Critical role for Nef in HIV-1-induced podocyte dedifferentiation

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.

Persistent NF-kappaB activation in renal epithelial cells in a mouse model of HIV-associated nephropathy

Human immunodeficiency virus (HIV)-associated nephropathy (HIVAN) is caused, in part, by direct infection of kidney epithelial cells by HIV-1. In the spectrum of pathogenic host-virus interactions, abnormal activation or suppression of host transcription factors is common. NF-kappaB is a necessary host transcription factor for HIV-1 gene expression, and it has been shown that NF-kappaB activity is dysregulated in many naturally infected cell types. We show here that renal glomerular epithelial cells (podocytes) expressing the HIV-1 genome, similar to infected immune cells, also have a dysregulated and persistent activation of NF-kappaB. Although podocytes produce p50, p52, RelA, RelB, and c-Rel, electrophoretic mobility shift assays and immunocytochemistry showed a predominant nuclear accumulation of p50/RelA-containing NF-kappaB dimers in HIV-1-expressing podocytes compared with normal. In addition, the expression level of a transfected NF-kappaB reporter plasmid was significantly higher in HIVAN podocytes. The mechanism of NF-kappaB activation involved increased phosphorylation of IkappaBalpha, resulting in an enhanced turnover of the IkappaBalpha protein. There was no evidence for regulation by IkappaBbeta or the alternate pathway of NF-kappaB activation. Altered activation of this key host transcription factor likely plays a role in the well-described cellular phenotypic changes observed in HIVAN, such as proliferation. Studies with inhibitors of proliferation and NF-kappaB suggest that NF-kappaB activation may contribute to the proliferative mechanism in HIVAN. In addition, because NF-kappaB regulates many aspects of inflammation, this dysregulation may also contribute to disease severity and progression through regulation of proinflammatory processes in the kidney microenvironment.

NF-kappaB regulates Fas-mediated apoptosis in HIV-associated nephropathy

Renal parenchymal injury in HIV-associated nephropathy (HIVAN) is characterized by epithelial proliferation, dedifferentiation, and apoptosis along the entire length of the nephron. Although apoptotic cell death in HIVAN has been well documented, the mechanism for HIV-induced apoptosis is poorly understood. Whether the epithelial apoptosis in HIVAN is mediated by NF-kappaB-activated Fas ligand expression was investigated here. In human HIVAN and HIV-1 transgenic mouse kidney specimens, the expression of Fas receptor and ligand proteins were markedly upregulated on epithelium in diseased glomerular and tubulointerstitial compartments when compared with normal. Podocyte cell lines that were derived from HIV-1 transgenic mice showed a similar upregulation of Fas receptor expression and de novo expression of Fas ligand by semiquantitative reverse transcription-PCR and Western blotting. In cultured podocytes, cross-linking of the Fas receptor to mimic ligand binding induced caspase 8 activity and apoptosis in both normal and HIVAN podocytes. Because constitutive NF-kappaB activity has been demonstrated in HIVAN epithelia, evidence for transcriptional control of the Fas ligand expression by NF-kappaB was sought. With the use of cultured podocytes, expression of a Fas ligand promoter reporter plasmid was higher in HIVAN podocytes, indicating increased transcriptional activity. In addition, chromatin immunoprecipitation assays were performed to demonstrate that p65-containing (RelA) complexes bound the Fas ligand promoter and that suppression of activated NF-kappaB with a peptide inhibitor could reduce the expression of Fas ligand mRNA in HIVAN podocytes. These results suggest that NF-kappaB may regulate Fas-mediated apoptosis in HIVAN by controlling the expression of Fas ligand in renal epithelium.

Highly active antiretroviral therapy and the epidemic of HIV+ end-stage renal disease

The rise in the number of patients with HIV-associated nephropathy and HIV-infection with end-stage renal disease (HIV+ ESRD) continues to be a substantial concern for the ESRD program. In order to assess the impact of highly active antiretroviral therapy (HAART) on the progression of patients with AIDS to the development of ESRD and to project the prevalence of HIV+ ESRD through 2020, a mathematical model of the dynamics of HIV+ infection in the ESRD population was developed. Epidemiologic data on AIDS and HIV+ ESRD among black individuals in the United States were obtained since 1991 from the Centers for Disease Control and Prevention and US Renal Data System, respectively. The model was constructed to predict the prevalence of HIV+ ESRD incorporating the current rate of growth in AIDS prevalence. Two possible trends were considered: linear AIDS growth and exponential AIDS growth. The likely effectiveness of HAART in slowing progression to HIV+ ESRD was estimated from the best fit of the model to the data after 1995, when HAART was introduced. The model was then used to evaluate recent data and to project the prevalence of HIV+ ESRD through 2020. The model suggested that HAART has reduced the rate of progression from AIDS to HIV+ ESRD by 38%. The model projected an increase in HIV+ ESRD prevalence in the future as a result of the increase in the AIDS population among black individuals. This increase was predicted even assuming a 95% reduction in the progression from AIDS to HIV+ ESRD. Despite the potential benefit of HAART, the prevalence of HIV+ ESRD in the United States is expected to rise in the future as a result of the expansion of the AIDS population among black individuals. It is concluded that prevention of progression to ESRD should focus on early antiretroviral treatment of HIV-infected patients who have evidence of HIV-associated nephropathy.

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.