Expression of the candidate circadian photopigment melanopsin (Opn4) in the mouse retinal pigment epithelium

Mar 30, 2004Brain research. Molecular brain research

Levels of the circadian light-sensing pigment melanopsin in mouse eye support cells

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Abstract

Opn4 expression is higher in the retinal pigment epithelium than in the retina.

Melanopsin (Opn4) is known for its role in light responses, particularly in circadian rhythm and pupil constriction.
Intrinsic photosensitivity in a subset of retinal ganglion cells (RGCs) is associated with Opn4.
Opn4 knock-out mice exhibit reduced responses to light in both circadian rhythms and pupil constriction.
This study identifies Opn4 expression in the retinal pigment epithelium (RPE), a tissue traditionally not linked to light sensing.
Retinal contamination of RPE extracts was excluded, indicating the specificity of Opn4 expression in RPE.

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A number of responses to light, including circadian entrainment and pupillary constriction, are preserved in mammals that lack rod and cone photoreceptors. Recent studies have demonstrated that a subset of retinal ganglion cells (RGCs) are intrinsically photosensitive, and that these RGCs project to regions of the brain associated with the regulation of the circadian clock and pupil constriction. The photopigment gene(s) that mediate these effects of irradiance remain unidentified, although melanopsin (Opn4) has emerged as a strong candidate. For example, Opn4 is expressed within intrinsically photosensitive RGCs, and Opn4 knock-out mice show attenuated circadian and pupillary responses to light. In this study we provide the first clear evidence that Opn4 expression is not confined to these photosensitive RGCs, but is also expressed in the retinal pigment epithelium (RPE), a tissue with no known photosensensory role. We can preclude retinal contamination of RPE extracts as levels of Opn4 expression were higher in the RPE than in the retina, and the expression of rod opsin and Thy1 (a marker of the RGC layer) were barely detectable in RPE extracts. Our results raise questions about the presumed function of melanopsin, and highlight the need for biochemical studies on this protein.

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Expression of the candidate circadian photopigment melanopsin (Opn4) in the mouse retinal pigment epithelium

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