Receptor‐mediated uptake of labeled transferrin by embryonic chicken dorsal root ganglion neurons in culture

Transferrin is a growth‐promoting plasma protein which is known to occur within developing neurons. Since little information exists on the process by which transferrin is internalized by neurons, we studied this process using dissociated embryonic chicken dorsal root ganglion neurons in culture. Cultured dorsal root ganglion neurons were incubated in the presence of 3.75 nM 125 I‐transferrin at 37°C, the cultures were extensively washed, the neurons were solubilized in a Triton‐containing buffer and internalized 125 I‐transferrin was quantified with a gamma counter. 125 I‐transferrin was internalized in a linear fashion for at least 60 min, and this uptake was abolished by the presence of 1.25 μM unlabeled transferrin. No competition for the uptake of 125 I‐transferrin was observed in the presence of 1.25 μM ovalbumin, cytochrome c , hemoglobin, insulin, horseradish peroxidase, aldolase or the carboxyl‐terminal fragment (‘half‐site’) of transferrin. By contrast, uptake was inhibited by approximately 50% in the presence of the amino‐terminal fragment (‘half‐site’) of transferrin (1.25 μM) or in the presence of concanavalin A (1.25 μM). The binding of transferrin conjugated to fluorescein isothiocyanate to neurons at 4°C and its subsequent internalization at 37°C was demonstrated by fluorescence microscopy of unfixed cells following incubation of the neurons in the presence of the fluorescently labeled protein. Furthermore, the transferrin receptors were visualized immunocytochemically on the surface membranes of dorsal root ganglion neurons using rabbit antibodies directed against transferrin receptors from chicken reticulocytes. From these data, we conclude that transferrin is internalized by neurons via receptor‐mediated endocytosis, and suggest that this protein may serve an important role in the development and survival of dorsal root ganglion neurons.