Ephrin-B1 is a ligand from Eph/Ephrin family involved in cell-cell interactions. If the role of ephrine molecules is well known in embryonic tissue, their expressions/functions in adults remain unclear and to date, no study have paid attention to their potential implication in heart physiopathology.

To characterize the cardiac phenotype of ephrin-B1 knockout mice.

Two months old-mice were analyzed. Heart tissue was fixed (formaldehyde 10 %) and embedded in paraffin for immunohistochemistry or frozen for western-blot (WB) analysis. Immunofluorescence (IF) studies were performed on heart cryosections. ECG was recorded (PowerLab, DSI) under isoflurane anaesthesia and heart rate spectral variability (HRV) was performed (FFT) in low frequency (LF: 0.15-1.5 Hz) and high frequency (HF: 1.5-5 Hz) ranges ; LH/HF ratio was also calculated.

We first assessed expression of Ephrin-B1 by WB in heart from WT animals. A specific band around 47 kDa was detected in WT heart total protein extracts that was lost in KO mice. Further IF studies demonstrated broad expression of Ephrin-B1 protein throughout all heart compartments with different cellular localizations (cardiomyocytes and micro/macrocirculation). Hematoxylin-eosin (HE) staining of paraffinembedded heart sections from KO mice revealed loss of organized cardiac tissue characterized by the presence of wavy cardiomyocytes in both septum and ventricles. Myocytes intersected at various angles with bundles wavy appearance. No inflammation, interstitial fibrosis or necrosis were noticed. These pathological observations correlated well with the lack of stiffness of hearts from KO mice compared with controls. When we examined ANS-dependent heart rate variability, LFHRV was significantly reduced in KO mice (16.3±1.2 %) when compared to controls (48.5±6.2 %) without any change in HF, suggesting a specific loss of cardiac sympathetic innervation in these animals. LF/HF ratio was lower in KO mice (0.6±0.2 vs 1.3±0.2 in controls).

This study provides the first evidence for the presence of ephrin molecules in the adult heart tissue with a specific expression of Ephrin-B1 ligand. The use of ephrin-B1 genetic mouse model highly suggests a role for ephrin-B1 in heart tissue architecture and in sympathetic control of heart rate variability.