Eyal and Six1 are essential for early steps of sensory neurogenesis in mammalian cranial placodes

Eya1 encodes a transcriptional co-activator and is expressed in cranial sensory placodes. It interacts with and functions upstream of the homeobox gene Six1 during otic placodal development. Here, we have examined their role in cranial sensory neurogenesis. Our data show that the initial cell fate determination for the vestibuloacoustic neurons and their delamination appeared to be unaffected in the absence of Eya1 or Six1 as judged by the expression of the basic helix-loop-helix genes, Neurog1 that specifies the neuroblast cell lineage, and Neurod that controls neuronal differentiation and survival. However, both genes are necessary for normal maintenance of neurogenesis. During the development of epibranchial placode-derived distal cranial sensory ganglia, while the phenotype appears less severe in Six1 than in Eya1 mutants, an early arrest of neurogenesis was observed in the mutants. The mutant epibranchial progenitor cells fail to express Neurog2 that is required for the determination of neuronal precursors, and other basic helix-loop-helix as well as the paired homeobox Phox2 genes that are essential for neural differentiation and maintenance. Failure to activate their normal differentiation program resulted in abnormal apoptosis of the progenitor cells. Furthermore, we show that disruption of viable ganglion formation leads to pathfinding errors of branchial motoneurons. Finally, our results suggest that the Eya-Six regulatory hierarchy also operates in the epibranchial placodal development. These findings uncover an essential function for Eya1 and Six1 as critical determination factors in acquiring both neuronal fate and neuronal subtype identity from epibranchial placodal progenitors. These analyses define a specific role for both genes in early differentiation and survival of the placodally derived cranial sensory neurons.