title: Neurotrophic activities of trk receptors conserved over 600 million years of evolution.
abstract: The trk family of receptor tyrosine kinases is crucial for neuronal survival in the vertebrate nervous system, however both C. elegans and Drosophila lack genes encoding trks or their ligands. The only invertebrate representative of this gene family identified to date is Ltrk from the mollusk Lymnaea. Did trophic functions of trk receptors originate early in evolution, or were they an innovation of the vertebrates? Here we show that the Ltrk gene conserves a similar exon/intron order as mammalian trk genes in the region encoding defined extracellular motifs, including one exon encoding a putative variant immunoglobulin-like domain. Chimeric receptors containing the intracellular and transmembrane domains of Ltrk undergo ligand-induced autophosphorylation followed by MAP kinase activation in transfected cells. The chimeras are internalized similarly to TrkA in PC12 cells, and their stimulation leads to differentiation and neurite extension. Knock-down of endogenous Ltrk expression compromises outgrowth and survival of Lymnaea neurons cultured in CNS-conditioned medium. Thus, Ltrk is required for neuronal survival, suggesting that trophic activities of the trk receptor family originated before the divergence of molluscan and vertebrate lineages approximately 600 million years ago.
authors.affiliation: Department of Biological Chemistry, Molecular Neurobiology Group, Weizmann Institute of Science, 76100 Rehovot, Israel.
authors.forename: Gad
authors.initials: G
authors.lastname: Beck
authors.forename: David W
authors.initials: DW
authors.lastname: Munno
authors.forename: Zehava
authors.initials: Z
authors.lastname: Levy
authors.forename: Helga M
authors.initials: HM
authors.lastname: Dissel
authors.forename: Jan
authors.initials: J
authors.lastname: Van-Minnen
authors.forename: Naweed I
authors.initials: NI
authors.lastname: Syed
authors.forename: Mike
authors.initials: M
authors.lastname: Fainzilber
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D005075:Biological Evolution; D015153:Blotting, Western; D002454:Cell Differentiation / Q000502:physiology; D002470:Cell Survival / Q000502:physiology; D002478:Cells, Cultured; D006801:Humans; D007150:Immunohistochemistry; D017403:In Situ Hybridization; D008195:Lymnaea; D008969:Molecular Sequence Data; D009474:Neurons / Q000166:cytology / Q000378:metabolism*; D016133:Polymerase Chain Reaction; D020836:Protein Structure, Quaternary; D020917:Receptor, trkA / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D011993:Recombinant Fusion Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D017385:Sequence Homology
chemical_list: D011993:Recombinant Fusion Proteins; D020917:Receptor, trkA