Photoreceptor Dystrophy Research Articles

Photoreceptor Differentiation following Transplantation of Allogeneic Retinal Progenitor Cells to the Dystrophic Rhodopsin Pro347Leu Transgenic Pig

Purpose. Transplantation of stem, progenitor, or precursor cells has resulted in photoreceptor replacement and evidence of functional efficacy in rodent models of retinal degeneration. Ongoing work has been directed toward the replication of these results in a large animal model, namely, the pig. Methods. Retinal progenitor cells were derived from the neural retina of GFP-transgenic pigs and transplanted to the subretinal space of rhodopsin Pro347Leu-transgenic allorecipients, in the early stage of the degeneration and the absence of immune suppression. Results. Results confirm the survival of allogeneic porcine RPCs without immune suppression in the setting of photoreceptor dystrophy. The expression of multiple photoreceptor markers by grafted cells included the rod outer segment-specific marker ROM-1. Further evidence of photoreceptor differentiation included the presence of numerous photoreceptor rosettes within GFP-positive grafts, indicative of the development of cellular polarity and self-assembly into rudiments of outer retinal tissue. Conclusion. Together, these data support the tolerance of RPCs as allografts and demonstrate the high level of rod photoreceptor development that can be obtained from cultured RPCs following transplantation. Strategies for further progress in this area, together with possible functional implications, are discussed.

Mutations in C8orf37, Encoding a Ciliary Protein, are Associated with Autosomal-Recessive Retinal Dystrophies with Early Macular Involvement

Cone-rod dystrophy (CRD) and retinitis pigmentosa (RP) are clinically and genetically overlapping heterogeneous retinal dystrophies. By using homozygosity mapping in an individual with autosomal-recessive (ar) RP from a consanguineous family, we identified three sizeable homozygous regions, together encompassing 46 Mb. Next-generation sequencing of all exons, flanking intron sequences, microRNAs, and other highly conserved genomic elements in these three regions revealed a homozygous nonsense mutation (c.497T>A [p.Leu166(∗)]) in C8orf37, located on chromosome 8q22.1. This mutation was not present in 150 ethnically matched control individuals, single-nucleotide polymorphism databases, or the 1000 Genomes database. Immunohistochemical studies revealed C8orf37 localization at the base of the primary cilium of human retinal pigment epithelium cells and at the base of connecting cilia of mouse photoreceptors. C8orf37 sequence analysis of individuals who had retinal dystrophy and carried conspicuously large homozygous regions encompassing C8orf37 revealed a homozygous splice-site mutation (c.156-2A>G) in two siblings of a consanguineous family and homozygous missense mutations (c.529C>T [p.Arg177Trp]; c.545A>G [p.Gln182Arg]) in siblings of two other consanguineous families. The missense mutations affect highly conserved amino acids, and in silico analyses predicted that both variants are probably pathogenic. Clinical assessment revealed CRD in four individuals and RP with early macular involvement in two individuals. The two CRD siblings with the c.156-2A>G mutation also showed unilateral postaxial polydactyly. These results underline the importance of disrupted ciliary processes in the pathogenesis of retinal dystrophies.

Electrophysiology of patients with nyctalopia

AbstractThe lecture will use a case‐based approach to demonstrate the value of electrophysiological testing in the diagnosis and management of patients with nyctalopia. The diagnostic features of various inherited and acquired diseases will be described, including photoreceptor dystrophies, congenital stationary night blindness, fundus albipunctatus, melanoma associated retinopathy, vitamin A deficiency and others.

Evidence for RPGRIP1 gene as risk factor for primary open angle glaucoma

Glaucoma is a genetically heterogeneous disorder and is the second cause of blindness worldwide owing to the progressive degeneration of retinal ganglion neurons. Very few genes causing glaucoma were identified to this date. In this study, we screened 10 candidate genes of glaucoma between the D14S261 and D14S121 markers of chromosome 14q11, a critical region previously linked to primary open-angle glaucoma (POAG). Mutation analyses of two large cohorts of patients with POAG, normal tension glaucoma (NTG) and juvenile open-angle glaucoma (JOAG), and control subjects, found only association of non-synonymous heterozygous variants of the retinitis pigmentosa GTPase regulator-interacting protein 1 (RPGRIP1) with POAG, NTG and JOAG. The 20 non-synonymous variants identified in RPGRIP1 were all distinct from variants causing photoreceptor dystrophies and were found throughout all but one domain (RPGR-interacting domain) of RPGRIP1. Among them, 14 missense variants clustered within or around the C2 domains of RPGRIP1. Yeast two-hybrid analyses of a subset of the missense mutations within the C2 domains of RPGRIP1 shows that five of them (p.R598Q, p.A635G, p.T806I, p.A837G and p.I838V) decrease the association of the C2 domains with nephrocystin-4 (NPHPH). When considering only these five confirmed C2-domain mutations, the association remains statistically significant (P=0.001). Altogether, the data support that heterozygous non-synonymous variants of RPGRIP1 may cause or increase the susceptibility to various forms of glaucoma and that among other factors, physical impairment of the interaction of RPGRIP1with different proteins may contribute to the pathogenesis of forms of glaucoma.

Acquired loss of chromatic sensitivity

Abstract Purpose A range of ophthalmic and neurological conditions cause diminished visual performance, even when the subject is often unaware of any problems and the loss of vision remains undetected in conventional perimetry and visual acuity tests. The extent to which detection of acquired colour vision loss can revealed in subclinical cases and distinguished from congenital loss has been investigated. Methods Over 400 subjects with congenital and acquired colour vision loss have been examined using conventional colour screening methods. In addition, the loss of yellow / blue and red / green chromatic sensitivity has been quantified using the CAD test (http://www.caa.co.uk/docs/33/200904.pdf). Those investigated included subjects with diseases of the retina and / or the optic nerve as well as patients with selective damage to central visual pathways. Patients with various stages of glaucoma, photoreceptor dystrophies, diabetes, optic neuritis, age‐related macular degeneration as well as tobacco and alcohol toxicity have been examined. Results Algorithms developed for analysis of colour vision loss and automatic classification of congenital and / or acquired colour deficiency will be described. In acquired deficiency, the loss of chromatic sensitivity tends to affect both the rg and the yb channels. Significant differential effects have, however, been observed in relation to stimulus size, retinal location and state of light adaptation. Conclusion The findings from these studies show that in the majority of these conditions, the loss of chromatic sensitivity is the most sensitive measure of early changes in diseases of the eye.

Cone-Rod Dependence in the Rat Retina: Variation with the Rate of Rod Damage

To assess the effect of accelerated rod damage on the integrity of cones in the rat retina. Rhodopsin-mutant P23H-3 and Sprague-Dawley (SD) rats were raised in scotopic ambient conditions (12 hours dark, 12 hours 5 lux) and then exposed to photopic conditions (12 hours dark, 12 hours 300 lux). Rods and cones were assessed for cell death, outer segment (OS) morphology, and electroretinogram (ERG) responses. Cones in the P23H-retina were affected rapidly by photopic exposure. Exposure for 2 days caused 50% reductions in LM- and S-cone OS length and cone ERG responses, associated with and preceded by reductions in rod OS length and ERG responses. Although 2 days' exposure increased the rate of rod death, outer nuclear layer thinning was minimal, and no evidence of cone death was detected. In the SD retina, the same photopic exposure had no measurable effects on death rates, OS length, or ERG responses in either rods or cones. Longer (7 days) photopic exposure reduced cone and rod OS length and ERG responses in SD, as well as P23H-3 retinas, but less severely than in the P23H-3 strain. Cones are damaged rapidly in the P23H-3 retina when rod damage is accelerated by raised ambient illumination. This close dependence of cone integrity on rod integrity contrasts with the life-long persistence of cone function in the scotopic reared P23H-3 rat. In humans suffering comparable photoreceptor dystrophies, the maintenance of steady, low ambient light may, by minimizing acute rod damage, optimize the function of surviving cones.

Síndrome de Alström Hallgren

espanolIntroduccion: El Sindrome de Alstrom es una enfermedad autosomica recesiva. Se conoce un gen (ALMS1) asociado al sindrome, caracterizado por ceguera causada por distrofia de conos y bastones, sordera sensorial, resistencia a la insulina, obesidad, y cardiopatias. Caso clinico: Nino de cuatro anos que tras una parada cardiorrespiratoria es diagnosticado de cardiomiopatia dilatada. Comienza con nistagmus y fotofobia. Se le realiza exploracion oftalmologica y ERG, siendo los resultados compatibles con un Sindrome de Alstrom. Discusion: El diagnostico en estos casos es clinico. El diagnostico de certeza con tecnicas moleculares es posible en un 25-40% de casos. El tratamiento es sintomatico. EnglishIntroduction: Alstrom-Hallgren syndrome is an inherited condition in which the transmission of a double dose of a mutated gene leads to specific clinical findings. To the present time there has been only one gene detected which leads to this syndrome, the ALMS1 gene. Mutation of this gene leads to progressive blindness due to photoreceptor dystrophy, progressive sensorineural hearing loss, insulin resistant diabetes, morbid obesity and cardiologic abnormalities. Clinical Case: We present the case of a four-year-old male who, after a cardiopulmonary shutdown in the fourth month of life, was diagnosed with a dilated cardiomyopathy. Nystagmus and photophobia followed, and, after ophthalmologic exploration and an electroretinogram, the results were consistent with Alstrom syndrome. The appropriate genetic studies were then performed. Discussion: The diagnosis in this case was considered basically because of its uncommon clinical features, and the fact a multidisciplinary approach was used in its evaluation. We were only able to confirm the diagnosis by molecular biology techniques, with this resulting in the correct diagnosis in 25-40% of cases. Treatment is symptomatic and the prognosis extremely variable.

RPE transplantation and its role in retinal disease

Retinal pigment epithelial (RPE) transplantation aims to restore the subretinal anatomy and re-establish the critical interaction between the RPE and the photoreceptor, which is fundamental to sight. The field has developed over the past 20 years with advances coming from a large body of animal work and more recently a considerable number of human trials. Enormous progress has been made with the potential for at least partial restoration of visual function in both animal and human clinical work. Diseases that have been treated with RPE transplantation demonstrating partial reversal of vision loss include primary RPE dystrophies such as the merTK dystrophy in the Royal College of Surgeons (RCS) rat and in humans, photoreceptor dystrophies as well as complex retinal diseases such as atrophic and neovascular age-related macular degeneration (AMD). Unfortunately, in the human trials the visual recovery has been limited at best and full visual recovery has not been demonstrated. Autologous full-thickness transplants have been used most commonly and effectively in human disease but the search for a cell source to replace autologous RPE such as embryonic stem cells, marrow-derived stem cells, umbilical cord-derived cells as well as immortalised cell lines continues. The combination of cell transplantation with other modalities of treatment such as gene transfer remains an exciting future prospect. RPE transplantation has already been shown to be capable of restoring the subretinal anatomy and improving photoreceptor function in a variety of retinal diseases. In the near future, refinements of current techniques are likely to allow RPE transplantation to enter the mainstream of retinal therapy at a time when the treatment of previously blinding retinal diseases is finally becoming a reality.

Utility in clinical practice of standard vs. high-intensity ERG a-waves

Standard ERG a-waves represent contributions from both photoreceptor and inner retinal cells, while the leading edge of the high-intensity a-wave is produced only by photoreceptors. This has raised questions about the value of the a-wave as an indicator of photoreceptor disease, and has led to suggestions for standardizing higher-intensity stimuli. Our objective was to compare the behavior of standard and high-intensity a-waves in clinical practice. Standard ISCEV (International Society for Clinical Electrophysiology of Vision) a-waves and high-intensity a-wave responses were recorded under scotopic and photopic conditions from normal subjects and from patients with photoreceptor dystrophies and other diseases. The standard scotopic a-wave amplitude followed the high-intensity a-wave closely among patients with different diagnoses, and the results did not change significantly when cone a-waves were subtracted to isolate rod signals. The only exception was one patient with the enhanced S cone syndrome (ESCS) whose dark-adapted responses were cone-driven. Initial peak times clustered in a small range for both standard and high-intensity responses, and were not very sensitive to disease. High-intensity a-waves can show photoreceptor characteristics directly, and may help analyze some rare disorders. However, in our study the amplitude of conventional scotopic a-waves mirrored that of the high-intensity responses quite closely over a wide range of patients. This suggests that for practical purposes even if it is not perfect, the standard ERG is an excellent indicator of photoreceptor disease.

Multiple, parallel cellular suicide mechanisms participate in photoreceptor cell death

Photoreceptor degeneration in human photoreceptor dystrophies and in the relevant animal models has been thought to be executed by one common mechanism – caspase-mediated apoptosis. However, recent experiments have challenged this concept. In previous experiments, analyzing gene expression in the degenerating rd/rd mouse retina, we have suggested that the gene defect leads to oxidative stress and altered metabolism, which may induce caspase-dependent and caspase-independent cell death mechanisms such as the activation of cystein-proteases, lysosomal proteases, autophagy and complement-mediated lysis. In this study we asked two questions. First, whether a temporal analysis of these different mechanisms during the course of degeneration would enable us to establish a causal relationship between these events; and second, whether photoreceptor degeneration in different models of photoreceptor dystrophies occurs by activating the same mechanisms. Three models of photoreceptor degeneration were chosen in which photoreceptor degeneration is caused by different events: the rd/rd mouse (calcium overload); the rds/rds mouse (structural defect); and light-damage (LD; oxidative stress). Marker genes were selected for the identified processes. PCR-analysis on laser capture microdissection samples was used to verify the expression of these genes in the rod photoreceptor layer. A temporal relationship between the processes was established at the mRNA level, using quantitative RT-PCR. The time course of gene expression was compared to that of cell loss (loss of rows of photoreceptor nuclei) and apoptosis (TUNEL labeling). Apoptosis and autophagy was analyzed using enzymatic assays. The time course of apoptosis and TUNEL labeling coincide in all three models. Complement-activated lysis was found to either parallel ( rd/rd and rds/rds) or precede (LD) the development of TUNEL-positive cells. Autophagy was determined to parallel ( rd/rd and LD) or lag ( rds/rds) behind the development of TUNEL-positive cells. In all three models, glucose metabolism was found to be increased significantly prior to the onset of cell death, but then dropped in parallel with the loss of cells. The presence of the marker genes was verified by laser capture microdissection, and apoptosis (caspase activity) and autophagy (lysozyme and cathepsin activity) were verified in retina extracts. These results provide evidence that irrespective of whether photoreceptor degeneration is triggered by gene defects (lack of β-PDE or rds/peripherin) or environmental stress (light-damage), a number of pro-apoptotic mechanisms are triggered leading to the degeneration of the photoreceptor cells. The temporal pattern of the different pathways suggests that the non-caspase-dependent mechanisms may actively participate in the demise of the photoreceptors, rather than represent a passive response of the retina to the presence of dying cells. Thus, unless the common upstream initiator for a given photoreceptor dystrophy is found, multiple rescue paradigms need to be used to target all active pathways.

High-resolution ultrasonography of subretinal structure and assessment of retina degeneration in rat

The purpose of this work was to evaluate the ability of 80 MHz ultrasonography to differentiate intra-retinal layers and quantitatively assess photoreceptor dystrophy in small animal models. Four groups of 10 RCS rats each (five dystrophic and five controls) were explored at 25, 35, 45 and 55 days post-natal (PN). A series of retina cross-sections were obtained ex vivo from outside intact eyes using an 80 MHz three-dimensional ultrasound backscatter microscope (20-μm-axial resolution). Ultrasound features of normal retina were correlated to those of corresponding histology and thickness measurements of photoreceptor segment and nuclear layers were performed on all groups. To show the ability of 80 MHz ultrasonography to distinguish the retinal degeneration in vivo, one RCS rat was explored at 25 and 55 days post-natal. Ultrasound image of normal retina displayed four distinct layers marked by reflections at neurites/nuclei interfaces and permitted to differentiate the photoreceptor segment and nuclear layers. The backscatter level from the retina was shown to be related to the size, density and organization of the intra-layer structure. Ultrasound thickness measurements highly correlated with histologic measurements. A thinning ( p<0.05) of outer nuclear layer (ONL) was detected over time for controls and was thought to be assigned to retina maturation. Retinal degeneration started at PN35 and resulted in a more pronounced ONL thinning ( p<0.05) over time. ONL degeneration was accompanied by segment layer thickening ( p<0.05) at PN35 and thinning thereafter. These changes may indicate accumulation of outer segment debris at PN35 then progressive destruction. In vivo images of rat intra-retinal structure showed the ability of the method to distinguish the photoreceptor layer changes. Our results indicate that 80 MHz ultrasonography reveals intra-retinal layers and is sensitive to age and degenerative changes of photoreceptors. This technique has great potential to follow-up retinal dystrophy and therapeutic effects in vivo.

Minocycline delays photoreceptor death in the rds mouse through a microglia-independent mechanism

Purpose. Minocycline, a semi-synthetic tetracycline antibiotic is reported to be neuroprotective in degenerative and ischaemic models of central nervous system disease, via mechanisms involving suppression of both cytotoxic microglial activity and caspase-dependent apoptosis. We have investigated the effect of minocycline treatment on a mouse model of retinitis pigmentosa, an inherited photoreceptor neurodegenerative disorder, and contrasted this with the effect of depleting retinal microglia using liposomal clodronate. Methods. rds mice were treated intraperitoneally from the second postnatal day (P2) with either daily minocycline until P16, P18, P21, P24 and P27 or alternative day clodronate liposomes until P16. Immunohistochemical and immunofluorescent methods were applied for the detection of microglia (F4/80) and apoptosis (TUNEL and caspase 3 activation). Results. Photoreceptor apoptosis was delayed by minocycline treatment but not, ultimately, prevented. Markedly reduced expression of activated caspase 3 was observed in photoreceptors at the early time point, corresponding with the reduced level of apoptosis. Delayed photoreceptor apoptosis due to minocycline treatment was associated with a 50% reduction in the numbers of microglia at early timepoints. Liposomal clodronate treatment also resulted in a marked reduction in the number of microglia (63% reduction in microglia), but in contrast to minocycline treatment, this had no effect on photoreceptor apoptosis. Conclusions. Minocycline appears to delay photoreceptor apoptosis through a microglia-independent action. Although microglial cytotoxicity has been implicated during other models of neurodegeneration, microglia are unlikely to play such a role in this model of photoreceptor dystrophy.

Long-term evaluation of retinal function in Prph2Rd2/Rd2 mice following AAV-mediated gene replacement therapy.

Prph2(Rd2/Rd2) mice have a retinal degeneration due to a null mutation for the Prph2 gene which encodes a photoreceptor-specific glycoprotein, peripherin2, essential for outer-segment formation. We have previously shown improvement of photoreceptor function at a single time point following AAV-mediated gene replacement therapy. Here we quantify the functional rescue over a 15-week time course and present a detailed analysis of the improvement in retinal function. An AAV2 vector, AAV.rho.rds, carrying a peripherin2 c-DNA, was in injected subretinally into 10-day-old Prph2(Rd2/Rd2) mice. One group was injected at a single time point while in a second group the injections were repeated after 5 days. The effect of treatment was analysed histologically using electron microscopy and electroretinography (ERG) was used to assess functional changes. Treated mice were recorded at regular intervals over 15 weeks. Untreated contralateral eyes served as internal control. A significant increase in b-wave amplitude was first noted 3 weeks after treatment of 10-day-old Prph2(Rd2/Rd2) mice and persisted for up to 14 weeks. An increase in the area of retina exposed to vector resulted in a significant increase in both b-wave amplitude and persistence. In this study AAV-mediated gene replacement in Prph2(Rd2/Rd2) mice resulted in a significant functional improvement over a period of 14 weeks. These results support the utility of gene therapy approaches as treatment for photoreceptor dystrophies.

Analysis of 180 patients with sensory defect nystagmus (SDN) and congenital idiopathic nystagmus (CIN)

Analysis of the diseases underlying congenital nystagmus in a series of patients registered during 6 years as a prerequisite for adequate counselling of the families. Retrospective study of all patients that presented between 1992 and 1998 with congenital nystagmus not related to visual deprivation or acquired pathologies of the visual pathways. The patients were examined clinically and in dependence on the findings also by electrophysiological (Ganzfeld ERG and VEP, Albino-flash-VEP), psychophysical (colour vision, dark adaptation, spectral sensitivity), and molecular genetic methods. When estimated necessary, family members affected by history and unaffected family members were also examined. In cases of complex neuroophthalmological diseases a neuropaediatric examination including neuroimaging was initiated. In total, 180 patients could be analysed. A sensory defect nystagmus (SDN) was present in 142 patients (79%). The diagnoses were as follows: albinism (any form) in 56 patients (30%), progressive photoreceptor dystrophy in 20 patients (11%), stationary cone dysfunction in 18 patients (10%), bilateral optic nerve hypoplasia in 15 patients (8%), chorioretinal or optic nerve colobomata in 10 patients (6%), aniridia and its variants in 10 patients (6%), familial isolated nystagmus in 8 patients (5%), and congenital stationary night blindness in 5 patients (3%). 38 patients (21%) could not (yet) be classified. The prevalence of SDN as the manifesting symptom of a variety of well defined diseases in the present series of at least 79% is similar to that of 90% reported earlier. The precise diagnosis is a prerequisite for counselling the families as to functional prognosis and recurrence risk. Unnecessary neurological examinations including neuroimaging can be avoided.

Restoration of photoreceptor ultrastructure and function in retinal degeneration slow mice by gene therapy.

The gene Prph2 encodes a photoreceptor-specific membrane glycoprotein, peripherin-2 (also known as peripherin/rds), which is inserted into the rims of photoreceptor outer segment discs in a complex with rom-1 (ref. 2). The complex is necessary for the stabilization of the discs, which are renewed constantly throughout life, and which contain the visual pigments necessary for photon capture. Mutations in Prph2 have been shown to result in a variety of photoreceptor dystrophies, including autosomal dominant retinitis pigmentosa and macular dystrophy. A common feature of these diseases is the loss of photoreceptor function, also seen in the retinal degeneration slow (rds or Prph2 Rd2/Rd2) mouse, which is homozygous for a null mutation in Prph2. It is characterized by a complete failure to develop photoreceptor discs and outer segments, downregulation of rhodopsin and apoptotic loss of photoreceptor cells. The electroretinograms (ERGs) of Prph2Rd2/Rd2 mice have greatly diminished a-wave and b-wave amplitudes, which decline to virtually undetectable concentrations by two months. Subretinal injection of recombinant adeno-associated virus (AAV) encoding a Prph2 transgene results in stable generation of outer segment structures and formation of new stacks of discs containing both perpherin-2 and rhodopsin, which in many cases are morphologically similar to normal outer segments. Moreover, the re-establishment of the structural integrity of the photoreceptor layer also results in electrophysiological correction. These studies demonstrate for the first time that a complex ultrastructural cell defect can be corrected both morphologically and functionally by in vivo gene transfer.

Growth Factors in Combination, but Not Individually, Rescue rd Mouse Photoreceptors in Organ Culture

The rd mouse retina is an animal model for human retinal dystrophy in which the rod photoreceptors undergo apoptosis during the first 4 weeks in vivo or in organ culture. We have examined the effect of different families of trophic factors on the survival of rd mouse photoreceptors in organ culture. Retinas were harvested from rd mice at postnatal day 2 and grown in organ culture for 27 days in vitro (DIV) in DMEM with 10% fetal calf serum. Ciliary neurotrophic factor (CNTF), brain-derived neurotrophic factor (BDNF), fibroblast growth factor-2 (FGF2), glial cell line-derived neurotrophic factor (GDNF), neurturin, and persephon were added individually or in combination to the medium at a dose of 50 ng/ml or less. CNTF + BDNF in combination resulted in photoreceptor survival comparable to wild-type retinas after 27 DIV. CNTF + FGF2 or CNTF + GDNF produced a partial prevention of photoreceptor death. Photoreceptor degeneration was not blocked by any of the trophic factors added individually. A significant increase in photoreceptor survival was seen with forskolin added to CNTF, but not to BDNF, FGF2, or GDNF. These results demonstrate that trophic factors promote photoreceptor survival through a synergistic interaction. Increased understanding of receptor interactions and signaling pathways may lead to a potential therapeutic role for combinatorial trophic factors in treatment of photoreceptor dystrophies.

Mechanisms of photoreceptor death and survival in mammalian retina.

The mammalian retina, like the rest of the central nervous system, is highly stable and can maintain its structure and function for the full life of the individual, in humans for many decades. Photoreceptor dystrophies are instances of retinal instability. Many are precipitated by genetic mutations and scores of photoreceptor-lethal mutations have now been identified at the codon level. This review explores the factors which make the photoreceptor more vulnerable to small mutations of its proteins than any other cell of the body, and more vulnerable to environmental factors than any other retinal neurone. These factors include the highly specialised structure and function of the photoreceptors, their high appetite for energy, their self-protective mechanisms and the architecture of their energy supply from the choroidal circulation. Particularly important are the properties of the choroidal circulation, especially its fast flow of near-arterial blood and its inability to autoregulate. Mechanisms which make the retina stable and unstable are then reviewed in three different models of retinal degeneration, retinal detachment, photoreceptor dystrophy and light damage. A two stage model of the genesis of photoreceptor dystrophies is proposed, comprising an initial "depletion" stage caused by genetic or environmental insult and a second "late" stage during which oxygen toxicity damages and eventually destroys any photoreceptors which survive the initial depletion. It is a feature of the model that the second "late" stage of retinal dystrophies is driven by oxygen toxicity. The implications of these ideas for therapy of retinal dystrophies are discussed.

Retinal rod photoreceptor-specific gene mutation perturbs cone pathway development.

Rod-specific photoreceptor dystrophies are complicated by the delayed death of genetically normal neighboring cones. In transgenic (Tg) swine with a rod-specific (rhodopsin) gene mutation, cone photoreceptor physiology was normal for months but later declined, consistent with delayed cone cell death. Surprisingly, cone postreceptoral function was markedly abnormal when cone photoreceptor physiology was still normal. The defect was localized to hyperpolarizing cells postsynaptic to the middle wavelength-sensitive cones. Recordings throughout postnatal development indicated a failure of cone circuitry maturation, a novel mechanism of secondary cone abnormality in rod dystrophy. The results have implications for therapy for human retinal dystrophies and raise the possibility that rod afferent activity plays a role in the postnatal maturation of cone retinal circuitry.

Embryonic dysplasia and adult retinal dystrophy in boron-deficient zebrafish

Boron (B) has been identified as a growth stimulator in embryonic trout and shown to be essential for embryogenesis in zebrafish. In the present study we describe adverse health endpoints of boron deficiency during two different stages of the lifecycle in zebrafish. The pathological manifestations of boron deficiency involved membrane alterations at both stages. Deficiency during the embryonic period was seen as dysplasia during the zygote and cleavage periods. In adults deficiency presented as retinal dystrophy. Boron deficient embryos were obtained from zebrafish maintained in water containing <0.2 μM B for six months. The initial stages of embryonic development, the zygote and cleavage periods, were marked by membrane blebbing and extrusion of cytoplasm in deficient, but not boron supplemented embryos (45 μM B). About 5 to 7% of the boron deficient embryos survived to produce offspring. The adult F1 boron deficient zebrafish were photophobic and histological evaluation of their eyes determined that the boron deficient adults had photoreceptor dystrophy. Compared to boron supplemented controls, the photoreceptor cells of the boron deficient adults were shortened due to a reduction in the myoid and outer segment regions. In boron deficient embryos and adults, the pathological changes were observed in cells that produce prodigious quantities of membrane. This outcome suggests that boron is required for the synthesis or processing of membrane or cytoskeletal proteins that function in the maintenance of membrane shape and integrity. J. Trace Elem. Exp. Med. 12:213–219, 1999. © 1999 Wiley-Liss, Inc.

A new family of Greek origin maps to the CRD locus for autosomal dominant cone-rod dystrophy on 19q.

Retinal photoreceptor dystrophies (RD) are a highly heterogeneous group of genetic disorders of the retina, representing the most frequently inherited form of visual handicap, affecting approximately 1.5 million people world...