Multiscale photoacoustic tomography using reversibly switchable thermochromics.

Significance: Based on acoustic detection of optical absorption, photoacoustic tomography (PAT) allows functional and molecular imaging beyond the optical diffusion limit with high spatial resolution. However, multispectral functional and molecular PAT is often limited by decreased spectroscopic accuracy and reduced detection sensitivity in deep tissues, mainly due to wavelength-dependent optical attenuation and inaccurate acoustic inversion. Aim: Previous work has demonstrated that reversible color-shifting can drastically improve the detection sensitivity of PAT by suppressing nonswitching background signals. We aim to develop a new color switching-based PAT method using reversibly switchable thermochromics (ReST). Approach: We developed a family of ReST with excellent water dispersion, biostability, and temperature-controlled color changes by surface modification of commercial thermochromic microcapsules with the hydrophilic polysaccharide alginate. Results: The optical absorbance of the ReST was switched on and off repeatedly by modulating the surrounding temperature, allowing differential photoacoustic detection that effectively suppressed the nonswitching background signal and substantially improved image contrast and detection sensitivity. We demonstrate reversible thermal-switching imaging of ReST in vitro and in vivo using three PAT modes at different length scales. Conclusions: ReST-enabled PAT is a promising technology for high-sensitivity deep tissue imaging of molecular activity in temperature-related biomedical applications, such as cancer thermotherapy.

Focused Ultrasound Stimulates ER Localized Mechanosensitive PANNEXIN-1 to Mediate Intracellular Calcium Release in Invasive Cancer Cells.

Focused ultrasound (FUS) is a rapidly developing stimulus technology with the potential to uncover novel mechanosensory dependent cellular processes. Since it is non-invasive, it holds great promise for future therapeutic applications in patients used either alone or as a complement to boost existing treatments. For example, FUS stimulation causes invasive but not non-invasive cancer cell lines to exhibit marked activation of calcium signaling pathways. Here, we identify the membrane channel PANNEXIN1 (PANX1) as a mediator for activation of calcium signaling in invasive cancer cells. Knockdown of PANX1 decreases calcium signaling in invasive cells, while PANX1 overexpression enhances calcium elevations in non-invasive cancer cells. We demonstrate that FUS may directly stimulate mechanosensory PANX1 localized in endoplasmic reticulum to evoke calcium release from internal stores. This process does not depend on mechanosensory stimulus transduction through an intact cytoskeleton and does not depend on plasma membrane localized PANX1. Plasma membrane localized PANX1, however, plays a different role in mediating the spread of intercellular calcium waves via ATP release. Additionally, we show that FUS stimulation evokes cytokine/chemokine release from invasive cancer cells, suggesting that FUS could be an important new adjuvant treatment to improve cancer immunotherapy.