Familial focal segmental glomerulosclerosis

Development of biomarkers for predicting recurrence by determining the metastatic ability of cancer cells

Adjuvant chemotherapy has been carried out for patients with cancer who underwent curative resection, but it is basically not needed for patients without micro-metastatic lesions who undergo a perfectly curative surgical operation. The patients who need adjuvant chemotherapy are defined as those whose micro-metastases cannot be detected by imaging modalities in the other sites of the resective areas, despite curative resection for the primary sites. If biomarkers to efficiently evaluate the metastatic potential of each patient could be developed, we may be able to provide personalized adjuvant chemotherapy in the clinical setting. Actinin-4 (ACTN4, gene name ACTN4) is an actin-bundling protein that we identified in 1998 as a novel molecule involved in cancer invasion and metastasis. Protein overexpression of actinin-4 in cancer cells leads to the invasive phenotype, and patients with gene amplification of ACTN4 have a worse prognosis than patients with a normal copy number in some cancers, including pancreas, lung, and salivary gland cancers. In this review, the biological roles of actinin-4 for cancer invasion and metastasis are summarized, and the potential usefulness of actinin-4 as a biomarker for evaluation of metastatic ability is examined.

The biological role of actinin-4 (ACTN4) in malignant phenotypes of cancer

Invasion and metastasis are malignant phenotypes in cancer that lead to patient death. Cell motility is involved in these processes. In 1998, we identified overexpression of the actin-bundling protein actinin-4 in several types of cancer. Protein expression of actinin-4 is closely associated with the invasive phenotypes of cancers. Actinin-4 is predominantly expressed in the cellular protrusions that stimulate the invasive phenotype in cancer cells and is essential for formation of cellular protrusions such as filopodia and lamellipodia. ACTN4 (gene name encoding actinin-4 protein) is located on human chromosome 19q. ACTN4 amplification is frequently observed in patients with carcinomas of the pancreas, ovary, lung, and salivary gland, and patients with ACTN4 amplifications have worse outcomes than patients without amplification. In addition, nuclear distribution of actinin-4 is frequently observed in small cell lung, breast, and ovarian cancer. Actinin-4, when expressed in cancer cell nuclei, functions as a transcriptional co-activator. In this review, we summarize recent developments regarding the biological roles of actinin-4 in cancer invasion.

Identification of nephropathy candidate genes by comparing sclerosis-prone and sclerosis-resistant mouse strain kidney transcriptomes

BACKGROUND

The genetic architecture responsible for chronic kidney disease (CKD) remains incompletely described. The Oligosyndactyly (Os) mouse models focal and segmental glomerulosclerosis (FSGS), which is associated with reduced nephron number caused by the Os mutation. The Os mutation leads to FSGS in multiple strains including the ROP-Os/+. However, on the C57Bl/6J background the mutation does not cause FSGS, although nephron number in these mice are equivalent to those in ROP-Os/+ mice. We exploited this phenotypic variation to identify genes that potentially contribute to glomerulosclerosis.

METHODS

To identify such novel genes, which regulate susceptibility or resistance to renal disease progression, we generated and compared the renal transcriptomes using serial analysis of gene expression (SAGE) from the sclerosis-prone ROP-Os/+ and sclerosis resistant C57-Os/+ mouse kidneys. We confirmed the validity of the differential gene expression using multiple approaches. We also used an Ingenuity Pathway Analysis engine to assemble differentially regulated molecular networks. Cell culture techniques were employed to confirm functional relevance of selected genes.

RESULTS

A comparative analysis of the kidney transcriptomes revealed multiple genes, with expression levels that were statistically different. These novel, candidate, renal disease susceptibility/resistance genes included neuropilin2 (Nrp2), glutathione-S-transferase theta (Gstt1) and itchy (Itch). Of 34 genes with the most robust statistical difference in expression levels between ROP-Os/+ and C57-Os/+ mice, 13 and 3 transcripts localized to glomerular and tubulointerstitial compartments, respectively, from micro-dissected human FSGS biopsies. Network analysis of all significantly differentially expressed genes identified 13 connectivity networks. The most highly scored network highlighted the roles for oxidative stress and mitochondrial dysfunction pathways. Functional analyses of these networks provided evidence for activation of transforming growth factor beta (TGFβ) signaling in ROP-Os/+ kidneys despite similar expression of the TGFβ ligand between the tested strains.

CONCLUSIONS

These data demonstrate the complex dysregulation of normal cellular functions in this animal model of FSGS and suggest that therapies directed at multiple levels will be needed to effectively treat human kidney diseases.

Palladin is a dynamic actin-associated protein in podocytes

Palladin, a cytoskeletal protein with essential functions for stress fiber formation, is found in developing and mature tissues, including the kidney. To define its role in the kidney, we measured its expression in mouse kidney and found it co-localized with F-actin in smooth muscle cells of renal arterial vessels, mesangial cells, and podocytes but not in tubular epithelium. Using immunoelectron microscopy, we confirmed that palladin was present in podocytes. In cultured mouse podocytes, palladin co-localized with F-actin in dense regions of stress fibers, focal adhesions, cell-cell contacts and motile cell margins. Transfection with the N-terminal half of palladin targeted it to F-actin-containing structures in podocytes while the C-terminal half accumulated in the nucleus, a result also found for endogenous palladin in cultured cells after leptomycin B was used to block nuclear export. Green fluorescent protein (GFP)-tagged palladin was found in dynamic ring-like F-actin structures and ruffles in cultured podocytes after stimulation with epidermal growth factor. Inhibition of palladin expression by transfection of an antisense construct reduced the formation of ring-like structures. Photo-bleaching analysis showed that GFP-palladin turned over with a half-time of 10 s in focal adhesions and dense regions of stress fibers, suggesting that palladin is a dynamic scaffolding protein. Our study shows that palladin is expressed in podocytes and plays an important role in actin dynamics.

Genetic predisposition for glomerulonephritis-induced glomerulosclerosis in rats is linked to chromosome 1

Genetic factors influence renal disease progression, and several loci have been linked to the spontaneous development of proteinuria and glomerulosclerosis in animal models. However, the role of genetic susceptibility in glomerulonephritis-induced progressive glomerulosclerosis is unknown. In a rat model of mesangial proliferative glomerulonephritis, anti-Thy-1 glomerulonephritis (antiThy1GN), Lewis/Maastricht (Lew/Maa) rats exhibit progression to glomerulosclerosis, whereas in genetically related Lewis/Møllegard (Lew/Moll) rats, glomerular lesions are repaired within 3 wk. The genetic factors underlying this strain-related difference are not known. To identify novel quantitative trait loci (QTL) involved in progression or repair in Lewis rats, 145 female backcross rats [F1(Lew/Maa x Lew/Moll) x Lew/Maa] were studied. After induction of antiThy1GN proteinuria, we determined mesangial activation, the percentage of microaneurysms, and the glomerular damage score for each animal; a genome scan using 187 microsatellite markers was performed. QTL mapping revealed a significant QTL for glomerular damage score on chromosome 1 with a logarithm of odds (LOD) score of 3.9. Homozygosity for Lew/Maa DNA in this region was associated with a higher percentage of damaged glomeruli on day 21. Furthermore, suggestive linkage was found for the percentage of glomeruli with microaneurysms on day 3 on chromosome 1, 6, and 11; for mesangial activation on day 7 on chromosome 18, while proteinuria was suggestively linked to chromosome 5 (day 0), 4 (day 3), and 6 (day 7). This study identifies a QTL on rat chromosome 1 that is significantly linked to progressive glomerulosclerosis after acute glomerulonephritis.

The Cytoskeletal Connection to Ion Channels as a Potential Mechanosensory Mechanism: Lessons from Polycystin-2 (TRPP2)

Mechanosensitivity of ion channels, or the ability to transfer mechanical forces into a gating mechanism of channel regulation, is split into two main working (not mutually exclusive) hypotheses. One is that elastic and/or structural changes in membrane properties act as a transducing mechanism of channel regulation. The other hypothesis involves tertiary elements, such as the cytoskeleton which, itself by dynamic interactions with the ion channel, may convey conformational changes, including those ascribed to mechanical forces. This hypothesis is supported by numerous instances of regulatory changes in channel behavior by alterations in cytoskeletal structures/interactions. However, only recently, the molecular nature of these interactions has slowly emerged. Recently, a surge of evidence has emerged to indicate that transient receptor potential (TRP) channels are key elements in the transduction of a variety of environmental signals. This chapter describes the molecular linkage and regulatory elements of polycystin-2 (PC2), a TRP-type (TRPP2) nonselective cation channel whose mutations cause autosomal dominant polycystic kidney disease (ADPKD). The chapter focuses on the involvement of cytoskeletal structures in the regulation of PC2 and discusses how these connections are the transducing mechanism of environmental signals to its channel function.

Alpha-actinin associates with polycystin-2 and regulates its channel activity

Polycystin-2 (PC2) is the product of the PKD2 gene, which is mutated in 10-15% patients of autosomal dominant polycystic kidney disease (ADPKD). PC2 is an integral transmembrane protein and acts as a calcium-permeable cation channel. The functional modulation of this channel by other protein partners remains largely unknown. In the present study, using a yeast two-hybrid approach, we discovered that both intracellular N- and C-termini of PC2 associate with alpha-actinins, actin-binding and actin-bundling proteins important in cytoskeleton organization, cell adhesion, proliferation and migration. The PC2-alpha-actinin association was confirmed by in vitro glutathione S-transferase pull-down and dot blot overlay assays. In addition, the in vivo interaction between endogenous PC2 and alpha-actinins was demonstrated by co-immunoprecipitation in human embryonic kidney 293 and Madin-Darby canine kidney (MDCK) cells, rat kidney and heart tissues and human syncytiotrophoblast (hST) apical membrane vesicles. Immunofluorescence experiments showed that PC2 and alpha-actinin were partially co-localized in epithelial MDCK and inner medullary collecting duct cells, NIH 3T3 fibroblasts and hST vesicles. We studied the functional modulation of PC2 by alpha-actinin in a lipid bilayer electrophysiology system using in vitro translated PC2 and found that alpha-actinin substantially stimulated the channel activity of reconstituted PC2. A similar stimulatory effect of alpha-actinin on PC2 was also observed when hST vesicles were reconstituted in lipid bilayer. Thus, physical and functional interactions between PC2 and alpha-actinin may play an important role in abnormal cell adhesion, proliferation and migration observed in ADPKD.

Molecular basis of proteinuria

The glomerular filtration barrier is composed of endothelial cells, basement membrane, and podocytes. In recent years, remarkable progress has been made in our understanding of the molecular structure of the filtration barrier and its relation to the effectiveness of the barrier function. The glomerular basement membrane is composed of a multitude of proteins, including collagen IV, heparan sulfate proteoglycans, and laminin, among others. The slit diaphragm, which is seen as a membrane covering the space between adjacent foot processes close to the basement membrane, is an extremely important structure with a crucial role in permselectivity of the filtration barrier. Its composition is now understood to consist primarily of a unique protein called nephrin. Mutations in the gene-encoding nephrin are known to result in the Finnish type of nephrotic syndrome. The exact mechanism by which nephrin controls permselectivity is not yet clear, but it is known to interact with several podocyte proteins including CD2AP, podocin, and alpha-actinin-4. Abnormalities of any of these proteins may result in proteinuria. The role of nephrin and its associated proteins in the pathogenesis of common acquired glomerulopathies in humans is still under investigation. Normal function of podocyte also depends upon maintaining a fully mature and terminally differentiated phenotype. A host of transcription factors, especially WT1 and PAX2, play a significant role in modulating podocyte function.

Podocyte biology and the emerging understanding of podocyte diseases

The understanding of the unique molecular apparatus of the podocyte has increased dramatically in recent years. This new knowledge has improved the diagnosis and classification of the diseases that have been termed podocytopathies. Podocyte injury frequently leads to reorganization of the slit diaphragm and reorganization of the foot process structure. Four major causes of foot process effacement can be identified, with some due to genetic mutations and others due to acquired conditions: (1) impaired formation of the slit diaphragm complex; (2) abnormalities of the glomerular basement membrane or the adhesion of podocytes to the glomerular basement membrane; (3) abnormalities of the actin cytoskeleton and associated proteins, and (4) alterations in the apical membrane domain of the podocyte. The major podocytopathies can also be organized into four categories, including those with a normal glomerular histology, diffuse mesangial sclerosis, focal segmental glomerulosclerosis, and collapsing glomerulopathy.

Inducible podocyte-specific gene expression in transgenic mice

The podocyte plays a key role in glomerular function and glomerular disease. To facilitate studies of podocyte function, we have developed a transgenic mouse model with inducible expression in the podocyte. The tetracycline-inducible transgenic system facilitates gene expression with restricted cellular distribution and tight temporal control. Recently, Bujard and colleagues have developed a functionally improved reverse tetracycline-controlled transcriptional activator (rtTA) with substantially lower background in the off state (the absence of tetracycline) and greater inducibility in the on state (the presence of tetracycline). We used the human podocin (NPHS2) gene promoter to control expression of the rtTA cassette and bred these mice with a reporter mouse line that contains the cytomegalovirus minimal promoter and tetO promoter elements together with LacZ, encoding beta-galactosidase. Dual transgenic mice, bearing both podocin-rtTA and tetO-LacZ transgenes, had no detectable expression in kidney or other organs in the absence of tetracycline. Administration of tetracycline in the drinking water was associated with podocyte expression of beta-galactosidase, in a fashion that was time dependent (maximal at 1 wk) and dose-dependent (maximal at 2 mg/ml). Podocyte expression was confirmed in two ways: histochemical staining for beta-galactosidase and double-immunostaining using the podocyte marker WT-1 and beta-galactosidase. This transgenic system should aid future investigations of podocyte function.

Pathobiochemistry of nephrotic syndrome

Nephrotic syndrome is a clinical and laboratory syndrome caused by the increased permeability of the glomerular capillary wall for macromolecules. Nephrotic syndrome is a potentially life-threatening state and persistent nephrotic syndrome has a poor prognosis with a high risk of progression to end-stage renal failure and a high risk of cardiovascular complications due to severe hyperlipidemia. Pathogenesis of increased glomerular permeability in different glomerular diseases has not been fully elucidated. Recently, identification of the mutated genes for some podocyte proteins (nephrin, podocin, alpha-actinin-4) in rare familial forms of nephrotic syndrome shed has new light on the molecular mechanisms of glomerular permselectivity. Gradually it becomes apparent that sporadic mutations of podocyte proteins (e.g., podocin) may be present even in some patients with acquired nephrotic syndrome. Expression of other podocyte proteins may change during the course of experimental nephrotic syndrome, possibly as a response to podocyte damage resulting either in apoptosis or stimulation of proliferation and some form of repair, including glomerular sclerosis. Better understanding of these mechanisms could clearly also have therapeutic implications. Glomerular permeability factors are believed to play a role in some noninflammatory glomerular diseases, mainly minimal change disease and focal segmental glomerulosclerosis, but their molecular identification remains elusive, possibly due to the nonhomogeneous nature of the underlying diseases. As an example, focal segmental glomerulosclerosis possibly can be caused by the sporadic mutation of some genes for podocyte proteins, increased production of glomerular permeability factor (possibly by T lymphocytes), or the loss of inhibitors of glomerular permeability factors in nephrotic urine. Clearly the factors causing increased glomerular permeability and factors perpetuating glomerular sclerosis are not necessarily the same. Proteinuria does not seem to be only the consequence of glomerular damage, but it may possibly cause tubular damage and initiate interstitial fibrosis and thus contribute to the progression of chronic renal failure in proteinuric renal diseases. Recent insights into the mechanisms of tubular protein reabsorption may give new tools for preventing the progression of chronic renal disease. Cubilin inhibitors could potentially ameliorate tubular and interstitial damage in patients with heavy proteinuria refractory to treatment. Nephrotic hyperlipidemia is accompanied with increased risk of cardiovascular complications and should be treated in all patients with persistent nephrotic syndrome. The putative positive effect of hypolipidemic drugs (namely statins) on the cardiovascular risk and potentially also on the rate of progression of chronic renal failure remains to be demonstrated in prospective controlled studies. Recent progress in understanding podocyte biology in rare inherited glomerular diseases gives the chance to understand in the near future the molecular pathogenesis of increased glomerular permeability in the much more common acquired forms of nephrotic syndrome.

Cross-reactivity and pathogenicity of anti-DNA autoantibodies in systemic lupus erythematosus

Autoantibodies to DNA were discovered over 40 years ago following the discovery a few years earlier of the 'LE' cell phenomenon by Hargraves and colleagues in 1948. These investigators noted that, when leucocytes were incubated with serum from lupus patients, changes in the nucleus could be seen together with phagocytosis of nuclear remnants by polymorphonuclear leucocytes. Since that time numerous studies in many laboratories have investigated almost every aspect of anti-DNA antibodies, partly to identify what determines their pathology. Whilst a subset of anti-DNA antibodies, especially anti-native, or double-stranded DNA (dsDNA) antibodies constitutes a hallmark of lupus disease and a diagnostic criterion, it is now clear that not all anti-DNA autoantibodies are of pathogenic relevance. Moreover, anti-DNA autoantibodies may also be found in other connective tissue disorders. Here we briefly review studies presented at the fifth international workshop on anti-DNA autoantibodies held in London to highlight relevant properties of pathogenic anti-DNA antibodies.

CD2-associated protein haploinsufficiency is linked to glomerular disease susceptibility

Loss of CD2-associated protein (CD2AP), a component of the filtration complex in the kidney, causes death in mice at 6 weeks of age. Mice with CD2AP haploinsufficiency developed glomerular changes at 9 months of age and had increased susceptibility to glomerular injury by nephrotoxic antibodies or immune complexes. Electron microscopic analysis of podocytes revealed defects in the formation of multivesicular bodies, suggesting an impairment of the intracellular degradation pathway. Two human patients with focal segmental glomerulosclerosis had a mutation predicted to ablate expression of one CD2AP allele, implicating CD2AP as a determinant of human susceptibility to glomerular disease.

Focal and segmental glomerulosclerosis in mice with podocyte-specific expression of mutant alpha-actinin-4

Mutations in the gene encoding alpha-actinin-4 (ACTN4), an actin crosslinking protein, are associated with a form of autosomal dominant focal segmental glomerulosclerosis (FSGS). To better study its progression, a transgenic mouse model was developed by expressing murine alpha-actinin-4 containing a mutation analogous to that affecting a human FSGS family in a podocyte-specific manner using the murine nephrin promoter. Consistent with human ACTN4-associated FSGS, which shows incomplete penetrance, a proportion of the transgenic mice exhibited significant albuminuria (8 of 18), while the overall average systolic BP was elevated in both proteinuric and non-proteinuric ACTN4-mutant mice. Immunofluorescence confirmed podocyte-specific expression of mutant alpha-actinin-4, and real-time RT-PCR revealed that HA-ACTN4 mRNA levels were higher in proteinuric versus non-proteinuric ACTN4-mutant mice. Only proteinuric mice exhibited histologic features consistent with human ACTN4-associated FSGS, including segmental sclerosis and tuft adhesion of some glomeruli, tubular dilatation, mesangial matrix expansion, as well as regions of podocyte vacuolization and foot process fusion. Consistent with such podocyte damage, proteinuric ACTN4-mutant kidneys exhibited significantly reduced mRNA and protein levels of the slit diaphragm component, nephrin. This newly developed mouse model of human ACTN4-associated FSGS suggests a cause-and-effect relationship between actin cytoskeleton dysregulation by mutant alpha-actinin-4 and the deterioration of the nephrin-supported slit diaphragm complex.

Proteinuria and glomerulosclerosis in the Sabra genetic rat model of salt susceptibility

In search of an experimental model that would simulate the association between proteinuria and salt sensitivity in humans, we studied protein excretion in the Sabra rat model of salt susceptibility. Monthly measurements of urinary protein excretion in animals fed standard rat chow revealed that normotensive salt-sensitive SBH/y developed proteinuria that averaged 65 +/- 7 mg/day (n = 10) at 9 mo, whereas proteinuria in normotensive salt-resistant SBN/y was 39 +/- 4 mg/day (n = 10) (P < 0.01). Histopathological evaluation revealed focal and segmental glomerulosclerosis (FSGS) lesions grade 2 in SBH/y and normal histology in SBN/y. To amplify the differences between the strains, uninephrectomy was performed. At 9 mo, proteinuria in SBH/y with one kidney (SBH/y-1K) was 195 +/- 12 mg/day (n = 10) and in SBN/y was 128 +/- 10 mg/day (n = 10) (P < 0.001); histopathology revealed FSGS grade 3 in SBH/y-1K and grade 1-2 in SBN/y-1K. To determine the effect of salt loading, animals were provided with 8% NaCl in chow, causing hypertension in SBH/y but not in SBN/y. Proteinuria markedly increased in both SBH/y with two kidneys (SBH/y-2K) and SBH/y-1K, but not in SBN/y; histopathology revealed FSGS grade 1-2 in SBH/y-2K, grade 2 in SBH/y-1K, no lesions in SBN/y-2K, and grade 0-1 in SBN/y-1K. We concluded that the SBH/y strain is more susceptible to develop proteinuria and glomerulosclerosis than SBN/y. In search for the genetic basis of this phenomenon, we investigated the role of candidate proteinuric gene loci. Consomic strains were constructed by introgressing chromosome 1 (which harbors the rf-1 and rf-2 proteinuric loci) or chromosome 17 (which harbors rf-5) from SBH/y onto the SBN/y genomic background. The resulting consomic strains developed marked proteinuria that was severalfold higher than in SBN/y-1K; histopathological evaluation, however, revealed FSGS lesions grade 1-2, similar to those found in SBN/y-1K and less severe than in SBH/y-1K. These results suggest a functional role of gene systems located on chromosomes 1 and 17 in inducing proteinuria in the salt-susceptible Sabra rat strain. These genetic loci do not appear to harbor major genes for glomerulosclerosis.