Effects of cardiomyocyte- or myofibroblast-specific connexin 43 overexpression in the healthy and infarcted heart

Electrical coupling of cardiomyocytes (CM) in the heart is enabled by connexins (Cx), with Cx43 being the predominant isoform in the working myocardium. Cx are transmembrane proteins, that establish cell-to-cell contacts through gap junctions (GJ). GJ are crucial for the propagation of electrical excitation in the heart (Rodríguez-Sinovas et al., 2021). Following a myocardial infarction (MI), the lost CM are replaced by scar tissue resulting in prominent changes in electrical conduction in the heart and an increase in the risk of potentially life-threatening ventricular tachycardias (VT). These re-entry-based arrhythmias originate predominantly in the cardiac border zone (BZ) and/or scar area. Herein we interrogate the role of the lack or the overexpression (oe) of Cx43 in CM or myofibroblasts (mFB) for post-MI VT, in particular, whether re-expression of Cx43 in the BZ/scar area reduces the electrical vulnerability post-MI.

For this purpose we generated an inducible and conditional Cx43 knockin mouse model (Gt(ROSA)26Sor^{tm1(CAG-Cx43-P2A-mCherry)BF}) by injecting murine blastocysts with transgenic G4 ES cells (Niemann et al., 2022). Crossing our mouse model with tamoxifen-inducible Cre mice (B6.129S-Postn^{tm2.1(cre/Esr1*)Jmol}/J or A1cf^{Tg(Myh6-cre/Esr1*)1Jmk}/J) allowed us an inducible and cell specific oe of Cx43 and the two markers mCherry (mCh)/P2A in CM or mFB. With the help of digital PCR (dPCR), Western blotting (WB), and immunohistochemistry (IHC) transgene expression was analyzed in hearts of healthy and cryo-infarcted tamoxifen-treated mice. We also assessed cardiac function in transgenic and control mice using in vivo electrophysiological testing (EPU) to study VT vulnerability, echocardiography (EC) and magnetic resonance imaging (MRI) measurements were performed to examine left ventricular (lv) function under inhalation anesthesia.

Cell biological analysis of tamoxifen-treated mice confirmed CM and mFB-specific oe of Cx43 in the heart. We found that the transgenic mice did not display an overt phenotype, when tamoxifen-induction occurred after birth, but in mice with CM-specific Cx43 oe we found strong lateralization of Cx43 in CM. Interestingly, this did not affect VT vulnerability or pump function compared to controls. However, preliminary data suggest that post-MI Cx43 oe in mFB can decrease VT incidence and improves lv function in mice. IHC of heart slices provides hints for heterocellular coupling of CM and Cx43 oe mFB in the border zone and also beyond this area in the healthy myocardium.

In conclusion, we established an inducible Cx43 oe mouse model to explore the impact of Cx43 oe in CM and mFB, and its therapeutic potential as a biological post-MI anti-VT device. Indeed, mFB-specific oe of Cx43 decreases post-MI VT incidence and improves lv function. The impact of CM-specific Cx43 oe is under further investigation.