Oligodendrocyte myelin glycoprotein (OMgp): evolution, structure and function

The oligodendrocyte myelin glycoprotein (OMgp) is a glycosylphosphatidylinositol-anchored protein expressed by neurons and oligodendrocytes in the central nervous system (CNS). Although the precise function of OMgp is yet to be determined in vivo, recent in vitro studies suggested roles for this protein in both the developing and adult central nervous system. In vitro experiments demonstrated the participation of OMgp in growth cone collapse and inhibition of neurite outgrowth through its interaction with NgR, the receptor for Nogo. This function requires its leucine-rich repeat domain, a highly conserved region in OMgp during mammal evolution. OMgp leucine-rich repeat domain is also implicated in the inhibition of cell proliferation. Based on its developmental expression, localization and structure, OMgp may also be involved in the formation and maintenance of myelin sheaths. Cell proliferation, neuronal sprouting and myelination are crucial processes involved in brain development and regeneration after injury. Here, we review the information available on the structure and evolution of OMgp, summarize its tissue expression and discuss its putative role(s) during the development and in adult CNS.     

SCIRR39 Promotes Differentiation of Oligodendrocyte Precursor Cells and Regulates Expression of Myelin-Associated Inhibitory Factors

SCIRR39 is an identified upregulated gene in rat primary neuron injury and/or regeneration process. However, roles of SCIRR39 in the regeneration of central nervous system (CNS) injury are still largely unexplored. Using real-time quantitative PCR and Western blotting, SCIRR39 expression was detected in oligodendrocyte precursor cells (OPCs) and oligodendrocytes. Moreover, the results from cell proliferation and cell cycle indicated that SCIRR39 inhibited OPCs proliferation and induced cell cycle arrest in G0/G1 and G2/M phases. Importantly, SCIRR39 positively regulated OPC differentiation and the expression of myelin basic protein. We also examined the effect of SCIRR39 on expression of myelin-associated inhibitory factors, including myelin-associated glycoprotein (MAG), oligodendrocyte myelin glycoprotein (OMgp), and Nogo A. Nogo A level was markedly regulated by SCIRR39 overexpression or knockdown in oligodendrocytes and cortical neurons co-cultures, while the expression of MAG and OMgp was not obviously changed by SCIRR39 overexpression or knockdown. Taken together, our results indicate the important role of SCIRR39 either in OPC differentiation or in axon myelination, and may provide a new therapeutic target for the treatment of CNS injury.     

Oligodendrocyte differentiation and myelination defects in OMgp null mice

OMgp is selectively expressed in CNS by oligodendrocyte. However, its potential role(s) in oligodendrocyte development and myelination remain unclear. We show that OMgp null mice are hypomyelinated in their spinal cords, resulting in slower ascending and descending conduction velocities compared to wild-type mice. Consistent with the hypomyelination, in the MOG induced EAE model, OMgp null mice show a more severe EAE clinical disease and slower nerve conduction velocity compared to WT animals. The contribution of OMgp to oligodendrocyte differentiation and myelination was verified using cultured oligodendrocytes from null mice. Oligodendrocytes isolated from OMgp null mice show a significant decrease in the number of MBP(+) cells and in myelination compared to wild-type mice. The dramatic effects of the OMgp KO in oligodendrocyte maturation in vivo and in vitro reveal a new and important function for OMgp in regulating CNS myelination.     

Dysmyelination and reduced myelin basic protein gene expression by oligodendrocytes of SHP-1-deficient mice

We have shown previously that myelin-forming oligodendrocytes express the protein tyrosine phosphatase SHP-1 and that myelin formation was decreased in SHP-1-deficient motheaten mice compared to that in normal littermates. These studies suggested a potential importance for SHP-1 in oligodendrocyte and myelin development. To address further this possibility, we analyzed myelin formation by microscopy and myelin basic protein (MBP) gene expression in motheaten mice at ages when myelination occurs in the developing central nervous system (CNS). Furthermore, we correlate these findings with MBP gene expression in oligodendrocytes grown in vitro. We have found that CNS myelination was significantly reduced in SHP-1-deficient mice relative to their normal littermates at multiple times during the active period of myelination. Under electron microscopy, greater numbers of axons in spinal cords of motheaten mice were either unmyelinated or had thinner myelin sheathes compared to those in matched areas of normal littermates. Accordingly, MBP protein and mRNA levels were reduced in SHP-1-deficient mice compared to that in the CNS of normal littermates. In vitro, O1(+) oligodendrocytes from motheaten mice expressed much less MBP than O1(+) oligodendrocytes of normal littermates indicating an alteration in oligodendrocyte differentiation. The latter correlated with reduced MBP mRNA relative to cerebroside galactosyl transferase (CGT) gene mRNA in SHP-1-deficient oligodendrocytes in purified cultures. We propose that SHP-1 is a critical regulator of developmental signals leading to terminal differentiation and myelin sheath formation by oligodendrocytes.     

Differential expression patterns of messenger RNAs encoding Nogo receptors and their ligands in the rat central nervous system

Nogo receptors (NgR1, -2, and -3) and their ligands, i.e., myelin-derived neurite outgrowth inhibitor (Nogo)-A, myelin-associated glycoprotein (MAG), and oligodendrocyte myelin glycoprotein (OMgp), have been considered to play pivotal roles in controlling axonal regeneration and neuronal plasticity. We show here that NgR1-3 mRNAs were differentially expressed exclusively in neurons situated in the telencephalon, diencephalons, and cerebellum, whereas we could not detect any NgR1-3 mRNA expression in the mesencephalon, pons, medulla oblongata, and spinal cord. On the other hand, Nogo-A mRNA was abundantly expressed in both neurons and oligodendrocytes throughout the central nervous system (CNS). MAG and OMgp mRNAs were also abundantly expressed in oligodendrocytes throughout the CNS. Interestingly, we did not detect NgR1-3 mRNAs in monoaminergic neurons in the substantia nigra, ventral tegmental area, locus caeruleus, and raphe nuclei, which are known to have high regenerative capacity. In addition, although neurons in the reticular thalamus and cerebellar nuclei are also known to show high capacity for regeneration, NgR1-3 mRNAs were not detected there. These data indicate that NgR1-3, Nogo-A, MAG, and OMgp mRNAs are differentially expressed in the rat CNS and suggest that the level of NgR1-3 expression in a neuron might determine its regenerative capacity.     

PAK1 Positively Regulates Oligodendrocyte Morphology and Myelination

The actin cytoskeleton is crucial for oligodendrocyte differentiation and myelination. Here we show that p21-activated kinase 1 (PAK1), a well-known actin regulator, promotes oligodendrocyte morphologic change and myelin production in the CNS. A combination of in vitro and in vivo models demonstrated that PAK1 is expressed throughout the oligodendrocyte lineage with highest expression in differentiated oligodendrocytes. Inhibiting PAK1 early in oligodendrocyte development decreased oligodendrocyte morphologic complexity and altered F-actin spreading at the tips of oligodendrocyte progenitor cell processes. Constitutively activating AKT in oligodendrocytes in male and female mice, which leads to excessive myelin wrapping, increased PAK1 expression, suggesting an impact of PAK1 during active myelin wrapping. Furthermore, constitutively activating PAK1 in oligodendrocytes in zebrafish led to an increase in myelin internode length while inhibiting PAK1 during active myelination decreased internode length. As myelin parameters influence conduction velocity, these data suggest that PAK1 may influence communication within the CNS. These data support a model in which PAK1 is a positive regulator of CNS myelination.SIGNIFICANCE STATEMENT Myelin is a critical component of the CNS that provides metabolic support to neurons and also facilitates communication between cells in the CNS. Recent data demonstrate that actin dynamics drives myelin wrapping, but how actin is regulated during myelin wrapping is unknown. The authors investigate the role of the cytoskeletal modulator PAK1 during differentiation and myelination by oligodendrocytes, the myelinating cells of the CNS. They demonstrate that PAK1 promotes oligodendrocyte differentiation and myelination by modulating the cytoskeleton and thereby internode length, thus playing a critical role in the function of the CNS.     

Characterization of an intronic enhancer that regulates myelin proteolipid protein (Plp) gene expression in oligodendrocytes

The myelin proteolipid protein (Plp) gene is expressed in oligodendrocytes and encodes the most abundant protein (approximately 50%) present in mature myelin from the central nervous system (CNS). Plp gene activity is low to nonexistent early in development but sharply increases, concurrently with the active myelination period of CNS development. Work from our laboratory suggests that the temporal regulation of Plp gene expression in mice is mediated by a positive regulatory element located within Plp intron 1 DNA. We have termed this regulatory element/region ASE (for antisilencer/enhancer). The ASE is situated approximately 1 kb downstream of exon 1 DNA and encompasses nearly 100 bp. To understand the mechanisms by which the ASE augments Plp gene expression in oligodendrocytes, Plp-lacZ constructs were generated and transfected into a mouse oligodendroglial cell line (N20.1). Results presented here demonstrate that upstream regulatory elements in the Plp promoter/5'-flanking DNA are not required for ASE activity; the ASE worked perfectly well when the thymidine kinase (TK) promoter was substituted for the Plp promoter. However, the relative location of the ASE appears to be important. When placed upstream of 2.4 kb of Plp 5'-flanking DNA, or downstream of the lacZ expression cassette, the ASE was no longer effective. Thus, the ASE might have to be in the context of the intron in order to function. To begin to identify the crucial nucleotides within the ASE, orthologous sequences from rat, human, cow, and pig Plp genes were swapped for the mouse sequence. Results presented here demonstrate that the orthologous sequence from rat can substitute for the mouse ASE, unlike those from human, cow, or pig.     

Neurofibromatosis 1 and multiple sclerosis.

Neurofibromatosis 1 is a common autosomal dominant disease that principally involves the skin and peripheral nervous system. The gene for the disorder has been located on chromosome 17q11.2 and there are three embedded genes within the neurofibrosis gene. One of these genes codes for oligodendrocyte-myelin glycoprotein, is found in the CNS during myelination, and may have a role in myelin formation. The case histories of five patients, including two siblings, who have both neurofibromatosis 1 and multiple sclerosis are reported. All five had the primary progressive form of multiple sclerosis, which forms only 15% of multiple sclerosis in population surveys. The coincidence of neurofibromatosis 1 and multiple sclerosis might be due to a mutation in the embedded oligodendrocyte-myelin glycoprotein gene.     

Distribution of adrenocorticoid receptors in the rat CNS measured by competitive PCR and cytosolic binding

A growing number of glycoproteins have been identified and characterized in myelin and myelin-forming cells. In addition to the major P0 glycoprotein of compact PNS myelin and the myelin-associated glycoprotein (MAG) in the periaxonal membranes of myelin-forming oligodendrocytes and Schwann cells, the list now includes peripheral myelin protein-22 (PMP-22), a 170 kDa glycoprotein associated with PNS myelin and Schwann cells (P170k/SAG), Schwann cell myelin protein (SMP), myelin/oligodendrocyte glycoprotein (MOG), and oligodendrocytemyelin glycoprotein (OMgp). Many of these glycoproteins are members of the immunoglobulin superfamily and express the adhesion-related HNK-1 carbohydrate epitope. This review summarizes recent findings concerning the structure and function of these glycoproteins of myelin sheaths with emphasis on the physiological roles of oligosaccharide moieties.     

Gene expression and oligodendrocyte development in the myelin deficient rat

The proteolipid proteins play a major role in the structure of the CNS myelin sheath, but they have also been implicated in the oligodendrocyte development leading to myelination. Mutations in the PLP gene result in severe dysmyelination and a paucity of mature oligodendrocytes. The myelin deficient (md) rat, carrying a Thr75? Pro substitution present in both isoforms of proteolipid protein (PLP and DM20), is the most severely affected of the PLP mutants described to date. The expression of myelin associated genes was quantitated to determine the effect of the mutation on oligodendrocyte development in vivo. At 5 days postnatal, gene expression in the and rat approximated that in age-matched control rats, but as they matured, there was a progressive inhibition of gene expression in the and rats. The genes expressed late in the myelination program (PLP and MBP) were affected more dramatically than those expressed earlier in oligodendrocyte development (CNP and GPDH). The results indicate that the later stages of oligodendrocyte maturation and myelin elaboration are inhibited. © 1995 Wiley-Liss, Inc.     

In vivo actions of fibroblast growth factor-2 and insulin-like growth factor-I on oligodendrocyte development and myelination in the central nervous system.

The in vivo effects of fibroblast growth factor-2 (FGF-2) and insulin-like growth factor-I (IGF-I) on oligodendrocytes and CNS myelination were determined in the postnatal rat anterior medullary velum (AMV) following injection of both cytokines into the cerebrospinal fluid. Either FGF-2, IGF-I, or saline were administered via the lateral ventricle, twice daily commencing at postnatal day (P) 6. At P9, AMV were immunohistochemically labeled with the Rip antibody, to enable analysis of the numbers of myelin sheaths and of promyelinating and myelinating oligodendrocytes; promyelinating oligodendrocytes are a recognisable immature phenotype which express myelin-related proteins prior to forming myelin sheaths. In parallel experiments, AMV were treated for Western blot analysis to determine relative changes in expression of the myelin proteins 2', 3'-cyclic nucleotide 3'-phosphohydrolase (CNP) and myelin oligodendrocyte glycoprotein (MOG), which, respectively, characterise early and late stages of myelin maturation. In FGF-2-treated AMV, the number of promyelinating oligodendrocytes increased by 87% compared to saline-injected controls. The numbers of myelinating oligodendrocytes and myelin sheaths were not decreased, but conspicuous unmyelinated gaps within fibre tracts were indications of retarded myelination following FGF-2 treatment. Western blot analysis demonstrated decreased expression of CNP and a near-total loss of MOG, confirming that FGF-2 decreased myelin maturation. In contrast, IGF-I had no effect on the number of promyelinating oligodendrocytes, but increased the numbers of myelinating oligodendrocytes and myelin sheaths by 100% and 93%, respectively. Western blot analysis showed that the amount of CNP was increased following IGF-I treatment, correlating with the greater number of oligodendrocytes, but that MOG expression was lower than in controls, suggesting that the increased number of myelin sheaths in IGF-I was not matched by increased myelin maturation. The results provide in vivo evidence that FGF-2 and IGF-I control the numbers of oligodendrocytes in the brain and, respectively, retard and promote myelination.     

Myelin acquisition in the central nervous system of the mouse revealed by an MBP-Lac Z transgene.

Myelin has pronounced effects upon the morphology, function, and growth of axons in the mammalian CNS. Consequently, oligodendrocyte development and myelination have been investigated using a wide variety of histological, immunocytochemical, ultrastructural, and biochemical techniques. While many of the spatial and temporal features of myelin appearance have been characterized, for any one species only limited regions of the CNS have been investigated. To address this limitation, we have derived transgenic mice in which the bacterial Lac Z gene is regulated by promoter elements of the myelin basic protein gene. When differentiating oligodendrocytes begin to elaborate recognizable myelin, they initiate expression of the MBP-Lac Z transgene and accumulate readily detectable levels of beta-galactosidase. Here, we exploit the sensitivity, resolution, and ease of beta-galactosidase histochemical assays to characterize the temporal and spatial patterns of CNS myelination in the mouse. Many features of the myelination program revealed by this approach were predicted by the immunocytochemical and ultrastructural data derived from other species. Nonetheless, previously undocumented patterns were also encountered. beta-Galactosidase was expressed first by oligodendrocytes in the ventral spinal cord, 1 d prior to birth. There, myelination proceeded in a strictly rostral-caudal direction, whereas in the dorsal cord, myelination initiated in the cervical enlargement and proceeded in both rostral and caudal directions. In the cerebellum, deep regions myelinated first, and in the optic nerve, myelination initiated at the retinal end. In contrast, the lateral olfactory tracts, pons, and optic chiasm initiated myelination along their entire course.(ABSTRACT TRUNCATED AT 250 WORDS)     

White Matter Injury Induced by Perinatal Exposure to Glutaric Acid

Glutaric acid (GA) is a neurotoxic metabolite that accumulates in the CNS of patients with glutaric acidemia-I (GA-I), a neurometabolic disease caused by deficient activity of glutaryl-CoA dehydrogenase. Most GA-I patients display characteristic CNS lesions, mainly in the gray and white matter of basal ganglia and cerebral cortex. Neurons and astrocytes are believed to be vulnerable to millimolar concentrations of GA. However, little is known about the effects of GA on oligodendrocytes (OL) and the myelination process in the postnatal brain. Here, we show that a single intracerebroventricular administration of GA to rat neonatal pups induced a selective and long-lasting myelination failure in the striatum but no deleterious effect in the myelination of the corpus callosum. At 45 days post-GA injection, the myelinated area of striatal axonal bundles was decreased by 35 %, and the expression of myelin basic protein and myelin-associated glycoprotein (MAG) reduced by 25 and 60 %, respectively. This was accompanied by long lasting cytopathology features in MAG and CC-1-expressing OLs, which was confirmed by transmission electron microscopy. Remarkably, GA did not induce acute loss of pre-OLs in the striatum as assessed by NG2 or PDGFRα immunohistochemistry, suggesting an indirect and progressive mechanism for OL damage. In accordance, GA-induced white matter injury was restricted to the striatum and associated to GA-induced astrocytosis and neuronal loss. In conclusion, the current evidence indicates a pathogenic mechanism by which GA can permanently affect myelin status.     

White matter inhibitors in CNS axon regeneration failure

Multiple lines of evidence have indicated that the inability of adult mammalian central nervous system (CNS) axons to regenerate after injury is partly due to the growth inhibitory property of central myelin. Three prototypical myelin-associated inhibitors of neurite outgrowth have been identified, including Nogo, myelin-associated glycoprotein (MAG) and oligodendrocyte-myelin glycoprotein (OMgp). These inhibitory ligands, their receptors and signaling pathways are being intensively investigated for their roles in CNS axon regeneration failure. In addition, several members of the axon guidance molecules have been implicated in restricting CNS axon regeneration, some of which are expressed by mature oligodendrocytes. Here we review in vitro and in vivo studies of these molecules in neurite growth and in axon regeneration failure and discuss the implications of these studies. While the increasing number of potential axon regeneration inhibitors highlights the complexity of the restrictive CNS environment, it provides new windows of opportunity as well as new challenges for therapeutic development for spinal cord injury and related neurological conditions.     

Ablation of neuronal ADAM17 impairs oligodendrocyte differentiation and myelination

Myelin, one of the most important adaptations of vertebrates, is essential to ensure efficient propagation of the electric impulse in the nervous system and to maintain neuronal integrity. In the central nervous system (CNS), the development of oligodendrocytes and the process of myelination are regulated by the coordinated action of several positive and negative cell-extrinsic factors. We and others previously showed that secretases regulate the activity of proteins essential for myelination. We now report that the neuronal α-secretase ADAM17 controls oligodendrocyte differentiation and myelin formation in the CNS. Ablation of Adam17 in neurons impairs in vivo and in vitro oligodendrocyte differentiation, delays myelin formation throughout development and results in hypomyelination. Furthermore, we show that this developmental defect is, in part, the result of altered Notch/Jagged 1 signaling. Surprisingly, in vivo conditional loss of Adam17 in immature oligodendrocytes has no effect on myelin formation. Collectively, our data indicate that the neuronal α-secretase ADAM17 is required for proper CNS myelination. Further, our studies confirm that secretases are important post-translational regulators of myelination although the mechanisms controlling CNS and peripheral nervous system (PNS) myelination are distinct.     

Identification of Tmem10/Opalin as an oligodendrocyte enriched gene using expression profiling combined with genetic cell ablation

Oligodendrocytes form an insulating multilamellar structure of compact myelin around axons, which allows efficient and rapid propagation of action potentials. However, little is known about the molecular mechanisms operating at the onset of myelination and during maintenance of the myelin sheath in the adult. Here we use a genetic cell ablation approach combined with Affymetrix GeneChip microarrays to identify a number of oligodendrocyte-enriched genes that may play a key role in myelination. One of the "oligogenes" we cloned using this approach is Tmem10/Opalin, which encodes for a novel transmembrane glycoprotein. In situ hybridization and RT-PCR analysis revealed that Tmem10 is selectively expressed by oligodendrocytes and that its expression is induced during their differentiation. Developmental immunofluorescence analysis demonstrated that Tmem10 starts to be expressed in the white matter tracks of the cerebellum and the corpus callosum at the onset of myelination after the appearance of other myelin genes such as MBP. In contrast to the spinal cord and brain, Tmem10 was not detected in myelinating Schwann cells, indicating that it is a CNS-specific myelin protein. In mature oligodendrocytes, Tmem10 was present at the cell soma and processes, as well as along myelinated internodes, where it was occasionally concentrated at the paranodes. In myelinating spinal cord cultures, Tmem10 was detected in MBP-positive cellular processes that were aligned with underlying axons before myelination commenced. These results suggest a possible role of Tmem10 in oligodendrocyte differentiation and CNS myelination.