Binding of soluble myelin-associated glycoprotein to specific gangliosides induces the association of p75NTR to lipid rafts and signal transduction

One Raft to Guide Them All, and in Axon Regeneration Inhibit Them

Central nervous system damage caused by traumatic injuries, iatrogenicity due to surgical interventions, stroke and neurodegenerative diseases is one of the most prevalent reasons for physical disability worldwide. During development, axons must elongate from the neuronal cell body to contact their precise target cell and establish functional connections. However, the capacity of the adult nervous system to restore its functionality after injury is limited. Given the inefficacy of the nervous system to heal and regenerate after damage, new therapies are under investigation to enhance axonal regeneration. Axon guidance cues and receptors, as well as the molecular machinery activated after nervous system damage, are organized into lipid raft microdomains, a term typically used to describe nanoscale membrane domains enriched in cholesterol and glycosphingolipids that act as signaling platforms for certain transmembrane proteins. Here, we systematically review the most recent findings that link the stability of lipid rafts and their composition with the capacity of axons to regenerate and rebuild functional neural circuits after damage.

Lipids in regulating oligodendrocyte structure and function

Oligodendrocytes enwrap central nervous system axons with myelin, a lipid enriched highly organized multi-layer membrane structure that allows for fast long-distance saltatory conduction of neuronal impulses. Myelin has an extremely high lipid content (∼80 % of its dry weight) and a peculiar lipid composition, with a 2:2:1 cholesterol:phospholipid:glycolipid ratio. Inherited neurodegenerative diseases of the lipids (caused by mutations in lipogenic enzymes) often present oligodendrocyte and/or myelin defects which contribute to the overall disease pathophysiology. These phenomena triggered an increasing number of studies over the functions lipid exert to shape and maintain myelin, and brought to the finding that lipids are more than only structural building blocks. They act as signaling molecules to drive proliferation and differentiation of oligodendrocyte progenitor cells, as well as proliferation of premyelinating oligodendrocytes, and their maturation into myelinating ones. Here, we summarize key findings in these areas, while presenting the main related human diseases. Despite many advances in the field, various questions remain open which we briefly discuss. This article is part of a special issue entitled "Role of Lipids in CNS Cell Physiology and Pathology".

Nogo-66 Restricts Synaptic Strengthening via Lingo1 and the ROCK2-Cofilin Pathway to Control Actin Dynamics

Nogo-A restricts long-term potentiation (LTP) at the Schaffer collateral-CA1 pathway in the adult hippocampus via 2 extracellular domains: Nogo-A-Δ20 and Nogo-66. Nogo-66 signals via Nogo Receptor 1 (NgR1) to regulate synaptic function. Whether the NgR1 coreceptors Lingo1 and p75NTR are involved in the signaling in this context is still not known. Moreover, the intracellular cascade mediating the activity of Nogo-66 in restricting LTP is unexplored. We combine electrophysiology and biochemistry in acute hippocampal slices and demonstrate that a loss of function for Lingo1 results in a significant increase in LTP levels at the Schaffer collateral-CA1 pathway, and that Lingo1 is the NgR1 coreceptor mediating the role of Nogo-66 in restricting LTP. Our data show that p75NTR is not involved in mediating the Nogo-66 effect on LTP. Moreover, loss of function for p75NTR and NgR1 equally attenuate LTD, suggesting that p75NTR might mediate the NgR1-dependent regulation of LTD, independently of Nogo-66. Finally, our results indicate that Nogo-66 signaling limits LTP via the ROCK2-Cofilin pathway to control the dynamics of the actin cytoskeleton. The present results elucidate the signaling pathway activated by Nogo-66 to control LTP and contribute to the understanding of how Nogo-A stabilizes the neural circuits to limit activity-dependent plasticity events in the mature hippocampus.

The Involvement of the Myelin-Associated Inhibitors and Their Receptors in CNS Plasticity and Injury

The limited capacity for the central nervous system (CNS) to repair itself was first described over 100 years ago by Spanish neuroscientist Ramon Y. Cajal. However, the exact mechanisms underlying this failure in neuronal regeneration remain unclear and, as such, no effective therapeutics yet exist. Numerous studies have attempted to elucidate the biochemical and molecular mechanisms that inhibit neuronal repair with increasing evidence suggesting that several inhibitory factors and repulsive guidance cues active during development actually persist into adulthood and may be contributing to the inhibition of repair. For example, in the injured adult CNS, there are various inhibitory factors that impede the outgrowth of neurites from damaged neurons. One of the most potent of these neurite outgrowth inhibitors is the group of proteins known as the myelin-associated inhibitors (MAIs), present mainly on the membranes of oligodendroglia. Several studies have shown that interfering with these proteins can have positive outcomes in CNS injury models by promoting neurite outgrowth and improving functional recovery. As such, the MAIs, their receptors, and downstream effectors are valid drug targets for the treatment of CNS injury. This review will discuss the current literature on MAIs in the context of CNS development, plasticity, and injury. Molecules that interfere with the MAIs and their receptors as potential candidates for the treatment of CNS injury will additionally be introduced in the context of preclinical and clinical trials.

The Role of Neurotrophin Signaling in Gliomagenesis: A Focus on the p75 Neurotrophin Receptor (p75NTR/CD271)

The p75 neurotrophin receptor (p75^NTR, a.k.a. CD271), a transmembrane glycoprotein and a member of the tumor necrosis family (TNF) of receptors, was originally identified as a nerve growth factor receptor in the mid-1980s. While p75^NTR is recognized to have important roles during neural development, its presence in both neural and nonneural tissues clearly supports the potential to mediate a broad range of functions depending on cellular context. Using an unbiased in vivo selection paradigm for genes underlying the invasive behavior of glioma, a critical characteristic that contributes to poor clinical outcome for glioma patients, we identified p75^NTR as a central regulator of glioma invasion. Herein we review the expanding role that p75^NTR plays in glioma progression with an emphasis on how p75^NTR may contribute to the treatment refractory nature of glioma. Based on the observation that p75^NTR is expressed and functional in two critical glioma disease reservoirs, namely, the highly infiltrative cells that evade surgical resection, and the radiation- and chemotherapy-resistant brain tumor-initiating cells (also referred to as brain tumor stem cells), we propose that p75^NTR and its myriad of downstream signaling effectors represent rationale therapeutic targets for this devastating disease. Lastly, we provide the provocative hypothesis that, in addition to the well-documented cell autonomous signaling functions, the neurotrophins, and their respective receptors, contribute in a cell nonautonomous manner to drive the complex cellular and molecular composition of the brain tumor microenvironment, an environment that fuels tumorigenesis.

Detection of Naturally Occurring Human Antibodies Against Gangliosides by ELISA

Gangliosides are sialic acid-containing glycolipids that have been considered attractive targets for cancer immunotherapy, based on the qualitative and quantitative changes they suffer during malignant transformation and due to their importance for tumor biology. Natural antibodies against gangliosides have been detected not only in cancer patients but also in healthy donors. The presence of these antibodies can be used as diagnostic or prognostic factor. However, these responses are difficult to detect because anti-ganglioside antibodies are usually of IgM isotype and low affinity. Enzyme Linked Immunosorbent Assay (ELISA) is an immunoassay based on the specific binding of antibodies to antigens bound to a solid phase. These antigens can be glycolipids like gangliosides. An enzyme linked to the last reactant allows the detection of specific binding through the development of color after the addition of a suitable substrate. ELISA combines the specificity of antibodies with the sensitivity of enzyme reactions. The ELISA method described herein can be used to detect antibody responses against gangliosides not only related to cancer but also to autoimmune diseases and infections, both in healthy donors, and patients, untreated or receiving specific immunotherapy.

A monoclonal natural human IgM protects axons in the absence of remyelination

BACKGROUND

Whereas demyelination underlies early neurological symptoms in multiple sclerosis (MS), axonal damage is considered critical for permanent chronic deficits. Intracerebral infection of susceptible mouse strains with Theiler's murine encephalomyelitis virus (TMEV) results in chronic induced demyelinating disease (TMEV-IDD) with progressive axonal loss and neurologic dysfunction similar to progressive forms of MS. We previously reported that treatment of chronic TMEV-IDD mice with a neurite outgrowth-promoting natural human antibody, HIgM12, improved brainstem NAA concentrations and preserved functional motor activity. In order to translate this antibody toward clinical trial, we generated a fully human recombinant form of HIgM12, rHIgM12, determined the optimal in vivo dose for functional improvement in TMEV-IDD, and evaluated the functional preservation of descending spinal cord axons by retrograde labeling.

FINDINGS

SJL/J mice at 45 to 90 days post infection (dpi) were studied. A single intraperitoneal dose of 0.25 mg/kg of rHIgM12 per mouse is sufficient to preserve motor function in TMEV-IDD. The optimal dose was 10 mg/kg. rHIgM12 treatment protected the functional transport in spinal cord axons and led to 40 % more Fluoro-Gold-labeled brainstem neurons in retrograde transport studies. This suggests that axons are not only present but also functionally competent. rHIgM12-treated mice also contained more mid-thoracic (T6) spinal cord axons than controls.

CONCLUSIONS

This study confirms that a fully human recombinant neurite outgrowth-promoting monoclonal IgM is therapeutic in a model of progressive MS using multiple reparative readouts. The minimum effective dose is similar to that of a remyelination-promoting monoclonal human IgM discovered by our group that is presently in clinical trials for MS.

Sialic Acid Is Required for Neuronal Inhibition by Soluble MAG but not for Membrane Bound MAG

Myelin-Associated Glycoprotein (MAG), a major inhibitor of axonal growth, is a member of the immunoglobulin (Ig) super-family. Importantly, MAG (also known as Siglec-4) is a member of the Siglec family of proteins (sialic acid-binding, immunoglobulin-like lectins), MAG binds to complex gangliosides, specifically GD1a and/or GT1b. Therefore, it has been proposed as neuronal receptors for MAG inhibitory effect of axonal growth. Previously, we showed that MAG binds sialic acid through domain 1 at Arg118 and is able to inhibit axonal growth through domain 5. We developed a neurite outgrowth (NOG) assay, in which both wild type MAG and mutated MAG (MAG Arg118) are expressed on cells. In addition we also developed a soluble form NOG in which we utilized soluble MAG-Fc and mutated MAG (Arg118-Fc). Only MAG-Fc is able to inhibit NOG, but not mutated MAG (Arg118)-Fc that has been mutated at its sialic acid binding site. However, both forms of membrane bound MAG- and MAG (Arg118)- expressing cells still inhibit NOG. Here, we review various results from different groups regarding MAG’s inhibition of axonal growth. Also, we propose a model in which the sialic acid binding is not necessary for the inhibition induced by the membrane form of MAG, but it is necessary for the soluble form of MAG. This finding highlights the importance of understanding the different mechanisms by which MAG inhibits NOG in both the soluble fragmented form and the membrane-bound form in myelin debris following CNS damage.

Glioma invasion mediated by the p75 neurotrophin receptor (p75(NTR)/CD271) requires regulated interaction with PDLIM1

The invasive nature of glioblastoma renders them incurable by current therapeutic interventions. Using a novel invasive human glioma model, we previously identified the neurotrophin receptor p75^NTR (aka CD271) as a mediator of glioma invasion. Herein, we provide evidence that preventing phosphorylation of p75^NTR on S303 by pharmacological inhibition of PKA, or by a mutational strategy (S303G), cripples p75^NTR-mediated glioma invasion resulting in serine phosphorylation within the C-terminal PDZ-binding motif (SPV) of p75^NTR. Consistent with this, deletion (ΔSPV) or mutation (SPM) of the PDZ motif results in abrogation of p75^NTR-mediated invasion. Using a peptide-based strategy, we identified PDLIM1 as a novel signaling adaptor for p75^NTR and provide the first evidence for a regulated interaction via S425 phosphorylation. Importantly, PDLIM1 was shown to interact with p75^NTR in highly invasive patient-derived glioma stem cells/tumor-initiating cells and shRNA knockdown of PDLIM1 in vitro and in vivo results in complete ablation of p75^NTR-mediated invasion. Collectively, these data demonstrate a requirement for a regulated interaction of p75^NTR with PDLIM1 and suggest that targeting either the PDZ domain interactions and/or the phosphorylation of p75^NTR by PKA could provide therapeutic strategies for patients with glioblastoma.

Attractive and repulsive factors act through multi-subunit receptor complexes to regulate nerve fiber growth

In the nervous system, attractive and repulsive factors guide neuronal growth, pathfinding and target innervation during development, learning and regeneration after injury. Repulsive and growth-inhibitory factors, such as some ephrins, semaphorins, netrins and myelin-associated growth inhibitors, restrict nerve fiber growth, whereas neurotrophins, and other ephrins, semaphorins and netrins attract fibers and promote neurite growth. Several of these guidance molecules also play crucial roles in vasculogenesis, and regulate cell migration and tissue formation in different organs. Precise and highly specific signal transduction in space and time is required in all these cases, which primarily depends on the presence and function of specific receptors. Interestingly, many of these ligands act through multi-subunit receptor complexes. In this Commentary, we review the current knowledge of how complexes of the receptors for attractive and repulsive neurite growth regulatory factors are reorganized in a spatial and temporal manner, and reveal the implications that such dynamics have on the signaling events that coordinate neurite fiber growth.

A natural human IgM that binds to gangliosides is therapeutic in murine models of amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a devastating, fatal neurological disease that primarily affects spinal cord anterior horn cells and their axons for which there is no treatment. Here we report the use of a recombinant natural human IgM that binds to the surface of neurons and supports neurite extension, rHIgM12, as a therapeutic strategy in murine models of human ALS. A single 200 µg intraperitoneal dose of rHIgM12 increases survival in two independent genetic-based mutant SOD1 mouse strains (SOD1G86R and SOD1G93A) by 8 and 10 days, delays the onset of neurological deficits by 16 days, delays the onset of weight loss by 5 days, and preserves spinal cord axons and anterior horn neurons. Immuno-overlay of thin layer chromatography and surface plasmon resonance show that rHIgM12 binds with high affinity to the complex gangliosides GD1a and GT1b. Addition of rHIgM12 to neurons in culture increases α-tubulin tyrosination levels, suggesting an alteration of microtubule dynamics. We previously reported that a single peripheral dose of rHIgM12 preserved neurological function in a murine model of demyelination with axon loss. Because rHIgM12 improves three different models of neurological disease, we propose that the IgM might act late in the cascade of neuronal stress and/or death by a broad mechanism.

Summary: A single peripheral dose of a recombinant natural human IgM increases lifespan and delays neurological deficits in mouse models of human ALS.

Sialic acids in the brain: gangliosides and polysialic acid in nervous system development, stability, disease, and regeneration

Every cell in nature carries a rich surface coat of glycans, its glycocalyx, which constitutes the cell's interface with its environment. In eukaryotes, the glycocalyx is composed of glycolipids, glycoproteins, and proteoglycans, the compositions of which vary among different tissues and cell types. Many of the linear and branched glycans on cell surface glycoproteins and glycolipids of vertebrates are terminated with sialic acids, nine-carbon sugars with a carboxylic acid, a glycerol side-chain, and an N-acyl group that, along with their display at the outmost end of cell surface glycans, provide for varied molecular interactions. Among their functions, sialic acids regulate cell-cell interactions, modulate the activities of their glycoprotein and glycolipid scaffolds as well as other cell surface molecules, and are receptors for pathogens and toxins. In the brain, two families of sialoglycans are of particular interest: gangliosides and polysialic acid. Gangliosides, sialylated glycosphingolipids, are the most abundant sialoglycans of nerve cells. Mouse genetic studies and human disorders of ganglioside metabolism implicate gangliosides in axon-myelin interactions, axon stability, axon regeneration, and the modulation of nerve cell excitability. Polysialic acid is a unique homopolymer that reaches >90 sialic acid residues attached to select glycoproteins, especially the neural cell adhesion molecule in the brain. Molecular, cellular, and genetic studies implicate polysialic acid in the control of cell-cell and cell-matrix interactions, intermolecular interactions at cell surfaces, and interactions with other molecules in the cellular environment. Polysialic acid is essential for appropriate brain development, and polymorphisms in the human genes responsible for polysialic acid biosynthesis are associated with psychiatric disorders including schizophrenia, autism, and bipolar disorder. Polysialic acid also appears to play a role in adult brain plasticity, including regeneration. Together, vertebrate brain sialoglycans are key regulatory components that contribute to proper development, maintenance, and health of the nervous system.

Spatiotemporal intracellular dynamics of neurotrophin and its receptors. Implications for neurotrophin signaling and neuronal function

Neurons possess a polarized morphology specialized to contribute to neuronal networks, and this morphology imposes an important challenge for neuronal signaling and communication. The physiology of the network is regulated by neurotrophic factors that are secreted in an activity-dependent manner modulating neuronal connectivity. Neurotrophins are a well-known family of neurotrophic factors that, together with their cognate receptors, the Trks and the p75 neurotrophin receptor, regulate neuronal plasticity and survival and determine the neuronal phenotype in healthy and regenerating neurons. Is it now becoming clear that neurotrophin signaling and vesicular transport are coordinated to modify neuronal function because disturbances of vesicular transport mechanisms lead to disturbed neurotrophin signaling and to diseases of the nervous system. This chapter summarizes our current understanding of how the regulated secretion of neurotrophin, the distribution of neurotrophin receptors in different locations of neurons, and the intracellular transport of neurotrophin-induced signaling in distal processes are achieved to allow coordinated neurotrophin signaling in the cell body and axons.

Density-dependent lectin-glycan interactions as a paradigm for conditional regulation by posttranslational modifications

Mice with null mutations in specific Golgi glycosyltransferases show evidence of glycan compensation where missing carbohydrate epitopes are found on biosynthetically related structures. Repetitive saccharide sequences within the larger glycan structures are functional epitopes recognized by animal lectins. These studies provide the first in vivo support for the existence of a feedback system that maintains and regulates glycan epitope density in cells. Receptor regulation by lectin-glycan interactions and the Golgi provides a mechanism for the adaptation of cell surface receptors and solute transporters in response to environmental cues and intracellular signaling. We suggest that other posttranslational modification systems might have similar conditional features regulated by density-dependent ligand-epitope interactions.

Anti-ganglioside anti-idiotypic vaccination: more than molecular mimicry

Surgery, chemotherapy, and radiation therapy are standard modalities for cancer treatment, but the effectiveness of these treatments has reached a plateau. Thus, other strategies are being explored to combine with the current treatment paradigms in order to reach better clinical results. One of these approaches is the active immunotherapy based on the induction of anti-tumor responses by anti-idiotypic vaccination. This approach arose from Jerne’s idiotypic network theory, which postulates that B lymphocytes forms a functional network, with a role in the establishment of the immune repertoires, in the regulation of natural antibody production and even in the establishment of natural tolerance. Due to the large potential diversity of the immunoglobulin variable regions, the idiotypes repertoire can mimic the universe of self and foreign epitopes, even those of non-protein nature, like gangliosides. Gangliosides are sialic acid-containing glycolipids that have been considered attractive targets for cancer immunotherapy, based on the qualitative and quantitative changes they suffer during malignant transformation and due to their importance for tumor biology. Although any idiotype could be able to mimic any antigen, only those related to antigens involved in functions relevant for organism homeostasis, and that in consequence has been fixed by evolution, would be able not only to mimic, but also to activate the idiotypic cascades related with the nominal antigen. The present review updates the results, failures and hopes, obtained with ganglioside mimicking anti-idiotypic antibodies and presents evidences of the existence of a natural response against gangliosides, suggesting that these glycolipids could be idiotypically relevant antigens.

Specificity of peripheral nerve regeneration: interactions at the axon level

Peripheral nerves injuries result in paralysis, anesthesia and lack of autonomic control of the affected body areas. After injury, axons distal to the lesion are disconnected from the neuronal body and degenerate, leading to denervation of the peripheral organs. Wallerian degeneration creates a microenvironment distal to the injury site that supports axonal regrowth, while the neuron body changes in phenotype to promote axonal regeneration. The significance of axonal regeneration is to replace the degenerated distal nerve segment, and achieve reinnervation of target organs and restitution of their functions. However, axonal regeneration does not always allows for adequate functional recovery, so that after a peripheral nerve injury, patients do not recover normal motor control and fine sensibility. The lack of specificity of nerve regeneration, in terms of motor and sensory axons regrowth, pathfinding and target reinnervation, is one the main shortcomings for recovery. Key factors for successful axonal regeneration include the intrinsic changes that neurons suffer to switch their transmitter state to a pro-regenerative state and the environment that the axons find distal to the lesion site. The molecular mechanisms implicated in axonal regeneration and pathfinding after injury are complex, and take into account the cross-talk between axons and glial cells, neurotrophic factors, extracellular matrix molecules and their receptors. The aim of this review is to look at those interactions, trying to understand if some of these molecular factors are specific for motor and sensory neuron growth, and provide the basic knowledge for potential strategies to enhance and guide axonal regeneration and reinnervation of adequate target organs.

Autoantobodies activate small GTPase RhoA to modulate neurite outgrowth

This review illustrates an example of adaptive immune responses (auto-antibodies) modulating growth/repair behavior of neurons in the disease context of Guillain-Barré syndrome (GBS), which is a prototypic autoimmune, acute monophasic disorder of the peripheral nerves that is the commonest cause of acute flaccid paralysis worldwide. Anti-ganglioside antibodies (Abs) are the most commonly recognized autoimmune markers in all forms of GBS and these Abs are associated with poor recovery. Extent of axonal injury and failure of axonal regeneration are critical determinants of recovery after GBS. In this clinical context, our group examined the hypothesis that anti-ganglioside Abs adversely affect axon regeneration after peripheral nerve injury. We show that anti-ganglioside Abs inhibit axon regeneration in preclinical cell culture and animal models. This inhibition is mediated by activation of small GTPase RhoA and its downstream effector Rho kinase (ROCK) by modulation of growth cone extension and associated neurite elongation in neuronal cultures. Our studies suggest that RhoA and ROCK are potential targets for development of novel therapeutic strategies to enhance nerve repair.

Anti-ganglioside antibody-mediated activation of RhoA induces inhibition of neurite outgrowth

Anti-ganglioside antibodies (Abs) are strongly associated with axonal forms of Guillain Barré syndrome (GBS). Some studies indicate that these Abs, including those with GD1a reactivity, are associated with poor prognosis and/or incomplete recovery. We recently demonstrated that a disease-relevant anti-ganglioside Ab with GD1a reactivity inhibits axon regeneration after PNS injury in an animal model (Lehmann et al., 2007). An implication of these findings is that anti-GD1a Abs can mediate inhibition of axon regeneration and limit recovery in some patients with GBS. The downstream inhibitory intracellular signaling that mediates anti-ganglioside Ab-induced axon inhibition remains unclear. In the current study, we show that disease-relevant and GBS patient's anti-ganglioside Abs can inhibit neurite outgrowth in dissociated primary neuronal cultures. Activation of small GTPase RhoA and its key downstream effector Rho kinase (ROCK) are critical mediators of growth cone and neurite outgrowth inhibition. Therefore, we examined the role of these intracellular signaling molecules in our primary neuronal cultures by molecular and pharmacologic approaches. Our results show that the Ab-mediated inhibition of neurite outgrowth involves the activation of RhoA and ROCK pathway and this activation is through the engagement of specific cell-surface gangliosides by Abs. In summary, these studies directly link patient autoantibodies to an intracellular inhibitory signaling pathway involved in anti-ganglioside Ab-mediated inhibition of neurite outgrowth.

Myelin-associated glycoprotein protects neurons from excitotoxicity

In addition to supporting rapid nerve conduction, myelination nurtures and stabilizes axons and protects them from acute toxic insults. One myelin molecule that protects and sustains axons is myelin-associated glycoprotein (MAG). MAG is expressed on the innermost wrap of myelin, apposed to the axon surface, where it interacts with axonal receptors that reside in lateral membrane domains including gangliosides, the glycosylphosphatidylinositol-anchored Nogo receptors, and β1-integrin. We report here that MAG protection extends beyond the axon to the neurons from which those axons emanate, protecting them from excitotoxicity. Compared to wild type mice, Mag-null mice displayed markedly increased seizure activity in response to intraperitoneal injection of kainic acid, an excitotoxic glutamate receptor agonist. Mag-null mice also had larger lesion volumes in response to intrastriatal injection of the excitotoxin NMDA. Prior injection of a soluble form of MAG partially protected Mag-null mice from NMDA-induced lesions. Hippocampal neurons plated on proteins extracted from wild-type rat or mouse myelin were resistant to kainic acid-induced excitotoxicity, whereas neurons plated on proteins from Mag-null myelin were not. Protection was reversed by anti-MAG antibody and replicated by addition of soluble MAG. MAG-mediated protection from excitotoxicity was dependent on Nogo receptors and β1-integrin. We conclude that MAG engages membrane-domain resident neuronal receptors to protect neurons from excitotoxicity, and that soluble MAG mitigates excitotoxic damage in vivo.

Myelin-associated glycoprotein and its axonal receptors

Myelin-associated glycoprotein (MAG) is expressed on the innermost myelin membrane wrap, directly apposed to the axon surface. Although it is not required for myelination, MAG enhances long-term axon-myelin stability, helps to structure nodes of Ranvier, and regulates the axon cytoskeleton. In addition to its role in axon-myelin stabilization, MAG inhibits axon regeneration after injury; MAG and a discrete set of other molecules on residual myelin membranes at injury sites actively signal axons to halt elongation. Both the stabilizing and the axon outgrowth inhibitory effects of MAG are mediated by complementary MAG receptors on the axon surface. Two MAG receptor families have been described, sialoglycans (specifically gangliosides GD1a and GT1b) and Nogo receptors (NgRs). Controversies remain about which receptor(s) mediates which of MAG's biological effects. Here we review the findings and challenges in associating MAG's biological effects with specific receptors.

Role of gangliosides in brain aging and neurodegeneration

Gangliosides are membrane glycosphingolipids bearing sialic acid residues. Within membranes, gangliosides are specifically enriched in highly organized domains, lipid rafts, and are attributed with diverse functions such as intercellular interactions, cell recognition, neurotransmission, and signal transduction. The highest concentration and variability of ganglioside structures are found in the human brain. Specific temporal and regional distribution of brain gangliosides has been reported; moreover, gangliosides may serve as markers of neurodevelopmental stages, aging and neurodegeneration. Brain ganglioside content and composition as well as ganglioside metabolism are altered in Alzheimer’s disease. It appears that the alterations of ganglioside metabolism leading to changes in membrane physico-chemical properties are not merely a consequence of primary pathology, but may also be involved in the early pathogenesis of Alzheimer’s disease through documented effects on APP proteolytic processing and amyloid aggregation. Investigations of glycolipid metabolic alterations which accompany neurodegenerative disorders provide insight into pathogenetic mechanisms and enable recognition of diagnostic markers as well as molecular structures acting as therapeutic tools interfering with cascade of pathological events.

Promoting axonal rewiring to improve outcome after stroke

A limited amount of functional recovery commonly occurs in the weeks and months after stroke, and a number of studies show that such recovery is associated with changes in the brain's functional organization. Measures that augment this reorganization in a safe and effective way may therefore help improve outcome in stroke patients. Here we review some of the evidence for functional and anatomical reorganization under normal physiological conditions, along with strategies that augment these processes and improve outcome after brain injury in animal models. These strategies include counteracting inhibitors of axon growth associated with myelin, activating neurons' intrinsic growth state, enhancing physiological activity, and having behavioral therapy. These approaches represent a marked departure from the recent focus on neuroprotection and may provide a more effective way to improve outcome after stroke.

Developmental expression of the oligodendrocyte myelin glycoprotein in the mouse telencephalon

The oligodendrocyte myelin glycoprotein is a glycosylphosphatidylinositol-anchored protein expressed by neurons and oligodendrocytes in the central nervous system. Attempts have been made to identify the functions of the myelin-associated inhibitory proteins (MAIPs) after axonal lesion or in neurodegeneration. However, the developmental roles of some of these proteins and their receptors remain elusive. Recent studies indicate that NgR1 and the recently discovered receptor PirB restrict cortical synaptic plasticity. However, the putative factors that trigger these effects are unknown. Because Nogo-A is mostly associated with the endoplasmic reticulum and myelin associated glycoprotein appears late during development, the putative participation of OMgp should be considered. Here, we examine the pattern of development of OMgp immunoreactive elements during mouse telencephalic development. OMgp immunoreactivity in the developing cortex follows the establishment of the thalamo-cortical barrel field. At the cellular level, we located OMgp neuronal membranes in dendrites and axons as well as in brain synaptosome fractions and axon varicosities. Lastly, the analysis of the barrel field in OMgp-deficient mice revealed that although thalamo-cortical connections were formed, their targeting in layer IV was altered, and numerous axons ectopically invaded layers II-III. Our data support the idea that early expressed MAIPs play an active role during development and point to OMgp participating in thalamo-cortical connections.

Roles of channels and receptors in the growth cone during PNS axonal regeneration

Neurons in the peripheral nervous system (PNS) are known to maintain a regenerative capacity and will normally regenerate their axons within a permissive growth environment. The success of regeneration in the PNS largely depends on maintenance of the supportive basal lamina membrane, efficient removal of axonal and myelin debris by macrophages and Schwann cells, expression of neurotrophic factors by Schwann cells, and up-regulation of the intrinsic growth program in PNS neurons. The PNS regenerative process is well characterized through initial Wallerian degeneration followed by axonal sprouting, formation of neuronal growth cones, active axonal growth to the target, and finally sensory and motor functional recovery. The initiation and maintenance of active growth cones during peripheral nerve regeneration recapitulate many aspects of early neural development and are achieved through the activation of complex signaling cascades, involving various receptors, channels, cytoplasmic signaling cascades, as well as transcriptional and translational programs. This review focuses on roles of cell surface ion channels and receptors in the growth cone during Wallerian degeneration and axon regeneration in the PNS.

The myelin-associated glycoprotein inhibitor BENZ induces outgrowth and survival of rat dorsal root ganglion cell cultures

The novel myelin-associated glycoprotein (MAG) inhibitor BENZ binds to the N-acetylneuraminic acid (Neu5Ac) portion of the N-terminal Ig-like domain of MAG. Treatment of rat dorsal root ganglion (DRG) cell cultures with BENZ-induced outgrowth of neurofilament 200-positive neurites improved survival of neurons and increased the number of GFAP-positive cells, as determined by fluorescence and confocal laser microscopy and by Western immunoblotting. Furthermore, treatment of DRG cell cultures with BENZ repressed gene and protein expression of the small GTPase RhoA but induced expression of Rho GTP-activating proteins 5 and 24, likely to counteract protein kinase A activity. Specifically, expression of inhibitors of neurite outgrowth, for example, Rock2 and PAK4, was repressed, but cofilin 1, a promoter of axonal growth, was induced. We propose that the MAG inhibitor BENZ abrogates the RhoA-ROCK-cofilin pathway to promote neurite outgrowth. Our findings require confirmation by in vivo animal studies.

Regulation of axonal development by plasma membrane gangliosides

Gangliosides present in the plasma membrane participate in fundamental processes during neuronal development. From the determination and the outgrowth of the axon, to the growth inhibitory activity produced after CNS injury, local interconversion of these glycosphingolipids regulate actin dynamics in a spatially restricted manner by modulating membrane receptors and their downstream signaling pathways. Here, we will review the possible mechanisms underlying these modulations and the potential importance of gangliosides and ganglioside-transforming enzymes as therapeutic targets.

Gangliosides and Nogo receptors independently mediate myelin-associated glycoprotein inhibition of neurite outgrowth in different nerve cells

In the injured nervous system, myelin-associated glycoprotein (MAG) on residual myelin binds to receptors on axons, inhibits axon outgrowth, and limits functional recovery. Conflicting reports identify gangliosides (GD1a and GT1b) and glycosylphosphatidylinositol-anchored Nogo receptors (NgRs) as exclusive axonal receptors for MAG. We used enzymes and pharmacological agents to distinguish the relative roles of gangliosides and NgRs in MAG-mediated inhibition of neurite outgrowth from three nerve cell types, dorsal root ganglion neurons (DRGNs), cerebellar granule neurons (CGNs), and hippocampal neurons. Primary rat neurons were cultured on control substrata and substrata adsorbed with full-length native MAG extracted from purified myelin. The receptors responsible for MAG inhibition of neurite outgrowth varied with nerve cell type. In DRGNs, most of the MAG inhibition was via NgRs, evidenced by reversal of inhibition by phosphatidylinositol-specific phospholipase C (PI-PLC), which cleaves glycosylphosphatidylinositol anchors, or by NEP1-40, a peptide inhibitor of NgR. A smaller percentage of MAG inhibition of DRGN outgrowth was via gangliosides, evidenced by partial reversal by addition of sialidase to cleave GD1a and GT1b or by P4, an inhibitor of ganglioside biosynthesis. Combining either PI-PLC and sialidase or NEP1-40 and P4 was additive. In contrast to DRGNs, in CGNs MAG inhibition was exclusively via gangliosides, whereas inhibition of hippocampal neuron outgrowth was mostly reversed by sialidase or P4 and only modestly reversed by PI-PLC or NEP1-40 in a non-additive fashion. A soluble proteolytic fragment of native MAG, dMAG, also inhibited neurite outgrowth. In DRGNs, dMAG inhibition was exclusively NgR-dependent, whereas in CGNs it was exclusively ganglioside-dependent. An inhibitor of Rho kinase reversed MAG-mediated inhibition in all nerve cells, whereas a peptide inhibitor of the transducer p75(NTR) had cell-specific effects quantitatively similar to NgR blockers. Our data indicate that MAG inhibits axon outgrowth via two independent receptors, gangliosides and NgRs.

Shedding of the p75NTRneurotrophin receptor is modulated by lipid rafts

Protein ectodomain shedding is the proteolytic release of the extracellular domain of membrane-bound proteins. Neurotrophin receptor p75(NTR) is known to be affected by shedding. The present work provides evidence, in rat brain synaptosomes, that p75(NTR) is present in detergent-resistant membranes (DRM), also known as lipid rafts, only in its full-length form. Disrupting the integrity of lipid rafts causes solubilization of p75(NTR) after detergent treatment and enhancement of the shedding. Analyses of the enzymes described as being responsible for p75(NTR) shedding, i.e. tumor necrosis factor alpha convertase (TACE) and presenilin-1 (PS1), revealed that TACE is absent in DRM, while variable proportions of the C-terminal and N-terminal fragments of PS1 are found. In summary, our results point to a role of lipid rafts in the modulation of the shedding of the p75(NTR) receptor.

Myelin-associated glycoprotein-mediated signaling in central nervous system pathophysiology

The myelin-associated glycoprotein (MAG) is a type I membrane-spanning protein expressed exclusively in oligodendrocytes and Schwann cells. It has two generally known pathophysiological roles in the central nervous system (CNS): maintenance of myelin integrity and inhibition of CNS axonal regeneration. The subtle CNS phenotype resulting from genetic ablation of MAG expression has made mechanistic analysis of its functional role in these difficult. However, the past few years have brought some major revelations, particularly in terms of mechanisms of MAG signaling through the Nogo-66 receptor (NgR) complex. Although apparently converging through NgR, a readily noticeable fact is that the neuronal growth inhibitory effect of MAG differs from that of Nogo-66. This may result from the influence of coreceptors in the form of gangliosides or from MAG-specific neuronal receptors such as NgR2. MAG has several other neuronal binding partners, and some of these may modulate its interaction with the NgR complex or downstream signaling. This article discusses new findings in MAG-forward and -reverse signaling and its role in CNS pathophysiology.

Passive immunization with anti-ganglioside antibodies directly inhibits axon regeneration in an animal model

Recent studies have proposed that neurite outgrowth is influenced by specific nerve cell surface gangliosides, which are sialic acid-containing glycosphingolipids highly enriched in the mammalian nervous system. For example, the endogenous lectin, myelin-associated glycoprotein (MAG), is reported to bind to axonal gangliosides (GD1a and GT1b) to inhibit neurite outgrowth. Clustering of gangliosides in the absence of inhibitors such as MAG is also shown to inhibit neurite outgrowth in culture. In some human autoimmune PNS and CNS disorders, autoantibodies against GD1a or other gangliosides are implicated in pathophysiology. Because of neurobiological and clinical relevance, we asked whether anti-GD1a antibodies inhibit regeneration of injured axons in vivo. Passive transfer of anti-GD1a antibody severely inhibited axon regeneration after PNS injury in mice. In mutant mice with altered ganglioside or complement expression, inhibition by antibodies was mediated directly through GD1a and was independent of complement-induced cytolytic injury. The impaired regenerative responses and ultrastructure of injured peripheral axons mimicked the abortive regeneration typically seen after CNS injury. These data demonstrate that inhibition of axon regeneration is induced directly by engaging cell surface gangliosides in vivo and imply that circulating autoimmune antibodies can inhibit axon regeneration through neuronal gangliosides independent of endogenous regeneration inhibitors such as MAG.

Recognition molecules and neural repair

Neural recognition molecules were discovered and characterized initially for their functional roles in cell adhesion as regulators of affinity between cells and the extracellular matrix in vitro. They were then recognized as mediators or co-receptors which trigger signal transduction mechanisms affecting cell adhesion and de-adhesion. Their involvement in contact attraction and repulsion relies on cell-intrinsic properties that are modulated by the spatial contexts of their expression at particular stages of ontogenetic development, in synaptic plasticity and during regeneration after injury. The functional roles of recognition molecules in cell proliferation and migration, determination of developmental fate, growth cone guidance, and synapse formation, stabilization and modulation have been well documented not only by in vitro, but also by in vivo studies that have been greatly aided by generation of genetically altered mice. More recently, the functions of recognition molecules have been investigated under conditions of neural repair and manipulated using a broad range of genetic and pharmacological approaches to achieve a beneficial outcome. The principal aim of most therapeutically oriented approaches has been to neutralize inhibitory factors. However, less attention has been paid to enhancing repair by stimulating the stimulatory factors. When considering potential therapeutic strategies, it is worth considering that a single recognition molecule can possess domains that are conducive or repellent and that the spatial distribution of recognition molecules can determine the overall function: Recognition molecules may be repellent for neurite outgrowth when presented as barriers or steep-concentration gradients and conducive when presented as uniform substrates. The focus of this review will be on the more recent attempts to study the conducive mechanisms with the expectation that they may be able to tip the balance from a regeneration inhospitable to a hospitable environment. It is likely that a combination of the two principles, as multifactorial as each principle may be in itself, will be of therapeutic value in humans.

Ganglioside GD1a is an essential coreceptor for Toll-like receptor 2 signaling in response to the B subunit of type IIb enterotoxin

Innate recognition and signaling by Toll-like receptors (TLRs) is facilitated by functionally associated coreceptors, although the cooperativity mechanisms involved are poorly understood. As a model we investigated TLR2 interactions with the GD1a ganglioside binding subunit of type IIb Escherichia coli enterotoxin (LT-IIb-B(5)). Both LT-IIb-B(5) and a GD1a binding-defective mutant (LT-IIb-B(5)(T13I)) could modestly bind to TLR2, but only the wild-type molecule displayed a dramatic increase in TLR2 binding activity in the presence of GD1a (although not in the presence of irrelevant gangliosides). Moreover, fluorescence resonance energy transfer experiments indicated that LT-IIb-B(5) induces lipid raft recruitment of TLR2 and TLR1 and their clustering with GD1a, in contrast to the GD1a binding-defective mutant, which moreover fails to activate TLR2 signaling. LT-IIb-B(5)-induced cell activation was critically dependent upon the Toll/IL-1 receptor domain-containing adaptor protein, which was induced to colocalize with TLR2 and GD1a, as shown by confocal imaging. Therefore, GD1a provides TLR2 coreceptor function by enabling the ligand to recruit, bind, and activate TLR2. These findings establish a model of TLR2 coreceptor function and, moreover, suggest novel mechanisms of adjuvanticity by non-toxic derivatives of type II enterotoxins dependent upon GD1a/TLR2 cooperative activity.

Myelin-associated glycoprotein (MAG): past, present and beyond

The myelin-associated glycoprotein (MAG) is a type I transmembrane glycoprotein localized in periaxonal Schwann cell and oligodendroglial membranes of myelin sheaths where it functions in glia-axon interactions. It contains five immunoglobulin (Ig)-like domains and is in the sialic acid-binding subgroup of the Ig superfamily. It appears to function both as a ligand for an axonal receptor that is needed for the maintenance of myelinated axons and as a receptor for an axonal signal that promotes the differentiation, maintenance and survival of oligodendrocytes. Its function in the maintenance of myelinated axons may be related to its role as one of the white matter inhibitors of neurite outgrowth acting through a receptor complex involving the Nogo receptor and/or gangliosides containing 2,3-linked sialic acid. MAG is expressed as two developmentally regulated isoforms with different cytoplasmic domains that may activate different signal transduction pathways in myelin-forming cells. MAG contains a carbohydrate epitope shared with other glycoconjugates that is a target antigen in autoimmune peripheral neuropathy associated with IgM gammopathy and has been implicated in a dying back oligodendrogliopathy in multiple sclerosis.

Gangliosides as immunomodulators

Gangliosides are glycosphingolipids expressed at the outer leaflet of the plasmatic membrane of cells from vertebrate organisms. These molecules exert diverse biological functions including modulation of the immune system responses. Aberrant expression of gangliosides has been demonstrated on malignant cells. Besides expression on tumor cell membranes, gangliosides are also shed in the tumor microenvironment and eventually circulate in patients blood. Gangliosides derived from tumors posses the capability to affect the immune system responses by altering the function of lymphocytes and antigen-presenting cells and promoting tumor growth. These molecules can be considered as tumor weapons directed to attack and destroy immunosurveillance mechanisms devoted to control cancer progression.

White Matter Rafting––Membrane Microdomains in Myelin

The myelin membrane comprises a plethora of regions that are compositionally, ultrastructurally, and functionally distinct. Biochemical dissection of oligodendrocytes, Schwann cells, and central and peripheral nervous system myelin by means such as cold-detergent extraction and differential fractionation has led to the identification of a variety of detergent-resistant membrane assemblies, some of which represent putative signalling platforms. We review here the different microdomains that have hitherto been identified in the myelin membrane, particularly lipid rafts, caveolae, and cellular junctions such as the tight junctions that are found in the radial component of the CNS myelin sheath.