Pathways that regulate renal development, fibrosis, and metabolic disease in mouse models

Abstract

The kidney is an essential organ that maintains homeostasis, maintains water and mineral balance, and removes metabolic waste products from the body. In mammals, the kidney derives from the intermediate mesoderm (IM) and develops through a multistep process where undifferentiated mesenchyme is converted into a highly complex organ. Several transcriptional regulators, including the Pax2 gene, have been identified in the specification and maintenance of this multistep process. The Pax2 gene marks the IM shortly after gastrulation, when the mesoderm becomes compartmentalized into paraxial, intermediate, and lateral plate. Pax2 expression in the IM distinguishes all of the cells fated to become epithelia in the urogenital tract and is necessary to establish and maintain this phenotype. Pax2 null mutants do develop a nephric duct (Brophy et al., 2001; Soofi et al., 2012), but the duct is completely absent in a Pax2/8 double mutant, suggesting that these Pax genes function redundantly in this early IM domain; however, in Pax2 homozygous mutant mice, the metanephric mesenchyme neither responds to inductive signals nor does the mutant mesenchyme aggregate into early renal vesicles resulting in a lack of kidneys, ureters, and genital track. We describe two new alleles of Pax2 created by inserting the Enhanced Green Fluorescent Protein coding region into the 5' untranslated leader sequence. One allele is a hypomorph that generates less protein and exhibits structural defects in kidneys and ureters upon homozygosity. A second allele is a true null that can be used to image Pax2 expressing cells in a mutant background. Organ culture and embryo analyses point to a loss of epithelial cell polarity and increased mobility in cells that have deleted Pax2 function. These experiments provide new insight into the role of Pax2 protein levels in determining correct renal architecture and cell fate. The prevalence of chronic kidney disease (CKD) worldwide is reflected by the increasing number of people with end stage renal disease (ESRD) requiring some form of renal replacement therapy. The overall incidence of ESRD is increasing at an alarming rate and is correlated with the rise of diabetes, obesity, and hypertension. Yet, effective therapies for chronic fibrosis in the kidney and other tissues are still awaited. Among the most extensively studied signaling pathways in renal fibrotic disease are those of the TGFb superfamily (TGFb and BMPs). Given the critical roles for TGFb and BMP proteins in enhancing or suppressing renal interstitial fibrosis, respectively, the results of this thesis will show how the expression of this secreted protein KCP could diminished renal fibrosis in mouse models of chronic and acute kidney disease. In vivo, KCP-KO mice are viable and fertile but are more sensitive to tubular injury and exhibit significant pathology after recovery. Also, deletion of KCP sensitized mice to developing obesity and associated complications such as liver steatosis and glucose intolerance. In contrast, transgenic mice that expressed KCP in the kidney, liver, and brown adipose tissues were resistant to developing high fat diet induced obesity and had significantly reduced white adipose tissue. This data demonstrates that modulation of the TGFβ signaling with secreted inhibitors or enhancers can alter the profile of adipose tissue, which reduces obesity and impaired the progression of metabolic disease. The Metabolic Syndrome is reaching epidemic proportions in the developed world, primarily due to the increased availability of high caloric foods and the decrease in daily physical activity. Energy balance is critical for maintaining normal body weight and homeostasis. When caloric intake chronically exceeds energy expenditure, white adipose tissue stores excess energy in the form of triglycerides, leading to obesity and related complications such as type-2 diabetes, a condition also referred to as metabolic syndrome which is a condition of chronic sub-clinical inflammation. In mice, the TGFβ superfamily has been implicated not only in the development and differentiation of white and brown adipose tissues, but also in the induction of the pro-inflammatory state that accompanies (Tseng et al., 2008). The work outlined in this thesis suggests that altering the TGFβ superfamily signaling pathway by a secreted protein (KCP) can attenuate renal fibrosis and the negative effects of obesity-associated metabolic syndrome. Providing a conceptual basis for the use of small molecule analogues of KCP to attenuate profibrotic pathways that depend on continued TGFβ signaling and/or counteraction by BMPs may potentially provide a novel approach to translating the protective role of specific BMPs (e.g. BMP-7) into clinical benefit