Molecular Characterization of Striated Muscle-Specific Gab1 Isoform as a Critical Signal Transducer for Neuregulin-1/ErbB Signaling in Cardiomyocytes

Abstract

Grb2-associated binder (Gab) docking proteins regulate signals downstream of a variety of growth factors and receptor tyrosine kinases. Neuregulin-1 (NRG-1), a member of epidermal growth factor family, plays a critical role for cardiomyocyte proliferation and prevention of heart failure via ErbB receptors. We previously reported that Gab1 and Gab2 in the myocardium are essential for maintenance of myocardial function in the postnatal heart via transmission of NRG-1/ErbB-signaling through analysis of Gab1/Gab2 cardiomyocyte-specific double knockout mice. In that study, we also found that there is an unknown high-molecular weight (high-MW) Gab1 isoform (120 kDa) expressed exclusively in the heart, in addition to the ubiquitously expressed low-MW (100 kDa) Gab1. However, the high-MW Gab1 has been molecularly ill-defined to date. Here, we identified the high-MW Gab1 as a striated muscle-specific isoform. The high-MW Gab1 has an extra exon encoding 27 amino acid residues between the already-known 3rd and 4th exons of the ubiquitously expressed low-MW Gab1. Expression analysis by RT-PCR and immunostaining with the antibody specific for the high-MW Gab1 demonstrate that the high-MW Gab1 isoform is exclusively expressed in striated muscle including heart and skeletal muscle. The ratio of high-MW Gab1/ total Gab1 mRNAs increased along with heart development. The high-MW Gab1 isoform in heart underwent tyrosine-phosphorylation exclusively after intravenous administration of NRG-1, among several growth factors. Adenovirus-mediated overexpression of the high-MW Gab1 induces more sustained activation of AKT after stimulation with NRG-1 in cardiomyocytes compared with that of β-galactosidase. On the contrary, siRNA-mediated knockdown of the high-MW Gab1 significantly attenuated AKT activation after stimulation with NRG-1 in cardiomyocytes. Taken together, these findings suggest that the striated muscle-specific high-MW isoform of Gab1 has a crucial role for NRG-1/ErbB signaling in cardiomyocytes.

Neuregulin-1 attenuates experimental cerebral malaria (ECM) pathogenesis by regulating ErbB4/AKT/STAT3 signaling

“…The neuregulin family of ligands consist of four members, neuregulin 1β (NRG-1), NRG-2, NRG-3, and NRG-4. While little is known about the biological functions of NRG-2, NRG-3, and NRG-4 [ 10 ], NRG-1 has been widely studied in stroke [ 11 , 12 ], cardiovascular diseases [ 13 , 14 ], and tumors [ 15 , 16 ]. NRG-1, a secreted trophic factor, is encoded by the neuregulin/NRG-1 gene located on the short arm of chromosome 8 [ 17 , 18 ].…”

The Role of GAB1 in Cancer

“…The GAB1 gene is located on chromosome 4q31.1 in humans [ 12 ]. Two isoforms of GAB1, lacking catalytic activity, have been described, one high molecular weight (120 kDa) expressed exclusively in the heart and one low molecular weight (100 kDa) expressed ubiquitously [ 4 , 11 , 13 ]. However, the role of different GAB1 isoforms in different cellular contexts and under different stimuli remains unknown.…”