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
Light triggered signalling cascade in photoreceptor cells
Absorption of light by the visual pigment rhodopsin leads to its active form metarhodopsin II (Rho*). Rho* catalyses the GDP/GTP-exchange at the heterotrimeric G-protein transducin (Tαβγ). The active Tα-subunit interacts with its target, a cGMP-specific phosphodiesterase (PDE), that hydrolyses cGMP with a high turnover rate.The intracellular target of cGMP is a cyclic nucleotide-gated ion channel in the plasma membrane, which is opened by binding of cGMP (dark state of the cell) and closed by dissociation of cGMP. Closing of channels diminshes or stops the influx of Ca2+ leading to a decrease in cytoplasmic Ca2+ due to the continous operation of a Na+/K+, Ca2+-exchanger.
Fig. 2: Ca2+ - feedback
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.
Koch, K.-W. (2006) Different biochemical properties of guanylate cyclase-activating proteins allow fine tuning of phototransduction. Neuronal Calcium Sensor Proteins (Philippov, P.P. and Koch, K.-W. Eds.) Nova Publishers, pp229-241
Koch, K.-W., Duda, T. and Sharma R. K. (2010) Ca2+ -modulated vision-linked ROS-GC guanylate cyclase transduction machinery. Mol Cell Biochem. 334, 105-115
Behnen, P., Dell'Orco, D. and Koch, K.-W. (2010) Involvement of the calcium sensor GCAP1 in hereditary cone dystrophies. Biol Chem. June, 391(6):631-637