# Soluble adenylyl cyclase in nonmammalian sperm is directly controlled by pH, not by HCO3− or Ca2+

## TL;DR

This paper shows that in nonmammalian sperm, soluble adenylyl cyclase is activated by pH rather than bicarbonate, which has implications for reproduction in changing ocean conditions.

## Contribution

The study reveals that sAC in nonmammalian sperm is pH-regulated, challenging the assumption of bicarbonate control across species.

## Key findings

- Soluble adenylyl cyclase in echinoderms and fish is activated by alkaline pH, not bicarbonate.
- pH-regulated sACs across 13 phyla have neutral residues that prevent bicarbonate activation.
- Alkaline pH during spawning stimulates cAMP synthesis in sea urchin sperm, crucial for motility activation.

## Abstract

Cyclic AMP (cAMP) is a cellular messenger vital for key sperm functions, including motility and fertilization capacity. The enzyme soluble adenylyl cyclase (sAC) synthesizes cAMP. While it was thought that bicarbonate regulation of sAC was conserved across species, from corals to humans, this does not apply to nonmammalian metazoans. In mammals, bicarbonate activates sAC, whereas in marine invertebrates and fish, sAC is unresponsive to bicarbonate but is instead activated by alkaline pH, functioning as a direct pH sensor. These findings help build a comprehensive model of cAMP signaling in aquatic animal sperm. The pH regulation of sAC in aquatic animals makes their reproductive success vulnerable to increasing ocean acidification driven by climate change.

In mammalian sperm, high HCO3− concentrations in semen and the oviduct activate soluble adenylyl cyclase (sAC), which synthesizes cyclic AMP to regulate sperm motility. Here, we demonstrate that sAC orthologs in echinoderms and fish, species whose sperm fertilize eggs in aquatic environments with low HCO3−, are activated by alkaline pH rather than HCO3−. In human sAC, two charged residues coordinate HCO3− and are essential for HCO3−-mediated activation. In contrast, pH-regulated sACs and orthologs from 13 phyla have these residues replaced by neutral ones. These substitutions abolish the enzyme’s responsiveness to HCO3− and suggest that pH regulation of sAC is widespread in nonmammalian metazoans. Furthermore, we show that in sea urchin sperm, a rise of pH during spawning stimulates cAMP synthesis, a key step in the activation of motility. An evolutionarily significant pattern is emerging: Across phyla, sAC regulation by pH or HCO3− represents an adaptation to environments with low or high HCO3−, respectively.