Ureteric Mesenchyme Research Articles

75: Programmed alterations in nephrogenic growth factor expression contribute to reduced glomerular number in IUGR offspring

CONTRIBUTE TO REDUCED GLOMERULAR NUMBER IN IUGR OFFSPRING AHMED ABDEL-HAKEEM, TASMIA HENRY, THOMAS MAGEE, MINA DESAI, CYNTHIA NAST, ROY Z. MANSANO, JOHN TORDAY, MICHAEL G. ROSS, Harbor-UCLA Med. Ctr. (LA BioMed), Dept. of Ob/Gyn, Torrance, California, Cedars-Sinai Medical Center, Dept. of Pathology & Lab. Med., Los Angeles, California OBJECTIVE: Intrauterine growth restricted (IUGR) humans and animals have reduced renal glomerular number, contributing in part to programmed offspring hypertension. Fetal nephron formation processes, including ureteric bud branching and mesenchyme signaling, is modulated by putative growth factors, including bone morphogenic proteins (BMPs), fibroblast growth factors (FGFs) and glial cell derived neurotrophic factor (GDNF) and the RET receptor. We thus investigated the effect of IUGR on the expression of nephogenic growth factors in the developing kidney. STUDY DESIGN: Sprague Dawley pregnant rats (n 12) were randomly allocated to 50% maternal food restricted (MFR) from day 10 of gestation (e10) or ad libitum feeding, in a protocol demonstrated to produce IUGR offspring. Embryonic kidneys were collected at e20 (n 6/group) and mRNA expression determined (using real time RT-PCR) of BMP4 (ureteric bud growth), BMP7 (ureteric bud inhibition), FGF2 (mesenchyme maintenance) FGF7 (ureteric bud growth and nephron number), and GDNF-RET (inductive signaling between ureteric bud and mesenchyme). RESULTS: Among IUGR fetuses, renal growth factors promoting ureteric bud growth and nephron development showed decreased mRNA expression, as evident by down-regulation of BMP4 (0.2-fold) and FGF7 (0.5-fold). The inhibitory growth factor BMP7 showed significantly increased mRNA expression being upregulated by 7-fold. FGF2 also was up-regulated (3-fold) while GDNF and RET gene expression were unchanged. CONCLUSION: The decreased expression of stimulatory growth factors (BMP4 and FGF7) and increased expression of an inhibitory factor (BMP7), suggests that reduced ureteric bud branching contributes to reduced nephron number in IUGR fetuses. The up-regulation of FGF2 may relate to attempted mesenchymal survival in this model. Thus, altered developmental nephrogenesis may be a key underlying mechanism for programmed hypertension in IUGR offspring.

Tbx18 regulates the development of the ureteral mesenchyme

Congenital malformations of the urinary tract are a major cause of renal failure in children and young adults. They are often caused by physical obstruction or by functional impairment of the peristaltic machinery of the ureter. The underlying molecular and cellular defects are, however, poorly understood. Here we present the phenotypic characterization of a new mouse model for congenital ureter malformation that revealed the molecular pathway important for the formation of the functional mesenchymal coating of the ureter. The gene encoding the T-box transcription factor Tbx18 was expressed in undifferentiated mesenchymal cells surrounding the distal ureter stalk. In Tbx18-/- mice, prospective ureteral mesenchymal cells largely dislocalized to the surface of the kidneys. The remaining ureteral mesenchymal cells showed reduced proliferation and failed to differentiate into smooth muscles, but instead became fibrous and ligamentous tissue. Absence of ureteral smooth muscles resulted in a short hydroureter and hydronephrosis at birth. Our analysis also showed that the ureteral mesenchyme derives from a distinct cell population that is separated early in kidney development from that of other mesenchymal cells of the renal system.

Going in circles: conserved mechanisms control radial patterning in the urinary and digestive tracts

Radial patterning in the urinary tract and gut depends on reciprocal signaling between epithelial cells, which form mucosa, and mesenchyme, which forms smooth muscle and connective tissue. These interactions depend on sonic hedgehog (Shh), which is secreted by epithelial cells and induces expression of bone morphogenetic protein 4 (Bmp4), a signaling molecule required for differentiation of smooth muscle progenitors. Patterning of the specialized mucosa lining the anterior-posterior (A-P) axis may be controlled independently by regionally expressed mesenchymal transcription factors. A study by Airik et al. in this issue of the JCI reveals that T-box 18 (Tbx18), a transcription factor selectively expressed in ureteral mesenchyme, regulates smooth muscle differentiation by maintaining Shh1 responsiveness in mesenchymal progenitors (see the related article beginning on page 663). Deletion of Tbx18 resulted in defective urothelial differentiation at the level of the ureter, suggesting that Tbx18 acts via mesenchyme as an important regulator of A-P patterning in the urinary tract.

Differential gene expression in the developing mouse ureter

In many instances, kidney dysgenesis results as a secondary consequence to defects in the development of the ureter. Through the use of mouse genetics a number of genes associated with such malformations have been identified, however, the cause of many other abnormalities remain unknown. In order to identify novel genes involved in ureter development we compared gene expression in embryonic day (E) 12.5, E15.5 and postnatal day (P) 75 ureters using the Compugen mouse long oligo microarrays. A total of 248 genes were dynamically upregulated and 208 downregulated between E12.5 and P75. At E12.5, when the mouse ureter is comprised of a simple cuboidal epithelium surrounded by ureteric mesenchyme, genes previously reported to be expressed in the ureteric mesenchyme, foxC1 and foxC2 were upregulated. By E15.5 the epithelial layer develops into urothelium, impermeable to urine, and smooth muscle develops for the peristaltic movement of urine towards the bladder. The development of these two cell types coincided with the upregulation of UPIIIa, RAB27b and PPARγ reported to be expressed in the urothelium, and several muscle genes, Acta1, Tnnt2, Myocd, and Tpm2. In situ hybridization identified several novel genes with spatial expression within the smooth muscle, Acta1; ureteric mesenchyme and smooth muscle, Thbs2 and Col5a2; and urothelium, Kcnj8 and Adh1. This study marks the first known report defining global gene expression of the developing mouse ureter and will provide insight into the molecular mechanisms underlying kidney and lower urinary tract malformations.

Sonic hedgehog regulates proliferation and differentiation of mesenchymal cells in the mouse metanephric kidney.

Signaling by the ureteric bud epithelium is essential for survival, proliferation and differentiation of the metanephric mesenchyme during kidney development. Most studies that have addressed ureteric signaling have focused on the proximal, branching, ureteric epithelium. We demonstrate that sonic hedgehog is expressed in the ureteric epithelium of the distal, non-branching medullary collecting ducts and continues into the epithelium of the ureter -- the urinary outflow tract that connects the kidney with the bladder. Upregulation of patched 1, the sonic hedgehog receptor and a downstream target gene of the signaling pathway in the mesenchyme surrounding the distal collecting ducts and the ureter suggests that sonic hedgehog acts as a paracrine signal. In vivo and in vitro analyses demonstrate that sonic hedgehog promotes mesenchymal cell proliferation, regulates the timing of differentiation of smooth muscle progenitor cells, and sets the pattern of mesenchymal differentiation through its dose-dependent inhibition of smooth muscle formation. In addition, we also show that bone morphogenetic protein 4 is a downstream target gene of sonic hedgehog signaling in kidney stroma and ureteral mesenchyme, but does not mediate the effects of sonic hedgehog in the control of mesenchymal proliferation.