Misregulation of cell adhesion molecules in the Ciona neural tube closure mutant bugeye.

Neural tube closure (NTC) is a complex multi-step morphogenetic process that transforms the flat neural plate found on the surface of the post-gastrulation embryo into the hollow and subsurface central nervous system (CNS). Errors in this process underlie some of the most prevalent human birth defects, and occur in about 1 out of every 1000 births. Previously, we discovered a mutant in the basal chordate Ciona savignyi (named bugeye) that revealed a novel role for a T-Type Calcium Channel (Cav3) in this process. Moreover, the requirement for CAV3s in Xenopus NTC suggests a conserved function among the chordates. Loss of CAV3 leads to defects restricted to anterior NTC, with the brain apparently fully developed, but protruding from the head. Here we report first on a new Cav3 mutant in the related species C. robusta. RNAseq analysis of both C. robusta and C. savignyi bugeye mutants reveals misregulation of a number of transcripts including ones that are involved in cell-cell recognition and adhesion. Two in particular, Selectin and Fibronectin leucine-rich repeat transmembrane, which are aberrantly upregulated in the mutant, are expressed in the closing neural tube, and when disrupted by CRISPR gene editing lead to the open brain phenotype displayed in bugeye mutants. We speculate that these molecules play a transient role in tissue separation and adhesion during NTC and failure to downregulate them leads to an open neural tube.