Microtubule-dependent apical polarization of basement membrane matrix mRNAs in mouse epithelial cells

The basement membrane (BM) demarcating epithelial tissues undergoes rapid expansion to accommodate tissue growth and morphogenesis during embryonic development. To facilitate the secretion of bulky BM proteins, their mRNAs are polarized basally in the  follicle epithelial cells of the Drosophila egg chamber to position their sites of production close to their deposition. In contrast, we observed the apical rather than basal polarization of all major BM mRNAs in the outer epithelial cells adjacent to the BM of mouse embryonic salivary glands using single-molecule RNA fluorescence in situ hybridization (smFISH). Moreover, electron microscopy and immunofluorescence revealed apical polarization of both the endoplasmic reticulum (ER) and Golgi apparatus, indicating that the site of BM component production was opposite to the site of deposition. At the apical side, BM mRNAs colocalized with ER, suggesting they may be co-translationally tethered. After microtubule inhibition, the BM mRNAs and ER became uniformly distributed rather than apically polarized, but they remained unchanged after inhibiting myosin II, ROCK, or F-actin, or after enzymatic disruption of the BM. Because Rab6 is generally required for Golgi-to-plasma membrane trafficking of BM components, we used lentivirus to express an mScarlet-tagged Rab6a in salivary gland epithelial cultures to visualize vesicle trafficking dynamics. We observed extensive bidirectional vesicle movements between Golgi at the apical side and the basal plasma membrane adjacent to the BM. Moreover, we showed that these vesicle movements depend on the microtubule motor kinesin-1 because very few vesicles remained motile after treatment with kinesore to compete for cargo-binding sites on kinesin-1. Overall, our work highlights the diverse strategies that different organisms use to secrete bulky matrix proteins: while Drosophila follicle epithelial cells strategically place their sites of BM protein production close to their deposition, mouse embryonic epithelial cells place their sites of production at the opposite end. Instead of spatial proximity, they use the microtubule cytoskeleton to mediate this organization as well as for the apical-to-basal transport of BM proteins.