Revisiting Basement Membrane Pathology in Renal Cystic Disease
Authors: Yashpal S. Kanwar
Journal: Journal of the American Society of Nephrology (2010)

Efforts to delineate the mechanisms of polycystic kidney disease (PKD) during the past decade have focused on the primary cilium of renal tubular epithelia.1–3 The current consensus is that genes mutated in PKD code for proteins that somehow modulate the ciliary pathobiology. Similarly, the same issue arises in patients with nephronophthisis (NPH), a distinct cystic disease whereby mutations in NPHP1, NPHP2, and NPHP3 associate with tubular disruption and basement membrane thickening at the corticomedullary junction, leading consequentially to polyuria and salt wasting.4 These NPHP genes are expressed in the cilia located on the apical surface of the tubular epithelial cells. Thus, the prevailing hypothesis is that cilia act as mechanosensors responding to the flow of glomerular filtrate along the tubular lumen of the nephron. This sensor function results in signal transduction from the flow of Ca2+ as a result of the opening of cation channels formed in part by polycystins-1 and -2 transmembrane proteins that are affected by mutations in the autosomal dominant form of PKD (ADPKD),1–4 the net result being that abnormalities in ciliary function cause changes in epithelial cell signaling, polarity, proliferation, and fluid accumulation, ultimately resulting in cystogenesis. Despite this highly attractive notion, not all forms of cystic kidney disease have a cause that is unswervingly related to defects in cilium. In 2006, Miner and colleagues5 reported in a landmark article that mice with a hypomorphic mutation in the laminin α5 gene (Lamα5neo allele) develop PKD-like renal disease along with thickened tubular basement membranes (TBM) in addition to proteinuria and hematuria, the latter being characteristic of glomerular dysfunction. Incidentally, the homozygous mutants (Lamα5neo/neo) die of apparent renal failure at approximately 4 weeks of age. Lamα5 is an extracellular matrix (ECM) protein, and it is not a constituent of the apical ciliary protein complex. It is an integral component of both glomerular basement membrane (GBM) and TBM.6 In the design of the hypomorphic mutation, there is a deliberate insertion of an FLP recombinase-recognition target (FRT) site flanking the neo gene in a Lamα5 intron that facilitates the generation of a conditional null allele, which seems to cause frequent abnormal splicing of the primary Lamα5 transcript, reducing synthesis of full-length Lamα5 protein.5 Reduced Lamα5 levels were observed in many nephron segments. From these seminal studies, the authors concluded that reduced Lamα5 in the GBM causes proteinuria and hematuria, whereas reduced Lamα5 in the TBM induces PKD by altering tubular epithelial cell behavior and perhaps by impairing cilia-mediated signaling events. In a follow-up study that appears in this issue of JASN, Goldberg et al.7 investigate the role of Lamα5 in the GBM and its relationship to the pathogenesis of PKD. Mutating Lamα5 selectively in glomerular podocytes using a fully functional but conditionally null allele and a podocyte-specific Cre causes proteinuria that has a variable onset but invariably progresses to nephrotic range. Intriguingly, there were no tubular abnormalities and no evidence of PKD. This is consistent with the fact that in these mutant mice, the TBMs are unaffected. Moreover, these results reemphasize the point that GBM components are vital for the integrity of the filtration barrier, a concept advanced by Farquhar and Kanwar8 decades ago. In a complementary set of studies, Goldberg et al.7 rescue by two different methods the Lamα5 expression defect in the hypomorphic Lamα5neo mutants specifically targeting podocytes. In the first approach, they removed the FRT site flanking the neo insertion using a novel FLP recombinase transgene expressed only in podocytes using the nephrin promoter. Although the authors did not document the timing of recombination, it seems to be early enough to ensure that podocytes can make the required amount of Lamα5. In the second approach, they expressed a human Lamα5 transgene only in podocytes using a doxycycline-inducible system. Both approaches rescue the glomerular proteinuria and hematuria without affecting the abnormal TBM composition. In addition, in both instances, cystogenesis is inhibited. This means that the glomerular filtration barrier defects somehow initiate cystogenesis in the hypomorphic mutants. In these experiments, it seems difficult to invoke the involvement of cilia in the pathogenesis of PKD in this unique murine model. Moreover, there are no precedents for congenital proteinuria causing cystogenesis. The authors thus suggest that in the precystic kidneys, some of the excessively filtered proteins in the glomerular ultrafiltrate damage the tubular epithelial cells, whose abnormal basement membrane and consequent abnormal cell–matrix interactions might synergize with this injury to promote cystogenesis. Long before the cilium became a focus of PKD research, earlier studies of polycystin-1 suggested that its large extracellular domain is involved in interactions with ECM proteins.1–3 Although such interactions have not yet proved relevant to disease pathogenesis, the notion that ciliary function in epithelial cells might be influenced by some aspect of cell–matrix interactions, which are clearly important for cell polarity, remain a possibility. Relevant to cystogenesis also are studies from the 1980s in which initially the notion of perturbations in cell–matrix interactions were deduced from observations made in a diphenylthiazole-induced murine model of PKD.9 Interestingly, as in nephronophthisis, the cystogenesis was associated with TBM thickening and salt wasting into the urine, and it seems that filtered diphenylthiazole targeted the tubular epithelia selectively without affecting the glomerular cells. The TBMs were noted to be deficient in sulfated proteoglycans, although the status of Lamα5 or the ciliary proteins was not investigated. In line with these studies are the observations made in mice deficient in xylosyltransferase 2 (XylT2), an enzyme that attaches glycosaminoglycan chains onto proteoglycans by O-xylosyl-serine linkage.10 The XylT2−/− mice develop renal cysts with thickened TBMs, suggesting aberrant ECM probably has some role in the pathogenesis of cystogenesis, and this notion is further reinforced by the current elegant Lamα5 genetic studies. Finally, this study raises a number of questions that could be the subject of future investigations. Although the authors previously showed that cilia are present in the mutant, do they function properly? Would the cystic kidney disease still occur in the hypomorphs if Lamα5 were restored in the TBM rather than in the GBM? Can tubular cell injury be documented in the precystic kidney? Would a cilium-relevant mutation exacerbate the cystic phenotype in the Lamα5 mutant? Addressing these questions might allow the cystic phenotype of these mice to be explained in the context of what is currently known about the pathogenesis of PKD. Disclosures None. This work was supported by National Institutes of Health grants DK28492 and DK606035.