Endocytosis of NBD-sphingolipids in neurons: exclusion from degradative compartments and transport to the Golgi complex.

Sphingolipids are abundant constituents of neuronal membranes that have been implicated in intracellular signaling, neurite outgrowth and differentiation. Differential localization and trafficking of lipids to membrane domains contribute to the specialized functions. In non-neuronal cultured cell lines, plasma membrane short-chain sphingomyelin and glucosylceramide are recycled via endosomes or sorted to degradative compartments. However, depending on cell type and lipid membrane composition, short-chain glucosylceramide can also be diverted to the Golgi complex. Here, we show that NBD-labeled glucosylceramide and sphingomyelin are transported from the plasma membrane to the Golgi complex in cultured rat hippocampal neurons irrespective of the stage of neuronal differentiation. Golgi complex localization was confirmed by colocalization and Golgi disruption studies, and importantly did not result from conversion of NBD-glucosylceramide or NBD-sphingomyelin to NBD-ceramide. Double-labeling experiments with transferrin or wheat-germ agglutinin showed that NBD-sphingolipids are first internalized to early/recycling endosomes, and subsequently transported to the Golgi complex. The internalization of these two sphingolipid analogs was energy and temperature dependent, and their intracellular transport was insensitive to the NBD fluorescence quencher sodium dithionite. These results indicate that vesicles mediate the transport of internalized NBD-glucosylceramide and NBD-sphingomyelin to the Golgi complex.