A question of identity

The mammalian Pax genes are homologues of the Drosophila gene paired, and are a family of transcription factors harbouring a highly conserved DNA-binding paired-box domain. The Pax genes have been found to be involved in organizing numerous aspects of development, including development of the central nervous system and diverse organs such as the pancreas and the kidneys. Pax2 belongs to this family and has for a long time been known to be important in mammalian renal development (as well as other tissues).  It is expressed in the pro- and mesonephros as well as the earliest condensing metanephric mesenchyme, and thereafter in the cap mesenchyme, renal vesicles, developing nephrons and ureteric bud. The essential requirement for Pax2 in kidney development is underlined by renal agenesis in Pax2 null mice, and by human mutations that cause pathology including renal malformations. Although known to be involved in normal renal development, the function of Pax2 in different renal cell types has not been established.

Two recent papers, one in mouse and one in human iPSC-derived organoids, now begin to shed light on the function of Pax2 in the metanephric mesenchyme.

A loss of function mutation for Pax2 leads to renal agenesis, making it impossible to dissect the contribution of Pax2 to the specification of different renal cell types. To solve this, Naiman et al created a floxed conditional compound heterozygote, with a non-functional truncated second allele. By crossing to a Six2-GFP-Cre line, Pax2 was conditionally removed only in the Six2-positive population that marks a subset of the metanephric mesenchyme – the cap mesenchyme. In this system, the kidneys were severely hypoplastic and only developed a small ureteric bud tree, underlining the importance of Pax2. Perhaps surprisingly, however, the cap mesenchyme did initially form, and CM cells persisted for 2 days. It could be that Pax2 is not directly required for specifying this subset of metanephric mesenchymal cells. However, deleting Pax2 in cells that are expressing Six2 means that the nephron progenitor cells have already begun to be specified as a subset of the MM when Pax2 is removed (because Six2 is a marker of the cap mesenchyme). This leads to a kind of chicken and egg question and therefore this study does not yet answer whether or not Pax2 is required for specification of the cap mesenchymal cells.

Nevertheless, the persistence of the cap cells for some time before apparently disappearing begged the question what was happening to the cap mesenchyme cells when Pax2 was absent. The authors surmised that there were two possibilities: either they were dying, or they were changing their identity. Transdifferentiation is a process whereby cells that are already committed to a cell fate can be persuaded to take on the identity of a different cell type, without first being taken back to an earlier state. To test whether the Six2-positive cap mesenchyme cells were transdifferentiating or simply dying, Naiman et al used a LacZ-Cre reporter to lineage trace specifically the Pax2-deficient, Six2-expressing cells. What they found was that the cap mesenchymal cells in the Pax2 mutants were not simply dying: they were changing their identity towards renal interstitial cell fate. This is striking because previous work has shown that the lineage of Six2+ nephron progenitor population and the Foxd1+ stromal progenitor population are committed very soon after the onset of metanephric development. Furthermore, the authors show that in wild type metanephric mesenchyme, Pax2 and Foxd1 are usually not co-expressed. Yet, analysis of the expression pattern of the transdifferentiating cells showed that cells starting out as Six2+Foxd1-, are briefly Six2+Foxd1+ before settling into their new identities as stromal progenitors and gaining a Six2-Foxd1+ signature.

So what does this tell us about the role of Pax2 in renal development, at least in mice? Clearly, Pax2 is required for maintenance of the nephron progenitor population. Six2 is also required for maintenance of this population by repressing premature MET. Unlike Six2, however, Pax2 appears to maintain this population by repressing stromal identity. With Waddington’s epigenetic landscape for cell fate determination in mind, this means that removal of Pax2, even after the marble has rolled down the cap mesenchyme “trough” leads to cells reverting to a default identity of renal interstitium. It seems as though cap mesenchyme cells are desperate to leave their identity behind, but this is thwarted by Pax2 and Six2 as they block escape to a new identity, one stromal and one epithelial.

Whatever light has been shed by the Naiman paper, however, some questions remain about the role of Pax2 in the MM. Hot on the heels of the work by Naiman et al is another paper, demonstrating that PAX2 appears to be dispensable for nephron formation, and therefore nephron progenitor cell maintenance, in human iPS cells. Kaku et al have taken human iPS cells, knocked out PAX2 (replacing it with GFP), and used their protocol to differentiate to metanephric mesenchyme. This mesenchyme exhibited robust tubulogenesis when co-cultured with murine spinal cord, despite the absence of PAX2. Using cell type specific markers, the authors then show that this differentiated cell population is indeed metanephric mesenchyme and identify two distinct sub-populations within it: ITGa8+/PDGFRA-, and ITGa8-/PDGFRA-. PDGFRα is a cell surface receptor that is associated with renal interstitial cells and which has been identified previously by the authors as a negative selection marker for nephron progenitors. The ITGa8+/PDGFRA- population sorted from both wild type and PAX2-deleted hiPSCs-derived MM was shown to induce tubulogensis whilst the ITGa8- population did not. Proximo-distal polarity was not impaired, and glomerular structures were observed. The only phenotype they observed in their PAX2 knockout organoids was a morphological change in the parietal epithelial cells of the glomerulus.

On the one hand, Naiman et al show quite convincingly that Pax2 is essential for maintaining the cap mesenchyme population and repressing a shift towards a different identity. On the other hand, Kaku et al suggest that it might be dispensable for nephron progenitor maintenance in human cells in vitro. One possible explanation to reconcile these opposing findings is that there is a species difference. It would be interesting to see if Pax2 remains dispensable for nephron formation from mouse iPS or ES cell-derived MM. Another possibility raised by Kaku et al is that PAX8, which is thought to have some redundancy with PAX2 based on the more severe phenotype of the double Pax2/Pax8 knockout mice, might be compensating for the lack of PAX2. They show that PAX8 expression is expanded in the epithelia of their hiPSC-derived PAX2 null organoids. If this is the case then there must indeed be some species differences since PAX8 remained intact in the Naiman conditional knockout. And in humans with mutations in PAX2, the serious kidney hypoplasia seen does not support a strong compensatory role for PAX8.

Clearly, in vitro generation from iPS cells and in vivo development are very different developmental processes. Another possible explanation for the different outcomes upon removal of PAX2 is that an organoid derived from a single renal cell type is not equal to an organ developed in vivo. In the human iPSC PAX2 knockout, the protocol differentiates only MM without UB and spinal cord is used to induce tubulogenesis. Could there be a signal from the UB telling the MM to become stromal progenitors, with PAX2 repressing this signal? If so, then in the absence of UB, PAX2 would indeed be dispensable for nephron formation since the message to follow the stromal lineage would be non-existent. Or perhaps the protocol for nephron formation in vitro does not faithfully reproduce the situation in vivo; there may be compensatory mechanisms at play due to this difference and therefore extrapolation to human in vivo development should be approached cautiously.

What is clear is that the role of Pax2 in mammalian renal development in vivo is critical. Its role in maintaining the identity of the nephron progenitor population within the developing renal environment in mice is a fascinating developmental mechanism, without which the cells undergo a sort of identity crisis leading to failure of the kidney to develop. Many questions persist including possible species difference, the mechanism of repression of stromal identity by Pax2 in mice, and the role of Pax2 in the collecting duct. It will be interesting to see how the story of Pax2 unfolds and the differences between species and in vitro/in vivo development.

Melanie Lawrence

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