Involvement of Tbx5 in triiodothyronine’s reparative effect after pharmacological myocardial injury

Introduction In adult mammals, myocardial injury (ΜΙ) results in a permanent loss of cardiomyocytes due to their limited proliferative capacity. However, organisms like amphibians and neonatal rodents exhibit cardiac regenerative potential, suggesting that mammalian heart may retain reparative programs, silenced with maturation. Evidence implicates the transcription factor Tbx5 as a critical mediator of injury-induced regenerative responses. Similarly, acute high-dose administration of triiodothyronine (T3) has been shown to modulate cardiomyocyte cell-cycle activity and enhance aspects of cardiac repair. These findings raise the possibility that thyroid hormone (TH) signalling may intersect with Tbx5-dependent pathways to influence myocardial regeneration. Purpose Our aim was to determine whether T3 treatment modulates Tbx5 function during cardiac repair process following ΜΙ. Methods A well-defined tamoxifen-induced Tbx5Cre;Rosa26ReYFP/eYFP transgenic mouse model was used. Animals (3-4 months old) were subjected to pharmacological ΜΙ via isoproterenol (ISO) administration (3 doses of 10mg/Kg/day) (n=18). Another group was treated with T3 (50μg/kg/day) for 2 days following ISO administration (n=17). Animals treated with vehicle were used as control (n=10). Echocardiography was performed, while nuclear and cytoplasmic protein expression as well as kinase activation was assessed in the left ventricle by immunoblotting and immunohistochemistry on days 6 and 28. Results T3 improved cardiac function, as assessed by echocardiographic analysis, and restored SERCA/Phospholamban ratio. Immunohistochemistry showed reduced fibrotic tissue deposition, upon T3 treatment, after 28 days. Western blot analysis revealed differential nuclear localisation between Tbx5 (5-fold decrease) and TRα1 (3-fold increase) within the early reparative stage (day 6). Regarding kinase signalling, activation of Akt did not change, whereas T3 treatment significantly reduced p38 MAPK activation at day 6. Interestingly, in the late reparative stage (day 28), T3 treatment resulted in increased nuclear localisation of both Tbx5 and TRα1 in ISO+T3 hearts. More importantly, T3 augmented YFPTbx5 signal in cardiac tissue indicating increased transcriptional activity of Tbx5. Our in silico analysis based on recent literature indicated a possible Tbx5 feedback loop and cooperative binding of Tbx5 and TRα1 in adult mammalian cardiomyocytes. Conclusions These findings indicate that TH signalling exerts stage-specific regulatory effects on key transcriptional and signalling pathways involved in adult cardiac repair. T3 facilitates a coordinated transition from early injury-induced plasticity toward later redifferentiation and functional restoration, potentially through a Tbx5–TRα1 regulatory axis. This work positions TH signalling as a modulatory mechanism capable of re-engaging latent Tbx5-dependent regenerative pathways in the adult mammalian heart.