Oxidative stress in the RPE is widely accepted as a contributing factor to AMD. We have previously shown that ribozyme-mediated reduction in the antioxidant enzyme manganese superoxide dismutase (MnSOD) leads to some of the features of geographic atrophy in mice. To develop a mouse model independent of viral injection, we used a conditional knockout of the Sod2 gene in the RPE to elevate mitochondrial oxidative stress in that cell layer.
Experimental mice in which exon 3 of Sod2 was flanked by loxP sites were also transgenic for PVMD2-rtTA and tetO-PhCMV
cre, so that cre recombinase was expressed only in the RPE. Pups of this genotype (Sod2flox/floxVMD2cre) were induced to express cre recombinase by feeding doxycycline-laced chow to nursing dams. Controls included mice of this genotype not treated with doxycycline and doxycycline-treated Sod2flox/flox mice lacking the cre transgene. Expression of cre in the RPE was verified by immunohistochemistry, and deletion of Sod2 exon 3 in the RPE was confirmed by PCR. Mice were followed up over a period of 9 months by spectral-domain optical coherence tomography (SD-OCT), digital fundus imaging, and full-field ERG. Following euthanasia, retinas were examined by light and electron microscopy or by immunohistochemistry. Contour length of rod outer segments and thickness of the RPE layer were measured by unbiased stereology.
Following doxycycline induction of cre, Sod2flox/flox cre mice demonstrated increased signs of oxidative stress in the RPE and accumulation of autofluorescent material by age 2 months. They showed a gradual decline in the ERG response and thinning of the outer nuclear layer (by SD-OCT), which were statistically significant by 6 months. In addition, OCT and electron microscopy revealed increased porosity of the choroid. At the same interval, hypopigmented foci appeared in fundus micrographs, and vascular abnormalities were detected by fluorescein angiography. By 9 months, the RPE layer in Sod2flox/flox cre mice was thicker than in nontransgenic littermates, and the rod outer segments were significantly longer over most of the retina, although localized atrophy of photoreceptors was also obvious in some eyes.
Conditional tissue-specific reduction in MnSOD induced oxidative stress in mouse RPE, leading to RPE dysfunction, damage to the choroid, and death of photoreceptor cells. The RPE oxidative stress did not cause drusen-like deposits, but the model recapitulated certain key aspects of the pathology of dry AMD and may be useful in testing therapies.
The RPE-specific deletion of mitochondrial superoxide dismutase in mice leads to oxidative stress in the RPE, accumulation of autofluorescent material, a decline in the ERG response, and localized death of photoreceptors. This model may be useful in understanding the mechanism of geographic atrophy.
retinal degeneration; mouse model; reactive oxygen species; manganese superoxide dismutase; retinal pigment epithelium
We developed a novel strategy for treatment of Leber hereditary optic neuropathy (LHON) caused by a mutation in the nicotinamide adenine dinucleotide dehydrogenase subunit IV (ND4) mitochondrial gene.
To demonstrate the safety and effects of the gene therapy vector to be used in a proposed gene therapy clinical trial.
DESIGN AND SETTING
In a series of laboratory experiments, we modified the mitochondrial ND4 subunit of complex I in the nuclear genetic code for import into mitochondria. The protein was targeted into the organelle by agency of a targeting sequence (allotopic expression). The gene was packaged into adeno-associated viral vectors and then vitreally injected into rodent, nonhuman primate, and ex vivo human eyes that underwent testing for expression and integration by immunohistochemical analysis and blue native polyacrylamide gel electrophoresis. During serial follow-up, the animal eyes underwent fundus photography, optical coherence tomography, and multifocal or pattern electroretinography. We tested for rescue of visual loss in rodent eyes also injected with a mutant G11778A ND4 homologue responsible for most cases of LHON.
Ocular infection with recombinant adeno-associated viral vectors containing a wild-type allotopic human ND4 gene.
MAIN OUTCOMES AND MEASURES
Expression of human ND4 and rescue of optic neuropathy induced by mutant human ND4.
We found human ND4 expressed in almost all mouse retinal ganglion cells by 1 week after injection and ND4 integrated into the mouse complex I. In rodent eyes also injected with a mutant allotopic ND4, wild-type allotopic ND4 prevented defective adenosine triphosphate synthesis, suppressed visual loss, reduced apoptosis of retinal ganglion cells, and prevented demise of axons in the optic nerve. Injection of ND4 in the ex vivo human eye resulted in expression in most retinal ganglion cells. Primates undergoing vitreal injection with the ND4 test article and followed up for 3 months had no serious adverse reactions.
CONCLUSIONS AND RELEVANCE
Expression of our allotopic ND4 vector in the ex vivo human eye, safety of the test article, rescue of the LHON mouse model, and the severe irreversible loss of visual function in LHON support clinical testing with mutated G11778A mitochondrial DNA in our patients.
Mutations in the gene for rhodopsin, RHO, cause autosomal dominant retinitis pigmentosa, a disease characterized by death of rod photoreceptor cells. At the end stage, when most rods are gone, cones die too, taking central vision with them. One goal of gene therapy, therefore, is to preserve central vision by promoting rod survival in the vicinity of the macula. Dominance in RHO mutations is associated with two phenomena: interference with the function of normal rhodopsin and intrinsic toxicity of the mutant protein. In the case of interference, increased production of the wild-type protein may be therapeutic, but in the case of toxicity, suppression of the mutant protein may also be needed. RHO augmentation has made use of advances in gene delivery to the retina using adeno-associated virus (AAV). Several strategies have been developed for suppression of rhodopsin expression, but because of the heterogeneity of RHO mutations they are not specific for the mutant allele: They suppress both mutant and wild-type RHO. Experiments in autosomal dominant retinitis pigmentosa (adRP) mouse models suggest that both RHO augmentation and supplementation plus suppression preserve the survival of rod cells.
drug delivery; gene therapy; retina; nanoparticle; translational medicine
The cell utilizes the Keap1/Nrf2-ARE signaling pathway to detoxify harmful chemicals in order to protect itself from oxidative stress and to maintain its reducing environment. When exposed to oxidative stress and xenobiotic inducers, the redox sensitive Keap1 is covalently modified at specific cysteine residues. Consequently, the latent transcription factor Nrf2 is stabilized and translocates into the nucleus, where it transactivates the expression of detoxification genes through binding to the antioxidant response element (ARE). In the pursuit of potent and bioavailable activators of the ARE, we validated hits from a pathway-directed high-throughput screening campaign by testing them in cell culture and a reporter strain of a whole animal model, Caenorhabditis elegans. These studies allowed us to identify AI-3 as an ARE activator that induces cytoprotective genes in human cells and in worms, which also translated into in vivo activity in mice. AI-3 is an electrophilic ARE activator with two thiol sensitive sites towards a nucleophilic aromatic substitution, and SAR studies indicated the tunability of the system. Tandem LC-MS analysis revealed that AI-3 alkylates Keap1 primarily at Cys151, while AI-3 is reactive towards additional cysteine residues at higher doses in vitro and in vivo. The immediate effects of such alkylation included the disruption of Keap1-Cul3 (low [AI-3]) and/or Keap1-Nrf2 (high [AI-3]) interactions that both led to the stabilization of Nrf2. This further translated into the downstream Nrf2-ARE regulated cytoprotective gene activation. Collectively, AI-3 may become a valuable biological tool and may even provide therapeutic benefits in oxidative stress related diseases.
The goal of this study is to validate whether reprogramming of the UPR via modulation of pro-apoptotic caspase-7 and CHOP proteins could be an effective approach to slow down the rate of retinal degeneration in ADRP mice. In order to pursue our goal we created the T17M RHO CASP7 and T17M RHO CHOP mice to study the impact of the CASP7 or CHOP ablations in T17M RHO retina by ERG, SD-OCT, histology and western blot analysis. The scotopic ERG demonstrated that the ablation of the CASP7 in T17M RHO retina leads to significant preservation of the function of photoreceptors compared to control. Surprisingly, the ablation of pro-apoptotic CHOP protein in T17M RHO mice led to a more severe form of retinal degeneration. Results of the SD-OCT and histology were in agreement with the ERG data. The further analysis demonstrated that the preservation of the structure and function or the acceleration of the onset of the T17M RHO photoreceptor degeneration occurred via reprogramming of the UPR. In addition, the CASP7 ablation leads to the inhibition of cJUN mediated apoptosis, while the ablation of CHOP induces an increase in the HDAC. Thus, manipulation with the UPR requires careful examination in order to achieve a therapeutic effect.
ADRP; UPR; Caspase-7; CHOP; apoptosis
Transforming growth factor β1 (TGFβ1), TGFβ receptor (TGFβR2), and connective tissue growth factor (CTGF) are key regulators of fibrosis in the cornea and in other tissues, including liver, skin, and kidney. We developed an antifibrotic treatment targeting these three critical scarring genes by using a combination of small interfering RNAs (siRNAs) and assessed its effect on downstream scarring genes, collagen I, and α smooth muscle actin (SMA).
Up to six individual siRNAs for each of the three target gene mRNAs were transfected into cultures of rabbit corneal fibroblasts at concentrations from 15 to 90 nM. The knockdown of target gene proteins was measured by ELISA, and the two most effective siRNAs were tested in dual combinations. Knockdown percentages of both individual and dual siRNA combinations were analyzed for synergy by using combination index to predict “effective” and “ineffective” triple siRNA combinations. Effects of both triple siRNA combinations on target and downstream mRNAs were measured by using quantitative RT-PCR, and levels of SMA protein were assessed by immunohistochemistry.
Single and dual siRNA combinations produced a wide range of protein knockdown of target genes (5%–80%). The effective triple siRNA combination significantly reduced mRNA levels of target genes (>80%) and downstream scarring genes (>85%), and of SMA protein (>95%), and significantly reduced cell migration without reducing cell viability.
Simultaneous targeting of TGFβ1, TGFβR2, and CTGF genes by effective triple siRNA combination produced high knockdown of target and downstream scarring genes without cell toxicity, which may have clinical applications in reducing corneal fibrosis and scarring in other tissues.
An optimized triple combination of siRNAs targeting TGFβ1, TGFβR2, and CTGF genes produced very high knockdown of target and downstream scarring genes (collagen and alpha smooth muscle actin) without cell toxicity, suggesting a new approach for reducing fibrosis in the cornea and other tissues.
siRNA; RNA interference; combination therapy; CTGF; TGFβ1
Retinal ischemia/reperfusion (I/R) injury results in the generation of reactive oxygen species (ROS). The aim of this study was to investigate whether delivery of the manganese superoxide dismutase gene (SOD2) or the catalase gene (CAT) could rescue the retinal vascular damage induced by I/R in mice.
I/R injury to the retina was induced in mice by elevating intraocular pressure for 2 hours, and reperfusion was established immediately afterward. One eye of each mouse was pretreated with plasmids encoding manganese superoxide dismutase or catalase complexed with cationic liposomes and delivered by intravitreous injection 48 hours before initiation of the procedure. Superoxide ion, hydrogen peroxide, and 4-hydroxynonenal (4-HNE) protein modifications were measured by fluorescence staining, immunohistochemistry, and Western blot analysis 1 day after the I/R injury. At 7 days after injury, retinal vascular cell apoptosis and acellular capillaries were quantitated.
Superoxide ion, hydrogen peroxide, and 4-HNE protein modifications increased at 24 hours after I/R injury. Administration of plasmids encoding SOD2 or CAT significantly reduced levels of superoxide ion, hydrogen peroxide, and 4-HNE. Retinal vascular cell apoptosis and acellular capillary numbers increased greatly by 7 days after the injury. Delivery of SOD2 or CAT inhibited the I/R-induced apoptosis of retinal vascular cell and retinal capillary degeneration.
Delivery of antioxidant genes inhibited I/R-induced retinal capillary degeneration, apoptosis of vascular cells, and ROS production, suggesting that antioxidant gene therapy might be a treatment for I/R-related disease.
Oxidative stress in the retinal pigment epithelium (RPE) is hypothesized to be a major contributor to the development of age-related macular degeneration (AMD). Mitochondrial manganese superoxide dismutase (MnSOD) is a critical antioxidant protein that scavenges the highly reactive superoxide radical. We speculated that specific reduction of MnSOD in the RPE will increase the level of reactive oxygen species in the retina/RPE/choroid complex leading to pathogenesis similar to geographic atrophy. To test this hypothesis, an Sod2-specific hammerhead ribozyme (Rz), delivered by AAV2/1 and driven by the human VMD2 promoter was injected subretinally into C57BL/6J mice. Dark-adapted full field electroretinogram (ERG) detected a decrease in the response to light. We investigated the age-dependent phenotypic and morphological changes of the outer retina digital fundus imaging and SD-OCT measurement of ONL thickness. Fundus microscopy revealed pigmentary abnormalities in the retina and these corresponded to sub-retinal and sub-RPE deposits seen in SD-OCT B-scans. Light and electron microscopy documented the localization of apical deposits and thickening of the RPE. In RPE flat-mounts we observed abnormally displaced nuclei and regions of apparent fibrosis in the central retina of the oldest mice. This region was surrounded by enlarged and irregular RPE cells that have been observed in eyes donated by AMD patients and in other mouse models of AMD.
retinal pigment epithelium; oxidative stress; superoxide dismutase; mitochondria; mouse model; age related macular degeneration
Connective tissue growth factor (CTGF) is a fibrogenic cytokine that is up-regulated by TGF-β and mediates most key fibrotic actions of TGF-β, including stimulation of synthesis of extracellular matrix and differentiation of fibroblasts into myofibroblasts. This study addresses the role of proteolytic processing of CTGF in human corneal fibroblasts (HCF) stimulated with TGF-β, normal ocular tissues and wounded corneas.
Proteolytic processing of CTGF in HCF cultures, normal animal eyes, and excimer laser wounded rat corneas were examined by Western blot. The identity of a 21-kDa band was determined by tandem mass spectrometry, and possible alternative splice variants of CTGF were assessed by 5′ Rapid Amplification of cDNA Ends (RACE).
HCF stimulated by TGF-β contained full length 38-kDa CTGF and fragments of 25, 21, 18, and 13 kDa, while conditioned medium contained full length 38- and a 21-kDa fragment of CTGF that contained the middle “hinge” region of CTGF. Fragmentation of recombinant CTGF incubated in HCF extracts was blocked by the aspartate protease inhibitor, pepstatin. Normal mouse, rat, and rabbit whole eyes and rabbit ocular tissues contained abundant amounts of C-terminal 25- and 21-kDa fragments and trace amounts of 38-kDa CTGF, although no alternative transcripts were detected. All forms of CTGF (38, 25, and 21 kDa) were detected during healing of excimer ablated rat corneas, peaking on day 11.
Proteolytic processing of 38-kDa CTGF occurs during corneal wound healing, which may have important implications in regulation of corneal scar formation.
Proteolytic processing of connective tissue growth factor (CTGF, 38 kDa) alters its biological effects. C-terminal fragments of CTGF (21 kDa and 25 kDa) are abundant in normal ocular tissues, and peak 11 days after excimer laser ablation in rat corneas. Processing of CTGF during corneal wound healing may impact scar formation.
Excessive scarring (fibrosis) is a major cause of pathologies in multiple tissues, including lung, liver, kidney, heart, cornea, and skin. The transforming growth factor- β (TGF- β) system has been shown to play a key role in regulating the formation of scar tissue throughout the body. Furthermore, connective tissue growth factor (CTGF) has been shown to mediate most of the fibrotic actions of TGF- β, including stimulation of synthesis of extracellular matrix and differentiation of fibroblasts into myofibroblasts. Currently, no approved drugs selectively and specifically regulate scar formation. Thus, there is a need for a drug that selectively targets the TGF- β cascade at the molecular level and has minimal off-target side effects. This chapter focuses on the design of hammerhead ribozymes, measurement of kinetic activity, and assessment of knockdown mRNAs of TGF- β and CTGF in cell cultures.
Ribozymes; TGF- β; CTGF; Scar formation; Transduction; Oligonucleotides
Many mutations in the human rhodopsin gene (RHO) cause autosomal dominant retinitis pigmentosa (ADRP). Our previous studies with a P23H (proline-23 substituted by histidine) RHO transgenic mouse model of ADRP demonstrated significant improvement of retinal function and preservation of retinal structure after transfer of wild-type rhodopsin by AAV. In this study we demonstrate long-term rescue of retinal structure and function by a single virus expressing both RHO replacement cDNA and small interfering RNA (siRNA) to digest mouse Rho and human P23H RHO mRNA. This combination should prevent overexpression of rhodopsin, which can be deleterious to photoreceptors. On the basis of the electroretinogram (ERG) response, degeneration of retinal function was arrested at 2 months postinjection, and the response was maintained at this level until termination at 9 months. Preservation of the ERG response in P23H RHO mice reflected survival of photoreceptors: both the outer nuclear layer (ONL) and outer segments of photoreceptor cells maintained the same thickness as in nontransgenic mice, whereas the control injected P23H eyes exhibited severe thinning of the ONL and outer segments. These findings suggest that delivery of both a modified cDNA and an siRNA by a single adeno-associated viral vector provided long-term rescue of ADRP in this model. Because the siRNA targets human as well as mouse rhodopsin mRNAs, the combination vector may be useful for the treatment of human disease.
Mao and colleagues show that a single subretinal injection of AAV serotype 5 (AAV5) vector encoding both a small interfering RNA targeted against a mutant form of rhodopsin (P23H RHO) and a replacement cDNA for the nonmutant form of RHO preserves retinal structure and leads to full recovery of retinal function in a mouse model of autosomal dominant retinitis pigmentosa.
Small interfering RNA (siRNA) is a promising tool for the treatment of dominant diseases. Autosomal dominant eye disease like retinitis pigmentosa, are a leading cause of blindness. Mutations in rds/peripherin lead to the degeneration of photoreceptors and are associated with several autosomal retinal diseases. Our goal is to develop a gene therapy for rds mutations. We describe a siRNA based mutation-independent approach, targeting rds in which levels of endogenous mutant and wild-type mRNA were reduced, and a siRNA-resistant version of rds gene was supplied simultaneously. siRNAs and resistant rds were delivered to the photoreceptors by recombinant adeno-associated virus (rAAV) vector through subretinal injections. The retinal phenotype was examined, both structurally and functionally at different time points after rAAV delivery. We demonstrate suppression of rds transcript by up to 50% with concomitant expression of replacement transcript in the retina of mice in vivo. These results validate the concept of suppression of rds and replacement strategies of gene therapy with rAAV vectors containing siRNA.
Autosomal dominant retinitis pigmentosa (ADRP) is frequently caused by mutations within the gene for the opsin of rod photoreceptor cells. Studies on transgenic mice, carrying mutated rhodopsin (RHO) transgene on different genetic backgrounds suggested that that an increased amount of wild-type RHO in ADRP photoreceptors attenuated the impact of the mutant transgene. Therefore, we employed a gene therapy approach with a help of Adeno-associated virus (AAV) to treat mice expressing a P23H mutant human RHO transgene. Knowing that AAV5 primarily transduces photoreceptor cells, we designed “hardened” form of the rhodopsin gene (RHO301) that expressed normal rhodopsin and was specifically resistant to degradation by the previously tested siRNA301. AAV5 RHO301 was subretinaly injected into the right eyes of P23H RHO mice at post-natal day 15. Animals were analyzed monthly by electroretinography (ERG) for 6 months. Analysis of the full field scotopic electroretinogram (ERG) demonstrated that increased expression of opsin slowed the rate of retinal degeneration in P23H mice with increased amplitudes in both a-wave and b-wave amplitudes compared to control eyes. An increase in the ERG amplitudes was correlated with improvement of retinal structure. The thickness of the outer nuclear layer in AAV-RHO301 injected eyes was increased by 80% compared to control eyes. This finding indicates that wild -type RHO could rescue the retinal degeneration in transgenic mice carrying a dominant RHO mutation and that increased production of normal rhodopsin could suppress the effect of the mutant protein. These findings suggest that wild-type RHO can used as a therapeutic agent to retard retinal degeneration in ADRP caused by different mutations of RHO via increased production of normal rhodopsin protein.
Barth's syndrome (BTHS) is an X-linked mitochondrial disease that is due to a mutation in the Tafazzin (TAZ) gene. Based on sequence homology, TAZ has been characterized as an acyltransferase involved in the metabolism of cardiolipin (CL), a unique phospholipid almost exclusively located in the mitochondrial inner membrane. Yeast, Drosophila, and zebrafish models have been invaluable in elucidating the role of TAZ in BTHS, but until recently a mammalian model to study the disease has been lacking. Based on in vitro evidence of RNA-mediated TAZ depletion, an inducible short hairpin RNA (shRNA)-mediated TAZ knockdown (TAZKD) mouse model has been developed (TaconicArtemis GmbH, Cologne, Germany), and herein we describe the assessment of this mouse line as a model of BTHS. Upon induction of the TAZ-specific shRNA in vivo, transgenic mouse TAZ mRNA levels were reduced by >89% in cardiac and skeletal muscle. TAZ deficiency led to the absence of tetralineoyl-CL and accumulation of monolyso-CL in cardiac muscle. Furthermore, mitochondrial morphology from cardiac and skeletal muscle was altered. Skeletal muscle mitochondria demonstrated disrupted cristae, and cardiac mitochondria were significantly enlarged and displace neighboring myofibrils. Physiological measurements demonstrated a reduction in isometric contractile strength of the soleus and a reduction in cardiac left ventricular ejection fraction of TAZKD mice compared with control animals. Therefore, the inducible TAZ-deficient model exhibits some of the molecular and clinical characteristics of BTHS patients and may ultimately help to improve our understanding of BTHS-related cardioskeletal myopathy as well as serve as an important tool in developing therapeutic strategies for BTHS.
Barth's syndrome (BTHS) is an X-linked mitochondrial disease that has been associated with loss-of-function mutations in the Tafazzin (TAZ) gene. There is no mammalian animal model of BTHS. In this study, Soustek and colleagues describe an inducible short hairpin RNA (shRNA)-mediated TAZ knockdown (TAZKD) mouse model. They report that this model exhibits some of the molecular and clinical characteristics of patients with BTHS and may ultimately serve as an important tool in developing therapeutic strategies for BTHS.
The mechanism of autosomal dominant retinitis pigmentosa (ADRP) caused by the P23H mutation in rhodopsin is tightly associated with misfolded rhodopsin (RHO) which causes endoplasmic reticulum overload (ER stress), activates the unfolded protein response (UPR) and triggers apoptosis. In efforts to create a therapy for ADRP caused by the P23H mutation, we have explored different approaches leading to survival of photoreceptor (PR) cells. The direct approach involves the modulation of the level of wild-type RHO, while the indirect approach involves reprogramming the UPR and increasing the expression of heat shock proteins (HSPs). Taking the direct approach, we found that over-expression of wild-type RHO rescues scotopic ERG responses partially. However, greater therapeutic effects were obtained by manipulation of the UPR in P23H RHO rat PRs treated with the endoplasmic reticulum protein BiP/Grp78. In vitro study revealed that the pro-survival effect of Bip gene was not associated with its function as a molecular chaperone, but rather with its regulation of the UPR. Another indirect approach was the over-expression of the Hsf-1 gene, a transcriptional regulator of the heat shock response. AAV-delivery of Hsf-1 resulted in an increase of scotopic ERG amplitudes by over 35%. Taken together these data suggest viable therapeutic treatments for ADRP.
Autosomal dominant retinitis pigmentosa (ADRP) is frequently caused by mutations in RHO, the gene for rod photoreceptor opsin. Earlier, a study on mice carrying mutated rhodopsin transgenes on either RHO + / + or RHO + /– backgrounds suggested that the amount of wild-type rhodopsin affected survival of photoreceptors. Therefore, we treated P23H RHO transgenic mice with adeno-associated virus serotype 5 (AAV5) expressing a cDNA clone of the rhodopsin gene (RHO301) that expressed normal opsin from the mouse opsin promoter. Analysis of the electroretinogram (ERG) demonstrated that increased expression of RHO301 slowed the rate of retinal degeneration in P23H mice: at 6 months, a-wave amplitudes were increased by 100% and b-wave amplitudes by 79%. In contrast, nontransgenic mice injected with AAV5 RHO301 demonstrated a decrease in the ERG, confirming the damaging effect of rhodopsin overproduction in normal photoreceptors. In P23H mice, the increase in the ERG amplitudes was correlated with improvement of retinal structure: the thickness of the outer nuclear layer in RHO301-treated eyes was increased by 80% compared with control eyes. These findings suggest that the wild-type RHO gene can be delivered to rescue retinal degeneration in mice carrying a RHO mutation and that increased production of normal rhodopsin can suppress the effect of the mutated protein. These findings make it possible to treat ADRP caused by different mutations of RHO with the expression of wild-type RHO.
In this study, Mao and colleagues examine the efficacy of AAV5-mediated gene transfer of the wild-type rhodopsin (RHO) gene in a mouse model of autosomal dominant retinitis pigmentosa. The authors find that this approach leads to the production of normal RHO protein and to the rescue of retinal degeneration.
Age-related macular degeneration (AMD), a major cause of blindness in the elderly, is associated with oxidative stress, lipofuscin accumulation and retinal degeneration. The aim of this study was to determine if a 5-HT1A receptor agonist can reduce lipofuscin accumulation, reduce oxidative damage and prevent retinal cell loss both in vitro and in vivo. Autophagy-derived and photoreceptor outer segment (POS)-derived lipofuscin formation was assessed using FACS analysis and confocal microscopy in cultured retinal pigment epithelial (RPE) cells in the presence or absence of the 5-HT1A receptor agonist, 8-OH DPAT. 8-OH DPAT treatment resulted in a dose-dependent reduction in both autophagy- and POS-derived lipofuscin compared to control. Reduction in autophagy-induced lipofuscin was sustained for 4 weeks following removal of the drug. The ability of 8-OH DPAT to reduce oxidative damage following exposure to 200 µM H2O2 was assessed. 8-OH DPAT reduced superoxide generation and increased mitochondrial superoxide dismutase (MnSOD) levels and the ratio of reduced glutathione to the oxidized form of glutathione in H2O2-treated cells compared to controls and protected against H2O2-initiated lipid peroxidation, nitrotyrosine levels and mitochondrial damage. SOD2 knockdown mice, which have an AMD-like phenotype, received daily subcutaneous injections of either saline, 0.5 or 5.0 mg/kg 8-OH DPAT and were evaluated at monthly intervals. Systemic administration of 8-OH DPAT improved the electroretinogram response in SOD2 knockdown eyes of mice compared to knockdown eyes receiving vehicle control. There was a significant increase in the ONL thickness in mice treated with 8-OH DPAT at 4 months past the time of MnSOD knockdown compared to untreated controls together with a 60% reduction in RPE lipofuscin. The data indicate that 5-HT1A agonists can reduce lipofuscin accumulation and protect the retina from oxidative damage and mitochondrial dysfunction. 5-HT1A receptor agonists may have potential as therapeutic agents in the treatment of retinal degenerative disease.
Although mutated G11778A NADH ubiquinone oxidoreductase subunit 4 (ND4) mitochondrial DNA (mtDNA) is firmly linked to the blindness of Leber hereditary optic neuropathy (LHON), a bona fide animal model system with mutated mtDNA complex I subunits that would enable probing the pathogenesis of optic neuropathy and testing potential avenues for therapy has yet to be developed.
The mutant human ND4 gene with a guanine to adenine transition at position 11778 with an attached FLAG epitope under control of the mitochondrial heavy strand promoter (HSP) was inserted into a modified self-complementary (sc) adeno-associated virus (AAV) backbone. The HSP-ND4FLAG was directed toward the mitochondria by adding the 23 amino acid cytochrome oxidase subunit 8 (COX8) presequence fused in frame to the N-terminus of green fluorescent protein (GFP) into the AAV2 capsid open reading frame. The packaged scAAV-HSP mutant ND4 was injected into the vitreous cavity of normal mice (OD). Contralateral eyes received scAAV-GFP (OS). Translocation and integration of mutant human ND4 in mouse mitochondria were assessed with PCR, reverse transcription–polymerase chain reaction (RT–PCR), sequencing, immunoblotting, and immunohistochemistry. Visual function was monitored with serial pattern electroretinography (PERG) and in vivo structure with spectral domain optical coherence tomography (OCT). Animals were euthanized at 1 year and processed for light and transmission electron microscopy.
The PCR products of the mitochondrial and nuclear DNA extracted from infected retinas and optic nerves gave the expected 500 base pair bands. RT–PCR confirmed transcription of the mutant human ND4 DNA in mice. DNA sequencing confirmed that the PCR and RT–PCR products were mutant human ND4 (OD only). Immunoblotting revealed the expression of mutant ND4FLAG (OD only). Pattern electroretinograms showed a significant decrement in retinal ganglion cell function OD relative to OS at 1 month and 6 months after AAV injections. Spectral domain optical coherence tomography showed optic disc edema starting at 1 month post injection followed by optic nerve head atrophy with marked thinning of the inner retina at 1 year. Histopathology of optic nerve cross sections revealed reductions in the optic nerve diameters of OD versus OS where transmission electron microscopy revealed significant loss of optic nerve axons in mutant ND4 injected eyes where some remaining axons were still in various stages of irreversible degeneration with electron dense aggregation. Electron lucent mitochondria accumulated in swollen axons where fusion of mitochondria was also evident.
Due to the UGA codon at amino acid 16, mutant G11778A ND4 was translated only in the mitochondria where its expression led to significant loss of visual function, loss of retinal ganglion cells, and optic nerve degeneration recapitulating the hallmarks of human LHON.
Gene therapy for dominantly inherited genetic disease is more difficult than gene-based therapy for recessive disorders, which can be treated with gene supplementation. Treatment of dominant disease may require gene supplementation partnered with suppression of the expression of the mutant gene either at the DNA level, by gene repair, or at the RNA level by RNA interference or transcriptional repression. In this review, we examine some of the gene delivery approaches used to treat animal models of autosomal dominant retinitis pigmentosa, focusing on those models associated with mutations in the gene for rhodopsin. We conclude that combinatorial approaches have the greatest promise for success.
Mitochondria are critical for ocular function as they represent the major source of a cell's supply of energy and play an important role in cell differentiation and survival. Mitochondrial dysfunction can occur as a result of inherited mitochondrial mutations (e.g. Leber's hereditary optic neuropathy and chronic progressive external ophthalmoplegia) or stochastic oxidative damage which leads to cumulative mitochondrial damage and is an important factor in age-related disorders (e.g. age-related macular degeneration, cataract and diabetic retinopathy). Mitochondrial DNA (mtDNA) instability is an important factor in mitochondrial impairment culminating in age-related changes and pathology, and in all regions of the eye mtDNA damage is increased as a consequence of aging and age-related disease. It is now apparent that the mitochondrial genome is a weak link in the defenses of ocular cells since it is susceptible to oxidative damage and it lacks some of the systems that protect the nuclear genome, such as nucleotide excision repair. Accumulation of mitochondrial mutations leads to cellular dysfunction and increased susceptibility to adverse events which contribute to the pathogenesis of numerous sporadic and chronic disorders in the eye.
Mitochondria; Ocular function; Reactive oxygen species
To evaluate the efficiency and safety of AAV-mediated gene delivery of a normal human ND4 complex I subunit in the mouse visual system.
A nuclear encoded human ND4 subunit fused to the ATPc mitochondrial targeting sequence and FLAG epitope were packaged in AAV2 capsids that were injected into the right eyes of mice. AAV-GFP was injected into the left eyes. One month later, pattern electroretinography (PERG), rate of ATP synthesis, gene expression, and incorporation of the human ND4 subunit into the murine complex I were evaluated. Quantitative analysis of ND4FLAG-injected eyes was assessed compared with green fluorescent protein (GFP)-injected eyes.
Rates of ATP synthesis and PERG amplitudes were similar in ND4FLAG- and GFP-inoculated eyes. PERG latency was shorter in eyes that received ND4FLAG. Immunoprecipitated murine complex I gave the expected 52-kDa band of processed human ND4FLAG. Confocal microscopy revealed perinuclear expression of FLAG colocalized with mitochondria-specific fluorescent dye. Transmission electron microscopy revealed FLAG immunogold within mitochondria. Compared with Thy1.2-positive retinal ganglion cells (RGCs), quantification was 38% for FLAG-positive RGCs and 65% for GFP-positive RGCs. Thy1.2 positive-RGC counts in AAV-ND4FLAG were similar to counts in control eyes injected with AAV-GFP.
Human ND4 was properly processed and imported into the mitochondria of RGCs and axons of mouse optic nerve after intravitreal injection. Although it had approximately two-thirds the efficiency of GFP, the expression of normal human ND4 in murine mitochondria did not induce the loss of RGCs, ATP synthesis, or PERG amplitude, suggesting that allotopic ND4 may be safe for the treatment of patients with Leber hereditary optic neuropathy.
A prerequisite for using corrective gene therapy to treat humans with inherited retinal degenerative diseases that affect primarily rods is to develop viral vectors that target specifically this population of photoreceptors. The delivery of a viral vector with photoreceptor tropism coupled with a rod-specific promoter is likely to be the safest and most efficient approach to target expression of the therapeutic gene to rods. Three promoters that included a fragment of the proximal mouse opsin promoter (mOP), the human G-protein coupled receptor protein kinase 1 promoter (hGRK1), or the cytomegalovirus immediate early enhancer combined with the chicken beta actin proximal promoter CBA) were evaluated for their specificity and robustness in driving GFP reporter gene expression in rods, when packaged in a recombinant adeno-associated viral vector of serotype 2/5 (AAV2/5), and delivered via subretinal injection to the normal canine retina. Photoreceptor specific promoters (mOP, hGRK1) targeted robust GFP expression to rods, while the ubiquitously expressed CBA promoter led to transgene expression in the retinal pigment epithelium, rods, cones and rare Müller, horizontal and ganglion cells. Late onset inflammation was frequently observed both clinically and histologically with all three constructs when the highest viral titers were injected. Cone loss in the injected regions of the retinas that received the highest titers occurred with both the hGRK1 and CBA promoters. Efficient and specific rod transduction, together with preservation of retinal structure was achieved with both mOP and hGRK1 promoters when viral titers in the order of 1011 vg/ml were used.
rAAV vectors; promoters; rod; retina; gene transfer; dog
Gene transfer strategies to reduce levels of mutant huntingtin (mHtt) mRNA and protein by targeting human Htt have shown therapeutic promise in vivo. Previously, we have reported that a specific, adeno-associated viral vector (rAAV)-delivered short-hairpin RNA (siHUNT-2) targeting human Htt mRNA unexpectedly decreased levels of striatal-specific transcripts in both wild-type and R6/1 transgenic HD mice. The goal of this study was to determine whether the siHUNT-2-mediated effect was due to adverse effects of RNA interference (RNAi) expression in the brain. To this end, we designed two catalytically active hammerhead ribozymes directed against the same region of human Htt mRNA targeted by siHUNT-2 and delivered them to wild-type and R6/1 transgenic HD mice. After 10 weeks of continuous expression, these ribozymes, like siHUNT-2, negatively impacted the expression of a subset of genes in the striatum. This effect was independent of rAAV transduction and specific to the targeting of a unique sequence in human Htt mRNA. After consideration of the known potential RNAi-specific toxic mechanisms, only cleavage of an unintended RNA target can account for the data reported herein. Thus, long-term rAAV-mediated RNAi in the brain does not, in and of itself, negatively affect striatal gene expression. These findings have important implications in the development of therapeutic RNAi for the treatment of neurological disease.