Browsing by Subject "NHERF family"

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Intrazelluläres Trafficking des intestinalen Anionenaustauschers Down-Regulated in Adenoma (DRA;SLC26A3)
(2011) Lissner, Simone; Graeve, Lutz
Electroneutral NaCl absorption occurs from the small intestine to the distal colon. This ion exchange is preferentially mediated by DRA and NHE3. Knockout mice, which suffer from chronic diarrhea, as well as the human genetic disorder congenital chloride diarrhea, in which a nonfunctional DRA leads to life-threatening diarrhea emphasize the importance of these two transporters. To elucidate this defective NaCl absorption it is necessary to understand the physiological regulation of these two transport proteins within enterocytes as well as the responsible extra- and intracellular signal transduction pathways. Both transport proteins interact with PDZ adaptor proteins of the NHERF family. Furthermore, both exchangers are partially localized within lipid rafts. The situation for NHE3 is complex in that its lipid raft localization is not only necessary for its normal activity but also for its basal and stimulated trafficking. Lipid rafts are involved in PI3-kinase dependent exocytosis of NHE3. Since the function of NHE3 and DRA appears to be regulated in parallel the function of DRA maybe depends on its rafts association as well. Thus the first objective of this thesis was to investigate whether the lipid raft association of DRA is essential for the surface expression and transport activity of DRA and also to analyze whether DRA is inserted into the plasma membrane in a PI3-kinase and lipid raft dependent manner. The present data show that: (A) Disruption of lipid raft integrity leads to functional inhibition and decreased cell surface expression of DRA. In HEK cells the inhibition of DRA activity as well as the decreased cell surface expression are entirely dependent on the presence of the PDZ interaction motif of DRA. In Caco-2/BBE cells on the other hand only part of the inhibition of DRA activity by disruption of raft integrity depends on the ability of DRA to interact with PDZ adaptor proteins. (B) Basal activity as well as basal surface expression of DRA depend on PI3-kinase activity in a way that requires the ability of DRA to interact with PDZ adaptor proteins. (C) Lipid rafts and PI3-kinase are situated along the same pathway, where DRA is present in lipid rafts before it is inserted into the plasma membrane. However, the inhibition of PI3-kinase has no influence on the raft association of DRA. Furthermore, the disruption of raft integrity does not inhibit the PI3-kinase activity. Based on these findings a model can be established as follows: DRA is present in lipid rafts in an intracellular fraction. Insertion into the plasma membrane from this intracellular compartment requires the interaction with one (or several) PDZ adaptor proteins, raft integrity and the action of PI3-kinase. To characterize the interplay between PI3-kinase, raft association and PDZ interaction of DRA with its insertion into the plasma membrane the recycling pathway of DRA was then investigated. The generated data show that the proteolytic degradation of DRA-ETKFminus occurs faster than the degradation of wild type DRA. Endosomal distribution of DRA depends on its PDZ-binding motif. The sorting process from early to recycling endosomes depends on the interaction of DRA with one or several PDZ adaptor proteins. Expression of dominant negative Rab11a leads to a decreased surface expression and transport activity of DRA. In conclusion, it was shown in this thesis that an intense interplay between PDZ interaction, lipid raft association, PI3-kinase and the activity and surface expression of DRA exists. It was also shown that the endosomal distribution of DRA depends on its PDZ-binding motif. Finally, it was demonstrated that DRA is recycled to the plasma membrane by Rab11a-enriched recycling endosomes.