A Novel GUCA1A Variant Associated with Cone Dystrophy Alters cGMP Signaling in Photoreceptors by Strongly Interacting with and Hyperactivating Retinal Guanylate Cyclase

Recombinant protein delivery enables modulation of the phototransduction cascade in mouse retina

Inherited retinal dystrophies are often associated with mutations in the genes involved in the phototransduction cascade in photoreceptors, a paradigmatic signaling pathway mediated by G protein-coupled receptors. Photoreceptor viability is strictly dependent on the levels of the second messengers cGMP and Ca2+. Here we explored the possibility of modulating the phototransduction cascade in mouse rods using direct or liposome-mediated administration of a recombinant protein crucial for regulating the interplay of the second messengers in photoreceptor outer segments. The effects of administration of the free and liposome-encapsulated human guanylate cyclase-activating protein 1 (GCAP1) were compared in biological systems of increasing complexity (in cyto, ex vivo, and in vivo). The analysis of protein biodistribution and the direct measurement of functional alteration in rod photoresponses show that the exogenous GCAP1 protein is fully incorporated into the mouse retina and photoreceptor outer segments. Furthermore, only in the presence of a point mutation associated with cone-rod dystrophy in humans p.(E111V), protein delivery induces a disease-like electrophysiological phenotype, consistent with constitutive activation of the retinal guanylate cyclase. Our study demonstrates that both direct and liposome-mediated protein delivery are powerful complementary tools for targeting signaling cascades in neuronal cells, which could be particularly important for the treatment of autosomal dominant genetic diseases.

cGMP Signaling in Photoreceptor Degeneration

Photoreceptors in the retina are highly specialized neurons with photosensitive molecules in the outer segment that transform light into chemical and electrical signals, and these signals are ultimately relayed to the visual cortex in the brain to form vision. Photoreceptors are composed of rods and cones. Rods are responsible for dim light vision, whereas cones are responsible for bright light, color vision, and visual acuity. Photoreceptors undergo progressive degeneration over time in many hereditary and age-related retinal diseases. Despite the remarkable heterogeneity of disease-causing genes, environmental factors, and pathogenesis, the progressive death of rod and cone photoreceptors ultimately leads to loss of vision/blindness. There are currently no treatments available for retinal degeneration. Cyclic guanosine 3', 5'-monophosphate (cGMP) plays a pivotal role in phototransduction. cGMP governs the cyclic nucleotide-gated (CNG) channels on the plasma membrane of the photoreceptor outer segments, thereby regulating membrane potential and signal transmission. By gating the CNG channels, cGMP regulates cellular Ca2+ homeostasis and signal transduction. As a second messenger, cGMP activates the cGMP-dependent protein kinase G (PKG), which regulates numerous targets/cellular events. The dysregulation of cGMP signaling is observed in varieties of photoreceptor/retinal degenerative diseases. Abnormally elevated cGMP signaling interferes with various cellular events, which ultimately leads to photoreceptor degeneration. In line with this, strategies to reduce cellular cGMP signaling result in photoreceptor protection in mouse models of retinal degeneration. The potential mechanisms underlying cGMP signaling-induced photoreceptor degeneration involve the activation of PKG and impaired Ca2+ homeostasis/Ca2+ overload, resulting from overactivation of the CNG channels, as well as the subsequent activation of the downstream cellular stress/death pathways. Thus, targeting the cellular cGMP/PKG signaling and the Ca2+-regulating pathways represents a significant strategy for photoreceptor protection in retinal degenerative diseases.

The Transition of Photoreceptor Guanylate Cyclase Type 1 to the Active State

Membrane-bound guanylate cyclases (GCs), which synthesize the second messenger guanosine-3', 5'-cyclic monophosphate, differ in their activation modes to reach the active state. Hormone peptides bind to the extracellular domain in hormone-receptor-type GCs and trigger a conformational change in the intracellular, cytoplasmic part of the enzyme. Sensory GCs that are present in rod and cone photoreceptor cells have intracellular binding sites for regulatory Ca²⁺-sensor proteins, named guanylate-cyclase-activating proteins. A rotation model of activation involving an α-helix rotation was described as a common activation motif among hormone-receptor GCs. We tested whether the photoreceptor GC-E underwent an α-helix rotation when reaching the active state. We experimentally simulated such a transitory switch by integrating alanine residues close to the transmembrane region, and compared the effects of alanine integration with the point mutation V902L in GC-E. The V902L mutation is found in patients suffering from retinal cone-rod dystrophies, and leads to a constitutively active state of GC-E. We analyzed the enzymatic catalytic parameters of wild-type and mutant GC-E. Our data showed no involvement of an α-helix rotation when reaching the active state, indicating a difference in hormone receptor GCs. To characterize the protein conformations that represent the transition to the active state, we investigated the protein dynamics by using a computational approach based on all-atom molecular dynamics simulations. We detected a swinging movement of the dimerization domain in the V902L mutant as the critical conformational switch in the cyclase going from the low to high activity state.

Molecular Properties of Human Guanylate Cyclase-Activating Protein 3 (GCAP3) and Its Possible Association with Retinitis Pigmentosa

The cone-specific guanylate cyclase-activating protein 3 (GCAP3), encoded by the GUCA1C gene, has been shown to regulate the enzymatic activity of membrane-bound guanylate cyclases (GCs) in bovine and teleost fish photoreceptors, to an extent comparable to that of the paralog protein GCAP1. To date, the molecular mechanisms underlying GCAP3 function remain largely unexplored. In this work, we report a thorough characterization of the biochemical and biophysical properties of human GCAP3, moreover, we identified an isolated case of retinitis pigmentosa, in which a patient carried the c.301G>C mutation in GUCA1C, resulting in the substitution of a highly conserved aspartate residue by a histidine (p.(D101H)). We found that myristoylated GCAP3 can activate GC1 with a similar Ca2+-dependent profile, but significantly less efficiently than GCAP1. The non-myristoylated form did not induce appreciable regulation of GC1, nor did the p.D101H variant. GCAP3 forms dimers under physiological conditions, but at odds with its paralogs, it tends to form temperature-dependent aggregates driven by hydrophobic interactions. The peculiar properties of GCAP3 were confirmed by 2 ms molecular dynamics simulations, which for the p.D101H variant highlighted a very high structural flexibility and a clear tendency to lose the binding of a Ca2+ ion to EF3. Overall, our data show that GCAP3 has unusual biochemical properties, which make the protein significantly different from GCAP1 and GCAP2. Moreover, the newly identified point mutation resulting in a substantially unfunctional protein could trigger retinitis pigmentosa through a currently unknown mechanism.