Soluble adenylyl cyclase as an evolutionarily conserved bicarbonate sensor.

Spermatozoa undergo a poorly understood activation process induced by bicarbonate and mediated by cyclic adenosine 3',5'-monophosphate (cAMP). It has been assumed that bicarbonate mediates its effects through changes in intracellular pH or membrane potential; however, we demonstrate here that bicarbonate directly stimulates mammalian soluble adenylyl cyclase (sAC) activity in vivo and in vitro in a pH-independent manner. sAC is most similar to adenylyl cyclases from cyanobacteria, and bicarbonate regulation of cyclase activity is conserved in these early forms of life. sAC is also expressed in other bicarbonate-responsive tissues, which suggests that bicarbonate regulation of cAMP signaling plays a fundamental role in many biological systems.

Specific expression of soluble adenylyl cyclase in male germ cells.

The cAMP signaling pathway is an important mediator of extracellular signals in organisms from prokaryotes to higher eukaryotes. In mammals two types of adenylyl cyclase synthesize cAMP; a ubiquitous family of transmembrane isoforms regulated by G proteins in response to extracellular signals, and a recently isolated soluble enzyme insensitive to heterotrimeric G protein modulation. Using the very sensitive reverse transcription-polymerase chain reaction (RT-PCR), soluble adenylyl cyclase (sAC) expression is detectable in almost all tissues examined; however, Northern analysis and in situ hybridization indicate that high levels of sAC message are unique to male germ cells. Elevated levels of sAC mRNA are first observed in pachytene spermatocytes and expression increases through spermiogenesis. The accumulation of high levels of message in round spermatids suggests sAC protein plays an important role in the generation of cAMP in spermatozoa, implying possible roles in sperm maturation through the epididymis, capacitation, hypermotility, and/or the acrosome reaction.

Cytosolic adenylyl cyclase defines a unique signaling molecule in mammals.

Mammals have nine differentially regulated isoforms of G protein-responsive transmembrane-spanning adenylyl cyclases. We now describe the existence of a distinct class of mammalian adenylyl cyclase that is soluble and insensitive to G protein or Forskolin regulation. Northern analysis indicates the gene encoding soluble adenylyl cyclase (sAC) is preferentially expressed in testis. As purified from rat testis cytosol, the active form of sAC appears to be a fragment derived from the full-length protein, suggesting a proteolytic mechanism for sAC activation. The two presumptive catalytic domains of sAC are closely related to cyanobacterial adenylyl cyclases, providing an evolutionary link between bacterial and mammalian signaling molecules.