Stimulation of the A2B Adenosine Receptor Subtype Enhances Connexin26 Hemichannel Activity in Small Airway Epithelial Cells.

BACKGROUND/AIMS: Adenosine release and connexin (Cx) hemichannel activity are enhanced in the respiratory epithelium during pathophysiological events such as inflammation. We analysed the interplay between Cx channels and adenosine signalling in human respiratory airway epithelium using the Calu-3 cell line as a model. METHODS: The Cx hemichannel activity in Calu-3 cells was evaluated by dye uptake assays. The expressed Cx isoforms and adenosine receptor subtypes were identified by PCR and western blot analysis. Pharmacological and molecular biological experiments were performed to analyse the involvement of the different adenosine receptor subtypes, the induced signalling pathways and the contribution of specific Cx isoforms to the hemichannel activity. RESULTS: The adenosine receptor agonist 5'-N-ethylcarboxamidoadenosine (NECA) increased the dye uptake rate in Calu-3 cells. The pannexon and Cx hemichannel inhibitor carbenoxolone (CBX) did not supress the dye uptake at pannexin-specific concentrations (100 µM). High CBX concentrations or the inhibitor La3+, both effective on Cx hemichannels, were needed to inhibit the dye uptake. The NECA-related increase of dye uptake depended on enhanced cAMP synthesis and subsequent activation of the protein kinase A (PKA) as shown by quantification of cAMP levels and pharmacological inhibition of the adenylyl cyclase and the PKA. Further pharmacological inhibition as well as knockdown experiments with specific siRNA showed that the A2B adenosine receptor was the subtype mainly responsible for the increased dye uptake. The NECA-related increase of the dye uptake rate correlated with a decrease of Cx43 mRNA and an increase of Cx26 mRNA content in the cells as well as Cx26 protein synthesis and was inhibited by Cx26 knockdown using Cx26 siRNA. Of note, a siRNA-induced knockdown of Cx43 increased the content of Cx26 mRNA and correspondingly the dye uptake rate. CONCLUSION: The Calu-3 cell model shows that stimulation of the A2B adenosine receptor subtype activates synthesis of cAMP. cAMP activates PKA and induces thereby an increase in Cx26 and a decrease in Cx43 mRNA levels. As a result, the synthesis of Cx26 is reinforced, leading to an enhanced Cx hemichannel activity. The report identifies a mechanism that integrates adenosine release and Cx hemichannel activity and shows how adenosine signalling and Cx channels may act together to promote persistent inflammation, which is observed in several chronic diseases of the respiratory airway.

The spatial distribution pattern of Connexin26 expression in supporting cells and its role in outer hair cell survival.

Mutations in the GJB2 gene (which encodes Connexin26 (Cx26)) account for about a quarter of all cases of non-syndromic deafness. Previous studies have indicated that knockout (KO) of Gjb2 gene during early postnatal days can cause outer hair cell (OHC) loss in mouse models. However, the postnatal spatial distribution pattern of Cx26 in different types of supporting cells (SCs) and the role of such distributions for the survival of OHCs is still obscure. In this study, the spatial distribution patterns of Cx26 in SCs were observed, and based on these observations different spatial Cx26-null mouse models were established in order to determine the effect of changes in the spatial distribution of Cx26 in SCs on the survival of OHCs. At postnatal day (P)3, unlike the synchronous expression of Cx26 along both longitudinal and radial boundaries of most types of SCs, Cx26 expression was primarily observed along the longitudinal boundaries of rows of Deiter's cells (DCs). From P5 to P7, radial expression of Cx26 was gradually observed between adjacent rows of DCs. When Gjb2 gene was knocked out at random in different types of SCs, about 40% of the total DCs lost Cx26 expression and these Cx26-null DCs were distributed randomly in all three rows of DCs. The mice in this randomly Cx26-null group showed normal hearing and no significant OHC loss. When using a longitudinal KO pattern to induce knockout of Gjb2 gene specifically in the third row of DCs, about 33% of the total DCs lost Cx26 expression in this specific longitudinally Cx26-null group. The mice in this group showed late-onset hearing loss and significant OHC loss, however, the morphology of corresponding DCs was slightly altered. In both experimental groups, no substantial DC loss was observed. These results indicate that longitudinal Cx26-based channels are predominant in DCs during P3-P5. The Cx26 expression along rows of DCs might play a key role in the survival of OHCs, but this longitudinal KO pattern in DCs has a limited effect on DC survival or on its postnatal development.