HETEROGENEITY OF OSTEOPONTIN EXPRESSION AMONG NEPHRONS IN MOUSE KIDNEYS AND ENHANCED EXPRESSION IN SCLEROTIC GLOMERULI

BACKGROUND: Osteopontin (OPN) is a secreted Ca2+-binding phosphoprotei n able to mediate cell attachment to bone via an RGD sequence and the alpha(v)beta3 integrin. OPN mRNA is found at high levels in the kidney , and the protein is found in the urine. Because published reports of where the protein is produced conflict, we undertook a comprehensive s tudy to localize OPN expression. EXPERIMENTAL DESIGN: In situ hybridiz ation with a mouse cDNA probe and immunohistochemical staining with th ree different antisera to mouse OPN were used to identify those cells that contained significant levels of mRNA and protein, respectively. R ESULTS: Both methods of analysis revealed that OPN expression in the n ormal mouse kidney was primarily restricted to the thick ascending lim bs of the loop of Henle and to the distal convoluted tubules. Protein was detected predominantly at the apical surface of cells lining the l umen of a subset of tubules. The alpha(v)beta3 integrin, which is a re ceptor for vitronectin and osteopontin, was uniformly localized by imm unostaining not on the apical surface but rather to the baso-lateral s urface of cells in the distal part of the tubule. OPN expression was n ot detected in healthy glomeruli, proximal tubules, thin limbs of the loop of Henle, collecting ducts, or interstitial fibroblasts. In contr ast to the localization of Tamm-Horsfall protein expression, in all di stal nephrons, expression of OPN was detected by both methods of analy sis in only some nephrons. OPN expression (relative to male mice) was somewhat increased in female, pregnant and lactating mice and markedly increased in the parietal epithelium of glomeruli undergoing sclerosi s in aging mice. OPN was also detected in the macula densa. CONCLUSION S: OPN is synthesized and secreted into the tubule fluid by the lumina l epithelia of the distal portions of a subset of kidney nephrons. As animals age expression is found in more proximal portions of the tubul e. OPN may contribute to, or be a consequence of, glomerular sclerosis , and may be an indicator of subclinical injury or infection.