Peripheral nerve injury modulates neurotrophin signaling in the peripheral and central nervous system

Autophagy-targeting modulation to promote peripheral nerve regeneration

Nerve regeneration following traumatic peripheral nerve injuries and neuropathies is a complex process modulated by diverse factors and intricate molecular mechanisms. Past studies have focused on factors that stimulate axonal outgrowth and myelin regeneration. However, recent studies have highlighted the pivotal role of autophagy in peripheral nerve regeneration, particularly in the context of traumatic injuries. Consequently, autophagy-targeting modulation has emerged as a promising therapeutic approach to enhancing peripheral nerve regeneration. Our current understanding suggests that activating autophagy facilitates the rapid clearance of damaged axons and myelin sheaths, thereby enhancing neuronal survival and mitigating injury-induced oxidative stress and inflammation. These actions collectively contribute to creating a favorable microenvironment for structural and functional nerve regeneration. A range of autophagy-inducing drugs and interventions have demonstrated beneficial effects in alleviating peripheral neuropathy and promoting nerve regeneration in preclinical models of traumatic peripheral nerve injuries. This review delves into the regulation of autophagy in cell types involved in peripheral nerve regeneration, summarizing the potential drugs and interventions that can be harnessed to promote this process. We hope that our review will offer novel insights and perspectives on the exploitation of autophagy pathways in the treatment of peripheral nerve injuries and neuropathies.

T cell death-associated gene 8-mediated distinct signaling pathways modulate the early and late phases of neuropathic pain

Peripheral nerve injury alters the transduction of nociceptive signaling. The coordination of neurons, glia, and immune cells results in persistent pain and inflammation. T cell death-associated gene 8 (TDAG8), located at nociceptors and immune cells, is involved in inflammatory pain and arthritis-induced pain. Here, we employed TDAG8-deficient mice, pharmacological approaches, and calcium/sodium imaging to elucidate how TDAG8-mediated signaling modulates neuron activities in a mouse model of chronic constriction injury-induced neuropathic pain. We demonstrated that TDAG8 participated alone in mechanical allodynia induced by constriction injury. (1) TDAG8-Nav1.8 signaling in small-diameter isolectin B4-positive [IB4(+)] neurons initiates mechanical allodynia; it also modulated substance P release from IB4(-) neurons to facilitate the development of early mechanical allodynia. (2) TDAG8-mediated signaling increased medium-to large-diameter IB4(-) neuron activity to maintain late mechanical allodynia; it also modulated substance P release in soma to reduce satellite glial number and Nav1.7 expression, thus attenuating chronic mechanical allodynia.

Therapeutic effect of exosomes derived from Schwann cells in the repair of peripheral nerve injury

Peripheral nerve injury (PNI) can cause nerve demyelination, neuronal apoptosis, axonal atrophy, inflammatory infiltration, glial scar formation, and other pathologies that can lead to sensory and motor dysfunction and seriously affect the psychosomatic health of patients. There is currently no effective treatment method, so exploring a promising treatment method is of great significance. Several studies have revealed the therapeutic roles of Schwann cells (SCs) and their exosomes in nerve injury repair. Exosomes are extracellular nanovesicles secreted by cells that act as key molecules in intercellular communication. Progress has been made in understanding the role of exosomes derived from SCs (SC-EXOs) in peripheral nerve regeneration, including the promotion of axonal regeneration and myelin formation, anti-inflammation, vascular regeneration, neuroprotection, and neuroregulation. Therefore, in this paper, we summarize the functional characteristics of SC-EXOs and discuss their potential therapeutic effects on PNI repair as well as some existing problems and future challenges.

Recent Advances in Biomolecular Patho-Mechanistic Pathways behind the Development and Progression of Diabetic Neuropathy

Diabetic neuropathy (DN) is a neurodegenerative disorder that is primarily characterized by distal sensory loss, reduced mobility, and foot ulcers that may potentially lead to amputation. The multifaceted etiology of DN is linked to a range of inflammatory, vascular, metabolic, and other neurodegenerative factors. Chronic inflammation, endothelial dysfunction, and oxidative stress are the three basic biological changes that contribute to the development of DN. Although our understanding of the intricacies of DN has advanced significantly over the past decade, the distinctive mechanisms underlying the condition are still poorly understood, which may be the reason behind the lack of an effective treatment and cure for DN. The present study delivers a comprehensive understanding and highlights the potential role of the several pathways and molecular mechanisms underlying the etiopathogenesis of DN. Moreover, Schwann cells and satellite glial cells, as integral factors in the pathogenesis of DN, have been enlightened. This work will motivate allied research disciplines to gain a better understanding and analysis of the current state of the biomolecular mechanisms behind the pathogenesis of DN, which will be essential to effectively address every facet of DN, from prevention to treatment.

Development and Application of an Advanced Biomedical Material-Silk Sericin

Sericin, a protein derived from silkworm cocoons, is considered a waste product derived from the silk industry for thousands of years due to a lack of understanding of its properties. However, in recent decades, a range of exciting properties of sericin are studied and uncovered, including cytocompatibility, low-immunogenicity, photo-luminescence, antioxidant properties, as well as cell-function regulating activities. These properties make sericin-based biomaterials promising candidates for biomedical applications. This review summarizes the properties and bioactivities of silk sericin and highlights the latest developments in sericin in tissue engineering and regenerative medicine. Furthermore, the extended application of sericin in developing flexible electronic devices and 3D bioprinting is also discussed. It is believed that sericin-based biomaterials have great potential of being developed into novel tissue engineering products and smart implantable devices for various medical applications toward improving clinical outcomes.

NT-3 contributes to chemotherapy-induced neuropathic pain through TrkC-mediated CCL2 elevation in DRG neurons

Cancer patients undergoing treatment with antineoplastic drugs often experience chemotherapy-induced neuropathic pain (CINP), and the therapeutic options for managing CINP are limited. Here, we show that systemic paclitaxel administration upregulates the expression of neurotrophin-3 (Nt3) mRNA and NT3 protein in the neurons of dorsal root ganglia (DRG), but not in the spinal cord. Blocking NT3 upregulation attenuates paclitaxel-induced mechanical, heat, and cold nociceptive hypersensitivities and spontaneous pain without altering acute pain and locomotor activity in male and female mice. Conversely, mimicking this increase produces enhanced responses to mechanical, heat, and cold stimuli and spontaneous pain in naive male and female mice. Mechanistically, NT3 triggers tropomyosin receptor kinase C (TrkC) activation and participates in the paclitaxel-induced increases of C-C chemokine ligand 2 (Ccl2) mRNA and CCL2 protein in the DRG. Given that CCL2 is an endogenous initiator of CINP and that Nt3 mRNA co-expresses with TrkC and Ccl2 mRNAs in DRG neurons, NT3 likely contributes to CINP through TrkC-mediated activation of the Ccl2 gene in DRG neurons. NT3 may be thus a potential target for CINP treatment.

Description of Novel Molecular Factors in Lumbar DRGs and Spinal Cord Factors Underlying Development of Neuropathic Pain Component in the Animal Model of Osteoarthritis

Osteoarthritis (OA) is one of the most common joint disorder, with pain accompanied by functional impairment, as the most pronounced clinical symptom. Currently used pharmacotherapy involves symptomatic treatment that do not always provide adequate pain relief. This may be due to concomitance of central sensitization and development of neuropathic features in OA patients. Here we performed studies in the animal model of OA to investigate of the neuropathic component. Intraarticular injection of monoiodoacetate (MIA, 1 mg) was used to induce OA in Wistar male rats. Development of pain phenotype was assessed by behavioral testing (PAM test and von Frey's test), while corresponding changes in dorsal root ganglia (DRGs L3-L5) and spinal cord (SC) gene expression were assessed by means of qRT-PCR technique. We also performed microtomography of OA-affected knee joints to correlate the level of bone degradation with observed behavioral and molecular changes. We observed gradually developing remote allodynia after MIA treatment, indicating the presence of neuropathic component. Our results showed that, among DRGs innervating knee joint, development of central sensitization is most likely due to peripheral input of stimuli through DRG L5. In SC, development of secondary hypersensitivity correlated with increased expression of TAC1 and NPY. Our studies provided molecular records on abnormal activation of pain transmission markers in DRG and SC during development of OA that are responsible for the manifestation of neuropathic features. The obtained results increase insight into molecular changes occurring in the neuronal tissue during OA development and may contribute to readdressing treatment paradigms.

Brain-derived neurotrophic factor (BDNF) in chronic pain research: A decade of bibliometric analysis and network visualization

Chronic pain research, with a specific focus on the brain-derived neurotrophic factor (BDNF), has made impressive progress in the past decade, as evident in the improved research quality and increased publications. To better understand this evolving landscape, a quantitative approach is needed. The main aim of this study is to identify the hotspots and trends of BDNF in chronic pain research. We screened relevant publications from 2013 to 2022 in the Scopus database using specific search subject terms. A total of 401 documents were selected for further analysis. We utilized several tools, including Microsoft Excel, Harzing's Publish or Perish, and VOSViewer, to perform a frequency analysis, citation metrics, and visualization, respectively. Key indicators that were examined included publication growth, keyword analyses, topmost influential articles and journals, networking by countries and co-citation of cited references. Notably, there was a persistent publication growth between 2015 and 2021. "Neuropathic pain" emerged as a prominent keyword in 2018, alongside "microglia" and "depression". The journal Pain® was the most impactful journal that published BDNF and chronic pain research, while the most influential publications came from open-access reviews and original articles. China was the leading contributor, followed by the United States (US), and maintained a leadership position in the total number of publications and collaborations. In conclusion, this study provides a comprehensive list of the most influential publications on BDNF in chronic pain research, thereby aiding in the understanding of academic concerns, research hotspots, and global trends in this specialized field.

Neurotrophin growth factors and their receptors as promising blood biomarkers for Alzheimer's Disease: a gene expression analysis study

BACKGROUND

Alzheimer's disease (AD) is a multifaceted neurological ailment affecting more than 50 million individuals globally, distinguished by a deterioration in memory and cognitive abilities. Investigating neurotrophin growth factors could offer significant contributions to understanding AD progression and prospective therapeutic interventions.

METHODS AND RESULTS

The present investigation collected blood samples from 50 patients diagnosed with AD and 50 healthy individuals serving as controls. The mRNA expression levels of neurotrophin growth factors and their receptors were measured using quantitative PCR. A Bayesian regression model was used in the research to assess the relationship between gene expression levels and demographic characteristics such as age and gender. The correlations between variables were analyzed using Spearman correlation coefficients, and the diagnostic potential was assessed using a Receiver Operating Characteristic curve. NTRK2, TrkA, TrkC, and BDNF expression levels were found to be considerably lower (p-value < 0.05) in the blood samples of AD patients compared to the control group. The expression of BDNF exhibited the most substantial decrease in comparison to other neurotrophin growth factors. Correlation analysis indicates a statistically significant positive association between the genes. The ROC analysis showed that BDNF exhibited the greatest Area Under the Curve (AUC) value of 0.76, accompanied by a sensitivity of 70% and specificity of 66%. TrkC, TrkA, and NTRK2 demonstrated considerable diagnostic potential in distinguishing between cases and controls.

CONCLUSION

The observed decrease in the expression levels of NTRK2, TrkA, TrkC, and BDNF in AD patients, along with the identified associations between specific genes and their diagnostic capacity, indicate that these expressions have the potential to function as biomarkers for the diagnosis and treatment of AD.

Rbfox1 regulates alternative splicing of Nrcam in primary sensory neurons to mediate peripheral nerve injury-induced neuropathic pain

The primary sensory neurons of the dorsal root ganglia (DRG) are subject to transcriptional alterations following peripheral nerve injury. These alterations are believed to play a pivotal role in the genesis of neuropathic pain. Alternative RNA splicing is a process that generates multiple transcript variants from a single gene, significantly contributing to the complexity of the transcriptome. However, little is known about the functional significance and control of alternative RNA splicing in injured DRG after spinal nerve ligation (SNL). In our study, we conducted a comprehensive transcriptome profiling and bioinformatic analysis to approach and identified a neuron-specific isoform of an RNA splicing regulator, RNA-binding Fox1 (Rbfox1, also known as A2BP1), as a crucial regulator of alternative RNA splicing in injured DRG after SNL. Notably, Rbfox1 expression is markedly reduced in injured DRG following peripheral nerve injury. Restoring this reduction effectively mitigates nociceptive hypersensitivity. Conversely, mimicking the downregulation of Rbfox1 expression generates neuropathic pain symptoms. Mechanistically, we uncovered that Rbfox1 may be a key factor influencing alternative RNA splicing of neuron-glial related cell adhesion molecule (NrCAM), a key neuronal cell adhesion molecule. In injured DRG after SNL, the downregulation of Rbfox1amplifies the insertion of exon 10 in Nrcam transcripts, leading to an increase in long Nrcam variants (L-Nrcam) and a corresponding decrease in short Nrcam variants (S-Nrcam) within injured DRG. In summary, our study supports the essential role of Rbfox1 in neuropathic pain within DRG, probably via the regulation of Nrcam splicing. These findings suggest that Rbfox1 could be a potential target for neuropathic pain therapy.

Understanding the role of hyperglycemia and the molecular mechanism associated with diabetic neuropathy and possible therapeutic strategies

Diabetic neuropathy is a neuro-degenerative disorder that encompasses numerous factors that impact peripheral nerves in the context of diabetes mellitus (DM). Diabetic peripheral neuropathy (DPN) is very prevalent and impacts 50% of diabetic patients. DPN is a length-dependent peripheral nerve lesion that primarily causes distal sensory loss, discomfort, and foot ulceration that may lead to amputation. The pathophysiology is yet to be fully understood, but current literature on the pathophysiology of DPN revolves around understanding various signaling cascades involving the polyol, hexosamine, protein-kinase C, AGE, oxidative stress, and poly (ADP ribose) polymerase pathways. The results of research have suggested that hyperglycemia target Schwann cells and in severe cases, demyelination resulting in central and peripheral sensitization is evident in diabetic patients. Various diagnostic approaches are available, but detection at an early stage remains a challenge. Traditional analgesics and opioids that can be used "as required" have not been the mainstay of treatment thus far. Instead, anticonvulsants and antidepressants that must be taken routinely over time have been the most common treatments. For now, prolonging life and preserving the quality of life are the ultimate goals of diabetes treatment. Furthermore, the rising prevalence of DPN has substantial consequences for occupational therapy because such therapy is necessary for supporting wellness, warding off other chronic-diseases, and avoiding the development of a disability; this is accomplished by engaging in fulfilling activities like yoga, meditation, and physical exercise. Therefore, occupational therapy, along with palliative therapy, may prove to be crucial in halting the onset of neuropathic-symptoms and in lessening those symptoms once they have occurred.

Low-intensity pulsed ultrasound ameliorates erectile dysfunction induced by bilateral cavernous nerve injury through enhancing Schwann cell-mediated cavernous nerve regeneration

BACKGROUND

Cavernous nerve injury-induced erectile dysfunction caused by pelvic surgery or trauma is refractory to conventional medications and required an alternative treatment. Low-intensity pulsed ultrasound is a noninvasive mechanical therapy that promotes nerve regeneration.

OBJECTIVES

To investigate the therapeutic effect and potential mechanism of low-intensity pulsed ultrasound in the treatment of neurogenic erectile dysfunction.

MATERIALS AND METHODS

Thirty rats were randomly divided into the sham-operated group, bilateral cavernous nerve injury group, and bilateral cavernous nerve injury + low-intensity pulsed ultrasound group. The erectile function was assessed 3 weeks after daily low-intensity pulsed ultrasound treatment. The penile tissues and cavernous nerve tissues were harvested and subjected to histologic analysis. Primary Schwann cells and explants were extracted from adult rats. The effects of low-intensity pulsed ultrasound on proliferation, migration, and nerve growth factor expression of Schwann cells and axonal elongation were examined in vitro. RNA sequencing and western blot assay were applied to predict and verify the molecular mechanism of low-intensity pulsed ultrasound-induced Schwann cell activation.

RESULTS

Our study showed that low-intensity pulsed ultrasound promoted Schwann cells proliferation, migration, and neurotrophic factor nerve growth factor expression. Meanwhile, low-intensity pulsed ultrasound exhibits a stronger ability to enhance Schwann cells-mediated neurite outgrowth of major pelvic ganglion neurons and major pelvic ganglion/cavernous nerve explants in vitro. In vivo experiments demonstrated that the erectile function of the rats in the bilateral cavernous nerve injury + low-intensity pulsed ultrasound group was significantly higher than those in the bilateral cavernous nerve injury groups. Moreover, the expression levels of smooth muscle and cavernous endothelium also increased significantly in the bilateral cavernous nerve injury + low-intensity pulsed ultrasound group. In addition, we observed the higher density and number of cavernous nerve regenerating axons in the bilateral cavernous nerve injury + low-intensity pulsed ultrasound group, indicating that low-intensity pulsed ultrasound promotes axonal regeneration following cavernous nerve injury in vivo. RNA sequencing analysis and bioinformatic analysis suggested that low-intensity pulsed ultrasound might trigger the activation of the PI3K/Akt pathway. Western blot assay confirmed that low-intensity pulsed ultrasound activated Schwann cells through TrkB/Akt/CREB signaling.

CONCLUSIONS

Low-intensity pulsed ultrasound promoted nerve regeneration and ameliorated erectile function by enhancing Schwann cells proliferation, migration, and neurotrophic factor nerve growth factor expression. The TrkB/Akt/CREB axis is the possible mechanism of low-intensity pulsed ultrasound-mediated Schwann cell activation. Low-intensity pulsed ultrasound-based therapy could be a novel potential treatment strategy for cavernous nerve injury-induced neurogenic erectile dysfunction.

Muscle denervation promotes functional interactions between glial and mesenchymal cells through NGFR and NGF

We performed scRNA-seq/snATAC-seq of skeletal muscles post sciatic nerve transection to delineate cell type-specific patterns of gene expression/chromatin accessibility at different time points post-denervation. Unlike myotrauma, denervation selectively activates glial cells and Thy1/CD90-expressing mesenchymal cells. Glial cells expressed Ngf receptor (Ngfr) and were located near neuromuscular junctions (NMJs), close to Thy1/CD90-expressing cells, which provided the main cellular source of NGF post-denervation. Functional communication between these cells was mediated by NGF/NGFR, as either recombinant NGF or co-culture with Thy1/CD90-expressing cells could increase glial cell number ex vivo. Pseudo-time analysis in glial cells revealed an initial bifurcation into processes related to either cellular de-differentiation/commitment to specialized cell types (e.g., Schwann cells), or failure to promote nerve regeneration, leading to extracellular matrix remodeling toward fibrosis. Thus, interactions between denervation-activated Thy1/CD90-expressing and glial cells represent an early abortive process toward NMJs repair, ensued by the conversion of denervated muscles into an environment hostile for NMJ repair.

Neuromodulation for Peripheral Nerve Regeneration: Systematic Review of Mechanisms and In Vivo Highlights

While denervation can occur with aging, peripheral nerve injuries are debilitating and often leads to a loss of function and neuropathic pain. Although injured peripheral nerves can regenerate and reinnervate their targets, this process is slow and directionless. There is some evidence supporting the use of neuromodulation to enhance the regeneration of peripheral nerves. This systematic review reported on the underlying mechanisms that allow neuromodulation to aid peripheral nerve regeneration and highlighted important in vivo studies that demonstrate its efficacy. Studies were identified from PubMed (inception through September 2022) and the results were synthesized qualitatively. Included studies were required to contain content related to peripheral nerve regeneration and some form of neuromodulation. Studies reporting in vivo highlights were subject to a risk of bias assessment using the Cochrane Risk of Bias tool. The results of 52 studies indicate that neuromodulation enhances natural peripheral nerve regeneration processes, but still requires other interventions (e.g., conduits) to control the direction of reinnervation. Additional human studies are warranted to verify the applicability of animal studies and to determine how neuromodulation can be optimized for the greatest functional restoration.

Effect of concurrent aerobic exercise and bone marrow stromal cell transplantation on time-dependent changes of myogenic differentiation-related cascades in soleus muscle after sciatic nerve injury

The purpose of this study was to investigate the time-dependent alteration in whether concurrent aerobic exercise and bone marrow stromal cell (BMSC) engraftment could regulate myogenic differentiation-related signaling pathway in the soleus up to 35 days after sciatic nerve injury (SNI). The rats were divided as follows: the normal control (CON, n=5), sedentary group (SED, n=20), treadmill exercise group (TEX, n=20), BMSC transplantation group (BMSC, n=20), TEX+BMSC transplantation group (TEX+BMSC, n=20) 7, 14, 21, and 35 days after SNI. SNI was applied into the thigh and treadmill exercise was comprised of walking at a speed of 4 to 8 m/min for 30 min once a day. Harvested BMSC at a density of 5×10⁶ in 50-μL phosphate-buff-ered saline was injected into the injury site. Phosphorylated (p) extracellular signal-regulated kinase 1/2 expression was dramatically upregulated in BMSC and BMSC+EX groups from 21 days after SNI compared to those in the SED group. P-ribosomal s6 kinase (RSK) was sharply increased 14 days later, and then rapidly downregulated from day 21, whereas TEX, BMSC and TEX+ BMSC groups significantly kept up expression levels of p-RSK until 35 days post injury than SED group. TEX+BMSC group significantly increased activation of protein kinase B-mammalian target of rapamycin in the soleus from day 14 and myoblast determination protein 1-myogen-in pathways was activated in TEX+BMSC group from day 21. Present findings provide information that combined intervention of aerobic exercise and BMSC transplantation might be a reliable therapeutic strategy for overcoming the morphological and functional problems in denervated soleus muscle.

Inhibiting apoptosis of Schwann cell under the high-glucose condition: A promising approach to treat diabetic peripheral neuropathy using Chinese herbal medicine

Diabetic peripheral neuropathy (DPN) is a common complication of diabetes. Glycemic control and lifestyle alterations cannot prevent the development of DPN; therefore, investigating effective treatments for DPN is crucial. Schwann cells (SCs) maintain the physiological function of peripheral nerves and promote the repair and regeneration of injured nerves. Inhibiting the apoptosis of SCs through various pathological pathways in a high-glucose environment plays an important role in developing DPN. Therefore, inhibiting the apoptosis of SCs can be a novel treatment strategy for DPN. Previous studies have indicated the potential of Chinese herbal medicine (CHM) in treating DPN. In this study, we have reviewed the effects of CHM (both monomers and extracts) on the apoptosis of SCs by interfering with the production of advanced glycation end products, oxidative stress, and endoplasmic reticulum stress pathological pathways. This review will demonstrate the potentialities of CHM in inhibiting apoptosis in SCs, providing new insights and perspectives for treating DPN.

The Effect of Electrical Stimulation on Nerve Regeneration Following Peripheral Nerve Injury

Peripheral nerve injuries (PNI) are common and often result in lifelong disability. The peripheral nervous system has an inherent ability to regenerate following injury, yet complete functional recovery is rare. Despite advances in the diagnosis and repair of PNIs, many patients suffer from chronic pain, and sensory and motor dysfunction. One promising surgical adjunct is the application of intraoperative electrical stimulation (ES) to peripheral nerves. ES acts through second messenger cyclic AMP to augment the intrinsic molecular pathways of regeneration. Decades of animal studies have demonstrated that 20 Hz ES delivered post-surgically accelerates axonal outgrowth and end organ reinnervation. This work has been translated clinically in a series of randomized clinical trials, which suggest that ES can be used as an efficacious therapy to improve patient outcomes following PNIs. The aim of this review is to discuss the cellular physiology and the limitations of regeneration after peripheral nerve injuries. The proposed mechanisms of ES protocols and how they facilitate nerve regeneration depending on timing of administration are outlined. Finally, future directions of research that may provide new perspectives on the optimal delivery of ES following PNI are discussed.

Exercise facilitates regeneration after severe nerve transection and further modulates neural plasticity

Patients with severe traumatic peripheral nerve injury (PNI) always suffer from incomplete recovery and poor functional outcome. Physical exercise-based rehabilitation, as a non-invasive interventional strategy, has been widely acknowledged to improve PNI recovery by promoting nerve regeneration and relieving pain. However, effects of exercise on chronic plastic changes following severe traumatic PNIs have been limitedly discussed. In this study, we created a long-gap sciatic nerve transection followed by autograft bridging in rats and tested the therapeutic functions of treadmill running with low intensity and late initiation. We demonstrated that treadmill running effectively facilitated nerve regeneration and prevented muscle atrophy and thus improved sensorimotor functions and walking performance. Furthermore, exercise could reduce inflammation at the injured nerve as well as prevent the overexpression of TRPV1, a pain sensor, in primary afferent sensory neurons. In the central nervous system, we found that PNI induced transcriptive changes at the ipsilateral lumber spinal dorsal horn, and exercise could reverse the differential expression for genes involved in the Notch signaling pathway. In addition, through neural imaging techniques, we found volumetric, microstructural, metabolite, and neuronal activity changes in supraspinal regions of interest (i.e., somatosensory cortex, motor cortex, hippocampus, etc.) after the PNI, some of which could be reversed through treadmill running. In summary, treadmill running with late initiation could promote recovery from long-gap nerve transection, and while it could reverse maladaptive plasticity after the PNI, exercise may also ameliorate comorbidities, such as chronic pain, mental depression, and anxiety in the long term.

The application of collagen in the repair of peripheral nerve defect

Collagen is a natural polymer expressed in the extracellular matrix of the peripheral nervous system. It has become increasingly crucial in peripheral nerve reconstruction as it was involved in regulating Schwann cell behaviors, maintaining peripheral nerve functions during peripheral nerve development, and being strongly upregulated after nerve injury to promote peripheral nerve regeneration. Moreover, its biological properties, such as low immunogenicity, excellent biocompatibility, and biodegradability make it a suitable biomaterial for peripheral nerve repair. Collagen provides a suitable microenvironment to support Schwann cells’ growth, proliferation, and migration, thereby improving the regeneration and functional recovery of peripheral nerves. This review aims to summarize the characteristics of collagen as a biomaterial, analyze its role in peripheral nerve regeneration, and provide a detailed overview of the recent advances concerning the optimization of collagen nerve conduits in terms of physical properties and structure, as well as the application of the combination with the bioactive component in peripheral nerve regeneration.

Peripheral Nerve Regeneration–Adipose-Tissue-Derived Stem Cells Differentiated by a Three-Step Protocol Promote Neurite Elongation via NGF Secretion

The lack of supportive Schwann cells in segmental nerve lesions seems to be one cornerstone for the problem of insufficient nerve regeneration. Lately, adipose-tissue-derived stem cells (ASCs) differentiated towards SC (Schwann cell)-like cells seem to fulfill some of the needs for ameliorated nerve recovery. In this study, three differentiation protocols were investigated for their ability to differentiate ASCs from rats into specialized SC phenotypes. The differentiated ASCs (dASCs) were compared for their expressions of neurotrophins (NGF, GDNF, BDNF), myelin markers (MBP, P0), as well as glial-marker proteins (S100, GFAP) by RT-PCR, ELISA, and Western blot. Additionally, the influence of the medium conditioned by dASCs on a neuron-like cell line was evaluated. The dASCs were highly diverse in their expression profiles. One protocol yielded relatively high expression rates of neurotrophins, whereas another protocol induced myelin-marker expression. These results were reproducible when the ASCs were differentiated on surfaces potentially used for nerve guidance conduits. The NGF secretion affected the neurite outgrowth significantly. It remains uncertain what features of these SC-like cells contribute the most to adequate functional recovery during the different phases of nerve recovery. Nevertheless, therapeutic applications should consider these diverse phenotypes as a potential approach for stem-cell-based nerve-injury treatment.

Overlapping transcriptional programs promote survival and axonal regeneration of injured retinal ganglion cells

Injured neurons in the adult mammalian central nervous system often die and seldom regenerate axons. To uncover transcriptional pathways that could ameliorate these disappointing responses, we analyzed three interventions that increase survival and regeneration of mouse retinal ganglion cells (RGCs) following optic nerve crush (ONC) injury, albeit not to a clinically useful extent. We assessed gene expression in each of 46 RGC types by single-cell transcriptomics following ONC and treatment. We also compared RGCs that regenerated with those that survived but did not regenerate. Each intervention enhanced survival of most RGC types, but type-independent axon regeneration required manipulation of multiple pathways. Distinct computational methods converged on separate sets of genes selectively expressed by RGCs likely to be dying, surviving, or regenerating. Overexpression of genes associated with the regeneration program enhanced both survival and axon regeneration in vivo, indicating that mechanistic analysis can be used to identify novel therapeutic strategies.

Neuroinflammation Involved in Diabetes-Related Pain and Itch

Diabetes mellitus (DM) is a global epidemic with increasing incidence, which results in diverse complications, seriously affects the patient quality of life, and brings huge economic burdens to society. Diabetic neuropathy is the most common chronic complication of DM, resulting in neuropathic pain and chronic itch. The precise mechanisms of diabetic neuropathy have not been fully clarified, hindering the exploration of novel therapies for diabetic neuropathy and its terrible symptoms such as diabetic pain and itch. Accumulating evidence suggests that neuroinflammation plays a critical role in the pathophysiologic process of neuropathic pain and chronic itch. Indeed, researchers have currently made significant progress in knowing the role of glial cells and the pro-inflammatory mediators produced from glial cells in the modulation of chronic pain and itch signal processing. Here, we provide an overview of the current understanding of neuroinflammation in contributing to the sensitization of the peripheral nervous system (PNS) and central nervous system (CNS). In addition, we also summarize the inflammation mechanisms that contribute to the pathogenesis of diabetic itch, including activation of glial cells, oxidative stress, and pro-inflammatory factors. Targeting excessive neuroinflammation may provide potential and effective therapies for the treatment of chronic neuropathic pain and itch in DM.

Peripheral nerve fibroblasts secrete neurotrophic factors to promote axon growth of motoneurons

Peripheral nerve fibroblasts play a critical role in nerve development and regeneration. Our previous study found that peripheral nerve fibroblasts have different sensory and motor phenotypes. Fibroblasts of different phenotypes can guide the migration of Schwann cells to the same sensory or motor phenotype. In this study, we analyzed the different effects of peripheral nerve-derived fibroblasts and cardiac fibroblasts on motoneurons. Compared with cardiac fibroblasts, peripheral nerve fibroblasts greatly promoted motoneuron neurite outgrowth. Transcriptome analysis results identified 491 genes that were differentially expressed in peripheral nerve fibroblasts and cardiac fibroblasts. Among these, 130 were significantly upregulated in peripheral nerve fibroblasts compared with cardiac fibroblasts. These genes may be involved in axon guidance and neuron projection. Three days after sciatic nerve transection in rats, peripheral nerve fibroblasts accumulated in the proximal and distal nerve stumps, and most expressed brain-derived neurotrophic factor. In vitro, brain-derived neurotrophic factor secreted from peripheral nerve fibroblasts increased the expression of β-actin and F-actin through the extracellular regulated protein kinase and serine/threonine kinase pathways, and enhanced motoneuron neurite outgrowth. These findings suggest that peripheral nerve fibroblasts and cardiac fibroblasts exhibit different patterns of gene expression. Peripheral nerve fibroblasts can promote motoneuron neurite outgrowth.

Neurotrophic factors combined with stem cells in the treatment of sciatic nerve injury in rats:a meta-analysis

Treatment of peripheral nerve regeneration with stem cells (SCs) alone has some limitations. For this reason, we evaluate the efficacy of neurotrophic factors combined with stem cell transplantation in the treatment of sciatic nerve injury (SNI) in rats. PubMed, Cochrane Library, Embase, WanFang, VIP and China National Knowledge Infrastructure databases were retrieved from inception to October 2021, and control experiments on neurotrophic factors combined with stem cells in the treatment of SNI in rats were searched. Nine articles and 551 rats were included in the meta-analysis. The results of meta-analysis confirmed that neurotrophic factor combined with stem cells for the treatment of SNI yielded more effective repair than normal rats with regard to sciatic nerve index, electrophysiological detection index, electron microscope observation index, and recovery rate of muscle wet weight. The conclusion is that neurotrophic factor combined with stem cells is more conducive to peripheral nerve regeneration and functional recovery than stem cells alone. However, due to the limitation of the quality of the included literature, the above conclusions need to be verified by randomized controlled experiments with higher quality and larger samples.

ZNF382 controls mouse neuropathic pain via silencer-based epigenetic inhibition of Cxcl13 in DRG neurons

Nerve injury-induced changes of gene expression in dorsal root ganglion (DRG) are critical for neuropathic pain genesis. However, how these changes occur remains elusive. Here we report the down-regulation of zinc finger protein 382 (ZNF382) in injured DRG neurons after nerve injury. Rescuing this down-regulation attenuates nociceptive hypersensitivity. Conversely, mimicking this down-regulation produces neuropathic pain symptoms, which are alleviated by C-X-C motif chemokine 13 (CXCL13) knockdown or its receptor CXCR5 knockout. Mechanistically, an identified cis-acting silencer at distal upstream of the Cxcl13 promoter suppresses Cxcl13 transcription via binding to ZNF382. Blocking this binding or genetically deleting this silencer abolishes the ZNF382 suppression on Cxcl13 transcription and impairs ZNF382-induced antinociception. Moreover, ZNF382 down-regulation disrupts the repressive epigenetic complex containing histone deacetylase 1 and SET domain bifurcated 1 at the silencer-promoter loop, resulting in Cxcl13 transcriptional activation. Thus, ZNF382 down-regulation is required for neuropathic pain likely through silencer-based epigenetic disinhibition of CXCL13, a key neuropathic pain player, in DRG neurons.

The Role of Formyl Peptide Receptors in Neurological Diseases via Regulating Inflammation

Formyl peptide receptors (FPRs) are a group of G protein-coupled cell surface receptors that play important roles in host defense and inflammation. Owing to the ubiquitous expression of FPRs throughout different cell types and since they interact with structurally diverse chemotactic agonists, they have a dual function in inflammatory processes, depending on binding with different ligands so that accelerate or inhibit key intracellular kinase-based regulatory pathways. Neuroinflammation is closely associated with the pathogenesis of neurodegenerative diseases, neurogenic tumors and cerebrovascular diseases. From recent studies, it is clear that FPRs are important biomarkers for neurological diseases as they regulate inflammatory responses by monitoring glial activation, accelerating neural differentiation, regulating angiogenesis, and controlling blood brain barrier (BBB) permeability, thereby affecting neurological disease progression. Given the complex mechanisms of neurological diseases and the difficulty of healing, we are eager to find new and effective therapeutic targets. Here, we review recent research about various mechanisms of the effects generated after FPR binding to different ligands, role of FPRs in neuroinflammation as well as the development and prognosis of neurological diseases. We summarize that the FPR family has dual inflammatory functional properties in central nervous system. Emphasizing that FPR2 acts as a key molecule that mediates the active resolution of inflammation, which binds with corresponding receptors to reduce the expression and activation of pro-inflammatory composition, govern the transport of immune cells to inflammatory tissues, and restore the integrity of the BBB. Concurrently, FPR1 is essentially related to angiogenesis, cell proliferation and neurogenesis. Thus, treatment with FPRs-modulation may be effective for neurological diseases.

Epitranscriptomic Analysis of m6A Methylome After Peripheral Nerve Injury

N6-methyladenosine (m6A) is one of the most plentiful internal RNA modifications, especially in eukaryotic messenger RNA (mRNA), which plays pivotal roles in the regulation of mRNA life cycle and nerve development. However, the mRNA m6A methylation pattern in peripheral nervous injury (PNI) has not been investigated. In this study, sciatic nerve samples were collected from 7 days after sciatic nerve injury (SNI) and control rats. Quantitative real-time PCR demonstrated that m6A-related methyltransferase/demethylase genes were remarkably upregulated in SNI group compared with control group. Methylated RNA immunoprecipitation sequencing (MeRIP-seq) was performed to reveal the m6A methylation landscape. The results showed that 4,014 m6A peaks were significantly altered, including 2,144 upregulated and 1,870 downregulated m6A peaks, which were corresponded to 1,858 genes. Moreover, 919 differentially expressed genes were identified by the conjoint analysis of MeRIP-seq and RNA-seq. GO and KEGG pathway analyses were performed to determine the biological functions and signaling pathways of the m6A-modified genes. Notably, these genes were mainly related to the immune system process, cell activation, and nervous system development in GO analysis. KEGG pathway analysis revealed that these genes were involved in the cell cycle, B cell receptor signaling pathway, axon guidance pathway, and calcium signaling pathway. Furthermore, the m6A methylation and protein expression levels of autophagy-related gene (Atg7) were increased, together with the activation of autophagy. These findings shed some light on the epigenetic regulation of gene expression, which may provide a new opinion to promote functional recovery after PNI.

A Review of Hand-Arm Vibration Studies Conducted by US NIOSH since 2000

Studies on hand-transmitted vibration exposure, biodynamic responses, and biological effects were conducted by researchers at the Health Effects Laboratory Division (HELD) of the National Institute for Occupational Safety and Health (NIOSH) during the last 20 years. These studies are systematically reviewed in this report, along with the identification of areas where additional research is needed. The majority of the studies cover the following aspects: (i) the methods and techniques for measuring hand-transmitted vibration exposure; (ii) vibration biodynamics of the hand-arm system and the quantification of vibration exposure; (iii) biological effects of hand-transmitted vibration exposure; (iv) measurements of vibration-induced health effects; (iv) quantification of influencing biomechanical effects; and (v) intervention methods and technologies for controlling hand-transmitted vibration exposure. The major findings of the studies are summarized and discussed.

The Expression of Chemokines Is Downregulated in a Pre-Clinical Model of TTR V30M Amyloidosis

Inflammation is a hallmark of several neurodegenerative disorders including hereditary amyloidogenic transthyretin amyloidosis (ATTRv). ATTRv is an autosomal dominant neurodegenerative disorder with extracellular deposition of mutant transthyretin (TTR) aggregates and fibrils, particularly in nerves and ganglia of the peripheral nervous system. Nerve biopsies from ATTRv patients show increased cytokine production, but interestingly no immune inflammatory cellular infiltrate is observed around TTR aggregates. Here we show that as compared to Wild Type (WT) animals, the expression of several chemokines is highly downregulated in the peripheral nervous system of a mouse model of the disease. Interestingly, we found that stimulation of mouse Schwann cells (SCs) with WT TTR results in the secretion of several chemokines, a process that is mediated by toll-like receptor 4 (TLR4). In contrast, the secretion of all tested chemokines is compromised upon stimulation of SCs with mutant TTR (V30M), suggesting that V30M TTR fails to activate TLR4 signaling. Altogether, our data shed light into a previously unappreciated mechanism linking TTR activation of SCs and possibly underlying the lack of inflammatory response observed in the peripheral nervous system of ATTRv patients.

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

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

Hedging against Neuropathic Pain: Role of Hedgehog Signaling in Pathological Nerve Healing

The peripheral nervous system has important regenerative capacities that regulate and restore peripheral nerve homeostasis. Following peripheral nerve injury, the nerve undergoes a highly regulated degeneration and regeneration process called Wallerian degeneration, where numerous cell populations interact to allow proper nerve healing. Recent studies have evidenced the prominent role of morphogenetic Hedgehog signaling pathway and its main effectors, Sonic Hedgehog (SHH) and Desert Hedgehog (DHH) in the regenerative drive following nerve injury. Furthermore, dysfunctional regeneration and/or dysfunctional Hedgehog signaling participate in the development of chronic neuropathic pain that sometimes accompanies nerve healing in the clinical context. Understanding the implications of this key signaling pathway could provide exciting new perspectives for future research on peripheral nerve healing.

Deletion of Acid-Sensing Ion Channel 3 Relieves the Late Phase of Neuropathic Pain by Preventing Neuron Degeneration and Promoting Neuron Repair

Neuropathic pain is one type of chronic pain that occurs as a result of a lesion or disease to the somatosensory nervous system. Chronic excessive inflammatory response after nerve injury may contribute to the maintenance of persistent pain. Although the role of inflammatory mediators and cytokines in mediating allodynia and hyperalgesia has been extensively studied, the detailed mechanisms of persistent pain or whether the interactions between neurons, glia and immune cells are essential for maintenance of the chronic state have not been completely elucidated. ASIC3, a voltage-insensitive, proton-gated cation channel, is the most essential pH sensor for pain perception. ASIC3 gene expression is increased in dorsal root ganglion neurons after inflammation and nerve injury and ASIC3 is involved in macrophage maturation. ASIC currents are increased after nerve injury. However, whether prolonged hyperalgesia induced by the nerve injury requires ASIC3 and whether ASIC3 regulates neurons, immune cells or glial cells to modulate neuropathic pain remains unknown. We established a model of chronic constriction injury of the sciatic nerve (CCI) in mice. CCI mice showed long-lasting mechanical allodynia and thermal hyperalgesia. CCI also caused long-term inflammation at the sciatic nerve and primary sensory neuron degeneration as well as increased satellite glial expression and ATF3 expression. ASIC3 deficiency shortened mechanical allodynia and attenuated thermal hyperalgesia. ASIC3 gene deletion shifted ATF3 expression from large to small neurons and altered the M1/M2 macrophage ratio, thereby preventing small neuron degeneration and relieved pain.

Modulation of Small RNA Signatures in Schwann-Cell-Derived Extracellular Vesicles by the p75 Neurotrophin Receptor and Sortilin

Schwann cells (SCs) are the main glial cells of the peripheral nervous system (PNS) and are known to be involved in various pathophysiological processes, such as diabetic neuropathy and nerve regeneration, through neurotrophin signaling. Such glial trophic support to axons, as well as neuronal survival/death signaling, has previously been linked to the p75 neurotrophin receptor (p75NTR) and its co-receptor Sortilin. Recently, SC-derived extracellular vesicles (EVs) were shown to be important for axon growth and nerve regeneration, but cargo of these glial cell-derived EVs has not yet been well-characterized. In this study, we aimed to characterize signatures of small RNAs in EVs derived from wild-type (WT) SCs and define differentially expressed small RNAs in EVs derived from SCs with genetic deletions of p75NTR (Ngfr-/-) or Sortilin (Sort1-/-). Using RNA sequencing, we identified a total of 366 miRNAs in EVs derived from WT SCs of which the most highly expressed are linked to the regulation of axonogenesis, axon guidance and axon extension, suggesting an involvement of SC EVs in axonal homeostasis. Signaling of SC EVs to non-neuronal cells was also suggested by the presence of several miRNAs important for regulation of the endothelial cell apoptotic process. Ablated p75NTR or sortilin expression in SCs translated into a set of differentially regulated tRNAs and miRNAs, with impact in autophagy and several cellular signaling pathways such as the phosphatidylinositol signaling system. With this work, we identified the global expression profile of small RNAs present in SC-derived EVs and provided evidence for a regulatory function of these vesicles on the homeostasis of other cell types of the PNS. Differentially identified miRNAs can pave the way to a better understanding of p75NTR and sortilin roles regarding PNS homeostasis and disease.

TrkA inhibitor promotes motor functional regeneration of recurrent laryngeal nerve by suppression of sensory nerve regeneration

Recurrent laryngeal nerve (RLN) injury, in which hoarseness and dysphagia arise as a result of impaired vocal fold movement, is a serious complication. Misdirected regeneration is an issue for functional regeneration. In this study, we demonstrated the effect of TrkA inhibitors, which blocks the NGF-TrkA pathway that acts on the sensory/automatic nerves thus preventing misdirected regeneration among motor and sensory nerves, and thereby promoting the regeneration of motor neurons to achieve functional recovery. RLN axotomy rat models were used in this study, in which cut ends of the nerve were bridged with polyglycolic acid-collagen tube with and without TrkA inhibitor (TrkAi) infiltration. Our study revealed significant improvement in motor nerve fiber regeneration and function, in assessment of vocal fold movement, myelinated nerve regeneration, compound muscle action potential, and prevention of laryngeal muscle atrophy. Retrograde labeling demonstrated fewer labeled neurons in the vagus ganglion, which confirmed reduced misdirected regeneration among motor and sensory fibers, and a change in distribution of the labeled neurons in the nucleus ambiguus. Our study demonstrated that TrkAi have a strong potential for clinical application in the treatment of RLN injury.

Augmented Buyang Huanwu Decoction facilitates axonal regeneration after peripheral nerve transection through the regulation of inflammatory cytokine production

ETHNOPHARMACOLOGICAL RELEVANCE

Herbal formulation Buyang Huanwu Decoction (BYHWD) has been used to treat cardiovascular disorders including cerebral ischemia. Recent studies showed its effects on promoting axonal regeneration after nerve injury. However, compositional reformulation supplemented with herbal components that regulates inflammation may increase its efficacy for nerve repair.

AIM OF THE STUDY

We prepared a new herbal decoction by adding selected herbal components to BYHWD (augmented BYHWD; ABHD) and investigated the effect of ABHD on the production of inflammatory cytokines and axonal regeneration using an animal model of nerve transection and coaptation (NTC).

MATERIALS AND METHODS

A rat model of NTC was performed on the sciatic nerve. The sciatic nerve and dorsal root ganglion (DRG) were isolated and used for immunofluorescence staining and western blot analysis. DRG tissue was also used to prepare primary neuron culture and the length of neurites was analyzed. Sensorimotor nerve activities were assessed by rotarod and von Frey tests.

RESULTS

Three herbal components that facilitated neurite outgrowth were chosen to formulate ABHD. ABHD administration into the sciatic nerve 1 week or 3 months after NTC facilitated axonal regeneration. Cell division cycle 2 (Cdc2) and brain-derived neurotrophic factor (BDNF) proteins were induced from the reconnected distal portion of the sciatic nerve and the levels were further elevated by in vivo administration of ABHD. Phospho-Erk1/2 level was increased by ABHD treatment as well, implying its role in mediating retrograde transport of BDNF signals into the neuronal cell body. Production of inflammatory cytokines IL-1β and TNF-α was induced in the reconnected nerve but attenuated by ABHD treatment. Behavioral tests revealed that ABHD treatment improved functional recovery of sensorimotor activities.

CONCLUSIONS

A newly formulated ABHD is effective at regulating the production of inflammatory cytokines and promoting axonal regeneration after nerve transection and may be considered to develop therapeutic strategies for peripheral nerve injury disorders.

Schwann Cell-Like Cells: Origin and Usability for Repair and Regeneration of the Peripheral and Central Nervous System

Functional recovery after neurotmesis, a complete transection of the nerve fiber, is often poor and requires a surgical procedure. Especially for longer gaps (>3 mm), end-to-end suturing of the proximal to the distal part is not possible, thus requiring nerve graft implantation. Artificial nerve grafts, i.e., hollow fibers, hydrogels, chitosan, collagen conduits, and decellularized scaffolds hold promise provided that these structures are populated with Schwann cells (SC) that are widely accepted to promote peripheral and spinal cord regeneration. However, these cells must be collected from the healthy peripheral nerves, resulting in significant time delay for treatment and undesired morbidities for the donors. Therefore, there is a clear need to explore the viable source of cells with a regenerative potential similar to SC. For this, we analyzed the literature for the generation of Schwann cell-like cells (SCLC) from stem cells of different origins (i.e., mesenchymal stem cells, pluripotent stem cells, and genetically programmed somatic cells) and compared their biological performance to promote axonal regeneration. Thus, the present review accounts for current developments in the field of SCLC differentiation, their applications in peripheral and central nervous system injury, and provides insights for future strategies.

Effect of Mycobacterium leprae on neurotrophins expression in human Schwann cells and mouse sciatic nerves

BACKGROUND

Although Mycobacterium leprae (ML) is well characterised as the causative agent of leprosy, the pathophysiological mechanisms underlying peripheral nerve damage still need further understanding. In vitro and in vivo studies have yielded insights into molecular mechanisms of ML interaction with Schwann cells (SC), indicating the regulation of genes and proteins crucial to neural plasticity.

OBJECTIVES

We aimed to investigate the effect of ML on neurotrophins expression in human SC (hSC) and mice sciatic nerves to better understand their role in leprosy neuropathy, and aiming to contribute to future therapeutic approaches.

METHODS

We evaluated mRNA and protein expression of BDNF, NGF, NT-3, NT-4 in hSC from amputation nerve fragments, as well as in athymic nude mice, infected by ML for eight months.

FINDINGS AND MAIN CONCLUSIONS

Our in vitro results showed a trend to decline in NGF and BDNF mRNA in ML-treated hSC, compared to controls. The immunodetection of BDNF and NT-4 was significantly downregulated in ML-treated hSC. Conversely, ML-infected mice demonstrated upregulation of NT-3, compared to non-infected animals. Our findings indicate that ML may be involved in neurotrophins regulation, suggesting that a pathogen-related imbalance of these growth factors may have a role in the neural impairment of leprosy.

Self-assembling peptide hydrogels functionalized with LN- and BDNF- mimicking epitopes synergistically enhance peripheral nerve regeneration

The regenerative capacity of the peripheral nervous system is closely related to the role that Schwann cells (SCs) play in construction of the basement membrane containing multiple extracellular matrix proteins and secretion of neurotrophic factors, including laminin (LN) and brain-derived neurotrophic factor (BDNF). Here, we developed a self-assembling peptide (SAP) nanofiber hydrogel based on self-assembling backbone Ac-(RADA)4-NH2 (RAD) dual-functionalized with laminin-derived motif IKVAV (IKV) and a BDNF-mimetic peptide epitope RGIDKRHWNSQ (RGI) for peripheral nerve regeneration, with the hydrogel providing a three-dimensional (3D) microenvironment for SCs and neurites. Methods: Circular dichroism (CD), atomic force microscopy (AFM), and scanning electron microscopy (SEM) were used to characterize the secondary structures, microscopic structures, and morphologies of self-assembling nanofiber hydrogels. Then the SC adhesion, myelination and neurotrophin secretion were evaluated on the hydrogels. Finally, the SAP hydrogels were injected into hollow chitosan tubes to bridge a 10-mm-long sciatic nerve defect in rats, and in vivo gene expression at 1 week, axonal regeneration, target muscular re-innervation, and functional recovery at 12 weeks were assessed. Results: The bioactive peptide motifs were covalently linked to the C-terminal of the self-assembling peptide and the functionalized peptides could form well-defined nanofibrous hydrogels capable of providing a 3D microenvironment similar to native extracellular matrix. SCs displayed improved cell adhesion on hydrogels with both IKV and RGI, accompanied by increased cell spreading and elongation relative to other groups. RSCs cultured on hydrogels with IKV and RGI showed enhanced gene expression of NGF, BDNF, CNTF, PMP22 and NRP2, and decreased gene expression of NCAM compared with those cultured on other three groups after a 7-day incubation. Additionally, the secretion of NGF, BDNF, and CNTF of RSCs was significantly improved on dual-functionalized peptide hydrogels after 3 days. At 1 week after implantation, the expressions of neurotrophin and myelin-related genes in the nerve grafts in SAP and Autograft groups were higher than that in Hollow group, and the expression of S100 in groups containing both IKV and RGI was significantly higher than that in groups containing either IKV or RGI hydrogels, suggesting enhanced SC proliferation. The morphometric parameters of the regenerated nerves, their electrophysiological performance, the innervated muscle weight and remodeling of muscle fibers, and motor function showed that RAD/IKV/RGI and RAD/IKV-GG-RGI hydrogels could markedly improve axonal regeneration with enhanced re-myelination and motor functional recovery through the synergetic effect of IKV and RGI functional motifs. Conclusions: We found that the dual-functionalized SAP hydrogels promoted RSC adhesion, myelination, and neurotrophin secretion in vitro and successfully bridged a 10-mm gap representing a sciatic nerve defect in rats in vivo. The results demonstrated the synergistic effect of IKVAV and RGI on axonal regrowth and function recovery after peripheral nerve injury.

Novel Analgesics with Peripheral Targets

A limited number of peripheral targets generate pain. Inflammatory mediators can sensitize these. The review addresses targets acting exclusively or predominantly on sensory neurons, mediators involved in inflammation targeting sensory neurons, and mediators involved in a more general inflammatory process, of which an analgesic effect secondary to an anti-inflammatory effect can be expected. Different approaches to address these systems are discussed, including scavenging proinflammatory mediators, applying anti-inflammatory mediators, and inhibiting proinflammatory or facilitating anti-inflammatory receptors. New approaches are contrasted to established ones; the current stage of progress is mentioned, in particular considering whether there is data from a molecular and cellular level, from animals, or from human trials, including an early stage after a market release. An overview of publication activity is presented, considering a IuPhar/BPS-curated list of targets with restriction to pain-related publications, which was also used to identify topics.

Application of conductive PPy/SF composite scaffold and electrical stimulation for neural tissue engineering

Electrical stimulation (ES) with conductive polymers can dramatically enhance neurite outgrowth and promote neural regeneration. However, besides ES, the practical applications of neural repair is also highly dependent on the nerve cell functionality and response to substrate conductivity. Therefore, the combination of the ES and suitable materials, such as tissue scaffolds, has been applied to facilitate treatment of neural injuries and demonstrated great potential in peripheral nerve regeneration. In this study, polypyrrole/silk fibroin (PPy/SF) conductive composite scaffold was fabricated by 3D bioprinting and electrospinning. Schwann cells seeded on these scaffolds were electrically stimulated and hence demonstrated enhanced viability, proliferation and migration, as well as upregulated expression of neurotrophic factors. Furthermore, the constructed PPy/SF conductive nerve guidance conduits accompanying with ES could effectively promote axonal regeneration and remyelination in vivo. Moreover, we found that the MAPKs signal transduction pathway was activated by ES at the conductive conduit. Our findings demonstrate that the PPy/SF conductive composite scaffolds with longitudinal guidance exhibit favorable properties for clinical use and promotes nerve regeneration and functional recovery.

Regeneration of adult rat sensory and motor neuron axons through chimeric peroneal nerve grafts containing donor Schwann cells engineered to express different neurotrophic factors

Large peripheral nerve (PN) defects require bridging substrates to restore tissue continuity and permit the regrowth of sensory and motor axons. We previously showed that cell-free PN segments repopulated ex vivo with Schwann cells (SCs) transduced with lentiviral vectors (LV) to express different growth factors (BDNF, CNTF or NT-3) supported the regeneration of axons across a 1 cm peroneal nerve defect (Godinho et al., 2013). Graft morphology, the number of regrown axons, the ratio of myelinated to unmyelinated axons, and hindlimb locomotor function differed depending on the growth factor engineered into SCs. Here we extend these observations, adding more LVs (expressing GDNF or NGF) and characterising regenerating sensory and motor neurons after injection of the retrograde tracer Fluorogold (FG) into peroneal nerve distal to grafts, 10 weeks after surgery. Counts were also made in rats with intact nerves and in animals receiving autografts, acellular grafts, or grafts containing LV-GFP transduced SCs. Counts and analysis of FG positive (⁺) DRG neurons were made from lumbar (L5) ganglia. Graft groups contained fewer labeled sensory neurons than non-operated controls, but this decrease was only significant in the LV-GDNF group. These grafts had a complex fascicular morphology that may have resulted in axon trapping. The proportion of FG⁺ sensory neurons immunopositive for calcitonin-gene related peptide (CGRP) varied between groups, there being a significantly higher percentage in autografts and most neurotrophic factor groups compared to the LV-CNTF, LV-GFP and acellular groups. Furthermore, the proportion of regenerating isolectin B₄⁺ neurons was significantly greater in the LV-NT-3 group compared to other groups, including autografts and non-lesion controls. Immunohistochemical analysis of longitudinal graft sections revealed that all grafts contained a reduced number of choline acetyltransferase (ChAT) positive axons, but this decrease was significant only in the GDNF and NT-3 graft groups. We also assessed the number and phenotype of regrowing lumbar FG⁺ motor neurons in non-lesioned animals, and in rats with autografts, acellular grafts, or in grafts containing SCs expressing GFP, CNTF, NGF or NT-3. The overall number of FG⁺ motor neurons per section was similar in all groups; however in tissue immunostained for NeuN (expressed in α- but not γ-motor neurons) the proportion of NeuN negative FG⁺ neurons ranged from about 40-50% in all groups except the NT-3 group, where the percentage was 82%, significantly more than the SC-GFP group. Immunostaining for the vesicular glutamate transporter VGLUT-1 revealed occasional proprioceptive terminals in 'contact' with regenerating FG⁺ α-motor neurons in PN grafted animals, the acellular group having the lowest counts. In sum, while all graft types supported sensory and motor axon regrowth, there appeared to be axon trapping in SC-GDNF grafts, and data from the SC-NT-3 group revealed greater regeneration of sensory CGRP⁺ and IB₄⁺ neurons, preferential regeneration of γ-motor neurons and perhaps partial restoration of monosynaptic sensorimotor relays.

Overexpression of Foxc1 regenerates crushed rat facial nerves by promoting Schwann cells migration via the Wnt/β-catenin signaling pathway

Facial paralysis can result in severe implications for patients. A good prognosis depends on the degree of nerve regeneration. Schwann cells (SCs) play an important role in facial nerve development and regeneration through migration. Forkhead box C1 (Foxc1), a member of the forkhead transcription factor family, is implicated in cell migration. However, the role of Foxc1 in the progression after facial nerve crush remains unknown. Our aim was to evaluate the effect of Foxc1 overexpression on SC migration and recovery of facial nerves after crush injury. The rat facial nerve crush injury model was established through the use of unilateral surgery. The results showed that the expression of Foxc1 was increased in the surgery group compared to that of the control group. SCs were isolated from the sciatic nerves and cultured. Foxc1, delivered by an adeno‐associated virus in vivo, or adenovirus in vitro, both induced overexpression of Foxc1, and increased the expression of CXCL12 and β‐catenin. After the transfection of Foxc1, the migration of SC was increased both in vitro and in vivo, was reduced by the inhibition of CXCL12 or β‐catenin. The facial nerve function and the nerve axon remyelination of the rats transfected with Foxc1 were significantly improved after nerve crush injury. Overall, the results demonstrated that overexpression of Foxc1 promoted SC migration by regulating CXCL12 via the Wnt/β‐catenin pathway, thus contributing to improved facial nerve function after crush injury.

Changes in the transcriptional fingerprint of satellite glial cells following peripheral nerve injury

Satellite glial cells (SGCs) are homeostatic cells enveloping the somata of peripheral sensory and autonomic neurons. A wide variety of neuronal stressors trigger activation of SGCs, contributing to, for example, neuropathic pain through modulation of neuronal activity. However, compared to neurons and other glial cells of the nervous system, SGCs have received modest scientific attention and very little is known about SGC biology, possibly due to the experimental challenges associated with studying them in vivo and in vitro. Utilizing a recently developed method to obtain SGC RNA from dorsal root ganglia (DRG), we took a systematic approach to characterize the SGC transcriptional fingerprint by using next-generation sequencing and, for the first time, obtain an overview of the SGC injury response. Our RNA sequencing data are easily accessible in supporting information in Excel format. They reveal that SGCs are enriched in genes related to the immune system and cell-to-cell communication. Analysis of SGC transcriptional changes in a nerve injury-paradigm reveal a differential response at 3 days versus 14 days postinjury, suggesting dynamic modulation of SGC function over time. Significant downregulation of several genes linked to cholesterol synthesis was observed at both time points. In contrast, regulation of gene clusters linked to the immune system (MHC protein complex and leukocyte migration) was mainly observed after 14 days. Finally, we demonstrate that, after nerve injury, macrophages are in closer physical proximity to both small and large DRG neurons, and that previously reported injury-induced proliferation of SGCs may, in fact, be proliferating macrophages.

NeuroD1 overexpression in spinal neurons accelerates axonal regeneration after sciatic nerve injury

Neurogenic differentiation 1 (NeuroD1) is mainlyexpressed in developing neurons where it plays critical roles in neuronal maturation and neurite elongation. The potential role and mechanism of NeuroD1 in adult axonal regeneration is not clear. The present study used synapsin (SYN) Cre and AAV9-Flex vectors to conditionally overexpress NeuroD1 in adult spinal neurons and found that NeuroD1 overexpression significantly accelerated axonal regeneration and functional recovery after sciatic nerve injury. Further in vitro and in vivo experiments suggested that the mechanism of NeuroD1 promotion on axonal regeneration was related to its regulation of the expression of neurotrophin BDNF and its receptor TrkB as well as a microtubule severing protein spastin.

Frequency-dependent changes in mitochondrial number and generation of reactive oxygen species in a rat model of vibration-induced injury

Regular use of vibrating hand tools results in cold-induced vasoconstriction, finger blanching, and a reduction in tactile sensitivity and manual dexterity. Depending upon the length and frequency, vibration induces regeneration, or dysfunction and apoptosis, inflammation and an increase in reactive oxygen species (ROS) levels. These changes may be associated with mitochondria, this study examined the effects of vibration on total and functional mitochondria number. Male rats were exposed to restraint or tail vibration at 62.5, 125, or 250 Hz. The frequency-dependent effects of vibration on mitochondrial number and generation of oxidative stress were examined. After 10 days of exposure at 125 Hz, ventral tail arteries (VTA) were constricted and there was an increase in mitochondrial number and intensity of ROS staining. In the skin, the influence of vibration on arterioles displayed a similar but insignificant response in VTA. There was also a reduction in the number of small nerves with exposure to vibration at 250 Hz, and a reduction in mitochondrial number in nerves in restrained and all vibrated conditions. There was a significant rise in the size of the sensory receptors with vibration at 125 Hz, and an elevation in ROS levels. Based upon these results, mitochondria number and activity are affected by vibration, especially at frequencies at or near resonance. The influence of vibration on the vascular system may either be adaptive or maladaptive. However, the effects on cutaneous nerves might be a precursor to loss of innervation and sensory function noted in workers exposed to vibration.

The electrostimulation and scar inhibition effect of chitosan/oxidized hydroxyethyl cellulose/reduced graphene oxide/asiaticoside liposome based hydrogel on peripheral nerve regeneration in vitro

The application of hollow nerve conduits in the repair of peripheral nerve defects is effected by inferior recovery, and nerve extension is hampered by the scar tissue generated during the repair process. In this study, the filler in hollow nerve conduit, chitosan/oxidized hydroxyethyl cellulose (CS/OHEC) hydrogel loaded asiaticoside liposome and the conductive reduced graphene oxide (rGO) were developed and used to reform the microenvironment for peripheral nerve regeneration. The physiochemical properties of CS/OHEC/rGO/asiaticoside liposome hydrogel were characterized by Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and compressive modulus, porosity, swelling ratio, degradation and conductivity. In addition, the asiaticoside release profiles in vitro were investigated. The hydrogel had a continuous porous network structure with pore size distribution in the range of 50-250 μm. The majority of the hydrogels had porosities above 70%, and a compressive modulus of 0.45 MPa. The weight loss rate of hydrogel reached 76.14 ± 4.45% within 8 weeks. The conductivity of the hydrogel was 5.27 ± 0.42 × 10^-4 S/cm. The hydrogel was non-toxic and suitable for adhesion and proliferation of nerve cells in vitro. In addition, the application of electrical stimulation after the addition of rGO can promote the differentiation and proliferation of nerve cells, accelerating nerve regeneration. The asiaticoside released from the hydrogel had a significant inhibitory effect on the growth and collagen secretion of fibroblasts, eliminating scars for regenerative nerves, which can promote the function recovery of defected peripheral nerve. Together, these positive results indicate that the hydrogel would be a promising candidate for peripheral nerve regeneration.