IQGAP1 interacts with components of the slit diaphragm complex in podocytes and is involved in podocyte migration and permeability in vitro

Towards an in vitro model of the glomerular barrier unit with an innovative bioassembly method

BACKGROUND

The development of an artificial glomerular unit may be pivotal for renal pathophysiology studies at a multicellular scale. Using a tissue engineering approach, we aimed to reproduce in part the specific glomerular barrier architecture by manufacturing a glomerular microfibre (Mf).

METHODS

Immortalized human glomerular cell lines of endothelial cells (GEnCs) and podocytes were used. Cells and a three-dimensional (3D) matrix were characterized by immunofluorescence with confocal analysis, Western blot and polymerase chain reaction. Optical and electron microscopy were used to study Mf and cell shapes. We also analysed cell viability and cell metabolism within the 3D construct at 14 days.

RESULTS

Using the Mf manufacturing method, we repeatedly obtained a cellularized Mf sorting human glomerular cells in 3D. Around a central structure made of collagen I, we obtained an internal layer composed of GEnC, a newly formed glomerular basement membrane rich in α5 collagen IV and an external layer of podocytes. The cell concentration, optimal seeding time and role of physical stresses were modulated to obtain the Mf. Cell viability and expression of specific proteins (nephrin, synaptopodin, vascular endothelial growth factor receptor 2 (VEGFR2) and von Willebrandt factor (vWF)) were maintained for 19 days in the Mf system. Mf ultrastructure, observed with EM, had similarities with the human glomerular barrier.

CONCLUSION

In summary, with our 3D bio-engineered glomerular fibre, GEnC and podocytes produced a glomerular basement membrane. In the future, this glomerular Mf will allow us to study cell interactions in a 3D system and increase our knowledge of glomerular pathophysiology.

Knockdown of IQGAP-1 Enhances Tight Junctions and Prevents P. aeruginosa Invasion of Human Corneal Epithelial Cells

PURPOSE

To determine the role of IQ-domain GTPase-activating protein1 (IQGAP-1) in tight junctions of human corneal epithelial cells (HCECs) and its effect against P. aeruginosa (PAK) invasion.

MATERIAL AND METHODS

Primary human corneal epithelial cells (HCECs), immortalized HCECs, and IQGAP-1 RNA knockdown HCECs (siHCECs) were used. Confocal microscopy, transepithelial electrical resistance (TER), trypan blue exclusion assay and gentamicin invasion assay were done.

RESULTS

In primary and immortalized HCECs, IQGAP-1 co-localized with zonular occludin-1 (ZO-1) and actin. Enhanced actin and ZO-1 aggregation were seen in siHCECs. IQGAP-1 knockdown significantly increased TER of immortalized HCECs (P < .0001). Cell viability after PAK infection increased for siHCECs for up to 4 h after infection. PAK intracellular invasion was significantly lowered by 50% in siHCECs at 1 h post-infection.

CONCLUSION

IQGAP-1 knockdown increased the strength and integrity of tight junctions and may provide an early protective effect against P. aeruginosa invasion.

MAGI1 Mediates eNOS Activation and NO Production in Endothelial Cells in Response to Fluid Shear Stress

Fluid shear stress stimulates endothelial nitric oxide synthase (eNOS) activation and nitric oxide (NO) production through multiple kinases, including protein kinase A (PKA), AMP-activated protein kinase (AMPK), AKT and Ca²⁺/calmodulin-dependent protein kinase II (CaMKII). Membrane-associated guanylate kinase (MAGUK) with inverted domain structure-1 (MAGI1) is an adaptor protein that stabilizes epithelial and endothelial cell-cell contacts. The aim of this study was to assess the unknown role of endothelial cell MAGI1 in response to fluid shear stress. We show constitutive expression and co-localization of MAGI1 with vascular endothelial cadherin (VE-cadherin) in endothelial cells at cellular junctions under static and laminar flow conditions. Fluid shear stress increases MAGI1 expression. MAGI1 silencing perturbed flow-dependent responses, specifically, Krüppel-like factor 4 (KLF4) expression, endothelial cell alignment, eNOS phosphorylation and NO production. MAGI1 overexpression had opposite effects and induced phosphorylation of PKA, AMPK, and CAMKII. Pharmacological inhibition of PKA and AMPK prevented MAGI1-mediated eNOS phosphorylation. Consistently, MAGI1 silencing and PKA inhibition suppressed the flow-induced NO production. Endothelial cell-specific transgenic expression of MAGI1 induced PKA and eNOS phosphorylation in vivo and increased NO production ex vivo in isolated endothelial cells. In conclusion, we have identified endothelial cell MAGI1 as a previously unrecognized mediator of fluid shear stress-induced and PKA/AMPK dependent eNOS activation and NO production.

Site-specific expression of IQGAP1 in human nephrons

IQGAP1 is a multifunctional, 190-kDa scaffolding protein that plays an important role in the regulation of cell adhesion, migration, proliferation, differentiation, polarization and cytoskeletal remodeling. IQGAP1 is ubiquitously expressed in human organs and is highly expressed in the kidney. Currently, the site-specific expression of IQGAP1 in the human nephrons is unclear. We performed Western blotting analysis, immunohistochemistry and double-immunolabeling confocal microscopic analysis of IQGAP1 with specific biomarkers of each nephron segment to study the expression and distribution of IQGAP1 in human nephrons. We found that IQGAP1 was strongly expressed in human podocytes and glomerular endothelial cells, but weakly expressed in glomerular mesangial cells. In human renal tubules, IQGAP1 was strongly expressed in the collecting duct, moderately expressed in the proximal tubule, medullary loop, distal convoluted tubule and connecting tubule. IQGAP1 staining was much stronger in the apical membrane in the proximal tubule, thick descending limb and thick ascending limb of medullary loop and collecting duct. However, the expression of IQGAP1 was mainly in the basolateral membrane of the connecting tubule, and diffusely in the thin limb of medullary loop and distal convoluted tubule. The interaction between IQGAP1 and F-actin suggested that cytoskeleton regulation may be the underlying mechanism mediating the effect of IQGAP1 in human nephrons. To the best of our knowledge, this is the first report of specific expression and differential subcellular location of IQGAP1 in human nephrons. The site-specific expression pattern of IQGAP1 suggests that IQGAP1 may play diverse roles in various human nephron segments.

Long-Term Cell Fate Tracking of Individual Renal Cells Using Serial Intravital Microscopy

Intravital multiphoton microscopy of the kidney is a powerful technique to study alterations in tissue morphology and function simultaneously in the living animal and represents a dynamic and developing research tool in the field. Recent technological advances include serial intravital multiphoton microscopy of the same kidney regions over several weeks and combined with ex vivo histology for cellular biomarker expression of the same cells, which had been subject to serial imaging before. Thus, serial intravital multiphoton microscopy followed by ex vivo histology provides unique tools to perform long-term cell fate tracing of the same renal cells during physiological and pathophysiological conditions, thereby allowing the detection of structural changes of the same renal cells over time. Examples include renal cell migration and proliferation while linking these events to local functional alterations and eventually to the expression of distinct cellular biomarkers. Here, we provide a detailed step-by-step protocol to facilitate serial intravital multiphoton microscopy for long-term in vivo tracking of renal cells and subsequent ex vivo histology for immunohistological staining of the same cells in the fixed tissue.

[Up to date of pathophysiology mechanism of idiopathic nephrotic syndromes: Minimal change disease and focal and segmental glomerulosclerosis]

Idiopathic nephrotic syndrome represents up to 30% of adult glomerulopathies. However, its prognosis according to remission, relapse and renal failure remains unchanged since the 80s and prediction remains difficult. Physiopathology of adult idiopathic nephrotic syndrome is complex and multifactorial, including immunologic and environmental factors and a putative permeability-circulating factor, still unknown. In this point of view, we propose to summarize actual knowledge about idiopathic minimal change disease and focal and segmental glomerulosclerosis physiopathology.

ShcA Adaptor Protein Promotes Nephrin Endocytosis and Is Upregulated in Proteinuric Nephropathies

Nephrin is a key structural component of the podocyte slit diaphragm, and proper expression of nephrin on the cell surface is critical to ensure integrity of the blood filtration barrier. Maintenance of nephrin within this unique cell junction has been proposed to require dynamic phosphorylation events and endocytic recycling, although the molecular mechanisms that control this interplay are poorly understood. Here, we investigated the possibility that the phosphotyrosine adaptor protein ShcA regulates nephrin turnover. Western blotting and immunostaining analysis confirmed that ShcA is expressed in podocytes. In immunoprecipitation and pulldown assays, ShcA, via its SH2 domain, was associated with several phosphorylated tyrosine residues on nephrin. Overexpression of ShcA promoted nephrin tyrosine phosphorylation and reduced nephrin signaling and cell surface expression in vitro In a rat model of reversible podocyte injury and proteinuria, phosphorylated nephrin temporally colocalized with endocytic structures coincident with upregulation of ShcA expression. In vivo biotinylation assays confirmed that nephrin expression decreased at the cell surface and correspondingly increased in the cytosol during the injury time course. Finally, immunostaining in kidney biopsy specimens demonstrated overexpression of ShcA in several human proteinuric kidney diseases compared with normal conditions. Our results suggest that increases in ShcA perturb nephrin phosphosignaling dynamics, leading to aberrant nephrin turnover and slit diaphragm disassembly.

The WW domain of the scaffolding protein IQGAP1 is neither necessary nor sufficient for binding to the MAPKs ERK1 and ERK2

Mitogen-activated protein kinase (MAPK) scaffold proteins, such as IQ motif containing GTPase activating protein 1 (IQGAP1), are promising targets for novel therapies against cancer and other diseases. Such approaches require accurate information about which domains on the scaffold protein bind to the kinases in the MAPK cascade. Results from previous studies have suggested that the WW domain of IQGAP1 binds to the cancer-associated MAPKs ERK1 and ERK2, and that this domain might thus offer a new tool to selectively inhibit MAPK activation in cancer cells. The goal of this work was therefore to critically evaluate which IQGAP1 domains bind to ERK1/2. Here, using quantitative in vitro binding assays, we show that the IQ domain of IQGAP1 is both necessary and sufficient for binding to ERK1 and ERK2, as well as to the MAPK kinases MEK1 and MEK2. Furthermore, we show that the WW domain is not required for ERK-IQGAP1 binding, and contributes little or no binding energy to this interaction, challenging previous models of how WW-based peptides might inhibit tumorigenesis. Finally, we show that the ERK2-IQGAP1 interaction does not require ERK2 phosphorylation or catalytic activity and does not involve known docking recruitment sites on ERK2, and we obtain an estimate of the dissociation constant (K_d ) for this interaction of 8 μm These results prompt a re-evaluation of published findings and a refined model of IQGAP scaffolding.

2017 update on the relationship between diabetes and colorectal cancer: epidemiology, potential molecular mechanisms and therapeutic implications

Worldwide deaths from diabetes mellitus (DM) and colorectal cancer increased by 90% and 57%, respectively, over the past 20 years. The risk of colorectal cancer was estimated to be 27% higher in patients with type 2 DM than in non-diabetic controls. However, there are potential confounders, information from lower income countries is scarce, across the globe there is no correlation between DM prevalence and colorectal cancer incidence and the association has evolved over time, suggesting the impact of additional environmental factors. The clinical relevance of these associations depends on understanding the mechanism involved. Although evidence is limited, insulin use has been associated with increased and metformin with decreased incidence of colorectal cancer. In addition, colorectal cancer shares some cellular and molecular pathways with diabetes target organ damage, exemplified by diabetic kidney disease. These include epithelial cell injury, activation of inflammation and Wnt/β-catenin pathways and iron homeostasis defects, among others. Indeed, some drugs have undergone clinical trials for both cancer and diabetic kidney disease. Genome-wide association studies have identified diabetes-associated genes (e.g. TCF7L2) that may also contribute to colorectal cancer. We review the epidemiological evidence, potential pathophysiological mechanisms and therapeutic implications of the association between DM and colorectal cancer. Further studies should clarify the worldwide association between DM and colorectal cancer, strengthen the biological plausibility of a cause-and-effect relationship through characterization of the molecular pathways involved, search for specific molecular signatures of colorectal cancer under diabetic conditions, and eventually explore DM-specific strategies to prevent or treat colorectal cancer.

BRAF Signaling Pathway Inhibition, Podocyte Injury, and Nephrotic Syndrome

Dabrafenib and trametinib, BRAF and MEK inhibitors, respectively, are effective targeted metastatic melanoma therapies, but little is known about their nephrotoxicity. Although tubulointerstitial injury has been the most widely reported renal side effect of targeted melanoma therapy, nephrotic syndrome has not been reported before. We report on a patient with metastatic melanoma who developed nephrotic syndrome during dabrafenib and trametinib treatment. Kidney biopsy showed diffuse loss of podocyte cytoarchitecture, extensive foot-process effacement, and glomerular endothelial injury. Kidney function and glomerular ultrastructural changes recovered fully after drug withdrawal. In vitro, BRAF inhibition decreased PLCε1 expression in podocytes, accompanied by a reduction in nephrin expression and an increase in permeability to albumin. Additionally, these drugs inhibited the podocyte-vascular endothelial growth factor (VEGF) system. In addition to implications for nephrotic syndrome pathophysiology, we suggest that patients given dabrafenib and trametinib be monitored closely for potential glomerular damage.

High glucose repatterns human podocyte energy metabolism during differentiation and diabetic nephropathy

Podocytes play a key role in diabetic nephropathy pathogenesis, but alteration of their metabolism remains unknown in human kidney. By using a conditionally differentiating human podocyte cell line, we addressed the functional and molecular changes in podocyte energetics during in vitro development or under high glucose conditions. In 5 mM glucose medium, we observed a stepwise activation of oxidative metabolism during cell differentiation that was characterized by peroxisome proliferator-activated receptor-γ coactivator 1α (PGC-1α)–dependent stimulation of mitochondrial biogenesis and function, with concomitant reduction of the glycolytic enzyme content. Conversely, when podocytes were cultured in high glucose (20 mM), stepwise oxidative phosphorylation biogenesis was aborted, and a glycolytic switch occurred, with consecutive lactic acidosis. Expression of the master regulators of oxidative metabolism transcription factor A mitochondrial, PGC-1α, AMPK, and serine–threonine liver kinase B1 was altered by high glucose, as well as their downstream signaling networks. Focused transcriptomics revealed that myocyte-specific enhancer factor 2C (MEF2C) and myogenic factor 5 (MYF5) expression was inhibited by high glucose levels, and endoribonuclease-prepared small interfering RNA–mediated combined inhibition of those transcription factors phenocopied the glycolytic shift that was observed in high glucose conditions. Accordingly, a reduced expression of MEF2C, MYF5, and PGC-1α was found in kidney tissue sections that were obtained from patients with diabetic nephropathy. These findings obtained in human samples demonstrate that MEF2C-MYF5–dependent bioenergetic dedifferentiation occurs in podocytes that are confronted with a high-glucose milieu.—Imasawa, T., Obre, E., Bellance, N., Lavie, J., Imasawa, T., Rigothier, C., Delmas, Y., Combe, C., Lacombe, D., Benard, G., Claverol, S., Bonneu, M., Rossignol, R. High glucose repatterns human podocyte energy metabolism during differentiation and diabetic nephropathy.

Deep insights: intravital imaging with two-photon microscopy

Intravital multiphoton microscopy is widely used to assess the structure and function of organs in live animals. Although different tissues vary in their accessibility for intravital multiphoton imaging, considerable progress has been made in the imaging quality of all tissues due to substantial technical improvements in the relevant imaging components, such as optics, excitation laser, detectors, and signal analysis software. In this review, we provide an overview of the technical background of intravital multiphoton microscopy. Then, we note a few seminal findings that were made through the use of multiphoton microscopy. Finally, we address the technical limitations of the method and provide an outlook for how these limitations may be overcome through future technical developments.

Podocytes

Podocytes are highly specialized cells of the kidney glomerulus that wrap around capillaries and that neighbor cells of the Bowman’s capsule. When it comes to glomerular filtration, podocytes play an active role in preventing plasma proteins from entering the urinary ultrafiltrate by providing a barrier comprising filtration slits between foot processes, which in aggregate represent a dynamic network of cellular extensions. Foot processes interdigitate with foot processes from adjacent podocytes and form a network of narrow and rather uniform gaps. The fenestrated endothelial cells retain blood cells but permit passage of small solutes and an overlying basement membrane less permeable to macromolecules, in particular to albumin. The cytoskeletal dynamics and structural plasticity of podocytes as well as the signaling between each of these distinct layers are essential for an efficient glomerular filtration and thus for proper renal function. The genetic or acquired impairment of podocytes may lead to foot process effacement (podocyte fusion or retraction), a morphological hallmark of proteinuric renal diseases. Here, we briefly discuss aspects of a contemporary view of podocytes in glomerular filtration, the patterns of structural changes in podocytes associated with common glomerular diseases, and the current state of basic and clinical research.

The Rho-GTPase binding protein IQGAP2 is required for the glomerular filtration barrier

Podocyte dysfunction impairs the size selectivity of the glomerular filter, leading to proteinuria, hypoalbuminuria, and edema, clinically defined as nephrotic syndrome. Hereditary forms of nephrotic syndrome are linked to mutations in podocyte-specific genes. To identify genes contributing to podocyte dysfunction in acquired nephrotic syndrome, we studied human glomerular gene expression data sets for glomerular-enriched gene transcripts differentially regulated between pretransplant biopsy samples and biopsies from patients with nephrotic syndrome. Candidate genes were screened by in situ hybridization for expression in the zebrafish pronephros, an easy-to-use in vivo assay system to assess podocyte function. One glomerulus-enriched product was the Rho-GTPase binding protein, IQGAP2. Immunohistochemistry found a strong presence of IQGAP2 in normal human and zebrafish podocytes. In zebrafish larvae, morpholino-based knockdown of iqgap2 caused a mild foot process effacement of zebrafish podocytes and a cystic dilation of the urinary space of Bowman's capsule upon onset of urinary filtration. Moreover, the glomerulus of zebrafish morphants showed a glomerular permeability for injected high-molecular-weight dextrans, indicating an impaired size selectivity of the glomerular filter. Thus, IQGAP2 is a Rho-GTPase binding protein, highly abundant in human and zebrafish podocytes, which controls normal podocyte structure and function as evidenced in the zebrafish pronephros.

The biology of IQGAP proteins: beyond the cytoskeleton

IQGAP scaffold proteins are evolutionarily conserved in eukaryotes and facilitate the formation of complexes that regulate cytoskeletal dynamics, intracellular signaling, and intercellular interactions. Fungal and mammalian IQGAPs are implicated in cytokinesis. IQGAP1, IQGAP2, and IQGAP3 have diverse roles in vertebrate physiology, operating in the kidney, nervous system, cardio-vascular system, pancreas, and lung. The functions of IQGAPs can be corrupted during oncogenesis and are usurped by microbial pathogens. Therefore, IQGAPs represent intriguing candidates for novel therapeutic agents. While modulation of the cytoskeletal architecture was initially thought to be the primary function of IQGAPs, it is now clear that they have roles beyond the cytoskeleton. This review describes contributions of IQGAPs to physiology at the organism level.

Genetic causes of proteinuria and nephrotic syndrome: impact on podocyte pathobiology

In the past 20 years, multiple genetic mutations have been identified in patients with congenital nephrotic syndrome (CNS) and both familial and sporadic focal segmental glomerulosclerosis (FSGS). Characterization of the genetic basis of CNS and FSGS has led to the recognition of the importance of podocyte injury to the development of glomerulosclerosis. Genetic mutations induce injury due to effects on the podocyte's structure, actin cytoskeleton, calcium signaling, and lysosomal and mitochondrial function. Transgenic animal studies have contributed to our understanding of podocyte pathobiology. Podocyte endoplasmic reticulum stress response, cell polarity, and autophagy play a role in maintenance of podocyte health. Further investigations related to the effects of genetic mutations on podocytes may identify new pathways for targeting therapeutics for nephrotic syndrome.

IQGAP1 regulates actin cytoskeleton organization in podocytes through interaction with nephrin

Increasing data has shown that the cytoskeletal reorganization of podocytes is involved in the onset of proteinuria and the progression of glomerular disease. Nephrin behaves as a signal sensor of the slit diaphragm to transmit cytoskeletal signals to maintain the unique structure of podocytes. However, the nephrin signaling cascade deserves further study. IQGAP1 is a scaffolding protein with the ability to regulate cytoskeletal organization. It is hypothesized that IQGAP1 contributes to actin reorganization in podocytes through interaction with nephrin. IQGAP1 expression and IQGAP1-nephrin colocalization in glomeruli were progressively decreased and then gradually recovered in line with the development of foot process fusion and proteinuria in puromycin aminonucleoside-injected rats. In cultured human podocytes, puromycin aminonucleoside-induced disruption of F-actin and disorders of migration and spreading were aggravated by IQGAP1 siRNA, and these effects were partially restored by a wild-type IQGAP1 plasmid. Furthermore, the cytoskeletal disorganization stimulated by cytochalasin D in COS7 cells was recovered by cotransfection with wild-type IQGAP1 and nephrin plasmids but was not recovered either by single transfection of the wild-type IQGAP1 plasmid or by cotransfection of mutant IQGAP1 [△1443(S→A)] and wild-type nephrin plasmids. Co-immunoprecipitation analysis using lysates of COS7 cells overexpressing nephrin and each derivative-domain molecule of IQGAP1 demonstrated that the poly-proline binding domain and RasGAP domain in the carboxyl terminus of IQGAP1 are the target modules that interact with nephrin. Collectively, these findings showed that activated IQGAP1, as an intracellular partner of nephrin, is involved in actin cytoskeleton organization and functional regulation of podocytes.

IQGAPs choreograph cellular signaling from the membrane to the nucleus

Since its discovery in 1994, recognized cellular functions for the scaffold protein IQGAP1 have expanded immensely. Over 100 unique IQGAP1-interacting proteins have been identified, implicating IQGAP1 as a critical integrator of cellular signaling pathways. Initial research established functions for IQGAP1 in cell-cell adhesion, cell migration, and cell signaling. Recent studies have revealed additional IQGAP1 binding partners, expanding the biological roles of IQGAP1. These include crosstalk between signaling cascades, regulation of nuclear function, and Wnt pathway potentiation. Investigation of the IQGAP2 and IQGAP3 homologs demonstrates unique functions, some of which differ from those of IQGAP1. Summarized here are recent observations that enhance our understanding of IQGAP proteins in the integration of diverse signaling pathways.

Sirtuin1 Maintains Actin Cytoskeleton by Deacetylation of Cortactin in Injured Podocytes

Recent studies have highlighted the renoprotective effect of sirtuin1 (SIRT1), a deacetylase that contributes to cellular regulation. However, the pathophysiologic role of SIRT1 in podocytes remains unclear. Here, we investigated the function of SIRT1 in podocytes. We first established podocyte-specific Sirt1 knockout (SIRT1(pod-/-)) mice. We then induced glomerular disease by nephrotoxic serum injection. The increase in urinary albumin excretion and BUN and the severity of glomerular injury were all significantly greater in SIRT1(pod-/-) mice than in wild-type mice. Western blot analysis and immunofluorescence showed a significant decrease in podocyte-specific proteins in SIRT1(pod-/-) mice, and electron microscopy showed marked exacerbation of podocyte injury, including actin cytoskeleton derangement in SIRT1(pod-/-) mice compared with wild-type mice. Protamine sulfate-induced podocyte injury was also exacerbated by podocyte-specific SIRT1 deficiency. In vitro, actin cytoskeleton derangement in H2O2-treated podocytes became prominent when the cells were pretreated with SIRT1 inhibitors. Conversely, this H2O2-induced derangement was ameliorated by SIRT1 activation. Furthermore, SIRT1 activation deacetylated the actin-binding and -polymerizing protein cortactin in the nucleus and facilitated deacetylated cortactin localization in the cytoplasm. Cortactin knockdown or inhibition of the nuclear export of cortactin induced actin cytoskeleton derangement and dissociation of cortactin from F-actin, suggesting the necessity of cytoplasmic cortactin for maintenance of the actin cytoskeleton. Taken together, these findings indicate that SIRT1 protects podocytes and prevents glomerular injury by deacetylating cortactin and thereby, maintaining actin cytoskeleton integrity.

The Ras GTPase-activating-like protein IQGAP1 is downregulated in human diabetic nephropathy and associated with ERK1/2 pathway activation

Podocyte injury may contribute to the pathogenesis of diabetic nephropathy (DN), but the underlying mechanism of hyperglycemia induced podocyte damage is not fully understood. The Ras GTPase-activating-like protein IQGAP1 is associated to the slit diaphragm proteins and the actin cytoskeleton in podocyte. Here, we studied IQGAP1 expression alterations in human DN biopsies and extracellular signal-regulated kinase (ERK)-dependent pathways of IQGAP1 expression in podocyte under high glucose (HG) media. In vivo, analysis of renal biopsies from patients with DN revealed a significant reduction in IQGAP1 expression compared to controls. In vitro, IQGAP1 mRNA and protein expression were observed to decline under HG media at 48 h. But phosphorylation of ERK1/2 was activated under HG media at 24 h and 48 h. However, HG-induced downregulation of IQGAP1 protein was attenuated by specific ERK1/2 activation inhibitor PD98059. Taken together, these results highlight the importance of IQGAP1 in DN, and suggest that IQGAP1 expression in podocyte under HG media is modulated by the ERK1/2 pathway, which may lead to the future development of therapies targeting IQGAP1 dysfunction in podocytes in DN.

IQ domain GTPase-activating protein 1 is involved in shear stress-induced progenitor-derived endothelial cell alignment

Shear stress is one of mechanical constraints which are exerted by blood flow on endothelial cells (ECs). To adapt to shear stress, ECs align in the direction of flow through adherens junction (AJ) remodeling. However, mechanisms regulating ECs alignment under shear stress are poorly understood. The scaffold protein IQ domain GTPase activating protein 1 (IQGAP1) is a scaffold protein which couples cell signaling to the actin and microtubule cytoskeletons and is involved in cell migration and adhesion. IQGAP1 also plays a role in AJ organization in epithelial cells. In this study, we investigated the potential IQGAP1 involvement in the endothelial cells alignment under shear stress. Progenitor-derived endothelial cells (PDECs), transfected (or not) with IQGAP1 small interfering RNA, were exposed to a laminar shear stress (1.2 N/m(2)) and AJ proteins (VE-cadherin and β-catenin) and IQGAP1 were labeled by immunofluorescence. We show that IQGAP1 is essential for ECs alignment under shear stress. We studied the role of IQGAP1 in AJs remodeling of PDECs exposed to shear stress by studying cell localization and IQGAP1 interactions with VE-cadherin and β-catenin by immunofluorescence and Proximity Ligation Assays. In static conditions, IQGAP1 interacts with VE-cadherin but not with β-catenin at the cell membrane. Under shear stress, IQGAP1 lost its interaction from VE-cadherin to β-catenin. This "switch" was concomitant with the loss of β-catenin/VE-cadherin interaction at the cell membrane. This work shows that IQGAP1 is essential to ECs alignment under shear stress and that AJ remodeling represents one of the mechanisms involved. These results provide a new approach to understand ECs alignment under to shear stress.

Podocyte energy metabolism and glomerular diseases

Mitochondria are crucial organelles that produce and deliver adenosine triphosphate (ATP), by which all cellular processes are driven. Although the mechanisms that control mitochondrial biogenesis, function and dynamics are complex process and vary among different cell types, recent studies provided many new discoveries in this field. Podocyte injury is a crucial step in the development of a large number of glomerular diseases. Glomerular podocytes are unique cells with complex foot processes that cover the outer layer of the glomerular basement membrane, and are the principle cells composing filtration barriers of glomerular capillaries. Little is known on the modalities and the regulation of podocyte's energetics as well as the type of energy substrate primarily used for their activity, recent studies revealed that dysfunction of energy transduction in podocytes may underlie the podocyte injury associated with numerous glomerular diseases. We herein review and discuss the importance of a fine regulation of energy metabolism in podocytes for maintaining their cellular structure and related kidney function. In the future, understanding these mechanisms will open up new areas of treatment for glomerular diseases.

IQGAP1 mediates angiotensin II-induced apoptosis of podocytes via the ERK1/2 MAPK signaling pathway

BACKGROUND/AIMS

The mechanism underlying angiotensin II (AngII)-promoted podocyte apoptosis has not been established. IQ domain GTPase-activating protein 1 (IQGAP1) is a scaffolding protein of the mitogen-activated protein kinases (MAPK) signaling pathway, and plays a significant role in apoptosis. The present study evaluates the role of IQGAP1 in AngII-induced podocyte apoptosis.

METHODS

We randomly assigned 36 male Wistar rats to a normal saline-infused group, an AngII-infused group, or a normal control group, and measured podocyte apoptosis by the terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labeling (TUNEL) assay and transmission electron microscopic analysis. In addition, we exposed differentiated mouse podocytes to AngII and then assessed apoptosis by flow cytometry and Hoechst-33258 staining. Expression of IQGAP1 was measured by Western blotting, real-time PCR and immunofluorescence assay in vivo and in vitro. IQGAP1 siRNA and MAPK pathway inhibitors were further introduced to investigate the role of IQGAP1 and MAPK signaling in the process. Coimmunoprecipitation was used to evaluate the interaction between ERK1/2 and IQGAP1.

RESULTS

AngII promoted podocyte apoptosis in vivo and in vitro. IQGAP1 had a linear distribution along the capillary loops of glomeruli in vivo, and was in the cellular membrane and cytoplasm of cultured podocytes. AngII stimulated IQGAP1 expression and increased phosphorylation of P38, JNK, and ERK1/2. Knockdown of IQGAP1 with siRNA prevented AngII-induced apoptosis of podocytes and reduced AngII-induced phosphorylation of ERK1/2, but not that of P38, JNK. This was accompanied by a reduced interaction between ERK1/2 and IQGAP1.

CONCLUSION

IQGAP1 contributes to AngII-induced apoptosis of podocytes by interacting with the ERK1/2 signaling protein.