Adaptation of a deep-sea cephalopod to the photic environment. Evidence for three visual pigments

Watasenia scintillans, a bioluminescent deep-sea squid, has a specially developed eye with a large open pupil and three visual pigments. Photoreceptor cells (outer segment: 476 micron; inner segment: 99 micron) were long in the small area of the ventral retina receiving downwelling light, whereas they were short (outer segment: 207 micron; inner segment: 44 micron) in the other regions of the retina. The short photoreceptor cells contained the visual pigment with retinal (lambda max approximately 484 nm), probably for the purpose of adapting to their environmental light. The outer segment of the long photoreceptor cells consisted of two strata, a pinkish proximal area and a yellow distal area. The visual pigment with 3-dehydroretinal (lambda max approximately 500 nm) was located in the pinkish proximal area, giving high sensitivity at longer wavelengths. A newly found pigment (lambda max approximately 471 nm) was in the yellow distal area. The small area of the ventral retina containing two visual pigments is thought to have a high and broad spectral sensitivity, which is useful for distinguishing the bioluminescence of squids of the same species in their environmental downwelling light. These findings were obtained by partial bleaching of the extracted pigment from various areas of the retina and by high-performance liquid chromatographic analysis of the chromophore, complemented by microscopic observations.     

Localization of Usher 1 proteins to the photoreceptor calyceal processes,which are absent from mice

The mechanisms underlying retinal dystrophy in Usher syndrome type I (USH1) remain unknown because mutant mice lacking any of the USH1 proteins—myosin VIIa, harmonin, cadherin-23, protocadherin-15, sans—do not display retinal degeneration. We found here that, in macaque photoreceptor cells, all USH1 proteins colocalized at membrane interfaces (i) between the inner and outer segments in rods and (ii) between the microvillus-like calyceal processes and the outer segment basolateral region in rods and cones. This pattern, conserved in humans and frogs, was mediated by the formation of an USH1 protein network, which was associated with the calyceal processes from the early embryonic stages of outer segment growth onwards. By contrast, mouse photoreceptors lacked calyceal processes and had no USH1 proteins at the inner–outer segment interface. We suggest that USH1 proteins form an adhesion belt around the basolateral region of the photoreceptor outer segment in humans, and that defects in this structure cause the retinal degeneration in USH1 patients.     

The basolateral targeting signal of CD147 (EMMPRIN) consists of a single leucine and is not recognized by retinal pigment epithelium

CD147, a type I integral membrane protein of the immunoglobulin superfamily, exhibits reversed polarity in retinal pigment epithelium (RPE). CD147 is apical in RPE in contrast to its basolateral localization in extraocular epithelia. This elicited our interest in understanding the basolateral sorting signals of CD147 in prototypic Madin-Darby canine kidney (MDCK) cells. The cytoplasmic domain of CD147 has basolateral sorting information but is devoid of well-characterized basolateral signals, such as tyrosine and di-leucine motifs. Hence, we carried out systematic site-directed mutagenesis to delineate basolateral targeting information in CD147. Our detailed analysis identified a single leucine (252) as the basolateral targeting motif in the cytoplasmic tail of CD147. Four amino acids (243-246) N-terminal to leucine 252 are also critical basolateral determinants of CD147, because deletion of these amino acids leads to mistargeting of CD147 to the apical membranes. We ruled out the involvement of adaptor complex 1B (AP1B) in the basolateral trafficking of CD147, because LLC-PK1 cells lacking AP1B, target CD147 basolaterally. At variance with MDCK cells, the human RPE cell line ARPE-19 does not distinguish between CD147 (WT) and CD147 with leucine 252 mutated to alanine and targets both proteins apically. Thus, our study identifies an atypical basolateral motif of CD147, which comprises a single leucine and is not recognized by RPE cells. This unusual basolateral sorting signal will be useful in unraveling the specialized sorting machinery of RPE cells.     

Immunoassay of rod visual pigment (opsin) in the eyes of rds mutant mice lacking receptor outer segments

In 020/A mice, homozygous for the retinal degeneration slow (rds) gene, the photoreceptor cells fail to develop outer segments, and in the absorption spectra of retinal extracts the rhodopsin peak is lacking. Application of an enzyme-linked immunoassay using antisera against bovine opsin shows, however, that opsin is present in the homozygous mutant retina (0.010 nmol/eye) at 3% of the level of the normal retina (0.38 nmol/eye) of Balb/c mice. In the retina of heterozygous mice the opsin level (0.19 nmol/eye) is about half of the normal. Detection of opsin in the rds mutant retina demonstrates the functional basis for the reported electroretinographic response and light-mediated reduction in cyclic nucleotide levels in this mutant.     

Oxygen distribution and consumption in the macaque retina

The oxygen distribution in the retina of six anesthetized macaques was investigated as a model for retinal oxygenation in the human retina in and adjacent to the fovea. P(O2) was measured as a function of retinal depth under normal physiological conditions in light and dark adaptation with O(2) microelectrodes. Oxygen consumption (Q(O2)) of the photoreceptors was extracted by fitting a steady-state diffusion model to P(O2) measurements. In the perifovea, the P(O2) was 48 +/- 13 mmHg (mean and SD) at the choroid and fell to a minimum of 3.8 +/- 1.9 mmHg around the photoreceptor inner segments in dark adaptation, rising again toward the inner retina. The P(O2) in the inner half of the retina in darkness was 17.9 +/- 7.8 mmHg. When averaged over the outer retina, photoreceptor Q(O2) (called Q(av)) was 4.6 +/- 2.3 ml O(2).100 g(-1).min(-1) under dark-adapted conditions. Illumination sufficient to saturate the rods reduced Q(av) to 72 +/- 11% of the dark-adapted value. Both perifoveal and foveal photoreceptors received most of their O(2) from the choroidal circulation. While foveal photoreceptors have more mitochondria, the Q(O2) of photoreceptors in the fovea was 68% of that in the perifovea. Oxygenation in macaque retina was similar to that previously found in cats and other mammals, reinforcing the relevance of nonprimate animal models for the study of retinal oxygenation, but there was a smaller reduction in Q(O2) with light than observed in cats, which may have implications for understanding the influence of light under some clinical conditions.     

A photoreceptor-specific cadherin is essential for the structural integrity of the outer segment and for photoreceptor survival.

A cadherin family member, prCAD, was identified in retina cDNA by subtractive hybridization and high throughput sequencing. prCAD is expressed only in retinal photoreceptors, and the prCAD protein is localized to the base of the outer segment of both rods and cones. In prCAD(-/-) mice, outer segments are disorganized and fragmented, and there is progressive death of photoreceptor cells. prCAD is unlikely to be involved in protein trafficking between inner and outer segments, since phototransduction proteins appear to be correctly localized and the light responses of both rods and cones are only modestly compromised in prCAD(-/-) mice. These experiments imply a highly specialized cell biological function for prCAD and suggest that localized adhesion activity is essential for outer segment integrity.     

Outer Segment Formation of Transplanted Photoreceptor Precursor Cells

Transplantation of photoreceptor precursor cells (PPCs) into the retina represents a promising treatment for cell replacement in blinding diseases characterized by photoreceptor loss. In preclinical studies, we and others demonstrated that grafted PPCs integrate into the host outer nuclear layer (ONL) and develop into mature photoreceptors. However, a key feature of light detecting photoreceptors, the outer segment (OS) with natively aligned disc membrane staples, has not been studied in detail following transplantation. Therefore, we used as donor cells PPCs isolated from neonatal double transgenic reporter mice in which OSs are selectively labeled by green fluorescent protein while cell bodies are highlighted by red fluorescent protein. PPCs were enriched using CD73-based magnetic associated cell sorting and subsequently transplanted into either adult wild-type or a model of autosomal-dominant retinal degeneration mice. Three weeks post-transplantation, donor photoreceptors were identified based on fluorescent-reporter expression and OS formation was monitored at light and electron microscopy levels. Donor cells that properly integrated into the host wild-type retina developed OSs with the formation of a connecting cilium and well-aligned disc membrane staples similar to the surrounding native cells of the host. Surprisingly, the majority of not-integrated PPCs that remained in the sub-retinal space also generated native-like OSs in wild-type mice and those affected by retinal degeneration. Moreover, they showed an improved photoreceptor maturation and OS formation by comparison to donor cells located on the vitreous side suggesting that environmental cues influence the PPC differentiation and maturation. We conclude that transplanted PPCs, whether integrated or not into the host ONL, are able to generate the cellular structure for effective light detection, a phenomenon observed in wild-type as well as in degenerated retinas. Given that patients suffering from retinitis pigmentosa lose almost all photoreceptors, our findings are of utmost importance for the development of cell-based therapies.     

Hyperoxia causes decreased expression of vascular endothelial growth factor and endothelial cell apoptosis in adult retina

Mice or humans with photoreceptor degenerations experience permeability and dropout of retinal capillaries. Loss of photoreceptors results in decreased oxygen usage and thinning of the retina with increased oxygen delivery to the inner retina. To investigate the possibility that increased tissue oxygen plays a role in the vascular damage, we exposed adult mice to hyperoxia, which also increases oxygen in the retina. After 1, 2, or 3 weeks of hyperoxia, there was a statistically significant decrease in retinal vascular density that was not reversible, and endothelial cell apoptosis was demonstrated by TUNEL staining. Mice exposed to hyperoxia and mice with photoreceptor degeneration both showed decreased expression of VEGF in the retina. After complete or near-complete degeneration of photoreceptors, there was increased expression of VEGF in RPE cells, which may explain the association of photoreceptor degeneration and neovascularization in or around the RPE. Increased expression of VEGF in photoreceptors of transgenic mice failed to prevent hyperoxia-induced retinal capillary dropout. These data suggest that increased oxygen in the retina, either by increased inspired oxygen or by photoreceptor degeneration, results in endothelial cell death and dropout of capillaries. Decreased expression of VEGF may be a contributing factor, but the situation may be more complicated for mature retinal vessels than it is for immature vessels, because VEGF replacement does not rescue mature retinal vessels, suggesting that other factors may also be involved.     

Retinal degeneration progression changes lentiviral vector cell targeting in the retina

In normal mice, the lentiviral vector (LV) is very efficient to target the RPE cells, but transduces retinal neurons well only during development. In the present study, the tropism of LV has been investigated in the degenerating retina of mice, knowing that the retina structure changes during degeneration. We postulated that the viral transduction would be increased by the alteration of the outer limiting membrane (OLM). Two different LV pseudotypes were tested using the VSVG and the Mokola envelopes, as well as two animal models of retinal degeneration: light-damaged Balb-C and Rhodopsin knockout (Rho-/-) mice. After light damage, the OLM is altered and no significant increase of the number of transduced photoreceptors can be obtained with a LV-VSVG-Rhop-GFP vector. In the Rho-/- mice, an alteration of the OLM was also observed, but the possibility of transducing photoreceptors was decreased, probably by ongoing gliosis. The use of a ubiquitous promoter allows better photoreceptor transduction, suggesting that photoreceptor-specific promoter activity changes during late stages of photoreceptor degeneration. However, the number of targeted photoreceptors remains low. In contrast, LV pseudotyped with the Mokola envelope allows a wide dispersion of the vector into the retina (corresponding to the injection bleb) with preferential targeting of Müller cells, a situation which does not occur in the wild-type retina. Mokola-pseudotyped lentiviral vectors may serve to engineer these glial cells to deliver secreted therapeutic factors to a diseased area of the retina.     

Cultured retinal neuronal cells and Müller cells both show net production of lactate

Glucose has long been considered the substrate for energy metabolism in the retina. Recently, an alternative hypothesis (metabolic coupling) suggested that mitochondria in retinal neurons utilize preferentially the lactate produced specifically by Müller cells, the principal glial cell in the retina. These two views of retinal metabolism were examined using confluent cultures of photoreceptor cells, Müller cells, ganglion cells, and retinal pigment epithelial cells incubated in modified Dulbecco's minimal essential medium containing glucose or glucose and lactate. The photoreceptor and ganglion cells represented neural elements, and the Müller and pigment epithelial cells represented non-neural cells. The purpose of the present experiments was two-fold: (1) to determine whether lactate is a metabolic product or substrate in retinal cells, and (2) to examine the evidence that supports the two views of retinal energy metabolism. Measurements were made of lactic acid production, cellular ATP levels, and cellular morphology over 4 h. Results showed that all cell types incubated with 5 mM glucose produced lactate aerobically and anaerobically at linear rates, the anaerobic rate being 2-3-fold higher (Pasteur effect). Cells incubated with both 5 mM glucose and 10 mM lactate produced lactate aerobically and anaerobically at rates similar to those found when cells were incubated with glucose alone. Anaerobic ATP content in the cells was maintained at greater than 50% of the control, aerobic value, and cellular morphology was well preserved under all conditions. The results show that the cultured retinal cells produce lactate, even in the presence of a high starting ambient concentration of lactate. Thus, the net direction of the lactic dehydrogenase reaction is toward lactate formation rather than lactate utilization. It is concluded that retinal cells use glucose, and not glial derived lactate, as their major substrate.     

Endothelin receptors in light-induced retinal degeneration

Excessive light exposure leads to retinal degeneration in albino animals and exacerbates the rate of photoreceptor apoptosis in several retinal diseases. In previous studies we have described the presence of endothelin-1 (ET-1) and its receptors (ET-A and ET-B) in different sites of the mouse retina, including the retinal pigment epithelium, the outer plexiform layer (OPL), astrocytes, the ganglion cell layer (GCL), and vascular endothelia. After light-induced degeneration of photoreceptors, endothelinergic structures disappear from the OPL, but ET-1 and ET-B immunoreactivities increase in astrocytes. Here, we present novel observations about the course of light-induced retinal degeneration in BALB-c mice exposed to 1500 lux during 4 days with or without treatment with tezosentan, a mixed endothelinergic antagonist. Retinal whole mounts were immunostained with anticleaved caspase-3 (CC-3) serum to identify apoptotic photoreceptor cells within the outer nuclear layer (ONL). Glial activation was measured as glial fibrillary acidic protein (GFAP) immunoreactivity in retinal whole mounts and in Western blots from retinal extracts. Tezosentan treatment significantly reduced both the number of CC3-immunoreactive cells and GFAP levels, suggesting that inhibition of endothelinergic receptors could play a role in photoreceptor survival. Using confocal double immunofluorescence, we have observed that ET-A seems to be localized in bipolar cell dendrites, whereas ET-B is localized in horizontal cells. Our observations suggest the existence of an endothelinergic mechanism modulating synaptic transmission in the OPL. This mechanism could perhaps explain the effects of tezosentan treatment on photoreceptor survival.     

Lysophosphatidylcholine acyltransferase 1 controls mitochondrial reactive oxygen species generation and survival of retinal photoreceptor cells

Due to their high energy demands and characteristic morphology, retinal photoreceptor cells require a specialized lipid metabolism for survival and function. Accordingly, dysregulation of lipid metabolism leads to the photoreceptor cell death and retinal degeneration. Mice bearing a frameshift mutation in the gene encoding lysophosphatidylcholine acyltransferase 1 (Lpcat1), which produces saturated phosphatidylcholine (PC) composed of two saturated fatty acids, has been reported to cause spontaneous retinal degeneration in mice; however, the mechanism by which this mutation affects degeneration is unclear. In this study, we performed a detailed characterization of LPCAT1 in the retina and found that genetic deletion of Lpcat1 induces light-independent and photoreceptor-specific apoptosis in mice. Lipidomic analyses of the retina and isolated photoreceptor outer segment (OS) suggested that loss of Lpcat1 not only decreased saturated PC production but also affected membrane lipid composition, presumably by altering saturated fatty acyl-CoA availability. Furthermore, we demonstrated that Lpcat1 deletion led to increased mitochondrial reactive oxygen species levels in photoreceptor cells, but not in other retinal cells, and did not affect the OS structure or trafficking of OS-localized proteins. These results suggest that the LPCAT1-dependent production of saturated PC plays critical roles in photoreceptor maturation. Our findings highlight the therapeutic potential of saturated fatty acid metabolism in photoreceptor cell degeneration–related retinal diseases.     

Loss of MAP3K1 enhances proliferation and apoptosis during retinal development

Precise coordination of progenitor cell proliferation and differentiation is essential for proper organ morphogenesis and function during mammalian development. The mitogen-activated protein kinase kinase kinase 1 (MAP3K1) has a well-established role in anterior eyelid development, as Map3k1-knockout mice have defective embryonic eyelid closure and an `eye-open at birth' (EOB) phenotype. Here, we show that MAP3K1 is highly expressed in the posterior of the developing eye and is required for retina development. The MAP3K1-deficient mice exhibit increased proliferation and apoptosis, and Müller glial cell overproduction in the developing retinas. Consequently, the retinas of these mice show localized rosette-like arrangements in the outer nuclear layer, and develop abnormal vascularization, broken down retinal pigment epithelium, photoreceptor loss and early onset of retinal degeneration. Although the retinal defect is associated with increased cyclin D1 and CDK4/6 expression, and RB phosphorylation and E2F-target gene upregulation, it is independent of the EOB phenotype and of JNK. The retinal developmental defect still occurs in knockout mice that have undergone tarsorrhaphy, but is absent in compound mutant Map3k1(+/ΔKD)Jnk1(-/-) and Map3k1(+/ΔKD)Jnk(+/-)Jnk2(+/-) mice that have EOB and reduced JNK signaling. Our results unveil a novel role for MAP3K1 in which it crosstalks with the cell cycle regulatory pathways in the prevention of retina malformation and degeneration.     

Light-induced oxidation of photoreceptor outer segment phospholipids generates ligands for CD36-mediated phagocytosis by retinal pigment epithelium: a potential mechanism for modulating outer segment phagocytosis under oxidant stress conditions

Clearance by the retinal pigment epithelium (RPE) of shed photoreceptor outer segments (OSs), a tissue with one of the highest turnover rates in the body, is critical to the maintenance and normal function of the retina. We hypothesized that there is a potential role for photo-oxidation in OS uptake by RPE via scavenger receptor-mediated recognition of structurally defined lipid peroxidation products. We now demonstrate that specific structurally defined oxidized species derived from arachidonyl, linoleoyl, and docosahexanoyl phosphatidylcholine may serve as endogenous ligands on OSs for uptake by RPE via the scavenger receptor CD36. Mass spectrometry studies of retinal lipids recovered from dark-adapted rats following physiological light exposure demonstrate in vivo formation of specific oxidized phosphatidylcholine molecular species possessing a CD36 recognition motif, an oxidatively truncated sn-2 acyl group with a terminal gamma-hydroxy(or oxo)-alpha,beta-unsaturated carbonyl. Cellular studies using RPE isolated from wild-type versus CD36 null mice suggest that CD36 plays a role in engulfment, but not initial binding, of OSs via these oxidized phospholipids. Parallel increases in OS protein-bound nitrotyrosine, a post-translational modification by nitric oxide (NO)-derived oxidants, were also observed, suggesting a possible role for light-induced generation of NO-derived oxidants in the initiation of OS lipid peroxidation. Collectively, these studies suggest that intense light exposure promotes "oxidative tagging" of photoreceptor outer segments with structurally defined choline glycerophospholipids that may serve as a physiological signal for CD36-mediated phagocytosis under oxidant stress conditions.     

Calmodulin gene expression in the neural retina of the adult rat

Calmodulin (CaM) mRNAs are expressed with low abundancy in the adult rat neural retina. However, when digoxigenin (DIG)-labeled cRNA probes specific for each CaM mRNA population were hybridized at slightly alkaline pH (pH 8.0), the widespread distribution of CaM mRNA-expressing cells was revealed, with similar abundance for all three CaM genes. The CaM genes displayed a uniquely similar, layer-specific expression throughout the retina, and no significant differences were found in the distribution patterns of the CaM mRNA populations or the labeled cell types. The strongest signal for all CaM mRNAs was demonstrated in the ganglion cell layer and the inner nuclear layer, where the highest signal intensity was found within the inner sublamina. Similarly intermediate signal intensities for all CaM genes were detected in the inner and outer plexiform layers, within the vicinity of the outer limiting membrane and in the retinal pigment epithelium. A very low specific signal was characteristic in the outer nuclear layer and the photoreceptor inner segment layer, while no specific hybridization signal was observed in the photoreceptor outer segment layer. In summary, all CaM genes exhibited a similar and a characteristically layer-specific expression pattern in the adult rat retina.     

Photoreceptor cell death, proliferation and formation of hybrid rod/S-cone photoreceptors in the degenerating STK38L mutant retina

A homozygous mutation in STK38L in dogs impairs the late phase of photoreceptor development, and is followed by photoreceptor cell death (TUNEL) and proliferation (PCNA, PHH3) events that occur independently in different cells between 7-14 weeks of age. During this period, the outer nuclear layer (ONL) cell number is unchanged. The dividing cells are of photoreceptor origin, have rod opsin labeling, and do not label with markers specific for macrophages/microglia (CD18) or Müller cells (glutamine synthetase, PAX6). Nestin labeling is absent from the ONL although it labels the peripheral retina and ciliary marginal zone equally in normals and mutants. Cell proliferation is associated with increased cyclin A1 and LATS1 mRNA expression, but CRX protein expression is unchanged. Coincident with photoreceptor proliferation is a change in the photoreceptor population. Prior to cell death the photoreceptor mosaic is composed of L/M- and S-cones, and rods. After proliferation, both cone types remain, but the majority of rods are now hybrid photoreceptors that express rod opsin and, to a lesser extent, cone S-opsin, and lack NR2E3 expression. The hybrid photoreceptors renew their outer segments diffusely, a characteristic of cones. The results indicate the capacity for terminally differentiated, albeit mutant, photoreceptors to divide with mutations in this novel retinal degeneration gene.     

Intra- and interspecific changes in retinal morphology among mesopelagic and demersal teleosts from the slope waters of New Zealand

Synopsis Retinae from mesopelagic teleosts with adult ranges in the shallow, mid and deep mesopelagic zones, respectively, were examined by light microscopy. Retinal characteristics were described, and photoreceptor densities, outer segment dimensions, and convergence ratios measured from transverse sections. Juveniles of all species had lower photoreceptor densities, outer segment lengths and convergence ratios than adults. In species with multiple banks of photoreceptors, additional banks were added as the retina increased in size. A positive correlation was found between the degree of retinal specialisation for vision in dim light, and the depth of occurrence. The retina of each specimen was given a rank based on log unit changes in photoreceptor density and convergence ratio, the length of photoreceptor outer segments and the presence or absence of multiple banks of photoreceptors. Higher ranks (indicating greater retinal specialisation) were found among species occurring at greater depths. Among species showing a change in depth preference with growth, there was a corresponding increase in retinal rank. It is suggested that the proposed system of ranks has application in predicting the depth of occurrence of a species with a given pattern of retinal morphology.     

Subretinal Transplantation of MACS Purified Photoreceptor Precursor Cells into the Adult Mouse Retina

Vision impairment and blindness due to the loss of the light-sensing cells of the retina, i.e. photoreceptors, represents the main reason for disability in industrialized countries. Replacement of degenerated photoreceptors by cell transplantation represents a possible treatment option in future clinical applications. Indeed, recent preclinical studies demonstrated that immature photoreceptors, isolated from the neonatal mouse retina at postnatal day 4, have the potential to integrate into the adult mouse retina following subretinal transplantation. Donor cells generated a mature photoreceptor morphology including inner and outer segments, a round cell body located at the outer nuclear layer, and synaptic terminals in close proximity to endogenous bipolar cells. Indeed, recent reports demonstrated that donor photoreceptors functionally integrate into the neural circuitry of host mice. For a future clinical application of such cell replacement approach, purified suspensions of the cells of choice have to be generated and placed at the correct position for proper integration into the eye. For the enrichment of photoreceptor precursors, sorting should be based on specific cell surface antigens to avoid genetic reporter modification of donor cells. Here we show magnetic-associated cell sorting (MACS) - enrichment of transplantable rod photoreceptor precursors isolated from the neonatal retina of photoreceptor-specific reporter mice based on the cell surface marker CD73. Incubation with anti-CD73 antibodies followed by micro-bead conjugated secondary antibodies allowed the enrichment of rod photoreceptor precursors by MACS to approximately 90%. In comparison to flow cytometry, MACS has the advantage that it can be easier applied to GMP standards and that high amounts of cells can be sorted in relative short time periods. Injection of enriched cell suspensions into the subretinal space of adult wild-type mice resulted in a 3-fold higher integration rate compared to unsorted cell suspensions.     

PARP1 gene knock-out increases resistance to retinal degeneration without affecting retinal function

Retinitis pigmentosa (RP) is a group of inherited neurodegenerative diseases affecting photoreceptors and causing blindness in humans. Previously, excessive activation of enzymes belonging to the poly-ADP-ribose polymerase (PARP) group was shown to be involved in photoreceptor degeneration in the human homologous rd1 mouse model for RP. Since there are at least 16 different PARP isoforms, we investigated the exact relevance of the predominant isoform - PARP1 - for photoreceptor cell death using PARP1 knock-out (KO) mice. In vivo and ex vivo morphological analysis using optic coherence tomography (OCT) and conventional histology revealed no major alterations of retinal phenotype when compared to wild-type (wt). Likewise, retinal function as assessed by electroretinography (ERG) was normal in PARP1 KO animals. We then used retinal explant cultures derived from wt, rd1, and PARP1 KO animals to test their susceptibility to chemically induced photoreceptor degeneration. Since photoreceptor degeneration in the rd1 retina is triggered by a loss-of-function in phosphodiesterase-6 (PDE6), we used selective PDE6 inhibition to emulate the rd1 situation on non-rd1 genotypes. While wt retina subjected to PDE6 inhibition showed massive photoreceptor degeneration comparable to rd1 retina, in the PARP1 KO situation, cell death was robustly reduced. Together, these findings demonstrate that PARP1 activity is in principle dispensable for normal retinal function, but is of major importance for photoreceptor degeneration under pathological conditions. Moreover, our results suggest that PARP dependent cell death or PARthanatos may play a major role in retinal degeneration and highlight the possibility to use specific PARP inhibitors for the treatment of RP.