Cyclosporin A retards the wallerian degeneration of peripheral mammalian axons

Polyethylene Glycol Fusion and Nerve Repair Success: Practical Applications

Peripheral nerve injuries are potentially devastating injuries leading to pain and impairment in motor and sensory functions. Since the first published use of microsurgical epineural repair of peripheral nerves in 1964, a wide variety of adjuncts have been studied. Polyethylene glycol is a fusogen that has been shown to restore axolemmal membranes. The use of polyethylene glycol in nerve injuries was first described in 1986, and animal studies have shown fusion of transected sensory and motor nerves following early application at the time of surgical repair with improved motor and sensory outcomes. Early human clinical trials have shown promising results, although more data are needed to provide specific indications and protocols. This article summarizes the background, current evidence, and future directions as well as potential applications of polyethylene glycol-mediated nerve fusion.

The effects of graft source and orientation on outcomes after ablation of a branched peripheral nerve

Segmental peripheral nerve injuries (PNI) are the most common cause of enduring nervous system dysfunction. The peripheral nervous system (PNS) has an extensive and highly branching organization. While much is known about the factors that affect regeneration through sharp bisections and linear ablations of peripheral nerves, very little has been investigated or documented about PNIs that ablate branch points. Such injuries present additional complexity compared to linear segmental defects. This study compared outcomes following ablation of a branch point with branched grafts, specifically examining how graft source and orientation of the branched graft contributed to regeneration. The model system was Lewis rats that underwent a 2.5 cm ablation that started in the sciatic nerve trunk and included the peroneal/tibial branch point. Rats received grafts that were rat sciatic autograft, inbred sciatic allograft, and inbred femoral allograft, each of which was a branched graft of 2.5 cm. Allografts were obtained from Lewis rats, which is an inbred strain. Both branches of the sciatic grafts were mixed motor and sensory while the femoral grafts were smaller in diameter than sciatic grafts and one branch of the femoral graft is sensory and the other motor. All branched grafts were sutured into the defect in two orientations dictated by which branch in the graft was sutured to the tibial vs peroneal stumps in recipients. Outcome measures include compound muscle action potentials (CMAPs) and CatWalk gait analysis throughout the recovery period, with toluidine blue for intrinsic nerve morphometry and retrograde labeling conducted at the 36-week experimental end point. Results indicate that graft source and orientation does play a significant role earlier in the regenerative process but by 36 weeks all groups showed very similar indications of regeneration across multiple outcomes.

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.

Presynaptic protein synthesis and brain plasticity: From physiology to neuropathology

To form and maintain extremely intricate and functional neural circuitry, mammalian neurons are typically endowed with highly arborized dendrites and a long axon. The synapses that link neurons to neurons or to other cells are numerous and often too remote for the cell body to make and deliver new proteins to the right place in time. Moreover, synapses undergo continuous activity-dependent changes in their number and strength, establishing the basis of neural plasticity. The innate dilemma is then how a highly complex neuron provides new proteins for its cytoplasmic periphery and individual synapses to support synaptic plasticity. Here, we review a growing body of evidence that local protein synthesis in discrete sites of the axon and presynaptic terminals plays crucial roles in synaptic plasticity, and that deregulation of this local translation system is implicated in various pathologies of the nervous system.

Polyethylene Glycol Fusion of Nerve Injuries: Review of the Technique and Clinical Applicability

Traumatic peripheral nerve injuries present a particular challenge to hand surgeons as mechanisms of nerve-healing pose serious limitations to achieving complete functional recovery. The loss of distal axonal segments through Wallerian degeneration results in the loss of neuromuscular junctions and irreversible muscle atrophy. Current methods of repair depend on the outgrowth of proximal nerve fibers following direct end-to-end repair or gap repair techniques. Investigational techniques in nerve repair using polyethylene glycol (PEG) nerve fusion have been shown to bypass Wallerian degeneration by immediately restoring nerve axonal continuity, thus resulting in a rapid and more complete functional recovery. The purpose of this article is to review the current literature surrounding this novel technique for traumatic nerve repair, paying particular attention to the underlying physiology of nerve healing and the current applications of PEG fusion in the laboratory and clinical setting. This article also serves to identify areas of future investigation to further establish validity and feasibility and encourage the translation of PEG fusion into clinical use.

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 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.

Polyethylene glycol treated allografts not tissue matched nor immunosuppressed rapidly repair sciatic nerve gaps, maintain neuromuscular functions, and restore voluntary behaviors in female rats

Many publications report that ablations of segments of peripheral nerves produce the following unfortunate results: (1) Immediate loss of sensory signaling and motor control; (2) rapid Wallerian degeneration of severed distal axons within days; (3) muscle atrophy within weeks; (4) poor behavioral (functional) recovery after many months, if ever, by slowly-regenerating (∼1mm/d) axon outgrowths from surviving proximal nerve stumps; and (5) Nerve allografts to repair gap injuries are rejected, often even if tissue matched and immunosuppressed. In contrast, using a female rat sciatic nerve model system, we report that neurorrhaphy of allografts plus a well-specified-sequence of solutions (one containing polyethylene glycol: PEG) successfully addresses each of these problems by: (a) Reestablishing axonal continuity/signaling within minutes by nonspecific ally PEG-fusing (connecting) severed motor and sensory axons across each anastomosis; (b) preventing Wallerian degeneration by maintaining many distal segments of inappropriately-reconnected, PEG-fused axons that continuously activate nerve-muscle junctions; (c) maintaining innervation of muscle fibers that undergo much less atrophy than otherwise-denervated muscle fibers; (d) inducing remarkable behavioral recovery to near-unoperated levels within days to weeks, almost certainly by CNS and PNS plasticities well-beyond what most neuroscientists currently imagine; and (e) preventing rejection of PEG-fused donor nerve allografts with no tissue matching or immunosuppression. Similar behavioral results are produced by PEG-fused autografts. All results for Negative Control allografts agree with current neuroscience data 1-5 given above. Hence, PEG-fusion of allografts for repair of ablated peripheral nerve segments expand on previous observations in single-cut injuries, provoke reconsideration of some current neuroscience dogma, and further extend the potential of PEG-fusion in clinical practice.

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.

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.

Early cyclosporin A treatment retards axonal degeneration in an experimental peripheral nerve injection injury model

Injury to peripheral nerves during injections of therapeutic agents such as penicillin G potassium is common in developing countries. It has been shown that cyclosporin A, a powerful immunosuppressive agent, can retard Wallerian degeneration after peripheral nerve crush injury. However, few studies are reported on the effects of cyclosporin A on peripheral nerve drug injection injury. This study aimed to assess the time-dependent efficacy of cyclosporine-A as an immunosuppressant therapy in an experimental rat nerve injection injury model established by penicillin G potassium injection. The rats were randomly divided into three groups based on the length of time after nerve injury induced by cyclosporine-A administration (30 minutes, 8 or 24 hours). The compound muscle action potentials were recorded pre-injury, early post-injury (within 1 hour) and 4 weeks after injury and compared statistically. Tissue samples were taken from each animal for histological analysis. Compared to the control group, a significant improvement of the compound muscle action potential amplitude value was observed only when cyclosporine-A was administered within 30 minutes of the injection injury (P < 0.05); at 8 or 24 hours after cyclosporine-A administration, compound muscle action potential amplitude was not changed compared with the control group. Thus, early immunosuppressant drug therapy may be a good alternative neuroprotective therapy option in experimental nerve injection injury induced by penicillin G potassium injection.

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.

Signaling mechanisms regulating Wallerian degeneration

Wallerian degeneration (WD) occurs after an axon is cut or crushed and entails the disintegration and clearance of the severed axon distal to the injury site. WD was initially thought to result from the passive wasting away of the distal axonal fragment, presumably because it lacked a nutrient supply from the cell body. The discovery of the slow Wallerian degeneration (Wld(s)) mutant mouse, in which distal severed axons survive intact for weeks rather than only one to two days, radically changed our thoughts on the autonomy of axon survival. Wld(s) taught us that under some conditions the axonal compartment can survive for weeks after axotomy without a cell body. The phenotypic and molecular characterization of Wld(S) and current models for Wld(S) molecular function are reviewed herein-the mechanism(s) by which Wld(S) spares severed axons remains unresolved. However, recent studies inspired by Wld(s) have led to the identification of the first 'axon death' signaling molecules whose endogenous activities promote axon destruction during WD.

WldS prevents axon degeneration through increased mitochondrial flux and enhanced mitochondrial Ca2+ buffering

Wld(S) (slow Wallerian degeneration) is a remarkable protein that can suppress Wallerian degeneration of axons and synapses, but how it exerts this effect remains unclear. Here, using Drosophila and mouse models, we identify mitochondria as a key site of action for Wld(S) neuroprotective function. Targeting the NAD(+) biosynthetic enzyme Nmnat to mitochondria was sufficient to fully phenocopy Wld(S), and Wld(S) was specifically localized to mitochondria in synaptic preparations from mouse brain. Axotomy of live wild-type axons induced a dramatic spike in axoplasmic Ca(2+) and termination of mitochondrial movement-Wld(S) potently suppressed both of these events. Surprisingly, Wld(S) also promoted increased basal mitochondrial motility in axons before injury, and genetically suppressing mitochondrial motility in vivo dramatically reduced the protective effect of Wld(S). Intriguingly, purified mitochondria from Wld(S) mice exhibited enhanced Ca(2+) buffering capacity. We propose that the enhanced Ca(2+) buffering capacity of Wld(S+) mitochondria leads to increased mitochondrial motility, suppression of axotomy-induced Ca(2+) elevation in axons, and thereby suppression of Wallerian degeneration.

Axonal degeneration is mediated by the mitochondrial permeability transition pore

Axonal degeneration is an active process that has been associated with neurodegenerative conditions triggered by mechanical, metabolic, infectious, toxic, hereditary and inflammatory stimuli. This degenerative process can cause permanent loss of function, so it represents a focus for neuroprotective strategies. Several signaling pathways are implicated in axonal degeneration, but identification of an integrative mechanism for this self-destructive process has remained elusive. Here, we show that rapid axonal degeneration triggered by distinct mechanical and toxic insults is dependent on the activation of the mitochondrial permeability transition pore (mPTP). Both pharmacological and genetic targeting of cyclophilin D, a functional component of the mPTP, protects severed axons and vincristine-treated neurons from axonal degeneration in ex vivo and in vitro mouse and rat model systems. These effects were observed in axons from both the peripheral and central nervous system. Our results suggest that the mPTP is a key effector of axonal degeneration, upon which several independent signaling pathways converge. Since axonal and synapse degeneration are increasingly considered early pathological events in neurodegeneration, our work identifies a potential target for therapeutic intervention in a wide variety of conditions that lead to loss of axons and subsequent functional impairment.

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).

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.

Effects of the immunomodulator linomide on macrophage migration and myelin phagocytic activity in peripheral nerve trauma: an experimental study

Wallerian degeneration after peripheral nerve transection leads to the phagocytosis of degenerated myelin and axon components by macrophages. These phagocytes are recruited from the systemic circulation and Wallerian degeneration may therefore be used as a model for myelin removal by hematogenous macrophages, a feature that is also a hallmark of demyelinating diseases of the central and peripheral nervous system. The immunomodulator linomide has been shown to be effective in the treatment of experimental demyelinating diseases although the exact mode of its action is not yet defined. The present study investigated the effect of linomide on monocyte invasion and myelin phagocytosis after sciatic nerve transection. Linomide had a dual effect in Wallerian degeneration. Monocyte migration from the circulation to the damaged nervous system was significantly reduced. Additionally, the myelin phagocytic capacity of macrophages was impaired, finally resulting in a significant delay in the removal of myelin. The present experiments may provide an explanation for the effects of linomide during the course of demyelinating diseases of the nervous system.

Cyclosporin A affects axons and macrophages during Wallerian degeneration

A traumatic injury of a peripheral nerve leads to Wallerian degeneration. It includes the recruitment of macrophages and the phagocytosis of myelin and the remnants of axons. We have previously studied the recruitment of macrophages and now wished to determine if the immunosuppressant cyclosporin A (CsA) affects the number of macrophages at the site of nerve injury. The primary target of CsA is T-cells, but it may also have an effect on mononuclear phagocytes which exert a key role during Wallerian degeneration. Rats were divided into two groups: CsA-treated animals and control animals. Following transection of the sciatic nerve in the treatment group, the animals received 5 mg/kg CsA subcutaneously. The groups were further subdivided into a freely regenerating nerve group and a sutured nerve group. The number of macrophages and MHC class II positive cells were counted 3 days, 7 days, 2 weeks, 4 weeks, and 8 weeks posttransection; also CD4, CD8, IL-2 receptor positive cells, B cells, and the axonal sprouting were studied. In the CsA-treated group, there were more macrophages in the distal areas under 8 weeks than in the controls (p < 0.05); thus, the clearance of macrophages is delayed in the CsA-treated rats compared to the control rats. In the proximal area, the difference in macrophage number did not gain statistical significance. Additionally, CsA retarded axonal degeneration. CsA affects number of macrophages during Wallerian degeneration, while retarding axonal degeneration and subsequent reinnervation. Its mechanism of action appears to involve either direct or indirect via T-cells-mediated responses.

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

The inability to rapidly restore the loss of function that results from severance (cutting or crushing) of PNS and CNS axons is a severe clinical problem. As a novel strategy to help alleviate this problem, we have developed in vitro procedures using Ca2+-free solutions of polyethylene glycol (PEG solutions), which within minutes induce functional and morphological continuity (PEG-induced fusion) between the cut or crushed ends of myelinated sciatic or spinal axons in rats. Using a PEG-based hydrogel that binds to connective tissue to provide mechanical strength at the lesion site and is nontoxic to nerve tissues in earthworms and mammals, we have also developed in vivo procedures that permanently maintain earthworm myelinated medial giant axons whose functional and morphological integrity has been restored by PEG-induced fusion after axonal severance. In all these in vitro or in vivo procedures, the success of PEG-induced fusion of sciatic or spinal axons and myelinated medial giant axons is measured by the restored conduction of action potentials through the lesion site, the presence of intact axonal profiles in electron micrographs taken at the lesion site, and/or the intra-axonal diffusion of fluorescent dyes across the lesion site. These and other data suggest that the application of polymeric fusiogens (such as our PEG solutions), possibly combined with a tissue adherent (such as our PEG hydrogels), could lead to in vivo treatments that rapidly and permanently repair cut or crushed axons in the PNS and CNS of adult mammals, including humans.

Viral teratogenesis: brain developmental damage associated with maturation state at time of infection

The rat brain continues to mature after birth and is particularly vulnerable to developmental damage following perinatal insult. Borna disease virus (BDV) infection of postnatal day one (PND-1) rat brain causes a non-encephalitic, persistent infection associated with developmental neuroanatomical and behavioral abnormalities. To test the hypothesis that BDV infection during different brain developmental stages yields variable pathological and clinical disease sequelae, rats were examined for BDV-induced neuroanatomical and behavioral abnormalities following inoculation with BDV on PND-15, and the findings were compared to those resulting from inoculation on PND-1. Similar to rats inoculated with BDV on PND-1, PND-15 inoculated rats developed a persistent infection associated with body weight stunting, abnormal salt taste preference and hippocampal neuron degeneration. However, unlike rats infected with BDV on PND-1, PND-15 inoculated rats did not show signs of cerebellar hypoplasia or hyperactivity. Thus, the risk of BDV-induced damage to specific brain regions, and their associated behaviors, appears, in part, dependent upon the brain's developmental stage at time of BDV-infection. These studies provide evidence of the selective vulnerability of specific neuroanatomic regions and behaviors in developing nervous system to virus-induced damage.

Nerve injury and regeneration: basic insights and therapeutic interventions

Recent observations have provided new insight into neuronal responses to axotomy, signalling of the Schwann cell switch from 'operating' to 'proliferation' mode and temporal molecular changes in the responsiveness of Schwann cells to neuronal signals, as well as into the role of macrophages in Wallerian degeneration, nerve repair and neuropathic pain. Furthermore, promising therapeutic interventions have been developed to promote axon regeneration and to attenuate axotomy-induced neuronal cell death by means of pharmacological treatment or application of neurotrophic proteins using various strategies and routes of delivery.

Traumatic brain injury causes delayed motor and cognitive impairment in a mutant mouse strain known to exhibit delayed Wallerian degeneration

Delayed Wallerian degeneration after neuronal injury is a feature of the C57BL/Wld(s) mouse mutant. In the present study, we examined the effect of unilateral controlled cortical impact (CCI) on motor and cognitive performance in C57BL/6 and C57BL/Wld(s) mice. Performance on a beam-walking task was impaired in both injured groups over the first 3 weeks; however, between 28 and 35 days post injury, C57BL/6 mice continued to improve whereas C57BL/Wld(s) mice showed increased footfaults. In a spatial learning task, C57BL/Wld(s) animals performed consistently better than C57BL/6 mice when tested 7-10 days and 14-17 days following CCI. C57BL/Wld(s) mice also demonstrated improved working memory performance as compared with C57BL/6 mice when trained on days 21-22 after injury; this effect was lost on days 23 and 24, and was not evident in other animals tested in the same task at 28-31 days following injury. These results indicate a marked delay in motor and cognitive impairment following CCI in C57BL/Wld(s) mice compared with injured C57BL/6 controls. This is consistent with previous work showing delayed temporal evolution of neuronal degeneration in C57BL/Wld(s) mice and suggests CCI may be a suitable model for examining the functional consequences of traumatic brain injury (TBI) in genetically altered mice.

Concentration-dependent effects of pentoxifylline on migration and myelin phagocytosis by macrophages

The effects of pentoxifylline (POX) on macrophage migration and myelin uptake were studied in an in vitro model of myelin phagocytosis. The POX is a phosphodiesterase inhibitor which inhibits TNF-alpha (tumor necrosis factor alpha) production and reduces ICAM-1 (intercellular adhesion molecule-1) expression by macrophages. Both of these molecules have earlier been shown to be involved in the process of myelin recognition and degradation. In the present series of experiments, cocultured peripheral nerves and macrophages were treated with different concentrations of POX. Untreated controls were massively invaded by macrophages which ingested the degenerating myelin sheaths. High concentrations of POX (100 microg ml(-1)) inhibited macrophage invasion of the nerves. Lower POX concentrations (50 microg ml(-1)), in contrast, lead to an increased myelin uptake by phagocytic cells without affecting macrophage migration. These data indicate that POX may regulate different effector functions of macrophages such as migration and myelin phagocytosis during Wallerian degeneration. This is important for inflammatory demyelinating conditions in the central or peripheral nervous system (PNS) in which macrophages are also important effector cells. Since POX is used as an immunomodulatory drug in demyelinating diseases, its effects on the described macrophage functions may be of high relevance. An increased myelin uptake during Wallerian degeneration may also support a more efficient axonal regeneration by removing axonal outgrowth inhibitors.

Delaminating myelin membranes help seal the cut ends of severed earthworm giant axons

Transected axons are often assumed to seal by collapse and fusion of the axolemmal leaflets at their cut ends. Using photomicroscopy and electronmicroscopy of fixed tissues and differential interference contrast and confocal fluorescence imaging of living tissues, we examined the proximal and distal cut ends of the pseudomyelinated medial giant axon of the earthworm, Lumbricus terrestris, at 5-60 min post-transection in physiological salines and Ca2+-free salines. In physiological salines, the axolemmal leaflets at the cut ends do not completely collapse, much less fuse, for at least 60 min post-transection. In fact, the axolemma is disrupted for 20-100 microm from the cut end at 5-60 min post-transection. However, a barrier to dye diffusion is observed when hydrophilic or styryl dyes are placed in the bath at 15-30 min post-transection. At 30-60 min post-transection, this barrier to dye diffusion near the cut end is formed amid an accumulation of some single-layered and many multilayered vesicles and other membranous material, much of which resembles delaminated pseudomyelin of the glial sheath. In Ca2+-free salines, this single and multilayered membranous material does not accumulate, and a dye diffusion barrier is not observed. These and other data are consistent with the hypothesis that plasmalemmal damage in eukaryotic cells is repaired by Ca2+-induced vesicles arising from invaginations or evaginations of membranes of various origin which form junctional contacts or fuse with each other and/or the plasmalemma.