Cell death in retinitis pigmentosa: gap junctions and the 'bystander' effect

Electrophysiological and Histological Characterization of Rod-Cone Retinal Degeneration and Microglia Activation in a Mouse Model of Mucopolysaccharidosis Type IIIB

Sanfilippo syndrome Type B or Mucopolysaccharidosis IIIB (MPS IIIB) is a neurodegenerative autosomal recessive lysosomal storage disorder in which patients suffer severe vision loss from associated retinopathy. Here we sought to study the underlying retinal functional and morphological changes associated with MPS IIIB disease progression using the established model of MPS IIIB, the B6.129S6-Naglu(tm1Efn)/J mouse line. Electroretinogram (ERG) was recorded from MPS IIIB and wild-type (WT) mice at the age of 28 and 46 weeks, and retinal tissues were subsequently collected for immunohistochemistry analysis. At the 28th week, rod a- and b-wave amplitudes were significantly diminished in MPS IIIB compared to WT mice. The cone a- and b-waves of MPS IIIB mice were not significantly different from those of the control at the 28th week but were significantly diminished at the 46 th week, when MPS IIIB mice showed a major loss of rods and rod bipolar cells in both central and peripheral regions and a minor loss of cones in the periphery. Activation of microglia and neovascularization were also detected in the MPS IIIB retina. The new findings that cones and rod bipolar cells also undergo degeneration, and that retinal microglia are activated, will inform future development of therapeutic strategies.

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.

TIMP-1 affects the spatial distribution of dendritic processes of second-order neurons in a rat model of Retinitis Pigmentosa

Retinitis Pigmentosa (RP) is an inherited disorder that may lead to blindness. In the rhodopsin S334ter-line-3 rat model of RP, the death of rods induces spatial rearrangement of cones into regular ring mosaics. Using this model, we discovered that the ring mosaics are restored to a homogeneous distribution upon application of tissue inhibitor of metalloproteinase-1 (TIMP-1). In this study, we further investigated the cone migration and spatial distribution of second-order neurons and their connections to cones in the presence or absence of TIMP-1 using immunohistochemistry to identify retinal neurons and their connections with cones. M-opsin cell bodies and their outer segments were evaluated to determine whether TIMP-1 delays the degeneration of outer segments of cones. We observed that during cone rearrangement into ring mosaics in RP retina, dendritic processes of second-order neurons undergo remodeling to maintain their synaptic connections with the cones in the rings. TIMP-1 treatment induced the cones to rearrange and dendritic processes of second-order neurons to return to a more homogeneous spatial distribution. In addition, TIMP-1 treatment protected the outer segments of cones at later stages of retinal degeneration. Our findings clearly demonstrate that despite their dramatic spatial rearrangement, cones and second-order neuron processes maintain their synaptic connections before and after TIMP-1 treatment.

Microglial Activation Promotes Cell Survival in Organotypic Cultures of Postnatal Mouse Retinal Explants

The role of microglia during neurodegeneration remains controversial. We investigated whether microglial cells have a neurotoxic or neuroprotective function in the retina. Retinal explants from 10-day-old mice were treated in vitro with minocycline to inhibit microglial activation, with LPS to increase microglial activation, or with liposomes loaded with clodronate (Lip-Clo) to deplete microglial cells. Flow cytometry was used to assess the viability of retinal cells in the explants and the TUNEL method to show the distribution of dead cells. The immunophenotypic and morphological features of microglia and their distribution were analyzed with flow cytometry and immunocytochemistry. Treatment of retinal explants with minocycline reduced microglial activation and simultaneously significantly decreased cell viability and increased the presence of TUNEL-labeled cell profiles. This treatment also prevented the migration of microglial cells towards the outer nuclear layer, where cell death was most abundant. The LPS treatment increased microglial activation but had no effect on cell viability or microglial distribution. Finally, partial microglial removal with Lip-Clo diminished the cell viability in the retinal explants, showing a similar effect to that of minocycline. Hence, cell viability is diminished in retinal explants cultured in vitro when microglial cells are removed or their activation is inhibited, indicating a neurotrophic role for microglia in this system.

The p110α isoform of phosphoinositide 3-kinase is essential for cone photoreceptor survival

Phosphoinositide 3-kinases (PI3Ks) are a family of lipid kinases that phosphorylates the 3'OH of the inositol ring of phosphoinositides (PIs). They are responsible for coordinating a diverse range of cellular functions. Class IA PI3K is a heterodimeric protein composed of a regulatory p85 and a catalytic p110 subunit. In this study, we conditionally deleted the p110α-subunit of PI3K in cone photoreceptor cells using the Cre-loxP system. Cone photoreceptors allow for color vision in bright light (daylight vision). Cone-specific deletion of p110α resulted in cone degeneration. Our studies suggest that PI3K signaling is essential for cone photoreceptor functions.

Gap junction-mediated death of retinal neurons is connexin and insult specific: a potential target for neuroprotection

Secondary cell death via gap junctions (GJs) plays a role in the propagation of neuronal loss under a number of degenerative disorders. Here, we examined the role of GJs in neuronal death in the retina, which has arguably the most diverse expression of GJs in the CNS. Initially, we induced apoptotic death by injecting single retinal ganglion cells and glia with cytochrome C and found that this resulted in the loss of neighboring cells to which they were coupled via GJs. We next found that pharmacological blockade of GJs eradicated nearly all amacrine cell loss and reduced retinal ganglion cell loss by ∼70% after induction of either excitotoxic or ischemic insult conditions. These data indicate that the GJ-mediated secondary cell death was responsible for the death of most cells. Whereas genetic deletion of the GJ subunit Cx36 increased cell survivability by ∼50% under excitotoxic condition, cell loss in Cx45 knock-out mouse retinas was similar to that seen in wild-type mice. In contrast, ablation of Cx45 reduced neuronal loss by ∼50% under ischemic insult, but ablation of Cx36 offered no protection. Immunolabeling of the connexins showed differential changes in protein expression consistent with their differing roles in propagating death signals under the two insults. These data indicate that secondary cell death is mediated by different cohorts of GJs dependent on the connexins they express and the type of initial insult. Our results suggest that targeting specific connexins offers a novel therapeutic strategy to reduce progressive cell loss under different neurodegenerative conditions.

Microglial cells in organotypic cultures of developing and adult mouse retina and their relationship with cell death

Organotypic cultures of retinal explants allow the detailed analysis of microglial cells in a cellular microenvironment similar to that in the in situ retina, with the advantage of easy experimental manipulation. However, the in vitro culture causes changes in the retinal cytoarchitecture and induces a microglial response that may influence the results of these manipulations. The purpose of this study was to analyze the influence of the retinal age on changes in retinal cytoarchitecture, cell viability and death, and microglial phenotype and distribution throughout the in vitro culture of developing and adult retina explants. Explants from developing (3 and 10 postnatal days, P3 and P10) and adult (P60) mouse retinas were cultured for up to 10 days in vitro (div). Dead or dying cells were recognized by TUNEL staining, cell viability was determined by flow cytometry, and the numbers and distribution patterns of microglial cells were studied by flow cytometry and immunocytochemistry, respectively. The retinal cytoarchitecture was better preserved at prolonged culture times (10 div) in P10 retina explants than in P3 or adult explants. Particular patterns of cell viability and death were observed at each age: in general, explants from developing retinas showed higher cell viability and lower density of TUNEL-positive profiles versus adult retinas. The proportion of microglial cells relative to the whole population of retinal cells was higher in explants fixed immediately after their dissection (i.e., non-cultured) from adult retinas than in those from developing retinas. This proportion was always higher in non-cultured explants than in explants at 10 div, suggesting the death of some microglial cells during the culture. Activation of microglial cells, as revealed by their phenotypical appearance, was observed in both developing and adult retina explants from the beginning of the culture. Immunofluorescence with the anti-CD68 antibody showed that some activated microglial cells were CD68-positive but others were CD68-negative. Flow cytometry using CD68-labeling revealed that the percentage of CD68-positive microglial cells was much higher in developing than in adult retina explants, despite the activation of microglia in both types of explants, indicating that CD68-labeling was more closely related to the maturity degree of microglia than to their activation. Some swollen activated microglial cells entered the outer nuclear layer in developing and adult cultured retinal explants, whereas this layer was devoid of microglia in non-cultured explants. There was no apparent correlation between the distribution of microglia and that of TUNEL-labeled profiles. However, some swollen activated microglial cells in the outer and inner nuclear layers engulfed clusters of cell nuclei that were negative or weakly positive for TUNEL. This engulfment activity of microglia mimicked that observed in degenerative pathologies of the retina. We conclude that organotypic cultures from developing retinas show a higher rate of cell viability and better preservation of the normal cytoarchitecture in comparison to those obtained from adult retinas. In addition, the features of microglial response in cultured retinal explants show them to be a useful model for studying interactions between microglial cells and degenerating neurons in retinal diseases.

Neuroprotective effects of the cannabinoid agonist HU210 on retinal degeneration

Cannabinoids have been demonstrated to exert neuroprotective effects on different types of neuronal insults. Here we have addressed the therapeutic potential of the synthetic cannabinoid HU210 on photoreceptor degeneration, synaptic connectivity and functional activity of the retina in the transgenic P23H rat, an animal model for autosomal dominant retinitis pigmentosa (RP). In P23H rats administered with HU210 (100 μg/kg, i.p.) from P24 to P90, ERG recordings showed an amelioration of vision loss, as compared to vehicle-administered animals. Under scotopic conditions, the maximum a-wave amplitudes recorded at P60 and P90 were higher in HU210-treated animals, as compared to the values obtained in untreated animals. The scotopic b-waves were significantly higher in treated animals than in untreated rats at P30, P60 and P90. This attenuation of visual deterioration correlated with a delay in photoreceptor degeneration and the preservation of retinal cytoarchitecture. HU210-treated animals had 40% more photoreceptors than untreated animals. Presynaptic and postsynaptic elements, as well as the synaptic contacts between photoreceptors and bipolar or horizontal cells, were also preserved in HU210-treated P23H rats. These results indicate that HU210 preserves cone and rod structure and function, together with their contacts with postsynaptic neurons, in P23H rats. These data suggest that cannabinoids are potentially useful to delay retinal degeneration in RP patients.

Rip3 knockdown rescues photoreceptor cell death in blind pde6c zebrafish

Achromatopsia is a progressive autosomal recessive retinal disease characterized by early loss of cone photoreceptors and later rod photoreceptor loss. In most cases, mutations have been identified in CNGA3, CNGB3, GNAT2, PDE6C or PDE6H genes. Owing to this genetic heterogeneity, mutation-independent therapeutic schemes aimed at preventing cone cell death are very attractive treatment strategies. In pde6c(w59) mutant zebrafish, cone photoreceptors expressed high levels of receptor-interacting protein kinase 1 (RIP1) and receptor-interacting protein kinase 3 (RIP3) kinases, key regulators of necroptotic cell death. In contrast, rod photoreceptor cells were alternatively immunopositive for caspase-3 indicating activation of caspase-dependent apoptosis in these cells. Morpholino gene knockdown of rip3 in pde6c(w59) embryos rescued the dying cone photoreceptors by inhibiting the formation of reactive oxygen species and by inhibiting second-order neuron remodelling in the inner retina. In rip3 morphant larvae, visual function was restored in the cones by upregulation of the rod phosphodiesterase genes (pde6a and pde6b), compensating for the lack of cone pde6c suggesting that cones are able to adapt to their local environment. Furthermore, we demonstrated through pharmacological inhibition of RIP1 and RIP3 activity that cone cell death was also delayed. Collectively, these results demonstrate that the underlying mechanism of cone cell death in the pde6c(w59) mutant retina is through necroptosis, whereas rod photoreceptor bystander death occurs through a caspase-dependent mechanism. This suggests that targeting the RIP kinase signalling pathway could be an effective therapeutic intervention in retinal degeneration patients. As bystander cell death is an important feature of many retinal diseases, combinatorial approaches targeting different cell death pathways may evolve as an important general principle in treatment.

Distinct and atypical intrinsic and extrinsic cell death pathways between photoreceptor cell types upon specific ablation of Ranbp2 in cone photoreceptors

Non-autonomous cell-death is a cardinal feature of the disintegration of neural networks in neurodegenerative diseases, but the molecular bases of this process are poorly understood. The neural retina comprises a mosaic of rod and cone photoreceptors. Cone and rod photoreceptors degenerate upon rod-specific expression of heterogeneous mutations in functionally distinct genes, whereas cone-specific mutations are thought to cause only cone demise. Here we show that conditional ablation in cone photoreceptors of Ran-binding protein-2 (Ranbp2), a cell context-dependent pleiotropic protein linked to neuroprotection, familial necrotic encephalopathies, acute transverse myelitis and tumor-suppression, promotes early electrophysiological deficits, subcellular erosive destruction and non-apoptotic death of cones, whereas rod photoreceptors undergo cone-dependent non-autonomous apoptosis. Cone-specific Ranbp2 ablation causes the temporal activation of a cone-intrinsic molecular cascade highlighted by the early activation of metalloproteinase 11/stromelysin-3 and up-regulation of Crx and CoREST, followed by the down-modulation of cone-specific phototransduction genes, transient up-regulation of regulatory/survival genes and activation of caspase-7 without apoptosis. Conversely, PARP1⁺-apoptotic rods develop upon sequential activation of caspase-9 and caspase-3 and loss of membrane permeability. Rod photoreceptor demise ceases upon cone degeneration. These findings reveal novel roles of Ranbp2 in the modulation of intrinsic and extrinsic cell death mechanisms and pathways. They also unveil a novel spatiotemporal paradigm of progression of neurodegeneration upon cell-specific genetic damage whereby a cone to rod non-autonomous death pathway with intrinsically distinct cell-type death manifestations is triggered by cell-specific loss of Ranbp2. Finally, this study casts new light onto cell-death mechanisms that may be shared by human dystrophies with distinct retinal spatial signatures as well as with other etiologically distinct neurodegenerative disorders.

The secondary demise of healthy neurons upon the degeneration of neurons harboring primary genetic defect(s) is hallmark to neurodegenerative diseases. However, the factors and mechanisms driving these cell-death processes are not understood, a severe limitation which has hampered the therapeutic development of neuroprotective approaches. The neuroretina is comprised of two main types of photoreceptor neurons, rods and cones. These undergo degeneration upon heterogeneous mutations or environmental stressors and the underlying diseases present conspicuous spatiotemporal pathological signatures whose molecular bases are not understood. We employed the multifunctional protein, Ran-binding protein-2 (Ranbp2), which is implicated in cell-type and stress-dependent clinical manifestations, to examine its role(s) in primary and secondary photoreceptor death mechanisms upon its specific loss in cones. Contrary to prior findings, we found that dying cones can trigger the loss of healthy rods. This process arises by the immediate activation of novel Ranbp2-responsive factors and downstream cascade events in cones that promote extrinsically the demise of rods. The mechanisms of rod and cone demise are molecularly distinct. Collectively, the data uncover distinct Ranbp2 roles in intrinsic and extrinsic cell-death and will likely contribute to our understanding of the spatiotemporal onset and progression of diseases affecting photoreceptor mosaics and other neural networks.

Gap junctional communication promotes apoptosis in a connexin-type-dependent manner

Gap junctions (GJs) have been described to modulate cell death and survival. It still remains unclear whether this effect requires functional GJ channels or depends on channel-independent effects of connexins (Cx), the constituents of GJs. Therefore, we analysed the apoptotic response to streptonigrin (SN, intrinsic apoptotic pathway) or to α-Fas (extrinsic apoptotic pathway) in HeLa cells expressing Cx43 as compared with empty vector-transfected (CTL) cells. Apoptosis assessed by annexin V-fluorescein isothiocyanate/propidium iodide staining was significantly higher in HeLa-Cx43 compared with HeLa-CTL cells. Moreover, the cleavage of caspase-7 or Parp occurred earlier in HeLa-Cx43 than in HeLa-CTL cells. Comparative analysis of the effect of two further (endothelial) Cx (Cx37 and Cx40) on apoptosis revealed that apoptosis was highest in HeLa-Cx43 and lowest in HeLa-Cx37 cells, and correlated with the GJ permeability (assessed by spreading of a GJ-permeable dye and locally induced Ca(2+) signals). Pharmacologic inhibition of GJ formation in HeLa-Cx43 cells reduced apoptosis significantly. The role of GJ communication was further analysed by the expression of truncated Cx43 proteins with and without channel-forming capacity. Activation of caspases was higher in cells expressing the channel-building part (HeLa-Cx43NT-GFP) than in cells expressing the channel-incompetent C-terminal part of Cx43 (HeLa-Cx43CT-GFP) only. A hemichannel-dependent release and, hence, paracrine effect of proapoptotic signals could be excluded since the addition of a peptide (Pep)-blocking Cx43-dependent hemichannels (but not GJs) did not reduce apoptosis in HeLa-Cx43 cells. Treatment with SN resulted in a significant higher increase of the intracellular free Ca(2+) concentration in HeLa-Cx43 and HeLa-Cx43NT-GFP cells compared with HeLa-CTL or HeLa-Cx43CT-GFP cells, suggesting that Ca(2+) or a Ca(2+)-releasing agent could play a signalling role. Blocking of inositol triphosphate receptors reduced the SN-induced Ca(2+) increase as well as the increase in apoptosis. Our observations suggest that Cx43 and Cx40 but not Cx37 promote apoptosis via gap junctional transfer of pro-apoptotic signals between cells.

Retinal morphological and functional changes in an animal model of retinitis pigmentosa

The P23H-1 transgenic rat carries a mutated mouse opsin gene, in addition to endogenous opsin genes, and undergoes progressive photoreceptor loss that is generally characteristic of human autosomal dominant retinitis pigmentosa (RP). Here, we examined morphological changes correlated with visual function that is comparable to clinical application in the pigmented P23H-1 rat retina as photoreceptor degeneration progressed. We found that rod function was compromised as early as postnatal day 28 and was a good indicator for tracking retinal degeneration. Cone function was normal and did not change until the thickness of the photoreceptor layer was reduced by 75%. Similar to the threshold versus intensity curves used to evaluate vision of RP patients, light-adaptation curves showed that cone thresholds depended on the number of remaining functioning cones, but not on its length of outer segments (OS). By 1 year of age, both rod and cone functions were significantly compromised. Correlating with early abnormal rod function, rods and related secondary neurons also underwent progressive degeneration, including shortening of inner and OS of photoreceptors, loss of rod bipolar and horizontal cell dendrites, thickening of the outer Müller cell processes, and reduced density of pre- and postsynaptic markers. Similar early morphological modifications were also observed in cones and their related secondary neurons. However, cone function was maintained at nearly normal level for a long period. The dramatic loss of rods at late stage of degeneration may contribute to the dysfunction of cones. Attention has to be focused on preserving cone function and identifying factors that damage cones when therapeutic regimes are applied to treat retinal degeneration. As such, these findings provide a foundation for future studies involving treatments to counter photoreceptor loss.

Testing for a gap junction-mediated bystander effect in retinitis pigmentosa: secondary cone death is not altered by deletion of connexin36 from cones

Retinitis pigmentosa (RP) relates to a group of hereditary neurodegenerative diseases of the retina. On the cellular level, RP results in the primary death of rod photoreceptors, caused by rod-specific mutations, followed by a secondary degeneration of genetically normal cones. Different mechanisms may influence the spread of cell death from one photoreceptor type to the other. As one of these mechanisms a gap junction-mediated bystander effect was proposed, i.e., toxic molecules generated in dying rods and propagating through gap junctions induce the death of healthy cone photoreceptors. We investigated whether disruption of rod-cone coupling can prevent secondary cone death and reduce the spread of degeneration. We tested this hypothesis in two different mouse models for retinal degeneration (rhodopsin knockout and rd1) by crossbreeding them with connexin36-deficient mice as connexin36 represents the gap junction protein on the cone side and lack thereof most likely disrupts rod-cone coupling. Using immunohistochemistry, we compared the progress of cone degeneration between connexin36-deficient mouse mutants and their connexin36-expressing littermates at different ages and assessed the accompanied morphological changes during the onset (rhodopsin knockout) and later stages of secondary cone death (rd1 mutants). Connexin36-deficient mouse mutants showed the same time course of cone degeneration and the same morphological changes in second order neurons as their connexin36-expressing littermates. Thus, our results indicate that disruption of connexin36-mediated rod-cone coupling does not stop, delay or spatially restrict secondary cone degeneration and suggest that the gap junction-mediated bystander effect does not contribute to the progression of RP.

Gap junctions and Bystander Effects: Good Samaritans and executioners

The "Bystander" and "Good Samaritan" effects involve the transfer of toxic or beneficial compounds from one cell to a generally adjacent other through gap junction channels and through extracellular routes. The variety of injuries in which bystander cell killing or protection occurs has greatly expanded in the last decade to include infectious agents and therapeutic compounds, radiation injury, chaperones in cell therapy and apoptosis in development. This has been accompanied by the appreciation that both gap junction mediated and paracrine routes are used for the signaling of the "kiss of life" and the "kiss of death" and that manipulations of these pathways and the molecules that use them may find therapeutic utility in treatment of a variety of pathological conditions.

Fas ligand-Fas signaling participates in light-induced apoptotic death in photoreceptor cells

PURPOSE

We investigated the function of Fas in photoreceptors.

METHODS

Postmortem human eyes and mouse-derived photoreceptor cells (661W) were examined for Fas expression by in situ hybridization and immunofluorescence. 661W cells were treated with FasL or Fas agonistic antibody, or exposed to light with/without pharmacological manipulation of Fas signaling, followed by apoptosis detection by TUNEL, immunofluorescence and fluorescence activated cell scanning (FACS). Fractionated cellular extracts were used to detect protein expression or protein phosphorylation after immunoprecipitation by Western blot.

RESULTS

Fas was expressed in the photoreceptor layer of human retina. Fas and a cleaved form of FasL were found on the cell surface of 661W cells. Treatment with FasL or Fas agonistic antibody induced apoptosis in 661W cells. Blocking the activity of FasL or administration of caspase-8 inhibitor z-IETD inhibited light-induced apoptosis. However, it simultaneously caused induction of necroptosis, which could be blocked by the receptor-interacting protein 1 (RIP1) inhibitor, necrostatin-1. Light exposure in the presence of z-IETD caused hyper-phosphorylation of RIP1. Light exposure did not elevate the expression of Fas, FasL, or the Fas-associated death domain adaptor protein (FADD). Cells or conditioned medium after light exposure induced apoptosis in dark-adapted cells, which could be attenuated by blockade of Fas.

CONCLUSIONS

Fas has a pro-apoptotic role in photoreceptors. Under light stress, soluble and membrane-bound FasL can bind to Fas, inducing apoptosis via a paracrine mechanism. Although blocking Fas signaling inhibits apoptosis, it does not improve the overall photoreceptor survival due to a compensatory activation of necroptosis. Hence, prevention of photoreceptor loss from retinal photo-oxidative stress should target Fas and RIP1.

Rearrangement of the cone mosaic in the retina of the rat model of retinitis pigmentosa

In retinitis pigmentosa (RP), the death of cones normally follows some time after the degeneration of rods. Recently, surviving cones in RP have been studied and reported in detail. These cones undergo extensive remodeling in their morphology. Here we report an extension of the remodeling study to consider possible modifications of spatial-distribution patterns. For this purpose we used S334ter-line-3 transgenic rats, a transgenic model developed to express a rhodopsin mutation causing RP. In this study, retinas were collected at postnatal (P) days P5-30, 90, 180, and P600. We then immunostained the retinas to examine the morphology and distribution of cones and to quantify the total cone numbers. Our results indicate that cones undergo extensive changes in their spatial distribution to give rise to a mosaic comprising an orderly array of rings. These rings first begin to appear at P15 at random regions of the retina and become ubiquitous throughout the entire tissue by P90. Such distribution pattern loses its clarity by P180 and mostly disappears at P600, at which time the cones are almost all dead. In contrast, the numbers of cones in RP and normal conditions do not show significant differences at stages as late as P180. Therefore, rings do not form by cell death at their centers, but by cone migration. We discuss its possible mechanisms and suggest a role for hot spots of rod death and the remodeling of Müller cell process into zones of low density of photoreceptors.

Loss of daylight vision in retinal degeneration: are oxidative stress and metabolic dysregulation to blame?

Retinitis pigmentosa is characterized by loss of night vision, followed by complete blindness. Over 40 genetic loci for retinitis pigmentosa have been identified in humans, primarily affecting photoreceptor structure and function. The availability of excellent animal models allows for a mechanistic characterization of the disease. Metabolic dysregulation and oxidative stress have been found to correlate with the loss of vision, particularly in cones, the type of photoreceptors that mediate daylight and color vision. The evidence that these problems actually cause loss of vision and potential therapeutic approaches targeting them are discussed.

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.

Changes in physiological properties of rat ganglion cells during retinal degeneration

Retinitis pigmentosa (RP) is a leading cause of degenerative vision loss, yet its progressive effects on visual signals transmitted from the retina to the brain are not well understood. The transgenic P23H rat is a valuable model of human autosomal dominant RP, exhibiting extensive similarities to the human disease pathology, time course, and electrophysiology. In this study, we examined the physiological effects of degeneration in retinal ganglion cells (RGCs) of P23H rats aged between P37 and P752, and compared them with data from wild-type control animals. The strength and the size of visual receptive fields of RGCs decreased rapidly with age in P23H retinas. Light responses mediated by rod photoreceptors declined earlier (∼ P300) than cone-mediated light responses (∼ P600). Responses of ON and OFF RGCs diminished at a similar rate. However, OFF cells exhibited hyperactivity during degeneration, whereas ON cells showed a decrease in firing rate. The application of synaptic blockers abolished about half of the elevated firing in OFF RGCs, indicating that the remodeled circuitry was not the only source of degeneration-induced hyperactivity. These results advance our understanding of the functional changes associated with retinal degeneration.

The genomic, biochemical, and cellular responses of the retina in inherited photoreceptor degenerations and prospects for the treatment of these disorders

The association of more than 140 genes with human photoreceptor degenerations, together with studies of animal models of these monogenic diseases, has provided great insight into their pathogenesis. Here we review the responses of the retina to photoreceptor mutations, including mechanisms of photoreceptor death. We discuss the roles of oxidative metabolism, mitochondrial reactive oxygen species, metabolic stress, protein misfolding, and defects in ciliary proteins, as well as the responses of Müller glia, microglia, and the retinal vasculature. Finally, we report on potential pharmacologic and biologic therapies, the critical role of histopathology as a prerequisite to treatment, and the exciting promise of gene therapy in animal models and in phase 1 trials in humans.

Cone degeneration following rod ablation in a reversible model of retinal degeneration

PURPOSE

Amphibian retinas regenerate after injury, making them ideal for studying the mechanisms of retinal regeneration, but this leaves their value as models of retinal degeneration in question. The authors asked whether the initial cellular changes after rod loss in the regenerative model Xenopus laevis mimic those observed in nonregenerative models. They also asked whether rod loss was reversible.

METHODS

The authors generated transgenic X. laevis expressing the Escherichia coli enzyme nitroreductase (NTR) under the control of the rod-specific rhodopsin (XOP) promoter. NTR converts the antibiotic metronidazole (Mtz) into an interstrand DNA cross-linker. A visually mediated behavioral assay and immunohistochemistry were used to determine the effects of Mtz on the vision and retinas of XOPNTR F1 tadpoles.

RESULTS

NTR expression was detected only in the rods of XOPNTR tadpoles. Mtz treatment resulted in rapid vision loss and near complete ablation of rod photoreceptors by day 12. Müller glial cell hypertrophy and progressive cone degeneration followed rod cell ablation. When animals were allowed to recover, new rods were born and formed outer segments.

CONCLUSIONS

The initial secondary cellular changes detected in the rodless tadpole retina mimic those observed in other models of retinal degeneration. The rapid and synchronous rod loss in XOPNTR animals suggested this model may prove useful in the study of retinal degeneration. Moreover, the regenerative capacity of the Xenopus retina makes these animals a valuable tool for identifying the cellular and molecular mechanisms at work in lower vertebrates with the remarkable capacity of retinal regeneration.

A mathematical model for photoreceptor interactions

The interactions between rods and cones in the retina have been the focus of innumerable experimental and theoretical biological studies in previous decades yet the understanding of these interactions is still incomplete primarily due to the lack of a unified concept of cone photoreceptor organization and its role in retinal diseases. The low abundance of cones in many of the non-primate mammalian models that have been studied make conclusions about the human retina difficult. A more complete knowledge of the human retina is crucial for counteracting the events that lead to certain degenerative diseases, in particular those associated with photoreceptor cell death (e.g., retinitis pigmentosa). In an attempt to gain important insight into the role and interactions of the rods and the cones we develop and analyze a set of mathematical equations that model a system of photoreceptors and incorporate a direct rod-cone interaction. Our results show that the system can exhibit stable oscillations, which correspond to the rhythmic renewal and shedding of the photoreceptors. In addition, our results show the mathematical necessity of this rod-cone direct interaction for survival of both and gives insight into this mechanism.

Mechanisms of photoreceptor death during retinal degeneration

The development of treatments that slow photoreceptor death could profoundly improve patient wellbeing in those with inherited retinal degenerations. Over recent years, it has emerged that extracellular adenosine-tri-phosphate (ATP) regulates the function of photoreceptors in rodents and primates. Moreover, when the retina is exposed to high levels of ATP, rapid death of photoreceptors occurs, which can be blocked by pretreatment with antagonists to P2X receptors. Compounds that inhibit the action of extracellular ATP slow photoreceptor loss in an animal model of inherited retinal degeneration. In this article, I provide an overview of our work in relation to other research in this area and suggest a model by which ATP contributes to photoreceptor death in inherited retinal degenerations.

A stress survival response in retinal cells mediated through inhibition of the serine/threonine phosphatase PP2A

Cell survival signalling involving the PI3K/Akt survival pathway can be negatively regulated by several phosphatases including PP2A. When retinal-derived 661W cells were subjected to trophic factor deprivation this initiated a survival response through inhibition of the activity of PP2A and subsequent upregulation of the Erk and Akt survival pathways. We show this survival response via inhibition of PP2A activity was due in part to increased reactive oxygen species production when retinal cells were deprived of trophic factors. Inhibition of PP2A activity was mediated by a rapid and transient increase in phosphorylation at Tyr307, accompanied by an increase in demethylation and a decrease in the methylated form. Pre-treatment with N-acetyl-L-cysteine, which is involved in scavenging reactive oxygen species, prevented PP2A inhibition and subsequent upregulation of survival pathways. Pre-treatment with the Src family kinase inhibitor PP2 resulted in approximately 50% reduction in cellular levels of phospho-PP2A in trophic factor-deprived 661W cells, suggesting an Src tyrosine kinase had a role to play in this redox regulation of cell survival. We observed similar events in the rd10 mouse retina where there was an increased survival response prior to retinal cell death mediated through an increase in both phospho-PP2A and phospho-Gsk. Together, these results demonstrate that when retinal cells are stressed there is an initial struggle to survive, mediated through inhibition of PP2A and subsequent upregulation of survival pathways, and that these events occur simultaneously with production of reactive oxygen species, thus suggesting an important cell-signalling role for reactive oxygen species.

Effect of para-aminobenzoic acid on the course of retinal degeneration in the rd10 mouse

PURPOSE

Recent evidence suggests that oxidative injury plays a significant role in the pathogenesis of retinal degenerative diseases. Para-aminobenzoic acid (PABA) is a cyclic amino acid, which may act to decrease lipid peroxidation and oxidative injury. Our aim was to evaluate the efficacy of PABA in attenuating oxidative injury and rate of retinal degeneration in the rd10 mouse.

METHODS

PABA (50 mg/kg) was administered intraperitoneally six times per week in 28 rd10 mice from postnatal day 3. Twenty-four littermate control mice were similarly injected with saline. At 3, 4.5, and 6 weeks of age, electrophysiological (full field electroretinogram-ERG), quantitative histological, and immunohistochemical techniques were used to assess the course and extent of retinal degeneration. Degree of lipid peroxidation was determined by the measurement of thiobarbituric acid reactive species (TBARS) and retinal carbonyl content was quantified using the 2,4-dinitrophenylhydrazine method.

RESULTS

Dark adapted mixed rod-cone ERG responses at 3 weeks of age were higher in the PABA-treated group as compared to saline control (P < 0.05). By 4.5 weeks, this protective effect was largely abolished and by 6 weeks ERG was unrecordable in both groups. However, at both 3 and 4.5 weeks of age, light-adapted cone ERG amplitudes were better preserved in PABA-treated animals. At 4.5 weeks, thickness of the outer nuclear layer was 28.6% higher in the peripheral retina of PABA-treated mice as compared to controls (P < 0.05). Quantitative immunohistochemistry revealed 2.4-fold higher red/green cone opsin content in the retinas of PABA-treated mice (P < 0.005). At both 3 and 4.5 weeks, levels of TBARS and protein carbonyls were 49%-69% lower in PABA-treated retinas (P < 0.05-0.0005), suggesting less oxidative injury.

CONCLUSIONS

PABA treatment may protect retinal function and attenuate the course of retinal degeneration in rd10 mice. Biochemical parameters indicate a lower degree of oxidative injury in PABA-treated retinas. PABA may potentially serve as an addition to antioxidative treatment for retinal and macular degenerations.

Wild-type cone photoreceptors persist despite neighboring mutant cone degeneration

In many retinal diseases, the malfunction that results in photoreceptor loss occurs only in either rods or cones, but degeneration can progress from the affected cell type to its healthy neighbors. Specifically, in human and mouse models of Retinitis Pigmentosa the loss of rods results in the death of neighboring healthy cones. Significantly less is known about cone-initiated degenerations and their affect on neighboring cells. Sometimes rods remain normal after cone death, whereas other patients experience a loss of scotopic vision over time. The affect of cone death on neighboring cones is unknown. The zebrafish is a cone-rich animal model in which the potential for dying cones to kill neighboring healthy cones can be evaluated. We previously reported that the zebrafish cone phosphodiesterase mutant (pde6c(w59)) displays a rapid death of cones soon after their formation and a subsequent loss of rods in the central retina. In this study we examine morphological changes associated with cone death in vivo in pde6c(w59) fish. We then use blastulae transplantations to create chimeric fish with a photoreceptor layer of mixed wild-type (WT) and pde6c(w59) cones. We find that the death of inoperative cones does not cause neighboring WT cone loss. The survival of WT cones is independent of transplant size and location within the retina. Furthermore, transplanted WT cones persist at least several weeks after the initial death of dysfunctional mutant cones. Our results suggest a potential for the therapeutic transplantation of healthy cones into an environment of damaged cones.

The Leber congenital amaurosis protein, AIPL1, is needed for the viability and functioning of cone photoreceptor cells

Leber congenital amaurosis (LCA) caused by mutations in Aryl hydrocarbon receptor interacting protein like-1 (Aipl1) is a severe form of childhood blindness. At 4 weeks of age, a mouse model of LCA lacking AIPL1 exhibits complete degeneration of both rod and cone photoreceptors. Rod cell death occurs due to rapid destabilization of rod phosphodiesterase, an enzyme essential for rod survival and function. However, little is understood regarding the role of AIPL1 in cone photoreceptors. Cone degeneration observed in the absence of AIPL1 could be due to an indirect 'bystander effect' caused by rod photoreceptor death or a direct role for AIPL1 in cones. To understand the importance of AIPL1 in cone photoreceptor cells, we transgenically expressed hAIPL1 exclusively in the rod photoreceptors of the Aipl1(-/-) mouse. Transgenic expression of hAIPL1 restored rod morphology and the rod-derived electroretinogram response, but cone photoreceptors were non-functional in the absence of AIPL1. In addition, the cone photoreceptors degenerate, but at a slower rate compared with Aipl1(-/-) mice. This degeneration is linked to the highly reduced levels of cone PDE6 observed in the hAIPL1 transgenic mice. Our studies demonstrate that AIPL1 is needed for the proper functioning and survival of cone photoreceptors. However, rod photoreceptors also provide support that partially preserves cone photoreceptors from rapid death in the absence of AIPL1.

Retinal light damage: mechanisms and protection

By its action on rhodopsin, light triggers the well-known visual transduction cascade, but can also induce cell damage and death through phototoxic mechanisms - a comprehensive understanding of which is still elusive despite more than 40 years of research. Herein, we integrate recent experimental findings to address several hypotheses of retinal light damage, premised in part on the close anatomical and metabolic relationships between the photoreceptors and the retinal pigment epithelium. We begin by reviewing the salient features of light damage, recently joined by evidence for retinal remodeling which has implications for the prognosis of recovery of function in retinal degenerations. We then consider select factors that influence the progression of the damage process and the extent of visual cell loss. Traditional, genetically modified, and emerging animal models are discussed, with particular emphasis on cone visual cells. Exogenous and endogenous retinal protective factors are explored, with implications for light damage mechanisms and some suggested avenues for future research. Synergies are known to exist between our long term light environment and photoreceptor cell death in retinal disease. Understanding the molecular mechanisms of light damage in a variety of animal models can provide valuable insights into the effects of light in clinical disorders and may form the basis of future therapies to prevent or delay visual cell loss.

Rhodopsin-mediated retinitis pigmentosa

Retinitis pigmentosa (RP) is a genetically and phenotypically heterogeneous group of diseases that cause blindness. Mutations within the rhodopsin gene account for approximately 25% of autosomal dominantly inherited RP cases. Therefore, understanding the mechanisms causing rhodopsin-mediated RP has a significant health impact. To date, results from multiple labs indicate that rhodopsin-mediated RP pathogenesis does not share a common mechanism of degeneration. There is strong evidence that multiple mechanisms are involved, including protein misfolding, mislocalization, release of toxic products, and aberrant signaling. Development of effective treatments requires investigation of the mechanism involved in the different rhodopsin mutations. This chapter focuses on the mechanisms by which rhodopsin mutations cause retinal degeneration, as well as potential therapeutic strategies to treat the disease.

CREB1/ATF1 activation in photoreceptor degeneration and protection

PURPOSE

The cAMP response element binding protein 1 (CREB1) and activating transcription factor 1 (ATF1) are closely related members of the bZIP superfamily of transcription factors. Both are activated in response to a wide array of stimuli, including cellular stress. This study was conducted to assess the CREB1/ATF1 pathway in photoreceptor disease and protection.

METHODS

The expression levels of p-CREB1, CREB1, and ATF1 were examined by immunoblot and immunohistochemistry in normal canine retina and retinas of several canine models of retinal degeneration (rcd1, rcd2, erd, prcd, XLPRA1, XLPRA2, T4R RHO). Humans retinas affected with age-related macular degeneration (AMD) were also examined. p-CREB1/ATF1 immunolabeling was assessed in normal and rcd1 dogs treated with ciliary neurotrophic factor (CNTF), to examine the effect of a neuroprotective stimulus on activation of CREB1/ATF1.

RESULTS

Native CREB1 and ATF1 as well as phosphorylated CREB1/ATF1 was examined in normal canine retina by immunoblot. The p-CREB1 antibody identified phosphorylated CREB1 and ATF1 and labeled the inner retina only in normal dogs. In degenerate canine and human retinas, strong immunolabeling appeared in rod and cone photoreceptors, indicating increased expression of native CREB1 and ATF1, as well as increased phosphorylation of these proteins. Retinal protection by CNTF in rcd1 dogs was accompanied by a significant increase in the number of p-CREB1/ATF1-labeled photoreceptor nuclei.

CONCLUSIONS

Positive association of CREB1/ATF1 phosphorylation with photoreceptor protection suggests that it may contribute to an innate protective response. These data identify a signaling mechanism in rods and cones of potential importance for therapies of RP and AMD.

Loss of protein kinase Cgamma in knockout mice and increased retinal sensitivity to hyperbaric oxygen

OBJECTIVE

To determine if loss of protein kinase Cgamma (PKCgamma) results in increased structural damage to the retina by hyperbaric oxygen (HBO), a treatment used for several ocular disorders.

METHODS

Six-week-old mice were exposed in vivo to 100% HBO 3 times a week for 8 weeks. Eyes were dissected, fixed, embedded in Epon, sectioned, stained with toluidine blue O, and examined by light microscopy.

RESULTS

The thicknesses of the inner nuclear and ganglion cell layers were increased. Destruction of the outer plexiform layer was observed in the retinas of the PKCgamma-knockout mice relative to control mice. Exposure to HBO caused significant degradation of the retina in knockout mice compared with control mice. Damage to the outer segments of the photoreceptor layer and ganglion cell layer was apparent in central retinas of HBO-treated knockout mice.

CONCLUSIONS

Protein kinase Cgamma-knockout mice had increased retinal sensitivity to HBO. Results demonstrate that PKCgamma protects retinas from HBO damage.

CLINICAL RELEVANCE

Care should be taken in treating patients with HBO, particularly if they have a genetic disease, such as spinocerebellar ataxia type 14, a condition in which the PKCgamma is mutated and nonfunctional.

Photoreceptor cell death mechanisms in inherited retinal degeneration

Photoreceptor cell death is the major hallmark of a group of human inherited retinal degenerations commonly referred to as retinitis pigmentosa (RP). Although the causative genetic mutations are often known, the mechanisms leading to photoreceptor degeneration remain poorly defined. Previous research work has focused on apoptosis, but recent evidence suggests that photoreceptor cell death may result primarily from non-apoptotic mechanisms independently of AP1 or p53 transcription factor activity, Bcl proteins, caspases, or cytochrome c release. This review briefly describes some animal models used for studies of retinal degeneration, with particular focus on the rd1 mouse. After outlining the major features of different cell death mechanisms in general, we then compare them with results obtained in retinal degeneration models, where photoreceptor cell death appears to be governed by, among other things, changes in cyclic nucleotide metabolism, downregulation of the transcription factor CREB, and excessive activation of calpain and PARP. Based on recent experimental evidence, we propose a putative non-apoptotic molecular pathway for photoreceptor cell death in the rd1 retina. The notion that inherited photoreceptor cell death is driven by non-apoptotic mechanisms may provide new ideas for future treatment of RP.

The circadian clock in the retina controls rod-cone coupling

Although rod and cone photoreceptor cells in the vertebrate retina are anatomically connected or coupled by gap junctions, a type of electrical synapse, rod-cone electrical coupling is thought to be weak. Using tracer labeling and electrical recording in the goldfish retina and tracer labeling in the mouse retina, we show that the retinal circadian clock, and not the retinal response to the visual environment, controls the extent and strength of rod-cone coupling by activating dopamine D(2)-like receptors in the day, so that rod-cone coupling is weak during the day but remarkably robust at night. The results demonstrate that circadian control of rod-cone electrical coupling serves as a synaptic switch that allows cones to receive very dim light signals from rods at night, but not in the day. The increase in the strength and extent of rod-cone coupling at night may facilitate the detection of large dim objects.

Rod and rod-driven function in achromatopsia and blue cone monochromatism

PURPOSE

To evaluate rod photoreceptor and postreceptor retinal function in pediatric patients with achromatopsia (ACHR) and blue cone monochromatism (BCM) using contemporary electroretinographic (ERG) procedures.

METHODS

Fifteen patients (age range, 1-20 years) with ACHR and six patients (age range, 4-22 years) with BCM were studied. ERG responses to full-field stimuli were obtained in scotopic and photopic conditions. Rod photoreceptor (S(rod), R(rod)) and rod-driven postreceptor (log sigma, V(max)) response parameters were calculated from the a-wave and b-wave. ERG records were digitally filtered to demonstrate the oscillatory potentials (OPs); a sensitivity parameter, log SOPA(1/2), and an amplitude parameter, SOPA(max), were used to characterize the OP response. Response parameters were compared with those of 12 healthy control subjects.

RESULTS

As expected, photopic responses were nondetectable in patients with ACHR and BCM. In addition, mean scotopic photoreceptor (R(rod)) and postreceptor (V(max) and SOPA(max)) amplitude parameters were significantly reduced compared with those in healthy controls. The flash intensity required to evoke a half-maximum b-wave amplitude (log sigma) was significantly increased.

CONCLUSIONS

Results of this study provide evidence that deficits in rod and rod-mediated function occur in the primary cone dysfunction syndromes ACHR and BCM.

Inherited diseases of photoreceptors and prospects for gene therapy

The photoreceptor cells of the retina are subject to a wide range of genetic diseases. This review summarizes current knowledge regarding an important group of retinal diseases caused by mutations in photoreceptor-enriched genes. In addition, progress toward treatment of a variety of these diseases in animal models via adeno-associated virus gene therapy is described. Although no human trials have yet been initiated to treat diseases caused by mutations in photoreceptor-enriched genes, there is a great deal of optimism regarding the prospects of treating these diseases using adeno-associated virus gene therapy.

The phagocytic capacity of neurones

Phagocytosis is defined as the ingestion of particulates over 0.5 microm in diameter and is associated with cells of the immune system such as macrophages or monocytes. Neurones are not generally recognized to be phagocytic. Using light, confocal, time-lapse and electron microscopy, we carried out a wide range of in-vitro and in-vivo experiments to examine the phagocytic capacity of different neuronal cell types. We demonstrated phagocytosis of material by neurones, including cell debris and synthetic particles up to 2.8 microm in diameter. We showed phagocytosis in different neuronal types, and demonstrated that debris can be transported from neurite extremities to cell bodies and persist within neurones. Flow cytometry analysis demonstrated the lack of certain complement receptors on neurones but the presence of others, including integrin receptors known to mediate macrophage phagocytosis, indicating that a restricted set of phagocytosis receptors may mediate this process. Neuronal phagocytosis occurs in vitro and in vivo, and we propose that this is a more widespread and significant process than previously recognized. Neuronal phagocytosis may explain certain inclusions in neurones during disease, cell-to-cell spread of disease, neuronal death during disease progression and provide a potential mechanism for therapeutic intervention through the delivery of particulate drug carriers.

Oleamide synthesizing activity from rat kidney: identification as cytochrome c

Oleamide (cis-9-octadecenamide) is the prototype member of an emerging class of lipid signaling molecules collectively known as the primary fatty acid amides. Current evidence suggests that oleamide participates in the biochemical mechanisms underlying the drive to sleep, thermoregulation, and antinociception. Despite the potential importance of oleamide in these physiologic processes, the biochemical pathway for its synthesis in vivo has not been established. We report here the discovery of an oleamide synthetase found in rat tissues using [(14)C]oleoyl-CoA and ammonium ion. Hydrogen peroxide was subsequently found to be a required cofactor. The enzyme displayed temperature and pH optima in the physiologic range, a remarkable resistance to proteolysis, and specificity for long-chain acyl-CoA substrates. The reaction demonstrated Michaelis-Menten kinetics with a K(m) for oleoyl-CoA of 21 microm. Proteomic, biochemical, and immunologic analyses were used to identify the source of the oleamide synthesizing activity as cytochrome c. This identification was based upon peptide mass fingerprinting of isolated synthase protein, a tight correlation between enzymatic activity and immunoreactivity for cytochrome c, and identical functional properties shared by the tissue-derived synthetase and commercially obtained cytochrome c. The ability of cytochrome c to catalyze the formation of oleamide experimentally raises the possibility that cytochrome c may mediate oleamide biosynthesis in vivo.

Seeing the light: New insights into the molecular pathogenesis of retinal diseases

In the past, most treatments for retinal diseases have been empirical. Steroids and/or laser photocoagulation and/or surgery have been tried for almost every condition with little or no understanding of the underlying disease. Over the past several years vision researchers have uncovered molecular components of processes, such as visual transduction and the visual cycle, that are critical for visual function, and identified other molecules that lead to dysfunction and disease processes such as neovascularization and macular edema. It is becoming clear that dysregulation of certain molecules can have major effects on retinal structure and function. Studies in animal models have suggested that inhibiting or augmenting levels of a single molecule can have major effects in complex disease processes. Although several molecules probably contribute to neovascularization and excessive vascular permeability in the eye, blockade of vascular endothelial growth factor (VEGF) has remarkable beneficial effects in animal models that have now been proven to apply to human diseases in clinical trials. Intraocular injection of VEGF antagonists has revolutionized the treatment of choroidal neovascularization (CNV) and macular edema and serves as a model of targeted ocular pharmacotherapy. Significant progress elucidating the molecular pathogenesis of several disease processes in the eye may soon lead to new treatments following the lead of VEGF antagonists. Initial treatments that provide benefit from frequent intraocular injections are likely to be followed by sustained delivery of drugs and/or prolonged protein delivery by gene transfer. The eye has entered the era of molecular therapy.

Antioxidants reduce cone cell death in a model of retinitis pigmentosa

Retinitis pigmentosa (RP) is a label for a group of diseases caused by a large number of mutations that result in rod photoreceptor cell death followed by gradual death of cones. The mechanism of cone cell death is uncertain. Rods are a major source of oxygen utilization in the retina and, after rods die, the level of oxygen in the outer retina is increased. In this study, we used the rd1 mouse model of RP to test the hypothesis that cones die from oxidative damage. A mixture of antioxidants was selected to try to maximize protection against oxidative damage achievable by exogenous supplements; alpha-tocopherol (200 mg/kg), ascorbic acid (250 mg/kg), Mn(III)tetrakis (4-benzoic acid) porphyrin (10 mg/kg), and alpha-lipoic acid (100 mg/kg). Mice were treated with daily injections of the mixture or each component alone between postnatal day (P)18 and P35. Between P18 and P35, there was an increase in two biomarkers of oxidative damage, carbonyl adducts measured by ELISA and immunohistochemical staining for acrolein, in the retinas of rd1 mice. The staining for acrolein in remaining cones at P35 was eliminated in antioxidant-treated rd1 mice, confirming that the treatment markedly reduced oxidative damage in cones; this was accompanied by a 2-fold increase in cone cell density and a 50% increase in medium-wavelength cone opsin mRNA. Antioxidants also caused some preservation of cone function based upon photopic electroretinograms. These data support the hypothesis that gradual cone cell death after rod cell death in RP is due to oxidative damage, and that antioxidant therapy may provide benefit.

Lentiviral expression of retinal guanylate cyclase-1 (RetGC1) restores vision in an avian model of childhood blindness

BACKGROUND

Leber congenital amaurosis (LCA) is a genetically heterogeneous group of retinal diseases that cause congenital blindness in infants and children. Mutations in the GUCY2D gene that encodes retinal guanylate cyclase-1 (retGC1) were the first to be linked to this disease group (LCA type 1 [LCA1]) and account for 10%-20% of LCA cases. These mutations disrupt synthesis of cGMP in photoreceptor cells, a key second messenger required for function of these cells. The GUCY1*B chicken, which carries a null mutation in the retGC1 gene, is blind at hatching and serves as an animal model for the study of LCA1 pathology and potential treatments in humans.

METHODS AND FINDINGS

A lentivirus-based gene transfer vector carrying the GUCY2D gene was developed and injected into early-stage GUCY1*B embryos to determine if photoreceptor function and sight could be restored to these animals. Like human LCA1, the avian disease shows early-onset blindness, but there is a window of opportunity for intervention. In both diseases there is a period of photoreceptor cell dysfunction that precedes retinal degeneration. Of seven treated animals, six exhibited sight as evidenced by robust optokinetic and volitional visual behaviors. Electroretinographic responses, absent in untreated animals, were partially restored in treated animals. Morphological analyses indicated there was slowing of the retinal degeneration.

CONCLUSIONS

Blindness associated with loss of function of retGC1 in the GUCY1*B avian model of LCA1 can be reversed using viral vector-mediated gene transfer. Furthermore, this reversal can be achieved by restoring function to a relatively low percentage of retinal photoreceptors. These results represent a first step toward development of gene therapies for one of the more common forms of childhood blindness.

Cone survival despite rod degeneration in XOPS-mCFP transgenic zebrafish

PURPOSE

In animal models of retinitis pigmentosa, rod photoreceptor degeneration eventually leads to loss of cone photoreceptors. The purpose of this study was to characterize a transgenic model of rod degeneration in zebrafish.

METHODS

Zebrafish transgenic for XOPS-mCFP, a membrane-targeted form of cyan fluorescent protein driven by the Xenopus rhodopsin promoter, were generated by plasmid injection. Immunohistochemistry was used to detect cell type, proliferation, and TUNEL markers in larval and adult retinas. Rod- and cone-specific transcripts were detected by RT-PCR. Visual responses in transgenic adults were measured by electroretinogram.

RESULTS

The XOPS promoter directed specific expression of mCFP in rods by 55 hours post fertilization (hpf). Rods in XOPS-mCFP heterozygotes began dying at 3.5 days post fertilization (dpf) and were almost completely absent by 5 dpf. A few rods were observed at the retinal margin, and numerous immature rods were observed in the outer nuclear layer (ONL) of transgenic adults. Apoptosis was increased in the ONL of larval and adult transgenic animals, and an elevation of rod precursor proliferation in adults was observed. ERG analysis confirmed that rod responses were absent in this line. Cone morphology and electrophysiology appeared normal in transgenic animals up to 7 months of age.

CONCLUSIONS

The XOPS-mCFP transgene causes selective degeneration of rods without secondary loss of cones in animals up to 7 months of age. This raises important questions about the significance of rod-cone interactions in zebrafish and their potential as a model of human inherited retinal degenerations.

Microdamage and apoptosis

Microdamage of healthy bone leads to targeted removal and repair of the damage. This process must involve the production of specific targeting signals. The identity of these signals is unknown but constitutes a legitimate research goal since it is this targeting process which appears to become impaired in ageing and disease. Here we discuss the potential role of the matrix bound osteocyte in the sensing and targeting of microdamage. In particular we will review current understanding concerning the apoptotic death of osteocytes at sites of microdamage and discuss the potential physiological significance of these findings in the light of knowledge of the significance of apoptosis in other cell systems.

The spread of apoptosis through gap-junctional channels in BHK cells transfected with Cx32

Programmed cell death (apoptosis) occurs both during normal development and as a result of various pathological conditions. An in vitro system was used to explore the transmission of death signals from apoptotic cells to cells with which they were coupled via gap junctions. Confluent cultures of baby hamster kidney (BHK) cells, stably transfected with the gap-junctional protein connexin32, were scrape loaded with cytochrome C (cyC), a mitochondria-derived apoptotic agent, to introduce the protein into cells injured by the cut. The cultures were subsequently analyzed for the presence of activated caspases, the distribution of TUNEL staining, and the binding of annexin V. Although cyC is too large to traverse the gap junctional channel, each of the assays revealed that apoptosis had spread from dying cells at the margin of the scrape to otherwise healthy neighboring cells to which they were coupled. This "bystander effect" was significantly reduced in the presence of agents that block gap junctional intercellular communication.

Oxidative damage is a potential cause of cone cell death in retinitis pigmentosa

Retinitis pigmentosa (RP) is a prevalent cause of blindness caused by a large number of different mutations in many different genes. The mutations result in rod photoreceptor cell death, but it is unknown why cones die. In this study, we tested the hypothesis that cones die from oxidative damage by performing immunohistochemical staining for biomarkers of oxidative damage in a transgenic pig model of RP. The presence of acrolein- and 4-hydroxynonenal-adducts on proteins is a specific indicator that lipid peroxidation has occurred, and there was strong immunofluorescent staining for both in cone inner segments (IS) of two 10-month-old transgenic pigs in which almost all rods had died, compared to faint staining in two 10-month-old control pig retinas. In 22- and 24-month-old transgenic pigs in which all rods and many cones had died, staining was strong in cone axons and some cell bodies as well as IS indicating progression in oxidative damage between 10 and 22 months. Biomarkers for oxidative damage to proteins and DNA also showed progressive oxidative damage to those macromolecules in cones during the course of RP. These data support the hypothesis that the death of rods results in decreased oxygen consumption and hyperoxia in the outer retina resulting in gradual cone cell death from oxidative damage. This hypothesis has important therapeutic implications and deserves rapid evaluation.