Golgi apparatus fragmentation as a mechanism responsible for uniform delivery of uroplakins to the apical plasma membrane of uroepithelial cells

Abstract

The GA (Golgi apparatus) has an essential role in membrane trafficking, determining the assembly and delivery of UPs (uroplakins) to the APM (apical plasma membrane) of superficial UCs (uroepithelial cells) of urinary bladder. UPs are synchronously and uniformly delivered from the GA to the APM by DFVs (discoidal- or fusiform-shaped vesicles); however, the mechanism of UP delivery is not known. We have used the culture model of UCs with the capacity to undergo terminal differentiation to study the process of uniform delivery of DFVs to the APM and to elucidate the mechanisms involved. By three-dimensional localization using confocal microscopy of immunofluorescence-labelled GA-related markers [GM130 (cis-Golgi matrix protein of 130 kDa), GS15 (Golgi Snare 15 kDa), GS28 and giantin], uroepithelial differentiation-related markers (UPs), MTs (microtubules; α-tubulin) and intermediate filaments [CK7 (cytokeratin 7) and CK20], we found that in non-differentiated, UP-negative UCs the GA is mostly organized as a single ribbon-like structure close to the nucleus, whereas in differentiated, UP-positive UCs the GA is fragmented and spread almost through the entire cell. The FRAP (fluorescence recovery after photobleaching) experiments on the UCs transfected with GalT (trans-Golgi/TGN enzyme β1,4-galactosyltransferase) fused to fluorescent protein showed that Golgi-resident enzyme cycles freely within ribbon-like GA but not within fragmented GA. By CLEM (correlative light-electron microscopy), we examined the GA fragments in cells expressing UPs. We found that GA fragments are fully functional and similar to the GA fragments that are formed after nocodazole treatment. Furthermore, we demonstrated that the reorganization of GA into a fragmented form is associated with the impairment of the MT organization in the basal, central and subapical cytoplasm and the accumulation of intermediate filaments in the apical cytoplasm that could affect the kinetics of MT star leading to the peripheral fragmentation of the GA in the differentiated UCs. The fragmentation of the GA and the subsequent spreading of GA to the cell periphery represent one of the key events that promote the uniform delivery of UPs over the entire APM of differentiating UCs and thus are of major importance in the final proper formation and maintenance of the blood-urine barrier.