Recent advances in elucidating the genetic mechanisms of nephrogenesis using zebrafish

Abstract

The kidney is comprised of working units known as nephrons, which are epithelial tubules that contain a series of specialized cell types organized into a precise pattern of functionally distinct segment domains. There is a limited understanding of the genetic mechanisms that establish these discrete nephron cell types during renal development. The zebrafish embryonic kidney serves as a simplified yet conserved vertebrate model to delineate how nephron segments are patterned from renal progenitors. Here, we provide a concise review of recent advances in this emerging field, and discuss how continued research using zebrafish genetics can be applied to gain insights about nephrogenesis.

Keywords: aPKC; hnf1ba/b; kidney development; mecom; nephron segmentation; renal progenitor; sim1a; tubulogenesis; vertebrate; zebrafish.

Figure 1

Anatomy of the zebrafish pronephros. (A) The zebrafish pronephros develops from bilateral fields of renal progenitors (purple) that emerge from the intermediate mesoderm (IM) and are located lateral to the paraxial mesoderm (PM, red) that gives rise to the somites (denoted by their number along the embryo axis respective to each nephron segment in panel B); (B) At 24 hours post fertilization (hpf), the two nephrons have formed and consist of a series of segments that include: podocytes (P, light green) that contribute to the blood filter, neck (N, dark green), proximal convoluted tubule (PCT, orange), proximal straight tubule (PST, yellow), distal early (DE, sky blue), corpuscles of Stannius (CS, red), distal late (DL, dark blue), and pronephric duct (PD, black). The numbers indicate the position of each nephron segment with respect to the somites. [Figure adapted from [45], through terms of the Creative Commons License of the Authors].

Figure 2

Tubulogenesis of the zebrafish pronephros. (A) The timing of tubulogenesis is coincident with the stages of somitogenesis and organogenesis of the embryo; (B–B”) At 24 hours post fertilization (hpf), the two nephrons have formed distinct tubule lumens that can be detected by immunofluorescence to detect green fluorescent protein (GFP), acetylated tubulin (light blue), Prkc ι/ξ (red) and nuclei (DAPI, blue); (C, D) The precise onset of tubulogenesis occurs at the 20 ss, indicated by white arrowheads, along the proximal, central, and distal regions of the nephron territory, with progressive enlargement of the luminal space at 22 and 26 ss. Abbreviations: aorta (a), lateral plate mesoderm (lpm) notochord (n), somite stage (ss) cardinal vein (v), [Figure adapted from [43,57], through terms of the Creative Commons License of the Authors].

Figure 3

Deficiency of Prkcι/ξ disrupts nephron epithelial polarity. (A) At the 22 ss, actin is mislocalized in Prkcι/ξ deficient embryos, though lumen formation eventually occurs by 36 hpf, accompanied by normal laminin distribution at the basal lamina though Na⁺/K⁺ ATPase distribution remains mislocalized at this time point. (B) Summary of phenotypes in Prkcι/ξ deficient embryos compared to wild-type embryos. Abbreviations: hours post fertilization (hpf), mesenchymal to epithelial transition (MET), morpholino (MO) somite stage (ss). [Figure adapted from Reference [57], through terms of the Creative Commons License of the Authors].

Figure 4

Model of mecom functions in nephron segmentation. mecom expression is proposed to promote DL formation and inhibit PST fates during proximodistal segment patterning. These activities act in opposition to RA signaling, which promotes PCT and PST formation and has been speculated to inhibit distal fates. Abbreviations: distal early (DE), distal late (DL), intermediate mesoderm (IM), proximal convoluted tubule (PCT), pronephric duct (PD), proximal straight tubule (PST), retinoic acid (RA). [Figure adapted from [68], through terms of the Creative Commons License of the Authors].

Figure 5

Model of sim1a functions in nephron segmentation. sim1a is necessary to promote PST and CS formation and inhibit PCT fates during segment patterning in the zebrafish pronephros. These activities occur downstream of RA signaling, which promotes PCT and PST formation and expression of sim1a in these respective renal progenitor domains. Further, RA signaling inhibits sim1a expression in CS progenitors, placing it as a negative regulator upstream of sim1a during establishment of the CS lineage. Abbreviations: corpuscle of Stannius (CS), distal early (DE), distal late (DL), neck (N), podocytes (P), proximal convoluted tubule (PCT), pronephric duct (PD), proximal straight tubule (PST), retinoic acid (RA). [Figure adapted from [45], through terms of the Creative Commons License of the Authors].