Teashirt 3 is necessary for ureteral smooth muscle differentiation downstream of SHH and BMP4

Abstract

Ureteric contractions propel foetal urine from the kidney to the urinary bladder. Here, we show that mouse ureteric smooth muscle cell (SMC) precursors express the transcription factor teashirt 3 (TSHZ3), and that Tshz3-null mutant mice have congenital hydronephrosis without anatomical obstruction. Ex vivo, the spontaneous contractions that occurred in proximal segments of wildtype embryonic ureter explants were absent in Tshz3 mutant ureters. In vivo, prior to the onset of hydronephrosis, mutant proximal ureters failed to express contractile SMC markers, whereas these molecules were detected in controls. Mutant embryonic ureters expressed Shh and Bmp4 transcripts as normal, with appropriate expression of Ptch1 and pSMAD1/5/8 in target SM precursors, whereas myocardin, a key regulator for SMC differentiation, was not expressed in Tshz3-null ureters. In wild-type embryonic renal tract explants, exogenous BMP4 upregulated Tshz3 and myocardin expression. More interestingly, in Tshz3 mutant renal tract explants, exogenous BMP4 did not improve the Tshz3 phenotype. Thus, Tshz3 is required for proximal ureteric SMC differentiation downstream of SHH and BMP4. Furthermore, the Tshz3 mutant mouse model of 'functional' urinary obstruction resembles congenital pelvi-ureteric junction obstruction, a common human malformation, suggesting that TSHZ, or related, gene variants may contribute to this disorder.

Cell Biology of Ureter Development

“…42 This transcription factor is normally expressed in mesenchymal cells surrounding the urothelial stalk and its absence is associated with decreased epithelial proliferation. 42 Once initiated, further longitudinal growth occurs in isolated wild-type embryonic ureters maintained in organ culture 43 and in ureters of certain mutant embryos lacking kidneys. 44 Both observations show that exposure to fetal urine is not needed for longitudinal growth, although these experiments do not rule out a more subtle, differentiationoptimizing influence conferred by urine flow that, in mice, probably begins several days after UB initiation when the metanephros has formed its first layers of vascularized glomeruli (Figure 3).…”

“…53 Peri-urothelial mesenchymal cells are also stimulated to express BMP4, which itself effects their own differentiation into SM. 51,53 Here, BMP4 enhances intracellular levels of phospho-SMADs 43,54 and upregulates TSHZ3, a transcription factor-like protein. TSHZ3 is needed for MYOCD expression within nascent ureteric SM cells.…”

“…MYOCD, a transcriptional coactivator, then upregulates genes coding for muscle contractile proteins, such as smooth muscle actin and myosin heavy chains. 43,52 Lack of another transcription factor, SOX9, which like TSHZ3, is normally expressed by mesenchyme aggregating around the urothelial ureteric tube, also leads to failed SM differentiation. 55 The aggregation of SM precursor cells around urothelia depends on mesenchymal expression of TBX18 and, in mice engineered to lack this transcription factor, prospective SM precursors become mislocalized to the surface of the metanephros.…”

“…60 Notably, the explanted fetal ureter, even when physically disconnected from the kidney and bladder, undergoes regular peristalsis in a proximal to distal direction. 43 In vivo, peristalsis is triggered by HCN3, a hyperpolarization-activated cation channel expressed in the renal pelvis/kidney junction. 61 When hedgehog signaling is downregulated experimentally in this region, expression of KIT and HCN3 are compromised and contractions are perturbed, even though SM cells themselves appear intact.…”

Brg1 Determines Urothelial Cell Fate during Ureter Development

“…2,3 Expression of sonic hedgehog (Shh) by the basal cells has been shown to activate Shh receptor patched 1 (Ptch1) on the adjacent stromal cells and induce smooth muscle differentiation and investment of the ureter. 4,5 In contrast, the exact identity of the urothelial stem cell and factors that control urothelial stratification and differentiation are less well understood. In vitro studies suggest that the nuclear receptor Pparg can induce uroplakin expression and thus promote urothelial differentiation.…”