Cytoskeleton of intestinal goblet cells: role of microtubules in baseline secretion

To determine the involvement of microtubules (MTs) in granule translocation, autoradiographic analysis of maximal granule movement in the absence or presence of MT inhibitors was performed. Rabbit colonic mucosal explants were pulse-labeled with [3H]glucosamine for 30 min in organ culture, then maintained on nonradioactive medium for 1-6 h. Radio-labeled mucin granules appear in the apical granule mass in 1-2 h, then they migrate to the apical plasma membrane, with a total transit time of 4-6 h. Mucosal explants were treated with either nocodazole or taxol for 30 min, pulse-labeled for 30 min, then maintained in organ culture with the same drug for up to 6 h. Nocodazole binds tubulin, preventing polymerization. In response, granule movement out of the supranuclear region and along the apical granule mass is significantly impeded. Taxol stabilizes MTs, preventing depolymerization. In response, supranuclear MTs are misoriented, but thecal MTs maintain normal orientation. Taxol treatment impedes granule migration out of the supranuclear region of the cell but not migration along the theca. These data suggest that the organization of MTs dictate the spatial organization of the baseline secretory pathway. Microtubules are necessary for granule translocation by providing directed tracks for granule movement, but microtubule dynamics are not the motile mechanism transporting mucin granules to the apical plasma membrane for secretion.