Recoverin negative photoreceptor cells

Recoverin, a calcium-binding protein, is unique with respect to its cellular regulation. It is present in retinal rods, cones, cone bipolar cells, and in a rare population of cells in the ganglion cell layer. Inappropriate turn-on or turn-off of recoverin expression has been reported both in small cell lung carcinoma cells from patients with cancer-associated retinopathy (Matsusara et al. [1996] Br. J. Cancer 74:1419-1422; Adamus et al. [1998] J. Autoimmun. 11: 523-533; Ohguro et al. [1999] Invest. Ophthalmol. Vis. Sci. 40:82-89) and in cultured retinal neurons (McGinnis et al. [1999] J. Neursci. Res. 55:252-260). In a recent report using double labeling immunofluorescence microscopy methods with antibodies against either rhodopsin and recoverin or arrestin and recoverin, two unique cell phenotypes, rhodopsin-positive and recoverin-negative, and arrestin-positive, and recoverin-negative were observed in vitro. These two unique cell types could be nonphotoreceptor cells in which rhodopsin and arrestin are inappropriately turned on or they are photoreceptor cells in which the recoverin gene is inappropriately turned off. In this study, multiple antibodies were used to study, on a single-cell basis, whether the photoreceptor cell-specific marker, rhodopsin, is inappropriately expressed in nonphotoreceptor cells in our retinal neuronal culture system. We also examined the hypothesis that the two unique cell phenotypes represent the same population of cells. A triple labeling method has been established to visualize recoverin, rhodopsin, and arrestin protein expression simultaneously in cultured retinal neurons. Our data clearly and directly demonstrate that the previously described unique cell phenotypes are the same population of cells, rod photoreceptors. The existence of recoverin-negative photoreceptors demonstrates that the recoverin gene can be regulated independently of other photoreceptor cell-specific proteins and suggests that this primary cell culture may be useful as a model system for investigating the illicit expression of the recoverin gene in cancer associated retinopathy.