Alternative Pathway Dysregulation and the Conundrum of Complement Activation by IgG4 Immune Complexes in Membranous Nephropathy

Membranous nephropathy (MN), a major cause of nephrotic syndrome, is a non-inflammatory immune kidney disease mediated by IgG antibodies that form glomerular subepithelial immune complexes. In primary MN, auto-antibodies target proteins expressed on the podocyte surface, often phospholipase A2 receptor (PLA2R1). Pathology is driven by complement activation, leading to podocyte injury and proteinuria. This article overviews the mechanisms of complement activation and regulation in MN, addressing the paradox that anti-PLA2R1 and other antibodies causing primary MN are predominantly (but not exclusively) IgG4, an IgG subclass that does not fix complement. Besides immune complexes, alterations of the glomerular basement membrane (GBM) in MN may lead to impaired regulation of the alternative pathway. The alternative pathway amplifies complement activation on surfaces insufficiently protected by complement regulatory proteins. Whereas podocytes are protected by cell-bound regulators, the GBM must recruit plasma factor H, which inhibits the alternative pathway on host surfaces carrying certain polyanions, such as heparan sulfate chains. Because heparan sulfate chains present in the normal GBM are lost in MN, we posit that the local complement regulation by factor H may be impaired as a result. Thus, the loss of GBM heparan sulfate in MN creates a micro-environment that promotes local amplification of complement activation, which in turn may be initiated via the classical or lectin pathways by subsets of IgG in immune complexes. A detailed understanding of the mechanisms of complement activation and dysregulation in MN is important for designing more effective therapies

Increased Expression of SVCT2 in a New Mouse Model Raises Ascorbic Acid in Tissues and Protects against Paraquat-Induced Oxidative Damage in Lung

A new transgenic mouse model for global increases in the Sodium Dependent Vitamin C transporter 2 (SVCT2) has been generated. The SVCT2-Tg mouse shows increased SVCT2 mRNA levels in all organs tested and correspondingly increased ascorbic acid (ASC) levels in all organs except liver. The extent of the increase in transporter mRNA expression differed among mice and among organs. The increased ASC levels did not have any adverse effects on behavior in the SVCT2-Tg mice, which did not differ from wild-type mice on tests of locomotor activity, anxiety, sensorimotor or cognitive ability. High levels of SVCT2 and ASC were found in the kidneys of SVCT2-Tg mice and urinary albumin excretion was lower in these mice than in wild-types. No gross pathological changes were noted in kidneys from SVCT2-Tg mice. SVCT2 immunoreactivity was detected in both SVCT2 and wild-type mice, and a stronger signal was seen in tubules than in glomeruli. Six treatments with Paraquat (3x10 and 3x15 mg/kg i.p.) were used to induce oxidative stress in mice. SVCT2-Tg mice showed a clear attenuation of Paraquat-induced oxidative stress in lung, as measured by F2-isoprostanes. Paraquat also decreased SVCT2 mRNA signal in liver, lung and kidney in SVCT2-Tg mice

Integrin alpha 3 beta 1, a Novel Receptor for alpha 3(IV) noncollagenous domain and a trans-dominant inhibitor for integrin alpha v beta 3

Exogenous soluble human alpha 3 noncollagenous (NC1) domain of collagen IV inhibits angiogenesis and tumor growth. These biological functions are attributed to the binding of alpha 3NC1 to integrin alpha v beta 3. However, in some tumor cells that express integrin alpha v beta 3, the alpha 3NC1 domain does not inhibit proliferation, suggesting that integrin alpha v beta 3 expression is not sufficient to mediate the anti-tumorigenic activity of this domain. Therefore, in the present study, we searched for novel binding receptors for the soluble alpha 3NC1 domain in cells lacking alpha v beta 3 integrin. In these cells, soluble alpha 3NC1 bound integrin alpha 3 beta 1; however, unlike alpha v beta 3, alpha 3 beta 1 integrin did not mediate cell adhesion to immobilized alpha 3NC1 domain. Interestingly, in cells lacking integrin alpha 3 beta 1, adhesion to the alpha 3NC1 domain was enhanced due to activation of integrin alpha v beta 3. These findings indicate that integrin alpha 3 beta 1 is a receptor for the alpha 3NC1 domain and transdominantly inhibits integrin alpha v beta 3 activation. Thus integrin alpha 3 beta 1, in conjunction with integrin alpha v beta 3, modulates cellular responses to the alpha 3NC1 domain, which may be pivotal in the mechanism underpinning its anti-angiogenic and anti-tumorigenic activities

Identification of Noncollagenous Sites Encoding Specific Interactions and Quaternary Assembly of α3α4α5(IV) Collagen: IMPLICATIONS FOR ALPORT GENE THERAPY*

Defective assembly of α3α4α5(IV) collagen in the glomerular basement membrane causes Alport syndrome, a hereditary glomerulonephritis progressing to end-stage kidney failure. Assembly of collagen IV chains into heterotrimeric molecules and networks is driven by their noncollagenous (NC1) domains, but the sites encoding the specificity of these interactions are not known. To identify the sites directing quaternary assembly of α3α4α5(IV) collagen, correctly folded NC1 chimeras were produced, and their interactions with other NC1 monomers were evaluated. All α1/α5 chimeras containing α5NC1 residues 188-227 replicated the ability of α5NC1 to bind to α3NC1 and co-assemble into NC1 hexamers. Conversely, substitution of α5NC1 residues 188-227 by α1NC1 abolished these quaternary interactions. The amino-terminal 58 residues of α3NC1 encoded binding to α5NC1, but this interaction was not sufficient for hexamer co-assembly. Because α5NC1 residues 188-227 are necessary and sufficient for assembly into α3α4α5NC1 hexamers, whereas the immunodominant alloantigenic sites of α5NC1 do not encode specific quaternary interactions, the findings provide a basis for the rational design of less immunogenic α5(IV) collagen constructs for the gene therapy of X-linked Alport patients