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CELL BIOLOGY
One-way Calcium Channel Pinpointed Within the CellMitochondria's Calcium Portal Found to Be an Ion ChannelA bad batch of antibodies from a company led researchers at Children's Hospital to discover how mitochondria absorb calcium to rev up the cellular energy needed to think, breathe, and grow.
Counterclockwise from right, Yuriy Kirichok, assistant professor Grigory Krapivinsky, and David Clapham found an elusive calcium-selective channel in mitochondria. (Photo by Graham Ramsay) They pinpointed the discriminating one-way portals that calcium uses to slip into mitochondria, perhaps the first calcium-selective ion channel to be described inside a cell. The study, from the lab of Howard Hughes investigator David Clapham, the Aldo R. Castaneda professor of cardiovascular research and professor of neurobiology and of pediatrics at HMS, appears in the Jan. 22 Nature. Long known as the power plants of the cell, mitochondria only recently have been appreciated for their ability to respond to and sop up excess calcium. The team's discovery may enable scientists to understand calcium handling in mitochondria and the energetics of these organelles, which some believe play an essential role in health and aging-related diseases. It may also help researchers investigate the cause of progressive and degenerative mitochondrial disease in children.
But calcium can quickly become too much of a good thing. The cell must pump calcium back out in preparation for the next influx. A calcium build-up can trigger apoptosis, which works through the mitochondria in childhood to weed out excess neurons and refine the brain architecture, but in older people, the same process may kill diseased nerves and cause the dementia of Alzheimer's. A Clear ChannelIn Clapham's lab, postdoctoral fellow Yuriy Kirichok began the project in search of another class of ion channels that normally sense temperature, pain, and taste. Kirichok ordered antibodies that were supposed to tag the sensory ion channel. Strangely, in whole cells, the fluorescent-labeled antibodies from what was later seen as a bad batch lit up the mitochondria, making them look like glowing green snakes tangled throughout the cell. A peek through the electron microscope at antibodies labeled with gold particles narrowed the location to the inner mitochondrial membrane.It was an odd place for a sensory channel, but Kirichok went hunting for it with an innovative use of the standard ion channel research protocol known as patch clamping. His poster with the preliminary results at the Biophysical Society meeting last spring drew crowds who may have been as impressed with the technical feat of patch clamp recordings from a tiny mitoplast (a mitochondrion without its outer membrane) as with the actual findings. When it works, a tiny glass pipette seals around a membrane, or patch, and detects a channel opening and closing. "The thing that's beautiful about a patch clamp is that you're directly measuring the current flowing through channels in real time," Clapham said. "If a pipette is a football stadium, a channel is a tire's inner tube on the 50-yard line. If you know what turns it on, you can watch it open and close in real time." It took Kirichok a year and plenty of patience, but when he made his first recording of the current flow through the channel, he knew right away it was not the one he had been seeking. Instead, it matched what scientists had called the calcium uniporter, an elusive mechanism responsible for calcium transport into mitochondria. Clapham was not convinced. "I had to pile up more evidence to convince him," said Kirichok. So he meticulously documented the channel's stubborn preference for calcium, its one-way flow, and its inactivation by drugs known to block the uniporter. "It's now safe to say that the mitochondrial calcium uniporter is an ion channel," Kirichok said. "Interestingly enough, this is the only known intracellular calcium-selective channel." The Responsible GeneThe human genome encodes hundreds of channels that broker the passage of charged ions across impermeable lipid membranes, Clapham said, but the gene for the calcium uniporter has eluded researchers. The newly described properties of the mitochondrial calcium channel may help them track down the gene and identify the responsible molecule.The uniporter channels are liberally sprinkled throughout the inner membrane, which makes sense in light of other research that shows mitochondria absorb significant amounts of calcium locally at hot spots of activity in the cell. In nerve cells, for example, mitochondria seem to sense and converge where a cell needs more energy and is generating more calcium, such as at the ends of active synapses, said Thomas Schwarz, HMS professor of neurology and of neurobiology at Children's, who is dissecting these intracellular travel arrangements though is not an author on the paper. Several years ago, other researchers showed that the calcium that activates the mitochondria seems to come from the endoplasmic reticulum, long known to store calcium away from the sensitive cytosol. The endoplasmic reticulum network is intertwined with the mitochondrial network. The uniporter channel sucks calcium into the mitochondria, which in true multitasking fashion both sequesters the calcium and bolsters the ATP production to fuel whatever cellular activity was triggered by the calcium release. "I cannot imagine cellular activity without increasing the cellular concentration of calcium in the cytosol," said Kirichok. From the point of view of the mitochondria, "if you feel the calcium concentration is increasing in the cytoplasm, you need more energy. It's that simple." --Carol Cruzan Morton |
