Abstract 3340: Endothelial Neuregulin Expression is Essential for Maintaining Capillaries in the Heart and Preserving Cardiac Function
Neuregulin (NRG)/ErbB signaling has been shown to play a prominent role in maintaining cardiac function and protecting against heart failure. Because endothelium is an important source of NRG in the heart, we assessed the role of endothelial specific NRG expression in maintaining cardiac function and myocyte viability in vivo. A mouse expressing tamoxifen-inducible Cre-recombinase under the control of the tie2 promoter with homozygously floxed NRG sites (CFF) was generated to allow regulated and endothelial-selective NRG deletion. NRG deletion was carried out by placing CFF mice on a tamoxifen diet for up to 8 weeks (n=5– 8 per group). Cre expressing animals with wild type NRG alleles (C) were also fed tamoxifen and served as controls. At baseline, C and CFF animals showed comparable left ventricular ejection fraction (EF, 57±2.33 vs. 52.6±0.78 %, NS). After 4 weeks of tamoxifen, a significant decline in EF in the CFF (knockdown) vs. the C (control) animals was observed (36.2±1.16 vs. 51.78±2.59 %, P<0.05). This was accompanied by a significant reduction in heart weight to tibial length ratio in the CFF vs. C animals (6.0±0.32 vs. 7.1±0.2 mg/mm, P<0.05) and a significant increase in TUNEL-positive cells (0.18±0.01 vs. 0.04±0.01 TUNEL-positive cells/μm2, P<0.05). Using anti-PECAM staining, we observed that 70% of the TUNEL-positive cells in the CFF animals were endothelial cells. This increase in TUNEL-positive endothelial cells resulted in a significant reduction in capillary density in the CFF vs. C animals (1.63±0.42 vs. 3.83±0.52 capillaries/μm2, P<0.05). Furthermore, endothelial NRG deletion also resulted in a significant increase in protein expression of hypoxia inducible transcription factor (Hif1α) (136.7±5.6 vs. 81.1±22.8 AU, P<0.05), indicating the presence of tissue hypoxia, and a decrease in phosphorylation of the pro-survival kinase, AKT (84.0±9.8 vs. 141.1±14 AU, P<0.01) in the CFF vs. C animals. These findings demonstrate an important role for endothelial-specific NRG expression in preserving myocardial function, maintaining endothelial survival and capillary density in the heart, and suggest that the NRG/erbB pathway may represent a novel target for treating ischemic cardiomyopathies.