Characterising the mode of action of Connexin43 mimetic Peptide5 to target retinal ischemia-reperfusion injury

Abstract

Purpose : Connexin43 (Cx43) mimetic Peptide5 is a 12-residue sequence (VDCFLSRPTEKT) derived from the second extracellular loop of the human Cx43 protein. We have previously demonstrated in a preclinical model of retinal ischemia-reperfusion injury that Peptide5 prevents endothelial cell loss and protects against vascular permeability. However, the molecular mode of action of Peptide5 remains unknown. We hypothesised that Peptide5 binds to the second extracellular loop (EL) of Cx43 and inhibits Cx43 hemichannel mediated ATP release that contribute to ischemia-reperfusion injury.

Methods : Human cerebral endothelial cells (hCMVEC) were exposed to a hypoxic, acidic, ion-shifted Ringer solution for 2 hours to simulate ischemia, or further exposed to 2 hours of standard ringer solution to simulate ischemia-reperfusion in vitro. ATP released from hemichannels was quantified in a bioluminescence assay. Peptide5 (100µM) was tested under ischemic and reperfusion conditions (n=6). To determine the binding site, ischemic hCMVEC cells were incubated in equimolar concentrations of Peptide5 and synthetic Cx43 EL fragments in competition assays (n=6). To identify binding motifs crucial for Peptide5 efficacy, ischemic hCMVEC cells were incubated in modified and truncated analogues of Peptide5 (100µM)(n=8). One-way ANOVA and Tukey’s statistical test was used.

Results : Peptide5 significantly reduced ATP release when compared to ischemia (73.9 ± 2.1%, p<0.0001) and ischemia-reperfusion (70.3 ± 4.4%, p=0.02) injuries. Equimolar concentrations of Peptide5 and EL1 fragments were not statistically different (p > 0.5), but EL2 fragments significantly inhibited Peptide5 function by 42.2 ± 5.9% (p < 0.0001). Modified Peptide5 with single alanine substitutions significantly increased ATP release by at least 33.3 ± 10.9% when compared to native Peptide5 (p < 0.035). Truncated Peptide5, SRPTEKT (21.5 ± 4.6%, p = 0.0002), VDCFLSRP (21.7 ± 5.9%, p = 0.0084) and VDCFLS (26.3 ± 5.1%, p < 0.0001) also showed reduced efficacy when compared to the full length Peptide5.

Conclusions : Peptide5 inhibits ATP released from open Cx43 hemichannels under ischemic-reperfusion conditions by binding to the EL2 of Cx43. Results from modified analogues suggest that Peptide5 displays tight binding specificity. An understanding of the mode of action of Peptide5 supports its development as a treatment for retinal injury and disease.

This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.