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

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.

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.

Lipid Order Degradation in Autoimmune Demyelination Probed by Polarized Coherent Raman Microscopy

Myelin around axons is currently widely studied by structural analyses and large-scale imaging techniques, with the goal to decipher its critical role in neuronal protection. Although there is strong evidence that in myelin, lipid composition, and lipid membrane morphology are affected during the progression of neurodegenerative diseases, there is no quantitative method yet to report its ultrastructure in tissues at both molecular and macroscopic levels, in conditions potentially compatible with in vivo observations. In this work, we study and quantify the molecular order of lipids in myelin at subdiffraction scales, using label-free polarization-resolved coherent anti-Stokes Raman, which exploits coherent anti-Stokes Raman sensitivity to coupling between light polarization and oriented molecular vibrational bonds. Importantly, the method does not use any a priori parameters in the sample such as lipid type, orientational organization, and composition. We show that lipid molecular order of myelin in the mouse spinal cord is significantly reduced throughout the progression of experimental autoimmune encephalomyelitis, a model for multiple sclerosis, even in myelin regions that appear morphologically unaffected. This technique permits us to unravel molecular-scale perturbations of lipid layers at an early stage of the demyelination progression, whereas the membrane architecture at the mesoscopic scale (here ∼100 nm) seems much less affected. Such information cannot be brought by pure morphological observation and, to our knowledge, brings a new perspective to molecular-scale understanding of neurodegenerative diseases.

Heterogeneous and self-organizing mineralization of bone matrix promoted by hydroxyapatite nanoparticles

The mineralization process is crucial to the load-bearing characteristics of the bone extracellular matrix. In this work, we have studied the spatiotemporal dynamics of mineral deposition by human bone marrow mesenchymal stem cells differentiating toward osteoblasts promoted by the presence of exogenous hydroxyapatite nanoparticles. At the molecular level, the added nanoparticles positively modulated the expression of bone-specific markers and enhanced calcified matrix deposition during osteogenic differentiation. The nucleation, growth and spatial arrangement of newly deposited hydroxyapatite nanocrystals have been evaluated using scanning micro X-ray diffraction and scanning micro X-ray fluorescence. As leading results, we have found the emergence of a complex scenario where the spatial organization and temporal evolution of the process exhibit heterogeneous and self-organizing dynamics. At the same time the possibility of controlling the differentiation kinetics, through the addition of synthetic nanoparticles, paves the way to empower the generation of more structured bone scaffolds in tissue engineering and to design new drugs in regenerative medicine.

Myelin structure in unfixed, single nerve fibers: Scanning X-ray microdiffraction with a beam size of 200nm

Previous raster-scanning with a 1μm X-ray beam of individual, myelinated fibers from glutaraldehyde-fixed rat sciatic nerve revealed a spatially-dependent variation in the diffraction patterns from single fibers. Analysis indicated differences in the myelin periodicity, membrane separations, distribution of proteins, and orientation of membrane lamellae. As chemical fixation is known to produce structural artifacts, we sought to determine in the current study whether the structural heterogeneity is intrinsic to unfixed myelin. Using a 200nm-beam that was about five-fold smaller than before, we raster-scanned individual myelinated fibers from both the peripheral (PNS; mouse and rat sciatic nerves) and central (CNS; rat corpus callosum) nervous systems. As expected, the membrane stacking in the internodal region was nearly parallel to the fiber axis and in the paranodal region it was perpendicular to the axis. A myelin lattice was also frequently observed when the incident beam was injected en face to the sheath. Myelin periodicity and diffracted intensity varied with axial position along the fiber, as did the calculated membrane profiles. Raster-scanning with an X-ray beam at sub-micron resolution revealed for the first time that the individual myelin sheaths in unfixed nerve are heterogeneous in both membrane structure and packing.

Imaging collagen packing dynamics during mineralization of engineered bone tissue

The structure and organization of the Type I collagen microfibrils during mineral nanoparticle formation appear as the key factor for a deeper understanding of the biomineralization mechanism and for governing the bone tissue physical properties. In this work we investigated the dynamics of collagen packing during ex-vivo mineralization of ceramic porous hydroxyapatite implant scaffolds using synchrotron high resolution X-ray phase contrast micro-tomography (XPCμT) and synchrotron scanning micro X-ray diffraction (SμXRD). While XPCμT provides the direct 3D image of the collagen fibers network organization with micrometer spatial resolution, SμXRD allows to probe the structural statistical fluctuations of the collagen fibrils at nanoscale. In particular we imaged the lateral spacing and orientation of collagen fibrils during the anisotropic growth of mineral nanocrystals. Beyond throwing light on the bone regeneration multiscale process, this approach can provide important information in the characterization of tissue in health, aging and degeneration conditions.

STATEMENT OF SIGNIFICANCE

BONE grafts are the most common transplants after the blood transfusions. This makes the bone-tissue regeneration research of pressing scientific and social impact. Bone is a complex hierarchical structure, where the interplay of organic and inorganic mineral phases at different length scale (from micron to atomic scale) affect its functionality and health. Thus, the understanding of bone tissue regeneration requires to image its spatial-temporal evolution (i) with high spatial resolution and (ii) at different length scale. We exploited high spatial resolution X-ray Phase Contrast micro Tomography and Scanning micro X-ray Diffraction in order to get new insight on the engineered tissue formation mechanisms. This approach could open novel routes for the early detection of different degenerative conditions of tissue.