Developmental regulation and partial-length cloning of tubulointerstitial nephritis antigen of murine metanephros

Abstract

Tubulointerstitial nephritis antigen (TIN-ag) is an extracellular matrix (ECM) glycoprotein that has been recently isolated and cloned from the rabbit kidney. It is an integral component of the basal lamina, and unlike other basement membrane proteins it is exclusively expressed in the tubular basement membranes (TBMs). Since other ECM glycoproteins have been shown to regulate development of various organ systems, studies were initiated to ascertain its developmental regulation in renal tubulogenesis and glomerulogenesis. Embryonic (day-13 and -17 of gestation), newborn and one- week-old mice kidneys were harvested for expression of TIN-ag as well as cDNA cloning studies. Immunostaining with polyclonal anti-TIN-ag antibody revealed its localization to the basal lamina of ureteric bud branches and epithelial elements of developing nephrons in day-13 embryonic kidneys. Interestingly, it was heavily expressed at the tips of the ureteric bud branches, and was not expressed in the distal convolutions of the S-shaped body stage of the nephrons, the region which forms the future glomerulus. At day-17, TIN-ag expression was less, and the immuno-reactivity was mainly localized to the cortex. In the newborn and one-week-old mice kidneys, the cortical expression of TIN-ag increased progressively, but was absent in the glomeruli. The TIN- ag expression was confined to the cortical TBMs, while absent in the medullary tubules, the latter included segments of the collecting ducts and loop of Henle. Immunoprecipitation studies on [35S]methionine-labeled metanephroi revealed a single band of ~58 kDa at day-13, and the incorporated radioactivity decreased at day-17. No high molecular weight isoforms were observed. A partial-length mouse TIN-ag cDNA of ~530 bp PCR product was generated, and it had ~88% and ~93% nucleotide and amine acid sequence homology, respectively, with rabbit TIN-ag. Utilizing this cDNA, Northern blot analyses revealed a single transcript of ~2 Kb in fetal and postnatal mice kidneys. mRNA expression initially decreased at day-17, and then progressively increased by one week. Utilizing a mouse TIN-ag riboprobe, in situ hybridization studies revealed a generalized diffuse expression of TIN-ag in the epithelial elements of developing nephrons and ureteric bud branches at day-13. Gene expression decreased by day-17, and became confined to the renal cortex, and then progressively increased during the neo- and post-natal periods, but remained absent in the renal medulla and glomeruli. These data indicate that TIN-ag is expressed in the metanephros early in embryonic life in the absence of any detectable isoforms, and it exhibits spatio-temporal characteristics during metanephric development. Being concentrated at the tips of the ureteric bud branches, it is conceivably involved in epithelial:mesenchymal interactions which are highly prevalent during renal organogenesis, and its role in tubulogenesis diverges from glomerulogenesis at the S-shaped body stage of the nephron.