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First published online 30 March 2005
doi: 10.1242/dev.01803
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1 Departament de Genética, Facultat de Biologia, Universitat de
Barcelona, Avinguda Diagonal, 645, E-08028, Barcelona, Spain
2 Departament de Ciéncies Médiques Bàsiques, Facultat de
Medicina, Universitat de Lleida, Avinguda Rovira Roure 44, E-25198, Lleida,
Spain
Author for correspondence (e-mail:
jordigarcia{at}ub.edu)
Accepted 24 February 2005
Neurotrophins (Nt) and their tyrosine kinase Trk receptors play an
essential role in the development and maintenance of the complex vertebrate
nervous system. Invertebrate genome sequencing projects have suggested that
the Nt/Trk system is a vertebrate innovation. We describe the isolation and
characterisation of the amphioxus Trk receptor, AmphiTrk. Its
ancestral link to vertebrate Trk receptors is supported by phylogenetic
analysis and domain characterisation. The genomic structure of
AmphiTrk strongly suggests that a ProtoTrk gene emerged by
means of exon-shuffling prior to the cephalochordate/vertebrate split. We also
examined the physiological response of AmphiTrk to vertebrate neurotrophins,
and found that despite 500 million years of divergence, AmphiTrk transduces
signals mediated by NGF, BDNF, NT3 and NT4. Markedly, AmphiTrk is able to
activate survival and differentiation pathways, but fails to activate the
PLC
pathway, which is involved in synaptic plasticity in higher
vertebrates. AmphiTrk is expressed during amphioxus embryogenesis in
sensory neural precursors in the epidermis, which possesses single migratory
cells. We propose that the duplication and divergence of the Nt/Trk system, in
tandem with recruitment of the PLC
pathway, may have provided the
genetic basis for a key aspect of vertebrate evolution: the complexity of the
nervous system.
Key words: Amphioxus, Exon shuffling, Vertebrate transition, Nervous system, Neurotrophic activity
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