Genetic network underlying the induction and formation of cranial placodes in the Preplacodal Region

Vertebrate cranial sensory organs and their ganglia originate from thickened ectoderm called cranial placodes. Despite the cellular and functional diversity of cranial sense organs, their precursors derive from the ectoderm adjacent to the anterior neural plate region called preplacodal region (PPR). The PPR is characterized by the expression of a unique set of transcription factors referred as PPR genes, which include Foxi, ERNI, Datch, GATA, Dlx, Six, and Eya. The expression analysis of these genes does not correlate precisely with phenotypes in the PPR. Knockouts of all PPR genes do not show complete loss of any cranial placodes. However, Foxi3 and Dlx5 genes are expressed early in the PPR in complementary fashion, with Dlx5 localized anteriorly and Foxi3 localized posteriorly. In addition, mutants of Dlx gene family members show defective anterior cranial members do not show defects in the induction of cranial placodes. All these genes are considered as PPR genes because of their expression in the PPR, but not all of them contribute to PPR induction. Placodes, and Foxi3 knockout mice show defects in posterior placodes. On the other hand, Six and Eya gene family members, mentioned as definitive PPR genes in previous studies, are expressed after Foxi3 and Dlx5. Moreover, mutants of Six and Eya gene family Vertebrate cranial sense organs largely arise from ectodermal thickening, the placodes. Albeit diverse in function, precursors of cranial sensory placodes are intermingled initially and derive from a common region called preplacodal region (PPR) (Baker and Bronner-Fraser, 2001; Couly and Le Douarin, 1985; Couly and Le Douarin, 1988; D’Amico-Martel and Noden, 1983; Litsiou et al., 2005; Streit, 2004). Several transcription factors have been identified as fate determining genes for the PPR and/ or cranial placodes, but the gene regulatory network that establishes PPR is not well understood. Based on knockout studies of different PPR genes, I have selected to discuss Foxi, Dlx, Six, and Eya gene families in this review. I will refer to Foxi and Dlx as competence transcription factors (CTFs) and the Six and Eya as definitive placodal genes (DPGs). The purpose of this review is to understand the relationship of CTFs and DPGs during the induction of PPR and cranial placodes.