Chimeric ferritin as a reporter for MRI

Introduction The iron storage protein ferritin is a heteropolymer composed of H and L subunits (1). The H subunit contains a ferroxidase site, whereas the L chain provides the nucleation centers for deposition of the ferrihydrite core (1). The subunit ratio H:L varies across different tissues and cell types (2). Recent reports used the paramagnetic ferritin core as a probeless MRI reporter (3, 4); however ferritin detection depends on the amount of stored iron and often has relatively low sensitivity. The iron loading capacity of ferritin increases with the fraction of H-chains in the protein shell and reaches a saturation point ~40% (5). Pathologic consequences of long-term ferritin H-chain overexpression have been reported in aging mice (6). Conversely, protection from oxidative stress has been shown in mice that conditionally overexpress ferritin H-chain, because of the iron-chelating properties of ferritin (7). These data underscore the importance of controlling the H and L subunit ratio for sensitive and nontoxic use of ferritin as an MR reporter. In this study, we fixed the ferritin subunit stoichiometry by engineering chimeric molecules H*L and L*H by fusing the H and L subunits. We report chimeric shell size distribution, iron loading and NMR relaxation rates of these constructs expressed in human cell lines and in mouse brain. We show that one specific design, L*H, exhibits significantly stronger R2 effects than H*L and wild-type ferritin.