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Researchers discover which
organs in Antarctic fish produce antifreeze
Life Sciences Editor
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by L. Brian Stauffer
“Christina” C. Cheng, a professor of animal
biology at Illinois, and colleagues have discovered
that antifreeze glycoproteins originate in the exocrine
pancreas and the stomach of Antarctic fish, not the
liver as long presumed.
— Thirty-five years ago Arthur DeVries of the University of Illinois
at Urbana-Champaign first documented antifreeze glycoproteins (AFGPs)
in Antarctic notothenioid fishes. This month three colleagues report
they’ve solved the ensuing, long-running mystery of where these
AFGPs, which allow the fish to survive in icy waters, are produced.
“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. “It turns out that the liver has no role in the freezing
avoidance in these fishes at all.”
Instead, antifreeze glycoproteins (AFGP) originate primarily from the
exocrine pancreas and the stomach, say Cheng, Paul A. Cziko and Clive
W. Evans in a paper appearing online this week ahead of regular publication
in the Proceedings of the National Academy of Sciences. Cziko is a research
specialist at Illinois. Evans is a professor of molecular genetics and
development at the University of Auckland in New Zealand.
The liver-synthesis mindset dominated earlier studies even though results
appeared to be at odds, Cheng said. The first radioactive-tracer characterization
of liver AFGP biosynthesis, in fact, suggested another source of production
was possible. Later on, Northern-blot studies had shown very low expression
levels of antifreeze messenger RNA in the liver, but this was inconsistent
with high levels of production of the protein, the researchers noted.
Cheng and colleagues
used Northern blots of total RNA from various tissues to hybridize with
an AFGP gene probe. A clear picture of strong AFGP mRNA expression came
into focus in the pancreatic tissues in all notothenioids tested. The
use of cDNA cloning and sequencing showed that the mRNA all encode secreted
An RNA analysis from tissues of a single notothenioid unveiled the anterior
portion of the stomach, next to the esophagus-stomach junction, as being
the only other site with strong AFGP mRNA expression. Using antibodies,
the researchers found the absence of liver synthesis and strong pancreas
expression in newly hatched fish larvae and young juveniles.
The exocrine pancreas is the larger of the two parts that make up the
pancreas. It consists of tubuloacinar glands that primarily manufacture
and secrete digestive enzymes that break down food in the intestine
so it can be absorbed.
In this case, AFGPs
are secreted into the intestinal lumen where they protect the intestinal
fluid from being frozen by ice crystals that come in with seawater and
food. Internal fluids in notothenioids are about one-half as salty as
seawater. While seawater reaches its freezing point at –1.91 degrees
Celsius, fish fluids freeze at about –1 degree Celsius. These
species dwell in water that rarely rises above the freezing point and
is regularly filled with ice crystals.
From the intestine, the AFGPs are, apparently, absorbed into the blood.
This hypothesis is based on the near-identical composition and abundance
of AFGPs found in the fish serum.
“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 Arctic 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.
The National Science Foundation funded the research through two grants
Cheng, while Evans received funding from the University of Auckland