Exosome formation during maturation of mammalian and avian reticulocytes: Evidence that exosome release is a major route for externalization of obsolete membrane proteins

Abstract We have assessed whether exosome formation is a significant route for loss of plasma membrane functions during sheep reticulocyte maturation in vitro. Although the recovery of transferrin binding activity in exosomes is at best ∼25–30% of the lost activity, recoveries of over 50% of the lost receptor can be obtained if 125 I‐labelled transferrin receptor is measured using an immunological approach. Degradation products of the transferrin receptor in the medium suggest that receptor instability may contribute to the less than quantitative recovery of the transferrin receptor. Significantly higher (75–80%) levels of the nucleoside transporter can be recovered in exosomes during red cell maturation using 3 H‐nitrobenzylthioinosine binding to measure the nucleoside transporter. These data suggest that exosome formation is a major route for removal of plasma membrane proteins during reticulocyte maturation and plasma membrane remodelling. We have also shown that both in vivo and in vitro, embryonic chicken reticulocytes form exosomes which contain the transferrin receptor. Thus, exosome formation is not restricted to mammalian red cells, but also occurs in red cells, which retain organelles, such as nuclei and mitochondria, into the mature red cell stage.