Glomerulosclerosis and viral gene expression in HIV-transgenic mice: role of nef

HIV-1 Nef acts in synergy with APOL1-G1 to induce nephrocyte cell death in a new Drosophila model of HIV-related kidney diseases

Background: People carrying two APOL1 risk alleles (RA) G1 or G2 are at greater risk of developing HIV-associated nephropathy (HIVAN). Studies in transgenic mice showed that the expression of HIV-1 genes in podocytes, and nef in particular, led to HIVAN. However, it remains unclear whether APOL1-RA and HIV-1 Nef interact to induce podocyte cell death. Method: We generated transgenic (Tg) flies that express APOL1-G1 (derived from a child with HIVAN) and HIV-1 nef specifically in the nephrocytes, the fly equivalent of mammalian podocytes, and assessed their individual and combined effects on the nephrocyte filtration structure and function. Results: We found that HIV-1 Nef acts in synergy with APOL1-G1 resulting in nephrocyte structural and functional defects. Specifically, HIV-1 Nef itself can induce endoplasmic reticulum (ER) stress without affecting autophagy. Furthermore, Nef exacerbates the organelle acidification defects and autophagy reduction induced by APOL1-G1. The synergy between HIV-1 Nef and APOL1-G1 is built on their joint effects on elevating ER stress, triggering nephrocyte dysfunction and ultimately cell death. Conclusions: Using a new Drosophila model of HIV-1-related kidney diseases, we identified ER stress as the converging point for the synergy between HIV-1 Nef and APOL1-G1 in inducing nephrocyte cell death. Given the high relevance between Drosophila nephrocytes and human podocytes, this finding suggests ER stress as a new therapeutic target for HIV-1 and APOL1-associated nephropathies.

Recent advances of animal model of focal segmental glomerulosclerosis

In the last decade, great advances have been made in understanding the genetic basis for focal segmental glomerulosclerosis (FSGS). Animal models using specific gene disruption of the slit diaphragm and cytoskeleton of the foot process mirror the etiology of the human disease. Many animal models have been developed to understand the complex pathophysiology of FSGS. Therefore, we need to know the usefulness and exact methodology of creating animal models. Here, we review classic animal models and newly developed genetic animal models. Classic animal models of FSGS involve direct podocyte injury and indirect podocyte injury due to adaptive responses. However, the phenotype depends on the animal background. Renal ablation and direct podocyte toxin (PAN, adriamycin) models are leading animal models for FSGS, which have some limitations depending on mice background. A second group of animal models were developed using combinations of genetic mutation and toxin, such as NEP25, diphtheria toxin, and Thy1.1 models, which specifically injure podocytes. A third group of animal models involves genetic engineering techniques targeting podocyte expression molecules, such as podocin, CD2-associated protein, and TRPC6 channels. More detailed information about podocytopathy and FSGS can be expected in the coming decade. Different animal models should be used to study FSGS depending on the specific aim and sometimes should be used in combination.

Src family kinases in chronic kidney disease

Src family kinases (SFKs) belong to nonreceptor protein tyrosine kinases and have been implicated in the regulation of numerous cellular processes, including cell proliferation, differentiation, migration and invasion, and angiogenesis. The role and mechanisms of SFKs in tumorgenesis have been extensively investigated, and some SFK inhibitors are currently under clinical trials for tumor treatment. Recent studies have also demonstrated the importance of SFKs in regulating the development of various fibrosis-related chronic diseases (e.g., idiopathic pulmonary fibrosis, liver fibrosis, renal fibrosis, and systemic sclerosis). In this article, we summarize the roles of SFKs in various chronic kidney diseases, including glomerulonephritis, diabetic nephropathy, human immunodeficiency virus-associated nephropathy, autosomal dominant form of polycystic kidney disease, and obesity-associated kidney disease, and discuss the mechanisms involved.

Update on current management of chronic kidney disease in patients with HIV infection

The prevalence of HIV-associated chronic kidney disease (CKD) varies geographically and depends on the definition of CKD used, ranging from 4.7% to 38% globally. The incidence, however, has decreased with the use of effective combined antiretroviral therapy (cART). A wide variety of histological patterns are seen in HIV-associated kidney diseases that include glomerular and tubulointerstitial pathology. In resource-rich settings, there has been a plateau in the incidence of end-stage renal disease secondary to HIV-associated nephropathy (HIVAN). However, the prevalence of end-stage renal disease in HIV-positive individuals has risen, mainly due to increased longevity on cART. There is a disparity in the occurrence of HIVAN among HIV-positive individuals such that there is an 18- to 50-fold increased risk of developing kidney disease among HIV-positive individuals of African descent aged between 20 and 64 years and who have a poorer prognosis compared with their European descent counterparts, suggesting that genetic factors play a vital role. Other risk factors include male sex, low CD4 counts, and high viral load. Improvement in renal function has been observed after initiation of cART in patients with HIV-associated CKD. Treatment with an angiotensin-converting enzyme inhibitor/angiotensin receptor blocker is recommended, when clinically indicated in patients with confirmed or suspected HIVAN or clinically significant albuminuria. Other standard management approaches for patients with CKD are recommended. These include addressing other cardiovascular risk factors (appropriate use of statins and aspirin, weight loss, cessation of smoking), avoidance of nephrotoxins, and management of serum bicarbonate and uric acid, anemia, calcium, and phosphate abnormalities. Early diagnosis of kidney disease by screening of HIV-positive individuals for the presence of kidney disease is critical for the optimal management of these patients. Screening for the presence of kidney disease upon detection of HIV infection and annually thereafter in high-risk populations is recommended.

Podocyte injury: the role of proteinuria, urinary plasminogen, and oxidative stress

Podocytes are the key target for injury in proteinuric glomerular diseases that result in podocyte loss, progressive focal segmental glomerular sclerosis (FSGS), and renal failure. Current evidence suggests that the initiation of podocyte injury and associated proteinuria can be separated from factors that drive and maintain these pathogenic processes leading to FSGS. In nephrotic urine aberrant glomerular filtration of plasminogen (Plg) is activated to the biologically active serine protease plasmin by urokinase-type plasminogen activator (uPA). In vivo inhibition of uPA mitigates Plg activation and development of FSGS in several proteinuric models of renal disease including 5/6 nephrectomy. Here, we show that Plg is markedly increased in the urine in two murine models of proteinuric kidney disease associated with podocyte injury: Tg26 HIV-associated nephropathy and the Cd2ap^-/- model of FSGS. We show that human podocytes express uPA and three Plg receptors: uPAR, tPA, and Plg-RKT. We demonstrate that Plg treatment of podocytes specifically upregulates NADPH oxidase isoforms NOX2/NOX4 and increases production of mitochondrial-dependent superoxide anion (O₂^-) that promotes endothelin-1 synthesis. Plg via O₂^- also promotes expression of the B scavenger receptor CD36 and subsequent increased intracellular cholesterol uptake resulting in podocyte apoptosis. Taken together, our findings suggest that following disruption of the glomerular filtration barrier at the onset of proteinuric disease, podocytes are exposed to Plg resulting in further injury mediated by oxidative stress. We suggest that chronic exposure to Plg could serve as a "second hit" in glomerular disease and that Plg is potentially an attractive target for therapeutic intervention.

HIV and chronic kidney disease

Chronic kidney disease (CKD) is a frequent complication of HIV infection, occurring in 3.5 - 48.5%, and occurs as a complication of HIV infection, other co-morbid disease and infections and as a consequence of therapy of HIV infection and its complications. The classic involvement of the kidney by HIV infection is HIV-associated nephropathy (HIVAN), occurring typically in young adults of African ancestry with advanced HIV disease in association with APOL1 high-risk variants. HIV-immune complex disease is the second most common diagnosis obtained from biopsies of patients with HIV-CKD. CKD is mediated by factors related to the virus, host genetic predisposition and environmental factors. The host response to HIV infection may influence disease phenotype through activation of cytokine pathways. With the introduction of antiretroviral therapy (ART), there has been a decline in the incidence of HIVAN, with an increasing prevalence of focal segmental glomerulosclerosis. Several studies have demonstrated the overall improvement in kidney function when initiating ART for HIV CKD. Progression to end stage kidney disease has been reported to be more likely when high grade proteinuria, severely reduced eGFR, hepatitis B and/C co-infection, diabetes mellitus, extensive glomerulosclerosis, and chronic interstitial fibrosis are present. Improved renal survival is associated with use of renin angiotensin system blockers and viral suppression. Many antiretroviral medications are partially or completely eliminated by the kidney and require dose adjustment in CKD. Certain drug classes, such as the protease inhibitors and the non-nucleoside reverse transcriptase inhibitors, are metabolized by the liver and do not require dose adjustment. HIV-infected patients requiring either hemo- or peritoneal dialysis, who are stable on ART, are achieving survival rates comparable to those of dialysis patients without HIV infection. Kidney transplantation has been performed successfully in HIV-infected patients; graft and patient survival appears to be similar to that of HIV-uninfected recipients. Early detection of kidney disease by implementation of screening on diagnosis of HIV infection and annual screening thereafter will have an impact on the burden of disease, together with access to ART to those who require it. Programs for prevention of HIV infection are essential to prevent this lethal disease.

HIV-associated immune complex kidney disease

The introduction in the late 20(th) century of combination antiretroviral therapy (cART) to treat patients infected with HIV has changed the natural history of the disease from an acute illness that rapidly culminates in death, to a chronic condition that can be managed with medications. Over the past decade the epidemiology of kidney disease in US patients infected with HIV has changed, perhaps because of the increased availability and use of cART. Patients with HIV infection exhibit unique immunologic characteristics, including immunodeficiency and dysregulation of immunoglobulin synthetic responses and T-cell function, which can result in glomerular immune complex deposition and subsequent kidney injury. This Review examines the differential diagnoses of HIV-associated immune complex kidney diseases (HIVICD), and discusses the clinical manifestations and mechanisms underlying their development. We address the issues associated with treatment, clinical outcomes, and research needs to enhance our ability to diagnose and optimally treat patients with HIVICD.

Hallmarks of HIV-1 pathogenesis are modulated by Nef's Secretion Modification Region

CD4⁺ T cell depletion and immune activation are hallmarks of HIV infection. Despite extensive studies, the mechanisms underlying immune modulation remain elusive. HIV-1 Nef protein is secreted in exosomes from infected cells and is abundant in the plasma of HIV+ individuals. Exosomal Nef (exNef) was also shown to induce apoptosis in bystander CD4⁺ T cells. We hypothesized that exNef contributes to HIV pathogenesis. A HIV-1 NL4-3 virus containing alanine substitutions in the secretion modification region (SMR; amino acids 66 to 70; HIVNefsmr5a) was developed. Nef protein containing this modified SMR was shown to be deficient in exNef secretion in nef-transfected cells. Using both HIV-1 NL4-3 wild type (HIVwt) and HIVNefsmr5a, correlates of pathogenesis were evaluated in cell-lines, human peripheral blood mononuclear cells, and humanized NOD-RAG1^-/- IL2r^-/- double mutant (NRG) mice. Disruption of the SMR did not affect viral replication or exNef secretion from infected cell cultures as compared with nef-transfected cells. However, T cell apoptosis was reduced in HIVNefsmr5a infected cell cultures and CD4⁺ T cell depletion was reduced in the spleen and peripheral blood of similarly infected NRG mice. Inflammatory cytokine release was also decreased in the sera of HIVNefsmr5a infected mice relative to HIVwt infected controls. These findings demonstrate the importance of Nef and the SMR motif in HIV pathogenesis and suggest a potential role for exNef in HIV-driven immune modulation.

Apolipoprotein L1 (APOL1) Variants (Vs) a possible link between Heroin-associated Nephropathy (HAN) and HIV-associated Nephropathy (HIVAN)

In 1970s, Heroin-associated Nephropathy (HAN), one form of focal and segmental glomerulosclerosis (FSGS), was a predominant cause of End-stage Kidney Disease (ESKD) in African-Americans (AAs). In 1980s, with the surge of Acquired Immune Deficiency Syndrome (AIDS) in AAs, HAN more or less disappeared, and the incidence of Human Immunodeficiency Virus associated Nephropathy (HIVAN) markedly increased. Recent studies in AAs have identified APOL1 variants (Vs) as a major risk factor for the development and progression of non-diabetic kidney diseases including idiopathic FSGS and hypertension-attributed nephrosclerosis. These observations have also offered partial insights into the mechanisms of development, and higher rate of occurrence of both HAN and HIVAN in AAs. AAs with APOL1Vs develop idiopathic FSGS at four-fold higher rate compared to European Americans (EAs). Similarly, HIV infected AAs with APOL1Vs (if not on antiviral therapy), risk a 50% (10-fold greater) chance of developing HIVAN. It has been suggested that APOL1Vs expression may render podocytes more vulnerable to various types of injury: bacterial, viral, and others. However, in addition to genetic variants, additional factors such as persistence of a second hit may determine the nature and severity of glomerular disease. In patients with HAN, heroin or contaminants may have been the offending second insult(s) which caused renal disease in susceptible AA patients. In the 80's, since heroin-induced second hit was neither consistent nor sustained (depending on drug availability in the street), the disease was masked or replaced HIV infected patients (especially in untreated subjects), by an overwhelming second hit by the virus which was both intense as well as persistent. It appears that APOL1Vs may be one of the links between the disappearance of HAN and emergence of HIVAN in AA patients.

Podocyte pathology and nephropathy - sphingolipids in glomerular diseases

Sphingolipids are components of the lipid rafts in plasma membranes, which are important for proper function of podocytes, a key element of the glomerular filtration barrier. Research revealed an essential role of sphingolipids and sphingolipid metabolites in glomerular disorders of genetic and non-genetic origin. The discovery that glucocerebrosides accumulate in Gaucher disease in glomerular cells and are associated with clinical proteinuria initiated intensive research into the function of other sphingolipids in glomerular disorders. The accumulation of sphingolipids in other genetic diseases including Tay-Sachs, Sandhoff, Fabry, hereditary inclusion body myopathy 2, Niemann-Pick, and nephrotic syndrome of the Finnish type and its implications with respect to glomerular pathology will be discussed. Similarly, sphingolipid accumulation occurs in glomerular diseases of non-genetic origin including diabetic kidney disease (DKD), HIV-associated nephropathy, focal segmental glomerulosclerosis (FSGS), and lupus nephritis. Sphingomyelin metabolites, such as ceramide, sphingosine, and sphingosine-1-phosphate have also gained tremendous interest. We recently described that sphingomyelin phosphodiesterase acid-like 3b (SMPDL3b) is expressed in podocytes where it modulates acid sphingomyelinase activity and acts as a master modulator of danger signaling. Decreased SMPDL3b expression in post-reperfusion kidney biopsies from transplant recipients with idiopathic FSGS correlates with the recurrence of proteinuria in patients and in experimental models of xenotransplantation. Increased SMPDL3b expression is associated with DKD. The consequences of differential SMPDL3b expression in podocytes in these diseases with respect to their pathogenesis will be discussed. Finally, the role of sphingolipids in the formation of lipid rafts in podocytes and their contribution to the maintenance of a functional slit diaphragm in the glomerulus will be discussed.

Secretion modification region-derived peptide disrupts HIV-1 Nef's interaction with mortalin and blocks virus and Nef exosome release

Nef is secreted from infected cells in exosomes and is found in abundance in the sera of HIV-infected individuals. Secreted exosomal Nef (exNef) induces apoptosis in uninfected CD4⁺ T cells and may be a key component of HIV pathogenesis. The exosomal pathway has been implicated in HIV-1 virus release, suggesting a possible link between these two viral processes. However, the underlying mechanisms and cellular components of exNef secretion have not been elucidated. We have previously described a Nef motif, the secretion modification region (SMR; amino acids 66 to 70), that is required for exNef secretion. In silico modeling data suggest that this motif can form a putative binding pocket. We hypothesized that the Nef SMR binds a cellular protein involved in protein trafficking and that inhibition of this interaction would abrogate exNef secretion. By using tandem mass spectrometry and coimmunoprecipitation with a novel SMR-based peptide (SMRwt) that blocks exNef secretion and HIV-1 virus release, we identified mortalin as an SMR-specific cellular protein. A second set of coimmunoprecipitation experiments with full-length Nef confirmed that mortalin interacts with Nef via Nef's SMR motif and that this interaction is disrupted by the SMRwt peptide. Overexpression and microRNA knockdown of mortalin revealed a positive correlation between exNef secretion levels and mortalin protein expression. Using antibody inhibition we demonstrated that the Nef/mortalin interaction is necessary for exNef secretion. Taken together, this work constitutes a significant step in understanding the underlying mechanism of exNef secretion, identifies a novel host-pathogen interaction, and introduces an HIV-derived peptide with antiviral properties.

HIV-1 and kidney cells: better understanding of viral interaction

HIV-associated nephropathy (HIVAN) is the most common disease affecting untreated seropositive patients of African descent. Besides genetic (African descent) and HIV-1 infection (environmental), specific host factors such as activation of renin-angiotensin-aldosterone system (RAAS) have also been demonstrated to play a role in the manifestation of HIVAN. The recent identification of MYH9 as susceptible allele is a key step forward in our understanding for the pathogenesis of focal glomerulosclerosis in people of African-American descent. HIV-1 transgenic models have significantly advanced our knowledge base in terms of role of HIV-1 genes in general and individual gene in particular in the development of renal lesions mimicking HIVAN. These studies suggest that viral replication is not needed for the development of renal lesions. Renal biopsy data from HIVAN patients suggest that renal epithelial cells express HIV-1 genes and thus it may be sufficient to invoke HIVAN phenotype in the presence of specific host and genetic factors. On the other hand, immune response to infection may be required to induce HIV-1 associated immune complex kidney disease (HIVICK). Since renal cell lack conventional HIV-1 receptors, HIV-1 entry into renal cells has been a mystery. Recently, non-conventional pathways have been demonstrated to facilitate HIV-1 entry into renal cells in in vitro studies. These include presence of DEC-205 receptors in renal tubular cells and lipid rafts in podocytes. However, HIV-1 entry through these pathways only allows non-productive infection. It appears that the presence of specific genetic and host factors in in vivo conditions may be facilitating the development of the productive HIV-1 infection in kidney cells.

Controversies in the pathogenesis of HIV-associated renal diseases

The two most common HIV-associated renal diseases, HIV-associated nephropathy and HIV immune-complex kidney disease, share the common pathologic finding of hyperplasia within the glomerulus. Podocyte injury is central to the pathogenesis of these diseases; however, the source of the proliferating glomerular epithelial cell remains a topic of debate. Parenchymal injury has been linked to direct infection of renal epithelial cells by HIV-1, although the mechanism of viral entry into this non-lymphoid compartment is unclear. Although transgenic rodent models have provided insight into viral proteins responsible for inducing renal disease, such models have substantial limitations. Rodent HIV-1 models, for instance, cannot replicate all features of immune activation, a process that could have an important role in the pathogenesis of the HIV-associated renal diseases.

Genetic characterization of HIV type 1 Nef-induced vesicle secretion

The HIV-1 Nef protein is known to be secreted, and our group has shown that Nef is secreted from nef-transfected and HIV-1-infected cells in small exosome-like vesicles (d. 40-100 nm). The role of secreted Nef remains to be fully characterized. Thus, it is important to characterize the nature of and the mechanisms regulating Nef secretion. We hypothesized that specific structural domains on the Nef protein interact with components of the endosomal trafficking machinery, sorting Nef into multivesicular bodies (MVB) and packaging it in exosome-like vesicles. To identify those domains, a series of mutants spanning the entire nef sequence were made and cloned into the expression vector pQB1, which expresses the mutants as Nef-GFP fusion proteins. These constructs were used in transient transfection assays to identify sequences necessary for secretion of the Nef-GFP fusion protein. N-terminal domains were identified as critical for Nef-induced vesicle secretion: (1) a basic cluster of four arginine residues (aa 17, 19, 21, 22), (2) the phosphofurin acidic cluster sequence (PACS; Glu62-65), and (3) a previously uncharacterized domain spanning amino acid residues 66-70 (VGFPV), which we named the secretion modification region (SMR). Additional amino acids P25, 29GVG31, and T44 were identified in HIV-1 Nef as regulating its secretion. These residues have not been associated with other reported Nef functions. The myristoylation domain, ubiquitination lysine residues, and the C-terminal portion of Nef (aa 71-206) had no effect on secretion. A minimal HIV-1 Nef sequence, comprising the identified motifs, was sufficient for Nef-induced vesicle secretion.

Selective expression of human immunodeficiency virus Nef in specific immune cell populations of transgenic mice is associated with distinct AIDS-like phenotypes

We previously reported that CD4C/human immunodeficiency virus (HIV)(Nef) transgenic (Tg) mice, expressing Nef in CD4(+) T cells and cells of the macrophage/dendritic cell (DC) lineage, develop a severe AIDS-like disease, characterized by depletion of CD4(+) T cells, as well as lung, heart, and kidney diseases. In order to determine the contribution of distinct populations of hematopoietic cells to the development of this AIDS-like disease, five additional Tg strains expressing Nef through restricted cell-specific regulatory elements were generated. These Tg strains express Nef in CD4(+) T cells, DCs, and macrophages (CD4E/HIV(Nef)); in CD4(+) T cells and DCs (mCD4/HIV(Nef) and CD4F/HIV(Nef)); in macrophages and DCs (CD68/HIV(Nef)); or mainly in DCs (CD11c/HIV(Nef)). None of these Tg strains developed significant lung and kidney diseases, suggesting the existence of as-yet-unidentified Nef-expressing cell subset(s) that are responsible for inducing organ disease in CD4C/HIV(Nef) Tg mice. Mice from all five strains developed persistent oral carriage of Candida albicans, suggesting an impaired immune function. Only strains expressing Nef in CD4(+) T cells showed CD4(+) T-cell depletion, activation, and apoptosis. These results demonstrate that expression of Nef in CD4(+) T cells is the primary determinant of their depletion. Therefore, the pattern of Nef expression in specific cell population(s) largely determines the nature of the resulting pathological changes.

Transgenic and infectious animal models of HIV-associated nephropathy

HIV-associated nephropathy (HIVAN) is a major cause of HIV-related morbidity and mortality. Transgenic and infectious models of HIVAN faithfully recapitulate the human disease and are important tools in advancing our understanding of disease pathogenesis, genetic susceptibility, and therapeutic intervention beyond the inhibition of viral replication. This review discusses the available transgenic murine models and infectious models of HIVAN in mice, rats, nonhuman primates, and felines. Particular emphasis is given to cell type-specific HIV expression as well as partial HIV genome expression used to map HIV-1 Nef and Vpr as pathologic determinants.

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.

HIV-1 Vpr inhibits cytokinesis in human proximal tubule cells

Transgenic mouse models of HIV-associated nephropathy (HIVAN) show that expression of HIV-1 genes in kidney cells produces collapsing focal segmental glomerulosclerosis and microcystic tubular disease typical of the human disease. HIV-1 vpr plays an important role in the glomerulosclerosis of HIVAN, especially when it is associated with nef expression in podocytes. Further, Vpr is reported to exacerbate tubular pathology. Here we determined effects of vpr expression on renal tubular epithelial cell function by transducing them with a pseudotyped lentivirus vector carrying HIV-1 vpr and control genes. Vpr expression in the cultured cells impaired cytokinesis causing cell enlargement and multinucleation. This profound in vitro phenotype caused us to reexamine the HIVAN mouse model and human HIVAN biopsies to see if similar changes occur in vivo. Both showed abundant hypertrophic tubule cells similar to the in vitro finding that represents a previously unappreciated aspect of the human disease. Additionally, multinucleated tubular cells were identified in the murine HIVAN model and increased chromosome number was detected in tubular cells of human HIVAN biopsies. Our study provides evidence of a new clinical phenotype in HIVAN that may result from the ability of Vpr to impair cytokinesis.

HIV-1 upregulates VEGF in podocytes

HIV-associated nephropathy (HIVAN) is characterized by collapsing FSGS. Because transgenic mice with podocyte-specific overexpression of the vascular endothelial growth factor 164 (VEGF164) isoform also develop collapsing FSGS, we sought to determine whether VEGF plays a role in HIVAN. Compared with controls, immunohistochemistry revealed that kidneys from HIV-1-transgenic mice (Tg26) and from patients with HIVAN had greater expression of both VEGF and its transcriptional regulator, hypoxia-inducible factor 2alpha (HIF-2alpha). Similarly, mRNA and protein levels of VEGF and HIF-2alpha were increased in HIV-infected podocytes in vitro, and this transcriptional upregulation was found to be stimulated by the HIV viral protein Nef in a Src kinase-and Stat3-dependent manner. HIV-1 also upregulated VEGFR2 and its co-receptor neuropilin-1 and suppressed the expression of semaphorin 3a in the podocyte. Exogenous VEGF stimulated proliferation and de-differentiation of podocytes, which are features of collapsing FSGS, and VEGFR2 neutralizing antibodies reversed these features in podocytes infected with HIV-1 or isolated from Tg26 mice. In conclusion, HIV-1 induces VEGF and VEGFR2 expression in podocytes, and this may be a critical step in the pathogenesis of HIVAN.

Suppression of HIV-1 replication by antiretroviral therapy improves renal function in persons with low CD4 cell counts and chronic kidney disease

OBJECTIVE

To examine the association between changes in glomerular filtration rates (GFR) and antiretroviral therapy (ART)-mediated suppression of plasma HIV-1 viremia.

DESIGN

: Observational, prospective, multicenter cohort study.

INTERVENTION

ART regimens or treatment strategies in HIV-1-infected subjects were implemented through randomized clinical trials; 1776 ambulatory subjects from these trials also enrolled in this cohort study.

METHOD

The association between suppression of viremia and GFR changes from baseline was examined using the abbreviated Modification of Diet and Renal Disease equation in mixed effects linear models.

RESULTS

GFR improvement was associated with ART-mediated suppression of plasma viremia in subjects with both chronic kidney disease stage > or = 2 and low baseline CD4 cell counts (< 200 cells/microl). In this subset, viral suppression (by > 1.0 log10 copies/ml or to < 400 copies/ml) was associated with an average increase in GFR of 9.2 ml/min per 1.73 m(2) from baseline (95% confidence interval, 1.6-16.8; P = 0.02) over a median follow-up of 160 weeks. The magnitude of this association increased in subjects who had greater baseline impairment of renal function, and it did not depend on race or sex.

CONCLUSIONS

Viral suppression was associated with GFR improvements in those with both low CD4 cell counts and impaired baseline renal function, supporting an independent contribution of HIV-1 replication to chronic renal dysfunction in advanced HIV disease. GFR improvement not associated with viral suppression also was observed in subjects with higher CD4 cell counts.

Macrophage colony-stimulating factor in the pathogenesis of HIV infection: potential target for therapeutic intervention

Macrophage colony stimulating factor (M-CSF) appears to play a major role in promoting and maintaining reservoirs of human immunodeficiency virus type 1 (HIV-1) in infected individuals. HIV-1 infection induces production of M-CSF by macrophages, which in turn promotes further infection of macrophages via increases in CD4 and CCR5 receptors, as well as increases in virus gene expression. M-CSF promotes the ontogeny and survival of macrophages, contributing to both the number and longevity of these infected cells. M-CSF dysregulation promotes the differentiation of monocytes toward macrophages and osteoclasts and at the same time may inhibit differentiation toward dendritic cells, resulting in immune impairment. The potential role of M-CSF in HIV-associated end organ diseases including HIV-associated dementia, HIV-associated nephropathy, and osteoporosis is discussed. This review emphasizes the need for developing M-CSF antagonists for treatment of HIV-1-infected patients.

Viral subversion mechanisms in chronic kidney disease pathogenesis

Viruses cannot autonomously replicate but must rely on the host cellular machinery to support their life cycle. Through natural selection, viruses have evolved strategies to co-opt the host organism to be a better site for their propagation. Some of these strategies are directed at the cellular machinery and involve complicated and ingenious solutions to optimize infection, replication, viral gene expression, and new virion assembly and shedding. Other strategies are directed at the host's innate and adaptive immune systems that permit the virus to evade clearance mechanisms. The more common pathogenic viral infections in nephrology-cytomegalovirus, HIV-1, hepatitis C virus, polyomavirus BK, and parvovirus B19-all have acquired subversion strategies that benefit the virus but because they interfere with normal cellular and immune processes also have become pathogenic to the host. In addition, the highly prevalent viruses cytomegalovirus, BK, and B19 cause severe disease only in the setting of immunosuppression, revealing the very delicate balance that some viruses have achieved with their host's immune system. Thus, selective pressure for survival drives both the evolution of more sophisticated viruses and the host immune system as it evolves to combat the environment of adapting and emerging infectious agents. Understanding the molecular mechanisms of these viral subversion strategies may reveal new targets for the development of highly specific antiviral therapies and also aid vaccine development.

HIV-1 and HIV-Associated Nephropathy 25 Years Later

Twenty-five years after the first published description of AIDS, HIV-associated nephropathy (HIVAN) remains an important cause of kidney disease in HIV-infected patients. The pathogenesis of HIVAN involves direct HIV infection of the kidney, with both viral and host genetic factors playing an important role. The widespread use of antiretroviral therapy has influenced the epidemiology of HIV-related kidney disease, and the nephrology community should support efforts to improve access to therapy and limit HIV transmission in susceptible minority populations. This article reviews the history of HIV and HIVAN, focusing on advances in the understanding of pathogenesis, epidemiology, and treatment.

HIV-1 infection and the kidney: an evolving challenge in HIV medicine

With the advent of highly active antiretroviral therapy (HAART), the incidence of opportunistic infections has declined substantially, and cardiovascular, liver, and renal diseases have emerged as major causes of morbidity and mortality in individuals with human immunodeficiency virus (HIV). Acute renal failure is common in HIV-infected patients and is associated with acute infection and medication-related nephrotoxicity. HIV-associated nephropathy is the most common cause of chronic kidney disease in HIV-positive African American populations and may respond to HAART. Other important HIV-associated renal diseases include HIV immune complex kidney diseases and thrombotic microangiopathy. The increasing importance of non-HIV-associated diseases, such as diabetes mellitus, hypertension, and vascular disease, to the burden of chronic kidney disease has been recognized, focusing attention on prevention and control of these diseases in HIV-positive individuals. HIV-positive individuals who experience progression to end-stage renal disease and who have undetectable HIV-1 viral loads while receiving HAART should be evaluated for renal transplant. Emerging evidence suggests that HIV-positive individuals may have graft and patient survival comparable to HIV-negative individuals. Several studies suggest that HIV-1 can potentially infect renal cells, and HIV transgenic mice have clarified the roles of a number of HIV proteins in the pathogenesis of HIV-associated renal disease. Host factors may modify disease expression at the level of cytokine networks and the renal microvasculature and contribute to the pathogenic effects of HIV-1 infection on the kidney.

Emerging Paradigms in the Renal Pathology of Viral Diseases

This review considers recent information that illuminates pathogenetic mechanisms that involve three of the major viral infections that cause renal injury in the form of HIV-associated nephropathy, polyoma virus nephropathy, and hepatitis C virus-associated glomerulonephritis.

Primary human immunodeficiency virus type 1 nef alleles show major differences in pathogenicity in transgenic mice

We previously reported that the human immunodeficiency virus type 1 NL4-3 Nef is necessary and sufficient to induce a severe AIDS-like disease in transgenic (Tg) mice when the protein is expressed under the regulatory sequences of the human CD4 gene. We have now assayed additional Nef alleles (SF2, JR-CSF, YU10x, and NL4-3 [T71R] Nef alleles), including some from long-term nonprogressors (AD-93, 032an, and 039nm alleles) in the same Tg system and compared their pathogenicities. All these Nef alleles downregulated cell surface CD4 in human cells in vitro and also, with the exception of Nef(YU10x), in Tg CD4(+) T cells. Depletion of double-positive and single-positive thymocytes occurred with all alleles but was less pronounced in Nef(YU10x) Tg mice. A loss of peripheral CD4(+) T cells was observed with all alleles but was minimal in Nef(YU10x) Tg mice. In Nef(032an) and Nef(SF2) Tg mice, T-cell loss was severe despite lower levels of Tg expression, suggesting a higher virulence of these alleles. All Nef alleles except the Nef(YU10x) and Nef(NL4-3(T71R)) alleles induced an enhanced activated memory (CD25(+) CD69(+) CD44(high) CD45RB(low) CD62L(low)) and apoptotic phenotype. Also, all could interact with and/or activate PAK2 except the Nef(JR-CSF) allele. Organ (lung and kidney) diseases were present in Nef(NL4-3(T71R)), Nef(032an), Nef(039nm), and Nef(SF2) Tg mice, despite very low levels of Tg expression for the last strain. However, no organ disease or minimal organ disease developed in Nef(YU10x) and Nef(AD-93) Tg mice and Nef(JR-CSF) Tg mice, respectively, despite high levels of Tg expression. Our data show that important differences in the pathogenicities of various Nef alleles can be scored in Tg mice. Interestingly, our results also revealed that some phenotypes can segregate independently, such as CD4(+) T-cell depletion and activation, as well as severe depletion of thymic CD4(+) T cells and peripheral CD4(+) T cells. Therefore, expression of Nef alleles in Tg mice under the CD4C regulatory elements represents a novel assay for measuring their pathogenicity. Because of the very high similarity of this murine AIDS-like disease to human AIDS, this assay may have a predictive value regarding the behavior of Nef in infected humans.

Update on HIV-associated nephropathy

PURPOSE OF REVIEW

HIV-associated nephropathy is characterized by a constellation of pathologic findings including a collapsing glomerulopathy, tubular dilatation, and interstitial infiltration with leukocytes. This review summarizes some of the recent advances in our understanding of the gene products and signaling pathways that contribute to the pathogenesis of HIV-associated nephropathy.

RECENT FINDINGS

Podocytes infected with HIV-associated nephropathy exhibit podocyte proliferation and de-differentiation. Restriction of HIV-1 transgene expression to the podocyte in a murine model supports the belief that podocyte infection is pivotal to the development of the disease. Recent studies have provided compelling in-vitro and in-vivo evidence that expression of the HIV-1 accessory gene nef is critical in altering the phenotype of mature podocytes and causing injury to these cells. An in-vitro study suggests that nef's effects in the podocyte appear to be mediated through Src kinase-dependent activation of the signal transducer and activator of transcription 3 and mitogen-activated protein kinase 1,2 signaling pathways.

SUMMARY

Recent evidence demonstrates that the viral protein nef plays a critical role in the development of HIV-associated nephropathy and provides a foundation for developing new therapeutic strategies for patients afflicted with this disease.

Animal models of HIV-associated nephropathy

PURPOSE OF REVIEW

HIV-1-associated nephropathy is characterized clinically by proteinuria with azotemia and pathologically by collapsing focal segmental glomerulosclerosis with tubulointerstitial nephritis and microcystic tubular dilatation. This review summarizes the manner in which different transgenic animal models contribute to our knowledge of the pathogenesis of HIV-1-associated nephropathy.

RECENT FINDINGS

The most widely studied has been a transgenic mouse model bearing a gag and pol-deleted proviral construct that develops renal disease with many of the clinical and pathologic characteristics seen in HIV-1-associated nephropathy. Studies using this model have helped to highlight the role of HIV-1 viral gene expression in renal cells, podocyte dysregulation, and genetic host factors in the pathogenesis of HIV-1-associated nephropathy. This model has provided the key insights that led to detection of HIV-1 in human kidney epithelial cells. Other transgenic models have helped define critical roles for individual HIV gene products (Nef and Vpr) in the pathogenesis of HIV-1-associated nephropathy. Transgenic mouse models have also provided a method to discover new treatments targeting various steps in the pathogenesis of this disease.

SUMMARY

Transgenic animal models of HIV-1-associated nephropathy have contributed greatly to the progress made toward understanding the pathogenesis of this disease.

HIV-1 genes vpr and nef synergistically damage podocytes, leading to glomerulosclerosis

This study aimed to identify the causative gene for HIV-1 associated nephropathy, a paradigmatic podocytopathy. A previous study demonstrated that transgenic expression of nonstructural HIV-1 genes selectively in podocytes in mice with FVB/N genetic background resulted in podocyte injury and glomerulosclerosis. In this study, transgenic mice that expressed individual HIV-1 genes in podocytes were generated. Five of six transgenic mice that expressed vpr developed podocyte damage and glomerulosclerosis. Analysis of an established vpr transgenic line revealed that transgenic mice on FVB/N but not on C57BL/6 genetic background developed podocyte injury by 8 wk of age, with later glomerulosclerosis. Four of 11 transgenic mice that expressed nef also developed podocyte injury. One transgenic line was established from the nef founder mouse with the mildest phenotype. Transgenic mice in this line developed mesangial expansion at 3 wk of age and mild focal podocyte damage at 10 wk of age. Mating with FVB/N mice did not augment nephropathy. None of the transgenic mice that expressed vif, tat, rev, or vpu in podocytes, even with the FVB/N genetic background, developed podocyte injury. For testing effects of simultaneous expression of vpr and nef, these two lines were mated. All nef:vpr double-transgenic mice showed severe podocyte injury and glomerulosclerosis by 4 wk of age. In contrast, all vpr or nef single-transgenic mice in the same litter uniformly showed no or much milder podocyte injury. These findings indicate that vpr and nef each can induce podocyte injury with a prominent synergistic interaction.

Collapsing glomerulopathy

Collapsing glomerulopathy (CG) has become an important cause of ESRD. First delineated from other proteinuric glomerular lesions in the 1980s, CG is now recognized as a common, distinct pattern of proliferative parenchymal injury that portends a rapid loss of renal function and poor responses to empiric therapy. Notwithstanding, the rise in disorders that are associated with CG, the identification of the first susceptibility genes for CG, the remarkable increase in murine modeling of CG, and promising preclinical testing of new therapeutic strategies suggest that the outlook for CG as a poorly understood and therapeutically resistant renal disease is set to change in the future. This focused review highlights recent advances in research into the pathogenesis and treatment of CG.

Direct effects of dexamethasone on human podocytes

Glucocorticoids are widely used in the treatment of human glomerular diseases, but their mode of action is poorly understood particularly in steroid-sensitive nephrotic syndrome, which is most common in childhood and is characterized by a lack of inflammation in the kidney. The podocyte is a key cell in the glomerulus in health and disease: until recently, human podocytes have been difficult to study in vitro. We have developed a conditionally immortalized human podocyte cell line transfected with a temperature-sensitive simian virus 40 transgene: when the transgene is inactivated in vitro, these cells adopt the phenotype of differentiated podocytes. We have used these cells to evaluate, using immunocytochemistry, reverse transcriptase-polymerase chain reaction, and Western blotting, direct effects of the glucocorticoid dexamethasone at concentrations designed to mimic in vivo therapeutic corticosteroid levels. Dexamethasone upregulated expression of nephrin and tubulin-alpha, and downregulated vascular endothelial growth factor. Effects on cell cycle were complex with downregulation of cyclin kinase inhibitor p21 and augmentation of podocyte survival, without any effect on apoptosis. We report cytokine production by human podocytes, especially interleukin (IL)-6 and -8; IL-6 expression was suppressed by dexamethasone. These potent direct effects on podocytes illustrate a novel mode of action of glucocorticoids and suggest potential new therapeutic strategies for glomerular disease.

Expression of HIV-1 genes in podocytes alone can lead to the full spectrum of HIV-1-associated nephropathy

BACKGROUND

Human immunodeficiency virus (HIV)-1-associated nephropathy (HIVAN) is characterized by collapsing focal and segmental glomerulosclerosis (FSGS) and microcystic tubular dilatation. HIV-1 infection is also associated with other forms of nephropathy, including mesangial hyperplasia. Since HIV-1 gene products are detected in podocytes and other renal cells, it remains uncertain whether podocyte-restricted HIV-1 gene expression can account for the full spectrum of renal lesions involving nonpodocytes.

METHODS

To define the role of podocyte-restricted HIV-1 gene expression in the progression of HIVAN, we generated transgenic mice that express nonstructural HIV-1 genes selectively in podocytes.

RESULTS

Four of the seven founder mice developed proteinuria and nephropathy. In a subsequently established transgenic line, reverse transcription-polymerase chain reaction (RT-PCR) analysis detected mRNAs for vif, vpr, nef, and spliced forms of tat and rev, but not vpu, in the kidney. In situ hybridization localized HIV-1 RNA to the podocyte. Transgenic mice on FVB/N genetic background exhibited cuboidal morphology of podocytes with reduced extension of primary and foot processes at 2 weeks of age. After 3 weeks of age, these mice developed massive and nonselective proteinuria with damage of podocytes and other glomerular cells and, after 4 weeks of age, collapsing FSGS and microcystic tubular dilatation. In marked contrast, transgenic mice with C57BL/6 genetic background showed either normal renal histology or only mild mesangial expansion without overt podocyte damage.

CONCLUSION

The present study demonstrates that podocyte-restricted expression of HIV-1 gene products is sufficient for the development of collapsing glomerulosclerosis in the setting of susceptible genetic background.

HIV and the kidney: a status report after 20 years

The first association between HIV-1 infection and kidney disease was made in 1984 and much has been learned over the past 20 years. In recent years, more effective therapies for HIV-1 infection and its associated opportunistic infections have led to improved patient survival. However, with prolonged survival, morbidity associated with renal disease has also increased. Among the multiple glomerulopathies that can affect patients with HIV, focal segmental glomerulosclerosis (FSGS) is most common and frequently leads to end-stage renal disease. Although the precise mechanisms of HIV-associated FSGS remain to be elucidated, it appears that host genetic susceptibility, direct infection of the renal epithelium, and toxicity of one or more viral accessory protein contribute. Therapy for HIV-associated FSGS includes control of blood pressure and the use of angiotensin antagonist therapy. A randomized trial of angiotensin receptor blocker will be initiated shortly. Drug-related nephropathies are also common, manifesting as acute renal failure, nephrolithiasis, and interstitial nephritis. Tenofovir, a newer nucleoside analogue, has recently been implicated in causing tubular toxicity, although the incidence is low. Appropriate screening for renal dysfunction can minimize the likelihood of progressive renal injury in all patients with HIV-1 infection.

HIV-1 Nef induces dedifferentiation of podocytes in vivo: a characteristic feature of HIVAN

OBJECTIVE

To determine the specific role of Nef in the pathogenesis of HIV-associated nephropathy.

DESIGN

Podocytes are highly differentiated non-dividing cells in the normal glomerulus, however, they undergo dedifferentiation and acquire a proliferative phenotype in HIVAN patients, in HIV-transgenic mice and if infected by HIV-1 in vitro. These changes are accompanied by loss of the maturation markers synaptopodin and WT1, and expression of the proliferation marker Ki-67. Previously, we mapped the gene responsible for these changes in vitro to HIV-1 Nef. To determine the role of Nef in vivo, we developed a transgenic mouse model in which Nef was exclusively expressed in podocytes.

METHODS

Transgenic mice were generated using a construct in which Nef expression was blocked by a floxed lacZ intervening gene. When crossed with another transgenic mice expressing Cre under the Podocin promoter (a podocyte specific gene), the intervening lacZ gene was removed activating the expression of Nef in podocytes. The in vivo expression profiles of the Nef, the proliferation marker Ki-67, the differentiation markers synaptopodin and WT1, and phospho-Stat3, were determined by immunohistochemistry.

RESULTS

Podocyte-specific expression of Nef induced loss of synaptopodin and WT1, and expression of Ki-67 in podocytes. Furthermore, Nef activated expression of phospho-Stat3, one of the downstream signaling pathways for cell proliferation.

CONCLUSIONS

We conclude that Nef induces the early molecular changes in podocytes that are essential for the dedifferentiation and proliferation of podocytes in HIVAN pathogenesis. These data provide the first clear molecular evidence that Nef alters the podocyte phenotype in vivo.

Factors associated with chronic renal failure in HIV-infected ambulatory patients

OBJECTIVE

Renal disease is an increasingly common manifestation among HIV-positive persons, particularly during late stages of HIV disease. We performed a cohort-based, nested case-control study to examine the role of several factors in developing HIV-related chronic renal disease, including HIV viral load and CD4+ cell count.

DESIGN

Incident cases of chronic renal disease were identified from a cohort of 6361 prospectively followed HIV-1 positive persons. Controls were selected using incidence density sampling and matched 4:1 on age, race/ethnicity, and gender.

METHODS

Odds ratios (OR) and 95% confidence intervals (CI) were obtained using conditional logistic regression.

RESULTS

One hundred and eight cases of chronic renal disease were identified; 80 (74.1%) were eligible for the current analysis. Nadir CD4+ cell count < 200 x 10(6) cells/l (OR = 4.3; 95% CI, 2.1-8.7), highly active antiretroviral therapy (HAART) use for 56 days or more (OR = 0.5; 95% CI, 0.3-1.0), and hypertension [treated with angiotensin-converting enzyme (ACE) inhibitors: OR = 4.6; 95% CI, 1.8-11.6; treated with non-ACE inhibitors: OR = 2.5; 95% CI, 1.0-6.2; not treated: OR = 4.2; 95% CI, 0.8-21.6] were associated with disease. HAART use for 56 days or more modified the associations for nadir CD4+ cell count and hypertension.

CONCLUSIONS

Our findings suggest that advanced HIV-disease, as indicated by low CD4+ cell count, is associated with subsequently developing chronic renal disease and treatment with HAART may reduce the risk of developing chronic renal disease.

The clinical epidemiology and course of the spectrum of renal diseases associated with HIV infection

BACKGROUND

While an understanding of the epidemiology and clinical course of HIV-associated nephropathy (HIVAN) is growing, little is known about the risk factors and clinical course of the other renal diseases that may also occur as a complication of HIV infection. This study was undertaken to compare HIVAN to the spectrum of other kidney diseases seen among HIV-infected patients.

METHODS

This retrospective cohort study included all HIV-infected patients who underwent renal biopsy during the course of their clinical care at six major medical centers. Demographic and clinical information were abstracted from each patient's clinical record. Time to initiation of renal replacement therapy was compared for patients with lesions other than HIVAN to patients with HIVAN using Cox proportional hazards regression.

RESULTS

Eighty-nine patients (47 with lesions other than HIVAN and 42 with HIVAN) were available for inclusion. Patients with lesions other than HIVAN were less likely to be black (37/47 vs. 42/42, P= 0.02), more likely to have a positive hepatitis B surface antigen (10/37 vs. 4/42, P= 0.04), less likely to have the diagnosis of hypertension (24/46 vs. 31/42, P= 0.03), more likely to have a greater creatinine clearance at time of biopsy (60.6 vs. 39.0 cc/min, P= 0.008), and have a greater CD4 lymphocyte count at time of biopsy (287 vs. 187 cells/mL, P= 0.04) compared to patients with HIVAN. Lesions other than HIVAN were associated with a longer time to initiation of renal replacement therapy compared with HIVAN (HR 0.33, 95% CI 0.15-0.71, P= 0.005). Other factors associated with a longer time to renal replacement therapy included higher creatinine clearance at time of biopsy, greater CD4(+) lymphocyte count, the absence of hepatitis C antibody, and the use of an angiotensin-converting enzyme inhibitor or angiotensin receptor blocker. The type of renal disease (HIVAN vs. other) interacted significantly with HIV-1 RNA level and the use of antiretroviral therapy (P= 0.0001 and 0.006, respectively). Among patients with lesions other than HIVAN, the presence of nondetectable HIV-1 RNA was not associated with a greater risk of progression of renal disease (HR 0.27, P= 0.24). Among patients with HIVAN, because all patients had detectable virus at the time of institution of renal replacement therapy, this highly significant association could not be quantified. Among patients with lesions other than HIVAN, the use of antiretroviral therapy was not associated with the progression to renal replacement therapy (HR 3.29, P= 0.06). Among patients with HIVAN, the use of antiretroviral therapy was associated with a slower progression to renal replacement therapy (HR 0.24, P= 0.03).

CONCLUSION

Among HIV-infected patients with renal disease other than HIVAN, viral suppression and the use of antiretroviral therapy are not associated with a beneficial effect on renal function; thus, additional therapeutic strategies may need to be utilized. Because renal histology is associated with prognostic differences, these data provide outcomes information that will improve the clinical utility of renal biopsy among HIV-infected patients with renal disease.

Nef stimulates proliferation of glomerular podocytes through activation of Src-dependent Stat3 and MAPK1,2 pathways

In collapsing focal segmental glomerulosclerosis (FSGS) of HIV-associated nephropathy (HIVAN), podocytes exhibit a high proliferation rate and loss of differentiation markers. We have found previously that the nef gene of HIV-1 is responsible for these changes. Here, we investigated the signaling pathways induced by Nef and its role in the pathogenesis of HIVAN. Using conditionally immortalized podocytes after differentiation, we found that infection of podocytes with nef increased Src kinase activity and signal transducer and activator of transcription 3 (Stat3) phosphorylation and activated the Ras-c-Raf-MAPK1,2 pathway. A dominant negative mutant of Src abolished the Nef effect, whereas inhibition of MAPK1,2 or dominant negative Stat3 reduced Nef effects partially. Reducing the expression of Nef with small interference RNA reversed the Nef effect. Mutation of Nef in the PxxP or R105R106 motifs diminished Nef signaling and the phenotypic changes in podocytes. Both phospho-MAPK1,2 and phospho-Stat3 staining increased in podocytes of kidneys from HIV-1 transgenic mice compared with their littermates and in podocytes of kidneys from HIVAN patients compared with HIV patients with non-HIVAN kidney diseases or non-HIV patients with idiopathic FSGS, classic FSGS, or minimal-change disease. These data suggest that Nef-induced activation of Stat3 and Ras-MAPK1,2 via Src-dependent pathways is responsible for podocyte proliferation and dedifferentiation, a characteristic finding in collapsing FSGS of HIVAN.

A 20-year history of childhood HIV-associated nephropathy

In 1984, physicians in New York and Miami reported HIV-infected adult patients with heavy proteinuria and rapid progression to end-stage renal disease. These patients showed large edematous kidneys with a combination of focal segmental glomerulosclerosis (FSGS) and tubulointerstitial lesions. This renal syndrome, named HIV-associated nephropathy (HIVAN), was found predominantly in African Americans. Subsequent studies confirmed the presence of HIVAN in children, who frequently develop nephrotic syndrome in association with FSGS and/or mesangial hyperplasia with microcystic tubular dilatation. Since then, substantial progress has been made in our understanding of the etiology and pathogenesis of HIVAN. This article reviews 20 years of research into the pathogenesis of HIVAN and discusses how these concepts could be applied to the treatment of children with HIVAN. HIV-1 infection plays a direct role in the pathogenesis of childhood HIVAN, at least partially by affecting the growth and differentiation of glomerular and tubular epithelial cells and enhancing the renal recruitment of infiltrating mononuclear cells and cytokines. An up-regulation of renal heparan sulfate proteoglycans seems to play a relevant role in this process, by increasing the recruitment of heparin-binding growth factors (i.e., FGF-2), chemokines, HIV-infected cells, and viral proteins (i.e., gp120, Tat). These changes enhance the infectivity of HIV-1 in the kidney and induce injury and proliferation of intrinsic renal cells. Highly active anti-retroviral therapy (HAART) appears to be the most promising treatment to prevent the progression of childhood HIVAN. Hopefully, in the near future, better education, prevention, and treatment programs will lead to the eradication of this fatal childhood disease.

Focal glomerulosclerosis in proviral and c-fms transgenic mice links Vpr expression to HIV-associated nephropathy

Clinical and morphologic features of human immunodeficiency virus (HIV)-associated nephropathy (HIVAN), such as proteinuria, sclerosing glomerulopathy, tubular degeneration, and interstitial disease, have been modeled in mice bearing an HIV proviral transgene rendered noninfectious through a deletion in gag/pol. Exploring the genetic basis of HIVAN, HIV transgenic mice bearing mutations in either or both of the accessory genes nef and vpr were created. Proteinuria and focal glomerulosclerosis (FGS) only developed in mice with an intact vpr gene. Transgenic mice bearing a simplified proviral DNA (encoding only Tat and Vpr) developed renal disease characterized by FGS in which Vpr protein was localized to glomerular and tubular epithelia by immunohistochemistry. The dual transgenic progeny of HIV[Tat/Vpr] mice bred to HIV[DeltaVpr] proviral transgenic mice displayed a more severe nephropathy with no apparent increase in Vpr expression, implying that multiple viral genes contribute to HIVAN. However, the unique contribution of macrophage-specific Vpr expression in the development of glomerular disease was underscored by the induction of FGS in multiple murine lines bearing a c-fms/vpr transgene.

Differential expression of d-type cyclins in podocytes in vitro and in vivo

The proliferative response of podocytes to injury determines the histological phenotype. Moreover, an apparent lack of podocyte proliferation may underlie the development of glomerulosclerosis. Podocyte proliferation is closely linked with its state of differentiation. However, the mechanisms regulating these processes are not fully elucidated. Because D-type cyclins have been shown to be important in the regulation of proliferation and differentiation, we examined their expression in podocytes in vitro and in vivo. The glomerular expression of cyclins D1 and D3 was examined in vitro in cultured immortalized podocytes by immunostaining and Western blot analysis, and in embryonic mice and rats, the passive Heymann nephritis model of experimental membranous nephropathy in rats, and human immunodeficiency virus (HIV)-transgenic mice. Kidneys from cyclin D1 knockout mice were also examined. Cyclin D1 was abundant in cultured proliferating podocytes, but not in quiescent differentiated podocytes. In contrast, cyclin D3 was abundant in differentiated, but not proliferating podocytes. Cyclin D1 was expressed in embryonic mouse and rat glomeruli during the S- and comma-shaped stages, and was absent in podocytes at the capillary loop stage and in mature rodent glomeruli. Cyclin D1 protein increased after injury in passive Heymann nephritis rats and in HIV-transgenic mice. Cyclin D3 was constitutively and specifically expressed in podocytes in normal rodent glomeruli, and decreases during dedifferentiation and proliferation in HIV-transgenic mice. Kidneys from cyclin D1-/- mice were normal with the podocytes expressing specific differentiation markers. Cyclin D1 is not necessary for the terminal differentiation of podocytes, and expression coincides with cell-cycle entry. In contrast, cyclin D3 expression coincides with podocyte differentiation and quiescence.

Mapping a locus for susceptibility to HIV-1-associated nephropathy to mouse chromosome 3

HIV-1-associated nephropathy (HIVAN) is a major complication of HIV-1 infection with distinct pathologic features. Introduction of the HIV-1 genome into mice results in a renal disease with all of the histologic and clinical hallmarks of HIVAN on the FVB/N genetic background (TgFVB). We assessed the influence of genetic background on the development or progression of HIVAN by making F1 hybrids of TgFVB with five other inbred strains (CBA, DBA/2, CAST/Ei, C3H/He, BALB/c) and determining phenotypes relevant to renal failure among transgenic offspring (histology, blood urea nitrogen, proteinuria, serum albumin, and serum cholesterol). We found striking variation in phenotypes among F1s, ranging from severe renal disease to no renal disease whatsoever (P<0.001 for ANOVA across all groups). To map genes responsible for this variation, we produced a backcross of TgFVB/CAST F1 x TgFVB. By genome-wide analysis of linkage in 185 heterozygous transgenic backcross mice, we identified a locus on chromosome 3A1-3, HIVAN1, that showed highly significant linkage to renal disease [logarithm of odds (lod) score 4.9 at D3Mit203, accounting for 15% of the variance in renal disease]. Other loci on chromosomes 11, 14, and 16 were suggestive of linkage to renal disease, and a locus on chromosome 9 influenced serum cholesterol but not nephropathy. Interestingly, HIVAN1 is syntenic to human chromosome 3q25-27, an interval showing suggestive evidence of linkage to various nephropathies. These findings demonstrate a strong genetic influence on HIVAN and demonstrate a major renal disease susceptibility locus on mouse chromosome 3A1-3.

The HIV-associated renal diseases: current insight into pathogenesis and treatment

Since the description of a new renal syndrome in patients with the acquired immunodeficiency syndrome (AIDS) in the middle 1980s, much has been learned regarding the association of human immunodeficiency virus (HIV) infection and renal disease. The HIV-associated renal diseases represent a spectrum of clinical and histopathologic conditions. In this review, epidemiologic and clinical aspects of HIV-associated renal diseases are presented. Particular attention is placed on the pathologic and pathophysiologic mechanisms involved in HIV-associated focal glomerulosclerosis, immune complex-mediated disease, and thrombotic microangiopathies. Pharmaceutical treatment options, including the use of glucocorticoids, angiotensin-converting enzyme (ACE) inhibitors, and highly active antiretroviral therapy, are discussed. The therapeutic option of renal transplantation is presented, with insight into new clinical and basic research supporting a possible role of immunosuppressive therapy in this already immunocompromised patient population.

Mitotic cell cycle proteins increase in podocytes despite lack of proliferation

BACKGROUND

Podocyte proliferation is an uncommon response to glomerular injury and its lack may underlie the development of glomerulosclerosis. However, whether podocytes have the capacity to enter and finish mitosis and cytokinesis is not known.

METHODS

The expression of mitotic cell cycle proteins (phosphorylated Histone 3, Cdc2, cyclin B1 and B2) was examined by immunohistochemistry in kidneys of embryonal mice, transgenic HIV-mice, and rats with experimental membranous nephropathy (passive Heymann nephritis, PHN). Mitotic proteins also were measured by Western blot in glomerular protein from PHN-rats and the activity of mitotic cyclins was quantified by histone kinase assay.

RESULTS

Mitotic proteins were increased in embryonal mouse glomeruli during the S- and comma-shaped stages and were absent at the capillary loop stage and in mature rodent glomeruli. There was an increase in podocyte expression of Cdc2, cyclin B1 and B2 and phosphorylated histone 3 in PHN rats, and in HIV transgenic mice.

CONCLUSIONS

Podocytes have the ability to increase cell cycle proteins required for mitosis. Without obvious differences in the expression of the major mitotic proteins in PHN- and HIV-nephropathy, a regulatory disturbance in cytokinesis might be responsible for the development of polynucleated cells and a lack of podocyte proliferation in experimental glomerular disease.

Renal Dysfunction in HIV-1-infected Patients

Improved therapy directed against opportunistic infection and HIV-1 itself has resulted in greatly enhanced patient survival in the past decade among patients infected with HIV-1. Since patients are living longer, HIV-1 infection is associated with a rising burden of kidney disease. Approximately 14% of black patients and 6% of white patients dying with HIV-1 infection in 1999 in the United States had renal disease. Overall, 10% of patients dying with HIV-1 infection had renal failure. The most common glomerular diseases are focal segmental glomerulosclerosis and immune complex glomerulonephritis. Appropriate therapy for focal segmental glomerulosclerosis includes effective antiretroviral therapy and angiotensin antagonist medication. Drug toxicity is also common, often manifesting as electrolyte abnormalities, acute renal failure, interstitial nephritis, or nephrolithiasis. In particular, indinavir is associated with crystalluria, nephrolithiasis, interstitial nephritis, and lower urinary tract inflammation. Appropriate screening for renal disease and appropriate intervention will likely reduce the morbidity and mortality associated with progressive renal disease.

HIV-1 Nef induces proliferation and anchorage-independent growth in podocytes

HIV-associated nephropathy (HIVAN) is now the third leading cause of end-stage renal disease in the African American population. HIV-1 infects renal tubular and glomerular epithelial cells or podocytes, cells that are a critical part of the filtration barrier. HIV-1 infection induces the loss of podocyte differentiation markers and increases podocyte proliferation. It has been previously shown that HIV-infection induces loss of contact inhibition. Here, the HIV-1 gene responsible for proliferative changes is identified by using cultured podocytes in vitro. The HIV-1 proviral construct, pNL4-3 was rendered noninfectious by replacing the HIV-1 gag/pol sequences with an EGFP reporter gene (pNL4-3: DeltaG/P-EGFP). This construct was then pseudotyped with VSV.G envelope to infect podocytes that were conditionally immortalized with SV-40 T antigen. In addition, mutated constructs were engineered with premature stop codons in the HIV-1 env, vif, vpr, vpu, nef, or rev genes. The parental construct and all the other mutated constructs, with the exception of nef, induced proliferation under nonpermissive conditions and anchorage-independent growth (colony formation in soft agar) under permissive conditions. In contrast, deletion of nef markedly reduced proliferation and colony formation. Although tat alone, or tat plus rev induced marginal levels of anchorage-independent growth, coexpression with nef significantly increased colony formation. Finally, stable expression of Nef in a retroviral vector, pBabe-puro, was sufficient to induce increased proliferation and colony formation. Moreover, nef induced saturation density and loss of contact inhibition. These data indicate that Nef induces multiple proliferative effects in podocytes in culture and that nef may therefore be an important gene in the pathogenesis of HIVAN in vivo.

Modulation of podocyte phenotype in collapsing glomerulopathies

Podocytes are well-differentiated postmitotic cells whose function is largely based on their complex cytoskeletal architecture. In diseases with proteinuria, podocytes undergo morphologic changes. Podocytes react to an injurious stimulus by a reorganization of their foot process architecture that is independent of the primary injury and the cause of the proteinuria. Collapsing glomerulopathies, including the idiopathic and secondary forms due to HIV infection, have been previously considered a part of the focal sclerosing glomerulosclerosis (FSGS) spectrum. However, in contrast to FSGS, both forms of collapsing glomerulopathy are characterized by segmental and global collapse of the glomerular basement membrane (GBM) and by characteristic ultrastructural alterations in podocytes. These alterations include loss of the actin-based cytoskeleton, a dysregulated/dedifferentiated phenotype, cellular hypertrophy, and cell proliferation. These observations raise the following questions: 1) What mechanism causes glomerular collapse and do podocytes have a role? We recently proposed that in collapsing glomerulopathies the composition of the GBM is altered and contains more immature forms of collagen IV. These observations suggest that dedifferentiated/dysregulated podocytes may participate in remodeling the GBM composition, producing fetal collagen isoforms. 2) What is the pathomechanism underlying podocyte dysregulation? Although it is still unclear which etiologic factors are responsible for the idiopathic forms of collapsing glomerulopathy, in situ hybridization studies in a transgenic mouse model of HIV-associated collapsing glomerulopathy and on renal biopsies of patients with HIV-associated collapsing glomerulopathy demonstrated the presence of the HIV-1 RNA in podocytes and tubular epithelial cells. These findings suggest a direct link between viral gene expression and the dysregulation of the podocyte phenotype. 3) Another open question is how podocytes become infected in HIV-associated collapsing glomerulopathy. HIV-1 typically uses CD4 and a co-receptor such as CCR5 or CXCR4 to enter cells. So far, there is no demonstration of the expression of these receptors in podocytes. These negative findings, however, do not exclude the possibility that in the kidney another, CD4 independent, co-receptor may be used for viral cell entry. Finally, is it important to mention that collapsing glomerulopathies have a high prevalence in black patients, suggesting a link between racial background and the virus-related podocyte injury.

Predictors of proteinuria and renal failure among women with HIV infection

BACKGROUND

Glomerular disease with proteinuria and renal failure are complications of human immunodeficiency virus (HIV) infection. While studies suggest risk factors for both include black race and lower CD4 lymphocyte count, they have not been established in population-based cohorts. This study examines the risk factors for proteinuria and renal failure in a large cohort of HIV-infected women not selected for the presence of renal disease.

METHODS

This prospective cohort includes 2059 women enrolled in the Women's Interagency HIV study (WIHS). WIHS is a longitudinal study of the clinical course of HIV infection in which subjects are followed biannually with a detailed exam including urine analysis, serum creatinine, CD4 lymphocyte count, and HIV RNA level. Proteinuria was defined as > or =+1 on urine dipstick exam on at least two consecutive urine analyses, and renal failure was defined as a doubling of serum creatinine. Multivariable logistic regression was used to estimate the associations between clinical variables and the presence of proteinuria on initial evaluation in a cross-sectional analysis. Cox proportional hazards regression was used to estimate the associations between clinical variables and time to renal failure among study participants with proteinuria in a prospective longitudinal analysis.

RESULTS

Of 2057 HIV-positive women, 32% (N=671) had proteinuria on initial evaluation. Predictors of proteinuria include increasing (log) HIV RNA level [odds ratio (OR)=1.05], black race (OR=2.0), absolute CD4 lymphocyte count < or =200 cells/mm3 (OR=1.41), and the presence of hepatitis C antibody (OR=1.27; all P < 0.0001). Absolute CD4 lymphocyte count < or =200 cells/mm3 [hazard ratio (HR)=3.57, P=0.001], detectable HIV RNA level (HR=2.33, P=0.02), increasing systolic blood pressure (HR=1.02, P=0.002), and decreasing albumin (HR=3.33, P=0.0001) and increasing creatinine (1.67, P=0.0001) were all associated with the development of renal failure.

CONCLUSIONS

This analysis establishes the associations between both increasing HIV RNA level and decreasing CD4 lymphocyte count with the presence of proteinuria and occurrence of renal failure. Additionally, it demonstrates an association between proteinuria and a positive hepatitis C antibody. To lessen the presence and progression of renal disease among HIV-infected patients, future research should focus on suppression of the HIV RNA level and improvement in CD4 lymphocyte count.