Neuregulin Signaling in Pieces—Evolution of the Gene Family

Paracrine and juxtacrine signaling via proteins expressed on the cell surface are an integral part of metazoan biology. More than one-half billion years ago epidermal growth factor (EGF) and its cognate receptor formed a functional binding partnership, which has been conserved through evolution in essentially all eubilaterate members of the animal kingdom. Early chordates spawned offspring of these seminal genes to begin the creation of new gene families and an expanded cell-cell signaling network, which included the Neuregulin (NRG) ligands and the erbB receptors. First appearance of ancestral NRG, represented in a NRG4-like gene in the lancelet Branchiostoma floridae, appears to have: 1) occurred in the common chordate ancestor prior to the divergence of lancelets (amphioxus), and; 2) antedated the formation of the receptor gene family. Orthologues of NRG1 and multiple erbB receptors found in the sea lamprey Petromyzon marinus suggest that several key events, which were required to expand and diversify these gene families, occurred in the common ancestor of agnathostomes and jawed vertebrates. These important inventions surely played major roles in the acquisition of multiple apomorphic features of the emerging vertebrate lineage.