Investigations into the role of the ErbB4 receptor in cardiomyocyte hypertrophy and adult cardiac function

Abstract

ErbB receptors (ErbB1 -ErbB4) are a subfamily of tyrosine kinase receptors that regulate cell proliferation and differentiation. It has been proposed that the ErbB1 subtype is transactivated by Ang II to mediate cardiac hypertrophy. However, whether other ErbB receptors, in particular the abundant subtype ErbB4, are involved in this process is not known. ErbB4 has four isoforms due to alternative splicing, each of which might play a distinct role in regulating cell activity. However, the role of individual ErbB4 isoforms in hypertrophic signalling has not been investigated. In addition, ErbB4 activation is critical for cardiac development, cardiomyocyte survival in various rodent models of cardiovascular pathologies. However, the physiological role of ErbB4 in the adult heart remains poorly understood. The overall aim of my PhD project is to examine the role of the ErbB4 receptor in mediating hypertrophic growth of cardiomyocytes in vitro, and maintenance of the adult heart in vivo.To investigate the role of ErbB1, ErbB2 and ErbB4 in mediating hypertrophy, I inhibited individual ErbB receptors in primary neonatal rat ventricular cardiomyocytes using RNA interference or a pharmacological inhibitor (AG1478). Hypertrophy induced with Ang II (100 nM) or NRG1 (10 nM) was assessed by measuring the promoter activity of hypertrophic genes, ERK1/2 activation, and hypertrophic growth. The NRG1-induced hypertrophy was reduced by down-regulation of Following the studies above, I investigated whether the different ErbB4 isoforms had any functional differences in the cardiomyocytes. Irrespective of any changes in total ErbB4 mRNA levels, expression of the non-cleavable JM-b isoform was always predominant in adult heart in both physiological and pathological conditions. Although the cleavable isoform JM-a was detectable, it is not cleaved in cardiomyocytes. I replaced the endogenous ErbB4 with exogenous individual isoform in cardiomyocytes and found that all four isoforms of ErbB4 could mediate the NRG1-induced hypertrophic signalling. This suggests that the hypertrophy is triggered by a common feature of the four isoforms (ostensibly the kinase activity), and appears independent of isoformspecific features, such as the cleavable domain.To investigate the physiological function of ErbB4 in adult heart, we adopted the tamoxifeniii inducible αMHC-MerCreMer/loxP system to induce ErbB4 deletion from cardiomyocytes in the adult mouse. The expression of ErbB4 was reduced by ~ 90% at 10 days after tamoxifen treatment. Echocardiography revealed no differences in cardiac function (fractional shortening) betweenErbB4-conditional knockout (ErbB4-cKO) and control groups at 3-4 months after deletion of ErbB4. However, the heart weight was increased in ErbB4-cKO animals. Interestingly, there is no change in cardiac structure (cardiomyocyte size and cardiac fibrosis) or the expression of genes associated with pathological hypertrophy. This suggests that the cardiac hypertrophy observed following ErbB4 deletion may be physiological, a...

Analysis of regulatory mechanism after ErbB4 gene mutation based on local modeling methodology

“…The protein functions in the maintenance of the heart, central nervous system, intestinal tract, stomach, and mammary tissue. Wang (2015) found that activated ErbB4 was necessary for heart development and cardiac muscle cells; Almohazey and Frey (2015) and Takahashi et al (2015) showed that ErbB4 gene mutations were closely linked with mental illness such as schizophrenia, bipolar disorder, and amyotrophic lateral sclerosis. Schumacher et al (2015) suggested that ErbB4 receptor tyrosine kinase could cause lesions in the colon epithelium in inflammatory bowels.…”