Electrical stimulation promotes sensory neuron regeneration and growth-associated gene expression

An update on addressing important peripheral nerve problems: challenges and potential solutions

From time to time it is thoughtful and productive to review a medical field and reflect upon what are the major issues that need to be addressed and what is being done to do so. This review article is not meant to be all-inclusive but rather focuses on four evolving areas in the field of peripheral nerve disorders and treatments: (1) nerve surgery under ultrasound guidance using a new ultra-minimally invasive thread technique; (2) evolving magnetic resonance imaging (MRI) and ultrasound imaging techniques that are helping to both diagnose and treat a variety of peripheral nerve problems including entrapment neuropathies, traumatic nerve injuries, and masses arising from nerves; (3) promoting recovery after nerve injury using electrical stimulation; and (4) developing animal models to reproduce a severe nerve injury (neurotmetic grade in continuity) that requires a surgical intervention and repair. In each area we first describe the current challenges and then discuss new and emerging techniques and approaches. It is our hope that this article will bring added attention and resources to help better address peripheral nerve problems that remain a challenge for both patients and physicians.

Validation of electrical stimulation models: intracellular calcium measurement in three-dimensional scaffolds

Peripheral nerve injury can be disabling. Regeneration is limited by the rate of axonal extension, and proximal injury to peripheral nerves can take over a year to reach target organs. Electrical stimulation (ES) has been shown to increase the rate of neurite growth, though the mechanism is not yet well understood. In our prior manuscript, we developed a computational model that demonstrates how ES can functionally elevate intracellular calcium concentration ([Ca²⁺]_i) based on ES intensity and duration. In this article, we validate the computation model for the [Ca²⁺]_i changes in neuron soma. Embryonic chicken dorsal root ganglion cells were suspended in 3-dimensional collagen scaffolds. Fura-2 was used to measure [Ca²⁺]_i in response to biphasic ES pulses ranging from 70 to 60,000 V/m in intensity and from 10 µs to 100 ms in duration. The computational model most closely matched the experimental data of the neurons with the highest [Ca²⁺]_i elevation for ES pulses 100 µs or greater in duration. Nickel (200 µM) and cadmium (200 µM) blocked 98-99% of the [Ca²⁺]_i rise, indicating that the rise in [Ca²⁺]_i in response to ES is via voltage-dependent calcium channels. The average [Ca²⁺]_i rise in response to ES was about one-tenth of the peak rise. Therefore, the computational model is validated for elevating [Ca²⁺]_i of neurons and can be used as a tool for designing efficacious ES protocols for improving neuronal regeneration.NEW & NOTEWORTHY Electrical stimulation is used to enhance neuron growth, and the role of neuronal intracellular calcium concentration ([Ca²⁺]_i) is an area of research interest. Widely varying stimulation parameters in the literature make it difficult to compare stimulation protocols. The results in this manuscript are the first to show neuronal [Ca²⁺]_i in response to a broad and defined range of electrical pulse durations and intensities. These results validate our previously published novel computational model of [Ca²⁺]_i.

Surgical repair in humans after traumatic nerve injury provides limited functional neural regeneration in adults

Traumatic nerve injuries result in devastating loss of neurologic function with unpredictable functional recovery despite optimal medical management. After traumatic nerve injury and denervation, regenerating axons must traverse a complex environment in which they encounter numerous barriers on the way to reinnervation of their target muscle. Outcomes of surgical intervention alone have unfortunately reached a plateau, resulting in often unsatisfactory functional recovery. Over the past few decades, many improvements were developed to supplement and boost the results of surgical repair. Biological optimization of Schwann cells, macrophages, and degradation enzymes have been studied due to the key roles of these components in axonal development, maintenance and response to injury. Moreover, surgical techniques such as nerve grafting, conduits, and growth factor supplementation are also employed to enhance the microenvironment and nerve regeneration. Yet, most of the roadblocks to recovery after nerve injury remain unsolved. These roadblocks include, but are not limited to: slow regeneration rates and specificity of target innervation, the presence of a segmental nerve defect, and degeneration of the target end-organ after prolonged periods of denervation. A recognition of these limitations is necessary so as to develop new strategies to improve functional regeneration for these life changing injuries.

The role of cAMP and its downstream targets in neurite growth in the adult nervous system

Injured neurons in the adult mammalian central nervous system (CNS) have a very limited capacity for axonal regeneration and neurite outgrowth. This inability to grow new axons or to regrow injured axons is due to the presence of molecules that inhibit axonal growth, and age related changes in the neuron's innate growth capabilities. Available levels of cAMP are thought to have an important role in linking both of these factors. Elevated levels of cAMP in the developing nervous system are important for the guidance and stability of growth cones. As the nervous system matures, cAMP levels decline and the growth promoting effects of cAMP diminish. It has frequently been demonstrated that increasing neuronal cAMP can enhance neurite growth and regeneration. Some methods used to increase cAMP include administration of cAMP agonists, conditioning lesions, or electrical stimulation. Furthermore, it has been proposed that multiple stages of cAMP induced growth exist, one directly caused by its downstream effector Protein Kinase A (PKA) and one caused by the eventual upregulation of gene transcription. Although the role cAMP in promoting axon growth is well accepted, the downstream pathways that mediate cAMP-mediated axonal growth are less clear. This is partly because various key studies that explored the link between PKA and axonal outgrowth relied on the PKA inhibitors KT5720 and H89. More recent studies have shown that both of these drugs are less specific than initially thought and can inhibit a number of other signalling molecules including the Exchange Protein Activated by cAMP (EPAC). Consequently, it has recently been shown that a number of intracellular signalling pathways previously attributed to PKA can now be attributed solely to activation of EPAC specific pathways, or the simultaneous co-activation of PKA and EPAC specific pathways. These new studies open the door to new potential treatments for repairing the injured spinal cord.

4-Aminopyridine promotes functional recovery and remyelination in acute peripheral nerve injury

Traumatic peripheral nerve damage is a major medical problem without effective treatment options. In repurposing studies on 4-aminopyridine (4-AP), a potassium channel blocker that provides symptomatic relief in some chronic neurological afflictions, we discovered this agent offers significant promise as a small molecule regenerative agent for acute traumatic nerve injury. We found, in a mouse model of sciatic crush injury, that sustained early 4-AP administration increased the speed and extent of behavioral recovery too rapidly to be explained by axonal regeneration. Further studies demonstrated that 4-AP also enhanced recovery of nerve conduction velocity, promoted remyelination, and increased axonal area post-injury. We additionally found that 4-AP treatment enables distinction between incomplete and complete lesions more rapidly than existing approaches, thereby potentially addressing the critical challenge of more effectively distinguishing injured individuals who may require mutually exclusive treatment approaches. Thus, 4-AP singularly provides both a new potential therapy to promote durable recovery and remyelination in acute peripheral nerve injury and a means of identifying lesions in which this therapy would be most likely to be of value.

Enhancement of neurite adhesion, alignment and elongation on conductive polypyrrole-poly(lactide acid) fibers with cell-derived extracellular matrix

Extracellular matrix (ECM) can promote peripheral nerve repair. In this study, a conductive fiber-film (CFF) with core-sheath structure and conductivity of ∼10Scm^-1 was prepared by electrospinning of aligned poly(l-lactide acid) (PLLA) fibers and electrochemical deposition of polypyrole (PPy) nanoparticles. Then the multiple components of ECM, including laminin, fibronectin and collagen, were coated on the surface of CFF by culturing and lysing L929 cells to fabricate the bioactive scaffold of ECM-linked CFF (ECM-CFF). The electrical stimulation (ES) of 100mV/cm for 14days and 2h per day did not significantly decrease the conductivity of ECM-CFF. The results of PC12 cells test indicated that, cells adhesion rate, neurite-bearing cell rate and neurite alignment rate on ECM-CFF were ∼95%, ∼77%, ∼70%, respectively, significantly larger than the corresponding values on bare CFF (17%, 29% and 14%, respectively). The neurites length on ECM-CFF (∼79mm) was also larger than that on bare CFF (∼25mm). ES of 100mV/cm onto PC12 cells through ECM-CFF could significantly promote neurite extension in first 3days of the neurite growth. These results indicated that, the combination of ECM-CFF with ES could improve the nerve regeneration by encouraging neural-cell adhesion, neurite growth and extension.

Dynamic impact of brief electrical nerve stimulation on the neural immune axis-polarization of macrophages toward a pro-repair phenotype in demyelinated peripheral nerve

Demyelinating peripheral nerves are infiltrated by cells of the monocyte lineage, including macrophages, which are highly plastic, existing on a continuum from pro-inflammatory M1 to pro-repair M2 phenotypic states. Whether one can therapeutically manipulate demyelinated peripheral nerves to promote a pro-repair M2 phenotype remains to be elucidated. We previously identified brief electrical nerve stimulation (ES) as therapeutically beneficial for remyelination, benefits which include accelerated clearance of macrophages, making us theorize that ES alters the local immune response. Thus, the impact of ES on the immune microenvironment in the zone of demyelination was examined. Adult male rat tibial nerves were focally demyelinated via 1% lysophosphatidyl choline (LPC) injection. Five days later, half underwent 1 hour 20 Hz sciatic nerve ES proximal to the LPC injection site. ES had a remarkable and significant impact, shifting the macrophage phenotype from predominantly pro-inflammatory/M1 toward a predominantly pro-repair/M2 one, as evidenced by an increased incidence of expression of M2-associated phenotypic markers in identified macrophages and a decrease in M1-associated marker expression. This was discernible at 3 days post-ES (8 days post-LPC) and continued at the 5 day post-ES (10 days post-LPC) time point examined. ES also affected chemokine (C-C motif) ligand 2 (CCL2; aka MCP-1) expression in a manner that correlated with increases and decreases in macrophage numbers observed in the demyelination zone. The data establish that briefly increasing neuronal activity favorably alters the immune microenvironment in demyelinated nerve, rapidly polarizing macrophages toward a pro-repair phenotype, a beneficial therapeutic concept that may extend to other pathologies. GLIA 2016;64:1546-1561.

Nerve Regeneration: Understanding Biology and Its Influence on Return of Function After Nerve Transfers

Poor functional outcomes are frequent after peripheral nerve injuries despite the regenerative support of Schwann cells. Motoneurons and, to a lesser extent, sensory neurons survive the injuries but outgrowth of axons across the injury site is slow. The neuronal regenerative capacity and the support of regenerating axons by the chronically denervated Schwann cells progressively declines with time and distance of the injury from the denervated targets. Strategies, including brief low-frequency electrical stimulation that accelerates target reinnervation and functional recovery, and the insertion of cross-bridges between a donor nerve and a recipient denervated nerve stump, are effective in promoting functional outcomes after complete and incomplete injuries.

Growth and follow-up of primary cortical neuron cells on nonfunctionalized graphene nanosheet film

BACKGROUND

Conductive biomaterials are an ideal biosubstrate for modifying cellular behaviors by conducting either internal or external electrical signals. In this study, based on a simple-preparation graphite exfoliation method in organic reagent, a nonfunctionalized graphene nanosheet film (NGNF) with high conductivity and large size was simply fabricated through spraying coating. The biocompatibility of the NGNF was carefully tested with primary cortical neuron cells, and its biocompatibility properties were compared with a chemical vapor deposition (CVD) graphene film.

METHODS

Nonfunctionalized graphene nanosheet (NGN) was first exfoliated from graphite with a flat-tip ultrasonicator probe, and then spray-coated onto glass slide substrate to form the film. The morphology of NGNF was observed with light microscopy and SEM. The morphology and neuronal network formation of primary cortical neuron cells onto NGNF, as shown by DAPI and Alexa Fluor® 488 staining, were observed with fluorescent microscopy. Cell viability and proliferation were measured with MTT.

RESULTS

NGNF had better cell biocompatibility than CVD graphene film. MTT test showed that NGNF exhibited no cytotoxicity. According to neuronal network formation at 7 days of cell culture, primary neuron cells aggregated into 50-μm "nuclei"; the average neurite number and length were 3 and 100 μm, respectively. However, these values were almost doubled after 14 days of cell culture.

CONCLUSIONS

These results may improve the use of NGNF as a conductive scaffold for nerve regeneration.

Electrical Stimulation to Enhance Axon Regeneration After Peripheral Nerve Injuries in Animal Models and Humans

Injured peripheral nerves regenerate their lost axons but functional recovery in humans is frequently disappointing. This is so particularly when injuries require regeneration over long distances and/or over long time periods. Fat replacement of chronically denervated muscles, a commonly accepted explanation, does not account for poor functional recovery. Rather, the basis for the poor nerve regeneration is the transient expression of growth-associated genes that accounts for declining regenerative capacity of neurons and the regenerative support of Schwann cells over time. Brief low-frequency electrical stimulation accelerates motor and sensory axon outgrowth across injury sites that, even after delayed surgical repair of injured nerves in animal models and patients, enhances nerve regeneration and target reinnervation. The stimulation elevates neuronal cyclic adenosine monophosphate and, in turn, the expression of neurotrophic factors and other growth-associated genes, including cytoskeletal proteins. Electrical stimulation of denervated muscles immediately after nerve transection and surgical repair also accelerates muscle reinnervation but, at this time, how the daily requirement of long-duration electrical pulses can be delivered to muscles remains a practical issue prior to translation to patients. Finally, the technique of inserting autologous nerve grafts that bridge between a donor nerve and an adjacent recipient denervated nerve stump significantly improves nerve regeneration after delayed nerve repair, the donor nerves sustaining the capacity of the denervated Schwann cells to support nerve regeneration. These reviewed methods to promote nerve regeneration and, in turn, to enhance functional recovery after nerve injury and surgical repair are sufficiently promising for early translation to the clinic.

The use of brief post-surgical low frequency electrical stimulation to enhance nerve regeneration in clinical practice

Despite efforts to enhance peripheral nerve regeneration, there has been little progress in improving clinical outcomes. Recently, a method of brief post-surgical low frequency electrical stimulation of surgically repaired nerves has been developed. It was shown to accelerate axon outgrowth across the repair site and it hastened target reinnervation. In this brief review, we describe the mechanistic insights and functional impacts of the post-surgical electrical stimulation that have been gained through animal studies. Brain-derived neurotrophic factor, cyclic AMP and regeneration-associated genes play a vital role in expediting the outgrowth of axons across the injury site. The method of stimulation has also been shown to be effective in patients with severe compressive neuropathy as well as those with digital nerve laceration. Its clinical feasibility and positive impact open the door of further clinical translation in other peripheral nerve injuries.

Therapeutic application of electrical stimulation and constraint induced movement therapy in perinatal brachial plexus injury: A case report

Infants and children with perinatal brachial plexus injury (PBPI) have motion limitations in the shoulder, elbow, forearm and hand that are dependent on the level of injury and degree of recovery. The injury and subsequent recovery period occur during critical periods of central and spinal neural development placing infants and children at-risk for developmental disregard and disuse of the affected arm and hand. A case report outlines the therapy and surgical interventions provided in the first 2 years of life for a child with global PBPI and a positive Horner's sign. Electrical stimulation and constraint induced movement therapy provided sequentially were effective therapy interventions. Neurosurgery to repair the brachial plexus was performed at an optimal time period.(2) The Assisting Hand Assessment,(12) Modified Mallet(13) and Active Movement Scale(14) are effective outcome measures in PBPI and served as valuable guides for therapy intervention. Oxford Level of Evidence: 3b; Individual Case Control Study.

Electrical stimulation promotes regeneration of injured oculomotor nerves in dogs

Functional recovery after oculomotor nerve injury is very poor. Electrical stimulation has been shown to promote regeneration of injured nerves. We hypothesized that electrical stimulation would improve the functional recovery of injured oculomotor nerves. Oculomotor nerve injury models were created by crushing the right oculomotor nerves of adult dogs. Stimulating electrodes were positioned in both proximal and distal locations of the lesion, and non-continuous rectangular, biphasic current pulses (0.7 V, 5 Hz) were administered 1 hour daily for 2 consecutive weeks. Analysis of the results showed that electrophysiological and morphological recovery of the injured oculomotor nerve was enhanced, indicating that electrical stimulation improved neural regeneration. Thus, this therapy has the potential to promote the recovery of oculomotor nerve dysfunction.

Pathways Mediating Activity-Induced Enhancement of Recovery From Peripheral Nerve Injury

This article outlines the novel hypothesis that exercise promotes axon regeneration after peripheral nerve injury through neuronal brain-derived neurotrophic factor (BDNF), and there are three required means of promoting BDNF expression: 1) increased signaling through androgen receptors, 2) increased cAMP-responsive element-binding protein expression, and 3) increased expression of the transcription factor SRY-box containing gene 11.

Ex Vivo Assay of Electrical Stimulation to Rat Sciatic Nerves: Cell Behaviors and Growth Factor Expression

Neurite outgrowth and axon regeneration are known to benefit from electrical stimulation. However, how neuritis and their surroundings react to electrical field is difficult to replicate by monolayer cell culture. In this work freshly harvested rat sciatic nerves were cultured and exposed to two types of electrical field, after which time the nerve tissues were immunohistologically stained and the expression of neurotrophic factors and cytokines were evaluated. ELISA assay was used to confirm the production of specific proteins. All cell populations survived the 48 h culture with little necrosis. Electrical stimulation was found to accelerate Wallerian degeneration and help Schwann cells to switch into migratory phenotype. Inductive electrical stimulation was shown to upregulate the secretion of multiple neurotrophic factors. Cellular distribution in nerve tissue was altered upon the application of an electrical field. This work thus presents an ex vivo model to study denervated axon in well controlled electrical field, bridging monolayer cell culture and animal experiment. It also demonstrated the critical role of electrical field distribution in regulating cellular activities.

Brain-controlled muscle stimulation for the restoration of motor function

Loss of the ability to move, as a consequence of spinal cord injury or neuromuscular disorder, has devastating consequences for the paralyzed individual, and great economic consequences for society. Functional electrical stimulation (FES) offers one means to restore some mobility to these individuals, improving not only their autonomy, but potentially their general health and well-being as well. FES uses electrical stimulation to cause the paralyzed muscles to contract. Existing clinical systems require the stimulation to be preprogrammed, with the patient typically using residual voluntary movement of another body part to trigger and control the patterned stimulation. The rapid development of neural interfacing in the past decade offers the promise of dramatically improved control for these patients, potentially allowing continuous control of FES through signals recorded from motor cortex, as the patient attempts to control the paralyzed body part. While application of these 'brain-machine interfaces' (BMIs) has undergone dramatic development for control of computer cursors and even robotic limbs, their use as an interface for FES has been much more limited. In this review, we consider both FES and BMI technologies and discuss the prospect for combining the two to provide important new options for paralyzed individuals.

Computational modeling of neurons: intensity-duration relationship of extracellular electrical stimulation for changes in intracellular calcium

In many instances of extensive nerve damage, the injured nerve never adequately heals, leaving lack of nerve function. Electrical stimulation (ES) has been shown to increase the rate and orient the direction of neurite growth, and is a promising therapy. However, the mechanism in which ES affects neuronal growth is not understood, making it difficult to compare existing ES protocols or to design and optimize new protocols. We hypothesize that ES acts by elevating intracellular calcium concentration ([Ca(2+)]i) via opening voltage-dependent Ca(2+) channels (VDCCs). In this work, we have created a computer model to estimate the ES Ca(2+) relationship. Using COMSOL Multiphysics, we modeled a small dorsal root ganglion (DRG) neuron that includes one Na(+) channel, two K(+) channels, and three VDCCs to estimate [Ca(2+)]i in the soma and growth cone. As expected, the results show that an ES that generates action potentials (APs) can efficiently raise the [Ca(2+)]i of neurons. More interestingly, our simulation results show that sub-AP ES can efficiently raise neuronal [Ca(2+)]i and that specific high-voltage ES can preferentially raise [Ca(2+)]i in the growth cone. The intensities and durations of ES on modeled growth cone calcium rise are consistent with directionality and orientation of growth cones experimentally shown by others. Finally, this model provides a basis to design experimental ES pulse parameters, including duration, intensity, pulse-train frequency, and pulse-train duration to efficiently raise [Ca(2+)]i in neuronal somas or growth cones.

Long-term effects of an intensive interventional training program based on activities for individuals with spinal cord injury: a pilot study

OBJECTIVES

To investigate the long-term effects of a rehabilitation program using activity-based therapies in daily activities and the participation of individuals with spinal cord injury (SCI).

METHOD

A descriptive study of case reports assessing the performance of daily activities and quality of life as a dependent variable, using the Functional Independence Measure (FIM) and the Short-Form Health Survey (SF-36), respectively. Seven individuals were included in the intervention composed of a multimodal intensive therapies program based on activities (activity-based therapy, ABT) conducted for 18 months.

RESULTS

It was possible to descriptively observe that the individual with the shortest time of injury and previous training obtained the largest variation in the FIM score. But no statistically significant difference was found in the assessments.

CONCLUSION

For trained individuals with chronic SCIs, classified "A" according to the American Spinal Injury Association (ASIA), an ABT program did not significantly affect the scores of the scales used to assess quality of life (SF-36) and functional independence (FIM).

Electrical Stimulation to Promote Peripheral Nerve Regeneration

Peripheral nerve injury afflicts individuals from all walks of life. Despite the peripheral nervous system’s intrinsic ability to regenerate, many patients experience incomplete functional recovery. Surgical repair aims to expedite this recovery process in the most thorough manner possible. However, full recovery is still rarely seen especially when nerve injury is compounded with polytrauma where surgical repair is delayed. Pharmaceutical strategies supplementary to nerve microsurgery have been investigated but surgery remains the only viable option. Brief low-frequency electrical stimulation of the proximal nerve stump after primary repair has been widely investigated. This article aims to review the currently known biological basis for the regenerative effects of acute brief low-frequency electrical stimulation on axonal regeneration and outline the recent clinical applications of the electrical stimulation protocol to demonstrate the significant translational potential of this modality for repairing peripheral nerve injuries. The review concludes with a discussion of emerging new advancements in this exciting area of research. The current literature indicates the imminent clinical applicability of acute brief low-frequency electrical stimulation after surgical repair to effectively promote axonal regeneration as the stimulation has yielded promising evidence to maximize functional recovery in diverse types of peripheral nerve injuries.

Exercise, neurotrophins, and axon regeneration in the PNS

Electrical stimulation and exercise are treatments to enhance recovery from peripheral nerve injuries. Brain-derived neurotrophic factor and androgen receptor signaling are requirements for the effectiveness of these treatments. Increased neuronal activity is adequate to promote regeneration in injured nerves, but the dosing of activity and its relationship to neurotrophins and sex steroid hormones is less clear. Translation of these therapies will require principles associated with their cellular mechanisms.

Peripheral neuron plasticity is enhanced by brief electrical stimulation and overrides attenuated regrowth in experimental diabetes

Peripheral nerve regrowth is less robust than commonly assumed, particularly when it accompanies common clinical scenarios such as diabetes mellitus. Brief extracellular electrical stimulation (ES) facilitates the regeneration of peripheral nerves in part through early activation of the conditioning injury response and BDNF. Here, we explored intrinsic neuronal responses to ES to identify whether ES might impact experimental diabetes, where regeneration is attenuated. ES altered several regeneration related molecules including rises in tubulin, Shh (Sonic hedgehog) and GAP43 mRNAs. ES was associated with rises in neuronal intracellular calcium but its strict linkage to regrowth was not confirmed. In contrast, we identified PI3K-PTEN involvement, an association previously linked to diabetic regenerative impairment. Following ES there were declines in PTEN protein and mRNA both in vitro and in vivo and a PI3K inhibitor blocked its action. In vitro, isolated diabetic neurons were capable of mounting robust responsiveness to ES. In vivo, ES improved electrophysiological and behavioral indices of nerve regrowth in a chronic diabetic model of mice with pre-existing neuropathy. Regrowth of myelinated axons and reinnervation of the epidermis were greater following ES than sham stimulation. Taken together, these findings identify a role for ES in supporting regeneration during the challenges of diabetes mellitus.

Strategies to promote peripheral nerve regeneration: electrical stimulation and/or exercise

Enhancing the regeneration of axons is often considered to be a therapeutic target for improving functional recovery after peripheral nerve injury. In this review, the evidence for the efficacy of electrical stimulation (ES), daily exercise and their combination in promoting nerve regeneration after peripheral nerve injuries in both animal models and in human patients is explored. The rationale, effectiveness and molecular basis of ES and exercise in accelerating axon outgrowth are reviewed. In comparing the effects of ES and exercise in enhancing axon regeneration, increased neural activity, neurotrophins and androgens are considered to be common requirements. Similarly, there are sex-specific requirements for exercise to enhance axon regeneration in the periphery and for sustaining synaptic inputs onto injured motoneurons. ES promotes nerve regeneration after delayed nerve repair in humans and rats. The effectiveness of exercise is less clear. Although ES, but not exercise, results in a significant misdirection of regenerating motor axons to reinnervate different muscle targets, the loss of neuromuscular specificity encountered has only a very small impact on resulting functional recovery. Both ES and exercise are promising experimental treatments for peripheral nerve injury that seem to be ready to be translated to clinical use.

Shocking therapy: Brief electrical stimulation for delayed nerve repair

This commentary provides perspective on a recent paper published in Experimental Neurology by Elzinga et al. where the authors investigated the effect of brief electrical stimulation (ES) on nerve regeneration after delayed nerve repair in a rodent model. Their results from a well controlled series of experiments indicated that brief ES promoted axonal outgrowth after chronic axotomy as well after chronic Schwann cell and muscle denervation. ES also increased chronically axotomized neurons regenerating into chronically denervated stumps, which represent a true delayed repair. The authors conclude that brief ES promotion of nerve regeneration after delayed nerve repair is as effective as after immediate repair. Given the prior experimental evidence, and the prior clinical data from patients with carpal tunnel syndrome and digital nerve repair, the implication of this new work is to consider a well designed clinical trial for use of brief ES in nerve graft and nerve transfer repairs.

Optogenetic control of nerve growth

Due to the limited regenerative ability of neural tissue, a diverse set of biochemical and biophysical cues for increasing nerve growth has been investigated, including neurotrophic factors, topography, and electrical stimulation. In this report, we explore optogenetic control of neurite growth as a cell-specific alternative to electrical stimulation. By investigating a broad range of optical stimulation parameters on dorsal root ganglia (DRGs) expressing channelrhodopsin 2 (ChR2), we identified conditions that enhance neurite outgrowth by three-fold as compared to unstimulated or wild-type (WT) controls. Furthermore, optogenetic stimulation of ChR2 expressing DRGs induces directional outgrowth in WT DRGs co-cultured within a 10 mm vicinity of the optically sensitive ganglia. This observed enhancement and polarization of neurite growth was accompanied by an increased expression of neural growth and brain derived neurotrophic factors (NGF, BDNF). This work highlights the potential for implementing optogenetics to drive nerve growth in specific cell populations.

Electrical stimulation enhances sensory recovery: a randomized controlled trial

OBJECTIVE

Brief postsurgical electrical stimulation (ES) has been shown to enhance peripheral nerve regeneration in animal models following axotomy and crush injury. However, whether this treatment is beneficial in humans with sensory nerve injury has not been tested. The goal of this study was to test the hypothesis that ES would enhance sensory nerve regeneration following digital nerve transection compared to surgery alone.

METHODS

Patients with complete digital nerve transection underwent epineurial nerve repair. After coaptation of the severed nerve ends, fine wire electrodes were implanted before skin closure. Postoperatively, patients were randomized to receiving either 1 hour of 20Hz continuous ES or sham stimulation in a double-blinded manner. Patients were followed monthly for 6 months by a blinded evaluator to monitor physiological recovery of spatial discrimination, pressure threshold, and quantitative small fiber sensory testing. Functional disability was measured using the Disability of Arm, Shoulder, and Hand questionnaire.

RESULTS

A total of 36 patients were recruited, with 18 in each group. Those in the ES group showed consistently greater improvements in all sensory modalities by 5 to 6 months postoperatively compared to the controls. Although there was a trend of greater functional improvements in the ES group, it was not statistically significant (p > 0.01).

INTERPRETATION

Postsurgical ES enhanced sensory reinnervation in patients who sustained complete digital nerve transection. The conferred benefits apply to a wide range of sensory functions.

Low-frequency electrical stimulation induces the proliferation and differentiation of peripheral blood stem cells into Schwann cells

BACKGROUND

Functional recovery after peripheral nerve injury remains a tough problem at present. Specifically, a type of glial cell exists in peripheral nerves that promotes axonal growth and myelin formation and secretes various active substances, such as neurotrophic factors, extracellular matrix and adherence factors. These substances have important significance for the survival, growth and regeneration of nerve fibers. Numerous recent studies have shown that electrical stimulation can increase the number of myelinated nerve fibers. However, whether electrical stimulation acts on neurons or Schwann cells has not been verified in vivo. This study investigates low-frequency electrical stimulation-induced proliferation and differentiation of peripheral blood stem cells into Schwann cells and explores possible mechanisms.

METHODS

Peripheral blood stem cells from Sprague-Dawley rats were primarily cultured. Cells in passage 3 were divided into 4 groups: a low-frequency electrical stimulation group (20 Hz, 100 μs, 3 V), a low-frequency electrical stimulation+PD98059 (blocking the extracellular signal-regulated kinase [ERK] signaling pathway) group, a PD98059 group and a control group (no treatment). After induction, the cells were characterized. A 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazoliumbromide assay was employed to measure the absorbance values at 570 nm in the 4 groups. A Western blot assay was used to detect the expression of cyclin D1 and cyclin-dependent kinase 4 (CDK4) in each group.

RESULTS

No significant difference in cell viability was detected before induction. Peripheral blood stem cells from the 4 groups differentiated into Schwann cells. Phosphorylated ERK 1/2, cyclin D1 and CDK4 protein levels were highest in the low-frequency electrical stimulation group and lowest in the ERK blockage group. Phosphorylated ERK 1/2, cyclin D1 and CDK4 protein levels in the low-frequency electrical stimulation+ERK blockage group were lower than those in the low-frequency electrical stimulation group but higher than those in the ERK blockage group.

CONCLUSIONS

Low-frequency electrical stimulation contributed to the proliferation of peripheral blood stem cells cultured in vitro and induced differentiation into Schwann cells. The ERK signaling pathway underlies cell proliferation and differentiation.

Brief electrical stimulation improves nerve regeneration after delayed repair in Sprague Dawley rats

Functional recovery after peripheral nerve injury and surgical repair declines with time and distance because the injured neurons without target contacts (chronic axotomy) progressively lose their regenerative capacity and chronically denervated Schwann cells (SCs) atrophy and fail to support axon regeneration. Findings that brief low frequency electrical stimulation (ES) accelerates axon outgrowth and muscle reinnervation after immediate nerve surgery in rats and human patients suggest that ES might improve regeneration after delayed nerve repair. To test this hypothesis, common peroneal (CP) neurons were chronically axotomized and/or tibial (TIB) SCs and ankle extensor muscles were chronically denervated by transection and ligation in rats. The CP and TIB nerves were cross-sutured after three months and subjected to either sham or one hour 20Hz ES. Using retrograde tracing, we found that ES significantly increased the numbers of both motor and sensory neurons that regenerated their axons after a three month period of chronic CP axotomy and/or chronic TIB SC denervation. Muscle and motor unit forces recorded to determine the numbers of neurons that reinnervated gastrocnemius muscle demonstrated that ES significantly increased the numbers of motoneurons that reinnervated chronically denervated muscles. We conclude that electrical stimulation of chronically axotomized motor and sensory neurons is effective in accelerating axon outgrowth into chronically denervated nerve stumps and improving target reinnervation after delayed nerve repair. Possible mechanisms for the efficacy of ES in promoting axon regeneration and target reinnervation after delayed nerve repair include the upregulation of neurotrophic factors.

Electrical stimuli in the central nervous system microenvironment

Electrical stimulation to manipulate the central nervous system (CNS) has been applied as early as the 1750s to produce visual sensations of light. Deep brain stimulation (DBS), cochlear implants, visual prosthetics, and functional electrical stimulation (FES) are being applied in the clinic to treat a wide array of neurological diseases, disorders, and injuries. This review describes the history of electrical stimulation of the CNS microenvironment; recent advances in electrical stimulation of the CNS, including DBS to treat essential tremor, Parkinson's disease, and depression; FES for the treatment of spinal cord injuries; and alternative electrical devices to restore vision and hearing via neuroprosthetics (retinal and cochlear implants). It also discusses the role of electrical cues during development and following injury and, importantly, manipulation of these endogenous cues to support regeneration of neural tissue.

Electrical stimulation does not enhance nerve regeneration if delayed after sciatic nerve injury: the role of fibrosis

Electrical stimulation has been shown to accelerate and enhance nerve regeneration in sensory and motor neurons after injury, but there is little evidence that focuses on the varying degrees of fibrosis in the delayed repair of peripheral nerve tissue. In this study, a rat model of sciatic nerve transection injury was repaired with a biodegradable conduit at 1 day, 1 week, 1 month and 2 months after injury, when the rats were divided into two subgroups. In the experimental group, rats were treated with electrical stimuli of frequency of 20 Hz, pulse width 100 ms and direct current voltage of 3 V; while rats in the control group received no electrical stimulation after the conduit operation. Histological results showed that stained collagen fibers comprised less than 20% of the total operated area in the two groups after delayed repair at both 1 day and 1 week but after longer delays, the collagen fiber area increased with the time after injury. Immunohistochemical staining revealed that the expression level of transforming growth factor β (an indicator of tissue fibrosis) decreased at both 1 day and 1 week after delayed repair but increased at both 1 and 2 months after delayed repair. These findings indicate that if the biodegradable conduit repair combined with electrical stimulation is delayed, it results in a poor outcome following sciatic nerve injury. One month after injury, tissue degeneration and distal fibrosis are apparent and are probably the main reason why electrical stimulation fails to promote nerve regeneration after delayed repair.

Proteomic study of retinal proteins associated with transcorneal electric stimulation in rats

Background. To investigate how transcorneal electric stimulation (TES) affects the retina, by identifying those proteins up- and downregulated by transcorneal electric stimulation (TES) in the retina of rats. Methods. Adult Wistar rats received TES on the left eyes at different electrical currents while the right eyes received no treatment and served as controls. After TES, the eye was enucleated and the retina was isolated. The retinas were analyzed by proteomics. Results. Proteomics showed that twenty-five proteins were upregulated by TES. The identified proteins included cellular signaling proteins, proteins associated with neuronal transmission, metabolic proteins, immunological factors, and structural proteins. Conclusions. TES induced changes in expression of various functional proteins in the retina.

Advances in diagnosis and non-surgical treatment of Bell's palsy

Bell's palsy is a commonly seen cranial nerve disease and can result in compromised facial appearance and functions. Its etiology, prognosis and treatment are still being debated. This paper is a review of recent development in the understanding of etiology, diagnosis and non-surgical treatment of Bell's palsy.

Hierarchical patterning of multifunctional conducting polymer nanoparticles as a bionic platform for topographic contact guidance

The use of programmed electrical signals to influence biological events has been a widely accepted clinical methodology for neurostimulation. An optimal biocompatible platform for neural activation efficiently transfers electrical signals across the electrode-cell interface and also incorporates large-area neural guidance conduits. Inherently conducting polymers (ICPs) have emerged as frontrunners as soft biocompatible alternatives to traditionally used metal electrodes, which are highly invasive and elicit tissue damage over long-term implantation. However, fabrication techniques for the ICPs suffer a major bottleneck, which limits their usability and medical translation. Herein, we report that these limitations can be overcome using colloidal chemistry to fabricate multimodal conducting polymer nanoparticles. Furthermore, we demonstrate that these polymer nanoparticles can be precisely assembled into large-area linear conduits using surface chemistry. Finally, we validate that this platform can act as guidance conduits for neurostimulation, whereby the presence of electrical current induces remarkable dendritic axonal sprouting of cells.

Short-term electrical stimulation to promote nerve repair and functional recovery in a rat model

PURPOSE

To evaluate the effect of duration of electrical stimulation on peripheral nerve regeneration and functional recovery. Based on previous work, we hypothesized that applying 10 minutes of electrical stimulation to a 10-mm rat sciatic nerve defect would significantly improve nerve regeneration and functional recovery compared with the non-electrical stimulation group.

METHODS

A silicone tube filled with a collagen gel was used to bridge a 10-mm nerve defect in rats, and either 10 minutes or 60 minutes of electrical stimulation was applied to the nerve during surgery. Controls consisted of a silicone tube with collagen gel and no electrical stimulation or an isograft. We analyzed recovery over a 12-week period, measuring sciatic functional index and extensor postural thrust scores and concluding with histological examination of the nerve.

RESULTS

Functional assessment scores at week 12 increased 24% in the 10-minute group as compared to the no stimulation control group. Electrical stimulation of either 10 or 60 minutes improved the number of nerve fibers over no stimulation. Additionally, the electrical stimulation group's histomorphometric analysis was not different from the isograft group.

CONCLUSIONS

Several previous studies have demonstrated the effectiveness of 60-minute stimulations on peripheral nerve regeneration. This study demonstrated that an electrical stimulation of 10 minutes enhanced several functional and histomorphometric outcomes of nerve regeneration and was overall similar to a 60-minute stimulation over 12 weeks.

CLINICAL RELEVANCE

Decreasing the electrical stimulation time from 60 minutes to 10 minutes provided a potential clinically feasible and safe method to enhance nerve regeneration and functional recovery.

Delaying the onset of treadmill exercise following peripheral nerve injury has different effects on axon regeneration and motoneuron synaptic plasticity

Transection of a peripheral nerve results in withdrawal of synapses from motoneurons. Some of the withdrawn synapses are restored spontaneously, but those containing the vesicular glutamate transporter 1 (VGLUT1), and arising mainly from primary afferent neurons, are withdrawn permanently. If animals are exercised immediately after nerve injury, regeneration of the damaged axons is enhanced and no withdrawal of synapses from injured motoneurons can be detected. We investigated whether delaying the onset of exercise until after synapse withdrawal had occurred would yield similar results. In Lewis rats, the right sciatic nerve was cut and repaired. Reinnervation of the soleus muscle was monitored until a direct muscle (M) response was observed to stimulation of the tibial nerve. At that time, rats began 2 wk of daily treadmill exercise using an interval training protocol. Both M responses and electrically-evoked H reflexes were monitored weekly for an additional seven wk. Contacts made by structures containing VGLUT1 or glutamic acid decarboxylase (GAD67) with motoneurons were studied from confocal images of retrogradely labeled cells. Timing of full muscle reinnervation was similar in both delayed and immediately exercised rats. H reflex amplitude in delayed exercised rats was only half that found in immediately exercised animals. Unlike immediately exercised animals, motoneuron contacts containing VGLUT1 in delayed exercised rats were reduced significantly, relative to intact rats. The therapeutic window for application of exercise as a treatment to promote restoration of synaptic inputs onto motoneurons following peripheral nerve injury is different from that for promoting axon regeneration in the periphery.

Reduced expression of regeneration associated genes in chronically axotomized facial motoneurons

Chronically axotomized motoneurons progressively fail to regenerate their axons. Since axonal regeneration is associated with the increased expression of tubulin, actin and GAP-43, we examined whether the regenerative failure is due to failure of chronically axotomized motoneurons to express and sustain the expression of these regeneration associated genes (RAGs). Chronically axotomized facial motoneurons were subjected to a second axotomy to mimic the clinical surgical procedure of refreshing the proximal nerve stump prior to nerve repair. Expression of α1-tubulin, actin and GAP-43 was analyzed in axotomized motoneurons using in situ hybridization followed by autoradiography and silver grain quantification. The expression of these RAGs by acutely axotomized motoneurons declined over several months. The chronically injured motoneurons responded to a refreshment axotomy with a re-increase in RAG expression. However, this response to a refreshment axotomy of chronically injured facial motoneurons was less than that seen in acutely axotomized facial motoneurons. These data demonstrate that the neuronal RAG expression can be induced by injury-related signals and does not require acute deprivation of target derived factors. The transient expression is consistent with a transient inflammatory response to the injury. We conclude that transient RAG expression in chronically axotomized motoneurons and the weak response of the chronically axotomized motoneurons to a refreshment axotomy provides a plausible explanation for the progressive decline in regenerative capacity of chronically axotomized motoneurons.

Delayed nerve stimulation promotes axon-protective neurofilament phosphorylation, accelerates immune cell clearance and enhances remyelination in vivo in focally demyelinated nerves

Rapid and efficient axon remyelination aids in restoring strong electrochemical communication with end organs and in preventing axonal degeneration often observed in demyelinating neuropathies. The signals from axons that can trigger more effective remyelination in vivo are still being elucidated. Here we report the remarkable effect of delayed brief electrical nerve stimulation (ES; 1 hour @ 20 Hz 5 days post-demyelination) on ensuing reparative events in a focally demyelinated adult rat peripheral nerve. ES impacted many parameters underlying successful remyelination. It effected increased neurofilament expression and phosphorylation, both implicated in axon protection. ES increased expression of myelin basic protein (MBP) and promoted node of Ranvier re-organization, both of which coincided with the early reappearance of remyelinated axons, effects not observed at the same time points in non-stimulated demyelinated nerves. The improved ES-associated remyelination was accompanied by enhanced clearance of ED-1 positive macrophages and attenuation of glial fibrillary acidic protein expression in accompanying Schwann cells, suggesting a more rapid clearance of myelin debris and return of Schwann cells to a nonreactive myelinating state. These benefits of ES correlated with increased levels of brain derived neurotrophic factor (BDNF) in the acute demyelination zone, a key molecule in the initiation of the myelination program. In conclusion, the tremendous impact of delayed brief nerve stimulation on enhancement of the innate capacity of a focally demyelinated nerve to successfully remyelinate identifies manipulation of this axis as a novel therapeutic target for demyelinating pathologies.

Electrical stimulation promotes regeneration of defective peripheral nerves after delayed repair intervals lasting under one month

Background

Electrical stimulation (ES) has been proven to be an effective means of enhancing the speed and accuracy of nerve regeneration. However, these results were recorded when the procedure was performed almost immediately after nerve injury. In clinical settings, most patients cannot be treated immediately. Some patients with serious trauma or contaminated wounds need to wait for nerve repair surgery. Delays in nerve repair have been shown to be associated with poorer results than immediate surgery. It is not clear whether electrical stimulation still has any effect on nerve regeneration after enough time has elapsed.

Methods

A delayed nerve repair model in which the rats received delayed nerve repair after 1 day, 1 week, 1 month, and 2 months was designed. At each point in time, the nerve stumps of half the rats were bridged with an absorbable conduit and the rats were given 1 h of weak electrical stimulation. The other half was not treated. In order to analyze the morphological and molecular differences among these groups, 6 ES rats and 6 sham ES rats per point in time were killed 5 days after surgery. The other rats in each group were allowed to recover for 6 weeks before the final functional test and tissue observation.

Results

The amounts of myelinated fibers in the distal nerve stumps decreased as the delay in repair increased for both ES rats and sham ES rats. In the 1-day-delay and 1-week-delay groups, there were more fibers in ES rats than in sham ES rats. And the compound muscle action potential (CMAP) and motor nerve conduction velocity (MNCV) results were better for ES rats in these two groups. In order to analyze the mechanisms underlying these differences, Masson staining was performed on the distal nerves and quantitative PCR on the spinal cords. Results showed that, after delays in repair of 1 month and 2 months, there was more collagen tissue hyperplasia in the distal nerve in all rats. The brain-derived neurotrophic factor (BDNF) and trkB expression levels in the spinal cords of ES rats were higher than in sham ES rats. However, these differences decreased as the delay in repair increased.

Conclusions

Electrical stimulation does not continue to promote nerve regeneration after long delays in nerve repair. The effective interval for nerve regeneration after delayed repair was found to be less than 1 month. The mechanism seemed to be related to the expression of nerve growth factors and regeneration environment in the distal nerves.

Low-level laser irradiation improves functional recovery and nerve regeneration in sciatic nerve crush rat injury model

The development of noninvasive approaches to facilitate the regeneration of post-traumatic nerve injury is important for clinical rehabilitation. In this study, we investigated the effective dose of noninvasive 808-nm low-level laser therapy (LLLT) on sciatic nerve crush rat injury model. Thirty-six male Sprague Dawley rats were divided into 6 experimental groups: a normal group with or without 808-nm LLLT at 8 J/cm² and a sciatic nerve crush injury group with or without 808-nm LLLT at 3, 8 or 15 J/cm². Rats were given consecutive transcutaneous LLLT at the crush site and sacrificed 20 days after the crush injury. Functional assessments of nerve regeneration were analyzed using the sciatic functional index (SFI) and hindlimb range of motion (ROM). Nerve regeneration was investigated by measuring the myelin sheath thickness of the sciatic nerve using transmission electron microscopy (TEM) and by analyzing the expression of growth-associated protein 43 (GAP43) in sciatic nerve using western blot and immunofluorescence staining. We found that sciatic-injured rats that were irradiated with LLLT at both 3 and 8 J/cm² had significantly improved SFI but that a significant improvement of ROM was only found in rats with LLLT at 8 J/cm². Furthermore, the myelin sheath thickness and GAP43 expression levels were significantly enhanced in sciatic nerve-crushed rats receiving 808-nm LLLT at 3 and 8 J/cm². Taken together, these results suggest that 808-nm LLLT at a low energy density (3 J/cm² and 8 J/cm²) is capable of enhancing sciatic nerve regeneration following a crush injury.

The parameters of transcutaneous electrical nerve stimulation are critical to its regenerative effects when applied just after a sciatic crush lesion in mice

We investigated the effect of two frequencies of transcutaneous electrical nerve stimulation (TENS) applied immediately after lesion on peripheral nerve regeneration after a mouse sciatic crush injury. The animals were anesthetized and subjected to crushing of the right sciatic nerve and then separated into three groups: nontreated, Low-TENS (4 Hz), and High-TENS (100 Hz). The animals of Low- and High-TENS groups were stimulated for 2 h immediately after the surgical procedure, while the nontreated group was only positioned for the same period. After five weeks the animals were euthanized, and the nerves dissected bilaterally for histological and histomorphometric analysis. Histological assessment by light and electron microscopy showed that High-TENS and nontreated nerves had a similar profile, with extensive signs of degeneration. Conversely, Low-TENS led to increased regeneration, displaying histological aspects similar to control nerves. High-TENS also led to decreased density of fibers in the range of 6-12 μm diameter and decreased fiber diameter and myelin area in the range of 0-2 μm diameter. These findings suggest that High-TENS applied just after a peripheral nerve crush may be deleterious for regeneration, whereas Low-TENS may increase nerve regeneration capacity.

Experimental study on the effect of electrostimulation on neural regeneration after oculomotor nerve injury

The oculomotor nerve can regenerate anatomically and histologically after injury; however, the degree of functional recovery of extraocular muscles and the pupil sphincter muscle was not satisfactory. Electrostimulation was one potential intervention that was increasingly being studied for use in nerve injury settings. However, the effect of electrostimulation on regeneration of the injured oculomotor nerve was still obscure. In this study, we studied the effects of electrostimulation on neural regeneration in terms of neurofunction, myoelectrophysiology, neuroanatomy, and neurohistology after oculomotor nerve injury and found that electrostimulation on the injured oculomotor nerve enhanced the speed and final level of its functional and electrophysiological recovery, promoted neural regeneration, and enhanced the selectivity and specificity of reinnervation of the regenerated neuron, the conformity among the electrophysiological and functional recovery of extraocular muscles, and neural regeneration, and that the function of extraocular muscles recovered slower than electrophysiology. Thus, we speculated that electrostimulation on the injured oculomotor nerve produced a marked effect on all phases of neural regeneration including neuronal survival, sprout formation, axonal elongation, target reconnection, and synaptogenesis. We think that neural electrostimulation can be used in oculomotor nerve injury.

Neural tissue engineering options for peripheral nerve regeneration

Tissue engineered nerve grafts (TENGs) have emerged as a potential alternative to autologous nerve grafts, the gold standard for peripheral nerve repair. Typically, TENGs are composed of a biomaterial-based template that incorporates biochemical cues. A number of TENGs have been used experimentally to bridge long peripheral nerve gaps in various animal models, where the desired outcome is nerve tissue regeneration and functional recovery. So far, the translation of TENGs to the clinic for use in humans has met with a certain degree of success. In order to optimize the TENG design and further approach the matching of TENGs with autologous nerve grafts, many new cues, beyond the traditional ones, will have to be integrated into TENGs. Furthermore, there is a strong requirement for monitoring the real-time dynamic information related to the construction of TENGs. The aim of this opinion paper is to specifically and critically describe the latest advances in the field of neural tissue engineering for peripheral nerve regeneration. Here we delineate new attempts in the design of template (or scaffold) materials, especially in the context of biocompatibility, the choice and handling of support cells, and growth factor release systems. We further discuss the significance of RNAi for peripheral nerve regeneration, anticipate the potential application of RNAi reagents for TENGs, and speculate on the possible contributions of additional elements, including angiogenesis, electrical stimulation, molecular inflammatory mediators, bioactive peptides, antioxidant reagents, and cultured biological constructs, to TENGs. Finally, we consider that a diverse array of physicochemical and biological cues must be orchestrated within a TENG to create a self-consistent coordinated system with a close proximity to the regenerative microenvironment of the peripheral nervous system.