Orientation of myelin proteolipid protein in the oligodendrocyte cell membrane

Correlation Between Anti-Myelin Proteolipid Protein (PLP) Antibodies and Disease Severity in Multiple Sclerosis Patients With PLP Response-Permissive HLA Types

The most prominent pathological features of multiple sclerosis (MS) are demyelination and neurodegeneration. The exact pathogenesis of MS is unknown, but it is generally regarded as a T cell-mediated autoimmune disease. Increasing evidence, however, suggests that other components of the immune system, particularly B cells and antibodies, contribute to the cumulative CNS damage and worsening disability that characterize the disease course in many patients. We have previously described strongly elevated T cell reactivity to an extracellular domain of the most abundant CNS myelin protein, myelin proteolipid protein (PLP) in people with MS. The current paper addresses the question of whether this region of PLP is also a target of autoantibodies in MS. Here we show that serum levels of isotype-switched anti-PLP_181-230 specific antibodies are significantly elevated in patients with MS compared to healthy individuals and patients with other neurological diseases. These anti-PLP_181-230 antibodies can also live-label PLP-transfected cells, confirming that they can recognize native PLP expressed at the cell surface. Importantly, the antibodies are only elevated in patients who carry HLA molecules that allow strong T cell responses to PLP. In that subgroup of patients, there is a positive correlation between the levels of anti-PLP_181-230 antibodies and the severity of MS. These results demonstrate that anti-PLP antibodies have potentially important roles to play in the pathogenesis of MS.

A common mechanism of PLP/DM20 misfolding causes cysteine-mediated endoplasmic reticulum retention in oligodendrocytes and Pelizaeus-Merzbacher disease

A large number of mutations in the human PLP1 gene lead to abnormal myelination and oligodendrocyte death in Pelizaeus-Merzbacher disease (PMD). Here we show that a major subgroup of PMD mutations that map into the extracellular loop region of PLP/DM20 leads to the failure of oligodendrocytes to form the correct intramolecular disulfide bridges. This leads to abnormal protein cross-links and endoplasmic reticulum retention and activates the unfolded protein response. Importantly, surface expression of mutant PLP/DM20 can be restored and the unfolded protein response can be reverted by the removal of two cysteines. Thus, covalent protein cross-links emerge as a cause, rather than as a consequence, of endoplasmic reticulum retention.

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.

Schwann cell expression of PLP1 but not DM20 is necessary to prevent neuropathy

Proteolipid protein (PLP1) and its alternatively spliced isoform, DM20, are the major myelin proteins in the CNS, but are also expressed in the PNS. The proteins have an identical sequence except for 35 amino acids in PLP1 (the PLP1-specific domain) not present in DM20. Mutations of PLP1/DM20 cause Pelizaeus-Merzbacher Disease (PMD), a leukodystrophy, and in some instances, a peripheral neuropathy. To identify which mutations cause neuropathy, we have evaluated a cohort of patients with PMD and PLP1 mutations for the presence of neuropathy. As shown previously, all patients with PLP1 null mutations had peripheral neuropathy. We also identified 4 new PLP1 point mutations that cause both PMD and peripheral neuropathy, three of which truncate PLP1 expression within the PLP1-specific domain, but do not alter DM20. The fourth, a splicing mutation, alters both PLP1 and DM20, and is probably a null mutation. Six PLP1 point mutations predicted to produce proteins with an intact PLP1-specific domain do not cause peripheral neuropathy. Sixty-one individuals with PLP1 duplications also had normal peripheral nerve function. These data demonstrate that expression of PLP1 but not DMSO is necessary to prevent neuropathy, and suggest that the 35 amino acid PLP1-specific domain plays an important role in normal peripheral nerve function.

Different mutations in the same codon of the proteolipid protein gene, PLP, may help in correlating genotype with phenotype in Pelizaeus-Merzbacher disease/X-linked spastic paraplegia (PMD/SPG2)

Pelizaeus-Merzbacher disease/X-linked spastic paraplegia (PMD/SPG2) comprises a spectrum of diseases that range from severe to quite mild. The reasons for the variation in severity are not obvious, but suggested explanations include the extent of disruption of the transmembrane portion of the proteolipid protein caused by certain amino acid substitutions and interference with the trafficking of the PLP molecule in oligodendrocytes. Four codons in which substitution of more than one amino acid has occurred are available for examination of clinical and potential structural manifestations: Valine165 to either glutamate or glycine, leucine 045 to either proline or arginine, aspartate 202 to asparagine or histidine, and leucine 223 to isoleucine or proline. Three of these mutations, Val165Gly, Leu045Pro, and Leu223Ile have not been described previously in humans. The altered amino acids appear in the A-B loop, C helix, and C-D loop, respectively. We describe clinically patients with the mutations T494G (Val165Gly), T134C (Leu045Pro), and C667A (Leu223Ile). We discuss also the previously reported mutations Asp202Asn and Asp202His. We have calculated the changes in hydrophobicity of short sequences surrounding some of these amino acids and compared the probable results of the changes in transmembrane structure of the proteolipid protein for the various mutations with the clinical data available on the patients. While the Val165Glu mutation, which is expected to produce disruption of a transmembrane loop of the protein, produces more severe disease than does Val165Gly, no particular correlation with hydrophobicity is found for the other mutations. As these are not in transmembrane domains, other factors such as intracellular transport or interaction between protein chains during myelin formation are probably at work.

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.

Intracellular transport of the DM-20 bearing shaking pup (shp) mutation and its possible phenotypic consequences

Paralytic tremor (pt) in rabbits and shaking pup (shp) in dogs are allelic dysmyelinated mutants of the proteolipid protein (Plp) gene. Both mutations affect the same amino acid, histidine36, which is replaced by glutamine in pt and by proline in shp. Phenotypic expression of these two mutations is very different. Paralytic tremor presents a much milder form of dysmyelination than shaking pup. The number of oligodendrocytes in the mutant rabbit is normal, while in the dog, the oligodendrocyte number is reduced due to early death or incomplete maturation. We have previously reported an abnormal intracellular transport of the PLPpt, whereas DM-20pt was normally transported to the cell membrane. In the present study, we show that the transport of the two isoforms containing the shp mutation is impaired in transfected Cos-7 cells. Cotransfecting cells with different ratios and combinations of mutated PLP and DM-20 cDNAs, we demonstrated that DM-20pt, but not DM-20shp, facilitates intracellular trafficking and integration into the plasma membrane of either of the two mutated PLPs. The phenotypic difference between these two allelic mutations can result from differences in DM-20 protein trafficking and sorting. These results show that the loss of function of PLP is not position-dependent but depends on the nature of the mutation.

Redefining the lipophilin family of proteolipid proteins

The past few years have seen a dramatic increase in our understanding, in molecular terms, of the involvement of the central nervous system proteolipid protein in myelinogenesis and X-linked genetic diseases. In addition, we have expanded our knowledge of the proteins that have been recruited into the vertebrate myelin membrane over the past 400 million years with the molecular cloning of several cDNAs encoding proteins which are homologous to the proteolipid protein gene. In searching for a name to distinguish these proteins from other "proteolipid" proteins of nonneural origin I propose that we resurrect the term "lipophilins" which describes a small family of unusually hydrophobic integral membrane proteins exhibiting identical topologies and similar physical properties. Two subgroups are distinguishable among the lipophilins based on the patterns of expression during development and the presence or absence of a small motif that is exposed to the extracellular space.

Conservation of topology, but not conformation, of the proteolipid proteins of the myelin sheath

The proteolipid protein gene products DM-20 and PLP are adhesive intrinsic membrane proteins that make up >/=50% of the protein in myelin and serve to stabilize compact myelin sheaths at the extracellular surfaces of apposed membrane lamellae. To identify which domains of DM-20 and PLP are positioned topologically in the extracellular space to participate in adhesion, we engineered N-glycosylation consensus sites into the hydrophilic segments and determined the extent of glycosylation. In addition, we assessed the presence of two translocation stop-transfer signals and, finally, mapped the extracellular and cytoplasmic dispositions of four antibody epitopes. We find that the topologies of DM-20 and PLP are identical, with both proteins possessing four transmembrane domains and N and C termini exposed to the cytoplasm. Consistent with this notion, DM-20 and PLP contain within their N- and C-terminal halves independent stop-transfer signals for insertion into the bilayer of the rough endoplasmic reticulum during de novo synthesis. Surprisingly, the conformation (as opposed to topology) of DM-20 and PLP may differ, which has been inferred from the divergent effects that many missense mutations have on the intracellular trafficking of these two isoforms. The 35 amino acid cytoplasmic peptide in PLP, which distinguishes this protein from DM-20, imparts a sensitivity to mutations in extracellular domains. This peptide may normally function during myelinogenesis to detect conformational changes originating across the bilayer from extracellular PLP interactions in trans and trigger intracellular events such as membrane compaction in the cytoplasmic compartment.