# Reciprocal inhibition of Wnt signaling pathways pattern the interconnection of epithelial tubules in the regenerating zebrafish kidney

## TL;DR

The study shows how Wnt signaling pathways work together and in opposition to guide kidney tubule connections during zebrafish kidney regeneration.

## Contribution

The novel contribution is the discovery that a single Wnt ligand can activate both canonical and non-canonical pathways in the same cells to control tubule interconnection.

## Key findings

- Canonical Wnt signaling induces invasive cell protrusions in new nephrons.
- Non-canonical Wnt signaling via frizzled9b restricts canonical Wnt and drives apical constriction.
- Wnt ligands wnt9b and wnt4 are essential for new nephron formation and tubule interconnection.

## Abstract

In the adult zebrafish kidney, nephrogenesis occurs as a regenerative response to injury and provides a model to explore cell signaling pathways required for nephron formation and engraftment. Differentiating kidney tubules interconnect with collecting system epithelia to generate a pathway for fluid excretion. We show that canonical Wnt signaling induces a mesenchymal, invasive cell phenotype and is required, along with Src kinase and Rac1, to generate basal cell protrusions on new nephrons. The Wnt ligands wnt9b and wnt4 are both required for new nephron formation after injury. Mutation in wnt4 and wnt9b, or treatment with the canonical Wnt inhibitor IWR1 blocks the formation of basal protrusions in forming nephrons. Mutation in the Wnt receptor frizzled9b reveals a fusion-associated non-canonical Wnt pathway that acts to (1) restrict canonical Wnt gene expression, (2) drive Rho kinase-dependent apical constriction of epithelial cells and (3) position basal protrusions to generate orthogonal tubule lumenal connections. Mutation in wnt9b phenocopies fzd9b mutants, indicating that a single ligand can induce canonical and non-canonical Wnt signaling in the same cells to orient and drive tubule interconnection in the regenerating zebrafish kidney.

Summary: Canonical and non-canonical Wnt signaling pathways operate simultaneously and antagonistically in the same cells to orient and drive invasive cell behavior and signal cell shape changes that facilitate tubule interconnection.