No abstract

Imaging transgene expression in live tissues requires reporters that are detectable with deeply penetrant modalities, such as magnetic resonance imaging (MRI). Here, we show that LSAqp1, a water channel engineered from aquaporin-1, can be used to create background-free, drug-gated, and multiplex images of gene expression using MRI. LSAqp1 is a fusion protein composed of aquaporin-1 and a degradation tag that is sensitive to a cell-permeable ligand, which allows for dynamic small molecule modulation of MRI signals. LSAqp1 improves specificity for imaging gene expression by allowing reporter signals to be conditionally activated and distinguished from the tissue background by difference imaging. In addition, by engineering destabilized aquaporin-1 variants with different ligand requirements, it is possible to image distinct cell types simultaneously. Finally, we expressed LSAqp1 in a tumor model and showed successful in vivo imaging of gene expression without background activity. LSAqp1 provides a conceptually unique approach to accurately measure gene expression in living organisms by combining the physics of water diffusion and biotechnology tools to control protein stability.

Detecting bacterial cells with high specificity in deep tissues is challenging. Optical probes provide specificity, but are limited by the scattering and absorption of light in biological tissues. Conversely, magnetic...

The ability to noninvasively detect bacteria at any depth inside opaque tissues has important applications ranging from infection diagnostics to tracking therapeutic microbes in their mammalian host. Current examples of probes for detecting bacteria with strain-type specificity are largely based on optical dyes, which cannot be used to examine bacteria in deep tissues due to the physical limitation of light scattering. Here, we describe a new biomolecular probe for visualizing bacteria in a cell-type specific fashion using magnetic resonance imaging (MRI). The probe is based on a peptide that selectively binds manganese and is attached in high numbers to the capsid of filamentous phage. By genetically engineering phage particles to display this peptide, we are able to bring manganese ions to specific bacterial cells targeted by the phage, thereby producing MRI contrast. We show that this approach allows MRI-based detection of targeted E. coli strains while discriminating against non-target bacteria as well as mammalian cells. By engineering the phage coat to display a protein that targets cell surface receptors in V. cholerae, we further show that this approach can be applied to image other bacterial targets with MRI. Finally, as a preliminary example of in vivo applicability, we demonstrate MR imaging of phage-labeled V. cholerae cells implanted subcutaneously in mice. The nanomaterial developed here thus represents a path towards noninvasive detection and tracking of bacteria by combining the programmability of phage architecture with the ability to produce three-dimensional images of biological structures at any arbitrary depth with MRI.