Dec. 22, 2005 Rutgers geneticists have reported groundbreaking research on the genetics of fertility. They have discovered two genes, aptly named egg-1 and egg-2, required for fertilization to take place. The proteins encoded by these genes are similar to low density lipoprotein (LDL) receptors, known from cholesterol and fat metabolism but never before specifically implicated in fertilization.
One in six couples is experiencing fertility problems worldwide, and people are asking why. This is a question of great medical, social and economic importance -- one that cannot be answered until the process of fertilization is more fully understood.
A team led by Andrew Singson, an assistant professor and Pavan Kadandale, a graduate student in the Singson lab at the Waksman Institute of Microbiology at Rutgers, The State University of New Jersey, has taken a new and productive approach in this quest. The researchers found that in the absence of these two genes, the vital process of fertilization came to a halt. "What we learn in studying fertilization is not only important for this event, but also for the functioning of other cells in our bodies and for understanding many of those processes," Singson said.
Fertilization can be a paradigm for gaining insight into how cells interact over the life and development of multicellular organisms because it is one of the most basic of cell-cell interactions. The underlying cell biology is going to be universal with applications even in infectious diseases, such as AIDS, where the virus passes its genetic material to the cells it infects just as fertilization transmits sperm DNA to the egg, Singson explained.
Fertilization has primarily been studied in mammals or select marine invertebrates; but Singson and his group have instead turned to the lowly roundworm Caenorhabditis elegans (C. elegans), the first multicellular organism to have had its genome completely sequenced.
In addition to having its genome sequence, C. elegans offers particular advantages as a model system -- one from which results can be extrapolated to other organisms including humans. The millimeter-long worm is transparent, allowing a clear view of its internal workings, and its short life cycle permits researchers to chronicle developmental and hereditary factors over generations. These properties have enabled researchers to use C. elegans for fundamental discoveries in other fields ranging from cell death and life span regulation to nervous system structure and function.
But the worm's most important attribute as a model for this work may be its curious reproductive biology. Worms exist as males or hermaphrodites. When hermaphrodites are young they produce sperm and switch to produce eggs as adults. The Rutgers researchers were thus able to alter eggs in the hermaphrodites and use sperm from young males to test fertilization.
Genetic tools such as RNA interference (a way of removing gene function) and gene "knockout" mutants were used to see what would happen if worms lacked the function of egg-1 or egg-2 genes. The results were that the worms became sterile because fertilization had failed to occur. Normal sperm could no longer enter the eggs produced by egg-1 and egg-2 mutant hermaphrodites.
The traditional biochemical approach to studying fertilization has been to collect sperm and eggs and try to separate all the molecules or components of the cells, then discern how they might function in fertility. Singson admits that this has been productive, but he says that the definitive test for a role in fertilization or any biological process is to completely remove that molecule, or the gene that codes for it, and watch what happens.
"If you get infertility, then you know that the molecule is required for fertility, and this is our 'smoking gun.' Basically, we are asking the animal to tell us what it requires for its fertility, and then we try to understand how it works on a molecular level," he said. "Our use of this genetic approach, which hasn't been generally done in the past, is, indeed, groundbreaking."
Singson's group picked the two "egg" genes as an educated guess based on research Singson had previously conducted with a group of sperm genes. Mutations in sperm genes prevented the sperm from fertilizing normal eggs. The hope was that the sperm genes, together with the newly discovered egg genes, would be the "lock and key" that mediate normal fertilization.
For a sperm to enter an egg, the sperm has to recognize the egg and ignore other sperm or cells in the environment, Singson explained. Then there are interactions needed to get the surface membranes of both sperm and egg to fuse, a critical initial step in fertilization.
The results described in the Dec. 20 issue of Current Biology by Kadandale et al. confirm that along with the previously identified sperm genes, the egg-1 and egg-2 encoded molecules are a key component of the cellular machinery required for successful fertilization in worms.
"Ultimately," Singson said, "it will be exciting to determine if defects in similar molecules can lead to human infertility."
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