SIX1 acts synergistically with TBX18 in mediating ureteral smooth muscle formation

Abstract

SUMMARYDysfunction of the ureter often leads to urine flow impairment from the kidney to the bladder, causing dilation of the ureter and/or renal pelvis. Six1 is a crucial regulator of renal development: mutations in human SIX1 cause branchio-oto-renal (BOR) syndrome and Six1 -/-mice exhibit renal agenesis, although the ureter is present. It remains unclear whether Six1 plays a role in regulating ureter morphogenesis. We demonstrate here that Six1 is differentially expressed during ureter morphogenesis. It was expressed in undifferentiated smooth muscle (SM) progenitors, but was downregulated in differentiating SM cells (SMCs) and had disappeared by E18.5. In Six1 -/-mice, the ureteral mesenchymal precursors failed to condense and differentiate into normal SMCs and showed increased cell death, indicating that Six1 is required for the maintenance and normal differentiation of SM progenitors. A delay in SMC differentiation was observed in Six1 -/-ureters. A lack of Six1 in the ureter led to hydroureter and hydronephrosis without anatomical obstruction when kidney formation was rescued in Six1 -/-embryos by specifically expressing Six1 in the metanephric mesenchyme, but not the ureter, under control of the Eya1 promoter. We show that Six1 and Tbx18 genetically interact to synergistically regulate SMC development and ureter function and that their gene products form a complex in cultured cells and in the developing ureter. Two missense mutations in SIX1 from BOR patients reduced or abolished SIX1-TBX18 complex formation. These findings uncover an essential role for Six1 in establishing a functionally normal ureter and provide new insights into the molecular basis of urinary tract malformations in BOR patients.

Sall1-dependent signals affect Wnt signaling and ureter tip fate to initiate kidney development

“…However, unlike Six1 (Nie et al, 2010), Sall1 is not required for proper formation of the smooth muscle layer. Sall1 expression is not preserved in Six1 -/-mutant mm and it has been suggested that Sall1 is a direct transcriptional target of Six1 regulation .…”

Identification of molecular compartments and genetic circuitry in the developing mammalian kidney

“…Several genes are documented to show specific expression in inner condensed mesenchyme (Airik et al, 2006;Nie et al, 2010;Yu et al, 2002), but no specific markers have been reported for the outer loose mesenchyme. Interestingly, Prrx1, a paired-related homeobox gene, displayed a whole-mount in situ hybridization expression pattern indicative of superficial expression in the ureter (Fig.…”

Genetics of congenital anomalies of the kidney and urinary tract

“…Congenital hydronephrosis has an overall prevalence of 1 in 1,000 live births [45]. Molecular mechanisms leading to hydronephrosis/hydroureter in the absence of anatomic obstruction include T-box transcription factor 18 (Tbx18) or Six-1-dependent delay in differentiation of ureteral smooth muscle cells [46,47]. Aberrant interactions between the UB and MM or stroma result in renal hypodysplasia, which is observed in 0.027% of fetuses by ultrasonography [42] (Fig.…”

“…Phenotypic heterogeneity of CAKUT may result from differences in genetic background (variation of gene expression between alleles) [81], genetic [47,82,83] or epigenetic modifiers [84][85][86], and environment [87,88]. In monogenic diseases, phenotypic outcome and disease severity depend on the mode of inheritance and type of mutation.…”

“…In contrast, a combination of the same mutations in NPHP1 and NPHP6 causes an additional extrarenal disease phenotype [91]. Other RDGs that interact genetically and may influence renal phenotype include Pax2 and LMX1B or Six1 and Tbx18 [47,82]. Noteworthy, two missense mutations in Six1 identified in patients with BOR syndrome reduce Six1-Tbx18 complex formation [47].…”