DOMAIN-SPECIFIC GENE DISRUPTION REVEALS CRITICAL REGULATION OF NEUREGULIN SIGNALING BY ITS CYTOPLASMIC TAIL

Neuregulins are a multi-isoform family of growth factors that activate members of the erbB family of receptor tyrosine kinases, The membrane -anchored isoforms contain the receptor-activating ligand in their ext racellular domain, a single membrane-spanning region, and a long cytop lasmic tail, To evaluate the potential biological role of the intracel lular domain of the membrane-anchored neuregulin isoforms, we used a d omain-specific gene disruption approach to produce a mouse line in whi ch only the region of the neuregulin gene encoding almost the entire i ntracellular domain was disrupted. Consistent with previous reports in which all neuregulin isoforms were disrupted, the resulting homozygou s neuregulin mutants died at E10.5 of circulatory failure and displaye d defects in neural and cardiac development, To further understand the se in vivo observations, we evaluated a similarly truncated neuregulin construct after transient expression in COS-7 cells. This cytoplasmic tail-deleted mutant, unlike wild-type neuregulin isoforms, was resist ant to proteolytic release of its extracellular-domain ligand, a proce ss required for erbB receptor activation. Thus proteolytic processing of the membrane-bound neuregulin isoforms involved in cranial ganglia and heart embryogenesis is likely developmentally regulated and is cri tically controlled by their intracellular domain, This observation ind icates that erbB receptor activation by membrane-bound neuregulins mos t likely involves a unique temporally and spatially regulated ''inside -out'' signaling process that is critical for processing and release o f the extracellular-domain ligand.