Generating mouse models of retinal disease using ENU mutagenesis

Leaves of Acer palmatum thumb. Rescues N-ethyl-N-nitrosourea (ENU)-Induced retinal degeneration in mice

BACKGROUND

In the East Asia, the genus Acer (Aceraceae) is a herbal medicine that is used to treat various diseases, including hemostasis, hepatic disorders, traumatic bleeding and poor eyesight. However, the effects of Acer palmatum thumb. on retinal degeneration are unknown.

AIM

In this study, we investigated whether Acer palmatum thumb.ethanol extract (KIOM-2015E) can protect eyes from retinal degeneration. Our research investigated whether KIOM-2015E could have a protective effect in the retinal degenerating mouse model induced by N-ethyl-N-nitrosourea (ENU).

MATERIALS AND METHODS

Retinal degeneration was induced by a single intraperitoneal injection of ENU in ICR mice. KIOM-2015E (100, 200 mg/kg) was orally administered once per day. The eyeballs were embedded and lysed after drug administration to examine the histological changed and protein expression levels.

RESULTS

The ENU-induced retinal degeneration model exhibited increased photoreceptor cell death and a loss of the outer nuclear layer. Additionally, the expression of PKCα and OPN1SW was reduced, and that of GFAP and Nestin was increased in ENU-treated retinal tissues.

CONCLUSION

KIOM-2015E treatment ameliorated the ENU-induced retinal degeneration. KIOM-2015E prevents ENU-induced retinal degeneration by modulating protein expression and the thickness of the outer nuclear layer in the retina.

N -ethyl- N -nitrosourea induces retinal photoreceptor damage in adult rats

Seven-week-old male Lewis rats received a single intraperitoneal injection of N-ethyl-N-nitrosourea (ENU) (100, 200, 400 or 600 mg/kg), and retinal damage was evaluated 7 days after the treatment. Sequential morphological features of the retina and retinal DNA damage, as determined by a TUNEL assay and phospho-histone H2A.X (γ-H2AX), were analyzed 3, 6, 12, 24 and 72 hr, 7 days, and/or 30 days after 400 mg/kg ENU treatment. Activation of the nuclear enzyme poly (ADP-ribose) polymerase (PARP) was analyzed immunohistochemically by poly (ADP-ribose) (PAR) expression in response to DNA damage of the retina. All rats that received ≥ 400 mg/kg of ENU developed retinal degeneration characterized by the loss of photoreceptor cells in both the central and peripheral retina within 7 days. In the 400 mg/kg ENU-treated rats, TUNEL-positive signals were only located in the photoreceptor cells and peaked 24 hr after ENU treatment. The γ-H2AX signals in inner retinal cells appeared at 24 hr and peaked at 72 hr after ENU treatment, and the PAR signals selectively located in the photoreceptor cell nuclei appeared at 12 hr and peaked at 24 hr after ENU treatment. However, degeneration was restricted to photoreceptor cells, and no degenerative changes in inner retinal cells were seen at any time points. Retinal thickness and the photoreceptor cell ratio in the central and peripheral retina were significantly decreased, and the retinal damage ratio was significantly increased 7 days after ENU treatment. In conclusion, ENU induced retinal degeneration in adult rats that was characterized by photoreceptor cell apoptosis through PARP activity.

Translational vision research models program

ENU mutagenesis is an efficient method to identify new animal models of ocular disease. The new alleles described herein will be a useful resource to further examine the role of the affected molecules and the effects of their disruption within the retina.

Retinal degeneration induced in adult mice by a single intraperitoneal injection of N-ethyl-N-nitrosourea

Seven-week-old female BALB/c mice received a single intraperitoneal injection of N-ethyl-N-nitrosourea (ENU) (50, 100, 200, 400, or 600 mg/kg), and retinal damage was evaluated after 7 days. Sequential morphological features of the retina and retinal apoptosis, as determined by the TUNEL assay, were analyzed 6, 12, 24, and 72 hr and 7 days after treatment with 600 mg/kg of ENU. Moreover, older mice (25 to 34 weeks of age) received an intraperitoneal injection of 600 mg/kg ENU and were sacrificed 7 days later. All animals were necropsied, and both eyes were examined histopathologically. Two of the 5 mice that received 600 mg/kg ENU died during the experimental period. Histopathologically, all mice that received 600 mg/kg of ENU experienced retinal degeneration characterized by the loss of photoreceptor cells (disappearance of the outer nuclear layer and photoreceptor layer) in both the central and peripheral retina within 7 days. One of 5 mice treated with 400 mg/kg ENU exhibited retinal damage that was restricted to the central retina. Older mice treated with 600 mg/kg ENU exhibited retinal damage that was similar to the retinal damage in younger mice. In the 600 mg/kg ENU-treated mice, TUNEL-positive photoreceptor cells peaked 72 hr after ENU treatment. Retinal thickness and the photoreceptor cell ratio in the central and peripheral retina were significantly decreased, and the retinal damage ratio was significantly increased 7 days after treatment. In conclusion, ENU induces retinal degeneration in adult mice that is characterized by photoreceptor cell apoptosis.

Mouse model resources for vision research

The need for mouse models, with their well-developed genetics and similarity to human physiology and anatomy, is clear and their central role in furthering our understanding of human disease is readily apparent in the literature. Mice carrying mutations that alter developmental pathways or cellular function provide model systems for analyzing defects in comparable human disorders and for testing therapeutic strategies. Mutant mice also provide reproducible, experimental systems for elucidating pathways of normal development and function. Two programs, the Eye Mutant Resource and the Translational Vision Research Models, focused on providing such models to the vision research community are described herein. Over 100 mutant lines from the Eye Mutant Resource and 60 mutant lines from the Translational Vision Research Models have been developed. The ocular diseases of the mutant lines include a wide range of phenotypes, including cataracts, retinal dysplasia and degeneration, and abnormal blood vessel formation. The mutations in disease genes have been mapped and in some cases identified by direct sequencing. Here, we report 3 novel alleles of Crx^tvrm65, Rp1^tvrm64, and Rpe65^tvrm148 as successful examples of the TVRM program, that closely resemble previously reported knockout models.

A novel mutation in Prph2, a gene regulated by Nr2e3, causes retinal degeneration and outer-segment defects similar to Nr2e3 ( rd7/rd7 ) retinas

The nmf193 mutant was generated by a large-scale ENU mutagenesis screen and originally described as having a dominantly inherited phenotype characterized by fundus abnormalities. We determined that nmf193 mice exhibit outer-segment defects and progressive retinal degeneration. Clinical examination revealed retinal spotting apparent at 6 weeks of age. Histologic analysis of homozygous mutant mice at 6 weeks indicated an absence of outer segments (OS) and a 50% reduction of photoreceptor cells which progressed to complete loss of photoreceptors by 10 months. Mice heterozygous for the nmf193 mutation had a less severe phenotype of shortened outer segments at 2 months with progressive loss of photoreceptor cells to 50% by 10 months. A positional cloning approach using a DNA pooling strategy was performed to identify the causative mutation in nmf193 mice. The nmf193 mutation was linked to chromosome 17 and fine mapped to an interval containing the peripherin/rds (Prph2) gene. Mutation analysis identified a single base change in Prph2 that causes aberrant splicing between exons 1 and 2. Interestingly, a comparative histologic analysis demonstrated that Prph2 ( nmf193/+ ) mutants have similar photoreceptor degeneration to that of Nr2e3 ( rd7/rd7 ). We show that Prph2 mRNA and protein levels are reduced in the Nr2e3 ( rd7/rd7 ) mutant compared to control littermates. Chromatin immunoprecipitation analysis shows that Prph2 is a direct target of NR2E3. In addition, the downregulation of Prph2 gene expression is similar in both the Nr2e3 ( rd7/rd7 ) and Prph2 ( nmf193/+ ) mutants, suggesting that the reduction of Prph2 may contribute to the degenerative pathology seen in Nr2e3 ( rd7/rd7 ).

Screening for novel ENU-induced rhythm, entrainment and activity mutants

Chemical mutagenesis has provided an opportunity to develop and expand the repertoire of behavioural mutants for gene function studies. With this in mind, we have established a screen in mice for mutations affecting circadian rhythms, entrainment to light and other wheel-running parameters. The screen consists of an assessment of mouse wheel-running activity in a 12:12 h light/dark cycle for 7-10 days followed by assessment in constant darkness for up to 20 days. Responses to light are assessed using two protocols; a 15 minute light pulse given at circadian time 16 on the tenth day in constant darkness and an additional 12 h of light upon transition from light/dark conditions to constant darkness. To date, approximately 1300 progeny of chemically mutagenised mice have been screened. Computer-aided assessment of wheel-running parameters has helped in identifying abnormal phenotypes in approximately 5% of all animals screened. Inheritance testing of mice with abnormal phenotypes has confirmed the number of robustly inherited mutant phenotypes to be 1% of the total screened. Confirmed mutants including those affecting free-running period, light-responsiveness and wheel-running endurance have been identified. Thus far, low-resolution map positions have been established for four mutants by completing genome scans in backcross progeny. Mutant loci do not correspond with those previously associated with wheel-running behaviour. This result confirms that phenotype-driven approaches such as this should continue to provide material for mammalian gene function studies.

ENU large-scale mutagenesis and quantitative trait linkage (QTL) analysis in mice: novel technologies for searching polygenetic determinants of craniofacial abnormalities

Discrepancies in size and shape of the jaws are the underlying etiology in many orthodontic and orthognathic surgery patients. Genetic factors combined with environmental interactions have been postulated to play a causal or contributory role in these craniofacial abnormalities. Along with the soon-to-be-available complete human and mouse genomic sequence data, mouse mutants have become a valuable tool in the functional mapping of genes involved in the development of human maxillofacial dysmorphologies. We review two powerful methods in such efforts: N-ethyl-N-nitrosourea (ENU) large-scale mutagenesis and quantitative trait linkage (QTL) analysis. The former aims at producing a plethora of novel variants of particular trait(s), and ultimately mapping the point mutations responsible for the appearance of these new traits. In contrast, the latter applies intensive breeding and mapping techniques to identify multiple loci (and, subsequently, genes) contributing to the phenotypic difference between the tested strains. A prerequisite for either approach to studying variations in the traits of interest is the application of effective mouse cephalometric phenotype analysis and rapid DNA mapping techniques. These approaches will produce a wealth of new data on critical genes that influence the size and shape of the human face.

Mouse models for neurological disease

The mouse has many advantages over human beings for the study of genetics, including the unique property that genetic manipulation can be routinely carried out in the mouse genome. Most importantly, mice and human beings share the same mammalian genes, have many similar biochemical pathways, and have the same diseases. In the minority of cases where these features do not apply, we can still often gain new insights into mouse and human biology. In addition to existing mouse models, several major programmes have been set up to generate new mouse models of disease. Alongside these efforts are new initiatives for the clinical, behavioural, and physiological testing of mice. Molecular genetics has had a major influence on our understanding of the causes of neurological disorders in human beings, and much of this has come from work in mice.