Findings may provide insight into biological basis of inner ear function and sense of touch
Researchers at the Howard Hughes Medical Institute and Rockefeller University have identified a molecule in vertebrates that senses osmotic pressure -- the measure of saltiness essential for living cells -- and may provide an inroad into understanding inner ear function and the sense of touch. The findings are reported in the October 27 issue of Cell.
The molecule, called VR-OAC (Vanilloid Receptor-related Osmotically Activated Channel), is an ion channel that responds to changes in osmotic pressure of extracellular fluid. Osmotic pressure is the pressure exerted by salts and proteins dissolved in bodily fluids. When osmotic pressure outside a cell decreases, this leads to an increased tension in a cell's membrane, like the tension in a balloon as it is inflated, a physical stimulus which in turn opens up the channel, allowing ions, among them calcium ions, to pass through, which elicits a cascade of intracellular events. In the case of sensory cells and nerve cells, this is converted into electrical signals to the central nervous system. The scientists cloned VR-OAC from rat, mouse, chicken and human gene libraries. The ion channel was detected in cells from the inner ear, the osmoregulator centers of the brain in the hypothalamus and from cells surrounding whiskers, a rodent's snout hair known to be very sensitive to touch. The presence of VR-OAC in the latter cells suggests that this receptor may also be involved in the sense of touch.
The research was led by Wolfgang Liedtke, M.D., a research associate in the Laboratory of Molecular Genetics headed by Jeffrey M. Friedman, M.D., Ph.D., an investigator at the Howard Hughes Medical Institute, and Stefan Heller, Ph.D., a former postdoctoral researcher in the Laboratory of Sensory Neuroscience at Rockefeller headed by A. James Hudspeth, M.D., Ph.D., also an investigator at the Howard Hughes Medical Institute. Heller is now an assistant professor at Harvard Medical School. Liedtke discovered VR-OAC while looking in a region of the brain called the hypothalamus for genes operative in the regulation of body temperature. The hypothalamus monitors and regulates a score of vital parameters in the body, including osmotic pressure, energy balance and temperature. Of those, systemic osmotic pressure is the most aggressively defended setpoint value in vertebrate animals including humans. An increase in osmotic pressure leads to the sense of thirst and drinking behavior, and osmoreceptors in the hypothalamus send signals to nerve cells that manufacture antidiuretic hormone (ADH), the primary regulator of body water.
Liedtke found VR-OAC expressed in neurons in brain structures called the circumventricular organs. Most of the brain is protected by the blood-brain barrier, a gateway that allows only certain substances to pass into the brain. The circumventricular organs are located inside the brain, but outside the blood-brain barrier. Circumventricular organs are recognized as important sites for communication of the central nervous system with the remainder of the organism through the blood/serum.
Liedtke collaborated with Heller and colleagues in the Laboratory of Sensory Neuroscience who had cloned VR-OAC from a chicken inner ear gene library. Liedtke thought that VR-OAC could be an ion channel that responds to osmotic pressure. They showed that VR-OAC was also present in hair cells of the inner ear, which Hudspeth's lab has studied and functionally characterized. These cells are the principal mechanotransductory cells essential for the perception of sound and the sensing of acceleration in the inner ear's sense of equilibrium. VR-OAC may be key in helping to regulate the salt and water household of these cells.
"As an alternative model and a more general concept, VR-OAC could be part of sensory cells' mechanosensitive molecular apparatus, an exciting possibility," says Liedtke. "We are very eager to examine this in follow-up studies."
Working with hamster cells driven to express VR-OAC, the researchers showed that VR-OAC is exquisitely sensitive to subtle changes in osmotic pressure. VR-OAC does not respond to changes in temperature, but it is "fine-tuned" by temperature, with maximum sensitivity at body temperature in mammals (37?C/98.6?F) and in birds (40?C/104?F).
"Our laboratories then began to characterize the properties of what we now know to be an osmotically gated channel," says Friedman. "These studies showed that, indeed, the channel opens in response to decreased salt concentration, or osmolarity. This opening admits a small amount of calcium, which in turn triggers the release of a burst of calcium from storage depots inside the cell. When this happens in key nerve cells in the brain's osmotic regulatory region, we suspect that these receptors would fire and produce a host of responses that affect thirst, salt intake and perhaps even salt excretion."
VR-OAC is a member of the new family of vanilloid receptor-related genes, which have several exciting functions, namely sensitivity to osmotic pressure, and possibly to mechanical stimuli, painful heat and to capsaicin, the pungent ingredient of hot peppers.
Liedtke's, Heller's, Hudspeth's and Friedman's co-authors are graduate student Yong Choe, Andrea M. Bell and Charlotte S. Denis in the Laboratory of Sensory Neuroscience; and postdoctoral associate Marc A. Martí-Renom, Ph.D., and Associate Professor Andrej Sali, Ph.D., head of a Laboratory of Molecular Biophysics, all at Rockefeller.
This research was supported in part by the National Institute of General Medical Sciences, the National Institute of Diabetes and Digestive and Kidney Diseases, and the National Institute on Deafness and Other Communication Disorders, all part of the U.S. National Institutes of Health. Choe was supported in part by a graduate fellowship from the National Science Foundation. Sali is a fellow of the Alfred P. Sloan Foundation.
John D. Rockefeller founded Rockefeller University in 1901 as The Rockefeller Institute for Medical Research. Rockefeller scientists have made significant achievements, including the discovery that DNA is the carrier of genetic information. The University has ties to 21 Nobel laureates, six of whom are on campus. Rockefeller University scientists have received the award in two consecutive years: neurobiologist Paul Greengard, Ph.D., in 2000 and cell biologist Günter Blobel, M.D., Ph.D., in 1999, both in physiology or medicine. At present, 32 faculty are elected members of the U.S. National Academy of Sciences, including the president, Arnold J. Levine, Ph.D. Celebrating its centennial anniversary in 2001, Rockefeller -- the nation’s first biomedical research center -- continues to lead the field in both scientific inquiry and the development of tomorrow’s scientists.
Journal
Cell