Evolutionary adaptation of the sensitivity of connexin26 hemichannels to CO ₂

Abstract

CO2 readily combines with H2O to form and H+. Because an increase of only 100 nM in the concentration of H+ (a decrease of 0.1 unit of pH) in blood can prove fatal, the regulated excretion of CO2 during breathing is an essential life-preserving process. In rodents and humans, this vital process is mediated in part via the direct sensing of CO2 via connexin26 (Cx26). CO2 binds to hemichannels of Cx26 causing them to open and allow release of the neurotransmitter ATP. If Cx26 were to be a universal and important CO2 sensor across all homeothermic animals, then a simple hypothesis would posit that it should exhibit evolutionary adaptation in animals with different homeostatic set points for the regulation of partial pressure of arterial CO2 (PaCO2). In humans and rats, PaCO2 is regulated around a set point of 40 mmHg. By contrast, birds are able to maintain cerebral blood flow and breathing at much lower levels of PaCO2. Fossorial mammals, such as the mole rat, live exclusively underground in burrows that are both hypoxic and hypercapnic and can thrive under very hypercapnic conditions. We have therefore compared the CO2 sensitivity of Cx26 from human, chicken, rat and mole rat (Heterocephalus glaber). We find that both the affinity and cooperativity of CO2 binding to Cx26 have been subjected to evolutionary adaption in a manner consistent with the homeostatic requirements of these four species. This is analogous to the evolutionary adaptation of haemoglobin to the needs of O2 transport across the animal kingdom and supports the hypothesis that Cx26 is an important and universal CO2 sensor in homeotherms.

Opposing modulation of Cx26 gap junctions and hemichannels by CO2

“…The key result from our study is that Cx26 gap junctions are closed by a direct action of CO2 on the protein. Our prior publications have demonstrated the opening action of CO2 on Cx26 hemichannels (Huckstepp et al, 2010a;Meigh et al, 2013;Meigh et al, 2014;de Wolf et al, 2016;de Wolf et al, 2017;Cook et al, 2019;Dospinescu et al, 2019). This paper further shows that these diametrically opposite actions of CO2 on gap junctions and hemichannels depend on the same residues and presumably the same carbamate bridging mechanism.…”

“…The action of CO2 and pH on the gap junction are therefore mechanistically independent at the moderate levels of CO2 used in this study. This independence of mechanistic action of pH and CO2 is also true for Cx26 hemichannels as acidification causes hemichannel closure (Yu et al, 2007), whereas a modest increase in PCO2 causes hemichannel opening (Huckstepp et al, 2010a;Meigh et al, 2013;de Wolf et al, 2017;Cook et al, 2019;Hill et al, 2020).…”

Connexin26 mediates CO2-dependent regulation of breathing via glial cells of the medulla oblongata

“…There was no detectable effect of Cx26 DN on the ventilatory response to 3% inspired CO 2 . This is unlikely to reflect the properties of Cx26, as it is sensitive to changes in PCO 2 from 20 to 70 mmHg 27,33 , so would very likely be capable of detecting the small change in systemic PCO 2 resulting from a 3% CO 2 challenge. Cx26 contributes only to the adaptive change in tidal volume during hypercapnia (Fig.…”

Connexin26 mediates CO2-dependent regulation of breathing via glial cells of the medulla oblongata

“…There was no detectable effect of Cx26 DN on the ventilatory response to 3% inspired CO2. This is unlikely to reflect the properties of Cx26, as it is sensitive to changes in PCO2 from 20 to 70 mmHg (de Wolf et al, 2017;Huckstepp et al, 2010a), so would very likely be capable of detecting the small change in systemic PCO2 resulting from a 3% CO2 challenge. Cx26 contributes only to the adaptive change in tidal volume during hypercapnia ( Figure 5).…”

“…Like all connexins, Cx26 hemichannels can be opened by removal of extracellular Ca 2+ (Muller et al, 2002). However, the CO2-dependent opening of Cx26 hemichannels can occur in the absence of membrane depolarisation and at physiological levels of extracellular Ca 2+ (de Wolf et al, 2017;Dospinescu et al, 2019;Huckstepp et al, 2010a;Meigh et al, 2013).…”