Development in a naturally acidified environment: Na⁺/H⁺-exchanger 3-based proton secretion leads to CO₂ tolerance in cephalopod embryos

Background: Regulation of pH homeostasis is a central feature of all animals to cope with acid-base disturbances caused by respiratory CO₂. Although a large body of knowledge is available for vertebrate and mammalian pH regulatory systems, the mechanisms of pH regulation in marine invertebrates remain largely unexplored.Results: We used squid (Sepioteuthis lessoniana), which are known as powerful acid-base regulators to investigate the pH regulatory machinery with a special focus on proton secretion pathways during environmental hypercapnia. We cloned a Rhesus protein (slRhP), V-type H⁺-ATPase (slVHA) and the Na⁺/H⁺ exchanger 3 (slNHE3) from S. lessoniana, which are hypothesized to represent key players in proton secretion pathways among different animal taxa. Specifically designed antibodies for S. lessoniana demonstrated the sub-cellular localization of NKA, VHA (basolateral) and NHE3 (apical) in epidermal ionocytes of early life stages. Gene expression analyses demonstrated that slNHE3, slVHA and slRhP are up regulated in response to environmental hypercapnia (pH 7.31; 0.46 kPa pCO₂) in body and yolk tissues compared to control conditions (pH 8.1; 0.045 kPa pCO₂). This observation is supported by H⁺ selective electrode measurements, which detected increased proton gradients in CO₂ treated embryos. This compensatory proton secretion is EIPA sensitive and thus confirms the central role of NHE based proton secretion in cephalopods.Conclusion: The present work shows that in convergence to teleosts and mammalian pH regulatory systems, cephalopod early life stages have evolved a unique acid-base regulatory machinery located in epidermal ionocytes. Using cephalopod molluscs as an invertebrate model this work provides important insights regarding the unifying evolutionary principles of pH regulation in different animal taxa that enables them to cope with CO₂ induced acid-base disturbances.