Lipid Trafficking in Neurons and Schwann Cells

Abstract

Axons in the peripheral nervous system are capable of extending thousands of times the length of their cell bodies. These projections possess high surface area to volume ratios and thus require large quantities of lipids to maintain their membrane structure. During initial development this demand for lipids can be quite high as axons are capable of extending at rates of 1 mm/day. Studies suggest that long distance transport of lipids occurs in membranous vesicles with insertion occurring only at the growth cone. Other sources of membrane lipids have been demonstrated including locally synthesized lipids in axons, transfer of lipids from myelin, and addition of lipids internalized along the axon. Robust analysis and quantification of lipid trafficking has yet to be well characterized. To quantify these phenomena, a model of developmental sensory axons utilizing dorsal root ganglia (DRG) of day 1 to 5 rats were used. Cells were harvested and plated overnight. Cultures were then treated with cytosine arabinoside to remove fibroblasts. Two days post-harvest, cultures were treated with a fluorescent ceramide analogue labeled with boron dipyrromethene difluoride (BODIPY). One hour post-treatment the cultures were observed for twenty minute time intervals using fluorescent microscopy. Preliminary results indicate multiple transport processes may play a role in the trafficking of lipids in the axons of neurons. At least two distinct populations seem to exist; one composed of bright, punctuate fluorescence that may be attributable to vesicles and a second diffuse fluorescence that may be attributable to labeled plasma membrane lipids. Quantification of the traffic patterns seem to initially suggest the bright, punctuate population are more motile than the diffusely fluorescent populations. Future experiments will compare patterns observed in neurons with those in Schwann cell projections, which may be a source of lipids transferred to axons.