By David N. Leff
The sexual life of the nematode Caenorhabditis elegans is stranger than most people think.
Nematodes are the most abundant of all multicellular animals on our planet. Some 20,000 species have been named, and taxonomists are still counting. Typically, 1 square meter of mud flat at the mouth of a river harbors 20 million copies of C. elegans.
This tiny, transparent speck is 1 millimeter long by 0.1 mm thick. Its genome, sequenced late in 1998, numbers 97 million base pairs of DNA, comprising 19,099 protein-coding genes on six chromosomes in its 959 cells. C. elegans, like the fruit fly Drosophila melanogaster, is the darling of cell biologists and geneticists. (See BioWorld Today, Dec. 16, 1998, p. 1.)
Like Homo sapiens, C. elegans possesses a nervous system, digests food (typically bacteria) and in its 3.5-day life span, it manages to give birth to some 300 progeny per day. Finding a sex partner poses no problem because the minute worm is equipped with both male spermatozoa and female oocytes (eggs), which permit self-fertilization.
"C. elegans will ovulate an oocyte from each of its two gonad arms," observed biotech polymath David Greenstein, an associate professor of cell biology at Vanderbilt University in Nashville, Tenn. "Each gonad is like an assembly line for the production of worms. Each arm will ovulate oocytes every 23 minutes. One of the things that nematodes do best," he noted, "is eat and reproduce.
"The C. elegans we study is a hermaphrodite," Greenstein told BioWorld Today, "but many nematodes in the wild are male/female. In them, the sperm can get into the female only by mating, going through the vulva, and then crawling into the spermatheca, which is a sperm storage compartment. It's just a place in the worm's plumbing where the oocytes develop in the gonad arms, and get ovulated into.
"So what we think is happening in this male/female situation," he continued, "the sperm enter the vulva, cross the spermatheca, and release their major sperm protein, MSP, in some mysterious way. That signals to the oocyte, which is located something like 30 microns away."
Females Manufacture Own Sperm
"The nematodes in our lab," Greenstein went on, "are either free-living hermaphrodites or males. Males can mate with hermaphrodites, but hermaphrodites can be self-fertile. In evolution, the ancestral state was separate male/female animal sexes mating. But what happened during evolution was that the females figured out how to make a little bit of sperm, so they weren't dependent on males. They live in the dirt, and one animal can crawl away and start a whole population. Then males will arise in that population, and can mate to get genetic diversity. The C. elegans adult is self-fertile for four or five days," Greenstein recounted, "and then it runs out of sperm, because it's a hermaphrodite, so it can't have any more progeny. But if we mate it in the lab, it can.
"This is part of the sperm-sensing mechanism," he pointed out. "When a hermaphrodite runs out of sperm, it doesn't want to waste its oocytes. So it goes, 'Aha, I'd better keep my oocytes and not mature them.' If we ovulate them and they get mated to a male, there'll be sperm in its reproductive tract, there'll be MSP, and it will signal the oocytes: 'Now we can start!' Oocyte maturation comes before ovulation, and ovulation comes before fertilization.
"We've discovered the cell-to-cell MSP signal by which the nematode oocytes smell that there's a sperm around," Greenstein said, "and that it's safe to ovulate and produce progeny."
That discovery is reported in today's issue of Science, dated March 16, 2001, titled: "A sperm cytoskeletal protein that signals oocyte meiotic maturation and ovulation."
Greenstein explained the MSP's double life: "People have been studying this protein for 20 years, because it's involved in motility as well as reproduction. Nematode sperm crawl. Most crawling or migrating cells depend on the actin cytoskeleton, which is vital to cell locomotion. Surprisingly, nematode sperm don't have any actin, because when they complete meiotic division, they throw away their actin, their ribosomes and their microtubules.
"What they use instead," he continued, "is this MSP - major sperm protein - which forms filaments. Functionally, it's like an actin analogue, but it doesn't look like actin at all. Its crystal structure is great for a grad student. She can actually refer to the model on the computer, then figure out what's important for binding the MSP receptor. And this is also very nice for drug discovery, because we can use all this information to accelerate our research."
Greenstein observed, "For a basic biologist it's fascinating that through evolution the sperm cytoskeletal protein can acquire separate intracellular and extracellular signaling functions for reproduction - a discovery we consider without precedent. For parasitology," he added, "it constitutes an ideal drug target, because, for example in our Science paper, we show an alignment of the C terminus of this protein, which signals the gonad muscle cells that contract for ovulation. It's similar to the Onchocerca volvulus nematodes that cause river blindness in Africa, and it's also highly conserved in Ascaridia roundworm pathogens." He pointed out, "This region of the MSP protein is not conserved in other phyla, so it might be a really good anti-helminthic fertility target."
Two Ways To Make It Work
As a putative mechanism for such drugs, Greenstein proposed, "Either bind the MSP receptor and block signaling so the oocytes wouldn't mature, and you wouldn't get fertilization, or find molecules that bound to the MSP receptor itself.
"In terms of biotech," Greenstein added, "we hope this MSP, the major sperm cytoskeleton protein, could be an ideal birth-control agent for parasitic infections, but could be used in conjunction with other anti-helminthic agents. There are drugs on the market for parasitic nematodes," he noted, "but there is always the problem of drug resistance. So we're wondering whether as a combination agent - like AIDS three-drug cocktails - a drug that targets fertility in nematodes universally may be a great thing to develop."
The university has applied for patent protection on Greenstein's discovery, and contacts with potentially interested biotech companies are "just at the very beginning stage." n