Expression and function of new candidate regulators of placodal neurogenesis in Xenopus laevis

The transcription factor Six1 and its co-activator Eya1 play central and varied roles during the development of sensory neurons derived from the cranial placodes in vertebrates. Previous studies suggested that these proteins promote both the maintenance of proliferative neuronal progenitors and neuronal differentiation. Context-specific interactions of Six1 and/or Eya1 with different cofactors are likely to contribute to the activation of distinct target genes during different stages of placodal neurogenesis. However, while some protein binding partners of Six1 have recently been identified, little is known about Eya1 interaction partners. To uncover additional Eya1 interaction partners with potential roles in placodal neurogenesis, we here conduct a yeast two-hybrid screen in Xenopus laevis. Apart from confirming Six1 (Six1.L) and Six4 (Six4.L and Six4.S) as known protein interaction partners of Eya1, this screen identified 25 additional candidate binding partners including several transcription factors, chromatin modifiers and enzymes involved in posttranslational modifications. Analysis of the expression of genes encoding five of these candidates (garre1, msh6, zmym3, pias4, smarce1) during X. laevis embryogenesis by in situ hybridization, revealed extensive co-expression of each of these with eya1 during placodal development. This suggests potential roles of these candidates in the regulation of placodal neurogenesis possibly in conjunction with Eya1. In gain and loss of function studies of the E3 SUMO-ligase Pias4 and the chromatin modifier Smarce1, we confirm essential roles of both of these proteins for placodal neurogenesis but their mode of interaction with Eya1 remains to be further clarified.