Regulation of the V-ATPase subunit ATP6V0D2 and its role in demyelination after peripheral nerve injury

After peripheral nerve injury, demyelinating Schwann cells discharge myelin debris and macrophages execute myelin degradation, leading to demyelination of degenerating axons, which is essential for efficient nerve regeneration. In this study, we show that vacuolar-type proton ATPase subunit d2 (Atp6v0d2) is among the most highly upregulated genes in degenerating mouse sciatic nerves after nerve injury using microarray analysis. ATP6V0D2 is mostly expressed in macrophages of injured nerves. Atp6v0d2 knockout mice display delayed peripheral nerve demyelination and significantly attenuated myelin lipid digestion after nerve injury. However, macrophage recruitment and Schwann cell dedifferentiation are unaffected by loss of Atp6v0d2 expression. Taken together, these data demonstrate that ATP6V0D2 in macrophages is specifically required for demyelination during Wallerian degeneration.

Very Early Involvement of Innate Immunity in Peripheral Nerve Degeneration in SOD1-G93A Mice

Recent preclinical and clinical evidence suggest that immune system has a role in the progression and prognosis of Amyotrophic Lateral Sclerosis (ALS), but the identification of a clear mechanism and immune players remains to be elucidated. Here, we have investigated, in 30 and 60 days (presymptomatic) and 120 days (symptomatic) old SOD1-G93A mice, systemic, peripheral, and central innate and adaptive immune and inflammatory response, correlating it with the progression of the neurodegeneration in neuromuscular junction, sciatic nerves, and spinal cord. Surprisingly, we found a very initial (45-60 days) presence of IgG in sciatic nerves together with a gradual enhancement of A20/TNFAIP3 (protein controlling NF-κB signalling) and a concomitantly significant increase and activation of circulating mast cells (MCs) as well as MCs and macrophages in sciatic nerve and an enhancement of IL-6 and IL-10. This immunological frame coincided with a myelin aggregation. The 30-60 days old SOD1-G93A mice didn't show real elements of neuroinflammation and neurodegeneration in spinal cord. In 120 days old mice macrophages and monocytes are widely diffused in sciatic nerves, peripheral neurodegeneration reaches the tip, high circulating levels of TNFα and IL-2 were found and spinal cord exhibits clear signs of neural damage and infiltrating immune cells. Our results underpin a clear immunological disorder at the origin of ALS axonopathy, in which MCs are involved in the initiation and sustaining of inflammatory events. These data cannot be considered a mere epiphenomenon of motor neuron degeneration and reveal new potential selective immune targets in ALS therapy.

Necroptosis is SARMful to your health

Necroptosis is a cell death pathway involved in inflammation and disease. In this issue, Ko et al. (2020. J. Cell Biol.https://doi.org/10.1083/jcb.201912047) link SARM1, the executioner of Wallerian degeneration of axons, to necroptosis, revealing a unique form of axonal disassembly likely involved in neurodegenerative disorders.

NLRP3 inflammasome is involved in nerve recovery after sciatic nerve injury

The activation of the inflammasome plays an important role in the central nervous system. However, only a few studies have investigated the effects of inflammasome activation in the peripheral nerve, especially in the sciatic nerve, and the mechanism of this activation remains elusive. Moreover, how interleukin-1 beta (IL-1β) is produced after sciatic nerve injury is also unknown. In our study, we aimed to investigate whether the nucleotide-binding oligomerization domain-like pyrin domain containing protein 3 (NLRP3) inflammasome is activated after sciatic nerve injury and to explore its role in sciatic nerve injury. The results of immunoblotting and immunofluorescence microscopy indicate that the NLRP3 inflammasome was activated after sciatic nerve injury in wild-type (WT) mice, as demonstrated by upregulated inflammasome-related components, e.g., NLRP3, procaspase-1 and ASC. Furthermore, upregulated inflammasome-related components cis-cleavage precursor IL-1β (proIL-1β) and precursor interleukin-18 (proIL-18) to IL-1β and IL-18, contributing to the inflammatory response. Consequently, the inflammatory response after sciatic nerve injury in NLRP3 knockout (NLRP3-KO) mice was less severe than that in WT mice. Moreover, NLRP3-KO mice exhibited an increased sciatic functional index (SFI), which was determined by footprint analysis, suggesting that NLRP3 deficiency is beneficial to sciatic nerve recovery after injury. Therefore, our results indicate that NLRP3 is involved in the recovery from sciatic nerve injury and mediates the production of inflammatory factors, such as IL-1β, after sciatic nerve injury.

Mechanisms of injury-induced axon degeneration

Injury-induced axon degeneration in model organisms and cell culture has emerged as an area of growing interest due to its experimental tractability and to the promise of identifying conserved mechanisms that mediate axon loss in human disease. Injury-induced axon degeneration is also observed within the well-studied process of Wallerian degeneration, a complex phenomenon triggered by axon injury to peripheral nerves in mammals. Recent studies have led to the identification of key molecular components of injury-induced axon degeneration. Axon survival factors, such as NMNAT2, act to protect injured axons from degeneration. By contrast, factors such as SARM1, MAPK, and PHR1 act to promote degeneration. The coordinated activity of these factors determines axon fate after injury. Since axon loss is an early feature of neurodegenerative diseases, it is possible that understanding the molecular mechanism of injury-induced degeneration will lead to new treatments for axon loss in neurodegenerative disease. Here, we discuss the critical pathways for injury-induced axon degeneration across species with an emphasis on their interactions in an integrated signaling network.

Rapid depletion of ESCRT protein Vps4 underlies injury-induced autophagic impediment and Wallerian degeneration

Injured axons undergo a controlled, self-destruction process, known as Wallerian degeneration. However, the underlying mechanism remains elusive. Using the Drosophila wing nerve as a model, we identify the ESCRT component Vps4 as a previously unidentified essential gene for axonal integrity. Up-regulation of Vps4 remarkably delays degeneration of injured axons. We further reveal that Vps4 is required and sufficient to promote autophagic flux in axons and mammalian cells. Moreover, using both in vitro and in vivo models, we show that the function of Vps4 in maintaining axonal autophagy and suppressing Wallerian degeneration is conserved in mammals. Last, we uncover that Vps4 protein is rapidly depleted in injured mouse axons, which may underlie the injury-induced autophagic impediment and the subsequent axonal degeneration. Together, Vps4 and ESCRT may represent a novel signal transduction mechanism in axon injury and Wallerian degeneration.

Peripheral neuropathy in monoclonal gammapathy with cryoglobulinemia and arteritis

We report a patient with IgM gammapathy, cryoglobulinemia, Raynaud's phenomena, purpura hyperglobulinemica of the legs and polyneuropathy. Endoneural vasculitis with infiltrations of eosinophilic and neutrophilic granulocytes and an extensive loss or Wallerian degeneration of myelinated nerve fibers were seen on histopathologic examination of a sural nerve biopsy specimen. The microscopic picture differed somewhat from that observed previously in cryoglobulinemic vasculitis. Although vasculitis is most often believed to represent an immunologically mediated lesion, we propose an alternate explanation, namely, that the disease manifestations in the present case were secondary to cold-induced effects of the cryoglobulin on the microcirculation.

Phagocytes of the transected feline optic nerve: an ultrastructural study

Following eye enucleation, there is a spatio-temporal pattern of degeneration along the length of the feline optic nerve, and this depends on the proximity of the segment or zone to the transected end. Thus, the rate of axonal degeneration depends on how close it is to the point of excision of the eye. In the proximal and medial segments, degeneration is rapid and the phagocytes originate from extrinsic haematogenous cells, which have invaded the optic nerve from surrounding meningeal blood vessels and capillaries. These phagocytes appear initially around blood vessels and then progressively spread to the parenchyma. In the distal segments, where Wallerian degeneration has occurred, the corresponding process is relatively slow and the phagocytes originate intrinsically. In contrast, these phagocytes are first seen in the parenchyma and then later spread to the blood vessels. Ultrastructurally, these phagocytes share fine features specifically identified with neuroglial cells such as microglia, oligodendrocytes but not astrocytes. For example, these phagocytes are rich in organelles such as mitochondria, ribosomes, microtubules, and rough and smooth endoplasmic reticulum including the Golgi apparatus. Cytoplasmic inclusions such as dense laminar bodies, lipid droplets, lysosomes and lipofuscin granules are also present. Intermediate filaments, seen only in astrocytes, are not found in these phagocytes. Although the study has established that the phagocytes are intrinsic and neuroglial in origin, their true identity is still unresolved. Secondarily, the study has shown that oligodendrocytes continue to survive and remain viable in an axon-free environment. The question about their new role in Wallerian degeneration remains.

A morphologically conserved nonapoptotic program promotes linker cell death in Caenorhabditis elegans

Apoptosis, cell death characterized by stereotypical morphological features, requires caspase proteases. Nonapoptotic, caspase-independent cell death pathways have been postulated; however, little is known about their molecular constituents or in vivo functions. Here, we show that death of the Caenorhabditis elegans linker cell during development is independent of the ced-3 caspase and all known cell death genes. The linker cell employs a cell-autonomous death program, and a previously undescribed engulfment program is required for its clearance. Dying linker cells display nonapoptotic features, including nuclear crenellation, absence of chromatin condensation, organelle swelling, and accumulation of cytoplasmic membrane-bound structures. Similar features are seen during developmental death of neurons in the vertebrate spinal cord and ciliary ganglia. Linker cell death is controlled by the microRNA let-7 and Zn-finger protein LIN-29, components of the C. elegans developmental timing pathway. We propose that the program executing linker cell death is conserved and used during vertebrate development.

Spontaneous pain, both neuropathic and inflammatory, is related to frequency of spontaneous firing in intact C-fiber nociceptors

Spontaneous pain, a poorly understood aspect of human neuropathic pain, is indicated in animals by spontaneous foot lifting (SFL). To determine whether SFL is caused by spontaneous firing in nociceptive neurons, we studied the following groups of rats: (1) untreated; (2) spinal nerve axotomy (SNA), L5 SNA 1 week earlier; (3) mSNA (modified SNA), SNA plus loose ligation of the adjacent L4 spinal nerve with inflammation-inducing chromic gut; and (4) CFA (complete Freund's adjuvant), intradermal complete Freund's adjuvant-induced hindlimb inflammation 1 and 4 d earlier. In all groups, recordings of SFL and of spontaneous activity (SA) in ipsilateral dorsal root ganglion (DRG) neurons (intracellularly) were made. Evoked pain behaviors were measured in nerve injury (SNA/mSNA) groups. Percentages of nociceptive-type C-fiber neurons (C-nociceptors) with SA increased in intact L4 but not axotomized L5 DRGs in SNA and mSNA (to 35%), and in L4/L5 DRGs 1-4 d after CFA (to 38-25%). SFL occurred in mSNA but not SNA rats. It was not correlated with mechanical allodynia, extent of L4 fiber damage [ATF3 (activation transcription factor 3) immunostaining], or percentage of L4 C-nociceptors with SA. However, L4 C-nociceptors with SA fired faster after mSNA (1.8 Hz) than SNA (0.02 Hz); estimated L4 total firing rates were approximately 5.0 and approximately 0.6 kHz, respectively. Similarly, after CFA, faster L4 C-nociceptor SA after 1 d was associated with SFL, whereas slower SA after 4 d was not. Thus, inflammation causes L4 C-nociceptor SA and SFL. Overall, SFL was related to SA rate in intact C-nociceptors. Both L5 degeneration and chromic gut cause inflammation. Therefore, both SA and SFL/spontaneous pain after nerve injury (mSNA) may result from cumulative neuroinflammation.

Genetic approaches to autonomic dysreflexia

Autonomic dysreflexia is a potentially life-threatening condition in which episodic hypertension occurs after injuries above the mid-thoracic segments of the spinal cord. Despite the seriousness of this condition, little is known of the molecular mechanisms that lead to its development. The completed sequencing of the mouse genome, its dense genetic map, and the large repository of engineered and spontaneous mouse mutants, make the mouse an ideal model organism in which to study the molecular mechanisms underlying autonomic dysreflexia. We subjected two wild-type strains of mice, 129Sv and C57BL/6, and one spontaneous mouse mutant, Wallerian degeneration slow (Wld s), to spinal cord transection and clip-compression injury. We found that the incidence of autonomic dysreflexia is greatly reduced, compared to spinal cord-transected wild-type mice, in Wld s mice after both injury paradigms and in 129Sv and C57BL/6 that have undergone the clip-compression injury. We also found that the amplitude of the dysreflexic response was greater in cord-compressed 129Sv than in C57BL/6 mice. These results implicate axonal degeneration as an important source of signals that trigger the development of autonomic dysreflexia and are discussed in the context of mouse genetics, interstrain differences and possible molecular mechanisms underlying autonomic dysreflexia after spinal cord injury.

Noxa is a critical mediator of p53-dependent motor neuron death after nerve injury in adult mouse

Axotomy-induced motor neuron death occurs within a week in the neonatal rat and mouse. However, slowly progressive motor neuron death, which takes more than a month, occurs after axotomy in the adult mouse (C57BL/6) but not in the adult rat (Wistar). Here we demonstrate that expression of a p53-inducible Bcl-2 homology domain 3 (BH3)-only protein, Noxa, is enhanced in axotomized neurons of the adult mouse but not in the adult rat. In p53-deficient mice, slowly progressive neuronal death was suppressed and accompanied by reduced Noxa expression after axotomy. However, a minor response of Noxa expression was still observed after axotomy in p53-deficient mice, suggesting that p53-independent Noxa expression occurs to a minor extent. Noxa-deficient mice were used to confirm the consequence of Noxa expression in nerve-injured mouse motor neurons. In Noxa-deficient mice, axotomy-induced motor neuron death was suppressed. Furthermore, among the BH3-only protein members examined, Noxa exhibited the most marked upregulation after axotomy in the mouse. In conclusion, motor neuron death seen in the adult mouse is mainly p53 dependent, and Noxa is a major executor for axotomy-induced motor neuron death in the adult mouse, as a mediator located downstream of p53.

Degeneration of myelinated efferent fibers prompts mitosis in Remak Schwann cells of uninjured C-fiber afferents

The factors inducing normally innervated Schwann cells in peripheral nerve to divide are poorly understood. Transection of the fourth and fifth lumbar ventral roots (L4/5 ventral rhizotomy) of the rat is highly selective, sparing unmyelinated axons and myelinated sensory axons; Wallerian degeneration is restricted to myelinated efferent fibers. We found that L4/5 ventral rhizotomy prompted many normally innervated nonmyelinating (Remak) Schwann cells to enter cell cycle; myelinating Schwann cells of intact (sensory) axons did not. Three days after L4/5 ventral rhizotomy, [3H]thymidine incorporation into Remak Schwann cells increased 30-fold. Schwann cells of degenerating efferents and endoneurial cells also incorporated label. Increased [3H]thymidine incorporation persisted at least 10 d after ventral rhizotomy. Despite Remak Schwann cell proliferation, the morphology of unmyelinated nerve (Remak) bundles was static. Seven days after L5 ventral rhizotomy, Remak Schwann cells in the L5-predominant lateral plantar nerve increased slightly; endoneurial cells doubled. Terminal deoxynucleotidyl transferase-mediated biotinylated UTP nick end labeling-positive nuclei increased dramatically in peripheral nerve after L5 ventral rhizotomy; many of these were macrophage nuclei. In summary, we find that the degeneration of myelinated motor axons produced signals that were mitogenic for nonmyelinating Schwann cells with intact axons but not for myelinating Schwann cells with intact axons.

Wallerian degeneration in the optic nerve of the rabbit

Progressive anterograde axonal degeneration is known to follow after transection of the axon from the soma, which to some extent correlates with the passage of time after the lesion. However, the minimum time required for such changes to begin remains unresolved. In this study, 20 young adult rabbits of either sex underwent experimental monocular enucleation (left eye) under general anaesthesia. Left optic nerves from such animals were treated as experimental and those from either side of non-operated animals served as controls. Animals were sacrificed postoperatively at periods ranging from 12 h to 3 months. Brains were fixed with 10% formalin and Karnovsky fixatives by an intracardiac perfusion method. Light microscopy of 8-microm paraffin sections and 0.5-microm araldite sections from the optic nerves did not reveal any changes at 12 h. At 24 h, focal minute cavities appeared across the optic nerves. Those nerves from late postoperative stages revealed such cavities with increasing dimensions, disarray of fascicular organization, fragmentation, ovoid formation and finally dissolution of the myelin sheaths. There was an appreciable increase in the number, size and aggregation of glia cells. The debris of degeneration remained prominent even 3 months after enucleation. Electron microscopy revealed splitting of myelin, intramyelinic and periaxonal oedema and occurrence of amorphous and electron-dense materials in the degenerating nerve fibres. It was concluded that while the optic nerve showed degenerative changes as early as 24 h after enucleation, debris of degeneration was only partly removed even after 3 months.

Axon Reactive B Cells Clonally Expanded in the Cerebrospinal Fluid of Patients with Multiple Sclerosis

Demyelination and axonal loss have been described as the histological hallmarks of inflammatory lesions of multiple sclerosis (MS) and are the pathological correlates of persistent disability. However, the immune mechanisms underlying axonal damage in MS remain unknown. Here, we report the use of single chain-variable domain fragments (scFv) from clonally expanded cerebrospinal fluid (CSF) B cells to show the role of an anti-axon immune response in the central nervous system (CNS) in MS. The cellular and subcellular distribution of the antigen(s) recognized by these CSF-derived clonal scFv antibodies (CSFC-scFv Abs) was studied by immunochemical staining of brain tissues obtained at autopsy from patients with MS. Immunochemistry showed specific binding of CSFC-scFv Abs to axons in acute MS lesions. The stained axons showed three major types of axonal pathological changes: 1) linear axons, axonal ovoid formation, and axonal transection were seen in the myelinated white matter adjacent to the lesion; 2) accumulation of axonal ovoid formations and Wallerian degeneration were seen at the border between demyelinated lesions and the adjacent white matter; and 3) Wallerian degeneration occurred at the center and edge of acute demyelinated lesions. These findings suggest a B cell axonal specific immune response in the CNS in MS.

Pathophysiology of peripheral nerve injury: a brief review

Clinicians caring for patients with brachial plexus and other nerve injuries must possess a clear understanding of the peripheral nervous system's response to trauma. In this article, the authors briefly review peripheral nerve injury (PNI) types, discuss the common injury classification schemes, and describe the dynamic processes of degeneration and reinnervation that characterize the PNI response.

[Neuronal regeneration and functional recovery following peripheral nervous system lesions]

INTRODUCTION

The complete traumatic sectioning of peripheral nerves start subcellular and molecular processes in the involved sensory and motor neurons that ends, in many cases, with a complete reinnervation of the sensory or muscular target. Nevertheless, the process is frequently disturbed, from a functional point of view, by the improper reinnervation of targets different from the original ones, a fact implying a partial or total lost of the involved sensory or motor function.

METHOD AND AIMS

Results obtained with several types of axotomy and of experimental anastomosis carried out with the different brainstem motor nerves are shown. The aim was to analyze the capabilities of the different brainstem centers to adapt their physiology to the functional characteristics of a new motor target, with respect to their affinity with the motor tasks carried out by the new target.

CONCLUSIONS

It is concluded that there is a gradient of functional adaptability in motoneurons to the role of new motor targets depending on their affinity in embryologic origins and functional properties. It is remarked the importance that, for a proper recovery of the lost function, have the compensatory processes started by synergistic motor systems not affected directly by the lesion.

Wallerian degeneration in the rat; a chemical study

The wet weight of the sciatic nerve of the rat increased as the nerve degenerated after nerve section. At the same time the concentration of nucleic acid increased and the concentration of total phospholipid, sphingomyelin (phosphosphingoside), and free cholesterol decreased. Cholesterol ester, absent from intact nerves, appeared during the degeneration. These findings are in agreement with those previously reported for the sciatic nerve of the cat (3, 5), with the exception that all the changes took place more rapidly in the rat than in the cat. It would thus appear that the changes previously described for the cat are general in nature and not merely characteristic of one particular species. These experiments indicate that the rat is a suitable experimental animal for the study of the effect of dietary and other factors on Wallerian degeneration.

Response of Schwann cells in the inferior alveolar nerve to distraction osteogenesis: an ultrastructural and immunohistochemical study

The biological mechanisms of nerve adaptation to distraction osteogenesis have not yet been elucidated. This study observed response of Schwann cells in the inferior alveolar nerve (IAN) following mandibular lengthening by electron microscopy and immunohistochemistry of S-100 protein, a specific marker of Schwann cells. Unilateral mandibular distraction (10mm elongation) was performed in nine young adult goats. Three animals were sacrificed at 7, 14 and 28 days after completion of distraction, respectively. The distracted IAN specimens and control nerves (from the contralateral sides) were harvested and processed for histological, ultrastructural and immunohistochemical examinations. Wallerian degeneration was observed in the distracted IAN, and Signs of axonal regeneration, as well as many activated Schwann cells were seen in the lengthened nerves. The expression of S-100 protein increased significantly at early stage of distraction osteogenesis, but almost returned to the normal level at 28 days after distraction. This study suggests that Wallerian degeneration caused by mechanical stretching may stimulate Schwann cells to enter a proliferated and activated state. Schwann cells and S-100 protein appear to play crucial roles in axonal regeneration that contributes to nerve adaptation to gradual distraction. Therefore, the IAN injury caused by mandibular gradual distraction was not serious; it seems to recover totally through a complicated repair mechanism.

Classical infantile spinal muscular atrophy with SMN deficiency causes sensory neuronopathy

OBJECTIVE

Classic infantile spinal muscular atrophy (SMA) is believed to be a purely motor disorder, affecting neurons of the spinal anterior horn and nuclei of the lower cranial nerves. Other organ malformations or peripheral nerve involvement have been regarded as exclusion criteria for infantile SMA. Whether SMN protein deficiency can also lead to loss of sensory neurons has not been systematically addressed.

METHODS

The authors evaluated the sural nerve biopsies of 19 patients with infantile SMA of varying severity. The diagnosis of SMA was confirmed by the presence of a homozygous deletion of the SMN1 gene in all patients.

RESULTS

In seven unrelated infants with SMA type I, axonal degeneration of the sural nerve was noted. Five patients showed abnormal sensory conduction, thus prompting sural nerve biopsy. Sural nerves showed different degrees of axonal loss: fiber density ranged from 3.482 to 22.076/mm2 and was markedly reduced in four patients. There was no evidence of primary demyelination: the ratio of total myelinated fiber thickness to axon diameter (g-ratio) was normal in the patients examined. In seven patients with SMA II and five patients with SMA III, no sural nerve alterations were seen, and conduction velocity was normal. In addition to SMN1 gene deletions, homozygous NAIP gene deletions were detected in six out of seven infants with peripheral neuropathy, whereas there was no evidence of a large deletion including the multicopy markers C212 and Ag1-CA in two out of three families tested.

CONCLUSIONS

In this series of patients with SMA I through III who underwent sural nerve biopsy, there was significant sensory nerve pathology in severely affected patients with SMA type I, whereas there were no sensory nerve alterations clinically or morphologically in patients with milder SMA type II or III.

Unique developmental patterns of GABAergic neurons in rat spinal cord

Gamma-aminobutyric acid (GABA)ergic neurons have been postulated to compose an important component of local circuits in the adult spinal cord, yet their identity and axonal projections have not been well defined. We have found that, during early embryonic ages (E12-E16), both glutamic acid decarboxylase 65 (GAD65) and GABA were expressed in cell bodies and growing axons, whereas at older ages (E17-P28), they were localized primarily in terminal-like structures. To determine whether these developmental changes in GAD65 and GABA were due to an intracellular shift in the distribution pattern of GAD proteins, we used a spinal cord slice model. Initial experiments demonstrated that the pattern of GABAergic neurons within organotypic cultures mimicked the expression pattern seen in embryos. Sixteen-day-old embryonic slices grown 1 day in vitro contained many GAD65- and GAD67-labeled somata, whereas those grown 4 days in vitro contained primarily terminal-like varicosities. When isolated E14-E16 slices were grown for 4 days in vitro, the width of the GAD65-labeled ventral marginal zone decreased by 40-50%, a finding that suggests these GABAergic axons originated from sources both intrinsic and extrinsic to the slices. Finally, when axonal transport was blocked in vitro, the developmental subcellular localization of GAD65 and GAD67 was reversed, so that GABAergic cell bodies were detected at all ages examined. These data indicate that an intracellular redistribution of both forms of GAD underlie the developmental changes observed in GABAergic spinal cord neurons. Taken together, our findings suggest a rapid translocation of GAD proteins from cell bodies to synaptic terminals following axonal outgrowth and synaptogenesis.

Nerve repair and grafting in the upper extremity

Knowledge of peripheral nerve anatomy, coupled with available clinical tools for examination, provides the foundation for initial evaluation. Use of this information may be related to the specific injury and a reconstructive plan formulated to maximize the potential for recovery. Realistic expectations can be presented to the patient based on information from summarized clinical experience. The field of nerve repair continues to change rapidly, and clinicians must utilize all resources available for planning, evaluation, and management. As the cellular behaviors of nerve regeneration are delineated, new avenues of research will open.

Schwann cell-enriched cultures from adult human peripheral nerve: a technique combining short enzymatic dissociation and treatment with cytosine arabinoside (Ara-C)

Attempts to design the nerve cellular prostheses have focused on the production of autologous Schwann cells expanded in vitro as the essential component in the regeneration process of injured peripheral nerves. To obtain human Schwann cells of high quality we tested a short enzymatic dissociation protocol that optimized cellular viability levels. We also assessed patterns of bromodeoxyuridine (BrdU) incorporation in both Schwann cells and fibroblasts in the presence or absence of the antimitotic Ara-C, an enrichment option for adult human Schwann cell cultures. The Ara-C treated cultures showed a significantly higher Schwann cell percentage (95%), compared with that obtained in the absence of Ara-C (70%), indicating that this antimitotic acts to eliminate fibroblasts in each one of the applied pulses (four pulses). However, we have observed that the use of this antimitotic during prolonged periods of time produced a cumulative effect causing Schwann cell cytotoxicity. Therefore, we consider that our enzymatic dissociation technique and the application of only two pulses of Ara-C to the cultures are enough to achieve enrichment of adult human Schwann cells in culture.

Rehabilitation of peripheral nerve injuries

Traumatic injuries to peripheral nerves pose complex challenges to both military and civilian physicians. Treatment of nerve injuries must consider all aspects of the inherent disability. Pain control is of paramount importance. Little will be accomplished until pain is brought down to tolerable levels. Rehabilitation needs to be instituted as first-line treatment. Focus must be first placed on protection of the affected area from complications stemming from disuse and immobility and then on enhancement of strength, flexibility, sensory discrimination, and dexterity. Early intervention sets the stage for optimal physiologic and functional recovery.

Neurosurgical management of neuropathic pain

Neuropathic pain is a very difficult problem with which the neurosurgeon frequently must deal. The neurosurgical methods to be considered are: (1) modulative, by using neurostimulation or implanted drug delivery systems, and (2) ablative, by making selective therapeutic lesions in well-defined and identified targets proven to sustain pain mechanisms (especially DREZotomy).

Neurofilamentous hypertrophy of intramedullary axonal arbors in intact spinal motoneurons undergoing peripheral sprouting

An incomplete motor nerve injury or a partial loss of motoneurons leads to a partial denervation of skeletal muscle. As part of a compensatory response, the remaining intact motoneurons undergo peripheral sprouting and increase their motor unit size. Our knowledge about the responses in the more proximal parts of these sprouting motoneurons is sparse, however. We investigated the effects of an incomplete transection of the medial gastrocnemius (MG) nerve in the adult cat on the morphology of the intramedullary axon and axon collateral systems of the remaining intact MG motoneurons. At twelve weeks following the partial transection of the MG nerve, intracellular recording and labeling techniques were used to deposit horseradish peroxidase into single intact MG motoneurons for detailed morphological studies. The light microscopic appearance and caliber of the intramedullary stem motor axons of the intact MG motoneurons were indistinguishable from controls. The number and size of the intramedullary motoraxon collateral systems were also unchanged. However, frequent and marked hypertrophy of the distal portions of the motoraxon collaterals was encountered. Electron microscopic studies of the hypertrophied collaterals demonstrated abnormal accumulations of disorganized neurofilaments arranged in bundles or whorls. The morphological changes were indistinguishable from the neurofilamentous hypertrophy that has previously been reported in Wallerian degeneration, in experimental and human motor neuron disease and in some regenerating axonal processes of spinal motoneurons. We conclude that, neurofilamentous hypertrophy of the intramedullary arbors of motor axons may also be part of a reactive and non-degenerative response in intact motoneurons undergoing compensatory peripheral sprouting.

Endogenous BDNF is required for myelination and regeneration of injured sciatic nerve in rodents

Following a peripheral nerve injury, brain-derived neurotrophic factor (BDNF) and the p75 neurotrophin receptor are upregulated in Schwann cells of the Wallerian degenerating nerves. However, it is not known whether the endogenous BDNF is critical for the functions of Schwann cells and regeneration of injured nerve. Treatment with BDNF antibody was shown to retard the length of the regenerated nerve from injury site by 24%. Histological and ultrastructural examination showed that the number and density of myelinated axons in the distal side of the lesion in the antibody-treated mice was reduced by 83%. In the BDNF antibody-treated animals, there were only distorted and disorganized myelinated fibres in the injured nerve where abnormal Schwann cells and phagocytes were present. As a result of nerve degeneration in BDNF antibody-treated animals, subcellular organelles, such as mitochondria, disappeared or were disorganized and the laminal layers of the myelin sheath were loosened, separated or collapsed. Our in situ hybridization revealed that BDNF mRNA was expressed in Schwann cells in the distal segment of lesioned nerve and in the denervated muscle fibres. These results indicate that Schwann cells and muscle fibres may contribute to the sources of BDNF during regeneration and that the deprivation of endogenous BDNF results in an impairment in regeneration and myelination of regenerating axons. It is concluded that endogenous BDNF is required for peripheral nerve regeneration and remyelination after injury.

Heme oxygenase1 (HSP-32) is induced in myelin-phagocytosing Schwann cells of injured sciatic nerves in the rat

Schwann cells participate in myelin phagocytosis in the early stage of Wallerian degeneration, prior to the recruitment of macrophages. This is the first report that Schwann cells induce heme oxygenase-1 (HO-1), a 32-kDa heat shock protein, only when they have transformed into myelin-phagocytosing cells from myelinating cells (days 2-3) immediately after crush injury of rat sciatic nerves. Double immunofluorescent labelling for HO-1 and transferrin receptors revealed that HO-1-immunoreactive Schwann cells also expressed transferrin receptors suggesting activation of iron metabolism. The transient induction of HO-1 in Schwann cells may contribute to the adaptive function in an altered environment when the cells have lost contact with axons, and may play a crucial role in the ensuing regeneration.

A Ufd2/D4Cole1e chimeric protein and overexpression of Rbp7 in the slow Wallerian degeneration (WldS) mouse

Exons of three genes were identified within the 85-kilobase tandem triplication unit of the slow Wallerian degeneration mutant mouse, C57BL/Wld(S). Ubiquitin fusion degradation protein 2 (Ufd2) and a previously undescribed gene, D4Cole1e, span the proximal and distal boundaries of the repeat unit, respectively. They have the same chromosomal orientation and form a chimeric gene when brought together at the boundaries between adjacent repeat units in Wld(S). The chimeric mRNA is abundantly expressed in the nervous system and encodes an in-frame fusion protein consisting of the N-terminal 70 amino acids of Ufd2, the C-terminal 302 amino acids of D4Cole1e, and an aspartic acid formed at the junction. Antisera raised against synthetic peptides detect the expected 43-kDa protein specifically in Wld(S) brain. This expression pattern, together with the previously established role of ubiquitination in axon degeneration, makes the chimeric gene a promising candidate for Wld. The third gene altered by the triplication, Rbp7, is a novel member of the cellular retinoid-binding protein family and is highly expressed in white adipose tissue and mammary gland. The whole gene lies within the repeat unit leading to overexpression of the normal transcript in Wld(S) mice. However, it is undetectable on Northern blots of Wld(S) brain and seems unlikely to be the Wld gene. These data reveal both a candidate gene for Wld and the potential of the Wld(S) mutant for studies of ubiquitin and retinoid metabolism.

Expression of type I and III collagen and laminin beta1 after rat sciatic nerve crush injury

Extracellular matrix changes are thought to be essential to the regeneration of peripheral nerves. The production of this matrix is believed to be regulated by interactions between axons and their supporting cells. In this study matrix production and cell proliferation were studied during rat sciatic nerve regeneration after a crush injury, and compared to that after rat sciatic nerve transection. Expression of proalpha1(I) and proalpha1(III) collagen and laminin beta1 mRNAs was followed in isolated endoneuria by Northern and in situ hybridization both proximally and distally to the site of either a crush injury or transection of rat sciatic nerve up to 18 weeks. Changes in the Schwann cell and fibroblast populations were monitored by morphometric analysis of endoneurial cross-sections immunostained for S-100 protein. The process of axonal regeneration was followed by Bielschowsky's silver staining. A crush injury initially resulted in increased expression of all mRNAs studied in the endoneurial cells. However, with progressing axonal regeneration the amount of collagen mRNAs returned to control levels, whereas the amount of laminin beta1 mRNA in the distal site of the crush remained elevated throughout the study period. The expression of type I collagen mRNA was enhanced after nerve transection injury compared to that after the crush injury. The epineurial fibroblasts actively expressed both type I and III collagen mRNAs after the injury. The proliferation of Schwann cells and the expression of collagen mRNAs are not, at least directly, related to the axonal regeneration. However, the long-lasting and strong expression of laminin beta1 mRNA after a nerve crush injury may be related to good axonal regeneration. The expression of type I collagen in the epineurium may lead to clinically well-recognized epineurial scarring and thus impede axonal regeneration.

The early changes in the axoplasm during wallerian degeneration

Axoplasmic changes were studied in the saphenous nerve of the albino rat during the early stages of Wallerian degeneration. The axons were examined at 0, 24, and 48 hours after the surgical transection of the nerve. The material was fixed in 2 per cent OsO₄ in phosphate buffer (pH 7.2–7.5) with sucrose (added to a final osmolar concentration of ∼0.37 M). The earliest changes were seen in the endoplasmic reticulum which became fragmented into rows of small vesicles. Then, between 24 and 48 hours, the neurofilaments underwent complete disintegration and the axoplasm became filled with finely granular material which later formed irregular clumps surrounded by a structureless matrix, probably fluid in vivo. The fragmentation of the neurofilaments was accompanied by pronounced swelling of the mitochondria.

The macrophage in acute neural injury: changes in cell numbers over time and levels of cytokine production in mammalian central and peripheral nervous systems

We evaluated the timing and density of ED-1-positive macrophage accumulation (ED 1 is the primary antibody for the macrophage) and measured cytokine production by macrophages in standardized compression injuries to the spinal cord and sciatic nerves of individual rats 3, 5, 10 and 21 days post-injury. The actual site of mechanical damage to the nervous tissue, and a more distant site where Wallerian degeneration had occurred, were evaluated in both the peripheral nervous system (PNS) and the central nervous system (CNS) at these time points. The initial accumulation of activated macrophages was similar at both the central and peripheral sites of damage. Subsequently, macrophage densities at all locations studied were statistically significantly higher in the spinal cord than in the sciatic nerve at every time point but one. The peak concentrations of three cytokines, tumor necrosis factor α (TNF α), interleukin-1 (IL-1) and interleukin-6 (IL-6), appeared earlier and were statistically significantly higher in injured spinal cord than in injured sciatic nerve. We discuss the meaning of these data relative to the known differences in the reparative responses of the PNS and CNS to injury.

Classification of teased nerve fibers for multicenter clinical trials

Teased nerve fibers are used widely in both clinical and experimental neuropathology, but anecdotal evidence indicates that even experienced readers find little agreement on categories for teased fiber classification. To develop a classification scheme that could be used and understood by both experienced and naive readers, specific criteria were developed for normal fibers and those exhibiting Wallerian degeneration, demyelination, hypomyelination, remyelination, and abnormal paranodal myelination. Twenty fibers teased from human sural nerve biopsies were selected as examples of one or more of these categories. Ten readers, including seven having no previous experience with teased fibers, were given a set of instructions and asked to score each fiber for all matching categories. These readers averaged high rates of true positive (56-85%) classifications, while average false positive (3-18%) rates were much lower. Among the three experienced readers, true positive agreements averaged between 75 and 100% across the fiber classifications. False positives were correspondingly low, ranging between 0 and 8%. These results suggest that it is possible to design an easily learned, meaningful scheme for classifying teased nerve fibers.

Viability of cultured nerve grafts: An assessment of proliferation of Schwann cells and fibroblasts

Previous studies demonstrated that the viability of nerve grafts had a positive effect on nerve regeneration, while the cold storage of nerve grafts obtained few viable cells at the later stage. The purpose of this study was to examine the cellular activities of Schwann cells and fibroblasts in cultured nerve grafts prior to transplantation. 2.5-cm long sciatic nerve grafts were harvested from 75 male Lewis rats. Two different media were utilized to culture the nerve grafts up to 3 weeks: Dulbecco's modified eagle medium (DMEM) only or DMEM supplemented with 2 microM forskolin and 10 microg/ml pituitary exact (mitogen medium for Schwann cells). In vivo predegenerated and normal nerve grafts were used as positive and negative controls, respectively. We employed a 5-bromo-2'-deoxyuridine (BrdU) incorporation method to evaluate the proliferating cells in the cultured nerve grafts. S-100 and vimentin immunostaining were used to estimate the presence of Schwann cells and fibroblasts in all nerve grafts at different intervals. The results showed that the proliferating cells increased progressively under culture conditions. The proliferating cells distributed evenly in small fascicles (average diameter 251 +/- 71.5 microm), whereas they appeared mainly in the margin of large fascicles (average diameter 624 +/- 87.3 microm). The mitogen medium stimulated Schwann cell multiplication more significantly in comparison with DMEM after 3 days of culture (P < 0.01), however, there were fewer fibroblasts present in the mitogen medium than in DMEM after 2 days of culture (P < 0.01). It is suggested that the viability of nerve grafts can be preserved under culture conditions. Furthermore, the cellular activity of the Schwann cells and fibroblasts in nerve grafts can be manipulated in in vitro Wallerian degeneration.

Recovery of cutaneous pain sensitivity after end-to-side nerve repair in the rat

OBJECT

The hypothesis that collaterally sprouting axons from an uninjured donor nerve may provide recovery of pain sensitivity in the skin after end-to-side nerve repair was investigated in rats. In addition, the effect of this technique on the donor nerve was examined.

METHODS

The distal stump of the transected peroneal nerve was sutured end to side to the intact sural nerve. No epineurial window or perineurial slit was made in the sural nerve at the site of coaptation. Other nerves in the leg were transected and ligated. Eighteen weeks later, the sural nerve was transected at a site distal from the coaptation site. The residual pain sensitivity in the peroneal innervation field in the instep was documented using the skin pinch test in three of 11 animals. The area of sensitivity encompassed 19 to 40% of the maximum nociceptive innervation area of the normal peroneal nerve. The nerve pinch test revealed functional sensory axons in all communicating peroneal nerves, in which 277+/-119 myelinated axons (mean +/- standard deviation) were found by histological investigation.

CONCLUSIONS

The authors conclude that at least partial recovery of sensory function due to collateral sprouting of axons after end-to-side nerve repair is possible in principle. However, the presence of functional sensory axons in the peroneal nerve stumps did not guarantee the recovery of skin sensitivity to pain in all animals. No functional or morphological evidence of an untoward effect of collateral sprouting into the end-to-side communicating nerve was detected in the axons of the donor nerve itself.

Serial determination of tumor necrosis factor-alpha content in rat sciatic nerve after chronic constriction injury

Wallerian degeneration, induced after injury to a peripheral nerve, is associated with upregulation of proinflammatory cytokines, which are suggested to contribute to the development of lesion-induced neuropathic pain. In chronic constrictive injury (CCI), an animal model of injury-induced painful mononeuropathy, inhibition of synthesis, release, or function of the cytokine tumor necrosis factor-alpha (TNF) results in reduced pain-associated behavior. Here, changes of TNF content in rat sciatic nerves after CCI (days 0, 0.5, 1, 3, 7 and 14) were investigated by enzyme-linked-immunoassay. Low levels of TNF were already detectable in control nerves. Concentrations increased rapidly after CCI, with a maximum (2.7-fold) at 12 h, and remained elevated on a lower level until day 3. Baseline levels were reached again at day 14. These results indicate that TNF is produced at an early time point in the cascade of events resulting in Wallerian degeneration and hyperalgesia following peripheral nerve injury. Given that only prophylactic treatment with TNF inhibitors efficiently reduces hyperalgesia in CCI, TNF seems to contribute to the initiation of neuropathic pain in this model.

"Pseudo-conduction block" in vasculitic neuropathy

The predominant electrophysiologic feature of vasculitic mononeuropathy multiplex is axonal loss. Electrophysiologic findings interpreted as conduction block have, however, also been reported to occur in neuropathy secondary to necrotizing vasculitis. We report 3 patients with mononeuropathy multiplex and biopsy proven vasculitis in whom eight nerves met criteria for conduction block. In each circumstance, serial study demonstrated conversion of the electrophysiologic findings to those most consistent with severe axonal loss. "Conduction block" in vasculitic mononeuropathy multiplex is secondary to focal axonal conduction failure presumably related to infarctive axonal injury. The term conduction block should be used with caution in this disorder and only if serial studies demonstrate findings consistent with this electrophysiologic diagnosis.

Transient, focal accumulation of axonal mitochondria during the early stages of wallerian degeneration

Wallerian degeneration was produced in guinea pig sciatic nerves by a crush injury. At intervals of 2, 12, 24, 36, 48, 72, and 96 hours after the crush, the nerves were fixed in osmium tetroxide, and blocks from the distal, degenerating segment identified topographically prior to embedding in Araldite or Epon. Phase and electron microscopic study of serial cross- and longitudinal sections reveals a striking, localized accumulation of axonal mitochondria which precedes or accompanies the swelling and fragmentation previously reported by others. These focal accumulations of mitochondria are transient and are most frequently observed in the paranodal axoplasm of large myelinated fibers 24 to 36 hours after crush injury, but are also occasionally identified in small myelinated fibers and unmyelinated axons. Migration and proliferation of axonal mitochondria are considered as possible explanations of these observations.

Complement depletion reduces macrophage infiltration and activation during Wallerian degeneration and axonal regeneration

After peripheral nerve injury, macrophages infiltrate the degenerating nerve and participate in the removal of myelin and axonal debris, in Schwann cell proliferation, and in axonal regeneration. In vitro studies have demonstrated the role serum complement plays in both macrophage invasion and activation during Wallerian degeneration of peripheral nerve. To determine its role in vivo, we depleted serum complement for 1 week in adult Lewis rats, using intravenously administered cobra venom factor. At 1 d after complement depletion the right sciatic nerve was crushed, and the animals were sacrificed 4 and 7 d later. Macrophage identification with ED-1 and CD11a monoclonal antibodies revealed a significant reduction in their recruitment into distal degenerating nerve in complement-depleted animals. Complement depletion also decreased macrophage activation, as indicated by their failure to become large and multivacuolated and their reduced capacity to clear myelin, which was evident at both light and electron microscopic levels. Axonal regeneration was delayed in complement-depleted animals. These findings support a role for serum complement in both the recruitment and activation of macrophages during peripheral nerve degeneration as well as a role for macrophages in promoting axonal regeneration.