Expression of beta-carotene 15,15' monooxygenase in retina and RPE-choroid

Beta-carotene 15,15' monooxygenase (beta-CM) catalyzes the central cleavage of beta-carotene to all-trans-retinal, the first step in vitamin A synthesis. This study was conducted to determine the expression of beta-CM in the mammalian retina and RPE, to assess its relevance in carotenoid-retinoid metabolism in the retina and RPE.

RT-PCR was used to detect expression of beta-CM mRNA in the retina and RPE-choroid of the mouse, cow, human, and monkey and in RPE cells and other cell lines. Immunofluorescence microscopy was used to localize beta-CM in mouse and monkey retina with an anti-peptide antibody specific for beta-CM.

By RT-PCR, beta-CM mRNA was detected at a low level in mouse and monkey retina and in the RPE-choroid of the monkey but not of the mouse. Conversely, beta-CM mRNA was expressed at a low level in both human and bovine RPE-choroid, but not in the retina of either. RPE primary cultured cells of the monkey also showed beta-CM mRNA expression, although the three human lines did not. In addition, of nine other cell lines tested, only COS-7 was positive for beta-CM. Immunofluorescence microscopy showed weak immunoreactivity in the inner retina in both the mouse and monkey. beta-CM immunoreactivity was not detectable in RPE of the mouse. Use of a long-wavelength exciting and emitting secondary probe to mitigate lipofuscin autofluorescence, facilitated the detection of a low level of beta-CM immunoreactivity in monkey RPE.

Beta-CM mRNA and protein are expressed at low levels in the mammalian retina and RPE-choroid. Given the low and variable expression of beta-CM in the retina and RPE, it can be concluded that beta-CM is not necessary for a conserved retina or RPE-specific function, but may be necessary for a species-specific function.