Integrin α1β1 and Transforming Growth Factor-β1 Play Distinct Roles in Alport Glomerular Pathogenesis and Serve as Dual Targets for Metabolic Therapy

Abstract

Alport syndrome is a genetic disorder resulting from mutations in type IV collagen genes. The defect results in pathological changes in kidney glomerular and inner-ear basement membranes. In the kidney, progressive glomerulonephritis culminates in tubulointerstitial fibrosis and death. Using gene knockout-mouse models, we demonstrate that two different pathways, one mediated by transforming growth factor (TGF)-beta1 and the other by integrin alpha1beta1, affect Alport glomerular pathogenesis in distinct ways. In Alport mice that are also null for integrin alpha1 expression, expansion of the mesangial matrix and podocyte foot process effacement are attenuated. The novel observation of nonnative laminin isoforms (laminin-2 and/or laminin-4) accumulating in the glomerular basement membrane of Alport mice is markedly reduced in the double knockouts. The second pathway, mediated by TGF-beta1, was blocked using a soluble fusion protein comprising the extracellular domain of the TGF-beta1 type II receptor. This inhibitor prevents focal thickening of the glomerular basement membrane, but does not prevent effacement of the podocyte foot processes. If both integrin alpha1beta1 and TGF-beta1 pathways are functionally inhibited, glomerular foot process and glomerular basement membrane morphology are primarily restored and renal function is markedly improved. These data suggest that integrin alpha1beta1 and TGF-beta1 may provide useful targets for a dual therapy aimed at slowing disease progression in Alport glomerulonephritis.

αvβ6 Integrin Regulates Renal Fibrosis and Inflammation in Alport Mouse

“…The inability to affect kidney function was attributed to the failure to inhibit effacement of the podocyte processes caused by improper composition of the Col4A3-deficient glomerular basement membrane (GBM). 7 Consistent with these findings, we observed podocyte effacement and GBM alterations by transmission electron microscopy in kidneys of the Col4A3 Ϫ/Ϫ ;␤6 Ϫ/Ϫ mice (data not shown). These findings indicate that inhibition of the TGF-␤ pathway does not alter the structural integrity of the GBM in Alport mice.…”

“…Cloned heavy and light chain variable regions were ligated into mammalian expression vectors with human IgG1 constant regions. Recombinant soluble murine TGF-␤ receptor type II-Ig fusion protein (rsTGF-␤RII-Ig) was generated as previously described 7 and purchased from R&D Systems (532-R2; Minneapolis, MN). Antibodies were purchased as indicated: fluorescein isothiocyanate-conjugated pan anti-cytokeratin mAb (C-11; F3418) from Sigma-Aldrich (St. Louis, MO); anti-laminin B1 chain mAb (LT3; MAB1928) from Chemicon (Temecula, CA); phycoerythrin (PE)-conjugated anti-␣v mAb (RMV7; CBL1346P) from Chemicon; rabbit anti-␣v from Chemicon (AB1930); PE-rat IgG1 (553925) from BD Biosciences (San Jose, CA); and anti-smooth muscle actin (SMA)-Cy3 from Sigma-Aldrich (C-6198).…”

“…30,31 It has been previously reported that treatment of Col4A3 Ϫ/Ϫ mice with rsTGF-␤RII-Ig leads to inhibition of kidney fibrosis. 7 Kidneys from Col4A3 Ϫ/Ϫ mice begin to show histological signs of fibrosis at approximately 5 to 6 weeks of age. The disease progresses rapidly with age, and the mice die of renal failure at approximately 11 weeks.…”

Global Gene Expression Analysis Reveals a Role for the α1 Integrin in Renal Pathogenesis

“…The first pathway is TGF-␤ 1 -stimulated and has been well established in many other disease models as well (7)(8)(9)(10)(11). The second pathway is ␣ 1 ␤ 1 integrin-dependent (12). The ␣ 1 ␤ 1 integrin is a major integrin receptor expressed on mesangial cells, and Cosgrove et al (12) hypothesized that loss of ␣ 1 integrin expression on the mesangial cells resulted in the abnormal expression of specific laminin isoforms.…”

“…The second pathway is ␣ 1 ␤ 1 integrin-dependent (12). The ␣ 1 ␤ 1 integrin is a major integrin receptor expressed on mesangial cells, and Cosgrove et al (12) hypothesized that loss of ␣ 1 integrin expression on the mesangial cells resulted in the abnormal expression of specific laminin isoforms. Focusing on glomerular pathogenesis, these authors demonstrated that mesangial expansion and podocyte foot process effacement are attenuated in Alport's animals lacking the ␣ 1 ␤ 1 integrin.…”