title: Methylmercury exposure during early Xenopus laevis development affects cell proliferation and death but not neural progenitor specification.
abstract: Methylmercury (MeHg) is a widespread environmental toxin that preferentially and adversely affects developing organisms. To investigate the impact of MeHg toxicity on the formation of the vertebrate nervous system at physiologically relevant concentrations, we designed a graded phenotype scale for evaluating Xenopus laevis embryos exposed to MeHg in solution. Embryos displayed a range of abnormalities in response to MeHg, particularly in brain development, which is influenced by both MeHg concentration and the number of embryos per ml of exposure solution. A TC50 of ~50μg/l and LC50 of ~100μg/l were found when maintaining embryos at a density of one per ml, and both increased with increasing embryo density. In situ hybridization and microarray analysis showed no significant change in expression of early neural patterning genes including sox2, en2, or delta; however a noticeable decrease was observed in the terminal neural differentiation genes GAD and xGAT, but not xVGlut. PCNA, a marker for proliferating cells, was negatively correlated with MeHg dose, with a significant reduction in cell number in the forebrain and spinal cord of exposed embryos by tadpole stages. Conversely, the number of apoptotic cells in neural regions detected by a TUNEL (terminal deoxynucleotidyl transferase dUTP nick end labeling) assay was significantly increased. These results provide evidence that disruption of embryonic neural development by MeHg may not be directly due to a loss of neural progenitor specification and gene transcription, but to a more general decrease in cell proliferation and increase in cell death throughout the developing nervous system.
authors.affiliation: Department of Applied Science, College of William and Mary, McGlothlin-Street Hall, 251 Jamestown Road, Williamsburg, VA 23185, United States.
authors.forename: Ryan W
authors.initials: RW
authors.lastname: Huyck
authors.affiliation: Department of Biology, College of William and Mary, Integrated Science Center, 540 Landrum Drive, Williamsburg, VA 23185, United States.
authors.forename: Maitreyi
authors.initials: M
authors.lastname: Nagarkar
authors.affiliation: Department of Biology, College of William and Mary, Integrated Science Center, 540 Landrum Drive, Williamsburg, VA 23185, United States.
authors.forename: Nina
authors.initials: N
authors.lastname: Olsen
authors.affiliation: Department of Biology, College of William and Mary, Integrated Science Center, 540 Landrum Drive, Williamsburg, VA 23185, United States.
authors.forename: Samuel E
authors.initials: SE
authors.lastname: Clamons
authors.affiliation: Department of Biology, College of William and Mary, Integrated Science Center, 540 Landrum Drive, Williamsburg, VA 23185, United States. Electronic address: mssaha@wm.edu.
authors.forename: Margaret S
authors.initials: MS
authors.lastname: Saha
mesh_terms: D000818:Animals; D017209:Apoptosis / Q000187:drug effects*; D049109:Cell Proliferation / Q000187:drug effects*; D004305:Dose-Response Relationship, Drug; D004625:Embryo, Nonmammalian; D047108:Embryonic Development / Q000187:drug effects; D020869:Gene Expression Profiling; D018507:Gene Expression Regulation, Developmental / Q000187:drug effects*; D008767:Methylmercury Compounds / Q000633:toxicity*; D055495:Neurogenesis / Q000187:drug effects*; D020411:Oligonucleotide Array Sequence Analysis; D009955:Ornithine Decarboxylase / Q000235:genetics / Q000378:metabolism; D018809:Proliferating Cell Nuclear Antigen / Q000235:genetics / Q000378:metabolism; D012333:RNA, Messenger / Q000378:metabolism; D014982:Xenopus laevis
chemical_list: D008767:Methylmercury Compounds; D018809:Proliferating Cell Nuclear Antigen; D012333:RNA, Messenger; D009955:Ornithine Decarboxylase; C004925:methylmercuric chloride
keywords: Methylmercury; Neurodevelopment; Teratology; Toxicity; Xenopus