Introduction: Thematic Papers Issue on Peripheral Nerve Regeneration and Repair

Immunity to the microbiota promotes sensory neuron regeneration

Tissue immunity and responses to injury depend on the coordinated action and communication among physiological systems. Here, we show that, upon injury, adaptive responses to the microbiota directly promote sensory neuron regeneration. At homeostasis, tissue-resident commensal-specific T cells colocalize with sensory nerve fibers within the dermis, express a transcriptional program associated with neuronal interaction and repair, and promote axon growth and local nerve regeneration following injury. Mechanistically, our data reveal that the cytokine interleukin-17A (IL-17A) released by commensal-specific Th17 cells upon injury directly signals to sensory neurons via IL-17 receptor A, the transcription of which is specifically upregulated in injured neurons. Collectively, our work reveals that in the context of tissue damage, preemptive immunity to the microbiota can rapidly bridge biological systems by directly promoting neuronal repair, while also identifying IL-17A as a major determinant of this fundamental process.

Preclinical nerve conduction: Nerve battery options for primate studies

Electrophysiological neurodiagnostic tests of nerve conduction (NC) are key assays included in preclinical safety and toxicology programs to assess the peripheral neuropathy (PN) liability of a new drug. Despite their increased use, standardization of nerve conduction studies (NCS) is lacking in the preclinical space, with limited regulatory guidelines stipulating type and number of nerves or minimum combinations appropriate for each stage of drug development or indication. Detection of subtle peripheral toxicities depends on choosing appropriate nerve targets for testing, especially when functional changes remain above the lower limit of normal values. To support robust preclinical toxicology study designs, the current short communication provides options and recommendations for selecting peripheral nerves for clinically translatable nerve conduction batteries applicable to toxicology and gene therapy, with a focus on clinically translatable primate models. A comprehensive compilation of accessible nerve locations is offered including lower and upper extremity motor nerves, and sensory nerves with origin at multiple DRG levels. Rankings of technique difficulty and repeatability across serial collections are presented for each assay informed by serial nerve conduction from 500 adult primates. The goal of this communication is to support the standardization and preclinical implementation of this important assay.

Emerging Role of HDACs in Regeneration and Ageing in the Peripheral Nervous System: Repair Schwann Cells as Pivotal Targets

The peripheral nervous system (PNS) has a remarkable regenerative capacity in comparison to the central nervous system (CNS), a phenomenon that is impaired during ageing. The ability of PNS axons to regenerate after injury is due to Schwann cells (SC) being reprogrammed into a repair phenotype called Repair Schwann cells. These repair SCs are crucial for supporting axonal growth after injury, myelin degradation in a process known as myelinophagy, neurotropic factor secretion, and axonal growth guidance through the formation of Büngner bands. After regeneration, repair SCs can remyelinate newly regenerated axons and support nonmyelinated axons. Increasing evidence points to an epigenetic component in the regulation of repair SC gene expression changes, which is necessary for SC reprogramming and regeneration. One of these epigenetic regulations is histone acetylation by histone acetyl transferases (HATs) or histone deacetylation by histone deacetylases (HDACs). In this review, we have focused particularly on three HDAC classes (I, II, and IV) that are Zn²⁺-dependent deacetylases. These HDACs are important in repair SC biology and remyelination after PNS injury. Another key aspect explored in this review is HDAC genetic compensation in SCs and novel HDAC inhibitors that are being studied to improve nerve regeneration.

Parallels between the Developing Vascular and Neural Systems: Signaling Pathways and Future Perspectives for Regenerative Medicine

Neurovascular disorders, which involve the vascular and nervous systems, are common. Research on such disorders usually focuses on either vascular or nervous components, without looking at how they interact. Adopting a neurovascular perspective is essential to improve current treatments. Therefore, comparing molecular processes known to be involved in both systems separately can provide insight into promising areas of future research. Since development and regeneration share many mechanisms, comparing signaling molecules involved in both the developing vascular and nervous systems and shedding light to those that they have in common can reveal processes, which have not yet been studied from a regenerative perspective, yet hold great potential. Hence, this review discusses and compares processes involved in the development of the vascular and nervous systems, in order to provide an overview of the molecular mechanisms, which are most promising with regards to treatment for neurovascular disorders. Vascular endothelial growth factor, semaphorins, and ephrins are found to hold the most potential, while fibroblast growth factor, bone morphogenic protein, slits, and sonic hedgehog are shown to participate in both the developing vascular and nervous systems, yet have not been studied at the neurovascular level, therefore being of special interest for future research.

PP-1β and PP-2Aα modulate cAMP response element-binding protein (CREB) functions in aging control and stress response through de-regulation of αB-crystallin gene and p300-p53 signaling axis

The function of the transcription factor, cAMP response element‐binding protein (CREB), is activated through S133 phosphorylation by PKA and others. Regarding its inactivation, it is not well defined. cAMP response element‐binding protein plays an essential role in promoting cell proliferation, neuronal survival and the synaptic plasticity associated with long‐term memory. Our recent studies have shown that CREB is an important player in mediating stress response. Here, we have demonstrated that CREB regulates aging process through suppression of αB‐crystallin and activation of the p300‐p53‐Bak/Bax signaling axis. First, we determined that two specific protein phosphatases, PP‐1β and PP‐2Aα, can inactivate CREB through S133 dephosphorylation. Subsequently, we demonstrated that cells expressing the S133A‐CREB, a mutant mimicking constant dephosphorylation at S133, suppress CREB functions in aging control and stress response. Mechanistically, S133A‐CREB not only significantly suppresses CREB control of αB‐crystallin gene, but also represses CREB‐mediated activation of p53 acetylation and downstream Bak/Bax genes. cAMP response element‐binding protein suppression of αB‐crystallin and its activation of p53 acetylation are major molecular events observed in human cataractous lenses of different age groups. Together, our results demonstrate that PP‐1β and PP‐2Aα modulate CREB functions in aging control and stress response through de‐regulation of αB‐crystallin gene and p300‐p53‐Bax/Bak signaling axis, which regulates human cataractogenesis in the aging lens.

Identification of sensory and motor nerve fascicles by immunofluorescence staining after peripheral nerve injury

Background

Inappropriate matching of motor and sensory fibers after nerve repair or nerve grafting can lead to failure of nerve recovery. Identification of motor and sensory fibers is important for the development of new approaches that facilitate neural regeneration and the next generation of nerve signal-controlled neuro-prosthetic limbs with sensory feedback technology. Only a few methods have been reported to differentiate sensory and motor nerve fascicles, and the reliability of these techniques is unknown. Immunofluorescence staining is one of the most commonly used methods to distinguish sensory and motor nerve fibers, however, its accuracy remains unknown.

Methods

In this study, we aim to determine the efficacy of popular immunofluorescence markers for motor and sensory nerve fibers. We harvested the facial (primarily motor fascicles) and sural (primarily sensory fascicles) nerves in rats, and examined the immunofluorescent staining expressions of motor markers (choline acetyltransferase (ChAT), tyrosine kinase (TrkA)), and sensory markers [neurofilament protein 200 kDa (NF-200), calcitonin gene-related peptide (CGRP) and Transient receptor potential vanillic acid subtype 1 (TRPV1)]. Three methods, including the average area percentage, the mean gray value, and the axon count, were used to quantify the positive expression of nerve markers in the immunofluorescence images.

Results

Our results suggest the mean gray value method is the most reliable method. The mean gray value of immunofluorescence in ChAT (63.0 ± 0.76%) and TRKA (47.6 ± 0.43%) on the motor fascicles was significantly higher than that on the sensory fascicles (ChAT: 49.2 ± 0.72%, P < 0.001; and TRKA: 29.1 ± 0.85%, P < 0.001). Additionally, the mean gray values of TRPV1 (51.5 ± 0.83%), NF-200 (61.5 ± 0.62%) and CGRP (37.7 ± 1.22%) on the motor fascicles were significantly lower than that on the sensory fascicles respectively (71.9 ± 2.32%, 69.3 ± 0.46%, and 54.3 ± 1.04%) (P < 0.001). The most accurate cutpoint occurred using CHAT/CRCP ratio, where a value of 0.855 had 100% sensitivity and 100% specificity to identify motor and sensory nerve with an area under the ROC curve of 1.000 (P < 0.001).

Conclusions

A combination of ChAT and CGRP is suggested to distinguish motor and sensory nerve fibers.

Variation in expression of small ubiquitin-like modifiers in injured sciatic nerve of mice

Small ubiquitin-like modifiers (SUMOs) have been shown to regulate axonal regeneration, signal transduction, neuronal migration, and myelination, by covalently and reversibly attaching to the protein substrates during neuronal cell growth, development, and differentiation. It has not been reported whether SUMOs play a role in peripheral nerve injury and regeneration. To investigate any association between SUMOylation and potential neuroprotective effects during peripheral nerve injury and regeneration, C57/BL mice were randomly divided into sham and experimental groups. The sciatic nerve was exposed only in the sham group. The experimental group underwent neurotomy and epineurial neurorrhaphy. Real-time quantitative polymerase chain reaction and western blot assay results revealed different mRNA and protein expression levels of SUMO1, SUMO2, SUMO3 and UBC9 in sciatic nerve tissue (containing both 5 mm of proximal and distal stumps at the injury site) at various time points after injury. Compared with the sham group, protein levels of SUMO1 and SUMO2/3 increased in both their covalent and free states after sciatic nerve injury in the experimental group, especially in the covalent state. UBC9 protein levels showed similar changes to those of SUMO1 and SUMO2/3 in the covalent states. Immunohistochemical staining demonstrated that SUMO1 and SUMO2/3 immunopositivities were higher in the experimental group than in the sham group. Our results verified that during the repair of sciatic nerve injury, the mRNA and protein expression of SUMO1, SUMO2, SUMO3 and UBC9 in injured nerve tissues changed in varying patterns and there were clear changes in the expression of SUMO-related proteins. These findings reveal that SUMOs possibly play an important role in the repair of peripheral nerve injury. All animal protocols were approved by the Institutional Animal Care and Use Committee of Tianjin Fifth Central Hospital, China (approval No. TJWZXLL2018041) on November 8, 2018.