Cell surface targeting accounts for the difference in iodide uptake activity between human Na+/I- symporter and rat Na+/I- symporter

Context: The Na (+)/I- symporter (NIS) has been proposed to serve as an imaging reporter gene to optimize vector delivery, monitor therapeutic gene expression, and map the tissue/organ sites of repopulated progenitor cells in vivo. In addition, NIS can serve as a therapeutic gene to facilitate targeted radionuclide therapy for various cancers. Objective: It was reported that rat NIS (rNIS) confers higher radioactive iodide uptake (RAIU) activity than human NIS (hNIS). We aim to investigate the mechanism underlying this difference. Results: We showed that the open reading frames (ORF) of hNIS and rNIS, although encoding for proteins with 83% amino acid identity, exhibit a significant difference in RAIU activity in transfected cells. The ORF rNIS confers four to five times higher RAIU activity as well as cell surface NIS accumulation than ORF hNIS despite similar total NIS protein levels. Multiple regions appear to play roles in the difference in NIS cell surface levels between ORF hNIS and ORF rNIS, indicating that proper folding of NIS in tertiary structure is critical for NIS cell surface targeting. We also showed that the kinetics of Na+ binding are different between ORF hNIS and ORF rNIS, and that site-directed mutation changing Ser200 to other uncharged amino acid significantly increased RAIU activity in ORF hNIS. Conclusions: NIS transgene could be optimized for cell surface trafficking and RAIU activity to improve its clinical applications.