N-linked glycosylation is critical for the plasma membrane localization of nephrin

Partner, Neighbor, Housekeeper and Dimension: 3D versus 2D Glomerular Co-Cultures Reveal Drawbacks of Currently Used Cell Culture Models

Signaling-pathway analyses and the investigation of gene responses to different stimuli are usually performed in 2D monocultures. However, within the glomerulus, cells grow in 3D and are involved in direct and paracrine interactions with different glomerular cell types. Thus, the results from 2D monoculture experiments must be taken with caution. We cultured glomerular endothelial cells, podocytes and mesangial cells in 2D/3D monocultures and 2D/3D co-cultures and analyzed cell survival, self-assembly, gene expression, cell-cell interaction, and gene pathways using live/dead assay, time-lapse analysis, bulk-RNA sequencing, qPCR, and immunofluorescence staining. Without any need for scaffolds, 3D glomerular co-cultures self-organized into spheroids. Podocyte- and glomerular endothelial cell-specific markers and the extracellular matrix were increased in 3D co-cultures compared to 2D co-cultures. Housekeeping genes must be chosen wisely, as many genes used for the normalization of gene expression were themselves affected in 3D culture conditions. The transport of podocyte-derived VEGFA to glomerular endothelial cells confirmed intercellular crosstalk in the 3D co-culture models. The enhanced expression of genes important for glomerular function in 3D, compared to 2D, questions the reliability of currently used 2D monocultures. Hence, glomerular 3D co-cultures might be more suitable in the study of intercellular communication, disease modelling and drug screening ex vivo.

Characterizing Intraindividual Podocyte Morphology In Vitro with Different Innovative Microscopic and Spectroscopic Techniques

Podocytes are critical components of the glomerular filtration barrier, sitting on the outside of the glomerular basement membrane. Primary and secondary foot processes are characteristic for podocytes, but cell processes that develop in culture were not studied much in the past. Moreover, protocols for diverse visualization methods mostly can only be used for one technique, due to differences in fixation, drying and handling. However, we detected by single-cell RNA sequencing (scRNAseq) analysis that cells reveal high variability in genes involved in cell type-specific morphology, even within one cell culture dish, highlighting the need for a compatible protocol that allows measuring the same cell with different methods. Here, we developed a new serial and correlative approach by using a combination of a wide variety of microscopic and spectroscopic techniques in the same cell for a better understanding of podocyte morphology. In detail, the protocol allowed for the sequential analysis of identical cells with light microscopy (LM), Raman spectroscopy, scanning electron microscopy (SEM) and atomic force microscopy (AFM). Skipping the fixation and drying process, the protocol was also compatible with scanning ion-conductance microscopy (SICM), allowing the determination of podocyte surface topography of nanometer-range in living cells. With the help of nanoGPS Oxyo^®, tracking concordant regions of interest of untreated podocytes and podocytes stressed with TGF-β were analyzed with LM, SEM, Raman spectroscopy, AFM and SICM, and revealed significant morphological alterations, including retraction of podocyte process, changes in cell surface morphology and loss of cell-cell contacts, as well as variations in lipid and protein content in TGF-β treated cells. The combination of these consecutive techniques on the same cells provides a comprehensive understanding of podocyte morphology. Additionally, the results can also be used to train automated intelligence networks to predict various outcomes related to podocyte injury in the future.

Translation rate underpins specific targeting of N-terminal transmembrane proteins to mitochondria

Protein biogenesis is a complex process, and complexity is greatly increased in eukaryotic cells through specific targeting of proteins to different organelles. To direct targeting, organellar proteins carry an organelle-specific targeting signal for recognition by organelle-specific import machinery. However, the situation is confusing for transmembrane domain (TMD)-containing signal-anchored (SA) proteins of various organelles because TMDs function as an endoplasmic reticulum (ER) targeting signal. Although ER targeting of SA proteins is well understood, how they are targeted to mitochondria and chloroplasts remains elusive. Here, we investigated how the targeting specificity of SA proteins is determined for specific targeting to mitochondria and chloroplasts. Mitochondrial targeting requires multiple motifs around and within TMDs: a basic residue and an arginine-rich region flanking the N- and C-termini of TMDs, respectively, and an aromatic residue in the C-terminal side of the TMD that specify mitochondrial targeting in an additive manner. These motifs play a role in slowing down the elongation speed during translation, thereby ensuring mitochondrial targeting in a co-translational manner. By contrast, the absence of any of these motifs individually or together causes at varying degrees chloroplast targeting that occurs in a post-translational manner.

Mass spectrometry-based N-glycosylation analysis in kidney disease

Kidney disease is a global health concern with an enormous expense. It is estimated that more than 10% of the population worldwide is affected by kidney disease and millions of patients would progress to death prematurely and unnecessarily. Although creatinine detection and renal biopsy are well-established tools for kidney disease diagnosis, they are limited by several inevitable defects. Therefore, diagnostic tools need to be upgraded, especially for the early stage of the disease and possible progression. As one of the most common post-translational modifications of proteins, N-glycosylation plays a vital role in renal structure and function. Deepening research on N-glycosylation in kidney disease provides new insights into the pathophysiology and paves the way for clinical application. In this study, we reviewed recent N-glycosylation studies on several kidney diseases. We also summarized the development of mass spectrometric methods in the field of N-glycoproteomics and N-glycomics.

Inhibition of p38 MAPK decreases hyperglycemia-induced nephrin endocytosis and attenuates albuminuria

Abstract

Chronic hyperglycemia, as in diabetes mellitus, may cause glomerular damage with microalbuminuria as an early sign. Noteworthy, even acute hyperglycemia can increase glomerular permeability before structural damage of the glomerular filter can be detected. Despite intensive research, specific antiproteinuric therapy is not available so far. Thus, a deeper understanding of the molecular mechanisms of albuminuria is desirable. P38 MAPK signaling is involved in the development of hyperglycemia-induced albuminuria. However, the mechanism of increased p38 MAPK activity leading to increased permeability and albuminuria remained unclear. Recently, we demonstrated that acute hyperglycemia triggers endocytosis of nephrin, the key molecule of the slit diaphragm, and induces albuminuria. Here, we identify p38 MAPK as a pivotal regulator of hyperglycemia-induced nephrin endocytosis. Activated p38 MAPK phosphorylates the nephrin c-terminus at serine 1146, facilitating the interaction of PKCα with nephrin. PKCα phosphorylates nephrin at threonine residues 1120 and 1125, mediating the binding of β-arrestin2 to nephrin. β-arrestin2 triggers endocytosis of nephrin by coupling it to the endocytic machinery, leading to increased glomerular permeability. Pharmacological inhibition of p38 MAPK preserves nephrin surface expression and significantly attenuates albuminuria.

Key messages

Acute hyperglycemia triggers endocytosis of nephrin.

Activated p38 MAPK phosphorylates the nephrin c-terminus at serine 1146, facilitating the interaction of PKCα with nephrin.

PKCα phosphorylates nephrin at threonine residues 1120 and 1125, mediating the binding of β-arrestin2 to nephrin.

β-arrestin2 triggers endocytosis of nephrin by coupling it to the endocytic machinery, leading to a leaky glomerular filter.

Pharmacological inhibition of p38 MAPK preserves nephrin surface expression and significantly attenuates albuminuria under hyperglycemic conditions.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00109-022-02184-5.

Renal cell markers: lighthouses for managing renal diseases

Kidneys, one of the vital organs in our body, are responsible for maintaining whole body homeostasis. The complexity of renal function (e.g., filtration, reabsorption, fluid and electrolyte regulation, and urine production) demands diversity not only at the level of cell types but also in their overall distribution and structural framework within the kidney. To gain an in depth molecular-level understanding of the renal system, it is imperative to discern the components of kidney and the types of cells residing in each of the subregions. Recent developments in labeling, tracing, and imaging techniques have enabled us to mark, monitor, and identify these cells in vivo with high efficiency in a minimally invasive manner. In this review, we summarize different cell types, specific markers that are uniquely associated with those cell types, and their distribution in the kidney, which altogether make kidneys so special and different. Cellular sorting based on the presence of certain proteins on the cell surface allowed for the assignment of multiple markers for each cell type. However, different studies using different techniques have found contradictions in cell type-specific markers. Thus, the term "cell marker" might be imprecise and suboptimal, leading to uncertainty when interpreting the data. Therefore, we strongly believe that there is an unmet need to define the best cell markers for a cell type. Although the compendium of renal-selective marker proteins presented in this review is a resource that may be useful to researchers, we acknowledge that the list may not be necessarily exhaustive.

Crumbs2 Is an Essential Slit Diaphragm Protein of the Renal Filtration Barrier

BACKGROUND

Crumbs2 is expressed at embryonic stages as well as in the retina, brain, and glomerular podocytes. Recent studies identified CRB2 mutations as a novel cause of steroid-resistant nephrotic syndrome (SRNS).

METHODS

To study the function of Crb2 at the renal filtration barrier, mice lacking Crb2 exclusively in podocytes were generated. Gene expression and histologic studies as well as transmission and scanning electron microscopy were used to analyze these Crb2^podKO knockout mice and their littermate controls. Furthermore, high-resolution expansion microscopy was used to investigate Crb2 distribution in murine glomeruli. For pull-down experiments, live cell imaging, and transcriptome analyses, cell lines were applied that inducibly express fluorescent protein-tagged CRB2 wild type and mutants.

RESULTS

Crb2^podKO mice developed proteinuria directly after birth that preceded a prominent development of disordered and effaced foot processes, upregulation of renal injury and inflammatory markers, and glomerulosclerosis. Pull-down assays revealed an interaction of CRB2 with Nephrin, mediated by their extracellular domains. Expansion microscopy showed that in mice glomeruli, Crb2 and Nephrin are organized in adjacent clusters. SRNS-associated CRB2 protein variants and a mutant that lacks a putative conserved O-glycosylation site were not transported to the cell surface. Instead, mutants accumulated in the ER, showed altered glycosylation pattern, and triggered an ER stress response.

CONCLUSIONS

Crb2 is an essential component of the podocyte's slit diaphragm, interacting with Nephrin. Loss of slit diaphragm targeting and increasing ER stress are pivotal factors for onset and progression of CRB2-related SRNS.

Endoplasmic reticulum-associated degradation is required for nephrin maturation and kidney glomerular filtration function

Podocytes are key to the glomerular filtration barrier by forming a slit diaphragm between interdigitating foot processes; however, the molecular details and functional importance of protein folding and degradation in the ER remain unknown. Here, we show that the SEL1L-HRD1 protein complex of ER-associated degradation (ERAD) is required for slit diaphragm formation and glomerular filtration function. SEL1L-HRD1 ERAD is highly expressed in podocytes of both mouse and human kidneys. Mice with podocyte-specific Sel1L deficiency develop podocytopathy and severe congenital nephrotic syndrome with an impaired slit diaphragm shortly after weaning and die prematurely, with a median lifespan of approximately 3 months. We show mechanistically that nephrin, a type 1 membrane protein causally linked to congenital nephrotic syndrome, is an endogenous ERAD substrate. ERAD deficiency attenuated the maturation of nascent nephrin, leading to its retention in the ER. We also show that various autosomal-recessive nephrin disease mutants were highly unstable and broken down by SEL1L-HRD1 ERAD, which attenuated the pathogenicity of the mutants toward the WT allele. This study uncovers a critical role of SEL1L-HRD1 ERAD in glomerular filtration barrier function and provides insights into the pathogenesis associated with autosomal-recessive disease mutants.

Loss of slit protein nephrin is associated with reduced antioxidant superoxide dismutase expression in podocytes shed from women with preeclampsia

Recent findings of podocyte shedding/podocyturia highlight the central significance of podocyte injury in preeclampsia, a hypertensive disorder unique to human pregnancy. To test the hypothesis that oxidative stress contributes to kidney podocyte injury in preeclampsia, we specifically examined expression and distribution of antioxidant CuZn‐SOD with nephrin and podoplanin in shed podocytes from women with preeclampsia. Human podocyte AB 8/13 cells served as control. We found that CuZn‐SOD was localized at the front/outreach region of nephrin at the cell periphery (foot process areas) in control podocytes and expression of CuZn‐SOD, nephrin, and podoplanin were all dislocated or lost in shed podocytes from preeclamptic patients. We further tested oxidative stress‐induced nephrin shedding in podocytes, in which AB 8/13 podocytes were cultured under lowered oxygen condition (2%O₂) or treated with hypoxic mimicking agent cobalt chloride. Our results showed that reduced nephrin and podoplanin expression were associated with downregulation of CuZn‐SOD expression in podocytes when cells were cultured under lowered oxygen or hypoxic conditions. Nephrin shed in urinary specimen from preeclamptic women was also determined by immunoprecipitation/immunoblotting. The molecular sizes of nephrin that corresponded to that were lost when cells were cultured under hypoxic conditions. We concluded that increased oxidative stress plays a significant role in inducing podocyte protein shedding in preeclampsia.

Glycosylation in health and disease

The glycome describes the complete repertoire of glycoconjugates composed of carbohydrate chains, or glycans, that are covalently linked to lipid or protein molecules. Glycoconjugates are formed through a process called glycosylation and can differ in their glycan sequences, the connections between them and their length. Glycoconjugate synthesis is a dynamic process that depends on the local milieu of enzymes, sugar precursors and organelle structures as well as the cell types involved and cellular signals. Studies of rare genetic disorders that affect glycosylation first highlighted the biological importance of the glycome, and technological advances have improved our understanding of its heterogeneity and complexity. Researchers can now routinely assess how the secreted and cell-surface glycomes reflect overall cellular status in health and disease. In fact, changes in glycosylation can modulate inflammatory responses, enable viral immune escape, promote cancer cell metastasis or regulate apoptosis; the composition of the glycome also affects kidney function in health and disease. New insights into the structure and function of the glycome can now be applied to therapy development and could improve our ability to fine-tune immunological responses and inflammation, optimize the performance of therapeutic antibodies and boost immune responses to cancer. These examples illustrate the potential of the emerging field of 'glycomedicine'.

Modulation of Endocannabinoid-Binding Receptors in Human Neuroblastoma Cells by Tunicamycin

Endocannabinoid (eCB)-binding receptors can be modulated by several ligands and membrane environment, yet the effect of glycosylation remains to be assessed. In this study, we used human neuroblastoma SH-SY5Y cells to interrogate whether expression, cellular localization, and activity of eCB-binding receptors may depend on N-linked glycosylation. Following treatment with tunicamycin (a specific inhibitor of N-linked glycosylation) at the non-cytotoxic dose of 1 µg/mL, mRNA, protein levels and localization of eCB-binding receptors, as well as N-acetylglucosamine (GlcNAc) residues, were evaluated in SH-SY5Y cells by means of quantitative real-time reverse transcriptase-polymerase chain reaction (qRT-PCR), fluorescence-activated cell sorting (FACS), and confocal microscopy, respectively. In addition, the activity of type-1 and type-2 cannabinoid receptors (CB₁ and CB₂) was assessed by means of rapid binding assays. Significant changes in gene and protein expression were found upon tunicamycin treatment for CB₁ and CB₂, as well as for GPR55 receptors, but not for transient receptor potential vanilloid 1 (TRPV1). Deglycosylation experiments with N-glycosidase-F and immunoblot of cell membranes derived from SH-SY5Y cells confirmed the presence of one glycosylated form in CB₁ (70 kDa), that was reduced by tunicamycin. Morphological studies demonstrated the co-localization of CB₁ with GlcNAc residues, and showed that tunicamycin reduced CB₁ membrane expression with a marked nuclear localization, as confirmed by immunoblotting. Cleavage of the carbohydrate side chain did not modify CB receptor binding affinity. Overall, these results support N-linked glycosylation as an unprecedented post-translational modification that may modulate eCB-binding receptors’ expression and localization, in particular for CB₁.

Complex N-glycan promotes CD133 mono-ubiquitination and secretion

CD133 is a widely used cell surface marker of cancer stem cells that plays an important role in tumor initiation and metastasis. Increasing evidence shows that CD133 is secreted to the extracellular space. However, the underlying mechanisms of CD133 secretion remain largely unknown. In this study, we report that secreted CD133 has a complex-type N-glycosylation and is modified by beta1,6GlcNAc N-glycan. We found that inhibition of CD133 complex-type N-glycosylation by swainsonine does not affect the membrane localization of CD133, but significantly reduces CD133 secretion and promotes its accumulation in early endosomes. Moreover, swainsonine reduces CD133 secretion by reducing its mono-ubiquitination and inhibiting the interaction between CD133 and Tsg101. These findings reveal a new mechanism of glycosylation-dependent secretion of CD133.

Mitotic Catastrophe Causes Podocyte Loss in the Urine of Human Diabetics

Mitotic catastrophe (MC) is a major cause of podocyte loss in vitro and in vivo. We evaluated urine samples (n = 184 urine samples from diabetic patients; n = 41 patients) from diabetic patients and determined the presence of podocytes in the urine and studied their characteristics, specifically asking whether apoptosis versus MC is present. We also evaluated diabetic glomeruli in renal biopsy specimens by electron microscopy (n = 54). A battery of stains including the antibody to podocalyxin (PCX) were used. PCX and podocytes (PCX+podo) showed nuclear morphologies such as a i) mononucleated normal shape (8.7%), ii) large and abnormal shape (3.8%), iii) multinucleated with or without micronucleoli (31.2%), iv) mitotic spindles (8.2%), v) single nucleus and denucleation combined (10.3%), and vi) denucleation only (37.0%). Large size/abnormal shape, multinucleation, mitotic spindles, and a combination of single nucleus and denucleation were considered features of MC (53.5%). Dual staining of PCX+podo was positive for Glepp 1 (50%), whereas none of PCX+podo were positive for nephrin, podocin, leukocyte, or parietal epithelial cell markers (cytokeratin 8), annexin V, cleaved caspase-3, and terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling. Ten percent of PCX+podo were positive for phosphorylated vimentin. Electron microscopy identified cellular and nuclear podocyte changes characteristic of MC. The majority of urine podocytes in diabetic patients showed MC, not apoptosis. This noninvasive approach may be clinically useful in determining progressive diabetic nephropathy or response to therapeutic intervention.

Association of crumbs homolog-2 with mTORC1 in developing podocyte

The evidence that gene mutations in the polarity determinant Crumbs homologs-2 (CRB2) cause congenital nephrotic syndrome suggests the functional importance of this gene product in podocyte development. Because another isoform, CRB3, was reported to repress the mechanistic/mammalian target of the rapamycin complex 1 (mTORC1) pathway, we examined the role of CRB2 function in developing podocytes in relation to mTORC1. In HEK-293 and MDCK cells constitutively expressing CRB2, we found that the protein localized to the apicolateral side of the cell plasma membrane and that this plasma membrane assembly required N-glycosylation. Confocal microscopy of the neonate mouse kidney revealed that both the tyrosine-phosphorylated form and non-phosphorylated form of CRB2 commence at the S-shaped body stage at the apicolateral side of podocyte precursor cells and move to foot processes in a capillary tuft pattern. The pattern of phosphorylated mTOR in developing podocytes was similar to that of CRB2 tyrosine phosphorylation. Additionally, the lack of a tyrosine phosphorylation site on CRB2 led to the reduced sensitivity of mTORC1 activation in response to energy starvation. CRB2 may play an important role in the mechanistic pathway of developing podocytes through tyrosine phosphorylation by associating with mTORC1 activation.

Taurine Supplementation Alleviates Puromycin Aminonucleoside Damage by Modulating Endoplasmic Reticulum Stress and Mitochondrial-Related Apoptosis in Rat Kidney

Taurine (TAU) is a sulfur-containing beta amino acid that is not involved in protein composition and anabolism, conditionally essential in mammals provided through diet. Growing evidence supports a protective role of TAU supply in osmoregulation, calcium flux, and reduction of inflammation and oxidant damage in renal diseases like diabetes. Endoplasmic reticulum (ER) stress, due to abnormal proteostasis, is a contributor to nephrotic syndrome and related renal damage. Here, we investigated the effect of dietary TAU (1.5% in drinking water for 15 days) in an established rat model that mimics human minimal change nephrosis, consisting of a single puromycin aminonucleoside (PAN) injection (intraperitoneally 15 mg/100 g body weight), with sacrifice after eight days. TAU limited proteinuria and podocytes foot processes effacement, and balanced slit diaphragm nephrin and glomerular claudin 1 expressions. In cortical proximal tubules, TAU improved lysosomal density, ER perimeter, restored proper ER-mitochondria tethering and mitochondrial cristae, and decreased inflammation. Remarkably, TAU downregulated glomerular ER stress markers (GRP78, GRP94), pro-apoptotic C/EBP homologous protein, activated caspase 3, tubular caspase1, and mitochondrial chaperone GRP75, but maintained anti-apoptotic HSP25. In conclusion, TAU, by targeting upstream ER stress separate from mitochondria dysfunctions at crucial renal sites, might be a promising dietary supplement in the treatment of the drug-resistant nephrotic syndrome.

A novel in vivo method to quantify slit diaphragm protein abundance in murine proteinuric kidney disease

Injury of the glomerular filter causes proteinuria by disrupting the sensitive interplay of the glomerular protein network. To date, studies of the expression and trafficking of glomerular proteins have been mostly limited to in vitro or histologic studies. Here, we report a novel in vivo biotinylation assay that allows the quantification of surface expression of glomerular proteins in mice. Kidneys were perfused in situ with biotin before harvest. Afterwards glomeruli were isolated and lyzed. The protein of interest was separated by immunoprecipitation and the amount of surface-expressed protein was quantified by Western blot analysis with streptavidin staining. As proof-of-concept, we examined the presence of nephrin in the slit diaphragm in two well-established murine models of proteinuric kidney disease: nephrotoxic nephritis and adriamycin nephropathy. In proteinuric animals, significantly less nephrin was detected in the slit diaphragm. When proteinuria decreased once again during the course of disease, the amount of surface nephrin returned to the baseline. Our present results suggest that our assay is a valuable tool to study the glomerular filter in proteinuric kidney diseases. Note that the assay is not limited to proteins expressed in the slit diaphragm, and all surface proteins that are accessible to biotin perfusion and immunoprecipitation qualify for this analysis.

Wolf-Hirschhorn syndrome candidate 1-like 1 epigenetically regulates nephrin gene expression

Altered expression of nephrin underlies the pathophysiology of proteinuria in both congenital and acquired nephrotic syndrome. However, the epigenetic mechanisms of nephrin gene regulation remain elusive. Here, we show that Wolf-Hirschhorn syndrome candidate 1-like 1 long form (WHSC1L1-L) is a novel epigenetic modifier of nephrin gene regulation. WHSC1L1-L was associated with histone H3K4 and H3K36 in human embryonic kidney cells. WHSC1L1-L gene was expressed in the podocytes, and functional protein product was detected in these cells. WHSC1L1-L was found to bind nephrin but not other podocyte-specific gene promoters, leading to its inhibition/suppression, abrogating the stimulatory effect of WT1 and NF-κB. Gene knockdown of WHSC1L1-L in primary cultured podocytes accelerated the transcription of nephrin but not CD2AP. An in vivo zebrafish study involving the injection of Whsc1l1 mRNA into embryos demonstrated an apparent reduction of nephrin mRNA but not podocin and CD2AP mRNA. Immunohistochemistry showed that both WHSC1L1-L and nephrin emerged at the S-shaped body stage in glomeruli. Immunofluorescence and confocal microscopy displayed WHSC1L1 to colocalize with trimethylated H3K4 in the glomerular podocytes. Chromatin immunoprecipitation assay revealed the reduction of the association of trimethylated H3K4 at the nephrin promoter regions. Finally, nephrin mRNA was upregulated in the glomerulus at the early proteinuric stage of mouse nephrosis, which was associated with the reduction of WHSC1L1. In conclusion, our results demonstrate that WHSC1L1-L acts as a histone methyltransferase in podocytes and regulates nephrin gene expression, which may in turn contribute to the integrity of the slit diaphragm of the glomerular filtration barrier.

Properties of genes essential for mouse development

Essential genes are those that are critical for life. In the specific case of the mouse, they are the set of genes whose deletion means that a mouse is unable to survive after birth. As such, they are the key minimal set of genes needed for all the steps of development to produce an organism capable of life ex utero. We explored a wide range of sequence and functional features to characterise essential (lethal) and non-essential (viable) genes in mice. Experimental data curated manually identified 1301 essential genes and 3451 viable genes. Very many sequence features show highly significant differences between essential and viable mouse genes. Essential genes generally encode complex proteins, with multiple domains and many introns. These genes tend to be: long, highly expressed, old and evolutionarily conserved. These genes tend to encode ligases, transferases, phosphorylated proteins, intracellular proteins, nuclear proteins, and hubs in protein-protein interaction networks. They are involved with regulating protein-protein interactions, gene expression and metabolic processes, cell morphogenesis, cell division, cell proliferation, DNA replication, cell differentiation, DNA repair and transcription, cell differentiation and embryonic development. Viable genes tend to encode: membrane proteins or secreted proteins, and are associated with functions such as cellular communication, apoptosis, behaviour and immune response, as well as housekeeping and tissue specific functions. Viable genes are linked to transport, ion channels, signal transduction, calcium binding and lipid binding, consistent with their location in membranes and involvement with cell-cell communication. From the analysis of the composite features of essential and viable genes, we conclude that essential genes tend to be required for intracellular functions, and viable genes tend to be involved with extracellular functions and cell-cell communication. Knowledge of the features that are over-represented in essential genes allows for a deeper understanding of the functions and processes implemented during mammalian development.

Role of calcineurin (CN) in kidney glomerular podocyte: CN inhibitor ameliorated proteinuria by inhibiting the redistribution of CN at the slit diaphragm

Although calcineurin (CN) is distributed in many cell types and functions in regulating cell functions, the precise roles of CN remained in each type of the cells are not well understood yet. A CN inhibitor (CNI) has been used for steroid‐resistant nephrotic syndrome. A CNI is assumed to ameliorate proteinuria by preventing the overproduction of T‐cell cytokines. However, recent reports suggest that CNI has a direct effect on podocyte. It is accepted that a slit diaphragm (SD), a unique cell–cell junction of podocytes, is a critical barrier preventing a leak of plasma protein into urine. Therefore, we hypothesized that CNI has an effect on the SD. In this study, we analyzed the expression of CN in physiological and in the nephrotic model caused by the antibody against nephrin, a critical component of the SD. We observed that CN is expressed at the SD in normal rat and human kidney sections and has an interaction with nephrin. The staining of CN at the SD was reduced in the nephrotic model, while CN activity in glomeruli was increased. We also observed that the treatment with tacrolimus, a CNI, in this nephrotic model suppressed the redistribution of CN, nephrin, and other SD components and ameliorated proteinuria. These observations suggested that the redistribution and the activation of CN may participate in the development of the SD injury.

Diabetic kidney disease: a role for advanced glycation end-product receptor 1 (AGE-R1)?

Diabetic patients are postulated to be in a perpetual state of oxidative stress and inflammation at sites where chronic complications occur. The accumulation of AGEs derived from both endogenous and exogenous sources (such as the diet) have been implicated in the development and progression of diabetic complications, particularly nephropathy. There has been some interest in investigating the potential for reducing the AGE burden in chronic disease, through the action of AGE "clearance" receptors, such as the advanced glycation end-product receptor 1 (AGE-R1). Reducing the burden of AGEs has been linked to attenuation of inflammation, slower progression of diabetic complications (in particular vascular and renal complications) and has been shown to extend lifespan. To date, however, there have been no direct investigations into whether AGE-R1 has any role in modulating normal kidney function, or specifically during the development and progression of diabetes. This mini-review will focus on the recent advances in knowledge around the mechanistic function of AGE-R1 and the implications of this for the pathogenesis of diabetic kidney disease.

An update: the role of Nephrin inside and outside the kidney

Nephrin is a key molecule in podocytes to maintain normal slit diaphragm structure. Nephin interacts with many other podocyte and slit diaphragm protein and also mediates important cell signaling pathways in podocytes. Loss of nephrin during the development leads to the congenital nephrotic syndrome in children. Reduction of nephrin expression is often observed in adult kidney diseases including diabetic nephropathy and HIV-associated nephropathy. The critical role of nephrin has been confirmed by different animal models with nephrin knockout and knockdown. Recent studies demonstrate that knockdown of nephrin expression in adult mice aggravates the progression of unilateral nephrectomy and Adriamycin-induced kidney disease. In addition to its critical role in maintaining normal glomerular filtration unit in the kidney, nephrin is also expressed in other organs. However, the exact role of nephrin in kidney and extra-renal organs has not been well characterized. Future studies are required to determine whether nephrin could be developed as a drug target to treat patients with kidney disease.

Astragaloside IV attenuates proteinuria in streptozotocin-induced diabetic nephropathy via the inhibition of endoplasmic reticulum stress

Background

Diabetic nephropathy (DN) is a major cause of Chronic Kidney Disease and End-Stage Renal Disease throughout the world; however, the reversibility of diabetic nephropathy remains controversial. Endoplasmic reticulum (ER) stress plays an important role in the pathogenesis of DN. Astragaloside IV (AS-IV) is derived from Astragalus membranaceus (Fisch) Bge, a widely used traditional herbal medicine in China, and has diverse pharmacological activities including the attenuation of podocyte injury and amelioration of proteinuria in idiopathic nephrotic syndrome. The present study aimed to investigate the effect and mechanism of AS-IV on proteinuria in the rat streptozotocin (STZ)-induced model of diabetes.

Methods

Male Sprague–Dawley (SD) rats were randomly divided into four groups: normal control (Normal group), diabetic nephropathy (Model group), diabetic nephropathy plus AS-IV treatment (AS-IV group) and diabetic nephropathy plus 4-phenyl butyric acid treatment (PBA group). ER stress was induced in cultured human podocytes, pretreated with or without AS-IV, with tunicamycin (TM). At the end of 8 weeks, serum creatinine (Scr), blood urea nitrogen (BUN) and 24-hour urinary protein excretion rate (UAER) were determined. Renal morphology was examined after periodic acid-Schiff staining of kidney sections. Apoptosis of podocytes was measured by flow cytometry. The total expression and phosphorylation of eIF2α, PERK and JNK, and the expression of CHOP and cleaved caspase-3 were determined by western blotting. The expression of glucose-regulated protein 78 (GRP78) and 150 kDa oxygen-regulated protein (ORP150) mRNA and protein was determined by real-time PCR and western blotting respectively.

Results

AS-IV treatment significantly reduced urinary albumin excretion, plasma creatinine and blood urea nitrogen levels, and prevented the mesangial matrix expansion and increase in mean mesangial induced by STZ. AS-IV also prevented the phosphorylation of eIF2α, PERK and JNK, and inhibited the expression of GRP78 and ORP150 markedly, both in vivo and in vitro. AS-IV inhibited the TM-induced apoptosis of podocytes, concomitant with decreased CHOP expression and cleaved caspase-3.

Conclusions

This study supports the hypothesis that AS-IV reduces proteinuria and attenuates diabetes, which is associated with decreased ER stress. This might be an important mechanism in the renoprotective function of AS-IV in the pathogenesis of DN.

Glycosylation patterns of kidney proteins differ in rat diabetic nephropathy

Diabetic nephropathy often progresses to end-stage kidney disease and, ultimately, to renal replacement therapy. Hyperglycemia per se is expected to have a direct impact on the biosynthesis of N- and O-linked glycoproteins. This study aims to establish the link between protein glycosylation and progression of experimental diabetic kidney disease using orthogonal methods. Kidneys of streptozotocin-diabetic and control rats were harvested at three different time points post streptozotocin injection. A panel of 12 plant lectins was used in the screening of lectin blots. The lectins UEAI, PHA-E, GSI, PNA, and RCA identified remarkable disease-associated differences in glycoprotein expression. Lectin affinity chromatography followed by mass spectrometric analyses led to the identification of several glycoproteins involved in salt-handling, angiogenesis, and extracellular matrix degradation. Our data confirm a substantial link between glycosylation signature and diabetes progression. Furthermore, as suggested by our findings on dipeptidyl peptidase-IV, altered protein glycosylation may reflect changes in biochemical properties such as enzymatic activity. Thus, our study demonstrates the unexplored potential of protein glycosylation analysis in the discovery of molecules linked to diabetic kidney disease.

Stress in the kidney is the road to pERdition: is endoplasmic reticulum stress a pathogenic mediator of diabetic nephropathy?

The endoplasmic reticulum (ER) is an organelle that primarily functions to synthesise new proteins and degrade old proteins. Owing to the continual and variable nature of protein turnover, protein synthesis is inherently an error-prone process and is therefore tightly regulated. Fortunately, if this balance between synthesis and degradation is perturbed, an intrinsic response, the unfolded protein response (UPR) is activated to restore ER homoeostasis through the action of inositol-requiring protein 1, activating transcription factor 6 and PKR-like ER kinase transmembrane sensors. However, if the UPR is oversaturated and misfolded proteins accumulate, the ER can shift into a cytotoxic response, a physiological phenomenon known as ER stress. The mechanistic pathways of the UPR have been extensively explored; however, the role of this process in such a synthetic organ as the kidney requires further clarification. This review will focus on these aspects and will discuss the role of ER stress in specific resident kidney cells and how this may be integral in the pathogenesis and progression of diabetic nephropathy (DN). Given that diabetes is a perturbed state of protein turnover in most tissues, it is important to understand if ER stress is a secondary or tertiary response to other changes within the diabetic milieu or if it is an independent accelerator of kidney disease. Modulators of ER stress could provide a valuable tool for the treatment of DN and are under active investigation in other contexts.

3D-ultrastructure, functions and stress responses of gastropod (Biomphalaria glabrata) rhogocytes

Rhogocytes are pore cells scattered among the connective tissue of different body parts of gastropods and other molluscs, with great variation in their number, shape and size. They are enveloped by a lamina of extracellular matrix. Their most characteristic feature is the "slit apparatus", local invaginations of the plasma membrane bridged by cytoplasmic bars, forming slits of ca. 20 nm width. A slit diaphragm creates a molecular sieve with permeation holes of 20×20 nm. In blue-blooded gastropods, rhogocytes synthesize and secrete the respiratory protein hemocyanin, and it has been proposed-though not proven-that in the rare red-blooded snail species they might synthesize and secrete the hemoglobin. However, the cellular secretion pathway for respiratory proteins, and the functional role(s) of the enigmatic rhogocyte slit apparatus are still unclear. Additional functions for rhogocytes have been proposed, notably a role in protein uptake and degradation, and in heavy metal detoxification. Here we provide new structural and functional information on the rhogocytes of the red-blooded freshwater snail Biomphalaria glabrata. By in situ hybridization of mantle tissues, we prove that rhogocytes indeed synthesize hemoglobin. By electron tomography, the first three dimensional (3D) reconstructions of the slit apparatus are provided, showing detail of highly dense material in the cytoplasmic bars close to the slits. By immunogold labelling, we collected evidence that a major component of this material is actin. By genome databank mining, the complete sequence of a B. glabrata nephrin was obtained, and localized to the rhogocytes by immunofluorescence microscopy. The presence of both proteins fit the ultrastructure-based hypothesis that rhogocytes are related to mammalian podocytes and insect nephrocytes. Reactions of the rhogocytes to deprivation of food and cadmium toxification are also documented, and a possible secretion pathway of newly synthesized respiratory proteins through the slit apparatus is discussed.

SUMOylation determines turnover and localization of nephrin at the plasma membrane

Podocyte effacement and the reformation of foot processes and slit diaphragms can be induced within minutes experimentally. Therefore, it seems likely that the slit diaphragm proteins underlie orchestrated recycling mechanisms under the control of posttranslational modifiers. One of these modifiers, SUMO (small ubiquitin-like modifier), is an ubiquitin-like protein with a 20% corresponding identity to ubiquitin. Modification by SUMOs to proteins on lysine residues can block the ubiquitination of the same site leading to the stabilization of the target protein. Here we found in vitro and in vivo that nephrin is a substrate modified by SUMO proteins thereby increasing its steady-state level and expression at the plasma membrane. A conversion of lysines to arginines at positions 1114 and 1224 of the intracellular tail of murine nephrin led to decreased stability of nephrin, decreased expression at the plasma membrane, and decreased PI3K/AKT signaling. Furthermore, treatment of podocytes with the SUMOylation inhibitor ginkgolic acid led to reduced membrane expression of nephrin. Similarly, the conversion of lysine to arginine at position 1100 of human nephrin caused decreased stability and expression at the plasma membrane. As SUMOylation is a reversible process, our results suggest that SUMOylation participates in the tight orchestration of nephrin turnover at the slit diaphragm.

Yi qi qing re gao attenuates podocyte injury and inhibits vascular endothelial growth factor overexpression in puromycin aminonucleoside rat model

Proteinuria is the hallmark of chronic kidney disease. Podocyte damage underlies the formation of proteinuria, and vascular endothelial growth factor (VEGF) functions as an autocrine/paracrine regulator. Yi Qi Qing Re Gao (YQQRG) has been used to treat proteinuria for more than two decades. The objective of this study was to investigate the protective effect and possible mechanisms of YQQRG on puromycin aminonucleoside (PAN) rat model. Eighty male Sprague-Dawley rats were randomized into sham group, PAN group, PAN + YQQRG group, and PAN + fosinopril group. Treatments were started 7 days before induction of nephrosis (a single intravenous injection of 40 mg/kg PAN) until day 15. 24 h urinary samples were collected on days 5, 9, and 14. The animals were sacrificed on days 3, 10, and 15, respectively. Blood samples and renal tissues were obtained for detection of biochemical and molecular biological parameters. YQQRG significantly reduced proteinuria, elevated serum albumin, and alleviated renal pathological lesions. YQQRG inhibited VEGF-A, nephrin, podocin, and CD2AP mRNA expression and elevated nephrin, podocin, and CD2AP protein levels starting on day 3. In conclusion, YQQRG attenuates podocyte injury in the rat PAN model through downregulation of VEGF-A and restoration of nephrin, podocin, and CD2AP protein expression.

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.

Exome analysis resolves differential diagnosis of familial kidney disease and uncovers a potential confounding variant

A girl aged 6 presented with haematuria and her sister (aged 5) presented with haematuria and proteinuria. Family history showed multiple individuals suffering from end stage renal failure from the paternal side of the pedigree. Following kidney biopsy in the father and paternal grandmother, the pathological diagnosis was of focal segmental glomerulosclerosis (FSGS). Exome sequencing was undertaken in the proband's sister and grandmother. Genetic variants shared by both affected individuals were interrogated to identify the genetic cause of disease. Candidate variants were then sequenced in all the family members to determine segregation with the disease. A mutation of COL4A5 known to cause Alport syndrome segregated with disease from the paternal side of the pedigree and a variant in NPHS1 was present in both paediatric cases and inherited from their mother. This study highlights the advantages of exome sequencing over single gene testing; disease presentation can be heterogeneous with several genes representing plausible candidates; candidate gene(s) may be unavailable as a diagnostic test; consecutive, single gene testing typically concludes once a single causal mutation is identified. In this family, we were able to confirm a diagnosis of Alport syndrome, which will facilitate testing in other family members.

4-Phenylbutyrate inhibits tunicamycin-induced acute kidney injury via CHOP/GADD153 repression

Different forms of acute kidney injury (AKI) have been associated with endoplasmic reticulum (ER) stress; these include AKI caused by acetaminophen, antibiotics, cisplatin, and radiocontrast. Tunicamycin (TM) is a nucleoside antibiotic known to induce ER stress and is a commonly used inducer of AKI. 4-phenylbutyrate (4-PBA) is an FDA approved substance used in children who suffer from urea cycle disorders. 4-PBA acts as an ER stress inhibitor by aiding in protein folding at the molecular level and preventing misfolded protein aggregation. The main objective of this study was to determine if 4-PBA could protect from AKI induced by ER stress, as typified by the TM-model, and what mechanism(s) of 4-PBA's action were responsible for protection. C57BL/6 mice were treated with saline, TM or TM plus 4-PBA. 4-PBA partially protected the anatomic segment most susceptible to damage, the outer medullary stripe, from TM-induced AKI. In vitro work showed that 4-PBA protected human proximal tubular cells from apoptosis and TM-induced CHOP expression, an ER stress inducible proapoptotic gene. Further, immunofluorescent staining in the animal model found similar protection by 4-PBA from CHOP nuclear translocation in the tubular epithelium of the medulla. This was accompanied by a reduction in apoptosis and GRP78 expression. CHOP(-/-) mice were protected from TM-induced AKI. The protective effects of 4-PBA extended to the ultrastructural integrity of proximal tubule cells in the outer medulla. When taken together, these results indicate that 4-PBA acts as an ER stress inhibitor, to partially protect the kidney from TM-induced AKI through the repression of ER stress-induced CHOP expression.

Identification of the SPLUNC1 ENaC-inhibitory domain yields novel strategies to treat sodium hyperabsorption in cystic fibrosis airway epithelial cultures

The epithelial sodium channel (ENaC) is responsible for Na(+) and fluid absorption across colon, kidney, and airway epithelia. Short palate lung and nasal epithelial clone 1 (SPLUNC1) is a secreted, innate defense protein and an autocrine inhibitor of ENaC that is highly expressed in airway epithelia. While SPLUNC1 has a bactericidal permeability-increasing protein (BPI)-type structure, its NH2-terminal region lacks structure. Here we found that an 18 amino acid peptide, S18, which corresponded to residues G22-A39 of the SPLUNC1 NH2 terminus inhibited ENaC activity to a similar degree as full-length SPLUNC1 (∼2.5 fold), while SPLUNC1 protein lacking this region was without effect. S18 did not inhibit the structurally related acid-sensing ion channels, indicating specificity for ENaC. However, S18 preferentially bound to the βENaC subunit in a glycosylation-dependent manner. ENaC hyperactivity is contributory to cystic fibrosis (CF) lung disease. Unlike control, CF human bronchial epithelial cultures (HBECs) where airway surface liquid (ASL) height was abnormally low (4.2 ± 0.6 μm), addition of S18 prevented ENaC-led ASL hyperabsorption and maintained CF ASL height at 7.9 ± 0.6 μm, even in the presence of neutrophil elastase, which is comparable to heights seen in normal HBECs. Our data also indicate that the ENaC inhibitory domain of SPLUNC1 may be cleaved away from the main molecule by neutrophil elastase, suggesting that it may still be active during inflammation or neutrophilia. Furthermore, the robust inhibition of ENaC by the S18 peptide suggests that this peptide may be suitable for treating CF lung disease.

Nephrin missense mutations: induction of endoplasmic reticulum stress and cell surface rescue by reduction in chaperone interactions

Nephrin, an important component of the podocyte filtration slit diaphragm, plays a key role in the maintenance of glomerular permselectivity. Mutations in nephrin lead to proteinuria and congenital nephrotic syndrome. Nephrin undergoes posttranslational modifications in the endoplasmic reticulum (ER) prior to export to the plasma membrane. We examined the effects of human nephrin disease-associated missense mutations on nephrin folding in the ER and on cellular trafficking in cultured cells. Compared with wild-type (WT) nephrin, the mutants showed impaired glycosylation and enhanced association with the ER chaperone, calnexin, as well as accumulation in the ER. Nephrin mutants demonstrated enhanced ubiquitination, and they underwent ER-associated degradation. Certain nephrin mutants did not traffic to the plasma membrane. Expression of nephrin mutants resulted in the stimulation of the activating transcription factor-6 pathway of the unfolded protein response, and an increase in the ER chaperone, Grp94. We treated cells with castanospermine (an inhibitor of glucosidase I) in order to decrease the association of nephrin mutants with calnexin. Castanospermine increased plasma membrane expression of nephrin mutants; however, full glycosylation and signaling activity of the mutants were not restored. Modulation of ER quality control mechanisms represents a potential new approach to development of therapies for proteinuric kidney disease, including congenital nephrotic syndrome.

Hsf-1 affects podocyte markers NPHS1, NPHS2 and WT1 in a transgenic mouse model of TTRVal30Met-related amyloidosis

INTRODUCTION

Familial amyloid polyneuropathy is characterized by transthyretin (TTR) deposition in various tissues, including the kidneys. While deposition induces organ dysfunction, renal involvement in TTR-related amyloidosis could manifest from proteinuria to end-stage kidney failure. As proteinuria is considered result of glomerular filtration barrier injury we investigated whether TTR deposition affects either glomerular basement membrane (GBM) or podocytes.

MATERIALS AND METHODS

Immunohistochemistry, immunoblot and gene expression studies for nephrin, podocin and WT1 were run on renal tissue from human-TTRV30M transgenic mice hemizygous or homozygous for heat shock factor one (Hsf-1). Transmission electron microscopy was used for evaluation of podocyte foot process width (PFW) and GBM thickness in Hsf-1 hemizygous mice with or without TTRV30M or amyloid deposition.

RESULTS

Glomeruli of hsf-1 hemizygous transgenic mice showed lower nephrin and podocin protein levels but an increased podocyte number when compared to Hsf-1 homozygous transgenic mice. Nephrin, podocin and WT1 gene expression levels were unaffected by the Hsf-1 carrier status. TTRV30M deposition was associated with increased PFW and GBM thickness.

CONCLUSIONS

Under the effect of Hsf-1 hemizygosity, TTRV30M deposition has deleterious effects on GBM thickness, PFW and slit diaphragm composition, without affecting nephrin and podocin gene expression.

Identification of SPLUNC1's ENaC-inhibitory domain yields novel strategies to treat sodium hyperabsorption in cystic fibrosis airways

The epithelial sodium channel (ENaC) is responsible for Na+ and fluid absorption across colon, kidney, and airway epithelia. We have previously identified SPLUNC1 as an autocrine inhibitor of ENaC. We have now located the ENaC inhibitory domain of SPLUNC1 to SPLUNC1's N terminus, and a peptide corresponding to this domain, G22-A39, inhibited ENaC activity to a similar degree as full-length SPLUNC1 (∼2.5 fold). However, G22-A39 had no effect on the structurally related acid-sensing ion channels, indicating specificity for ENaC. G22-A39 preferentially bound to the β-ENaC subunit in a glycosylation-dependent manner. ENaC hyperactivity is contributory to cystic fibrosis (CF) lung disease. Addition of G22-A39 to CF human bronchial epithelial cultures (HBECs) resulted in an increase in airway surface liquid height from 4.2±0.6 to 7.9±0.6 μm, comparable to heights seen in normal HBECs, even in the presence of neutrophil elastase. Our data also indicate that the ENaC inhibitory domain of SPLUNC1 may be cleaved away from the main molecule by neutrophil elastase, which suggests that it may still be active during inflammation or neutrophilia. Furthermore, the robust inhibition of ENaC by the G22-A39 peptide suggests that this peptide may be suitable for treating CF lung disease.

Deficits in sialylation impair podocyte maturation

The role of sialylation in kidney biology is not fully understood. The synthesis of sialoglycoconjugates, which form the outermost structures of animal cells, requires CMP-sialic acid, which is a product of the nuclear enzyme CMAS. We used a knock-in strategy to create a mouse with point mutations in the canonical nuclear localization signal of CMAS, which relocated the enzyme to the cytoplasm of transfected cells without affecting its activity. Although insufficient to prevent nuclear entry in mice, the mutation led to a drastically reduced concentration of nuclear-expressed enzyme. Mice homozygous for the mutation died from kidney failure within 72 hours after birth. The Cmas(nls) mouse exhibited podocyte foot process effacement, absence of slit diaphragms, and massive proteinuria, recapitulating features of nephrin-knockout mice and of patients with Finnish-type congenital nephrotic syndrome. Although the Cmas(nls) mouse displayed normal sialylation in all organs including kidney, a critical shortage of CMP-sialic acid prevented sialylation of nephrin and podocalyxin in the maturing podocyte where it is required during the formation of foot processes. Accordingly, the sialylation defects progressed with time and paralleled the morphologic changes. In summary, sialylation is critical during the development of the glomerular filtration barrier and required for the proper function of nephrin. Whether altered sialylation impairs nephrin function in human disease requires further study.

Biochemical and pharmacological study of N-linked glycosylation of the human serotonin 5-HT₇a receptor

The 5-hydroxytryptamine (5-HT)(7(a)) receptor is a G-protein-coupled receptor critically involved in human psychiatric and neurological disorders. In the present study, we evaluate the presence and the functional role of N-glycosylation of the human 5-HT(7) receptor. Western blot analysis of HEK293T cells transiently expressing the 5-HT(7(a)) receptor in the presence of tunicamycin gave rise to a band shift, indicating the existence of an N-glycosylated form of the 5-HT(7(a)) receptor. To further investigate this, we mutated the two predicted N-glycosylation sites (N5Q and N66Q) and compared the molecular mass of the immunoreactive bands with those of the wild-type receptor, indicating that both asparagines were N-glycosylated. The mutant receptors had the same binding affinity for [(3) H]5-CT and the same potency and efficacy with regard to 5-HT-induced activation of adenylyl cyclase. However, there was a reduction in maximal ligand binding for the single and double mutants compared to the wild-type receptor. Next, membrane labelling and immunocytochemical studies demonstrated that the N-glycosylation mutants were expressed at the cell surface. We conclude that N-glycosylation is not important for cell surface expression of the 5-HT(7) receptor.

The struggle for energy in podocytes leads to nephrotic syndrome

Podocytes are terminally differentiated post-mitotic cells similar to neurons, and their damage leads to nephrotic syndrome, which is characterized by massive proteinuria associated with generalized edema. A recent functional genetic approach has identified the pathological relevance of several mutated proteins in glomerular podocytes to the mechanism of proteinuria in hereditary nephrotic syndrome. In contrast, the pathophysiology of acquired primary nephrotic syndrome, including minimal change disease, is still largely unknown. We recently demonstrated the possible linkage of an energy-consuming process in glomerular podocytes to the mechanism of proteinuria. Puromycin aminonucleoside nephrosis, a rat model of minimal change disease, revealed the activation of the unfolded protein response (UPR) in glomerular podocytes to be a cause of proteinuria. The pretreatment of puromycin aminonucleoside rat podocytes and cultured podocytes with the mammalian target of rapamycin (mTOR) inhibitor everolimus further revealed that mTOR complex 1 consumed energy, which was followed by UPR activation. In this paper, we will review nutritional transporters to summarize the energy uptake process in podocytes and review the involvement of the UPR in the pathogenesis of glomerular diseases. We will also present additional data that reveal how mTOR complex 1 acts upstream of the UPR. Finally, we will discuss the potential significance of targeting the energy metabolism of podocytes to develop new therapeutic interventions for acquired nephrotic syndrome.

The Gne M712T mouse as a model for human glomerulopathy

Pathological glomerular hyposialylation has been implicated in certain unexplained glomerulopathies, including minimal change nephrosis, membranous glomerulonephritis, and IgA nephropathy. We studied our previously established mouse model carrying a homozygous mutation in the key enzyme of sialic acid biosynthesis, N-acetylglucosamine 2-epimerase/N-acetylmannosamine kinase. Mutant mice died before postnatal day 3 (P3) from severe glomerulopathy with podocyte effacement and segmental glomerular basement membrane splitting due to hyposialylation. Administration of the sialic acid precursor N-acetylmannosamine (ManNAc) led to improved sialylation and survival of mutant pups beyond P3. We determined the onset of the glomerulopathy in the embryonic stage. A lectin panel, distinguishing normally sialylated from hyposialylated glycans, used WGA, SNA, PNA, Jacalin, HPA, and VVA, indicating glomerular hyposialylation of predominantly O-linked glycoproteins in mutant mice. The glomerular glycoproteins nephrin and podocalyxin were hyposialylated in this unique murine model. ManNAc treatment appeared to ameliorate the hyposialylation status of mutant mice, indicated by a lectin histochemistry pattern similar to that of wild-type mice, with improved sialylation of both nephrin and podocalyxin, as well as reduced albuminuria compared with untreated mutant mice. These findings suggest application of our lectin panel for categorizing human kidney specimens based on glomerular sialylation status. Moreover, the partial restoration of glomerular architecture in ManNAc-treated mice highlights ManNAc as a potential treatment for humans affected with disorders of glomerular hyposialylation.

Notch promotes dynamin-dependent endocytosis of nephrin

Notch signaling in podocytes causes proteinuria and glomerulosclerosis in humans and rodents, but the underlying mechanism remains unknown. Here, we analyzed morphologic, molecular, and cellular events before the onset of proteinuria in newborn transgenic mice that express activated Notch in podocytes. Immunohistochemistry revealed a loss of the slit diaphragm protein nephrin exclusively in podocytes expressing activated Notch. Podocyte-specific deletion of Rbpj, which is essential for canonical Notch signaling, prevented this loss of nephrin. Overexpression of activated Notch decreased cell surface nephrin and increased cytoplasmic nephrin in transfected HEK293T cells; pharmacologic inhibition of dynamin, but not depletion of cholesterol, blocked these effects on nephrin, suggesting that Notch promotes dynamin-dependent, raft-independent endocytosis of nephrin. Supporting an association between Notch signaling and nephrin trafficking, electron microscopy revealed shortened podocyte foot processes and fewer slit diaphragms among the transgenic mice compared with controls. These data suggest that Notch signaling induces endocytosis of nephrin, thereby triggering the onset of proteinuria.

mTORC1 activation triggers the unfolded protein response in podocytes and leads to nephrotic syndrome

Although podocyte damage is known to be responsible for the development of minimal-change disease (MCD), the underlying mechanism remains to be elucidated. Previously, using a rat MCD model, we showed that endoplasmic reticulum (ER) stress in the podocytes was associated with the heavy proteinuric state and another group reported that a mammalian target of rapamycin complex 1 (mTORC1) inhibitor protected against proteinuria. In this study, which utilized a rat MCD model, a combination of immunohistochemistry, dual immunofluorescence and confocal microscopy, western blot analysis, and quantitative real-time RT-PCR revealed co-activation of the unfolded protein response (UPR), which was induced by ER stress, and mTORC1 in glomerular podocytes before the onset of proteinuria and downregulation of nephrin at the post-translational level at the onset of proteinuria. Podocyte culture experiments revealed that mTORC1 activation preceded the UPR that was associated with a marked decrease in the energy charge. The mTORC1 inhibitor everolimus completely inhibited proteinuria through a reduction in both mTORC1 and UPR activity and preserved nephrin expression in the glomerular podocytes. In conclusion, mTORC1 activation may perturb the regulatory system of energy metabolism primarily by promoting energy consumption and inducing the UPR, which underlie proteinuria in MCD.

Fibroblast growth factor 2 and protein kinase C alpha are involved in syndecan-4 cytoplasmic domain modulation of turkey myogenic satellite cell proliferation

Syndecan-4 core protein is composed of extracellular, transmembrane, and cytoplasmic domains. The cytoplasmic domain functions in transmitting signals into the cell through the protein kinase C alpha (PKCα) pathway. The glycosaminoglycan (GAG) and N-linked glycosylated (N-glycosylated) chains attached to the extracellular domain influence cell proliferation. The current study investigated the function of syndecan-4 cytoplasmic domain in combination with GAG and N-glycosylated chains in turkey muscle cell proliferation, differentiation, fibroblast growth factor 2 (FGF2) responsiveness, and PKCα membrane localization. Syndecan-4 or syndecan-4 without the cytoplasmic domain and with or without the GAG and N-glycosylated chains were transfected or co-transfected with a small interfering RNA targeting syndecan-4 cytoplasmic domain into turkey muscle satellite cells. The overexpression of syndecan-4 mutants increased cell proliferation but did not change differentiation. Syndecan-4 mutants had increased cellular responsiveness to FGF2 during proliferation. Syndecan-4 increased PKCα cell membrane localization, whereas the syndecan-4 mutants decreased PKCα cell membrane localization compared to syndecan-4. However, compared to the cells without transfection, syndecan-4 mutants increased cell membrane localization of PKCα. These data indicated that the syndecan-4 cytoplasmic domain and the GAG and N-glycosylated chains are critical in syndecan-4 regulating satellite cell proliferation, responsiveness to FGF2, and PKCα cell membrane localization.

TM4SF10 and ADAP interaction in podocytes: role in Fyn activity and nephrin phosphorylation

TM4SF10 [transmembrane tetra(4)-span family 10] is a claudin-like cell junction protein that is transiently expressed during podocyte development where its expression is downregulated in differentiating podocytes coincident with the appearance of nephrin at the slit diaphragm. In a yeast two-hybrid screen, we identified adhesion and degranulation-promoting adaptor protein (ADAP), a well-known Fyn substrate and Fyn binding partner, as a TM4SF10 interacting protein in mouse kidney. Using coimmunoprecipitation and immunohistochemistry experiments in cultured human podocytes, we show that TM4SF10 colocalizes with Fyn and ADAP but does not form a stable complex with Fyn. Cytoskeletal changes and phosphorylation events mediated by Fyn activity were reversed by TM4SF10 overexpression, including a decrease in the activating tyrosine phosphorylation of Fyn (Y(421)), suggesting TM4SF10 may have a regulatory role in suppressing Fyn activity. In addition, TM4SF10 was reexpressed following podocyte injury by puromycin aminonucleoside treatment, and its expression enhanced the abundance of high-molecular-weight forms of nephrin indicating it may participate in a mechanism controlling nephrin's appearance at the plasma membrane. Therefore, these studies have identified ADAP as another Fyn adapter protein expressed in podocytes, and that TM4SF10, possibly through ADAP, may regulate Fyn activity. Since TM4SF10 expression is temporally regulated during kidney development, these studies may help define a mechanism by which the slit diaphragm matures as a highly specialized cell junction during podocyte differentiation.

Relevance of VEGF and nephrin expression in glomerular diseases

The glomerular filtration barrier is affected in a large number of acquired and inherited diseases resulting in extensive leakage of plasma albumin and larger proteins, leading to nephrotic syndrome and end-stage renal disease. Unfortunately, the molecular mechanisms governing the development of the nephrotic syndrome remain poorly understood. Here, I give an overview of recent investigations that have focused on characterizing the interrelationships between the slit diaphragm components and podocytes-secreted VEGF, which have a significant role for maintaining the normal podocyte structure and the integrity of the filtering barrier.

Role of syndecan-4 side chains in turkey satellite cell growth and development

Syndecan-4 is a cell membrane heparan sulfate proteoglycan that is composed of a core protein and covalently attached glycosaminoglycans (GAG) and N-linked glycosylated (N-glycosylated) chains. Syndecan-4 has been shown to function independent of its GAG chains. Syndecan-4 may derive its biological function from the N-glycosylated chains due to the biological role of N-glycosylated chains in protein folding and cell membrane localization. The objective of the current study was to investigate the role of syndecan-4 N-glycosylated chains and the interaction between GAG and N-glycosylated chains in turkey myogenic satellite cell proliferation, differentiation, and fibroblast growth factor 2 (FGF2) responsiveness. The wild type turkey syndecan-4 and the syndecan-4 without GAG chains were cloned into the expression vector pCMS-EGFP and used as templates to generate syndecan-4 N-glycosylated one-chain and no-chain mutants with or without GAG chains. The wild type syndecan-4, all of the syndecan-4 N-glycosylated chain mutants were transfected into turkey myogenic satellite cells. Cell proliferation, differentiation, and responsiveness to FGF2 were measured. The overexpression of syndecan-4 N-glycosylated mutants with or without GAG chains did not change cell proliferation, differentiation, and responsiveness to FGF2 compared to the wild type syndecan-4 except that the overexpression of syndecan-4 N-glycosylated mutants without GAG chains increased cell proliferation at 48 and 72 h post-transfection. These data suggest that syndecan-4 functions in an FGF2-independent manner, and the N-glycosylated and GAG chains are required for syndecan-4 to regulate turkey myogenic satellite cell proliferation, but not differentiation.

Antigen-binding abilities of anti-nephrin antibody are prescribed by signal sequence of expression vector in genetic immunization

BACKGROUND

Nephrin is an essential protein for maintaining the normal structure of podocyte foot processes and the glomerular filtration barrier. To analyze the mechanism of proteinuria and nephrotic syndrome, we previously reported on a new method of producing polyclonal anti-nephrin antibody by genetic immunization. In the present paper, we investigate the effect of signal peptide sequence cDNA on the characteristics of polyclonal anti-nephrin antibodies induced by genetic immunization.

METHODS

Five fragments of nephrin cDNA with or without signal peptide sequence were inserted into the pTARGET™ vector. Rats were immunized with these vectors by the gene gun method. Sera obtained from rats at 14 weeks following immunization were analyzed by Western blotting, immunoprecipitation, flow cytometry and immunohistochemistry.

RESULTS

Four different antibodies induced by cDNA encoding nephrin protein fragments without signal peptide showed antigen site-specific binding to fragmented glycosylation-disturbed nephrin proteins. These antibodies also reacted to a glycosylation-disturbed full-length nephrin protein (inhibited by tunicamycin), but did not react to either a native nephrin protein or a fully glycosylated conformational nephrin protein. Four different antibodies induced by cDNA encoding nephrin fragments with signal peptide showed an antigen site-specific binding to glycosylation-disturbed nephrin protein fragments. In addition, these antibodies reacted to both a native nephrin protein and a full-length glycosylated conformational nephrin protein.

CONCLUSIONS

The absence of signal peptide sequence cDNA in the expression vector produced antibodies specific for glycosylation-disturbed proteins, while its presence produced antibodies that bound to native or fully glycosylated conformational protein.

Glomerular sclerosis is prevented during urinary tract obstruction due to podocyte protection

Urine outflow obstruction activates a variety of profibrotic factors, including the intrarenal renin-angiotensin system. However, the obstruction also nullifies the transmural hydraulic pressure difference across the glomerular capillary wall, an established inducer of glomerulosclerosis. In the present study, we investigated whether, and by what mechanism, urine outflow obstruction affects the process of progressive glomerulosclerosis. For this purpose, we tested the effect of unilateral ureteral obstruction (UUO) of 7 days duration in two distinct mouse models of glomerulosclerosis. In the human immunodeficiency virus (HIV) nephropathy model, where HIV-1 genes are selectively expressed in podocytes and develop progressive podocyte damage and glomerulosclerosis, UUO protected against sclerosis with preservation of podocytes morphologically and immunohistochemically. In contrast, the nonobstructed contralateral kidneys of these mice, as well as sham-operated HIV-1 mouse kidneys, developed severe podocyte injury and glomerulosclerosis. The protection against glomerulosclerosis imparted by ureteral obstruction was also documented in the NEP25 model of podocyte injury, in which a single injection of immunotoxin, LMB2, triggers selective podocyte injury followed by glomerulosclerosis, both of which were protected by UUO. Notably, intervention with an angiotensin II type 1 receptor antagonist provided only a partial protective effect in each of the models. These results demonstrate that urine outflow obstruction protects the glomerulus from progressive sclerosis. The results further reveal that this protection occurs at a very early stage of the pathologic process, namely, damage of podocytes.

A novel method for isolating podocytes using magnetic activated cell sorting

BACKGROUND

A large body of accumulated data has now revealed that podocytes play a major role in the development of proteinuria. However, the mechanisms of podocyte injury, leading to foot process effacement and proteinuria, are still unclear partly due to the current lack of an appropriate strategy for preparing podocytes. In this study, we have developed a novel method of rapid isolation of podocytes from mice using magnetic activated cell sorting with an anti-nephrin antibody.

METHODS

After endothelial cell depletion using anti-CD31 antibody, nephrin-positive cells were prepared from mouse kidneys using magnetic activated cell sorting with polyclonal rabbit anti-nephrin antibody. Purity of the positively sorted cells was determined by confocal microscopy and fluorescence-activated cell sorting (FACS) analysis. Expression profiles of podocyte-specific molecules in the sorted fractions were characterized by qualitative PCR and immunoblot analysis.

RESULTS

Nephrin-positive cells, isolated from mouse kidneys within 6 h, showed dual positivity for synaptopodin and rabbit IgG on confocal microscopy. FACS analysis revealed that the purity of the positively sorted fractions was ∼75%. The nephrin-positive cells sorted by this approach showed a significantly higher expression of podocyte-specific molecules compared with nephrin-negative fractions.

CONCLUSIONS

These data strongly suggest that our novel method for isolating podocytes has great utility for various downstream applications such as genomic analysis, proteomics and transcriptomics to elucidate molecular profiling of podocyte biology in vivo compared with conventional methods as our approach requires only several hours to complete and no tissue culture.