Ultrabright and Fluorogenic Probes for Multicolor Imaging and Tracking of Lipid Droplets in Cells and Tissues

Lipid
droplets (LDs) are intracellular lipid-rich organelles that
regulate the storage of neutral lipids and were recently found to
be involved in many physiological processes, metabolic disorders,
and diseases including obesity, diabetes, and cancers. Herein we present
a family of new fluorogenic merocyanine fluorophores based on an indolenine
moiety and a dioxaborine barbiturate derivative. These so-called StatoMerocyanines
(SMCy) fluoresce from yellow to the near-infrared (NIR) in oil with
an impressive fluorescence enhancement compared to aqueous media.
Additionally, SMCy display remarkably high molar extinction coefficients
(up to 390 000 M–1 cm–1) and high quantum yield values (up to 100%). All the members of
this new family specifically stain the LDs in live cells with very
low background noise. Unlike Nile Red, a well-known lipid droplet
marker, SMCy dyes possess narrow absorption and emission bands in
the visible, thus allowing multicolor imaging. SMCy proved to be compatible
with fixation and led to high-quality 3D images of lipid droplets
in cells and tissues. Their high brightness allowed efficient tissue
imaging of adipocytes and circulating LDs. Moreover their remarkably
high two-photon absorption cross-section, especially SMCy5.5 (up to
13 300 GM), as well as their capacity to efficiently fluoresce
in the NIR region led to two-photon multicolor tissue imaging (liver).
Taking advantage of the available color palette, lipid droplet exchange
between cells was tracked and imaged, thus demonstrating intercellular
communication.