Ex vivo ERG analysis of photoreceptors using an in vivo ERG system

Leading edge of the a-wave of the electroretinogram and sodium iodate-induced age-related macular degeneration: A model

BACKGROUND

Iron-induced oxidative stress was thought to be the reason why the a-wave amplitude of the electroretinogram (ERG) dropped when iron ions were present. It is assumed that reactive oxygen species (ROS) are generated in the presence of iron ions, and this leads to a decrease in hyperpolarization of the photoreceptor. It is known that in age-related macular degeneration (AMD), sodium iodate can induce oxidative stress, apoptosis, and retinal damage, which mimic the effects of clinical AMD. Here, the reduction of the a-wave amplitude in mice with sodium iodate-induced age-related macular degeneration is explained.

METHODS

The leading edge of the a-wave is divided into voltages developed by cones and rods. The same oxidative stress model is applied here since sodium iodate causes the creation of ROS in a manner similar to that caused by iron ions, with the exception that the retina is treated as a circuit of various resistances when computing the photoresponse. Moreover, sodium iodate also leads to apoptosis and, hence, may cause misalignment in cones (not in rods) during the initial stage of apoptosis in AMD. To include the effects of apoptosis and shortening in cones and rods, we have used a factor representing the fraction of total cones and rods that are alive. To include the effect of misalignment of cones on the reduction of the a-wave amplitude, we have used the Stiles-Crawford function to calculate the number of photoisomerizations occurring in a photoreceptor misaligned at an angle θ. The results are compared with experimental data.

RESULTS

In sodium iodate-treated eyes, the ROS produced can attract calcium ions in the photoreceptor, which increases the calcium influx. In the case of the cones, the inclusion of the misalignment angle in the phototransduction process helps in determining the voltage and slope of the voltage vs. time graph.The smaller the fraction of active photoreceptors, the smaller the amplitude of the a-wave. The calcium influx, misaligned photoreceptors, and total photoreceptor loss all cause the amplitude of the a-wave to decrease, and at any time from the beginning of phototransduction cascade, the calcium influx causes the slope of the a-wave to increase.

CONCLUSION

The reduction in the a-wave amplitude in the eyes of sodium iodate-treated mice is attributed to oxidative stress in both cones and rods and cone misalignment, which ultimately lead to apoptosis and vision loss in AMD.

A combination treatment based on drug repurposing demonstrates mutation-agnostic efficacy in pre-clinical retinopathy models

Inherited retinopathies are devastating diseases that in most cases lack treatment options. Disease-modifying therapies that mitigate pathophysiology regardless of the underlying genetic lesion are desirable due to the diversity of mutations found in such diseases. We tested a systems pharmacology-based strategy that suppresses intracellular cAMP and Ca2+ activity via G protein-coupled receptor (GPCR) modulation using tamsulosin, metoprolol, and bromocriptine coadministration. The treatment improves cone photoreceptor function and slows degeneration in Pde6βrd10 and RhoP23H/WT retinitis pigmentosa mice. Cone degeneration is modestly mitigated after a 7-month-long drug infusion in PDE6A-/- dogs. The treatment also improves rod pathway function in an Rpe65-/- mouse model of Leber congenital amaurosis but does not protect from cone degeneration. RNA-sequencing analyses indicate improved metabolic function in drug-treated Rpe65-/- and rd10 mice. Our data show that catecholaminergic GPCR drug combinations that modify second messenger levels via multiple receptor actions provide a potential disease-modifying therapy against retinal degeneration.

The USH3A causative gene clarin1 functions in Müller glia to maintain retinal photoreceptors

Mutations in CLRN1 cause Usher syndrome type IIIA (USH3A), an autosomal recessive disorder characterized by hearing and vision loss, and often accompanied by vestibular balance issues. The identity of the cell types responsible for the pathology and mechanisms leading to vision loss in USH3A remains elusive. To address this, we employed CRISPR/Cas9 technology to delete a large region in the coding and untranslated (UTR) region of zebrafish clrn1. Retina of clrn1 mutant larvae exhibited sensitivity to cell stress, along with age-dependent loss of function and degeneration in the photoreceptor layer. Investigation revealed disorganization in the outer retina in clrn1 mutants, including actin-based structures of the Müller glia and photoreceptor cells. To assess cell-specific contributions to USH3A pathology, we specifically re-expressed clrn1 in either Müller glia or photoreceptor cells. Müller glia re-expression of clrn1 prevented the elevated cell death observed in larval clrn1 mutant zebrafish exposed to high-intensity light. Notably, the degree of phenotypic rescue correlated with the level of Clrn1 re-expression. Surprisingly, high levels of Clrn1 expression enhanced cell death in both wild-type and clrn1 mutant animals. However, rod- or cone-specific Clrn1 re-expression did not rescue the extent of cell death. Taken together, our findings underscore three crucial insights. First, clrn1 mutant zebrafish exhibit key pathological features of USH3A; second, Clrn1 within Müller glia plays a pivotal role in photoreceptor maintenance, with its expression requiring controlled regulation; third, the reliance of photoreceptors on Müller glia suggests a structural support mechanism, possibly through direct interactions between Müller glia and photoreceptors mediated in part by Clrn1 protein.

Light drives the developmental progression of outer retinal function

The complex nature of rod and cone photoreceptors and the light-evoked responsivity of bipolar cells in the mature rodent retina have been well characterized. However, little is known about the emergent light-evoked response properties of the mouse retina and the role light plays in shaping these emergent responses. We have previously demonstrated that the outer retina is responsive to green light as early as postnatal day 8 (P8). Here, we characterize the progression of both photoreceptors (rods and cones) and bipolar cell responses during development and into adulthood using ex vivo electroretinogram recordings. Our data show that the majority of photoreceptor response at P8 originates from cones and that these outputs drive second-order bipolar cell responses as early as P9. We find that the magnitude of the photoresponse increases concurrently with each passing day of postnatal development and that many functional properties of these responses, as well as the relative rod/cone contributions to the total light-evoked response, are age dependent. We compare these responses at eye opening and maturity to age-matched animals raised in darkness and found that the absence of light diminishes emergent and mature cone-to-bipolar cell signaling. Furthermore, we found cone-evoked responses to be significantly slower in dark-reared retinas. Together, this work characterizes the developmental photoresponsivity of the mouse retina while highlighting the importance of properly timed sensory input for the maturation of the first visual system synapse.

Prenylation is essential for the enrichment of cone phosphodiesterase-6 (PDE6) in outer segments and efficient cone phototransduction

Phosphodiesterase-6 (PDE6) is the key phototransduction effector enzyme residing in the outer segment (OS) of photoreceptors. Cone PDE6 is a tetrameric protein consisting of two inhibitory subunits (γ') and two catalytic subunits (α'). The catalytic subunit of cone PDE6 contains a C-terminus prenylation motif. Deletion of PDE6α' C-terminal prenylation motif is linked to achromatopsia (ACHM), a type of color blindness in humans. However, mechanisms behind the disease and roles for lipidation of cone PDE6 in vision are unknown. In this study, we generated two knock-in mouse models expressing mutant variants of cone PDE6α' lacking the prenylation motif (PDE6α'∆C). We find that the C-terminal prenylation motif is the primary determinant for the association of cone PDE6 protein with membranes. Cones from PDE6α'∆C homozygous mice are less sensitive to light, and their response to light is delayed, whereas cone function in heterozygous PDE6α'∆C/+ mice is unaffected. Surprisingly, the expression level and assembly of cone PDE6 protein were unaltered in the absence of prenylation. Unprenylated assembled cone PDE6 in PDE6α'∆C homozygous animals is mislocalized and enriched in the cone inner segment and synaptic terminal. Interestingly, the disk density and the overall length of cone OS in PDE6α'∆C homozygous mutants are altered, highlighting a novel structural role for PDE6 in maintaining cone OS length and morphology. The survival of cones in the ACHM model generated in this study bodes well for gene therapy as a treatment option for restoring vision in patients with similar mutations in the PDE6C gene.

An Ex Vivo Electroretinographic Apparatus for the mL-Scale Testing of Drugs to One Day and Beyond

When screening new drugs to treat retinal diseases, ex vivo electroretinography (ERG) potentially combines the experimental throughput of its traditional in vivo counterpart, with greater mechanistic insight and reproducible delivery. To date, this technique was used in experiments with open loop superfusion and lasting up to a few hours. Here, we present a compact apparatus that provides continuous and simultaneous recordings of the scotopic a-waves from four mouse retinas for much longer durations. Crucially, each retina can be incubated at 37 °C in only 2 mL of static medium, enabling the testing of very expensive drugs or nano devices. Light sensitivity and response kinetics of these preparations remain in the physiological range throughout incubation, displaying only very slow drifts. As an example application, we showed that barium, a potassium channel blocker used to abolish the glial component of the ERG, displayed no overt side effects on photoreceptors over several hours. In another example, we fully regenerated a partially bleached retina using a minimal quantity of 9-cis-retinal. Finally, we demonstrated that including antibiotic in the incubation medium extends physiological light responses to over one day. This system represents a necessary stepping stone towards the goal of combining ERG recordings with organotypically cultured retinas.

The origins of the full-field flash electroretinogram b-wave

The electroretinogram (ERG) measures the electrical activity of retinal neurons and glial cells in response to a light stimulus. Amongst other techniques, clinicians utilize the ERG to diagnose various eye diseases, including inherited conditions such as cone-rod dystrophy, rod-cone dystrophy, retinitis pigmentosa and Usher syndrome, and to assess overall retinal health. An ERG measures the scotopic and photopic systems separately and mainly consists of an a-wave and a b-wave. The other major components of the dark-adapted ERG response include the oscillatory potentials, c-wave, and d-wave. The dark-adapted a-wave is the initial corneal negative wave that arises from the outer segments of the rod and cone photoreceptors hyperpolarizing in response to a light stimulus. This is followed by the slower, positive, and prolonged b-wave, whose origins remain elusive. Despite a large body of work, there remains controversy around the mechanisms involved in the generation of the b-wave. Several hypotheses attribute the origins of the b-wave to bipolar or Müller glial cells or a dual contribution from both cell types. This review will discuss the current hypothesis for the cellular origins of the dark-adapted ERG, with a focus on the b-wave.

Bicarbonate boosts flash response amplitude to augment absolute sensitivity and extend dynamic range in murine retinal rods

Rod photoreceptors in the retina adjust their responsiveness and sensitivity so that they can continue to provide meaningful information over a wide range of light intensities. By stimulating membrane guanylate cyclases in the outer segment to synthesize cGMP at a faster rate in a Ca²⁺-dependent fashion, bicarbonate increases the circulating "dark" current and accelerates flash response kinetics in amphibian rods. Compared to amphibian rods, mammalian rods are smaller in size, operate at a higher temperature, and express visual cascade proteins with somewhat different biochemical properties. Here, we evaluated the role of bicarbonate in rods of cpfl3 mice. These mice are deficient in their expression of functional cone transducin, Gnat2, making cones very insensitive to light, so the rod response to light could be observed in isolation in electroretinogram recordings. Bicarbonate increased the dark current and absolute sensitivity and quickened flash response recovery in mouse rods to a greater extent than in amphibian rods. In addition, bicarbonate enabled mouse rods to respond over a range that extended to dimmer flashes. Larger flash responses may have resulted in part from a bicarbonate-induced elevation in intracellular pH. However, high pH alone had little effect on flash response recovery kinetics and even suppressed the accelerating effect of bicarbonate, consistent with a direct, modulatory action of bicarbonate on Ca²⁺- dependent, membrane guanylate cyclase activity.

Synaptic vesicle release during ribbon synapse formation of cone photoreceptors

Mammalian cone photoreceptors enable through their sophisticated synapse the high-fidelity transfer of visual information to second-order neurons in the retina. The synapse contains a proteinaceous organelle, called the synaptic ribbon, which tethers synaptic vesicles (SVs) at the active zone (AZ) close to voltage-gated Ca2+ channels. However, the exact contribution of the synaptic ribbon to neurotransmission is not fully understood, yet. In mice, precursors to synaptic ribbons appear within photoreceptor terminals shortly after birth as free-floating spherical structures, which progressively elongate and then attach to the AZ during the following days. Here, we took advantage of the process of synaptic ribbon maturation to study their contribution to SV release. We performed whole-cell patch-clamp recordings from cone photoreceptors at three postnatal (P) development stages (P8-9, P12-13, >P30) and measured evoked SV release, SV replenishment rate, recovery from synaptic depression, domain organization of voltage-sensitive Ca2+ channels, and Ca2+-sensitivity of exocytosis. Additionally, we performed electron microscopy to determine the density of SVs at ribbon-free and ribbon-occupied AZs. Our results suggest that ribbon attachment does not organize the voltage-sensitive Ca2+ channels into nanodomains or control SV release probability. However, ribbon attachment increases SV density at the AZ, increases the pool size of readily releasable SVs available for evoked SV release, facilitates SV replenishment without changing the SV pool refilling time, and increases the Ca2+- sensitivity of glutamate release.

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.

Deletion of Protein Phosphatase 2A Accelerates Retinal Degeneration in GRK1- and Arr1-Deficient Mice

Purpose

Light detection in retinal rod photoreceptors is initiated by activation of the visual pigment rhodopsin. A critical, yet often-overlooked, step enabling efficient perception of light is rhodopsin dephosphorylation mediated by protein phosphatase 2A (PP2A). PP2A deficiency has been reported to impair rhodopsin regeneration after phosphorylation by G protein receptor kinase 1 (GRK1) and binding of arrestin (Arr1), thereby delaying rod dark adaptation. However, its effects on the viability of photoreceptors in the absence of GRK1 and Arr1 remain unclear. Here, we investigated the effects of PP2A deficiency in the absence of GRK1 or Arr1, both of which have been implicated in Oguchi disease, a form of night blindness.

Methods

Rod-specific mice lacking the predominant catalytic Cα-subunit of PP2A were crossed with the Grk1^−/− or Arr1^−/− strains to obtain double knockout lines. Rod photoreceptor viability was analyzed in histological cross-sections of the retina stained with hematoxylin and eosin, and rod function was evaluated by ex vivo electroretinography.

Results

PP2A deficiency alone did not impair photoreceptor viability up to 12 months of age. Retinal degeneration was more pronounced in rods lacking GRK1 compared to rods lacking Arr1, and degeneration was accelerated in both Grk1^−/− or Arr1^−/− strains where PP2A was also deleted. In Arr1^−/− mice, rod maximal photoresponse amplitudes were reduced by 80% at 3 months, and this diminution was enhanced further with concomitant PP2A deficiency.

Conclusions

These results suggest that although PP2A is not required for the survival of rods, its deletion accelerates the degeneration induced by the absence of either GRK1 or Arr1.

Acyl-CoA:wax alcohol acyltransferase 2 modulates the cone visual cycle in mouse retina

The daylight and color vision of diurnal vertebrates depends on cone photoreceptors. The capability of cones to operate and respond to changes in light brightness even under high illumination is attributed to their fast rate of recovery to the ground photosensitive state. This process requires the rapid replenishing of photoisomerized visual chromophore (11-cis-retinal) to regenerate cone visual pigments. Recently, several gene candidates have been proposed to contribute to the cone-specific retinoid metabolism, including acyl-CoA wax alcohol acyltransferase 2 (AWAT2, aka MFAT). Here, we evaluated the role of AWAT2 in the regeneration of visual chromophore by the phenotypic characterization of Awat2^-/- mice. The global absence of AWAT2 enzymatic activity did not affect gross retinal morphology or the rate of visual chromophore regeneration by the canonical RPE65-dependent visual cycle. Analysis of Awat2 expression indicated the presence of the enzyme throughout the murine retina, including the retinal pigment epithelium (RPE) and Müller cells. Electrophysiological recordings revealed reduced maximal rod and cone dark-adapted responses in AWAT2-deficient mice compared to control mice. While rod dark adaptation was not affected by the lack of AWAT2, M-cone dark adaptation both in isolated retina and in vivo was significantly suppressed. Altogether, these results indicate that while AWAT2 is not required for the normal operation of the canonical visual cycle, it is a functional component of the cone-specific visual chromophore regenerative pathway.

Revival of light signalling in the postmortem mouse and human retina

Death is defined as the irreversible cessation of circulatory, respiratory or brain activity. Many peripheral human organs can be transplanted from deceased donors using protocols to optimize viability. However, tissues from the central nervous system rapidly lose viability after circulation ceases^1,2, impeding their potential for transplantation. The time course and mechanisms causing neuronal death and the potential for revival remain poorly defined. Here, using the retina as a model of the central nervous system, we systemically examine the kinetics of death and neuronal revival. We demonstrate the swift decline of neuronal signalling and identify conditions for reviving synchronous in vivo-like trans-synaptic transmission in postmortem mouse and human retina. We measure light-evoked responses in human macular photoreceptors in eyes removed up to 5 h after death and identify modifiable factors that drive reversible and irreversible loss of light signalling after death. Finally, we quantify the rate-limiting deactivation reaction of phototransduction, a model G protein signalling cascade, in peripheral and macular human and macaque retina. Our approach will have broad applications and impact by enabling transformative studies in the human central nervous system, raising questions about the irreversibility of neuronal cell death, and providing new avenues for visual rehabilitation.

Identification of small molecule allosteric modulators that act as enhancers/disrupters of rhodopsin oligomerization

The elongated cilia of the outer segment of rod and cone photoreceptor cells can contain concentrations of visual pigments of up to 5 mM. The rod visual pigments, G protein–coupled receptors called rhodopsins, have a propensity to self-aggregate, a property conserved among many G protein–coupled receptors. However, the effect of rhodopsin oligomerization on G protein signaling in native cells is less clear. Here, we address this gap in knowledge by studying rod phototransduction. As the rod outer segment is known to adjust its size proportionally to overexpression or reduction of rhodopsin expression, genetic perturbation of rhodopsin cannot be used to resolve this question. Therefore, we turned to high-throughput screening of a diverse library of 50,000 small molecules and used a novel assay for the detection of rhodopsin dimerization. This screen identified nine small molecules that either disrupted or enhanced rhodopsin dimer contacts in vitro. In a subsequent cell-free binding study, we found that all nine compounds decreased intrinsic fluorescence without affecting the overall UV-visible spectrum of rhodopsin, supporting their actions as allosteric modulators. Furthermore, ex vivo electrophysiological recordings revealed that a disruptive, hit compound #7 significantly slowed down the light response kinetics of intact rods, whereas compound #1, an enhancing hit candidate, did not substantially affect the photoresponse kinetics but did cause a significant reduction in light sensitivity. This study provides a monitoring tool for future investigation of the rhodopsin signaling cascade and reports the discovery of new allosteric modulators of rhodopsin dimerization that can also alter rod photoreceptor physiology.

Two-photon AMPK and ATP imaging reveals the bias between rods and cones in glycolysis utility

In vertebrates, retinal rod and cone photoreceptor cells rely significantly on glycolysis. Lactate released from photoreceptor cells fuels neighboring retinal pigment epithelium cells and Müller glial cells through oxidative phosphorylation. To understand this highly heterogeneous metabolic environment around photoreceptor cells, single-cell analysis is needed. Here, we visualized cellular AMP-activated protein kinase (AMPK) activity and ATP levels in the retina by two-photon microscopy. Transgenic mice expressing a hyBRET-AMPK biosensor were used for measuring the AMPK activity. GO-ATeam2 transgenic mice were used for measuring the ATP level. Temporal metabolic responses were successfully detected in the live retinal explants upon drug perfusion. A glycolysis inhibitor, 2-deoxy-d-glucose (2-DG), activated AMPK and reduced ATP. These effects were clearly stronger in rods than in cones. Notably, rod AMPK and ATP started to recover at 30 min from the onset of 2-DG perfusion. Consistent with these findings, ex vivo electroretinogram recordings showed a transient slowdown in rod dim flash responses during a 60-min 2-DG perfusion, whereas cone responses were not affected. Based on these results, we propose that cones surrounded by highly glycolytic rods become less dependent on glycolysis, and rods also become less dependent on glycolysis within 60 min upon the glycolysis inhibition.

A p97/Valosin-Containing Protein Inhibitor Drug CB-5083 Has a Potent but Reversible Off-Target Effect on Phosphodiesterase-6

CB-5083 is an inhibitor of p97/valosin-containing protein (VCP), for which phase I trials for cancer were terminated because of adverse effects on vision, such as photophobia and dyschromatopsia. Lower dose CB-5083 could combat inclusion body myopathy with early-onset Paget disease and frontotemporal dementia or multisystem proteinopathy caused by gain-of-function mutations in VCP. We hypothesized that the visual impairment in the cancer trial was due to CB-5083's inhibition of phosphodiesterase (PDE)-6, which mediates signal transduction in photoreceptors. To test our hypothesis, we used in vivo and ex vivo electroretinography (ERG) in mice and a PDE6 activity assay of bovine rod outer segment (ROS) extracts. Additionally, histology and optical coherence tomography were used to assess CB-5083's long-term ocular toxicity. A single administration of CB-5083 led to robust ERG signal deterioration, specifically in photoresponse kinetics. Similar recordings with known PDE inhibitors sildenafil, tadalafil, vardenafil, and zaprinast showed that only vardenafil had as strong an effect on the ERG signal in vivo as did CB-5083. In the biochemical assay, CB-5083 inhibited PDE6 activity with a potency higher than sildenafil but lower than that of vardenafil. Ex vivo ERG revealed a PDE6 inhibition constant of 80 nM for CB-5083, which is 7-fold smaller than that for sildenafil. Finally, we showed that the inhibitory effect of CB-5083 on visual function is reversible, and its chronic administration does not cause permanent retinal anomalies in aged VCP-disease model mice. Our results warrant re-evaluation of CB-5083 as a clinical therapeutic agent. We recommend preclinical ERG recordings as a routine drug safety screen. SIGNIFICANCE STATEMENT: This report supports the use of a valosin-containing protein (VCP) inhibitor drug, CB-5083, for the treatment of neuromuscular VCP disease despite CB-5083's initial clinical failure for cancer treatment due to side effects on vision. The data show that CB-5083 displays a dose-dependent but reversible inhibitory action on phosphodiesterase-6, an essential enzyme in retinal photoreceptor function, but no long-term consequences on retinal function or structure.

Temporal Contrast Sensitivity Increases despite Photoreceptor Degeneration in a Mouse Model of Retinitis Pigmentosa

The detection of temporal variations in amplitude of light intensity, or temporal contrast sensitivity (TCS), depends on the kinetics of rod photoresponse recovery. Uncharacteristically fast rod recovery kinetics are facets of both human patients and transgenic animal models with a P23H rhodopsin mutation, a prevalent cause of retinitis pigmentosa (RP). Here, we show that mice with this mutation (Rho^P23H/+) exhibit an age-dependent and illumination-dependent enhancement in TCS compared with controls. At retinal illumination levels producing ≥1000 R*/rod/s or more, postnatal day 30 (P30) Rho^P23H/+ mice exhibit a 1.2-fold to 2-fold increase in retinal and optomotor TCS relative to controls in response to flicker frequencies of 3, 6, and 12 Hz despite significant photoreceptor degeneration and loss of flash electroretinogram (ERG) b-wave amplitude. Surprisingly, the TCS of Rho^P23H/+ mice further increases as degeneration advances. Enhanced TCS is also observed in a second model (rhodopsin heterozygous mice, Rho^+/-) with fast rod recovery kinetics and no apparent retinal degeneration. In both mouse models, enhanced TCS is explained quantitatively by a comprehensive model that includes photoresponse recovery kinetics, density and collecting area of degenerating rods. Measurement of TCS may be a non-invasive early diagnostic tool indicative of rod dysfunction in some forms of retinal degenerative disease.

Rod Photoreceptors Avoid Saturation in Bright Light by the Movement of the G Protein Transducin

Rod photoreceptors can be saturated by exposure to bright background light, so that no flash superimposed on the background can elicit a detectable response. This phenomenon, called increment saturation, was first demonstrated psychophysically by Aguilar and Stiles and has since been shown in many studies to occur in single rods. Recent experiments indicate, however, that rods may be able to avoid saturation under some conditions of illumination. We now show in ex vivo electroretinogram and single-cell recordings that in continuous and prolonged exposure even to very bright light, the rods of mice from both sexes recover as much as 15% of their dark current and that responses can persist for hours. In parallel to recovery of outer segment current is an ∼10-fold increase in the sensitivity of rod photoresponses. This recovery is decreased in transgenic mice with reduced light-dependent translocation of the G protein transducin. The reduction in outer-segment transducin together with a novel mechanism of visual-pigment regeneration within the rod itself enable rods to remain responsive over the whole of the physiological range of vision. In this way, rods are able to avoid an extended period of transduction channel closure, which is known to cause photoreceptor degeneration.SIGNIFICANCE STATEMENT Rods are initially saturated in bright light so that no flash superimposed on the background can elicit a detectable response. Frederiksen and colleagues show in whole retina and single-cell recordings that, if the background light is prolonged, rods slowly recover and can continue to produce significant responses over the entire physiological range of vision. Response recovery occurs by translocation of the G protein transducin from the rod outer to the inner segment, together with a novel mechanism of visual-pigment regeneration within the rod itself. Avoidance of saturation in bright light may be one of the principal mechanisms the retina uses to keep rod outer-segment channels from ever closing for too long a time, which is known to produce photoreceptor degeneration.

Assessment of the Retina of Plp-α-Syn Mice as a Model for Studying Synuclein-Dependent Diseases

Purpose

Synucleinopathies such as multiple system atrophy (MSA) and Parkinson's disease are associated with a variety of visual symptoms. Functional and morphological retinal aberrations are therefore supposed to be valuable biomarkers for these neurodegenerative diseases. This study examined the retinal morphology and functionality resulting from human α-synuclein (α-Syn) overexpression in the transgenic Plp-α-Syn mouse model.

Methods

Immunohistochemistry on retinal sections and whole-mounts was performed on 8- to 11-week-old and 12-month-old Plp-α-Syn mice and C57BL/6N controls. Quantitative RT-PCR experiments were performed to study the expression of endogenous and human α-Syn and tyrosine hydroxylase (TH). We confirmed the presence of human α-Syn in the retina in western blot analyses. Multi-electrode array (MEA) analyses from light-stimulated whole-mounted retinas were used to investigate their functionality.

Results

Biochemical and immunohistochemical analyses showed human α-Syn in the retina of Plp-α-Syn mice. We found distinct staining in different retinal cell layers, most abundantly in rod bipolar cells of the peripheral retina. In the periphery, we also observed a trend toward a decline in the number of retinal ganglion cells. The number of TH+ neurons was unaffected in this human α-Syn overexpression model. MEA recordings showed that Plp-α-Syn retinas were functional but exhibited mild alterations in dim light conditions.

Conclusions

Together, these findings implicate an impairment of retinal neurons in the Plp-α-Syn mouse. The phenotype partly relates to retinal deficits reported in MSA patients. We further propose the suitability of the Plp-α-Syn retina as a biological model to study synuclein-mediated mechanisms.

Mitochondrial Calcium Uniporter (MCU) deficiency reveals an alternate path for Ca2+ uptake in photoreceptor mitochondria

Rods and cones use intracellular Ca2+ to regulate many functions, including phototransduction and neurotransmission. The Mitochondrial Calcium Uniporter (MCU) complex is thought to be the primary pathway for Ca2+ entry into mitochondria in eukaryotes. We investigate the hypothesis that mitochondrial Ca2+ uptake via MCU influences phototransduction and energy metabolism in photoreceptors using a mcu-/- zebrafish and a rod photoreceptor-specific Mcu-/- mouse. Using genetically encoded Ca2+ sensors to directly examine Ca2+ uptake in zebrafish cone mitochondria, we found that loss of MCU reduces but does not eliminate mitochondrial Ca2+ uptake. Loss of MCU does not lead to photoreceptor degeneration, mildly affects mitochondrial metabolism, and does not alter physiological responses to light, even in the absence of the Na+/Ca2+, K+ exchanger. Our results reveal that MCU is dispensable for vertebrate photoreceptor function, consistent with its low expression and the presence of an alternative pathway for Ca2+ uptake into photoreceptor mitochondria.

Homeostatic plasticity in the retina is associated with maintenance of night vision during retinal degenerative disease

Neuronal plasticity of the inner retina has been observed in response to photoreceptor degeneration. Typically, this phenomenon has been considered maladaptive and may preclude vision restoration in the blind. However, several recent studies utilizing triggered photoreceptor ablation have shown adaptive responses in bipolar cells expected to support normal vision. Whether such homeostatic plasticity occurs during progressive photoreceptor degenerative disease to help maintain normal visual behavior is unknown. We addressed this issue in an established mouse model of Retinitis Pigmentosa caused by the P23H mutation in rhodopsin. We show robust modulation of the retinal transcriptomic network, reminiscent of the neurodevelopmental state, and potentiation of rod – rod bipolar cell signaling following rod photoreceptor degeneration. Additionally, we found highly sensitive night vision in P23H mice even when more than half of the rod photoreceptors were lost. These results suggest retinal adaptation leading to persistent visual function during photoreceptor degenerative disease.

Ex vivo electroretinograms made easy: performing ERGs using 3D printed components

KEY POINTS

Rod and cone photoreceptors convert light into electrochemical signals that are transferred to second order cells, initiating image-forming visual processing. Electroretinograms (ERGs) can detect the associated light-induced extracellular transretinal events, allowing for physiological assessment of cellular activity from morphologically intact retinas. We outline a method for economically configuring a traditional patch-clamp rig for performing high signal-to-noise ex vivo ERGs. We accomplish this by incorporating various 3D printed components and by modifying existing light pathways in a typical patch-clamp rig. This methodology provides an additional set of tools to labs interested in studying the physiological function of neuronal populations in isolated retinal tissue.

ABSTRACT

Rod and cone photoreceptors of the retina are responsible for the initial stages in vision and convey sensory information regarding our visual world across a wide range of lighting conditions. These photoreceptors hyperpolarize in the presence of light and subsequently transmit signals to second-order bipolar and horizontal cells. The electrical components of these events are experimentally detectable, and in conjunction with pharmacological agents, can be further separated into their respective cellular contributions using electroretinograms (ERGs). Extracellular activity from populations of rods and cones generate the negative-going a-wave, while ON-bipolar cells generate positive-going b-waves. ERGs can be performed in vivo or alternatively using an ex vivo configuration, where retinas are isolated and transretinal photovoltages are recorded at high signal-to-noise ratios. However, most ERG set-ups require their own unique set of tools. We demonstrate how, at low cost, to reconfigure a typical patch-clamp rig for ERG recordings. The bulk of these modifications require implementation of various 3D printed components, which can alternatively aid in generating a stand-alone ERG set-up without a patch-rig. Further, we discuss how to configure an ERG system without a patch-clamp rig. Compared to in vivo ERGs, these are superior when measuring small responses, such as those that are cone-evoked or those from immature mouse retinae. This recording configuration provides high signal-to-noise detection of a-waves (300-600 µV) and b-waves (1-3 mV), and is ultimately capable of discerning small (1-2 µV) photovoltages from noise. These quick and economical modifications allow researchers to equip their technical arsenal with an interchangeable patch-clamp/ERG system.

Rod phototransduction and light signal transmission during type 2 diabetes

INTRODUCTION

Diabetic retinopathy is a major complication of diabetes recently associated with compromised photoreceptor function. Multiple stressors in diabetes, such as hyperglycemia, oxidative stress and inflammatory factors, have been identified, but systemic effects of diabetes on outer retina function are incompletely understood. We assessed photoreceptor physiology in vivo and in isolated retinas to better understand how alterations in the cellular environment compared with intrinsic cellular/molecular properties of the photoreceptors, affect light signal transduction and transmission in the retina in chronic type 2 diabetes.

RESEARCH DESIGN AND METHODS

Photoreceptor function was assessed in BKS.Cs-Dock7^m+/+Lepr ^db/J mice, using homozygotes for Lepr^db as a model of type 2 diabetes and heterozygotes as non-diabetic controls. In vivo electroretinogram (ERG) was recorded in dark-adapted mice at both 3 and 6 months of age. For ex vivo ERG, isolated retinas were superfused with oxygenated Ames' media supplemented with 30 mM glucose or mannitol as iso-osmotic control and electrical responses to light stimuli were recorded.

RESULTS

We found that both transduction and transmission of light signals by rod photoreceptors were compromised in 6-month-old (n=9-10 eyes from 5 animals, ***p<0.001) but not in 3-month-old diabetic mice in vivo (n=4-8 eyes from 2 to 4 animals). In contrast, rod signaling was similar in isolated retinas from 6-month-old control and diabetic mice under normoglycemic conditions (n=11). Acutely elevated glucose ex vivo increased light-evoked rod photoreceptor responses in control mice (n=11, ***p<0.001), but did not affect light responses in diabetic mice (n=11).

CONCLUSIONS

Our data suggest that long-term diabetes does not irreversibly change the ability of rod photoreceptors to transduce and mediate light signals. However, type 2 diabetes appears to induce adaptational changes in the rods that render them less sensitive to increased availability of glucose.

Mislocalization of cone nuclei impairs cone function in mice

The nuclei of cone photoreceptors are located on the apical side of the outer nuclear layer (ONL) in vertebrate retinas. However, the functional role of this evolutionarily conserved localization of cone nuclei is unknown. We previously showed that Linkers of the Nucleoskeleton to the Cytoskeleton (LINC complexes) are essential for the apical migration of cone nuclei during development. Here, we developed an efficient genetic strategy to disrupt cone LINC complexes in mice. Experiments with animals from both sexes revealed that disrupting cone LINC complexes resulted in mislocalization of cone nuclei to the basal side of ONL in mouse retina. This, in turn, disrupted cone pedicle morphology, and appeared to reduce the efficiency of synaptic transmission from cones to bipolar cells. Although we did not observe other developmental or phototransduction defects in cones with mislocalized nuclei, their dark adaptation was impaired, consistent with a deficiency in chromophore recycling. These findings demonstrate that the apical localization of cone nuclei in the ONL is required for the timely dark adaptation and efficient synaptic transmission in cone photoreceptors.

Functionally validated imaging endpoints in the Alabama study on early age-related macular degeneration 2 (ALSTAR2): design and methods

Background

Age-related macular degeneration (AMD), a leading cause of irreversible vision impairment in the United States and globally, is a disease of the photoreceptor support system involving the retinal pigment epithelium (RPE), Bruch’s membrane, and the choriocapillaris in the setting of characteristic extracellular deposits between outer retinal cells and their blood supply. Research has clearly documented the selective vulnerability of rod photoreceptors and rod-mediated (scotopic) vision in early AMD, including delayed rod-mediated dark adaptation (RMDA) and impaired rod-mediated light and pattern sensitivity. The unifying hypothesis of the Alabama Study on Early Macular Degeneration (ALSTAR2) is that early AMD is a disease of micronutrient deficiency and vascular insufficiency, due to detectable structural changes in the retinoid re-supply route from the choriocapillaris to the photoreceptors. Functionally this is manifest as delayed rod-mediated dark adaptation and eventually as rod-mediated visual dysfunction in general.

Methods

A cohort of 480 older adults either in normal macular health or with early AMD will be enrolled and followed for 3 years to examine cross-sectional and longitudinal associations between structural and functional characteristics of AMD. Using spectral domain optical coherence tomography, the association between (1) subretinal drusenoid deposits and drusen, (2) RPE cell bodies, and (3) the choriocapillaris’ vascular density and rod- and cone-mediated vision will be examined. An accurate map and timeline of structure-function relationships in aging and early AMD gained from ALSTAR2, especially the critical transition from aging to disease, will identify major characteristics relevant to future treatments and preventative measures.

Discussion

A major barrier to developing treatments and prevention strategies for early AMD is a limited understanding of the temporal interrelationships among structural and functional characteristics while transitioning from aging to early AMD. ALSTAR2 will enable the development of functionally valid, structural biomarkers for early AMD, suitable for use in forthcoming clinical trials as endpoint/outcome measures. The comprehensive dataset will also allow hypothesis-testing for mechanisms that underlie the transition from aging to AMD, one of which is a newly developed Center-Surround model of cone resilience and rod vulnerability.

Trial registration

ClinicalTrials.gov Identifier NCT04112667, October 7, 2019.

Diminished Cone Sensitivity in cpfl3 Mice Is Caused by Defective Transducin Signaling

Purpose

Cone photoreceptor function loss 3 (Gnat2cpfl3/cpfl3 or cpfl3) is a mouse model commonly used as a functional cones null from a naturally occurring mutation in the α-subunit of cone transducin (Gnat2). We nevertheless detected robust cone-mediated light responses from cpfl3 animals, which we now explore.

Methods

Recordings were made from whole retina and from identified cones with whole-cell patch clamp in retinal slices. Relative levels of GNAT2 protein and numbers of cones in isolated retinas were compared between cpfl3, rod transducin knockout (Gnat1-/-), cpfl3/Gnat1-/- double mutants, and control C57Bl/6J age-matched mice at 4, 9, and 14 weeks of age.

Results

Cones from cpfl3 and cpfl3/Gnat1-/- mice 2 to 3 months of age displayed normal dark currents but greatly reduced sensitivity and amplification constants. Responses decayed more slowly than in control (C57Bl/6J) mice, indicating an altered mechanism of inactivation. At dim light intensities rod responses could be recorded from cpfl3 cones, indicating intact rod/cone gap junctions. The cpfl3 and cpfl3/Gnat1-/- mice express two-fold less GNAT2 protein compared with C57 at 4 weeks, and a four-fold decrease by 14 weeks. This is accompanied by a small decrease in the number of cones.

Conclusions

Cplf3 cones can respond to light with currents of normal amplitude and cannot be assumed to be a Gnat2 null. The decreased sensitivity and amplification rate of cones is not explained by a reduction in GNAT2 protein level, but instead by abnormal interactions of the mutant transducin with rhodopsin and the effector molecule, cGMP phosphodiesterase.

Phosphorylation at Serine 21 in G protein-coupled receptor kinase 1 (GRK1) is required for normal kinetics of dark adaption in rod but not cone photoreceptors

Timely recovery of the light response in photoreceptors requires efficient inactivation of photoactivated rhodopsin. This process is initiated by phosphorylation of its carboxyl terminus by G protein-coupled receptor kinase 1 (GRK1). Previously, we showed that GRK1 is phosphorylated in the dark at Ser21 in a cAMP-dependent manner and dephosphorylated in the light. Results in vitro indicate that dephosphorylation of Ser21 increases GRK1 activity, leading to increased phosphorylation of rhodopsin. This creates the possibility of light-dependent regulation of GRK1 activity and its efficiency in inactivating the visual pigment. To address the functional role of GRK1 phosphorylation in rods and cones in vivo, we generated mutant mice in which Ser21 is substituted with alanine (GRK1-S21A), preventing dark-dependent phosphorylation of GRK1. GRK1-S21A mice had normal retinal morphology, without evidence of degeneration. The function of dark-adapted GRK1-S21A rods and cones was also unaffected, as demonstrated by the normal amplitude and kinetics of their responses obtained by ex vivo and in vivo ERG recordings. In contrast, rod dark adaptation following exposure to bright bleaching light was significantly delayed in GRK1-S21A mice, suggesting that the higher activity of this kinase results in enhanced rhodopsin phosphorylation and therefore delays its regeneration. In contrast, dark adaptation of cones was unaffected by the S21A mutation. Taken together, these data suggest that rhodopsin phosphorylation/dephosphorylation modulates the recovery of rhodopsin to the ground state and rod dark adaptation. They also reveal a novel role for cAMP-dependent phosphorylation of GRK1 in regulating the dark adaptation of rod but not cone photoreceptors.

Sensitivity of Mammalian Cone Photoreceptors to Infrared Light

Two-photon vision arises from the perception of pulsed infrared (IR) laser light as color corresponding to approximately half of the laser wavelength. The physical process responsible for two-photon vision in rods has been delineated and verified experimentally only recently. Here, we sought to determine whether IR light can also be perceived by mammalian cone photoreceptors via a similar activation mechanism. To investigate selectively mammalian cone signaling in mice, we used animals with disabled rod signal transduction. We found that, contrary to the expected progressive sensitivity decrease based on the one-photon cone visual pigment spectral template, the sensitivity of mouse cone photoreceptors decreases only up to 800 nm and then increases at 900 nm and 1000 nm. Similarly, in experiments with the parafoveal region of macaque retinas, we found that the spectral sensitivity of primate cones diverged above the predicted one-photon spectral sensitivity template beyond 800 nm. In both cases, efficient detection of IR light was dependent on minimizing the dispersion of the ultrashort light pulses, indicating a non-linear two-photon activation process. Together, our studies demonstrate that mammalian cones can be activated by near IR light by a nonlinear two-photon excitation. Our results pave the way for the creation of a two-photon IR-based ophthalmoscope for the simultaneous imaging and functional testing of human retinas as a novel tool for the diagnosis and treatment of a wide range of visual disorders.

Increasing Ca2+ in photoreceptor mitochondria alters metabolites, accelerates photoresponse recovery, and reveals adaptations to mitochondrial stress

Photoreceptors are specialized neurons that rely on Ca²⁺ to regulate phototransduction and neurotransmission. Photoreceptor dysfunction and degeneration occur when intracellular Ca²⁺ homeostasis is disrupted. Ca²⁺ homeostasis is maintained partly by mitochondrial Ca²⁺ uptake through the mitochondrial Ca²⁺ uniporter (MCU), which can influence cytosolic Ca²⁺ signals, stimulate energy production, and trigger apoptosis. Here we discovered that zebrafish cone photoreceptors express unusually low levels of MCU. We expected that this would be important to prevent mitochondrial Ca²⁺ overload and consequent cone degeneration. To test this hypothesis, we generated a cone-specific model of MCU overexpression. Surprisingly, we found that cones tolerate MCU overexpression, surviving elevated mitochondrial Ca²⁺ and disruptions to mitochondrial ultrastructure until late adulthood. We exploited the survival of MCU overexpressing cones to additionally demonstrate that mitochondrial Ca²⁺ uptake alters the distributions of citric acid cycle intermediates and accelerates recovery kinetics of the cone response to light. Cones adapt to mitochondrial Ca²⁺ stress by decreasing MICU3, an enhancer of MCU-mediated Ca²⁺ uptake, and selectively transporting damaged mitochondria away from the ellipsoid toward the synapse. Our findings demonstrate how mitochondrial Ca²⁺ can influence physiological and metabolic processes in cones and highlight the remarkable ability of cone photoreceptors to adapt to mitochondrial stress.

Stereospecific modulation of dimeric rhodopsin

The classic concept that GPCRs function as monomers has been challenged by the emerging evidence of GPCR dimerization and oligomerization. Rhodopsin (Rh) is the only GPCR whose native oligomeric arrangement was revealed by atomic force microscopy demonstrating that Rh exists as a dimer. However, the role of Rh dimerization in retinal physiology is currently unknown. In this study, we identified econazole and sulconazole, two small molecules that disrupt Rh dimer contacts, by implementing a cell-based high-throughput screening assay. Racemic mixtures of identified lead compounds were separated and tested for their stereospecific binding to Rh using UV-visible spectroscopy and intrinsic fluorescence of tryptophan (Trp) 265 after illumination. By following the changes in UV-visible spectra and Trp265 fluorescence in vitro, we found that binding of R-econazole modulates the formation of Meta III and quenches the intrinsic fluorescence of Trp265. In addition, electrophysiological ex vivo recording revealed that R-econazole slows photoresponse kinetics, whereas S-econazole decreased the sensitivity of rods without effecting the kinetics. Thus, this study contributes new methodology to identify compounds that disrupt the dimerization of GPCRs in general and validates the first active compounds that disrupt the Rh dimer specifically.-Getter, T., Gulati, S., Zimmerman, R., Chen, Y., Vinberg, F., Palczewski, K. Stereospecific modulation of dimeric rhodopsin.

Activation of Rod Input in a Model of Retinal Degeneration Reverses Retinal Remodeling and Induces Formation of Functional Synapses and Recovery of Visual Signaling in the Adult Retina

A major cause of human blindness is the death of rod photoreceptors. As rods degenerate, synaptic structures between rod and rod bipolar cells disappear and the rod bipolar cells extend their dendrites and occasionally make aberrant contacts. Such changes are broadly observed in blinding disorders caused by photoreceptor cell death and are thought to occur in response to deafferentation. How the remodeled retinal circuit affects visual processing following rod rescue is not known. To address this question, we generated male and female transgenic mice wherein a disrupted cGMP-gated channel (CNG) gene can be repaired at the endogenous locus and at different stages of degeneration by tamoxifen-inducible cre-mediated recombination. In normal rods, light-induced closure of CNG channels leads to hyperpolarization of the cell, reducing neurotransmitter release at the synapse. Similarly, rods lacking CNG channels exhibit a resting membrane potential that was ~10 mV hyperpolarized compared to WT rods, indicating diminished glutamate release. Retinas from these mice undergo stereotypic retinal remodeling as a consequence of rod malfunction and degeneration. Upon tamoxifen-induced expression of CNG channels, rods recovered their structure and exhibited normal light responses. Moreover, we show that the adult mouse retina displays a surprising degree of plasticity upon activation of rod input. Wayward bipolar cell dendrites establish contact with rods to support normal synaptic transmission, which is propagated to the retinal ganglion cells. These findings demonstrate remarkable plasticity extending beyond the developmental period and support efforts to repair or replace defective rods in patients blinded by rod degeneration.SIGNIFICANCE STATEMENT Current strategies for treatment of neurodegenerative disorders are focused on the repair of the primary affected cell type. However, the defective neurons function within a complex neural circuitry, which also becomes degraded during disease. It is not known whether rescued neurons and the remodeled circuit will establish communication to regain normal function. We show that the adult mammalian neural retina exhibits a surprising degree of plasticity following rescue of rod photoreceptors. The wayward dendrites of rod bipolar cells re-establish contact with rods to support normal synaptic transmission, which is propagated to the retinal ganglion cells. These findings support efforts to repair or replace defective rods in patients blinded by rod cell loss.

Photoreceptors in a mouse model of Leigh syndrome are capable of normal light-evoked signaling

Mitochondrial dysfunction is an important cause of heritable vision loss. Mutations affecting mitochondrial bioenergetics may lead to isolated vision loss or life-threatening systemic disease, depending on a mutation's severity. Primary optic nerve atrophy resulting from death of retinal ganglion cells is the most prominent ocular manifestation of mitochondrial disease. However, dysfunction of other retinal cell types has also been described, sometimes leading to a loss of photoreceptors and retinal pigment epithelium that manifests clinically as pigmentary retinopathy. A popular mouse model of mitochondrial disease that lacks NADH:ubiquinone oxidoreductase subunit S4 (NDUFS4), a subunit of mitochondrial complex I, phenocopies many traits of the human disease Leigh syndrome, including the development of optic atrophy. It has also been reported that ndufs4 ^-/- mice display diminished light responses at the level of photoreceptors or bipolar cells. By conducting electroretinography (ERG) recordings in live ndufs4 ^-/- mice, we now demonstrate that this defect occurs at the level of retinal photoreceptors. We found that this deficit does not arise from retinal developmental anomalies, photoreceptor degeneration, or impaired regeneration of visual pigment. Strikingly, the impairment of ndufs4 ^-/- photoreceptor function was not observed in ex vivo ERG recordings from isolated retinas, indicating that photoreceptors with complex I deficiency are intrinsically capable of normal signaling. The difference in electrophysiological phenotypes in vivo and ex vivo suggests that the energy deprivation associated with severe mitochondrial impairment in the outer retina renders ndufs4 ^-/- photoreceptors unable to maintain the homeostatic conditions required to operate at their normal capacity.

Light-Driven Regeneration of Cone Visual Pigments through a Mechanism Involving RGR Opsin in Müller Glial Cells

While rods in the mammalian retina regenerate rhodopsin through a well-characterized pathway in cells of the retinal pigment epithelium (RPE), cone visual pigments are thought to regenerate in part through an additional pathway in Müller cells of the neural retina. The proteins comprising this intrinsic retinal visual cycle are unknown. Here, we show that RGR opsin and retinol dehydrogenase-10 (Rdh10) convert all-trans-retinol to 11-cis-retinol during exposure to visible light. Isolated retinas from Rgr+/+ and Rgr-/- mice were exposed to continuous light, and cone photoresponses were recorded. Cones in Rgr-/- retinas lost sensitivity at a faster rate than cones in Rgr+/+ retinas. A similar effect was seen in Rgr+/+ retinas following treatment with the glial cell toxin, α-aminoadipic acid. These results show that RGR opsin is a critical component of the Müller cell visual cycle and that regeneration of cone visual pigment can be driven by light.

Rod Photoresponse Kinetics Limit Temporal Contrast Sensitivity in Mesopic Vision

The mammalian visual system operates over an extended range of ambient light levels by switching between rod and cone photoreceptors. Rod-driven vision is sluggish, highly sensitive, and operates in dim or scotopic lights, whereas cone-driven vision is brisk, less sensitive, and operates in bright or photopic lights. At intermediate or mesopic lights, vision transitions seamlessly from rod-driven to cone-driven, despite the profound differences in rod and cone response dynamics. The neural mechanisms underlying such a smooth handoff are not understood. Using an operant behavior assay, electrophysiological recordings, and mathematical modeling we examined the neural underpinnings of the mesopic visual transition in mice of either sex. We found that rods, but not cones, drive visual sensitivity to temporal light variations over much of the mesopic range. Surprisingly, speeding up rod photoresponse recovery kinetics in transgenic mice improved visual sensitivity to slow temporal variations, in the range where perceptual sensitivity is governed by Weber's law of sensation. In contrast, physiological processes acting downstream from phototransduction limit sensitivity to high frequencies and temporal resolution. We traced the paradoxical control of visual temporal sensitivity to rod photoresponses themselves. A scenario emerges where perceptual sensitivity is limited by: (1) the kinetics of neural processes acting downstream from phototransduction in scotopic lights, (2) rod response kinetics in mesopic lights, and (3) cone response kinetics as light levels rise into the photopic range.SIGNIFICANCE STATEMENT Our ability to detect flickering lights is constrained by the dynamics of the slowest step in the visual pathway. Cone photoresponse kinetics limit visual temporal sensitivity in bright (photopic) lights, whereas mechanisms in the inner retina limit sensitivity in dim (scotopic) lights. The neural mechanisms underlying the transition between scotopic and photopic vision in mesopic lights, when both rods are cones are active, are unknown. This study provides a missing link in this mechanism by establishing that rod photoresponse kinetics limit temporal sensitivity during the mesopic transition. Surprisingly, this range is where Weber's Law of Sensation governs temporal contrast sensitivity in mouse. Our results will help guide future studies of complex and dynamic interactions between rod-cone signals in the mesopic retina.

Noninvasive Electroretinographic Procedures for the Study of the Mouse Retina

Overall retinal function can be monitored by recording the light-evoked response of the eye at the corneal surface. The major components of the electroretinogram (ERG) provide important information regarding the functional status of many retinal cell types including rod photoreceptors, cone photoreceptors, bipolar cells, and the retinal pigment epithelium (RPE). The ERG can be readily recorded from mice, and this unit describes procedures for mouse anesthesia and the use of stimulation and recording procedures for measuring ERGs that reflect the response properties of different retinal cell types. Through these, the mouse ERG provides a noninvasive approach to measure multiple aspects of outer retinal function, including the status of the initial rod and cone pathways, rod photoreceptor deactivation, rod dark adaptation, the photoreceptor-to-bipolar cell synapse, and the RPE. © 2018 by John Wiley & Sons, Inc.

Examining the Role of Cone-expressed RPE65 in Mouse Cone Function

Efficient chromophore supply is paramount for the continuous function of vertebrate cone photoreceptors. It is well established that isomerization of all-trans- to 11-cis- retinoid in the retinal pigmented epithelium by RPE65 is a key reaction in this process. Mutations in RPE65 result in a disrupted chromophore supply, retinal degeneration, and blindness. Interestingly, RPE65 has recently been found to also be expressed in cone photoreceptors in several species, including mouse and human. However, the functional role of cone-expressed RPE65 has remained unknown. Here, we used loss and gain of function approaches to investigate this issue. First, we compared the function of cones from control and RPE65-deficient mice. Although we found that deletion of RPE65 partially suppressed cone dark adaptation, the interpretation of this result was complicated by the abnormal cone structure and function caused by the chromophore deficiency in the absence of RPE65 in the pigmented epithelium. As an alternative approach, we generated transgenic mice to express human RPE65 in the cones of mice where RPE65 expression is normally restricted to the pigmented epithelium. Comparison of control (RPE65-deficient) and transgenic (RPE65-expressing) cones revealed no morphological or functional changes, with only a slight delay in dark adaptation, possibly caused by the buffering of retinoids by RPE65. Together, our results do not provide any evidence for a functional role of RPE65 in mouse cones. Future studies will have to determine whether cone-expressed RPE65 plays a role in maintaining the long-term homeostasis of retinoids in cones and their function and survival, particularly in humans.

Guanylate cyclase-activating protein 2 contributes to phototransduction and light adaptation in mouse cone photoreceptors

Light adaptation of photoreceptor cells is mediated by Ca²⁺-dependent mechanisms. In darkness, Ca²⁺ influx through cGMP-gated channels into the outer segment of photoreceptors is balanced by Ca²⁺ extrusion via Na⁺/Ca²⁺, K⁺ exchangers (NCKXs). Light activates a G protein signaling cascade, which closes cGMP-gated channels and decreases Ca²⁺ levels in photoreceptor outer segment because of continuing Ca²⁺ extrusion by NCKXs. Guanylate cyclase-activating proteins (GCAPs) then up-regulate cGMP synthesis by activating retinal membrane guanylate cyclases (RetGCs) in low Ca²⁺ This activation of RetGC accelerates photoresponse recovery and critically contributes to light adaptation of the nighttime rod and daytime cone photoreceptors. In mouse rod photoreceptors, GCAP1 and GCAP2 both contribute to the Ca²⁺-feedback mechanism. In contrast, only GCAP1 appears to modulate RetGC activity in mouse cones because evidence of GCAP2 expression in cones is lacking. Surprisingly, we found that GCAP2 is expressed in cones and can regulate light sensitivity and response kinetics as well as light adaptation of GCAP1-deficient mouse cones. Furthermore, we show that GCAP2 promotes cGMP synthesis and cGMP-gated channel opening in mouse cones exposed to low Ca²⁺ Our biochemical model and experiments indicate that GCAP2 significantly contributes to the activation of RetGC1 at low Ca²⁺ when GCAP1 is not present. Of note, in WT mouse cones, GCAP1 dominates the regulation of cGMP synthesis. We conclude that, under normal physiological conditions, GCAP1 dominates the regulation of cGMP synthesis in mouse cones, but if its function becomes compromised, GCAP2 contributes to the regulation of phototransduction and light adaptation of cones.

The Development of Mid-Wavelength Photoresponsivity in the Mouse Retina

PURPOSE

Photoreceptors in the mouse retina express much of the molecular machinery necessary for phototransduction and glutamatergic transmission prior to eye opening at postnatal day 13 (P13). Light responses have been observed collectively from rod and cone photoreceptors via electroretinogram recordings as early as P13 in mouse, and the responses are known to become more robust with maturation, reaching a mature state by P30. Photocurrents from single rod outer segments have been recorded at P12, but no earlier, and similar studies on cone photoreceptors have been done, but only in the adult mouse retina. In this study, we wanted to document the earliest time point in which outer retinal photoreceptors in the mouse retina begin to respond to mid-wavelength light.

METHODS

Ex-vivo electroretinogram recordings were made from isolated mouse retinae at P7, P8, P9, P10, and P30 at seven different flash energies (561 nm). The a-wave was pharmacologically isolated and measured at each developmental time point across all flash energies.

RESULTS

Outer-retinal photoreceptors generated a detectable response to mid-wavelength light as early as P8, but only at photopic flash energies. a-wave intensity response curves and kinetic response properties are similar to the mature retina as early as P10.

CONCLUSION

These data represent the earliest recorded outer retinal light responses in the rodent. Photoreceptors are electrically functional and photoresponsive prior to eye opening, and much earlier than previously thought. Prior to eye opening, critical developmental processes occur that have been thought to be independent of outer retinal photic modulation. However, these data suggest light acting through outer-retinal photoreceptors has the potential to shape these critical developmental processes.

The Contribution of L-Type Cav1.3 Channels to Retinal Light Responses

L-type voltage-gated calcium channels (LTCCs) regulate tonic neurotransmitter release from sensory neurons including retinal photoreceptors. There are three types of LTCCs (Ca_v1.2, Ca_v1.3, and Ca_v1.4) expressed in the retina. While Ca_v1.2 is expressed in all retinal cells including the Müller glia and neurons, Ca_v1.3 and Ca_v1.4 are expressed in the retinal neurons with Ca_v1.4 exclusively expressed in the photoreceptor synaptic terminals. Mutations in the gene encoding Ca_v1.4 cause incomplete X-linked congenital stationary night blindness in humans. Even though Ca_v1.3 is present in the photoreceptor inner segments and the synaptic terminals in various vertebrate species, its role in vision is unclear, since genetic alterations in Ca_v1.3 are not associated with severe vision impairment in humans or in Ca_v1.3-null (Ca_v1.3^-/-) mice. However, a failure to regulate Ca_v1.3 was found in a mouse model of Usher syndrome, the most common cause of combined deafness and blindness in humans, indicating that Ca_v1.3 may contribute to retinal function. In this report, we combined physiological and morphological data to demonstrate the role of Ca_v1.3 in retinal physiology and function that has been undervalued thus far. Through ex vivo and in vivo electroretinogram (ERG) recordings and immunohistochemical staining, we found that Ca_v1.3 plays a role in retinal light responses and synaptic plasticity. Pharmacological inhibition of Ca_v1.3 decreased ex vivo ERG a- and b-wave amplitudes. In Ca_v1.3^-/- mice, their dark-adapted ERG a-, b-wave, and oscillatory potential amplitudes were significantly dampened, and implicit times were delayed compared to the wild type (WT). Furthermore, the density of ribbon synapses was reduced in the outer plexiform layer of Ca_v1.3^-/- mice retinas. Hence, Ca_v1.3 plays a more prominent role in retinal physiology and function than previously reported.

Dephosphorylation by protein phosphatase 2A regulates visual pigment regeneration and the dark adaptation of mammalian photoreceptors

Resetting of G-protein-coupled receptors (GPCRs) from their active state back to their biologically inert ground state is an integral part of GPCR signaling. This "on-off" GPCR cycle is regulated by reversible phosphorylation. Retinal rod and cone photoreceptors arguably represent the best-understood example of such GPCR signaling. Their visual pigments (opsins) are activated by light, transduce the signal, and are then inactivated by a GPCR kinase and arrestin. Although pigment inactivation by phosphorylation is well understood, the enzyme(s) responsible for pigment dephosphorylation and the functional significance of this reaction remain unknown. Here, we show that protein phosphatase 2A (PP2A) acts as opsin phosphatase in both rods and cones. Elimination of PP2A substantially slows pigment dephosphorylation, visual chromophore recycling, and ultimately photoreceptor dark adaptation. These findings demonstrate that visual pigment dephosphorylation regulates the dark adaptation of photoreceptors and provide insights into the role of this reaction in GPCR signaling.

The Na+/Ca2+, K+ exchanger NCKX4 is required for efficient cone-mediated vision

Calcium (Ca2+) plays an important role in the function and health of neurons. In vertebrate cone photoreceptors, Ca2+ controls photoresponse sensitivity, kinetics, and light adaptation. Despite the critical role of Ca2+ in supporting the function and survival of cones, the mechanism for its extrusion from cone outer segments is not well understood. Here, we show that the Na+/Ca2+, K+ exchanger NCKX4 is expressed in zebrafish, mouse, and primate cones. Functional analysis of NCKX4-deficient mouse cones revealed that this exchanger is essential for the wide operating range and high temporal resolution of cone-mediated vision. We show that NCKX4 shapes the cone photoresponse together with the cone-specific NCKX2: NCKX4 acts early to limit response amplitude, while NCKX2 acts late to further accelerate response recovery. The regulation of Ca2+ by NCKX4 in cones is a novel mechanism that supports their ability to function as daytime photoreceptors and promotes their survival.

The role of retinol dehydrogenase 10 in the cone visual cycle

Pigment regeneration is critical for the function of cone photoreceptors in bright and rapidly-changing light conditions. This process is facilitated by the recently-characterized retina visual cycle, in which Müller cells recycle spent all-trans-retinol visual chromophore back to 11-cis-retinol. This 11-cis-retinol is oxidized selectively in cones to the 11-cis-retinal used for pigment regeneration. However, the enzyme responsible for the oxidation of 11-cis-retinol remains unknown. Here, we sought to determine whether retinol dehydrogenase 10 (RDH10), upregulated in rod/cone hybrid retinas and expressed abundantly in Müller cells, is the enzyme that drives this reaction. We created mice lacking RDH10 either in cone photoreceptors, Müller cells, or the entire retina. In vivo electroretinography and transretinal recordings revealed normal cone photoresponses in all RDH10-deficient mouse lines. Notably, their cone-driven dark adaptation both in vivo and in isolated retina was unaffected, indicating that RDH10 is not required for the function of the retina visual cycle. We also generated transgenic mice expressing RDH10 ectopically in rod cells. However, rod dark adaptation was unaffected by the expression of RDH10 and transgenic rods were unable to use cis-retinol for pigment regeneration. We conclude that RDH10 is not the dominant retina 11-cis-RDH, leaving its primary function in the retina unknown.

cis Retinol oxidation regulates photoreceptor access to the retina visual cycle and cone pigment regeneration

KEY POINTS

This study explores the nature of the cis retinol that Müller cells in the retina provide to cones for the regeneration of their visual pigment. We report that the retina visual cycle provides cones exclusively with 11-cis chromophore in both salamander and mouse and show that this selectivity is dependent on the 11-cis-specific cellular retinaldehyde binding protein (CRALBP) present in Müller cells. Even though salamander blue cones and green rods share the same visual pigment, only blue cones but not green rods are able to dark-adapt in the retina following a bleach and to use exogenous 9-cis retinol for pigment regeneration, suggesting that access to the retina visual cycle is cone-specific and pigment-independent. Our results show that the retina produces 11-cis retinol that can be oxidized and used for pigment regeneration and dark adaptation selectively in cones and not in rods.

ABSTRACT

Chromophore supply by the retinal Müller cells (retina visual cycle) supports the efficient pigment regeneration required for cone photoreceptor function in bright light. Surprisingly, a large fraction of the chromophore produced by dihydroceramide desaturase-1, the putative all-trans retinol isomerase in Müller cells, appears to be 9-cis retinol. In contrast, the canonical retinal pigment epithelium (RPE) visual cycle produces exclusively 11-cis retinal. Here, we used the different absorption spectra of 9-cis and 11-cis pigments to identify the isoform of the chromophore produced by the visual cycle of the intact retina. We found that the spectral sensitivity of salamander and mouse cones dark-adapted in the isolated retina (with only the retina visual cycle) was similar to that of cones dark-adapted in the intact eye (with both the RPE and retina visual cycles) and consistent with pure 11-cis pigment composition. However, in mice lacking the cellular retinaldehyde binding protein (CRALBP), cone spectral sensitivity contained a substantial 9-cis component. Thus, the retina visual cycle provides cones exclusively with 11-cis chromophore and this process is mediated by the 11-cis selective CRALBP in Müller cells. Finally, despite sharing the same pigment, salamander blue cones, but not green rods, recovered their sensitivity in the isolated retina. Exogenous 9-cis retinol produced robust sensitivity recovery in bleached red and blue cones but not in red and green rods, suggesting that cis retinol oxidation restricts access to the retina visual cycle to cones.

Autophagy supports color vision

Cones comprise only a small portion of the photoreceptors in mammalian retinas. However, cones are vital for color vision and visual perception, and their loss severely diminishes the quality of life for patients with retinal degenerative diseases. Cones function in bright light and have higher demand for energy than rods; yet, the mechanisms that support the energy requirements of cones are poorly understood. One such pathway that potentially could sustain cones under basal and stress conditions is macroautophagy. We addressed the role of macroautophagy in cones by examining how the genetic block of this pathway affects the structural integrity, survival, and function of these neurons. We found that macroautophagy was not detectable in cones under normal conditions but was readily observed following 24 h of fasting. Consistent with this, starvation induced phosphorylation of AMPK specifically in cones indicating cellular starvation. Inhibiting macroautophagy in cones by deleting the essential macroautophagy gene Atg5 led to reduced cone function following starvation suggesting that cones are sensitive to systemic changes in nutrients and activate macroautophagy to maintain their function. ATG5-deficiency rendered cones susceptible to light-induced damage and caused accumulation of damaged mitochondria in the inner segments, shortening of the outer segments, and degeneration of all cone types, revealing the importance of mitophagy in supporting cone metabolic needs. Our results demonstrate that macroautophagy supports the function and long-term survival of cones providing for their unique metabolic requirements and resistance to stress. Targeting macroautophagy has the potential to preserve cone-mediated vision during retinal degenerative diseases.

The Na(+)/Ca(2+), K(+) exchanger 2 modulates mammalian cone phototransduction

Calcium ions (Ca²⁺) modulate the phototransduction cascade of vertebrate cone photoreceptors to tune gain, inactivation, and light adaptation. In darkness, the continuous current entering the cone outer segment through cGMP-gated (CNG) channels is carried in part by Ca²⁺, which is then extruded back to the extracellular space. The mechanism of Ca²⁺ extrusion from mammalian cones is not understood. The dominant view has been that the cone-specific isoform of the Na⁺/Ca²⁺, K⁺ exchanger, NCKX2, is responsible for removing Ca²⁺ from their outer segments. However, indirect evaluation of cone function in NCKX2-deficient (Nckx2^−/−) mice by electroretinogram recordings revealed normal photopic b-wave responses. This unexpected result suggested that NCKX2 may not be involved in the Ca²⁺ homeostasis of mammalian cones. To address this controversy, we examined the expression of NCKX2 in mouse cones and performed transretinal recordings from Nckx2^−/− mice to determine the effect of NCKX2 deletion on cone function directly. We found that Nckx2^−/− cones exhibit compromised phototransduction inactivation, slower response recovery and delayed background adaptation. We conclude that NCKX2 is required for the maintenance of efficient Ca²⁺ extrusion from mouse cones. However, surprisingly, Nckx2^−/− cones adapted normally in steady background light, indicating the existence of additional Ca²⁺-extruding mechanisms in mammalian cones.

Evaluation of micro Electroretinograms Recorded with Multiple Electrode Array to Assess Focal Retinal Function

Full-field electroretinograms (ERGs) are used to objectively assess the mass function of the retina, whereas focal ERGs are used to evaluate the focal retinal function. The purpose of this study was to determine the usefulness of a multiple electrode array (MEA) system for recording ex vivo micro ERGs (mERGs) together with multiunit spike responses of the retinal ganglion cells (RGCs) to assess focal retinal function in isolated mouse retinas. The a- and b-waves of the full-field ERGs were present in the mERG. The b-wave was blocked by L-AP4, an inhibitor of the mGluR6 receptor, and the OFF-component was blocked by exposure to PDA, an antagonist of ionotropic glutamate receptors, with a corresponding RGC responses. mERGs were also recorded from mice with progressive retinal degeneration, the C57BL/6J-Pde6b^rd1-2J/J (rd1) mice, from which conventional full-field ERGs are non-recordable. A blockade of the glutamate receptors indicated that the negative wave of rd1 mice do not originate from the photoreceptors but from the second or third order neurons. This technique of recording mERGs will be useful in assessing the focal properties of the retinas obtained from eyes with pathology and also to follow the recovery of the physiology of the retina in regenerative studies.

The B3 Subunit of the Cone Cyclic Nucleotide-gated Channel Regulates the Light Responses of Cones and Contributes to the Channel Structural Flexibility

Cone photoreceptor cyclic nucleotide-gated (CNG) channels play a pivotal role in cone phototransduction, which is a process essential for daylight vision, color vision, and visual acuity. Mutations in the cone channel subunits CNGA3 and CNGB3 are associated with human cone diseases, including achromatopsia, cone dystrophies, and early onset macular degeneration. Mutations in CNGB3 alone account for 50% of reported cases of achromatopsia. This work investigated the role of CNGB3 in cone light response and cone channel structural stability. As cones comprise only 2-3% of the total photoreceptor population in the wild-type mouse retina, we used Cngb3(-/-)/Nrl(-/-) mice with CNGB3 deficiency on a cone-dominant background in our study. We found that, in the absence of CNGB3, CNGA3 was able to travel to the outer segments, co-localize with cone opsin, and form tetrameric complexes. Electroretinogram analyses revealed reduced cone light response amplitude/sensitivity and slower response recovery in Cngb3(-/-)/Nrl(-/-) mice compared with Nrl(-/-) mice. Absence of CNGB3 expression altered the adaptation capacity of cones and severely compromised function in bright light. Biochemical analysis demonstrated that CNGA3 channels lacking CNGB3 were more resilient to proteolysis than CNGA3/CNGB3 channels, suggesting a hindered structural flexibility. Thus, CNGB3 regulates cone light response kinetics and the channel structural flexibility. This work advances our understanding of the biochemical and functional role of CNGB3 in cone photoreceptors.

Retinol dehydrogenase 8 and ATP-binding cassette transporter 4 modulate dark adaptation of M-cones in mammalian retina

KEY POINTS

This study explores the molecular mechanisms that regulate the recycling of chromophore required for pigment regeneration in mammalian cones. We report that two chromophore binding proteins, retinol dehydrogenase 8 (RDH8) and photoreceptor-specific ATP-binding cassette transporter (ABCA4) accelerate the dark adaptation of cones, first, directly, by facilitating the processing of chromophore in cones, and second, indirectly, by accelerating the turnover of chromophore in rods, which is then recycled and delivered to both rods and cones. Preventing competition with the rods by knocking out rhodopsin accelerated cone dark adaptation, demonstrating the interplay between rod and cone pigment regeneration driven by the retinal pigment epithelium (RPE). This novel interdependence of rod and cone pigment regeneration should be considered when developing therapies targeting the recycling of chromophore for rods, and evaluating residual cone function should be a critical test for such regimens targeting the RPE.

ABSTRACT

Rapid recycling of visual chromophore and regeneration of the visual pigment are critical for the continuous function of mammalian cone photoreceptors in daylight vision. However, the molecular mechanisms modulating the supply of visual chromophore to cones have remained unclear. Here we explored the roles of two chromophore-binding proteins, retinol dehydrogenase 8 (RDH8) and photoreceptor-specific ATP-binding cassette transporter 4 (ABCA4), in dark adaptation of mammalian cones. We report that young adult RDH8/ABCA4-deficient mice have normal M-cone morphology but reduced visual acuity and photoresponse amplitudes. Notably, the deletion of RDH8 and ABCA4 suppressed the dark adaptation of M-cones driven by both the intraretinal visual cycle and the retinal pigmented epithelium (RPE) visual cycle. This delay can be caused by two separate mechanisms: direct involvement of RDH8 and ABCA4 in cone chromophore processing, and an indirect effect from the delayed recycling of chromophore by the RPE due to its slow release from RDH8/ABCA4-deficient rods. Intriguingly, our data suggest that RDH8 could also contribute to the oxidation of cis-retinoids in cones, a key reaction of the retina visual cycle. Finally, we dissected the roles of rod photoreceptors and RPE for dark adaptation of M-cones. We found that rods suppress, whereas RPE promotes, cone dark adaptation. Thus, therapeutic approaches targeting the RPE visual cycle could have adverse effects on the function of cones, making the evaluation of residual cone function a critical test for regimens targeting the RPE.