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Researchers at Weill Cornell Medical College have begun to elucidate the actual molecular and cellular processes by which post-ejaculation sperm mature and are thereby able to fertilize an egg. Understanding these age-old processes, which have been hitherto largely unknown, may ultimately lead, on the one hand, to new clinical approaches to sterility and infertility and, on the other, to the development of new and improved male contraceptives.

The research, led by Drs. Jochen Buck and Lonny Levin of the Department of Pharmacology at Weill Cornell, has just been published in the July 28 issue of Science under the title "Soluble Adenylyl Cyclase as an Evolutionarily Conserved Bicarbonate Sensor."

The article defines the first molecular target of the poorly understood changes mammalian sperm undergo post-ejaculation. It is well established that the physiologically ubiquitous ion, bicarbonate, in prostatic and vaginal fluids induces sperm maturation via increasing intracellular cAMP (cyclic adenosine 3',5'-monophosphate); in vitro fertilization methods are absolutely dependent upon this fact. Thus far, it has been largely assumed that bicarbonate's effects are mediated via changes in intracellular pH or membrane potential. However, the researchers now demonstrate that a unique form of the enzyme adenylyl cyclase—known as mammalian soluble adenylyl cyclase (sAC) and found in elevated levels in the testis and sperm—functions as the specific bicarbonate sensor mediating spermatozoa maturation. They clearly show that bicarbonate directly stimulates sAC activity both in vitro and in vivo in a pH independent manner.

The new research also definitively distinguishes between sAC and the widely studied G-protein responsive transmembrane adenylyl cyclases (tmACs). Previously published research by these scientists indicated that sAC was insensitive to known regulators of tmACs; this study shows that bicarbonate regulation of mammalian adenylyl cyclase activity is specific to sAC. Therefore, mammals possess at least two, independently controlled cAMP signal transduction cascades, which is a surprising new aspect to what had been previously thought to be a very well understood signaling pathway.

The physiological role of bicarbonate (and carbon dioxide, because of their tight equilibrium) regulation of sAC and of second messenger signal transduction may not be restricted to mammalian sperm. As a major component of all extracellular buffers, serum and fluid bicarbonate levels must be tightly controlled, but the chemosensor(s) assessing its concentration in kidney and other tissues (such as choroid plexus, which regulates cerebrospinal fluid) has remained uncharacterized. This research demonstrates, for the first time, that sAC is present in these chemosensing tissues. This fact, coupled with the evidence that bicarbonate transduces signals via modulation of sAC, suggests a mechanism for bicarbonate-mediated chemosensation.

Finally, the authors show that bicarbonate regulation of cAMP synthesis is conserved in cyanobacteria, one of the oldest forms of life on this planet. Cyanobacteria, or blue-green algae, evolved in the carbon dioxide-rich environment of the early earth, and survive by generating complex sugars from bicarbonate (carbon dioxide and water) and sunlight.

Conservation of any signaling pathway from the earliest forms of life to man is extremely compelling. Other known signaling systems have disappeared during evolution. Thus, judging solely by its evolutionary conservation from cyanobacteria to mammals, sensing bicarbonate by modulating cAMP must be fundamentally important for life.

Key Summary Points and Future Possibilities

1. Freshly ejaculated sperm must undergo a number of critical physiological changes before they are competent and mature enough to fertilize an egg. These changes appear to be controlled by the unique enzyme Soluble Adenylyl Cyclase (sAC), which is found in elevated levels in the testis and sperm. Moreover, these changes are initiated by the high bicarbonate concentration in the seminal fluids and supported by the surrounding vaginal and uterine environment.
2. Since the enzyme sAC is the bicarbonate chemical sensor that regulates the essential maturation processes in mammalian sperm, sAC must be considered as a potential target for male contraceptives.
3. Regarding sterility/infertility, if one were to establish that infertile men have an altered or malfunctioning sAC protein, then theoretically one could treat them, or more precisely their sperm, with membrane permeable cAMP analogues to complement the sAC defect.
4. sAC may mediate carbon dioxide/bicarbonate chemosensing in other mammalian physiological systems and in a wide variety of biological processes. For example, sAC is present in high amounts in the kidneys, and the kidneys tightly control the amount of bicarbonate in the bloodstream. While it is known how the kidney raises and lowers bicarbonate levels in the blood, it is not known how the kidney senses when to raise or lower the circulating bicarbonate concentration. As the only known signaling enzyme regulated by bicarbonate, sAC appears to be the most likely candidate.
5. sAC may also be involved in fluid formation in the brain and eye (glaucoma results from too much fluid build up), in controlling the rate of breathing, and in the secretion of insulin in response to sugar levels in the bloodstream. The authors are currently testing sAC function in vivo in a variety of organs/systems.
6. Mammalian sAC defines an independent cAMP (cyclic adenosine 3',5'-monophosphate) signaling system from the widely studied G-protein responsive transmembrane Adenylyl Cyclases (tmACs). Thus, mammals possess at least two distinct, independently controlled cAMP signal transduction cascades.
7. Bicarbonate regulation of cAMP signaling evolved in cyanobacteria, or blue-green algae, and has been conserved through evolution in mammals. This suggests that carbon dioxide regulation of cAMP synthesis must be essential for life.

Dr. Jochen Buck, an Associate Professor of Pharmacology at Weill Cornell, and Dr. Lonny Levin, an Assistant Professor of Pharmacology, have been joined in this research by Weill Cornell co-authors Drs. Yanqiu Chen, Martin Cann, Vadim Iourgenko, and Meeghan Sinclair, and Tatiana Litvin.

The research was generously supported by grants from Weill Medical College of Cornell University Research Associates, the Irma T. Hirschl Trust, The L.W. Frohlich Charitable Trust, Edith C. Blum Foundation, and the National Institutes of Health.