Abstract
PurposeTo assess the potential of an MRI gene reporter based on the ferritin receptor Timd2 (T‐cell immunoglobulin and mucin domain containing protein 2), using T1‐ and T2‐weighted imaging.MethodsPellets of cells that had been modified to express the Timd2 transgene, and incubated with either iron‐loaded or manganese‐loaded ferritin, were imaged using T1‐ and T2‐weighted MRI. Mice were also implanted subcutaneously with Timd2‐expressing cells and the resulting xenograft tissue imaged following intravenous injection of ferritin using T2‐weighted imaging.ResultsTimd2‐expressing cells, but not control cells, showed a large increase in both R2 and R1 in vitro following incubation with iron‐loaded and manganese‐loaded ferritin, respectively. Expression of Timd2 had no effect on cell viability or proliferation; however, manganese‐loaded ferritin, but not iron‐loaded ferritin, was toxic to Timd2‐expressing cells. Timd2‐expressing xenografts in vivo showed much smaller changes in R2 following injection of iron‐loaded ferritin than the same cells incubated in vitro with iron‐loaded ferritin.ConclusionTimd2 has demonstrated potential as an MRI reporter gene, producing large increases in R2 and R1 with ferritin and manganese‐loaded ferritin respectively in vitro, although more modest changes in R2 in vivo. Manganese‐loaded apoferritin was not used in vivo due to the toxicity observed in vitro. Magn Reson Med, 2015. © 2015 The Authors. Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. Magn Reson Med 75:1697–1707, 2016. © 2015 The Authors. Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance.
Highlights
Previous studies have shown that ferritin is endocytosed by Timd2-expressing cells and processed by means of the endocytic pathway into an acidic perinuclear compartment within 30 min [40,41,51,52]
In cells incubated with a fixed concentration of FITC-ferritin (75 nM) there was an increase in fluorescence between 0 and 40 min of incubation, which remained relatively constant for the 20 min, before decreasing over the 30 min and steadily increasing again thereafter (Fig. 1b)
Previous investigations have described the endocytosis of ferritin as it goes through the endocytic pathway: first to the early endosome [51], which it can reach in as little as 2 min [40], the late endosome, and to the lysosome [57,60], where the iron core is broken down [61,62,63]
Summary
A variety of MR reporter genes have been developed that are detectable in T2- and T1-weighted 1H images [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17], in 19F images and spectra [18,19,20] and in 31P spectra [20,21,22,23,24]. Two of the more promising multimodal approaches use DMT1, which is a transporter for manganese [15], and Oatp, which transports a clinically approved gadolinium-based contrast agent [16]. These produce higher levels of contrast in T1-weighted images than reporters described previously, as well as functioning as radionuclide imaging reporters [16,36] and, in the case of Oatp, enhancing bioluminescence signal from luciferase-expressing cells as well [37]
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