Calcium-mediated breakdown of glial filaments and neurofilaments in rat optic nerve and spinal cord

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Abstract
Disruptive effects of calcium upon neurofilaments and glial filaments were studied in white matter of rat optic nerve and spinal cord and in rat peripheral nerve. Filament ultrastructure and tissue protein composition were compared following a calcium influx into excised tissues. A calcium influx was induced by freeze-thawing tissues in media containing calcium (5 mM) while control tissues were freeze-thawed in the presence of EGTA (5 mM). Experimental and control tissues were either fixed by immersion in glutaraldehyde and processed for electron microscopic examination or homogenized in a solubilizing buffer and analyzed for protein content by SDS-polyacrylamide gel electrophoresis. Morphological studies showed that calcium influxes led to the loss of neurofilaments and glial filaments and to their replacement by an amorphous granular material. These morphological changes were accompanied by the loss of neurofilament triplet proteins and glial fibrillary acidic (GFA) protein from whole-tissue homogenates. In addition, a calcium-sensitive 58,000-mol-wt protein was identified in rat optic and peripheral nerve. The findings indicate the widespread occurrence of neurofilament proteolysis following calcium influxes into CNS and PNS tissues. The parallel breakdown of glial filaments and loss of GFA protein subunits suggest the presence of additional calcium-activated proteases(s) in astroglial cells.