Sayoko Fujimura

The murine homolog of SALL4, a causative gene in Okihiro syndrome, is essential for embryonic stem cell proliferation, and cooperates with Sall1 in anorectal, heart, brain and kidney development

Mutations in SALL4, the human homolog of the Drosophila homeotic gene spalt (sal), cause the autosomal dominant disorder known as Okihiro syndrome. In this study, we show that a targeted null mutation in the mouse Sall4 gene leads to lethality during peri-implantation. Growth of the inner cell mass from the knockout blastocysts was reduced, and Sall4-null embryonic stem (ES) cells proliferated poorly with no aberrant differentiation. Furthermore, we demonstrated that anorectal and heart anomalies in Okihiro syndrome are caused by Sall4 haploinsufficiency and that Sall4/Sall1 heterozygotes exhibited an increased incidence of anorectal and heart anomalies, exencephaly and kidney agenesis. Sall4 and Sall1 formed heterodimers, and a truncated Sall1 caused mislocalization of Sall4 in the heterochromatin; thus, some symptoms of Townes-Brocks syndrome caused by SALL1 truncations could result from SALL4 inhibition.

Sall4 Is Essential for Stabilization, But Not for Pluripotency, of Embryonic Stem Cells by Repressing Aberrant Trophectoderm Gene Expression

Sall4 is a mouse homolog of a causative gene of the autosomal dominant disorder Okihiro syndrome. We previously showed that the absence of Sall4 leads to lethality during peri‐implantation and that Sall4‐null embryonic stem (ES) cells proliferate poorly with intact pluripotency when cultured on feeder cells. Here, we report that, in the absence of feeder cells, Sall4‐null ES cells express the trophectoderm marker Cdx2, but are maintained for a long period in an undifferentiated state with minimally affected Oct3/4 expression. Feeder‐free Sall4‐null ES cells contribute solely to the inner cell mass and epiblast in vivo, indicating that these cells still retain pluripotency and do not fully commit to the trophectoderm. These phenotypes could arise from derepression of the Cdx2 promoter, which is normally suppressed by Sall4 and the Mi2/NuRD HDAC complex. However, proliferation was impaired and G1 phase prolonged in the absence of Sall4, suggesting another role for Sall4 in cell cycle control. Although Sall1, also a Sall family gene, is known to genetically interact with Sall4 in vivo, Sall1‐null ES cells have no apparent defects and no exacerbation is observed in ES cells lacking both Sall1 and Sall4, compared with Sall4‐null cells. This suggests a unique role for Sall4 in ES cells. Thus, though Sall4 does not contribute to the central machinery of the pluripotency, it stabilizes ES cells by repressing aberrant trophectoderm gene expression. STEM CELLS 2009;27:796–805

Kif26b, a kinesin family gene, regulates adhesion of the embryonic kidney mesenchyme.

The kidney develops through reciprocal interactions between two precursor tissues: the metanephric mesenchyme and the ureteric bud. We previously demonstrated that the zinc finger protein Sall1 is essential for ureteric bud attraction toward the mesenchyme. Here, we show that Kif26b, a kinesin family gene, is a downstream target of Sall1 and that disruption of this gene causes kidney agenesis because of impaired ureteric bud attraction. In the Kif26b-null metanephros, compact adhesion between mesenchymal cells adjacent to the ureteric buds and the polarized distribution of integrin α8 were impaired, resulting in failed maintenance of Gdnf, a critical ureteric bud attractant. Overexpression of Kif26b in vitro caused increased cell adhesion through interactions with nonmuscle myosin. Thus, Kif26b is essential for kidney development because it regulates the adhesion of mesenchymal cells in contact with ureteric buds.

Notch2 Activation in the Embryonic Kidney Depletes Nephron Progenitors

Successive activation of Wnt4 and Notch2 generates nephrons from the metanephric mesenchyme. Mesenchymal-to-epithelial transition requires Wnt4, and normal development of the proximal nephron (epithelia of glomeruli and proximal tubules) requires Notch2. It is unknown, however, whether Notch2 dictates the fate of the proximal nephron directly. Here, we generated a mutant strain of mice with activated Notch2 in Six2-containing nephron progenitor cells of the metanephric mesenchyme. Notch2 activation did not skew the cell fate toward the proximal nephron but resulted in severe kidney dysgenesis and depletion of Six2-positive progenitors. We observed ectopic expression of Wnt4 and premature tubule formation, similar to the phenotype of Six2-deficient mice. Activation of Notch2 in the progenitor cells suppressed Pax2, an upstream regulator of Six2, possibly through Hesr genes. Taken together, these data suggest that a positive feedback loop exists between Notch2 and Wnt4, and that Notch2 stabilizes, rather than dictates, nephron fate by shutting down the maintenance of undifferentiated progenitor cells, thereby depleting this population.

Islet1 Deletion Causes Kidney Agenesis and Hydroureter Resembling CAKUT

Islet1 (Isl1) is a transcription factor transiently expressed in a subset of heart and limb progenitors. During studies of limb development, conditional Isl1 deletion produced unexpected kidney abnormalities. Here, we studied the renal expression of Isl1 and whether it has a role in kidney development. In situ hybridization revealed Isl1 expression in the mesenchymal cells surrounding the base of the ureteric bud in mice. Conditional deletion of Isl1 caused kidney agenesis or hypoplasia and hydroureter, a phenotype resembling human congenital anomalies of the kidney and urinary tract (CAKUT). The absence of Isl1 led to ectopic branching of the ureteric bud out from the nephric duct or to the formation of accessory buds, both of which could lead to obstruction of the ureter-bladder junction and consequent hydroureter. The abnormal elongation and poor branching of the ureteric buds were the likely causes of the kidney agenesis or hypoplasia. Furthermore, the lack of Isl1 reduced the expression of Bmp4, a gene implicated in the CAKUT-like phenotype, in the metanephric region before ureteric budding. In conclusion, Isl1 is essential for proper development of the kidney and ureter by repressing the aberrant formation of the ureteric bud. These observations call for further studies to investigate whether Isl1 may be a causative gene for human CAKUT.

Nephron progenitors in the metanephric mesenchyme

The kidney is formed by a reciprocally inductive interaction between two precursor tissues, the metanephric mesenchyme and the ureteric bud. This interaction can be divided into three processes: attraction of the ureteric bud toward the mesenchyme, maintenance of the mesenchyme in an undifferentiated state versus transition to an epithelial state, and further differentiation into multiple epithelial lineages, such as glomeruli and renal tubules. In this review we describe our recent findings related to each process. A mesenchymal nuclear zinc finger protein, Sall1, controls ureteric bud attraction by regulating a novel kinesin, Kif26b. The Sall1 gene is highly expressed in multipotent nephron progenitors in the mesenchyme, and these cells can partially reconstitute a three-dimensional structure in organ cultures following Wnt4 stimulation. While Notch2 is required for further differentiation of proximal nephron structures, ectopic Notch2 activation in the embryonic kidney depletes nephron progenitors, suggesting that Notch2 stabilizes—rather than dictates—nephron fate by shutting down the maintenance of undifferentiated progenitor cells.

A mouse line expressing Sall1-driven inducible Cre recombinase in the kidney mesenchyme.

Sall1 is expressed in the metanephric mesenchyme in the developing kidney, and mice deficient in Sall1 show kidney agenesis or dysgenesis. Sall1 is also expressed elsewhere, including in the limb buds, anus, heart, and central nervous system. Dominant‐negative mutations of Sall1 in mice and humans lead to developmental defects in these organs. Here, we generated a mouse line expressing tamoxifen‐inducible Cre recombinase (CreERT2) under the control of the endogenous Sall1 promoter. Upon tamoxifen treatment, these mice showed genomic recombination in the tissues where endogenous Sall1 is expressed. When CreERT2 mice were crossed with the floxed Sall1 allele, tamoxifen administration during gestation led to a significant decrease in Sall1 expression and small kidneys at birth, suggesting that Sall1 functions were disrupted. Furthermore, Sall1 expression in the kidney was significantly reduced by neonatal tamoxifen treatment. The Sall1CreERT2 mouse is a valuable tool for in vivo time‐dependent and region‐specific knockout and overexpression studies. genesis 48:207–212, 2010.

Sall1 in renal stromal progenitors non-cell autonomously restricts the excessive expansion of nephron progenitors.

The mammalian kidney develops from reciprocal interactions between the metanephric mesenchyme and ureteric bud, the former of which contains nephron progenitors. The third lineage, the stroma, fills up the interstitial space and is derived from distinct progenitors that express the transcription factor Foxd1. We showed previously that deletion of the nuclear factor Sall1 in nephron progenitors leads to their depletion in mice. However, Sall1 is expressed not only in nephron progenitors but also in stromal progenitors. Here we report that specific Sall1 deletion in stromal progenitors leads to aberrant expansion of nephron progenitors, which is in sharp contrast with a nephron progenitor-specific deletion. The mutant mice also exhibited cystic kidneys after birth and died before adulthood. We found that Decorin, which inhibits Bmp-mediated nephron differentiation, was upregulated in the mutant stroma. In contrast, the expression of Fat4, which restricts nephron progenitor expansion, was reduced mildly. Furthermore, the Sall1 protein binds to many stroma-related gene loci, including Decorin and Fat4. Thus, the expression of Sall1 in stromal progenitors restricts the excessive expansion of nephron progenitors in a non-cell autonomous manner, and Sall1-mediated regulation of Decorin and Fat4 might at least partially underlie the pathogenesis.

The phosphatase Dullard negatively regulates BMP signalling and is essential for nephron maintenance after birth

Most kidney nephron components, including glomeruli and renal tubules, derive from the metanephric mesenchyme. The overall differentiation into each component finishes at birth, but the molecular events linking the perinatal and adult kidneys remain elusive. Dullard was cloned from Xenopus kidneys, and encodes a phosphatase that negatively regulates BMP signalling. Here we report that Dullard deletion in the murine metanephric mesenchyme leads to failure of nephron maintenance after birth, resulting in lethality before adulthood. The nephron components are lost by massive apoptosis within 3 weeks after birth, leading to formation of a large hollow with a thin-layered cortex and medulla. Phosphorylated Smad1/5/8 is upregulated in the mutant nephrons, probably through cell-autonomous inhibitory effects of Dullard on BMP signalling. Importantly, administration of the BMP receptor kinase inhibitor LDN-193189 partially rescued the defects caused by Dullard deletion. Thus, Dullard keeps BMP signalling at an appropriate level, which is required for nephron maintenance in the postnatal period.

Nonmuscle Myosin II Regulates the Morphogenesis of Metanephric Mesenchyme–Derived Immature Nephrons

The kidney develops from reciprocal interactions between the metanephric mesenchyme and ureteric bud. The mesenchyme transforms into epithelia and forms complicated nephron structures, whereas the ureteric bud extends its pre-existing epithelial ducts. Although the roles are well established for extracellular stimuli, such as Wnt and Notch, it is unclear how the intracellular cytoskeleton regulates these morphogenetic processes. Myh9 and Myh10 encode nonmuscle myosin II heavy chains, and Myh9 mutations in humans are implicated in congenital kidney diseases and focal segmental glomerulosclerosis in adults. Here, we analyzed the roles of Myh9 and Myh10 in the developing kidney. Ureteric bud-specific depletion of Myh9 resulted in no apparent phenotypes, whereas mesenchyme-specific Myh9 deletion caused proximal tubule dilations and renal failure. Mesenchyme-specific Myh9/Myh10 mutant mice died shortly after birth and showed a severe defect in nephron formation. The nascent mutant nephrons failed to form a continuous lumen, which likely resulted from impaired apical constriction of the elongating tubules. In addition, nephron progenitors lacking Myh9/Myh10 or the possible interactor Kif26b were less condensed at midgestation and reduced at birth. Taken together, nonmuscle myosin II regulates the morphogenesis of immature nephrons derived from the metanephric mesenchyme and the maintenance of nephron progenitors. Our data also suggest that Myh9 deletion in mice results in failure to maintain renal tubules but not in glomerulosclerosis.