Cloche is a bHLH-PAS transcription factor that drives haemato-vascular specification

The zebrafish cloche gene is required for the formation of most endothelial and haematopoietic cells, however, it has been difficult to isolate; this study reveals that cloche encodes a PAS-domain-containing bHLH transcription factor, and a mammalian orthologue can partially rescue cloche mutants, indicating a possible conserved role in mammals. Cloche protein identified as transcription factor The cloche gene, when mutated, leads to loss of most endothelial and haematopoietic cells in zebrafish. It has proved difficult to establish the molecular identity of the gene because it sits so near to the end of chromosomes at the telomeres. Didier Stainier and colleagues have used a genome-editing approach in the region corresponding to cloche to identify the Cloche protein as a transcription factor that is essential for the induction of endothelial fate in the developing mesoderm. This master regulator acts upstream of the endothelial-related transcription factors identified so far. The mammalian orthologue of cloche partially rescues the zebrafish mutant, indicating a possible conserved role in mammals. Vascular and haematopoietic cells organize into specialized tissues during early embryogenesis to supply essential nutrients to all organs and thus play critical roles in development and disease. At the top of the haemato-vascular specification cascade lies cloche , a gene that when mutated in zebrafish leads to the striking phenotype of loss of most endothelial and haematopoietic cells 1 , 2 , 3 , 4 and a significant increase in cardiomyocyte numbers 5 . Although this mutant has been analysed extensively to investigate mesoderm diversification and differentiation 1 , 2 , 3 , 4 , 5 , 6 , 7 and continues to be broadly used as a unique avascular model, the isolation of the cloche gene has been challenging due to its telomeric location. Here we used a deletion allele of cloche to identify several new cloche candidate genes within this genomic region, and systematically genome-edited each candidate. Through this comprehensive interrogation, we succeeded in isolating the cloche gene and discovered that it encodes a PAS-domain-containing bHLH transcription factor, and that it is expressed in a highly specific spatiotemporal pattern starting during late gastrulation. Gain-of-function experiments show that it can potently induce endothelial gene expression. Epistasis experiments reveal that it functions upstream of etv2 and tal1 , the earliest expressed endothelial and