Toll-Like Receptors Regulate the Development and Progression of Renal Diseases

Abstract

Background: Stimulated by both microbial and endogenous ligands, toll-like receptors (TLRs) play an important role in the development and progression of renal diseases.

Summary: As a highly conserved large family, TLRs have 11 members in humans (TLR1∼TLR11) and 13 members in mouse (TLR1∼TLR13). It has been widely reported that TLR2 and TLR4 signaling, activated by both exogenous and endogenous ligands, promote disease progression in both renal ischemia-reperfusion injury and diabetic nephropathy. TLR4 also vitally functions in CKD and infection-associated renal diseases such as pyelonephritis induced by urinary tract infection. Stimulation of intracellular TLR7/8 and TLR9 by host-derived nucleic acids also plays a key role in systemic lupus erythematosus. Given that certain microRNAs with GU-rich sequence have recently been found to be able to serve as TLR7/8 ligands, these microRNAs may initiate pro-inflammatory signal via activating TLR signal. Moreover, as microRNAs can be transferred across different organs via cell-secreted exosomes or protein-RNA complex, the TLR signaling activated by the miRNAs released by other injured organs may also result in renal dysfunction.

Key messages: In this review, we sum up the recent progress in the role of TLRs in various forms of glomerulonephritis and discuss the possible prevention or therapeutic strategies for clinic treatment to renal diseases.

Keywords: Renal diseases; Toll-like receptors.

Fig. 1

Intracellular signal transduction of TLR family. All TLRs except TLR3 recruit MyD88 to activate IRAK complex and TRAF6, which acts on TAK1 to initiate downstream MAPK, NF-κB, IRF5, and IRF7, leading to the production of inflammatory cytokines and IFNs. TRAF6 also activates TBK1-IKKε-IRF3 axis to induce IFNs; yet, this signaling pathway is TRAF3-dependent. TLR3 particularly recruits TRIF to initiate downstream pathways through the activation of TRAF6 and TRAF3. TLR4 initiates signal via both MyD88-dependent and MyD88-independent pathways. TLR, Toll-like receptor; TRAF6, tumor necrosis factor receptor-associated factor 6; TAK1, transforming growth factor β-activated kinase 1, MAPK, mitogen-activated protein kinase; NF-κB, nuclear factor kappa-B; IRF, interferon regulatory factor; IFN, interferon; TRIF, TIR-domain-containing adaptor-inducing interferon-β; TIR, Toll receptor/interleukin (IL)-1 receptor.