Impact of the Sensory and Sympathetic Nervous System on Fracture Healing in Ovariectomized Mice

Neuronal regulation of bone and tendon injury repair: a focused review

Beyond the sensation of pain, peripheral nerves have been shown to play crucial roles in tissue regeneration and repair. As a highly innervated organ, bone can recover from injury without scar formation, making it an interesting model in which to study the role of nerves in tissue regeneration. As a comparison, tendon is a musculoskeletal tissue that is hypo-innervated, with repair often resulting in scar formation. Here, we reviewed the significance of innervation in 3 stages of injury repair (inflammatory, reparative, and remodeling) in 2 commonly injured musculoskeletal tissues: bone and tendon. Based on this focused review, we conclude that peripheral innervation is essential for phases of proper bone and tendon repair, and that nerves may dynamically regulate the repair process through interactions with the injury microenvironment via a variety of neuropeptides or neurotransmitters. A deeper understanding of neuronal regulation of musculoskeletal repair, and the crosstalk between nerves and the musculoskeletal system, will enable the development of future therapies for tissue healing.

Impact of sensory neuropeptide deficiency on behavioral patterns and gait in a murine surgical osteoarthritis model

Substance P (SP) and a calcitonin-related gene alpha (αCGRP-/-) are implicated in musculoskeletal pain perception and were shown to have different effects on the pathogenesis of osteoarthritis (OA). However, it has not been investigated, whether deficiency for SP or αCGRP impacts pain-related behavior and well-being as well as gait during development of experimental OA. We induced OA in the right knee of wild-type (WT) mice and mice either deficient for SP (tachykinin 1, Tac-1) or αCGRP (male, n = 8 per genotype) by destabilizing the medial meniscus (DMM). We monitored body weight and food and water intake as indicators of wellbeing, determined nest building and composite pain score, and performed CatWalk gait analysis over 12 weeks. Cartilage degeneration was determined by OARSI scoring. The 12-week post-DMM, cartilage degradation in the medial compartment was significantly reduced in Tac1-/- mice compared to the WT and to αCGRP-/- mice, coinciding with highest unloading of the operated limb in Tac1-/-. Behavioral and gait analysis revealed only minor differences between the genotypes. Paw print area was most prominently reduced in Tac1-/- over the observation period; at 12 weeks, we found a significant reduction in normalized print area in Tac1-/- compared to presurgery and to the WT at the same time-point. Calculated weight bearing was significantly reduced only in Tac1-/-. Overall, we observed minor impact of DMM on gait and behavior in the present study. The reduced cartilage damage in the absence of SP might be in part due to reduced loading, however, the mechanism is not clear yet.

3D printing of customized bioceramics for promoting bone tissue regeneration by regulating sympathetic nerve behavior

Numerous studies have shown that there are multiple neural activities involved in the process of bone resorption and bone regeneration, and promoting osteogenesis by promoting neural network reconstruction is an effective strategy for repairing critical size bone defects. However, traumatic bone defects often cause activation of the sympathetic nervous system (SNS) in the damaged area, releasing excess catecholamines (CAs), resulting in a decrease in the rate of bone formation. Herein, a 3D-printed scaffold loaded with propranolol (PRN) is proposed to reduce CA concentrations in bone defect areas and promote bone regeneration through drug release. For this purpose, PRN-loaded methacrylated gelatin (GelMA) microspheres were mixed with low-concentration GelMA and perfused into a 3D-printed porous hydroxyapatite (HAp) scaffold. By releasing PRN, which can block β-adrenergic receptors, it hinders the activation of sympathetic nerves and inhibits the release of excess CA by the SNS. At the same time, the composite scaffold recruits bone marrow mesenchymal stem cells (BMSCs) and promotes the differentiation of BMSCs in the direction of osteoblasts, which effectively promotes bone regeneration in the rabbit femoral condyle defect model. The results of the study showed that the release of PRN from the composite scaffold could effectively hinder the activation of sympathetic nerves and promote bone regeneration, providing a new strategy for the treatment of bone defects.

Biomaterial design for regenerating aged bone: materiobiological advances and paradigmatic shifts

China's aging demographic poses a challenge for treating prevalent bone diseases impacting life quality. As bone regeneration capacity diminishes with age due to cellular dysfunction and inflammation, advanced biomaterials-based approaches offer hope for aged bone regeneration. This review synthesizes materiobiology principles, focusing on biomaterials that target specific biological functions to restore tissue integrity. It covers strategies for stem cell manipulation, regulation of the inflammatory microenvironment, blood vessel regeneration, intervention in bone anabolism and catabolism, and nerve regulation. The review also explores molecular and cellular mechanisms underlying aged bone regeneration and proposes a database-driven design process for future biomaterial development. These insights may also guide therapies for other age-related conditions, contributing to the pursuit of 'healthy aging'.

Nociceptor mechanisms underlying pain and bone remodeling via orthodontic forces: toward no pain, big gain

Orthodontic forces are strongly associated with pain, the primary complaint among patients wearing orthodontic braces. Compared to other side effects of orthodontic treatment, orthodontic pain is often overlooked, with limited clinical management. Orthodontic forces lead to inflammatory responses in the periodontium, which triggers bone remodeling and eventually induces tooth movement. Mechanical forces and subsequent inflammation in the periodontium activate and sensitize periodontal nociceptors and produce orthodontic pain. Nociceptive afferents expressing transient receptor potential vanilloid subtype 1 (TRPV1) play central roles in transducing nociceptive signals, leading to transcriptional changes in the trigeminal ganglia. Nociceptive molecules, such as TRPV1, transient receptor potential ankyrin subtype 1, acid-sensing ion channel 3, and the P2X3 receptor, are believed to mediate orthodontic pain. Neuropeptides such as calcitonin gene-related peptides and substance P can also regulate orthodontic pain. While periodontal nociceptors transmit nociceptive signals to the brain, they are also known to modulate alveolar bone remodeling in periodontitis. Therefore, periodontal nociceptors and nociceptive molecules may contribute to the modulation of orthodontic tooth movement, which currently remains undetermined. Future studies are needed to better understand the fundamental mechanisms underlying neuroskeletal interactions in orthodontics to improve orthodontic treatment by developing novel methods to reduce pain and accelerate orthodontic tooth movement-thereby achieving "big gains with no pain" in clinical orthodontics.

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.

Evaluating rearing behaviour as a model-specific pain indicator in mouse osteotomy models

To assess pain in mouse models of bone fractures, currently applied assessment batteries use combinations of clinical signs with spontaneous behaviours and model-specific behaviours, including walking and weight-bearing behaviour. Rearing behaviour - an upright position on the hindlimbs - has a motivational and an ambulatory component. Thus, rearing behaviour might have the potential to be an indicator for model-specific pain in mouse fracture models. To date, the assessment of rearing behaviour in bone fracture models using mice is only scarcely described. In this study, we aimed to determine whether the duration of rearing behaviour is affected by osteotomy of the femur in male and female C57BL/6N mice with external fixation (rigid vs. flexible) and could be an additional sign for model-specific pain, such as the presence of limping. Rearing duration was significantly decreased after osteotomy in male and female mice at 24 h, 48 h and 72 h, but was not affected by anaesthesia/analgesia alone. In male mice, the relative rearing duration increased over 72 h (both fixations) and at 10 days in the rigid fixation group but remained significantly lower in the flexible fixation group. In contrast, in female mice, no increase in rearing duration was observed within 72 h and at 10 days post-osteotomy, independent of the fixation. We did not identify any association between relative rearing time and presence or absence of limping. In summary, our results do not provide sufficient evidence that altered rearing behaviour might be an indicative sign for pain in this model.

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.

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.

Reduced angiogenesis and delayed endochondral ossification in CD163-/- mice highlights a role of M2 macrophages during bone fracture repair

While recent studies showed that macrophages are critical for bone fracture healing, and lack of M2 macrophages have been implicated in models of delayed union, functional roles for specific M2 receptors have yet to be defined. Moreover, the M2 scavenger receptor CD163 has been identified as a target to inhibit sepsis following implant-associated osteomyelitis, but potential adverse effects on bone healing during blockage therapy have yet to be explored. Thus, we investigated fracture healing in C57BL/6 versus CD163-/- mice using a well-established closed, stabilized, mid-diaphyseal femur fracture model. While gross fracture healing in CD163-/- mice was similar to that of C57BL/6, plain radiographs revealed persistent fracture gaps in the mutant mice on Day 14, which resolved by Day 21. Consistently, 3D vascular micro-CT demonstrated delayed union on Day 21, with reduced bone volume (74%, 61%, and 49%) and vasculature (40%, 40%, and 18%) compared to C57BL/6 on Days 10, 14, and 21 postfracture, respectively (p < 0.01). Histology confirmed large amounts of persistent cartilage in CD163-/- versus C57BL/6 fracture callus on Days 7 and 10 that resolves over time, and immunohistochemistry demonstrated deficiencies in CD206+ M2 macrophages. Torsion testing of the fractures confirmed the delayed early union in CD163-/- femurs, which display decreased yield torque on Day 21, and a decreased rigidity with a commensurate increase in rotation at yield on Day 28 (p < 0.01). Collectively, these results demonstrate that CD163 is required for normal angiogenesis, callus formation, and bone remodeling during fracture healing, and raise potential concerns about CD163 blockade therapy.

Substance P promote macrophage M2 polarization to attenuate secondary lymphedema by regulating NF-kB/NLRP3 signaling pathway

Secondary lymphedema often occurs after filariasis, trauma, lymph node dissection and radiation therapy, which is manifested by infiltration of inflammatory cells and fibrosis formation in pathologically. Substance P is a widely used neuropeptide in the field of tissue repair, while the regenerative potential of the substance P has not been proven in the secondary lymphedema. In this study, animal model of secondary lymphedema was constructed by excising the skin and subcutaneous lymphatic network in the tail of mice, and the degree of swelling in the tail of mice was evaluated after 6 weeks under the treatment with substance P. Immunofluorescence staining was also performed to assess immune cell infiltration, subcutaneous fibrosis and lymphangiogenesis. The results revealed that substance P significantly alleviated post-surgical lymphedema in mice. Furthermore, we found that substance P promoted macrophages M2 polarization, a process associated with downregulation of the NF-kB/NLRP3 pathway. After application of disodium clodronate (macrophage scavenger, CLO), the positive effect of substance P in lymphedema is significantly inhibited. In vitro experiments, we further demonstrated the polarizing effect of substance P on bone marrow-derived macrophages (BMDMs), while substance P inhibited the activation of the NF-kB/NLRP3 pathway in BMDMs after the treatment of lipopolysaccharide (LPS). In addition, polarized macrophages were demonstrated to promote the proliferation, tube-forming and migratory functions of human lymphatic endothelial cells (hLEC). In conclusion, our study provides preliminary evidence that substance P alleviates secondary lymphedema by promoting macrophage M2 polarization, and this therapeutic effect may be associated with downregulation of the NF-kB/NLRP3 pathway.

Role of the Peripheral Nervous System in Skeletal Development and Regeneration: Controversies and Clinical Implications

PURPOSE OF REVIEW

This review examines the diverse functional relationships that exist between the peripheral nervous system (PNS) and bone, including key advances over the past century that inform our efforts to translate these discoveries for skeletal repair.

RECENT FINDINGS

The innervation of the bone during development, homeostasis, and regeneration is highly patterned. Consistent with this, there have been nearly 100 studies over the past century that have used denervation approaches to isolate the effects of the different branches of the PNS on the bone. Overall, a common theme of balance emerges whereby an orchestration of both local and systemic neural functions must align to promote optimal skeletal repair while limiting negative consequences such as pain. An improved understanding of the functional bidirectional pathways linking the PNS and bone has important implications for skeletal development and regeneration. Clinical advances over the next century will necessitate a rigorous identification of the mechanisms underlying these effects that is cautious not to oversimplify the in vivo condition in diverse states of health and disease.

Traumatic brain injury stimulates sympathetic tone-mediated bone marrow myelopoiesis to favor fracture healing

Traumatic brain injury (TBI) accelerates fracture healing, but the underlying mechanism remains largely unknown. Accumulating evidence indicates that the central nervous system (CNS) plays a pivotal role in regulating immune system and skeletal homeostasis. However, the impact of CNS injury on hematopoiesis commitment was overlooked. Here, we found that the dramatically elevated sympathetic tone accompanied with TBI-accelerated fracture healing; chemical sympathectomy blocks TBI-induced fracture healing. TBI-induced hypersensitivity of adrenergic signaling promotes the proliferation of bone marrow hematopoietic stem cells (HSCs) and swiftly skews HSCs toward anti-inflammation myeloid cells within 14 days, which favor fracture healing. Knockout of β3- or β2-adrenergic receptor (AR) eliminate TBI-mediated anti-inflammation macrophage expansion and TBI-accelerated fracture healing. RNA sequencing of bone marrow cells revealed that Adrb2 and Adrb3 maintain proliferation and commitment of immune cells. Importantly, flow cytometry confirmed that deletion of β2-AR inhibits M2 polarization of macrophages at 7th day and 14th day; and TBI-induced HSCs proliferation was impaired in β3-AR knockout mice. Moreover, β3- and β2-AR agonists synergistically promote infiltration of M2 macrophages in callus and accelerate bone healing process. Thus, we conclude that TBI accelerates bone formation during early stage of fracture healing process by shaping the anti-inflammation environment in the bone marrow. These results implicate that the adrenergic signals could serve as potential targets for fracture management.

Pathophysiological mechanism of acute bone loss after fracture

BACKGROUND

Acute bone loss after fracture is associated with various effects on the complete recovery process and a risk of secondary fractures among patients. Studies have reported similarities in pathophysiological mechanisms involved in acute bone loss after fractures and osteoporosis. However, given the silence nature of bone loss and bone metabolism complexities, the actual underlying pathophysiological mechanisms have yet to be fully elucidated.

AIM OF REVIEW

To elaborate the latest findings in basic research with a focus on acute bone loss after fracture. To briefly highlight potential therapeutic targets and current representative drugs. To arouse researchers' attention and discussion on acute bone loss after fracture.

KEY SCIENTIFIC CONCEPTS OF REVIEW

Bone loss after fracture is associated with immobilization, mechanical unloading, blood supply damage, sympathetic nerve regulation, and crosstalk between musculoskeletals among other factors. Current treatment strategies rely on regulation of osteoblasts and osteoclasts, therefore, there is a need to elucidate on the underlying mechanisms of acute bone loss after fractures to inform the development of efficacious and safe drugs. In addition, attention should be paid towards ensuring long-term skeletal health.

Research Progress in Calcitonin Gene-Related Peptide and Bone Repair

Calcitonin gene-related peptide (CGRP) has 37 amino acids. Initially, CGRP had vasodilatory and nociceptive effects. As research progressed, evidence revealed that the peripheral nervous system is closely associated with bone metabolism, osteogenesis, and bone remodeling. Thus, CGRP is the bridge between the nervous system and the skeletal muscle system. CGRP can promote osteogenesis, inhibit bone resorption, promote vascular growth, and regulate the immune microenvironment. The G protein-coupled pathway is vital for its effects, while MAPK, Hippo, NF-κB, and other pathways have signal crosstalk, affecting cell proliferation and differentiation. The current review provides a detailed description of the bone repair effects of CGRP, subjected to several therapeutic studies, such as drug injection, gene editing, and novel bone repair materials.

Hallmarks of peripheral nerve function in bone regeneration

Skeletal tissue is highly innervated. Although different types of nerves have been recently identified in the bone, the crosstalk between bone and nerves remains unclear. In this review, we outline the role of the peripheral nervous system (PNS) in bone regeneration following injury. We first introduce the conserved role of nerves in tissue regeneration in species ranging from amphibians to mammals. We then present the distribution of the PNS in the skeletal system under physiological conditions, fractures, or regeneration. Furthermore, we summarize the ways in which the PNS communicates with bone-lineage cells, the vasculature, and immune cells in the bone microenvironment. Based on this comprehensive and timely review, we conclude that the PNS regulates bone regeneration through neuropeptides or neurotransmitters and cells in the peripheral nerves. An in-depth understanding of the roles of peripheral nerves in bone regeneration will inform the development of new strategies based on bone-nerve crosstalk in promoting bone repair and regeneration.

Bone mass loss in chronic heart failure is associated with sympathetic nerve activation

PURPOSE

Chronic heart failure causes osteoporosis, but the mechanism remains unclear. The sympathetic nerve plays an important role in both bone metabolism and cardiovascular function.

METHODS

Thirty-six adult male SD rats were randomly divided into the following four groups: sham surgery (Sham) group, guanethidine (GD) group, abdominal transverse aorta coarctation-induced heart failure + normal saline (TAC) group, and TAC + guanethidine (TAC + GD) group. Normal saline (0.9 % NaCl) or guanethidine (40 mg/kg/ml) was intraperitoneally injected daily for 5 weeks. Then, DXA, micro-CT, ELISA and RT-PCR analyses were performed 12 weeks after treatment.

RESULTS

The bone loss in rats subjected to TAC-induced chronic heart failure and chemical sympathectomy with guanethidine was increased. Serum norepinephrine levels were increased in rats with TAC-induced heart failure but were decreased in TAC-induced heart failure rats treated with guanethidine. The expression of α2A adrenergic receptor, α2C adrenergic receptor, osteoprotegerin (OPG), and osteocalcin in the tibia decreased in the TAC-induced heart failure group, and the expression of β1 adrenergic receptor, β2 adrenergic receptor, receptor activator of nuclear factor-κ B ligand (RANKL), and RANKL/OPG in the tibia increased in the heart failure group. In addition, these changes in gene expression levels were rescued by chemical sympathectomy with guanethidine.

CONCLUSIONS

TAC-induced chronic heart failure is associated with bone mass loss, and the sympathetic nerve plays a significant role in heart failure-related bone mass loss.

MINI ABSTRACT

The present study supports the hypothesis that heart failure is related to bone loss, and the excessive activation of sympathetic nerves participates in this pathophysiological process. The present study suggests a potential pathological mechanism of osteoporosis associated with heart failure and new perspectives for developing strategies for heart failure-related bone loss.

Influence of the Peripheral Nervous System on Murine Osteoporotic Fracture Healing and Fracture-Induced Hyperalgesia

Osteoporotic fractures are often linked to persisting chronic pain and poor healing outcomes. Substance P (SP), α-calcitonin gene-related peptide (α-CGRP) and sympathetic neurotransmitters are involved in bone remodeling after trauma and nociceptive processes, e.g., fracture-induced hyperalgesia. We aimed to link sensory and sympathetic signaling to fracture healing and fracture-induced hyperalgesia under osteoporotic conditions. Externally stabilized femoral fractures were set 28 days after OVX in wild type (WT), α-CGRP- deficient (α-CGRP -/-), SP-deficient (Tac1-/-) and sympathectomized (SYX) mice. Functional MRI (fMRI) was performed two days before and five and 21 days post fracture, followed by µCT and biomechanical tests. Sympathectomy affected structural bone properties in the fracture callus whereas loss of sensory neurotransmitters affected trabecular structures in contralateral, non-fractured bones. Biomechanical properties were mostly similar in all groups. Both nociceptive and resting-state (RS) fMRI revealed significant baseline differences in functional connectivity (FC) between WT and neurotransmitter-deficient mice. The fracture-induced hyperalgesia modulated central nociception and had robust impact on RS FC in all groups. The changes demonstrated in RS FC in fMRI might potentially be used as a bone traumata-induced biomarker regarding fracture healing under pathophysiological musculoskeletal conditions. The findings are of clinical importance and relevance as they advance our understanding of pain during osteoporotic fracture healing and provide a potential imaging biomarker for fracture-related hyperalgesia and its temporal development. Overall, this may help to reduce the development of chronic pain after fracture thereby improving the treatment of osteoporotic fractures.

Neurovascular coupling in bone regeneration

The mammalian skeletal system is densely innervated by both neural and vascular networks. Peripheral nerves in the skeleton include sensory and sympathetic nerves. The crosstalk between skeletal and neural tissues is critical for skeletal development and regeneration. The cellular processes of osteogenesis and angiogenesis are coupled in both physiological and pathophysiological contexts. The cellular and molecular regulation of osteogenesis and angiogenesis have yet to be fully defined. This review will provide a detailed characterization of the regulatory role of nerves and blood vessels during bone regeneration. Furthermore, given the importance of the spatial relationship between nerves and blood vessels in bone, we discuss neurovascular coupling during physiological and pathological bone formation. A better understanding of the interactions between nerves and blood vessels will inform future novel therapeutic neural and vascular targeting for clinical bone repair and regeneration.

Inhibition of PGE2 in Subchondral Bone Attenuates Osteoarthritis

Aberrant subchondral bone architecture is a crucial driver of the pathological progression of osteoarthritis, coupled with increased sensory innervation. The sensory PGE2/EP4 pathway is involved in the regulation of bone mass accrual by the induction of differentiation of mesenchymal stromal cells. This study aimed to clarify whether the sensory PGE2/EP4 pathway induces aberrant structural alteration of subchondral bone in osteoarthritis. Destabilization of the medial meniscus (DMM) using a mouse model was combined with three approaches: the treatment of celecoxib, capsaicin, and sensory nerve-specific prostaglandin E2 receptor 4 (EP4)-knockout mice. Cartilage degeneration, subchondral bone architecture, PGE2 levels, distribution of sensory nerves, the number of osteoprogenitors, and pain-related behavior in DMM mice were assessed. Serum and tissue PGE2 levels and subchondral bone architecture in a human sample were measured. Increased PGE2 is closely related to subchondral bone’s abnormal microstructure in humans and mice. Elevated PGE2 concentration in subchondral bone that is mainly derived from osteoblasts occurs in early-stage osteoarthritis, preceding articular cartilage degeneration in mice. The decreased PGE2 levels by the celecoxib or sensory denervation by capsaicin attenuate the aberrant alteration of subchondral bone architecture, joint degeneration, and pain. Selective EP4 receptor knockout of the sensory nerve attenuates the aberrant formation of subchondral bone and facilitates the prevention of cartilage degeneration in DMM mice. Excessive PGE2 in subchondral bone caused a pathological alteration to subchondral bone in osteoarthritis and maintaining the physiological level of PGE2 could potentially be used as an osteoarthritis treatment.

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.

Clinically relevant preclinical animal models for testing novel cranio-maxillofacial bone 3D-printed biomaterials

Bone tissue engineering is a rapidly developing field with potential for the regeneration of craniomaxillofacial (CMF) bones, with 3D printing being a suitable fabrication tool for patient-specific implants. The CMF region includes a variety of different bones with distinct functions. The clinical implementation of tissue engineering concepts is currently poor, likely due to multiple reasons including the complexity of the CMF anatomy and biology, and the limited relevance of the currently used preclinical models. The 'recapitulation of a human disease' is a core requisite of preclinical animal models, but this aspect is often neglected, with a vast majority of studies failing to identify the specific clinical indication they are targeting and/or the rationale for choosing one animal model over another. Currently, there are no suitable guidelines that propose the most appropriate animal model to address a specific CMF pathology and no standards are established to test the efficacy of biomaterials or tissue engineered constructs in the CMF field. This review reports the current clinical scenario of CMF reconstruction, then discusses the numerous limitations of currently used preclinical animal models employed for validating 3D-printed tissue engineered constructs and the need to reduce animal work that does not address a specific clinical question. We will highlight critical research aspects to consider, to pave a clinically driven path for the development of new tissue engineered materials for CMF reconstruction.

Benefits and Costs of Mixed-Species Aggregations in Harvestmen (Arachnida: Opiliones)

Many animals form aggregations with individuals of the same species (single-species aggregations, SSA). Less frequently, individuals may also aggregate with individuals of other species (mixed-species aggregations, MSA). Although the benefits and costs of SSA have been intensively studied, the same is not true for MSA. Here, we first review the cases of MSA in harvestmen, an arachnid order in which the records of MSA are more frequent than other arthropod orders. We then propose several benefits and costs of MSA in harvestmen, and contrast them with those of SSA. Second, using field-gathered data we describe gregariousness in seven species of Prionostemma harvestmen from Costa Rica. These species form MSA, but individuals are also found solitarily or in SSA. We tested one possible benefit and one possible cost of gregariousness in Prionostemma harvestmen. Regarding the benefit, we hypothesized that individuals missing legs would be more exposed to predation than eight-legged individuals and thus they should be found preferentially in aggregations, where they would be more protected from predators. Our data, however, do not support this hypothesis. Regarding the cost, we hypothesized that gregariousness increases the chances of parasitism. We found no support for this hypothesis either because both mite prevalence and infestation intensity did not differ between solitary or aggregated individuals. Additionally, the type of aggregation (SSA or MSA) was not associated with the benefit or the cost we explored. This lack of effect may be explained by the fluid membership of the aggregations, as we found high turnover over time in the number of individuals and species composition of the aggregations. In conclusion, we hope our review and empirical data stimulate further studies on MSA, which remains one of the most elusive forms of group living in animals.

Effect and mechanism of propranolol on promoting osteogenic differentiation and early implant osseointegration

The present study aimed to investigate the effect of β‑receptor blocker propranolol on early osseointegration of pure titanium implants and the underlying molecular regulatory mechanisms. An implant osseointegration model using the tibial metaphysis of New Zealand rabbits was established. The rabbits were divided into control and low‑, medium‑ and high‑dose propranolol groups. The formation of implant osseointegration was detected by X‑ray scanning. Mesenchymal stem cells (MSCs) and osteoblasts (OBs) were isolated and cultured in vitro, isoproterenol was supplemented to simulate sympathetic action and propranolol was subsequently administrated. The effect of propranolol on cell proliferation and osteogenic differentiation were assessed by EdU, flow cytometry, alizarin red staining and alkaline phosphatase (ALP) detection. The expression levels of bone morphogenetic protein (BMP)2, RUNX family transcription factor (RunX)2, collagen (COL)‑1, osteocalcin (OCN) and β2‑adrenergic receptor (AR) were detected by immunofluorescence, reverse transcription‑quantitative PCR and western blot assay. Propranolol effectively promoted implant osseointegration in vivo, facilitated proliferation of OBs, inhibited proliferation of MSCs and enhanced osteogenic differentiation of OBs and MSCs. The calcium content and ALP activity of cells treated with propranolol were markedly higher than in the control group. Propranolol also elevated mRNA and protein expression levels of BMP2, RunX2, COL‑1 and OCN in tissue and cells, and decreased the expression of β2‑AR. The present study demonstrated that the β‑receptor blocker propranolol promoted osteogenic differentiation of OBs and MSCs and enhanced implant osseointegration. The present study provided a novel insight into the application and regulatory mechanisms of propranolol.

Vasoactive Intestinal Peptide Promotes Fracture Healing in Sympathectomized Mice

Vasoactive intestinal peptide (VIP) as a neuromodulator and neurotransmitter played a significant role in modulating bone homeostasis. Our previous study reported an essential role of VIP in in vitro BMSCs osteogenesis and in vivo bone defect repair. VIP was also revealed to have a promoting effect on embryonic skeletal element development. However, the role of VIP in fracture healing is not known yet. We hypothesized that the disorder of sympathetic nervous system impairs bone structure and fracture healing, whereas VIP may rescue the sympathetic inhibition effects and promote fracture healing. We employed a 6-hydroxydopamine (6-OHDA) induced sympathectomy mice model (sympathectomized mice), in which successful sympathetic inhibition was confirmed by a decreased level of norephedrine (NE) in the spleen. In the sympathectomized mice, the femoral micro-architecture, bone density and mechanical properties were all impaired compared to the vehicle control mice. The femoral fracture was created in the vehicle or sympathectomized mice. Vehicle mice were locally injected with PBS as a negative control, and the sympathectomized mice were treated with injection of PBS or VIP. VIP expression at the fracture site was significantly decreased in sympathectomized mice. The fracture healing was repressed upon 6-OHDA treatment and rescued by VIP treatment. Micro-CT examination showed that the femoral bone micro-architecture at the fracture sites and mechanical properties were all impaired. Simultaneously, the expression level of osteogenic markers OCN and OPN were reduced in sympathectomized mice compared with vehicle group. While the VIP treatment rescued the repression effects of 6-OHDA on bone remodeling and significantly promoted bone quality and mechanical properties as well as increased osteogenesis marker expression in the sympathectomized mice. VIP administration promoted bone fracture healing by inhibiting bone resorption, making it a putative new alternative treatment strategy for fracture healing.

The Effects of Calcitonin Gene-Related Peptide on Bone Homeostasis and Regeneration

PURPOSE OF REVIEW

The goals of this review are two folds: (1) to describe the recent understandings on the roles of calcitonin gene-related peptide-α (CGRP) in bone homeostasis and the underlying mechanisms of related neuronal regulation and (2) to propose innovative CGRP-modulated approaches for enhancing bone regeneration in challenging bone disorders.

RECENT FINDINGS

CGRP is predominantly produced by the densely distributed sensory neuronal fibers in bone, declining with age. Under mechanical and biochemical stimulations, CGRP releases and exerts either physiological or pathophysiological roles. CGRP at physiological level orchestrates the communications of bone cells with cells of other lineages, affecting not only osteogenesis, osteoclastogenesis, and adipogenesis but also angiogenesis, demonstrating with pronounced anabolic effect, thus is essential for maintaining bone homeostasis, with tuned nerve-vessel-bone network. In addition, its effects on immunity and cell recruitment are also crucial for bone fracture healing. Binding to the G protein-coupled receptor composited by calcitonin receptor-like receptor (CRLR) and receptor activity modifying protein 1 (RAMP1) on cellular surface, CGRP triggers various intracellular signaling cascades involving cyclic adenosine monophosphate (cAMP) and cAMP response element-binding protein (CREB). Peaking at early stage post-fracture, CGRP promotes bone formation, displaying with larger callus. Then CGRP gradually decreases over time, allowing normal or physiological bone remodeling. By elevating CGRP at early stage, low-intensity pulsed ultrasound (LIPUS), electrical stimulation, and magnesium-based bio-mineral products may promisingly accelerate bone regeneration experimentally in medical conditions like osteoporosis, osteoporotic fracture, and spine fusion. Excess CGRP expression is commonly observed in pathological conditions including cancer metastatic lesions in bone and fracture delayed- or non-healing, resulting in persistent chronic pain. To date, these discoveries have largely been limited to animal models. Clinical applications are highly desirable. Compelling evidence show the anabolic effects of CGRP on bone in animals. However, further validation on the role of CGRP and the underlying mechanisms in human skeletons is required. It remains unclear if it is type H vessel connecting neuronal CGRP to osteogenesis, and if there is only specific rather than all osteoprogenitors responsible to CGRP. Clear priority should be put to eliminate these knowledge gaps by integrating with high-resolution 3D imaging of transparent bulk bone and single-cell RNA-sequencing. Last but not the least, given that small molecule antagonists such as BIBN4096BS can block the beneficial effects of CGRP on bone, concerns on the potential side effects of humanized CGRP-neutralizing antibodies when systemically administrated to treat migraine in clinics are arising.

The neuropeptide calcitonin gene-related peptide alpha is essential for bone healing

Background

Impaired fracture healing represents an ongoing clinical challenge, as treatment options remain limited. Calcitonin gene-related peptide (CGRP), a neuropeptide targeted by emerging anti-migraine drugs, is also expressed in sensory nerve fibres innervating bone tissue.

Method

Bone healing following a femoral osteotomy stabilized with an external fixator was analysed over 21 days in αCGRP-deficient and WT mice. Bone regeneration was evaluated by serum analysis, µCT analysis, histomorphometry and genome-wide expression analysis. Bone-marrow-derived osteoblasts and osteoclasts, as well as the CGRP antagonist olcegepant were employed for mechanistic studies.

Findings

WT mice with a femoral fracture display increased CGRP serum levels. αCGRP mRNA expression after skeletal injury is exclusively induced in callus tissue, but not in other organs. On protein level, CGRP and its receptor, calcitonin receptor-like receptor (CRLR) complexing with RAMP1, are differentially expressed in the callus during bone regeneration. On the other hand, αCGRP-deficient mice display profoundly impaired bone regeneration characterised by a striking reduction in the number of bone-forming osteoblasts and a high rate of incomplete callus bridging and non-union. As assessed by genome-wide expression analysis, CGRP induces the expression of specific genes linked to ossification, bone remodeling and adipogenesis. This suggests that CGRP receptor-dependent PPARγ signaling plays a central role in fracture healing.

Interpretation

This study demonstrates an essential role of αCGRP in orchestrating callus formation and identifies CGRP receptor agonism as a potential approach to stimulate bone regeneration. Moreover, as novel agents blocking CGRP or its receptor CRLR are currently introduced clinically for the treatment of migraine disorders, their potential negative impact on bone regeneration warrants clinical investigation.

Funding

This work was funded by grants from the Else-Kröner-Fresenius-Stiftung (EKFS), the Deutsche Forschungsgemeinschaft (DFG), and the Berlin Institute of Health (BIH).