Differential microtubule requirements for transcytosis in MDCK cells

Regulation of cell surface polarity from bacteria to mammals

The generation of unique domains on the cell, cell surface polarity, is critical for differentiation into the diversity of cell structures and functions found in a wide variety of organisms and cells, including the bacterium Caulobacter crescentus, the budding yeast Saccharomyces cerevisiae, and mammalian polarized epithelial cells. Comparison of the mechanisms for establishing polarity in these cells indicates that restricted membrane protein distributions are generated by selective protein targeting to, and selective protein retention at, the cell surface. Initiation of these mechanisms involves reorientation of components of the cytoskeleton and protein transport pathways toward restricted sites at the cell surface and formation of a targeting patch at those sites for selective recruitment and retention of proteins.

Antimicrotubule drugs inhibit the polarized insertion of an intracellular glycoprotein pool into the apical membrane of Madin-Darby canine kidney (MDCK) cells

Previous experiments on MDCK cells have demonstrated that the polarized appearance of a 135 kDa glycoprotein (gp135) on the apical plasma membrane can occur through the insertion of both newly synthesized gp135 as well as a pre-existing gp135 intracellular pool. In this study, anticytoskeletal drugs were utilized to determine the role of the cytoskeleton in the polarized delivery of gp135. Colchicine and nocodazole produced a 15–20% inhibition in the apical surface accumulation of newly synthesized gp135 and inhibited the appearance of the gp135 pool by approximately 33%, while cytochalasin D had no affect on the apical accumulation of either newly synthesized gp135 or the gp135 pool. These results indicate that microtubules, but not microfilaments, are involved in the intracellular targeting of gp135. Quantitative immunogold electron microscopy of nocodazole-treated cells demonstrated that gp135 was not mistargeted to the basolateral membrane, suggesting the possibility that some vesicles containing gp135 did not fuse with the apical membrane and remained in the cells. These experiments demonstrate that microtubules are an important component of gp135 insertion into the apical membrane. They also suggest that gp135 resides within vesicles which have an apical membrane recognition signal and cannot fuse with the basolateral membrane. The possibility that these data, and those of others, could support a hypothesis for the presence of two constitutive apical transport pathways is discussed.

Selective inhibition of transcytosis by brefeldin A in MDCK cells

Treatment of most cells with brefeldin A (BFA) leads to the retrieval of the Golgi complex to the endoplasmic reticulum, presumably reflecting an inhibition of cytoplasmic coat protein binding to Golgi membranes. Although BFA has been thought to act only on biosynthetic organelles, we now show that this drug also reversibly blocks polymeric immunoglobulin receptor-mediated transcytosis in MDCK cells. The action of BFA on transcytosis was selective, since internalization, recycling, and intracellular degradation were unaffected. The block occurred early on the transcytotic pathway, probably before the translocation of IgA-containing vesicles from the basal to the apical cytoplasm. Although BFA caused MDCK cell endosomes to become more tubular, the organization of the Golgi and binding of the 110 kd Golgi coat protein beta-COP was surprisingly unaffected. These results suggest that in MDCK cells, endocytic organelles contain a BFA-sensitive coat that regulates their organization and function even though the Golgi coat is BFA resistant.

Basolateral sorting in MDCK cells requires a distinct cytoplasmic domain determinant

In MDCK cells, Golgi to basolateral transport of several membrane proteins has been found to involve a cytoplasmic domain determinant. In some cases (Fc receptor, lysosomal glycoprotein Igp120), the determinant appears similar to that required for endocytosis via clathrin-coated pits; for Igp120, elimination of a single cytoplasmic domain tyrosine both blocks internalization and results in apical transport. In other cases (LDL receptor), the determinant does not involve the cytoplasmic domain tyrosine required for endocytosis. Thus, contrary to current models, basolateral transport in MCDK cells occurs not by default but depends on one or more cytoplasmic domain determinants, the precise nature of which is unknown. For some proteins, it is closely related to coated pit determinants. The fact that many membrane proteins can reach the apical surface in the absence of this determinant suggests that signals for apical transport are widely distributed.

Sorting of plasma membrane proteins in epithelial cells

Proteins follow two routes to reach the correct surface (apical or basolateral) of a polarized epithelial cell: direct sorting from the trans-Golgi network and transcytosis from early endosomes. Several signals have been identified recently that control these sorting events, namely a glycosyl-phosphatidylinositol anchor for apical targeting, a 14-residue cytoplasmic segment of the polymeric immunoglobulin receptor for basolateral targeting, and phosphorylation of a Ser residue for transcytosis of this receptor. The machinery involved is still poorly understood.

Tumor cell autocrine motility factor receptor

The ability to locomote and migrate is fundamental to the acquisition of invasive and metastatic properties by tumor cells. Autocrine motility factor (AMF) is a cytokine produced by various tumor cells which stimulates their in vitro motility and in vivo lung-colonizing ability. AMF stimulates cell motility via a receptor-mediated signalling pathway. Signal transduction following binding of AMF to its receptor, a cell surface glycoprotein of 78 kD (gp78), is mediated by a pertussis toxin sensitive G protein, inositol phosphate production and the phosphorylation of gp78. AMF induces gp78 internalization to intracellular tubulovesicles and transport to the leading edge stimulating pseudopodial protrusion and cell motility.