Fibrinogen that appears in Bowman's space of proteinuric kidneys in vivo activates podocyte Toll-like receptors 2 and 4 in vitro.

Background: Composition of nonselective proteinuria includes several endogenous ligands of Toll-like receptors (TLRs) not normally present in Bowman's space, thus raising the possibility that TLRs are involved in proteinuria-mediated podocyte injury.

Methods: Kidneys of NEP25 mice, a model of glomerular sclerosis induced by podocyte-specific injury, were immunohistochemically evaluated for the presence of fibrin/fibrinogen, which are potent ligands for TLRs. A podocyte cell line was treated with fibrinogen or lipopolysaccharides and examined for expression of cytokines. siRNAs were used to knockdown components of TLR signaling.

Results: We found deposits of fibrin/fibrinogen only in the damaged podocytes of proteinuric kidneys, indicating that podocytes are exposed to these potent TLR ligands in proteinuric state. In cultured podocytes, we confirmed mRNA expressions of TLR2, TLR4, as well as their major TLR signal transducer, MyD88. Fibrinogen and lipopolysaccharides dose-dependently upregulated mRNA expressions of MCP-1, TNF-alpha and TLR2 in podocytes as well as increased the MCP-1 protein in the medium. Knockdown of TLR2 and TLR4 inhibited the fibrinogen-induced MCP-1 mRNA upregulation. Knockdown of MyD88 also inhibited the upregulation.

Conclusion: These results suggest that plasma macromolecules that appear in Bowman's space in proteinuric conditions have the capacity to induce podocyte cytokines through TLRs, and thereby accelerate podocyte injury.