Photoreceptor “Retinoid Pumps” in Health and Disease

complicated by the involvement of cell–cell interactions and the importance of trophic effects. The field of genet-of the cells that express the mutated gene. However, it can, A., et al. throughout the body can also cause specific photore-Stargardt disease the genetic defect in rod cells apparently affects RPE cells, which in turn affect cone cells. Even the situation with rhodopsin mutations is complex, as demonstrated by the finding of apparent non–cell autonomous cell death. In chimeric animals containing a mixture of wild-type photorecep-Photoreceptor " Retinoid Pumps " tors as well as cells that carry a mutant rhodopsin in Health and Disease transgene, the wild-type photoreceptors die and they die just as fast as the transgenic cells (Huang et al., 1993). The Banin et al. (1999) paper thus adds complexity to Since its original documentation by Dowling in 1960, the a field that is already complicated. It is in fact one of visual cycle (or retinoid cycle) has been the subject of those important pieces of work that raises more ques-considerable scrutiny. The visual cycle involves the con-tions than it answers. It will cause investigators in the version of vitamin A (known as all-trans-retinol or all-field to rethink many of their treasured paradigms. As a trans-ROL) into 11-cis-retinaldehyde (11-cis-RAL) by the first step, especially due to complications in the extrac-retinal pigment epithelium (RPE), the transport of 11-tion of cellular information from ERG recordings, it will cis-RAL to photoreceptors for the generation of pho-be important to reproduce and extend the findings using topigments, the photoisomerization and reduction of 11-similar as well as complementary approaches. Since it cis-RAL back to all-trans-ROL, and the transport of is becoming increasingly clear that there are significant all-trans-ROL back to the RPE for participation in another differences between vertebrate retinae, it will be impor-cycle. Although initial progress toward understanding all tant to test whether the observed phenomena are unique of the molecular players in the visual cycle was relatively to the pig retina or are common to other mammalian, slow, recombinant DNA technology has enhanced the and perhaps also other vertebrate, retinae. Are species pace considerably. The results have in some cases been in which there is prominent postnatal retinal develop-surprising and rich in relevance to human disease. The ment more likely to demonstrate these changes? Al-latest example is the characterization of an ATP binding though data are presented to suggest similar postsyn-cassette (ABC) transporter that is …