Degradation of basic protein and Wolfgram protein in central nervous system by soluble enzymes of human peripheral polymorphonuclear leucocytes

Absence of Wallerian Degeneration does not Hinder Regeneration in Peripheral Nerve

Wallerian degeneration of the distal stump of a severed peripheral nerve involves invasion by myelomonocytic cells, whose presence is necessary for destruction of myelin and for initiating mitosis in Schwann cells (Beuche and Friede, 1984). Degeneration of the distal ends of the axons themselves is assumed to occur by autolytic mechanisms. We describe a strain of mice (C57BL/6/Ola) in which leucocyte invasion is slow and sparse. In these mice, confirming Beuche and Friede, myelin removal is extremely slow. A new finding is that axon degeneration is also very slow. This is a consequence of lack of recruitment of myelomonocytic cells for if such recruitment is prevented in other mouse strains by a monoclonal antibody against the complement type 3 receptor (Rosen and Gordon, 1987) axon degeneration is again slowed. We have also, surprisingly, found that nerve regeneration in the C57BL/6/Ola mice is not impeded by the presence of largely intact axons in the distal stump and absence of recruited cells, myelin debris and the absence of Schwann cell mitosis.

Ibotenic acid induced demyelination in the central nervous system: a consequence of a local inflammatory response

We examined the effects of injections of ibotenic acid into the medial septum (MS), dorsal lateral geniculate nucleus (dLGN), caudate-putamen (CP) or fornix-fimbria (FF) on fibres close to the injection site. Injections into the MS and dLGN resulted in demyelination. The area of demyelination correlated with the area of maximal gliosis. The major cell type in the area of gliosis were cells of haemopoietic origin, as revealed by using monoclonal antibodies (Mabs). Demyelination was not observed in the CP or FF. Axonal transport in fibres en passage through the dLGN lesion was also disrupted following injections of horseradish peroxidase (HRP) into the eye contralateral to the lesion. These results show that when ibotenic acid induces cell death it can secondarily damage myelin sheaths and disrupt axonal transport in areas containing diffuse fibre systems. We suggest that this is due to a non-specific effect of the inflammatory response.

Cation-mediated release and proteolytic cleavage of basic protein of isolated human myelin at acid pH

Myelin from human brain was incubated at pH 4.4 with metal salts, including KCl, NaCl, CaCl2, and MgSO4, to elicit cation-dependent autoproteolysis of myelin proteins. Incubation of myelin resulted in soluble proteolytic breakdown products of Mr smaller than those of the three original myelin basic proteins (MBPs). Comparable polypeptides were essentially absent from residual myelin. Proteolysis was strongly stimulated by increasing millimolar concentrations of K+, Na+, and Mg2+ and only moderately by Ca2+. Breakdown products were traced to MBP by immunostaining. Their origin from MBP was also indicated by identical electrophoretic cleavage patterns from endogenous myelin protein and exogenous MBP. All four metal salts, in addition to activating endogenous proteolysis, also caused a biphasic extraction of MBP. Electrophoresis of myelin revealed a quick initial and a slow further loss of protein, eventually leading to the removal of up to 78% of original MBP. The results are consistent with a concurrent extraction of MBP and activation of latent-bound acid protease activity by metal cations. It is therefore suggested that, in particular disease states, unfavorable changes in electrolytes and pH of white matter may cause a selective loss and proteolytic cleavage of MBP.

Susceptibility of the Wolfgram proteins and stability of 2',3'-cyclic nucleotide 3'-phosphodiesterase of rat brain myelin to limited proteolytic digestion

The susceptibility of proteins in the myelin membrane to proteases was studied. Lyophilized rat brain myelin suspended in water was subjected to controlled proteolytic digestion with pure trypsin (N-tosyl-L-phenylalanine chloromethyl ketone treated, 5 units/mg of myelin), and proteins remaining in the pellet were analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Under these conditions, large basic protein (LBP) was completely hydrolyzed in 5-10 min, proteolipid proteins remained largely intact until 60 min, whereas Wolfgram protein (WP) was progressively degraded from 10 min onward with the simultaneous appearance of a new protein band with a molecular weight of 35K. A similar pattern was obtained on treatment with chymotrypsin or subtilisin. The 35K protein band was shown to be derived from WP by its immunological cross-reactivity with WP antibodies. Western blot analysis showed that 35K protein is the only major breakdown product of WP under these conditions. Treatment with higher concentrations of trypsin (greater than 20 units/mg of myelin) resulted in the degradation of all myelin proteins. Essentially all the 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNP) activity was observed in the myelin pellet after controlled or drastic digestion with trypsin. It is concluded that the major fragment of WP (35K) is located in the hydrophobic milieu of the bilayer, relatively inaccessible to trypsin, whereas a portion (20K) of the WP is exposed to the cytoplasmic side (major dense line), like LBP, and that peptide fragments (less than 14K) that remained in the myelin membrane lipid bilayer after trypsin digestion could exhibit CNP activity.

The role of non-resident cells in Wallerian degeneration

Wallerian degeneration was studied in the phrenic or sciatic nerves of mice following transplantation into Millipore diffusion chambers of 0.22 micron pore size which were implanted in the peritoneal cavity and kept for up to eight weeks. This method positively eliminates the access of nonresident cells to the tissue, at the same time providing proper conditions for tissue survival. Such nerves showed no proliferation of Schwann cells and no evidence for their active role in the removal or digestion of myelin. Schwann cells rejected their sheaths and the latter persisted for weeks, leading either to sheath distension (the sheath becoming wider and thinner) or to collapse (the sheath becoming thicker, collapsing upon the empty axis cylinder). The outer envelope of Schwann cytoplasm separated into pseudopodia rich in microtubules. Sheath rejection led to a slow decay of the myelin in the absence of active phagocytosis. There was profuse fibroblastic proliferation from the epineurium and perineurium, from which cells migrated into the chambers developing fatty change. No evidence was found to link the fatty change in fibroblasts to sheath decay. Diffusion chambers of 5.0 micron pore size were invaded by leukocytes and monocytes. Nerves kept in such chambers showed active phagocytosis of myelin leading to its removal, similar to Wallerian degeneration in situ. Phagocytes were shown to attack selectively the rejected myelin sheaths, distinguishing the latter from the surviving Schwann cells, even though both structures derive from the same cell. The activity of phagocytes in digesting myelin was mediated by a signal which diminished in intensity with time; there was very little active phagocytosis of myelin in nerves that had been predegenerated in 0.22 micron pore chambers. Various modifications of the experiment, including studies with co-cultured peritoneal macrophages or bone marrow, indicate a need for additional activating factors to induce myelin phagocytosis.

Degradation of myelin proteins by brain endogenous neutral protease

A proteolytic enzyme isolated from calf brain cytosol, degraded purified myelin basic protein in the presence of Ca2+ at pH 7.0 (Singh, I. and Singh, A.K., Trans. Amer. Soc. Neurochem., 13 (1982) 119). This proteolytic enzyme also degrades basic proteins when incubated with intact myelin in the presence of Ca2+ at a neutral pH. Three hour treatment of purified myelin with this protease resulted in degradation of large basic protein and small basic protein by 73 and 89%, respectively. This proteolytic activity was inhibited by EDTA, leupeptin and low pH (pH 4.0), but was not affected by phenylmethylsulfonylfluoride, p-nitrophenylguanidinobenzoate and pepstatin A. Purified myelin preparations also contain small amounts of Ca2+-activated proteolytic activity. The fact that this is a neutral protease, endogenous to the brain, suggests that it may play a role in the degradation of myelin under physiological and pathological conditions.

Membrane protein antigens in multiple sclerosis

Rabbits were immunized with white matter (WM) membrane fractions isolated from autopsy brain specimens of three patients with multiple sclerosis (MS), and three controls. All the rabbits developed high serum antibody titers to the MS and control WM fractions, as tested by enzyme immunoassay. Antibodies against WM membrane components were analyzed further by immunoprecipitation of radio-labeled WM proteins and subsequent polyacrylamide gradient gel electrophoresis. Antigenic membrane components with molecular weights of 138 000, 111 000, 86 500, 79 600, 69 000, 63 000, 58 000, 53 400, 45 700, 24 500 and 22 300 were found in both MS and control WM. Although there may have been some quantitative differences in these immunogenic proteins of MS and normal WM, no multiple sclerosis-specific membrane antigen could be demonstrated. The hyperimmune anti-WM sera did not precipitate 35S-labeled polypeptides from cells infected with herpes simplex type 1, adeno type 5, measles, mumps, rubella, respiratory syncytial, parainfluenza type 2 or cytomegaloviruses, which suggests that the MS brain WM membrane proteins do not share common antigenic determinants with the viral polypeptides.