worms' noses sense oxygen
Robert Sanders, Media Relations
professor Michael Marletta
ranging from bacteria to humans navigate environments that can
contain dangerously too little or too much oxygen. Yet, scientists
know little about how animals sense oxygen levels around them.
from the Berkeley and San Francisco campuses of the University
of California have now discovered how the nematode C. elegans
senses oxygen levels in order to steer clear of surrounding
areas that are too low or too high in oxygen.
the process, the researchers also discovered that the worm doesn't
like as much fresh air as people thought. While nematodes grown
in laboratory Petri dishes are kept at the same oxygen concentration
humans breathe in ambient air - 21 percent - nematodes appear
to prefer only 6 percent oxygen.
was totally unexpected that they would actually prefer 6 percent.
We don't know why, though it probably gives them some survival
advantage," said Michael A. Marletta, professor of
chemistry and of molecular and cell biology at UC Berkeley, and
a faculty scientist at Lawrence Berkeley National Laboratory (LBNL).
"And the bordering and clumping that worm experts refer to
as social behavior is really the worms, in an artificial setting
like a Petri dish, trying to get to an area of 6 percent oxygen,
which they like. It's a laboratory phenomenon."
and clumping is a peculiar behavior in which worms cluster around
the border of the Petri dish instead of spreading evenly around
the surface. Marletta and his colleagues, members of the California
Institute for Quantitative Biomedical Research (QB3), determined
that the bacteria the worms feed on are at a higher density around
the border of the dish, consuming oxygen along with the worms.
Apparently, when oxygen levels are high, the worms pile onto the
densest clumps of bacteria, because that's where oxygen levels
move through a MEMS chamber to find the optimum oxygen level,
with 21% oxygen at one end and none at the other. (Jesse
Gray, Marletta Lab/UC Berkeley)
swarm of worms and density of bacteria together lower the oxygen
concentration in that immediate environment," he said. "We
found that when we lower the oxygen concentration to six percent,
the worms disperse in three minutes."
high concentrations, oxygen is toxic and corrosive. Worms avoid
high oxygen presumably because the oxygen creates highly reactive
chemicals that damage cells, though oxygen sensors also may help
them find food, Marletta said.
Cori Bargmann, a UC San Francisco professor of anatomy and
biochemistry and biophysics, and their colleagues reported their
results on June 27 in the Early Online Edition of Nature.
The paper will be published in the July 15 issue of Nature.
the worm's oxygen sensors, which are actually enzymes that bind
oxygen, are similar to enzymes used in humans and other animals
to detect the signaling molecule nitric oxide, or NO. NO plays
a major role in the cardiovascular system, activating an enzyme
that triggers dilation of blood vessels and thereby controls blood
was work on this NO-sensing enzyme that led Marletta into C.
elegans research. The enzyme, guanylate cyclase, is found
in smooth muscle, like that encircling blood vessels. NO binds
and activates guanylate cyclase, triggering a cascade of chemical
reactions that make the muscle relax, opening up the vessel and
lowering blood pressure. NO also activates guanylate cyclase in
the brain, where it is involved in learning in memory.
is important in maintaining blood vessel homeostasis, and so is
critical to cardiovascular function, gut motility and penile erection,
among other things," Marletta said.
students have painstakingly picked apart the guanylate cyclase
protein, in particular the exact binding site for NO. It turns
out to be a heme molecule nearly identical to the heme that binds
oxygen in hemoglobin to carry it through the blood stream to muscle.
The heme in hemoglobin cannot discriminate between oxygen, carbon
monoxide (CO) or NO, which is why CO is toxic: it replaces oxygen.
puzzle has been why the heme in guanylate cyclase is able to exclude
oxygen from its binding site and reserve it only for NO. This
molecular question - the key to understanding how NO works - has
been a focus of the Marletta lab, and pursuit of the answer led
directly to oxygen sensing in worms.
discovery of a similar enzyme in the nematode, but one that binds
oxygen instead of NO, will help Marletta and his colleagues discover
the tricks used by the enzyme to let in or screen out oxygen from
the heme binding site to selectively detect one or the other.
experiment helps us understand how NO receptors in muscle and
brain are able to bind selectively to NO in low concentrations
even when oxygen is present in far greater concentrations,"
he said. "This could have implications across a wide swath
of biology, in cases where organisms need to bind NO in low concentrations
and very selectively."
order to understand how guanylate cyclase is put together, Marletta
and his students went in search of similar enzymes in the genomes
of other organisms. Patricia Pellicena, a UC Berkeley postdoctoral
fellow with collaborator John Kuriyan, a UC Berkeley professor
of chemistry, found homologues not only in the nematode, but also
in more primitive organisms called bacterial prokaryotes. One
group of these, the obligate anaerobes, die in the presence of
oxygen, so they evidently require a sensitive oxygen detector.
insight helped graduate students David S. Karow of UC Berkeley
and Jesse M. Gray of UCSF make sense of puzzling data they
were obtaining about C. elegans' response to NO. Perhaps
this enzyme was serving as an oxygen detector, not as a NO detector,
in C. elegans?
manipulating oxygen levels in Petri dishes filled with worms feeding
on a lawn of bacteria, the researchers were able to show that
bordering and clumping was actually a response to high oxygen
levels. Karow and Gray employed a custom MEMS (microelectromechanical
systems) chamber - only 100 microns (1/10 millimeter or 4 thousandths
of an inch) thick - to study worm reactions to oxygen levels.
Built at UC Berkeley by UCSF postdoctoral fellow Hang Lu,
it has speeded behavioral studies of these denizens of the soil
and served as a prototype for further nematode studies.
knocking out several genes coding for parts of the guanylate cyclase
enzyme, they were able to show that this enzyme was acting as
an oxygen detector, primarily steering worms away from too much
oxygen. The enzyme is found in three separate neurons that innervate
the worm's nose.
speculates that the oxygen-sensing system used by C. elegans
may be used by other animals who must avoid low-oxygen environments,
including fish. Humans may also have such a detector to trigger
hyperventilation during exercise or exposure to anoxic environments.
are immersed in a 21 percent oxygen atmosphere all the time, and
our blood stream and lungs maintain the optimum oxygen levels
in our tissues. So, we take oxygen levels for granted," Bargmann
said. "But most other animals on the planet live in water
or the soil, such as C. elegans. And since oxygen diffuses
much more slowly in those environments, they must evolve ways
to sense oxygen and react to changes in oxygen levels."
students continue to take apart the guanylate cyclase enzyme and,
working with Kuriyan, are trying to crystallize the pieces in
order to get X-ray diffraction data to determine the 3-D structure.
has learned to use NO in cell-to-cell signaling, evolving a system
to generate and use it at very low concentrations. But what kind
of receptors can work at very low concentrations of NO?"
Marletta said. "We want to find out how nature engineered
a guanylate cyclase protein that doesn't bind oxygen but still
surprising," he added, "that 225 years after Lavoisier
discovered oxygen, we're still finding out how organisms sense
and use it, and little did we imagine that studying NO would lead
us closer to understanding the fundamentals of oxygen utilization."
work was supported by the Howard Hughes Medical Institute and
LBNL. Other coauthors of the paper are Andy J. Chang of
UCSF, Jennifer S. Chang of the University of Michigan and
Ronald E. Ellis of the University of Medicine and Dentistry
of New Jersey.
Marletta research group
Kuriyan research group