Adult Rat Dorsal Root Ganglion Neurons Extend Neurites on Predegenerated But Not on Normal Peripheral Nerves In Vitro

Accumulation of F-spondin in injured peripheral nerve promotes the outgrowth of sensory axons

F-spondin, an extracellular matrix protein, is present in peripheral nerve during embryonic development, but its amount diminishes by birth. Axotomy of adult rat sciatic nerve, however, causes a massive upregulation of both F-spondin mRNA and protein distal to the lesion. F-spondin in the distal stump of axotomized nerve promotes neurite outgrowth of sensory neurons, as revealed by protein neutralization with F-spondin-specific antibodies. Thus, F-spondin is likely to play a role in promoting axonal regeneration after nerve injury.

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.

Restricted inflammatory reaction in the CNS: a key impediment to axonal regeneration?

Axons in the central nervous system (CNS) of adult mammals do not regenerate after injury. Mammalian CNS differs in this respect from other mammalian tissues, including the peripheral nervous system (PNS), and from the CNS of lower vertebrates. In most parts of the body, including the nervous system, injury triggers an inflammatory reaction involving macrophages. This reaction is needed for tissue healing; when it is delayed or insufficient, healing is incomplete. The CNS, although needing an efficient inflammatory reaction resembling that in the periphery for tissue healing, appears to have lost the ability to supply it. We suggest that restricted CNS recruitment and activation of macrophages are linked to regeneration failure and might reflect the immune privilege that characterizes the mammalian CNS. As macrophages play a critical role in tissue restoration, and because their recruitment and activation are among the most upstream of the events leading to tissue healing, overcoming the deficiencies in these steps might trigger a self-repair processing leading to recovery after CNS injury.

Neuronal matrix metalloproteinase-2 degrades and inactivates a neurite-inhibiting chondroitin sulfate proteoglycan

Chondroitin sulfate proteoglycans (CSPGs) are implicated in the regulation of axonal growth. We previously reported that the neurite-promoting activity of laminin is inhibited by association with a Schwann cell-derived CSPG and that endoneurial laminin may be inhibited by this CSPG as well [Zuo J, Hernandez YJ, Muir D (1998) Chondroitin sulfate proteoglycan with neurite-inhibiting activity is upregulated after peripheral nerve injury. J Neurobiol 34:41-54]. Mechanisms regulating axonal growth were studied by using an in vitro bioassay in which regenerating embryonic dorsal root ganglionic neurons (DRGn) were grown on sections of normal adult nerve. DRGn achieved slow neuritic growth on sections of normal nerve, which was reduced significantly by treatment with metalloproteinase inhibitors. Similar results were obtained on a synthetic substratum composed of laminin and inhibitory CSPG. DRGn expressed the matrix metalloproteinase, MMP-2, which was transported to the growth cone. Recombinant MMP-2 inactivated the neurite-inhibiting CSPG without hindering the neurite-promoting potential of laminin. Similarly, neuritic growth by DRGn cultured on normal nerve sections was increased markedly by first treating the nerve sections with MMP-2. The proteolytic deinhibition by MMP-2 was equivalent to and nonadditive with that achieved by chondroitinase, suggesting that both enzymes inactivated inhibitory CSPG. Additionally, the increases in neuritic growth resulting from treating nerve sections with MMP-2 or chondroitinase were blocked by anti-laminin antibodies. From these results we conclude that MMP-2 provides a mechanism for the deinhibition of laminin in the endoneurial basal lamina and may play an important role in the regeneration of peripheral nerve.

Retinal axon regeneration in the lizard Gallotia galloti in the presence of CNS myelin and oligodendrocytes

Retinal ganglion cell (RGC) axons in lizards (reptiles) were found to regenerate after optic nerve injury. To determine whether regeneration occurs because the visual pathway has growth-supporting glia cells or whether RGC axons regrow despite the presence of neurite growth-inhibitory components, the substrate properties of lizard optic nerve myelin and of oligodendrocytes were analyzed in vitro, using rat dorsal root ganglion (DRG) neurons. In addition, the response of lizard RGC axons upon contact with rat and reptilian oligodendrocytes or with myelin proteins from the mammalian central nervous system (CNS) was monitored. Lizard optic nerve myelin inhibited extension of rat DRG neurites, and lizard oligodendrocytes elicited DRG growth cone collapse. Both effects were partially reversed by antibody IN-1 against mammalian 35/250 kD neurite growth inhibitors, and IN-1 stained myelinated fiber tracts in the lizard CNS. However, lizard RGC growth cones grew freely across oligodendrocytes from the rat and the reptilian CNS. Mammalian CNS myelin proteins reconstituted into liposomes and added to elongating lizard RGC axons caused at most a transient collapse reaction. Growth cones always recovered within an hour and regrew. Thus, lizard CNS myelin and oligodendrocytes possess nonpermissive substrate properties for DRG neurons--like corresponding structures and cells in the mammalian CNS, including mammalian-like neurite growth inhibitors. Lizard RGC axons, however, appear to be far less sensitive to these inhibitory substrate components and therefore may be able to regenerate through the visual pathway despite the presence of myelin and oligodendrocytes that block growth of DRG neurites.

Selective inhibition of early axonal regeneration by myelin-associated glycoprotein

When the distal stump of a transected peripheral nerve is brought into the vicinity of the proximal nerve stump, the regenerating axons advance toward it across the gap. Similar results are obtained when a predegenerated nerve segment is used. However, when a nerve segment subjected to proximal axotomy 7 days earlier (7-day nerve segment) was placed close to the proximal end of a freshly cut nerve at a distance of less than 1.5 mm, there were neither regenerating axons nor sprouts. The same inhibition of axonal regeneration was also exhibited when a nerve segment subjected to axotomy 9 to 14 days earlier was used. To examine the inhibitory effect of the nerve segments on established regenerating axons, we positioned a 7-day nerve segment in close apposition to a proximal nerve end at 2 or 3 days after transection. The growth of the 3-day-old regenerating axons, already ensheathed by Schwann cells, was not disturbed, but the 2-day-old regenerating axons, consisting of naked axons, were eliminated by the 7-day nerve segment. It is assumed that the findings reflect a mechanism serving to eliminate abundant sprouts and immature axons, probably conferring optimum regeneration and maturation of outgrowing pioneer axons. The inhibitory effect on abundant sprouts and immature axons was completely blocked by local application of antibodies to myelin-associated glycoprotein (MAG). The MAG-containing cells appeared at 6 to 12 days after axotomy.

Use of explant cultures of peripheral nerves of adult vertebrates to study axonal regeneration in vitro

Explanted preparations of peripheral nerves with attached dorsal root ganglia of adult mammals and amphibia survive for several days in serum-free medium and can be used to study axonal regeneration in vitro. This review outlines the methods which we routinely use and how they may be applied to study different aspects of axonal regeneration. When the peripheral nerves are crushed in vitro, axons regenerate through the crush site into the distal stump within 1 day (mouse) or 3 days (frog). The outgrowth distance of the leading sensory axons can be determined with the use of a simple method based on axonal transport of labelled proteins. A compartmentalised system permits selective application of drugs and other agents to either ganglia or peripheral nerve containing the regenerating axons and has been used to study selected aspects of regeneration including influence of non-neuronal cells, retrograde signalling, axonal release of proteins during regeneration and the role of phospholipase A2 activity. Explanted preparations may also be cultured in a layer of extracellular matrix material (matrigel), in which spontaneous outgrowth of a large number of naked axons from the cut ends of nerves starts within 1 day and continues for several days. This provides an opportunity to study the direct effects of different agents on axonal elongation. Preparations cultured in collagen gels show sparse spontaneous axonal growth, but this can be increased by addition of certain growth factors. The phenotype of the regenerating axons can be studied using immunohistochemical methods.

Changes in expression and localization of hemopexin and its transcripts in injured nervous system: a comparison of central and peripheral tissues

The recent demonstration of hemopexin synthesis in the adult rat sciatic nerve and its accumulation after injury has raised the question of the possible role of this acute phase protein during the process of nerve repair. To gain insight into its function, we have compared the distribution of both hemopexin and its messenger RNA in the peripheral and the central nervous systems. We find that hemopexin is present in all types of peripheral nerves and ganglia, confined to the extracellular matrix and basement membranes of the endoneurium, blood vessels and connective tissues. After injury, hemopexin messenger RNA is overexpressed by Schwann cells, fibroblasts and invading macrophages. The content in hemopexin protein increases in all nerves studied, without changes in localization. Therefore, hemopexin does not appear to be associated with the fate of myelin or with the regeneration of a particular type of nerve fibre. In the central nervous system, hemopexin messenger RNA cannot be detected and the protein is only found in basement membranes of the vascular system (capillaries, meninges and choroid plexus). Furthermore, hemopexin and its messenger RNA remain absent from the distal part of the injured optic nerves. Our results further support the idea that hemopexin plays specific roles during nerve repair, and that it may be associated with the endoneurial extracellular matrix.

Effects of macrophage transplantation in the injured adult rat spinal cord: a combined immunocytochemical and biochemical study

Early and robust invasion by macrophages may be one of the reasons why axonal regeneration is more effective in the PNS than in the CNS. Therefore, we have grafted autologous peritoneal macrophages labeled with fluorescent latex microspheres into spinal cord compression lesions. At various survival times, we have studied their effect on the expression of neuronal (neurofilaments [NF], calcitonin gene-related peptide [CGRP], 5-hydroxytryptamine [5-HT]) and nonneuronal markers (myelin-associated glycoprotein [MAG], glial fibrillary acidic protein [GFAP], laminin) by using semiquantitative Western blot and immunohistochemical techniques. After 1 month, we observed a significant decrease of the expression of MAG as well as an important invasion of the lesion site by neurites, chiefly peptidergic axons of presumed dorsal root origin, in macrophage-grafted animals compared with controls. In addition, angiogenesis and Schwann cell infiltration were more pronounced after macrophage grafts, providing an increase in laminin, a favorable substrate for axonal regrowth. By using reverse transcription-polymerase chain reaction (RT-PCR), mRNAs for tumor necrosis factor-alpha (TNF-alpha) were detected in the transplanted cells, whereas results were negative for nerve growth factor (NGF), neurotrophin-3 (NT-3), brain-derived neurotrophic factor (BDNF), or acidic fibroblast growth factor (aFGF) and basic fibroblast growth factor (bFGF). Thus, macrophage grafts may represent an interesting strategy to promote axonal regeneration in the CNS. Our study suggests that they may exert their beneficial effects by degrading myelin products, which inhibit axonal regrowth, and by promoting a permissive extracellular matrix containing notably laminin. No evidence for a direct synthesis of neurotrophic factors by the transplanted macrophages was found in this study, but resident glial cells could secrete such factors as a result of stimulation by macrophage-released cytokines.

Comparison of neurite outgrowth induced by intact and injured sciatic nerves: a confocal and functional analysis

Mechanisms regulating axon growth in the peripheral nervous system have been studied by means of an in vitro bioassay, the tissue section culture, in which regenerating neurons are grown on substrata made up of tissue sections. Sections from intact and degenerated sciatic nerves proved to be different in their ability to support neurite outgrowth of embryonic chick sensory neurons from both qualitative and quantitative points of view. On denervated nerve sections, the total length of neurites elaborated per neuron was almost twice that found on intact nerve sections. In addition, confocal microscopy revealed a striking difference between intact and denervated nerve substrata: on denervated nerve sections, neurites grew inside the internal structures of endoneurial Schwann cell tubes, within the underlying tissue sections, whereas on intact nerve sections neurites extended along endoneurial basal laminae but never entered Schwann cell tubes. Perturbation experiments were used to analyze some of the molecular determinants that control neurite outgrowth in this system. Antibodies directed against the beta1-integrin subunit inhibited neurite extension on both normal and degenerated rat sciatic nerve tissue. Strikingly, however, differential inhibition was observed using antibodies directed against extracellular matrix molecules. Anti-laminin-2 (merosin) antibodies drastically reduced both the percentage of growing neurons and the total length of neurites on denervated nerve sections, but they did not modify these parameters on sections of normal nerve. Taken together, these results suggest that laminin-2/merosin promotes neurite outgrowth in peripheral nerve environments but only after Wallerian degeneration, which is when axons are allowed to extend within endoneurial tubes.

Effects of extracellular matrix components on axonal outgrowth from peripheral nerves of adult animals in vitro

Relatively little is known of the growth requirements for regenerating axons of the peripheral nervous system of adult animals. In the present study, we show that extracellular matrix material secreted by the Engelbreth-Holm-Swarm tumor cell line (matrigel) supports axonal growth from explanted peripheral nerve-dorsal root ganglia (DRG) preparations of adult mice and amphibia in serum-free media, without addition of growth factors. Axonal growth in matrigel was much more profuse than that in the more commonly used gels of type 1 collagen and, after some days in culture, was accompanied by migration of Schwann cells along axons. The most abundant protein in matrigel is laminin, which has been shown in many studies to support axonal growth but, surprisingly, antisera to laminin did not inhibit axonal growth in matrigel. To determine the ability of the major components of matrigel, laminin, type IV collagen, and heparan sulfate proteoglycan (HSPG), to support axonal growth, these proteins were added to preparations of mouse peripheral nerve-DRGs in type I collagen gels. Regenerating axons were significantly longer in the presence of laminin and type IV collagen than in control cultures, while HSPG had a slight inhibitory effect. In this assay system, however, diluted matrigel solution was even more effective in stimulating axonal growth than laminin or type IV collagen, either alone or in combination. The results suggest that in addition to laminin and type IV collagen, other components within matrigel may contribute to its ability to support axonal growth.

Differential regulation of mRNAs for GDNF and its receptors Ret and GDNFR alpha after sciatic nerve lesion in the mouse

Glial cell line-derived neurotrophic factor (GDNF), first characterized for its effect on dopamine uptake in central dopaminergic neurons, appears to be a powerful neurotrophic factor for motor neurons. GDNF has recently been shown to signal through a multisubunit receptor. This receptor is composed of a ligand-binding subunit, called GDNF receptor alpha (GDNFR alpha), and a signalling tyrosine kinase subunit, Ret. To gain further insight into GDNF function, we investigated the expression of GDNF and its receptors after nerve lesion in adult mice. Analysis of expression in muscle, nerve and spinal cord by RNase protection assay and in situ hydridization revealed that, in adult non-lesioned mice, GDNF mRNA was expressed in the nerve and GDNFR alpha mRNA in the nerve and the spinal cord, while the expression of Ret was restricted to spinal cord motor neurons. After a sciatic nerve crush a rapid increase in GDNF mRNA was observed in the distal part of the nerve and a delayed elevation in the muscle, while GDNFR alpha mRNA was up-regulated in the distal part of the sciatic nerve but not in proximal nerve or spinal cord. The lesion also induced a rapid increase in Ret mRNA expression, but the increase was observed only in spinal cord motor neurons and in dorsal root ganglion neurons. A pattern of expression of GDNF and its receptors similar to that seen after lesion in the adult was detected during embryonic development. Administration of GDNF enhanced sciatic nerve regeneration measured by the nerve pinch test. Taken together, these results suggest that GDNF has an important role during regeneration after nerve damage in the adult.

The cellular and molecular basis of peripheral nerve regeneration

Functional recovery from peripheral nerve injury and repair depends on a multitude of factors, both intrinsic and extrinsic to neurons. Neuronal survival after axotomy is a prerequisite for regeneration and is facilitated by an array of trophic factors from multiple sources, including neurotrophins, neuropoietic cytokines, insulin-like growth factors (IGFs), and glial-cell-line-derived neurotrophic factors (GDNFs). Axotomized neurons must switch from a transmitting mode to a growth mode and express growth-associated proteins, such as GAP-43, tubulin, and actin, as well as an array of novel neuropeptides and cytokines, all of which have the potential to promote axonal regeneration. Axonal sprouts must reach the distal nerve stump at a time when its growth support is optimal. Schwann cells in the distal stump undergo proliferation and phenotypical changes to prepare the local environment to be favorable for axonal regeneration. Schwann cells play an indispensable role in promoting regeneration by increasing their synthesis of surface cell adhesion molecules (CAMs), such as N-CAM, Ng-CAM/L1, N-cadherin, and L2/HNK-1, by elaborating basement membrane that contains many extracellular matrix proteins, such as laminin, fibronectin, and tenascin, and by producing many neurotrophic factors and their receptors. However, the growth support provided by the distal nerve stump and the capacity of the axotomized neurons to regenerate axons may not be sustained indefinitely. Axonal regenerations may be facilitated by new strategies that enhance the growth potential of neurons and optimize the growth support of the distal nerve stump in combination with prompt nerve repair.

Change in the molecular phenotype of Schwann cells upon transplantation into the central nervous system: down-regulation of c-jun

Activated Schwann cells such as those in the distal stump of a cut peripheral nerve, or those cultured in vitro, develop a molecular phenotype very different from that of quiescent Schwann cells, and express high levels of the transcription factor c-jun. We studied the expression of c-jun messenger RNA, by in situ hybridization, and Jun-like immunoreactivity of Schwann cells in segments of peripheral nerve, or in cell suspensions grafted into the adult rat brain. Schwann cells rapidly lost their Jun immuno-positivity, and down-regulated expression of c-jun messenger RNA once implanted into the brain, and only the Schwann cells contained in the portion of peripheral nerve which remained outside the brain maintained Jun-like immunopositivity. c-jun messenger RNA was also down-regulated in the grafts, but more slowly than the protein; however, a proximodistal gradient in the level of expression of c-jun messenger RNA along the graft, comparable to that found for Jun immunoreactivity, was not detected. Schwann cells transplanted into the lesioned central nervous system promote regeneration of some injured central nervous system axons, but this regenerative response is always much more limited than peripheral nervous system regeneration. We suggest a correlation between the limited regeneration of central nervous system axons into peripheral nerve grafts and the loss of c-jun expression in Schwann cells following exposure to the central nervous system environment.

Disruption of the gene for the myelin-associated glycoprotein improves axonal regrowth along myelin in C57BL/Wlds mice

The myelin-associated glycoprotein (MAG) has been shown to be inhibitory for certain neurons in vitro (Mukhopadhyay et al., 1994; McKerracher et al., 1994). To investigate whether MAG is an inhibitory component in peripheral myelin in vivo, MAG-deficient mutant mice were cross-bred with C57BL/Wlds mice that have delayed lesion-induced myelin degeneration and axon regrowth. While in crushed nerves of C57BL/Wlds mice expressing MAG, only 16% of myelin sheaths were associated with regrowing axons, this number was doubled in MAG-deficient C57BL/Wlds mice. These observations suggest that the absence of MAG may contribute to the improved axonal regrowth in the double mutants. Therefore, degeneration of MAG-containing myelin might be an important prerequisite to optimize axonal regrowth after peripheral nerve injury.

Axonal regeneration

Axons damaged in a peripheral nerve are often able to regenerate from the site of injury along the degenerate distal segment of the nerve to reform functional synapses. Schwann cells play a central role in this process. However, in the adult mammalian central nervous system, from which Schwann cells are absent, axonal regeneration does not progress to allow functional recovery. This is due to inhibitors of axonal growth produced by both oligodendrocytes and astrocytes and also to the decreased ability of adult neurons to extend axons during regeneration compared to embryonic neurons during development. However once provided with a substrate conducive to axonal growth, such as a peripheral nerve graft, many central neurons are able to regenerate axons over long distances. Over the past year this response has been utilised in experimental models to produce a degree of behavioural recovery.

Myelination and axonal regeneration in the central nervous system of mice deficient in the myelin-associated glycoprotein

The myelin-associated glycoprotein, a member of the immunoglobulin superfamily, has been implicated in the formation and maintenance of myelin sheaths. In addition, recent studies have demonstrated that myelin-associated glycoprotein is inhibitory for neurite elongation in vitro and it has therefore been suggested that myelin-associated glycoprotein prevents axonal regeneration in lesioned nervous tissue. The generation of mice deficient in the expression of myelin-associated glycoprotein by targeted disruption of the mag gene via homologous recombination in embryonic stem cells has allowed the study of the functional role of this molecule in vivo. This review summarizes experiments aimed at answering the following questions: (i) is myelin-associated glycoprotein involved in the formation and maintenance of myelin in the CNS? and (ii) does myelin-associated glycoprotein restrict axonal regeneration in the adult mammalian CNS? Analysis of optic nerves from mutant mice revealed a delay in myelination when compared to optic nerves of wild-type animals, a lack of a periaxonal cytoplasmic collar from most myelin sheaths, and the presence of some doubly and multiply myelinated axons. Axonal regeneration in the CNS of adult myelin-associated glycoprotein deficient mice was not improved when compared to wild-type animals. These observations indicate that myelin-associated glycoprotein is functionally involved in the recognition of axons by oligodendrocytes and in the morphological maturation of myelin sheaths. However, results do not support a role of myelin-associated glycoprotein as a potent inhibitor of axonal regeneration in the adult mammalian CNS.

Sprouting and abnormal contacts of nonmedullated axons, and deposition of extracellular material induced by the amyloid precursor protein (APP) and other protease inhibitors

We have reported that the local administration of serine protease inhibitors (amyloid precursor protein with the Kunitz insert (APP K+), aprotinin, and leupeptin) to the rat sciatic nerve determines a sprouting response of myelinated axons, proliferation of Schwann cells, and demyelination, 5 to 7 days later. Further study of these nerves with the electron microscope revealed (i) a sprouting response of nonmedullated axons, (ii) the appearance of fine axons with a few turns of compact myclin, (iii) abnormal contracts of axons with basal laminae, with fibroblast-like cells, and between them, (iv) the occurrence of hemidesmosome- and desmosome-like junctions between Schwann cell processes, and between Schwann cells and axons, and (v) the appearance of amorphous and fibrillary extracellular deposits alongside the axolemma. The adjacent proximal and distal segments were normal, i.e., axons remained continuous, and the alterations were confined to the segment exposed to the protease inhibitors. Heated APP Kappa +, APP without the Kunitz insert (APP K-), bovine serum albumin, and saline, did not elicit cytological alterations. Our results suggest that these inhibitors of serine proteases (i) set free a sprouting drive of axons by disrupting an ongoing repressive mechanism: (ii) modify the adhesive properties of axons and Schwann cells, and (iii) alter the natural history of an extracellular material. The imbalance of an extracellular protease system may participate in the pathogenesis of Alzheimer's disease.

Use of chimeric F3-NCAM molecules to explore the properties of VASE exon in modulating polysialylation and neurite outgrowth

Differential splicing of VASE exon in the fourth immunoglobulin (Ig) domain and attachment to the fifth Ig domain of alpha 2-8 linked sialic acid (PSA) both dramatically change, in opposite manner, Neural Cell Adhesion Molecule (NCAM) functional properties. Reciprocal patterns of VASE and PSA expression suggest that they might be mutually exclusive. Here, we tested whether informations conferring polysialylation reside in NCAM-Ig domains 4 and 5 and the influence of the VASE exon encoded sequence on this process. We also examined if the VASE sequence was still able to inhibit neurite outgrowth when presented out of its normal NCAM context. Constructs have been prepared encoding NCAM-Ig domains 4 (with or without the VASE exon) and 5 fused to the F3 molecule. Stable clones expressing the chimeric molecules or wild type F3 were then obtained in the AtT-20 cell line. Although the chimeric molecules were expressed on the cell surface none of them was bearing PSA. Thus, polysialylation cannot be conferred to proteins by addition of the NCAM-Ig domains 4 and 5 modular motif and in this molecular context, the VASE sequence is not influencing the process. These chimeric molecules, either expressed at the surface of RIN or COS cells or presented as soluble forms, were examined for their effect on neurite outgrowth. In all cases, the length of neurites of sensory neurons was significantly reduced when grown in presence of the VASE containing chimera by comparison with the chimera without VASE or wild type F3. When neurons from NCAM knock-out mice were used for the assay, the VASE inhibition could not be detected. Thus VASE is able to act as a modular motif and NCAM expressed on neurons participates in transducing its effect.

Preferential growth of neonatal rat dorsal root ganglion cells on homotypic peripheral nerve substrates in vitro

Developing sensory neurons interact with molecular signals in the local environment to generate stereotypic nerve pathways. Regenerating neurons seem to lose the ability to reinnervate their original sites in the targets, resulting in abnormal sensory input and consequent clinical pathophysiology. The specificity of reinnervation of peripheral targets by regenerating axons is thus crucial for normal recovery of function. In this study, we have examined evidence for selectivity of interactions between primary afferent neurons from identified levels of the spinal cord and different peripheral nerve environments by culturing these neurons on sections of nerves to muscle and viscera. We have compared the growth of a population of sensory afferents normally innervating somatic targets (dorsal root ganglion cells from L4 and L5) with populations containing many afferents innervating visceral targets (L6 and S1 dorsal root ganglia and nodose ganglion). These neurons, from newly born rats, were cultured on unfixed cryostat sections of normal and prelesioned gastrocnemius nerve, pelvic spinal nerve and vagus nerve from adult rats. Normal muscle nerve was seen to support the regeneration of a significantly greater proportion of somatic neurons, with longer neurites, than the visceral nerves. Similarly, much higher proportions of the 'visceral' population of afferent neurons were seen to extend neurites on the normal visceral nerve substrates, with longer neurites, than on the muscle nerve substrate. The selectivity displayed by the sensory neurons for their normal nerve substrates was abolished when they were cultured on prelesioned nerve substrates subjected to Wallerian degeneration, which was apparent from the equivalent and increased proportions of growing neurons having comparable neurite lengths, on all the nerve substrates. We conclude that sensory neurons recognize and respond to substrate-specific and substrate-bound molecules present in normal adult peripheral nerves, and that these differences are lost in prelesioned nerves following Wallerian degeneration.

Antibodies directed against the beta 1-integrin subunit and peptides containing the IKVAV sequence of laminin perturb neurite outgrowth of peripheral neurons on immature spinal cord substrata

Neuron-substratum interactions regulating axon growth in the developing central nervous system of the rat have been studied by means of an in vitro bioassay: the tissue section culture. We have previously shown that purified chicken sensory or sympathetic neurons grown on natural substrata consisting of cryostat sections of neonatal rat spinal cord elaborate numerous long neurites [Sagot et al. (1991) Brain Res. 543, 25-35]. Perturbation experiments, in which neuron-substratum interactions are modified by antibodies and peptides, have allowed us to analyse some of the molecular determinants which control neurite outgrowth in this system. Antibodies directed against the beta 1-integrin subunit, one of the neuronal receptors for extracellular matrix molecules, reduced the percentage of growing neurons by about 30% and the length of neurites by about 50%. In contrast, antibodies directed against laminin-1 or fibronectin, two extracellular matrix proteins transiently expressed in various areas of the developing central nervous system, were unable to block neurite outgrowth. Paradoxically, a peptide containing the IKVAV sequence, which mimics an active sequence of the laminin alpha 1 chain responsible for neurite extension, also blocked neurite outgrowth on neonatal spinal cord substrata. These results indicate that integrin receptors containing the beta 1 subunit may play a role in regulating axon growth in the developing nervous system. Among the putative extracellular matrix ligands for these receptors, laminin and fibronectin do not appear as prominent candidates in the neonatal spinal cord. However, our data also suggest that the developing central nervous system may contain neurite outgrowth-promoting proteins carrying the IKVAV sequence, different from laminin-1.

Granulocyte macrophage colony stimulating factor produced in lesioned peripheral nerves induces the up-regulation of cell surface expression of MAC-2 by macrophages and Schwann cells

Peripheral nerve injury is followed by Wallerian degeneration which is characterized by cellular and molecular events that turn the degenerating nerve into a tissue that supports nerve regeneration. One of these is the removal, by phagocytosis, of myelin that contains molecules which inhibit regeneration. We have recently documented that the scavenger macrophage and Schwann cells express the galactose-specific lectin MAC-2 which is significant to myelin phagocytosis. In the present study we provide evidence for a mechanism leading to the augmented expression of cell surface MAC-2. Nerve lesion causes noneuronal cells, primarily fibroblasts, to produce the cytokine granulocyte macrophage-colony stimulating factor (GM-CSF). In turn, GM-CSF induces Schwann cells and macrophages to up-regulate surface expression of MAC-2. The proposed mechanism is based on the following novel observations. GM-CSF mRNA was detected by PCR in in vitro and in vivo degenerating nerves, but not in intact nerves. The GM-CSF molecule was detected by ELISA in medium conditioned by in vitro and in vivo degenerating peripheral nerves as of the 4th h after injury. GM-CSF activity was demonstrated by two independent bioassays, and repressed by activity blocking antibodies. Significant levels of GM-CSF were produced by nerve derived fibroblasts, but neither by Schwann cells nor by nerve derived macrophages. Mouse rGM-CSF enhanced MAC-2 production in nerve explants, and up-regulated cell surface expression of MAC-2 by Schwann cells and macrophages. Interleukin-1 beta up-regulated GM-CSF production thus suggesting that injury induced GM-CSF production may be mediated by interleukin-1 beta. Our findings highlight the fact that fibroblasts, by producing GM-CSF and thereby affecting macrophage and Schwann function, play a significant role in the cascade of molecular events and cellular interactions of Wallerian degeneration.

Lack of evidence that myelin-associated glycoprotein is a major inhibitor of axonal regeneration in the CNS

The MAG-deficient mouse was used to test whether MAG acts as a significant inhibitor of axonal regeneration in the adult mammalian CNS, as suggested by cell culture experiments. Cell spreading, neurite elongation, or growth cone collapse of different cell types in vitro was not significantly different when myelin preparations or optic nerve cryosections from either MAG-deficient or wild-type mice were used as a substrate. More importantly, the extent of axonal regrowth in lesioned optic nerve and corticospinal tract in vivo was similarly poor in MAG-deficient and wild-type mice. However, axonal regrowth increased significantly and to a similar extent in both genotypes after application of the IN-1 antibody directed against the neurite growth inhibitors NI-35 and NI-250. These observations do not support the view that MAG is a significant inhibitor of axonal regeneration in the adult CNS.

Laminin overrides the inhibitory effects of peripheral nervous system and central nervous system myelin-derived inhibitors of neurite growth

Axon growth inhibitory proteins associated with central nervous system (CNS) myelin are responsible in part for the absence of long distance axon regeneration in the adult mammalian CNS. We have recently reported that myelin-associated glycoprotein (MAG), which is also present in peripheral nerves, is a potent inhibitor of neurite growth. This was surprising given the robust regenerative capacity of peripheral nerves. We now provide evidence that myelin purified from peripheral nerve also has neurite growth inhibitory activity. However, this activity can be masked by laminin, which is a constituent of the Schwann cell basal lamina. We also report that laminin, which is largely absent from the normal adult mammalian CNS, when added to purified CNS myelin, can override the neurite growth inhibitory activity in CNS myelin. These results have important implications for the development of strategies to foster axon regeneration in the adult mammalian CNS where multiple growth inhibitors exist.

Myelin-associated glycoprotein: a role in myelination and in the inhibition of axonal regeneration?

Inhibitory molecules in CNS myelin affect axonal regeneration after injury. In the past year, myelin-associated glycoprotein (MAG), a well-characterized myelin protein, has been identified as an inhibitor of axonal regeneration. This finding, together with its established ability to promote outgrowth, defines MAG as a bifunctional molecule. MAG has also been included in a family of sialic acid binding proteins, providing a clue to the identity of the MAG receptor. MAG knockout mice reveal that MAG is not essential for the initiation of myelination; however, it plays an important role in maintaining a stable interaction between axons and myelin.

Cell body response to injury in motoneurons and primary sensory neurons of a mutant mouse, Ola (Wld), in which Wallerian degeneration is delayed

We examined the response to axon injury in the facial motoneurons and dorsal root ganglion (DRG) neurons of C57BL/Ola (Wld) mice, compared with the responses of C57BL/6J mice. The peripheral nerves of Ola mutants undergo remarkably slowed and muted Wallerian degeneration after injury. The increase in GAP-43 mRNA levels in facial motoneurons and DRG neurons was similar in both strains of mice, as was the initial decrease in medium-weight neurofilament (NFM) mRNA in facial motoneurons, and the increase in JUN immunoreactivity in both types of neurons. However, the subsequent recovery to normal low levels of JUN and GAP-43 mRNA expression and high levels of NFM mRNA was delayed in Ola motoneurons. We ascribe this delay to the slow regeneration and target reinnervation of facial axons in the Ola mice. These results show that absence of rapid Wallerian degeneration does not affect the initial cell body response to axonal injury. They also provide further evidence that restoration of normal levels of expression of GAP-43 and NFM mRNAs is dependent on target reinnervation and/or trophic factors provided by the distal nerve. Impaired regeneration in the Ola mouse does not seem to be a consequence of a defective cell body response to injury, and our results illustrate the general principle that, even if there is a vigorous cell body response to injury, normal axonal regeneration requires the additional provision of a favorable environment for growth.

Predegeneration enhances regeneration into acellular nerve grafts

In the present study, we determined the regeneration rate and the initial delay in rat sciatic nerve grafts first made hypercellular by predegeneration then acellular by freeze-thawing. 7-day predegenerated nerve pieces from the distal nerve stump on the right side were made acellular by repeated freeze-thawing and inserted as grafts into a 10-mm long freshly created defect on the left contralateral side. Freshly made (no predegeneration period) acellular nerve grafts were used as controls. Both types of grafts supported outgrowth of regenerating axons as demonstrated by the sensory pinch test. However, the predegenerated acellular nerve grafts had a significantly shorter initial delay period (2.7 days) as compared with freshly made acellular nerve grafts (9.5 days). The initial delay period for predegenerated acellular nerve grafts was similar to that for fresh cellular nerve grafts but significantly longer than that for predegenerated cellular nerve grafts [24]. The rate of regeneration appeared independent of the type of grafts used. We suggest that modifications of the basal lamina and/or factors produced during the predegeneration period by non-neuronal cells survive the freeze-thawing cycle and account for the decrease in the initial delay period.

Transforming growth factor-beta 1 regulates axon/Schwann cell interactions

We have investigated the potential regulatory role of TGF-beta in the interactions of neurons and Schwann cells using an in vitro myelinating system. Purified populations of neurons and Schwann cells, grown alone or in coculture, secrete readily detectable levels of the three mammalian isoforms of TGF-beta; in each case, virtually all of the TGF-beta activity detected is latent. Expression of TGF-beta 1, a major isoform produced by Schwann cells, is specifically and significantly downregulated as a result of axon/Schwann cell interactions. Treatment of Schwann cells or Schwann cell/neuron cocultures with TGF-beta 1, in turn, has dramatic effects on proliferation and differentiation. In the case of purified Schwann cells, treatment with TGF-beta 1 increases their proliferation, and it promotes a pre- or nonmyelinating Schwann cell phenotype characterized by increased NCAM expression, decreased NGF receptor expression, inhibition of the forskolin-mediated induction of the myelin protein P0, and induction of the Schwann cell transcription factor suppressed cAMP-inducible POU protein. Addition of TGF-beta 1 to the cocultures inhibits many of the effects of the axon on Schwann cells, antagonizing the proliferation induced by contact with neurons, and, strikingly, blocking myelination. Ultrastructural analysis of the treated cultures confirmed the complete inhibition of myelination and revealed only rudimentary ensheathment of axons. Associated defects of the Schwann cell basal lamina and reduced expression of laminin were also detected. These effects of TGF-beta 1 on Schwann cell differentiation are likely to be direct effects on the Schwann cells themselves which express high levels of TGF-beta 1 receptors when cocultured with neurons. The regulated expression of TGF-beta 1 and its effects on Schwann cells suggest that it may be an important autocrine and paracrine mediator of neuron/Schwann cell interactions. During development, TGF-beta 1 could serve as an inhibitor of Schwann cell proliferation and myelination, whereas after peripheral nerve injury, it may promote the transition of Schwann cells to a proliferating, nonmyelinating phenotype, and thereby enhance the regenerative response.

Glial cell types, lineages, and response to injury in rat and fish: implications for regeneration

Axons of the mammalian central nervous system do not regenerate spontaneously after axonal injury, unlike the central nervous system axons of fish and amphibians and the peripheral nervous system of mammals, which possess a good regenerative ability (Grafstein: The Retina: A Model for Cell Biology Studies, Part II, 1986; Kiernan: Biol Rev 54:155-197, 1979; Murray: J Comp Neurol 168:175-196, 1976; Ramón y Cajal: Degeneration and Regeneration of the Nervous System, 1928; Reier and Webster: J Neurocytol 3:591-618, 1974; Sperry: Physiol Zool 23:351-361, 1948). It was previously believed that intrinsic differences between the central nervous system neurons of mammals and fish account for their differences in regenerative ability. The past decade, however, has seen an accumulation of evidence, indicating that mammalian central nervous system neurons are able to regenerate injured axons, at least to some extent. This was first demonstrated by Aguayo and colleagues (David and Aguayo: Science 214:931-933, 1981; Kierstead et al: Science 246:255-257, 1989), who showed that injured mammalian central nervous system axons can grow for a considerable distance into an autograft of a peripheral nerve. It was also demonstrated that injured rabbit optic axons can regenerate into their own environment (i.e., into the distal part of the injured optic nerve), if the injured nerve is treated so as to make it conducive for growth (Lavie et al: J Comp Neurol 298:293-314, 1990; Eitan et al: Science 264:1764-1768, 1994).(ABSTRACT TRUNCATED AT 250 WORDS)

Tenascin-C expression during wallerian degeneration in C57BL/Wlds mice: possible implications for axonal regeneration

Schwann cells in the distal stumps of lesioned peripheral nerves strongly express the extracellular matrix glycoprotein tenascin-C. To gain insights into the relationship between Wallerian degeneration, lesion induced tenascin-C upregulation and regrowth of axons we have investigated C57BL/Wlds (C57BL/Ola) mice, a mutant in which Wallerian degeneration is considerably delayed. Since we found a distinct difference in the speed of Wallerian degeneration between muscle nerves and cutaneous nerves in 16-week-old C57BL/Wlds mice, as opposed to 6-week-old animals in which Wallerian degeneration is delayed in both, we chose the older animals for closer investigation. Five days post lesion tenascin-C was upregulated in the muscle branch (quadriceps) but not in the cutaneous branch (saphenous) of the femoral nerve in 16-week-old animals. In addition myelomonocytic cells displaying endogenous peroxidase activity invaded the muscle branch readily whereas they were absent from the cutaneous branch at this time. We could further show that it is only a subpopulation of axon-Schwann cell units (mainly muscle efferents) in the muscle branch which undergo Wallerian degeneration and upregulate tenascin-C at normal speed and that the remaining axon-Schwann cell units (mainly afferents) displayed delayed Wallerian degeneration and no tenascin-C expression. Regrowing axons could only be found in the tenascin-C-positive muscle branch where they always grew in association with axon-Schwann cell units undergoing Wallerian degeneration. These observations indicate a tight relationship between Wallerian degeneration, upregulation of tenascin-C expression and regrowth of axons, suggesting an involvement of tenascin-C in peripheral nerve regeneration.

Pre-degenerated nerve grafts enhance regeneration by shortening the initial delay period

In the present study we tested how nerve grafts with different pre-degeneration periods (1-28 days) influenced the early regenerative response in the rat sciatic nerve. The sciatic nerve on the right side was crushed and after 1-28 days of pre-degeneration, a 10 mm segment was used as an autologous nerve graft and transposed to a freshly made 10 mm long nerve defect on the left side. The regeneration distance was measured by the sensory pinch test 2-10 days after nerve repair. A newly developed mathematical model was used to calculate regeneration rates and initial delay periods from the measured regeneration distances. Pre-degenerated nerve grafts improved nerve regeneration by decreasing the initial delay period as compared to fresh nerve grafts without affecting the regeneration rate. Only one day of pre-degeneration was sufficient to reduce the initial delay period from 3.6 days to 1.7 days. The maximal effect on the initial delay period was achieved after 3 days of pre-degeneration. The initial delay period at later pre-degeneration intervals (7-14 days) was about 1 day. The effect persisted for at least 28 days of pre-degeneration. The regeneration rate was 1.5 mm/day for fresh nerve grafts and between 1.8-2.1 mm/day for pre-degenerated grafts. The results suggest that the effects of pre-degeneration are not only due to the increased cell proliferation in the graft, but that also trophic and/or inflammatory mechanisms may be of importance. Grafts pre-degenerated by crush may have clinical implications since they are easy to perform if an elective nerve grafting procedure is planned.

Neurite outgrowth and GAP-43 mRNA expression in cultured adult rat dorsal root ganglion neurons: effects of NGF or prior peripheral axotomy

Adult dorsal root ganglion (DRG) cells are capable of neurite outgrowth in vivo and in vitro after axotomy. We have investigated, in cultured adult rat DRG cells, the relative influence of nerve growth factor (NGF) or a prior peripheral nerve lesion on the capacity of these neurons to produce neurites. Since there is evidence suggesting that the growth-associated protein GAP-43 may play a crucial role in axon elongation during development and regeneration, we have also compared the effect of these treatments on GAP-43 mRNA expression. NGF increased the early neurite outgrowth in a subpopulation of DRG cells. This effect was substantially less, however, than that resulting from preaxotomy, which initiated an early and profuse neurite outgrowth in almost all cells. No difference in the expression of GAP-43 mRNA was found between neurons grown in the presence or absence of NGF over 1 week of culture, in spite of the increased growth produced by NGF. In contrast, cultures of neurons that had been preaxotomized showed substantial increases in GAP-43 mRNA and NGF had, as expected, a significant effect on substance P mRNA levels. Two forms of growth may be present in adult DRG neurons: an NGF-independent, peripheral nerve injury-provoked growth associated with substantial GAP-43 upregulation, and an NGF-dependent growth that may underlie branching or sprouting of NGF-sensitive neurons, but which is not associated with increased levels of GAP-43 mRNA.

Quantification of the mononuclear phagocyte response to Wallerian degeneration of the optic nerve

We investigated the numbers, origin and phenotype of mononuclear phagocytes (macrophages/microglia) responding to Wallerian degeneration of the mouse optic nerve in order to compare it with the response to Wallerian degeneration in the PNS, already described. We found macrophage/microglial numbers elevated nearly four fold in the distal segments of crushed optic nerves and their projection areas in the contralateral superior colliculus 1 week after unilateral optic nerve crush. This relative increase in mononuclear phagocyte numbers compared well with the four-to-five-fold increases reported in the distal segments of transected saphenous or sciatic nerves. Moreover, maximum numbers are reached at 3, 5 and 7 days in the saphenous, sciatic and optic nerves respectively, suggesting that the very slow clearance of axonal debris and myelin in CNS undergoing Wallerian degeneration is not simply due to a slow or small mononuclear phagocyte response. The apparent delay in the response in the CNS occurs because the mononuclear phagocytes respond to the Wallerian degeneration of axons, which is slightly slower in the CNS than the PNS, rather than to events associated with the crush itself, such as the abolition of normal electrical activity in the distal segment. This was demonstrated by the protracted time course of the mononuclear phagocyte response in the distal segment following optic nerve crush in mice carrying the Wlds mutation which dramatically slows the rate at which the axons undergo Wallerian degeneration. By [3H]-Thymidine labelling or by blocking microglial proliferation by X-irradiation of the head prior to optic nerve crush, we showed that the majority of macrophages/microglia initiating the response to Wallerian degeneration were of local, CNS origin but these cells rapidly (from 3 days post crush) upregulate endocytic and phagocytic functional markers although they do not resemble rounded myelin-phagocytosing macrophages observed in degenerating peripheral nerves. We speculate that the poor clearance of myelin in CNS fibre tracts undergoing Wallerian degeneration compared to the PNS, in the face of a mononuclear phagocyte response which is similar in relative magnitude and time course, is because Schwann cells in degenerating peripheral nerves promptly modify their myelin sheaths such that they can be recognized and phagocytosed by macrophages, whilst in the CNS oligodendrocytes do not.

Long-lasting accumulation of hemopexin in permanently transected peripheral nerves and its down-regulation during regeneration

We have previously demonstrated that hemopexin is present in the intact sciatic nerve and is overproduced in the distal stump after nerve transection (Swerts et al.: J Biol Chem 267:10596-10600, 1992). To get further insight into the function of this hemoprotein in nervous tissue, we have documented long-term changes in hemopexin levels in permanently degenerated (transected) and regenerating (crush-lesioned) sciatic nerves of adult rats, using immunochemical techniques. As early as a couple of days after nerve transection, the amount of hemopexin was raised in the distal stump and at the end of the proximal stump. Similarly, after a crush lesion hemopexin was rapidly increased at the injury site and in the distal part of the nerve. Subsequently, in transected nerves the level of hemopexin rose steadily and remained elevated, representing, three months after injury, over 20 times the amount found in intact contralateral nerves. In contrast, in crush-lesioned nerves, hemopexin level declined progressively in a proximodistal direction and returned to basal values 2 months after injury, together with axonal regeneration. This long-term increase in hemopexin in permanently degenerated nerves and its progressive return to normal levels during nerve regeneration suggests that hemopexin content could be regulated negatively, directly or indirectly, by growing axons. In turn, these results support the idea that hemopexin could be involved in the process of Wallerian degeneration and/or in nerve repair.

A novel role for myelin-associated glycoprotein as an inhibitor of axonal regeneration

Following nerve injury, axons in the CNS do not normally regenerate. It has been shown that CNS myelin inhibits neurite outgrowth, though the nature of the molecules responsible for this effect are not known. Here, we demonstrate that the myelin-associated glycoprotein (MAG), a transmembrane protein of both CNS and PNS myelin, strongly inhibits neurite outgrowth from both developing cerebellar and adult dorsal root ganglion (DRG) neurons in vitro. This inhibition is reversed by an anti-MAG antibody. In contrast, MAG promotes neurite outgrowth from newborn DRG neurons. These results suggest that MAG may be responsible, in part, for the lack of CNS nerve regeneration in vivo and may influence, both temporally and spatially, regeneration in the PNS.

Presence of growth inhibitors in fish optic nerve myelin: postinjury changes

This study shows that the fish optic nerve, which is able to regenerate after injury, contains myelin-associated growth inhibitors similar to the growth inhibitors present in mammalian central nervous system (CNS) myelin. The ability of nerves to regenerate was previously correlated with the ability of sections from these nerves to support neuronal attachment and axonal growth in vitro. Thus neuroblastoma cells or embryonic neurons became attached to and grew axons on sections of rat sciatic nerve or fish optic nerve, which are spontaneously regenerating systems, but not on sections of rat optic nerve, a nonregenerating system. Failure of the latter to support axonal growth has been attributed, at least in part, to growth inhibitors. Recently it was shown that adult neurons, which differ in their growth requirement from embryonic neurons, are unable to extend neurites on sections of normal sciatic nerve but are able to extend neurites on sections of sciatic nerve that was injured prior to its excision. We found a similar situation in the fish optic nerve, i.e., that the nerve is normally not permissive to growth of adult retinal axons but becomes growth permissive after injury. The nonpermissiveness of the normal fish optic nerve was found to correlate with the presence of myelin-associated growth-inhibitory molecules. This inhibitory activity of fish myelin was neutralized by IN-1 antibodies, known to neutralize rat myelin growth inhibitors. The results thus demonstrate that fish optic nerve myelin contains inhibitors apparently similar or even identical to those of rat, but possibly present in lower amounts than in the rat. Results are discussed with respect to the possibility that fish optic nerve, like the rat sciatic nerve and unlike the rat optic nerve, undergoes certain changes after injury that support regeneration of adult neurons. Such changes might include elimination or neutralization of growth inhibitors.

A monoclonal antibody (IN-1) which neutralizes neurite growth inhibitory proteins in the rat CNS recognizes antigens localized in CNS myelin

In previous studies two neurite growth inhibiting protein fractions of 35 and 250 kDa were identified in myelin preparations of the rat CNS. These activities were not found in the myelin of PNS. A monoclonal antibody (mAb IN-1) was raised against the 250 kDa protein fraction and selected for its ability to neutralize the inhibitory effect of CNS myelin and of both isolated protein fractions. IN-1 has been shown both in vitro and in vivo to neutralize the inhibitory effect of differentiated oligodendrocytes and CNS white matter. In the present study, the antigens of IN-1 were localized by immunohistochemistry on cryostat sections of the adult rat nervous system. The staining pattern of IN-1 was compared to that of mAbs specific for proteins found in CNS and PNS myelin. These proteins include myelin basic protein, myelin oligodendrocyte glycoprotein, and myelin associated glycoprotein. IN-1 stained white matter and myelinated fibre tracts in the CNS on sections of fresh frozen tissue fixed with 95% ethanol: 5% acetic acid (Clark's solution). Sciatic nerve myelin and spinal roots remained unstained. The staining pattern of IN-1 corresponded most closely to that of a mAb against myelin oligodendrocyte glycoprotein, a protein which occurs exclusively in CNS myelin and on differentiated oligodendrocytes.

Regeneration of axons into the trochlear rootlet after anterior medullary lesions in the rat is specific for ipsilateral IVth nerve motoneurones

The fibre projection from the IVth nerve nucleus to the superior oblique muscle was determined quantitatively in the normal rat by defining fibre numbers in transverse sections of the IVth nerve, and neurone numbers after retrograde labelling by horseradish peroxidase (HRP) injection into the muscle. There were 183 +/- 27 (S.E.) labelled neurones in the nucleus contralateral to the injected muscle and only 2 +/- 1 ipsilateral. The ipsilateral fibre number was 234 +/- 7 and the cell/axon ratio 0.8 +/- 0.1. Extensive analysis of all HRP retrogradely labelled material revealed no central fibre contribution to the IVth nerve other than from neurones resident in the trochlear nucleus. The central portion of the trochlear nerve tract was severed at its point of decussation in the anterior medullary velum. Ninety days after lesion, 10 +/- 4 (6% of control) neurones were labelled in the ipsilateral trochlear nucleus; none were labelled in the contralateral nucleus or in any other part of the midbrain, pons, medulla, or cerebellum. The number of myelinated fibres in the IVth nerve had decreased to 21 +/- 5 (9% of control) so that the cell/axon ratio was 0.4 +/- 0.2, thus suggesting that a single motoneurone has more fibres after lesion. In electron micrographs of the IVth nerve, larger than normal numbers of unmyelinated fibres were seen. Many myelinated fibres displayed signs of abnormal myelination. After regeneration, the projection was exclusively ipsilateral and not crossed as in the normal. These findings establish that there is a high degree of specificity after regeneration since no myelinated central nervous system axons other than trochlear fibres select the IVth nerve root as a trajectory over which to regenerate.

Further studies on motor and sensory nerve regeneration in mice with delayed Wallerian degeneration

The axons of both peripheral and central neurons in C57BL/Wlds (C57BL/Ola) mice are unique among mammals in degenerating extremely slowly after axotomy. Motor and sensory axons attempting to regenerate are thus confronted with an intact distal nerve stump rather than axon- and myelin-free Schwann cell-filled endoneurial tubes. Surprisingly, however, motor axons in the sciatic nerve innervating the soleus muscle regenerate rapidly, and there is evidence that they may use Schwann cells associated with unmyelinated fibres as a pathway. If this is so, motor axon regeneration might be impaired in C57BL/Wlds mice in the phrenic nerve, which has very few unmyelinated fibres. We found that as long as the myelinated axons in the distal stump of the phrenic nerve remained intact (up to 10 days), regeneration of motor axons did not occur, in spite of vigorous production of sprouts at the crush site. In contrast to motor axons, myelinated sensory axons regenerate very poorly in C57BL/Wlds mice, even in the presence of unmyelinated axons. We showed that this was also due to adverse local conditions confronting nerve sprouts, for the dorsal root ganglion cell bodies responded normally to injury with a rapid induction of Jun protein-like immunoreactivity and when the saphenous nerve was forced to degenerate more rapidly by multiple crush lesions sensory axons regrew much more successfully. The findings show that motor and sensory axons in C57BL/Wlds mice, although very atypical in the way that they degenerate, are able to regenerate normally but only in an appropriate environment.(ABSTRACT TRUNCATED AT 250 WORDS)

Early regeneration in vitro of adult mouse sciatic axons is dependent on local protein synthesis but may not involve neurotrophins

The sensory axons of the adult mouse sciatic nerve were shown to regenerate after a local test crush lesion in vitro in a serum-free medium. The average outgrowth distance of the leading axons after culturing for 3 days was 2.8 +/- 0.1 mm, which was shorter than in vivo (3.8 +/- 0.2 mm). With the use of a compartmentalised culture system we could show that regeneration was partially dependent on local protein synthesis in the injury region. The initial stages of regeneration did not seem to involve neurotrophins since both K252a and K252b, selective and nontoxic inhibitors of the neurotrophin actions, failed to inhibit axonal growth. The present in vitro model system offers favourable conditions to investigate the early events of the regeneration process in an adult mammalian peripheral nerve.

Inflammation after axonal injury has conflicting consequences for recovery of function: rescue of spared axons is impaired but regeneration is supported

Neural injury leads to tissue damage beyond that caused by the initial lesion, mainly as a result of a chain of autodestructive events triggered by the trauma. These events apparently include the activation of immune-derived cells and their products, as treatment with anti-inflammatory agents, such as corticosteroids, limits the damage and thus improves recovery. On the other hand, immune-derived substances, such as cytokines, are thought to play an important role in post-traumatic axonal regeneration. Thus, the need to reduce inflammation to limit the spread of damage appears to be in conflict with the need to permit inflammation to promote regeneration. Comprehension and resolution of this apparent conflict may lead to the development of treatment protocols aimed at rescuing axons spared by the initial injury, without hampering the potential regeneration of directly and indirectly injured axons. In this study, carried out on rats with crushed optic nerves, daily intraperitoneal injections of dexamethasone commencing prior to the injury significantly attenuated the injury-induced decrease in electrophysiological activity and reduced the area of tissue damage. On the other hand, dexamethasone treatment reduced the permissiveness of the injured nerves to neural adhesion and regrowth in vitro. This latter phenomenon was also observed in injured peripheral nerves. Results are discussed with respect to the possible establishment of an appropriate protocol for corticosteroid treatment of nerve injuries aimed at promoting neuronal rescue without compromising neuronal regeneration.

Effects of predegeneration on nerve regeneration through silicone Y-chambers

The effects of nerve predegeneration on the preferential growth of regenerating axons were studied using a silicone Y-chamber model. This system provided a choice for axons to grow towards two distal nerve options, either a 7-day predegenerated nerve segment (PNS) or a fresh nerve segment (FNS). The rat peroneal or tibial nerve was inserted into the proximal intlet and the PNS and FNS of the corresponding nerve were inserted into the distal outlets. At 28 days postoperative, the size of the distal regenerate was significantly greater (26%) towards the PNS for the tibial nerve group. The density and number of regenerated myelinated axons in the distal nerve segment was greater on the PNS for both the tibial (97 and 88%, respectively) and peroneal (221 and 221%, respectively) nerve groups. In contrast, the elevated density and number of nonvascular nuclei was relatively constant for both PNS and FNS. Immunocytochemical and ultrastructural evidence support the hypothesis that the early activation of Schwann cells is primarily responsible for the enhanced regeneration and maturation observed in PNS. It is suggested that PNS might improve the outcome after clinical repair of injured peripheral nerves.