Cultured Müller cells from mammals can synthesize and accumulate retinyl esters

Abstract

Research reports from our laboratory, as well as those fromothers, have shown that cone-dominant species such as chicken,ground squirrel and zebrafish possess a novel cone cycle which isdependent on the production of 11-cis retinal from Muller cells intheretina(Munizetal.,2006;Munizetal.,2007;WangandKefalov,2011). This visual cycle operates differently from the classical(canonical) visual cycle, which is dependent on the synthesis of11- cis retinal by an isomerohydrolase (RPE65 or Isomerase 1)located in the retinal pigment epithelium (RPE) (Fleisch andNeuhauss, 2010; Takahashi et al., 2011). Using eyecup and iso-lated retina preparations, Kolesnikov et al. recently reported thatthe mouse retina promoted M/L-cone dark adaptation eightfoldfaster than the RPE alone (Kolesnikov et al., 2011) suggesting thatthe initial rapid cone pigment regeneration is highly dependent onthe synthesis of 11-cis retinal from Muller cells in the retina(Kolesnikov et al., 2011). However, it is not clear if IRBP (Inter-photoreceptor Retinoid Binding Protein) is essential for normalretinoidprocessingintheretinalconecycle(Kolesnikovetal.,2011;Parker et al., 2011). Furthermore, it has recently been reported thatdifferent variants of isomerases are expressed in mouse conephotoreceptors (Tang et al., 2011) as well as zebrafish Muller cellsand ganglion cells (RPE65a and RPE65c) (Takahashi et al., 2011).Mullercellsfromthechickenretinahavebeen shownto possessretinol isomerase (Das et al., 1992; Mata et al., 2005; Muniz et al.,2009) and 11-cis retinyl ester synthase (Muniz et al., 2006) activi-ties. Although limited biochemical data are now available on thesevisual cycle enzymes, they are collected from the retina or Mullercellsin cone-dominant species. Atpresent, it is notknown if Mullercellsfromrod-dominantretinasalsopossessaconevisualcycleandspecific visual proteins for the cone cycle.Spontaneously immortalized human Muller cells were obtainedfrom Dr. Astrid Limb (University College; London, UK). Cells werecultured in T25 flasks using Dulbecco’smodified essential medium(DMEM) with 4.5 g/L glucose and 10% fetal bovine serum (FBS) at37 C þ 5% CO