Expression and regulation of golli products of myelin basic protein gene during in vitro development of oligodendrocytes

Generation and characterization of cerebellar granule neurons specific knockout mice of Golli-MBP

Golli-myelin basic proteins, encoded by the myelin basic protein gene, are widely expressed in neurons and oligodendrocytes in the central nervous system. Further, prior research has shown that Golli-myelin basic protein is necessary for myelination and neuronal maturation during central nervous system development. In this study, we established Golli-myelin basic protein-floxed mice to elucidate the cell-type-specific effects of Golli-myelin basic protein knockout through the generation of conditional knockout mice (Golli-myelin basic proteinsfl/fl; E3CreN), in which Golli-myelin basic proteins were specifically deleted in cerebellar granule neurons, where Golli-myelin basic proteins are expressed abundantly in wild-type mice. To investigate the role of Golli-myelin basic proteins in cerebellar granule neurons, we further performed histopathological analyses of these mice, with results indicating no morphological changes or degeneration of the major cellular components of the cerebellum. Furthermore, behavioral analysis showed that Golli-myelin basic proteinsfl/fl; E3CreN mice were healthy and did not display any abnormal behavior. These results suggest that the loss of Golli-myelin basic proteins in cerebellar granule neurons does not lead to cerebellar perturbations or behavioral abnormalities. This mouse model could therefore be employed to analyze the effect of Golli-myelin basic protein deletion in specific cell types of the central nervous system, such as other neuronal cells and oligodendrocytes, or in lymphocytes of the immune system.

Intrinsic Disorder as a Natural Preservative: High Levels of Intrinsic Disorder in Proteins Found in the 2600-Year-Old Human Brain

Proteomic analysis revealed the preservation of many proteins in the Heslington brain (which is at least 2600-year-old brain tissue uncovered within the skull excavated in 2008 from a pit in Heslington, Yorkshire, England). Five of these proteins-"main proteins": heavy, medium, and light neurofilament proteins (NFH, NFM, and NFL), glial fibrillary acidic protein (GFAP), and myelin basic (MBP) protein-are engaged in the formation of non-amyloid protein aggregates, such as intermediate filaments and myelin sheath. We used a wide spectrum of bioinformatics tools to evaluate the prevalence of functional disorder in several related sets of proteins, such as the main proteins and their 44 interactors, all other proteins identified in the Heslington brain, as well as the entire human proteome (20,317 manually curated proteins), and 10,611 brain proteins. These analyses revealed that all five main proteins, half of their interactors and almost one third of the Heslington brain proteins are expected to be mostly disordered. Furthermore, most of the remaining Heslington brain proteins are expected to contain sizable levels of disorder. This is contrary to the expected substantial (if not complete) elimination of the disordered proteins from the Heslington brain. Therefore, it seems that the intrinsic disorder of NFH, NFM, NFL, GFAP, and MBP, their interactors, and many other proteins might play a crucial role in preserving the Heslington brain by forming tightly folded brain protein aggregates, in which different parts are glued together via the disorder-to-order transitions.

Multiple sclerosis and myelin basic protein: insights into protein disorder and disease

Myelin basic protein (MBP) is an abundant protein in central nervous system (CNS) myelin. MBP has long been studied as a factor in the pathogenesis of the autoimmune neurodegenerative disease multiple sclerosis (MS). MS is characterized by CNS inflammation, demyelination, and axonal loss. One of the main theories on the pathogenesis of MS suggests that exposure to foreign antigens causes the activation of cross-reactive T cells in genetically susceptible individuals, with MBP being a possible autoantigen. While a direct role for MBP as a primary antigen in human MS is unclear, it is clear that MBP and its functions in myelin formation and long-term maintenance are linked to MS. This review looks at some key molecular characteristics of MBP and its relevance to MS, as well as the mechanisms of possible molecular mimicry between MBP and some viral antigens. We also discuss the use of serum anti-myelin antibodies as biomarkers for disease. MBP is a prime example of an apparently simple, but in fact biochemically and structurally complex molecule, which is closely linked to both normal nervous system development and neurodegenerative disease.

Identification of a novel variant of Golli myelin basic protein BG21 in the uniquely neuroprotective white-footed mouse

The myelin basic protein (MBP) gene is a complex gene which codes for several distinct forms of MBP. The various forms of MBP are functionally involved in the development of the nervous system, T-cell regulation, and myelination. Several neurological disorders have been linked to MBP abnormality, further demonstrating its functional significance in the nervous system. The white-footed mouse (Peromyscus leucopus) exhibits profound neuroprotective characteristics, is asymptomatic to various disease-states, and has a lifespan twice that of the house mouse (Mus musculus). We used M. musculus mouse MBP as a reference to explore MBP in P. leucopus mice. Through genetic and downstream proteomic data analysis, we identified a novel variant of the BG21 isoform of MBP in P. leucopus mice. Variation in this isoform is present at the genetic level between the two species of mice. Our results show differences within the open reading frame of the transcripts accompanied by corresponding differences in protein structure prediction. These data introduce the potential of MBP variation as one of many causal variables contributing to the unique presentation of enhanced neuroprotection and longevity in P. leucopus mice.

Myelin in cartilaginous fish

Myelin is probably one of the most fascinating and innovative biological acquisition: a glia plasma membrane tightly wrapped around an axon and insulating it. Chondrichthyans (cartilaginous fishes) form a large group of vertebrates, and they are among oldest extant jawed vertebrate lineage. It has been known from studies 150 years ago, that they are positioned at the root of the successful appearance of compact myelin and main adhesive proteins in vertebrates. More importantly, the ultrastructure of their compact myelin is indistinguishable from the one observed in tetrapods and the first true myelin basic protein (MBP) and myelin protein zero (MPZ) seem to have originated on cartilaginous fish or their ancestors, the placoderms. Thus, the study of their myelin formation would bring new insights in vertebrate׳s myelin evolution. Chondrichthyans central nervous system (CNS) myelin composition is also very similar to peripheral nervous system (PNS) myelin composition. And while they lack true proteolipid protein (PLP) like tetrapods, they express a DM-like protein in their myelin. This article is part of a Special Issue entitled SI: Myelin Evolution.

Dysfunction of platelet-derived growth factor receptor α (PDGFRα) represses the production of oligodendrocytes from arylsulfatase A-deficient multipotential neural precursor cells

The membrane-bound receptor for platelet-derived growth factor A (PDGFRα) is crucial for controlling the production of oligodendrocytes (OLs) for myelination, but regulation of its activity during OL differentiation is largely unknown. We have examined the effect of increased sulfated content of galactosylceramides (sulfatides) on the regulation of PDGFRα in multipotential neural precursors (NPs) that are deficient in arylsulfatase A (ASA) activity. This enzyme is responsible for the lysosomal hydrolysis of sulfatides. We show that sulfatide accumulation significantly impacts the formation of OLs via deregulation of PDGFRα function. PDGFRα is less associated with detergent-resistant membranes in ASA-deficient cells and showed a significant decrease in AKT phosphorylation. Rescue experiments with ASA showed a normalization of the ratio of long versus short sulfatides, restored PDGFRα levels, corrected its localization to detergent-resistant membranes, increased AKT phosphorylation, and normalized the production of OLs in ASA-deficient NPs. Moreover, our studies identified a novel mechanism that regulates the secretion of PDGFRα in NPs, in glial cells, and in the brain cortex via exosomal shedding. Our study provides a first step in understanding the role of sulfatides in regulating PDGFRα levels in OLs and its impact in myelination.

Over-expression in E. coli and purification of functional full-length murine small C-terminal domain phosphatase (SCP1, or Golli-interacting protein)

During myelination in the central nervous system, proteins arising from the gene in the oligodendrocyte lineage (golli) participate in diverse events in signal transduction and gene regulation. One of the interacting partners of the Golli-isoform BG21 was discovered by yeast-2-hybrid means and was denoted the Golli-interacting-protein (GIP). In subsequent in vitro studies of recombinant murine GIP, it was not possible to produce a full-length version of recombinant murine rmGIP in functional form under native conditions, primarily because of solubility issues, necessitating the study of a hexahistidine-tagged, truncated form ΔN-rmGIP. This protein is an acidic phosphatase belonging to the family of RNA-polymerase-2, small-subunit, C-terminal phosphatases (SCP1), and studies of the human ortholog hSCP1 have also been performed on truncated forms. Here, a new SUMO-expression and purification protocol has been developed for the preparation of a functional, full-length mSCP1/GIP (our nomenclature henceforth), with no additional purification tags. Both full-length mSCP1/GIP and the truncated murine form (now denoted ΔN-rmSCP1/GIP) had similar melting temperatures, indicating that the integrity of the catalytic core per se was minimally affected by the N-terminus. Characterization of mSCP1/GIP activity with the artificial substrate p-NPP (p-nitrophenylphosphate) yielded kinetic parameters comparable to those of ΔN-rmSCP1/GIP and the truncated human ortholog ΔN-hSCP1. Similarly, mSCP1/GIP dephosphorylated a more natural CTD-peptide substrate (but not protein kinase C-phosphorylated BG21) with comparable kinetics to ΔN-hSCP1. The successful production of an active, full-length mSCP1/GIP will enable future evaluation of the functional role of its N-terminus in protein-protein interactions (e.g., BG21) that regulate its phosphatase activity.

Regulatory effect of the glial Golli-BG21 protein on the full-length murine small C-terminal domain phosphatase (SCP1, or Golli-interacting protein)

The gene in the oligodendrocyte lineage (golli) encodes a number of proteins essential for myelination, comprising Golli and classic isoforms that are expressed in a developmentally-regulated manner. The Golli-interacting-protein (GIP) was previously discovered in a search for potential interacting partners of the Golli-isoform BG21, and was realised to be an acidic phosphatase belonging to the family of RNA-polymerase-2, small-subunit, C-terminal phosphatases (viz., SCP1). Here, we refer to this protein as mSCP1/GIP. In subsequent in vitro studies of recombinant murine SCP1/GIP, the inability to produce an active full-length version of the protein under native conditions necessitated the study of a truncated form ΔN-rmSCP1/GIP, but with inconclusive results regarding its interaction with BG21 [13]. We have since developed a new SUMO-expression and purification protocol for the preparation of a functional, full-length mGIP/SCP1, with no additional purification tags. Here, the interaction between mSCP1/GIP (with intact N-terminus) and BG21 is shown to be different than for the truncation mutant studied previously. Specifically, this interaction shows a dual effect on the enzymatic activity of mSCP1/GIP by BG21: BG21 enhanced mSCP1/GIP phosphatase activity (Ka = 30 μM), whereas PKCα-phosphorylated BG21 inhibited its activity (Ki = 2.9 μM), suggesting a potential role of BG21 as a molecular switch ("quick-brake mechanism") on mSCP1/GIP. The successful production of an active, full-length mSCP1/GIP thus demonstrates a role for its N-terminus in regulation of phosphatase activity, in events such as the regulation of transcription in oligodendrocytes.

Modulation of the Wnt/beta-catenin pathway in human oligodendroglioma cells by Sox17 regulates proliferation and differentiation

Oligodendrogliomas originate from oligodendrocyte progenitor cells (OPCs), whose development is regulated by the Sonic hedgehog and Wnt/beta-catenin pathways. We investigated the contribution of these pathways in the proliferation and differentiation of human oligodendroglioma cells (HOG). Inhibition of Hedgehog signaling with cyclopamine decreased cell survival and increased phosphorylated beta-catenin without altering myelin protein levels. Conversely, treatment of HOG with the Wnt antagonist secreted frizzled related protein (SFRP1), led to increased myelin protein levels and reduced cell proliferation, suggesting cell cycle arrest and differentiation. Unlike normal primary human OPCs, beta-catenin in HOG cells is not associated with endogenous Sox17 protein despite high levels of both proteins. Retroviral overexpression of recombinant Sox17 increased HOG cell cycle exit and apoptosis, and raised myelin protein levels and the percentage of O4(+) cells, indicating increased differentiation. Recombinant Sox17 also increased beta-catenin-TCF4-Sox17 complex formation and decreased total cellular levels of beta-catenin. These changes were associated with increased SFRP1, and reduced expression of Wnt-1 and Frizzled-1, -3 and -7 RNA, indicating that Sox17 induced a Hedgehog target, and regulated Wnt signaling at multiple levels. Our studies indicate that Wnt signaling regulates HOG cell cycle arrest and differentiation, and that recombinant Sox17 mediates modulation of the Wnt pathway through changes in beta-catenin, SFRP1 and Wnt/Frizzled expression. Our results thus identify Sox17 as a potential molecular target to include in HOG therapeutic strategies.

Myelin management by the 18.5-kDa and 21.5-kDa classic myelin basic protein isoforms

The classic myelin basic protein (MBP) splice isoforms range in nominal molecular mass from 14 to 21.5 kDa, and arise from the gene in the oligodendrocyte lineage (Golli) in maturing oligodendrocytes. The 18.5-kDa isoform that predominates in adult myelin adheres the cytosolic surfaces of oligodendrocyte membranes together, and forms a two-dimensional molecular sieve restricting protein diffusion into compact myelin. However, this protein has additional roles including cytoskeletal assembly and membrane extension, binding to SH3-domains, participation in Fyn-mediated signaling pathways, sequestration of phosphoinositides, and maintenance of calcium homeostasis. Of the diverse post-translational modifications of this isoform, phosphorylation is the most dynamic, and modulates 18.5-kDa MBP's protein-membrane and protein-protein interactions, indicative of a rich repertoire of functions. In developing and mature myelin, phosphorylation can result in microdomain or even nuclear targeting of the protein, supporting the conclusion that 18.5-kDa MBP has significant roles beyond membrane adhesion. The full-length, early-developmental 21.5-kDa splice isoform is predominantly karyophilic due to a non-traditional P-Y nuclear localization signal, with effects such as promotion of oligodendrocyte proliferation. We discuss in vitro and recent in vivo evidence for multifunctionality of these classic basic proteins of myelin, and argue for a systematic evaluation of the temporal and spatial distributions of these protein isoforms, and their modified variants, during oligodendrocyte differentiation.

Nucleus-localized 21.5-kDa myelin basic protein promotes oligodendrocyte proliferation and enhances neurite outgrowth in coculture, unlike the plasma membrane-associated 18.5-kDa isoform

The classic myelin basic protein (MBP) family of central nervous system (CNS) myelin arises from transcription start site 3 of the Golli (gene of oligodendrocyte lineage) complex and comprises splice isoforms ranging in nominal molecular mass from 14 kDa to (full-length) 21.5 kDa. We have determined here a number of distinct functional differences between the major 18.5-kDa and minor 21.5-kDa isoforms of classic MBP with respect to oligodendrocyte (OLG) proliferation. We have found that, in contrast to 18.5-kDa MBP, 21.5-kDa MBP increases proliferation of early developmental immortalized N19-OLGs by elevating the levels of phosphorylated ERK1/2 and Akt1 kinases and of ribosomal protein S6. Coculture of N2a neuronal cells with N19-OLGs transfected with the 21.5-kDa isoform (or conditioned medium from), but not the 18.5-kDa isoform, caused the N2a cells to have increased neurite outgrowth and process branching complexity. These roles were dependent on subcellular localization of 21.5-kDa MBP to the nucleus and on the exon II-encoded segment, suggesting that the nuclear localization of early minor isoforms of MBP may play a crucial role in regulating and/or initiating myelin and neuronal development in the mammalian CNS.

The 21.5-kDa isoform of myelin basic protein has a non-traditional PY-nuclear-localization signal

The predominant 18.5-kDa classic myelin basic protein (MBP) is mainly responsible for compaction of the myelin sheath in the central nervous system, but is multifunctional, having numerous interactions with Ca(2+)-calmodulin, actin, tubulin, and SH3-domains, and can tether these proteins to a lipid membrane in vitro. The full-length 21.5-kDa MBP isoform has an additional 26 residues encoded by exon-II of the classic gene, which causes it to be trafficked to the nucleus of oligodendrocytes (OLGs). We have performed site-directed mutagenesis of selected residues within this segment in red fluorescent protein (RFP)-tagged constructs, which were then transfected into the immortalized N19-OLG cell line to view protein localization using epifluorescence microscopy. We found that 21.5-kDa MBP contains two non-traditional PY-nuclear-localization signals, and that arginine and lysine residues within these motifs were involved in subcellular trafficking of this protein to the nucleus, where it may have functional roles during myelinogenesis.

Classic 18.5- and 21.5-kDa myelin basic protein isoforms associate with cytoskeletal and SH3-domain proteins in the immortalized N19-oligodendroglial cell line stimulated by phorbol ester and IGF-1

The 18.5-kDa classic myelin basic protein (MBP) is an intrinsically disordered protein arising from the Golli (Genes of Oligodendrocyte Lineage) gene complex and is responsible for compaction of the myelin sheath in the central nervous system. This MBP splice isoform also has a plethora of post-translational modifications including phosphorylation, deimination, methylation, and deamidation, that reduce its overall net charge and alter its protein and lipid associations within oligodendrocytes (OLGs). It was originally thought that MBP was simply a structural component of myelin; however, additional investigations have demonstrated that MBP is multi-functional, having numerous protein-protein interactions with Ca²⁺-calmodulin, actin, tubulin, and proteins with SH3-domains, and it can tether these proteins to a lipid membrane in vitro. Here, we have examined cytoskeletal interactions of classic 18.5-kDa MBP, in vivo, using early developmental N19-OLGs transfected with fluorescently-tagged MBP, actin, tubulin, and zonula occludens 1 (ZO-1). We show that MBP redistributes to distinct 'membrane-ruffled' regions of the plasma membrane where it co-localizes with actin and tubulin, and with the SH3-domain-containing proteins cortactin and ZO-1, when stimulated with PMA, a potent activator of the protein kinase C pathway. Moreover, using phospho-specific antibody staining, we show an increase in phosphorylated Thr98 MBP (human sequence numbering) in membrane-ruffled OLGs. Previously, Thr98 phosphorylation of MBP has been shown to affect its conformation, interactions with other proteins, and tethering of other proteins to the membrane in vitro. Here, MBP and actin were also co-localized in new focal adhesion contacts induced by IGF-1 stimulation in cells grown on laminin-2. This study supports a role for classic MBP isoforms in cytoskeletal and other protein-protein interactions during membrane and cytoskeletal remodeling in OLGs.

Proline substitutions and threonine pseudophosphorylation of the SH3 ligand of 18.5-kDa myelin basic protein decrease its affinity for the Fyn-SH3 domain and alter process development and protein localization in oligodendrocytes

The developmentally regulated myelin basic proteins (MBPs), which arise from the golli (gene of oligodendrocyte lineage) complex, are highly positively charged, intrinsically disordered, multifunctional proteins having several alternatively spliced isoforms and posttranslational modifications, and they play key roles in myelin compaction. The classic 18.5-kDa MBP isoform has a proline-rich region comprising amino acids 92-99 (murine sequence -T(92)PRTPPPS(99)-) that contains a minimal SH3 ligand domain. We have previously shown that 18.5-kDa MBP binds to several SH3 domains, including that of Fyn, a member of the Src family of tyrosine kinases involved in a number of signaling pathways during CNS development. To determine the physiological role of this binding as well as the role of phosphorylation of Thr92 and Thr95, in the current study we have produced several MBP variants specifically targeting phosphorylation sites and key structural regions of MBP's SH3 ligand domain. Using isothermal titration calorimetry, we have demonstrated that, compared with the wild-type protein, these variants have lower affinity for the SH3 domain of Fyn. Moreover, overexpression of N-terminal-tagged GFP versions in immortalized oligodendroglial N19 and N20.1 cell cultures results in aberrant elongation of membrane processes and increased branching complexity and inhibits the ability of MBP to decrease Ca(2+) influx. Phosphorylation of Thr92 can also cause MBP to traffic to the nucleus, where it may participate in additional protein-protein interactions. Coexpression of MBP with a constitutively active form of Fyn kinase resulted in membrane process elaboration, a phenomenon that was abolished by point amino acid substitutions in MBP's SH3 ligand domain. These results suggest that MBP's SH3 ligand domain plays a key role in intracellular protein interactions in vivo and may be required for proper membrane elaboration of developing oligodendrocytes and, further, that phosphorylation of Thr92 and Thr95 can regulate this function.

Classical 18.5-and 21.5-kDa isoforms of myelin basic protein inhibit calcium influx into oligodendroglial cells, in contrast to golli isoforms

The myelin basic protein (MBP) family arises from different transcription start sites of the golli (gene of oligodendrocyte lineage) complex, with further variety generated by differential splicing. The "classical" MBP isoforms are peripheral membrane proteins that facilitate compaction of the mature myelin sheath but also have multiple protein interactions. The early developmental golli isoforms have previously been shown to promote process extension and enhance Ca(2+) influx into primary and immortalized oligodendrocyte cell lines. Here, we have performed similar studies with the classical 18.5- and 21.5-kDa isoforms of MBP. In contrast to golli proteins, overexpression of classical MBP isoforms significantly reduces Ca(2+) influx in the oligodendrocyte cell line N19 as well as in primary cultures of oligodendroglial progenitor cells. Pharmacological experiments demonstrate that this effect is mediated by voltage-operated Ca(2+) channels (VOCCs) and not by ligand-gated Ca(2+) channels or Ca(2+) release from intracellular stores. The pseudo-deiminated 18.5-kDa and the full-length 21.5-kDa isoforms do not reduce Ca(2+) influx as much as the unmodified 18.5-kDa isoform. However, more efficient membrane localization (of overexpressed, pseudo-deiminated 18.5-kDa and 21.5-kDa isoforms of classical MBP containing the 21-nt 3'-untranslated region transit signal) further reduces the Ca(2+) response after plasma membrane depolarization, suggesting that binding of classical MBP isoforms to the plasma membrane is important for modulation of Ca(2+) homeostasis. Furthermore, we have found that the mature 18.5-kDa isoform expressed in oligodendrocytes colocalizes with VOCCs, particularly at the leading edge of extending membrane processes. In summary, our findings suggest a key role for classical MBP proteins in regulating voltage-gated Ca(2+) channels at the plasma membrane of oligodendroglial cells and thus also in regulation of multiple developmental stages in this cell lineage.

Myelin sheaths are formed with proteins that originated in vertebrate lineages

All vertebrate nervous systems, except those of agnathans, make extensive use of the myelinated fiber, a structure formed by coordinated interplay between neuronal axons and glial cells. Myelinated fibers, by enhancing the speed and efficiency of nerve cell communication allowed gnathostomes to evolve extensively, forming a broad range of diverse lifestyles in most habitable environments. The axon-covering myelin sheaths are structurally and biochemically novel as they contain high portions of lipid and a few prominent low molecular weight proteins often considered unique to myelin. Here we searched genome and EST databases to identify orthologs and paralogs of the following myelin-related proteins: (1) myelin basic protein (MBP), (2) myelin protein zero (MPZ, formerly P0), (3) proteolipid protein (PLP1, formerly PLP), (4) peripheral myelin protein-2 (PMP2, formerly P2), (5) peripheral myelin protein-22 (PMP22) and (6) stathmin-1 (STMN1). Although widely distributed in gnathostome/vertebrate genomes, neither MBP nor MPZ are present in any of nine invertebrate genomes examined. PLP1, which replaced MPZ in tetrapod CNS myelin sheaths, includes a novel 'tetrapod-specific' exon (see also Möbius et al., 2009). Like PLP1, PMP2 first appears in tetrapods and like PLP1 its origins can be traced to invertebrate paralogs. PMP22, with origins in agnathans, and STMN1 with origins in protostomes, existed well before the evolution of gnathostomes. The coordinated appearance of MBP and MPZ with myelin sheaths and of PLP1 with tetrapod CNS myelin suggests interdependence - new proteins giving rise to novel vertebrate structures.

Targeted overexpression of a golli-myelin basic protein isoform to oligodendrocytes results in aberrant oligodendrocyte maturation and myelination

Recently, several in vitro studies have shown that the golli–myelin basic proteins regulate Ca²⁺ homoeostasis in OPCs (oligodendrocyte precursor cells) and immature OLs (oligodendrocytes), and that a number of the functions of these cells are affected by cellular levels of the golli proteins. To determine the influence of golli in vivo on OL development and myelination, a transgenic mouse was generated in which the golli isoform J37 was overexpressed specifically within OLs and OPCs. The mouse, called JOE (J37-overexpressing), is severely hypomyelinated between birth and postnatal day 50. During this time, it exhibits severe intention tremors that gradually abate at later ages. After postnatal day 50, ultrastructural studies and Northern and Western blot analyses indicate that myelin accumulates in the brain, but never reaches normal levels. Several factors appear to underlie the extensive hypomyelination. In vitro and in vivo experiments indicate that golli overexpression causes a significant delay in OL maturation, with accumulation of significantly greater numbers of pre-myelinating OLs that fail to myelinate axons during the normal myelinating period. Immunohistochemical studies with cell death and myelin markers indicate that JOE OLs undergo a heightened and extended period of cell death and are unable to effectively myelinate until 2 months after birth. The results indicate that increased levels of golli in OPC/OLs delays myelination, causing significant cell death of OLs particularly in white matter tracts. The results provide in vivo evidence for a significant role of the golli proteins in the regulation of maturation of OLs and normal myelination.

Solution NMR and CD spectroscopy of an intrinsically disordered, peripheral membrane protein: evaluation of aqueous and membrane-mimetic solvent conditions for studying the conformational adaptability of the 18.5 kDa isoform of myelin basic protein (MBP)

The stability and secondary structure propensity of recombinant murine 18.5 kDa myelin basic protein (rmMBP, 176 residues) was assessed using circular dichroic and nuclear magnetic resonance spectroscopy (1H-15N HSQC experiments) to determine the optimal sample conditions for further NMR studies (i.e., resonance assignments and protein-protein interactions). Six solvent conditions were selected based on their ability to stabilise the protein, and their tractability to currently standard solution NMR methodology. Selected solvent conditions were further characterised as functions of concentration, temperature, and pH. The results of these trials indicated that 30% TFE-d2 in H2O (v/v), pH 6.5 at 300 K, and 100 mM KCl, pH 6.5 at 277 K were the best conditions to use for future solution NMR studies of MBP. Micelles of DPC were found to be inappropriate for backbone resonance assignments of rmMBP in this instance.

Transcriptional control of oligodendrogenesis

Oligodendrocytes (OGs) assemble the myelin sheath around axons in the central nervous system. Specification of cells into the OG lineage is largely the result of interplay between bone morphogenetic protein, sonic hedgehog and Notch signaling pathways, which regulate expression of transcription factors (TFs) dictating spatial and temporal aspects of oligodendrogenesis. Many of these TFs and others then direct OG development through to a mature myelinating OG. Here we describe signaling pathways and TFs that are inductive, inhibitory, and/or permissive to OG specification and maturation. We develop a basic transcriptional network and identify similarities and differences between regulation of oligodendrogenesis in the spinal cord and brain.

Transgenic mice expressing a dual, CRE-inducible reporter for the analysis of axon guidance and synaptogenesis

Improved and modular tools are needed for the neuroanatomical dissection of CNS axonal tracts, and to study the cell-intrinsic and cell-extrinsic cues that govern their assembly and plasticity. Here we describe a general purpose transgenic tracer that can be used to visualize axonal tracts and synaptic terminals in any region of the embryonic neural tube or postnatal CNS, on any wild type or mutant genetic background. The construct permits CRE-inducible expression of a dicistronic axonal marker encoding two surface reporter proteins: a farnesylated GFP and the human Placental Alkaline Phosphatase (PLAP). Both proteins localize alongside the neuronal surface, permitting the concomitant detection of cell body, neurites, and presynaptic and postsynaptic sites in the same neuron. This provides a CRE-inducible dual system for imaging neural circuits in vivo, and to study their assembly and remodeling in cultured neurons, neural stem cells, and tissue explants derived from the reporter line. Unlike existing lines, this reporter does not encode a ubiquitously expressed, floxable LacZ gene, permitting the simultaneous analysis of beta galactosidase activity in mutant lines.

Molecular "negativity" may underlie multiple sclerosis: role of the myelin basic protein family in the pathogenesis of MS

Myelin basic protein (MBP) binds to negatively charged lipids on the cytosolic surface of oligodendrocyte membranes and is responsible for adhesion of these surfaces in the multilayered myelin sheath. The pattern of extensive posttranslational modifications of MBP is dynamic during normal central nervous system development and during myelin degeneration in multiple sclerosis (MS), affecting its interactions with the myelin membranes and other proteins. In particular, the degree of deimination (or citrullination) of MBP is correlated with the severity of MS, and may represent a primary defect that precedes neurodegeneration due to autoimmune attack. That MBP deimination also affects topological accessibility of an otherwise partially buried immunodominant epitope of the protein indicates that this modification may play a major role in the autoimmune pathogenesis of the disease. In this chapter, we describe the structural and functional consequences of MBP deimination in healthy and diseased myelin.

Partitioning of myelin basic protein into membrane microdomains in a spontaneously demyelinating mouse model for multiple sclerosisThis paper is one of a selection of papers published in this Special Issue, entitled CSBMCB — Membrane Proteins in Health and Disease

We have characterized the lipid rafts in myelin from a spontaneously demyelinating mouse line (ND4), and from control mice (CD1 background), as a function of age and severity of disease. Myelin was isolated from the brains of CD1 and ND4 mice at various ages, and cold lysed with 1.5% CHAPS (3-[(3-cholamidopropyl) dimethylammonio]-1-propanesulphonate). The lysate was separated by low-speed centrifugation into supernatant and pellet fractions, which were characterized by Western blotting for myelin basic protein (MBP) isoforms and their post-translationally modified variants. We found that, with maturation and with disease progression, there was a specific redistribution of the 14–21.5 kDa MBP isoforms (classic exon-II-containing vs exon-II-lacking) and phosphorylated forms into the supernatant and pellet. Further fractionation of the supernatant to yield detergent-resistant membranes (DRMs), representing coalesced lipid rafts, showed these to be highly enriched in exon-II-lacking MBP isoforms, and deficient in methylated MBP variants, in mice of both genotypes. The DRMs from the ND4 mice appeared to be enriched in MBP phosphorylated by MAP kinase at Thr95 (murine 18.5 kDa numbering). These studies indicate that different splice isoforms and post-translationally modified charge variants of MBP are targeted to different microdomains in the myelin membrane, implying multifunctionality of this protein family in myelin maintenance.

Axons mediate the distribution of arylsulfatase A within the mouse hippocampus upon gene delivery

Axonal transport of the lysosomal enzyme arylsulfatase A (ARSA) may be an additional mechanism of enzyme distribution after in vivo brain gene transfer in an animal model of metachromatic leukodystrophy (MLD). Direct molecular demonstration of the movement of this lysosomal enzyme within axonal networks was missing. We generated lentiviral vectors carrying the ARSA cDNA tagged with hemagglutinin or the green fluorescent protein and examined the subcellular localization and anatomical distribution of the tagged enzymes within the MLD hippocampus after in vivo lentiviral gene transfer. The use of tagged ARSA allowed direct real-time observation and tracking of axon-dendritic transport of the enzyme after lentiviral gene therapy. Tagged ARSA was expressed in transduced pyramidal, granule, and hilar neurons within the lentiviral-injected side and was robustly contained in vesicles within ipsilateral axon-dendritic processes as well as in vesicles associated with contralateral axons and commissural axons of the ventral hippocampal commissure. Axonal transport of tagged ARSA led to the correction of hippocampal defects in long-term treated MLD mice, which was accompanied by enzyme uptake in nontransduced contralateral neurons, enzyme accumulation within the lysosomal compartment, and clearance of sulfatide storage deposits in this region of the MLD brain. These results contribute to the understanding of the mechanisms of distribution of lysosomal enzymes within the mammalian brain after direct gene therapy, demonstrating the use of neural processes for enzyme transport.

A Tale of Two Citrullines—Structural and Functional Aspects of Myelin Basic Protein Deimination in Health and Disease

Myelin basic protein (MBP) binds to negatively charged lipids on the cytosolic surface of oligodendrocyte membranes and is responsible for adhesion of these surfaces in the multilayered myelin sheath. The pattern of extensive post-translational modifications of MBP is dynamic during normal central nervous system (CNS) development and during myelin degeneration in multiple sclerosis (MS), affecting its interactions with the myelin membranes and with other molecules. In particular, the degree of deimination (or citrullination) of MBP is correlated with the severity of MS, and may represent a primary defect that precedes neurodegeneration due to autoimmune attack. That the degree of MBP deimination is also high in early CNS development indicates that this modification plays major physiological roles in myelin assembly. In this review, we describe the structural and functional consequences of MBP deimination in healthy and diseased myelin.

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.

Myelin basic protein-diverse conformational states of an intrinsically unstructured protein and its roles in myelin assembly and multiple sclerosis

The 18.5 kDa isoform of myelin basic protein (MBP) is a major component of the myelin sheath in the central nervous system of higher vertebrates, and a member of a larger family of proteins with a multiplicity of forms and post-translational modifications (PTMs). The 18.5 kDa protein is the exemplar of the family, being most abundant in adult myelin, and thus the most-studied. It is peripherally membrane-associated, but has generally been investigated in isolated form. MBP is an 'intrinsically unstructured' protein with a high proportion (approximately 75%) of random coil, but postulated to have core elements of beta-sheet and alpha-helix. We review here the properties of the MBP family, especially of the 18.5 kDa isoform, and discuss how its three-dimensional (3D) structure may be resolved by direct techniques available to us, viz., X-ray and electron crystallography, and solution and solid-state NMR spectrometry. In particular, we emphasise that creating an appropriate environment in which the protein can adopt a physiologically relevant fold is crucial to such endeavours. By solving the 3D structure of 18.5 kDa MBP and the effects of PTMs, we will attain a better understanding of myelin architecture, and of the molecular mechanisms that transpire in demyelinating diseases such as multiple sclerosis.

Interaction of the 18.5-kD isoform of myelin basic protein with Ca2+ -calmodulin: effects of deimination assessed by intrinsic Trp fluorescence spectroscopy, dynamic light scattering, and circular dichroism

The effects of deimination (conversion of arginyl to citrullinyl residues) of myelin basic protein (MBP) on its binding to calmodulin (CaM) have been examined. Four species of MBP were investigated: unmodified recombinant murine MBP (rmMBP-Cit(0)), an engineered protein with six quasi-citrullinyl (i.e., glutaminyl) residues per molecule (rmMBP-qCit(6)), human component C1 (hMBP-Cit(0)), and human component C8 (hMBP-Cit(6)), both obtained from a patient with multiple sclerosis (MS). Both rmMBP-Cit(0) and hMBP-Cit(0) bound CaM in a Ca(2+)-dependent manner and primarily in a 1:1 stoichiometry, which was verified by dynamic light scattering. Circular dichroic spectroscopy was unable to detect any changes in secondary structure in MBP upon CaM-binding. Inherent Trp fluorescence spectroscopy and a single-site binding model were used to determine the dissociation constants: K(d) = 144 +/- 76 nM for rmMBP-Cit(0), and K(d) = 42 +/- 15 nM for hMBP-Cit(0). For rmMBP-qCit(6) and hMBP-Cit(6), the changes in fluorescence were suggestive of a two-site interaction, although the dissociation constants could not be accurately determined. These results can be explained by a local conformational change induced in MBP by deimination, exposing a second binding site with a weaker association with CaM, or by the existence of several conformers of deiminated MBP. Titration with the collisional quencher acrylamide, and steady-state and lifetime measurements of the fluorescence at 340 nm, showed both dynamic and static components to the quenching, and differences between the unmodified and deiminated proteins that were also consistent with a local conformational change due to deimination.

Experimental autoimmune encephalomyelitis relapses are reduced in heterozygous golli MBP knockout mice

Increased golli MBP (golli) expression has been observed in the peripheral immune system of mice in the relapsing phase of EAE, raising the possibility that golli MBP expression in the periphery may contribute to relapses. Here we describe the generation of golli MBP-deficient mice and a comparison of the clinical course of EAE between heterozygous (golli(+/-)) and wild-type (golli(+/+)) mice. There was no difference between the two groups in incidence of disease, severity of the first episode of disease, or remission after the first episode. However, there was a significant reduction in relapses in golli(+/-) mice vs. controls, suggesting a role for golli proteins in the relapses in EAE.

Expression of Golli proteins in adult human brain and multiple sclerosis lesions

It has been suggested that Golli proteins, structurally related to myelin basic proteins (MBPs), have a role in autoimmune processes. We studied the expression of these proteins in multiple sclerosis (MS) and determined that the number of Golli-immunoreactive (ir) cells was significantly higher around lesions of chronic MS than in control white matter. Golli proteins were expressed in the adult oligodendrocyte precursor cells (OPCs), activated microglia/macrophages, and some demyelinated axons around MS lesions. Their expression in adult OPCs indicates remyelination attempts, whereas the expression in the subpopulation of microglia/macrophages suggests roles in the immune processes of MS. In addition, Golli proteins may be markers of axonal transection, which is characteristic for MS.