Carbon dioxide detection in biological systems

1. Introduction

This issue of Interface Focus contains a collection of papers arising from a Theo Murphy international scientific meeting entitled ‘Carbon dioxide detection in biological systems' held in December 2019 at The Kavli Royal Society Centre, Chicheley Hall. In this first symposium of its kind, the meeting included researchers investigating experimental models from both plant and animal kingdoms with a common interest in understanding the molecular basis of carbon dioxide (CO₂) detection and its downstream signalling and physiological effects. Here, we summarize the meeting under four themes: CO₂ detection and transport, CO₂ in the animal kingdom, CO₂ in the plant kingdom and the role of CO₂ in immunity and inflammation.

2. Theme 1—carbon dioxide detection and transport

The meeting outlined the many effects which altered CO₂ levels can elicit in both the plant and animal kingdoms. While many pathways are implicated as sensitive to changes in CO₂, a discovery method is required to identify sites of CO₂ interaction within these pathways. Linthwaite & Cann [1] present research describing the use of state-of-the-art technology to discover CO₂ binding proteins within a soluble proteome and apply this methodology to Escherichia coli. This method has been previously deployed using the soluble leaf proteome of Arabidopsis thaliana with similar results: the discovery of a selection of proteins previously unknown to interact with CO₂ directly. The application of this method, therefore, demonstrates that CO₂ binding in proteins is more widespread than formerly known and can identify CO₂–protein interactions within cellular signalling pathways.

Blosser et al. [2] present a review discussing the nature of CO₂ transport within cells. The discovery of the channel proteins aquaporin-1 (AQP1) and rhesus (Rh) proteins offered new means of CO₂ transport, previously thought to occur only via the dissolution and diffusion of CO₂ through membranes. These proteins provide cells with an essential pathway for CO₂ movement. For example, preliminary work shows that AQP5 can block the transport of CO₂ through extracellular metal binding. These methods of control through CO₂-permeable proteins provide cellular regulation to CO₂ levels and can become future drug targets.

3. Theme 2—carbon dioxide in the animal kingdom

This theme begins with a comprehensive review of the literature concerning CO₂-dependent signalling in mammalian models provided by Phelan et al. [3]. The review discusses the CO₂-initiated signalling events that cause a range of physiological responses. The study starts with a discussion of pH, including pH homeostasis, systems to regulate breathing and chemosensing. It then introduces carbonic anhydrases and their importance to the relationship between $\mathrm{HC}{\mathrm{O}}_3^{-}$/CO₂ and pH regulation. Following that, the authors provide information on the role of CO₂ in mitochondrial function and the demonstration that CO₂ protects against oxidative stress. The review expands to describe the influence of CO₂ on the regulation of intracellular calcium as well as how CO₂ interacts with the cyclic adenosine monophosphate (cAMP) and AMP-activated ser/thr protein kinases pathways; further work on the relationship between cAMP and CO₂ is also expanded on in a later review [4] and research article [5]. The review then summarizes the various responses to CO₂ by transcription factors, including a discussion of hypercapnia and the NF-κB pathway, which links CO₂ to immunity and inflammation (further presented in theme 4). The review ends by highlighting the current literature on the role of CO₂ in protein post-translational modification.

A review by Dale [6] discusses the equilibrium between the rate of breathing and arterial blood pH. This relationship is proposed to be maintained by chemosensory cells which measure blood and tissue pH levels. This review highlights data which suggests a role for direct CO₂ sensing and then discusses a molecular mechanism by which direct detection of CO₂ via the gap junction protein connexin26 (Cx26) contributes to the regulation of breathing. This work highlights that direct measurements of CO₂ on a protein might be an essential factor during modest hypercapnia and adds a further dimension to the regulation of breathing. This work links to the later discussions of the effects of hypercapnia within the immunity and inflammation theme.

Soluble adenylyl cyclase (sAC) is a bicarbonate ($\mathrm{HC}{\mathrm{O}}_3^{-}$)- regulated enzyme that synthesizes cAMP from ATP. cAMP has roles in biological processes ranging from development through to apoptosis and provides a direct link between [$\mathrm{HC}{\mathrm{O}}_3^{-}$] and the adaptive response. Rosetti et al. have reviewed the relationship between sAC and the physiological $\mathrm{HC}{\mathrm{O}}_3^{-}$/CO₂/pH environment [4]. The review outlines the ways through which sAC functions as both $\mathrm{HC}{\mathrm{O}}_3^{-}$ sensor and CO₂/pH sensor due to the high abundance of carbonic anhydrase catalysing the equilibration of CO₂, $\mathrm{HC}{\mathrm{O}}_3^{-}$ and protons within the extracellular and intracellular environments. The review highlights the importance of sAC in mechanisms whereby cells can respond to changes in $\mathrm{HC}{\mathrm{O}}_3^{-}$/CO₂/pH.

Salmerón et al. [5] present research describing the discovery of sAC-like proteins in bony fish species. Following the discovery of sAC-encoding genes in various cartilaginous, ray-finned and lobe-finned fish species, they focused their research on rainbow trout sAC (rtsAC). They identified several possible rtsAC protein isoforms that are stimulated by $\mathrm{HC}{\mathrm{O}}_3^{-}$ at physiologically relevant levels. The rtsAC isoforms were also sensitive to established inhibitors of sAC from other organisms and insensitive to modulators of transmembrane adenylyl cyclases.

Fascinating research within the theme of CO₂ in the animal kingdom was also described by Chen et al. [7]. Their work centred on mosquito attraction to host animals via CO₂ plumes. A receptor senses CO₂ in the A neuron of the capitate peg sensilla type on the maxillary palps. This receptor also detects several other odorants and Chen et al. [7] have discovered that the standard skin odorant butyric acid can cause prolonged activity of the CO₂ receptor. They used machine learning to model other prolonged activators. This work provides an opportunity to discover novel ligands for the CO₂ receptor and therefore manipulate mosquito behaviour.

4. Theme 3—carbon dioxide in the plant kingdom

Movahedi et al. [8] explore the importance of abscisic acid (ABA) catabolism in the plant stomatal responses to CO₂ and light. The investigation was performed with the use of an ABA 8′-hydroxylase-deficient A. thaliana mutant which is unable to catabolize ABA. ABA is a regulator for plant stress responses and guard cell closure mechanisms; ABA levels are, therefore, essential for plant growth and survival. The work reveals the importance of the regulation of ABA levels in mediating stomatal responses to CO₂ and light, as well as highlighting the close relationship between ABA levels and CO₂ sensitivity. The paper also demonstrates that optimal stomatal opening in the absence of CO₂ requires ABA catabolism. When this catabolism is impaired, there is a markedly lower stomatal density than expected. These results suggest that ABA turnover could be a target for improving water use efficiency in crops for predicted higher CO₂ environments.

Clarke et al. [9] investigate how mesophyll conductance varies with leaf age and through a tobacco canopy. Mesophyll conductance is one of the main physiological processes limiting CO₂ uptake and fixation within plants; this becomes especially important in the light of efforts to improve and maximize crop yields. This study concludes that mesophyll conductance decreases as leaves age and is associated with a reduction in chloroplast leaf area exposed to intercellular airspace per unit leaf area. Increasing mesophyll conductance within crop plant species could therefore increase crop yields.

5. Theme 4—carbon dioxide in immunity and inflammation

Hypercapnia (HC) is the elevation of CO₂ in blood and tissues and usually occurs in severe acute and chronic respiratory diseases. Shigemura and Sznajder present a review of the effects of HC on the lung airways and our current understanding of the impact of high CO₂ modulation on airway contractility [10]. This paper discusses the literature concerning the effects of HC, which were previously thought to be inconsequential, but are now proposed to have harmful effects on the lungs, skeletal muscles and innate immunity. The review underlines the need for further understanding of the impact of elevated CO₂ on airway contractility in patients suffering from COPD or HC.

The review from Shigemura and Sznajder is complemented by Masterson et al. [11] who review HC in the critically ill. At the cellular level, CO₂ has profound effects on multiple signalling pathways via both CO₂ itself and the related acidosis. Studies are essential to understanding how HC may affect clinical outcomes in patients receiving mechanical ventilation for acute respiratory distress syndrome (ARDS). This review presents the current understanding of how CO₂ affects mammalian systems, especially with regards to inflammation and disease. The study focuses on the critically ill patient and how enhancing our knowledge of the physiological effects of CO₂ could lead to the preservation of life. The review concludes by noting that clinicians must tightly control HC to provide any clinical benefit and that researchers might target future work in this area towards small molecules that underlie these pathways, such as sAC and the NF-κB pathway constituents.

These two reviews are built upon by research by Casalino-Matsuda et al. [12] investigating the LPS-induced changes in innate immunity and DNA replication-related gene transcription in the macrophage. Recent studies have demonstrated that HC can inhibit select innate immune responses. This work investigated the effect of HC (20% (v/v) CO₂) on gene transcription in human THP-1, and mouse RAW 264.7 macrophages stimulated with lipopolysaccharide (LPS). The study confirmed that HC-downregulated genes are associated with innate immune responses as well as inflammatory pathways in both macrophage systems.

6. Conclusion

The collection of papers published in this special issue of Interface Focus address the state of the art concerning CO₂-dependent signalling and sensing. While much impressive work has been performed to identify critical CO₂-dependent signalling nodes in both the plant and animal kingdoms, future work is required to more precisely determine the CO₂-sensitive proteins within these pathways. Technological innovation in the area of CO₂-dependent post-translational modifications can advance our understanding in this area to the betterment of researchers in the plant, animal and clinical communities.

Data accessibility

This article has no additional data.

Competing interests

We declare we have no competing interests.

Funding

This study was funded by the Biotechnology and Biological Sciences Research Council (BB/S015132/1).