Rapid induction of functional and morphological continuity between severed ends of mammalian or earthworm myelinated axons

Polyethylene Glycol-Fusion Repair of Peripheral Nerve Injuries

Axons successfully repaired with polyethylene glycol (PEG) fusion tecnology restored axonal continuity thereby preventing their Wallerian degeneration and minimizing muscle atrophy. PEG fusion studies in animal models and preliminary clinical trials involving patients with digital nerve repair have shown promise for this therapeutic approach. PEG fusion is safe to perform, and given the enormous potential benefits, there is no reason not to explore its therapeutic potential.

Repair of traumatic lesions to the plasmalemma of neurons and other cells: Commonalities, conflicts, and controversies

Neuroscientists and Cell Biologists have known for many decades that eukaryotic cells, including neurons, are surrounded by a plasmalemma/axolemma consisting of a phospholipid bilayer that regulates trans-membrane diffusion of ions (including calcium) and other substances. Cells often incur plasmalemmal damage via traumatic injury and various diseases. If the damaged plasmalemma is not rapidly repaired within minutes, activation of apoptotic pathways by calcium influx often results in cell death. We review publications reporting what is less-well known (and not yet covered in neuroscience or cell biology textbooks): that calcium influx at the lesion sites ranging from small nm-sized holes to complete axonal transection activates parallel biochemical pathways that induce vesicles/membrane-bound structures to migrate and interact to restore original barrier properties and eventual reestablishment of the plasmalemma. We assess the reliability of, and problems with, various measures (e.g., membrane voltage, input resistance, current flow, tracer dyes, confocal microscopy, transmission and scanning electron microscopy) used individually and in combination to assess plasmalemmal sealing in various cell types (e.g., invertebrate giant axons, oocytes, hippocampal and other mammalian neurons). We identify controversies such as plug versus patch hypotheses that attempt to account for currently available data on the subcellular mechanisms of plasmalemmal repair/sealing. We describe current research gaps and potential future developments, such as much more extensive correlations of biochemical/biophysical measures with sub-cellular micromorphology. We compare and contrast naturally occurring sealing with recently-discovered artificially-induced plasmalemmal sealing by polyethylene glycol (PEG) that bypasses all natural pathways for membrane repair. We assess other recent developments such as adaptive membrane responses in neighboring cells following injury to an adjacent cell. Finally, we speculate how a better understanding of the mechanisms involved in natural and artificial plasmalemmal sealing is needed to develop better clinical treatments for muscular dystrophies, stroke and other ischemic conditions, and various cancers.

Rapid and effective fusion repair of severed digital nerves using neurorrhaphy and bioengineered solutions including polyethylene glycol: A case report

Peripheral nerve injuries (PNIs) that consist of simple nerve severance often result in severe motor impairment and permanent loss of function. Such patients face significant costs and pose major burdens to healthcare systems. Currently, the most promising surgical technique to achieve the best clinical outcome after such PNIs is immediate primary coaptation of severed nerve ends by microsutures (neurorrhaphy). However, recovery is often poor and delayed for many months due to Wallerian degeneration (WD) and slow (1-2 mm/day) axonal outgrowths from severed proximal axons that may not properly reinnervate denervated afferent/efferent targets that have atrophied. In contrast, recent pre-clinical studies using polyethylene glycol (PEG) to facilitate primary nerve repair have greatly improved the rate and extent of sensory and motor recovery and prevented much WD and muscle atrophy. That is, PEG-fused axons rapidly establish proximal-distal axoplasmic/axolemmal continuity, which do not undergo WD and maintain the structure and function of neuromuscular junction (NMJ). PEG-fused axons rapidly reinnervate denervated NMJs, thereby preventing muscle atrophy associated with monthslong denervation due to slowly regenerating axonal outgrowths. We now describe PEG-mediated fusion repair of a digital nerve in each of two patients presenting with a digital laceration resulting in total loss of sensation. The first patient's tactile perception improved markedly at 3 days postoperatively (PO). Two-point discrimination improved from greater than 10 mm at initial presentation to 4 mm at 11-week PO, and the Semmes-Weinstein monofilament score improved from greater than 6.65 to 2.83 mm, a near-normal level. The second patient had severe PO edema and scar development requiring a hand compression glove and scar massage, which began improving at 11-week PO. The sensory function then improved for 4 months PO, with both two-point discrimination and Semmes-Weinstein scores approaching near-normal levels at the final follow-up. These case study data are consistent with data from animal models. All these data suggest that PEG-fusion technologies could produce a paradigm shift from the current clinical practice of waiting days to months to repair ablation PNIs with autografts, anucleated nerve allografts, or conduits in which the patient outcome is solely dependent upon axon regeneration over months or years.

Innovations in Peripheral Nerve Injury: Current Concepts and Emerging Techniques to Improve Recovery

Despite the surgical advances in treatment of peripheral nerve injuries, consistent recovery of function is limited suggesting that a multimodal approach is required to optimize nerve regeneration. This approach should include advanced surgical repair techniques, as well as tissue engineering, cellular therapies, and application of local and systemic modulators of neuroregeneration. Further research is needed to advance these therapies from the laboratory to clinical practice, and to further understand how these treatments and techniques can act in concert to optimize functional nerve regeneration.

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.

Methylene blue enhances polyethylene glycol-fusion repair of completely severed rat sciatic nerves

Complete transection of peripheral mixed nerves immediately produces loss of sensory perception, muscle contractions and voluntary behavior mediated by the severed distal axons. In contrast to natural regeneration (~1 mm/d) of proximal axons that may eventually reinnervate denervated targets, re-innervation is restored within minutes by PEG-fusion that consists of neurorrhaphy and a sequence of well specified hypo- and isotonic calcium-free or calcium-containing solutions, the anti-oxidant methylene blue (MB) and the membrane fusogen polyethylene glycol (PEG). In this study, we examined the relative efficacy of PEG-fusion with no MB (0%), 0.5% MB, or 1% MB on the recovery of voluntary behaviors by female Sprague-Dawley rats with a complete mid-thigh severance of their sciatic nerve bathed in extracellular fluid or calcium-containing isotonic saline. The recovery of voluntary behaviors is the most relevant measure of success of any technique to repair peripheral nerve injuries. We assessed recovery by the sciatic functional index, a commonly used measure of voluntary hindlimb behaviors following complete sciatic transections. We reported that both 1% MB and 0.5% MB in sterile distilled water in our PEG-fusion protocol with neurorrhaphy significantly increased the rate and extent of behavioral recovery compared to PEG plus neurorrhaphy alone. Furthermore, 0.5% MB was as effective as 1% MB in voluntary behavioral recovery as assessed by the sciatic functional index. Since sterile 1% MB is no longer clinically available, we therefore recommend that 0.5% MB be included in upcoming human clinical trials to evaluate the safety and efficacy of PEG-fusion. All animal procedures were approved by the University of Texas Institutional Animal Care and Use Committee (AUP-2019-00225) on September 9, 2020.

Behavioral recovery and spinal motoneuron remodeling after polyethylene glycol fusion repair of singly cut and ablated sciatic nerves

Polyethylene glycol repair (PEG-fusion) of severed sciatic axons restores their axoplasmic and membrane continuity, prevents Wallerian degeneration, maintains muscle fiber innervation, and greatly improves recovery of voluntary behaviors. We examined alterations in spinal connectivity and motoneuron dendritic morphology as one potential mechanism for improved behavioral function after PEG-fusion. At 2–112 days after a single-cut or allograft PEG-fusion repair of transected or ablated sciatic nerves, the number, size, location, and morphology of motoneurons projecting to the tibialis anterior muscle were assessed by retrograde labeling. For both lesion types, labeled motoneurons were found in the appropriate original spinal segment, but also in inappropriate segments, indicating mis-pairings of proximal-distal segments of PEG-fused motor axons. Although the number and somal size of motoneurons was unaffected, dendritic distributions were altered, indicating that PEG-fusion preserves spinal motoneurons but reorganizes their connectivity. This spinal reorganization may contribute to the remarkable behavioral recovery seen after PEG-fusion repair.

Natural mechanisms and artificial PEG-induced mechanism that repair traumatic damage to the plasmalemma in eukaryotes

Eukaryotic tissues are composed of individual cells surrounded by a plasmalemma that consists of a phospholipid bilayer with hydrophobic heads that bind cell water. Bound-water creates a thermodynamic barrier that impedes the fusion of a plasmalemma with other membrane-bound intracellular structures or with the plasmalemma of adjacent cells. Plasmalemmal damage consisting of small or large holes or complete transections of a cell or axon results in calcium influx at the lesion site. Calcium activates fusogenic pathways that have been phylogenetically conserved and that lower thermodynamic barriers for fusion of membrane-bound structures. Calcium influx also activates phylogenetically conserved sealing mechanisms that mobilize the gradual accumulation and fusion of vesicles/membrane-bound structures that seal the damaged membrane. These naturally occurring sealing mechanisms for different cells vary based on the type of lesion, the type of cell, the proximity of intracellular membranous structures to the lesion and the relation to adjacent cells. The reliability of different measures to assess plasmalemmal sealing need be carefully considered for each cell type. Polyethylene glycol (PEG) bypasses calcium and naturally occurring fusogenic pathways to artificially fuse adjacent cells (PEG-fusion) or artificially seal transected axons (PEG-sealing). PEG-fusion techniques can also be used to rapidly rejoin the closely apposed, open ends of severed axons. PEG-fused axons do not (Wallerian) degenerate and PEG-fused nerve allografts are not immune-rejected, and enable behavioral recoveries not observed for any other clinical treatment. A better understanding of natural and artificial mechanisms that induce membrane fusion should provide better clinical treatment for many disorders involving plasmalemmal damage.

Polyethylene Glycol: The Future of Posttraumatic Nerve Repair? Systemic Review

Peripheral nerve injury is a common posttraumatic complication. The precise surgical repair of nerve lesion does not always guarantee satisfactory motor and sensory function recovery. Therefore, enhancement of the regeneration process is a subject of many research strategies. It is believed that polyethylene glycol (PEG) mediates axolemmal fusion, thus enabling the direct restoration of axon continuity. It also inhibits Wallerian degeneration and recovers nerve conduction. This systemic review, performed according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines, describes and summarizes published studies on PEG treatment efficiency in various nerve injury types and repair techniques. Sixteen original experimental studies in animal models and one in humans were analyzed. PEG treatment superiority was reported in almost all experiments (based on favorable electrophysiological, histological, or behavioral results). To date, only one study attempted to transfer the procedure into the clinical phase. However, some technical aspects, e.g., the maximal delay between trauma and successful treatment, await determination. PEG therapy is a promising prospect that may improve the surgical treatment of peripheral nerve injuries in the clinical practice.

Polyethylene glycol (PEG) and other bioactive solutions with neurorrhaphy for rapid and dramatic repair of peripheral nerve lesions by PEG-fusion

BACKGROUND

Nervous system injuries in mammals often involve transection or segmental loss of peripheral nerves. Such injuries result in functional (behavioral) deficits poorly restored by naturally occurring 1-2 mm/d axonal outgrowths aided by primary repair or reconstruction. "Neurorrhaphy" or nerve repair joins severed connective tissues, but not severed cytoplasmic/plasmalemmal extensions (axons) within the tissue.

NEW METHOD

PEG-fusion consists of neurorrhaphy combined with a well-defined sequence of four pharmaceutical agents in solution, one containing polyethylene glycol (PEG), applied directly to closely apposed viable ends of severed axons.

RESULTS

PEG-fusion of rat sciatic nerves: (1) restores axonal continuity across coaptation site(s) within minutes, (2) prevents Wallerian degeneration of many distal severed axons, (3) preserves neuromuscular junctions, (4) prevents target muscle atrophy, (5) produces rapid and improved recovery of voluntary behaviors compared with neurorrhaphy alone, and (6) PEG-fused allografts are not rejected, despite no tissue-matching nor immunosuppression.

COMPARISON WITH EXISTING METHODS

If PEG-fusion protocols are not correctly executed, the results are similar to that of neurorrhaphy alone: (1) axonal continuity across coaptation site(s) is not re-established, (2) Wallerian degeneration of all distal severed axons rapidly occurs, (3) neuromuscular junctions are non-functional, (4) target muscle atrophy begins within weeks, (5) recovery of voluntary behavior occurs, if ever, after months to levels well-below that observed in unoperated animals, and (6) allografts are either rejected or not well-accepted.

CONCLUSION

PEG-fusion produces rapid and dramatic recovery of function following rat peripheral nerve injuries.

Axonal fusion: An alternative and efficient mechanism of nerve repair

Injuries to the nervous system can cause lifelong morbidity due to the disconnect that occurs between nerve cells and their cellular targets. Re-establishing these lost connections is the ultimate goal of endogenous regenerative mechanisms, as well as those induced by exogenous manipulations in a laboratory or clinical setting. Reconnection between severed neuronal fibers occurs spontaneously in some invertebrate species and can be induced in mammalian systems. This process, known as axonal fusion, represents a highly efficient means of repair after injury. Recent progress has greatly enhanced our understanding of the molecular control of axonal fusion, demonstrating that the machinery required for the engulfment of apoptotic cells is repurposed to mediate the reconnection between severed axon fragments, which are subsequently merged by fusogen proteins. Here, we review our current understanding of naturally occurring axonal fusion events, as well as those being ectopically produced with the aim of achieving better clinical outcomes.

Immediate Enhancement of Nerve Function Using a Novel Axonal Fusion Device After Neurotmesis

Background

The management of peripheral nerve injuries remains a large challenge for plastic surgeons. With the inability to fuse axonal endings, results after microsurgical nerve repair have been inconsistent. Our current nerve repair strategies rely upon the slow and lengthy process of axonal regeneration (~1 mm/d). Polyethylene glycol (PEG) has been investigated as a potential axonal fusion agent; however, the percentage of axonal fusion has been inconsistent. The purpose of this study was to identify a PEG delivery device to standardize outcomes after attempted axonal fusion with PEG.

Materials and Methods

We used a rat sciatic nerve injury model in which we completely transected and repaired the left sciatic nerve to evaluate the efficacy of PEG fusion over a span of 12 weeks. In addition, we evaluated the effectiveness of a delivery device's ability to optimize results after PEG fusion.

Results

We found that PEG rapidly (within minutes) restores axonal continuity as assessed by electrophysiology, fluorescent retrograde tracer, and diffusion tensor imaging. Immunohistochemical analysis shows that motor axon counts are significantly increased at 1 week, 4 weeks, and 12 weeks postoperatively in PEG-treated animals. Furthermore, PEG restored behavioral functions up to 50% compared with animals that received the criterion standard epineurial repair (control animals).

Conclusions

The ability of PEG to rapidly restore nerve function after neurotmesis could have vast implications on the clinical management of traumatic injuries to peripheral nerves.

Polyethylene glycol solutions rapidly restore and maintain axonal continuity, neuromuscular structures, and behaviors lost after sciatic nerve transections in female rats

Complete severance of major peripheral mixed sensory-motor nerve proximally in a mammalian limb produces immediate loss of action potential conduction and voluntary behaviors mediated by the severed distal axonal segments. These severed distal segments undergo Wallerian degeneration within days. Denervated muscles atrophy within weeks. Slowly regenerating (∼1 mm/day) outgrowths from surviving proximal stumps that often nonspecifically reinnervate denervated targets produce poor, if any, restoration of lost voluntary behaviors. In contrast, in this study using completely transected female rat sciatic axons as a model system, we provide extensive morphometric, immunohistochemical, electrophysiological, and behavioral data to show that these adverse outcomes are avoided by microsuturing closely apposed axonal cut ends (neurorrhaphy) and applying a sequence of well-specified solutions, one of which contains polyethylene glycol (PEG). This "PEG-fusion" procedure within minutes reestablishes axoplasmic and axolemmal continuity and signaling by nonspecifically fusing (connecting) closely apposed open ends of severed motor and/or sensory axons at the lesion site. These PEG-fused axons continue to conduct action potentials and generate muscle action potentials and muscle twitches for months and do not undergo Wallerian degeneration. Continuously innervated muscle fibers undergo much less atrophy compared with denervated muscle fibers. Dramatic behavioral recovery to near-unoperated levels occurs within days to weeks, almost certainly by activating many central nervous system and peripheral nervous system synaptic and other plasticities, some perhaps to a greater extent than most neuroscientists would expect. Negative control transections in which neurorrhaphy and all solutions except the PEG-containing solution are applied produce none of these remarkably fortuitous outcomes observed for PEG-fusion.

The Effect of Axon Resealing on Retrograde Neuronal Death after Spinal Cord Injury in Lamprey

Failure of axon regeneration in the central nervous system (CNS) of mammals is due to both extrinsic inhibitory factors and to neuron-intrinsic factors. The importance of intrinsic factors is illustrated in the sea lamprey by the 18 pairs of large, individually identified reticulospinal (RS) neurons, whose axons are located in the same spinal cord tracts but vary greatly in their ability to regenerate after spinal cord transection (TX). The neurons that are bad regenerators also undergo very delayed apoptosis, signaled early by activation of caspases. We noticed that the neurons with a low probability of axon regeneration tend to be larger than the good regenerators. We postulate that the poorly regenerating larger neurons have larger caliber axons, which reseal more slowly, allowing more prolonged entry of toxic signals (e.g., Ca++) into the axon at the injury site. To test this hypothesis, we used a dye-exclusion assay, applying membrane-impermeable dyes to the cut ends of spinal cords at progressively longer post-TX intervals. Axons belonging to the very small neurons (not individually identified) of the medial inferior RS nucleus resealed within 15 min post-TX. Almost 75% of axons belonging to the medium-sized identified RS neurons resealed within 3 h. At this time, only 36% of the largest axons had resealed, often taking more than 24 h to exclude the dye. There was an inverse relationship between an RS neuron's size and the probability that its axon would regenerate (r = -0.92) and that the neuron would undergo delayed apoptosis, as indicated by staining with a fluorescently labeled inhibitor of caspases (FLICA; r = 0.73). The artificial acceleration of resealing with polyethylene glycol (PEG) reduced retrograde neuronal apoptosis by 69.5% at 2 weeks after spinal cord injury (SCI), suggesting that axon resealing is a critical determinant of cell survival. Ca++-free Ringer's solution with EGTA prolonged the sealing time and increased apoptotic signaling, suggesting that factors other than Ca++ diffusion into the injured tip contribute to retrograde death signaling. A longer distance of the lesion from the cell body reduced apoptotic signaling independent of the axon sealing time.

Polyethylene glycol fusion associated with antioxidants: A new promise in the treatment of traumatic facial paralysis

BACKGROUND

Recent studies in invertebrates have taught us that early cell membrane regeneration is determinant for axonal recovery and survival after trauma. Many authors obtained extraordinary results in neural regeneration using polyethylene glycol fusion protocols, which also involved microsutures and antioxidants.

METHODS

Sixty rats were evaluated with functional and histological protocol after facial nerve neurotmesis. Groups A and B had their stumps coapted with microsuture after 24 hours of neurotmesis and groups C and D after 72 hours. In addition to the microstructure, groups B and D used the polyethylene glycol-fusion protocol for the modulation of the Ca⁺² .

RESULTS

At the sixth week, the latency of group D and duration of group B was lower than groups A and C (P = .011). The axonal diameter of the groups that used polyethylene glycol-fusion was higher than those who did not use polyethylene glycol-fusion (P ≤ .001).

CONCLUSION

Although not providing a functional improvement, polyethylene glycol-fusion slowed down demyelination.

Evaluation of a Nerve Fusion Technique With Polyethylene Glycol in a Delayed Setting After Nerve Injury

PURPOSE

Polyethylene glycol (PEG) has been hypothesized to restore axonal continuity using an in vivo rat sciatic nerve injury model when nerve repair occurs within minutes after nerve injury. We hypothesized that PEG could restore axonal continuity when nerve repair was delayed.

METHODS

The left sciatic nerves of female Sprague-Dawley rats were transected and repaired in an end-to-end fashion using standard microsurgical techniques at 3 time points (1, 8, and 24 hours) after injury. Polyethylene glycol was delivered to the neurorrhaphy in the experimental group. Post-repair compound action potentials were immediately recorded after repair. Animals underwent behavioral assessments at 3 days and 1 week after surgery using the sciatic functional index test. The animals were sacrificed at 1 week to obtain axon counts.

RESULTS

The PEG-treated nerves had improved compound action potential conduction and animals treated with PEG had improved sciatic function index. Compound action potential conduction was restored in PEG-fused rats when nerves were repaired at 1, 8, and 24 hours. In the control groups, no compound action potential conduction was restored when nerves were repaired. Sciatic functional index was superior in PEG-fused rats at 3 and 7 days after surgery compared with control groups at all 3 time points of nerve repair. Distal motor and sensory axon counts were higher in the PEG-treated rats.

CONCLUSIONS

Polyethylene glycol fusion is a new adjunct for nerve repair that allows rapid restoration of axonal continuity. It effective when delayed nerve repair is performed.

CLINICAL RELEVANCE

Nerve repair with application of PEG is a potential therapy that may have efficacy in a clinical setting. It is an experimental therapy that needs more investigation as well as clinical trials.

A novel therapy to promote axonal fusion in human digital nerves

BACKGROUND

Peripheral nerve injury can have a devastating impact on our military and veteran population. Current strategies for peripheral nerve repair include techniques such as nerve tubes, nerve grafts, tissue matrices, and nerve growth guides to enhance the number of regenerating axons. Even with such advanced techniques, it takes months to regain function. In animal models, polyethylene glycol (PEG) therapy has shown to improve both physiologic and behavioral outcomes after nerve transection by fusion of a portion of the proximal axons to the distal axon stumps. The objective of this study was to show the efficacy of PEG fusion in humans and to retrospectively compare PEG fusion to standard nerve repair.

METHODS

Patients with traumatic lacerations involving digital nerves were treated with PEG after standard microsurgical neurorrhaphy. Sensory assessment after injury was performed at 1 week, 2 weeks, 1 month, and 2 months using static two-point discrimination and Semmes-Weinstein monofilament testing. The Medical Research Council Classification (MRCC) for Sensory Recovery Scale was used to evaluate the level of injury. The PEG fusion group was compared to patient-matched controls whose data were retrospectively collected.

RESULTS

Four PEG fusions were performed on four nerve transections in two patients. Polyethylene glycol therapy improves functional outcomes and speed of nerve recovery in clinical setting assessed by average MRCC score in week 1 (2.8 vs 1.0, p = 0.03). At 4 weeks, MRCC remained superior in the PEG fusion group (3.8 vs 1.3, p = 0.01). At 8 weeks, there was improvement in both groups with the PEG fusion cohort remaining statistically better (4.0 vs 1.7, p = 0.01).

CONCLUSION

Polyethylene glycol fusion is a novel therapy for peripheral nerve repair with proven effectiveness in animal models. Clinical studies are still in early stages but have had encouraging results. Polyethylene glycol fusion is a potential revolutionary therapy in peripheral nerve repair but needs further investigation.

LEVEL OF EVIDENCE

Therapeutic study, level IV.

Repair of traumatic plasmalemmal damage to neurons and other eukaryotic cells

The repair (sealing) of plasmalemmal damage, consisting of small holes to complete transections, is critical for cell survival, especially for neurons that rarely regenerate cell bodies. We first describe and evaluate different measures of cell sealing. Some measures, including morphological/ultra-structural observations, membrane potential, and input resistance, provide very ambiguous assessments of plasmalemmal sealing. In contrast, measures of ionic current flow and dye barriers can, if appropriately used, provide more accurate assessments. We describe the effects of various substances (calcium, calpains, cytoskeletal proteins, ESCRT proteins, mUNC-13, NSF, PEG) and biochemical pathways (PKA, PKC, PLC, Epac, cytosolic oxidation) on plasmalemmal sealing probability, and suggest that substances, pathways, and cellular events associated with plasmalemmal sealing have undergone a very conservative evolution. During sealing, calcium ion influx mobilizes vesicles and other membranous structures (lysosomes, mitochondria, etc.) in a continuous fashion to form a vesicular plug that gradually restricts diffusion of increasingly smaller molecules and ions over a period of seconds to minutes. Furthermore, we find no direct evidence that sealing occurs through the collapse and fusion of severed plasmalemmal leaflets, or in a single step involving the fusion of one large wound vesicle with the nearby, undamaged plasmalemma. We describe how increases in perikaryal calcium levels following axonal transection account for observations that cell body survival decreases the closer an axon is transected to the perikaryon. Finally, we speculate on relationships between plasmalemmal sealing, Wallerian degeneration, and the ability of polyethylene glycol (PEG) to seal cell membranes and rejoin severed axonal ends – an important consideration for the future treatment of trauma to peripheral nerves. A better knowledge of biochemical pathways and cytoplasmic structures involved in plasmalemmal sealing might provide insights to develop treatments for traumatic nerve injuries, stroke, muscular dystrophy, and other pathologies.

Degeneration, Trophic Interactions, and Repair of Severed Axons: A Reconsideration of Some Common Assumptions

We suggest that several interrelated properties of severed axons (degeneration, trophic dependencies, initial repair, and eventual repair) differ in important ways from commonly held assumptions about those properties. Specifically, (1) axotomy does not necessarily produce rapid degeneration of distal axonal segments because (2) the trophic maintenance of nerve axons does not necessarily depend entirely on proteins transported from the perikaryon—but instead axonal proteins can be trophically maintained by slowing their degradation and/or by acquiring new proteins via axonal synthesis or transfer from adjacent cells (e.g., glia). (3) The initial repair of severed distal or proximal segments occurs by barriers (seals) formed amid accumulations of vesicles and/or myelin delaminations induced by calcium influx at cut axonal ends—rather than by collapse and fusion of cut axolemmal leaflets. (4) The eventual repair of severed mammalian CNS axons does not necessarily have to occur by neuritic outgrowths, which slowly extend from cut proximal ends to possibly reestablish lost functions weeks to years after axotomy—but instead complete repair can be induced within minutes by polyethylene glycol to rejoin (fuse) the cut ends of surviving proximal and distal stumps. Strategies to repair CNS lesions based on fusion techniques combined with rehabilitative training and induced axonal outgrowth may soon provide therapies that can at least partially restore lost CNS functions.

Effects of extracellular calcium and surgical techniques on restoration of axonal continuity by polyethylene glycol fusion following complete cut or crush severance of rat sciatic nerves

Complete crush or cut severance of sciatic nerve axons in rats and other mammals produces immediate loss of axonal continuity. Loss of locomotor functions subserved by those axons is restored only after months, if ever, by outgrowths regenerating at ∼1 mm/day from the proximal stumps of severed axonal segments. The distal stump of a severed axon typically begins to degenerate in 1-3 days. We recently developed a polyethylene glycol (PEG) fusion technology, consisting of sequential exposure of severed axonal ends to hypotonic Ca(2+) -free saline, methylene blue, PEG in distilled water, and finally Ca(2+) -containing isotonic saline. This study examines factors that affect the PEG fusion restoration of axonal continuity within minutes, as measured by conduction of action potentials and diffusion of an intracellular fluorescent dye across the lesion site of rat sciatic nerves completely cut or crush severed in the midthigh. Also examined are factors that affect the longer-term PEG fusion restoration of lost behavioral functions within days to weeks, as measured by the sciatic functional index. We report that exposure of cut-severed axonal ends to Ca(2+) -containing saline prior to PEG fusion and stretch/tension of proximal or distal axonal segments of cut-severed axons decrease PEG fusion success. Conversely, trimming cut-severed ends in Ca(2+) -free saline just prior to PEG fusion increases PEG fusion success. PEG fusion prevents or retards the Wallerian degeneration of cut-severed axons, as assessed by measures of axon diameter and G ratio. PEG fusion may produce a paradigm shift in the treatment of peripheral nerve injuries. © 2016 Wiley Periodicals, Inc.

Local Polyethylene Glycol in Combination with Chitosan Based Hybrid Nanofiber Conduit Accelerates Transected Peripheral Nerve Regeneration

OBJECTIVE

The incapability to promptly improve behavioral function after discontinuation of peripheral nerves is a current problem in clinical practice. Effect of local polyethylene glycol in combination with chitosan-based hybrid nanofiber conduit was assessed.

STUDY DESIGN

A 10-mm sciatic nerve defect was bridged using a chitosan-based hybrid nanofiber conduit (Chitosan) filled with phosphate-buffered saline. In authograft group (AUTO), a segment of sciatic nerve was transected and reimplanted reversely. In polyethylene glycol-treated group (CHIT/PEG), the conduit was filled with polyethylene glycol solution. The regenerated fibers were studied within 12 weeks after surgery.

RESULTS

The behavioral and functional tests confirmed faster recovery of the regenerated axons in PEG-treated group compared to Chitosan group (p < .05). The mean ratios of gastrocnemius muscles weight were measured. There was statistically significant difference between the muscle weight ratios of CHIT/PEG and Chitosan groups (p < .05). Morphometric indices of regenerated fibers showed number and diameter of the myelinated fibers were significantly higher in CHIT/PEG than in Chitosan. In immuohistochemistry, the location of reactions to S-100 in CHIT/PEG was clearly more positive than Chitosan group.

CONCLUSION

polyethylene glycol solution when loaded in a chitosan-based hybrid nanofiber conduit resulted in acceleration of functional recovery and quantitative morphometric indices of sciatic nerve.

Axonal fusion via conduit-based delivery of hydrophilic polymers

BACKGROUND

Hydrophilic polymers have been shown to improve physiologic recovery following repair of transected nerves with microsutures. Our study was designed to combine hydrophilic polymer therapy with nerve tubes (NT) to enhance polymer delivery to the site of nerve injury.

METHODS

Using a rat sciatic nerve injury model, a single transection injury was repaired in an end-to-end fashion with NT + polyethylene glycol (PEG) to NT alone. Compound action potentials (CAPs) were recorded before nerve transection and after repair. Behavioral testing was performed for 5 weeks.

RESULTS

PEG therapy restored CAPS in all, but one, animals, while no CAPS were recorded in animals not receiving PEG. Behavioral nerve function was measured using the standardized functional assessment technique and foot fault asymmetry scores (FF). FF scores were improved for the PEG therapy groups on postoperative days 7, 14, and 21. However, after expected eventual axonal outgrowth, the benefit was less noticeable at days 28 and 35. Immunohistochemistry of the distal axon segments showed an increase number of sensory and motor axons in the NT + PEG group as compared to NT alone.

CONCLUSION

These data suggest that PEG delivery via a conduit may provide a simple, effective way to fuse severed axons and regain early nerve function. For proximal nerve injuries in large animals, recovery of axonal continuity could dramatically improve outcomes, even if fusion only occurs in a small percentage of axons.

The curious ability of polyethylene glycol fusion technologies to restore lost behaviors after nerve severance

Traumatic injuries to PNS and CNS axons are not uncommon. Restoration of lost behaviors following severance of mammalian peripheral nerve axons (PNAs) relies on regeneration by slow outgrowths and is typically poor or nonexistent when after ablation or injuries close to the soma. Behavioral recovery after severing spinal tract axons (STAs) is poor because STAs do not naturally regenerate. Current techniques to enhance PNA and/or STA regeneration have had limited success and do not prevent the onset of Wallerian degeneration of severed distal segments. This Review describes the use of a recently developed polyethylene glycol (PEG) fusion technology combining concepts from biochemical engineering, cell biology, and clinical microsurgery. Within minutes after microsuturing carefully trimmed cut ends and applying a well-specified sequence of solutions, PEG-fused axons exhibit morphological continuity (assessed by intra-axonal dye diffusion) and electrophysiological continuity (assessed by conduction of action potentials) across the lesion site. Wallerian degeneration of PEG-fused PNAs is greatly reduced as measured by counts of sensory and/or motor axons and maintenance of axonal diameters and neuromuscular synapses. After PEG-fusion repair, cut-severed, crush-severed, or ablated PNAs or crush-severed STAs rapidly (within days to weeks), more completely, and permanently restore PNA- or STA-mediated behaviors compared with nontreated or conventionally treated animals. PEG-fusion success is enhanced or decreased by applying antioxidants or oxidants, trimming cut ends or stretching axons, and exposure to Ca(2+) -free or Ca(2+) -containing solutions, respectively. PEG-fusion technology employs surgical techniques and chemicals already used by clinicians and has the potential to produce a paradigm shift in the treatment of traumatic injuries to PNAs and STAs.

Polyethylene glycol-fused allografts produce rapid behavioral recovery after ablation of sciatic nerve segments

Restoration of neuronal functions by outgrowths regenerating at ∼1 mm/day from the proximal stumps of severed peripheral nerves takes many weeks or months, if it occurs at all, especially after ablation of nerve segments. Distal segments of severed axons typically degenerate in 1-3 days. This study shows that Wallerian degeneration can be prevented or retarded, and lost behavioral function can be restored, following ablation of 0.5-1-cm segments of rat sciatic nerves in host animals. This is achieved by using 0.8-1.1-cm microsutured donor allografts treated with bioengineered solutions varying in ionic and polyethylene glycol (PEG) concentrations (modified PEG-fusion procedure), being careful not to stretch any portion of donor or host sciatic nerves. The data show that PEG fusion permanently restores axonal continuity within minutes, as initially assessed by action potential conduction and intracellular diffusion of dye. Behavioral functions mediated by the sciatic nerve are largely restored within 2-4 weeks, as measured by the sciatic functional index. Increased restoration of sciatic behavioral functions after ablating 0.5-1-cm segments is associated with greater numbers of viable myelinated axons within and distal to PEG-fused allografts. Many such viable myelinated axons are almost certainly spared from Wallerian degeneration by PEG fusion. PEG fusion of donor allografts may produce a paradigm shift in the treatment of peripheral nerve injuries.

Spinal cord regeneration: where fish, frogs and salamanders lead the way, can we follow?

Major trauma to the mammalian spinal cord often results in irreversible loss of function, i.e. paralysis, and current therapies ranging from drugs, implantations of stem cells and/or biomaterials, and electrically stimulated nerve regrowth, have so far offered very limited success in improving quality-of-life. However, in marked contrast with this basic shortcoming of ours, certain vertebrate species, including fish and salamanders, display the amazing ability to faithfully regenerate various complex body structures after injury or ablation, restoring full functionality, even in the case of the spinal cord. Despite the inherently strong and obvious translational potential for improving treatment strategies for human patients, our in-depth molecular-level understanding of these decidedly more advanced repair systems remains in its infancy. In the present review, we will discuss the current state of this field, focusing on recent progress in such molecular analyses using various regenerative species, and how these so far relate to the mammalian situation.

Synthesis and properties of caprolactone and ethylene glycol copolymers for neural regeneration

Copolymer networks from poly(ethylene glycol) methacrylate (PEGMA) and caprolactone 2-(methacryloyloxy) ethyl ester were synthesized and the resulting structure of the copolymer network was characterized by differential scanning calorimetry, thermogravimetry, Fourier transform infrared spectroscopy, equilibrium water gain and dynamic mechanical analysis, results which were employed to conclude about the network structure of the resulting copolymers. The new material is a random copolymer with a good miscibility and increasing hydrophilicity as the PEGMA content increases in the composition. Physical data suggest an excess free volume and synergistic interactions between the lateral chains of both comonomers. Olfactory ensheathing cells were cultured on the different networks, and cell viability and proliferation were assessed by MTS assay. The copolymers with a 30 wt% of PEGMA showed the best results compared with the other compositions in this respect, indicating the relevance for biological performance of a balance of hydrophilic and hydrophobic functionalities in the polymer chain.

Hydrophilic polymers enhance early functional outcomes after nerve autografting

BACKGROUND

Approximately 12% of operations for traumatic neuropathy are for patients with segmental nerve loss, and less than 50% of these injuries obtain meaningful functional recovery. Polyethylene glycol (PEG) therapy has been shown to improve functional outcomes after nerve severance, and we hypothesized this therapy could also benefit nerve autografting.

METHODS

We used a segmental rat sciatic nerve injury model in which we repaired a 0.5-cm defect with an autograft using microsurgery. We treated experimental animals with solutions containing methylene blue (MB) and PEG; control animals did not receive PEG. We recorded compound action potentials (CAPs) before nerve transection, after solution therapy, and at 72 h postoperatively. The animals underwent behavioral testing at 24 and 72 h postoperatively. After we euthanized the animals, we fixed the nerves, sectioned and immunostained them to allow for quantitative morphometric analysis.

RESULTS

The introduction of hydrophilic polymers greatly improved morphological and functional recovery of rat sciatic axons at 1-3 d after nerve autografting. Polyethylene glycol therapy restored CAPs in all animals, and CAPs were still present 72 h postoperatively. No CAPS were detectable in control animals. Foot Fault asymmetry scores and sciatic functional index scores were significantly improved for PEG therapy group at all time points (P < 0.05 and P < 0.001; P < 0.001 and P < 0.01). Sensory and motor axon counts were increased distally in nerves treated with PEG compared with control (P = 0.019 and P = 0.003).

CONCLUSIONS

Polyethylene glycol therapy improves early physiologic function, behavioral outcomes, and distal axonal density after nerve autografting.

Cellular mechanisms of plasmalemmal sealing and axonal repair by polyethylene glycol and methylene blue

Mammalian neurons and all other eukaryotic cells endogenously repair traumatic injury within minutes by a Ca²⁺-induced accumulation of vesicles that interact and fuse with each other and the plasmalemma to seal any openings. We have used uptake or exclusion of extracellular fluorescent dye to measure the ability of rat hippocampal B104 cells or rat sciatic nerves to repair (seal) transected neurites in vitro or transected axons ex vivo. We report that endogenous sealing in both preparations is enhanced by Ca²⁺-containing solutions and is decreased by Ca²⁺-free solutions containing antioxidants such as dithiothreitol (DTT), melatonin (MEL), methylene blue (MB), and various toxins that decrease vesicular interactions. In contrast, the fusogen polyethylene glycol (PEG) at 10-50 mM artificially seals the cut ends of B104 cells and rat sciatic axons within seconds and is not affected by Ca²⁺ or any of the substances that affect endogenous sealing. At higher concentrations, PEG decreases sealing of transected axons and disrupts the plasmalemma of intact cells. These PEG-sealing data are consistent with the hypothesis that lower concentrations of PEG directly seal a damaged plasmalemma. We have considered these and other data to devise a protocol using a well-specified series of solutions that vary in tonicity, Ca²⁺, MB, and PEG content. These protocols rapidly and consistently repair (PEG-fuse) rat sciatic axons in completely cut sciatic nerves in vivo rapidly and dramatically to restore long-lasting morphological continuity, action potential conduction, and behavioral functions.

Neurite extension of primary neurons on electrospun piezoelectric scaffolds

Neural tissue engineering may be a promising option for neural repair treatment, for which a well-designed scaffold is essential. Smart materials that can stimulate neurite extension and outgrowth have been investigated as potential scaffolding materials. A piezoelectric polymer polyvinylidene fluoride-trifluoroethylene (PVDF-TrFE) was used to fabricate electrospun aligned and random scaffolds having nano- or micron-sized fiber dimensions. The advantage of using a piezoelectric polymer is its intrinsic electrical properties. The piezoelectric characteristics of PVDF-TrFE scaffolds were shown to be enhanced by annealing. Dorsal root ganglion (DRG) neurons attached to all fibrous scaffolds. Neurites extended radially on random scaffolds, whereas aligned scaffolds directed neurite outgrowth for all fiber dimensions. Neurite extension was greatest on aligned, annealed PVDF-TrFE having micron-sized fiber dimensions in comparison with annealed and as-spun random PVDF-TrFE scaffolds. DRG on micron-sized aligned, as-spun and annealed PVDF-TrFE also had the lowest aspect ratio amongst all scaffolds, including non-piezoelectric PVDF and collagen-coated substrates. Findings from this study demonstrate the potential use of a piezoelectric fibrous scaffold for neural repair applications.

Axon repair: surgical application at a subcellular scale

Injury to the nervous system is a common occurrence after trauma. Severe cases of injury exact a tremendous personal cost and place a significant healthcare burden on society. Unlike some tissues in the body that exhibit self healing, nerve cells that are injured, particularly those in the brain and spinal cord, are incapable of regenerating circuits by themselves to restore neurological function. In recent years, researchers have begun to explore whether micro/nanoscale tools and materials can be used to address this major challenge in neuromedicine. Efforts in this area have proceeded along two lines. One is the development of new nanoscale tissue scaffold materials to act as conduits and stimulate axon regeneration. The other is the use of novel cellular-scale surgical micro/nanodevices designed to perform surgical microsplicing and the functional repair of severed axons. We discuss results generated by these two approaches and hurdles confronting both strategies.

Polyethylene glycol rapidly restores axonal integrity and improves the rate of motor behavior recovery after sciatic nerve crush injury

The inability to rapidly (within minutes to hours) improve behavioral function after severance of peripheral nervous system axons is an ongoing clinical problem. We have previously reported that polyethylene glycol (PEG) can rapidly restore axonal integrity (PEG-fusion) between proximal and distal segments of cut- and crush-severed rat axons in vitro and in vivo. We now report that PEG-fusion not only reestablishes the integrity of crush-severed rat sciatic axons as measured by the restored conduction of compound action potentials (CAPs) and the intraaxonal diffusion of fluorescent dye across the lesion site, but also produces more rapid recovery of appropriate hindlimb motor behaviors. Improvement in recovery occurred during the first few postoperative weeks for the foot fault (FF) asymmetry test and between week 2 and week 3 for the Sciatic Functional Index (SFI) based on analysis of footprints. That is, the FF test was the more sensitive indicator of early behavioral recovery, showing significant postoperative improvement of motor behavior in PEG-treated animals at 24-48 h. In contrast, the SFI more sensitively measured longer-term postoperative behavioral recovery and deficits at 4-8 wk, perhaps reflecting the development of fine (distal) motor control. These and other data show that PEG-fusion not only rapidly restores physiological and morphological axonal continuity, but also more quickly improves behavioral recovery.

Stimulation of neurite outgrowth using positively charged hydrogels

Autologous nerve grafts are currently the best option for the treatment of segmental peripheral nerve defects. However, autografts have several drawbacks including size mismatch and loss of sensation in the donor nerve's sensory distribution. In this work, we have investigated the development of a synthetic hydrogel that contains positive charge for use as a substrate for nerve cell attachment and neurite outgrowth in culture. We have demonstrated that modification of oligo-(polyethylene glycol) fumarate (OPF) with a positively charged monomer improves primary sensory rat neuron attachment and differentiation in a dose-dependent manner. Positively charged hydrogels also supported attachment of dorsal root ganglion (DRG) explants that contain sensory neurons, Schwann cells and neuronal support cells. Furthermore, charged hydrogels were analyzed for the appearance of myelinated structures in a co-culture containing DRG neurons and Schwann cells. DRGs and Schwann cells remained viable on charged hydrogels for a time period of three weeks and neurites extended from the DRGs. Sudan black staining revealed that neurites emerging from DRGs were accompanied by migrating Schwann cells. These findings suggest that charged OPF hydrogels are capable of sustaining both primary nerve cells and the neural support cells that are critical for regeneration.

On-line observation of cell growth in a three-dimensional matrix on surface-modified microelectrode arrays

Despite many successful applications of microelectrode arrays (MEAs), typical two-dimensional in-vitro cultures do not project the full scale of the cell growth environment in the three-dimensional (3D) in-vivo setting. This study aims to on-line monitor in-vitro cell growth in a 3D matrix on the surface-modified MEAs with a dynamic perfusion culture system. A 3D matrix consisting of poly(ethylene glycol) hydrogel supplemented with poly-D-lysine was subsequently synthesized in situ on the self-assembled monolayer modified MEAs. FTIR spectrum analysis revealed a peak at 2100 cm(-1) due to the degradation of the structure of the 3D matrix. After 2 wks, microscopic examination revealed that the non-degraded area was around 1500 microm(2) and provided enough space for cell growth. Fluorescence microscopy revealed that the degraded 3D matrix was non-cytotoxic allowing the growth of NIH3T3 fibroblasts and cortical neurons in vitro. Time-course changes of total impedance including resistance and reactance were recorded for 8 days to evaluate the cell growth in the 3D matrix on the MEA. A consistent trend reflecting changes of reactance and total impedance was observed. These in-vitro assays demonstrate that our 3D matrix can construct a biomimetic system for cell growth and analysis of cell surface interactions.

Effect of lead on survival, locomotion and sperm morphology of Asian earthworm, Pheretima guillelmi

To provide basic toxicity data for formulating risk characterization benchmarks, the effects of lead on survival, locomotion, and sperm morphology were investigated in the Asian earthworm Pheretima guillelmi. The LC50 of P. guillelmi for 7 and 14 d were 4285 +/- 339 mg/kg and 3207 +/- 248 mg/kg, which shows P. guillelmi can tolerate a higher concentration of lead nitrate. The average weight of the surviving earthworms decreased at concentration of 2800 mg Pb/kg soil, and the locomotor ability of earthworms exposed to a range of soil Pb concentrations showed a general decrease with increasing Pb concentrations. We also presented data depicting the sperm morphology of earthworms, which shows potential as a sensitive biomarker for measuring the effects of heavy metal on reproduction.

An in vitro protocol for recording from spinal motoneurons of adult rats

In vitro slice preparations of CNS tissue are invaluable for studying neuronal function. However, up to now, slice protocols for adult mammal spinal motoneurons--the final common pathway for motor behaviors--have been available for only limited portions of the spinal cord. In most cases, these preparations have not been productive due to the poor viability of motoneurons in vitro. This report describes and validates a new slice protocol that for the first time provides reliable intracellular recordings from lumbar motoneurons of adult rats. The key features of this protocol are: preexposure to 100% oxygen; laminectomy prior to perfusion; anesthesia with ketamine/xylazine; embedding the spinal cord in agar prior to slicing; and, most important, brief incubation of spinal cord slices in a 30% solution of polyethylene glycol to promote resealing of the many motoneuron dendrites cut during sectioning. Together, these new features produce successful recordings in 76% of the experiments and an average action potential amplitude of 76 mV. Motoneuron properties measured in this new slice preparation (i.e., voltage and current thresholds for action potential initiation, input resistance, afterhyperpolarization size and duration, and onset and offset firing rates during current ramps) are comparable to those recorded in vivo. Given the mechanical stability and precise control over the extracellular environment afforded by an in vitro preparation, this new protocol can greatly facilitate electrophysiological and pharmacological study of these uniquely important neurons and other delicate neuronal populations in adult mammals.

Melatonin enhances the in vitro and in vivo repair of severed rat sciatic axons

This study examines the effects of several experimental compounds [melatonin (MEL), cyclosporin A (CsA), glial-derived neurotrophic factor (GDNF), and methylprednisolone (MP)] on polyethylene glycol (PEG)-induced repair in vitro and/or in vivo by plasmalemmal fusion (PEG-fusion) of sciatic axons severed by crushing. As measured by conduction of compound action potentials (CAPs) through the lesion site, a significantly (p<0.025) higher percentage (75%) of crushed rat sciatic axons can be repaired in vitro by PEG-fusion following exposure to MEL compared to PEG-fusion of severed sciatic axons in control Krebs saline that contains calcium (CTL=20%). In contrast, no other experimental compound (GDNF: 45%; MP: 42%; CsA: 24%) produces a significant improvement in PEG-fusion success compared to CTL. Further, MEL produces significantly (p<0.001) larger peak CAP amplitudes conducted through the lesion site following PEG-fusion compared to CTL or any other experimental compound in vitro. Additionally, MEL significantly (p<0.025) increases the ability to PEG-fuse sciatic axons in vivo, compared to CTL. Finally, PEG-fusion success in vivo is significantly (p<0.01) greater in calcium-free CTL (CTL-Ca) compared to CTL.

Degeneration of mammalian PNS and CNS axons is accelerated by incubation with protein synthesis inhibitors

Protein synthesis inhibitors (PSIs) increase the rate of degeneration, as measured by compound action potential (CAP) conduction, in segments of rat PNS and CNS axons. Sciatic axonal segments maintained in vitro in Krebs at 37-38 degrees C generate CAPs for 24 h compared to 8 h for axons exposed to Krebs containing two PSIs, 100 microM anisomycin and/or 35 microM cycloheximide. Spinal axonal segments at 37-38 degrees C generate CAPs for 3 h compared to 2 h for axons exposed to Krebs containing PSIs. While cooling (6-9 degrees C) slows degeneration rate, cooled sciatic axons exposed to PSIs exhibit lower peak CAPs compared to cooled control segments (P<0.005).

Neural tissue engineering: strategies for repair and regeneration

Nerve regeneration is a complex biological phenomenon. In the peripheral nervous system, nerves can regenerate on their own if injuries are small. Larger injuries must be surgically treated, typically with nerve grafts harvested from elsewhere in the body. Spinal cord injury is more complicated, as there are factors in the body that inhibit repair. Unfortunately, a solution to completely repair spinal cord injury has not been found. Thus, bioengineering strategies for the peripheral nervous system are focused on alternatives to the nerve graft, whereas efforts for spinal cord injury are focused on creating a permissive environment for regeneration. Fortunately, recent advances in neuroscience, cell culture, genetic techniques, and biomaterials provide optimism for new treatments for nerve injuries. This article reviews the nervous system physiology, the factors that are critical for nerve repair, and the current approaches that are being explored to aid peripheral nerve regeneration and spinal cord repair.

Cooling enhances in vitro survival and fusion-repair of severed axons taken from the peripheral and central nervous systems of rats

Severed segments of rat peripheral (PNS; sciatic) and central nervous system (CNS; spinal) axons continue to conduct action potentials when maintained in vitro at 6-9 degrees C for up to 7 (sciatic axons) and 2 days (spinal axons), compared with only 36 h at 37-38 degrees C for sciatic axons and 6 h for spinal axons. These PNS and CNS axonal segments can be crushed and then treated with polyethylene glycol (PEG), resulting in a rapid reconnection (fusion) of the surviving axons at the crush site, as assessed by conduction of action potentials through the crush site within minutes after PEG administration. Severed PNS or CNS axons maintained in vitro at 6-9 degrees C prior to crushing can be successfully PEG-fused for up to 4 and 1.5 days, respectively, compared with only 24 (sciatic) and 3 h (spinal) at 37-38 degrees C. These data demonstrate that cooling significantly increases both the survival time of severed mammalian PNS and CNS axons and the time that severed axons can still be PEG-fused (rejoined) to rapidly re-establish axonal continuity in vitro.

A 25-year perspective of peripheral nerve surgery: evolving neuroscientific concepts and clinical significance

In spite of an enormous amount of new experimental laboratory data based on evolving neuroscientific concepts during the last 25 years peripheral nerve injuries still belong to the most challenging and difficult surgical reconstructive problems. Our understanding of biological mechanisms regulating posttraumatic nerve regeneration has increased substantially with respect to the role of neurotrophic and neurite-outgrowth promoting substances, but new molecular biological knowledge has so far gained very limited clinical applications. Techniques for clinical approximation of severed nerve ends have reached an optimal technical refinement and new concepts are needed to further increase the results from nerve repair. For bridging gaps in nerve continuity little has changed during the last 25 years. However, evolving principles for immunosuppression may open new perspectives regarding the use of nerve allografts, and various types of tissue engineering combined by bioartificial conduits may also be important. Posttraumatic functional reorganizations occurring in brain cortex are key phenomena explaining much of the inferior functional outcome following nerve repair, and increased knowledge regarding factors involved in brain plasticity may help to further improve the results. Implantation of microchips in the nervous system may provide a new interface between biology and technology and developing gene technology may introduce new possibilities in the manipulation of nerve degeneration and regeneration.