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Phagosome number and distribution in retinal pigment epithelial cells of vitiligo mutant mice
1New England Regional Primate Research Center, Harvard Medical School, Southborough, MA 01772-9102, USA.
Abstract:
The vitiligo mutant mouse has a disorder affecting the interaction of retinal pigment epithelium (RPE) and photoreceptor cells of the neural retina. Among the phenotypic features are patches of hyper- and hypopigmentation in the embryonic RPE, increased RPE cell production neonatally, and a later onset of progressive photoreceptor cell degeneration that continues for more than one year until all photoreceptor cells are gone. Failure of RPE microvilli to intertwine with rod outer segments (ROS) at any age, the accumulation of ROS membranous fragments in the subretinal space, and a relatively early retinal separation from the RPE suggested analysis of whether RPE phagocytosis might be impaired. Post-natal day 23 (P23) and P36 mutant and congenic control wild-type mice were kept in darkness overnight and eyes were examined by light and transmission electron microscopy 0.5 hr before, 1.5 hr after and 10.5 hr after lights turned on at 0700 hr. At these ages ROS have not yet degenerated, though they are shorter than normal and somewhat misoriented. The number of phagosomes per RPE cell was markedly reduced in mutants compared to controls at both ages and all time points. Nonetheless, the highest counts were obtained 1.5 hr after the lights turned on in mutant and control specimens. In the mutant eyes, the proportion of phagosomes in the microvillous zone of the RPE cells was consistently lower than in any other cellular compartment. Phagosome distribution in the apical and basal zones of the RPE cell cytoplasm was within normal limits. Macrophage-like cells become numerous in the subretinal space at older ages, but were already present at P23 and P36, and contained phagosomes in their cytoplasm. The hypothesis is proposed that binding of ROS to RPE cells might be defective in vitiligo mice, in contrast to the rdy rat, where the work of others indicates that binding is normal and the subsequent ingestion of phagosomes is impaired.
Insights
Vitiligo mutant mice exhibit impaired retinal pigment epithelium (RPE) phagocytosis of photoreceptor outer segments, leading to progressive vision loss. This defect may stem from defective binding, not ingestion, of outer segments by RPE cells.
Area of Science:
- Ophthalmology
- Cell Biology
- Genetics
Background:
- The vitiligo mouse model displays progressive photoreceptor degeneration.
- Key features include RPE abnormalities and failure of RPE microvilli to interact with rod outer segments (ROS).
- Previous studies suggest potential defects in RPE phagocytosis.
Purpose of the Study:
- To investigate whether RPE phagocytosis is impaired in vitiligo mutant mice.
- To analyze the interaction between RPE and photoreceptor outer segments in this model.
Main Methods:
- Light and transmission electron microscopy were used to examine eyes of mutant and control mice at post-natal days 23 and 36.
- Mice were subjected to a dark-light cycle to assess phagosome formation and distribution in RPE cells.
- Phagosome counts and distribution within RPE cells were quantified at various time points after light onset.
Main Results:
- Mutant mice showed a marked reduction in phagosomes per RPE cell compared to controls.
- Phagosomes in mutant RPE cells were found less frequently in the microvillous zone.
- Macrophage-like cells containing phagosomes were present in the subretinal space of mutants.
Conclusions:
- Vitiligo mutant mice have impaired phagocytosis of rod outer segments by RPE cells.
- The defect appears to be related to the binding or initial uptake of ROS by RPE, rather than subsequent ingestion.
- This contrasts with the rdy rat model, where binding is normal but ingestion is impaired.