By David N. Leff

In the beginning - say 3.5 billion years ago - cyanobacteria were among the first and foremost forms of life on earth.

"The fossil record shows," observed molecular geneticist Lonny Levin at Cornell University, "that the cyanobacteria that flourished all those billions of years ago are indistinguishable from the cyanobacteria of today - also known as 'blue-green algae.' So as some of the first organisms on the planet, they are widely attributed as being responsible for converting what used to be a carbon dioxide atmosphere into an oxygen atmosphere.

"The importance there is," Levin pointed out, "that this earliest form of life evolved a system for detecting carbon dioxide (CO₂) and translating it into a cyclic adenosine monophosphate (cAMP) second messenger signal in cells. And that seems to have been conserved all the way to mammals, up to what we selfishly call the highest form of life on the planet. So if those bacteria have a CO₂ or bicarbonate-sensing cyclase enzyme, which is retained all the way through evolution into mammals, it must be of fundamental importance to each living cell."

That importance takes off from the freshly ejaculated sperm cell and influences events in the brain, the eye, the lung, the kidney and the pancreas - to mention only the organs reached thus far by Levin's research. He and colleague Jochen Buck, at Cornell's Weill Medical College in New York, are co-senior authors of a paper in Science dated July 28, 2000. Its title: "Soluble adenylyl cyclase [sAC] as an evolutionarily conserved bicarbonate sensor."

"One significant aspect of this finding," Levin told BioWorld Today, "is that we have a candidate molecule for a male anti-fertility drug, or even a topically applied one, because, as described in the paper, the best evidence for what process is involved is the maturation and developmental processes that sperm undergo in storage in the epidydimis and post-ejaculation. The swimming, the capacitation - stripping its tip for action, and potentially the acrosome reaction - penetrating the ovum's wall - are all seemingly dependent upon soluble adenylyl activity. So a potential for a male contraceptive exists."

What Does The Inhibitor Inhibit?

That prospect, Levin explains, relies on developing an inhibitor to block sAC's activity. "Bicarbonate stimulates that activity," he pointed out, "and that makes cAMP, which initiates a cascade of prototypical second-messenger events. The inhibitor we are developing would be inhibiting the catalytic activity of sAC, so no matter how much bicarbonate came along, you wouldn't be making cAMP, and you wouldn't be initiating this subsequent cAMP cascade.

"Conceptually," he went on, "the dream that we have is that there would be a pill that we could give. But because sAC is so widely expressed - because it's so fundamentally important to every cell - we would have problems administering an inhibitor orally - systemically. However, it's fairly easy to envision a dosing paradigm where one could give a pill at a dose that would not affect most cells but only curb the sperm. By providing such an inhibitor, then sperm couldn't swim, couldn't capacitate and penetrate post-ejaculation - and would be shooting blanks."

Levin and his co-authors also conceive of a topical alternative to an oral drug. "It could be given as a vaginal cream," Levin suggested, "because if you could administer an inhibitor to sAC post-ejaculation in the female, it could be formulated so as not to be systemic, and you wouldn't have to worry about that dosing problem. As a topical ointment or cream it could be dosed to prevent sperm activity in the vagina, uterus and fallopian tubes."

Soluble sAC and bicarbonate are molecules joined at the hip. "Bicarbonate of soda," Levin pointed out, "is HCO_3. CO₂ and H₂O - water. So when the CO₂ dissolves in the water, it condenses to a bicarbonate ion and an H⁺ proton. And sAC is tightly regulated by bicarbonate throughout every cell and fluid in the body."

Levin and his co-authors purified the sAC protein from rat testes, cloned the gene that expressed it, and raised antibody against the protein to demonstrate the sAC-bicarbonate connection. Human sAC resides on the long arm of chromosome 1. "There's a huge amount of homology between rats and humans," he observed.

"There's a host of male infertile patients in the Cornell in vitro fertilization clinic," Levin noted. They have a whole bank of infertile sperm. One of the things we want to look at is to find out whether these have a defect in their sAC. And we're just developing the reagents to do that. One way is to look at the protein and see how it runs on a gel: Is it normal or different - strange - in these infertile males?"

A Patent For All Reasons

The university has applied for patents covering virtually all aspects of the sAC research thus far. "We've filed a patent on the gene and protein, as well as its potential uses as a target for male and female contraceptives," Levin said. "Also for male and female infertilities. If there is a defect in the gene, perhaps we can develop a better activator than just simple bicarbonate, and thereby help infertile sperm.

"And we've patented its potential use, for example, as a target for glaucoma treatment - its potential bicarbonate-carbon dioxide chemosensor in the eye. Another patent filing covers its potential use as a diabetes target because it's known that there's a cAMP component to how glucose is detected in some way by the pancreatic islet cells, and they secrete insulin in response to the glucose levels. Bicarbonate is necessary to induce or potentiate the insulin secretion. So it's possible that sAC is a mediator of that as well."

So far the team has not begun looking for potential commercial collaborators ."We haven't gone out to find a partner," Levin vouchsafed, "and no one has contacted us yet - although I don't think that will be the case for very much longer."

He concluded, "I have the feeling we're going to be approached."