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March 17, 2000
Bitter Taste Receptors Identified
Howard Hughes Medical Institute (HHMI) researchers at the University
of California, San Diego and their colleagues at the National Institute
of Dental and Craniofacial Research (NIDCR) have identified a new
family of genes that encode proteins that function as bitter taste
receptors.
The research, which is reported in two articles in the March 17,
2000, issue of the journal Cell, provides important insight into
the organization of the taste system.
“Bitterness clearly evolved with the sole purpose of warning you against the ingestion of toxic substances.”
Charles S. Zuker
"We've been trying for the past four years to understand how the
taste system works, focusing primarily on sweet and bitter signaling,"
says Charles
Zuker, an HHMI investigator at the University of California, San
Diego. Now, the researchers have conducted a series of experiments that
they say demonstrates conclusively that a single family of genes indeed
contains human and rodent taste receptors.
"We now have the means to really start to investigate how taste
works, not just in the tongue, but also in the brain," says Nicholas
Ryba of NIDCR.
The research group includes Zuker and colleagues Ken Mueller,
Jayaram Chandrashekar and Wei Guo of the University of California, San
Diego; Elliot Adler, Mark Hoon and Ryba of the NIDCR; and Luxin Feng of
Aurora Biosciences in La Jolla, Calif.
In 1999, a team led by Zuker and Ryba reported the discovery of two
genes, T1R1 and T1R2, which had
most of the characteristics expected of taste receptor genes. The genes
resembled other known sensory receptor genes and were expressed in the
appropriate places inside taste receptor cells on the tongue and
palate. But Zuker and Ryba hypothesized that two receptors seemed far
too few to handle the huge number of chemicals that produce sweet and
bitter substances. What's more, T1R1 and
T1R2 generally were not found in the same places as
gustducin, a coupling protein critical in sending the bitter signal
from taste buds to the brain.
"That meant that we were missing the family of taste receptors that
coupled with gustducin," say study co-authors Mueller and Adler, "so we
set out to identify those receptors." In their latest studies, the
researchers focused on a specific interval of DNA on one human
chromosome that was known to be associated with the ability to taste
the bitter compound PROP (6-n-propylthiouracil). They identified a
likely looking receptor sequence in that stretch of DNA, and showed
that it belonged to a family of some 80 genes, which they dubbed
T2Rs. Like T1R1 and T1R2,
the T2R genes were selectively expressed in taste receptor
cells, but there was even better news.
"If you look at the expression of this new family, you find that
every cell that expresses one of these receptors is a
gustducin-expressing cell," says Zuker.
Next, the researchers screened libraries of mouse genes in a search
for the mouse versions of the new gene family. Mice are useful in
studying taste because strains have been bred with the inborn ability
to taste or not taste certain bitter substances. Studies of these mice
have pinpointed a cluster of gene positions on mouse chromosome six
that are associated with the tasting of a number of bitter substances.
When Zuker's and Ryba's group mapped the mouse versions of their new
gene family to mouse chromosomes, "Bingo, a whole set of them sat right
on top of that bitter cluster!" says Zuker.
Everything so far hinted that T2Rs were bitter taste
receptors, but the researchers still did not have definitive proof. "To
get that, we needed to show that when we put in a bitter compound, the
compound binds to the receptor, and that triggers activity in the
receptor cells," Zuker explains. That's difficult to do in a living
system, so the researchers engineered laboratory-cultured cells to
"report" activity when properly triggered.
"We were able to show that three of the receptors - two mouse and
one human - specifically signaled in response to bitter taste," says
Chandrashekar. One of the mouse receptors responded to cyclohexamide, a
bitter compound for which there are mouse "taster" and "nontaster"
strains. "It turns out that the receptor gene from the nontasters
differed from that in the tasters," representing two alternate forms,
or alleles, of the gene, says Zuker. When the researchers compared
engineered cells containing the nontaster allele to those containing
the taster allele, "we saw a corresponding shift in their sensitivity
to cyclohexamide."
The new work helps explain, on a molecular level, the "logic" behind
the taste system and how it differs from the olfactory system. The
olfactory system is designed to recognize a wide range of odors and to
discriminate one odor from another — an essential ability if one
is to avoid such inappropriate responses as mistaking a mate for a
snack. The organization of the olfactory system reflects this need,
with each olfactory neuron expressing only one of the 1,000 or so
olfactory receptor genes.
Taste is a different matter, especially where bitter compounds are
concerned. Virtually every naturally occurring toxin tastes bitter, "so
bitterness clearly evolved with the sole purpose of warning you against
the ingestion of toxic substances," says Zuker. The important thing is
to recognize and reject anything bitter, not to get hung up on
distinctions among different compounds. Indeed, experimental evidence
indicates that humans are unable to discriminate one bitter substance
from another.
"This imposes an interesting contrast with the olfactory system, and
we now have found the logic behind it," says Hoon. Every cell that
expresses genes in the T2R family expresses nearly all the genes
in that family. "So rather than having one receptor per cell, like
olfaction, you have many. This dramatically increases the repertoire of
bitter things you can taste, but since the receptors are all in the
same cell and the cell simply fires when activated, you do not
discriminate."
Zuker is satisfied that the T2R family of genes represents at
least a subset of bitter taste receptors, but there's more work to be
done: tracing pathways from receptor cells to the brain, generating
"knockout" mice that lack T2Rs and studying their taste
deficits, and searching for more gustducin-linked receptors.
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