Using a small boron-based molecule, glucagon-a hormone that plays a key role
in pancreatic diseases-can now be imaged in live pancreatic cells. This breakthrough
discovery was made by a team led by Young-Tae Chang from the Singapore
Bioimaging Consortium of A*STAR*.
 | | Fluorescent microscope images of unstained pancreatic alpha cells (top) and the same cells 24 hours after staining with glucagon yellow (bottom) (scale bar, 20 µm). Reproduced from Ref. 1 © 2009 The American Chemical Society
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Produced in alpha cells of the pancreas, glucagon is a hormone peptide that,
in partnership with insulin, maintains glucose levels in the blood. Glucagon
production increases when the concentration of blood glucose decreases; an imbalance
between insulin and glucagon results in diabetic disease.
Glucagon secretion has been studied extensively, but progress has been hampered
by a lack of suitable probes for imaging the hormone in real time inside live
cells and tissues. The current method, using antibodies, can only be used to
take a snap shot of a dead cell. “Our glucagon probe can be used for imaging
live cells, over a long period of time,” says Chang.
In their alternative approach, the team first made a library of small molecules
consisting of a molecular component (4,4-difluoro-4-bora-3a,4a-diaza-s-indacene,
or BODIPY), which fluoresces when the molecule binds to glucagon directly attached
to different chemical entities. The team selected 1,3-dimethyl-BODIPY as their
fluorescent scaffold and reacted it with 238 different aromatic aldehydes in
a microwave. This produced 160 compounds that were pure enough to be screened
against live cells. This is the first report of a systematic construction of
a library based on BODIPY, notes Chang.
The researchers assessed the compounds using an in vitro high-throughput screen
against three different live cell lines-pancreatic alpha cells, pancreatic
beta cells (which produce insulin), and control HeLa cells. From fluorescent
images taken at different time intervals, they found that one of the compounds
exhibited a selective and highly fluorescent response to the glucagon-secreting
alpha cell line. They named this compound glucagon yellow, owing to its yellow
fluorescence signal and selective fluorescence increase upon binding to glucagon
(Fig. 1).
“Next we would like to find more alpha cell dyes, and also beta cell
selective dyes,” Chang says. Through collaborations with positron emission
tomography (PET) and single photon emission computed tomography (SPECT) experts
at the Singapore Bioimaging Consortium, Chang also plans to try imaging the
pancreas in situ using his team’s compounds. “The fluorescent molecule
itself may not be a robust imaging probe in clinical applications,” but
through team work he says “our dye will be converted in PET/SPECT probes,
and will be applied to animal and clinical usage.”
The A*STAR affiliated authors in this highlight are from the Singapore Bioimaging
Consortium
* Lee, J.-S., Kang, N.-Y., Kim, Y.K., Samanta, A., Feng, S., Kim, H.K., Vendrell,
M., Park, J.H. & Chang, Y.-T. Synthesis of a BODIPY library and its application
to the development of live cell glucagon imaging probe. Journal of the American
Chemical Society 131, 10077–10082 (2009).
Posted November 24th, 2009
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