Genetically Encoded Gas Nanostructures as Biophyically Tunable Molecular Reporters for MRI and Ultrasound

Abstract

The study of cellular and molecular processes occurring deep inside living organisms requires new technologies for non-invasive molecular imaging. In particular, there is a need for “magnetic” and “acoustic” analogs of the green fluorescent protein (GFP) that can be used to sensitively observe gene expression using magnetic resonance imaging (MRI) and ultrasound. We are developing genetic reporters for both of these modalities based on the unique biophysical properties of genetically encoded gas nanostructures known as gas vesicles (GVs). Expressed by aquatic microorganism as a means to control buoyancy, GVs are hollow protein-shelled nano-compartments that exclude water but are permeable to gas. We have adapted GVs as the first genetically encoded reporters for hyperpolarized MRI - a form of imaging in which nuclei such as the biocompatible noble gas xenon are introduced into tissues in a non-equilibrium state with 10^4 - 10^5 stronger polarization compared to conventional 1H-MRI. Xenon partitioning into GVs enables their detection using chemical exchange saturation transfer at sub-nanomolar reporter concentrations. In parallel, we have shown that GVs can be detected with high frequency ultrasound, their physical properties enabling linear, harmonic and collapse-mode imaging in vitro and in vivo. Furthermore, inter-species differences in the genetically encoded biophysical properties of gas vesicles enable multiplexed imaging with both MRI and ultrasound and provide clues for genetic-level biophysical tuning of these unique nanostructures.