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Potassium channel-based optogenetic tool development - Establishment and optimization of compartment-specific light-inducible silencing tools

Engel, Anja Jeannine (2020)
Potassium channel-based optogenetic tool development - Establishment and optimization of compartment-specific light-inducible silencing tools.
Technische Universität Darmstadt
doi: 10.25534/tuprints-00011782
Dissertation, Erstveröffentlichung

Kurzbeschreibung (Abstract)

Optogenetics offers unique possibilities to control cells with a high spatio-temporal precision using light-sensitive proteins. There are two groups of light-inducible proteins: the first comprises retinal-containing proteins, which are light-inducible ion pumps or channels. The second group includes modular photoactuators, in which light induces conformational changes which in turn cause interaction with an effector domain. Optogenetics is most widely used to study excitable cells and has the potential to serve as a treatment for a lot of diseases like neurological disorders or blindness due to degradation of photoreceptors. In this work, potassium channel-based optogenetic tools were developed, optimized and functionally characterized. The goal was to target the respective GFP-tagged channels to two distinct locations within the cell: (i) the plasma membrane, where potassium channels can serve as optogenetic silencing tools by inducing hyperpolarization, (ii) the inner mitochondrial membrane, where light-inducible depolarization by potassium channel activity offers the chance to study the so far unknown function of potassium channels in mitochondria. To achieve targeting to the plasma membrane, two candidate channels were examined: KcvPBCV-1 and KcvNTS. Kesv and Kmpv12T on the other hand served as candidate channels to study the effects of potassium channel expression in the inner mitochondrial membrane.

To allow a most effective sorting of these four candidate channels, two different approaches were pursued to optimize the sorting efficiency of these channels in cultured mammalian cells. One effective way of augmenting sorting of channels to the mitochondria was achieved by codon optimization. For this purpose, rare codons in the channel coding gene were replaced by frequently used codons. This procedure resulted, in particular in Kesv, in an enhanced sorting to the mitochondria. Another effect which markedly increased sorting efficiency to the mitochondria was to shorten the protein linker between the channel protein and its GFP-tag. The combination of these two steps, a codon optimized channel combined with a short protein linker, yielded a very efficient and exclusive sorting of Kesv to the mitochondria. This positive effect of codon optimization on mitochondrial targeting was reproducible in six different cell lines, suggesting that this is a general mechanism in mammalian cells. KcvPBCV-1, which is by default sorted to the secretary pathway and further to the plasma membrane, was insensitive to codon optimization. In contrast to the mitochondrial potassium channels, the two channels sorted to the plasma membrane favor low expression levels to achieve the most effective sorting. To design an optogenetic tool for short-term depolarization of the mitochondria, the existing light-gated potassium channel BLINK1 was reengineered for mitochondrial sorting. It occurred that the addition of six canonical mitochondrial targeting sequences to the N-terminus of the protein resulted in an effective sorting of this protein to the mitochondria. Its functionality in the mitochondria remains to be tested. In a different strategy, optogenetic tools for long-term activation of potassium channels in the plasma membrane or the mitochondria were developed. In this case, two different light-inducible gene expression systems were employed to regulate expression of suitable potassium channels by light. The first approach was based on dimerization of the bacterial light-oxygen-voltage domain from Erythrobacter litoralis (EL222). The second system was based on the light-regulated interaction of cryptochrome 2 (CRY2) with its interacting partner CIB1. In these experiments the expression of a GFP-tagged potassium channel was placed under control of the EL222 system. Based on the intensity of the GFP signal in transfected cells, it became clear that the channel of interest was already significantly expressed in the dark and required a very high light intensity (120 µE for 16 hours) for elevating expression above the dark level. Because of this combination of unspecific leak expression in the dark and high light requirement, the EL222 system was not further pursued. In an alternative system, potassium channel expression was controlled by the blue light receptor system CRY2/CIB1 from Arabidopsis thaliana. This light-sensitive transcription system proved to be effective for the purpose of light-inducible potassium channel expression. It required very low light intensities (6 µE for 1 hour) or short exposure times for a robust expression of the candidate channel. A 5 second pulse of 120 µE was already sufficient for a maximal stimulation of channel expression. Leak expression of the channel in the dark was barely above detection limit. It was also possible to simplify the handling of the system by placing all three necessary components (CRY2, CIB1, potassium channel) in one expression vector without affecting its efficacy. We found that one hour of pulsed light (6 µE) was sufficient to achieve full activation one hour after light-stimulation. The CRY2 CIB1 system was examined in different functional assays. The properties of channels which were expressed at the plasma membrane in a light-dependent manner, were examined by whole cell patch clamp measurements. The data show that the channels, which were expressed under control of the CRY2 CIB1 system reached the plasma membrane and there exhibited the same functional properties as the constitutively expressed control channels. The functionality of channels expressed in the inner mitochondrial membrane, were examined in planar lipid bilayer experiments as well as by a set of microscopy assays. By using fluorescent reporters of the mitochondrial membrane voltage and mitochondrial calcium, it occurs that expression of mitochondrially targeted potassium channels under control of the CRY2 CIB1 system caused an effective depolarization of theses organelles. Also, they did significantly lower the mitochondrial calcium levels, but did not induce apoptosis. Currently, the CRY2 CIB1 constructs undergo functional tests in vivo to examine whether they influence the development of zebrafish larvae if expressed in different organ systems.

Typ des Eintrags: Dissertation
Erschienen: 2020
Autor(en): Engel, Anja Jeannine
Art des Eintrags: Erstveröffentlichung
Titel: Potassium channel-based optogenetic tool development - Establishment and optimization of compartment-specific light-inducible silencing tools
Sprache: Englisch
Referenten: Thiel, Prof. Dr. Gerhard ; Bertl, Prof. Dr. Adam
Publikationsjahr: 10 Juli 2020
Ort: Darmstadt
Datum der mündlichen Prüfung: 10 Juli 2020
DOI: 10.25534/tuprints-00011782
URL / URN: https://tuprints.ulb.tu-darmstadt.de/11782
Kurzbeschreibung (Abstract):

Optogenetics offers unique possibilities to control cells with a high spatio-temporal precision using light-sensitive proteins. There are two groups of light-inducible proteins: the first comprises retinal-containing proteins, which are light-inducible ion pumps or channels. The second group includes modular photoactuators, in which light induces conformational changes which in turn cause interaction with an effector domain. Optogenetics is most widely used to study excitable cells and has the potential to serve as a treatment for a lot of diseases like neurological disorders or blindness due to degradation of photoreceptors. In this work, potassium channel-based optogenetic tools were developed, optimized and functionally characterized. The goal was to target the respective GFP-tagged channels to two distinct locations within the cell: (i) the plasma membrane, where potassium channels can serve as optogenetic silencing tools by inducing hyperpolarization, (ii) the inner mitochondrial membrane, where light-inducible depolarization by potassium channel activity offers the chance to study the so far unknown function of potassium channels in mitochondria. To achieve targeting to the plasma membrane, two candidate channels were examined: KcvPBCV-1 and KcvNTS. Kesv and Kmpv12T on the other hand served as candidate channels to study the effects of potassium channel expression in the inner mitochondrial membrane.

To allow a most effective sorting of these four candidate channels, two different approaches were pursued to optimize the sorting efficiency of these channels in cultured mammalian cells. One effective way of augmenting sorting of channels to the mitochondria was achieved by codon optimization. For this purpose, rare codons in the channel coding gene were replaced by frequently used codons. This procedure resulted, in particular in Kesv, in an enhanced sorting to the mitochondria. Another effect which markedly increased sorting efficiency to the mitochondria was to shorten the protein linker between the channel protein and its GFP-tag. The combination of these two steps, a codon optimized channel combined with a short protein linker, yielded a very efficient and exclusive sorting of Kesv to the mitochondria. This positive effect of codon optimization on mitochondrial targeting was reproducible in six different cell lines, suggesting that this is a general mechanism in mammalian cells. KcvPBCV-1, which is by default sorted to the secretary pathway and further to the plasma membrane, was insensitive to codon optimization. In contrast to the mitochondrial potassium channels, the two channels sorted to the plasma membrane favor low expression levels to achieve the most effective sorting. To design an optogenetic tool for short-term depolarization of the mitochondria, the existing light-gated potassium channel BLINK1 was reengineered for mitochondrial sorting. It occurred that the addition of six canonical mitochondrial targeting sequences to the N-terminus of the protein resulted in an effective sorting of this protein to the mitochondria. Its functionality in the mitochondria remains to be tested. In a different strategy, optogenetic tools for long-term activation of potassium channels in the plasma membrane or the mitochondria were developed. In this case, two different light-inducible gene expression systems were employed to regulate expression of suitable potassium channels by light. The first approach was based on dimerization of the bacterial light-oxygen-voltage domain from Erythrobacter litoralis (EL222). The second system was based on the light-regulated interaction of cryptochrome 2 (CRY2) with its interacting partner CIB1. In these experiments the expression of a GFP-tagged potassium channel was placed under control of the EL222 system. Based on the intensity of the GFP signal in transfected cells, it became clear that the channel of interest was already significantly expressed in the dark and required a very high light intensity (120 µE for 16 hours) for elevating expression above the dark level. Because of this combination of unspecific leak expression in the dark and high light requirement, the EL222 system was not further pursued. In an alternative system, potassium channel expression was controlled by the blue light receptor system CRY2/CIB1 from Arabidopsis thaliana. This light-sensitive transcription system proved to be effective for the purpose of light-inducible potassium channel expression. It required very low light intensities (6 µE for 1 hour) or short exposure times for a robust expression of the candidate channel. A 5 second pulse of 120 µE was already sufficient for a maximal stimulation of channel expression. Leak expression of the channel in the dark was barely above detection limit. It was also possible to simplify the handling of the system by placing all three necessary components (CRY2, CIB1, potassium channel) in one expression vector without affecting its efficacy. We found that one hour of pulsed light (6 µE) was sufficient to achieve full activation one hour after light-stimulation. The CRY2 CIB1 system was examined in different functional assays. The properties of channels which were expressed at the plasma membrane in a light-dependent manner, were examined by whole cell patch clamp measurements. The data show that the channels, which were expressed under control of the CRY2 CIB1 system reached the plasma membrane and there exhibited the same functional properties as the constitutively expressed control channels. The functionality of channels expressed in the inner mitochondrial membrane, were examined in planar lipid bilayer experiments as well as by a set of microscopy assays. By using fluorescent reporters of the mitochondrial membrane voltage and mitochondrial calcium, it occurs that expression of mitochondrially targeted potassium channels under control of the CRY2 CIB1 system caused an effective depolarization of theses organelles. Also, they did significantly lower the mitochondrial calcium levels, but did not induce apoptosis. Currently, the CRY2 CIB1 constructs undergo functional tests in vivo to examine whether they influence the development of zebrafish larvae if expressed in different organ systems.

Alternatives oder übersetztes Abstract:
Alternatives AbstractSprache

Optogenetik bietet einzigartige Möglichkeiten, zelluläre Funktionen durch die Verwendung von lichtsensitiven Proteinen mit hoher zeitlicher und räumlicher Präzision zu kontrollieren. Diese lichtsensitiven Proteine können in zwei Gruppen eingeteilt werden: Die erste Gruppe umfasst Proteine welche Retinal enthalten, die als lichtinduzierbare Pumpen oder Kanäle fungieren. Die zweite Gruppe umfasst modulare Photoaktivatoren in welchen durch Licht Konformationsänderungen verursacht werden, welche wiederum die Interaktion mit einer Effektor-Domäne verursachen. Hauptsächlich findet Optogenetik Verwendung in der Untersuchung von erregbaren Zellen und bietet großes Potential für neuartige Behandlungsmöglichkeiten für viele Krankheiten, wie neurologische Störungen oder durch Abbau von Photorezeptoren verursachte Blindheit. In der vorliegenden Arbeit wurden verschiedene optogenetische Werkzeuge, basierend auf Kaliumkanälen, entwickelt, optimiert und auf ihre Funktionalität untersucht. Das Ziel war die Expression GFP-markierter Kaliumkanäle in der Plasmamembran oder den Mitochondrien von Säugerzellen. In der Plasmamembran können Kaliumkanäle als optogenetische Silencer fungieren, da sie die Zellen hyperpolarisieren und damit das Feuern von Aktionspotentialen verhindern. Um diese Effekte zu untersuchen, wurden zwei Kaliumkanäle verwendet: KcvPBCV-1 und KcvNTS. Bei einer lichtinduzierten Expression von Kaliumkanälen in den Mitochondrien ermöglicht die induzierbare Depolarisation die Untersuchung der Wirkung von Kaliumkanälen in den Mitochondrien. Für die Untersuchung dieser Fragestellung wurden die Kaliumkanäle Kesv und Kmpv12T verwendet.

Um die Sortierung der vier verwendeten Kaliumkanäle zu optimieren, wurden zwei verschiedene Wege zur Maximierung der Sortierungseffizienz in Säugerzellen angewendet. Die Optimierung der verwendeten Codone, durch Austausch von Seltenen gegen häufig verwendete Codone, führte zu einer verbesserten Sortierung der Kanäle in die Mitochondrien. Vor allem bei Kesv führte dieses Vorgehen zu einer effektiveren Sortierung in die Mitochondrien. Auch eine Verkürzung des Proteinlinkers zwischen Kanal und GFP führte zu einer vermehrten Sortierung von Kesv in die Mitochondrien. Die Kombination dieser beiden Herangehensweisen – ein codonoptimierter Kanal in Verbindung mit einem verkürzten Linker – führte zu einer hundertprozentigen Sortierung von Kesv in die Mitochondrien. Der positive Effekt dieser beiden Herangehensweisen konnte in sechs verschiedenen Säugerzelllinien nachgewiesen werden und legt damit einen generellen Sortierungsmechanismus in Säugerzellen nahe. Für den an die Plasmamembran sortierten Kanal KcvPBCV-1 konnte kein Effekt von Codonoptimierung auf die Sortierungseffizienz nachgewiesen werden. Im Unterschied zu den mitochondrialen Kanälen, ist für eine optimale Sortierung der an der Plasmamembran exprimierten Kanäle ein niedriges Expressionslevel von Vorteil.

Für das Design eines optogenetischen Tools zur kurzzeitigen Depolarisation der Mitochondrien wurden verschiedene Änderungen am bestehenden lichtschaltbaren Kaliumkanal BLINK1 vorgenommen. Durch das Hinzufügen von sechs kanonischen mitochondrialen Sortierungssignalen am N-Terminus konnte eine effektive Sortierung des Kanals an die Mitochondrien erreicht werden. Die funktionellen Eingenschaften dieses Konstrukts müssen noch überprüft werden. In einer alternativen Strategie wurden optogenetische Werkzeuge zur Langzeitaktivierung von Kaliumkanälen in der Plasmamembran oder den Mitochondrien entwickelt. In diesem Fall wurden zwei verschiedene lichtinduzierbare Genexpressionssysteme eingesetzt, um die Expression geeigneter Kaliumkanäle durch Licht zu regulieren. Der erste Ansatz basierte auf der Dimerisierung der bakteriellen Light-Oxygen-Voltage-Domäne aus Erythrobacter litoralis (EL222). Das zweite System basierte auf der lichtgesteuerten Interaktion von Cryptochrome 2 (CRY2) mit seinem Interaktionspartner CIB1. In diesen Experimenten wurde die Expression eines GFP-markierten Kaliumkanals unter Kontrolle des EL222-Systems untersucht. Anhand der Intensität des GFP-Signals in den transfizierten Zellen wurde deutlich, dass der Kanal bereits im Dunkeln signifikant exprimiert wurde und eine sehr hohe Lichtintensität (120 µE für 16 Stunden) benötigte, um die Expression über das Dunkelniveau zu heben. Aufgrund dieser Kombination aus unspezifischer Leck-Expression im Dunkeln und hohem Lichtbedarf wurde das EL222-System nicht weiter untersucht. In einem alternativen System wurde die Kaliumkanalexpression durch das Blaulicht-induzierte Rezeptorsystem CRY2/CIB1 aus Arabidopsis thaliana kontrolliert. Dieses lichtempfindliche Transkriptionssystem erwies sich als wirksam für die lichtinduzierbare Kaliumkanalexpression. Es benötigte sehr niedrige Lichtintensitäten (6 µE für 1 Stunde) oder kurze Belichtungszeiten mit höherer Lichtintensität für eine robuste Expression des Kandidatenkanals. Ein 5-Sekunden-Impuls von 120 µE war bereits ausreichend für eine maximale Stimulation der Kanalexpression. Die unspezifische Expression des Kanals im Dunkeln lag kaum über dem Detektionsminimum. Es war auch möglich, die Handhabung des Systems zu vereinfachen, indem alle drei notwendigen Komponenten (CRY2, CIB1, Kaliumkanal) in einen Expressionsvektor kloniert wurden, ohne die Wirksamkeit des Systems zu beeinträchtigen. Eine Stunde gepulstes Licht (6 µE) war ausreichend, um eine Stunde nach der Lichtstimulation die volle Aktivierung der Kanalexpression zu erreichen. Das CRY2 CIB1-System wurde in verschiedenen funktionellen Assays untersucht. Die Eigenschaften von Kanälen, die an der Plasmamembran lichtabhängig exprimiert wurden, wurden durch whole-cell Patch-Clamp-Messungen bestätigt. Die Daten zeigen, dass die Kanäle, die unter Kontrolle des CRY2 CIB1-Systems exprimiert wurden, die Plasmamembran erreichten und dort die gleichen funktionellen Eigenschaften wie die konstitutiv exprimierten Kontrollkanäle zeigten. Die Funktionalität der Kanäle, die in der inneren Mitochondrienmembran exprimiert werden, wurde sowohl in planaren lipid bilayer Experimenten, als auch durch eine Reihe von Mikroskopie-Assays untersucht. Durch die Verwendung von Fluoreszenzreportern zur Untersuchung der mitochondrialen Membranspannung und der mitochondrialen Kalziumkonzentration konnte gezeigt werden, dass die Expression von mitochondrialen Kaliumkanälen unter Kontrolle des CRY2 CIB1-Systems eine effektive Depolarisierung dieser Organellen bewirkt. Auch der mitochondrialen Kalziumspiegel zeigte eine Verringerung, eine Induktion von Apoptose wurde aber nicht beobachtet. Derzeit werden die CRY2 CIB1-Konstrukte in vivo funktionellen Tests unterzogen, um zu untersuchen, ob sie die Entwicklung von Zebrafischlarven beeinflussen, wenn sie in verschiedenen Organsystemen exprimiert werden.

Deutsch
URN: urn:nbn:de:tuda-tuprints-117820
Sachgruppe der Dewey Dezimalklassifikatin (DDC): 500 Naturwissenschaften und Mathematik > 570 Biowissenschaften, Biologie
Fachbereich(e)/-gebiet(e): 10 Fachbereich Biologie
10 Fachbereich Biologie > Plant Membrane Biophyscis (am 20.12.23 umbenannt in Biologie der Algen und Protozoen)
Hinterlegungsdatum: 18 Sep 2020 13:18
Letzte Änderung: 22 Sep 2020 05:39
PPN:
Referenten: Thiel, Prof. Dr. Gerhard ; Bertl, Prof. Dr. Adam
Datum der mündlichen Prüfung / Verteidigung / mdl. Prüfung: 10 Juli 2020
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