Diversity of sensory guanylate cyclases in teleost fishes

Incorporating phototransduction proteins in zebrafish green cone with pressure-polished patch pipettes

The neuronal Ca²⁺-sensor guanylate cyclase-activating protein 3 (zGCAP3) is a major regulator of guanylate cyclase (GC) activity expressed in zebrafish cone cells. Here, the zGCAP3, or a monoclonal antibody directed against zGCAP3, was injected in the cone cytoplasm by employing the pressure-polished pipette technique. This technique allows to perform "real time" zGCAP3 (or of any other phototransduction protein) over-expression or knock-down, respectively, via the patch pipette. Photoresponses were not affected by purified zGCAP3, indicating that GC was already saturated with endogenous zGCAP3. The cytosolic injection of anti-zGCAP3 produced the slowing down kinetics of the flash response recovery, as theoretically expected by a minimal phototransduction model considering the antibody acting exclusively on the maximal GC activation by low Ca²⁺. However, the antibody produced a progressive current decay toward the zero level, as if the antibody affected also the basal GC activity in the dark.

Microscopic and Submicroscopic Gradient Variation of Olfactory Systems among Six Sinocyclocheilus Species Living in Different Environments

The fish genus Sinocyclocheilus contains many different species that inhabit diverse natural environments, such as surface water layer, cave, or intermediate. As a result of these different habitats there are some differences in their sensory systems. Microscopic and submicroscopic structures of olfactory systems in six representative species of Sinocyclocheilus were studied, including one surface-dwelling species (S. grahami), two intermediate species (S. jii and S. macrophthalmus) and three cave-dwelling species (S. brevibarbatus, S. anshuiensis, and S. tianlinensis). Due to adaptive evolution under extreme environmental conditions, cave-dwelling species have more developed olfactory systems. We observed that, compared with surface-dwelling species, the olfactory sac of the cave-dwelling Sinocyclocheilus species has the following characteristics: higher density of cilia, greater length of sensory cilia, many other special structures (micro-ridge, olfactory islet, rod cilia). These results reveal different levels of olfactory system development, consistent with the view that that cave-dwelling species have more developed olfactory systems than intermediate and surface-dwelling species.

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.

Zebrafish guanylate cyclase type 3 signaling in cone photoreceptors

The zebrafish guanylate cyclase type 3 (zGC3) is specifically expressed in cone cells. A specifc antibody directed against zGC3 revealed expression at the protein level at 3.5 dpf in outer and inner retinal layers, which increased in intensity between 3.5 and 7 dpf. This expression pattern differed from sections of the adult retina showing strong immunostaining in outer segments of double cones and short single cones, less intense immunoreactivity in long single cones, but no staining in the inner retina. Although transcription and protein expression levels of zGC3 are similar to that of the cyclase regulator guanylate cyclase-activating protein 3 (zGCAP3), we surprisingly found that zGCAP3 is present in a 28-fold molar excess over zGC3 in zebrafish retinae. Further, zGCAP3 was an efficient regulator of guanylate cyclases activity in native zebrafish retinal membrane preparations. Therefore, we investigated the physiological function of zGCAP3 by two different behavioral assays. Using the morpholino antisense technique, we knocked down expression of zGCAP3 and recorded the optokinetic and optomotor responses of morphants, control morphants, and wild type fish at 5-6 dpf. No significant differences in behavioral responses among wild type, morphants and control morphants were found, indicating that a loss of zGCAP3 has no consequences in primary visual processing in the larval retina despite its prominent expression pattern. Its physiological function is therefore compensated by other zGCAP isoforms.

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.

The guanylate cyclase signaling system in zebrafish photoreceptors

Zebrafish express in the retina a large variety of three different membrane-bound guanylate cyclases and six different guanylate cyclase-activating proteins (zGCAPs) belonging to the family of neuronal calcium sensor proteins. Although these proteins are predominantly localized in rod and cone photoreceptor cells of the retina, they differ in their spatial-temporal expression profiles. Further, each zGCAP has a different affinity for Ca(2+) and displays different Ca(2+)-sensitivities of guanylate cyclase activation. Thus, zGCAPs operate as cytoplasmic Ca(2+)-sensors that sense incremental changes of cytoplasmic Ca(2+)-concentration in rod and cone cells and control the activity of their target guanylate cyclases in a Ca(2+)-relay mode fashion.

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.

Differential calcium signaling by cone specific guanylate cyclase-activating proteins from the zebrafish retina

Zebrafish express in their retina a higher number of guanylate cyclase-activating proteins (zGCAPs) than mammalians pointing to more complex guanylate cyclase signaling systems. All six zGCAP isoforms show distinct and partial overlapping expression profiles in rods and cones. We determined critical Ca²⁺-dependent parameters of their functional properties using purified zGCAPs after heterologous expression in E.coli. Isoforms 1–4 were strong, 5 and 7 were weak activators of membrane bound guanylate cyclase. They further displayed different Ca²⁺-sensitivities of guanylate cyclase activation, which is half maximal either at a free Ca²⁺ around 30 nM (zGCAP1, 2 and 3) or around 400 nM (zGCAP4, 5 and 7). Zebrafish GCAP isoforms showed also differences in their Ca²⁺/Mg²⁺-dependent conformational changes and in the Ca²⁺-dependent monomer-dimer equilibrium. Direct Ca²⁺-binding revealed that all zGCAPs bound at least three Ca²⁺. The corresponding apparent affinity constants reflect binding of Ca²⁺ with high (≤100 nM), medium (0.1–5 µM) and/or low (≥5 µM) affinity, but were unique for each zGCAP isoform. Our data indicate a Ca²⁺-sensor system in zebrafish rod and cone cells supporting a Ca²⁺-relay model of differential zGCAP operation in these cells.

Ca(2+) sensor GCAP1: A constitutive element of the ONE-GC-modulated odorant signal transduction pathway

In a small subset of the olfactory sensory neurons, the odorant receptor ONE-GC guanylate cyclase is a central transduction component of the cyclic GMP signaling pathway. In a two-step transduction model, the odorant, uroguanylin, binds to the extracellular domain and activates its intracellular domain to generate the odorant second messenger, cyclic GMP. This study via comprehensive technology, including gene deletion, live cell Forster resonance energy transfer (FRET), and surface plasmon resonance (SPR) spectroscopy, documents the identity of a remarkably intriguing operation of a Ca(2+) sensor component of the ONE-GC transduction machinery, GCAP1. In the ciliary membranes, the sites of odorant transduction, GCAP1 is biochemically and physiologically coupled to ONE-GC. Strikingly, this coupling reverses its well- established function in ROS-GC1 signaling, linked with phototransduction. In response to the free Ca(2+) range from nanomolar to semimicromolar, it inhibits ROS-GC1, yet in this range, it incrementally stimulates ONE-GC. These two opposite modes of signaling two SENSORY processes by a single Ca(2+) sensor define a new transduction paradigm of membrane guanylate cyclases. This paradigm is pictorially presented.