G-protein betagamma-complex is crucial for efficient signal amplification in vision

Updates on protein-prenylation and associated inherited retinopathies

Membrane-anchored proteins play critical roles in cell signaling, cellular architecture, and membrane biology. Hydrophilic proteins are post-translationally modified by a diverse range of lipid molecules such as phospholipids, glycosylphosphatidylinositol, and isoprenes, which allows their partition and anchorage to the cell membrane. In this review article, we discuss the biochemical basis of isoprenoid synthesis, the mechanisms of isoprene conjugation to proteins, and the functions of prenylated proteins in the neural retina. Recent discovery of novel prenyltransferases, prenylated protein chaperones, non-canonical prenylation-target motifs, and reversible prenylation is expected to increase the number of inherited systemic and blinding diseases with aberrant protein prenylation. Recent important investigations have also demonstrated the role of several unexpected regulators (such as protein charge, sequence/protein-chaperone interaction, light exposure history) in the photoreceptor trafficking of prenylated proteins. Technical advances in the investigation of the prenylated proteome and its application in vision research are discussed. Clinical updates and technical insights into known and putative prenylation-associated retinopathies are provided herein. Characterization of non-canonical prenylation mechanisms in the retina and retina-specific prenylated proteome is fundamental to the understanding of the pathogenesis of protein prenylation-associated inherited blinding disorders.

Overexpression of Nfe2l1 increases proteasome activity and delays vision loss in a preclinical model of human blindness

Proteasomes are the central proteolytic machines that are critical for breaking down most of the damaged and abnormal proteins in human cells. Although universally applicable drugs are not yet available, the stimulation of proteasomal activity is being analyzed as a proof-of-principle strategy to increase cellular resistance to a broad range of proteotoxic stressors. These approaches have included the stimulation of proteasomes through the overexpression of individual proteasome subunits, phosphorylation, or conformational changes induced by small molecules or peptides. In contrast to these approaches, we evaluated a transcription-driven increase in the total proteasome pool to enhance the proteolytic capacity of degenerating retinal neurons. We show that overexpression of nuclear factor erythroid-2-like 1 (Nfe2l1) transcription factor stimulated proteasome biogenesis and activity, improved the clearance of the ubiquitin-proteasomal reporter, and delayed photoreceptor neuron loss in a preclinical mouse model of human blindness caused by misfolded proteins. The findings highlight Nfe2l1 as an emerging therapeutic target to treat neurodegenerative diseases linked to protein misfolding.

Gene-agnostic approaches to treating inherited retinal degenerations

Most patients with inherited retinal degenerations (IRDs) have been waiting for treatments that are "just around the corner" for decades, with only a handful of seminal breakthroughs happening in recent years. Highlighting the difficulties in the quest for curative therapeutics, Luxturna required 16 years of development before finally obtaining United States Food and Drug Administration (FDA) approval and its international equivalents. IRDs are both genetically and phenotypically heterogeneous. While this diversity offers many opportunities for gene-by-gene precision medicine-based approaches, it also poses a significant challenge. For this reason, alternative (or parallel) strategies to identify more comprehensive, across-the-board therapeutics for the genetically and phenotypically diverse IRD patient population are very appealing. Even when gene-specific approaches may be available and become approved for use, many patients may have reached a disease stage whereby these approaches may no longer be viable. Thus, alternate visual preservation or restoration therapeutic approaches are needed at these stages. In this review, we underscore several gene-agnostic approaches that are being developed as therapeutics for IRDs. From retinal supplementation to stem cell transplantation, optogenetic therapy and retinal prosthetics, these strategies would bypass at least in part the need for treating every individual gene or mutation or provide an invaluable complement to them. By considering the diverse patient population and treatment strategies suited for different stages and patterns of retinal degeneration, gene agnostic approaches are very well poised to impact favorably outcomes and prognosis for IRD patients.

Investigating the Role of Rhodopsin F45L Mutation in Mouse Rod Photoreceptor Signaling and Survival

Rhodopsin is the critical receptor molecule which enables vertebrate rod photoreceptor cells to detect a single photon of light and initiate a cascade of molecular events leading to visual perception. Recently, it has been suggested that the F45L mutation in the transmembrane helix of rhodopsin disrupts its dimerization in vitro To determine whether this mutation of rhodopsin affects its signaling properties in vivo, we generated knock-in mice expressing the rhodopsin F45L mutant. We then examined the function of rods in the mutant mice versus wild-type controls, using in vivo electroretinography and transretinal and single cell suction recordings, combined with morphologic analysis and spectrophotometry. Although we did not evaluate the effect of the F45L mutation on the state of dimerization of the rhodopsin in vivo, our results revealed that F45L-mutant mice exhibit normal retinal morphology, normal rod responses as measured both in vivo and ex vivo, and normal rod dark adaptation. We conclude that the F45L mutation does not affect the signaling properties of rhodopsin in its natural setting.

Transcriptome Analysis of Retinal and Choroidal Pathologies in Aged BALB/c Mice Following Systemic Neonatal Murine Cytomegalovirus Infection

Our previous studies have shown that systemic neonatal murine cytomegalovirus (MCMV) infection of BALB/c mice spread to the eye with subsequent establishment of latency in choroid/RPE. In this study, RNA sequencing (RNA-Seq) analysis was used to determine the molecular genetic changes and pathways affected by ocular MCMV latency. MCMV (50 pfu per mouse) or medium as control were injected intra-peritoneally (i.p.) into BALB/c mice at <3 days after birth. At 18 months post injection, the mice were euthanized, and the eyes were collected and prepared for RNA-Seq. Compared to three uninfected control eyes, we identified 321 differentially expressed genes (DEGs) in six infected eyes. Using the QIAGEN Ingenuity Pathway Analysis (QIAGEN IPA), we identified 17 affected canonical pathways, 10 of which function in neuroretinal signaling, with the majority of DEGs being downregulated, while 7 pathways function in upregulated immune/inflammatory responses. Retinal and epithelial cell death pathways involving both apoptosis and necroptosis were also activated. MCMV ocular latency is associated with upregulation of immune and inflammatory responses and downregulation of multiple neuroretinal signaling pathways. Cell death signaling pathways are also activated and contribute to the degeneration of photoreceptors, RPE, and choroidal capillaries.

Regulation of rod photoreceptor function by farnesylated G-protein γ-subunits

Heterotrimeric G-protein transducin, Gt, is a key signal transducer and amplifier in retinal rod and cone photoreceptor cells. Despite similar subunit composition, close amino acid identity, and identical posttranslational farnesylation of their Gγ subunits, rods and cones rely on unique Gγ1 (Gngt1) and Gγc (Gngt2) isoforms, respectively. The only other farnesylated G-protein γ-subunit, Gγ11 (Gng11), is expressed in multiple tissues but not retina. To determine whether Gγ1 regulates uniquely rod phototransduction, we generated transgenic rods expressing Gγ1, Gγc, or Gγ11 in Gγ1-deficient mice and analyzed their properties. Immunohistochemistry and Western blotting demonstrated the robust expression of each transgenic Gγ in rod cells and restoration of Gαt1 expression, which is greatly reduced in Gγ1-deficient rods. Electroretinography showed restoration of visual function in all three transgenic Gγ1-deficient lines. Recordings from individual transgenic rods showed that photosensitivity impaired in Gγ1-deficient rods was also fully restored. In all dark-adapted transgenic lines, Gαt1 was targeted to the outer segments, reversing its diffuse localization found in Gγ1-deficient rods. Bright illumination triggered Gαt1 translocation from the rod outer to inner segments in all three transgenic strains. However, Gαt1 translocation in Gγ11 transgenic mice occurred at significantly dimmer background light. Consistent with this, transretinal ERG recordings revealed gradual response recovery in moderate background illumination in Gγ11 transgenic mice but not in Gγ1 controls. Thus, while farnesylated Gγ subunits are functionally active and largely interchangeable in supporting rod phototransduction, replacement of retina-specific Gγ isoforms by the ubiquitous Gγ11 affects the ability of rods to adapt to background light.

Applying Protein–Protein Interactions and Complex Networks to Identify Novel Genes in Retinitis Pigmentosa Pathogenesis

Retinitis Pigmentosa (RP) is a hereditary retinal disorder that causes the atrophy of photoreceptor rod cells. Since individual defective genes converge on the same disease, we hypothesized that all causal genes of RP belong in a complex network. To explore this hypothesis, we conducted a gene connection analysis using 161 genes attributed to RP, compiled from the Retinal Information Network, RetNet. We then examined the protein interaction network (PIN) of these genes. In line with our hypothesis, using STRING, we directly connected 149 genes out of the recognized 159 genes. To uncover the association between the PIN and the ten unrecalled genes, we developed an algorithm to pinpoint the best candidate genes to connect the uncalled genes to the PIN and identified ten such genes. We propose that mutations within these ten genes may also cause RP; this notion is supported by analyzing and categorizing the known causal genes based on cellular locations and related functions. The successful establishment of the PIN among all documented genes and the discovery of novel genes for RP strongly suggest an interconnectedness that causes the disease on the molecular level. In addition, our computational gene search protocol can help identify the genes and loci responsible for genetic diseases, not limited to RP.

Stratified Test Accurately Identifies Differentially Expressed Genes Under Batch Effects in Single-Cell Data

Analyzing single-cell sequencing data from large cohorts is challenging. Discrepancies across experiments and differences among participants often lead to omissions and false discoveries in differentially expressed genes. We find that the Van Elteren test, a stratified version of the widely used Wilcoxon rank-sum test, elegantly mitigates the problem. We also modified the common language effect size to supplement this test, further improving its utility. On both simulated and real patient data we show the ability of Van Elteren test to control for false positives and false negatives. A comprehensive assessment using receiver operating characteristic (ROC) curve shows that Van Elteren test achieves higher sensitivity and specificity on simulated datasets, compared with nine state-of-the-art differential expression analysis methods. The effect size also estimates the differences between cell types more accurately.

Arabidopsis cryptochrome 1 promotes stomatal development through repression of AGB1 inhibition of SPEECHLESS DNA-binding activity

Cryptochromes are blue light photoreceptors that mediate various light responses in plants and mammals. The heterotrimeric G-protein is known to regulate various physiological processes in plants and mammals. In Arabidopsis, cryptochrome 1 (CRY1) and the G-protein β subunit AGB1 act antagonistically to regulate stomatal development. The molecular mechanism by which CRY1 and AGB1 regulate this process remains unknown. Here, we show that Arabidopsis CRY1 acts partially through AGB1, and AGB1 acts through SPEECHLESS (SPCH), a master transcription factor that drives stomatal initiation and proliferation, to regulate stomatal development. We demonstrate that AGB1 physically interacts with SPCH to block the bHLH DNA-binding domain of SPCH and inhibit its DNA-binding activity. Moreover, we demonstrate that photoexcited CRY1 represses the interaction of AGB1 with SPCH to release AGB1 inhibition of SPCH DNA-binding activity, leading to the expression of SPCH-target genes promoting stomatal development. Taken together, our results suggest that the mechanism by which CRY1 promotes stomatal development involves positive regulation of the DNA-binding activity of SPCH mediated by CRY1 inhibition of the AGB1-SPCH interaction. We propose that the antagonistic regulation of SPCH DNA-binding activity by CRY1 and AGB1 may allow plants to balance light and G-protein signaling and optimize stomatal density and pattern.

Gene network analyses unveil possible molecular basis underlying drug-induced glaucoma

BACKGROUND

Drug-induced glaucoma (DIG) is a kind of serious adverse drug reaction that can cause irreversible blindness. Up-to-date, the molecular mechanism of DIG largely remains unclear yet due to the medical complexity of glaucoma onset.

METHODS

In this study, we conducted data mining of tremendous historical adverse drug events and genome-wide drug-regulated gene signatures to identify glaucoma-associated drugs. Upon these drugs, we carried out serial network analyses, including the weighted gene co-expression network analysis (WGCNA), to illustrate the gene interaction network underlying DIG. Furthermore, we applied pathogenic risk assessment to discover potential biomarker genes for DIG.

RESULTS

As the results, we discovered 13 highly glaucoma-associated drugs, a glaucoma-related gene network, and 55 glaucoma-susceptible genes. These genes likely played central roles in triggering DIGs via an integrative mechanism of phototransduction dysfunction, intracellular calcium homeostasis disruption, and retinal ganglion cell death. Further pathogenic risk analysis manifested that a panel of nine genes, particularly OTOF gene, could serve as potential biomarkers for early-onset DIG prognosis.

CONCLUSIONS

This study elucidates the possible molecular basis underlying DIGs systematically for the first time. It also provides prognosis clues for early-onset glaucoma and thus assists in designing better therapeutic regimens.

NRL-/- gene edited human embryonic stem cells generate rod-deficient retinal organoids enriched in S-cone-like photoreceptors

Organoid cultures represent a unique tool to investigate the developmental complexity of tissues like the human retina. NRL is a transcription factor required for the specification and homeostasis of mammalian rod photoreceptors. In Nrl-deficient mice, photoreceptor precursor cells do not differentiate into rods, and instead follow a default photoreceptor specification pathway to generate S-cone-like cells. To investigate whether this genetic switch mechanism is conserved in humans, we used CRISPR/Cas9 gene editing to engineer an NRL-deficient embryonic stem cell (ESC) line (NRL-/- ), and differentiated it into retinal organoids. Retinal organoids self-organize and resemble embryonic optic vesicles (OVs) that recapitulate the natural histogenesis of rods and cone photoreceptors. NRL-/- OVs develop comparably to controls, and exhibit a laminated, organized retinal structure with markers of photoreceptor synaptogenesis. Using immunohistochemistry and quantitative polymerase chain reaction (qPCR), we observed that NRL-/- OVs do not express NRL, or other rod photoreceptor markers directly or indirectly regulated by NRL. On the contrary, they show an abnormal number of photoreceptors positive for S-OPSIN, which define a primordial subtype of cone, and overexpress other cone genes indicating a conserved molecular switch in mammals. This study represents the first evidence in a human in vitro ESC-derived organoid system that NRL is required to define rod identity, and that in its absence S-cone-like cells develop as the default photoreceptor cell type. It shows how gene edited retinal organoids provide a useful system to investigate human photoreceptor specification, relevant for efforts to generate cells for transplantation in retinal degenerative diseases.

Retinal homeostasis and metformin-induced protection are not affected by retina-specific Pparδ knockout

Peroxisome proliferator-activated receptors (PPARs) are a family of three nuclear hormone receptors (PPARα, PPARδ, and PPARγ) that are known to regulate expression of lipid metabolism and oxidative stress genes. Given their role in reducing oxidative stress in a variety of tissues, these genes are likely important for retinal homeostasis. This hypothesis has been further supported by recent studies suggesting that PPAR-activating drugs are protective against retinal degenerations. The objective of the present study was to determine the role of PPARδ in the neuroretina. RNA-seq data show that Pparα and Pparδ are both expressed in the retina, but that Pparδ is expressed at 4-fold higher levels. Single-cell RNAseq data show that Pparδ is broadly expressed in all retinal cell types. To determine the importance of Pparδ to the retina, we generated retina-specific Pparδ knockout mice. We found that deletion of Pparδ had a minimal effect on retinal function or morphology out to 12 months of age and did not increase retinal sensitivity to oxidative stress induced by exposure to bright light. While data show that PPARδ levels were increased by the drug metformin, PPARδ was not necessary for metformin-induced protection from light damage. These data suggest that Pparδ either has a redundant function with Pparα or is not essential for normal neuroretina function or resistance to oxidative stress.

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PPARδ is not essential for neuroretinal development or retinal structure or function.

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Deletion of PPARδ in the neuroretina does not enhance degeneration due to light-induced damage.

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Metformin-induced neuroprotection is not mediated by PPARδ activity.

Mouse Models of Inherited Retinal Degeneration with Photoreceptor Cell Loss

Inherited retinal degeneration (RD) leads to the impairment or loss of vision in millions of individuals worldwide, most frequently due to the loss of photoreceptor (PR) cells. Animal models, particularly the laboratory mouse, have been used to understand the pathogenic mechanisms that underlie PR cell loss and to explore therapies that may prevent, delay, or reverse RD. Here, we reviewed entries in the Mouse Genome Informatics and PubMed databases to compile a comprehensive list of monogenic mouse models in which PR cell loss is demonstrated. The progression of PR cell loss with postnatal age was documented in mutant alleles of genes grouped by biological function. As anticipated, a wide range in the onset and rate of cell loss was observed among the reported models. The analysis underscored relationships between RD genes and ciliary function, transcription-coupled DNA damage repair, and cellular chloride homeostasis. Comparing the mouse gene list to human RD genes identified in the RetNet database revealed that mouse models are available for 40% of the known human diseases, suggesting opportunities for future research. This work may provide insight into the molecular players and pathways through which PR degenerative disease occurs and may be useful for planning translational studies.

Stratified Test Alleviates Batch Effects in Single-Cell Data

Analyzing single-cell sequencing data across batches is challenging. We find that the Van Elteren test, a stratified version of Wilcoxon rank-sum test, elegantly mitigates the problem. We also modified the common language effect size to supplement this test, further improving its utility. On both simulated and real patient data we show the ability of Van Elteren test to control for false positives and false negatives. The effect size also estimates the differences between cell types more accurately.

Single-nuclei RNA-seq on human retinal tissue provides improved transcriptome profiling

Single-cell RNA-seq is a powerful tool in decoding the heterogeneity in complex tissues by generating transcriptomic profiles of the individual cell. Here, we report a single-nuclei RNA-seq (snRNA-seq) transcriptomic study on human retinal tissue, which is composed of multiple cell types with distinct functions. Six samples from three healthy donors are profiled and high-quality RNA-seq data is obtained for 5873 single nuclei. All major retinal cell types are observed and marker genes for each cell type are identified. The gene expression of the macular and peripheral retina is compared to each other at cell-type level. Furthermore, our dataset shows an improved power for prioritizing genes associated with human retinal diseases compared to both mouse single-cell RNA-seq and human bulk RNA-seq results. In conclusion, we demonstrate that obtaining single cell transcriptomes from human frozen tissues can provide insight missed by either human bulk RNA-seq or animal models.

AAA+ Protein-Based Technologies to Counter Neurodegenerative Disease

Protein misfolding and overloaded proteostasis networks underlie a range of neurodegenerative diseases. No cures exist for these diseases, but developing effective therapeutic agents targeting the toxic, misfolded protein species in disease is one promising strategy. AAA+ (ATPases associated with diverse cellular activities) protein translocases, which naturally unfold and translocate substrate proteins, could be potent therapeutic agents to disassemble toxic protein conformers in neurodegenerative disease. Here, we discuss repurposing AAA+ protein translocases Hsp104 and proteasome-activating nucleotidase (PAN) to alleviate the toxicity from protein misfolding in neurodegenerative disease. Hsp104 effectively protects various animal models from neurodegeneration underpinned by protein misfolding, and enhanced Hsp104 variants strongly counter neurodegenerative disease-associated protein misfolding toxicity in yeast, Caenorhabditis elegans, and mammalian cells. Similarly, a recently engineered PAN variant (PANet) mitigates photoreceptor degeneration instigated by protein misfolding in a mouse model of retinopathy. Further study and engineering of AAA+ translocases like Hsp104 and PAN will reveal promising agents to combat protein misfolding toxicity in neurodegenerative disease.

Phytochrome B and AGB1 Coordinately Regulate Photomorphogenesis by Antagonistically Modulating PIF3 Stability in Arabidopsis

Phytochrome B (phyB), the primary red light photoreceptor, promotes photomorphogenesis in Arabidopsis by interacting with the basic helix-loop-helix transcriptional factor PIF3 and inducing its phosphorylation and degradation. Heterotrimeric G proteins are known to regulate various developmental processes in plants and animals. In Arabidopsis, the G-protein β subunit AGB1 is known to repress photomorphogenesis. However, whether and how phyB and AGB1 coordinately regulate photomorphogenesis are largely unknown. Here we show that phyB physically interacts with AGB1 in a red light-dependent manner and that AGB1 interacts directly with PIF3. Moreover, we demonstrate that the AGB1-PIF3 interaction inhibits the association of PIF3 with phyB, leading to reduced phosphorylation and degradation of PIF3, whereas the phyB-AGB1 interaction represses the association of PIF3 with AGB1, resulting in enhanced phosphorylation and degradation of PIF3. Our results suggest that phyB and AGB1 antagonistically regulate PIF3 stability by dynamically interacting with each other and PIF3. This dynamic mechanism may allow plants to balance phyB and G-protein signaling to optimize photomorphogenesis.

Molecular Classification and Comparative Taxonomics of Foveal and Peripheral Cells in Primate Retina

High-acuity vision in primates, including humans, is mediated by a small central retinal region called the fovea. As more accessible organisms lack a fovea, its specialized function and its dysfunction in ocular diseases remain poorly understood. We used 165,000 single-cell RNA-seq profiles to generate comprehensive cellular taxonomies of macaque fovea and peripheral retina. More than 80% of >60 cell types match between the two regions but exhibit substantial differences in proportions and gene expression, some of which we relate to functional differences. Comparison of macaque retinal types with those of mice reveals that interneuron types are tightly conserved. In contrast, projection neuron types and programs diverge, despite exhibiting conserved transcription factor codes. Key macaque types are conserved in humans, allowing mapping of cell-type and region-specific expression of >190 genes associated with 7 human retinal diseases. Our work provides a framework for comparative single-cell analysis across tissue regions and species.

Archaeal Unfoldase Counteracts Protein Misfolding Retinopathy in Mice

Deregulation of cellular proteostasis due to the failure of the ubiquitin proteasome system to dispose of misfolded aggregation-prone proteins is a hallmark of various neurodegenerative diseases in humans. Microorganisms have evolved to survive massive protein misfolding and aggregation triggered by heat shock using their protein-unfolding ATPases (unfoldases) from the Hsp100 family. Because the Hsp100 chaperones are absent in homoeothermic mammals, we hypothesized that the vulnerability of mammalian neurons to misfolded proteins could be mitigated by expressing a xenogeneic unfoldase. To test this idea, we expressed proteasome-activating nucleotidase (PAN), a protein-unfolding ATPase from thermophilic Archaea, which is homologous to the 19S eukaryotic proteasome and similar to the Hsp100 family chaperones in rod photoreceptors of mice. We found that PAN had no obvious effect in healthy rods; however, it effectively counteracted protein-misfolding retinopathy in Gγ1 knock-out mice. We conclude that archaeal PAN can rescue a protein-misfolding neurodegenerative disease, likely by recognizing misfolded mammalian proteins.SIGNIFICANCE STATEMENT This study demonstrates successful therapeutic application of an archaeal molecular chaperone in an animal model of neurodegenerative disease. Introducing the archaeal protein-unfolding ATPase proteasome-activating nucleotidase (PAN) into the retinal photoreceptors of mice protected these neurons from the cytotoxic effect of misfolded proteins. We propose that xenogeneic protein-unfolding chaperones could be equally effective against other types of neurodegenerative diseases of protein-misfolding etiology.

Transducin β-Subunit Can Interact with Multiple G-Protein γ-Subunits to Enable Light Detection by Rod Photoreceptors

The heterotrimeric G-protein transducin mediates visual signaling in vertebrate photoreceptor cells. Many aspects of the function of transducin were learned from knock-out mice lacking its individual subunits. Of particular interest is the knockout of its rod-specific γ-subunit (Gγ₁). Two studies using independently generated mice documented that this knockout results in a considerable >60-fold reduction in the light sensitivity of affected rods, but provided different interpretations of how the remaining α-subunit (Gα_t) mediates phototransduction without its cognate Gβ₁γ₁-subunit partner. One study found that the light sensitivity reduction matched a corresponding reduction in Gα_t content in the light-sensing rod outer segments and proposed that Gα_t activation is supported by remaining Gβ₁ associating with other Gγ subunits naturally expressed in photoreceptors. In contrast, the second study reported the same light sensitivity loss but a much lower, only approximately sixfold, reduction of Gα_t and proposed that the light responses of these rods do not require Gβγ at all. To resolve this controversy and elucidate the mechanism driving visual signaling in Gγ₁ knock-out rods, we analyzed both mouse lines side by side. We first determined that the outer segments of both mice have identical Gα_t content, which is reduced ∼65-fold from the wild-type (WT) level. We further demonstrated that the remaining Gβ₁ is present in a complex with endogenous Gγ₂ and Gγ₃ subunits and that these complexes exist in wild-type rods as well. Together, these results argue against the idea that Gα_t alone supports light responses of Gγ₁ knock-out rods and suggest that Gβ₁γ₁ is not unique in its ability to mediate vertebrate phototransduction.

LRIT1 Modulates Adaptive Changes in Synaptic Communication of Cone Photoreceptors

Cone photoreceptors scale dynamically the sensitivity of responses to maintain responsiveness across wide range of changes in luminance. Synaptic changes contribute to this adaptation, but how this process is coordinated at the molecular level is poorly understood. Here, we report that a cell adhesion-like molecule, LRIT1, is enriched selectively at cone photoreceptor synapses where it engages in a trans-synaptic interaction with mGluR6, the principal receptor in postsynaptic ON-bipolar cells. The levels of LRIT1 are regulated by the neurotransmitter release apparatus that controls photoreceptor output. Knockout of LRIT1 in mice increases the sensitivity of cone synaptic signaling while impairing its ability to adapt to background light without overtly influencing the morphology or molecular composition of photoreceptor synapses. Accordingly, mice lacking LRIT1 show visual deficits under conditions requiring temporally challenging discrimination of visual signals in steady background light. These observations reveal molecular mechanisms involved in scaling synaptic communication in the retina.

Farnesylation of the Transducin G Protein Gamma Subunit Is a Prerequisite for Its Ciliary Targeting in Rod Photoreceptors

Primary cilia are microtubule-based organelles, which protrude from the plasma membrane and receive a wide range of extracellular signals. Various cilia use G protein-coupled receptors (GPCRs) for the detection of these signals. For instance, vertebrate rod photoreceptors use their cilia (also called outer segments) as antennae detecting photons by GPCR rhodopsin. Rhodopsin recognizes incoming light and activates its G protein, transducin, which is composed of three subunits α, β, and γ. Similar to all G protein γ subunits, the transducin Gγ1 subunit undergoes C-terminal prenylation resulting in the addition of an isoprenoid farnesyl; however, the significance of this posttranslational modification is unclear. To study the role of the farnesyl group, we genetically introduced a mutant Gγ1 that lacked the prenylation site into the retinal photoreceptors of mice. The biochemical and physiological analyses of these mice revealed that mutant Gγ1 dimerizes with the endogenous transducin Gβ1 subunit and that the resulting Gβγ dimers display reduced hydrophobicity. Although mutant Gβγ dimers could form a heterotrimeric G protein, they could not mediate phototransduction. This deficiency was due to a strong exclusion of non-farnesylated Gβγ complexes from the cilia (rod outer segments). Our results provide the first evidence that farnesylation is required for trafficking of G-protein βγ subunits to the cilium of rod photoreceptors.

Deficiency of Isoprenylcysteine Carboxyl Methyltransferase (ICMT) Leads to Progressive Loss of Photoreceptor Function

Retinal neurons use multiple strategies to fine-tune visual signal transduction, including post-translational modifications of proteins, such as addition of an isoprenyl lipid to a carboxyl-terminal cysteine in proteins that terminate with a "CAAX motif." We previously showed that RAS converting enzyme 1 (RCE1)-mediated processing of isoprenylated proteins is required for photoreceptor maintenance and function. However, it is not yet known whether the requirement for the RCE1-mediated protein processing is related to the absence of the endoproteolytic processing step, the absence of the subsequent methylation step by isoprenylcysteine methyltransferase (ICMT), or both. To approach this issue and to understand the significance of protein methylation, we generated mice lacking Icmt expression in the retina. In the absence of Icmt expression, rod and cone light-mediated responses diminished progressively. Lack of ICMT-mediated methylation led to defective association of isoprenylated transducin and cone phosphodiesterase 6 (PDE6α') with photoreceptor membranes and resulted in decreased levels of transducin, PDE6α', and cone G-protein coupled receptor kinase-1 (GRK1). In contrast to our earlier findings with retina-specific Rce1 knock-out mice, rod PDE6 in Icmt-deficient mice trafficked normally to the photoreceptor outer segment, suggesting that the failure to remove the -AAX is responsible for blocking the movement of PDE6 to the outer segment. Our findings demonstrate that carboxyl methylation of isoprenylated proteins is crucial for maintenance of photoreceptor function.

SIGNIFICANCE STATEMENT

In this report, we show that an absence of isoprenylcysteine methyltransferase-mediated protein methylation leads to progressive loss of vision. Photoreceptors also degenerate, although at a slower pace than the rate of visual loss. The reduction in photoresponses is due to defective association of crucial players in phototransduction cascade. Unlike the situation with RCE1 deficiency, where both methylation and removal of -AAX were affected, the transport of isoprenylated proteins in isoprenylcysteine methyltransferase-deficient retinas was not dependent on methylation. This finding implies that the retention of the -AAX in PDE6 catalytic subunits in Rce1(-/-) mice is responsible for impeding their transport to the rod photoreceptor outer segment. In conclusion, lack of methylation of isoprenylcysteines leads to age-dependent photoreceptor dysfunction.

Seed dormancy cycling and the regulation of dormancy mechanisms to time germination in variable field environments

Many molecular mechanisms that regulate dormancy have been identified individually in controlled laboratory studies. However, little is known about how the seed employs this complex suite of mechanisms during dormancy cycling in the variable environment of the soil seed bank. Nevertheless, this behaviour is essential to ensure germination takes place in a favourable habitat and climate space, and in the correct season for the resulting plant to complete its life cycle. During their time in the soil seed bank, seeds continually adjust their dormancy status by sensing a range of environmental signals. Those related to slow seasonal change (e.g. temperature) are used for temporal sensing to determine the time of year and depth of dormancy. This alters their sensitivity to signals related to their spatial environment (e.g. light, nitrate, and water potential) that indicate that conditions are suitable for germination, and so trigger the termination of dormancy. We review work on the physiological, molecular, and ecological aspects of seed dormancy in Arabidopsis and interpret it in the context of dormancy cycling in the soil seed bank. This approach has provided new insight into the co-ordination of mechanisms and signalling networks, and the multidimensional sensing that regulates dormancy cycling in a variable environment.

Ultraweak Photon Emission from the Seed Coat in Response to Temperature and Humidity-A Potential Mechanism for Environmental Signal Transduction in the Soil Seed Bank

Seeds beneath the soil sense the changing environment to time germination and seedling emergence with the optimum time of year for survival. Environmental signals first impact with the seed at the seed coat. To investigate whether seed coats have a role in environmental sensing we investigated their ultraweak photon emission (UPE) under the variable temperature, relative humidity and oxygen conditions they could experience in the soil seed bank. Using a custom‐built luminometer we measured UPE intensity and spectra (300–700 nm) from Phaseolus vulgaris seeds, seed coats and cotyledons. UPE was greatest from the internal surface of the seed coat. Seed coat UPE increased concomitantly with both increasing temperature and decreasing relative humidity. Emission was oxygen dependent and it was abolished by treatment with dinitrophenylhydrazine, demonstrating the key role of seed coat carbonyls in the phenomenon. We hypothesize that beneath the soil surface the attenuation of light (virtual darkness: low background noise) enables seeds to exploit UPE for transducing key environmental variables in the soil (temperature, humidity and oxygen) to inform them of seasonal and local temperature patterns. Overall, seed coats were found to have potential as effective transducers of key fluctuating environmental variables in the soil.

Lack of mGluR6-related cascade elements leads to retrograde trans-synaptic effects on rod photoreceptor synapses via matrix-associated proteins

Heterotrimeric G-proteins couple metabotropic receptors to downstream effectors. In retinal ON bipolar cells, Go couples the metabotropic receptor mGluR6 to the TRPM1 channel and closes it in the dark, thus hyperpolarizing the cell. Light, via GTPase-activating proteins, deactivates Go , opens TRPM1 and depolarizes the cell. Go comprises Gαo1 , Gβ3 and Gγ13; all are necessary for efficient coupling. In addition, Gβ3 contributes to trafficking of certain cascade proteins and to maintaining the synaptic structure. The goal of this study was to determine the role of Gαo1 in maintaining the cascade and synaptic integrity. Using mice lacking Gαo1 , we quantified the immunostaining of certain mGluR6-related components. Deleting Gαo1 greatly reduced staining for Gβ3, Gγ13, Gβ5, RGS11, RGS7 and R9AP. Deletion of Gαo1 did not affect mGluR6, TRPM1 or PCP2. In addition, deleting Gαo1 reduced the number of rod bipolar dendrites that invaginate the rod terminal, similar to the effect seen in the absence of mGluR6, Gβ3 or the matrix-associated proteins, pikachurin, dystroglycan and dystrophin, which are localized presynaptically to the rod bipolar cell. We therefore tested mice lacking mGluR6, Gαo1 and Gβ3 for expression of these matrix-associated proteins. In all three genotypes, staining intensity for these proteins was lower than in wild type, suggesting a retrograde trans-synaptic effect. We propose that the mGluR6 macromolecular complex is connected to the presynaptic rod terminal via a protein chain that includes the matrix-associated proteins. When a component of the macromolecular chain is missing, the chain may fall apart and loosen the dendritic tip adherence within the invagination.

Stable G protein-effector complexes in striatal neurons: mechanism of assembly and role in neurotransmitter signaling

In the striatum, signaling via G protein-coupled neurotransmitter receptors is essential for motor control. Critical to this process is the effector enzyme adenylyl cyclase type 5 (AC5) that produces second messenger cAMP upon receptor-mediated activation by G protein Golf. However, the molecular organization of the Golf-AC5 signaling axis is not well understood. In this study, we report that in the striatum AC5 exists in a stable pre-coupled complex with subunits of Golf heterotrimer. We use genetic mouse models with disruption in individual components of the complex to reveal hierarchical order of interactions required for AC5-Golf stability. We further identify that the assembly of AC5-Golf complex is mediated by PhLP1 chaperone that plays central role in neurotransmitter receptor coupling to cAMP production motor learning. These findings provide evidence for the existence of stable G protein-effector signaling complexes and identify a new component essential for their assembly.

A new mouse model for stationary night blindness with mutant Slc24a1 explains the pathophysiology of the associated human disease

Mutations that affect calcium homeostasis (Ca(2+)) in rod photoreceptors are linked to retinal degeneration and visual disorders such as retinitis pigmentosa and congenital stationary night blindness (CSNB). It is thought that the concentration of Ca(2+) in rod outer segments is controlled by a dynamic balance between influx via cGMP-gated (CNG) channels and extrusion via Na(+)/Ca(2+), K(+) exchangers (NCKX1). The extrusion-driven lowering of rod [Ca(2+)]i following light exposure controls their light adaptation and response termination. Mutant NCKX1 has been linked to autosomal-recessive stationary night blindness. However, whether NCKX1 contributes to light adaptation has not been directly tested and the mechanisms by which human NCKX1 mutations cause night blindness are not understood. Here, we report that the deletion of NCKX1 in mice results in malformed outer segment disks, suppressed expression and function of rod CNG channels and a subsequent 100-fold reduction in rod responses, while preserving normal cone responses. The compensating loss of CNG channel function in the absence of NCKX1-mediated Ca(2+) extrusion may prevent toxic Ca(2+) buildup and provides an explanation for the stationary nature of the associated disorder in humans. Surprisingly, the lack of NCKX1 did not compromise rod background light adaptation, suggesting additional Ca(2+)-extruding mechanisms exist in these cells.

Sensitivity and kinetics of signal transmission at the first visual synapse differentially impact visually-guided behavior

In the retina, synaptic transmission between photoreceptors and downstream ON-bipolar neurons (ON-BCs) is mediated by a GPCR pathway, which plays an essential role in vision. However, the mechanisms that control signal transmission at this synapse and its relevance to behavior remain poorly understood. In this study we used a genetic system to titrate the rate of GPCR signaling in ON-BC dendrites by varying the concentration of key RGS proteins and measuring the impact on transmission of signal between photoreceptors and ON-BC neurons using electroretinography and single cell recordings. We found that sensitivity, onset timing, and the maximal amplitude of light-evoked responses in rod- and cone-driven ON-BCs are determined by different RGS concentrations. We further show that changes in RGS concentration differentially impact visually guided-behavior mediated by rod and cone ON pathways. These findings illustrate that neuronal circuit properties can be modulated by adjusting parameters of GPCR-based neurotransmission at individual synapses.

DOI:http://dx.doi.org/10.7554/eLife.06358.001

At the back of the eye, a structure called the retina contains several types of cell that convert light into the electrical signals that the brain interprets to produce vision. Cells called rods and cones detect the light, and then signal to other neurons in the retina that relay this information to the brain. Rods and cones are specialized to respond best to different visual features: cones detect color and can track rapid movement; whereas rods are more sensitive to low light levels and so enable night vision.

All rods and cones communicate with particular types of neuron called an ‘ON bipolar cell’: rods send their information to rod-specific ON bipolar cells and cones to cone ON-bipolar cells. To maintain the differences in how visual features are detected, the signals sent by the rod or cone cells need to be tuned separately. Previous studies showed that bipolar cells rely on the action of proteins called RGSs to control how information is passed from rods and cones to ON bipolar cells. However, how the RGS proteins produce their effects is not well understood, and neither is their impact on vision or behavior.

Sarria et al. used a genetic approach to create mice that progressively lost RGS proteins from their retina over the course of several weeks. Recording the nerve impulses produced by the bipolar cells as light shone on the retina revealed that RGS depletion affects these neurons in three ways: how sensitive they are to the signals sent by the rod and cone cells, how quickly they respond to a signal, and the size of the electrical response that they produce.

Sarria et al. then investigated how these changes affected the behavior of the mice. To test the response of the rod cells, the mice performed tasks in dim light. This revealed that it was only when the sensitivity of the bipolar cells decreased that the mice performed worse. However, in a task involving fast-moving objects that investigated the response of cone cells, only changes to the speed of the response affected vision. Therefore, the RGS protein has different effects on the signals from rod cells and cone cells. These findings will be useful for understanding how different light sensitive cells in the retina communicate their signals to extract important visual features, allowing us to both see well at night and track rapid changes in scenery on a bright sunny day.

DOI:http://dx.doi.org/10.7554/eLife.06358.002

Activation and quenching of the phototransduction cascade in retinal cones as inferred from electrophysiology and mathematical modeling

PURPOSE

To experimentally identify and quantify factors responsible for the lower sensitivity of retinal cones compared to rods.

METHODS

Electrical responses of frog rods and fish (Carassius) cones to short flashes of light were recorded using the suction pipette technique. A fast solution changer was used to apply a solution that fixed intracellular Ca2+ concentration at the prestimulus level, thereby disabling Ca2+ feedback, to the outer segment (OS). The results were analyzed with a specially designed mathematical model of phototransduction. The model included all basic processes of activation and quenching of the phototransduction cascade but omitted unnecessary mechanistic details of each step.

RESULTS

Judging from the response versus intensity curves, Carassius cones were two to three orders of magnitude less sensitive than frog rods. There was a large scatter in sensitivity among individual cones, with red-sensitive cones being on average approximately two times less sensitive than green-sensitive ones. The scatter was mostly due to different signal amplification, since the kinetic parameters of the responses among cones were far less variable than sensitivity. We argue that the generally accepted definition of the biochemical amplification in phototransduction cannot be used for comparing amplification in rods and cones, since it depends on an irrelevant factor, that is, the cell's volume. We also show that the routinely used simplified parabolic curve fitting to an initial phase of the response leads to a few-fold underestimate of the amplification. We suggest a new definition of the amplification that only includes molecular parameters of the cascade activation, and show how it can be derived from experimental data. We found that the mathematical model with unrestrained parameters can yield an excellent fit to experimental responses. However, the fits with wildly different sets of parameters can be virtually indistinguishable, and therefore cannot provide meaningful data on underlying mechanisms. Based on results of Ca2+-clamp experiments, we developed an approach to strongly constrain the values of many key parameters that set the time course and sensitivity of the photoresponse (such as the dark turnover rate of cGMP, rates of turnoffs of the photoactivated visual pigment and phosphodiesterase, and kinetics of Ca2+ feedback). We show that applying these constraints to our mathematical model enables accurate determination of the biochemical amplification in phototransduction. It appeared that, contrary to many suggestions, maximum biochemical amplification derived for "best" Carassius cones was as high as in frog rods. On the other hand, all turnoff and recovery reactions in cones proceeded approximately 10 times faster than in rods.

CONCLUSIONS

The main cause of the differing sensitivity of rods and cones is cones' ability to terminate their photoresponse faster.

Retinal cone photoreceptors require phosducin-like protein 1 for G protein complex assembly and signaling

G protein β subunits (Gβ) play essential roles in phototransduction as part of G protein βγ (Gβγ) and regulator of G protein signaling 9 (RGS9)-Gβ5 heterodimers. Both are obligate dimers that rely on the cytosolic chaperone CCT and its co-chaperone PhLP1 to form complexes from their nascent polypeptides. The importance of PhLP1 in the assembly process was recently demonstrated in vivo in a retinal rod-specific deletion of the Phlp1 gene. To test whether this is a general mechanism that also applies to other cell types, we disrupted the Phlp1 gene specifically in mouse cones and measured the effects on G protein expression and cone visual signal transduction. In PhLP1-deficient cones, expression of cone transducin (Gt2) and RGS9-Gβ5 subunits was dramatically reduced, resulting in a 27-fold decrease in sensitivity and a 38-fold delay in cone photoresponse recovery. These results demonstrate the essential role of PhLP1 in cone G protein complex formation. Our findings reveal a common mechanism of Gβγ and RGS9-Gβ5 assembly in rods and cones, highlighting the importance of PhLP1 and CCT-mediated Gβ complex formation in G protein signaling.

Differential function of Gγ13 in rod bipolar and ON cone bipolar cells

Heterotrimeric G-proteins (comprising Gα and Gβγ subunits) are critical for coupling of metabotropic receptors to their downstream effectors. In the retina, glutamate released from photoreceptors in the dark activates metabotropic glutamate receptor 6 (mGluR6) receptors in ON bipolar cells; this leads to activation of Go , closure of transient receptor potential melastatin 1 channels and hyperpolarization of these cells. Go comprises Gαo , Gβ3 and a Gγ. The best Gγ candidate is Gγ13, although functional data to support this are lacking. Thus, we tested Gγ13 function by generating Gng13(-/-) knockout (KO) mice, recording electroretinograms (ERG) and performing immunocytochemical staining. The amplitude of scotopic ERG b-waves in KO mice was lower than in wild-type (WT) mice. Furthermore, in both KO and WT mice, the ERG b-wave decreased with age; this decrease was much more pronounced in KO mice. By contrast, the photopic ERG b-waves in KO mice were hardly affected at any age. In KO mice retinas, immunostaining for Gβ3 and for the GTPase activating proteins RGS7, RGS11, R9AP and Gβ5 decreased significantly in rod bipolar cells but not in ON cone bipolar cells. Staining for Gαo and certain other cascade elements decreased only slightly. Analysis of our ON bipolar cDNA library showed that these cells express mRNAs for Gγ5, Gγ10 and Gγ11. Quantitative RT-PCR of retinal cDNA showed greater values for these transcripts in retinas of KO mice, although the difference was not significant. Our results suggest that Gγ13 contributes to mGluR6 signalling in rod bipolar cells more than in ON cone bipolar cells, and that this contribution includes both coupling the receptor and maintaining a stable localization of the mGluR6-related cascade elements.

Pivotal role of extended linker 2 in the activation of Gα by G protein-coupled receptor

G protein-coupled receptors (GPCRs) relay extracellular signals mainly to heterotrimeric G-proteins (Gαβγ) and they are the most successful drug targets. The mechanisms of G-protein activation by GPCRs are not well understood. Previous studies have revealed a signal relay route from a GPCR via the C-terminal α5-helix of Gα to the guanine nucleotide-binding pocket. Recent structural and biophysical studies uncover a role for the opening or rotating of the α-helical domain of Gα during the activation of Gα by a GPCR. Here we show that β-adrenergic receptors activate eight Gαs mutant proteins (from a screen of 66 Gαs mutants) that are unable to bind Gβγ subunits in cells. Five of these eight mutants are in the αF/Linker 2/β2 hinge region (extended Linker 2) that connects the Ras-like GTPase domain and the α-helical domain of Gαs. This extended Linker 2 is the target site of a natural product inhibitor of Gq. Our data show that the extended Linker 2 is critical for Gα activation by GPCRs. We propose that a GPCR via its intracellular loop 2 directly interacts with the β2/β3 loop of Gα to communicate to Linker 2, resulting in the opening and closing of the α-helical domain and the release of GDP during G-protein activation.

The Gβγ-Src signaling pathway regulates TNF-induced necroptosis via control of necrosome translocation

Formation of multi-component signaling complex necrosomes is essential for tumor necrosis factor α (TNF)-induced programmed necrosis (also called necroptosis). However, the mechanisms of necroptosis are still largely unknown. We isolated a TNF-resistant L929 mutant cell line generated by retrovirus insertion and identified that disruption of the guanine nucleotide-binding protein γ 10 (Gγ10) gene is responsible for this phenotype. We further show that Gγ10 is involved in TNF-induced necroptosis and Gβ2 is the partner of Gγ10. Src is the downstream effector of Gβ2γ10 in TNF-induced necroptosis because TNF-induced Src activation was impaired upon Gγ10 knockdown. Gγ10 does not affect TNF-induced activation of NF-κB and MAPKs and the formation of necrosomes, but is required for trafficking of necrosomes to their potential functioning site, an unidentified subcellular organelle that can be fractionated into heterotypic membrane fractions. The TNF-induced Gβγ-Src signaling pathway is independent of RIP1/RIP3 kinase activity and necrosome formation, but is required for the necrosome to function.

Splice isoforms of phosducin-like protein control the expression of heterotrimeric G proteins

Heterotrimeric G proteins play an essential role in cellular signaling; however, the mechanism regulating their synthesis and assembly remains poorly understood. A line of evidence indicates that the posttranslational processing of G protein β subunits begins inside the protein-folding chamber of the chaperonin containing t-complex protein 1. This process is facilitated by the ubiquitously expressed phosducin-like protein (PhLP), which is thought to act as a CCT co-factor. Here we demonstrate that alternative splicing of the PhLP gene gives rise to a transcript encoding a truncated, short protein (PhLPs) that is broadly expressed in human tissues but absent in mice. Seeking to elucidate the function of PhLPs, we expressed this protein in the rod photoreceptors of mice and found that this manipulation caused a dramatic translational and posttranslational suppression of rod heterotrimeric G proteins. The investigation of the underlying mechanism revealed that PhLPs disrupts the folding of Gβ and the assembly of Gβ and Gγ subunits, events normally assisted by PhLP, by forming a stable and apparently inactive tertiary complex with CCT preloaded with nascent Gβ. As a result, the cellular levels of Gβ and Gγ, which depends on Gβ for stability, decline. In addition, PhLPs evokes a profound and rather specific down-regulation of the Gα transcript, leading to a complete disappearance of the protein. This study provides the first evidence of a generic mechanism, whereby the splicing of the PhLP gene could potentially and efficiently regulate the cellular levels of heterotrimeric G proteins.

Heterotrimeric G protein subunit Gγ13 is critical to olfaction

The activation of G-protein-coupled olfactory receptors on the olfactory sensory neurons (OSNs) triggers a signaling cascade, which is mediated by a heterotrimeric G-protein consisting of α, β, and γ subunits. Although its α subunit, Gαolf, has been identified and well characterized, the identities of its β and γ subunits and their function in olfactory signal transduction, however, have not been well established yet. We, and others, have found the expression of Gγ13 in the olfactory epithelium, particularly in the cilia of the OSNs. In this study, we generated a conditional gene knock-out mouse line to specifically nullify Gγ13 expression in the olfactory marker protein-expressing OSNs. Immunohistochemical and Western blot results showed that Gγ13 subunit was indeed eliminated in the mutant mice's olfactory epithelium. Intriguingly, Gαolf, β1 subunits, Ric-8B and CEP290 proteins, were also absent in the epithelium whereas the presence of the effector enzyme adenylyl cyclase III remained largely unaltered. Electro-olfactogram studies showed that the mutant animals had greatly reduced responses to a battery of odorants including three presumable pheromones. Behavioral tests indicated that the mutant mice had a remarkably reduced ability to perform an odor-guided search task although their motivation and agility seemed normal. Our results indicate that Gαolf exclusively forms a functional heterotrimeric G-protein with Gβ1 and Gγ13 in OSNs, mediating olfactory signal transduction. The identification of the olfactory G-protein's βγ moiety has provided a novel approach to understanding the feedback regulation of olfactory signal transduction pathways as well as the control of subcellular structures of OSNs.

Cones respond to light in the absence of transducin β subunit

Mammalian cones respond to light by closing a cGMP-gated channel via a cascade that includes a heterotrimeric G-protein, cone transducin, comprising Gαt2, Gβ3 and Gγt2 subunits. The function of Gβγ in this cascade has not been examined. Here, we investigate the role of Gβ3 by assessing cone structure and function in Gβ3-null mouse (Gnb3(-/-)). We found that Gβ3 is required for the normal expression of its partners, because in the Gnb3(-/-) cone outer segments, the levels of Gαt2 and Gγt2 are reduced by fourfold to sixfold, whereas other components of the cascade remain unaltered. Surprisingly, Gnb3(-/-) cones produce stable responses with normal kinetics and saturating response amplitudes similar to that of the wild-type, suggesting that cone phototransduction can function efficiently without a Gβ subunit. However, light sensitivity was reduced by approximately fourfold in the knock-out cones. Because the reduction in sensitivity was similar in magnitude to the reduction in Gαt2 level in the cone outer segment, we conclude that activation of Gαt2 in Gnb3(-/-) cones proceeds at a rate approximately proportional to its outer segment concentration, and that activation of phosphodiesterase and downstream cascade components is normal. These results suggest that the main role of Gβ3 in cones is to establish optimal levels of transducin heteromer in the outer segment, thereby indirectly contributing to robust response properties.

Proteasome overload is a common stress factor in multiple forms of inherited retinal degeneration

Inherited retinal degenerations, caused by mutations in over 100 individual genes, affect approximately 2 million people worldwide. Many of the underlying mutations cause protein misfolding or mistargeting in affected photoreceptors. This places an increased burden on the protein folding and degradation machinery, which may trigger cell death. We analyzed how these cellular functions are affected in degenerating rods of the transducin γ-subunit (Gγ1) knockout mouse. These rods produce large amounts of transducin β-subunit (Gβ1), which cannot fold without Gγ1 and undergoes intracellular proteolysis instead of forming a transducin βγ-subunit complex. Our data revealed that the most critical pathobiological factor leading to photoreceptor cell death in these animals is insufficient capacity of proteasomes to process abnormally large amounts of misfolded protein. A decrease in the Gβ1 production in Gγ1 knockout rods resulted in a significant reduction in proteasomal overload and caused a striking reversal of photoreceptor degeneration. We further demonstrated that a similar proteasomal overload takes place in photoreceptors of other mutant mice where retinal degeneration has been ascribed to protein mistargeting or misfolding, but not in mice whose photoreceptor degenerate as a result of abnormal phototransduction. These results establish the prominence of proteasomal insufficiency across multiple degenerative diseases of the retina, thereby positioning proteasomes as a promising therapeutic target for treating these debilitating conditions.

Phosducin-like protein 1 is essential for G-protein assembly and signaling in retinal rod photoreceptors

G-protein β subunits perform essential neuronal functions as part of G-protein βγ and Gβ5-regulators of G-protein signaling (RGS) complexes. Both Gβγ and Gβ5-RGS are obligate dimers that are thought to require the assistance of the cytosolic chaperonin CCT and a cochaperone, phosducin-like protein 1 (PhLP1) for dimer formation. To test this hypothesis in vivo, we deleted the Phlp1 gene in mouse (Mus musculus) retinal rod photoreceptor cells and measured the effects on G-protein biogenesis and visual signal transduction. In the PhLP1-depleted rods, Gβγ dimer formation was decreased 50-fold, resulting in a >10-fold decrease in light sensitivity. Moreover, a 20-fold reduction in Gβ5 and RGS9-1 expression was also observed, causing a 15-fold delay in the shutoff of light responses. These findings conclusively demonstrate in vivo that PhLP1 is required for the folding and assembly of both Gβγ and Gβ5-RGS9.

Reprogramming of adult rod photoreceptors prevents retinal degeneration

A prime goal of regenerative medicine is to direct cell fates in a therapeutically useful manner. Retinitis pigmentosa is one of the most common degenerative diseases of the eye and is associated with early rod photoreceptor death followed by secondary cone degeneration. We hypothesized that converting adult rods into cones, via knockdown of the rod photoreceptor determinant Nrl, could make the cells resistant to the effects of mutations in rod-specific genes, thereby preventing secondary cone loss. To test this idea, we engineered a tamoxifen-inducible allele of Nrl to acutely inactivate the gene in adult rods. This manipulation resulted in reprogramming of rods into cells with a variety of cone-like molecular, histologic, and functional properties. Moreover, reprogramming of adult rods achieved cellular and functional rescue of retinal degeneration in a mouse model of retinitis pigmentosa. These findings suggest that elimination of Nrl in adult rods may represent a unique therapy for retinal degeneration.

cAMP controls rod photoreceptor sensitivity via multiple targets in the phototransduction cascade

In early studies, both cyclic AMP (cAMP) and cGMP were considered as potential secondary messengers regulating the conductivity of the vertebrate photoreceptor plasma membrane. Later discovery of the cGMP specificity of cyclic nucleotide–gated channels has shifted attention to cGMP as the only secondary messenger in the phototransduction cascade, and cAMP is not considered in modern schemes of phototransduction. Here, we report evidence that cAMP may also be involved in regulation of the phototransduction cascade. Using a suction pipette technique, we recorded light responses of isolated solitary rods from the frog retina in normal solution and in the medium containing 2 µM of adenylate cyclase activator forskolin. Under forskolin action, flash sensitivity rose more than twofold because of a retarded photoresponse turn-off. The same concentration of forskolin lead to a 2.5-fold increase in the rod outer segment cAMP, which is close to earlier reported natural day/night cAMP variations. Detailed analysis of cAMP action on the phototransduction cascade suggests that several targets are affected by cAMP increase: (a) basal dark phosphodiesterase (PDE) activity decreases; (b) at the same intensity of light background, steady background-induced PDE activity increases; (c) at light backgrounds, guanylate cyclase activity at a given fraction of open channels is reduced; and (d) the magnitude of the Ca²⁺ exchanger current rises 1.6-fold, which would correspond to a 1.6-fold elevation of [Ca²⁺]_in. Analysis by a complete model of rod phototransduction suggests that an increase of [Ca²⁺]_in might also explain effects (b) and (c). The mechanism(s) by which cAMP could regulate [Ca²⁺]_in and PDE basal activity is unclear. We suggest that these regulations may have adaptive significance and improve the performance of the visual system when it switches between day and night light conditions.

Functional comparison of rod and cone Gα(t) on the regulation of light sensitivity

The signaling cascades mediated by G protein-coupled receptors (GPCRs) exhibit a wide spectrum of spatial and temporal response properties to fulfill diverse physiological demands. However, the mechanisms that shape the signaling response of the GPCR are not well understood. In this study, we replaced cone transducin α (cTα) for rod transducin α (rTα) in rod photoreceptors of transgenic mice, which also express S opsin, to evaluate the role of Gα subtype on signal amplification from different GPCRs in the same cell; such analysis may explain functional differences between retinal rod and cone photoreceptors. We showed that ectopically expressed cTα 1) forms a heterotrimeric complex with rod Gβ(1)γ(1), 2) substitutes equally for rTα in generating photoresponses initiated by either rhodopsin or S-cone opsin, and 3) exhibited similar light-activated translocation as endogenous rTα in rods and endogenous cTα in cones. Thus, rTα and cTα appear functionally interchangeable. Interestingly, light sensitivity appeared to correlate with the concentration of cTα when expression is reduced below 35% of normal. However, quantification of endogenous cTα concentration in cones showed a higher level to rTα in rods. Thus, reduced sensitivity in cones cannot be explained by reduced coupling efficiency between the GPCR and G protein or a lower concentration of G protein in cones versus rods.

Gβ3 is required for normal light ON responses and synaptic maintenance

Heterotrimeric G-proteins, comprising Gα and Gβγ subunits, couple metabotropic receptors to various downstream effectors and contribute to assembling and trafficking receptor-based signaling complexes. A G-protein β subunit, Gβ(3), plays a critical role in several physiological processes, as a polymorphism in its gene is associated with a risk factor for several disorders. Retinal ON bipolar cells express Gβ(3), and they provide an excellent system to study its role. In the ON bipolar cells, mGluR6 inverts the photoreceptor's signal via a cascade in which glutamate released from photoreceptors closes the TRPM1 channel. This cascade is essential for vision since deficiencies in its proteins lead to complete congenital stationary night blindness. Here we report that Gβ(3) participates in the G-protein heterotrimer that couples mGluR6 to TRPM1. Gβ(3) deletion in mouse greatly reduces the light response under both scotopic and photopic conditions, but it does not eliminate it. In addition, Gβ(3) deletion causes mislocalization and downregulation of most cascade elements and modulators. Furthermore, Gβ(3) may play a role in synaptic maintenance since in its absence, the number of invaginating rod bipolar dendrites is greatly reduced, a deficit that was not observed at 3 weeks, the end of the developmental period.

Signaling states of rhodopsin in rod disk membranes lacking transducin βγ-complex

PURPOSE

To characterize the possible role of transducin Gtβγ-complex in modulating the signaling properties of photoactivated rhodopsin and its lifetime in rod disc membranes and intact rods.

METHODS

Rhodopsin photolysis was studied using UV-visible spectroscopy and rapid scanning spectroscopy in the presence of hydroxylamine in highly purified wild-type and Gtγ-deficient mouse rod disc membranes. Complex formation between photoactivated rhodopsin and transducin was measured by extra-metarhodopsin (meta) II assay. Recovery of dark current and flash sensitivity in individual intact wild-type and Gtγ-deficient mouse rods was measured by single-cell suction recordings.

RESULTS

Photoconversion of rhodopsin to meta I/meta II equilibrium proceeds normally after elimination of the Gtβγ-complex. The meta I/meta II ratio, the rate of meta II decay, the reactivity of meta II toward hydroxylamine, and the rate of meta III formation in Gtγ-deficient rod disc membranes were identical with those observed in wild-type samples. Under low-intensity illumination, the amount of extra-meta II in Gtγ-deficient discs was significantly reduced. The initial rate of dark current recovery after 12% rhodopsin bleach was three times faster in Gtγ-deficient rods, whereas the rate of the late current recovery was largely unchanged. Mutant rods also exhibited faster postbleach recovery of flash sensitivity.

CONCLUSIONS

Photoactivation and thermal decay of rhodopsin proceed similarly in wild-type and Gtγ-deficient mouse rods, but the complex formation between photoactivated rhodopsin and transducin is severely compromised in the absence of Gtβγ. The resultant lower transduction activation contributes to faster photoresponse recovery after a moderate pigment bleach in Gtγ-deficient rods.