Neutron scattering from myelin revisited: bilayer asymmetry and water-exchange kinetics

Incorporation and localisation of alkanes in a protomembrane model by neutron diffraction

Protomembranes at the origin of life were likely composed of short-chain lipids, readily available on the early Earth. Membranes formed by such lipids are less stable and more permeable under extreme conditions, so a novel membrane architecture was suggested to validate the accuracy of this assumption. The model membrane includes the presence of a layer of alkanes in the mid-plane of the protomembrane in between the two monolayer leaflets and lying perpendicular to the lipid acyl chains. Here, we investigated such a possibility experimentally for membranes formed by the short-chain phospholipid 1,2-didecanoyl-sn-glycero-3-phophocholine, including or not the alkanes eicosane, squalane or triacontane by means of neutron membrane diffraction and contrast variation. We found strong indications for incorporation of two of the three alkanes in the membrane mid-plane through the determination of neutron scattering length density profiles with hydrogenated vs deuterated alkanes and membrane swelling at various relative humidities indicating a slightly increased bilayer thickness when the alkanes are incorporated into the bilayers. The selectivity of the incorporation points out the role of the length of the n-alkanes with respect to the capacity of the membrane to incorporate them.

Distribution and orientation of nerve fibers and myelin assembly in a brain section retrieved by small-angle neutron scattering

The structural connectivity of the brain has been addressed by various imaging techniques such as diffusion weighted magnetic resonance imaging (DWMRI) or specific microscopic approaches based on histological staining or label-free using polarized light (e.g., three-dimensional Polarized Light Imaging (3D-PLI), Optical Coherence Tomography (OCT)). These methods are sensitive to different properties of the fiber enwrapping myelin sheaths i.e. the distribution of myelin basic protein (histology), the apparent diffusion coefficient of water molecules restricted in their movements by the myelin sheath (DWMRI), and the birefringence of the oriented myelin lipid bilayers (3D-PLI, OCT). We show that the orientation and distribution of nerve fibers as well as myelin in thin brain sections can be determined using scanning small angle neutron scattering (sSANS). Neutrons are scattered from the fiber assembly causing anisotropic diffuse small-angle scattering and Bragg peaks related to the highly ordered periodic myelin multilayer structure. The scattering anisotropy, intensity, and angular position of the Bragg peaks can be mapped across the entire brain section. This enables mapping of the fiber and myelin distribution and their orientation in a thin brain section, which was validated by 3D-PLI. The experiments became possible by optimizing the neutron beam collimation to highest flux and enhancing the myelin contrast by deuteration. This method is very sensitive to small microstructures of biological tissue and can directly extract information on the average fiber orientation and even myelin membrane thickness. The present results pave the way toward bio-imaging for detecting structural aberrations causing neurological diseases in future.

Anomalous water dynamics in brain: a combined diffusion magnetic resonance imaging and neutron scattering investigation

Water diffusion is an optimal tool for investigating the architecture of brain tissue on which modern medical diagnostic imaging techniques rely. However, intrinsic tissue heterogeneity causes systematic deviations from pure free-water diffusion behaviour. To date, numerous theoretical and empirical approaches have been proposed to explain the non-Gaussian profile of this process. The aim of this work is to shed light on the physics piloting water diffusion in brain tissue at the micrometre-to-atomic scale. Combined diffusion magnetic resonance imaging and first pioneering neutron scattering experiments on bovine brain tissue have been performed in order to probe diffusion distances up to macromolecular separation. The coexistence of free-like and confined water populations in brain tissue extracted from a bovine right hemisphere has been revealed at the micrometre and atomic scale. The results are relevant for improving the modelling of the physics driving intra- and extracellular water diffusion in brain, with evident benefit for the diffusion magnetic resonance imaging technique, nowadays widely used to diagnose, at the micrometre scale, brain diseases such as ischemia and tumours.

X-ray diffraction reveals blunt-force loading threshold for nanoscopic structural change in ex vivo neuronal tissues

An ex vivo blunt-force loading experiment is reported that may, in the future, provide insight into the molecular structural changes occurring in load-induced conditions such as traumatic brain injury (TBI). TBI appears to manifest in changes in multiple structures and elements within the brain and nervous system. Individuals with a TBI may suffer from cognitive and/or behavioral impairments which can adversely affect their quality of life. Information on the injury threshold of tissue loading for mammalian neurons is critical in the development of a quantified neuronal-level dose-response model. Such a model could aid in the discovery of enhanced methods for TBI detection, treatment and prevention. Currently, thresholds of mechanical load leading to direct force-coupled nanostructural changes in neurons are unknown. In this study, we make use of the fact that changes in the structure and periodicity of myelin may indicate neurological damage and can be detected with X-ray diffraction (XRD). XRD allows access to a nanoscopic resolution range not readily achieved by alternative methods, nor does the experimental methodology require chemical sample fixation. In this study, XRD was used to evaluate the affects of controlled mechanical loading on myelin packing structure in ex vivo optic nerve samples. By using a series of crush tests on isolated optic nerves a quantified baseline for mechanical load was found to induce changes in the packing structure of myelin. To the authors' knowledge, this is the first report of its kind.

Cooling induces phase separation in membranes derived from isolated CNS myelin

Purified myelin membranes (PMMs) are the starting material for biochemical analyses such as the isolation of detergent-insoluble glycosphingolipid-rich domains (DIGs), which are believed to be representatives of functional lipid rafts. The normal DIGs isolation protocol involves the extraction of lipids under moderate cooling. Here, we thus address the influence of cooling on the structure of PMMs and its sub-fractions. Thermodynamic and structural aspects of periodic, multilamellar PMMs are examined between 4°C and 45°C and in various biologically relevant aqueous solutions. The phase behavior is investigated by small-angle X-ray scattering (SAXS) and differential scanning calorimetry (DSC). Complementary neutron diffraction (ND) experiments with solid-supported myelin multilayers confirm that the phase behavior is unaffected by planar confinement. SAXS and ND consistently show that multilamellar PMMs in pure water become heterogeneous when cooled by more than 10–15°C below physiological temperature, as during the DIGs isolation procedure. The heterogeneous state of PMMs is stabilized in physiological solution, where phase coexistence persists up to near the physiological temperature. This result supports the general view that membranes under physiological conditions are close to critical points for phase separation. In presence of elevated Ca²⁺ concentrations (> 10 mM), phase coexistence is found even far above physiological temperatures. The relative fractions of the two phases, and thus presumably also their compositions, are found to vary with temperature. Depending on the conditions, an “expanded” phase with larger lamellar period or a “compacted” phase with smaller lamellar period coexists with the native phase. Both expanded and compacted periods are also observed in DIGs under the respective conditions. The observed subtle temperature-dependence of the phase behavior of PMMs suggests that the composition of DIGs is sensitive to the details of the isolation protocol.

Claudin-11 Tight Junctions in Myelin Are a Barrier to Diffusion and Lack Strong Adhesive Properties

The radial component is a network of interlamellar tight junctions (TJs) unique to central nervous system myelin. Ablation of claudin-11, a TJ protein, results in the absence of the radial component and compromises the passive electrical properties of myelin. Although TJs are known to regulate paracellular diffusion, this barrier function has not been directly demonstrated for the radial component, and some evidence suggests that the radial component may also mediate adhesion between myelin membranes. To investigate the physical properties of claudin-11 TJs, we compared fresh, unfixed Claudin 11-null and control nerves using x-ray and neutron diffraction. In Claudin 11-null tissue, we detected no changes in myelin structure, stability, or membrane interactions, which argues against the notion that myelin TJs exhibit significant adhesive properties. Moreover, our osmotic stressing and D2O-H2O exchange experiments demonstrate that myelin lacking claudin-11 is more permeable to water and small osmolytes. Thus, our data indicate that the radial component serves primarily as a diffusion barrier and elucidate the mechanism by which TJs govern myelin function.

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.