Genetically encoded acousto-magnetic protein nanostructures for non-invasive imaging of cellular functions

Abstract

Genetically encoded optical reporters such as green fluorescent protein (GFP) have revolutionized biomedical research by enabling observations of specific biol. processes in engineered cells and transgenic animals. However, such optical agents are fundamentally limited by the ~ 1 mm penetration depth of light in opaque tissues. As cellular therapies advance towards rodent models and ultimately humans, this limitation becomes increasingly severe. Therefore, we aim to develop new classes of reporter genes for non-invasive imaging modalities that utilize deeply penetrant forms of energy, such as magnetic fields and sound waves. Here, we describe the first reporter genes for acoustically modulated magnetic resonance imaging (AM-MRI), a modality that combines MRI and ultrasound. These agents are based on gas vesicles (GVs), a class of gas-filled protein nanostructures evolved in photosynthetic microbes as a means to regulate their buoyancy. GVs are hundreds of nanometers in size, and transferring the gene cluster into E. coli enables their formation inside these bacteria. The air inside GVs allows them to be detected at nanomolar concns. by susceptibilitybased MRI. Uniquely, such contrast is "erasable" by ultrasound pulses at specific pressures, which permits selective imaging of these agents without background tissue contrast that has plagued the use of existing MRI contrast agents. Furthermore, gene orthologs encode GVs of different size, shape and ultrasound-responsive pressure, which in turn give rise to differential MRI contrast and "erasable" pressure thresholds. Thus, multiplexed imaging can be achieved by genetically encoding several types of GVs. Finally, the clustering of GVs induces a 10-fold enhancement of T2* contrast, which enables the potential design of sensors to dynamically report biol. signals. The ability of GVs to be genetically encoded and engineered opens the possibility of using this new form of imaging contrast in a wide range of applications, esp. in diagnosis and cellular therapeutics.