Tamm-Horsfall protein (THP) is a glycoprotein with unclear functions expressed exclusively in thick ascending limbs (TAL) of the kidney. Its role in ischemic acute kidney injury is uncertain, with previous data suggesting a possible negative effect by enhancing cast formation and promoting inflammation. Using a recently characterized THP knockout mouse (THP-/-), we investigated the role of THP in renal ischemia-reperfusion injury (IRI). In wild-type mice (THP+/+), THP expression was increased by injury. THP-/- mice developed more functional and histological renal damage after IRI compared with THP+/+. THP-/- kidneys showed more inflammation and tubular necrosis. Cast formation correlated with the severity of injury and was independent of THP presence. THP absence was associated with a more necrotic, rather than apoptotic, phenotype of cell death. The outer medulla was predominantly affected, where significant interstitial neutrophil infiltration was detected in proximity to injured S3 proximal tubular segments and TAL. This coincided with an enhanced expression of the innate immunity receptor Toll-like receptor 4 (TLR4) in S3 segments of THP-/- compared with THP+/+ mice. Specifically, a basolateral S3 expression of TLR4 was more evident in THP-/- kidneys compared with a more apical distribution in THP+/+. Such basolateral location for TLR4 allows a greater interaction with proinflammatory ligands present in the interstitium during ischemia. In conclusion, we are showing a completely novel role for a very old protein in the setting of renal injury. Our data suggest that THP stabilizes the outer medulla in the face of injury by decreasing inflammation, possibly through an effect on TLR4.

Townes-Brocks syndrome (TBS, OMIM #107480) is an autosomal dominant disorder that causes multiple birth defects including renal, ear, anal and limb malformations. Mutations in SALL1 have been postulated to cause TBS by haploinsufficiency; however, a mouse model carrying a sall1-null allele does not mimic the human syndrome. Since the mutations that cause TBS could express a truncated SALL1 protein containing the domain necessary for transcriptional repression but lacking the complete DNA binding domain, we hypothesized that TBS is due to dominant-negative or gain-of-function activity of a mutant protein. To test this hypothesis, we have created a mutant allele, sall1-DeltaZn2-10, that produces a truncated protein and recapitulates the abnormalities found in human TBS. Heterozygous mice mimic TBS patients by displaying high-frequency sensorineural hearing loss, renal cystic hypoplasia and wrist bone abnormalities. Homozygous sall1-DeltaZn2-10 mutant mice exhibit more severe defects than sall1-null mice including complete renal agenesis, exencephaly, limb and anal deformities. We demonstrate that truncated Sall1 mediates interaction with all Sall family members and could interfere with the normal function of all Sall proteins. These data support a model for the pathogenesis of TBS in which expression of a truncated SALL1 protein causes abnormal development of multiple organs.

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SUMMARYDevelopment of the metanephric kidney depends on precise control of branching of the ureteric bud. Branching events represent terminal bifurcations that are thought to depend on unique patterns of gene expression in the tip compared with the stalk and are influenced by mesenchymal signals. The metanephric mesenchyme-derived signals that control gene expression at the ureteric bud tip are not well understood. In mouse Sall1 mutants, the ureteric bud grows out and invades the metanephric mesenchyme, but it fails to initiate branching despite tip-specific expression of Ret and Wnt11. The stalk-specific marker Wnt9b and the -catenin downstream target Axin2 are ectopically expressed in the mutant ureteric bud tips, suggesting that upregulated canonical Wnt signaling disrupts ureter branching in this mutant. In support of this hypothesis, ureter arrest is rescued by lowering -catenin levels in the Sall1 mutant and is phenocopied by ectopic expression of a stabilized -catenin in the ureteric bud. Furthermore, transgenic overexpression of Wnt9b in the ureteric bud causes reduced branching in multiple founder lines. These studies indicate that Sall1-dependent signals from the metanephric mesenchyme are required to modulate ureteric bud tip Wnt patterning in order to initiate branching.