Developmental patterns of protein expression in photoreceptors implicate distinct environmental versus cell-intrinsic mechanisms

The present study has examined the spatial and temporal expression patterns of various proteins associated with the structure and function of mature p hotoreceptor outer segments in the developing ferret's retina using immunoc ytochemistry and RT-PCR. One set of proteins, including rod opsin, arrestin , and recoverin, was detected progressively in photoreceptors as they becam e postmitotic, bring expressed well before the differentiation of outer seg ments. A second set of proteins, including beta- and gamma -transducin, cGM P-phosphodiesterase, phosducin, rhodopsin kinase, rod cGMP-gated cation cha nnel protein, and peripherin, displayed a contrasting temporal onset and pa ttern of spatial emergence. These latter proteins first became detectable e ither shortly before or coincident with outer segment formation, and were e xpressed simultaneously in both older and younger photoreceptor cells. A th ird set, the short wavelength-sensitive (SWS) and medium wavelength-sensiti ve (MWS) cone opsin proteins, was the last to be detected, but materialized in a spatio-temporal pattern reminiscent of the neurogenetic gradient of t he cones. These different spatial and temporal patterns indicate that cellu lar maturation must play a primary role in regulating the onset of expressi on of some of these proteins, while extrinsic signals must act to coordinat e the expression of other proteins across photoreceptors of different ages.