After the
nuclear accidents at Chernobyl and Fukushima and due to a growing use of
radionuclides, radiation protection of the environment is a major concern.
Studies show an increased sensitivity to ionizing radiation (IR) during
development. However, there are few data on the molecular mechanisms leading to
the effects of IR on embryogenesis and moreover for chronic exposures. In this
context, the objective of this thesis is to better understand the effects of
exposure to IR on the development of zebrafish (Danio rerio), a model
organism in ecotoxicology and in biomedicine. Fertilized eggs were exposed to
gamma rays emitted by a 137Cs source at low to moderate dose rates
(from 0.005 mGy/h to 50 mGy/h), then analysed from the gastrula stage (6 hours)
to the larval stage (5 days) by an integrative approach from molecular effects
to behavioural effects.
Part of
this work has focused on studying the effects of IR at an early stage of
zebrafish development after exposure to IR. For this, a multi-omics approach
with the study of the transcriptome and the methylome has been implemented. A
transcriptomic study at the shield stage with dose rates ranging from 0.005
mGy/h to 50 mGy/h was carried out and demonstrated a moderate but significant
effect on the regulation of oxidative stress and mitochondrial activity. At 5
and 50 mGy/h, the genes involved in the development of the central nervous
system (CNS) and muscles are affected. The methylome study has revealed
hypomethylation of the promoter sequences of genes involved in the development
of the CNS and muscles, associated with a modification of gene expression.
These results on early embryonic stages, highlight potential effects on the
development of the CNS and muscles. The other part of this work focused on
assessing the effects of IR exposure at later developmental stages using a system
biology approach. For this, analyses were carried out from the molecular scale
to the phenotypic scale. At the molecular level, a multi-omics analysis of the
transcriptome and the proteome has highlighted the modulation of genes involved
in the retinoic acid pathway, the development of the CNS and muscles in embryos
and larvae exposed to 0.5; 5 and 50 mGy/h. Labelling by in-situ
hybridization confirmed the misregulation of the transcription factors her4.4
in the CNS and myog in the muscles of embryos exposed to IR. At the
tissue level, disruption of muscle myofibrils and alteration of neuromuscular
junctions are detected in embryos exposed to 5 and 50 mGy/h. In order to
assess an individual effect at phenotypic scale, the locomotor behaviour of the
larvae was assessed under stress conditions. A significant decrease in larval
motility was detected between 0.5 mGy/h and 50 mGy/h.
This work
has shown effects of IR on the development of the CNS and muscles from early
development at the molecular level. These effects are confirmed at later stages
of development. This study suggests that the molecular disturbances observed
during early development are predictive of the effects observed at later
developmental stages. In addition, this thesis work allows us to propose a
model of "Adverse Outcome Pathway" where the deregulation of the
retinoic acid pathway by exposure to IR will lead to effects on the development
of the CNS and muscles.