Proteolysis and myelin breakdown: a review of recent histochemical and biochemical studies

Synopsis Proteins are important constituents of the myelin sheath and serve to maintain its structural integrity. One of the protein components is susceptible to tryptic digestion and may be regarded as a particularly vulnerable part of the myelin sheath. The initial events in myelin breakdown may involve disruption of lipid-protein attachments followed later by chemical degradation of released lipids.In Wallerian degeneration the activity of proteolytic enzymes increases by 12 hr after nerve section. Proteolytic enzyme activity increases in the prodromal phase of diphtheritic neuropathy. Extracts of degenerating nerve cause proteolysis of normal myelin with loss of trypanophilic basic protein and lipid; selective loss of basic protein occurs very early in Wallerian degeneration and has also been found in and around plaques of multiple sclerosis. Proteolytic activity is increased at the edges of active multiple sclerosis lesions. It has been shown that the basic encephalitogenic protein is susceptible to digestion by neural proteases, yielding an active encephalitogenic fragment.It is inferred from these collective observations that proteases play an important role in early myelin breakdown and may also be implicated in the pathogenesis of multiple sclerosis plaques by digesting basic protein, by releasing lipid from its attachment to such protein, and by liberating an active encephalitogenic peptide. The factors responsible for the activation and release of proteases remain unknown.Research Associate supported by the Multiple Sclerosis Society.     

Comparative Study of Cultured Burkitt Tumor Cells by Immunofluorescence, Autoradiography, and Electron Microscopy

Cultured Burkitt cells were examined by immunofluorescence, autoradiography, and electron microscopy in an effort to identify the stainable cells with those harboring herpes-type virus particles. Immediately after a 2-hr pulse of (3)H-thymidine, from 30 to 60% of the cells revealed heavy nuclear labeling. In most cases the grains were evenly dispersed, but in about 3 to 5% the grains showed a focal distribution and occasionally they extended into the cytoplasm. Such nuclear foci were rarely seen at 8 hr after the pulse. When the analysis was restricted to preselected immunofluorescent cells, up to 80% showed label at 8 hr and cytoplasmic grains were prominent. To reduce cellular deoxyribonucleic acid (DNA) synthesis, cells were X-irradiated with 3,000 to 6,000 R, and the isotope pulse was applied 1, 4, or 7 days later. Whereas the total number of labeled cells decreased in roughly twofold steps at the respective intervals (from 40 to 10%), the incorporation of (3)H-thymidine into fluorescent cells was not affected by X irradiation. In each series, about 70% of the fluorescent cells contained label when they were examined at 24 and 48 hr after the pulse, whereas at 8 and 72 hr fewer were positive. At the earlier intervals, unlabeled fluorescent cells most likely represented cells which had completed viral DNA synthesis prior to the pulse; at the later intervals, unlabeled fluorescent cells were probably cells which commenced viral replication after the pulse. These data support the conclusion that the immunofluorescent cells are the ones which harbor virus, and also confirm the expectation that the virus is a DNA virus from a member of the herpes group. This conclusion was firmly established by sectioning and electron microscopic examination of individual fluorescent cells, all of which contained numerous virus particles, whereas the nonstained cells prepared in a similar manner were free of them.