Bone-protective Functions of Netrin 1 Protein

Peripheral nerves modulate the peri-implant osteogenesis under type 2 diabetes through exosomes derived from schwann cells via miR-15b-5p/Txnip signaling axis

Studies have shown that the prognosis of dental implant treatment in patients with diabetes is not as good as that in the non-diabetes population. The nerve plays a crucial role in bone metabolism, but the role and the mechanism of peripheral nerves in regulating peri-implant osteogenesis under Type 2 diabetes mellitus (T2DM) situation remains unclear. In this study, it was shown that high glucose-stimulated Schwann cells (SCs) inhibited peri-implant osteogenesis via their exosomes. SCs-derived exosomes were analyzed for their miRNA cargo, identifying miR-15b-5p as significantly downregulated in high glucose conditions. T2DM rats and patients exhibited decreased miR-15b-5p expression, correlating with impaired bone microarchitecture. Luciferase assays and Western blotting confirmed TXNIP as a direct miR-15b-5p target, implicating its involvement in ROS signaling and inflammation-related osteogenesis suppression. Furthermore, normal SCs exosomes improved bone parameters around dental implants in T2DM rats. These findings underscore the therapeutic potential of miR-15b-5p and normal SCs exosomes in mitigating poor peri-implant bone regeneration of T2DM patients, offering insights into the molecular mechanisms of peripheral nerves governing bone regeneration in diabetic conditions.

Skeletal interoception and prospective application in biomaterials for bone regeneration

Accumulating research has shed light on the significance of skeletal interoception, in maintaining physiological and metabolic homeostasis related to bone health. This review provides a comprehensive analysis of how skeletal interoception influences bone homeostasis, delving into the complex interplay between the nervous system and skeletal system. One key focus of the review is the role of various factors such as prostaglandin E2 (PGE2) in skeletal health via skeletal interoception. It explores how nerves innervating the bone tissue communicate with the central nervous system to regulate bone remodeling, a process critical for maintaining bone strength and integrity. Additionally, the review highlights the advancements in biomaterials designed to utilize skeletal interoception for enhancing bone regeneration and treatment of bone disorders. These biomaterials, tailored to interact with the body's interoceptive pathways, are positioned at the forefront of innovative treatments for conditions like osteoporosis and fractures. They represent a convergence of bioengineering, neuroscience, and orthopedics, aiming to create more efficient and targeted therapies for bone-related disorders. In conclusion, the review underscores the importance of skeletal interoception in physiological regulation and its potential in developing more effective therapies for bone regeneration. It emphasizes the need for further research to fully understand the mechanisms of skeletal interoception and to harness its therapeutic potential fully.

Emerging roles of nerve-bone axis in modulating skeletal system

Over the past decades, emerging evidence in the literature has demonstrated that the innervation of bone is a crucial modulator for skeletal physiology and pathophysiology. The nerve-bone axis sparked extensive preclinical and clinical investigations aimed at elucidating the contribution of nerve-bone crosstalks to skeleton metabolism, homeostasis, and injury repair through the perspective of skeletal neurobiology. To date, peripheral nerves have been widely reported to mediate bone growth and development and fracture healing via the secretion of neurotransmitters, neuropeptides, axon guidance factors, and neurotrophins. Relevant studies have further identified several critical neural pathways that stimulate profound alterations in bone cell biology, revealing a complex interplay between the skeleton and nerve systems. In addition, inspired by nerve-bone crosstalk, novel drug delivery systems and bioactive materials have been developed to emulate and facilitate the process of natural bone repair through neuromodulation, eventually boosting osteogenesis for ideal skeletal tissue regeneration. Overall, this work aims to review the novel research findings that contribute to deepening the current understanding of the nerve-bone axis, bringing forth some schemas that can be translated into the clinical scenario to highlight the critical roles of neuromodulation in the skeletal system.

Serum netrin-1 levels are high in Rheumatoid arthritis associated interstitial lung disease

BACKGROUND

Recent data show that netrin-1 has a role in development of pulmonary fibrosis. This study was aimed to investigate serum netrin-1 level and its relation to interstitial lung disease(ILD) in patients with rheumatoid arthritis (RA).

METHOD

42 RA patients with RA-ILD, 58 RA patients without RA-ILD (RA non-ILD group), and 61 healthy volunteers were included in this study. The modified DAS28-ESR score was used to calculate disease activity in RA patients. Using the quantitative immunoassay method, Serum netrin-1 levels were measured with an ELISA kit (Catalog number: E-EL-H2328; lab science, lot number: GZWTKZ5SWK, Texas, USA).

RESULTS

The median value of netrin-1 was found to be significantly higher in the RA-ILD group (82.9 [59.9-124]) compared to both the RA non-ILD group(52.9 [49.5-73.1])(B = -0.006, OR = 0.994, CI 95 %=0.989-0.999, P = 0.018) and the control group(53.5 [49.5-87.5]) (B: -0.005, OR: 0.994, CI 95 %: 0.990-0.999, p: 0.022). A cut-off value of 61.78 for netrin-1 was found to have a sensitivity of 73.8 % and a specificity of 69 % for the diagnosis of RA-ILD (AUC [95 %Cl] = 0.771 [0.679-0.862], p < 0.0001).It was found that high serum netrin-1 level was strongly associated with the RA-usual interstitial pneumonia(UIP) pattern and poorly related to the RA-nonspecific interstitial pneumonia(NSIP) pattern compared to the RA non-ILD group.

CONCLUSIONS

Netrin-1 is elevated in the serum of patients with RA-ILD, especially in the UIP pattern. Netrin-1 may be a potential candidate for predicting the development of RA-ILD that should be investigated in the pathophysiological and therapeutic fields..

Cinnamaldehyde attenuates streptozocin-induced diabetic osteoporosis in a rat model by modulating netrin-1/DCC-UNC5B signal transduction

Background: Cinnamaldehyde (CMD) is a major functional component of Cinnamomum verum and has shown treatment effects against diverse bone diseases. This study aimed to assess the anti-diabetic osteoporosis (DOP) potential of diabetes mellitus (DM) and to explore the underlying mechanism driving the activity of CMD. Methods: A DOP model was induced via an intraperitoneal injection of streptozocin (STZ) into Sprague-Dawley rats, and then two different doses of CMD were administered to the rats. The effects of CMD on the strength, remodeling activity, and histological structure of the bones were assessed. Changes in the netrin-1 related pathways also were detected to elucidate the mechanism of the anti-DOP activity by CMD. Results: CMD had no significant effect on the body weight or blood glucose level of the model rats. However, the data showed that CMD improved the bone strength and bone remodeling activity as well as attenuating the bone structure destruction in the DOP rats in a dose-dependent manner. The expression of netrin-1, DCC, UNC5B, RANKL, and OPG was suppressed, while the expression of TGF-β1, cathepsin K, TRAP, and RANK was induced by the STZ injection. CMD administration restored the expression of all of these indicators at both the mRNA and protein levels, indicating that the osteoclast activity was inhibited by CMD. Conclusion: The current study demonstrated that CMD effectively attenuated bone impairments associated with DM in a STZ-induced DOP rat model, and the anti-DOP effects of CMD were associated with the modulation of netrin-1/DCC/UNC5B signal transduction.

Impact of intestinal microenvironments in obesity and bariatric surgery on shaping macrophages

Obesity is associated with alterations in tissue composition, systemic cellular metabolism, and low-grade chronic inflammation. Macrophages are heterogenous innate immune cells ubiquitously localized throughout the body and are key components of tissue homeostasis, inflammation, wound healing, and various disease states. Macrophages are highly plastic and can switch their phenotypic polarization and change function in response to their local environments. Here, we discuss how obesity alters the intestinal microenvironment and potential key factors that can influence intestinal macrophages as well as macrophages in other organs, including adipose tissue and hematopoietic organs. As bariatric surgery can induce metabolic adaptation systemically, we discuss the potential mechanisms through which bariatric surgery reshapes macrophages in obesity.

Senso-immunology: the past, present, and future

Pain and mechanical stimulation are thought to be alarm systems that alert the brain to physical abnormalities. When we experience unpleasant feelings in infected or traumatized tissues, our awareness is directed to the afflicted region, prompting activities such as resting or licking the tissue. Despite extensive research into the molecular biology of nociceptors, it was unclear whether their role was limited to the generation and transmission of unpleasant feelings or whether they actively modulate the pathogenesis of infected or traumatized tissues. Recently, it has become clear how the sensory and immune systems interact with one another and share similar receptors and ligands to modify the pathogenesis of various diseases. In this paper, we summarize the mechanisms of crosstalk between the sensory and immune systems and the impact of this new interdisciplinary field, which should be dubbed 'senso-immunology,' on medical science.

Targeting small GTPases: emerging grasps on previously untamable targets, pioneered by KRAS

Small GTPases including Ras, Rho, Rab, Arf, and Ran are omnipresent molecular switches in regulating key cellular functions. Their dysregulation is a therapeutic target for tumors, neurodegeneration, cardiomyopathies, and infection. However, small GTPases have been historically recognized as "undruggable". Targeting KRAS, one of the most frequently mutated oncogenes, has only come into reality in the last decade due to the development of breakthrough strategies such as fragment-based screening, covalent ligands, macromolecule inhibitors, and PROTACs. Two KRAS^G12C covalent inhibitors have obtained accelerated approval for treating KRAS^G12C mutant lung cancer, and allele-specific hotspot mutations on G12D/S/R have been demonstrated as viable targets. New methods of targeting KRAS are quickly evolving, including transcription, immunogenic neoepitopes, and combinatory targeting with immunotherapy. Nevertheless, the vast majority of small GTPases and hotspot mutations remain elusive, and clinical resistance to G12C inhibitors poses new challenges. In this article, we summarize diversified biological functions, shared structural properties, and complex regulatory mechanisms of small GTPases and their relationships with human diseases. Furthermore, we review the status of drug discovery for targeting small GTPases and the most recent strategic progress focused on targeting KRAS. The discovery of new regulatory mechanisms and development of targeting approaches will together promote drug discovery for small GTPases.

Peripheral nerves in the tibial subchondral bone : the role of pain and homeostasis in osteoarthritis

Osteoarthritis (OA) is a highly prevalent degenerative joint disorder characterized by joint pain and physical disability. Aberrant subchondral bone induces pathological changes and is a major source of pain in OA. In the subchondral bone, which is highly innervated, nerves have dual roles in pain sensation and bone homeostasis regulation. The interaction between peripheral nerves and target cells in the subchondral bone, and the interplay between the sensory and sympathetic nervous systems, allow peripheral nerves to regulate subchondral bone homeostasis. Alterations in peripheral innervation and local transmitters are closely related to changes in nociception and subchondral bone homeostasis, and affect the progression of OA. Recent literature has substantially expanded our understanding of the physiological and pathological distribution and function of specific subtypes of neurones in bone. This review summarizes the types and distribution of nerves detected in the tibial subchondral bone, their cellular and molecular interactions with bone cells that regulate subchondral bone homeostasis, and their role in OA pain. A comprehensive understanding and further investigation of the functions of peripheral innervation in the subchondral bone will help to develop novel therapeutic approaches to effectively prevent OA, and alleviate OA pain.

Cite this article: Bone Joint Res 2022;11(7):439–452.

Is netrin-1 a reliable inflammatory marker for periodontitis?

Background

The current study tested netrin-1 as a reliable inflammatory marker of periodontal disease.

Methods

Gingival crevicular fluid (GCF) samples were taken at baseline from 30 systemically healthy individuals. Fifteen subjects had stage II grade A or B periodontitis, and 15 were periodontally and clinically healthy. Whole-mouth periodontal parameters [probing depth (PD), clinical attachment loss (CAL), plaque index (PI), and gingival index (GI)] were recorded. The GCF samples were re-collected, and clinical parameters were re-recorded after six weeks following full-mouth scaling and root planing (SRP) in the periodontitis group.

Results

Netrin-1 GCF levels were significantly lower in periodontitis patients than periodontally healthy individuals at baseline with a significant increase in netrin-1 GCF levels after SRP (P<0.05).

Conclusion

Netrin-1 may have a significant role in the inflammatory process of chronic periodontitis; thus, it could be a promising anti-inflammatory marker in periodontal disease.

Rho GTPase signaling in rheumatic diseases

Rho guanosine triphosphatase (GTPases), as molecular switches, have been identified to be dysregulated and involved in the pathogenesis of various rheumatic diseases, mainly including rheumatoid arthritis, osteoarthritis, systemic sclerosis, and systemic lupus erythematosus. Downstream pathways involving multiple types of cells, such as fibroblasts, chondrocytes, synoviocytes, and immunocytes are mediated by activated Rho GTPases to promote pathogenesis. Targeted therapy via inhibitors of Rho GTPases has been implicated in the treatment of rheumatic diseases, demonstrating promising effects. In this review, the effects of Rho GTPases in the pathogenesis of rheumatic diseases are summarized, and the Rho GTPase-mediated pathways are elucidated. Therapeutic strategies using Rho GTPase inhibitors in rheumatic diseases are also discussed to provide insights for further exploration of targeted therapy in preclinical studies and clinical practice. Future directions on studies of Rho GTPases in rheumatic diseases based on current understandings are provided.

Role of Slit/Robo Signaling pathway in Bone Metabolism

Slit/Robo signals were initially found to play an essential role in nerve development as axonal guidance molecules. In recent years, with in-depth study, the role of Slit/Robo in other life activities, such as tumor development, angiogenesis, cell migration, and bone homeostasis, has gradually been revealed. Bone is an organ with an active metabolism. Bone resorption and bone formation are closely related through precise spatiotemporal coordination. There is much evidence that slit, as a new bone coupling factor, can regulate bone formation and resorption. For example, Slit3 can promote bone formation and inhibit bone resorption through Robo receptors, which has excellent therapeutic potential in metabolic bone diseases. Although the conclusions of some studies are contradictory, they all affirm the vital role of Slit/Robo signaling in regulating bone metabolism. This paper reviews the research progress of Slit/Robo signaling in bone metabolism, briefly discusses the contradictions in the existing research, and puts forward the research direction of Slit/Robo in the field of bone metabolism in the future.

Netrin-1: A Modulator of Macrophage Driven Acute and Chronic Inflammation

Netrins belong to the family of laminin-like secreted proteins, which guide axonal migration and neuronal growth in the developing central nervous system. Over the last 20 years, it has been established that netrin-1 acts as a chemoattractive or chemorepulsive cue in diverse biological processes far beyond neuronal development. Netrin-1 has been shown to play a central role in cell adhesion, cell migration, proliferation, and cell survival in neuronal and non-neuronal tissue. In this context, netrin-1 was found to orchestrate organogenesis, angiogenesis, tumorigenesis, and inflammation. In inflammation, as in neuronal development, netrin-1 plays a dichotomous role directing the migration of leukocytes, especially monocytes in the inflamed tissue. Monocyte-derived macrophages have long been known for a similar dual role in inflammation. In response to pathogen-induced acute injury, monocytes are rapidly recruited to damaged tissue as the first line of immune defense to phagocyte pathogens, present antigens to initiate the adaptive immune response, and promote wound healing in the resolution phase. On the other hand, dysregulated macrophages with impaired phagocytosis and egress capacity accumulate in chronic inflammation sites and foster the maintenance-and even the progression-of chronic inflammation. In this review article, we will highlight the dichotomous roles of netrin-1 and its impact on acute and chronic inflammation.

The expression of Netrin-1 in the MIA-induced osteoarthritic temporomandibular joint in mice

Subchondral bone degeneration is the main pathological change during temporomandibular joint (TMJ) osteoarthritis (OA) development. Netrin-1, an axon-guiding factor, might play roles in OA development and pain. The purpose of this study was to investigate the expression of Netrin-1 in TMJ OA and its possible role in the progression of TMJ OA and pain. The synovial fluids of temporomandibular joint disorders (TMDs) patients were collected for Netrin-1 by enzyme linked immunosorbent assay (ELISA). TMJ OA model was built by MIA joint injection, and then the von Frey test, hematoxylin & eosin (H&E) staining, toluidine blue (TB) staining, immunohistochemical (IHC) staining and micro-CT were performed. After induction of osteoclast differentiation of raw264.7 cells, immunofluorescence (IF) was used to detect the Netrin-1 and its receptors on osteoclast membrane. The concentration of Netrin-1 increased in the synovial fluid of TMJ OA patients. After MIA injection to TMJ, the head withdrawal threshold (HWT) was significantly decreased. Microscopically, the structural disorder of subchondral bone was the most obvious at the 2nd week after MIA injection. In addition, Netrin-1 expression increased in the subchondral bone at the 2nd week after MIA injection. In vitro, the expressions of Netrin-1 and its receptor Unc5B were upregulated on the osteoclast membrane. Netrin-1 might be an important regulator during bone degeneration and pain in the process of TMJ OA.

Late Protective Effect of Netrin-1 in the Murine Acetaminophen Hepatotoxicity Model

Acetaminophen (APAP) overdose-induced acute liver failure is an important clinical problem in the United States and the current antidote N-acetylcysteine, has a short early therapeutic window. Since most patients present late to the clinic, there is need for novel late-acting therapeutic options. Though the neuronal guidance cue netrin-1, has been shown to promote hepatic repair and regeneration during liver ischemia/reperfusion injury, its effect in APAP-induced hepatotoxicity is unknown. In the quest for a late-acting therapeutic intervention in APAP-induced liver injury, we examined the role of netrin-1 in a mouse model of APAP overdose. Male C57BL/6J mice were cotreated with exogenous netrin-1 or vehicle control, along with 300 mg/kg APAP and euthanized at 6, 12, and 24 h. Significant elevations in alanine aminotransferase indicative of liver injury were seen in control mice at 6 h and this was not affected by netrin-1 administration. Also, netrin-1 treatment did not influence mitochondrial translocation of phospho-JNK, or peroxynitrite formation indicating that there was no interference with APAP-induced injury processes. Interestingly however, netrin-1 administration attenuated liver injury at 24 h, as seen by alanine aminotransferase levels and histology, at which time significant elevations in the netrin-1 receptor, adenosine A2B receptor (A2BAR) as well as macrophage infiltration was evident. Removal of resident macrophages with clodronate liposomes or treatment with the A2BAR antagonist PSB1115 blocked the protective effects of netrin-1. Thus, our data indicate a previously unrecognized role for netrin-1 in attenuation of APAP hepatotoxicity by enhancing recovery and regeneration, which is mediated through the A2BAR and involves resident liver macrophages.

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.

Osteoclast Multinucleation: Review of Current Literature

Multinucleation is a hallmark of osteoclast maturation. The unique and dynamic multinucleation process not only increases cell size but causes functional alterations through reconstruction of the cytoskeleton, creating the actin ring and ruffled border that enable bone resorption. Our understanding of the molecular mechanisms underlying osteoclast multinucleation has advanced considerably in this century, especially since the identification of DC-STAMP and OC-STAMP as “master fusogens”. Regarding the molecules and pathways surrounding these STAMPs, however, only limited progress has been made due to the absence of their ligands. Various molecules and mechanisms other than the STAMPs are involved in osteoclast multinucleation. In addition, several preclinical studies have explored chemicals that may be able to target osteoclast multinucleation, which could enable us to control pathogenic bone metabolism more precisely. In this review, we will focus on recent discoveries regarding the STAMPs and other molecules involved in osteoclast multinucleation.

Roles of Slit Ligands and Their Roundabout (Robo) Family of Receptors in Bone Remodeling

Slit guidance ligands (Slits) and their roundabout (Robo) family of receptors are well-known axon guidance molecules that were originally identified in Drosophila mutants with commissural axon pathfinding defects. However, Slit-Robo signaling has been shown to be involved in not only neurogenesis, but also the development of other organs such as the kidney and heart. Recently, it was also revealed that Slit-Robo signaling plays an important role in bone metabolism. For example, osteoclast-derived Slit3 plays an osteoprotective role by synchronously stimulating bone formation by osteoblasts and suppressing bone resorption by osteoclasts through Robo receptors expressed on osteoblastic and osteoclastic cell lineages, making it a potential therapeutic target for metabolic bone disorders. Furthermore, osteoblast-derived Slit3 promotes bone formation indirectly as a proangiogenic factor. This review summarizes the recent progress on defining the roles of the Slit-Robo signaling in bone metabolism, and discusses the possible roles of the interaction between Robo and neural epidermal growth factor-like (NEL)-like (NELL) proteins that are novel ligands for Robo receptors.

Netrin-1 and its receptor Unc5b as markers of periodontal disease

OBJECTIVES

The aim of this present study was to evaluate the levels of Netrin-1 and Unc5b in periodontal health and disease.

BACKGROUND

Netrin-1, acting via its receptor UNC5b, regulates the inflammatory response and takes apart in bone destructive diseases.

METHODS

Samples of gingival crevicular fluid (GCF), whole saliva, and serum were taken from systemically healthy, nonsmoking 20 periodontitis , 20 gingivitis patients, and 20 periodontally healthy subjects at baseline and 4 weeks after nonsurgical periodontal treatment (NSPT). Whole-mouth and site-specific clinical periodontal parameters were recorded. Netrin-1 and Unc5b levels were measured by enzyme-linked immunosorbent assay. Data were analyzed by nonparametric tests.

RESULTS

Total amount of Netrin-1 in GCF was significantly higher in periodontitis than the others, and the levels were significantly reduced after NSPT. Salivary and serum concentrations of Netrin-1 were significantly different among the study groups (P = .000), and NSPT significantly increased the concentration levels of both salivary and serum Netrin-1 (P < .05). Healthy subjects had significantly lower GCF (P = .001) and conversely, higher salivary and serum levels of Unc5b than the other groups (P = .002). The GCF levels of Unc5b were significantly reduced (P < .01), and conversely, serum concentrations were significantly increased after NSPT (P < .05). GCF Netrin-1 and Unc5b total amounts were positively correlated with clinical parameters (P < .01 and P < .05) whereas salivary Netrin-1 and Unc5b concentrations were negatively correlated with clinical parameters (P < .01 and P < .05).

CONCLUSIONS

The results of this study indicate that Netrin-1 and its receptor Unc5b may have essential roles in periodontal inflammation and those can be assumed as useful therapeutic agent to control inflammation and periodontal breakdown.

Selective patterning of netrin-1 as a novel guiding cue for anisotropic dendrogenesis in osteocytes

Although protein patterning approaches have found widespread applications in tuning surface characteristics of biomaterials, selective control of growth in cell body and dendrites utilizing such platforms remains difficult. The functional roles assumed by cell body and dendrites in a physiological milieu have extremely high specificity. In particular, osteocytes embedded inside the mineralized bone matrix are interconnected via dendritic cell processes characterized by an anisotropic arrangement of the lacunar-canalicular system, where the fluid-flow inside the canaliculi system regulates the mechanoresponsive functionalization of bone. Control of cellular networks connected by dendritic cell processes is, therefore, imperative for constructing artificially controlled bone-mimetic structures and as an extension, for gaining insights into the molecular mechanisms underlying dendrogenesis inside the mineralized bone matrix. Here, we report an innovative strategy to induce controlled elongation of cell body or dendritic process structures in selective directions by using the inkjet printing technique. Artificial runways employing netrin-1, inspired by neural architecture, were utilized to trigger controlled elongation in the osteocyte dendritic processes in desired directions. This is the first report, to the best of our knowledge, demonstrating that anisotropic dendrogenesis of osteocytes can be controlled with selective patterning of extracellular proteins, specifically via the axon guidance ligand netrin-1.

Flrt2 is involved in fine-tuning of osteoclast multinucleation

Osteoclasts are multinucleated giant cells derived from myeloid progenitors. Excessive bone resorption by osteoclasts can result in serious clinical outcomes for which better treatment options are needed. Here, we identified fibronectin leucine-rich transmembrane protein 2 (Flrt2), a ligand of the Unc5 receptor family for neurons, as a novel target associated with the late/maturation stage of osteoclast differentiation. Flrt2 expression is induced by stimulation with receptor activator of nuclear factor-kB ligand (RANKL). Flrt2 deficiency in osteoclasts results in reduced hyper-multinucleation, which could be restored by RNAi-mediated knockdown of Unc5b. Treatment with Netrin1, another ligand of Unc5b which negatively controls osteoclast multinucleation through down regulation of RANKL-induced Rac1 activation, showed no inhibitory effects on Flrt2-deficient cells. In addition, RANKL-induced Rac1 activation was attenuated in Flrt2-deficient cells. Taken together, these results suggest that Flrt2 regulates osteoclast multinucleation by interfering with Netrin 1-Unc5b interaction and may be a suitable therapeutic target for diseases associated with bone remodeling.

Subchondral bone osteoclasts induce sensory innervation and osteoarthritis pain

Joint pain is the defining symptom of osteoarthritis (OA) but its origin and mechanisms remain unclear. Here, we investigated an unprecedented role of osteoclast-initiated subchondral bone remodeling in sensory innervation for OA pain. We show that osteoclasts secrete netrin-1 to induce sensory nerve axonal growth in subchondral bone. Reduction of osteoclast formation by knockout of receptor activator of nuclear factor kappa-B ligand (Rankl) in osteocytes inhibited the growth of sensory nerves into subchondral bone, dorsal root ganglion neuron hyperexcitability, and behavioral measures of pain hypersensitivity in OA mice. Moreover, we demonstrated a possible role for netrin-1 secreted by osteoclasts during aberrant subchondral bone remodeling in inducing sensory innervation and OA pain through its receptor DCC (deleted in colorectal cancer). Importantly, knockout of Netrin1 in tartrate-resistant acid phosphatase-positive (TRAP-positive) osteoclasts or knockdown of Dcc reduces OA pain behavior. In particular, inhibition of osteoclast activity by alendronate modifies aberrant subchondral bone remodeling and reduces innervation and pain behavior at the early stage of OA. These results suggest that intervention of the axonal guidance molecules (e.g., netrin-1) derived from aberrant subchondral bone remodeling may have therapeutic potential for OA pain.

Netrin-4 Promotes Differentiation and Migration of Osteoblasts

BACKGROUND/AIM

While netrin-4 plays a vital role in the vascular system, the role of netrin-1 in osteoblast differentiation is not well understood. In this study we explored whether netrin-4 has functional roles in osteoblasts.

MATERIALS AND METHODS

Quantitative reverse-transcriptase polymerase chain reaction (PCR), RNA interference, the generation of plasmids, transfections, measurement of alkaline phosphatase activity, a mineralization assay, a migration assay and a cell proliferation assay were performed.

RESULTS

Netrin-4 expression was up-regulated during osteoblast differentiation and an RNA interference experiment showed that small interfering RNA used to silence netrin-4 inhibited osteoblast differentiation. Recombinant mouse netrin-4 promoted alkaline phosphatase (ALP) activity of osteoblasts and enhancement of calcium deposits. Moreover, we constructed a vector containing the netrin-4 gene on the basis of the plasmid pcDNA3.1/V5-His. Overexpression of netrin-4 enhanced differentiation of osteoblasts. Finally, recombinant mouse netrin-4 promoted cell migration of osteoblasts.

CONCLUSION

Netrin-4 promotes differentiation and migration of osteoblasts.

Functional Roles of Netrin-1 in Osteoblast Differentiation

AIM

Recent studies have demonstrated that netrin-1 plays a vital role in bone metabolism. Previous studies have shown that osteoblasts produce netrin-1 which affects osteoclast differentiation. However, the role of netrin-1 in osteoblast differentiation is not well understood. In this study, we explored the roles of netrin-1 in osteoblasts.

MATERIALS AND METHODS

Quantitative reverse-transcriptase polymerase chain reaction (qPCR), RNA interference for netrin receptors, the generation of netrin-1 plasmid, transfection of plasmids, and cell proliferation assay were performed.

RESULTS

During osteoblast differentiation by ascorbic acid, netrin-1 expression was significantly decreased. Gene expression related with osteoblast differentiation was down-regulated by netrin-1 treatment. We also found that osteoblast differentiation by bone morphogenetic protein-4 (BMP-4) was inhibited in the presence of recombinant netrin-1. Forced expression of both BMP-4 and netrin-1 significantly decreased alkaline phosphatase expression. On the other hand, Unc5b, neogenin, and A2b which belong to netrin receptors were expressed by osteoblasts. Moreover, alkaline phosphatase expression was significantly decreased by knockdown for the combination of two receptors among these receptors.

CONCLUSION

Netrin-1 is involved in the regulation of osteoblast differentiation.

Pulsed electromagnetic fields inhibit human osteoclast formation and gene expression via osteoblasts

Pulsed electromagnetic fields (PEMFs) can be effective in promoting the healing of delayed union or nonunion fractures. We previously reported that PEMF (Spinal-Stim® by Orthofix, Inc., Lewisville, TX) stimulated proliferation, differentiation and mineralization of rat calvarial osteoblastic cells in culture. In the present work we investigated the effects of PEMF (Physio-Stim® by Orthofix, Inc., Lewisville, TX) on human bone marrow macrophages (hBMMs) differentiated to osteoclasts. PEMF had striking inhibitory effects on formation of osteoclasts from hBMMs from both younger and older women. There were significantly greater changes in gene expression as ascertained by RNAseq from cells from older women. Interestingly, all of the genes identified by RNAseq were upregulated, and all were genes of mesenchymal or osteoblastic cells and included members of the TGF-β signaling pathway and many extracellular matrix proteins, as well as RANKL and osteoprotegerin, indicating the mixed nature of these cultures. From these results, we suggest that PEMF can inhibit osteoclast formation via action on osteoblasts. Thus, PEMF may be very effective for bone mass maintenance in subjects with osteoporosis.

Netrins as prophylactic targets in skeletal diseases: A double-edged sword?

The netrin family of proteins are involved in axon guidance during central nervous system development. In vertebrates, two membrane bound forms and five secreted forms of netrin have been reported. In addition to their critical role in neural morphogenesis, a growing number of reports suggest that netrin family proteins also play a role in inflammatory conditions, angiogenesis, and tumorigenesis. In these processes, Unc5 and DCC family proteins serve as receptors of netrin proteins. Recently, it was reported that some netrin family proteins may be involved in the pathogenesis of skeletal diseases including osteoporosis and arthritis. For example, administration of secreted netrin family proteins such as netrin 1 and netrin 4 has prophylactic potential in pathogenic bone degradation in mice. However, netrin 1 blocking antibody also protects mice from inflammatory bone destruction. Therefore, netrin family proteins are involved in the regulation of bone homeostasis, but their bona fide roles in the skeletal system remain controversial. In this review, we discuss the osteo-innate-immune functions of the netrin family of proteins, and summarize their therapeutic potential.

Low-intensity pulsed ultrasound (LIPUS) prevents periprosthetic inflammatory loosening through FBXL2-TRAF6 ubiquitination pathway

Previous studies have shown that Low intensity pulsed ultrasound(LIPUS) prevents polyethylene-debris-induced periprosthetic loosening in vivo, but the details of the mechanism by which it does so remain unclear. In this article, we used polyethylene debris induced RAW 264.7 cells as the in vitro model, and tested the effect of LIPUS on this model. Changes in the level of inflammatory cytokines, cell proliferation, and apoptosis were assessed. Gene overexpression and siRNA technique were applied, and the levels of expression of FBXL2, TRAF6, ERK, and related inflammatory cytokines were also measured. Results indicated that FBXL2-mediated TRAF6 ubiquitination and degradation also plays an important role in aseptic periprosthetic loosening process, and LIPUS prevents such loosening by strengthening this pathway.

Netrin-1 acts as a non-canonical angiogenic factor produced by human Wharton's jelly mesenchymal stem cells (WJ-MSC)

BACKGROUND

Angiogenesis, the process in which new blood vessels are formed from preexisting ones, is highly dependent on the presence of classical angiogenic factors. Recent evidence suggests that axonal guidance proteins and their receptors can also act as angiogenic regulators. Netrin, a family of laminin-like proteins, specifically Netrin-1 and 4, act via DCC/Neogenin-1 and UNC5 class of receptors to promote or inhibit angiogenesis, depending on the physiological context.

METHODS

Mesenchymal stem cells secrete a broad set of classical angiogenic factors. However, little is known about the expression of non-canonical angiogenic factors such as Netrin-1. The aim was to characterize the possible secretion of Netrin ligands by Wharton's jelly-derived mesenchymal stem cells (WJ-MSC). We evaluated if Netrin-1 presence in the conditioned media from these cells was capable of inducing angiogenesis both in vitro and in vivo, using human umbilical vein endothelial cells (HUVEC) and chicken chorioallantoic membrane (CAM), respectively. In addition, we investigated if the RhoA/ROCK pathway is responsible for the integration of Netrin signaling to control vessel formation.

RESULTS

The paracrine angiogenic effect of the WJ-MSC-conditioned media is mediated at least in part by Netrin-1 given that pharmacological blockage of Netrin-1 in WJ-MSC resulted in diminished angiogenesis on HUVEC. When HUVEC were stimulated with exogenous Netrin-1 assayed at physiological concentrations (10-200 ng/mL), endothelial vascular migration occurred in a concentration-dependent manner. In line with our determination of Netrin-1 present in WJ-MSC-conditioned media we were able to obtain endothelial tubule formation even in the pg/mL range. Through CAM assays we validated that WJ-MSC-secreted Netrin-1 promotes an increased angiogenesis in vivo. Netrin-1, secreted by WJ-MSC, might mediate its angiogenic effect through specific cell surface receptors on the endothelium, such as UNC5b and/or integrin α6β1, expressed in HUVEC. However, the angiogenic response of Netrin-1 seems not to be mediated through the RhoA/ROCK pathway.

CONCLUSIONS

Thus, here we show that stromal production of Netrin-1 is a critical component of the vascular regulatory machinery. This signaling event may have deep implications in the modulation of several processes related to a number of diseases where angiogenesis plays a key role in vascular homeostasis.