Localization and partial characterization of a 60 kDa citrulline-containing transport form of myelin basic protein from MO3-13 cells and human white matter

Myelin basic protein cleaves cell adhesion molecule L1 and promotes neuritogenesis and cell survival

The cell adhesion molecule L1 is a Lewis(x)-carrying glycoprotein that plays important roles in the developing and adult nervous system. Here we show that myelin basic protein (MBP) binds to L1 in a Lewis(x)-dependent manner. Furthermore, we demonstrate that MBP is released by murine cerebellar neurons as a sumoylated dynamin-containing protein upon L1 stimulation and that this MBP cleaves L1 as a serine protease in the L1 extracellular domain at Arg(687) yielding a transmembrane fragment that promotes neurite outgrowth and neuronal survival in cell culture. L1-induced neurite outgrowth and neuronal survival are reduced in MBP-deficient cerebellar neurons and in wild-type cerebellar neurons in the presence of an MBP antibody or L1 peptide containing the MBP cleavage site. Genetic ablation of MBP in shiverer mice and mutagenesis of the proteolytically active site in MBP or of the MBP cleavage site within L1 as well as serine protease inhibitors and an L1 peptide containing the MBP cleavage site abolish generation of the L1 fragment. Our findings provide evidence for novel functions of MBP in the nervous system.

Sirtuin 2, a mammalian homolog of yeast silent information regulator-2 longevity regulator, is an oligodendroglial protein that decelerates cell differentiation through deacetylating alpha-tubulin

Silent information regulator-2 (SIR2) proteins regulate lifespan of diverse organisms, but their distribution and roles in the CNS remain unclear. Here, we show that sirtuin 2 (SIRT2), a mammalian SIR2 homolog, is an oligodendroglial cytoplasmic protein and localized to the outer and juxtanodal loops in the myelin sheath. Among cytoplasmic proteins of OLN-93 oligodendrocytes, alpha-tubulin was the main substrate of SIRT2 deacetylase. In cultured primary oligodendrocyte precursors (OLPs), SIRT2 emergence accompanied elevated alpha-tubulin acetylation and OLP differentiation into the prematurity stage. Small interfering RNA knockdown of SIRT2 increased the alpha-tubulin acetylation, myelin basic protein expression, and cell arbor complexity of OLPs. SIRT2 overexpression had the opposite effects, and counteracted the cell arborization-promoting effect of overexpressed juxtanodin. SIRT2 mutation concomitantly reduced its deacetylase activity and its impeding effect on OLP arborization. These results demonstrated a counterbalancing role of SIRT2 against a facilitatory effect of tubulin acetylation on oligodendroglial differentiation. Selective SIRT2 availability to oligodendroglia may have important implications for myelinogenesis, myelin-axon interaction, and brain aging.

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.

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.

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.

Attenuation of Experimental Autoimmune Encephalomyelitis and Nonimmune Demyelination by IFN-β plus Vitamin B12: Treatment to Modify Notch-1/Sonic Hedgehog Balance

Interferon-beta is a mainstay therapy of demyelinating diseases, but its effects are incomplete in human multiple sclerosis and several of its animal models. In this study, we demonstrate dramatic improvements of clinical, histological, and laboratory parameters in in vivo mouse models of demyelinating disease through combination therapy with IFN-beta plus vitamin B(12) cyanocobalamin (B(12)CN) in nonautoimmune primary demyelinating ND4 (DM20) transgenics, and in acute and chronic experimental autoimmune encephalomyelitis in SJL mice. Clinical improvement (p values <0.0001) was paralleled by near normal motor function, reduced astrocytosis, and reduced demyelination. IFN-beta plus B(12)CN enhanced in vivo and in vitro oligodendrocyte maturation. In vivo and in vitro altered expression patterns of reduced Notch-1 and enhanced expression of sonic hedgehog and its receptor were consistent with oligodendrocyte maturation and remyelination. IFN-beta-B(12)CN combination therapy may be promising for the treatment of multiple sclerosis.

Immunohistochemical localization of citrullinated proteins in adult rat brain

By using hybridoma technology, an IgM monoclonal antibody (F95) against multiple citrullinated synthetic and natural peptides was recently developed and used to stain immunohistochemically subsets of astrocytes and myelin basic protein (MBP) from selected regions of human brain (Nicholas and Whitaker [2002] Glia 37:328-336). With this antibody, the present study provides a more detailed localization of citrullinated epitopes in the central nervous system (CNS) by examining immunohistochemical staining patterns for F95 in the normal adult rat brain. Thus, immunohistochemical labeling for citrullinated epitopes was seen in white matter areas consistent with myelin staining; however, in general, it was more prominent and uniform in the caudal CNS (spinal cord, medulla oblongata, pons, and cerebellum) than in more rostral areas. F95 staining was also seen in cells and fibers often intimately associated with blood vessels and/or ventricular surfaces. By using dual-color immunofluorescence, the vast majority of this latter staining was colocalized within a subset of astrocytes also immunoreactive for glial fibrillary acidic protein (GFAP). By using Western blot analysis of rat brain proteins, multiple GFAP- and MBP-immunoreactive proteins and peptide fragments were seen, and many of them were also reactive with the F95 antibody. Thus, the present study not only demonstrates that citrullinated epitopes in normal rat brain are most concentrated in subsets of myelin and astrocytes but also provides evidence that GFAP, like MBP, may be present as multiple citrullinated isoforms.

Citrulline immunohistochemistry may not necessarily identify nitric oxide synthase activity: the pitfall of peptidylarginine deiminase

Nitric oxide synthase (NOS) converts L-arginine as a substrate to form nitric oxide and the "by-product" citrulline. To characterize NOS activity in the nervous tissue at the single-cell level, citrulline immunostaining is considered to be a suitable means of working on the principle that in brain tissue, due to the incomplete urea cycle, citrulline is produced exclusively by NOS. This assumption is correct for free citrulline but it does not consider the conversion of arginine to citrulline residues of proteins by the calcium-dependent peptidylarginine deiminase (PAD). Using a polyclonal antiserum against citrulline we observed in cerebellar cell cultures immunopositivity in a few, mostly NOS-positive, neurons, in activated microglia, and in oligodendroglia (which under control conditions are in doubt to be able to express NOS), but not in astroglia. Treatment with the excitotoxin kainate substantially enhanced the staining intensity for citrulline in neurons and glial cells. To distinguish between free (NOS-related) and protein-bound (PAD-related) citrulline we blocked NOS activity by 7-nitroindazole or L-N5-(1-iminoethyl)lysine. The results provide evidence that citrulline immunolabeling results partly from PAD-mediated protein citrullination, enhanced pathophysiologically under stimulated conditions by exposure to kainate. Our immunocytochemical observations were corroborated by Western blot analysis showing several bands of citrulline-positive proteins, whose number and staining intensity depended on kainate treatment and calcium ions.

Infection of human oligodendroglioma cells by a recombinant measles virus expressing enhanced green fluorescent protein

One of the hallmarks of the human CNS disease subacute sclerosing panencephalitis (SSPE) is a high level of measles virus (MV) infection of oligodendrocytes. It is therefore surprising that there is only one previous report of MV infection of rat oligodendrocytes in culture and no reports of human oligodendrocyte infection in culture. In an attempt to develop a model system to study MV infection of oligodendrocytes, time-lapse confocal microscopy, immunocytochemistry, and electron microscopy (EM) were used to study infection of the human oligodendroglioma cell line, MO3.13. A rat oligodendrocyte cell line, OLN-93, was also studied as a control. MO3.13 cells were shown to be highly susceptible to MV infection and virus budding was observed from the surface of infected MO3.13 cells by EM. Analysis of the infection in real time and by immunocytochemistry revealed that virus spread occurred by cell-to-cell fusion and was also facilitated by virus transport in cell processes. MO3.13 cells were shown to express CD46, a MV receptor, but were negative for the recently discovered MV receptor, signaling leucocyte activation molecule (SLAM). Immunohistochemical studies on SSPE tissue sections demonstrated that CD46 was also expressed on populations of human oligodendrocytes. SLAM expression was not detected on oligodendrocytes. These studies, which are the first to show MV infection of human oligodendrocytes in culture, show that the cells are highly susceptible to MV infection and this model cell line has been used to further our understanding of MV spread in the CNS.

Analogous structural motifs in myelin basic protein and in MARCKS

Myelin basic protein (MBP) and myristoylated alanine-rich C-kinase substrate (MARCKS) are similar in terms of having extended conformations regulated by their environment (i.e., solubilised or lipid-associated), N-terminal modifications, a dual nature of interactions with lipids, binding to actin and Ca2+-calmodulin, and being substrates for different kinds of protein kinases. The further sequence similarities of segments of MBP with lipid effector regions of MARCKS, and numerous reports in the literature, support the thesis that some developmental isoform of MBP functions in signal transduction.