In vivo screen reveals specific roles of Hippo pathway components in development and regeneration

The Hippo signalling pathway is a major regulator of regeneration and development. However, the comparative importance and functional roles of individual Hippo pathway components in vivo is greatly unknown, particularly within the vertebrate lineage. To gain direct and comparable insights we took advantage of the zebrafish larvae model system. We generated individual and combined CRISPR/Cas9 F0 knockouts of a range of core Hippo pathway genes, including upstream regulators, the co-transcriptional regulators Yap1/Taz, and Yap1/Taz-target genes. We analysed and compared the resulting developmental and regenerative phenotypes. Our findings highlight that paralogues of core components have distinct, but in some instances overlapping, functions. Intriguingly, we find that Yap and Taz have differential roles during development and regeneration. In addition, we characterise and compare two tail fin regenerative paradigms: after both severe and mild injury. These injury paradigms are drastically different and elicit diverse resolution processes. We confirm critical roles of the immune system in the regenerative process and identify that macrophages are recruited to lesser extent in tail fin regeneration upon Yap1 and Taz loss, suggesting defective regenerative macrophage function. This defective macrophage involvement might therefore be one of the mediators of the deficient regeneration in these two Crispants. Overall, our analysis emphasises distinct requirements and responses of the Hippo pathway during development and across different regenerative paradigms. Overall design: Bulk RNA-seq of zebrafish tail fin tissue. Samples include uninjured and injured (severe tail fin injury paradigm tissue at 3dpf) tissue, from WT and Yap or Taz Crispant fish.