Developmentally Regulated Secreted Factors Control Expression of Muscarinic Receptor Subtypes in Embryonic Chick Retina

Abstract: Two molecular mass subtypes of muscarinic receptor are expressed by the chick retina (72 and 86 kDa). During development, the ratio of subtypes changes, with the 72‐kDa form becoming predominant. We have found that subtype switch can occur in retina cell culture, and have investigated factors that influence this in vitro increase in the 72‐kDa receptor. Increases similar to those in vivo occurred when cells were cultured at 10⁵ cells/cm², but not at 10‐fold lower density. High‐density cultures, maintained on coverslips, showed no receptor development when transferred to large volumes of fresh medium, indicating that cell‐cell contact alone was not responsible for induction. However, replacement of fresh medium with conditioned medium (from high‐density cultures) resulted in normal induction. There were no morphological differences between cultures with high and low levels of the 72‐kDa receptor. Conditioned medium also induced 72‐kDa receptors in low‐density cultures, consistent with a minimal role for cell‐cell contact. Efficacy of conditioned medium was markedly dependent on age. Media from cells cultured 1–4 days had no effect, but media from cells cultured 5–8 and 1–8 days elicited 1.6‐fold and fourfold increases in the 72‐kDa subtype, respectively. The data indicate that maturing retina cells secrete developmentally regulated factors that are necessary for abundant expression of the 72‐kDa muscarinic receptor subtype.