Retinal degenerations: from cell signaling to cell therapy; pre-clinical and clinical issues

Metabolic and Redox Signaling of the Nucleoredoxin-Like-1 Gene for the Treatment of Genetic Retinal Diseases

The loss of cone photoreceptor function in retinitis pigmentosa (RP) severely impacts the central and daily vision and quality of life of patients affected by this disease. The loss of cones follows the degeneration of rods, in a manner independent of the causing mutations in numerous genes associated with RP. We have explored this phenomenon and proposed that the loss of rods triggers a reduction in the expression of rod-derived cone viability factor (RdCVF) encoded by the nucleoredoxin-like 1 (NXNL1) gene which interrupts the metabolic and redox signaling between rods and cones. After providing scientific evidence supporting this mechanism, we propose a way to restore this lost signaling and prevent the cone vision loss in animal models of RP. We also explain how we could restore this signaling to prevent cone vision loss in animal models of the disease and how we plan to apply this therapeutic strategy by the administration of both products of NXNL1 encoding the trophic factor RdCVF and the thioredoxin enzyme RdCVFL using an adeno-associated viral vector. We describe in detail all the steps of this translational program, from the design of the drug, its production, biological validation, and analytical and preclinical qualification required for a future clinical trial that would, if successful, provide a treatment for this incurable disease.

Intranasal administration of erythropoietin rescues the photoreceptors in degenerative retina: a noninvasive method to deliver drugs to the eye

Inherited retinopathies typically lead to photoreceptor loss and severe visual impairments in the subjects. Intranasal administration is an efficient approach to deliver therapeutic agents to the targeted tissue. The present study is designed to deliver the erythropoietin (EPO) into the N-methyl-N-nitrosourea (MNU) induced mice, a pharmacological retinopathy model via intranasal or intravenous route. The mice were then subjected to bioavailability assay and therapeutic effects evaluation. Our results showed that the intranasal delivery of EPO is effective to alleviate the morphological disruptions in the MNU induced mice. The intranasal delivery of EPO also ameliorated the visual impairments in the MNU induced mice. Immunostaining experiment showed that both the M-cone and S-cone populations in the degenerative retinas are rescued by the intranasal delivery of EPO. In particular, the M-cone photoreceptors in dorsal-temporal (DT) quadrant and the S-cone photoreceptors in ventral-nasal (VN) quadrant were preferentially preserved by the intranasal delivery of EPO. Mechanism studies showed that the intranasal delivery of EPO could the modulate apoptosis and restrict oxidation in the degenerative retina. Compared with intravenous delivery, the intranasal delivery led to the significantly higher EPO concentration in the retina. The intranasal delivery resulted in more potent protection and had less erythropoiesis-stimulating activity than the intravenous delivery. Our results suggest that the intranasal administration is a noninvasive and efficient approach to deliver EPO into the retinas. These findings lay the groundwork for further intranasal administration of EPO in ophthalmological practice.

Requirement of the Mowat-Wilson Syndrome Gene Zeb2 in the Differentiation and Maintenance of Non-photoreceptor Cell Types During Retinal Development

Mutations in the human transcription factor gene ZEB2 cause Mowat-Wilson syndrome, a congenital disorder characterized by multiple and variable anomalies including microcephaly, Hirschsprung disease, intellectual disability, epilepsy, microphthalmia, retinal coloboma, and/or optic nerve hypoplasia. Zeb2 in mice is involved in patterning neural and lens epithelia, neural tube closure, as well as in the specification, differentiation and migration of neural crest cells and cortical neurons. At present, it is still unclear how Zeb2 mutations cause retinal coloboma, whether Zeb2 inactivation results in retinal degeneration, and whether Zeb2 is sufficient to promote the differentiation of different retinal cell types. Here, we show that during mouse retinal development, Zeb2 is expressed transiently in early retinal progenitors and in all non-photoreceptor cell types including bipolar, amacrine, horizontal, ganglion, and Müller glial cells. Its retina-specific ablation causes severe loss of all non-photoreceptor cell types, cell fate switch to photoreceptors by retinal progenitors, and elevated apoptosis, which lead to age-dependent retinal degeneration, optic nerve hypoplasia, synaptic connection defects, and impaired ERG (electroretinogram) responses. Moreover, overexpression of Zeb2 is sufficient to promote the fate of all non-photoreceptor cell types at the expense of photoreceptors. Together, our data not only suggest that Zeb2 is both necessary and sufficient for the differentiation of non-photoreceptor cell types while simultaneously inhibiting the photoreceptor cell fate by repressing transcription factor genes involved in photoreceptor specification and differentiation, but also reveal a necessity of Zeb2 in the long-term maintenance of retinal cell types. This work helps to decipher the etiology of retinal atrophy associated with Mowat-Wilson syndrome and hence will impact on clinical diagnosis and management of the patients suffering from this syndrome.

Predicting Progression of ABCA4-Associated Retinal Degenerations Based on Longitudinal Measurements of the Leading Disease Front

PURPOSE

To evaluate the progression of the earliest stage of disease in ABCA4-associated retinal degenerations (RDs).

METHODS

Near-infrared excited reduced-illuminance autofluorescence imaging was acquired across the retina up to 80 degrees eccentricity in 44 patients with two ABCA4 alleles. The eccentricity of the leading disease front (LDF) corresponding to the earliest stage of disease was measured along the four meridians. A mathematical model describing the expansion of the LDF was developed based on 6 years of longitudinal follow-up.

RESULTS

The extent of LDF along the superior, inferior, and temporal meridians showed a wide spectrum from 3.5 to 70 degrees. In patients with longitudinal data, the average centrifugal expansion rate was 2 degrees per year. The nasal extent of LDF between the fovea and ONH ranged from 4.3 to 16.5 degrees and expanded at 0.35 degrees per year. The extent of LDF beyond ONH ranged from 19 to 75 degrees and expanded on average at 2 degrees per year. A mathematical model fit well to the longitudinal data describing the expansion of the LDF.

CONCLUSIONS

The eccentricity of the LDF in ABCA4-RD shows a continuum from parafovea to far periphery along all four meridians consistent with a wide spectrum of severity observed clinically. The model of progression may provide a quantitative prediction of the LDF expansion based on the age and eccentricity of the LDF at a baseline visit, and thus contribute significantly to the enrollment of candidates appropriate for clinical trials planning specific interventions, efficacy outcomes, and durations.

Transcriptional regulation of nucleoredoxin-like genes takes place on a daily basis in the retina and pineal gland of rats

The nucleoredoxin-like gene Nxnl1 (Txnl6) and its paralogue Nxnl2 encode the rod-derived cone viability factors (RdCVF and RdCVF2), which increase the resistance to photooxidative damage and have therapeutic potential for the survival of cones in retinitis pigmentosa. In this study, the transcription of Nxnl genes was investigated as a function of the day/night cycle in rats. The transcript levels of Nxnl1 and Nxnl2 were seen to display daily rhythms with steadily increasing values during the light phase and peak expression around dark onset in preparations of whole retina, photoreceptor cells and—but only in regard to Nxnl1—in photoreceptor-related pinealocytes. The cycling of Nxnl1 but not that of Nxnl2 persisted in constant darkness in the retina. This suggests that daily regulation of Nxnl1 is driven by a circadian clock, whereas that of Nxnl2 is promoted by environmental light. The present data indicate clock- and light-dependent regulations of nucleoredoxin-like genes that may be part of a protective shield against photooxidative damage.

Vibratome sectioning mouse retina to prepare photoreceptor cultures

The retina is a part of the central nervous system that has organized architecture, with neurons in layers from the photoreceptors, both rods and cones in contact with the retinal pigmented epithelium in the most distant part on the retina considering the direction of light, and the ganglion cells in the most proximal distance. This architecture allows the isolation of the photoreceptor layer by vibratome sectioning. The dissected neural retina of a mouse aged 8 days is flat-embedded in 4% gelatin on top of a slice of 20% gelatin photoreceptor layer facing down. Using a vibratome and a double edged razor blade, the 100 µm thick inner retina is sectioned. This section contains the ganglion cells and the inner layer with notably the bipolar cells. An intermediary section of 15 µm is discarded before 200 µm of the outer retina containing the photoreceptors is recovered. The gelatin is removed by heating at 37 °C. Pieces of outer layer are incubated in 500 µl of Ringer's solution with 2 units of activated papain for 20 min at 37 °C. The reaction is stopped by adding 500 µl 10% fetal calf serum (FCS) in Dulbecco's Modified Eagle Medium (DMEM), then 25 units of DNAse I is added before centrifugation at RT, washed several times to remove serum and the cells are resuspended in 500 µl of DMEM and seeded at 1 x 10⁵ cells/cm². The cells are grown to 5 days in vitro and their viability scored using live/dead assay. The purity of the culture is first determined by microscopic observation during the experiment. The purity is then validated by seeding and fixing cells on a histological slide and analyzing using a rabbit polyclonal anti-SAG, a photoreceptor marker and mouse monoclonal anti-RHO, a rod photoreceptor specific marker. Alternatively, the photoreceptor layer (97% rods) can be used for gene or protein expression analysis and for transplantation.

Association between multifocal electroretinograms, optical coherence tomography and central visual sensitivity in advanced retinitis pigmentosa

PURPOSE

To investigate the association between automated perimetry, multifocal electroretinogram (mfERG) and optical coherence tomography (OCT) measurements in patients with advanced retinitis pigmentosa (RP).

METHODS

Twenty-five patients with advanced RP were included. Central visual field sensitivity (VFS) was evaluated using an average of visual sensitivity value at central four test points during central 30-2 static automated perimetry. OCT imaging was conducted, and the inner and outer segment (IS/OS) line was classified into three groups: Group 1, absence; Group 2, partially intact; and Group 3, intact. Central retinal thickness (CRT) that is the retinal thickness of central 3.0 mm was also evaluated. Average amplitude and implicit time of N1 and P1 in ring 1 and 2 were measured on mfERG. Comparisons of VFS, mfERG and OCT among the three subgroups were performed following IS/OS integrity. Relationship among VFS, mfERG and CRT was evaluated by regression analysis.

RESULTS

Group 3 patients with an intact IS/OS line showed a better VFS, and amplitude of mfERG response than those of Group 1 and 2. VFS and amplitudes of mfERG were correlated significantly with CRT in linear regression analysis.

CONCLUSIONS

Disrupted IS/OS integrity was associated with visual dysfunction which was shown by decreased amplitude of mfERG response and reduced central VFS. CRT was significantly correlated with amplitude of mfERG response and central VFS. An eye with the more reduced CRT was associated with the worse amplitude of mfERG response and central VFS.

Chemical chaperone TUDCA preserves cone photoreceptors in a mouse model of Leber congenital amaurosis

PURPOSE

Mutations in either retinoid isomerase (RPE65) or lecithin-retinol acyltransferase (LRAT) lead to Leber congenital amaurosis (LCA). By using the Lrat(-/-) mouse model, previous studies have shown that the rapid cone degeneration in LCA was caused by endoplasmic reticulum (ER) stress induced by S-opsin aggregation. The purpose of this study is to examine the efficacy of an ER chemical chaperone, tauroursodeoxycholic acid (TUDCA), in preserving cones in the Lrat(-/-) model.

METHODS

Lrat(-/-) mice were systemically administered with TUDCA and vehicle (0.15 M NaHCO(3)) every 3 days from P9 to P28. Cone cell survival was determined by counting cone cells on flat-mounted retinas. The expression and subcellular localization of cone-specific proteins were analyzed by western blotting and immunohistochemistry, respectively.

RESULTS

TUDCA treatment reduced ER stress and apoptosis in Lrat(-/-) retina. It significantly slowed down cone degeneration in Lrat(-/-) mice, resulting in a ∼3-fold increase in cone density in the ventral and central retina as compared with the vehicle-treated mice at P28. Furthermore, TUDCA promoted the degradation of cone membrane-associated proteins by enhancing the ER-associated protein degradation pathway.

CONCLUSIONS

Systemic injection of TUDCA is effective in reducing ER stress, preventing apoptosis, and preserving cones in Lrat(-/-) mice. TUDCA has the potential to lead to the development of a new class of therapeutic drugs for treating LCA.

℮-conome: an automated tissue counting platform of cone photoreceptors for rodent models of retinitis pigmentosa

Background

Retinitis pigmentosa is characterized by the sequential loss of rod and cone photoreceptors. The preservation of cones would prevent blindness due to their essential role in human vision. Rod-derived Cone Viability Factor is a thioredoxin-like protein that is secreted by rods and is involved in cone survival. To validate the activity of Rod-derived Cone Viability Factors (RdCVFs) as therapeutic agents for treating retinitis Pigmentosa, we have developed e-conome, an automated cell counting platform for retinal flat mounts of rodent models of cone degeneration. This automated quantification method allows for faster data analysis thereby accelerating translational research.

Methods

An inverted fluorescent microscope, motorized and coupled to a CCD camera records images of cones labeled with fluorescent peanut agglutinin lectin on flat-mounted retinas. In an average of 300 fields per retina, nine Z-planes at magnification X40 are acquired after two-stage autofocus individually for each field. The projection of the stack of 9 images is subject to a threshold, filtered to exclude aberrant images based on preset variables. The cones are identified by treating the resulting image using 13 variables empirically determined. The cone density is calculated over the 300 fields.

Results

The method was validated by comparison to the conventional stereological counting. The decrease in cone density in rd1 mouse was found to be equivalent to the decrease determined by stereological counting. We also studied the spatiotemporal pattern of the degeneration of cones in the rd1 mouse and show that while the reduction in cone density starts in the central part of the retina, cone degeneration progresses at the same speed over the whole retinal surface. We finally show that for mice with an inactivation of the Nucleoredoxin-like genes Nxnl1 or Nxnl2 encoding RdCVFs, the loss of cones is more pronounced in the ventral retina.

Conclusion

The automated platform ℮-conome used here for retinal disease is a tool that can broadly accelerate translational research for neurodegenerative diseases.

Deletion of the p85alpha regulatory subunit of phosphoinositide 3-kinase in cone photoreceptor cells results in cone photoreceptor degeneration

PURPOSE

Downregulation of the retinal insulin/mTOR pathway in mouse models of retinitis pigmentosa is linked to cone cell death, which can be delayed by systemic administration of insulin. A classic survival kinase linking extracellular trophic/growth factors with intracellular antiapoptotic pathways is phosphoinositide 3-kinase (PI3K), which the authors have shown to protect rod photoreceptors from stress-induced cell death. The role of PI3K in cones was studied by conditional deletion of its p85α regulatory subunit.

METHODS

Mice expressing Cre recombinase in cones were bred to mice with a floxed pi3k gene encoding the p85α regulatory subunit of the PI3K and were back-crossed to ultimately generate offspring with cone-specific p85α knockout (cKO). Cre expression and cone-specific localization were confirmed by Western blot analysis and immunohistochemistry (IHC), respectively. Cone structural integrity was determined by IHC using peanut agglutinin and an M-opsin-specific antibody. Electroretinography (ERG) was used to assess rod and cone photoreceptor function. Retinal structure was examined by light and electron microscopy.

RESULTS

An age-related cone degeneration was found in cKO mice, evidenced by a reduction in photopic ERG amplitudes and loss of cone cells. By 12 months of age, approximately 78% of cones had died, and progressive disorganization of synaptic ultrastructure was noted in surviving cone terminals in cKO retinas. Rod viability was unaffected in p85α cKO mice.

CONCLUSIONS

The present study suggests that PI3K signaling pathway is essential for cone survival in the mouse retina.

Expression of rod-derived cone viability factor: dual role of CRX in regulating promoter activity and cell-type specificity

BACKGROUND

RdCVF and RdCVF2, encoded by the nucleoredoxin-like genes NXNL1 and NXNL2, are trophic factors with therapeutic potential that are involved in cone photoreceptor survival. Studying how their expression is regulated in the retina has implications for understanding both their activity and the mechanisms determining cell-type specificity within the retina.

METHODOLOGY/PRINCIPAL FINDINGS

In order to define and characterize their promoters, a series of luciferase/GFP reporter constructs that contain various fragments of the 5'-upstream region of each gene, both murine and human, were tested in photoreceptor-like and non-photoreceptor cell lines and also in a biologically more relevant mouse retinal explant system. For NXNL1, 5'-deletion analysis identified the human -205/+57 bp and murine -351/+51 bp regions as having promoter activity. Moreover, in the retinal explants these constructs drove expression specifically to photoreceptor cells. For NXNL2, the human -393/+27 bp and murine -195/+70 bp regions were found to be sufficient for promoter activity. However, despite the fact that endogenous NXNL2 expression is photoreceptor-specific within the retina, neither of these DNA sequences nor larger upstream regions demonstrated photoreceptor-specific expression. Further analysis showed that a 79 bp NXNL2 positive regulatory sequence (-393 to 315 bp) combined with a 134 bp inactive minimal NXNL1 promoter fragment (-77 to +57 bp) was able to drive photoreceptor-specific expression, suggesting that the minimal NXNL1 fragment contains latent elements that encode cell-type specificity. Finally, based on bioinformatic analysis that suggested the importance of a CRX binding site within the minimal NXNL1 fragment, we found by mutation analysis that, depending on the context, the CRX site can play a dual role.

CONCLUSIONS/SIGNIFICANCE

The regulation of the Nucleoredoxin-like genes involves a CRX responsive element that can act as both as a positive regulator of promoter activity and as a modulator of cell-type specificity.

Trophic factors GDNF and BDNF improve function of retinal sheet transplants

The aim of this study was to compare glial-derived neurotrophic factor (GDNF) treatment with brain-derived neurotrophic factor (BDNF) treatment of retinal transplants on restoration of visual responses in the superior colliculus (SC) of the S334ter line 3 rat model of rapid retinal degeneration (RD). RD rats (age 4-6 weeks) received subretinal transplants of intact sheets of fetal retina expressing the marker human placental alkaline phosphatase (hPAP). Experimental groups included: (1) untreated retinal sheet transplants, (2) GDNF-treated transplants, (3) BDNF-treated transplants, (4) none surgical, age-matched RD rats, (5) sham surgery RD controls, (6) progenitor cortex transplant RD controls, and (7) normal pigmented rat controls. At 2-8 months after transplantation, multi-unit visual responses were recorded from the SC using a 40 ms full-field stimulus (-5.9 to +1 log cd/m(2)) after overnight dark-adaptation. Responses were analyzed for light thresholds, spike counts, response latencies, and location within the SC. Transplants were grouped into laminated or rosetted (more disorganized) transplants based on histological analysis. Visual stimulation of control RD rats evoked no responses. In RD rats with retinal transplants, a small area of the SC corresponding to the position of the transplant in the host retina, responded to light stimulation between -4.5 and -0.08 log cd/m(2), whereas the light threshold of normal rats was at or below -5 log cd/m(2) all over the SC. Overall, responses in the SC in rats with laminated transplants had lower response thresholds and were distributed over a wider area than rats with rosetted transplants. BDNF treatment improved responses (spike counts, light thresholds and responsive areas) of rats with laminated transplants whereas GDNF treatment improved responses from rats with both laminated and rosetted (more disorganized) transplants. In conclusion, treatment of retinal transplants with GDNF and BDNF improved the restoration of visual responses in RD rats; and GDNF appears to exert greater overall restoration than BDNF.

Genetic reactivation of cone photoreceptors restores visual responses in retinitis pigmentosa

Retinitis pigmentosa refers to a diverse group of hereditary diseases that lead to incurable blindness, affecting two million people worldwide. As a common pathology, rod photoreceptors die early, whereas light-insensitive, morphologically altered cone photoreceptors persist longer. It is unknown if these cones are accessible for therapeutic intervention. Here, we show that expression of archaebacterial halorhodopsin in light-insensitive cones can substitute for the native phototransduction cascade and restore light sensitivity in mouse models of retinitis pigmentosa. Resensitized photoreceptors activate all retinal cone pathways, drive sophisticated retinal circuit functions (including directional selectivity), activate cortical circuits, and mediate visually guided behaviors. Using human ex vivo retinas, we show that halorhodopsin can reactivate light-insensitive human photoreceptors. Finally, we identified blind patients with persisting, light-insensitive cones for potential halorhodopsin-based therapy.

Self-complementary AAV-mediated gene therapy restores cone function and prevents cone degeneration in two models of Rpe65 deficiency

To test whether fast-acting, self-complimentary (sc), adeno-associated virus-mediated RPE65 expression prevents cone degeneration and/or restores cone function, we studied two mouse lines: the Rpe65-deficient rd12 mouse and the Rpe65-deficient, rhodopsin null ('that is, cone function-only') Rpe65(-/-)::Rho(-/-) mouse. scAAV5 expressing RPE65 was injected subretinally into one eye of rd12 and Rpe65(-/-)::Rho(-/-) mice at postnatal day 14 (P14). Contralateral rd12 eyes were injected later, at P35. Rd12 behavioral testing revealed that rod vision loss was prevented with either P14 or P35 treatment, whereas cone vision was only detected after P14 treatment. Consistent with this observation, P35 treatment only restored rod electroretinogram (ERG) signals, a result likely due to reduced cone densities at this time point. For Rpe65(-/-)::Rho(-/-) mice in which there is no confounding rod contribution to the ERG signal, cone cells and cone-mediated ERGs were also maintained with treatment at P14. This work establishes that a self-complimentary AAV5 vector can restore substantial visual function in two genetically distinct models of Rpe65 deficiency within 4 days of treatment. In addition, this therapy prevents cone degeneration but only if administered before extensive cone degeneration, thus supporting continuation of current Leber's congenital amaurosis-2 clinical trials with an added emphasis on cone subtype analysis and early intervention.

Human RPE65 gene therapy for Leber congenital amaurosis: persistence of early visual improvements and safety at 1 year

Human gene therapy with rAAV2-vector was performed for the RPE65 form of childhood blindness called Leber congenital amaurosis. In three contemporaneous studies by independent groups, the procedure was deemed safe and there was evidence of visual gain in the short term. At 12 months after treatment, our young adult subjects remained healthy and without vector-related serious adverse events. Results of immunological assays to identify reaction to AAV serotype 2 capsid were unchanged from baseline measurements. Results of clinical eye examinations of study and control eyes, including visual acuities and central retinal structure by in vivo microscopy, were not different from those at the 3-month time point. The remarkable improvements in visual sensitivity we reported by 3 months were unchanged at 12 months. The retinal extent and magnitude of rod and cone components of the visual sensitivity between 3 and 12 months were also the same. The safety and efficacy of human retinal gene transfer with rAAV2-RPE65 vector extends to at least 1 year posttreatment.

Interleukin-4 blocks thapsigargin-induced cell death in rat rod photoreceptors: involvement of cAMP/PKA pathway

Although the photoreceptors cell death is the main cause of some retinopathies diseases, the mechanisms involved in this process are poorly understood. The neuroprotective effects of interleukin-4 (IL-4) have been shown in several tissues, including retina. We demonstrate that treatment of rat retinal explants with IL-4 completely inhibited the thapsigargin-induced rod photoreceptor cell death after 24 hr in culture. We also showed that IL-4 receptor alpha subunit (IL-4Ralpha) is abundantly present in retina. Colocalization of IL-4Ralpha and rhodopsin indicate a direct effect of this cytokine in rod photoreceptor cells. Moreover, IL-4 increased the intracellular levels of cAMP in 7.4-fold, indicating that the neuroprotective effect of this cytokine was completely blocked by RpcAMP, an inhibitor of protein kinase (PKA). Our data demonstrate, for the first time, the neuroprotective effect of IL-4 through cAMP/PKA pathway in thapsigargin-induced photoreceptor cell death.

Tyrosine hydroxylase-positive amacrine interneurons in the mouse retina are resistant against the application of various parkinsonian toxins

Toxins such as 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), 6-hydroxydopamine (6-OHDA), or rotenone have been used to induce degeneration of dopaminergic (DA) neurons in the nigrostriatal pathway and to reproduce pathological characteristics of Parkinson's disease (PD). DA neurons are also present in the retina, and visual impairments in PD patients have been reported. We examined the vulnerability of TH-positive (TH(+)) amacrine interneurons in the retina against MPTP, 6-OHDA, or rotenone-induced cell death. We intraperitoneally (i.p.) injected mice with MPTP, which induced degeneration of DA neurons in the midbrain. However, no death of TH(+) amacrine cells was detectable in the same mice. HPLC analysis revealed a 9 times lower level of the toxic metabolite of MPTP, MPP(+), in the eye compared with the striatum. Another membrane-permeable compound (Tat-Hsp70) could be delivered into the retina after i.p. application, suggesting that the blood-retina barrier (BRB) could be overcome after systemic application. Possible reason for the survival of retinal amacrine cells after systemic MPTP application was a less efficient conversion into toxic MPP(+) in the retina or a general higher resistance against toxic insults of retinal DA neurons compared with DA neurons in the substantia nigra pars compacta (SNpc). Therefore, we directly injected high doses of MPP(+), 6-OHDA, or rotenone into the eye. No loss of TH(+) amacrine cells in the retina was observed, suggesting different properties and less vulnerability of amacrine neurons compared with DA neurons in the midbrain.

The thioredoxin-like protein rod-derived cone viability factor (RdCVFL) interacts with TAU and inhibits its phosphorylation in the retina

Rod-derived cone viability factor (RdCVF) is produced by the Nxnl1 gene that codes for a second polypeptide, RdCVFL, by alternative splicing. Although the role of RdCVF in promoting cone survival has been described, the implication of RdCVFL, a putative thioredoxin enzyme, in the protection of photoreceptors is presently unknown. Using a proteomics approach we identified 90 proteins interacting with RdCVFL including the microtubule-binding protein TAU. We demonstrate that the level of phosphorylation of TAU is increased in the retina of the Nxnl1(-/-) mice as it is hyperphosphorylated in the brain of patients suffering from Alzheimer disease, presumably in some cases through oxidative stress. Using a cell-based assay, we show that RdCVFL inhibits TAU phosphorylation. In vitro, RdCVFL protects TAU from oxidative damage. Photooxidative stress is implicated in retinal degeneration, particularly in retinitis pigmentosa, where it is considered to be a contributor to secondary cone death. The functional interaction between RdCVFL and TAU described here is the first characterization of the RdCVFL signaling pathway involved in neuronal cell death mediated by oxidative stress.

Functional cone rescue by RdCVF protein in a dominant model of retinitis pigmentosa

In retinitis pigmentosa (RP), a majority of causative mutations affect genes solely expressed in rods; however, cone degeneration inevitably follows rod cell loss. Following transplantation and in vitro studies, we demonstrated the role of photoreceptor cell paracrine interactions and identified a Rod-derived Cone Viability Factor (RdCVF), which increases cone survival. In order to establish the clinical relevance of such mechanism, we assessed the functional benefit afforded by the injection of this factor in a frequent type of rhodopsin mutation, the P23H rat. In this model of autosomal dominant RP, RdCVF expression decreases in parallel with primary rod degeneration, which is followed by cone loss. RdCVF protein injections induced an increase in cone cell number and, more important, a further increase in the corresponding electroretinogram (ERG). These results indicate that RdCVF can not only rescue cones but also preserve significantly their function. Interestingly, the higher amplitude of the functional versus the survival effect of RdCVF on cones indicates that RdCVF is acting more directly on cone function. The demonstration at the functional level of the therapeutic potential of RdCVF in the most frequent of dominant RP mutations paves the way toward the use of RdCVF for preserving central vision in many RP patients.