Figure 3. Transcytosis as a regulator of epithelial cell shape.
In each of the models diagrammed, the apical and basal membrane surfaces are designated as red and blue, respectively. The hypothesized path that transcytosed membrane takes through each cell is designated by solid black, numbered arrows. The proposed contribution of directly secreted de novo synthesized membrane (DS) from the ER/Golgi (shaded orange) is indicated by segmented arrows. (A) Pelissier et al, (2003) suggested that endocytosed apical membrane contributes to lateral membrane expansion and the generation of a columnar epithelium during Drosophila cellularization. In this model, (1) clathrin (green) and dynamin internalize apical membrane (red), which is then trafficked through (2) Rab5‐ and Rab8/Rab11‐dependent endosomes (E) to contribute to lateral membrane expansion (Mavor et al, 2016) (3). The contribution of apically endocytosed membrane seems to be incomplete, suggesting that export of directly secreted membrane (DS) from the Golgi also supports lateral membrane growth as well. The apical membrane also fuels lateral membrane ingression through the flattening of its microvilli (red arrows) (Figard et al, 2013; Figard et al, 2016). (B) Soulavie et al (2018) proposed a transcytosis model for apical lumen elongation in the C. elegans excretory duct cell. As the duct cell elongates, the apical lumen grows to a length greater than the cell itself. In this model, internalization of basal membrane (1) is completed by AFF‐1 (teal)‐mediated membrane scission followed by incorporation into RAB‐11‐dependent secretory endosomes (2) and trafficking to the apical surface (3). Potentially, directly secreted membrane (DS) from the Golgi apparatus (or unidentified intracellular sources) may also be trafficked apically via these endosomes. (C) Mathew et al, (2020) provided evidence that synchronous apical and basal membrane growth during early Drosophila terminal cell elongation is achieved by transcytosis. In this model, newly synthesized membrane (DS) and proteins are first secreted at the apical lumen of the terminal cell. Apical plasma membrane (red) along with basally bound transmembrane proteins is endocytosed in a clathrin‐dependent (green) and dynamin‐dependent fashion (1). Internalized membrane is then trafficked through early (2) and late, multivesicular endosomes (3) before being transcytosed to the basal membrane (blue) (4). Concurrent apical recycling (R) from the late endosome balances apical and basal membrane allocation. (D) Basal‐to‐apical transcytosis is necessary for the development of bile canaliculi. In HepG2 cells, apically destined membrane proteins (purple) and membrane are initially secreted to the basolateral surface (gray‐blue) (DS). This cargo is then endocytosed (1) and incorporated into basal endosomes (E) (2). Vesicles and tubular projections containing MAL2 (orange) traffic from the subapical compartment (SAC) to fuse with basal cargo‐containing endosomes (3). After fusion, the MAL2+ cargo‐containing tubular/vesicular bodies aggregate and return to the SAC (4) for subsequent delivery of cargo to the apical, canalicular surface (5) (de Marco et al, 2002; de Marco et al, 2006; Madrid et al, 2010).