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

Identification of Myelin Basic Protein Proximity Interactome Using TurboID Labeling Proteomics

Myelin basic protein (MBP) is one of the key structural elements of the myelin sheath and has autoantigenic properties in multiple sclerosis (MS). Its intracellular interaction network is still partially deconvoluted due to the unfolded structure, abnormally basic charge, and specific cellular localization. Here we used the fusion protein of MBP with TurboID, an engineered biotin ligase that uses ATP to convert biotin to reactive biotin-AMP that covalently attaches to nearby proteins, to determine MBP interactome. Despite evident benefits, the proximity labeling proteomics technique generates high background noise, especially in the case of proteins tending to semi-specific interactions. In order to recognize unique MBP partners, we additionally mapped protein interaction networks for deaminated MBP variant and cyclin-dependent kinase inhibitor 1 (p21), mimicking MBP in terms of natively unfolded state, size and basic amino acid clusters. We found that in the plasma membrane region, MBP is colocalized with adhesion proteins occludin and myelin protein zero-like protein 1, solute carrier family transporters ZIP6 and SNAT1, Eph receptors ligand Ephrin-B1, and structural components of the vesicle transport machinery-synaptosomal-associated protein 23 (SNAP23), vesicle-associated membrane protein 3 (VAMP3), protein transport protein hSec23B and cytoplasmic dynein 1 heavy chain 1. We also detected that MBP potentially interacts with proteins involved in Fe²⁺ and lipid metabolism, namely, ganglioside GM2 activator protein, long-chain-fatty-acid-CoA ligase 4 (ACSL4), NADH-cytochrome b5 reductase 1 (CYB5R1) and metalloreductase STEAP3. Assuming the emerging role of ferroptosis and vesicle cargo docking in the development of autoimmune neurodegeneration, MBP may recruit and regulate the activity of these processes, thus, having a more inclusive role in the integrity of the myelin sheath.

The Relationship of the Mechanisms of the Pathogenesis of Multiple Sclerosis and the Expression of Endogenous Retroviruses

Simple Summary

Multiple sclerosis is a neurodegenerative disease of the central nervous system, develops at an early age and often leads to a disability. The etiological cause of the disease has not been fully elucidated, and as a result, no effective treatment is available. This review summarizes the current knowledge about the relationship between the expression of human endogenous retroviruses and the pathogenesis of multiple sclerosis. The epigenetic mechanisms of transcriptional regulation, the role of transcription factors, cytokines, and exogenous viruses are also addressed in this review. The elucidation of the mechanisms of an increase in endogenous retrovirus expression in multiple sclerosis could help to develop therapeutic strategies and novel methods for early diagnosis and treatment of the disease.

Abstract

Two human endogenous retroviruses of the HERV-W family can act as cofactors triggering multiple sclerosis (MS): MS-associated retrovirus (MSRV) and ERVWE1. Endogenous retroviral elements are believed to have integrated in our ancestors’ DNA millions of years ago. Their involvement in the pathogenesis of various diseases, including neurodegenerative pathologies, has been demonstrated. Numerous studies have shown a correlation between the deterioration of patients’ health and increased expression of endogenous retroviruses. The exact causes and mechanisms of endogenous retroviruses activation remains unknown, which hampers development of therapeutics. In this review, we will summarize the main characteristics of human endogenous W retroviruses and describe the putative mechanisms of activation, including epigenetic mechanisms, humoral factors as well as the role of the exogenous viral infections.

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.

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.

Myelin basic protein induces neuron-specific toxicity by directly damaging the neuronal plasma membrane

The central nervous system (CNS) insults may cause massive demyelination and lead to the release of myelin-associated proteins including its major component myelin basic protein (MBP). MBP is reported to induce glial activation but its effect on neurons is still little known. Here we found that MBP specifically bound to the extracellular surface of the neuronal plasma membrane and induced neurotoxicity in vitro. This effect of MBP on neurons was basicity-dependent because the binding was blocked by acidic lipids and competed by other basic proteins. Further studies revealed that MBP induced damage to neuronal membrane integrity and function by depolarizing the resting membrane potential, increasing the permeability to cations and other molecules, and decreasing the membrane fluidity. At last, artificial liposome vesicle assay showed that MBP directly disturbed acidic lipid bilayer and resulted in increased membrane permeability. These results revealed that MBP induces neurotoxicity through its direct interaction with acidic components on the extracellular surface of neuronal membrane, which may suggest a possible contribution of MBP to the pathogenesis in the CNS disorders with myelin damage.

Glatiramer acetate and nanny proteins restrict access of the multiple sclerosis autoantigen myelin basic protein to the 26S proteasome

We recently showed that myelin basic protein (MBP) is hydrolyzed by 26S proteasome without ubiquitination. The previously suggested concept of charge-mediated interaction between MBP and the proteasome led us to attempt to compensate or mimic its positive charge to inhibit proteasomal degradation. We demonstrated that negatively charged actin and calmodulin (CaM), as well as basic histone H1.3, inhibit MBP hydrolysis by competing with the proteasome and MBP, respectively, for binding their counterpart. Interestingly, glatiramer acetate (GA), which is used to treat multiple sclerosis (MS) and is structurally similar to MBP, inhibits intracellular and in vitro proteasome-mediated MBP degradation. Therefore, the data reported in this study may be important for myelin biogenesis in both the normal state and pathophysiological conditions.

Interaction of myelin basic protein with cytoskeletal and signaling proteins in cultured primary oligodendrocytes and N19 oligodendroglial cells

Background

The classic myelin basic protein (MBP) isoforms are intrinsically-disordered proteins of 14–21.5 kDa in size arising from the Golli (Gene in the Oligodendrocyte Lineage) gene complex, and are responsible for formation of the multilayered myelin sheath in the central nervous system. The predominant membrane-associated isoform of MBP is not simply a structural component of compact myelin but is highly post-translationally modified and multi-functional, having interactions with numerous proteins such as Ca²⁺-calmodulin, and with actin, tubulin, and proteins with SH3-domains, which it can tether to a lipid membrane in vitro. It co-localizes with such proteins in primary oligodendrocytes (OLGs) and in early developmental N19-OLGs transfected with fluorescently-tagged MBP.

Results

To provide further evidence for MBP associations with these proteins in vivo, we show here that MBP isoforms are co-immunoprecipitated from detergent extracts of primary OLGs together with actin, tubulin, zonula occludens 1 (ZO-1), cortactin, and Fyn kinase. We also carry out live-cell imaging of N19-OLGs co-transfected with fluorescent MBP and actin, and show that when actin filaments re-assemble after recovery from cytochalasin D treatment, MBP and actin are rapidly enriched and co-localized at certain sites at the plasma membrane and in newly-formed membrane ruffles. The MBP and actin distributions change similarly with time, suggesting a specific and dynamic association.

Conclusions

These results provide more direct evidence for association of the predominant 18.5-kDa MBP isoform with these proteins in primary OLGs and in live cells than previously could be inferred from co-localization observations. This study supports further a role for classic MBP isoforms in protein-protein interactions during membrane and cytoskeletal extension and remodeling in OLGs.

Effects of an anticarcinogenic Bowman-Birk protease inhibitor on purified 20S proteasome and MCF-7 breast cancer cells

Proteasome inhibitors have been described as an important target for cancer therapy due to their potential to regulate the ubiquitin-proteasome system in the degradation pathway of cellular proteins. Here, we reported the effects of a Bowman-Birk-type protease inhibitor, the Black-eyed pea Trypsin/Chymotrypsin Inhibitor (BTCI), on proteasome 20S in MCF-7 breast cancer cells and on catalytic activity of the purified 20S proteasome from horse erythrocytes, as well as the structural analysis of the BTCI-20S proteasome complex. In vitro experiments and confocal microscopy showed that BTCI readily crosses the membrane of the breast cancer cells and co-localizes with the proteasome in cytoplasm and mainly in nucleus. Indeed, as indicated by dynamic light scattering, BTCI and 20S proteasome form a stable complex at temperatures up to 55°C and at neutral and alkaline pHs. In complexed form, BTCI strongly inhibits the proteolytic chymotrypsin-, trypsin- and caspase-like activities of 20S proteasome, indicated by inhibition constants of 10⁻⁷ M magnitude order. Besides other mechanisms, this feature can be associated with previously reported cytostatic and cytotoxic effects of BTCI in MCF-7 breast cancer cells by means of apoptosis.

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.

Recognition pliability is coupled to structural heterogeneity: a calmodulin intrinsically disordered binding region complex

Protein interactions within regulatory networks should adapt in a spatiotemporal-dependent dynamic environment, in order to process and respond to diverse and versatile cellular signals. However, the principles governing recognition pliability in protein complexes are not well understood. We have investigated a region of the intrinsically disordered protein myelin basic protein (MBP(145-165)) that interacts with calmodulin, but that also promiscuously binds other biomolecules (membranes, modifying enzymes). To characterize this interaction, we implemented an NMR spectroscopic approach that calculates, for each conformation of the complex, the maximum occurrence based on recorded pseudocontact shifts and residual dipolar couplings. We found that the MBP(145-165)-calmodulin interaction is characterized by structural heterogeneity. Quantitative comparative analysis indicated that distinct conformational landscapes of structural heterogeneity are sampled for different calmodulin-target complexes. Such structural heterogeneity in protein complexes could potentially explain the way that transient and promiscuous protein interactions are optimized and tuned in complex regulatory networks.

Effect of phosphorylation of phosphatidylinositol on myelin basic protein-mediated binding of actin filaments to lipid bilayers in vitro

Myelin basic protein (MBP) binds to negatively charged lipids on the cytosolic surface of oligodendrocytes and is believed to be responsible for adhesion of these surfaces in the multilayered myelin sheath. It can also assemble actin filaments and tether them to lipid bilayers through electrostatic interactions. Here we investigate the effect of increased negative charge of the lipid bilayer due to phosphorylation of phosphatidylinositol (PI) on MBP-mediated binding of actin to the lipid bilayer, by substituting phosphatidylinositol 4-phosphate or phosphatidylinositol 4,5-bisphosphate for PI in phosphatidylcholine/phosphatidylglycerol lipid vesicles. Phosphorylation of PI caused dissociation of the MBP/actin complex from the lipid vesicles due to repulsion of the negatively charged complex from the negatively charged membrane surface. An effect of phosphorylation could be detected even if the inositol lipid was only 2mol% of the total lipid. Calcium-calmodulin dissociated actin from the MBP-lipid vesicles and phosphorylation of PI increased the amount dissociated. These results show that changes to the lipid composition of myelin, which could occur during signaling or other physiological events, could regulate the ability of MBP to act as a scaffolding protein and bind actin filaments to the lipid bilayer.

Structured functional domains of myelin basic protein: cross talk between actin polymerization and Ca(2+)-dependent calmodulin interaction

The 18.5-kDa myelin basic protein (MBP), the most abundant isoform in human adult myelin, is a multifunctional, intrinsically disordered protein that maintains compact assembly of the sheath. Solution NMR spectroscopy and a hydrophobic moment analysis of MBP's amino-acid sequence have previously revealed three regions with high propensity to form strongly amphipathic α-helices. These regions, located in the central, N- and C-terminal parts of the protein, have been shown to play a role in the interactions of MBP with cytoskeletal proteins, Src homology 3-domain-containing proteins, Ca(2+)-activated calmodulin (Ca(2+)-CaM), and myelin-mimetic membrane bilayers. Here, we have further characterized the structure-function relationship of these three domains. We constructed three recombinant peptides derived from the 18.5-kDa murine MBP: (A22-K56), (S72-S107), and (S133-S159) (which are denoted α1, α2, and α3, respectively). We used a variety of biophysical methods (circular dichroism spectroscopy, isothermal titration calorimetry, transmission electron microscopy, fluorimetry, and solution NMR spectroscopy and chemical shift index analysis) to characterize the interactions of these peptides with actin and Ca(2+)-CaM. Our results show that all three peptides can adopt α-helical structure inherently even in aqueous solution. Both α1- and α3-peptides showed strong binding with Ca(2+)-CaM, and both adopted an α-helical conformation upon interaction, but the binding of the α3-peptide appeared to be more dynamic. Only the α1-peptide exhibited actin polymerization and bundling activity, and the addition of Ca(2+)-CaM resulted in depolymerization of actin that had been polymerized by α1. The results of this study proved that there is an N-terminal binding domain in MBP for Ca(2+)-CaM (in addition to the primary site located in the C-terminus), and that it is sufficient for CaM-induced actin depolymerization. These three domains of MBP represent molecular recognition fragments with multiple roles in both membrane- and protein-association.

Myelin basic protein binds microtubules to a membrane surface and to actin filaments in vitro: effect of phosphorylation and deimination

Myelin basic protein (MBP) is a multifunctional protein involved in maintaining the stability and integrity of the myelin sheath by a variety of interactions with membranes and other proteins. It assembles actin filaments and microtubules, can bind actin filaments and SH3-domains to a membrane surface, and may be able to tether them to the oligodendrocyte membrane and participate in signal transduction in oligodendrocytes/myelin. In the present study, we have shown that the 18.5 kDa MBP isoform can also bind microtubules to lipid vesicles in vitro. Phosphorylation of MBP at Thr94 and Thr97 (bovine sequence) by MAPK, and deimination of MBP (using a pseudo-deiminated recombinant form), had little detectable effect on its ability to polymerize and bundle microtubules, in contrast to the effect of these modifications on MBP-mediated assembly of actin. However, these modifications dramatically decreased the ability of MBP to tether microtubules to lipid vesicles. MBP and its phosphorylated and pseudo-deiminated variants were also able to bind microtubules to actin filaments. These results suggest that MBP may be able to tether microtubules to the cytoplasmic surface of the oligodendrocyte membrane, and that this binding can be regulated by post-translational modifications to MBP. We further show that MBP appears to be co-localized with actin filaments and microtubules in cultured oligodendrocytes, and also at the interface between actin filaments at the leading edge of membrane processes and microtubules behind them. Thus, MBP may also cross-link microtubules to actin filaments in vivo.

Fuzzy complexes of myelin basic protein: NMR spectroscopic investigations of a polymorphic organizational linker of the central nervous system

The classic 18.5 kDa isoform of myelin basic protein (MBP) is central to maintaining the structural homeostasis of the myelin sheath of the central nervous system. It is an intrinsically disordered, promiscuous, multifunctional, peripheral membrane protein, whose conformation adapts to its particular environment. Its study requires the selective and complementary application of diverse approaches, of which solution and solid-state NMR spectroscopy are the most powerful to elucidate site-specific features. We review here several recent solution and solid-state NMR spectroscopic studies of 18.5 kDa MBP, and the induced partial disorder-to-order transitions that it has been demonstrated to undergo when complexed with calmodulin, actin, and phospholipid membranes.

Influence of membrane surface charge and post-translational modifications to myelin basic protein on its ability to tether the Fyn-SH3 domain to a membrane in vitro

Myelin basic protein (MBP) is a highly post-translationally modified, multifunctional structural component of central nervous system myelin, adhering to phospholipid membranes and assembling cytoskeletal proteins, and has previously been shown to bind SH3 domains in vitro and tether them to a membrane surface [Polverini, E., et al. (2008) Biochemistry 47, 267-282]. Since molecular modeling shows that the Fyn-SH3 domain has a negative surface charge density even after binding the MBP ligand, we have investigated the influence of negative membrane surface charge and the effects of post-translational modifications to MBP on the interaction of the Fyn-SH3 domain with membrane-associated MBP. Using a sedimentation assay with multilamellar vesicles consisting of neutral phosphatidylcholine (PC) and negatively charged phosphatidylinositol (PI), we demonstrate that increasing the negative surface charge of the membrane by increasing the proportion of PI reduces the amount of Fyn-SH3 domain that binds to membrane-associated MBP, due to electrostatic repulsion. When one of the phosphoinositides, PI(4)P or PI(4,5)P(2) was substituted for PI in equal proportion, none of the Fyn-SH3 domain bound to MBP under the conditions that were used. Post-translational modifications of MBP which reduced its net positive charge, i.e., phosphorylation or arginine deimination, increased the degree of repulsion of Fyn-SH3 from the membrane surface, an effect further modulated by the lipid charge. This study suggests that changes in membrane negative surface charge due to protein or lipid modifications, which could occur during cell signaling, can regulate the binding of the Fyn-SH3 domain to membrane-associated MBP and thus could regulate the activity of Fyn at the oligodendrocyte membrane surface.

Probing the interaction between recombinant human myelin basic protein and caseins using surface plasmon resonance and diffusing wave spectroscopy

An intrinsically unstructured human myelin basic protein (hMBP) was expressed in the milk of transgenic cows (TGmilk) and found exclusively associated with the casein micellar phase. The interaction between the recombinant protein and milk caseins was investigated using surface plasmon resonance (SPR). An anti-human myelin basic protein antibody was covalently immobilized to the surface of the sensor chip. Subsequently the interaction between the recombinant protein (captured by this antibody) and caseins was studied in comparison to that noted with its human counterpart. Results showed a calcium-mediated interaction between the recombinant protein and caseins. The order of magnitude of this interaction was in agreement with the number of phosphorylated residues carried by each type of casein (alpha(s)- > beta- > kappa-casein). This selective interaction was not noted between the human protein and milk caseins indicating that the recombinant protein was phosphorylated to a higher extent than the human protein. The obtained results indicated that the co-expression of the recombinant protein and caseins by the mammary gland along with the recombinant protein's ability to form calcium bridges played a key role in the association of the recombinant human myelin basic protein (rhMBP) with the casein micelles of milk. Despite this association between the recombinant protein and milk caseins, light scattering investigations using diffusing wave spectroscopy (DWS) showed no significant differences between the milks of the transgenic and the non-transgenic control cows, with respect to both the average micelle size and surface charges. This was attributed to the low expression levels of the recombinant protein in milk.

Induced secondary structure and polymorphism in an intrinsically disordered structural linker of the CNS: solid-state NMR and FTIR spectroscopy of myelin basic protein bound to actin

The 18.5 kDa isoform of myelin basic protein (MBP) is a peripheral membrane protein that maintains the structural integrity of the myelin sheath of the central nervous system by conjoining the cytoplasmic leaflets of oligodendrocytes and by linking the myelin membrane to the underlying cytoskeleton whose assembly it strongly promotes. It is a multifunctional, intrinsically disordered protein that behaves primarily as a structural stabilizer, but with elements of a transient or induced secondary structure that represent binding sites for calmodulin or SH3-domain-containing proteins, inter alia. In this study we used solid-state NMR (SSNMR) and Fourier transform infrared (FTIR) spectroscopy to study the conformation of 18.5 kDa MBP in association with actin microfilaments and bundles. FTIR spectroscopy of fully (13)C,(15)N-labeled MBP complexed with unlabeled F-actin showed induced folding of both protein partners, viz., some increase in beta-sheet content in actin, and increases in both alpha-helix and beta-sheet content in MBP, albeit with considerable extended structure remaining. Solid-state NMR spectroscopy revealed that MBP in MBP-actin assemblies is structurally heterogeneous but gains ordered secondary structure elements (both alpha-helical and beta-sheet), particularly in the terminal fragments and in a central immunodominant epitope. The overall conformational polymorphism of MBP is consistent with its in vivo roles as both a linker (membranes and cytoskeleton) and a putative signaling hub.

Structural analysis of the complex between calmodulin and full-length myelin basic protein, an intrinsically disordered molecule

Myelin basic protein (MBP) is present between the cytoplasmic leaflets of the compact myelin membrane in both the peripheral and central nervous systems, and characterized to be intrinsically disordered in solution. One of the best-characterized protein ligands for MBP is calmodulin (CaM), a highly acidic calcium sensor. We pulled down MBP from human brain white matter as the major calcium-dependent CaM-binding protein. We then used full-length brain MBP, and a peptide from rodent MBP, to structurally characterize the MBP-CaM complex in solution by small-angle X-ray scattering, NMR spectroscopy, synchrotron radiation circular dichroism spectroscopy, and size exclusion chromatography. We determined 3D structures for the full-length protein-protein complex at different stoichiometries and detect ligand-induced folding of MBP. We also obtained thermodynamic data for the two CaM-binding sites of MBP, indicating that CaM does not collapse upon binding to MBP, and show that CaM and MBP colocalize in myelin sheaths. In addition, we analyzed the post-translational modifications of rat brain MBP, identifying a novel MBP modification, glucosylation. Our results provide a detailed picture of the MBP-CaM interaction, including a 3D model of the complex between full-length proteins.

EIN2, the central regulator of ethylene signalling, is localized at the ER membrane where it interacts with the ethylene receptor ETR1

Genetic studies have identified the membrane protein EIN2 (ethylene insensitive 2) as a central component of ethylene signalling in Arabidopsis. In addition, EIN2 might take part in multiple hormone signalling pathways and in response to pathogens as demonstrated by recent genetic and biochemical studies. Here we show, by an integrated approach using in vivo and in vitro fluorescence techniques, that EIN2 is localized at the ER (endoplasmic reticulum) membrane where it shows specific interaction with the ethylene receptor protein ETR1.

Modulation of Peptidyl Arginine Deiminase 2 and Implication for Neurodegeneration

PURPOSE

To demonstrate that elevated pressure increases the peptidyl arginine deiminase 2 (PAD2) expression in cultured astrocytes in vitro that can be modulated by pharmacological agents modulating intracellular calcium.

METHODS

Isolated rat brain astrocytes were subjected to pressure treatment. Western and immunohistochemical analyses detected PAD2 protein expression. Calcium measurements were achieved employing fluorescence-based microscopic imaging and quantification system. Experiments were repeated with human optic nerve head-derived astrocytes.

RESULTS

PAD2 has recently been shown to be associated with glaucomatous optic nerve. Astrocytes subjected to pressure (25-100 mmHg) show elevated level of PAD2, increased intracellular calcium, and concomitant citrullination but not significant cell death. PAD2 expression in response to elevated pressure may play a role in glaucomatous neurodegeneration. Pressure-treated astrocytes were also subjected to thapsigargin (50-250 nM) treatment, but it is unclear whether this had any further effect in increasing PAD2 expression. Conversely, treatment with calcium chelating agent BAPTA-AM (50-250 nM) results in decreased intracellular calcium concentration and PAD2.

CONCLUSIONS

These results suggest calcium modulation could be exploited as therapeutic strategy to modulate pressure-induced PAD2 expression and citrullination.

Naturally presented peptides on major histocompatibility complex I and II molecules eluted from central nervous system of multiple sclerosis patients

Tandem mass spectrometry was used to identify naturally processed peptides bound to major histocompatibility complex (MHC) I and MHC II molecules in central nervous system (CNS) of eight patients with multiple sclerosis (MS). MHC molecules were purified from autopsy CNS material by immunoaffinity chromatography with monoclonal antibody directed against HLA-A, -B, -C, and -DR. Subsequently peptides were separated by reversed-phase HPLC and analyzed by mass spectrometry. Database searches revealed 118 amino acid sequences from self-proteins eluted from MHC I molecules and 191 from MHC II molecules, corresponding to 174 identified source proteins. These sequences define previously known and potentially novel autoantigens in MS possibly involved in disease induction and antigen spreading. Taken together, we have initiated the characterization of the CNS-expressed MHC ligandome in CNS diseases and were able to demonstrate the presentation of naturally processed myelin basic protein peptides in the brain of MS patients.

Thermodynamic study of the binding of calcium and magnesium ions with myelin basic protein using the extended solvation theory

The interaction of myelin basic protein (MBP) from the bovine central nervous system with Ca2+ and Mg2+ ions, named as M2+, was studied by isothermal titration calorimetry at 27 degrees C in aqueous solution. The extended solvation model was used to reproduce the enthalpies of MBP+M2+ interactions. The solvation parameters recovered from the extended solvation model were attributed to the structural change of MBP due to the metal ion interaction. It was found that there is a set of two identical and noninteracting binding sites for Ca2+ and Mg2+ ions.

Myelin basic protein as a "PI(4,5)P2-modulin": a new biological function for a major central nervous system protein

The 18.5 kDa isoform of myelin basic protein (MBP) is multifunctional and has previously been shown to have structural and phenomenological similarities with domains of other membrane- and cytoskeleton-associated proteins such as MARCKS (myristoylated alanine-rich C kinase substrate). Here, we have investigated whether 18.5 kDa MBP can sequester phosphatidylinositol-(4,5)-bis-phosphate (PI(4,5)P 2) in membranes, like MARCKS and other "PIPmodulins" do. Using fluorescence-quenching and electron paramagnetic resonance (EPR) spectroscopy, and model membranes containing BODIPY-FL- or proxyl-labeled PI(4,5)P 2, respectively, we have demonstrated that MBP laterally sequesters PI(4,5)P 2. The MBP-PI(4,5)P 2 interactions are electrostatic, partially cholesterol-dependent, and sensitive to phosphorylation, deimination, and Ca (2+)-CaM binding. Confocal microscopy of cultured oligodendrocytes also revealed patched colocalization of MBP and PI(4,5)P 2, indicating the spatial clustering of PI(4,5)P 2 in the plasma membrane. On the basis of these findings as well as the overwhelming convergence of functional properties, modifying enzymes, and interaction partners, we propose that MBP is mechanistically related to GAP-43, MARCKS, and CAP-23. During myelinogenesis, it may mediate calcium and phosphorylation-sensitive plasma membrane availability of PI(4,5)P 2. This regulation of PI(4,5)P 2 availability at the cell cortex may be coupled to the elaboration and outgrowth of the membranous cellular processes by oligodendrocytes.

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

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

Backbone dynamics of the 18.5 kDa isoform of myelin basic protein reveals transient alpha-helices and a calmodulin-binding site

The 18.5 kDa isoform of myelin basic protein (MBP) is the predominant form in adult human central nervous system myelin. It is an intrinsically disordered protein that functions both in membrane adhesion, and as a linker connecting the oligodendrocyte membrane to the underlying cytoskeleton; its specific interactions with calmodulin and SH3-domain containing proteins suggest further multifunctionality in signaling. Here, we have used multidimensional heteronuclear nuclear magnetic resonance spectroscopy to study the conformational dependence on environment of the protein in aqueous solution (100 mM KCl) and in a membrane-mimetic solvent (30% TFE-d(2)), particularly to analyze its secondary structure using chemical shift indexing, and to investigate its backbone dynamics using (15)N spin relaxation measurements. Collectively, the data revealed three major segments of the protein with a propensity toward alpha-helicity that was stabilized by membrane-mimetic conditions: T33-D46, V83-T92, and T142-L154 (murine 18.5 kDa sequence numbering). All of these regions corresponded with bioinformatics predictions of ordered secondary structure. The V83-T92 region comprises a primary immunodominant epitope that had previously been shown by site-directed spin labeling and electron paramagnetic resonance spectroscopy to be alpha-helical in membrane-reconstituted systems. The T142-L154 segment overlapped with a predicted calmodulin-binding site. Chemical shift perturbation experiments using labeled MBP and unlabeled calmodulin demonstrated a dramatic conformational change in MBP upon association of the two proteins, and were consistent with the C-terminal segment of MBP being the primary binding site for calmodulin.

Interaction between the C-terminal region of human myelin basic protein and calmodulin: analysis of complex formation and solution structure

BACKGROUND

The myelin sheath is a multilamellar membrane structure wrapped around the axon, enabling the saltatory conduction of nerve impulses in vertebrates. Myelin basic protein, one of the most abundant myelin-specific proteins, is an intrinsically disordered protein that has been shown to bind calmodulin. In this study, we focus on a 19-mer synthetic peptide from the predicted calmodulin-binding segment near the C-terminus of human myelin basic protein.

RESULTS

The interaction of native human myelin basic protein with calmodulin was confirmed by affinity chromatography. The binding of the myelin basic protein peptide to calmodulin was tested with isothermal titration calorimetry (ITC) in different temperatures, and Kd was observed to be in the low muM range, as previously observed for full-length myelin basic protein. Surface plasmon resonance showed that the peptide bound to calmodulin, and binding was accompanied by a conformational change; furthermore, gel filtration chromatography indicated a decrease in the hydrodynamic radius of calmodulin in the presence of the peptide. NMR spectroscopy was used to map the binding area to reside mainly within the hydrophobic pocket of the C-terminal lobe of calmodulin. The solution structure obtained by small-angle X-ray scattering indicates binding of the myelin basic protein peptide into the interlobal groove of calmodulin, while calmodulin remains in an extended conformation.

CONCLUSION

Taken together, our results give a detailed structural insight into the interaction of calmodulin with a C-terminal segment of a major myelin protein, the myelin basic protein. The used 19-mer peptide interacts mainly with the C-terminal lobe of calmodulin, and a conformational change accompanies binding, suggesting a novel mode of calmodulin-target protein interaction. Calmodulin does not collapse and wrap around the peptide tightly; instead, it remains in an extended conformation in the solution structure. The observed affinity can be physiologically relevant, given the high abundance of both binding partners in the nervous system.

Identification and characterization of a MBP isoform specific to hypothalamus in orange-spotted grouper (Epinephelus coioides)

Myelin basic protein (MBP), as a major component of the myelin sheath, has been revealed to play an important role in forming and maintaining myelin structure in vertebrate nervous system. In teleost, hypothalamus is an instinctive brain center and plays significant roles in many physiological functions, such as energy metabolism, growth, reproduction, and stress response. In comparison with other MBP identified in vertebrates, a smallest MBP is cloned and identified from the orange-spotted grouper hypothalamic cDNA plasmid library in this study. RT-PCR analysis and Western blot detection indicate that the EcMBP is specific to hypothalamus, and expresses mainly in the tuberal hypothalamus in adult grouper. Immunofluorescence localization suggests that EcMBP should be expressed by oligodendrocytes, and the expressing cells should be concentrated in hypothalamus and the area surrounding hypothalamus, such as NPOpc, VC, DP, NLTm, and NDLI. The studies on EcMBP expression pattern and developmental behaviour in the brains of grouper embryos and larvae reveal that the EcMBP-expressing cells are only limited in a defined set of cells on the border of hypothalamus, and suggest that the EcMBP-expressing cells might be a subpopulation of oligodendrocyte progenitor cells. This study not only identifies a smallest MBP isoform specific to hypothalamus that can be used as a molecular marker of oligodendrocytes in fish, but also provides new insights for MBP evolution and cellular distribution.

Myelin basic protein as a binding partner and calmodulin adaptor for the BKCa channel

The activity and localization of large-conductance Ca2+ -activated K+ (BKCa) channels are known to be modulated by several different proteins. Although many binding partners have been identified via yeast two-hybrid screening, this method may not detect certain classes of interacting proteins such as low affinity binding proteins or multi-component protein complexes. In this study, we employed mass spectrometry to identify proteins that interact with BKCa channels. We expressed and purified the 'tail domain' of the rat BKCa channel alpha-subunit, a 54-kDa region that is crucial for expression and functional activity of the channel. Using rat brain lysate and purified 'tail domain', we identified several novel proteins that interact with the BKCa channel. These included the myelin basic protein (MBP), upon which we performed subsequent biochemical and electrophysiological studies. Interaction between the BKCa channel and MBP was confirmed in vivo and in vitro. MBP co-expression affected the Ca2+ -dependent activation of the BKCa channel by increasing its Ca2+ sensitivity. Moreover, we showed that calmodulin (CaM) interacts with the BKCa channel via MBP. Since CaM is a key regulator of many Ca2+ -dependent processes, it may be recruited by MBP to the vicinity of the BKCa channel, modulating its functional activity.

NMR assignment of an intrinsically disordered protein under physiological conditions: the 18.5 kDa isoform of murine myelin basic protein

We report the NMR assignment of 18.5 kDa recombinant murine myelin basic protein (MBP) in 100 mM KCl as a prerequisite to structural analyses of its Ca2+-dependent interaction with calmodulin.

Purification and spectroscopic characterization of the recombinant BG21 isoform of murine golli myelin basic protein

A recombinant form of the murine Golli-myelin basic protein (MBP) isoform BG21 (rmBG21) has been expressed in E. coli, and isolated to 96% purity via metal chelation chromatography. Characteristic yields were 6-8 mg protein per liter of culture in either minimal M9 or standard Luria-Bertani media. Circular dichroism spectroscopy showed that rmBG21 had a large proportion of random coil in aqueous solution, but gained alpha-helix in the presence of monosialoganglioside G(M1) and PI(4)P, as well as in the membrane-mimetic solvent trifluoroethanol. Bioinformatics analyses of the amino acid sequence of rmBG21 predicted an N-terminal calmodulin (CaM)-binding site. It was determined by fluorescence spectroscopy and dynamic light scattering that rmBG21 and CaM interacted weakly in a 1:1 ratio in a Ca(2+)-dependent manner. Solution NMR spectra of uniformly [(13)C(15)N]-labeled protein in aqueous buffer were consistent with it being an extended protein; spectral quality was independent of temperature. Thus, like "classic" MBP and the Golli-MBP isoform J37, rmBG21 is intrinsically disordered, implying multi functionality, and that its conformation depends on its environment and bound ligands.

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.

Calmodulin isoform-specific activation of a rice calmodulin-binding kinase conferred by only three amino-acids of OsCaM61

The kinase activity of a Ca(2+)/calmodulin (CaM)-binding serine/threonine protein kinase from rice (Oryza sativa) (OsCBK) has been reported to be unaffected by OsCaM1 binding. In this study, we examined whether other rice CaMs can stimulate OsCBK. It was observed that OsCaM61 stimulated OsCBK in a Ca(2+)-dependent manner. In addition, Ala(111), Gly(123) and Ser(127) were identified as critical residues for OsCBK activation. Mutational study and fluorescent spectroscopy analysis indicated that CaM-binding affinity does not correlate with the kinase activity and that these key amino-acids in OsCaM61 play a vital role in suitable changes of OsCBK conformation for kinase activation.

A novel fluorescent probe for protein binding and folding studies:p-cyano-phenylalanine

Recently, it is has been shown that the C=N stretching vibration of a non-natural amino acid, p-cyano-phenylalanine (PheCN), could be used as an infrared reporter of local environment. Here, we further showed that the fluorescence emission of PheCN is also sensitive to solvent and, therefore, could be used as a novel optical probe for protein binding and folding studies. Moreover, we found that the fluorescence quantum yield of PheCN is nearly five times larger than that of phenylalanine and, more importantly, can be selectively excited even when other aromatic amino acids are present, thus making it a more versatile fluorophore. To test the feasibility of using PheCN as a practical fluorescent probe, we studied the binding of calmodulin (CaM) to a peptide derived from the CaM-binding domain of skeletal muscle myosin light chain kinase (MLCK). The peptide (MLCK3CN) contains a single PheCN residue and has been shown to bind to CaM with high affinity. As expected, addition of CaM into a MLCK3CN solution resulted in quenching of the PheCN fluorescence. A series of stochiometric titrations further allowed us to determine the binding affinity (Kd) of this peptide to CaM. Taken together, these results indicated that the PheCN fluorescence is sensitive to environment and could be applicable to a wide variety of biological problems.

Charge effects modulate actin assembly by classic myelin basic protein isoforms

Myelin basic protein (MBP), a highly cationic structural protein of the myelin sheath, is believed to be associated with the cytoskeleton in vivo and interacts with actin in vitro, but little is known about the regulation of this interaction. The rate and extent of actin polymerization induced by 18.5 kDa MBP charge isomers were correlated to charge reduction by post-translational modifications. Increased ionic strength attenuated the initial rate but not the final extent of polymerization achieved. Reduced pH enhanced the rate and extent of polymerization, presumably via partial protonation of intrinsic histidyl residues. The polymerizing activities of the 21.5, 17, and 14 kDa MBP splice variants were not proportionate to their net charges or charge densities. The presence of at least one region derived from exon II or VI of the "classic" MBP gene was required for effective bundling as assessed by light scattering and transmission electron microscopy.

Electron paramagnetic resonance spectroscopy and molecular modelling of the interaction of myelin basic protein (MBP) with calmodulin (CaM)—diversity and conformational adaptability of MBP CaM-targets

The classic 18.5 kDa isoform of murine myelin basic protein (mMBP) has been shown to bind calmodulin (CaM) strongly and specifically in vitro. Here, we have used site-directed spin labelling (SDSL) and electron paramagnetic resonance (EPR) spectroscopy to map more precisely the sites of interaction of recombinant mMBP (rmMBP) with CaM. On the basis of these and previous experimental data, and the predictions of CaM-binding motifs using the Calmodulin Target Database (), three main segments of MBP were suggested for the interaction. The first site is located at the C-terminus; the second one lies in the central portion of the protein and forms an amphipathic alpha-helix in reconstituted myelin-mimetic systems; the third is quite close to the N-terminus. The murine Golli-MBP isoform J37 has also been shown to bind CaM in vitro, and an interaction site was predicted in the N-terminal Golli-specific portion of the protein. From these four segments, we selected peptide fragments of 12-14 residues in length, chosen on the bases of their amphipathicity and CaM-target characteristics. We modelled each of these peptides as alpha-helices, and performed docking simulations to investigate their interactions with the CaM peptide-binding tunnel. Different yet almost equally favourable CaM-binding modes were found for each of them. The experimental SDSL/EPR and theoretical modelling results were in good agreement, and supported the conjecture that there are several plausible CaM-binding sites in MBP, that could be induced into an alpha-helical conformation by their interaction with CaM and account for strong immobilisation of spin-labeled residues in all three segments. Phosphorylation and deimination were also emulated and simulated for known sites of MBP post-translational modification. The results obtained confirmed the appropriate utilisation of simple residue substitutions to mimic the natural modifications, and demonstrated molecular mechanisms by which MBP-CaM interactions could be modulated in vivo.

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.

Myelin basic protein has multiple calmodulin-binding sites

Myelin basic protein (MBP) has been shown to bind calmodulin (CaM) in a specific Ca(2+)-dependent manner via a primary target sequence at its C-terminus [Protein Sci. 12 (2003) 1507]. Upon deimination of MBP, the nature of the interaction changed significantly, suggesting either a new binding site or different conformers with different affinities for CaM. In order to resolve this issue, we investigated here the CaM-binding properties of N- and C-terminal deletion mutants of MBP using Trp fluorescence spectroscopy and mass spectrometry. We conclude that there is an additional CaM-binding site on MBP in a central segment (we posit murine residues 82-93) that forms an amphipathic alpha-helix.