Immunocytochemical localization of phosphatidylinositol-anchored acetylcholinesterase in excitable membranes of Torpedo ocellata

Abstract

In Torpedo electric organ much of the acetylcholinesterase is a ‘globular’ dimer (G 2), anchored to the plasma membrane via covalently attached phosphatidylinositol and solubilized by a bacterial phosphatidylinositol-specific phospholipase C. This suggested that selective solubilization with phosphatidylinositol-specific phospholipase C, coupled with immunocytochemistry, might be used to localize G 2 acetylcholinesterase in excitable tissues of Torpedo. Cryostat sections of electric organ, electromotor nerve, electric lobe and back muscle from Torpedo ocellata were labelled, using three different antibody preparations to Torpedo acetylcholinesterase, followed by a fluorescent second antibody, before and after exposure to the phospholipase. Sites of innervation on electrocytes and myofibers were labelled selectively, as were motor and electromotor nerves. In all these cases labelling was substantially diminished by prior exposure to the phospholipase. The results support our previous assignment, based on biochemical evidence, for a neuronal and synaptic localization of the G 2 acetylcholinesterase in Torpedo. Electric lobe acetylcholinesterase appears insensitive to the phospholipase treatment and lacks certain epitopes present in both electric organ and electromotor nerve enzyme. This suggests that substantial processing of the G 2 form occurs concomitantly with its movement from the electric lobe into the electromotor nerve.