Genetically encoded nanostructures for non-invasive imaging of biological systems

Abstract

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 limited. Technologies such as fluorescent and luminescent proteins enable exquisitely precise imaging of genetically specific cellular function in small and translucent specimens, but are limited by the poor penetration of 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, I will describe molecular reporters for non-invasive imaging using MRI and ultrasound developed based on a unique class of genetically encoded gas-filled nanostructures called gas vesicles (GVs). These nanostructures evolved in photosynthetic microbes as a means to regulate buoyancy, and comprise a thin self-assembled protein shell enclosing a hollow interior with dimensions of approximately 250 nm. We have shown that the unique properties of GVs enable them to serve as sensitive molecular reporters in ultrasound, hyperpolarized ^(129)Xe MRI and susceptibility weighted MRI, representing the first genetically encodable reporters for each of these modalities. Furthermore, by engineering GVs at the genetic level, we can modify their contrast properties, mechanics and surface functionalization to enable new modes of imaging. In addition, by adapting GV-encoding gene clusters for expression in heterologous hosts, we are now able to use GVs as reporter genes to image engineered cells in vivo.