Nephrin phosphorylation regulates podocyte adhesion through the PINCH-1-ILK-α-parvin complex

How immunosuppressive drugs may directly target podocytes in glomerular diseases

Podocytes are the direct target of immunologic injury in many immune-mediated glomerular diseases, leading to proteinuria and subsequent kidney failure. Immunosuppressive agents such as steroids, calcineurin inhibitors, and rituximab are the commonly used treatment strategies in this context for their immunotherapeutic or anti-inflammatory properties. However, in recent years, studies have demonstrated that immunosuppressive agents can have a direct effect on podocytes, introducing the concept of the non-immunologic mechanism of kidney protection by immunomodulators. In this review, we focus on the mechanisms by which these agents may directly target the podocyte independent of their systemic effects and examine their clinical significance.

Pulmonary surfactants and the respiratory-renal connection in steroid-sensitive nephrotic syndrome of childhood

Steroid-sensitive nephrotic syndrome (SSNS) in childhood is usually due to minimal change disease (MCD). Unlike many glomerular conditions, SSNS/MCD is commonly precipitated by respiratory infections. Of interest, pulmonary inflammation releases surfactants in circulation which are soluble agonists of SIRPα, a podocyte receptor that regulates integrin signaling. Here, we characterized this pulmonary-renal connection in MCD and performed studies to determine its importance. Children with SSNS/MCD in relapse but not remission had elevated plasma surfactants and urinary SIRPα. Sera from relapsing subjects triggered podocyte SIRPα signaling via tyrosine phosphatase SHP-2 and nephrin dephosphorylation, a marker of podocyte activation. Further, addition of surfactants to MCD sera from patients in remission replicated these findings. Similarly, nasal instillation of toll-like receptor 3 and 4 agonists in mice resulted in elevated serum surfactants and their binding to glomeruli triggering proteinuria. Together, our data document a critical pulmonary-podocyte signaling pathway involving surfactants and SIRPα signaling in SSNS/MCD.

Nephrinuria and podocytopathies

The discovery of nephrin in 1998 has launched a new era in glomerular diseases research, emphasizing its crucial role in the structure and function of the glomerular filtration barrier. In the past 20 years, substantial advances have been made in understanding podocyte structure and function as well as the discovery of several podocyte-related proteins including nephrin. The glomerular filtration barrier is comprised of podocytes, the glomerular basement membrane and endothelial cells. Podocytes, with their specialized slit diaphragm, form the essential backbone of the glomerular filtration barrier. Nephrin is a crucial structural and functional feature of the slit diaphragm that prevents plasma protein, blood cell and macromolecule leakage into the urine. Podocyte damage results in nephrin release. Podocytopathies are kidney diseases in which podocyte damage drives proteinuria, i.e., nephrotic syndrome. Many kidney diseases involve podocytopathy including congenital nephrotic syndrome of Finnish type, diffuse mesangial sclerosis, minimal change disease, focal segmental glomerulosclerosis, collapsing glomerulonephropathy, diabetic nephropathy, lupus nephropathy, hypertensive nephropathy and preeclampsia. Recently, urinary nephrin measurement has become important in the early detection of podocytopathies. In this chapter, we elaborate the main structural and functional features of nephrin as a podocyte-specific protein, pathomechanisms of podocytopathies which result in nephrinuria, highlight the most commonly used methods for detecting urinary nephrin and investigate the diagnostic, prognostic and potential therapeutic relevance of urinary nephrin in primary and secondary proteinuric kidney diseases.

Patients with Proliferative Lupus Nephritis Have Autoantibodies That React to Moesin and Demonstrate Increased Glomerular Moesin Expression

Kidney involvement in systemic lupus erythematosus (SLE)—termed lupus nephritis (LN)—is a severe manifestation of SLE that can lead to end-stage kidney disease (ESKD). LN is characterized by immune complex deposition and inflammation in the glomerulus. We tested the hypothesis that autoantibodies targeting podocyte and glomerular cell proteins contribute to the development of immune complex formation in LN. We used Western blotting with SLE sera from patients with and without LN to identify target antigens in human glomerular and cultured human-derived podocyte membrane proteins. Using liquid chromatography-tandem mass spectrometry (LC-MS/MS), we identified the proteins in the gel regions corresponding to reactive bands observed with sera from LN patients. We identified 102 proteins that were present in both the podocyte and glomerular samples. We identified 10 high-probability candidates, including moesin, using bioinformatic analysis. Confirmation of moesin as a target antigen was conducted using immunohistochemical analysis (IHC) of kidney biopsy tissue and enzyme-linked immunosorbent assay (ELISA) to detect circulating antibodies. By IHC, biopsies from patients with proliferative lupus nephritis (PLN, class III/IV) demonstrated significantly increased glomerular expression of moesin (p < 0.01). By ELISA, patients with proliferative LN demonstrated significantly increased antibodies against moesin (p < 0.01). This suggests that moesin is a target glomerular antigen in lupus nephritis.

[Size exclusion-reverse liquid column chromatography-mass spectrometry and its application in the identification of post-translationally modified proteins in rat kidney]

Proteomics is an emerging field that has been shown to play a crucial role in unveiling the mechanisms underlying physiological and pathological processes, and liquid chromatography-mass spectrometry (LC-MS) is one of the most important methods employed in this field. However, in complex biological systems, such as eukaryotes, it is challenging to perform a comprehensive and unbiased proteome analysis due to the high complexity of biological samples and enormous differences in sample contents. For example, post-translational modifications (PTMs) in proteins are imperative for cell signaling, but post-translationally modified proteins account for about 1% of the total proteins in a single cell, making their identification extremely difficult. Therefore, chromatographic separation methods based on different principles are generally applied to reduce the complexity of biological samples and enrich trace proteins for their identification through mass spectrometry (MS). In this study, we developed a new proteomics method by combining size exclusion chromatography (SEC) and reversed-phase chromatography (RPLC), to separate and identify trace proteins in complex systems. SEC was used to separate and enrich kidney-specific proteins. After optimization of the method, it was found that 30 mmol/L of ammonium acetate could efficiently separate rat kidney proteins from the total protein fraction so that they could be eluted based on their relative molecular mass. Sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis and LC-MS results showed that our SEC separation method not only refined the protein composition of the biological sample but also enhanced the relative contents of trace proteins through multiple injections. The collected protein fractions were further concentrated through ultrafiltration centrifugation followed by freeze-drying, which further improved the recovery of trace proteins by approximately 90% and largely decreased the time required with the use of freeze-drying alone. Thereafter, five protein fractions were separately digested using trypsin, and the resultant peptides were further analyzed by reverse phase chromatography-MS analysis. In the RPLC column, the peptides were isolated mainly based on their hydrophobicity. As a result, by combining SEC and RPLC, 23621 peptides and 1345 proteins were identified from the kidney, with an increase in numbers by 69% and 27%, respectively, when compared to those obtained using the common 2D strong cation exchange (SCX)-RPLC-MS method. However, no significant difference was observed in the pI and grand average of hydropathicity (GRAVY) values. Gene ontology (GO) analysis revealed an increase in the number of proteins in each cell component, especially the membrane. Furthermore, identification of a higher rate of identified peptides than proteins suggested that the protein coverage was also improved, thereby facilitating the detection of PTM proteins. Consequently, five common PTMs in biological processes, including methylation, acetylation, carbamylation, oxidation, and phosphorylation, were examined and compared between the two methods. As expected, the number of post-translationally modified peptides identified using SEC-RPLC-MS were 1.7-1.9 times more than those determined using the SCX-RPLC-MS method. Especially for the identification of phosphorylated peptides, we could achieve the level of the targeted enrichment strategy; however no significant difference was observed in the extents of phosphorylation among serine, threonine, and tyrosine. These results further indicate that upon combining SEC and RPLC, high efficiency could be achieved by decreasing the complexity of the protein sample, and the identification was unbiased. Finally, the phosphorylation of some kidney proteins, such as spectrin, L-lactate dehydrogenase, and ATPases, was found, which is critical for their functions. In summary, the SEC-RPLC-MS approach was developed for the identification of rat kidney proteins and is especially applicable for the identification of PTM proteins. Using this method, the identification efficiency for PTM peptides increased significantly. Therefore, this method has potential for better understanding the impact of PTM on kidney proteins and further elucidating the potential mechanisms underlying its physiological and pathological functions.

Inflammation-induced PINCH expression leads to actin depolymerization and mitochondrial mislocalization in neurons

BACKGROUND

Diseases and disorders with a chronic neuroinflammatory component are often linked with changes in brain metabolism. Among neurodegenerative disorders, people living with human immunodeficiency virus (HIV) and Alzheimer's disease (AD) are particularly vulnerable to metabolic disturbances, but the mechanistic connections of inflammation, neurodegeneration and bioenergetic deficits in the central nervous system (CNS) are poorly defined. The particularly interesting new cysteine histidine-rich-protein (PINCH) is nearly undetectable in healthy mature neurons, but is robustly expressed in tauopathy-associated neurodegenerative diseases including HIV infection and AD. Although robust PINCH expression has been reported in neurons in the brains of patients with HIV and AD, the molecular mechanisms and cellular consequences of increased PINCH expression in CNS disease remain largely unknown.

METHODS

We investigated the regulatory mechanisms responsible for PINCH protein-mediated changes in bioenergetics, mitochondrial subcellular localization and bioenergetic deficits in neurons exposed to physiological levels of TNFα or the HIV protein Tat. Changes in the PINCH-ILK-Parvin (PIP) complex association with cofilin and TESK1 were assessed to identify factors responsible for actin depolymerization and mitochondrial mislocalization. Lentiviral and pharmacological inhibition experiments were conducted to confirm PINCH specificity and to reinstate proper protein-protein complex communication.

RESULTS

We identified MEF2A as the PINCH transcription factor in neuroinflammation and determined the biological consequences of increased PINCH in neurons. TNFα-mediated activation of MEF2A via increased cellular calcium induced PINCH, leading to disruption of the PIP ternary complex, cofilin activation by TESK1 inactivation, and actin depolymerization. The disruption of actin led to perinuclear mislocalization of mitochondria by destabilizing the kinesin-dependent mitochondrial transport machinery, resulting in impaired neuronal metabolism. Blocking TNFα-induced PINCH expression preserved mitochondrial localization and maintained metabolic functioning.

CONCLUSIONS

This study reported for the first time the mechanistic and biological consequences of PINCH expression in CNS neurons in diseases with a chronic neuroinflammation component. Our findings point to the maintenance of PINCH at normal physiological levels as a potential new therapeutic target for neurodegenerative diseases with impaired metabolisms.

The impact of steroids on the injured podocytes in nephrotic syndrome

Nephrotic syndrome (NS), a common chronic kidney disease, embraces a variety of kidney disorders. Though Glucocorticoids (GCs) are generally used in the treatment of NS, their mechanism of action is poorly understood. A plethora of evidence indicates that podocytes are considered as the main target cells for the therapeutic strategies to prevent NS. GCs regulate the transactivation and transrepression of genes in podocytes that affect their morphological and cytoskeletal features, motility, apoptosis and survival rate. Moreover, they prevent protein leakage through the glomerular barrier membrane by affecting the synthesis, trafficking and posttranslational modifications of slit diaphragms components, podocytes' intercellular junctions. The response to the treatment is variable among different ethnics and populations and resistance to the steroids is detected in almost 50% of adult patients. Not only do pharmacokinetics and pharmacogenetics of steroids play a role in GC resistance but also the genetic variations in one or more podocyte related genes are connected with the steroid resistance in cases with NS. The focus of this review is to explain the underlying cellular and molecular mechanisms of GCs in podocytes. Understanding the mechanisms by which the GCs and GCs receptors in podocytes regulate the gene expression network and crosstalk with other molecular pathways would guarantee an optimum therapeutic benefit of steroid treatment.

Original Research: Potential of urinary nephrin as a biomarker reflecting podocyte dysfunction in various kidney disease models

Urinary nephrin is a potential non-invasive biomarker of disease. To date, however, most studies of urinary nephrin have been conducted in animal models of diabetic nephropathy, and correlations between urinary nephrin-to-creatinine ratio and other parameters have yet to be evaluated in animal models or patients of kidney disease with podocyte dysfunction. We hypothesized that urinary nephrin-to-creatinine ratio can be up-regulated and is negatively correlated with renal nephrin mRNA levels in animal models of kidney disease, and that increased urinary nephrin-to-creatinine ratio levels are attenuated following administration of glucocorticoids. In the present study, renal nephrin mRNA, urinary nephrin-to-creatinine ratio, urinary protein-to-creatinine ratio, and creatinine clearance ratio were measured in animal models of adriamycin nephropathy, puromycin aminonucleoside nephropathy, anti-glomerular basement membrane glomerulonephritis, and 5/6 nephrectomy. The effects of prednisolone on urinary nephrin-to-creatinine ratio and other parameters in puromycin aminonucleoside (single injection) nephropathy rats were also investigated. In all models tested, urinary nephrin-to-creatinine ratio and urinary protein-to-creatinine ratio increased, while renal nephrin mRNA and creatinine clearance ratio decreased. Urinary nephrin-to-creatinine ratio exhibited a significant negative correlation with renal nephrin mRNA in almost all models, as well as a significant positive correlation with urinary protein-to-creatinine ratio and a significant negative correlation with creatinine clearance ratio. Urinary protein-to-creatinine ratio exhibited a significant negative correlation with renal nephrin mRNA. Following the administration of prednisolone to puromycin aminonucleoside (single injection) nephropathy rats, urinary nephrin-to-creatinine ratio was significantly suppressed and exhibited a significant positive correlation with urinary protein-to-creatinine ratio. In addition, the decrease in number of glomerular Wilms tumor antigen-1-positive cells was attenuated, and urinary nephrin-to-creatinine ratio exhibited a significant negative correlation in these cells. In conclusion, these results suggest that urinary nephrin-to-creatinine ratio level is a useful and reliable biomarker for predicting the amelioration of podocyte dysfunction by candidate drugs in various kidney disease models with podocyte dysfunction. This suggestion will also be validated in a clinical setting in future studies.

Nephrin Tyrosine Phosphorylation Is Required to Stabilize and Restore Podocyte Foot Process Architecture

Podocytes are specialized epithelial cells of the kidney blood filtration barrier that contribute to permselectivity via a series of interdigitating actin-rich foot processes. Positioned between adjacent projections is a unique cell junction known as the slit diaphragm, which is physically connected to the actin cytoskeleton via the transmembrane protein nephrin. Evidence indicates that tyrosine phosphorylation of the intracellular tail of nephrin initiates signaling events, including recruitment of cytoplasmic adaptor proteins Nck1 and Nck2 that regulate actin cytoskeletal dynamics. Nephrin tyrosine phosphorylation is altered in human and experimental renal diseases characterized by pathologic foot process remodeling, prompting the hypothesis that phosphonephrin signaling directly influences podocyte morphology. To explore this possibility, we generated and analyzed knockin mice with mutations that disrupt nephrin tyrosine phosphorylation and Nck1/2 binding (nephrin(Y3F/Y3F) mice). Homozygous nephrin(Y3F/Y3F) mice developed progressive proteinuria accompanied by structural changes in the filtration barrier, including podocyte foot process effacement, irregular thickening of the glomerular basement membrane, and dilated capillary loops, with a similar but later onset phenotype in heterozygous animals. Furthermore, compared with wild-type mice, nephrin(Y3F/Y3F) mice displayed delayed recovery in podocyte injury models. Profiling of nephrin tyrosine phosphorylation dynamics in wild-type mice subjected to podocyte injury indicated site-specific differences in phosphorylation at baseline, injury, and recovery, which correlated with loss of nephrin-Nck1/2 association during foot process effacement. Our results define an essential requirement for nephrin tyrosine phosphorylation in stabilizing podocyte morphology and suggest a model in which dynamic changes in phosphotyrosine-based signaling confer plasticity to the podocyte actin cytoskeleton.

Autoantibodies targeting glomerular annexin A2 identify patients with proliferative lupus nephritis

PURPOSE

Patients with systemic lupus erythematosus (SLE) frequently develop lupus nephritis (LN), a complication frequently leading to end stage kidney disease. Immune complex deposition in the glomerulus is central to the development of LN. Using a targeted proteomic approach, we tested the hypothesis that autoantibodies targeting glomerular antigens contribute to the development of LN.

EXPERIMENTAL DESIGN

Human podocyte and glomerular proteins were separated by SDS-PAGE and immunoblotted with sera from SLE patients with and without LN. The regions of those gels corresponding to reactive bands observed with sera from LN patients were analyzed using LC-MS/MS.

RESULTS

LN reactive bands were seen at approximately 50 kDa in podocyte extracts and between 36 and 50 kDa in glomerular extracts. Those bands were analyzed by LC-MS/MS and 102 overlapping proteins were identified. Bioinformatic analysis determined that 36 of those proteins were membrane associated, including a protein previously suggested to contribute to glomerulonephritis and LN, annexin A2. By ELISA, patients with proliferative LN demonstrated significantly increased antibodies against annexin A2.

CONCLUSION AND CLINICAL RELEVANCE

Proteomic approaches identified multiple candidate antigens for autoantibodies in patients with LN. Serum antibodies against annexin A2 were significantly elevated in subjects with proliferative LN, validating those antibodies as potential biomarkers.

Deep proteome mapping of mouse kidney based on OFFGel prefractionation reveals remarkable protein post- translational modifications

Performing a comprehensive nonbiased proteome analysis is an extraordinary challenge due to sample complexity and wide dynamic range, especially in eukaryotic tissues. Thus, prefractionation steps conducted prior to mass spectrometric analysis are critically important to reduce complex biological matrices and allow in-depth analysis. Here we demonstrated the use of OFFGel prefractionation to identify more low abundant and hydrophobic proteins than in a nonfractionated sample. Moreover, OFFGel prefractionation of a kidney protein sample was able to unveil protein functional relevance by detecting PTMs, especially when prefractionation was augmented with a targeted enrichment strategy such as TiO₂ phospho-enrichment. The OFFGel-TiO₂ combination used in this study was comparable to other global phosphoproteomics approaches (SCX-TiO₂, ERLIC-TiO₂, or HILIC-TiO₂). The detailed mouse kidney proteome with the phosphopeptide enrichment presented here serves as a useful platform for a better understanding of how the renal protein modification machinery works and, ultimately, will contribute to our understanding of pathological processes as well as normal physiological renal functions.