The calcium-sensor guanylate cyclase activating protein type 2 specific site in rod outer segment membrane guanylate cyclase type 1

Feedforward and feedback mechanisms cooperatively regulate rapid experience-dependent response adaptation in a single thermosensory neuron type

Sensory adaptation allows neurons to adjust their sensitivity and responses based on recent experience. The mechanisms that mediate continuous adaptation to stimulus history over seconds- to hours-long timescales, and whether these mechanisms can operate within a single sensory neuron type, are unclear. The single pair of AFD thermosensory neurons in Caenorhabditis elegans exhibits experience-dependent plasticity in their temperature response thresholds on both minutes- and hours-long timescales upon a temperature upshift. While long-term response adaptation requires changes in gene expression in AFD, the mechanisms driving rapid response plasticity are unknown. Here, we show that rapid thermosensory response adaptation in AFD is mediated via cGMP and calcium-dependent feedforward and feedback mechanisms operating at the level of primary thermotransduction. We find that either of two thermosensor receptor guanylyl cyclases (rGCs) alone is sufficient to drive rapid adaptation, but that each rGC drives adaptation at different rates. rGC-driven adaptation is mediated in part via phosphorylation of their intracellular domains, and calcium-dependent feedback regulation of basal cGMP levels via a neuronal calcium sensor protein. In turn, cGMP levels feedforward via cGMP-dependent protein kinases to phosphorylate a specific subunit of the cGMP-gated thermotransduction channel to further regulate rapid adaptation. Our results identify multiple molecular pathways that act in AFD to ensure rapid adaptation to a temperature change and indicate that the deployment of both transcriptional and nontranscriptional mechanisms within a single sensory neuron type can contribute to continuous sensory adaptation.

Feedforward and feedback mechanisms cooperatively regulate rapid experience-dependent response adaptation in a single thermosensory neuron type

Sensory adaptation allows neurons to adjust their sensitivity and responses based on recent experience. The mechanisms that mediate continuous adaptation to stimulus history over seconds to hours long timescales, and whether these mechanisms can operate within a single sensory neuron type, are unclear. The single pair of AFD thermosensory neurons in C. elegans exhibits experience-dependent plasticity in their temperature response thresholds on both minutes- and hours-long timescales upon a temperature upshift. While long-term response adaptation requires changes in gene expression in AFD, the mechanisms driving rapid response plasticity are unknown. Here, we show that rapid thermosensory response adaptation in AFD is mediated via cGMP and calcium-dependent feedforward and feedback mechanisms operating at the level of primary thermotransduction. We find that either of two thermosensor receptor guanylyl cyclases (rGCs) alone is sufficient to drive rapid adaptation, but that each rGC drives adaptation at different rates. rGC-driven adaptation is mediated in part via phosphorylation of their intracellular domains, and calcium-dependent feedback regulation of basal cGMP levels via a neuronal calcium sensor protein. In turn, cGMP levels feedforward via cGMP-dependent protein kinases to phosphorylate a specific subunit of the cGMP-gated thermotransduction channel to further regulate rapid adaptation. Our results identify multiple molecular pathways that act in AFD to ensure rapid adaptation to a temperature change, and indicate that the deployment of both transcriptional and non-transcriptional mechanisms within a single sensory neuron type can contribute to continuous sensory adaptation.

Multilimbed membrane guanylate cyclase signaling system, evolutionary ladder

One monumental discovery in the field of cell biology is the establishment of the membrane guanylate cyclase signal transduction system. Decoding its fundamental, molecular, biochemical, and genetic features revolutionized the processes of developing therapies for diseases of endocrinology, cardio-vasculature, and sensory neurons; lastly, it has started to leave its imprints with the atmospheric carbon dioxide. The membrane guanylate cyclase does so via its multi-limbed structure. The inter-netted limbs throughout the central, sympathetic, and parasympathetic systems perform these functions. They generate their common second messenger, cyclic GMP to affect the physiology. This review describes an historical account of their sequential evolutionary development, their structural components and their mechanisms of interaction. The foundational principles were laid down by the discovery of its first limb, the ACTH modulated signaling pathway (the companion monograph). It challenged two general existing dogmas at the time. First, there was the question of the existence of a membrane guanylate cyclase independent from a soluble form that was heme-regulated. Second, the sole known cyclic AMP three-component-transduction system was modulated by GTP-binding proteins, so there was the question of whether a one-component transduction system could exclusively modulate cyclic GMP in response to the polypeptide hormone, ACTH. The present review moves past the first question and narrates the evolution and complexity of the cyclic GMP signaling pathway. Besides ACTH, there are at least five additional limbs. Each embodies a unique modular design to perform a specific physiological function; exemplified by ATP binding and phosphorylation, Ca²⁺-sensor proteins that either increase or decrease cyclic GMP synthesis, co-expression of antithetical Ca²⁺ sensors, GCAP1 and S100B, and modulation by atmospheric carbon dioxide and temperature. The complexity provided by these various manners of operation enables membrane guanylate cyclase to conduct diverse functions, exemplified by the control over cardiovasculature, sensory neurons and, endocrine systems.

Structural Insights into Retinal Guanylate Cyclase Activator Proteins (GCAPs)

Retinal guanylate cyclases (RetGCs) promote the Ca²⁺-dependent synthesis of cGMP that coordinates the recovery phase of visual phototransduction in retinal rods and cones. The Ca²⁺-sensitive activation of RetGCs is controlled by a family of photoreceptor Ca²⁺ binding proteins known as guanylate cyclase activator proteins (GCAPs). The Mg²⁺-bound/Ca²⁺-free GCAPs bind to RetGCs and activate cGMP synthesis (cyclase activity) at low cytosolic Ca²⁺ levels in light-activated photoreceptors. By contrast, Ca²⁺-bound GCAPs bind to RetGCs and inactivate cyclase activity at high cytosolic Ca²⁺ levels found in dark-adapted photoreceptors. Mutations in both RetGCs and GCAPs that disrupt the Ca²⁺-dependent cyclase activity are genetically linked to various retinal diseases known as cone-rod dystrophies. In this review, I will provide an overview of the known atomic-level structures of various GCAP proteins to understand how protein dimerization and Ca²⁺-dependent conformational changes in GCAPs control the cyclase activity of RetGCs. This review will also summarize recent structural studies on a GCAP homolog from zebrafish (GCAP5) that binds to Fe²⁺ and may serve as a Fe²⁺ sensor in photoreceptors. The GCAP structures reveal an exposed hydrophobic surface that controls both GCAP1 dimerization and RetGC binding. This exposed site could be targeted by therapeutics designed to inhibit the GCAP1 disease mutants, which may serve to mitigate the onset of retinal cone-rod dystrophies.

Normal GCAPs partly compensate for altered cGMP signaling in retinal dystrophies associated with mutations in GUCA1A

Missense mutations in the GUCA1A gene encoding guanylate cyclase-activating protein 1 (GCAP1) are associated with autosomal dominant cone/cone-rod (CORD) dystrophies. The nature of the inheritance pattern implies that a pool of normal GCAP proteins is present in photoreceptors together with the mutated variant. To assess whether human GCAP1 and GCAP2 may similarly regulate the activity of the retinal membrane guanylate cyclase GC-1 (GC-E) in the presence of the recently discovered E111V-GCAP1 CORD-variant, we combined biochemical and in silico assays. Surprisingly, human GCAP2 does not activate GC1 over the physiological range of Ca²⁺ whereas wild-type GCAP1 significantly attenuates the dysregulation of GC1 induced by E111V-GCAP1. Simulation of the phototransduction cascade in a well-characterized murine system, where GCAP2 is able to activate the GC1, suggests that both GCAPs can act in a synergic manner to mitigate the effects of the CORD-mutation. We propose the existence of a species-dependent compensatory mechanism. In murine photoreceptors, slight increases of wild-type GCAPs levels may significantly attenuate the increase in intracellular Ca²⁺ and cGMP induced by E111V-GCAP1 in heterozygous conditions. In humans, however, the excess of wild-type GCAP1 may only partly attenuate the mutant-induced dysregulation of cGMP signaling due to the lack of GC1-regulation by GCAP2.

Molecular Details of Retinal Guanylyl Cyclase 1/GCAP-2 Interaction

The rod outer segment guanylyl cyclase 1 (ROS-GC1) is an essential component of photo-transduction in the retina. In the light-induced signal cascade, membrane-bound ROS-GC1 restores cGMP levels in the dark in a calcium-dependent manner. With decreasing calcium concentration in the intracellular compartment, ROS-GC1 is activated via the intracellular site by guanylyl cyclase-activating proteins (GCAP-1/-2). Presently, the exact activation mechanism is elusive. To obtain structural insights into the ROS-GC1 regulation by GCAP-2, chemical cross-linking/mass spectrometry studies using GCAP-2 and three ROS-GC1 peptides were performed in the presence and absence of calcium. The majority of cross-links were identified with the C-terminal lobe of GCAP-2 and a peptide comprising parts of ROS-GC1's catalytic domain and C-terminal extension. Consistently with the cross-linking results, surface plasmon resonance and fluorescence measurements confirmed specific binding of this ROS-GC peptide to GCAP-2 with a dissociation constant in the low micromolar range. These results imply that a region of the catalytic domain of ROS-GC1 can participate in the interaction with GCAP-2. Additional binding surfaces upstream of the catalytic domain, in particular the juxtamembrane domain, can currently not be excluded.

Membrane Guanylate Cyclase catalytic Subdomain: Structure and Linkage with Calcium Sensors and Bicarbonate

Membrane guanylate cyclase (MGC) is a ubiquitous multi-switching cyclic GMP generating signaling machine linked with countless physiological processes. In mammals it is encoded by seven distinct homologous genes. It is a single transmembrane spanning multi-modular protein; composed of integrated blocks and existing in homo-dimeric form. Its core catalytic domain (CCD) module is a common transduction center where all incoming signals are translated into the production of cyclic GMP, a cellular signal second messenger. Crystal structure of the MGC's CCD does not exist and its precise identity is ill-defined. Here, we define it at a sub-molecular level for the phototransduction-linked MGC, the rod outer segment guanylate cyclase type 1, ROS-GC1. (1) The CCD is a conserved 145-residue structural unit, represented by the segment V820-P964. (2) It exists as a homo-dimer and contains seven conserved catalytic elements (CEs) wedged into seven conserved motifs. (3) It also contains a conserved 21-residue neurocalcin δ-modulated structural domain, V836-L857. (4) Site-directed mutagenesis documents that each of the seven CEs governs the cyclase's catalytic activity. (5) In contrast to the soluble and the bacterium MGC which use Mn2+-GTP substrate for catalysis, MGC CCD uses the natural Mg2+-GTP substrate. (6) Strikingly, the MGC CCD requires anchoring by the Transmembrane Domain (TMD) to exhibit its major (∼92%) catalytic activity; in isolated form the activity is only marginal. This feature is not linked with any unique sequence of the TMD; there is minimal conservation in TMD. Finally, (7) the seven CEs control each of four phototransduction pathways- -two Ca2+-sensor GCAPs-, one Ca2+-sensor, S100B-, and one bicarbonate-modulated. The findings disclose that the CCD of ROS-GC1 has built-in regulatory elements that control its signal translational activity. Due to conservation of these regulatory elements, it is proposed that these elements also control the physiological activity of other members of MGC family.

Label-free quantification of calcium-sensor targeting to photoreceptor guanylate cyclase and rhodopsin kinase by backscattering interferometry

Quantification of protein binding to membrane proteins is challenging and a limited set of methods is available to study such systems. Here we employed backscattering interferometry (BSI), a free-solution label-free method with high sensitivity, to quantify the interaction of neuronal Ca²⁺-Sensor proteins with their targets operating in phototransduction. We tested direct binding of guanylate cyclase–activating proteins (GCAP1 and GCAP2) to their membrane target guanylate cyclase 1. The regulatory mechanism of GCAPs including their binding interface in the target is unresolved. Here we used a label-free, free-solution assay method based on BSI to determine binding constants of GCAP1 and GCAP2 to the full-length membrane-bound guanylate cyclase type 1. GCAP1 and GCAP2 bound to different regions on the target guanylate cyclase with submicromolar affinity (apparent K_D-values of 663 ± 121 nM and 231 ± 63 nM for Ca²⁺-free GCAP1 and GCAP2, respectively). A guanylate cyclase construct containing the juxta-membrane and kinase homology domain harbored an exclusive binding site for GCAP1 with similar affinities as the full-length protein, whereas GCAP2 did not bind to this region. We provide a model in which GCAP1 and GCAP2 do not share a single binding site to the target, thus cannot exchange upon fluctuating Ca²⁺ levels.

Integrative Signaling Networks of Membrane Guanylate Cyclases: Biochemistry and Physiology

This monograph presents a historical perspective of cornerstone developments on the biochemistry and physiology of mammalian membrane guanylate cyclases (MGCs), highlighting contributions made by the authors and their collaborators. Upon resolution of early contentious studies, cyclic GMP emerged alongside cyclic AMP, as an important intracellular second messenger for hormonal signaling. However, the two signaling pathways differ in significant ways. In the cyclic AMP pathway, hormone binding to a G protein coupled receptor leads to stimulation or inhibition of an adenylate cyclase, whereas the cyclic GMP pathway dispenses with intermediaries; hormone binds to an MGC to affect its activity. Although the cyclic GMP pathway is direct, it is by no means simple. The modular design of the molecule incorporates regulation by ATP binding and phosphorylation. MGCs can form complexes with Ca²⁺-sensing subunits that either increase or decrease cyclic GMP synthesis, depending on subunit identity. In some systems, co-expression of two Ca²⁺ sensors, GCAP1 and S100B with ROS-GC1 confers bimodal signaling marked by increases in cyclic GMP synthesis when intracellular Ca²⁺ concentration rises or falls. Some MGCs monitor or are modulated by carbon dioxide via its conversion to bicarbonate. One MGC even functions as a thermosensor as well as a chemosensor; activity reaches a maximum with a mild drop in temperature. The complexity afforded by these multiple limbs of operation enables MGC networks to perform transductions traditionally reserved for G protein coupled receptors and Transient Receptor Potential (TRP) ion channels and to serve a diverse array of functions, including control over cardiac vasculature, smooth muscle relaxation, blood pressure regulation, cellular growth, sensory transductions, neural plasticity and memory.

Bicarbonate and Ca(2+) Sensing Modulators Activate Photoreceptor ROS-GC1 Synergistically

Photoreceptor ROS-GC1, a prototype subfamily member of the membrane guanylate cyclase family, is a central component of phototransduction. It is a single transmembrane-spanning protein, composed of modular blocks. In rods, guanylate cyclase activating proteins (GCAPs) 1 and 2 bind to its juxtamembrane domain (JMD) and the C-terminal extension, respectively, to accelerate cyclic GMP synthesis when Ca(2+) levels are low. In cones, the additional expression of the Ca(2+)-dependent guanylate cyclase activating protein (CD-GCAP) S100B which binds to its C-terminal extension, supports acceleration of cyclic GMP synthesis at high Ca(2+) levels. Independent of Ca(2+), ROS-GC1 activity is also stimulated directly by bicarbonate binding to the core catalytic domain (CCD). Several enticing molecular features of this transduction system are revealed in the present study. In combination, bicarbonate and Ca(2+)-dependent modulators raised maximal ROS-GC activity to levels that exceeded the sum of their individual effects. The F(514)S mutation in ROS-GC1 that causes blindness in type 1 Leber's congenital amaurosis (LCA) severely reduced basal ROS-GC1 activity. GCAP2 and S100B Ca(2+) signaling modes remained functional, while the GCAP1-modulated mode was diminished. Bicarbonate nearly restored basal activity as well as GCAP2- and S100B-stimulated activities of the F(514)S mutant to normal levels but could not resurrect GCAP1 stimulation. We conclude that GCAP1 and GCAP2 forge distinct pathways through domain-specific modules of ROS-GC1 whereas the S100B and GCAP2 pathways may overlap. The synergistic interlinking of bicarbonate to GCAPs- and S100B-modulated pathways intensifies and tunes the dependence of cyclic GMP synthesis on intracellular Ca(2+). Our study challenges the recently proposed GCAP1 and GCAP2 "overlapping" phototransduction model (Peshenko et al., 2015b).

Protein and Signaling Networks in Vertebrate Photoreceptor Cells

Vertebrate photoreceptor cells are exquisite light detectors operating under very dim and bright illumination. The photoexcitation and adaptation machinery in photoreceptor cells consists of protein complexes that can form highly ordered supramolecular structures and control the homeostasis and mutual dependence of the secondary messengers cyclic guanosine monophosphate (cGMP) and Ca²⁺. The visual pigment in rod photoreceptors, the G protein-coupled receptor rhodopsin is organized in tracks of dimers thereby providing a signaling platform for the dynamic scaffolding of the G protein transducin. Illuminated rhodopsin is turned off by phosphorylation catalyzed by rhodopsin kinase (GRK1) under control of Ca²⁺-recoverin. The GRK1 protein complex partly assembles in lipid raft structures, where shutting off rhodopsin seems to be more effective. Re-synthesis of cGMP is another crucial step in the recovery of the photoresponse after illumination. It is catalyzed by membrane bound sensory guanylate cyclases (GCs) and is regulated by specific neuronal Ca²⁺-sensor proteins called guanylate cyclase-activating proteins (GCAPs). At least one GC (ROS-GC1) was shown to be part of a multiprotein complex having strong interactions with the cytoskeleton and being controlled in a multimodal Ca²⁺-dependent fashion. The final target of the cGMP signaling cascade is a cyclic nucleotide-gated (CNG) channel that is a hetero-oligomeric protein located in the plasma membrane and interacting with accessory proteins in highly organized microdomains. We summarize results and interpretations of findings related to the inhomogeneous organization of signaling units in photoreceptor outer segments.

Dimerization Domain of Retinal Membrane Guanylyl Cyclase 1 (RetGC1) Is an Essential Part of Guanylyl Cyclase-activating Protein (GCAP) Binding Interface

The photoreceptor-specific proteins guanylyl cyclase-activating proteins (GCAPs) bind and regulate retinal membrane guanylyl cyclase 1 (RetGC1) but not natriuretic peptide receptor A (NPRA). Study of RetGC1 regulation in vitro and its association with fluorescently tagged GCAP in transfected cells showed that R822P substitution in the cyclase dimerization domain causing congenital early onset blindness disrupted RetGC1 ability to bind GCAP but did not eliminate its affinity for another photoreceptor-specific protein, retinal degeneration 3 (RD3). Likewise, the presence of the NPRA dimerization domain in RetGC1/NPRA chimera specifically disabled binding of GCAPs but not of RD3. In subsequent mapping using hybrid dimerization domains in RetGC1/NPRA chimera, multiple RetGC1-specific residues contributed to GCAP binding by the cyclase, but the region around Met(823) was the most crucial. Either positively or negatively charged residues in that position completely blocked GCAP1 and GCAP2 but not RD3 binding similarly to the disease-causing mutation in the neighboring Arg(822). The specificity of GCAP binding imparted by RetGC1 dimerization domain was not directly related to promoting dimerization of the cyclase. The probability of coiled coil dimer formation computed for RetGC1/NPRA chimeras, even those incapable of binding GCAP, remained high, and functional complementation tests showed that the RetGC1 active site, which requires dimerization of the cyclase, was formed even when Met(823) or Arg(822) was mutated. These results directly demonstrate that the interface for GCAP binding on RetGC1 requires not only the kinase homology region but also directly involves the dimerization domain and especially its portion containing Arg(822) and Met(823).

Evaluating the role of retinal membrane guanylyl cyclase 1 (RetGC1) domains in binding guanylyl cyclase-activating proteins (GCAPs)

Retinal membrane guanylyl cyclase 1 (RetGC1) regulated by guanylyl cyclase-activating proteins (GCAPs) controls photoreceptor recovery and when mutated causes blinding disorders. We evaluated the principal models of how GCAP1 and GCAP2 bind RetGC1: through a shared docking interface versus independent binding sites formed by distant portions of the cyclase intracellular domain. At near-saturating concentrations, GCAP1 and GCAP2 activated RetGC1 from HEK293 cells and RetGC2(-/-)GCAPs1,2(-/-) mouse retinas in a non-additive fashion. The M26R GCAP1, which binds but does not activate RetGC1, suppressed activation of recombinant and native RetGC1 by competing with both GCAP1 and GCAP2. Untagged GCAP1 displaced both GCAP1-GFP and GCAP2-GFP from the complex with RetGC1 in HEK293 cells. The intracellular segment of a natriuretic peptide receptor A guanylyl cyclase failed to bind GCAPs, but replacing its kinase homology and dimerization domains with those from RetGC1 restored GCAP1 and GCAP2 binding by the hybrid cyclase and its GCAP-dependent regulation. Deletion of the Tyr(1016)-Ser(1103) fragment in RetGC1 did not block GCAP2 binding to the cyclase. In contrast, substitutions in the kinase homology domain, W708R and I734T, linked to Leber congenital amaurosis prevented binding of both GCAP1-GFP and GCAP2-GFP. Our results demonstrate that GCAPs cannot regulate RetGC1 using independent primary binding sites. Instead, GCAP1 and GCAP2 bind with the cyclase molecule in a mutually exclusive manner using a common or overlapping binding site(s) in the Arg(488)-Arg(851) portion of RetGC1, and mutations in that region causing Leber congenital amaurosis blindness disrupt activation of the cyclase by both GCAP1 and GCAP2.

Bicarbonate Modulates Photoreceptor Guanylate Cyclase (ROS-GC) Catalytic Activity

By generating the second messenger cGMP in retinal rods and cones, ROS-GC plays a central role in visual transduction. Guanylate cyclase-activating proteins (GCAPs) link cGMP synthesis to the light-induced fall in [Ca(2+)]i to help set absolute sensitivity and assure prompt recovery of the response to light. The present report discloses a surprising feature of this system: ROS-GC is a sensor of bicarbonate. Recombinant ROS-GCs synthesized cGMP from GTP at faster rates in the presence of bicarbonate with an ED50 of 27 mM for ROS-GC1 and 39 mM for ROS-GC2. The effect required neither Ca(2+) nor use of the GCAPs domains; however, stimulation of ROS-GC1 was more powerful in the presence of GCAP1 or GCAP2 at low [Ca(2+)]. When applied to retinal photoreceptors, bicarbonate enhanced the circulating current, decreased sensitivity to flashes, and accelerated flash response kinetics. Bicarbonate was effective when applied either to the outer or inner segment of red-sensitive cones. In contrast, bicarbonate exerted an effect when applied to the inner segment of rods but had little efficacy when applied to the outer segment. The findings define a new regulatory mechanism of the ROS-GC system that affects visual transduction and is likely to affect the course of retinal diseases caused by cGMP toxicity.

Reliable identification of cross-linked products in protein interaction studies by 13C-labeled p-benzoylphenylalanine

We describe the use of the (13)C-labeled artificial amino acid p-benzoyl-L-phenylalanine (Bpa) to improve the reliability of cross-linked product identification. Our strategy is exemplified for two protein-peptide complexes. These studies indicate that in many cases the identification of a cross-link without additional stable isotope labeling would result in an ambiguous assignment of cross-linked products. The use of a (13)C-labeled photoreactive amino acid is considered to be preferred over the use of deuterated cross-linkers as retention time shifts in reversed phase chromatography can be ruled out. The observation of characteristic fragment ions additionally increases the reliability of cross-linked product assignment. Bpa possesses a broad reactivity towards different amino acids and the derived distance information allows mapping of spatially close amino acids and thus provides more solid structural information of proteins and protein complexes compared to the longer deuterated amine-reactive cross-linkers, which are commonly used for protein 3D-structure analysis and protein-protein interaction studies.

Membrane guanylate cyclase, a multimodal transduction machine: history, present, and future directions

A sequel to these authors' earlier comprehensive reviews which covered the field of mammalian membrane guanylate cyclase (MGC) from its origin to the year 2010, this article contains 13 sections. The first is historical and covers MGC from the year 1963–1987, summarizing its colorful developmental stages from its passionate pursuit to its consolidation. The second deals with the establishment of its biochemical identity. MGC becomes the transducer of a hormonal signal and founder of the peptide hormone receptor family, and creates the notion that hormone signal transduction is its sole physiological function. The third defines its expansion. The discovery of ROS-GC subfamily is made and it links ROS-GC with the physiology of phototransduction. Sections ROS-GC, a Ca²⁺-Modulated Two Component Transduction System to Migration Patterns and Translations of the GCAP Signals Into Production of Cyclic GMP are Different cover its biochemistry and physiology. The noteworthy events are that augmented by GCAPs, ROS-GC proves to be a transducer of the free Ca²⁺ signals generated within neurons; ROS-GC becomes a two-component transduction system and establishes itself as a source of cyclic GMP, the second messenger of phototransduction. Section ROS-GC1 Gene Linked Retinal Dystrophies demonstrates how this knowledge begins to be translated into the diagnosis and providing the molecular definition of retinal dystrophies. Section Controlled By Low and High Levels of [Ca²⁺]_i, ROS-GC1 is a Bimodal Transduction Switch discusses a striking property of ROS-GC where it becomes a “[Ca²⁺]_i bimodal switch” and transcends its signaling role in other neural processes. In this course, discovery of the first CD-GCAP (Ca²⁺-dependent guanylate cyclase activator), the S100B protein, is made. It extends the role of the ROS-GC transduction system beyond the phototransduction to the signaling processes in the synapse region between photoreceptor and cone ON-bipolar cells; in section Ca²⁺-Modulated Neurocalcin δ ROS-GC1 Transduction System Exists in the Inner Plexiform Layer (IPL) of the Retinal Neurons, discovery of another CD-GCAP, NCδ, is made and its linkage with signaling of the inner plexiform layer neurons is established. Section ROS-GC Linkage With Other Than Vision-Linked Neurons discusses linkage of the ROS-GC transduction system with other sensory transduction processes: Pineal gland, Olfaction and Gustation. In the next, section Evolution of a General Ca²⁺-Interlocked ROS-GC Signal Transduction Concept in Sensory and Sensory-Linked Neurons, a theoretical concept is proposed where “Ca²⁺-interlocked ROS-GC signal transduction” machinery becomes a common signaling component of the sensory and sensory-linked neurons. Closure to the review is brought by the conclusion and future directions.

RNA interference gene therapy in dominant retinitis pigmentosa and cone-rod dystrophy mouse models caused by GCAP1 mutations

RNA interference (RNAi) knockdown is an efficacious therapeutic strategy for silencing genes causative for dominant retinal dystrophies. To test this, we used self-complementary (sc) AAV2/8 vector to develop an RNAi-based therapy in two dominant retinal degeneration mouse models. The allele-specific model expresses transgenic bovine GCAP1(Y99C) establishing a rapid RP-like phenotype, whereas the nonallele-specific model expresses mouse GCAP1(L151F) producing a slowly progressing cone-rod dystrophy (CORD). The late onset GCAP1(L151F)-CORD mimics the dystrophy observed in human GCAP1-CORD patients. Subretinal injection of scAAV2/8 carrying shRNA expression cassettes specific for bovine or mouse guanylate cyclase-activating protein 1 (GCAP1) showed strong expression at 1 week post-injection. In both allele-specific [GCAP1(Y99C)-RP] and nonallele-specific [GCAP1(L151F)-CORD] models of dominant retinal dystrophy, RNAi-mediated gene silencing enhanced photoreceptor survival, delayed onset of degeneration and improved visual function. Such results provide a "proof of concept" toward effective RNAi-based gene therapy mediated by scAAV2/8 for dominant retinal disease based on GCAP1 mutation. Further, nonallele-specific RNAi knockdown of GCAP1 may prove generally applicable toward the rescue of any human GCAP1-based dominant cone-rod dystrophy.

ROS-GC interlocked Ca(2+)-sensor S100B protein signaling in cone photoreceptors: review

Photoreceptor rod outer segment membrane guanylate cyclase (ROS-GC) is central to visual transduction; it generates cyclic GMP, the second messenger of the photon signal. Photoexcited rhodopsin initiates a biochemical cascade that leads to a drop in the intracellular level of cyclic GMP and closure of cyclic nucleotide gated ion channels. Recovery of the photoresponse requires resynthesis of cyclic GMP, typically by a pair of ROS-GCs, 1 and 2. In rods, ROS-GCs exist as complexes with guanylate cyclase activating proteins (GCAPs), which are Ca(2+)-sensing elements. There is a light-induced fall in intracellular Ca(2+). As Ca(2+) dissociates from GCAPs in the 20-200 nM range, ROS-GC activity rises to quicken the photoresponse recovery. GCAPs then progressively turn down ROS-GC activity as Ca(2+) and cyclic GMP levels return to baseline. To date, GCAPs mediate the only known mechanism of ROS-GC regulation in the photoreceptors. However, in mammalian cone outer segments, cone synapses and ON bipolar cells, another Ca(2+) sensor protein, S100B, complexes with ROS-GC1 and senses the Ca(2+) signal with a K1/2 of 400 nM. Unlike GCAPs, S100B stimulates ROS-GC activity when Ca(2+) is bound. Thus, the ROS-GC system in cones functions as a Ca(2+) bimodal switch; with rising intracellular Ca(2+), its activity is first turned down by GCAPs and then turned up by S100B. This presentation provides a historical perspective on the role of S100B in the photoreceptors, offers a pictorial model for the "bimodal" operation of the ROS-GC switch and projects future tasks that are needed to understand its operation. Some accounts of this review have been adopted from the original publications of these authors.

Structural diversity of neuronal calcium sensor proteins and insights for activation of retinal guanylyl cyclase by GCAP1

Neuronal calcium sensor (NCS) proteins, a sub-branch of the calmodulin superfamily, are expressed in the brain and retina where they transduce calcium signals and are genetically linked to degenerative diseases. The amino acid sequences of NCS proteins are highly conserved but their physiological functions are quite different. Retinal recoverin controls Ca²⁺-dependent inactivation of light-excited rhodopsin during phototransduction, guanylyl cyclase activating proteins 1 and 2 (GCAP1 and GCAP2) promote Ca²⁺-dependent activation of retinal guanylyl cyclases, and neuronal frequenin (NCS-1) modulates synaptic activity and neuronal secretion. Here we review the molecular structures of myristoylated forms of NCS-1, recoverin, and GCAP1 that all look very different, suggesting that the attached myristoyl group helps to refold these highly homologous proteins into different three-dimensional folds. Ca²⁺-binding to both recoverin and NCS-1 cause large protein conformational changes that ejects the covalently attached myristoyl group into the solvent exterior and promotes membrane targeting (Ca²⁺-myristoyl switch). The GCAP proteins undergo much smaller Ca²⁺-induced conformational changes and do not possess a Ca²⁺-myristoyl switch. Recent structures of GCAP1 in both its activator and Ca²⁺-bound inhibitory states will be discussed to understand structural determinants that control their Ca²⁺-dependent activation of retinal guanylyl cyclases.

Dysfunction of outer segment guanylate cyclase caused by retinal disease related mutations

Membrane bound guanylate cyclases are expressed in rod and cone cells of the vertebrate retina and mutations in several domains of rod outer segment guanylate cyclase 1 (ROS-GC1 encoded by the gene GUCY2D) correlate with different forms of retinal degenerations. In the present work we investigated the biochemical consequences of three point mutations, one is located in position P575L in the juxtamembrane domain close to the kinase homology domain and two are located in the cyclase catalytic domain at H1019P and P1069R. These mutations correlate with various retinal diseases like autosomal dominant progressive cone degeneration, e.g., Leber Congenital Amaurosis and a juvenile form of retinitis pigmentosa. Wildtype and mutant forms of ROS-GC1 were heterologously expressed in HEK cells, their cellular distribution was investigated and activity profiles in the presence and absence of guanylate cyclase-activating proteins were measured. The mutant P575L was active under all tested conditions, but it displayed a twofold shift in the Ca²⁺-sensitivity, whereas the mutant P1069R remained inactive despite normal expression levels. The mutation H1019P caused the cyclase to become more labile. The different biochemical consequences of these mutations seem to reflect the different clinical symptoms. The mutation P575L induces a dysregulation of the Ca²⁺-sensitive cyclase activation profile causing a slow progression of the disease by the distortion of the Ca²⁺-cGMP homeostasis. In contrast, a strong reduction in cGMP synthesis due to an inactive or structurally unstable ROS-GC1 would trigger more severe forms of retinal diseases.

A calcium-relay mechanism in vertebrate phototransduction

Calcium-signaling in cells requires a fine-tuned system of calcium-transport proteins involving ion channels, exchangers, and ion-pumps but also calcium-sensor proteins and their targets. Thus, control of physiological responses very often depends on incremental changes of the cytoplasmic calcium concentration, which are sensed by calcium-binding proteins and are further transmitted to specific target proteins. This Review will focus on calcium-signaling in vertebrate photoreceptor cells, where recent physiological and biochemical data indicate that a subset of neuronal calcium sensor proteins named guanylate cyclase-activating proteins (GCAPs) operate in a calcium-relay system, namely, to make gradual responses to small changes in calcium. We will further integrate this mechanism in an existing computational model of phototransduction showing that it is consistent and compatible with the dynamics that are characteristic for the precise operation of the phototransduction pathways.

Differential Ca(2+) sensor guanylate cyclase activating protein modes of photoreceptor rod outer segment membrane guanylate cyclase signaling

Photoreceptor ROS-GC1 (rod outer segment membrane guanylate cyclase) is a vital component of phototransduction. It is a bimodal Ca(2+) signal transduction switch, operating between 20 and ∼1000 nM. Modulated by Ca(2+) sensors guanylate cyclase activating proteins 1 and 2 (GCAP1 and GCAP2, respectively), decreasing [Ca(2+)](i) from 200 to 20 nM progressively turns it "on", as does the modulation by the Ca(2+) sensor S100B, increasing [Ca(2+)](i) from 100 to 1000 nM. The GCAP mode plays a vital role in phototransduction in both rods and cones and the S100B mode in the transmission of neural signals to cone ON-bipolar cells. Through a programmed domain deletion, expression, in vivo fluorescence spectroscopy, and in vitro reconstitution experiments, this study demonstrates that the biochemical mechanisms modulated by two GCAPs in Ca(2+) signaling of ROS-GC1 activity are totally different. (1) They involve different structural domains of ROS-GC1. (2) Their signal migratory pathways are opposite: GCAP1 downstream and GCAP2 upstream. (3) Importantly, the isolated catalytic domain, translating the GCAP-modulated Ca(2+) signal into the generation of cyclic GMP, in vivo, exists as a homodimer, the two subunits existing in an antiparallel conformation. Furthermore, the findings demonstrate that the N-terminally placed signaling helix domain is not required for the catalytic domain's dimeric state. The upstream GCAP2-modulated pathway is the first of its kind to be observed for any member of the membrane guanylate cyclase family. It defines a new model of Ca(2+) signal transduction.

Ca(2+)-sensors and ROS-GC: interlocked sensory transduction elements: a review

From its initial discovery that ROS-GC membrane guanylate cyclase is a mono-modal Ca(2+)-transduction system linked exclusively with the photo-transduction machinery to the successive finding that it embodies a remarkable bimodal Ca(2+) signaling device, its widened transduction role in the general signaling mechanisms of the sensory neuron cells was envisioned. A theoretical concept was proposed where Ca(2+)-modulates ROS-GC through its generated cyclic GMP via a nearby cyclic nucleotide gated channel and creates a hyper- or depolarized sate in the neuron membrane (Ca(2+) Binding Proteins 1:1, 7-11, 2006). The generated electric potential then becomes a mode of transmission of the parent [Ca(2+)](i) signal. Ca(2+) and ROS-GC are interlocked messengers in multiple sensory transduction mechanisms. This comprehensive review discusses the developmental stages to the present status of this concept and demonstrates how neuronal Ca(2+)-sensor (NCS) proteins are the interconnected elements of this elegant ROS-GC transduction system. The focus is on the dynamism of the structural composition of this system, and how it accommodates selectivity and elasticity for the Ca(2+) signals to perform multiple tasks linked with the SENSES of vision, smell, and possibly of taste and the pineal gland. An intriguing illustration is provided for the Ca(2+) sensor GCAP1 which displays its remarkable ability for its flexibility in function from being a photoreceptor sensor to an odorant receptor sensor. In doing so it reverses its function from an inhibitor of ROS-GC to the stimulator of ONE-GC membrane guanylate cyclase.

Synergetic effect of recoverin and calmodulin on regulation of rhodopsin kinase

Phosphorylation of photoactivated rhodopsin by rhodopsin kinase (RK or GRK1), a first step of the phototransduction cascade turnoff, is under the control of Ca²⁺/recoverin. Here, we demonstrate that calmodulin, a ubiquitous Ca²⁺-sensor, can inhibit RK, though less effectively than recoverin does. We have utilized the surface plasmon resonance technology to map the calmodulin binding site in the RK molecule. Calmodulin does not interact with the recoverin-binding site within amino acid residues M1-S25 of the enzyme. Instead, the high affinity calmodulin binding site is localized within a stretch of amino acid residues V150-K175 in the N-terminal regulatory region of RK. Moreover, the inhibitory effect of calmodulin and recoverin on RK activity is synergetic, which is in agreement with the existence of separate binding sites for each Ca²⁺-sensing protein. The synergetic inhibition of RK by both Ca²⁺-sensors occurs over a broader range of Ca²⁺-concentration than by recoverin alone, indicating increased Ca²⁺-sensitivity of RK regulation in the presence of both Ca²⁺-sensors. Taken together, our data suggest that RK regulation by calmodulin in photoreceptor cells could complement the well-known inhibitory effect of recoverin on RK.

EF hand-mediated Ca- and cGMP-signaling in photoreceptor synaptic terminals

Photoreceptors, the light-sensitive receptor neurons of the retina, receive and transmit a plethora of visual informations from the surrounding world. Photoreceptors capture light and convert this energy into electrical signals that are conveyed to the inner retina. For synaptic communication with the inner retina, photoreceptors make large active zones that are marked by synaptic ribbons. These unique synapses support continuous vesicle exocytosis that is modulated by light-induced, graded changes of membrane potential. Synaptic transmission can be adjusted in an activity-dependent manner, and at the synaptic ribbons, Ca²⁺- and cGMP-dependent processes appear to play a central role. EF-hand-containing proteins mediate many of these Ca²⁺- and cGMP-dependent functions. Since continuous signaling of photoreceptors appears to be prone to malfunction, disturbances of Ca²⁺- and cGMP-mediated signaling in photoreceptors can lead to visual defects, retinal degeneration (rd), and even blindness. This review summarizes aspects of signal transmission at the photoreceptor presynaptic terminals that involve EF-hand-containing Ca²⁺-binding proteins.

Retinal degeneration 3 (RD3) protein inhibits catalytic activity of retinal membrane guanylyl cyclase (RetGC) and its stimulation by activating proteins

Retinal membrane guanylyl cyclase (RetGC) in the outer segments of vertebrate photoreceptors is controlled by guanylyl cyclase activating proteins (GCAPs), responding to light-dependent changes of the intracellular Ca(2+) concentrations. We present evidence that a different RetGC binding protein, retinal degeneration 3 protein (RD3), is a high-affinity allosteric modulator of the cyclase which inhibits RetGC activity at submicromolar concentrations. It suppresses the basal activity of RetGC in the absence of GCAPs in a noncompetitive manner, and it inhibits the GCAP-stimulated RetGC at low intracellular Ca(2+) levels. RD3 opposes the allosteric activation of the cyclase by GCAP but does not significantly change Ca(2+) sensitivity of the GCAP-dependent regulation. We have tested a number of mutations in RD3 implicated in human retinal degenerative disorders and have found that several mutations prevent the stable expression of RD3 in HEK293 cells and decrease the affinity of RD3 for RetGC1. The RD3 mutant lacking the carboxy-terminal half of the protein and associated with Leber congenital amaurosis type 12 (LCA12) is unable to suppress the activity of the RetGC1/GCAP complex. Furthermore, the inhibitory activity of the G57V mutant implicated in cone-rod degeneration is strongly reduced. Our results suggest that inhibition of RetGC by RD3 may be utilized by photoreceptors to block RetGC activity during its maturation and/or incorporation into the photoreceptor outer segment rather than participate in dynamic regulation of the cyclase by Ca(2+) and GCAPs.

Enzymatic properties and regulation of the native isozymes of retinal membrane guanylyl cyclase (RetGC) from mouse photoreceptors

Mouse photoreceptor function and survival critically depend on Ca(2+)-regulated retinal membrane guanylyl cyclase (RetGC), comprised of two isozymes, RetGC1 and RetGC2. We characterized the content, catalytic constants, and regulation of native RetGC1 and RetGC2 isozymes using mice lacking guanylyl cyclase activating proteins GCAP1 and GCAP2 and deficient for either GUCY2F or GUCY2E genes, respectively. We found that the characteristics of both native RetGC isozymes were considerably different from other reported estimates made for mammalian RetGCs: the content of RetGC1 per mouse rod outer segments (ROS) was at least 3-fold lower, the molar ratio (RetGC2:RetGC1) 6-fold higher, and the catalytic constants of both GCAP-activated isozymes between 12- and 19-fold higher than previously measured in bovine ROS. The native RetGC isozymes had different basal activity and were accelerated 5-28-fold at physiological concentrations of GCAPs. RetGC2 alone was capable of contributing as much as 135-165 μM cGMP s(-1) or almost 23-28% to the maximal cGMP synthesis rate in mouse ROS. At the maximal level of activation by GCAP, this isozyme alone could provide a significantly high rate of cGMP synthesis compared to what is expected for normal recovery of a mouse rod, and this can help explain some of the unresolved paradoxes of rod physiology. GCAP-activated native RetGC1 and RetGC2 were less sensitive to inhibition by Ca(2+) in the presence of GCAP1 (EC(50Ca) ∼132-139 nM) than GCAP2 (EC(50Ca) ∼50-59 nM), thus arguing that Ca(2+) sensor properties of GCAP in a functional RetGC/GCAP complex are defined not by a particular target isozyme but the intrinsic properties of GCAPs themselves.

657WTAPELL663 motif of the photoreceptor ROS-GC1: a general phototransduction switch

This study documents the identity of an intriguing transduction mechanism of the [Ca(2+)](i) signals by the photoreceptor ROS-GC1. Despite their distal residences and operational modes in phototransduction, the two GCAPs transmit and activate ROS-GC1 through a common Ca(2+) transmitter switch (Ca(2+)TS). A combination of immunoprecipitation, fluorescent spectroscopy, mutational analyses and reconstitution studies has been used to demonstrate that the structure of this switch is (657)WTAPELL(663). The two Ca(2+) signaling GCAP pathways converge in Ca(2+)TS, get transduced, activate ROS-GC1, generate the LIGHT signal second messenger cyclic GMP and yet functionally perform divergent operations of the phototransduction machinery. The findings define a new Ca(2+)-modulated photoreceptor ROS-GC transduction model; it is depicted and discussed for its application to processing the different shades of LIGHT.

Neurocalcin delta modulation of ROS-GC1, a new model of Ca(2+) signaling

ROS-GC1 membrane guanylate cyclase is a Ca(2+) bimodal signal transduction switch. It is turned "off" by a rise in free Ca(2+) from nanomolar to the semicromolar range in the photoreceptor outer segments and the olfactory bulb neurons; by a similar rise in the bipolar and ganglion retinal neurons it is turned "on". These opposite operational modes of the switch are specified by its Ca(2+) sensing devices, respectively termed GCAPs and CD-GCAPs. Neurocalcin delta is a CD-GCAP. In the present study, the neurocalcin delta-modulated site, V(837)-L(858), in ROS-GC1 has been mapped. The location and properties of this site are unique. It resides within the core domain of the catalytic module and does not require the alpha-helical dimerization domain structural element (amino acids 767-811) for activating the catalytic module. Contrary to the current beliefs, the catalytic module is intrinsically active; it is directly regulated by the neurocalcin delta-modulated Ca(2+) signal and is dimeric in nature. A fold recognition based model of the catalytic domain of ROS-GC1 was built, and neurocalcin delta docking simulations were carried out to define the three-dimensional features of the interacting domains of the two molecules. These findings define a new transduction model for the Ca(2+) signaling of ROS-GC1.

Activation of retinal guanylyl cyclase RetGC1 by GCAP1: stoichiometry of binding and effect of new LCA-related mutations

Retinal membrane guanylyl cyclase (RetGC) and Ca(2+)/Mg(2+) sensor proteins (GCAPs) control the recovery of the photoresponse in vertebrate photoreceptors, through their molecular interactions that remain rather poorly understood and controversial. Here we have determined the main RetGC isozyme (RetGC1):GCAP1 binding stoichiometry at saturation in cyto, using fluorescently labeled RetGC1 and GCAP1 coexpressed in HEK293 cells. In a striking manner, the equimolar binding of RetGC1 with GCAP1 in transfected HEK293 cells typical for wild-type RetGC1 was eliminated by a substitution, D639Y, in the kinase homology domain of RetGC1 found in a patient with a severe form of retinal dystrophy, Leber congenital amaurosis (LCA). A similar effect was observed with another LCA-related mutation, R768W, in the same domain of RetGC1. In contrast to the completely suppressed binding and activation of RetGC1 by Mg(2+)-liganded GCAP1, neither of these two mutations eliminated the GCAP1-independent activity of RetGC stimulated by Mn(2+). These results directly implicate the D639 (and possibly R768)-containing portion of the RetGC1 kinase homology domain in its primary recognition by the Mg(2+)-bound activator form of GCAP1.

Membrane guanylate cyclase is a beautiful signal transduction machine: overview

This article is a sequel to the four earlier comprehensive reviews which covered the field of membrane guanylate cyclase from its origin to the year 2002 (Sharma in Mol Cell Biochem 230:3-30, 2002) and then to the year 2004 (Duda et al. in Peptides 26:969-984, 2005); and of the Ca(2+)-modulated membrane guanylate cyclase to the year 1997 (Pugh et al. in Biosci Rep 17:429-473, 1997) and then to 2004 (Sharma et al. in Curr Top Biochem Res 6:111-144, 2004). This article contains three parts. The first part is "Historical"; it is brief, general, and freely borrowed from the earlier reviews, covering the field from its origin to the year 2004 (Sharma in Mol Cell Biochem, 230:3-30, 2002; Duda et al. in Peptides 26:969-984, 2005). The second part focuses on the "Ca(2+)-modulated ROS-GC membrane guanylate cyclase subfamily". It is divided into two sections. Section "Historical" and covers the area from its inception to the year 2004. It is also freely borrowed from an earlier review (Sharma et al. in Curr Top Biochem Res 6:111-144, 2004). Section "Ca(2+)-modulated ROS-GC membrane guanylate cyclase subfamily" covers the area from the year 2004 to May 2009. The objective is to focus on the chronological development, recognize major contributions of the original investigators, correct misplaced facts, and project on the future trend of the field of mammalian membrane guanylate cyclase. The third portion covers the present status and concludes with future directions in the field.

Mg2+/Ca2+ cation binding cycle of guanylyl cyclase activating proteins (GCAPs): role in regulation of photoreceptor guanylyl cyclase

Photon absorption by photoreceptors activates hydrolysis of cGMP, which shuts down cGMP-gated channels and decreases free Ca(2+) concentrations in outer segment. Suppression of Ca(2+) influx through the cGMP channel by light activates retinal guanylyl cyclase through guanylyl cyclase activating proteins (GCAPs) and thus expedites photoreceptors recovery from excitation and restores their light sensitivity. GCAP1 and GCAP2, two ubiquitous among vertebrate species isoforms of GCAPs that activate retGC during rod response to light, are myristoylated Ca(2+)/Mg(2+)-binding proteins of the EF-hand superfamily. They consist of one non-metal binding EF-hand-like domain and three other EF-hands, each capable of binding Ca(2+) and Mg(2+). In the metal binding EF-hands of GCAP1, different point mutations can selectively block binding of Ca(2+) or both Ca(2+) and Mg(2+) altogether. Activation of retGC at low Ca(2+) (light adaptation) or its inhibition at high Ca(2+) (dark adaptation) follows a cycle of Ca(2+)/Mg(2+) exchange in GCAPs, rather than release of Ca(2+) and its binding by apo-GCAPs. The Mg(2+) binding in two of the EF-hands controls docking of GCAP1 with retGC1 in the conditions of light adaptation and is essential for activation of retGC. Mg(2+) binding in a C-terminal EF-hand contributes to neither retGC1 docking with the cyclase nor its subsequent activation in the light, but is specifically required for switching the cyclase off in the conditions of dark adaptation by binding Ca(2+). The Mg(2+)/Ca(2+) exchange in GCAP1 and 2 operates within different range of intracellular Ca(2+) concentrations and provides a two-step activation of the cyclase during rod recovery.

Ca(2+)-modulated vision-linked ROS-GC guanylate cyclase transduction machinery

Vertebrate phototransduction depends on the reciprocal relationship between two-second messengers, cyclic GMP and Ca(2+). The concentration of both is reciprocally regulated including the dynamic synthesis of cyclic GMP by a membrane bound guanylate cyclase. Different from hormone receptor guanylate cyclases, the cyclases operating in phototransduction are regulated by the intracellular Ca(2+)-concentration via small Ca(2+)-binding proteins. Based on the site of their expression and their Ca(2+) modulation, this sub-branch of the cyclase family was named sensory guanylate cyclases, of which the retina specific forms are named ROS-GCs (rod outer segment guanylate cyclases). This review focuses on the structure and function of the ROS-GC subfamily present in the mammalian retinal neurons: photoreceptors and inner layers of the retinal neurons. Portions and excerpts of the review are from a previous chapter (Curr Top Biochem Res 6:111-144, 2004).

A role for GCAP2 in regulating the photoresponse. Guanylyl cyclase activation and rod electrophysiology in GUCA1B knock-out mice

Cyclic GMP serves as the second messenger in visual transduction, linking photon absorption by rhodopsin to the activity of ion channels. Synthesis of cGMP in photoreceptors is supported by a pair of retina-specific guanylyl cyclases, retGC1 and -2. Two neuronal calcium sensors, GCAP1 and GCAP2, confer Ca(2+) sensitivity to guanylyl cyclase activity, but the importance and the contribution of each GCAP is controversial. To explore this issue, the gene GUCA1B, coding for GCAP2, was disrupted in mice, and the capacity for knock-out rods to regulate retGC and generate photoresponses was tested. The knock-out did not compromise rod viability or alter outer segment ultrastructure. Levels of retGC1, retGC2, and GCAP-1 expression did not undergo compensatory changes, but the absence of GCAP2 affected guanylyl cyclase activity in two ways; (a) the maximal rate of cGMP synthesis at low [Ca(2+)] dropped 2-fold and (b) the half-maximal rate of cGMP synthesis was attained at a higher than normal [Ca(2+)]. The addition of an antibody raised against mouse GCAP2 produced similar effects on the guanylyl cyclase activity in wild type retinas. Flash responses of GCAP2 knock-out rods recovered more slowly than normal. Knock-out rods became more sensitive to flashes and to steps of illumination but tended to saturate at lower intensities, as compared with wild type rods. Therefore, GCAP2 regulation of guanylyl cyclase activity quickens the recovery of flash and step responses and adjusts the operating range of rods to higher intensities of ambient illumination.

Binding of guanylyl cyclase activating protein 1 (GCAP1) to retinal guanylyl cyclase (RetGC1). The role of individual EF-hands

Guanylyl cyclase activating protein 1 (GCAP1), after substitution of Ca(2+) by Mg(2+) in its EF-hands, stimulates photoreceptor guanylyl cyclase, RetGC1, in response to light. We inactivated metal binding in individual EF-hands of GCAP1 tagged with green fluorescent protein to assess their role in GCAP1 binding to RetGC1 in co-transfected HEK293 cells. When expressed alone, GCAP1 was uniformly distributed throughout the cytoplasm and the nuclei of the cells, but when co-expressed with either fluorescently tagged or non-tagged RetGC1, it co-localized with the cyclase in the membranes. The co-localization did not occur when the C-terminal portion of RetGC1, containing its regulatory and catalytic domains, was removed. Mutations that preserved Mg(2+) binding in all three metal-binding EF-hands did not affect GCAP1 association with the cyclase in live cells. Locking EF-hand 4 in its apo-conformation, incapable of binding either Ca(2+) or Mg(2+), had no effect on GCAP1 association with the cyclase. In contrast to EF-hand 4, inactivation of EF-hand 3 reduced the efficiency of the co-localization, and inactivation of EF-hand 2 drastically suppressed GCAP1 binding to the cyclase. These results directly demonstrate that metal binding in EF-hand 2 is crucial for GCAP1 attachment to RetGC1, and that in EF-hand 3 it is less critical, although it enhances the efficiency of the GCAP1 docking on the target enzyme. Metal binding in EF-hand 4 has no role in the primary attachment of GCAP1 to the cyclase, and it only triggers the activator-to-inhibitor functional switch in GCAP1.

Calcium-dependent conformational changes in guanylate cyclase-activating protein 2 monitored by cysteine accessibility

Guanylate cyclase-activating protein 2 (GCAP2) is expressed in vertebrate photoreceptors cells where it regulates the activity of membrane bound guanylate cyclases in a Ca(2+)-dependent manner. The essential trigger step involves a Ca(2+)-induced conformational change in GCAP2. We investigated these Ca(2+)-dependent changes by probing the cysteine accessibility in wild type and mutant GCAP2 forms with the thiol-modifying reagent 5,5'-dithio-bis-(2-nitrobenzoic acid) (DTNB). Cysteine residues in position 35 and 111 displayed a restricted accessibility in the presence of Ca(2+), whereas cysteine in position 131 reacted with DTNB in the presence and absence of Ca(2+). Our data indicate that the Ca(2+)-sensitivity of GCAP2 is significantly controlled by its third Ca(2+)-binding site, EF-hand 3.

Novel frequenin-modulated Ca2+-signaling membrane guanylate cyclase (ROS-GC) transduction pathway in bovine hippocampus

Frequenin is a member of the neuronal Ca(2+) sensor protein family, implicated in being the modulator of the neurotransmitter release, potassium channels, phosphatidylinositol signaling pathway and the Ca(2+)-dependent exocytosis of dense-core granules in the PC12 cells. Frequenin exhibits these biological activities through its Ca(2+) myristoyl switch, yet the switch is functionally inactive. These structural and functional traits of frequenin have been derived through the use of recombinant frequenin. In the present study, frequenin (BovFrq) native to the bovine hippocampus has been purified, sequenced for its 9 internal fragments, cloned, and studied. The findings show that structure of the BovFrq is identical to its form present in chicken, rat, mouse and human, indicating its evolutionary conservation. Its Ca(2+) myristoyl switch is active in the hippocampus. And, BovFrq physically interacts and turns on yet undisclosed ONE-GC-like ROS-GC membrane guanylate cyclase transduction machinery in the hippocampal neurons. This makes BovFrq a new Ca(2+)-sensor modulator of a novel ROS-GC transduction machinery. The study demonstrates the presence and mechanistic features of this cyclic GMP signaling pathway in the hippocampal neurons, and also provides one more support for the evolving concept where the Ca(2+)-modulated membrane guanylate cyclase transduction machinery in its variant forms is a central operational component of all neurons.

Survey of the year 2005 commercial optical biosensor literature

We identified 1113 articles (103 reviews, 1010 primary research articles) published in 2005 that describe experiments performed using commercially available optical biosensors. While this number of publications is impressive, we find that the quality of the biosensor work in these articles is often pretty poor. It is a little disappointing that there appears to be only a small set of researchers who know how to properly perform, analyze, and present biosensor data. To help focus the field, we spotlight work published by 10 research groups that exemplify the quality of data one should expect to see from a biosensor experiment. Also, in an effort to raise awareness of the common problems in the biosensor field, we provide side-by-side examples of good and bad data sets from the 2005 literature.