MRI measurements of reporter-mediated increases in transmembrane water exchange enable detection of a gene reporter

Abstract

Non-invasive imaging of gene expression can be used to track implanted cells in vivo but often requires the addition of an exogenous contrast agent that may have limited tissue access1. We show that the urea transporter (UT-B) can be used as a gene reporter, where reporter expression was detected using 1H MRI measurements of UT-B-mediated increases in plasma membrane water exchange. HEK cells transfected with the reporter showed an increased apparent water exchange rate (AXR), which increased in line with UT-B expression. AXR values measured in vivo, in UT-B-expressing HEK cell xenografts, were significantly higher (~ 2-fold, p < 0.0001), compared with non-expressing controls and fluorescence imaging of a red fluorescent protein (mStrawberry), co-expressed with UT-B, showed that UT-B expression correlated in a linear fashion with AXR. Transduction of rat brain cells in situ with a lentiviral vector expressing UT-B resulted in a ~ 2-fold increase in AXR at the site of virus injection.

Proteins, air and water: reporter genes for ultrasound and magnetic resonance imaging

“…3C). Together with another water channel published shortly after this work *[33], aquaporin represents a new category of MRI reporter genes that is sensitive, compact, autologous to its host, nonmetallic and non-toxic.…”

Biomolecular MRI reporters: Evolution of new mechanisms

“…In light of the importance of transmembrane diffusion, it was recently proposed that facilitating water exchange across the membrane by overexpressing water-permeable channels would result in enhanced contrast in DWI or more specialized sequences [56, 57]. The most natural class of molecules for achieving this goal is aquaporins, a highly conserved family of tetrameric integral membrane proteins that mediate the selective exchange of water molecules across the plasma membrane in a wide range of cell types [58–63].…”

“…In addition to aquaporin, a separate study recently reported that the urea transporter (UT-B) could also be used as a reporter gene based on its ability to increase the permeability of cell membranes [57]. In addition to urea, UT-B is able to transport water at rates similar to aquaporin.…”

Molecular Imaging in Synthetic Biology, and Synthetic Biology in Molecular Imaging

“…Pioneering examples include enzymes that alter the relaxivity of gadolinium chelates 39 , human iron storage and transport genes such as ferritin 40, 41 and transferrin 42 , and natural and engineered proteins with large numbers of chemically labile protons for chemical exchange saturation transfer (CEST) imaging 43–48 . Recent developments have also included reporter genes causing accumulation of MRI-detectable compounds 49 , proteins interacting with hyperpolarized molecules 50, 51 , channels that alter the diffusion of water across cell membranes 52, 53 and vasodilators altering hemodynamic signals 54 . Several of these reporter genes are covered in detail in previous review articles 55–57 .…”