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i. Investigating the molecular and cellular events important for formation of connections between the ureter and the bladder. The ureters and bladder form independently, but must connect during development to generate a patent outflow tract. Initially, ureters are joined to the nephric duct, the primordium of the male genital tract, via a wedge of epithelial tissue called the common nephric duct. In order to join the bladder, ureters much detach from the nephric duct, move to the bladder and fuse with the bladder epithelium. Our studies suggest that this process is divided into at least 3 steps all of which are likely to depend on interactions with the bladder: expansion of the common nephric duct, apoptosis of the common nephric duct, and fusion of ureters with the bladder epithelium. We are exploring these processes, by analyzing caspase mutants to determine whether apoptosis is indeed required for ureter insertion. A second line of study is aimed identifying common nephric derived and bladder-derived signals that control common nephric duct remodeling and apoptosis.
ii. Understanding the cause of urethral valves. Urethral valves is a common malformation found almost exclusively in males, about which little is understood. In affected newborns, the urethra is blocked at the bladder neck or at posterior levels by symmetrical pieces of tissue whose origin is unknown, resulting in obstructed urine flow. There are several possible abnormalities that could lead to urethral obstruction. For example failure in common nephric remodeling or apoptosis could leave remnants at the level of the bladder neck that block urine flow. In females, failure in common nephric duct apoptosis would be rescued by apoptosis, as the common nephric duct is part of the male genital duct system which normally degenerates in females. However in males, there is no mechanism outside of normal remodeling that can induce common nephric duct apoptosis. An alternate possibility is the urethral valves are caused by abnormalities intrinsic to the male urethra, as male and female urethral development are quite different. We are using mouse models lacking Sall1, a transcription factor, to study these malformations. In humans, mutations in Sall1 lead to Towne-Brocks syndrome, which is associated with spectrum of lower urinary tract abnormalities including urethral valves. We find that in Sall1 mutants, common nephric duct remodeling is abnormal and urethral malformations are also present. To determine which of these defects leads to valves, we are using Sall1 conditional alleles to inactivate Sall1 in a tissue specific manner.
iii. Studying the cellular and developmental mechanisms important for bladder formation. The bladder grows out from the anterior region of the cloaca at E12. In animals with impaired retinoid signaling, the cloaca differentiates into the urethra, but the bladder is rudimentary, failing to extend properly after E12. We find that inhibition of retinoid signaling in the bladder epithelium disrupts its elongation, while inhibition of retinoid signaling in the mesenchyme has little if any effect. We are currently analyzing mutants and wild type animals to determine by which mechanisms bladder elongation normally occurs, and to identify retinoid targets that are normally important for driving this process.
iv. Identification of urothelial progenitors that give rise to the normal epithelium, and progenitors that give rise to bladder cancer. The urothelium is a stratified epithelium composed of basal, intermediate and umbrella cell layers that lines the renal pelvis, ureters and bladder, which is important for preventing leakage of urine and damage to the underlying tissue. In addition to bladder cancer, a number of diseases in humans are linked to urothelial abnormalities, however little is known about how this specialized epithelium forms. We are currently in the process of performing lineage studies to identify cell types that are progenitors of the normal urothelium. In addition, in collaboration with Cory Abate-Shen and Carlos Cardon-Cordo we will apply our findings from normal lineage to identify urothelial progenitors that give rise to cancer.
