The Interaction of Biological Factors with Mechanical Signals in Bone Adaptation: Recent Developments

Mechanotransduction in bone is fundamental to proper skeletal development. Deficiencies in signaling mechanisms that transduce physical forces to effector cells can have severe consequences for skeletal integrity. Therefore, a solid understanding of the cellular and molecular components of mechanotransduction is crucial for correcting skeletal modeling and remodeling errors and designing effective therapies. In recent years, progress has been made on many fronts regarding our understanding of bone cell mechanotransduction, including subcellular localization of mechanosensitive components in bone cells, the discovery of mechanosensitive G-protein- coupled receptors, identification of new ion channels and larger pores (eg, hemichannels) involved in physical signal transduction, and cell adhesion proteins, among others. These and other recent mechanisms are reviewed to provide a synthesis of recent experimental findings, in the larger context of whole bone adaptation

Osteocytes and mechanical loading: The Wnt connection

Bone adapts to the mechanical forces that it experiences. Orthodontic tooth movement harnesses the cell‐ and tissue‐level properties of mechanotransduction to achieve alignment and reorganization of the dentition. However, the mechanisms of action that permit bone resorption and formation in response to loads placed on the teeth are incompletely elucidated, though several mechanisms have been identified. Wnt/Lrp5 signalling in osteocytes is a key pathway that modulates bone tissue's response to load. Numerous mouse models that harbour knock‐in, knockout and transgenic/overexpression alleles targeting genes related to Wnt signalling point to the necessity of Wnt/Lrp5, and its localization to osteocytes, for proper mechanotransduction in bone. Alveolar bone is rich in osteocytes and is a highly mechanoresponsive tissue in which components of the canonical Wnt signalling cascade have been identified. As Wnt‐based agents become clinically available in the next several years, the major challenge that lies ahead will be to gain a more complete understanding of Wnt biology in alveolar bone so that improved/expedited tooth movement becomes a possibility

New Insights into Wnt-Lrp5/6-β-Catenin Signaling in Mechanotransduction

Mechanical loading is essential to maintain normal bone metabolism and the balance between bone formation and resorption. The cellular mechanisms that control mechanotransduction are not fully defined, but several key pathways have been identified. We discuss the roles of several components of the Wnt signaling cascade, namely Lrp5, Lrp6, and β-catenin in mechanical loading-induced bone formation. Lrp5 is an important Wnt co-receptor for regulating bone mass and mechanotransduction, and appears to function principally by augmenting bone formation. Lrp6 also regulates bone mass but its action might involve resorption as well as formation. The role of Lrp6 in mechanotransduction is unclear. Studies addressing the role of β-catenin in bone metabolism and mechanotransduction highlight the uncertainties in downstream modulators of Lrp5 and Lrp6. Taken together, these data indicate that mechanical loading might affect bone regulation triggering the canonical Wnt signaling (and perhaps other pathways) not only via Lrp5 but also via Lrp6. Further work is needed to clarify the role of the Wnt signaling pathway in Lrp5 and/or Lrp6-mediated mechanotransduction, which could eventually lead to powerful therapeutic agents that might mimic the anabolic effects of mechanical stimulation

WNT-mediated Modulation of Bone Metabolism: Implications for WNT Targeting to Treat Extraskeletal Disorders

The WNT-signaling pathway is involved in cellular and tissue functions that control such diverse processes as body axis patterning, cellular proliferation, differentiation, and life span. The long list of molecules that can participate or modify WNT signaling makes this pathway one of the most complex in cell biology. In bone tissues, WNT signaling is required for proper skeletal development, and human mutations in various components of the cascade revealed insights into pharmacologic targeting that can be harnessed to improve skeletal health. In particular, mutations in genes that code for the WNT-signaling inhibitor sclerostin or the WNT coreceptor lipoprotein receptor-related protein 5 have highlighted the potential therapeutic value of recapitulating those effects in patients with low bone mass. A constant challenge in this area is selectively modifying WNT components in the tissue of interest, as WNT has manifold effects in nearly every tissue

Skeletal loading in animals

A number of in vivo skeletal loading models have been developed to test specific hypotheses addressing the key mechanical and biochemical signals involved in bone’s adaptive response to loading. Exercise protocols, osteotomy procedures, loading of surgically implanted pins, and force application through the soft tissues are common approaches to alter the mechanical environment of a bone. Although each animal overload model has a number of assets and limitations, models employing extrinsic forces allow greater control of the mechanical environment. Sham controls, for both surgical intervention (when performed) and loading, are required to unequivocally demonstrate that responses to loading are mechanically adaptive. Collectively, extrinsic loading models have fostered a greater understanding of the mechanical signals important for stimulating bone cells, and highlighted the roles of key signaling molecules in the adaptive response

Postnatal β-catenin deletion from Dmp1-expressing osteocytes/osteoblasts reduces structural adaptation to loading, but not periosteal load-induced bone formation

Mechanical signal transduction in bone tissue begins with load-induced activation of several cellular pathways in the osteocyte population. A key pathway that participates in mechanotransduction is Wnt/Lrp5 signaling. A putative downstream mediator of activated Lrp5 is the nucleocytoplasmic shuttling protein β-catenin (βcat), which migrates to the nucleus where it functions as a transcriptional co-activator. We investigated whether osteocytic βcat participates in Wnt/Lrp5-mediated mechanotransduction by conducting ulnar loading experiments in mice with or without chemically induced βcat deletion in osteocytes. Mice harboring βcat floxed loss-of-function alleles (βcat(f/f)) were bred to the inducible osteocyte Cre transgenic (10)(kb)Dmp1-CreERt2. Adult male mice were induced to recombine the βcat alleles using tamoxifen, and intermittent ulnar loading sessions were applied over the following week. Although adult-onset deletion of βcat from Dmp1-expressing cells reduced skeletal mass, the bone tissue was responsive to mechanical stimulation as indicated by increased relative periosteal bone formation rates in recombined mice. However, load-induced improvements in cross sectional geometric properties were compromised in recombined mice. The collective results indicate that the osteoanabolic response to loading can occur on the periosteal surface when β-cat levels are significantly reduced in Dmp1-expressing cells, suggesting that either (i) only low levels of β-cat are required for mechanically induced bone formation on the periosteal surface, or (ii) other additional downstream mediators of Lrp5 might participate in transducing load-induced Wnt signaling

The Osteocyte: New Insights

Osteocytes are an ancient cell, appearing in fossilized skeletal remains of early fish and dinosaurs. Despite its relative high abundance, even in the context of nonskeletal cells, the osteocyte is perhaps among the least studied cells in all of vertebrate biology. Osteocytes are cells embedded in bone, able to modify their surrounding extracellular matrix via specialized molecular remodeling mechanisms that are independent of the bone forming osteoblasts and bone-resorbing osteoclasts. Osteocytes communicate with osteoclasts and osteoblasts via distinct signaling molecules that include the RankL/OPG axis and the Sost/Dkk1/Wnt axis, among others. Osteocytes also extend their influence beyond the local bone environment by functioning as an endocrine cell that controls phosphate reabsorption in the kidney, insulin secretion in the pancreas, and skeletal muscle function. These cells are also finely tuned sensors of mechanical stimulation to coordinate with effector cells to adjust bone mass, size, and shape to conform to mechanical demands

Finite Element Analysis of the Mouse Distal Femur with Tumor Burden in Response to Knee Loading

Breast cancer-associated bone metastasis induces bone loss, followed by an increased risk of bone fracture. To develop a strategy for preventing tumor growth and protecting bone, an understanding of the mechanical properties of bone under tumor burden is indispensable. Using a mouse model of mammary tumor, we conducted finite element analysis (FEA) of two bone samples from the distal femur. One sample was from a placebo-treated mouse, and the other was from a mouse treated with the investigational drug candidate, PD407824, an inhibitor of checkpoint kinases. Mechanical testing and microCT images revealed that bone strength is improved by administration of PD407824. In response to loading to the knee, FEA predicted that the peaks of von Mises stress, an indicator of fracture yielding, as well as the third principal compressive stress, were higher in the placebo-treated femur than the drug-treated femur. Higher peak stresses in trabecular segments were observed in the lateral condyle, a critical region for integrity of the knee joint. Collectively, this FE study supports the notion that mechanical weakening of the femur was observed in the tumor-invaded trabecular bone, and chemical agents such as PD407824 may potentially assist in preventing bone loss and bone fracture

Improving bone properties and fracture susceptibility: experimental models of genetic manipulation, pharmacologic intervention, and cellular perturbation reveal new approaches for improving bone health

poster abstractBone, a crucial support structure in the human body, is often taken for granted for its lightweight properties and unparalleled strength. Skeletal fracture is a major clinical condition affecting millions of Americans, which results from abnormal aging, hormonal imbalance, genetic conditions, and lifestyle choices (e.g., exercise). Because fractures are caused by a number of different factors, reducing fracture incidence requires a multifactorial approach to unraveling the underlying biology of bone metabolism, in order to discover new ways to improve bone properties and prevent fractures. We have taken such an approach by conducting (1) genetic manipulation experiments in mice, where genes predicted to be involved in bone mass regulation were mutated; (2) pharmacologic experiments to quantify the dose-response effect of an agent that inhibits bone loss, and (3) cell culture experiments, aimed at revealing molecular pathways activated by mechanical stimulation. METHODS: Mice with mutations in two genes, likely to regulate bone mass (SOST, DKK1) were generated and subjected to in vivo dual energy x-ray absorptiometry (DEXA) scans at 6-wk old. Whole body scans were analyzed for bone mineral density (BMD) using Lunar Piximus II v2.10 software. Mice (6-wk) were also dosed (0, 1, 10, 100, or 1000 mg/kg) with daily alendronate HCl, a bisphosphonate that inhibits osteoclast activity. Six wks later, the mice were sacrificed, and the femurs were dissected and sectioned for histological analysis of bone formation parameters, including mineralizing surface (MS/BS), mineral apposition rate (MAR), and bone formation rate (BFR/BS). To understand the cellular signaling events in response to mechanical loading, bone marrow mesenchymal stem cells (MSCs) were treated with 10, 20, 30, or 40μM PF7408671, an S6 kinase inhibitor. Cells then were subject to 100 cycles of biaxial mechanical strain (2%, 10 cycles/min). Protein lysates were separated by electrophoresis and probed for phosphorylation of Rictor and Akt by Western blot. RESULTS: Mice harboring mutations in either the SOST gene or the DKK1gene exhibited significantly increased BMD compared to wild-type control mice, though the SOST mutation had a stronger effect on BMD than DKK1. Mice with compound mutations (SOST and DKK1 mutations) had significantly greater BMD than mice with either single mutation, suggesting that inhibition of SOST and DKK1 might be an effective means to increase bone mass in patients susceptible to fracture. Mice treated with high-dose alendronate (100 or 1,000 mg/kg) exhibited significant decreases in bone formation parameters (MS/BS, MAR, and BFR/BS) compared to untreated (0 mg) mice, suggesting that while this compound might be beneficial for inhibiting bone loss, it also inhibits bone formation. The signaling hub, mTORC2, is a critical regulator of mechanical force in MSC progenitors. Our data demonstrate that S6 kinase is an upstream activator of mTORC2 in response to mechanical strain. CONCLUSION: Our experiments suggest that genetic manipulation of mice reveal viable protein targets (e.g., SOST, DKK1) that could ultimately be manipulated pharmacologically to improve bone mass. We also found that an FDA-approved class of drugs inhibits bone formation even at very low doses, suggesting that additional pro-anabolic compounds might benefit patients taking bisphosphonates. On a cell signaling level, we found that the mTORC2 pathway shows considerable promise for pharmacologic manipulation to simulate the effects of exercise. Taken together, these experiments highlight the utility of a broad approach to solving bone metabolism challenges that can affect fracture susceptibility