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 are greatly unknown, particularly within the vertebrate lineage. To gain direct and comparable insights, we took advantage of the zebrafish larva 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 Yap1 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. Macrophage recruitment is reduced during severe tail fin regeneration after Yap1 and Taz loss, appearing earlier in yap1 than wwtr1 Crispants and correlating with defective regenerative 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.