Acoustic Reporter Genes for Non-Invasive Deep Tissue Imaging of Engineered Cells

Genetically encoded fluorescent proteins are used in virtually every study in basic and synthetic biology to observe specific cellular and molecular processes. However, the poor penetration of visible light into opaque tissues limits the utility of optical imaging in living animals. Conversely, non-invasive imaging modalities such as ultrasound are able to image deep tissues but lack genetically encoded molecular reporters. If such reporters could be developed, they would transform our ability to study the function of natural and engineered cells and genetic circuits in vivo. To address this need, we recently discovered that a unique class of gas-filled protein nanostructures, called gas vesicles (GVs), are able to serve as biomolecular reporters for ultrasound¹. Naturally expressed in buoyant waterborne microbes, GVs comprise all-protein shells with sizes on the order of 250 nm that are filled with gas. To use these biomolecules as genetically encoded reporters, it is necessary to transfer the 8-14 gene operons encoding them from their native hosts into heterologous cells. Here we describe how we have used synthetic biology techniques to achieve this goal in bacteria and mammalian cells. First, we describe a 6-kilobase gene cluster that we engineered for functional GV expression in E. coli and Salmonella, allowing the use of this “acoustic reporter gene” for non-invasive ultrasound imaging of microbes in mammalian hosts². We then describe our progress in developing GVs as acoustic reporter genes for mammalian cells. To enable this challenging transfer of a large operon from prokaryotes to eukaryotes, we have used a combination of viral elements and engineered promoters to design a minimized mammalian gene cluster with appropriate inter-gene stoichiometry for GV expression. We anticipate that the resulting acoustic reporter genes will enable previously impossible approaches to monitoring the location, viability and function of a large variety of engineered cells in vivo with applications in basic biology, cellular diagnostics and therapeutics.

1. Shapiro MG, Goodwill PW, Neogy A, Yin M, Foster FS, Schaffer DV, et al. Biogenic gas nanostructures as ultrasonic molecular reporters. Nat Nanotechnol 2014, 9(4): 311-316.

2. Bourdeau RW, Lee-Gosselin A, Lakshmanan A, Kumar SR, Farhadi A, Shapiro MG. Acoustic reporter genes for non-invasive imaging of microbes in mammalian hosts. In revision.