Transferrin 1 is not essential for iron transport in Drosophila melanogaster

Transferrin 1 (Tsf1) is an insect hemolymph protein with a high affinity for iron.  It is homologous to mammalian serum transferrin, which transports iron into cells via receptor-mediated endocytosis; however, whether Tsf1 plays a significant role in iron transport is unknown, and no Tsf1 receptor has been identified. The goal of this study was to determine whether Tsf1 is essential for iron transport in Drosophila melanogaster. To answer this question, we took two approaches. First, we performed phenotypic analyses to determine whether insects lacking Tsf1 have differences in iron content or iron distribution. Second, we used immunohistochemistry to evaluate whether Tsf1 is present in the endosomes of fat body cells and other tissues. We found that insects that lack Tsf1 are viable and fertile, and they have no obvious changes in iron content or iron distribution. These results indicate that Tsf1 is not essential for iron transport. In addition, we failed to detect Tsf1 in endosomes in most tissues, suggesting that Tsf1 does not deliver iron to most cells through endocytic uptake. One exception was oocytes. Our data suggest that Tsf1 from hemolymph is transported into developing oocytes via endocytosis and stored in yolk granules. We predicted that Tsf1 may provide oocytes with iron; however, newly laid eggs from Tsf1 null mothers were not iron deficient. Tsf1 may function in immune-related iron sequestration in eggs rather than in iron provisioning. This study suggests that iron transport in insects is likely to be mediated primarily by non-transferrin based mechanisms.