Amanda E. Faulkner

High Susceptibility to Experimental Myopia in a Mouse Model with a Retinal ON Pathway Defect

PURPOSE Nob mice share the same mutation in the Nyx gene that is found in humans with complete congenital stationary night blindness (CSNB1). Nob mutant mice were studied to determine whether this defect resulted in myopia, as it does in humans. METHODS Refractive development was measured in unmanipulated wild-type C57BL/6J (WT) and nob mice from 4 to 12 weeks of age by using an infrared photorefractor. The right eye was form deprived by means of a skull-mounted goggling apparatus at 4 weeks of age. Refractive errors were recorded every 2 weeks after goggling. The content of dopamine and the dopamine metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) were measured by HPLC with electrochemical detection (HPLC-ECD) in retinas of nob and WT mice under light- and dark-adapted conditions. RESULTS The nob mice had greater hyperopic refractive errors than did the WT mice under normal visual conditions, until 12 weeks of age when both strains had similar refractions. At 6 weeks of age, refractions became less hyperopic in the nob mice but continued to become more hyperopic in the WT mice. After 2 weeks of form deprivation (6 weeks of age), the nob mice displayed a significant myopic shift (~4 D) in refractive error relative to the opposite and control eyes, whereas WT mice required 6 weeks of goggling to elicit a similar response. As expected with loss of ON pathway transmission, light exposure did not alter DOPAC levels in the nob mice. However, dopamine and DOPAC levels were significantly lower in the nob mice compared with WT. CONCLUSIONS Under normal laboratory visual conditions, only minor differences in refractive development were observed between the nob and WT mice. The largest myopic shift in the nob mice resulted after form deprivation, suggesting that visual pathways dependent on nyctalopin and/or abnormally low dopaminergic activity play a role in regulating refractive development. These findings demonstrate an interaction of genetics and environment in refractive development.

Tauroursodeoxycholic acid preserves photoreceptor structure and function in the rd10 mouse through post-natal day 30

PURPOSE Retinitis pigmentosa (RP) is a progressive neurodegenerative disease resulting in blindness for which there is no current treatment. Although the members of the family of RP diseases differ in etiology, their outcomes are the same: apoptosis of rods and then by cones. Recently, the bile acid tauroursodeoxycholic acid (TUDCA) has been shown to have antiapoptotic properties in neurodegenerative diseases, including those of the retina. In this study the authors examined the efficacy of TUDCA on preserving rod and cone function and morphology at postnatal day 30 (P30) in the rd10 mouse, a model of RP. METHODS Wild-type C57BL/6J and rd10 mice were systemically injected with TUDCA (500 mg/kg) every 3 days from P6 to P30 and were compared with vehicle (0.15 M NaHCO(3)). At P30, retinal function was measured with electroretinography, and morphologic preservation of the rods and cones was assessed with immunohistochemistry. RESULTS Dark-adapted electroretinographic (ERG) responses were twofold greater in rd10 mice treated with TUDCA than with vehicle, likewise light-adapted responses were twofold larger in TUDCA-treated mice than in controls at the brightest ERG flash intensities. TUDCA-treated rd10 retinas had fivefold more photoreceptors than vehicle-treated retinas. TUDCA treatments did not alter retinal function or morphology of wild-type mice when administered to age-matched mice. CONCLUSIONS TUDCA is efficacious and safe in preserving vision in the rd10 mouse model of RP when treated between P6 and P30. At P30, a developmental stage at which nearly all rods are absent in the rd10 mouse model of RP, TUDCA treatment preserved rod and cone function and greatly preserved overall photoreceptor numbers.

Exaggerated eye growth in IRBP-deficient mice in early development

PURPOSE Because interphotoreceptor retinoid-binding protein (IRBP) is expressed before being needed in its presumptive role in the visual cycle, we tested whether it controls eye growth during development. METHODS The eyes of congenic IRBP knockout (KO) and C57BL/6J wild-type (WT) mice ranging in age from postnatal day (P)2 to P440 were compared by histology, laser micrometry, cycloplegic photorefractions, and partial coherence interferometry. RESULTS The size and weight of IRBP KO mouse eyes were greater than those of the WT mouse, even before eye-opening. Excessive ocular enlargement started between P7 and P10, with KO retinal arc lengths becoming greater compared with WT from P10 through P30 (18%; P < 0.01). The outer nuclear layer (ONL) of KO retinas became 20% thinner between P12 to P25, and progressed to 38% thinner at P30. At P30, there were 30% fewer cones per vertical section in KO than in WT retinas. Bromodeoxyuridine (BrdU) labeling indicated the same number of retinal cells were born in KO and WT mice. A spike in apoptosis was observed in KO outer nuclear layer at P25. These changes in size were accompanied by a large decrease in hyperopic refractive error, which reached -4.56 ± 0.70 diopters (D) versus +9.98 ± 0.993 D (mean ± SD) in WT, by postnatal day 60 (P60). CONCLUSIONS; In addition to its role in the visual cycle, IRBP is needed for normal eye development. How IRBP mediates ocular development is unknown.

Head-mounted goggles for murine form deprivation myopia.

Recently a murine model has been developed for use in form deprivation myopia experiments. Due to the small size of the head and eye, methods to blur visual input to the mouse eye are challenging. Previous methods to induce form deprivation include lid suture and gluing diffuser goggles directly to the fur around the eye. In this paper we describe a new method of goggling using a head pedestal and goggle, which improves compliance and allows for better ocular health. Nob mice, previously shown to be highly susceptible to form deprivation myopia, were used for these experiments. Immediately following baseline refraction using an infrared automated photorefractor, mice were either goggled with a diffuser attached directly to the fur or with a head-mounted goggling apparatus. The goggle apparatus consists of five main components: goggle and frame, head pedestal, acrylic cube for stabilization, and balancing bar. Mice were goggled for 2 weeks in which ocular health and goggle position was monitored and then had a final refraction. The use of head-mounted goggles resulted in 75% fewer instances of goggle loss and 55% fewer ocular complications compared to goggles glued to the fur. Both goggling methods induced a myopic shift of approximately 5 diopters. The head-mounted goggle apparatus provides an improved method for inducing form deprivation in mice and offers the ability to easily take repeated refractive measurements as well as allowing for the use of defocusing lenses.

Status of the feline retina 5 years after subretinal implantation.

Retinal prosthetics are designed to restore functional vision to patients with photoreceptor degeneration by detecting light and stimulating the retina. Since devices are surgically implanted into the eye, long-term biocompatibility and durability are critical for viable treatment of retinal disease. To extend our previous work, which demonstrated the biocompatibility of a microphotodiode array (MPA) for 10 to 27 months in the normal feline retina, we implanted normal cats with an MPA implant backed with either an iridium oxide or platinum electrode and examined retinal function and biocompatibility for 3 to 5 years. All implants functioned throughout the study period. Retinal function remained steady and normal with a less than 15 percent decrease in electroretinogram response. The retinas had normal laminar structure with no signs of inflammation or rejection in areas adjacent to or distant from the implants. Directly over the implants, a loss of photoreceptor nuclei and remodeling of inner retinal layers existed. These results indicate that the subretinal MPA device is durable and well tolerated by the retina 5 years postimplantation.

Effects of Subretinal Electrical Stimulation in Mer-KO Mice

PURPOSE Subretinal electrical stimulation (SES) from microphotodiode arrays protects photoreceptors in the RCS rat model of retinitis pigmentosa. The authors examined whether mer(kd) mice, which share a Mertk mutation with RCS rats, showed similar neuroprotective effects from SES. METHODS Mer(kd) mice were implanted with a microphotodiode array at postnatal day (P) 14. Weekly electroretinograms (ERGs) followed by retinal histology at week 4 were compared with those of age-matched controls. RT-PCR for fibroblast growth factor beta (Fgf2), ciliary nerve trophic factor (Cntf), glial-derived neurotrophic factor (Gdnf), insulin growth factor 1 (Igf1), and glial fibrillary acidic protein (Gfap) was performed on retinas at 1 week after surgery. Rates of degeneration using ERG parameters were compared between mer(kd) mice and RCS rats from P28 to P42. RESULTS SES-treated mer(kd) mice showed no differences in ERG a- and b-wave amplitudes or photoreceptor numbers compared with controls. However, the expression of Fgf2 and Cntf was greater (6.5 ± 1.9- and 2.5 ± 0.5-fold, respectively; P < 0.02) in SES-treated mer(kd) retinas. Rates of degeneration were faster for dark-adapted maximal b-wave, log σ, and oscillatory potentials in mer(kd) mice than in RCS rats. CONCLUSIONS Although SES upregulated Fgf2 in mer(kd) retinas, as reported previously for RCS retinas, this was not accompanied by neuroprotection of photoreceptors. Comparisons of ERG responses from mer(kd) mice and RCS rats across different ages showed inner retinal dysfunction in mer(kd) mice but not in RCS rats. This inner retinal dysfunction and the faster rate of degeneration in mer(kd) mice may produce a retinal environment that is not responsive to neuroprotection from SES.

Retinal Function and Structure in Ant1-Deficient Mice

PURPOSE Mutations in ANT, a mitochondrial ATP transporter, are typically associated with myopathy. Because of the high metabolic demands of the retina, the authors examined whether elimination of the Ant1 isoform in a transgenic mouse affects retinal function or morphology. METHODS RT-PCR was used to confirm Ant1 expression in retinas of wild-type (WT) or Ant1(-/-) mice. Full-field ERGs were used to test retinal function under dark- and light-adapted conditions and the recovery of the photoresponse to a bright flash. Using histologic methods, the authors assessed the retinal location of ANT and ANT1-β-gal reporter protein, mitochondrial activity with cytochrome c oxidase (COX) and succinate dehydrogenase (SDH) staining, retinal layer thickness, and bipolar cell types using Chx10 and recoverin. RESULTS Ant1(-/-) mice had supernormal ERG b-waves under both dark- and light-adapted conditions. X-Gal staining was detected in a subset of cells within the inner retina. The following characteristics were normal in Ant1(-/-) mice compared with age-matched WT mice: recovery of the photoresponse, COX and SDH activity, retinal morphology, and bipolar cell morphology. CONCLUSIONS The presence of ANT1 in a subset of inner retinal cells accompanied by supernormal ERG responses suggests that ANT1 may be localized to hyperpolarizing bipolar cells. However, the immunohistochemical techniques used here did not show any differences in bipolar cells. Moderate functional changes coupled with a lack of detectable morphologic changes suggest that ANT1 is not essential for ATP transport in the retina.

Implantation of Mouse Eyes with a Subretinal Microphotodiode Array

Retinal prosthetics are designed to restore vision in patients with photoreceptor degenerative diseases, such as retinitis pigmentosa (RP) and macular degeneration. Subretinal microphotodiode arrays (MPAs), which response to incident light in a gradient fashion, have been designed to replace degenerating photoreceptors. Such devices have been implanted into rats (Ball et al., 2001), cats (Chow et al., 2001) and humans (Chow et al., 2004). These studies have revealed that implantation of a MPA device is capable of restoring some visual function in patients (Chow et al., 2004) and eliciting a superior colliculus response in normal and degenerating rats (DeMarco et al., 2007). Furthermore, the low level electrical stimulation produced by the MPA device has been shown to have neuroprotective properties (Pardue et al., 2005). When RCS rats are implanted with an MPA device at the beginning of the degenerative process, photoreceptor function and morphology are preserved (Pardue et al., 2005). Subretinal electrical stimulation may provide protection to the photoreceptors by stimulating the selective expression of FGF-2 in the RCS rat (Ciavatta et al., 2006). While these studies show promise for subretinal electrical stimulation as a treatment of RP, implantation of MPA devices in S334ter rats does not preserve photoreceptors (Walker et al., 2005). We hypothesize that this may be due to the underlying mutations between RCS and S334ter rats. RCS rats have a recessive mutation in a tyrosine kinase gene, Mertk, which results in failed phagocytosis of shed outer segments by the retinal pigment epithelium (Mullen et al., 1976; D’Cruz et al., 2000); while S334ter rats have a rhodopsin mutation which leads to photoreceptor death (Lee et al., 2003). Rat models with photoreceptor degeneration are few while there are numerous mouse models of RP that have been described (Chang et al., 2002; Dalke and Graw, 2005). Thus, to further elucidate whether the neuroprotective effect of subretinal electrical stimulation is generalized to all types of photoreceptor degeneration, implantation of mouse models of RP would be advantageous. This study describes the development of surgical techniques and the success of implanting a small mouse eye with a subretinal MPA device.

Neuroprotective Dose Response in RCS Rats Implanted with Microphotodiode Arrays

Neuropreservation of retinal function and structure in RCS rats following implantation of a microphotodiode array (MPA) has been shown in previous studies (Pardue et al. J Neural Eng 2005;2:S39–47; Pardue et al. Invest Ophthalmol Vis Sci 2005;46:674–682). Since microphotodiodes produce electrical currents in proportion to the intensity of incident light, increased light exposure may result in greater neuroprotective effects. Our previous studies suggested that the frequency of light exposure to electroretinogram (ERG) flash stimuli might provide increased neuroprotection. Thus, in this study, we examined the dose response of subretinal electrical stimulation by exposing RCS rats implanted with MPAs to variable durations and combinations of two different lighting regimens: pulsing incandescent bulbs and xenon stimuli from an ERG Ganzfeld. While incandescent light regimens did not produce any significant differences in ERG function, we found significantly greater dark-adapted ERG b-wave amplitudes in RCS rats that received weekly vs. biweekly ERGs over the course of 8 weeks of follow-up. These results suggest that subretinal electrical stimulation may be optimized to produce greater neuroprotective effects by dosing with periodic higher current.