Compartmentation of Fyn kinase with glycosylphosphatidylinositol-anchored molecules in oligodendrocytes facilitates kinase activation during myelination

The function of neurofascin155 in oligodendrocytes is regulated by metalloprotease-mediated cleavage and ectodomain shedding

Formation of the paranodal axo-glial junction requires the oligodendrocyte-specific 155-kDa isoform of neurofascin (NF155). Here, we report the presence of two peptides in cultured oligodendrocytes, which are recognized by distinct NF155-specific antibodies and correspond to a membrane anchor of 30 kDa and a 125 kDa peptide, which is shed from the cells, indicating that it consists of the NF155 ectodomain. Transfection of OLN-93 cells with NF155 verified that both peptides originate from NF155 cleavage, and we present evidence that metalloproteases mediate NF155 processing. Interestingly, metalloprotease activity is required for NF155 transport into oligodendrocyte processes supporting the functional significance of NF155 cleavage. To further characterize NF155 cleavage and function, we transfected MDCK cells with NF155. Although ectodomain shedding was observed in polarized and non-polarized MDCK cells, surface localization of NF155 was restricted to the lateral membrane of polarized cells consistent with a role in cell-cell adhesion. Aggregation assays performed with OLN-93 cells confirmed that NF155 accelerates cell-cell adhesion in a metalloprotease-dependent manner. The physiological relevance of NF155 processing is corroborated by the presence of NF155 cleavage products in heavy myelin, suggesting a role of NF155 ectodomain shedding for the generation and/or stabilization of the nodal/paranodal architecture.

Rafts in oligodendrocytes: Evidence and structure–function relationship

The plasma membrane of eukaryotic cells exhibits lateral inhomogeneities, mainly containing cholesterol and sphingomyelin, which provide liquid-ordered microdomains (lipid "rafts") that segregate membrane components. Rafts are thought to modulate the biological functions of molecules that become associated with them, and as such, they appear to be involved in a variety of processes, including signal transduction, membrane sorting, cell adhesion and pathogen entry. Although still a matter of ongoing debate, evidence in favor of the presence of these microdomains is gradually accumulating but a consensus on issues like their size, lifetime, composition, and biological significance has yet to be reached. Here, we provide an overview of the evidence supporting the presence of rafts in oligodendrocytes, the myelin-producing cells of the central nervous system, and discuss their functional significance. The myelin membrane differs fundamentally from the plasma membrane, both in lipid and protein composition. Moreover, since myelin membranes are unusually enriched in glycosphingolipids, questions concerning the biogenesis and functional relevance of microdomains thus appear of special interest in oligodendrocytes. The current picture of rafts in oligodendrocytes is mainly based on detergent methods. The robustness of such data is discussed and alternative methods that may provide complementary data are indicated.

Identification and functional characterization of mouse TPO1 as a myelin membrane protein

TPO1 is a member of the AIGP family, a unique group of proteins that contains 11 putative transmembrane domains. Expression of the rat TPO1 gene is upregulated in cultured oligodendrocytes (OLs) during development from pro-oligodendroblasts to postmitotic OLs. However, the distribution of native TPO1 protein in cultured OLs and in the brain has not been elucidated. We investigated the distribution and cellular function of TPO1 in myelinating cells of the nervous system. In mice, TPO1 gene expression was detected in the central (CNS) and peripheral (PNS) nervous systems and was markedly upregulated at postnatal days 10-20, an early phase of myelination in the mouse brain. To investigate TPO1 localization, we generated affinity-purified antibodies to synthetic peptides derived from mouse TPO1. Immunohistochemical analysis showed that TPO1 was expressed in OLs and Schwann cells but not in neurons and astrocytes. Schwann cells from trembler mice, which lack PNS myelin, had significantly decreased TPO1 expression and an altered localization pattern, suggesting that TPO1 is a functional myelin membrane protein. In OL lineage cell cultures, TPO1 was detected in A2B5+ bipolar early progenitors, A2B5+ multipolar Pro-OLs, GalC+ immature OLs and MBP+ mature OLs. The subcellular localization of TPO1 in OL lineage cells was mapped to the GM130+ Golgi in cell bodies and Fyn+ cell processes and myelin-like sheets. Furthermore, TPO1 selectively colocalized with non-phosphorylated Fyn and promoted Fyn autophosphorylation in COS7 cells, suggesting that TPO1 may play a role in myelin formation via Fyn kinase activation in the PNS and CNS.

The membrane environment of endogenous cellular prion protein in primary rat cerebellar neurons

We studied the membrane environment of cellular prion protein in primary cultured rat cerebellar neurons differentiated in vitro. In these cells, about 45% of total cellular prion protein (corresponding to a 35-fold enrichment) is associated with a low-density, sphingolipid- and cholesterol-enriched membrane fraction, that can be separated by flotation on sucrose gradient. Biotinylation experiments indicated that almost all prion protein recovered in this fraction was exposed at the cell surface. Prion protein was efficiently separated from this fraction by a monoclonal antibody immuno-separation procedure. Under conditions designed to preserve lipid-mediated membrane organization, several proteins were found in the prion protein-enriched membrane domains (i.e. the non-receptor tyrosine kinases Lyn and Fyn and the neuronal glycosylphosphatidylinositol-anchored protein Thy-1). The prion protein-rich membrane domains contained, as well, about 50% of the sphingolipids, cholesterol and phosphatidylcholine present in the sphingolipid-enriched membrane fraction. All main sphingolipids, including sphingomyelin, neutral glycosphingolipids and gangliosides, were similarly enriched in the prion protein-rich membrane domains. Thus, prion protein plasma membrane environment in differentiated neurons resulted to be a complex entity, whose integrity requires a network of lipid-mediated non-covalent interactions.

Role of Mayven, a kelch-related protein in oligodendrocyte process formation

Oligodendrocyte function is central to the maintenance of the normal nervous system in health and disease. In particular, process formation and the generation of large sheets of myelin are important components of their biological properties. We have investigated the role of Mayven, a recently identified member of the kelch family of proteins, in process extension in oligodendrocyte-lineage cells. The kelch superfamily consists of a large number of structurally diverse proteins characterized by the presence of a kelch-repeat domain. Other members of this family associate with the actin cytoskeleton and regulate process length. Mayven is expressed predominantly in the CNS, has six kelch repeats, and is an actin-binding protein, associating with actin through its kelch-repeat domain. We have cloned rat Mayven and examined its role in the oligodendrocyte lineage by using RT-PCR, RNA interference, and a truncated, dominant-negative myc-tagged Mayven. Oligodendrocyte precursors treated with siRNA directed to Mayven have reduced process length, but there was no change in migration or expression of differentiation markers. Immunocytochemistry demonstrated that Mayven associated with F-actin at cell tips. Finally, overexpression of truncated Mayven lacking the SH3 ligand binding domain in oligodendrocyte-lineage cells resulted in shorter process formation, which was augmented when the cells were plated on laminin and fibronectin. These data suggest a role for Mayven in oligodendrocyte precursor cell process formation.

Two types of detergent-insoluble, glycosphingolipid/cholesterol-rich membrane domains from isolated myelin

Two different types of low-density detergent-insoluble glycosphingolipid-enriched membrane domain (DIG) fractions were isolated from myelin by extraction with Triton X-100 (TX-100) in 50 mM sodium phosphate buffer at room temperature (20 degrees C) (procedure 1), in contrast to a single low-density fraction obtained by extraction with TX-100 in Tris buffer containing 150 mM NaCl and 5 mM EDTA at 4 degrees C (procedure 2). Procedure 1 has been used in the past by others for myelin extraction to preserve the cytoskeleton and/or radial component of oligodendrocytes and myelin, whereas procedure 2 is now more commonly used to isolate myelin DIG fractions. The two DIG fractions obtained by procedure 1 gave opaque bands, B1 and B2, at somewhat lower and higher sucrose density respectively than myelin itself. The single DIG fraction obtained by procedure 2 gave a single opaque band at a similar sucrose density to B1. Both B1 and B2 had characteristics of lipid rafts, i.e. high galactosylceramide and cholesterol content and enrichment in GPI-linked 120-kDa neural cell adhesion molecule (NCAM)120, as found by others for the single low-density DIG fraction obtained by procedure 2. However, B2 had most of the myelin GM1 and more of the sulfatide than B1, and they differed significantly in their protein composition. B2 contained 41% of the actin, 100% of the tubulin, and most of the flotillin-1 and caveolin in myelin, whereas B1 contained more NCAM120 and other proteins than B2. The single low-density DIG fraction obtained by procedure 2 contained only low amounts of actin and tubulin. B1 and B2 also had size-isoform selectivity for some proteins, suggesting specific interactions and different functions of the two membrane domains. We propose that B1 may come from non-caveolar raft domains whereas B2 may derive from caveolin-containing raft domains associated with cytoskeletal proteins. Some kinases present were active on myelin basic protein suggesting that the DIGs may come from signaling domains.

The neuronal growth and regeneration associated Cntn1 (F3/F11/Contactin) gene is duplicated in fish: expression during development and retinal axon regeneration

The Cntn1 (Contactin/F3/F11) cell adhesion molecule is involved in axon growth and guidance, fasciculation, synapse formation, and myelination in birds and mammals. We identified Cntn1 genes in goldfish, zebrafish, and fugu, and provide evidence for a fish-specific duplication leading to Cntn1a and Cntn1b. Our analyses suggest a subfunctionalization for the Cntn1 paralogs in zebrafish compared to other vertebrates which have a single Cntn1 gene. Similar to Cntn1a, Cntn1b transcripts are found in subsets of sensory and motor neurons. However, Cntn1b is detected later and more restricted than Cntn1a. This spatio-temporal expression pattern of the two zebrafish Cntn1 paralogs suggests functions related to those of mammalian Cntn1. In adult goldfish, Cntn1b is expressed in oligodendrocytes and is upregulated in retinal ganglion cells after optic nerve transection, which is consistent with an additional role during regeneration.

Myelin-associated glycoprotein is altered in a familial late-onset orthochromatic leukodystrophy

Adult-onset dominant leukodystrophies are a heterogeneous group of rare disorders, whose etiology, pathogenesis and molecular background are still unknown. We report the neuropathological and biochemical investigations of the brains and their myelin proteins components in 2 members of an Italian family affected by an adult-onset autosomal dominant leukoencephalopathy. Clinical signs included spastic paraparesis, pseudobulbar syndrome, action tremor of head and hands, and moderate memory impairment. No mental deterioration or neuropathy was present. Onset was subacute (range 42-53 years) and progression spanned 4 to 7 years. The neuropathological phenotype overlapped that of orthochromatic leukodystrophies. The biochemical analysis revealed an abnormal myelin-associated glycoprotein (MAG); the defect was localized at the C-terminal domain of the L-MAG isoform, resulting in a protein approximately 5 kDa shorter than the normal counterpart. No mutation in the MAG gene-coding regions was uncovered, and linkage analysis formally excluded the entire MAG locus. We show that the identified MAG protein alteration is probably due to an abnormal post-translational event. Considering MAG function in the maintenance of myelin, the abnormal protein may have a role in the pathogenesis of this disease. This is the first report of a possible pathogenetic role of MAG in a hereditary disease affecting the central white matter.

C18:3-GM1a induces apoptosis in Neuro2a cells: enzymatic remodeling of fatty acyl chains of glycosphingolipids

GM1a [Gal beta1-3GalNAc beta1-4(NeuAc alpha2-3)Gal beta1-4Glc beta1-1Cer] is known to support and protect neuronal functions. However, we report that alpha-linolenic acid-containing GM1a (C18:3-GM1a), which was prepared using the reverse hydrolysis reaction of sphingolipid ceramide N-deacylase, induced apoptosis in neuronal cells. Intranucleosomal DNA fragmentation, chromatin condensation, and caspase activation, all typical features of apoptosis, were observed when mouse neuroblastoma Neuro2a cells were cultured with C18:3-GM1a but not GM1a containing stearic acid (C18:0) or oleic acid (C18:1). The phenotype of Neuro2a cells induced by C18:3-GM1a was similar to that evoked by lyso-GM1a. However, lyso-GM1a caused a complete disruption of lipid microdomains of Neuro2a cells and hemolysis of sheep erythrocytes, whereas C18:3-GM1a did neither. C18:3-GM1a, but not lyso-GM1a, was found to be abundant in lipid microdomains after the removal of loosely bound GM1a by BSA. The activation of stress-activated protein kinase/c-Jun N-terminal kinase in Neuro2a cells was observed with lyso-GM1a but not C18:3-GM1a. These results indicate that the mechanism of apoptosis induced by C18:3-GM1a is distinct from that caused by lyso-GM1a. This study also clearly shows that fatty acid composition of gangliosides significantly affected their pharmacological activities when added to the cell cultures and suggests why naturally occurring gangliosides do not possess polyunsaturated fatty acids as a major constituent.

LINGO-1 negatively regulates myelination by oligodendrocytes

The control of myelination by oligodendrocytes in the CNS is poorly understood. Here we show that LINGO-1 is an important negative regulator of this critical process. LINGO-1 is expressed in oligodendrocytes. Attenuation of its function by dominant-negative LINGO-1, LINGO-1 RNA-mediated interference (RNAi) or soluble human LINGO-1 (LINGO-1-Fc) leads to differentiation and increased myelination competence. Attenuation of LINGO-1 results in downregulation of RhoA activity, which has been implicated in oligodendrocyte differentiation. Conversely, overexpression of LINGO-1 leads to activation of RhoA and inhibition of oligodendrocyte differentiation and myelination. Treatment of oligodendrocyte and neuron cocultures with LINGO-1-Fc resulted in highly developed myelinated axons that have internodes and well-defined nodes of Ranvier. The contribution of LINGO-1 to myelination was verified in vivo through the analysis of LINGO-1 knockout mice. The ability to recapitulate CNS myelination in vitro using LINGO-1 antagonists and the in vivo effects seen in the LINGO-1 knockout indicate that LINGO-1 signaling may be critical for CNS myelination.

Process elongation of oligodendrocytes is promoted by the Kelch-related actin-binding protein Mayven

Rearrangement of the cytoskeleton leading to the extension of cellular processes is essential for the myelination of axons by oligodendrocytes. We observed that the actin-binding protein, Mayven, is expressed during all stages of the oligodendrocyte lineage, and that its expression is up-regulated during oligodendrocyte differentiation. Mayven is localized in the cytoplasm and along the cell processes. Mayven also binds actin, and is involved in the cytoskeletal reorganization in oligodendrocyte precursor cells (O-2A cells) that leads to process elongation. Mayven overexpression resulted in an increase in the process outgrowth of O-2A cells and in the lengths of the processes, while microinjection of Mayven-specific antibodies inhibited process extension in these cells. Furthermore, O-2A cells transduced with recombinant retroviral sense Mayven (pMIG-W-Mayven) showed an increase in the number of oligodendrocyte processes with outgrowth, while recombinant retroviral antisense Mayven (pMIG-W-Mayven-AS) blocked O-2A process extension. Interestingly, co-localization and association of Mayven with Fyn kinase were found in O-2A cells, and these interactions were increased during the outgrowth of oligodendrocyte processes. This association was mediated via the SH3 domain ligand (a.a. 1-45) of Mayven and the SH3 domain of Fyn, suggesting that Mayven may act as a linker to bind Fyn, via its N-terminus. Thus, Mayven plays a role in the dynamics of cytoskeletal rearrangement leading to the process extension of oligodendrocytes.

A glycosynapse in myelin?

Myelin, the multilayered membrane which surrounds nerve axons, is the only example of a membranous structure where contact between extracellular surfaces of membrane from the same cell occurs. The two major glycosphingolipids (GSLs) of myelin, galactosylceramide (GalC) and its sulfated form, galactosylceramide I(3)-sulfate (SGC), can interact with each other by trans carbohydrate-carbohydrate interactions across apposed membranes. They occur in detergent-insoluble lipid rafts containing kinases and thus may be located in membrane signaling domains. These signaling domains may contact each other across apposed extracellular membranes, thus forming glycosynapses in myelin. Multivalent forms of these carbohydrates, GalC/SGC-containing liposomes, or galactose conjugated to albumin, have been added to cultured oligodendrocytes (OLs) to mimic interactions which might occur between these signaling domains when OL membranes or the extracellular surfaces of myelin come into contact. These interactions between multivalent carbohydrate and the OL membrane cause co-clustering or redistribution of myelin GSLs, GPI-linked proteins, several transmembrane proteins, and signaling proteins to the same membrane domains. They also cause depolymerization of the cytoskeleton, indicating that they cause transmission of a signal across the membrane. Their effects have similarities to those of anti-GSL antibodies on OLs, shown by others, suggesting that the multivalent carbohydrate interacts with GalC/SGC in the OL membrane. Communication between the myelin sheath and the axon regulates both axonal and myelin function and is necessary to prevent neurodegeneration. Participation of transient GalC and SGC interactions in glycosynapses between the apposed extracellular surfaces of mature compact internodal myelin might allow transmission of signals throughout the myelin sheath and thus facilitate myelin-axonal communication.

Differential gene expression in neural stem cells and oligodendrocyte precursor cells: a cDNA microarray analysis

The use of neural stem cells (NSCs) or their progeny oligodendrocyte precursor cells (OPCs) represents a promising repair strategy for many neurological disorders. However, the molecular events and biological features during the transition from NSCs to OPCs remain unclear. In the present study, we isolated NSCs from the embryonic rat forebrain and induced them into OPCs by using B104 conditioned medium (B104CM) in vitro. We then employed cDNA array technology to compare changes in gene expression between the two cell populations. Among 1,176 genes examined, 40 were differentially expressed, and some of them may be involved in OPC differentiation from NSCs. Our findings thus provide new insights into the molecular basis of differentiation of OPCs from NSCs.

Alteration of the extracellular matrix interferes with raft association of neurofascin in oligodendrocytes. Potential significance for multiple sclerosis?

Remyelination, as potential treatment for demyelinating diseases like multiple sclerosis (MS), requires the formation of new axoglial interactions by differentiating oligodendrocyte progenitor cells. Since the oligodendrocyte-specific isoform of neurofascin, NF155 (neurofascin isoform of 155 kDa), may be important for establishing axoglial interactions, we analyzed whether its expression is changed in chronic relapsing experimental allergic encephalomyelinitis (EAE). Although overall expression of NF155 was not changed, immunoreactivity of NF155 was dramatically increased in EAE lesion sites indicating an enhanced accessibility of NF155 epitopes. As this may be due to infiltrating plasma components, for example, fibronectin, we analyzed whether fibronectin affects the intracellular distribution and membrane association of NF155 in primary oligodendrocytes. In oligodendrocytes cultivated on polylysine, NF155 was recruited to membrane microdomains (rafts) during development and became enriched in secondary and tertiary processes. Fibronectin perturbed localization and raft association of NF155 and inhibited the morphological differentiation of oligodendrocytes. Consistent with the in vitro data, raft association of NF155 was reduced in spinal cord of EAE rats. The results suggest that the association of NF155 to microdomains in the oligodendrocyte membrane is required for its participation in intermolecular interactions, which are important for myelination and/or myelin integrity.

Signaling from integrins to Fyn to Rho family GTPases regulates morphologic differentiation of oligodendrocytes

Differentiation of oligodendrocyte progenitor cells requires activation of the Src family kinase Fyn. The signals that are upstream and downstream of Fyn in oligodendrocytes remain essentially unknown. Here we show that extracellular matrix engagement regulates the morphology of oligodendrocytes and activates Fyn. Infection of primary oligodendrocyte cultures with recombinant adenovirus revealed that expression of Fyn or its downstream target p190RhoGAP induced process extension. This phenotypic change was not observed when kinase-inactive Fyn or GAP-defective p190 mutants were expressed. Because Rho family proteins are regulated by p190, we monitored the effects of introducing dominant-negative (DN) or constitutively activated (CA) versions of Rho, Rac1, or Cdc42 into primary oligodendrocyte cultures. Expression of DN Rho, CA Rac1, or CA Cdc42 induced outgrowth of oligodendrocyte processes, whereas introduction of CA Rho, DN Rac1, or DN cdc42 inhibited oligodendrocyte differentiation, indicating that Rho and Cdc42-Rac1 exert opposing effects on oligodendrocyte differentiation. Direct measurement of Rho family activity revealed that RhoA was downregulated, and Cdc42 and Rac1 were upregulated during differentiation of primary oligodendrocytes. Moreover, inhibition of integrin engagement or of Fyn activation blocked activation of Rac1 and cdc42 as well as myelin basic protein expression. Taken together, these results suggest a linear signal transduction pathway of integrin-Fyn-Rho family GTPases that controls morphologic differentiation of oligodendrocytes.

Comparison of myelin, axon, lipid, and immunopathology in the central nervous system of differentially myelin-compromised mutant mice: a morphological and biochemical study

The present study was carried out to compare different myelin-compromised mouse mutants with regard to myelin morphology in relation to axon-, lipid-, and immunopathology as a function of age. Mouse mutants deficient in the myelin-associated glycoprotein (MAG) and myelin basic protein (MBP) display subtle and severe myelin pathologies in the central nervous system (CNS), respectively. Animals doubly deficient in MAG and the neural cell adhesion molecule (NCAM) show defects similar to those present in MAG single mutants while mice deficient in MAG and the nonreceptor type tyrosine kinase Fyn are severely hypomyelinated, in addition to the MAG-specific myelin abnormalities. These mutant mice showed distinct myelin pathologies in different regions of the central nervous system and generally displayed a decrease in axonal integrity with age. Myelin pathology did not correlate locally with axon transection and with an involvement of the immune system as seen by numbers of CD3-positive lymphocytes and MAC-3-positive macrophages. Interestingly, the degree of these cellular abnormalities also did not correlate with abnormalities in levels of phospholipids, arachidonic acid, cholesterol, and apolipoprotein E (apoE). Moreover, these changes in lipid metabolism, including immune system-related arachidonic acid, preceded cellular pathology. The combined observations point to differences, but also similarities in the relation of myelin, axon, and immunopathology with genotype, and to a common aggravation of the phenotype with age.

Signaling cascades activated upon antibody cross-linking of myelin oligodendrocyte glycoprotein: potential implications for multiple sclerosis

Antibody-induced demyelination is an important component of pathology in multiple sclerosis. In particular, antibodies to myelin oligodendrocyte glycoprotein (MOG) are elevated in multiple sclerosis patients, and they have been implicated as mediators of demyelination. We have shown previously that antibody cross-linking of MOG in oligodendrocytes results in the repartitioning of MOG into glycosphingolipid-cholesterol membrane microdomains ("lipid rafts"), followed by changes in the phosphorylation of specific proteins, including dephosphorylation of beta-tubulin and the beta subunit of the trimeric G protein and culminating in rapid and dramatic morphological alterations. In order to further elucidate the mechanism of anti-MOG-mediated demyelination, we have carried out a proteomic analysis to identify the set of proteins for which the phosphorylation states or expression levels are altered upon anti-MOG treatment. We demonstrate that treatment of oligodendrocytes with anti-MOG alone leads to an increase in calcium influx and activation of the MAPK/Akt pathways that is independent of MOG repartitioning. However, further cross-linking of anti-MOG.MOG complexes with a secondary anti-IgG results in the lipid raft-dependent phosphorylation of specific proteins related to cellular stress response and cytoskeletal stability. Oligodendrocyte survival is not compromised by these treatments. We discuss the possible significance of the anti-MOG-induced signaling cascade in relation to the initial steps of MOG-mediated demyelination.

Developmental partitioning of myelin basic protein into membrane microdomains

Specific membrane microdomains (including lipid rafts) exist in myelin but have not been fully characterized. Myelin basic protein (MBP) maintains the compactness of the myelin sheath and is highly posttranslationally modified. Thus, it has been suggested that MBP might also have other functions, e.g., in signal transduction. Here, the distribution of MBP and its modified forms was studied, spatially and temporally, by detailed characterization of membrane microdomains from developing and mature bovine myelin. Myelin membranes were extracted with three different detergents (Brij 96V, CHAPS, or Triton X-100) at 4 degrees C. The detergent-resistant membranes (DRMs), representing coalesced lipid rafts, were isolated as low-buoyant-density fractions on a sucrose density gradient. These myelin rafts were disrupted when cholesterol was depleted with methyl-beta-cyclodextrin. The use of CHAPS detergent led to enrichment of several myelin proteins, including phospho-Thr97-MBP, in the DRMs from mature myelin. Citrullinated and methylated MBP remained in "nonraft" microdomains. In contrast, the DRMs from early myelin were enriched in Golli-MBP, Fyn, Lyn, and CNP. The localization of various proteins in DRMs was further supported by the colocalization of these lipid raft components in cultured mouse oligodendrocytes. Thus, there is a developmental regulation of posttranslationally modified forms of MBP into specific membrane microdomains.

RPTPalpha is essential for NCAM-mediated p59fyn activation and neurite elongation

The neural cell adhesion molecule (NCAM) forms a complex with p59^fyn kinase and activates it via a mechanism that has remained unknown. We show that the NCAM140 isoform directly interacts with the intracellular domain of the receptor-like protein tyrosine phosphatase RPTPα, a known activator of p59^fyn. Whereas this direct interaction is Ca²⁺ independent, formation of the complex is enhanced by Ca²⁺-dependent spectrin cytoskeleton–mediated cross-linking of NCAM and RPTPα in response to NCAM activation and is accompanied by redistribution of the complex to lipid rafts. Association between NCAM and p59^fyn is lost in RPTPα-deficient brains and is disrupted by dominant-negative RPTPα mutants, demonstrating that RPTPα is a link between NCAM and p59^fyn. NCAM-mediated p59^fyn activation is abolished in RPTPα-deficient neurons, and disruption of the NCAM–p59^fyn complex in RPTPα-deficient neurons or with dominant-negative RPTPα mutants blocks NCAM-dependent neurite outgrowth, implicating RPTPα as a major phosphatase involved in NCAM-mediated signaling.

Integrins direct Src family kinases to regulate distinct phases of oligodendrocyte development

Specific integrins expressed on oligodendrocytes, the myelin-forming cells of the central nervous system, promote either differentiation and survival or proliferation by amplification of growth factor signaling. Here, we report that the Src family kinases (SFKs) Fyn and Lyn regulate each of these distinct integrin-driven behaviors. Fyn associates with alpha6beta1 and is required to amplify platelet-derived growth factor survival signaling, to promote myelin membrane formation, and to switch neuregulin signaling from a phosphatidylinositol 3-kinase to a mitogen-activated protein kinase pathway (thereby changing the response from proliferation to differentiation). However, earlier in the lineage Lyn, not Fyn, is required to drive alphaVbeta3-dependent progenitor proliferation. The two SFKs respond to integrin ligation by different mechanisms: Lyn, by increased autophosphorylation of a catalytic tyrosine; and Fyn, by reduced Csk phosphorylation of the inhibitory COOH-terminal tyrosine. These findings illustrate how different SFKs can act as effectors for specific cell responses during development within a single cell lineage, and, furthermore, provide a molecular mechanism to explain similar region-specific hypomyelination in laminin- and Fyn-deficient mice.

Developmental abnormalities of myelin basic protein expression in fyn knock-out brain reveal a role of Fyn in posttranscriptional regulation

Fyn protein-tyrosine kinase (PTK), a member of the Src-PTK family, is essential for myelin development in the central nervous system (CNS). The absence of Fyn activity results in defects in the morphogenesis of oligodendrocyte precursors (OPCs) and CNS hypomyelination. However, molecular mechanisms for Fyn to control CNS myelinogenesis remain elusive. Here we show that Fyn-PTK is significantly up-regulated in early OPC differentiation, concentrated in the compact myelin, and declines during myelin development. Despite the high levels of Fyn-PTK expression during early OPC differentiation, Fyn deficiency does not affect the expression of mRNAs that encode myelin structural proteins, including that for the myelin basic protein (MBP), until postnatal day 13 (P13). However, the accumulation rate of MBP mRNA is significantly attenuated during the most active period of myelinogenesis (P13 and P20). Interestingly, the absence of Fyn causes a preferential reduction of the exon-2 containing MBP mRNA isoforms derived from alternative splicing, providing the first evidence that Fyn is required for posttranscriptional regulation of MBP. Consistent with this idea, Fyn phosphorylates the selective RNA-binding protein QKI, which likely modulates the activity of QKI in binding and stabilizing the MBP mRNA. Furthermore, Fyn deficiency exerts an opposing influence on MBP isoform patterning in comparison to that by QKI deficiency. These observations collectively suggest that Fyn plays critical roles in promoting accelerated MBP expression during myelinogenesis in a MBP isoform-preferential manner, and QKI may act in the same pathway downstream of Fyn for MBP mRNA homeostasis.

The raft-associated protein MAL is required for maintenance of proper axon--glia interactions in the central nervous system

The myelin and lymphocyte protein (MAL) is a tetraspan raft-associated proteolipid predominantly expressed by oligodendrocytes and Schwann cells. We show that genetic ablation of mal resulted in cytoplasmic inclusions within compact myelin, paranodal loops that are everted away from the axon, and disorganized transverse bands at the paranode–axon interface in the adult central nervous system. These structural changes were accompanied by a marked reduction of contactin-associated protein/paranodin, neurofascin 155 (NF155), and the potassium channel Kv1.2, whereas nodal clusters of sodium channels were unaltered. Initial formation of paranodal regions appeared normal, but abnormalities became detectable when MAL started to be expressed. Biochemical analysis revealed reduced myelin-associated glycoprotein, myelin basic protein, and NF155 protein levels in myelin and myelin-derived rafts. Our results demonstrate a critical role for MAL in the maintenance of central nervous system paranodes, likely by controlling the trafficking and/or sorting of NF155 and other membrane components in oligodendrocytes.

Lipid rafts and integrin activation regulate oligodendrocyte survival

Newly formed oligodendrocytes in the CNS derive survival cues from their target axons. These cues are provided in part by laminins expressed on the axon, which are recognized by alpha6beta1 integrin on the oligdendrocyte and amplify platelet-derived growth factor (PDGF) signaling through the phosphatidylinositol 3'-kinase (PI3K) pathway. The alpha6beta1 integrin is localized in oligodendrocyte lipid rafts. We show here using the sphingolipid synthesis inhibitor fumonisin-B1 to deplete rafts that this localization is important for normal survival signaling, because depletion increases oligodendrocyte apoptosis and inhibits PI3K signaling. We have shown previously that PDGF-mediated integrin activation is an important component of oligodendrocyte proliferation signaling, and here we present evidence that a similar mechanism operates in survival signaling. Integrin activation using manganese increases raft localization and rescues the effects of both raft depletion and PDGF removal on survival and PI3K signaling. Together, these results point to an essential role for rafts in oligodendrocyte survival signaling on the basis of the provision of a favorable environment for growth factor-mediated integrin activation.

Specific downregulation and mistargeting of the lipid raft-associated protein MAL in a glycolipid storage disorder

Metachromatic leukodystrophy (MLD) is a lysosomal lipid storage disease caused by arylsulfatase A deficiency. In MLD patients the sphingolipid sulfatide increasingly accumulates leading to progressive demyelination. We have analysed arylsulfatase A-deficient mice, a MLD mouse model, and we show that accumulation of sulfatide is not restricted to the lysosomal compartment but also occurs in myelin itself. Although, this sulfatide storage did not affect the overall composition of most myelin proteins, it specifically caused a severe reduction of MAL. This demonstrates a regulatory link between sulfatide accumulation and MAL expression and indicates the existence of regulatory mechanisms between lipid and myelin protein synthesis in oligodendrocytes. In addition, in cultured renal epithelial cells, sulfatide accumulation diverts MAL to the late endosomal/lysosomal compartment and thus also affects the intracellular distribution of MAL. The specific reduction and mistargeting of MAL protein as a reaction to sulfatide overload may contribute to the pathogenic mechanisms in metachromatic leukodystrophy.

Myelin associated glycoprotein cross-linking triggers its partitioning into lipid rafts, specific signaling events and cytoskeletal rearrangements in oligodendrocytes

Myelin-associated glycoprotein (MAG) has been implicated in inhibition of nerve regeneration in the CNS. This results from interactions between MAG and the Nogo receptor and gangliosides on the apposing axon, which generates intracellular inhibitory signals in the neuron. However, because myelin-axon signaling is bidirectional, we undertook an analysis of potential MAG-activated signaling in oligodendrocytes (OLs). In this study, we show that antibody cross-linking of MAG on the surface of OLs (to mimic axonal binding) leads to the redistribution of MAG into detergent (TX-100)-insoluble complexes, hyperphosphorylation of Fyn, dephosphorylation of serine and threonine residues in specific proteins, including lactate dehydrogenase and the beta subunit of the trimeric G-protein-complex, and cleavage of alpha-fodrin followed by a transient depolymerization of actin. We propose that these changes are part of a signaling cascade in OLs associated with MAG function as a mediator of axon-glial communication which might have implications for the mutual regulation of the formation and stability of axons and myelin.

The myelin-axolemmal complex: biochemical dissection and the role of galactosphingolipids

Myelin-axolemmal interactions regulate many cellular and molecular events, including gene expression, oligodendrocyte survival and ion channel clustering. Here we report the biochemical fractionation and enrichment of distinct subcellular domains from myelinated nerve fibers. Using antibodies against proteins found in compact myelin, non-compact myelin and axolemma, we show that a rigorous procedure designed to purify myelin also results in the isolation of the myelin-axolemmal complex, a high-affinity protein complex consisting of axonal and oligodendroglial components. Further, the isolation of distinct subcellular domains from galactolipid-deficient mice with disrupted axoglial junctions is altered in a manner consistent with the delocalization of axolemmal proteins observed in these animals. These results suggest a paradigm for identification of proteins involved in neuroglial signaling.

Sulfatide is a negative regulator of oligodendrocyte differentiation: Development in sulfatide-null mice

Galactosylceramide (GalC) and its sulfated analogue, sulfatide, are major galactosphingolipid components of myelin and oligodendrocyte plasma membranes in the nervous system. We previously hypothesized that these galactolipids play functional roles in the regulation of oligodendrocyte terminal differentiation by acting as sensors/transmitters of environmental information. Evidence strongly supports this idea. First, these molecules are initially expressed on the cell surface at the interface at which oligodendrocyte progenitors first enter terminal differentiation. Second, exposure of oligodendrocyte progenitors to anti-GalC/-sulfatide (RmAb) or antisulfatide (O4), but not anti-GalC (O1), antibodies leads to the reversible arrest of oligodendrocyte lineage progression at this interface. Third, in cerebroside galactosyl transferase-null mice (Cgt(-/-)) that are unable to synthesize either GalC or sulfatide, terminal differentiation and morphological maturation of oligodendrocytes are enhanced. In the present study, we examined oligodendrocytes differentiation in cerebroside sulfotransferase-null mice (Cst(-/-)) that lack sulfatide but express GalC. We show that cerebroside sulfotransferase mRNA expression begins already in the embryonic spinal cord and progressively increases with age, that the late progenitor marker POA is not synthesized in the absence of this enzyme, and that, most notably, there is a two- to threefold enhancement in the number of terminally differentiated oligodendrocytes both in culture and in vivo, similar to that in mice lacking both GalC and sulfatide. We conclude that primarily sulfatide, rather than GalC, is a key molecule for the negative regulation of oligodendrocyte terminal differentiation.

Co-clustering of galactosylceramide and membrane proteins in oligodendrocyte membranes on interaction with polyvalent carbohydrate and prevention by an intact cytoskeleton

We have shown previously that addition of liposomes containing the two major glycosphingolipids of myelin, galactosylceramide (GalC) and cerebroside sulfate (CBS), to cultured oligodendrocytes (OLs) caused clustering of GalC on the extracellular surface and myelin basic protein (MBP) on the cytosolic surface to the same membrane domains. It also caused depolymerization of actin microfilaments and microtubules, indicating that interaction of the liposomes with the OL surface induces transmembrane signal transmission. We show that a multivalent form of galactose conjugated to bovine serum albumin has a similar effect as the multivalent GalC/CBS-containing liposomes. Because GalC and CBS can interact with each other across apposed membranes and because anti-GalC and anti-CBS antibodies also cause redistribution of GalC/CBS and depolymerization of microtubules, we believe that the multivalent carbohydrate interacts with GalC and CBS in the OL membrane. Several myelin-specific transmembrane proteins could be involved in this transmembrane signal transmission from GalC/CBS. We looked at co-clustering of several myelin constituents by confocal microscopy to determine if they are located in or redistribute to GalC/MBP-containing domains. Myelin oligodendrocyte glycoprotein (MOG), proteolipid protein (PLP), MAPK, and some phosphotyrosine-containing proteins were found to co-cluster with GalC and MBP, but myelin-associated glycoprotein (MAG) and phosphatidylinositol-4,5-bisphosphate (PIP(2)) did not. These results suggest that MOG and PLP, but probably not MAG, are possible candidates for transmembrane transmission of the signal received by GalC/CBS. To determine if depolymerization of actin microfilaments was required for co-clustering, or was secondary to clustering, we stabilized F-actin with jasplakinolide. This also prevented depolymerization of the microtubules and prevented clustering of all constituents, including GalC. The prevention of clustering or redistribution of these glycolipids and proteins by an intact cytoskeleton is consistent with the picket fence model.

Ig superfamily cell adhesion molecules in the brain

Cell adhesion molecules of the immunoglobulin superfamily (IgSF CAMs) were discovered 25 years ago based on their role in cell-cell adhesion. Ever since, they have played a major role in developmental neuroscience research. The elucidation of IgSF CAM structure and function has been tightly linked to the establishment of new areas of research. Over the years, our view of the role of the IgSF CAMs has changed. First, they were thought to provide "specific glue" segregating subtypes of cells in the nervous system. Soon it became clear that IgSF CAMs can do much more. The focus shifted from simple adhesion to CAM-associated signaling that was shown to be involved in the promotion of axon growth and the regulation of cell migration. From there it was a small step to axon guidance, a field that has been given a lot of attention during the last decade. More recently, the involvement of IgSF CAMs in synapse formation and maturation has been discovered, although this last step in the formation of neural circuits was thought to be the domain of other families of cell adhesion molecules, such as the neuroligins, the neurexins, and the cadherins. Certainly, the most striking discovery in the context of IgSF CAMs has been the diversity of signaling mechanisms that are associated with them. The versatility of signals and their complexity make IgSF CAMs a perfect tool for brain development.

The local differentiation of myelinated axons at nodes of Ranvier

Efficient and rapid propagation of action potentials in myelinated axons depends on the molecular specialization of the nodes of Ranvier. The nodal region is organized into several distinct domains, each of which contains a unique set of ion channels, cell-adhesion molecules and cytoplasmic adaptor proteins. Voltage-gated Na+ channels - which are concentrated at the nodes - are separated from K+ channels - which are clustered at the juxtaparanodal region - by a specialized axoglial contact that is formed between the axon and the myelinating cell at the paranodes. This local differentiation of myelinated axons is tightly regulated by oligodendrocytes and myelinating Schwann cells, and is achieved through complex mechanisms that are used by another specialized cell-cell contact - the synapse.

Metachromatic leukodystrophy: consequences of sulphatide accumulation

Metachromatic leukodystrophy is a lysosomal lipid storage disorder. It is caused by mutations in the gene for arylsulphatase A, an enzyme involved in the degradation of the sphingolipid 3'-O-sulphogalactosylceramide (sulphatide). This membrane lipid can be found in various cell types, but in particularly high concentrations in the myelin of the nervous system. Patients suffer from progressive, finally lethal, demyelination due to accumulation of sulphatide. In the nervous system, lipid storage not only affects oligodendrocytes but also neurons and, in addition, leads to astrogliosis and activation of microglia. At the cellular level, lysosomal sulphatide storage also affects the lipid composition of myelin itself and has consequences for the amount and localization of particular myelin membrane-associated proteins. Here we review data, largely based on an arylsulphatase A knock-out mouse model of metachromatic leukodystrophy.

CONCLUSION

The knock-out mouse model of metachromatic leukodystrophy has provided insights into the histopathological and cellular consequences of sulphatide storage.

F3/contactin acts as a functional ligand for Notch during oligodendrocyte maturation

Axon-derived molecules are temporally and spatially required as positive or negative signals to coordinate oligodendrocyte differentiation. Increasing evidence suggests that, in addition to the inhibitory Jagged1/Notch1 signaling cascade, other pathways act via Notch to mediate oligodendrocyte differentiation. The GPI-linked neural cell recognition molecule F3/contactin is clustered during development at the paranodal region, a vital site for axoglial interaction. Here, we show that F3/contactin acts as a functional ligand of Notch. This trans-extracellular interaction triggers gamma-secretase-dependent nuclear translocation of the Notch intracellular domain. F3/Notch signaling promotes oligodendrocyte precursor cell differentiation and upregulates the myelin-related protein MAG in OLN-93 cells. This can be blocked by dominant negative Notch1, Notch2, and two Deltex1 mutants lacking the RING-H2 finger motif, but not by dominant-negative RBP-J or Hes1 antisense oligonucleotides. Expression of constitutively active Notch1 or Notch2 does not upregulate MAG. Thus, F3/contactin specifically initiates a Notch/Deltex1 signaling pathway that promotes oligodendrocyte maturation and myelination.

Lipid-dependent recruitment of neuronal Src to lipid rafts in the brain

Although most Src family tyrosine kinases are modified by palmitoylation as well as myristoylation, Src itself is only myristoylated. Dual acylation is important for attachment to liquid-ordered microdomains or lipid rafts. Accordingly, Src is excluded from lipid rafts in fibroblasts. Evidence of partial genetic redundancy between Src and Fyn for brain-specific targets suggests that these two kinases may occupy overlapping subcellular locations. Neuronal Src (NSrc), an alternative isoform of Src with a 6-amino acid insert in the Src homology 3 domain, is highly expressed in neurons. We investigated whether this structural difference in NSrc allows it to associate with lipid rafts. We found that perinatal mouse brains express predominantly NSrc, which is partly (10-20%) in a lipid raft fraction from brain but not fibroblasts. The association of Src with brain lipid rafts does not depend on the NSrc insert but depends on the amino-terminal myristoylation signal. A crude lipid fraction from brain promotes NSrc entry into rafts in vitro. Moreover, lipid raft-localized NSrc is more catalytically active than NSrc from the soluble fraction, possibly because raft localization alters access to other tyrosine kinases and phosphatases. These findings suggest that NSrc may be involved in signaling from lipid rafts in mouse brain.

Metachromatic leukodystrophy: recent research developments

Metachromatic leukodystrophy is a lysosomal storage disorder caused by the deficiency of arylsulfatase A, causing the storage of the sphingolipid sulfatide. The disease is characterized by a progressive demyelination, which results in severe, finally lethal, neurologic symptoms. Genetically, the disease is heterogeneous, most mutant alleles are private, and only three have a frequency worth mentioning. An arylsulfatase A-deficient knockout mouse displays some of the disease features seen in patients but does not show the widespread demyelination characteristic of the disease. Nevertheless, this animal model was used to investigate the therapeutic potential of bone marrow stem cell-based gene therapy. Although treated animals show considerable arylsulfatase A activity in many tissues, including the brain, the effect on sulfatide storage was disappointing. Only in the kidney and liver of animals with very high enzyme levels was lipid storage positively affected. In particular, no effect was seen in the brain. These results suggest that bone marrow stem cell-based gene therapy could be of limited value in the treatment of this disease. The pathogenesis of the disease is poorly understood. It is not yet known how sulfatide storage negatively affects the metabolism of the oligodendrocyte. The amount and distribution of various proteins in the myelin of arylsulfatase A-deficient mice were investigated. It was shown that the myelin protein, myelin and lymphocyte protein (MAL), is reduced in deficient animals and that the myelin protein proteolipid displays an altered distribution within the myelin membranes. Possibly, these alterations contribute to the development of demyelination in this disease.

Antibody cross-linking of myelin oligodendrocyte glycoprotein leads to its rapid repartitioning into detergent-insoluble fractions, and altered protein phosphorylation and cell morphology

Myelin oligodendrocyte glycoprotein (MOG) is, quantitatively, a relatively minor component of the myelin membrane. Nevertheless, peritoneal administration of MOG evokes potent cellular and humoral immunoreactivity, resulting in an experimental allergic encephalitis with immunopathology similar to multiple sclerosis. Moreover, antibodies against MOG cause myelin destruction in situ. Therefore, it appears that MOG-related demyelination is dependent on anti-MOG antibody, but the mechanism(s) by which it occurs is unclear. Of potential significance are observations that some proteins are selectively partitioned into specialized plasma membrane microdomains rich in glycosphingolipids and cholesterol ("lipid rafts"). In particular, during ligand or antibody cross-linking, various plasma membrane receptors undergo enhanced partitioning into rafts as an obligatory first step toward participation in early signal transduction events. In contrast to mature myelin, in oligodendrocytes (OLs) in culture MOG is not raft associated [Triton X-100 (TX-100) soluble, 4 degrees C]. However, in this study we show that antibody cross-linking (anti-MOG plus secondary antibody) of MOG on the surface of OLs results in the repartitioning of approximately 95% of MOG into the TX-100-insoluble fraction. This repartitioning of MOG is rapid (<or=1 min), antibody dose dependent, requires an intact cytoskeleton, leads to phosphorylation or dephosphorylation of tyrosine, serine, and threonine residues in specific proteins (e.g., beta-tubulin, Gbeta1-2), and invokes a rapid retraction of OL processes. After removal of the cross-linking antibodies, these events are reversed. We hypothesize that antibody-mediated repartitioning of MOG into TX-100-insoluble glycosphingolipid-cholesterol-rich microdomains initiates specific cellular signaling that could be related to initial steps of MOG-mediated demyelination.

Inactivation of ether lipid biosynthesis causes male infertility, defects in eye development and optic nerve hypoplasia in mice

Although known for almost 80 years, the physiological role of plasmalogens (PLs), the major mammalian ether lipids (ELs), is still enigmatic. Humans that lack ELs suffer from rhizomelic chondrodysplasia punctata (RCDP), a peroxisomal disorder usually resulting in death in early childhood. In order to learn more about the functions of ELs, we generated a mouse model for RCDP by a targeted disruption of the dihydroxyacetonephosphate acyltransferase gene. The mutant mice revealed multiple abnormalities, such as male infertility, defects in eye development, cataract and optic nerve hypoplasia, some of which were also observed in RCDP. Mass spectroscopic analysis demonstrated the presence of highly unsaturated fatty acids including docosahexaenoic acid (DHA) in brain PLs and the occurrence of PLs in lipid raft microdomains (LRMs) isolated from brain myelin. In mutants, PLs were completely absent and the concentration of brain DHA was reduced. The marker proteins flotillin-1 and F3/contactin were found in brain LRMs in reduced concentrations. In addition, the gap junctional protein connexin 43, known to be recruited to LRMs and essential for lens development and spermatogenesis, was down-regulated in embryonic fibroblasts of the EL-deficient mice. Free cholesterol, an important constituent of LRMs, was found in these fibroblasts to be accumulated in a perinuclear compartment. These data suggest that the EL-deficient mice allow the identification of new phenotypes not related so far to EL-deficiency (male sterility, defects in myelination and optic nerve hypoplasia) and indicate that PLs are required for the correct assembly and function of LRMs.

Neural cell adhesion molecule (NCAM) association with PKCbeta2 via betaI spectrin is implicated in NCAM-mediated neurite outgrowth

In hippocampal neurons and transfected CHO cells, neural cell adhesion molecule (NCAM) 120, NCAM140, and NCAM180 form Triton X-100-insoluble complexes with betaI spectrin. Heteromeric spectrin (alphaIbetaI) binds to the intracellular domain of NCAM180, and isolated spectrin subunits bind to both NCAM180 and NCAM140, as does the betaI spectrin fragment encompassing second and third spectrin repeats (betaI2-3). In NCAM120-transfected cells, betaI spectrin is detectable predominantly in lipid rafts. Treatment of cells with methyl-beta-cyclodextrin disrupts the NCAM120-spectrin complex, implicating lipid rafts as a platform linking NCAM120 and spectrin. NCAM140/NCAM180-betaI spectrin complexes do not depend on raft integrity and are located both in rafts and raft-free membrane domains. PKCbeta2 forms detergent-insoluble complexes with NCAM140/NCAM180 and spectrin. Activation of NCAM enhances the formation of NCAM140/NCAM180-spectrin-PKCbeta2 complexes and results in their redistribution to lipid rafts. The complex is disrupted by the expression of dominant-negative betaI2-3, which impairs binding of spectrin to NCAM, implicating spectrin as the bridge between PKCbeta2 and NCAM140 or NCAM180. Redistribution of PKCbeta2 to NCAM-spectrin complexes is also blocked by a specific fibroblast growth factor receptor inhibitor. Furthermore, transfection with betaI2-3 inhibits NCAM-induced neurite outgrowth, showing that formation of the NCAM-spectrin-PKCbeta2 complex is necessary for NCAM-mediated neurite outgrowth.

Nicotine modulates the expression of a diverse set of genes in the neuronal SH-SY5Y cell line

Nicotine exposure can have long lasting effects on nervous system function, some of which must contribute to nicotine dependence. Up-regulation, an increase in numbers of radioligand-binding nicotinic acetylcholine receptors (nAChR), occurs on exposure to nicotine at high concentrations. To determine whether altered gene expression might account for long term changes and up-regulation following nicotine exposure, we assessed effects of 1 h of 1 mm nicotine exposure on alteration of gene expression in the neuron-like SH-SY5Y neuroblastoma clonal line. Repeat and cross-controlled microarray analyses yielded a list of 17 genes from the initially screened approximately 5,000 whose expression was consistently altered following nicotine treatment. Subsequent quantitative, real time reverse transcriptase PCR analyses confirmed altered expression in 14 of 16 genes tested. Further, the general nAChR antagonist, d-tubocurarine, blocked all but two of the observed changes in gene expression, indicating that these changes are dependent on nAChR activation. Use of other antagonists revealed that nAChR subtypes can differentially affect gene expression. The genes affected code for proteins that may be broadly categorized into four groups: transcription factors, protein processing factors, RNA-binding proteins, and plasma membrane-associated proteins. Our results suggest that nicotinic activation of nAChR may have a broad role in affecting cellular physiology through modulating gene expression.

Tyrosine phosphorylation of QKI mediates developmental signals to regulate mRNA metabolism

The selective RNA-binding protein QKI is essential for myelination in the central nervous system (CNS). QKI belongs to the family of signal transduction activators of RNA (STARs), characteristic of binding RNA and signaling molecules, therefore is postulated to regulate RNA homeostasis in response to developmental signals. Here we report that QKI acts downstream of the Src family protein tyrosine kinases (Src-PTKs) during CNS myelination. QKI selectively interacted with the mRNA encoding the myelin basic protein (MBP). Such interaction stabilized MBP mRNA and was required for the rapid accumulation of MBP mRNA during active myelinogenesis. We found that the interaction between QKI and MBP mRNA was negatively regulated by Src-PTK-dependent phosphorylation of QKI. During early myelin development, tyrosine phosphorylation of QKI in the developing myelin drastically declined, presumably leading to enhanced interactions between QKI and MBP mRNA, which was associated with the rapid accumulation of MBP mRNA and accelerated myelin production. Therefore, developmental regulation of Src-PTK-dependent tyrosine phosphorylation of QKI suggests a novel mechanism for accelerating CNS myelinogenesis via regulating mRNA metabolism.

Ceramide in rafts (detergent-insoluble fraction) mediates cell death in neurotumor cell lines

Detergent-resistant lipid microdomains (Rafts) were isolated from human oligodendroglioma (HOG), human neuroblastoma (LA-N-5), and immortalized dorsal root ganglion (F-11) cell lines by sucrose-density gradient ultracentrifugation and shown to be enriched in cholesterol, sphingomyelin, and ceramide. [(3)H]palmitate labeling allowed the Raft fraction to be easily identified as a sharp peak of (3)H radioactivity in the 5-30% sucrose interphase. Treatment of [(3)H]palmitate-labeled cells with staurosporine (to activate caspase 8 and induce apoptosis) or exogenous sphingomyelinase specifically increased the [(3)H]ceramide content of the Raft fraction. Depletion of cholesterol with beta-methylcyclodextran decreased Raft formation and partially blocked staurosporine-induced apoptosis. Similarly, treatment of cells with Fumonisin B1 to inhibit de novo sphingolipid synthesis by 50% reduced the labeling of the Raft fraction and partially blocked staurosporine-induced apoptosis. Staurosporine treatment activated neutral sphingomyelinase but had no effect on acid sphingomyelinase activity or on other lysosomal hydrolases, such as alpha-L-fucosidase. Most of the neutral sphingomyelinase activity is in the Raft fraction, suggesting that the conversion of sphingomyelin to ceramide in Rafts is an important event in neural cell apoptosis.

The proteoglycan NG2 is complexed with alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors by the PDZ glutamate receptor interaction protein (GRIP) in glial progenitor cells. Implications for glial-neuronal signaling

The proteoglycan NG2 is expressed by immature glial cells in the developing and adult central nervous system. Using the COOH-terminal region of NG2 as bait in a yeast two-hybrid screen, we identified the glutamate receptor interaction protein GRIP1, a multi-PDZ domain protein, as an interacting partner. NG2 exhibits a PDZ binding motif at the extreme COOH terminus which binds to the seventh PDZ domain of GRIP1. In addition to the published expression in neurons, GRIP1 is expressed by immature glial cells. GRIP1 is known to bind to the GluRB subunit of the AMPA glutamate receptor expressed by subpopulations of neurons and immature glial cells. In cultures of primary oligodendrocytes, cells coexpress GluRB and NG2. A complex of NG2, GRIP1, and GluRB can be precipitated from transfected mammalian cells and from cultures of primary oligodendrocytes. Furthermore, NG2 and GRIP can be coprecipitated from developing brain tissue. These data suggest that GRIP1 acts as a scaffolding molecule clustering NG2 and AMPA receptors in immature glia. In view of the presence of synaptic contacts between neurons and NG2-positive glial cells in the hippocampus and the close association of NG2-expressing glial cells with axons, we suggest a role for the NG2.AMPA receptor complex in glial-neuronal recognition and signaling.

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.

Regulation of integrin growth factor interactions in oligodendrocytes by lipid raft microdomains

Individual growth factors can regulate multiple aspects of behavior within a single cell during differentiation, with each signaling pathway controlled independently and also responsive to other receptors such as cell surface integrins. The mechanisms by which this is achieved remain poorly understood. Here we use myelin-forming oligodendrocytes and their precursors to examine the role of lipid rafts, cholesterol and sphingolipid-rich microdomains of the cell membrane implicated in cell signaling. In these cells, the growth factor PDGF has sequential and independent roles in proliferation and survival. We show that the oligodendrocyte PDGFalpha receptor becomes sequestered in a raft compartment at the developmental stage when PDGF ceases to promote proliferation, but is now required for survival. We also show that laminin-2, which is expressed on axons in the CNS and which provides a target-dependent signal for oligodendrocyte survival by amplification of PDGFalphaR signaling, induces clustering of the laminin binding integrin alpha6beta1 with the PDGFalphaR-containing lipid raft domains. This extracellular matrix-induced colocalization of integrin and growth factor receptor generates a signaling environment within the raft for survival-promoting PI3K/Akt activity. These results demonstrate novel signaling roles for lipid rafts that ensure the separation and amplification of growth factor signaling pathways during development.

Interleukin-2 receptors and interleukin-2-mediated signaling in myelin: activation of diacylglycerol kinase and phosphatidylinositol 3-kinase

Myelin was previously shown to possess neurotransmitter and cytokine receptors that trigger well-defined signaling mechanisms within the multilamellar structure. The present study reveals the presence of an interleukin-2 (IL-2) receptor in isolated mouse CNS myelin that responds to recombinant mouse IL-2 by activating diacylglycerol kinase (DAGK) and phosphoinositide 3-kinase (PI3K); additional evidence suggests participation by protein tyrosine kinase. Activation of myelin DAGK by IL-2 occurred in brain stem tissue mince and was blocked by chelerythrin chloride, indicating an essential role for myelin-localized protein kinase C. Two inhibitors of PI3K, wortmannin and LY294002, blocked endogenous PI3K as well as that enhanced by IL-2. Activation of PI3K by IL-2 was also blocked by tyrphostin A25, a selective inhibitor of PTK, suggesting activation of the latter by IL-2 is upstream to PI3K activation. This reaction resulted in tyrosine phosphorylation of a protein tentatively identified as the p85 subunit of PI3K. Developmental changes were noted in that receptor density and signaling activity were robust during the period of rapid myelination and declined rapidly thereafter.

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.

CNS integrins switch growth factor signalling to promote target-dependent survival

Depending on the stage of development, a growth factor can mediate cell proliferation, survival or differentiation. The interaction of cell-surface integrins with extracellular matrix ligands can regulate growth factor responses and thus may influence the effect mediated by the growth factor. Here we show, by using mice lacking the alpha(6) integrin receptor for laminins, that myelin-forming oligodendrocytes activate an integrin-regulated switch in survival signalling when they contact axonal laminins. This switch alters survival signalling mediated by neuregulin from dependence on the phosphatidylinositol-3-OH kinase (PI(3)K) pathway to dependence on the mitogen-activated kinase pathway. The consequent enhanced survival provides a mechanism for target-dependent selection during development of the central nervous system. This integrin-regulated switch reverses the capacity of neuregulin to inhibit the differentiation of precursors, thereby explaining how neuregulin subsequently promotes differentiation and survival in myelinating oligodendrocytes. Our results provide a general mechanism by which growth factors can exert apparently contradictory effects at different stages of development in individual cell lineages.

Reciprocal raft-receptor interactions and the assembly of adhesion complexes

Cell adhesion complexes are critical for the physical coordination of cell-cell interactions and the morphogenesis of tissues and organs. Many adhesion receptors are anchored to the plasma membrane by a glycosylphosphatidylinositol (GPI) moiety and are thereby partitioned into membrane rafts. In this review, we focus on reciprocal interactions between rafts and adhesion molecules, leading to receptor clustering and raft expansion and stability. A model for a three-stage adhesion complex assembly process is also proposed. First, GPI-anchored adhesion molecules are recruited into rafts, which in turn promote receptor cis-oligomerization and thereby produce precursory complexes primed for avid trans-interactions. Second, trans-interactions of the receptors cross-link and stabilize large amalgams of rafts at sites of adhesion complex assembly. Finally, the enlarged and stabilized rafts acquire enhanced abilities to recruit the cytoskeleton and induce signaling. This process exemplifies how the domain structure of the plasma membrane can impact the function of its receptors.

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.

Molecular constituents of the node of Ranvier

The interaction between neurons and glial cells that results in myelin formation represents one of the most remarkable intercellular events in development. This is especially evident at the primary functional site within this structure, the node of Ranvier. Recent experiments have revealed a surprising level of complexity within this zone, with several components, including ion channels, sequestered with a very high degree of precision and sharply demarcated borders. We discuss the current state of knowledge of the cellular and molecular mechanisms responsible for the formation and maintenance of the node. In normal axons, Na+ channels are present at high density within the nodal gap, and voltage-dependent K+ channels are sequestered on the internodal side of the paranode--a region known as the juxtaparanode. Modifying the expression of certain surface adhesion molecules that have been recently identified, markedly alters this pattern. There is a special emphasis on contactin, a protein with multiple roles in the nervous system. In central nervous system (CNS) myelinated fibers, contactin is localized within both the nodal gap and paranodes, and appears to have unique functions in each zone. New experiments on contactin-null mutant mice help to define these mechanisms.