Worm sex receptor identified
by Leigh MacMillan
January 17, 2003
David Greenstein quips that his group’s research can
be summed up by the phrase “it takes two to tango.”
The “two,” in this case, are sperm and egg, whose
lively dance launches an organism’s developmental
routine.
Despite the importance of the sperm-egg interaction,
“we don’t know at the molecular level how this works
in any organism,” said Greenstein, Ph.D., associate
professor of Cell & Developmental Biology.
Greenstein and colleagues have now identified a
sperm-sensing receptor in the eggs of a microscopic
worm. Their work, reported Jan. 15 in Genes &
Development, is the first to find a receptor that
participates in egg maturation and ovulation.
Courtesy of David Greenstein
The current work is actually the second chapter of a
story that started several years ago when Michael A.
Miller, Ph.D., a postdoctoral fellow in Greenstein’s
laboratory, began searching for the signal that lets
worm eggs know it’s time to mature and be ovulated.
The image shows an electron micrograph of a
single sperm cell in the microscopic worm C.
elegans. Major sperm protein (MSP), the signal
that tells worm eggs to mature and be ovulated,
appears as tiny dots
In most animals, including worms and human beings,
eggs are arrested in an immature state until they
receive a signal to reenter the cell cycle and mature.
In human beings, the trigger for this egg maturation
process is unknown. In worms, it is a signaling protein released by sperm. Miller and Greenstein
identified the worm maturation signal as MSP (major sperm protein) and reported their findings in
Science in 2001. And now they’ve found the receptor and signaling pathway that MSP uses to
promote egg maturation and ovulation.
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Worm sex receptor identified at VUMC
Greenstein hopes their research will shed light on
problems with egg maturation in human beings.
Conservative estimates suggest that about five
percent of human pregnancies result in embryos with
an extra chromosome, he said, most likely due to
failures in egg maturation — specifically, failures in a
cell division process called meiosis. These
pregnancies usually end in miscarriage.
“Human beings are really bad at meiosis,” Greenstein
said. “We hope that by studying how eggs mature —
meiosis — in a simple system like C. elegans (a type
of microscopic worm), we will be able to glean
general principles about the signals important to
reproduction. We need to understand the basic
biology before we can attempt to fix the problem.”
Greenstein and colleagues are following a rich history
of studying egg maturation as a way to discover
signaling pathways that control the cell cycle and cell
division, he said. “We are a bit unusual in studying C.
elegans oocytes (eggs), but we’re interested in the
same fundamental questions of how the oocyte
makes a crucial cell cycle transition and how that is
coordinated with fertilization.”
Miller and Greenstein got a welcome boost in their
search for the receptor that worm eggs use to “hear”
the sperm signal.
David Greenstein
“The real breakthrough for us was the realization that there were data available that could help
us,” Greenstein said, referring to the genomic studies of a group at Stanford University that
defined worm egg- or sperm-enriched gene products. Miller and Greenstein narrowed the list of
egg-enriched genes to a handful of candidates and used a series of biochemical and genetic
tests to identify an MSP receptor. It is not the only MSP receptor, Greenstein said, and its identity
— it is a so-called Eph receptor — was a surprise.
Eph receptors are well characterized, and they are known to regulate multiple aspects of
mammalian development, including cell migration, nerve cell connections, and vascular and heart
development. They are receptors with tyrosine kinase enzyme activity and are, Greenstein said,
the largest class of this type of receptor in the human genome. The first tyrosine kinase receptor,
the epidermal growth factor receptor, was characterized at Vanderbilt by Nobel laureate Stanley
Cohen, Ph.D. and Graham Carpenter, Ph.D., professor of Biochemistry.
“Human beings are really bad at meiosis. We hope that by studying how eggs
mature — meiosis — in a simple system like C. elegans, we will be able to glean
general principles about the signals important to reproduction. We need to
understand the basic biology before we can attempt to fix the problem.”
– David Greenstein
The way that MSP and its Eph receptor (a protein called VAB-1 in C. elegans) regulate egg
maturation was also unexpected, Greenstein said. Instead of simply turning on a signaling
pathway that tells the egg to mature, MSP actually blocks the Eph receptor. The Eph receptor, it
turns out, normally acts to keep eggs in an immature state, and MSP’s block of its action stops
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Worm sex receptor identified at VUMC
this negative signaling pathway and lets maturation proceed. This type of action amounts to
what’s called a “checkpoint,” Greenstein said, a surveillance mechanism that determines whether
or not sperm are present. If sperm — and MSP — are around, the eggs are freed to mature and
be ovulated.
This kind of surveillance mechanism is especially important for the worm, because ovulation and
fertilization need to be closely coupled in time, Greenstein said. “Otherwise the worms would be
throwing away all of their oocytes, and that would be very bad for the species.”
Checkpoints are fundamental to cell growth control, Greenstein added. Cells must pass through
several important checkpoints, for example those that assess DNA damage and spindle integrity,
before they can divide. “It turns out that many cancer cells are defective in checkpoints, and this
leads to chromosome instability,” he said. So by studying worm reproductive signaling,
Greenstein and colleagues may have identified a new checkpoint mechanism that has relevance
to cancer-causing pathways.
The antagonistic action of MSP at the Eph receptor is also intriguing, Greenstein said. Eph
receptors are known to respond to signals called ephrins, and MSP may be the first of a group of
signals that block ephrin action. “By studying sex signaling in C. elegans, we have discovered a
new class of ligands for an ancient and widespread receptor signaling pathway,” Greenstein said.
The team’s findings, in addition to their relevance to fundamental questions of developmental
biology and cell cycle progression, could be exploited to develop new parasite-fighting drugs,
Greenstein said. Nematode worms related to C. elegans are the culprits in diseases like
elephantiasis and river blindness, and they destroy billions of dollars worth of crops every year.
These worms share similar reproductive strategies, making MSP or proteins in the Eph receptor
signaling pathway attractive targets for anti-parasite therapeutics, he said.
Other authors of the Genes & Development paper include Paul J. Ruest, Mary Kosinski, and
Steven K. Hanks. The research was supported by the National Institutes of Health, the
Vanderbilt-Ingram Cancer Center, and the American Cancer Society.
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Worm sex receptor identified at VUMC
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