Targeting mTOR signaling mitigates atrial remodeling and fibrosis in a murine model of LKB1-related atrial disease

Background

Atrial fibrosis is a key driver of atrial remodeling and arrhythmias, including atrial fibrillation (AF). Cardiac-specific LKB1 knockout (csLkb1-KO) and atrial-specific AMPK KO mice develop a phenotype resembling human atrial cardiomyopathy, characterized by electrical instability and structural remodeling leading to AF as early as two weeks of age [1–5], supporting LKB1–AMPK signaling as a critical upstream regulator of atrial structural and electrical integrity. mTOR is a key downstream effector of this pathway and contributes to pathological hypertrophy, impaired autophagy, and fibrosis [6].

Purpose

To evaluate whether mTOR inhibition (everolimus) or antifibrotic therapy with pirfenidone, an inhibitor of fibrogenic pathways, attenuates atrial remodeling in csLkb1-KO mice.

Method

Six-week-old csLkb1-KO mice were compared with littermate LKB1floxed/floxed controls (WT) using ECG, echocardiography, and histology. The primary endpoint was percentage of atrial interstitial fibrosis assessed by Masson’s trichrome staining. Secondary endpoints included ECG parameters, atrial and ventricular size/function by echocardiography, and α-smooth muscle actin (α-SMA) expression, a marker of myocardial hypertrophy. The same endpoints were applied in a therapeutic study in which KO mice received placebo (n = 7), everolimus (5 mg/kg/day, n = 6), or pirfenidone (300–400 mg/kg/day, n = 5) from day 22 to day 42. Data are presented as median (95% CI). WT vs KO comparisons used Mann–Whitney U test; treatment groups were compared using Kruskal–Wallis with Dunn’s post hoc test. P<0.05 was considered statistically significant.

Results

At six weeks, KO mice showed reduced heart rate (HR), increased HR standard deviation (SD), prolonged PR interval, increased PR SD, and loss of p waves consistent with atrioventricular conduction variability and atrial arrhythmia (Figure 1A–B). Echocardiography demonstrated biatrial dilation and reduced left atrial (LA) function in KO mice (Figure 1C–D), with preserved LVEF. Histology revealed marked interstitial fibrosis in both atria (Figure 1E) and increased α-SMA expression, more pronounced in the right atrium (RA) (Figure 1F).

In the therapeutic study (Figure 2A), everolimus reduced LA length (2.5 vs 2.8 mm, p=0.02) and RA area (7.7 vs 9.4 mm², p=0.02) compared with placebo, whereas pirfenidone did not significantly modify atrial size (Figure 2C). Both everolimus and pirfenidone reduced RA fibrosis (11.5% and 10.2% vs 14.6%; p=0.02 and p=0.03, respectively) (Figure 2B). No differences were observed between treatment groups in body weight, ECG parameters, LVEF, or α-SMA expression.

Conclusion

Cardiac-specific LKB1 deletion induces atrial cardiomyopathy characterized by electrical instability, atrial dilation, and interstitial fibrosis. mTOR inhibition with everolimus attenuates atrial remodeling and reduces atrial fibrosis, supporting mTOR signaling as a therapeutic target in atrial cardiomyopathy.