Podocin-Related Mechanisms in Posttransplantation Recurrence of Focal Segmental Glomerulsclerosis

Posttransplantation recurrence of focal segmental glomerulosclerosis (FSGS) is one of the most disarming events in human pathology with important social and psychological consequences. It usually occurs in 30% to 50% of patients affected by the primary form of the disease with an abrupt onset in the majority of cases occurring within 1 month of the transplantation. Prediction of recurrent cases and early therapy with plasmapheresis are the main goals of the therapy. Although the mechanism of posttransplantation recurrence is still obscure, it has been proposed to be of a multifactorial origin, in which plasma factors determine the shedding of proteins of the slit-diaphragm, such as nephrin and podocin, with structural alterations of the ultra-filtering unit of the glomerulus. Low resynthesis of podocin and/or haplo-insufficiency due to heterozygous mutations should represent significant predisposing factors to proteinuria. In this review, the role of podocin in posttransplantation recurrence will be evaluated focusing on the possibility that resynthesis of the protein could represent a key step also for stable normalization of the renal filter. The recent characterization of the podocin promoter cis- and trans- acting elements and the possibility to characterize low- and high-podocin producer haplotypes offer opportunities to evaluate the capacity for podocin resynthesis in the donor kidney. A review of the literature on posttransplantation recurrence of FSGS in patients originally carrying homozygous and/or heterozygous NPHS2 mutations supports the general idea of a multifactorial origin of the primary disease that can be extended to the pathogenesis of posttransplantation recurrence.

Cis and trans regulatory elements in NPHS2 promoter: implications in proteinuria and progression of renal diseases

Podocin (NPHS2) expression in podocytes is associated with variable degrees of proteinuria and progression to renal failure in different glomerular diseases that suggests different expression profiles in NPHS2 promoter. Three functional polymorphisms in NPHS2 promoter (-51T, -116T, and -535 insCTTTTTT(3)) were found determining strong downregulation (-73, -59, and -82%, respectively) of the reporter gene expression when transfected in podocytes. Electrophoretic mobility shift assay experiments showed that all wild-type variants (-51G, -116C, and -535 insCTTTTTT(2)) formed specific DNA-protein complexes with podocyte nuclear extracts that were abolished by the presence of the rare forms (-51T, -116T, and -535 insCTTTTTT(3)). In the case of -51G, upstream stimulatory factor-1 (USF1) was identified as the specific trans element in accord to binding inhibition experiments and USF1 RNAi silencing. Haplotype analysis of 204 normal controls and 545 patients with renal diseases (308 immunoglobulin (Ig)A nephropathy and 237 focal segmental glomerulosclerosis) evidenced that -116/-51 and -535/P2OL formed two blocks in strong linkage disequilibrium in both normal and pathological cohorts. The high NPHS2 promoter profile -116C/-51G haplotype was more frequent in patients with IgA nephropathy (P-value=0.005) and was associated with a better clinical outcome in terms of proteinuria and creatinine levels. Overall our study describes functional variants of NPHS2 promoter and characterizes trans-acting elements that modulate podocin expression in the kidney. High producer NPHS2 promoter haplotypes seem protective in patients with chronic glomerular diseases.

Familial aggregation of primary glomerulonephritis in an Italian population isolate: Valtrompia study

Hereditary factors are suspected to contribute to the pathogenesis of sporadic primary glomerulonephritis, but their contribution is difficult to delineate in the general population. We studied the prevalence of primary glomerulonephritis in an isolated population from the extreme northern Valtrompia valley, Northern Italy. Investigation of medical records, community urinary screening program and molecular characterization of the population's ancestry were performed; genealogies of affected individuals were researched. Forty-three patients with primary glomerulonephritis were identified: 25 had biopsy-proven disease (11 immunoglobulin A (IgA) nephropathy; eight mesangial proliferative glomerulonephritis without IgA deposits; four focal segmental glomerular sclerosis; two membranous nephropathy), and 18 had clinical glomerulonephritis. All 43 patients originated from three mountain villages (Collio, San Colombano, and Bovegno). In contrast, we found only four cases of primary glomerulonephritis in two nearby villages (Pezzaze and Tavernole) that shared similar population histories and lifestyles, demonstrating heterogeneity of risk factors for glomerulonephritis (P=3 x 10(-5)). All 43 affected individuals could be traced back to common ancestors (XVI-XVII centuries), enabling the construction of three large pedigree including three parent-child affected pairs and five affected siblings pairs. Molecular data showed lower genetic diversity and increased inbreeding in the Valtrompia population compared to the control population. Molecular and genealogical evidence of limited set of founders and the absence of shared nephritogenic environmental factors suggest that our patients share a common genetic susceptibility to the development of primary glomerulonephritis. Further molecular study of our families will offer the possibility to shed light on the genetic background underlying these glomerular disorders.

Glomerular clusterin is associated with PKC-alpha/beta regulation and good outcome of membranous glomerulonephritis in humans

Mechanisms for human membranous glomerulonephritis (MGN) remain elusive. Most up-to-date concepts still rely on the rat model of Passive Heymann Nephritis that derives from an autoimmune response to glomerular megalin, with complement activation and membrane attack complex assembly. Clusterin has been reported as a megalin ligand in immunodeposits, although its role has not been clarified. We studied renal biopsies of 60 MGN patients by immunohistochemistry utilizing antibodies against clusterin, C5b-9, and phosphorylated-protien kinase C (PKC) isoforms (pPKC). In vitro experiments were performed to investigate the role of clusterin during podocyte damage by MGN serum and define clusterin binding to human podocytes, where megalin is known to be absent. Clusterin, C5b-9, and pPKC-alpha/beta showed highly variable glomerular staining, where high clusterin profiles were inversely correlated to C5b-9 and PKC-alpha/beta expression (P=0.029), and co-localized with the low-density lipoprotein receptor (LDL-R). Glomerular clusterin emerged as the single factor influencing proteinuria at multivariate analysis and was associated with a reduction of proteinuria after a follow-up of 1.5 years (-88.1%, P=0.027). Incubation of podocytes with MGN sera determined strong upregulation of pPKC-alpha/beta that was reverted by pre-incubation with clusterin, serum de-complementation, or protein-A treatment. Preliminary in vitro experiments showed podocyte binding of biotinilated clusterin, co-localization with LDL-R and specific binding inhibition with anti-LDL-R antibodies and with specific ligands. These data suggest a central role for glomerular clusterin in MGN as a modulator of inflammation that potentially influences the clinical outcome. Binding of clusterin to the LDL-R might offer an interpretative key for the pathogenesis of MGN in humans.

Circulating anti-actin and anti-ATP synthase antibodies identify a sub-set of patients with idiopathic nephrotic syndrome

Idiopathic nephrotic syndrome (iNS) with resistance or dependence to steroids is a common disease in children but in spite of an increasing clinical impact its pathogenesis is unknown. We screened for the presence of circulating antibodies against glomerular (podocytes, mesangium) and tubular cells (tubular epithelia) a cohort of 60 children with iNS including 8 patients with a familial trait of iNS or with proven mutation of NPHS1-NPHS2 and 12 with good sensitivity to steroids. Positive sera were found in 8 cases, all belonging to the category without familial trait/molecular defects. The targets of antibodies were characterized with Western blot and MALDI-Mass utilizing beta-hexyl cell extracts separated with two-dimensional electrophoresis. In all cases antibodies of the IgM class were directed against ATP synthase beta chain alone (4 cases) or in combination with actin (3 cases); one child presented IgG against aldose reductase. The clinical picture was nephrotic syndrome with steroid resistance or dependence and variable cyclosporin sensitivity; 3 patients developed end stage renal failure. The basic pathology picture was focal segmental glomerulosclerosis (FSGS) in 4 cases and mesangial proliferative glomerulonephrites with deposition of IgM in 2. Overall, patients with circulating auto-antibodies could not be readely differentiated on clinical grounds with the exception of 3 children who developed positivity for antinuclear antibodies during the follow-up. Affinity-purified IgM from one patient who underwent plasmapheresis for therapeutical pourposes (but not from a normal pool) induced proteinuria in Sprague-Dawley rats and concomitant human IgM deposition within glomeruli. This is the first report of circulating anti-actin/ATP synthase beta chain antibodies in a subset of patients with iNS. Both pathological significance and clinical impact given by the presence of these antibodies and the relationship with other conditions such as lupus-erythematosus, characterized by their presence, must be defined.