The function of guanylate cyclase 1 and guanylate cyclase 2 in rod and cone photoreceptors

Biology and therapy of inherited retinal degenerative disease: insights from mouse models

Retinal neurodegeneration associated with the dysfunction or death of photoreceptors is a major cause of incurable vision loss. Tremendous progress has been made over the last two decades in discovering genes and genetic defects that lead to retinal diseases. The primary focus has now shifted to uncovering disease mechanisms and designing treatment strategies, especially inspired by the successful application of gene therapy in some forms of congenital blindness in humans. Both spontaneous and laboratory-generated mouse mutants have been valuable for providing fundamental insights into normal retinal development and for deciphering disease pathology. Here, we provide a review of mouse models of human retinal degeneration, with a primary focus on diseases affecting photoreceptor function. We also describe models associated with retinal pigment epithelium dysfunction or synaptic abnormalities. Furthermore, we highlight the crucial role of mouse models in elucidating retinal and photoreceptor biology in health and disease, and in the assessment of novel therapeutic modalities, including gene- and stem-cell-based therapies, for retinal degenerative diseases.

Organization of cGMP sensing structures on the rod photoreceptor outer segment plasma membrane

A diffusion barrier segregates the plasma membrane of the rod photoreceptor outer segment into 2 domains; one which is optimized for the conductance of ions in the phototransduction cascade and another for disk membrane synthesis. We propose the former to be named "phototransductive plasma membrane domain," and the latter to be named "disk morphogenic plasma membrane domain." Within the phototransductive plasma membrane, cGMP-gated channels are concentrated in striated membrane features, which are proximally located to the sites of active cGMP production within the disk membranes. For proper localization of cGMP-gated channel to the phototransductive plasma membrane, the glutamic acid-rich protein domain encoded in the β subunit plays a critical role. Quantitative study suggests that the disk morphogenic domain likely plays an important role in enriching rhodopsin prior to its sequestration into closed disk membranes. Thus, this and our previous studies provide new insight into the mechanism that spatially organizes the vertebrate phototransduction cascade.

Gene Therapy Fully Restores Vision to the All-Cone Nrl(-/-) Gucy2e(-/-) Mouse Model of Leber Congenital Amaurosis-1

Mutations in GUCY2D are the cause of Leber congenital amaurosis type 1 (LCA1). GUCY2D encodes retinal guanylate cyclase-1 (retGC1), a protein expressed exclusively in outer segments of photoreceptors and essential for timely recovery from photoexcitation. Recent clinical data show that, despite a high degree of visual disturbance stemming from a loss of cone function, LCA1 patients retain normal photoreceptor architecture, except for foveal cone outer segment abnormalities and, in some patients, foveal cone loss. These results point to the cone-rich central retina as a target for GUCY2D replacement. LCA1 gene replacement studies thus far have been conducted in rod-dominant models (mouse) or with vectors and organisms lacking clinical translatability. Here we investigate gene replacement in the Nrl(-/-) Gucy2e(-/-) mouse, an all-cone model deficient in retGC1. We show that AAV-retGC1 treatment fully restores cone function, cone-mediated visual behavior, and guanylate cyclase activity, and preserves cones in treated Nrl(-/-) Gucy2e(-/-) mice over the long-term. A novel finding was that retinal function could be restored to levels above that in Nrl(-/-) controls, contrasting results in other models of retGC1 deficiency. We attribute this to increased cyclase activity in treated Nrl(-/-) Gucy2e(-/-) mice relative to Nrl(-/-) controls. Thus, Nrl(-/-) Gucy2e(-/-) mice possess an expanded dynamic range in ERG response to gene replacement relative to other models. Lastly, we show that a candidate clinical vector, AAV5-GRK1-GUCY2D, when delivered to adult Nrl(-/-) Gucy2e(-/-) mice, restores retinal function that persists for at least 6 months. Our results provide strong support for clinical application of a gene therapy targeted to the cone-rich, central retina of LCA1 patients.

Heterotrimeric kinesin-2 (KIF3) mediates transition zone and axoneme formation of mouse photoreceptors

Anterograde intraflagellar transport (IFT) employing kinesin-2 molecular motors has been implicated in trafficking of photoreceptor outer segment proteins. We generated embryonic retina-specific (prefix "emb") and adult tamoxifen-induced (prefix "tam") deletions of KIF3a and IFT88 in adult mice to study photoreceptor ciliogenesis and protein trafficking. In (emb)Kif3a(-/-) and in (emb)Ift88(-/-) mice, basal bodies failed to extend transition zones (connecting cilia) with outer segments, and visual pigments mistrafficked. In contrast, (tam)Kif3a(-/-) and (tam)Ift88(-/-) photoreceptor axonemes disintegrated slowly post-induction, starting distally, but rhodopsin and cone pigments trafficked normally for more than 2 weeks, a time interval during which the outer segment is completely renewed. The results demonstrate that visual pigments transport to the retinal outer segment despite removal of KIF3 and IFT88, and KIF3-mediated anterograde IFT is responsible for photoreceptor transition zone and axoneme formation.

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.

Targeted ablation of the Pde6h gene in mice reveals cross-species differences in cone and rod phototransduction protein isoform inventory

Phosphodiesterase-6 (PDE6) is a multisubunit enzyme that plays a key role in the visual transduction cascade in rod and cone photoreceptors. Each type of photoreceptor utilizes discrete catalytic and inhibitory PDE6 subunits to fulfill its physiological tasks, i.e. the degradation of cyclic guanosine-3',5'-monophosphate at specifically tuned rates and kinetics. Recently, the human PDE6H gene was identified as a novel locus for autosomal recessive (incomplete) color blindness. However, the three different classes of cones were not affected to the same extent. Short wave cone function was more preserved than middle and long wave cone function indicating that some basic regulation of the PDE6 multisubunit enzyme was maintained albeit by a unknown mechanism. To study normal and disease-related functions of cone Pde6h in vivo, we generated Pde6h knock-out (Pde6h(-/-)) mice. Expression of PDE6H in murine eyes was restricted to both outer segments and synaptic terminals of short and long/middle cone photoreceptors, whereas Pde6h(-/-) retinae remained PDE6H-negative. Combined in vivo assessment of retinal morphology with histomorphological analyses revealed a normal overall integrity of the retinal organization and an unaltered distribution of the different cone photoreceptor subtypes upon Pde6h ablation. In contrast to human patients, our electroretinographic examinations of Pde6h(-/-) mice suggest no defects in cone/rod-driven retinal signaling and therefore preserved visual functions. To this end, we were able to demonstrate the presence of rod PDE6G in cones indicating functional substitution of PDE6. The disparities between human and murine phenotypes caused by mutant Pde6h/PDE6H suggest species-to-species differences in the vulnerability of biochemical and neurosensory pathways of the visual signal transduction system.

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.

Genetic deletion of S-opsin prevents rapid cone degeneration in a mouse model of Leber congenital amaurosis

Mutations in RPE65 or lecithin-retinol acyltransferase (LRAT) disrupt 11-cis-retinal synthesis and cause Leber congenital amaurosis (LCA), a severe hereditary blindness occurring in early childhood. The pathology is attributed to a combination of 11-cis-retinal deficiency and photoreceptor degeneration. The mistrafficking of cone membrane-associated proteins including cone opsins (M- and S-opsins), cone transducin (Gαt2), G-protein-coupled receptor kinase 1 (GRK1) and guanylate cyclase 1 (GC1) has been suggested to play a role in cone degeneration. However, their precise role in cone degeneration is unclear. Here we investigated the role of S-opsin (Opn1sw) in cone degeneration in Lrat(-) (/-), a murine model for LCA, by genetic ablation of S-opsin. We show that deletion of just one allele of S-opsin from Lrat(-) (/-) mice is sufficient to prevent the rapid cone degeneration for at least 1 month. Deletion of both alleles of S-opsin prevents cone degeneration for an extended period (at least 12 months). This genetic prevention is accompanied by a reduction of endoplasmic reticulum (ER) stress in Lrat(-) (/-) photoreceptors. Despite cone survival in Opn1sw(-/-)Lrat(-) (/-) mice, cone membrane-associated proteins (e.g. Gαt2, GRK1 and GC1) continue to have trafficking problems. Our results suggest that cone opsins are the 'culprit' linking 11-cis-retinal deficiency to cone degeneration in LCA. This result has important implications for the current gene therapy strategy that emphasizes the need for a combinatorial therapy to both improve vision and slow photoreceptor degeneration.

Leber congenital amaurosis caused by mutations in GUCY2D

Leber congenital amaurosis (LCA) is a clinically and genetically heterogeneous group of diseases that account for the most severe form of early-onset retinal dystrophy. Mutations in retinal guanylate cyclase-1 (GUCY2D) are associated with LCA1, a prevalent form. GUCY2D encodes guanylate cyclase-1 (GC1), a protein expressed in rod and cone photoreceptors that regulates cGMP and Ca(2+) levels within these cells. LCA1 patients present with severely impaired vision, reduced, or ablated electroretinogram and nystagmus. Despite a high degree of visual disturbance, LCA1 patients retain normal photoreceptor laminar architecture, except for foveal cone outer segment abnormalities and, in some patients, foveal cone loss. This article will summarize clinical characterization of patients and proof of concept gene replacement studies in several animal models of GC1 deficiency, both of which have laid the groundwork for clinical application of a gene therapy for treatment of LCA1.

Functional EF-hands in neuronal calcium sensor GCAP2 determine its phosphorylation state and subcellular distribution in vivo, and are essential for photoreceptor cell integrity

The neuronal calcium sensor proteins GCAPs (guanylate cyclase activating proteins) switch between Ca²⁺-free and Ca²⁺-bound conformational states and confer calcium sensitivity to guanylate cyclase at retinal photoreceptor cells. They play a fundamental role in light adaptation by coupling the rate of cGMP synthesis to the intracellular concentration of calcium. Mutations in GCAPs lead to blindness. The importance of functional EF-hands in GCAP1 for photoreceptor cell integrity has been well established. Mutations in GCAP1 that diminish its Ca²⁺ binding affinity lead to cell damage by causing unabated cGMP synthesis and accumulation of toxic levels of free cGMP and Ca²⁺. We here investigate the relevance of GCAP2 functional EF-hands for photoreceptor cell integrity. By characterizing transgenic mice expressing a mutant form of GCAP2 with all EF-hands inactivated (EF⁻GCAP2), we show that GCAP2 locked in its Ca²⁺-free conformation leads to a rapid retinal degeneration that is not due to unabated cGMP synthesis. We unveil that when locked in its Ca²⁺-free conformation in vivo, GCAP2 is phosphorylated at Ser201 and results in phospho-dependent binding to the chaperone 14-3-3 and retention at the inner segment and proximal cell compartments. Accumulation of phosphorylated EF⁻GCAP2 at the inner segment results in severe toxicity. We show that in wildtype mice under physiological conditions, 50% of GCAP2 is phosphorylated correlating with the 50% of the protein being retained at the inner segment. Raising mice under constant light exposure, however, drastically increases the retention of GCAP2 in its Ca²⁺-free form at the inner segment. This study identifies a new mechanism governing GCAP2 subcellular distribution in vivo, closely related to disease. It also identifies a pathway by which a sustained reduction in intracellular free Ca²⁺ could result in photoreceptor damage, relevant for light damage and for those genetic disorders resulting in “equivalent-light” scenarios.

Visual perception is initiated at retinal photoreceptor cells, where light activates an enzymatic cascade that reduces free cGMP. As cGMP drops, cGMP-channels close and reduce the inward current –including Ca²⁺ influx– so that photoreceptors hyperpolarize and emit a signal. As the light extinguishes, cGMP levels are restored to reestablish sensitivity. cGMP synthesis relies on guanylate cyclase/guanylate cyclase activating protein (RetGC/GCAP) complexes. GCAPs link the rate of cGMP synthesis to intracellular Ca²⁺ levels, by switching between a Ca²⁺-free state that activates cGMP synthesis during light exposure, and a Ca²⁺-bound state that arrests cGMP synthesis in the dark. It is established that GCAP1 mutations linked to adCORD disrupt this tight Ca²⁺ control of the cGMP levels. We here show that a GCAP2 functional transition from the Ca²⁺-free to the Ca²⁺-loaded form is essential for photoreceptor cell integrity, by a non-related mechanism. We show that GCAP2 locked in its Ca²⁺-free form is retained by phosphorylation and 14-3-3 binding to the proximal rod compartments, causing severe cell damage. This study identifies a pathway by which a sustained reduction in intracellular free Ca²⁺ could result in photoreceptor damage, relevant for light damage and for those genetic disorders resulting in “equivalent-light” scenarios.

Functional architecture of the retina: development and disease

Structure and function are highly correlated in the vertebrate retina, a sensory tissue that is organized into cell layers with microcircuits working in parallel and together to encode visual information. All vertebrate retinas share a fundamental plan, comprising five major neuronal cell classes with cell body distributions and connectivity arranged in stereotypic patterns. Conserved features in retinal design have enabled detailed analysis and comparisons of structure, connectivity and function across species. Each species, however, can adopt structural and/or functional retinal specializations, implementing variations to the basic design in order to satisfy unique requirements in visual function. Recent advances in molecular tools, imaging and electrophysiological approaches have greatly facilitated identification of the cellular and molecular mechanisms that establish the fundamental organization of the retina and the specializations of its microcircuits during development. Here, we review advances in our understanding of how these mechanisms act to shape structure and function at the single cell level, to coordinate the assembly of cell populations, and to define their specific circuitry. We also highlight how structure is rearranged and function is disrupted in disease, and discuss current approaches to re-establish the intricate functional architecture of the retina.

Membrane guanylyl cyclase complexes shape the photoresponses of retinal rods and cones

In vertebrate rods and cones, photon capture by rhodopsin leads to the destruction of cyclic GMP (cGMP) and the subsequent closure of cyclic nucleotide gated ion channels in the outer segment plasma membrane. Replenishment of cGMP and reopening of the channels limit the growth of the photon response and are requisite for its recovery. In different vertebrate retinas, there may be as many as four types of membrane guanylyl cyclases (GCs) for cGMP synthesis. Ten neuronal Ca²⁺ sensor proteins could potentially modulate their activities. The mouse is proving to be an effective model for characterizing the roles of individual components because its relative simplicity can be reduced further by genetic engineering. There are two types of GC activating proteins (GCAPs) and two types of GCs in mouse rods, whereas cones express one type of GCAP and one type of GC. Mutant mouse rods and cones bereft of both GCAPs have large, long lasting photon responses. Thus, GCAPs normally mediate negative feedback tied to the light-induced decline in intracellular Ca²⁺ that accelerates GC activity to curtail the growth and duration of the photon response. Rods from other mutant mice that express a single GCAP type reveal how the two GCAPs normally work together as a team. Because of its lower Ca²⁺ affinity, GCAP1 is the first responder that senses the initial decrease in Ca²⁺ following photon absorption and acts to limit response amplitude. GCAP2, with a higher Ca²⁺ affinity, is recruited later during the course of the photon response as Ca²⁺ levels continue to decline further. The main role of GCAP2 is to provide for a timely response recovery and it is particularly important after exposure to very bright light. The multiplicity of GC isozymes and GCAP homologs in the retinas of other vertebrates confers greater flexibility in shaping the photon responses in order to tune visual sensitivity, dynamic range and frequency response.

Insights into the role of RD3 in guanylate cyclase trafficking, photoreceptor degeneration, and Leber congenital amaurosis

Retinal degeneration 3 (RD3) is an evolutionarily conserved 23 kDa protein expressed in rod and cone photoreceptor cells. Mutations in the gene encoding RD3 resulting in unstable non-functional C-terminal truncated proteins are responsible for early onset photoreceptor degeneration in Leber Congenital Amaurosis 12 patients, the rd3 mice, and the rcd2 collies. Recent studies have shown that RD3 interacts with guanylate cyclases GC1 and GC2 in retinal cell extracts and HEK293 cells co-expressing GC and RD3. This interaction inhibits GC catalytic activity and promotes the exit of GC1 and GC2 from the endoplasmic reticulum and their trafficking to photoreceptor outer segments. Adeno-associated viral vector delivery of the normal RD3 gene to photoreceptors of the rd3 mouse restores GC1 and GC2 expression and outer segment localization and leads to the long-term recovery of visual function and photoreceptor cell survival. This review focuses on the genetic and biochemical studies that have provided insight into the role of RD3 in photoreceptor function and survival.

Eyes underground: regression of visual protein networks in subterranean mammals

Regressive evolution involves the degeneration of formerly useful structures in a lineage over time, and may be accompanied by the molecular decay of phenotype-specific genes. The mammalian eye has repeatedly undergone degeneration in taxa that occupy dim-light environments including subterranean habitats. Here we assess whether a decrease in the amount of light that reaches the retina is associated with increased regression of retinal genes, whether the phototransduction and visual cycle pathways degrade in a predictable pattern, and if the timing of retinal gene loss is associated with the entrance of mammalian lineages into subterranean environments. Sequence data were obtained from the publically available genomes of the Cape golden mole (Chrysochloris asiatica), naked mole-rat (Heterocephalus glaber) and star-nosed mole (Condylura cristata) for 65 genes associated with phototransduction, the visual cycle, and other retinal functions. Gene sequences were inspected for inactivating mutations and, when present, pseudogene sequences were compared to sequences from subaerial outgroup species. To test whether retinal degeneration is correlated with historical entrances into subterranean environments, estimated dates of retinal gene inactivation were compared to the fossil record and phylogenetic inferences of ancestral fossoriality. Our results show that (1) lower levels of light available to the retina correspond with an increase in the number of retinal pseudogenes, (2) retinal protein networks generally degrade in a predictable manner, although the extensive loss of cone phototransduction genes in Heterocephalus raises further questions regarding SWS1-cone monochromacy versus functional rod monochromacy in this species, and (3) inactivation dates of retinal genes usually post-date inferred entrances into subterranean habitats.

Insights gained from gene therapy in animal models of retGC1 deficiency

Vertebrate species possess two retinal guanylate cyclases (retGC1 and retGC2) and at least two guanylate cyclase activating proteins (GCAPs), GCAP1 and GCAP2. GCAPs function as Ca²⁺ sensors that regulate the activity of guanylate cyclases. Together, these proteins regulate cGMP and Ca²⁺ levels within the outer segments of rod and cone photoreceptors. Mutations in GUCY2D, the gene that encodes retGC1, are a leading cause of the most severe form of early onset retinal dystrophy, Leber congenital amaurosis (LCA1). These mutations, which reduce or abolish the ability of retGC1 to replenish cGMP in photoreceptors, are thought to lead to the biochemical equivalent of chronic light exposure in these cells. In spite of this, the majority of LCA1 patients retain normal photoreceptor laminar architecture aside from foveal cone outer segment abnormalities, suggesting they may be good candidates for gene replacement therapy. Work began in the 1980s to characterize multiple animal models of retGC1 deficiency. 34 years later, all models have been used in proof of concept gene replacement studies toward the goal of developing a therapy to treat GUCY2D-LCA1. Here we use the results of these studies as well as those of recent clinical studies to address specific questions relating to clinical application of a gene therapy for treatment of LCA1.

Identification of target binding site in photoreceptor guanylyl cyclase-activating protein 1 (GCAP1)

Retinal guanylyl cyclase (RetGC)-activating proteins (GCAPs) regulate visual photoresponse and trigger congenital retinal diseases in humans, but GCAP interaction with its target enzyme remains obscure. We mapped GCAP1 residues comprising the RetGC1 binding site by mutagenizing the entire surface of GCAP1 and testing the ability of each mutant to bind RetGC1 in a cell-based assay and to activate it in vitro. Mutations that most strongly affected the activation of RetGC1 localized to a distinct patch formed by the surface of non-metal-binding EF-hand 1, the loop and the exiting helix of EF-hand 2, and the entering helix of EF-hand 3. Mutations in the binding patch completely blocked activation of the cyclase without affecting Ca(2+) binding stoichiometry of GCAP1 or its tertiary fold. Exposed residues in the C-terminal portion of GCAP1, including EF-hand 4 and the helix connecting it with the N-terminal lobe of GCAP1, are not critical for activation of the cyclase. GCAP1 mutants that failed to activate RetGC1 in vitro were GFP-tagged and co-expressed in HEK293 cells with mOrange-tagged RetGC1 to test their direct binding in cyto. Most of the GCAP1 mutations introduced into the "binding patch" prevented co-localization with RetGC1, except for Met-26, Lys-85, and Trp-94. With these residues mutated, GCAP1 completely failed to stimulate cyclase activity but still bound RetGC1 and competed with the wild type GCAP1. Thus, RetGC1 activation by GCAP1 involves establishing a tight complex through the binding patch with an additional activation step involving Met-26, Lys-85, and Trp-94.

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.

cGMP accumulation causes photoreceptor degeneration in CNG channel deficiency: evidence of cGMP cytotoxicity independently of enhanced CNG channel function

Photoreceptor cyclic nucleotide-gated (CNG) channels regulate Ca(2+) influx in rod and cone photoreceptors. cGMP, the native ligand of the photoreceptor CNG channels, has been associated with cytotoxicity when its levels rise above normal due to defects in photoreceptor phosphodiesterase (PDE6) or regulation of retinal guanylyl cyclase (retGC). We found a massive accumulation of cGMP in CNGA3-deficient retina and investigated whether cGMP accumulation plays a role in cone degeneration in CNG channel deficiency. The time course study showed that the retinal cGMP level in Cnga3(-/-);Nrl(-/-) mice with CNGA3 deficiency on a cone-dominant background was sharply increased at postnatal day 8 (P8), peaked around P10-P15, remained high through P30-P60, and returned to near control level at P90. This elevation pattern correlated with photoreceptor apoptotic death, which peaked around P15-P20. In Cnga3(-/-);Gucy2e(-/-) mice lacking retGC1, cone density and expression levels of cone-specific proteins were significantly increased compared with Cnga3(-/-), consistent with a role of cGMP accumulation as the major contributor to cone death caused by CNG channel deficiency. The activity and expression levels of cGMP-dependent protein kinase G (PKG) were significantly increased in Cnga3(-/-);Nrl(-/-) retina compared with Nrl(-/-), suggesting an involvement of PKG regulation in cell death. Our results indicate that cGMP accumulation in photoreceptors can itself exert cytotoxic effect in cones, independently of CNG channel activity and Ca(2+) influx.

The GAFa domain of phosphodiesterase-6 contains a rod outer segment localization signal

Photoreceptor phosphodiesterase-6 (PDE6) is a peripheral membrane protein synthesized in the inner segment of photoreceptor cells. Newly synthesized PDE6 is transported to the outer segment (OS) where it serves as a key effector enzyme in the phototransduction cascade. Proper localization of PDE6 in photoreceptors is critically important to the function and survival of photoreceptor cells. The mechanism of PDE6 transport to the OS remains largely unknown. In this study, we investigated potential OS targeting signals of PDE6 by constructing cGMP-binding, cGMP-specific phosphodiesterase-5/PDE6 chimeric proteins and analyzing their localization in rods of transgenic Xenopus laevis. We found that efficient OS localization of chimeric isoprenylated PDE enzymes required the presence of a targeting motif within the PDE6 GAFa domain. Furthermore, the GAFa-dependent localization signal was sufficient to target GAFa fusion protein to the OS. Our results support the idea that effective trafficking of the peripheral membrane proteins to the OS of photoreceptor cells requires a sorting/targeting motif in addition to a membrane-binding signal.

Combined rod and cone transduction by adeno-associated virus 2/8

Gene transfer to both cone and rod photoreceptors (PRs) is essential for gene therapy of inherited retinal degenerations that are caused by mutations in genes expressed in both PR types. Vectors based on the adeno-associated virus (AAV) efficiently transduce PRs of different species. However, these are predominantly rods and little is known about the ability of the AAV to transduce cones in combination with rods. Here we show that AAV2/8 transduces pig cones to levels that are similar to AAV2/9, and the outer nuclear layer (mainly rods) to levels that are on average higher, although not statistically significant, than both AAV2/5 and AAV2/9. We additionally found that the ubiquitous cytomegalovirus (CMV), but not the PR-specific GRK1 promoter, transduced pig cones efficiently, presumably because GRK1 is not expressed in pig cones as observed in mice and humans. Indeed, the GRK1 and CMV promoters transduce a similar percentage of murine cones with the CMV reaching the highest expression levels. Consistent with this, the AAV2/8 vectors with either the CMV or the GRK1 promoter restore cone function in a mouse model of Leber congenital amaurosis type 1 (LCA1), supporting the use of AAV2/8 for gene therapy of LCA1 as well as of other retinal diseases requiring gene transfer to both PR types.

AIPL1, A protein linked to blindness, is essential for the stability of enzymes mediating cGMP metabolism in cone photoreceptor cells

Defects in the photoreceptor-specific gene encoding aryl hydrocarbon receptor interacting protein like-1 (AIPL1) are linked to blinding diseases, including Leber congenital amaurosis (LCA) and cone dystrophy. While it is apparent that AIPL1 is needed for rod and cone function, the role of AIPL1 in cones is not clear. In this study, using an all-cone animal model lacking Aipl1, we show a light-independent degeneration of M- and S-opsin containing cones that proceeds in a ventral-to-dorsal gradient. Aipl1 is needed for stability, assembly and membrane association of cone PDE6, an enzyme crucial for photoreceptor function and survival. Furthermore, RetGC1, a protein linked to LCA that is needed for cGMP synthesis, was dramatically reduced in cones lacking Aipl1. A defect in RetGC1 is supported by our finding that cones lacking Aipl1 exhibited reduced levels of cGMP. These findings are in contrast to the role of Aipl1 in rods, where destabilization of rod PDE6 results in an increase in cGMP levels, which drives rapid rod degeneration. Our results illustrate mechanistic differences behind the death of rods and cones in retinal degenerative disease caused by deficiencies in AIPL1.

AAV-mediated gene therapy in the guanylate cyclase (RetGC1/RetGC2) double knockout mouse model of Leber congenital amaurosis

Mutations in GUCY2D are associated with recessive Leber congenital amaurosis-1 (LCA1). GUCY2D encodes photoreceptor-specific, retinal guanylate cyclase-1 (RetGC1). Reports of retinal degeneration in LCA1 are conflicting; some describe no obvious degeneration and others report loss of both rods and cones. Proof of concept studies in models representing the spectrum of phenotypes is warranted. We have previously demonstrated adeno-associated virus (AAV)-mediated RetGC1 is therapeutic in GC1ko mice, a model exhibiting loss of cones only. The purpose of this study was to characterize AAV-mediated gene therapy in the RetGC1/RetGC2 double knockout (GCdko) mouse, a model lacking rod and cone function and exhibiting progressive loss of both photoreceptor subclasses. Use of this model also allowed for the evaluation of the functional efficiency of transgenic RetGC1 isozyme. Subretinal delivery of AAV8(Y733F) vector containing the human rhodopsin kinase (hGRK1) promoter driving murine Gucy2e was performed in GCdko mice at various postnatal time points. Treatment resulted in restoration of rod and cone function at all treatment ages and preservation of retinal structure in GCdko mice treated as late as 7 weeks of age. Functional gains and structural preservation were stable for at least 1 year. Treatment also conferred cortical- and subcortical-based visually-guided behavior. Functional efficiency of transgenic RetGC1 was indistinguishable from that of endogenous isozyme in congenic wild-type (WT) mice. This study clearly demonstrates AAV-mediated RetGC1 expression restores function to and preserves structure of rod and cone photoreceptors in a degenerative model of retinal guanylate cyclase deficiency, further supporting development of an AAV-based vector for treatment of LCA1.

RD3 gene delivery restores guanylate cyclase localization and rescues photoreceptors in the Rd3 mouse model of Leber congenital amaurosis 12

RD3 is a 23 kDa protein implicated in the stable expression of guanylate cyclase in photoreceptor cells. Truncation mutations are responsible for photoreceptor degeneration and severe early-onset vision loss in Leber congenital amaurosis 12 (LCA12) patients, the rd3 mouse and the rcd2 collie. To further investigate the role of RD3 in photoreceptors and explore gene therapy as a potential treatment for LCA12, we delivered adeno-associated viral vector (AAV8) with a Y733F capsid mutation and containing the mouse Rd3 complementary DNA (cDNA) under the control of the human rhodopsin kinase promoter to photoreceptors of 14-day-old Rb(11.13)4Bnr/J and In (5)30Rk/J strains of rd3 mice by subretinal injections. Strong RD3 transgene expression led to the translocation of guanylate cyclase from the endoplasmic reticulum (ER) to rod and cone outer segments (OSs) as visualized by immunofluorescence microscopy. Guanylate cyclase expression and localization coincided with the survival of rod and cone photoreceptors for at least 7 months. Rod and cone visual function was restored in the In (5)30Rk/J strain of rd3 mice as measured by electroretinography (ERG), but only rod function was recovered in the Rb(11.13)4Bnr/J strain, suggesting that the latter may have another defect in cone phototransduction. These studies indicate that RD3 plays an essential role in the exit of guanylate cyclase from the ER and its trafficking to photoreceptor OSs and provide a 'proof of concept' for AAV-mediated gene therapy as a potential therapeutic treatment for LCA12.

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.

Changes in gene expression associated with retinal degeneration in the rd3 mouse

PURPOSE

To identify and characterize changes in gene expression associated with photoreceptor degeneration in the rd3 mouse model of Leber congenital amaurosis (LCA) type 12.

METHODS

Global genome expression profiling using microarray technology was performed on total RNA extracts from rd3 and wild-type control mouse retinas at postnatal day 21. Quantitative PCR analysis of selected transcripts was performed to validate the microarray results.

RESULTS

Functional annotation of differentially regulated genes in the rd3 mouse defined key canonical pathways, including phototransduction, glycerophospholipid metabolism, tumor necrosis factor receptor 1 signaling, and endothelin signaling. Overall, 1,140 of approximately 55,800 transcripts were differentially represented. In particular, a large percentage of the upregulated transcripts encode proteins involved in the immune response; whereas the downregulated transcripts encode proteins involved in phototransduction and lipid metabolism.

CONCLUSIONS

This analysis has elucidated several candidate genes and pathways, thus providing insight into the pathogenic mechanisms underlying the photoreceptor degeneration in the rd3 mouse retina and indicating directions for future studies.

Germline deletion of Cetn1 causes infertility in male mice

Centrins are calmodulin-like Ca(2+)-binding proteins that can be found in all ciliated eukaryotic cells from yeast to mammals. Expressed in male germ cells and photoreceptors, centrin 1 (CETN1) resides in the photoreceptor transition zone and connecting cilium. To identify its function in mammals, we deleted Cetn1 by homologous recombination. Cetn1(-/-) mice were viable and showed no sign of retina degeneration suggesting that CETN1 is nonessential for photoreceptor ciliogenesis or structural maintenance. Phototransduction components localized normally to the Cetn1(-/-) photoreceptor outer segments, and loss of CETN1 had no effect on light-induced translocation of transducin to the inner segment. Although Cetn1(-/-) females and Cetn1(+/-) males had normal fertility, Cetn1(-/-) males were infertile. The Cetn1(-/-) testes size was normal, and spermatogonia as well as spermatocytes developed normally. However, spermatids lacked tails suggesting severe defects at the late maturation phase of spermiogenesis. Viable sperm cells were absent and the few surviving spermatozoa were malformed. Light and electron microscopy analyses of Cetn1(-/-) spermatids revealed failures in centriole rearrangement during basal body maturation and in the basal-body-nucleus connection. These results confirm an essential role for CETN1 in late steps of spermiogenesis and spermatid maturation.

Protein sorting, targeting and trafficking in photoreceptor cells

Vision is the most fundamental of our senses initiated when photons are absorbed by the rod and cone photoreceptor neurons of the retina. At the distal end of each photoreceptor resides a light-sensing organelle, called the outer segment, which is a modified primary cilium highly enriched with proteins involved in visual signal transduction. At the proximal end, each photoreceptor has a synaptic terminal, which connects this cell to the downstream neurons for further processing of the visual information. Understanding the mechanisms involved in creating and maintaining functional compartmentalization of photoreceptor cells remains among the most fascinating topics in ocular cell biology. This review will discuss how photoreceptor compartmentalization is supported by protein sorting, targeting and trafficking, with an emphasis on the best-studied cases of outer segment-resident proteins.

RNAi-mediated gene suppression in a GCAP1(L151F) cone-rod dystrophy mouse model

Dominant mutations occurring in the high-affinity Ca(2+)-binding sites (EF-hands) of the GUCA1A gene encoding guanylate cyclase-activating protein 1 (GCAP1) cause slowly progressing cone-rod dystrophy (CORD) in a dozen families worldwide. We developed a nonallele-specific adeno-associated virus (AAV)-based RNAi knockdown strategy to rescue the retina degeneration caused by GCAP1 mutations. We generated three genomic transgenic mouse lines expressing wildtype (WT) and L151F mutant mouse GCAP1 with or without a C-terminal GFP fusion. Under control of endogenous regulatory elements, the transgenes were expressed specifically in mouse photoreceptors. GCAP1(L151F) and GCAP1(L151F)-GFP transgenic mice presented with a late onset and slowly progressive photoreceptor degeneration, similar to that observed in human GCAP1-CORD patients. Transgenic expression of WT GCAP1-EGFP in photoreceptors had no adverse effect. Toward therapy development, a highly effective anti-mGCAP1 shRNA, mG1hp4, was selected from four candidate shRNAs using an in-vitro screening assay. Subsequently a self-complementary (sc) AAV serotype 2/8 expressing mG1hp4 was delivered subretinally to GCAP1(L151F)-GFP transgenic mice. Knockdown of the GCAP1(L151F)-GFP transgene product was visualized by fluorescence live imaging in the scAAV2/8-mG1hp4-treated retinas. Concomitant with the mutant GCAP1-GFP fusion protein, endogenous GCAP1 decreased as well in treated retinas. We propose nonallele-specific RNAi knockdown of GCAP1 as a general therapeutic strategy to rescue any GCAP1-based dominant cone-rod dystrophy in human patients.

A comprehensive review of retinal gene therapy

Blindness, although not life threatening, is a debilitating disorder for which few, if any treatments exist. Ocular gene therapies have the potential to profoundly improve the quality of life in patients with inherited retinal disease. As such, tremendous focus has been given to develop such therapies. Several factors make the eye an ideal organ for gene-replacement therapy including its accessibility, immune privilege, small size, compartmentalization, and the existence of a contralateral control. This review will provide a comprehensive summary of (i) existing gene therapy clinical trials for several genetic forms of blindness and (ii) preclinical efficacy and safety studies in a variety of animal models of retinal disease which demonstrate strong potential for clinical application. To be as comprehensive as possible, we include additional proof of concept studies using gene replacement, neurotrophic/neuroprotective, optogenetic, antiangiogenic, or antioxidative stress strategies as well as a description of the current challenges and future directions in the ocular gene therapy field to this review as a supplement.

Union makes strength: a worldwide collaborative genetic and clinical study to provide a comprehensive survey of RD3 mutations and delineate the associated phenotype

Leber congenital amaurosis (LCA) is the earliest and most severe retinal degeneration (RD), and the most common cause of incurable blindness diagnosed in children. It is occasionally the presenting symptom of multisystemic ciliopathies which diagnosis will require a specific care of patients. Nineteen LCA genes are currently identified and three of them account for both non-syndromic and syndromic forms of the disease. RD3 (LCA12) was implicated as a LCA gene based on the identification of homozygous truncating mutations in two LCA families despite the screening of large cohorts of patients. Here we provide a comprehensive survey of RD3 mutations and of their clinical expression through the screening of a cohort of 852 patients originating worldwide affected with LCA or early-onset and severe RD. We identified three RD3 mutations in seven unrelated consanguineous LCA families - i.e., a 2 bp deletion and two nonsense mutations – predicted to cause complete loss of function. Five families originating from the Southern Shores of the Mediterranean segregated a similar mutation (c.112C>T, p.R38*) suggesting that this change may have resulted from an ancient founder effect. Considering the low frequency of RD3 carriers, the recurrence risk for LCA in non-consanguineous unions is negligible for both heterozygote and homozygote RD3 individuals. The LCA12 phenotype in our patients is highly similar to those of patients with mutant photoreceptor-specific guanylate cyclase (GUCY2D/LCA1). This observation is consistent with the report of the role of RD3 in trafficking of GUCYs and gives further support to a common mechanism of photoreceptor degeneration in LCA12 and LCA1, i.e., inability to increase cytoplasmic cGMP concentration in outer segments and thus to recover the dark-state. Similar to LCA1, LCA12 patients have no extraocular symptoms despite complete inactivation of both RD3 alleles, supporting the view that extraocular investigations in LCA infants with RD3 mutations should be avoided.

The prenyl-binding protein PrBP/δ: a chaperone participating in intracellular trafficking

Expressed ubiquitously, PrBP/δ functions as chaperone/co-factor in the transport of a subset of prenylated proteins. PrBP/δ features an immunoglobulin-like β-sandwich fold for lipid binding, and interacts with diverse partners. PrBP/δ binds both C-terminal C15 and C20 prenyl side chains of phototransduction polypeptides and small GTP-binding (G) proteins of the Ras superfamily. PrBP/δ also interacts with the small GTPases, ARL2 and ARL3, which act as release factors (GDFs) for prenylated cargo. Targeted deletion of the mouse Pde6d gene encoding PrBP/δ resulted in impeded trafficking to the outer segments of GRK1 and cone PDE6 which are predicted to be farnesylated and geranylgeranylated, respectively. Rod and cone transducin trafficking was largely unaffected. These trafficking defects produce progressive cone-rod dystrophy in the Pde6d(-/-) mouse.

Enzymatic relay mechanism stimulates cyclic GMP synthesis in rod photoresponse: biochemical and physiological study in guanylyl cyclase activating protein 1 knockout mice

Regulation of cGMP synthesis by retinal membrane guanylyl cyclase isozymes (RetGC1 and RetGC2) in rod and cone photoreceptors by calcium-sensitive guanylyl cyclase activating proteins (GCAP1 and GCAP2) is one of the key molecular mechanisms affecting the response to light and is involved in congenital retinal diseases. The objective of this study was to identify the physiological sequence of events underlying RetGC activation in vivo, by studying the electrophysiological and biochemical properties of mouse rods in a new genetic model lacking GCAP1. The GCAP1^−/− retinas expressed normal levels of RetGC isozymes and other phototransduction proteins, with the exception of GCAP2, whose expression was elevated in a compensatory fashion. RetGC activity in GCAP1^−/− retinas became more sensitive to Ca²⁺ and slightly increased. The bright flash response in electroretinogram (ERG) recordings recovered quickly in GCAP1^−/−, as well as in RetGC1^−/−GCAP1^−/−, and RetGC2^−/−GCAP1^−/− hybrid rods, indicating that GCAP2 activates both RetGC isozymes in vivo. Individual GCAP1^−/− rod responses varied in size and shape, likely reflecting variable endogenous GCAP2 levels between different cells, but single-photon response (SPR) amplitude and time-to-peak were typically increased, while recovery kinetics remained faster than in wild type. Recovery from bright flashes in GCAP1^−/− was prominently biphasic, because rare, aberrant SPRs producing the slower tail component were magnified. These data provide strong physiological evidence that rod photoresponse recovery is shaped by the sequential recruitment of RetGC isozyme activation by GCAPs according to the different GCAP sensitivities for Ca²⁺ and specificities toward RetGC isozymes. GCAP1 is the ‘first-response’ sensor protein that stimulates RetGC1 early in the response and thus limits the SPR amplitude, followed by activation of GCAP2 that adds stimulation of both RetGC1 and RetGC2 to speed-up photoreceptor recovery.

Determining consequences of retinal membrane guanylyl cyclase (RetGC1) deficiency in human Leber congenital amaurosis en route to therapy: residual cone-photoreceptor vision correlates with biochemical properties of the mutants

The GUCY2D gene encodes retinal membrane guanylyl cyclase (RetGC1), a key component of the phototransduction machinery in photoreceptors. Mutations in GUCY2D cause Leber congenital amaurosis type 1 (LCA1), an autosomal recessive human retinal blinding disease. The effects of RetGC1 deficiency on human rod and cone photoreceptor structure and function are currently unknown. To move LCA1 closer to clinical trials, we characterized a cohort of patients (ages 6 months-37 years) with GUCY2D mutations. In vivo analyses of retinal architecture indicated intact rod photoreceptors in all patients but abnormalities in foveal cones. By functional phenotype, there were patients with and those without detectable cone vision. Rod vision could be retained and did not correlate with the extent of cone vision or age. In patients without cone vision, rod vision functioned unsaturated under bright ambient illumination. In vitro analyses of the mutant alleles showed that in addition to the major truncation of the essential catalytic domain in RetGC1, some missense mutations in LCA1 patients result in a severe loss of function by inactivating its catalytic activity and/or ability to interact with the activator proteins, GCAPs. The differences in rod sensitivities among patients were not explained by the biochemical properties of the mutants. However, the RetGC1 mutant alleles with remaining biochemical activity in vitro were associated with retained cone vision in vivo. We postulate a relationship between the level of RetGC1 activity and the degree of cone vision abnormality, and argue for cone function being the efficacy outcome in clinical trials of gene augmentation therapy in LCA1.

Retinal guanylyl cyclase isozyme 1 is the preferential in vivo target for constitutively active GCAP1 mutants causing congenital degeneration of photoreceptors

Two calcium-sensitive guanylyl cyclase activating proteins (GCAP1 and GCAP2) activate cGMP synthesis in photoreceptor by retinal membrane guanylyl cyclase isozymes (RetGC1 and RetGC2) to expedite recovery, but calcium-insensitive constitutively active GCAP1 mutants cause photoreceptor degeneration in human patients and transgenic mice. Although GCAP1 and GCAP2 can both activate RetGC1 and RetGC2 in vitro, we find that GCAP1 selectively regulates RetGC1 in vivo. Furthermore, elimination of RetGC1 but not RetGC2 isozyme reverses abnormal calcium sensitivity of cGMP synthesis and rescues mouse rods in transgenic mice expressing GCAP1 mutants causing photoreceptor disease. Rods expressing mutant GCAP1 not only survive in the absence of RetGC1 but also remain functional, albeit with reduced electroretinography (ERG) amplitudes typical of RetGC1-/- genotype. The rod ERG recovery from a strong flash, only slightly affected in both RetGC1-/- and RetGC2-/- mice, becomes very slow in RetGC1-/- but not RetGC2-/- mice when GCAP2 is not available to provide Ca²⁺ feedback to the remaining RetGC isozyme. The intrinsic biochemical properties of RetGC and GCAP determined in vitro do not explain the observed phenomena. Instead, our results argue that there must be a cellular mechanism that limits GCAP1 access to RetGC2 and makes RetGC1 isozyme a preferential target for the disease-causing GCAP1 mutants. A more general conclusion from our findings is that nondiscriminatory interactions between homologous effector enzymes and their regulatory proteins permitted by their intrinsic biochemical properties can be effectively restricted in a living photoreceptor.

Speed, adaptation, and stability of the response to light in cone photoreceptors: the functional role of Ca-dependent modulation of ligand sensitivity in cGMP-gated ion channels

The response of cone photoreceptors to light is stable and reproducible because of the exceptional regulation of the cascade of enzymatic reactions that link visual pigment (VP) excitation to the gating of cyclic GMP (cGMP)-gated ion channels (cyclic nucleotide–gated [CNG]) in the outer segment plasma membrane. Regulation is achieved in part through negative feedback control of some of these reactions by cytoplasmic free Ca²⁺. As part of the control process, Ca²⁺ regulates the phosphorylation of excited VP, the activity of guanylate cyclase, and the ligand sensitivity of the CNG ion channels. We measured photocurrents elicited by stimuli in the form of flashes, steps, and flashes superimposed on steps in voltage-clamped single bass cones isolated from striped bass retina. We also developed a computational model that comprises all the known molecular events of cone phototransduction, including all Ca-dependent controls. Constrained by available experimental data in bass cones and cone transduction biochemistry, we achieved an excellent match between experimental photocurrents and those simulated by the model. We used the model to explore the physiological role of CNG ion channel modulation. Control of CNG channel activity by both cGMP and Ca²⁺ causes the time course of the light-dependent currents to be faster than if only cGMP controlled their activity. Channel modulation also plays a critical role in the regulation of the light sensitivity and light adaptation of the cone photoresponse. In the absence of ion channel modulation, cone photocurrents would be unstable, oscillating during and at the offset of light stimuli.

Ciliary transition zone (TZ) proteins RPGR and CEP290: role in photoreceptor cilia and degenerative diseases

INTRODUCTION

Primary cilia are microtubule-based extensions of the plasma membrane in nearly all cell types. In vertebrate photoreceptors, the sensory cilium develops as outer segment (OS) that contains the photopigment rhodopsin and other proteins necessary for phototransduction. The distinct composition of proteins and lipids in the OS membrane is maintained by the selective barrier located at the border between the basal body and the ciliary compartment, called the transition zone (TZ).

AREAS COVERED

In this review, we will discuss the identification and function of two ciliary TZ proteins, RPGR (retinitis pigmentosa GTPase regulator) and CEP290. Mutations in these proteins account for a majority of retinopathies due to ciliary dysfunction. We will also discuss the potential of such information in designing therapeutic approaches to treat cilia-dependent photoreceptor degenerative diseases.

EXPERT OPINION

RPGR and CEP290 perform overlapping yet distinct functions in regulating trafficking of cargo via the TZ of photoreceptors. While RPGR modulates the trafficking by acting as a GEF for the small GTPase RAB8A, CEP290 may be involved in maintaining the polarized distribution of proteins in the OS by modulating intracellular levels of selected proteins involved in inhibiting OS formation.

shRNA knockdown of guanylate cyclase 2e or cyclic nucleotide gated channel alpha 1 increases photoreceptor survival in a cGMP phosphodiesterase mouse model of retinitis pigmentosa

In vertebrate rods, dark and light conditions produce changes in guanosine 3′,5′-cyclic monophosphate (cGMP) and calcium (Ca²⁺) levels, which are regulated by the opposing function of several proteins. During the recovery of a bright flash, guanylate cyclase (GUCY) helps raise cGMP to levels that open cGMP-gated calcium sodium channels (CNG) to increase Na⁺ and Ca²⁺ influx in the outer segment. In contrast, light activates cGMP phosphodiesterase 6 (PDE6) causing rapid hydrolysis of cGMP, CNG closure, and reduced Na⁺ and Ca²⁺ levels. In Pde6b mouse models of retinitis pigmentosa (RP), photoreceptor death is preceded by abnormally high cGMP and Ca²⁺ levels, likely because of continued synthesis of cGMP by guanylate cyclases and unregulated influx of Ca²⁺ to toxic levels through CNG channels. To reverse the effects of Pde6b loss of function, we employed an shRNA knockdown approach to reduce the expression of Gucy2e or Cnga1 in Pde6b^H620Q photoreceptors prior to degeneration. Gucy2e- or Cnga1-shRNA lentiviral-mediated knockdown GUCY2E and CNGA1 expression increase visual function and photoreceptor survival in Pde6b^H620Q mice. We demonstrated that effective knockdown of GUCY2E and CNGA1 expression to counteract loss of PDE6 function may develop into a valuable approach for treating some patients with RP.

Photoreceptor signaling: supporting vision across a wide range of light intensities

For decades, photoreceptors have been an outstanding model system for elucidating basic principles in sensory transduction and biochemistry and for understanding many facets of neuronal cell biology. In recent years, new knowledge of the kinetics of signaling and the large-scale movements of proteins underlying signaling has led to a deeper appreciation of the photoreceptor's unique challenge in mediating the first steps in vision over a wide range of light intensities.

Lentivirus-mediated expression of cDNA and shRNA slows degeneration in retinitis pigmentosa

Mutations in Pde6b lead to high levels of signaling molecules cyclic guanosine monophosphate (cGMP) and Ca(2+), which ultimately result in photoreceptor cell death in certain forms of retinitis pigmentosa (RP). The level of cGMP, which is controlled by opposing activities of guanylate cyclase (GUCY) and photoreceptor phosphodiesterase-6 (PDE6), regulates the opening of cyclic nucleotide-gated ion channels [CNG] and thereby controls Ca(2+) influx into the outer segments. Using a lentiviral gene therapy approach, we have previously shown that degeneration can be temporarily slowed either by introducing wild-type PDE6β or knocking down expression of GUCY2E and CNGA1 in photoreceptors of Pde6b(H620Q), a mouse model for RP. Rescue was transient with either approach. Therefore, we tested a novel combination therapy using bipartite lentiviral vectors designed to both introduce wild-type PDE6β expression and knockdown GUCY2E or CNGA1. Immunoblot analysis shows simultaneous increases in PDE6β and decreases in GUCY2E or CNGA1 in retinas transduced by the vectors, indicating successful transduction. In Pde6b(H620Q) mutants, we observe rescue of photoreceptor function and an increase in photoreceptor rows as compared with untreated controls. However, no evidence of prolonged rescue beyond the limit of the previously tested single therapy was observed.

Targeting of mouse guanylate cyclase 1 (Gucy2e) to Xenopus laevis rod outer segments

Photoreceptor guanylate cyclase (GC1) is a transmembrane protein and responsible for synthesis of cGMP, the secondary messenger of phototransduction. It consists of an extracellular domain, a single transmembrane domain, and an intracellular domain. It is unknown how GC1 targets to the outer segments where it resides. To identify a putative GC1 targeting signal, we generated a series of peripheral membrane and transmembrane constructs encoding extracellular and intracellular mouse GC1 fragments fused to EGFP. The constructs were expressed in Xenopus laevis rod photoreceptors under the control of the rhodopsin promoter. We examined the localization of GFP-GC1 fusion proteins containing the complete GC1 sequence, or partial GC1 sequences, which were membrane-associated via either the GC1 transmembrane domain or the rhodopsin C-terminal palmitoyl chains. Full-length GFP-GC1 targeted to the rod outer segment disk rims. As a group, fusion proteins containing the entire cytoplasmic domain of GC1 targeted to the OS, whereas other fusion proteins containing portions of the cytoplasmic or the extracellular domains did not. We conclude that GC1 likely has no single linear peptide-based OS targeting signal. Our results suggest targeting is due to either multiple weak signals in the cytoplasmic domain of GC1, or co-transport to the OS with an accessory protein.

Guanylyl cyclase structure, function and regulation

Nitric oxide, bicarbonate, natriuretic peptides (ANP, BNP and CNP), guanylins, uroguanylins and guanylyl cyclase activating proteins (GCAPs) activate a family of enzymes variously called guanyl, guanylyl or guanylate cyclases that catalyze the conversion of guanosine triphosphate to cyclic guanosine monophosphate (cGMP) and pyrophosphate. Intracellular cyclic GMP is a second messenger that modulates: platelet aggregation, neurotransmission, sexual arousal, gut peristalsis, blood pressure, long bone growth, intestinal fluid secretion, lipolysis, phototransduction, cardiac hypertrophy and oocyte maturation. This review briefly discusses the discovery of cGMP and guanylyl cyclases, then nitric oxide, nitric oxide synthase and soluble guanylyl cyclase are described in slightly greater detail. Finally, the structure, function, and regulation of the individual mammalian single membrane-spanning guanylyl cyclases GC-A, GC-B, GC-C, GC-D, GC-E, GC-F and GC-G are described in greatest detail as determined by biochemical, cell biological and gene-deletion studies.

Long-term preservation of cone photoreceptors and restoration of cone function by gene therapy in the guanylate cyclase-1 knockout (GC1KO) mouse

PURPOSE

The authors previously showed that subretinal delivery of AAV5 vectors containing murine guanylate cyclase-1 (GC1) cDNA driven by either photoreceptor-specific (hGRK1) or ubiquitous (smCBA) promoters was capable of restoring cone-mediated function and visual behavior and preserving cone photoreceptors in the GC1 knockout (GC1KO) mouse for 3 months. Here, the authors compared therapy conferred by the aforementioned vectors to that achieved with the highly efficient capsid tyrosine mutant AAV8(Y733F) and asked whether long-term therapy is achievable in this model.

METHODS

AAV5-hGRK1-mGC1, AAV5-smCBA-mGC1, or AAV8(Y733F)-hGRK1-mGC1 was delivered subretinally to GC1KO mice between postnatal day (P)14 and P25. Retinal function was assayed by electroretinography. Localization of AAV-mediated GC1 expression and cone survival were assayed with immunohistochemistry, and the spread of vector genomes beyond the retina was quantified by PCR of optic nerve and brain tissue.

RESULTS

Cone function was restored with all vectors tested, with AAV8(Y733F) being the most efficient. Electroretinographic responses were clearly measurable out to 1 year after treatment. AAV-mediated expression of GC1 was found exclusively in photoreceptors out to 15 months after injection. Cones were preserved for at least 11 months after treatment. AAV5- and AAV8(733)-delivered vector genomes were recovered primarily from optic nerve of the treated eye and, in only instance, from brain (1 of 20 samples).

CONCLUSIONS

The authors demonstrate for the first time that long-term therapy (∼1 year) is achievable in a mammalian model of GC1 deficiency. These data provide additional justification for the development of an AAV-based gene therapy vector for the clinical treatment of Leber congenital amaurosis-1.

Immunocytochemical evidence of Tulp1-dependent outer segment protein transport pathways in photoreceptor cells

Tulp1 is a protein of unknown function exclusive to rod and cone photoreceptor cells. Mutations in the gene cause autosomal recessive retinitis pigmentosa in humans and photoreceptor degeneration in mice. In tulp1-/- mice, rod and cone opsins are mislocalized, and rhodopsin-bearing extracellular vesicles accumulate around the inner segment, indicating that Tulp1 is involved in protein transport from the inner segment to the outer segment. To investigate this further, we sought to define which outer segment transport pathways are Tulp1-dependent. We used immunohistochemistry to examine the localization of outer segment proteins in tulp1-/- photoreceptors, prior to retinal degeneration. We also surveyed the condition of inner segment organelles and rhodopsin transport machinery proteins. Herein, we show that guanylate cyclase 1 and guanylate cyclase activating proteins 1 and 2 are mislocalized in the absence of Tulp1. Furthermore, arrestin does not translocate to the outer segment in response to light stimulation. Additionally, data from the tulp1-/- retina adds to the understanding of peripheral membrane protein transport, indicating that rhodopsin kinase and transducin do not co-transport in rhodopsin carrier vesicles and phosphodiesterase does not co-transport in guanylate cyclase carrier vesicles. These data implicate Tulp1 in the transport of selective integral membrane outer segment proteins and their associated proteins, specifically, the opsin and guanylate cyclase carrier pathways. The exact role of Tulp1 in outer segment protein transport remains elusive. However, without Tulp1, two rhodopsin transport machinery proteins exhibit abnormal distribution, Rab8 and Rab11, suggesting a role for Tulp1 in vesicular docking and fusion at the plasma membrane near the connecting cilium.

Long-term preservation of cones and improvement in visual function following gene therapy in a mouse model of leber congenital amaurosis caused by guanylate cyclase-1 deficiency

Leber congenital amaurosis (LCA) is a severe retinal dystrophy manifesting from early infancy as poor vision or blindness. Loss-of-function mutations in GUCY2D cause LCA1 and are one of the most common causes of LCA, accounting for 20% of all cases. Human GUCY2D and mouse Gucy2e genes encode guanylate cyclase-1 (GC1), which is responsible for restoring the dark state in photoreceptors after light exposure. The Gucy2e(-/-) mouse shows partially diminished rod function, but an absence of cone function before degeneration. Although the cones appear morphologically normal, they exhibit mislocalization of proteins involved in phototransduction. In this study we tested the efficacy of an rAAV2/8 vector containing the human rhodopsin kinase promoter and the human GUCY2D gene. Following subretinal delivery of the vector in Gucy2e(-/-) mice, GC1 protein was detected in the rod and cone outer segments, and in transduced areas of retina cone transducin was appropriately localized to cone outer segments. Moreover, we observed a dose-dependent restoration of rod and cone function and an improvement in visual behavior of the treated mice. Most importantly, cone preservation was observed in transduced areas up to 6 months post injection. To date, this is the most effective rescue of the Gucy2e(-/-) mouse model of LCA and we propose that a vector, similar to the one used in this study, could be suitable for use in a clinical trial of gene therapy for LCA1.

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.

Early-onset, slow progression of cone photoreceptor dysfunction and degeneration in CNG channel subunit CNGB3 deficiency

PURPOSE

To investigate the progression of cone dysfunction and degeneration in CNG channel subunit CNGB3 deficiency.

METHODS

Retinal structure and function in CNGB3(-/-) and wild-type (WT) mice were evaluated by electroretinography (ERG), lectin cytochemistry, and correlative Western blot analysis of cone-specific proteins. Cone and rod terminal integrity was assessed by electron microscopy and synaptic protein immunohistochemical distribution.

RESULTS

Cone ERG amplitudes (photopic b-wave) in CNGB3(-/-) mice were reduced to approximately 50% of WT levels by postnatal day 15, decreasing further to approximately 30% of WT levels by 1 month and to approximately 20% by 12 months of age. Rod ERG responses (scotopic a-wave) were not affected in CNGB3(-/-) mice. Average CNGB3(-/-) cone densities were approximately 80% of WT levels at 1 month and declined slowly thereafter to only approximately 50% of WT levels by 12 months. Expression levels of M-opsin, cone transducin α-subunit, and cone arrestin in CNGB3(-/-) mice were reduced by 50% to 60% by 1 month and declined to 35% to 45% of WT levels by 9 months. In addition, cone opsin mislocalized to the outer nuclear layer and the outer plexiform layer in the CNGB3(-/-) retina. Cone and rod synaptic marker expression and terminal ultrastructure were normal in the CNGB3(-/-) retina.

CONCLUSIONS

These findings are consistent with an early-onset, slow progression of cone functional defects and cone loss in CNGB3(-/-) mice, with the cone signaling deficits arising from disrupted phototransduction and cone loss rather than from synaptic defects.

Light prevents exogenous 11-cis retinal from maintaining cone photoreceptors in chromophore-deficient mice

PURPOSE

To determine the effect of light/dark cycles on the cones of 11-cis retinal-treated RPE65/rhodopsin double knockout (Rpe65(-/-)Rho(-/-)) mice. Studies have shown that cones degenerate in chromophore-deficient mouse models for Leber Congenital Amaurosis (LCA), but exogenous supplementation of the native 11-cis retinal chromophore can inhibit this degeneration, suggesting that 11-cis retinal could be used as a therapeutic agent for preserving functional cones in patients with LCA. However, these treated mice were maintained in the dark.

METHODS

11-cis Retinal was introduced into Rpe65(-/-)Rho(-/-) mice at postnatal day 10 as a single subcutaneous injection mixed with a basement membrane matrix. The mice were maintained in either normal light/dark cycles or constant dark conditions. Fluorescence microscopy was used to assess retinal morphology. Cone cell survival was determined by counting cone opsin-containing cells on flat-mounted P30 retinas. Cross-sections of P21 mouse retina were used to assess cone cell integrity by visualizing opsin localization. Cone function was determined by electroretinography (ERG).

RESULTS

Previous studies have shown that 11-cis retinal-treated mice lacking RPE65 and raised in constant dark have higher cone photoreceptor cell number, improved cone opsin localization, and enhanced cone ERG signals when compared with untreated mice. However, in this study the authors show that 11-cis retinal-treated Rpe65(-/-)Rho(-/-) mice raised in cyclic light did not show the improvements seen with the dark-reared mice.

CONCLUSIONS

Thus, 11-cis retinal by itself, as well as other agents that form photosensitive pigments, will not be good therapeutic candidates for preserving cones in LCA.