No pain, no gain? The effects of pain-promoting neuropeptides and neurotrophins on fracture healing

The Role of Neuromodulation and Potential Mechanism in Regulating Heterotopic Ossification

Heterotopic ossification (HO) is a pathological process characterized by the aberrant formation of bone in muscles and soft tissues. It is commonly triggered by traumatic brain injury, spinal cord injury, and burns. Despite a wide range of evidence underscoring the significance of neurogenic signals in proper bone remodeling, a clear understanding of HO induced by nerve injury remains rudimentary. Recent studies suggest that injury to the nervous system can activate various signaling pathways, such as TGF-β, leading to neurogenic HO through the release of neurotrophins. These pathophysiological changes lay a robust groundwork for the prevention and treatment of HO. In this review, we collected evidence to elucidate the mechanisms underlying the pathogenesis of HO related to nerve injury, aiming to enhance our understanding of how neurological repair processes can culminate in HO.

Nerve Growth Factor-Preconditioned Mesenchymal Stem Cell-Derived Exosome-Functionalized 3D-Printed Hierarchical Porous Scaffolds with Neuro-Promotive Properties for Enhancing Innervated Bone Regeneration

The essential role of the neural network in enhancing bone regeneration has often been overlooked in biomaterial design, leading to delayed or compromised bone healing. Engineered mesenchymal stem cells (MSCs)-derived exosomes are becoming increasingly recognized as potent cell-free agents for manipulating cellular behavior and improving therapeutic effectiveness. Herein, MSCs are stimulated with nerve growth factor (NGF) to regulate exosomal cargoes to improve neuro-promotive potential and facilitate innervated bone regeneration. In vitro cell experiments showed that the NGF-stimulated MSCs-derived exosomes (N-Exos) obviously improved the cellular function and neurotrophic effects of the neural cells, and consequently, the osteogenic potential of the osteo-reparative cells. Bioinformatic analysis by miRNA sequencing and pathway enrichment revealed that the beneficial effects of N-Exos may partly be ascribed to the NGF-elicited multicomponent exosomal miRNAs and the subsequent regulation and activation of the MAPK and PI3K-Akt signaling pathways. On this basis, N-Exos were delivered on the micropores of the 3D-printed hierarchical porous scaffold to accomplish the sustained release profile and extended bioavailability. In a rat model with a distal femoral defect, the N-Exos-functionalized hierarchical porous scaffold significantly induced neurovascular structure formation and innervated bone regeneration. This study provided a feasible strategy to modulate the functional cargoes of MSCs-derived exosomes to acquire desirable neuro-promotive and osteogenic potential. Furthermore, the developed N-Exos-functionalized hierarchical porous scaffold may represent a promising neurovascular-promotive bone reparative scaffold for clinical translation.

Metformin-grafted polycaprolactone nanoscaffold targeting sensory nerve controlled fibroblasts reprograming to alleviate epidural fibrosis

Epidural fibrosis (EF), associated with various biological factors, is still a major troublesome clinical problem after laminectomy. In the present study, we initially demonstrate that sensory nerves can attenuate fibrogenic progression in EF animal models via the secretion of calcitonin gene-related peptide (CGRP), suggesting a new potential therapeutic target. Further studies showed that CGRP could inhibit the reprograming activation of fibroblasts through PI3K/AKT signal pathway. We subsequently identified metformin (MET), the most widely prescribed medication for obesity-associated type 2 diabetes, as a potent stimulator of sensory neurons to release more CGRP via activating CREB signal way. We copolymerized MET with innovative polycaprolactone (PCL) nanofibers to develop a metformin-grafted PCL nanoscaffold (METG-PCLN), which could ensure stable long-term drug release and serve as favorable physical barriers. In vivo results demonstrated that local implantation of METG-PCLN could penetrate into dorsal root ganglion cells (DRGs) to promote the CGRP synthesis, thus continuously inhibit the fibroblast activation and EF progress for 8 weeks after laminectomy, significantly better than conventional drug loading method. In conclusion, this study reveals the unprecedented potential of sensory neurons to counteract EF through CGRP signaling and introduces a novel strategy employing METG-PCLN to obstruct EF by fine-tuning sensory nerve-regulated fibrogenesis.

Role of the Neurologic System in Fracture Healing: An Extensive Review

PURPOSE OF REVIEW

Despite advances in orthopedics, there remains a need for therapeutics to hasten fracture healing. However, little focus is given to the role the nervous system plays in regulating fracture healing. This paucity of information has led to an incomplete understanding of fracture healing and has limited the development of fracture therapies that integrate the importance of the nervous system. This review seeks to illuminate the integral roles that the nervous system plays in fracture healing.

RECENT FINDINGS

Preclinical studies explored several methodologies for ablating peripheral nerves to demonstrate ablation-induced deficits in fracture healing. Conversely, activation of peripheral nerves via the use of dorsal root ganglion electrical stimulation enhanced fracture healing via calcitonin gene related peptide (CGRP). Investigations into TLR-4, TrkB agonists, and nerve growth factor (NGF) expression provide valuable insights into molecular pathways influencing bone mesenchymal stem cells and fracture repair. Finally, there is continued research into the connections between pain and fracture healing with findings suggesting that anti-NGF may be able to block pain without affecting healing. This review underscores the critical roles of the central nervous system (CNS), peripheral nervous system (PNS), and autonomic nervous system (ANS) in fracture healing, emphasizing their influence on bone cells, neuropeptide release, and endochondral ossification. The use of TBI models contributes to understanding neural regulation, though the complex influence of TBI on fracture healing requires further exploration. The review concludes by addressing the neural connection to fracture pain. This review article is part of a series of multiple manuscripts designed to determine the utility of using artificial intelligence for writing scientific reviews.

Dual aldehyde cross-linked hyaluronic acid hydrogels loaded with PRP and NGF biofunctionalized PEEK interfaces to enhance osteogenesis and vascularization

Polyetheretherketone (PEEK) material has become a potential bone replacement material due to its elastic modulus, which is close to that of human bone, and stable chemical properties. However, its biological inertness has hindered its clinical application. To improve the biological inertia of PEEK material, a hyaluronic acid (HA) hydrogel coating loaded with platelet-rich plasma (PRP) and nerve growth factor (NGF) was constructed on the surface of PEEK material in this study. After the hybrid hydrogel coating was constructed, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), Fourier transform infrared spectroscopy (FT-IR), degradation tests, and enzyme-linked immunosorbent assays (ELISAs) were used to evaluate its characteristics and biological properties. The osteogenic and angiogenic potentials were also investigated in vitro and in vivo. Our results showed that the HA hydrogel loaded with RPP and NGF on the PEEK surface degraded slowly and could sustainably release various growth factors, including NGF. The results of in vitro tests showed that the hybrid hydrogel on the surface of PEEK effectively promoted osteogenesis and angiogenesis. The in vivo experiment also confirmed that the PEEK surface hydrogel could promote osseointegration of the implant and the integration of new bone and neovascularization. Our results suggest that the cross-linked hyaluronic acid hydrogel loaded with PRP and NGF can significantly improve the biological inertia of PEEK material, endowing PEEK material with good osteogenic and angiogenic ability.

Do Not Lose Your Nerve, Be Callus: Insights Into Neural Regulation of Fracture Healing

PURPOSE OF REVIEW

Fractures are a prominent form of traumatic injury and shall continue to be for the foreseeable future. While the inflammatory response and the cells of the bone marrow microenvironment play significant roles in fracture healing, the nervous system is also an important player in regulating bone healing.

RECENT FINDINGS

Considerable evidence demonstrates a role for nervous system regulation of fracture healing in a setting of traumatic injury to the brain. Although many of the impacts of the nervous system on fracture healing are positive, pain mediated by the nervous system can have detrimental effects on mobilization and quality of life. Understanding the role the nervous system plays in fracture healing is vital to understanding fracture healing as a whole and improving quality of life post-injury. This review article is part of a series of multiple manuscripts designed to determine the utility of using artificial intelligence for writing scientific reviews.

Cracking the Code: The Role of Peripheral Nervous System Signaling in Fracture Repair

PURPOSE OF REVIEW

The traditionally understated role of neural regulation in fracture healing is gaining prominence, as recent findings underscore the peripheral nervous system's critical contribution to bone repair. Indeed, it is becoming more evident that the nervous system modulates every stage of fracture healing, from the onset of inflammation to repair and eventual remodeling.

RECENT FINDINGS

Essential to this process are neurotrophins and neuropeptides, such as substance P, calcitonin gene-related peptide, and neuropeptide Y. These molecules fulfill key roles in promoting osteogenesis, influencing inflammation, and mediating pain. The sympathetic nervous system also plays an important role in the healing process: while local sympathectomies may improve fracture healing, systemic sympathetic denervation impairs fracture healing. Furthermore, chronic activation of the sympathetic nervous system, often triggered by stress, is a potential impediment to effective fracture healing, marking an important area for further investigation. The potential to manipulate aspects of the nervous system offers promising therapeutic possibilities for improving outcomes in fracture healing. This review article is part of a series of multiple manuscripts designed to determine the utility of using artificial intelligence for writing scientific reviews.

Crosstalk Between the Neuroendocrine System and Bone Homeostasis

The homeostasis of bone microenvironment is the foundation of bone health and comprises 2 concerted events: bone formation by osteoblasts and bone resorption by osteoclasts. In the early 21st century, leptin, an adipocytes-derived hormone, was found to affect bone homeostasis through hypothalamic relay and the sympathetic nervous system, involving neurotransmitters like serotonin and norepinephrine. This discovery has provided a new perspective regarding the synergistic effects of endocrine and nervous systems on skeletal homeostasis. Since then, more studies have been conducted, gradually uncovering the complex neuroendocrine regulation underlying bone homeostasis. Intriguingly, bone is also considered as an endocrine organ that can produce regulatory factors that in turn exert effects on neuroendocrine activities. After decades of exploration into bone regulation mechanisms, separate bioactive factors have been extensively investigated, whereas few studies have systematically shown a global view of bone homeostasis regulation. Therefore, we summarized the previously studied regulatory patterns from the nervous system and endocrine system to bone. This review will provide readers with a panoramic view of the intimate relationship between the neuroendocrine system and bone, compensating for the current understanding of the regulation patterns of bone homeostasis, and probably developing new therapeutic strategies for its related disorders.

Neuro-bone tissue engineering: emerging mechanisms, potential strategies, and current challenges

The skeleton is a highly innervated organ in which nerve fibers interact with various skeletal cells. Peripheral nerve endings release neurogenic factors and sense skeletal signals, which mediate bone metabolism and skeletal pain. In recent years, bone tissue engineering has increasingly focused on the effects of the nervous system on bone regeneration. Simultaneous regeneration of bone and nerves through the use of materials or by the enhancement of endogenous neurogenic repair signals has been proven to promote functional bone regeneration. Additionally, emerging information on the mechanisms of skeletal interoception and the central nervous system regulation of bone homeostasis provide an opportunity for advancing biomaterials. However, comprehensive reviews of this topic are lacking. Therefore, this review provides an overview of the relationship between nerves and bone regeneration, focusing on tissue engineering applications. We discuss novel regulatory mechanisms and explore innovative approaches based on nerve-bone interactions for bone regeneration. Finally, the challenges and future prospects of this field are briefly discussed.

The ratio of alpha-calcitonin gene-related peptide to substance P is associated with the transition of bone metabolic states during aging and healing

Alpha-calcitonin gene-related peptide (αCGRP) and substance P (SP) are functionally correlated sensory neuropeptides deeply involved in bone homeostasis. However, they are usually studied individually rather than as an organic whole. To figure out whether they are interdependent, we firstly recorded the real-time αCGRP and SP levels in aging bone and healing fracture, which revealed a moderate to high level of αCGRP coupled with a low αCGRP/SP ratio in an anabolic state, and a high level of αCGRP coupled with a high αCGRP/SP ratio in a catabolic state, suggesting the importance of αCGRP/SP ratio in driving aging and healing scenarios. During facture healing, increase in αCGRP/SP ratio by adding αCGRP led to better callus formation and faster callus remodeling, while simultaneous addition of αCGRP and SP resulted in hypertrophic callus and delayed remodeling. The characteristics in inflammation and osteoclast activation further confirmed the importance of high αCGRP/SP ratio during catabolic bone remodeling. In vitro assays using different mixtures of αCGRP-SP proved that the osteogenic potential of the mixtures depended mostly on αCGRP, while their effects on osteoclasts and neutrophils relied on both peptides. These results demonstrated that αCGRP and SP were spatiotemporally interdependent. The αCGRP/SP ratio may be more important than the dose of a single neuropeptide in managing age-related and trauma-related bone diseases.

Effects of NGF and Photobiomodulation Therapy on Crush Nerve Injury and Fracture Healing: A Stereological and Histopathological Study in an Animal Model

Study Design

A stereological and histopathological study in an animal model.

Objective

This study explores the effects of the nerve growth factor and photobiomodulation therapy on the damaged nerve tissue and fracture healing.

Methods

A total of 24 rabbits were divided into 4 groups: control group (n = 5), nerve growth factor (NGF) group (n = 7), photobiomodulation (PBMT) group (n = 6), and nerve growth factor and photobiomodulation therapy (NGF+PBMT) group (n = 6). The vertical fracture was performed between the mental foramen and the first premolar, and the mental nerve was crushed for 30 seconds with a standard serrated clamp with a force of approximately 50 N in all groups. The control group received an isotonic solution (.02 mL, .09% NaCl) to the operation site locally. The NGF group received 1 μg human NGF-β/.9% .2 mL NaCl solution for 7 days locally. The PBMT group received PBMT treatment (GaAlAs laser, 810 nm, .3 W, 18 J/cm2) every 48 hours for 14 sessions following the surgery. The NGF+PBMT group received both NGF and PBMT treatment as described above. After 28 days, the bone tissues and mental nerves from all groups were harvested and histologically and stereologically analyzed.

Results

According to the stereological results, the volume of the new vessel and the volume of the new bone were significantly higher in the PBMT group than in other groups (P < .001). According to the histopathological examinations, higher myelinated axons were observed in experimental groups than in the control group.

Conclusions

As a result, PBMT has beneficial effects on bone regeneration. Based on the light microscopic evaluation, more regenerated axon populations were observed in the NGF group than in the PBMT and PBMT + NGF groups in terms of myelinated axon content.

Multi-Functional Small Molecule Alleviates Fracture Pain and Promotes Bone Healing

Bone injuries such as fractures are one major cause of morbidities worldwide. A considerable number of fractures suffer from delayed healing, and the unresolved acute pain may transition to chronic and maladaptive pain. Current management of pain involves treatment with NSAIDs and opioids with substantial adverse effects. Herein, we tested the hypothesis that the purine molecule, adenosine, can simultaneously alleviate pain and promote healing in a mouse model of tibial fracture by targeting distinctive adenosine receptor subtypes in different cell populations. To achieve this, a biomaterial-assisted delivery of adenosine is utilized to localize and prolong its therapeutic effect at the injury site. The results demonstrate that local delivery of adenosine inhibited the nociceptive activity of peripheral neurons through activation of adenosine A1 receptor (ADORA1) and mitigated pain as demonstrated by weight bearing and open field movement tests. Concurrently, local delivery of adenosine at the fracture site promoted osteogenic differentiation of mesenchymal stromal cells through adenosine A2B receptor (ADORA2B) resulting in improved bone healing as shown by histological analyses and microCT imaging. This study demonstrates the dual role of adenosine and its material-assisted local delivery as a feasible therapeutic approach to treat bone trauma and associated pain.

Musculoskeletal Biomaterials: Stimulated and Synergized with Low Intensity Pulsed Ultrasound

Clinical biophysical stimulating strategies, which have significant effects on improving the function of organs or treating diseases by causing the salutary response of body, have shown many advantages, such as non-invasiveness, few side effects, and controllable treatment process. As a critical technique for stimulation, the low intensity pulsed ultrasound (LIPUS) has been explored in regulating osteogenesis, which has presented great promise in bone repair by delivering a combined effect with biomaterials. This review summarizes the musculoskeletal biomaterials that can be synergized with LIPUS for enhanced biomedical application, including bone regeneration, spinal fusion, osteonecrosis/osteolysis, cartilage repair, and nerve regeneration. Different types of biomaterials are categorized for summary and evaluation. In each subtype, the verified biological mechanisms are listed in a table or graphs to prove how LIPUS was effective in improving musculoskeletal tissue regeneration. Meanwhile, the acoustic excitation parameters of LIPUS that were promising to be effective for further musculoskeletal tissue engineering are discussed, as well as their limitations and some perspectives for future research. Overall, coupled with biomimetic scaffolds and platforms, LIPUS may be a powerful therapeutic approach to accelerate musculoskeletal tissue repair and even in other regenerative medicine applications.

Polyhedron-Like Biomaterials for Innervated and Vascularized Bone Regeneration

Neural-vascular networks are densely distributed through periosteum, cortical bone, and cancellous bone, which is of great significance for bone regeneration and remodeling. Although significant progress has been made in bone tissue engineering, ineffective bone regeneration, and delayed osteointegration still remains an issue due to the ignorance of intrabony nerves and blood vessels. Herein, inspired by space-filling polyhedra with open architectures, polyhedron-like scaffolds with spatial topologies are prepared via 3D-printing technology to mimic the meshwork structure of cancellous bone. Benefiting from its spatial topologies, polyhedron-like scaffolds greatly promoted the osteogenic differentiation of bone mesenchymal stem cells (BMSCs) via activating PI3K-Akt signals, and exhibiting satisfactory performance on angiogenesis and neurogenesis. Computational fluid dynamic (CFD) simulation elucidates that polyhedron-like scaffolds have a relatively lower area-weighted average static pressure, which is beneficial to osteogenesis. Furthermore, in vivo experiments further demonstrate that polyhedron-like scaffolds obviously promote bone formation and osteointegration, as well as inducing vascularization and ingrowth of nerves, leading to innervated and vascularized bone regeneration. Taken together, this work offers a promising approach for fabricating multifunctional scaffolds without additional exogenous seeding cells and growth factors, which holds great potential for functional tissue regeneration and further clinical translation.

Persistent Changes in Mechanical Nociception in Rats With Traumatic Brain Injury Involving Polytrauma

Traumatic brain injury (TBI) survivors often experience debilitating consequences. Due to the high impact nature of TBI, patients often experience concomitant peripheral injuries (ie, polytrauma). A common, yet often overlooked, comorbidity of TBI is chronic pain. Therefore, this study investigated how common concomitant peripheral injuries (ie, femoral fracture and muscle crush) can affect long-term behavioral and structural TBI outcomes with a particular focus on nociception. Rats were randomly assigned to 1 of 4 groups: polytrauma (POLY; ie, fracture + muscle crush + TBI), peripheral injury (PERI; ie, fracture + muscle crush + sham TBI), TBI (ie, sham fracture + sham muscle crush + TBI), and sham-injured (SHAM; ie, sham fracture + sham muscle crush + sham TBI). Rats underwent behavioral testing at 3-, 6-, and 11-weeks postinjury, and were then euthanized for postmortem magnetic resonance imaging (MRI). POLY rats had a persisting increase in pain sensitivity compared to all groups on the von Frey test. MRI revealed that POLY rats also had abnormalities in the cortical and subcortical brain structures involved in nociceptive processing. These findings have important implications and provide a foundation for future studies to determine the underlying mechanisms and potential treatment strategies for chronic pain in TBI survivors. PERSPECTIVE: Rats with TBI and concomitant peripheral trauma displayed chronic nociceptive pain and MRI images also revealed damaged brain structures/pathways that are involved in chronic pain development. This study highlights the importance of polytrauma and the affected brain regions for developing chronic pain.

CC chemokines and receptors in osteoarthritis: new insights and potential targets

Osteoarthritis (OA) is a prevalent degenerative disease accompanied by the activation of innate and adaptive immune systems-associated inflammatory responses. Due to the local inflammation, the expression of various cytokines was altered in affected joints, including CC motif chemokine ligands (CCLs) and their receptors (CCRs). As essential members of chemokines, CCLs and CCRs played an important role in the pathogenesis and treatment of OA. The bindings between CCLs and CCRs on the chondrocyte membrane promoted chondrocyte apoptosis and the release of multiple matrix-degrading enzymes, which resulted in cartilage degradation. In addition, CCLs and CCRs had chemoattractant functions to attract various immune cells to osteoarthritic joints, further leading to the aggravation of local inflammation. Furthermore, in the nerve endings of joints, CCLs and CCRs, along with several cellular factors, contributed to pain hypersensitivity by releasing neurotransmitters in the spinal cord. Given this family's diverse and complex functions, targeting the functional network of CCLs and CCRs is a promising strategy for the prognosis and treatment of OA in the future.

Safety of current treatment options for NTRK fusion-positive cancers

INTRODUCTION

Oncogenic NTRK fusions have been found in multiple cancer types affecting adults and/or children, including rare tumors with pathognomonic fusions and common cancers in which fusions are rare. The tropomyosin receptor kinase inhibitors (TRKi) larotrectinib and entrectinib are among the first agents with tissue agnostic FDA approvals for cancer treatment, and additional TRKi are undergoing development. As experience with TRKi grow, novel mechanisms of resistance and on/off target side effects have become increasingly important considerations.

AREAS COVERED

Authors reviewed literature published through July 2023 on platforms such as PubMed, clinicaltrials.gov, and manufacturer/FDA drug labels, focusing on the development of TRKi, native functions of TRK, phenotype of congenital TRK aberrancies, efficacy, and safety profile of TRKi in clinical trials and investigator reports, and on/off target adverse effects associated with TRKi (Appendix A).

EXPERT OPINION

TRKi have histology-agnostic activity against tumors with NTRK gene fusions. TRKi are generally well tolerated with a side effect profile that compares favorably to cytotoxic chemotherapy. There are numerous ongoing studies investigating TRKi as frontline, adjuvant, and salvage therapy. It will be critical to continue to gather long-term safety data on the use of these agents, particularly in children.

Inhibition of CGRP signaling impairs fracture healing in mice

Calcitonin gene-related peptide (CGRP) is a neuropeptide produced by sensory nerves and functions as a pain sensor. It acts by binding to the calcitonin-like receptor (CLR, protein; Calcrl, gene). CGRP inhibition has been recently introduced as therapeutic treatment of migraine-associated pain. Previous studies have shown that CGRP stimulates bone formation. The aim of our study is to determine whether the inhibition of CGRP signaling negatively impacted fracture healing. Using α-smooth muscle actin (αSMA) Cre animals crossed with Ai9 reporter mice, we showed that CGRP-expressing nerves are near αSMA + cells in the periosteum. In vitro experiments revealed that periosteal cells express Calcrl and receptor activity modifying protein 1; and CGRP stimulation increased periosteal cell proliferation. Using a tamoxifen-inducible model αSMACre/CLRfl/fl , we targeted the deletion of CLR to periosteal progenitor cells and examined fracture healing. Microcomputed tomography of fractured femurs showed a reduction in bone mass in αSMACre+/CLRfl/fl female mice relative to controls and callus volume in males. Pharmacological CGRP-CLR inhibition was achieved by subcutaneous delivery of customized pellets with small molecule inhibitor olcegepant (BIBN-4096) at a dose of 10 μg/day. BIBN-4096-treated C57BL/6J mice had a higher latency toward thermal nociception than placebo-treated mice, indicating impaired sensory function through CGRP inhibition. CGRP inhibition also resulted in reduced callus volume, bone mass, and bone strength compared to placebo controls. These results indicate that inhibiting CGRP by deleting CLR or by using BIBN-4096, contributes to delayed bone healing.

Substance P Exacerbates the Inflammatory and Pro-osteoclastogenic Responses of Murine Osteoclasts and Osteoblasts to Staphylococcus aureus

Staphylococcus aureus infections of bone tissue are associated with inflammatory bone loss. Resident bone cells, including osteoblasts and osteoclasts, can perceive S. aureus and produce an array of inflammatory and pro-osteoclastogenic mediators, thereby contributing to such damage. The neuropeptide substance P (SP) has been shown to exacerbate microbially induced inflammation at sites such as the gut and the brain and has previously been shown to affect bone cell differentiation and activity. Here we demonstrate that the interaction of SP with its high affinity receptor, neurokinin-1 receptor (NK-1R), expressed on murine osteoblasts and osteoclasts, augments the inflammatory responses of these cells to S. aureus challenge. Additionally, SP alters the production of pro- and anti-osteoclastogenic factors by bacterially challenged bone cells and their proteolytic functions in a manner that would be anticipated to exacerbate inflammatory bone loss at sites of infection. Furthermore, we have demonstrated that the clinically approved NK-1R antagonist, aprepitant, attenuates local inflammatory and pro-osteoclastogenic mediator expression in an in vivo mouse model of post-traumatic staphylococcal osteomyelitis. Taken together, these results indicate that SP/NK-1R interactions could play a significant role in the initiation and/or progression of damaging inflammation in S. aureus bone infections and suggest that the repurposing of currently approved NK-1R antagonists might represent a promising new adjunct therapy for such conditions.

Bioactive semaphorin 3A promotes sequential formation of sensory nerve and type H vessels during in situ osteogenesis

Introduction: Sensory nerves and vessels are critical for skeletal development and regeneration, but crosstalk between neurovascular network and mineralization are not clear. The aim of this study was to explore neurovascular changes and identify bioactive regulators during in situ osteogenesis. Method: In situ osteogenesis model was performed in male rats following Achilles tenotomy. At 3, 6 and 9 weeks after surgery, mineralization, blood vessels, sensory innervation, and bioactive regulators expression were evaluated via micro-computed tomography, immunofluorescent staining, histology and reverse transcriptase-polymerase chain reaction analyses. Result: In the process of in situ osteogenesis, the mineral density increased with time, and the locations of minerals, nerves and blood vessels were highly correlated at each time point. The highest density of sensory nerve was observed in the experimental group at the 3rd week, and then gradually decreased with time, but still higher than that in the sham control group. Among many regulatory factors, semaphorin 3A (Sema3A) was highly expressed in experimental model and its expression was temporally sequential and spatially correlated sensory nerve. Conclusion: The present study showes that during in situ osteogenesis, innervation and angiogenesis are highly correlated, and Sema3A is associated with the position and expression of the sensory nerve.

Neural Regeneration in Regenerative Endodontic Treatment: An Overview and Current Trends

Pulpal and periapical diseases are the most common dental diseases. The traditional treatment is root canal therapy, which achieves satisfactory therapeutic outcomes-especially for mature permanent teeth. Apexification, pulpotomy, and pulp revascularization are common techniques used for immature permanent teeth to accelerate the development of the root. However, there are obstacles to achieving functional pulp regeneration. Recently, two methods have been proposed based on tissue engineering: stem cell transplantation, and cell homing. One of the goals of functional pulp regeneration is to achieve innervation. Nerves play a vital role in dentin formation, nutrition, sensation, and defense in the pulp. Successful neural regeneration faces tough challenges in both animal studies and clinical trials. Investigation of the regeneration and repair of the nerves in the pulp has become a serious undertaking. In this review, we summarize the current understanding of the key stem cells, signaling molecules, and biomaterials that could promote neural regeneration as part of pulp regeneration. We also discuss the challenges in preclinical or clinical neural regeneration applications to guide deep research in the future.

Skeletal interoception in bone homeostasis and pain

Accumulating evidence indicates that interoception maintains proper physiological status and orchestrates metabolic homeostasis by regulating feeding behaviors, glucose balance, and lipid metabolism. Continuous skeletal remodeling consumes a tremendous amount of energy to provide skeletal scaffolding, support muscle movement, store vital minerals, and maintain a niche for hematopoiesis, which are processes that also contribute to overall metabolic balance. Although skeletal innervation has been described for centuries, recent work has shown that skeletal metabolism is tightly regulated by the nervous system and that skeletal interoception regulates bone homeostasis. Here, we provide a general discussion of interoception and its effects on the skeleton and whole-body metabolism. We also discuss skeletal interoception-mediated regulation in the context of pathological conditions and skeletal pain as well as future challenges to our understanding of these process and how they can be leveraged for more effective therapy.

Aptamer engineering exosomes loaded on biomimetic periosteum to promote angiogenesis and bone regeneration by targeting injured nerves via JNK3 MAPK pathway

Repairing critical bone defects is a complex problem in the clinic. The periosteum rich in nerve plays a vital role in initiating and regulating bone regeneration. However, current studies have paid little attention to repairing nerves in the periosteum to promote bone regeneration. Thus, it is essential to construct bionic periosteum with the targeted injured nerves in the periosteum. We coupled phosphatidylserine (PS) targeted aptamers with repair Schwann cell exosomes to construct exosome@aptamer (EA). Then through PEI, EA was successfully built on the surface of the electrospun fiber, which was PCL@PEI@exosome@aptamer (PPEA). Through SEM, TEM, and other technologies, PPEA was characterized. Experiments prove in vivo and in vitro that it has an excellent repair effect on damaged nerves and regeneration of vascular and bones. In vivo, we confirmed that biomimetic periosteum has an apparent ability to promote nerve and bone regeneration by using Microcomputer tomography, hematoxylin-eosin, Masson, and Immunofluorescence. In vitro, we used Immunofluorescence, Real-Time Quantitative PCR, Alkaline phosphatase staining, and other tests to confirm that it has central nerve, blood vessel, and bone regeneration ability. The PPEA biomimetic periosteum has apparent neurogenic, angiogenic, and osteogenic effects. The PPEA biomimetic periosteum will provide a promising method for treating bone defects.

•

To construct a biomimetic periosteum that can target injured axons and bone regeneration.

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PS targeted aptamer is coupled with repair Schwann cell exosomes.

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PEI self-assembly was used for the PCL electrospun biomimetic membrane loading.

•

It targeted and repaired the injured axons and promoted the secretion of CGRP and SP.

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Biomimetic periosteum promotes vascular regeneration and bone regeneration.

Nervous System-Driven Osseointegration

Implants are essential therapeutic tools for treating bone fractures and joint replacements. Despite the in-depth study of osseointegration for more than fifty years, poor osseointegration caused by aseptic loosening remains one of the leading causes of late implant failures. Osseointegration is a highly sophisticated and spatiotemporal process in vivo involving the immune response, angiogenesis, and osteogenesis. It has been unraveled that the nervous system plays a pivotal role in skeletal health via manipulating neurotrophins, neuropeptides, and nerve cells. Herein, the research related to nervous system-driven osseointegration was systematically analyzed and reviewed, aiming to demonstrate the prominent role of neuromodulation in osseointegration. Additionally, it is indicated that the implant design considering the role of neuromodulation might be a promising way to prevent aseptic loosening.

Contemporary perspectives on heterotopic ossification

Heterotopic ossification (HO) is the formation of ectopic bone that is primarily genetically driven (fibrodysplasia ossificans progressiva [FOP]) or acquired in the setting of trauma (tHO). HO has undergone intense investigation, especially over the last 50 years, as awareness has increased around improving clinical technologies and incidence, such as with ongoing wartime conflicts. Current treatments for tHO and FOP remain prophylactic and include NSAIDs and glucocorticoids, respectively, whereas other proposed therapeutic modalities exhibit prohibitive risk profiles. Contemporary studies have elucidated mechanisms behind tHO and FOP and have described new distinct niches independent of inflammation that regulate ectopic bone formation. These investigations have propagated a paradigm shift in the approach to treatment and management of a historically difficult surgical problem, with ongoing clinical trials and promising new targets.

Neural Peptide α-CGRP Coregulated Angiogenesis and Osteogenesis via Promoting the Cross-Talk between Mesenchymal Stem Cells and Endothelial Cells

Background

The coupled vascularization and bone remodeling are key steps during bone healing, during which the cross-talk between mesenchymal stem cells (MSCs) and endothelial cells plays vital roles. Evidence indicates the well-characterized neuropeptide Calcitonin Gene-Related Peptide-α (CGRP) is proven to play an important role during bone regeneration. However, the regulatory effects of αCGRP on angiogenesis and osteogenesis, as well as underlying cellular and molecular mechanisms, remain unclear.

Aim

The present study was performed to verify the availability of the CGRP for osteogenic capacity in MSCs and explore its potential underlying molecular mechanism. After that, the promoted angiogenic effect of CGRP as well as its underlying mechanisms was studied.

Methods and Results

The results showed that CGRP could significantly increase the cyclic adenosine monophosphate (cAMP) level and promote the osteogenesis ability of MSCs via cAMP/PKA signaling pathway. Direct exposure to CGRP increased nitric oxide synthase expression, the release of NO, tube formation, and wound healing of human umbilical vein endothelial cells (HUVEC). The CGRP-treated MSCs were observed with high expression levels of angiogenic factors, such as bFGF and VEGF-α; the conditioned medium derived from CGRP-treated MSCs was also able to promote tube formation and transmembrane migration of HUVECs.

Conclusion

These findings demonstrate the coregulated angiogenesis and osteogenesis effects of CGRP, especially for its regulation effects on the cross-talk between mesenchymal stem cells and endothelial cells.

Mechanisms of bone pain: Progress in research from bench to bedside

The field of research on pain originating from various bone diseases is expanding rapidly, with new mechanisms and targets asserting both peripheral and central sites of action. The scope of research is broadening from bone biology to neuroscience, neuroendocrinology, and immunology. In particular, the roles of primary sensory neurons and non-neuronal cells in the peripheral tissues as important targets for bone pain treatment are under extensive investigation in both pre-clinical and clinical settings. An understanding of the peripheral mechanisms underlying pain conditions associated with various bone diseases will aid in the appropriate application and development of optimal strategies for not only managing bone pain symptoms but also improving bone repairing and remodeling, which potentially cures the underlying etiology for long-term functional recovery. In this review, we focus on advances in important preclinical studies of significant bone pain conditions in the past 5 years that indicated new peripheral neuronal and non-neuronal mechanisms, novel targets for potential clinical interventions, and future directions of research.

The role of interleukin (IL)-23 in regulating pain in arthritis

Current understanding of IL-23 biology, with its link to other pro-inflammatory cytokines, for example, IL-17 and granulocyte macrophage-colony stimulating factor (GM-CSF), is primarily focused on T lymphocyte-mediated inflammation/autoimmunity. Pain is a significant symptom associated with many musculoskeletal conditions leading to functional impairment and poor quality of life. While the role of IL-23 in arthritis has been studied in mouse models of adaptive immune-mediated arthritis using targeted approaches (e.g., monoclonal antibody (mAb) neutralization), the literature on IL-23 and arthritis pain is limited. Encouragingly, the anti-IL-23p19 mAb, guselkumab, reduces pain in psoriatic arthritis patients. Recent evidence has suggested a new biology for IL-23, whereby IL-23 is required in models of innate immune-mediated arthritis and its associated pain with its action being linked to a GM-CSF-dependent pathway (the so-called GM-CSF➔CCL17 pathway). This Commentary discusses the current understanding of potential cytokine networks involving IL-23 in arthritis pain and provides a rationale for future clinical studies targeting IL-23p19 in arthritis pain.

Development of a rat model with lumbar vertebral endplate lesion

PURPOSE

Vertebral endplate lesion (EPL) caused by severe disc degeneration is associated with low back pain. However, there is no suitable animal model to elucidate the pathophysiology of EPL. This study aimed to develop a rat model of EPL and evaluate rat behavior and imaging and histological findings.

METHODS

The L4-5 intervertebral discs of Sprague-Dawley rats were transperitoneally removed, except for the outer annulus fibrosus and cartilage endplate, in the EPL group. The L4-5 discs were not removed and simply exposed in the sham group. Changes around the vertebral endplate on magnetic resonance imaging (MRI) and computed tomography (CT) were evaluated. Additionally, pain-related behavioral and histological assessments were performed.

RESULTS

In the EPL group, a low-signal area around the vertebral endplate was observed on T1-weighted and T2-weighted fat-saturated MRI at 8 weeks or later after surgery. In the same group, CT showed osteosclerosis around the vertebral endplate at 12 weeks after surgery. The sham group did not show abnormal imaging features on the MRI and CT. Behavioral evaluation showed that the EPL group had a significantly longer grooming time than the sham group. Conversely, the 12-week postoperative locomotion time and the 1- and 12-week postoperative standing times were significantly shorter in the EPL group than in the sham group. Histological evaluation showed a high degree of vertebral endplate degeneration and an increased number of osteoclasts and proportion of nerve fibers expressing calcitonin gene-related peptide in the EPL group compared to those in the sham group.

CONCLUSION

Our rat EPL model showed pain-related behavioral patterns and an increased expression of pain-related neuropeptide. This model could contribute to the study of the pathophysiology of EPL and will help in the treatment of low back pain in the future.

Supramolecular Peptide Nanofiber Hydrogels for Bone Tissue Engineering: From Multihierarchical Fabrications to Comprehensive Applications

Bone tissue engineering is becoming an ideal strategy to replace autologous bone grafts for surgical bone repair, but the multihierarchical complexity of natural bone is still difficult to emulate due to the lack of suitable biomaterials. Supramolecular peptide nanofiber hydrogels (SPNHs) are emerging biomaterials because of their inherent biocompatibility, satisfied biodegradability, high purity, facile functionalization, and tunable mechanical properties. This review initially focuses on the multihierarchical fabrications by SPNHs to emulate natural bony extracellular matrix. Structurally, supramolecular peptides based on distinctive building blocks can assemble into nanofiber hydrogels, which can be used as nanomorphology-mimetic scaffolds for tissue engineering. Biochemically, bioactive motifs and bioactive factors can be covalently tethered or physically absorbed to SPNHs to endow various functions depending on physiological and pharmacological requirements. Mechanically, four strategies are summarized to optimize the biophysical microenvironment of SPNHs for bone regeneration. Furthermore, comprehensive applications about SPNHs for bone tissue engineering are reviewed. The biomaterials can be directly used in the form of injectable hydrogels or composite nanoscaffolds, or they can be used to construct engineered bone grafts by bioprinting or bioreactors. Finally, continuing challenges and outlook are discussed.

Development and Trial of a Prototype Device for Sensorimotor Therapy in Patients with Distal Radius Fractures

This study examined the clinical feasibility of a prototype device (development name: Ghost) for facilitating range of motion (RoM) recovery in the acute phase in patients with distal radius fractures (DRF). The Ghost device involves the administration of a combination of vibratory and visual stimuli. We divided the patients into the Ghost (n = 10) and control group (n = 4; tendon vibration only) groups. The experimental interventions were administered between the day after surgery and day 7 postoperatively. Traditional hand therapy was provided to both groups once daily from day 7 until day 84 postoperatively and once a week from day 84 until the end of the intervention period. Because vibratory stimulation makes the patient focus on wrist flexion, the primary outcome was the arc of wrist flexion-extension on the injured side, which was measured on days 7, 14, 28, 42, 56, 70, and 84. Analysis of covariance was applied using a bootstrap method to evaluate changes over time and compare them between the groups. Analyses was performed after stratification by age and body mass index. Both interventions improved RoM over time in patients with DRF. Results showed that Ghost has greater efficacy for improving wrist RoM in DRF patients than vibration alone. Treatment with Ghost can result in good RoM improvement during the acute phase of DRF in young patients and those with and normal or low body mass index. Further study is needed to verify our findings and assess the extent of RoM recovery.

Implantable Electrical Stimulation at Dorsal Root Ganglions Accelerates Osteoporotic Fracture Healing via Calcitonin Gene-Related Peptide

The neuronal engagement of the peripheral nerve system plays a crucial role in regulating fracture healing, but how to modulate the neuronal activity to enhance fracture healing remains unexploited. Here it is shown that electrical stimulation (ES) directly promotes the biosynthesis and release of calcitonin gene-related peptide (CGRP) by activating Ca2+ /CaMKII/CREB signaling pathway and action potential, respectively. To accelerate rat femoral osteoporotic fracture healing which presents with decline of CGRP, soft electrodes are engineered and they are implanted at L3 and L4 dorsal root ganglions (DRGs). ES delivered at DRGs for the first two weeks after fracture increases CGRP expression in both DRGs and fracture callus. It is also identified that CGRP is indispensable for type-H vessel formation, a biological event coupling angiogenesis and osteogenesis, contributing to ES-enhanced osteoporotic fracture healing. This proof-of-concept study shows for the first time that ES at lumbar DRGs can effectively promote femoral fracture healing, offering an innovative strategy using bioelectronic device to enhance bone regeneration.

Exploring the effect of the "quaternary regulation" theory of "peripheral nerve-angiogenesis-osteoclast-osteogenesis" on osteoporosis based on neuropeptides

Osteoporosis is a common bone metabolic disease among the middle-aged and elderly, with its high incidence rate and a major cause of disability and mortality. Early studies found that bone metabolic homeostasis is achieved through osteogenesis-osteoclast coupling. Although current anti-osteoporosis drugs can attenuate bone loss caused by aging, they present specific side effects. With the discovery of CD31^hi Emcn^hi blood vessels in 2014, the effect of H-type blood vessels on bone metabolism has been valued by researchers, and the ternary regulation theory of bone metabolism of "Angiogenesis-Osteoclast-Osteogenesis" has also been recognized. Nowadays, more studies have confirmed that peripheral nerves substantially impact bone metabolism. However, due to the complex function of peripheral nerves, the crosstalk mechanism of "Peripheral nerve-Angiogenesis-Osteoclast-Osteogenesis" has not yet been fully revealed. Neuropeptide serves as signaling molecules secreted by peripheral nerves that regulate blood vessels, osteoblasts, and osteoclasts' functions. It is likely to be the breakthrough point of the quaternary regulation theory of "Peripheral nerve-Angiogenesis-Osteoclast-Osteogenesis". Here, we discuss the effect of peripheral nerves on osteoporosis based on neuropeptides.

What Did We Learn About Fracture Pain from Animal Models?

Progress in bone fracture repair research has been made possible due to the development of reproducible models of fracture in rodents with more clinically relevant fracture fixation, where there is considerably better assessment of the factors that affect fracture healing and/or novel therapeutics. However, chronic or persistent pain is one of the worst, longest-lasting and most difficult symptoms to manage after fracture repair, and an ongoing challenge remains for animal welfare as limited information exists regarding pain scoring and management in these rodent fracture models. This failure of adequate pre-clinical pain assessment following osteotomy in the rodent population may not only subject the animal to severe pain states but may also affect the outcome of the bone healing study. Animal models to study pain were also mainly developed in rodents, and there is increasing validation of fracture and pain models to quantitatively evaluate fracture pain and to study the factors that generate and maintain fracture pain and develop new therapies for treating fracture pain. This review aims to discuss the different animal models for fracture pain research and characterize what can be learned from using animal models of fracture regarding behavioral pain states and new molecular targets for future management of these behaviors.

Macrophage Polarization and Alveolar Bone Healing Outcome: Despite a Significant M2 Polarizing Effect, VIP and PACAP Treatments Present a Minor Impact in Alveolar Bone Healing in Homeostatic Conditions

Host inflammatory immune response comprises an essential element of the bone healing process, where M2 polarization allegedly contributes to a favorable healing outcome. In this context, immunoregulatory molecules that modulate host response, including macrophage polarization, are considered potential targets for improving bone healing. This study aims to evaluate the role of the immunoregulatory molecules VIP (Vasoactive intestinal peptide) and PACAP (Pituitary adenylate cyclase activating polypeptide), which was previously described to favor the development of the M2 phenotype, in the process of alveolar bone healing in C57Bl/6 (WT) mice. Experimental groups were submitted to tooth extraction and maintained under control conditions or treated with VIP or PACAP were evaluated by microtomographic (µCT), histomorphometric, immunohistochemical, and molecular analysis at 0, 3, 7, and 14 days to quantify tissue healing and host response indicators at the healing site. Gene expression analysis demonstrates the effectiveness of VIP or PACAP in modulating host response, evidenced by the early dominance of an M2-type response, which was paralleled by a significant increase in M2 (CD206+) in treated groups. However, despite the marked effect of M1/M2 balance in the healing sites, the histomorphometric analysis does not reveal an equivalent/corresponding modulation of the healing process. µCT reveals a slight increase in bone matrix volume and the trabecular thickness number in the PACAP group, while histomorphometric analyzes reveal a slight increase in the VIP group, both at a 14-d time-point; despite the increased expression of osteogenic factors, osteoblastic differentiation, activity, and maturation markers in both VIP and PACAP groups. Interestingly, a lower number of VIP and PACAP immunolabeled cells were observed in the treated groups, suggesting a reduction in endogenous production. In conclusion, while both VIP and PACAP treatments presented a significant immunomodulatory effect with potential for increased healing, no major changes were observed in bone healing outcome, suggesting that the signals required for bone healing under homeostatic conditions are already optimal, and additional signals do not improve an already optimal process. Further studies are required to elucidate the role of macrophage polarization in the bone healing process.

Increased Calcitonin Gene-Related Peptide Expression in DRG and Nerve Fibers Proliferation Caused by Nonunion Fracture in Rats

Purpose

Nonunion bone fracture can be a cause of persistent pain, but the pathophysiology remains largely unknown. The objective of this study was to identify how nonunion affect persistent pain after fracture. Specifically, we evaluated the association of neuropeptide change in dorsal root ganglia (DRG) and nerve proliferation at fracture sites with pain.

Methods

Rat union and nonunion fracture models were created. A piece of latex glove was placed at the fracture site to create a nonunion model. At 6 weeks after surgery, bone healing was assessed using radiography. In addition, the presence of calcitonin gene-related peptide-immunoreactive (CGRP-IR) DRG at the level of L3 and anti-growth associated protein 43-immunoreactive (GAP43-IR) nerve fibers in the scar tissue between the bone fragments were evaluated. Pain-related behavior was assessed using forced treadmill running.

Results

In radiological images at 6 weeks after surgery, callus formation was formed continuously between bone fragments in the union models. On the one hand, a clear gap was detected between fragments in nonunion models. The percentage of CGRP-IR DRG cells and the density of GAP43-IR nerve fibers in the scar tissue between the bone fragments in nonunion models was significantly higher than that in union models (p < 0.05). An increase in inflammatory cell infiltrate was observed in scar tissues in the nonunion models. During forced treadmill running, rats in the union model could run significantly longer than those in the nonunion models.

Conclusion

Increased CGRP expression in DRG cells and abnormal nerve proliferation secondary to prolonged inflammation could lead to persistent pain after bone fracture. In clinical practice, early achievement of bone union may minimize the development of persistent pain after fractures.

Sensory nerves: A driver of the vicious cycle in bone metastasis?

Bone is one of the preferential target organs of cancer metastasis. Bone metastasis is associated with various complications, of which bone pain is most common and debilitating. The cancer-associated bone pain (CABP) is induced as a consequence of increased neurogenesis, reprogramming and axonogenesis of sensory nerves (SNs) in harmony with sensitization and excitation of SNs in response to the tumor microenvironment created in bone. Importantly, CABP is associated with increased mortality, of which precise cellular and molecular mechanism remains poorly understood. Bone is densely innervated by autonomic nerves (ANs) (sympathetic and parasympathetic nerves) and SNs. Recent studies have shown that the nerves innervating the tumor microenvironment establish intimate communications with tumors, producing various stimuli for tumors to progress and disseminate. In this review, our current understanding of the role of SNs innervating bone in the pathophysiology of CABP will be overviewed. Then the hypothesis that SNs facilitate cancer progression in bone will be discussed in conjunction with our recent findings that SNs play an important role not only in the induction of CABP but also the progression of bone metastasis using a preclinical model of CABP. It is suggested that SNs are a critical component of the bone microenvironment that drives the vicious cycle between bone and cancer to progress bone metastasis. Suppression of the activity of bone-innervating SNs may have potential therapeutic effects on the progression of bone metastasis and induction of CABP.

Nerve growth factor promotes osteogenic differentiation of MC3T3-E1 cells via BMP-2/Smads pathway

BACKGROUND

Exogenous nerve growth factor (NGF) can induce osteogenic precursor cell differentiation and promote fracture healing. However, the molecular mechanism by which NGF induces osteogenesis is not well understood. BMP-2 has good osteogenic efficacy and is one of the most osteogenic-inducing growth factors known. Therefore, this study aimed to determine whether NGF induces osteogenic differentiation of mouse embryonic osteogenic precursor cell line MC3T3-E1 by BMP-2 and search further mechanisms of NGF on BMP-2.

METHODS

MC3T3-E1 cells were treated with NGF at a concentration gradient for indicated times, after which the cell viability was measured by CCK-8 kit. Osteogenic differentiation was detected with quantification of alkaline phosphatase (ALP) activity also visualized with ALP staining. The transcription and expression of relevant genes were detected by qPCR and western blotting, respectively. NGF's effect on BMP2 was studied with qPCR and luciferase reporter assay. The phosphorylation of Smads was probed with specific antibodies by western blotting, and the location of Smads was observed through immunofluorescence.

RESULTS

We found that NGF promoted proliferation and osteogenic differentiation of MC3T3-E1, increased the expression level of BMP-2, as well as the phosphorylation and nuclear translocation of Smad1/5/8. However, neutralization of BMP-2 with si-BMP-2 or BMP-2 signal inhibitors reversed NGF induced phosphorylation and nuclear translocation of Smad1/5/8, as well as the expression of Runx2, type I collagen, osteocalcin and osteopontin. In addition, si-BMP-2 abrogated NGF-induced ALP activity.

CONCLUSION

NGF induced osteogenic differentiation of MC3T3-E1 cells through BMP-2/Smads pathway and induction of Runx2. Our study would provide a theoretical basis for clinical treatment of fractures using NGF.

Crosstalk between Bone and Nerves within Bone

For the past two decades, the function of intrabony nerves on bone has been a subject of intense research, while the function of bone on intrabony nerves is still hidden in the corner. In the present review, the possible crosstalk between bone and intrabony peripheral nerves will be comprehensively analyzed. Peripheral nerves participate in bone development and repair via a host of signals generated through the secretion of neurotransmitters, neuropeptides, axon guidance factors and neurotrophins, with additional contribution from nerve-resident cells. In return, bone contributes to this microenvironmental rendezvous by housing the nerves within its internal milieu to provide mechanical support and a protective shelf. A large ensemble of chemical, mechanical, and electrical cues works in harmony with bone marrow stromal cells in the regulation of intrabony nerves. The crosstalk between bone and nerves is not limited to the physiological state, but also involved in various bone diseases including osteoporosis, osteoarthritis, heterotopic ossification, psychological stress-related bone abnormalities, and bone related tumors. This crosstalk may be harnessed in the design of tissue engineering scaffolds for repair of bone defects or be targeted for treatment of diseases related to bone and peripheral nerves.

Luminescent net-like inorganic scaffolds with europium-doped hydroxyapatite for enhanced bone reconstruction

Bone reconstruction is an urgent problem during clinical treatment. In the past few decades, the construction of composite scaffolds has been a hot spot in the research field of bone tissue engineering (BTE). However, the disadvantages of composite materials raise our awareness to explore the potential application of hydroxyapatite (HAp) in bone substitutes due to the closest properties of HAp to natural bone tissue. In our study, we synthesized Eu³⁺-doped HAp (HAp:Eu³⁺) ultralong nanowires, which can be transformed to hydrophilic net-like scaffolds via a thiol-ene click reaction. The property of luminescence of HAp from Eu³⁺ is beneficial for identifying the relative position of materials and bone marrow mesenchymal stem cells (BMSCs). HAp:Eu³⁺ scaffolds with excellent cell biocompatibility could promote the expression of early bone formation markers (ALP and ARS) and enhance the expression of genes and proteins associated with osteogenesis (Runx 2, OCN, and OPN). In the end, the results of the in vivo osteogenesis experiment showed that pure HAp scaffolds presented different effects of bone tissue reconstruction compared with the composite scaffolds with HAp nanorods and polymer materials. The superior osteogenic effect could be observed in net-like pure HAp scaffold groups. Furthermore, the absorption of HAp:Eu³⁺ scaffolds could be monitored due to the luminescence property of Eu³⁺. This strategy based on ultralong HAp nanowires proved to be a new method for the construction of simple reticular scaffolds for potential osteogenic applications.

The effect of nerve growth factor on corneal nerve regeneration and dry eye after LASIK

Dry eye is the most common complication after refractive surgery, especially after laser in situ keratomileusis (LASIK), in which nerves may be cut when making the corneal flap. Nerve growth factor (NGF) has been demonstrated to stimulate corneal sensitivity and nerve regeneration and NGF has been suggested as a potential treatment for dry eye disease (DED). Hence, this study aimed to investigate the effect of NGF on corneal nerve regeneration, sensitivity and dry eye symptoms after LASIK, compared to hycosan and normal saline (NS) treatments. Thirty-eight New Zealand white rabbits that underwent LASIK procedures were randomly assigned to three groups. Each group underwent NGF, hycosan, and NS treatment. The nerve densities and the number of corneal sub-basal and superficial stromal nerves were measured with confocal microscopy, and the results were compared before surgery and at one month and three months postoperatively. Corneal sensitivity was assessed with an esthesiometer. The tear breakup time (TBUT) was recorded to check for signs of dry eye. The whole corneas of the experimental animals were excised at three months after the surgery for immunohistochemically analysis. After LASIK, treatment with NGF significantly accelerated the recovery of sub-basal and superficial stromal nerve densities and the numbers, compared to hycosan and NS treatments at one month and three months postoperatively (NGF vs. hycosan, P < 0.01 each; NGF vs. NS, P < 0.01 each). The recovery of corneal sensitivity was significantly enhanced in the NGF group compared to the hycosan or NS treatment groups after surgery (P < 0.05). Also, the TBUT data showed a statistically significant longer time in the NGF group at one month, and three months postoperatively (P < 0.05). Immunofluorescence analysis showed the nerve fiber quantity of the NGF group was larger than in the hycosan and NS groups. Taken together, the experimental results suggested that mNGF had an obvious effect on promoting corneal nerve repairing and the potential to improve dry eye in different periods following LASIK.

Local injections of β-NGF accelerates endochondral fracture repair by promoting cartilage to bone conversion

There are currently no pharmacological approaches in fracture healing designed to therapeutically stimulate endochondral ossification. In this study, we test nerve growth factor (NGF) as an understudied therapeutic for fracture repair. We first characterized endogenous expression of Ngf and its receptor tropomyosin receptor kinase A (TrkA) during tibial fracture repair, finding that they peak during the cartilaginous phase. We then tested two injection regimens and found that local β-NGF injections during the endochondral/cartilaginous phase promoted osteogenic marker expression. Gene expression data from β-NGF stimulated cartilage callus explants show a promotion in markers associated with endochondral ossification such as Ihh, Alpl, and Sdf-1. Gene ontology enrichment analysis revealed the promotion of genes associated with Wnt activation, PDGF- and integrin-binding. Subsequent histological analysis confirmed Wnt activation following local β-NGF injections. Finally, we demonstrate functional improvements to bone healing following local β-NGF injections which resulted in a decrease in cartilage and increase of bone volume. Moreover, the newly formed bone contained higher trabecular number, connective density, and bone mineral density. Collectively, we demonstrate β-NGF’s ability to promote endochondral repair in a murine model and uncover mechanisms that will serve to further understand the molecular switches that occur during cartilage to bone transformation.

Dexamethasone-Loaded Thermosensitive Hydrogel Suppresses Inflammation and Pain in Collagen-Induced Arthritis Rats

Purpose

To overcome negative adverse effects and improve therapeutic index of dexamethasone (Dex) in rheumatoid arthritis (RA), we developed a novel sustained release formulation-intra-articular injectable dexamethasone-loaded thermosensitive hydrogel (DLTH) with chitosan-glycerin-borax as carrier for the remission of inflammation and pain. The focus of this article is to explore both anti-inflammatory and pain-relieving effects of DLTH joint injection in bovine type-II collagen-induced arthritis (CIA) rats.

Methods

Wistar rats were randomized into three groups, including the normal group (n=6), the model group (n=6) and the DLTH group (n=10). Joint injection of DLTH (1mg/kg Dex per rat) was injected on day 12 in the DLTH group twice a week for three weeks. Clinical signs of body weight, paw swelling and arthritis scores, histologic analysis, hind paw mechanical withdrawal threshold (MWT), plantar pressure pain threshold (PPT) were taken into consideration. Serum contents of IL-17A, prostaglandin E2 (PGE2), prostacyclin 2 (PGI2) and prostaglandin D2 (PGD2), real-time polymerase chain reaction (PCR) analysis of inflammatory factors and pain-related mediators in synovium and dorsal root ganglia (DRG), Western blotting of NF-κB in synovium were all evaluated.

Results

Paw swelling, arthritis scores and joint inflammation destruction were all attenuated in the DLTH-treated group. Results showed that DLTH not only down-regulated serum IL-17A, but also mRNA levels of inflammatory factors and NGF, and key proteins contents of the NF-κB pathway in synovium. Increases of MWT and PPT in DLTH-treated rats elucidated pain-reducing effects of DLTH. Elevated serum PGD2 levels and declines of serum PGE2 and PGI2, and inflammatory and pain-related genes in DRGs in the DLTH group were also recorded.

Conclusion

These data elucidated that DLTH joint injection impeded synovial inflammation processes through down-regulating transcription activity of NF-κB pathway, and intra-articular DLTH may aid in the regulation of RA pain through regulating inflammation and pain conduction process.

The emerging role of the BDNF-TrkB signaling pathway in the modulation of pain perception

The brain derived neurotrophic factor (BDNF) is a crucial neuromodulator in pain transmission both in peripheral and central nervous system (CNS). Despite evidence of a pro-nociceptive role of BDNF, recent studies have reported contrasting results, including anti-nociceptive and anti-inflammatory activities. Moreover, BDNF polymorphisms can interfere with BDNF role in pain perception. In Val66Met carriers, the Met allele may have a dual role, with anti-nociceptive actions in normal condition and pro-nociceptive effects during chronic pain. In order to elucidate the main effects of BDNF in nociception, we reviewed the main characteristics of this neurotrophin, focusing on its involvement in pain.

Physical Exercise and Myokines: Relationships with Sarcopenia and Cardiovascular Complications

Skeletal muscle is capable of secreting different factors in order to communicate with other tissues. These mediators, the myokines, show potentially far-reaching effects on non-muscle tissues and can provide a molecular interaction between muscle and body physiology. Sarcopenia is a chronic degenerative neuromuscular disease closely related to cardiomyopathy and chronic heart failure, which influences the production and release of myokines. Our objective was to explore the relationship between myokines, sarcopenia, and cardiovascular diseases (CVD). The autocrine, paracrine, and endocrine actions of myokines include regulation of energy expenditure, insulin sensitivity, lipolysis, free fatty acid oxidation, adipocyte browning, glycogenolysis, glycogenesis, and general metabolism. A sedentary lifestyle accelerates the aging process and is a risk factor for developing sarcopenia, metabolic syndrome, and CVD. Increased adipose tissue resulting from the decrease in muscle mass in patients with sarcopenia may also be involved in the pathology of CVD. Myokines are protagonists in the complex condition of sarcopenia, which is associated with adverse clinical outcomes in patients with CVD. The discovery of new pathways and the link between myokines and CVD remain a cornerstone toward multifaceted interventions and perhaps the minimization of the damage resulting from muscle loss induced by factors such as atherosclerosis.

Targeting neurotrophic factors: Novel approaches to musculoskeletal pain

Chronic pain represents a substantial unmet medical need globally. In recent years, the quest for a new generation of novel, safe, mechanism-based analgesic treatments has focused on neurotrophic factors, a large group of secreted proteins that control the growth and survival of different populations of neurons, but that postnatally are involved in the genesis and maintenance of pain, with biological activity in both the periphery and the central nervous system. In this narrative review, we discuss the two families of neurotrophic proteins that have been extensively studied for their role in pain: first, the neurotrophins, nerve growth factor (NGF) and brain-derived growth factor (BDNF), and secondly, the GDNF family of ligands (GFLs). We provide an overview of the pain pathway, and the pain-producing effects of these different proteins. We summarize accumulating preclinical and clinical findings with a focus on musculoskeletal pain, and on osteoarthritis in particular, because the musculoskeletal system is the most prevalent source of chronic pain and of disability, and clinical testing of these novel agents - often biologics- is most advanced in this area.