Aedes aegypti ferritin: A cytotoxic protector against iron and oxidative challenge?

Diseases transmitted by hematophagous (blood-feeding) insects are responsible for millions of human deaths worldwide. In hematophagous insects, the blood meal is important for regulating egg maturation. Although a high concentration of iron is toxic for most organisms, hematophagous insects seem unaffected by the iron load in a blood meal. One means by which hematophagous insects handle this iron load is, perhaps, by the expression of iron-binding proteins, specifically the iron storage protein ferritin. In vertebrates, ferritin is an oligomer composed of two types of subunits called heavy and light chains, and is part of the constitutive antioxidant response. Previously, we found that the insect midgut, a main site of iron load, is also a primary site of ferritin expression and that, in the yellow fever mosquito, Aedes aegypti, the expression of the ferritin heavy-chain homologue (HCH) is induced following blood feeding. We now show that the expression of the Aedes ferritin light-chain homologue (LCH) is also induced with blood-feeding, and that the genes of the LCH and HCH are tightly clustered. mRNA levels for both LCH- and HCH-genes increase with iron, H₂O₂ and heroin treatment, and the temporal expression of the genes is very similar. These results confirm that ferritin could serve as the cytotoxic protector in mosquitoes against the oxidative challenge of the blood-meal. Finally, although the Aedes LCH has no iron responsive element (IRE) at its 5′-untranslated region (UTR), the 5′-UTR contains several introns that are alternatively spliced, and this alternative splicing event is different from any ferritin message seen to date.