HIV-1 expression induces cyclin D1 expression and pRb phosphorylation in infected podocytes: cell-cycle mechanisms contributing to the proliferative phenotype in HIV-associated nephropathy

NEF-Induced HIV-Associated Nephropathy Through HCK/LYN Tyrosine Kinases

HIV-1-associated nephropathy (HIVAN) is a severe complication of HIV-1 infection. To gain insight into the pathogenesis of kidney disease in the setting of HIV, a transgenic (Tg) mouse model [CD4C/HIV-negative regulator factor (Nef)] was used in which HIV-1 nef expression is under control of regulatory sequences (CD4C) of the human CD4 gene, thus allowing expression in target cells of the virus. These Tg mice develop a collapsing focal segmental glomerulosclerosis associated with microcystic dilatation, similar to human HIVAN. To identify kidney cells permissive to the CD4C promoter, CD4C reporter Tg lines were used. They showed preferential expression in glomeruli, mainly in mesangial cells. Breeding CD4C/HIV Tg mice on 10 different mouse backgrounds showed that HIVAN was modulated by host genetic factors. Studies of gene-deficient Tg mice revealed that the presence of B and T cells and that of several genes was dispensable for the development of HIVAN: those involved in apoptosis (Trp53, Tnfsf10, Tnf, Tnfrsf1b, and Bax), in immune cell recruitment (Ccl3, Ccl2, Ccr2, Ccr5, and Cx3cr1), in nitric oxide (NO) formation (Nos3 and Nos2), or in cell signaling (Fyn, Lck, and Hck/Fgr). However, deletion of Src partially and that of Hck/Lyn largely abrogated its development. These data suggest that Nef expression in mesangial cells through hematopoietic cell kinase (Hck)/Lck/Yes novel tyrosine kinase (Lyn) represents important cellular and molecular events for the development of HIVAN in these Tg mice.

SUMO specific peptidase 6 regulates the crosstalk between podocytes and glomerular endothelial cells in diabetic kidney disease

There is increasing evidence that the crosstalk between podocytes and glomerular endothelial cells (GECs) exacerbates the progression of diabetic kidney disease (DKD). Here, we investigated the underlying role of SUMO specific peptidase 6 (SENP6) in this crosstalk. In the diabetic mice, SENP6 was decreased in glomerular tissues and its knockdown further exacerbated glomerular filtration barrier injury. In the mouse podocyte cell line MPC5 cells, SENP6 overexpression reversed HG-induced podocyte loss by suppressing the activation of Notch1 signaling. Notch1 intracellular domain (N1ICD) is the active form of Notch1. SENP6 upregulated the ubiquitination of N1ICD by deSUMOylating Notch1, thereby reducing N1ICD and suppressing Notch1 signaling activation in MPC5 cells. Endothelin-1 (EDN1) is a protein produced by podocytes and has been reported to promote GEC dysfunction. The supernatant from HG-treated MPC5 cells induced mitochondrial dysfunction and surface layer injury in GECs, and the supernatant from SENP6-deficient podocytes further exacerbated the above GEC dysfunction, while this trend was reversed by an EDN1 antagonist. The following mechanism study showed that SENP6 deSUMOylated KDM6A (a histone lysine demethylase) and then decreased the binding potency of KDM6A to EDN1. The latter led to the upregulation of H3K27me2 or H3K27me3 of EDN1 and suppressed its expression in podocytes. Taken together, SENP6 suppressed the HG-induced podocyte loss and ameliorated GEC dysfunction caused by crosstalk between podocytes and GECs, and the protective effect of SENP6 on DKD is attributed to its deSUMOylation activity.

Impact of human immunodeficiency virus on pulmonary vascular disease

With the advent of anti-retroviral therapy, non-AIDS-related comorbidities have increased in people living with HIV. Among these comorbidities, pulmonary hypertension (PH) is one of the most common causes of morbidity and mortality. Although chronic HIV-1 infection is independently associated with the development of pulmonary arterial hypertension, PH in people living with HIV may also be the outcome of various co-morbidities commonly observed in these individuals including chronic obstructive pulmonary disease, left heart disease and co-infections. In addition, the association of these co-morbidities and other risk factors, such as illicit drug use, can exacerbate the development of pulmonary vascular disease. This review will focus on these complex interactions contributing to PH development and exacerbation in HIV patients. We also examine the interactions of HIV proteins, including Nef, Tat, and gp120 in the pulmonary vasculature and how these proteins alter the endothelial and smooth muscle function by transforming them into susceptible PH phenotype. The review also discusses the available infectious and non-infectious animal models to study HIV-associated PAH, highlighting the advantages and disadvantages of each model, along with their ability to mimic the clinical manifestations of HIV-PAH.

Yes-associated protein regulates podocyte cell cycle re-entry and dedifferentiation in adriamycin-induced nephropathy

Podocytes are terminally differentiated cells with little proliferative capacity. The high expression levels of cell cycle inhibitory proteins, including p21, p27, and p57, play an important role in maintaining the low level of proliferation of mature podocytes. In the present study, we aimed to explore the role of yes-associated protein (YAP) signalling in adriamycin-induced podocyte re-entry into the cell cycle and dedifferentiation. Proliferating cell nuclear antigen (PCNA)-, cyclin-dependent kinase 4 (CDK4)-, and Cyclin D1-positive podocytes were found in mice with adriamycin-induced nephropathy. In vitro, adriamycin administration increased the percentage of cells in S phase and the upregulation of mesenchymal-related marker proteins. CDK4 and cyclin D1 were significantly up-regulated after incubation with adriamycin. Overexpression of YAP in podocytes promoted their entry into the cell cycle; up-regulated cyclin D1, desmin, and snail2 expression and down-regulated Wilms’ tumour 1 (WT1) and nephrin production. Recombinant murine FGF-basic induced podocytes to re-enter the cell cycle, inhibited WT1 and nephrin, and increased desmin and snail2 expression. Pretreating podocytes with verteporfin, an inhibitor of YAP/ TEA domain transcription factor (TEAD), decreased the adriamycin-induced overexpression of cyclin D1 and reduced the ratio of S-phase podocytes. This result was further verified by knocking down YAP expression using RNA interference. In conclusion, adriamycin induced podocytes to re-enter the cell cycle via upregulation of CDK4 and cyclin D1 expression, which was at least partly mediated by YAP signalling. Re-entry into the cell cycle induced the over-expression of mesenchymal markers in podocytes.

The Neuropeptides Vasoactive Intestinal Peptide and Pituitary Adenylate Cyclase-Activating Polypeptide Control HIV-1 Infection in Macrophages Through Activation of Protein Kinases A and C

Vasoactive intestinal peptide (VIP) and pituitary adenylate cyclase-activating polypeptide (PACAP) are highly similar neuropeptides present in several tissues, endowed with immunoregulatory functions and other systemic effects. We previously reported that both neuropeptides reduce viral production in HIV-1-infected primary macrophages, with the participation of β-chemokines and IL-10, and now we describe molecular mechanisms engaged in this activity. Macrophages exposed to VIP or PACAP before HIV-1 infection showed resistance to viral replication, comparable to that observed when the cells were treated after infection. Also, multiple treatments with a suboptimal dose of VIP or PACAP after macrophage infection resulted in a decline of virus production similar to the inhibition promoted by a single exposure to the optimal inhibitory concentration. Cellular signaling pathways involving cAMP production and activation of protein kinases A and C were critical components of the VIP and PACAP anti-HIV-1 effects. Analysis of the transcription factors and the transcriptional/cell cycle regulators showed that VIP and PACAP induced cAMP response element-binding protein activation, inhibited NF-kB, and reduced Cyclin D1 levels in HIV-1-infected cells. Remarkably, VIP and PACAP promoted G-to-A mutations in the HIV-1 provirus, matching those derived from the activity of the APOBEC family of viral restriction factors, and reduced viral infectivity. In conclusion, our findings strengthen the antiretroviral potential of VIP and PACAP and point to new therapeutic approaches to control the progression of HIV-1 infection.

TNFR2 interposes the proliferative and NF-κB-mediated inflammatory response by podocytes to TNF-α

The development of proliferative podocytopathies has been linked to ligation of tumor necrosis factor receptor 2 (TNFR2) expressed on the renal parenchyma; however, the TNFR2-positive cells within the kidney responsible for podocyte injury are unknown. We detected de novo expression of TNFR2 on podocytes before hyperplastic injury in crescentic glomerulonephritis of mice with nephrotoxic nephritis, and in collapsing glomerulopathy of Tg26(HIV/nl) mice, kd/kd mice, and human beings. We further found that serum levels of soluble TNF-α and TNFR2 correlated significantly with renal injury in Tg26(HIV/nl) mice. Thus, we asked whether ligand binding of TNFR2 on podocytes ex vivo precipitates the characteristic proliferative and pro-inflammatory diseased podocyte phenotypes. Soluble TNF-α activated NF-κB and dose-dependently induced podocyte proliferation, marked by the expression of the podocyte G(1) cyclin and NF-κB target gene, cyclin D1. Microarray gene and chemokine protein expression profiling showed a marked pro-inflammatory NF-κB signature, and activated podocytes secreting CCL2- and CCL5-induced macrophage migration in transwell assays. Neutralization of TNFR2 on podocytes with blocking antibodies abrogated NF-κB activation and the induction of cyclin D1 by TNF-α, and identified TNFR2 as the primary receptor that induced IκBα degradation, the initiating event in NF-κB activation. These results suggest that TNFR2 expressed on podocytes and its canonical NF-κB signaling may directly interpose the compound pathogenic responses by podocytes to TNF-α, in the absence of other TNFR2-positive renal cell types in proliferative podocytopathies.

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.

The cell cycle and acute kidney injury

Acute kidney injury (AKI) activates pathways of cell death and cell proliferation. Although seemingly discrete and unrelated mechanisms, these pathways can now be shown to be connected and even to be controlled by similar pathways. The dependence of the severity of renal-cell injury on cell cycle pathways can be used to control and perhaps to prevent acute kidney injury. This review is written to address the correlation between cellular life and death in kidney tubules, especially in acute kidney injury.

CDK/GSK-3 inhibitors as therapeutic agents for parenchymal renal diseases

Drug discovery to lessen the burden of chronic renal failure and end-stage renal disease remains a principle goal of translational research in nephrology. In this review, we provide an overview of the current development of small molecule cyclin-dependent kinase (CDK)/glycogen synthase kinase-3 (GSK-3) inhibitors as therapeutic agents for parenchymal renal diseases. The emergence of this drug family has resulted from the recognition that CDKs and GSK-3s play critical roles in the progression and regression of many kidney diseases. CDK/GSK-3 inhibitors suppress pathogenic proliferation, apoptosis, and inflammation, and promote regeneration of injured tissue. Preclinical efficacy has now been demonstrated in mesangial proliferative glomerulonephritis, crescentic glomerulonephritis, collapsing glomerulopathy, proliferative lupus nephritis, polycystic kidney diseases, diabetic nephropathy, and several forms of acute kidney injury. Novel biomarkers of therapy are aiding the process of drug development. This review will highlight these advancements in renal therapeutics.

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.

Cell Cycle Regulatory Proteins in Podocyte Health and Disease

The glomerular visceral epithelial cell, or podocyte, is a highly specialized and terminally differentiated cell that is fundamental to the integrity of the glomerular filtration barrier and functions to prevent urinary protein leakage and to oppose intracapillary hydrostatic pressure. Common to many human kidney diseases and experimental animal models is a strong association between podocyte injury and the development of progressive kidney disease. Studies have shown that a decline in podocyte number strongly correlates with, and likely underlies, proteinuria and the progression to glomerulosclerosis. Maintenance of podocyte differentiation, essential to its normal structure and function, is challenged in the setting of glomerular injury, with very divergent outcomes dependent upon the inciting injury. In response to injury, podocytes may undergo several cell fates, including proliferation, de-differentiation, hypertrophy, apoptosis, or necrosis. Common to these potential outcomes of renal injury is their ultimate regulation at the level of the cell cycle. Positive regulators (cyclins and cyclin-dependent kinases) and negative regulators (cyclin-dependent kinase inhibitors) coordinate the cell cycle. There is now a large body of literature confirming the importance of cell cycle regulatory proteins in the cellular response to injury. Emerging lessons from mouse knockout experiments highlight that the cell cycle machinery operates differently in distinct cell types. Recent studies focusing on the roles of cell cycle regulatory proteins specifically in podocytes have provided important clues on how these proteins operate to constrain cell proliferation and preserve differentiation in health, and how they modulate the dysregulated phenotype in diseased states. In disease, both a failure to regenerate lost podocytes and an inappropriate proliferative response can have profound consequences for glomerular structure and function. Here, we will review the latest advances in understanding the roles of cell cycle regulatory proteins in diseases of the podocyte.

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.

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.

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

The notable glomerular feature of human immunodeficiency virus (HIV)-associated nephropathy (HIVAN) is the collapse of the capillary tuft with marked glomerular epithelial cell hyperplasia. These data suggest a loss of normal podocyte function, which is associated with a loss of the podocyte differentiation markers, Wilm's tumor (WT-1), synaptopodin, podocalyxin, and common acute lymphoblastic leukemia antigen (CALLA). We have previously shown that HIV-1 expression can induce these changes in HIV-1 transgenic mice. To identify which HIV-1 gene product(s) are responsible for the phenotypic changes in podocytes, we created multiple mutated HIV-1 constructs and then pseudotyped them with vesticular stomatitis virus glycoprotein (VSVG) envelope to enhance the tropism of these mutant viruses. In addition to gag/pol, the mutant viruses lacked one of the following, env, nef, rev, vif, vpr, or vpu. In addition, we generated single gene expressing pseudotyped viruses to complement the scanning mutation approach of our viral parental construct. Murine podocytes were then infected with one of the viral constructs either lacking or expressing the various HIV-1 genes. We found that HIV-1 nef was necessary and sufficient for proliferation of podocytes and down-regulation of synaptopodin and CALLA. These data suggest that Nef induces many of the changes we observe in HIV transgenic model and, as a result, this now defines the pathway for exploration of host responses to HIV-1 infection.

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.

The role of cell cycle proteins in Glomerular disease

Although initially identified and characterized as regulators of the cell cycle and hence proliferation, an extended role for cell cycle proteins has been appreciated more recently in a number of physiologic and pathologic processes, including development, differentiation, hypertrophy, and apoptosis. Their precise contribution to the cellular response to injury appears to be dependent on both the cell type and the nature of the initiating injury. The glomerulus offers a remarkable situation in which to study the cell cycle proteins, as each of the 3 major resident cell types (the mesangial cell, podocyte, and glomerular endothelial cell) has a specific pattern of cell cycle protein expression when quiescent and responds uniquely after injury. Defining their roles may lead to potential therapeutic strategies in glomerular disease.

Update in podocyte biology: putting one's best foot forward

PURPOSE OF REVIEW

The rapidly developing field of podocyte cell biology is reviewed, focusing on papers published in the last 12 months.

RECENT FINDINGS

Four areas of particular progress can be discerned. First, podocytes proliferate during early metanephric development, are quiescent after the capillary loop stage, and re-enter the cell cycle only in the disease group termed collapsing glomerulopathy. We have learned that control of the podocyte cell cycle involves both expression of cell-cycle regulating proteins and the process of cytokinesis. Second, the podocyte slit diaphragm is the final component of the filtration barrier. The structure and maintenance of the slit diaphragm has been a major focus of research activity, and a multiplicity of relevant molecular interactions have been defined. Significant advances have been made in understanding the complex and interacting role of nephrin and podocin mutations in the genesis of clinical glomerular disease. Third, several proteins essential to controlling discrete podocyte transcriptional programs have been defined. Finally, conditionally-immortalized podocyte cell lines, derived from mouse and human tissue, have proven their worth as models to advance investigations of podocyte biology.

SUMMARY

Podocyte injury occurs as a consequence of genetic mutation, immunological injury, viral infection, or abnormal hemodynamic forces within the glomerulus. As we understand more about the podocyte proteome and cell biology, we gain an increasingly detailed molecular understanding of podocyte structure and function. In this drama we have many molecular players and increasing stretches of molecular dialogue, but the script remains largely to be deciphered. Nevertheless, we do understand the consequences that arise when the podocyte cannot put its best foot (processes) forward.