Crystal structure of human peptidylarginine deiminase type VI (PAD6) provides insights into its inactivity

Human peptidylarginine deiminase isoform VI (PAD6), which is predominantly limited to cytoplasmic lattices in the mammalian oocytes in ovarian tissue, is essential for female fertility. It belongs to the peptidylarginine deiminase (PAD) enzyme family that catalyzes the conversion of arginine residues to citrulline in proteins. In contrast to other members of the family, recombinant PAD6 was previously found to be catalytically inactive. We sought to provide structural insight into the human homologue to shed light on this observation. We report here the first crystal structure of PAD6, determined at 1.7 Å resolution. PAD6 follows the same domain organization as other structurally known PAD isoenzymes. Further structural analysis and size-exclusion chromatography show that PAD6 behaves as a homodimer similar to PAD4. Differential scanning fluorimetry suggests that PAD6 does not coordinate Ca2+ which agrees with acidic residues found to coordinate Ca2+ in other PAD homologs not being conserved in PAD6. The crystal structure of PAD6 shows similarities with the inactive state of apo PAD2, in which the active site conformation is unsuitable for catalytic citrullination. The putative active site of PAD6 adopts a non-productive conformation that would not allow protein-substrate binding due to steric hindrance with rigid secondary structure elements. This observation is further supported by the lack of activity on the histone H3 and cytokeratin 5 substrates. These findings suggest a different mechanism for enzymatic activation compared with other PADs; alternatively, PAD6 may exert a non-enzymatic function in the cytoplasmic lattice of oocytes and early embryos.

Role of Amyloidogenic and Non-Amyloidogenic Protein Spaces in Neurodegenerative Diseases and their Mitigation Using Theranostic Agents

Neurodegenerative diseases (NDDs) refer to a complex heterogeneous group of diseases which are associated with the accumulation of amyloid fibrils or plaques in the brain leading to progressive loss of neuronal functions. Alzheimer's disease is one of the major NDD responsible for 60-80 % of all dementia cases. Currently, there are no curative or disease-reversing/modifying molecules for many of the NDDs except a few such as donepezil, rivastigmine, galantamine, carbidopa and levodopa which treat the disease-associated symptoms. Similarly, there are very few FDA-approved tracers such as flortaucipir (Tauvid) for tau fibril imaging and florbetaben (Neuraceq), flutemetamol (Vizamyl), and florbetapir (Amyvid) for amyloid imaging available for diagnosis. Recent advances in the cryogenic electron microscopy reported distinctly different microstructures for tau fibrils associated with different tauopathies highlighting the possibility to develop tauopathy-specific imaging agents and therapeutics. In addition, it is important to identify the proteins that are associated with disease development and progression to know about their 3D structure to develop various diagnostics, therapeutics and theranostic agents. The current article discusses in detail the disease-associated amyloid and non-amyloid proteins along with their structural insights. We comprehensively discussed various novel proteins associated with NDDs and their implications in disease pathology. In addition, we document various emerging chemical compounds developed for diagnosis and therapy of different NDDs with special emphasis on theranostic agents for better management of NDDs.

Identifying mRNAs Residing in Myelinating Oligodendrocyte Processes as a Basis for Understanding Internode Autonomy

In elaborating and maintaining myelin sheaths on multiple axons/segments, oligodendrocytes distribute translation of some proteins, including myelin basic protein (MBP), to sites of myelin sheath assembly, or MSAS. As mRNAs located at these sites are selectively trapped in myelin vesicles during tissue homogenization, we performed a screen to identify some of these mRNAs. To confirm locations, we used real-time quantitative polymerase chain reaction (RT-qPCR), to measure mRNA levels in myelin (M) and ‘non-myelin’ pellet (P) fractions, and found that five (LPAR1, TRP53INP2, TRAK2, TPPP, and SH3GL3) of thirteen mRNAs were highly enriched in myelin (M/P), suggesting residences in MSAS. Because expression by other cell-types will increase p-values, some MSAS mRNAs might be missed. To identify non-oligodendrocyte expression, we turned to several on-line resources. Although neurons express TRP53INP2, TRAK2 and TPPP mRNAs, these expressions did not invalidate recognitions as MSAS mRNAs. However, neuronal expression likely prevented recognition of KIF1A and MAPK8IP1 mRNAs as MSAS residents and ependymal cell expression likely prevented APOD mRNA assignment to MSAS. Complementary in situ hybridization (ISH) is recommended to confirm residences of mRNAs in MSAS. As both proteins and lipids are synthesized in MSAS, understanding myelination should not only include efforts to identify proteins synthesized in MSAS, but also the lipids.

Role of Helical Structure in MBP Immunodominant Peptides for Efficient IgM Antibody Recognition in Multiple Sclerosis

The involvement of Myelin Basic Protein (MBP) in Multiple Sclerosis (MS) has been widely discussed in the literature. This intrinsically disordered protein has an interesting α-helix motif, which can be considered as a conformational epitope. In this work we investigate the importance of the helical structure in antibody recognition by MBP peptides of different lengths. Firstly, we synthesized the peptide MBP (81–106) (1) and observed that its elongation at both N- and C-termini, to obtain the peptide MBP (76–116) (2) improves IgM antibody recognition in SP-ELISA, but destabilizes the helical structure. Conversely, in competitive ELISA, MBP (81–106) (1) is recognized more efficiently by IgM antibodies than MBP (76–116) (2), possibly thanks to its more stable helical structure observed in CD and NMR conformational experiments. These results are discussed in terms of different performances of peptide antigens in the two ELISA formats tested.

The Expanding Role of Extracellular Traps in Inflammation and Autoimmunity: The New Players in Casting Dark Webs

The first description of a new form of neutrophil cell death distinct from that of apoptosis or necrosis was discovered in 2004 and coined neutrophil extracellular traps "(NETs)" or "NETosis". Different stimuli for NET formation, and pathways that drive neutrophils to commit to NETosis have been elucidated in the years that followed. Critical enzymes required for NET formation have been discovered and targeted therapeutically. NET formation is no longer restricted to neutrophils but has been discovered in other innate cells: macrophages/monocytes, mast Cells, basophils, dendritic cells, and eosinophils. Furthermore, extracellular DNA can also be extruded from both B and T cells. It has become clear that although this mechanism is thought to enhance host defense by ensnaring bacteria within large webs of DNA to increase bactericidal killing capacity, it is also injurious to innocent bystander tissue. Proteases and enzymes released from extracellular traps (ETs), injure epithelial and endothelial cells perpetuating inflammation. In the context of autoimmunity, ETs release over 70 well-known autoantigens. ETs are associated with pathology in multiple diseases: lung diseases, vasculitis, autoimmune kidney diseases, atherosclerosis, rheumatoid arthritis, cancer, and psoriasis. Defining these pathways that drive ET release will provide insight into mechanisms of pathological insult and provide potential therapeutic targets.

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.

Neutrophils: Underestimated Players in the Pathogenesis of Multiple Sclerosis (MS)

Neutrophils are the most abundant circulating and first-responding innate myeloid cells and have so far been underestimated in the context of multiple sclerosis (MS). MS is the most frequent, immune-mediated, inflammatory disease of the central nervous system. MS is treatable but not curable and its cause(s) and pathogenesis remain elusive. The involvement of neutrophils in MS pathogenesis has been suggested by the use of preclinical animal disease models, as well as on the basis of patient sample analysis. In this review, we provide an overview of the possible mechanisms and functions by which neutrophils may contribute to the development and pathology of MS. Neutrophils display a broad variety of effector functions enabling disease pathogenesis, including (1) the release of inflammatory mediators and enzymes, such as interleukin-1β, myeloperoxidase and various proteinases, (2) destruction and phagocytosis of myelin (as debris), (3) release of neutrophil extracellular traps, (4) production of reactive oxygen species, (5) breakdown of the blood–brain barrier and (6) generation and presentation of autoantigens. An important question relates to the issue of whether neutrophils exhibit a predominantly proinflammatory function or are also implicated in the resolution of chronic inflammatory responses in MS.

Myelin basic protein (MBP) charge variants show different sphingomyelin-mediated interactions with myelin-like lipid monolayers

Multiple sclerosis (MS) is correlated with increased deimination of myelin basic protein (MBP) in the central nervous system. Here, the interaction of MBP C1 (charge: +19) and MBP C8 (charge: +13) with the major lipids of the cytoplasmic side of the oligodendrocyte membrane is analysed using monolayer adsorption experiments and epifluorescence microscopy. Our findings show that the electrostatic attraction between the positively charged proteins and negatively charged lipids in the myelin-like monolayers competes with the incorporation of MBP into regions directly bordering cholesterol-rich domains. The latter is favoured to avoid additional lipid condensation and reduction in fluidity of the phospholipid layer. We find that MBP C1 does not incorporate at the cholesterol-rich domains if sphingomyelin (SM) is absent from the lipid composition. In contrast, MBP C8 is still incorporated near cholesterol-enriched regions without SM. Thus, the highly charged C1 variant needs a specific interaction with SM, whereas for C8 the incorporation at the cholesterol-rich regions is ensured due to its reduced net positive charge. This phenomenon may be relevant for the correlation of higher amounts of MBP C8 in brains of adult MS patients and healthy children, in which the amount of SM is reduced compared to healthy adults.

An Overview of the Intrinsic Role of Citrullination in Autoimmune Disorders

A protein undergoes many types of posttranslation modification. Citrullination is one of these modifications, where an arginine amino acid is converted to a citrulline amino acid. This process depends on catalytic enzymes such as peptidylarginine deiminase enzymes (PADs). This modification leads to a charge shift, which affects the protein structure, protein-protein interactions, and hydrogen bond formation, and it may cause protein denaturation. The irreversible citrullination reaction is not limited to a specific protein, cell, or tissue. It can target a wide range of proteins in the cell membrane, cytoplasm, nucleus, and mitochondria. Citrullination is a normal reaction during cell death. Apoptosis is normally accompanied with a clearance process via scavenger cells. A defect in the clearance system either in terms of efficiency or capacity may occur due to massive cell death, which may result in the accumulation and leakage of PAD enzymes and the citrullinated peptide from the necrotized cell which could be recognized by the immune system, where the immunological tolerance will be avoided and the autoimmune disorders will be subsequently triggered. The induction of autoimmune responses, autoantibody production, and cytokines involved in the major autoimmune diseases will be discussed.

Molecular Interplay between the Dimer Interface and the Substrate-Binding Site of Human Peptidylarginine Deiminase 4

Our previous studies suggest that the fully active form of Peptidylarginine deiminase 4 (PAD4) should be a dimer and not a monomer. This paper provides a plausible mechanism for the control of PAD4 catalysis by molecular interplay between its dimer-interface loop (I-loop) and its substrate-binding loop (S-loop). Mutagenesis studies revealed that two hydrophobic residues, W347 and V469, are critical for substrate binding at the active site; mutating these two residues led to a severe reduction in the catalytic activity. We also identified several hydrophobic amino acid residues (L6, L279 and V283) at the dimer interface. Ultracentrifugation analysis revealed that interruption of the hydrophobicity of this region decreases dimer formation and, consequently, enzyme activity. Molecular dynamic simulations and mutagenesis studies suggested that the dimer interface and the substrate-binding site of PAD4, which consist of the I-loop and the S-loop, respectively, are responsible for substrate binding and dimer stabilization. We identified five residues with crucial roles in PAD4 catalysis and dimerization: Y435 and R441 in the I-loop, D465 and V469 in the S-loop, and W548, which stabilizes the I-loop via van der Waals interactions with C434 and Y435. The molecular interplay between the S-loop and the I-loop is crucial for PAD4 catalysis.

Human Deiminases: Isoforms, Substrate Specificities, Kinetics, and Detection

Peptidylarginine deiminase (PAD) enzymes are of enormous interest in biomedicine. They catalyze the conversion of a positively-charged guanidinium at an arginine side chain into a neutral ureido group. As a result of this conversion, proteins acquire the non-ribosomally encoded amino acid "citrulline". This imposes critical influences on the structure and function of the target molecules. In multiple sclerosis, myelin hyper-citrullination promotes demyelination by reducing its compaction and triggers auto-antibody production. Immune responses to citrulline-containing proteins play a central role in the pathogenesis of autoimmune diseases. Moreover, auto-antibodies, specific to citrullinated proteins, such as collagen type I and II and filaggrin, are early detectable in rheumatoid arthritis, serving as diagnostic markers of the disease. Despite their significance, little is understood about the role in demyelinating disorders, diversified cancers, and auto-immune diseases. To impart their biological and pathological effects, it is crucial to better understand the reaction mechanism, kinetic properties, substrate selection, and specificities of peptidylarginine deiminase isoforms.Many aspects of PAD biochemistry and physiology have been ignored in past, but, herein is presented a comprehensive survey to improve our current understandings of the underlying mechanism and regulation of PAD enzymes.

Myelin Basic Protein Citrullination in Multiple Sclerosis: A Potential Therapeutic Target for the Pathology

Multiple sclerosis (MS) is a multifactorial demyelinating disease characterized by neurodegenerative events and autoimmune response against myelin component. Citrullination or deimination, a post-translational modification of protein-bound arginine into citrulline, catalyzed by Ca(2+) dependent peptidylarginine deiminase enzyme (PAD), plays an essential role in physiological processes include gene expression regulation, apoptosis and the plasticity of the central nervous system, while aberrant citrullination can generate new epitopes, thus involving in the initiation and/or progression of autoimmune disorder like MS. Myelin basic protein (MBP) is the major myelin protein and is generally considered to maintain the stability of the myelin sheath. This review describes the MBP citrullination and its consequence, as well as offering further support for the "inside-out" hypothesis that MS is primarily a neurodegenerative disease with secondary inflammatory demyelination. In addition, it discusses the role of MBP citrullination in the immune inflammation and explores the potential of inhibition of PAD enzymes as a therapeutic strategy for the disease.

MyelStones: the executive roles of myelin basic protein in myelin assembly and destabilization in multiple sclerosis

The classic isoforms of myelin basic protein (MBP, 14–21.5 kDa) are essential to formation of the multilamellar myelin sheath of the mammalian central nervous system (CNS). The predominant 18.5-kDa isoform links together the cytosolic surfaces of oligodendrocytes, but additionally participates in cytoskeletal turnover and membrane extension, Fyn-mediated signalling pathways, sequestration of phosphoinositides and maintenance of calcium homoeostasis. All MBP isoforms are intrinsically disordered proteins (IDPs) that interact via molecular recognition fragments (MoRFs), which thereby undergo local disorder-to-order transitions. Their conformations and associations are modulated by environment and by a dynamic barcode of post-translational modifications, particularly phosphorylation by mitogen-activated and other protein kinases and deimination [a hallmark of demyelination in multiple sclerosis (MS)]. The MBPs are thus to myelin what basic histones are to chromatin. Originally thought to be merely structural proteins forming an inert spool, histones are now known to be dynamic entities involved in epigenetic regulation and diseases such as cancer. Analogously, the MBPs are not mere adhesives of compact myelin, but active participants in oligodendrocyte proliferation and in membrane process extension and stabilization during myelinogenesis. A central segment of these proteins is pivotal in membrane-anchoring and SH3 domain (Src homology 3) interaction. We discuss in the present review advances in our understanding of conformational conversions of this classic basic protein upon membrane association, including new thermodynamic analyses of transitions into different structural ensembles and how a shift in the pattern of its post-translational modifications is associated with the pathogenesis and potentially onset of demyelination in MS.

Peptidylarginine deiminases in citrullination, gene regulation, health and pathogenesis

Peptidylarginine deiminases are a family of enzymes that mediate post-translational modifications of protein arginine residues by deimination or demethylimination to produce citrulline. In vitro, the activity of PADs is dependent on calcium and reductive reagents carrying a free sulfhydryl group. The discovery that PAD4 can target both arginine and methyl-arginine for citrullination about 10years ago renewed our interest in studying this family of enzymes in gene regulation and their physiological functions. The deregulation of PADs is involved in the etiology of multiple human diseases, including cancers and autoimmune disorders. There is a growing effort to develop isoform specific PAD inhibitors for disease treatment. However, the regulation of the activity of PADs in vivo remains largely elusive, and we expect that much will be learned about the role of these enzymes in a normal life cycle and under pathology conditions.

Adhesive properties and inflammatory potential of citrullinated myelin basic protein peptide 45-89

Deimination of arginyl residue of myelin basic protein (MBP) reduces cationicity of MBP and impedes the normal myelin membrane assembly. Less ordered structure of MBP is more susceptible to proteolytic attack that may lead to the release of highly immunogenic deiminated peptides into extracellular milieu. We have studied the association of peptides 45-89 derived from citrullinated MBP (C8 isomer) and phosphorylated MBP (C3 isomer) with the myelin lipids in a model membrane system using optical waveguide lightmode spectrometry. The analysis of association/dissociation kinetics to planar lipids under controlled hydrodynamic conditions has shown that MBP 45-89 peptide from citrullinated C8 isomer is less effectively adsorbed on the lipid membrane, than peptide from phosphorylated C3 isomer and packing densities for phosphorylated 45-89 MBP peptide is higher than for citrullinated forms. On the other hand, our results shown that continuous (24 h) exposure of mixed oligodendrocyte/microglial cells to peptides 45-89 from MBP-C8 induces apoptosis via mitochondrial pathway. In addition, peptides 45-89 stimulated the secretion of nitric oxide from microglial cells via induction of iNOS and decreased the level of the inhibitory protein IkB, indicating involvement of the transcription factor NF-kB in these processes. Our results suggest that some citrullinated peptides, initially released from oligodendrocytes, might activate microglia, which produces reactive nitrogen species and generates in turn fatal feedbacks that kill oligodendrocytes.

Citrullination of CNS proteins in the pathogenesis of multiple sclerosis

Multiple sclerosis is a chronic immune-mediated disease of the CNS. Although it is a predominantly T-cell mediated condition, B cells and autoreactive antibodies play an important role in its pathogenesis, with the presence of oligoclonal immunoglobulins in the cerebrospinal fluid being an important diagnostic indicator. The target of these immunoglobulins has not yet been fully characterized. However, post-translational modifications of CNS-specific proteins are thought to contribute to their production through the generation of novel epitopes. One post-translational modification in particular, the conversion of the amino acid arginine to the nonstandard amino acid, citrulline, has been increasingly described in the literature as a factor in the pathogenesis of this condition. In this article, we summarize and discuss the current knowledge on citrullination in multiple sclerosis, the importance of this in relation to its pathogenesis and, potentially, its diagnosis.

Peptidyl arginine deiminase: A novel immunohistochemical marker for liver fibrosis in patients with chronic hepatitis

Peptidylarginine deiminase (PAD) is an enzyme known to be involved in the pathogenesis of rheumatoid arthritis (RA). Since many of the molecular events present in the joints in RA also take place in the injured liver, we postulated in this study that PAD may be involved in liver fibrosis. The objectives of this study therefore were to find out if PAD could be demonstrated immunohistochemically in liver biopsies of patients with chronic hepatitis and if it is associated with METAVIR activity and fibrosis scores. Liver biopsies were obtained from 100 patients with chronic liver diseases between September 2006 and 2007. The biopsies were scored by two histopathologists according to the METAVIR activity and fibrosis scores after histological preparation. Immunohistochemistry for PAD was performed on the biopsies using a monoclonal antibody against PAD. PAD could not be demonstrated in normal liver biopsies but was found in the hepatocytes of patients with chronic hepatitis. PAD labeling could distinguish patients with no fibrosis from either F1 or F2 or F3 or F4 fibrosis. Similarly, PAD labeling could separate patients with no inflammatory activity from those with mild or moderate or severe activity. We concluded that PAD could be demonstrated immunohistochemically in liver biopsies of patients with chronic hepatitis and that its immunodetection was significantly associated with Metavir activity and fibrosis scores.

Myelin basic protein and myelin protein 2 act synergistically to cause stacking of lipid bilayers

Saltatory conduction of nerve impulses along axonal membranes depends on the presence of a multilayered membrane, myelin, that wraps around the axon. Myelin basic protein (MBP) and myelin protein 2 (P2) are intimately involved in the generation of the myelin sheath. They are also implicated in a number of neurological diseases, including autoimmune diseases of both the central and peripheral nervous systems. Here, we have used atomic force microsopy (AFM) to study the effects of MBP and P2 on lipid bilayers. MBP in association with a mica substrate appeared unstructured, and tended to coat the mica surface in the form of a monolayer. In contrast, P2 appeared as discrete particles, with molecular volumes consistent with the formation of both monomers and dimers. Either MBP or P2, at micromolar concentrations, caused stacking of brain lipid bilayers. This stacking effect was significantly potentiated when both proteins were added together. Bilayers composed of phosphatidylcholine (PC) and phosphatidylserine (PS) were stacked by MBP, provided that cholesterol was also present; in contrast, P2 did not stack PC/PS/cholesterol bilayers. Hence, the bilayer stacking effects of the two proteins have different lipid requirements.

Effect of enzymatic deimination on the conformation of recombinant prion protein

Deimination is the post-translational conversion of arginine residues to citrulline. It has been implicated as a causative factor in autoimmune diseases such as multiple sclerosis and rheumatoid arthritis and more recently, as a marker of neurodegeneration. We have investigated the effect of the post-translational modification of arginine residues on the structure of recombinant ovine prion protein. Deiminated prion protein exhibited biophysical properties characteristic of the scrapie-associated conformer of prion protein viz. an increased beta-sheet secondary structure, congophilic structures indicative of amyloid and proteinase K resistance which could be templated onto normal unmodified prion protein. In the light of these findings, a potential role of post-translational modifications to prion protein in disease initiation or propagation is discussed.

Temperature-sensitive nonionic vesicles prepared from Span 80 (sorbitan monooleate)

Different types of nonionic vesicles were prepared from commercial Span 80 (also called sorbitan monooleate), as an inexpensive, biocompatible alternative to conventional phospholipid-based vesicles (liposomes). The vesicles were characterized by different techniques and comparison was made with vesicles formed from POPC (1-palmitoyl-2-oleoyl- sn-glycero-3-phosphocholine) or DOPC (1,2-dioleoyl- sn-glycero-3-phosphocholine). Dynamic light scattering measurements, electron microscopy analyses, and two types of fusion assays indicate that Span 80 vesicles are stable for at least 7 days at 4 or 25 degrees C, while storage at 42 degrees C causes irreversible vesicle fusion. This indicates that Span 80 vesicles are thermoresponsive with vesicle fusion occurring at elevated temperature. This property may be related to headgroup dehydration and is certainly not directly linked to the phase transition temperature (Tm) of the vesicles, since the Tm is below -30 degrees C, as determined by differential scanning calorimetry (DSC). The measured Tm value for Span 80 vesicles is lower than in the case of DOPC or POPC, correlating with a higher fluidity of Span 80 vesicles as compared to POPC or DOPC vesicles, as determined with DPH (1,6-diphenyl-1,3,5-hexatriene) as fluorescent membrane probe. High fluidity correlates with increased leakage of entrapped water-soluble dye molecules. Addition of cholesterol and soybean phosphatidylcholine lowers the extent of leakage, allowing a tuning of the bilayer permeability.

Mast cell activation by myelin through scavenger receptor

A role for mast cells (MC) in the pathogenesis of multiple sclerosis (MS) has been suggested, based on the analysis of human lesions and on an animal model of the disease (EAE). What role MC play in the development of MS is not well understood. We hypothesized that the link connecting MC with demyelinating diseases may be represented by their interaction with myelin. Here we show that myelin can activate mast cells. This process could be a key event in the mast cell function required for inducing EAE in mice and possibly in MS in man.

Peptidyl argininedeiminase 2 CpG island in multiple sclerosis white matter is hypomethylated

In previous studies, we documented increased citrullinated myelin basic protein (MBP) was present in MBP isolated from multiple sclerosis (MS) normal appearing white matter (NAWM). This increase was due to the myelin enzyme peptidyl argininedeiminase 2 (PAD2). In this study, we show that methylation of cytosine of the PAD2 promoter in DNA from MS NAWM was decreased to one-third of the level of that in DNA from normal white matter. The PAD2 promoter in DNA from thymus obtained from the same MS patients and white matter DNA from Alzheimer's, Huntington's, and Parkinson's was not hypomethylated. DNA demethylase activity in supernatants prepared from NAWM of MS patients was 2-fold higher than the DNA demethylase from normal, Alzheimer's, Huntington's and Parkinson's disease white matter. The amount of PAD2 enzyme and citrullinated MBP was increased in MS NAWM. The decreased methylation of cytosines in the PAD2 promoter may explain the increased synthesis of PAD2 protein that is responsible for the increased amount of citrullinated MBP, which in turn results in loss of myelin stability in MS brain.

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.

The role of citrullinated proteins suggests a novel mechanism in the pathogenesis of multiple sclerosis

The pathogenesis of MS is unknown. In our studies, we have demonstrated an important role for citrullinated myelin basic protein (MBP). The accompanying loss of positive charge compromises the ability of MBP to interact with the lipid bilayer. The conversion of arginine to citrulline in brain is carried out by an enzyme peptidyl arginine deiminase (PAD) 2. The amount of PAD 2 in brain was increased in MS normal-appearing white matter. The mechanism responsible for this increase involved hypomethylation of the promoter region in the PAD 2 gene in MS, but no change (compared to normal) was found in thymus tissue DNA from the same MS patients. In addition, no change was observed in other neurological diseases, including Alzheimer’s, Parkinson’s, and Huntington’s. We propose that citrullinated MBP, resulting from elevated levels of PAD 2 represents an important biochemical pathway in the pathogenesis of MS.

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.

Citrullination: a posttranslational modification in health and disease

Posttranslational modifications are chemical changes to proteins that take place after synthesis. One such modification, peptidylarginine to peptidylcitrulline conversion, catalysed by peptidylarginine deiminases, has recently received significant interest in biomedicine. Introduction of citrulline dramatically changes the structure and function of proteins. It has been implicated in several physiological and pathological processes. Physiological processes include epithelial terminal differentiation, gene expression regulation, and apoptosis. Rheumatoid arthritis, multiple sclerosis, and Alzheimer's disease are examples of human diseases where protein citrullination involvement has been demonstrated. In this review, we discuss our current understanding on the importance of protein deimination in these processes. We describe the enzymes catalyzing the reaction, as well as their known protein substrates. We review the citrullinated peptide epitopes that are proposed as disease markers, specifically recognized in certain human autoimmune disorders. The potential autopathogenic role of citrullinated epitopes is also discussed.

Deimination of membrane-bound myelin basic protein in multiple sclerosis exposes an immunodominant epitope

The degradation of myelin in the CNS is the hallmark of multiple sclerosis. Reduction in the net positive charge of myelin basic protein (MBP), through deimination, correlates strongly with disease severity and may mediate myelin instability and loss of compaction. Using Cys scanning, spin labeling, EPR spectroscopy, and site-specific proteolysis, we show that in the membrane-bound state the primary immunodominant epitope, V83-T92, of the less cationic recombinant murine MBP C8 mimic (rmC8) forms a more highly surface-exposed and shorter amphipathic alpha-helix than in the unmodified form, recombinant murine MBP C1 mimic (rmC1), analogous to the most cationic and abundant isomer of MBP in normal myelin. Moreover, cathepsin D digested lipid-associated rmC8 3-fold faster than rmC1, and cleavage at F86-F87 occurred more readily in rmC8 than rmC1. These findings suggest a mechanism for initial loss of myelin stability and the autoimmune pathogenesis of multiple sclerosis.

Interaction of native Bowman-Birk soybean protease inhibitor and its hydrophobized derivative with multilamellar vesicles of soybean phospholipids

The interaction of native Bowman-Birk soybean protease inhibitor (BBI) and its hydrophobized derivative with multilamellar vesicles of various soybean phospholipids was investigated. Decrease in pH and introduction of negatively charged components to the lipid mixture increased BBI content in the protein-lipid complex. This suggests a contribution of electrostatic forces in the protein-lipid interaction. Protein hydrophobization insignificantly influenced BBI binding to lipids. In the complex with lipids, both proteins (BBI and its hydrophobized derivative) retained high anti-chymotrypsin activity (75-100%), which was not influenced by the presence of the ionic detergent sodium deoxycholate.

Experimental autoimmune encephalomyelitis induction in peptidylarginine deiminase 2 knockout mice

During the development of multiple sclerosis the destruction of the myelin sheath surrounding the neurites is accompanied by citrullination of several central nervous system (CNS) proteins, including myelin basic protein and glial fibrillary acidic protein. In experimental autoimmune encephalomyelitis (EAE), a disease induced in animals by immunization with proteins or peptides from the CNS, the animals develop symptoms similar to multiple sclerosis (MS). The increased levels of citrullinated CNS proteins associated with MS are also observed during the development of EAE. To study the role of CNS protein citrullination in EAE development, we induced EAE with a peptide derived from myelin oligodendrocyte glycoprotein (MOG(35-55)) in mice lacking the peptidylarginine deiminase 2 (PAD2) protein, because this enzyme was the most likely candidate to be involved in catalyzing CNS protein citrullination in the diseased state. Even though the PAD2 knockout mice displayed a dramatic reduction in the amount of citrullination present in the CNS, indicating that PAD2 is indeed responsible for the majority of detectable citrullination observed in EAE, the development of EAE was not impaired by genetic deletion of PAD2, suggesting that PAD2 catalyzed citrullination is not essential to the development of EAE.

Citrullination of central nervous system proteins during the development of experimental autoimmune encephalomyelitis

Immunization of mammals with central nervous system (CNS)-derived proteins or peptides induces experimental autoimmune encephalomyelitis (EAE), a disease resembling the human autoimmune disease multiple sclerosis (MS). Both diseases are accompanied by destruction of a part of the of the myelin sheaths, which surround neurites in the CNS. Previous studies in MS have described alterations in the citrullination of myelin basic protein, one of the main protein constituents of the myelin sheath. Here, we show that, also during the development of EAE in mice, hypercitrullination occurs in the areas of the spinal cord that show the highest degree of inflammation and that myelin basic protein and glial fibrillary acidic protein are among the hypercitrullinated proteins. We conclude that hypercitrullination of myelin proteins in the CNS is a common phenomenon in demyelinating disease. Hypercitrullination may cause conformational changes in proteins, so the affected proteins may be involved in the pathogenesis of CNS autoimmune disease by acting as autoreactive T-cell epitopes. This is the first report in which hypercitrullination of CNS proteins in EAE is described and in which proteins other than myelin basic protein are reported to be citrullinated during autoimmune-mediated CNS inflammation.

Expression of citrullinated proteins in murine experimental autoimmune encephalomyelitis

In this study, we demonstrate for the first time the immunohistochemical expression of citrullinated proteins in the central nervous system (CNS) of mice with myelin oligodendrocyte glycoprotein (MOG)-induced experimental autoimmune encephalomyelitis (EAE). By using an established monoclonal antibody (F95) against natural and synthetic citrullinated proteins (Nicholas and Whitaker [2002] Glia 37:328-336), numerous, small, previously unrecognized "patches" of citrullinated proteins were discovered throughout EAE brains, whereas EAE spinal cords showed similar but much larger lesions. On dual color immunofluorescence, these lesions were found to contain citrullinated myelin basic protein (MBP) and were surrounded by astrocytes immunoreactive for both glial fibrillary acidic protein (GFAP) and F95. These lesions became evident about the time when EAE mice became symptomatic and increased in size and number with increasing disease severity. In some sections of spinal cord but not brains of severely debilitated EAE mice, a widespread gliotic response was seen, with astrocytes containing citrullinated GFAP spread throughout the gray and white matter. Western blot analysis of acidic proteins from the brains and spinal cords of EAE mice had higher levels of multiple citrullinated GFAP isoforms compared with controls, with more F95-positive bands in the EAE brains vs. spinal cords. These results raise the possibility that citrullination of both GFAP and MBP may contribute to the pathophysiology of EAE and that the brains of EAE mice may contain more pathology than previously realized.

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.

Delayed neuropathology after carbon monoxide poisoning is immune-mediated

The neuropathological sequelae of carbon monoxide (CO) poisoning cannot be explained by hypoxic stress alone. CO poisoning also causes adduct formation between myelin basic protein (MBP) and malonylaldehyde, a reactive product of lipid peroxidation, resulting in an immunological cascade. MBP loses its normal cationic characteristics, and antibody recognition of MBP is altered. Immunohistochemical evidence of degraded MBP occurs in brain over days, along with influx of macrophages and CD-4 lymphocytes. Lymphocytes from CO-poisoned rats subsequently exhibit an auto-reactive proliferative response to MBP, and there is a significant increase in the number of activated microglia in brain. Rats rendered immunologically tolerant to MBP before CO poisoning exhibit acute biochemical changes in MBP but no lymphocyte proliferative response or brain microglial activation. CO poisoning causes a decrement in learning that is not observed in immunologically tolerant rats. These results demonstrate that delayed CO-mediated neuropathology is linked to an adaptive immunological response to chemically modified MBP.

PAD, a growing family of citrullinating enzymes: genes, features and involvement in disease

Peptidylarginine deiminase (PAD, EC 3.5.3.15) enzymes catalyze the conversion of protein-bound arginine to citrulline. This post-translational modification may have a big impact on the structure and function of the target protein. In this review, we will discuss the effects of citrullination and its involvement in several human diseases, including rheumatoid arthritis and multiple sclerosis. So far, four isotypes of PAD have been described in mammals. We describe the existence of PAD in non-mammalian vertebrates and the existence of a fifth mammalian PAD. In addition, tissue-specific expression, genomic organization and evolutionary conservation of the different PAD isotypes will be discussed in detail. This article contains supplementary material which may be viewed at the BioEssays website at http://www.interscience.wiley.com/jpages/0265-9247/suppmat/2003/25/v25.1106.html.

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.

Increased citrullinated glial fibrillary acidic protein in secondary progressive multiple sclerosis

In this study, we demonstrate that grossly unaffected white matter from secondary progressive multiple sclerosis (SP-MS) patients is heavily citrullinated, as compared to normal white matter from control patients. Citrullination was most pronounced at plaque interfaces and was shown to colocalize with glial fibrillary acidic protein (GFAP)-immunoreactivity using dual color immunofluorescence. In contrast, the plaques themselves weakly stained for citrullinated proteins compared to control white matter and usually contained a blood vessel with surrounding astrocytes that were positive both for citrullinated proteins and GFAP. In SP-MS brain samples, but not in normal brains, long fibers of colocalized GFAP- and citrullinated proteins extended into the gray matter. Increased numbers of astrocytes containing citrullinated proteins and GFAP were also present at the junction between the gray and white matter in SP-MS brains. Western blot analysis of acidic brain proteins from nonplaque-containing white matter showed upregulation of multiple citrullinated GFAP proteins in SP-MS brains as compared to controls. Our results demonstrate that increased amounts of citrullinated GFAP are present in SP-MS brains, but also shows that these proteins are present in areas of MS brains that were grossly normal appearing. These data raise the possibility that citrullination of GFAP contributes to the pathophysiology of MS.

Adhesion and clustering of charge isomers of myelin basic protein at model myelin membranes

The association of myelin basic protein charge isomers with the lipid part of the myelin membrane was investigated at the microscopic (molecular) level in a model membrane system, using optical waveguide lightmode spectrometry to determine with high precision the kinetics of association and dissociation to planar phospholipid membranes under controlled hydrodynamic conditions and over a range of protein concentrations. Detailed analysis of the data revealed a rich and intricate behaviour and clearly showed that the membrane protein affinity is characterized by at least four independent parameters: (i) the association rate coefficient characterizing the protein-membrane interaction energy as the protein approaches the fluid-membrane interface; (ii) the protein-membrane adhesion, i.e., the probability that it will remain at the membrane after arrival; (iii) the protein conformation at the membrane; and (iv) the protein's tendency to cluster at the membrane. Some of these parameters varied in characteristic ways as the bulk solution concentration of the protein was varied, giving further clues to the detailed molecular comportment of the protein. The parameters and their characteristic variations with bulk concentration were markedly different for the different isomers. Implications of these results for neurological disorders involving demyelination, such as multiple sclerosis, are discussed.

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.

Multiple sclerosis: an important role for post-translational modifications of myelin basic protein in pathogenesis

Myelin basic protein (MBP) represents a candidate autoantigen in multiple sclerosis (MS). We isolated MBP from normal and MS human white matter and purified six components (charge isomers) to compare the post-translational modifications on each. The sites and extent of methylation, deimination, and phosphorylation were documented for all tryptic peptides by mass spectrometry. We found that mono and dimethylated arginine 107 was increased in MS samples; deimination of arginine occurred at a number of sites and was elevated in MS; phosphorylation was observed in 10 peptides in normal samples but was greatly reduced or absent in most peptides from MS samples. Data obtained with MBP isolated from fresh brain obtained from a spontaneously demyelinating mouse model supported the view that the changes observed in human brain were probably related to pathogenesis of demyelination, i.e. we found decreased phosphorylation and decreased amounts of glycogen synthesis kinase in brain homogenates using specific antibodies. This study represents the first to define post-translational modifications in demyelinating disease and suggest an important role in pathogenesis.

Characterization of loaded liposomes by size exclusion chromatography

This review focuses on the use of conventional (SEC) and high performance (HPSEC) size exclusion chromatography for the analysis of liposomes. The suitability of both techniques is examined regarding the field of liposome applications. The potentiality of conventional SEC is strongly improved by using a HPLC system associated to gel columns with a size selectivity range allowing liposome characterization in addition to particle fractionation. Practical aspects of size exclusion chromatography are described and a methodology based on HPSEC coupled to multidetection modes for on-line analysis of liposomes via label or substance encapsulation is presented. Examples of conventional SEC and HPSEC applications are described which concern polydispersity, size and encapsulation stability, bilayer permeabilization, liposome formation and reconstitution, incorporation of amphiphilic molecules. Size exclusion chromatography is a simple and powerful technique for investigation of encapsulation, insertion/interaction of substances from small solutes (ions, surfactants, drugs, etc.) up to large molecules (proteins, peptides and nucleic acids) in liposomes.

AMPA/kainate receptors in mouse spinal cord cell-specific display of receptor subunits by oligodendrocytes and astrocytes and at the nodes of Ranvier

Spinal cord white matter is susceptible to AMPA/kainate (KA)-type glutamate receptor-mediated excitotoxicity. To understand this vulnerability, it is important to characterize the distribution of AMPA/KA receptor subunits in this tissue. Using immunohistochemistry and laser confocal microscopy, we studied the expression sites of AMPA/KA receptor subunits in mouse spinal cord. The white matter showed consistent immunoreactivity for AMPA receptor subunit GluR2/3 and KA receptor subunits GluR6/7 and KA2. In contrast, antibodies against GluR1, GluR2, GluR4 (AMPA), and GluR5 (KA) subunits showed only weak and occasional labeling of white matter. However, gray matter neurons did express GluR1 and GluR2, as well as GluR2/3. The white matter astrocytes were GluR2/3 and GluR6/7 immunopositive, while the gray matter astrocytes displayed primarily GluR6/7. Both exclusively and abundantly, KA2 labeled oligodendrocytes and myelin, identified by CNPase expression. Interestingly, myelin basic protein, another myelin marker, showed less correlation with KA2 expression, placing KA2 at specific CNPase-containing subdomains. Focal points of dense KA2 labeling showed colocalization with limited, but distinct, axonal regions. These regions were identified as nodes of Ranvier by coexpressing the nodal marker, ankyrin G. Overall, axonal tracts showed little, if any, AMPA/KA receptor expression. The proximity of oligodendrocytic KA2 to the axonal node and the paucity of axonal AMPA/kainate receptor expression suggest that excitotoxic axonal damage may be secondary and, possibly, mediated by oligodendrocytes. Our data demonstrate differential expression of glutamate AMPA and KA receptor subunits in mouse spinal cord white matter and point to astrocytes and oligodendrocytes as potential targets for pharmacological intervention in white matter glutamate excitotoxicity.

Nuclear localization of peptidylarginine deiminase V and histone deimination in granulocytes

Peptidylarginine deiminase (PAD) deiminates arginine residues in proteins to citrulline residues Ca(2+) dependently. There are four types of PADs, I, II, III, and V, in humans. We studied the subcellular distribution of PAD V in HL-60 granulocytes and peripheral blood granulocytes. Expression of green fluorescent protein-tagged PADs in HeLa cells revealed that PAD V is localized in the nucleus, whereas PAD I, II, and III are localized in the cytoplasm. PAD V deletion mutants indicated that the sequence residues 45-74 have a nuclear localization signal (NLS). A sequence feature of this NLS is a three-lysine residue cluster preceded by a proline residue and is not found in the three other PADs. Substitution of the lysine cluster by an alanine cluster abrogated the nuclear import activity. These results suggested that the NLS is a classical monopartite NLS. HL-60 granulocytes, neutrophils, and eosinophils stained with antibody specific for PAD V exhibited distinct positive signals in the nucleus. Subcellular fractionation of HL-60 granulocytes also showed the nuclear localization of the enzyme. When neutrophils were stimulated with calcium ionophore, protein deimination occurred in the nucleus. The major deiminated proteins were identified as histones H2A, H3, and H4. The implication of PAD V in histone modifications is discussed.

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.

Peptidylarginine deiminase: a candidate factor in demyelinating disease

In earlier studies we demonstrated that an increase in the relative amounts of citrullinated myelin basic protein (MBP) was found in multiple sclerosis (Moscarello et al. 1994). To determine the temporal relationship between the citrullinated MBP and peptidylarginine deiminase (PAD), the enzyme responsible for deiminating arginyl residues in proteins, we studied enzyme activity, enzyme protein, PAD mRNA in a spontaneously demyelinating transgenic mouse model and we correlated the amount of PAD with citrullinated MBP. Both PAD protein as measured in an immunoslot blot method and PAD RNA were elevated. In fractionation studies we showed that the increase in PAD enzyme was due to an increase in the PAD found in membrane fractions and not the soluble PAD (PADII). From our data we concluded that up-regulation of myelin-associated PAD was responsible for the increase in citrullinated MBP in our transgenic mice prior to onset of clinical or pathological signs of demyelination. We postulate that a similar mechanism may be responsible for the increase in citrullinated MBP in multiple sclerosis.

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.

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.