An acidic peptide sequence of nucleolin-related protein can mediate the attachment of calcium oxalate to renal tubule cells

Abstract

Crystals that form in tubular fluid must be retained in the kidney to become stones. Nucleolin-related protein (NRP) is found on the surface of inner medullary collecting duct (IMCD) cells in culture (cIMCD) and selectively adsorbs to calcium oxalate (CaOx). We proposed that NRP mediates attachment to the renal tubular epithelium of Ca stone crystals through an electrostatic interaction with a highly acidic region (acidic fragment [AF]) similar to those of other proteins that have been reported to affect urinary crystal formation. The current studies demonstrate that nucleolin is expressed on both apical and basolateral cell surfaces of cIMCD, reaching a peak in the late stages of mitosis and gradually declining to undetectable levels with maturation of the polarized epithelium. Scraping areas of mature monolayers stimulated the cells surrounding the defects to migrate and proliferate so as to repair them, and these areas demonstrate surface NRP expression and enhanced attachment of CaOx monohydrate crystals. Surface expression of the NRP AF was produced by cloning the NRP AF into a display vector. Transfected cIMCD demonstrating copious surface expression of AF enhanced CaOx attachment 6.7-fold compared with control cIMCD, whereas cells transfected with a vector without the AF did not differ from control. AF was also cloned into a replication-deficient adenovirus and expressed in 293 cells, resulting in AF secretion into the nutrient medium. This medium inhibited CaOx attachment to cIMCD, compared with conditioned medium from cells infected with wild-type virus. These results demonstrate that surface-bound AF can mediate CaOx attachment and that secreted AF can inhibit attachment. These results support the notion that surface-associated NRP could mediate attachment of CaOx to the renal tubule epithelium, thereby causing retention of crystals that might eventually become kidney stones.