Whole-Cell Photoacoustic Sensor Based on Pigment Relocalization

Photoacoustic (optoacoustic) imaging can extract molecular information with deeper tissue penetration than possible by fluorescence microscopy techniques. However, there is currently still a lack of robust genetically controlled contrast agents and molecular sensors that can dynamically detect biological analytes of interest with photoacoustics. In a biomimetic approach, we took inspiration from cuttlefish who can change their color by relocalizing pigment-filled organelles in so-called chromatophore cells under neurohumoral control. Analogously, we tested the use of melanophore cells from Xenopus laevis, containing compartments (melanosomes) filled with strongly absorbing melanin, as whole-cell sensors for optoacoustic imaging. Our results show that pigment relocalization in these cells, which is dependent on binding of a ligand of interest to a specific G protein-coupled receptor (GPCR), can be monitored in vitro and in vivo using photoacoustic mesoscopy. In addition to changes in the photoacoustic signal amplitudes, we could furthermore detect the melanosome aggregation process by a change in the frequency content of the photoacoustic signals. Using bioinspired engineering, we thus introduce a photoacoustic pigment relocalization sensor (PaPiReS) for molecular photoacoustic imaging of GPCR-mediated signaling molecules

Calcium Sensor for Photoacoustic Imaging

We introduce a selective and cell-permeable calcium sensor for photo­acoustics (CaSPA), a versatile imaging technique that allows for fast volumetric mapping of photo­absorbing molecules with deep tissue penetration. To optimize for Ca²⁺-dependent photo­acoustic signal changes, we synthesized a selective metallo­chromic sensor with high extinction coefficient, low quantum yield, and high photo­bleaching resistance. Micromolar concentrations of Ca²⁺ lead to a robust blue­shift of the absorbance of CaSPA, which translated into an accompanying decrease of the peak photo­acoustic signal. The acetoxy­methyl esterified sensor variant was readily taken up by cells without toxic effects and thus allowed us for the first time to perform live imaging of Ca²⁺ fluxes in genetically unmodified cells and heart organoids as well as in zebrafish larval brain via combined fluorescence and photo­acoustic imaging