A gradient of basic fibroblast growth factor in rod photoreceptors in the normal human retina

Inverse Problem Reveals Conditions for Characteristic Retinal Degeneration Patterns in Retinitis Pigmentosa Under the Trophic Factor Hypothesis

Retinitis pigmentosa (RP) is the most common inherited retinal dystrophy with a prevalence of about 1 in 4,000, affecting approximately 1.5 million people worldwide. Patients with RP experience progressive visual field loss as the retina degenerates, destroying light-sensitive photoreceptor cells (rods and cones), with rods affected earlier and more severely than cones. Spatio-temporal patterns of retinal degeneration in human RP have been well characterised; however, the mechanism(s) giving rise to these patterns have not been conclusively determined. One such mechanism, which has received a wealth of experimental support, is described by the trophic factor hypothesis. This hypothesis suggests that rods produce a trophic factor necessary for cone survival; the loss of rods depletes this factor, leading to cone degeneration. In this article, we formulate a partial differential equation mathematical model of RP in one spatial dimension, spanning the region between the retinal centre (fovea) and the retinal edge (ora serrata). Using this model we derive and solve an inverse problem, revealing for the first time experimentally testable conditions under which the trophic factor mechanism will qualitatively recapitulate the spatio-temporal patterns of retinal regeneration observed in human RP.

Trophic factors in the pathogenesis and therapy for retinal degenerative diseases

Trophic factors are endogenously secreted proteins that act in an autocrine and/or paracrine fashion to affect vital cellular processes such as proliferation, differentiation, and regeneration, thereby maintaining overall cell homeostasis. In the eye, the major contributors of these molecules are the retinal pigment epithelial (RPE) and Müller cells. The primary paracrine targets of these secreted proteins include the photoreceptors and choriocapillaris. Retinal degenerative diseases such as age-related macular degeneration and retinitis pigmentosa are characterized by aberrant function and/or eventual death of RPE cells, photoreceptors, choriocapillaris, and other retinal cells. We discuss results of in vitro and in vivo animal studies in which candidate trophic factors, either singly or in combination, were used in an attempt to ameliorate photoreceptor and/or retinal degeneration. We also examine current trophic factor therapies as they relate to the treatment of retinal degenerative diseases in clinical studies.

Viral delivery of heme oxygenase-1 attenuates photoreceptor apoptosis in an experimental model of retinal detachment

This study was designed to evaluate the efficacy of subretinal injection of recombinant adeno-associated virus vector expressing heme oxygenase-1 (rAAV-HO-1) in attenuating photoreceptor apoptosis induced by experimental retinal detachment (RD) in Sprague-Dawley rats. Our results disclosed that subretinal rAAV-HO-1 delivery achieved localized high HO-1 gene expression in retinal outer nuclear layer (ONL) compared with rAAV-lacZ-injected eyes and eyes with RD left untreated both at 2 (p=0.003) and 28 (p=0.007) days of RD. The ONL thickness (p=0.018) and mean photoreceptor nuclei count (p=0.009) in eyes receiving rAAV-HO-1 injection was significantly higher than in rAAV-lacZ-injected or eyes with RD left untreated at 28 days of RD. There were fewer apoptotic photoreceptor nuclei at 2 (p=0.008) and 5 (p=0.018) days of RD and less activated caspase-3 expression (p=0.008) at 2 days of RD in rAAV-HO-1 treated eyes than in control eyes. These data supported that gene transfer approach might attenuate photoreceptor apoptosis caused by RD with a resultant better ONL preservation.

ABCA4-associated retinal degenerations spare structure and function of the human parapapillary retina

PURPOSE

To study the parapapillary retinal region in patients with ABCA4-associated retinal degenerations.

METHODS

Patients with Stargardt disease or cone-rod dystrophy and disease-causing variants in the ABCA4 gene were included. Fixation location was determined under fundus visualization, and central cone-mediated vision was measured. Intensity and texture abnormalities of autofluorescence (AF) images were quantified. Parapapillary retina of an eye donor with ungenotyped Stargardt disease was examined microscopically.

RESULTS

AF images ranged from normal, to spatially homogenous abnormal increase of intensity, to a spatially heterogenous speckled pattern, to variably sized patches of low intensity. A parapapillary ring of normal-appearing AF was visible at all disease stages. Quantitative analysis of the intensity and texture properties of AF images showed the preserved region to be an annulus, at least 0.6 mm wide, surrounding the optic nerve head. A similar region of relatively preserved photoreceptor nuclei was apparent in the donor retina. In patients with foveal fixation, there was better cone sensitivity at a parapapillary locus in the nasal retina than at the same eccentricity in the temporal retina. In patients with eccentric fixation, approximately 30% had a preferred retinal locus in the parapapillary retina.

CONCLUSIONS

Human retinal degenerations caused by ABCA4 mutations spare the structure of retina and RPE in a circular parapapillary region that commonly serves as the preferred fixation locus when central vision is lost. The retina between fovea and optic nerve head could serve as a convenient, accessible, and informative region for structural and functional studies to determine natural history or outcome of therapy in ABCA4-associated disease.

Gradients of cone differentiation and FGF expression during development of the foveal depression in macaque retina

Cones in the foveola of adult primate retina are narrower and more elongated than cones on the foveal rim, which in turn, are narrower and more elongated than those located more eccentric. This gradient of cone morphology is directly correlated with cone density and acuity. Here we investigate the hypothesis that fibroblast growth factor (FGF) signaling mediates the morphological differentiation of foveal cones--in particular, the mechanism regulating the elongation of foveal cones. We used immunoreactivity to FGF receptor (R) 4, and quantitative analysis to study cone elongation on the horizontal meridian of macaque retinae, aged between foetal day (Fd) 95 and 2.5 years postnatal (P 2.5 y). We also used in situ hybridization and immunohistochemistry to investigate the expression patterns of FGF2 and FGFR1-4 at the developing fovea, and three other sample locations on the horizontal meridian. Labeled RNA was detected using the fluorescent marker "Fast Red" (Roche) and levels of expression in cone inner segments and in the ganglion cell layer (GCL) were compared using confocal microscopy, optical densitometry, and tested for statistical significance. Our results show that morphological differentiation of cones begins near the optic disc around Fd 95, progressing toward the developing fovea up until birth, approximately. Levels of FGF2 and FGFR4 mRNAs expression are low in foveal cones, compared with cones closer to the optic disc, during this period. There is no similar gradient of FGF2 mRNA expression in the ganglion cell layer of the same sections. Maturation of foveal cones is delayed until the postnatal period. The results suggest that a wave of cone differentiation spreads from the disc region toward the developing fovea during the second half of gestation in the macaque. A gradient of expression of FGFR4 and FGF2 associated with the wave of differentiation suggests that FGF signalling mediates cone narrowing and elongation.

Retinal Detachment Neuropathology and Potential Strategies for Neuroprotection

Understanding the neuropathology of retinal detachment from postmortem and animal models allows identification of cellular targets, receptors and mediators for pharmacological manipulation. In this review, concepts of retinal detachment and neuropathology are examined at cellular and structural anatomical levels using postmortem and animal model data. Possible neuroprotective strategies are reviewed in the setting of the new environment created by successful retinal reattachment surgery.

PROGRESSTOWARDUNDERSTANDING THEGENETIC ANDBIOCHEMICALMECHANISMS OFINHERITEDPHOTORECEPTORDEGENERATIONS

More than 80 genes associated with human photoreceptor degenerations have been identified. Attention must now turn toward defining the mechanisms that lead to photoreceptor death, which occurs years to decades after the birth of the cells. Consequently, this review focuses on topics that offer insights into such mechanisms, including the one-hit or constant risk model of photoreceptor death; topological patterns of photoreceptor degeneration; mutations in ubiquitously expressed splicing factor genes associated only with photoreceptor degeneration; disorders of the retinal pigment epithelium; modifier genes; and global gene expression analysis of the retina, which will greatly increase our understanding of the downstream events that occur in response to a mutation.

Concentric retinitis pigmentosa: clinicopathologic correlations

Progressive concentric (centripetal) loss of vision is one pattern of visual field loss in retinitis pigmentosa. This study provides the first clinicopathologic correlations for this form of retinitis pigmentosa. A family with autosomal dominant concentric retinitis pigmentosa was examined clinically and with visual function tests. A post-mortem eye of an affected 94 year old family member was processed for histopathology and immunocytochemistry with retinal cell specific antibodies. Unrelated simplex/multiplex patients with concentric retinitis pigmentosa were also examined. Affected family members of the eye donor and patients from the other families had prominent peripheral pigmentary retinopathy with more normal appearing central retina, good visual acuity, concentric field loss, normal or near normal rod and cone sensitivity within the preserved visual field, and reduced rod and cone electroretinograms. The eye donor, at age 90, had good acuity and function in a central island. Grossly, the central region of the donor retina appeared thinned but otherwise normal, while the far periphery contained heavy bone spicule pigment. Microscopically the central retina showed photoreceptor outer segment shortening and some photoreceptor cell loss. The mid periphery had a sharp line of demarcation where more central photoreceptors were near normal except for very short outer segments and peripheral photoreceptors were absent. Rods and cones showed abrupt loss of outer segments and cell death at this interface. It is concluded that concentric retinitis pigmentosa is a rare but recognizable phenotype with slowly progressive photoreceptor death from the far periphery toward the central retina. The disease is retina-wide but shows regional variation in severity of degeneration; photoreceptor death is severe in the peripheral retina with an abrupt edge between viable and degenerate photoreceptors. Peripheral to central gradients of unknown retinal molecule(s) may be defective or modify photoreceptor degeneration in concentric retinitis pigmentosa.

Photoreceptor rescue

Photoreceptor cell death is the final, irreversible event in many blinding diseases including retinitis pigmentosa, age-related macular disease and retinal detachment. This paper examines the potential strategies for preventing photoreceptor cell death in the context of current understanding of the mechanisms of cell death. There is evidence to suggest that photoreceptor cells are inherently vulnerable, apoptosis is the final common pathway of photoreceptor cell loss, and other retinal cells play an important role in the survival of rods and cones. Furthermore, the rationale of using neurotrophic factors as therapeutic agents in retinal degeneration is discussed in detail. Photoreceptor rescue by manipulation of genes involved in apoptosis and some pharmacological agents is also described.

Basic fibroblast growth factor is neither necessary nor sufficient for the development of retinal neovascularization

Basic fibroblast growth factor (FGF2) is constitutively expressed in the retina and its expression is increased by a number of insults, but its role in the retina is still uncertain. This study was designed to test the hypothesis that altered expression of FGF2 in the retina affects the development of retinal neovascularization. Mice with targeted disruption of the Fgf2 gene had no detectable expression of FGF2 in the retina by Western blot, but retinal vessels were not different in appearance or total area from wild-type mice. When FGF2-deficient mice were compared with wild-type mice in a murine model of oxygen-induced ischemic retinopathy, they developed the same amount of retinal neovascularization. Transgenic mice with a rhodopsin promoter/Fgf2 gene fusion expressed high levels of FGF2 in retinal photoreceptors but developed no retinal neovascularization or other abnormalities of retinal vessels; in the ischemic retinopathy model, they showed no significant difference in the amount of retinal neovascularization compared with wild-type mice. These data indicate that FGF2 expression is not necessary nor sufficient for the development of retinal neovascularization. This suggests that agents that specifically antagonize FGF2 are not likely to be useful adjuncts in the treatment of retinal neovascularization and therapies designed to increase FGF2 expression are not likely to be complicated by retinal neovascularization.

Disease sequence from mutant rhodopsin allele to rod and cone photoreceptor degeneration in man

Mutations in the gene encoding rhodopsin, the visual pigment in rod photoreceptors, lead to retinal degeneration in species from Drosophila to man. The pathogenic sequence from rod cell-specific mutation to degeneration of rods and cones remains unclear. To understand the disease process in man, we studied heterozygotes with 18 different rhodopsin gene mutations by using noninvasive tests of rod and cone function and retinal histopathology. Two classes of disease expression were found, and there was allele-specificity. Class A mutants lead to severely abnormal rod function across the retina early in life; topography of residual cone function parallels cone cell density. Class B mutants are compatible with normal rods in adult life in some retinal regions or throughout the retina, and there is a slow stereotypical disease sequence. Disease manifests as a loss of rod photoreceptor outer segments, not singly but in microscopic patches that coalesce into larger irregular areas of degeneration. Cone outer segment function remains normal until >75% of rod outer segments are lost. The topography of cone loss coincides with that of rod loss. Most class B mutants show an inferior-nasal to superior-temporal retinal gradient of disease vulnerability associated with visual cycle abnormalities. Class A mutant alleles behave as if cytotoxic; class B mutants can be relatively innocuous and epigenetic factors may play a major role in the retinal degeneration.

Rod outer segment maintenance is enhanced in the presence of bFGF, CNTF and GDNF

We employed a morphological assay of outer segment collapse to determine if growth factors or other supplements directly affect dissociated rod photoreceptors in vitro. The morphological changes in outer segments were correlated with the light responsiveness of rods. Time-lapse video microscopy was used to observe the collapse of rod outer segments from isolated single cells and small clumps of cells. A consistent pattern of outer segment collapse into the inner segment was observed, yielding a convenient assay of the effects of neurotrophic factors on photoreceptor functional maintenance. The functional state of rods, defined as light-responsiveness, was measured with suction electrode recordings and matched with the various stages of outer segment collapse. Ciliary neurotrophic factor (CNTF) and glial cell-line-derived neurotrophic factor (GDNF) at a high concentration, yielded statistically significant improvements in rat outer segment survival times. Basic fibroblast growth factor (bFGF), which rescues photoreceptors in several rodent models of retinal degeneration, produced a significant increase in survival time in the presence of the cofactor heparin. In 4 out of 10 cases using human tisue, bFGF also yielded a significant increase in survival times. When brain-derived neurotrophic factor (BDNF) was applied to rat rods, outer segment survival times did not change. Outer segments collapsed more quickly when either pigment epithelial cell derived factor (PEDF) or sugar N-acetyl D-galactosamine (NAD-gal) were present. Our results show that rod photoreceptors can respond to bFGF, GDNF and CNTF in vitro and provide evidence for a direct effect of these neurotrophic factors on rods. The rapid collapse of isolated photoreceptors in this model provides a convenient means for testing various neurotrophic agents and the induced cellular responses.