(7b) Genetically Encoded Gas Nanostructures As Reporters for Non-Invasive Molecular Imaging

Many important biological processes – ranging from simple metabolism to complex cognition – take place deep inside living organisms, yet our ability to study them in this context is very limited. Technologies such as fluorescent proteins and optogenetics enable exquisitely precise imaging and control of cellular function in small, translucent specimens using visible light, but are limited by the poor penetration of such light into larger tissues. In contrast, most non-invasive technologies such as magnetic resonance imaging (MRI) and ultrasound – while based on energy forms that penetrate tissue effectively – lack the needed molecular precision. Our work attempts to bridge this gap by engineering new molecular technologies that connect penetrant energy to specific aspects of cellular function in vivo. 

Here, we descrbe the development of highly sensitive molecular reporters for MRI¹ and ultrasound² based on genetically encoded gas nanostructures from buoyant microorganisms. Gas vesicles (GVs) are gas-filled protein-shelled compartments ~200 nm in size that exclude water but are permeable to gas. We show that GVs produce ultrasound contrast based on the echogenicity of their hollow interiors, enabling them to be sensitively detected in vitro and in vivo. Whereas the physics of conventional microbubble contrast agents dictates sizes larger than one micron and half-lives shorter than a few hours, GVs are nano-sized and inherently stable, enabling a broader range of biological applications. Furthermore, the unique mechanical properties and genetic diversity of GVs enable multiplexed imaging and dynamic biosensing.

In addition, we have adapted GVs as the first genetically encoded reporters for hyperpolarized MRI, a highly sensitive technique that uses non-proton nuclei such as the biocompatible noble gas xenon to achieve orders of magnitude greater molecular sensitivity than conventional ¹H-MRI. We show that the gas permeability of GVs enables them to interact with dissolved hyperpolarized xenon and alter its magnetic properties. As a result, GVs can be detected at picomolar concentrations, a 10,000-fold improvement over comparable reporter genes for conventional MRI. We have shown that GVs can be used to detect tumor cells and act as reporters of gene expression. As with ultrasound, we have also taken advantage of the genetic diversity of GVs to enable multiplexed imaging.

Our findings establish a promising new class of molecular reporters for two non-invasive imaging modalities and demonstrate the biochemical engineering potential of a unique family of naturally evolved nanostructures.

The presented work was conducted by the Shapiro group in collaboration with the Schaffer, Pines and Conolly groups at UC Berkeley.

1. Shapiro MG*, Ramirez ME, Sperling LJ, Sun G, Pines A , Schaffer DV, Bajaj VS*. Genetically encoded reporters for hyperpolarized MRI. In revision.  2. Shapiro MG*, Goodwill PW, Neogy A, Schaffer DV, Conolly SM. Genetically encoded gas nanostructures as ultrasonic molecular reporters. In revision. (*corresponding)