Research
Our lab is focused primarily on signaling pathways that control epithelial-mesenchymal interactions during the early stages of kidney development. In the mammalian kidney, interactions between the ureteric bud, an epithelial structure, and the metanephric blastema, a mesenchymal structure, give rese, respectively, to ureteric bud and collecting duct branches and to the nephron. In our published work we used in vitro and in vivo models of renal branching morphogenesis to demonstrate that BMPs modulate tubular growth and branching via an inhibitory Smad 1-dependent pathway. These results provide a basis for our current work aimed at identifying the actions of these pathways in vivo.
In related work, we analyzed the renal phenotype of Glypican-3 deficient mice. The Glypicain-3 (GPC3) gene encodes a cell surface heparin sulfate proteoglycan and is mutated in humans with Simpson-Golabi-Behmel Syndrome characterized by somatic overgrowth and renal dysplasia. Our analysis of GPC3 deficient mice demonstrated that ureteric bud cell proliferation is markedly increased. Moreover, medullary collecting duct cells undergo massive rates of apoptosis at the subsequent stage of cortico medullary differentiation. To determine mechanisms that control abnormal rates of ureteric bud cell proliferation, we demonstrated that BMP2 does not inhibit ureteric bud branching in vitro and that deficiency of both BMP2 and Glypican-3 increase ureteric bud cell proliferation in vivo.
While studying the functions of BMP-depended Smad signaling during renal branching morphogenesis in vivo, we generated a model of renal medullary cystic dysplasia. Our investigations of the molecular mechanisms controlling this phenotype revealed misexpress of beta-catenin and C-MYC in cystic epithelium of duct origin and induction of molecular interactions between beta-catenin and Smads. Our finding that both Phospho-Smad1 and beta-catenin are impregnated in human dysplastic renal tissue demonstrated the relevance of these finds to human disease. In subsequent studies, we elucidated mechanisms by which Smad1, beta-catenin and TCF4 cooperate to control the transcription of C-MYC. Our findings related to the deleterious effects of beta-catenin over expression in the embryonic kidney have motivated our ongoing studies focused on the functions of beta-catenin during epithelial mesenchymal interactions in vivo.
Sonic Hedgehog (SHH), a secreted morphogen, controls tissue development across a variety of species. SHH signals via a family GLI proteins. As intact proteins, GLIs activate transcription. However, when processed to shorter forms, GLIs repress transcription. Humans with Pallister-Hall Syndrome have a mutation in GLI3 that generates a short (truncated) form. To determine the pathogenesis of renal dysplasia in affected individuals, we investigated SHH functions during murine renal development in vivo. Our results demonstrate that SHH deficiency causes either renal aplasia & formation of a single ectopic dysplastic kidney in which expression of Pax2, Sall1, N-myc, Cyclin D1, GLI1 and GLI2 are markedly decreased and the ratio of GLI3 activator: repressor is greatly decreased in favor of the repressor. Deficiency of both SHH and GLI3 rescues these renal abnormalities. Studies of GLI protein interactions with the 5 UTRs of SHH-target genes using chromatin immunoprecipitation demonstrated that GLI2 associates with these genes in a SHH sufficient state. However, in SHH deficient mice, GLI3 replaces GLI2. These results provide a basis for our current studies aimed at determining the lineage-specific functions of SHH during kidney development.
- Bone Mrphogenetic Proteins (BMPs)
- Beta-catenin
- Sonic Hodgehog
- Genetic Mechanisms and Human Renal Dysplasia