Purification of P0 myelin glycoprotein by a Cu2+-immobilized metal affinity chromatography

GlcNAc6ST-1 regulates sulfation of N-glycans and myelination in the peripheral nervous system

Highly specialized glial cells wrap axons with a multilayered myelin membrane in vertebrates. Myelin serves essential roles in the functioning of the nervous system. Axonal degeneration is the major cause of permanent neurological disability in primary myelin diseases. Many glycoproteins have been identified in myelin, and a lack of one myelin glycoprotein results in abnormal myelin structures in many cases. However, the roles of glycans on myelin glycoproteins remain poorly understood. Here, we report that sulfated N-glycans are involved in peripheral nervous system (PNS) myelination. PNS myelin glycoproteins contain highly abundant sulfated N-glycans. Major sulfated N-glycans were identified in both porcine and mouse PNS myelin, demonstrating that the 6-O-sulfation of N-acetylglucosamine (GlcNAc-6-O-sulfation) is highly conserved in PNS myelin between these species. P0 protein, the most abundant glycoprotein in PNS myelin and mutations in which at the glycosylation site cause Charcot-Marie-Tooth neuropathy, has abundant GlcNAc-6-O-sulfated N-glycans. Mice deficient in N-acetylglucosamine-6-O-sulfotransferase-1 (GlcNAc6ST-1) failed to synthesize sulfated N-glycans and exhibited abnormal myelination and axonal degeneration in the PNS. Taken together, this study demonstrates that GlcNAc6ST-1 modulates PNS myelination and myelinated axonal survival through the GlcNAc-6-O-sulfation of N-glycans on glycoproteins. These findings may provide novel insights into the pathogenesis of peripheral neuropathy.

Glycans of myelin proteins

Human P0 is the main myelin glycoprotein of the peripheral nervous system. It can bind six different glycans, all linked to Asn(93) , the unique glycosylation site. Other myelin glycoproteins, also with a single glycosylation site (PMP22 at Asn(36) , MOG at Asn(31) ), bind only one glycan. The MAG has 10 glycosylation sites; the glycoprotein OMgp has 11 glycosylation sites. Aside from P0, no comprehensive data are available on other myelin glycoproteins. Here we review and analyze all published data on the physicochemical structure of the glycans linked to P0, PMP22, MOG, and MAG. Most data concern bovine P0, whose glycan moieties have an MW ranging from 1,294.56 Da (GP3) to 2,279.94 Da (GP5). The pI of glycosylated P0 protein varies from pH 9.32 to 9.46. The most charged glycan is MS2 containing three sulfate groups and one glucuronic acid; whereas the least charged one is the BA2 residue. All glycans contain one fucose and one galactose. The most mannose rich are the glycans MS2 and GP4, each of them has four mannoses; OPPE1 contains five N-acetylglucosamines and one sulfated glucuronic acid; GP4 contains one sialic acid. Furthermore, human P0 variants causing both gain and loss of glycosylation have been described and cause peripheral neuropathies with variable clinical severity. In particular, the substitution T(95) →M is a very common in Europe and is associated with a late-onset axonal neuropathy. Although peripheral myelin is made up largely of glycoproteins, mutations altering glycosylation have been described only in P0. This attractive avenue of research requires further study.

High-resolution structural model of porcine P2 myelin membrane protein with associated fatty acid ligand: fact or artifact?

Myelin membrane is a biological complex of glial cells origin; it is composed of 25% (w/w) proteins and 75% lipids, and more than 300 proteins are associated with central nervous system myelin (for peripheral nervous system myelin, such data are lacking). Myelin plays an important role in maintaining propagation of nerve signals. To uncover the nature of propagation phenomena, it is essential to study biochemistry of myelin proteins and lipids, myelin composition, and myelin structure. Nearly all myelin proteins are like antigens, causing clinically well-defined devastating diseases; multiple sclerosis and Guillain-Barré syndrome are two of them. In this article, a high-resolution study (1.8 Å) of porcine myelin P2 protein is presented. Myelin was purified from porcine intradural spinal roots, which were stored at -80°C for 10 years before myelin and P2 protein were purified (spinal roots were a gift of Prof. Kunio Kitamura, Saitama Medical School). The three-dimensional structural analysis uncovered embedded 18-carbons-long fatty acid. Some speculative interpretation is presented, to uncover how this ligand of fatty acid may form cholesterol ester and stabilize the myelin structure or form simple raft microdomain. Protein crystallography indicates that the ligand may be 18-carbons-long fatty acid. This is unlike previous work with mass spectrometry, in which three ligands were determined. In other protein crystallography-based studies of P2 (bovine), an oleic fatty acid was suggested, but, for recombinant (human) protein, palmitic acid was found. There is no fatty acid ligand in equine P2 protein.

N,N-diethyldithiocarbamate promotes oxidative stress prior to myelin structural changes and increases myelin copper content

Dithiocarbamates are a commercially important class of compounds that can produce peripheral neuropathy in humans and experimental animals. Previous studies have supported a requirement for copper accumulation and enhanced lipid peroxidation in dithiocarbamate-mediated myelinopathy. The study presented here extends previous investigations in two areas. Firstly, although total copper levels have been shown to increase within the nerve it has not been determined whether copper is increased within the myelin compartment, the primary site of lesion development. Therefore, the distribution of copper in sciatic nerve was characterized using synchrotron X-ray fluorescence microscopy to determine whether the neurotoxic dithiocarbamate, N,N-diethyldithiocarbamate, increases copper levels in myelin. Secondly, because lipid peroxidation is an ongoing process in normal nerve and the levels of lipid peroxidation products produced by dithiocarbamate exposure demonstrated an unusual cumulative dose response in previous studies the biological impact of dithiocarbamate-mediated lipid peroxidation was evaluated. Experiments were performed to determine whether dithiocarbamate-mediated lipid peroxidation products elicit an antioxidant response through measuring the protein expression levels of three enzymes, superoxide dismutase 1, heme oxygenase 1, and glutathione transferase alpha, that are linked to the antioxidant response element promoter. To establish the potential of oxidative injury to contribute to myelin injury the temporal relationship of the antioxidant response to myelin injury was determined. Myelin structure in peripheral nerve was assessed using multi-exponential transverse relaxation measurements (MET(2)) as a function of exposure duration, and the temporal relationship of protein expression changes relative to the onset of changes in myelin integrity were determined. Initial assessments were also performed to explore the potential contribution of dithiocarbamate-mediated inhibition of proteasome function and inhibition of cuproenzyme activity to neurotoxicity, and also to assess the potential of dithiocarbamates to promote oxidative stress and injury within the central nervous system. These evaluations were performed using an established model for dithiocarbamate-mediated demyelination in the rat utilizing sciatic nerve, spinal cord and brain samples obtained from rats exposed to N,N-diethyldithiocarbamate (DEDC) by intra-abdominal pumps for periods of 2, 4, and 8 weeks and from non exposed controls. The data supported the ability of DEDC to increase copper within myelin and to enhance oxidative stress prior to structural changes detectable by MET(2). Evidence was also obtained that the excess copper produced by DEDC in the central nervous system is redox active and promotes oxidative injury.

The peripheral neuropathy-linked Trembler and Trembler-J mutant forms of peripheral myelin protein 22 are folding-destabilized

Dominant mutations in the tetraspan membrane protein peripheral myelin protein 22 (PMP22) are known to result in peripheral neuropathies such as Charcot-Marie-Tooth type 1A (CMT1A) disease via mechanisms that appear to be closely linked to misfolding of PMP22 in the membrane of the endoplasmic reticulum (ER). To characterize the molecular defects in PMP22, we examined the structure and stability of two human disease mutant forms of PMP22 that are also the basis for mouse models of peripheral neuropathies: G150D ( Trembler phenotype) and L16P ( Trembler-J phenotype). Circular dichroism and NMR spectroscopic studies indicated that, when folded, the three-dimensional structures of these disease-linked mutants are similar to that of the folded wild-type protein. However, the folded forms of the mutants were observed to be destabilized relative to the wild-type protein, with the L16P mutant being particularly unstable. The rate of refolding from an unfolded state was observed to be very slow for the wild-type protein, and no refolding was observed for either mutant. These results lead to the hypothesis that ER quality control recognizes the G150D and L16P mutant forms of PMP22 as defective through mechanisms closely related to their conformational instability and/or slow folding. It was also seen that wild-type PMP22 binds Zn(II) and Cu(II) with micromolar affinity, a property that may be important to the stability and function of this protein. Zn(II) was able to rescue the stability defect of the Tr mutant.

Mycobacterium leprae binds to a major human peripheral nerve glycoprotein myelin P zero (P0)

We have previously shown that a major phosphorylated 25-kDa glycoprotein of the human peripheral nerve binds to Mycobacterium leprae. In the present study, we confirm that the 25-kDa glycoprotein of the human peripheral nerve is myelin P zero (P0) by immunoprecipitation and Western blot experiments using monoclonal antibodies to myelin P0. Immunohistochemical studies on human nerve using these antibodies to myelin P0 exhibited a strong immunoreactivity to the myelin and Schwann cells. Myelin P0 is a peripheral nerve specific protein; therefore it could likely be one of the key target molecules for M. leprae binding/internalization or even contact-dependent demyelination. This finding of M. leprae binding to myelin P0 adds to the present understanding on neural predilection of M. leprae.

Towards crystallization of hydrophobic myelin glycoproteins: P0 and PASII/PMP22

The preparation of a pure and homogeneous protein sample at proper concentration is a prerequisite for success when attempting their crystallization for structural determination. The detergents suitable for solubilization particularly of membrane proteins are not always the best for crystallization. Myelin of the peripheral nervous system of vertebrates is the example of a membrane for which neutral or "gentle" detergents are not even strong enough to solubilize its proteins. In contrast, sodium- or lithium-dodecyl sulfate is very effective. We solubilized myelin membrane in 2%(w/v) sodium dodecyl sulfate, followed by chromatographic purification of the hydrophobic myelin glycoproteins P0 and PASII/PMP22, and finally, we have exchanged the sodium dodecyl sulfate bound to protein for other neutral detergents using ceramic hydroxyapatite column. Theoretically, we should easily exchange sodium dodecyl sulfate for any neutral detergent, but for some of them, the solubility of myelin glycoproteins is low. To monitor the potential variability in the secondary structure of glycoproteins, we have used circular dichroism. Sodium dodecyl sulfate seems to be the appropriate detergent for the purpose of purification of very hydrophobic glycoproteins, since it can be easily exchanged for another neutral detergent.

Solubilization of PNS myelin membrane proteins by detergents

We have applied 25 different detergents to the PNS myelin membrane. The extracts produced were analyzed regarding the total amount of protein solubilized, and which myelin glycoproteins were solubilized. The degree of protein extraction was correlated with the critical micelle concentration. For some detergents the protein extraction ability depends clearly on critical micelle concentration. These detergents are more potent if critical micelle concentration is small. The other type of extraction is independent on the critical micelle concentration, but extractability of proteins is low. The best detergents for solubilization of myelin proteins are neutral of the alkyl chain length n =9-11, as follows: dodecyl-beta-D-maltopyranoside, decyl-beta-D-maltopyranoside, n-dodecylsulfobetaine. We expect that these detergents will also be suitable for crystallization of P0 and PASII/PMP22 glycoproteins.