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CD13 as a Biomarker and Functional Regulator of Congenital Ureteral Obstruction
Claire Gerber, BS1, Renee M. Silvas, BS2, Christine Kim, MD2, Jaganathan Subramani, PhD1, Katherine Herbst, MS2, Fernando Ferrer, MD2, Linda H. Shapiro, PhD1.
1University of Connecticut Health Center, Farmington, CT, USA, 2Connecticut Children's Medical Center, Hartford, CT, USA.

Background: Congenital ureteral obstruction is a primary cause of renal injury in children. Increased intrarenal pressure due to obstruction initiates an injury-signaling cascade resulting in renal dysfunction. While disease progression has been well studied, early diagnostic markers that reliably detect obstruction prior to significant renal damage are lacking. The CD13 peptidase is abundantly expressed on the luminal surface of the renal proximal tubule epithelium and is shed into the urine post renal injury. CD13 is also an inflammatory adhesion molecule that mediates immune cell trafficking to injured tissues. Thus, we hypothesized that CD13 may be an early urinary marker of renal damage and may also contribute to healing processes following congenital ureteral obstruction.
Methods: The murine unilateral ureteral obstruction (UUO) model was used to experimentally correlate renal damage with urinary CD13 levels. To assess the functional contribution of CD13 in congenital ureteral obstruction, wild type (WT) and CD13 global knockout (CD13KO) mice were subjected to UUO for various time intervals. Urine, kidney protein and RNA, and histological samples from ligated and contralateral control kidneys were collected and assessed for parameters of relative tissue damage. Urine CD13 and creatinine and kidney CD13 protein levels were measured. Flow cytometric analysis profiled inflammatory differences between WT and CD13KO mice.
Results: Murine CD13 protein levels were markedly increased in the urine of WT animals by the earliest time point following ligation but reduced in the obstructed kidneys when compared to controls. Importantly, these changes precede overt renal pathology, strongly supporting our hypothesis that CD13 will be a useful marker of early renal damage. Initial histological observations of the ligated kidney indicated increased damage in CD13KO mice at day 10-post UUO, with markedly enhanced tubule dilation and more atubular glomeruli. At day 3, significant differences between genotypes were evident as assessed by flow cytometry, real time PCR and western blot analysis. Specifically, flow analysis indicated higher numbers of CD45+ hematopoietic cells in obstructed kidneys of CD13KO mice, which may underlie the increased damage in the absence of CD13. In agreement with the increased inflammation, real time PCR analysis demonstrated higher levels of collagen 1α2 mRNA and increased α-smooth muscle actin protein by western blot analysis in the damaged kidneys, consistent with increases in fibroblasts and augmented fibrosis in the CD13KO mice.
Conclusions: Experimental evidence utilizing a murine UUO obstruction model supports our hypothesis that CD13 may be a useful and early urinary biomarker of ureteral obstruction prior to overt renal damage. We plan to translate these studies to samples from pediatric urology patients and compare CD13 protein levels across three groups; healthy patients (controls, group A), patients with suspected obstruction (group B), and patients with confirmed obstruction (group C). Finally, CD13 may play a protective role in the response to kidney injury by mediating immune cell trafficking and fibrosis and may be a promising therapeutic target.


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