Endocytic depletion of L-MAG from CNS myelin in quaking mice

Myelination by signaling through Arf guanine nucleotide exchange factor

During myelination, large quantities of proteins are synthesized and transported from the endoplasmic reticulum (ER)-trans-Golgi network (TGN) to their appropriate locations within the intracellular region and/or plasma membrane. It is widely believed that oligodendrocytes uptake neuronal signals from neurons to regulate the endocytosis- and exocytosis-mediated intracellular trafficking of major myelin proteins such as myelin-associated glycoprotein (MAG) and proteolipid protein 1 (PLP1). The small GTPases of the adenosine diphosphate (ADP) ribosylation factor (Arf) family constitute a large group of signal transduction molecules that act as regulators for intracellular signaling, vesicle sorting, or membrane trafficking in cells. Studies on mice deficient in Schwann cell-specific Arfs-related genes have revealed abnormal myelination formation in peripheral nerves, indicating that Arfs-mediated signaling transduction is required for myelination in Schwann cells. However, the complex roles in these events remain poorly understood. This review aims to provide an update on signal transduction, focusing on Arf and its activator ArfGEF (guanine nucleotide exchange factor for Arf) in oligodendrocytes and Schwann cells. Future studies are expected to provide important information regarding the cellular and physiological processes underlying the myelination of oligodendrocytes and Schwann cells and their function in modulating neural activity.

Neural stem cells restore myelin in a demyelinating model of Pelizaeus-Merzbacher disease

Pelizaeus-Merzbacher disease is a fatal X-linked leukodystrophy caused by mutations in the PLP1 gene, which is expressed in the CNS by oligodendrocytes. Disease onset, symptoms and mortality span a broad spectrum depending on the nature of the mutation and thus the degree of CNS hypomyelination. In the absence of an effective treatment, direct cell transplantation into the CNS to restore myelin has been tested in animal models of severe forms of the disease with failure of developmental myelination, and more recently, in severely affected patients with early disease onset due to point mutations in the PLP1 gene, and absence of myelin by MRI. In patients with a PLP1 duplication mutation, the most common cause of Pelizaeus-Merzbacher disease, the pathology is poorly defined because of a paucity of autopsy material. To address this, we examined two elderly patients with duplication of PLP1 in whom the overall syndrome, including end-stage pathology, indicated a complex disease involving dysmyelination, demyelination and axonal degeneration. Using the corresponding Plp1 transgenic mouse model, we then tested the capacity of transplanted neural stem cells to restore myelin in the context of PLP overexpression. Although developmental myelination and axonal coverage by endogenous oligodendrocytes was extensive, as assessed using electron microscopy (n = 3 at each of four end points) and immunostaining (n = 3 at each of four end points), wild-type neural precursors, transplanted into the brains of the newborn mutants, were able to effectively compete and replace the defective myelin (n = 2 at each of four end points). These data demonstrate the potential of neural stem cell therapies to restore normal myelination and protect axons in patients with PLP1 gene duplication mutation and further, provide proof of principle for the benefits of stem cell transplantation for other fatal leukodystrophies with 'normal' developmental myelination.

Oligodendroglial defects during quakingviable cerebellar development

The selective RNA-binding protein Quaking I (QKI) has previously been implicated in RNA localization and stabilization, alternative splicing, cell proliferation, and differentiation. The spontaneously-occurring quakingviable (qkv) mutant mouse exhibits a sharply attenuated level of QKI in myelin-producing cells, including oligodendrocytes (OL) because of the loss of an OL-specific promoter. The disruption of QKI in OLs results in severe hypomyelination of the central nervous system, but the underlying cellular mechanisms remain to be fully elucidated. In this study, we used the qkv mutant mouse as a model to study myelination defects in the cerebellum. We found that oligodendroglial development and myelination are adversely affected in the cerebellum of qkv mice. Specifically, we identified an increase in the total number of oligodendroglial precursor cells in qkv cerebella, a substantial portion of which migrated into the grey matter. Furthermore, these mislocalized oligodendroglial precursor cells retained their migratory morphology late into development. Interestingly, a number of these presumptive oligodendrocyte precursors were found at the Purkinje cell layer in qkv cerebella, resembling Bergman glia. These findings indicate that QKI is involved in multiple aspects of oligodendroglial development. QKI disruption can impact the cell fate of oligodendrocyte precursor cells, their migration and differentiation, and ultimately myelination in the cerebellum. © 2015 Wiley Periodicals, Inc. Develop Neurobiol 76: 972-982, 2016.

Transcriptional upregulation of myelin components in spontaneous myelin basic protein-deficient mice

Myelin is essential for efficient signal transduction in the nervous system comprising of multiple proteins. The intricacies of the regulation of the formation of myelin, and its components, are not fully understood. Here, we describe the characterization of a novel myelin basic protein (Mbp) mutant mouse, mbp(jive), which spontaneously occurred in our mouse colony. These mice displayed the onset of a shaking gait before 3 weeks of age and seizure onset before 2 months of age. Due to a progressive increase of seizure intensity, mbp(jive) mice experienced premature lethality at around 3 months of age. Mbp mRNA transcript or protein was undetectable and, accordingly, genetic analysis demonstrated a homozygous loss of exons 3 to 6 of Mbp. Peripheral nerve conductance was mostly unimpaired. Additionally, we observed grave structural changes in white matter predominant structures were detected by T1, T2 and diffusion weighted magnetic resonance imaging. We additionally observed that Mbp-deficiency results in an upregulation of Qkl, Mag and Cnp, suggestive of a regulatory feedback mechanism whereby compensatory increases in Qkl have downstream effects on Mag and Cnp. Further research will clarify the role and specifications of this myelin feedback loop, as well as determine its potential role in therapeutic strategies for demyelinating disorders.

Common themes in PrP signaling: the Src remains the same

The ability of the cellular prion protein (PrP^C) to trigger intracellular signals appears central to neurodegeneration pathways, yet the physiological significance of such signals is rather puzzling. For instance, PrP^C deregulation disrupts phenomena as diverse as synaptic transmission in mammals and cell adhesion in zebrafish. Although unrelated, the key proteins in these events -the NMDA receptor (NMDAR) and E-cadherin, respectively- are similarly modulated by the Src family kinase (SFK) Fyn. These observations highlight the importance of PrP^C-mediated Fyn activation, a finding reported nearly two decades ago. Given their complex functions and regulation, SFKs may hold the key to intriguing aspects of PrP biology such as its seemingly promiscuous functions and the lack of strong phenotypes in knockout mice. Here we provide a mechanistic perspective on how SFKs might contribute to the uncertain molecular basis of neuronal PrP phenotypes affecting ion channel activity, axon myelination and olfactory function. In particular, we discuss SFK target proteins involved in these processes and the role of tyrosine phosphorylation in the regulation of their activity and cell surface expression.

Loss of p53 in quaking viable mice leads to Purkinje cell defects and reduced survival

The qk(v) mutation is a one megabase deletion resulting in abnormal expression of the qkI gene. qk(v) mice exhibit hypomyelination of the central nervous system and display rapid tremors and seizures as adults. The qkI locus on 6q26-27 has also been implicated as a candidate tumor suppressor gene as the qkI locus maps to a region of genetic instability in Glioblastoma Multiforme (GBM), an aggressive brain tumor of astrocytic lineage. As GBM frequently harbors mutations affecting p53, we crossbred qk(v) and p53 mutant mice to examine whether qk(v) mice on a p53(-/-) background have an increased incidence of GBM. qk(v) (/v); p53(-/-) mice had a reduced survival rate compared to p53(-/-) littermates, and the cause of death of the majority of the mice remains unknown. In addition, immunohistochemistry revealed Purkinje cell degeneration in the cerebellum. These results suggest that p53 and qkI are genetically linked for neuronal maintenance and survival.

Comparing peripheral glial cell differentiation in Drosophila and vertebrates

In all complex organisms, the peripheral nerves ensure the portage of information from the periphery to central computing and back again. Axons are in part amazingly long and are accompanied by several different glial cell types. These peripheral glial cells ensure electrical conductance, most likely nature the long axon, and establish and maintain a barrier towards extracellular body fluids. Recent work has revealed a surprisingly similar organization of peripheral nerves of vertebrates and Drosophila. Thus, the genetic dissection of glial differentiation in Drosophila may also advance our understanding of basic principles underlying the development of peripheral nerves in vertebrates.

Distinct endocytic recycling of myelin proteins promotes oligodendroglial membrane remodeling

The central nervous system myelin sheath is a multilayered specialized membrane with compacted and non-compacted domains of defined protein composition. How oligodendrocytes regulate myelin membrane trafficking and establish membrane domains during myelination is largely unknown. Oligodendroglial cells respond to neuronal signals by adjusting the relative levels of endocytosis and exocytosis of the major myelin protein, proteolipid protein (PLP). We investigated whether endocytic trafficking is common to myelin proteins and analyzed the endocytic fates of proteins with distinct myelin subdomain localization. Interestingly, we found that PLP, myelin-associated glycoprotein (MAG) and myelin-oligodendrocyte glycoprotein (MOG), which localize to compact myelin, periaxonal loops and abaxonal loops, respectively, exhibit distinct endocytic fates. PLP was internalized via clathrin-independent endocytosis, whereas MAG was endocytosed by a clathrin-dependent pathway, although both proteins were targeted to the late-endosomal/lysosomal compartment. MOG was also endocytosed by a clathrin-dependent pathway, but in contrast to MAG, trafficked to the recycling endosome. Endocytic recycling resulted in the association of PLP, MAG and MOG with oligodendroglial membrane domains mimicking the biochemical characteristics of myelin domains. Our results suggest that endocytic sorting and recycling of myelin proteins may assist plasma membrane remodeling, which is necessary for the morphogenesis of myelin subdomains.

Effects of detergents on the redistribution of gangliosides and GPI-anchored proteins in brain tissue sections

Gangliosides and glycosylphosphatidylinositol (GPI)-anchored proteins contain lipid tails that tether them to the outer side of the cell membrane. This mode of association with the cell membrane enables them to take part in the organization of lipid rafts, but it also permits gangliosides and GPI-anchored proteins to be actively released from one cell and inserted into the membrane of another cell. Recently, we reported that under conditions of lipid raft isolation, Triton X-100 causes significant redistribution of both gangliosides and GPI-anchored proteins. Aiming to find a less disruptive detergent, we evaluated the effects of CHAPS, Saponin, deoxycholic acid, Trappsol, Tween 20, Triton X-100, Brij 96V, Brij 98, and SDS on brain tissue sections. At room temperature, all detergents (1% concentration) extracted significant amounts of both gangliosides and Thy-1. At 4C, the extraction was weaker, but Triton X-100, CHAPS, and deoxycholic acid caused significant redistribution of GD1a and Thy-1 from gray matter into the white matter. Both redistribution and extraction were significantly augmented when sections were incubated with detergents in the presence of primary antibodies. Of the nine tested detergents, none is the ideal choice. However, Brij 96V appears to be able to sufficiently reveal myelin epitopes while causing the least amount of artifacts. This manuscript contains online supplemental material at http://www.jhc.org. Please visit this article online to view these materials.

A dual tyrosine-leucine motif mediates myelin protein P0 targeting in MDCK cells

Differential targeting of myelin proteins to multiple, biochemically and functionally distinct Schwann cell plasma membrane domains is essential for myelin formation. In this study, we investigated whether the myelin protein P0 contains targeting signals using Madin-Darby canine kidney (MDCK) cells. By confocal microscopy, P0 was localized to MDCK cell basolateral membranes. C-terminal deletion resulted in apical accumulation, and stepwise deletions defined a 15-mer region that was required for basolateral targeting. Alanine substitutions within this region identified the YAML sequence as a functional tyrosine-based targeting signal, with the ML sequence serving as a secondary leucine-based signal. Replacement of the P0 ectodomain with green fluorescent protein altered the distribution of constructs lacking the YAML signal. Coexpression of the myelin-associated glycoprotein did not alter P0 distribution in MDCK cells. The results indicate that P0 contains a hierarchy of targeting signals, which may contribute to P0 localization in myelinating Schwann cells and the pathogenesis in human disease.

Vesicle transport in oligodendrocytes: probable role of Rab40c protein

Intracellular membrane trafficking plays an essential role in the structural and functional organization of oligodendrocytes, which synthesize a large amount of membrane to form myelin. Rab proteins are key components in intracellular vesicular transport. We cloned a novel Rab protein from an oligodendrocyte cDNA library, designating it Rab40c because of its homology with Rab40a and Rab40b. The DNA sequence of Rab40c shows an 843-base pair open reading frame. The deduced amino acid sequence is a protein with 281 amino acids, with a molecular weight of 31,466 Da and an isoelectric point of 9.83. Rab40c presents a number of distinct structural features including a carboxyl terminal extension and amino acid substitutions in the consensus sequence of the GTP-binding motifs. The carboxyl terminal region contains motifs that permit isoprenylation and palmitoylation. Binding studies indicate that Rab40c binds guanosine 5'-0-(3-thiotriphosphate) (GTP gamma S) with a K(d) of 21 microM and has a higher affinity for guanosine triphosphate (GTP) than for guanosine diphosphate (GDP). Rab40c is localized in the perinuclear recycling compartment, suggesting its involvement in endocytic events such as receptor recycling. The importance of this recycling in myelin formation is suggested by the increase in both Rab40c mRNA and Rab40c protein as oligodendrocytes differentiate.

Nuclear retention of MBP mRNAs in the quaking viable mice

Quaking viable (qk(v)) mice fail to properly compact myelin in their central nervous systems. Although the defect in the qk(v) mice involves a mutation affecting the expression of the alternatively spliced qk gene products, their roles in myelination are unknown. We show that the QKI RNA binding proteins regulate the nuclear export of MBP mRNAs. Disruption of the QKI nucleocytoplasmic equilibrium in oligodendrocytes results in nuclear and perikaryal retention of the MBP mRNAs and lack of export to cytoplasmic processes, as it occurs in qk(v) mice. MBP mRNA export defect leads to a reduction in the MBP levels and their improper cellular targeting to the periphery. Our findings suggest that QKI participates in myelination by regulating the mRNA export of key protein components.

Myelin biogenesis: vesicle transport in oligodendrocytes

Intracellular trafficking of membranes plays an essential role in the biogenesis and maintenance of myelin. The requisite proteins and lipids are transported from their sites of synthesis to myelin via vesicles. Vesicle transport is tightly coordinated with synthesis of lipids and proteins. To maintain the structural and functional organization of oligodendrocytes it is essential synchronize the various pathways of vesicle transport and to coordinate vesicle transport with reorganization of cytoskeleton. The systems that regulate the targeting of protein to myelin by vesicle transport are now being described. Here we review the current knowledge of these systems including those involved in (a) protein folding, (b) protein sorting and formation of carrier vesicles, (c) vesicle transport along elements of the cytoskeleton, and (d) vesicle targeting/fusion.

Identification of single nucleotide variations in the coding and regulatory regions of the myelin-associated glycoprotein gene and study of their association with multiple sclerosis

The myelin-associated glycoprotein (MAG) gene is an appealing candidate in the 19q13 Multiple Sclerosis (MS) candidate region. Using denaturing high performance liquid chromatography (DHPLC), we identified 14 single nucleotide polymorphisms (SNPs) in MAG coding and regulatory regions, and we tested their possible association with MS in Italian patient and control DNA pools. Eight variations had a frequency <0.05, i.e. below the detection limit in the pools. Of these, Arg537Cys was further studied with individually genotyped individuals and was detected in 1/189 patients and 0/85 controls. The frequency of the six remaining SNPs were not significantly different in pools including a total of 1266 patient and 1612 control chromosomes. Considering the statistical power of the experimental design, these results exclude the MAG gene as an MS susceptibility factor with an odds ratio (OR) equal or higher than 1.3.

Ethidium bromide-induced demyelination of the sciatic nerve of adult Wistar rats

Peripheral nerve ultrastructure was assessed after single or multiple local injections of the intercalating dye ethidium bromide. Thirty-four adult Wistar rats of both sexes were divided into five groups and maintained in a controlled environment with rat chow and water ad libitum throughout the experiment. The experimental animals were injected with 1 microl of 0.1% ethidium bromide in 0.9% saline into the central third of the left sciatic nerve 1 (group 1), 2 (group 2), 4 (group 3), 6 (group 4) or 8 (group 5) times. In groups 2 to 5 the injections were made at 28-day intervals. Control animals received the same amount of 0.9% saline. The animals were killed at different times after injection: group 1 at 7 days (2 rats) and 15 days (2 rats); for groups 2, 3, 4 and 5, all rats were killed 10 days after the last injection and the lesions were investigated by light and transmission electron microscopy. In the acute lesions, intoxicated Schwann cells showed a vacuolated cytoplasm and separation of the sheaths from the axon. Myelin sheaths underwent progressive vesiculation and subsequent segmental demyelination. Myelin debris were withdrawn by macrophages and remyelination by Schwann cells was prominent. With the increase in the number of injections collagen fibers also increased in number and progressively enveloped smaller numbers of remyelinated axons composing new fascicles. Wallerian degeneration of fibers apparently not affected by ethidium bromide was more intense in the nerves from groups 4 and 5. The peripheral nerve repairs itself after demyelinating challenges with a profusion of collagen fibers and new fasciculations. This experimental model is valid to mimic recurrent demyelinating neuropathies.

Role of rRAB22b, an oligodendrocyte protein, in regulation of transport of vesicles from trans Golgi to endocytic compartments

Intracellular membrane trafficking plays an essential role in the biogenesis and maintenance of myelin. Members of the Rab protein family are important components of the systems that regulate intracellular vesicle transport. We examine the function of rRab22b, a novel rat Rab protein cloned from an oligodendrocyte cDNA library, by visualizing and identifying in living Hela cells the organelles that contain rRab22b. Our results show that rRab22b is present in the trans Golgi/TGN and endocytic compartments. Trafficking of membranes from trans Golgi to endocytic compartments takes place via small tubulo vesicular organelles containing rRab22b. The formation of vesicles in the trans Golgi also appears to be regulated by rRab22b. Additionally, our results suggest that rRab22b controls the transport of vesicles from the trans Golgi to endocytic compartments that localize in oligodendrocyte processes. That rRab22b is involved in the transport of certain proteins from trans Golgi to myelin is suggested by the evidence that certain proteins being targeted to the plasma membrane are first transported from trans Golgi to endocytic compartments.

Biology of oligodendrocyte and myelin in the mammalian central nervous system

Oligodendrocytes, the myelin-forming cells of the central nervous system (CNS), and astrocytes constitute macroglia. This review deals with the recent progress related to the origin and differentiation of the oligodendrocytes, their relationships to other neural cells, and functional neuroglial interactions under physiological conditions and in demyelinating diseases. One of the problems in studies of the CNS is to find components, i.e., markers, for the identification of the different cells, in intact tissues or cultures. In recent years, specific biochemical, immunological, and molecular markers have been identified. Many components specific to differentiating oligodendrocytes and to myelin are now available to aid their study. Transgenic mice and spontaneous mutants have led to a better understanding of the targets of specific dys- or demyelinating diseases. The best examples are the studies concerning the effects of the mutations affecting the most abundant protein in the central nervous myelin, the proteolipid protein, which lead to dysmyelinating diseases in animals and human (jimpy mutation and Pelizaeus-Merzbacher disease or spastic paraplegia, respectively). Oligodendrocytes, as astrocytes, are able to respond to changes in the cellular and extracellular environment, possibly in relation to a glial network. There is also a remarkable plasticity of the oligodendrocyte lineage, even in the adult with a certain potentiality for myelin repair after experimental demyelination or human diseases.

Membrane traffic in myelinating oligodendrocytes

In the central nervous system (CNS), the myelin sheath is synthesised by oligodendrocytes as a specialised subdomain of an extended plasma membrane, reminiscent of the segregated membrane domains of polarised cells. Myelination takes place within a relatively short period of time and oligodendrocytes must have adapted membrane sorting and transport mechanisms to achieve such a high rate of myelin synthesis and to maintain the unique organisation of the myelin membrane. In adult life, maintenance of the functional myelin sheath requires a carefully orchestrated balance of myelin synthesis and turnover. Imbalance in these processes may cause dys- or demyelination and disease. This review summarises what is currently known about myelin protein trafficking and mistrafficking in oligodendrocytes. We also present data demonstrating distinct transport pathways for myelin structural proteins and the expression of SNARE proteins in differentiating oligodendrocytes. Myelinating glial cells may well serve as a model system for studying general aspects of membrane trafficking and organisation of membrane domains.

Intracellular distribution of myelin protein gene products is altered in oligodendrocytes of the taiep rat

Hypomyelination and subsequent demyelination of the taiep rat CNS are thought to result from the abnormal accumulation of microtubules (MTs) in oligodendrocytes that disrupts intracellular transport of components needed to form and maintain the myelin sheath. In this study, myelin gene expression was evaluated in mutant and age-matched controls to determine if MT abnormalities affect the distribution of myelin proteins and their mRNAs. Immunohistochemical analysis of taiep brains and spinal cords revealed a gradual decrease in levels of several myelin proteins including myelin basic protein (MBP), proteolipid protein (PLP), myelin-associated glycoprotein (MAG), and 2',3'-cyclic nucleotide 3'-phosphodiesterase. Accompanying early declines in MAG and PLP, accumulations of immunoreactive products were detected within oligodendrocytes, consistent with a defect in protein trafficking. Northern blot analysis indicated that diminishing protein levels could not be attributed to changes in transcriptional activity, except for MBP of which mRNA levels decreased with age. Cellular localization of MBP mRNA by in situ hybridization further revealed that transcripts were concentrated within oligodendrocyte cell bodies instead of uniformly distributed throughout processes. These results demonstrate that changes in expression and intracellular localization of myelin gene products are concurrent with increases in MT mass in taiep oligodendrocytes and support our hypothesis that cytoskeletal defects prevent the normal transport of elements required for the formation and maintenance of the myelin sheath.

Down-regulation of cell surface receptors is modulated by polar residues within the transmembrane domain

How recycling receptors are segregated from down-regulated receptors in the endosome is unknown. In previous studies, we demonstrated that substitutions in the transferrin receptor (TR) transmembrane domain (TM) convert the protein from an efficiently recycling receptor to one that is rapidly down regulated. In this study, we demonstrate that the "signal" within the TM necessary and sufficient for down-regulation is Thr(11)Gln(17)Thr(19) (numbering in TM). Transplantation of these polar residues into the wild-type TR promotes receptor down-regulation that can be demonstrated by changes in protein half-life and in receptor recycling. Surprisingly, this modification dramatically increases the TR internalization rate as well ( approximately 79% increase). Sucrose gradient centrifugation and cross-linking studies reveal that propensity of the receptors to self-associate correlates with down-regulation. Interestingly, a number of cell surface proteins that contain TM polar residues are known to be efficiently down-regulated, whereas recycling receptors for low-density lipoprotein and transferrin conspicuously lack these residues. Our data, therefore, suggest a simple model in which specific residues within the TM sequences dramatically influence the fate of membrane proteins after endocytosis, providing an alternative signal for down-regulation of receptor complexes to the well-characterized cytoplasmic tail targeting signals.

Oligodendrocytes and the control of myelination in vivo: new insights from the rat anterior medullary velum

The rat anterior medullary velum (AMV) is representative of the brain and spinal cord, overall, and provides an almost two-dimensional preparation for investigating axon-glial interactions in vivo. Here, we review some of our findings on axon-oligodendrocyte unit relations in our adult, development, and injury paradigms: (1) adult oligodendrocytes are phenotypically heterogeneous, conforming to Del Rio Hortega's types I-IV, whereby differences in oligodendrocyte morphology, metabolism, myelin sheath radial and longitudinal dimensions, and biochemistry correlate with the diameters of axons in the unit; (2) oligodendrocytes derive from a common premyelinating oligodendrocyte phenotype, and divergence of types I-IV is related to the age they emerge and the presumptive diameter of axons in the unit; (3) during myelination, axon-oligodendrocyte units progress through a sequence of maturation phases, related to axon contact, ensheathment, establishment of internodal myelin sheaths, and finally the radial growth and compaction of the myelin sheath; (4) we provide direct in vivo evidence that platelet-derived growth factor-AA (PDGF-AA), fibroblast growth factor (FGF-2), and insulin-like growth factor-I (IGF-I) differentially regulate these events, by injecting the growth factors into the cerebrospinal fluid of neonatal rat pups; (5) in lesioned adult AMV, transected central nervous system (CNS) axons regenerate through the putatively inhibitory environment of the glial scar, but remyelination by oligodendrocytes is incomplete, indicating that axon-oligodendrocyte interactions are defective; and (6) in the adult AMV, cells expressing the NG2 chondroitin sulphate have a presumptive adult oligodendrocyte progenitor antigenic phenotype, but are highly complex cells and send processes to contact axolemma at nodes of Ranvier, suggesting they subserve a specific perinodal function. Thus, axons and oligodendrocyte lineage cells form interdependent functional units, but oligodendrocyte numbers, differentiation, phenotype divergence, and myelinogenesis are governed by axons in the units, mediated by growth factors and contact-dependent signals.

Myelin dysfunction/degradation in the central nervous system: why are myelin sheaths susceptible to damage?

In the central nervous system, myelin sheaths are produced to electrically insulate axons and to increase the velocity of axonal conduction. They are highly complex structures, which are often destructed in neurological disorders. One possible reason for the vulnerability of myelin sheaths to damage became apparent from analyses of animals with altered amounts of otherwise normal myelin components: Due to limited redundance in function between different myelin proteins, dysfunction or loss of one protein may cause loss of function and instability of the entire myelin sheath.

Structure-function analysis of Qk1: a lethal point mutation in mouse quaking prevents homodimerization

Qk1 is a member of the KH domain family of proteins that includes Sam68, GRP33, GLD-1, SF1, and Who/How. These family members are RNA binding proteins that contain an extended KH domain embedded in a larger domain called the GSG (for GRP33-Sam68-GLD-1) domain. An ethylnitrosourea-induced point mutation in the Qk1 GSG domain alters glutamic acid 48 to a glycine and is known to be embryonically lethal in mice. The function of Qk1 and the GSG domain as well as the reason for the lethality are unknown. Here we demonstrate that the Qk1 GSG domain mediates RNA binding and Qk1 self-association. By using in situ chemical cross-linking studies, we showed that the Qk1 proteins exist as homodimers in vivo. The Qk1 self-association region was mapped to amino acids 18 to 57, a region predicted to form coiled coils. Alteration of glutamic acid 48 to glycine (EG) in the Qk1 GSG domain (producing protein Qk1:EG) abolishes self-association but has no effect on the RNA binding activity. The expression of Qk1 or Qk1:EG in NIH 3T3 cells induces cell death by apoptosis. Approximately 90% of the remaining transfected cells are apoptotic 48 h after transfection. Qk1:EG was consistently more potent at inducing apoptosis than was wild-type Qk1. These results suggest that the mouse quaking lethality (EG) occurs due to the absence of Qk1 self-association mediated by the GSG domain.

The cytoplasmic domain of the large myelin-associated glycoprotein isoform is needed for proper CNS but not peripheral nervous system myelination

The myelin-associated glycoprotein (MAG) is a member of the immunoglobulin gene superfamily and is thought to play a critical role in the interaction of myelinating glial cells with the axon. Myelin from mutant mice incapable of expressing MAG displays various subtle abnormalities in the CNS and degenerates with age in the peripheral nervous system (PNS). Two distinct isoforms, large MAG (L-MAG) and small MAG (S-MAG), are produced through the alternative splicing of the primary MAG transcript. The cytoplasmic domain of L-MAG contains a unique phosphorylation site and has been shown to associate with the fyn tyrosine kinase. Moreover, L-MAG is expressed abundantly early in the myelination process, possibly indicating an important role in the initial stages of myelination. We have adapted the gene-targeting approach in embryonic stem cells to generate mutant mice that express a truncated form of the L-MAG isoform, eliminating the unique portion of its cytoplasmic domain, but that continue to express S-MAG. Similar to the total MAG knockouts, these animals do not express an overt clinical phenotype. CNS myelin of the L-MAG mutant mice displays most of the pathological abnormalities reported for the total MAG knockouts. In contrast to the null MAG mutants, however, PNS axons and myelin of older L-MAG mutant animals do not degenerate, indicating that S-MAG is sufficient to maintain PNS integrity. These observations demonstrate a differential role of the L-MAG isoform in CNS and PNS myelin.

The cytoplasmic domain of the large myelin-associated glycoprotein isoform is needed for proper CNS but not peripheral nervous system myelination

The myelin-associated glycoprotein (MAG) is a member of the immunoglobulin gene superfamily and is thought to play a critical role in the interaction of myelinating glial cells with the axon. Myelin from mutant mice incapable of expressing MAG displays various subtle abnormalities in the CNS and degenerates with age in the peripheral nervous system (PNS). Two distinct isoforms, large MAG (L-MAG) and small MAG (S-MAG), are produced through the alternative splicing of the primary MAG transcript. The cytoplasmic domain of L-MAG contains a unique phosphorylation site and has been shown to associate with the fyn tyrosine kinase. Moreover, L-MAG is expressed abundantly early in the myelination process, possibly indicating an important role in the initial stages of myelination. We have adapted the gene-targeting approach in embryonic stem cells to generate mutant mice that express a truncated form of the L-MAG isoform, eliminating the unique portion of its cytoplasmic domain, but that continue to express S-MAG. Similar to the total MAG knockouts, these animals do not express an overt clinical phenotype. CNS myelin of the L-MAG mutant mice displays most of the pathological abnormalities reported for the total MAG knockouts. In contrast to the null MAG mutants, however, PNS axons and myelin of older L-MAG mutant animals do not degenerate, indicating that S-MAG is sufficient to maintain PNS integrity. These observations demonstrate a differential role of the L-MAG isoform in CNS and PNS myelin.

Reciprocal expression of myelin-associated glycoprotein splice variants in the adult human peripheral and central nervous systems

The L- and S-MAG isoforms differ only at their C-terminus and are believed to be functionally distinct. To obtain information on the relative expression of these alternatively spliced isoforms in humans, we cloned an S-MAG cDNA fragment. The deduced amino-acid sequence of the human S-MAG C-terminus shows fairly conservative substitutions of 4 out of the 10 residues compared to the rodent peptide. Using reverse transcription and a competitive polymerase chain reaction, we show that, in contrast to rodents, the L-MAG splice variant predominates in adult human brain while, like in rodents, S-MAG transcripts are most abundant in peripheral nerve. The results obtained by Western blot analysis and immunohistochemistry are in good agreement with the findings at the mRNA level. Animal experiments may thus be more representative for the role of MAG in human nerve than in brain.

Neural cell type-specific expression of QKI proteins is altered in quakingviable mutant mice

qkI, a newly cloned gene lying immediately proximal to the deletion in the quakingviable mutation, is transcribed into three messages of 5, 6, and 7 kb. Antibodies raised to the unique carboxy peptides of the resulting QKI proteins reveal that, in the nervous system, all three QKI proteins are expressed strongly in myelin-forming cells and also in astrocytes. Interestingly, individual isoforms show distinct intracellular distributions: QKI-6 and QKI-7 are localized to perikaryal cytoplasm, whereas QKI-5 invariably is restricted to the nucleus, consistent with the predicted role of QKI as an RNA-binding protein. In quakingviable mutants, which display severe dysmyelination, QKI-6 and QKI-7 are absent exclusively from myelin-forming cells. By contrast, QKI-5 is absent only in oligodendrocytes of severely affected tracts. These observations implicate QKI proteins as regulators of myelination and reveal key insights into the mechanisms of dysmyelination in the quakingviable mutant.

Neural cell type-specific expression of QKI proteins is altered in quakingviable mutant mice

qkI, a newly cloned gene lying immediately proximal to the deletion in the quakingviable mutation, is transcribed into three messages of 5, 6, and 7 kb. Antibodies raised to the unique carboxy peptides of the resulting QKI proteins reveal that, in the nervous system, all three QKI proteins are expressed strongly in myelin-forming cells and also in astrocytes. Interestingly, individual isoforms show distinct intracellular distributions: QKI-6 and QKI-7 are localized to perikaryal cytoplasm, whereas QKI-5 invariably is restricted to the nucleus, consistent with the predicted role of QKI as an RNA-binding protein. In quakingviable mutants, which display severe dysmyelination, QKI-6 and QKI-7 are absent exclusively from myelin-forming cells. By contrast, QKI-5 is absent only in oligodendrocytes of severely affected tracts. These observations implicate QKI proteins as regulators of myelination and reveal key insights into the mechanisms of dysmyelination in the quakingviable mutant.