Human proteolipid protein (PLP) mediates winding and adhesion of phospholipid membranes but prevents their fusion

Human myelin proteolipid protein structure and lipid bilayer stacking

The myelin sheath is an essential, multilayered membrane structure that insulates axons, enabling the rapid transmission of nerve impulses. The tetraspan myelin proteolipid protein (PLP) is the most abundant protein of compact myelin in the central nervous system (CNS). The integral membrane protein PLP adheres myelin membranes together and enhances the compaction of myelin, having a fundamental role in myelin stability and axonal support. PLP is linked to severe CNS neuropathies, including inherited Pelizaeus-Merzbacher disease and spastic paraplegia type 2, as well as multiple sclerosis. Nevertheless, the structure, lipid interaction properties, and membrane organization mechanisms of PLP have remained unidentified. We expressed, purified, and structurally characterized human PLP and its shorter isoform DM20. Synchrotron radiation circular dichroism spectroscopy and small-angle X-ray and neutron scattering revealed a dimeric, α-helical conformation for both PLP and DM20 in detergent complexes, and pinpoint structural variations between the isoforms and their influence on protein function. In phosphatidylcholine membranes, reconstituted PLP and DM20 spontaneously induced formation of multilamellar myelin-like membrane assemblies. Cholesterol and sphingomyelin enhanced the membrane organization but were not crucial for membrane stacking. Electron cryomicroscopy, atomic force microscopy, and X-ray diffraction experiments for membrane-embedded PLP/DM20 illustrated effective membrane stacking and ordered organization of membrane assemblies with a repeat distance in line with CNS myelin. Our results shed light on the 3D structure of myelin PLP and DM20, their structure-function differences, as well as fundamental protein-lipid interplay in CNS compact myelin.

Peripheral myelin protein 22 alters membrane architecture

Peripheral myelin protein 22 (PMP22) is highly expressed in myelinating Schwann cells of the peripheral nervous system. PMP22 genetic alterations cause the most common forms of Charcot-Marie-Tooth disease (CMTD), which is characterized by severe dysmyelination in the peripheral nerves. However, the functions of PMP22 in Schwann cell membranes remain unclear. We demonstrate that reconstitution of purified PMP22 into lipid vesicles results in the formation of compressed and cylindrically wrapped protein-lipid vesicles that share common organizational traits with compact myelin of peripheral nerves in vivo. The formation of these myelin-like assemblies depends on the lipid-to-PMP22 ratio, as well as on the PMP22 extracellular loops. Formation of the myelin-like assemblies is disrupted by a CMTD-causing mutation. This study provides both a biochemical assay for PMP22 function and evidence that PMP22 directly contributes to membrane organization in compact myelin.

Remodeling myelination: implications for mechanisms of neural plasticity

One of the most significant paradigm shifts in membrane remodeling is the emerging view that membrane transformation is not exclusively controlled by cytoskeletal rearrangement, but also by biophysical constraints, adhesive forces, membrane curvature and compaction. One of the most exquisite examples of membrane remodeling is myelination. The advent of myelin was instrumental in advancing the nervous system during vertebrate evolution. With more rapid and efficient communication between neurons, faster and more complex computations could be performed in a given time and space. Our knowledge of how myelin-forming oligodendrocytes select and wrap axons has been limited by insufficient spatial and temporal resolution. By virtue of recent technological advances, progress has clarified longstanding controversies in the field. Here we review insights into myelination, from target selection to axon wrapping and membrane compaction, and discuss how understanding these processes has unexpectedly opened new avenues of insight into myelination-centered mechanisms of neural plasticity.

Myelin architecture: zippering membranes tightly together

Rapid nerve conduction requires the coating of axons by a tightly packed multilayered myelin membrane. In the central nervous system, myelin is formed from cellular processes that extend from oligodendrocytes and wrap in a spiral fashion around an axon, resulting in the close apposition of adjacent myelin membrane bilayers. In this review, we discuss the physical principles underlying the zippering of the plasma membrane of oligodendrocytes at the cytoplasmic and extracellular leaflet. We propose that the interaction of the myelin basic protein with the cytoplasmic leaflet of the myelin bilayer triggers its polymerization into a fibrous network that drives membrane zippering and protein extrusion. In contrast, the adhesion of the extracellular surfaces of myelin requires the down-regulation of repulsive components of the glycocalyx, in order to uncover weak and unspecific attractive forces that bring the extracellular surfaces into close contact. Unveiling the mechanisms of myelin membrane assembly at the cytoplasmic and extracelluar sites may help to understand how the myelin bilayers are disrupted and destabilized in the different demyelinating diseases.

Loss of electrostatic cell-surface repulsion mediates myelin membrane adhesion and compaction in the central nervous system

During the development of the central nervous system (CNS), oligodendrocytes wrap their plasma membrane around axons to form a multilayered stack of tightly attached membranes. Although intracellular myelin compaction and the role of myelin basic protein has been investigated, the forces that mediate the close interaction of myelin membranes at their external surfaces are poorly understood. Such extensive bilayer-bilayer interactions are usually prevented by repulsive forces generated by the glycocalyx, a dense and confluent layer of large and negatively charged oligosaccharides. Here we investigate the molecular mechanisms underlying myelin adhesion and compaction in the CNS. We revisit the role of the proteolipid protein and analyze the contribution of oligosaccharides using cellular assays, biophysical tools, and transgenic mice. We observe that differentiation of oligodendrocytes is accompanied by a striking down-regulation of components of their glycocalyx. Both in vitro and in vivo experiments indicate that the adhesive properties of the proteolipid protein, along with the reduction of sialic acid residues from the cell surface, orchestrate myelin membrane adhesion and compaction in the CNS. We suggest that loss of electrostatic cell-surface repulsion uncovers weak and unspecific attractive forces in the bilayer that bring the extracellular surfaces of a membrane into close contact over long distances.

Actin-independent behavior and membrane deformation exhibited by the four-transmembrane protein M6a

M6a is a four-transmembrane protein that is abundantly expressed in the nervous system. Previous studies have shown that over-expression of this protein induces various cellular protrusions, such as neurites, filopodia, and dendritic spines. In this detailed characterization of M6a-induced structures, we found their varied and peculiar characteristics. Notably, the M6a-induced protrusions were mostly devoid of actin filaments or microtubules and exhibited free random vibrating motion. Moreover, when an antibody bound to M6a, the membrane-wrapped protrusions were suddenly disrupted, leading to perturbation of the surrounding membrane dynamics involving phosphoinositide signaling. During single-molecule analysis, M6a exhibited cytoskeleton-independent movement and became selectively entrapped along the cell perimeter in an actin-independent manner. These observations highlight the unusual characteristics of M6a, which may have a significant yet unappreciated role in biological systems.

Critical and off-critical miscibility transitions in model extracellular and cytoplasmic myelin lipid monolayers

Monolayers based on the composition of the cytoplasmic (CYT) or extracellular (EXT) sides of the myelin bilayer form coexisting immiscible liquid phases similar to the liquid-ordered/liquid-disordered phases in phospholipid/cholesterol monolayers. Increasing the temperature or surface pressure causes the two liquid phases to mix, although in significantly different fashion for the CYT and EXT monolayers. The cerebroside-rich EXT monolayer is near a critical composition and the domains undergo coalescence and a circle-to-stripe transition along with significant roughening of the domain boundaries before mixing. The phase transition in the cerebroside-free cytoplasmic side occurs abruptly without domain coalescence; hence, the cytoplasmic monolayer is not near a critical composition, although the domains exhibit shape instabilities within 1-2 mN/m of the transition. The change in mixing pressure decreases significantly with temperature for the EXT monolayer, with dΠ(crit)/dT ∼ 1.5 mN/m/°C, but the mixing pressure of the CYT monolayer varies little with temperature. This is due to the differences in the nonideality of cholesterol interactions with cerebrosides (EXT) relative to phospholipids (CYT). EXT monolayers based on the composition of white matter from marmosets with experimental allergic encephalomyelitis (EAE), an animal model of multiple sclerosis, remain phase-separated at higher surface pressures than control, while EAE CYT monolayers are similar to control. Myelin basic protein, when added to the CYT monolayer, increases lipid miscibility in CYT monolayers; likely done by altering the dipole density difference between the two phases.

Evolution of myelin proteolipid proteins: Gene duplication in teleosts and expression pattern divergence

The coevolution of neurons and their supporting glia to the highly specialized axon-myelin unit included the recruitment of proteolipids as neuronal glycoproteins (DMbeta, DMgamma) or myelin proteins (DMalpha/PLP/DM20). Consistent with a genome duplication at the root of teleosts, we identified three proteolipid pairs in zebrafish, termed DMalpha1 and DMalpha2, DMbeta1 and DMbeta2, DMgamma1 and DMgamma2. The paralogous amino acid sequences diverged remarkably after gene duplication, indicating functional specialization. Each proteolipid has adopted a distinct spatio-temporal expression pattern in neural progenitors, neurons, and in glia. DMalpha2, the closest homolog to mammalian PLP/DM20, is coexpressed with P0 in oligodendrocytes and upregulated after optic nerve lesion. DMgamma2 is expressed in multipotential stem cells, and the other four proteolipids are confined to subsets of CNS neurons. Comparing protein sequences and gene structures from birds, teleosts, one urochordate species, and four invertebrates, we have reconstructed major steps in the evolution of proteolipids.

Myelin proteolipid protein-induced aggregation of lipid vesicles: efficacy of the various molecular species

The different molecular species that form the myelin proteolipid protein family were isolated by size-exclusion and ion-exchange chromatography in organic solvents and their adhesive properties were tested using a vesicle aggregation assay. Addition of the major proteolipid (PLP) to phosphatidylcholine-cholesterol vesicles caused their clustering as determined by increase in O.D.(450 nm) and by transmission electron microscopy. A small fraction of the aggregated vesicles underwent fusion as determined by resonance energy transfer experiments. Vesicle aggregation by PLP, but not the dissociation of the aggregates, was influenced by pH suggesting that electrostatic interactions are important only during cluster formation. Cleavage of disulfide bonds and methylation of carboxyl groups in PLP greatly reduced the aggregating activity, indicating that the process is dependent on the protein's conformation. Unexpectedly, the proteolipid DM-20 was also effective at inducing the clustering of neutral lipid vesicles. In contrast, three protein fractions comprising the naturally-occurring PLP fragments 1-107/112, 113/125-276 and 129/131-276, bearing different net charges, displayed a much lower activity. In addition, trypsin digestion of PLP resulted in a progressive decrease in the protein's ability to induce vesicle aggregation which coincided with the disappearance of the full-length molecule. Together, these results suggest that even large PLP fragments cannot fulfill the adhesive function of the intact protein.

Chemical deacylation reduces the adhesive properties of proteolipid protein and leads to decompaction of the myelin sheath

Myelin proteolipid protein (PLP) contains thioester-bound, long-chain fatty acids which are known to influence the structure of the molecule. To gain further insights into the role of this post-translational modification, we studied the effect that chemical deacylation of PLP had on the morphology of myelin and on the protein's ability to mediate the clustering of lipid vesicles. Incubation of rat optic nerves in isoosmotic solutions containing 100 mM hydroxylamine (HA) pH 7.4 led to deacylation of PLP and decompaction of myelin lamellae at the level of the intraperiod line. Incubation of nerves with milder nucleophilic agents (Tris and methylamine) or diluted HA, conditions that do not remove protein-bound fatty acids, caused no alterations in myelin structure. Other possible effects of HA which could have affected myelin compaction indirectly were ruled out. Incubation of optic nerves with 50 mM dithioerythritol (DTE) also led to the splitting of the myelin intraperiod line and this change again coincided with the removal of fatty acids. In addition, the apparently compacted CNS myelin in the PLP-less myelin-deficient rat, like that in tissue containing deacylated PLP, was readily decompacted upon incubation in isoosmotic buffers, suggesting that the function of PLP as a stabilizer of the interlamellar attachment is, at least in part, mediated by fatty acylation. Furthermore, in contrast to the native protein, PLP deacylated with either HA or DTE failed to induce the clustering of phosphatidylcholine/cholesterol vesicles in vitro. This phenomenon is not due to side-effects of the deacylation procedure since, upon partial repalmitoylation, the protein recovered most of its original vesicle-clustering activity. Collectively, these findings suggest that palmitoylation, by influencing the adhesive properties of PLP, is important for stabilizing the multilamellar structure of myelin.

Myelin basic protein component C1 in increasing concentrations can elicit fusion, aggregation, and fragmentation of myelin-like membranes

Myelin basic protein (MBP) is considered to have a primary role in the formation and maintenance of the myelin sheath. Many studies using artificial vesicle systems of simple lipid composition, and generally small size, have shown that MBP can elicit vesicle fusion, aggregation, or even fragmentation under different conditions. Here, we have studied the effects of increasing concentrations of bovine MBP charge isomer C1 (MBP/C1) on large unilamellar vesicles (LUVs) composed of phosphatidylcholine and phosphatidylserine (92:8 molar ratio), or with a lipid composition similar to that of the myelin membrane in vivo (Cyt-LUVs). Using absorbance spectrophotometry, fluorescence resonance energy transfer, dynamic light scattering and transmission electron microscopy, we have shown that vesicle aggregation and some vesicle fusion occurred upon addition of MBP/C1, and as the molar protein-lipid ratio increased. Fragmentation of Cyt-LUVs was observed at very high protein concentrations. These results showed that the phenomena of vesicle fusion, aggregation, and fragmentation can all be observed in one in vitro system, but were dependent on lipid composition and on the relative proportions of protein and lipid.