Five carboxyl-terminal residues of neuregulin2 are critical for stimulation of signaling by the ErbB4 receptor tyrosine kinase.

The neuregulins (NRGs) are members of the epidermal growth factor (EGF) family of peptide growth factors. These hormones are agonists for the ErbB family of receptor tyrosine kinases, a family that includes the epidermal growth factor receptor (EGFR/ErbB1), ErbB2/Neu/HER2, ErbB3/HER3, and ErbB4/HER4. We recently observed that the EGF family hormone NRG2beta is a potent agonist for ErbB4. In contrast, NRG2alpha, a splicing isoform of the same gene that encodes NRG2beta, is a poor ErbB4 agonist. We hypothesized that carboxyl-terminal residues of NRG2beta are critical for stimulation of ErbB4 tyrosine phosphorylation and coupling to downstream signaling events. Here, we demonstrate that the substitution of a lysine residue for Phe45 in NRG2beta results in reduced ligand potency. We also demonstrate that substitution of a phenylalanine for Lys45 in NRG2alpha results in increased ligand potency. Finally, analyses of the gain-of-function NRG2alpha Chg5 mutant demonstrate that Gln43, Met47, Asn49, and Phe50 regulate ligand efficacy. Thus, these data indicate that carboxyl-terminal residues of NRG2beta are critical for activation of ErbB4 signaling. Moreover, these NRG2alpha and NRG2beta mutants reveal new insights into models for ligand-induced ErbB family receptor tyrosine phosphorylation and coupling to downstream signaling events.

Neuregulin isoforms exhibit distinct patterns of ErbB family receptor activation.

During the last decade, several novel members of the Epidermal Growth Factor family of peptide growth factors have been identified. Most prominent among these are the Neuregulins or Heregulins. To date, four different Neuregulin genes have been identified (Neuregulin1-4) and several different splicing isoforms have been identified for at least two of these genes (Neuregulin1 and Neuregulin2). While Neuregulin1 isoforms have been extensively studied, comparatively little is known about Neuregulin3, Neuregulin4, or the Neuregulin2 isoforms. Indeed, there has been no systematic comparison of the activities of these molecules. Here we demonstrate that Neuregulin2alpha and Neuregulin2beta stimulate ErbB3 tyrosine phosphorylation and coupling to biological responses. In contrast, Neuregulin3 and Neuregulin4 fail to activate ErbB3 signaling. Furthermore, Neuregulin2beta, but not Neuregulin2alpha, stimulates ErbB4 tyrosine phosphorylation and coupling to biological responses. Finally, both Neuregulin3 and Neuregulin4 stimulate modest amounts of ErbB4 tyrosine phosphorylation. However, whereas Neuregulin3 stimulates a modest amount of ErbB4 coupling to biological responses, Neuregulin4 fails to stimulate ErbB4 coupling to biological responses. This suggests that there are qualitative as well as quantitative differences in ErbB family receptor activation by Neuregulin isoforms.