Carbon dioxide (CO2) is a physiological gas found at low levels in the atmosphere and produced in cells during the process of aerobic respiration. Consequently, the levels of CO2 within tissues are usually significantly higher than those found externally. Shifts in tissue levels of CO2 (leading to either hypercapnia or hypocapnia) are associated with a number of pathophysiological conditions in humans and can occur naturally in niche habitats such as those of burrowing animals. Clinical studies have indicated that such altered CO2 levels can impact upon disease progression. Recent advances in our understanding of the biology of CO2 has shown that like other physiological gases such as molecular oxygen (O2) and nitric oxide (NO), CO2 levels can be sensed by cells resulting in the initiation of physiological and pathophysiological responses. Acute CO2 sensing in neurons and peripheral and central chemoreceptors is important in rapidly activated responses including olfactory signalling, taste sensation and cardiorespiratory control. Furthermore, a role for CO2 in the regulation of gene transcription has recently been identified with exposure of cells and model organisms to high CO2 leading to suppression of genes involved in the regulation of innate immunity and inflammation. This latter, transcriptional regulatory role for CO2, has been largely attributed to altered activity of the NF-κB family of transcription factors. Here, we review our evolving understanding of how CO2 impacts upon gene transcription.

The natural history of CO2

During the history of metazoan evolution in the Phanerozoic aeon, atmospheric levels of CO2 in dry air ranged from over 6000 ppmv (0.6%) around 600–400 million years ago to 284 ppmv (0.0284%) in the mid 1800s (Berner & Kothavala, 2001; Berner, 2003; Beerling & Berner, 2005;Royer et al. 2007; Vandenbroucke et al. 2010). Current atmospheric  levels are approximately 387 ppmv (0.0387%), representing an increase of approximately 36% since the advent of human industrial activity. While relatively low, this level of CO2 is key in regulation of the Earth’s temperature and climate (Lacis et al. 2010).

In respiring metazoans, the main source of CO2 is the electron transport chain of mitochondria where the chemical reduction of molecular oxygen is responsible for the generation of CO2 as a by-product. Thus, in contrast to molecular oxygen, the levels of CO2 found in tissues of the body are significantly higher than those found in the external atmosphere. A number of enzymes utilise CO2during their activity including carbonic anhydrases, a family of ubiquitously expresses metallo-enzymes which are responsible for catalysing the reversible hydration of CO2 and H2O to HCO3−and H+ (De Simone & Supuran, 2010). Remaining CO2 is primarily removed by the blood and is exhaled or diffuses through the skin. Recent advances have demonstrated that organisms contain distinct mechanisms capable of sensing changes in CO2 and eliciting distinct acute responses or changes in gene expression through transcriptional regulation.

The ability of metazoan cells to sense CO2 acutely and initiate rapid neuronal responses is analogous in nature to the acute oxygen-sensing pathways which exist in specialized tissues such as the carotid body (Weir et al. 2005; Lopez-Barneo et al. 2009) leading to neuronal signalling to control rate and depth of breathing. It is likely that in vivo such changes in neuronal activity will lead indirectly to CO2-induced changes in gene transcription as a consequence of altered neuronal activity.

CO2 and gene expression

In studies investigating the mechanisms underpinning the protective effects of ‘permissive hypercapnia’ in pulmonary disease, gene array analysis experiments were carried out on neonatal mice exposed to atmospheric hypercapnia (Li et al. 2006). This study identified altered levels of pulmonary genes related to cell adhesion, growth, signal transduction and innate immunity (Li et al. 2006).

NF-κB is a master regulator of the genes involved in innate immunity and inflammation. The NF-κB pathway is complex and has been expertly reviewed recently (Gilmore 2006).

While the effects of in vivo hypercapnia on gene expression are likely to occur in part through indirect mechanisms such as altered neuronal activity or the release of stress hormones, recent evidence suggests that CO2 may also directly regulate gene expression through the NF-κB pathway (Cummins et al. 2010). Some insight into a possible mechanism underpinning the suppression of NF-κB activity by hypercapnia was recently provided by the demonstration of CO2-induced nuclear localization of the IKKα subunit (Cummins et al. 2010).

In summary, the studies outlined above provide evidence that metazoan cells possess the capability to sense changes in microenvironmental CO2 levels and activate a transcriptional response which results in the suppression of innate immunity and inflammatory signalling.

Additionally, altered CO2 levels are likely to impact upon metabolic processes such as glycolysis.

Table 1

In normal conditions,  levels in the body are likely to vary between tissues and individual cells. Typical arterial blood  values are in the range of 35–45 mmHg. A thorough review of the contribution of CO2 to physiological and pathophysiological processes has recently been published elsewhere (Curley et al. 2010).

Hypercapnia arises when the mean arterial  is elevated above normal levels and can occur as a consequence of respiratory failure (e.g. in chronic obstructive pulmonary disease), but clinically it is commonly seen as a consequence of a low tidal volume ventilation strategy for acute respiratory distress syndrome (ARDS). Environmental hypercapnia may also occur in the natural habitats of burrowing animals (Lechner, 1976).

Hypercapnic acidosis (HCA), which can be a consequence of patient hypoventilation, was also identified as being associated with decreased mortality in a subset of the ARDSnet patient cohort (patients receiving 12 ml kg−1 tidal volumes who were defined as having hypercapnic acidosis on day 1 of the study) independent of changes in mechanical ventilation (Kregenow et al. 2006). Taken together these data are suggestive of elevated CO2 levels being protective in the critically ill patient.

Therapeutic hypercapnia has been reported to be of benefit in ischaemia–reperfusion injury in the mesentery (Laffey et al. 2003) and recently in the liver (Li et al. 2010). The mechanisms for this protection are not yet fully elucidated in vivo, but the latter study reports attenuated IRI-mediated pro-inflammatory gene expression (TNFα), enhanced anti-inflammatory cytokine production (IL-10), decreased apoptosis and decreased immunohistochemical staining for NF-κB in the hypercapnia treated groups. These studies are consistent with the observations described above for CO2 (independent of extracellular pH) having a suppressive effect on NF-κB signalling (Cumminset al. 2010; Wang et al. 2010) and of hypercapnic acidosis blunting endotoxin-stimulated NF-κB signalling, resulting in decreased ICAM-1 and IL-8 expression in pulmonary endothelial cells (Takeshita et al. 2003).

CO2 through its modulation of NF-κB signalling has the ability to both suppress inflammatory signalling and diminish innate immune responses. Depending on the nature of the challenge, CO2 and/or HCA can both blunt inflammation driven tissue damage as in the case of LPS-induced lung injury and exacerbate lung damage in response to pathogen infection. This has clear implications for the potential therapeutic applications of CO2 in the clinic where CO2 suppresses inflammation but also the ability to fight infection.