The homophilic adhesion molecule sidekick-1 contributes to augmented podocyte aggregation in HIV-associated nephropathy

State of the art in childhood nephrotic syndrome: concrete discoveries and unmet needs

Nephrotic syndrome (NS) is a clinical entity characterized by proteinuria, hypoalbuminemia, and peripheral edema. NS affects about 2-7 per 100,000 children aged below 18 years old yearly and is classified, based on the response to drugs, into steroid sensitive (SSNS), steroid dependent, (SDNS), multidrug dependent (MDNS), and multidrug resistant (MRNS). Forms of NS that are more difficult to treat are associated with a worse outcome with respect to renal function. In particular, MRNS commonly progresses to end stage renal failure requiring renal transplantation, with recurrence of the original disease in half of the cases. Histological presentations of NS may vary from minimal glomerular lesions (MCD) to focal segmental glomerulosclerosis (FSGS) and, of relevance, the histological patterns do not correlate with the response to treatments. Moreover, around half of MRNS cases are secondary to causative pathogenic variants in genes involved in maintaining the glomerular structure. The pathogenesis of NS is still poorly understood and therapeutic approaches are mostly based on clinical experience. Understanding of pathogenetic mechanisms of NS is one of the 'unmet needs' in nephrology and represents a significant challenge for the scientific community. The scope of the present review includes exploring relevant findings, identifying unmet needs, and reviewing therapeutic developments that characterize NS in the last decades. The main aim is to provide a basis for new perspectives and mechanistic studies in NS.

Sidekick dynamically rebalances contractile and protrusive forces to control tissue morphogenesis

Contractile actomyosin and protrusive branched F-actin networks interact in a dynamic balance, repeatedly contracting and expanding apical cell contacts to organize the epithelium of the developing fly retina. Previously we showed that the immunoglobulin superfamily protein Sidekick (Sdk) contributes to contraction by recruiting the actin binding protein Polychaetoid (Pyd) to vertices. Here we show that as tension increases during contraction, Sdk progressively accumulates at vertices, where it toggles to recruit the WAVE regulatory complex (WRC) to promote actin branching and protrusion. Sdk alternately interacts with the WRC and Pyd using the same C-terminal motif. With increasing protrusion, levels of Sdk and the WRC decrease at vertices while levels of Pyd increase paving the way for another round of contraction. Thus, by virtue of dynamic association with vertices and interchangeable associations with contractile and protrusive effectors, Sdk is central to controlling the balance between contraction and expansion that shapes this epithelium.

Structure and Functions of Sidekicks

Many of the immunoglobulin superfamily (IgSF) molecules play pivotal roles in cell communication. The Sidekick (Sdk) gene, first described in Drosophila, encodes the single-pass transmembrane protein, Sdk, which is one of the largest among IgSF membrane proteins. Sdk first appeared in multicellular animals during the Precambrian age and later evolved to Sdk1 and Sdk2 in vertebrates by gene duplication. In flies, a single Sdk is involved in positioning photoreceptor neurons and their axons in the visual system and is responsible for dynamically rearranging cell shapes by strictly populating tricellular adherens junctions in epithelia. In vertebrates, Sdk1 and Sdk2 are expressed by unique sets of cell types and distinctively participate in the formation and/or maintenance of neural circuits in the retina, indicating that they are determinants of synaptic specificity. These functions are mediated by specific homophilic binding of their ectodomains and by intracellular association with PDZ scaffold proteins. Recent human genetic studies as well as animal experiments implicate that Sdk genes may influence various neurodevelopmental and psychiatric disorders, such as autism spectrum disorders, attention-deficit hyperactivity disorder, addiction, and depression. The gigantic Sdk1 gene is susceptible to erratic gene rearrangements or mutations in both somatic and germ-line cells, potentially contributing to neurological disorders and some types of cancers. This review summarizes what is known about the structure and roles of Sdks.

Genome-wide association study of multisite chronic pain in UK Biobank

Chronic pain is highly prevalent worldwide and represents a significant socioeconomic and public health burden. Several aspects of chronic pain, for example back pain and a severity-related phenotype 'chronic pain grade', have been shown previously to be complex heritable traits with a polygenic component. Additional pain-related phenotypes capturing aspects of an individual's overall sensitivity to experiencing and reporting chronic pain have also been suggested as a focus for investigation. We made use of a measure of the number of sites of chronic pain in individuals within the UK general population. This measure, termed Multisite Chronic Pain (MCP), is a complex trait and its genetic architecture has not previously been investigated. To address this, we carried out a large-scale genome-wide association study (GWAS) of MCP in ~380,000 UK Biobank participants. Our findings were consistent with MCP having a significant polygenic component, with a Single Nucleotide Polymorphism (SNP) heritability of 10.2%. In total 76 independent lead SNPs at 39 risk loci were associated with MCP. Additional gene-level association analyses identified neurogenesis, synaptic plasticity, nervous system development, cell-cycle progression and apoptosis genes as enriched for genetic association with MCP. Genetic correlations were observed between MCP and a range of psychiatric, autoimmune and anthropometric traits, including major depressive disorder (MDD), asthma and Body Mass Index (BMI). Furthermore, in Mendelian randomisation (MR) analyses a causal effect of MCP on MDD was observed. Additionally, a polygenic risk score (PRS) for MCP was found to significantly predict chronic widespread pain (pain all over the body), indicating the existence of genetic variants contributing to both of these pain phenotypes. Overall, our findings support the proposition that chronic pain involves a strong nervous system component with implications for our understanding of the physiology of chronic pain. These discoveries may also inform the future development of novel treatment approaches.

Risedronate Attenuates Podocyte Injury in Phosphate Transporter-Overexpressing Rats

Osteoporosis patients with chronic kidney disease (CKD) are becoming common in our superaging society. Renal dysfunction causes phosphorus accumulation in the circulating plasma and leads to the development of CKD-mineral bone disorder (MBD). We have previously reported that type III Pi transporter-overexpressing transgenic (Pit-1 TG) rats manifest phosphate (Pi)-dependent podocyte injury. In the present study, we explored the effect of risedronate on Pi-induced podocyte injury in vivo. Pit-1 TG rats and wild-type rats at 5 weeks old were divided into a risedronate-treated group and an untreated group. We subcutaneously administered 5 μg/kg body weight of risedronate or saline twice a week during the experimental period. Risedronate did not alter serum creatinine levels at 5, 8, and 12 weeks of age. However, electron microscopy images showed that thickening of the glomerular basement membrane was improved in the risedronate treatment group. Furthermore, immunostaining for podocyte injury markers revealed that both desmin- and connexin43-positive areas were smaller in the risedronate-treated group than in the untreated group, suggesting that bisphosphonates could rescue Pi-induced podocyte injury. In conclusion, our findings suggest that risedronate could maintain glomerular barrier function by rescuing Pi-induced podocyte injury.

Architecture of cell-cell adhesion mediated by sidekicks

Cell-cell adhesion is important for cell growth, tissue development, and neural network formation. Structures of cell adhesion molecules have been widely studied by crystallography, revealing the molecular details of adhesion interfaces. However, due to technical limitations, the overall structure and organization of adhesion molecules at cell adhesion interfaces has not been fully investigated. Here, we combine electron microscopy and other biophysical methods to characterize the structure of cell-cell adhesion mediated by the cell adhesion molecule Sidekick (Sidekick-1 and Sidekick-2) and obtain 3D views of the Sidekick-mediated adhesion interfaces as well as the organization of Sidekick molecules between cell membranes by electron tomography. The results suggest that the Ig-like domains and the fibronectin III (FnIII) domains of Sidekicks play different roles in cell adhesion. The Ig-like domains mediate the homophilic transinteractions bridging adjacent cells, while the FnIII domains interact with membranes, resulting in a tight adhesion interface between cells that may contribute to the specificity and plasticity of cell-cell contacts during cell growth and neural development.

Molecular Mechanisms of Injury in HIV-Associated Nephropathy

HIV-associated nephropathy (HIVAN) is an important cause of secondary focal glomerulosclerosis that occurs primarily in persons of African ancestry with advanced HIV disease. Although HIVAN is characterized by severe proteinuria and rapid progression to end stage renal disease without treatment, the phenotype is markedly attenuated by treatment with antiretroviral medications. HIV infection of glomerular and tubular epithelial cells and subsequent viral gene expression is a key contributor to HIVAN pathogenesis and the kidney can serve as reservoir for HIV strains that differ those in blood. HIV gene expression in renal epithelial cells leads to dysregulation of cellular pathways including cell cycle, inflammation, cell death, and cytoskeletal homeostasis. Polymorphisms in the APOL1 gene explain the marked predilection of HIVAN to occur in persons of African descent and HIVAN. Since HIVAN has the strongest association with APOL1 genotype of any of the APOL1-associated nephropathies, studies to determine the mechanisms by which HIV and APOL1 risk variants together promote kidney injury hold great promise to improve our understanding of the pathogenesis of APOL1-mediated kidney diseases.

Drosophila Sidekick is required in developing photoreceptors to enable visual motion detection

The assembly of functional neuronal circuits requires growth cones to extend in defined directions and recognize the correct synaptic partners. Homophilic adhesion between vertebrate Sidekick proteins promotes synapse formation between retinal neurons involved in visual motion detection. We show here that Drosophila Sidekick accumulates in specific synaptic layers of the developing motion detection circuit and is necessary for normal optomotor behavior. Sidekick is required in photoreceptors, but not in their target lamina neurons, to promote the alignment of lamina neurons into columns and subsequent sorting of photoreceptor axons into synaptic modules based on their precise spatial orientation. Sidekick is also localized to the dendrites of the direction-selective T4 and T5 cells, and is expressed in some of their presynaptic partners. In contrast to its vertebrate homologs, Sidekick is not essential for T4 and T5 to direct their dendrites to the appropriate layers or to receive synaptic contacts. These results illustrate a conserved requirement for Sidekick proteins in establishing visual motion detection circuits that is achieved through distinct cellular mechanisms in Drosophila and vertebrates.

Molecular basis of sidekick-mediated cell-cell adhesion and specificity

Sidekick (Sdk) 1 and 2 are related immunoglobulin superfamily cell adhesion proteins required for appropriate synaptic connections between specific subtypes of retinal neurons. Sdks mediate cell-cell adhesion with homophilic specificity that underlies their neuronal targeting function. Here we report crystal structures of Sdk1 and Sdk2 ectodomain regions, revealing similar homodimers mediated by the four N-terminal immunoglobulin domains (Ig1-4), arranged in a horseshoe conformation. These Ig1-4 horseshoes interact in a novel back-to-back orientation in both homodimers through Ig1:Ig2, Ig1:Ig1 and Ig3:Ig4 interactions. Structure-guided mutagenesis results show that this canonical dimer is required for both Sdk-mediated cell aggregation (via trans interactions) and Sdk clustering in isolated cells (via cis interactions). Sdk1/Sdk2 recognition specificity is encoded across Ig1-4, with Ig1-2 conferring the majority of binding affinity and differential specificity. We suggest that competition between cis and trans interactions provides a novel mechanism to sharpen the specificity of cell-cell interactions.

HIV-associated nephropathies: epidemiology, pathology, mechanisms and treatment

HIV is a highly adaptive, rapidly evolving virus, which is associated with renal diseases including collapsing glomerulopathy-the classic histomorphological form of HIV-associated nephropathy. Other nephropathies related to viral factors include HIV-immune-complex kidney disease and thrombotic microangiopathy. The distribution of HIV-associated kidney diseases has changed over time and continues to vary across geographic regions worldwide. The reasons for this diversity are complex and include a critical role of APOL1 variants and possibly other genetic factors, disparities in access to effective antiviral therapies, and likely other factors that we do not yet fully understand. The mechanisms responsible for HIVAN, including HIV infection of podocytes and tubular epithelial cells, the molecules responsible for HIV entry, and diverse mechanisms of cell injury, have been the focus of much study. Although combined antiretroviral therapy is effective at preventing and reversing HIVAN, focal segmental glomerulosclerosis, arterionephrosclerosis and diabetic nephropathy are increasingly common in individuals who have received such therapy for many years. These diseases are associated with metabolic syndrome, obesity and premature ageing. Future directions for HIV-related kidney disease will involve regular screening for drug nephrotoxicity and incipient renal disease, as well as further research into the mechanisms by which chronic inflammation can lead to glomerular disease.

The WAVE regulatory complex links diverse receptors to the actin cytoskeleton

The WAVE regulatory complex (WRC) controls actin cytoskeletal dynamics throughout the cell by stimulating the actin-nucleating activity of the Arp2/3 complex at distinct membrane sites. However, the factors that recruit the WRC to specific locations remain poorly understood. Here, we have identified a large family of potential WRC ligands, consisting of ∼120 diverse membrane proteins, including protocadherins, ROBOs, netrin receptors, neuroligins, GPCRs, and channels. Structural, biochemical, and cellular studies reveal that a sequence motif that defines these ligands binds to a highly conserved interaction surface of the WRC formed by the Sra and Abi subunits. Mutating this binding surface in flies resulted in defects in actin cytoskeletal organization and egg morphology during oogenesis, leading to female sterility. Our findings directly link diverse membrane proteins to the WRC and actin cytoskeleton and have broad physiological and pathological ramifications in metazoans.

Visualization of cytoskeletal dynamics in podocytes using adenoviral vectors

Glomerular visceral epithelial cells (podocytes) play a key role in maintaining selective protein filtration in the kidney. Podocytes have a complex cell shape characterized by the presence of numerous actin-rich processes, which cover the surface of glomerular capillaries and are connected by specialized cell-cell adhesion complexes (slit diaphragms). Human genetic studies and experiments in knockout mouse models show that actin filaments and actin-associated proteins are indispensable for the maintenance of podocyte shape, slit diaphragm integrity, and normal glomerular filtration. The ability to examine cytoskeletal protein organization and dynamics in podocytes and to test the effects of disease-associated mutations on protein localization provides valuable information for researchers aiming to dissect the molecular mechanisms of podocyte dysfunction. We describe how adenovirus-mediated transduction of cultured podocytes with DNA constructs can be used to reliably introduce fluorescently tagged cytoskeletal markers for live cell imaging with high efficiency and low toxicity. This technique can be used to study the dynamic reorganization of the podocyte cytoskeleton and to test the effects of novel mutations on podocyte cytoskeletal dynamics.

Essential role of poly(ADP-ribosyl)ation in cocaine action

Many of the long-term effects of cocaine on the brain's reward circuitry have been shown to be mediated by alterations in gene expression. Several chromatin modifications, including histone acetylation and methylation, have been implicated in this regulation, but the effect of other histone modifications remains poorly understood. Poly(ADP-ribose) polymerase-1 (PARP-1), a ubiquitous and abundant nuclear protein, catalyzes the synthesis of a negatively charged polymer called poly(ADP-ribose) or PAR on histones and other substrate proteins and forms transcriptional regulatory complexes with several other chromatin proteins. Here, we identify an essential role for PARP-1 in cocaine-induced molecular, neural, and behavioral plasticity. Repeated cocaine administration, including self-administration, increased global levels of PARP-1 and its mark PAR in mouse nucleus accumbens (NAc), a key brain reward region. Using PARP-1 inhibitors and viral-mediated gene transfer, we established that PARP-1 induction in NAc mediates enhanced behavioral responses to cocaine, including increased self-administration of the drug. Using chromatin immunoprecipitation sequencing, we demonstrated a global, genome-wide enrichment of PARP-1 in NAc of cocaine-exposed mice and identified several PARP-1 target genes that could contribute to the lasting effects of cocaine. Specifically, we identified sidekick-1--important for synaptic connections during development--as a critical PARP-1 target gene involved in cocaine's behavioral effects as well as in its ability to induce dendritic spines on NAc neurons. These findings establish the involvement of PARP-1 and PARylation in the long-term actions of cocaine.

Identification of hypertension susceptibility loci on rat chromosome 12

Previous studies have identified multiple blood pressure and renal disease quantitative trait loci located on rat chromosome 12. In the present study, we narrowed blood pressure loci using a series of overlapping Dahl salt-sensitive/Mcwi (SS)-12 Brown Norway (BN) congenic lines. We found that transferring 6.1 Mb of SS chromosome 12 (13.4-19.5 Mb) onto the consomic SS-12BN background significantly elevated blood pressure on 1% NaCl (146±6 versus 127±1 mm Hg; P<0.001) and 8% NaCl diets (178±7 versus 144±2 mm Hg; P<0.001). Compared with the SS-12BN consomic, these animals also had significantly elevated albumin (218±31 versus 104±8 mg/d; P<0.001) and protein excretion (347±41 versus 195±12 mg/d; P<0.001) on a 1% NaCl diet. Elevated blood pressure, albuminuria, and proteinuria coincided with greater renal and cardiac damage, demonstrating that SS allele(s) within the 6.1 Mb congenic interval are associated with strong cardiovascular disease phenotypes. Sequence analysis of the 6.1 Mb congenic region revealed 12 673 single nucleotide polymorphisms between SS and BN rats. Of these polymorphisms, 293 lie within coding regions, and 18 resulted in nonsynonymous changes in conserved genes, of which 5 were predicted to be potentially damaging to protein function. Syntenic regions in human chromosome 7 have also been identified in multiple linkage and association studies of cardiovascular disease, suggesting that genetic variants underlying cardiovascular phenotypes in this congenic strain can likely be translated to a better understanding of human hypertension.

Recent progress in HIV-associated nephropathy

The classic kidney disease of HIV infection, HIV-associated nephropathy (HIVAN), is an aggressive form of collapsing focal segmental glomerulosclerosis with accompanying tubular and interstitial lesions. HIVAN was first described among African-Americans and Haitian immigrants with advanced HIV disease, an early suggestion of a strong genetic association. This genetic susceptibility was recently linked to polymorphisms on chromosome 22 in individuals of African descent. The association with advanced HIV infection and evidence from HIV-transgenic mice suggested the possibility that HIV directly infects the kidney and that specific HIV gene expression induces host cellular pathways that are responsible for HIVAN pathogenesis. Although combination antiretroviral therapy has substantially reduced the impact of HIVAN in the United States, continued growth of the HIV epidemic in susceptible African populations may have important public health implications. This article reviews recent progress in the pathogenesis and treatment of HIVAN and describes the changing epidemiology of HIV-related kidney disease.

HIV-associated nephropathy: pathogenesis

PURPOSE OF REVIEW

HIV-associated nephropathy (HIVAN) is characterized histologically by a collapsing form of focal segmental glomerulosclerosis (FSGS), microcystic tubular dilation, interstitial inflammation and fibrosis. In this review, we provide a summary of the current state of knowledge about the mechanisms involved in the pathogenesis of HIVAN.

RECENT FINDINGS

Two variants in the ApoL1 gene have been identified as the susceptibility alleles that account for a majority of the increased risk of FSGS and nondiabetic end-stage renal disease in blacks. HIVAN1 and HIVAN2 are the other host susceptibility genes that have been identified in animal models for HIVAN. HIV infects renal tubular epithelial cells likely through direct cell-cell transmission. Both in-vivo and in-vitro evidence suggests that Nef and Vpr are the key viral genes mediating HIVAN. Nef induces podocyte dysfunction, whereas Vpr induces renal tubular epithelial cell apoptosis.

SUMMARY

HIVAN results from direct infection by HIV-1 and expression of viral genes, especially Nef and Vpr, in renal epithelial cells in a genetically susceptible host. The infected renal epithelium acts as a separate viral compartment from the blood and facilitates evolution of strains distant from blood. Dysregulation of several host cellular pathways, including those involved in cell cycle and apoptosis, ultimately results in the unique histopathological syndrome of HIVAN.

Phosphate overload induces podocyte injury via type III Na-dependent phosphate transporter

Uptake of P(i) at the cellular membrane is essential for the maintenance of cell viability. However, phosphate overload is also stressful for cells and can result in cellular damage. In the present study, we investigated the effects of the transgenic overexpression of type III P(i) transporter Pit-1 to explore the role of extracellular P(i) in glomerular sclerosis during chronic renal disease. Pit-1 transgenic (TG) rats showed progressive proteinuria associated with hypoalbuminemia and dyslipidemia. Ultrastructural analysis of TG rat kidney by transmission electron microscopy showed a diffuse effacement of the foot processes of podocytes and a thickening of the glomerular basement membrane, which were progressively exhibited since 8 wk after birth. TG rats died at 32 wk of age due to cachexia. At this time, more thickening of the glomerular basement membrane and segmental sclerosis were observed in glomeruli of the TG rats. Immunohistochemical examination using anti-connexin 43 and anti-desmin antibodies suggested the progressive injury of podocytes in TG rats. TG rats showed higher P(i) uptake in podocytes than wild-type rats, especially under low P(i) concentration. When 8-wk-old wild-type and TG rats were fed a 0.6% normal phosphate (NP) or 1.2% phosphate (HP) diet for 12 wk, HP diet-treated TG rats showed more progressive proteinuria and higher serum creatinine levels than NP diet-treated TG rats. In conclusion, our findings suggest that overexpression of Pit-1 in rats induces phosphate-dependent podocyte injury and damage to the glomerular barrier, which result in the progression of glomerular sclerosis in the kidney.

Up-regulation of the homophilic adhesion molecule sidekick-1 in podocytes contributes to glomerulosclerosis

Focal segmental glomerulosclerosis (FSGS) is a leading cause of nephrotic syndrome and end-stage renal disease worldwide. Although the mechanisms underlying this important disease are poorly understood, the glomerular podocyte clearly plays a central role in disease pathogenesis. In the current work, we demonstrate that the homophilic adhesion molecule sidekick-1 (sdk-1) is up-regulated in podocytes in FSGS both in rodent models and in human kidney biopsy samples. Transgenic mice that have podocyte-specific overexpression of sdk-1 develop gradually progressive heavy proteinuria and severe FSGS. We also show that sdk-1 associates with the slit diaphragm linker protein MAGI-1, which is already known to interact with several critical podocyte proteins including synaptopodin, alpha-actinin-4, nephrin, JAM4, and beta-catenin. This interaction is mediated through a direct interaction between the carboxyl terminus of sdk-1 and specific PDZ domains of MAGI-1. In vitro expression of sdk-1 enables a dramatic recruitment of MAGI-1 to the cell membrane. Furthermore, a truncated version of sdk-1 that is unable to bind to MAGI-1 does not induce podocyte dysfunction when overexpressed. We conclude that the up-regulation of sdk-1 in podocytes is an important pathogenic factor in FSGS and that the mechanism involves disruption of the actin cytoskeleton possibly via alterations in MAGI-1 function.

Assessment of a polymorphism of SDK1 with hypertension in Japanese Individuals

BACKGROUND

Hypertension is a major risk factor for cardiovascular disease. Although genetic studies have suggested that several genetic variants increase the risk for hypertension, the genes that underlie genetic susceptibility to this condition remain to be identified definitively. The purpose of the present study was to identify genetic variants that confer susceptibility to hypertension in Japanese individuals.

METHODS

A total of 5,734 Japanese individuals from two independent populations were examined: subject panel A comprised 2,066 hypertensive individuals and 824 controls; and subject panel B comprised 834 hypertensive individuals and 2,010 controls. The 150 polymorphisms examined in the present study were selected by genome-wide association studies of myocardial infarction and ischemic stroke with the use of the GeneChip Human Mapping 500K Array Set (Affymetrix).

RESULTS

The chi(2)-test revealed that 10 polymorphisms were significantly (P < 0.05) related to the prevalence of hypertension in subject panel A. To validate the relations, these polymorphisms were examined in subject panel B. The A-->G polymorphism (rs645106) of SDK1 and the C-->G polymorphism (rs12078839) of RABGAP1L were significantly associated with hypertension in subject panel B. Multivariable logistic regression analysis with adjustment for covariates, as well as a stepwise forward selection procedure revealed that the A-->G polymorphism of SDK1 was significantly associated with hypertension in both subject panels A and B, with the G allele protecting against this condition.

CONCLUSIONS

SDK1 may be a susceptibility gene for hypertension in Japanese individuals, although the functional relevance of the identified polymorphism was not determined.

The pathogenesis of HIV-associated nephropathy

HIV-associated nephropathy (HIVAN) is one of the leading causes of ESRD in HIV-1-seropositive patients. Patients typically present with heavy proteinuria and chronic renal failure with pathologic findings of collapsing focal segmental glomerulosclerosis (FSGS). The disease is caused by direct infection of renal epithelial cells by HIV-1 in a genetically susceptible host. The genetic factors responsible for the susceptibility to HIVAN among blacks include a noncoding variant in the podocyte-expressed gene nonmuscle myosin, heavy chain 9 (MYH9) as well as other genes yet to be identified. Podocyte and tubular dysfunction results from the expression of viral genes, in particular nef and vpr, and the subsequent dysregulation of numerous host factors, including critical signaling pathways, inflammatory mediators, and others. The identification of these factors has the potential to provide novel therapeutic targets to prevent and treat this important disease.

Pathogenesis of HIV-associated nephropathy

Human immunodeficiency virus-associated nephropathy (HIVAN) is a leading cause of end-stage renal disease in the HIV-1-seropositive population. HIVAN, which is characterized by heavy proteinuria and a rapid decline in renal function, is caused by infection and subsequent expression of viral genes in renal epithelial cells, although the exact mechanism of viral entry into these cells is unknown. The infected renal epithelium is a distinct compartment that supports the evolution of viral strains that may diverge from those found in the patient's blood. Research using animal models and in vitro studies has shown that vpr and nef are the HIV-1 genes most responsible for inducing the characteristic clinical and histopathologic syndrome of HIVAN. Dysregulation of several host factors, including mediators of inflammation, apoptosis, proliferation, transcription, and cell-cell interactions, are also critical factors in determining whether infection of the renal epithelium will lead to HIVAN. Additional research is required to delineate the mechanisms of HIVAN pathogenesis further so that more effective interventions can be implemented to prevent and treat this disease.

The spectrum of focal segmental glomerulosclerosis: new insights

PURPOSE OF REVIEW

Focal segmental glomerulosclerosis (FSGS) is a disease with diverse histologic patterns and etiologic associations. Genetic, toxic, infectious and inflammatory mediators have been identified. This review will focus on new evidence supporting the potential mechanistic basis underlying the histologic variants and their clinical relevance.

RECENT FINDINGS

Evidence from animal models and in-vitro studies suggests that injury inherent within or directed to the podocyte is a central pathogenetic factor. Disruption of signaling from any of the podocyte's specialized membrane domains, including slit diaphragm, apical and basal membranes, or originating at the level of the actin cytoskeleton, may promote the characteristic response of foot process effacement. Irreversible podocyte stress leading to podocyte depletion through apoptosis or detachment is a critical mechanism in most forms of FSGS. In the collapsing variant, podocyte dysregulation leads to podocyte dedifferentiation and glomerular epithelial cell proliferation.

SUMMARY

Translation studies in humans and new evidence from animal models have provided mechanistic insights into the diverse phenotypes of FSGS.

Functional genetic variation in aminopeptidase A (ENPEP): lack of clear association with focal and segmental glomerulosclerosis (FSGS)

The aminopeptidase A (APA) ectopeptidase is an integral membrane-bound zinc metalloprotease that cleaves aspartic and glutamic acidic residues from the N-terminus of a number of protein substrates that includes angiotensin II. Angiotensin II, the most vasoactive component of the renin-angiotensin-aldosterone (RAAS) pathway, can contribute to renal disease by causing an increase in arterial blood pressure leading to glomerular injury and fibrosis. APA is expressed in many organs, including the kidney where it localizes mainly to the podocyte cell membrane and brush borders of the proximal tubule cells. Antibodies directed to the APA peptide can induce an acute massive albuminuria in wild-type BALB/c mice after intravenous injection. We examined whether variants in the APA encoding gene (ENPEP) are more frequent in individuals with the proteinuric disease focal and segmental glomerulosclerosis (FSGS) compared to control individuals. The ENPEP coding sequence was re-sequenced in 188 FSGS patients and 48 controls. Genetic variants were further genotyped in 181 individuals without any known kidney disease. We then examined the effect of the non-synonymous coding variants identified on their cell surface APA activity after transfection in COS-1 cells. Several of these ENPEP variants lead to reproducibly altered APA activity. However, we did not see a clear correlation between the presence of a functional ENPEP variant and FSGS. However, the existence of these variants with marked effect on APA activity suggests that both rare and common variation in ENPEP may contribute to the development of renal and hypertensive disorders and warrants further study.

Infection and glomerulonephritis

Glomerular injury, occurring either as primary glomerular disease or as part of a systemic disease process, is usually a result of immune-mediated mechanisms. The morphologic reaction pattern has a diverse spectrum of appearance, ranging from normal by light microscopy in minimal change disease to crescentic forms of glomerulonephritis, with conspicuous disruption of the normal glomerular morphology. The mechanisms of glomerular immune deposit formation include trapping of circulating antigen-antibody complexes and the in situ formation of immune complexes within the glomerulus. While the majority of postinfectious immune-complex-mediated glomerulonephritides are believed to result from the deposition of circulating antigen-antibody complexes, preformed outside of the kidney and secondarily deposited in the kidney, the notion of forming in situ antigen-antibody complexes to either planted antigens or to integral structural components of the glomerulus, through "cross-reacting" autoimmune reactions, is gaining popularity in a variety of forms of glomerulonephritides. Patients with HIV infection may develop a spectrum of renal pathology, the glomerular manifestations of which include both antigen-antibody complex and nonimmune-complex-mediated pathogenetic mechanisms. Similarly, patients with Streptococcal infections, Hepatitis B virus, or Hepatitis C virus infection may develop a spectrum of glomerulonephritides, which are predominantly immune-complex-mediated. Therapy for glomerular diseases due to HIV, hepatitis B, or C virus infections remains a challenge.