Hippo-Yap/Taz signalling in zebrafish regeneration

Abstract

The extent of tissue regeneration varies widely between species. Mammals have a limited regenerative capacity whilst lower vertebrates such as the zebrafish (Danio rerio), a freshwater teleost, can robustly regenerate a range of tissues, including the spinal cord, heart, and fin. The molecular and cellular basis of this altered response is one of intense investigation. In this review, we summarise the current understanding of the association between zebrafish regeneration and Hippo pathway function, a phosphorylation cascade that regulates cell proliferation, mechanotransduction, stem cell fate, and tumorigenesis, amongst others. We also compare this function to Hippo pathway activity in the regenerative response of other species. We find that the Hippo pathway effectors Yap/Taz facilitate zebrafish regeneration and that this appears to be latent in mammals, suggesting that therapeutically promoting precise and temporal YAP/TAZ signalling in humans may enhance regeneration and hence reduce morbidity.

Fig. 1. Summary of the Hippo pathway signalling cascade and its stimuli.

The Hippo pathway is regulated by the integration of a range of upstream stimuli. This includes mechanotransductive elements (such as caveolae and Piezo signalling), metabolism, extracellular matrix and integrin signalling, transduction of extracellular stimuli via mitogenic growth factor signalling and GPCRs, cell polarity and cell–cell contacts. Activation of the Hippo pathway triggers a phosphorylation cascade that leads to the phosphorylation of the Hippo pathway effectors YAP/TAZ. Phosphorylation of YAP/TAZ redistributes YAP/TAZ to the cytoplasm, blocking TEAD-mediated gene expression. Hippo pathway inactivation prevents YAP/TAZ phosphorylation, allowing their nuclear translocation and hence TEAD-mediated gene expression. Note that MST1/2 (mammalian STE20-like kinase1/2) are encoded by STK4/3, and TAZ by WWTR1. Figure 1 is created in BioRender.com.

Fig. 2. Similarity between selected human and zebrafish Hippo pathway genes.

Direct gene sequence comparison between a sample of human and zebrafish Hippo pathway members and transcriptional targets shows a range of similarity scores, emphasizing a high degree of similarities between fish and human genes, while also highlighting that some Hippo pathway components appear to have no direct orthologs present in both species. WWTR1 encodes TAZ. STK4 encodes MST1 and STK3 encodes MST2 (in accordance with the consensus of the Hippo pathway field). CYR61 is also known as CCN1 and CTGF as CCN2. % gene sequence similarity identified using ensembl.org under orthology tab. ctgfb, nf2b, map4k2, and rhoaa-c could not be identified as orthologues in this manner, so manual BLAST comparison of genomic sequence (from GRCz11) was performed to give the values indicated.

Fig. 3. Overview of zebrafish heart regeneration.

a Structure of the uninjured zebrafish adult heart. b Injury at the ventricle apex induces collagen and fibronectin deposition and scar formation. yap1, ctgfa, and cav-1 promote appropriate and transient scar formation. c Heart epicardium undergoes EMT and inflammatory cells (blue) infiltrate into the scar. yap1 and ctgfa inhibit inflammatory cell infiltration. d New coronary vessels form to revascularize the injury site. e Mature cardiomyocytes (CMs) (pink) dedifferentiate into progenitor cells (yellow) and migrate along the new coronary vessels into the injury site. ctgfa promotes CM migration. f CM progenitors proliferate to create a progenitor cell pool, which matures back to CMs to reform the heart muscle. ctgfa and cav-1 promote cell proliferation.

Fig. 4. Overview of zebrafish spinal cord regeneration.

a Structure of the uninjured spinal cord, with ependymal radial glia (ERG) (green) lining the central canal and motor neurons (yellow). b Spinal cord transection disrupts neuronal processes. c ERGs undergo EMT to form ERG progenitors (blue) and migrate to the site of injury. yap1 promotes EMT of ERGs, and yap1 and ctgfa promote progenitor proliferation. d ERG progenitors extend processes across the injury site to form a glial bridge (grey). yap1 and ctgfa promote the formation of the glial bridge. e Neuronal processes extend across the injury site, guided by the glial bridge to promote remodelling and reformation of the spinal cord.

Fig. 5. Overview of zebrafish tail fin regeneration (adult), focussing on osteoblast regeneration of bony rays.

a The uninjured tail fin of the adult zebrafish is formed of many bony rays, which each consist of epidermis surrounding mature osteoblasts (purple) in the mesenchyme. b Amputation of the tail fin disrupts the bony ray segment. c In the initial stages of tail fin regeneration the epidermis covers the wound. d Osteoblasts and other mature cells dedifferentiate and proliferate at the wound tip to form a blastema with osteoprogenitors (green). yap1 inhibits osteoblast dedifferentiation and bmp4 enhances blastema cell proliferation. e The bony ray segment extends through maturation of the progenitor cells back to their original cell type. yap1 promotes osteoprogenitor maturation.

Fig. 6. Overview of neuromast regeneration.

a Uninjured neuromasts consist of hair cells (green) with cilia projecting into the external liquid, support cells (blue), mantle cells (orange), and afferent sensory neurons (red) that project to the brain. b Administration of aminoglycosides or Cu²⁺ causes specific hair cell death. c Support cell proliferation increases and cells transdifferentiate into hair cells. yap1 promotes support cell transdifferentiation. d Hair cell cilia regrowth restores neuromast function.

Fig. 7. Overview of liver regeneration after minor (b, c) and severe (b’, c’, d’) injury.

a Healthy (uninjured) zebrafish liver consists of multiple cell types hepatocytes (orange) and bile ducts comprising of biliary ductal cells (green). b Minor liver injury such as partial hepatectomy removes portions of the liver and the associated cells. c Liver recovery after minor liver damage involves hypertrophy and increased proliferation of remaining cells. Yap1 promotes hepatocyte proliferation. b’ Chronic or severe liver damage causes widespread cell death and necrosis. c’ Remaining cells dedifferentiate into liver progenitor cells, promoted by Yap1. d’ Progenitor cells proliferate then differentiate into mature hepatocytes and biliary ductal cells.