THE INTERNALIZATION OF A SHORT ACYL-CHAIN ANALOG OF GANGLIOSIDE GM(1)IN POLARIZED NEURONS

In order to study the endocytosis of membrane lipids during the develo pment of neuronal polarity, we examined the internalization of a short acyl chain fluorescent derivative of ganglioside GM(1), obenz-2-oxa-1 ,3-diazole-7-yl)-aminohexanoyl)-GM(1) (C-6-NBD-GM(1)), in hippocampal neurons cultured at low density, C-6-NBD-GM(1) was internalized by tem perature- and energy-dependent mechanisms, and after short times of in cubation, accumulated in endosomes in the axon, cell body and dendrite s of neurons maintained for up to 4-5 days in culture, C-6-NBD-GM(1) w as subsequently transported in a retrograde direction to a pool of rec ycling endosomes in the cell body, with little transport to lysosomes, as indicated by the lack of degradation of C-6-NBD-GM(1) even after l ong times, and the re-appearance of intact C-6-NBD-GM(1) at the cell s urface after recycling; similarly, little degradation of C-6-NBD-GM(1) was detected in N18TG-2 neuroblastoma cells, In hippocampal neurons m aintained for longer than 6 days in culture, there was little internal ization of C-6-NBD-GM(1) along the length of axons, but the amount of endocytosis from dendrites was similar to that observed in younger neu rons, These results demonstrate that gangliosides turnover rapidly in dendritic membranes at all stages of neuronal development, whereas gan glioside turnover in axons is much less rapid, at least in mature, pol arized neurons.