Listening for Gene Expression

Take-Away Points

• ■ Major focus: Developing genetically encoded acoustic reporter gene technology for mammalian cells.

• ■ Key result: Optimized a suite of nine bacterial genes to produce nanoscale gas vesicles as ultrasound reporters for gene expression in mammalian cells.

• ■ Impact: Established the first US imaging reporter gene system for human cells, enabling molecular imaging studies in cell-based assays and preclinical models.

While US captures images with superb spatial and temporal resolution in deep tissues, use of this imaging modality for studies of cellular functions remains limited by lack of genetically encoded reporters. Some species of bacteria, such as Bacillus megaterium, produce protein-encapsulated, nanoscale gas vesicles for buoyancy. Gas vesicles isolated from these and similar bacteria produce US contrast in aqueous solutions when injected systemically into mice. Bacteria produce gas vesicles from clusters of eight to 14 different genes that function collectively. While investigators previously transferred genes for gas vesicles to different types of bacteria, no functional gene cluster this large had ever been transferred successfully into mammalian cells. Farhadi and colleagues optimized a suite of nine different bacterial genes that allowed human cells to produce intracellular gas vesicles. Each cell expressed approximately 45 gas vesicles with average dimensions of 64 × 274 nm, occupying less than 0.003% of total intracellular volume. Gas vesicles did not cause toxicity when expressed in human cells and only minimally reduced growth. Turning “on” expression of gas vesicle genes in cells progressively increased US contrast. In mixtures of cells without or with gas vesicles, US showed remarkable sensitivity, detecting the reporter gene expressed in as few as 2.5% of total cells. When implanted into mice, the research team successfully used US to detect expression and spatial localization of human cells with an acoustic reporter gene for the first time. A large design space exists to enhance applications of this technology for molecular and cellular imaging, including decreasing the overall size of genes needed to produce gas vesicles and reducing time needed to detect changes in gene expression. Despite these limitations, this research represents a pioneering advance that opens new horizons for US as a technology for imaging gene expression and other cellular functions.

Highlighted Article

• Farhadi A, Ho GH, Sawyer DP, Bourdeau RW, Shapiro MG. Ultrasound imaging of gene expression in mammalian cells. Science 2019;365: 1469–1475. doi: 10.1126/science.aax4804.