Two forms of 1B236/myelin-associated glycoprotein, a cell adhesion molecule for postnatal neural development, are produced by alternative splicing

Discovery, classification, evolution and diversity of Siglecs

Immunoglobulin (Ig) superfamily proteins play diverse roles in vertebrates, including regulation of cellular responses by sensing endogenous or exogenous ligands. Siglecs are a family of glycan-recognizing proteins belonging to the Ig superfamily (i.e., I-type lectins). Siglecs are expressed on various leukocyte types and are involved in diverse aspects of immunity, including the regulation of inflammatory responses, leukocyte proliferation, host-microbe interaction, and cancer immunity. Sialoadhesin/Siglec-1, CD22/Siglec-2, and myelin-associated glycoprotein/Siglec-4 were among the first to be characterized as members of the Siglec family, and along with Siglec-15, they are relatively well-conserved among tetrapods. Conversely, CD33/Siglec-3-related Siglecs (CD33rSiglecs, so named as they show high sequence similarity with CD33/Siglec-3) are encoded in a gene cluster with many interspecies variations and even intraspecies variations within some lineages such as humans. The rapid evolution of CD33rSiglecs expressed on leukocytes involved in innate immunity likely reflects the selective pressure by pathogens that interact and possibly exploit these Siglecs. Human Siglecs have several additional unique and/or polymorphic properties as compared with closely related great apes, changes possibly related to the loss of the sialic acid Neu5Gc, another distinctly human event in sialobiology. Multiple changes in human CD33rSiglecs compared to great apes include many examples of human-specific expression in non-immune cells, coinciding with human-specific diseases involving such cell types. Some Siglec gene polymorphisms have dual consequences-beneficial in a situation but detrimental in another. The association of human Siglec gene polymorphisms with several infectious and non-infectious diseases likely reflects the ongoing competition between the host and microbial pathogens.

Myelin-associated glycoprotein activation triggers glutamate uptake by oligodendrocytes in vitro and contributes to ameliorate glutamate-mediated toxicity in vivo

BACKGROUND

Myelin-associated glycoprotein (MAG) is a key molecule involved in the nurturing effect of myelin on ensheathed axons. MAG also inhibits axon outgrowth after injury. In preclinical stroke models, administration of a function-blocking anti-MAG monoclonal antibody (mAb) aimed to improve axon regeneration demonstrated reduced lesion volumes and a rapid clinical improvement, suggesting a mechanism of immediate neuroprotection rather than enhanced axon regeneration. In addition, it has been reported that antibody-mediated crosslinking of MAG can protect oligodendrocytes (OLs) against glutamate (Glu) overload by unknown mechanisms.

PURPOSE

To unravel the molecular mechanisms underlying the protective effect of anti-MAG therapy with a focus on neuroprotection against Glu toxicity.

RESULTS

MAG activation (via antibody crosslinking) triggered the clearance of extracellular Glu by its uptake into OLs via high affinity excitatory amino acid transporters. This resulted not only in protection of OLs but also nearby neurons. MAG activation led to a PKC-dependent activation of factor Nrf2 (nuclear-erythroid related factor-2) leading to antioxidant responses including increased mRNA expression of metabolic enzymes from the glutathione biosynthetic pathway and the regulatory chain of cystine/Glu antiporter system xc^- increasing reduced glutathione (GSH), the main antioxidant in cells. The efficacy of early anti-MAG mAb administration was demonstrated in a preclinical model of excitotoxicity induced by intrastriatal Glu administration and extended to a model of Experimental Autoimmune Encephalitis showing axonal damage secondary to demyelination.

CONCLUSIONS

MAG activation triggers Glu uptake into OLs under conditions of Glu overload and induces a robust protective antioxidant response.

Siglecs as Therapeutic Targets in Cancer

Hypersialylation is a common post-translational modification of protein and lipids found on cancer cell surfaces, which participate in cell-cell interactions and in the regulation of immune responses. Sialic acids are a family of nine-carbon α-keto acids found at the outermost ends of glycans attached to cell surfaces. Given their locations on cell surfaces, tumor cells aberrantly overexpress sialic acids, which are recognized by Siglec receptors found on immune cells to mediate broad immunomodulatory signaling. Enhanced sialylation exposed on cancer cell surfaces is exemplified as "self-associated molecular pattern" (SAMP), which tricks Siglec receptors found on leukocytes to greatly down-regulate immune responsiveness, leading to tumor growth. In this review, we focused on all 15 human Siglecs (including Siglec XII), many of which still remain understudied. We also highlighted strategies that disrupt the course of Siglec-sialic acid interactions, such as antibody-based therapies and sialic acid mimetics leading to tumor cell depletion. Herein, we introduced the central roles of Siglecs in mediating pro-tumor immunity and discussed strategies that target these receptors, which could benefit improved cancer immunotherapy.

A Novel Siglec-4 Derived Spacer Improves the Functionality of CAR T Cells Against Membrane-Proximal Epitopes

A domain that is often neglected in the assessment of chimeric antigen receptor (CAR) functionality is the extracellular spacer module. However, several studies have elucidated that membrane proximal epitopes are best targeted through CARs comprising long spacers, while short spacer CARs exhibit highest activity on distal epitopes. This finding can be explained by the requirement to have an optimal distance between the effector T cell and target cell. Commonly used long spacer domains are the CH2-CH3 domains of IgG molecules. However, CARs containing these spacers generally show inferior in vivo efficacy in mouse models compared to their observed in vitro activity, which is linked to unspecific Fcγ-Receptor binding and can be abolished by mutating the respective regions. Here, we first assessed a CAR therapy targeting membrane proximal CD20 using such a modified long IgG1 spacer. However, despite these mutations, this construct failed to unfold its observed in vitro cytotoxic potential in an in vivo model, while a shorter but less structured CD8α spacer CAR showed complete tumor clearance. Given the shortage of well-described long spacer domains with a favorable functionality profile, we designed a novel class of CAR spacers with similar attributes to IgG spacers but without unspecific off-target binding, derived from the Sialic acid-binding immunoglobulin-type lectins (Siglecs). Of five constructs tested, a Siglec-4 derived spacer showed highest cytotoxic potential and similar performance to a CD8α spacer in a CD20 specific CAR setting. In a pancreatic ductal adenocarcinoma model, a Siglec-4 spacer CAR targeting a membrane proximal (TSPAN8) epitope was efficiently engaged in vitro, while a membrane distal (CD66c) epitope did not activate the T cell. Transfer of the TSPAN8 specific Siglec-4 spacer CAR to an in vivo setting maintained the excellent tumor killing characteristics being indistinguishable from a TSPAN8 CD8α spacer CAR while outperforming an IgG4 long spacer CAR and, at the same time, showing an advantageous central memory CAR T cell phenotype with lower release of inflammatory cytokines. In summary, we developed a novel spacer that combines cytotoxic potential with an advantageous T cell and cytokine release phenotype, which make this an interesting candidate for future clinical applications.

CLARITY-BPA: Bisphenol A or Propylthiouracil on Thyroid Function and Effects in the Developing Male and Female Rat Brain

The CLARITY-BPA experiment, a large collaboration between the National Institute of Environmental Health Sciences, the National Toxicology Program, and the US Food and Drug Administration, is designed to test the effects of bisphenol A (BPA) on a variety of endocrine systems and end points. The specific aim of this subproject was to test the effect of BPA exposure on thyroid functions and thyroid hormone action in the developing brain. Timed-pregnant National Center for Toxicological Research Sprague-Dawley rats (strain code 23) were dosed by gavage with vehicle control (0.3% carboxymethylcellulose) or one of five doses of BPA [2.5, 25, 250, 2500, or 25,000 µg/kg body weight (bw) per day] or ethinyl estradiol (EE) at 0.05 or 0.50 µg/kg bw/d (n = 8 for each group) beginning on gestational day 6. Beginning on postnatal day (PND) 1 (day of birth is PND 0), the pups were directly gavaged with the same dose of vehicle, BPA, or EE. We also obtained a group of animals treated with 3 ppm propylthiouracil in the drinking water and an equal number of concordant controls. Neither BPA nor EE affected serum thyroid hormones or thyroid hormone‒sensitive end points in the developing brain at PND 15. In contrast, propylthiouracil (PTU) reduced serum T4 to the expected degree (80% reduction) and elevated serum TSH. Few effects of PTU were observed in the male brain and none in the female brain. As a result, it is difficult to interpret the negative effects of BPA on the thyroid in this rat strain because the thyroid system appears to respond differently from that of other rat strains.

Enhanced remyelination during late pregnancy: involvement of the GABAergic system

Pregnant women with MS experience fewer relapses, especially during the third trimester. In this study, we explore the cellular and molecular events that bring about the protective effect of late pregnancy on the course of de/remyelination in rats. Using cellular, molecular, and ultrastructural methods, we explored remyelination in response to a focal demyelination in the corpus callosum of late pregnant, virgin, and postpartum rats. We further explored the role of GABA_A receptor (GABA_AR) in the promyelinating effect observed during late pregnancy. Remyelination in response to a gliotoxin-induced demyelination in the corpus callosum was enhanced in late pregnant rats when compared to that seen in virgin and postpartum rats. This pregnancy-associated promyelinating effect was lost when either the GABA_AR was blocked or when 5α-reductase, the rate limiting enzyme for the endogenous GABA_AR activator allopregnanolone, was inhibited. Taken together, these data suggest that the pregnancy-associated pro-myelination operates, at least in part, through a GABAergic activated system.

High-affinity heterotetramer formation between the large myelin-associated glycoprotein and the dynein light chain DYNLL1

The close association of myelinated axons and their myelin sheaths involves numerous intercellular molecular interactions. For example, myelin-associated glycoprotein (MAG) mediates myelin-to-axon adhesion and signalling via molecules on the axonal surface. However, knowledge about intracellular binding partners of myelin proteins, including MAG, has remained limited. The two splice isoforms of MAG, S- and L-MAG, display distinct cytoplasmic domains and spatiotemporal expression profiles. We used yeast two-hybrid screening to identify interaction partners of L-MAG and found the dynein light chain DYNLL1 (also termed dynein light chain 8). DYNLL1 homodimers are known to facilitate dimerization of target proteins. L-MAG and DYNLL1 associate with high affinity, as confirmed with recombinant proteins in vitro. Structural analyses of the purified complex indicate that the DYNLL1-binding segment is localized close to the L-MAG C terminus, next to the Fyn kinase Tyr phosphorylation site. The crystal structure of the complex between DYNLL1 and its binding segment on L-MAG shows 2 : 2 binding in a parallel arrangement, indicating a heterotetrameric complex. The homology between L-MAG and previously characterized DYNLL1-ligands is limited, and some details of binding site interactions are unique for L-MAG. The structure of the complex between the entire L-MAG cytoplasmic domain and DYNLL1, as well as that of the extracellular domain of MAG, were modelled based on small-angle X-ray scattering data, allowing structural insights into L-MAG interactions on both membrane surfaces. Our data imply that DYNLL1 dimerizes L-MAG, but not S-MAG, through the formation of a specific 2 : 2 heterotetramer. This arrangement is likely to affect, in an isoform-specific manner, the functions of MAG in adhesion and myelin-to-axon signalling. OPEN SCIENCE BADGES: This article has received a badge for *Open Materials* because it provided all relevant information to reproduce the study in the manuscript. The complete Open Science Disclosure form for this article can be found at the end of the article. More information about the Open Practices badges can be found at https://cos.io/our-services/open-science-badges/. Read the Editorial Highlight for this article on page 712.

Structural and biochemical studies of the extracellular domain of Myelin protein zero-like protein 1

Myelin protein zero-like protein 1 (MPZL1) is a member of the immunoglobulin superfamily, and is also a receptor of concanavalin A (ConA). MPZL1 is upregulated in hepatocellular carcinoma (HCC) and accelerates migration of HCC cells. However, function of MPZL1 as a receptor of ConA and its role in HCC development are largely unknown. To elucidate the functional basis, we have determined the crystal structure of the extracellular domain of MPZL1 at 2.7 Å resolution. Overall, it folds like a typical immunoglobulin variable-like domain that is much like MPZ. Unexpectedly, we found Asn50 is a unique glycosylation site and the glycosylation mediates its interaction with ConA. Furthermore, we also found that MPZL1 exists as a homodimer in the crystal, in which hydrogen bonds between Ser86 and Val145 play an important role. Our results demonstrate that glycosylation of Asn50 is essential for its function as a receptor of ConA. We propose that dimerization of MPZL1 participates in control of its signal transmission in cell adhesion.

Dihydrotestosterone Treatment Accelerates Autograft Reversal Sciatic Nerve Regeneration in Rats

Neuroactive steroids such as progesterone, testosterone, and their derivatives have been widely studied for their neuroprotective roles in the nervous system. Autologous nerve transplantation is considered as the gold standard repair technique when primary suture is impossible; nevertheless, this method is far from ideal. In this study, we aimed to explore the impact of dihydrotestosterone (DHT), a 5α-reduced derivative of testosterone, on the recovery of peripheral nerve injury treated with autologous nerve transplantation. Sprague-Dawley rats were subjected to a 10-mm right side sciatic nerve reversed autologous nerve transplantation and randomly divided into groups that received DHT or DHT + flutamide (an androgen receptor blocker) daily for 8 weeks after operation. Our results demonstrated that DHT could speed up the rate of axonal regeneration and increase the expression of myelin protein zero (P0) in autograft reversal sciatic nerves. Thus, our study provided new insights into improving the prognosis of patients with long gap peripheral nerve defects.

Genetic variation is associated with RTN4R expression and working memory processing in healthy humans

The Nogo receptor (NgR) is implicated in neurodevelopmental processes and it participates in inhibiting axonal growth. Consistent with its high levels of expression in the prefrontal cortex, animal studies indicate that NgR is relevant for prefrontal-related cognitive processing. Given that genetic variation may alter mechanisms of gene expression impacting molecular and systems-level phenotypes, we investigated the association of genetic variation with the expression of the NgR coding gene (RTN4R), as well as with prefrontal correlates at progressively greater biological distance from gene effects. First, we studied the association of single nucleotide polymorphisms (SNPs) with RTN4R mRNA expression in postmortem prefrontal cortex of humans without psychiatric illnesses. Then, we probed in peripheral blood mononuclear cells (PBMCs) the association that we found in prefrontal tissue. Thus, we investigated whether functional genetic variation affecting RTN4R expression is also associated with prefrontal activity during working memory. We found that rs696884 (A/G) predicted these phenotypes. Specifically, the AA genotype was associated with lower RTN4R mRNA expression levels in the prefrontal cortex and PBMCs and inefficient prefrontal activity during working memory compared to the GG genotype. These results suggest that genetic variation associated with RTN4R mRNA expression influences prefrontal physiology in healthy individuals. Furthermore, they highlight the need for further investigations of the role of NgR in the pathophysiology of brain disorders associated with prefrontal dysfunction.

Neurodegeneration in Multiple Sclerosis: Relationship to Neurological Disability

Multiple sclerosis (MS) is an inflammatory demyelinating disease of the CNS. Most MS patients follow a relapsing-remitting course (RR-MS) for 8 to 15 years that transforms into a secondary progressive disease course (SP-MS). In this review, we discuss current data that describe MS as a neurodegenerative disease in which axonal loss is the major cause of irreversible neurological disability in MS patients. Neurological deficits in MS patients have two pathogeneses: acute inflammatory demyelination and axonal degeneration. Disability caused by inflammatory demyelination clinically dominates the early stages of RR-MS and is reversible. Axonal transection occurs at sites of inflammation and begins at disease onset but is clinically silent in RR-MS because the CNS compensates for neuronal loss. Once a threshold of axon loss is ex ceeded, MS patients enter an irreversible secondary progressive stage. In SP-MS, axonal degeneration is caused by chronic demyelination and may be irreversibly progressive. This view of MS provides a concep tional framework that explains conversion of RR-MS to SP-MS and provides a rationale for early aggressive anti-inflammatory and neuroprotective therapies. NEUROSCIENTIST 5:48-57, 1999

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.

hnRNP A1 and secondary structure coordinate alternative splicing of Mag

Myelin-associated glycoprotein (MAG) is a major component of myelin in the vertebrate central nervous system. MAG is present in the periaxonal region of the myelin structure, where it interacts with neuronal proteins to inhibit axon outgrowth and protect neurons from degeneration. Two alternatively spliced isoforms of Mag mRNA have been identified. The mRNA encoding the shorter isoform, known as S-MAG, contains a termination codon in exon 12, while the mRNA encoding the longer isoform, known as L-MAG, skips exon 12 and produces a protein with a longer C-terminal region. L-MAG is required in the central nervous system. How inclusion of Mag exon 12 is regulated is not clear. In a previous study, we showed that heteronuclear ribonucleoprotein A1 (hnRNP A1) contributes to Mag exon 12 skipping. Here, we show that hnRNP A1 interacts with an element that overlaps the 5' splice site of Mag exon 12. The element has a reduced ability to interact with the U1 snRNP compared with a mutant that improves the splice site consensus. An evolutionarily conserved secondary structure is present surrounding the element. The structure modulates interaction with both hnRNP A1 and U1. Analysis of splice isoforms produced from a series of reporter constructs demonstrates that the hnRNP A1-binding site and the secondary structure both contribute to exclusion of Mag exon 12.

Biology of Schwann cells

The fundamental roles of Schwann cells during peripheral nerve formation and regeneration have been recognized for more than 100 years, but the cellular and molecular mechanisms that integrate Schwann cell and axonal functions continue to be elucidated. Derived from the embryonic neural crest, Schwann cells differentiate into myelinating cells or bundle multiple unmyelinated axons into Remak fibers. Axons dictate which differentiation path Schwann cells follow, and recent studies have established that axonal neuregulin1 signaling via ErbB2/B3 receptors on Schwann cells is essential for Schwann cell myelination. Extracellular matrix production and interactions mediated by specific integrin and dystroglycan complexes are also critical requisites for Schwann cell-axon interactions. Myelination entails expansion and specialization of the Schwann cell plasma membrane over millimeter distances. Many of the myelin-specific proteins have been identified, and transgenic manipulation of myelin genes have provided novel insights into myelin protein function, including maintenance of axonal integrity and survival. Cellular events that facilitate myelination, including microtubule-based protein and mRNA targeting, and actin based locomotion, have also begun to be understood. Arguably, the most remarkable facet of Schwann cell biology, however, is their vigorous response to axonal damage. Degradation of myelin, dedifferentiation, division, production of axonotrophic factors, and remyelination all underpin the substantial regenerative capacity of the Schwann cells and peripheral nerves. Many of these properties are not shared by CNS fibers, which are myelinated by oligodendrocytes. Dissecting the molecular mechanisms responsible for the complex biology of Schwann cells continues to have practical benefits in identifying novel therapeutic targets not only for Schwann cell-specific diseases but other disorders in which axons degenerate.

Inferring transcript phylogenies

Alternative splicing, an unknown mechanism 20 years ago, is now recognized as a major mechanism for proteome and transcriptome diversity, particularly in mammals--some researchers conjecture that up to 90% of human genes are alternatively spliced. Despite much research on exon and intron evolution, little is known about the evolution of transcripts.

In this paper, we present a model of transcript evolution and an associated algorithm to reconstruct transcript phylogenies. The evolution of the gene structure--exons and introns--is used as basis for the reconstruction of transcript phylogenies. We apply our model and reconstruction algorithm on two well-studied genes, MAG and PAX6, obtaining results consistent with current knowledge and thereby providing evidence that a phylogenetic analysis of transcripts is feasible and likely to be informative.

Effects of polychlorinated biphenyls on the development of neuronal cells in growth period; structure-activity relationship

Polychlorinated biphenyls (PCBs) are accumulated in our body through food chain and cause a variety of adverse health effects including neurotoxicities such as cognitive deficits and motor dysfunction. In particular, neonates are considered as a high risk group for the neurotoxicity of PCBs exposure. The present study attempted to analyze the structure-activity relationship among PCB congeners and the mechanism of PCBs-induced neurotoxicity. We measured total protein kinase C (PKC) activities, PKC isoforms, reactive oxygen species (ROS), and induction of neurogranin (RC-3) and growth associated protein-43 (GAP-43) mRNA in cerebellar granule cells of neonatal rats with phorbol 12, 13-dibutyrate ([(3)H]PDBu) binding assay, western blot, ROS assay, and reverse transcription PCR (RT-PCR) analysis respectively following the different structural PCBs exposure. Only non-coplanar PCBs showed a significant increase of total PKC-α and βII activity as measured with [(3)H]PDBu binding assay. ROS were more increased with non-coplanar PCBs than coplanar PCBs. The mRNA levels of RC-3 and GAP-43 were more induced with non-coplanar PCBs than coplanar PCBs, indicating that these factors may be useful biomarkers for differentiating non-coplanar PCBs from coplanar PCBs. Non-coplanar PCBs may be more potent neurotoxic congeners than coplanar PCBs. This study provides evidences that non-coplanar PCBs, which have been neglected in the risk assessment processes, should be added in the future to improve the quality and accuracy of risk assessment on the neuroendocrinal adverse effects of PCBs exposures.

Myelin associated inhibitors: a link between injury-induced and experience-dependent plasticity

In the adult, both neurologic recovery and anatomical growth after a CNS injury are limited. Two classes of growth inhibitors, myelin associated inhibitors (MAIs) and extracellular matrix associated inhibitors, limit both functional recovery and anatomical rearrangements in animal models of spinal cord injury. Here we focus on how MAIs limit a wide spectrum of growth that includes regeneration, sprouting, and plasticity in both the intact and lesioned CNS. Three classic myelin associated inhibitors, Nogo-A, MAG, and OMgp, signal through their common receptors, Nogo-66 Receptor-1 (NgR1) and Paired-Immunoglobulin-like-Receptor-B (PirB), to regulate cytoskeletal dynamics and inhibit growth. Initially described as inhibitors of axonal regeneration, subsequent work has demonstrated that MAIs also limit activity and experience-dependent plasticity in the intact, adult CNS. MAIs therefore represent a point of convergence for plasticity that limits anatomical rearrangements regardless of the inciting stimulus, blurring the distinction between injury studies and more "basic" plasticity studies.

Opioid addiction and pregnancy: perinatal exposure to buprenorphine affects myelination in the developing brain

Buprenorphine is a mu-opioid receptor partial agonist and kappa-opioid receptor antagonist currently on trials for the management of pregnant opioid-dependent addicts. However, little is known about the effects of buprenorphine on brain development. Oligodendrocytes express opioid receptors in a developmentally regulated manner and thus, it is logical to hypothesize that perinatal exposure to buprenorphine could affect myelination. To investigate this possibility, pregnant rats were implanted with minipumps to deliver buprenorphine at 0.3 or 1 mg/kg/day. Analysis of their pups at different postnatal ages indicated that exposure to 0.3 mg/kg/day buprenorphine caused an accelerated and significant increase in the brain expression of all myelin basic protein (MBP) splicing isoforms. In contrast, treatment with the higher dose caused a developmental delay in MBP expression. Examination of corpus callosum at 26-days of age indicated that both buprenorphine doses cause a significant increase in the caliber of the myelinated axons. Surprisingly, these axons have a disproportionately thinner myelin sheath, suggesting alterations at the level of axon-glial interactions. Analysis of myelin associated glycoprotein (MAG) expression and glycosylation indicated that this molecule may play a crucial role in mediating these effects. Co-immunoprecipitation studies also suggested a mechanism involving a MAG-dependent activation of the Src-family tyrosine kinase Fyn. These results support the idea that opioid signaling plays an important role in regulating myelination in vivo and stress the need for further studies investigating potential effects of perinatal buprenorphine exposure on brain development.

A novel function of RING finger protein 10 in transcriptional regulation of the myelin-associated glycoprotein gene and myelin formation in Schwann cells

Myelin-associated glycoprotein (MAG) has been detected in Schwann cells prior to the onset of myelination, suggesting its functions in the initiation of myelination. However, transcriptional regulatory mechanisms of MAG remain to be elucidated. Here, we analyzed the promoter of the MAG gene by using luciferase reporter systems in the primary rat Schwann cells. We identified a novel cis-acting element located 160 bp upstream from the MAG transcription initiation site. Using the identified cis-element as a bait, we performed yeast one-hybrid screening and isolated a cDNA encoding a RNF10 as a putative trans-acting protein. When overexpressed in Schwann cells, RNF10 enhanced the activity of the MAG promoter. When RNF10 expression in Schwann cells was knocked down by siRNA, endogenous MAG mRNA and protein expression decreased. Furthermore, we evaluated myelin synthesis using Schwann cell-DRG neuron cocultures. When Schwann cells were infected with retrovirus expressing RNF10 siRNA, myelin formation was inhibited. These data suggest that RNF10 regulates MAG expression and is required for myelin formation.

Axon-glial signaling and the glial support of axon function

Oligodendrocytes and Schwann cells are highly specialized glial cells that wrap axons with a multilayered myelin membrane for rapid impulse conduction. Investigators have recently identified axonal signals that recruit myelin-forming Schwann cells from an alternate fate of simple axonal engulfment. This is the evolutionary oldest form of axon-glia interaction, and its function is unknown. Recent observations suggest that oligodendrocytes and Schwann cells not only myelinate axons but also maintain their long-term functional integrity. Mutations in the mouse reveal that axonal support by oligodendrocytes is independent of myelin assembly. The underlying mechanisms are still poorly understood; we do know that to maintain axonal integrity, mammalian myelin-forming cells require the expression of some glia-specific proteins, including CNP, PLP, and MAG, as well as intact peroxisomes, none of which is necessary for myelin assembly. Loss of glial support causes progressive axon degeneration and possibly local inflammation, both of which are likely to contribute to a variety of neuronal diseases in the central and peripheral nervous systems.

Leucine-rich repeat C4 protein is involved in nervous tissue development and neurite outgrowth, and induction of glioma cell differentiation

LRRC4, leucine-rich repeat C4 protein, has been identified in human (GenBank accession No. AF196976), mouse (GenBank accession No. DQ177325), rat (GenBank accession No. DQ119102) and bovine (GenBank accession No. DQ164537) with identical domains. In terms of their similarity, the genes encoding LRRC4 in these four mammalian species are orthogs and therefore correspond to the same gene entity. Based on previous research, and using in situ hybridization, we found that LRRC4 had the strongest expression in hippocampal CA1 and CA2, the granule cells of the dentate gyrus region, the mediodoral thalamic nucleus, and cerebella Purkinje cell layers. Using a P19 cell model, we also found that LRRC4 participates in the differentiation of neuron and glia cells. In addition, extracellular proteins containing both an LRR cassette and immunoglobulin domains have been shown to participate in axon guidance. Our data from neurite outgrowth assays indicated that LRRC4 promoted neurite extension of hippocampal neurons, and induced differentiation of glioblastoma U251 cells into astrocyte-like cells, confirmed by morphology observation and glial fibrillary acidic protein expression.

Recapitulate development to promote axonal regeneration: good or bad approach?

In the past decade there has been an explosion in our understanding, at the molecular level, of why axons in the adult, mammalian central nervous system (CNS) do not spontaneously regenerate while their younger counterparts do. Now a number of inhibitors of axonal regeneration have been described, some of the receptors they interact with to transduce the inhibitory signal are known, as are some of the steps in the signal transduction pathway that is responsible for inhibition. In addition, developmental changes in the environment and in the neurons themselves are also now better understood. This knowledge in turn reveals novel, putative sites for drug development and therapeutic intervention after injury to the brain and spinal cord. The challenge now is to determine which of these putative treatments are the most effective and if they would be better applied in combination rather than alone. In this review I will summarize what we have learnt about these molecules and how they signal. Importantly, I will also describe approaches that have been shown to block inhibitors and encourage regeneration in vivo. I will also speculate on what the differences are between the neonatal and adult CNS that allow the former to regenerate and the latter not to.

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.

The Inherited Neuropathies

Neuropathy is one of the most common referrals to neurologic clinics. Patients often undergo extensive testing for acquired etiologies; inherited causes are common. Increasingly, genetic causes are becoming known and commercial testing available. The rate of recent discovery has been rapid and relates to the extent of single gene disorders of nerve, the ease of peripheral nervous system functional examination, and readily accessible pathologic tissue. Foremost in the rate of recent discoveries is the work and tools of the human genome project. the rapidity of the ongoing discovery requires clinicians to be familiar with molecular biologic discoveries and consider wisely which testing should be performed.

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.

Polymorphisms of myelin-associated glycoprotein gene are associated with schizophrenia in the Chinese Han population

Results of gene expression microarray and quantitative PCR studies have suggested abnormalities in the expression of myelin-related genes including myelin-associated glycoprotein (MAG) in schizophrenic patients. Research provides strong evidence for oligodendrocyte dysfunction in schizophrenics. In order to further assess the role of MAG in schizophrenia, we examined four single nucleotide polymorphisms (SNPs), namely rs2301600, rs3746248, rs720309 and rs720308, of this gene in Chinese schizophrenic patients (n=470) and healthy controls (n=470). The distribution of rs720309 T/A genotypes showed a strong association with schizophrenia (chi(2)=14.58, d.f.=2, P=0.0008). A haplotype constructed of rs720309-rs720308 also revealed a significant association with schizophrenia (chi(2)=11.914, d.f.=3, P=0.0084). Our findings of a significant associations between schizophrenia and the MAG gene suggest that this gene may be involved in susceptibility to schizophrenia in the Chinese Han population.

Oligodendrocytes and progenitors become progressively depleted within chronically demyelinated lesions

To understand mechanisms that may underlie the progression of a demyelinated lesion to a chronic state, we have used the cuprizone model of chronic demyelination. In this study, we investigated the fate of oligodendrocytes during the progression of a demyelinating lesion to a chronic state and determined whether transplanted adult oligodendrocyte progenitors could remyelinate the chronically demyelinated axons. Although there is rapid regeneration of the oligodendrocyte population following an acute lesion, most of these newly regenerated cells undergo apoptosis if mice remain on a cuprizone diet. Furthermore, the oligodendrocyte progenitors also become progressively depleted within the lesion, which appears to contribute to the chronic demyelination. Interestingly, even if the mice are returned to a normal diet following 12 weeks of exposure to cuprizone, remyelination and oligodendrocyte regeneration does not occur. However, if adult O4+ progenitors are transplanted into the chronically demyelinated lesion of mice treated with cuprizone for 12 weeks, mature oligodendrocyte regeneration and remyelination occurs after the mice are returned to a normal diet. Thus, the formation of chronically demyelinated lesions induced by cuprizone appears to be the result of oligodendrocyte depletion within the lesion and not due to the inability of the chronically demyelinated axons to be remyelinated.

Actions of progesterone and its 5alpha-reduced metabolites on the major proteins of the myelin of the peripheral nervous system

The sciatic nerve, and the Schwann cells in particular, are able to synthesize progesterone and possess the enzymes forming the 5alpha-reduced and the 3alpha-5alpha-reduced derivatives of progesterone: dihydroprogesterone and tetrahydroprogesterone. Moreover, the progesterone receptor (PR) is present in the sciatic nerve and in Schwann cell cultures. These facts suggest that progesterone and its derivatives might play a role in the control of the synthesis of the two major proteins of the peripheral nervous system (PNS): the glycoprotein Po (Po) and peripheral myelin protein 22 (PMP22). We have shown that: (a) dihydroprogesterone enhances the low mRNA levels of Po in the sciatic nerve of aged male rats; (b) progesterone and its derivatives stimulate the gene expression of Po in the sciatic nerve of adult rats and in Schwann cell cultures; (c) tetrahydroprogesterone increases PMP22 gene expression in the sciatic nerve of adult rats and in Schwann cell cultures. In additional experiments, utilizing agonists and antagonists of PR and GABAA receptor, we have observed that progesterone and its derivatives control Po gene expression via the PR, while tetrahydroprogesterone modulates the expression of PMP22 through the GABAA receptor.

Serum IgG antibodies to P0 dimer and 35 kDa P0 related protein in neuropathy associated with monoclonal gammopathy

BACKGROUND

Peripheral neuropathies (PN) associated with monoclonal gammopathy (MG) are widely considered as autoimmune disorders, but the putative role of incriminated antigens is still not understood.

OBJECTIVE

Fifty five patients with PN associated with MG were studied to investigate whether new antigens could be found, and to evaluate their relation to clinical manifestations.

METHODS

An immunological study was conducted on patient sera to identify autoreactivities against nerve proteins by western blotting. Antigen proteins were purified and analysed by proteomic tools. Correlation with ultrastrucural and clinical features was then studied.

RESULTS

Of the 55 patients suffering from PN associated with MG, 17 exhibited IgG autoantibodies directed against peripheral nerve proteins of 35, 58, and 60 kDa. N-terminal microsequencing and mass spectrometry analyses of the 35 kDa protein revealed perfect peptidic matching with 47% of the amino acid sequence of P0, whereas the 58 and 60 kDa proteins were identified as the reduced and non-reduced forms of a P0 dimer. Deglycosylation did not affect IgG binding to the 35 kDa P0 related protein, suggesting a peptidic epitope. In contrast, deglycosylation abolished IgG recognition of the P0 dimer protein, so that a carbohydrate moiety may be implicated in the epitope formation. This confirmed the existence of two different types of IgG, one recognising the 58 and 60 kDa proteins and one directed against the 35 kDa protein.

CONCLUSIONS

This is the first report of antibody activity directed against the dimeric association of P0. Although P0 oligomerisation and adhesion properties play a crucial part in the myelin sheath compaction, the pathogenic significance of these autoantibodies needs further investigations to be elucidated.

Mouse NG2+ oligodendrocyte precursors express mRNA for proteolipid protein but not its DM-20 variant: a study of laser microdissection-captured NG2+ cells

Despite recent advances in our understanding of lineage of oligodendrocytes, detailed molecular characterization of this lineage in vivo is limited, primarily because of our inability to obtain a pure population of cells in situ. To define the molecular characteristics of oligodendrocyte lineage cells during development and their response to injury, we developed a strategy that uses laser capture microdissection (LCM) to isolate cells from sections and reverse transcription-PCR to determine mRNA expression. As a first step, we examined the expression of myelin-specific protein genes in NG2+ cells in cerebral cortex. We demonstrate that NG2+ cells in both developing and adult mice express NG2 mRNA but not mRNA for proteins specific for astrocytes, neurons, or microglia, indicating that a highly pure population of antigen-specific cells of the oligodendrocyte lineage can be obtained using LCM. Furthermore, we show that NG2+ cells express mRNAs for proteolipid protein (PLP), myelin basic protein, and 2',3'-cyclic nucleotide 3'-phosphodiesterase, but they dot not express DM-20 mRNA, a PLP mRNA splicing variant. Our data demonstrate that antigen-specific cells of oligodendrocyte lineage differentially express mRNA for myelin-specific proteins and their variants in vivo, partly define the gene expression in NG2+ cells, and raise questions about the cellular sites of DM-20 expression. This work also shows that LCM is a valuable tool to define and analyze gene expression in the cells of the oligodendrocyte lineage.

Mouse NG2+ oligodendrocyte precursors express mRNA for proteolipid protein but not its DM-20 variant: a study of laser microdissection-captured NG2+ cells

Despite recent advances in our understanding of lineage of oligodendrocytes, detailed molecular characterization of this lineage in vivo is limited, primarily because of our inability to obtain a pure population of cells in situ. To define the molecular characteristics of oligodendrocyte lineage cells during development and their response to injury, we developed a strategy that uses laser capture microdissection (LCM) to isolate cells from sections and reverse transcription-PCR to determine mRNA expression. As a first step, we examined the expression of myelin-specific protein genes in NG2+ cells in cerebral cortex. We demonstrate that NG2+ cells in both developing and adult mice express NG2 mRNA but not mRNA for proteins specific for astrocytes, neurons, or microglia, indicating that a highly pure population of antigen-specific cells of the oligodendrocyte lineage can be obtained using LCM. Furthermore, we show that NG2+ cells express mRNAs for proteolipid protein (PLP), myelin basic protein, and 2',3'-cyclic nucleotide 3'-phosphodiesterase, but they dot not express DM-20 mRNA, a PLP mRNA splicing variant. Our data demonstrate that antigen-specific cells of oligodendrocyte lineage differentially express mRNA for myelin-specific proteins and their variants in vivo, partly define the gene expression in NG2+ cells, and raise questions about the cellular sites of DM-20 expression. This work also shows that LCM is a valuable tool to define and analyze gene expression in the cells of the oligodendrocyte lineage.

Characterization of two novel proteins, NgRH1 and NgRH2, structurally and biochemically homologous to the Nogo-66 receptor

Nogo-66 receptor (NgR) has recently been identified as the neuronal receptor of the myelin-associated proteins Nogo-A, oligodendrocyte protein (OMgp) and myelin-associated glycoprotein (MAG), and mediates inhibition of axonal regeneration both in vitro and in vivo. Through database searches, we have identified two novel proteins (NgRH1 and NgRH2) that turned out to be homologous in their primary structures, biochemical properties and expression patterns to NgR. Like NgR, the homologues contain eight leucine-rich repeats (LRR) flanked by a leucine-rich repeat C-terminus (LRRCT) and a leucine-rich repeat N-terminus (LRRNT), and also have a C-terminal GPI signal sequence. Northern blot analysis showed predominant expression of NgRH1 and NgRH2 mRNA in the brain. In situ hybridization and immunohistochemistry on rat brain slices revealed neuronal expression of the genes. NgRH1 and NgRH2 were detected on the cell surface of recombinant cell lines as N-glycosylated GPI anchored proteins and, consistent with other GPI anchored proteins, were localized within the lipid rafts of cellular membranes. In addition, an N-terminal proteolytic fragment of NgR comprising the majority of the ectodomain was found to be constitutively secreted from cells. Our data indicate that NgR, NgRH1 and NgRH2 constitute a novel receptor protein family, which may play related roles within the CNS.

Differential expression of L- and S-MAG upon cAMP stimulated differentiation in oligodendroglial cells

Myelin-associated glycoprotein (MAG), an immunoglobulin-like cell signaling protein involved in axon-glial interactions, displays two intracellular C-termini as a result of alternative mRNA splicing. During brain development, the two MAG mRNAs that encode L-MAG and S-MAG differ in their relative abundance. We have investigated the differential expression of L- and S-MAG upon cAMP treatment in the oligodendroglial cell line Oli-neu, a cell line able to differentiate in vitro. We have engineered GFP and VSVG fusions by small insertions into the alternatively spliced exons of the cloned MAG gene and reintroduced them into Oli-neu cells. The individually tagged MAG isoforms were expressed under the control of the MAG promoter and regulatory region. In this system, L-MAG was the predominant isoform before the stimulation of cells with cAMP, whereas upon cAMP treatment the S-MAG isoform was predominantly expressed in cells with a high degree of morphological differentiation. We suggest that the regulation of the MAG alternative splicing and the morphological differentiation in oligodendrocytes are controlled both by the same cAMP-responsive differentiation step.

Degraded myelin-associated glycoprotein (dMAG) formation from pure human brain myelin-associated glycoprotein (MAG) is not mediated by calpain or cathepsin L-like activities

The myelin-associated glycoprotein (MAG) is a transmembrane cell adhesion molecule participating in myelin formation and maintenance. Calcium-activated/-dependent proteolysis of myelin-associated glycoprotein by calpain and cathepsin L-like activities has already been detected in purified myelin fractions, producing a soluble fragment, called degraded (d)MAG, characterized by the loss of the transmembrane and cytoplasmic domains. Here, we demonstrate and analyze dMAG formation from pure human brain myelin-associated glycoprotein. The activity never exhibited the high rate previously reported in human myelin fractions. Degradation is time-, temperature-, buffer- and structure-dependent, is inhibited at 4 degrees C and by denaturation of the sample, and is mediated by a trans-acting factor. There is no strict pH dependency of the proteolysis. Degradation was inhibited by excess aprotinin, but not by 1-10 micro g/mL aprotinin and was not eliminated by the use of an aprotinin-sepharose matrix during the purification. dMAG formation was not enhanced by calcium, nor inhibited by a wide variety of protease inhibitors, including specific calpain and cathepsin L inhibitors. Therefore, while cysteine proteases may be present in human myelin membrane fractions, they are not involved in dMAG formation from highly purified human brain myelin-associated glycoprotein preparations.

Galactolipids are molecular determinants of myelin development and axo-glial organization

Myelination is a developmentally regulated process whereby myelinating glial cells elaborate large quantities of a specialized plasma membrane that ensheaths axons. The myelin sheath contains an unusual lipid composition in that the glycolipid galactosylceramide (GalC) and its sulfated form sulfatide constitute a large proportion of the total lipid mass. These glycolipids have been implicated in a range of developmental processes such as cell differentiation and myelination initiation, but analyses of mice lacking UDP-galactose:ceramide galactosyltransferase (CGT), the enzyme required for myelin galactolipid synthesis, have more recently demonstrated that the galactolipids more subtly regulate myelin formation. The CGT mutants display a delay in myelin maturation and axo-glial interactions develop abnormally. By interbreeding the CGT mutants with mice that lack myelin-associated glycoprotein, it has been shown that these specialized myelin lipids and proteins act in concert to promote axo-glial adhesion during myelinogenesis. The analysis of the CGT mutants is helping to clarify the roles myelin galactolipids play in regulating the development, and ultimately the function of the myelin sheath.

Myelin proteolipid protein, basic protein, the small isoform of myelin-associated glycoprotein, and p42MAPK are associated in the Triton X-100 extract of central nervous system myelin

To further our understanding of the functions of the major myelin proteins, myelin basic protein (MBP) and proteolipid protein (PLP), and other myelin proteins, such as 2'3'-cyclic nucleotide 3'-phosphodiesterase (CNP) and myelin-associated glycoprotein (MAG), bovine brain myelin was extracted with Triton X-100, and protein complexes in the detergent-soluble fraction were isolated by coimmunoprecipitation and sucrose density gradient sedimentation. MBP, PLP, and the small isoform of MAG (S-MAG) were coimmunoprecipitated from the detergent-soluble fraction by anti-PLP, anti-MBP or anti-MAG monoclonal antibodies. Additionally, a 30 kDa phosphoserine-containing protein and two phosphotyrosine-containing proteins (M(r) 30 and 42 kDa) were found in the coimmunoprecipitates. The 42 kDa protein is probably p42MAPK, in that MAPK was shown also to be present in the immunoprecipitated complex. CNP, the small PLP isoform DM20, the large MAG isoform L-MAG, MOG, CD44, MEK, p44MAPK, and actin were not present in the immunoprecipitates, although they were present in the detergent-soluble fraction. Lipid analysis revealed that the PLP-MBP-S-MAG coimmunoprecipitated with some phospholipids and sulfatide but not cholesterol or galactosylceramide. However, the complex had a high density, indicating that the lipid/protein ratio is low, and it was retained on a Sepharose CL6B column, indicating that it is not a large membrane fragment. Given that MAG is localized mainly in the periaxonal region of myelin, where it interacts with axonal ligands, the PLP-MBP-S-MAG complex may come from these regions, where it could participate in dynamic functions in the myelin sheath and myelin-axonal interactions.

CNS myelinogenesis in vitro: myelin basic protein deficient shiverer oligodendrocytes

The shiverer mutant mouse is an autosomal recessive mutant characterized by incomplete myelin sheath formation in the central nervous system (CNS). Such mice contain a deletion in the MBP gene, do not produce MBP proteins, and have little or no compact myelin in the CNS. To investigate the myelin sheath formation in shiverer mutant mice resulting from the absence of compact myelin, firstly we developed new methods for generating oligodendrocyte precursor cells (OPCs) from an E17 mouse brain, and examined homozygous shiverer (shi/shi) OPCs with respect to myelinogenesis in vitro. After treatment of shi/shi OPCs in vitro with PDGF or bFGF, proliferation of shi/shi OPCs was enhanced similar to that observed in wild-type OPCs. The majority of cells from the shiverer mutant mouse, however, remained as A2B5-immunoreactive early OPCs. To determine which molecular events affect the differentiation of shi/shi OPCs, we determined the signaling pathway that could be responsible for activating myelin sheath-specific proteins. We found that the developmental schedule of shi/shi OPCs in vitro was accelerated by the addition of cyclic AMP analogs, dibutyryl cAMP (dbcAMP). Treatment of shi/shi OPCs with dbcAMP had significant effect on the differentiation of OPCs that became MAG-expressing oligodendrocytes. To further determine the possible mechanism involved in the activation of MAG by dbcAMP, we examined the cAMP-dependent signaling cascades. The activation of JNK was markedly stimulated by treatment with dbcAMP, and the phosphorylation of transcription factor ATF-2 was also stimulated by dbcAMP. We demonstrated that the MAG-positive shi/shi oligodendrocytes extend processes around axons and finally covered the axon, this was clearly observed by immunocytochemistry of shi/shi oligodendrocyte-DRG cocultures. These results suggest that ATF-2 coupled to specific signal transduction cascades plays an important regulatory role in MAG expression at a specific stage of shi/shi oligodendrocyte differentiation, and OPCs grow to become myelin-forming cells with numerous cell processes that wraps around an axon to form a thin myelin sheath.

Neuroactive steroids and peripheral myelin proteins

The present review summarizes observations obtained in our laboratories which underline the importance of neuroactive steroids (i.e., progesterone (PROG), dihydroprogesterone (5alpha-DH PROG), tetrahydroprogesterone (3alpha, 5alpha-TH PROG), testosterone (T), dihydrotestosterone (DHT) and 5alpha-androstan-3alpha,17beta-diol (3alpha-diol)) in the control of the gene expression of myelin proteins (i.e. glycoprotein Po (Po) and the peripheral myelin protein 22 (PMP22)) in the peripheral nervous system. Utilizing different in vivo (aged and adult male rats) and in vitro (Schwann cell cultures) experimental models, we have observed that neuroactive steroids are able to stimulate the mRNA levels of Po and PMP22. The effects of these neuroactive steroids, which are able to interact with classical (progesterone receptor, PR, and androgen receptor, AR) and non-classical (GABA(A) receptor) steroid receptors is further supported by our demonstration in sciatic nerve and/or Schwann cells of the presence of these receptors. On the basis of the observations obtained in the Schwann cells cultures, we suggest that the stimulatory effect of neuroactive steroids on Po is acting through PR, while that on PMP22 needs the GABA(A) receptor. The present findings might be of importance for the utilization of specific receptor ligands as new therapeutical approaches for the rebuilding of the peripheral myelin, particularly in those situations in which the synthesis of Po and PMP22 is altered (i.e. demyelinating diseases like Charcot-Marie-Tooth type 1A and type 1B, hereditary neuropathy with liability to pressure palsies and the Déjérine-Sottas syndrome, aging, and after peripheral injury).

Cloning and functional characterization of DSCAML1, a novel DSCAM-like cell adhesion molecule that mediates homophilic intercellular adhesion

DSCAM, a conserved gene involved in neuronal differentiation, is a member of the Ig superfamily of cell adhesion molecules. Herein, we report the functional characterization of a human DSCAM (Down syndrome cell adhesion molecule) paralogue, DSCAML1, located on chromosome 11q23. The deduced DSCAML1 protein contains 10 Ig domains, six fibronectin-III domains, and an intracellular domain, all of which are structurally identical to DSCAM. When compared to DSCAM, DSCAML1 protein showed 64% identity to the extracellular domain and 45% identity to the cytoplasmic domain. In the mouse brain, DSCAML1 is predominantly expressed in Purkinje cells of the cerebellum, granule cells of the dentate gyrus, and in neurons of the cerebral cortex and olfactory bulb. Biochemical and immunofluorescence analyses indicated that DSCAML1 is a cell surface molecule that targets axonal features in differentiated PC12 cells. DSCAML1 exhibits homophilic binding activity that does not require divalent cations. Based on its structural and functional properties and similarities to DSCAM, we suggest that DSCAML1 may be involved in formation and maintenance of neural networks. The chromosomal locus for DSCAML1 makes it an ideal candidate for neuronal disorders (such as Gilles de la Tourette and Jacobsen syndromes) that have been mapped on 11q23.

Identification and characterization of S2V, a novel putative siglec that contains two V set Ig-like domains and recruits protein-tyrosine phosphatases SHPs

We describe the molecular cloning and characterization of S2V, a novel sialic acid binding immunoglobulin-like lectin. The cDNA of S2V encodes a type 1 transmembrane protein with four extracellular immunoglobulin-like (Ig-like) domains and a cytoplasmic tail bearing a typical immunoreceptor tyrosine-based inhibitory motif (ITIM) and an ITIM-like motif. A unique feature of S2V is the presence of two V-set Ig-like domains responsible for the binding to sialic acid, whereas all other known siglecs possess only one. S2V is predominantly expressed in macrophage. In vivo S2V was tyrosine-phosphorylated when co-expressed with exogenous c-Src kinase. Upon tyrosine phosphorylation, S2V recruits both Src homology 2 (SH2) domain-containing protein-tyrosine phosphatases SHP-1 and SHP-2, two important inhibitory regulators of immunoreceptor signal transduction. These findings suggest that S2V is involved in the negative regulation of the signaling in macrophage by functioning as an inhibitory receptor. When expressed in COS-7 cells, S2V was able to mediate sialic acid-dependent binding to human red blood cells, suggesting that S2V may function through cell-cell interaction.

Rapid functional analysis in Xenopus oocytes of Po protein adhesive interactions

We have developed a coupled Xenopus oocyte expression system for evaluating the functional effects of mutations in known or suspected adhesion molecules, which allows for a very rapid assessment of intercellular adhesion. As a model protein, we first used Protein zero (Po), an adhesion molecule that mediates self-adhesion of the Schwann cell plasma membrane to form compact myelin in the mammalian PNS. A wide variety of mutations in Po cause certain human peripheral neuropathies, such as the Charcot-Marie-Tooth disease (CMT) type 1B and Dejerine-Sottas syndrome (DSS). After wild-type Po mRNA is injected, the protein is synthesized and correctly targeted to the oocyte cell surface. When two oocytes are paired, wild-type Po redistributes and concentrates at the cell-cell apposition region, and by electron microscopy, the oocyte pairs show close cell-cell appositions and are devoid of the microvilli that are observed in uninjected oocyte pairs. These are hallmark features of highly adhesive cell:cell interfaces. Several point mutations in Po were engineered, corresponding to the molecular defects in the CMT type 1B or DSS. The proteins encoded by these mutations reached the cell surface but failed to concentrate at the oocyte interface. Po carrying a point mutation that is found in DSS is not targeted on the plasma membrane and fail to accumulate at the cell-cell contact site.

Structure, heterologous expression, and adhesive properties of the P(0)-like myelin glycoprotein IP1 of trout CNS

The IP1 protein of trout CNS myelin as well as an IP1/P(0) chimeric protein were stably expressed in CHO cells. Successful targeting of the recombinant proteins to the membrane surface was verified by immunofluorescence staining. Full-length expression of IP1 could be confirmed by Western blot analysis of proteins extracted from stably transfected CHO-cells. The adhesive properties of IP1 were studied by an in vitro aggregation assay in which microscopic examination was combined with electronic particle counting. While IP1 conveyed only a weak increase in cell aggregation of transfected CHO cells, the IP1/P0 chimera was much more effective. In the presence of specific antibodies, cell aggregation was strongly reduced. The adhesive properties of P(0)-like proteins are discussed considering recent crystallographic data on the atomic structure of the extracellular domain of mammalian P(0).

Severe hypomyelination of the murine CNS in the absence of myelin-associated glycoprotein and fyn tyrosine kinase

The analysis of mice deficient in the myelin-associated glycoprotein (MAG) or Fyn, a nonreceptor-type tyrosine kinase proposed to act as a signaling molecule downstream of MAG, has revealed that both molecules are involved in the initiation of myelination. To obtain more insights into the role of the MAG-Fyn signaling pathway during initiation of myelination and formation of morphologically intact myelin sheaths, we have analyzed optic nerves of MAG-, Fyn- and MAG/Fyn-deficient mice. We observed a slight hypomyelination in optic nerves of MAG mutants that was significantly increased in Fyn mutants and massive in MAG/Fyn double mutants. The severe morphological phenotype of MAG/Fyn mutants, accompanied by behavioral deficits, substantiates the importance of both molecules for the initiation of myelination. The different severity of the phenotype of different genotypes indicates that the MAG-Fyn signaling pathway is complex and suggests the presence of compensatory mechanisms in the single mutants. However, data are also compatible with the possibility that MAG and Fyn act independently to initiate myelination. Hypomyelination of optic nerves was not related to a loss of oligodendrocytes, indicating that the phenotype results from impaired interactions between oligodendrocyte processes and axons and/or impaired morphological maturation of oligodendrocytes. Finally, we demonstrate that Fyn, unlike MAG, is not involved in the formation of ultrastructurally intact myelin sheaths.

The small myelin-associated glycoprotein binds to tubulin and microtubules

The myelin-associated glycoprotein (MAG) exists as two isoforms, differing only by their respective cytoplasmic domains, that have been suggested to function in the formation and maintenance of myelin. In the present study, a 50 kDa protein binding directly to the small MAG (S-MAG) cytoplasmic domain was detected and identified as tubulin, the core component of the microtubular cytoskeleton. In vitro, the S-MAG cytoplasmic domain slowed the polymerization rate of tubulin and co-purified with assembled microtubules. A significant sequence homology was found between the tau family tubulin-binding repeats and the carboxy-terminus of S-MAG. Our results indicate that S-MAG is the first member of the Ig superfamily that can be classified as a microtubule-associated protein, and place S-MAG in a dynamic structural complex that could participate in linking the axonal surface and the myelinating Schwann cell cytoskeleton.

mSiglec-E, a novel mouse CD33-related siglec (sialic acid-binding immunoglobulin-like lectin) that recruits Src homology 2 (SH2)-domain-containing protein tyrosine phosphatases SHP-1 and SHP-2

The sialic acid-binding immunoglobulin-like lectins (siglecs) represent a recently defined distinct subset of the immunoglobulin superfamily. By using the Src homology 2 (SH2)-domain-containing protein tyrosine phosphatase SHP-1 as bait in a yeast two-hybrid screen, we have identified a new member of the mouse siglec family, mSiglec-E. The mSiglec-E cDNA encodes a protein of 467 amino acids that contains three extracellular immunoglobulin-like domains, a transmembrane region and a cytoplasmic tail bearing two immunoreceptor tyrosine-based inhibitory motifs (ITIMs). mSiglec-E is highly expressed in mouse spleen, a tissue rich in leucocytes. The ITIMs of mSiglec-E can recruit SHP-1 and SHP-2, two inhibitory regulators of immunoreceptor signal transduction. This suggests that the function of mSiglec-E is probably an involvement in haematopoietic cells and the immune system as an inhibitory receptor. When expressed in COS-7 cells, mSiglec-E was able to mediate sialic acid-dependent binding to human red blood cells, suggesting that mSiglec-E may function through cell-cell interactions. In comparison with the known members of the siglec family, mSiglec-E exhibits a high degree of sequence similarity to both human siglec-7 and siglec-9. The gene encoding mSiglec-E is localized in the same chromosome as that encoding mouse CD33. Phylogenetic analysis reveals that neither mouse mSiglec-E nor CD33 shows a clear relationship with any human siglecs so far identified.

Transcriptional repression of neurotrophin receptor trkB by thyroid hormone in the developing rat brain

Expression of the neurotrophin receptor trkB is regulated by thyroid hormone (T3) during development of the rat brain. trkB mRNA levels, coding for the full-length and the truncated isoforms, are increased in the cerebral cortex of neonatal experimental hypothyroid animals. Run-on transcription assays with nuclei from postnatal day 15, hypothyroid, and control cerebral cortices demonstrated that an increase in the transcription rate of the trkB gene accounts for the observed effect. Transient transfection experiments using a reporter plasmid containing a 7-kilobase pair DNA fragment upstream of the mouse trkB gene showed that unliganded thyroid hormone receptor (T3R) increases promoter activity, whereas addition of T3 reverses that activity below basal levels. Deletion analysis shows that the T3-dependent repression requires binding of the T3R to a specific region located downstream of the transcription start site. This region, at nucleotide position -465/-432, contains an array of thyroid hormone response half-sites that bind preferentially T3R as heterodimers with retinoid X receptor and whose deletion causes loss of the T3-dependent repression. These half-sites are able to confer negative regulation by T3 to a heterologous promoter, thus indicating the functionality of these sequences. These results demonstrate that, in the developing rat brain, T3 down-regulates the expression of the trkB gene through the active repression of a novel negative response element located downstream of its transcription initiation site.

Severe hypomyelination of the murine CNS in the absence of myelin-associated glycoprotein and fyn tyrosine kinase

The analysis of mice deficient in the myelin-associated glycoprotein (MAG) or Fyn, a nonreceptor-type tyrosine kinase proposed to act as a signaling molecule downstream of MAG, has revealed that both molecules are involved in the initiation of myelination. To obtain more insights into the role of the MAG-Fyn signaling pathway during initiation of myelination and formation of morphologically intact myelin sheaths, we have analyzed optic nerves of MAG-, Fyn- and MAG/Fyn-deficient mice. We observed a slight hypomyelination in optic nerves of MAG mutants that was significantly increased in Fyn mutants and massive in MAG/Fyn double mutants. The severe morphological phenotype of MAG/Fyn mutants, accompanied by behavioral deficits, substantiates the importance of both molecules for the initiation of myelination. The different severity of the phenotype of different genotypes indicates that the MAG-Fyn signaling pathway is complex and suggests the presence of compensatory mechanisms in the single mutants. However, data are also compatible with the possibility that MAG and Fyn act independently to initiate myelination. Hypomyelination of optic nerves was not related to a loss of oligodendrocytes, indicating that the phenotype results from impaired interactions between oligodendrocyte processes and axons and/or impaired morphological maturation of oligodendrocytes. Finally, we demonstrate that Fyn, unlike MAG, is not involved in the formation of ultrastructurally intact myelin sheaths.