Implications of the sequence similarities between tau and myelin basic protein

Myelin structure in unfixed, single nerve fibers: Scanning X-ray microdiffraction with a beam size of 200nm

Previous raster-scanning with a 1μm X-ray beam of individual, myelinated fibers from glutaraldehyde-fixed rat sciatic nerve revealed a spatially-dependent variation in the diffraction patterns from single fibers. Analysis indicated differences in the myelin periodicity, membrane separations, distribution of proteins, and orientation of membrane lamellae. As chemical fixation is known to produce structural artifacts, we sought to determine in the current study whether the structural heterogeneity is intrinsic to unfixed myelin. Using a 200nm-beam that was about five-fold smaller than before, we raster-scanned individual myelinated fibers from both the peripheral (PNS; mouse and rat sciatic nerves) and central (CNS; rat corpus callosum) nervous systems. As expected, the membrane stacking in the internodal region was nearly parallel to the fiber axis and in the paranodal region it was perpendicular to the axis. A myelin lattice was also frequently observed when the incident beam was injected en face to the sheath. Myelin periodicity and diffracted intensity varied with axial position along the fiber, as did the calculated membrane profiles. Raster-scanning with an X-ray beam at sub-micron resolution revealed for the first time that the individual myelin sheaths in unfixed nerve are heterogeneous in both membrane structure and packing.

An Alzheimer's disease-relevant presenilin-1 mutation augments amyloid-beta-induced oligodendrocyte dysfunction

White matter pathology has been documented in the brains of familial Alzheimer's disease (FAD)-afflicted individuals during presymptomatic and preclinical stages of AD. How these defects in myelination integrity arise and what roles they may play in AD pathophysiology have yet to be fully elucidated. We previously demonstrated that triple-transgenic AD (3xTg-AD) mice, which harbor the human amyloid precursor Swedish mutation, presenilin-1 M146V (PS1(M146V) ) knock-in mutation, and tau(P301L) mutation, exhibit myelin abnormalities analogous to FAD patients and that Aβ(1-42) contributes to these white matter deficits. Herein, we demonstrate that the PS1(M146V) mutation predisposes mouse oligodendrocyte precursor (mOP) cells to Aβ(1-42) -induced alterations in cell differentiation in vitro. Furthermore, PS1(M146V) expression compromised mOP cell function and MBP protein distribution, a process that is further aggravated with exposure to Aβ(1-42) . We found that the myelination defect and MBP subcellular mislocalization triggered by PS1(M146V) and Aβ(1-42) can be effectively prevented by treatment with the GSK-3β inhibitor, TWS119, thereby implicating GSK-3β kinase activity in this pathogenic cascade. Overall, this work provides further mechanistic insights into PS1(M146V) and Aβ(1-42) -driven oligodendrocyte dysfunction andmyelin damage during early presymptomatic stages of AD, and provides a new target in oligodendrocytes for developing therapies designed to avert AD-related white matter pathology.

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.

Multiple sclerosis: etiological mechanisms and future directions

Multiple sclerosis (MS) is a complex human autoimmune-type disease with a predominantly unknown etiology. Immunologic destruction of myelin basic protein (MBP) throughout the nervous system is the major pathology of multiple sclerosis. This review will attempt to update new information about basic mechanisms and therapeutic management of the disease. The significance of the structure of MBP is discussed with respect to the contribution of such structures to the disease process. A number of MBP peptides that serve as the immunodominant antigens in MS patients have been identified. These peptides have been studied in animal models for their antigenic characteristics and ability to induce disease. Evidence for genetic contributions is reviewed with multigenerational twin studies providing the best evidence for susceptible haplotypes. The role of microorganisms/viruses and environmental agents are discussed as potential etiological factors but are now thought to be of minor importance to the primary causal development of the disease. Of major consideration are immunological mechanisms that contribute to the development of autoimmunity. In particular, antigen expression, cytokine and leukocyte interactions, and regulatory T-cells are discussed. Particular attention is given to regulatory T-cells (Treg), which help balance/modulate other T-cells such as Th1 and Th2 cells, and how such Treg regulate autoimmunity is addressed. The importance of the role of Tregs is exemplified by the demonstration that administration of oral antigens can induce specific Tregs that counteract experimental autoimmune encephalomyelitis in animal models. The significance of animal studies to human multiple sclerosis is discussed. A potential role for natural antibodies and innate immune mechanisms to help provide resistance to disease development is also reviewed. Finally, a variety of therapeutic agents that have been and continue to be utilized for multiple sclerosis is reviewed. Trials with oral antigens, such as glatirmer acetate (copolymer 1) especially in combination with interferon-beta, have shown promise. Antibody therapy and bone marrow transplantation are also briefly discussed.

Interactions of the 18.5-kDa isoform of myelin basic protein with Ca(2+)-calmodulin: in vitro studies using fluorescence microscopy and spectroscopy

The interactions of the 18.5-kDa isoform of myelin basic protein (MBP) with calmodulin (CaM) in vitro have been investigated using fluorescence microscopy and spectroscopy. Two forms of MBP were used: the natural bovine C1 charge isomer (bMBP/C1) and a hexahistidine-tagged recombinant murine product (rmMBP), with only minor differences in behaviour being observed. Fragments of each protein generated by digestion with cathepsin D (EC 3.4.23.5) were also evaluated. Using fluorescence microscopy, it was shown that MBP and CaM interacted in the presence of Ca2+ under a variety of conditions, including high urea and salt concentrations, indicating that the interaction was specific and not merely electrostatic in nature. Using cathepsin D digestion fragments of MBP, it was further shown that the carboxyl-terminal domain of MBP interacted with Ca(2+)-CaM, consistent with our theoretical prediction. Spectroscopy of the intrinsic fluorescence of the sole Trp residue of MBP showed that binding was cooperative in nature. The dissociation constants for formation of a 1:1 MBP-Ca(2+)-CaM complex were determined to be 2.1 +/- 0.1 and 2.0 +/- 0.2 microM for bMBP/C1 and rmMBP, respectively. Fluorescence spectroscopy using cathepsin D digestion fragments indicated also that the carboxyl-terminal region of each protein interacted with Ca(2+)-CaM, with dissociation constants of 1.8 +/- 0.2 and 2.8 +/- 0.9 microM for the bMBP/C1 and rmMBP fragments, respectively. These values show a roughly 1000-fold lower affinity of MBP for CaM than other CaM-binding peptides, such as myristoylated alanine-rich C-kinase substrate, that are involved in signal transduction.

The formation of helical tubular vesicles by binary monolayers containing a nickel-chelating lipid and phosphoinositides in the presence of basic polypeptides

Binary lipid monolayers consisting of equimolar proportions of a phosphoinositide and a nickel-chelating lipid formed helical tubular vesicular structures, which appeared to be induced and/or stabilized by myelin basic protein (MBP). Another basic polypeptide, poly-L-lysine, had a similar effect but not to as great a degree as MBP; the proteins thus appeared to act as polycations. Although, the nickel-chelating lipid is a synthetic product, other endogenous divalent cations such as Zn(2+), as well as phosphoinositides, are integral and dynamic components of the myelin sheath in vivo. There, comparable helical tubular structures might represent a means for sequestration of these lipids into domains of high local concentration, perhaps in regions where the membrane is greatly curved.

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.

Three-dimensional structure of myelin basic protein. II. Molecular modeling and considerations of predicted structures in multiple sclerosis

A computational model of myelin basic protein (MBP) has been constructed based on the premise of a phylogenetically conserved beta-sheet backbone and on electron microscopical three-dimensional reconstructions. Many residues subject to post-translational modification (phosphorylation, methylation, or conversion of arginines to citrullines) were located in loop regions and thus accessible to modifying enzymes. The triproline segment (residues 99-101) is fully exposed on the back surface of the protein in a long crossover connection between two parallel beta-strands. The proximity of this region to the underlying beta-sheet suggests that post-translational modifications here might have potential synergistic effects on the entire structure. Post-translational modifications that lead to a reduced surface charge could result first in a weakened attachment to the myelin membrane rather than in a gross conformational change of the protein itself. Such mechanisms could be operative in demyelinating diseases such as multiple sclerosis.

Immunolocalization of 17 and 21.5 kDa MBP isoforms in compact myelin and radial component

Our previous biochemical analyses revealed that the levels of the minor MBP isoforms 21.5 and 17 kDa are elevated relative to the 14 and 18.5 kDa MBP isoforms in the fraction of isolated myelin of murine CNS that is enriched in interlamellar junctions (or radial component). To substantiate the localization of 21.5 and 17 kDa MBP in the myelin sheath, we used immunoelectron microscopy on thin-sections of mouse optic nerve. Two different polyclonal antibodies were used to distinguish 21.5 and 17 kDa MBP from 14 and 18.5 kDa MBP: Ab-MBP21.5, which was raised against a synthetic peptide corresponding to the exon II amino acid sequence 61-83 of mouse 21.5 kDa MBP (LKQSRSPLPSHARSRPGLCHMYK), and Ab-MBP14, which is immunoreactive to all four isoforms of mouse MBP. Our SDS-PAGE/immunoblotting demonstrated that Ab-MBP21.5, unlike Ab-MBP14, recognized only the 21.5 and 17 kDa MBP isoforms from isolated mouse CNS myelin. Immunolabelling of tissue sections indicated that Ab-MBP14 bound tenfold more to junction-free compact myelin than to radial component, whereas Ab-MBP21.5 bound about equally to the two regions of the myelin sheath. In addition, within the junction-free compact myelin, both antibodies bound nearly three fold more to the major dense line than to the intraperiod line.