Evidence for Activation of the Unfolded Protein Response in Collagen IV Nephropathies

Abstract

Thin-basement-membrane nephropathy (TBMN) and Alport syndrome (AS) are progressive collagen IV nephropathies caused by mutations in COL4A3/A4/A5 genes. These nephropathies invariably present with microscopic hematuria and frequently progress to proteinuria and CKD or ESRD during long-term follow-up. Nonetheless, the exact molecular mechanisms by which these mutations exert their deleterious effects on the glomerulus remain elusive. We hypothesized that defective trafficking of the COL4A3 chain causes a strong intracellular effect on the cell responsible for COL4A3 expression, the podocyte. To this end, we overexpressed normal and mutant COL4A3 chains (G1334E mutation) in human undifferentiated podocytes and tested their effects in various intracellular pathways using a microarray approach. COL4A3 overexpression in the podocyte caused chain retention in the endoplasmic reticulum (ER) that was associated with activation of unfolded protein response (UPR)-related markers of ER stress. Notably, the overexpression of normal or mutant COL4A3 chains differentially activated the UPR pathway. Similar results were observed in a novel knockin mouse carrying the Col4a3-G1332E mutation, which produced a phenotype consistent with AS, and in biopsy specimens from patients with TBMN carrying a heterozygous COL4A3-G1334E mutation. These results suggest that ER stress arising from defective localization of collagen IV chains in human podocytes contributes to the pathogenesis of TBMN and AS through activation of the UPR, a finding that may pave the way for novel therapeutic interventions for a variety of collagenopathies.

Essential Role of X-Box Binding Protein-1 during Endoplasmic Reticulum Stress in Podocytes

“…4 Recent evidence has suggested a robust increase in podocyte BiP due to ER stress caused by Pierson syndrome (laminin mutations), Alport syndrome, thin membrane basement disease (collagen IV mutations), Heymann nephritis, puromcyin-induced nephrosis, and mesangioproliferative GN. [5][6][7][8] Therefore, in the current study, we first verified that treatment of cultured podocytes with tunicamycin, an agent that inhibits N-glycosylation of proteins in the ER, results in an increase in ER stress and activation of the UPR. This effect was further validated in vivo by glomeruli isolated from mice injected with rabbit-anti-mouse glomerular basement membrane nephrotoxic sera (NTS).…”

Endoplasmic reticulum stress in kidney function and disease

“…Deltas and colleagues found that a genetic mutant of collagen IV (G1334E) also accumulated in the ER and evoked ER stress-mediated renal injury in podocytes, resulting in thin-basement-membrane nephropathy and Alport syndrome [16].…”

Mesencephalic astrocyte-derived neurotrophic factor (MANF), a new player in endoplasmic reticulum diseases: structure, biology, and therapeutic roles

“…In cell culture studies, certain nephrin or podocin missense mutants are trapped inside the ER and activate ER stress 9–11 . Moreover, in mouse models, podocyte ER stress induced by LAMB2 C321R 12 , ACTN4 K256E 13 , or COL4A3 G1334E 14 leads to podocytopathy. Meanwhile, in diverse sporadic nephropathies including focal segmental glomerulosclerosis, membranous nephropathy, minimal change disease, and diabetic nephropathy, multiple studies have linked podocyte ER stress to the pathogenesis of these diseases in both experimental models 15–17 and human kidney biopsies 18, 19 .…”