Short communication Expression of the candidate circadian photopigment melanopsin (Opn4) in the mouse retinal pigment epithelium Stuart N. Peirson a, * , Petra H.M. Bovee-Geurts b , Daniela Lupi a , Glen Jeffery c , Willem J. DeGrip b , Russell G. Foster a a Department of Visual Neuroscience, Division of Neuroscience and Psychological Medicine, Faculty of Medicine, Imperial College London, Charing Cross Hospital, Fulham Palace Road, London W6 8RF, UK b Department of Biochemistry, NCMLS, University of Nijmegen, The Netherlands c Department of Visual Science, Institute of Ophthalmology, University College London, 11-43 Bath Street, London EC1V 9EL, UK Accepted 29 January 2004 Abstract 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. D 2004 Elsevier B.V. All rights reserved. Theme: Neural basis of behavior Topic: Biological rhythms and sleep Keywords: Opn4; Melanopsin; RPE; Circadian; Photopigment Photoreception within the vertebrate eye is not confined to the rods and cones. This heretical idea originated from studies on both teleost fish and retinally degenerate mice. Studies on mice that lack all functional rods and cones have shown that a range of responses to light, including circadian entrainment and pupillary constriction, are preserved in the absence of classical photoreceptors [7]. The photopig- ment(s) that mediate these effects of irradiance remain unidentified, although melanopsin (Opn4) has emerged as a strong candidate. Melanopsin was originally isolated from photosensitive melanophores of Xenopus laevis [15], and a homolog was then identified in mammals and shown to be expressed in a subset of retinal ganglion cells (RGCs) [16]. Studies in rats and mice have identified a population of RGCs that are directly light sensitive [3,17], and express melanopsin. Most recently, three different laboratories have generated mela- nopsin knock-out mice and shown that circadian and pupil- lary responses to light are attenuated in these animals, and completely abolished in animals lacking melanopsin and functional rods and cones [10,13]. Collectively these results have led to the assumption that melanopsin forms the photopigment of the intrinsically photosensitive ganglion cells. The mammalian RPE possesses no known photosensory function, but it does express a number of opsin-like proteins, such as RGR-opsin and peropsin, which are involved in photopigment regeneration [2]. A brief report suggested that 0169-328X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.molbrainres.2004.01.007 * Corresponding author. Tel.: +44-208-8467524; fax: +44-208- 8467511. E-mail address: s.peirson@imperial.ac.uk (S.N. Peirson). www.elsevier.com/locate/molbrainres Molecular Brain Research 123 (2004) 132 – 135