Neurotrophins and their tyrosine kinase Trk receptors are central to the development of the complex vertebrate nervous system. The neurotrophin/Trk system is absent in non-vertebrates and is generally regarded as a vertebrate innovation. Benito-Gutiérrez and colleagues now show that Trk receptors originated at least 550 million years ago, before the cephalochordate/vertebrate split (see p. ). Amphioxus(lancelets) have a vertebrate-like body plan but lack true vertebrae or a complex nervous system. In their study, the researchers isolate the single amphioxus Trk receptor gene AmphiTrk and describe its ancestral link to vertebrate Trks. In cultured rat cells, AmphiTrk transduces signals mediated by mammalian neurotrophins and activates survival and differentiation pathways but not the PLCγ pathway that is involved in synaptic plasticity. The researchers suggest that the duplication and divergence of the neurotrophin/Trk system, together with recruitment of the PLCγ pathway,underlies the evolution of vertebrate neural complexity.
