Imaging β-cell function in vivo

In live animals, non-invasive imaging of β-cell function has previously not been achieved owing to substantial technical challenges. A study published in eLife used a newly developed high-resolution two-photon light-sheet microscope (2P3A-DSLM) for the first in vivo imaging of β-cell function and development in transgenic embryonic zebrafish.

In embryonic zebrafish pancreatic islets, the acquisition of β-cell functionality is coordinated by islet vascularization. hpf, hours post-fertilization.

Insulin secretion is triggered from β-cells by a glucose-induced influx of Ca²⁺, which serves as a functional marker. The new microscopy technique was able to visualize Ca²⁺ flux in response to glucose in every single β-cell present in transparent transgenic embryonic zebrafish, which were engineered to have β-cells labelled with a fluorescent Ca²⁺ indicator.

Interestingly, the researchers observed two waves of β-cell functionality that propagated from the islet mantle to the core during zebrafish development. Visualization of blood vessels indicated that the acquisition of β-cell functionality was coordinated by islet vascularization, which was required for the delivery of optimal glucose concentrations.

Calcineurin–nuclear factor of activated T cells (NFAT) signalling is thought to have a role in β-cell function in mice. Glucose-induced Ca²⁺ influx in β-cells was analysed in zebrafish embryos that were preincubated with calcineurin–NFAT inhibitors or activators. These experiments showed that calcineurin–NFAT signalling acts downstream of glucose to induce β-cell functionality.

Importantly, these findings could be partially replicated in mammals. In ex vivo embryonic mouse islets, direct activation of calcineurin induced secretion of insulin in response to stimulation with a high, but not a low, concentration of glucose.

“The calcineurin–NFAT signalling pathway has never been directly manipulated in the differentiation of stem cells into functionally mature β-cells in vitro,” explain corresponding authors Liangyi Chen and Yanmei Liu. “Only by imaging the spatiotemporal profile of β-cell functional acquisition in vivo, we highlighted that this pathway is a maturation factor that was previously overlooked.”

the researchers observed two waves of β-cell functionality that propagated from the islet mantle to the core

This study presents the first imaging of the functionality of individual β-cells in live animals and reveals important biology. “We plan to use these findings to generate matured β-cells derived from stem cells in vitro, which will be important for β-cell regenerative approaches for diabetes therapy,” conclude Chen and Liu. “In the meantime, we will continue screening new compounds that promote β-cell maturation by using the transgenic zebrafish model developed in this study.”