Title: Distinct turnover pathways for connexin 43 gap junction channels and hemichannels in corneal endothelial cells
Authors: D'hondt, Catheleyne
Iyyathurai, Jegan
Welkenhuyzen, Kirsten
Himpens, Bernard
Bultynck, Geert
Issue Date: Jun-2013
Conference: International Cell Death Society location:Fuengirola date:6-9 June 2013
Abstract: Connexin (Cx) proteins form large conductance channels, mediating communication between neighboring cells via gap junctions (GJs) and hemichannels. Intercellular communication (IC) coordinates cellular responses in tissues and organs, controlling signaling, survival and cell death spreading.
Cxs have half-lives of only a few hours, and due to their fast turnover, degradation can rapidly modulate their function. Different degradation pathways including autophagy and the proteasome have been shown to control Cx43 gap-junction turnover. Yet, the contribution of the pathway for Cx43 hemichannel degradation remains largely unknown. Therefore, we studied the role of starvation-induced autophagy on Cx level and IC in bovine corneal endothelial cells (BCEC), a model system for hemichannel-mediated signaling.
In BCEC, starvation caused a time-dependent decrease of the Cx43 protein level and a concomitant reduction of the active area of the Ca2+ wave, correlating with the increase of LC3-II, an autophagic marker. Strikingly, bafilomycin A, an inhibitor of autophagic flux, only partially alleviated the starvation-induced decrease of the Cx43 protein level, suggesting that the two pools of the Cx43 protein (i.e. gap junctions and hemichannels) may have different turnover pathways, one that is mediated by autophagy and one that is independent of autophagy (e.g. proteasome). While bafilomycin A did not alleviate the starvation-induced reduction of active area, the proteasome inhibitor MG132 completely prevented the starvation-induced inhibition of the active area. Given the fact that in BCEC intercellular Ca2+-waves mainly propagate through Cx43 hemichannels, this may indicate that turnover of Cx43 hemichannels not occurs by autophagy but by the proteasome system. The fact that bafilomycin A did not affect the Ca2+-wave propagation while partially stabilizing Cx43 protein may be due to the relative minor contribution of Cx43 gap junctions to intercellular Ca2+-wave propagation in BCEC. Upon selective inhibition of Cx43 hemichannels using TAT-L2, starvation-induced inhibition of Cx43-gap junctional communication could be (partially) alleviated by bafilomycin A, but not by MG132. On the other hand, upon inhibition of Cx43 gap junction channels using Gap27, which was previously shown to mainly inhibit Cx43 gap junctions in BCEC, starvation-induced inhibition of Cx43-hemichannel communication could be (partially) alleviated by MG132, but not by bafilomycin A. These findings indicate that Cx43 hemichannels may be predominantly turned over by the proteasome and not by autophagy. The partial effect of MG132 in this case may be due to putative inhibition of Cx43 hemichannels by the Gap27 peptide.
Collectively, this set of data indicates that in BCEC Cx43 gap junctions and hemichannels follow distinct turnover pathways. Cx43 gap junctions are degraded by autophagy, while Cx43 hemichannels are degraded by the proteasome.
Publication status: published
KU Leuven publication type: IMa
Appears in Collections:Laboratory of Molecular and Cellular Signaling

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