Inherited Photoreceptor Degeneration Research Articles

Molecular mechanisms underlying inherited photoreceptor degeneration as targets for therapeutic intervention.

Retinitis pigmentosa (RP) is a form of retinal degeneration characterized by primary degeneration of rod photoreceptors followed by a secondary cone loss that leads to vision impairment and finally blindness. This is a rare disease with mutations in several genes and high genetic heterogeneity. A challenging effort has been the characterization of the molecular mechanisms underlying photoreceptor cell death during the progression of the disease. Some of the cell death pathways have been identified and comprise stress events found in several neurodegenerative diseases such as oxidative stress, inflammation, calcium imbalance and endoplasmic reticulum stress. Other cell death mechanisms appear more relevant to photoreceptor cells, such as high levels of cGMP and metabolic changes. Here we review some of the cell death pathways characterized in the RP mutant retina and discuss preclinical studies of therapeutic approaches targeting the molecular outcomes that lead to photoreceptor cell demise.

Neuroprotective effects of syngeneic bone marrow‐derived mononuclear stem cell transplantation and intraperitoneal minocycline in the treatment of inherited photoreceptor retinal degeneration

Inherited retinal degenerations are diseases characterized by photoreceptor loss secondary to genetic defects and Retinitis Pigmentosa (RP) is the most frequent inherited retinal degeneration. This disease has a prevalence of around 1 in 4000 individuals and is caused by mutations in more than 300 genes. RP is characterized by progressive loss of photoreceptors (fist rods following by cones), activation of microglial cells and gliosis increases of macroglial cell.The Royal college of Surgeon (RCS) rat is a well characterized animal models of RP which suffer a mutation of the MERTK gene, that impedes the normal photoreceptor outer segment phagocytosis by RPE cells thus triggers the simultaneous degeneration of rods and cones. Recent studies have shown that microglial activation starts simultaneously with the beginning of photoreceptor death in the RCS retinas, suggesting that microglial cells play an important role in the retinal degeneration progression.Therapies for inherited photoreceptor degeneration aim to prevent or delay death of the photoreceptors, or in specific circumstances try to replace them.The Bone marrow derived‐mononuclear cells (BM‐MNCs) are a mononuclear CD34+ subpopulation of bone marrow‐derived cells that can be separated from the other cellular fractions of the bone marrow aspirate using a Ficoll density gradient‐based protocol. These cells have shown promising results in different animal models of neuronal degeneration, including inherited photoreceptor degenerations such us: improve the morphology of the photoreceptors outer segment, decrease synaptic degeneration in the outer plexiform layer and reduce glial fibrillary acidic protein expression in Müller cells.On the other hands minocycline is a broad‐spectrum tetracycline antibiotic, of which it has been demonstrated has antimicrobial, anti‐inflammatory, antiapoptotic, neuroprotective, and microglial inhibition effects in central nervous system diseases.In RCS rat retina the neuroprotective effect of minocycline against photoreceptor apoptosis is can be explained by mainly two different mechanisms: (1) a direct antiapoptotic effect on photoreceptors and/or (2) an effect through its anti‐inflammatory properties reducing microglial cells activation and migration.The question arises whether the combination of intravitreal BM‐MNCs transplantation with minocycline, substances that increase survival presumably through different mechanisms, could further increase photoreceptor survival and therefore be used for the treatment of retinal degenerations.Our results showed that intravitreal transplantation of BM‐MNCs increase photoreceptor survival, improved the morphology of the photoreceptors outer segment, and reduce retinal thinning (in vivo). On the other hands intraperitoneally minocycline treatment, and reduced activation and migration of microglia and produced a significant rescue of photoreceptors. Finally, our result show that combination both treatments had higher neuroprotective effects in the retina, increasing photoreceptor survival, than each of the treatments alone.In conclusion in RCS rat animal model of photoreceptor degeneration, minocycline reduces microglial activation and migration, and intravitreal transplantation of BM‐MNCs and minocycline increase photoreceptor survival. The combination of both treatments shows greater neuroprotective effects than their isolated effects, and therefore could be indicated for treatment of the early stages of photoreceptor degenerations.

Taurine influences the phagocytic function of retinal pigment epithelium cells in healthy and dystrophic animals

Aims/Purpose: To investigate the influence of taurine on the phagocytic function of the retinal pigment epithelium (RPE) in normal and dystrophic rats.Methods: Normal albino Sprague–Dawley (SD) and pigmented dystrophic Royal College of Surgeons (RCS‐p+) rats that suffer an inherited photoreceptor degeneration due to RPE phagocytosis impairment were used for the study. Taurine depletion was induced in SD rats (n = 8) by providing β‐alanine in drinking water (3%) from postnatal day 21 (P21). Taurine supplementation was administered to RCS‐p+ rats (n = 8) in drinking water (0.2 M) from P21. Untreated RCSp+ (n = 8) and SD (n = 8) rats served as control groups. Rats were processed at P81 (SD) and at P45 (RCS) and received an intravitreal injection (IV) of. 1.5 μL of Fluorogold (FG, 3%) diluted in saline 24 h before processing. The RPE was dissected, flat‐mounted and immunodetected with anti‐S opsin antibodies to label the RPE phagosomes which were quantified using ImageJ and size criteria.Results: The mean numbers (± SD) of phagosomes per RPE whole mount was 414 ± 118 in control SD rats, 1034 ± 528 in taurine‐depleted SD rats 88 ± 12 in untreated RCS and 119 ± 9 in taurine‐treated RCS rats. The numbers of phagosomes were significantly higher in SD rats than in RCS‐p+ rats (t‐test, p < 0.0001) documenting the impaired phagocytic function of RCS‐p+ rats. In SD rats, the number of phagosomes were significantly higher in taurine‐depleted rats evidencing that taurine depletion decreases the RPE phagocytic capacity (t‐test, p < 0.0059). Finally, in RCSp+ rats, the numbers of phagosomes were significantly higher in taurine‐treated than in non‐treated RCS rats (t‐test, p < 0.0001) indicating that taurine supplementation ameliorates the phagocytic capacity of the RPE cells in RCS‐p+ rats.Conclusions: Taurine influences the phagocytic capacity of the RPE cells in normal rats and in dystrophic RCS‐p+ rats.

Late gene therapy limits the restoration of retinal function in a mouse model of retinitis pigmentosa

Retinitis pigmentosa is an inherited photoreceptor degeneration that begins with rod loss followed by cone loss. This cell loss greatly diminishes vision, with most patients becoming legally blind. Gene therapies are being developed, but it is unknown how retinal function depends on the time of intervention. To uncover this dependence, we utilize a mouse model of retinitis pigmentosa capable of artificial genetic rescue. This model enables a benchmark of best-case gene therapy by removing variables that complicate answering this question. Complete genetic rescue was performed at 25%, 50%, and 70% rod loss (early, mid and late, respectively). Early and mid treatment restore retinal output to near wild-type levels. Late treatment retinas exhibit continued, albeit slowed, loss of sensitivity and signal fidelity among retinal ganglion cells, as well as persistent gliosis. We conclude that gene replacement therapies delivered after 50% rod loss are unlikely to restore visual function to normal. This is critical information for administering gene therapies to rescue vision.

The Phosphoproteome of the Rd1 Mouse Retina, a Model of Inherited Photoreceptor Degeneration, Changes after Protein Kinase G Inhibition.

Retinitis pigmentosa (RP) is a frequent cause of blindness among the working population in industrial countries due to the inheritable death of photoreceptors. Though gene therapy was recently approved for mutations in the RPE65 gene, there is in general no effective treatment presently. Previously, abnormally high levels of cGMP and overactivation of its dependent protein kinase (PKG) have been suggested as causative for the fatal effects on photoreceptors, making it meaningful to explore the cGMP-PKG downstream signaling for more pathological insights and novel therapeutic target development purposes. Here, we manipulated the cGMP-PKG system in degenerating retinas from the rd1 mouse model pharmacologically via adding a PKG inhibitory cGMP-analogue to organotypic retinal explant cultures. A combination of phosphorylated peptide enrichment and mass spectrometry was then applied to study the cGMP-PKG-dependent phosphoproteome. We identified a host of novel potential cGMP-PKG downstream substrates and related kinases using this approach and selected the RAF1 protein, which may act as both a substrate and a kinase, for further validation. This showed that the RAS/RAF1/MAPK/ERK pathway may be involved in retinal degeneration in a yet unclarified mechanism, thus deserving further investigation in the future.

Does taurine have a role in the phagocytic function of the retinal pigment epithelium?

AbstractPurpose: To study the role of taurine in the morphology and phagocytic function of the retinal pigment epithelium (RPE) in normal and dystrophic rats.Methods: Albino Sprague–Dawley (SD) rats and pigmented dystrophic Royal College of Surgeons (RCS‐p+) rats that suffer a severe form of inherited photoreceptor degeneration due to RPE phagocytosis impairment, were used for this study. SD rats received β‐alanine in drinking water (3%) for 2 months from P21 to induce taurine depletion and RCS‐p + rats received taurine supplementation in drinking water (0.2 M) for 24 days from P21. Groups of untreated RCS and SD rats were used as control. SD rats were processed 2 months after the start of treatment, while RCS rats were processed at P45. Twenty‐four hours before processing, the animals received an intravitreal injection of 1.5 μl of 3% fluorogold (FG) diluted in saline 24 to label the RPE cells. Animals were processed and the RPE was dissected flat‐mounted and observed using a confocal microscope (Leica SP8).Results: In control SD animals, Fluorogold (FG) accumulates homogeneously in the cytoplasm of the RPE cells. Taurine‐depleted SD animals showed less FG accumulation in the cytoplasm in all cells and, and some cells had FG accumulation only in part of the cytoplasm or no FG accumulation at all. Untreated RCS‐p + animals showed an inverted accumulation of FG because it appeared mainly in the membrane of the RPE but not in the cytoplasm. Finally, taurine‐treated RCS‐p + animals showed FG accumulation in the cytoplasm of the RPE cells, although the accumulation of FG was smaller than that found in SD rats.Conclusions: Taurine influences the phagocytic capacity of the RPE cells. Taurine depletion impairs the phagocytic capacity of the RPE cells in healthy retinas, and taurine supplementation improves the phagocytic function of RPE cells in dystrophic RCS‐p + rats.

Neuroprotective effects of intraocular bone marrow‐derived mononuclear cell transplants in inherited retinal degenerations

AbstractInherited retinal degenerations are diseases characterized by photoreceptor loss secondary to genetic defects. At present, only a minority are susceptible of genetic treatment and therefore are a frequent cause of blindness in infancy and adulthood.Retinitis Pigmentosa (RP) is the most frequent inherited retinal degeneration. This disease has a prevalence of around 1 in 4000 individuals and is caused by mutations in more than 300 genes. RP is characterized by progressive loss of photoreceptors, rods and then cones that may die secondarily to rod loss.The rats P23H and Royal college of Surgeon (RCS) are two well characterized animal models of RP because they bear genetic defects observed in RP patients. The P23H rat suffers an autosomal dominant mutation of the rhodopsin gene that causes protein misfolding and endoplasmic reticulum stress in rods, which die first followed by cones. The RCS rat has a mutation of MERTK gene, that impedes the normal photoreceptor outer segment phagocytosis by RPE cells thus causing simultaneous rod and cone degeneration.Therapies for inherited photoreceptor degeneration aim to prevent or delay death of the photoreceptors, or in specific circumstances try to replace them.The Bone marrow derived‐mononuclear cells (BM‐MNCs) are a mononuclear CD34+ subpopulation of bone marrow‐derived cells that can be separated from the other cellular fractions of the bone marrow aspirate using a Ficoll density gradient‐based protocol. The BM‐MNCs is a mixture of specific bone marrow cell subtypes in which is possible to find cells: Lin+, Lin−, CD34+, c‐Kit+, CD133+ or Sca‐1+ HSCs, endothelial precursor cells, and Mesenchymal Stem Cells.BM‐MNCs transplantation has shown promising results in different animal models of neuronal degeneration, including inherited photoreceptor degenerations.In our studies we have performed a syngeneic intravitreal or subretinal transplantation of BM‐MNCs in a P23H‐1 and RCS rats, and we have documented that both increase photoreceptor survival. They also improve the morphology of the photoreceptors outer segment, decrease synaptic degeneration in the outer plexiform layer and reduce glial fibrillary acidic protein (GFAP) expression in Müller cells. However, the transplanted cells did not penetrate the retina, only survived for up to 15 days, and did not modify microglial cell activation and migration or the electroretinographic responses. Their beneficial effects are maybe due to a paracrine trophic effect achieved through release of neurotrophic, anti‐angiogenic and/or immunomodulatory factors.In conclusion, the intra ocular transplantation BM‐MNCs shows photoreceptor neuroprotective actions in inherited retinal degenerations. These cells may have the potential to slow down photoreceptor loss, and therefore could be indicated for treatment of the early stages of photoreceptor degenerations.

Wolfberry-derived zeaxanthin dipalmitate delays retinal degeneration in a mouse model of retinitis pigmentosa through modulating STAT3, CCL2 and MAPK pathways.

Retinitis pigmentosa (RP) is a group of inherited photoreceptor degeneration diseases that causes blindness without effective treatment. The pathogenesis of retinal degeneration involves mainly oxidative stress and inflammatory responses. Zeaxanthin dipalmitate (ZD), a wolfberry-derived carotenoid, has anti-inflammatory and anti-oxidative stress effects. Here we investigated whether these properties of ZD can delay the retinal degeneration in rd10 mice, a model of RP, and explored its underlying mechanism. One shot of ZD or control vehicle was intravitreally injected into rd10 mice on postnatal day 16 (P16). Retinal function and structure of rd10 mice were assessed at P25, when rods degenerate substantially, using a visual behavior test, multi-electrode-array recordings and immunostaining. Retinal pathogenic gene expression and regulation of signaling pathways by ZD were explored using transcriptome sequencing and western blotting. Our results showed that ZD treatment improved the visual behavior of rd10 mice and delayed the degeneration of retinal photoreceptors. It also improved the light responses of photoreceptors, bipolar cells and retinal ganglion cells. The expression of genes that are involved in inflammation, apoptosis and oxidative stress were up-regulated in rd10 mice, and were reduced by ZD. ZD further reduced the activation of two key factors, signal transducer and activator of transcription 3 and chemokine (C-C motif) ligand 2, down-regulated the expression of the inflammatory factor GFAP, and inhibited extracellular signal regulated protein kinases and P38, but not the JNK pathways. In conclusion, ZD delays the degeneration of the rd10 retina both morphologically and functionally. Its anti-inflammatory function is mediated primarily through the signal transducer and activator of transcription 3, chemokine (C-C motif) ligand 2 and MAPK pathways. Thus, ZD may serve as a potential clinical candidate to treat RP.

Developmental Dynamics of the Functional State of the Retina in Mice with Inherited Photoreceptor Degeneration

Developmental Dynamics of the Functional State of the Retina in Mice with Inherited Photoreceptor Degeneration

Minocycline decreases CCR2-positive monocytes in the retina and ameliorates photoreceptor degeneration in a mouse model of retinitis pigmentosa.

Retinal inflammation accelerates photoreceptor cell death caused by retinal degeneration. Minocycline, a semisynthetic broad-spectrum tetracycline antibiotic, has been previously reported to rescue photoreceptor cell death in retinal degeneration. We examined the effect of minocycline on retinal photoreceptor degeneration using c-mer proto-oncogene tyrosine kinase (Mertk)-/-Cx3cr1GFP/+Ccr2RFP/+ mice, which enabled the observation of CX3CR1-green fluorescent protein (GFP)- and CCR2-red fluorescent protein (RFP)-positive macrophages by fluorescence. Retinas of Mertk-/-Cx3cr1GFP/+Ccr2RFP/+ mice showed photoreceptor degeneration and accumulation of GFP- and RFP-positive macrophages in the outer retina and subretinal space at 6 weeks of age. Mertk-/-Cx3cr1GFP/+Ccr2RFP/+ mice were intraperitoneally administered minocycline. The number of CCR2-RFP positive cells significantly decreased after minocycline treatment. Furthermore, minocycline administration resulted in partial reversal of the thinning of the outer nuclear layer and decreased the number of apoptotic cells, as assessed by the TUNEL assay, in Mertk-/-Cx3cr1GFP/+Ccr2RFP/+ mice. In conclusion, we found that minocycline ameliorated photoreceptor cell death in an inherited photoreceptor degeneration model due to Mertk gene deficiency and has an inhibitory effect on CCR2 positive macrophages, which is likely to be a neuroprotective mechanism of minocycline.

Retinal Oxygenation in Inherited Diseases of the Retina.

(1) Background: The aim of our study was to investigate the relationship between retinal metabolic alterations (retinal vessel oximetry, RO) and structural findings (retinal vessel diameter, central retinal thickness and retinal nerve fiber layer thickness, RNFL) in patients with inherited retinal diseases (IRDs). (2) Methods: A total of 181 eyes of 92 subjects were examined: 121 eyes of 62 patients with IRDs were compared to 60 eyes of 30 healthy age-matched controls. The retinal vessel oximetry was performed with the oxygen saturation measurement tool of the Retinal Vessel Analyser (RVA; IMEDOS Systems UG, Jena, Germany). The oxygen saturation in all four major peripapillary retinal arterioles (A-SO2; %) and venules (V-SO2; %) were measured and their difference (A-V SO2; %) was calculated. Additionally, retinal vessel diameters of the corresponding arterioles (D-A; µm) and venules (D-V; µm) were determined. The peripapillary central retinal thickness and the RNFL thickness were measured using spectral domain optical coherence tomography (SD-OCT) (Carl Zeiss Meditec, Dublin, CA, USA). Moreover, we calculated the mean central retinal oxygen exposure (cO2-E; %/µm) and the mean peripapillary oxygen exposure (pO2-E; %/µm) per micron of central retinal thickness and nerve fiber layer thickness by dividing the mean central retinal thickness (CRT) and the RNFL thickness with the mean A-V SO2. (3) Results: Rod-cone dystrophy patients had the highest V-SO2 and A-SO2, the lowest A-V SO2, the narrowest D-A and D-V and the thickest RNFL, when compared not only to controls (p ≤ 0.040), but also to patients with other IRDs. Furthermore, in rod-cone dystrophies the cO2-E and the pO2-E were higher in comparison to controls and to patients with other IRDs (p ≤ 0.005). Cone-rod dystrophy patients had the lowest cO2-E compared to controls and patients with other IRDs (p ≤ 0.035). Evaluated in central zones, the cO2-E was significantly different when comparing cone-rod dystrophy (CRD) against rod-cone dystrophy (RCD) patients in all zones (p < 0.001), whereas compared with controls and patients with inherited macular dystrophy this was observed only in zones 1 and 2 (p ≤ 0.018). The oxygen exposure was also the highest in the RCD group for both the nasal and the temporal peripapillary area, among all the evaluated groups (p ≤ 0.025). (4) Conclusions: The presented metabolic-structural approach enhances our understanding of inherited photoreceptor degenerations. Clearly demonstrated through the O2-E comparisons, the central and the peripapillary retina in rod-cone dystrophy eyes consume less oxygen than the control-eyes and eyes with other IRDs. Rod-cone dystrophy eyes seem to be proportionally more exposed to oxygen, the later presumably leading to more pronounced oxidative damage-related remodeling.

Bone Marrow-Derived Mononuclear Cell Transplants Decrease Retinal Gliosis in Two Animal Models of Inherited Photoreceptor Degeneration.

Inherited photoreceptor degenerations are not treatable diseases and a frequent cause of blindness in working ages. In this study we investigate the safety, integration and possible rescue effects of intravitreal and subretinal transplantation of adult human bone-marrow-derived mononuclear stem cells (hBM-MSCs) in two animal models of inherited photoreceptor degeneration, the P23H-1 and the Royal College of Surgeons (RCS) rat. Immunosuppression was started one day before the injection and continued through the study. The hBM-MSCs were injected in the left eyes and the animals were processed 7, 15, 30 or 60 days later. The retinas were cross-sectioned, and L- and S- cones, microglia, astrocytes and Müller cells were immunodetected. Transplantations had no local adverse effects and the CD45+ cells remained for up to 15 days forming clusters in the vitreous and/or a 2–3-cells-thick layer in the subretinal space after intravitreal or subretinal injections, respectively. We did not observe increased photoreceptor survival nor decreased microglial cell numbers in the injected left eyes. However, the injected eyes showed decreased GFAP immunoreactivity. We conclude that intravitreal or subretinal injection of hBM-MSCs in dystrophic P23H-1 and RCS rats causes a decrease in retinal gliosis but does not have photoreceptor neuroprotective effects, at least in the short term. However, this treatment may have a potential therapeutic effect that merits further investigation.

Noninvasive Electrical Stimulation Improves Photoreceptor Survival and Retinal Function in Mice with Inherited Photoreceptor Degeneration.

PurposeNeurons carry electrical signals and communicate via electrical activities. The therapeutic potential of electrical stimulation (ES) for the nervous system, including the retina, through improvement of cell survival and function has been noted. Here we investigated the neuroprotective and regenerative potential of ES in a mouse model of inherited retinal degeneration.MethodsRhodopsin-deficient (Rho−/−) mice received one or two sessions of transpalpebral ES or sham treatments for 7 consecutive days. Intraperitoneal injection of 5-ethynyl-2′-deoxyuridine was used to label proliferating cells. Weekly electroretinograms were performed to monitor retinal function. Retinal morphology, photoreceptor survival, and regeneration were evaluated in vivo using immunohistochemistry and genetic fate-mapping techniques. Müller cell (MC) cultures were employed to further define the optimal conditions of ES application.ResultsNoninvasive transpalpebral ES in Rho−/− mice improved photoreceptor survival and electroretinography function in vivo. ES also triggered residential retinal progenitor-like cells such as MCs to reenter the cell cycle, possibly producing new photoreceptors, as shown by immunohistochemistry and genetic fate-mapping techniques. ES directly stimulated cell proliferation and the expression of progenitor cell markers in MC cultures, at least partially through bFGF signaling.ConclusionsOur study showed that transpalpebral ES improved photoreceptor survival and retinal function and induced the proliferation, probably photoreceptor regeneration, of MCs; this occurs via stimulation of the bFGF pathways. These results suggest the exciting possibility of applying noninvasive ES as a versatile tool for preventing photoreceptor loss and mobilizing endogenous progenitors for reversing vision loss in patients with photoreceptor degeneration.

Electrical Stimulation Induces Retinal Müller Cell Proliferation and Their Progenitor Cell Potential.

Non-invasive electrical stimulation (ES) is increasingly applied to improve vision in untreatable eye conditions, such as retinitis pigmentosa and age-related macular degeneration. Our previous study suggested that ES promoted retinal function and the proliferation of progenitor-like glial cells in mice with inherited photoreceptor degeneration; however, the underlying mechanism remains obscure. Müller cells (MCs) are thought to be dormant residential progenitor cells that possess a high potential for retinal neuron repair and functional plasticity. Here, we showed that ES with a ramp waveform of 20 Hz and 300 µA of current was effective at inducing mouse MC proliferation and enhancing their expression of progenitor cell markers, such as Crx (cone–rod homeobox) and Wnt7, as well as their production of trophic factors, including ciliary neurotrophic factor. RNA sequencing revealed that calcium signaling pathway activation was a key event, with a false discovery rate of 5.33 × 10−8 (p = 1.78 × 10−10) in ES-mediated gene profiling changes. Moreover, the calcium channel blocker, nifedipine, abolished the observed effects of ES on MC proliferation and progenitor cell gene induction, supporting a central role of ES-induced Ca2+ signaling in the MC changes. Our results suggest that low-current ES may present a convenient tool for manipulating MC behavior toward neuroregeneration and repair.

Long‐term effects of inherited and acquired retinal degenerations on the retinal ganglion cell population

AbstractInherited photoreceptor degenerations cause loss of photoreceptors and retinal remodeling which, when severe, is accompanied of retinal vessel displacements that cause retinal ganglion cell (RGC) axonal compression and death. Light‐induced photoreceptor degenerations cause similar pathologic events and ultimately RGC loss. RGC death is secondary to photoreceptor degeneration and is a delayed event that occurs in retinal sectors where there is maximal photoreceptor degeneration. Other types of acquired retinal diseases, such as taurine deficiency‐induced retinal degeneration, which may be similar to vigabatrine‐induced retinopathy, can also cause photoreceptor degeneration and RGC loss. However, in taurine deficiency, RGC loss may not be secondary to photoreceptor degeneration, but occur as an independent event and thus, RGC loss is diffuse. The combination of two treatments: taurine deficiency and light toxicity increases photoreceptor loss but not RGC loss (García‐Ayuso et al., 2018). The population of intrinsically photosensitive RGCs (ipRGCs) may respond differently than the normal population of RGCs to different types of photoreceptor degeneration because they die in similar proportions to the other RGCs in inherited photoreceptor degenerations (García‐Ayuso et al., 2015), but are selectively affected by taurine‐deficiency and may show only a temporary decrease of melanopsin expression after light‐induced damage. The investigation of RGC loss in different retinal degenerations provides us with information on the mechanisms of cell death in order to plan strategies for its prevention.

The cGMP Pathway and Inherited Photoreceptor Degeneration: Targets, Compounds, and Biomarkers.

Photoreceptor physiology and pathophysiology is intricately linked to guanosine-3’,5’-cyclic monophosphate (cGMP)-signaling. Here, we discuss the importance of cGMP-signaling for the pathogenesis of hereditary retinal degeneration. Excessive accumulation of cGMP in photoreceptors is a common denominator in cell death caused by a variety of different gene mutations. The cGMP-dependent cell death pathway may be targeted for the treatment of inherited photoreceptor degeneration, using specifically designed and formulated inhibitory cGMP analogues. Moreover, cGMP-signaling and its down-stream targets may be exploited for the development of novel biomarkers that could facilitate monitoring of disease progression and reveal the response to treatment in future clinical trials. We then briefly present the importance of appropriate formulations for delivery to the retina, both for drug and biomarker applications. Finally, the review touches on important aspects of future clinical translation, highlighting the need for interdisciplinary cooperation of researchers from a diverse range of fields.

Role of microglial cells in photoreceptor degeneration.

Inherited photoreceptor degeneration in humans constitutes a major cause of irreversible blindness in the world. They comprise various diseases, but retinitis pigmentosa is the most frequently observed. Retinitis pigmentosa is commonly limited to the eye, where there is progressive photoreceptor degeneration, rods and secondarily cones. The mechanisms of cone and rod degeneration continue to be investigated, since most of the mutations causing retinitis pigmentosa affect rods and thus, the secondary death of cones is an intriguing question but, ultimately, the cause of blindness. Understanding the mechanisms of rod and cone degeneration could help us to develop therapies to stop or, at least, slow down the degeneration process. Secondary cone degeneration has been attributed to the trophic dependence between rods and cones, but microglial cell activation could also have a role. In this review, based on previous work carried out in our laboratory in early stages of photoreceptor degeneration in two animal models of retinitis pigmentosa, we show that microglial cell activation is observed prior to the the initiation of photoreceptor death. We also show that there is an increase of the retinal microglial cell densities and invasion of the outer retinal layers by microglial cells. The inhibition of the microglial cells improves photoreceptor survival and morphology, documenting a role for microglial cells in photoreceptor degeneration. Furthermore, these results indicate that the modulation of microglial cell reactivity can be used to prevent or diminish photoreceptor death in inherited photoreceptor degenerations.

Release of Retinal Extracellular Vesicles in aModel of Retinitis Pigmentosa.

Extracellular vesicles (EVs) are membranous structures released by cells, including those of the retinal pigment epithelium (RPE) and photoreceptors. The cargo of EVs includes genetic material and proteins, making these vesicles essential in cell communication. Among the genetic materials, we find a large number of microRNAs (miRNAs), small chains of noncoding RNA. In the case of EVs from the retina, changes have also been observed in the number and cargo of EVs.Our group confirmed that damaged RPE cells in vitro release a greater number of EVs with a higher pro-angiogenic factor (VEGFR-1 and VEGFR-2) than control non-damaged cells, thus increasing neovascularization in endothelial cell cultures. This indicates that something similar could happen in patients suffering from some types of retinal degeneration that occur with angiogenesis, such as wet AMD or RD.Here, we investigated the role of EVs in photoreceptor degeneration, and we report for the first time on CD9 and CD81, closely related tetraspanins, in wild-type and rd1 retinae. Our study demonstrates the involvement of EVs in the process of inherited photoreceptor degeneration in a PDE6 mutation.

Light-Dependent OCT Structure Changes in Photoreceptor Degenerative rd 10 Mouse Retina.

PurposeUsing optical coherence tomography (OCT) to analyze the effects of light/dark adaptation in a mouse model of inherited photoreceptor degeneration (rd10), and to study dynamics of subretinal fluid during the progress of retinal degeneration.Methodsrd10 and wild-type (WT) C57BL/6J mice were reared in cyclic light or darkness and imaged with Bioptigen UHR-OCT or Spectralis HRA+OCT after adaptation to either light or darkness.ResultsOCT images from rd10 mice were analyzed at three progressive stages of degeneration. After light-adaptation, stage I (postnatal age [P]26–29) eyes demonstrated no apparent subretinal fluid. At stage II (P32–38), subretinal fluid was present and restricted to parapapillary area, while at stage III (P44–45) extensive subretinal fluid was present across many retinal areas. Following overnight dark-adaptation, WT eyes showed a large reduction in outer retinal thickness (4.6 ± 1.4 μm, n = 16), whereas this change was significantly smaller in stage I rd10 eyes (1.5 ± 0.5 μm, n = 14). In stage II rd10 eyes, dark-adaptation significantly reduced the extent of subretinal fluid, with the amount of reduction correlating with the amount of fluid pre-existing in the light-adapted state. However, dark-adaptation did not significantly alter the amount of subretinal fluid observed in stage III rd10 mice. In addition, dark-rearing of rd10 mice from P6 to P30 slowed retinal degeneration.ConclusionsVisual experience in the form of light/dark adaptation exerts a significant effect on outer retinal structure in the context of photoreceptor degeneration. This effect may arise from light-dependent alterations in fluid transport across the RPE monolayer, and promote photoreceptor survival as induced by dark-rearing.

Involvement of Innate Immune System in Late Stages of Inherited Photoreceptor Degeneration

Retinitis pigmentosa (RP) is a group of inherited retinal degenerations that lead to progressive vision loss. Many mutations in 60 different genes have been shown to cause RP. Given the diversity of genes and mutations that cause RP, corrective gene therapy approaches currently in development may prove both time-consuming and cost-prohibitive for treatment of all forms of RP. An alternative approach is to find common biological pathways that cause retinal degeneration in various forms of RP, and identify new molecular targets. With this goal, we analyzed the retinal transcriptome of two non-allelic forms of RP in dogs, rcd1 and xlpra2, at clinically relevant advanced stages of the two diseases. Both diseases showed very similar trends in changes in gene expression compared to control normal dogs. Pathway analysis revealed upregulation of various components of the innate immune system in both diseases, including inflammasome and complement pathways. Our results show that the retinal transcriptome at advanced stages of RP is very similar to that of other retinal degenerative diseases such as age-related macular degeneration and diabetic retinopathy. Thus, drugs and therapeutics already in development for targeting these retinopathies may also prove useful for the treatment of many forms of RP.