POLARIZED SORTING OF NICOTINIC ACETYLCHOLINE-RECEPTORS TO THE POSTSYNAPTIC MEMBRANE IN TORPEDO ELECTROCYTE

Several regulatory mechanisms contribute to the accumulation and maint enance of high concentrations of acetylcholine receptors (AChR) at the postsynaptic membrane of the neuromuscular junction, including compar tmentalized gene transcription, targeting, clustering and anchoring to the cytoskeleton. The targeting of the AChR to the postsynaptic membr ane is likely to involve a polarized sorting in the exocytic pathway. In this work, we used the electrocyte of Torpedo marmorata electric or gan to study the intracellular trafficking of neosynthesized AChR and its delivery to the postsynaptic membrane. Gradient centrifugation and immunoisolation techniques have led to the isolation of two populatio ns of post-Golgi transport vesicles (PGVs) enriched in proteins of eit her the innervated (AChR) or non-innervated (Na,K-ATPase) membrane dom ains of the cell. Immunolabelling of these vesicles at the EM level di sclosed that very few PGVs contained both proteins. In AChR-enriched v esicles, high sialylation of AchR molecules, an expected post-translat ional modification of proteins exiting the trans-Golgi network, and th e presence of a marker of the exocytic pathway (Rab6p), indicate that these vesicles are carriers engaged in the Golgi-to-plasma membrane tr ansport. These data suggest that AChR and Na,K-ATPase are sorted intra cellularly most likely within the trans-Golgi network. Furthermore, EM analysis and immunogold-labelling experiments provided in situ eviden ce that the AChR-containing PGVs are conveyed to the postsynaptic memb rane, possibly by a microtubule-dependent transport mechanism. Our dat a therefore provide the first evidence that the targeting of receptors for neurotransmitters to synaptic sites could be contributed by intra cellular sorting and polarized delivery in the exocytic pathway.