osr1 Is Required for Podocyte Development Downstream of wt1a

Abstract

Odd-skipped related 1 (Osr1) encodes a zinc finger transcription factor required for kidney development. Osr1 deficiency in mice results in metanephric kidney agenesis, whereas knockdown or mutation studies in zebrafish revealed that pronephric nephrons require osr1 for proximal tubule and podocyte development. osr1-deficient pronephric podocyte progenitors express the Wilms' tumor suppressor wt1a but do not undergo glomerular morphogenesis or express the foot process junctional markers nephrin and podocin. The function of osr1 in podocyte differentiation remains unclear, however. Here, we found by double fluorescence in situ hybridization that podocyte progenitors coexpress osr1 and wt1a. Knockdown of wt1a disrupted podocyte differentiation and prevented expression of osr1. Blocking retinoic acid signaling, which regulates wt1a, also prevented osr1 expression in podocyte progenitors. Furthermore, unlike the osr1-deficient proximal tubule phenotype, which can be rescued by manipulation of endoderm development, podocyte differentiation was not affected by altered endoderm development, as assessed by nephrin and podocin expression in double osr1/sox32-deficient embryos. These results suggest a different, possibly cell-autonomous requirement for osr1 in podocyte differentiation downstream of wt1a. Indeed, osr1-deficient embryos did not exhibit podocyte progenitor expression of the transcription factor lhx1a, and forced expression of activated forms of the lhx1a gene product rescued nephrin expression in osr1-deficient podocytes. Our results place osr1 in a framework of transcriptional regulators that control the expression of podocin and nephrin and thereby mediate podocyte differentiation.

Glomerular development – Shaping the multi-cellular filtration unit

“…Osr1 knockout mice display a lack of the MM while the nephric duct is still formed [155]. Interestingly, a very recent study placed Osr1 in the context of podocyte differentiation in the zebrafish pronephros, demonstrating that Osr1 acts downstream of Wt1a regulating the expression of podocyte slit diaphragm genes Nphs1 and Podocin [156]. While not confirmed in the mouse, the study on osr1 function in zebrafish provides a novel putative transcriptional network involving Wt1→Osr1 and Lhx1 (Lim1 -a homeodomain containing transcription factor) in modulating the expression of essential podocyte genes.…”

The zebrafish kidney mutant zeppelin reveals that brca2/fancd1 is essential for pronephros development

“…During early development these groups of cells coalesce and migrate toward the midline, recruiting vasculature and forming a fused glomerulus (Drummond et al, 1998). A number of transcription factors and signaling molecules are known to be expressed in podocytes, like lhx1a and mafba , and several have been identified to play important roles in podocyte development, such as wt1a , wt1b, foxc1a, lmx1b.1, osr1 and the Notch pathway effector encoded by rbpj (Bollig et al, 2006; Bollig et al, 2009; Gerlach and Wingert, 2013; Krauss et al, 1991; He et al, 2014; O'Brien et al, 2011; Picker et al, 2002; Swanhart et al, 2010; Tomar et al, 2014; Toyama and Dawid, 1997). Further, these crucial genetic factors are associated with various renal defects and disease states.…”

Zebrafish Renal Development and Regeneration

“…Knockdown of osr1 leads to a glomerular phenotype similar to that of wt1a-deficient embryos, including fewer podocytes that remain in a progenitor state and fail to express nephrin and podocin [17,31]. Expression of wt1a is unaffected in osr1-deficient embryos, ruling out that osr1 acts as an upstream inducer of wt1a [17], but it may act downstream or parallel to wt1a and the Notch pathway [31]. Support for the latter pathway comes from Drosophila work, where the oddskipped gene family, downstream of Notch, has been implicated in promoting morphological changes associated with joint formation during leg development [39].…”

“…Knockdown of the wt1b paralog alone does affect early podocyte development, but at later stages it leads to the formation of neck cysts in some animals [30]. Embryos deficient in wt1a and wt1b seem to lack all podocytes, neck cells, and a portion of the proximal tubule [30,31]. While these data suggest that wt1a and wt1b may act redundantly during podocyte, neck, and proximal tubule development, a more detailed analysis of the doubly deficient phenotype is needed to understand the cause of these defects, given that endogenous transcripts for wt1a and wt1b have not been reported in proximal tubule progenitors.…”

“…Osr1, encoding a zinc finger transcription factor belonging to the odd-skipped family of genes, has recently been shown to play a role in zebrafish podocyte formation [31]. Knockdown of osr1 leads to a glomerular phenotype similar to that of wt1a-deficient embryos, including fewer podocytes that remain in a progenitor state and fail to express nephrin and podocin [17,31].…”