The lipid composition of light and heavy myelin subfractions isolated from rabbit sciatic nerve

The N-glycan profile of mouse myelin, a specialized central nervous system membrane

Understanding the rich complement of sugar chains found in cellular membranes is impeded by the complexity of cell types and membrane diversity. To overcome this, we have analyzed the N-linked sugar chain composition of the glycoproteins of CNS myelin, an elaboration of the plasma membranes of oligodendrocytes (OLs) that result in a multilamellar wrapping of neuronal axons, facilitating nerve conduction with dramatic savings of space and energy. Due to an usually high lipid to protein ratio, myelin can be separated readily from other heavier membranes on sucrose gradients and further fractionated into subdomains related to myelin structure and function, including compact myelin and myelin-associated axolemmal membrane (Menon et al. 2003). We analyzed these fractions for N-linked sugar chains, using 2D HPLC following hydrazinolysis and pyridylamination. Our results indicate that compared with total brain homogenate, the amount of N-glycans is 1.3-fold higher in the myelin-associated axolemmal membranes, but it is 0.5-fold less in CM. M5 [Manalpha1-3((Manalpha1-3)(Manalpha1-6)Manalpha1-6)Manbeta1-4GlcNAcbeta1-4GlcNAc] is the most abundant sugar chain in total brain homogenate, compact myelin, and myelin-associated axolemma, constituting approximately 20% of sugar chains. Although the types of sugar chains are similar among the fractions, their expression levels vary significantly. In addition to high mannose type oligosaccharides, the core fucosylated, biantennary N-glycans with bisecting N-acetylglucosamine (GlcNAc) residue, A2G1(3)FB [Galbeta1-4GlcNAcbeta1-2Manalpha1-3(GlcNAcbeta1-2Manalpha1-6)(GlcNAcbeta1-4)Manbeta1-4GlcNAcbeta1-4(Fucalpha1-6)GlcNAc], A2G1(6)FB [GlcNAcbeta1-2Manalpha1-3(Galbeta1-4GlcNAcbeta1-2Manalpha1-6)(GlcNAcbeta1-4)Manbeta1-4GlcNAcbeta1-4 (Fucalpha1-6)GlcNAc] and BA-1 [Manalpha1-3(GlcNAcbeta1-2Manalpha1-6)(GlcNAcbeta1-4)Manbeta1-4GlcNAcbeta1-4(Fucalpha1-6)GlcNAc], and A1(6)G0F [Manalpha1-3(GlcNAcbeta1-2Manalpha1-6)Manbeta1-4GlcNAcbeta1-4(Fucalpha1-6) GlcNAc] are also present in relatively large proportions in compact myelin. We suggest that these differences may be related to myelin-axolemmal function.

Abundance of triacylglycerols in ganglia and their depletion in diabetic mice: implications for the role of altered triacylglycerols in diabetic neuropathy

Herein, we report the first study on the mass distribution and molecular species composition of abundant triacylglycerols (TAG) in ganglia. This study demonstrates five novel findings. First, unanticipated high levels of TAG were present in all examined ganglia from multiple species (e.g. mouse, rat and rabbit). Second, ganglial TAG mass content is location-dependent. Third, the TAG mass levels in ganglia are species-specific. Fourth, dorsal root ganglial TAG mass levels in streptozotocin-induced diabetic mice are dramatically depleted relative to those found in untreated control mice. Fifth, mouse ganglial TAG mass levels decrease with age although molecular species composition is not changed. Collectively, these results indicate that TAG is an important component of ganglia and may potentially contribute to pathological alterations in peripheral neuronal function in diabetic neuropathy.

Relationship of ATP turnover, polyphosphoinositide metabolism, and protein phosphorylation in sciatic nerve and derived peripheral myelin subfractions from normal and streptozotocin diabetic rats

Sciatic nerve from streptozotocin-induced diabetic rats has previously been shown to incorporate more 32P into phosphatidylinositol-4,5-bisphosphate (PIP2) and the principal myelin proteins than normal nerve. In the present study, labeling of ATP and PIP2 was compared. Using nerve segments, [gamma-32P]ATP specific activity reached a plateau after incubation for 4 h with [32P]orthophosphate, whereas the specific activity of [32P]PIP2 rose much more slowly and was still increasing after 8 h. The rate of disappearance of radioactivity from prelabeled ATP was biphasic, with 75% being lost within 30 min and the remainder declining much more slowly for several hours thereafter. In contrast, no decrease in prelabeled PIP2 radioactivity could be detected for up to 4 h. The kinetics of ATP metabolism were not appreciably different for normal and diabetic nerve. However, after incubation with [32P]orthophosphate for 2 h, the specific activity of PIP2 was 50-120% higher in diabetic nerve. This phenomenon, therefore, cannot be ascribed to altered specific activity of the ATP precursor pool. Greater labeling of PIP2 in 32P-labeled diabetic nerve was present in purified myelin isolated using a simple discontinuous sucrose density gradient, but not in a "nonmyelin" fraction. When nerve homogenate was fractionated on a more complex gradient, three myelin-enriched subfractions were obtained which were heterogeneous as judged by morphological appearance, protein profile, and lipid metabolic activity. The proportion of total lipid radioactivity accounted for by PIP2 was elevated in all the subfractions relative to the homogenate. As compared to myelin subfractions from normal nerve, an increased percentage of 32P in PIP2 was obtained only in the major myelin subfraction from diabetic nerve. The phosphorylation of P0 relative to the other myelin proteins was also enhanced in this subfraction in nerve from diabetic animals.

Demyelination of sciatic nerve in acute experimental allergic neuritis of the rat. A gel electrophoresis investigation

Acute experimental allergic neuritis (EAN) of Lewis rats was monitored by clinical and electrophysiological tests for 60 days. Sciatic nerve myelin proteins were analyzed by quantitative microgel electrophoresis and electroimmunoblotting at different clinical stages of the disease. The clinical severity of EAN and the demyelination as measured by electrophysiological tests and myelin protein concentrations in sciatic nerve did not correspond during the course of the disease. Demyelination reached its peak after partial clinical recovery and was still present on day 60. The major peripheral nervous system (PNS) myelin proteins P0, P1 and P2 decreased at different rates during the course of the disease, indicating possible differences in their proteolytic degradation. Treatment with Freund's complete adjuvant (CFA) alone resulted in alterations of membrane and myelin protein patterns challenging the widely held belief that pure CFA is inert with regard to demyelination.

Membrane interactions in nerve myelin: II. Determination of surface charge from biochemical data

In our accompanying paper (Inouye and Kirschner, 1988) we calculated the surface charge density at the extracellular surfaces in peripheral and central nervous system (PNS; CNS) myelins from observations on the dependency of the width of the extracellular space on pH and ionic strength. Here, we have determined the surface charge density of the membrane surfaces in myelin from its chemical composition and the localization of some of its molecular components. We then analyzed the attractive and repulsive forces between the apposed surfaces and calculated equilibrium periods for comparison with the measured values. The biochemical model accounts for the observed isoelectric range of the myelin period and, with the surface charge reduced (possibly by divalent cation binding or a space charge approximation), the model also accounts for the dependency of period on pH above the isoelectric range. At the extracellular (and cytoplasmic) surfaces the contribution of lipid (with pI approximately 2) to the net surface charge is about the same in both PNS and CNS myelin, whereas the contribution of protein depends on which ones are exposed at the two surfaces. The protein conformation and localization modulate the surface charge of the lipid, resulting in positively-charged cytoplasmic surfaces (pI approximately 9) and negatively-charged extracellular surfaces (pI approximately 2-4). The net negative charge at the extracellular surface is due in CNS myelin to lipid, and in PNS myelin to both lipid and (PO) glycoprotein. The net positive charge at the cytoplasmic surface is due in CNS myelin mostly to basic protein, and in PNS myelin to PO glycoprotein and basic protein. The invariance of the cytoplasmic packing may be due to specific short-range interactions. Our models demonstrate how the particular myelin proteins and their localization and conformation can account for the differences in inter-membrane interactions in CNS and PNS myelins.

Identification of an axolemma-enriched fraction from peripheral nerve

A method has been devised for the fractionation of whole peripheral nerve. The procedure utilizes differential centrifugation and separation on a linear sucrose gradient (10-40%, wt/wt). A membrane fraction localized between 26% and 29% sucrose was not only enriched for the plasma membrane markers, 5'-nucleotidase and acetylcholinesterase (AChE), but also possessed the highest binding of [3H]saxitoxin, a specific marker for sodium channels. Neurons in the lumbar dorsal roots and ventral horns of rats were injected with [3H]fucose to label glycoproteins associated with the axolemma from sciatic nerve. Fractionation of the labeled nerves demonstrated a coincidence in the distribution of [3H]fucose-labeled material and AChE activity in the sucrose density gradient. The increase in the specific activity of marker enzymes for plasma membrane, sodium channels, and labeled membrane, previously demonstrated to be of axolemmal origin, identified the 26-29% region of the sucrose gradient as enriched for axolemma derived from peripheral nerve.

Myelin subfractions isolated from mouse brain: analysis of the lipid composition at three developmental stages

Lipids were examined in whole myelin and 8 myelin subfractions isolated from mouse brain at 18-24, 44-48 and 80-90 days of age. Relative to protein, total lipid was lowest in whole myelin isolated from the oldest animals as well as from subfractions isolated at greater sucrose densities, thus partially accounting for the observed myelin subfraction distribution pattern which shifted during development and an average peak density between 0.55 and 0.65 M sucrose to one banding between 0.60 and 0.70 M sucrose. Whole myelin and each myelin subfraction isolated at one age contained nearly the same ratio of sterol and phospholipid to galactolipid; these ratios decreased uniformly during development suggesting enrichment with galactolipid in all myelin subfractions. Sulfatide, as percentage of total galactolipid, was relatively constant during development and appeared to be slightly enriched in the denser myelin subfractions. The findings suggest that regardless of the origin(s) of the subfractions, an age-related mechanism exists in the central nervous system which modified myelin lipid composition relatively uniformly.

The glycoprotein composition of peripheral nervous system myelin subfractions

Two fractions were isolated by continuous density gradient centrifugation from total particulate matter of rabbit sciatic nerves: a minor fraction, B, consisting of small-sized membrane fragments and a major fraction, C, of characteristic multilayered myelin figures, with maxima at 0.33 and 0.58 M-sucrose, respectively. In comparison with C, fraction B was enriched in CNPase and alkaline phosphatase activities and the P0, 23K and Z proteins, but was virtually devoid of basic protein. The glycoprotein composition of all fractions was examined with four fluorescein isothiocyanate-labelled lectins (WGA, Con A, RCA-60, U.E.). These revealed the presence of six glycoproteins in all fractions with similar lectin binding capacities and molecular weights ranging from 35,500 to 16,000, of which P0 was the predominant component. Material found on the heavy side of fraction C was characterized by the presence of a multitude of glycoproteins which bound variable proportions of the four different lectins, suggesting substantial variations in their carbohydrate moieties. Their absence from the central portion of fraction C points to a location other than that of compact PNS myelin.