Phototransduction
Contact
Prof. Dr. Karl-Wilhelm Koch
Tel.: +49-(0)441-798-3640
Office: W4 1-137
E-mail: karl.w.koch@uol.de
Phototransduction
Phototransduction
Photoreceptor cells of the vertebrate retina are exquisite light detectors. They can detect single photons and thereby reach the limit of light detection. Absorption of light and the first steps of information processing in the retina occur in the outer segments of rod and cone cells. Light is absorbed by rhodopsin (in rods), which triggers the activation of a signalling cascade and leads within a few hundred milliseconds to the hydrolysis of the intracellular second messenger cyclic GMP (Figure 1). After a short delay the cytoplasmic Ca2+-concentration also decreases. The concentrations of cyclic GMP and Ca2+ are interdependently regulated by several feedback loops (Ca2+-feedback, Figure 2). These crucially important regulatory mechanisms are mediated by neuronal calcium binding proteins as for example recoverin, isoforms of GCAPs (guanlyate cyclase-activating proteins) or by ubiquitously expressed proteins as calmodulin and S100b. A change in the cytoplasmic Ca2+-concentration is sensed by calcium binding proteins and transferred to downstream target proteins. These events take place on the surface of the disk and plasma membrane in rod and cone cells and contribute significantly to the molecular mechanisms of light adaptation.
Our group tries to decipher the molecular mechanisms of these signalling events, in particular the physiological role of calcium binding proteins in light adaptation. Our interest is focused on protein-protein interactions and structure/function-relationships of retina specific protein complexes. Current research projects and aims are:
Molecular interaction studies of neuronal calcium sensors (recoverin, GCAPs) and their target proteins (rhodopsin kinase, membrane bound guanylate cyclases) by using surface plasmon resonance spectroscopy, fluorescence spectroscopy and isothermal titration calorimetry
Structural determination of recoverin mutants and protein complexes
Synthesis and screening of peptide libraries to identify interaction domains
Isolation and biochemical characterization of unknown photoreceptor proteins; cellular lokalization and investigation of their function
Cone vision in zebrafish retina
Signaling cascade in phototransduction
Figure 1: Signaling cascade in phototransduction. Light triggers activation of rhodopsin (Rh to Rh*) leading to GDP/GTP exchange at the G protein transducin (T). The active form of GTP-bound form of T activates its effector PDE switching from the inactive (PDEi) to the active state (PDEa). Activated PDE hydrolyzes cGMP with high turnover rates. Steps highlighted in green contribute to the amplification of the cascade. Resynthesis of cGMP by guanylate cyclase (GC) is under control of a Ca2+-feedback loop involving Ca2+-sensor proteins (guanylate cyclase-activating proteins, GCAP). The dark current through the CNG-channel is carried by Na+ and Ca2+, the latter is extruded by the Na+/Ca2+, K+-exchanger leading to a decrease of cytoplasmic Ca2+. Ca2+-bound recoverin (Rec) inhibits GRK1 (rhodopsin kinase) thereby preventing phosphorylation of Rh*. Details of the deactivation and regeneration processes of Rh* are not shown (modified from Koch and Dell’Orco, 2015).
Ca2+ - feedback
Figure 2: The decrease of intracellular Ca2+ after illumination is sensed by Ca2+-sensor proteins. GCAP-1 and GCAP-2 belong to these sensor molecules, they activate membrane bound guanylate cyclases in rod and cone cells at low Ca2+-concentrations. Another Ca2+-sensor is recoverin (Rec) that inhibits rhodopsin kinase (RhK) at high Ca2+-concentration. Decreasing of intracellular Ca2+ leads to the dissociation of the Rec/RhK-complex and rhodopsin can be phosphorylated and thereby becomes deactivated.
Selected publications
Koch, K.-W. and Dell'Orco, D. (2013) A calcium relay mechanism in vertebrate phototransduction. ACS Chem Neurosci. 4, 909-917.
Koch KW (2013) The guanylate cyclase signaling system in zebrafish photoreceptors. FEBS Lett. 587, 2055-2059.
Koch, K.-W. and Dell'Orco, D. (2015) Protein and signaling networks in vertebrate photoreceptor cells. Front. Mol. Neurosci. Vol.8, Article 67