A protocadherin mediates cell-cell adhesion and integrity of the oral placode in the tunicate Ciona

In chordate embryos, placodes are ectodermal thickenings around the borders of the neural plate that give rise to various sensory organs and cell types. While generally thought to be a vertebrate-specific innovation, homologous placodes are proposed to exist in non-vertebrate chordates as well. In Ciona robusta, a solitary tunicate, the adult mouth (the oral siphon) is derived from one such "cranial-like" placode in the larva, which we term the oral siphon placode (OSP). At embryonic and larval stages, the OSP consists of a small rosette of cells that forms from the neuropore at the anteriormost extent of neural tube closure. While the morphogenesis of the OSP and its physical separation from other surface ectoderm structures have been described in detail, how this is regulated at the molecular level is currently unknown. Here we show the involvement of protocadherin-mediated cell-cell adhesion in the segregation and structural cohesiveness of the OSP. Protocadherin.e (Pcdhe.e) is expressed specifically in the OSP but not in other surface ectoderm cells. CRISPR/Cas9-mediated disruption of Pcdh.e in these cells results in loss of OSP structural integrity and ability to physically separate from other structures derived from the same cell lineage. Overexpression of Pcdh.e throughout the anterior surface ectoderm results in similar loss of a physically separate and distinct OSP territory. Furthermore, we show that Pcdh.e expession in the OSP depends on oral placode-specific transcription factors such as Six1/2 and Pitx. Our results suggest that OSP integrity and morphogenesis require precise regulation of a homotypic cell-cell adhesion molecule, which might reflect a conserved mechanism for placode formation in chordates.