The role of wingless‐type MMTV integration site family member 7Ba in zebrafish cloaca development

Abstract

The cloaca is the combined posterior exit portal for the intestinal, reproductive and urinary tracts of certain animal species, and exists in placental mammals as a primordial structure preceding development of the rectum, bladder and reproductive organs. In the zebrafish, the cloaca persists through embryonic and adult stages of development. The initial stages of zebrafish cloacal development involve fusion of the epithelial tubule of the early pronephric kidney with ventrally located, ectodermally derived cloacal cells, to form an opening to the exterior environment. Many aspects of this process are conserved both with human cloaca development, and fusion of epithelial tubules in other developmental contexts. The zebrafish cloaca therefore provides an excellent model system in which to investigate this process. In this study, we identify the origin of two discrete cellular populations during early stages of development, that give rise to distal kidney and cloacal progenitors. Previous studies have indicated that fusion of these populations to form a cloacal pore may involve programmed apoptotic cell death, Surprisingly, we demonstrate that if cell death is involved, it does not take the form of caspase dependent, apoptotic death. Interestingly, we also show a requirement for Wnt signalling in normal cloaca development. Antisense morpholino mediated knockdown of the wnt7ba gene gave a cystic cloaca phenotype with effects on expression of downstream targets, mmp9, and nfkbiaa. Pharmacological inhibition of c-Jun N-terminal kinase (JNK) gives a similar phenotype and similar effects on these downstream marker genes, suggesting an interlinked pathway. Taken together, data demonstrate a novel wnt7b mediated pathway is required for fusion of two discrete cellular populations of independent origin, in order for development of a normal, functional cloaca in the zebrafish. These observations may hold relevance for understanding development of the equivalent region in mammalian models, as well as Wnt7b mediated signaling in other contexts.