Low affinity nerve growth factor receptor p75NTR is highly increased in human left atria during atrial fibrillation

JZ Černe2, K Geršak2, M Šoštarič1, T Mele3, DM Jurič3, G Drevenšek3, B Geršak1. 1University Clinical Centre Ljubljana, Department of Cardiovascular Surgery, Slovenia, 2University Clinical Centre Ljubljana, Department of Obstetrics and Gynaecology, Slovenia, 3University of Ljubljana, Faculty of Medicine, Institute of Pharmacology and Experimental Toxicology, Slovenia

Atrial fibrillation is the most common sustained arrhythmia, characterized by rapid, irregular and uncoordinated activation of the atria. It is often triggered by ectopic foci and it eventually leads to atrial electric and structural remodelling. Neurotrophins has recently been recognized for their regulative role in remodelling of cardiovascular system and participate in cardiac myocyte survival, angiogenesis, hypertrophy and are probably one of the potential therapeutic targets in arrhythmia. Considering their involvement in atrial fibrillation occurrence, we aimed to identify the role of neurotrophins and gene expression in human atrial tissue related to arrhythmia.

National Medical Ethics Committee approved the research (No. 186/12/08). We obtained serum and part of auricula samples from 34 patients with valvular disease during the heart surgery. The first group consist of 19 patients, suffering paroxysmal or chronic atrial fibrillation whereas in the control group 15 patients were involved with regular sinus heart rhythm. Tissue samples were dissected from the atrial incision area due to introduction of the extracorporal circulation. Samples were sunk in liquid nitrogen or into RNAlater. We quantified nerve growth factor (NGF) and neurotrophin-3 (NT-3) protein levels using enzyme immunoassay (ELISA) while gene expression levels of neurotrophins NGF and NT-3 and neurotrophin receptors p75NTR, TrkA and TrkB was determined by qRT-PCR. Statistical significance was evaluated by two-tailed, unpaired Student’s t-test or Mann-Whitney test.

Serum levels of NGF or NT-3 from patients with atrial fibrillation and without signs of arrhythmia revealed no significant differences. Measuring neurotrophin protein concentration in homogenates of human atria confirmed for the first time the presence of significant amounts of all three neurotrophins. Basal protein levels of the studied neurotrophins were comparable between left and right atria of patients without arrhythmia and did not significantly change in the presence of paroxysmal or chronic atrial fibrillation. Gene expression analysis, expressed as ratio of protein mRNA vs. PPIA mRNA levels, confirmed the expression of NT-3 and NGF, and all three receptors in atrial samples. The expression of NGF (1.04 ± 0.04) and p75NTR (1.13 ± 0.19) in the right atria of patients with regular rhythm was significantly higher (2-fold and 11-fold, respectively) when compared to the values in left atria (0.55±0.04 and 0.19±0.01). Atrial fibrillation strongly affected the expression of p75NTR in the left atria, and its expression was greatly increased (7.17 ± 1.06, by 6.5 fold). Expression level of TrkA in the right atria in the tissues from patients suffering atrial fibrillation was reduced (0.73±0.08) by 1.6-fold, whereas the expression of TrkB (1.52±0.25) was increased by 1.4-fold. There were no changes in gene expression of neurotrophins NGF and NT-3 in the tissues from the patients suffering atrial fibrillation.

Trk receptors in nervous tissues promote survival or growth of the cells and p75NTR promotes death of the cells. In atrial fibrillation gene expression of TrkA in right atria was decreased, indicating decreased cell survival, and p75NTR was increased, indicating facilitated cell death. Both processes probably influence myocardial remodelling and heart nerve impulse conduction in the heart, suffering from atrial fibrillation.