Functional consequences of a rod outer segment membrane guanylate cyclase (ROS-GC1) gene mutation linked with Leber's congenital amaurosis

A natural history study of autosomal dominant GUCY2D-associated cone-rod dystrophy

PURPOSE

To describe the natural history of autosomal dominant (AD) GUCY2D-associated cone-rod dystrophies (CRDs), and evaluate associated structural and functional biomarkers.

METHODS

Retrospective analysis was conducted on 16 patients with AD GUCY2D-CRDs across two sites. Assessments included central macular thickness (CMT) and length of disruption to the ellipsoid zone (EZ) via optical coherence tomography (OCT), electroretinography (ERG) parameters, best corrected visual acuity (BCVA), and fundus autofluorescence (FAF).

RESULTS

At first visit, with a mean age of 30 years (range 5-70 years), 12 patients had a BCVA below Australian driving standard (LogMAR ≥ 0.3 bilaterally), and 1 patient was legally blind (LogMAR ≥ 1). Longitudinal analysis demonstrated a deterioration of LogMAR by - 0.019 per year (p < 0.001). This accompanied a reduction in CMT of - 1.4 µm per year (p < 0.0001), lengthened EZ disruption by 42 µm per year (p =  < 0.0001) and increased area of FAF by 0.05 mm2 per year (p = 0.027). Similarly, cone function decreased with increasing age, as demonstrated by decreasing b-wave amplitude of the light-adapted 30 Hz flicker and fused flicker (p = 0.005 and p = 0.018, respectively). Reduction in CMT and increased EZ disruption on OCT were associated with functional changes including poorer BCVA and decreased cone function on ERG.

CONCLUSION

We have described the natural long-term decline in vision and cone function associated with mutations in GUCY2D and identified a set of functional and structural biomarkers that may be useful as outcome parameters for future therapeutic clinical trials.

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.

First 3D-Structural Data of Full-Length Guanylyl Cyclase 1 in Rod-Outer-Segment Preparations of Bovine Retina by Cross-Linking/Mass Spectrometry

The rod-outer-segment guanylyl cyclase 1 (ROS-GC1) is a key transmembrane protein for retinal phototransduction. Mutations of ROS-GC1 correlate with different retinal diseases that often lead to blindness. No structural data are available for ROS-GC1 so far. We performed a 3D-structural analysis of native ROS-GC1 from bovine retina by cross-linking/mass spectrometry (XL-MS) and computational modeling. Absolute quantification and activity measurements of native ROS-GC1 were performed by MS-based assays directly in bovine retina samples. Our data present the first 3D-structural analysis of active, full-length ROS-GC1 derived from bovine retina. We propose a novel domain organization for the intracellular domain ROS-GC1. Our XL-MS data of native ROS-GC1 from rod-outer-segment preparations of bovine retina agree with a dimeric architecture. Our integrated approach can serve as a blueprint for conducting 3D-structural studies of membrane proteins in their native environment.

Novel GUCY2D Variant (E843Q) at Mutation Hotspot Associated with Macular Dystrophy in a Japanese Patient

BACKGROUND

The GUCY2D (guanylate cyclase 2D) gene encodes a photoreceptor guanylate cyclase (GC-E), that is predominantly expressed in the cone outer segments. Mutations in the GUCY2D lead to severe retinal disorders such as autosomal dominant cone-rod dystrophy (adCRD) and autosomal recessive Leber congenital amaurosis type 1. The purpose of this study was to identify the phenotype of a Japanese patient with a probably pathogenic GUCY2D variant.

METHODS

Detailed ophthalmic examinations were performed, and whole exome sequencing was performed on DNA obtained from the patient. The variants identified by exome sequencing and targeted analysis were further confirmed by direct sequencing.

RESULTS

A 47-year-old man had atrophic and pigmentary changes in the macula of both eyes. Amplitudes and implicit times on full-field electroretinograms (ERGs) were within normal limits; however, the densities of multifocal ERGs in the central area were reduced in both eyes. Whole exome sequencing identified heterozygous variant c.2527G>C, p.Glu843Gln in the GUCY2D gene within the mutation hot spot for adCRD. The allelic frequencies of this variant are extremely low and, according to American College of Medical Genetics and Genomics standards and guidelines, the variants are classified as likely pathogenic.

CONCLUSIONS

This is the first report of a heterozygous variant, c.2527G>C, p.Glu843Gln, in the GUCY2D, in a patient presenting with mild macular dystrophy without a general reduction in cone function. Our findings expand the spectrum of the clinical phenotypes of GUCY2D-adCRD and help clarify the morphological and functional changes caused by defects of dimerization of GC-E in the phototransduction cascade.

GUCY2D-Associated Leber Congenital Amaurosis: A Retrospective Natural History Study in Preparation for Trials of Novel Therapies

PURPOSE

To describe the natural history of Leber congenital amaurosis (LCA) associated with GUCY2D variants (GUCY2D-LCA) in a cohort of children and adults, in preparation for trials of novel therapies.

DESIGN

Retrospective case series.

METHODS

Participants: Patients with GUCY2D-LCA at a single referral center.

PROCEDURES

Review of clinical notes, retinal imaging including fundus autofluorescence (FAF) and optical coherence tomography (OCT), electroretinography (ERG), and molecular genetic testing.

MAIN OUTCOME MEASURES

Demographic data, symptoms at presentation, visual acuity, evidence of progression, OCT and FAF findings, ERG assessment, and molecular genetics.

RESULTS

Twenty-one subjects with GUCY2D-LCA were included, with a mean follow-up ± standard deviation (SD) of 10 ± 11.85 years. Marked reduction in visual acuity (VA) and nystagmus was documented in all patients within the first 3 years of life. Fifty-seven percent (n = 12) exhibited photophobia and 38% (n = 8) had nyctalopia. VA was worse than hand motion in 71% of the patients (n = 15). Longitudinal assessment of VA showed stability in all patients, except 1 patient who experienced deterioration over a follow-up of 44 years. Hyperopia was reported in 13 of the 17 subjects (71%) with available refraction data. Eighteen subjects had either normal fundus appearance (n = 14) or a blond fundus (n = 3), while only 4 of the eldest subjects had mild retinal pigment epithelium (RPE) atrophy (mean, 49 years; range 40-54 years). OCT data were available for 11 subjects and 4 different grades of ellipsoid zone (EZ) integrity were identified: (1) continuous/intact EZ (n = 6), (2) focally disrupted EZ (n = 2), (3) focally disrupted with RPE changes (n = 2), and (4) diffuse EZ disruption with RPE changes (n = 1). All examined subjects had stable OCT findings over the long follow-up period. Full-field ERGs showed evidence of a severe cone-rod dystrophy in 5 of 6 patients and undetectable ERGs in 1 subject. Novel genotype-phenotype correlations are also reported.

CONCLUSION

GUCY2D-LCA is a severe early-onset retinal dystrophy associated with very poor VA from birth. Despite the severely affected photoreceptor function, the relatively preserved photoreceptor structure based on EZ integrity until late in the disease in the majority of subjects suggests a wide therapeutic window for gene therapy trials.

Experimental Approaches for Defining the Role of the Ca2+-Modulated ROS-GC System in Retinal Rods of Mouse

Our ability to see is based on the activity of retinal rod and cone photoreceptors. Rods function when there is very little light, while cones operate at higher light levels. Photon absorption by rhodopsin activates a biochemical cascade that converts photic energy into a change in the membrane potential of the cell by decreasing the levels of a second messenger, cGMP, that control the gating of cation channels. But just as important as the activation of the cascade are the shut-off and recovery processes. The timing of shutoff and recovery ultimately affects sensitivity, temporal resolution and even the capacity for counting single photons. An important part of the recovery is restoration of cGMP through the action of rod outer segment membrane guanylate cyclases (ROS-GCs) and guanylate cyclase-activating proteins (GCAPs). In darkness, ROS-GCs catalyze the conversion of GTP to cGMP at a low rate, due to inhibition of cyclase activity by GCAPs. In the light, GCAP enhances ROS-GC activity. Mutations in the ROS-GC system can cause problems in vision, and even result in blindness due to photoreceptor death. The mouse has emerged as a particularly useful subject to study the role of ROS-GC because the technology for the manipulation of their genetics is advanced, making production of mice with targeted mutations much easier. Here we describe some experimental procedures for studying the retinal rods of wild-type and genetically engineered mice: biochemical assays of ROS-GC activity, immunohistochemistry, and single cell recording.

Ca2+-Sensor Neurocalcin δ and Hormone ANF Modulate ANF-RGC Activity by Diverse Pathways: Role of the Signaling Helix Domain

Prototype member of the membrane guanylate cyclase family, ANF-RGC (Atrial Natriuretic Factor Receptor Guanylate Cyclase), is the physiological signal transducer of two most hypotensive hormones ANF and BNP, and of the intracellular free Ca²⁺. Both the hormonal and the Ca²⁺-modulated signals operate through a common second messenger, cyclic GMP; yet, their operational modes are divergent. The hormonal pathways originate at the extracellular domain of the guanylate cyclase; and through a cascade of structural changes in its successive domains activate the C-terminal catalytic domain (CCD). In contrast, the Ca²⁺ signal operating via its sensor, myristoylated neurocalcin δ both originates and is translated directly at the CCD. Through a detailed sequential deletion and expression analyses, the present study examines the role of the signaling helix domain (SHD) in these two transduction pathways. SHD is a conserved 35-amino acid helical region of the guanylate cyclase, composed of five heptads, each meant to tune and transmit the hormonal signals to the CCD for their translation and generation of cyclic GMP. Its structure is homo-dimeric and the molecular docking analyses point out to the possibility of antiparallel arrangement of the helices. Contrary to the hormonal signaling, SHD has no role in regulation of the Ca²⁺- modulated pathway. The findings establish and define in molecular terms the presence of two distinct non-overlapping transduction modes of ANF-RGC, and for the first time demonstrate how differently they operate, and, yet generate cyclic GMP utilizing common CCD machinery.

Photoreceptor Guanylate Cyclase (GUCY2D) Mutations Cause Retinal Dystrophies by Severe Malfunction of Ca2+-Dependent Cyclic GMP Synthesis

Over 100 mutations in GUCY2D that encodes the photoreceptor guanylate cyclase GC-E are known to cause two major diseases: autosomal recessive Leber congenital amaurosis (arLCA) or autosomal dominant cone-rod dystrophy (adCRD) with a poorly understood mechanism at the molecular level in most cases. Only few mutations were further characterized for their enzymatic and molecular properties. GC-E activity is under control of neuronal Ca²⁺-sensor proteins, which is often a possible route to dysfunction. We investigated five recently-identified GC-E mutants that have been reported in patients suffering from arLCA (one large family) and adCRD/maculopathy (four families). Microsatellite analysis revealed that one of the mutations, c.2538G > C (p.K846N), occurred de novo. To better understand the mechanism by which mutations that are located in different GC-E domains develop different phenotypes, we investigated the molecular consequences of these mutations by expressing wildtype and mutant GC-E variants in HEK293 cells. Analyzing their general enzymatic behavior, their regulation by Ca²⁺ sensor proteins and retinal degeneration protein 3 (RD3) dimerization domain mutants (p.E841K and p.K846N) showed a shift in Ca²⁺-sensitive regulation by guanylate cyclase-activating proteins (GCAPs). Mutations in the cyclase catalytic domain led to a loss of enzyme function in the mutant p.P873R, but not in p.V902L. Instead, the p.V902L mutation increased the guanylate cyclase activity more than 20-fold showing a high GCAP independent activity and leading to a constitutively active mutant. This is the first mutation to be described affecting the GC-E catalytic core in a complete opposite way.

CO2/bicarbonate modulates cone photoreceptor ROS-GC1 and restores its CORD6-linked catalytic activity

This study with recombinant reconstituted system mimicking the cellular conditions of the native cones documents that photoreceptor ROS-GC1 is modulated by gaseous CO₂. Mechanistically, CO₂ is sensed by carbonic anhydrase (CAII), generates bicarbonate that, in turn, directly targets the core catalytic domain of ROS-GC1, and activates it to increased synthesis of cyclic GMP. This, then, functions as a second messenger for the cone phototransduction. The study demonstrates that, in contrast to the Ca²⁺-modulated phototransduction, the CO₂ pathway is Ca²⁺-independent, yet is linked with it and synergizes it. It, through R⁷⁸⁷C mutation in the third heptad of the signal helix domain of ROS-GC1, affects cone-rod dystrophy, CORD6. CORD6 is caused firstly by lowered basal and GCAP1-dependent ROS-GC1 activity and secondly, by a shift in Ca²⁺ sensitivity of the ROS-GC1/GCAP1 complex that remains active in darkness. Remarkably, the first but not the second defect disappears with bicarbonate thus explaining the basis for CORD6 pathological severity. Because cones, but not rods, express CAII, the excessive synthesis of cyclic GMP would be most acute in cones.

The Human Odontoblast Cell Layer and Dental Pulp Proteomes and N-Terminomes

The proteome and N-terminome of the human odontoblast cell layer were identified for the first time by shotgun proteomic and terminal amine isotopic labeling of substrates (TAILS) N-terminomic analyses, respectively, and compared with that of human dental pulp stroma from 26 third molar teeth. After reverse-phase liquid chromatography-tandem mass spectrometry, >170,000 spectra from the shotgun and TAILS analyses were matched by 4 search engines to 4,888 and 12,063 peptides in the odontoblast cell layer and pulp stroma, respectively. Within these peptide groups, 1,543 and 5,841 protein N-termini, as well as 895 and 2,423 unique proteins, were identified with a false discovery rate of ≤1%. Thus, the human dental pulp proteome was expanded by 974 proteins not previously identified among the 4,123 proteins in our 2015 dental pulp study. Further, 222 proteins of the odontoblast cell layer were not found in the pulp stroma, suggesting many of these proteins are synthesized only by odontoblasts. When comparing the proteomes of older and younger donors, differences were more apparent in the odontoblast cell layer than in the dental pulp stroma. In the odontoblast cell layer proteome, we found proteomic evidence for dentin sialophosphoprotein, which is cleaved into dentin sialoprotein and dentin phosphoprotein. By exploring the proteome of the odontoblast cell layer and expanding the known dental pulp proteome, we found distinct proteome differences compared with each other and with dentin. Moreover, between 61% and 66% of proteins also occurred as proteoforms commencing with a neo-N-terminus not annotated in UniProt. Hence, TAILS increased proteome coverage and revealed considerable proteolytic processing, by identifying stable proteoforms in these dynamic dental tissues. All mass spectrometry raw data have been deposited to ProteomeXchange with the identifier <PXD006557>, with the accompanying metadata at Mendeley Data ( https://data.mendeley.com/datasets/b57zfh6wmy/1 ).

Leber congenital amaurosis, from darkness to light: An ode to Irene Maumenee

This article is dedicated to Irene Hussels Maumenee, Professor of Human Genetics and Ophthalmology, Johns Hopkins' Wilmer Eye Institute, Ocular Genetics Fellowship director in 1994-1995. Leber congenital amaurosis (LCA) has almost come full circle, from a profound and molecularly uncharacterized form of congenital retinal blindness to one in which a large number of causative genes and disease pathways are known, and the world's first human retinal disease to be treated by gene therapy. Dr. Maumenee's insights, efforts, and leadership have contributed significantly to this remarkable scientific journey. In this manuscript, we present a short summary of the known LCA genes, LCA disease subtypes, and emerging treatment options. Our manuscript consolidates previous knowledge with current findings in an attempt to provide a more comprehensive understanding of LCA.

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).

Electrophoretic mobility shift in native gels indicates calcium-dependent structural changes of neuronal calcium sensor proteins

In proteins of the neuronal calcium sensor (NCS) family, changes in structure as well as function are brought about by the binding of calcium. In this article, we demonstrate that these structural changes, solely due to calcium binding, can be assessed through electrophoresis in native gels. The results demonstrate that the NCS proteins undergo ligand-dependent conformational changes that are detectable in native gels as a gradual decrease in mobility with increasing calcium but not other tested divalent cations such as magnesium, strontium, and barium. Surprisingly, such a gradual change over the entire tested range is exhibited only by the NCS proteins but not by other tested calcium-binding proteins such as calmodulin and S100B, indicating that the change in mobility may be linked to a unique NCS family feature--the calcium-myristoyl switch. Even within the NCS family, the changes in mobility are characteristic of the protein, indicating that the technique is sensitive to the individual features of the protein. Thus, electrophoretic mobility on native gels provides a simple and elegant method to investigate calcium (small ligand)-induced structural changes at least in the superfamily of NCS proteins.

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.

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.

Impaired association of retinal degeneration-3 with guanylate cyclase-1 and guanylate cyclase-activating protein-1 leads to leber congenital amaurosis-1

One-fifth of all cases of Leber congenital amaurosis are type 1 (LCA1). LCA1 is a severe form of retinal dystrophy caused by loss-of-function mutations in guanylate cyclase 1 (GC1), a key member of the phototransduction cascade involved in modulating the photocurrents. Although GC1 has been studied for some time, the mechanisms responsible for its regulation and membrane targeting are not fully understood. We reported earlier that retinal degeneration 3 (RD3) protein interacts with GC1 and promotes its targeting to the photoreceptor outer segments (POS). Here, we extend our studies to show a direct association between RD3 and guanylate cyclase activating protein 1 (GCAP1). Furthermore, we demonstrate that this functional interaction is important for GC1 targeting to POS. We also show that most LCA1-causing mutations in GC1 result in lost GC1 interaction with RD3 or GC1 being targeted to the plasma membrane. Our data suggest that GC1, GCAP1, and RD3 form a complex in the endoplasmic reticulum that targets GC1 to POS. Interruption of this assembly is likely the underlying mechanism for a subset of LCA1. This study offers insights for the development of therapeutic strategies to treat this severe form of blindness.

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.

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.

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.

The dimerization domain in outer segment guanylate cyclase is a Ca²⁺-sensitive control switch module

Membrane-bound guanylate cyclases harbor a region called the dimerization or linker domain, which aids the enzymes in adopting an optimal monomer-monomer arrangement for catalysis. One subgroup of these guanylate cyclases is expressed in rod and cone cells of vertebrate retina, and mutations in the dimerization domain of rod outer segment guanylate cyclase 1 (ROS-GC1, encoded by the GUCY2D gene) correlate with retinal cone-rod dystrophies. We investigate how a Q847L/K848Q double mutation, which was found in patients suffering from cone-rod dystrophy, and the Q847L and K848Q single-point mutations affect the regulatory mechanism of ROS-GC1. Both the wild type and mutants of heterologously expressed ROS-GC1 were present in membranes. However, the mutations affected the catalytic properties of ROS-GC1 in different manners. All mutants had higher basal guanylate cyclase activities but lower levels of activation by Ca²⁺-sensing guanylate cyclase-activating proteins (GCAPs). Further, incubation with wild-type GCAP1 and GCAP2 revealed for all ROS-GC1 mutants a shift in Ca²⁺ sensitivity, but activation of the K848Q mutant by GCAPs was severely impaired. Apparent affinities for GCAP1 and GCAP2 were different for the double mutant and the wild type. Circular dichroism spectra of the dimerization domain showed that the wild type and mutants adopt a prevalently α-helical structure, but mutants exhibited lower thermal stability. Our results indicate that the dimerization domain serves as a Ca²⁺-sensitive control module. Although it is per se not a Ca²⁺-sensing unit, it seems to integrate and process information regarding Ca²⁺ sensing by sensor proteins and regulator effector affinity.

Transgenic zebrafish expressing mutant human RETGC-1 exhibit aberrant cone and rod morphology

Cone-rod dystrophy 6 (CORD6) is an inherited blindness that presents with defective cone photoreceptor function in childhood, followed by loss of rod function. CORD6 results from mutations in GUCY2D, the human gene encoding retinal guanylate cyclase 1 (RETGC-1). RETGC-1 functions in phototransduction, synthesising cGMP to open ion channels in photoreceptor outer segments. As there is limited histopathological data on the CORD6 retina, our goal was to generate a CORD6 model by expressing mutant human RETGC-1 in zebrafish cone photoreceptors and to investigate effects on retinal morphology and function. cDNAs encoding wildtype and mutant (E837D R838S) RETGC-1 were cloned under the control of the cone-specific gnat2 promoter and microinjected into zebrafish embryos to generate transgenic lines. RETGC-1 mRNA expression in zebrafish eyes was confirmed by RT-PCR. Fluorescent microscopy analysed retinal morphology and visual behaviour was quantified by the optokinetic response (OKR). Stable transgenic lines expressing mutant or wildtype human RETGC-1 in zebrafish eyes were generated. OKR assays of 5-day-old larvae did not uncover any deficits in visual behaviour. However, transgenic (E837D R838S) RETGC-1 expression results in aberrant cone morphology and a reduced cone density. A reduction in the number of photoreceptor nuclei, the thickness of the outer nuclear layer and the labelling of rod outer segments, particularly in the central retina, was evident. Expression of mutant human RETGC-1 leads to a retinal phenotype that includes aberrant photoreceptor morphology and a reduced number of photoreceptors. This phenotype likely explains the compromised visual function, characteristic of CORD6.

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.

Antithetical modes of and the Ca(2+) sensors targeting in ANF-RGC and ROS-GC1 membrane guanylate cyclases

The membrane guanylate cyclase family has been branched into three subfamilies: natriuretic peptide hormone surface receptors, Ca²⁺-modulated neuronal ROS-GC, and Ca²⁺-modulated odorant surface receptor ONE-GC. The first subfamily is solely modulated by the extracellularly generated hormonal signals; the second, by the intracellularly generated sensory and sensory-linked signals; and the third, by combination of these two. The present study defines a new paradigm and a new mechanism of Ca²⁺ signaling. (1) It demonstrates for the first time that ANF-RGC, the prototype member of the surface receptor subfamily, is stimulated by free [Ca²⁺]_i. The stimulation occurs via myristoylated form of neurocalcin δ, and both the guanylate cyclase and the calcium sensor neurocalcin δ are present in the glomerulosa region of the adrenal gland. (2) The EF-2, EF-3 and EF-4 hands of GCAP1 sense the progressive increment of [Ca²⁺]_i and with a K_1/2 of 100 nM turn ROS-GC1 “OFF.” In total reversal, the same EF hands upon sensing the progressive increment of [Ca²⁺]_i with K_1/2 turn ONE-GC “ON.” The findings suggest a universal Ca²⁺-modulated signal transduction theme of the membrane guanylate cyclase family; demonstrate that signaling of ANF-RGC occurs by the peptide hormones and also by [Ca²⁺]_i signals; that for the Ca²⁺ signal transduction, ANF-RGC functions as a two-component transduction system consisting of the Ca²⁺ sensor neurocalcin δ and the transducer ANF-RGC; and that the neurocalcin δ in this case expands beyond its NCS family. Furthermore, the study shows a novel mechanism of the [Ca²⁺]_i sensor GCAP1 where it acts as an antithetical NCS for the signaling mechanisms of ROS-GC1 and ONE-GC.

Involvement of the calcium sensor GCAP1 in hereditary cone dystrophies

Progressive visual impairment leading to blindness is often associated with inherited retinal dystrophies. These disorders correlate in most cases with mutations in genes that code for proteins of the visual transduction system in rod and cone photoreceptor cells. Recent progress has highlighted the involvement of a neuronal calcium sensor protein that is specifically expressed in rod and cone cells and operates as a guanylate cyclase-activating protein (GCAP). A group of patients suffering from cone or cone-rod dystrophies carry mutations in the GCAP1 gene, and biochemical analysis of GCAP1 function revealed that for most of these mutations GCAP1 exhibits a disturbance in its Ca(2+)-sensing and its guanylate cyclase-activating properties. Cellular consequences of different GCAP1 mutations are compared and discussed.

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.

Novel functions of photoreceptor guanylate cyclases revealed by targeted deletion

Targeted deletion of membrane guanylate cyclases (GCs) has yielded new information concerning their function. Here, we summarize briefly recent results of laboratory generated non-photoreceptor GC knockouts characterized by complex phenotypes affecting the vasculature, heart, brain, kidney, and other tissues. The main emphasis of the review, however, addresses the two GCs expressed in retinal photoreceptors, termed GC-E and GC-F. Naturally occurring GC-E (GUCY2D) null alleles in human and chicken are associated with an early onset blinding disorder, termed "Leber congenital amaurosis type 1" (LCA-1), characterized by extinguished scotopic and photopic ERGs, and retina degeneration. In mouse, a GC-E null genotype produces a recessive cone dystrophy, while rods remain functional. Rod function is supported by the presence of GC-F (Gucy2f), a close relative of GC-E. Deletion of Gucy2f has very little effect on rod and cone physiology and survival. However, a GC-E/GC-F double knockout (GCdko) phenotypically resembles human LCA-1 with extinguished ERGs and rod/cone degeneration. In GCdko rods, PDE6 and GCAPs are absent in outer segments. In contrast, GC-E(-/-) cones lack proteins of the entire phototransduction cascade. These results suggest that GC-E may participate in transport of peripheral membrane proteins from the endoplasmic reticulum (ER) to the outer segments.

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.

ROS-GC subfamily membrane guanylate cyclase-linked transduction systems: taste, pineal gland and hippocampus

In the continuous efforts to test the validity of the theme that the Ca(2+)-modulated ROS-GC subfamily system is a universal transduction component of the sensory and sensory-linked network of neurons, this article focuses on the presence and variant biochemical forms of this transduction system in the gustatory epithelium, the site of gustatory transduction; in the pineal, a light-sensitive gland; and in the hippocampus neurons, linked with the perception of all SENSES.

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).

Guanylate cyclases and associated activator proteins in retinal disease

Two isoforms of guanylate cyclase, GC1 and GC2 encoded by GUCY2D and GUCY2F, are responsible for the replenishment of cGMP in photoreceptors after exposure to light. Both are required for the normal kinetics of photoreceptor sensitivity and recovery, although disease mutations are restricted to GUCY2D. Recessive mutations in this gene cause the severe early-onset blinding disorder Leber congenital amaurosis whereas dominant mutations result in a later onset less severe cone-rod dystrophy. Cyclase activity is regulated by Ca(2+) which binds to the GC-associated proteins, GCAP1 and GCAP2 encoded by GUCA1A and GUCA1B, respectively. No recessive mutations in either of these genes have been reported. Dominant missense mutations are largely confined to the Ca(2+)-binding EF hands of the proteins. In a similar fashion to the disease mechanism for the dominant GUCY2D mutations, these mutations generally alter the sensitivity of the cyclase to inhibition as Ca(2+) levels rise following a light flash.

Leber congenital amaurosis: genes, proteins and disease mechanisms

Leber congenital amaurosis (LCA) is the most severe retinal dystrophy causing blindness or severe visual impairment before the age of 1 year. Linkage analysis, homozygosity mapping and candidate gene analysis facilitated the identification of 14 genes mutated in patients with LCA and juvenile retinal degeneration, which together explain approximately 70% of the cases. Several of these genes have also been implicated in other non-syndromic or syndromic retinal diseases, such as retinitis pigmentosa and Joubert syndrome, respectively. CEP290 (15%), GUCY2D (12%), and CRB1 (10%) are the most frequently mutated LCA genes; one intronic CEP290 mutation (p.Cys998X) is found in approximately 20% of all LCA patients from north-western Europe, although this frequency is lower in other populations. Despite the large degree of genetic and allelic heterogeneity, it is possible to identify the causative mutations in approximately 55% of LCA patients by employing a microarray-based, allele-specific primer extension analysis of all known DNA variants. The LCA genes encode proteins with a wide variety of retinal functions, such as photoreceptor morphogenesis (CRB1, CRX), phototransduction (AIPL1, GUCY2D), vitamin A cycling (LRAT, RDH12, RPE65), guanine synthesis (IMPDH1), and outer segment phagocytosis (MERTK). Recently, several defects were identified that are likely to affect intra-photoreceptor ciliary transport processes (CEP290, LCA5, RPGRIP1, TULP1). As the eye represents an accessible and immune-privileged organ, it appears to be uniquely suitable for human gene replacement therapy. Rodent (Crb1, Lrat, Mertk, Rpe65, Rpgrip1), avian (Gucy2D) and canine (Rpe65) models for LCA and profound visual impairment have been successfully corrected employing adeno-associated virus or lentivirus-based gene therapy. Moreover, phase 1 clinical trials have been carried out in humans with RPE65 deficiencies. Apart from ethical considerations inherently linked to treating children, major obstacles for the treatment of LCA could be the putative developmental deficiencies in the visual cortex in persons blind from birth (amblyopia), the absence of sufficient numbers of viable photoreceptor or RPE cells in LCA patients, and the unknown and possibly toxic effects of overexpression of transduced genes. Future LCA research will focus on the identification of the remaining causal genes, the elucidation of the molecular mechanisms of disease in the retina, and the development of gene therapy approaches for different genetic subtypes of LCA.

Expression level and activity profile of membrane bound guanylate cyclase type 2 in rod outer segments

Rod and cone cells of the mammalian retina harbor two types of a membrane bound guanylate cyclase (GC), rod outer segment guanylate cyclase type 1 (ROS-GC1) and ROS-GC2. Both enzymes are regulated by small Ca(2+)-binding proteins named GC-activating proteins that operate as Ca2+ sensors and enable cyclases to respond to changes of intracellular Ca2+after illumination. We determined the expression level of ROS-GC2 in bovine ROS preparations and compared it with the level of ROS-GC1 in ROSs. The molar ratio of a ROS-GC2 dimer to rhodopsin was 1 : 13 200. The amount of ROS-GC1 was 25-fold higher than the amount of ROS-GC2. Heterologously expressed ROS-GC2 was differentially activated by GC-activating protein 1 and 2 at low free Ca2+ concentrations. Mutants of GC-activating protein 2 modulated ROS-GC2 in a manner different from their action on ROS-GC1 indicating that the Ca2+ sensitivity of the Ca2+ sensor is controlled by the mode of target-sensor interaction.

Leber congenital amaurosis: a genetic paradigm

Leber congenital amaurosis (LCA; estimated prevalence 1 : 50,000-100,000) is an early-onset inherited cause of childhood blindness characterized by a severe retinal dystrophy immediately after birth. Variants in at least six genes, AIPL1, CRB1, CRX, GUCY2D, RPE65, and RPGRIP1, have been associated with a diagnosis consistent with LCA or early-onset retinitis pigmentosa and together account for less than 50% of all LCA cases. Genetically heterogeneous inheritance has complicated the molecular analysis of LCA cases, especially sporadic ones where conventional methods are of limited value. Until recently, the management of patients with LCA relied mainly on clinical examination, electrophysiology, and other ancillary tests. Genotyping, i.e., determining the exact genetic defect causing LCA in each specific case, was not routinely performed since the comprehensive screening of six genes by SSCP and/or direct sequencing is relatively inefficient and cost-prohibitive. Patients, therefore, were often left with no specific information on their disease status. Recent advances in genotyping technologies have allowed the introduction of comprehensive and affordable screening procedures to determine causal genetic variation, resulting in precise molecular diagnosis, more accurate visual prognosis, and suggestions towards treatment options.

Reciprocal regulation and integration of signaling by intracellular calcium and cyclic GMP

Calcium and guanosine-3',5'-cyclic monophosphate (cGMP) are second messenger molecules that regulate opposing physiological functions, reflected in the reciprocal regulation of their intracellular concentrations, in many systems. Indeed, cGMP and Ca2+ constitute discrete points of integration between multiple cell signaling cascades in both convergent and parallel pathways. This chapter describes the molecular mechanisms regulating intracellular Ca2+ and cGMP, and their integration in specific cellular responses.

An overview of Leber congenital amaurosis: a model to understand human retinal development

Leber congenital amaurosis is a congenital retinal dystrophy described almost 150 years ago. Today, Leber congenital amaurosis is proving instrumental in our understanding of the molecular events that determine normal and aberrant retinal development. Six genes have been shown to be mutated in Leber congenital amaurosis, and they participate in a wide variety of retinal pathways: retinoid metabolism (RPE65), phototransduction (GUCY2D), photoreceptor outer segment development (CRX), disk morphogenesis (RPGRIP1), zonula adherens formation (CRB1), and cell-cycle progression (AIPL1). Longitudinal studies of visual performance show that most Leber congenital amaurosis patients remain stable, some deteriorate, and rare cases exhibit improvements. Histopathological analyses reveal that most cases have extensive degenerative retinal changes, some have an entirely normal retinal architecture, whereas others have primitive, poorly developed retinas. Animal models of Leber congenital amaurosis have greatly added to understanding the impact of the genetic defects on retinal cell death, and response to rescue. Gene therapy for RPE65 deficient dogs partially restored sight, and provides the first real hope of treatment for this devastating blinding condition.

Regulatory modes of rod outer segment membrane guanylate cyclase differ in catalytic efficiency and Ca(2+)-sensitivity

In rod phototransduction, cyclic GMP synthesis by membrane bound guanylate cyclase ROS-GC1 is under Ca(2+)-dependent negative feedback control mediated by guanylate cyclase-activating proteins, GCAP-1 and GCAP-2. The cellular concentration of GCAP-1 and GCAP-2 approximately sums to the cellular concentration of a functional ROS-GC1 dimer. Both GCAPs increase the catalytic efficiency (kcat/Km) of ROS-GC1. However, the presence of a myristoyl group in GCAP-1 has a strong impact on the regulation of ROS-GC1, this is in contrast to GCAP-2. Catalytic efficiency of ROS-GC1 increases 25-fold when it is reconstituted with myristoylated GCAP-1, but only by a factor of 3.4 with nonmyristoylated GCAP-1. In contrast to GCAP1, myristoylation of GCAP-2 has only a minor effect on kcat/Km. The increase with both myristoylated and nonmyristoylated GCAP-2 is 10 to 13-fold. GCAPs also confer different Ca(2+)-sensitivities to ROS-GC1. Activation of the cyclase by GCAP-1 is half-maximal at 707 nM free [Ca(2+)], while that by GCAP-2 is at 100 nM. The findings show that differences in catalytic efficiency and Ca(2+)-sensitivity of ROS-GC1 are conferred by GCAP-1 and GCAP-2. The results further indicate the concerted operation of two 'GCAP modes' that would extend the dynamic range of cyclase regulation within the physiological range of free cytoplasmic Ca(2+) in photoreceptor cells.

Guanylate cyclase activating proteins, guanylate cyclase and disease

A range of cone and cone-rod dystrophies (CORD) have been observed in man, caused by mutations in retinal guanylate cyclase 1 (RetGC1) and guanylate cyclase activating protein 1 (GCAP 1). The CORD causing mutations in RetGC1 are located at a mutation "hot spot" within the dimerisation domain, where R838 is the key residue. Three disease causing mutations have been found in human GCAP1, resulting in cone or cone-rod degeneration. All three mutations are dominant in their effect although the mechanism by which the P50L mutation exerts its influence remains unclear although it might act due to a haplo-insufficiency, arising from increased susceptibility to protease activity and increased thermal instability. In contrast, loss of Ca2+ sensitivity appears to be the main cause of the diseased state for the Y99C and E155G mutations. The cone and cone-rod dystrophies that are caused by mutations in RetGC1 or GCAP1 arise from a perturbation of the delicate balance of Ca2+ and cGMP within the photoreceptor cells and it is this disruption that is believed to cause cell death. The diseases caused by mutations in RetGC1 and GCAP1 prominently affect cones, consistent with the higher concentrations of these proteins in cone cells.

Target recognition of guanylate cyclase by guanylate cyclase-activating proteins

Guanylate cyclase-activating proteins (GCAPs) control the activity of membrane bound guanylate cyclases in vertebrate photoreceptor cells. They form a permanent complex with guanylate cyclase 1 (ROS-GC1) at low and high Ca2+-concentrations. Five different target regions of GCAP-1 have been identified in ROS-GC1 at rather distant sites. These findings could indicate a multipoint attachment site for GCAP-1 or, alternatively, the presence of transient binding sites with short contact to GCAP-1. In addition some data are consistent with the operation of one or more transducer units, that represent regulatory regions without being direct binding sites. A permanent ROS-GC1/GCAP-1 complex is physiologically significant, since it allows a very short response time of cyclase activity when the intracellular Ca2+-concentration changes. Thereby, activation of cyclase participates in speeding up the recovery of the photoresponse after illumination and restores the circulating dark current.

Identification of functional regions of guanylate cyclase-activating protein 1 (GCAP1) using GCAP1/GCIP chimeras

Guanylate cyclase-activating protein 1 (GCAP1) and guanylate cyclase-inhibitory protein (GCIP) are calmodulin-related Ca2+-binding proteins expressed in vertebrate photoreceptor cells. GCAP1 activates photoreceptor guanylate cyclase 1 (GC1) at low free [Ca2+] (<50 nM, in the light), but inhibits it at physiological high [Ca2+] (1 microM, in the dark). GCIP, a Ca2+-binding protein from frog retina, inhibits GC1 at approximately 1 microM [Ca2+], but is unable to stimulate cyclase at low [Ca2+]. In this study, we probed the interaction between GCAP1 and GC1 by producing GCAP1/GCIP chimeras and tested their capability to stimulate GC1. We prepared eight pairs of constructs in which the N-terminal portions of GCIP and GCAP1 were successively replaced by corresponding domains of GCAP1, and GCIP, respectively. The expressed proteins were purified and tested for stimulation of GC1 at 50 nM [Ca2+], and their ability to competitively inhibit GC1 stimulation by a Ca2+-insensitive GCAP1 mutant, GCAP1-tm, at high [Ca2+]. While all GCAP1/GCIP chimeras competitively inhibited GC1 stimulation at high [Ca2+] by GCAP1-tm, several of the GCIP/GCAP1 chimeras had no effect. A chimera consisting of residues 1-20 of GCIP and 21-205 of GCAP1 had no effect on GC1 at low [Ca2+], suggesting that the N-terminal region MGNIMDGKSVEELSSTECHQ, which has no sequence similarity to GCIP, is among the key components necessary for GC1 stimulation. A GCAP1/GCIP chimera consisting of residues 1-43 (including nonfunctional EF1) of GCAP1 and residues 56-206 of GCIP stimulated GC1 at low [Ca2+] and inhibited GC1 at high [Ca2+], suggesting that the essential components required to transform an inhibitory to an activating protein are contained within the N-terminal region of GCAP1 (residues 1-43).

The guanylyl cyclase family at Y2K

During the 1980s the purification, cloning, and expression of various forms of guanylyl cyclase (GC) revealed that they served as receptors for extracellular signals. Seven membrane forms, which presumably exist as homodimers, and four subunits of apparent heterodimers (commonly referred to as the soluble forms) are known, but in animals such as nematodes, much larger numbers of GCs are expressed. The number of transmembrane segments (none, one, or multiple) divide the GC family into three groups. Those with no or one transmembrane segment bind nitric oxide/carbon monoxide (NO/CO) or peptides. There are no known ligands for the multiple transmembrane segment class of GCs. Mutational and structural analyses support a model where catalysis requires a shared substrate binding site between the subunits, whether homomeric or heteromeric in nature. Because some cyclases or cyclase ligand genes lack specific GC inhibitors, disruption of either has been used to define the functions of individual cyclases, as well as to define human genetic disease counterparts.

Regions in vertebrate photoreceptor guanylyl cyclase ROS-GC1 involved in Ca(2+)-dependent regulation by guanylyl cyclase-activating protein GCAP-1

The membrane bound guanylyl cyclase (GC) photoreceptor membrane GC1 (ROS-GCI) of photoreceptor cells synthesizes cGMP, the intracellular transmitter of vertebrate phototransduction. The activity of ROS-GCI is controlled by small Ca(2+)-binding proteins, named GC-activating proteins (GCAPs). We identified and characterized two short regulatory regions (M445-L456 and L503-1522) in the juxtamembrane domain (JMD) of ROS-GC1 by peptide competition and mutagenesis studies. Both regions are critical for the activation of ROS-GCI by GCAP-1.