Rhodopsin expressed in Chinese hamster ovary cells regulates adenylyl cyclase activity

Rhodopsin signaling mediates light-induced photoreceptor cell death in rd10 mice through a transducin-independent mechanism

Retinitis pigmentosa (RP) is a debilitating blinding disease affecting over 1.5 million people worldwide, but the mechanisms underlying this disease are not well understood. One of the common models used to study RP is the retinal degeneration-10 (rd10) mouse, which has a mutation in Phosphodiesterase-6b (Pde6b) that causes a phenotype mimicking the human disease. In rd10 mice, photoreceptor cell death occurs with exposure to normal light conditions, but as demonstrated in this study, rearing these mice in dark preserves their retinal function. We found that inactivating rhodopsin signaling protected photoreceptors from degeneration suggesting that the pathway activated by this G-protein-coupled receptor is causing light-induced photoreceptor cell death in rd10 mice. However, inhibition of transducin signaling did not prevent the loss of photoreceptors in rd10 mice reared under normal light conditions implying that the degeneration caused by rhodopsin signaling is not mediated through its canonical G-protein transducin. Inexplicably, loss of transducin in rd10 mice also led to photoreceptor cell death in darkness. Furthermore, we found that the rd10 mutation in Pde6b led to a reduction in the assembled PDE6αβγ2 complex, which was corroborated by our data showing mislocalization of the γ subunit. Based on our findings and previous studies, we propose a model where light activates a non-canonical pathway mediated by rhodopsin but independent of transducin that sensitizes cyclic nucleotide gated channels to cGMP and causes photoreceptor cell death. These results generate exciting possibilities for treatment of RP patients without affecting their vision or the canonical phototransduction cascade.

Genomic form of rhodopsin DNA nanoparticles rescued autosomal dominant Retinitis pigmentosa in the P23H knock-in mouse model

Retinitis pigmentosa (RP) is a group of inherited retinal degenerative conditions and a leading cause of irreversible blindness. 25%-30% of RP cases are caused by inherited autosomal dominant (ad) mutations in the rhodopsin (Rho) protein of the retina, which impose a barrier for developing therapeutic treatments for this genetically heterogeneous disorder, as simple gene replacement is not sufficient to overcome dominant disease alleles. Previously, we have explored using the genomic short-form of Rho (sgRho) for gene augmentation therapy of RP in a Rho knockout mouse model. We have shown improved gene expression and fewer epigenetic modifications compared with the use of a Rho cDNA expression construct. In the current study, we altered our strategy by delivering a codon-optimized genomic form of Rho (co-sgRho) (for gene replacement) in combination with an RNAi-based inactivation of endogenous Rho alleles (gene suppression of both mutant Rho alleles, but mismatched with the co-sgRho) into a homozygous Rho^P23H/P23H knock-in (KI) RP mouse model, which has a severe phenotype of adRP. In addition, we have conjugated a cell penetrating TAT peptide sequence to our previously established CK30PEG10 diblock co-polymer. The DNAs were compacted with CK30PEG10-TAT diblock co-polymer to form DNA nanoparticles (NPs). These NPs were injected into the sub-retinal space of the KI mouse eyes. As a proof of concept, we demonstrated the efficiency of this strategy in the partial improvement of visual function in the Rho^P23H/P23H KI mouse model.

Phosphorylation of G protein-coupled receptor kinase 1 (GRK1) is regulated by light but independent of phototransduction in rod photoreceptors

Phosphorylation of rhodopsin by G protein-coupled receptor kinase 1 (GRK1, or rhodopsin kinase) is critical for the deactivation of the phototransduction cascade in vertebrate photoreceptors. Based on our previous studies in vitro, we predicted that Ser(21) in GRK1 would be phosphorylated by cAMP-dependent protein kinase (PKA) in vivo. Here, we report that dark-adapted, wild-type mice demonstrate significantly elevated levels of phosphorylated GRK1 compared with light-adapted animals. Based on comparatively slow half-times for phosphorylation and dephosphorylation, phosphorylation of GRK1 by PKA is likely to be involved in light and dark adaptation. In mice missing the gene for adenylyl cyclase type 1, levels of phosphorylated GRK1 were low in retinas from both dark- and light-adapted animals. These data are consistent with reports that cAMP levels are high in the dark and low in the light and also indicate that cAMP generated by adenylyl cyclase type 1 is required for phosphorylation of GRK1 on Ser(21). Surprisingly, dephosphorylation was induced by light in mice missing the rod transducin α-subunit. This result indicates that phototransduction does not play a direct role in the light-dependent dephosphorylation of GRK1.

Point mutations in bovine opsin can be classified in four groups with respect to their effect on the biosynthetic pathway of opsin

Expression in vitro with the recombinant baculovirus expression system showed correct biosynthesis and post-translational processing of "wild-type' bovine opsin with regard to translocation, glycosylation, palmitoylation and targeting. However, several of these processes were severely affected by point mutations. From the overall results of 16 mutants reported here, four groups were distinguished. One group significantly affected neither biosynthesis nor folding of opsin (D83N, P291A, A299C-V300A-P303G). A second group produced a truncated protein (R69H, Y301F), suggesting that these positions are essential for a correct translational process. A third group affected membrane translocation as well as glycosylation, which can be interpreted as interference with the function of a transfer signal. Substitutions at positions Glu-113, Glu-122, Glu-134, Arg-135 and Lys-248 belong to this category. A fourth group induced structural changes in the protein that led to heterogeneous distribution in the plasma membrane (E113Q/D, W265F, Y268S). Taking any functional consequences of these mutations into consideration, it seems that point mutations can have mosaic effects and therefore should be examined at several levels (folding, targeting, functional parameters).

The effect of carboxyl-terminal mutagenesis of Gt alpha on rhodopsin and guanine nucleotide binding

The carboxyl terminus of G protein alpha subunits plays an important role in receptor recognition. To identify the amino acids that participate in this interaction, COOH-terminal mutants of alpha t (the transducin alpha subunit) were expressed in vitro and analyzed for their ability to interact with rhodopsin and to bind guanine nucleotide. Gly-348, the reported site of a beta turn, was replaced with other neutral amino acids without severely affecting rhodopsin binding. However, proline substitution abolished rhodopsin interaction, suggesting that flexibility is important at this site. A comparison between C347Y, which lost both rhodopsin and guanine nucleotide binding, and a mutant substituted with alpha q sequence (D346E/C347Y/G348N/F350V), in which guanine nucleotide binding was restored, implies that distinct motifs maintain the structure of the alpha subunit and are necessary for selective interaction with receptors. Surprisingly, mutants L344A, L349A, F350stop, and stop351A demonstrated a parallel loss of rhodopsin and guanine nucleotide binding. Altered profiles of L344A and F350stop on sucrose density gradients indicate that these mutants may undergo denaturation. The equivalent of alpha tL344A generated in alpha s and alpha i did not show such a severe loss of guanine nucleotide binding, revealing that the alpha t carboxyl terminus is unique in its susceptibility to changes in amino acid sequence.

Effects of carboxyl-terminal truncation on the stability and G protein-coupling activity of bovine rhodopsin

A number of studies have suggested that G protein-coupled receptors possess domains within the carboxyl terminus that are important for the catalytic activation of G proteins. To define these regions, truncation mutants were generated in the cDNA of bovine rhodopsin, the receptor responsible for visual signal transduction in the retinal rod cell. The mutants were expressed in HEK-293 cells and analyzed for their ability to bind the chromophore, 11-cis-retinal, and for activating Gt, the G protein of the rod cell regulated by rhodopsin. Removal of 38 carboxyl-terminal amino acids resulted in the production of a mutant (K311 stop) that does not bind 11-cis-retinal, has an abnormal pattern of glycosylation, and does not catalyze light-dependent binding of GTP gamma S to Gt, suggesting that it is unable to fold properly during biogenesis. However, a truncation mutant with only five additional amino acids (C316stop) coupled normally to Gt, using membranes from transfected cells, despite the fact that it lacked the "fourth cytoplasmic loop" formed by palmitoylation of cysteines-322 and -323. When C316stop is extracted from the membrane with detergent, only a fraction is able to bind 11-cis-retinal, but the fraction that binds retinal activates Gt normally. In contrast, detergent-solubilized wild-type rhodopsin and K325stop (a truncation mutant with the longest carboxyl terminus) both bind retinal and activate Gt normally. These data suggest that the proximal region of the carboxyl terminus is critical for the proper folding and stability of the rhodopsin molecule and that amino acids Cys316 to Ala348 are not necessary for the activation of Gt.