Opposing modulation of Cx26 gap junctions and hemichannels by CO₂

Abstract

Cx26 hemichannels are opened by modest levels of CO2 (PCO2 55 mmHg) acting via a novel carbamylation mechanism to bias the hemichannel to the open state. In the hemichannel, CO2 nonenzymatically carbamylates Lys125, and the resulting carbamate forms a salt bridge to Arg104 of the neighbouring subunit. By contrast, similar modest levels of CO2 cause Cx26 gap junction closure.Gap junctions of Cx31, a beta connexin that lacks the carbamylation motif, are insensitive to these levels of CO2. Mutations of the carbamylation motif, Lys125Arg and Arg104Ala, which abrogate hemichannel opening in Cx26, also prevent gap junction closing. Elastic network modelling suggests that the lowest entropy state, when CO2 is bound, is in the open configuration for the hemichannel and the closed configuration for the gap junction Surprisingly, we conclude that the opposing actions of CO2 on Cx26 gap junctions and hemichannels results from carbamylation of the same residues.

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

“…10). Cx26 WT gap junctions can be closed by CO 2 acting via the same carbamylation motif that opens the hemichannel 55 and Cx26 DN subunits form gap junctions that are insensitive to CO 2 (Supplementary Fig. 10).…”

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

“…This is presumably due to compensation within the respiratory network, which is known to be highly plastic (Johnson and Mitchell, 2013). For example: the effects of carotid body denervation are largely reversed over the course of 2 weeks (Pan et al, 1998); mice engineered to express a mutant Phox2b gene selectively in neurons of the RTN have no CO2 chemosensitivity for the first 9 days postnatally, but recover about 40% of their CO2 chemosensitivity by 4 months (Ramanantsoa et al, 2011). Interestingly, the recovered chemosensitivity in these mice was mediated almost exclusively via an adaptive increase in tidal volume, rather than frequency (Ramanantsoa et al, 2011) -it is therefore tempting to speculate that this could have arisen via the progressive postnatal expression of Cx26 in glial cells of the caudal parapyramidal area.…”

Conformational changes and channel gating induced by CO2binding to Connexin26

“…Historically, the physiological actions of CO 2 have been regarded as being mediated exclusively via changes in pH. Here, we change this consensus by showing that the gap junction protein Connexin26 (Cx26) acts as a receptor for CO 2 showing sensitivity to modest changes in PCO 2 around the physiological norm 3-6 . Mass spectrometry analysis 7 shows that CO 2 carbamylates specific lysines on a regulatory loop of Cx26 at high, but not at low levels of PCO 2 .…”

“…from 35mmHg to 55 mmHg) can be mechanistically dissociated from the closing action of intracellular acidification. Mutation of Lys125 to an arginine in Cx26 gap junctions abolishes CO 2 dependent closure, but has no effect on closure to intracellular acidification 6 .…”

“…The effect of CO 2 on gap junctions and hemichannels is diametrically opposite: in hemichannels CO 2 causes hemichannel opening 3-5,27 , whereas in gap junctions CO 2 causes channel closure 6 . The closure of Cx26 gap junctions by a modest increase of PCO 2 ( e.g.…”