Multiomics analysis of O-GlcNAcylation in podocytes of diabetic kidney disease

AIM

To investigate the role of O-GlcNAc transferase (OGT)-mediated protein O-GlcNAcylation in podocyte injury during the progression of diabetic kidney disease (DKD).

MATERIALS AND METHODS

Proteomic and O-glycoproteomic analyses were conducted on high glucose (HG)-stimulated podocytes with OGT knockdown. Differentially expressed proteins and O-GlcNAcylated peptides/proteins were identified, and functional enrichment (GO, KEGG, COG/KOG) and motif analysis (motif-x) were performed using bioinformatics analysis. Co-immunoprecipitation (Co-IP) was used to validate O-GlcNAcylation of candidate proteins.

RESULTS

OGT knockdown in HG-treated podocytes resulted in 128 upregulated and 45 downregulated proteins. Glycoproteomics revealed 32 glycopeptides/21 glycoproteins upregulated and 37 glycopeptides/22 glycoproteins downregulated. The focus was on down-regulated glycosylated proteins without changes in their protein levels. These proteins are predominantly enriched in translation factor activity, RNA binding, and ECM-receptor interactions pathways. Among these proteins, Caprin1, Lrp1, and Sil1 were modified by O-GlcNAcylation.

CONCLUSION

OGT-driven O-GlcNAcylation exacerbates podocyte injury in DKD by post-translationally modifying key regulators of translational machinery and ECM signalling. Precision targeting of O-GlcNAc dynamics represents a promising therapeutic strategy to attenuate DKD.

O-linked β-N-acetylglucosamine (O-GlcNAc) modification: Emerging pathogenesis and a therapeutic target of diabetic nephropathy

AIMS

O-Linked β-N-acetylglucosamine (O-GlcNAc) modification, a unique post-translational modification of proteins, is elevated in diabetic nephropathy. This review aims to summarize the current knowledge on the mechanisms by which O-GlcNAcylation of proteins contributes to the pathogenesis and progression of diabetic nephropathy, as well as the therapeutic potential of targeting O-GlcNAc modification for its treatment.

METHODS

Current evidence in the literature was reviewed and synthesized in a narrative review.

RESULTS

Hyperglycemia increases glucose flux into the hexosamine biosynthesis pathway, which activates glucosamino-fructose aminotransferase expression and activity, leading to the production of O-GlcNAcylation substrate UDP-GlcNAc and an increase in protein O-GlcNAcylation in kidney cells. Protein O-GlcNAcylation regulates the function of kidney cells including mesangial cells, podocytes, and proximal tubular cells, and promotes renal interstitial fibrosis, resulting in kidney damage. Current treatments for diabetic nephropathy, such as sodium-glucose cotransporter 2 (SGLT-2) inhibitors and renin-angiotensin-aldosterone system (RAAS) inhibitors, delay disease progression, and suppress protein O-GlcNAcylation.

CONCLUSIONS

Increased protein O-GlcNAcylation mediates renal cell damage and promotes renal interstitial fibrosis, leading to diabetic nephropathy. Although the full significance of inhibition of O-GlcNAcylation is not yet understood, it may represent a novel target for treating diabetic nephropathy.

T-2 Toxin-Mediated β-Arrestin-1 O-GlcNAcylation Exacerbates Glomerular Podocyte Injury via Regulating Histone Acetylation

T-2 toxin causes renal dysfunction with proteinuria and glomerular podocyte damage. This work explores the role of metabolic disorder/reprogramming-mediated epigenetic modification in the progression of T-2 toxin-stimulated podocyte injury. A metabolomics experiment is performed to assess metabolic responses to T-2 toxin infection in human podocytes. Roles of protein O-linked-N-acetylglucosaminylation (O-GlcNAcylation) in regulating T-2 toxin-stimulated podocyte injury in mouse and podocyte models are assessed. O-GlcNAc target proteins are recognized by mass spectrometry and co-immunoprecipitation experiments. Moreover, histone acetylation and autophagy levels are measured. T-2 toxin infection upregulates glucose transporter type 1 (GLUT1) expression and enhances hexosamine biosynthetic pathway in glomerular podocytes, resulting in a significant increase in β-arrestin-1 O-GlcNAcylation. Decreasing β-arrestin-1 or O-GlcNAc transferase (OGT) effectively prevents T-2 toxin-induced renal dysfunction and podocyte injury. Mechanistically, O-GlcNAcylation of β-arrestin-1 stabilizes β-arrestin-1 to activate the mammalian target of rapamycin (mTOR) pathway as well as to inhibit autophagy during podocyte injury by promoting H4K16 acetylation. To sum up, OGT-mediated β-arrestin-1 O-GlcNAcylation is a vital regulator in the development of T-2 toxin-stimulated podocyte injury via activating the mTOR pathway to suppress autophagy. Targeting β-arrestin-1 or OGT can be a potential therapy for T-2 toxin infection-associated glomerular injury, especially podocyte injury.

Cytoskeleton Rearrangement in Podocytopathies: An Update

Podocyte injury can disrupt the glomerular filtration barrier (GFB), leading to podocytopathies that emphasize podocytes as the glomerulus's key organizer. The coordinated cytoskeleton is essential for supporting the elegant structure and complete functions of podocytes. Therefore, cytoskeleton rearrangement is closely related to the pathogenesis of podocytopathies. In podocytopathies, the rearrangement of the cytoskeleton refers to significant alterations in a string of slit diaphragm (SD) and focal adhesion proteins such as the signaling node nephrin, calcium influx via transient receptor potential channel 6 (TRPC6), and regulation of the Rho family, eventually leading to the disorganization of the original cytoskeletal architecture. Thus, it is imperative to focus on these proteins and signaling pathways to probe the cytoskeleton rearrangement in podocytopathies. In this review, we describe podocytopathies and the podocyte cytoskeleton, then discuss the molecular mechanisms involved in cytoskeleton rearrangement in podocytopathies and summarize the effects of currently existing drugs on regulating the podocyte cytoskeleton.

Nutritional and metabolic control of germ cell fate through O-GlcNAc regulation

Fate determination of primordial germ cells (PGCs) is regulated in a multi-layered manner, involving signaling pathways, epigenetic mechanisms, and transcriptional control. Chemical modification of macromolecules, including epigenetics, is expected to be closely related with metabolic mechanisms but the detailed molecular machinery linking these two layers remains poorly understood. Here, we show that the hexosamine biosynthetic pathway controls PGC fate determination via O-linked β-N-acetylglucosamine (O-GlcNAc) modification. Consistent with this model, reduction of carbohydrate metabolism via a maternal ketogenic diet that decreases O-GlcNAcylation levels causes repression of PGC formation in vivo. Moreover, maternal ketogenic diet intake until mid-gestation affects the number of ovarian germ cells in newborn pups. Taken together, we show that nutritional and metabolic mechanisms play a previously unappreciated role in PGC fate determination.

Fetal Reprogramming of Nutrient Surplus Signaling, O-GlcNAcylation, and the Evolution of CKD

ABSTRACT

Fetal kidney development is characterized by increased uptake of glucose, ATP production by glycolysis, and upregulation of mammalian target of rapamycin (mTOR) and hypoxia-inducible factor-1 alpha (HIF-1 α ), which (acting in concert) promote nephrogenesis in a hypoxic low-tubular-workload environment. By contrast, the healthy adult kidney is characterized by upregulation of sirtuin-1 and adenosine monophosphate-activated protein kinase, which enhances ATP production through fatty acid oxidation to fulfill the needs of a normoxic high-tubular-workload environment. During stress or injury, the kidney reverts to a fetal signaling program, which is adaptive in the short term, but is deleterious if sustained for prolonged periods when both oxygen tension and tubular workload are heightened. Prolonged increases in glucose uptake in glomerular and proximal tubular cells lead to enhanced flux through the hexosamine biosynthesis pathway; its end product-uridine diphosphate N -acetylglucosamine-drives the rapid and reversible O-GlcNAcylation of thousands of intracellular proteins, typically those that are not membrane-bound or secreted. Both O-GlcNAcylation and phosphorylation act at serine/threonine residues, but whereas phosphorylation is regulated by hundreds of specific kinases and phosphatases, O-GlcNAcylation is regulated only by O-GlcNAc transferase and O-GlcNAcase, which adds or removes N-acetylglucosamine, respectively, from target proteins. Diabetic and nondiabetic CKD is characterized by fetal reprogramming (with upregulation of mTOR and HIF-1 α ) and increased O-GlcNAcylation, both experimentally and clinically. Augmentation of O-GlcNAcylation in the adult kidney enhances oxidative stress, cell cycle entry, apoptosis, and activation of proinflammatory and profibrotic pathways, and it inhibits megalin-mediated albumin endocytosis in glomerular mesangial and proximal tubular cells-effects that can be aggravated and attenuated by augmentation and muting of O-GlcNAcylation, respectively. In addition, drugs with known nephroprotective effects-angiotensin receptor blockers, mineralocorticoid receptor antagonists, and sodium-glucose cotransporter 2 inhibitors-are accompanied by diminished O-GlcNAcylation in the kidney, although the role of such suppression in mediating their benefits has not been explored. The available evidence supports further work on the role of uridine diphosphate N -acetylglucosamine as a critical nutrient surplus sensor (acting in concert with upregulated mTOR and HIF-1 α signaling) in the development of diabetic and nondiabetic CKD.

The role of O-GlcNAcylation in development

O-GlcNAcylation is a dynamic post-translational modification performed by two opposing enzymes: O-GlcNAc transferase and O-GlcNAcase. O-GlcNAcylation is generally believed to act as a metabolic integrator in numerous signalling pathways. The stoichiometry of this modification is tightly controlled throughout all stages of development, with both hypo/hyper O-GlcNAcylation resulting in broad defects. In this Primer, we discuss the role of O-GlcNAcylation in developmental processes from stem cell maintenance and differentiation to cell and tissue morphogenesis.

O-GlcNAcylation in Renal (Patho)Physiology

Kidneys maintain internal milieu homeostasis through a well-regulated manipulation of body fluid composition. This task is performed by the correlation between structure and function in the nephron. Kidney diseases are chronic conditions impacting healthcare programs globally, and despite efforts, therapeutic options for its treatment are limited. The development of chronic degenerative diseases is associated with changes in protein O-GlcNAcylation, a post-translation modification involved in the regulation of diverse cell function. O-GlcNAcylation is regulated by the enzymatic balance between O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA) which add and remove GlcNAc residues on target proteins, respectively. Furthermore, the hexosamine biosynthetic pathway provides the substrate for protein O-GlcNAcylation. Beyond its physiological role, several reports indicate the participation of protein O-GlcNAcylation in cardiovascular, neurodegenerative, and metabolic diseases. In this review, we discuss the impact of protein O-GlcNAcylation on physiological renal function, disease conditions, and possible future directions in the field.

Renal Tubular Handling of Glucose and Fructose in Health and Disease

The proximal tubule of the kidney is programmed to reabsorb all filtered glucose and fructose. Glucose is taken up by apical sodium-glucose cotransporters SGLT2 and SGLT1 whereas SGLT5 and potentially SGLT4 and GLUT5 have been implicated in apical fructose uptake. The glucose taken up by the proximal tubule is typically not metabolized but leaves via the basolateral facilitative glucose transporter GLUT2 and is returned to the systemic circulation or used as an energy source by distal tubular segments after basolateral uptake via GLUT1. The proximal tubule generates new glucose in metabolic acidosis and the postabsorptive phase, and fructose serves as an important substrate. In fact, under physiological conditions and intake, fructose taken up by proximal tubules is primarily utilized for gluconeogenesis. In the diabetic kidney, glucose is retained and gluconeogenesis enhanced, the latter in part driven by fructose. This is maladaptive as it sustains hyperglycemia. Moreover, renal glucose retention is coupled to sodium retention through SGLT2 and SGLT1, which induces secondary deleterious effects. SGLT2 inhibitors are new anti-hyperglycemic drugs that can protect the kidneys and heart from failing independent of kidney function and diabetes. Dietary excess of fructose also induces tubular injury. This can be magnified by kidney formation of fructose under pathological conditions. Fructose metabolism is linked to urate formation, which partially accounts for fructose-induced tubular injury, inflammation, and hemodynamic alterations. Fructose metabolism favors glycolysis over mitochondrial respiration as urate suppresses aconitase in the tricarboxylic acid cycle, and has been linked to potentially detrimental aerobic glycolysis (Warburg effect). © 2022 American Physiological Society. Compr Physiol 12:2995-3044, 2022.

Insulin resistance in glomerular podocytes: Potential mechanisms of induction

Glomerular podocytes are a target for the actions of insulin. Accumulating evidence indicates that exposure to nutrient overload induces insulin resistance in these cells, manifested by abolition of the stimulatory effect of insulin on glucose uptake. Numerous recent studies have investigated potential mechanisms of the induction of insulin resistance in podocytes. High glucose concentrations stimulated reactive oxygen species production through NADPH oxidase activation, decreased adenosine monophosphate-activated protein kinase (AMPK) phosphorylation, and reduced deacetylase sirtuin 1 (SIRT1) protein levels and activity. Calcium signaling involving transient receptor potential cation channel C, member 6 (TRPC6) also was demonstrated to play an essential role in the regulation of insulin-dependent signaling and glucose uptake in podocytes. Furthermore, podocytes exposed to diabetic environment, with elevated insulin levels become insulin resistant as a result of degradation of insulin receptor (IR), resulting in attenuation of insulin signaling responsiveness. Also elevated levels of palmitic acid appear to be an important factor and contributor to podocytes insulin resistance. This review summarizes cellular and molecular alterations that contribute to the development of insulin resistance in glomerular podocytes.

Role of O-linked N-acetylglucosamine in the homeostasis of metabolic organs, and its potential links with diabetes and its complications

Recent studies using genetically manipulated mouse models have shown the pivotal role of O-linked N-acetylglucosamine modification (O-GlcNAcylation) in the metabolism of multiple organs. The molecular mechanism involves the sensing of glucose flux by the hexosamine biosynthesis pathway, which leads to the adjustment of cellular metabolism to protect against changes in the environment of each organ through O-GlcNAcylation. More recently, not only glucose, but also fluxes of amino acids and fatty acids have been reported to induce O-GlcNAcylation, affecting multiple cellular processes. In this review, we discuss how O-GlcNAcylation maintains homeostasis in organs that are affected by diabetes mellitus: skeletal muscle, adipose tissue, liver and pancreatic β-cells. Furthermore, we discuss the importance of O-GlcNAcylation in the pathogenesis of diabetic complications. By elucidating the molecular mechanisms whereby cellular homeostasis is maintained, despite changes in metabolic flux, these studies might provide new targets for the treatment and prevention of diabetes and its complications.

Decreased KAT5 Expression Impairs DNA Repair and Induces Altered DNA Methylation in Kidney Podocytes

Altered DNA methylation plays an important role in the onset and progression of kidney disease. However, little is known about how the changes arise in disease states. Here, we report that KAT5-mediated DNA damage repair is essential for the maintenance of kidney podocytes and is associated with DNA methylation status. Podocyte-specific KAT5-knockout mice develop severe albuminuria with increased DNA double-strand breaks (DSBs), increased DNA methylation of the nephrin promoter region, and decreased nephrin expression. Podocyte KAT5 expression is decreased, whereas DNA DSBs and DNA methylation are increased in diabetic nephropathy; moreover, KAT5 restoration by gene transfer attenuates albuminuria. Furthermore, KAT5 decreases DNA DSBs and DNA methylation at the same nephrin promoter region, which indicates that KAT5-mediated DNA repair may be related to DNA methylation status. These results suggest a concept in which an environment of DNA damage repair, which occurs with decreased KAT5, may affect DNA methylation status.

O-GlcNAcylation and phosphorylation of β-actin Ser199 in diabetic nephropathy

The function of actin is regulated by various posttranslational modifications. We have previously shown that in the kidneys of nonobese type 2 diabetes model Goto-Kakizaki rats, increased O-GlcNAcylation of β-actin protein is observed. It has also been reported that both O-GlcNAcylation and phosphorylation occur on Ser199 of β-actin. However, their roles are not known. To elucidate their roles in diabetic nephropathy, we examined the rat kidney for changes in O-GlcNAcylation of Ser199 (gS199)-actin and in the phosphorylation of Ser199 (pS199)-actin. Both gS199- and pS199-actin molecules had an apparent molecular weight of 40 kDa and were localized as nonfilamentous actin in both the cytoplasm and nucleus. Compared with the normal kidney, the immunostaining intensity of gS199-actin increased in podocytes of the glomeruli and in proximal tubules of the diabetic kidney, whereas that of pS199-actin did not change in podocytes but decreased in proximal tubules. We confirmed that the same results could be observed in the glomeruli of the human diabetic kidney. In podocytes of glomeruli cultured in the presence of the O-GlcNAcase inhibitor Thiamet G, increased O-GlcNAcylation was accompanied by a concomitant decrease in the amount of filamentous actin and in morphological changes. Our present results demonstrate that dysregulation of O-GlcNAcylation and phosphorylation of Ser199 occurred in diabetes, which may contribute partially to the causes of the morphological changes in the glomeruli and tubules. gS199- and pS199-actin will thus be useful for the pathological evaluation of diabetic nephropathy.

A preclinical overview of emerging therapeutic targets for glomerular diseases

Introduction: Animal models have provided significant insights into the mechanisms responsible for the development of glomerular lesions and proteinuria; they have also helped to identify molecules that control the podocyte function as suitable target-specific therapeutics. Areas covered: We discuss putative therapeutic targets for proteinuric glomerular diseases. An exhaustive search for eligible studies was performed in PubMed/MEDLINE. Most of the selected reports were published in the last decade, but we did not exclude older relevant milestone publications. We consider the molecules that regulate podocyte cytoskeletal dynamics and the transcription factors that regulate the expression of slit-diaphragm proteins. There is a focus on SGLT2 and sirtuins which have recently emerged as mediators of podocyte injury and repair. We also examine paracrine signallings involved in the cross-talk of injured podocytes with the neighbouring glomerular endothelial cells and parietal epithelial cells. Expert opinion: There is a need to discover novel therapeutic moleecules with renoprotective effects for those patients with glomerular diseases who do not respond completely to standard therapy. Emerging strategies targeting components of the podocyte cytoskeleton or signallings that regulate cellular communication within the glomerulus are promising avenues for treating glomerular diseases.

From podocyte biology to novel cures for glomerular disease

The podocyte is a key component of the glomerular filtration barrier. Podocyte dysfunction is central to the underlying pathophysiology of many common glomerular diseases, including diabetic nephropathy, glomerulonephritis and genetic forms of nephrotic syndrome. Collectively, these conditions affect millions of people worldwide, and account for the majority of kidney diseases requiring dialysis and transplantation. The 12th International Podocyte Conference was held in Montreal, Canada from May 30 to June 2, 2018. The primary aim of this conference was to bring together nephrologists, clinician scientists, basic scientists and their trainees from all over the world to present their research and to establish networks with the common goal of developing new therapies for glomerular diseases based on the latest advances in podocyte biology. This review briefly highlights recent advances made in understanding podocyte structure and metabolism, experimental systems in which to study podocytes and glomerular disease, disease mediators, genetic and immune origins of glomerulopathies, and the development of novel therapeutic agents to protect podocyte and glomerular injury.

Protein O-GlcNAcylation Is Essential for the Maintenance of Renal Energy Homeostasis and Function via Lipolysis during Fasting and Diabetes

BACKGROUND

Energy metabolism in proximal tubular epithelial cells (PTECs) is unique, because ATP production largely depends on lipolysis in both the fed and fasting states. Furthermore, disruption of renal lipolysis is involved in the pathogenesis of diabetic tubulopathy. Emerging evidence suggests that protein O-GlcNAcylation, an intracellular nutrient-sensing system, may regulate a number of metabolic pathways according to changes in nutritional status. Although O-GlcNAcylation in PTECs has been demonstrated experimentally, its precise role in lipolysis in PTECs is unclear.

METHODS

To investigate the mechanism of renal lipolysis in PTECs-specifically, the role played by protein O-GlcNAcylation-we generated mice with PTECs deficient in O-GlcNAc transferase (Ogt). We analyzed their renal phenotypes during ad libitum feeding, after prolonged fasting, and after mice were fed a high-fat diet for 16 weeks to induce obesity and diabetes.

RESULTS

Although PTEC-specific Ogt-deficient mice lacked a marked renal phenotype during ad libitum feeding, after fasting 48 hours, they developed Fanconi syndrome-like abnormalities, PTEC apoptosis, and lower rates of renal lipolysis and ATP production. Proteomic analysis suggested that farnesoid X receptor-dependent upregulation of carboxylesterase-1 is involved in O-GlcNAcylation's regulation of lipolysis in fasted PTECs. PTEC-specific Ogt-deficient mice with diabetes induced by a high-fat diet developed severe tubular cell damage and enhanced lipotoxicity.

CONCLUSIONS

Protein O-GlcNAcylation is essential for renal lipolysis during prolonged fasting and offers PTECs significant protection against lipotoxicity in diabetes.

Increased dynamin expression precedes proteinuria in glomerular disease

Dynamin plays an essential role in maintaining the structure and function of the glomerular filtration barrier. Specifically, dynamin regulates the actin cytoskeleton and the turnover of nephrin in podocytes, and knocking down dynamin expression causes proteinuria. Moreover, promoting dynamin oligomerization with Bis-T-23 restores podocyte function and reduces proteinuria in several animal models of chronic kidney disease. Thus, dynamin is a promising therapeutic target for treating chronic kidney disease. Here, we investigated the pathophysiological role of dynamin under proteinuric circumstances in a rat model and in humans. We found that glomerular Dnm2 and Dnm1 mRNA levels are increased prior to the onset of proteinuria in a rat model of spontaneous proteinuria. Also, in zebrafish embryos, we confirm that knocking down dynamin translation results in proteinuria. Finally, we show that the glomerular expression of dynamin and cathepsin L protein is increased in several human proteinuric kidney diseases. We propose that the increased expression of glomerular dynamin reflects an exhausted attempt to maintain and/or restore integrity of the glomerular filtration barrier. These results confirm that dynamin plays an important role in maintaining the glomerular filtration barrier, and they support the notion that dynamin is a promising therapeutic target in proteinuric kidney disease. © 2018 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of Pathological Society of Great Britain and Ireland.

Modulation of dynamin function by small molecules

Dynamins are essential as membrane remodelers in various cellular processes, like receptor-mediated endocytosis, synaptic vesicle recycling and spermatogenesis. Moreover, dynamin is involved in the internalization of numerous viruses and in the motility of several cancer cell lines. As tools for dissecting the underlying mechanisms of these important biological processes and as potential future therapeutics, small molecules have been developed in the last two decades that modulate the functions of dynamin. In this review we give an overview of the compound classes that are currently in use and describe how they affect dynamin function.

Emerging therapeutic strategies for minimal change disease and focal and segmental glomerulosclerosis

INTRODUCTION

Minimal change disease (MCD) and Focal and segmental glomerulosclerosis (FSGS) are two of the major causes of nephrotic syndrome (NS) in children and adults. According to KDIGO (Kidney Disease: Improving Global Outcomes) guidelines, the treatment of adult primary MCD and FSGS should be based on immunosuppressants and antiproteinuric drugs. Recently, Rituximab, a humanized monoclonal antibody (mAb) has emerged as a potential treatment for steroid or calcineurin inhibitor-dependent patients; it has however demonstrated lower efficacy in those with nephrotic syndrome that is resistant to the above indicated drugs.

AREAS COVERED

Analysis of ongoing and already completed clinical trials, retrieved from clinicaltrials.gov, clinicaltrialsregister.eu and PubMed involving new therapies for nephrotic syndrome secondary to MCD and FSGS.

EXPERT OPINION

The most promising drugs under investigation for MCD and FSGS are mAbs. We are hopeful that new therapeutic options to treat multi-drug resistant MCD and FSGS will emerge from currently ongoing studies. What appears certain is the difficulty in enrolling patients affected by orphan renal diseases and the selection of valid endpoints in clinical trials, such as kidney failure.

O-GlcNAcylation reduces proximal tubule protein reabsorption and promotes proteinuria in spontaneously hypertensive rats

Hypertensive individuals are at greater risk for developing chronic kidney disease (CKD). Reducing proteinuria has been suggested as a possible therapeutic approach to treat CKD. However, the mechanisms underlying the development of proteinuria in hypertensive conditions are incompletely understood. Cardiac and vascular dysfunction is associated with changes in the O-GlcNAcylation pathway in hypertensive models. We hypothesized that O-GlcNAcylation is also involved in renal damage, especially development of proteinuria, associated with hypertension. Using the spontaneously hypertensive rat (SHR) model, we observed higher renal cortex O-GlcNAcylation, glutamine-fructose aminotransferase (GFAT), and O-GlcNAc transferase (OGT) protein expression, which positively correlated with proteinuria. Interestingly, this was observed in hypertensive, but not pre-hypertensive, rats. Pharmacological inhibition of GFAT decreased renal cortex O-GlcNAcylation, proteinuria, and albuminuria in SHR. Using a proximal tubule cell line, we observed that increased O-GlcNAcylation reduced megalin surface expression and albumin endocytosis in vitro, and the effects were correlated in vivo Moreover, megalin is O-GlcNAcylated both in vitro and in vivo In conclusion, our results demonstrate a new mechanism involved in hypertension-associated proteinuria.

Actin dynamics at focal adhesions: a common endpoint and putative therapeutic target for proteinuric kidney diseases

Proteinuria encompasses diverse causes including both genetic diseases and acquired forms such as diabetic and hypertensive nephropathy. The basis of proteinuria is a disturbance in size selectivity of the glomerular filtration barrier, which largely depends on the podocyte: a terminally differentiated epithelial cell type covering the outer surface of the glomerulus. Compromised podocyte structure is one of the earliest signs of glomerular injury. The phenotype of diverse animal models and podocyte cell culture firmly established the essential role of the actin cytoskeleton in maintaining functional podocyte structure. Podocyte foot processes, actin-based membrane extensions, contain 2 molecularly distinct "hubs" that control actin dynamics: a slit diaphragm and focal adhesions. Although loss of foot processes encompasses disassembly of slit diaphragm multiprotein complexes, as long as cells are attached to the glomerular basement membrane, focal adhesions will be the sites in which stress due to filtration flow is counteracted by forces generated by the actin network in foot processes. Numerous studies within last 20 years have identified actin binding and regulatory proteins as well as integrins as essential components of signaling and actin dynamics at focal adhesions in podocytes, suggesting that some of them may become novel, druggable targets for proteinuric kidney diseases. Here we review evidence supporting the idea that current treatments for chronic kidney diseases beneficially and directly target the podocyte actin cytoskeleton associated with focal adhesions and suggest that therapeutic reagents that target the focal adhesion-regulated actin cytoskeleton in foot processes have potential to modernize treatments for chronic kidney diseases.

Podocytes from the diagnostic and therapeutic point of view

The central role of podocytes in glomerular diseases makes this cell type an interesting diagnostic tool as well as a therapeutic target. In this review, we discuss the current literature on the use of podocytes and podocyte-specific markers as non-invasive diagnostic tools in different glomerulopathies. Furthermore, we highlight the direct effects of drugs currently used to treat primary glomerular diseases and describe their direct cellular effects on podocytes. A new therapeutic potential is seen in drugs targeting the podocytic actin cytoskeleton which is essential for podocyte foot process structure and function. Incubation of cultured human podocyte cell lines with sera from patients with active glomerular diseases is currently also used to identify novel circulating factors with pathophysiological relevance for the glomerular filtration barrier. In addition, treatment of detached urinary podocytes from patients with substances that restore their cytoskeleton might serve as a novel personalized tool to estimate their potential for podocyte recovery ex vivo.