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Biomedical subjects

A E Blaurock

Publications and source records attributed to A E Blaurock.

At least 19 recordsLinked to original sources
PubMed1995

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.

Animals↗
PubMed1991

Membrane flow within the myelin sheath in IDPN neuropathy.

This report describes some aspects of beta,beta'-iminodipropionitrile (IDPN) neuropathy in rats as observed by ultrastructural methods and X-ray diffraction. Light microscopy shows gross swelling of the axons in proximal lumbar spinal roots 8 days after intraperitoneal injection of IDPN. Mean axon cross-sectional area and mean axon perimeter increased to 280% and 160% of their control values, respectively. At the same time, myelin membrane packing was not visibly disturbed. In addition, X-ray diffraction patterns, recorded under physiological conditions, demonstrate that the myelin lipid bilayer thickness and widths of the aqueous spaces between bilayers did not change. Related observations are made on posterior tibial nerve (PNS myelin) and ventral spinal cord (CNS myelin). The various observations together are interpreted in terms of a fluid myelin membrane. It is proposed that the myelin membrane flows during axon swelling even though normal membrane-membrane contacts are maintained within the sheath. Membrane flow and slippage between membranes are explained in terms of a molecular model of the myelin multilayer.

Animals↗
PubMed1987

Calcium ions trigger the expansion in bistable myelin.

Ultrastructural studies of nerve myelin emphasize its static aspects. However, current work indicates that the myelin multilayer is poised for a supramolecular transition from a compact to an expanded form. In teleosts, the membrane pair becomes 27 A thicker. The trigger for this expansion is physiological levels of Ca. Varying proportions of the myelin can be trapped in the expanded state by transferring an excised specimen from Ca-containing to Ca-free medium in order to prevent expansion of the remaining compact myelin. Images of the myelin multilayer show that the expansion occurs in the aqueous spaces between lipid bilayers. Previously we demonstrated the reversal of expansion by removing Ca under special conditions. These results together lead us to suggest that myelin may expand locally in normal nerve function, while wholesale expansion may be an early step in myelin breakdown in vivo.

Animals↗
PubMed1986

Structure of the crystalline bilayer in the subgel phase of dipalmitoylphosphatidylglycerol.

The structure of the subgel phase of dipalmitoylphosphatidylglycerol (DPPG) has been analyzed by X-ray diffraction techniques. Diffraction recorded from highly oriented DPPG specimens in the subgel phase extends to 2-A resolution. There are sharp lamellar reflections on the meridian, and other reflections lie on a series of wide-angle lattice lines parallel to the meridian and crossing the equator in the range of 8-2 A. The wide-angle lattice lines consist of radially sharp reflections centered on the equator of the X-ray film and also a series of broader, off-equatorial maxima. The lattice lines indicate that the DPPG molecules in each bilayer crystallize in a two-dimensional oblique lattice with dimensions a = 5.50 A, b = 7.96 A, and gamma = 100.5 degrees. These oblique lattices are not regularly aligned from bilayer to bilayer. Analysis of the lamellar diffraction shows that the bilayer has about the same thickness in the subgel and gel (L beta') phases. In the direction normal to the hydrocarbon chains, the chains are significantly closer together in the subgel phase as compared to the normal L beta' gel phase but have about the same separation as the chains in polyethylene and the crystalline n-alkanes. The bilayer thickness, area per lipid molecule, and intensity distribution along the lattice lines all indicate that in the subgel phase the hydrocarbon chains are tilted between 30 and 35 degrees from the normal to the bilayer plane.(ABSTRACT TRUNCATED AT 250 WORDS)

Crystallization↗
PubMed1986

Ca-controlled, reversible structural transition in myelin.

When excised goldfish spinal cord is kept in physiological saline at room temperature, the myelin multilayers swell. As studied by X-ray diffraction, the original repeating distance of 150 A (AS myelin) swells to 177 A (AL myelin); no intermediate distances are seen. At least 70% of the myelin can undergo this gradual conversion. Omitting glucose from the saline, or substituting 2-deoxy-D-glucose for the glucose, or adding NaCN to the saline all promote conversion. Cooling retards the rate but does not prevent conversion. Omitting Ca does prevent conversion, however, and substituting Mg, Sr, or Ba for Ca also does so. Moreover, agents that increase the rate of conversion in Ca-containing saline by up to 5 fold (NaCN, colchicine, A23187) fail to convert myelin in Ca-free saline. We then converted AL myelin back to AS by withdrawing Ca. After converting in NaCN-containing saline, up to 3/4 of the AL myelin recompacts in Ca-free saline; however, none of the myelin recompacts if NaCN is present in the Ca-free saline. Little or no recompaction occurs after conversion in saline without NaCN. Based on our results, we suggest that the oligodendrocytes may maintain AS myelin in vivo by pumping Ca out of the myelin sheath. The myelin in a human PNS nerve has been induced to undergo a similar cycle of swelling and recompaction.

Animals↗
PubMed1985

Surprising thermal transition in fish myelin.

A new structural transition in nerve myelin has been discovered by means of X-ray diffraction of excised teleost nerves in physiological saline. The reversible transition is between two structures, designated AS and AL, with repeating distances (d spacings) differing by 25-35 A. When the temperature of bream spinal cord is lowered from room temperature to 4 degrees C, much but not all of the AS (short spacing) myelin changes into AL (long spacing) myelin. The change is reversed when the temperature is raised back to 22 degrees C, and it occurs a second time when the temperature is lowered again to 4 degrees C. The myelin in bream optic nerve undergoes a similar thermal transition, but the myelin in brachial plexus does not. The thermal transition does not involve the liquid crystal-to-gel transition observed in lipids and natural membranes. When a specimen is kept at constant temperature, there is a gradual conversion from AS to AL myelin which is not thermally reversible, suggesting the existence of two distinct subclasses of AL. Similarly, two subclasses are indicated for AS myelin since part of it does not transform thermally. The observations reported here may have significance for the evolutionary development of myelin.

Animals↗
PubMed1985

Reconstituted P2/myelin-lipid multilayers.

A complex forms when bovine P2 protein is added to single-bilayer vesicles created by sonicating myelin lipids. The complex was studied by biochemical analysis, freeze-fracture (FF) and thin-section electron microscopy (EM), and by X-ray diffraction. Smaller amounts of P2 cause the vesicles to aggregate and fuse whereas larger amounts (greater than or equal to 4 wt%) cause multilayers to form. Binding saturates at 15 wt% P2. FF EM shows that large, flat multilayers form within 15 min of addition of P2. Only smooth fracture faces are seen, as expected for a peripheral membrane protein. X-ray diffraction shows a constant repeating distance in the multilayers: 86.0 +/- 0.7 A between the centers of bilayers in the range 4 wt% less than or equal to P2/(P2 + lipid) less than or equal to 15 wt%. Assuming a 53 A-thick bilayer, the space between bilayers is 33 A wide. This is a wider space than for myelin basic protein (MBP) (20-25 A wide). The respective widths are consistent with a compact, globular structure for P2 and a flattened shape for MBP. Calculated electron-density profiles of the lipids with and without P2 reveal the protein largely in the interbilayer spaces, with a small part possibly inserted into the lipid headgroup layers. The different proportions of P2 in the sciatic nerve of various species are tentatively correlated with the different average widths observed by X-ray diffraction for the cytoplasmic space (major period line) between bilayers in the respective sciatic myelins.

Animals↗
PubMed1984

Lipid/myelin basic protein multilayers. A model for the cytoplasmic space in central nervous system myelin.

A multilayered complex forms when a solution of myelin basic protein is added to single-bilayer vesicles formed by sonicating myelin lipids. Vesicles and multilayers have been studied by electron microscopy, biochemical analysis, and X-ray diffraction. Freeze-fracture electron microscopy shows well-separated vesicles before myelin basic protein is added, but afterward there are aggregated, possibly multilayered, vesicles and extensive planar multilayers. The vesicles aggregate and fuse within seconds after the protein is added, and the multilayers form within minutes. No intra-bilayer particles are seen, with or without the protein. Some myelin basic protein, but no lipid, remains in the supernatant after the protein is added and the complex sedimented for X-ray diffraction. A rather variable proportion of the protein is bound. X-ray diffraction patterns show that the vesicles are stable in the absence of myelin basic protein, even under high g-forces. After the protein is added, however, lipid/myelin basic protein multilayers predominate over single-bilayer vesicles. The protein is in every space between lipid bilayers. Thus the vesicles are torn open by strong interaction with myelin basic protein. The inter-bilayer spaces in the multilayers are comparable to the cytoplasmic spaces in central nervous system myelins . The diffraction indicates the same lipid bilayer thickness in vesicles and multilayers, to within 1 A. By comparing electron-density profiles of vesicles and multilayers, most of the myelin basic protein is located in the inter-bilayer space while up to one-third may be inserted between lipid headgroups. When cytochrome c is added in place of myelin basic protein, multilayers also form. In this case the protein is located entirely outside the unchanged bilayer. Comparison of the various profiles emphasizes the close and extensive apposition of myelin basic protein to the lipid bilayer. Numerous bonds may form between myelin basic protein and lipids. Cholesterol may enhance binding by opening gaps between diacyl-lipid headgroups.

Animals↗
PubMed1984

Resistance to disruption of multilamellar fragments of central nervous system myelin.

Single-bilayer vesicles of myelin are desirable for studying myelin development and metabolism. Accordingly, our interest was drawn to a procedure for vesiculating myelin (Steck et al., Biochim, Biophys. Acta 509, 397-408, 1978). We used X-ray diffraction analysis to examine these putative vesicle preparations because much larger amounts of material can be surveyed by this method than by electron microscopy. The sharpness (width) of the rings in the X-ray diffraction pattern varies inversely with the number of bilayers per multilayer structure. We therefore expected to see the diffuse diffraction pattern characteristic of single bilayers. Diffraction patterns were recorded from isolated rat brain myelin before and after the vesiculation procedure. Both patterns showed sharp rings, indicating numerous multilayered structures. Average values ranging from 7 to 10 bilayers per multilayer were calculated in both cases. This procedure did produce a small fraction of single-bilayer structures, which were isolated by differential centrifugation; however, these accounted for only about 1% of the total myelin present. The diffraction pattern of this material showed the diffuse band typical of single-bilayer structures, and sodium dodecyl sulfate-polyacrylamide gel electrophoresis indicated it had the same protein composition as in normal myelin. Similar results were also obtained using either fresh or frozen bovine brain myelin. Variations of the published vesiculation procedure (incubation in 0.1 M NaCl or in buffers containing glycerol; disruption by sonication or use of a Tissumizer) also were not effective in breaking down the multilamellar fragments into thinner structures. The conclude that the multilamellar fragments of isolated CNS myelin resist disruption into single-bilayer structures.

Animals↗
PubMed1981

The spaces between membrane bilayers within PNS myelin as characterized by X-ray diffraction.

X-ray diffraction data from a wide range of animals characterize the two spaces in the peripheral nervous system (PNS) and the central nervous system (CNS) myelins. The 'extracellular space' is nearly constant from species to species while the 'cytoplasmic space' is more variable. A profile of swollen rat sciatic myelin shows material projecting from the bilayer into the extracellular space. Interdigitating Po molecules therefore are proposed to separate neighboring myelin bilayers across the extracellular space in PNS.

Animals↗
PubMed1981

A change observation of oriented diffraction from low-density serum lipoprotein.

The X-ray diffraction patterns have been recorded from concentrated preparations of low-density serum lipoprotein (LDL). In one case a specimen gave rise to oriented diffraction: the 36 A ring, which has previously been attributed to the cholesteryl esters in LDL, shows markedly oriented intensity; and of the other small-angle rings, which do not show oriented peak intensity, one nonetheless shows significant anisometry. The possibility cannot be ruled out that an artifactual structure has been created; however, it is calculated that the structure will, if exposed in an unoriented preparation, give rise to 36 A diffraction not obviously distinguishable from the native pattern. If valid for the native LDL particle, these observation suggest that the cholesteryl esters in the core of the particle may be arranged in simple planar, or else cylindrically concentric, layers and that the overall shape of the particle is not a uniform sphere.

Cholesterol Esters↗
PubMed1979

On phasing the small-angle x-ray diffraction pattern from nerve myelin.

Using a method they developed, Stamatoff and Krimm (1976) have phased swelling data from nerve myelin. Although most phases agree with those I determined previously, there are a few differences. In this letter the two different phasings, theirs and my own, are used to compute the corresponding electron-density profiles, which are then closely compared. For both phasings, small differences are seen in the membrane profile at different degrees of swelling. The explanation that these differences are due simply to errors in measuring intensity is shown to be quite improbable; thus the differences indicate a real change in the profile. It follows that the assumption of a constant membrane profile appears to be invalid in the case of myelin swelling. The differences therefore are assumed to indicate a real change in the profile. It is shown that this change can be attributed consistently to interdigitation of protein molecules at the surfaces of neighboring membranes, while the membrane structure itself remains unchanged. In this case, valid phases still can be determined by swelling, but the phases determined by Stamatoff and Krimm are not valid.

Animals↗
PubMed1978

Disorder is characteristic of nerve myelin.

An X-ray diffraction pattern from the myelin in frog sciatic nerve has been obtained using the intense synchrotron radiation from the storage ring, SPEAR. Data with good statistical accuracy are obtained in a few minutes by using a scintillation counter or position-sensitive detector. The same indications for stacking disorder are seen as in previous conventional exposures which required one to two days. Thus, the stacking disorder is characteristic of myelin in a freshly dissected nerve. The present data, obtained with a more nearly monochromatic X-ray beam than in the previous study, remove one of two ambiguities which bear on the possible phasing of the higher order Bragg reflections.

Animals↗
PubMed1977

Asymmetric structure of the purple membrane.

There is both functional and structural evidence that bacteriorhodopsin is oriented asymmetrically across the purple membrane of Halobacterium halobium. To assess the degree of asymmetry, the x-ray diffraction data from the membrane have been analyzed for possible electron-density profiles. A recent theory predicts that only a limited number of profiles are consistent with the continuous diffraction data, and two possible profiles have been found. Both profiles indicate that the protein molecules span a lipid bilayer in the membrane. Both profiles are asymmetric; there are more lipid molecules in one half of the membrane than in the other, and the bacteriorhodopsin molecule shows a slight complementary asymmetry.

Bacteriorhodopsins↗