Immunocytological localization of the major peripheral nervous system glycoprotein P0 and the L2/HNK-1 and L3 carbohydrate structures in developing and adult mouse sciatic nerve

Transcriptional profiling of mouse peripheral nerves to the single-cell level to build a sciatic nerve ATlas (SNAT)

Peripheral nerves are organ-like structures containing diverse cell types to optimize function. This interactive assembly includes mostly axon-associated Schwann cells, but also endothelial cells of supporting blood vessels, immune system-associated cells, barrier-forming cells of the perineurium surrounding and protecting nerve fascicles, and connective tissue-resident cells within the intra-fascicular endoneurium and inter-fascicular epineurium. We have established transcriptional profiles of mouse sciatic nerve-inhabitant cells to foster the fundamental understanding of peripheral nerves. To achieve this goal, we have combined bulk RNA sequencing of developing sciatic nerves up to the adult with focused bulk and single-cell RNA sequencing of Schwann cells throughout postnatal development, extended by single-cell transcriptome analysis of the full sciatic nerve both perinatally and in the adult. The results were merged in the transcriptome resource Sciatic Nerve ATlas (SNAT: https://www.snat.ethz.ch). We anticipate that insights gained from our multi-layered analysis will serve as valuable interactive reference point to guide future studies.

Neural glycomics: the sweet side of nervous system functions

The success of investigations on the structure and function of the genome (genomics) has been paralleled by an equally awesome progress in the analysis of protein structure and function (proteomics). We propose that the investigation of carbohydrate structures that go beyond a cell's metabolism is a rapidly developing frontier in our expanding knowledge on the structure and function of carbohydrates (glycomics). No other functional system appears to be suited as well as the nervous system to study the functions of glycans, which had been originally characterized outside the nervous system. In this review, we describe the multiple studies on the functions of LewisX, the human natural killer cell antigen-1 (HNK-1), as well as oligomannosidic and sialic (neuraminic) acids. We attempt to show the sophistication of these structures in ontogenetic development, synaptic function and plasticity, and recovery from trauma, with a view on neurodegeneration and possibilities to ameliorate deterioration. In view of clinical applications, we emphasize the need for glycomimetic small organic compounds which surpass the usefulness of natural glycans in that they are metabolically more stable, more parsimonious to synthesize or isolate, and more advantageous for therapy, since many of them pass the blood brain barrier and are drug-approved for treatments other than those in the nervous system, thus allowing a more ready access for application in neurological diseases. We describe the isolation of such mimetic compounds using not only Western NIH, but also traditional Chinese medical libraries. With this review, we hope to deepen the interests in this exciting field.

Functions of Small Organic Compounds that Mimic the HNK-1 Glycan

Because of the importance of the HNK-1 carbohydrate for preferential motor reinnervation after injury of the femoral nerve in mammals, we screened NIH Clinical Collection 1 and 2 Libraries and a Natural Product library comprising small organic compounds for identification of pharmacologically useful reagents. The reason for this attempt was to obviate the difficult chemical synthesis of the HNK-1 carbohydrate and its isolation from natural sources, with the hope to render such compounds clinically useful. We identified six compounds that enhanced neurite outgrowth from cultured spinal motor neurons at nM concentrations and increased their neurite diameter, but not their neurite branch points. Axons of dorsal root ganglion neurons did not respond to these compounds, a feature that is in agreement with their biological role after injury. We refer to the positive functions of some of these compounds in animal models of injury and delineate the intracellular signaling responses elicited by application of compounds to cultured murine central nervous system neurons. Altogether, these results point to the potential of the HNK-1 carbohydrate mimetics in clinically-oriented settings.

Survival and Integration of Transplanted Olfactory Ensheathing Cells are Crucial for Spinal Cord Injury Repair: Insights from the Last 10 Years of Animal Model Studies

Olfactory ensheathing cells (OECs), the glial cells of the primary olfactory nervous system, support the natural regeneration of the olfactory nerve that occurs throughout life. OECs thus exhibit unique properties supporting neuronal survival and growth. Transplantation of OECs is emerging as a promising treatment for spinal cord injury; however, outcomes in both animals and humans are variable and the method needs improvement and standardization. A major reason for the discrepancy in functional outcomes is the variability in survival and integration of the transplanted cells, key factors for successful spinal cord regeneration. Here, we review the outcomes of OEC transplantation in rodent models over the last 10 years, with a focus on survival and integration of the transplanted cells. We identify the key factors influencing OEC survival: injury type, source of transplanted cells, co-transplantation with other cell types, number and concentration of cells, method of delivery, and time of transplantation after the injury. We found that two key issues are hampering optimization and standardization of OEC transplantation: lack of (1) reliable methods for identifying transplanted cells, and (2) three-dimensional systems for OEC delivery. To develop OEC transplantation as a successful and standardized therapy for spinal cord injury, we must address these issues and increase our understanding of the complex parameters influencing OEC survival.

Why are olfactory ensheathing cell tumors so rare?

The glial cells of the primary olfactory nervous system, olfactory ensheathing cells (OECs), are unusual in that they rarely form tumors. Only 11 cases, all of which were benign, have been reported to date. In fact, the existence of OEC tumors has been debated as the tumors closely resemble schwannomas (Schwann cell tumors), and there is no definite method for distinguishing the two tumor types. OEC transplantation is a promising therapeutic approach for nervous system injuries, and the fact that OECs are not prone to tumorigenesis is therefore vital. However, why OECs are so resistant to neoplastic transformation remains unknown. The primary olfactory nervous system is a highly dynamic region which continuously undergoes regeneration and neurogenesis throughout life. OECs have key roles in this process, providing structural and neurotrophic support as well as phagocytosing the axonal debris resulting from turnover of neurons. The olfactory mucosa and underlying tissue is also frequently exposed to infectious agents, and OECs have key innate immune roles preventing microbes from invading the central nervous system. It is possible that the unique biological functions of OECs, as well as the dynamic nature of the primary olfactory nervous system, relate to the low incidence of OEC tumors. Here, we summarize the known case reports of OEC tumors, discuss the difficulties of correctly diagnosing them, and examine the possible reasons for their rare incidence. Understanding why OECs rarely form tumors may open avenues for new strategies to combat tumorigenesis in other regions of the nervous system.

Methods of olfactory ensheathing cell harvesting from the olfactory mucosa in dogs

Olfactory ensheathing cells are thought to support regeneration and remyelination of damaged axons when transplanted into spinal cord injuries. Following transplantation, improved locomotion has been detected in many laboratory models and in dogs with naturally-occurring spinal cord injury; safety trials in humans have also been completed. For widespread clinical implementation, it will be necessary to derive large numbers of these cells from an accessible and, preferably, autologous, source making olfactory mucosa a good candidate. Here, we compared the yield of olfactory ensheathing cells from the olfactory mucosa using 3 different techniques: rhinotomy, frontal sinus keyhole approach and rhinoscopy. From canine clinical cases with spinal cord injury, 27 biopsies were obtained by rhinotomy, 7 by a keyhole approach and 1 with rhinoscopy. Biopsy via rhinoscopy was also tested in 13 cadavers and 7 living normal dogs. After 21 days of cell culture, the proportions and populations of p75-positive (presumed to be olfactory ensheathing) cells obtained by the keyhole approach and rhinoscopy were similar (~4.5 x 10⁶ p75-positive cells; ~70% of the total cell population), but fewer were obtained by frontal sinus rhinotomy. Cerebrospinal fluid rhinorrhea was observed in one dog and emphysema in 3 dogs following rhinotomy. Blepharitis occurred in one dog after the keyhole approach. All three biopsy methods appear to be safe for harvesting a suitable number of olfactory ensheathing cells from the olfactory mucosa for transplantation within the spinal cord but each technique has specific advantages and drawbacks.

Transcriptional control of neural crest specification into peripheral glia

The neural crest is a transient migratory multipotent cell population that originates from the neural plate border and is formed at the end of gastrulation and during neurulation in vertebrate embryos. These cells give rise to many different cell types of the body such as chondrocytes, smooth muscle cells, endocrine cells, melanocytes, and cells of the peripheral nervous system including different subtypes of neurons and peripheral glia. Acquisition of lineage-specific markers occurs before or during migration and/or at final destination. What are the mechanisms that direct specification of neural crest cells into a specific lineage and how do neural crest cells decide on a specific migration route? Those are fascinating and complex questions that have existed for decades and are still in the research focus of developmental biologists. This review discusses transcriptional events and regulations occurring in neural crest cells and derived lineages, which control specification of peripheral glia, namely Schwann cell precursors that interact with peripheral axons and further differentiate into myelinating or nonmyelinating Schwann cells, satellite cells that remain tightly associated with neuronal cell bodies in sensory and autonomous ganglia, and olfactory ensheathing cells that wrap olfactory axons, both at the periphery in the olfactory mucosa and in the central nervous system in the olfactory bulb. Markers of the different peripheral glia lineages including intermediate multipotent cells such as boundary cap cells, as well as the functions of these specific markers, are also reviewed. Enteric ganglia, another type of peripheral glia, will not be discussed in this review. GLIA 2015;63:1883-1896.

The effect of glycomimetic functionalized collagen on peripheral nerve repair

Increasing evidence suggests that the improper synaptic reconnection of regenerating axons is a significant cause of incomplete functional recovery following peripheral nerve injury. In this study, we evaluate the use of collagen hydrogels functionalized with two peptide glycomimetics of naturally occurring carbohydrates-polysialic acid (PSA) and human natural killer cell epitope epitope (HNK-1)-that have been independently shown to encourage nerve regeneration and axonal targeting. Our novel biomaterial was used to bridge a critical gap size (5 mm) in a mouse femoral nerve injury model. Functional recovery was assessed using gait and hind limb extension, and was significantly better in all glycomimetic peptide-coupled collagen conditions versus non-functional scrambled peptide-coupled collagen, native collagen, and saline controls. Analysis of cross-sections of the regenerated nerve demonstrated that hydrogels coupled with the PSA glycomimetic, but not HNK, had significant increases in the number of myelinated axons over controls. Conversely, hydrogels coupled with HNK, but not PSA, showed improvement in myelination. Additionally, significantly more correctly projecting motoneurons were observed in groups containing coupled HNK-1 mimicking peptide, but not PSA mimicking peptide. Given the distinct morphological outcomes between the two glycomimetics, our study indicates that the enhancement of recovery following peripheral nerve injury induced by PSA- and HNK-functionalized collagen hydrogels likely occurs through distinct mechanisms.

The role of sulfoglucuronosyl glycosphingolipids in the pathogenesis of monoclonal IgM paraproteinemia and peripheral neuropathy

In IgM paraproteinemia and peripheral neuropathy, IgM M-protein secretion by B cells leads to a T helper cell response, suggesting that it is antibody-mediated autoimmune disease involving carbohydrate epitopes in myelin sheaths. An immune response against sulfoglucuronosyl glycosphingolipids (SGGLs) is presumed to participate in demyelination or axonal degeneration in the peripheral nervous system (PNS). SGGLs contain a 3-sulfoglucuronic acid residue that interacts with anti-myelin-associated glycoprotein (MAG) and the monoclonal antibody anti-HNK-1. Immunization of animals with sulfoglucuronosyl paragloboside (SGPG) induced anti-SGPG antibodies and sensory neuropathy, which closely resembles the human disease. These animal models might help to understand the disease mechanism and lead to more specific therapeutic strategies. In an in vitro study, destruction or malfunction of the blood-nerve barrier (BNB) was found, resulting in the leakage of circulating antibodies into the PNS parenchyma, which may be considered as the initial key step for development of disease.

Olfactory ensheathing cells from the nose: clinical application in human spinal cord injuries

Olfactory mucosa, the sense organ of smell, is an adult tissue that is regenerated and repaired throughout life to maintain the integrity of the sense of smell. When the sensory neurons of the olfactory epithelium die they are replaced by proliferation of stem cells and their axons grow from the nose to brain assisted by olfactory ensheathing cells located in the lamina propria beneath the sensory epithelium. When transplanted into the site of traumatic spinal cord injury in rat, olfactory lamina propria or purified olfactory ensheathing cells promote behavioural recovery and assist regrowth of some nerves in the spinal cord. A Phase I clinical trial demonstrated that autologous olfactory ensheathing cell transplantation is safe, with no adverse outcomes recorded for three years following transplantation. Autologous olfactory mucosa transplantation is also being investigated in traumatic spinal cord injury although this whole tissue contains many cells in addition to olfactory ensheathing cells, including stem cells. If olfactory ensheathing cells are proven therapeutic for human spinal cord injury there are several important practical issues that will need to be solved before they reach general clinical application. This article is part of a Special Issue entitled: Understanding olfactory ensheathing glia and their prospect for nervous system repair.

Peripheral nerve: what's new in basic science laboratories

Peripheral nerve regeneration research has unfolded a wealth of basic science knowledge in the last century. Today, that knowledge has become the fundamental groundwork for evolving clinical applications to treat peripheral nerve defects. This article discusses two clinical applications that have been investigated thoroughly in the laboratory setting for decades and recently tested in the clinical setting: nerve allotransplantation to graft nerve defects, and brief electrical stimulation to promote nerve regeneration. It also discusses the generation of Thy-1-XFP transgenic mice, which express fluorescent proteins in the nervous system and provide new avenues for investigating peripheral nerve regeneration.

Fate mapping of neural crest cells during eye development using a protein 0 promoter-driven transgenic technique

PURPOSE

To map neural crest cell fate during eye development.

METHODS

Neural crest cells were tracked in developing mouse eyes using a transgene expressing Cre recombinase controlled by the Protein 0 promoter and a Rosa26 Cre-responsive reporter gene that produced beta-galactosidase after Cre-mediated recombination.

RESULTS

beta-galactosidase-positive cells were detected in the periocular segment on embryonic day (E) 9.5. Several neural crest cell-derived tissues including corneal stroma, corneal endothelium, iridocorneal angle, ciliary body, primary vitreous and eyelid were strongly stained on E13.5-E18.5. The staining decreased in the corneal stroma after birth, but persisted in the presumptive iridocorneal angle.

CONCLUSIONS

Protein 0-Cre transgenic mice offer a conditional knock-out strategy to investigate anterior eye segment differentiation.

Families of neural adhesion molecules

The neural cell adhesion molecules L1 and N-CAM share a common carbohydrate epitope that is recognized by the monoclonal antibodies L2 and HNK-1. The L2/HNK-1 epitope is also present on the myelin-associated glycoprotein (MAG) and secreted J1 glycoprotein, both of which have been identified as cell adhesion molecules. Each of the four adhesion molecules is differentially expressed during development on distinct cell types. Expression of the L2/HNK-1 epitope is regulated independently of the protein backbone, is phylogenetically conserved, and plays a role in cell-cell and, particularly, cell-substrate interactions. Another set of glycoproteins shares a common carbohydrate epitope designated L3. This epitope is present on the novel adhesion molecule on glia (AMOG), L1 and MAG, but not on J1 and N-CAM. As in the L2/HNK-1 family, the number of glycoproteins expressing this epitope is not yet known. It is therefore possible that heterogeneities in carbohydrate structures are associated with different sets of adhesion molecules and may have functional implications.

Effects of motor and sensory nerve transplants on amount and specificity of sciatic nerve regeneration

Nerve regeneration after complete transection does not allow for adequate functional recovery mainly because of lack of selectivity of target reinnervation. We assessed if transplanting a nerve segment from either motor or sensory origin may improve specifically the accuracy of sensory and motor reinnervation. For this purpose, the rat sciatic nerve was transected and repaired with a silicone guide containing a predegenerated segment of ventral root (VR) or dorsal root (DR), compared to a silicone guide filled with saline. Nerve regeneration and reinnervation was assessed during 3 months by electrophysiologic and functional tests, and by nerve morphology and immunohistochemistry against choline acetyltransferase (ChAT) for labeling motor axons. Functional tests showed that reinnervation was successful in all the rats. However, the two groups with a root allotransplant reached higher degrees of reinnervation in comparison with the control group. Group VR showed the highest reinnervation of muscle targets, whereas Group DR had higher levels of sensory reinnervation than VR and saline groups. The total number of regenerated myelinated fibers was similar in the three groups, but the number of ChAT+ fibers was slightly lower in the VR group in comparison with DR and saline groups. These results indicate that a predegenerated root nerve allotransplant enhances axonal regeneration, leading to faster and higher levels of functional recovery. Although there is not clear preferential reinnervation, regeneration of motor axons is promoted at early times by a motor graft, whereas reinnervation of sensory pathways is increased by a sensory graft.

Myelin-associated glycoprotein reduces axonal branching and enhances functional recovery after sciatic nerve transection in rats

The mature peripheral nervous system (PNS) generally shows better regeneration of injured axons as opposed to the central nervous system (CNS). However, complete functional recovery is rarely achieved even in the PNS although morphologically good axonal regeneration often occurs. This mainly results from aberrant reinnervation due to extensive branching of cut axons with consequent failure of synchronized movements of the muscles. Myelin-associated glycoprotein (MAG), a well-characterized molecule existing both in the CNS and PNS myelin, is considered to be a potent inhibitor of axonal regeneration especially in the CNS. In the present study, we investigated whether MAG has any effects not only on axonal elongation, but also on axonal branching. We show herein that MAG minimized branching of the peripheral axons both in vitro and in vivo via activation of RhoA. Furthermore, after sciatic nerve transection in rats, focal and temporary application of MAG to the lesion dramatically enhanced the functional recovery. Using double retrograde labeling and preoperative/postoperative labeling of spinal neurons, reduced hyperinnervation and improved accuracy of target reinnervation was confirmed, respectively. In conclusion, as MAG significantly improves the quality of axonal regeneration, it can be used as a new therapeutic approach for peripheral nerve repair with possible focal and temporary application.

Schwann cells and endoneurial extracellular matrix molecules as potential cues for sorting of regenerated axons: a review

Besides very well elaborated microsurgical management of severed peripheral nerves, the clinical results of functional recovery following surgical repair of mixed nerves are disappointing. An improvement of functional recovery after peripheral nerve lesion requires the accurate regeneration of axons to their original target tissues and structures. Therefore, better clinical results could be obtained by a greater understanding of the cellular and molecular biology of selective nerve regeneration. The studies concerning Schwann cells and their endoneurial extracellular matrix as potent cues for selective promotion and influence of regenerating motor and sensory axons are reviewed. Knowledge of the sorting mechanisms of regenerated motor and sensory axons is needed not only for improvement of functional recovery, but also for the development of biocompatible nerve prostheses.

Antibodies to myelin-associated glycoprotein accelerate preferential motor reinnervation

Predegeneration of nerve enhances its ability to support axon regeneration. Trophic factors are upregulated by reactive Schwann cells while potentially inhibitory molecules are removed. These experiments isolate the effects of one such inhibitory molecule, the myelin-associated glycoprotein (MAG), to determine its role in modifying regeneration after nerve repair. Suture of the mouse femoral nerve was followed by daily intraperitoneal injection of antibodies to MAG, antibodies to HNK-1, a specific muscle pathway marker, or no further treatment. Regeneration was assayed by double-labeling the femoral cutaneous and muscle branches with horseradish peroxidase and fluoro-gold after 4 weeks or 6 weeks of regeneration. Four weeks after nerve repair, selective reinnervation of the muscle branch by motoneurons, or preferential motor reinnervation (PMR), was not seen in either controls or L2-antibody-treated animals. In contrast, treatment with MAG antibodies resulted in dramatic PMR. By 6 weeks, the controls had achieved borderline specificity, substantial PMR developed in the L2 antibody group and the MAG group changed little. Blocking access to MAG in the distal nerve stump thus accelerated and enhanced PMR. Sensory regeneration was depressed by both antibody treatments at 4 weeks but recovered by 6 weeks. Antibody administration has a generalized effect on sensory regeneration that is unrelated to the behavior of motoneurons in the same nerve.

Comparison of matrix metalloproteinase expression during Wallerian degeneration in the central and peripheral nervous systems

The matrix metalloproteinases (MMPs) are a large family of zinc-dependent enzymes which are able to degrade the protein components of the extracellular matrix. They can be placed into subgroups based on structural similarities and substrate specificity. Aberrant expression of these destructive enzymes has been implicated in the pathogenesis of immune-mediated neuroinflammatory disorders. In this study we investigate the involvement of MMPs, from each subgroup, in Wallerian degeneration in both the central and peripheral nervous systems. Wallerian degeneration describes the process initiated by transection of a nerve fibre and entails the degradation and removal of the axon and myelin from the distal stump. A similar degenerative process occurs as the final shared pathway contributing to most common neuropathies. MMP expression and localisation in the peripheral nervous system are compared with events in the CNS during Wallerian degeneration. Within 3 days after axotomy in the peripheral nervous system, MMP-9, MMP-7 and MMP-12 are elevated. These MMPs are produced by Schwann cells, endothelial cells and macrophages. The temporospatial expression of activated MMP-9 correlates with breakdown of the blood-nerve barrier. In the CNS, 1 week after optic nerve crush, four MMPs are induced and primarily localised to astrocytes, not microglia or oligodendrocytes. In the degenerating optic nerve, examined at later time points (4, 8, 12 and 18 weeks), MMP expression was down-regulated. The absence of MMPs in oligodendrocytes and mononuclear phagocytes during Wallerian degeneration may contribute to the slower removal of myelin debris observed in the CNS. The low level of the inactive pro-form of MMP-9 in the degenerating optic nerve may explain why the blood-brain barrier remains intact, while the blood-nerve barrier is rapidly broken down. We conclude that the difference in the level of expression, activation state and cellular distribution of MMPs may contribute to the different sequence of events observed during Wallerian degeneration in the peripheral compared to the CNS.

The L2/HNK-1 Carbohydrate Epitope is Involved in the Preferential Outgrowth of Motor Neurons on Ventral Roots and Motor Nerves

Based on the observation that in adult mice the carbohydrate epitope L2/HNK-1 is detectable on Schwann cells in ventral spinal roots, but only scarcely in dorsal roots (Martini et al., Dev. Biol., 129, 330 - 338, 1988), the possibility was investigated that the carbohydrate is involved in the outgrowth of regenerating motor neuron axons on peripheral nerve substrates expressing the epitope. To monitor whether the L2 carbohydrate remains present during the time periods in which regenerating axons penetrate the denervated distal nerve stumps, the expression of L2 in motor and sensory branches of the femoral nerve was investigated in normal animals and after a crush lesion. During the first two postoperative weeks, L2 immunoreactivity remained high in the myelinating Schwann cells of the motor branch, whereas L2 immunoreactivity was virtually absent in the sensory branch. In a first experimental approach, cryosections of ventral and dorsal spinal roots and of motor and sensory nerves of adult rats and mice were used as substrates for neurite outgrowth. Neurites of motor neurons from chicken embryos were approximately 35% longer after 30 h of maintenance on ventral roots than on dorsal roots. Neurites from sensory neurons had the same length on dorsal as on ventral motors and were as long as neurites from motor neurons grown on dorsal roots. L2 antibodies reduced neurite outgrowth of motor neurons on ventral roots but not on dorsal roots. Neurite outgrowth of sensory neurons on both roots was not altered by the antibodies. Neurite outgrowth of motor neurons on a mixture of the extracellular matrix glycoprotein laminin and the L2 carbohydrate-carrying glycolipid was significantly higher than on the laminin substrate mixture with GD1b ganglioside or sulphatide. L2 antibodies reduced neurite outgrowth of motor neurons by 50% on the L2 glycolipid, but not on GD1b or sulphatide. These observations indicate that the L2 carbohydrate promotes neurite outgrowth of motor neurons in vitro and may thus contribute to the preferential reinnervation of motor nerves by regenerating motor axons in vivo.

LR white post-embedding colloidal gold method to immunostain MBP, P0, NF and S100 in glutaraldehyde fixed peripheral nerve tissue

A variety of immunocytochemical techniques are now widely used for the electron and light microscopic examination of biological samples. They are employed routinely for investigating the role of certain proteins in nervous tissue. Immunoelectron microscopic studies require the tissue to be fixed and embedded in a solid support, which may disrupt cellular structures and destroy crucial antigens. A technique of post-embedding with LR white resin has been developed, and it has been shown that certain antigens tolerate fixation with glutaraldehyde. In this study, we optimized a previous post-embedding method using low-water-miscible low-temperature embedding resin (LR white) to immunostain MBP, P0, NF and S100 proteins in peripheral nerves fixed with a relatively high concentration of glutaraldehyde found to be compatible with the morphology of normally compacted nerve fibers from humans and adult animals. The main difference in the procedures described here from previous ones is the elimination of vibratome sectioning, rendering this immunostaining technique more accessible to neuropathological laboratories using standard equipment for the ultrastructural study of peripheral nerves. It may prove of value for localization and quantification of these proteins in normal and pathological conditions.

Epitope diversity of N-glycans from bovine peripheral myelin glycoprotein P0 revealed by mass spectrometry and nano probe magic angle spinning 1H NMR spectroscopy

The carbohydrate structures present on the glycoproteins in the central and peripheral nerve systems are essential in many cell adhesion processes. The P0 glycoprotein, expressed by myelinating Schwann cells, plays an important role during the formation and maintenance of myelin, and it is the most abundant constituent of myelin. Using monoclonal antibodies, the homophilic binding of the P0 glycoprotein was shown to be mediated via the human natural keller cell (HNK)-1 epitope (3-O-SO(3)H-GlcUA(beta1-3)Gal(beta1-4)GlcNAc) present on the N-glycans. We recently described the structure of the N-glycan carrying the HNK-1 epitope, present on bovine peripheral myelin P0 (Voshol, H., van Zuylen, C. W. E. M., Orberger, G., Vliegenthart, J. F. G., and Schachner, M. (1996) J. Biol. Chem. 271, 22957-22960). In this study, we report on the structural characterization of the detectable glycoforms, present on the single N-glycosylation site, using state-of-the-art NMR and mass spectrometry techniques. Even though all structures belong to the hybrid- or biantennary complex-type structures, the variety of epitopes is remarkable. In addition to the 3-O-sulfate present on the HNK-1-carrying structures, most of the glycans contain a 6-O-sulfated N-acetylglucosamine residue. This indicates the activity of a 6-O-sulfo-GlcNAc-transferase, which has not been described before in peripheral nervous tissue. The presence of the disialo-, galactosyl-, and 6-O-sulfosialyl-Lewis X epitopes provides evidence for glycosyltransferase activities not detected until now. The finding of such an epitope diversity triggers questions related to their function and whether events, previously attributed merely to the HNK-1 epitope, could be mediated by the structures described here.

Morphometric analysis of early regeneration of motor axons through motor and cutaneous nerve grafts

Peripheral nerve damage is a frequent consequence of trauma, tumor surgery or diseases. Clinical results of functional reinnervation after the application of cutaneous grafts are still unsatisfactory. Differences in the extracellular matrix are considered to be one of the factors responsible for poor results of motor axon reinnervation through the cutaneous graft. To verify these differences, we compared morphological features of the motor axons regenerating through the graft prepared from the saphenous nerve and the motor branch of the femoral nerve. Eighteen female adult rats (Wistar) were used in experiments. The saphenous nerve, the femoral nerve, and its main motor branch were exposed under deep anesthesia with ketamine and xylazine. The nerve graft (10 mm) prepared from the saphenous nerve was applied between the stumps of the transected motor branch of the femoral nerve in the 6 rats. In the next 6 rats, the nerve graft (10 mm) harvested from the motor branch of the femoral nerve was inserted between stumps of the transected motor branch of the femoral nerve on the contralateral side. All rats were perfused with Zamboni's fixative solution 14 days after grafting. The samples of grafts and the intact motor branch (n = 6) were dissected and embedded in Durcupan ACM. Semithin sections stained with Toluidine Blue were used for morphometric analysis of myelinated axons by means of computer-assisted image analysis system. Ultrathin sections counterstained with uranyl acetate were viewed and photographed in an electron microscope. The number of myelinated motor axons showing early regeneration under conditions of the cutaneous and motor nerve grafts was similar. The diameter of axons and thickness of their myelin sheaths were significantly smaller when the axons regenerated into the saphenous nerve in contrast to the motor graft. Morphometric analysis of early regeneration of myelinated motor axons suggests that the cutaneous and motor branches of the femoral nerve provide different conditions not for the growth but for the maturation of motor axons.

Myelin formation by Schwann cells in the absence of beta4 integrin

The interaction of the Schwann cell with its basal lamina has been hypothesized to be an important prerequisite for the formation of a myelin sheath in the peripheral nervous system. One possible player in this interaction is beta4 integrin; it is up-regulated during myelin formation and, in association with alpha6 integrin, can interact with particular components of the Schwann cell basal lamina. In order to characterize the functional roles of beta4 integrin during myelination, we investigated myelination in the absence of beta4 integrin, i.e., in peripheral nerve tissue from beta4 integrin-deficient mice. Because the mutants die within several hours after birth, we cultured dorsal root ganglia from neonatal mutants under conditions that promote myelination, quantified the myelin segments by immunofluorescence, and investigated the ultrastructure of the cultured myelin sheaths. In another approach, we quantified the few myelin sheaths that are detectable in femoral nerves of newborn animals. Based on both approaches, we conclude that myelination by Schwann cells can occur in the absence of beta4 integrin demonstrating that this Schwann cell component is dispensable for myelin formation in peripheral nerves.

A novel transgenic technique that allows specific marking of the neural crest cell lineage in mice

Neural crest cells are embryonic, multipotent stem cells that give rise to various cell/tissue types and thus serve as a good model system for the study of cell specification and mechanisms of cell differentiation. For analysis of neural crest cell lineage, an efficient method has been devised for manipulating the mouse genome through the Cre-loxP system. We generated transgenic mice harboring a Cre gene driven by a promoter of protein 0 (P0). To detect the Cre-mediated DNA recombination, we crossed P0-Cre transgenic mice with CAG-CAT-Z indicator transgenic mice. The CAG-CAT-Z Tg line carries a lacZ gene downstream of a chicken beta-actin promoter and a "stuffer" fragment flanked by two loxP sequences, so that lacZ is expressed only when the stuffer is removed by the action of Cre recombinase. In three different P0-Cre lines crossed with CAG-CAT-Z Tg, embryos carrying both transgenes showed lacZ expression in tissues derived from neural crest cells, such as spinal dorsal root ganglia, sympathetic nervous system, enteric nervous system, and ventral craniofacial mesenchyme at stages later than 9.0 dpc. These findings give some insights into neural crest cell differentiation in mammals. We believe that P0-Cre transgenic mice will facilitate many interesting experiments, including lineage analysis, purification, and genetic manipulation of the mammalian neural crest cells.

Inherited demyelinating neuropathies: from gene to disease

Hereditary peripheral neuropathies have traditionally been classified by the clinical disease pattern and mode of inheritance. It only recently became possible to provide a more precise subdivision of the diseases by the discovery of distinct genetic defects. Most inherited peripheral neuropathies are caused by distinct mutations in the genes of three well known myelin components, peripheral myelin protein 22, P0 and the gap junction protein connexin 32. The present review addresses the expression and functional roles of these myelin components, as well as the putative pathomechanisms caused by distinct mutations in the corresponding genes. Moreover, the suitability of mutant animals, such as knock-out mice and transgenic rodents, as artificial models for these diseases and their use in the study of possible treatment strategies are discussed.

The effect of the mouse mutation claw paw on myelination and nodal frequency in sciatic nerves

Despite the biophysical and clinical importance of differentiating nodal and internodal axolemma, very little is known about the process. We chose to study myelination and node of Ranvier formation in the hypomyelinating mouse mutant claw paw (clp). The phenotype of clp is delayed myelination in the peripheral nervous system. The specific defect is unknown but is thought to arise from a breakdown in the complex signaling mechanism between axon and Schwann cell. Myelination was assessed in sciatic nerve cross sections from adult and postnatal day 14 (P14) heterozygous and homozygous clp mice. Antibodies to P0, myelin-associated glycoprotein (MAG), and neural cell adhesion molecule were used to assess the stage of myelination. P14 homozygous clp mice showed an atypical staining pattern of immature myelin, which resolved into a relatively normal pattern by adulthood. Sodium channel clustering and node of Ranvier frequency were studied in whole-mount sciatic nerves with sodium channel and MAG antibodies. P14 homozygous clp nerves again showed an atypical, immature pattern with diffuse sodium channel clusters suggesting nodal formation was delayed. In the adult, homozygous clp sciatic nerves displayed dramatically shortened internodal distances. The data from this study support the hypotheses that node of Ranvier formation begins with the onset of myelination and that the number and location of nodes of Ranvier in the sciatic nerve are determined by myelinating Schwann cells.

The role of glycosphingolipids in neurological disorders. Mechanisms of immune action

Specific criteria that are required for understanding the significance of glycosphingolipid (GSL) antibodies, as well as mechanisms that may underlie the immunopathogenesis of these disorders, are proposed. These criteria are illustrated by describing the role of a unique family of acidic GSLs, the sulfated glucuronosyl glycolipids (SGGLs), in the pathogenic mechanisms of peripheral neuropathy with IgM paraproteinemia. High anti-SGGL antibody titers are detected in patients suffering from this disorder. It is demonstrated that SGGLs, which possess a common carbohydrate epitope with myelin-associated glycoprotein (MAG), several low-molecular-weight glycoproteins in the PNS, and a number of cell adhesion molecules, are potential target antigens for the neuropathy. Evidence is provided that sensitization of laboratory animals with pure SGGLs elicits experimental peripheral neuropathies that exhibit remarkable similarities with respect to antibody specificity, and electrophysiological and pathological features to the human conditions. By intraneural injection of antibodies into the sciatic nerve of rats, it is demonstrated that pathological changes consisting of demyelination and axonal degeneration are mediated by an antibody- and complement-dependent process. To elucidate the mechanisms of antibody penetration from circulation into the endoneurial space, it is further shown that brain microvascular endothelial cells express SGGLs. Moreover it has been found that inflammatory cytokines are capable of upregulating the expression of SGGLs on the endothelial cell surface, resulting in a greater attachment of leukocytes. This latter observation suggests that SGGLs may also participate in cell-mediated responses in certain inflammatory neurological disorders.

Formation and maintenance of the myelin sheath in the peripheral nerve: roles of cell adhesion molecules and the gap junction protein connexin 32

Based on previous in vitro studies, the cell adhesion molecules L1, N-CAM, MAG, and P0, which all belong to the immunoglobulin (Ig)-superfamily, have been suggested to mediate myelin formation in the peripheral nervous system. Unexpectedly, studies in mice deficient for the corresponding molecules revealed that only P0 plays pivotal roles during the formation of peripheral nerve myelin in vivo, while L1-, N-CAM-, and MAG-deficient mice develop myelin of normal ultrastructure. However, MAG turned out to be important for the maintenance of myelin, as reflected by degeneration of myelin and axons in MAG-deficient mice older than 6 months. The MAG-mediated maintenance of myelin is backed up by N-CAM, since mice deficient in both MAG and N-CAM show an earlier and more prominent myelin degeneration than MAG single mutants. Another peripheral nerve component involved in the maintenance of myelinated fibers is connexin 32 (Cx32), a gap junction channel protein that does not belong to the Ig-superfamily. Mice deficient in Cx32 initially form normal myelin, which then develops blown-up periaxonal collars and abnormally shaped non-compacted regions followed by myelin and axonal degeneration. Our findings strongly support the view that very few myelin components are necessary for myelin formation whereas the maintenance of myelin is much more sensitive to molecular alterations. In addition, it became evident that myelin molecules can fulfill functionally overlapping roles that ensure that myelination takes place even under conditions in which there is a deficiency in the normal molecular components of myelin.

Sodium channel distribution in axons of hypomyelinated and MAG null mutant mice

Na+ channel organization was studied with immunofluorescence in the peripheral nervous system of mice genetically altered to produce abnormal myelin. In two of these strains, transcription of inserted transgenes was targeted to myelinating Schwann cells through linkage to a promoter for the myelin protein P0. Adults of both of these strains had hindlimb paralysis and a tremor on lifting by the tail. In one case, Schwann cells were eliminated via expression of the diphtheria toxin A chain (DT-A). During postnatal days 3-7, Na+ channel clustering at forming nodes was dramatically reduced compared with that of normal animals. At 1-3 months of age, Na+ channel immunofluorescence was often found spread over long stretches of the axolemma, instead of being confined to nodal gaps. In the second P0-linked transgenic model, Schwann cell expression of the large T antigen tsA-1609 resulted in cell cycle dysfunction. Adult axons had regions of diffuse Na+ channel labeling. Focal clusters were rare within these zones, which were characterized by a series of cells of myelinating phenotype tightly apposed to the axon. Previous studies suggested that Schwann cells had to reach the stage of ensheathment characterized by periaxonal myelin associated glycoprotein (MAG) expression in order to induce Na+ channel clustering. However, in MAG-deficient mice, Na+ channel labeling patterns within sciatic nerves were normal.

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.

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.

Periaxin expression in myelinating Schwann cells: modulation by axon-glial interactions and polarized localization during development

Periaxin is a newly described protein that is expressed exclusively by myelinating Schwann cells. In developing nerves, periaxin is first detected as Schwann cells ensheathe axons, prior to the appearance of the proteins that characterize the myelin sheath. Periaxin is initially concentrated in the adaxonal membrane (apposing the axon) but, during development, as myelin sheaths mature, periaxin becomes predominately localized at the abaxonal Schwann cell membrane (apposing the basal lamina). In permanently axotomized adult nerves, periaxin is lost from the abaxonal and adaxonal membranes, becomes associated with degenerating myelin sheaths and is phagocytosed by macrophages. In crushed nerves, in which axons regenerate and are remyelinated, periaxin is first detected in the adoxonal membrane as Schwann cells ensheathe regenerating axons, but again prior to the appearance of other myelin proteins. Periaxin mRNA and protein levels change in parallel with those of other myelin-related genes after permanent axotomy and crush. These data demonstrate that periaxin is expressed by myelinating Schwann cells in a dynamic, developmentally regulated manner. The shift in localization of periaxin in the Schwann cell after completion of the spiralization phase of myelination suggests that periaxin participates in membrane-protein interactions that are required to stabilize the mature myelin sheath.

Several extracellular domains of the neural cell adhesion molecule L1 are involved in homophilic interactions

The neural cell adhesion molecule L1 is a multidomain protein that plays important roles in cell adhesion, migration, and neurite outgrowth. It can interact with itself by a self-binding, i.e., homophilic adhesion mechanism (Kadmon et al.: J Cell Biol 110: 193-208, 1990a). To determine the domains of L1 involved in homophilic binding, we have generated protein fragments of L1 in a prokaryotic and a eukaryotic expression system and used these covalently coupled to fluorescent microspheres to quantify aggregation between them by cytofluorometric analysis. Protein fragments containing the first and second Ig-like domains and the third fibronectin type III homologous repeat showed avid self-binding. Ig-like domains III and IV also showed some self-binding, whereas Ig-like domains V and VI and fibronectin type III homologous repeats 1 and 2 as well as 4 and 5 were less or not active. Binding between different domains was also observed: fibronectin type III homologous repeats 4 and 5 interacted with Ig-like domains I and II, and fibronectin type III homologous repeats 3-5 interacted with all Ig-like domains. These results were confirmed by experiments testing the binding of fragment-conjugated microspheres to substrate-coated L1 or to cell surface-expressed L1 on cultured neurons. Binding of L1 to itself was interfered with by all protein fragments tested, suggesting that also less avidly binding domains of L1 contribute to homophilic binding. These observations indicate prominent functional roles of both Ig-like domains and fibronectin type III homologous repeats in homophilic binding of L1.

Recognition molecules myelin-associated glycoprotein and tenascin-C inhibit integrin-mediated adhesion of neural cells to collagen

Because of the importance of collagens in mediating cell-substrate interactions and the association of collagens with neural recognition molecules in the peripheral nervous system, the ability of neural recognition molecules to modify the substrate properties of collagens, in particular collagen type I, for cell adhesion was determined. Two cell lines, the N2A neuroblastoma and PC12 pheochromocytoma, were investigated for their capacity to adhere to different collagen types in the absence or presence of several neural recognition molecules. Adhesion of N2A or PC12 cells and membrane vesicles from PC12 cells to collagen type I was reduced when the collagen had been preincubated prior to its application as substrate with the extracellular domain of myelin-associated glycoprotein (s-MAG) or, as control, fibroblast tenascin-C (F-tenascin). In mixture with other collagen types, s-MAG was only able to reduce the adhesiveness of collagen types III and V, but not of collagen types II and IV. F-tenascin reduced the adhesiveness of all collagen types tested. In contrast to F-tenascin, s-MAG had to be present during fibrillogenesis to exert its effect, indicating that it must be coassembled into the collagen fibril to block the binding site. Cell adhesion to collagen type I was dependent on Mg2+ or Mn2+ and inhibited by a monoclonal antibody to the alpha 1 integrin subunit. The combined observations indicate that s-MAG and F-tenascin interfere with cell binding, most probably by modifying the integrin binding site, and that the two molecules act by different mechanisms, both leading to reduction of adhesion.

Myelin glycoprotein P0 is expressed at early stages of chicken and rat embryogenesis

Although previous studies suggest that P0 is expressed only in myelinating Schwann cells, monoclonal antibody 1E8 reacts with P0, yet also stains early Schwann cell precursors and non-myelinating Schwann cells (Bhattacharyya et al.: Neuron 7:831-844, 1991). We therefore characterized the 1E8 epitope and analyzed P0 mRNA expression during development. Immunoblot analyses of P0 fusion proteins and of deglycosylated P0 indicated that the 1E8 epitope is polypeptide. Northern blot and polymerase chain reaction (PCR) analyses revealed that P0 is encoded by a single mRNA that is expressed in chicken embryos as early as E4 and in rat embryos as early as E14. These data indicate that the antigen recognized by 1E8 in early chicken embryos is P0 and that, during development of both chickens and rats, P0 mRNA is expressed long before myelination.

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.

The L2/HNK-1 carbohydrate is carried by the myelin associated glycoprotein and sulphated glucuronyl glycolipids in muscle but not cutaneous nerves of adult mice

We have previously shown that myelinating Schwann cells associated with motor, but not sensory, axons in peripheral nerves of adult mice express the L2/HNK-1 carbohydrate epitope. This carbohydrate structure carried by glycolipids and neural cell adhesion molecules has been suggested to specifically foster regrowth of motor as opposed to sensory axons after infliction of a lesion. To determine which molecular components may be the carriers of the L2 carbohydrate in motor axon-associated myelinating Schwann cells, we have isolated the purely sensory, cutaneous branch and the mixed sensory and motor muscle branch of the femoral nerve of adult mice, isolated the myelin fraction thereof and analysed the molecules expressing the L2 carbohydrate by several immunochemical methods. L2 immunoreactivity in myelin of the muscle branch was four to five times higher than that of the cutaneous branch. The 110 kDa L2-immunoreactive glycoprotein in myelin of the muscle branch, which is not L2-immunoreactive in the cutaneous branch, was identified as the myelin-associated glycoprotein by a combination of immunoprecipitation and Western blot analysis. Myelin extraction with organic solvents additionally revealed the two L2-carrying glycolipids, which amounted to approximately 40 ng glycolipid/mg dry weight in myelin of the muscle branch, whereas no significant amounts of the L2 glycolipids were found in myelin of the cutaneous branch. These observations suggest an astonishing degree of differential regulation of carbohydratesynthesizing activities in myelinating Schwann cells.

Expression and biological functions of sulfoglucuronyl glycolipids (SGGLs) in the nervous system--a review

Sulfoglucuronyl carbohydrate linked to neolactotetraose reacts with HNK-1 antibody. The HNK-1 carbohydrate epitope is found in two major glycolipids, several glycoproteins and in some proteoglycans of the nervous system. Most of the HNK-1 reactive glycoproteins so far identified are neural cell adhesion molecules and/or are involved in cell-cell interactions. HNK-1 carbohydrate is highly immunogenic. Several HNK-1-like antibodies, including IgM of some patients with plasma cell abnormalities and having peripheral neuropathy, have been described. This article summarizes published work mainly on sulfoglucuronyl glycolipids, SGGLs and covers: structural requirements of the carbohydrate epitope for binding to HNK-1 and human antibodies, expression of the lipids in various neural areas, stage and region specific developmental expression in CNS and PNS, immunocytochemical localization, loss of expression in Purkinje cell abnormality murine mutations, biosynthetic regulation of expression by a single enzyme N-acetylglucosaminyl transferase, identification of receptor-like carbohydrate binding neural proteins (lectins), and perceived role of the carbohydrate in physiological functions. The latter includes role in: pathogenesis of certain peripheral neuropathies, in migration of neural crest cells, as a ligand in cell-cell adhesion/interaction and as a promoter of neurite outgrowth for motor neurons. Multiple expression of HNK-1 carbohydrate in several molecules and in various neural cell types at specific stages of nervous system development has puzzled investigators as to its specific biological function, but this may also suggest its importance in multiple systems during cell differentiation and migration processes.

Mice deficient for the myelin-associated glycoprotein show subtle abnormalities in myelin

Using homologous recombination in embryonic stem cells, we have generated mice with a null mutation in the gene encoding the myelin-associated glycoprotein (MAG), a recognition molecule implicated in myelin formation. MAG-deficient mice appeared normal in motor coordination and spatial learning tasks. Normal myelin structure and nerve conduction in the PNS, with N-CAM overexpression at sites normally expressing MAG, suggested compensatory mechanisms. In the CNS, the onset of myelination was delayed, and subtle morphological abnormalities were detected in that the content of oligodendrocyte cytoplasm at the inner aspect of most myelin sheaths was reduced and that some axons were surrounded by two or more myelin sheaths. These observations suggest that MAG participates in the formation of the periaxonal cytoplasmic collar of oligodendrocytes and in the recognition between oligodendrocyte processes and axons.

Myelin-associated glycoprotein is not detectable in perikaryal myelin of spiral ganglion neurons of adult mice

The expression of the adhesion molecules N-CAM (neural cell adhesion molecule), L1, myelin-associated glycoprotein (MAG), and P0 has been investigated by postembedding-immunoelectron microscopy in spiral ganglia of adult mice in which both axons and neuronal cell bodies are myelinated. L1 was absent from both axonal and perikaryal myelin of spiral ganglion neurons but was expressed on unmyelinated nerve fibers. P0 was expressed in compacted parts of both types of myelin but was absent from non-compacted myelin. MAG was not detectable in perikaryal myelin but was strongly expressed in axonal myelin both periaxonally and in non-compacted regions. Conversely, N-CAM was detectable at the interface between myelin and neuronal perikarya and in non-compacted regions of perikaryal myelin, whereas it was hardly detectable in axonal myelin. This inverse expression of MAG and N-CAM in perikaryal and axonal myelin points to the possibility that N-CAM may functionally replace MAG in perikaryal myelin of spiral ganglion neurons.

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.

Expression and functional roles of neural cell surface molecules and extracellular matrix components during development and regeneration of peripheral nerves

By combining both immunocytochemical and functional investigations, a hypothetical framework will be developed for the molecular mechanisms underlying neuron-glia interactions during development and regeneration of peripheral nerves. In particular, the immunoglobulin-like molecules L1, N-CAM, MAG and P0, the extracellular matrix molecules laminin and tenascin, and the carbohydrates PSA and L2/HNK-1 will be considered. During early stages of limb bud innervation in embryos, L1 and N-CAM are expressed on axons and Schwann cells and are involved in axonal fasciculation, whereas tenascin is thought to be involved in forming a scaffold around the nerve possibly preventing axons and/or Schwann cells from leaving the nerve. PSA has been shown to be involved in pathway selection at initial stages of limb bud innervation. Later on, when motor axons enter muscles, the carbohydrates determine the branching pattern of the nerves. During myelination, L1 appears to play a pivotal role during the formation of the first Schwann cell loops around the prospective myelin-containing axons. MAG and P0 appear also to be functionally involved at initial stages of myelin formation. Additionally, MAG may contribute to the formation and maintenance of non-compacted myelin and axon-Schwann cell apposition whereas P0 is involved in myelin compaction. Under regenerative conditions, L1, N-CAM, laminin, and tenascin are strongly up-regulated by denervated Schwann cells. In vitro observations strongly suggest that these molecules might foster axonal regeneration. The carbohydrate PSA is confined to regrowing axons and is also a candidate to support axonal regrowth. L2/HNK-1, which is found on motor axon-associated Schwann cells, may provide regenerating motor axons with a selective advantage over others resulting in appropriate reinnervation of motor pathways. Since many of the functional studies this review refers to have been performed in vitro, some of the conclusions drawn need reexamination in vivo. Gene manipulations, such as the generation of null mutants followed by a thorough morphological and immunocytochemical investigation may be a powerful tool to resolve this problem.

Transgenic and knock-out mice: models of neurological disease

Besides providing useful model systems for basic science, studies based on modification of the mammalian germ line are changing our understanding of pathogenetic principles. In this article, we review the most popular techniques for generating specific germ line mutations in vivo and discuss the impact of various transgenic models on the study of neurodegenerative diseases. The "gain of function" approach, i.e., ectopic expression of exogenous genes in neural structures, has deepened our understanding of neurodegeneration resulting from infection with papova viruses, picorna viruses, and human retroviruses. Further, inappropriate expression of mutated cellular molecules in the nervous system of transgenic mice is proving very useful for studying conditions whose pathogenesis is controversial, such as Alzheimer's disease and motor neuron diseases. As a complementary approach, ablation of entire cell lineages by tissue-specific expression of toxins has been useful in defining the role of specific cellular compartments. Modeling of recessive genetic diseases, such as Lesch-Nyhan syndrome, was helped by the development of techniques for targeted gene deletion (colloquially termed "gene knock-out"). Introduction of subtle homozygous mutations in the mouse genome was made possible by the latter approach. Such "loss of function" mutants have been used for clarifying the role of molecules thought to be involved in development and structural maintenance of the nervous system, such as the receptors for nerve growth factor and the P0 protein of peripheral myelin. In addition, these models are showing their assets also in the study of enigmatic diseases such as spongiform encephalopathies.

Expression of a P0-like glycoprotein in central nervous system myelin of amphibians (Ambystoma mexicanus, Xenopus laevis and Rana catesbeiana)

1. The myelin protein profiles in the CNS and PNS of three species of amphibians were analyzed by biochemical and immunohistochemical methods. 2. The CNS myelin of the African clawed frog (Xenopus) and the Mexican salamander (axolotl) contained, in addition to proteolipid protein, a unique protein zero (P0)-like protein, whereas the adult bullfrog did not. 3. A strong expression of the P0-like protein in the bullfrog CNS myelin was found transiently at ontogenetically early phases including at the time of metamorphosis. 4. The CNS P0-like protein and the PNS P0 protein showed a difference in reactivity with lectins and anti-L2/HNK-1 antibodies, suggesting that the two proteins differ in some aspects of their carbohydrate structures.

Glycosphingolipids of human peripheral nervous system myelins isolated from cauda equina

Compositions of neutral and sulfated glucuronyl glycosphingolipids purified from human motor and sensory nerves and myelins were studied. Higher neutral glycosphingolipids (fraction B), which were separated from GalCer (fraction A), were analyzed by TLC and TLC-immunostaining. Both nerve myelins contained paragloboside (nLc4Cer) and nLc6Cer dominantly as major higher glycosphingolipids and very little globoside (Gb4Cer), whereas both nerves contained Gb4Cer and nLc4Cer. Besides these major glycosphingolipids, a neutral glycolipid containing asialoGM1 (Gg4Cer) epitope and other minor components such as ceramide trihexoside and ceramide dihexoside were detected in both nerves and their myelins. Furthermore, sulfated glucuronyl nLc4Cer and nLc6Cer, which were monoclonal antibody HNK-1 reactive glycolipids, were detected in both nerves and myelins.