Glial cell line-derived neurotrophic factor gene transfer exerts protective effect on axons in sciatic nerve following constriction-induced peripheral nerve injury

Engineered hydrogels for peripheral nerve repair

Peripheral nerve injury (PNI) is a complex disease that often appears in young adults. It is characterized by a high incidence, limited treatment options, and poor clinical outcomes. This disease not only causes dysfunction and psychological disorders in patients but also brings a heavy burden to the society. Currently, autologous nerve grafting is the gold standard in clinical treatment, but complications, such as the limited source of donor tissue and scar tissue formation, often further limit the therapeutic effect. Recently, a growing number of studies have used tissue-engineered materials to create a natural microenvironment similar to the nervous system and thus promote the regeneration of neural tissue and the recovery of impaired neural function with promising results. Hydrogels are often used as materials for the culture and differentiation of neurogenic cells due to their unique physical and chemical properties. Hydrogels can provide three-dimensional hydration networks that can be integrated into a variety of sizes and shapes to suit the morphology of neural tissues. In this review, we discuss the recent advances of engineered hydrogels for peripheral nerve repair and analyze the role of several different therapeutic strategies of hydrogels in PNI through the application characteristics of hydrogels in nerve tissue engineering (NTE). Furthermore, the prospects and challenges of the application of hydrogels in the treatment of PNI are also discussed.

Treatment of postherpetic neuralgia by bone marrow aspirate injection: A case report

BACKGROUND

Postherpetic neuralgia (PHN) is the most frequent and a difficult-to-treat complication of herpes zoster (HZ). Its symptoms include allodynia, hyperalgesia, burning, and an electric shock-like sensation stemming from the hyperexcitability of damaged neurons and varicella-zoster virus-mediated inflammatory tissue damage. HZ-related PHN has an incidence of 5%–30%, and in some patients, the pain is intolerable and can lead to insomnia or depression. In many cases, the pain is resistant to pain-relieving drugs, necessitating radical therapy.

CASE SUMMARY

We present the case of a patient with PHN whose pain was not cured by conventional treatments, such as analgesics, block injections, or Chinese medicines, but by bone marrow aspirate concentrate (BMAC) injection containing bone marrow mesenchymal stem cells. BMAC has already been used for joint pains. However, this is the first report on its use for PHN treatment.

CONCLUSION

This report reveals that bone marrow extract can be a radical therapy for PHN.

Implantable Biomaterials for Peripheral Nerve Regeneration-Technology Trends and Translational Tribulations

The use of autografted nerve in surgical repair of peripheral nerve injuries (PNI) is severely limited due to donor site morbidity and restricted tissue availability. As an alternative, synthetic nerve guidance channels (NGCs) are available on the market for surgical nerve repair, but they fail to promote nerve regeneration across larger critical gap nerve injuries. Therefore, such injuries remain unaddressed, result in poor healing outcomes and are a limiting factor in limb reconstruction and transplantation. On the other hand, a myriad of advanced biomaterial strategies to address critical nerve injuries are proposed in preclinical literature but only few of those have found their way into clinical practice. The design of synthetic nerve grafts should follow rational criteria and make use of a combination of bioinstructive cues to actively promote nerve regeneration. To identify the most promising NGC designs for translation into applicable products, thorough mode of action studies, standardized readouts and validation in large animals are needed. We identify design criteria for NGC fabrication according to the current state of research, give a broad overview of bioactive and functionalized biomaterials and highlight emerging composite implant strategies using therapeutic cells, soluble factors, structural features and intrinsically conductive substrates. Finally, we discuss translational progress in bioartificial conduits for nerve repair from the surgeon's perspective and give an outlook toward future challenges in the field.

Interplay of BDNF and GDNF in the Mature Spinal Somatosensory System and Its Potential Therapeutic Relevance

The growth factors BDNF and GDNF are gaining more and more attention as modulators of synaptic transmission in the mature central nervous system (CNS). The two molecules undergo a regulated secretion in neurons and may be anterogradely transported to terminals where they can positively or negatively modulate fast synaptic transmission. There is today a wide consensus on the role of BDNF as a pro-nociceptive modulator, as the neurotrophin has an important part in the initiation and maintenance of inflammatory, chronic, and/or neuropathic pain at the peripheral and central level. At the spinal level, BDNF intervenes in the regulation of chloride equilibrium potential, decreases the excitatory synaptic drive to inhibitory neurons, with complex changes in GABAergic/glycinergic synaptic transmission, and increases excitatory transmission in the superficial dorsal horn. Differently from BDNF, the role of GDNF still remains to be unraveled in full. This review resumes the current literature on the interplay between BDNF and GDNF in the regulation of nociceptive neurotransmission in the superficial dorsal horn of the spinal cord. We will first discuss the circuitries involved in such a regulation, as well as the reciprocal interactions between the two factors in nociceptive pathways. The development of small molecules specifically targeting BDNF, GDNF and/or downstream effectors is opening new perspectives for investigating these neurotrophic factors as modulators of nociceptive transmission and chronic pain. Therefore, we will finally consider the molecules of (potential) pharmacological relevance for tackling normal and pathological pain.

Partial high frequency nerve block decreases neuropathic signaling following chronic sciatic nerve constriction injury

OBJECTIVE

High frequency (HF) block can quickly and reversibly stop nerve conduction. We hypothesized HF block at the sciatic nerve would minimize nociception by preventing neuropathic signals from reaching the central nervous system.

APPROACH

Lewis rats were implanted with a constriction cuff and a distal cuff electrode around their right sciatic nerve. Tactile sensitivity was evaluated using the 50% paw withdrawal threshold determined using Chaplan's method for von Frey monofilaments. Over the course of 49 days, the 50% paw withdrawal threshold was measured 1) before HF block, 2) during HF block (50 kHz, 3 Vpp), and 3) after HF block. Gait was observed and scored before and during block. At end point, HF block efficacy was directly evaluated using additional cuff electrodes to elicit and record compound neural action potentials across the HF blocking cuff.

MAIN RESULTS

At days 7 and 14 days post-operation, tactile sensitivity was significantly lower during HF block compared to before and after block (p < 0.005). Additionally, an increase in gait disability was not visually observed during HF block.

SIGNIFICANCE

HF block can reduce tactile sensitivity in a limb with a neuropthic injury in a rapidly reversible fashion.

Melatonin promotes Schwann cell dedifferentiation and proliferation through the Ras/Raf/ERK and MAPK pathways, and glial cell-derived neurotrophic factor expression

Upon peripheral nerve injury (PNI), continuous proliferation of Schwann cells is critical for axon regeneration and tubular reconstruction for nerve regeneration. Melatonin is a hormone that is able to induce proliferation in various cell types. In the present study, the effects of melatonin on promoting Schwann cell proliferation and the molecular mechanism involved were investigated. The present results showed that melatonin enhanced the melatonin receptors (MT1 and MT2) expression in Schwann cells. Melatonin induced Schwann cell dedifferentiation into progenitor-like Schwann cells, as observed by immunofluorescence staining, which showed Sox2 marker expression. In addition, melatonin enhanced Schwann cell proliferation, mediated by the upregulation of glial cell-derived neurotropic factor (GNDF) and protein kinase C (PKC). Furthermore, the Ras/Raf/ERK and MAPK signaling pathways were also involved in Schwann cell dedifferentiation and proliferation. In conclusion, melatonin induced Schwann cell dedifferentiation and proliferation via the Ras/Raf/ERK, MAPK and GDNF/PKC pathways. The present results suggested that melatonin could be used to enhance the recovery of PNI.

Brain-derived neurotrophic factor and Glial cell line-derived neurotrophic factor expressions in the trigeminal root entry zone and trigeminal ganglion neurons of a trigeminal neuralgia rat model

Microvascular compression on the trigeminal root entry zone (TREZ) is the main etiology of trigeminal neuralgia (TN) patients. To investigate brain-derived neurotrophic factor (BDNF) and glial cell line-derived neurotrophic factor (GDNF) in the trigeminal ganglion (TG) and TREZ, immunofluorescence staining and Western blot were used in a rat TN model. Both BDNF and GDNF were observed in the TG neurons and TREZ. The expression of the BDNF dimer in the TG was increased in the TN group, while GDNF expression was decreased after compression injury. The BDNF dimer/pro-BDNF ratio in the TREZ of the TN group was higher than that in the sham group, but the GDNF expression in the TREZ was significantly lower than that in the sham group. These results suggested that compression injury in the TREZ of rats induced dynamic changes in BDNF and GDNF in both the TG and TREZ, and these changes are involved in the nociceptive transmission of the TN animal model.

Enhanced expression of gene coding for β-endorphin in human monocytic cells exposed to pulsed radio frequency electric fields through thermal and non-thermal effects

Background

The enhanced expression of endogenous opioid peptides, including β-endorphin, has been implicated in the mechanism of action of pulsed radio frequency (PRF) application in pain modulation. Because thermal effects cannot be separated from the physical property of PRF application to biological tissues, we evaluated whether temperatures higher than that of the normal body temperature (37°C) modulate mRNA expression for the precursor of β-endorphin, proopiomelanocortin (POMC) in human monocytic cells THP-1. We also attempted to examine whether mechanisms other than thermal effects also modulate such gene expression.

Methods and results

The mRNA for POMC in THP-1 cells increased by a 15-minutes incubation at 42°C, 45°C, or 70°C without PRF application as compared with that in cells incubated at 37°C. On the other hand, gene expression for POMC in cells incubated at 20°C as well as at 37°C with PRF application for 15 minutes increased as compared to that in cells incubated at 37°C without PRF application. Continuous radio frequency at 70°C but not PRF provoked apoptotic cell death at 1–2 hour, and necrotic cell death at 24 hours after the RF application.

Conclusion

A simple experimental system using human monocytic cells in culture demonstrated that a 15 minute elevation of temperature above 37°C enhanced gene expression for POMC in THP-1 cells, while a 15 minute application of PRF to these cells incubated at 37°C or lower, also enhanced gene expression, indicating that temperature-independent mechanisms as well as thermal effects may be involved in such gene expression.

The role of undifferentiated adipose-derived stem cells in peripheral nerve repair

Peripheral nerve injuries impose significant health and economic consequences, yet no surgical repair can deliver a complete recovery of sensory or motor function. Traditional methods of repair are less than ideal: direct coaptation can only be performed when tension-free repair is possible, and transplantation of nerve autograft can cause donor-site morbidity and neuroma formation. Cell-based therapy delivered via nerve conduits has thus been explored as an alternative method of nerve repair in recent years. Stem cells are promising sources of the regenerative core material in a nerve conduit because stem cells are multipotent in function, abundant in supply, and more accessible than the myelinating Schwann cells. Among different types of stem cells, undifferentiated adipose-derived stem cell (uASC), which can be processed from adipose tissue in less than two hours, is a promising yet underexplored cell type. Studies of uASC have emerged in the past decade and have shown that autologous uASCs are non-immunogenic, easy to access, abundant in supply, and efficacious at promoting nerve regeneration. Two theories have been proposed as the primary regenerative mechanisms of uASC: in situ trans-differentiation towards Schwann cells, and secretion of trophic and anti-inflammatory factors. Future studies need to fully elucidate the mechanisms, side effects, and efficacy of uASC-based nerve regeneration so that uASCs can be utilized in clinical settings.

Photobiomodulation of the dorsal root ganglion for the treatment of low back pain: A pilot study

BACKGROUND AND OBJECTIVE

Chronic low back pain is a worldwide public health issue with high socioeconomic impact. The aim of this study was to determine the efficacy of laser irradiation of the dorsal root ganglion of the second lumbar spinal nerve for chronic axial low back pain compared to lidocaine injection and radiofrequency treatment.

STUDY DESIGN/MATERIALS AND METHODS

Twenty-eight patients were randomly divided into three treatment groups: lidocaine injection, radiofrequency, or laser. The second intervertebral foramen between the second and third lumbar vertebrae was accessed by percutaneous needle puncture bilaterally, guided by fluoroscopy. In the local anesthetic group, injection of 1 ml lidocaine without epinephrine was applied through a 20-gauge (G20) Quincke tip spinal needle inserted in the second lumbar intervertebral foramen. In the radiofrequency group, the probe (150 mm long with a 5 mm active tip) was directed through a G20 needle placed in the second lumbar intervertebral foramen and neuromodulation was done with a radiofrequency of Cosman G4® in pulses of 20 ms with wash-out period of 480 ms, for 300 seconds at 42°C. A single treatment was used. In the laser treatment group, a continuous wave, 808 nm wavelength diode laser (Photon Lase III® DCM, Brazil), with an output power of 100 mW was used for a single treatment. An 18 gauge needle was placed in the second lumbar intervertebral foramen guided by fluoroscopy. Light was delivered through a 600 µm optical fiber placed in the G18 needle. The tip of the fiber extended 5 mm beyond the tip of the needle in the second lumbar intervertebral foramen. The beam spot size was 0.003 cm(2) , irradiance = 35W/cm(2) , exposure time = 84 seconds, energy density = 2800J/cm(2) , total energy was 8.4 J. The low back pain score was assessed by the visual analog scale (VAS) and Pain Relief Scale (PRS) pre, post procedure and in 1 month follow up. Temperature was measured using a digital thermometer.

RESULTS

All patients in the local anesthetic and laser treatment groups reported a pain reduction of at least 50% immediately post-procedure and 10 out of 11 patients in the radiofrequency group reported a pain reduction of at least 50%. At 1 month post-treatment, the laser treatment group had the greatest number of patients who reported more than 50% pain relief based on PRS (7 out of 10 patients) while only 2 out of 7 patients and 3 out of 11 patients in the lidocaine and radiofrequency treatment groups respectively reported more than a 50% pain relief.

CONCLUSION

Laser irradiation caused an immediate decrease in low back pain post-procedure similar to pain reduction caused by lidocaine injection. Both lidocaine injection and laser irradiation were more effective than radiofrequency treatment for immediate and longer term (1 month post-treatment) chronic back pain. Lasers Surg. Med. 48:653-659, 2016. © 2016 Wiley Periodicals, Inc.

Demyelination/remyelination and expression of interleukin-1β, substance P, nerve growth factor, and glial-derived neurotrophic factor during trigeminal neuropathic pain in rats

The etiology of trigeminal neuropathic pain is not clear, but there is evidence that demyelination, expression of cytokines, neuropeptides, and neurotrophic factors are crucial contributors. In order to elucidate mechanisms underlying trigeminal neuropathic pain, we evaluated the time course of morphological changes in myelinated and unmyelinated trigeminal nerve fibers, expression of cytokine IL-1β, neuropeptide substance P (SP), nerve growth factor (NGF), and glial derived neurotrophic factor (GDNF) in peripheral and ganglion tissues, using a rat model of trigeminal neuropathic pain. Chronic constriction injury (CCI) of the infraorbital nerve (IoN), or a sham surgery, was performed. Mechanical allodynia was evaluated from day 3 to day 15 post-surgery. Trigeminal nerves were divided into 2 sections - distal to CCI and ganglion - for morphological analyses, immunohistochemistry (IL-1β, SP), and protein quantification by ELISA (NGF, GDNF). At early postoperative time points, decreased mechanical responses were observed, which were associated with demyelination, glial cell proliferation, increased immunoexpression of IL-1 β and SP, and impaired GDNF production. In the late postoperative period, mechanical allodynia was present with partial recovery of myelination, glial cell proliferation, and increased immunoreactivity of IL-1β and SP. Our data show that demyelination/remyelination processes are related to the development of pain behavior. IL-1β may have effects both in ganglia and nerves, while SP may be an important mediator at the nerve endings. Additionally, low levels of GDNF may produce impaired signaling, which may be involved in generation of pain.

Upregulation of p‑Akt by glial cell line‑derived neurotrophic factor ameliorates cell apoptosis in the hippocampus of rats with streptozotocin‑induced diabetic encephalopathy

The loss of neurotrophic factor support has been shown to contribute to the development of the central nervous system. Glial cell line‑derived neurotrophic factor (GDNF), a potent neurotrophic factor, is closely associated with apoptosis and exerts neuroprotective effects on numerous populations of cells. However, the underlying mechanisms of these protective effects remain unknown. In the present study, a significant increase in Bax levels and DNA fragmentation was observed in the hippocampus obtained from the brains of diabetic rats 60 days after diabetes had been induced. The apoptotic changes were correlated with the loss of GDNF/Akt signaling. GDNF administration was found to reverse the diabetes‑induced Bax and DNA fragmentation changes. This was associated with an improvement in the level of p‑Akt/Akt. In addition, combination of GDNF with a specific inhibitor of the phosphoinositide 3‑kinase (PI3K)/Akt pathway, Wortmannin, significantly abrogated the effects of GDNF on the levels of p‑Akt/Akt, Bax and DNA fragmentation. However, a p38 mitogen‑activated proten kinase (MAPK) inhibitor, SB203580, had no effect on the expression of p‑Akt/Akt, Bax or DNA fragmentation. These results demonstrate the pivotal role of GDNF as well as the PI3K/Akt pathway, but not the MAPK pathway, in the prevention of diabetes‑induced neuronal apoptosis in the hippocampus.

Involvement of phosphatase and tensin homolog deleted from chromosome 10 in rodent model of neuropathic pain

Background

Many cancer research studies have extensively examined the phosphatase and tensin homolog deleted from chromosome 10 (PTEN) pathway. There are only few reports that suggest that PTEN might affect pain; however, there is still a lack of evidence to show the role of PTEN for modulating pain. Here, we report a role for PTEN in a rodent model of neuropathic pain.

Results

We found that chronic constriction injury (CCI) surgery in rats could elicit downregulation of spinal PTEN as well as upregulation of phosphorylated PTEN (phospho-PTEN) and phosphorylated mammalian target of rapamycin (phospho-mTOR). After examining such changes in endogenous PTEN in neuropathic rats, we explored the effects of modulating the spinal PTEN pathway on nociceptive behaviors. The normal rats exhibited mechanical allodynia after intrathecal (i.t.) injection of adenovirus-mediated PTEN antisense oligonucleotide (Ad-antisense PTEN). These data indicate the importance of downregulation of spinal PTEN for nociception. Moreover, upregulation of spinal PTEN by i.t. adenovirus-mediated PTEN (Ad-PTEN) significantly prevented CCI-induced development of nociceptive sensitization, thermal hyperalgesia, mechanical allodynia, cold allodynia, and weight-bearing deficits in neuropathic rats. Furthermore, upregulation of spinal PTEN by i.t. Ad-PTEN significantly attenuated CCI-induced microglia and astrocyte activation, upregulation of tumor necrosis factor-α (TNF-α) and phospho-mTOR, and downregulation of PTEN in neuropathic rats 14 days post injury.

Conclusions

These findings demonstrate that PTEN plays a key, beneficial role in a rodent model of neuropathic pain.

Current progress in use of adipose derived stem cells in peripheral nerve regeneration

Unlike central nervous system neurons; those in the peripheral nervous system have the potential for full regeneration after injury. Following injury, recovery is controlled by schwann cells which replicate and modulate the subsequent immune response. The level of nerve recovery is strongly linked to the severity of the initial injury despite the significant advancements in imaging and surgical techniques. Multiple experimental models have been used with varying successes to augment the natural regenerative processes which occur following nerve injury. Stem cell therapy in peripheral nerve injury may be an important future intervention to improve the best attainable clinical results. In particular adipose derived stem cells (ADSCs) are multipotent mesenchymal stem cells similar to bone marrow derived stem cells, which are thought to have neurotrophic properties and the ability to differentiate into multiple lineages. They are ubiquitous within adipose tissue; they can form many structures resembling the mature adult peripheral nervous system. Following early in vitro work; multiple small and large animal in vivo models have been used in conjunction with conduits, autografts and allografts to successfully bridge the peripheral nerve gap. Some of the ADSC related neuroprotective and regenerative properties have been elucidated however much work remains before a model can be used successfully in human peripheral nerve injury (PNI). This review aims to provide a detailed overview of progress made in the use of ADSC in PNI, with discussion on the role of a tissue engineered approach for PNI repair.

Sciatic nerve injury repair: a visualized analysis of research fronts and development trends

A total of 3,446 publications regarding sciatic nerve injury repair and protection indexed by Web of Science during 2000–2004 were used for a detailed analysis of temporal-spatial distribution characteristics. Reference co-citation networks of the 100 top-cited publications as per the number of total citations were created using the Web of Science database and the information visualization tool, CiteSpaceIII. The key words that showed high frequency in these publications were included for analyzing the research fronts and development trends for sciatic nerve injury repair and protection. Through word frequency trend analysis, studies on bone marrow mesenchymal stem cells, adipose-derived stem cells, and skeletal muscle-derived multipotent stem cells combined with tissue-engineered scaffold material will become the forefronts in the field of sciatic nerve injury repair and protection in the near future.

Treating small fiber neuropathy by topical application of a small molecule modulator of ligand-induced GFRα/RET receptor signaling

Small-fiber neuropathy (SFN) is a disorder of peripheral nerves commonly found in patients with diabetes mellitus, HIV infection, and those receiving chemotherapy. The complexity of disease etiology has led to a scarcity of effective treatments. Using two models of progressive SFN, we show that overexpression of glial cell line-derived neurotrophic factor (GDNF) in skin keratinocytes or topical application of XIB4035, a reported nonpeptidyl agonist of GDNF receptor α1 (GFRα1), are effective treatments for SFN. We also demonstrate that XIB4035 is not a GFRα1 agonist, but rather it enhances GFRα family receptor signaling in conjunction with ligand stimulation. Taken together, our results indicate that topical application of GFRα/RET receptor signaling modulators may be a unique therapy for SFN, and we have identified XIB4035 as a candidate therapeutic agent.

Gamma knife irradiation of injured sciatic nerve induces histological and behavioral improvement in the rat neuropathic pain model

We examined the effects of gamma knife (GK) irradiation on injured nerves using a rat partial sciatic nerve ligation (PSL) model. GK irradiation was performed at one week after ligation and nerve preparations were made three weeks after ligation. GK irradiation is known to induce immune responses such as glial cell activation in the central nervous system. Thus, we determined the effects of GK irradiation on macrophages using immunoblot and histochemical analyses. Expression of Iba-1 protein, a macrophage marker, was further increased in GK-treated injured nerves as compared with non-irradiated injured nerves. Immunohistochemical study of Iba-1 in GK-irradiated injured sciatic nerves demonstrated Iba-1 positive macrophage accumulation to be enhanced in areas distal to the ligation point. In the same area, myelin debris was also more efficiently removed by GK-irradiation. Myelin debris clearance by macrophages is thought to contribute to a permissive environment for axon growth. In the immunoblot study, GK irradiation significantly increased expressions of βIII-tubulin protein and myelin protein zero, which are markers of axon regeneration and re-myelination, respectively. Toluidine blue staining revealed the re-myelinated fiber diameter to be larger at proximal sites and that the re-myelinated fiber number was increased at distal sites in GK-irradiated injured nerves as compared with non-irradiated injured nerves. These results suggest that GK irradiation of injured nerves facilitates regeneration and re-myelination. In a behavior study, early alleviation of allodynia was observed with GK irradiation in PSL rats. When GK-induced alleviation of allodynia was initially detected, the expression of glial cell line-derived neurotrophic factor (GDNF), a potent analgesic factor, was significantly increased by GK irradiation. These results suggested that GK irradiation alleviates allodynia via increased GDNF. This study provides novel evidence that GK irradiation of injured peripheral nerves may have beneficial effects.

Importance of glial activation in neuropathic pain

Glia plays a crucial role in the maintenance of neuronal homeostasis in the central nervous system. The microglial production of immune factors is believed to play an important role in nociceptive transmission. Pain may now be considered a neuro-immune disorder, since it is known that the activation of immune and immune-like glial cells in the dorsal root ganglia and spinal cord results in the release of both pro- and anti-inflammatory cytokines, as well as algesic and analgesic mediators. In this review we presented an important role of cytokines (IL-1alfa, IL-1beta, IL-2, IL-4, IL-6, IL-10, IL-15, IL-18, TNFalpha, IFNgamma, TGF-beta 1, fractalkine and CCL2); complement components (C1q, C3, C5); metaloproteinases (MMP-2,-9) and many other factors, which become activated on spinal cord and DRG level under neuropathic pain. We discussed the role of the immune system in modulating chronic pain. At present, unsatisfactory treatment of neuropathic pain will seek alternative targets for new drugs and it is possible that anti-inflammatory factors like IL-10, IL-4, IL-1alpha, TGF-beta 1 would fulfill this role. Another novel approach for controlling neuropathic pain can be pharmacological attenuation of glial and immune cell activation. It has been found that propentofylline, pentoxifylline, minocycline and fluorocitrate suppress the development of neuropathic pain. The other way of pain control can be the decrease of pro-nociceptive agents like transcription factor synthesis (NF-kappaB, AP-1); kinase synthesis (MEK, p38MAPK, JNK) and protease activation (cathepsin S, MMP9, MMP2). Additionally, since it is known that the opioid-induced glial activation opposes opioid analgesia, some glial inhibitors, which are safe and clinically well tolerated, are proposed as potential useful ko-analgesic agents for opioid treatment of neuropathic pain. This review pointed to some important mechanisms underlying the development of neuropathic pain, which led to identify some possible new approaches to the treatment of neuropathic pain, based on the more comprehensive knowledge of the interaction between the nervous system and glial and immune cells.

Trigeminal nerve injury ErbB3/ErbB2 promotes mechanical hypersensitivity

BACKGROUND

Chronic constriction injury of the trigeminal infraorbital nerve results in transient analgesia followed by whisker pad mechanical allodynia in rats. Neuregulin 1 expressed on axonal membranes binds receptor tyrosine kinase ErbB, promoting Schwann cell development and remyelination. This study investigated whether orofacial mechanical allodynia is signaled by ErbB3-ErbB2 heterodimers in injured nerves.

METHODS

Whisker pad mechanical allodynia (von Frey stimuli) was quantified in wild type rats and in transgenic rats with Sleeping Beauty transposon mutation for neuregulin 1 transgene. Pain-related behavior was retested after intraperitoneal injection of the ErbB2 inhibitor Lapatinib, an agent shown by others to reduce breast cancer pain. Infraorbital nerve injury was evaluated histologically with myelin and neuronal biomarkers. ErbB3 changes over time were measured with western blots.

RESULTS

Whisker pad mechanical hypersensitivity began in week 2 in wild type rats (3.11 ± 5.93 g vs. 18.72 ± 0.00 g after sham surgery, n = 9, P < 0.001), indicating trigeminal neuropathic pain, but was not evident in transgenic rats (odds ratio: 1.12, 95% confidence interval: 0.38-3.35). Initiation of statistically significant mechanohypersensitivity was delayed until week 6 after surgery in transgenic rats (3.44 ± 4.60 g vs. 18.72 ± 0.00 g, n = 4, P < 0.001). Mechanical allodynia, which persisted 8 weeks in wild type rats was alleviated by Lapatinib (15 ± 3.89 g vs. 2.45 ± 1.13 g, n = 6, P < 0.001). Infraorbital nerve damage was verified histologically. Statistically significant ErbB3 increases (weeks 5 and 10) in wild type and transgenic rats (week 10) coincided with time points when mechanical hypersensitivity was present.

CONCLUSION

The Neuregulin 1-ErbB3-ErbB2 complex is a causal mechanism in nerve injury-induced trigeminal neuropathic pain. Understanding peripheral glial mechanisms after nerve injury will improve neuropathic pain treatment.

GDNF induces mechanical hyperalgesia in muscle by reducing I(BK) in isolectin B4-positive nociceptors

We have assessed the mechanism underlying glial cell-derived neurotrophic factor (GDNF)-induced mechanical hyperalgesia in the gastrocnemius muscle, using patch clamp electrophysiology, in vivo electrophysiology and behavioral studies. Cultured isolectin B4-positive (IB4+) dorsal root ganglion neurons that innervated this muscle were held under current clamp; the majority developed an increase in action potential duration (a factor of increase of 2.29±0.24, compared to 1.13±0.17 in control, P<0.01) in response to GDNF (200 ng/ml) by 15 min after application. They also demonstrated a depolarization of resting membrane potential, but without significant changes in rheobase, action potential peak, or after-hyperpolarization. Large-conductance voltage- and calcium-activated potassium (BK) channels, which have recently been shown to play a role in the repolarization of IB4+ nociceptors, were inhibited under voltage clamp, as indicated by a significant reduction in the iberiotoxin-sensitive current. In vivo single-fiber recording from muscle afferents revealed that injection of iberiotoxin into their peripheral nociceptive field caused an increase in nociceptor firing in response to a 60s suprathreshold stimulus (an increase from 392.2±119.8 spikes to 596.1±170.8 spikes, P<0.05). This was observed in the absence of changes in the mechanical threshold. Finally, injection of iberiotoxin into the gastrocnemius muscle produced dose-dependent mechanical hyperalgesia. These data support the suggestion that GDNF induces nociceptor sensitization and mechanical hyperalgesia, at least in part, by inhibiting BK current in IB4+ nociceptors.

Human adipose-derived mesenchymal stem cells systemically injected promote peripheral nerve regeneration in the mouse model of sciatic crush

Mesenchymal stem cells (MSCs) represent a promising therapeutic approach in nerve tissue engineering. To date, the local implantation of MSC in injured nerves has been the only route of administration used. In case of multiple sites of injury, the systemic administration of cells capable of reaching damaged nerves would be advisable. In this regard, we found that an intravenous administration of adipose-derived MSC (ASC) 1 week after sciatic nerve crush injury, a murine model of acute axonal damage, significantly accelerated the functional recovery. Sciatic nerves from ASC-treated mice showed the presence of a restricted number of undifferentiated ASC together with a significant improvement in fiber sprouting and the reduction of inflammatory infiltrates for up to 3 weeks. Besides the immune modulatory effect, our results show that ASC may contribute to peripheral nerve regeneration because of their ability to produce in culture neuroprotective factors such as insulin-like growth factor I, brain-derived neurotrophic factor, or basic fibroblast growth factor. In addition to this production in vitro, we interestingly found that the concentration of glial-derived neurotrophic factor (GDNF) was significantly increased in the sciatic nerves in mice treated with ASC. Since no detectable levels of GDNF were observed in ASC cultures, we hypothesize that ASC induced the local production of GDNF by Schwann cells. In conclusion, we show that systemically injected ASC have a clear therapeutic potential in an acute model of axonal damage. Among the possible mechanisms promoting nerve regeneration, our results rule out a process of trans-differentiation and rather suggest the relevance of a bystander effect, including the production of in situ molecules, which, directly or indirectly through a cross-talk with local glial cells, may modulate the local environment with the down-regulation of inflammation and the promotion of axonal regeneration.

Translational strategies in peripheral neuroinflammation and neurovascular repair

Current therapies for immune-mediated inflammatory disorders in peripheral nerves are non-specific, and partly efficacious. Peripheral nerve regeneration following axonal degeneration or injury is suboptimal, with current therapies focused on modulating the underlying etiology and treating the consequences, such as neuropathic pain and weakness. Despite significant advances in understanding mechanisms of peripheral nerve inflammation, as well as axonal degeneration and regeneration, there has been limited translation into effective new drugs for these disorders. A major limitation in the field has been the unavailability of reliable disease models or research tools that mimic some key essential features of these human conditions. A relatively overlooked aspect of peripheral nerve regeneration has been neurovascular repair required to restore the homeostatic microenvironment necessary for normal function. Using Guillain-Barré syndrome (GBS) and chronic inflammatory demyelinating polyradiculoneuropathy (CIDP) as examples of human acute and chronic immune-mediated peripheral neuroinflammatory disorders respectively, we have performed detailed studies in representative mouse models to demonstrate essential features of the human disorders. These models are important tools to develop and test treatment strategies using realistic outcomes measures applicable to affected patients. In vitro models of the human blood-nerve barrier using endothelial cells derived by endoneurial microvessels provide insights into pro-inflammatory leukocyte-endothelial cell interactions relevant to peripheral neuroinflammation, as well as potential mediators and signaling pathways required for vascular proliferation, angiogenesis, remodeling and tight junction specialization necessary to restore peripheral nerve function following injury. This review discusses the progress we are making in translational peripheral neurobiology and our future directions.