Lopez C A, Hoyer J R, Wilson P D, Waterhouse P, Denhardt D T
Department of Biological Sciences, Nelson Biological Laboratories, Rutgers University, Piscataway, New Jersey.
Lab Invest. 1993 Sep;69(3):355-63.
Osteopontin (OPN) is a secreted Ca(2+)-binding phosphoprotein able to mediate cell attachment to bone via an RGD sequence and the alpha v beta 3 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 study to localize OPN expression.
In situ hybridization with a mouse cDNA probe and immunohistochemical staining with three different antisera to mouse OPN were used to identify those cells that contained significant levels of mRNA and protein, respectively.
Both methods of analysis revealed that OPN expression in the normal mouse kidney was primarily restricted to the thick ascending limbs of the loop of Henle and to the distal convoluted tubules. Protein was detected predominantly at the apical surface of cells lining the lumen of a subset of tubules. The alpha v beta 3 integrin, which is a receptor for vitronectin and osteopontin, was uniformly localized by immunostaining not on the apical surface but rather to the baso-lateral surface of cells in the distal part of the tubule. OPN expression was not detected in healthy glomeruli, proximal tubules, thin limbs of the loop of Henle, collecting ducts, or interstitial fibroblasts. In contrast to the localization of Tamm-Horsfall protein expression, in all distal nephrons, expression of OPN was detected by both methods of analysis 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 sclerosis in aging mice. OPN was also detected in the macula densa.
OPN is synthesized and secreted into the tubule fluid by the luminal epithelia of the distal portions of a subset of kidney nephrons. As animals age expression is found in more proximal portions of the tubule. OPN may contribute to, or be a consequence of, glomerular sclerosis, and may be an indicator of subclinical injury or infection.
骨桥蛋白（OPN）是一种分泌型的钙离子结合磷蛋白，能够通过RGD序列和αvβ3整合素介导细胞与骨的附着。OPN mRNA在肾脏中高水平表达，且该蛋白存在于尿液中。由于已发表的关于该蛋白产生部位的报道相互矛盾，我们进行了一项全面研究以定位OPN的表达。
使用小鼠cDNA探针进行原位杂交，并使用三种不同的抗小鼠OPN抗血清进行免疫组织化学染色，分别鉴定那些含有大量mRNA和蛋白的细胞。
两种分析方法均显示，正常小鼠肾脏中的OPN表达主要局限于髓袢升支粗段和远曲小管。蛋白主要在一部分小管管腔衬里细胞的顶端表面检测到。作为玻连蛋白和骨桥蛋白受体的αvβ3整合素，通过免疫染色均匀地定位于小管远端细胞的基底外侧表面，而非顶端表面。在健康的肾小球、近端小管、髓袢细段、集合管或间质成纤维细胞中未检测到OPN表达。与Tamm-Horsfall蛋白表达的定位不同，在所有远曲小管中，仅在一些肾单位中通过两种分析方法检测到OPN表达。雌性、怀孕和哺乳期小鼠的OPN表达（相对于雄性小鼠）略有增加，而在衰老小鼠中发生硬化的肾小球壁层上皮中则明显增加。在致密斑中也检测到了OPN。
OPN由一部分肾单位远端的管腔上皮合成并分泌到小管液中。随着动物年龄增长，在小管更靠近近端的部分也可发现其表达。OPN可能促成肾小球硬化或为其结果，并且可能是亚临床损伤或感染的指标。
