Toll-like Receptor 9 Can be Activated by Endogenous Mitochondrial DNA to Induce Podocyte Apoptosis

Abstract

Toll-like receptor 9 (TLR9) senses bacterial DNA characteristic of unmethylated CpG motifs to induce innate immune response. TLR9 is de novo expressed in podocytes of some patients with glomerular diseases, but its role in podocyte injury remains undetermined. Since TLR9 activates p38 MAPK and NFkB that are known to mediate podocyte apoptosis, we hypothesized that TLR9 induces podocyte apoptosis in glomerular diseases. We treated immortalized podocytes with puromycin aminonucleosides (PAN) and observed podocyte apoptosis, accompanied by TLR9 upregulation. Prevention of TLR9 upregulation by siRNA significantly attenuated NFκB p65 or p38 activity and apoptosis, demonstrating that TLR9 mediates podocyte apoptosis. We next showed that endogenous mitochondrial DNA (mtDNA), whose CpG motifs are also unmethylated, is the ligand for TLR9, because PAN induced mtDNA accumulation in endolysosomes where TLR9 is localized, overexpression of endolysosomal DNase 2 attenuated PAN-induced p38 or p65 activity and podocyte apoptosis, and DNase 2 silencing was sufficient to activate p38 or p65 and induce apoptosis. In PAN-treated rats, TLR9 was upregulated in the podocytes, accompanied by increase of apoptosis markers. Thus, de novo expressed TLR9 may utilize endogenous mtDNA as the ligand to facilitate podocyte apoptosis, a novel mechanism underlying podocyte injury in glomerular diseases.

Biomarker prediction for membranous nephropathy prognosis by microarray analysis

“…22 Other studies also confirm the activation of NFKB signal pathway in the immune cells of IgAN patients. 23,24 However, different from these findings in IgAN, in our study, NFKB1 was found to be downregulated in MN samples compared with the control. It might be inferred that the NFKB pathway might be inhibited by other regulators in MN development.…”

“…TLR are demonstrated to be associated with IgAN, and activation of TLR could trigger the transcription factor NFKB . Other studies also confirm the activation of NFKB signal pathway in the immune cells of IgAN patients . However, different from these findings in IgAN, in our study, NFKB1 was found to be downregulated in MN samples compared with the control.…”

The Impact of Circulating Mitochondrial DNA on Cardiomyocyte Apoptosis and Myocardial Injury After TLR4 Activation in Experimental Autoimmune Myocarditis

“…Our study presented that lowering ROS level could inhibit LPS mediated mtDNA damage in cardiomyocytes, which was consistent with the study that peroxyauraptenol(PXT) significantly inhibited inflammation and NLRP3 inflammasome in LPS activated macrophages by preventing ROS generation and the release of mtDNA [37]. MtDNA can also directly induce cell apoptosis by targeting TLR9, as showed in podocyte, HUVECs and cardiomyocytes [21, 23, 38]. In our EAM model, TLR9 expression in myocardium was obviously increased.…”

The rOX‐stars of inflammation: links between the inflammasome and mitochondrial meltdown

“…93 TLR9 recognises unmethylated CpG in the endosomal system, 94,95 including that of mitochondrial origin following mitochondrial disruption. [96][97][98][99] CpG recognition by TLR9 provides a signal 1 for NLRP3 activation, transcriptionally upregulating inflammasome-required genes under NF-jB control 100,101 (Figure 3b). The mitochondrial dynamics that drive disruption and exposure of mtDNA to TLR9 are varied and unclear, yet some evidence implicates defective fission/fusion dynamics.…”