Rhodopsin ist der Photorezeptor der Stäbchenzellen in der Retina von Vertebraten und wird als Prototyp für die gesamte Gruppe der GPCRs beforscht. Trifft ein Photon auf das Protein, so wird der über eine Schiffbase kovalent gebundene Chromophor von seiner 11-cis- in die All-trans-Konfiguration isomerisiert und setzt infolgedessen den Aktivierungsprozess in Gang. Dieser mündet in der aktiven Rezeptorkonformation, die das G-Protein Transducin aktivieren kann und dadurch eine Kaskade weiterer Aktivierungsschritten einleitet, die letztlich ein Nervensignal verursachen. Das Ziel dieser Arbeit war die Aufklärung der späten Aktivierungsschritte und ihrer Ursache-Wirkungs-Beziehungen. Zu diesem Zweck wurden Blitzlichtphotolyse, Elektronenspinresonanz (EPR) mit Spinlabeling (SDSL), UV/vis-Spektroskopie, FTIR-Spektroskopie und Fluoreszenzspektroskopie angewandt. Kinetische Messungen wurden unter identischen Bedingungen durchgeführt, um die Abfolge der mit den unterschiedlichen Techniken zugänglichen Aktivierungsschritte aufzuklären. Nach der Bildung des absorptionsspektroskopisch definierten Meta-II-Zustands bewegt sich die Helix TM6 in einem späteren Schritt als ganzes nach außen und markiert damit den Übergang von Meta-IIa zu Meta-IIb. Dadurch wird die bis dahin in der Membran verborgene D(E)RY-Region für das Umgebungsmedium zugänglich und nimmt ohne Zeitverzögerung ein Proton auf, wodurch der Meta-IIb*H+-Zustand gebildet wird. Die verfügbaren Daten sprechen dafür, dass das D(E)RY-Motiv bei der Aktivierung des Transducins sowohl die Alpha- als auch die Gamma-Untereinheit desselben bindet. Die Bindung von zu Transducin-Abschnitten analogen Peptiden kann dann erfolgen, wenn die Helix TM6 im nach außen bewegten Zustand ist, und führt zur Abgabe von bis zu zwei Protonen vom aktivierten Rhodopsin. Sowohl das D(E)RY- und das NPxxY(x)5,6F-Motiv als auch die beiden Zustände Meta-IIb und Meta-IIb*H+ könnten relevant für den sequenziellen Transducin-Aktivierungsmechanismus sein. / Rhodopsin is the photoreceptor in the rod cells of the vertebrate retina. It is considered as a prototype of the whole group of GPCRs. Upon absorption of a photon the chromophore, which is covalently bound through a Schiff base, is isomerized from its 11-cis into the all-trans configuration. This initiates the activation process and finally results in the active receptor conformation which is capable of activating the G protein transducin and thereby triggers a cascade of further activation steps which finally cause a nerve signal. The aim of this work was the clarification of the late activation steps and their cause-and-effect chain. For this purpose flash photolysis, electron paramagnetic resonance (EPR) with spin labeling (SDSL), UV/vis spectroscopy, FTIR spectroscopy and fluorescence spectroscopy were applied. Kinetic measurements were executed under identical conditions in order to elucidate the sequence of activation steps, which are accessible with the different techniques. After formation of the spectroscopically defined Meta-II state helix TM6 moves outward as a rigid body, thereby marking the transition from Meta-IIa to Meta-IIb. Therefore the D(E)RY region, which is until then buried in the membrane, gets accessible to the surrounding solution. It consequently takes up a proton without delay, thus forming the Meta-IIb*H+ state. Available data argue for the D(E)RY motif binding both the Alpha and the Gamma subunit of transducin during activation of the latter. The binding of peptides which are analogous to sections of transducin is possible when helix TM6 is in the outward position. It causes the release of up to two protons from the activated rhodopsin. Both the D(E)RY motif and the NPxxY(x)5,6F motif as well as both the states Meta-IIb and Meta-IIb*H+ are potentially relevant for the sequential transducin activation mechanism.

Photoreceptors are of prime importance for humans, since vision is one of the most important senses for us. In our daily life, where nearly every action is dependent on visual input, an impairment or a loss of eyesight leads to severe disability. With a non-syndromic prevalence of 1:4000, retinitis pigmentosa, a collective term for a group of inherited retinal eye diseases, represents, together with age-related macula degeneration, one of the main causes for visual impairment and blindness in industrialized countries. The dominant reason for vision loss is, in both cases, the irreversible loss of photoreceptor cells located in the outer nuclear layer of the retina. To date, no effective treatment is available to preserve or regain visual function in affected patients. Recent promising strategies for new retinal therapeutical approaches focus on one hand on the development of gene therapies, where an introduced wild-type allele compensates a mutated gene, and on the other hand on cell therapies, where stem or photoreceptor precursor cells (PPCs) are transplanted to the sub-retinal space to replace degenerated host photoreceptors. The current study is subdivided into three parts, addressing the issue of non-reversible photoreceptor cell loss due to retinal degenerative diseases by investigating in the first two parts new qualitative as well as quantitative approaches in the field of retinal cell therapy, while in the third part an ocular gene therapeutical approach targeting prominin-1, a gene involved in retinal degenerative disorders, was investigated. Briefly, this study shows in the first part, a significant enhancement of the integration rate of PPCs in wild-type host retinas, achieved by pre-transplantational sorting, using the recently discovered PPC - specific cell surface marker CD73. This sets another step further towards retinal cell therapy by increasing the effectiveness of such treatment. Next to this quantitative approach, it is also shown that the quality of transplanted photoreceptor precursor cells is comparable to native photoreceptors by demonstrating, that an indispensable prerequisite of every photoreceptor cell, the outer segment, is developed by transplanted PPCs after proper integration. Importantly, transplanted PPCs develop native outer segments even when not integrated in the host tissue but located in the sub-retinal space, as it is predominantly observed after transplantation into severely degenerated retinas. These results substantiate the feasibility of cell therapeutical treatment of severely degenerated retinas. At the end of this part, it is demonstrated, that outer segments are not formed properly by PPCs transplanted to the vitreal side of the retina. This suggests an influence of signaling molecules, presumably secreted by retinal pigment epithelial cells into the sub-retinal space, on transplanted PPC final differentiation. Since intensive research is done to differentiate stem cells into PPCs for cell therapeutical transplantation, these results may contribute significantly to this research by demonstrating, that factors secreted by the retinal pigment epithelium might play a crucial role for successful stem cell to PPC differentiation. The last part of my work investigates a gene therapeutical approach to cure inherited retinal degenerative diseases. One gene, where reported mutations cause retinal degeneration in humans is prominin-1, a protein expressed at cell membrane evaginations in a variety of cell types. Interestingly, the prominin-1 knock-out mouse is characterized exclusively by disorganized photoreceptor outer segment formation and progressive retinal degeneration. Successful delivery of a wild-type form of mouse prominin-1 using adeno-associated viral vector transfer, into the photoreceptors of prominin-1 - deficient mice is demonstrated. The divergent results show on one hand a rescue of the thickness of the photoreceptor outer nuclear layer on a short time period (3 weeks post treatment), and on the other hand long-term data (8-10 weeks post treatment) suggests histologically as well as functionally a negative effect on treated photoreceptors. This might be due to effects caused by an over-expression of prominin-1 and will be investigated in future studies. In conclusion, distinct and important investigations were made which contribute significant puzzle pieces to new cell- as well as gene therapeutical approaches for the treatment of retinal degenerative disorders.

Photoreceptors are of prime importance for humans, since vision is one of the most important senses for us. In our daily life, where nearly every action is dependent on visual input, an impairment or a loss of eyesight leads to severe disability. With a non-syndromic prevalence of 1:4000, retinitis pigmentosa, a collective term for a group of inherited retinal eye diseases, represents, together with age-related macula degeneration, one of the main causes for visual impairment and blindness in industrialized countries. The dominant reason for vision loss is, in both cases, the irreversible loss of photoreceptor cells located in the outer nuclear layer of the retina. To date, no effective treatment is available to preserve or regain visual function in affected patients. Recent promising strategies for new retinal therapeutical approaches focus on one hand on the development of gene therapies, where an introduced wild-type allele compensates a mutated gene, and on the other hand on cell therapies, where stem or photoreceptor precursor cells (PPCs) are transplanted to the sub-retinal space to replace degenerated host photoreceptors. The current study is subdivided into three parts, addressing the issue of non-reversible photoreceptor cell loss due to retinal degenerative diseases by investigating in the first two parts new qualitative as well as quantitative approaches in the field of retinal cell therapy, while in the third part an ocular gene therapeutical approach targeting prominin-1, a gene involved in retinal degenerative disorders, was investigated. Briefly, this study shows in the first part, a significant enhancement of the integration rate of PPCs in wild-type host retinas, achieved by pre-transplantational sorting, using the recently discovered PPC - specific cell surface marker CD73. This sets another step further towards retinal cell therapy by increasing the effectiveness of such treatment. Next to this quantitative approach, it is also shown that the quality of transplanted photoreceptor precursor cells is comparable to native photoreceptors by demonstrating, that an indispensable prerequisite of every photoreceptor cell, the outer segment, is developed by transplanted PPCs after proper integration. Importantly, transplanted PPCs develop native outer segments even when not integrated in the host tissue but located in the sub-retinal space, as it is predominantly observed after transplantation into severely degenerated retinas. These results substantiate the feasibility of cell therapeutical treatment of severely degenerated retinas. At the end of this part, it is demonstrated, that outer segments are not formed properly by PPCs transplanted to the vitreal side of the retina. This suggests an influence of signaling molecules, presumably secreted by retinal pigment epithelial cells into the sub-retinal space, on transplanted PPC final differentiation. Since intensive research is done to differentiate stem cells into PPCs for cell therapeutical transplantation, these results may contribute significantly to this research by demonstrating, that factors secreted by the retinal pigment epithelium might play a crucial role for successful stem cell to PPC differentiation. The last part of my work investigates a gene therapeutical approach to cure inherited retinal degenerative diseases. One gene, where reported mutations cause retinal degeneration in humans is prominin-1, a protein expressed at cell membrane evaginations in a variety of cell types. Interestingly, the prominin-1 knock-out mouse is characterized exclusively by disorganized photoreceptor outer segment formation and progressive retinal degeneration. Successful delivery of a wild-type form of mouse prominin-1 using adeno-associated viral vector transfer, into the photoreceptors of prominin-1 - deficient mice is demonstrated. The divergent results show on one hand a rescue of the thickness of the photoreceptor outer nuclear layer on a short time period (3 weeks post treatment), and on the other hand long-term data (8-10 weeks post treatment) suggests histologically as well as functionally a negative effect on treated photoreceptors. This might be due to effects caused by an over-expression of prominin-1 and will be investigated in future studies. In conclusion, distinct and important investigations were made which contribute significant puzzle pieces to new cell- as well as gene therapeutical approaches for the treatment of retinal degenerative disorders.

Histidinkinase-Rhodopsine (HKRs) können als besondere Gruppe der Hybrid-Histidinkinasen beschrieben werden, deren N-terminale sensorische Domäne ein mikrobielles Rhodopsin ist. HKR-codierende Sequenzen konnten in den Genomen verschiedener Algen, Pilze und Amoeben gefunden werden doch ihre Aufgaben und Wirkungsweisen sind bisher ungeklärt. Im Rahmen dieser Arbeit wurden die rekombinanten Rhodopsin-Domänen von zwei HKRs mit verschiedenen spektroskopischen Techniken charakterisiert. Sie zeigten mehrere Besonderheiten. Das Rhodopsin-Fragment von Cr-HKR1 aus Chlamydomonas reinhardtii kann durch alternierende kurzwellige und langwellige Belichtung zwischen zwei stabilen Absorptionsformen konvertiert werden: einer Blaulicht-absorbierenden (Rh-Bl) und einer UVA-Licht-absorbierenden Form (Rh-UV). Dies resultiert aus der ungewöhnlichen thermischen Stabilität des Zustandes mit deprotonierter Schiff’scher Base. Das zweite charakterisierte HKR, die Os-HKR-Rhodopsin-Domäne aus der marinen Picoalge Ostreococcus tauri, zeigt eine Dunkelabsorption von 505 nm. Auch Os-HKR ist photochrom und die deprotonierte Spezies kann effizient akkumuliert werden. Diese P400-Absorptionsform ist jedoch nicht völlig stabil sondern es kommt nach Belichtungsende zur langsamen Dunkelzustands-Regeneration. Überraschenderweise konnte die Bindung sowie die transiente Abgabe eines Anions während des Os-HKR-Photozyklus festgestellt werden. Somit beeinflusst nicht nur das Licht, sondern auch das Salz in der Umgebung die Os-HKR-Reaktionen. Aufgrund ihrer photochromen Eigenschaften werden die HKRs als wirksame lichtinduzierte Schalter für die C-terminalen Signaltransduktionsdomänen postuliert. Schwingungsspektroskopische Analysen deckten eine Heterogenität hinsichtlich der im Protein gebundenen Retinal‐Konfiguration sowie die Existenz von zwei parallelen Photozyklen auf. Jeder dieser Photozyklen geht aus einer der beiden Retinal-Isomere hervor. / Histidine kinase rhodopsins (HKRs) can be described as hybrid histidine kinases with a microbial rhodopsin as N-terminal sensory domain. HKR-encoding sequences were found in the genomes of various unicellular organisms such as algae, fungi and amoeba but their mechanistic and physiologic function is unknown. During this work the absorptive properties of the recombinant rhodopsin domains of two HKRs were studied by the usage of different spectroscopic techniques. Both HKRs showed unusual characteristics. The rhodopsin fragment of Cr‐HKR1 from Chlamydomonas reinhardtii can be interconverted between two stable absorbance forms by the alternate application of short‐ and long‐wavelength light: a blue light-absorbing dark form (Rh-Bl) and a UVA light-absorbing form (Rh-UV). This unusual photocycle results from the uncommon thermal stability of the absorbance state with a deprotonated retinal Schiff base. The second studied HKR, the Os‐HKR rhodopsin domain from the marine picoalga Ostreococcus tauri, shows an absorbance maximum at 505 nm in darkness. Likewise Cr‐HKR1 the Os‐HKR is photochromic and the deprotonated form P400 can be efficiently accumulated. But the Os-HKR P400-form is not completely stable. A slow dark state recovery occurs. Surprisingly the dark state absorbance of Os‐HKR was found to be dependent on anion binding in the protein. Furthermore during the photocycle the transient anion release occurs and therefore not only light but also salt impacts the Os-HKR-reactions. Due to their pronounced photochromic properties, the HKRs are postulated to act as effective molecular switches for the C-terminal signal transduction domains in response to the light conditions. Vibrational spectroscopy revealed the heterogeneity with regard to the retinal configuration bound in the HKRs suggesting the existence of two parallel photocycles. Either of these photocycles originates from one of the two retinal isoforms.