Peripheral myelin of Xenopus laevis: role of electrostatic and hydrophobic interactions in membrane compaction

Myelin basic protein dynamics from out-of-equilibrium functional state to degraded state in myelin

Living matter is a quasi-stationary out-of-equilibrium system; in this physical condition, structural fluctuations at nano- and meso-scales are needed to understand the physics behind its biological functionality. Myelin has a simple ultrastructure whose fluctuations show correlated disorder in its functional out-of-equilibrium state. However, there is no information on the relationship between this correlated disorder and the dynamics of the intrinsically disordered Myelin Basic Protein (MBP) which is expected to influence the membrane structure and overall functionality. In this work, we have investigated the role of this protein structural dynamics in the myelin ultrastructure fluctuations in various conditions, by using synchrotron Scanning micro X Ray Diffraction and Small Angle X ray Scattering. We have induced the crossover from out-of-equilibrium functional state to in-equilibrium degeneration changing the pH to values far from physiological condition. The observed compression of the cytosolic layer thickness probes that the intrinsic large MBP fluctuations preserve the cytosol structure also in the degraded state. Thus, the transition of myelin ultrastructure from correlated to uncorrelated disordered state, is principally affected by the deformation of the membrane and extracellular domain.

Acetyl-11-keto-β-boswellic acid regulates the repair of rat sciatic nerve injury by promoting the proliferation of Schwann cells

AIMS

This study aimed to study the effects of acetyl-11-keto-β-boswellic acid (AKBA) on the regeneration of injured peripheral nerves and the ability of the extracellular signal-regulated kinase (ERK) signaling pathway to regulate the proliferation of Schwann cells and the formation of myelin.

MAIN METHODS

A sciatic nerve crush injury model rats were randomly divided into the model control, low-, medium-, and high-dose AKBA groups. The repair of myelin damage was observed through Luxol Fast Blue staining and the expression of neurofilament-200 (NF200) protein was detected through immunohistochemical tests. The relative expression levels of ERK, Phosphorylated-ERK (p-ERK), c-Jun N-terminal Kinase (JNK), and Phosphorylated-JNK (p-JNK) proteins were detected in vitro in Schwann cells treated with AKBA. The effect of AKBA on P0 and P75 protein expression in Schwann cells was detected through siRNA-mediated ERK gene knockout.

KEY FINDINGS

AKBA promotes the repair of rat sciatic nerve injury by elevating the phosphorylation of the ERK signaling pathway and by regulating the proliferation and myelination of Schwann cells.

SIGNIFICANCE

This test can provide data support for AKBA to repair sciatic nerve injury, provide a theoretical basis for further revealing AKBA repair mechanism, and provide reference for clinical development of sciatic nerve injury drugs.

Nanoscale Correlated Disorder in Out-of-Equilibrium Myelin Ultrastructure

Ultrastructural fluctuations at nanoscale are fundamental to assess properties and functionalities of advanced out-of-equilibrium materials. We have taken myelin as a model of supramolecular assembly in out-of-equilibrium living matter. Myelin sheath is a simple stable multilamellar structure of high relevance and impact in biomedicine. Although it is known that myelin has a quasi-crystalline ultrastructure, there is no information on its fluctuations at nanoscale in different states due to limitations of the available standard techniques. To overcome these limitations, we have used scanning micro X-ray diffraction, which is a unique non-invasive probe of both reciprocal and real space to visualize statistical fluctuations of myelin order of the sciatic nerve of Xenopus laevis. The results show that the ultrastructure period of the myelin is stabilized by large anticorrelated fluctuations at nanoscale, between hydrophobic and hydrophilic layers. The ratio between the total thickness of hydrophilic and hydrophobic layers defines the conformational parameter, which describes the different states of myelin. Our key result is that myelin in its out-of-equilibrium functional state fluctuates point-to-point between different conformations showing a correlated disorder described by a Levy distribution. As the system approaches the thermodynamic equilibrium in an aged state, the disorder loses its correlation degree and the structural fluctuation distribution changes to Gaussian. In a denatured state at low pH, it changes to a completely disordered stage. Our results aim to clarify the degradation mechanism in biological systems by associating these states with ultrastructural dynamic fluctuations at nanoscale.

Imaging Myelination In Vivo Using Transparent Animal Models

Myelination by oligodendrocytes in the central nervous system (CNS) and Schwann cells in the peripheral nervous system is essential for nervous system function and health. Despite its importance, we have a relatively poor understanding of the molecular and cellular mechanisms that regulate myelination in the living animal, particularly in the CNS. This is partly due to the fact that myelination commences around birth in mammals, by which time the CNS is complex and largely inaccessible, and thus very difficult to image live in its intact form. As a consequence, in recent years much effort has been invested in the use of smaller, simpler, transparent model organisms to investigate mechanisms of myelination in vivo. Although the majority of such studies have employed zebrafish, the Xenopus tadpole also represents an important complementary system with advantages for investigating myelin biology in vivo. Here we review how the natural features of zebrafish embryos and larvae and Xenopus tadpoles make them ideal systems for experimentally interrogating myelination by live imaging. We outline common transgenic technologies used to generate zebrafish and Xenopus that express fluorescent reporters, which can be used to image myelination. We also provide an extensive overview of the imaging modalities most commonly employed to date to image the nervous system in these transparent systems, and also emerging technologies that we anticipate will become widely used in studies of zebrafish and Xenopus myelination in the near future.

Evolution of myelin ultrastructure and the major structural myelin proteins

Myelin sheaths, as the specialized tissue wrapping the nerve fibers in the central and peripheral nervous systems (CNS and PNS), are responsible for rapid conduction of electrical signals in these fibers. We compare the nerve myelin sheaths of different phylogenetic origins-including mammal, rodent, bird, reptile, amphibian, lungfish, teleost, and elasmobranch-with respect to periodicities and inter-membrane separations at their cytoplasmic and extracellular appositions, and correlate these structural parameters with biochemical composition. P0 glycoprotein and P0-like proteins are present in PNS of terrestrial species or land vertebrates (Tetrapod) and in CNS and PNS of aquatic species. Proteolipid protein (PLP) is a major component only in the CNS myelin of terrestrial species and is involved in compaction of the extracellular apposition. The myelin structures of aquatic garfish and lungfish, which contain P0-like protein both in CNS and PNS, are similar to those of terrestrial species, indicating that they may be transitional organisms between water and land species. This article is part of a Special Issue entitled SI: Myelin Evolution.

Changes of statistical structural fluctuations unveils an early compacted degraded stage of PNS myelin

Degradation of the myelin sheath is a common pathology underlying demyelinating neurological diseases from Multiple Sclerosis to Leukodistrophies. Although large malformations of myelin ultrastructure in the advanced stages of Wallerian degradation is known, its subtle structural variations at early stages of demyelination remains poorly characterized. This is partly due to the lack of suitable and non-invasive experimental probes possessing sufficient resolution to detect the degradation. Here we report the feasibility of the application of an innovative non-invasive local structure experimental approach for imaging the changes of statistical structural fluctuations in the first stage of myelin degeneration. Scanning micro X-ray diffraction, using advances in synchrotron x-ray beam focusing, fast data collection, paired with spatial statistical analysis, has been used to unveil temporal changes in the myelin structure of dissected nerves following extraction of the Xenopus laevis sciatic nerve. The early myelin degeneration is a specific ordered compacted phase preceding the swollen myelin phase of Wallerian degradation. Our demonstration of the feasibility of the statistical analysis of SµXRD measurements using biological tissue paves the way for further structural investigations of degradation and death of neurons and other cells and tissues in diverse pathological states where nanoscale structural changes may be uncovered.

Myelin sheaths are formed with proteins that originated in vertebrate lineages

All vertebrate nervous systems, except those of agnathans, make extensive use of the myelinated fiber, a structure formed by coordinated interplay between neuronal axons and glial cells. Myelinated fibers, by enhancing the speed and efficiency of nerve cell communication allowed gnathostomes to evolve extensively, forming a broad range of diverse lifestyles in most habitable environments. The axon-covering myelin sheaths are structurally and biochemically novel as they contain high portions of lipid and a few prominent low molecular weight proteins often considered unique to myelin. Here we searched genome and EST databases to identify orthologs and paralogs of the following myelin-related proteins: (1) myelin basic protein (MBP), (2) myelin protein zero (MPZ, formerly P0), (3) proteolipid protein (PLP1, formerly PLP), (4) peripheral myelin protein-2 (PMP2, formerly P2), (5) peripheral myelin protein-22 (PMP22) and (6) stathmin-1 (STMN1). Although widely distributed in gnathostome/vertebrate genomes, neither MBP nor MPZ are present in any of nine invertebrate genomes examined. PLP1, which replaced MPZ in tetrapod CNS myelin sheaths, includes a novel 'tetrapod-specific' exon (see also Möbius et al., 2009). Like PLP1, PMP2 first appears in tetrapods and like PLP1 its origins can be traced to invertebrate paralogs. PMP22, with origins in agnathans, and STMN1 with origins in protostomes, existed well before the evolution of gnathostomes. The coordinated appearance of MBP and MPZ with myelin sheaths and of PLP1 with tetrapod CNS myelin suggests interdependence - new proteins giving rise to novel vertebrate structures.

Myelin structure and composition of myelinated tissue in the African lungfish

To analyze myelin structure and the composition of myelinated tissue in the African lungfish(Protopterus dolloi), we used a combination of ultrastructural and biochemical techniques. Electron microscopy showed typical multilamellar myelin: CNS sheaths abutted one another, and PNS sheaths were separated by endoneurial collagen. The radial component, prominent in CNS myelin of higher vertebrates, was suggested by the pattern of staining but was poorly organized. The lipid and myelin protein compositions of lungfish tissues more closely resembled those of teleost than those of higher vertebrates (frog, mouse). Of particular note, for example, lungfish glycolipids lacked hydroxy fatty acids. Native myelin periodicities from unfixed nerves were in the range of those for higher vertebrates rather than for teleost fish. Lungfish PNS myelin had wider inter-membrane spaces compared with other vertebrates, and lungfish CNS myelin had spaces that were closer in value to those in mammalian than to amphibian or teleost myelins. The membrane lipid bilayer was narrower in lungfish PNS myelin compared to other vertebrates, whereas in the CNS myelin the bilayer was in the typical range. Lungfish PNS myelin showed typical compaction and swelling responses to incubation in acidic or alkaline hypotonic saline. The CNS myelin, by contrast, did not compact in acidic saline but did swell in the alkaline solution. This lability was more similar to that for the higher vertebrates than for teleost.

Analysis of peripheral nerve expression profiles identifies a novel myelin glycoprotein, MP11

The myelin sheath insulates axons and allows for rapid salutatory conduction in the nervous system of all vertebrates. The formation of peripheral myelin requires expression of the transcription factor Egr2, which is responsible for inducing such essential myelin-associated genes as Mpz, Mbp, Pmp22, and Mag. Using microarray analysis to compare gene expression patterns in peripheral nerve during development, during remyelination after nerve injury, and in a congenital hypomyelinating mouse model, we identified an evolutionarily conserved novel component of myelin called Mp11 (myelin protein of 11 kDa). The Mp11 genomic locus contains multiple conserved Egr binding sites, and Mp11 induction is regulated by the expression of Egr2. Similar to other Egr2-dependent genes, it is induced during developmental myelination and remyelination after nerve injury. Mp11 is a glycoprotein expressed preferentially in the myelin of the peripheral nervous system versus CNS and is specifically localized to the Schmidt-Lanterman incisures and paranodes of peripheral nerve. The Mp11 protein contains no identifiable similarity to other known protein domains or motifs. However, like other myelin genes, strict Mp11 expression levels are a requirement for the in vitro myelination of DRG neurons, indicating that this previously uncharacterized gene product is a critical component of peripheral nervous system myelin.