Valproic Acid silencing ofascl1b/ascl1results in the failure of serotonergic differentiation in a zebrafish model of Fetal Valproate Syndrome

Abstract

Fetal valproate syndrome (FVS) is caused by in utero exposure to the drug sodium valproate. Valproate is used worldwide for the treatment of epilepsy, as a mood stabiliser and for its pain-relieving properties. In addition to birth defects, FVS is associated with an increased risk of autism spectrum disorder (ASD), which is characterised by abnormal behaviours. Valproate perturbs multiple biochemical pathways and alters gene expression through its inhibition of histone deacetylases. Which, if any, of these mechanisms is relevant to the genesis of its behavioural side effects is unclear. Neuroanatomical changes associated with FVS have been reported and, among these, altered serotonergic neuronal differentiation is a consistent finding. Altered serotonin homeostasis is also associated with autism. Here we have used a chemical-genetics approach to investigate the underlying molecular defect in a zebrafish FVS model. Valproate causes the selective failure of zebrafish central serotonin expression. It does so by downregulating the proneural gene ascl1b, an ortholog of mammalian Ascl1, which is a known determinant of serotonergic identity in the mammalian brainstem. ascl1b is sufficient to rescue serotonin expression in valproate-treated embryos. Chemical and genetic blockade of the histone deacetylase Hdac1 downregulates ascl1b, consistent with the Hdac1-mediated silencing of ascl1b expression by valproate. Moreover, tonic Notch signalling is crucial for ascl1b repression by valproate. Concomitant blockade of Notch signalling restores ascl1b expression and serotonin expression in both valproate-exposed and hdac1 mutant embryos. Together, these data provide a molecular explanation for serotonergic defects in FVS and highlight an epigenetic mechanism for genome-environment interaction in disease.

Zebrafish as a systems toxicology model for developmental neurotoxicity testing

“…They found that the index could distinguish between neurotoxic and non-neurotoxic chemicals. Recently, Jacob et al used the antibody to label serotonergic neurons and revealed that serotonergic differentiation failed in a zebrafish model of fetal valproate syndrome (Jacob et al 2014), which is highly consistent with rodent models of fetal valproate syndrome (Miyazaki et al 2005;Kuwagata et al 2009;Oyabu et al 2013).…”

“…used the antibody to label serotonergic neurons and revealed that serotonergic differentiation failed in a zebrafish model of fetal valproate syndrome (Jacob et al. ), which is highly consistent with rodent models of fetal valproate syndrome (Miyazaki et al. ; Kuwagata et al.…”

Developmental profiling of ASD-related shank3 transcripts and their differential regulation by valproic acid in zebrafish

“…VPA is a typical example of HDAC inhibitor that was recently found to have teratogenic (interfering with early development) and neuropsychiatric side effects related to in utero exposure (Jacob et al, 2014). Moreover, fetal VPA exposure has been associated with a 3- to 46-fold increased risk of ASD (Dufour-Rainfray et al, 2010; Bromley et al, 2013; Christensen et al, 2013; Wood, 2014).…”

Valproic acid, a histone deacetylase inhibitor, regulates cell proliferation in the adult zebrafish optic tectum

“…Other studies have shown that HDAC1 mutant zebrafish as well as those treated with antimorpholino oligo against HDAC1 exhibited impaired neurogenesis in the hindbrain (Cunliffe, 2004;Harrison et al, 2011;Lightman et al, 2011;Jacob et al, 2014) and retina (Yamaguchi et al, 2005). Studies have also revealed that HDAC1 promotes the transformation of neural progenitors into differentiated neurons and glia (Cunliffe, 2004;Yamaguchi et al, 2005;Harrison et al, 2011;Lightman et al, 2011;Jacob et al, 2014). Mice lacking HDAC1 and HDAC2 have exhibited impaired migration of neural precursors during brain development (Montgomery et al, 2009).…”

“…Studying the retina of HDAC1 mutant zebrafish, Yamaguchi et al (2005) showed that HDAC1 antagonizes Notch as well as Wnt pathways to promote cell-cycle exit and subsequent inhibited neurogenesis in the zebrafish retina. HDAC1 has also been shown to be necessary for the repression of Notch target gene expression in the hindbrain of embryonic zebrafish (Cunliffe, 2004;Harrison et al, 2011;Lightman et al, 2011;Jacob et al, 2014).…”