Wnt/β-catenin signaling regulates cardiac Cx43 in a metabolic substrate-dependent manner

Background

Both Na_v1.5 and Cx43 are critical for the fast electrical impulse conduction in the myocardium and their reductions create the arrhythmogenic substrate. Wnt/β-catenin signaling is activated in arrhythmogenic myocardium, and although this signaling is known to downregulate cardiac Na_v1.5, its regulation of Cx43 is unclear as conflicting results have been reported. The present study investigated how Wnt/β-catenin signaling regulates Cx43 in rat and human cardiomyocytes and if it is dependent on the sex of the cells or the metabolic substrates.

Methods

Male and female neonatal rat ventricular myocytes (NRVMs) were treated with CHIR-99021 (CHIR) or Wnt3a protein, two different activators of the Wnt/β-catenin signaling, either in a medium rich in glucose (a preferred metabolic substrate in heart failure) or in a medium rich in lipid (∼150 μM fatty acid, a preferred substrate in healthy hearts). Both healthy and Brugada Syndrome human iPSC-derived cardiomyocytes (iPSC-CMs) were used to confirm observations in NRVMs.

Results

When maintained in a glucose-rich medium, Gja1 mRNA (encoding Cx43) was reduced by a low concentration of CHIR (1 μM) in female NRVMs but only at a high concentration of CHIR (10 μM) in male NRVMs. However, reductions in Cx43 protein were observed at 1 μM CHIR in both male and female NRVMs, suggesting the involvement of both transcriptional and post-transcriptional mechanisms. When maintained in a lipid-rich medium, neither Gja1 mRNA nor Cx43 protein was altered by CHIR at 1 or 3 μM. In contrast, CHIR-induced reductions in Scn5a mRNA and Na_v1.5 protein were observed in both glucose-rich and lipid-rich media, with no significant sex-specific differences detected. Consistent with studies using CHIR, which is a Wnt receptor-independent activator, Wnt3a protein also reduced both Gja1 mRNA and Cx43 protein in NRVMs in the glucose-rich medium but not in the lipid-rich medium. In human iPSC-CMs from two healthy volunteers and one Brugada Syndrome patient, Wnt/β-catenin signaling activation reduced GJA1 mRNA and Cx43 protein in a standard, glucose-containing medium.

Conclusions

These data demonstrate that metabolic substrates regulate the effects of Wnt/β-catenin signaling in cardiomyocytes, with reductions in Cx43 mRNA and protein only observed when glucose is the primary metabolic substrate, which occurs in arrhythmogenic conditions such as cardiac hypertrophy and heart failure.