Intermediate filaments in nervous tissues

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Abstract
Intermediate filaments were isolated from rabbit intradural spinal nerve roots by the axonal flotation method. This method was modified to avoid exposure of axons to low ionic strength medium. The purified filaments are morphologically 75-80% pure. The gel electrophoretogram shows 4 major bands migrating at 200,000, 145,000, 68,000 and 60,000 daltons, respectively. A similar preparation from rabbit brain shows 4 major polypeptides with MW of 200,000, 145,000, 68,000, and 51,000 daltons. The neurofilament is composed of a triplet of polypeptides with MW of 200,000, 145,000 and 68,000 daltons. The 51,000 dalton band that appears in brain filament preparations as the major polypeptide seems to be of glial origin. The significance of the 60,000 dalton band in the nerve root filament preparation is unclear at this time. Antibodies raised against 2 of the triplet proteins isolated from calf brain localize by immunofluorescence to neurons in central and peripheral nerve, but an antibody to the 51,000 dalton polypeptide gives only glial staining in the brain and very weak peripheral nerve staining. Prolonged exposure of axons to low ionic strength medium solubilizes almost all the triplet polypeptides, leaving behind only the 51,000 dalton component. The neurofilament is probably soluble at low ionic strength, whereas the glial filament is not. Probably, neurofilaments and glial filaments are composed of different polypeptides and have different solubility characteristics.