Insulin signaling to the glomerular podocyte is critical for normal kidney function

Albumin inhibits the insulin-mediated ACE2 increase in cultured podocytes

Podocytes are key cells in the glomerular filtration barrier with a major role in the development of diabetic nephropathy. Podocytes are insulin-sensitive cells and have a functionally active local renin-angiotensin system. The presence and activity of angiotensin-converting enzyme 2 (ACE2), the main role of which is cleaving profibrotic and proinflammatory angiotensin-II into angiotensin-(1–7), have been demonstrated in podocytes. Conditionally immortalized mouse podocytes were cultured with insulin in the presence and absence of albumin. We found that insulin increases ACE2 gene and protein expression, by real-time PCR and Western blotting, respectively, and enzymatic activity within the podocyte and these increases were maintained over time. Furthermore, insulin favored an “anti-angiotensin II” regarding ACE/ACE2 gene expression balance and decreased fibronectin gene expression as a marker of fibrosis in the podocytes, all studied by real-time PCR. Similarly, insulin incubation seemed to protect podocytes from cell death, studied by a terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling assay. However, all these effects disappeared in the presence of albumin, which may mimic albuminuria, a main feature of DN pathophysiology. Our results suggest that modulation of renin-angiotensin system balance, fibrosis, and apoptosis by insulin in the podocyte may be an important factor in preventing the development and progression of diabetic kidney disease, but the presence of albuminuria seems to block these beneficial effects.

Roles of renal proximal tubule transport in acid/base balance and blood pressure regulation

Sodium-coupled bicarbonate absorption from renal proximal tubules (PTs) plays a pivotal role in the maintenance of systemic acid/base balance. Indeed, mutations in the Na⁺-HCO₃⁻ cotransporter NBCe1, which mediates a majority of bicarbonate exit from PTs, cause severe proximal renal tubular acidosis associated with ocular and other extrarenal abnormalities. Sodium transport in PTs also plays an important role in the regulation of blood pressure. For example, PT transport stimulation by insulin may be involved in the pathogenesis of hypertension associated with insulin resistance. Type 1 angiotensin (Ang) II receptors in PT are critical for blood pressure homeostasis. Paradoxically, the effects of Ang II on PT transport are known to be biphasic. Unlike in other species, however, Ang II is recently shown to dose-dependently stimulate human PT transport via nitric oxide/cGMP/ERK pathway, which may represent a novel therapeutic target in human hypertension. In this paper, we will review the physiological and pathophysiological roles of PT transport.

Fatty kidney: emerging role of ectopic lipid in obesity-related renal disease

The global increase in chronic kidney disease (CKD) parallels the obesity epidemic. Obesity conveys a gradual but independent risk of progression of CKD that seems irrespective of the underlying nephropathy. Obesity has been associated with a secondary focal segmental glomerulosclerosis coined obesity-related glomerulopathy (ORG). Pathways through which obesity might cause renal disease are not well understood, and early clinical biomarkers for incipient ORG or renal relevant obesity are currently lacking. Recent human and experimental studies have associated ectopic lipid accumulation in the kidney (fatty kidney) with obesity-related renal disease. There is enough growing insight that ectopic lipid--the accumulation of lipid in non-adipose tissue--is associated with structural and functional changes of mesangial cells, podocytes, and proximal tubular cells to propose the development of ORG as a maladaptive response to hyperfiltration and albuminuria. Recent advances in metabolic imaging might validate ectopic lipid as a biomarker and research aid, to help translate novel therapeutics from experimental models to patients.

Podocytes, signaling pathways, and vascular factors in diabetic kidney disease

Alterations and injury to glomerular podocytes play a key role in the initiation and progression of diabetic kidney disease (DKD). Multiple factors in diabetes cause abnormalities in podocyte signaling that lead to podocyte foot process effacement, hypertrophy, detachment, loss, and death. Alterations in insulin action and mammalian target of rapamycin activation have been well documented to lead to pathology. Reduced insulin action directly leads to albuminuria, increased glomerular matrix accumulation, thickening of the glomerular basement membrane, podocyte apoptosis, and glomerulosclerosis. In addition, podocytes generate factors that alter signaling in other glomerular cells. Prominent among these is vascular endothelial growth factor-A, which maintains glomerular endothelium viability but causes endothelial cell pathology when generated at too high a level. Finally, circulating vascular factors (eg, activated protein C) have a profound effect on podocyte stability and survival. This cytoprotective factor is critical for podocyte health, and its deficiency promotes podocyte injury and apoptosis. Thus, the podocyte sits in the center of a network of paracrine and hormonal signaling systems that in health keep the podocyte adaptable and viable, but in diabetes they can lead to pathologic changes, detachment, and death.

Vascular endothelial growth factor-receptor 1 inhibition aggravates diabetic nephropathy through eNOS signaling pathway in db/db mice

The manipulation of vascular endothelial growth factor (VEGF)-receptors (VEGFRs) in diabetic nephropathy is as controversial as issue as ever. It is known to be VEGF-A and VEGFR2 that regulate most of the cellular actions of VEGF in experimental diabetic nephropathy. On the other hand, such factors as VEGF-A, -B and placenta growth factor bind to VEGFR1 with high affinity. Such notion instigated us to investigate on whether selective VEGFR1 inhibition with GNQWFI hexamer aggravates the progression of diabetic nephropathy in db/db mice. While diabetes suppressed VEGFR1, it did increase VEGFR2 expressions in the glomerulus. Db/db mice with VEGFR1 inhibition showed more prominent features with respect to, albuminuria, mesangial matrix expansion, inflammatory cell infiltration and greater numbers of apoptotic cells in the glomerulus, and oxidative stress than that of control db/db mice. All these changes were related to the suppression of diabetes-induced increases in PI3K activity and Akt phosphorylation as well as the aggravation of endothelial dysfunction associated with the inactivation of FoxO3a and eNOS-NOx. In cultured human glomerular endothelial cells (HGECs), high-glucose media with VEGFR1 inhibition induced more apoptotic cells and oxidative stress than did high-glucose media alone, which were associated with the suppression of PI3K-Akt phosphorylation, independently of the activation of AMP-activated protein kinase, and inactivation of FoxO3a and eNOS-NOx pathway. In addition, transfection with VEGFR1 siRNA in HGECs also suppressed PI3K-Akt-eNOS signaling. In conclusion, the specific blockade of VEGFR1 with GNQWFI caused severe renal injury related to profound suppression of the PI3K-Akt, FoxO3a and eNOS-NOx pathway, giving rise to the oxidative stress-induced apoptosis of glomerular cells in type 2 diabetic nephropathy.

Hydrogen sulfide alleviates cardiac contractile dysfunction in an Akt2-knockout murine model of insulin resistance: role of mitochondrial injury and apoptosis

Hydrogen sulfide (H2S) is a toxic gas now being recognized as an endogenous signaling molecule in multiple organ systems, in particular, the cardiovascular system. H2S is known to regulate cardiac function and protect against ischemic injury. However, little information is available regarding the effect of H2S on cardiac function in insulin resistance. This study was designed to examine the impact of H2S supplementation on cardiac function using an Akt2 knockout model of insulin resistance. Wild-type and Akt2 knockout mice were treated with NaHS (50 μM·kg(-1)·day(-1) ip for 10 days) prior to evaluation of echocardiographic, cardiomyocyte contractile, and intracellular Ca(2+) properties, apoptosis, and mitochondrial damage. Our results revealed that Akt2 ablation led to overtly enlarged ventricular end-systolic diameter, reduced myocardial and cardiomyocyte contractile function, and disrupted intracellular Ca(2+) homeostasis and apoptosis, the effects of which were ameliorated by H2S. Furthermore, Akt2 knockout displayed upregulated apoptotic protein markers (Bax, caspase-3, caspase-9, and caspace-12) and mitochondrial damage (reduced aconitase activity and NAD(+), elevated cytochrome-c release from mitochondria) along with reduced phosphorylation of PTEN, Akt, and GSK3β in the absence of changes in pan protein expression, the effects of which were abolished or significantly ameliorated by H2S treatment. In vitro data revealed that H2S-induced beneficial effect against Akt2 ablation was obliterated by mitochondrial uncoupling. Taken together, our findings suggest the H2S may reconcile Akt2 knockout-induced myocardial contractile defect and intracellular Ca(2+) mishandling, possibly via attenuation of mitochondrial injury and apoptosis.

Insulin stimulates glucose transport via protein kinase G type I alpha-dependent pathway in podocytes

Podocyte resistance to the actions of insulin on glucose transport could contribute to the pathogenesis of diabetic podocytopathy (DP) via disturbances in cyclic-dependent protein kinase signaling. To determine whether cGMP-dependent protein kinase (PKG) is involved in the insulin regulation of glucose transport, we measured insulin-dependent glucose uptake into cultured rat podocytes under conditions of modified PKG activity using pharmacological (PKG activator or inhibitor) and biochemical (siRNA PKGIα, siRNA insulin receptor β) means. Our findings indicate the participation of PKG in insulin-stimulated transport and provide new insights into how PKG may trigger the resistance of glucose transport to insulin in DP.

Podocyte-specific GLUT4-deficient mice have fewer and larger podocytes and are protected from diabetic nephropathy

Podocytes are a major component of the glomerular filtration barrier, and their ability to sense insulin is essential to prevent proteinuria. Here we identify the insulin downstream effector GLUT4 as a key modulator of podocyte function in diabetic nephropathy (DN). Mice with a podocyte-specific deletion of GLUT4 (G4 KO) did not develop albuminuria despite having larger and fewer podocytes than wild-type (WT) mice. Glomeruli from G4 KO mice were protected from diabetes-induced hypertrophy, mesangial expansion, and albuminuria and failed to activate the mammalian target of rapamycin (mTOR) pathway. In order to investigate whether the protection observed in G4 KO mice was due to the failure to activate mTOR, we used three independent in vivo experiments. G4 KO mice did not develop lipopolysaccharide-induced albuminuria, which requires mTOR activation. On the contrary, G4 KO mice as well as WT mice treated with the mTOR inhibitor rapamycin developed worse adriamycin-induced nephropathy than WT mice, consistent with the fact that adriamycin toxicity is augmented by mTOR inhibition. In summary, GLUT4 deficiency in podocytes affects podocyte nutrient sensing, results in fewer and larger cells, and protects mice from the development of DN. This is the first evidence that podocyte hypertrophy concomitant with podocytopenia may be associated with protection from proteinuria.

Signal transduction in podocytes--spotlight on receptor tyrosine kinases

The mammalian kidney filtration barrier is a complex multicellular, multicomponent structure that maintains homeostasis by regulating electrolytes, acid-base balance, and blood pressure (via maintenance of salt and water balance). To perform these multiple functions, podocytes--an important component of the filtration apparatus--must process a series of intercellular signals. Integrating these signals with diverse cellular responses enables a coordinated response to various conditions. Although mature podocytes are terminally differentiated and cannot proliferate, they are able to respond to growth factors. It is possible that the initial response of podocytes to growth factors is beneficial and protective, and might include the induction of hypertrophic cell growth. However, extended and/or uncontrolled growth factor signalling might be maladaptive and could result in the induction of apoptosis and podocyte loss. Growth factors signal via the activation of receptor tyrosine kinases (RTKs) on their target cells and around a quarter of the 58 RTK family members that are encoded in the human genome have been identified in podocytes. Pharmacological inhibitors of many RTKs exist and are currently used in experimental and clinical cancer therapy. The identification of pathological RTK-mediated signal transduction pathways in podocytes could provide a starting point for the development of novel therapies for glomerular disorders.

Global analysis reveals the complexity of the human glomerular extracellular matrix

The glomerulus contains unique cellular and extracellular matrix (ECM) components, which are required for intact barrier function. Studies of the cellular components have helped to build understanding of glomerular disease; however, the full composition and regulation of glomerular ECM remains poorly understood. We used mass spectrometry-based proteomics of enriched ECM extracts for a global analysis of human glomerular ECM in vivo and identified a tissue-specific proteome of 144 structural and regulatory ECM proteins. This catalog includes all previously identified glomerular components plus many new and abundant components. Relative protein quantification showed a dominance of collagen IV, collagen I, and laminin isoforms in the glomerular ECM together with abundant collagen VI and TINAGL1. Protein network analysis enabled the creation of a glomerular ECM interactome, which revealed a core of highly connected structural components. More than one half of the glomerular ECM proteome was validated using colocalization studies and data from the Human Protein Atlas. This study yields the greatest number of ECM proteins relative to previous investigations of whole glomerular extracts, highlighting the importance of sample enrichment. It also shows that the composition of glomerular ECM is far more complex than previously appreciated and suggests that many more ECM components may contribute to glomerular development and disease processes. The mass spectrometry proteomics data have been deposited to the ProteomeXchange Consortium with the dataset identifier PXD000456.

Free Fatty acids and their metabolism affect function and survival of podocytes

Podocyte injury and loss critically contribute to the pathogenesis of proteinuric kidney diseases including diabetic nephropathy. Deregulated lipid metabolism with disturbed free fatty acid (FFA) metabolism is a characteristic of metabolically unhealthy obesity and type 2 diabetes and likely contributes to end-stage kidney disease irrespective of the underlying kidney disease. In the current review, we summarize recent findings related to FFAs and altered renal FFA metabolism with a special focus on podocytes. We will outline the opposing effects of saturated and monounsaturated FFAs and a particular emphasis will be given to the underlying molecular mechanisms involving insulin resistance and endoplasmic reticulum homeostasis. Finally, recent data suggesting a critical role of renal FFA metabolism to adapt to an altered lipid environment will be discussed.

The importance of podocyte adhesion for a healthy glomerulus

Podocytes are specialized epithelial cells that cover the outer surfaces of glomerular capillaries. Unique cell junctions, known as slit diaphragms, which feature nephrin and Neph family proteins in addition to components of adherens, tight, and gap junctions, connect adjacent podocyte foot processes. Single gene disorders affecting the slit diaphragm result in nephrotic syndrome in humans, characterized by massive loss of protein across the capillary wall. In addition to specialized cell junctions, interconnecting podocytes also adhere to the glomerular basement membrane (GBM) of the capillary wall. The GBM is a dense network of secreted, extracellular matrix (ECM) components and contains tissue-restricted isoforms of collagen IV and laminin in addition to other structural proteins and ECM regulators such as proteases and growth factors. The specialized niche of the GBM provides a scaffold for endothelial cells and podocytes to support their unique functions and human genetic mutations in GBM components lead to renal failure, thus highlighting the importance of cell-matrix interactions in the glomerulus. Cells adhere to ECM via adhesion receptors, including integrins, syndecans, and dystroglycan and in particular the integrin heterodimer α3β1 is required to maintain barrier integrity. Therefore, the sophisticated function of glomerular filtration relies on podocyte adhesion both at cell junctions and at the interface with the ECM. In health, the podocyte coordinates signals from cell junctions and cell-matrix interactions, in response to environmental cues in order to regulate filtration and as our understanding of mechanisms that control cell adhesion in the glomerulus develops, then insight into the effects of disease will improve. The ultimate goal will be to develop targeted therapies to prevent or repair defects in the filtration barrier and to restore glomerular function.

Molecular and cellular events mediating glomerular podocyte dysfunction and depletion in diabetes mellitus

The essential function of the kidney is to ensure formation of a relatively protein-free ultra-filtrate, urine. The rate of filtration and composition of the primary renal filtrate is determined by the transport of fluid and solutes across the glomerular filtration barrier consisting of endothelial cells, the glomerular basement membrane, and podocyte foot processes. In diabetes mellitus (DM), components of the kidney that enable renal filtration get structurally altered and functionally compromised resulting in proteinuria that often progresses to end-stage renal disease. Histological alterations in DM include early hypertrophy of glomerular and tubular components, subsequent thickening of basement membrane in glomeruli and tubules, progressive accumulation of extracellular matrix proteins in the glomerular mesangium and loss of podocytes, together constituting a clinical condition referred to as diabetic nephropathy (DN). The glomerulus has become the focus of research investigating the mechanism of proteinuria. In particular, the progressive dysfunction and/or loss of podocytes that is contemporaneous with proteinuria in DN have attracted intense scientific attention. The absolute number of podocytes predicts glomerular function and podocyte injury is a hallmark of various glomerular diseases. This review discusses the importance of podocytes in normal renal filtration and details the molecular and cellular events that lead to podocyte dysfunction and decreased podocyte count in DN.

Urinary miR-29 correlates with albuminuria and carotid intima-media thickness in type 2 diabetes patients

BACKGROUND

Cell-free microRNAs stably and abundantly exist in body fluids and emerging evidence suggests cell-free microRNAs as novel and non-invasive disease biomarker. Deregulation of miR-29 is involved in the pathogenesis of diabetic nephropathy and insulin resistance thus may be implicated in diabetic vascular complication. Therefore, we investigated the possibility of urinary miR-29 as biomarker for diabetic nephropathy and atherosclerosis in patients with type 2 diabetes.

METHODS

83 patients with type 2 diabetes were enrolled in this study, miR-29a, miR-29b and miR-29c levels in urine supernatant was determined by TaqMan qRT-PCR, and a synthetic cel-miR-39 was added to the urine as a spike-in control before miRNAs extraction. Urinary albumin excretion rate and urine albumin/creatinine ratio, funduscopy and carotid ultrasound were used for evaluation of diabetic vascular complication. The laboratory parameters indicating blood glucose level, renal function and serum lipids were also collected.

RESULTS

Patients with albuminuria (n = 42, age 60.62 ± 12.00 yrs) showed significantly higher comorbidity of diabetic retinopathy (p = 0.015) and higher levels of urinary miR-29a (p = 0.035) compared with those with normoalbuminuria (n = 41, age 58.54 ± 14.40 yrs). There was no significant difference in urinary miR-29b (p = 0.148) or miR-29c level (p = 0.321) between groups. Urinary albumin excretion rate significantly correlated with urinary miR-29a level (r = 0.286, p = 0.016), while urinary miR-29b significantly correlated with carotid intima-media thickness (cIMT) (r = 0.286, p = 0.046).

CONCLUSION

Urinary miR-29a correlated with albuminuria while urinary miR-29b correlated with carotid intima-media thickness (cIMT) in patients with type 2 diabetes. Therefore, they may have the potential to serve as alternative biomarker for diabetic nephropathy and atherosclerosis in type 2 diabetes.

Recent advances in our understanding of insulin signalling to the podocyte

It is becoming increasingly clear that the insulin responses of a number of different cell types within the kidney are important in the maintenance of normal renal function. This review summarizes our current understanding of renal insulin signalling, with specific focus on the podocyte, presenting recent evidence that suggests these responses are altered in systemic insulin-resistant states and chronic kidney disease via a number of different mechanisms.

Obesity-mediated autophagy insufficiency exacerbates proteinuria-induced tubulointerstitial lesions

Obesity is an independent risk factor for renal dysfunction in patients with CKDs, including diabetic nephropathy, but the mechanism underlying this connection remains unclear. Autophagy is an intracellular degradation system that maintains intracellular homeostasis by removing damaged proteins and organelles, and autophagy insufficiency is associated with the pathogenesis of obesity-related diseases. We therefore examined the role of autophagy in obesity-mediated exacerbation of proteinuria-induced proximal tubular epithelial cell damage in mice and in human renal biopsy specimens. In nonobese mice, overt proteinuria, induced by intraperitoneal free fatty acid-albumin overload, led to mild tubular damage and apoptosis, and activated autophagy in proximal tubules reabsorbing urinary albumin. In contrast, diet-induced obesity suppressed proteinuria-induced autophagy and exacerbated proteinuria-induced tubular cell damage. Proximal tubule-specific autophagy-deficient mice, resulting from an Atg5 gene deletion, subjected to intraperitoneal free fatty acid-albumin overload developed severe proteinuria-induced tubular damage, suggesting that proteinuria-induced autophagy is renoprotective. Mammalian target of rapamycin (mTOR), a potent suppressor of autophagy, was activated in proximal tubules of obese mice, and treatment with an mTOR inhibitor ameliorated obesity-mediated autophagy insufficiency. Furthermore, both mTOR hyperactivation and autophagy suppression were observed in tubular cells of specimens obtained from obese patients with proteinuria. Thus, in addition to enhancing the understanding of obesity-related cell vulnerability in the kidneys, these results suggest that restoring the renoprotective action of autophagy in proximal tubules may improve renal outcomes in obese patients.

Resistance to insulin and kidney disease in the cardiorenal metabolic syndrome; role for angiotensin II

The presence of insulin resistance is increasingly recognized as an important contributor to early stage kidney disease independent of the contribution of diabetes. Important in this relationship is the strong correlation between hyperinsulinemia and low levels of albuminuria (e.g. microalbuminuria). Recent work highlight mechanisms for glomerular/tubulointerstitial injury with excess insulin and emerging evidence identifies a unique role for insulin metabolic signaling and altered handling of salt reabsorption at the level of the proximal tubule. Evidence is also emerging for the role of insulin signaling in the glomerulus both epithelial and endothelial. Central to the mechanism of injury is inappropriate activation of the RAAS.

AKT2 is essential to maintain podocyte viability and function during chronic kidney disease

In chronic kidney disease (CKD), loss of functional nephrons results in metabolic and mechanical stress in the remaining ones, resulting in further nephron loss. Here we show that Akt2 activation has an essential role in podocyte protection after nephron reduction. Glomerulosclerosis and albuminuria were substantially worsened in Akt2(-/-) but not in Akt1(-/-) mice as compared to wild-type mice. Specific deletion of Akt2 or its regulator Rictor in podocytes revealed that Akt2 has an intrinsic function in podocytes. Mechanistically, Akt2 triggers a compensatory program that involves mouse double minute 2 homolog (Mdm2), glycogen synthase kinase 3 (Gsk3) and Rac1. The defective activation of this pathway after nephron reduction leads to apoptosis and foot process effacement of the podocytes. We further show that AKT2 activation by mammalian target of rapamycin complex 2 (mTORC2) is also required for podocyte survival in human CKD. More notably, we elucidate the events underlying the adverse renal effect of sirolimus and provide a criterion for the rational use of this drug. Thus, our results disclose a new function of Akt2 and identify a potential therapeutic target for preserving glomerular function in CKD.

Diabetic nephropathy: protective factors and a new therapeutic paradigm

Diabetic nephropathy (DN) is the most common cause of chronic kidney disease (CKD) and its number has been increasing. CKD is a worldwide threat to health but the precise mechanism of this problem is not fully appreciated. It is believed that hyperglycemia is one of the most important metabolic factors in the development of DN. Multiple molecular mechanisms have been proposed to mediate hyperglycemia's adverse effects on kidney. To identify targets for therapeutic intervention, most studies have focused on understanding how abnormal levels of such metabolities cause DN. However, there have been few reports regarding endogenous renal protective factors. Thus, recognition of the importance of this could be providing a new perspective for understanding the development of DN and a new therapeutic paradigm to combat DN.

Associations between structural and functional changes to the kidney in diabetic humans and mice

Type 1 and Type 2 diabetic patients are at high risk of developing diabetic nephropathy (DN). Renal functional decline is gradual and there is high variability between patients, though the reason for the variability is unknown. Enough diabetic patients progress to end stage renal disease to make diabetes the leading cause of renal failure. The first symptoms of DN do not appear for years or decades after the onset of diabetes. During and after the asymptomatic period structural changes develop in the diabetic kidney. Typically, but not always, the first symptom of DN is albuminuria. Loss of renal filtration rate develops later. This review examines the structural abnormalities of diabetic kidneys that are associated with and possibly the basis for advancing albuminuria and declining GFR. Mouse models of diabetes and genetic manipulations of these models have become central to research into mechanisms underlying DN. This article also looks at the value of these mouse models to understanding human DN as well as potential pitfalls in translating the mouse results to humans.

IGF-1 receptor deficiency in thyrocytes impairs thyroid hormone secretion and completely inhibits TSH-stimulated goiter

Although thyroid-stimulating hormone (TSH) is known to be a major regulator of thyroid hormone biosynthesis and thyroid growth, insulin-like growth factor 1 (IGF-1) is required for mediating thyrocyte growth in concert with TSH in vitro. We generated mice with thyrocyte-selective ablation of IGF-1 receptor (TIGF1RKO) to explore the role of IGF-1 receptor signaling on thyroid function and growth. In 5-wk-old TIGF1RKO mice, serum thyroxine (T4) concentrations were decreased by 30% in concert with a 43% down-regulation of the monocarboxylate transporter 8 (MCT8), which is involved in T4 secretion. Despite a 3.5-fold increase in circulating concentrations of TSH, thyroid architecture and size were normal. Furthermore, thyrocyte area was increased by 40% in WT thyroids after 10 d TSH injection, but this effect was absent in TSH-injected TIGF1RKO mice. WT mice treated with methimazole and sodium perchlorate for 2 or 6 wk exhibited pronounced goiter development (2.0 and 5.4-fold, respectively), but in TIGF1RKO mice, goiter development was completely abrogated. These data reveal an essential role for IGF-1 receptor signaling in the regulation of thyroid function and TSH-stimulated goitrogenesis.

Short-term changes after a weight reduction intervention in advanced diabetic nephropathy

BACKGROUND AND OBJECTIVES

Obesity precedes and is strongly linked to the development of type 2 diabetic nephropathy in most patients, yet little is known about the effects of weight reduction on this disease. This study aimed to establish proof of concept for the hypothesis that weight reduction ameliorates diabetic nephropathy.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

Six obese individuals with advanced diabetic nephropathy (estimated GFR <40 ml/min per 1.73 m(2), urine albumin excretion >30 mg/d) currently taking a renin-aldosterone axis inhibitor underwent a 12-week very low calorie ketogenic weight reduction diet with encouragement of exercise between March and September 2012. Albuminuria and other parameters of kidney health were the main outcome measures.

RESULTS

There was a 12% reduction in weight (median 118.5 versus 104.3 kg, P=0.03). The intervention was associated with a 36% reduction in albuminuria that did not reach statistical significance (2124 versus 1366 mg/24 h, P=0.08) and significant reductions in the filtration markers serum creatinine (3.54 versus 3.13 mg/dl, P<0.05) and cystatin C (2.79 versus 2.46 mg/l, P<0.05). Improvements were also noted for the diabetes markers fasting glucose (166 versus 131 mg/dl, P<0.05), fasting insulin (26.9 versus 10.4 μU/ml, P<0.05), and insulin resistance (9.6 versus 4.2, P=0.03). Physical function, general health, and the number of diabetes medications also showed statistically significant signs of improvement.

CONCLUSIONS

After a short-term intensive weight reduction intervention in patients with advanced diabetic nephropathy, improvements were observed in markers of glomerular filtration, diabetes status, and risk factors for kidney disease progression, as well as other general indicators of health and well-being.

Ectopic lipid accumulation: A potential cause for metabolic disturbances and a contributor to the alteration of kidney function

Ectopic lipid accumulation is now known to be a mechanism that contributes to organ injury in the context of metabolic diseases. In muscle and liver, accumulation of lipids impairs insulin signaling. This hypothesis accounts for the mechanism of insulin resistance in obesity, type 2 diabetes, aging and lipodystrophy. Increasing data suggest that lipid accumulation in the kidneys could also contribute to the alteration of kidney function in the context of metabolic syndrome and obesity. Furthermore and more unexpectedly, animal models of kidney disease exhibit a decreased adiposity and ectopic lipid redistribution suggesting that kidney disease may be a state of lipodystrophy. However, whether this abnormal lipid partitioning during chronic kidney disease (CKD) may have any functional impact in these tissues needs to be investigated. Here, we provide a perspective by defining the problem and analyzing the possible causes and consequences. Further human studies are required to strengthen these observations, and provide novel therapeutic approaches.

Altered expression and localization of insulin receptor in proximal tubule cells from human and rat diabetic kidney

Diabetes is the major cause of end stage renal disease, and tubular alterations are now considered to participate in the development and progression of diabetic nephropathy (DN). Here, we report for the first time that expression of the insulin receptor (IR) in human kidney is altered during diabetes. We detected a strong expression in proximal and distal tubules from human renal cortex, and a significant reduction in type 2 diabetic patients. Moreover, isolated proximal tubules from type 1 diabetic rat kidney showed a similar response, supporting its use as an excellent model for in vitro study of human DN. IR protein down-regulation was paralleled in proximal and distal tubules from diabetic rats, but prominent in proximal tubules from diabetic patients. A target of renal insulin signaling, the gluconeogenic enzyme phosphoenolpyruvate carboxykinase (PEPCK), showed increased expression and activity, and localization in compartments near the apical membrane of proximal tubules, which was correlated with activation of the GSK3β kinase in this specific renal structure in the diabetic condition. Thus, expression of IR protein in proximal tubules from type 1 and type 2 diabetic kidney indicates that this is a common regulatory mechanism which is altered in DN, triggering enhanced gluconeogenesis regardless the etiology of the disease.

Importance of insulin resistance to vascular repair and regeneration

Metabolic insulin resistance is apparent across a spectrum of clinical disorders, including obesity and diabetes, and is characterized by an adverse clustering of cardiovascular risk factors related to abnormal cellular responses to insulin. These disorders are becoming increasingly prevalent and represent a major global public health concern because of their association with significant increases in atherosclerosis-related mortality. Endogenous repair mechanisms are thought to retard the development of vascular disease, and a growing evidence base supports the adverse impact of the insulin-resistant phenotype upon indices of vascular repair. Beyond the impact of systemic metabolic changes, emerging data from murine studies also provide support for abnormal insulin signaling at the level of vascular cells in retarding vascular repair. Interrelated pathophysiological factors, including reduced nitric oxide bioavailability, oxidative stress, altered growth factor activity, and abnormal intracellular signaling, are likely to act in conjunction to impede vascular repair while also driving vascular damage. Understanding of these processes is shaping novel therapeutic paradigms that aim to promote vascular repair and regeneration, either by recruiting endogenous mechanisms or by the administration of cell-based therapies.

The insulin receptor and the kidney

PURPOSE OF REVIEW

In recent years, it has become clear that the insulin receptor is important in a variety of renal cell types. It is through this transmembrane receptor that insulin, and to a lesser extent insulin-like growth factor, hormones bind and can control important cellular functions. This review will summarize the advances in our understanding of the role of the insulin receptor and insulin signalling in the glomeruli and tubules of the kidney.

RECENT FINDINGS

The insulin receptor is important for podocyte function and when lost results in a number of features resembling diabetic nephropathy. Exciting recent data also highlight the importance of mammalian target of rapamycin in nutrient sensing and protein biosynthesis in the podocyte, which may also be regulated by the insulin receptor. The insulin receptor has also been shown to perform an important role in the distal regions of the renal tubules, regulating sodium excretion and blood pressure control here.

SUMMARY

The insulin receptor is crucial for renal function in glomeruli and tubules. When signalling is diminished here, as may occur in insulin-resistant states, it may be responsible for a number of important renal complications including albuminuric glomerular disease and hypertension.

Expression of SHP-1 induced by hyperglycemia prevents insulin actions in podocytes

Renal podocyte apoptosis is an early event of diabetic nephropathy progression. Insulin action is critical for podocyte survival. Previous studies demonstrated that Src homology-2 domain-containing phosphatase-1 (SHP-1) is elevated in renal cortex of type 1 diabetic mice; we hypothesized that hyperglycemia-induced SHP-1 expression may affect insulin actions in podocytes. Type 1 diabetic Akita mice (Ins2(+/C96Y)) developed elevated foot process effacement and podocyte apoptosis compared with control littermate mice (Ins2(+/+)). In contrast to Ins2(+/+) mice, insulin-stimulated protein kinase B (Akt) and extracellular signal-regulated kinase (ERK) phosphorylation were remarkably reduced in renal podocytes of Akita mice. This renal insulin resistance was associated with elevated SHP-1 expression in the glomeruli. Cultured podocytes exposed to high glucose concentration (HG; 25 mM) for 96 h exhibited high levels of apoptotic markers and caspase-3/7 enzymatic activity. HG exposure raised mRNA and protein levels of SHP-1 and reduced the insulin-signaling pathway in podocytes. Overexpression of dominant-negative SHP-1 in podocytes prevented HG effects and restored insulin actions. Elevated SHP-1 expression induced by high glucose levels was directly associated with insulin receptor-β in vitro and in vivo to prevent insulin-stimulated Akt and ERK phosphorylation. In conclusion, our results showed that high levels of SHP-1 expression in glomeruli cause insulin resistance and podocyte loss, thereby contributing to diabetic nephropathy.

Insulin receptor substrate-2 is expressed in kidney epithelium and up-regulated in diabetic nephropathy

Diabetic nephropathy (DN) is a progressive fibrotic condition that may lead to end-stage renal disease and kidney failure. Transforming growth factor-β1 and bone morphogenetic protein-7 (BMP7) have been shown to induce DN-like changes in the kidney and protect the kidney from such changes, respectively. Recent data identified insulin action at the level of the nephron as a crucial factor in the development and progression of DN. Insulin requires a family of insulin receptor substrate (IRS) proteins for its physiological effects, and many reports have highlighted the role of insulin and IRS proteins in kidney physiology and disease. Here, we observed IRS2 expression predominantly in the developing and adult kidney epithelium in mouse and human. BMP7 treatment of human kidney proximal tubule epithelial cells (HK-2 cells) increases IRS2 transcription. In addition, BMP7 treatment of HK-2 cells induces an electrophoretic shift in IRS2 migration on SDS/PAGE, and increased association with phosphatidylinositol-3-kinase, probably due to increased tyrosine/serine phosphorylation. In a cohort of DN patients with a range of chronic kidney disease severity, IRS2 mRNA levels were elevated approximately ninefold, with the majority of IRS2 staining evident in the kidney tubules in DN patients. These data show that IRS2 is expressed in the kidney epithelium and may play a role in the downstream protective events triggered by BMP7 in the kidney. The specific up-regulation of IRS2 in the kidney tubules of DN patients also indicates a novel role for IRS2 as a marker and/or mediator of human DN progression.

Report on ISN Forefronts, Melbourne, Australia, 4-7 October 2012: tubulointerstitial disease in diabetic nephropathy

The mechanisms involved in expansion of the tubulointerstitial compartment of the kidney in individuals with diabetes are not well understood. Given that tubulointerstitial damage is an important predictor of progression to end-stage kidney disease in most forms of chronic kidney disease it is imperative to gain a greater understanding of the processes involved. With this in mind, a very clear objective for the scientific content of this meeting was to spend more than half the program outside the comfort zone of nephrology, gaining insights from sources such as neurodegenerative and mitochondrial diseases, stem cells, cancer and high-level computing to reconstruct organ systems. The meeting also aimed to place the new concepts presented in the context of current knowledge in diabetic kidney disease and the milestones achieved to date in this area. The presenters were all extremely generous, giving not only their time, but also showing a large proportion of unpublished data to stimulate discussions, questions and innovation.

Insulin directly stimulates VEGF-A production in the glomerular podocyte

Podocytes are critically important for maintaining the integrity of the glomerular filtration barrier and preventing albuminuria. Recently, it has become clear that to achieve this, they need to be insulin sensitive and produce an optimal amount of VEGF-A. In other tissues, insulin has been shown to regulate VEGF-A release, but this has not been previously examined in the podocyte. Using in vitro and in vivo approaches, in the present study, we now show that insulin regulates VEGF-A in the podocyte in both mice and humans via the insulin receptor (IR). Insulin directly increased VEGF-A mRNA levels and protein production in conditionally immortalized wild-type human and murine podocytes. Furthermore, when podocytes were rendered insulin resistant in vitro (using stable short hairpin RNA knockdown of the IR) or in vivo (using transgenic podocyte-specific IR knockout mice), podocyte VEGF-A production was impaired. Importantly, in vivo, this occurs before the development of any podocyte damage due to podocyte insulin resistance. Modulation of VEGF-A by insulin in the podocyte may be another important factor in the development of glomerular disease associated with conditions in which insulin signaling to the podocyte is deranged.

Insulin-like growth factor-II is produced by, signals to and is an important survival factor for the mature podocyte in man and mouse

Podocytes are crucial for preventing the passage of albumin into the urine and, when lost, are associated with the development of albuminuria, renal failure and cardiovascular disease. Podocytes have limited capacity to regenerate, therefore pro-survival mechanisms are critically important. Insulin-like growth factor-II (IGF-II) is a potent survival and growth factor; however, its major function is thought to be in prenatal development, when circulating levels are high. IGF-II has only previously been reported to continue to be expressed in discrete regions of the brain into adulthood in rodents, with systemic levels being undetectable. Using conditionally immortalized human and ex vivo adult mouse cells of the glomerulus, we demonstrated the podocyte to be the major glomerular source and target of IGF-II; it signals to this cell via the IGF-I receptor via the PI3 kinase and MAPK pathways. Functionally, a reduction in IGF signalling causes podocyte cell death in vitro and glomerular disease in vivo in an aged IGF-II transgenic mouse that produces approximately 60% of IGF-II due to a lack of the P2 promoter of this gene. Collectively, this work reveals the fundamental importance of IGF-II in the mature podocyte for glomerular health across mammalian species.

NOD2 promotes renal injury by exacerbating inflammation and podocyte insulin resistance in diabetic nephropathy

An increasing number of clinical and animal model studies indicate that activation of the innate immune system and inflammatory mechanisms are important in the pathogenesis of diabetic nephropathy. Nucleotide-binding oligomerization domain containing 2 (NOD2), a member of the NOD-like receptor family, plays an important role in innate immune response. Here we explore the contribution of NOD2 to the pathogenesis of diabetic nephropathy and found that it was upregulated in kidney biopsies from diabetic patients and high-fat diet/streptozotocin-induced diabetic mice. Further, NOD2 deficiency ameliorated renal injury in diabetic mice. In vitro, NOD2 induced proinflammatory response and impaired insulin signaling and insulin-induced glucose uptake in podocytes. Moreover, podocytes treated with high glucose, advanced glycation end-products, tumor necrosis factor-α, or transforming growth factor-β (common detrimental factors in diabetic nephropathy) significantly increased NOD2 expression. NOD2 knockout diabetic mice were protected from the hyperglycemia-induced reduction in nephrin expression. Further, knockdown of NOD2 expression attenuated high glucose-induced nephrin downregulation in vitro, supporting an essential role of NOD2 in mediating hyperglycemia-induced podocyte dysfunction. Thus, NOD2 is one of the critical components of a signal transduction pathway that links renal injury to inflammation and podocyte insulin resistance in diabetic nephropathy.

Implications of treatment that target protective mechanisms against diabetic nephropathy

Diabetes results in vascular changes and dysfunction, and vascular complications are the leading cause of morbidity and mortality in diabetic patients. There has been a continual increase in the number of diabetic nephropathy patients and epidemic increases in the number of patients progressing to end-stage renal diseases. To identify targets for therapeutic intervention, most studies have focused on understanding how abnormal levels of glucose metabolites cause diabetic nephropathy, which is of paramount importance in devising strategies to combat the development and progression of diabetic nephropathy. However, less studied than the systemic toxic mechanisms, hyperglycemia and dyslipidemia might inhibit the endogenous vascular protective factors such as insulin, vascular endothelial growth factor, and platelet-derived growth factor. In this review, we highlight the importance of enhancing endogenous protective factors to prevent or delay diabetic nephropathy.

How many ways can a podocyte die?

Podocytes are highly specialized epithelial cells that line the urinary surface of the glomerular capillary tuft. To maintain kidney filtration, podocytes oppose the high intraglomerular hydrostatic pressure, form a molecular sieve, secrete soluble factors to regulate other glomerular cell types, and provide synthesis and maintenance of the glomerular basement membrane. Impairment of any of these functions after podocyte injury results in proteinuria and possibly renal failure. Loss of glomerular podocytes is a key feature for the progression of renal diseases, and detached podocytes can be retrieved in the urine of patients with progressive glomerular diseases. Thus, the concept of podocyte loss as a hallmark of progressive glomerular disease has been widely accepted. However, the nature of events that promote podocyte detachment and whether detachment is preceded by any kind of podocyte cell death, such as apoptosis, necroptosis, or necrosis, still remains unclear and is discussed in this review.

VEGF and podocytes in diabetic nephropathy

Vascular endothelial growth factor-A (VEGF-A) is a protein secreted by podocytes that is necessary for survival of endothelial cells, podocytes, and mesangial cells. VEGF-A regulates slit-diaphragm signaling and podocyte shape via VEGF-receptor 2-nephrin-nck-actin interactions. Chronic hyperglycemia-induced excess podocyte VEGF-A and low endothelial nitric oxide drive the development and the progression of diabetic nephropathy. The abnormal cross-talk between VEGF-A and nitric oxide pathways is fueled by the diabetic milieu, resulting in increased oxidative stress. Recent findings on these pathogenic molecular mechanisms provide new potential targets for therapy for diabetic renal disease.

Mechanisms of diabetic complications

It is increasingly apparent that not only is a cure for the current worldwide diabetes epidemic required, but also for its major complications, affecting both small and large blood vessels. These complications occur in the majority of individuals with both type 1 and type 2 diabetes. Among the most prevalent microvascular complications are kidney disease, blindness, and amputations, with current therapies only slowing disease progression. Impaired kidney function, exhibited as a reduced glomerular filtration rate, is also a major risk factor for macrovascular complications, such as heart attacks and strokes. There have been a large number of new therapies tested in clinical trials for diabetic complications, with, in general, rather disappointing results. Indeed, it remains to be fully defined as to which pathways in diabetic complications are essentially protective rather than pathological, in terms of their effects on the underlying disease process. Furthermore, seemingly independent pathways are also showing significant interactions with each other to exacerbate pathology. Interestingly, some of these pathways may not only play key roles in complications but also in the development of diabetes per se. This review aims to comprehensively discuss the well validated, as well as putative mechanisms involved in the development of diabetic complications. In addition, new fields of research, which warrant further investigation as potential therapeutic targets of the future, will be highlighted.

Coming full circle in diabetes mellitus: from complications to initiation

Glycaemic control, reduction of blood pressure using agents that block the renin-angiotensin system and control of dyslipidaemia are the major strategies used in the clinical management of patients with diabetes mellitus. Each of these approaches interrupts a number of pathological pathways, which directly contributes to the vascular complications of diabetes mellitus, including renal disease, blindness, neuropathy and cardiovascular disease. However, research published over the past few years has indicated that many of the pathological pathways important in the development of the vascular complications of diabetes mellitus are equally relevant to the initiation of diabetes mellitus itself. These pathways include insulin signalling, generation of cellular energy, post-translational modifications and redox imbalances. This Review will examine how the development of diabetes mellitus has come full circle from initiation to complications and suggests that the development of diabetes mellitus and the progression to chronic complications both require the same mechanistic triggers.

Insulin signalling to the kidney in health and disease

Ninety-one years ago insulin was discovered, which was one of the most important medical discoveries in the past century, transforming the lives of millions of diabetic patients. Initially insulin was considered only important for rapid control of blood glucose by its action on a restricted number of tissues; however, it has now become clear that this hormone controls an array of cellular processes in many different tissues. The present review will focus on the role of insulin in the kidney in health and disease.

Vascular complications of diabetes: mechanisms of injury and protective factors

In patients with diabetes, atherosclerosis is the main reason for impaired life expectancy, and diabetic nephropathy and retinopathy are the largest contributors to end-stage renal disease and blindness, respectively. An improved therapeutic approach to combat diabetic vascular complications might include blocking mechanisms of injury as well as promoting protective or regenerating factors, for example by enhancing the action of insulin-regulated genes in endothelial cells, promoting gene programs leading to induction of antioxidant or anti-inflammatory factors, or improving the sensitivity to vascular cell survival factors. Such strategies could help prevent complications despite suboptimal metabolic control.

Tissue-specific insulin signaling, metabolic syndrome, and cardiovascular disease

Impaired insulin signaling is central to development of the metabolic syndrome and can promote cardiovascular disease indirectly through development of abnormal glucose and lipid metabolism, hypertension, and a proinflammatory state. However, insulin's action directly on vascular endothelium, atherosclerotic plaque macrophages, and in the heart, kidney, and retina has now been described, and impaired insulin signaling in these locations can alter progression of cardiovascular disease in the metabolic syndrome and affect development of microvascular complications of diabetes mellitus. Recent advances in our understanding of the complex pathophysiology of insulin's effects on vascular tissues offer new opportunities for preventing these cardiovascular disorders.

Role of insulin resistance in kidney dysfunction: insights into the mechanism and epidemiological evidence

Several lines of evidence suggest a pathogenic role of insulin resistance on kidney dysfunction. Potential mechanisms are mostly due to the effect of single abnormalities related to insulin resistance and clustering into the metabolic syndrome. Hyperinsulinemia, which is inevitably associated to insulin resistance in non diabetic states, also appears to play a role on kidney function by inducing glomerular hyperfiltration and increased vascular permeability. More recently, adipocytokine which are linked to insulin resistance, low grade inflammation, endothelial dysfunction and vascular damage have been proposed as additional molecules able to modulate kidney function. In addition, recent evidences point also to a role of insulin resistance at the level of the podocyte, an important player in early phases of diabetic kidney damage, thus suggesting a new mechanism through which a reduction of insulin action can affect kidney function. In fact, mouse models not expressing the podocyte insulin receptor develop podocytes apoptosis, effacement of its foot processes along with thickening of the glomerular basement membrane, increased glomerulosclerosis and albuminuria. A great number of epidemiological studies have repeatedly reported the association between insulin resistance and kidney dysfunction in both non diabetic and diabetic subjects. Among these, studies addressing the impact of insulin resistance genes on kidney dysfunction have played the important role to help establish a cause-effect relationship between these two traits. Finally, numerous independent intervention studies have shown that a favourable modulation of insulin resistance has a positive effect also on urinary albumin and total protein excretion. In conclusion, several data of different nature consistently support the role of insulin resistance and related abnormalities on kidney dysfunction. Intervention trials designed to investigate whether treating insulin resistance ameliorates also hard renal end-points are both timely and needed.

Accelerated renal disease is associated with the development of metabolic syndrome in a glucolipotoxic mouse model

Individuals with metabolic syndrome are at high risk of developing chronic kidney disease (CKD) through unclear pathogenic mechanisms. Obesity and diabetes are known to induce glucolipotoxic effects in metabolically relevant organs. However, the pathogenic role of glucolipotoxicity in the aetiology of diabetic nephropathy is debated. We generated a murine model, the POKO mouse, obtained by crossing the peroxisome proliferator-activated receptor gamma 2 (PPARγ2) knockout (KO) mouse into a genetically obese ob/ob background. We have previously shown that the POKO mice showed: hyperphagia, insulin resistance, hyperglycaemia and dyslipidaemia as early as 4 weeks of age, and developed a complete loss of normal β-cell function by 16 weeks of age. Metabolic phenotyping of the POKO model has led to investigation of the structural and functional changes in the kidney and changes in blood pressure in these mice. Here we demonstrate that the POKO mouse is a model of renal disease that is accelerated by high levels of glucose and lipid accumulation. Similar to ob/ob mice, at 4 weeks of age these animals exhibited an increased urinary albumin:creatinine ratio and significantly increased blood pressure, but in contrast showed a significant increase in the renal hypertrophy index and an associated increase in p27(Kip1) expression compared with their obese littermates. Moreover, at 4 weeks of age POKO mice showed insulin resistance, an alteration of lipid metabolism and glomeruli damage associated with increased transforming growth factor beta (TGFβ) and parathyroid hormone-related protein (PTHrP) expression. At this age, levels of proinflammatory molecules, such as monocyte chemoattractant protein-1 (MCP-1), and fibrotic factors were also increased at the glomerular level compared with levels in ob/ob mice. At 12 weeks of age, renal damage was fully established. These data suggest an accelerated lesion through glucolipotoxic effects in the renal pathogenesis in POKO mice.

Mild electrical stimulation and heat shock ameliorates progressive proteinuria and renal inflammation in mouse model of Alport syndrome

Alport syndrome is a hereditary glomerulopathy with proteinuria and nephritis caused by defects in genes encoding type IV collagen in the glomerular basement membrane. All male and most female patients develop end-stage renal disease. Effective treatment to stop or decelerate the progression of proteinuria and nephritis is still under investigation. Here we showed that combination treatment of mild electrical stress (MES) and heat stress (HS) ameliorated progressive proteinuria and renal injury in mouse model of Alport syndrome. The expressions of kidney injury marker neutrophil gelatinase-associated lipocalin and pro-inflammatory cytokines interleukin-6, tumor necrosis factor-α and interleukin-1β were suppressed by MES+HS treatment. The anti-proteinuric effect of MES+HS treatment is mediated by podocytic activation of phosphatidylinositol 3-OH kinase (PI3K)-Akt and heat shock protein 72 (Hsp72)-dependent pathways in vitro and in vivo. The anti-inflammatory effect of MES+HS was mediated by glomerular activation of c-jun NH₂-terminal kinase 1/2 (JNK1/2) and p38-dependent pathways ex vivo. Collectively, our studies show that combination treatment of MES and HS confers anti-proteinuric and anti-inflammatory effects on Alport mice likely through the activation of multiple signaling pathways including PI3K-Akt, Hsp72, JNK1/2, and p38 pathways, providing a novel candidate therapeutic strategy to decelerate the progression of patho-phenotypes in Alport syndrome.

Plekhh2, a novel podocyte protein downregulated in human focal segmental glomerulosclerosis, is involved in matrix adhesion and actin dynamics

Pleckstrin homology domain-containing, family H (with MyTH4 domain), member 2 (Plekhh2) is a 1491-residue intracellular protein highly enriched in renal glomerular podocytes for which no function has been ascribed. Analysis of renal biopsies from patients with focal segmental glomerulosclerosis revealed a significant reduction in total podocyte Plekhh2 expression compared to controls. Sequence analysis indicated a putative α-helical coiled-coil segment as the only recognizable domain within the N-terminal half of the polypeptide, while the C-terminal half contains two PH, a MyTH4, and a FERM domain. We identified a phosphatidylinositol-3-phosphate consensus-binding site in the PH1 domain required for Plekhh2 localization to peripheral regions of cell lamellipodia. The N-terminal half of Plekkh2 is not necessary for lamellipodial targeting but mediates self-association. Yeast two-hybrid screening showed that Plekhh2 directly interacts through its FERM domain with the focal adhesion protein Hic-5 and actin. Plekhh2 and Hic-5 coprecipitated and colocalized at the soles of podocyte foot processes in situ and Hic-5 partially relocated from focal adhesions to lamellipodia in Plekhh2-expressing podocytes. In addition, Plekhh2 stabilizes the cortical actin cytoskeleton by attenuating actin depolymerization. Our findings suggest a structural and functional role for Plekhh2 in the podocyte foot processes.

Septin 7 forms a complex with CD2AP and nephrin and regulates glucose transporter trafficking

Podocytes are insulin-sensitive and take up glucose in response to insulin. This requires nephrin, which interacts with vesicle-associated membrane protein 2 (VAMP2) on GLUT4 storage vesicles (GSVs) and facilitates their fusion with the plasma membrane. In this paper, we show that the filament-forming GTPase septin 7 is expressed in podocytes and associates with CD2-associated protein (CD2AP) and nephrin, both essential for glomerular ultrafiltration. In addition, septin 7 coimmunoprecipitates with VAMP2. Subcellular fractionation of cultured podocytes revealed that septin 7 is found in both cytoplasmic and membrane fractions, and immunofluorescence microscopy showed that septin 7 is expressed in a filamentous pattern and is also found on vesicles and the plasma membrane. The filamentous localization of septin 7 depends on CD2AP and intact actin organization. A 2-deoxy-d-glucose uptake assay indicates that depletion of septin 7 by small interfering RNA or alteration of septin assembly by forchlorfenuron facilitates glucose uptake into cells and further, knockdown of septin 7 increased the interaction of VAMP2 with nephrin and syntaxin 4. The data indicate that septin 7 hinders GSV trafficking and further, the interaction of septin 7 with nephrin in glomeruli suggests that septin 7 may participate in the regulation of glucose transport in podocytes.