Xath5 regulates neurogenesis in the Xenopus olfactory placode

Abstract

Helix-loop-helix (HLH) genes function as important regulators of neurogenesis in both the peripheral and central nervous systems. The olfactory system is an ideal tissue in which to study the role of these genes in regulating the acquisition of neuronal cell fate, particularly that of the olfactory receptor neuron (ORN). Here we describe the expression of several basic HLH (bHLH) and repeat HLH (rHLH) factors during olfactory placode development in Xenopus laevis. Our work reveals that a combination of both bHLH and rHLH genes are sequentially expressed within the nascent olfactory placode during normal development. Moreover, overexpression of the bHLH factor, Xenopus atonal homologue 5 (Xath5), promotes olfactory neural fate independent of cellular proliferation within a restricted domain at the anterior of the embryo. Collectively, our data argue that HLH genes are expressed in a cascade during olfactory placode development and that the activity of an atonal homologue, Xath5, can promote ORN fate but only in the appropriate developmental context.

Evolutionary origins of vertebrate placodes: insights from developmental studies and from comparisons with other deuterostomes

“…In contrast, the single ascidian atonal homologue is expressed in other primary epidermal sensory cells including presumptive mechanoreceptors in the tail (http://ghost.zool.kyoto-u.ac. jp/tfst.html), while the vertebrate atonal homologues regulate the differentiation of mechanoreceptive hair cells (Ath1; see above), photoreceptive retinal ganglion cells (Ath5; Kanekar et al, '97;Brown et al, 2001;Wang et al, 2001), and chemoreceptive olfactory receptor neurons (Ath5; Burns and Vetter, 2002). The single Neurogenin orthologue of the vertebrate Neurogenin genes in ascidians (Imai et al, 2004; and amphioxus is expressed in the CNS and in some ectodermal domains (rostral preoral ectoderm in amphioxus; atrial primordia in ascidians), from which sensory neurons are known to develop.…”

The Xenopus doublesex-related gene Dmrt5 is required for olfactory placode neurogenesis

“…5L). To determine whether other neuronal differentiation markers are also reduced after Dmrt5 knockdown in neuralized animal caps, we also analysed at stage 21 the effect of the depletion of Dmrt5 on the general neuronal marker Myt1 (Bellefroid et al, 1996) and Ath5 whose expression is restricted at that stage to the olfactory placodes (Kanekar et al, 1997;Burns and Vetter, 2002). Both markers were also reduced in caps derived from Dmrt5-MO injected embryos and the phenotype could be rescued by the overexpression of mDmrt5 (Suppl.…”

“…In Xenopus, the first HLH gene to be expressed in the presumptive olfactory placode is Ngnr1, followed by Ebf2, Ebf3 and Ath5 factors (Burns and Vetter, 2002). Overexpression studies in Xenopus have shown that those HLH factors can promote neurogenesis Burns and Vetter, 2002;Green and Vetter, 2011). We therefore tested whether Dmrt5 and Dmrt4 expression is regulated by those HLH proneural factors.…”

“…MT-Dmrt5, MT-mDmrt5, MT-NLSDmrt5DC and MT-NLS-Dmrt5DDM were linearized with NotI and transcribed with Sp6. Templates described previously include: MT-Ngnr1 , Noggin (Smith and Harland, 1992), MT-hGR-EBF2 and MT-hGR-EBF3 (Green and Vetter, 2011), MTAth5 (Burns and Vetter, 2002), MT-Dmrt4 (Huang et al, 2005); MT-Otx2-hGR (Gammill and Sive, 2001) and MT-hGR-Su(H)Ank-MT (Wettstein et al, 1997). The Dmrt5 antisense morpholino (5 0 -ACC ATT CAG CTC CAT TGT ACA GTT G-3 0 ) and control morpholino oligonucleotides were obtained from Genetools.…”

“…5I and data not shown). As Ebf2 is expanded within the developing olfactory placode in MT-Ath5 mRNA injected embryos (Burns and Vetter, 2002), we also asked whether Dmrt5 is required for the ability of Ath5 to expand Ebf2. Fig.…”