The effects of trypsin on purified myelin

Cytochemical demonstration of negative surface charges in central myelin

Homogenates of central myelin were treated with ferritin derivatives having different isoelectric points. It was found that considerable amounts of cationic ferritin (pI 8.5-9.5) had access to the extracellular space, but that anionic ferritin (pI 4.0) and native ferritin (pI 4.5) did not. The electrostatic nature of the binding of cationic ferritin was demonstrated by treating the homogenates with poly-L-lysine and 1 M NaCl:both reagents led to a complete displacement of the bound cationic ferritin. Neither extensive trypsination nor neuraminidase treatment showed a significant effect on the intralamellar distribution of the bound cationic ferritin molecules. This suggests that the net negative charge on the extracellular myelin face stems primarily from acidic lipid groups in the membrane.

Isolation of a product from the trypsin-digested glycoprotein of sciatic nerve myelin

When purified rabbit sciatic nerve myelin, whether lyophilized or not, is treated with low amounts of trypsin (25 microgram/ml) for 0.5, 3, or 24 h the resulting protein patterns viewed on sodium dodecyl sulfate (SDS) gel electrophoresis are similar. The most striking feature of the trypsinized myelin is the accumulation of a heavy band at the basic protein position, molecular weight 19 000, which is accounted for as a degradation product of the PO protein, referred to as the TPO protein. The PO protein, the major glycoprotein of sciatic nerve myelin, as well as the 23K and P2 proteins and albumin, an absorbed component, are all partially degraded; most high molecular weight bands are lost. The TPO protein, isolated by gel filtration in 2% SDS on an agarose column, like the PO protein, is highly insoluble in aqueous solvents. It is a glycoprotein (8% carbohydrate), staining with periodic acid-Schiff reagent; containing 3 mannose, 1 galactose, 3 N-acetylglucosamine, 1 sialic acid, and 1 fucose residues and is identical to the nonasaccharide of the parent PO protein. The amino acid composition of the TPO protein, is similar to the PO protein, but has a much higher content of hydrophobic residues and begins with NH2-methionine. This suggests that the PO protein is an amphipathic membrane protein in which its more polar character is confined to the first third of its NH2-terminus. This polar domain is probably positioned above the lipid leaflet where it is accessible to trypsin which cleaves a sensitive lysinyl (or argininyl)-methionine linkage. The more hydrophobic domain (the TPO protein) is buried in the myelin bilayer where it is protected from further tryptic attack. Thus trypsin can serve as a useful probe of myelin structure.

Neutral proteinases secreted by macrophages degrade basic protein: a possible mechanism of inflammatory demyelination

In the inflammatory demyelinating diseases, such as multiple sclerosis, Landry-Guillain-Barré syndrome and experimental allergic encephalomyelitis, demyelination occurs in the vicinity of infiltrating mononuclear cells. Although the histopathology is characteristic of each disease, the general observation that myelin destruction in inflammatory lesions begins prior to phagocytosis suggests a common mechanism for myelinolysis in these diseases. Recent studies show that stimulated macrophages secrete several neutral proteinases, including plasminogen (Plg) activator. We have tested the possibility that these proteinases could, directly or indirectly, initiate myelin destruction. Isolated brain myelin was incubated with supernatant media from cultures of stimulated mouse peritoneal macrophages in the presence and absence of Plg. Cell supernatants alone caused some degradation of basic protein (BP) in myelin. The amount degraded was considerably enhanced in the presence of Plg. The other myelin proteins remained essentially intact. While the Plg-independent proteolytic activity in the supernatants was abolished by EDTA, known to inhibit the neutral proteinases, the Plg-dependent hydrolysis was inhibited by p-nitrophenylguanidinobenzoate, an inhibitor of Plg activator and plasmin. These results suggested that the Plg activator secreted by the macrophages generated plasmin, which selectively degraded BP. This interpretation was confirmed by the observation that urokinase, a Plg activator, plus Plg was effective in degrading BP in myelin. We propose that the action of neutral proteinases released by stimulated macrophages, and its amplification by the Plg-plasmin system, may play a significant role in several inflammatory demyelinating diseases; and that the relative specificity of these reactions for myelin lies in the extreme susceptibility of BP to proteolysis.

Turnover of myelin proteins in mouse brain in vivo

The incorporation of tyrosine into proteins was measured after the subcutaneous implantation of a pellet of [14C]tyrosine in mice. This method keeps the specific radioactivity of free tyrosine fairly constant and makes it possible to follow incorporation up to a 10-day period. At the end of 10 days most of the protein-bound tyrosine was replaced (i.e. most protein turned over) in lung, liver, heart, kidney and spleen; about half was replaced in brain, one-quarter in muscle. The rate of protein turnover in myelin was approx. 40% of that of whole brain proteins; at 10 days one-fifth of the myelin proteins were replaced. All protein components of myelin measured were in a dynamic state; incorporation decreased in the following order, Wolfgram greater than DM-20 greater than basic greater than proteolipid proteins. The incorporation of tyrosine into each protein fraction was greater in the 0-5-day than in the 5-10-day period, indicating heterogeneity of metabolic rates. The results show that after myelination at least a portion of each protein component of myelin is undergoing significant metabolic turnover. In the adult, myelin components are not stable, but turnover is heterogeneous, and each protein may be compartmentalized. Turnover can be influenced by a variety of factors.

Neurochemical findings in adreno-leukodystrophy

The results of neurochemical examination of brain tissue derived from 2 cases of adreno-leukodystrophy have been presented. Both white and grey matter contained suadanophilic material. Although free fatty acid was also present, cholesteryl ester accounted for the bulk of the sudanophilic material. Total cholesterol, galactolipid and phospholipid content was reduced in both white and grey matter. The lipid loss was particularly severe from white matter indicative of considerable demyelination. Cholesterol was found to be the only major sterol present in white or grey matter. Subcellular fractionation of the diseased white matter resulted in myelin and two related fractions, one of which was very fatty and was rich in steryl ester. Morphological examination of myelin indicated loosely-packed lamellae. All of 3 fractions had adenosine 2',6'-cyclic nucleotide-3'-phosphohydrolase activity. Myein and the fraction not rich in cholesteryl ester had discernable basic protein bands when examined by polyacrylamide gel electrophoresis. Analysis of the fatty acid composition of choline and ethanolamine glycerophospholipids indicated a general increase of saturated fatty acids, relative to control values and a decrease in long-chain fatty acids. Examination of sphingomyelin fatty acids also demonstrated a loss of long-chain fatty acids. The fatty acid composition of the cholesteryl esters from white and grey matter differed. The findings indicate generalized damage to the brain, both of white and grey matter, with the damage to the white matter being much more severe. No abnormal sterol or other lipid was isolated.

Electron microscopy of trypsin-digested peripheral nerve myelin

Sciatic nerves from mice were removed and soaked in either PBS (phosphate buffered saline) or PBS plus I% trypsin (Sigma Type III) for various periods of time. Specimens were soaked at either room temperature or 37-degrees C at pH's ranging from 7.5 to 8.0. The epineural and perineural sheaths were split to allow the trypsin to penetrate the nerve. Tissue was prepared for electron microscopy by fixation in cacodylate buffered formaldehyde-glutaraldehyde solutions, post-fixed in OSO4 and embedded in Epon 812 or in glutaraldehyde-urea resin without osmication. After four h incubation at 37-degrees C or eight h at room temperature, the basement membranes of the Schwann cells became fragmented and detached and the myelin intraperiod band lost some density. After 18 h, myelin with swollen intraperiod bands displaying a loss of electron density and split main period bands was noted adjacent to normal myelin. Other areas had been transformed into vesicles indicating that the membranes of these vesicles appeared to have been derived from the detachment of both the intraperiod and main period bands within the myelin. Evidence is presented for the presence of trypsin digestable proteins in both the main period and intraperiod bands of peripheral nervouse system myelin.

Lipid and basic protein interaction of myelin

1. Purified myelin labelled with [(3)H]myo-inositol or [1-(14)C]acetate was incubated with trypsin or acetylated trypsin at 37 degrees C, pH8.0 for 30min. 2. After incubation and centrifugation analysis of the myelin pellet showed marked digestion of basic protein on polyacrylamide-gel electrophoresis. Proteolipid and Wolfgram proteins remained unchanged. 3. A loss of 15% of total protein and loss of all classes of lipids was also found. Most significant lipid losses were phosphoinositides, phosphatidylserine and sulphatide. 4. A low-density material containing more phospholipid than cholesterol and galactolipid was isolated from the supernatant obtained after centrifugation of trypsin-treated myelin. 5. Interaction of sulphatide and myelin basic protein was shown to take place in a biphasic system. Basic protein does not form any complex either with cerebroside or cholesterol in the same solvent system. 6. The release of acidic lipids from myelin suggests that they may be linked to basic protein by ionic forces and the neutral lipids may be by lipid-lipid interactions. 7. The relevance of these studies as a model of brain degeneration is discussed.