Myelin composition

Table 2 Effective diffusion coefficients and iam(O) (myelin composition) for assumed values of Deiam m the lamellar phase for AOT/water semi-infinite experiment...

Niebroj-Dobos I, Rafalowska J, Fisziariska A, Gadamski R, Grieb P (2007) Myelin composition of spinal cord in a model of amyotrophic lateral sclerosis (ALS) in SOD1G93A transgenic rats. Foha Neuropathol 45 236-241... 

Norton WT, Poduslo SE. Myelination in rat brain changes in myelin composition during brain matiuation. 

Norton, W. T. Poduslo, S. E. Myelination in rat brain Changes in myelin composition during brain maturation. J. Neurochem. 1973, 21 759 773. 

Exposures of 10 weeks (5 days/week) to 2,500 mg/kg/day trichloroethylene in com oil by gavage resulted in altered myelin thickness in the rat mental nerve, a branch of the trigeminal nerve (Barret et al. 1991). Effects of similar exposures on the rat trigeminal nerve included decreased fiber diameter and altered fatty acid composition in total lipid extracts, indicative of demyelination (Barret et al. 1992). Stronger effects were seen with the trichloroethylene decomposition product dichloroacetylene. 

Lipids have critical roles in nervous system structure and function. Synaptic complexes and myelin are characterized by unique lipid compositions that contribute to the specialized properties of these nervous system structures. Multiple signaling pathways involving lipid intermediates regulate cell differentiation and synaptic transmission. 

O Brien, J. S. and Sampson, E. L. Lipid composition of the normal human brain gray matter, white matter, and myelin. /. Lipid Res. 6 537-544.1965. 

The composition of myelin is well characterized because it can be isolated in high yield and purity by subcellular fractionation 56 Central nervous system myelin is enriched in certain lipids 56 Peripheral and central nervous system myelin lipids are qualitatively similar 58... 

The composition of myelin changes during development 68 Spontaneous mutations in experimental animals provide insights about the structure and assembly of myelin 68 Myelin components exhibit great heterogeneity of metabolic turnover 69... 

Myelin in situ has a water content of about 40%. The dry mass of both CNS and PNS myelin is characterized by a high proportion of lipid (70-85%) and, consequently, a low proportion of protein (15-30%). By comparison, most biological membranes have a higher ratio of proteins to lipids. The currently accepted view of membrane structure is that of a lipid bilayer with integral membrane proteins embedded in the bilayer and other extrinsic proteins attached to one surface or the other by weaker linkages. Proteins and lipids are asymmetrically distributed in this bilayer, with only partial asymmetry of the lipids. The proposed molecular architecture of the layered membranes of compact myelin fits such a concept (Fig. 4-11). Models of compact myelin are based on data from electron microscopy, immunostaining, X-ray diffraction, surface probes studies, structural abnormalities in mutant mice, correlations between structure and composition in various species, and predictions of protein structure from sequencing information [4]. 

Central nervous system myelin is enriched in certain lipids. Table 4-1 lists the composition of bovine, rat, and human myelin compared to bovine and human white matter, human gray matter, and rat whole brain [1] (see Ch. 3). While there are no absolutely myelin-specific lipids, cerebroside (galactosyl ceramide) is the most typical of myelin. With the exception of early development,... 

TABLE 4-1 Composition of central nervous system myelin and brain... 

The data in Table 4-1 indicate that myelin accounts for much of the total lipid of white matter, and that the lipid composition of gray matter is quite different from that of myelin. The composition of brain myelin from all mammalian species studied is very much the same. There are, however, some species differences for example, myelin of rat has less sphingomyelin than does that of bovine or human (Table 4-1). Although not shown in the table, there are also regional variations for example, myelin isolated from the spinal cord has a higher lipid-to-protein ratio than brain myelin from the same species. 

Peripheral and central nervous system myelin lipids are qualitatively similar. However, there are quantitative differences. PNS myelin has less cerebroside and sulfatide and considerably more sphingomyelin than CNS myelin. Of interest is the presence of the LM1 ganglioside, sialosyl-lactoneotetraosylceramide, as a characteristic component of myelin in the PNS of some species. These differences in lipid composition between CNS and PNS myelin are not, however, as dramatic as the differences in protein composition discussed below. 

The principal difference in the overall protein composition of PNS and CNS myelin is that P0 replaces PLP as the major protein, although myelin-forming Schwann cells do express very low levels of PLP. It is interesting to note that PLP and P0 proteins, which are so different in sequence, post-translational modifications and membrane topology, may have similar roles in the formation of structures as closely related as myelin of the CNS and PNS respectively. Expression of P0 in transfected cells results in cell-cell interactions that are due to homophilic binding... 

Other leukodystrophies are associated with the lysosomal and peroxisomal disorders in which specific lipids or other substances accumulate due to a deficiency in a catabolic enzyme - for example Krabbe s disease, meta-chromatic leukodystrophy (MLD) and adrenoleuko-dystrophy (ALD) [1,2]. (These are discussed in detail in Ch. 40.) Similarly, disorders of amino acid metabolism can lead to hypomyelination - for example phenylketonuria and Canavan s disease (spongy degeneration) [1, 2, 25] (Ch. 40). The composition of myelin in the genetically... 

Biological membranes consist of lipids, proteins, and carbohydrates (see p. 214). These components occur in varying proportions (left). Proteins usually account for the largest proportion, at around half. By contrast, carbohydrates, which are only found on the side facing away from the cytoplasm, make up only a few percent. An extreme composition is seen in myelin, the insulating material in nerve cells, three-quarters of which consists of lipids. By contrast, the inner mitochondrial membrane is characterized by a very low proportion of lipids and a particularly high proportion of proteins. 

For the experiments of Fig. 13, Sm can be measured, so that it is not necessary to have independent information on (kAp). Moreover, with the known interfacial compositions given by Fig. 15 and the measured velocities of the two interfaces below the base of the myelins, it is possible to solve for the effective diffusivities DeVi and and iam(O) in terms of Heiam- Results of such calculations are shown in Table 2. 

There may be limitations in applying the above model to other systems. For instance, the initial surfactant often exists as a lamellar phase as for phospholipids, so that there are no interfaces between various liquid crystalline phases whose velocities can be measured and used to determine effective dif-fusivities as in the AOT analysis above. As a result, the base of the myelinic figures must approach the base of the vertical cell as the volume of the lamellar phase shrinks, and the assumption made above that the composition of the myelins is independent of time may not be valid. 

Eto Y, Suzuki K, Suzuki K. 1971. Lipid composition of rat brain myelin in triethyl tin-induced edema. J Lipid Res 12 570-579. 

Selivonchick, D.P. and Roots, B.J. (1976). Variation in myelin lipid composition induced by change in environmental temperature of goldfish. Journal of Chemical Biology 1,131-135. 

The lipid, water, and protein contents of certain tissues vary markedly as a function of age. For example, the adipose tissues of neonates contain about 55% water and 35% lipids, whereas the corresponding figures for the adult are about 25% and 70%, respectively (Friis-Hansen, 1971). The proportion of water in skin falls as a function of age, due to an increase in collagen. The water contents of liver, brain, and kidneys decrease from birth to adulthood by 5-15%. The decrease in water contents of liver and kidneys is primarily due to an increase in protein, whereas this change in the brain is due to an increase in myelin. The overall composition of muscle in terms of lipid and water does not seem to vary with age (Dickerson Widdowson, 1960). 

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