title: Tryptophan hydroxylase Is Required for Eye Melanogenesis in the Planarian Schmidtea mediterranea.
abstract: Melanins are ubiquitous and biologically important pigments, yet the molecular mechanisms that regulate their synthesis and biochemical composition are not fully understood. Here we present a study that supports a role for serotonin in melanin synthesis in the planarian Schmidtea mediterranea. We characterize the tryptophan hydroxylase (tph) gene, which encodes the rate-limiting enzyme in serotonin synthesis, and demonstrate by RNA interference that tph is essential for melanin production in the pigment cups of the planarian photoreceptors. We exploit this phenotype to investigate the biological function of pigment cups using a quantitative light-avoidance behavioral assay. Planarians lacking eye pigment remain phototactic, indicating that eye pigmentation is not essential for light avoidance in S. mediterranea, though it improves the efficiency of the photophobic response. Finally, we show that the eye pigmentation defect observed in tph knockdown animals can be rescued by injection of either the product of TPH, 5-hydroxytryptophan (5-HTP), or serotonin. Together, these results highlight a role for serotonin in melanogenesis, perhaps as a regulatory signal or as a pigment substrate. To our knowledge, this is the first example of this relationship to be reported outside of mammalian systems.
authors.affiliation: Howard Hughes Medical Institute and Department of Cell and Developmental Biology, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States of America.
authors.forename: Bramwell G
authors.initials: BG
authors.lastname: Lambrus
authors.affiliation: Division of Biological Sciences and Department of Physics, UC San Diego, San Diego, California, United States of America.
authors.forename: Olivier
authors.initials: O
authors.lastname: Cochet-Escartin
authors.affiliation: Howard Hughes Medical Institute and Department of Cell and Developmental Biology, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States of America.
authors.forename: Jiarong
authors.initials: J
authors.lastname: Gao
authors.affiliation: Howard Hughes Medical Institute and Department of Cell and Developmental Biology, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States of America.
authors.forename: Phillip A
authors.initials: PA
authors.lastname: Newmark
authors.affiliation: Division of Biological Sciences and Department of Physics, UC San Diego, San Diego, California, United States of America.
authors.forename: Eva-Maria S
authors.initials: EM
authors.lastname: Collins
authors.affiliation: Howard Hughes Medical Institute and Department of Cell and Developmental Biology, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States of America; Neuroscience Program, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States of America.
authors.forename: James J
authors.initials: JJ
authors.lastname: Collins
mesh_terms: D006916:5-Hydroxytryptophan / Q000494:pharmacology; D000818:Animals; D005123:Eye / Q000166:cytology / Q000187:drug effects / Q000648:ultrastructure; D055785:Gene Knockdown Techniques; D006257:Head / Q000502:physiology; D008027:Light; D008543:Melanins / Q000378:metabolism*; D009799:Ocular Physiological Phenomena; D010932:Planarians / Q000235:genetics / Q000502:physiology*; D034622:RNA Interference; D012038:Regeneration / Q000502:physiology; D012168:Retinal Pigments / Q000378:metabolism*; D012701:Serotonin / Q000494:pharmacology; D014365:Tryptophan Hydroxylase / Q000235:genetics / Q000378:metabolism*
chemical_list: D008543:Melanins; D012168:Retinal Pigments; D012701:Serotonin; D006916:5-Hydroxytryptophan; D014365:Tryptophan Hydroxylase