Expression of neuropilin-1 by human glomerular epithelial cells in vitro and in vivo

Vascular endothelial growth factor (VEGF) is a potent promoter of endothelial mitogenesis and of endothelial permeability. Within the kidney it is synthesized primarily in the visceral glomerular epithelial cells (vGECs); however, the role of VEGF in the glomerulus remains unknown, as does the target cell upon which it acts. Although the target cells may be those of the glomerular endothelium, there are micro-anatomical reasons why this might not be the case. This, therefore, led us to consider the possibility that glomerular VEGF may bind to the vGECs themselves. Since it has been shown that vGECs do not express the main tyrosine kinase VEGF receptors, we chose to study vGEC expression of the more recently described VEGF isoform-specific receptors, the neuropilins. The expression of mRNAs for neuropilin-1, neuropilin-2 and soluble neuropilin was studied in whole kidney, sieved glomeruli and cultured podocytes by reverse transcription-PCR, and neuropilin-1 mRNA expression in isolated single glomeruli was analysed by nested reverse transcription-PCR. The expression of neuropilin-1 protein was investigated in cultured vGECs by Western blotting and immunocytochemistry, and in normal kidney sections by immunohistochemistry. Neuropilin-1 mRNA was detected in whole kidney, single and sieved glomeruli and cultured vGECs. Neuropilin-1 protein was detected in cultured vGECs and in vGECs in normal kidney sections by immunohistochemistry. Thus the present study suggests that vGECs may have the potential to bind the VEGF that they secrete. Functional studies will be required to address the potential significance of this finding in terms of an autocrine loop or VEGF sequestration.