A Novel Experimental Model to Determine the Axon-Promoting Effects of Grafted Cells After Peripheral Nerve Injury

GFRα1 Promotes Axon Regeneration after Peripheral Nerve Injury by Functioning as a Ligand

The neurotrophic factor, Glial cell line derived neurotrophi factor (GDNF), exerts a variety of biological effects through binding to its receptors, GDNF family receptor alpha-1 (GFRα1), and RET. However, the existence of cells expressing GFRα1 but not RET raises the possibility that GFRα1 can function independently from RET. Here, it is shown that GFRα1 released from repair Schwann cells (RSCs) functions as a ligand in a GDNF-RET-independent manner to promote axon regeneration after peripheral nerve injury (PNI). Local administration of GFRα1 into injured nerve promoted axon regeneration, even more when combined with GDNF blockade. GFRα1 bound to a receptor complex consisting of NCAM and integrin α7β1 of dorsal root ganglion neurons in a GDNF-RET independent manner. This is further confirmed by the Ret Y1062F knock-in mice, which cannot transmit most of GDNF-RET signaling. Finally, local administration of GFRα1 into injured sciatic nerve promoted functional recovery. These findings reveal a novel role of GFRα1 as a ligand, the molecular mechanism supporting axon regeneration by RSCs, and a novel therapy for peripheral nerve repair.

Rab32 facilitates Schwann cell pyroptosis in rats following peripheral nerve injury by elevating ROS levels

BACKGROUND

Peripheral nerve injury (PNI) is commonly observed in clinical practice, yet the underlying mechanisms remain unclear. This study investigated the correlation between the expression of a Ras-related protein Rab32 and pyroptosis in rats following PNI, and potential mechanisms have been explored by which Rab32 may influence Schwann cells pyroptosis and ultimately peripheral nerve regeneration (PNR) through the regulation of Reactive oxygen species (ROS) levels.

METHODS

The authors investigated the induction of Schwann cell pyroptosis and the elevated expression of Rab32 in a rat model of PNI. In vitro experiments revealed an upregulation of Rab32 during Schwann cell pyroptosis. Furthermore, the effect of Rab32 on the level of ROS in mitochondria in pyroptosis model has also been studied. Finally, the effects of knocking down the Rab32 gene on PNR were assessed, morphology, sensory and motor functions of sciatic nerves, electrophysiology and immunohistochemical analysis were conducted to assess the therapeutic efficacy.

RESULTS

Silencing Rab32 attenuated PNI-induced Schwann cell pyroptosis and promoted peripheral nerve regeneration. Furthermore, our findings demonstrated that Rab32 induces significant oxidative stress by damaging the mitochondria of Schwann cells in the pyroptosis model in vitro.

CONCLUSION

Rab32 exacerbated Schwann cell pyroptosis in PNI model, leading to delayed peripheral nerve regeneration. Rab32 can be a potential target for future therapeutic strategy in the treatment of peripheral nerve injuries.

Promotion effect of TGF-β-Zfp423-ApoD pathway on lip sensory recovery after nerve sacrifice caused by nerve collateral compensation

Resection of oral and maxillofacial tumors is often accompanied by the inferior alveolar nerve neurectomy, resulting in abnormal sensation in lower lip. It is generally believed that spontaneous sensory recovery in this nerve injury is difficult. However, during our follow-up, patients with inferior alveolar nerve sacrifice showed different degrees of lower lip sensory recovery. In this study, a prospective cohort study was conducted to demonstrate this phenomenon and analyze the factors influencing sensory recovery. A mental nerve transection model of Thy1-YFP mice and tissue clearing technique were used to explore possible mechanisms in this process. Gene silencing and overexpression experiments were then conducted to detect the changes in cell morphology and molecular markers. In our follow-up, 75% of patients with unilateral inferior alveolar nerve neurectomy had complete sensory recovery of the lower lip 12 months postoperatively. Patients with younger age, malignant tumors, and preservation of ipsilateral buccal and lingual nerves had a shorter recovery time. The buccal nerve collateral sprouting compensation was observed in the lower lip tissue of Thy1-YFP mice. ApoD was demonstrated to be involved in axon growth and peripheral nerve sensory recovery in the animal model. TGF-β inhibited the expression of STAT3 and the transcription of ApoD in Schwann cells through Zfp423. Overall, after sacrificing the inferior alveolar nerve, the collateral compensation of the ipsilateral buccal nerve could innervate the sensation. And this process was regulated by TGF-β-Zfp423-ApoD pathway.

Peripheral nerve-derived fibroblasts promote neurite outgrowth in adult dorsal root ganglion neurons more effectively than skin-derived fibroblasts

NEW FINDINGS

What is the central question of this study? Although fibroblasts are involved in the regenerative process associated with peripheral nerve injury, detailed information regarding their characteristics is largely lacking. What is the main finding and its importance? Nerve-derived fibroblasts have a greater neurite-promoting effect than skin-derived fibroblasts, and epineurium-derived fibroblasts can promote neurite outgrowth more effectively than parenchyma-derived fibroblasts. The epineurium-derived fibroblasts and parenchyma-derived fibroblasts have distinctly different molecular profiles, including genes of soluble factors to promote axonal growth. Fibroblasts are molecularly and functionally different depending on their localization in nerve tissue, and epineurium-derived fibroblasts might be involved in axon regeneration after peripheral nerve injury more than previously thought.

ABSTRACT

Although fibroblasts (Fb) are components of a peripheral nerve involved in the regenerative process associated with peripheral nerve injury, detailed information regarding their characteristics is largely lacking. The objective of the present study was to investigate the capacity of Fb derived from peripheral nerves to stimulate the outgrowth of neurites from adult dorsal root ganglion neurons and to clarify their molecular characteristics. Fibroblasts were prepared from the epineurium and parenchyma of rat sciatic nerves and skin. The Fb derived from epineurium showed the greatest effect on neurite outgrowth, followed by the Fb derived from parenchyma, indicating that Fb derived from nerves promote neurite outgrowth more effectively than skin-derived Fb. Although both soluble and cell-surface factors contributed evenly to the neurite-promoting effect of nerve-derived Fb, in crush and transection injury models, Fb were not closely associated with regenerating axons, indicating that only soluble factors from Fb are available to regenerating axons. A transcriptome analysis revealed that the molecular profiles of these Fb were distinctly different and that the gene expression profiles of soluble factors that promote axonal growth are unique to each Fb. These findings indicate that Fb are molecularly and functionally different depending on their localization in nerve tissue and that Fb derived from epineurium might be involved more than was previously thought in axon regeneration after peripheral nerve injury.

Biomimetic Strategies for Peripheral Nerve Injury Repair: An Exploration of Microarchitecture and Cellularization

Injuries to the nervous system present formidable challenges to scientists, clinicians, and patients. While regeneration within the central nervous system is minimal, peripheral nerves can regenerate, albeit with limitations. The regenerative mechanisms of the peripheral nervous system thus provide fertile ground for clinical and scientific advancement, and opportunities to learn fundamental lessons regarding nerve behavior in the context of regeneration, particularly the relationship of axons to their support cells and the extracellular matrix environment. However, few current interventions adequately address peripheral nerve injuries. This article aims to elucidate areas in which progress might be made toward developing better interventions, particularly using synthetic nerve grafts. The article first provides a thorough review of peripheral nerve anatomy, physiology, and the regenerative mechanisms that occur in response to injury. This is followed by a discussion of currently available interventions for peripheral nerve injuries. Promising biomaterial fabrication techniques which aim to recapitulate nerve architecture, along with approaches to enhancing these biomaterial scaffolds with growth factors and cellular components, are then described. The final section elucidates specific considerations when developing nerve grafts, including utilizing induced pluripotent stem cells, Schwann cells, nerve growth factors, and multilayered structures that mimic the architectures of the natural nerve.

Behavioral and axon histomorphometric analyses after intraneural transplantation of human skin- and nerve-derived Schwann cells in nude rats

INTRODUCTION/AIMS

We have recently isolated and expanded skin-derived Schwann cells (Sk-SCs) from human skin and showed that they are largely similar to nerve-derived Schwann cells (N-SCs). Here, we extend our investigation into functional assessments of the nude rats that received human Sk-SCs and N-SCs after intraneural delivery into crushed and decellularized tibial nerve in adult nude rats.

METHODS

Sk-SCs, N-SCs, dermal fibroblasts, or control culture medium was injected into the crushed and decellularized tibial nerve using in situ repeated freeze-thaw cycles. Animals were then subjected to a ladder rung walking test, nociceptive von Frey testing, and walking gait analysis weekly. Animals were euthanized 6 weeks after surgery, gastrocnemius and soleus muscles were weighed, distal nerves were harvested, and whole semithin cross-sections were analyzed using segmentation software.

RESULTS

N-SC-injected and dermal fibroblast-injected animals improved significantly at 4 to 6 weeks postinjury in nociceptive assessment compared with medium-injected controls. Sk-SCs recovered more rapidly in tibial functional index at 2 weeks postinjury compared with medium-injected controls. No significant difference was observed for the ladder rung walking test or muscle weight ratio. Histologically, the number of myelinated axons was significantly higher in all cell injection groups compared with medium-injected controls. No significant difference was observed in g ratio, axon diameter, or myelin thickness.

DISCUSSION

Cell injection significantly improved axon regeneration across an in situ decellularized nerve segment. However, a more human cell-permissive animal model is required to delineate functional differences between cell types for preclinical transplantation studies.

Molecular and Regenerative Characterization of Repair and Non-repair Schwann Cells

Although evidence has accumulated to indicate that Schwann cells (SCs) differentiate into repair SCs (RSCs) upon injury and that the unique phenotype of these cells allow them to provide support for peripheral nerve regeneration, the details of the RSCs are not fully understood. The findings of the current study indicate that the RSCs have enhanced adherent properties and a greater capability to promote neurite outgrowth and axon regeneration after peripheral nerve injury, compared to the non-RSCs. Further, transcriptome analyses have demonstrated that the molecular signature of the RSCs is distinctly different from that of the non-RSCs. The RSCs upregulate a group of genes that are related to inflammation, repair, and regeneration, whereas non-RSCs upregulate genes related to myelin maintenance, Notch, and aging. These findings indicate that the RSCs have markedly different cellular, regenerative, and molecular characteristics compared to the non-RSCs, even though the RSCs were just derived from non-RSCs upon injury, thus providing the basis for understanding the mechanisms related to SC mediated repair after peripheral nerve injury.

Neutrophils delay repair process in Wallerian degeneration by releasing NETs outside the parenchyma

Although inflammation is indispensable for the repair process in Wallerian degeneration (WD), the role of neutrophils in the WD repair process remains unclear. After peripheral nerve injury, neutrophils accumulate at the epineurium but not the parenchyma in the WD region because of the blood-nerve barrier. An increase or decrease in the number of neutrophils delayed or promoted macrophage infiltration from the epineurium into the parenchyma and the repair process in WD. Abundant neutrophil extracellular traps (NETs) were formed around neutrophils, and its inhibition dramatically increased macrophage infiltration into the parenchyma. Furthermore, inhibition of either MIF or its receptor, CXCR4, in neutrophils decreased NET formation, resulting in enhanced macrophage infiltration into the parenchyma. Moreover, inhibiting MIF for just 2 h after peripheral nerve injury promoted the repair process. These findings indicate that neutrophils delay the repair process in WD from outside the parenchyma by inhibiting macrophage infiltration via NET formation and that neutrophils, NETs, MIF, and CXCR4 are therapeutic targets for peripheral nerve regeneration.

IL4 stimulated macrophages promote axon regeneration after peripheral nerve injury by secreting uPA to stimulate uPAR upregulated in injured axons

Accumulating evidences suggest that M2 macrophages are involved with repair processes in the nervous system. However, whether M2 macrophages can promote axon regeneration by directly stimulating axons nor its precise molecular mechanism remains elusive. Here, the current study demonstrated that typical M2 macrophages, which were generated by IL4 simulation, had the capacity to stimulate axonal growth by their direct effect on axons and that the graft of IL4 stimulated macrophages into the region of Wallerian degeneration enhanced axon regeneration and improved functional recovery after PNI. Importantly, uPA (urokinase plasminogen activator)-uPA receptor (uPAR) was identified as the central axis underlying the axon regeneration effect of IL4 stimulated macrophages. IL4 stimulated macrophages secreted uPA, and its inhibition abolished their axon regeneration effect. Injured but not intact axons expressed uPAR to be sensitive to uPA. These results unveil a cellular and molecular mechanism underlying the macrophage related axon regeneration and provide a basis of a novel therapy for PNI.

Mature but not developing Schwann cells promote axon regeneration after peripheral nerve injury

Since Schwann cells (SCs) support axonal growth at development as well as after peripheral nerve injury (PNI), developing SCs might be able to promote axon regeneration after PNI. The purpose of the current study was to elucidate the capability of developing SCs to induce axon regeneration after PNI. SC precursors (SCPs), immature SCs (ISCs), repair SCs (RSCs) from injured nerves, and non-RSCs from intact nerves were tested by grafting into acellular region of rat sciatic nerve with crush injury. Both of developing SCs completely failed to support axon regeneration, whereas both of mature SCs, especially RSCs, induced axon regeneration. Further, RSCs but not SCPs promoted neurite outgrowth of adult dorsal root ganglion neurons. Transcriptome analysis revealed that the gene expression profiles were distinctly different between RSCs and SCPs. These findings indicate that developing SCs are markedly different from mature SCs in terms of functional and molecular aspects and that RSC is a viable candidate for regenerative cell therapy for PNI.

Repairing whole facial nerve defects with xenogeneic acellular nerve grafts in rhesus monkeys

Acellular nerve allografts conducted via chemical extraction have achieved satisfactory results in bridging whole facial nerve defects clinically, both in terms of branching a single trunk and in connecting multiple branches of an extratemporal segment. However, in the clinical treatment of facial nerve defects, allogeneic donors are limited. In this experiment, we exposed the left trunk and multiple branches of the extratemporal segment in six rhesus monkeys and dissected a gap of 25 mm to construct a monkey model of a whole left nerve defect. Six monkeys were randomly assigned to an autograft group or a xenogeneic acellular nerve graft group. In the autograft group, the 25-mm whole facial nerve defect was immediately bridged using an autogenous ipsilateral great auricular nerve, and in the xenogeneic acellular nerve graft group, this was done using a xenogeneic acellular nerve graft with trunk-branches. Examinations of facial symmetry, nerve-muscle electrophysiology, retrograde transport of labeled neuronal tracers, and morphology of the regenerated nerve and target muscle at 8 months postoperatively showed that the faces of the monkey appeared to be symmetrical in the static state and slightly asymmetrical during facial movement, and that they could actively close their eyelids completely. The degree of recovery from facial paralysis reached House-Brackmann grade II in both groups. Compound muscle action potentials were recorded and orbicularis oris muscles responded to electro-stimuli on the surgical side in each monkey. FluoroGold-labeled neurons could be detected in the facial nuclei on the injured side. Immunohistochemical staining showed abundant neurofilament-200-positive axons and soluble protein-100-positive Schwann cells in the regenerated nerves. A large number of mid-graft myelinated axons were observed via methylene blue staining and a transmission electron microscope. Taken together, our data indicate that xenogeneic acellular nerve grafts from minipigs are safe and effective for repairing whole facial nerve defects in rhesus monkeys, with an effect similar to that of autologous nerve transplantation. Thus, a xenogeneic acellular nerve graft may be a suitable choice for bridging a whole facial nerve defect if no other method is available. The study was approved by the Laboratory Animal Management Committee and the Ethics Review Committee of the Affiliated Wuxi No. 2 People's Hospital of Nanjing Medical University, China (approval No. 2018-D-1) on March 15, 2018.