| Cell surface receptors make attractive targets for providing cell-type specific recognition and for facilitating cellular internalization. Among them, transferrin receptor (TfR) is a well-characterized target with implications for research in cancer, brain delivery, viral infections, and basic research into endosomal machinery. Although they have been around for 25+ years, there has been a resurgence of interest in aptamers (nucleic acid ligands) for the targeted delivery of therapeutic and diagnostic cargoes to cells. A major limitation for TfR ligands that are destined for in vivo use is the saturating concentration of the receptor's natural ligand, transferrin (Tf), in serum. Drawing insight from TfR-binding viruses, which have evolved to recognize an epitope of TfR that does not compete with the Tf binding site, we utilized Tf competition to tune an aptamer selection for non-competitive ligands. This selection identified a novel aptamer to human TfR (hTfR), Waz, which does not compete with Tf for binding and is able to localize to human xenograft tumors in vivo. We further demonstrate that Waz competes with pathogenic New World mammarenaviruses (NWM) for binding to the apical domain of hTfR and is able to inhibit wild-type virus infection in human cells. Finally, multimerization of Waz results in dramatically increased affinity, which is proportional to enhanced inhibition of viral infection. In sum, our results demonstrate the validation of ligand competition as a useful tool in aptamer selections, and we present a novel aptamer as a potentially useful tool in future TfR investigations. |
