Effect of pH on interlamellar spacing in rat sciatic myelin

Structure and Stability of Internodal Myelin in Mouse Models of Hereditary Neuropathy

Peripheral neuropathies often result in abnormalities in the structure of internodal myelin, including changes in period and membrane packing, as observed by electron microscopy (EM). Mutations in the gene that encodes the major adhesive structural protein of internodal myelin in the peripheral nervous system of humans and mice--P0 glycoprotein--correlate with these defects. The mechanisms by which P0 mutations interfere with myelin packing and stability are not well understood and cannot be provided by EM studies that give static and qualitative information on fixed material. To gain insights into the pathogenesis of mutant P0, we used x-ray diffraction, which can detect more subtle and dynamic changes in native myelin, to investigate myelin structure in sciatic nerves from murine models of hereditary neuropathies. We used mice with disruption of one or both copies of the P0 gene (models of Charcot-Marie-Tooth-like neuropathy [CMT1B] or Dejerine-Sottas-like neuropathy) and mice with a CMT1B resulting from a transgene encoding P0 with an amino terminal myc-tag. To directly test the structural role of P0, we also examined a mouse that expresses P0 instead of proteolipid protein in central nervous system myelin. To link our findings on unfixed nerves with EM results, we analyzed x-ray patterns from unembedded, aldehyde-fixed nerves and from plastic-embedded nerves. From the x-ray patterns recorded from whole nerves, we assessed the amount of myelin and its quality (i.e. relative thickness and regularity). Among sciatic nerves having different levels of P0, we found that unfixed nerves and, to a lesser extent, fixed but unembedded nerves gave diffraction patterns of sufficient quality to distinguish periods, sometimes differing by a few Angstroms. Certain packing abnormalities were preserved qualitatively by aldehyde fixation, and the relative amount and structural integrity of myelin among nerves could be distinguished. Measurements from the same nerve over time showed that the amount of P0 affected myelin's stability against swelling, thus directly supporting the hypothesis that packing defects underlie instability in "live" or intact myelin. Our findings demonstrate that diffraction can provide a quantitative basis for understanding, at a molecular level, the membrane packing defects that occur in internodal myelin in demyelinating peripheral neuropathies.

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.

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.

Bovine brain cathepsin D: inhibition by pepstatin and binding to concanavalin A

1. Cathepsin D from bovine brain has been purified 1100-fold in 46% recovery. Three isozymes are present with pI (+/- 0.05) = 6.10, 6.30 and 6.40. 2. The isozymes are single polypeptide chains with apparent Mr = 42,000 and are similar with respect to substrate binding and cleavage; the pH-optimum is 3.5 with virtually no activity at neutral pH. 3. Pepstatin inhibits the enzyme and kinetic data are consistent with a "tight binding" mechanism. 4. The dissociation constant for the concanavalin A-enzyme complex is Kd = 19 nM at pH 5.0. 5. Under conditions where 90% of the enzyme is bound to soluble concanavalin A, full enzymatic activity is observed.

Cryo-electron microscopy of nervous tissue. A review

The present work attempts to demonstrate that cryofixation is a valuable method for the study of the nervous tissue. The use of the newly developed methods of cryofixation and freeze-etching without fixatives or cryoprotectants allows new exciting perspectives for the electron microscopical observation of cellular components, emphasizing their three-dimensional morphological structures. Significant contributions have been made on the fine structure of the cytoskeleton, cell membranes and cell organelles. The components of the cytoskeleton are distributed in different composition through the perikarya, dendrites and axon. The ubiquitous presence of the cytoskeleton suggests a crucial role in the functional activities of the neurons, especially in relation to the intracellular communication and to developmental and regeneration processes. Vitrified cellular membranes of myelin sheaths and rod outer segments have been observed in hydrated state by using cryofixation and cryotransfer techniques. These procedures allow new insights into the supramolecular structure and an approximation of morphological data to the present biophysical membrane model including a critical comparison with the current descriptions gained by conventional electron microscopy.

The intermediate dense line of the myelin sheath is preferentially accessible to cations and is stabilized by cations

Biophysical studies have shown that the narrow slit between the turns of the myelin leaflet includes a water space lined by strongly negative, fixed charges on the faces of the myelin leaflet. The accessibility of this slit to a marker should depend largely on the interaction between the marker charges and the surface charges on the myelin leaflet. This premise was explored in vitro by comparing the redistribution of anionic ferritin with highly cationized ferritin under a variety of experimental conditions. Cationized ferritin stained the basal lamina and penetrated it. It also bound to Schwann cell membranes, and it entered mesaxons and lodged between myelin lamellae. There was evidence of facilitated particle redistribution due to attractive forces between the cationized ferritin particles and the membrane surfaces. Anionic ferritin did not enter sheaths under identical experimental conditions. Additional experiments reconfirmed X-ray spectrographic data on a loosening of lamellar coherence upon elution of Ca2+ and recompaction of myelin by small amounts of Ca2+. If cationic ferritin was substituted for Ca2+ in these experiments, it also caused recompaction of myelin which had been loosened by previous Ca2+ elution. The cationic ferritin particles sandwiched between the recompacted myelin lamellae. These observations show that the slit between the turns of the myelin leaflet is preferentially accessible to cations, that cations can redistribute along it and that their presence is important for maintaining myelin periodicity. They also throw light on the significance of wide-spaced myelin in pathological conditions.

Triton X-100 extractions of central nervous system myelin indicate a possible role for the minor myelin proteins in the stability in lamellae

Isolated CNS myelin membranes were extracted with Triton X-100 under conditions previously established for the isolation of cytoskeletal proteins. Treated myelin retained much of its characteristic lamellar structure despite the removal of most of the major myelin basic protein (18.5 kDa) and the proteolipid protein, which together normally constitute 60% of the total myelin protein. The SDS-PAGE profile of this extract residue demonstrated an enrichment in proteins of Mr 30 to 60 kilodaltons (the Wolfgram group). The major myelin proteins were identified by antibodies on Western immunoblots, as were the 2'3'-cyclic nucleotide 3'-phosphodiesterase (CNP), actin, tubulin, myelin-associated glycoprotein (MGP) and the 21.5 kDA MBP. The overall behavior of CNP, the 21.5 kDa MBP, MGP and tubulin towards Triton extraction is reminiscent of the behavior of other membrane-skeletal complexes, supporting the idea that these and other minor myelin proteins might be part of heteromolecular complexes with interactions spanning several lamellae of the myelin sheath.