Abstract
Non-invasive imaging of gene expression in live, optically opaque animals is important for multiple applications, including monitoring of genetic circuits and tracking of cell-based therapeutics. Magnetic resonance imaging (MRI) could enable such monitoring with high spatiotemporal resolution. However, existing MRI reporter genes based on metalloproteins or chemical exchange probes are limited by their reliance on metals or relatively low sensitivity. Here we introduce a new class of MRI reporters based on the human water channel aquaporin 1. We show that aquaporin overexpression produces contrast in diffusion-weighted MRI by increasing tissue water diffusivity without affecting viability. Low aquaporin levels or mixed populations comprising as few as 10% aquaporin-expressing cells are sufficient to produce MRI contrast. We characterize this new contrast mechanism through experiments and simulations, and demonstrate its utility in vivo by imaging gene expression in tumours. Our results establish an alternative class of sensitive, metal-free reporter genes for non-invasive imaging.
Highlights
Non-invasive imaging of gene expression in live, optically opaque animals is important for multiple applications, including monitoring of genetic circuits and tracking of cell-based therapeutics
aquaporin 1 (AQP1)-dependent contrast is readily observed in cell cultures, including cells known to have higher levels of endogenous aquaporins as well as in vivo tumour xenografts, and AQP1 expression has no adverse effect on cell proliferation and viability
AQP1-dependent contrast can be detected at reasonably low concentrations (B0.5 mM), which makes it a sensitive Magnetic resonance imaging (MRI) reporter gene
Summary
Non-invasive imaging of gene expression in live, optically opaque animals is important for multiple applications, including monitoring of genetic circuits and tracking of cell-based therapeutics. Noting the strong influence of the last factor[46,51,52], we hypothesized that facilitating the transmembrane diffusion of water by overexpressing waterpermeable channels would result in enhanced contrast in DWI Towards this end, aquaporins are 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[53,54,55,56,57,58]. We introduce human aquaporin 1 (AQP1) as a new genetically encoded reporter for diffusionweighted MRI This reporter requires no metals, is non-toxic in several cell lines and in vivo tumours, produces contrast orthogonal to paramagnetic and CEST reporters and is detectable when expressed at low levels and in small subsets of cells. We characterize the imaging performance and mechanisms of AQP1 through live-cell experiments and Monte Carlo models, and demonstrate its utility by imaging tumour gene expression in vivo
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