The Pancreas of Antarctic Fish Produces Antifreeze

The resilient fishes of the Antarctic - known as Antarctic notothenioids - keep from freezing solid in the cold waters thanks to a special "antifreeze protein" that prevents their bodily fluids from turning into crystals. The antifreeze glycoprotein, called AFGP for short, was discovered 35 years ago by Arthur DeVries of the University of Illinois at Urbana-Champaign, but is wasn't clear whether the fish produced the special molecules or not.

"Ever since the discovery of these antifreeze proteins, it was assumed they had to be produced in the liver, since the vertebrate liver is well known as a source of secreted plasma protein, so there was no reason to think otherwise," said Chi-Hing "Christina" C. Cheng, a professor of animal biology at the University of Illinois. "It turns out that the liver has no role in the freezing avoidance in these fishes at all."

In the Southern Ocean, sea water temperatures rarely rise above -2 degrees Celsius and the water is often filled with tiny ice crystals. But fish fluids freeze at about -1 degrees Celsius and ingesting the ice crystals, as they eat, the fishes risk freezing from the inside out.

Cheng, Paul A. Cziko and Clive W. Evans have now found that the antifreeze glycoproteins originate primarily from the exocrine pancreas and the stomach. The protein is then absorbed into the blood preventing the freezing of the internal fluids.

"In this comprehensive study, we confirm that the exocrine pancreas is the major AFGP synthesis site in Antarctic notothenioid fishes from hatching through adulthood, while the liver is AFGP-expression null in all life stages," the researchers conclude. "Because the notothenioids are confined to chronically icy Antarctic waters, and face high risks of ice inoculation from frequent seawater drinking, the evolution of AFGPs in these fishes was probably driven first and foremost by the need to prevent the hyposmotic intestinal fluid from freezing."

The researchers also studied a variety of fishes from Antarctic waters that have liver expression of AFGPs, and found that all of them also express antifreeze in the pancreas. The findings, they wrote, bring a new perspective to teleost freeze-avoidance physiology and "reveals that the long-held paradigm of hepatic-based AF synthesis and secretion is no longer universally applicable." Instead, pancreatic antifreeze expression is universal.

Photo Credit: PNAS