Adult motor axons preferentially reinnervate predegenerated muscle nerve

Typical and atypical properties of peripheral nerve allografts enable novel strategies to repair segmental-loss injuries

We review data showing that peripheral nerve injuries (PNIs) that involve the loss of a nerve segment are the most common type of traumatic injury to nervous systems. Segmental-loss PNIs have a poor prognosis compared to other injuries, especially when one or more mixed motor/sensory nerves are involved and are typically the major source of disability associated with extremities that have sustained other injuries. Relatively little progress has been made, since the treatment of segmental loss PNIs with cable autografts that are currently the gold standard for repair has slow and incomplete (often non-existent) functional recovery. Viable peripheral nerve allografts (PNAs) to repair segmental-loss PNIs have not been experimentally or clinically useful due to their immunological rejection, Wallerian degeneration (WD) of anucleate donor graft and distal host axons, and slow regeneration of host axons, leading to delayed re-innervation and producing atrophy or degeneration of distal target tissues. However, two significant advances have recently been made using viable PNAs to repair segmental-loss PNIs: (1) hydrogel release of Treg cells that reduce the immunological response and (2) PEG-fusion of donor PNAs that reduce the immune response, reduce and/or suppress much WD, immediately restore axonal conduction across the donor graft and re-innervate many target tissues, and restore much voluntary behavioral functions within weeks, sometimes to levels approaching that of uninjured nerves. We review the rather sparse cellular/biochemical data for rejection of conventional PNAs and their acceptance following Treg hydrogel and PEG-fusion of PNAs, as well as cellular and systemic data for their acceptance and remarkable behavioral recovery in the absence of tissue matching or immune suppression. We also review typical and atypical characteristics of PNAs compared with other types of tissue or organ allografts, problems and potential solutions for PNA use and storage, clinical implications and commercial availability of PNAs, and future possibilities for PNAs to repair segmental-loss PNIs.

The Chimeric Scapulodorsal Vascularized Latissimus Dorsi Nerve Flap for Immediate Reconstruction of Total Parotidectomy Defects With Facial Nerve Sacrifice: Building a New Program and Preliminary Results From 25 Cases

BACKGROUND

Total parotidectomy with facial nerve sacrifice creates 2 challenging reconstructive problems: restoration of facial contour and facial nerve rehabilitation. Strong evidence suggesting that vascularized nerve grafts are superior to nonvascularized nerve grafts motivated our team to develop a chimeric scapulodorsal flap combining the usual harvestable local tissues with the vascularized latissimus dorsi motor nerve (SD-LDVxN). We present our experiences developing a new program at University of California, San Diego, highlighting our first case here, and present preliminary retrospective results focusing on the functional outcomes of facial nerve reanimation.

MATERIALS AND METHODS

The first case performed in the United States was a 57-year-old woman with stage IVA left parotid adenoid cystic carcinoma and House-Brackmann grade 6 facial palsy. She underwent total parotidectomy with facial nerve sacrifice and a free chimeric SD-LDVxN flap reconstruction. She had an unremarkable postoperative course, and 3- and 6-month follow-up functional results are reported. Preliminary functional results from our total series of 25 patients were reported.

RESULTS

At her 3-month follow-up, she was a House-Brackmann 5 with a static eFACE score of 37, dynamic eFACE score of 31, and smile eFACE score of 48. At her 6-month follow-up, she was a House-Brackmann 5 with a static eFACE score of 50, dynamic eFACE score of 27, and smile eFACE score of 53. Preliminary results from our total series of 25 patients with an average of 5 years of follow-up were a House-Brackmann 2.5 and eFACE scores of 83.1 for static facial symmetry, 67.5 for dynamic facial symmetry, and 77.7 for smile score. Twenty of the 25 patients had postoperative radiotherapy. No local tumor recurrence had been reported. The average reinnervation time was 9 months and ranged from 3 to 15 months.

CONCLUSIONS

The SD-LDVxN flap is a highly resourceful solution to reconstruct complex parotid defects, especially those that sacrifice the facial nerve. The vascularized nerve graft allows for primary facial reanimation. Nerve recovery may be superior to what could be expected with a conventional nerve graft.

Reconstruction of Extensive Composite Parotid Region Oncologic Defects with Immediate Facial Nerve Reconstruction Using a Chimeric Scapulodorsal Vascularized Nerve Free Flap

BACKGROUND

 Cancer involving the parotid gland region may originates from parotid parenchyma itself or from locoregional organs and in rare cases, the facial nerve (FN) has to be sacrificed during tumor resection. In these cases, cancer extension often goes beyond the parotid compartment and requires extensive local resection responsible for complex multitissular defects. The goals of reconstruction may be summarized in the following two components: (1) restoration of the volumetric tissue defect and (2) FN reconstruction. The aim of this study is to describe our surgical technique and our cosmetic results using the chimeric scapulodorsal vascularized nerve (SDVN) flap to reconstruct extensive maxillofacial defects associated with FN sacrifice.

METHODS

 All patients undergone an extensive maxillofacial resection with FN sacrifice and primarily reconstructed with a SDVN flap were included. We classified the maxillofacial defects into six groups based on the type of resection. Intraoperative data including flap composition, topography of FN injury, length of nerve gap, and number of nervous anastomosis were recorded.

RESULTS

 Twenty-nine patients were included. Mean follow-up was 38.7 months. The harvested flaps included the SDVN combined with different components according to the defect group. A satisfactory volumetric restoration was obtained in 93% of cases. The mean number of distal nervous anastomosis was 4.5. The length of the vascularized grafted nerve ranged from 7 to 10 cm.

CONCLUSION

 This is largest series presented in literature on primary FN reconstruction utilizing a vascularized nerve graft. We believe that the chimeric SDVN flap should be highly considered for these cases due to its versatility. The surgeon is able to use single donor site available soft and hard tissues components along with a vascular motor nerve graft, which offers a great length and number of distal branches, and easily matches with the extracranial FN trunk and its peripheral ramifications.

Different protein expression patterns in rat spinal nerves during Wallerian degeneration assessed using isobaric tags for relative and absolute quantitation proteomics profiling

Sensory and motor nerve fibers of peripheral nerves have different anatomies and regeneration functions after injury. To gain a clear understanding of the biological processes behind these differences, we used a labeling technique termed isobaric tags for relative and absolute quantitation to investigate the protein profiles of spinal nerve tissues from Sprague-Dawley rats. In response to Wallerian degeneration, a total of 626 proteins were screened in sensory nerves, of which 368 were upregulated and 258 were downregulated. In addition, 637 proteins were screened in motor nerves, of which 372 were upregulated and 265 were downregulated. All identified proteins were analyzed using the Gene Ontology and Kyoto Encyclopedia of Genes and Genomes analysis of bioinformatics, and the presence of several key proteins closely related to Wallerian degeneration were tested and verified using quantitative real-time polymerase chain reaction analyses. The differentially expressed proteins only identified in the sensory nerves were mainly relevant to various biological processes that included cell-cell adhesion, carbohydrate metabolic processes and cell adhesion, whereas differentially expressed proteins only identified in the motor nerves were mainly relevant to biological processes associated with the glycolytic process, cell redox homeostasis, and protein folding. In the aspect of the cellular component, the differentially expressed proteins in the sensory and motor nerves were commonly related to extracellular exosomes, the myelin sheath, and focal adhesion. According to the Kyoto Encyclopedia of Genes and Genomes, the differentially expressed proteins identified are primarily related to various types of metabolic pathways. In conclusion, the present study screened differentially expressed proteins to reveal more about the differences and similarities between sensory and motor nerves during Wallerian degeneration. The present findings could provide a reference point for a future investigation into the differences between sensory and motor nerves in Wallerian degeneration and the characteristics of peripheral nerve regeneration. The study was approved by the Ethics Committee of the Chinese PLA General Hospital, China (approval No. 2016-x9-07) in September 2016.

GDNF pretreatment overcomes Schwann cell phenotype mismatch to promote motor axon regeneration via sensory graft

In the clinic, severe motor nerve injury is commonly repaired by autologous sensory nerve bridging, but the ability of Schwann cells (SCs) in sensory nerves to support motor neuron axon growth is poor due to phenotype mismatch. In vitro experiments have demonstrated that sensory-derived SCs overcome phenotypic mismatch-induced growth inhibition after pretreatment with exogenous glial cell-derived neurotrophic factor (GDNF) and induce motor neuron axonal growth. Thus, we introduced a novel staging surgery: In the first stage of surgery, the denervated sensory nerve was pretreated with sustained-release GDNF, which was encapsulated into a self-assembling peptide nanofiber scaffold (SAPNS) RADA-16I in the donor area in vivo. In the second stage of surgery, the pretreated sensory grafts were transplanted to repair motor nerve injury. Motor axon regeneration and remyelination and muscle functional recovery after the second surgery was compared to those in the control groups. The expression of genes previously shown to be differently expressed in motor and sensory SCs was also analyzed in pretreated sensory grafts by qRT-PCR to explore possible changes after exogenous GDNF application. Exogenous GDNF acted directly on the denervated sensory nerve graft in vivo, increasing the expression of endogenous GDNF and sensory SC-derived marker brain-derived neurotrophic factor (BDNF). After transplantation to repair motor nerve injury, exogenous GDNF pretreatment promoted the regeneration and remyelination of proximal motor axons and the recovery of muscle function. Further research into how phenotype, gene expression and changes in neurotrophic factors in SCs are affected by GDNF will help us design more effective methods to treat peripheral nerve injury.

Spatiotemporal Differences in Gene Expression Between Motor and Sensory Autografts and Their Effect on Femoral Nerve Regeneration in the Rat

To improve the outcome after autologous nerve grafting in the clinic, it is important to understand the limiting variables such as distinct phenotypes of motor and sensory Schwann cells. This study investigated the properties of phenotypically different autografts in a 6 mm femoral nerve defect model in the rat, where the respective femoral branches distally of the inguinal bifurcation served as homotopic, or heterotopic autografts. Axonal regeneration and target reinnervation was analyzed by gait analysis, electrophysiology, and wet muscle mass analysis. We evaluated regeneration-associated gene expression between 5 days and 10 weeks after repair, in the autografts as well as the proximal, and distal segments of the femoral nerve using qRT-PCR. Furthermore we investigated expression patterns of phenotypically pure ventral and dorsal roots. We identified highly significant differences in gene expression of a variety of regeneration-associated genes along the central – peripheral axis in healthy femoral nerves. Phenotypically mismatched grafting resulted in altered spatiotemporal expression of neurotrophic factor BDNF, GDNF receptor GFRα1, cell adhesion molecules Cadm3, Cadm4, L1CAM, and proliferation associated Ki67. Although significantly higher quadriceps muscle mass following homotopic nerve grafting was measured, we did not observe differences in gait analysis, and electrophysiological parameters between treatment paradigms. Our study provides evidence for phenotypic commitment of autologous nerve grafts after injury and gives a conclusive overview of temporal expression of several important regeneration-associated genes after repair with sensory or motor graft.

Comparative transcriptomic profiling of peripheral efferent and afferent nerve fibres at different developmental stages in mice

Peripheral nerve injury impairs motor and sensory function in humans, and its functional recovery largely depends on the axonal outgrowth required for the accurate reinnervation of appropriate targets. To better understand how motor and sensory nerve fibres select their terminal pathways, an unbiased cDNA microarray analysis was conducted to examine differential gene expression patterns in peripheral efferent and afferent fibres at different developmental stages in mice. Gene ontology (GO) and Kyoto Enrichment of Genes and Genomes (KEGG) analyses revealed common and distinct features of enrichment for differentially expressed genes during motor and sensory nerve fibre development. Ingenuity Pathway Analysis (IPA) further indicated that the key differentially expressed genes were associated with trans-synaptic neurexin-neuroligin signalling components and a variety of gamma-aminobutyric acid (GABA) receptors. The aim of this study was to generate a framework of gene networks regulated during motor and sensory neuron differentiation/maturation. These data may provide new clues regarding the underlying cellular and molecular mechanisms that determine the intrinsic capacity of neurons to regenerate after peripheral nerve injury. Our findings may thus facilitate further development of a potential intervention to manipulate the therapeutic efficiency of peripheral nerve repair in the clinic.

Experimental Models of Spinal Cord Injury in Laboratory Rats

Pathologies associated with spinal cord injury are some of the leading diseases in the world. The search for new therapeutic agents and 3D biodegradable materials for the recovery of spinal cord functions is a topical issue. In this review, we have summarized the literature data on the most common experimental models of spinal cord injury in laboratory rats and analyzed the experience of using 3D biodegradable materials (scaffolds) in experimental studies of spinal trauma. The advantages and disadvantages of the described models are systematically analyzed in this review.

Femoral nerve regeneration and its accuracy under different injury mechanisms

Surgical accuracy has greatly improved with the advent of microsurgical techniques. However, complete functional recovery after peripheral nerve injury has not been achieved to date. The mechanisms hindering accurate regeneration of damaged axons after peripheral nerve injury are in urgent need of exploration. The present study was designed to explore the mechanisms of peripheral nerve regeneration after different types of injury. Femoral nerves of rats were injured by crushing or freezing. At 2, 3, 6, and 12 weeks after injury, axons were retrogradely labeled using 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (Dil) and True Blue, and motor and sensory axons that had regenerated at the site of injury were counted. The number and percentage of Dil-labeled neurons in the anterior horn of the spinal cord increased over time. No significant differences were found in the number of labeled neurons between the freeze and crush injury groups at any time point. Our results confirmed that the accuracy of peripheral nerve regeneration increased with time, after both crush and freeze injury, and indicated that axonal regeneration accuracy was still satisfactory after freezing, despite the prolonged damage.

Femoral nerve regeneration and its accuracy under different injury mechanisms

Surgical accuracy has greatly improved with the advent of microsurgical techniques. However, complete functional recovery after peripheral nerve injury has not been achieved to date. The mechanisms hindering accurate regeneration of damaged axons after peripheral nerve injury are in urgent need of exploration. The present study was designed to explore the mechanisms of peripheral nerve regeneration after different types of injury. Femoral nerves of rats were injured by crushing or freezing. At 2, 3, 6, and 12 weeks after injury, axons were retrogradely labeled using 1,1′-dioctadecyl-3,3,3′,3′-tetramethylindocarbocyanine perchlorate (Dil) and True Blue, and motor and sensory axons that had regenerated at the site of injury were counted. The number and percentage of Dil-labeled neurons in the anterior horn of the spinal cord increased over time. No significant differences were found in the number of labeled neurons between the freeze and crush injury groups at any time point. Our results confirmed that the accuracy of peripheral nerve regeneration increased with time, after both crush and freeze injury, and indicated that axonal regeneration accuracy was still satisfactory after freezing, despite the prolonged damage.

Specificity of motor axon regeneration: a comparison of recovery following biodegradable conduit small gap tubulization and epineurial neurorrhaphy

Functional recovery is often unsatisfactory after lesions in the peripheral nervous system despite the strong potential for regeneration and advances in microsurgical techniques. Axonal regeneration in mixed nerve into inappropriate pathways is a major contributing factor to this failure. In this study, the rat femoral nerve model of transection and surgical repair was used to evaluate the specificity of motor axon regeneration as well as functional and morphological recovery using biodegradable conduit small gap tubulization compared to epineurial neurorrhaphy. 12 weeks after nerve repair, the specificity was assessed using the retrograde neurotracers TB and DiI to backlabel motor neurons that regenerate axons into muscle and cutaneous pathways. To evaluate the functional recovery of the quadriceps muscle, the quadriceps muscle forces were examined. The quadriceps muscle and myelinated axons were assessed using electrophysiology and histology. The results showed that the specificity of motor axon regeneration (preferential reinnervation) was significantly higher when the nerve transection was treated by biodegradable conduit small gap tubulization and there was no significant difference between the two suture methods with respect to the functional and morphological recovery. This study demonstrated that the quicker and easier biodegradable conduit small gap tubulization may get more accurate reinnervation than traditional epineurial neurorrhaphy and produced functional and morphological recovery equal to traditional epineurial neurorrhaphy.