Selective loss of cone function in mice lacking the cyclic nucleotide-gated channel CNG3

Promotion of endoplasmic reticulum retrotranslocation by overexpression of E3 ubiquitin-protein ligase synoviolin 1 reduces endoplasmic reticulum stress and preserves cone photoreceptors in cyclic nucleotide-gated channel deficiency

Cone photoreceptor cyclic nucleotide-gated (CNG) channels play an essential role in phototransduction and cellular Ca2+ homeostasis. Mutations in genes encoding the channel subunits CNGA3 and CNGB3 are associated with achromatopsia, progressive cone dystrophy, and early-onset macular degeneration. Cone loss in patients with achromatopsia and cone dystrophy associated with CNG channel mutations has been documented by optical coherence tomography and in mouse models of CNG channel deficiency. Cone death in CNG channel-deficient retinas involves endoplasmic reticulum (ER) stress-associated apoptosis, dysregulation of cellular/ER Ca2+ homeostasis, impaired protein folding/processing, and impaired ER-associated degradation (ERAD). The E3 ubiquitin-protein ligase synoviolin 1 (SYVN1) is the primary component of the SYVN1/SEL1L ER retrotranslocon responsible for ERAD. Previous studies have shown that manipulations that protect cones and reduce ER stress/cone death in CNG channel deficiency, such as increasing ER Ca2+ preservation or treatment with an ER chaperone, increase the expression of SYVN1 and other components of the ER retrotranslocon. The present work investigated the effects of SYVN1 overexpression. Intraocular injection of AAV5-IRBP/GNAT2-Syvn1 resulted in overexpression of SYVN1 in cones of CNG channel-deficient mice. Following treatment, cone density in Cnga3-/- mice was significantly increased, compared with untreated controls, outer segment localization of cone opsin was improved, and ER stress/apoptotic cell death was reduced. Overexpression of SYVN1 also led to increased expression levels of the retrotranslocon components, degradation in ER protein 1 (DERL1), ERAD E3 ligase adaptor subunit (SEL1L), and homocysteine inducible ER protein with ubiquitin-like domain 1 (HERPUD1). Moreover, overexpression of SYVN1 likely enhanced protein ubiquitination/proteasome degradation in CNG channel-deficient retinas. This study demonstrates the role of SYVN1/ERAD in cone preservation in CNG channel deficiency and supports the strategy of promoting ERAD for cone protection.

Molecular Mechanisms Governing Sight Loss in Inherited Cone Disorders

Inherited cone disorders (ICDs) are a heterogeneous sub-group of inherited retinal disorders (IRDs), the leading cause of sight loss in children and working-age adults. ICDs result from the dysfunction of the cone photoreceptors in the macula and manifest as the loss of colour vision and reduced visual acuity. Currently, 37 genes are associated with varying forms of ICD; however, almost half of all patients receive no molecular diagnosis. This review will discuss the known ICD genes, their molecular function, and the diseases they cause, with a focus on the most common forms of ICDs, including achromatopsia, progressive cone dystrophies (CODs), and cone-rod dystrophies (CORDs). It will discuss the gene-specific therapies that have emerged in recent years in order to treat patients with some of the more common ICDs.

Analysis of Suspected Achromatopsia by Multimodal Diagnostic Testing

BACKGROUND

Achromatopsia (ACHM) as a hereditary cone disease might manifest in a stationary and progressive manner. The proper clinical and genetic diagnosis may allow an individual prognosis, accurate genetic counselling, and the optimal choice of low vision aids. The primary aim of the study was to determine the spectrum of clinical and genetic diagnostics required to characterize the ACHM.

METHODS

A retrospective analysis was performed in 8 patients from non-related families (5 ♀,3 ♂); age at diagnosis: 3 - 56 y, mean 18.13 (SD ± 18.22). Clinical phenotyping, supported by colour vision test, fundus photography-, autofluorescence- (FAF), infra-red- (IR), OCT imaging and electroretinography provided information on the current status and the course of the disease over the years. In addition, genetic examinations were performed with ACHM relevant testing (CNGA3, CNGB3, GNAT2, PDE6C, PDE6H and the transcription factor ATF6).

RESULTS

All patients suffered photophobia and reduced visual acuity (mean: 0.16 [SD ± 0.08]). Nystagmus was identified in 7 from 8 subjects and in one patient a head-turn right helped to reduce the nystagmus amplitude. Colour vision testing confirmed complete achromatopsia in 7 out of 8 patients. Electrophysiology found severely reduced photopic- but also scotopic responses. Thinning and interruption of the inner segment ellipsoid (ISe) line within the macula but also FAF- and IR abnormalities in the fovea and/or parafovea were characteristic in all ACHM patients. Identification of pathogenic mutations in 7 patients helped to confirm the diagnosis of ACHM (3 adults, 4 children; 3 ♀ and 4 ♂). Achromatopsia was linked to CNGA3 (2 ♀, 1 ♂) and CNGB3 variants (2 ♀, 3 ♂). The youngest patient (♀, 10 y) had 3 different CNGB3 variants on different alleles. In a patient (♂, 29 y) carrying 2 pathogenic digenic-triallelic CNGA3- and CNGB3-mutations, a severe progression of ISe discontinuity to coloboma-like macular atrophy was observed during the 12-year follow-up. The oldest female (67 y) showed a compound homozygous CNGA3- and heterozygous CNGB3-, as well as a heterozygous GUCY2D variants. The destruction of her ISe line was significantly enlarged and represented a progressive cone-rod phenotype in comparison to other ACHM patients. In a patient (♂, 45 y) carrying a pathogenic CNGB3 and USH2 mutation, a severe macular oedema and a rod-cone phenotype was observed. In addition, two variants in C2ORF71 considered as VOS were found. One patient showed the rare ATF6 mutation, where a severe coloboma-like macular atrophy was observed on the left eye as early as at the age of three years.

CONCLUSION

Combining multimodal ophthalmological diagnostics and molecular genetics when evaluating patients with ACHM helps in characterizing the disease and associated modifiers, and is therefore strongly recommended for such patients.

Regulation of mouse exploratory behaviour by irradiance and cone-opponent signals

BACKGROUND

Animal survival depends on the ability to adjust behaviour according to environmental conditions. The circadian system plays a key role in this capability, with diel changes in the quantity (irradiance) and spectral content ('colour') of ambient illumination providing signals of time-of-day that regulate the timing of rest and activity. Light also exerts much more immediate effects on behaviour, however, that are equally important in shaping daily activity patterns. Hence, nocturnal mammals will actively avoid light and dramatically reduce their activity when light cannot be avoided. The sensory mechanisms underlying these acute effects of light are incompletely understood, particularly the importance of colour.

RESULTS

To define sensory mechanisms controlling mouse behaviour, we used photoreceptor-isolating stimuli and mice with altered cone spectral sensitivity (Opn1mwR), lacking melanopsin (Opn1mwR; Opn4-/-) or cone phototransduction (Cnga3-/-) in assays of light-avoidance and activity suppression. In addition to roles for melanopsin-dependent irradiance signals, we find a major influence of spectral content in both cases. Hence, remarkably, selective increases in S-cone irradiance (producing a blue-shift in spectrum replicating twilight) drive light-seeking behaviour and promote activity. These effects are opposed by signals from longer-wavelength sensitive cones, indicating a true spectrally-opponent mechanism. Using c-Fos-mapping and multielectrode electrophysiology, we further show these effects are associated with a selective cone-opponent modulation of neural activity in the key brain site implicated in acute effects of light on behaviour, the subparaventricular zone.

CONCLUSIONS

Collectively, these data reveal a mechanism whereby blue-shifts in the spectrum of environmental illumination, such as during twilight, promote mouse exploratory behaviour.

Colour opponency is widespread across the mouse subcortical visual system and differentially targets GABAergic and non-GABAergic neurons

Colour vision plays many important roles in animal behaviour but the brain pathways processing colour remain surprisingly poorly understood, including in the most commonly used laboratory mammal, mice. Indeed, particular features of mouse retinal organisation present challenges in defining the mechanisms underlying colour vision in mice and have led to suggestions that this may substantially rely on 'non-classical' rod-cone opponency. By contrast, studies using mice with altered cone spectral sensitivity, to facilitate application of photoreceptor-selective stimuli, have revealed widespread cone-opponency across the subcortical visual system. To determine the extent to which such findings are truly reflective of wildtype mouse colour vision, and facilitate neural circuit mapping of colour-processing pathways using intersectional genetic approaches, we here establish and validate stimuli for selectively manipulating excitation of the native mouse S- and M-cone opsin classes. We then use these to confirm the widespread appearance of cone-opponency (> 25% of neurons) across the mouse visual thalamus and pretectum. We further extend these approaches to map the occurrence of colour-opponency across optogenetically identified GABAergic (GAD2-expressing) cells in key non-image forming visual centres (pretectum and intergeniculate leaflet/ventral lateral geniculate; IGL/vLGN). Strikingly, throughout, we find S-ON/M-OFF opponency is specifically enriched in non-GABAergic cells, with identified GABAergic cells in the IGL/VLGN entirely lacking this property. Collectively, therefore, we establish an important new approach for studying cone function in mice, confirming a surprisingly extensive appearance of cone-opponent processing in the mouse visual system and providing new insight into functional specialisation of the pathways processing such signals.

Retinal Phenotyping of a Murine Model of Lafora Disease

Lafora disease (LD) is a progressive neurologic disorder caused by biallelic pathogenic variants in EPM2A or EPM2B, leading to tissue accumulation of polyglucosan aggregates termed Lafora bodies (LBs). This study aimed to characterize the retinal phenotype in Epm2a−/− mice by examining knockout (KO; Epm2a−/−) and control (WT) littermates at two time points (10 and 14 months, respectively). In vivo exams included electroretinogram (ERG) testing, optical coherence tomography (OCT) and retinal photography. Ex vivo retinal testing included Periodic acid Schiff Diastase (PASD) staining, followed by imaging to assess and quantify LB deposition. There was no significant difference in any dark-adapted or light-adapted ERG parameters between KO and WT mice. The total retinal thickness was cFigure mparable between the groups and the retinal appearance was normal in both groups. On PASD staining, LBs were observed in KO mice within the inner and outer plexiform layers and in the inner nuclear layer. The average number of LBs within the inner plexiform layer in KO mice were 1743 ± 533 and 2615 ± 915 per mm2, at 10 and 14 months, respectively. This is the first study to characterize the retinal phenotype in an Epm2a−/− mouse model, demonstrating significant LB deposition in the bipolar cell nuclear layer and its synapses. This finding may be used to monitor the efficacy of experimental treatments in mouse models.

Biology, Pathobiology and Gene Therapy of CNG Channel-Related Retinopathies

The visual process begins with the absorption of photons by photopigments of cone and rod photoreceptors in the retina. In this process, the signal is first amplified by a cyclic guanosine monophosphate (cGMP)-based signaling cascade and then converted into an electrical signal by cyclic nucleotide-gated (CNG) channels. CNG channels are purely ligand-gated channels whose activity can be controlled by cGMP, which induces a depolarizing Na⁺/Ca²⁺ current upon binding to the channel. Structurally, CNG channels belong to the superfamily of pore-loop cation channels and share structural similarities with hyperpolarization-activated cyclic nucleotide (HCN) and voltage-gated potassium (KCN) channels. Cone and rod photoreceptors express distinct CNG channels encoded by homologous genes. Mutations in the genes encoding the rod CNG channel (CNGA1 and CNGB1) result in retinitis-pigmentosa-type blindness. Mutations in the genes encoding the cone CNG channel (CNGA3 and CNGB3) lead to achromatopsia. Here, we review the molecular properties of CNG channels and describe their physiological and pathophysiological roles in the retina. Moreover, we summarize recent activities in the field of gene therapy aimed at developing the first gene therapies for CNG channelopathies.

Photoreceptor Ion Channels in Signaling and Disease

Photoreceptors (PRs) in the neural retina convert photon capture into an electrical signal that is communicated across a chemical synapse to second-order neurons in the retina and on through the rest of the visual pathway. This information is decoded in the visual cortex to create images. The activity of PRs depends on the concerted action of several voltage-gated ion channels that will be discussed in this chapter.

Electroretinogram (ERG) to Evaluate the Retina Using Mouse Models

Electroretinogram (ERG) is a sensitive and useful tool for the measurement of the retina's electrical response to flash stimuli. It provides a functional evaluation of the photoreceptors and downstream associated retinal cells. Similar to those conducted on humans, mouse ERGs include the amplitudes of a- and b-waves as well as the implicit time from those ERGs. Applications of ERGs include identification of retinal phenotypes, measurement of retinal function (at one and various time points), and evaluation of treatment efficacy. However, there are some differences between the manifestation of disease in patients as compared to mouse models that should be taken into consideration when implementing mouse ERGs. Herein, this chapter will introduce how to perform and obtain mouse ERGs.

IL-4 induces reparative phenotype of RPE cells and protects against retinal neurodegeneration via Nrf2 activation

Retinal degeneration is a kind of neurodegeneration characterized by progressive neuronal death and dysfunction of retinal pigment epithelium (RPE) cells, leading to permanent visual impairment. It still lacks effective therapeutic options and new drugs are highly warranted. In this study, we found the expression of IL-4, a critical regulator of immunity, was reduced in both patients and mouse models. Importantly, exogenous intravitreal IL-4 application could exert a novel neuroprotective effect, characterized by well-preserved RPE layer and neuroretinal structure, as well as amplified wave-amplitudes in ERG. The RNA-seq analysis revealed that IL-4 treatment suppressed the essential oxidative and pro-inflammatory pathways in the degenerative retina. Particularly, IL-4 upregulated the IL-4Rα on RPE cells and induced a reparative phenotype via the activation of Nrf2 both in vitro and in vivo. Furthermore, the Nrf2-/- mice displayed no recovery in response to IL-4 application, highlighting a significant role of Nrf2 in IL-4-mediated protection. Our data provides evidence that IL-4 protects against retinal neurodegeneration by its antioxidant and anti-inflammatory property through IL-4Rα upregulation and Nrf2 activation in RPE cells. The IL-4/IL-4Rα-Nrf2 axis maybe the potential targets for the development of novel therapies for neurodegenerative diseases.

Three-year results of phase I retinal gene therapy trial for CNGA3-mutated achromatopsia: results of a non randomised controlled trial

AIMS

To determine long-term safety and efficacy outcomes of a subretinal gene therapy for CNGA3-associated achromatopsia. We present data from an open-label, nonrandomised controlled trial (NCT02610582).

METHODS

Details of the study design have been previously described. Briefly, nine patients were treated in three escalating dose groups with subretinal AAV8.CNGA3 gene therapy between November 2015 and October 2016. After the first year, patients were seen on a yearly basis. Safety assessment constituted the primary endpoint. On a secondary level, multiple functional tests were carried out to determine efficacy of the therapy.

RESULTS

No adverse or serious adverse events deemed related to the study drug occurred after year 1. Safety of the therapy, as the primary endpoint of this trial, can, therefore, be confirmed. The functional benefits that were noted in the treated eye at year 1 were persistent throughout the following visits at years 2 and 3. While functional improvement in the treated eye reached statistical significance for some secondary endpoints, for most endpoints, this was not the case when the treated eye was compared with the untreated fellow eye.

CONCLUSION

The results demonstrate a very good safety profile of the therapy even at the highest dose administered. The small sample size limits the statistical power of efficacy analyses. However, trial results inform on the most promising design and endpoints for future clinical trials. Such trials have to determine whether treatment of younger patients results in greater functional gains by avoiding amblyopia as a potential limiting factor.

Homeostatic plasticity in the retina

Vision begins in the retina, whose intricate neural circuits extract salient features of the environment from the light entering our eyes. Neurodegenerative diseases of the retina (e.g., inherited retinal degenerations, age-related macular degeneration, and glaucoma) impair vision and cause blindness in a growing number of people worldwide. Increasing evidence indicates that homeostatic plasticity (i.e., the drive of a neural system to stabilize its function) can, in principle, preserve retinal function in the face of major perturbations, including neurodegeneration. Here, we review the circumstances and events that trigger homeostatic plasticity in the retina during development, sensory experience, and disease. We discuss the diverse mechanisms that cooperate to compensate and the set points and outcomes that homeostatic retinal plasticity stabilizes. Finally, we summarize the opportunities and challenges for unlocking the therapeutic potential of homeostatic plasticity. Homeostatic plasticity is fundamental to understanding retinal development and function and could be an important tool in the fight to preserve and restore vision.

Melanopsin retinal ganglion cells mediate light-promoted brain development

During development, melanopsin-expressing intrinsically photosensitive retinal ganglion cells (ipRGCs) become light sensitive much earlier than rods and cones. IpRGCs project to many subcortical areas, whereas physiological functions of these projections are yet to be fully elucidated. Here, we found that ipRGC-mediated light sensation promotes synaptogenesis of pyramidal neurons in various cortices and the hippocampus. This phenomenon depends on activation of ipRGCs and is mediated by the release of oxytocin from the supraoptic nucleus (SON) and the paraventricular nucleus (PVN) into cerebral-spinal fluid. We further characterized a direct connection between ipRGCs and oxytocin neurons in the SON and mutual projections between oxytocin neurons in the SON and PVN. Moreover, we showed that the lack of ipRGC-mediated, light-promoted early cortical synaptogenesis compromised learning ability in adult mice. Our results highlight the importance of light sensation early in life on the development of learning ability and therefore call attention to suitable light environment for infant care.

Comprehensive variant spectrum of the CNGA3 gene in patients affected by achromatopsia

Achromatopsia (ACHM) is a congenital cone photoreceptor disorder characterized by impaired color discrimination, low visual acuity, photosensitivity, and nystagmus. To date, six genes have been associated with ACHM (CNGA3, CNGB3, GNAT2, PDE6C, PDE6H, and ATF6), the majority of these being implicated in the cone phototransduction cascade. CNGA3 encodes the CNGA3 subunit of the cyclic nucleotide-gated ion channel in cone photoreceptors and is one of the major disease-associated genes for ACHM. Herein, we provide a comprehensive overview of the CNGA3 variant spectrum in a cohort of 1060 genetically confirmed ACHM patients, 385 (36.3%) of these carrying "likely disease-causing" variants in CNGA3. Compiling our own genetic data with those reported in the literature and in public databases, we further extend the CNGA3 variant spectrum to a total of 316 variants, 244 of which we interpreted as "likely disease-causing" according to ACMG/AMP criteria. We report 48 novel "likely disease-causing" variants, 24 of which are missense substitutions underlining the predominant role of this mutation class in the CNGA3 variant spectrum. In addition, we provide extensive in silico analyses and summarize reported functional data of previously analyzed missense, nonsense and splicing variants to further advance the pathogenicity assessment of the identified variants.

Structural and functional characterization of an achromatopsia-associated mutation in a phototransduction channel

Numerous missense mutations in cyclic nucleotide-gated (CNG) channels cause achromatopsia and retinitis pigmentosa, but the underlying pathogenic mechanisms are often unclear. We investigated the structural basis and molecular/cellular effects of R410W, an achromatopsia-associated, presumed loss-of-function mutation in human CNGA3. Cryo-EM structures of the Caenorhabditis elegans TAX-4 CNG channel carrying the analogous mutation, R421W, show that most apo channels are open. R421, located in the gating ring, interacts with the S4 segment in the closed state. R421W disrupts this interaction, destabilizes the closed state, and stabilizes the open state. CNGA3_R410W/CNGB3 and TAX4_R421W channels are spontaneously active without cGMP and induce cell death, suggesting cone degeneration triggered by spontaneous CNG channel activity as a possible cause of achromatopsia. Our study sheds new light on CNG channel allosteric gating, provides an impetus for a reevaluation of reported loss-of-function CNG channel missense disease mutations, and has implications for mutation-specific treatment of retinopathy.

Achromatopsia: Genetics and Gene Therapy

Achromatopsia (ACHM), also known as rod monochromatism or total color blindness, is an autosomal recessively inherited retinal disorder that affects the cones of the retina, the type of photoreceptors responsible for high-acuity daylight vision. ACHM is caused by pathogenic variants in one of six cone photoreceptor-expressed genes. These mutations result in a functional loss and a slow progressive degeneration of cone photoreceptors. The loss of cone photoreceptor function manifests at birth or early in childhood and results in decreased visual acuity, lack of color discrimination, abnormal intolerance to light (photophobia), and rapid involuntary eye movement (nystagmus). Up to 90% of patients with ACHM carry mutations in CNGA3 or CNGB3, which are the genes encoding the alpha and beta subunits of the cone cyclic nucleotide-gated (CNG) channel, respectively. No authorized therapy for ACHM exists, but research activities have intensified over the past decade and have led to several preclinical gene therapy studies that have shown functional and morphological improvements in animal models of ACHM. These encouraging preclinical data helped advance multiple gene therapy programs for CNGA3- and CNGB3-linked ACHM into the clinical phase. Here, we provide an overview of the genetic and molecular basis of ACHM, summarize the gene therapy-related research activities, and provide an outlook for their clinical application.

Preservation of endoplasmic reticulum (ER) Ca2+ stores by deletion of inositol-1,4,5-trisphosphate receptor type 1 promotes ER retrotranslocation, proteostasis, and protein outer segment localization in cyclic nucleotide-gated channel-deficient cone photoreceptors

Endoplasmic reticulum (ER) Ca²⁺ homeostasis relies on an appropriate balance between efflux- and influx-channel activity responding to dynamic changes of intracellular Ca²⁺ levels. Dysregulation of this complex signaling network has been shown to contribute to neuronal and photoreceptor death in neuro- and retinal degenerative diseases, respectively. In mice with cone cyclic nucleotide-gated (CNG) channel deficiency, a model of achromatopsia/cone dystrophy, cones display early-onset ER stress-associated apoptosis and protein mislocalization. Cones in these mice also show reduced cytosolic Ca²⁺ level and subsequent elevation in the ER Ca²⁺ -efflux-channel activity, specifically the inositol-1,4,5-trisphosphate receptor type 1 (IP₃ R1), and deletion of IP₃ R1 results in preservation of cones. This work investigated how preservation of ER Ca²⁺ stores leads to cone protection. We examined the effects of cone specific deletion of IP₃ R1 on ER stress responses/cone death, protein localization, and ER proteostasis/ER-associated degradation. We demonstrated that deletion of IP₃ R1 improves trafficking of cone-specific proteins M-/S-opsin and phosphodiesterase 6C to cone outer segments and reduces localization to cone inner segments. Consistent with the improved protein localization, deletion of IP₃ R1 results in increased ER retrotranslocation protein expression, reduced proteasome subunit expression, reduced ER stress/cone death, and reduced retinal remodeling. We also observed the enhanced ER retrotranslocation in mice that have been treated with a chemical chaperone, supporting the connection between improved ER retrotranslocation/proteostasis and alleviation of ER stress. Findings from this work demonstrate the importance of ER Ca²⁺ stores in ER proteostasis and protein trafficking/localization in photoreceptors, strengthen the link between dysregulation of ER Ca²⁺ homeostasis and ER stress/cone degeneration, and support an involvement of improved ER proteostasis in ER Ca²⁺ preservation-induced cone protection; thereby identifying IP₃ R1 as a critical mediator of ER stress and protein mislocalization and as a potential target to preserve cones in CNG channel deficiency.

Novel AAV capsids for intravitreal gene therapy of photoreceptor disorders

Gene therapy using recombinant adeno-associated virus (rAAV) vectors to treat blinding retinal dystrophies has become clinical reality. Therapeutically impactful targeting of photoreceptors still relies on subretinal vector delivery, which detaches the retina and harbours substantial risks of collateral damage, often without achieving widespread photoreceptor transduction. Herein, we report the development of novel engineered rAAV vectors that enable efficient targeting of photoreceptors via less invasive intravitreal administration. A unique in vivo selection procedure was performed, where an AAV2-based peptide-display library was intravenously administered in mice, followed by isolation of vector DNA from target cells after only 24 h. This stringent selection yielded novel vectors, termed AAV2.GL and AAV2.NN, which mediate widespread and high-level retinal transduction after intravitreal injection in mice, dogs and non-human primates. Importantly, both vectors efficiently transduce photoreceptors in human retinal explant cultures. As proof-of-concept, intravitreal Cnga3 delivery using AAV2.GL lead to cone-specific expression of Cnga3 protein and rescued photopic cone responses in the Cnga3-/- mouse model of achromatopsia. These novel rAAV vectors expand the clinical applicability of gene therapy for blinding human retinal dystrophies.

The cGMP-Dependent Protein Kinase 2 Contributes to Cone Photoreceptor Degeneration in the Cnga3-Deficient Mouse Model of Achromatopsia

Mutations in the CNGA3 gene, which encodes the A subunit of the cyclic guanosine monophosphate (cGMP)-gated cation channel in cone photoreceptor outer segments, cause total colour blindness, also referred to as achromatopsia. Cones lacking this channel protein are non-functional, accumulate high levels of the second messenger cGMP and degenerate over time after induction of ER stress. The cell death mechanisms that lead to loss of affected cones are only partially understood. Here, we explored the disease mechanisms in the Cnga3 knockout (KO) mouse model of achromatopsia. We found that another important effector of cGMP, the cGMP-dependent protein kinase 2 (Prkg2) is crucially involved in cGMP cytotoxicity of cones in Cnga3 KO mice. Virus-mediated knockdown or genetic ablation of Prkg2 in Cnga3 KO mice counteracted degeneration and preserved the number of cones. Analysis of markers of endoplasmic reticulum stress and unfolded protein response confirmed that induction of these processes in Cnga3 KO cones also depends on Prkg2. In conclusion, we identified Prkg2 as a novel key mediator of cone photoreceptor degeneration in achromatopsia. Our data suggest that this cGMP mediator could be a novel pharmacological target for future neuroprotective therapies.

Cones Support Alignment to an Inconsistent World by Suppressing Mouse Circadian Responses to the Blue Colors Associated with Twilight

In humans, short-wavelength light evokes larger circadian responses than longer wavelengths [1-3]. This reflects the fact that melanopsin, a key contributor to circadian assessments of light intensity, most efficiently captures photons around 480 nm [4-8] and gives rise to the popular view that "blue" light exerts the strongest effects on the clock. However, in the natural world, there is often no direct correlation between perceived color (as reported by the cone-based visual system) and melanopsin excitation. Accordingly, although the mammalian clock does receive cone-based chromatic signals [9], the influence of color on circadian responses to light remains unclear. Here, we define the nature and functional significance of chromatic influences on the mouse circadian system. Using polychromatic lighting and mice with altered cone spectral sensitivity (Opn1mw^R), we generate conditions that differ in color (i.e., ratio of L- to S-cone opsin activation) while providing identical melanopsin and rod activation. When biased toward S-opsin activation (appearing "blue"), these stimuli reliably produce weaker circadian behavioral responses than those favoring L-opsin ("yellow"). This influence of color (which is absent in animals lacking cone phototransduction; Cnga3^-/-) aligns with natural changes in spectral composition over twilight, where decreasing solar angle is accompanied by a strong blue shift [9-11]. Accordingly, we find that naturalistic color changes support circadian alignment when environmental conditions render diurnal variations in light intensity weak/ambiguous sources of timing information. Our data thus establish how color contributes to circadian entrainment in mammals and provide important new insight to inform the design of lighting environments that benefit health.

Potential contribution of ryanodine receptor 2 upregulation to cGMP/PKG signaling-induced cone degeneration in cyclic nucleotide-gated channel deficiency

Photoreceptor cyclic nucleotide-gated (CNG) channels regulate Ca2+ influx in rod and cone photoreceptors. Mutations in cone CNG channel subunits CNGA3 and CNGB3 are associated with achromatopsia and cone dystrophies. Mice lacking functional cone CNG channel show endoplasmic reticulum (ER) stress-associated cone degeneration. The elevated cyclic guanosine monophosphate (cGMP)/cGMP-dependent protein kinase (PKG) signaling and upregulation of the ER Ca2+ channel ryanodine receptor 2 (RyR2) have been implicated in cone degeneration. This work investigates the potential contribution of RyR2 to cGMP/PKG signaling-induced ER stress and cone degeneration. We demonstrated that the expression and activity of RyR2 were highly regulated by cGMP/PKG signaling. Depletion of cGMP by deleting retinal guanylate cyclase 1 or inhibition of PKG using chemical inhibitors suppressed the upregulation of RyR2 in CNG channel deficiency. Depletion of cGMP or deletion of Ryr2 equivalently inhibited unfolded protein response/ER stress, activation of the CCAAT-enhancer-binding protein homologous protein, and activation of the cyclic adenosine monophosphate response element-binding protein, leading to early-onset cone protection. In addition, treatment with cGMP significantly enhanced Ryr2 expression in cultured photoreceptor-derived Weri-Rb1 cells. Findings from this work demonstrate the regulation of cGMP/PKG signaling on RyR2 in the retina and support the role of RyR2 upregulation in cGMP/PKG signaling-induced ER stress and photoreceptor degeneration.

CNGA3 acts as a cold sensor in hypothalamic neurons

Most mammals maintain their body temperature around 37°C, whereas in hibernators it can approach 0°C without triggering a thermogenic response. The remarkable plasticity of the thermoregulatory system allowed mammals to thrive in variable environmental conditions and occupy a wide range of geographical habitats, but the molecular basis of thermoregulation remains poorly understood. Here we leverage the thermoregulatory differences between mice and hibernating thirteen-lined ground squirrels (Ictidomys tridecemlineatus) to investigate the mechanism of cold sensitivity in the preoptic area (POA) of the hypothalamus, a critical thermoregulatory region. We report that, in comparison to squirrels, mice have a larger proportion of cold-sensitive neurons in the POA. We further show that mouse cold-sensitive neurons express the cyclic nucleotide-gated ion channel CNGA3, and that mouse, but not squirrel, CNGA3 is potentiated by cold. Our data reveal CNGA3 as a hypothalamic cold sensor and a molecular marker to interrogate the neuronal circuitry underlying thermoregulation.

AAV-mediated human CNGB3 restores cone function in an all-cone mouse model of CNGB3 achromatopsia

Complete congenital achromatopsia is a devastating hereditary visual disorder. Mutations in the CNGB3 gene account for more than 50% of all known cases of achromatopsia. This work investigated the efficiency of subretinal (SR) delivered AAV8 (Y447, 733F) vector containing a human PR2.1 promoter and a human CNGB3 cDNA in Cngb3 ^-/-/ Nrl ^-/- mice. The Cngb3 ^-/-/ Nrl ^-/- mouse was a cone-dominant model with Cngb3 channel deficiency, which partially mimicked the all-cone foveal structure of human achromatopsia with CNGB3 mutations. Following SR delivery of the vector, AAV-mediated CNGB3 expression restored cone function which was assessed by the restoration of the cone-mediated electroretinogram (ERG) and immunohistochemistry. This therapeutic rescue resulted in long-term improvement of retinal function with the restoration of cone ERG amplitude. This study demonstrated an AAV-mediated gene therapy in a cone-dominant mouse model using a human gene construct and provided the potential to be utilized in clinical trials.

Melanopsin Retinal Ganglion Cells Regulate Cone Photoreceptor Lamination in the Mouse Retina

Newborn neurons follow molecular cues to reach their final destination, but whether early life experience influences lamination remains largely unexplored. As light is among the first stimuli to reach the developing nervous system via intrinsically photosensitive retinal ganglion cells (ipRGCs), we asked whether ipRGCs could affect lamination in the developing mouse retina. We show here that ablation of ipRGCs causes cone photoreceptors to mislocalize at different apicobasal positions in the retina. This effect is partly mediated by light-evoked activity in ipRGCs, as dark rearing or silencing of ipRGCs leads a subset of cones to mislocalize. Furthermore, ablation of ipRGCs alters the cone transcriptome and decreases expression of the dopamine receptor D4, while injection of L-DOPA or D4 receptor agonist rescues the displaced cone phenotype observed in dark-reared animals. These results show that early light-mediated activity in ipRGCs influences neuronal lamination and identify ipRGC-elicited dopamine release as a mechanism influencing cone position.

Ryanodine Receptor 2 Contributes to Impaired Protein Localization in Cyclic Nucleotide-Gated Channel Deficiency

The photoreceptor cyclic nucleotide-gated (CNG) channel plays a pivotal role in phototransduction and cellular calcium homeostasis. Mutations in the cone photoreceptor CNG channel subunits CNGA3 and CNGB3 are associated with achromatopsia and cone dystrophies. CNG channel deficiency leads to endoplasmic reticulum (ER) stress-associated cone apoptosis, protein mislocalization, and ER calcium dysregulation. This work investigated the potential mechanisms of protein mislocalization associated with ER calcium dysregulation using Cnga3-/- mice lacking ER Ca2+ channel ryanodine receptor 2 (RyR2) specifically in cones. Deletion of Ryr2 improved outer segment (OS) localization of the cone proteins M-opsin, S-opsin, and cone phosphodiesterase subunit α' (PDE6C) and decreased inner segment localization. One-month-old Cnga3-/- mice showed ∼30% of M-opsin, 55% of S-opsin, and 50% of PDE6C localized to the OS. Cnga3-/- mice with Ryr2 deletion at the same age showed almost 60% of M-opsin, 70% of S-opsin, and 70% of PDE6C localized to the OS. Deletion of Ryr2 nearly completely reversed elevations of the ER stress markers phospho-IRE1α and phospho-eIF2α and suppressed cone apoptosis. Consistent with the improved cone protein localization and reduced ER stress/cone apoptosis, cone survival was improved by deletion of Ryr2 The number of cones was increased by ∼28% in 2- to 4-month-old Cnga3-/- mice with Ryr2 deletion compared with age-matched Cnga3-/- mice. This work demonstrates a role of RyR2/ER calcium dysregulation in protein mislocalization, ER stress, and cone death. The findings provide novel insights into the mechanisms of photoreceptor degeneration and support strategies targeting ER calcium regulation to manage retinal degeneration.

Tie2 activation promotes choriocapillary regeneration for alleviating neovascular age-related macular degeneration

Choriocapillary loss is a major cause of neovascular age-related macular degeneration (NV-AMD). Although vascular endothelial growth factor (VEGF) blockade for NV-AMD has shown beneficial outcomes, unmet medical needs for patients refractory or tachyphylactic to anti-VEGF therapy exist. In addition, the treatment could exacerbate choriocapillary rarefaction, necessitating advanced treatment for fundamental recovery from NV-AMD. In this study, Tie2 activation by angiopoietin-2-binding and Tie2-activating antibody (ABTAA) presents a therapeutic strategy for NV-AMD. Conditional Tie2 deletion impeded choriocapillary maintenance, rendering eyes susceptible to NV-AMD development. Moreover, in a NV-AMD mouse model, ABTAA not only suppressed choroidal neovascularization (CNV) and vascular leakage but also regenerated the choriocapillaris and relieved hypoxia. Conversely, VEGF blockade degenerated the choriocapillaris and exacerbated hypoxia, although it suppressed CNV and vascular leakage. Together, we establish that angiopoietin-Tie2 signaling is critical for choriocapillary maintenance and that ABTAA represents an alternative, combinative therapeutic strategy for NV-AMD by alleviating anti-VEGF adverse effects.

Accessory heterozygous mutations in cone photoreceptor CNGA3 exacerbate CNG channel-associated retinopathy

Mutations in CNGA3 and CNGB3, the genes encoding the subunits of the tetrameric cone photoreceptor cyclic nucleotide-gated ion channel, cause achromatopsia, a congenital retinal disorder characterized by loss of cone function. However, a small number of patients carrying the CNGB3/c.1208G>A;p.R403Q mutation present with a variable retinal phenotype ranging from complete and incomplete achromatopsia to moderate cone dysfunction or progressive cone dystrophy. By exploring a large patient cohort and published cases, we identified 16 unrelated individuals who were homozygous or (compound-)heterozygous for the CNGB3/c.1208G>A;p.R403Q mutation. In-depth genetic and clinical analysis revealed a co-occurrence of a mutant CNGA3 allele in a high proportion of these patients (10 of 16), likely contributing to the disease phenotype. To verify these findings, we generated a Cngb3R403Q/R403Q mouse model, which was crossbred with Cnga3-deficient (Cnga3-/-) mice to obtain triallelic Cnga3+/- Cngb3R403Q/R403Q mutants. As in human subjects, there was a striking genotype-phenotype correlation, since the presence of 1 Cnga3-null allele exacerbated the cone dystrophy phenotype in Cngb3R403Q/R403Q mice. These findings strongly suggest a digenic and triallelic inheritance pattern in a subset of patients with achromatopsia/severe cone dystrophy linked to the CNGB3/p.R403Q mutation, with important implications for diagnosis, prognosis, and genetic counseling.

Detecting significant genotype-phenotype association rules in bipolar disorder: market research meets complex genetics

Background

Disentangling the etiology of common, complex diseases is a major challenge in genetic research. For bipolar disorder (BD), several genome-wide association studies (GWAS) have been performed. Similar to other complex disorders, major breakthroughs in explaining the high heritability of BD through GWAS have remained elusive. To overcome this dilemma, genetic research into BD, has embraced a variety of strategies such as the formation of large consortia to increase sample size and sequencing approaches. Here we advocate a complementary approach making use of already existing GWAS data: a novel data mining procedure to identify yet undetected genotype–phenotype relationships. We adapted association rule mining, a data mining technique traditionally used in retail market research, to identify frequent and characteristic genotype patterns showing strong associations to phenotype clusters. We applied this strategy to three independent GWAS datasets from 2835 phenotypically characterized patients with BD. In a discovery step, 20,882 candidate association rules were extracted.

Results

Two of these rules—one associated with eating disorder and the other with anxiety—remained significant in an independent dataset after robust correction for multiple testing. Both showed considerable effect sizes (odds ratio ~ 3.4 and 3.0, respectively) and support previously reported molecular biological findings.

Conclusion

Our approach detected novel specific genotype–phenotype relationships in BD that were missed by standard analyses like GWAS. While we developed and applied our method within the context of BD gene discovery, it may facilitate identifying highly specific genotype–phenotype relationships in subsets of genome-wide data sets of other complex phenotype with similar epidemiological properties and challenges to gene discovery efforts.

Electronic supplementary material

The online version of this article (10.1186/s40345-018-0132-x) contains supplementary material, which is available to authorized users.

Cyclic-Nucleotide- and HCN-Channel-Mediated Phototransduction in Intrinsically Photosensitive Retinal Ganglion Cells

Non-image-forming vision in mammals is mediated primarily by melanopsin-expressing, intrinsically photosensitive retinal ganglion cells (ipRGCs). In mouse M1-ipRGCs, by far the best-studied subtype, melanopsin activates PLCβ4 (phospholipase C-β4) to open TRPC6,7 channels, mechanistically similar to phototransduction in fly rhabdomeric (microvillous) photoreceptors. We report here that, surprisingly, mouse M4-ipRGCs rely on a different and hitherto undescribed melanopsin-driven, ciliary phototransduction mechanism involving cyclic nucleotide as the second messenger and HCN channels rather than CNG channels as the ion channel for phototransduction. Even more surprisingly, within an individual mouse M2-ipRGC, this HCN-channel-dependent, ciliary phototransduction pathway operates in parallel with the TRPC6,7-dependent rhabdomeric pathway. These findings reveal a complex heterogeneity in phototransduction among ipRGCs and, more importantly, break a general dogma about segregation of the two phototransduction motifs, likely with strong evolutionary implications.

A Robust Optomotor Assay for Assessing the Efficacy of Optogenetic Tools for Vision Restoration

Purpose

To develop an animal behavioral assay for the quantitative assessment of the functional efficacy of optogenetic therapies.

Methods

A triple-knockout (TKO) mouse line, Gnat1^−/−Cnga3^−/−Opn4^−/−, and a double-knockout mouse line, Gnat1^−/−Cnga3^−/−, were employed. The expression of channelrhodopsin-2 (ChR2) and its three more light-sensitive mutants, ChR2-L132C, ChR2-L132C/T159C, and ChR2-132C/T159S, in inner retinal neurons was achieved using rAAV2 vectors via intravitreal delivery. Pupillary constriction was assessed by measuring the pupil diameter. The optomotor response (OMR) was examined using a homemade optomotor system equipped with light-emitting diodes as light stimulation.

Results

A robust OMR was restored in the ChR2-mutant-expressing TKO mice; however, significant pupillary constriction was observed only for the ChR2-L132C/T159S mutant. The ability to evoke an OMR was dependent on both the light intensity and grating frequency. The most light-sensitive frequency for the three ChR2 mutants was approximately 0.042 cycles per degree. Among the three ChR2 mutants, ChR2-L132C/T159S was the most light sensitive, followed by ChR2-L132C/T159C and ChR2-L132C. Melanopsin-mediated pupillary constriction resulted in a substantial reduction in the light sensitivity of the ChR2-mediated OMR.

Conclusions

The OMR assay using TKO mice enabled the quantitative assessment of the efficacy of different optogenetic tools and the properties of optogenetically restored vision. Thus, the assay can serve as a valuable tool for developing effective optogenetic therapies.

Additive contributions of melanopsin and both cone types provide broadband sensitivity to mouse pupil control

Background

Intrinsically photosensitive retinal ganglion cells (ipRGCs) drive an array of non-image-forming (NIF) visual responses including circadian photoentrainment and the pupil light reflex. ipRGCs integrate extrinsic (rod/cone) and intrinsic (melanopsin) photoreceptive signals, but the contribution of cones to ipRGC-dependent responses remains incompletely understood. Given recent data revealing that cone-derived colour signals influence mouse circadian timing and pupil responses in humans, here we set out to investigate the role of colour information in pupil control in mice.

Results

We first recorded electrophysiological activity from the pretectal olivary nucleus (PON) of anaesthetised mice with a red-shifted cone population (Opn1mw^R) and mice lacking functional cones (Cnga3^−/−) or melanopsin (Opn1mw^R; Opn4^−/−). Using multispectral stimuli to selectively modulate the activity of individual opsin classes, we show that PON cells which receive ipRGC input also exhibit robust S- and/or L-cone opsin-driven activity. This population includes many cells where the two cone opsins drive opponent responses (most commonly excitatory/ON responses to S-opsin stimulation and inhibitory/OFF responses to L-opsin stimulation). These cone inputs reliably tracked even slow (0.025 Hz) changes in illuminance/colour under photopic conditions with melanopsin contributions becoming increasingly dominant for higher-contrast/lower temporal frequency stimuli. We also evaluated consensual pupil responses in awake animals and show that, surprisingly, this aspect of physiology is insensitive to chromatic signals originating with cones. Instead, by contrast with the situation in humans, signals from melanopsin and both cone opsins combine in a purely additive manner to drive pupil constriction in mice.

Conclusion

Our data reveal a key difference in the sensory control of the mouse pupil relative to another major target of ipRGCs—the circadian clock. Whereas the latter uses colour information to help estimate time of day, the mouse pupil instead sums signals across cone opsin classes to provide broadband spectral sensitivity to changes in illumination. As such, while the widespread co-occurrence of chromatic responses and melanopsin input in the PON supports a close association between colour discrimination mechanisms and NIF visual processing, our data suggest that colour opponent PON cells in the mouse contribute to functions other than pupil control.

Electronic supplementary material

The online version of this article (10.1186/s12915-018-0552-1) contains supplementary material, which is available to authorized users.

Shedding light on the role of cAMP in mammalian sperm physiology

Mammalian fertilization relies on sperm finding the egg and penetrating the egg vestments. All steps in a sperm's lifetime crucially rely on changes in the second messenger cAMP (cyclic adenosine monophosphate). In recent years, it has become clear that signal transduction in sperm is not a continuum, but rather organized in subcellular domains, e.g. the sperm head and the sperm flagellum, with the latter being further separated into the midpiece, principal piece, and endpiece. To understand the underlying signaling pathways controlling sperm function in more detail, experimental approaches are needed that allow to study sperm signaling with spatial and temporal precision. Here, we will give a comprehensive overview on cAMP signaling in mammalian sperm, describing the molecular players involved in these pathways and the sperm functions that are controlled by cAMP. Furthermore, we will highlight recent advances in analyzing and manipulating sperm signaling with spatio-temporal precision using light.

Regulation of calcium homeostasis in the outer segments of rod and cone photoreceptors

Calcium plays important roles in the function and survival of rod and cone photoreceptor cells. Rapid regulation of calcium in the outer segments of photoreceptors is required for the modulation of phototransduction that drives the termination of the flash response as well as light adaptation in rods and cones. On a slower time scale, maintaining proper calcium homeostasis is critical for the health and survival of photoreceptors. Decades of work have established that the level of calcium in the outer segments of rods and cones is regulated by a dynamic equilibrium between influx via the transduction cGMP-gated channels and extrusion via rod- and cone-specific Na⁺/Ca²⁺, K⁺ exchangers (NCKXs). It had been widely accepted that the only mechanism for extrusion of calcium from rod outer segments is via the rod-specific NCKX1, while extrusion from cone outer segments is driven exclusively by the cone-specific NCKX2. However, recent evidence from mice lacking NCKX1 and NCKX2 have challenged that notion and have revealed a more complex picture, including a NCKX-independent mechanism in rods and two separate NCKX-dependent mechanisms in cones. This review will focus on recent findings on the molecular mechanisms of extrusion of calcium from the outer segments of rod and cone photoreceptors, and the functional and structural changes in photoreceptors when normal extrusion is disrupted.

Retinal Cyclic Nucleotide-Gated Channels: From Pathophysiology to Therapy

The first step in vision is the absorption of photons by the photopigments in cone and rod photoreceptors. After initial amplification within the phototransduction cascade the signal is translated into an electrical signal by the action of cyclic nucleotide-gated (CNG) channels. CNG channels are ligand-gated ion channels that are activated by the binding of cyclic guanosine monophosphate (cGMP) or cyclic adenosine monophosphate (cAMP). Retinal CNG channels transduce changes in intracellular concentrations of cGMP into changes of the membrane potential and the Ca²⁺ concentration. Structurally, the CNG channels belong to the superfamily of pore-loop cation channels and share a common gross structure with hyperpolarization-activated cyclic nucleotide-gated (HCN) channels and voltage-gated potassium channels (KCN). In this review, we provide an overview on the molecular properties of CNG channels and describe their physiological role in the phototransduction pathways. We also discuss insights into the pathophysiological role of CNG channel proteins that have emerged from the analysis of CNG channel-deficient animal models and human CNG channelopathies. Finally, we summarize recent gene therapy activities and provide an outlook for future clinical application.

Gene therapy for achromatopsia

The present review summarizes the current status of achromatopsia (ACHM) gene therapy-related research activities and provides an outlook for their clinical application. ACHM is an inherited eye disease characterized by a congenital absence of cone photoreceptor function. As a consequence, ACHM is associated with strongly impaired daylight vision, photophobia, nystagmus and a lack of color discrimination. Currently, six genes have been linked to ACHM. Up to 80% of the patients carry mutations in the genes CNGA3 and CNGB3 encoding the two subunits of the cone cyclic nucleotide-gated channel. Various animal models of the disease have been established and their characterization has helped to increase our understanding of the pathophysiology associated with ACHM. With the advent of adeno-associated virus vectors as valuable gene delivery tools for retinal photoreceptors, a number of promising gene supplementation therapy programs have been initiated. In recent years, huge progress has been made towards bringing a curative treatment for ACHM into clinics. The first clinical trials are ongoing or will be launched soon and are expected to contribute important data on the safety and efficacy of ACHM gene supplementation therapy.

Achromatopsia: clinical features, molecular genetics, animal models and therapeutic options

Achromatopsia is an autosomal recessive condition, characterised by reduced visual acuity, impaired colour vision, photophobia and nystagmus. The symptoms can be profoundly disabling, and there is no cure currently available. However, the recent development of gene-based interventions may lead to improved outcomes in the future. This article aims to provide a comprehensive review of the clinical features of the condition, its genetic basis and the underlying pathogenesis. We also explore the insights derived from animal models, including the implications for gene supplementation approaches. Finally, we discuss current human gene therapy trials.

Elovl4 5-bp deletion does not accelerate cone photoreceptor degeneration in an all-cone mouse

Mutations in the elongation of very long chain fatty acid 4 (ELOVL4) gene cause Stargardt macular dystrophy 3 (STGD3), a rare, juvenile-onset, autosomal dominant form of macular degeneration. Although several mouse models have already been generated to investigate the link between the three identified disease-causing mutations in the ELOVL4 gene, none of these models recapitulates the early-onset cone photoreceptor cell death observed in the macula of STGD3 patients. To address this specifically, we investigated the effect of mutant ELOVL4 in a mouse model with an all-cone retina. Hence, we bred mice carrying the heterozygously mutated Elovl4 gene on the R91W;Nrl^-/- all-cone background and analyzed the retinal lipid composition, morphology, and function over the course of 1 year. We observed a reduction of total phosphatidylcholine-containing very long chain-polyunsaturated fatty acids (PC-VLC-PUFAs) by 39% in the R91W;Nrl^-/-;Elovl4 mice already at 6 weeks of age with a pronounced decline of the longest forms of PC-VLC-PUFAs. Total levels of shorter-chain fatty acids (< C26) remained unaffected. However, this reduction in PC-VLC-PUFA content in the all-cone retina had no impact on morphology or function and did not accelerate retinal degeneration in the R91W;Nrl^-/-;Elovl4 mice. Taken together, mutations in the ELOVL4 gene lead to cone degeneration in humans, whereas mouse models expressing the mutant Elovl4 show predominant rod degeneration. The lack of a phenotype in the all-cone retina expressing the mutant form of the protein supports the view that aberrant function of ELOVL4 is especially detrimental for rods in mice and suggests a more subtle role of VLC-PUFAs for cone maintenance and survival.

Gene Therapy for Color Blindness

Achromatopsia is a rare congenital cause of vision loss due to isolated cone photoreceptor dysfunction. The most common underlying genetic mutations are autosomal recessive changes in CNGA3, CNGB3, GNAT2, PDE6H, PDE6C, or ATF6. Animal models of Cnga3, Cngb3, and Gnat2 have been rescued using AAV gene therapy; showing partial restoration of cone electrophysiology and integration of this new photopic vision in reflexive and behavioral visual tests. Three gene therapy phase I/II trials are currently being conducted in human patients in the USA, the UK, and Germany. This review details the AAV gene therapy treatments of achromatopsia to date. We also present novel data showing rescue of a Cnga3^-/- mouse model using an rAAV.CBA.CNGA3 vector. We conclude by synthesizing the implications of this animal work for ongoing human trials, particularly, the challenge of restoring integrated cone retinofugal pathways in an adult visual system. The evidence to date suggests that gene therapy for achromatopsia will need to be applied early in childhood to be effective.

Rods progressively escape saturation to drive visual responses in daylight conditions

Rod and cone photoreceptors support vision across large light intensity ranges. Rods, active under dim illumination, are thought to saturate at higher (photopic) irradiances. The extent of rod saturation is not well defined; some studies report rod activity well into the photopic range. Using electrophysiological recordings from retina and dorsal lateral geniculate nucleus of cone-deficient and visually intact mice, we describe stimulus and physiological factors that influence photopic rod-driven responses. We find that rod contrast sensitivity is initially strongly reduced at high irradiances, but progressively recovers to allow responses to moderate contrast stimuli. Surprisingly, rods recover faster at higher light levels. A model of rod phototransduction suggests that phototransduction gain adjustments and bleaching adaptation underlie rod recovery. Consistently, exogenous chromophore reduces rod responses at bright background. Thus, bleaching adaptation renders mouse rods responsive to modest contrast at any irradiance. Paradoxically, raising irradiance across the photopic range increases the robustness of rod responses.

Rod photoreceptors are thought to be saturated under bright light. Here, the authors describe the physiological parameters that mediate response saturation of rod photoreceptors in mouse retina, and show that rods can drive visual responses in photopic conditions.

Orexin-A Suppresses Signal Transmission to Dopaminergic Amacrine Cells From Outer and Inner Retinal Photoreceptors

Purpose

The neuropeptides orexin-A and orexin-B are widely expressed in the vertebrate retina; however, their role in visual function is unclear. This study investigates whether and how orexins modulate signal transmission to dopaminergic amacrine cells (DACs) from both outer retinal photoreceptors (rods and cones) and inner retinal photoreceptors (melanopsin-expressing intrinsically photosensitive retinal ganglion cells [ipRGCs]).

Methods

A whole-cell voltage-clamp technique was used to record light-induced responses from genetically labeled DACs in flat-mount mouse retinas. Rod and cone signaling to DACs was confirmed pharmacologically (in wild-type retinas), whereas retrograde melanopsin signaling to DACs was isolated either pharmacologically (in wild-type retinas) or by genetic deletion of rod and cone function (in transgenic mice).

Results

Orexin-A attenuated rod/cone-mediated light responses in the majority of DACs and inhibited all DACs that exhibited melanopsin-based light responses, suggesting that exogenous orexin suppresses signal transmission from rods, cones, and ipRGCs to DACs. In addition, orexin receptor 1 antagonist SB334867 and orexin receptor 2 antagonist TCS OX229 enhanced melanopsin-based DAC responses, indicating that endogenous orexins inhibit signal transmission from ipRGCs to DACs. We further found that orexin-A inhibits melanopsin-based DAC responses via orexin receptors on DACs, whereas orexin-A may modulate signal transmission from rods and cones to DACs through activation of orexin receptors on DACs and their upstream neurons.

Conclusions

Our results suggest that orexins could influence visual function via the dopaminergic system in the mammalian retina.

The sensory substrate of multimodal communication in brown-headed cowbirds: are females sensory 'specialists' or 'generalists'?

Many animals communicate with multimodal signals. While we have an understanding of multimodal signal production, we know relatively less about receiver filtering of multimodal signals and whether filtering capacity in one modality influences filtering in a second modality. Most multimodal signals contain a temporal element, such as change in frequency over time or a dynamic visual display. We examined the relationship in temporal resolution across two modalities to test whether females are (1) sensory 'specialists', where a trade-off exists between the sensory modalities, (2) sensory 'generalists', where a positive relationship exists between the modalities, or (3) whether no relationship exists between modalities. We used female brown-headed cowbirds (Molothrus ater) to investigate this question as males court females with an audiovisual display. We found a significant positive relationship between female visual and auditory temporal resolution, suggesting that females are sensory 'generalists'. Females appear to resolve information well across multiple modalities, which may select for males that signal their quality similarly across modalities.

M1 ipRGCs Influence Visual Function through Retrograde Signaling in the Retina

Melanopsin-expressing intrinsically photosensitive retinal ganglion cells (ipRGCs, with five subtypes named M1-M5) are a unique subclass of RGCs with axons that project directly to many brain nuclei involved in non-image-forming functions such as circadian photoentrainment and the pupillary light reflex. Recent evidence suggests that melanopsin-based signals also influence image-forming visual function, including light adaptation, but the mechanisms involved are unclear. Intriguingly, a small population of M1 ipRGCs have intraretinal axon collaterals that project toward the outer retina. Using genetic mouse models, we provide three lines of evidence showing that these axon collaterals make connections with upstream dopaminergic amacrine cells (DACs): (1) ipRGC signaling to DACs is blocked by tetrodotoxin both in vitro and in vivo, indicating that ipRGC-to-DAC transmission requires voltage-gated Na(+) channels; (2) this transmission is partly dependent on N-type Ca(2+) channels, which are possibly expressed in the axon collateral terminals of ipRGCs; and (3) fluorescence microscopy reveals that ipRGC axon collaterals make putative presynaptic contact with DACs. We further demonstrate that elimination of M1 ipRGCs attenuates light adaptation, as evidenced by an impaired electroretinogram b-wave from cones, whereas a dopamine receptor agonist can potentiate the cone-driven b-wave of retinas lacking M1 ipRGCs. Together, the results strongly suggest that ipRGCs transmit luminance signals retrogradely to the outer retina through the dopaminergic system and in turn influence retinal light adaptation.

SIGNIFICANCE STATEMENT

Melanopsin-expressing intrinsically photosensitive retinal ganglion cells (ipRGCs) comprise a third class of retinal photoreceptors that are known to mediate physiological responses such as circadian photoentrainment. However, investigation into whether and how ipRGCs contribute to vision has just begun. Here, we provide convergent anatomical and physiological evidence that axon collaterals of ipRGCs constitute a centrifugal pathway to DACs, conveying melanopsin-based signals from the innermost retina to the outer retina. We further demonstrate that retrograde signals likely influence visual processing because elimination of axon collateral-bearing ipRGCs impairs light adaptation by limiting dopamine-dependent facilitation of the cone pathway. Our findings strongly support the hypothesis that retrograde melanopsin-based signaling influences visual function locally within the retina, a notion that refutes the dogma that RGCs only provide physiological signals to the brain.

Determination of Rod and Cone Influence to the Early and Late Dynamic of the Pupillary Light Response

PURPOSE

This study aims to identify which aspects of the pupil light reflex are most influenced by rods and cones independently by analyzing pupil recordings from different mouse models of photoreceptor deficiency.

METHODS

One-month-old wild type (WT), rodless (Rho-/-), coneless (Cnga3-/-), or photoreceptor less (Cnga3-/-; Rho-/- or Gnat1-/-) mice were subjected to brief red and blue light stimuli of increasing intensity. To describe the initial dynamic response to light, the maximal pupillary constriction amplitudes and the derivative curve of the first 3 seconds were determined. To estimate the postillumination phase, the constriction amplitude at 9.5 seconds after light termination was related to the maximal constriction amplitude.

RESULTS

Rho-/- mice showed decreased constriction amplitude but more prolonged pupilloconstriction to all blue and red light stimuli compared to wild type mice. Cnga3-/- mice had constriction amplitudes similar to WT however following maximal constriction, the early and rapid dilation to low intensity blue light was decreased. To high intensity blue light, the Cnga3-/- mice demonstrated marked prolongation of the pupillary constriction. Cnga3-/-; Rho-/- mice had no pupil response to red light of low and medium intensity.

CONCLUSIONS

From specific gene defective mouse models which selectively voided the rod or cone function, we determined that mouse rod photoreceptors are highly contributing to the pupil response to blue light stimuli but also to low and medium red stimuli. We also observed that cone cells mainly drive the partial rapid dilation of the initial response to low blue light stimuli. Thus photoreceptor dysfunction can be derived from chromatic pupillometry in mouse models.

AAV-Mediated Gene Supplementation Therapy in Achromatopsia Type 2: Preclinical Data on Therapeutic Time Window and Long-Term Effects

Achromatopsia type 2 (ACHM2) is a severe, inherited eye disease caused by mutations in the CNGA3 gene encoding the α subunit of the cone photoreceptor cyclic nucleotide-gated (CNG) channel. Patients suffer from strongly impaired daylight vision, photophobia, nystagmus, and lack of color discrimination. We have previously shown in the Cnga3 knockout (KO) mouse model of ACHM2 that gene supplementation therapy is effective in rescuing cone function and morphology and delaying cone degeneration. In our preclinical approach, we use recombinant adeno-associated virus (AAV) vector-mediated gene transfer to express the murine Cnga3 gene under control of the mouse blue opsin promoter. Here, we provide novel data on the efficiency and permanence of such gene supplementation therapy in Cnga3 KO mice. Specifically, we compare the influence of two different AAV vector capsids, AAV2/5 (Y719F) and AAV2/8 (Y733F), on restoration of cone function, and assess the effect of age at time of treatment on the long-term outcome. The evaluation included in vivo analysis of retinal function using electroretinography (ERG) and immunohistochemical analysis of vector-driven Cnga3 transgene expression. We found that both vector capsid serotypes led to a comparable rescue of cone function over the observation period between 4 weeks and 3 months post treatment. In addition, a clear therapeutic effect was present in mice treated at 2 weeks of age as well as in mice treated at 3 months of age at the first assessment at 4 weeks after treatment. Importantly, the effect extended in both cases over the entire observation period of 12 months post treatment. However, the average ERG amplitude levels differed between the two groups, suggesting a role of the absolute age, or possibly, the associated state of the degeneration, on the achievable outcome. In summary, we found that the therapeutic time window of opportunity for AAV-mediated Cnga3 gene supplementation therapy in the Cnga3 KO mouse model extends at least to an age of 3 months, but is presumably limited by the condition, number and topographical distribution of remaining cones at the time of treatment. No impact of the choice of capsid on the therapeutic success was detected.

Endoplasmic reticulum (ER) Ca2+-channel activity contributes to ER stress and cone death in cyclic nucleotide-gated channel deficiency

Endoplasmic reticulum (ER) stress and mislocalization of improperly folded proteins have been shown to contribute to photoreceptor death in models of inherited retinal degenerative diseases. In particular, mice with cone cyclic nucleotide-gated (CNG) channel deficiency, a model for achromatopsia, display both early-onset ER stress and opsin mistrafficking. By 2 weeks of age, these mice show elevated signaling from all three arms of the ER-stress pathway, and by 1 month, cone opsin is improperly distributed away from its normal outer segment location to other retinal layers. This work investigated the role of Ca²⁺-release channels in ER stress, protein mislocalization, and cone death in a mouse model of CNG-channel deficiency. We examined whether preservation of luminal Ca²⁺ stores through pharmacological and genetic suppression of ER Ca²⁺ efflux protects cones by attenuating ER stress. We demonstrated that the inhibition of ER Ca²⁺-efflux channels reduced all three arms of ER-stress signaling while improving opsin trafficking to cone outer segments and decreasing cone death by 20-35%. Cone-specific gene deletion of the inositol-1,4,5-trisphosphate receptor type I (IP₃R1) also significantly increased cone density in the CNG-channel-deficient mice, suggesting that IP₃R1 signaling contributes to Ca²⁺ homeostasis and cone survival. Consistent with the important contribution of organellar Ca²⁺ signaling in this achromatopsia mouse model, significant differences in dynamic intraorganellar Ca²⁺ levels were detected in CNG-channel-deficient cones. These results thus identify a novel molecular link between Ca²⁺ homeostasis and cone degeneration, thereby revealing novel therapeutic targets to preserve cones in inherited retinal degenerative diseases.

Unravelling the genetics of inherited retinal dystrophies: Past, present and future

The identification of the genes underlying monogenic diseases has been of interest to clinicians and scientists for many years. Using inherited retinal dystrophies as an example of monogenic disease we describe the history of molecular genetic techniques that have been pivotal in the discovery of disease causing genes. The methods that were developed in the 1970's and 80's are still in use today but have been refined and improved. These techniques enabled the concept of the Human Genome Project to be envisaged and ultimately realised. When the successful conclusion of the project was announced in 2003 many new tools and, as importantly, many collaborations had been developed that facilitated a rapid identification of disease genes. In the post-human genome project era advances in computing power and the clever use of the properties of DNA replication has allowed the development of next-generation sequencing technologies. These methods have revolutionised the identification of disease genes because for the first time there is no need to define the position of the gene in the genome. The use of next generation sequencing in a diagnostic setting has allowed many more patients with an inherited retinal dystrophy to obtain a molecular diagnosis for their disease. The identification of novel genes that have a role in the development or maintenance of retinal function is opening up avenues of research which will lead to the development of new pharmacological and gene therapy approaches. Neither of which can be used unless the defective gene and protein is known. The continued development of sequencing technologies also holds great promise for the advent of truly personalised medicine.

Purification of cone outer segment for proteomic analysis on its membrane proteins in carp retina

Rods and cones are both photoreceptors in the retina, but they are different in many aspects including the light response characteristics and, for example, cell morphology and metabolism. These differences would be caused by differences in proteins expressed in rods and cones. To understand the molecular bases of these differences between rods and cones, one of the ways is to compare proteins expressed in rods and cones, and to find those expressed specifically or dominantly. In the present study, we are interested in proteins in the outer segment (OS), the site responsible for generation of rod- or cone-characteristic light responses and also the site showing different morphology between rods and cones. For this, we established a method to purify the OS and the inner segment (IS) of rods and also of cones from purified carp rods and cones, respectively, using sucrose density gradient. In particular, we were interested in proteins tightly bound to the membranes of cone OS. To identify these proteins, we analyzed proteins in some selected regions of an SDS-gel of washed membranes of the OS and the IS obtained from both rods and cones, with Liquid Chromatography-tandem Mass Spectrometry (LC-MS/MS) using a protein database constructed from carp retina. By comparing the lists of the proteins found in the OS and the IS of both rods and cones, we found some proteins present in cone OS membranes specifically or dominantly, in addition to the proteins already known to be present specifically in cone OS.

Cone Genesis Tracing by the Chrnb4-EGFP Mouse Line: Evidences of Cellular Material Fusion after Cone Precursor Transplantation

The cone function is essential to mediate high visual acuity, color vision, and daylight vision. Inherited cone dystrophies and age-related macular degeneration affect a substantial percentage of the world population. To identify and isolate the most competent cells for transplantation and integration into the retina, cone tracing during development would be an important added value. To that aim, the Chrnb4-EGFP mouse line was characterized throughout retinogenesis. It revealed a sub-population of early retinal progenitors expressing the reporter gene that is progressively restricted to mature cones during retina development. The presence of the native CHRNB4 protein was confirmed in EGFP-positive cells, and it presents a similar pattern in the human retina. Sub-retinal transplantations of distinct subpopulations of Chrnb4-EGFP-expressing cells revealed the embryonic day 15.5 high-EGFP population the most efficient cells to interact with host retinas to provoke the appearance of EGFP-positive cones in the photoreceptor layer. Importantly, transplantations into the DsRed retinas revealed material exchanges between donor and host retinas, as >80% of transplanted EGFP-positive cones also were DsRed positive. Whether this cell material fusion is of significant therapeutic advantage requires further thorough investigations. The Chrnb4-EGFP mouse line definitely opens new research perspectives in cone genesis and retina repair.

In vivo imaging of a cone mosaic in a patient with achromatopsia associated with a GNAT2 variant

PURPOSE

The 2 most common causative genes for achromatopsia (ACHM) are CNGA3 and CNGB3; other genes including GNAT2 account for only a small portion of ACHM cases. The cone mosaics in eyes with CNGA3 and CNGB3 variants are severely disrupted; the cone mosaics in patients with GNAT2-associated ACHM; however, have been reported to show a contiguous pattern in adaptive optics (AO) retinal images. The purpose of this study was to analyze the cone mosaic of another case of GNAT2-associated ACHM.

PATIENT AND METHODS

The patient was a 17-year-old Japanese boy. Comprehensive ocular examinations including fundus photography, electroretinography (ERGs), optical coherence tomography (OCT), and whole-exome analysis were performed. The cone mosaic was recorded with a flood-illuminated AO fundus camera, and the cone density was compared with those of 10 normal control eyes.

RESULTS

The patient had the typical phenotype of ACHM, and a novel homozygous variant, c.730_743del, in GNAT2 was identified. The fundus did not show any specific abnormalities, and the OCT images showed the presence of the ellipsoid zone. The AO fundus image showed a clearly defined cone mosaic around the fovea. The cone density at 500 μm from the fovea was reduced by 15-30 % as compared with those of the normal eyes.

CONCLUSIONS

This is the first description of a Japanese patient with ACHM with a novel GNAT2 variant. The eyes of this patient had a preserved cone structure with loss of function.