April 23, 2001
Researchers Discover Human Gene that May Produce Sweet Taste Receptor
Two research groups led by Howard Hughes Medical Institute (HHMI)
investigators have independently identified a human gene that encodes a
likely receptor for sweet compounds. The researchers say finding the
gene, which is expressed by the tongue’s taste cells, opens an
important research pathway that may help answer fundamental questions
such as how the brain perceives sweet taste and why molecules with
dramatically different chemical structures can taste sweet. Discovery
of the candidate sweet taste receptor adds to a repertoire of recently
discovered receptors thought to be involved in the perception of bitter
and umami tastes.
Discovery of the candidate sweet receptor gene, called T1r3,
was reported in articles published in the May 2001 issue of Nature
Neuroscience by HHMI’s Linda
Buck and colleagues at Harvard Medical School; and in the May 2001
issue of Nature Genetics by HHMI’s Robert F. Margolskee
and colleagues at the Mount Sinai School of Medicine at New York
University.
“We find the co-expression of these two genes extremely intriguing. It implies that individual taste cells are probably recognizing at least two tastes.”
Linda B. Buck
Taste receptors are proteins that nestle in the surface of taste bud
cells and bind to specific chemicals. When the appropriate chemical
activates a taste receptor, it launches a cascade of molecular events
that culminates in the brain’s perception of taste
information.
The starting point for both research groups was the Sac locus
in the mouse genome —a region of mouse chromosome 4 known to
govern the preference for sweet tasting substances.
"The Sac locus in mice was known to be the most important
determinant for differentiating ‘taster’ strains of mice
that preferred sweetened solutions from ‘non-taster’
strains that didn’t prefer a sweetened solution over plain
water," said Margolskee. "But we didn’t know whether the
Sac locus was one gene or multiple genes, or a genetic control
element."
Margolskee’s group built on earlier studies by Alexander
Bachmanov and Gary Beauchamp of the Monell Chemical Senses Center that
had narrowed the location of the Sac locus to a small chromosome
region near a known marker. Once Margolskee’s team located the
homologous marker in human DNA, "we put together a contiguous region of
about a million base pairs of DNA around this marker using both
finished human sequence and unfinished high throughput human
sequences," said Margolskee. In fact, since the human genome sequence
in that region had not yet been completed, the scientists had to fit
the pieces together themselves, like a jigsaw puzzle, to create an
organized search region.
Within this contiguous human genome sequence, the researchers
discovered numerous genes. But only one coded for a protein that fit
their working assumption that the Sac gene product should be a
signal transduction component such as a G protein-coupled receptor
(GPCR).
Buck and her colleagues used a different approach in searching for a
GPCR near the Sac locus. "We first determined where the region
corresponding to the mouse Sac locus was in humans and looked
for genes encoding G protein-coupled receptors in that region," said
Buck. "In the finished human genome database, we didn’t find
anything, but in the draft sequence, we found a piece of DNA that would
fit in that region, and which had a gene encoding what appeared to be a
GPCR." Studies by Buck and her colleagues revealed that the gene was
related to similar previously identified genes, called T1r1 and
T1r2 that had been found in taste cells, but whose
taste-reception function was unknown. T1r1 and T1r2 had
been previously identified by HHMI investigator Charles
Zuker at the University of California, San Diego and Nicholas Ryba
at the National Institutes of Health.
Both Buck's and Margolskee’s groups named the potential sweet
receptor gene product T1r3, for "taste receptor family 1, member
3."
"This receptor really shouted out to us as a strong candidate to be
the Sac gene and to be a sweet receptor, because of its
similarity to T1r1 and T1r2," said Margolskee. Also, the
T1r3 protein has a large extracellular loop that juts outside
the cell membrane, as would be required for a receptor that had evolved
to attach to large carbohydrate sugar molecules, said Margolskee.
Importantly, studies of T1r3 expression by both groups confirmed
that the gene was selectively expressed in the membranes of taste cells
and not in other cells in the tongue or elsewhere in the body.
"This protein is physically in the taste cell where it ought to be
if it is the Sac protein," said Margolskee. "And within the taste
cells, it’s in the apical membranes, where a sweet receptor
should be."
Both teams of scientists had predicted that the T1r3 gene
would show sequence differences between sweet-preferring taster mice
and sweet-indifferent non-taster mice. And, indeed, they did find such
differences in gene sequence between the strains that argued for the
gene’s role in sweet tasting. In a molecular model of the
structure of T1r3, Margolskee and colleagues identified a specific
amino acid sequence in the non-taster T1r3 protein that is predicted to
add a new carbohydrate group at a portion of the receptor likely to be
critical for its function.
In a discovery that hinted at how very different molecules might
trigger the sweet taste sensation, Buck and her colleagues found that
the majority of mouse taste bud cells that expressed T1r3 also
expressed T1r2.
"We find the co-expression of these two genes extremely intriguing,"
said Buck. "It implies that individual taste cells are probably
recognizing at least two tastes. They might be functioning
independently as taste receptors; or the receptor proteins might
interact to form various combinations that give the opportunity to
recognize different sweeteners."
Buck and Margolskee—whose laboratories are now exploring the
molecular function of the candidate sweet receptor—emphasize that
such work could have important clinical benefits.
"A large percentage of people in the United States and other western
countries are overweight. And the artificial sweeteners now used in an
attempt to control weight just don’t do a very good job of
mimicking the taste of the natural sweetener," said Margolskee. "Until
now, developing those sweeteners has been a hit-or-miss proposition;
but if we understood the sweet receptor and its binding mechanism, we
could design a sweetener molecule that would fit perfectly and be a
million times more potent than sugar yet have the same sweetness as a
natural sugar sweetener.
"Also, the loss of the sense of taste is a major quality-of-life
problem for the elderly, and one that can contribute to malnutrition,"
said Margolskee. "If we could enhance the activity of taste receptors,
including those for sweet and amino acids, it might help patients with
nutritional problems."
Buck emphasized that identifying the sweet receptor offers an
important pathway for exploring how the brain processes taste
information. "If you have genes that code for receptors that recognize
particular tastes such as sweet versus bitter, you can use those genes
as tools to actually visualize what’s happening inside the brain.
For example, is there a sweet spot in the brain, a bitter spot, or a
sour spot?" Furthermore, researchers would be able to explore whether
the olfactory and taste senses—long known to function in
concert—share neural circuitry.
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