Microtubules and the endoplasmic reticulum are highly interdependent structures.

TLDR

The authors used fluorescent dye staining and immunofluorescence to map ER, microtubules, and intermediate filaments in peripheral regions of cultured fibroblasts, epithelial, and endothelial cells, and perturbed microtubules with nocodazole, cold, or hypotonic shock to assess effects on ER distribution. Microtubules and the ER co‑extend into lamellipodia, showing nearly identical distributions, while intermediate filaments lag; perturbation experiments revealed that microtubule polymerization and ER extension occur together, and that sustained microtubule loss causes gradual ER retraction toward the cell center, indicating a long‑term dependence of the ER network on microtubules.

Abstract

The interrelationships of the endoplasmic reticulum (ER), microtubules, and intermediate filaments were studied in the peripheral regions of thin, spread fibroblasts, epithelial, and vascular endothelial cells in culture. We combined a fluorescent dye staining technique to localize the ER with immunofluorescence to localize microtubules or intermediate filaments in the same cell. Microtubules and the ER are sparse in the lamellipodia, but intermediate filaments are usually completely absent. These relationships indicate that microtubules and the ER advance into the lamellipodia before intermediate filaments. We observed that microtubules and tubules of the ER have nearly identical distributions in lamellipodia, where new extensions of both are taking place. We perturbed microtubules by nocodazole, cold temperature, or hypotonic shock, and observed the effects on the ER distribution. On the basis of our observations in untreated cells and our experiments with microtubule perturbation, we conclude that microtubules and the ER are highly interdependent in two ways: (a) polymerization of individual microtubules and extension of individual ER tubules occur together at the level of resolution of the fluorescence microscope, and (b) depolymerization of microtubules does not disrupt the ER network in the short term (15 min), but prolonged absence of microtubules (2 h) leads to a slow retraction of the ER network towards the cell center, indicating that over longer periods of time, the extended state of the entire ER network requires the microtubule system.