Polarized sorting of nicotinic acetylcholine receptors to the postsynaptic membrane in Torpedo electrocyte

Several regulatory mechanisms contribute to the accumulation and maintenance of high concentrations of acetylcholine receptors (AChR) at the postsynaptic membrane of the neuromuscular junction, including compartmentalized gene transcription, targeting, clustering and anchoring to the cytoskeleton. The targeting of the AChR to the postsynaptic membrane is likely to involve a polarized sorting in the exocytic pathway. In this work, we used the electrocyte of Torpedo marmorata electric organ 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 populations of post-Golgi transport vesicles (PGVs) enriched in proteins of either the innervated (AChR) or non-innervated (Na,K-ATPase) membrane domains of the cell. Immunolabelling of these vesicles at the EM level disclosed that very few PGVs contained both proteins. In AChR-enriched vesicles, high sialylation of AchR molecules, an expected post-translational modification of proteins exiting the trans-Golgi network, and the presence of a marker of the exocytic pathway (Rab6p), indicate that these vesicles are carriers engaged in the Golgi-to-plasma membrane transport. These data suggest that AChR and Na,K-ATPase are sorted intracellularly most likely within the trans-Golgi network. Furthermore, EM analysis and immunogold-labelling experiments provided in situ evidence that the AChR-containing PGVs are conveyed to the postsynaptic membrane, possibly by a microtubule-dependent transport mechanism. Our data therefore provide the first evidence that the targeting of receptors for neurotransmitters to synaptic sites could be contributed by intracellular sorting and polarized delivery in the exocytic pathway.