Modulation of Wnt signaling influences fracture repair

Single-nucleus transcriptomics reveal the differentiation trajectories of periosteal skeletal/stem progenitor cells in bone regeneration

Bone regeneration is mediated by skeletal stem/progenitor cells (SSPCs) that are mainly recruited from the periosteum after bone injury. The composition of the periosteum and the steps of SSPC activation and differentiation remain poorly understood. Here, we generated a single-nucleus atlas of the periosteum at steady state and of the fracture site during the early stages of bone repair (https://fracture-repair-atlas.cells.ucsc.edu). We identified periosteal SSPCs expressing stemness markers (Pi16 and Ly6a/SCA1) and responding to fracture by adopting an injury-induced fibrogenic cell (IIFC) fate, prior to undergoing osteogenesis or chondrogenesis. We identified distinct gene cores associated with IIFCs and their engagement into osteogenesis and chondrogenesis involving Notch, Wnt, and the circadian clock signaling, respectively. Finally, we show that IIFCs are the main source of paracrine signals in the fracture environment, suggesting a crucial paracrine role of this transient IIFC population during fracture healing. Overall, our study provides a complete temporal topography of the early stages of fracture healing and the dynamic response of periosteal SSPCs to injury, redefining our knowledge of bone regeneration.

Single nuclei transcriptomics reveal the differentiation trajectories of periosteal skeletal/stem progenitor cells in bone regeneration

Bone regeneration is mediated by skeletal stem/progenitor cells (SSPCs) that are mainly recruited from the periosteum after bone injury. The composition of the periosteum and the steps of SSPC activation and differentiation remain poorly understood. Here, we generated a single-nuclei atlas of the periosteum at steady-state and of the fracture site during early stages of bone repair ( https://fracture-repair-atlas.cells.ucsc.edu ). We identified periosteal SSPCs expressing stemness markers ( Pi16 and Ly6a /SCA1) and responding to fracture by adopting an injury-induced fibrogenic cell (IIFC) fate, prior to undergoing osteogenesis or chondrogenesis. We identified distinct gene cores associated with IIFCs and their engagement into osteogenesis and chondrogenesis involving Notch, Wnt and the circadian clock signaling respectively. Finally, we show that IIFCs are the main source of paracrine signals in the fracture environment, suggesting a crucial paracrine role of this transient IIFC population during fracture healing. Overall, our study provides a complete temporal topography of the early stages of fracture healing and the dynamic response of periosteal SSPCs to injury, redefining our knowledge of bone regeneration.

The role of Wnt signaling on Tooth Extraction Wound Healing: Narrative review

Compared to an incisional skin or mucosal wound, a tooth extraction wound results in far more soft tissue loss. A blood clot instantly fills the gap left by the extracted tooth. An embryonic type of bone forms during the healing of extraction wounds, and mature bone only later replaces it. Osteocytes in embryonic bone, also known as coarse fibrillar bone or immature bone, differ from those in adult bone in terms of number, size, and irregular arrangement. This immature bone is more radiolucent than mature bone due to the higher cell density and the smaller volume of calcified intercellular material. The Wnt gene family contains genes that encode secreted signaling proteins that have good promise for promoting bone regeneration. However, we still have a limited understanding the interplay of the molecular elements of the Wnt pathway in signal transduction, from ligand detection on the cell surface to transcription of target genes in the nucleus. We discuss the function of Wnt signaling molecules in this review, in tissue repair following tooth extraction and present recent results about these molecules. Conclusions: Wnt signaling activity helps to hasten bone regeneration while bone healing is slowed down by mutations in LRP5/6 or β-catenin.

Antisclerostin Effect on Osseointegration and Bone Remodeling

Objective: This study reviewed the literature on local or systemic administration of antisclerostin, presenting results associated with osseointegration of dental/orthopedic implants and stimulation of bone remodeling. Materials and Methods: An extensive electronic search was conducted through MED-LINE/PubMed, PubMed Central, Web of Science databases and specific peer-reviewed journals to identify case reports, case series, randomized controlled trials, clinical trials and animal studies comparing either the systemic or local administration of antisclerostin and its effect in osseointegration and bone remodeling. Articles in English and with no restriction on period were included. Results: Twenty articles were selected for a full-text, and one was excluded. Finally, 19 articles were included in the study (16 animal studies and 3 randomized control trials). These studies were divided into two groups, which evaluated (i) osseointegration and (ii) bone remodeling potential. Initially 4560 humans and 1191 animals were identified. At least 1017 were excluded from the studies (981 humans and 36 animals), totaling 4724 subjects who completed (3579 humans and 1145 animals). (a) Osseointegration: 7 studies described this phenomenon; 4 reported bone-implant contact, which increased in all included studies. Similar results were found for bone mineral density, bone area/volume and bone thickness. (b) Bone remodeling: 13 studies were used for description. The studies reported an increase in BMD with sclerostin antibody treatment. A similar effect was found for bone mineral density/area/volume, trabecular bone and bone formation. Three biomarkers of bone formation were identified: bone-specific alkaline phosphatase (BSAP), osteocalcin and procollagen type 1 N-terminal Pro-peptide (P1NP); and markers for bone resorption were: serum C-telopeptide (sCTX), C-terminal telopeptides of type I collagen (CTX-1), β-isomer of C-terminal telopeptides of type I collagen (β-CTX) and tartrate-resistant acid phosphatase 5b (TRACP-5b). There were limitations: low number of human studies identified; high divergence in the model used (animal or human); the variance in the type of Scl-Ab and doses of administration; and the lack of reference quantitative values in the parameters analyzed by authors' studies (many articles only reported qualitative information). Conclusion: Within the limitations of this review and carefully observing all data, due to the number of articles included and the heterogeneity existing, more studies must be carried out to better evaluate the action of the antisclerostin on the osseointegration of dental implants. Otherwise, these findings can accelerate and stimulate bone remodeling and neoformation.

Combined methylphenidate and fluoxetine treatment in adolescent rats significantly impairs weight gain with minimal effects on skeletal development

Methylphenidate (MP) is frequently prescribed to treat Attention-Deficit/Hyperactivity Disorder (ADHD); however, many patients with ADHD experience depression and anxiety. As such, concomitant administration of selective serotonin reuptake inhibitors such as fluoxetine (FLX) is common. Our laboratory and others have shown that MP impairs skeletal development in preclinical and clinical settings, and FLX has also been linked to skeletal deficits. Unfortunately, little is known about the effects of combined MP and FLX treatment on skeletal development. The objective of this study was to investigate the effects of MP and FLX on bone morphology and biomechanical properties in adolescent rats. Four-week-old male Sprague-Dawley rats were randomly divided into the following 4 groups: Water, MP, FLX, and MP + FLX. As body weights in the MP, FLX, and MP + FLX groups were all lower than Water, the data were compared directly and after adjusting to body weight via linear regression. The direct comparison revealed that MP + FLX rats had significantly shorter (~12 %) and narrower femora and tibiae (~10 %) compared to most other groups, along with shorter (26-35 %), disorganized tibial growth plates. MicroCT analyses of the trabecular compartment of the proximal tibia identified reductions of 47 % for TV, 86 % for BV, 74 % for BV/TV, 68 % for Tb.N, 25 % in Tb.Th, and 74 % in vBMD concomitant with increases of 44 % for Tb.Sp for MP + FLX compared to Water. Similar analyses of femoral midshaft cortical bone identified reductions of 29 % for Ct.V, 30 % for Ps.V, 30 % for Ec. V, and 51 % for pMOI, as well as increases of 17 % for Ct.Th and 2 % for TMD for MP + FLX compared to Water. Biomechanically, MP + FLX femora were weaker, as indicated by a reduction in ultimate force (14 %) in MP + FLX compared to Water. The microstructural and biomechanical effects of MP + FLX were eliminated after adjustment for body weight, though the detrimental effects on growth plate morphology remained. We conclude that while the adverse microstructural and biomechanical effects of MP + FLX seen via direct comparison are predominantly attributable to reductions in body weight rather than direct effects on bone, MP and FLX, particularly in combination show detrimental effects on growth plate structure and chondrocyte morphology. These findings warrant further research into the effect of these drugs on weight gain, skeletal development and growth plate morphology, as well as consideration by physicians treating children and adolescents with ADHD.

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.

Fracture hematoma micro-architecture influences transcriptional profile and plays a crucial role in determining bone healing outcomes

The hematoma that forms between broken fragments of bone serves as a natural fibrin scaffold, and its removal from the defect site delays bone healing. The hypothesis of this study is that the microarchitectural and mechanical properties of the initially formed hematoma has a significant effect on the regulation of the biological process, which ultimately determines the outcome of bone healing. To mimic three healing conditions in the rat femur (normal, delayed, and non-healing bone defects), three different defect sizes of 0.5, 1.5, and 5.0 mm, are respectively used. The analysis of 3-day-old hematomas demonstrates clear differences in fibrin clot micro-architecture in terms of fiber diameter, fiber density, and porosity of the formed fibrin network, which result in different mechanical properties (stiffness) of the hematoma in each model. Those differences directly affect the biological processes involved. Specifically, RNA-sequencing reveals almost 700 differentially expressed genes between normally healing and non-healing defects, including significantly up-regulated essential osteogenic genes in normally healing defects, also differences in immune cell populations, activated osteogenic transcriptional regulators as well as potential novel marker genes. Most importantly, this study demonstrates that the healing outcome has already been determined during the hematoma phase of bone healing, three days post-surgery.

Activation of canonical Wnt signaling accelerates intramembranous bone regeneration in male mice

Canonical Wnt signaling plays an important role in skeletal development, homeostasis, and both endochondral and intramembranous repair. While studies have demonstrated that the inhibition of Wnt signaling impairs intramembranous bone regeneration, how its activation affects intramembranous bone regeneration has been underexplored. Therefore, we sought to determine the effects of activation of canonical Wnt signaling on intramembranous bone regeneration by using the well-established marrow ablation model. We hypothesized that mice with a mutation in the Wnt ligand coreceptor gene Lrp5 would have accelerated intramembranous bone regeneration. Male and female wild-type and Lrp5-mutant mice underwent unilateral femoral bone marrow ablation surgery in the right femur at 4 weeks of age. Both the left intact and right operated femurs were assessed at Days 3, 5, 7, 10, and 14. The intact femur of Lrp5 mutant mice of both sexes had higher bone mass than wild-type littermates, although to a greater degree in males than females. Overall, the regenerated bone volume in Lrp5 mutant male mice was 1.8-fold higher than that of littermate controls, whereas no changes were observed between female Lrp5 mutant and littermate control mice. In addition, the rate of intramembranous bone regeneration (from Day 3 to Day 7) was higher in Lrp5 mutant male mice compared to their same-sex littermate controls with no difference in the females. Thus, activation of canonical Wnt signaling increases bone mass in intact bones of both sexes, but accelerates intramembranous bone regeneration following an injury challenge only in male mice.

Hormone sensitive lipase ablation promotes bone regeneration

There is an inverse relationship between the differentiation of mesenchymal stem cells (MSCs) along either an adipocyte or osteoblast lineage, with lineage differentiation known to be mediated by transcription factors PPARγ and Runx2, respectively. Endogenous ligands for PPARγ are generated during the hydrolysis of triacylglycerols to fatty acids through the actions of lipases such as hormone sensitive lipase (HSL). To examine whether reduced production of endogenous PPARγ ligands would influence bone regeneration, we examined the effects of HSL knockout on fracture repair in mice using a tibial mono-cortical defect as a model. We found an improved rate of fracture repair in HSL-ko mice documented by serial μCT and bone histomorphometry compared to wild-type (WT) mice. Similarly, accelerated rates of bone regeneration were observed with a calvarial model where implantation of bone grafts from HSL-ko mice accelerated bone regeneration at the injury site. Further analysis revealed improved MSC differentiation down osteoblast and chondrocyte lineage with inhibition of HSL. MSC recruitment to the injury site was greater in HSL-ko mice than WT. Finally, we used single cell RNAseq to understand the osteoimmunological differences between WT and HSL-ko mice and found changes in the pre-osteoclast population. Our study shows HSL-ko mice as an interesting model to study improvements to bone injury repair. Furthermore, our study highlights the potential importance of pre-osteoclasts and osteoclasts in bone repair.

Drug Discovery of DKK1 Inhibitors

Dickkopf-1 (DKK1) is a well-characterized Wnt inhibitor and component of the Wnt/β-catenin signaling pathway, whose dysregulation is associated with multiple abnormal pathologies including osteoporosis, Alzheimer's disease, diabetes, and various cancers. The Wnt signaling pathway has fundamental roles in cell fate determination, cell proliferation, and survival; thus, its mis-regulation can lead to disease. Although DKK1 is involved in other signaling pathways, including the β-catenin-independent Wnt pathway and the DKK1/CKAP4 pathway, the inhibition of DKK1 to propagate Wnt/β-catenin signals has been validated as an effective way to treat related diseases. In fact, strategies for developing DKK1 inhibitors have produced encouraging clinical results in different pathological models, and many publications provide detailed information about these inhibitors, which include small molecules, antibodies, and nucleic acids, and may function at the protein or mRNA level. However, no systematic review has yet provided an overview of the various aspects of their development and prospects. Therefore, we review the DKK1 inhibitors currently available or under study and provide an outlook on future studies involving DKK1 and drug discovery.

Enhancing fracture repair: cell-based approaches

Fracture repair is based both on the macrolevel modulation of fracture fragments and the subsequent cellular activity. Surgeons have also long recognized other influences on cellular behavior: the effect of the fracture or subsequent surgery on the available pool of cells present locally in the periosteum, the interrelated effects of fragment displacement, and construct stiffness on healing potential, patient pathophysiology and systemic disease conditions (such as diabetes), and external regulators of the skeletal repair (such as smoking or effect of medications). A wide variety of approaches have been applied to enhancing fracture repair by manipulation of cellular biology. Many of these approaches reflect our growing understanding of the cellular physiology that underlies skeletal regeneration. This review focuses on approaches to manipulating cell lineages, influencing paracrine and autocrine cell signaling, or applying other strategies to influence cell surface receptors and subsequent behavior. Scientists continue to evolve new approaches to pharmacologically enhancing the fracture repair process.

The interaction of Notch and Wnt signaling pathways in vertebrate regeneration

Regeneration is an evolutionarily conserved process in animal kingdoms, however, the regenerative capacities differ from species and organ/tissues. Mammals possess very limited regenerative potential to replace damaged organs, whereas non-mammalian species usually have impressive abilities to regenerate organs. The regeneration process requires proper spatiotemporal regulation from key signaling pathways. The canonical Notch and Wnt signaling pathways, two fundamental signals guiding animal development, have been demonstrated to play significant roles in the regeneration of vertebrates. In recent years, increasing evidence has implicated the cross-talking between Notch and Wnt signals during organ regeneration. In this review, we summarize the roles of Notch signaling and Wnt signaling during several representative organ regenerative events, emphasizing the functions and molecular bases of their interplay in these processes, shedding light on utilizing these two signaling pathways to enhance regeneration in mammals and design legitimate therapeutic strategies.

MicroRNAs and fracture healing: Pre-clinical studies

During the past several years, pre-clinical experiments have established that microRNAs (miRNAs), small non-coding RNAs, serve as key regulatory molecules of fracture healing. Their easy modulation with agonists and antagonists make them highly desirable targets for future therapeutic strategies, especially for pathophysiologic fractures that either do not heal (nonunions) or are delayed. It is now well documented that these problematic fractures lead to human suffering and impairment of life quality. Additionally, financial difficulties are also encountered as work productivity decreases and income is reduced. Moreover, targeting miRNAs may also be an avenue to enhancing normal physiological fracture healing. Herein we present the most current knowledge of the involvement of miRNAs during fracture healing in pre-clinical studies. Following a brief description on the nature of miRNAs and of the fracture healing process, we present data from studies focusing specifically, on miRNA regulation of osteoblast differentiation and osteogenesis (within the context of known signaling pathways), chondrocytes, angiogenesis, and apoptosis, all critical to successful bone repair. Further, we also discuss miRNAs and exosomes. We hope that this manuscript serves as a comprehensive review that will facilitate basic/translational scientists in the orthopaedic arena to realize and further decipher the biological and future therapeutic impact of these small regulatory RNA molecules, especially as they relate to the molecular events of each of the major phases of fracture healing.

Prx1-expressing cells contributing to fracture repair require primary cilia for complete healing in mice

Bone is a dynamic organ that is continuously modified during development, load-induced adaptation, and fracture repair. Understanding the cellular and molecular mechanisms for natural fracture healing can lead to therapeutics that enhance the quality of newly formed tissue, advance the rate of healing, or replace the need for invasive surgical procedures. Prx1-expressing cells in the periosteum are thought to supply the majority of osteoblasts and chondrocytes in the fracture callus, but the exact mechanisms for this behavior are unknown. The primary cilium is a sensory organelle that is known to mediate several signaling pathways involved in fracture healing and required for Prx1-expressing cells to contribute to juvenile bone development and adult load-induced bone formation. We therefore investigated the role of Prx1-expressing cell primary cilia in fracture repair by developing a mouse model that enabled us to simultaneously track Prx1 lineage cell fate and disrupt Prx1-expressing cell primary cilia in vivo. The cilium KO mice exhibited abnormally large calluses with significantly decreased bone formation and persistent cartilage nodules. Analysis of mRNA expression in the early soft callus revealed downregulation of osteogenesis, Hh signaling, and Wnt signaling, and upregulation of chondrogenesis and angiogenesis. The mutant mice also exhibited decreased Osx and Periostin but increased αSMA and PECAM-1 protein expression in the hard callus. We further used a Gli1LacZ reporter and found that Hh signaling was significantly upregulated in the mutant callus at later stages of healing. Interestingly, altered protein expression and Hh signaling did not correlate with labeled Prx1-lineage cells, suggesting loss of cilia altered Hh signaling non-autonomously. Overall, cilium KO mice demonstrated severely delayed and incomplete fracture healing, and our findings suggest Prx1-expressing cell primary cilia are necessary to tune Hh signaling for proper fracture repair.

Hematopoietic Wnts Modulate Endochondral Ossification During Fracture Healing

Wnt signaling plays a critical role in bone formation, homeostasis, and injury repair. Multiple cell types in bone have been proposed to produce the Wnts required for these processes. The specific role of Wnts produced from cells of hematopoietic origin has not been previously characterized. Here, we examined if hematopoietic Wnts play a role in physiological musculoskeletal development and in fracture healing. Wnt secretion from hematopoietic cells was blocked by genetic knockout of the essential Wnt modifying enzyme PORCN, achieved by crossing Vav-Cre transgenic mice with Porcn^flox mice. Knockout mice were compared with their wild-type littermates for musculoskeletal development including bone quantity and quality at maturation. Fracture healing including callus quality and quantity was assessed in a diaphyseal fracture model using quantitative micro computer-assisted tomographic scans, histological analysis, as well as biomechanical torsional and 4-point bending stress tests. The hematopoietic Porcn knockout mice had normal musculoskeletal development, with normal bone quantity and quality on micro-CT scans of the vertebrae. They also had normal gross skeletal dimensions and normal bone strength. Hematopoietic Wnt depletion in the healing fracture resulted in fewer osteoclasts in the fracture callus, with a resultant delay in callus remodeling. All calluses eventually progressed to full maturation. Hematopoietic Wnts, while not essential, modulate osteoclast numbers during fracture healing. These osteoclasts participate in callus maturation and remodeling. This demonstrates the importance of diverse Wnt sources in bone repair.

Systemic DKK1 neutralization enhances human adipose-derived stem cell mediated bone repair

Progenitor cells from adipose tissue are able to induce bone repair; however, inconsistent or unreliable efficacy has been reported across preclinical and clinical studies. Soluble inhibitory factors, such as the secreted Wnt signaling antagonists Dickkopf-1 (DKK1), are expressed to variable degrees in human adipose-derived stem cells (ASCs), and may represent a targetable "molecular brake" on ASC mediated bone repair. Here, anti-DKK1 neutralizing antibodies were observed to increase the osteogenic differentiation of human ASCs in vitro, accompanied by increased canonical Wnt signaling. Human ASCs were next engrafted into a femoral segmental bone defect in NOD-Scid mice, with animals subsequently treated with systemic anti-DKK1 or isotype control during the repair process. Human ASCs alone induced significant but modest bone repair. However, systemic anti-DKK1 induced an increase in human ASC engraftment and survival, an increase in vascular ingrowth, and ultimately improved bone repair outcomes. In summary, anti-DKK1 can be used as a method to augment cell-mediated bone regeneration, and could be particularly valuable in the contexts of impaired bone healing such as osteoporotic bone repair.

Growth Factors, Carrier Materials, and Bone Repair

This chapter provides an overview of the growth factors active in bone regeneration and healing. Both normal and impaired bone healing are discussed, with a focus on the spatiotemporal activity of the various growth factors known to be involved in the healing response. The review highlights the activities of most important growth factors impacting bone regeneration, with a particular emphasis on those being pursued for clinical translation or which have already been marketed as components of bone regenerative materials. Current approaches the use of bone grafts in clinical settings of bone repair (including bone grafts) are summarized, and carrier systems (scaffolds) for bone tissue engineering via localized growth factor delivery are reviewed. The chapter concludes with a consideration of how bone repair might be improved in the future.

Unleashing β-catenin with a new anti-Alzheimer drug for bone tissue regeneration

The Wnt/β-catenin signaling pathway is critical for bone differentiation and regeneration. Tideglusib, a selective FDA approved glycogen synthase kinase-3β (GSK-3β) inhibitor, has been shown to promote dentine formation, but its effect on bone has not been examined. Our objective was to study the effect of localized Tideglusib administration on bone repair. Bone healing between Tideglusib treated and control mice was analysed at 7, 14 and 28 days postoperative (PO) with microCT, dynamic histomorphometry and immunohistology. There was a local downregulation of GSK-3β in Tideglusib animals, resulting in a significant increase in the amount of new bone formation with both enhanced cortical bone bridging and medullary bone deposition. The bone formation in the Tideglusib group was characterized by early osteoblast differentiation with down-regulation of GSK-3β at day 7 and 14, and higher accumulation of active β-catenin at day 14. Here, for the first time, we show a positive effect of Tideglusib on bone formation through the inactivation of GSK-3β. Furthermore, the findings suggest that Tideglusib does not interfere with precursor cell recruitment and commitment, contrary to other GSK-3β antagonists such as lithium chloride. Taken together, the results indicate that Tideglusib could be used directly at a fracture site during the initial intraoperative internal fixation without the need for further surgery, injection or drug delivery system. This FDA-approved drug may be useful in the future for the prevention of non-union in patients presenting with a high risk for fracture-healing.

Biomedical research models in the science of fracture healing - Pitfalls & promises

Development of intervention strategies to stimulate fracture healing has long been a focus of musculoskeletal research. Considerable investment in empirical research has led to the discovery of several genes and signaling pathways that are involved in skeletal development and regeneration. However, there are currently very few biologic interventions that can efficiently be used to enhance fracture healing in clinical practice. This translational barrier is due in part to experimental barriers to mechanism discovery. Animal models, biomechanical models, finite element models, and mathematical models are a few examples of models that aid in the discovery of mechanisms. Understanding the advantages, limitations, and specialized uses of each model type is critical to our ability to interpret mechanistic insights from such research and to help bridge the translation gap between pre-clinical research and clinical practice. In this review, we look at specific modeling methods used in the study of the fracture healing mechanism. We also discuss the strength and limitations to translation of each method, hopefully leading to a better understanding of how we can use models to advance the study of fracture healing.

Wnt modulation in bone healing

Genetic studies have been instrumental in the field of orthopaedics for finding tools to improve the standard management of fractures and delayed unions. The Wnt signaling pathway that is crucial for development and maintenance of many organs also has a very promising pathway for enhancement of bone regeneration. The Wnt pathway has been shown to have a direct effect on stem cells during bone regeneration, making Wnt a potential target to stimulate bone repair after trauma. A more complete view of how Wnt influences animal bone regeneration has slowly come to light. This review article provides an overview of studies done investigating the modulation of the canonical Wnt pathway in animal bone regeneration models. This not only includes a summary of the recent work done elucidating the roles of Wnt and β-catenin in fracture healing, but also the results of thirty transgenic studies, and thirty-eight pharmacological studies. Finally, we discuss the discontinuation of sclerostin clinical trials, ongoing clinical trials with lithium, the results of Dkk antibody clinical trials, the shift into combination therapies and the future opportunities to enhance bone repair and regeneration through the modulation of the Wnt signaling pathway.

Dkk1 acts as a negative regulator in the osteogenic differentiation of the posterior longitudinal ligament cells

Ossification of the posterior longitudinal ligament (OPLL) is a spinal disorder characterized by progressive ectopic bone formation in the PLL of the spine. Dickkopf-1 (Dkk1) is a secreted inhibitor of the Wnt pathway that negatively regulates bone formation during skeletal development. However, whether Dkk1 impacts the pathogenesis of OPLL has not been reported. This study is to investigate the role of Dkk1 in the development of OPLL. Our results show that the serum levels of Dkk1 are decreased in OPLL patients compared with non-OPLL controls. The expression of Dkk1 is also reduced in OPLL ligament cells. Downregulation of Dkk1 in ligament cells is associated with activation of the Wnt/β-catenin signaling, as indicated by stabilized β-catenin and increased T-cell factor-dependent transcriptional activity. Functionally, Dkk1 exerts a growth-inhibitory effect by repressing proliferation but promoting apoptosis of ligament cells. Dkk1 also suppresses bone morphogenetic protein 2-induced entire osteogenic differentiation of ligament cells, and this suppression is mediated via its inhibition of the Wnt pathway. Our results demonstrate for the first time that Dkk1 acts as an important negative regulator in the ossification of the PLL. Targeting the Wnt pathway using Dkk1 may represent a potential therapeutic strategy for the treatment of OPLL.

Anti-DKK1 Enhances the Early Osteogenic Differentiation of Human Adipose-Derived Stem/Stromal Cells

Adipose-derived stem/stromal cells (ASCs) have been previously used for bone repair. However, significant cell heterogeneity exists within the ASC population, which has the potential to result in unreliable bone tissue formation and/or low efficacy. Although the use of cell sorting to lower cell heterogeneity is one method to improve bone formation, this is a technically sophisticated and costly process. In this study, we tried to find a simpler and more deployable solution-blocking antiosteogenic molecule Dickkopf-1 (DKK1) to improve osteogenic differentiation. Human adipose-derived stem cells were derived from = 5 samples of human lipoaspirate. In vitro, anti-DKK1 treatment, but not anti-sclerostin (SOST), promoted ASC osteogenic differentiation, assessed by alizarin red staining and real-time polymerase chain reaction (qPCR). Increased canonical Wnt signaling was confirmed after anti-DKK1 treatment. Expression levels of DKK1 peaked during early osteogenic differentiation (day 3). Concordantly, anti-DKK1 supplemented early (day 3 or before), but not later (day 7) during osteogenic differentiation positively regulated osteoblast formation. Finally, anti-DKK1 led to increased transcript abundance of the Wnt inhibitor SOST, potentially representing a compensatory cellular mechanism. In sum, DKK1 represents a targetable "molecular brake" on the osteogenic differentiation of human ASC. Moreover, release of this brake by neutralizing anti-DKK1 antibody treatment at least partially rescues the poor bone-forming efficacy of ASC.

The PI3K/AKT pathway promotes fracture healing through its crosstalk with Wnt/β-catenin

PI3K/AKT is one of the key pathways that regulate cell behaviors including apoptosis, proliferation, and differentiation. Although previous studies have demonstrated that this pathway is a crucial regulator of osteoblasts, the role of PI3K/AKT in fracture healing remains unclear. It is well known that the Wnt/β-catenin pathway plays an essential role in bone regeneration. However, whether there exists crosstalk between Wnt/β-catenin and PI3K/AKT in regulating osteoblasts and bone repair has not been reported. To address these issues, we establish a stabilized fracture model in mice and show that PI3K inhibitor LY294002 substantially inhibits the bone healing process, suggesting that PI3K/AKT promotes fracture repair. More importantly, we report that PI3K/AKT increases phosphorylation of GSK-3β at Ser9 and phosphorylation of β-catenin at Ser552 in fracture callus and murine osteoblastic MC3T3-E1 cells, both of which lead to β-catenin stabilization, nuclear translocation, as well as β-catenin-mediated TCF-dependent transcription, suggesting that β-catenin is activated downstream of PI3K/AKT. Furthermore, we show that ICG001, the inhibitor of β-catenin transcriptional activity, attenuates PI3K/AKT-induced osteoblast proliferation, differentiation, and mineralization, indicating that the PI3K/AKT/β-catenin axis is functional in regulating osteoblasts. Notably, the PI3K/AKT pathway is also activated by Wnt3a and is involved in Wnt3a-induced osteoblast proliferation and differentiation. Hence, our results reveal the existence of a Wnt/PI3K/AKT/β-catenin signaling nexus in osteoblasts, highlighting complex crosstalk between PI3K/AKT and Wnt/β-catenin pathways that are critically implicated in fracture healing.

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.

MiR-128 promotes osteogenic differentiation of bone marrow mesenchymal stem cells in rat by targeting DKK2

Bone loss caused by inflammatory disease, such as peri-implantitis, poses a great challenge to clinicians for restoration. Emerging evidence indicates that microRNAs (miRNAs) are indispensable regulators of bone growth, development, and formation. In the present study, we found that microRNA-128 (miR-128) was differentially up-regulated during the osteogenic differentiation of rat bone marrow stem cells (rBMSCs). Overexpression of miR-128 promoted osteogenic differentiation of rBMSCs by up-regulating alkaline phosphatase (ALP), matrix mineralization, mRNA, and protein levels of osteogenic makers (e.g. RUNX2, BMP-2, and COLIA1), whereas inhibition of miR-128 suppressed osteoblastic differentiation in vitro. Mechanistically, miR-128 directly and functionally targeted Dickkopf2 (DKK2), which is a Wnt signaling pathway antagonist, and enhanced Wnt/β-catenin signaling activity. Furthermore, the positive effect of miR-128 on osteogenic differentiation was apparently abrogated by DKK2 overexpression. Collectively, these results indicate that miR-128 promotes osteogenic differentiation of rBMSCs by targeting DKK2, which may provide a promising approach to the treatment of peri-implantitis.

WNT Signaling in Disease

Developmental signaling pathways control a vast array of biological processes during embryogenesis and in adult life. The WNT pathway was discovered simultaneously in cancer and development. Recent advances have expanded the role of WNT to a wide range of pathologies in humans. Here, we discuss the WNT pathway and its role in human disease and some of the advances in WNT-related treatments.

Bone growth as the main determinant of mouse digit tip regeneration after amputation

Regeneration is classically demonstrated in mammals using mice digit tip. In this study, we compared different amputation plans and show that distally amputated digits regrow with morphology close to normal but fail to regrow the fat pad. Proximally amputated digits do not regrow the phalangeal bone, but the remaining structures (nail, skin and connective tissue), all with intrinsic regenerative capacity, re-establishing integrity indistinguishably in distally and proximally amputated digits. Thus, we suggest that the bone growth promoted by signals and progenitor cells not removed by distal amputations is responsible for the re-establishment of a drastically different final morphology after distal or proximal digit tip amputations. Despite challenging the use of mouse digit tip as a model system for limb regeneration in mammals, these findings evidence a main role of bone growth in digit tip regeneration and suggest that mechanisms that promote joint structures formation should be the main goal of regenerative medicine for limb and digit regrowth.

Postoperative Tendon Loading With Treadmill Running Delays Tendon-to-Bone Healing: Immunohistochemical Evaluation in a Murine Rotator Cuff Repair Model

Mechanical stress has an important effect on tendon-to-bone healing. The purpose of the present study was to compare tendon-to-bone healing in animals exposed to either tendon unloading (botulinum toxin injection) or excessive loading (treadmill running) in a murine rotator cuff repair model. Forty-eight C57BL/6 mice underwent unilateral supraspinatus tendon detachment and repair. Mice in the unloaded group were injected with botulinum toxin to the supraspinatus muscle. The contralateral shoulder of the unloaded group was used as a control. Mice were euthanized at 1, 2, and 4 weeks after surgery and evaluated with hematoxylin-eosin and immunohistochemical (IHC) staining for Ihh, Gli1, Wnt3a, and β-catenin. The positive staining area on IHC and the Modified Tendon Maturing Score were measured. The score of the unloaded group was significantly higher (better healing) than that of the treadmill group at 4 weeks. Ihh and the glioma-associated oncogene homolog 1 (Gli1) positive area in the unloaded group were significantly higher than those of the control group at 1 week. The peak time-points of the Ihh and Gli1 positive area was 1 week for the unloaded group and 2 weeks for the treadmill group. The Wnt3a positive area in the unloaded group was significantly higher than that of the control group at 2 weeks. The β-catenin positive area in the unloaded group was significantly higher than that of the treadmill group and the control group at 1 week. Our data indicated that the unloaded group has superior tendon maturation compared to the treadmill running group. Excessive tendon loading may delay the tendon healing process by affecting the activity of Ihh and Wnt/β-Catenin pathways. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 37:1628-1637, 2019.

The Investigation of LRP5-Loaded Composite with Sustained Release Behavior and Its Application in Bone Repair

Low-density lipoprotein receptor-related protein 5 (LRP5) plays a vital role in bone formation and regeneration. In this study, we developed an injectable and sustained-release composite loading LRP5 which could gelatinize in situ. The sustained release of the composite and its efficacy in bone regeneration were evaluated. Sodium alginate, collagen, hydroxyapatite, and LRP5 formed the composite LRP5-Alg/Col/HA. It was found that the initial setting time and final setting time of LRP5-Alg/Col/HA containing 4% alginate were suitable for surgical operation. When the composite was loaded with 40 μg/mL LRP5, LRP5-Alg/Col/HA did not exhibit a burst-release behavior and could sustainably release LRP5 up to 21 days. Up to 18 days, LRP5 released from LRP5-Alg/Col/HA still present the binding activity with DKK1 (Wnt signaling pathway antagonist) and could increase the downstream β-catenin mRNA in bone marrow mesenchymal stem cells. Moreover, LRP5-Alg/Col/HA was found to significantly increase bone mineral density in the defect area after 6 weeks’ implantation of LRP5-Alg/Col/HA into the rats’ calvarial defect area. H&E staining detection demonstrated that LRP5-Alg/Col/HA could mediate the formation of a new bone tissue. Therefore, we concluded that Alg/Col/HA was a suitable sustained-release carrier for LRP5 and LRP5-Alg/Col/HA had a significant effect on repairing bone defects and could be a good bone regeneration material.

Mechanisms Underlying Normal Fracture Healing and Risk Factors for Delayed Healing

PURPOSE OF REVIEW

Substantial advances have been made in understanding the biological basis of fracture healing. Yet, it is unclear whether the presence of osteoporosis or prior or current osteoporosis therapy influences the healing process or is associated with impaired healing. This review discusses the normal process of fracture healing and the role of osteoporosis and patient-specific factors in relation to fracture repair.

RECENT FINDINGS

The definitive association of osteoporosis to impaired fracture healing remains inconclusive because of limited evidence addressing this point. eStudies testing anabolic agents in preclinical models of ovariectomized animals with induced fractures have produced mostly positive findings showing enhanced fracture repair. Prospective human clinical trials, although few in number and limited in design and to testing only one anabolic agent, have similarly yielded modestly favorable results. Interest is high for exploring currently available osteoporosis therapies for efficacy in fracture repair. Definitive data supporting their efficacy are essential in achieving approval for this indication.

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 promote bone healing after trauma. This review summarizes our current understanding of the essential role of the Wnt pathway in bone regeneration and repair.

Cellular biology of fracture healing

The biology of bone healing is a rapidly developing science. Advances in transgenic and gene-targeted mice have enabled tissue and cell-specific investigations of skeletal regeneration. As an example, only recently has it been recognized that chondrocytes convert to osteoblasts during healing bone, and only several years prior, seminal publications reported definitively that the primary tissues contributing bone forming cells during regeneration were the periosteum and endosteum. While genetically modified animals offer incredible insights into the temporal and spatial importance of various gene products, the complexity and rapidity of healing-coupled with the heterogeneity of animal models-renders studies of regenerative biology challenging. Herein, cells that play a key role in bone healing will be reviewed and extracellular mediators regulating their behavior discussed. We will focus on recent studies that explore novel roles of inflammation in bone healing, and the origins and fates of various cells in the fracture environment. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res.

Sclerostin neutralization unleashes the osteoanabolic effects of Dkk1 inhibition

The WNT pathway has become an attractive target for skeletal therapies. High-bone-mass phenotypes in patients with loss-of-function mutations in the LRP5/6 inhibitor Sost (sclerosteosis), or in its downstream enhancer region (van Buchem disease), highlight the utility of targeting Sost/sclerostin to improve bone properties. Sclerostin-neutralizing antibody is highly osteoanabolic in animal models and in human clinical trials, but antibody-based inhibition of another potent LRP5/6 antagonist, Dkk1, is largely inefficacious for building bone in the unperturbed adult skeleton. Here, we show that conditional deletion of Dkk1 from bone also has negligible effects on bone mass. Dkk1 inhibition increases Sost expression, suggesting a potential compensatory mechanism that might explain why Dkk1 suppression lacks anabolic action. To test this concept, we deleted Sost from osteocytes in, or administered sclerostin neutralizing antibody to, mice with a Dkk1-deficient skeleton. A robust anabolic response to Dkk1 deletion was manifest only when Sost/sclerostin was impaired. Whole-body DXA scans, μCT measurements of the femur and spine, histomorphometric measures of femoral bone formation rates, and biomechanical properties of whole bones confirmed the anabolic potential of Dkk1 inhibition in the absence of sclerostin. Further, combined administration of sclerostin and Dkk1 antibody in WT mice produced a synergistic effect on bone gain that greatly exceeded individual or additive effects of the therapies, confirming the therapeutic potential of inhibiting multiple WNT antagonists for skeletal health. In conclusion, the osteoanabolic effects of Dkk1 inhibition can be realized if sclerostin upregulation is prevented. Anabolic therapies for patients with low bone mass might benefit from a strategy that accounts for the compensatory milieu of WNT inhibitors in bone tissue.

Forces associated with launch into space do not impact bone fracture healing

Segmental bone defects (SBDs) secondary to trauma invariably result in a prolonged recovery with an extended period of limited weight bearing on the affected limb. Soldiers sustaining blast injuries and civilians sustaining high energy trauma typify such a clinical scenario. These patients frequently sustain composite injuries with SBDs in concert with extensive soft tissue damage. For soft tissue injury resolution and skeletal reconstruction a patient may experience limited weight bearing for upwards of 6 months. Many small animal investigations have evaluated interventions for SBDs. While providing foundational information regarding the treatment of bone defects, these models do not simulate limited weight bearing conditions after injury. For example, mice ambulate immediately following anesthetic recovery, and in most cases are normally ambulating within 1-3 days post-surgery. Thus, investigations that combine disuse with bone healing may better test novel bone healing strategies. To remove weight bearing, we have designed a SBD rodent healing study in microgravity (µG) on the International Space Station (ISS) for the Rodent Research-4 (RR-4) Mission, which launched February 19, 2017 on SpaceX CRS-10 (Commercial Resupply Services). In preparation for this mission, we conducted an end-to-end mission simulation consisting of surgical infliction of SBD followed by launch simulation and hindlimb unloading (HLU) studies. In brief, a 2 mm defect was created in the femur of 10 week-old C57BL6/J male mice (n = 9-10/group). Three days after surgery, 6 groups of mice were treated as follows: 1) Vivarium Control (maintained continuously in standard cages); 2) Launch Negative Control (placed in the same spaceflight-like hardware as the Launch Positive Control group but were not subjected to launch simulation conditions); 3) Launch Positive Control (placed in spaceflight-like hardware and also subjected to vibration followed by centrifugation); 4) Launch Positive Experimental (identical to Launch Positive Control group, but placed in qualified spaceflight hardware); 5) Hindlimb Unloaded (HLU, were subjected to HLU immediately after launch simulation tests to simulate unloading in spaceflight); and 6) HLU Control (single housed in identical HLU cages but not suspended). Mice were euthanized 28 days after launch simulation and bone healing was examined via micro-Computed Tomography (µCT). These studies demonstrated that the mice post-surgery can tolerate launch conditions. Additionally, forces and vibrations associated with launch did not impact bone healing (p = .3). However, HLU resulted in a 52.5% reduction in total callus volume compared to HLU Controls (p = .0003). Taken together, these findings suggest that mice having a femoral SBD surgery tolerated the vibration and hypergravity associated with launch, and that launch simulation itself did not impact bone healing, but that the prolonged lack of weight bearing associated with HLU did impair bone healing. Based on these findings, we proceeded with testing the efficacy of FDA approved and novel SBD therapies using the unique spaceflight environment as a novel unloading model on SpaceX CRS-10.

Wnt/β-catenin signaling is required for distraction osteogenesis in rats

OVERVIEW

The Wnt signaling pathway plays crucial roles in embryonic skeletal development and postnatal bone regeneration. However, mechanisms of Wnt signaling functioning in distraction osteogenesis (DO) haven't been well characterized.

MATERIALS AND METHODS

We established a DO model using Sprague-Dawley rat tibia. And a Wnt signaling blocking agent, recombinant rat Dickkopf-related protein 1 (rrDkk1), was locally applied in the distracted gap to study the role of Wnt signaling during DO process. Animals in the experimental group received rrDkk1 injections (dose = 25 μg/kg) once daily during distraction period and every third day during consolidation stage (n = 48). Animals in the control group received saline under the same injection strategy (n = 48). Animals at different time points during DO process (1, 3, 6, 12 days after distraction, 10 days and 6 weeks after consolidation) were killed and tissues in the distraction region were harvested for radiography, dual energy X-ray absorptiometry, micro-computed tomography (micro-CT), and histological analyses.

RESULTS

Most Wnt ligands, cofactors, receptors, and antagonists were widely expressed in the distraction callus and were significantly upregulated during DO process. After rrDkk1 administration, the majority of these factors were downregulated at the mRNA level, except sFRP and GSK-3β. At the protein level, both β-catenin and Lef-1 were also suppressed by rrDkk1. In the long term, restricted bone healing was observed in the distracted callus in the rrDkk1 injection group. These findings were confirmed by histological and micro-CT analyses.

CONCLUSIONS

Our findings suggest that Wnt signaling participates in the process of DO, and clinical therapeutic approaches of DO may do well to avoid Wnt pathway suppression.

Endothelin signaling regulates mineralization and posttranscriptionally regulates SOST in TMOb cells via miR 126-3p

Previously, our laboratory identified ECE‐1, encoding endothelin‐converting enzyme‐1 (ECE‐1), as a positional candidate for a pleiotropic quantitative trait locus affecting femoral size, shape, and biomechanical performance. We hypothesized that endothelin‐1 (ET‐1) signaling promotes osteogenesis. Exposure of immortalized mouse osteoblast (TMOb) cells to big ET‐1 increased mineralization. Following big ET‐1 treatment, we measured the secretion of insulin‐like‐growth factor‐1 (IGF1), dickkopf‐homolog‐1 protein 1 (DKK1), and sclerostin (SOST). In each case, big ET‐1 signaling changed secretion in a manner that favored increased osteogenic activity. Treatment with ECE‐1, endothelin receptor A (EDNRA), or WNT receptor antagonists inhibited the big ET‐1‐mediated increase in mineralization. In the presence of big ET‐1, message levels of Runx2, Igf1, Dkk1, and Sost are uncoupled from protein production, suggesting posttranscriptional regulation. To evaluate the role of big ET‐1 in normal bone physiology, we inhibited EDNRA signaling during mineralization in the absence of exogenous ET‐1. EDNRA blockade reduced mineralization, decreased IGF1, and increased DKK1 and SOST secretion, responses opposite to those induced by exogenous big ET‐1. Pharmacological and siRNA knockdown to inhibit ECE‐1 reduced mineralization and IGF1 secretion with decreasing DKK1 and decreasing or stable SOST secretion, suggesting a further, unknown role of ECE‐1 in osteoblast maturation. Previously we identified miR 126‐3p as a potential ET‐1‐responsive regulator of SOST in murine cells. Overexpression of miR126‐3p increased mineralization in TMOb cells and decreased SOST secretion. Osteoblasts express the ET‐1 signaling pathway and ET‐1 signaling is necessary for normal osteoblast differentiation and mineralization, acting through regulation of miRs that target osteogenic molecules.

Low-Density Lipoprotein Receptor-Related Proteins in Skeletal Development and Disease

The identification of the low-density lipoprotein receptor (LDLR) provided a foundation for subsequent studies in lipoprotein metabolism, receptor-mediated endocytosis, and many other fundamental biological functions. The importance of the LDLR led to numerous studies that identified homologous molecules and ultimately resulted in the description of the LDL-receptor superfamily, a group of proteins that contain domains also found in the LDLR. Subsequent studies have revealed that members of the LDLR-related protein family play roles in regulating many aspects of signal transduction. This review is focused on the roles of selected members of this protein family in skeletal development and disease. We present background on the identification of this subgroup of receptors, discuss the phenotypes associated with alterations in their function in human patients and mouse models, and describe the current efforts to therapeutically target these proteins to treat human skeletal disease.

Exploiting the WNT Signaling Pathway for Clinical Purposes

PURPOSE OF REVIEW

The goal of this paper is to evaluate critically the literature published over the past 3 years regarding the Wnt signaling pathway. The Wnt pathway was found to be involved in bone biology in 2001-2002 with the discovery of a (G171V) mutation in the lipoprotein receptor-related protein 5 (LRP5) that resulted in high bone mass and another mutation that completely inactivated Lrp5 function and resulted in osteoporosis pseudoglioma syndrome (OPPG). The molecular biology has been complex, and very interesting. It has provided many opportunities for exploitation to develop new clinical treatments, particularly for osteoporosis. More clinical possibilities include: treatments for fracture healing, corticosteroid osteoporosis, osteogenesis imperfecta, and others. In addition, we wish to provide historical information coming from distant publications (~350 years ago) regarding bone biology that have been confirmed by study of Wnt signaling.

RECENT FINDINGS

A recent finding is the development of an antibody to sclerostin that is under study as a treatment for osteoporosis. Development of treatments for other forms of osteoporosis, such as corticosteroid osteoporosis, is also underway. The full range of the applications of the work is not yet been achieved.

Novel Spiked-Washer Repair Is Biomechanically Superior to Suture and Bone Tunnels for Arcuate Fracture Repair

OBJECTIVES

Injuries to the posterolateral corner of the knee can lead to chronic degenerative changes, external rotation instability, and varus instability if not repaired adequately. A proximal fibula avulsion fracture, referred to as an arcuate fracture, has been described in the literature, but a definitive repair technique has yet to be described. The objective of this study was to present a novel arcuate fracture repair technique, using a spiked-washer with an intramedullary screw, and to compare its biomechanical integrity to a previously described suture and bone tunnel method.

METHODS

Ten fresh-frozen cadaveric knees underwent a proximal fibula osteotomy to simulate a proximal fibula avulsion fracture. The lateral knee capsule and posterior cruciate ligament were also sectioned to create maximal varus instability. Five fibulas were repaired using a novel spiked-washer technique and the other 5 were repaired using the suture and bone tunnel method. The repaired knees were subjected to a monotonic varus load using a mechanical testing system instrument until failure of the repair or associated posterolateral corner structures.

RESULTS

Compared with the suture repair group, the spiked-washer repair group demonstrated a 100% increase in stiffness, 100% increase in yield, 110% increase in failure force, and 108% increase in energy to failure.

CONCLUSIONS

The spiked-washer technique offers superior quasi-static biomechanical performance compared with suture repair with bone tunnels for arcuate fractures of the proximal fibula. Further clinical investigation of this technique is warranted and the results of this testing may lead to improved outcomes and patient satisfaction for proximal fibula avulsion fractures.

Mice with a heterozygous Lrp6 deletion have impaired fracture healing

Bone fracture non-unions, the failure of a fracture to heal, occur in 10%–20% of fractures and are a costly and debilitating clinical problem. The Wnt/β-catenin pathway is critical in bone development and fracture healing. Polymorphisms of linking low-density lipoprotein receptor-related protein 6 (LRP6), a Wnt-binding receptor, have been associated with decreased bone mineral density and fragility fractures, although this remains controversial. Mice with a homozygous deletion of Lrp6 have severe skeletal abnormalities and are not viable, whereas mice with a heterozygous deletion have a combinatory effect with Lrp5 to decrease bone mineral density. As fracture healing closely models embryonic skeletal development, we investigated the process of fracture healing in mice heterozygous for Lrp6 (Lrp6^+/−) and hypothesized that the heterozygous deletion of Lrp6 would impair fracture healing. Mid-diaphyseal femur fractures were induced in Lrp6^+/− mice and wild-type controls (Lrp6^+/+). Fractures were analyzed using micro-computed tomography (μCT) scans, biomechanical testing, and histological analysis. Lrp6^+/− mice had significantly decreased stiffness and strength at 28 days post fracture (PF) and significantly decreased BV/TV, total density, immature bone density, and mature area within the callus on day-14 and -21 PF; they had significantly increased empty callus area at days 14 and 21 PF. Our results demonstrate that the heterozygous deletion of Lrp6 impairs fracture healing, which suggests that Lrp6 has a role in fracture healing.

Effects of Sclerostin Antibody on the Healing of Femoral Fractures in Ovariectomised Rats

The inhibition of sclerostin by the systemic administration of a monoclonal antibody (Scl-Ab) significantly increased bone mass and strength in fractured bones in animal models and non-fractured bones in ovariectomised (OVX) rats. In this study, the effects of Scl-Ab on healing were examined in a closed fracture model in OVX rats. Sixty Sprague-Dawley rats underwent an ovariectomy or a sham operation at 4 months of age, and a closed fracture of the right femur was performed 3 months later. Subcutaneous injections with Scl-Ab (25 mg/kg) or saline were then administered on day 1 after the fracture and twice a week for 8 weeks (n = 20 per group), at which time the fractured femurs were harvested for micro-computed tomography analysis, four-point bending mechanical testing and histomorphometric analysis to examine bone mass, bone strength and dynamic bone formation at the fracture site. The angiogenesis at the fracture site was also examined. Bone marrow stem cells were also isolated from the fractured bone to perform a colony-forming unit (CFU) assay and an alkaline phosphatase-positive (ALP(+)) CFU assay. OVX rats treated with Scl-Ab for 8 weeks had significantly increased bone mineral density and relative bone volume compared with OVX rats treated with saline. Similarly, maximum loading, energy to maximum load and stiffness in Scl-Ab-treated OVX rats were significantly higher than those in saline controls. The mineral apposition rate (MAR), mineralising surface (MS/BS) and bone formation rate (BFR/BS) were also significantly increased in Scl-Ab-treated group compared with the saline-treated group in OVX rats. Furthermore, the Scl-Ab-treated group had more CFUs and ALP(+) CFUs than the saline-treated group in OVX rats. No significant difference in angiogenesis at the fracture site was found between the groups. Our study demonstrated that Scl-Ab helped to increase bone mass, bone strength and bone formation at the fracture site in a closed femoral fracture model in OVX rats. Bone marrow stem cells in OVX rats injected with Scl-Ab also had increased CFUs and ALP(+) CFUs.

Extracellular signaling molecules to promote fracture healing and bone regeneration

To date, the delivery of signaling molecules for bone regeneration has focused primarily on factors that directly affect the bone formation pathways (osteoinduction) or that serve to increase the number of bone forming progenitor cells. The first commercialized growth factors approved for bone regeneration, Bone Morphogenetic Protein 2 and 7 (BMP2 and BMP7), are direct inducers of osteoblast differentiation. As well, newer generations of potential therapeutics that target the Wnt signaling pathway are also direct osteoinducers. On the other hand, some signaling molecules may play a role as mitogens and serve to increase the number of bone producing cells or may increase vascularization. This is true for factors such as Platelet Derived Growth Factor (PDGF) or Fibroblast Growth Factor (FGF). Vascular Endothelial Growth Factor (VEGF) likely has a special role. Not only does it induce new blood vessel formation, it also has direct effects on osteoblasts through endothelial cell-based BMP production. In addition to these pathways that classically have targeted bone production, there are also opportunities to target other aspects of the bone healing process such as inflammation, vascularization, and cell ingress to the fracture site. Bone regeneration is highly complex with defined, yet overlapping stages of healing. We will review established and novel extracellular signaling factors associated with various stages of fracture healing that could be targeted to promote enhanced bone regeneration. Importantly, multiple potential cell and tissues could be targeted to enhance healing in addition to focusing solely on osteoinductive therapeutics.

Role of Wnt signaling in fracture healing

The Wnt signaling pathway is well known to play major roles in skeletal development and homeostasis. In certain aspects, fracture repair mimics the process of bone embryonic development. Thus, the importance of Wnt signaling in fracture healing has become more apparent in recent years. Here, we summarize recent research progress in the area, which may be conducive to the development of Wnt-based therapeutic strategies for bone repair. [BMB Reports 2014; 47(12): 666-672]

Role of Wnt/β-catenin and RANKL/OPG in bone healing of diabetic Charcot arthropathy patients

BACKGROUND AND PURPOSE

Charcot neuropathy is characterized by bone destruction in a foot leading to deformity, instability, and risk of amputation. Little is known about the pathogenic mechanisms. We hypothesized that the bone-regulating Wnt/β-catenin and RANKL/OPG pathways have a role in Charcot arthropathy.

PATIENTS AND METHODS

24 consecutive Charcot patients were treated by off-loading, and monitored for 2 years by repeated foot radiography, MRI, and circulating levels of sclerostin, dickkopf-1, Wnt inhibitory factor-1, Wnt ligand-1, OPG, and RANKL. 20 neuropathic diabetic controls and 20 healthy controls served as the reference.

RESULTS

Levels of sclerostin, Dkk-1 and Wnt-1, but not of Wif-1, were significantly lower in Charcot patients than in the diabetic controls at inclusion. Dkk-1 and Wnt-1 levels responded to off-loading by increasing. Sclerostin levels were significantly higher in the diabetic controls than in the other groups whereas Wif-1 levels were significantly higher in the healthy controls than in the other groups. OPG and RANKL levels were significantly higher in the Charcot patients than in the other groups at inclusion, but decreased to the levels in healthy controls at 2 years. OPG/RANKL ratio was balanced in all groups at inclusion, and it remained balanced in Charcot patients on repeated measurement throughout the study.

INTERPRETATION

High plasma RANKL and OPG levels at diagnosis of Charcot suggest that there is high bone remodeling activity before gradually normalizing after off-loading treatment. The consistently balanced OPG/RANKL ratio in Charcot patients suggests that there is low-key net bone building activity by this pathway following diagnosis and treatment. Inter-group differences at diagnosis and changes in Wnt signaling following off-loading treatment were sufficiently large to be reflected by systemic levels, indicating that this pathway has a role in bone remodeling and bone repair activity in Charcot patients. This is of particular clinical relevance considering the recent emergence of promising drugs that target this system.

Dkk1 haploinsufficiency requires expression of Bmp2 for bone anabolic activity

Bone fractures remain a serious health burden and prevention and enhanced healing of fractures have been obtained by augmenting either BMP or Wnt signaling. However, whether BMP and Wnt signaling are both required or are self-sufficient for anabolic and fracture healing activities has never been fully elucidated. Mice haploinsufficient for Dkk1 (Dkk1(+/-)) exhibit a high bone mass phenotype due to an up-regulation of canonical Wnt signaling while mice lacking Bmp2 expression in the limbs (Bmp2(c/c);Prx1::cre) succumb to spontaneous fracture and are unable to initiate fracture healing; combined, these mice offer an opportunity to examine the requirement for activated BMP signaling on the anabolic and fracture healing activity of Wnts. When Dkk1(+/-) mice were crossed with Bmp2(c/c);Prx1::cre mice, the offspring bearing both genetic alterations were unable to increase bone mass and heal fractures, indicating that increased canonical Wnt signaling is unable to exploit its activity in absence of Bmp2. Thus, our data suggest that BMP signaling is required for Wnt-mediated anabolic activity and that therapies aimed at preventing fractures and fostering fracture repair may need to target both pathways for maximal efficacy.