CNGA3 deficiency affects cone synaptic terminal structure and function and leads to secondary rod dysfunction and degeneration

Excessive Thyroid Hormone Signaling Induces Photoreceptor Degeneration in Mice

Rod and cone photoreceptors degenerate in inherited and age-related retinal degenerative diseases, ultimately leading to loss of vision. Thyroid hormone (TH) signaling regulates cell proliferation, differentiation, and metabolism. Recent studies have shown a link between TH signaling and retinal degeneration. This work investigates the effects of excessive TH signaling on photoreceptor function and survival in mice. C57BL/6, Thra1 -/-, Thrb2 -/-, Thrb -/-, and the cone dominant Nrl -/- mice received triiodothyronine (T3) treatment (5-20 μg/ml in drinking water) for 30 d, followed by evaluations of retinal function, photoreceptor survival/death, and retinal stress/damage. Treatment with T3 reduced light responses of rods and cones by 50-60%, compared with untreated controls. Outer nuclear layer thickness and cone density were reduced by ∼18% and 75%, respectively, after T3 treatment. Retinal sections prepared from T3-treated mice showed significantly increased numbers of TUNEL-positive, p-γH2AX-positive, and 8-OHdG-positive cells, and activation of Müller glial cells. Gene expression analysis revealed upregulation of the genes involved in oxidative stress, necroptosis, and inflammation after T3 treatment. Deletion of Thra1 prevented T3-induced degeneration of rods but not cones, whereas deletion of Thrb2 preserved both rods and cones. Treatment with an antioxidant partially preserved photoreceptors and reduced retinal stress responses. This study demonstrates that excessive TH signaling induces oxidative stress/damage and necroptosis, induces photoreceptor degeneration, and impairs retinal function. The findings provide insights into the role of TH signaling in retinal degeneration and support the view of targeting TH signaling for photoreceptor protection.

Delineating the Molecular and Phenotypic Spectrum of the CNGA3-Related Cone Photoreceptor Disorder in Pakistani Families

Cone photoreceptor dysfunction represents a clinically heterogenous group of disorders characterized by nystagmus, photophobia, reduced central or color vision, and macular dystrophy. Here, we described the molecular findings and clinical manifestations of achromatopsia, a partial or total absence of color vision, co-segregating with three known missense variants of CNGA3 in three large consanguineous Pakistani families. Fundus examination and optical coherence tomography (OCT) imaging revealed myopia, thin retina, retinal pigment epithelial cells loss at fovea/perifovea, and macular atrophy. Combination of Sanger and whole exome sequencing revealed three known homozygous missense variants (c.827A>G, p.(Asn276Ser); c.847C>T, p.(Arg283Trp); c.1279C>T, p.(Arg427Cys)) in CNGA3, the α-subunit of the cyclic nucleotide-gated cation channel in cone photoreceptor cells. All three variants are predicted to replace evolutionary conserved amino acids, and to be pathogenic by specific in silico programs, consistent with the observed altered membrane targeting of CNGA3 in heterologous cells. Insights from our study will facilitate counseling regarding the molecular and phenotypic landscape of CNGA3-related cone dystrophies.

Identification of a novel compound heterozygous CYP4V2 variant in a patient with autosomal recessive retinitis pigmentosa

Retinitis pigmentosa (RP) belongs to a family of retinal disorders that is characterized by the progressive degeneration of rod and cone photoreceptors. The aim of the present study was to screen for possible disease-causing genetic variants in a non-consanguineous Chinese family with non-syndromic autosomal recessive RP. Whole-exome sequencing (WES) was performed in samples from the affected individual (the proband) and those from the two children of the proband. A novel compound heterozygous variant of c.C958T (p.R320X) and c.G1355A (p.R452H) in the Cytochrome P450 family 4 subfamily V member 2 (CYP4V2) gene was identified through WES. Subsequently, this variant was validated by direct Sanger sequencing. This compound heterozygous variant was found to be absent from other unaffected family members and 400 ethnically-matched healthy control individuals. In addition, this compound variant was co-segregated with the RP phenotype in an autosomal recessive manner. In silico analysis revealed that both c.C958T (p.R320X) and c.G1355A (p.R452H) could compromise the protein function of CYP4V2. These results strongly suggest this compound variant to be a disease-causing variant, which expands upon the spectrum of currently known CYP4V2 genetic variants associated with retinal diseases.

Deficiency of thyroid hormone receptor protects retinal pigment epithelium and photoreceptors from cell death in a mouse model of age-related macular degeneration

Age-related macular degeneration (AMD) is the leading cause of vision loss in the elderly. Progressive dystrophy of the retinal pigment epithelium (RPE) and photoreceptors is the characteristic of dry AMD, and oxidative stress/damage plays a central role in the pathogenic lesion of the disease. Thyroid hormone (TH) regulates cell growth, differentiation, and metabolism, and regulates development/function of photoreceptors and RPE in the retina. Population-/patient-based studies suggest an association of high free-serum TH levels with increased risk of AMD. We recently showed that suppressing TH signaling by antithyroid treatment reduces cell damage/death of the RPE and photoreceptors in an oxidative-stress/sodium iodate (NaIO₃)-induced mouse model of AMD. This work investigated the effects of TH receptor (THR) deficiency on cell damage/death of the RPE and photoreceptors and the contribution of the receptor subtypes. Treatment with NaIO₃ induced RPE and photoreceptor cell death/necroptosis, destruction, and oxidative damage. The phenotypes were significantly diminished in Thrα1^−/−, Thrb^−/−, and Thrb2^−/− mice, compared with that in the wild-type (C57BL/6 J) mice. The involvement of the receptor subtypes varies in the RPE and retina. Deletion of Thrα1 or Thrb protected RPE, rods, and cones, whereas deletion of Thrb2 protected RPE and cones but not rods. Gene-expression analysis showed that deletion of Thrα1 or Thrb abolished/suppressed the NaIO₃-induced upregulation of the genes involved in cellular oxidative-stress responses, necroptosis/apoptosis signaling, and inflammatory responses. In addition, THR antagonist effectively protected ARPE-19 cells and hRPE cells from NaIO₃-induced cell death. This work demonstrates the involvement of THR signaling in cell damage/death of the RPE and photoreceptors after oxidative-stress challenge and the receptor-subtype contribution. Findings from this work support a role of THR signaling in the pathogenesis of AMD and the strategy of suppressing THR signaling locally in the retina for protection of the RPE/retina in dry AMD.

Impact of Photoreceptor Loss on Retinal Circuitry

Our sense of sight relies on photoreceptors, which transduce photons into the nervous system's electrochemical interpretation of the visual world. These precious photoreceptors can be disrupted by disease, injury, and aging. Once photoreceptors start to die, but before blindness occurs, the remaining retinal circuitry can withstand, mask, or exacerbate the photoreceptor deficit and potentially be receptive to newfound therapies for vision restoration. To maximize the retina's receptivity to therapy, one must understand the conditions that influence the state of the remaining retina. In this review, we provide an overview of the retina's structure and function in health and disease. We analyze a collection of observations on photoreceptor disruption and generate a predictive model to identify parameters that influence the retina's response. Finally, we speculate on whether the retina, with its remarkable capacity to function over light levels spanning nine orders of magnitude, uses these same adaptational mechanisms to withstand and perhaps mask photoreceptor loss.

BBSome Component BBS5 Is Required for Cone Photoreceptor Protein Trafficking and Outer Segment Maintenance

Purpose

To identify the role of the BBSome protein Bardet-Biedl syndrome 5 (BBS5) in photoreceptor function, protein trafficking, and structure using a congenital mutant mouse model.

Methods

Bbs5-/- mice (2 and 9 months old) were used to assess retinal function and morphology. Hematoxylin and eosin staining of retinal sections was performed to visualize histology. Electroretinography was used to analyze rod and cone photoreceptor function. Retinal protein localization was visualized using immunofluorescence (IF) within retinal cryosections. TUNEL staining was used to quantify cell death. Transmission electron microscopy (TEM) was used to examine retinal ultrastructure.

Results

In the Bbs5-/- retina, there was a significant loss of nuclei in the outer nuclear layer accompanied by an increase in cell death. Through electroretinography, Bbs5-/- mice showed complete loss of cone photoreceptor function. IF revealed mislocalization of the cone-specific proteins M- and S-opsins, arrestin-4, CNGA3, and GNAT2, as well as a light-dependent arrestin-1 mislocalization, although perpherin-2 was properly localized. TEM revealed abnormal outer segment disk orientation in Bbs5-/-.

Conclusions

Collectively, these data suggest that, although BBS5 is a core BBSome component expressed in all ciliated cells, its role within the retina mediates specific photoreceptor protein cargo transport. In the absence of BBS5, cone-specific protein mislocalization and a loss of cone photoreceptor function occur.

Forward genetic analysis using OCT screening identifies Sfxn3 mutations leading to progressive outer retinal degeneration in mice

Retinal disease and loss of vision can result from any disruption of the complex pathways controlling retinal development and homeostasis. Forward genetics provides an excellent tool to find, in an unbiased manner, genes that are essential to these processes. Using N-ethyl-N-nitrosourea mutagenesis in mice in combination with a screening protocol using optical coherence tomography (OCT) and automated meiotic mapping, we identified 11 mutations presumably causative of retinal phenotypes in genes previously known to be essential for retinal integrity. In addition, we found multiple statistically significant gene-phenotype associations that have not been reported previously and decided to target one of these genes, Sfxn3 (encoding sideroflexin-3), using CRISPR/Cas9 technology. We demonstrate, using OCT, light microscopy, and electroretinography, that two Sfxn3 ^-/- mouse lines developed progressive and severe outer retinal degeneration. Electron microscopy showed thinning of the retinal pigment epithelium and disruption of the external limiting membrane. Using single-cell RNA sequencing of retinal cells isolated from C57BL/6J mice, we demonstrate that Sfxn3 is expressed in several bipolar cell subtypes, retinal ganglion cells, and some amacrine cell subtypes but not significantly in Müller cells or photoreceptors. In situ hybridization confirmed these findings. Furthermore, pathway analysis suggests that Sfxn3 may be associated with synaptic homeostasis. Importantly, electron microscopy analysis showed disruption of synapses and synaptic ribbons in the outer plexiform layer of Sfxn3 ^-/- mice. Our work describes a previously unknown requirement for Sfxn3 in retinal function.

Inhibition of thyroid hormone signaling protects retinal pigment epithelium and photoreceptors from cell death in a mouse model of age-related macular degeneration

Age-related macular degeneration (AMD) is the leading cause of blindness in the elderly. Dry AMD is characterized by a progressive macular degeneration of the retinal pigment epithelium (RPE) and photoreceptors, and the RPE oxidative damage/dystrophy is at the core of the disease. Recent population/patients-based studies have shown an association of high free serum thyroid hormone (TH) levels with increased risk of AMD. This work investigated the effects of TH signaling inhibition on RPE and photoreceptor damage/cell death in an oxidative stress-induced mouse model of AMD. TH signaling inhibition was achieved by anti-thyroid drug treatment and oxidative stress was induced by sodium iodate (NaIO3) administration. Mice treated with NaIO3 showed severe RPE and photoreceptor cell death/necroptosis, destruction, oxidative damage, retinal stress, and reduced retinal function. Treatment with anti-thyroid drug protected RPE and photoreceptors from damage/cell death induced by NaIO3, reduced oxidative damage of RPE and photoreceptors, and preserved retinal function. Gene expression analysis showed that the NaIO3-induced RPE/photoreceptor damage/cell death involves multiple mechanisms, including cellular oxidative stress responses, activation of necroptosis/apoptosis signaling, and inflammatory responses. Treatment with anti-thyroid drug abolished these cellular stress/death responses. The findings of this study demonstrate a role of TH signaling in RPE and photoreceptor cell death after oxidative stress challenge, and support a role of TH signaling in the pathogenesis of AMD.

Diseases associated with mutations in CNGA3: Genotype-phenotype correlation and diagnostic guideline

Along with the molecular and functional characterization of CNGA3, knowledge about diseases associated with CNGA3 mutations has made great progress. So far, CNGA3 mutations are not only one of the most common causes of achromatopsia and cone dystrophy or cone-rod dystrophy but also one of the most commonly mutated genes among various forms of retinopathy. Understanding the clinical characteristics of CNGA3-associated retinal diseases may help clinical practice of infants or children with related diseases. Recognizing the importance of CNGA3 in inherited retinal diseases may enhance related research in searching for functional restoration or repair of CNGA3 defects.

ELECTRONEGATIVE ELECTRORETINOGRAM IN ACHROMATOPSIA

PURPOSE

To report novel electroretinographic findings in a genetically confirmed case of achromatopsia.

METHODS

A patient with a history of childhood nystagmus, photoaversion, and absent color vision was examined. Electroretinography and fundus examination were performed under anesthesia at the time of corrective surgery for nystagmus. Genomic DNA isolated from peripheral blood was directly sequenced for variations in the CNGA3 and CNGB3 genes.

RESULTS

Ophthalmoscopic examination revealed no distinct abnormalities. Electroretinography obtained under anesthesia at age three years revealed absent photopic responses. The dark-adapted combined responses had reduced b-wave amplitudes resulting in an electronegative configuration. Genetic testing revealed two heterozygous sequence variations present in the coding sequence of the CNGA3 gene (Arg223Trp and Pro372Ser), which have been previously described in the setting of achromatopsia. Sequencing of the patient's parents confirmed that these two variations lie on separate alleles.

CONCLUSION

Novel electroretinography findings in a patient with genetically confirmed achromatopsia are reported. The electronegative configuration in this clinical setting is of unclear etiology; however, it may suggest some component of inner retinal dysfunction.

Cell-based therapeutic strategies for replacement and preservation in retinal degenerative diseases

Cell-based therapeutics offer diverse options for treating retinal degenerative diseases, such as age-related macular degeneration (AMD) and retinitis pigmentosa (RP). AMD is characterized by both genetic and environmental risks factors, whereas RP is mainly a monogenic disorder. Though treatments exist for some patients with neovascular AMD, a majority of retinal degenerative patients have no effective therapeutics, thus indicating a need for universal therapies to target diverse patient populations. Two main cell-based mechanistic approaches are being tested in clinical trials. Replacement therapies utilize cell-derived retinal pigment epithelial (RPE) cells to supplant lost or defective host RPE cells. These cells are similar in morphology and function to native RPE cells and can potentially supplant the responsibilities of RPE in vivo. Preservation therapies utilize supportive cells to aid in visual function and photoreceptor preservation partially by neurotrophic mechanisms. The goal of preservation strategies is to halt or slow the progression of disease and maintain remaining visual function. A number of clinical trials are testing the safety of replacement and preservation cell therapies in patients; however, measures of efficacy will need to be further evaluated. In addition, a number of prevailing concerns with regards to the immune-related response, longevity, and functionality of the grafted cells will need to be addressed in future trials. This review will summarize the current status of cell-based preclinical and clinical studies with a focus on replacement and preservation strategies and the obstacles that remain regarding these types of treatments.

A review of the mechanisms of cone degeneration in retinitis pigmentosa

Retinitis pigmentosa (RP) is an inherited condition that features degeneration of rod and cone photoreceptors. In all forms of RP, the genetic mutation is expressed exclusively in rods; however, cones die too. The secondary death of cones in RP remains somewhat mysterious. A better understanding of the mechanisms that cause cone degeneration in RP could lead to novel treatments that preserve cones. There are a number of prevailing theories that attempt to explain cone degeneration in RP. One concept is that cone survival is dependent on trophic factors produced by rods. Another hypothesis is that cones suffer from a nutrient shortage after rods have been lost. Additionally, oxidative stress and pro-inflammatory microglial activation have also been suggested to play a role in cone death. The present review evaluates the evidence supporting these theories and provides an update on the mechanisms of cone degeneration in RP.

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.

How to make a synaptic ribbon: RIBEYE deletion abolishes ribbons in retinal synapses and disrupts neurotransmitter release

Synaptic ribbons are large proteinaceous scaffolds at the active zone of ribbon synapses that are specialized for rapid sustained synaptic vesicles exocytosis. A single ribbon-specific protein is known, RIBEYE, suggesting that ribbons may be constructed from RIBEYE protein. RIBEYE knockdown in zebrafish, however, only reduced but did not eliminate ribbons, indicating a more ancillary role. Here, we show in mice that full deletion of RIBEYE abolishes all presynaptic ribbons in retina synapses. Using paired recordings in acute retina slices, we demonstrate that deletion of RIBEYE severely impaired fast and sustained neurotransmitter release at bipolar neuron/AII amacrine cell synapses and rendered spontaneous miniature release sensitive to the slow Ca(2+)-buffer EGTA, suggesting that synaptic ribbons mediate nano-domain coupling of Ca(2+) channels to synaptic vesicle exocytosis. Our results show that RIBEYE is essential for synaptic ribbons as such, and may organize presynaptic nano-domains that position release-ready synaptic vesicles adjacent to Ca(2+) channels.

Early adaptive response of the retina to a pro-diabetogenic diet: Impairment of cone response and gene expression changes in high-fructose fed rats

The lack of plasticity of neurons to respond to dietary changes, such as high fat and high fructose diets, by modulating gene and protein expression has been associated with functional and behavioral impairments that can have detrimental consequences. The inhibition of high fat-induced rewiring of hypothalamic neurons induced obesity. Feeding rodents with high fructose is a recognized and widely used model to trigger obesity and metabolic syndrome. However the adaptive response of the retina to short term feeding with high fructose is poorly documented. We therefore aimed to characterize both the functional and gene expression changes in the neurosensory retina of Brown Norway rats fed during 3 and 8 days with a 60%-rich fructose diet (n = 16 per diet and per time point). Glucose, insulin, leptin, triacylglycerols, total cholesterol, HDL-cholesterol, LDL-cholesterol and fructosamine were quantified in plasma (n = 8 in each group). Functionality of the inner retina was studied using scotopic single flash electroretinography (n = 8 in each group) and the individual response of rod and cone photoreceptors was determined using 8.02 Hz Flicker electroretinography (n = 8 in each group). Analysis of gene expression in the neurosensory retina was performed by Affymetrix genechips, and confirmed by RT-qPCR (n = 6 in each group). Elevated glycemia (+13%), insulinemia (+83%), and leptinemia (+172%) was observed after 8 days of fructose feeding. The cone photoreceptor response was altered at day 8 in high fructose fed rats (Δ = 0.5 log unit of light stimulus intensity). Affymetrix analysis of gene expression highlighted significant modulation of the pathways of eIF2 signaling and endoplasmic reticulum stress, regulation of eIF4 and p70S6K signaling, as well as mTOR signaling and mitochondrial dysfunction. RT-qPCR analysis confirmed the down regulation of Crystallins, Npy, Nid1 and Optc genes after 3 days of fructose feeding, and up regulation of End2. Meanwhile, a trend towards an increased expression of αA- and αB-crystallin proteins was observed at day 8. Our results are consistent with early alterations of the functioning and gene expression in the retina in a pro diabetogenic environment.

Activated mTORC1 promotes long-term cone survival in retinitis pigmentosa mice

Retinitis pigmentosa (RP) is an inherited photoreceptor degenerative disorder that results in blindness. The disease is often caused by mutations in genes that are specific to rod photoreceptors; however, blindness results from the secondary loss of cones by a still unknown mechanism. Here, we demonstrated that the mammalian target of rapamycin complex 1 (mTORC1) is required to slow the progression of cone death during disease and that constitutive activation of mTORC1 in cones is sufficient to maintain cone function and promote long-term cone survival. Activation of mTORC1 in cones enhanced glucose uptake, retention, and utilization, leading to increased levels of the key metabolite NADPH. Moreover, cone death was delayed in the absence of the NADPH-sensitive cell death protease caspase 2, supporting the contribution of reduced NADPH in promoting cone death. Constitutive activation of mTORC1 preserved cones in 2 mouse models of RP, suggesting that the secondary loss of cones is caused mainly by metabolic deficits and is independent of a specific rod-associated mutation. Together, the results of this study address a longstanding question in the field and suggest that activating mTORC1 in cones has therapeutic potential to prolong vision in RP.

Mouse models for cone degeneration

Loss of cone vision has devastating effects on everyday life. Even though much effort has been made to understand cone physiology and pathophysiology, no successful therapies are available for patients suffering from cone disorders. As complex retinal interactions cannot be studied in vitro, utilization of different animal models is inevitable. Due to recent advances in transgenesis, mice became the most popular animal model to study human diseases, also in ophthalmology. While there are similarities in retinal anatomy and pathophysiology between mice and humans, there are also differences, most importantly the lack of a cone-rich macula in mice. Instead, cones in mice are rare and distributed over the whole retina, which makes the analysis of cone pathophysiology very difficult in these animals. This hindrance is one of the reasons why our understanding of rod pathophysiological processes is much more advanced. Recently, however, the sparseness of cones was overcome by the generation of the Nrl (- / -) mouse that expresses only cone photoreceptors in the retina. This paper will give a brief overview of some of the known mouse models to study cone degeneration and discuss the current knowledge gained from the analysis of these models.

Homozygous missense variant in the human CNGA3 channel causes cone-rod dystrophy

We assessed a large consanguineous Pakistani family (PKAB157) segregating early onset low vision problems. Funduscopic and electroretinographic evaluation of affected individuals revealed juvenile cone-rod dystrophy (CRD) with maculopathy. Other clinical symptoms included loss of color discrimination, photophobia and nystagmus. Whole-exome sequencing, segregation and haplotype analyses demonstrated that a transition variant (c.955T>C; p.(Cys319Arg)) in CNGA3 co-segregated with the CRD phenotype in family PKAB157. The ability of CNGA3 channel to influx calcium in response to agonist, when expressed either alone or together with the wild-type CNGB3 subunit in HEK293 cells, was completely abolished due to p.Cys319Arg variant. Western blotting and immunolocalization studies suggest that a decreased channel density in the HEK293 cell membrane due to impaired folding and/or trafficking of the CNGA3 protein is the main pathogenic effect of the p.Cys319Arg variant. Mutant alleles of the human cone photoreceptor cyclic nucleotide-gated channel (CNGA3) are frequently associated with achromatopsia. In rare cases, variants in CNGA3 are also associated with cone dystrophy, Leber's congenital amaurosis and oligo cone trichromacy. The identification of predicted p.(Cys319Arg) missense variant in CNGA3 expands the repertoire of the known genetic causes of CRD and phenotypic spectrum of CNGA3 alleles.

Loss of cone cyclic nucleotide-gated channel leads to alterations in light response modulating system and cellular stress response pathways: a gene expression profiling study

The cone photoreceptor cyclic nucleotide-gated (CNG) channel is essential for central and color vision and visual acuity. Mutations in the channel subunits CNGA3 and CNGB3 are associated with achromatopsia and cone dystrophy. We investigated the gene expression profiles in mouse retina with CNG channel deficiency using whole genome expression microarrays. As cones comprise only 2 to 3% of the total photoreceptor population in the wild-type mouse retina, the mouse lines with CNG channel deficiency on a cone-dominant background, i.e. Cnga3-/-/Nrl-/- and Cngb3-/-/Nrl-/- mice, were used in our study. Comparative data analysis revealed a total of 105 genes altered in Cnga3-/-/Nrl-/- and 92 in Cngb3-/-/Nrl-/- retinas, relative to Nrl-/- retinas, with 27 genes changed in both genotypes. The differentially expressed genes primarily encode proteins associated with cell signaling, cellular function maintenance and gene expression. Ingenuity pathway analysis (IPA) identified 26 and 9 canonical pathways in Cnga3-/-/Nrl-/- and Cngb3-/-/Nrl-/- retinas, respectively, with 6 pathways being shared. The shared pathways include phototransduction, cAMP/PKA-mediated signaling, endothelin signaling, and EIF2/endoplasmic reticulum (ER) stress, whereas the IL-1, CREB, and purine metabolism signaling were found to specifically associate with Cnga3 deficiency. Thus, CNG channel deficiency differentially regulates genes that affect cell processes such as phototransduction, cellular survival and gene expression, and such regulations play a crucial role(s) in the retinal adaptation to impaired cone phototransduction. Though lack of Cnga3 and Cngb3 shares many common pathways, deficiency of Cnga3 causes more significant alterations in gene expression. This work provides insights into how cones respond to impaired phototransduction at the gene expression levels.

Molecular genetics of achromatopsia in Newfoundland reveal genetic heterogeneity, founder effects and the first cases of Jalili syndrome in North America

Achromatopsia (ACHM) is a severe retinal disorder characterized by an inability to distinguish colors, impaired visual acuity, photophobia and nystagmus. This rare autosomal recessive disorder of the cone photoreceptors is best known for its increased frequency due to founder effect in the Pingelapese population of the Pacific islands. Sixteen patients from Newfoundland, Canada were sequenced for mutations in the four known achromatopsia genes CNGA3, CNGB3, GNAT2, and PDE6C. The majority (n = 12) of patients were either homozygotes or compound heterozygotes for known achromatopsia alleles, two in CNGB3 (p.T383fsX and p.T296YfsX9) and three in CNGA3 (p.R283Q, p.R427C and p.L527R). Haplotype reconstruction showed that recurrent mutations p.T383fsX and p.L527R were due to a founder effect. Aggregate data from exome sequencing, segregation analysis and archived medical records support a rediagnosis of Jalili syndrome in affected siblings (n = 4) from Family 0094, which to our knowledge is the first family identified with Jalili Syndrome in North America.

Bcl-xL-mediated remodeling of rod and cone synaptic mitochondria after postnatal lead exposure: electron microscopy, tomography and oxygen consumption

PURPOSE

Postnatal lead exposure produces rod-selective and Bax-mediated apoptosis, decreased scotopic electroretinograms (ERGs), and scotopic and mesopic vision deficits in humans and/or experimental animals. Rod, but not cone, inner segment mitochondria were considered the primary site of action. However, photoreceptor synaptic mitochondria were not examined. Thus, our experiments investigated the structural and functional effects of environmentally relevant postnatal lead exposure on rod spherule and cone pedicle mitochondria and whether Bcl-xL overexpression provided neuroprotection.

METHODS

C57BL/6N mice pups were exposed to lead only during lactation via dams drinking water containing lead acetate. The blood [Pb] at weaning was 20.6±4.7 µg/dl, which decreased to the control value by 2 months. To assess synaptic mitochondrial structural differences and vulnerability to lead exposure, wild-type and transgenic mice overexpressing Bcl-xL in photoreceptors were used. Electron microscopy, three-dimensional electron tomography, and retinal and photoreceptor synaptic terminal oxygen consumption (QO(2)) studies were conducted in adult control, Bcl-xL, lead, and Bcl-xL/lead mice.

RESULTS

The spherule and pedicle mitochondria in lead-treated mice were swollen, and the cristae structure was markedly changed. In the lead-treated mice, the mitochondrial cristae surface area and volume (abundance: measure correlated with ATP (ATP) synthesis) were decreased in the spherules and increased in the pedicles. Pedicles also had an increased number of crista segments per volume. In the lead-treated mice, the number of segments/crista and fraction of cristae with multiple segments (branching) similarly increased in spherule and pedicle mitochondria. Lead-induced remodeling of spherule mitochondria produced smaller cristae with more branching, whereas pedicle mitochondria had larger cristae with more branching and increased crista junction (CJ) diameter. Lead decreased dark- and light-adapted photoreceptor and dark-adapted photoreceptor synaptic terminal QO(2). Bcl-xL partially blocked many of the lead-induced alterations relative to controls. However, spherules still had partially decreased abundance, whereas pedicles still had increased branching, increased crista segments per volume, and increased crista junction diameter. Moreover, photoreceptor and synaptic QO(2) were only partially recovered.

CONCLUSIONS

These findings reveal cellular and compartmental specific differences in the structure and vulnerability of rod and cone inner segment and synaptic mitochondria to postnatal lead exposure. Spherule and pedicle mitochondria in lead-exposed mice displayed complex and distinguishing patterns of cristae and matrix damage and remodeling consistent with studies showing that synaptic mitochondria are more sensitive to Ca(2+) overload, oxidative stress, and ATP loss than non-synaptic mitochondria. The lead-induced decreases in QO(2) likely resulted from the decreased spherule cristae abundance and smaller cristae, perhaps due to Bax-mediated effects as they occurred in apoptotic rod inner segments. The increase in pedicle cristae abundance and CJ diameter could have resulted from increased Drp1-mediated fission, as small mitochondrial fragments were observed. The mechanisms of Bcl-xL-mediated remodeling might occur via interaction with formation of CJ protein 1 (Fcj1), whereas the partial protection of synaptic QO(2) might result from the enhanced efficiency of energy metabolism via Bcl-xL's direct interaction with the F1F0 ATP synthase and/or regulation of cellular redox status. These lead-induced alterations in photoreceptor synaptic terminal mitochondria likely underlie the persistent scotopic and mesopic deficits in lead-exposed children, workers, and experimental animals. Our findings stress the clinical and scientific importance of examining synaptic dysfunction following injury or disease during development, and developing therapeutic treatments that prevent synaptic degeneration in retinal and neurodegenerative disorders even when apoptosis is blocked.