Expression of the IL-2R in Human Podocytes and the Effect of Activation on Autophagy and Apoptosis

Interleukin 2 (IL-2) treatment is associated with proteinuria. Materials and Methods: A conditionally immortalized human podocyte cell line was used to investigate expression of the podocyte specific marker podocin, IL-2R alpha (IL-2Rα), apoptosis marker Bax, and autophagy markers LC3I AND LC3II, determined by quantitative immunoblotting, following 24, 48, and 72 hours of IL-2 stimulation, comparing them to unstimulated cells. Results: Podocin was expressed at all time points. IL-2Rα expression was increased after 24 and 72 hrs (p = 0.0014, p = 0.0139) and decreased after 48 hours (p = 0.0445). Bax, LC3I, and LC3II were increased after 24 hrs (p = 0.0094, p = 0.0016, p = 0.0004) and 48 hrs (p = 0.0072, p = 0.0024, p = 0.0087). Conclusion: Human podocytes express the IL-2R and activation results in increased autophagy and apoptosis.

YAP Translocation Precedes Cytoskeletal Rearrangement in Podocyte Stress Response: A Podometric Investigation of Diabetic Nephropathy

Podocyte loss plays a pivotal role in the pathogenesis of glomerular disease. However, the mechanisms underlying podocyte damage and loss remain poorly understood. Although detachment of viable cells has been documented in experimental Diabetic Nephropathy, correlations between reduced podocyte density and disease severity have not yet been established. YAP, a mechanosensing protein, has recently been shown to correlate with glomerular disease progression, however, the underlying mechanism has yet to be fully elucidated. In this study, we sought to document podocyte density in Diabetic Nephropathy using an amended podometric methodology, and to investigate the interplay between YAP and cytoskeletal integrity during podocyte injury. Podocyte density was quantified using TLE4 and GLEPP1 multiplexed immunofluorescence. Fourteen Diabetic Nephropathy cases were analyzed for both podocyte density and cytoplasmic translocation of YAP via automated image analysis. We demonstrate a significant decrease in podocyte density in Grade III/IV cases (124.5 per 10⁶ μm³) relative to Grade I/II cases (226 per 10⁶ μm³) (Student's t-test, p < 0.001), and further show that YAP translocation precedes cytoskeletal rearrangement following injury. Based on these findings we hypothesize that a significant decrease in podocyte density in late grade Diabetic Nephropathy may be explained by early cytoplasmic translocation of YAP.

Role of the Nox4/AMPK/mTOR signaling axe in adipose inflammation-induced kidney injury

Diabetic kidney disease is one of the most serious complications of diabetes worldwide and is the leading cause of end-stage renal disease. While research has primarily focused on hyperglycemia as a key player in the pathophysiology of diabetic complications, recently, increasing evidence have underlined the role of adipose inflammation in modulating the development and/or progression of diabetic kidney disease. This review focuses on how adipose inflammation contribute to diabetic kidney disease. Furthermore, it discusses in detail the underlying mechanisms of adipose inflammation, including pro-inflammatory cytokines, oxidative stress, and AMPK/mTOR signaling pathway and critically describes their role in diabetic kidney disease. This in-depth understanding of adipose inflammation and its impact on diabetic kidney disease highlights the need for novel interventions in the treatment of diabetic complications.

Effect of Tongxinluo on Podocyte Apoptosis via Inhibition of Oxidative Stress and P38 Pathway in Diabetic Rats

Diabetic nephropathy (DN) has been the leading cause of end-stage renal disease (ESRD). Podocyte apoptosis is a main mechanism of progression of DN. It has been demonstrated that activated P38 and caspase-3 induced by oxidative stress mainly account for increased podocyte apoptosis and proteinuria in DN. Meanwhile, Tongxinluo (TXL) can ameliorate renal structure disruption and dysfunction in DN patients in our clinical practice. However, the effect of TXL on podocyte apoptosis and P38 pathway remains unclear. To explore the effect of TXL on podocyte apoptosis and its molecular mechanism in DN, our in vivo and in vitro studies were performed. TXL attenuated oxidative stress in podocyte in DN in our in vivo and in vitro studies. Moreover, TXL inhibited the activation of P38 and caspase-3. Bcl-2 and Bax expression was partially restored by TXL treatment in our in vivo and in vitro studies. More importantly, TXL decreased podocyte apoptosis in diabetic rats and high glucose cultured podocyte. In conclusion, TXL protects podocyte from apoptosis in DN, partially through its antioxidant effect and inhibiting of the activation of P38 and caspase-3.

Upregulation of α3β1-Integrin in Podocytes in Early-Stage Diabetic Nephropathy

Background. Podocyte injury plays an important role in the onset and progression of diabetic nephropathy (DN). Downregulation of α3β1-integrin expression in podocytes is thought to be associated with podocyte detachment from the glomerular basement membrane, although the mechanisms remain obscure. To determine the mechanism of podocyte detachment, we analyzed the expression levels of α3β1-integrin in podocytes in early and advanced stages of DN. Methods. Surgical specimens from DN patients were examined by in situ hybridization, and the expression levels of α3- and β1-integrin subunits in glomeruli of early (n = 6) and advanced (n = 8) stages were compared with those of normal glomeruli (n = 5). Heat-sensitive mouse podocytes (HSMP) were cultured with TGF-β1 to reproduce the microenvironment of glomeruli of DN, and the expression levels of integrin subunits and the properties of migration and attachment were examined. Results. Podocytes of early-stage DN showed upregulation of α3- and β1-integrin expression while those of advanced stage showed downregulation. Real-time PCR indicated a tendency for upregulation of α3- and β1-integrin in HSMP cultured with TGF-β1. TGF-β1-stimulated HSMP also showed enhanced in vitro migration and attachment on collagen substrate. Conclusions. The results suggested that podocyte detachment during early stage of DN is mediated through upregulation of α3β1-integrin.

Early-onset diabetic E1-DN mice develop albuminuria and glomerular injury typical of diabetic nephropathy

The transgenic E1-DN mice express a kinase-negative epidermal growth factor receptor in their pancreatic islets and are diabetic from two weeks of age due to impaired postnatal growth of β-cell mass. Here, we characterize the development of hyperglycaemia-induced renal injury in the E1-DN mice. Homozygous mice showed increased albumin excretion rate (AER) at the age of 10 weeks; the albuminuria increased over time and correlated with blood glucose. Morphometric analysis of PAS-stained histological sections and electron microscopy images revealed mesangial expansion in homozygous E1-DN mice, and glomerular sclerosis was observed in the most hyperglycaemic mice. The albuminuric homozygous mice developed also other structural changes in the glomeruli, including thickening of the glomerular basement membrane and widening of podocyte foot processes that are typical for diabetic nephropathy. Increased apoptosis of podocytes was identified as one mechanism contributing to glomerular injury. In addition, nephrin expression was reduced in the podocytes of albuminuric homozygous E1-DN mice. Tubular changes included altered epithelial cell morphology and increased proliferation. In conclusion, hyperglycaemic E1-DN mice develop albuminuria and glomerular and tubular injury typical of human diabetic nephropathy and can serve as a new model to study the mechanisms leading to the development of diabetic nephropathy.

NADPH oxidase-induced NALP3 inflammasome activation is driven by thioredoxin-interacting protein which contributes to podocyte injury in hyperglycemia

Diabetic nephropathy (DN) is one of the major causes of end-stage renal disease, and previously we demonstrated that NALP3 inflammasome was involved in the pathogenesis of DN. Here we investigated the mechanisms of NALP3 inflammasome activation in podocyte injury during DN. We found that, besides the activation of NALP3 inflammasome and upregulated thioredoxin-interacting protein (TXNIP), the glomerular expression of gp91 (phox) , a subunit of NADPH oxidase, was enhanced in DN mice simultaneously. Inhibiting NADPH oxidase abrogated NALP3 inflammasome activation, and IL-1β production and eventually protected podocytes from high glucose- (HG-) induced injury. TXNIP, an inhibitor of thioredoxin, acts as a suppressor for antioxidant defense system. Our observation indicated that in HG-exposed podocytes genetic deletion of TXNIP by shRNA reversed gp91 (phox) overexpression and alleviated the injury of podocyte. Collectively, our findings proposed that HG-induced NADPH oxidase activation was driven by TXNIP which subsequently triggered NALP3 inflammasome activation in podocytes and ultimately led to podocyte injury, and blocking TXNIP/NADPH oxidase signaling may be a promising treatment for DN.

Podocytes … What's Under Yours? (Podocytes and Foot Processes and How They Change in Nephropathy)

Most of the described structures of podocytes in health and disease have been inferred from light and electron microscopic studies of rodent models. The variation in filtration barrier features is measured on micrographs, the aim being statistical significance. This is the technical campaign waged against kidney disease but this approach can be misleading. The signaling cascades and connectivity of the podocyte and foot processes (FPs) are inferred from in vitro studies that at best blurr the reality of the in vivo state. This review will outline actin signaling connectivity and the key differences in the structural and functional domains squeezed into the FPs and the relationship of these domains to other parts of the podocyte. It covers the changes in podocytes during nephropathy concentrating on FP and finally proposes an alternative interpretation of FP ultrastructure derived from articles published over the last 60 years.

p21(WAF1/CIP1) Expression is Differentially Regulated by Metformin and Rapamycin

The mammalian target of rapamycin (mTOR) pathway plays an important role in the development of diabetic nephropathy and other age-related diseases. One of the features of DN is the elevated expression of p21^WAF1/CIP1. However, the importance of the mTOR signalling pathway in p21 regulation is poorly understood. Here we investigated the effect of metformin and rapamycin on mTOR-related phenotypes in cell lines of epithelial origin. This study reports that metformin inhibits high glucose-induced p21 expression. High glucose opposed metformin in regulating cell size, proliferation, and protein synthesis. These effects were associated with reduced AMPK activation, affecting downstream mTOR signalling. However, the inhibition of the mTOR pathway by rapamycin did not have a negative effect on p21 expression, suggesting that metformin regulates p21 upstream of mTOR. These findings provide support for the hypothesis that AMPK activation may regulate p21 expression, which may have implications for diabetic nephropathy and other age-related pathologies.

Podocyte pathology and nephropathy - sphingolipids in glomerular diseases

Sphingolipids are components of the lipid rafts in plasma membranes, which are important for proper function of podocytes, a key element of the glomerular filtration barrier. Research revealed an essential role of sphingolipids and sphingolipid metabolites in glomerular disorders of genetic and non-genetic origin. The discovery that glucocerebrosides accumulate in Gaucher disease in glomerular cells and are associated with clinical proteinuria initiated intensive research into the function of other sphingolipids in glomerular disorders. The accumulation of sphingolipids in other genetic diseases including Tay-Sachs, Sandhoff, Fabry, hereditary inclusion body myopathy 2, Niemann-Pick, and nephrotic syndrome of the Finnish type and its implications with respect to glomerular pathology will be discussed. Similarly, sphingolipid accumulation occurs in glomerular diseases of non-genetic origin including diabetic kidney disease (DKD), HIV-associated nephropathy, focal segmental glomerulosclerosis (FSGS), and lupus nephritis. Sphingomyelin metabolites, such as ceramide, sphingosine, and sphingosine-1-phosphate have also gained tremendous interest. We recently described that sphingomyelin phosphodiesterase acid-like 3b (SMPDL3b) is expressed in podocytes where it modulates acid sphingomyelinase activity and acts as a master modulator of danger signaling. Decreased SMPDL3b expression in post-reperfusion kidney biopsies from transplant recipients with idiopathic FSGS correlates with the recurrence of proteinuria in patients and in experimental models of xenotransplantation. Increased SMPDL3b expression is associated with DKD. The consequences of differential SMPDL3b expression in podocytes in these diseases with respect to their pathogenesis will be discussed. Finally, the role of sphingolipids in the formation of lipid rafts in podocytes and their contribution to the maintenance of a functional slit diaphragm in the glomerulus will be discussed.

Advanced diabetic nephropathy with nephrotic range proteinuria: a pilot study of the long-term efficacy of subcutaneous ACTH gel on proteinuria, progression of CKD, and urinary levels of VEGF and MCP-1

BACKGROUND AND OBJECTIVE

Adrenocorticotropic hormone (ACTH) is able to reduce proteinuria in nondiabetic glomerulopathies through activation of melanocortin receptors (MCR) expressed in the podocyte. To determine the efficacy of ACTH, we conducted a randomized, open-label pilot trial of ACTH gel in patients with advanced diabetic nephropathy.

STUDY DESIGN

Twenty-three (23) patients with diabetic nephropathy were randomized to daily subcutaneous (SQ) injections of 16 or 32 units of ACTH gel for six months. Outcome. The primary endpoint was the percentage of patients achieving a complete remission (<300 mg/24 hours) within 6 months. Exploratory endpoints included the percentage of partial (50% reduction) remissions, changes in Cr, and urinary cytokine markers.

RESULTS

After 6 months of ACTH gel therapy, 8 of 14 (57%) patients achieved a complete (n = 1) or partial (n = 7) remission. In the low-dose ACTH gel group (16 units), urinary protein fell from 6709 + 953 to 2224 + 489 mg/24 hrs (P < 0.001). In contrast, 2 of 6 patients in the 32-unit group achieved partial remission, but aggregate proteinuria (5324 + 751 to 5154 + 853 mg/24 hours) did not change. Urinary VEGF increased from 388 to 1346 pg/mg urinary creatinine (P < 0.02) in the low-dose group but remained unchanged in the high-dose group.

CONCLUSION

ACTH gel stabilizes renal function and reduces urinary protein for up to 6 months after treatment. The ClinTrials.gov identifier is NCT01028287.

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.

Recent advancement of understanding pathogenesis of type 1 diabetes and potential relevance to diabetic nephropathy

Type 1 diabetes mellitus is an autoimmune disease characterized by progressive destruction of pancreatic beta cells by genetic and environmental factors which leads to an absolute dependence of insulin for survival and maintenance of health. Although the majority of mechanisms of beta cell destruction remain unclear, many molecules, including proinflammatory cytokines and chemokines such as tumor necrosis factor alpha and monocyte chemoattractant protein-1, are implicated in the development of beta cell damage. Furthermore, beta cell destruction is enhanced by the Th1 and Th17 subsets of CD4+ T cells. In contrast, there are mechanisms involved in the maintenance of peripheral tolerance by regulatory T cells, the function of which depends on the pleiotropic cytokine transforming growth factor beta. Development and progression of renal injuries in patients with diabetic nephropathy are also associated with several growth factors and proinflammatory cytokines, including tumor necrosis factor alpha, insulin-like growth factor-1, monocyte chemoattractant protein-1, vascular endothelial growth factor, and transforming growth factor beta. Although the pathogenic mechanisms underlying type 1 diabetes and diabetic nephropathy are principally different, i.e., autoimmunity and inflammation, some common factors, including susceptibility genes and proinflammatory cytokines, are involved in both mechanisms, including infiltrating cell recruitment, upregulation of other cytokines and chemokines, or apoptosis.

Podocyte damage resulting in podocyturia: a potential diagnostic marker to assess glomerular disease activity

A decrease in podocyte number contributes to the development of glomerulosclerosis in most forms of glomerular disease [1, 2, 3, 4, 5]. Traditionally, it has been argued that this decrease may be caused by the inability of podocytes to proliferate and replace those lost following immune, metabolic, toxic or hemodynamic injury. These data contrast with recent studies showing that podocytes are able to enter the cell cycle after injury, to progress through the different phases of the cell cycle and even enter mitosis. However, experimental and human data suggest that entry of podocytes into the cell cycle may result in reduced adhesion to the glomerular basement membrane with subsequent loss of podocytes into the urine and excretion of both viable and apoptotic podocytes. Viable urinary podocytes can be cultivated ex vivo for up to 2-3 weeks and in experimental models precede the onset of proteinuria. More importantly, podocyturia can decrease despite persistent proteinuria. The latter observation suggests that podocyturia may serve as the first non-invasive marker of 'active' glomerular damage and might thus drive therapeutic interventions in the future. However, at present technical issues still prevent a broad clinical application of podocyturia detection in clinical practice.

Beneficial Effect of TGFβ Antagonism in Treating Diabetic Nephropathy Depends on When Treatment Is Started

BACKGROUND

In diabetic rats with maximal activation of RAS induced by uninephrectomy, late treatment with anti-TGFbeta antibody limited renal injury only when combined with ACE inhibitor. We investigated whether in a two-kidney diabetic model the time at which treatment started predicted the response to TGFbeta antagonist.

METHODS

27 weeks after streptozotocin injection, animals had mild proteinuria and were randomized to receive irrelevant antibody, anti-TGFbeta antibody (1D11) or enalapril till 52 weeks (early treatment). The effect of agents alone or combined was also evaluated at the time of overt proteinuria (late treatment, 52-61 weeks).

RESULTS

When given early, 1D11 displayed marked antihypertensive and antiproteinuric effects. Glomerulosclerosis was reduced to the extent that a remarkable percentage of glomeruli without sclerosis appeared after treatment. Podocyte number was normalized. Renoprotection of 1D11 was comparable to enalapril. Despite control of blood pressure, in late treatment single agents did not reduce proteinuria significantly. Glomerulosclerosis and podocyte loss were partially limited by 1D11 or enalapril, but full protection was achieved by combination.

CONCLUSIONS

Renoprotective effect of TGFbeta antagonism crucially depends on the time at which treatment started. Effectiveness of early treatment with 1D11 would indicate that TGFbeta is a major mediator of damage in early diabetes. To tackle the renal damage in the phase of advanced disease, a combined treatment with ACE inhibitor is needed.

Research into the glomerular podocyte--is it relevant to diabetic nephropathy?

The cause of proteinuria in renal disease is the subject of intensive research and, latterly, the podocyte, a specialized epithelial cell of the kidney glomerulus, has been the focus of much of this endeavour. It is a complex cell with functions and structural features that have an important role in the development of proteinuria. This review explores some of the characteristics of the podocyte and how abnormalities of its structure and function may have particular relevance to the development and progression of clinical diabetic nephropathy.

Comparison of methods for counting cells in the mouse glomerulus

BACKGROUND

Researchers have long been interested in counting the number of cells within the glomerulus. Investigators using different techniques have yielded conflicting results. The most direct method is to count the cells in serial sections from the entire glomerulus. This Exhaustive Count method is not usually practical. The disector/fractionator method counts the number of cells in a fraction of sections. The total is estimated from the product of the cell count and the reciprocal of the fraction. The Weibel-Gomez method determines the density of cells per glomerulus, then multiplies this density by glomerular volume to obtain cell number. In this study, we compared the disector/fractionator and Weibel-Gomez methods to identify a practical alternative for the time-consuming Exhaustive Count method.

METHODS

Glomeruli from a normal mouse kidney were completely sectioned and images obtained. Appropriate images were used to count glomerular cell number using each method.

RESULTS

The Exhaustive Count method yielded 213 +/- 22 (mean +/- SD) cells/glomerulus vs. the disector/fractionator average of 211 +/- 29 cells/glomerulus (p = 0.82). The Weibel-Gomez method average of 235 +/- 26 cells/glomerulus was statistically different from the Exhaustive Count method (p = 0.003).

CONCLUSION

The Weibel-Gomez produced a 10% overestimation, whereas the disector/fractionator method was unbiased and thus a good substitute for the Exhaustive Count method.

Viable podocytes detach in experimental diabetic nephropathy: potential mechanism underlying glomerulosclerosis

BACKGROUND

A decrease in podocyte number contributes to the development of glomerulosclerosis in diabetic nephropathy. Although podocytes have been detected in the urine in certain glomerular diseases, their viability is poorly understood.

METHODS

Diabetes was induced in rats with streptozotocin. Urine was collected from control rats (given citrate), and rats with diabetic nephropathy, and cells obtained by centrifugation were resuspended in tissue culture media, and seeded onto collagen-coated tissue culture plates. Cells were grown under standard cell culture conditions ex vivo. Cell number was measured, the cell type in the urine was identified by immunostaining with specific antibodies, and morphology was assessed by light and electron microscopy.

RESULTS

Within 24 h, cells obtained from the urine of diabetic rats attached to tissue culture plates ex vivo. Cells were not detected in the urine from control rats. All cells from diabetic rats stained positive for the podocyte-specific proteins synaptopodin, nephrin, podocin and Glepp-1 and negative for mesangial (OX-7), tubular (Tamm-Horsfall protein) and endothelial (RECA) cell antigens. The cell number increased daily, which is consistent with cell growth ex vivo.

CONCLUSIONS

Rats with diabetic nephropathy shed podocytes into the urine that attach and grow ex vivo. These results are consistent with the detachment of viable podocytes in diabetes and add new perspectives into our understanding of development of glomerulosclerosis in diabetes mellitus.

Diabetic nephropathy: the proteinuria hypothesis

BACKGROUND/AIMS

Proteinuria, nearly a universal finding in progressive kidney disease, has been the subject of frequent recent analyses in the renal literature. Proteinuria is a hallmark of diabetic nephropathy: microalbuminuria is the principal early predictor for progression of diabetic glomerulopathy, and proteinuria may be viewed as a measure of the severity and promoter of progression of nephropathy.

METHODS

This article critically reviews for the first time the full scope of diabetic proteinuria--complex molecular mechanisms, natural history, and analysis of treatment trials--in order to address the validity of 'the proteinuria hypothesis', i.e., that diabetic proteinuria is a modifiable determinant of renal progression. This hypothesis is analyzed in detail, including recent studies on the primary therapy of diabetic nephropathy, renin-angiotensin blockade.

RESULTS

As fully developed, this hypothesis consists of three postulates: that higher amounts of proteinuria predict progressive loss of function, that proteinuria reduction correlates with slowing progression, and that proteinuria is a surrogate endpoint for clinical trials. The latter postulate has not before been adequately linked to growing information about the first two postulates as they apply to diabetic kidney disease.

CONCLUSION

While diabetic nephropathy is a disease model for the potential use of proteinuria as a surrogate marker for renal progression, this shift in perspective will require prospective data from additional clinical trials, particularly of non-renin-angiotensin blocking drugs, to be complete.