Martin Smith, professor of anatomy and neurobiology in the School of Medicine, and his UCI colleagues discovered that agrin -- a protein that directs synapse formation between nerve and muscle cells -- can also inhibit the function of "pumps" that control sodium and potassium levels within cells.
These pumps, called sodium-potassium ATPases -- or sodium pumps, for short -- are especially important in electrically excitable cells, where they provide the basis for electrical impulses, known as action potentials, which are responsible for muscle contraction and signaling between nerve cells in the brain. They do this by pumping sodium out of a cell and pumping potassium in, setting up an electrochemical gradient -- in a sense, turning the cell into a battery.
If this activity isn't properly moderated, uncontrollable electrical impulses can be triggered, which is one of the cellular mechanisms behind an epileptic seizure, for instance.
This is where agrin comes into action. The UCI researchers observed in laboratory tests that agrin controls the excitability of nerve cells in the brain by regulating sodium pump activity. Adding agrin caused nerve cells to fire electrical impulses uncontrollably. In turn, the researchers found that they could block these electrical impulses by introducing small fragments of agrin, which prevented the full agrin proteins from binding their sites on the sodium pump molecules and initiating action potentials.
"The ability of agrin to modulate nerve cell excitability suggests that the agrin-sodium pump interactions can be exploited as a novel therapeutic target for epilepsy and other brain disorders," Smith said.
Agrin proteins are also expressed in heart tissue, and Smith notes that sodium pump inhibitors, such as digoxin, are commonly used to treat congestive heart failure. Agrin may, therefore, have therapeutic value for the treatment of diseases affecting tissues and organs outside of the brain.
The study appears in the April 21 issue of Cell. Lutz Hilgenberg, Hailing Su, Huaiyu Gu and Diane O'Dowd of UCI collaborated on the study, which was supported by the National Institutes of Health.
UCI has filed for patents covering the use of agrin and its derivatives in treatment of epilepsy and other pathologies of the brain and as tools that could be used to screen for novel compounds that regulate sodium pump activity.
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