8.1. Bioluminescence imaging
Many tissues contain molecules with strong autofluorescence; therefore,
the use of fluorescence excitation with visible light in such samples is
complicated. Bioluminescence imaging allows background-free image
acquisition since no excitation light is required (Figure 2m).
Bioluminescence is produced by the chemical transformation of a
substrate by the luciferase enzyme. The absence of a background signal
allows for very long exposure times (minutes) to detect even very weak
luminescence intensities. However, this is also a major drawback of
bioluminescence imaging because such long exposure times are often
required for image acquisition, limiting its applicability for the study
of dynamic processes. A particularly promising approach is
bioluminescence complementation, which is based on the reconstruction of
a complete functional luciferase by binding its two split pieces
together (Remy & Michnick, 2006). Bioluminescence complementation has
been used in live mice to detect the interactions of β-arrestin2 with
β2AR (Takakura et al., 2012) and sphingosine-1-phosphate receptor 1
(Kono et al., 2017) and visualise β2AR activation by ligands in breast
cancer cells (Alcobia et al., 2018). Bioluminescence has been used to
report Gs activation in live mice using the cAMP response element
(Dressler et al., 2014). Bioluminescence imaging is also compatible with
high throughput screening reporting GPCR interactions with β-arrestins
in multi-well plates (Hattori & Ozawa, 2015).