Optogenetics and photopharmacology enable the spatiotemporal control of cell and animal behavior by light. Red light offers deep tissue penetration and minimal phototoxicity, but very few red light-sensitive optogenetic methods are currently available. Here, we developed a red light-induced homodimerization domain. We first showed that an optimized sensory domain of the cyanobacterial phytochrome 1 (CPH1) can be expressed robustly and without cytotoxicity in human cells. We then applied this domain to induce dimerization of two receptor tyrosine kinases, the fibroblast growth factor receptor 1 and the neurotrophin receptor trkB. With this new optogenetic method in hand, we activated the MAPK/ERK pathway non-invasively in mammalian tissue and developed multi-color cell signaling experiments. The light-controlled dimerizer and the red light-activated receptor tyrosine kinases will prove useful to regulate a variety of cellular processes with light.

Nature has incorporated small photochromic molecules, colloquially termed 'photoswitches', in photoreceptor proteins to sense optical cues in phototaxis and vision. While Nature's ability to employ light-responsive functionalities has long been recognized, it was not until recently that scientists designed, synthesized and applied synthetic photochromes to manipulate biological processes with the temporal and spatial resolution of light. Ion channels in particular have come to the forefront of proteins that can be put under the designer control of synthetic photochromes.Photochromic ion channel controllers are comprised of three classes, photochromic soluble ligands (PCLs), photochromic tethered ligands (PTLs) and photochromic crosslinkers (PXs), and in each class ion channel functionality is controlled through reversible changes in photochrome structure. By acting as light-dependent ion channel agonists, antagonist or modulators, photochromic controllers effectively converted a wide range of ion channels, including voltage-gated ion channels, 'leak channels', tri-, tetra-and pentameric ligand-gated ion channels, and temperaturesensitive ion channels, into man-made photoreceptors. Control by photochromes is reversible, unlike in the case of 'caged' compounds, and non-invasive with high spatial precision, unlike pharmacology and electrical manipulation. Here, we introduce design principles of emerging photochromic molecules that act on ion channels and discuss the impact that these molecules are beginning to have on ion channel biophysics and neuronal physiology.

In the vibrant field of optogenetics, optics and genetic targeting are combined to commandeer cellular functions, such as the neuronal action potential, by optically stimulating light-sensitive ion channels expressed in the cell membrane. One broadly applicable manifestation of this approach are covalently attached photochromic tethered ligands (PTLs) that allow activating ligand-gated ion channels with outstanding spatial and temporal resolution. Here, we describe all steps towards the successful development and application of PTL-gated ion channels in cell lines and primary cells. The basis for these experiments forms a combination of molecular modeling, genetic engineering, cell culture and electrophysiology. The light-gated glutamate receptor LiGluR, which consists of the PTLfunctionalized GluK2 receptor, serves as a model.

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Optogenetik und Photopharmakologie ermçglichen eine lichtgesteuerte räumlicheu nd zeitliche Manipulation von Prozessen in Zellen und in Tiermodellen. Obwohl rotes Licht durch minimale Toxizität und gute Gewebedurchdringung überzeugt, sind nur wenige für rotes Licht empfindliche optogenetische Hilfsmittel bekannt. Wirh aben eine neue,d urch rotes Licht aktivierte Homodimerisierungsdomäne in Form der Sensordomäne des cyanobakteriellen Phytochroms 1e ntwickelt, die robust und ohne Zytotoxizität in menschlichen Zellen exprimiert werden kann. Mithilfe dieser Domäne induzierten wir die Dimerisierung und Aktivierung von zwei Rezeptor-Tyrosinkinasen, dem Fibroblasten-Wachstumsfaktor Rezeptor 1u nd dem Neurotrophinrezeptor trkB.A uf diese Weise waren wir in der Lage,d en MAPK/ERK-Signalweg durch Gewebe hindurchz ua ktivieren und mehrfarbige Signaltransduktionsexperimente durchzuführen. Die Homodimerisierung von Proteinen und die Aktivierung von Rezeptor-Tyrosinkinasen auf Basis von rotem Lichts ind vielseitige Erweiterungen des optogenetischen Repertoires.