Role of the microtubule cytoskeleton in traffic of EGF through the lacrimal acinar cell endomembrane network.

We have previously documented a novel biphasic traffic pattern for epidermal growth factor (EGF) in the acinar epithelial cell of the lacrimal gland. Different from the typical paradigm observed in many other cell types, EGF initially accumulates in the acinar basal-lateral recycling endosome, then is re-directed to the prelysosomes and lysosomes and degraded. While the cellular content of intact EGF decreases by 40% between 20 and 120 m of continuous incubation at 37 degrees C, the EGF receptor (EGFR) content decreases only modestly [J. Cell Physiol. 199 (2004) 108]. The purpose of the present study was to investigate the role of the microtubule cytoskeleton in this traffic. Primary cultured rabbit lacrimocytes were incubated with [(125)I]-EGF, lysed, and analyzed by subcellular fractionation on sorbitol density gradients. Nocodazole treatment appeared to slightly decrease the initial uptake rate but to have no significant effect on the total amount of [(125)I] accumulation. However, it enhanced accumulation of [(125)I]-EGF and EGFR in the basal-lateral recycling endosome, and it enhanced accumulation of prepro- and pro- cathepsin B in fractions containing late endosomes and prelysosomes. Nocodazole permitted the time-dependent release of [(125)I]-EGF from the recycling endosome, but it partially inhibited [(125)I]-EGF degradation and decreased accumulation of [(125)I]-labeled degradation products in the lysosome. The microtubule-based molecular motors, cytoplasmic dynein and kinesin, were localized in compartments containing the late endosomes, prelysosomes, and lysosomes, consistent with the suggestion that microtubule-based molecular motors play important roles in traffic within the lysosomal pathway. Confocal fluorescence microscopy imaging of FITC-EGF substantiated the effects observed in biochemical studies by demonstrating that nocodazole increased accumulation in a peripheral compartment and decreased traffic to a perinuclear compartment. These data suggest that initial accumulation in the basal-lateral recycling endosome and subsequent release from the recycling endosome to the late endosomes and prelysosome are not microtubule-dependent. On the other hand, microtubule-based motors are more critical for traffic from the prelysosome to the lysosome.