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How Sweet It Is...
NATIONAL INSTITUTES OF HEALTH
National Institute on Deafness and Other Communication Disorders
Scientists in the intramural laboratories of the National Institute on Deafness and Other Communication Disorders (NIDCD) report in Journal of Neurochemistry April 27, 2001, the identification of a novel candidate receptor gene for taste. The gene, T1R3, maps to the distal region of mouse chromosome 4 within the genetically defined interval for the Sac (saccharin preferring) locus. The team, led by Dr. Susan F. Sullivan in the Laboratory of Molecular Biology of NIDCD, demonstrated that T1R3 is selectively expressed in taste receptor cells and also demonstrated the existence of several polymorphisms between the T1R3 genes of sweet-tasting and non-tasting strains of mice. Six of these are predicted to result in amino acid substitutions. This research is one of several studies released this month related to sweet receptors, marking a major breakthrough in understanding mammalian gustatory information processing. According to Sullivan, two other laboratories, one led by Robert F. Margolskee at Mt. Sinai School of Medicine and the other by Linda B. Buck at Harvard Medical School, announced similar findings. Margolskee and Buck are grantees of NIDCD and are also Howard Hughes Medical Institute investigators.
Gustatory or taste receptor cells respond to chemicals or "tastants" present in food and beverages. Taste receptor cells are clustered within taste buds that are widely distributed on the tongue and palate. Each taste receptor cell extends an apical process to the surface of the tongue where it comes into contact with tastants. The cell is subsequently activated and ultimately relays information about the tastant to the brain.
The human gustatory system is able to discriminate between at least five different taste qualities: sweet, sour, bitter, salty and umami (the taste elicited by glutamate). From recent research, molecules perceived as sweet are thought to activate G-protein-coupled receptors. Scientists are keenly interested in the molecular identification of sweet receptors and how the brain interprets information detected by these receptors. Susan F. Sullivan, Ph.D. and James F. Battey, Jr., M.D., Ph.D. noted, "Finding the taste receptors and identifying their selective capabilities and the cellular and spatial distributions will lead to understanding how mammals detect and process gustatory information both in the periphery and within the brain."
The team identified a novel candidate taste receptor gene, T1R3, which maps to the distal region of mouse chromosome 4 within the genetically defined interval for the Sac locus. Its homology to other chemosensory receptors, chromosomal mapping position, and allelic variation between sweet taster and non-taster strains of mice makes it a compelling candidate gene for the Sac locus. This would mean that its putative ligands would include sucrose and saccharin.
Deficits in chemosensation could have significant clinical applications and have been the focus of much scientific interest in the past several years. A person with faulty chemosensory ability is deprived of an early warning system that most take for granted. Taste and smell alert individuals to spoiled food and beverages. Taste losses can also lead to reduced quality of life and depression. Abnormalities in smell and taste functions frequently accompany and even signal the existence of several diseases or unhealthy conditions, including obesity, diabetes, hypertension, and malnutrition especially in older populations. Recently, chemosensory changes have given early signals of some degenerative diseases of the nervous system.
The National Institute on Deafness and Other Communication Disorders, one of the institutes of the National Institutes of Health, supports and conducts research and research training on the normal and disordered processes of hearing, balance, smell, taste, voice, speech and language. NIDCD also develops and disseminates health information, based upon scientific discovery, to the public.