Comparison of nephron segment composition between the zebrafish and the mouse. (A) The zebrafish embryo forms a pronephros that consists of two nephrons located on either side of the midline, which form by the 24 h post fertilization (hpf) stage and quickly undergo morphogenesis events that enable them to begin blood filtration by approximately 48 hpf. (B) Nephron composition in the mouse metanephros; note that the nephron is drawn here in a linear configuration, but it would display a folded/convoluted anatomical configuration within the native kidney. There is a striking conservation of proximal and distal segments, but the thin limb is one notable distinction between these nephron forms. Abbreviations are as follows: P = podocytes; N = neck; PCT = proximal convoluted tubule; PST = proximal straight tubule; DE = distal early; CS = Corpuscle of Stannius; DL = distal late; CD = collecting duct; TL = thin limb; TAL = thick ascending limb; MD = macula densa; DCT = distal convoluted tubule; CNT = connecting tubule.

Reverse genetic loss-of-function strategies in the zebrafish embryo. (A) Morpholino knockdown. (B) CRISPR-Cas9.

Reverse genetic gain-of-function strategies in the zebrafish embryo. (A) Overexpression through mRNA delivery. (B) Transgenic mediated overexpression.

Role of Tfap2 transcription factors in DE segment differentiation in the zebrafish pronephros. (A) trm mutants exhibit reductions (indicated by blue boxes) in the expression of transcripts that encode DE segment solute transporters, such as slc12a1. (B) Tfap2a is a requisite for discrete aspects of DE differentiation, but is not required for features such as polarity establishment and ciliogenesis. Adapted from Ref. [148].

Working model of components in the Tfap2 gene regulatory network that controls distal segment differentiation during zebrafish pronephros development. tfap2a is positioned as an intermediary factor within a cascade of the irx3-irx1a transcription factors that promote DE formation. Furthermore, tfap2a positively regulates the expression of tfap2b, and auto-regulates itself by modulating the expression of the kctd15a/b repressors. Adapted from Refs. [148,149].