Correction of the disease phenotype in the mouse model of Stargardt disease by lentiviral gene therapy

Autosomal recessive Stargardt disease (STGD1) is a macular dystrophy caused by mutations in the ABCA4 (ABCR) gene. The disease phenotype that is most recognized in STGD1 patients, and also in the Abca4-/- mouse (a disease model), is lipofuscin accumulation in retinal pigment epithelium. Here, we tested whether delivery of the normal (wt) human ABCA4 gene to the subretinal space of the Abca4 -/- mice via lentiviral vectors would correct the disease phenotype; that is, reduce accumulation of the lipofuscin pigment A2E. Equine infectious anemia virus (EIAV)-derived lentiviral vectors were constructed expressing either the human ABCA4 gene or the LacZ reporter gene under the control of the constitutive (CMV) or photoreceptor-specific (Rho) promoters. Abca4-/- mice were injected subretinally with 1 microl ( approximately 5.0 x 10(5) TU) of each EIAV vector in one eye at postnatal days 4 and 5. An injection of saline, an EIAV-null vector, or an uninjected contralateral eye served as a control. Mice were killed at various times after injection to determine photoreceptor (PR) transduction efficiency and A2E concentrations. EIAV-LacZ vectors transduced from 5 to 20% of the PRs in the injected area in mice. Most importantly, a single subretinal injection of EIAV-CMV-ABCA4 to Abca4-/- mouse eyes substantially reduced disease-associated A2E accumulation compared to untreated and mock-treated control eyes. Treated eyes of Abca4-/- mice accumulated 8-12 pmol per eye (s.d.=2.7) of A2E 1 year after treatment, amounts comparable to wt controls, whereas mock-treated or untreated eyes had 3-5 times more A2E (27-39 pmol per eye, s.d.=1.5; P=0.001-0.005). Although extrapolation to humans requires caution, the high transduction efficiency of both rod and cone photoreceptors and the statistically significant reduction of A2E accumulation in the mouse model of STGD1 suggest that lentiviral gene therapy is a potentially efficient tool for treating ABCA4-associated diseases.

Behavior of retinal epithelium to bleb detachment versus retinectomy

PURPOSE

To compare the behavior of the retinal pigment epithelium to a bleb detachment versus removal of a segment of neural retina.

METHODS

A bleb detachment was performed on 14 adult pigmented rabbits. In seven rabbits, the neural retina was removed within the bleb detachment. The rabbits were followed for months after surgery by scanning laser ophthalmoscopy and post mortem histology using light and electron microscopy.

RESULTS

Bleb detachment produces a transformation of the retinal epithelial layer that results in migration, enlargement of cell size and accumulation of large amounts of lysosomal material, resembling that found in Chediak-Higashi syndrome. Retinectomy causes migration of the epithelial cells, development of multiple layers with vacuoles but no accumulation of lysosomal material.

CONCLUSION

The neural retina appears to exert a strong influence on the behavior of the retinal epithelial layer. Brief separation of the neural retina from the epithelium provokes a rapid transformation of this cell layer leading to migration of the cells and apparent faulty digestion of phagosomes, causing an enormous buildup of lysosomal debris. Removal of the neural retina also provokes retinal epithelial cell migration but no buildup of lysosomal debris occurs, presumably due to the absence of photoreceptor outer segments and consequently phagosomes. This migratory tendency, which appears to alter lysosomal degradation, could lead to apoptosis of these epithelial cells.

Genotyping microarray (gene chip) for the ABCR (ABCA4) gene

Genetic variation in the ABCR (ABCA4) gene has been associated with five distinct retinal phenotypes, including Stargardt disease/fundus flavimaculatus (STGD/FFM), cone-rod dystrophy (CRD), and age-related macular degeneration (AMD). Comparative genetic analyses of ABCR variation and diagnostics have been complicated by substantial allelic heterogeneity and by differences in screening methods. To overcome these limitations, we designed a genotyping microarray (gene chip) for ABCR that includes all approximately 400 disease-associated and other variants currently described, enabling simultaneous detection of all known ABCR variants. The ABCR genotyping microarray (the ABCR400 chip) was constructed by the arrayed primer extension (APEX) technology. Each sequence change in ABCR was included on the chip by synthesis and application of sequence-specific oligonucleotides. We validated the chip by screening 136 confirmed STGD patients and 96 healthy controls, each of whom we had analyzed previously by single strand conformation polymorphism (SSCP) technology and/or heteroduplex analysis. The microarray was >98% effective in determining the existing genetic variation and was comparable to direct sequencing in that it yielded many sequence changes undetected by SSCP. In STGD patient cohorts, the efficiency of the array to detect disease-associated alleles was between 54% and 78%, depending on the ethnic composition and degree of clinical and molecular characterization of a cohort. In addition, chip analysis suggested a high carrier frequency (up to 1:10) of ABCR variants in the general population. The ABCR genotyping microarray is a robust, cost-effective, and comprehensive screening tool for variation in one gene in which mutations are responsible for a substantial fraction of retinal disease. The ABCR chip is a prototype for the next generation of screening and diagnostic tools in ophthalmic genetics, bridging clinical and scientific research.

Lentiviral transduction of green fluorescent protein in retinal epithelium: evidence of rejection

This paper demonstrates lentiviral transduction of the humanized form of the Aequoria victoria gene for green fluorescent protein (GFP) into human fetal retinal pigment epithelium (RPE) in vitro and rabbit RPE in vivo. In vitro GFP expression of cultured human fetal RPE begins within two to three days after 12-16 h of maintained exposure to the virus at titers of 10(8)-10(9) infectious units (IU)/ml. Both stationary and dividing cells are transduced using a lenti viral vector with a cytomegalovirus (CMV) promoter. Expression remains stable for at least three to four months without evidence of toxicity and continues through cell division. In vivo expression is followed non-invasively in rabbit eye using a scanning laser ophthalmoscope (SLO), which can detect single fluorescing retinal cells. In vivo expression begins within a few days after a viral solution is introduced into the subretinal space. A solution of 10(9) IU/ml produces fluorescence within three to four days. Less concentrated solutions lead to slower and less expression. No expression is detectable at concentrations of 10(6) IU/ml. Within one to two weeks after introduction of the viral solution, there is evidence of rejection seen by SLO as a loss of GFP fluorescence and disruption of the RPE. Histology shows damage to the RPE layer and monocytic cell infiltrates in the choroid and subretinal space within the area receiving the viral solution. Strong GFP expression leads to rejection within two weeks. With less expression, rejection is delayed and in some cases undetectable for at least six months. If the GFP gene is not included in the viral vector or if the viral concentration is insufficient to produce detectable GFP expression, rejection is not seen. Using a rhodopsin promoter or injecting the virus intra rather than subretinally produces weak expression and no rejection. Lentivirus can induce expression of a foreign gene in the RPE. Viral induced transduction and GFP expression have no effect on the viability of the RPE in vitro. Continued expression of GFP after cell division implies chromosomal integration of the gene. In vivo expression of GFP in RPE encounters rejection. Rejection may not occur with low GFP expression. The latter occurs with low viral titers, a rhodopsin promoter or intra-retinal injection of viral solution. The results are relevant to gene therapy in retina when gene transduction leads to the expression of foreign proteins.

Identifying UV-cone responses in the murine superior colliculus

A microelectrode was used to detect local visually evoked potentials from clusters of neurons in the murine superior colliculus. Chromatic stimuli and selective chromatic adaptation were used to identify responses of UV and middle-wavelength-sensitive (M) cones and/or rods. Three types of evoked potentials were found: those driven by UV and M cones and/or rods and those driven only by UV cone or only by M cones and/or rods. UV cone responses were more frequent in the medial and those from M cones more common in the lateral part of the superior colliculus. All three responses were found in the same area. UV cones provide a significant input to the murine superior colliculus. The spatial distribution of these responses in the superior colliculus reflects the organization of UV cones in the retina. Although synergistic inputs from UV- and M-cone and/or rod inputs appear to mix in local evoked responses in the superior colliculus, some areas are found to transmit only UV- or only M-cone and/or rod responses, indicating that there cannot be a widespread mixing of UV- and M-cone opsins in all murine cones.

Ultraviolet and middle wavelength sensitive cone responses in the electroretinogram (ERG) of normal and Rpe65 -/- mice

Ultra-violet (UV) and middle wavelength sensitive (M) cone responses were identified in the ERG of normal and Rpe65 -/- mice using chromatic flashes and selective chromatic adaptation. In normal mice, the UV-cone response was as large as, or larger, in the presence of a bright yellow adapting light than it is in the presence of a dim white light. The M-cone response became undetectable in the presence of the yellow adapting light. Yellow adapting light initially reduced the UV response, but it recovered in 8-10 min. The M-cone response did not recover. UV-cone responses were undetectable in Rpe65 -/- mice. The M-cone response of young Rpe65 -/- mice was almost as large as in normal mice. A yellow adapting light only diminished this M-cone response. With age, the M-cone response further decreased in Rpe -/- mice. We show a pronounced loss of UV-cone function in Rpe65 -/- mice, which may be related to a defect UV-cones share with rods. The M-cone function is also affected already in young Rpe65 -/- mice. The transient effect of a yellow adapting light on the UV-cone response of normal mice is suggested to be neural, because it disappears during maintained light adaptation.

In vivo studies of the gamma subunit of retinal cGMP-phophodiesterase with a substitution of tyrosine-84

The inhibitory rod cGMP phosphodiesterase gamma subunit (PDEgamma) is a major component of the photoresponse and is required to support rod integrity. Pdeg(tm1)/Pdeg(tm1) mice (which lack PDEgamma owing to a targeted disruption of the Pdeg gene) suffer from a very rapid and severe photoreceptor degeneration. The Y84G (Tyr(84)-->Gly) allele of PDEgamma has previously been shown in experiments carried out in vitro to reduce the regulatory control of the PDE catalytic core (PDEalphabeta) exerted by the wild-type gamma subunit. To determine the effects of this mutation on in vivo function, the murine opsin promoter was used to direct expression to the photoreceptors of +/Pdeg(tm1) mice of a mutant Y84G and a wild-type PDEgamma control transgene. The transgenic mice were crossed with Pdeg(tm1)/Pdeg(tm1) mice to generate animals able to synthesize only the transgenic PDEgamma. Our results showed that wild-type PDEgamma and Y84G transgenes could complement the Pdeg(tm1)/Pdeg(tm1) mutant for photoreceptor survival. The mutation caused a significant biochemical defect in PDE activation by transducin. However, the Y84G mutation did not fully eliminate the control of PDEgamma on the PDE catalytic core in vivo; the expression of the mutant subunit was associated with only a 10-fold reduction in the amplitude of the a-wave and a 1.5-fold decrease in the b-wave of the corneal electroretinogram. Unexpectedly, the mutation caused a much 'milder' phenotype in vivo than was predicted from the biochemical assays in vitro.

Isolation of human fetal cones

PURPOSE

To describe a method for isolating a monolayer of human fetal cone photoreceptors and to compare their structure and ultrastructure before and after preparation.

METHODS

Eyes from human fetuses (fetal week 20 to 24) were dissected and the neural retina of the developing fovea identified, cut out and placed on 10% gelatin. A VISX Star excimer laser was used to remove the inner retinal layers. The isolated cone monolayers were cultured for 18 hours and compared with untreated retinas by light microscopy and transmission electron microscopy.

RESULTS

Excimer laser ablation removed the inner nuclear and ganglion cell layers leaving a monolayer of fetal cones. These cones survive in culture for at least 18 hours. The laser ablation disorganized the ultrastructure of the synaptic pedicles of these cones, left their plasma membranes intact.

CONCLUSIONS

The developing central retina of human fetal eyes provides a source of fetal cones, which can be isolated from inner retinal cells using the excimer laser. Such a monolayer of human fetal cones may be useful for transplantation or biochemical studies.

Cone inputs to murine retinal ganglion cells

Responses of single retinal ganglion cells in different areas of mouse retina were studied to determine their cone inputs, using spectral sensitivity functions and chromatic adaptation. Spectral sensitivity curves were based on threshold response criteria to full field stimulation. The retina of the mouse was viewed through a dilated pupil with a surgical microscope. Ganglion cells were classified into three groups: one receiving inputs from short wave sensitive cones, a second receiving inputs from only middle wavelength sensitive cones and a third receiving inputs from both of these types of cones. The ventral retina, contained a large fraction of the first group of ganglion cells. The dorsal retina and the border between these two areas contained relatively more of the latter two groups. A small fraction of cells were found which displayed antagonistic-like interactions between photoreceptor systems. The results demonstrate that single ganglion cells in mouse retina can select responses from only one of the two cone mechanisms present in this retina, even in areas containing both types of cones.

Local immunosuppression prolongs survival of RPE xenografts labeled by retroviral gene transfer

PURPOSE

To determine whether local immunosuppression with Cyclosporin A can influence the survival of human fetal retinal pigment epithelium (RPE) xenografts in the rabbit's subretinal space.

METHODS

Cultured human fetal RPE cells were transduced with the gene for green fluorescent protein (GFP) using a lentiviral vector. The RPE was transplanted into the subretinal space of rabbits that received intravitreal cyclosporine either by weekly injections (0. 25-0.5 mg) or by slow release (approximately 2 microg/d) from a capsule sutured into the vitreal cavity after prior cryopexy. The transplanted RPE was followed by GFP fluorescence scanning laser ophthalmoscopy and by histology of the transplant site.

RESULTS

RPE xenografts in eyes receiving intravitreal cyclosporine survived longer (several months) than they did in control eyes without cyclosporine. Survival was as long with slow release capsules as it was with weekly intravitreal injections at much higher concentrations of cyclosporine.

CONCLUSIONS

Local immunosuppression of the eye with cyclosporine prolongs the survival of RPE xenografts in the subretinal space of rabbits, implying that rejection involves activated T lymphocytes. Local immunosuppression with slow release capsules is as effective as weekly injections at much higher concentrations.

A new component in the a-wave of the human cone electroretinogram

The human cone electroretinogram (ERG) to a full field flash has been examined on a rod saturating background (17,000 photopic trolands). With strong stimuli, a negative wavelet appears in the falling phase of the a-wave. This response has a latency of 10-12 milliseconds, about 6-8 milliseconds after the start of the a-wave and just before the rising phase of the corneal positive b-wave begins. We suggest that it may represent a hyperpolarizing response of second order retinal neurons.

Cone properties of the light-adapted murine ERG

PURPOSE

Because the mouse lacks a typical Purkinje shift, we have examined its light-adapted ERG to determine whether there was other evidence in addition to tolerance to background light, that could be used to identify cone function in the ERG.

METHODS

Full field corneal ERGs to white flashes, double flashes and flash trains were examined in the presence of a strong full field light adaptation and compared with the human cone ERG.

RESULTS

The following cone-like properties could be identified. (1) The light-adapted murine ERG increases in amplitude gradually during the first 10 minutes of light-adaptation; (2) It is capable of responding to a 50 Hz stimulus, although its overall frequency response is slower than that of the human cone ERG; (3) A corneal positive d-wave occurs to the termination of a flash train; (4) The response increases linearly with light intensity.

CONCLUSION

The light-adapted murine ERG has several properties of cones but it has a slower response than the human cone ERG.

Impaired retinal function and vitamin A availability in mice lacking retinol-binding protein

Retinol-binding protein (RBP) is the sole specific transport protein for retinol (vitamin A) in the circulation, and its single known function is to deliver retinol to tissues. Within tissues, retinol is activated to retinoic acid, which binds to nuclear receptors to regulate transcription of >300 diverse target genes. In the eye, retinol is also activated to 11-cis-retinal, the visual chromophore. We generated RBP knockout mice (RBP(-/-)) by gene targeting. These mice have several phenotypes. Although viable and fertile, they have reduced blood retinol levels and markedly impaired retinal function during the first months of life. The impairment is not due to developmental retinal defect. Given a vitamin A-sufficient diet, the RBP(-/-) mice acquire normal vision by 5 months of age even though blood retinol levels remain low. Deprived of dietary vitamin A, vision remains abnormal and blood retinol declines to undetectable levels. Another striking phenotype of the mutant mice is their abnormal retinol metabolism. The RBP(-/-) mice can acquire hepatic retinol stores, but these cannot be mobilized. Thus, their vitamin A status is extremely tenuous and dependent on a regular vitamin A intake. Unlike wild-type mice, serum retinol levels in adult RBP(-/-) animals become undetectable after only a week on a vitamin A-deficient diet and their retinal function rapidly deteriorates. Thus RBP is needed for normal vision in young animals and for retinol mobilization in times of insufficient dietary intake, but is otherwise dispensable for the delivery of retinol to tissues.

Tracking RPE transplants labeled by retroviral gene transfer with green fluorescent protein

PURPOSE

To determine whether human retinal pigment epithelium (RPE) can be modified by retroviral-mediated gene transfer and to monitor the human RPE cells in the subretinal space of living rabbits with scanning laser ophthalmoscopy (SLO).

METHODS

Cultured human fetal retinal pigment epithelium (HFRPE) was exposed to green fluorescent protein (GFP)-transducing retroviral vectors, Moloney murine leukemia virus, and lentivirus. The cultured cells were followed by fluorescence microscopy. Suspensions of GFP-expressing HFRPE were transplanted into the subretinal space of pigmented rabbits, and the transplant sites were examined by SLO for fluorescence, including fluorescein and indocyanine green angiography. The rabbits were euthanatized at different times after transplantation, and the retinas were studied histologically.

RESULTS

Retroviral gene transfer can introduce a foreign gene such as GFP into cultured HFRPE. Gene expression is maintained in cultured RPE for at least 3 months. The lentiviral vector transduced both nondividing and dividing cells; the Moloney vector only transduced the latter. GFP-expressing cells can be followed in the living retina. Their changes reflect the rejection response followed histologically.

CONCLUSIONS

Cultured HFRPE could be transduced to express GFP for long periods of time by retroviral gene transfer. GFP allowed retinal transplants and gene expression to be monitored in vivo. These results provide a model for potential ex vivo gene therapy in the subretinal space.

Long-term outcome of RPE allografts in non-immunosuppressed patients with AMD

PURPOSE

To determine the long-term outcome of human retinal pigment epithelium (RPE) transplants in patients with advanced age-related macular degeneration (AMD).

METHODS

Using pars plana microsurgical techniques, RPE allografts were transplanted subretinally to four groups of AMD patients: five patients received organized patch transplants after removal of choroidal neovascular membranes, four got small patch transplants in dry AMD; suspensions of RPE cells were transplanted in five cases with dry AMD, and two patients with RPE tears. None received immunosuppression. Transplants were followed for 24-38 months by biomicroscopy, fundus photography, SLO microperimetry, and fluorescein angiography. Rejection was defined as loss of visual function over the transplant, development of an exudative response (subretinal fluid with or without neovascularization), fluorescein leakage, and disruption, depigmentation, or encapsulation of the transplant.

RESULTS

Four of 16 transplants (25%) presented no clinical signs of rejection. Three of the four small patch transplants remained pigmented and essentially unchanged after 30-32 months. Clinical signs of graft rejection appeared within three months in all cases of neovascular AMD (disrupted blood-retinal barrier, BRB), but after 6-20 months in five of nine eyes with non-exudative AMD (intact BRB).

CONCLUSIONS

Subretinal human RPE allografts present a high rejection rate (75%) without immunosuppression. However, small extrafoveal transplants remained unchanged in shape, size and color for more than two years in non-exudative AMD. A disrupted BRB is likely to enhance graft rejection, which occurs earlier in exudative than in non-exudative AMD.

Photoreceptor allografts in a feline model of retinal degeneration

BACKGROUND

Photoreceptor transplants provide a potential means to restore function in a degenerate retina and/or rescue degenerating host photoreceptors by trophic influences. We have examined photoreceptor allografts in the Abyssinian cat model of hereditary photoreceptor degeneration to determine the viability and influence of such transplants on the host retina.

METHODS

Small pieces of 3- to 5-day-old normal kitten retina containing undifferentiated photoreceptors were injected into the subretinal space of adult Abyssinian cats at an early stage of retinal degeneration using standard vitreo-retinal surgical techniques. The retinas were examined by ophthalmoscopy and fundus photography, then by light and electron microscopy at different times after surgery.

RESULTS

Such allografts survive for at least 6 months after surgery. The photoreceptors develop outer segments, invariably in rosettes. The transplants gradually integrate with the host retina but detach the host photoreceptor layer from the retinal pigment epithelium (RPE), which tends to reduce the number of host photoreceptors over the transplant. There is no slowing of the photoreceptor degeneration in neighboring non-detached retina. Inflammation or rejection was not detected.

CONCLUSION

Undifferentiated, neonatal photoreceptor allografts survive and develop outer segments in the subretinal space of the Abyssinian mutant feline retina. The allografts gradually integrate with the host neural retina without inducing rejection. In the vicinity of the transplant there is increased loss of host photoreceptors, considered to be due to their detachment from the RPE layer. There is no evidence of any rescue of host photoreceptors elsewhere in this mutant retina.

Role for the target enzyme in deactivation of photoreceptor G protein in vivo

Heterotrimeric guanosine 5'-triphosphate (GTP)-binding proteins (G proteins) are deactivated by hydrolysis of the GTP that they bind when activated by transmembrane receptors. Transducin, the G protein that relays visual excitation from rhodopsin to the cyclic guanosine 3',5'-monophosphate phosphodiesterase (PDE) in retinal photoreceptors, must be deactivated for the light response to recover. A point mutation in the gamma subunit of PDE impaired transducin-PDE interactions and slowed the recovery rate of the flash response in transgenic mouse rods. These results indicate that the normal deactivation of transducin in vivo requires the G protein to interact with its target enzyme.

Cone ERG subnormality to red flash in central retinal vein occlusion: a predictor of ocular neovascularisation?

In patients with unilateral central retinal vein occlusion (CRVO), we retrospectively examined whether cone electroretinogram (ERG) subnormality to red flash (ratio of the b-wave amplitude in the CRVO eye to that in the normal fellow eye < 1) found at the time of diagnosis of the CRVO was a predictor of later ocular neovascularisation. Ganzfeld ERG cone and rod responses had initially been obtained in a consecutive series of 21 patients with unilateral CRVO. Patients were re-evaluated 6-55 months later to determine whether ocular neovascularisation had developed. Of the 21 CRVO eyes, 6 (29%) were subnormal to red in the affected compared with the normal fellow eye. At follow-up, all 6 (100%) patients had developed ocular neovascularisation compared with 1 (7%) of the 15 patients who were supernormal to red (p = 0.00013). Cone ERG subnormality to red flash in CRVO eyes compared with normal fellow eyes may be a predictor of later development of ocular neovascularisation.

Uniqueness of the S-cone pedicle in the human retina and consequences for color processing

The purpose of this study was to investigate more fully the shape and content of ribbons and synapses to second-order neurons in the short-wavelength cone (S-cone, blue cone) pedicle and to learn more concerning the uniqueness of the S-cone system in the primate retina. A piece of well-fixed peripheral human retina (10 mm, 35 degrees nasal to the fovea) was serially thick sectioned in the tangential plane from the level of the outer segments to the tops of the cone pedicles. Then serial electron microscope (EM) sections were collected through the whole depth of the pedicle-occupying region into the neuropil of the outer plexiform layer (OPL). The resultant EM micrograph montages of a large field of cone pedicles were perused, and S-cone pedicles were identified. Serial micrographs of a single S-cone pedicle, picked out of the montages, were digitized and reconstructed by computer three-dimensional methods. The S-cone pedicle arose from a slightly oblique axon and projected 0.5-1 microm more vitread in the OPL than other cone pedicles. It was bilobed in shape, with synaptic invaginations and ribbons in both lobes. No cone-contacting telodendria projected from the S-cone pedicle itself, but a small number of neighboring cones sent telodendria to its surface to make small gap junctions. Neighboring rod spherules also made small gap junctions. Four robust bipolar cell dendrites, most likely from S-cone-specific bipolar cells, made synapses at ribbons and basal (distal) junctions. A small number of other bipolar cell dendrites made narrow-cleft basal junction only. The majority of lateral elements were thought to be from HII horizontal cells, and a minority from HI horizontal cells. We conclude that the S-cone pedicle has a unique morphology and connectivity to second-order neurons that makes it quite different from the other two longer wavelength cone systems, and we speculate on the consequences for color processing in the visual system in general.

Retarding photoreceptor degeneration in Pdegtm1/Pdegtml mice by an apoptosis suppressor gene

PURPOSE

Mice (Pdegtm1/Pdegtm1) homozygous for a mutant allele of the gamma subunit of retinal cyclic guanosine monophosphate phosphodiesterase (PDE gamma) suffer a severe photoreceptor degeneration. To determine whether the antiapoptotic BCL2 gene is effective in delaying the cell death pathway in this new strain of mutant mice, a transgene encoding the BCL2 gene product was introduced by mating into the mutant background, and the resulting mice were examined for possible rescue of the retinal degeneration.

METHODS

Electroretinograms (ERGs) of the Pdegtm1/Pdegtm1 mice carrying BCL2 were taken to monitor the responses to light. Light and electron microscopy of sections were used to examine degeneration at different times after birth.

RESULTS

The ERGs of the mutants with the transgene were larger than those without the transgene at 2 and 3 weeks after birth. The maximum differences occurred at 2 weeks postpartum. At 4 weeks after birth, no ERG could be detected in either strain. Histologic analysis showed a greater preservation of photoreceptor nuclei in the Pdegtm1/Pdegtm1 mice containing the BCL2 transgene, which paralleled the electroretinography.

CONCLUSIONS

The introduction of an antiapoptotic transgene BCL2 can delay temporarily and partially the degeneration of photoreceptors in a new autosomal-recessive murine model of retinal degeneration.

Transplantation of RPE in age-related macular degeneration: observations in disciform lesions and dry RPE atrophy

A study was carried out to investigate whether human RPE allografts are tolerated or rejected in the subretinal space and to determine the feasibility of RPE transplantation in subjects with age-related macular degeneration (AMD).

METHODS

Patches of human fetal RPE (13-20 weeks of gestational age) were transplanted into the subretinal space of five patients after surgical removal of subfoveal fibrovascular membranes, and to four subjects with dry geographic atrophy. Suspensions of RPE cells were transplanted to four other patients with nonexudative AMD. Results were evaluated with clinical ophthalmological examination, SLO microperimetry and fluorescein angiography over 8-20 months.

RESULTS

In disciform lesions, RPE transplants developed macular edema and fluorescein leakage concomitant with gradual reduction of visual acuity, implying host-graft rejection, over 1-6 months. In geographic atrophy, three of four transplants showed little change in shape and size after 12 months (one transplant was slowly rejected). In non-exudative AMD, RPE suspension transplants showed no evidence of rejection and were associated with the disappearance of drusen; visual acuity remained stable and SLO microperimetry confirmed retinal function over the transplanted area.

CONCLUSION

Human RPE allografts are not invariably rejected in the subretinal space without immunosuppression. The rejection rate is lower in nonexudative than in neovascular AMD. An intact blood-retinal barrier is likely to protect against rejection. It is technically feasible to transplant human RPE into the submacular space without adversely affecting visual function in nonexudative AMD over relatively long periods of time.

Tolerance of human fetal retinal pigment epithelium xenografts in monkey retina

BACKGROUND

RPE transplantation offers the possibility of treating certain forms of retinal degeneration. Understanding how to optimize the surgical technique for performing RPE transplantation, especially in primates, is therefore of considerable interest.

METHODS

Fifteen patch RPE transplants were performed in six monkeys. The transplant sites were examined at follow-up by ophthalmoscopy, biomicroscopy, fluorescein angiography and histology. Foveal and peripheral retinal transplants were compared.

RESULTS

Human fetal RPE xenografts can survive without rejection for at least 6 months after transplantation in monkey retina. Such grafts form a basal lamina and make intimate contacts with the outer segments of the host. Both rods and cones retain a normal appearance when in contact with unrejected transplants. Rejection occurred in only 30% (3/10) of the peripheral but in 60% (3/5) of the foveal transplants.

CONCLUSIONS

Cultured human fetal RPE patch transplants can survive and maintain local photoreceptor integrity for relatively long periods of time in monkey subretinal space without immunosuppression. Rejection, when it occurs, is more frequent near the fovea.

Retinal cell transplantation in the macula: new techniques

Transplantation of RPE allografts into the submacular space after the removal of neovascular membranes in "wet" forms of ARMD usually leads to chronic leakage around the transplant, whereas this seldom occurs in transplants in "dry" forms of ARMD. The former may be the result of chronic rejection because of a compromised blood/retinal barrier. This is supported by the fact that visual function remains over the transplant in the dry forms of ARMD at 6 months after transplantation. Whether this function can slow or reverse the progression of ARMD remains to be seen. The possibility of transplantation of photoreceptors in retinal degenerations is also reviewed.

Cone ERGs to flash trains: the antagonism of a later flash

The human cone electroretinogram (ERG) to flash trains and double flashes at different interflash intervals was studied. A double ganzfeld stimulator was used in which computer controlled flashes were presented independently in the presence of strong rod saturating backgrounds. Corneal ERGs were examined at different frequencies of flash train presentations. Flash trains with individual flashes of high frequency (100 Hz) simulate a cone ERG to light pulses of long duration by producing a corneal positive off-response (d-wave) time locked to the cessation of the train. A second flash can reduce and delay the cone b-wave produced by a first flash. This effect is maximal when the second flash occurs 10-12 msec later. There is an antagonistic mechanism in the cone system of the retina which can catch and reduce the cone b-wave produced by an earlier flash.

Photoreceptor cell rescue in retinal degeneration (rd) mice by in vivo gene therapy

Mutations in the beta subunit of the cGMP phosphodiesterase gene (beta PDE) can cause a recessively inherited retinal degeneration in several species, including mice, dogs and humans. We tested the possibility of altering the course of retinal degeneration in the rd mouse through subretinal injection of a recombinant replication-defective adenovirus that contains the murine cDNA for wild-type (beta PDE, Ad.CMV beta PDE. Subretinal injection of Ad.CMV beta PDE results in beta PDE transcripts and increased PDE activity and delays photoreceptor cell death by six weeks. The findings demonstrate cell rescue by in vivo gene transfer, thus supporting the feasibility of treating an inherited retinal degeneration by somatic gene therapy.

Retinal degeneration in mice lacking the gamma subunit of the rod cGMP phosphodiesterase

The retinal cyclic guanosine 3',5'-monophosphate (cGMP) phosphodiesterase (PDE) is a key regulator of phototransduction in the vertebrate visual system. PDE consists of a catalytic core of alpha and beta subunits associated with two inhibitory gamma subunits. A gene-targeting approach was used to disrupt the mouse PDEgamma gene. This mutation resulted in a rapid retinal degeneration resembling human retinitis pigmentosa. In homozygous mutant mice, reduced rather than increased PDE activity was apparent; the PDEalphabeta dimer was formed but lacked hydrolytic activity. Thus, the inhibitory gamma subunit appears to be necessary for integrity of the photoreceptors and expression of PDE activity in vivo.

Digitalis-induced visual disturbances with therapeutic serum digitalis concentrations

OBJECTIVE

To assess the role of digitalis in the development of visual symptoms severe enough to warrant ophthalmologic consultation in patients who received digitalis and who had no other clinical or laboratory evidence of digitalis toxicity.

DESIGN

Clinical case study.

SETTING

Neuro-ophthalmology referral practice.

PATIENTS

Six elderly patients (aged 66 to 85 years) who received digitalis were referred to ophthalmologists for evaluation of photopsia (five patients) or decreased visual acuity (one patient). No patient had chromatopsia or nonvisual clinical manifestations of digitalis intoxication at the time of examination.

MEASUREMENTS

All patients had serum digitalis concentrations within or below the therapeutic range. In most patients, the electroretinographic cone b-wave implicit time was longer than normal.

RESULTS

Discontinuation of digitalis therapy, which was possible in five patients, was followed by resolution of visual symptoms and by shortening of the b-wave implicit time. Characteristic features of digitalis-induced photopsia were its dependence on illumination and its tendency to be localized in peripheral visual fields.

CONCLUSIONS

In an elderly patient receiving digitalis, the development of photopsia characterized by innumerable points of light in the peripheral visual fields or a decrease in visual acuity raises the possibility that the patient's visual disturbance may have been digitalis induced. Digitalis-induced visual disturbances other than chromatopsia or disturbances of color vision may occur in elderly patients who have no other clinical manifestations of digitalis intoxication and who have a serum digitalis concentration within or below the therapeutic range.

Retroviral gene transfer into retinal pigment epithelial cells followed by transplantation into rat retina

In this preliminary report, we describe a technique for gene transfer into the retina using a retrovirus vector. We transferred the bacterial LacZ gene and the neomycin-resistance gene into pigmented wild-type rat retinal pigment epithelial (RPE) cells in culture. The RPE culture was exposed to retrovirus, and infected cells were selected with a neomycin analog (G418). The LacZ gene product was detected by X-Gal histochemistry in 95-100% of drug-resistant cells. These genetically labeled cells were transplanted into the subretinal space of two 15- to 25-day-old albino RCS rats, which have an inherited retinal degeneration syndrome. The retinas were fixed and stained with X-Gal at 3 and 6 weeks after transplantation. At both time points, pigmented, LacZ-containing cells were seen in the subretinal space. Further, there were several rows of photoreceptor nuclei in the transplant area of the approximately 2-month-old rats, while in the control contralateral eye the photoreceptor nuclei were virtually absent, as for untreated animals, suggesting that the transplanted LacZ-marked, wild-type RPE cells may have helped preserve photoreceptors. The technique for gene transfer into RPEs followed by transplantation thus provides a means for gene therapy in organisms with a genetic defect in RPE cells.

Clinical electroretinography in the dog with ganzfeld stimulation: a practical method of examining rod and cone function

We used a simple and reproducible technique with full-field electroretinography and a special-purpose computer system to test and evaluate outer retinal function in dogs. The standardized protocol included the following five basic responses: (1) a stable initial light-adapted, mainly cone derived response, (2) a dark-adapted rod response, (3) chromatically separated rod and cone responses, (4) a maximal rod and cone response and (5) an isolated cone flicker response. For evaluating the electroretinographic responses, a graphic-presentation was used that included data from the tested animal as well as normative data from dogs of the specific breed and age group, presented as the percentage of the median in which limits of normality were depicted in percentiles.

The effects of bFGF on RCS rat eyes

Basic fibroblast growth factor (bFGF) has been implicated as a factor in retinal differentiation and disease. Recent studies have shown that subretinal or intravitreal injections of bFGF delay the retinal degeneration of the RCS rat but the global nature of this effect has been quantified for few test animals and the mechanism underlying this effect is not understood. In order to determine more accurately the global effects of intravitreal bFGF and to further elucidate the mechanism of bFGF promoted photoreceptor cell saving, we injected one of three bFGF doses into the vitreal cavities of young RCS rats. Using measurements from several eyes, we confirmed that a single intravitreal bFGF injection globally delays the RCS dystrophy. Test eyes contained fewer debris zone macrophages and more inner retinal macrophages than did control eyes at 1 month post injection. As bFGF's saving effect waned, the number of inner retinal macrophages decreased and the number of debris zone macrophages increased toward control levels. Dose-dependent cataract formation occurred in 100% of test eyes. Eyes that received the highest bFGF dose showed increased retinal vascularization at 1, 2 and 3 months post injection. The possible relationships between bFGF promoted photoreceptor survival and our findings are discussed.

The focal cone electroretinogram

The focal cone electroretinogram (ERG) in monkey retina has been examined with a 3 deg pulse of laser light (544 and 633 nm) centered on a 25 deg steady white rod saturating field. The stimuli were viewed simultaneously through a slit lamp and corneal contact lens. Cone ERGs were studied at different eccentricities from the fovea and compared with full-field corneal and intraretinal ERGs. The cone ERG is maximum at the fovea. There are two components to the on- (b-wave) and off- (d-wave) response, one slower, more long wavelength sensitive and more foveally oriented than the faster response. This makes the foveal cone ERG slower and more longer wavelength sensitive than the perifoveal ERG. This difference disappears at high rates (> 20 Hz) of stimulation. The foveal cone ERG is larger and slower than that of more peripheral retina. The slowness appears to be due to a subcomponent of the response which is especially prominent in the fovea and has a slightly greater long wavelength sensitivity than the more peripherally generated ERG. It may depend on a unique difference in L-M cone bipolar systems or in L-M cone interactions that are more prominent near the fovea.

Patch transplants of human fetal retinal pigment epithelium in rabbit and monkey retina

PURPOSE

To transplant human fetal retinal pigment epithelium (RPE) into the subretinal space of rabbits and monkeys as an organized monolayer without artificial support.

METHODS

The method involves dissecting small patches of cultured RPE monolayers in sheets (1 to 5 mm2), sucking them into a glass pipette and injecting them into the subretinal space after producing a bleb detachment of the neural retina.

RESULTS

These patches unfold and survive as a quasi-monolayer under the reattached neural retina intimately associated with the host photoreceptors and phagocytizing host outer segment material. Graft rejection is observed in most rabbits at 1 month but not in monkeys at 3 months after transplantation.

CONCLUSIONS

Monolayer patches of cultured human fetal RPE can be transplanted to the subretinal space, where they survive in contiguity with healthy host outer segments. In primates, but not in rabbits, host-graft rejection does not occur for at least 2 to 3 months.

Transplantation of fetal retinal pigment epithelium in age-related macular degeneration with subfoveal neovascularization

BACKGROUND

Age-related macular degeneration (ARMD) is caused by abnormal retinal pigment epithelium (RPE) and may be complicated by choroidal neovascularization. The object of treatment would be to replace the diseased RPE with normal human RPE.

METHOD

Five patients with ARMD (preoperative visual acuity 0.08-0.2) underwent removal of subretinal fibrovascular membranes using pars plana vitrectomy techniques. Human fetal RPE (15-17 weeks gestational age) was cultured and transplanted as a monolayer patch into the subretinal space. Transplants were followed by funduscopy and fluorescein angiography. Macular function was assessed using scanning laser ophthalmoscopic (SLO) microperimetry.

RESULTS

Three RPE transplants were placed in the fovea; two were placed parafoveally. All transplants have survived for 3 months. They have grown and increased in size covering part of the epithelial defect caused by removal of the fibrovascular membrane. SLO microperimetry indicated that visual function was present in four of the transplants at 1 month but in only two at 3 months after surgery. Function over the transplants, especially those in the fovea, was compromised by cystoidlike macular edema.

CONCLUSIONS

Human fetal RPE transplants survive well in the macula for as long as 3 months. They are capable of growing to cover epithelial defects caused by removal of subretinal neovascular membranes. The causes for development of macular edema in transplants directly in the fovea warrant further evaluation.

Reporter gene expression in cones in transgenic mice carrying bovine rhodopsin promoter/lacZ transgenes

Rhodopsin gene expression has been used as a model system to study the mechanisms regulating photoreceptor gene expression. Previous transgenic experiments using rhodopsin promoter/lacZ fusion constructs identified some of the cis-acting DNA elements responsible for photoreceptor cell-specific expression. However, the issue of rod specificity vs. photoreceptor (rod and cone) specificity of the elements was not resolved. To address this issue, the specificity of reporter gene expression in the retinas of transgenic mice carrying bovine rhodopsin promoter/lacZ (beta-galactosidase) fusion genes was assessed using X-gal staining and electron microscopy. Two independent transgenic lines, one carrying a rhodopsin promoter fragment extending from -2174 to +70 base pairs (bp) relative to the messenger RNA start site and another line carrying a fragment from -222 to +70 bp, both showed reporter gene expression in cones as well as rods, although the level of staining appeared to be less in the cones than in the rods. These results demonstrate that the -2174 to +70 bp and -222 to +70 bp bovine rhodopsin promoter fragments are not rod-specific in transgenic mice and indicate that the existence of rod promoter mediated-expression in cones must be considered when interpreting results from transgenic experiments utilizing the rhodopsin promoter.

Patch culturing and transfer of human fetal retinal epithelium

Human retinal pigmented epithelium (RPE) can be cultured by removing small patches of this layer from the choroid of the fetal eye. Such RPE patches give rise to healthy, epithelioid monolayers in vitro within 1-2 weeks without contamination from retinal or choroidal cells. The viability and proliferative capacity of these cultures is independent of the initial polarity of the patch. These RPE monolayers develop apical/basal polarity and a basal lamina and rest on a field of collagen fibers; they are capable of phagocytizing outer segments. A patch can be lifted off a confluent monolayer and transferred to another culture dish without risking the viability of either the old or the new culture. This provides a means of transplanting an organized, polarized patch of human RPE from one place to another.

Long-term photoreceptor transplants in dystrophic and normal mouse retina

PURPOSE

To determine the long-term status of transgenic photoreceptors transplanted to the subretinal space of both rd mutant (receptorless) and normal mouse retina.

METHODS

Microaggregates of neural retina from transgenic mice containing lacZ-labeled photoreceptors were transplanted to the subretinal space of adult rd mutant and normal mice. The transplant site was examined by light and electron microscopy at monthly intervals up to 9 months after transplantation surgery.

RESULTS

Photoreceptors develop and survive well if transplanted with the proper orientation to the retinal pigment epithelium (RPE). The status of the photoreceptors, including outer segments and synaptic terminals, appear normal for at least 9 months after transplantation; they continue to express the lacZ reporter gene. Cones survive as well as rods. Transplants to the normal mouse develop normally, whereas the host photoreceptors displaced from the RPE degenerate. A barrier, formed by Müller cell processes, develops after photoreceptor degeneration in both normal and rd mouse retina and demarcates host from transplant tissue. Areas can be found in which neural processes have penetrated this barrier. There is no evidence of host-graft rejection.

CONCLUSION

Transplanted progenitor photoreceptors develop and survive well for long periods of time in either the rd mutant or normal retina if they are properly positioned. In the former, they reconstitute a photoreceptor layer; in the latter, they replace the host photoreceptor layer, which degenerates after being displaced from the RPE. Areas of potential contact between donor and host neurons exist in these transplants.

Retinal pigment epithelial transplants and retinal function in RCS rats

PURPOSE

To determine if retinal pigment epithelium (RPE) transplantation maintains visual function in Royal College of Surgeons (RCS) strain of rats.

METHODS

Twelve RCS rats received RPE transplants at 16 to 20 days after birth. The retinas were studied electrophysiologically and histologically from 3 to 10 months after transplantation and compared with 11 RCS controls and 11 normal rats of comparable ages. A microelectrode was guided to the transplant site visible by its pigmentation in the albinotic RCS retina to detect responses.

RESULTS

Spontaneous ganglion cell activity was present in all retinas. Ganglion cell responses to light were detected in 9 of the 12 transplant eyes but not in any of the 11 controls. 96, 44, 140 units were encountered and 30%, 0%, 97% were driven by light respectively in transplant, control, and normal retinas. In transplants 36%, 29%, and 28% were driven at 3 to 4, 6 to 7, and 10 months after transplantation, respectively. Intraretinal ERGs with both a- and b-waves were recorded in 5 of the 8 transplants studied. None of the RCS controls studied had an IERG. The average IERG was 2.5 microV (SD = 1.9) in transplants and 59 microV (SD = 19) in normal retinas. The electrode track was traced to the transplant site in six of the seven retinas that were responsive to light and examined histologically.

CONCLUSION

RPE transplants to RCS rats maintain retinal function in the transplant site for long periods of time.

The human S-cone electroretinogram and its variation among subjects with and without L and M-cone function

PURPOSE

To examine the S-cone ERG in subjects with and without L and M-cone function.

METHODS

Ganzfeld spectral flashes in the presence of strong Ganzfeld adapting fields are used to elicit S-cone ERGs.

RESULTS

The S-cone ERG b-wave ranges from 0.2 to 4 mV in amplitude and 38-45 msec in implicit time. There is a progressive decrease in amplitude with age. The response is similar in subjects with or without L and M cone function.

CONCLUSION

The S-cone ERG is detectable in subjects of all ages, but intersubject variability limits its diagnostic usefulness. The S-cone ERG is slightly later than but does not appear to be obviously influenced by the L and M-cone ERG.

Reconstruction of degenerate rd mouse retina by transplantation of transgenic photoreceptors

Photoreceptors from neonatal transgenic mice with normally developing retinas were transplanted to the subretinal spaces of 2-3-month-old rd mutant mice that lack photoreceptors. The transgenic mouse photoreceptors express high levels of the lac Z reporter gene product, beta-galactosidase, which facilitated tracking the transplanted cells. Two sources were used for these cells: (1) dissection of retinal microaggregates containing photoreceptors and (2) papain-dissociated photoreceptors. Host retinas were examined after transplantation. Both methods led to survival of photoreceptors for at least 2 mo after transplantation. Relatively mature outer segments were found only in transplanted microaggregates; this occurred optimally when the cells were adjacent to the retinal pigment epithelium (RPE). beta-galactosidase-labeled outer segments associated closely with the apical processes of the host RPE, which, together with labeled phagosomes in the RPE cells, suggested functional interaction between the transplanted photoreceptors and the host RPE. This study is the first to the authors' knowledge to show electron microscopically that a morphologically normal-appearing photoreceptor layer can be reconstructed in an otherwise photoreceptorless retina.

Muller cells of chicken retina synthesize 11-cis-retinol

The amounts of endogenous retinyl palmitate, retinol and retinaldehyde were measured in the neural retina and retinal pigment epithelium (RPE) of predominantly cone (chicken), rod (rat) and more mixed (cat, human) retinae. The ratio of 11-cis to all-trans isomers of retinyl palmitate and retinol in the neural retina and the RPE increases progressively with the increase in diurnality of the species from rat to chicken. The membrane fractions of both chicken and bovine RPE enzymically isomerize all-trans retinol to 11-cis-retinol. Chicken neural retina membranes enzymically form 11-cis-retinol and all-trans-retinyl palmitate from all-trans-retinol. Light and electron microscopy revealed no contamination of chicken neural retina by RPE. Muller cells from chicken retina were isolated, cultured and characterized by immunocytochemical localization of cellular retinaldehyde-binding protein. Cultured chicken Muller cells form all-trans-retinyl palmitate, 11-cis-retinol and 11-cis-retinyl palmitate from all-trans-retinol and release most of the 11-cis-retinol into the medium. The results indicate that chicken neural retina and Muller cells in particular synthesize 11-cis-retinoids from all-trans-retinol.