Molecular cloning, expression, and distribution of glomerular epithelial protein 1 in developing mouse kidney

Background. Glomerular epithelial protein 1 (GLEPP1) is a receptor-like mem brane protein tyrosine phosphatase (RPTP) with a large ectodomain consistin g of multiple fibronectin type III repeats, a single transmembrane segment, and a single cytoplasmic phosphatase active site sequence. In adult human and rabbit kidneys, GLEPP1 is found exclusively on apical membranes of podo cytes and especially on surfaces of foot processes. Although neither ligand nor function for this protein is known, other RPTPs with similar topologie s have been implicated in mediating adherence behavior of cells. Methods. To evaluate potential roles of GLEPP1 further, we cloned the full- length mouse GLEPP1 cDNA and examined its expression patterns in developing kidney by Northern blot analysis, in situ hybridization, and immunofluores cence microscopy. Results. Nucleotide sequencing showed that mouse GLEPP1 was approximately 8 0% identical to rabbit and human GLEPP1 and approximately 91% identical at the amino acid Bevel. The membrane-spanning and phosphatase domains of mous e GLEPP1 shared >99% homology with PTP phi, a murine macrophage cytoplasmic phosphatase. Northern analysis identified a single GLEPP1 transcript of ap proximately 5.5 kb in fetal kidney that became approximately threefold more abundant in adults. In situ hybridization of newborn mouse kidney revealed GLEPP1 mRNA in visceral epithelial cells (developing podocytes) of comma- and S-shaped nephric figures, and expression increased in capillary loop an d maturing stage glomeruli. Beginning on embryonic day 14, GLEPP1 protein w as first observed on cuboidal podocytes of capillary loop stage glomeruli, but nascent podocytes of earlier comma- and S-shaped nephric figures were n egative. At later stages of glomerular maturation, where foot process elong ation and interdigitation occurs, GLEPP1 immunolabeling intensified on podo cytes and then persisted at high levels in fully developed glomeruli. Conclusion. Our findings are consistent with a role for GLEPP1 in mediating and maintaining podocyte differentiation specifically.