A small molecule chaperone rescues keratin-8 mediated trafficking of misfolded podocin to correct genetic Nephrotic Syndrome.

Podocin is a key membrane scaffolding protein of the kidney podocyte essential for intact glomerular filtration. Mutations in NPHS2, the podocin-encoding gene, represent the commonest form of inherited nephrotic syndrome (NS), with early, intractable kidney failure. The most frequent podocin gene mutation in European children is R138Q, causing retention of the misfolded protein in the endoplasmic reticulum. Here, we provide evidence that podocin R138Q (but not wild-type podocin) complexes with the intermediate filament protein keratin 8 (K8) thereby preventing its correct trafficking to the plasma membrane. We have also identified a small molecule (c407), a compound that corrects the Cystic Fibrosis Transmembrane Conductance Regulator protein defect, that interrupts this complex and rescues mutant protein mistrafficking. This results in both the correct localization of podocin at the plasma membrane and functional rescue in both human patient R138Q mutant podocyte cell lines, and in a mouse inducible knock-in model of the R138Q mutation. Importantly, complete rescue of proteinuria and histological changes was seen when c407 was administered both via osmotic minipumps or delivered orally prior to induction of disease or crucially via osmotic minipump two weeks after disease induction. Thus, our data constitute a therapeutic option for patients with NS bearing a podocin mutation, with implications for other misfolding protein disorders. Further studies are necessary to confirm our findings.

IGFBP-1 expression is reduced in human type 2 diabetic glomeruli and modulates ß1-integrin/FAK signalling in human podocytes.

AIMS/HYPOTHESIS: Podocyte loss or injury is one of the earliest features observed in the pathogenesis of diabetic kidney disease (DKD), which is the leading cause of end-stage renal failure worldwide. Dysfunction in the IGF axis, including in IGF binding proteins (IGFBPs), is associated with DKD, particularly in the early stages of disease progression. The aim of this study was to investigate the potential roles of IGFBPs in the development of type 2 DKD, focusing on podocytes. METHODS: IGFBP expression was analysed in the Pima DKD cohort, alongside data from the Nephroseq database, and in ex vivo human glomeruli. Conditionally immortalised human podocytes and glomerular endothelial cells were studied in vitro, where IGFBP-1 expression was analysed using quantitative PCR and ELISAs. Cell responses to IGFBPs were investigated using migration, cell survival and adhesion assays; electrical cell-substrate impedance sensing; western blotting; and high-content automated imaging. RESULTS: Data from the Pima DKD cohort and from the Nephroseq database demonstrated a significant reduction in glomerular IGFBP-1 in the early stages of human type 2 DKD. In the glomerulus, IGFBP-1 was predominantly expressed in podocytes and controlled by phosphoinositide 3-kinase (PI3K)-forkhead box O1 (FoxO1) activity. In vitro, IGFBP-1 signalled to podocytes via ß1-integrins, resulting in increased phosphorylation of focal-adhesion kinase (FAK), increasing podocyte motility, adhesion, electrical resistance across the adhesive cell layer and cell viability. CONCLUSIONS/INTERPRETATION: This work identifies a novel role for IGFBP-1 in the regulation of podocyte function and that the glomerular expression of IGFBP-1 is reduced in the early stages of type 2 DKD, via reduced FoxO1 activity. Thus, we hypothesise that strategies to maintain glomerular IGFBP-1 levels may be beneficial in maintaining podocyte function early in DKD.

TRPC6 Binds to and Activates Calpain, Independent of Its Channel Activity, and Regulates Podocyte Cytoskeleton, Cell Adhesion, and Motility.

BACKGROUND: Mutations in the transient receptor potential channel 6 (TRPC6) gene are associated with an inherited form of FSGS. Despite widespread expression, patients with TRPC6 mutations do not present with any other pathologic phenotype, suggesting that this protein has a unique yet unidentified role within the target cell for FSGS, the kidney podocyte. METHODS: We generated a stable TRPC6 knockout podocyte cell line from TRPC6 knockout mice. These cells were engineered to express wild-type TRPC6, a dominant negative TRPC6 mutation, or either of two disease-causing mutations of TRPC6, G109S or K874*. We extensively characterized these cells using motility, detachment, and calpain activity assays; immunofluorescence; confocal or total internal reflection fluorescence microscopy; and western blotting. RESULTS: Compared with wild-type cells, TRPC6-/- podocytes are less motile and more adhesive, with an altered actin cytoskeleton. We found that TRPC6 binds to ERK1/2 and the actin regulatory proteins, caldesmon (a calmodulin- and actin-binding protein) and calpain 1 and 2 (calcium-dependent cysteine proteases that control the podocyte cytoskeleton, cell adhesion, and motility via cleavage of paxillin, focal adhesion kinase, and talin). Knockdown or expression of the truncated K874* mutation (but not expression of the gain-of-function G019S mutation or dominant negative mutant of TRPC6) results in the mislocalization of calpain 1 and 2 and significant downregulation of calpain activity; this leads to altered podocyte cytoskeleton, motility, and adhesion-characteristics of TRPC6-/- podocytes. CONCLUSIONS: Our data demonstrate that independent of TRPC6 channel activity, the physical interaction between TRPC6 and calpain in the podocyte is important for cell motility and detachment and demonstrates a scaffolding role of the TRPC6 protein in disease.

MAGI2 Mutations Cause Congenital Nephrotic Syndrome.

Steroid-resistant nephrotic syndrome (SRNS), a heterogeneous disorder of the renal glomerular filtration barrier, results in impairment of glomerular permselectivity. Inheritance of genetic SRNS may be autosomal dominant or recessive, with a subset of autosomal recessive SRNS presenting as congenital nephrotic syndrome (CNS). Mutations in 53 genes are associated with human SRNS, but these mutations explain ≤30% of patients with hereditary cases and only 20% of patients with sporadic cases. The proteins encoded by these genes are expressed in podocytes, and malfunction of these proteins leads to a universal end point of podocyte injury, glomerular filtration barrier disruption, and SRNS. Here, we identified novel disease-causing mutations in membrane-associated guanylate kinase, WW, and PDZ domain-containing 2 (MAGI2) through whole-exome sequencing of a deeply phenotyped cohort of patients with congenital, childhood-onset SRNS. Although MAGI2 has been shown to interact with nephrin and regulate podocyte cytoskeleton and slit diaphragm dynamics, MAGI2 mutations have not been described in human SRNS. We detected two unique frameshift mutations and one duplication in three patients (two families); two siblings shared the same homozygous frameshift mutation, whereas one individual with sporadic SRNS exhibited compound heterozygosity. Two mutations were predicted to introduce premature stop codons, and one was predicted to result in read through of the normal translational termination codon. Immunohistochemistry in kidney sections from these patients revealed that mutations resulted in lack of or diminished podocyte MAGI2 expression. Our data support the finding that mutations in the MAGI2 gene are causal for congenital SRNS.

Prolonged exposure of mouse and human podocytes to insulin induces insulin resistance through lysosomal and proteasomal degradation of the insulin receptor.

AIMS/HYPOTHESIS: Podocytes are insulin-responsive cells of the glomerular filtration barrier and are key in preventing albuminuria, a hallmark feature of diabetic nephropathy. While there is evidence that a loss of insulin signalling to podocytes is detrimental, the molecular mechanisms underpinning the development of podocyte insulin resistance in diabetes remain unclear. Thus, we aimed to further investigate podocyte insulin responses early in the context of diabetic nephropathy. METHODS: Conditionally immortalised human and mouse podocyte cell lines and glomeruli isolated from db/db DBA/2J mice were studied. Podocyte insulin responses were investigated with western blotting, cellular glucose uptake assays and automated fluorescent imaging of the actin cytoskeleton. Quantitative (q)RT-PCR was employed to investigate changes in mRNA. Human cell lines stably overproducing the insulin receptor (IR) and nephrin were also generated, using lentiviral constructs. RESULTS: Podocytes exposed to a diabetic environment (high glucose, high insulin and the proinflammatory cytokines TNF-α and IL-6) become insulin resistant with respect to glucose uptake and activation of phosphoinositide 3-kinase (PI3K) and mitogen-activated protein kinase (MAPK) signalling. These podocytes lose expression of the IR as a direct consequence of prolonged exposure to high insulin concentrations, which causes an increase in IR protein degradation via a proteasome-dependent and bafilomycin-sensitive pathway. Reintroducing the IR into insulin-resistant human podocytes rescues upstream phosphorylation events, but not glucose uptake. Stable expression of nephrin is also required for the insulin-stimulated glucose uptake response in podocytes and for efficient insulin-stimulated remodelling of the actin cytoskeleton. CONCLUSIONS/INTERPRETATION: Together, these results suggest that IR degradation, caused by high levels of insulin, drives early podocyte insulin resistance, and that both the IR and nephrin are required for full insulin sensitivity of this cell. This could be highly relevant for the development of nephropathy in individuals with type 2 diabetes, who are commonly hyperinsulinaemic in the early phases of their disease.

CD317/tetherin is an organiser of membrane microdomains.

The integral membrane protein tetherin has been associated with an eclectic mix of cellular processes, including restricting the release of a range of enveloped viruses from infected cells. The unusual topology of tetherin (it possesses both a conventional transmembrane domain and a glycosylphosphatidylinositol anchor), its localisation to membrane microdomains (lipid rafts) and the fact that its cytosolic domain can be linked (indirectly) to the actin cytoskeleton, led us to speculate that tetherin might form a 'tethered picket fence' and thereby play a role in the organisation of lipid rafts. We now show that knocking down expression of tetherin leads to changes in the distribution of lipid raft-localised proteins and changes in the organisation of lipids in the plasma membrane. These changes can be reversed by re-expression of wild-type tetherin, but not by any of a range of tetherin-based constructs, indicating that no individual feature of the tetherin sequence is dispensable in the context of its lipid raft organising function.

Adeno-associated virus gene therapy prevents progression of kidney disease in genetic models of nephrotic syndrome.

Gene therapy for kidney diseases has proven challenging. Adeno-associated virus (AAV) is used as a vector for gene therapy targeting other organs, with particular success demonstrated in monogenic diseases. We aimed to establish gene therapy for the kidney by targeting a monogenic disease of the kidney podocyte. The most common cause of childhood genetic nephrotic syndrome is mutations in the podocyte gene NPHS2, encoding podocin. We used AAV-based gene therapy to rescue this genetic defect in human and mouse models of disease. In vitro transduction studies identified the AAV-LK03 serotype as a highly efficient transducer of human podocytes. AAV-LK03-mediated transduction of podocin in mutant human podocytes resulted in functional rescue in vitro, and AAV 2/9-mediated gene transfer in both the inducible podocin knockout and knock-in mouse models resulted in successful amelioration of kidney disease. A prophylactic approach of AAV 2/9 gene transfer before induction of disease in conditional knockout mice demonstrated improvements in albuminuria, plasma creatinine, plasma urea, plasma cholesterol, histological changes, and long-term survival. A therapeutic approach of AAV 2/9 gene transfer 2 weeks after disease induction in proteinuric conditional knock-in mice demonstrated improvement in urinary albuminuria at days 42 and 56 after disease induction, with corresponding improvements in plasma albumin. Therefore, we have demonstrated successful AAV-mediated gene rescue in a monogenic renal disease and established the podocyte as a tractable target for gene therapy approaches.

Release of filamentous and spherical influenza A virus is not restricted by tetherin.

The cellular protein tetherin is thought to act as a 'leash' that anchors many enveloped viruses to the plasma membrane and prevents their release. We found that replication of multiple strains of influenza A virus was generally insensitive to alteration of tetherin levels, as assessed by output titre or scanning electron microscopy of cell-associated virions. This included human, swine, avian and equine isolates, strains that form filamentous or spherical particles and viruses that lack the M2 or NS1 proteins. Levels of cell-surface tetherin were not reduced by influenza infection, but tetherin and the viral haemagglutinin co-localized on the plasma membrane. However, tetherin could not be detected in filamentous virions, suggesting that influenza may possess a mechanism to exclude it from virions. Overall, if influenza does encode a specific antagonist of tetherin, it is not M2 or NS1 and we find no evidence for a role in host range specificity.

Disease causing mutations in inverted formin 2 regulate its binding to G-actin, F-actin capping protein (CapZ α-1) and profilin 2.

Focal segmental glomerulosclerosis (FSGS) is a devastating form of nephrotic syndrome which ultimately leads to end stage renal failure (ESRF). Mutations in inverted formin 2 (INF2), a member of the formin family of actin-regulating proteins, have recently been associated with a familial cause of nephrotic syndrome characterized by FSGS. INF2 is a unique formin that can both polymerize and depolymerize actin filaments. How mutations in INF2 lead to disease is unknown. In the present study, we show that three mutations associated with FSGS, E184K, S186P and R218Q, reduce INF2 auto-inhibition and increase association with monomeric actin. Furthermore using a combination of GFP-INF2 expression in human podocytes and GFP-Trap purification coupled with MS we demonstrate that INF2 interacts with profilin 2 and the F-actin capping protein, CapZ α-1. These interactions are increased by the presence of the disease causing mutations. Since both these proteins are involved in the dynamic turnover and restructuring of the actin cytoskeleton these changes strengthen the evidence that aberrant regulation of actin dynamics underlies the pathogenesis of disease.

Expression of HIV-1 Vpu leads to loss of the viral restriction factor CD317/Tetherin from lipid rafts and its enhanced lysosomal degradation.

CD317/tetherin (aka BST2 or HM1.24 antigen) is an interferon inducible membrane protein present in regions of the lipid bilayer enriched in sphingolipids and cholesterol (often termed lipid rafts). It has been implicated in an eclectic mix of cellular processes including, most notably, the retention of fully formed viral particles at the surface of cells infected with HIV and other enveloped viruses. Expression of the HIV viral accessory protein Vpu has been shown to lead to intracellular sequestration and degradation of tetherin, thereby counteracting the inhibition of viral release. There is evidence that tetherin interacts directly with Vpu, but it remains unclear where in the cell this interaction occurs or if Vpu expression affects the lipid raft localisation of tetherin. We have addressed these points using biochemical and cell imaging approaches focused on endogenous rather than ectopically over-expressed tetherin. We find i) no evidence for an interaction between Vpu and endogenous tetherin at the cell surface, ii) the vast majority of endogenous tetherin that is at the cell surface in control cells is in lipid rafts, iii) internalised tetherin is present in non-raft fractions, iv) expression of Vpu in cells expressing endogenous tetherin leads to the loss of tetherin from lipid rafts, v) internalised tetherin enters early endosomes, and late endosomes, in both control cells and cells expressing Vpu, but the proportion of tetherin molecules destined for degradation rather than recycling is increased in cells expressing Vpu vi) lysosomes are the primary site for degradation of endogenous tetherin in cells expressing Vpu. Our studies underlie the importance of studying endogenous tetherin and let us propose a model in which Vpu intercepts newly internalised tetherin and diverts it for lysosomal destruction rather than recycling to the cell surface.

A CD317/tetherin-RICH2 complex plays a critical role in the organization of the subapical actin cytoskeleton in polarized epithelial cells.

CD317/tetherin is a lipid raft-associated integral membrane protein with a novel topology. It has a short N-terminal cytosolic domain, a conventional transmembrane domain, and a C-terminal glycosyl-phosphatidylinositol anchor. We now show that CD317 is expressed at the apical surface of polarized epithelial cells, where it interacts indirectly with the underlying actin cytoskeleton. CD317 is linked to the apical actin network via the proteins RICH2, EBP50, and ezrin. Knocking down expression of either CD317 or RICH2 gives rise to the same phenotype: a loss of the apical actin network with concomitant loss of apical microvilli, an increase in actin bundles at the basal surface, and a reduction in cell height without any loss of tight junctions, transepithelial resistance, or the polarized targeting of apical and basolateral membrane proteins. Thus, CD317 provides a physical link between lipid rafts and the apical actin network in polarized epithelial cells and is crucial for the maintenance of microvilli in such cells.

Clathrin-mediated endocytosis of a lipid-raft-associated protein is mediated through a dual tyrosine motif.

We have previously shown that the integral membrane protein CD317 has both a conventional transmembrane domain near its N-terminus and a C-terminal glycosyl-phosphatidylinositol (GPI) anchor. With the possible exception of a minor topological variant of the prion protein, there remain no other convincing examples of a mammalian protein with such a topology. CD317 is localised to cholesterol-rich lipid microdomains ('lipid rafts') in the plasma membrane and is internalised from the cell surface for delivery to a juxta-nuclear compartment (most probably the TGN). We have now investigated the mechanism by which CD317 is internalised and find that this raft-associated integral membrane protein is internalised through a clathrin-dependent pathway, internalisation is dependent upon a novel dual-tyrosine-based motif in the cytosolic domain of CD317, the cytosolic domain of CD317 can interact with the mu subunits of the AP2 and AP1 adaptor complexes, interaction with AP1 is required for delivery of CD317 back to the TGN, and removal of the GPI anchor from CD317 reduces the efficiency of CD317 internalisation. Collectively, these data indicate that CD317 is internalised and delivered back to the TGN by the sequential action of AP2 and AP1 adaptor complexes and that, surprisingly, the clathrin-mediated internalisation of CD317 occurs more efficiently if CD317 is localised to lipid rafts.

Bst-2/HM1.24 is a raft-associated apical membrane protein with an unusual topology.

An expression screen of a rat cDNA library for sequences encoding Golgi-localized integral membrane proteins identified a protein with an apparent novel topology, i.e. with both an N-terminal transmembrane domain and a C-terminal glycosyl-phosphatidylinositol (GPI) anchor. Our data are consistent with this. Thus, the protein would have a topology that, in mammalian cells, is shared only by a minor, but pathologically important, topological isoform of the prion protein (PrP). The human orthologue of this protein has been described previously (BST-2 or HM1.24 antigen) as a cell surface molecule that appears to be involved in early pre-B-cell development and which is present at elevated levels at the surface of myeloma cells. We show that rat BST-2/HM1.24 has both a cell surface and an intracellular (juxtanuclear) location and is efficiently internalized from the cell surface. We also show that the cell surface pool of BST-2/HM1.24 is predominantly present in the apical plasma membrane of polarized cells. The fact that rat BST-2/HM1.24 apparently possesses a GPI anchor led us to speculate that it might exist in cholesterol-rich lipid microdomains (lipid rafts) at the plasma membrane. Data from several experiments are consistent with this localization. We present a model in which BST-2/HM1.24 serves to link adjacent lipid rafts within the plasma membrane.