Effect of low-intensity pulsed ultrasound on the expression of neurotrophin-3 and brain-derived neurotrophic factor in cultured Schwann cells

Neuromodulatory Responses Elicited by Intermittent versus Continuous Transcranial Focused Ultrasound Stimulation of the Motor Cortex in Rats

Transcranial focused ultrasound stimulation (tFUS) has emerged as a promising neuromodulation technique that delivers acoustic energy with high spatial resolution for inducing long-term potentiation (LTP)- or depression (LTD)-like plasticity. The variability in the primary effects of tFUS-induced plasticity could be due to different stimulation patterns, such as intermittent versus continuous, and is an aspect that requires further detailed exploration. In this study, we developed a platform to evaluate the neuromodulatory effects of intermittent and continuous tFUS on motor cortical plasticity before and after tFUS application. Three groups of rats were exposed to either intermittent, continuous, or sham tFUS. We analyzed the neuromodulatory effects on motor cortical excitability by examining changes in motor-evoked potentials (MEPs) elicited by transcranial magnetic stimulation (TMS). We also investigated the effects of different stimulation patterns on excitatory and inhibitory neural biomarkers, examining c-Fos and glutamic acid decarboxylase (GAD-65) expression using immunohistochemistry staining. Additionally, we evaluated the safety of tFUS by analyzing glial fibrillary acidic protein (GFAP) expression. The current results indicated that intermittent tFUS produced a facilitation effect on motor excitability, while continuous tFUS significantly inhibited motor excitability. Furthermore, neither tFUS approach caused injury to the stimulation sites in rats. Immunohistochemistry staining revealed increased c-Fos and decreased GAD-65 expression following intermittent tFUS. Conversely, continuous tFUS downregulated c-Fos and upregulated GAD-65 expression. In conclusion, our findings demonstrate that both intermittent and continuous tFUS effectively modulate cortical excitability. The neuromodulatory effects may result from the activation or deactivation of cortical neurons following tFUS intervention. These effects are considered safe and well-tolerated, highlighting the potential for using different patterns of tFUS in future clinical neuromodulatory applications.

Methylcobalamin in Combination with Early Intervention of Low-Intensity Pulsed Ultrasound Potentiates Nerve Regeneration and Functional Recovery in a Rat Brachial Plexus Injury Model

This study evaluated and compared the functional recovery and histopathological outcomes of treatment involving low-intensity pulsed ultrasound (LIPUS) and methylcobalamin (B12) on brachial plexus injury (BPI) in an experimental rat model. Three days after BPI, the rats were assigned to receive either LIPUS or methylcobalamin alone or in combination consecutively for 12 days. Serial changes in sensory and motor behavioral responses, as well as morphological and immunohistochemical changes for substance P (SP), ionized calcium-binding adapter molecule 1 (iba1), brain-derived neurotrophic factor (BDNF), and S100 were examined 28 days after BPI as the outcome measurements. Early intervention of LIPUS and methylcobalamin, whether alone or in combination, augmented the sensory and motor behavioral recovery as well as modulated SP and iba1 expression in spinal dorsal horns, BDNF, and S100 in the injured nerve. Moreover, the combined therapy with its synergistic effect gave the most beneficial effect in accelerating functional recovery. In view of the effective initiation of early recovery of sensory and motor functions, treatment with LIPUS and methylcobalamin in combination has a potential role in the clinical management of early-phase BPI.

Research Progress of Low-Intensity Pulsed Ultrasound in the Repair of Peripheral Nerve Injury

Peripheral nerve injury (PNI) is a common disease that has profound impact on the health of patients, but has poor prognosis. The gold standard for the treatment of peripheral nerve defects is autologous nerve grafting; notwithstanding, due to the extremely high requirement for surgeons and medical facilities, there is great interest in developing better treatment strategies for PNI. Low-intensity pulsed ultrasound (LIPUS) is a noninterventional stimulation method characterized by low-intensity pulsed waves. It has good therapeutic effect on fractures, inflammation, soft tissue regeneration, and nerve regulation, and can participate in PNI repair from multiple perspectives. This review concentrates on the effects and mechanisms of LIPUS in the repair of PNI from the perspective of LIPUS stimulation of neural cells and stem cells, modulation of neurotrophic factors, signaling pathways, proinflammatory cytokines, and nerve-related molecules. In addition, the effects of LIPUS on nerve conduits are reviewed, as nerve conduits are expected to be a successful alternative treatment for PNI with the development of tissue engineering. Overall, the application advantages and prospects of LIPUS in the repair of PNI are highlighted by summarizing the effects of LIPUS on seed cells, neurotrophic factors, and nerve conduits for neural tissue engineering.

Hippo Pathway in Schwann Cells and Regeneration of Peripheral Nervous System

Hippo pathway is an evolutionarily conserved signaling pathway comprising a series of MST/LATS kinase complexes. Its key transcriptional coactivators YAP and TAZ regulate transcription factors such as TEAD family to direct gene expression. The regulation of Hippo pathway, especially the nuclear level change of YAP and TAZ, significantly influences the cell fate switching from proliferation to differentiation, regeneration, and postinjury repair. This review outlines the main findings of Hippo pathway in peripheral nerve development, regeneration, and tumorigenesis, especially the studies in Schwann cells. We also summarize other roles of Hippo pathway in damage repair of the peripheral nerve system and discuss the potential future research which probably contributes to novel therapeutic strategies.

Low-intensity pulsed ultrasound promotes proliferation and myelinating genes expression of Schwann cells through NRG1/ErbB signaling pathway

Schwann cells (SCs) are the major component of myelin sheath in the peripheral nervous system, which are necessary in the development, function maintenance, and repair of peripheral nerves. This study aimed to investigate the potential mechanism of low-intensity pulsed ultrasound (LIPUS) affecting the proliferation and myelinating activity of SCs. Rat Schwann cell line RSC96 were cultured and exposed to LIPUS of different duty ratios (control, 20 %, 50 %, 80 %). Results demonstrated that LIPUS with a duty ratio of 50 % showing the maximal effect in facilitating proliferation of SCs. The expressions of Krox20 and myelin basic protein (MBP), the key molecules of SC myelination, and the potent inducer of myelination neuregulin 1 (NRG1) and its receptors ErbB2 and ErbB3 increased significantly by LIPUS. The reaction of these factors to LIPUS were both time- and duty ratio-dependent: namely LIPUS with higher duty ratios took effects when applied repeatedly over more consecutive days. These observations indicated that NRG1/ErbB signaling pathway might contribute to the effects of LIPUS on the proliferation and myelinating status of SCs, which could be one of the mechanisms in the protective role of LIPUS in nerve repair and regeneration. Our work provided novel insights for promising strategies of nerve repair therapy.

Low intensity focused ultrasound: a new prospect for the treatment of Parkinson's disease

Background: As a chronic and progressive neurodegenerative disease, Parkinson's disease (PD) still lacks effective and safe targeted drug therapy. Low-intensity focused ultrasound (LIFU), a new method to stimulate the brain and open the blood-brain barrier (BBB), has been widely concerned by PD researchers due to its non-invasive characteristics.Methods: PubMed was searched for the past 10 years using the terms 'focused ultrasound', 'transcranial ultrasound', 'pulse ultrasound', and 'Parkinson's disease'. Relevant citations were selected from the authors' references. After excluding articles describing high-intensity focused ultrasound or non-Parkinson's disease applications, we found more than 100 full-text analyses for pooled analysis.Results: Current preclinical studies have shown that LIFU could improve PD motor symptoms by regulating microglia activation, increasing neurotrophic factors, reducing oxidative stress, and promoting nerve repair and regeneration, while LIFU combined with microbubbles (MBs) can promote drugs to cross the BBB, which may become a new direction of PD treatment. Therefore, finding an efficient drug carrier system is the top priority of applying LIFU with MBs to deliver drugs.Conclusions: This article aims to review neuro-modulatory effect of LIFU and the possible biophysical mechanism in the treatment of PD, summarize the latest progress in delivering vehicles with MBs, and discuss its advantages and limitations.

The impact of physical, biochemical, and electrical signaling on Schwann cell plasticity

Peripheral nervous system (PNS) injuries are an ongoing health care concern. While autografts and allografts are regarded as the current clinical standard for traumatic injury, there are inherent limitations that suggest alternative remedies should be considered for therapeutic purposes. In recent years, nerve guidance conduits (NGCs) have become increasingly popular as surgical repair devices, with a multitude of various natural and synthetic biomaterials offering potential to enhance the design of conduits or supplant existing technologies entirely. From a cellular perspective, it has become increasingly evident that Schwann cells (SCs), the primary glia of the PNS, are a predominant factor mediating nerve regeneration. Thus, the development of severe nerve trauma therapies requires a deep understanding of how SCs interact with their environment, and how SC microenvironmental cues may be engineered to enhance regeneration. Here we review the most recent advancements in biomaterials development and cell stimulation strategies, with a specific focus on how the microenvironment influences the behavior of SCs and can potentially lead to functional repair. We focus on microenvironmental cues that modulate SC morphology, proliferation, migration, and differentiation to alternative phenotypes. Promotion of regenerative phenotypic responses in SCs and other non-neuronal cells that can augment the regenerative capacity of multiple biomaterials is considered along with innovations and technologies for traumatic injury.

Investigating the Optimal Initiation Time of Ultrasound Therapy for Peripheral Nerve Regeneration after Axonotmesis in Rats

This study was aimed at identifying the optimal initiation time of ultrasound (US) therapy for peripheral nerve regeneration after axonotmesis. Thirty-six rats with sciatic nerve crush injury were divided into four groups that received US irradiation initiated 1, 7 or 14 d after injury, or sham stimulation for 4 wk. Motor function analysis was conducted weekly; however, there was no significant improvement attributed to US treatment. Four weeks after injury, compound muscle action potential amplitude values of the group in which US irradiation was initiated 1 d after the injury exhibited significant improvement compared with the sham stimulation group. In addition, myelin sheath thickness was significantly greater in the 1-d group than in other groups. These results indicate that US treatment initiated 1 d after peripheral nerve injury promotes maximum regeneration.

Therapeutic Low-Intensity Ultrasound for Peripheral Nerve Regeneration – A Schwann Cell Perspective

Peripheral nerve injuries are common conditions that can arise from trauma (e.g., compression, severance) and can lead to neuropathic pain as well as motor and sensory deficits. Although much knowledge exists on the mechanisms of injury and nerve regeneration, treatments that ensure functional recovery following peripheral nerve injury are limited. Schwann cells, the supporting glial cells in peripheral nerves, orchestrate the response to nerve injury, by converting to a “repair” phenotype. However, nerve regeneration is often suboptimal in humans as the repair Schwann cells do not sustain their repair phenotype long enough to support the prolonged regeneration times required for successful nerve regrowth. Thus, numerous strategies are currently focused on promoting and extending the Schwann cells repair phenotype. Low-intensity ultrasound (LIU) is a non-destructive therapeutic approach which has been shown to facilitate peripheral nerve regeneration following nerve injury in rodents. Still, clinical trials in humans are scarce and limited to small population sizes. The benefit of LIU on nerve regeneration could possibly be mediated through the repair Schwann cells. In this review, we discuss the known and possible molecular mechanisms activated in response to LIU in repair Schwann cells to draw support and attention to LIU as a compelling regenerative treatment for peripheral nerve injury.

Effect of low-intensity pulsed ultrasound on orofacial sensory disturbance following inferior alveolar nerve injury: Role of neurotrophin-3 signaling

Percutaneous treatment of low-intensity pulsed ultrasound (LIPUS) to the site of inferior alveolar nerve (IAN) transection promotes functional regeneration, but the detailed mechanism is unknown. We examined the involvement of neurotrophin-3 (NT-3), which primarily binds with tropomyosin receptor kinase C (TrkC), in functional transected IAN regeneration following LIPUS treatment in rats. Daily LIPUS treatment to the transected IAN was performed, and the mechanical sensitivity of the facial skin was measured for 14 d. On day 5 after IAN transection, the expression of NT-3 in the transected IAN and TrkC-positive trigeminal ganglion neurons were immunohistochemically examined. Further, the effect of TrkC neutralization on the acceleration of facial mechanosensory disturbance restoration due to LIPUS treatment was analyzed. LIPUS treatment to the site of IAN transection significantly facilitated functional recovery from sensory disturbance on facial skin. Schwann cells in the transected IAN expressed NT-3, and LIPUS treatment increased the amount of NT-3. The facilitated recovery from the mechanosensory disturbance by continuous LIPUS treatment was inhibited by the ongoing TrkC neutralization at the IAN transection site. These results suggest that LIPUS treatment accelerates the recovery of orofacial mechanosensory function following IAN transection through the enhancement of NT-3 signaling in the transected IAN.

Low-intensity pulsed ultrasound for regenerating peripheral nerves: potential for penile nerve

Peripheral nerve damage, such as that found after surgery or trauma, is a substantial clinical challenge. Much research continues in attempts to improve outcomes after peripheral nerve damage and to promote nerve repair after injury. In recent years, low-intensity pulsed ultrasound (LIPUS) has been studied as a potential method of stimulating peripheral nerve regeneration. In this review, the physiology of peripheral nerve regeneration is reviewed, and the experiments employing LIPUS to improve peripheral nerve regeneration are discussed. Application of LIPUS following nerve surgery may promote nerve regeneration and improve functional outcomes through a variety of proposed mechanisms. These include an increase of neurotrophic factors, Schwann cell (SC) activation, cellular signaling activations, and induction of mitosis. We searched PubMed for articles related to these topics in both in vitro and in vivo animal research models. We found numerous studies, suggesting that LIPUS following nerve surgery promotes nerve regeneration and improves functional outcomes. Based on these findings, LIPUS could be a novel and valuable treatment for nerve injury-induced erectile dysfunction.

Low-Intensity Pulsed Ultrasound Prompts Both Functional and Histologic Improvements While Upregulating the Brain-Derived Neurotrophic Factor Expression after Sciatic Crush Injury in Rats

The aim of this study was to determine that low-intensity pulsed ultrasound (LIPUS) at an intensity of 140 mW/cm² promotes functional and histologic improvements in sciatic nerve crush injury in a rat model and to investigate changes over time in relevant growth factors and receptors, exploring the mechanism of LIPUS in the recovery process after injury. Toe angle in the toe-off phase, regenerative axonal length, myelinated nerve fiber density, diameter of myelinated nerve fiber, axon diameter and myelin sheath thickness were significantly higher in the LIPUS group than in the sham group. Gene and protein expression of brain-derived neurotrophic factor (BDNF) was upregulated in the LIPUS group. In conclusion, LIPUS contributed to rapid functional and histologic improvement and upregulated BDNF expression after sciatic nerve crush injury in rats.

The Hippo pathway: Horizons for innovative treatments of peripheral nerve diseases

Initially identified in Drosophila, the Hippo signaling pathway regulates how cells respond to their environment by controlling proliferation, migration and differentiation. Many recent studies have focused on characterizing Hippo pathway function and regulation in mammalian cells. Here, we present a brief overview of the major components of the Hippo pathway, as well as their regulation and function. We comprehensively review the studies that have contributed to our understanding of the Hippo pathway in the function of the peripheral nervous system and in peripheral nerve diseases. Finally, we discuss innovative approaches that aim to modulate Hippo pathway components in diseases of the peripheral nervous system.

[A review of mechanisms by which low-intensity pulsed ultrasound affects bone regeneration]

Low-intensity pulsed ultrasound (LIPUS) is a common physical therapy to accelerate the healing of bone fracture and treat delayed union of bone fracture. Vessels, nerves, and bone tissue are essential constituents of bone system. Recently, increasing evidence has been revealed that LIPUS can not only promote bone regeneration by directly regulating osteoblasts, osteoblasts, mesenchymal stem cells, but also have a positive impact on the repair of bone healing through vessels and nerves. Thus, we reviewed and summarized the latest published literature about the molecular mechanism for the effects of LIPUS on bone regeneration, which might offer a promising therapy for bone-related diseases.

Ultrasound therapy with optimal intensity facilitates peripheral nerve regeneration in rats through suppression of pro-inflammatory and nerve growth inhibitor gene expression

Background

Therapeutic ultrasound (US) is a promising physical therapy modality for peripheral nerve regeneration. However, it is necessary to identify the most effective US parameters and clarify the underlying mechanisms before its clinical application. The intensity of US is one of the most important parameters. However, the optimum intensity for the promotion of peripheral nerve regeneration has yet to be determined.

Objectives

To identify the optimum intensity of US necessary for the promotion of peripheral nerve regeneration after crush injuries in rats and to clarify the underlying mechanisms of US by mRNA expression analysis.

Methods

We inflicted sciatic nerve crush injuries on adult Lewis rats and performed ultrasound irradiation using 4 different US intensities: 0 (sham stimulation), 30, 140, and 250 mW/cm² with frequency (5 days/week) and duration (5 min/day). We evaluated peripheral nerve regeneration by quantitative real-time PCR one week after injury. Histomorphometric analyses and motor function analysis were evaluated 3 weeks after injury.

Results

US stimulation enhanced re-myelination as well as sprouting of axons, especially at an intensity of 140 mW/cm². mRNA expression revealed that US suppressed the expression of the inflammatory cytokines TNF and IL-6 and the axonal growth inhibitors SEMA3A and GSK3β.

Conclusions

An intensity of 140 mW/cm² was optimal to support regeneration of the sciatic nerve after a crush injury in rats by, in part, the suppression of pro-inflammatory and nerve growth inhibitor gene expression.

Preventive potential of low intensity pulsed ultrasound for chronic traumatic encephalopathy after repetitive head collisions in contact sports

Chronic traumatic encephalopathy (CTE), a disease process well-recognized in boxers, American football players and military personnel, is a progressive neurodegenerative disease caused by repetitive blows to the head. Subjects with CTE can have a wide range of emotional, cognitive and physical symptoms. The cognitive group patients had a significantly higher probability of developing dementia in later years. Currently, there are no disease modifying regimen for CTE. Timely intervention of head blow could diminish the development of CTE. Low-intensity pulsed ultrasound (LIPUS) is a common adjunct used to promote bone healing for fresh fracture. Recent reports suggest that LIPUS can noninvasively modulate the cortical function and have neuroprotective effect in various animal models of traumatic brain injury, stroke, Alzheimer's disease and major depressive disorder. The multifunctional mechanisms of LIPUS neuroprotective effect include several trophic factor stimulations, anti-inflammatory properties and reduction of brain edema. From the above evidence, LIPUS intervention could be a strategy for the prevention of the clinical CTE sequelae of repetitive head blows. We hypothesized that due to its neuroprotective effects, the non-invasive and easy-to-use method of LIPUS brain stimulation could have a preventive effect on players who have head blows during the match. The development of a time sensitive protocol, resembling the therapeutic algorithm for traumatic brain injury, would potentially prevent the development of subsequent CTE adverse outcome. Further long-term longitudinal studies of LIPUS stimulation are warranted to verify the prevention efficacy of this intervention for CTE.

Protective effects of low-intensity pulsed ultrasound on aluminum overload-induced cerebral damage through epigenetic regulation of brain-derived neurotrophic factor expression

In consideration of its noninvasive administration and endogenous stimulation property, the enhancement of brain-derived neurotrophic factor (BDNF) via low-intensity pulsed ultrasound (LIPUS) could be a novel strategy for aluminum (Al) overload-induced cerebral damage. LIPUS was pre-treated 7 days before concomitantly given with aluminum chloride (AlCl₃) daily for a period of 42 days. Morris water maze and elevated plus maze were performed to analyze spatial learning and memory. Western Blot and immunoprecipitation were used to detect BDNF and histone acetylation of histone H3 lysine 9 (H3K9) and histone H4 lysine 12 (H4K12) in the hippocampus. Assay of malondialdehyde (MDA), superoxide dismutase (SOD), glutathione (GSH), and glutathione peroxidase (GSH-Px) indicated the extent of oxidative damages. Aluminium exposure in rats can cause attenuated spatial learning and memory, followed by up-regulated histone deacetylase 6 (HDAC6) expression, down-regulated H3K9 and H4K12 acetylation at the PIII and PIV promoter of BDNF, all of which will eventually inhibit BDNF expression. LIPUS can recover reduced cognitive function by restoring histone acetylation and BDNF expression, accompanied with increased SOD, GSH, and GSH-Px activity. LIPUS treatment might alleviate aluminium exposure-induced cognitive decline by acetylation regulation of BDNF expression and reducing oxidative stress in the hippocampus.

Clinically Available Low Intensity Ultrasound Devices do not Promote Axonal Regeneration After Peripheral Nerve Surgery-A Preclinical Investigation of an FDA-Approved Device

The slow axonal regeneration and consecutive delayed muscle reinnervation cause persistent functional deficits following peripheral nerve injury, even following sufficient surgical nerve reconstruction. Preclinically, adjunct ultrasound therapy has shown to significantly accelerate nerve regeneration and thereby improve muscle function compared to nerve reconstruction alone. However, although FDA-approved and clinically well-tested ultrasound devices for other conditions such as delayed-healing fractures are available, they have not been investigated for peripheral nerve injury yet. Aiming to provide a fast clinical translation, we evaluated EXOGEN (Bioventus LLC, Durham, USA), a clinical device for low-intensity ultrasound therapy in various treatment intervals following peripheral nerve surgery. Sixty rats, randomized to five groups of twelve animals each, underwent median nerve transection and primary epineural nerve reconstruction. Post-surgically the ultrasound therapy (duration: 2 min, frequency: 1.5 MHz, pulsed SATA-intensity: 30 mW/cm², repetition-rate: 1.0 kHz, duty-cycle: 20%) was applied either weekly, 3 times a week or daily. A daily sham-therapy and a control-group served as references. Functional muscle testing, electrodiagnostics and histological analyses were used to evaluate nerve regeneration. The post-surgically absent grip strength recovered in all groups and increased from week four on without any significant differences among groups. The weekly treated animals showed significantly reduced target muscle atrophy compared to sham-treated animals (p = 0.042), however, with no significant differences to three-times-a-week-, daily treated and control animals. The number of myelinated axons distal to the lesion site increased significantly in all groups (p < 0.001) without significant difference among groups (p > 0.05). A full recovery of distal latency was achieved in all groups and muscle function and CMAP recurred with insignificant differences among groups. In conclusion, the clinically available FDA-approved ultrasound device did not promote the axonal regeneration following nerve injury in comparison to control and sham groups. This is in contrast to a conclusive preclinical evidence base and likely due to the insufficient ultrasound-intensity of 30 mW/cm². We recommend the clinical investigation of 200–300 mW/cm².

A potential treatment of low intensity pulsed ultrasound on cavernous nerve injury for erectile dysfunction

Erectile dysfunction after nerve injury is a common disease after radical prostatectomy. Brain-derived neurotrophic factor (BDNF) is a member of the neurotrophin family, which plays an important role in the survival of existing neurons, the differentiation of new neurons, and synaptic plasticity. It has been demonstrated that low-intensity pulsed ultrasound (LIPUS) accelerates bone healing and axonal regeneration after injury. LIPUS may also be able to stimulate neuronal activity and enhance the levels of neurotrophic factors. Evidence suggests that elevated levels of BDNF in the brain have protective effects against neurodegenerative diseases. Previous studies have shown that the treatment on cavernous nerve injury repair, and protective effect plus neuro-regeneration effect by low-intensity pulsed ultrasound. They shared the similar mechanism including several trophic factors stimulation, Pl3K/akt pathway activation, and anti-fibrosis mechanism. We hypothesized that due to its combined neuroregenerative and protective effects, the non-invasive and easy-to-use method of LIPUS stimulation could have a therapeutic effect on erectile dysfunction stemming from cavernous nerve injury.

Low-intensity focused ultrasound stimulator using focal depth controller for improved targeting in neuromuscular rehabilitation

Low-intensity ultrasound can be used for noninvasive neuromuscular stimulation to enhance the recovery of injured peripheral nerves and muscles. In order to improve the stimulation efficiency, focused ultrasound (FUS) with a high energy density should be used. Because the FUS is concentrated at a miniaturized focal area, deliberate targeting with focal depth control is required to achieve the accurate stimulation. In this study, a low-intensity focused ultrasound (LIFU) stimulator with a focal depth controller is presented as an effective ultrasound stimulation system for the neuromuscular rehabilitation. In the developed stimulator, the target depth is adjusted by the focal depth controller with the brief structure. The experimental results show that the developed stimulator can generate the high quality LIFU stimulation with the spatial average-pulse average intensity (I_SAPA) of 496 mW/cm², which is sufficient to improve the rehabilitation of injured peripheral nerves and muscles. In the in vivo experiment, a rat's sciatic nerve was successfully targeted by the developed stimulator. These results indicate that the developed stimulator can be used to improve the quality of the ultrasound stimulation for neuromuscular rehabilitation by targeting a specific area.

Low-intensity pulsed ultrasound promotes Schwann cell viability and proliferation via the GSK-3β/β-catenin signaling pathway

Background: It has been reported that ultrasound enhances peripheral nerve regeneration, but the mechanism remains elusive. Low-intensity pulsed ultrasound (LIPUS) has been reported to enhance proliferation and alter protein production in various types of cells. In this study, we detected the effects of LIPUS on Schwann cells. Material and methods: Schwann cells were separated from new natal Sprague-Dawley rat sciatic nerves and were cultured and purified. The Schwann cells were treated by LIPUS for 10 minutes every day, with an intensity of 27.37 mW/cm2. After treatment for 5 days, MTT, EdU staining, and flow cytometry were performed to examine cell viability and proliferation. Neurotrophic factors, including FGF, NGF, BDNF, and GDNF, were measured by western blot and real-time PCR. GSK-3β, p-GSK-3β, β-catenin and Cyclin D1 protein levels were detected using a western blot analysis. The expression of Cyclin D1 was also detected by immunofluorescence. Results: MTT and EdU staining showed that LIPUS increased the Schwann cells viability and proliferation. Compared to the control group, LIPUS increased the expression of growth factors and neurotrophic factors, including FGF, NGF, BDNF, GDNF, and Cyclin D1. Meanwhile, GSK-3β activity was inhibited in the LIPUS group as demonstrated by the increased level of p-GSK-3β and the ratio of the p-GSK-3β/GSK-3β level. The mRNA and protein expressions of β-catenin were increased in the LIPUS group. However, SB216763, a GSK-3β inhibitor, reversed the effects of LIPUS on Schwann cells. Conclusion: LIPUS promotes Schwann cell viability and proliferation by increasing Cyclin D1 expression via enhancing the GSK-3β/β-catenin signaling pathway.

Preventive Effect of Low Intensity Pulsed Ultrasound against Experimental Cerebral Ischemia/Reperfusion Injury via Apoptosis Reduction and Brain-derived Neurotrophic Factor Induction

Stroke is known as the top 10 causes of death worldwide. Development of effectively neuroprotective or preventive strategies for ischemia stroke is imperative. For the purpose of stroke prevention, we tested the neuroprotective effects of low-intensity pulsed ultrasound (LIPUS) on ischemic stroke. Adult C57BL/6 mice were used to daily treatment with LIPUS for 5 days on left hemisphere before middle cerebral artery occlusion (MCAO)-induced cerebral ischemia/reperfusion injury. Western blotting and immunohistochemistry were performed to assess the protein expressions of signaling molecules. Pretreatment with LIPUS significantly ameliorated the brain ischemic damage, including the reduction of neurological deficit score, infarct area, histopathological score, and showed a better performance in neurological and behavior functions. LIPUS pretreatment could also significantly decrease the neuronal cell apoptosis and upregulation of apoptosis-related signaling molecules and downregulation of brain-derived neurotrophic factor (BDNF) in brain tissues of MCAO-treated mice. Furthermore, LIPUS significantly prevented the decreased cell viability, the increased caspase-3 cleavage, and the decreased BDNF expression in ischemia/reperfusion-treated microglial cells. These results demonstrate that LIPUS effectively prevented the cerebral ischemia/reperfusion injury through apoptosis reduction and BDNF induction in a MCAO mouse model. The neuroprotective potential of LIPUS may provide a novel preventive strategy for ischemic stroke in high-risk patients.

Ultrasound and shock-wave stimulation to promote axonal regeneration following nerve surgery: a systematic review and meta-analysis of preclinical studies

Limited regeneration after nerve injury often leads to delayed or incomplete reinnervation and consequently insufficient muscle function. Following nerve surgery, application of low-intensity ultrasound or extracorporeal shock waves may promote nerve regeneration and improve functional outcomes. Because currently clinical data is unavailable, we performed a meta-analysis following the PRISMA-guidelines to investigate the therapeutic effect of ultrasound and shock wave therapies on motor nerve regeneration. Ten ultrasound-studies (N = 445 rats) and three shock-wave studies (N = 110 rats) were identified from multiple databases. We calculated the difference in means or standardized mean difference with 95% confidence intervals for motor function, nerve conduction velocity and histomorphological parameters of treated versus sham or non-treated animals. Ultrasound treatment showed significantly faster nerve conduction, increased axonal regeneration with thicker myelin and improved motor function on sciatic functional index scale (week two: DM[95%CI]: 19,03[13,2 to 25,6], 71 animals; week four: 7,4[5,4 to 9,5], 47 animals). Shock wave induced recovery improvements were temporarily significant. In conclusion, there is significant evidence for low-intensity ultrasound but not for extracorporeal shock wave treatment to improve nerve regeneration. Prospective clinical trials should therefore investigate available FDA-approved ultrasound devices as adjunct postoperative treatment following nerve surgery.

Noninvasive, targeted gene therapy for acute spinal cord injury using LIFU-mediated BDNF-loaded cationic nanobubble destruction

Various gene delivery systems have been widely studied for the acute spinal cord injury (SCI) treatment. In the present study, a novel type of brain-derived neurotrophic factor (BDNF)-loaded cationic nanobubbles (CNBs) conjugated with MAP-2 antibody (mAb_MAP-2/BDNF/CNBs) was prepared to provide low-intensity focused ultrasound (LIFU)-targeted gene therapy. In vitro experiments, the ultrasound-targeted tranfection to BDNF overexpressioin in neurons and efficiently inhibition neuronal apoptosis have been demonstrated, and the elaborately designed mAb_MAP-2/BDNF/CNBs can specifically target to the neurons. Furthermore, in a acute SCI rat model, LIFU-mediated mAb_MAP-2/BDNF/CNBs transfection significantly increased BDNF expression, attenuated histological injury, decreased neurons loss, inhibited neuronal apoptosis in injured spinal cords, and increased BBB scores in SCI rats. LIFU-mediated mAb_MAP-2/BDNF/CNBs destruction significantly increase transfection efficiency of BDNF gene both in vitro and in vivo, and has a significant neuroprotective effect on the injured spinal cord. Therefore, the combination of LIFU irradiation and gene therapy through mAb_MAP-2/BDNF/CNBs can be considered as a novel non-invasive and targeted treatment for gene therapy of SCI.

Promoting axonal regeneration following nerve surgery: a perspective on ultrasound treatment for nerve injuries

Nerve injury is often associated with limited axonal regeneration and thus leads to delayed or incomplete axonal reinnervation. As a consequence of slow nerve regeneration, target muscle function is often insufficient and leads to a lifelong burden. Recently, the diagnosis of nerve injuries has been improved and likewise surgical reconstruction has undergone significant developments. However, the problem of slow nerve regeneration has not been solved. In a recent meta-analysis, we have shown that the application of low-intensity ultrasound promotes nerve regeneration experimentally and thereby can improve functional outcomes. Here we want to demonstrate the experimental effect of low intensity ultrasound on nerve regeneration, the current state of investigations and its possible future clinical applications.

Influence of Mechanical Stimuli on Schwann Cell Biology

Schwann cells are the glial cells of the peripheral nervous system (PNS). They insulate axons by forming a specialized extension of plasma membrane called the myelin sheath. The formation of myelin is essential for the rapid saltatory propagation of action potentials and to maintain the integrity of axons. Although both axonal and extracellular matrix (ECM) signals are necessary for myelination to occur, the cellular and molecular mechanisms regulating myelination continue to be elucidated. Schwann cells in peripheral nerves are physiologically exposed to mechanical stresses (i.e., tensile, compressive and shear strains), occurring during development, adulthood and injuries. In addition, there is a growing body of evidences that Schwann cells are sensitive to the stiffness of their environment. In this review, we detail the mechanical constraints of Schwann cells and peripheral nerves. We explore the regulation of Schwann cell signaling pathways in response to mechanical stimulation. Finally, we provide a comprehensive overview of the experimental studies addressing the mechanobiology of Schwann cells. Understanding which mechanical properties can interfere with the cellular and molecular biology of Schwann cell during development, myelination and following injuries opens new insights in the regulation of PNS development and treatment approaches in peripheral neuropathies.

Low-intensity pulsed ultrasound upregulates pro-myelination indicators of Schwann cells enhanced by co-culture with adipose-derived stem cells

OBJECTIVES

Peripheral nerve injuries are a common occurrence, resulting in considerable patient suffering; it also represents a major economic burden on society. To improve treatment options following peripheral nerve injuries, scientists aim to find a way to promote Schwann cell (SC) myelination to help nerves to carry out their functions effectively. In this study, we investigated myelination ability of SCs, regulated by co-culture with adipose-derived stem cells (ASCs) or low-intensity pulsed ultrasound (LIPUS), and synergistic effects of combined treatments.

MATERIALS AND METHODS

Schwann cells were co-cultured with or without ASCs, and either left untreated or treated with LIPUS for 10 min/d for 1, 4 or 7 days. Effects of LIPUS and ASC co-culture on pro-myelination indicators of SCs were analysed by real-time PCR (RT-PCR), Western blotting and immunofluorescence staining (IF).

RESULTS

Our results indicate that ASC-SC co-culture and LIPUS, together or individually, promoted mRNA levels of epidermal growth factor receptor 3 (EGFR3/ErbB3), neuregulin1 (NRG1), early growth response protein 2 (Egr2/Krox20) and myelin basic protein (MBP), with corresponding increases in protein levels of ErbB3, NRG1 and Krox20. Interestingly, combination of ASC-SC co-culture and LIPUS displayed the most remarkable effects.

CONCLUSION

We demonstrated that ASCs upregulated pro-myelination indicators of SCs by indirect contact (through co-culture) and that effects could be potentiated by LIPUS. We conclude that LIPUS, as a mechanical stress, may have potential in nerve regeneration with potential clinical relevance.

Low-Intensity Pulsed Ultrasound Enhances Nerve Growth Factor-Induced Neurite Outgrowth through Mechanotransduction-Mediated ERK1/2-CREB-Trx-1 Signaling

Enhancing the action of nerve growth factor (NGF) is a potential therapeutic approach to neural regeneration. To facilitate neural regeneration, we investigated whether combining low-intensity pulsed ultrasound (LIPUS) and NGF could promote neurite outgrowth, an essential process in neural regeneration. In the present study, PC12 cells were subjected to a combination of LIPUS (1 MHz, 30 or 50 mW/cm², 20% duty cycle and 100-Hz pulse repetition frequency, 10 min every other day) and NGF (50 ng/mL) treatment, and then neurite outgrowth was compared. Our findings indicated that the combined treatment with LIPUS (50 mW/cm²) and NGF (50 ng/mL) promotes neurite outgrowth that is comparable to that achieved by NGF (100 ng/mL) treatment alone. LIPUS significantly increased NGF-induced neurite length, but not neurite branching. These effects were attributed to the enhancing effects of LIPUS on NGF-induced phosphorylation of ERK1/2 and CREB and the expression of thioredoxin (Trx-1). Furthermore, blockage of stretch-activated ion channels with Gd³⁺ suppressed the stimulating effects of LIPUS on NGF-induced neurite outgrowth and the downstream signaling activation. Taken together, our findings suggest that LIPUS enhances NGF-induced neurite outgrowth through mechanotransduction-mediated signaling of the ERK1/2-CREB-Trx-1 pathway. The combination of LIPUS and NGF could potentially be used for the treatment of nerve injury and neurodegenerative diseases.

Combined effect of pulsed electromagnetic field and sound wave on In vitro and In vivo neural differentiation of human mesenchymal stem cells

Biophysical wave stimulus has been used as an effective tool to promote cellular maturation and differentiation in the construction of engineered tissue. Pulsed electromagnetic fields (PEMFs) and sound waves have been selected as effective stimuli that can promote neural differentiation. The aim of this study was to investigate the synergistic effect of PEMFs and sound waves on the neural differentiation potential in vitro and in vivo using human bone marrow mesenchymal stem cells (hBM-MSCs). In vitro, neural-related genes in hBM-MSCs were accelerated by the combined exposure to both waves more than by individual exposure to PEMFs or sound waves. The combined wave also up-regulated the expression of neural and synaptic-related proteins in a three-dimensional (3-D) culture system through the phosphorylation of extracellular signal-related kinase. In a mouse model of photochemically induced ischemia, exposure to the combined wave reduced the infarction volume and improved post-injury behavioral activity. These results indicate that a combined stimulus of biophysical waves, PEMFs and sound can enhance and possibly affect the differentiation of MSCs into neural cells. Our study is meaningful for highlighting the potential of combined wave for neurogenic effects and providing new therapeutic approaches for neural cell therapy. © 2016 American Institute of Chemical Engineers Biotechnol. Prog., 33:201-211, 2017.

Low-intensity pulsed ultrasound treatment improved the rate of autograft peripheral nerve regeneration in rat

Low intensity pulsed ultrasound (LIPUS) has been widely used in clinic for the treatment of repairing pseudarthrosis, bone fractures and of healing in various soft tissues. Some reports indicated that LIPUS accelerated peripheral nerve regeneration including Schwann cells (SCs) and injured nerves. But little is known about its appropriate intensities on autograft nerves. This study was to investigate which intensity of LIPUS improved the regeneration of gold standard postsurgical nerves in experimental rat model. Sprague-Dawley rats were made into 10 mm right side sciatic nerve reversed autologous nerve transplantation and randomly treated with 250 mW/cm², 500 mW/cm² or 750 mW/cm² LIPUS for 2–12 weeks after operation. Functional and pathological results showed that LIPUS of 250 mW/cm² significantly induced faster rate of axonal regeneration. This suggested that autograft nerve regeneration was improved.

Noninvasive, Targeted, and Non-Viral Ultrasound-Mediated GDNF-Plasmid Delivery for Treatment of Parkinson's Disease

Glial cell line-derived neurotrophic factor (GDNF) supports the growth and survival of dopaminergic neurons. CNS gene delivery currently relies on invasive intracerebral injection to transit the blood-brain barrier. Non-viral gene delivery via systematic transvascular route is an attractive alternative because it is non-invasive, but a high-yield and targeted gene-expressed method is still lacking. In this study, we propose a novel non-viral gene delivery approach to achieve targeted gene transfection. Cationic microbubbles as gene carriers were developed to allow the stable formation of a bubble-GDNF gene complex, and transcranial focused ultrasound (FUS) exposure concurrently interacting with the bubble-gene complex allowed transient gene permeation and induced local GDNF expression. We demonstrate that the focused ultrasound-triggered GDNFp-loaded cationic microbubbles platform can achieve non-viral targeted gene delivery via a noninvasive administration route, outperform intracerebral injection in terms of targeted GDNF delivery of high-titer GDNF genes, and has a neuroprotection effect in Parkinson’s disease (PD) animal models to successfully block PD syndrome progression and to restore behavioral function. This study explores the potential of using FUS and bubble-gene complexes to achieve noninvasive and targeted gene delivery for the treatment of neurodegenerative disease.

Emerging neural stimulation technologies for bladder dysfunctions

In the neural engineering field, physiological dysfunctions are approached by identifying the target nerves and providing artificial stimulation to restore the function. Neural stimulation and recording technologies play a central role in this approach, and various engineering devices and stimulation techniques have become available to the medical community. For bladder control problems, electrical stimulation has been used as one of the treatments, while only a few emerging neurotechnologies have been used to tackle these problems. In this review, we introduce some recent developments in neural stimulation technologies including microelectrode array, closed-loop neural stimulation, optical stimulation, and ultrasound stimulation.

Differentiation of neural stem/progenitor cells using low-intensity ultrasound

Herein, we report the evaluation of apoptosis, cell differentiation, neurite outgrowth and differentiation of neural stem/progenitor cells (NSPCs) in response to low-intensity ultrasound (LIUS) exposure. NSPCs were cultured under different conditions, with and without LIUS exposure, to evaluate the single and complex effects of LIUS. A lactic dehydrogenase assay revealed that the cell viability of NSPCs was maintained with LIUS exposure at an intensity range from 100 to 500 mW/cm(2). Additionally, in comparison with no LIUS exposure, the cell survival rate was improved with the combination of medium supplemented with nerve growth factor and LIUS exposure. Our results indicate that LIUS exposure promoted NSPC attachment and differentiation on a glass substrate. Neurite outgrowth assays revealed the generation of longer, thicker neurites after LIUS exposure. Furthermore, LIUS stimulation substantially increased the percentage of differentiating neural cells in NSPCs treated with nerve growth factor in comparison with the unstimulated group. The high percentage of differentiated neural cells indicated that LIUS induced neuronal networks denser than those observed in the unstimulated groups. Furthermore, the release of nitric oxide, an important small-molecule neurotransmitter, was significantly upregulated after LIUS exposure. It is therefore reasonable to suggest that LIUS promotes the differentiation of NSPCs into neural cells, induces neurite outgrowth and regulates nitric oxide production; thus, LIUS may be a potential candidate for NSPC induction and neural cell therapy.

Ultrasound effect on neural differentiation of gingival stem/progenitor cells

Dental pulp loss due to caries or pulpitis can affect the longevity of teeth. Dental pulp tissue engineering necessitates the use of progenitor cells that has the potential to differentiate into neural, vascular and odontoblasts like cells. Previous reports have shown that human gingival progenitor cells (HGPCs) can be differentiated into different cell types; however neural differentiation of these cells, to the best of our knowledge, has not been reported. Low intensity pulsed ultrasound (LIPUS) has been reported to enhance cell differentiation. The aims of this study were (1) to explore the potential neural differentiation of HGPCs and (2) to investigate the effect of LIPUS on the differentiation of HGPCs when incubated under neuroinductive conditions. The HGPCs were isolated from human interdental papilla proximal to the premolar teeth that were extracted for orthodontic purpose. The HGPCs were induced to differentiate into neural lineage using a neuroinductive culture medium. HGPCs were divided into four groups; control group, neuro-induction (NI) group, ultrasound group (LIPUS), and a combined NI+LIPUS group. HGPCs were harvested for immunostaining and q-PCR after 1 day. Immunostaining for neuron specific antigens and q-PCR suggested that HGPCs can be differentiated into neural lineage and that selected neurodifferentiation markers can be enhanced by LIPUS.

The use of phototherapy in peripheral nerve regeneration: an updated critical review

This review describes the possible factors that may have contributed to the variability of the results between studies that have assessed the effects of phototherapy on peripheral nerve regeneration. Furthermore, it aims to make recommendations to overcome the methodological shortcomings identified. A search of the literature was conducted. In vitro and in vivo experimental studies and clinical trials were included. Twenty five studies were critically reviewed and showed considerable variability in irradiation parameters, techniques, approaches, length of irradiation courses, experimental injury tools and procedures. Many studies that have investigated the use of phototherapy in nerve regeneration produce positive results. However, the majority of these studies suffered from a number of shortcomings: no evidence of blinding and/or randomizing procedures, lack of specification of irradiation parameters, unspecified and/or inadequate tests in their experimental injury procedures, inappropriate irradiation parameters and/or poor experimental conditions.

Acceleration of peripheral nerve regeneration through asymmetrically porous nerve guide conduit applied with biological/physical stimulation

Sufficient functional restoration of damaged peripheral nerves is a big clinical challenge. In this study, a nerve guide conduit (NGC) with selective permeability was prepared by rolling an asymmetrically porous polycaprolactone/Pluronic F127 membrane fabricated using a novel immersion precipitation method. Dual stimulation (nerve growth factor [NGF] as a biological stimulus and low-intensity pulse ultrasound [US] as a physical stimulus) was adapted to enhance nerve regeneration through an NGC. The animal study revealed that each stimulation (NGF or US) has a positive effect to promote the peripheral nerve regeneration through the NGC, however, the US-stimulated NGC group allowed more accelerated nerve regeneration compared with the NGF-stimulated group. The NGC group that received dual stimulation (NGF and US) showed more effective nerve regeneration behavior than the groups that received a single stimulation (NGF or US). The asymmetrically porous NGC with dual NGF and US stimulation may be a promising strategy for the clinical treatment of delayed and insufficient functional recovery of a peripheral nerve.

The effects of altered ultrasound parameters on the recovery of sciatic nerve injury

BACKGROUND

Initial studies have shown that low-energy ultrasound stimulates living tissue cells to reduce regeneration or speed up their recovery. The purpose of this study was to examine the effects of various ultrasound parameters on the speed of recovery in injured sciatic nerves.

METHODS

NMRI mice (n = 200) with injured left paw, caused by crushing their sciatic nerves, were randomly selected. The animals were exposed to ultrasound radiation with various frequencies, intensities, and exposure time. They were allocated into 20 groups (19 treatment and 1 control groups). Sciatic functional index (SFI) test was used to evaluate the difference between the groups with respect to functional efficiency of the sciatic nerve and its recovery.

RESULTS

The results of SFI test obtained from the 14th day showed a significant difference among the groups (P<0.05). On the 14th day after treatment, one of the groups (US11) recovered up to 90%.

CONCLUSION

Altered ultrasound exposure parameters had more favorable outcomes compared with our previous work.

Thermally-mediated ultrasound-induced contraction of equine muscular arteries in vitro and an investigation of the associated cellular mechanisms

We have previously shown that MHz frequency ultrasound causes contraction of the carotid artery in vitro. We now extend this investigation to equine mesenteric arteries and investigate the cellular mechanisms. In vitro exposure of the large lateral cecal mesenteric artery to 4-min periods of 3.2 MHz continuous wave ultrasound at acoustic powers up to 145 mW induced reversible repeatable contraction. The magnitude of the response was linearly dependent on acoustic power and, at 145 mW, the mean increase in wall stress was 0.020 ± 0.017 mN/mm(2) (n = 34). These results are consistent with our previous study and the effect was hypothesised to be thermally mediated. A 2°C temperature rise produced an increase in intracellular calcium, measured by Fluo-4 fluorescence. Inhibition of the inward-rectifier potassium ion channel with BaCl(2) (4 μM) increased the response to ultrasound by 55% ± 49%, indicating a similar electrophysiologic basis to the response to mild hyperthermia. In small mesenteric arteries (0.5-1.0 mm diameter) mounted in a perfusion myograph, neither ultrasound exposure nor heating produced measureable vasoconstriction or a rise in intracellular calcium and we conclude that temperature-sensitive channels are absent or inactive in these small vessels. It, therefore, appears that response of blood vessels to ultrasound depends not only on the thermal properties of the vessels and surrounding tissues but also on the electrophysiology of the smooth muscle cells.

The development and validation of a LIPUS system with preliminary observations of ultrasonic effects on human adult stem cells

To study the potential effects of low-intensity pulsed ultrasound (LIPUS) on cell response in vitro, the ability to alter LIPUS parameters is required. However, commercial LIPUS systems have very little control over parameter selection. In this study, a custom LIPUS system was designed and validated by exploring the effects of using different pulse repetition frequency (PRF) parameters on human adipose derived adult stem cells (hASCs) and bone marrow derived mesenchymal stem cells (hMSCs), two common stem cell sources for creating bone constructs in vitro. Changing the PRF was found to affect cellular response to LIPUS stimulation for both cell types. Proliferation of LIPUS-stimulated cells was found to decrease for hASCs by d 7 for all three groups compared with unstimulated control cells (P = 0.008, 0.011, 0.014 for 1 Hz, 100 Hz and 1 kHz PRF, respectively) and for hMSCs by d 14 (donor 1: P = 0.0005, 0.0002, 0.0003; donor 2: P = 0.0003, 0.0002, 0.0001; for PRFs of 1 Hz, 100 Hz, and 1 kHz, respectively). Additionally, LIPUS was shown to strongly accelerate osteogenic differentiation of hASCs based on amount of calcium accretion normalized by total DNA (P = 0.003, 0.001, 0.003, and 0.032 between control/100 Hz, control/1 kHz, 1 Hz/1 kHz, and 100 Hz/1 kHz pulse repetition frequencies, respectively). These findings promote the study of using LIPUS to induce osteogenic differentiation and further encourage the exploration of LIPUS parameter optimization. The custom LIPUS system was successfully designed to allow extreme parameter variation, specifically PRF, and encourages further studies.

Upgraded nerve growth factor expression induced by low-intensity continuous-wave ultrasound accelerates regeneration of neurotometicly injured sciatic nerve in rats

Low-intensity ultrasound (LIU) can stimulate injured nerve regeneration but the mechanism is still unclear. We investigated the stimulating effect and its mechanism of continuous-wave LIU on neurotometic injury of sciatic nerve. The right sciatic nerves of 64 adult Wistar rats were first crushed and then exposed (32 rats) or sham-exposed (32 rats) to LIU at the crush site. The LIU had a spatial averaged and temporal averaged intensity of 0.25 W/cm(2) operated at 1.0 MHz for 1 min every other day. At various stages (the second, fourth, sixth and eighth weeks) after LIU exposure, the sciatic nerve function index (SFI), the sensory nerve conduction velocity (SNCV), the expression of nerve growth factor (NGF) and sample histology were studied. It was found that the density of nerve fibers with myelin sheath, SFI, SNCV and NGF expression of the treatment group were higher than that of control group (p < 0.05). It has been determined that LIU treatment can accelerate the regeneration and functional recovery of neurotometic injured sciatic nerve at earlier stages after injury, the upgraded expression of NGF induced by LIU may be the primary mechanism of the acceleration effects.