By David N. Leff
If all the sperm cells churned out by one average male in a calendar year were divvied up among all 6 billion people on earth, there'd be more than 10 spermatozoa for each person alive today.
More specifically, that average individual's output of sperm per day amounts to 200 million cells. Assuming intercourse every day, that means a considerable back-up so that one single sperm cell can reach and fertilize a single female ovum.
In practice, only a fraction of that total output gets as far as ejaculation, much less fertilization. Like other mass-production manufacturing processes, the sperm assembly line undergoes quality control and culling en route from the birth to the maturation of sperm cells during spermatogenesis. (See BioWorld Today, May 14, 1996, p. 1.)
Those rejects include sperm with more than one head, misshapen flagella and other structural defects, not to mention cells missing chromosomes. Developmental biologists know that surveillance mechanisms along the sperm assembly line get rid of these anomalous specimens, but whether this rejection takes the form of simple necrosis (lethal damage) or programmed cell death (apoptosis) is a subject of lively research.
Two back-to-back articles in the March 1998 issue of Nature Genetics tackle the question from opposite ends:
"The meiotic checkpoint monitoring synapsis eliminates spermatocytes via p-53-independent apoptosis" is the title of a paper by developmental geneticist Paul Burgoyne and co-authors at Britain's National Institute for Medical Research, in London.
"Testicular degeneration in Bclw-deficient mice" heads the companion article. Its senior author is developmental geneticist Grant MacGregor, at Emory University, in Atlanta.
Potential Spinoffs: Male Contraceptive; Cancer Curb
"We were looking for genes that have an essential function in regulating male gametogenesis," MacGregor told BioWorld Today. "Apart from basic biological aspects, we saw two potential practical spinoffs."
Spelling these out, he pointed out: "If you want to identify potential routes for making male contraceptives, one way to do this is to find the gene product that's essential for that process. Obviously, if one could develop a drug that's an antagonist to the process, that drug could possibly create a temporary case of male infertility."
MacGregor continued: "Another spinoff from our work may include finding molecular mechanisms to abrogate testicular cancer. That's a very aggressive tumor type. If we could find the molecular basis for it, we might go ahead and devise treatments to try to stop that cancer."
Toward these goals, MacGregor and his co-authors looked at 40 lines of mice that had been the subjects of a mutagenic screen. They pinpointed four of them as sterile. In one of these lines, the responsible mutant gene they identified was Bclw, belonging to the Bcl2 family. (See BioWorld Today, Dec. 5, 1997, p.1.)
"It's composed of two types of genes," MacGregor explained. "We can conceptualize them as yin and yang. They act in concert, and current dogma has it that they antagonize each other. Essentially," he went on, "each cell receives a signal, which these gene products then decide whether or not to integrate into a death-promoting process. So they really act like a control center for apoptosis, meaning that its gene would be expressed in places where one might wish cells to survive."
Bclw is expressed in the testicular Sertoli somatic cells, which sustain and nurture the germ cells on their way down the seminiferous tubules to maturation. "You can conceptualize them," MacGregor suggested, "as being the boat yard. If you think of the germ cells as ships being prepared for some wondrous voyage, then the Sertoli cell is the boat yard in which these germ cells develop."
In pre-pubertal transgenic mice with mutated Bclw genes, MacGregor and his co-authors discovered a high level of apoptotic spermatocytes and an arrest in spermatid differentiation.
"The final phenotype of that mouse," MacGregor recounted, "is loss of all germ cells, and of their supporting Sertoli cells. What we're currently doing is evaluating whether the gene function is required within the germ cells or the somatic Sertoli cells, or both."
Letdown: p53 Cell-Death Protection
Burgoyne, in London, started his sperm studies by asking whether the quality control system picked out, and threw out, cells dying of necrosis or apoptosis.
"In the course of this work," he told BioWorld Today, "we became convinced, for three reasons, that p53 was a very likely candidate for the lead into the apoptotic pathway. First," he pointed out, "p53 is known to trigger apoptosis where there is DNA damage.
"Second, p53, we know, is expressed at high levels in the testis, in spermatocytes at the stage where their chromosomes [inherited from both parents] are pairing. We believe that a failure to pair generates that death signal.
"Third, we knew from the mechanics of meiosis [part of the sperm cells' cascade of divisions] that you nick the DNA as part of the recombination process. You have to exchange genes between the two copies of the chromosomes — that is what meiosis is about — and reshuffling your genes. That involves nicking the DNA, and if you have no pairing partner, you're not able to repair that nick."
Burgoyne and his co-authors generated mice which had their X and Y chromosomes stuck together end-to-end to make a single chromosome, "so they can't satisfy the requirements of pairing."
When those mouse models reached the metaphase 1 stage of meiosis, they went into cell arrest. "And then," Burgoyne observed, "we show that they undergo apoptosis."
Assuming that p53 would work its death-protective magic, they then stripped their X-to-Y-joined mice of their p53 gene to see if that would overcome the arrest block. "That was our expectation," Burgoyne said. "And we felt that if we did this we would become quite famous. But to our surprise, it did not overcome the block."
Nor did it in a second, more informative, mouse model. Again a negative p53 result.
"But that's only the beginning of the story," he added. "Now we have to find out what the pathway actually is. If it's not p53 that's leading in, what is it? And that's what we've been doing now for the last year and a bit." *