Effects of neurotoxic and neuroprotective agents on peripheral nerve regeneration assayed by time-lapse imaging in vivo

Advances of Schwann cells in peripheral nerve regeneration: From mechanism to cell therapy

Peripheral nerve injuries (PNIs) frequently occur due to various factors, including mechanical trauma such as accidents or tool-related incidents, as well as complications arising from diseases like tumor resection. These injuries frequently result in persistent numbness, impaired motor and sensory functions, neuropathic pain, or even paralysis, which can impose a significant financial burden on patients due to outcomes that often fall short of expectations. The most frequently employed clinical treatment for PNIs involves either direct sutures of the severed ends or bridging the proximal and distal stumps using autologous nerve grafts. However, autologous nerve transplantation may result in sensory and motor functional loss at the donor site, as well as neuroma formation and scarring. Transplantation of Schwann cells/Schwann cell-like cells has emerged as a promising cellular therapy to reconstruct the microenvironment and facilitate peripheral nerve regeneration. In this review, we summarize the role of Schwann cells and recent advances in Schwann cell therapy in peripheral nerve regeneration. We summarize current techniques used in cell therapy, including cell injection, 3D-printed scaffolds for cell delivery, cell encapsulation techniques, as well as the cell types employed in experiments, experimental models, and research findings. At the end of the paper, we summarize the challenges and advantages of various cells (including ESCs, iPSCs, and BMSCs) in clinical cell therapy. Our goal is to provide the theoretical and experimental basis for future treatments targeting peripheral nerves, highlighting the potential of cell therapy and tissue engineering as invaluable resources for promoting nerve regeneration.

Neural regeneration in the human central nervous system-from understanding the underlying mechanisms to developing treatments. Where do we stand today?

Mature neurons in the human central nervous system (CNS) fail to regenerate after injuries. This is a common denominator across different aetiologies, including multiple sclerosis, spinal cord injury and ischemic stroke. The lack of regeneration leads to permanent functional deficits with a substantial impact on patient quality of life, representing a significant socioeconomic burden worldwide. Great efforts have been made to decipher the responsible mechanisms and we now know that potent intra- and extracellular barriers prevent axonal repair. This knowledge has resulted in numerous clinical trials, aiming to promote neuroregeneration through different approaches. Here, we summarize the current understanding of the causes to the poor regeneration within the human CNS. We also review the results of the treatment attempts that have been translated into clinical trials so far.

Translational bioengineering strategies for peripheral nerve regeneration: opportunities, challenges, and novel concepts

Peripheral nerve injuries remain a challenging problem in need of better treatment strategies. Despite best efforts at surgical reconstruction and postoperative rehabilitation, patients are often left with persistent, debilitating motor and sensory deficits. There are currently no therapeutic strategies proven to enhance the regenerative process in humans. A clinical need exists for the development of technologies to promote nerve regeneration and improve functional outcomes. Recent advances in the fields of tissue engineering and nanotechnology have enabled biomaterial scaffolds to modulate the host response to tissue repair through tailored mechanical, chemical, and conductive cues. New bioengineered approaches have enabled targeted, sustained delivery of protein therapeutics with the capacity to unlock the clinical potential of a myriad of neurotrophic growth factors that have demonstrated promise in enhancing regenerative outcomes. As such, further exploration of combinatory strategies leveraging these technological advances may offer a pathway towards clinically translatable solutions to advance the care of patients with peripheral nerve injuries. This review first presents the various emerging bioengineering strategies that can be applied for the management of nerve gap injuries. We cover the rationale and limitations for their use as an alternative to autografts, focusing on the approaches to increase the number of regenerating axons crossing the repair site, and facilitating their growth towards the distal stump. We also discuss the emerging growth factor-based therapeutic strategies designed to improve functional outcomes in a multimodal fashion, by accelerating axonal growth, improving the distal regenerative environment, and preventing end-organs atrophy.

Long-Duration and Non-Invasive Photoacoustic Imaging of Multiple Anatomical Structures in a Live Mouse Using a Single Contrast Agent

Long-duration in vivo simultaneous imaging of multiple anatomical structures is useful for understanding physiological aspects of diseases, informative for molecular optimization in preclinical models, and has potential applications in surgical settings to improve clinical outcomes. Previous studies involving simultaneous imaging of multiple anatomical structures, for example, blood and lymphatic vessels as well as peripheral nerves and sebaceous glands, have used genetically engineered mice, which require expensive and time-consuming methods. Here, an IgG4 isotype control antibody is labeled with a near-infrared dye and injected into a mouse ear to enable simultaneous visualization of blood and lymphatic vessels, peripheral nerves, and sebaceous glands for up to 3 h using photoacoustic microscopy. For multiple anatomical structure imaging, peripheral nerves and sebaceous glands are imaged inside the injected dye-labeled antibody mass while the lymphatic vessels are visualized outside the mass. The efficacy of the contrast agent to label and localize deep medial lymphatic vessels and lymph nodes using photoacoustic computed tomography is demonstrated. The capability of a single injectable contrast agent to image multiple structures for several hours will potentially improve preclinical therapeutic optimization, shorten discovery timelines, and enable clinical treatments.

A new phenotype identification method with the fluorescent expression in cross-sectioned tails in Thy1-GCaMP6s transgenic mice

PURPOSE

Unless the phenotype of the transgenic mice is distinguishable, genotyping in each mouse is required prior to experiments. This study aimed to establish a new identification method for the phenotype in Thy1-GCaMP6s transgenic mice to reduce the cost and time.

METHODS

Tail biopsies (2 mm) were performed under general anesthesia with isoflurane in 3 to 4-week-old mice. Then, the resected tail was cut again with a sharp razor, and the cross-sections were observed with two-photon microscopy (excitation wavelength = 940 nm). The emitted light was split into green and red light by a dichroic mirror (570 nm) with bandpass filters (495-540 nm for green, 575-645 nm for red).

RESULTS

Two types of expressed fluorescent pattern were found in the tail tissue: the presence of green fluorescent structures (type 1) and the absence of the structures (type 2). Cortical imaging confirmed that type 1 expressed the cortical GCaMP6s, while type 2 did not.

CONCLUSION

These results suggest that observation of the cross-sectioned tail in Thy1-GCaMP6s mice enabled to identify the phenotype within approximately 10 min/mouse, which reduces the cost and time for genotyping.

Thy-1 (CD90), Integrins and Syndecan 4 are Key Regulators of Skin Wound Healing

Acute skin wound healing is a multistage process consisting of a plethora of tightly regulated signaling events in specialized cells. The Thy-1 (CD90) glycoprotein interacts with integrins and the heparan sulfate proteoglycan syndecan 4, generating a trimolecular complex that triggers bi-directional signaling to regulate diverse aspects of the wound healing process. These proteins can act either as ligands or receptors, and they are critical for the successful progression of wound healing. The expression of Thy-1, integrins, and syndecan 4 is controlled during the healing process, and the lack of expression of any of these proteins results in delayed wound healing. Here, we review and discuss the roles and regulatory events along the stages of wound healing that support the relevance of Thy-1, integrins, and syndecan 4 as crucial regulators of skin wound healing.

Effect of herbal extracts on peripheral nerve regeneration after microsurgery of the sciatic nerve in rats

Background

Recent experimental studies using herbal extracts have shown the possibility of peripheral nerve regeneration. This study aimed to investigate the effects of herbal extracts on peripheral nerve regeneration in a rat sciatic nerve injury model.

Methods

A total of 53 rats were randomly assigned to a control group or one of four experimental groups. In all rats, the sciatic nerve was completely severed and microscopic epineural end-to-end neurorrhaphy was performed. Normal saline (2 mL) was topically applied to the site of nerve repair in the control group, whereas four different herbal extracts – 2 mL each of Astragalus mongholicus Bunge, Coptis japonica (Thunb.) Makino, Aconitum carmichaelii Debeaux, or Paeonia lactiflora Pall. – were topically applied to the site of nerve repair in each experimental group. Nerve conduction studies were performed at an average of 11.9 weeks after the operation, and conduction velocity and proximal and distal amplitudes were measured. Biopsies were performed at an average of 13.2 weeks after the initial neurorrhaphy. The quality of nerve anastomosis and perineural adhesion to the surrounding soft tissues was macroscopically evaluated. The neuroma size at the site of the neurorrhaphy was microscopically measured, whereas the size of the scar tissue was evaluated relative to the diameter of the repaired nerve.

Results

The nerve conduction study results showed the highest nerve conduction velocity in the experimental group that used the Coptis japonica (Thunb.) Makino extract and the highest proximal and distal amplitudes in the experimental group that used the Aconitum carmichaelii Debeaux extract. Macroscopic evaluations after the second operation showed that grade 2 perineural adhesion was found in 70.8% of rats. The mean neuroma size in the Coptis japonica (Thunb.) Makino, Aconitum carmichaelii Debeaux, and Paeonia lactiflora Pall. groups showed statistically significant decreases relative to the control group. The mean scar tissue formation index in the Paeonia lactiflora Pall. group showed a statistically significant decrease relative to the control group.

Conclusions

The peripheral nerve regeneration effect of the herbal extracts was confirmed through decreased neuroma and scar tissue formation.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12906-021-03335-w.

Ultrastructural and morphometric alterations to the peripheral nerve following the administration of immunosuppressive agent tacrolimus (FK506)

Tacrolimus, a widely used immunosuppressive drug for preventing graft rejection following organ transplantation, was reported to develop neurotoxic side effects ranging from mild to severe symptoms in the literature. Rats were randomly divided into three groups as control and 2-week and 3-week treatment groups and received a 2 mg/kg/day tacrolimus by oral gavage. Animals were sacrificed and sciatic nerves obtained from all groups were fixed and processed for light and electron microscopic investigations. The myelinated fiber diameter, axon diameter, G-ratio (axon diameter/myelinated fiber diameter), and myelin thickness were also determined. The data obtained in the control and tacrolimus-treated groups were compared.The control group sciatic nerve fascicles showed normal morphology with myelinated and unmyelinated fibers. Experimental groups exhibited axonal dilatation, irregularly thickened and vacuolated myelin sheaths with separation of myelin layers. The morphometric analysis showed that the myelinated fibers of the 2-week tacrolimus-treated group displayed a moderate increase in the myelin thickness and axon and fiber diameter in comparison with the control and 3-week tacrolimus-treated groups. The G-ratio was found to be in normal range in all groups and there were no statistically significant difference.The present study indicates that the treatment with tacrolimus may produce a mild degenerative change but prolonged drug administration for 3 weeks led to improvement in morphometric and morphologic data and the normal G-ratio values, suggesting that the regeneration capacity of the myelinated fibers maintains their normal function to transmit nerve impulses.

Self-assembling multidomain peptide hydrogels accelerate peripheral nerve regeneration after crush injury

Multidomain peptide (MDP) hydrogels are a class of self-assembling materials that have been shown to elicit beneficial responses for soft tissue regeneration. However, their capacity to promote nervous system regeneration remains unknown. The peripheral nervous system (PNS) substantially recovers after injury, partly due to the abundance of extracellular matrix (ECM) components in its basal lamina. However, severe peripheral nerve injuries that significantly damage the ECM continue to be a major clinical challenge as they occur at a high rate and can be extremely detrimental to patients' quality of life. In this study, a panel of eight MDPs were designed to contain various motifs mimicking extracellular matrix components and growth factors and successfully self-assembled into injectable, nanofibrous hydrogels. Using an in vitro screening system, various lysine based MDPs were found to enhance neurite outgrowth. To test their capacity to promote nerve regeneration in vivo, rat sciatic nerve crush injury was performed with MDP hydrogels injected directly into the injury sites. MDP hydrogels were found to enhance macrophage recruitment to the injury site and degrade efficiently over time. Rats that were injected with the MDP hydrogel K2 and laminin motif-containing MDPs K2-IIKDI and K2-IKVAV were found to have significantly accelerated functional recovery and remyelination compared to those injected with HBSS or other MDPs. These results demonstrate that MDPs enhance neurite outgrowth and promote a multicellular pro-regenerative response in peripheral nerve injury. This study provides important insights into the potential of MDPs as biomaterials for nerve regeneration and other clinical applications.

Adult Mouse Retina Explants: From ex vivo to in vivo Model of Central Nervous System Injuries

In mammals, adult neurons fail to regenerate following any insult to adult central nervous system (CNS), which leads to a permanent and irreversible loss of motor and cognitive functions. For a long time, much effort has been deployed to uncover mechanisms of axon regeneration in the CNS. Even if some cases of functional recovery have been reported, there is still a discrepancy regarding the functionality of a neuronal circuit upon lesion. Today, there is a need not only to identify new molecules implicated in adult CNS axon regeneration, but also to decipher the fine molecular mechanisms associated with regeneration failure. Here, we propose to use cultures of adult retina explants to study all molecular and cellular mechanisms that occur during CNS regeneration. We show that adult retinal explant cultures have the advantages to (i) recapitulate all the features observed in vivo, including axon regeneration induced by intrinsic factors, and (ii) be an ex vivo set-up with high accessibility and many downstream applications. Thanks to several examples, we demonstrate that adult explants can be used to address many questions, such as axon guidance, growth cone formation and cytoskeleton dynamics. Using laser guided ablation of a single axon, axonal injury can be performed at a single axon level, which allows to record early and late molecular events that occur after the lesion. Our model is the ideal tool to study all molecular and cellular events that occur during CNS regeneration at a single-axon level, which is currently not doable in vivo. It is extremely valuable to address unanswered questions of neuroprotection and neuroregeneration in the context of CNS lesion and neurodegenerative diseases.

Assessing Axonal Degeneration in Embryonic Dorsal Root Ganglion Neurons In Vitro

The molecular players regulating the axon degeneration pathway have been identified using in vitro experimental models. Here, we describe an in vitro assay to assess the axonal fragmentation induced by mechanical injury to axons in cultured mouse embryonic dorsal root ganglion (DRG) neurons. DRG neurons are pseudounipolar and therefore suitable for an assay of axonal degeneration after injury. In addition, the time course of the axonal fragmentation is stereotyped, enabling the identification of reagents that either expedite or impede the degeneration process. With an image-based quantification method, the in vitro degeneration assay can be utilized as a platform supporting high-throughput screens for pharmacological or genetic reagents delaying axon degeneration.

High resolution three-dimensional imaging of the ocular surface and intact eyeball using tissue clearing and light sheet microscopy

PURPOSE

High resolution visualization of the ocular surface and intact eyeball is critical and essential for our understanding and treatment of eye diseases. This study is to achieve this goal using advanced tissue clearing and three-dimensional (3D) imaging technologies.

METHODS

Wild type and fluorescently labeled transgenic mice of Prox-1-GFP (green fluorescent protein) or Thy1-YFP (yellow fluorescent protein) were used in the study. Eyeballs were harvested from normal or a disease model of corneal inflammation. Samples were infused with hydrogel monomers and heated for polymerization. Lipids were removed by electrophoresis. The transparent tissue-hydrogel hybrids of the anterior segments or intact eyeballs with immunolabeling or endogenous fluorescence were imaged by an advanced light sheet fluorescent microscope. High resolution 3D images and videos were captured for a wide array of structures and cell types.

RESULTS

Optical transparency was achieved from intact eyeballs of both normal and diseased conditions. A variety of important structures and cell types, such as blood and lymphatic vessels, Schlemm's canal, nerves and endothelial cells, were detected with their natural morphology, location and organizational network.

CONCLUSIONS

This study provides the first comprehensive and 3D high resolution imaging of the intact eyeball using tissue clearing and advanced light sheet microscopy. Given that the eye is the window of the body, we anticipate this advanced technology will facilitate diverse applications in biomedical research inside and outside the eye.

DLK regulates a distinctive transcriptional regeneration program after peripheral nerve injury

Following damage to a peripheral nerve, injury signaling pathways converge in the cell body to generate transcriptional changes that support axon regeneration. Here, we demonstrate that dual leucine zipper kinase (DLK), a central regulator of injury responses including axon regeneration and neuronal apoptosis, is required for the induction of the pro-regenerative transcriptional program in response to peripheral nerve injury. Using a sensory neuron-conditional DLK knockout mouse model, we show a time course for the dependency of gene expression changes on the DLK pathway after sciatic nerve injury. Gene ontology analysis reveals that DLK-dependent gene sets are enriched for specific functional annotations such as ion transport and immune response. A series of comparative analyses shows that the DLK-dependent transcriptional program is distinct from that promoted by the importin-dependent retrograde signaling pathway, while it is partially shared between PNS and CNS injury responses. We suggest that DLK-dependency might provide a selective filter for regeneration-associated genes among the injury-responsive transcriptome.

Macroporous nanofiber wraps promote axonal regeneration and functional recovery in nerve repair by limiting fibrosis

Functional outcomes following nerve repair remain suboptimal. Scarring at the repair site is a major impediment to regeneration. A biomaterial scaffold applied around the coaptation site that decreases inflammation holds great potential in reducing scarring, enhancing axonal growth, and improving functional recovery. In this study, we evaluated the effect of a macroporous nanofiber wrap, comprised of nonwoven electrospun poly-ε-caprolactone (PCL), in improving axonal regeneration in a rat sciatic nerve cut and direct repair model. Controls consisted of conventional epineurial repair. We also evaluated our wrap against the commercially available AxoGuard wrap. At five weeks following repair, the nanofiber wrap group showed a significantly decreased intraneural macrophage invasion and collagen deposition at the repair site. This was associated with increased expression of the anti-inflammatory cytokine (IL-10), decreased expression of the pro-inflammatory cytokine (TNF-α), and a decrease in the M1:M2 macrophage phenotype ratio. These findings suggest that this nanofiber wrap, with its unique macroporosity, is modulating the inflammatory response at the repair site by polarizing macrophages towards a pro-regenerative M2 phenotype. Concomitantly, a higher number of regenerated axons was noted. At sixteen weeks, the nanofiber wrap resulted in enhanced functional recovery as demonstrated by electrophysiology, neuromuscular re-innervation, and muscle histology. When compared to the AxoGuard wrap, the nanofiber wrap showed similar inflammation at the repair site and similar nerve morphometric findings, but there was a trend towards a lower overall number of macrophages invading the wrap wall. These results demonstrate favorable outcomes of the macroporous nanofiber wrap in promoting neuroregeneration and functional recovery following nerve repair. STATEMENT OF SIGNIFICANCE: Electrospun nanofiber scaffolds, with specific fiber and pore sizes, were shown to modulate the immune response and create a regenerative environment. In this paper, we present a macroporous nanofiber wrap, made of poly-ε-caprolactone, to be applied at the coaptation site in primary nerve repair. We show that it regulates the inflammatory response at the repair site and decreases scarring/fibrosis. This results in enhanced axonal regeneration, allowing a higher number of axons to cross the suture line and reach the target muscle in a timely fashion. Functional outcomes are thus improved.

A novel approach to 32-channel peripheral nervous system myelin imaging in vivo, with single axon resolution

OBJECTIVE Intravital spectral imaging of the large, deeply situated nerves in the rat peripheral nervous system (PNS) has not been well described. Here, the authors have developed a highly stable platform for performing imaging of the tibial nerve in live rodents, thus allowing the capture of high-resolution, high-magnification spectral images requiring long acquisition times. By further exploiting the qualities of the topically applied myelin dye Nile red, this technique is capable of visualizing the detailed microenvironment of peripheral nerve demyelination injury and recovery, while allowing us to obtain images of exogenous Schwann cell myelination in a living animal. METHODS The authors caused doxorubicin-induced focal demyelination in the tibial nerves of 25 Thy-1 GFP rats, of which 2 subsets (n = 10 each) received either BFP-labeled SKP-SCs or SCs to the zone of injury. Prior to acquiring images of myelin recovery in these nerves, a tibial nerve window was constructed using a silicone hemitube, a fast drying silicone polymer, and a small coverslip. This construct was then affixed to a 3D-printed nerve stage, which in turn was affixed to an external fixation/microscope stage device. Myelin visualization was facilitated by the topical application of Nile red. RESULTS The authors reliably demonstrated intravital peripheral nerve myelin imaging with micron-level resolution and magnification, and minimal movement artifact. The detailed microenvironment of nerve remyelination can be vividly observed, while exogenously applied Schwann cells and skin-derived precursor Schwann cells can be seen myelinating axons. CONCLUSIONS Topically applied Nile red enables intravital study of myelin in the living rat PNS. Furthermore, the use of a tibial nerve window facilitates stable intravital peripheral nerve imaging, making possible high-definition spectral imaging with long acquisition times.

Diffusion MRI in acute nervous system injury

Diffusion weighted magnetic resonance imaging (DWI) and related techniques such as diffusion tensor imaging (DTI) are uniquely sensitive to the microstructure of the brain and spinal cord. In the acute aftermath of nervous system injury, for example, DWI reveals changes caused by injury that remains invisible on other MRI contrasts such as T₂-weighted imaging. This ability has led to a demonstrated clinical utility in cerebral ischemia. However, despite strong promise in preclinical models and research settings, DWI has not been as readily adopted for other acute injuries such as traumatic spinal cord, brain, or peripheral nerve injury. Furthermore, the precise biophysical mechanisms that underlie DWI and DTI changes are not fully understood. In this report, we review the DWI and DTI changes that occur in acute neurological injury of cerebral ischemia, spinal cord injury, traumatic brain injury, and peripheral nerve injury. Their associations with the underlying biology are examined with an emphasis on the role of acute axon and dendrite beading. Lastly, emerging DWI techniques to overcome the limitations of DTI are discussed as these may offer the needed improvements to translate to clinical settings.

Comparative analysis of the transcriptome of injured nerve segments reveals spatiotemporal responses to neural damage in mice

Nerves are particularly vulnerable to damage due to their unique structure with meter-long axons. In the peripheral nervous system neurons and Schwann cells can activate the injury-response program that directs axons to either regenerate or degenerate after traumatic nerve injury. However, the differences between the genetic programs driving nerve regeneration and degeneration have not yet been described extensively. To understand these differences, in this study we have compared the injury-induced transcriptomic changes between the regenerating proximal segment and the degenerating distal segment of a transected nerve, at different post-injury time points. We analyzed the spatiotemporal dynamics of the mouse transcriptome using a sciatic nerve-injury model by means of RNA sequencing. The results of the differentially regulated genes (DEGs) analysis show that some DEG groups are similarly regulated in both proximal and distal segments, and primarily display a positive correlation. However, some DEG groups are exclusively regulated in either the proximal or the distal segment, suggesting that these DEG groups constitute a genetic network for distinguishing the regenerative and degenerative responses. In addition, our gene ontology analysis revealed an enrichment of particular biological processes in different phases and locations. Thus, our data provide a spatiotemporal profile of the transcriptomes that are differentially regulated in either regenerating or degenerating nerves, in vivo. The specific biological processes enriched in the DEG groups might delineate the injury-responsive program that induces contrasting regenerative and degenerative responses in different nerve segments.

Tubulation repair mitigates misdirection of regenerating motor axons across a sciatic nerve gap in rats

The repair of peripheral nerve laceration injury to obtain optimal function recovery remains a big challenge in the clinic. Misdirection of regenerating axons to inappropriate target, as a result of forced mismatch of endoneurial sheaths in the case of end-to-end nerve anastomosis or nerve autografting, represents one major drawback that limits nerve function recovery. Here we tested whether tubulation repair of a nerve defect could be beneficial in terms of nerve regeneration accuracy and nerve function. We employed sequential retrograde neuronal tracing to assess the accuracy of motor axon regeneration into the tibial nerve after sciatic nerve laceration and entubulation in adult Sprague-Dawley rats. In a separate cohort of rats with the same sciatic nerve injury/repair protocols, we evaluated nerve function recovery behaviorally and electrophysiologically. The results showed that tubulation repair of the lacerated sciatic nerve using a 3-6-mm-long bioabsorbable guidance conduit significantly reduced the misdirection of motor axons into the tibial nerve as compared to nerve autografting. In addition, tubulation repair ameliorated chronic flexion contracture. This study suggests that tubulation repair of a nerve laceration injury by utilizing a bioresorbable nerve guidance conduit represents a potential substitute for end-to-end epineurial suturing and nerve autografting.

High-Intensity Ultrasound Treatment for Vincristine-Induced Neuropathic Pain

BACKGROUND

Vincristine is a commonly used chemotherapeutic agent that results in debilitating untreatable peripheral neuropathy.

OBJECTIVE

To determine the effects of pulsed high-intensity focused ultrasound (HIFU) on sensory thresholds in a validated vincristine-induced neuropathy (VIN) rodent model.

METHODS

VIN was induced and mechanical allodynia was confirmed by nociceptive testing. von Frey fibers and Randall-Sellito test were used as measures of innocuous and noxious mechanical thresholds, respectively, and the hot plate test for thermal thresholds. Tests were performed before VIN, after 2 wk of vincristine, at 24, 48, 72, and 120 h after HIFU applied to the left L5 dorsal root ganglia at 3 Watts for 3 min. Comparisons were made between a VIN cohort who underwent HIFU, a VIN cohort who underwent sham HIFU, and naïve rodents who underwent HIFU.

RESULTS

VIN HIFU rats had significantly increased mechanical thresholds at 24 h (P < .001), 48 h (P = .008), 72 h (P = .003), and 120 h (P = .03) after treatment, when compared to pre-HIFU thresholds. Furthermore, at 24 and 48 h following treatment, VIN HIFU rats had significantly higher innocuous and noxious mechanical thresholds and thermal thresholds than VIN sham HIFU rats (P < .001). Thresholds were not altered in naïve rodents who underwent HIFU. Histological data of L5 dorsal root ganglia of VIN HIFU rats suggest that transient cellular edema resolves by 48 h.

CONCLUSION

Our data suggest that HIFU increases mechanical and thermal thresholds in VIN rodents. Whether HIFU can preclude the development of reduced thresholds in the VIN model warrants further study.

Lost in elimination: mechanisms of axonal loss

Axonal loss is an important process both dur­ing development and diseases of the ner­vous system. While the molecular mecha­nisms that mediate axonal loss are largely elusive, modern imaging technology affords an increasingly clear view of the cellular processes that allow nerve cells to shed individiual axon branches or even dismantle entire parts of their axonal projections. The present review discusses the characteristics of post-traumatic Wallerian degeneration, the process of axonal loss currently best understood. Subsequently, the properties of a number of recently discovered axonal loss phenome­na are described. These phenomena explain some of the axonal loss that occurs locally after axon transection, during neuro-inflammatory insults, and as part of normal neurode­velopment.

Epigenetic Regulation of Axon Regeneration after Neural Injury

When peripheral axons are damaged, neuronal injury signaling pathways induce transcriptional changes that support axon regeneration and consequent functional recovery. The recent development of bioinformatics techniques has allowed for the identification of many of the regeneration-associated genes that are regulated by neural injury, yet it remains unclear how global changes in transcriptome are coordinated. In this article, we review recent studies on the epigenetic mechanisms orchestrating changes in gene expression in response to nerve injury. We highlight the importance of epigenetic mechanisms in discriminating efficient axon regeneration in the peripheral nervous system and very limited axon regrowth in the central nervous system and discuss the therapeutic potential of targeting epigenetic regulators to improve neural recovery.

Doublecortin-Like Kinases Promote Neuronal Survival and Induce Growth Cone Reformation via Distinct Mechanisms

After axotomy, neuronal survival and growth cone re-formation are required for axon regeneration. We discovered that doublecortin-like kinases (DCLKs), members of the doublecortin (DCX) family expressed in adult retinal ganglion cells (RGCs), play critical roles in both processes, through distinct mechanisms. Overexpression of DCLK2 accelerated growth cone re-formation in vitro and enhanced the initiation and elongation of axon re-growth after optic nerve injury. These effects depended on both the microtubule (MT)-binding domain and the serine-proline-rich (S/P-rich) region of DCXs in-cis in the same molecules. While the MT-binding domain is known to stabilize MT structures, we show that the S/P-rich region prevents F-actin destabilization in injured axon stumps. Additionally, while DCXs synergize with mTOR to stimulate axon regeneration, alone they can promote neuronal survival possibly by regulating the retrograde propagation of injury signals. Multifunctional DCXs thus represent potential targets for promoting both survival and regeneration of injured neurons.

Macroscopic in vivo imaging of facial nerve regeneration in Thy1-GFP rats

IMPORTANCE

Facial nerve injury leads to severe functional and aesthetic deficits. The transgenic Thy1-GFP rat is a new model for facial nerve injury and reconstruction research that will help improve clinical outcomes through translational facial nerve injury research.

OBJECTIVE

To determine whether serial in vivo imaging of nerve regeneration in the transgenic rat model is possible, facial nerve regeneration was imaged under the main paradigms of facial nerve injury and reconstruction.

DESIGN, SETTING, AND PARTICIPANTS

Fifteen male Thy1-GFP rats, which express green fluorescent protein (GFP) in their neural structures, were divided into 3 groups in the laboratory: crush-injury, direct repair, and cross-face nerve grafting (30-mm graft length). The distal nerve stump or nerve graft was predegenerated for 2 weeks. The facial nerve of the transgenic rats was serially imaged at the time of operation and after 2, 4, and 8 weeks of regeneration. The imaging was performed under a GFP-MDS-96/BN excitation stand (BLS Ltd).

INTERVENTION OR EXPOSURE

Facial nerve injury.

MAIN OUTCOME AND MEASURE

Optical fluorescence of regenerating facial nerve axons.

RESULTS

Serial in vivo imaging of the regeneration of GFP-positive axons in the Thy1-GFP rat model is possible. All animals survived the short imaging procedures well, and nerve regeneration was followed over clinically relevant distances. The predegeneration of the distal nerve stump or the cross-face nerve graft was, however, necessary to image the regeneration front at early time points. Crush injury was not suitable to sufficiently predegenerate the nerve (and to allow for degradation of the GFP through Wallerian degeneration). After direct repair, axons regenerated over the coaptation site in between 2 and 4 weeks. The GFP-positive nerve fibers reached the distal end of the 30-mm-long cross-face nervegrafts after 4 to 8 weeks of regeneration.

CONCLUSIONS AND RELEVANCE

The time course of facial nerve regeneration was studied by serial in vivo imaging in the transgenic rat model. Nerve regeneration was followed over clinically relevant distances in a small number of experimental animals, as they were subsequently imaged at multiple time points. The Thy1-GFP rat model will help improve clinical outcomes of facial reanimation surgery through improving the knowledge of facial nerve regeneration after surgical procedures.

LEVEL OF EVIDENCE

NA.

Visualization of Sensory Neurons and Their Projections in an Upper Motor Neuron Reporter Line

Visualization of peripheral nervous system axons and cell bodies is important to understand their development, target recognition, and integration into complex circuitries. Numerous studies have used protein gene product (PGP) 9.5 [a.k.a. ubiquitin carboxy-terminal hydrolase L1 (UCHL1)] expression as a marker to label sensory neurons and their axons. Enhanced green fluorescent protein (eGFP) expression, under the control of UCHL1 promoter, is stable and long lasting in the UCHL1-eGFP reporter line. In addition to the genetic labeling of corticospinal motor neurons in the motor cortex and degeneration-resistant spinal motor neurons in the spinal cord, here we report that neurons of the peripheral nervous system are also fluorescently labeled in the UCHL1-eGFP reporter line. eGFP expression is turned on at embryonic ages and lasts through adulthood, allowing detailed studies of cell bodies, axons and target innervation patterns of all sensory neurons in vivo. In addition, visualization of both the sensory and the motor neurons in the same animal offers many advantages. In this report, we used UCHL1-eGFP reporter line in two different disease paradigms: diabetes and motor neuron disease. eGFP expression in sensory axons helped determine changes in epidermal nerve fiber density in a high-fat diet induced diabetes model. Our findings corroborate previous studies, and suggest that more than five months is required for significant skin denervation. Crossing UCHL1-eGFP with hSOD1^G93A mice generated hSOD1^G93A-UeGFP reporter line of amyotrophic lateral sclerosis, and revealed sensory nervous system defects, especially towards disease end-stage. Our studies not only emphasize the complexity of the disease in ALS, but also reveal that UCHL1-eGFP reporter line would be a valuable tool to visualize and study various aspects of sensory nervous system development and degeneration in the context of numerous diseases.

An in vitro assay to study induction of the regenerative state in sensory neurons

After injury, peripheral neurons activate a pro-regenerative program that facilitates axon regeneration. While many regeneration-associated genes have been identified, the mechanism by which injury activates this program is less well understood. Furthermore, identifying pharmacological methods to induce a pro-regenerative state could lead to novel treatments to repair the injured nervous system. Therefore, we have developed an in vitro assay to study induction of the pro-regenerative state following injury or pharmacological treatment. First, we took advantage of the observation that dissociating and culturing sensory neurons from dorsal root ganglia activates a pro-regenerative program. We show that cultured neurons activate transcription factors and upregulate regeneration-associated genes common to the pro-regenerative program within the first hours after dissection. In a paradigm similar to pre-conditioning, neurons injured by dissociation display enhanced neurite outgrowth when replated as early as 12h after being removed from the animal. Furthermore, stimulation of the pro-regenerative state improves growth on inhibitory substrates and requires DLK/JNK signaling, both hallmarks of the pro-regeneration response in vivo. Finally, we modified this assay in order to identify new methods to activate the pro-regenerative state in an effort to mimic the pre-conditioning effect. We report that after several days in culture, neurons down-regulate many molecular hallmarks of injury and no longer display enhanced neurite outgrowth after replating. Hence, these neurons are functionally naïve and are a useful tool for identifying methods to induce the pro-regenerative state. We show that both injury and pre-treatment with forskolin reactivate the pro-regenerative state in this paradigm. Hence, this assay is useful for identifying pharmacological agents that induce the pro-regenerative state in the absence of injury.

Besides neuro-imaging, the Thy1-YFP mouse could serve for visualizing experimental tumours, inflammation and wound-healing

The B6.Cg-Tg(Thy1-YFP)16Jrs/J transgenic mouse strain, widely used to study neuronal development and regeneration, expresses the yellow fluorescent protein (YFP) in the peripheral nerves and the central nervous system under the control of regulatory sequences of the Thy1 gene. The Thy1 (CD90) cell surface glycoprotein is present on many cell types besides neurons, and is known to be involved in cell adhesion, migration and signal transduction. We hypothesized that Thy1-activating conditions could probably activate the truncated Thy1 regulatory sequences used in the Thy1-YFP construct, resulting in YFP transgene expression outside the nervous system. We demonstrated that the stroma of subcutaneous tumours induced by the injection of 4T1 or MC26 carcinoma cells into BALB/c(Thy1-YFP) mice, carrying the same construct, indeed expressed the YFP transgene. In the tumour mass, the yellow-green fluorescent stromal cells were clearly distinguishable from 4T1 carcinoma cells stably transfected with red fluorescent protein. Local inflammation induced by subcutaneous injection of complete Freund's adjuvant, as well as the experimental wound-healing milieu, also triggered YFP fluorescence in both the BALB/c(Thy1-YFP) and B6.Cg-Tg(Thy1-YFP)16Jrs/J mice, pointing to eventual overlapping pathways of wound-healing, inflammation and tumour growth.

Peripheral nerve reconstruction after injury: a review of clinical and experimental therapies

Unlike other tissues in the body, peripheral nerve regeneration is slow and usually incomplete. Less than half of patients who undergo nerve repair after injury regain good to excellent motor or sensory function and current surgical techniques are similar to those described by Sunderland more than 60 years ago. Our increasing knowledge about nerve physiology and regeneration far outweighs our surgical abilities to reconstruct damaged nerves and successfully regenerate motor and sensory function. It is technically possible to reconstruct nerves at the fascicular level but not at the level of individual axons. Recent surgical options including nerve transfers demonstrate promise in improving outcomes for proximal nerve injuries and experimental molecular and bioengineering strategies are being developed to overcome biological roadblocks limiting patient recovery.

Syndecan promotes axon regeneration by stabilizing growth cone migration

Growth cones facilitate the repair of nervous system damage by providing the driving force for axon regeneration. Using single-neuron laser axotomy and in vivo time-lapse imaging, we show that syndecan, a heparan sulfate (HS) proteoglycan, is required for growth cone function during axon regeneration in C. elegans. In the absence of syndecan, regenerating growth cones form but are unstable and collapse, decreasing the effective growth rate and impeding regrowth to target cells. We provide evidence that syndecan has two distinct functions during axon regeneration: (1) a canonical function in axon guidance that requires expression outside the nervous system and depends on HS chains and (2) an intrinsic function in growth cone stabilization that is mediated by the syndecan core protein, independently of HS. Thus, syndecan is a regulator of a critical choke point in nervous system repair.

Misdirection and guidance of regenerating axons after experimental nerve injury and repair

Misdirection of regenerating axons is one of the factors that can explain the limited results often found after nerve injury and repair. In the repair of mixed nerves innervating different distal targets (skin and muscle), misdirection may, for example, lead to motor axons projecting toward skin, and vice versa—that is, sensory axons projecting toward muscle. In the repair of motor nerves innervating different distal targets, misdirection may result in reinnervation of the wrong target muscle, which might function antagonistically. In sensory nerve repair, misdirection might give an increased perceptual territory. After median nerve repair, for example, this might lead to a dysfunctional hand.

Different factors may be involved in the misdirection of regenerating axons, and there may be various mechanisms that can later correct for misdirection. In this review the authors discuss these different factors and mechanisms that act along the pathway of the regenerating axon. The authors review recently developed evaluation methods that can be used to investigate the accuracy of regeneration after nerve injury and repair (including the use of transgenic fluorescent mice, retrograde tracing techniques, and motion analysis). In addition, the authors discuss new strategies that can improve in vivo guidance of regenerating axons (including physical guidance with multichannel nerve tubes and biological guidance accomplished using gene therapy).

A chronic window imaging device for the investigation of in vivo peripheral nerves

Chronic imaging of the peripheral nervous system with contemporary techniques requires repetitive surgical procedures to reopen an area of interest in order to see underlying biological processes over time. The recurrence of surgical openings on an animal increases trauma, stress, and risk of infection. Such effects can greatly lessen the physiological relevance of any data recorded in this manner. In order to bypass repetitive surgery, a Peripheral Nerve Window (PNW) device has been created for chronic in vivo imaging purposes. Intravital imaging window devices have been used previously to image parts of the rodent model such as the brain, spinal cord, and mammary tissue, but currently have not been used in the peripheral nervous system because of lack of bone anchoring and access to deep nerve tissue. We demonstrate a novel surgical technique in a rat which transposes the sciatic nerve above the surrounding muscle tissue allowing the PNW access to an 8mm section of the nerve. Subsequent days of observation revealed increased vasculature development primarily around the nerve, showing that this preparation can be used to image nerve tissue and surrounding vasculature for up to one week post-implantation.

Functional regeneration of recurrent laryngeal nerve injury during thyroid surgery using an asymmetrically porous nerve guide conduit in an animal model

BACKGROUND

Vocal cord paralysis (VCP) caused by recurrent laryngeal nerve (RLN) damage during thyroidectomy commonly results in serious medico-legal problems. The purpose of this study was to evaluate the usefulness of an asymmetrically porous polycaprolactone (PCL)/Pluronic F127 nerve guide conduit (NGC) for functional regeneration in a RLN injury animal model.

METHODS

A biodegradable, asymmetrically porous PCL/F127 NGC with selective permeability was fabricated for use in this study. A 10-mm segment of left RLN was resected in 28 New Zealand white rabbits, and then an asymmetrically porous NGC or a nonporous silicone tube was interposed between both stumps and securely fixed. Vocal cord mobility was endoscopically evaluated at one, four, and eight weeks postoperatively. Nerve growth through NGCs was assessed by toluidine blue staining, and thyroarytenoid (TA) muscle atrophy was evaluated by hematoxylin and eosin staining. Immunohistochemical stainings for acetylcholinesterase (AchE), anti-neurofilament (NF), and anti-S100 protein were also conducted, and transmission electron microscopy (TEM) was used to evaluate functional nerve regeneration.

RESULTS

At eight weeks postoperatively, endoscopic evaluations showed significantly better recovery from VCP in the asymmetrically porous PCL/F127 NGC group (6 of 10 rabbits) than in the silicone tube group (1 of 10 rabbits). Continued nerve growth on the damaged nerve endings was observed with time in the asymmetrically porous PCL/F127 NGC-interposed RLNs. TA muscle dimensions and AchE expressions in TA muscle were significantly greater in the asymmetrically porous PCL/F127 NGC group than in the silicone tube group. Furthermore, immunohistochemical staining revealed the expression of NF and S100 protein in the regenerated nerves in the asymmetrically porous PCL/F127 NGC group at eight weeks postoperatively, and at this time, TEM imaging showed myelinated axons in the regenerated RLNs.

CONCLUSION

The study shows that asymmetrically porous PCL/F127 NGC provides a favorable environment for RLN regeneration and that it has therapeutic potential for the regeneration of RLN damage.

Dynamic regulation of SCG10 in regenerating axons after injury

Peripheral axons can re-extend robustly after nerve injury. Soon after a nerve crush regenerating axons grow through the nerve segment distal to the lesion in close proximity to distal axons that are still morphologically and molecularly preserved. Hence, following the progress of regenerating axons necessitates markers that can distinguish between regenerating and degenerating axons. Here, we show that axonal levels of superior cervical ganglion 10 (SCG10) are dynamically regulated after axonal injury and provide an efficient method to label regenerating axons. In contrast to the rapid loss of SCG10 in distal axons (Shin et al., 2012b), we report that SCG10 accumulates in the proximal axons within an hour after injury, leading to a rapid identification of the lesion site. The increase in SCG10 levels is maintained during axon regeneration after nerve crush or nerve repair and allows for more selective labeling of regenerating axons than the commonly used markers growth-associated protein 43 (GAP43) and YFP. SCG10 is preferentially expressed in regenerating sensory axons rather than motor axons in the sciatic nerve. In a mouse model of slow Wallerian degeneration, SCG10 labeling remains selective for regenerating axons and allows for a quantitative analysis of delayed regeneration in this mutant. Taken together, these data demonstrate the utility of SCG10 as an efficient and selective marker of sensory axon regeneration.

Drosophila growth cones advance by forward translocation of the neuronal cytoskeletal meshwork in vivo

In vitro studies conducted in Aplysia and chick sensory neurons indicate that in addition to microtubule assembly, long microtubules in the C-domain of the growth cone move forward as a coherent bundle during axonal elongation. Nonetheless, whether this mode of microtubule translocation contributes to growth cone motility in vivo is unknown. To address this question, we turned to the model system Drosophila. Using docked mitochondria as fiduciary markers for the translocation of long microtubules, we first examined motion along the axon to test if the pattern of axonal elongation is conserved between Drosophila and other species in vitro. When Drosophila neurons were cultured on Drosophila extracellular matrix proteins collected from the Drosophila Kc167 cell line, docked mitochondria moved in a pattern indicative of bulk microtubule translocation, similar to that observed in chick sensory neurons grown on laminin. To investigate whether the C-domain is stationary or advances in vivo, we tracked the movement of mitochondria during elongation of the aCC motor neuron in stage 16 Drosophila embryos. We found docked mitochondria moved forward along the axon shaft and in the growth cone C-domain. This work confirms that the physical mechanism of growth cone advance is similar between Drosophila and vertebrate neurons and suggests forward translocation of the microtubule meshwork in the axon underlies the advance of the growth cone C-domain in vivo. These results highlight the need for incorporating en masse microtubule translocation, in addition to assembly, into models of axonal elongation.

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

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

Diagnosis of brachial and lumbosacral plexus lesions

To most doctors, brachial and lumbosacral plexopathies are known as difficult disorders, because of their complicated anatomy and relatively rare occurrence. Both the brachial, lumbar, and sacral plexuses are extensive PNS structures stretching from the neck to axillary region and running in the paraspinal lumbar and pelvic region, containing 100000-200000 axons with 12-15 major terminal branches supplying almost 50 muscles in each limb. The most difficult part in diagnosing a plexopathy is probably that it requires an adequate amount of clinical suspicion combined with a thorough anatomical knowledge of the PNS and a meticulous clinical examination. Once a set of symptoms is recognized as a plexopathy the patients' history and course of the disorder will often greatly limit the differential diagnosis. The most common cause of brachial plexopathy is probably neuralgic amyotrophy and the most common cause of lumbosacral plexopathy is diabetic amyotrophy. Traumatic and malignant lesions are fortunately rarer but just as devastating. This chapter provides an overview of both common and rarer brachial and lumbosacral plexus disorders, focusing on clinical examination, the use of additional investigative techniques, prognosis, and treatment.

A systematic review of animal models used to study nerve regeneration in tissue-engineered scaffolds

Research on biomaterial nerve scaffolds has been carried out for 50 years. Only three materials (collagen, polycaprolactone and polyglycollic acid) have progressed to clinical use. Pre-clinical animal models are critical for testing nerve scaffolds prior to implementation in clinical practice. We have conducted a systematic review of 416 reports in which animal models were used for evaluation of nerve regeneration into synthetic conduits. A valid animal model of nerve regeneration requires it to reproduce the specific processes that take place in regeneration after human peripheral nerve injury. No distinct animal species meets all the requirements for an ideal animal model. Certain models are well suited for understanding regenerative neurobiology while others are better for pre-clinical evaluation of efficacy. The review identified that more than 70 synthetic materials were tested in eight species using 17 different nerves. Nerve gaps ranged from 1 to 90 mm. More than 20 types of assessment methodology were used with no standardization of methods between any of the publications. The review emphasizes the urgent need for standardization or rationalization of animal models and evaluation methods for studying nerve repair.