Soluble Adenylyl Cyclase as an Evolutionarily Conserved Bicarbonate Sensor

Abstract

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.

Bicarbonate activation of adenylyl cyclase via promotion of catalytic active site closure and metal recruitment

“…Its absence, along with the observations that bicarbonate (i) does not disrupt microcrystals of free sAC, and (ii) is very specific (Supplementary Table 1 online), yet requires high concentrations (physiological concentrations between 5 and 25 mM) to activate sAC 5,10,24 (corresponding to an estimated off-rate of ~10 6 s −1 , much higher than the turnover rate k cat of ~1 to 4 s −1 ; Supplementary Methods online), might suggest that bicarbonate interacts only very transiently with the enzyme during each catalytic cycle.…”

“…Cocrystallization of sAC with bicarbonate did not produce any crystals and soaking bicarbonate into preformed crystals dissolved them, suggesting that bicarbonate caused a structural change. Like sAC activation in solution 5 (Supplementary Table 1 online), the bicarbonate effect on sAC crystals is specific and pH-independent and cannot be induced with other anions, such as nitrate or acetate (see Supplementary Fig. 3 and Supplementary Table 1 Bicarbonate 'closes' the active site mainly by inducing a 4-5 Å movement of the β7-β8 loop toward the dimer center and a shift of the α1 helix in the same direction.…”

CFTR mediates bicarbonate-dependent activation of miR-125b in preimplantation embryo development

“…More importantly, CFTR is expressed in many different tissues, including the epithelium [51], endothelium [52], heart [53], neurons [54] and germ cells [55]. On the other hand, HCO 3 − is also abundant in extracellular fluid and acts as a bioactive molecule, with sAC as its sensor, which is also known to be evolutionally conserved and widely expressed in different tissues [56]. The conserved wide distribution of CFTR and sAC in various cell types suggests that the presently demonstrated CFTR-HCO 3 − -dependent signaling pathway may have implications in other cellular processes far beyond embryo development.…”

Protein biomarkers for male artificial insemination subfertility in bovine spermatozoa

“…A soluble type of adenylyl cyclase, “ADCY10,”94, 95, 96, 97, 98 is unique in its lack of transmembrane domains, independence of G‐protein‐coupled receptors, and direct stimulation by the interaction with bicarbonate and Ca 2+ , compared with the transmembrane types of the adenylyl cyclase, “ADCY1‐9.” This isoform is synthesized most abundantly in mouse testes as either the 189 kDa full‐length form or the 48 kDa truncated form from the same Adcy10 gene by alternative splicing. Although both forms contain two cyclase domains that catalyze the conversion from ATP to cAMP, the specific activity of the truncated form is 20‐fold higher than that of the full‐length form.…”