Zebrafish Recoverin Isoforms Display Differences in Calcium Switch Mechanisms

Molecular tuning of calcium dependent processes by neuronal calcium sensor proteins in the retina

Vertebrate photoreceptor cells are exquisite light detectors operating under very dim and bright illumination mediated by phototransduction, which is under control of the two secondary messengers cGMP and Ca²⁺. Feedback mechanisms enable photoreceptor cells to regain their responsiveness after light stimulation and involve neuronal Ca²⁺-sensor proteins, named GCAPs (guanylate cyclase-activating proteins) and recoverins. This review compares the diversity in Ca²⁺-related signaling mediated by GCAP and recoverin variants that exhibit differences in Ca²⁺-sensing, protein conformational changes, myristoyl switch mechanisms, diversity in divalent cation binding and dimer formation. In summary, both subclasses of neuronal Ca²⁺-sensor proteins contribute to a complex signaling network in rod and cone cells, which is perfectly suited to match the requirements for sensitive cell responses and maintaining this responsiveness in the presence of different background light intensities.

Kinetics of cone specific G-protein signaling in avian photoreceptor cells

Cone photoreceptor cells of night-migratory songbirds seem to process the primary steps of two different senses, vision and magnetoreception. The molecular basis of phototransduction is a prototypical G protein-coupled receptor pathway starting with the photoexcitation of rhodopsin or cone opsin thereby activating a heterotrimeric G protein named transducin. This interaction is well understood in vertebrate rod cells, but parameter describing protein-protein interactions of cone specific proteins are rare and not available for migratory birds. European robin is a model organism for studying the orientation of birds in the earth magnetic field. Recent findings showed a link between the putative magnetoreceptor cryptochrome 4a and the cone specific G-protein of European robin. In the present work, we investigated the interaction of European robin cone specific G protein and cytoplasmic regions of long wavelength opsin. We identified the second loop in opsin connecting transmembrane regions three and four as a critical binding interface. Surface plasmon resonance studies using a synthetic peptide representing the second cytoplasmic loop and purified G protein α-subunit showed a high affinity interaction with a K _D value of 21 nM. Truncation of the G protein α-subunit at the C-terminus by six amino acids slightly decreased the affinity. Our results suggest that binding of the G protein to cryptochrome can compete with the interaction of G protein and non-photoexcited long wavelength opsin. Thus, the parallel presence of two different sensory pathways in bird cone photoreceptors is reasonable under dark-adapted conditions or during illumination with short wavelengths.

Farnesylation of Zebrafish G-Protein-Coupled Receptor Kinase Using Bio-orthogonal Labeling

G-protein-coupled receptors are deactivated or desensitized by phosphorylation by respective G-protein-coupled receptor kinases (GRKs). In zebrafish rod and cone photoreceptor cells, four orthologous GRKs are expressed participating in the deactivation of rod and cone opsins. An important feature of GRKs in general is the consensus sites for lipid modification, which would allow the posttranslational attachment of isoprenoids facilitating membrane association and enzymatic performance. Because direct proof is missing for isoprenoid modification of zebrafish GRKs, we used a semichemical approach to study the incorporation of a farnesyl moiety into a GRK and its cellular consequences. The approach involves organic synthesis of a functionalized farnesyl derivative that is suitable for a subsequent alkyne-azide cycloaddition (click reaction). For this purpose, zebrafish GRK was expressed in HEK293 cells and modified in situ with the synthetic farnesyl moiety. Successful farnesylation by an endogenous farnesyltransferase was detected by immunoblotting and immunocytochemistry using a biotin-streptavidin-coupled assay and ligation with a fluorescence dye, respectively. Immunocytochemical detection of farnesylated GRK in different cell compartments indicates the applicability of the approach for studying the transport of cellular components.

Biochemistry and physiology of zebrafish photoreceptors

All vertebrates share a canonical retina with light-sensitive photoreceptors in the outer retina. These photoreceptors are of two kinds: rods and cones, adapted to low and bright light conditions, respectively. They both show a peculiar morphology, with long outer segments, comprised of ordered stacks of disc-shaped membranes. These discs host numerous proteins, many of which contribute to the visual transduction cascade. This pathway converts the light stimulus into a biological signal, ultimately modulating synaptic transmission. Recently, the zebrafish (Danio rerio) has gained popularity for studying the function of vertebrate photoreceptors. In this review, we introduce this model system and its contribution to our understanding of photoreception with a focus on the cone visual transduction cascade.

Interaction of G protein-coupled receptor kinases and recoverin isoforms is determined by localization in zebrafish photoreceptors

The zebrafish retina expresses four recoverin genes (rcv1a, rcv1b, rcv2a and rcv2b) and four opsin kinase genes (grk1a, grk1b, grk7a and grk7b) coding for recoverin and G protein-coupled receptor kinase (opsin kinase) paralogs, respectively. Both protein groups are suggested to form regulatory complexes in rod and cone outer segments, but at present, we lack information about co-localization of recoverin and opsin kinases in zebrafish retinae and which protein-protein interacting pairs form. We analyzed the distribution and co-localization of recoverin and opsin kinase expression in the zebrafish retina. For this purpose, we used custom-tailored monospecific antibodies revealing that the amount of recoverin paralogs in a zebrafish retina can differ by more than one order of magnitude with the highest amount for recoverin 1a and 2b. Further, immunohistochemical labelling showed presence of recoverin 1a in all rod cell compartments, but it only co-localized with opsin kinase 1a in rod outer segments. In contrast, recoverin 2b was only detected in double cones and co-localized with opsin kinases 1b, 7a and 7b. Further, we investigated the interaction between recoverin and opsin kinase variants by surface plasmon resonance spectroscopy indicating interaction of recoverin 1a and recoverin 2b with all opsin kinases. However, binding kinetics for recoverin 1a differed from those observed with recoverin 2b that showed slower association and dissociation processes. Our results indicate diverse recoverin and opsin kinase properties due to differential expression and interaction profiles.

Quantitative Determination of Ca2+-binding to Ca2+-sensor Proteins by Isothermal Titration Calorimetry

Diverse and complex molecular recognitions are central elements of signal transduction cascades. The strength and nature of these interaction modes can be determined by different experimental approaches. Among those, Isothermal titration calorimetry (ITC) offers certain advantages by providing binding constants and thermodynamic parameters from titration series without a need to label or immobilize one or more interaction partners. Furthermore, second messenger homeostasis involving Ca²⁺-ions requires in particular knowledge about stoichiometries and affinities of Ca²⁺-binding to Ca²⁺-sensor proteins or Ca²⁺-dependent regulators, which can be obtained by employing ITC. We used ITC to measure these parameters for a set of neuronal Ca²⁺-sensor proteins operating in photoreceptor cells. Here, we present a step wise protocol to (a) measure Ca²⁺ interaction with the Ca²⁺-sensor guanylate cyclase-activating protein 1, (b) to design an ITC experiment and prepare samples, (c) to remove Ca²⁺ nearly completely from Ca²⁺ binding proteins without using a chelating agent like EGTA.

The Binding Properties and Physiological Functions of Recoverin

Recoverin (Rcv) is a low molecular-weight, neuronal calcium sensor (NCS) primarily located in photoreceptor outer segments of the vertebrate retina. Calcium ions (Ca²⁺)-bound Rcv has been proposed to inhibit G-protein-coupled receptor kinase (GRKs) in darkness. During the light response, the Ca²⁺-free Rcv releases GRK, which in turn phosphorylates visual pigment, ultimately leading to the cessation of the visual transduction cascade. Technological advances over the last decade have contributed significantly to a deeper understanding of Rcv function. These include both biophysical and biochemical approaches that will be discussed in this review article. Furthermore, electrophysiological experiments uncovered additional functions of Rcv, such as regulation of the lifetime of Phosphodiesterase-Transducin complex. Recently, attention has been drawn to different roles in rod and cone photoreceptors.This review article focuses on Rcv binding properties to Ca²⁺, disc membrane and GRK, and its physiological functions in phototransduction and signal transmission.