The P2 protein of bovine root myelin: partial chemical characterization

Myelin basic protein dynamics from out-of-equilibrium functional state to degraded state in myelin

Living matter is a quasi-stationary out-of-equilibrium system; in this physical condition, structural fluctuations at nano- and meso-scales are needed to understand the physics behind its biological functionality. Myelin has a simple ultrastructure whose fluctuations show correlated disorder in its functional out-of-equilibrium state. However, there is no information on the relationship between this correlated disorder and the dynamics of the intrinsically disordered Myelin Basic Protein (MBP) which is expected to influence the membrane structure and overall functionality. In this work, we have investigated the role of this protein structural dynamics in the myelin ultrastructure fluctuations in various conditions, by using synchrotron Scanning micro X Ray Diffraction and Small Angle X ray Scattering. We have induced the crossover from out-of-equilibrium functional state to in-equilibrium degeneration changing the pH to values far from physiological condition. The observed compression of the cytosolic layer thickness probes that the intrinsic large MBP fluctuations preserve the cytosol structure also in the degraded state. Thus, the transition of myelin ultrastructure from correlated to uncorrelated disordered state, is principally affected by the deformation of the membrane and extracellular domain.

Biology of Schwann cells

The fundamental roles of Schwann cells during peripheral nerve formation and regeneration have been recognized for more than 100 years, but the cellular and molecular mechanisms that integrate Schwann cell and axonal functions continue to be elucidated. Derived from the embryonic neural crest, Schwann cells differentiate into myelinating cells or bundle multiple unmyelinated axons into Remak fibers. Axons dictate which differentiation path Schwann cells follow, and recent studies have established that axonal neuregulin1 signaling via ErbB2/B3 receptors on Schwann cells is essential for Schwann cell myelination. Extracellular matrix production and interactions mediated by specific integrin and dystroglycan complexes are also critical requisites for Schwann cell-axon interactions. Myelination entails expansion and specialization of the Schwann cell plasma membrane over millimeter distances. Many of the myelin-specific proteins have been identified, and transgenic manipulation of myelin genes have provided novel insights into myelin protein function, including maintenance of axonal integrity and survival. Cellular events that facilitate myelination, including microtubule-based protein and mRNA targeting, and actin based locomotion, have also begun to be understood. Arguably, the most remarkable facet of Schwann cell biology, however, is their vigorous response to axonal damage. Degradation of myelin, dedifferentiation, division, production of axonotrophic factors, and remyelination all underpin the substantial regenerative capacity of the Schwann cells and peripheral nerves. Many of these properties are not shared by CNS fibers, which are myelinated by oligodendrocytes. Dissecting the molecular mechanisms responsible for the complex biology of Schwann cells continues to have practical benefits in identifying novel therapeutic targets not only for Schwann cell-specific diseases but other disorders in which axons degenerate.

Systematic approaches to central nervous system myelin

Rapid signal propagation along vertebrate axons is facilitated by their insulation with myelin, a plasma membrane specialization of glial cells. The recent application of 'omics' approaches to the myelinating cells of the central nervous system, oligodendrocytes, revealed their mRNA signatures, enhanced our understanding of how myelination is regulated, and established that the protein composition of myelin is much more complex than previously thought. This review provides a meta-analysis of the > 1,200 proteins thus far identified by mass spectrometry in biochemically purified central nervous system myelin. Contaminating proteins are surprisingly infrequent according to bioinformatic prediction of subcellular localization and comparison with the transcriptional profile of oligodendrocytes. The integration of datasets also allowed the subcategorization of the myelin proteome into functional groups comprising genes that are coregulated during oligodendroglial differentiation. An unexpectedly large number of myelin-related genes cause-when mutated in humans-hereditary diseases affecting the physiology of the white matter. Systematic approaches to oligodendrocytes and myelin thus provide valuable resources for the molecular dissection of developmental myelination, glia-axonal interactions, leukodystrophies, and demyelinating diseases.

Experimental allergic neuritis in the SJL/J mouse: induction of severe and reproducible disease with bovine peripheral nerve myelin and pertussis toxin with or without interleukin-12

We report a reproducible model of experimental allergic neuritis (EAN) with severe clinical signs and consistent pathological features in mice. Pertussis toxin (PT) in the presence or absence of murine recombinant interleukin-12 (mrIL-12) was used as an adjuvant with bovine peripheral nerve myelin (BPNM) to induce clinical EAN in SJL/J mice. After immunization with a combination of BPNM in complete Freund's adjuvant (CFA) and PT, mice developed severe consistent signs of EAN. The additional treatment of immunized mice with mrIL-12 prolonged the course of EAN characterized by earlier clinical signs of the disease and delayed the recovery stage. Mice injected with BPNM and CFA without PT developed mild clinical signs. Histological examination of the caudae equinae and the sciatic nerves taken from mice with clinical signs of EAN during the recovery stage revealed severe demyelination, remyelination and remnants of mononuclear cell infiltration. Moderate to severe EAN can be induced in SJL/J mice by the injection of a combination of BPNM in CFA and PT. This model can provide a better understanding of mechanism of demyelination in infiltrating peripheral neuropathy.

Purification of bovine P2 myelin protein with bound lipids

The P2 protein is a neuritogenic, small basic protein present in PNS myelin. It belongs to the family of the cytoplasmic lipid-binding proteins and can be incorporated in lipidic bilayers. P2 has been purified and crystallized only in the lipid-free form. Here we show that the P2 protein can be purified with bound lipids by applying to PNS myelin the same procedure that as used to purify lipid-bound myelin basic protein from CNS myelin. SDS-PAGE showed a single band of 16.5 kDa, and TLC showed the presence of most of the myelin lipids associated with the protein. Lipid-bound P2 revealed different circular dichroism spectra from the corresponding lipid-free form, indicating that lipids influence P2 conformation.

Experimental allergic neuritis in the SJL/J mouse: dysfunction of peripheral nerve without clinical signs

Experimental allergic neuritis (EAN) was studied in the SJL/J mouse and compared to EAN in the Lewis rat. The Lewis rat developed hind limb weakness and weight loss while the SJL/J mouse had no discernible clinical abnormalities. The SJL/J mouse, however, suffered subclinical damage to peripheral nerve (PN) myelin. Both species reproducibly developed electrophysiologic dysfunction of PN and histopathology confined to the peripheral nervous system (PNS). Understanding of autoimmune demyelination in the central nervous system was greatly enhanced by the development of experimental allergic encephalomyelitis in the SJL/J mouse. We believe that EAN in the SJL/J mouse could lead to a similar increase in our understanding of autoimmune demyelination in the PNS.

The secondary structure of myelin P2 protein derived by secondary structure prediction methods, circular dichroism, and 400-MHz 1H-NMR spectroscopy: implications for tertiary structure

The various methods used to study the secondary and tertiary structure of myelin P2 protein, and one of its peptides (CN1), in aqueous solution, indicate that the native protein contains a significant fraction of alpha-helix, suggesting that the current prediction of an all-beta tertiary structure requires revision.

Cellular retinoid binding proteins

The cellular retinol-binding protein (CRBP) and the cellular retinoic acid binding protein (CRABP) have similar physicochemical characteristics. The amino acid sequences of rat CRBP and bovine CRABP have been elucidated and they display 40% sequence identity. Both protein sequences appear to be evolutionarily highly conserved. The amino acid sequence of human CRBP, deduced from a cDNA-clone, is 96% identical to the rat CRBP sequence. CRBP and CRABP are members of a protein family, all members of which may bind hydrophobic ligands and interact with membrane components. All members of the protein family are probably related in tertiary structure and might interact with membrane components through two regions with a high probability for alpha-helix. The tissue distribution of CRBP and CRABP, together with their relation to lipid transporting proteins suggests that CRBP and CRABP are cellular transporting proteins for retinol and retinoic acid, respectively.

Ultrastructural localization of P2 protein in actively myelinating rat Schwann cells

The myelin specific protein, P2, was localized immunocytochemically in electron micrographs of 4-day-old rat peripheral nerve by a preembedding technique. P2 staining was restricted to Schwann cells that had established a one-to-one relationship with an axon. P2 antiserum produced a diffuse staining throughout the entire cytosol of myelinating Schwann cells. In addition, the cytoplasmic side of Schwann cell plasma membranes and the membranes of cytoplasmic organelles that were exposed to cytosol were stained by P2 antiserum. This cytoplasmic localization of P2 protein is similar to that described for soluble or peripheral membrane proteins that are synthesized on free ribosomes. P2 antiserum stained the cytoplasmic side of Schwann cell membranes that formed single or multiple loose myelin spirals around an axon. In the region of the outer mesaxon, P2 antiserum stained the major dense line of compact myelin. These results demonstrate that P2 protein is located on the cytoplasmic side of compact myelin membranes and are consistent with biochemical studies demonstrating P2 to be a peripheral membrane protein.

Expression of specific mRNAs during adipose differentiation: identification of an mRNA encoding a homologue of myelin P2 protein

To identify and characterize specific mRNAs that increase in abundance during differentiation of mouse 3T3-L1 preadipocytes, a cDNA library was constructed from poly(A)+RNA isolated from differentiated 3T3-L1 adipocytes. Mixed probe isotope ratio selection and RNA blot analyses have identified several unique cDNA clones that represent mRNA species expressed either exclusively or at dramatically increased levels in differentiated cells. Further characterization of one such clone (pAL422) revealed that the corresponding mRNA, detectable only after differentiation, is approximately the same length (600 +/- 150 bases) as the cDNA insert (672 bases). The complete nucleotide sequence of the cDNA insert in pAL422 revealed a single long open reading frame that encodes a 132 amino acid polypeptide (the 422 protein) of 14.6 kDa. These and other results suggest that this cDNA may represent a nearly full-length copy of the mRNA. Computer-assisted analyses showed that the 422 protein shares 69% and 64% homology with myelin P2 proteins from rabbit and bovine peripheral nerves, respectively, as well as 23% and 30% homology with fatty-acid binding proteins from rat liver and intestine, respectively. Moreover, the mRNA hybrid selected by pAL422 DNA directs the in vitro translation of an approximately equal to 13 kDa polypeptide, and this protein is specifically immunoprecipitated by antiserum against bovine myelin P2. These observations strongly suggest that the 422 protein is a structural, and possibly functional, analog of myelin P2.

A general model of the P2 protein of peripheral nervous system myelin based on secondary structure predictions, tertiary folding principles, and experimental observations

The amino acid sequence of the P2 protein of peripheral myelin was analyzed with regard to regions of probable alpha-helix, beta-structure, beta-turn, and unordered conformation by means of several algorithms commonly used to predict secondary structure in proteins. Because of the high beta-sheet content and virtual absence of alpha-helix shown by the circular dichroic spectra of the protein, a bias was introduced into the algorithms to favor the beta-structure over the alpha-helical conformation. In order to define those beta-sheet residues that could lie on the external hydrophilic surface of the protein and those that could lie in its hydrophobic interior, the predicted beta-strands were examined for charged and uncharged amino acids located at alternating positions in the sequence. The sequential beta-strands in the predicted secondary structure were then ordered into beta-sheets and aligned according to generally accepted tertiary folding principles and certain chemical properties peculiar to the P2 protein. The general model of the P2 protein that emerged was a "Greek key" beta-barrel, consisting of eight antiparallel beta-strands with a two-stranded ribbon of antiparallel beta-structure emerging from one end. The model has an uncharged, hydrophobic core and a highly hydrophilic surface. The two Cys residues, which form a disulfide, occur in a loop connecting two adjacent antiparallel strands. Two hydrophilic loops, each containing a cluster of acidic residues and a single Phe, protrude from one end of the molecule. The general model is consistent with many of the properties of the actual protein, including the relatively weak nature of its association with myelin lipids and the positions of amino acid substitutions. Alternative beta-strand orderings yield three specific models having different interstrand connections across the barrel ends.

Conformation of bovine nerve root P2 protein: characteristics of the molecule from circular dichroism spectra

Circular dichroism (CD) was used to study the conformations of bovine nerve root P2 basic protein, its reduced and carboxymethylated derivative (RCM-P2), and its large cyanogen bromide fragment (CN1). Data in the far UV show that both the parent protein and RCM-P2 have conformations dominated by a large amount of beta structure. However, the CN1 peptide appears to exist in a largely unordered conformation. Since CN1 lacks short (20 residue) amino- and carboxy-terminal segments of the P2 protein, the spectral data suggest that these regions are important for determining and/or maintaining folding of the P2 protein in aqueous solutions. The P2 protein was found to have a distinctive CD spectrum in the near UV. The characteristics of molar ellipticities indicate that the spectrum contains significant contributions from tyrosine residues, and that these contributions suggest different environments for the two tyrosines in P2 protein. Both environments depend on protein conformation, since CD side chain absorptions are lost when P2 protein is denatured with 5 M urea.

An immunological characterization of the basic proteins of rodent sciatic nerve myelin

Radioimmunoassays (RIA) for the myelin basic protein (MBP) and P2 protein together with sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) were used to establish the identities of and relationships between the basic proteins (BP) of rodent peripheral nervous system (PNS) myelin. The PNS myelin proteins studied, in order of increasing mobility of SDS-PAGE, are P1, PR (R = rodent) and PB (B = breakdown). The majority of the acid extractable proteins of rodent PNS myelin are MBP related as shown by MBP-RIA. When tested individually, rodents P1, PR and PB were each found to cross-react in the RIA for MBP but not that for P2. The acid extracts of rodent PNS myelin were found to contain P2, although in minute quantities. P2 accounts for approximately 0.05-1.0% of the acid extractable protein in rodent PNS myelin.

The protein antigens of peripheral nerve myelin

Peripheral nervous system (PNS) tissue contains a variety of proteins, lipids, and carbohydrates that may serve as immunogens in proving immune responses, as antigens participating in immunological reactions, or as both types of agents. Three proteins P0, P1, and P2, account for approximately 70% of PNS myelin proteins. P0 is the major PNS myelin protein and is restricted to the PNS. P1 is similar to, if not identical with, myelin basic protein, the component of central nervous system myelin which induces experimental allergic encephalomyelitis. P2 has neuritogenic properties for inducing experimental allergic neuritis and may be involved in immune-mediated PNS myelin injury in humans. The complete amino acid sequence for P2 has recently been delineated, and its neuritogenic, immunogenic, and antigenic features can now be further characterized.

Bovine peripheral nervous system myelin P2 protein: chemical and immunological characterization of the cyanogen bromide peptides

Cleavage of bovine P2 protein by cyanogen bromide (CNBr) produced peptide fractions CN1, CN2, and CN3 which were isolated by gel filtration chromatography. CN2 was found to contain two NH2-terminals (lysine and valine) and accounted for both of the cysteine residues of P2. When reduced carboxymethylated P2 (RCM-P2) was digested with CNBr, peptides CN1 and CN3 were obtained as were (1) a peptide with NH2-terminal lysine (Lys) that contained no homoserine and only one cysteine residue and (2) a peptide with NH2-terminal valine (Val) that was co-eluted with CN3. These data and the chemical characterization of all the CNBr peptides obtained from P2 and RCM-P2 suggest that isolated P2 protein has a structure composed of the CNBr peptides in the order CN3-CN1-CN2(Val)-CN2(Lys) with an intrachain disulfide bond between the cysteine residues located in the two constituent peptides of CN2, CN2(Lys) and CN2(Val). To locate the neuritogenic region(s) within the P2 protein structure, CN1, CN2, and CN3 were tested for the ability to induced experimental allergic neuritis (EAN) in Lewis rats. The disease-inducing sites of P2 protein were found only in CN1; neither CN2 nor CN3 produced disease. EAN induced by CN1 was comparable to that induced with P2 protein as determined by disease onset, clinical symptoms, and histologic lesions.

Bovine P2 protein: sequence at the NH2-terminal of the protein

Sequence data from key fragments of the P2 protein established the order of cyanogen bromide (CNBr) peptides in the structure of the protein and the primary structure for approximately one-half of the molecule. Data were obtained from the three tryptic peptides of blocked NH2-terminal CNBr peptide (CN3), the large CNBr peptide of P2 protein (CN1), and a fragment obtained from P2 by cleavage at tryptophan with 2-(2-nitrophenylsulfenyl)-3-methyl-3'-bromoindolenine. This last fragment was found to contain an over-lapping sequence that proved the juxtaposition of CN1 and CN3 in P2 protein. Thus, based on this fact and the characteristics of the CNBr peptides, the P2 structure is composed of CNBr peptides in the order: CN3-CN1-CN2(Val)-CN2(Lys). A comparison was made between the partial sequence of P2 protein and the equivalent portion of the structure of bovine myelin basic protein. The structures of these two proteins were found to be distinctly different although certain similarities are found.

Immunocytochemical localization of rat peripheral nervous system myelin proteins: P2 protein is not a component of all peripheral nervous system myelin sheaths

Specific antibodies have been developed against P1, P2, and P0 myelin proteins and were used to study the localization of these proteins in the rat peripheral nervous system. Both peripheral and central nervous system myelin sheaths contain P1 protein. P0 and P2 proteins are found exclusively in peripheral nervous system myelin sheaths. Antisera to P1 and P0 proteins stain all peripheral nervous system myelin sheaths uniformly. P2 protein is not a component of all peripheral nervous system myelin sheaths. In sheaths that do contain P2 protein, it is concentrated in the area of the Schmidt-Lanterman incisures.

Proteins from sciatic-nerve myelin in quaking and jimpy mice

Myelin from two neurological mutants in mice was isolated from sciatic nerves and its protein composition analysed. In Quaking mice, two intrinsic myelin proteins P1 and P2 were drastically decreased, whereas the major myelin protein P0 was unaffected. A normal protein composition was found in sciatic myelin from Jimpy mice.

Studies on myelin proteins in human peripheral nerve

The myelin fraction from human peripheral nerve was prepared. Two basic protein fractions (BF-P2 and PB) were isolated from acid extracts of the myelin fraction and three glycoproteins (BR-PO, PASII and Y protein) were purified from its acid-insoluble residue. In biochemical analysis the human BF-P2 protein (M.W.13,000) showed similar but not identical properties to bovine BF-P2 protein. The PB fraction was suggested to include the encephalitogenic CNS-BP (M.W.18,000) and another, new protein of similar molecular weight. Both the human BR-PO protein (M.W.28,000) and PASII protein (M.W.13,000) showed similar biochemical properties to the corresponding myelin proteins of bovine peripheral nerve, while they both are clearly different from other myelin proteins. Close relationship between the BR-PO protein and the Y protein (M.W.22,000) was suggested by amino acid analysis. Injection of the myelin fraction of bovine peripheral nerve with the complete adjuvant produced EAN while the CNS-BP induced EAE in laboratory animals. However, all three purified proteins, BF-P2, BR-PO and PASII, from bovine peripheral nerve myelin were inactive in inducing demyelinating diseases.

Conformational changes induced by ionic strength and pH in two bovine myelin basic proteins

The structures of two biologically different myelin proteins, A1 from the central nervous system and P2 from the peripheral nervous system, were investigated. Both proteins were isolated from nerve tissues. Conformational changes in the homogeneous proteins were examined in aqueous solutions by means of circular dichroism measurements. The secondary structures of both proteins proved to be very stable between pH 2.5 and pH 11.7. Unlike the P2 protein, the A1 protein is stable up to pH 13 without detectable conformational changes. The stereochemistry of the polypeptide chains of both proteins is markedly different in the presence of urea. While the value of theta222 for the A1 protein changes linearly with increasing urea concentration, a sigmoidal curve was obtained for the P2 protein. The observed differences in the dichroic properties of the basic myelin proteins A1 and P2 indicate the possibility of further structure - function correlations.

Experimental allergic neuritis induced by a basic neuritogenic protein (P1L) of human peripheral nerve origin

Experimental allergic neuritis (EAN) in the peripheral nervous system, without involvement of the central nervous system, was produced in laboratory animals by the injection of a basic neuritogenic protein, P1L, purified from human peripheral nerves. The animals manifested a positive skin test with P1L, and their lymphocytes were found to be transformed in vitro in the presence of this protein several days before the appearance of the clinical signs. Passive transfer of the disease was performed with lymph node cells from donor guinea pigs immunized with P1L protein. EAN, the experimental model for the human disease Guillaain-Barré syndrome, was shown to be a transient disease and could be suppressed by the administration of hydrocortisone.