Netrin-1 induces proliferation of Schwann cells through Unc5b receptor

Fabrication of GDNF-Gel/HA-Mg nerve conduit and its role in repairing peripheral nerve defects

Background

Magnesium (Mg) and its alloys are receiving increasing attention in peripheral nerve regeneration, but they were limited due to the low corrosion resistance and rapid degradation. In this study, GDNF-Gel/HA-Mg was prepared and its value in peripheral nerve defects repairment was explored both in vitro and in vivo.

Methods

A hydroxyapatite (HA) coating was first applied to the pure Mg surface, followed by the formation of gelatin methacrylate (GelMA) loaded with glial cell-derived neurotrophic factor (GDNF) on the HA-coated Mg surface. GDNF-Gel/HA-Mg corrosion resistance was explored. The effect of GDNF-Gel/HA-Mg conduit on Schwann cell proliferation and migration abilities were investigated. And sciatic nerve defects models were established to explored the role of GDNF-Gel/HA-Mg conduit in peripheral nerve defects repairment.

Findings

The electrochemical, immersion, and hydrogen evolution experiments indicated that the corrosion resistance in phosphate buffer saline (PBS) of pure Mg was significantly improved by the GDNF-Gel/HA coating. Cell cycle, Cell Count Kit-8 (CCK-8), and clone formation assays indicated that GDNF-Gel/HA-Mg promoted the proliferation of Schwann cells. Scratch and Transwell assay results demonstrated that GDNF-Gel/HA-Mg promoted Schwann cell migration ability dose-dependently. GDNF-Gel/HA-Mg was found to enhance the secretion of nerve growth factor (NGF) and the expression of p75NTR. Flow cytometry results showed that GDNF-Gel/HA-Mg could reduce H2O2-induced oxidative stress and Schwann cell apoptosis. GDNF-Gel/HA-Mg inhibited M1 macrophage polarization while facilitated M2 macrophage polarization in a concentration-dependent manner. The in vivo studies demonstrated that GDNF-Gel/HA-Mg conduit could significantly promote the regeneration and myelination of sciatic nerve, as well as the recovery of denervated gastrocnemius atrophy.

Interpretation

The GDNF-Gel/HA-Mg conduit prepared in this study exhibited good hydrophilicity and corrosion resistance and greatly enhanced the proliferation, migration, and invasion abilities of Schwann cells, as well as peripheral nerve regeneration.

NTN-1 attenuates amyloid-β-mediated microglial neuroinflammation and memory impairment via the NF-κB pathway and NLRP3 inflammasome in a rat model of Alzheimer's disease

Introduction

Neuroinflammation driven by microglial activation represents a pivotal pathological mechanism underlying brain injury in Alzheimer's disease (AD), with NLRP3 inflammasome activation being a hallmark feature of this process. Netrin-1 (NTN-1) was recently shown to have potent anti-inflammatory and anti-apoptotic properties in a range of inflammatory diseases; however, its potential effect on neuroinflammation in AD treatment has not been well examined. Accordingly, this study aimed to investigate the effects of NTN-1 on cognitive impairment and to explore the anti-inflammatory properties related to the NLRP3 inflammasome and NF-κB signaling in Aβ1-42-induced rat models.

Methods

We assessed the effects of NTN-1 on neurobehavioral function, microglial activation and neuroinflammation mechanisms in Aβ1-42-treated rats using the Morris water maze test and Western blotting.

Results

Our results indicated that microinjections of NTN-1 attenuated Aβ1-42-induced memory and cognitive dysfunction and significantly inhibited microglial proliferation and NLRP3 inflammasome activation in the hippocampus and cortex of AD rats. Additionally, NTN-1 effectively prevented proinflammatory factor (IL1β and IL18) release and NF-κB signaling upstream activation.

Discussion

Overall, the results of the present study indicated that exogenous NTN-1 treatment prevented neuroinflammation and cognitive deficits by inhibiting microglial activation, which is possibly mediated by the NF-κB signaling pathway and NLRP3 inflammasome activation in Aβ1-42-simulated rat models. NTN-1 emerges as a promising therapeutic candidate for mitigating microglia-mediated neuropathology in AD through its anti-inflammatory properties.

The phenotypic changes of Schwann cells promote the functional repair of nerve injury

Functional recovery after nerve injury is a significant challenge due to the complex nature of nerve injury repair and the non-regeneration of neurons. Schwann cells (SCs), play a crucial role in the nerve injury repair process because of their high plasticity, secretion, and migration abilities. Upon nerve injury, SCs undergo a phenotypic change and redifferentiate into a repair phenotype, which helps in healing by recruiting phagocytes, removing myelin fragments, promoting axon regeneration, and facilitating myelin formation. However, the repair phenotype can be unstable, limiting the effectiveness of the repair. Recent research has found that transplantation of SCs can be an effective treatment option, therefore, it is essential to comprehend the phenotypic changes of SCs and clarify the related mechanisms to develop the transplantation therapy further.

Exosomes derived from human umbilical cord mesenchymal stem cells loaded with RVG-Lamp2b and Netrin-1 promotes Schwann cell invasion and migration

BACKGROUND

Netrin-1 has a neuroprotective effect by regulating angiogenesis, autophagy, apoptosis, and neuroinflammation. This study investigated the effects of netrin-1 delivery to mouse Schwann cells and vascular endothelial cells using exosomes modified with rabies virus glycoprotein (RVG) peptides.

MATERIALS AND METHODS

RVG-Lamp2b and/or Netrin-1 were overexpressed in human umbilical cord mesenchymal stem cells to obtain exosomes modified with RVG-Lamp2b and/or loaded with Netrin-1. Then, exosomes were labeled with carboxyfluorescein diacetate succinimidyl ester and co-cultured with mouse Schwann cells and endothelial cells. Netrin-1 expression in Schwann cells and endothelial cells was measured using quantitative polymerase chain reaction and immunoblotting. Moreover, methyl thiazolyl tetrazolium assays and Transwell assays were used to detect proliferation, migration, and invasion of Schwann cells and endothelial cells.

RESULTS

Exosomes with RVG-Lamp2b entered Schwann cells more readily compared with the exosomes without RVG-Lamp2b. Meanwhile, this was not the case in endothelial cells. Netrin-1-loaded exosomes significantly promoted Netrin-1 expression, cell proliferation, migration, invasion, and epithelial-mesenchymal transition in Schwann cells and endothelial cells. These effects were further enhanced by Netrin-1-loaded exosomes modified with RVG-Lamp2b in Schwann cells, but not in endothelial cells.

CONCLUSION

HucMSC-derived exosomes loaded with RVG-Lamp2b and Netrin-1 promote proliferation, migration, and invasion of Schwann cells.

Magnetic resonance imaging assessment of the therapeutic effect of combined electroacupuncture and stem cells in acute peripheral nerve injury

Objectives: This study aimed to evaluate the therapeutic effect of a combination of Bone Mesenchymal stem cells (BMSCs) transplantation and Electroacupuncture (EA) for acute sciatic nerve injury in rats using magnetic resonance. Methods: Ninety-two male adult healthy Sprague-Dawley rats were randomly divided into the EA+BMSCs group, EA group, MSCs group, and PBS group (control). Electroacupuncture was performed on a rat receiving EA treatment at Huantiao (GB30) and Zusanli (ST36). T2 values and diffusion tensor imaging (DTI) derived from multiparametric magnetic resonance imaging (MRI), histological assessments, and immunohistochemistry was used to monitor nerve regeneration. Walking track analysis was used to assess nerve functional recovery. Repeated-measures one-way analysis of variance was used to evaluate the significance of T2, DTI, and SFI values among the four groups. One-way analysis of variance was used for comparing the histological characteristics. Bonferroni test was used for multiple pairwise comparisons at each time point. Results: In terms of FA, the EA+BMSCs and EA groups had faster recovery than PBS (control) in all time points after surgery, and the EA+BMSCs group recovered better than the BMSCs group at 3 weeks (P ≤ 0.008). FA values were higher in the EA group than in the BMSCs group at 4 weeks (P ≤ 0.008). In terms of RD, the EA+BMSCs group recovered better than the BMSCs group at 2 and 4 weeks (P ≤ 0.008). Immunofluorescence staining for axon guidance molecule netrin-1 revealed that it was significantly higher in the EA+BMSCs subgroup and EA subgroup than it was in the control (PBS) subgroup at 1-3 weeks (P < 0.001). Immunofluorescence staining for S100 showed the continuity of nerve fibers recovered more quickly in the EA+BMSCs subgroup than in the BMSCs subgroup. Conclusion: Our research revealed that a combination of MSCs and EA can provide both topological and biomolecular guidance to promote axonal extension, myelin regeneration, and functional recovery after PNI. EA not only promotes nerve repair on its own, but also enhanced the beneficial effects of stem cell treatment and the secretion of netrin 1, a guidance regeneration factor, and promotes the orderly growth of nerve fibers. These PNI repairs could be monitored non-invasively and in situ by MRI. The FA and RD values derived from MRI could be sensitive biomarkers to reflect the PNI repair process.

Relationship between circulating netrin-1 levels, obesity, prediabetes and newly diagnosed type 2 diabetes

BACKGROUND

Netrin-1 is presumed to have regenerative, angiogenic and anti-inflammatory properties, thus it could play a substantial role in the development of insulin resistance and T2DM.

OBJECTIVE

The aim of this study was to evaluate the relationship between serum netrin-1 levels and carbohydrate disturbances in patients with obesity.

METHODS

Sample size consisted of 163 patients, divided into four groups: obesity without carbohydrate disturbances prediabetes and diabetes and healthy controls Netrin-1 level was determined using ELISA method.

RESULTS

Circulating serum Netrin-1 was significantly lower in patients only with obesity, as well as with those with prediabetes and diabetes in comparison to the control group. Correlation analysis revealed that netrin-1 correlates negatively with BMI, waist, WSR, LDL and positive with sudomotor function. Netrin-1 ≤ 0.17 ng/ml has about 3 fold higher risk for carbohydrate disturbances (OR 3.06, 95% CI 1.48-6.34, p = .003).

CONCLUSION

Netrin-1 is associated with an increased risk for glycaemic disorders in patients with obesity.

UNC5B Overexpression Alleviates Peripheral Neuropathic Pain by Stimulating Netrin-1-Dependent Autophagic Flux in Schwann Cells

Lesions or diseases of the somatosensory system can cause neuropathic pain (NP). Schwann cell (SC) autophagy plays an important role in NP. Uncoordinated gene 5 homolog B (UNC5B), the canonical dependent receptor of netrin-1, is known to be exclusively expressed in SCs and involved in NP; however, the underlying mechanisms were unclear. A rat model of sciatic nerve chronic constriction injury (CCI) was used to induce peripheral neuropathic pain. Adeno-associated virus (AAV) overexpressing UNC5B was applied to the injured nerve, and an autophagy inhibitor, 3-mechyladenine (3-MA), was intraperitoneally injected in some animals. Behavioral tests were performed to evaluate NP, the morphology of the injured nerves was analyzed, and autophagy-related proteins were detected. A rat SC line (RSC96) undergoing oxygen and glucose deprivation (OGD) was used to mimic an ischemic setting to examine the role of UNC5B in autophagy. Local UNC5B overexpression alleviated CCI-induced NP and rescued myelin degeneration. Meanwhile, UNC5B overexpression improved CCI-induced impairment of autophagic flux, while the autophagy inhibitor 3-MA reversed the analgesic effect of UNC5B. In cultured SCs, UNC5B helped recruit netrin-1 to the cell membrane. UNC5B overexpression promoted autophagic flux while inhibiting apoptosis, which was further augmented with exogenous netrin-1 and reversed by netrin-1 knockdown. The enhanced phosphorylation of AMP-activated protein kinase (AMPK) and Unc51-like autophagy activating kinase 1 (ULK1) by UNC5B overexpression was also correlated with netrin-1. Our results suggest that UNC5B facilitates autophagic flux in SCs via phosphorylation of AMPK and ULK1, dependent on its ligand netrin-1, protecting myelin and partly preventing injury-induced NP.

Netrin-1 as a Multitarget Barrier Stabilizer in the Peripheral Nerve after Injury

The blood–nerve barrier and myelin barrier normally shield peripheral nerves from potentially harmful insults. They are broken down during nerve injury, which contributes to neuronal damage. Netrin-1 is a neuronal guidance protein with various established functions in the peripheral and central nervous systems; however, its role in regulating barrier integrity and pain processing after nerve injury is poorly understood. Here, we show that chronic constriction injury (CCI) in Wistar rats reduced netrin-1 protein and the netrin-1 receptor neogenin-1 (Neo1) in the sciatic nerve. Replacement of netrin-1 via systemic or local administration of the recombinant protein rescued injury-induced nociceptive hypersensitivity. This was prevented by siRNA-mediated knockdown of Neo1 in the sciatic nerve. Mechanistically, netrin-1 restored endothelial and myelin, but not perineural, barrier function as measured by fluorescent dye or fibrinogen penetration. Netrin-1 also reversed the decline in the tight junction proteins claudin-5 and claudin-19 in the sciatic nerve caused by CCI. Our findings emphasize the role of the endothelial and myelin barriers in pain processing after nerve damage and reveal that exogenous netrin-1 restores their function to mitigate CCI-induced hypersensitivity via Neo1. The netrin-1-neogenin-1 signaling pathway may thus represent a multi-target barrier protector for the treatment of neuropathic pain.

Differential impact of keratinocytes and fibroblasts on nociceptor degeneration and sensitization in small fiber neuropathy

ABSTRACT

Peripheral denervation and pain are hallmarks of small fiber neuropathy (SFN). We investigated the contribution of skin cells on nociceptor degeneration and sensitization. We recruited 56 patients with SFN and 31 healthy controls and collected skin punch biopsies for immunohistochemical and immunocytochemical analysis of netrin-1 (NTN1) and proinflammatory and anti-inflammatory cytokine expression patterns. We further applied coculture systems with murine dorsal root ganglion (DRG) neurons for skin cell-nerve interaction studies and patch-clamp analysis. Human keratinocytes attract murine DRG neuron neurites, and the gene expression of the axon guidance cue NTN1 is higher in keratinocytes of patients with SFN than in controls. NTN1 slows and reduces murine sensory neurite outgrowth in vitro, but does not alter keratinocyte cytokine expression. In the naive state, keratinocytes of patients with SFN show a higher expression of transforming growth factor-β1 (P < 0.05), while fibroblasts display higher expression of the algesic cytokines interleukin (IL)-6 (P < 0.01) and IL-8 (P < 0.05). IL-6 incubation of murine DRG neurons leads to an increase in action potential firing rates compared with baseline (P < 0.01). Our data provide evidence for a differential effect of keratinocytes and fibroblasts on nociceptor degeneration and sensitization in SFN compared with healthy controls and further supports the concept of cutaneous nociception.

Local uncoordinated gene 5H2 contributes to nerve injury-induced mechanical allodynia associated to its role in autophagy

Lesions of the peripheral nerves can lead to lifelong neuropathic pain (NP). Autophagic deficiency in the Schwann cells (SCs) is an early event in the origin of NP chronification. Uncoordinated gene 5H2 (UNC5H2), one of the repulsive netrin receptors, mediated the effect of netrin-1 on autophagic activation and cell survival in endothelial cells. However, its role on autophagy regulation in peripheral nerves during NP process remains unidentified. Chronic constriction injury (CCI) of the left sciatic nerve was induced in Sprague-Dawley rats, and UNC5H2 small interfering RNA was transfected to the ipsilateral sciatic nerve immediately after injury. Mechanical allodynia was assessed. Sciatic UNC5H2 and netrin-1 protein levels were investigated. Autophagy in the ipsilateral sciatic nerves was evaluated by detecting punctate light chain 3(LC3) and autophagosomes, as well as the levels of LC3 II, p62 and phosphorylated UNC51-like kinase (ULK1). After CCI, UNC5H2 of the sciatic nerves was upregulated, exclusively expressed in SCs. Small interfering RNA transfection resulted in significant decrease of UNC5H2 and netrin-1 protein, leading to exaggeration of mechanical allodynia through 14 days after CCI. Autophagy was activated but autophagic influx was interfered within a week after CCI, shown by the elevated levels of both LC3II and p62, which was further deteriorated with UNC5H2 knockdown. In addition, the injury-induced augmentation of phosphorylated ULK1 was significantly diminished by UNC5H2 knockdown. Altogether, the results suggest that local UNC5H2 of the peripheral nerve plays a significant role in the process of injury-induced mechanical allodynia, probably associated to its contribution to autophagic regulation.

Macrophage-derived netrin-1 contributes to endometriosis-associated pain

Background

Endometriosis-associated pain can be considered a type of neuropathic pain. Netrin-1 is an axon guidance cue that regulates axonal attraction or rejection in neural injury and regeneration. However, whether netrin-1 plays a role in endometriosis-associated pain remains unclear. This study aimed to determine the role of netrin-1 in endometriosis-related pain.

Methods

Peripheral blood, peritoneal fluid, and endometrial tissues were sampled from women with (n=37) and without endometriosis (n=23). Lipopolysaccharide (LPS) and interferon gamma (IFN-γ) were used to stimulate human monocytic cell lines (THP-1) and rat alveolar macrophage-derived cell lines (NR8383) to induce M1 phenotype macrophages. Serum netrin-1 concentrations, endometrial expression levels of netrin-1, and its receptors including deleted in colorectal cancer (DCC), A2B adenosine receptor (A2BAR), uncoordinated B receptor (UNC5B), uncoordinated C receptor (UNC5C) and Down’s syndrome cell adhesion molecule (DSCAM) were assessed. The polarization phenotypes of the peritoneal macrophages were identified by detecting the marker expression of M1/M2 macrophages via flow cytometry. The expression levels of M1 markers and netrin-1 in THP-1/NR8383 cells were determined.

Results

The expression levels of netrin-1 in serum and endometriotic lesions were significantly higher in women with endometriosis, and were positively correlated with the severity of endometriosis-associated pain. Netrin-1 was co-expressed with CD68 (a macrophage marker) in endometriotic lesions and was synthesized and secreted by THP-1 and NR8383 cells in the process of M1 polarization. In women with endometriosis, peritoneal macrophages were polarized towards the M1 phenotype. In addition, increased expression of DCC and A2BAR, and decreased expression of UNC5B, UNC5C and DSCAM were found in endometriotic lesions.

Conclusions

These results suggest that netrin-1 production by macrophages in endometriotic lesions may play an important role in endometriosis-associated pain.

A Subtle Network Mediating Axon Guidance: Intrinsic Dynamic Structure of Growth Cone, Attractive and Repulsive Molecular Cues, and the Intermediate Role of Signaling Pathways

A fundamental feature of both early nervous system development and axon regeneration is the guidance of axonal projections to their targets in order to assemble neural circuits that control behavior. In the navigation process where the nerves grow toward their targets, the growth cones, which locate at the tips of axons, sense the environment surrounding them, including varies of attractive or repulsive molecular cues, then make directional decisions to adjust their navigation journey. The turning ability of a growth cone largely depends on its highly dynamic skeleton, where actin filaments and microtubules play a very important role in its motility. In this review, we summarize some possible mechanisms underlying growth cone motility, relevant molecular cues, and signaling pathways in axon guidance of previous studies and discuss some questions regarding directions for further studies.

Netrin-1 attenuates brain injury after middle cerebral artery occlusion via downregulation of astrocyte activation in mice

Background

Netrin-1 functions largely via combined receptors and downstream effectors. Evidence has shown that astrocytes express netrin-1 receptors, including DCC and UNC5H2. However, whether netrin-1 influences the function of astrocytes was previously unknown.

Methods

Lipopolysaccharide was used to stimulate the primary cultured astrocytes; interleukin release was used to track astrocyte activation. In vivo, shRNA and netrin-1 protein were injected in the mouse brain. Infarct volume, astrocyte activation, and interleukin release were used to observe the function of netrin-1 in neuroinflammation and brain injury after middle cerebral artery occlusion.

Results

Our results demonstrated that netrin-1 reduced lipopolysaccharide-induced interleukin-1β and interleukin-12β release in cultured astrocytes, and blockade of the UNC5H2 receptor with an antibody reversed this effect. Additionally, netrin-1 increased p-AKT and PPAR-γ expression in primary cultured astrocytes. In vivo studies showed that knockdown of netrin-1 increased astrocyte activation in the mouse brain after middle cerebral artery occlusion (p < 0.05). Moreover, injection of netrin-1 attenuated GFAP expression (netrin-1 0.27 ± 0.06 vs. BSA 0.62 ± 0.04, p < 0.001) and the release of interleukins and reduced infarct volume after brain ischemia (netrin-1 0.27 ± 0.06 vs. BSA 0.62 ± 0.04 mm³, p < 0.05).

Conclusion

Our results indicate that netrin-1 is an important molecule in regulating astrocyte activation and neuroinflammation in cerebral ischemia and provides a potential target for ischemic stroke therapy.

Electronic supplementary material

The online version of this article (10.1186/s12974-018-1291-5) contains supplementary material, which is available to authorized users.

Regeneration of sciatic nerve crush injury by a hydroxyapatite nanoparticle-containing collagen type I hydrogel

The current study aimed to enhance the efficacy of peripheral nerve regeneration using a hydroxyapatite nanoparticle-containing collagen type I hydrogel. A solution of type I collagen, extracted from the rat tails, was incorporated with hydroxyapatite nanoparticles (with the average diameter of ~212 nm) and crosslinked with 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide (EDC) to prepare the hydrogel. The Schwann cell cultivation on the prepared hydrogel demonstrated a significantly higher cell proliferation than the tissue culture plate, as positive control, after 48 h (n = 3, P < 0.005) and 72 h (n = 3, P < 0.01). For in vivo evaluation, the prepared hydrogel was administrated on the sciatic nerve crush injury in Wistar rats. Four groups were studied: negative control (with injury but without interventions), positive control (without injury), collagen hydrogel and hydroxyapatite nanoparticle-containing collagen hydrogel. After 12 weeks, the administration of hydroxyapatite nanoparticle-containing collagen significantly (n = 4, P < 0.005) enhanced the functional behavior of the rats compared with the collagen hydrogel and negative control groups as evidenced by the sciatic functional index, hot plate latency and compound muscle action potential amplitude measurements. The overall results demonstrated the applicability of the produced hydrogel for the regeneration of peripheral nerve injuries.

Netrin as a Novel Biomarker and Its Therapeutic Implications in Diabetes Mellitus and Diabetes-Associated Complications

Diabetes is a multifactorial metabolic syndrome and is one of the shared long-lasting illnesses globally. It is linked to long-term microvascular and macrovascular complications that contribute to disability, compromised quality of life, and reduction in lifespan, which eventually leads to death. This disease is not only incurring significant economic burden but also adversely affects the patients, caregivers, communities, and the society at large. The interruption of diabetes progress and its complications is a primary focus of scientific communities. In spite of various diagnostic modalities for diabetes, there is a limited marker to investigate the risk and progress of its complications. Netrin has recently received more attention as a biomarker of diabetes and a broader range of long-term complication. Therefore, the impetus of this review is to exhaustively discuss the role of Netrin as a potential biomarker and its therapeutic implication in diabetes and diverse sets of microvascular and macrovascular complications of diabetes. It also discourses the possible mechanisms of Netrin for the said pharmacological effect for a better understanding of the development and progression of diabetes and its complications in relation to this protein. It enables protective measures to be applied at the subclinical stage and the responses to preventive or therapeutic measures to be scrutinized. Besides, it might also facilitate the appraisal of novel therapeutic options for diabetes and various complications through modifying the endogenous Netrin and provide surrogate endpoints for intervention.

Netrin 1 mediates protective effects exerted by insulin-like growth factor 1 on cochlear hair cells

Sensorineural hearing loss (SNHL) is mainly caused by the damage of cochlear hair cells (HCs). As HCs and supporting cells (SCs) do not proliferate in postnatal mammals, the loss of HCs and SCs is irreversible, emphasizing the importance of preserving their numbers to prevent SNHL. It is known that insulin-like growth factor 1 (IGF1) is instrumental in the treatment of SNHL. Our previous study indicates that IGF1 protects HCs against aminoglycoside by activating IGF1 receptor and its two major downstream pathways, PI3K/AKT and MEK/ERK, in SCs, which results in the upregulation of the expression of the Netrin1-encoding gene (Ntn1). However, the mechanisms underlying IGF1-induced protection of HCs via SC activation as well as the role of NTN1 in this process have not been elucidated. Here, we demonstrated that NTN1, similar to IGF1, promoted HC survival. NTN1 blocking antibody attenuated IGF1-induced HC protection from aminoglycoside, indicating that NTN1 is the effector molecule of IGF1 signaling during HC protection. In situ hybridization demonstrated that IGF1 potently induced Ntn1 expression in SCs. NTN1 receptors were abundantly expressed in the cochlea; among them, UNC5B mediated IGF1 protective effects on HCs, as NTN1 binding to UNC5B inhibited HC apoptosis. These results provide new insights into the mechanisms underlying IGF1 protection of cochlear HCs, suggesting a possibility of using NTN1 as a new treatment for SNHL.

Role of Netrin-1 Signaling in Nerve Regeneration

Netrin-1 was the first axon guidance molecule to be discovered in vertebrates and has a strong chemotropic function for axonal guidance, cell migration, morphogenesis and angiogenesis. It is a secreted axon guidance cue that can trigger attraction by binding to its canonical receptors Deleted in Colorectal Cancer (DCC) and Neogenin or repulsion through binding the DCC/Uncoordinated (Unc5) A–D receptor complex. The crystal structures of Netrin-1/receptor complexes have recently been revealed. These studies have provided a structure based explanation of Netrin-1 bi-functionality. Netrin-1 and its receptor are continuously expressed in the adult nervous system and are differentially regulated after nerve injury. In the adult spinal cord and optic nerve, Netrin-1 has been considered as an inhibitor that contributes to axon regeneration failure after injury. In the peripheral nervous system, Netrin-1 receptors are expressed in Schwann cells, the cell bodies of sensory neurons and the axons of both motor and sensory neurons. Netrin-1 is expressed in Schwann cells and its expression is up-regulated after peripheral nerve transection injury. Recent studies indicated that Netrin-1 plays a positive role in promoting peripheral nerve regeneration, Schwann cell proliferation and migration. Targeting of the Netrin-1 signaling pathway could develop novel therapeutic strategies to promote peripheral nerve regeneration and functional recovery.

Netrin-1 overexpression in bone marrow mesenchymal stem cells promotes functional recovery in a rat model of peripheral nerve injury

Transplantation of bone marrow mesenchymal stem cells (BMSCs) has been developed as a new method of treating diseases of the peripheral nervous system. While netrin-1 is a critical molecule for axonal path finding and nerve growth, it may also affect vascular network formation. Here, we investigated the effect of transplanting BMSCs that produce netrin-1 in a rat model of sciatic nerve crush injury. We introduced a sciatic nerve crush injury, and then injected 1×10(6) BMSCs infected by a recombinant adenovirus expressing netrin-1 Ad5-Netrin-1-EGFP or culture medium into the injured part in the next day. At day 7, 14 and 28 after injection, we measured motor nerve conduction and detected mRNA expressions of netrin-1 receptors UNC5B and Deleted in Colorectal Cancer (DCC), and neurotrophic factors brain-derived neurotrophic factor (BDNF) and nerve growth factor (NGF) by real-time PCR. We also detected protein expressions of BDNF and NGF by Western blotting assays and examined BMSCs that incorporated into myelin and vascellum. The results showed that BMSCs infected by Ad5-Netrin-1-EGFP significantly improved the function of the sciatic nerve, and led to increased expression of BDNF and NGF (P<0.05). Moreover, 28 days after injury, more Schwann cells were found in BMSCs infected by Ad5-Netrin-1-EGFP compared to control BMSCs. In conclusion, transplantation of BMSCs that produce netrin-1 improved the function of the sciatic nerve after injury. This method may be a new treatment of nerve injury.

Netrin-1 induces the migration of Schwann cells via p38 MAPK and PI3K-Akt signaling pathway mediated by the UNC5B receptor

Schwann cells (SCs) play an essentially supportive role in the regeneration of injured peripheral nerve system (PNS). As Netrin-1 is crucial for the normal development of nervous system (NS) and can direct the process of damaged PNS regeneration, our study was designed to determine the role of Netrin-1 in RSC96 Schwann cells (an immortalized rat Schwann cell line) proliferation and migration. Our studies demonstrated that Netrin-1 had no effect on RSC96 cells proliferation, while significantly promoted RSC96 cells migration. The Netrin-1-induced RSC96 cells migration was significantly attenuated by inhibition of p38 and PI3K through pretreatment with SB203580 and LY294002 respectively, but not inhibition of MEK1/2 and JNK by U0126-EtOH and SP600125 individually. Treatment with Netrin-1 enhanced the phosphorylation of p38 and Akt. QRT-PCR indicated that Netrin-1 and only its receptors Unc5a, Unc5b and Neogenin were expressed in RSC96 cells, among which Unc5b expressed the most. And UNC5B protein was significantly increased after stimulated by Netrin-1. In conclusion, we show here that Netrin-1-enhanced SCs migration is mediated by activating p38 MAPK and PI3K-Akt signal cascades via receptor UNC5B, which suggests that Netrin-1 could serve as a new therapeutic strategy and has potential application value for PNS regeneration.

Netrin-1 overexpression promotes white matter repairing and remodeling after focal cerebral ischemia in mice

Damage of oligodendrocytes after ischemia has negative impact on white matter integrity and neuronal function. In this work, we explore whether Netrin-1 (NT-1) overexpression facilitates white matter repairing and remodeling. Adult CD-1 mice received stereotactic injection of adeno-associated virus carrying NT-1 gene (AAV-NT-1). One week after gene transfer, mice underwent 60 minutes of middle cerebral artery occlusion. The effect of NT-1 on neural function was evaluated by neurobehavioral tests. Proliferated oligodendrocyte progenitor cells (OPCs), newly matured oligodendrocytes, and remyelination were semi-quantified by immunohistochemistry. The role of NT-1 in oligodendrogenesis was further explored by examining specific NT-1 receptors and their function. Netrin-1 overexpression was detected in neurons and astrocytes 2 weeks after AAV-NT-1 gene transfer and significantly improved the neurobehavioral outcomes compared with the control (P<0.05). In comparison with the control, proliferated OPCs, newly matured oligodendrocytes, and remyelination were greatly increased in the ipsilateral hemisphere of AAV-NT-1-transduced mice. Furthermore, both NT-1 receptors deleted in colorectal carcinoma and UNC5H2 were expressed on OPCs whereas only UNC5H2 was expressed in myelinated axons. Our study indicated that NT-1 promoted OPC proliferation, differentiation, and increased remyelination, suggesting that NT-1 is a promising factor for white matter repairing and remodeling after ischemia.

The roles of PIKE in tumorigenesis

Tumorigenesis is the process by which normal cells evolve the capacity to evade and overcome the constraints usually placed upon their growth and survival. To ensure the integrity of organs and tissues, the balance of cell proliferation and cell death is tightly maintained. The proteins controlling this balance are either considered oncogenes, which promote tumorigenesis, or tumor suppressors, which prevent tumorigenesis. Phosphoinositide 3-kinase enhancer (PIKE) is a family of GTP-binding proteins that possess anti-apoptotic functions and play an important role in the central nervous system. Notably, accumulating evidence suggests that PIKE is a proto-oncogene involved in tumor progression. The PIKE gene (CENTG1) is amplified in a variety of human cancers, leading to the resistance against apoptosis and the enhancement of invasion. In this review, we will summarize the functions of PIKE proteins in tumorigenesis and discuss their potential implications in cancer therapy.

Expression patterns and functional evaluation of the UNC5b receptor during the early phase of peripheral nerve regeneration using the mouse median nerve model

INTRODUCTION

In this study, we evaluated the role of the Netrin-1 receptor UNC5b (Uncoordinated), a neuronal guidance molecule, during peripheral nerve regeneration using the mouse median nerve model.

MATERIALS AND METHODS

Using Western blot analysis, we examined the expression changes of UNC5b after transection and microsurgical repair of the mouse median nerve distal to the transection site. We evaluated the histomorphometrical changes and functional recovery of the grasping force after median nerve transection and repair in wild-type (WT) mice and UNC5b(+/-) heterozygous mice.

RESULTS

In Western blot analysis, we could show a high increase of UNC5b in the nerve segment distal to the injury site at day 14. Histomorphometrical analysis did not show any significant differences between WT animals and heterozygous animals. Using the functional grasping test, we could demonstrate that peripheral nerve regeneration is significantly diminished in heterozygous UNC5b(+/-) mice.

CONCLUSION

By using the mouse median nerve model in transgenic animals, we demonstrate that the Netrin-1 receptor UNC5b plays an important role during peripheral nerve regeneration.

Netrin-1 improves spatial memory and synaptic plasticity impairment following global ischemia in the rat

Cerebral ischemia, which is the second and most common cause of mortality, affects millions of individuals worldwide. The present study was performed to investigate whether intrahippocampal administration of netrin-1 could improve spatial memory impairment in radial arm maze task and restore long-term potentiation (LTP) in 4-vessel occlusion model of global ischemia. The results showed that intrahippocampal infusion of nerin-1 24 h after ischemia (at both doses of 400 and 800 ng) significantly ameliorated spatial memory impairment and at a dose of 800 ng was capable to improve synaptic dysfunction as observed by recovery of population spike component of basal evoked potential and LTP through enhancement of excitability and normalization of paired pulse response. Taken together, the present study shows that netrin-1 dose-dependently ameliorates spatial memory impairment and improves synaptic dysfunction as observed by recovery of population spike component of basal evoked potential and LTP in rats with global ischemia.

Paracrine signalling loops in adult human and mouse pancreatic islets: netrins modulate beta cell apoptosis signalling via dependence receptors

AIMS/HYPOTHESIS

Adult pancreatic islets contain multiple cell types that produce and secrete well characterised hormones, including insulin, glucagon and somatostatin. Although it is increasingly apparent that islets release and respond to more secreted factors than previously thought, systematic analyses are lacking. We therefore sought to identify potential autocrine and/or paracrine islet growth factor loops, and to characterise the function of the netrin family of islet-secreted factors and their receptors, which have been previously unreported in adult islets.

METHODS

Gene expression databases, islet-specific tag sequencing libraries and microarray datasets of FACS purified beta cells were used to compile a list of secreted factors and receptors present in mouse or human islets. Netrins and their receptors were further assessed using RT-PCR, Western blot analysis and immunofluorescence staining. The roles of netrin-1 and netrin-4 in beta cell function, apoptosis and proliferation were also examined.

RESULTS

We identified 233 secreted factors and 234 secreted factor receptors in islets. The presence of netrins and their receptors was further confirmed. Downregulation of caspase-3 activation was observed when MIN6 cells were exposed to exogenous netrin-1 and netrin-4 under hyperglycaemic conditions. Reduction in caspase-3 cleavage was linked to the decrease in dependence receptors, neogenin and unc-5 homologue A, as well as the activation of Akt and extracellular signal-regulated protein kinase (ERK) signalling.

CONCLUSIONS/INTERPRETATION

Our results highlight the large number of potential islet growth factors and point to a context-dependent pro-survival role for netrins in adult beta cells. Since diabetes results from a deficiency in functional beta cell mass, these studies are important steps towards developing novel therapies to improve beta cell survival.

Netrin-1 overexpression protects kidney from ischemia reperfusion injury by suppressing apoptosis

Netrin-1, a diffusible laminin-related protein, is highly expressed in the kidney. However, the pathophysiological roles of netrin-1 in the kidney are unknown. To address this question directly, we used transgenic mice that overexpress chicken netrin-1 in the kidney. Netrin-1 overexpression was confirmed by real-time RT-PCR and Western blot analysis. Eight-week-old wild-type and transgenic mice were subjected to 26 minutes of renal ischemia followed by reperfusion for 72 hours. Wild-type mice developed more severe renal dysfunction by 24 hours than netrin-1 transgenic mice. Functional improvement was associated with better preservation of morphology, reduced cytokine expression, and reduced oxidative stress in the kidney of transgenic mice as compared with wild-type mice. In addition, both basal and reperfusion-induced cell proliferation were dramatically increased in transgenic kidneys as determined by Ki-67 staining. Interestingly, ischemia reperfusion induced a large increase in apoptosis in wild-type mice but not in netrin-1 transgenic mice that was associated with reduced caspase-3 activation in the transgenic kidney. These results suggest that netrin-1 protects renal tubular epithelial cells against ischemia reperfusion-induced injury by increasing proliferation and suppressing apoptosis.

Unc5B associates with LARG to mediate the action of repulsive guidance molecule

Neuronal axons are guided by attractive and repulsive cues in their local environment. Because the repulsive guidance molecule A (RGMa) was originally identified as an axon repellent in the visual system, diverse functions in the developing and adult central nervous system have been ascribed to it. RGMa binding to its receptor neogenin induces RhoA activation, leading to inhibitory/repulsive behavior and collapse of the neuronal growth cone. However, the precise mechanisms that regulate RhoA activation are poorly understood. In this study, we show that Unc5B, a member of the netrin receptor family, interacts with neogenin as a coreceptor for RGMa. Moreover, leukemia-associated guanine nucleotide exchange factor (LARG) associates with Unc5B to transduce the RhoA signal. Focal adhesion kinase (FAK) is involved in RGMa-induced tyrosine phosphorylation of LARG as well as RhoA activation. These findings uncover the molecular basis for diverse functions mediated by RGMa.

Netrin-1 increases proliferation and migration of renal proximal tubular epithelial cells via the UNC5B receptor

The cellular hallmark of kidney repair is a rapid proliferation of renal tubular epithelial cells ultimately leading to the restoration of nephron structure and function. Netrin-1 was discovered as a neural guidance cue and found to be expressed outside the nervous system, including in kidney. Previous work showed that netrin-1 is upregulated in response to ischemic injury and ameliorates ischemic injury. The objectives of this study were to determine the role of netrin-1 in renal tubular epithelial cell proliferation and migration in vitro. Real-time RT-PCR analysis showed that netrin-1 and its receptors UNC5B and neogenin are highly expressed in cultured mouse renal epithelial cells (TKPTS), whereas the expression of the Deleted in Colon Cancer (DCC), UNC5A, UNC5C, and UNC5D receptors is negligible or undetectable. Netrin-1 protein was induced in the edges of mechanical wounds in vitro. Netrin-1 increased TKPTS cell proliferation in a dose-dependent manner. The netrin-1-induced increase in TKPTS cell proliferation was completely prevented by small interfering RNA (siRNA) inhibition of UNC5B receptor but not UNC5C receptor expression. Netrin-1 also increased TKPTS cell migration in vitro, and this was also mediated through the UNC5B receptor. Netrin-1 increased the phosphorylation of Akt and ERK. Inhibition of phosphatidylinositol 3-kinase and MEK1/2 completely inhibited netrin-1-induced cell proliferation but not migration. These results indicate that netrin-1 increases renal tubular epithelial cell proliferation and migration through the UNC5B receptor. Moreover, the increase in cell proliferation, but not migration, was mediated via activation of Akt and ERK pathways.

A novel mechanism of methylglyoxal cytotoxicity in neuroglial cells

Methylglyoxal (MGO) is an endogenous dicarbonyl compound that is highly produced in hyperglycemic conditions. It forms advanced glycation endproducts that are believed to contribute, as etiological factors, to the pathophysiology of diabetic complications. In addition, MGO suppresses cell viability through the induction of apoptosis in vitro. In this study, we have, for the first time, demonstrated the effect of MGO on the gp130 cytokine-induced signal transducer and activator of transcription 3 (STAT3) responses in RT4 schwannoma, PC12 pheochromocytoma and U87MG glioma cells. At dose that very mildly affects cell viability, MGO rapidly induces endocytotic degradation of gp130, which involves the di-leucine internalization motif in the cytoplasmic domain of gp130, without affecting other growth factor receptors. Concomitant inhibition of basal and interleukin-6-induced STAT3 activation was observed following pre-treatment with MGO. The inhibitory effect of MGO on the gp130/STAT3 signaling was prevented by the pre-treatment with an advanced glycation endproduct scavenger aminoguanidine. Finally, these deleterious effects of MGO on STAT3 signaling led to down-regulation of a STAT3 target gene, Bcl-2, and sensitized cellular toxicity induced by H(2)O(2) and etoposide. Our data indicate that MGO affects cell viability via desensitization of gp130/STAT3 signaling, which is the key signaling pathway for cell survival, and thereby promotes cytotoxicity.

Tyrphostin ErbB2 Inhibitors AG825 and AG879 Have Non-specific Suppressive Effects on gp130/ STAT3 Signaling

Although the interaction between gp130 and the ErbB family has frequently been shown in cancer cells, the mechanism of this interaction remains unclear and controversial. In the present study, we found that specific tyrphostin inhibitors of ErbB2 (AG825 and AG879), but not ErbB1 inhibitor (AG1478), suppressed IL-6-induced tyrosine phosphorylation of STAT3 in schwannoma cells. However, biochemical evidence for transactivation of ErbB2 by IL-6 was not observed. Additionally, the inhibition of ErbB2 expression, with either a specific RNAi or transfection of an ErbB2 mutant lacking the intracellular domain did not inhibit the IL-6-induced tyrosine phosphorylation of STAT3. Thus, it seems that tyrphostins, which are known as specific inhibitors of the ErbB2 kinase, may have non-specific suppressive effects on the IL-6/STAT3 pathway.

Netrin-1 and its receptors in tumour growth promotion

BACKGROUND

Netrin-1 belongs to a family of secreted proteins that act as migration and adhesion cues in the developing CNS and in a number of non-neural tissues. Netrin-1 is the ligand of deleted in colorectal cancer (DCC) and the uncoordinated family member 5 (UNC5) orthologues of the dependence receptor family. Over the past ten years, a novel mechanism has emerged, that a receptor unoccupied by its ligand is not necessarily inactive. Rather, such a receptor can mediate two signalling pathways, depending on whether it is bound to its ligand or not. In the absence of ligand, an active signalling pathway results in cell death through apoptosis.

OBJECTIVE

Coupled netrin-1 receptors have been shown to regulate diverse processes such as maintenance, integrity, migration and renewal of many tissues. We propose that netrin-1 receptors can regulate tumour development.

METHODS

We review the properties of netrin-1 and present netrin-1 receptors as regulators of tumourigenesis.

RESULTS/CONCLUSION

Netrin-1 and its receptors are unexplored critical targets in cancer.