Untersuchung von Galectin-3 im apikalen Transport der Beta-Untereinheit der H,K-ATPase in MDCK-Zellen

Like many other cells at the boundary to environment, the gastric parietal cell is a polar epithelial cell. The membrane of these cells is subdivided into an apical and a basolateral part. Hence, single cells, organs and finally the whole apparatus are able to correctly perform necessary functions. Currently, two different sorting platforms are discussed that direct lipids and proteins to the apical membrane. On the one hand this sorting is mediated by lipid rafts. This mechanism concentrates proteins and lipids in microdomains of the membrane by forming clusters. These clusters are then transported to the apical cell surface. On the other hand there is a raft-independent route mediated by lectins. By cross linking the proteins and forming different clusters, Galectin-3 initializes the transport to the apical surface. The transport route guiding the gastric H,K-ATPase to the apical surface is still unknown. With respect to the transportation, the β-subunit seems to be of crucial relevance. Due to the strong glycosylation of the β-subunit it is the ideal target for lectins like galcetin-3. The scope of this work is to investigate the influence of galectin-3 on the transport of the H,K-ATPase’s β-subunit. Microscopic analysis suggests the coexistence of galectin-3 and the β-subunit in subapical structures. The number of co-localisations decreased with increasing cultivation time. Furthermore, the co-immunoprecipitations do not show an interaction between both proteins. Galectin-3 knockdown using siRNA does not significantly change polarized sorting of the β-subunit either. Only a slightly decreased fraction in the membrane and an increased intracellular fraction are detected. Hence, it is unlikely that Galectin-3 plays an important role in apical sorting of the β-subunit. Furthermore, microscopic analysis indicates the co-localisation of β-subunits with rab11. Rab11 is a marker of recycling endosomes which have noticeable similarities with the tubulovesicular system found in parietal cells. In the same way, a co-localisation of the β-subunit and the lipid raft marker caveolin-1 is detected. Subsequent raft-isolation confirms the microscopic observations. For the first time, the β-subunit is detected in raft-dependent transport routes. However, the direction of transportation and the existence of alternative transport routes call for further research. Summing up, the results presented in this work lead to the conclusion that a governing role of Galectin-3 in the sorting of the gastric H,K-ATPase is very unlikely. The experimental data rather suggest a raft-dependent transport of the β-subunit.