Orientation of the myelin proteolipid protein C-terminus in oligodendroglial membranes

Monoclonal antibodies to distinct regions of human myelin proteolipid protein simultaneously recognize central nervous system myelin and neurons of many vertebrate species

Myelin proteolipid protein (PLP), the major protein of mammalian CNS myelin, is a member of the proteolipid gene family (pgf). It is an evolutionarily conserved polytopic integral membrane protein and a potential autoantigen in multiple sclerosis (MS). To analyze antibody recognition of PLP epitopes in situ, monoclonal antibodies (mAbs) specific for different regions of human PLP (50-69, 100-123, 139-151, 178-191, 200-219, 264-276) were generated and used to immunostain CNS tissues of representative vertebrates. mAbs to each region recognized whole human PLP on Western blots; the anti-100-123 mAb did not recognize DM-20, the PLP isoform that lacks residues 116-150. All of the mAbs stained fixed, permeabilized oligodendrocytes and mammalian and avian CNS tissue myelin. Most of the mAbs also stained amphibian, teleost, and elasmobranch CNS myelin despite greater diversity of their pgf myelin protein sequences. Myelin staining was observed when there was at least 40% identity of the mAb epitope and known pgf myelin proteins of the same or related species. The pgf myelin proteins of teleosts and elasmobranchs lack 116-150; the anti-100-123 mAb did not stain their myelin. In addition to myelin, the anti-178-191 mAb stained many neurons in all species; other mAbs stained distinct neuron subpopulations in different species. Neuronal staining was observed when there was at least approximately 30% identity of the PLP mAb epitope and known pgf neuronal proteins of the same or related species. Thus, anti-human PLP epitope mAbs simultaneously recognize CNS myelin and neurons even without extensive sequence identity. Widespread anti-PLP mAb recognition of neurons suggests a novel potential pathophysiologic mechanism in MS patients, i.e., that anti-PLP antibodies associated with demyelination might simultaneously recognize pgf epitopes in neurons, thereby affecting their functions.

Co-clustering of galactosylceramide and membrane proteins in oligodendrocyte membranes on interaction with polyvalent carbohydrate and prevention by an intact cytoskeleton

We have shown previously that addition of liposomes containing the two major glycosphingolipids of myelin, galactosylceramide (GalC) and cerebroside sulfate (CBS), to cultured oligodendrocytes (OLs) caused clustering of GalC on the extracellular surface and myelin basic protein (MBP) on the cytosolic surface to the same membrane domains. It also caused depolymerization of actin microfilaments and microtubules, indicating that interaction of the liposomes with the OL surface induces transmembrane signal transmission. We show that a multivalent form of galactose conjugated to bovine serum albumin has a similar effect as the multivalent GalC/CBS-containing liposomes. Because GalC and CBS can interact with each other across apposed membranes and because anti-GalC and anti-CBS antibodies also cause redistribution of GalC/CBS and depolymerization of microtubules, we believe that the multivalent carbohydrate interacts with GalC and CBS in the OL membrane. Several myelin-specific transmembrane proteins could be involved in this transmembrane signal transmission from GalC/CBS. We looked at co-clustering of several myelin constituents by confocal microscopy to determine if they are located in or redistribute to GalC/MBP-containing domains. Myelin oligodendrocyte glycoprotein (MOG), proteolipid protein (PLP), MAPK, and some phosphotyrosine-containing proteins were found to co-cluster with GalC and MBP, but myelin-associated glycoprotein (MAG) and phosphatidylinositol-4,5-bisphosphate (PIP(2)) did not. These results suggest that MOG and PLP, but probably not MAG, are possible candidates for transmembrane transmission of the signal received by GalC/CBS. To determine if depolymerization of actin microfilaments was required for co-clustering, or was secondary to clustering, we stabilized F-actin with jasplakinolide. This also prevented depolymerization of the microtubules and prevented clustering of all constituents, including GalC. The prevention of clustering or redistribution of these glycolipids and proteins by an intact cytoskeleton is consistent with the picket fence model.

Cotranslational integration of myelin proteolipid protein (PLP) into the membrane of endoplasmic reticulum: analysis of topology by glycosylation scanning and protease domain protection assay

The four transmembrane domain topology of the proteolipid protein (PLP) in the myelin membrane of the central nervous system (CNS) has been further substantiated by biochemical studies. We have analyzed the cotranslational polytopic integration of nascent PLP during protein synthesis into the membrane of the endoplasmic reticulum (ER) on two routes. Consensus sequences for N-glycosylation were introduced by site directed mutagenesis into the PLP sequence as reporter sites, which upon glycosylation monitor the intraluminal location of the respective domains corresponding to the extracellular side of the plasma membrane. Single, double, and triple mutant cDNAs were constructed for transcription/translation in vitro in the presence of ER-membranes. The glycosylation pattern of the translation products revealed that hydrophilic extramembrane regions 2 and 4 (EMR2/EMR4) and EMR3 of PLP are exposed on opposite sides of the ER membrane. Their localization either at the cytosolic or luminal side of the ER membrane leads to two different topologies. The two modes of membrane integration during in vitro cotranslational translocation were confirmed by protease protection assays with wild-type and truncated PLP polypeptides with either one, two, or three putative transmembrane domains integrated into the ER-membrane. The fragment pattern of the [35S]methionine- or [3H]leucine-labeled polypeptides revealed that EMR3 and EMR4 were exposed with opposite orientation either on the cytosolic or luminal side of the ER membrane supporting the 4-transmembrane helix (TMH) N(in) model with the N and C termini on the cytoplasmic side, as established for the myelin membrane (plasma membrane); the other inversely integrated PLP constructs indicate the 4-TMH-Nout profile. These results are discussed with regard to the PLP biogenesis and the plasma membrane topology in PLP-expressing cells.

A history of proteolipids: a personal memoir

This review is a personal memoir of the history of proteolipids and is limited to aspects of the field with which the author has been involved in one way or another. The discovery of proteolipids was a serendipitous observation made in the course of the study of sulfatides. Initial focus was on the chemical characterization of brain proteolipids, their behavior under different conditions and their identification as the major protein of CNS myelin. The sequence of PLP was obtained using solid phase protein sequencing techniques. This, in turn, made possible a new era in which biochemical, cellular and molecular approaches could be applied to address new questions about PLP. Identification of genetic defects in the PLP molecule and its regulation has contributed to understanding myelin biology. Studies of the encephalitogenic activity of PLP in animal models have influenced the views of inflammatory processes in multiple sclerosis. Despite remarkable progress, much remains to be learned about the structure and function of PLP.

Orientation of myelin proteolipid protein in the oligodendrocyte cell membrane

The orientation of proteins within a cell membrane can often be difficult to determine. A number of models have been proposed for the orientation of the myelin protein, proteolipid protein (PLP), each of which includes exposed domains on the intracellular and extracellular membrane faces. Immunolabeling experiments have localized the C-terminus and the region spanning amino acids 103-116 to the cytoplasmic face of the membrane, but no well characterized antibodies have been available that label extracellular PLP domains. In this report, we describe the generation and characterization of mouse monoclonal antibodies (mAb) against putative extramembrane domains. Three of the mAb, specific for PLP peptides 40-59, 178-191, or 215-232, immunostain live oligodendrocytes, indicating that these regions of the molecule are exposed on the external surface of the cell. In addition, we have used these mAb to study the time-course of incorporation of PLP into the oligodendrocyte membrane. These studies increase our knowledge of the orientation of PLP in the lipid bilayer and are relevant for understanding myelin function.

Identification and characterization of a B-cell determinant within the amphipathic domain (residues 178-238) of the myelin proteolipid protein

Pooled polyclonal rabbit anti-rat myelin and mouse anti-human proteolipid protein (PLP) antisera were screened against a panel of PLP synthetic peptides spanning residues 178-238 of the protein. Cross-reactivity against one determinant defined by PLP(200-219) was particularly prominent in both the anti-myelin and anti-PLP antisera and was chosen for further study. Competitive inhibition studies, utilizing a panel of overlapping synthetic peptides, demonstrated that the C-terminal portion of PLP(200-219), specifically residues comprising PLP(200-217), was important for antibody recognition of this region. Immunohistochemical analyses with an affinity-purified rabbit anti-PLP(200-219) antiserum demonstrated antibody cross-reactivity with PLP in both paraffin- and gelatin-embedded brain sections and immunocytochemical staining of mouse oligodendrocyte-enriched cultures demonstrated antibody binding with native PLP in situ. Staining of living non-permeabilized cells localized binding to the extracellular face of the myelin membrane. Collectively, these data argue for the presence of an immunodominant B-cell determinant defined by PLP residues 200-219. Furthermore, the structural conformation of this determinant in native PLP can be mimicked by the synthetic peptide, resulting in the generation of an antibody reagent that has considerable utility for immunohistochemical and immunocytochemical investigations of PLP expression and localization within the central nervous system myelin membrane.

Minireview: autoimmune responses to myelin proteolipid protein

The authors present a brief historical sketch of the development of our understanding of immune responses to myelin proteolipid protein (PLP) and the acceptance of PLP as a potent antigen in the induction of experimental allergic encephalomyelitis (EAE). The distinct characteristics of the PLP molecule that may contribute to complex immune responses to this protein are reviewed and these responses are compared with those to MBP, both in the pathology of EAE and at the level of the T cell. Recent evidence demonstrating differences between T cell responses to PLP and MBP is reviewed. Finally, the potential contribution of immune responses to PLP in human diseases, particularly multiple sclerosis (MS), that have been identified to date are then summarized.

Mechanisms of myelin basic protein and proteolipid protein targeting in oligodendrocytes (review)

The segregation of proteins to specific cellular membranes is recognized as a common phenomenon. In oligodendrocytes of the central nervous system, localization of certain proteins to select regions of the plasma membrane gives rise to the myelin membrane. Whilst the fundamental structure and composition of myelin is well understood, less is known of the mechanisms by which the constituent proteins are specifically recruited to those regions of plasma membrane that are forming myelin. The two principal proteins of myelin, the myelin basic protein and proteolipid protein, differ greatly in character and sites of synthesis. The message for myelin basic protein is selectively translocated to the ends of the cell processes, where it is translated on free ribosomes and is incorporated directly into the membrane. Proteolipid protein synthesized at the rough endoplasmic reticulum, processed through the Golgi apparatus, and presumably transported via vesicles to the myelin membrane. This review examines the mechanisms by which these two proteins are targeted to the myelin membrane.

Immunolocalization of proteolipid protein peptide 103-116 in myelin

Determination of the topographic orientation of proteolipid protein (PLP) within myelin is part of an overall understanding of the functions of PLP and the roles of its multiple domains in diseases that primarily affect central nervous system (CNS) myelin. As part of an analysis of PLP orientation, two mouse monoclonal antibodies (mAb) and a rabbit antiserum against a synthetic peptide corresponding to PLP residues 103-116 (YKTTICGKGLSATV) were tested for their reactivity on compact CNS myelin. By ELISA, the antibodies react with intact PLP and PLP residues 103-116, but not with other PLP peptides. Ultrathin cryosections of adult rat optic nerve were immunostained and antibody binding was localized using appropriate second antibodies coupled to 1 nm gold particles that were visualized by silver enhancement. Localization of the particles on the major or intermediate dense lines was determined by three independent observers. Using the PLP peptide mAb and the polyclonal antibody, we demonstrated that > or = 71% of the particles were localized on the major dense line. At least 66% of particles directed against myelin basic protein, which is known to occur on the major dense line, were also found in that location. These semiquantitative morphologic observations suggest that PLP residues 103-116 occur on the cytoplasmic face of the myelin membrane.

Humoral immune recognition of proteolipid protein (PLP)-specific encephalitogenic epitopes in the SJL/J mouse

SJL/J mice were immunized with human PLP as well as encephalitogenic PLP peptides 139-151 and 178-191 and the resulting antibody responses examined for immunochemical specificity employing a panel of 17 synthetic PLP peptide ligands. All animals had demonstrable circulating titers of antibodies early in the humoral immune response to their respective encephalitogens, however, there was no clear qualitative correlation between antibody responses and the induction of EAE. In the majority of PLP immunized animals, determinant-specific antibody populations, including those against encephalitogenic centers, were not detectable in the presence of an anti-PLP antibody response. Antiencephalitogenic peptide antibodies were present in both 139-151 and 178-191 immunized animals regardless of clinical/histologic status. Neither group produced cross-reactive anti-PLP antibodies as detected by ELISA. In animals immunized with peptide 139-151, only anti-139-151 antibody specificities were noted. In contrast, all animals immunized with peptide 178-191 had an antibody population cross-reactive with three other PLP peptides: 97-110, 209-217, and 215-228. As humoral immune responses can be demonstrated against PLP-specific encephalitogenic epitopes, the significance of these B cell responses should be considered in the context of their potential role in the development, modulation, and/or potentiation of EAE.

The genetics of myelin

Myelin formation and maintenance requires complex interactions between neurons and glia, and between the integral protein and lipid components of the myelin sheath. Many of the underlying mechanisms may be examined by studying the perturbations caused by spontaneous and targeted mutations in myelin protein genes. This review summarizes the progress in our understanding of these mutations with an emphasis on integrating the recent advances in the genetics of myelin into a more generalized view of myelin organization and function.

The structure and function of central nervous system myelin

Multiple sclerosis (MS) is characterized by the active degradation of central nervous system myelin, a multilamellar membrane system that insulates nerve axons. MS arises from complex interactions between genetic, immunological, infective, and biochemical mechanisms. Although the circumstances of MS etiology remain hypothetical, one persistent theme involves immune system recognition of myelin-specific antigens derived from myelin basic protein, the most abundant extrinsic myelin membrane protein, and/or another equally suitable myelin protein or lipid. Knowledge of the biochemical and physical-chemical properties of myelin proteins, and lipids, particularly their composition, organization, structure, and accessibility with respect to the compacted myelin multilayers, thus becomes central to understanding how and why these antigens become selected during the development of MS. This article focuses on the current understanding of the molecular basis of MS as it may relate to the protein and lipid components of myelin, which dictate myelin morphology on the basis of protein-lipid and lipid-lipid interactions, and the relationship, if any, between the protein/lipid components and the destruction of myelin in pathological situations.