Serum IgG antibodies to a 35-kDa P0-related glycoprotein in motor neuron disease

[Introduction to Myelin Research]

Myelin is a multilamellar membrane structure formed by oligodendrocytes in the central nervous system (CNS) and Schwann cells in the peripheral nervous system (PNS). It has been recognized as an insulator that is essential for the rapid and efficient propagation of action potentials by saltatory conduction. However, recently many studies have shown that myelin and myelin-forming cells interact with axons and regulate the nervous system far more actively than previously thought. For example, myelination changes axons dynamically and divides them into four distinct functional domains: node of Ranvier, paranode, juxtaparanode, and internode. Voltage-gated Na⁺ channels are clustered at the node, while K⁺ channels are at the juxtaparanode, and segregation of these channels by paranodal axoglial junction is necessary for proper axonal function. My research experience began at the neurology ward of the Niigata University Medical Hospital, where I saw a patient with peripheral neuropathy of unknown etiology more than 37 years ago. In the patient's serum, we found an autoantibody against a glycolipid enriched in the PNS. Since then, I have been interested in myelin because of its beautiful structure and unique roles in the nervous system. In this review, our recent studies related to CNS and PNS myelin are presented.

Chronic inflammatory demyelinating polyradiculoneuropathy and variants: where we are and where we should go

Chronic inflammatory demyelinating polyradiculoneuropathy (CIDP) is a chronic and often disabling sensory motor neuropathy postulated as caused by an immune attack against peripheral nerve myelin. In addition to a classic sensory–motor polyneuropathy, other phenotypes of CIDP have been described including the Lewis- Sumner syndrome, distal acquired demyelinating symmetric (DADS) neuropathy, pure motor CIDP, pure sensory CIDP including chronic immune sensory polyradiculopathy (CISP), and focal CIDP. These phenotypes are currently considered to be variants of CIDP, even if the possibility that they represent different demyelinating neuropathies cannot be fully excluded considering differences in their response to therapy. Several data support the role of the immune system in the pathogenesis of CIDP even if the precise targets and actors (antibodies and lymphocytes) of this immune response remain uncertain. Recent studies have shown that the therapeutic response may differ in patients with peculiar clinical presentations supporting the hypothesis that different pathogenetic mechanisms may underlie the heterogeneity of CIDP. The majority of patients with CIDP show improvement after immune therapies including corticosteroids, plasma exchange, and high-dose intravenous immunoglobulin (IVIg). It remains unclear why none of the other immune therapies that were reported to be variably effective in other immune disorders proved to be effective also in CIDP.

L-MPZ, a novel isoform of myelin P0, is produced by stop codon readthrough

Myelin protein zero (P0 or MPZ) is a major myelin protein (∼30 kDa) expressed in the peripheral nervous system (PNS) in terrestrial vertebrates. Several groups have detected a P0-related 36-kDa (or 35-kDa) protein that is expressed in the PNS as an antigen for the serum IgG of patients with neuropathy. The molecular structure and function of this 36-kDa protein are, however, still unknown. We hypothesized that the 36-kDa protein may be derived from P0 mRNA by stop codon readthrough. We found a highly conserved region after the regular stop codon in predicted sequences from the 3'-UTR of P0 in higher animals. MS of the 36-kDa protein revealed that both P0 peptides and peptides deduced from the P0 3'-UTR sequence were found among the tryptic fragments. In transfected cells and in an in vitro transcription/translation system, the 36-kDa molecule was also produced from the identical mRNA that produced P0. We designated this 36-kDa molecule as large myelin protein zero (L-MPZ), a novel isoform of P0 that contains an additional domain at the C terminus. In the PNS, L-MPZ was localized in compact myelin. In transfected cells, just like P0, L-MPZ was localized at cell-cell adhesion sites in the plasma membrane. These results suggest that L-MPZ produced by the stop codon readthrough mechanism is potentially involved in myelination. Since this is the first finding of stop codon readthrough in a common mammalian protein, detailed analysis of L-MPZ expression will help to understand the mechanism of stop codon readthrough in mammals.

Serum IgG antibodies to P0 dimer and 35 kDa P0 related protein in neuropathy associated with monoclonal gammopathy

BACKGROUND

Peripheral neuropathies (PN) associated with monoclonal gammopathy (MG) are widely considered as autoimmune disorders, but the putative role of incriminated antigens is still not understood.

OBJECTIVE

Fifty five patients with PN associated with MG were studied to investigate whether new antigens could be found, and to evaluate their relation to clinical manifestations.

METHODS

An immunological study was conducted on patient sera to identify autoreactivities against nerve proteins by western blotting. Antigen proteins were purified and analysed by proteomic tools. Correlation with ultrastrucural and clinical features was then studied.

RESULTS

Of the 55 patients suffering from PN associated with MG, 17 exhibited IgG autoantibodies directed against peripheral nerve proteins of 35, 58, and 60 kDa. N-terminal microsequencing and mass spectrometry analyses of the 35 kDa protein revealed perfect peptidic matching with 47% of the amino acid sequence of P0, whereas the 58 and 60 kDa proteins were identified as the reduced and non-reduced forms of a P0 dimer. Deglycosylation did not affect IgG binding to the 35 kDa P0 related protein, suggesting a peptidic epitope. In contrast, deglycosylation abolished IgG recognition of the P0 dimer protein, so that a carbohydrate moiety may be implicated in the epitope formation. This confirmed the existence of two different types of IgG, one recognising the 58 and 60 kDa proteins and one directed against the 35 kDa protein.

CONCLUSIONS

This is the first report of antibody activity directed against the dimeric association of P0. Although P0 oligomerisation and adhesion properties play a crucial part in the myelin sheath compaction, the pathogenic significance of these autoantibodies needs further investigations to be elucidated.

Ultrastructural identification of peripheral myelin proteins by a pre-embedding immunogold labeling method

Ultrastructural immunolabeling of peripheral nervous system components is an important tool to study the relation between structure and function. Owing to the scarcity of certain antigens and the dense structure of the peripheral nerve, a pre-embedding technique is likely appropriate. After several investigations on procedures for pre-embedding immunolabeling, we propose a method that offers a good compromise between detection of antigenic sites and preservation of morphology at the ultrastructural level, and that is easy to use and suitable for investigations on peripheral nerve biopsies from humans. Pre-fixation by immersion in paraformaldehyde/glutaraldehyde is necessary to stabilize the ultrastructure. Then, ultrasmall gold particles with silver enhancement are advised. Antibodies against myelin protein zero and myelin basic protein were chosen for demonstration. The same technique was applied to localize a 35 kDa myelin protein.

A possible novel isoform of peripheral myelin P0 protein: a target antigen recognized by an autoantibody in a patient with malignant lymphoma and peripheral neuropathy

We tried to characterize a 35-kD antigen recognized by the serum IgG of a patient with malignant lymphoma and peripheral neuropathy. By Western blotting, the serum IgG reacted with the 35-kD antigen in the human, bovine and mouse peripheral nerve (PN) but not with other neural and non-neural tissues. Immunohistochemical analysis showed immunoreactivity of the IgG in the compact myelin of PN. We constructed a human sciatic nerve cDNA library and screened it using IgG of the patient. Three independent clones were obtained. Sequence alignment indicated that the inserts of these clones were homologous to the P0 cDNA, but that all three corresponded to the 3'-untranslational region of the P0 cDNA. To biochemically analyze the 35-kD antigen, myelin fractions of the human and bovine PN were prepared. The 35-kD antigen was purified from the crude myelin fraction by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis. When the immunoreactivities of the 35-kD antigen for the IgG of the patient and a monoclonal anti-P0 antibody were compared with those of P0 protein for these antibodies, the 35-kD antigen reacted with both antibodies, but the P0 protein reacted with only the monoclonal anti-P0 antibody. These results suggest that the 35-kD antigen is an isoform of P0 protein. Although it is unlikely that the autoantibody may be the primary cause of neuropathy, because they were also detected in patients with lymphoma without overt neuropathy, they appear to be a modifying factor in the progression of neuropathy.

Clinical features and anti-neural reactivity in neuropathy associated with IgG monoclonal gammopathy of undetermined significance

Neuropathy has been frequently reported in patients with IgG monoclonal gammopathy of undetermined significance (MGUS) but it is still unclear whether this association has clinical or pathogenetic relevance. In order to clarify the possible role of IgG MGUS in the neuropathy we correlated the clinical and electrophysiological features of the neuropathy with the duration and anti-neural activity of the M-protein in 17 patients with neuropathy and IgG MGUS. Ten patients (59%) had a chronic demyelinating neuropathy clinically indistinguishable from chronic inflammatory demyelinating polyneuropathy (CIDP) while 7 (41%) had a predominantly sensory axonal or mixed neuropathy. In 80% of patients in the CIDP-like and 28% in the sensory group the IgG M-protein became manifest several months to years after onset of the neuropathy. Antibodies to one or more neural antigens (including tubulin, a 35KD P0-like nerve myelin glycoprotein, GD1a, GM1 and chondrotin sulfate C) were found in 40% of patients with CIDP-like and 43% with sensory neuropathy but also in 37% patients with IgG MGUS without neuropathy. Neuropathy associated with IgG MGUS is probably less heterogeneous than previously considered suggesting that this association may not be merely casual. The evidence for primary pathogenetic role of IgG M-proteins in the neuropathy remains however elusive.

Characterization and partial purification of a novel 36 kDa peripheral myelin protein recognized by the sera of patients with neurological disorders

Sera of some patients with acquired sensory neuropathy, chronic inflammatory demyelinating polyradiculoneuropathy and motor neuron disease have high titres of IgG autoantibodies to a minor human peripheral nerve glycoprotein of approximately 36 kDa. This protein cofractionated with PNS myelin and was also found in bovine and rat nerve but not in CNS myelin or other nonneural human tissues. The N-terminal sequence revealed that this protein is related to the major myelin protein P0. Monoclonal antibodies to P0 and to the carbohydrate epitope HNK-1 did not recognize the 36-kDa protein, and the human anti-36-kDa antibodies did not bind to P0. IgG binding to this protein was not abolished after periodate oxidation or deglycosylation, suggesting that the epitope recognized by the human antibodies is peptidic. Differential glycosylation did not account for the differences in the apparent molecular weight between these two proteins. Overall our results indicate that the 36-kDa protein is a variant of P0.

Immunological investigation of chronic inflammatory demyelinating polyradiculoneuropathy

In order to investigate the hypothesis that chronic inflammatory demyelinating polyradiculoneuropathy (CIDP) is an autoimmune disease related to the acute inflammatory form of Guillain-Barre Syndrome (GBS), we studied 40 patients, 40 age and sex matched controls with other forms of peripheral neuropathy (ONP) and 37 controls from the same family or household (FC). We sought antibodies to gangliosides GM1 and LM1 by enzyme linked immunoassay (ELISA) confirmed by immuno-overlay. Only 6 (15%) CIDP patients had IgM antibodies to ganglioside GM1 (GM1) and none had IgG antibodies. We found IgM antibodies to ganglioside LM1 in 2 (5%) and IgG antibodies in 4 (10%) CIDP patients. Antibodies of IgG or IgM class were detected by ELISA to chondroitin sulphate C or sulfatide in up to 7.5% of CIDP patients. There were IgM antibodies in 3 (7.5%) and IgG in 4 (10%) patients against 25, 28 or 36 kD myelin proteins identified by immunoblot. Antibodies to any of these candidate myelin autoantigens were not significantly more frequent in CIDP than FC or ONP controls. Sera from 5 CIDP patients with active disease which subsequently responded to plasma exchange did not induce more demyelination upon intraneural injection into rat sciatic nerve than ONP sera. Serum tumor necrosis factor alpha (TNFalpha) concentrations were not increased in any of the CIDP patients. Serological evidence of Campylobacter jejuni (Cj) infection was present in 4 (10%) CIDP patients. IgM antibodies to cytomegalovirus (CMV) were not detected in any sera. CIDP is not commonly associated with either of these infections or with an antibody-mediated response to any of these glycolipid or myelin autoantigens.