Morphologic characteristics of N-methyl-N-nitrosourea-induced retinal degeneration in C57BL mice

Cd9 Protects Photoreceptors from Injury and Potentiates Edn2 Expression

Purpose

Cd9 is a tetraspanin membrane protein that plays various roles in tissue development and disease pathogenesis, especially in cancer, but the expression patterns and function of Cd9 in retinal development and disease are not well understood. We asked its roles during retinal photoreceptor degeneration by using CD9-knockout mice.

Methods

Cd9 knockout mice and rd1 mice were used to examine roles of Cd9 for progression of photoreceptor degeneration. Reverse transcription-polymerase chain reaction and immunohistochemistry were mainly used as analytical methods.

Results

Cd9 transcripts were only weakly expressed in retina at embryonic day 14, but its expression level subsequently increased and peaked at around postnatal day 12. In 6-week-old female mice derived retina, mRNA expression decreased slightly but was maintained at a significant level. Published RNA-sequencing data and immunohistochemistry indicated that Cd9 was expressed abundantly in Müller glia and weakly in other retinal neurons. Notably, when photoreceptors were damaged, Cd9 expression was increased in rod photoreceptors and decreased in Müller glia. Cd9 knockout mice retinas developed normally; however, once the retina suffered damage, degeneration of photoreceptors was more severe in Cd9 knockout retinas than control retinas. Induction of Edn2, which is known to protect against photoreceptor damage, was severely hampered. In addition, induction of Socs3, which is downstream of gp130 (Il6st), was weaker in Cd9 knockout retinas.

Conclusions

Taken together, these findings indicate that, although Cd9 was dispensable for normal gross morphological development, it protected rod photoreceptors and enhanced Edn2 expression, possibly through modulation of gp130 signaling.

Ultrastructural changes in the choriocapillaris of N-methyl-N-nitrosourea-induced retinal degeneration in C57BL/6 mice

N-methyl-N-nitrosourea (MNU) is known to cause apoptosis of photoreceptor cells and changes in retinal pigment epithelium (RPE). However, the changes in choriocapillaris, which nourishes photoreceptor cells by diffusing tissue fluid through RPE, have not been reported in detail. Therefore, we studied the ultrastructural transformation in and around the choriocapillaris to characterize the interdependence between choriocapillaris and surrounding tissue components in a mouse model. Seven-week-old male C57BL/6 mice were given a single intraperitoneal injection of MNU (60 mg/kg of body weight). Perfusion-fixed eyeballs were examined chronologically using immunohistochemistry and electron microscopy until the photoreceptor cells were lost. Sequential ultrastructural changes were observed in photoreceptor cells, RPE, Bruch's membrane, choriocapillaris, and choroidal melanocytes after an MNU injection. The lumens of the choriocapillaris narrowed following dilation, and the vascular endothelium showed structural alterations. When the photoreceptor cells were completely lost, the choriocapillaris appeared to be in a recovery process. Our results suggest that transport abnormality through Bruch's membrane and structural changes in the choroid might have influenced the morphology of choriocapillaris. The thin wall of the choriocapillaris appears to be the cause of the vulnerability with its altered morphology.

Phosphatidylserine recognition and Rac1 activation are required for Müller glia proliferation, gliosis and phagocytosis after retinal injury

Müller glia, the principal glial cell type in the retina, have the potential to reenter the cell cycle after retinal injury. In mammals, proliferation of Müller glia is followed by gliosis, but not regeneration of neurons. Retinal injury is also accompanied by phagocytic removal of degenerated cells. We here investigated the possibility that proliferation and gliosis of Müller glia and phagocytosis of degenerated cells may be regulated by the same molecular pathways. After N-methyl-N–nitrosourea-induced retinal injury, degenerated photoreceptors were eliminated prior to the infiltration of microglia/macrophages into the outer nuclear layer, almost in parallel with cell cycle reentry of Müller glia. Inhibition of microglia/macrophage activation with minocycline did not affect the photoreceptor clearance. Accumulation of lysosomes and rhodopsin-positive photoreceptor debris within the cytoplasm of Müller glia indicated that Müller glia phagocytosed most photoreceptor debris. Pharmacological inhibition of phosphatidylserine and Rac1, key regulators of the phagocytic pathway, prevented cell cycle reentry, migration, upregulation of glial fibrillary acidic protein, and phagocytic activity of Müller glia. These data provide evidence that phosphatidylserine and Rac1 may contribute to the crosstalk between different signaling pathways activated in Müller glia after injury.

Development of a Post-vitrectomy Injection of N-methyl-N-nitrosourea as a Localized Retinal Degeneration Rabbit Model

Since genetic models for retinal degeneration (RD) in animals larger than rodents have not been firmly established to date, we sought in the present study to develop a new rabbit model of drug-induced RD. First, intravitreal injection of N-methyl-N-nitrosourea (MNU) without vitrectomy in rabbits was performed with different doses. One month after injection, morphological changes in the retinas were identified with ultra-wide-field color fundus photography (FP) and fundus autofluorescence (AF) imaging as well as spectral-domain optical coherence tomography (OCT). Notably, the degree of RD was not consistently correlated with MNU dose. Then, to check the effects of vitrectomy on MNU-induced RD, the intravitreal injection of MNU after vitrectomy in rabbits was also performed with different doses. In OCT, while there were no significant changes in the retinas for injections up to 0.1 mg (i.e., sham, 0.05 mg, and 0.1 mg), outer retinal atrophy and retinal atrophy of the whole layer were observed with MNU injections of 0.3 mg and 0.5 mg, respectively. With this outcome, 0.2 mg MNU was chosen to be injected into rabbit eyes (n=10) at two weeks after vitrectomy for further study. Six weeks after injection, morphological identification with FP, AF, OCT, and histology clearly showed localized outer RD - clearly bordered non-degenerated and degenerated outer retinal area - in all rabbits. We suggest our post-vitrectomy MNU-induced RD rabbit model could be used as an interim animal model for visual prosthetics before the transition to larger animal models.

Inflammation, necrosis, and the kinase RIP3 are key mediators of AAG-dependent alkylation-induced retinal degeneration

DNA-alkylating agents are commonly used to kill cancer cells, but the base excision repair (BER) pathway they trigger can also produce toxic intermediates that cause tissue damage, such as retinal degeneration (RD). Apoptosis, a process of programmed cell death, is assumed to be the main mechanism of this alkylation-induced photoreceptor (PR) cell death in RD. Here, we studied the involvement of necroptosis (another programmed cell death process) and inflammation in alkylation-induced RD. Male mice exposed to a methylating agent exhibited a reduced number of PR cell rows, active gliosis, and cytokine induction and macrophage infiltration in the retina. Dying PRs exhibited a necrotic morphology, increased 8-hydroxyguanosine abundance (an oxidative damage marker), and overexpression of the necroptosis-associated genes Rip1 and Rip3 The activity of PARP1, which mediates BER, cell death, and inflammation, was increased in PR cells and associated with the release of proinflammatory chemokine HMGB1 from PR nuclei. Mice lacking the anti-inflammatory cytokine IL-10 exhibited more severe RD, whereas deficiency of RIP3 (also known as RIPK3) conferred partial protection. Female mice were partially protected from alkylation-induced RD, showing reduced necroptosis and inflammation compared to males. PRs in mice lacking the BER-initiating DNA glycosylase AAG did not exhibit alkylation-induced necroptosis or inflammation. Our findings show that AAG-initiated BER at alkylated DNA bases induces sex-dependent RD primarily by triggering necroptosis and activating an inflammatory response that amplifies the original damage and, furthermore, reveal new potential targets to prevent this side effect of chemotherapy.

Correlations between specific patterns of spontaneous activity and stimulation efficiency in degenerated retina

Retinal prostheses that are currently used to restore vision in patients suffering from retinal degeneration are not adjusted to the changes occurring during the remodeling process of the retina. Recent studies revealed abnormal rhythmic activity in the retina of genetic mouse models of retinitis pigmentosa. Here we describe this abnormal activity also in a pharmacologically-induced (MNU) mouse model of retinal degeneration. To investigate how this abnormal activity affects the excitability of retinal ganglion cells, we recorded the electrical activity from whole mounted retinas of rd10 mice and MNU-treated mice using a microelectrode array system and applied biphasic current pulses of different amplitude and duration to stimulate ganglion cells electrically. We show that the electrical stimulation efficiency is strongly reduced in degenerated retinas, in particular when abnormal activity such as oscillations and rhythmic firing of bursts of action potentials can be observed. Using a prestimulus pulse sequence, we could abolish rhythmic retinal activity. Under these conditions, the stimulation efficiency was enhanced in a few cases but not in the majority of tested cells. Nevertheless, this approach supports the idea that modified stimulation protocols could help to improve the efficiency of retinal prostheses in the future.

Visual Responses of Photoreceptor-Degenerated Rats Expressing Two Different Types of Channelrhodopsin Genes

Optogenetic technologies are expected to be applicable for clinical use in restoring vision. However, the degree of recovered visual function is highly dependent on the function of the chosen optogenetic gene. To investigate the effect on visual function of dual expression of genes with different wavelength sensitivities, we transduced a modified Volvox-derived channelrhodopsin gene (mVChR1) via an adeno-associated virus vector into transgenic rats harbouring the ChR2 gene in retinal ganglion cells. These transgenic rats were given an intraperitoneal injection of N-methyl-N-nitrosourea to induce the degeneration of native photoreceptor cells prior to transduction of mVChR1. Optical coherence tomography images indicated the degeneration of the native photoreceptor cells after the N-methyl-N-nitrosourea injection. Complete loss of function of the native photoreceptor cells was confirmed using electroretinograms. In the ChR2 transgenic rats, visually evoked potentials were clearly detectable in spite of native photoreceptor function abolishment; however the responses were limited to within blue wavelengths. In contrast, the limited wavelength sensitivities were improved by the additional transduction of mVChR1, which exhibited sensitivities to green and red. Thus, the transductions of dual genes encoding channelrhodopsins that exhibit different wavelength sensitivities represents a promising candidate method to expand and to enhance rescued wavelength sensitivities in blind subjects.

Classical Photoreceptors Are Primarily Responsible for the Pupillary Light Reflex in Mouse

Pupillary light reflex (PLR) is an important clinical tool to assess the integrity of visual pathways. The available evidence suggests that melanopsin-expressing retinal ganglion cells (mRGCs) mediate PLR—driven by the classical photoreceptors (rods and cones) at low irradiances and by melanopsin activation at high irradiances. However, genetic or pharmacological elimination of melanopsin does not completely abolish PLR at high irradiances, raising the possibility that classical photoreceptors may have a role even at high irradiances. Using an inducible mouse model of photoreceptor degeneration, we asked whether classical photoreceptors are responsible for PLR at all irradiances, and found that the PLR was severely attenuated at all irradiances. Using multiple approaches, we show that the residual PLR at high irradiances in this mouse was primarily from the remnant rods and cones, with a minor contribution from melanopsin activation. In contrast, in rd1 mouse where classical photoreceptor degeneration occurs during development, the PLR was absent at low irradiances but intact at high irradiances, as reported previously. Since mRGCs receive inputs from classical photoreceptors, we also asked whether developmental loss of classical photoreceptors as in rd1 mouse leads to compensatory takeover of the high-irradiance PLR by mRGCs. Specifically, we looked at a distinct subpopulation of mRGCs that express Brn3b transcription factor, which has been shown to mediate PLR. We found that rd1 mouse had a significantly higher proportion of Brn3b-expressing M1 type of mRGCs than in the inducible model. Interestingly, inducing classical photoreceptor degeneration during development also resulted in a higher proportion of Brn3b-expressing M1 cells and partially rescued PLR at high irradiances. These results suggest that classical photoreceptors are primarily responsible for PLR at all irradiances, while melanopsin activation makes a minor contribution at very high irradiances.

N-methyl-N-nitrosourea-induced neuronal cell death in a large animal model of retinal degeneration in vitro

N-methyl-N-nitrosourea (MNU) has been reported to induce photoreceptor-specific degeneration with minimal inner retinal impact in small animals in vivo. Pending its use within a retinal transplantation paradigm, we here explore the effects of MNU on outer and inner retinal neurons and glia in an in vitro large animal model of retinal degeneration. The previously described degenerative culture explant model of adult porcine retina was used and compared with explants receiving 10 or 100 μg/ml MNU (MNU10 and MNU100) supplementation. All explants were kept for 5 days in vitro, and examined for morphology as well as for glial and neuronal immunohistochemical markers. Rhodopsin-labeled photoreceptors were present in all explants. The number of cone photoreceptors (transducin), rod bipolar cells (PKC) and horizontal cells (calbindin) was significantly lower in MNU treated explants (p < 0.001). Gliosis was attenuated in MNU10 treated explants, with expression of vimentin, glial fibrillary protein (GFAP), glutamine synthetase (GS), and bFGF comparable to in vivo controls. In corresponding MNU100 counterparts, the expression of Müller cell proteins was almost extinguished. We here show that MNU causes degeneration of outer and inner retinal neurons and glia in the adult porcine retina in vitro. MNU10 explants display attenuation of gliosis, despite decreased neuronal survival compared with untreated controls. Our results have impact on the use of MNU as a large animal photoreceptor degeneration model, on tissue engineering related to retinal transplantation, and on our understanding of gliosis related neuronal degenerative cell death.

Loss of photoreceptors results in upregulation of synaptic proteins in bipolar cells and amacrine cells

Deafferentation is known to cause significant changes in the postsynaptic neurons in the central nervous system. Loss of photoreceptors, for instance, results in remarkable morphological and physiological changes in bipolar cells and horizontal cells. Retinal ganglion cells (RGCs), which send visual information to the brain, are relatively preserved, but show aberrant firing patterns, including spontaneous bursts of spikes in the absence of photoreceptors. To understand how loss of photoreceptors affects the circuitry presynaptic to the ganglion cells, we measured specific synaptic proteins in two mouse models of retinal degeneration. We found that despite the nearly total loss of photoreceptors, the synaptophysin protein and mRNA levels in retina were largely unaltered. Interestingly, the levels of synaptophysin in the inner plexiform layer (IPL) were higher, implying that photoreceptor loss results in increased synaptophysin in bipolar and/or amacrine cells. The levels of SV2B, a synaptic protein expressed by photoreceptors and bipolar cells, were reduced in whole retina, but increased in the IPL of rd1 mouse. Similarly, the levels of syntaxin-I and synapsin-I, synaptic proteins expressed selectively by amacrine cells, were higher after loss of photoreceptors. The upregulation of syntaxin-I was evident as early as one day after the onset of photoreceptor loss, suggesting that it did not require any massive or structural remodeling, and therefore is possibly reversible. Together, these data show that loss of photoreceptors results in increased synaptic protein levels in bipolar and amacrine cells. Combined with previous reports of increased excitatory and inhibitory synaptic currents in RGCs, these results provide clues to understand the mechanism underlying the aberrant spiking in RGCs.

N -methyl- N -nitrosourea-induced retinal degeneration in mice

Mouse retinal degeneration models have been investigated for many years in the hope of understanding the mechanism of photoreceptor cell death. N -methyl- N -nitrosourea (MNU) has been previously shown to induce outer retinal degeneration in mice. After MNU was intraperitoneally injected in C57/BL mice, we observed a gradual decrease in the outer nuclear layer (ONL) thickness associated with photoreceptor outer segment loss, bipolar cell dendritic retraction and reactive gliosis. Reactive gliosis was confirmed by increased GFAP protein levels. More serious damage to the central retina as opposed to the peripheral retina was found in the MNU-induced retinal degeneration model. Retinal ganglion cells (RGC) appear to be spared for at least two months after MNU treatment. Following retinal vessel labelling, we observed vascular complexes in the distal vessels, indicating retinal vessel damage. In the remnant retinal photoreceptor of the MNU-treated mouse, concentrated colouring nuclei were detected by electron microscopy, together with the loss of mitochondria and displaced remnant synaptic ribbons in the photoreceptor. We also observed decreased mitochondrial protein levels and increased amounts of nitrosylation/nitration in the photoreceptors. The mechanism of MNU-induced apoptosis may result from oxidative stress or the loss of retinal blood supply. MNU-induced mouse retinal degeneration in the outer retina is a useful animal model for photoreceptor degeneration diseases, such as age-related macular degeneration (AMD) and retinitis pigmentosa (RP).

Anthocyanins from the seed coat of black soybean reduce retinal degeneration induced by N-methyl-N-nitrosourea

Anthocyanins are known to have antioxidant effects and thus may play an important role in preventing various degenerative diseases. In this study, we examined the effect of anthocyanins extracted from the seed coat of black soybean on an animal model of retinal degeneration (RD), a leading cause of photoreceptor cell death resulting in blindness. RD was induced in rats by an intraperitoneal injection of N-methyl-N-nitrosourea (MNU) (50mg/kg), a DNA-methylating agent that causes photoreceptor damage. Anthocyanins extracted from black soybean seed coat (50mg/kg) were daily administered, orally, for 1, 2, and 4 weeks after MNU injection. Electroretinographic (ERG) recordings and morphological analyses were performed. In control rats with MNU-induced retinal damage, the ERG recordings showed a gradual significant time-dependent reduction in both a- and b-wave amplitudes compared with those of normal animals. In the MNU-induced RD rats given anthocyanins for 4 weeks, ERG responses were significantly increased compared with untreated RD rats, more apparently in scotopic stimulation than in the photopic condition. However, in the MNU-injected rats given anthocyanins for 1 and 2 weeks, the increase in ERG responses was not significant. Morphologically, the outer nuclear layer, where photoreceptors reside, was well preserved in the anthocyanin-treated rat retinas throughout the experimental period. In addition, retinal injury, evaluated by immunolabeling with an antibody against glial fibrillary acidic protein, was markedly reduced in anthocyanin-treated retinas. These results demonstrate that anthocyanins extracted from black soybean seeds can protect retinal neurons from MNU-induced structural and functional damages, suggesting that anthocyanins from black soybean seed coat may be used as a useful supplement to modulate RD.

Role of oxidative stress in retinal photoreceptor cell death in N-methyl-N-nitrosourea-treated mice

This study aimed to investigate whether oxidative stress contributes to retinal cell death in a mouse model of photoreceptor degeneration induced by N-methyl-N-nitrosourea (MNU). We measured in vitro MNU-induced radical production in retinal cell cultures of murine 661W photoreceptor-derived cells; RGC-5, a mouse ganglion cell line; and primary retinal cells. The addition of MNU induced oxidative radical generation in 661W and primary retinal cells, but not in RGC-5 cells. Edaravone, a free radical scavenger, at 1 µM reduced MNU-induced radical production in 661W and primary retinal cells. To induce in vivo retinal photoreceptor degeneration in mice, we administered 60 mg/kg MNU by intraperitoneal injection. We intravenously administered 1 mg/kg edaravone immediately and at 6 h after the MNU injection. Retinal photoreceptor degeneration was evaluated by measuring the thickness of the outer nuclear layer (ONL) by terminal deoxynucleotidyl transferase dUTP nick-end labeling (TUNEL) staining and by oxidative stress markers. MNU caused photoreceptor cell loss at 7 days after administration. Edaravone inhibited ONL thinning and reduced TUNEL-positive cells and the oxidative stress markers. These findings indicate that MNU leads to selective photoreceptor degradation via oxidative stress in vitro and in vivo and may help to understand the pathogenic mechanism of retinitis pigmentosa.

Morphological and functional evaluation of an animal model for the retinal degeneration induced by N-methyl-N-nitrosourea

The retinal degeneration (RD) is a general cause of blindness. To study its pathophysiology and evaluate the effects of new therapeutic agents before clinical trials, it is essential to establish reliable and stable animal models. This study evaluated a RD animal model in which blindness was induced by N-methyl-N-nitrosourea (MNU), a potent retinotoxin leading to apoptosis of photoreceptors. MNU was applied to the Sprague-Dawley rats by a single intraperitoneal injection in different doses (40, 50, and 60 mg/kg). The retinal functions were examined at 1 week after MNU injection by electroretinogram (ERG). Afterwards, each retina was examined by hematoxylin and eosin stain and immunohistochemistry with anti-glial fibrillary acidic protein antibody. Upon MNU injection of 40, 50 and 60 mg/kg, the ERG amplitude of a-waves showed significant reductions of 7, 26, and 44%, respectively, when compared to that of normal a-waves. The b-wave amplitudes were about 89, 65, and 58% of normal b-waves in the response to scotopic light stimulus. At 1 week, 2 weeks, and 4 weeks after MNU injection (50 mg/kg), all scotopic ERG components decreased progressively. In addition, degeneration of retinal neurons was observed in a time- and dose-dependent manner after MNU injection. Taken together, functional reduction following RD induced by MNU correlates with morphological changes. Thus, this RD rat model may be a useful model to study its pathophysiology and to evaluate the effects of new therapeutic agents before clinical trials.

Evaluation of retinal degeneration in P27KIP1 null mouse

PURPOSE

p27kip1 is well-known as a cell cycle inhibitor and also plays an important role for cell differentiation. We hypothesized that if we caused retinal degeneration in a p27(-/-) mouse, then the appropriate method of restoration may be different from that of wild mice and therefore suggest a therapeutic methodology for retinal regeneration.

METHODS

Histological and electrophysiological (ERG) examination was performed on p27(-/-) mice retina. We injected N-methy-N-nitrosourea (MNU) to induce retinal degeneration. BrdU was used to identify the dividing cells in the retina.

RESULTS

Thicker retina were observed in the p27(-/-) mice when compared to those of the p27(-/+) mice or wild type mice. Almost all retinal layers were thick and optic nerves were also enlarged. A statistically significant decrease of a and b waves amplitudes of ERG was observed in p27(-/-) mice when compared to those of the other mice. BrdU and nestin positive cells were present at the outer nuclear layer with no difference between p27(-/-) and wild type mice after MNU injection.

CONCLUSION

p27(-/-) mice showed thicker retina and less retinal function than those of other mice. The MNU-induced retinal degeneration in p27(-/-) mice closely resembled the reaction of the other mice with no retinal regeneration observed in our experimental condition.

Caspase-3-independent photoreceptor degeneration by N-methyl-N-nitrosourea (MNU) induces morphological and functional changes in the mouse retina

BACKGROUND

Retinal degeneration is followed by significant changes in the structure and function of photoreceptors in humans and several genetic animal models. However, it is not clear whether similar changes occur when the degeneration is induced pharmacologically. Therefore, our aim was to investigate the influence of retinotoxic N-methyl-N-nitrosourea (MNU) on the function, morphology and underlying molecular pathways of programmed cell death.

METHODS

C57/BL6 mice were injected with different doses of MNU, and function was determined by analysing optokinetic reflex measurements and cued water maze results at several time points post-injection. Morphometric measurements were also taken from H&E-stained paraffin eye sections. TUNEL-positive cells and caspase-3 and -6 were detected by immunohistochemistry. To assess the molecular changes leading to cell death, qRT-PCR from neurosensory retina mRNA was performed.

RESULTS

The application of MNU led to an instant decrease in function and a delayed decrease in the thickness of the retinal outer nuclear layer. These responses were observed in the absence of any structural changes in the retinal pigment epithelium. The degeneration of the photoreceptor cell layer was highest with 60 mg/kg MNU. The assessment of TUNEL-positive cells visualised cell death after treatment, but no detectable caspase-3 activity was observed concomitant with these changes. qRT-PCR revealed the possible involvement of the inflammatory mediator caspase-1 and endoplasmic reticulum stress-mediated apoptosis by caspase-12.

CONCLUSION

MNU leads to the dose-dependent degeneration of photoreceptor cells in mice by caspase-3-independent pathways and is, therefore, a suitable model to study retinal degeneration in an animal model.

Early remodeling in an inducible animal model of retinal degeneration

Photoreceptor degeneration is followed by significant morphological changes in the second-order retinal neurons in humans and in several genetic animal models. However, it is not clear whether similar changes occur when photoreceptor degeneration is induced nongenetically, raising the question whether these changes are a general effect of deafferentation independent of the cause of degeneration. We addressed this by inducing selective photoreceptor degeneration with N-methyl-N-nitrosourea (MNU) and studying its effects on inner retinal neurons in a mouse for up to 3 months, using immunocytochemistry and iontophoretic labeling. To develop objective measures of photoreceptor degeneration and of retinal remodeling, we measured several retinal proteins using immunoblot analysis, and quantified gross visual ability of the animal in a visual cliff test. The MNU-induced progressive degeneration of rods and cones was associated with declining levels of postsynaptic density 95 protein in the retina, and with deteriorating visual performance of the animal. Müller glial cells showed enhanced reactivity for glial fibrillary acidic protein as demonstrated by immunocytochemistry, which also reflected in increased levels of the protein as demonstrated by immunoblotting. Horizontal cells and rod bipolar cells progressively lost their dendritic processes, which correlated with a slight decline in the levels of calbindin and protein kinase C alpha respectively. Horizontal cell axons, immunoreactive for nonphosphorylated neurofilaments, showed sprouting into the inner nuclear layer. Ganglion cells and their synaptic inputs, probed by immunolocalizing beta-III-tubulin, neurofilaments, bassoon and synaptophysin, appeared to be unaffected. These results demonstrate that MNU-induced photoreceptor degeneration leads to retinal remodeling similar to that observed in genetic models, suggesting that the remodeling does not depend on the etiopathology that underlies photoreceptor degeneration.

Light deprivation slows but does not prevent the loss of photoreceptors in taurine transporter knockout mice

Taurine transporter knockout mice show severe retinal degeneration at an early age. The study was designed to determine whether degeneration also takes place in the absence of light. Mice were maintained up to 6 weeks of age in cyclic lighting or in total darkness. Degeneration took place in both groups, but was more rapid in animals exposed to standard cyclic illumination. At the ultrastructural level the retinas showed features characteristic of apoptosis but not of necrosis.

CONCLUSIONS

Cell differentiation is not seriously affected by the lack of a functional taurine transporter but mature photoreceptor cells do not survive without an intact transporter, even in the dark.

Caspase-independent photoreceptor apoptosis in mouse models of retinal degeneration

Apoptosis is the mode of cell death in retinitis pigmentosa, a group of retinal degenerative disorders primarily affecting rod photoreceptors. Although caspases have been demonstrated to play a central role in many incidences of apoptosis, accumulating evidence suggests that they may not be required for all forms of apoptotic cell death. The present study examined the mechanism of cell death in two in vivo models of photoreceptor apoptosis: the retinal degeneration (rd) mouse, a naturally occurring mutant model, and N-methyl-N-nitrosourea-induced retinal degeneration. Specifically, we examined the activation status of caspase-9, -8, -7, -3, and -2 and determined the caspase requirements for cytochrome c release, DNA fragmentation, and apoptosis-associated proteolysis of specific caspase substrates. We show that apoptosis in both in vivo models is independent of caspase-9, -8, -7, -3, and -2 activation. DNA fragmentation occurs in the absence of caspase-mediated ICAD (inhibitor of caspase-activated DNase) proteolysis, suggesting that an alternative endonuclease is responsible for DNA cleavage in these models. Importantly, we show that apoptosome activation is prevented because of an absence of mitochondrial cytochrome c release. Experiments performed using a cell-free system indicate that cytochrome c-dependent proteolysis and activation of caspase-9 can be restored in a neonatal cell-free system. However, we found that cytochrome c-dependent proteolysis and activation of caspase-9 could not be restored in an adult cell-free system because of an age-related decrease in the expression of Apaf-1 in the normal developing mouse retina. In the rd mouse, however, this age-related downregulation of apoptotic proteins was not observed, highlighting a critical feature of this model and the prevention of cytochrome c release as an apical event in caspase-independent apoptosis in this system.

Dietary docosahexaenoic acid protects against N-methyl-N-nitrosourea-induced retinal degeneration in rats

The effect of dietary intake of specific types of fatty acids on retinal degeneration due to N-methyl-N-nitrosourea (MNU)-induced photoreceptor cell apoptosis was evaluated. Fifty-day-old female Sprague-Dawley rats were given a single intraperitoneal injection of 50 mg kg(-1) body weight of MNU, and were then switched to one of five different diets containing the following fatty acids at the following weight percentages: 10% linoleic acid (LA); 9.5% palmitic acid (PA) and 0.5% LA; 9.5% eicosapentaenoic acid (EPA) and 0.5% LA; 4.75% EPA, 4.75% docosahexaenoic acid (DHA) and 0.5% LA; or 9.5% DHA and 0.5% LA. When rats developed MNU-induced mammary tumors with a diameter of > or =1 cm, or at the termination of the experiment (20 weeks after MNU injection), retinal tissue samples were obtained and examined. Incidence and severity of retinal damage were compared by histologic examination. MNU-induced retinal degeneration was prevented in rats fed the diet containing 9.5% DHA (4.75% DHA was less effective), whereas it was accelerated in rats fed the 10% LA diet. Over the course of the 20-week experimental period, the fatty acid composition of serum reflected differences in dietary fatty acids. The present results indicate that a diet containing 9.5% DHA can counteract MNU retinotoxicity in the rat retina. DHA may play a role in protection against MNU-induced photoreceptor cell apoptosis in the rat retina.

Functional rescue of N-methyl-N-nitrosourea-induced retinopathy by nicotinamide in Sprague-Dawley rats

PURPOSE

A single intraperitoneal injection of 60 mg/kg body weight of N-methyl-N-nitrosourea (MNU) into rats results in retinal degeneration over a 7-day period in all treated animals. The purpose of this study was to determine whether nicotinamide (NAM) can lead to a functional rescue of the MNU-induced retinopathy.

METHODS

NAM, a water-soluble B-group vitamin (vitamin B( 3)), was administered immediately after MNU injection, and retinas were examined morphologically and functionally.

RESULTS

Morphologically, 1000 mg/kg NAM completely suppressed and 50 mg/kg NAM partially suppressed the photoreceptor cell loss. Functionally, scotopic and photopic electroretinographic (ERG) recordings showed that both rod and cone photoreceptor cells were well protected from MNU damage by 1000 mg/kg NAM and partially protected by 50 mg/kg NAM.

CONCLUSIONS

NAM can protect photoreceptor cells from MNU-induced retinopathy both structurally and functionally.

CI-1010 induced opening of the mitochondrial permeability transition pore precedes oxidative stress and apoptosis in SY5Y neuroblastoma cells

The hetero-bifunctional nitroimidazole radiosensitizer CI-1010, R-alpha-[[(2-bromoethyl)-amino]methyl]-2-nitro-1H-imidazole-1-ethanol monohydrobromide, causes selective irreversible apoptotic loss of retinal photoreceptor cells in vivo. The human neuroblastoma cell line, SH-SY5Y, was used as a neuronotypic model of CI-1010-mediated retinal degeneration. Exposure to CI-1010 for 24 h induced apoptosis in neuroblastoma cells, as determined by histopathological and ultrastructural analysis and by TUNEL technique. CI-1010 causes a dose-dependent decrease in cell viability in SY5Y cells, as measured by the reduction of MTT, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide. Superoxide dismutase reduced loss of cell viability following CI-1010 treatment suggesting an oxidative stress-mediated mechanism of toxicity. The effects of CI-1010 on mitochondrial membrane potential and intracellular levels of reactive oxygen species were assessed in live SY5Y cells by confocal microscopy using the fluorescent dyes, tetramethylrhodamine methyl ester and 5,6-carboxy-2',7'-dihydrodichlorofluorescein diacetate. CI-1010 caused a rapid depolarization of mitochondria in SY5Y cells followed by an increase in ROS. Both CI-1010-induced mitochondrial depolarization and subsequent increases in ROS were prevented by pretreatment with either the permeability transition pore inhibitor, cyclosporin A (CsA), and by the antioxidant, alpha-tocopherol. However, CsA and alpha-tocopherol were unable to prevent apoptosis in CI-1010-treated cells, suggesting the influence of additional mechanism(s) of CI-1010-induced toxicity. This study evaluates intracellular oxidative stress associated with pore opening prior to apoptosis and provides evidence in support of a mitochondrial mechanism of CI-1010-induced neuronal cell death.

Nicotinamide prevents N-methyl-N-nitrosourea-induced photoreceptor cell apoptosis in Sprague-Dawley rats and C57BL mice

In previous studies, it was found that a single systemic administration of N-methyl-N-nitrosourea (MNU) to rats and mice resulted in the retinal degeneration in all treated animals over a 7 day period. Retinal degeneration was due to photoreceptor cell apoptosis that was identical to the apoptosis seen in human retinitis pigmentosa (RP). In the present study, nicotinamide (NAM), a water-soluble B-group vitamin (vitamin B(3)), suppressed photoreceptor cell loss in a dose-dependent manner when administered immediately after MNU treatment. In rats, a dose of NAM >or=25 mg kg(-1) completely suppressed photoreceptor cell loss, and 10 mg kg(-1) partially suppressed photoreceptor cell loss. In mice, doses of 1000 and >or=100 mg kg(-1) were needed for complete and partial suppression, respectively. Thus, rats were more responsive to NAM than mice. The retinoprotective effect of 1000 mg kg(-1) NAM lasted throughout the long-term (35 days) observation period, with no apparent toxicity. Also, in rats, 1000 mg kg(-1) NAM completely suppressed photoreceptor cell loss when administered up to 4 hr after MNU treatment, and partially suppressed photoreceptor cell loss when administered 6 hr after MNU treatment. In mice, administration of NAM 2-6 hr after MNU resulted in partial suppression. NAM did not reduce levels of 7-methyldeoxyguanosine DNA adduct, but did reduce photoreceptor cell apoptosis. Although the mechanism of action underlying this retinoprotection remains to be clarified, NAM may be a potential therapeutic agent for the treatment of retinal degeneration.

AP-1 mediated retinal photoreceptor apoptosis is independent of N-terminal phosphorylation of c-Jun

Apoptosis is essential for retinal development but it is also a major mode of cell loss in many human retinal dystrophies. High levels of visible light induce retinal apoptosis in mice and rats. This process is dependent on the induction of the transcription factor AP-1, a dimeric complex composed of c-Fos and c-Jun/JunD phosphoproteins. While c-Fos is essential, JunD is dispensable for light-induced photoreceptor apoptosis. Here we show that N-terminal phosphorylation of c-Jun, the other main partner of c-Fos in induced AP-1 complexes is not required for programmed cell death during retinal development in vivo and is also dispensable for photoreceptor apoptosis induced by the exogenous stimuli "excessive light" and N-nitroso-N-methylurea (MNU). Mice expressing a mutant c-Jun protein (JunAA) that cannot be phosphorylated at its N-terminus are apoptosis competent and their retina is not distinguishable from wild-type mice. Accordingly, Jun kinase, responsible for phosphorylation of wild-type c-Jun protein is at best only marginally induced by the apoptotic stimuli "light" and MNU. Complex composition of light-induced AP-1 complexes is similar in wild-type and JunAA mice. This shows that the mutant c-Jun protein can be part of the DNA binding complex AP-1 and demonstrates that induction of the DNA binding activity of AP-1 after light insult does not depend on N-terminal phosphorylation of c-Jun. Our results suggest that transactivation of target genes by phosphorylated c-jun/AP-1 is not required for MNU- or light-induced apoptosis of photoreceptor cells.

Caspase-3 inhibitor rescues N -methyl- N -nitrosourea-induced retinal degeneration in Sprague-Dawley rats

The effect of a caspase-3 inhibitor on N -methyl- N -nitrosourea (MNU)-induced retinal degeneration was investigated. Sixty mg kg(-1)MNU was given intraperitoneally to 50 day old female Sprague-Dawley rats, and 4000 ng Ac-DEVD-CHO, a caspase-3 inhibitor, was injected intravitreally twice at 0 and 10 hr after MNU. In both peripheral and central retina, an apoptotic index of the photoreceptor cells 24 hr after MNU treatment was calculated by TUNEL labeling, and retinal damage 7 days after MNU treatment was evaluated from retinal thickness and a retinal damage ratio (length of damaged retina : whole retinal length). In MNU-treated rats, the TUNEL index 24 hr post-MNU was 79.5% in the peripheral and 83.7% in the central retina, while the Ac-DEVD-CHO injection significantly reduced it to 59.7 and 71.8%, respectively. Total retinal thickness 7 days after MNU was 38 microm in the peripheral and 75 microm in the central retina. Ac-DEVD-CHO injection increased these values to 72 and 77 microm, respectively. The retinal damage ratio 7 days after MNU was 98.5%. Ac-DEVD-CHO injection significantly reduced this value to 54.4%. The use of a caspase-3 inhibitor was effective in the suppression of MNU-induced retinal apoptosis and may be a therapeutic intervention in human retinitis pigmentosa.

Cataractogenesis in neonatal Sprague-Dawley rats by N-methyl-N-nitrosourea

Cataract was induced by a single intraperitoneal injection of 100 mg/kg N-methyl-N-nitrosourea (MNU) to 0-, 5-, 10-, 15-, or 20-day-old male and female Sprague-Dawley rats. In day 0, 5, 10, and 15 MNU-treated rats, mature cataracts were constantly seen 7, 14, 14, and 30 days after dosing, respectively. In the day 20 MNU-treated rats, only subcapsular cataract was seen 30 days after dosing. Therefore, the rats exposed to MNU at an earlier age caused cataract more rapidly and severely. In the day 0 MNU-treated rats, 7-methyldeoxyguanosine DNA adduct was detected in the lens epithelial nuclei 12 hours after MNU dosing, followed by apoptosis, which was confirmed by morphology, by TUNEL signals, and by DNA ladder and peaked 3 days after MNU dosing. In the apoptosis cascade, upregulation of Bax, downregulation of Bcl-2, and increased CPP32 protease (caspase-3) activity were seen 12 hours after MNU dosing. Therefore, the pathogenesis of MNU-induced cataract was associated with DNA adduct formation in the lens epithelial cell nuclei leading to apoptosis by upregulation of Bax protein, downmodulation of Bcl-2 protein, and activation of caspase-3.

Hypoxia-dependent retinal toxicity of bioreductive anticancer prodrugs in mice

The bioreductive anticancer prodrug CI-1010 ((2R)-1-[(2-bromoethyl)amino]-3-(2-nitro-1H-imidazol-1-yl)-2-propanol hydrobromide) is an alkylating nitroimidazole which shows selective toxicity against hypoxic cells in murine tumors, but causes extensive apoptosis in the outer retina in rodents and monkeys. This irreversible retinal toxicity has terminated preclinical development of CI-1010. We have investigated whether such toxicity is due to physiological hypoxia in the retina, and whether it is a general feature of hypoxia-selective bioreductive drugs. Retinal damage was quantified by morphometric analysis of histological sections following treatment of female C57Bl6 mice. Both CI-1010 and tirapazamine (TPZ, 1,2,4-benzotriazin-3-amine 1,4-dioxide), a bioreductive drug in Phase III clinical trial, caused a time and dose-dependent loss of photoreceptor cells of the outer retina following administration of single intraperitoneal doses. The lesion caused by TPZ was qualitatively similar to that with CI-1010, but was less severe at equivalent fractions of the maximum tolerated dose (as defined by lethality). With both bioreductive drugs, lesion severity was increased if animals breathed 10% O(2) for 3 h after drug administration, while breathing 95% O(2)/5% CO(2) was protective. Other hypoxia-selective bioreductive drugs tested (the quinone porfiromycin, the anthraquinone N-oxide AQ4N and the nitrogen mustard prodrugs SN 23816 and SN 25341) did not cause retinal damage at their maximum tolerated doses. This study suggests that the retinal toxicity of bioreductive drugs might be avoided by manipulation of tissue hypoxia using 95% O(2)/5% CO(2), although this intervention could suppress antitumor activity. The finding that not all bioreductive drugs cause retinal toxicity suggests this toxicity can be avoided through appropriate drug design.

c-fos controls the "private pathway" of light-induced apoptosis of retinal photoreceptors

White light (5 klux for 2 hr) induces apoptosis of rod photoreceptors in wild-type mice (c-fos(+/+)) within 24 hr, whereas rods of c-fos knock-out mice (c-fos(-/-)) are protected (). The range of this protection was tested by analyzing retinas of c-fos(+/+) and c-fos(-/-) mice up to 10 d after exposure to threefold increased light intensities (15 klux for 2 hr). In c-fos(-/-) mice, rods were unaffected, whereas they were destroyed in c-fos(+/+) mice. After light exposure, mitochondrial damage in rods was observed exclusively in c-fos(+/+) mice. Electroretinograms recorded 48 hr after exposure revealed a decrease of all components in c-fos(+/+) mice but indicated no light-induced loss of function in c-fos(-/-) mice. Thus, in c-fos(-/-) mice, light-induced apoptosis is blocked or its threshold is elevated more than threefold. Increased activity of the transcription factor activator protein-1 (AP-1) in retinas of light-exposed c-fos(+/+) mice indicated an acute contribution of AP-1 to apoptosis induction. AP-1 activity increased already during exposure and peaked approximately 6 hr thereafter, coinciding with the appearance of major morphological signs of apoptosis. Activated AP-1 mainly consisted of c-Fos/Jun heterodimers. In c-fos(-/-) mice, AP-1 activity remained unchanged, indicating that no other Jun- or Fos-family member could substitute for c-Fos. Like damaging light, N-methyl-N-nitrosourea (MNU) induced AP-1 containing c-Fos in c-fos(+/+) mice and did not induce AP-1 in c-fos(-/-) mice. In contrast to light, however, MNU induced apoptosis in rods of c-fos(-/-) mice. Thus, c-Fos is essential for a specific premitochondrial "private apoptotic pathway" induced by light but not for the execution of apoptosis induced by other stimuli.

Time-specific action of N-methyl-N-nitrosourea in the occurrence of retinal dysplasia and retinal degeneration in neonatal mice

The morphologic response of neonatal mouse retina to the alkylating agent N-methyl-N-nitrosourea (MNU) was examined at different periods of retinal development. A dose of 60 mg/kg N-methyl-N-nitrosourea was injected intraperitoneally to neonatal C57BL mice at 0, 3, 5, 8, 11, 14, 17, and 20 days of age and to C3H mice at 0 days of age, and the retinas were examined sequentially. In the C57BL mice, MNU evoked a time-dependent occurrence of retinal dysplasia and retinal degeneration. With MNU treatment at day 0 and day 3 (the stage of retinal cell proliferation), retinal dysplasia characterized by the progressive disorganization of neuroblasts, which led to the formation of rosettes, was found in the outer neuroblastic/nuclear layer above the normal pigment epithelial cells during days 8-20, but decreased at day 50. The rosettes were surrounded by photoreceptor segments and Müller cell processes, and by photoreceptor nuclei. The MNU response was related to retinal differentiation; following MNU treatment at day 5 or 8 (the stage of retinal cell differentiation) the cells were much less sensitive (i.e. no retinal response was found). However, with MNU treatment at days 11, 14, 17, and 20 (after cellular differentiation), retinal degeneration characterized by selective photoreceptor apoptosis was seen. These results suggest that there is a critical period for the time of MNU administration in the development of mouse retinal lesions. In C3H (rd/rd) mice, MNU treatment at day 0 resulted in retinal degeneration with only slight rosette formation at the peripheral retina.