1Medizinische Hochschule Hannover Molekulare und Translationale Kardiologie Hannover, Deutschland; 2Hannover Medical School Department of Pathology Hannover, Deutschland; 3Medizinische Hochschule Hannover Kardiologie und Angiologie Hannover, Deutschland
Objective: The synergistic effects of heart failure (HF) and iron deficiency (ID) lead to exercise intolerance and poor quality of life. To explore whether skeletal muscle (SkM) ID impacts cardiac adaptation to hemodynamic stress we generated gene-targeted mice with SkM-selective ID and subjected them to transverse aortic constriction (TAC).
Model: To selectively induce SkM-ID, we crossed iron-regulatory protein (Irp) 1 and 2-floxed mice (Irp1/2f/f) with mice expressing Cre recombinase under the control of the myosin light chain promoter 1. These mice (SkM-Irp1/2-KO), which developed skeletal muscle atrophy, enabled us to investigate the impact of SkM-ID on the heart independent of systemic or cardiac ID. SkM-Irp1/2-KO and Irp1/2f/f (control) mice were subjected to TAC at the age of 8-12 weeks using a 26G needle. Cardiac function was assessed by pressure-volume (PV) loop catheterization.
Results: Cardiac morphology and function were indistinguishable in SkM-Irp1/2-KO and control mice under baseline conditions. Early mortality after TAC (day 3: 32% vs. 8.1%, P=0.0018) was increased in SkM-Irp1/2-KO mice. One day after TAC, maximal cardiac contractility (dP/dtmax 7412 ± 638 vs. 11498 ± 605 mmHg/s, P˂0.001), ejection fraction (45 ± 3 vs. 58 ± 4%, P=0.02) and cardiac output (10.4 ± 0.4 vs. 13.6 ± 0.7 mL/min, P=0.001) were reduced in SkM-Irp1/2-KO mice. The number of apoptotic (TUNEL+) cells was increased in the left ventricle (LV) of SkM-Irp1/2-KO (1.3 ± 0.1% vs. 0.6 ± 0.09%, P=0.002) on day 1. One week after TAC, SkM-Irp1/2-KO mice had developed more pronounced cardiomyocyte hypertrophy (increase in cross sectional area by 112 ± 4% vs. Irp1/2f/f controls, P=0.03) and displayed more interstitial LV fibrosis (201±32% vs. Irp1/2f/f, P=0.03).
To identify the underlying mechanisms we performed a metabolomic screening of LV myocardium. This revealed a significant downregulation of acetylcholine (ACh) in the LV of SkM- Irp1/2-KO mice under baseline conditions (29±2% vs. Irp1/2f/f, P=0.008. ACh has been shown to exert protective effects in the heart under several pathological conditions, therefore we investigated the underlying mechanism for reduced cardiac acetylcholine in our model. Plasma AChE (acetylcholine degrading enzyme, acetylcholinesterase) activity was high in the Irp-targeted mice at baseline (12.2 ± 0.8 vs. 9.48 ± 0.3, P=0.0072). Higher plasma AChE activity was found to be associated with higher expression of AChE derived from M. quadriceps (123 ± 8% vs. Irp1/2f/f, P=0.026).
Mechanistically we observed AChE expression in doxorubicin treated C2C12 muscle cell line as a consequence of muscle atrophy. In addition, neo-natal rat cardiomyocytes (NRCM) treated with AChE showed higher activated caspase 3 expression compared to untreated NRCM indicating the role of AChE in apoptosis induction, as reported by various studies as well.
Conclusion: We show that SkM-selective Irp1 and 2 inactivation promotes SkM-ID and impairs cardiac adaptation to pressure overload. We identify cardiac ACh deficiency and increased AChE as a potential mechanism for SkM-aggravated cardiac dysfunction in pressure overload.
Data are presented as Mean + SEM