Membrane interactions are altered in myelin isolated from central and peripheral nervous system tissues

Statistical field theory of the transmission of nerve impulses

Background

Stochastic processes leading voltage-gated ion channel dynamics on the nerve cell membrane are a sufficient condition to describe membrane conductance through statistical mechanics of disordered and complex systems.

Results

Voltage-gated ion channels in the nerve cell membrane are described by the Ising model. Stochastic circuit elements called “Ising Neural Machines” are introduced. Action potentials are described as quasi-particles of a statistical field theory for the Ising system.

Conclusions

The particle description of action potentials is a new point of view and a powerful tool to describe the generation and propagation of nerve impulses, especially when classical electrophysiological models break down.

The particle description of action potentials allows us to develop a new generation of devices to study neurodegenerative and demyelinating diseases as Multiple Sclerosis and Alzheimer’s disease, even integrated by connectomes. It is also suitable for the study of complex networks, quantum computing, artificial intelligence, machine and deep learning, cryptography, ultra-fast lines for entanglement experiments and many other applications of medical, physical and engineering interest.

Phase Diagram of Purified CNS Myelin Reveals Continuous Transformation between Expanded and Compacted Lamellar States

Purified myelin membranes (PMMs) are the starting material for biochemical studies, from individual components up to the isolation of detergent-resistant membrane (DRM) fractions or detergent-insoluble glycosphingolipid (DIG) fractions, which are commonly believed to resemble physiological lipid rafts. The normal DIG isolation protocol involves the extraction of lipids under moderate cooling. The isolation of PMMs also involves the cooling of myelin as well as exposure to low ionic strength (IS). Here, we addressed the combined influence of cooling and IS on the structure of PMMs. The phase behaviour was investigated by small angle X-ray diffraction. Analysis of the diffraction peaks revealed the lamellar periodicity (d), the number of periodically correlated bilayers (N), and the relatives fractions of each phase. Departure from physiological conditions induced a phase separation in myelin. The effect of monovalent and divalent ions was also compared at equivalent IS, showing a differential effect, and phase diagrams for both ion types were established—Ca²⁺ induced the well-known over-compacted phase, but additionally we also found an expanded phase at low IS. Na⁺ promoted phase separation, and also induced over-compaction at sufficiently high IS. Finally, exploring the whole phase diagram, we found evidence for the direct isothermal transformation from the expanded to the compacted phase, suggesting that both phases could in fact originate from the identical primary lateral phase separation, whereas the apparent difference lies in the inter-bilayer interaction that is modulated by the ionic milieu.

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.

Interaction of myelin basic protein and 2',3'-cyclic nucleotide phosphodiesterase with mitochondria

The content and distribution of myelin basic protein (MBP) isoforms (17 and 21.5 kDa) as well as 2',3'-cyclic nucleotide-3'-phosphodiesterase (CNPase) were determined in mitochondrial fractions (myelin fraction, synaptic and nonsynaptic mitochondria) obtained after separation of brain mitochondria by Percoll density gradient. All the fractions could accumulate calcium, maintain membrane potential, and initiate the opening of the permeability transition pore (mPTP) in response to calcium overloading. Native mitochondria and structural contacts between membranes of myelin and mitochondria were found in the myelin fraction associated with brain mitochondria. Using Western blot, it was shown that addition of myelin fraction associated with brain mitochondria to the suspension of liver mitochondria can lead to binding of CNPase and MBP, present in the fraction with liver mitochondria under the conditions of both closed and opened mPTP. However, induction of mPTP opening in liver mitochondria was prevented in the presence of myelin fraction associated with brain mitochondria (Ca2+ release rate was decreased 1.5-fold, calcium retention time was doubled, and swelling amplitude was 2.8-fold reduced). These results indicate possible protective properties of MBP and CNPase.

Equivalent aqueous phase modulation of domain segregation in myelin monolayers and bilayer vesicles

Purified myelin can be spread as monomolecular films at the air/aqueous interface. These films were visualized by fluorescence and Brewster angle microscopy, showing phase coexistence at low and medium surface pressures (<20-30 mN/m). Beyond this threshold, the film becomes homogeneous or not, depending on the aqueous subphase composition. Pure water as well as sucrose, glycerol, dimethylsulfoxide, and dimethylformamide solutions (20% in water) produced monolayers that become homogeneous at high surface pressures; on the other hand, the presence of salts (NaCl, CaCl(2)) in Ringer's and physiological solution leads to phase domain microheterogeneity over the whole compression isotherm. These results show that surface heterogeneity is favored by the ionic milieu. The modulation of the phase-mixing behavior in monolayers is paralleled by the behavior of multilamellar vesicles as determined by small-angle and wide-angle x-ray scattering. The correspondence of the behavior of monolayers and multilayers is achieved only at high surface pressures near the equilibrium adsorption surface pressure; at lower surface pressures, the correspondence breaks down. The equilibrium surface tension on all subphases corresponds to that of the air/alkane interface (27 mN/m), independently on the surface tension of the clean subphase.

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.

Multilamellar packing of myelin modeled by lipid-bound MBP

Membrane compaction and adhesion at the major dense line (cytoplasmic apposition) of myelin, particularly in the central nervous system (CNS), is typically attributed to myelin basic protein (MBP). To explore the role of MBP in myelin membrane adhesion, we attempted to reconstitute the major dense line of myelin from purified lipid-bound MBP, which is a detergent-soluble form of MBP that retains the binding of all the myelin lipids. Removal of detergent by long-term dialysis yielded a precipitate, which, when analyzed by sodium dodecylsulfate polyacrylamide gel electrophoresis (SDS-PAGE) and thin-layer chromatography, contained MBP that was still associated with myelin lipids, but in different proportions than in the native membrane. Comparison of lipid composition among isolated myelin, MBP-free myelin lipids, and lipid-bound MBP aggregates showed that the lipid-bound form of the protein was specifically enriched in phosphatidylethanolamine, phosphatidylcholine, sphingomyelin, phosphatidylinositol, and phosphatidylserine. Electron microscopy and x-ray diffraction demonstrated that the lipid-MBP complexes formed multilayers having periods of 70-85 A, which correspond in width to individual myelin membranes. By contrast, the lipids alone assembled as multilayers having a period of approximately 40 A. Thus, the detergent-soluble form of MBP, which is bound to lipids, might serve as a simple model for the cytoplasmic apposition of myelin.

Morphology of cryofixed myelin sheath

The myelin sheath is formed by concentrically apposed membrane pairs and shows a regularly layered pattern of alternating light lines and dense lines. Observation of cryofixed myelin demonstrated that the structures represent aqueous spaces. All lamellae of the myelin sheath show globular aggregates of particles and these particles are corresponding with aggregates observed after detergent extraction of the myelin. Experimental fusion of myelin lamellae shows an intermixing of the globular particles or subunits. The interaction of these structural units in the bilayers may provide the stability of the myelin lamellae and their lamination.

Spinal cord myelin is vulnerable to decompression

Spinal cord white matter is the major site of tissue damage resulting from decompression sickness (DCS or "the bends"). Damage is thought to result from bubble nucleation within the tissue. Why DCS occurs predominantly in the spinal cord and not in the brain is not known; neither is the exact pathological mechanism by which the spinal cord is damaged, nor how multiple sclerosis (MS)-like symptoms may ensue. To investigate the molecular basis of white matter damage, we subjected myelinated mouse tissues to varying durations of decompression, and then after recompression to one atmosphere, examined them for changes in myelin structure and composition. X-ray diffraction showed that the myelin period in spinal cord decreased by 4%, whereas those of optic and sciatic nerves were stable. The change in period was accompanied by a change in membrane bilayer profile--i.e., relative to control, the width of the bilayer decreased by approximately 6 A, whereas the interbilayer spaces each increased by approximately 3 A. The changes in electron density levels suggested a redistribution of matter from the interbilayer spaces into the lipid headgroup layers. By contrast with these structural changes, sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and high-performance thin layer chromatography (HPTLC) revealed no noticeable change in myelin composition--i.e., there was no release of myelin-specific proteins or lipids. Our findings indicate that spinal cord myelin has an inherent structural vulnerability that may facilitate the targeting of this tissue during pressure changes.

Myelin vesicles: what we know and what we do not know

In the following review, we address difficulties that have arisen when attempting to convert the myelin multilayers into vesicles. The emphasis is on CNS myelin of adult mammals although both central nervous system (CNS) and peripheral nervous system (PNS) myelin are considered. The ability to prepare vesicle of myelin membrane has yet not been feasible. We hope to clarify some aspect of this problem and offer some possible approaches. Special attention is paid to myelin swelling phenomena because these indicate ways in which the myelin multilayer can break down. Images of isolated myelin are reviewed with special attention to the ways in which the multilayer actually breaks down. Attempts at reproducing a procedure for vesiculating myelin are summarized, and a critique is given to account for the inability to reproduce the published results. Finally, novel approaches for vesiculating myelin are proposed, which are based on well-characterized swelling phenomena.

Cryo-electron microscopy of myelin treated with detergents

After detergent extraction, myelin lamellae disaggregate into diverse globular fragments that correspond to the globular structures observed in normal cryofixed myelin. The interaction of these structural units in the bilayer is probably the morphological basis that provides the stability of the myelin lamellae and their lamination.

In vitro changes in the fine structure and protein composition of light myelin fractions isolated from guinea pig brain

To find out if in vitro maintenance produces changes in the electron microscopic appearance, protein composition and phosphorylation properties of guinea pig CNS myelin fractions, we incubated them for 10 min, 4 hr, 24 hr, and 48 hr in phosphate-buffered saline (pH 7.4) or in 20 mM Hepes, 2 mM EDTA, 0.5 mM EGTA, 0.5 mM dithiothreitol, and 20 mM NaCl at 4 and 30 degree C. Aliquots were processed for electron microscopic study, were analyzed for protein content by gel electrophoresis, and were assayed for endogenous protein phosphorylation. Before incubation, electron micrographs of fractions contained two types of multilamellar whorls with the periodicity of CNS myelin sheaths. The first type of whorl was separated from nearby whorls; the other type had surface lamellae that were connected to other multilayered membrane fragments. After incubation at 4 degree C for 24 hr, the number of both types of multilamellar whorls in micrographs had increased approximately 3- to 4- fold. Counts per unit area showed that the observed increase was both time- and temperature-dependent. In aliquots studied by gel electrophoresis, only minor degradation of myelin proteins was observed. The endogenous protein phosphorylation properties of the myelin fragments also remained functional, suggesting that the activities of protein phosphotransferases were not altered. We conclude that the incubation conditions described here favor interactions of proteins and lipids that lead to the formation of multilayered aggregates of CNS myelin membranes.

Radial component of CNS myelin: junctional subunit structure and supramolecular assembly

The radial component is a structural specialization within CNS myelin that is believed to stabilize the apposition of membranes in the internode. Previous observations on thin sections and freeze-fracture replicas show that this junctional complex consists of linear, particulate strands that run parallel to the nerve fibre axis and radially through the myelin sheath, but details on its molecular organization are lacking. The objective of our current study was to gain further insight into its arrangement and composition by examining its fine-structure and incidence in: myelin with known deficits in protein composition (e.g., shiverer, transgenic shiverer, myelin deficient and jimpy mutant mice); isolated CNS myelin, which has been shown by X-ray diffraction to be more stable than intact CNS myelin; and human white matter, in which this junctional complex has not yet been described. Our results confirm the localization and general appearance of the radial component as previously reported. In addition, we found that: (1) the radial component occurs abundantly in human CNS myelin where it has a complex subunit structure; (2) the constituent junctional unit of this structure is organized as a pair of globular domains (each approximately 40 A diameter) at the extracellular apposition which is linked by approximately 15 A diameter filaments extending through the bilayer to approximately 25 A globular domains in the adjacent cytoplasmic apposition; (3) the radial component is present with apparently normal structure in the sparse, compact myelin of murine mutants containing either different amounts of MBP or no PLP which indicates that neither of these proteins is necessary for junctional integrity; (4) the radial component is present in purified CNS myelin membranes which may account for the stability of these membranes; and (5) the radial component is structurally resistant to Triton, which suggests a method for its further biochemical characterization. Finally, from an analysis of images from tilted transverse and longitudinal sections, we have reconstructed a model of its three-dimensional, supramolecular organization.

The dysmyelinating mouse mutations shiverer (shi) and myelin deficient (shimld)

Shiverer (shi/shi) is an autosomal recessive mouse mutation that produces a shivering phenotype in affected mice. A shivering gait can be seen from a few weeks after birth until their early death, which occurs between 50 and 100 days. The central nervous system of the mutant mouse is hypomyelinated but the peripheral nervous system appears normal. The myelin of the CNS, wherever present, is not well compacted and lacks the major dense line. Myelin basic protein (MBP), which is associated with the major dense line, is absent, and this is due to a deletion of the major part of the gene encoding MBP. Transgenic shiverer mice that have integrated and express the wild-type mouse MBP transgene no longer shiver and have normal life spans. Conversely, normal mice that have integrated an antisense MBP transgene, shiver. Myelin deficient shimld/shimld is allelic to shiverer (shi/shi) but the mutant mouse is less severely affected. Although MBP is present in the CNS, it is low in quantity and is not developmentally regulated. The gene encoding MBP has been both duplicated and inverted. Transgenic shimld/shimld mice with the wild-type MBP transgene have normal phenotypes.

Membrane structure in isolated and intact myelins

The biochemical composition of myelin and the topology of its constituent lipids and proteins are typically studied using membranes that have been isolated from whole, intact tissue using procedures involving hypotonic shock and sucrose density gradient centrifugation. To what extent, however, are the structure and intermembrane interactions of isolated myelin similar to those of intact myelin? We have previously reported that intact and isolated myelins do not always show identical myelin periods, indicating a difference in membrane-membrane interactions. The present study addresses the possibility that this is due to altered membrane structure. Because x-ray scattering from isolated myelin sometimes consists of overlapping Bragg reflections or is continuous, we developed nonlinear least squares procedures for analyzing the total intensity distribution after film scaling, background subtraction, and Lorentz correction. We calculated electron density profiles of isolated myelin for comparison with membrane profiles from intact myelin. The change in the width of the extracellular space and the relative invariance of the cytoplasmic space as a function of pH and ionic strength that we previously found for intact nerve was largely paralleled by isolated myelin. There were two exceptions: isolated CNS myelin was resistant to swelling under all conditions, and isolated PNS myelin in hypotonic saline showed indefinite swelling at the extracellular apposition. However, electron density profiles of isolated myelins, calculated to 30 A resolution, did not show any major change in structure compared with intact myelin that could account for the differences in interactions.

Buoyant density and lipid composition of purified myelin of aging human brain

Purified myelin of human brain from 15 young adult (below 50 years of age) and old (above 70 years of age) autopsy cases each was examined by isopycnic centrifugation in continuous sucrose gradients, and for lipid composition. The mean buoyant density of myelin was the same in both groups. Apparent features of old age were a wide range of density values, less compact myelin bands, and the dissociation of myelin into two bands in six of 15 old cases. Lipid analyses of randomly selected myelin samples of each group revealed an inverse relationship between the total lipid to protein ratio and density of myelin. In old age total lipids decreased by an average 10 mol lipid per mol protein. This decrease was accounted for by cholesterol, phosphatidylserine and cerebrosides. Changes in fatty acid moieties mainly affected sphingolipids. C20:0 and C24:0 of sphingomyelin increased, as did even more markedly the more hydrophilic OH-fatty acids of cerebrosides. Correlations with buoyant density existed for the ratios of cholesterol to protein in young adult cases, and those of galactolipids to protein in old cases. The results suggest that old age is associated with impaired stability and altered lipid composition of myelin.