Human myelin proteolipid protein structure and lipid bilayer stacking

Abstract

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.

Roadmap for Postnatal Brain Maturation: Changes in Gray and White Matter Composition during Development Measured by Fourier Transformed Infrared Microspectroscopy

“…Furthermore, a shift from the α-helix to the intramolecular β-sheet has been detected by μFTIR in the peri-infarct zone of rat brain ischemic lesions or in the CC after traumatic brain injury, coinciding, in the first study, with myelin loss detected by electron microscopy. , Changes in protein structure have been related to different functions, and indeed, its alteration has been associated with demyelination. – This early evidence suggests that properly compacted and functional myelin favored the α-helix conformation of the main compacting proteins MBP and PLP. In support of this data, PLP, the most abundant protein in CNS myelin, contains several domains that adopt an α-helical structure in lipid bilayers . MBP has specific domains with α-helix and β-sheet conformational states, but the α-helix domain increases with lipid interactions, especially more with saturated than unsaturated fatty acids. ,– Therefore, in this study, we suggest that myelin bundle formation and its alterations could be detected by measuring intramolecular β-sheet/α-helix ratios in WM.…”

“…In support of this data, PLP, the most abundant protein in CNS myelin, contains several domains that adopt an α- helical structure in lipid bilayers. 60 MBP has specific domains with α-helix and β-sheet conformational states, but the α-helix domain increases with lipid interactions, especially more with saturated than unsaturated fatty acids. 54,61−64 Therefore, in this study, we suggest that myelin bundle formation and its alterations could be detected by measuring intramolecular βsheet/α-helix ratios in WM.…”

Overview of myelin, major myelin lipids, and myelin-associated proteins

“…The larger isoform weights 30 kDa and is 277 amino acids long, and the shorter isoform, PLP/DM20, is 26 kDa and is identical in sequence to the longer version, except for a deletion of 35 amino acids in the intracellular loop ( Spörkel et al, 2002 ). A recent publication shows that both full-length human PLP and its shorter DM20 isoform have a dimeric, α-helical conformation and discusses structural differences between the isoforms in terms of their impact on protein function and interaction with lipids ( Ruskamo et al, 2022 ).…”

Proteomic profiling of aging brains identifies key proteins by which cognitively healthy centenarians defy their age by decades

“…2′,3′-cyclic nucleotide 3′-phosphodiesterase (CNP) likely protects axons by removing toxic 2′,3′-cAMP and to replace it with axonal protectant adenosine 41 . Myelin-associated glycoprotein (MAG) and Myelin 10 oligodendrocyte glycoprotein (MOG) are critical for the formation and maintenance of myelin sheaths [42][43][44][45][46] . In line with the resistance to age-related axonal loss, centenarians also resisted decrease in neurofilaments.…”