’ signaling pathways (for example, Wnt, Hedgehog, Notch, PTHrP, and FGF) to coordinate osteogenesis, skeletal development, and bone homeostasis. This review summarizes the recent advances in our understanding of TGF-β/BMP signaling in osteoblast differentiation, chondrocyte differentiation, skeletal development, cartilage formation, bone formation, bone homeostasis, and related human bone diseases caused by the disruption of TGF-β/BMP signaling.

BIOMEDICAL ENGINEERING OF SCLEROSTIN ACTION IN THE BONE REMODELING

Mathematical modeling of biological processes has bridged the fields of experimental as well as theoretical research and has carried forward remarkable innovation. Sclerostin is a fundamental communication element for bone remodeling and its activity regulates the reabsorption and deposition of new bone tissue. During this research, we have presented several studies, which illustrate the function of sclerostin in communication with the Wnt signaling pathways. This article features the sclerostin-based pathological patterns related to diseases such as bone cancer. To have a good remodeling process, the osteocytes must recruit the pre-osteoblast cells from the mesenchymal stem cells with the help of the signal mechanism given by the Wnt pathway. The Wnt signal pathway is a complex transduction of a pool of well-conserved genes whose expression regulates various activities like gene translation, cell adhesion, cell differentiation, mitogenic stimulation and polarity cell. The complexity of the interaction of the Wnt pathway is due to the ligands of Wnt itself, to the proteins R-spondin and norrin. The receptors on the surface of the cell, then, activate a process of transduction of the intracellular signal that initiates gene transduction. The hypothesis of a sort of “steady state” has therefore proved indispensable to establish a sort of common base on which the two phases. This paper seeks to give a qualitative view of the action of sclerostin through a simple mathematical model. We use a logic related to stimulation and inhibition signals of new tissue production and illustrate the role of sclerostin in the mechanical and biochemical interaction during the bone remodeling process.

The bone remodelling cycle

The bone remodelling cycle replaces old and damaged bone and is a highly regulated, lifelong process essential for preserving bone integrity and maintaining mineral homeostasis. During the bone remodelling cycle, osteoclastic resorption is tightly coupled to osteoblastic bone formation. The remodelling cycle occurs within the basic multicellular unit and comprises five co-ordinated steps; activation, resorption, reversal, formation and termination. These steps occur simultaneously but asynchronously at multiple different locations within the skeleton. Study of rare human bone disease and animal models have helped to elucidate the cellular and molecular mechanisms that regulate the bone remodelling cycle. The key signalling pathways controlling osteoclastic bone resorption and osteoblastic bone formation are receptor activator of nuclear factor-κB (RANK)/RANK ligand/osteoprotegerin and canonical Wnt signalling. Cytokines, growth factors and prostaglandins act as paracrine regulators of the cycle, whereas endocrine regulators include parathyroid hormone, vitamin D, calcitonin, growth hormone, glucocorticoids, sex hormones, and thyroid hormone. Disruption of the bone remodelling cycle and any resulting imbalance between bone resorption and formation leads to metabolic bone disease, most commonly osteoporosis. The advances in understanding the cellular and molecular mechanisms underlying bone remodelling have also provided targets for pharmacological interventions which include antiresorptive and anabolic therapies. This review will describe the remodelling process and its regulation, discuss osteoporosis and summarize the commonest pharmacological interventions used in its management.

Physiological and pathophysiological bone turnover — role of the immune system

Receptor activator of NF-κB ligand (RANKL) and osteoprotegerin (OPG) are key downstream effectors of bone resorption; tumour necrosis factor (TNF) might synergize with RANKL to superinduce osteoclastic bone resorptionB cells, regulated by T cells, are a key source of basal OPG whereas activated T cells and B cells are key sources of TNF and RANKL in inflammatory conditionsShort-term antiresorptive therapies might safely prevent bone loss associated with combination antiretroviral therapyAnergic and parathyroid-hormone-treated T cells secrete the protein Wnt-10b, which promotes bone formationNovel therapeutic strategies targeting the immune system might promote bone formation and decrease bone resorption to manage osteoporotic bone loss and prevent fracture

Mathematical Modeling of the Gut–Bone Axis and Implications of Butyrate Treatment on Osteoimmunology

Butyrate, a short-chain fatty acid produced by the gut microbiota, has pivotal roles in the regulation of the immune system. Recent studies have revealed that butyrate increases the differentiation of peripheral regulatory T cells in the gut–bone axis and promotes osteoblasts’ bone forming activity. However, the mechanism of the therapeutic benefit of butyrate in bone remodeling remains incompletely understood. Here, we develop a multicompartment mathematical model to quantitatively predict the contribution of butyrate on the expansion of regulatory T cells in the gut, blood, and bone compartments. We investigate the interplay between regulatory T cell-derived TGF-β and CD8+ T cell-derived Wnt-10b with changes in gut butyrate concentration. In addition, we connect our model to a detailed model of bone metabolism to study the impacts of butyrate and Wnt-10b on trabecular bone volume. Our results indicate both direct and indirect immune-mediated impacts of butyrate on bone metabolism.

Osteoblast-Osteoclast Communication and Bone Homeostasis

Bone remodeling is tightly regulated by a cross-talk between bone-forming osteoblasts and bone-resorbing osteoclasts. Osteoblasts and osteoclasts communicate with each other to regulate cellular behavior, survival and differentiation through direct cell-to-cell contact or through secretory proteins. A direct interaction between osteoblasts and osteoclasts allows bidirectional transduction of activation signals through EFNB2-EPHB4, FASL-FAS or SEMA3A-NRP1, regulating differentiation and survival of osteoblasts or osteoclasts. Alternatively, osteoblasts produce a range of different secretory molecules, including M-CSF, RANKL/OPG, WNT5A, and WNT16, that promote or suppress osteoclast differentiation and development. Osteoclasts also influence osteoblast formation and differentiation through secretion of soluble factors, including S1P, SEMA4D, CTHRC1 and C3. Here we review the current knowledge regarding membrane bound- and soluble factors governing cross-talk between osteoblasts and osteoclasts.

BMP–Smad1/9 signaling plays a critical role in regulating zebrafish PGC proliferation

The germ cell fate in zebrafish is determined by germ plasm, whereas mammalian germ cell fate is induced by bone morphogenetic protein (BMP) signaling. It remains elusive whether BMP signaling is implicated in zebrafish germ cell development. Here, we demonstrate that BMP–Smad1/9 signaling plays a critical role in zebrafish primordial germ cell (PGC) maintenance rather than fate determination. BMP inhibition or smad1/9 knockdown reduces PGC numbers. Furthermore, we generated PGC-specific smad1/9 knockouts using a transgenic approach with PGC-specifically expressed Cas9 and ubiquitously expressed guide RNAs. Smad1/9 deficiency in PGCs leads to impaired PGC proliferation and increased apoptosis, consequently reducing PGC numbers. Transcriptome analysis revealed unchanged PGC-specific gene expression, but a marked upregulation of DNA damage response-related genes, which is validated by ectopic ATR–pChk1 activation in PGCs and PGC restoring by ATR inhibition. Collectively, these findings underscore conserved but functionally distinct roles of BMP signaling in vertebrate PGC development.

Osteoclast differentiation and activation

Osteoclasts are specialized cells derived from the monocyte/macrophage haematopoietic lineage that develop and adhere to bone matrix, then secrete acid and lytic enzymes that degrade it in a specialized, extracellular compartment. Discovery of the RANK signalling pathway in the osteoclast has provided insight into the mechanisms of osteoclastogenesis and activation of bone resorption, and how hormonal signals impact bone structure and mass. Further study of this pathway is providing the molecular basis for developing therapeutics to treat osteoporosis and other diseases of bone loss.