Retinal degeneration in the nervous mutant mouse. III. Electrophysiological studies of the visual pathway

Complement anaphylatoxin receptors C3aR and C5aR are required in the pathogenesis of experimental autoimmune uveitis

Recent studies have suggested that reagents inhibiting complement activation could be effective in treating T cell mediated autoimmune diseases such as autoimmune uveitis. However, the precise role of the complement anaphylatoxin receptors (C3a and C5a receptors) in the pathogenesis of autoimmune uveitis remains elusive and controversial. We induced experimental autoimmune uveitis in mice deficient or sufficient in both C3a and C5a receptors and rigorously compared their retinal phenotype using various imaging techniques, including indirect ophthalmoscopy, confocal scanning laser ophthalmoscopy, spectral domain optical coherence tomography, topical endoscopic fundus imaging, and histopathological analysis. We also assessed retinal function using electroretinography. Moreover, we performed Ag-specific T cell recall assays and T cell adoptive transfer experiments to compare pathogenic T cell activity between wild-type and knockout mice with experimental autoimmune uveitis. These experiments showed that C3a receptor/C5a receptor-deficient mice developed much less severe uveitis than did control mice using all retinal examination methods and that these mice had reduced pathogenic T cell responses. Our data demonstrate that both complement anaphylatoxin receptors are important for the development of experimental autoimmune uveitis, suggesting that targeting these receptors could be a valid approach for treating patients with autoimmune uveitis.

Spontaneous optic nerve compression in the osteopetrotic (op/op) mouse: a novel model of myelination failure

We report a focal disturbance in myelination of the optic nerve in the osteopetrotic (op/op) mouse, which results from a spontaneous compression of the nerve resulting from stenosis of the optic canal. The growth of the op/op optic nerve was significantly affected, being maximally suppressed at postnatal day 30 (P30; 33% of age matched control). Myelination of the nerve in the optic canal was significantly delayed at P15, and myelin was almost completely absent at P30. The size of nerves and myelination were conserved both in the intracranial and intraorbital segments at P30, suggesting that the axons in the compressed site are spared in all animals at P30. Interestingly, we observed recovery both in the nerve size and the density of myelinated axons at 7 months in almost half of the optic nerves examined, although some nerves lost axons and became atrophic. In vivo and ex vivo electrophysiological examinations of P30 op/op mice showed that nerve conduction was significantly delayed but not blocked with partial recovery in some mice by 7 months. Transcardial perfusion of FITC-labeled albumin suggested that local ischemia was at least in part the cause of this myelination failure. These results suggest that the primary abnormality is dysmyelination of the optic nerve in early development. This noninvasive model system will be a valuable tool to study the effects of nerve compression on the function and survival of oligodendrocyte progenitor cells/oligodendrocytes and axons and to explore the mechanism of redistribution of oligodendrocyte progenitor cells with compensatory myelination.

The retina of the PCD/PCD mouse as a model of photoreceptor degeneration. A structural and functional study

In this work, we used the pcd (Purkinje cell degeneration) mutant mouse with a slow temporal progression of photoreceptor degeneration in order to analyze the structural and functional modifications in the neuronal populations of the outer and inner retina. Retinal immunocytochemistry and functional electroretinography were performed on the pcd/pcd mutant mice and control wild type animals of the C57/DBA strain at 45, 90, 180 and 270 post-natal days. Immunohistochemical studies were performed for a series of protein markers: calbindin, calretinin, PKCα, bassoon, synapsin, syntaxin and islet1. Full field electroretinography recordings were performed on control and dystrophic mice. Rod and mixed responses, and oscillatory potentials, were recorded in dark adapted conditions; cone and flicker responses were recorded under light adaptation. Our results show significant structural modifications in the photoreceptor populations and neurons of the inner retina. Changes in cell morphology affect mainly to the bipolar cells, which gradually lose their dendritic tufts. The electroretinography records reveal that in the pcd retinas the rod and cone systems show a reduction in the amplitude of the electrical signals. This decrease progresses slowly with the passage of time, although for the most advanced stage of photoreceptor degeneration considered, 270 post-natal days, it is still possible to record light induced responses. We conclude that pcd mice experience a loss of retinal function in correlation with the loss of photoreceptors with age, and significant changes in retinal synaptic processes.

Visual abnormalities associated with enhanced optic nerve myelination

Expression of the constitutively active serine/threonine kinase Akt in oligodendrocytes results in enhanced myelination in the CNS. Here, we have examined the effects of this Akt overexpression on optic nerve structure and on optic nerve function, assessed using the visual evoked potential (VEP). Transgenic mice have been generated with the Plp promoter driving expression of a modified form of Akt, in which aspartic acids are substituted for Thr308 and Ser473. These Plp-Akt-DD (Akt-DD) mice, and littermate controls, were studied at different ages. Optic nerves were examined anatomically at 2 and 6 months of age. At 2 months of age, optic nerves were substantially thicker in Akt-DD mice, reflecting an increase in myelination of optic nerve axons. By electron microscopy, myelin thickness was increased in Akt-DD optic nerve, with extended paranodal domains having excess paranodal loops, and the density of nodes of Ranvier was reduced, relative to control mice. We recorded VEPs in response to strobe flash ganzfeld stimuli presented after overnight dark- and light-adapted conditions at ages ranging from 1 to 10 months. It was possible to record a clear VEP from Akt-DD mice at all ages examined. At 1 month of age, VEP implicit times were somewhat shorter in Akt-DD transgenic mice than in control animals. Beyond 6months of age, VEP latencies were consistently delayed in Akt-DD transgenic mice. These abnormalities did not reflect an alteration in retinal function as there were no significant differences between ERGs obtained from control or Akt-DD transgenic mice. In young mice, the somewhat faster responses may reflect improved transmission due to increased myelination of optic nerve axons. In older mice, where the Akt-DD optic nerve is markedly thicker than control, it is remarkable that optic nerves continue to function.

The mouse model of Down syndrome Ts65Dn presents visual deficits as assessed by pattern visual evoked potentials

PURPOSE

The Ts65Dn mouse is the most complete widely available animal model of Down syndrome (DS). Quantitative information was generated about visual function in the Ts65Dn mouse by investigating their visual capabilities by means of electroretinography (ERG) and patterned visual evoked potentials (pVEPs).

METHODS

pVEPs were recorded directly from specific regions of the binocular visual cortex of anesthetized mice in response to horizontal sinusoidal gratings of different spatial frequency, contrast, and luminance generated by a specialized video card and presented on a 21-in. computer display suitably linearized by gamma correction.

RESULTS

ERG assessments indicated no significant deficit in retinal physiology in Ts65Dn mice compared with euploid control mice. The Ts65Dn mice were found to exhibit deficits in luminance threshold, spatial resolution, and contrast threshold, compared with the euploid control mice. The behavioral counterparts of these parameters are luminance sensitivity, visual acuity, and the inverse of contrast sensitivity, respectively.

CONCLUSIONS

DS includes various phenotypes associated with the visual system, including deficits in visual acuity, accommodation, and contrast sensitivity. The present study provides electrophysiological evidence of visual deficits in Ts65Dn mice that are similar to those reported in persons with DS. These findings strengthen the role of the Ts65Dn mouse as a model for DS. Also, given the historical assumption of integrity of the visual system in most behavioral assessments of Ts65Dn mice, such as the hidden-platform component of the Morris water maze, the visual deficits described herein may represent a significant confounding factor in the interpretation of results from such experiments.

Early synaptic defects in tulp1-/- mice

PURPOSE

Mutations in the photoreceptor-specific tubby-like protein 1 (TULP1) underlie a form of autosomal recessive retinitis pigmentosa. To investigate the role of Tulp1 in the photoreceptor synapse, the authors examined the presynaptic and postsynaptic architecture and retinal function in tulp1(-/-) mice

METHODS

The authors used immunohistochemistry to examine tulp1(-/-) mice before retinal degeneration and made comparisons with wild-type (wt) littermates and retinal degeneration 10 (rd10) mice, another model of photoreceptor degeneration that has a comparable rate of degeneration. Retinal function was characterized with the use of electroretinography.

RESULTS

In wt mice, Tulp1 is localized to the photoreceptor synapse. In the tulp1(-/-) synapse, the spatial relationship between the ribbon-associated proteins Bassoon and Piccolo are disrupted, and few intact ribbons are present. Furthermore, bipolar cell dendrites are stunted. Comparable abnormalities are not seen in rd10 mice. The leading edge of the a-wave had normal kinetics in tulp1(-/-) mice but reduced gain in rd10 mice. The b-wave intensity-response functions of tulp1(-/-) mice are shifted to higher intensities than in wt mice, but those of rd10 mice are not.

CONCLUSIONS

Photoreceptor synapses and bipolar cell dendrites in tulp1(-/-) mice display abnormal structure and function. A malformation of the photoreceptor synaptic ribbon is likely the cause of the dystrophy in bipolar cell dendrites. The association of early-onset, severe photoreceptor degeneration preceded by synaptic abnormalities appears to represent a phenotype not previously described. Not only is Tulp1 critical for photoreceptor function and survival, it is essential for the proper development of the photoreceptor synapse.

Isoflurane is an effective alternative to ketamine/xylazine/acepromazine as an anesthetic agent for the mouse electroretinogram

The electroretinogram (ERG) is an essential measure of retinal function for studying mouse models of retinal disease. Ketamine, in combination with xylazine and/or acepromazine, is the most commonly used anesthetic agent. Although it works well in most situations, some fragile mouse strains have high mortality rates with this ketamine cocktail. We compared isoflurane with the ketamine cocktail in a longitudinal study of light-adapted and dark-adapted ERGs in C57BL/6J mice. Waveforms were averaged, oscillatory potentials (OPs) were extracted by digital filtration, and key ERG parameters were analyzed. The ERG waveforms were qualitatively similar with both anesthetics, and the male and female ERG parameters did not show significant differences. For light-adapted ERGs, b-wave amplitude and implicit time, and wavelet index were decreased under isoflurane anesthesia, whereas for dark-adapted ERGs, a- and b-wave implicit times were decreased and wavelet index was increased. The dark-adapted b-wave amplitude showed a significant inverse correlation with animal weight and age. Rod phototransduction gain and the Naka-Rushton n and R (max) parameters were the same for both anesthetics, and only the Naka-Rushton log k parameter was significantly elevated for isoflurane anesthesia. We propose that isoflurane is a satisfactory alternative to the ketamine cocktail for anesthesia in the mouse ERG. Precise quantitative comparisons, however, should only employ study designs using isoflurane versus isoflurane, or ketamine versus ketamine. Moreover, in light of the effects of both isoflurane and the ketamine cocktail on blood glucose levels, it would be prudent to control the fasting state of the animals in quantitative ERG studies.

Neurochemical, morphological, and neurophysiological abnormalities in retinas of Sandhoff and GM1 gangliosidosis mice

Retinal abnormalities are well documented in patients with ganglioside storage diseases. The total content and distribution of retinal glycosphingolipids was studied for the first time in control mice and in Sandhoff disease (SD) and GM1 gangliosidosis mice. Light and electron microscopy of the SD and the GM1 retinas revealed storage in ganglion cells. Similar to previous findings in rat retina, GD3 was the major ganglioside in mouse retina, while GM2 and GM1 were minor species. Total ganglioside content was 44% and 40% higher in the SD and the GM1 retinas, respectively, than in the control retinas. Furthermore, GM2 and GM1 content were 11-fold and 51-fold higher in the SD and the GM1 retinas than in the control retinas, respectively. High concentrations of asialo-GM2 and asialo-GM1 were found in the SD and the GM1 retinas, respectively, but were undetectable in the control retinas. The GSL abnormalities in the SD and the GM1 retinas reflect significant reductions in beta-hexosaminidase and beta-galactosidase enzyme activities, respectively. Although electroretinograms appeared normal in the SD and the GM1 mice, visual evoked potentials were subnormal in both mutants, indicating visual impairments. Our findings present a model system for assessing retinal pathobiology and therapies for the gangliosidoses.

Ciliary margin transdifferentiation from neural retina is controlled by canonical Wnt signaling

The epithelial layers of the ciliary body (CB) and iris are non-neural structures that differentiate from the anterior region of the eyecup, the ciliary margin (CM). We show here that activation of the canonical Wnt signaling pathway is sufficient and necessary for the normal development of anterior eye structures. Pharmacological activation of beta-catenin signaling with lithium (Li(+)) treatment in retinal explants in vitro induced the ectopic expression of the CM markers Otx1 and Msx1. Cre-mediated stabilization of beta-catenin expression in the peripheral retina in vivo induced a cell autonomous upregulation of CM markers at the expense of neural retina (NR) markers and inhibited neurogenesis. Consistent with a cell autonomous conversion to peripheral eye fates, the proliferation index in the region of the retina that expressed stabilized beta-catenin was identical to the wild-type CM and there was an expansion of CB-like structures at later stages. Conversely, Cre-mediated inactivation of beta-catenin reduced CM marker expression as well as the size of the CM and CB/iris. Aberrant CB development in both mouse models was also associated with a reduction in the number of retinal stem cells in vitro. In summary, activation of canonical Wnt signaling is sufficient to promote the development of peripheral eyecup fates at the expense of the NR and is also required for the normal development of anterior eyecup structures.

Electrophysiological responses of the mouse retina to 12C ions

Phosphenes ("light flashes") have been reported by most astronauts on space missions and by healthy subjects whose eyes were exposed to ionizing radiation in early experiments in particle accelerators. The conditions of occurrence suggested retinal effects of heavy ions. To develop an in vivo animal model, we irradiated the eyes of anesthetized wild-type mice with repeated bursts of 12C ions delivered under controlled conditions in accelerator. 12C ions evoked electrophysiological retinal mass responses and activated the visual system as indicated by responses recorded from the visual cortex. No retinal immunohistological damage was detected. Mice proved a suitable animal model to study radiation-induced phosphenes in vivo and our findings are consistent with an origin of phosphenes in radiation activating the retina.

Neuronal pentraxins mediate synaptic refinement in the developing visual system

Neuronal pentraxins (NPs) define a family of proteins that are homologous to C-reactive and acute-phase proteins in the immune system and have been hypothesized to be involved in activity-dependent synaptic plasticity. To investigate the role of NPs in vivo, we generated mice that lack one, two, or all three NPs. NP1/2 knock-out mice exhibited defects in the segregation of eye-specific retinal ganglion cell (RGC) projections to the dorsal lateral geniculate nucleus, a process that involves activity-dependent synapse formation and elimination. Retinas from mice lacking NP1 and NP2 had cholinergically driven waves of activity that occurred at a frequency similar to that of wild-type mice, but several other parameters of retinal activity were altered. RGCs cultured from these mice exhibited a significant delay in functional maturation of glutamatergic synapses. Other developmental processes, such as pathfinding of RGCs at the optic chiasm and hippocampal long-term potentiation and long-term depression, appeared normal in NP-deficient mice. These data indicate that NPs are necessary for early synaptic refinements in the mammalian retina and dorsal lateral geniculate nucleus. We speculate that NPs exert their effects through mechanisms that parallel the known role of short pentraxins outside the CNS.

Positive visual phenomena in space: A scientific case and a safety issue in space travel

Most astronauts on Apollo, Skylab, and MIR reported 'flashes of light' occurring in different shapes and apparently moving across the visual field, in the absence of auditory, somatosensory, or olfactory abnormal percepts. A temporal correlation with heavy nuclei or protons has been documented in space and comparable phosphenes were observed by volunteers whose eyes were exposed to accelerated heavy ions at intensities below the threshold for Cerenkov visible radiation. An interaction between heavy ions and the retina was suggested. However, the biophysics of heavy ions or protons action remains undefined, the effects on photoreceptors and neuroretina have not been differentiated, and some direct action on the visual cortex never ruled out. Phosphenes are common in migraine and are known to occur also in response to the electrical stimulation of ganglion cells (in retinas without photoreceptors), optic pathways or visual cortex, with mechanisms that bypass the chemically gated channels. Intrinsic photosensitive ganglion cells exist in the retina of teleost fish and mammals. In the hypothesis of a peculiar sensitivity to subatomic particles of the visual system, phosphenes due to the activation of processes by-passing the photoreceptors would raise questions about human safety in space. The issue is particularly relevant with experiments of increasing duration being now operative in the International Space Station (ISS) and with plans of space travel outside the geomagnetic shield. Research is in progress both in the ISS and on animal models, in the framework of the NASA/ESA actions to improve the astronauts' health in space.

The visual evoked potential in the mouse--origins and response characteristics

The visual evoked potential (VEP) in the mouse is characterized and compared to responses obtained with the electroretinogram (ERG). The results indicate that: 1, the VEP originates in the visual cortex; 2, the rod and cone pathways contribute separately to the VEP; 3, temporal tuning functions for rod and cone ERGs are low pass and band pass, respectively; VEP tuning functions are both band pass; and 4, VEP acuity is 0.62+/-0.156 cycles/degree. The differences in the spatial and temporal tuning functions obtained from the retina and visual cortex provides a tool to investigate signal processing through the visual system.

Loss of bipolar cells resulting from the expression of bcl-2 directed by the IRBP promoter

We have recently noted marked reductions in the electroretinographic (ERG) b-wave in HIBA transgenic mice expressing bcl-2 under control of the human IRBP promoter. These electrophysiological results are unexpected as this promoter has been shown to specifically target transgene expression to the rod and cone photoreceptors. Here, we have carried out a series of studies to better understand this result. ERGs were recorded from three lines of HIBA transgenic mice. Mice with higher levels of transgene expression developed progressive photoreceptor degeneration, and an associated reduction in the ERG a-wave. These higher-expressing lines also exhibited a severe reduction in the ERG b-wave that affected both rod- and cone-mediated responses. These mice were mated to L7 transgenic mice, which express beta-galactosidase in bipolar cells. In double transgenic mice, the ERG b-wave reduction was associated with a decrease in the number of bipolar cells in the inner retina. These results indicate that bcl-2, targeted to photoreceptors, can induce bipolar cell degeneration, and indicate that the potential benefit for bcl-2 in treating hereditary retinal disease appears limited.

Electrophysiological analysis of visual function in mutant mice

The mouse has become a key animal model for ocular research. This situation reflects the fact that genes implicated in human retinal disorders or in mammalian retinal function may be readily manipulated in the mouse. Visual electrophysiology provides a means to examine retinal function in mutant mice, and stimulation and recording protocols have been developed that allow the activity of many classes of retinal neurons to be examined and which take into account unique features of the mouse retina. Here, we review the mouse visual electrophysiology literature, covering techniques used to record the mouse electroretinogram and visual evoked potential, and how these have been applied to characterize the functional implications of gene mutation or manipulation in the mouse retina.

Progressive age-related changes similar to age-related macular degeneration in a transgenic mouse model

Age-related macular degeneration (AMD) is the major cause of blindness in the developed world. Its pathomechanism is unknown and its late onset, complex genetics and strong environmental components have all hampered investigations. Here we demonstrate the development of an animal model for AMD that reproduces features associated with geographic atrophy; a transgenic mouse line (mcd/mcd) expressing a mutated form of cathepsin D that is enzymatically inactive thus impairing processing of phagocytosed photoreceptor outer segments in the retinal pigment epithelial (RPE) cells. Pigmentary changes indicating RPE cell atrophy and a decreased response to flash electroretinograms were observed in 11- to 12-month-old mcd/mcd mice. Histological studies showed RPE cell proliferation, photoreceptor degeneration, shortening of photoreceptor outer segments, and accumulation of immunoreactive photoreceptor breakdown products in the RPE cells. An accelerated photoreceptor cell death was detected in 12-month-old mcd/mcd mice. Transmission electron microscopy demonstrated presence of basal laminar and linear deposits that are considered to be the hallmarks of AMD. Small hard drusen associated with human age-related maculopathy were absent in the mcd/mcd mouse model at the ages analyzed. In summary, this model presents several features of AMD, thus providing a valuable tool for investigating the underlying biological processes and pathomechanism of AMD.

Temporal properties of the mouse cone electroretinogram

To determine the temporal response characteristics of the mouse cone electroretinogram (ERG), we recorded responses to high contrast sinusoidal stimuli ranging from 2 to 52 Hz. The largest response amplitudes obtained from wild-type (WT) mice occurred at stimulus frequencies below 10 Hz, and cone ERG amplitude declined progressively with increasing stimulus frequency above that level. In comparison, human responses recorded under the same stimulus and recording conditions displayed maximal responses to stimulus frequencies near 4 and 40 Hz, and a pronounced dip at 12 Hz. Responses were also obtained from nob (no b-wave) mice, which lack ERG contributions from depolarizing bipolar cells (DBCs). At low temporal frequencies, nob cone ERGs were smaller than those of WT mice and had a different waveform. As temporal frequency increased, nob and WT responses became more similar and came into register at the highest temporal frequencies. To evaluate the contribution of the DBC pathway to the mouse cone ERG, nob responses were vector-subtracted from those of WT mice. The derived DBC response was maximal at low stimulus frequencies and fell sharply as stimulus frequency increased. These results indicate that the mouse cone ERG is more linear than the primate response and that the temporal response of the mouse outer retina is tuned to much lower frequencies than that of primate.

Age-related changes in the mouse outer retina

PURPOSE

To define the physiological and structural changes that may accompany aging in the normal mouse retina.

METHODS

C57BL/6 mice were maintained under cyclic light for either 2, 6, or 12 months. After rod- and cone-mediated corneal electroretinograms (ERG's) were recorded from anesthetized animals, the retinal structure was quantitatively examined. Photoreceptor cell density was measured within 100-microm regions of the central superior and inferior retina. Cone photoreceptor subtypes were identified by immunocytochemistry.

RESULTS

The amplitudes of rod- and cone-mediated ERG's were reduced in older mice, although the overall ERG wave-form did not change appreciably and implicit times were not changed in an age-dependent fashion. In comparison, there was no significant age-related decline in rod or cone photoreceptor density.

CONCLUSIONS

The amplitude of the mouse ERG declines with age. This change does not appear to reflect a change in the structural integrity of the photoreceptor cells. In functional studies of murine models of late-onset retinal disorders, it will be important to take these changes into consideration.

Implantation of silicon chip microphotodiode arrays into the cat subretinal space

There are currently no therapies to restore vision to patients blinded by photoreceptor degeneration. This project concerns an experimental approach toward a semiconductor-based subretinal prosthetic designed to electrically stimulate the retina. The present study describes surgical techniques for implanting a silicon microphotodiode array in the cat subretinal space and subsequent studies of implant biocompatibility and durability. Using a single-port vitreoretinal approach, implants were placed into the subretinal space of the right eye of normal cats. Implanted retinas were evaluated post-operatively over a 10 to 27 month period using indirect ophthalmoscopy, fundus photography, electroretinography, and histology. Infrared stimulation was used to isolate the electrical response of the implant from that of the normal retina. Although implants continued to generate electrical current in response to light, the amplitude of the implant response decreased gradually due to dissolution of the implant's gold electrode. Electroretinograms recorded from implanted eyes had normal waveforms but were typically 10-15% smaller in amplitude than those in unimplanted left eyes. The nonpermeable silicon disks blocked choroidal nourishment to the retina, producing degeneration of the photoreceptors. The laminar structure of the inner retinal layers was preserved. Retinal areas located away from the implantation site appeared normal in all respects. These results demonstrate that silicon-chip microphotodiode-based implants can be successfully placed into the subretinal space. Gold electrode-based subretinal implants, however, appear to be unsuitable for long-term use due to electrode dissolution and subsequent decreased electrical activity.

Retinal degeneration in the nervous mutant mouse. IV. Inner retinal changes

We have recently noted that the inner nuclear layer (INL) and the inner plexiform layer (IPL) were significantly thinner in mice homozygous for the nervous defect (nr / nr) than in control (nr /+ or +/+) littermates. Here, we have carried out a series of anatomical studies to further understand these inner retinal changes. At postnatal day (P) 13, there was no difference in the inner retina between nervous and control mice, while a significant difference was observed at P30. Similar changes were not seen in other mouse models of photoreceptor degeneration. There was a significant reduction in the density of cells in the INL that were stained by antibodies against the inhibitory neurotransmitters GABA and glycine. These results indicate that the nervous defect causes a degeneration of one or more sub-types of amacrine cells, in addition to the loss of cerebellar Purkinje cells and retinal photoreceptors that is known to occur in these mutant animals. Finally, evidence is provided that photoreceptors die by an apoptotic pathway in nervous mice.