Imaging in cells and bodies
Fluorescence has been used for many years to visualize cellular com-
ponents. Originally, small organic dyes were attached through the use of
antibodies and by the use of fluorophores that recognized organelles.
More recently, fluorescent proteins have provided the opportunity to probe
gene expression, protein trafficking, and responses to signals. Spectroscopy
using fluorescence has expanded rapidly as these tools have become more
commonplace, allowing scientists to target specific sites even at the single-
molecule level. Reporters for many key metabolites have been developed
and the ability to perform genetic tagging provides scientists with many
opportunities to probe cellular processes at a molecular level. One of the
most versatile fluorescent probes is green fluorescent protein, which is
described in the first section of this chapter. In addition to these develop-
ments at a cellular level, imaging techniques at the level of an organism are
also becoming widely used. The second part of this chapter discusses how
techniques such as positron emission tomography are giving healthcare
providers the opportunity to probe cellular activities and identify markers
for brain disorders that continue to be very difficult to treat.
Green fluorescent protein
Green fluorescent protein (GFP) was discovered as a companion protein
to aequorin, a chemiluminescent protein from the jellyfish Aequorea, and
was found to be composed of a single polypeptide with 238 amino acid
residues. GFP emits a green fluorescence when irradiated with UV light.
The proper expression of the chromophore of GFP was found to require
molecular oxygen but to be independent of any enzymes. Biosynthesis
of the chromophore was found to be an exception compared to most
chromophore-containing proteins as the protein was found to catalyze the