San Diego, CA (2/18/99)- Mammals depend on their sense of test for
their very survival. Researchers now report the first identification of the
genes underlying the molecular basis of taste, a finding with far-reaching
implication in many areas of research.
A
team of researchers have identified genes that encode two novel proteins expressed
in cells specifically geared to the sense of taste. The proteins are members
of a new group of "G protein" receptors. They are expressed in taste buds
of the tongue and palate.
Graphic: Unlocking
the Mysteries of Taste (click for large version).
"The identity of the receptor molecules for the different sensory modalities,
like vision, olfaction and taste, represents the Holy Grail of the sensory
field. These receptor molecules provide the unique specificity and selectivity
of each sensory system. The color receptors in our retinas allow us to see
in color and the olfactory receptors in our nose endow us with great olfactory
discrimination. In the case of taste, they are what make sweet cells respond
to sweet substances, bitter cells to bitter compounds, and so on," notes Charles
Zuker, professor of biology and neurosciences at the University of California
at San Diego.
Utilizing DNA screening techniques, the researchers scanned for candidate
receptors in the taste buds of rats and mice. This led to the isolation of
two novel receptors, TR1 and TR2. The molecules belong to a large family of
receptors called G protein-coupled receptors. The new proteins are beleivd
to be related to the receptors for pheromones, another family of sensory receptors.
Each taste bud contains roughly 50 to 150 taste receptor cells. When they
encounter a piece of food, proteins on the surface of these cells bind to
the food, recognize them and switch the cells "on" by prompting them into
an active state. The cells then transmit information to nerve cells that relay
the data to the taste centers of the cortex through synapses in the brain
stem and thalamus.
"The identification of candidate mammalian taste receptors makes it possible
to understand how the different taste cells differ from each other (for example
what makes a sweet cell a sweet cell, etc.) and how taste information is encoded
so that the brain can interpret and respond to the presentation of taste stimuli
on our tongue. We may be able to mark the different cells and use the marks
as a map of the pathway to the brain. Pharmacologically it could be used to
identify-using biochemical and biological assays-high potency for agonists
and antagonists of taste function."said Zuker.
This research provides the basis for future efforts at manipulating the perception
of taste and devising methods to stimulate or block taste cell function. The
identification of these genes should also lead to a better understanding of
how the sense of taste functions neurologically. The research could also lead
to ways to manipulate taste receptors, possibly eliminating bitter and sour
tastes from such products as children's medicine.
"These two molecules have the hallmarks we expect of taste receptors. They
may be the key to unlocking our understanding of how we detect taste, which
is unclear at the moment. We must now demonstrate that functionally they can
do the job," said co-investigator Nicholas Ryba of the National Institute
of Dental and Craniofacial Research.
The research appears in the February 19, 1999 issue of the scientific journal
Cell.
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