Membrane adhesion and other functions for the myelin basic proteins

Diverse functions of DEAD-box proteins in oligodendrocyte development, differentiation, and homeostasis

Oligodendrocytes, a type of glial cell in the central nervous system, have a critical role in the formation of myelin around axons, facilitating saltatory conduction, and maintaining the integrity of nerve axons. The dysregulation of oligodendrocyte differentiation and homeostasis have been implicated in a wide range of neurological diseases, including dysmyelinating disorders (e.g., Pelizaeus-Merzbacher disease), demyelinating diseases (e.g., multiple sclerosis), Alzheimer's disease, and psychiatric disorders. Therefore, unraveling the mechanisms of oligodendrocyte development, differentiation, and homeostasis is essential for understanding the pathogenesis of these diseases and the development of therapeutic interventions. Numerous studies have identified and analyzed the functions of transcription factors, RNA metabolic factors, translation control factors, and intracellular and extracellular signals involved in the series of processes from oligodendrocyte fate determination to terminal differentiation. DEAD-box proteins, multifunctional RNA helicases that regulate various intracellular processes, including transcription, RNA processing, and translation, are increasingly recognized for their diverse roles in various aspects of oligodendrocyte development, differentiation, and maintenance of homeostasis. This review introduces the latest insights into the regulatory networks of oligodendrocyte biology mediated by DEAD-box proteins.

Advances in Amyloid-β Clearance in the Brain and Periphery: Implications for Neurodegenerative Diseases

This review examines the role of impaired amyloid-β clearance in the accumulation of amyloid-β in the brain and the periphery, which is closely associated with Alzheimer's disease (AD) and cerebral amyloid angiopathy (CAA). The molecular mechanism underlying amyloid-β accumulation is largely unknown, but recent evidence suggests that impaired amyloid-β clearance plays a critical role in its accumulation. The review provides an overview of recent research and proposes strategies for efficient amyloid-β clearance in both the brain and periphery. The clearance of amyloid-β can occur through enzymatic or non-enzymatic pathways in the brain, including neuronal and glial cells, blood-brain barrier, interstitial fluid bulk flow, perivascular drainage, and cerebrospinal fluid absorption-mediated pathways. In the periphery, various mechanisms, including peripheral organs, immunomodulation/immune cells, enzymes, amyloid-β-binding proteins, and amyloid-β-binding cells, are involved in amyloid-β clearance. Although recent findings have shed light on amyloid-β clearance in both regions, opportunities remain in areas where limited data is available. Therefore, future strategies that enhance amyloid-β clearance in the brain and/or periphery, either through central or peripheral clearance approaches or in combination, are highly encouraged. These strategies will provide new insight into the disease pathogenesis at the molecular level and explore new targets for inhibiting amyloid-β deposition, which is central to the pathogenesis of sporadic AD (amyloid-β in parenchyma) and CAA (amyloid-β in blood vessels).

A novel myelin basic protein transcript variant in the murine central nervous system

Myelin basic protein is a multifunctional protein whose primary role is to adhere membranes of the myelin sheath. There are various isoforms that have been identified, 6 distinct isoforms in human and 13 distinct isoforms in mice. These distinct isoforms are the product of alternative splicing of a single gene. The present study sought out to identify the different isoforms found in the murine central nervous system. Neuronal tissue (brain) from five different C57BL6/J mice at 2 months of age was harvested and used for mRNA extraction. mRNA was reversed transcribed to cDNA and transcripts were detected through PCR amplification and DNA agarose gel separation. Primers for exon 1, exon 5b and exon 11 of the myelin basic protein gene were used to capture all the possible transcripts that are naturally found in the murine central nervous system. Unknown transcript was sequenced at Genewiz facilities (South Plainfield, NJ) and mass spectrometry protein sequence analysis demonstrated the presence of a novel myelin basic protein transcript variant. We identified a novel transcript variant of myelin basic protein. This novel transcript variant corresponds to a myelin basic protein of 32.5 kDa which has not been previously reported. This novel transcript variant presents relevant clinical significance to various demyelinating diseases due to its contribution to the understanding of the natural state of the murine central nervous system.

Dual role of the RNA helicase DDX5 in post-transcriptional regulation of Myelin Basic Protein in oligodendrocytes

In the central nervous system, oligodendroglial expression of Myelin Basic Protein (MBP) is crucial for the assembly and structure of the myelin sheath. MBP synthesis is tightly regulated in space and time, particularly on the post-transcriptional level. We have identified the DEAD-box RNA helicase DDX5 (alias p68) in a complex with Mbp mRNA in oligodendroglial cells. Expression of DDX5 is highest in progenitor cells and immature oligodendrocytes, where it localizes to heterogeneous populations of cytoplasmic ribonucleoprotein (RNP) complexes associated with Mbp mRNA in the cell body and processes. Manipulation of DDX5 protein amounts inversely affects levels of MBP protein. We present evidence that DDX5 is involved in post-transcriptional regulation of MBP protein synthesis, with implications for oligodendroglial development. In addition, DDX5 knockdown results in an increased abundance of MBP exon 2-positive isoforms in immature oligodendrocytes, most likely by regulating alternative splicing of Mbp. Our findings contribute to the understanding of the complex nature of MBP post-transcriptional control in immature oligodendrocytes where DDX5 appears to affect the abundance of MBP proteins via distinct but converging mechanisms.

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.

Oligodendrogenesis: the role of iron

Iron seems to be an essential factor in myelination and oligodendrocyte (OLGc) biology. However, the specific role of iron in these processes remains to be elucidated. Iron deficiency (ID) imposed to developing rats has been a relevant model to understand the role of iron in oligodendrogenesis and myelination. During early development ID causes specific changes in myelin composition, including a lower relative content of cholesterol, proteolipid protein (PLP), and myelin basic protein 21 (MBP21). These changes could be a consequence of the adverse effects of ID on OLGc development and function. We subsenquently studied the possible corrective effect of a single intracranial injection (ICI) of apotransferrin (aTf) on myelin formation in ID rats OLGc migration and differentiation after an ICI of aTf was evaluated at 3 days of age. ID increased the number of proliferating and undifferentiated cells in the corpus callosum (CC), while a single aTf injection reverts these effects, increasing the number of mature cells and myelin formation. Overall, results of a series of studies supports the concept that iron may affect OLGc development at early stages of embryogenesis rather than during late development. Myelin composition is altered by a limited iron supply, changes that can be reverted by a single injection of aTf.

The multiple roles of myelin protein genes during the development of the oligodendrocyte

It has become clear that the products of several of the earliest identified myelin protein genes perform functions that extend beyond the myelin sheath. Interestingly, these myelin proteins, which comprise proteolipid protein, 2′,3′-cyclic nucleotide 3′-phosphodiesterase and the classic and golli MBPs (myelin basic proteins), play important roles during different stages of oligodendroglial development. These non-myelin-related functions are varied and include roles in the regulation of process outgrowth, migration, RNA transport, oligodendrocyte survival and ion channel modulation. However, despite the wide variety of cellular functions performed by the different myelin genes, the route by which they achieve these many functions seems to converge upon a common mechanism involving Ca²⁺ regulation, cytoskeletal rearrangements and signal transduction. In the present review, the newly emerging functions of these myelin proteins will be described, and these will then be discussed in the context of their contribution to oligodendroglial development.

Degradation of amyloid beta protein by purified myelin basic protein

The progressive accumulation of beta-amyloid (Abeta) in senile plaques and in the cerebral vasculature is the hallmark of Alzheimer disease and related disorders. Impaired clearance of Abeta from the brain likely contributes to the prevalent sporadic form of Alzheimer disease. Several major pathways for Abeta clearance include receptor-mediated cellular uptake, blood-brain barrier transport, and direct proteolytic degradation. Myelin basic protein (MBP) is the major structural protein component of myelin and plays a functional role in the formation and maintenance of the myelin sheath. MBP possesses endogenous serine proteinase activity and can undergo autocatalytic cleavage liberating distinct fragments. Recently, we showed that MBP binds Abeta and inhibits Abeta fibril formation (Hoos, M. D., Ahmed, M., Smith, S. O., and Van Nostrand, W. E. (2007) J. Biol. Chem. 282, 9952-9961; Hoos, M. D., Ahmed, M., Smith, S. O., and Van Nostrand, W. E. (2009) Biochemistry 48, 4720-4727). Here we show that Abeta40 and Abeta42 peptides are degraded by purified human brain MBP and recombinant human MBP, but not an MBP fragment that lacks autolytic activity. MBP-mediated Abeta degradation is inhibited by serine proteinase inhibitors. Similarly, Cos-1 cells expressing MBP degrade exogenous Abeta40 and Abeta42. In addition, we demonstrate that purified MBP also degrades assembled fibrillar Abeta in vitro. Mass spectrometry analysis identified distinct degradation products generated from Abeta digestion by MBP. Lastly, we demonstrate in situ that purified MBP can degrade parenchymal amyloid plaques as well as cerebral vascular amyloid that form in brain tissue of Abeta precursor protein transgenic mice. Together, these findings indicate that purified MBP possesses Abeta degrading activity in vitro.

CNTF protects oligodendrocytes from ammonia toxicity: intracellular signaling pathways involved

In pediatric patients, hyperammonemia can provoke irreversible damages to developing CNS like cortical atrophy, ventricular enlargement, demyelination or gray and white matter hypodensities which are concordant with alterations of neurons and oligodendrocytes. Cerebral injury triggers endogenous protective mechanisms that can prevent or limit brain damage. Understanding these mechanisms may lead to new therapeutic strategies. We investigated whether ciliary neurotrophic factor (CNTF), a cytokine-like protein expressed by astrocytes and described as an injury-associated survival factor, was up-regulated by ammonia in developing reaggregated 3D brain cell cultures. We showed that CNTF is up-regulated by ammonia exposure, through mediation of p38 MAPK activation in astrocytes. We also observed that SAPK/JNK and Erk1/2 activations in oligodendrocytes and neurons, respectively, also play indirect roles in CNTF synthesis by astrocytes. Co-treatment with exogenous CNTF demonstrated strong protective effects on oligodendrocytes, but not on neurons, against ammonia toxicity. These protective effects involved JAK/STAT, SAPK/JNK and c-jun proteins.

Lysophosphatidic acid can support the formation of membranous structures and an increase in MBP mRNA levels in differentiating oligodendrocytes

During development, differentiating oligodendrocytes progress in distinct maturation steps from premyelinating to myelinating cells. Such maturing oligodendrocytes express both the receptors mediating signaling via extracellular lysophosphatidic acid (LPA) and the major enzyme generating extracellular LPA, namely phosphodiesterase-Ialpha/autotaxin (PD-Ialpha/ATX). However, the biological role of extracellular LPA during the maturation of differentiating oligodendrocytes is currently unclear. Here, we demonstrate that application of exogenous LPA induced an increase in the area occupied by the oligodendrocytes' process network, but only when PD-Ialpha/ATX expression was down-regulated. This increase in network area was caused primarily by the formation of membranous structures. In addition, LPA increased the number of cells positive for myelin basic protein (MBP). This effect was associated by an increase in the mRNA levels coding for MBP but not myelin oligodendrocyte glycoprotein (MOG). Taken together, these data suggest that LPA may play a crucial role in regulating the later stages of oligodendrocyte maturation.

Effect of manipulation of iron storage, transport, or availability on myelin composition and brain iron content in three different animal models

Several observations suggest that iron is an essential factor in myelination and oligodendrocyte biology. However, the specific role of iron in these processes remains to be elucidated. This role could be as an essential cofactor in metabolic processes or as a transcriptional or translational regulator. In this study, we used animals models each with a unique defect in iron availability, storage, or transfer to test the hypothesis that disruptions in these mechanisms affect myelinogenesis and myelin composition. Disruption of iron availability either by limiting dietary iron or by altering iron storage capacity resulted in a decrease in myelin proteins and lipids but not the iron content of myelin. Among the integral myelin proteins, proteolipid protein was most consistently affected, suggesting that limiting iron to oligodendrocytes results not only in hypomyelination but also in a decrease in myelin compaction. Mice deficient in transferrin must receive transferrin injections beginning at birth to remain viable, and these mice had increases in all of the myelin components and in the iron content of the myelin. This finding indicates that the loss of endogenous iron mobility in oligodendrocytes could be overcome by application of exogenous transferrin. Overall, the results of this study demonstrate how myelin composition can be affected by loss of iron homeostasis and reveal specific chronic changes in myelin composition that may affect behavior and attempts to rescue myelin deficits.

NMR and molecular dynamics studies of an autoimmune myelin basic protein peptide and its antagonist: structural implications for the MHC II (I-Au)-peptide complex from docking calculations

Experimental autoimmune encephalomyelitis can be induced in susceptible animals by immunodominant determinants of myelin basic protein (MBP). To characterize the molecular features of antigenic sites important for designing experimental autoimmune encephalomyelitis suppressing molecules, we report structural studies, based on NMR experimental data in conjunction with molecular dynamic simulations, of the potent linear dodecapeptide epitope of guinea pig MBP, Gln74-Lys75-Ser76-Gln77-Arg78-Ser79-Gln80-Asp81-Glu82-Asn83-Pro84-Val85 [MBP(74-85)], and its antagonist analogue Ala81MBP(74-85). The two peptides were studied in both water and Me(2)SO in order to mimic solvent-dependent structural changes in MBP. The agonist MBP(74-85) adopts a compact conformation because of electrostatic interactions of Arg78 with the side chains of Asp81 and Glu82. Arg78 is 'locked' in a well-defined conformation, perpendicular to the peptide backbone which is practically solvent independent. These electrostatic interactions are, however, absent from the antagonist Ala81MBP(74-85), resulting in great flexibility of the side chain of Arg78. Sequence alignment of the two analogues with several species of MBP suggests a critical role for the positively charged residue Arg78, firstly, in the stabilization of the local microdomains (epitopes) of the integral protein, and secondly, in a number of post-translational modifications relevant to multiple sclerosis, such as the conversion of charged arginine residues to uncharged citrullines. Flexible docking calculations on the binding of the MBP(74-85) antigen to the MHC class II receptor site I-A(u) using haddock indicate that Gln74, Ser76 and Ser79 are MHC II anchor residues. Lys75, Arg78 and Asp81 are prominent, solvent-exposed residues and, thus, may be of importance in the formation of the trimolecular T-cell receptor-MBP(74-85)-MHC II complex.

Small angle x-ray scattering from lipid-bound myelin basic protein in solution

The structure of myelin basic protein (MBP), purified from the myelin sheath in both lipid-free (LF-MBP) and lipid-bound (LB-MBP) forms, was investigated in solution by small angle x-ray scattering. The water-soluble LF-MBP, extracted at pH < 3.0 from defatted brain, is the classical preparation of MBP, commonly regarded as an intrinsically unfolded protein. LB-MBP is a lipoprotein-detergent complex extracted from myelin with its native lipidic environment at pH > 7.0. Under all conditions, the scattering from the two protein forms was different, indicating different molecular shapes. For the LB-MBP, well-defined scattering curves were obtained, suggesting that the protein had a unique, compact (but not globular) structure. Furthermore, these data were compatible with earlier results from molecular modeling calculations on the MBP structure which have been refined by us. In contrast, the LF-MBP data were in accordance with the expected open-coil conformation. The results represent the first direct structural information from x-ray scattering measurements on MBP in its native lipidic environment in solution.

Identification and characterisation of a cDNA encoding a 17-kDa isoform of rat myelin basic protein

The myelin basic proteins (MBPs) are the major proteins of the myelin membrane. Multiple MBP mRNAs and protein isoforms are generated by alternative RNA splicing. Here we describe the isolation and characterisation of a cDNA clone encoding a 17-kDa MBP isoform from the rat (Rattus norvegicus). The isoform is a 158-amino acid protein consisting of exons 1, 3, 4, 6 and 7 of the MBP gene. RT-PCR analysis of brain mRNA showed that transcripts encoding the 17-kDa isoform were expressed at higher levels early in post-natal development, up to 7 days post-partum.

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.

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.

Expression and properties of the recombinant murine Golli-myelin basic protein isoform J37

A recombinant form of the murine Golli-myelin basic protein (MBP) isoform J37 (rmJ37) has been expressed in Escherichia coli and isolated to 95% purity via metal chelation and ion exchange chromatography. The protein did not aggregate lipid vesicles containing acidic phospholipids, unlike the 18.5 kDa isoform of MBP. This result is consistent with J37 having a functional role prior to the assembly of compact myelin. Circular dichroic spectroscopy showed that rmJ37 had a large proportion of random coil in aqueous solution but gained alpha-helix and beta-sheet in the presence of monosialoganglioside G(M1) and PI(4)P. Thus, like "classic" MBP, J37 is intrinsically unstructured, and its conformation depends on its environment and bound ligands. Analyses of the amino acid sequence of rmJ37 predicted an N-terminal calmodulin (CaM)-binding site. It was determined via a gel-shift assay and fluorescence spectroscopy that rmJ37 and CaM interacted in a 1:1 ratio in a Ca(2+)-dependent manner. However, the interaction was weak compared with 18.5 kDa MBP.

Interactions of the 18.5 kDa isoform of myelin basic protein with Ca2+-calmodulin: in vitro studies using gel shift assays

The interactions of the 18.5 kDa isoform of myelin basic protein (MBP) with calmodulin (CaM) in vitro have been investigated using glutaraldehyde or dithiobis[succinimidylpropionate] (DSP) cross-linking, and SDS-polyacrylamide gel electrophoresis. The following forms of MBP were used: the natural bovine C1 charge isomer (bMBP/C1) and a recombinant murine product (rmMBP), and their fragments generated by digestion with cathepsin D (EC 3.4.23.5). In physiological buffers (10 mM HEPES-NaOH, pH 7.4, 5 mM CaCl2, 0.0035% glutaraldehyde; or 50 mM HEPES-NaOH, pH 7.4, 100 mM NaCl, 1 mM CaCl2, 0.0035% DSP), MBP and CaM interacted primarily in a 1:1 molar ratio, consistent with previous studies that used 6 M urea, i.e. denaturing conditions. Moreover, the appearance of higher-order bands (not previously observed) suggested that the mechanism of interaction of the two proteins involved a series of relatively complex equilibria, resulting in 2:1 ratios of MBP to CaM. This observation would explain the cooperativity of association inferred from fluorescence studies [13]. Our results demonstrated further that the interaction involved the C-terminal domain of MBP, again in a primarily 1:1 molar ratio with CaM, consistent with our identification of a CaM-binding motif at the C-terminus.

Decreased GFAP-mRNA expression in spinal cord of cobalamin-deficient rats

We have demonstrated previously that chronic vitamin B12 [cobalamin (Cbl)] deficiency preferentially affects glial cells in the rat central nervous system (CNS) and severely affects peripheral glial cells independently of and concomitantly with the central neuropathy. In this study, we determined the mRNA levels for myelin basic protein (MBP) and glial fibrillary acidic protein (GFAP) in different CNS areas of rats made Cbl-deficient by total gastrectomy, as well as the mRNA levels for glycoprotein Po and peripheral myelin protein (PMP)22 in the sciatic nerve. GFAP-mRNA levels were significantly decreased in the spinal cord (SC) and hypothalamus, but not in the cortex, hippocampus, or striatum of totally gastrectomized (TGX) rats. No differences in GFAP protein levels were found in the SC and hypothalamus of the TGX rats treated or not with Cbl. MBP-mRNA levels were significantly decreased only in the hypothalamus, and the levels of mRNA for both glial markers returned to normal with Cbl replacement therapy. The levels of mRNA for the various myelin proteins in the sciatic nerve were not modified by Cbl deficiency. These results demonstrate that: a) the neurotrophic action of Cbl in rat CNS occurs in a zonal manner; and b) Cbl deficiency does not affect myelin synthesis (with the sole exception of the hypothalamus).

Two separate domains in the golli myelin basic proteins are responsible for nuclear targeting and process extension in transfected cells

The golli products of the myelin basic protein (MBP) gene are expressed in neurons and oligodendrocytes (OLs). In certain neuronal populations, golli proteins undergo translocation between the nucleus and cytoplasm/processes during development. The proteins consist of two domains, a golli domain of 133 amino acids and an MBP domain of variable length. One objective of this study was to identify the sequences responsible for nuclear targeting. Site-directed mutagenesis and deletion analyses were used to generate a series of golli-green fluorescent protein (GFP) DNA constructs that were transfected into OL and neuronal cell lines to follow localization by confocal microscopy. The results indicated that a 36-residue stretch in the MBP domain is essential for nuclear targeting, and the sequence appears to be a nontraditional localization signal motif. The studies also revealed that overexpression of golli proteins could induce dramatic changes in cell morphology. In OL lines, overexpression of intact golli proteins, or golli peptide alone, caused an increase in the length and number of processes, and the elaboration of membrane sheets. In the neuronal lines, there was a dramatic increase in number and length of extensions. The results, consistent with the timing of golli expression in cells during neural development, suggest that golli proteins may be involved in process formation/extension in OLs and neurons during development. These studies have defined two functional domains in the golli protein. Sequences in the MBP domain target the protein into the nucleus and sequences within the golli domain induce process sheet extension in OLs and neurons.

Effects of the osmolyte trimethylamine-N-oxide on conformation, self-association, and two-dimensional crystallization of myelin basic protein

The osmolyte trimethylamine-N-oxide (TMAO) is a naturally in vivo occurring "chemical chaperone" that has been shown to stabilise the folding of numerous proteins. Myelin basic protein (MBP) is a molecule that has not yet been suitably crystallized either in three dimensions for X-ray crystallography or in two dimensions for electron crystallography. Here, we describe lipid monolayer crystallization experiments of two species of recombinant murine MBP in the presence of TMAO. One protein was unmodified, whereas the other contained six Arg/Lys-->Gln substitutions to mimic the effects of deimination (i.e., the enzymatic modification of Arg to citrulline), which reduces the net positive charge. Planar arrays of both proteins were formed on binary lipid monolayers containing a nickel-chelating lipid and a phosphoinositide. In the presence of TMAO, the diffraction spots of these arrays became sharper and more distinct than in its absence, indicating some improvement of crystallinity. The osmolyte also induced the formation of epitaxial growth of protein arrays, especially with the mutant protein. However, none of these assemblies was sufficiently ordered to extract high-resolution structural information. Circular dichroic spectroscopy showed that MBP gained no increase in ordered secondary structure in the presence of TMAO in bulk solution, whereas it did in the presence of lipids. Dynamic light-scattering experiments confirmed that the MBP preparations were monomodal under the optimal crystallization conditions determined by electron microscopy trials. The salt and osmolyte concentrations used were shown to result in a largely unassociated population of MBP. The amino acid composition of MBP overwhelmingly favours a disordered state, and a neural-network-based scheme predicted large segments that would be unlikely to adopt a regular conformation. Thus, this protein has an inherently disordered nature, which mitigates strongly against its crystallization for high-resolution structure determination.

An Arg/Lys-->Gln mutant of recombinant murine myelin basic protein as a mimic of the deiminated form implicated in multiple sclerosis

The degree of post-translational enzymatic deimination (conversion of arginyl to citrullinyl residues) of myelin basic protein (MBP) is correlated with the severity of the human autoimmune disease multiple sclerosis (MS). It is difficult to obtain large quantities of deiminated MBP from natural sources (autopsy material), and in vitro deimination using peptidylarginine deiminase (EC 3.5.3.15) is both non-specific and irreproducible. Since there is no known codon for citrulline, we have constructed a mutant form of recombinant murine MBP (rmMBP) in which 5 Arg and 1 Lys residues have been replaced by Gln as the most reasonable analogue of Cit. The residues were chosen to correspond to the 6 Arg residues in human MBP which are most commonly deiminated in chronic MS. The mutant species, rmMBP-qCit(6) where the "q" represents "quasi-," was probed by numerous biochemical and biophysical techniques. Highly homogeneous protein preparations were obtained using a modified expression system which minimised spurious misincorporation of Lys for Arg, as ascertained by electrospray ionisation mass spectrometry. The mutant form rmMBP-qCit(6) had a reduced ability to aggregate lipid vesicles, a slightly greater susceptibility to digestion by cathepsin D, a greater proportion of random secondary structure, and different conformational responses to lipids, compared with the unmodified rmMBP. Overall, the mutant protein's properties were consistent with the effects of deimination and support its use as a model for evaluating the effects of this modification.

The effects of deimination of myelin basic protein on structures formed by its interaction with phosphoinositide-containing lipid monolayers

The recombinant 18.5-kDa charge isoform of murine myelin basic protein (rmMBP) is unmodified posttranslationally and was used to study the effects of deimination, i.e., the conversion of arginyl to citrullinyl residues, on the protein's interactions with itself and with lipids. The unmodified species rmMBP-Cit(0) (i.e., containing no citrullinyl residues) interacted with binary monolayers containing acidic (phosphatidylinositol) and nickel-chelating lipids to form paracrystalline arrays with 4.8-nm spacing. A sample of protein was deiminated to an average of 9 citrullinyl residues per molecule of protein, yielding rmMBP-Cit(9). Under both low- and high-salt conditions, this species formed better-ordered domains than rmMBP-Cit(0), viz., planar crystalline assemblies. Thus, deimination of MBP resulted in a significant alteration of its lipid-organizing and self-interaction properties that might be operative in myelin in vivo, especially in progression of the autoimmune disease multiple sclerosis. Comparisons of amino acid sequences indicated significant similarities of MBP with filaggrin, a protein that is deiminated in another autoimmune disease, rheumatoid arthritis, suggesting that comparable epitopes could be targeted in both pathologies. In contrast, binary lipid monolayers consisting of phosphatidylinositol-4-phosphate (or phosphatidylinositol-4,5-bisphosphate) and a nickel-chelating lipid formed helical tubular vesicular structures, which appeared to be induced and/or stabilized by rmMBP, especially in its deiminated form. Sequence comparisons with other actin- and phosphoinositide-binding proteins (vinculin, ActA, MARCKS) suggested that the carboxyl-terminal segment of MBP could form an amphipathic alpha helix and was the phosphoinositide binding site.

Characterization of a recombinant murine 18.5-kDa myelin basic protein

A recombinant hexahistidine-tagged 18.5-kDa isoform of murine myelin basic protein has been characterized biochemically and immunogenically, by mass spectrometry, by circular dichroism under various conditions (in aqueous solution, with monosialoganglioside G(M1), and in 89% 2-propanol), and by transmission electron microscopy. The preparations of this protein indicated a high degree of purity and homogeneity, with no significant posttranslational modifications. Circular dichroic spectra showed that this preparation had the same degree of secondary structure as the natural bovine 18.5-kDa isoform of myelin basic protein. Incubation of the recombinant protein with lipid monolayers containing a nickel-chelating lipid resulted in the formation of fibrous assemblies that formed paracrystals of spacings 4.8 nm between fibers and 3-4 nm along them.

Fluorescent myelin proteins provide new tools to study the myelination process

We present here a new approach which permits us to follow myelin proteins within living, actively myelinating cells. We have developed probes to study the spatial and temporal incorporation of proteins into the myelin sheath by expressing myelin proteins fused to the green fluorescent protein (GFP). GFP from the jellyfish Aequorea victoria and its derivatives, e.g., blue fluorescent protein (BFP) were used as molecular reporters to monitor the intracellular distribution of myelin proteins. Fusion proteins (14 kD myelin basic protein [MBP]-GFP, 21 kD MBP-GFP) were expressed in primary Schwann cells (SCs) and their distribution was monitored by confocal microscopy. The autofluorescent chimeric proteins were readily visualized and their subcellular localization was unaffected by the GFP reporter. However, because of the length of culturing time necessary to establish permanent cell lines, we found that it was not possible to obtain MBP-GFP stable SCs that also were capable of myelinating neuronal axons. We therefore devised a way of introducing vectors under conditions where cells are dividing in response to endogenous stimuli, and therefore are still capable of myelinating. We designed a protocol in which SCs cocultured with dorsal root ganglion (DRG) neurons are transfected while they are actively dividing. SCs transfected in this way exhibit a good level of protein expression and retain their myelinating phenotype. The fusion protein expression lasts long enough to observe "green myelin. " These fluorescently tagged myelin proteins will allow high-resolution examination of the protein and membrane traffic in normal myelinating cells.

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.

Myelin basic protein component C1 in increasing concentrations can elicit fusion, aggregation, and fragmentation of myelin-like membranes

Myelin basic protein (MBP) is considered to have a primary role in the formation and maintenance of the myelin sheath. Many studies using artificial vesicle systems of simple lipid composition, and generally small size, have shown that MBP can elicit vesicle fusion, aggregation, or even fragmentation under different conditions. Here, we have studied the effects of increasing concentrations of bovine MBP charge isomer C1 (MBP/C1) on large unilamellar vesicles (LUVs) composed of phosphatidylcholine and phosphatidylserine (92:8 molar ratio), or with a lipid composition similar to that of the myelin membrane in vivo (Cyt-LUVs). Using absorbance spectrophotometry, fluorescence resonance energy transfer, dynamic light scattering and transmission electron microscopy, we have shown that vesicle aggregation and some vesicle fusion occurred upon addition of MBP/C1, and as the molar protein-lipid ratio increased. Fragmentation of Cyt-LUVs was observed at very high protein concentrations. These results showed that the phenomena of vesicle fusion, aggregation, and fragmentation can all be observed in one in vitro system, but were dependent on lipid composition and on the relative proportions of protein and lipid.

Cryoelectron microscopy of protein-lipid complexes of human myelin basic protein charge isomers differing in degree of citrullination

Myelin basic protein (MBP) is considered to be essential for the maintenance of stability of the myelin sheath. Reduction in cationicity of MBP, especially due to conversion of positively charged arginine residues to uncharged citrulline (Cit), has been found to be associated with multiple sclerosis (MS). Here, the interactions of an anionic phosphatidylserine/monosialoganglioside-G(M1) (4:1, w:w) lipid monolayer with 18.5-kDa MBP preparations from age-matched adult humans without MS (no Cit residues), with chronic MS (6 Cit), and with acute Marburg-type MS (18 Cit) were studied by transmission and ultralow dose scanning transmission electron microscopy under cryogenic conditions. Immunogold labeling and single particle electron crystallography were used to define the nature of the complexes visualized. These electron microscopical analyses showed that the three different MBP charge isomers all formed uniformly sized and regularly shaped protein-lipid complexes with G(M1), probably as hexamers, but exhibited differential association with and organization of the lipid. The least cationic Marburg MBP-Cit(18) formed the most open protein-lipid complex. The data show a disturbance in lipid-MBP interactions at the ultrastructural level that is related to degree of citrullination, and which may be involved in myelin degeneration in multiple sclerosis.

Adhesion of acidic lipid vesicles by 21.5 kDa (recombinant) and 18.5 kDa isoforms of myelin basic protein

Myelin basic protein (MBP) is thought to be responsible for adhesion of the intracellular surfaces of compact myelin to give the major dense line. The 17 and 21.5 kDa isoforms containing exon II have been reported by others to localize to the cytoplasm and nucleus of murine oligodendrocytes and HeLa cells while the 14 and 18.5 kDa isoforms lacking exon II are confined to the plasma membrane. However, we show that the exon II(-) 18.5 kDa form and a recombinant exon II(+) 21.5 kDa isoform both caused similar aggregation of acidic lipid vesicles, indicating that they should have similar abilities to bind to the intracellular lipid surface of the plasma membrane and to cause adhesion of those surfaces to each other. The circular dichroism spectra of the two isoforms indicated that both had a similar secondary structure. Thus, both isoforms should be able to bind to and cause adhesion of the cytosolic surfaces of compact myelin. The fact that they do not could be due to differences in post-translational modification in vivo, trafficking through the cell and/or subcellular location of synthesis, but it is not due to differences in their lipid binding.

Force measurements on myelin basic protein adsorbed to mica and lipid bilayer surfaces done with the atomic force microscope

The mechanical and adhesion properties of myelin basic protein (MBP) are important for its function, namely the compaction of the myelin sheath. To get more information about these properties we used atomic force microscopy to study tip-sample interaction of mica and mixed dioleoylphosphatidylserine (DOPS) (20%)/egg phosphatidylcholine (EPC) (80%) lipid bilayer surfaces in the absence and presence of bovine MBP. On mica or DOPS/EPC bilayers a short-range repulsive force (decay length 1.0-1.3 nm) was observed during the approach. The presence of MBP always led to an attractive force between tip and sample. When retracting the tip again, force curves on mica and on lipid layers were different. While attached to the mica surface, the MBP molecules exhibited elastic stretching behavior that agreed with the worm-like chain model, yielding a persistence length of 0.5 +/- 0.25 nm and an average contour length of 53 +/- 19 nm. MBP attached to a lipid bilayer did not show elastic stretching behavior. This shows that the protein adopts a different conformation when in contact with lipids. The lipid bilayer is strongly modified by MBP attachment, indicating formation of MBP-lipid complexes and possibly disruption of the original bilayer structure.

Distribution of cells bearing the HNK-1 epitope in the human placenta

HNK-1 (Leu-7 antigen or CD57) is a unique carbohydrate moiety found in certain glycosphingolipids and several cell adhesion glycoproteins on the cell membrane. Previous studies have suggested that HNK-1 carbohydrates act as adhesive ligands in cell-cell interactions. Using a monoclonal antibody reactive to the HNK-1 moiety and an immunoperoxidase method on formalin-fixed paraffin-embedded tissue, the expression of the HNK-1 epitope in human placentae was confined to the intermediate trophoblast (IT) in trophoblastic columns. The number of HNK-1 immunoreactive IT cells increased from the proximal to the midportion of the trophoblastic column, and then disappeared at the junction of the column with the basal plate where IT infiltrates the endomyometrium and becomes extravillous IT. Neither cytotrophoblast nor syncytiotrophoblast reacted with the HNK-1 antibody. In hydatidiform moles, HNK-1 immunoreactivity was localized to areas that structurally resembled trophoblastic columns. In contrast, placental site trophoblastic tumours which do not contain structures analogous to trophoblastic columns did not express HNK-1 epitope. Expression of HNK-1 was only rarely observed in choriocarcinomas, being present in less than 5 per cent of trophoblastic cells in two of 13 cases. The murine placenta, which lacks trophoblastic columns, was negative for HNK-1. Thin-layer chromatography immunostaining demonstrated the HNK-1 reactive glycosphingolipids in placental lipid extracts, whereas Western blot analysis from placental protein extract failed to reveal detectable glycoproteins that demonstrated HNK-1 immunoreactivity. In conclusion, the specific localization of HNK-1 reactive glycosphingolipids in trophoblastic columns of the human placenta suggests that the HNK-1 moiety may play an important role in maintaining cohesion between intermediate trophoblastic cells in the trophoblastic columns thereby contributing to the structural integrity of the villi that anchor the placenta to the basal plate.

Three-dimensional structure of myelin basic protein. I. Reconstruction via angular reconstitution of randomly oriented single particles

Myelin basic protein (MBP) plays an integral role in the structure and function of the myelin sheath. In humans and cattle, an 18.5-kDa isoform of MBP predominates and exists as a multitude of charge isomers resulting from extensive and varied post-translational modifications. We have purified the least modified isomer (named C1) of the 18.5-kDa isoform of MBP from fresh bovine brain and imaged this protein as negatively stained single particles adsorbed to a lipid monolayer. Under these conditions, MBP/C1 presented diverse projections whose relative orientations were determined using an iterative quaternion-assisted angular reconstitution scheme. In different buffers, one with a low salt and the other with a high salt concentration, the conformation of the protein was slightly different. In low salt buffer, the three-dimensional reconstruction, solved to a resolution of 4 nm, had an overall "C" shape of outer radius 5.5 nm, inner radius 3 nm, overall circumference 15 nm, and height 4.7 nm. The three-dimensional reconstruction of the protein in high salt buffer, solved to a resolution of 2.8 nm, was essentially the same in terms of overall dimensions but had a somewhat more compact architecture. These results are the first structures achieved directly for this unusual macromolecule, which plays a key role in the development of multiple sclerosis.

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.