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Charakterisierung der lichtinduzierten Internalisierung des Ionenkanals TRPL aus Drosophila melanogaster
(2012) Oberegelsbacher, Claudia; Huber, Armin
The light-dependent isomerization of rhodopsin (Rh1), which takes place in the compound eyes of Drosophila, leads to the activation of the visual signaling cascade. The result is a depolarizing receptor potential caused by the Ca2+-influx through the two cation channels TRP and TRPL. This Ca2+ influx subsequently mediates a change in the subcellular localization of the TRPL channel by inducing its translocation. TRPL of dark-adapted flies is located inside the rhabdomeres, whereas upon illumination, TRPL translocates to a yet unidentified storage compartment in the cell body of the photoreceptor cell. The translocation is reversible; however, the underlying mechanism remains largely unclear. Based on the observation of TRPL-containing vesicles on immunocytochemical sections of illuminated flies a vesicular transport mechanism has been proposed for TRPL translocation. In the present work, the mechanism underlying light-dependent TRPL internalization was studied. Using immunocytochemical techniques, a co-localization of rhodopsin and TRPL was observed in endocytic vesicles. Like many other G-protein coupled receptors, Rhodopsin undergoes endocytosis following activation. The rate of Rh1 internalization depends on the amount of metarhodopsin and, therefore, on the light quality used for illumination. The internalization rate was determined by counting Rh1- and TRPL-positive vesicles observed upon illumination with different light qualities. Surprisingly, the light quality that induced the highest number of Rh1-positive vesicles (i.e. blue light) caused the lowest number of TRPL-positive vesicles, while illumination with orange light induced strong TRPL internalization, but poor Rh1 endocytosis. Likewise, time courses of TRPL internalization were significantly faster in orange light compared to blue light. These findings may indicate a competition between TRPL and Rh1 for a common internalization factor. Analysis of endocytosis in different mutants showed that the internalization of TRPL required Ca2+ influx mediated by the activation of the phototransduction cascade, whereas internalization of Rhodopsin was Ca2+-independent. Therefore, the trigger for activating TRPL and Rh1 endocytosis seems to be different, although both types of internalization were mechanistically similar and depended on dynamin function. The internalization of Rhodopsin is mediated by Rab5. A screen of dominant negative Rab mutants revealed that the light-induced internalization of TRPL is mediated by Rab5 and RabX4. Accordingly, the involvement of Rab5 constitutes another common feature in the endocytosis of TRPL and Rh1. Arrestins play an important role in regulating the endocytosis of rhodopsin. Whereas arrestin2 mediates the inactivation of metarhodopsin, arrestin1 is responsible for subsequent rhodopsin endocytosis. The endocytosis of TRPL is independent of arrestins, but arrestin2 fulfills an important function regarding the stability of the TRPL protein in the rhabdomere. In the present work, the analysis of different arr2 alleles revealed a complete degradation of the TRPL protein after ten days in darkness, but not in light. This finding suggests that arrestin2 has a possible function as a scaffolding protein in the rhabdomer of dark-adapted flies, but not of light-adapted flies, when TRPL is located in a storage compartment in the cell body. There is another fundamental difference between the two transport mechanisms regarding the fate of the protein after it has been internalized. Rhodopsin undergoes rapid lysosomal degradation whereas the trafficking of TRPL is described as a recycling mechanism. In this work, it was possible to show colocalization of TRPL with recycling endosomes indicating an involvement of these compartments in TRPL trafficking. Furthermore, rhodopsin but not TRPL showed colocalization with a lysosomal marker in light-adapted flies, providing additional evidence for the recycling of the TRPL channel.