Optical reporters based on bioluminescence and fluorescence have long dominated preclinical biomedical research because of their high accuracy in tracking biological processes. Green fluorescent protein (GFP), for example, can be expressed in cells and is a fantastic “reporter” for cells and cell processes. However, when target cells are deeper than a few mm, scatter and absorption severely limit the usefulness of optical reporters.
We are developing an Acoustic version of GFP that will enable well localized detection of cells expressing acoustic reporter genes (ARGs) deep in living tissue. Further development of ARGs – and specific methods for ultrasound imaging of ARG expression – will enable deeper, higher resolution molecular reporting to advance preclinical biomedical research. This would allow a much more detailed analysis of disease processes and responses to therapy
Gas vesicles (GVs) are a class of bio-derived genetically encoded gas nanostructures. expressed by aquatic microorganisms as a means to control buoyancy as illustrated above. GVs are hollow protein-shelled compartments that exclude water but are permeable to gas. Unlike artificial microbubbles, GVs are not pressurized and allow gases to freely exchange with the surrounding medium. This results in a very stable nanoscale configuration enabling a broader range of molecular imaging applications.
In 2014, working in collaboration with the Shapiro lab at Caltech (https://shapirolab.caltech.edu/), we were the first to demonstrate that GVs from multiple species produce stable ultrasound contrast that is readily detected in vitro, inside cells and in vivo. We subsequently developed standardized protocols for their production, evaluated in vivo bio distribution of GVs pre-clinical models, investigated methods to increase circulation times and researched the unique physics of these amazing nanoparticles.
The fact that GVs are genetically encoded provided an unprecedented opportunity to engineer and translate them to eukaryotic “ARG” cell lines suitable for research models of cancer. The Shapiro lab has subsequently accomplished this, and we are now focussing on the development of microultrasound imaging of acoustic reporter genes to enable more detailed investigations of cancer metastases. Below, images of GV contrast in a tumor model are shown.
Biogenic gas nanostructures as ultrasonic molecular reporters.
Shapiro MG, Goodwill PW, Neogy A, Yin M, Foster FS, Schaffer DV, Connolly SM.
Nature Nanotech9, 311–316 (2014). https://doi.org/10.1038/nnano.2014.32