Balancing cell numbers during organogenesis: Six1a differentially affects neurons and sensory hair cells in the inner ear

Abstract

While genes involved in the differentiation of the mechanosensory hair cells and the neurons innervating them have been identified, genes involved in balancing their relative numbers remain unknown. Six1a plays a dual role by promoting hair cell fate while inhibiting neuronal fate in these two lineages. Genes homologous to six1a act as either transcriptional activators or repressors, depending on the partners with which they interact. By assaying the in vivo and in vitro effects of mutations in presumptive protein-protein interacting and DNA-binding domains of Six1a, we show that, in the developing zebrafish inner ear, Six1a promotes hair cell fate by acting as a transcriptional activator and inhibits neuronal fate by acting as a transcriptional repressor. We also identify several potential partners for Six1a that differ between these two lineages. The dual role of Six1a in the developing otocyst provides a mechanism for balancing the relative number of hair cells and neurons during organogenesis of the inner ear.

MicroRNA-30a regulates zebrafish myogenesis via targeting the transcription factor Six1

“…In addition to contributing to early muscle development, six1b (formerly six1a) participates in both inner ear and pituitary development in zebrafish embryos (Bricaud and Collazo, 2011;Pogoda and Hammerschmidt, 2009), with analogous functions for six1a currently not reported. In the inner ear, six1b has divergent roles, as it promotes hair cell fate but inhibits neuronal fate (Bricaud and Collazo, 2011).…”

“…In the inner ear, six1b has divergent roles, as it promotes hair cell fate but inhibits neuronal fate (Bricaud and Collazo, 2011). Additional tissues in which SIX1 plays an important embryonic role have been identified in several species, including several embryonic placodes and the kidney, amongst others (Brugmann et al, 2004;Grifone et al, 2005;Ikeda et al, 2007;Laclef et al, 2003b;Sato et al, 2010).…”

Pa2G4 is a novel Six1 co-factor that is required for neural crest and otic development

“…Each well of a 6-well plate was transfected with 100 ng of a reporter of Six1/SO transcriptional activity, ARE-luciferase (Silver et al, 2003; Bricaud and Collazo, 2011) and 10 pg of a normalizer plasmid containing Renilla luciferase under the control of a CMV promoter (pRL-CMV). The same assay was performed with a Tfap2α -luciferase reporter that we constructed; 2500 bp of Xenopus laevis genomic sequence upstream of the Tfap2α transcription start site (Karpinka et al, 2015; Vize and Zorn, 2016) was cloned by PCR upstream of the luciferase sequence in the pGl3 plasmid (Promega) using J-Strain Xenopus laevis genomic DNA (National Xenopus Resource; Pearl et al, 2012) and the In-Fusion Kit (Clontech).…”

“…Six1 loss-of-function in Xenopus and chick results in reduced expression of several placode genes and defects in otic development (Brugmann et al, 2004; Christophorou et al, 2009; Schlosser et al, 2008). Six1 knock-down in zebrafish results in loss of inner ear hair cells (Bricaud and Collazo, 2006, 2011), and Six1 -null mice show defects in the olfactory placode, inner ear and cranial sensory ganglia (Chen et al, 2009; Ikeda et al, 2007, 2010; Konishi et al, 2006; Laclef et al, 2003; Ozaki et al, 2004; Zheng et al, 2003; Zou et al, 2004). Mutations in SIX1 associated with BOS are not truncations leading to loss-of-function (reviewed in Moody et al, 2015).…”

Segregating neural and mechanosensory fates in the developing ear: patterning, signaling, and transcriptional control

“…Interestingly, it affects these two processes differently in the two lineages: six1 regulates proliferation in hair cell progenitors and regulates survival of neuroblasts (Bricaud and Collazo, 2006). It does so by having a different mode of transcriptional regulation in each lineage: in the sensory lineage it partners with eya proteins to activate genes involved in cell proliferation, whereas in the neuronal lineage it partners with groucho co-repressors to properly restrict the domain of neurogenesis (Bricaud and Collazo, 2011). Other evidence suggests that transcriptional events occurring very early in zebrafish ear development biases the neuronal-sensory fate decision.…”