The phenotypes of podocytes and parietal epithelial cells may overlap in diabetic nephropathy

Continuous increase in podocyte numbers in the first 36 months of life-insights from forensic autopsies in Japanese children

BACKGROUND

Podocyte depletion is a critical factor in glomerulosclerosis development. While podocyte numbers per glomerulus typically decline with age in adults, they are hypothesized to increase during childhood. However, studies on podocyte number progression in childhood have been limited.

METHODS

This retrospective analysis examined forensic autopsy cases of Japanese children without kidney disease, aged under 192 months, between April 2010 and March 2023. Podocytes were identified using immunostaining with an anti-transducin-like enhancer of split 4 antibody and p57. Podometric parameters were estimated using the correction factor method, allowing estimation from a single histologic section.

RESULTS

This study included 68 cases with a median age of 9 months (interquartile range [IQR], 4-78). All podometric parameters correlated with age. Children younger than 36 months displayed significantly fewer podocyte numbers per glomerulus (median, 517; IQR, 483-546) compared to those aged 36 months and older (median, 616; IQR, 595-649; p < 0.001). Regression analysis revealed a significant age-related increase in podocyte numbers per glomerulus in children under 36 months (slope, 3.76; p < 0.001; 95% confidence interval [CI], 2.34-5.19), but not in those aged 36 months and older (slope, 0.25; p = 0.16; 95% CI, - 0.10-0.61). Additionally, the change in the slope at 36 months was significant (p < 0.001; 95% CI, 1.02-2.49); however, this increase did not appear linked to podocyte division.

CONCLUSIONS

Podocyte numbers per glomerulus increased from birth until 36 months and then stabilized. These findings could facilitate the development of novel treatments for chronic kidney disease caused by glomerulosclerosis and contribute to pediatric kidney health research.

The trans-differentiation promotion of parietal epithelial cells by magnesium isoglycyrrhizinate to improve podocyte injury induced by high fructose consumption

BACKGROUND

Podocytes have limited proliferative capacity, which leads to irreversible glomerular injury in diverse kidney diseases. Magnesium isoglycyrrhizinate (MgIG), a hepatoprotective agent in clinic, has been reported to improve glomerular podocyte injury. However, the underlying mechanism of MgIG in ameliorating podocyte injury remains unclear.

PURPOSE

Glomerular parietal epithelial cells (PECs) are recognized as podocyte progenitors and play a pivotal role in the recovery following glomerular injury. This work aims to investigate the protective mechanisms of MgIG in mitigating glomerular injury by promoting PEC trans-differentiation.

STUDY DESIGN

A rat model of progressive glomerular podocyte injury, and in vitro models using the primary podocytes and primary PECs, were established to further explore the pharmacological mechanism of MgIG.

METHODS

Four-week-old male Sprague-Dawley (SD) rats were fed a 10 % fructose solution for 3, 6, 9 and 12 weeks to induce glomerular injury. The effects of MgIG on the progressive changes in podocytes and PECs, and the correlation between PEC density and podocyte loss, were analyzed. The mechanism of MgIG in triggering PEC trans-differentiation was investigated, by examining adenosine secretion in injured podocytes, as well as the expression of cluster of differentiation 44 (CD44), nephrin, adenosine receptor A2B (ARA2B) and glucocorticoid receptor (GR) in PECs both in vivo and in vitro.

RESULTS

Rats fed a high fructose diet exhibited progressive changes in glomerular PECs, including increased cell density and a preference for trans-differentiation. A positive correlation was observed between PEC density and podocyte loss. Co-culture experiments demonstrated that extracellular adenosine accumulation from injured podocytes induced by high fructose exposure promoted PEC trans-differentiation via ARA2B. MgIG significantly improved podocyte injury and exhibited effects similar to dexamethasone on nephrin upregulation and CD44 inhibition. Moreover, the effect of MgIG on PEC ARA2B activation was more effective than that of dexamethasone. The co-expression of paired box 2 (PAX2)+-Nephrin+ in glomeruli indicated that MgIG induced PEC trans-differentiation and podocyte regeneration in model rats. Accordingly, podocyte loss and increased urine albumin-to-creatinine ratio (UACR) were also alleviated. Moreover, MgIG, which acts as a GR agonist to activate GR, reversed the upregulation of CD44 and decreased ARA2B induced by tumor necrosis factor-α (TNF-α) in primary PECs. The siRNA interference experiment manifested that MgIG exhibited a more pronounced enhancement of GR upregulation, in contrast to ARA2B activation, to promote PEC trans-differentiation.

CONCLUSION

This work reports for the first time that PECs respond to the accumulation of extracellular adenosine from injured podocytes via activating ARA2B and focuses on the role of adenosine and adenosine receptors in the trans-differentiation of PECs. Furthermore, this study provides the first evidence that MgIG may promote podocyte regeneration by enhancing PEC trans-differentiation through GR activation, providing a research basis for investigating the glucocorticoid-like activity of MgIG in ameliorating glomerular podocyte injury.

How Stem and Progenitor Cells Can Affect Renal Diseases

Stem and progenitor cells have been observed to contribute to regenerative processes in acute renal failure and chronic kidney disease. Recent research has delved into the intricate mechanisms by which stem and progenitor cells exert their influence on kidney diseases. Understanding how these cells integrate with the existing renal architecture and their response to injury could pave the way for innovative treatment strategies aimed at promoting kidney repair and regeneration. Overall, the role of stem and progenitor cells in kidney diseases is multifaceted, with their ability to contribute to tissue regeneration, immune modulation, and the maintenance of renal homeostasis. Here, we review the studies that we have available today about the involvement of stem and progenitor cells both in regenerative therapies and in the causes of renal diseases, as well as in natural healing mechanisms, taking into account the main kidney disorders, such as IgA nephropathy, lupus nephritis, diabetic nephropathy, C3 glomerulopathy, focal segmental glomerulosclerosis, idiopathic membranous nephropathy, anti-glomerular basement membrane glomerulonephritis, and ANCA-associated crescentic glomerulonephritis. Moreover, based on the comprehensive data available in the framework of the specific kidney diseases on stem cells and renal progenitors, we hypothesize a possible role of adult renal progenitors in exacerbating or recovering the illness.

Preclinical Detection of Early Glomerular Injury in Children with Kidney Diseases-Independently of Usual Markers of Kidney Impairment and Inflammation

Glomerular kidney diseases typically begin insidiously and can progress to end stage kidney failure. Early onset of therapy can slow down disease progression. Early diagnosis is required to ensure such timely therapy. The goal of our study was to evaluate protein biomarkers (BMs) for common nephropathies that have been described for children with Alport syndrome. Nineteen candidate BMs were determined by commercial ELISA in children with congenital anomalies of the kidneys and urogenital tract, inflammatory kidney injury, or diabetes mellitus. It is particularly essential to search for kidney disease BMs in children because they are a crucial target group that likely exhibits early disease stages and in which misleading diseases unrelated to the kidney are rare. Only minor differences in blood between affected individuals and controls were found. However, in urine, several biomarker candidates alone or in combination seemed to be promising indicators of renal injury in early disease stages. The BMs of highest sensitivity and specificity were collagen type XIII, hyaluronan-binding protein 2, and complement C4-binding protein. These proteins are unrelated to inflammation markers or to risk factors for and signs of renal failure. In conclusion, our study evaluated several strong candidates for screening for early stages of kidney diseases and can help to establish early nephroprotective regimens.

Mice with a Pax2 missense variant display impaired glomerular repair

PAX2 regulates kidney development, and its expression persists in parietal epithelial cells (PECs), potentially serving as a podocyte reserve. We hypothesized that mice with a Pax2 pathogenic missense variant (Pax2A220G/+) have impaired PEC-mediated podocyte regeneration. Embryonic wild-type mouse kidneys showed overlapping expression of PAX2/Wilms' tumor-1 (WT-1) until PEC and podocyte differentiation, reflecting a close lineage relationship. Embryonic and adult Pax2A220G/+ mice have reduced nephron number but demonstrated no glomerular disease under baseline conditions. Pax2A220G/+ mice compared with wild-type mice were more susceptible to glomerular disease after adriamycin (ADR)-induced podocyte injury, as demonstrated by worsened glomerular scarring, increased podocyte foot process effacement, and podocyte loss. There was a decrease in PAX2-expressing PECs in wild-type mice after adriamycin injury accompanied by the occurrence of PAX2/WT-1-coexpressing glomerular tuft cells. In contrast, Pax2A220G/+ mice showed no changes in the numbers of PAX2-expressing PECs after adriamycin injury, associated with fewer PAX2/WT-1-coexpressing glomerular tuft cells compared with injured wild-type mice. A subset of PAX2-expressing glomerular tuft cells after adriamycin injury was increased in Pax2A220G/+ mice, suggesting a pathological process given the worse outcomes observed in this group. Finally, Pax2A220G/+ mice have increased numbers of glomerular tuft cells expressing Ki-67 and cleaved caspase-3 compared with wild-type mice after adriamycin injury, consistent with maladaptive responses to podocyte loss. Collectively, our results suggest that decreased glomerular numbers in Pax2A220G/+ mice are likely compounded with the inability of their mutated PECs to regenerate podocyte loss, and together these two mechanisms drive the worsened focal segmental glomerular sclerosis phenotype in these mice.NEW & NOTEWORTHY Congenital anomalies of the kidney and urinary tract comprise some of the leading causes of kidney failure in children, but our previous study showed that one of its genetic causes, PAX2, is also associated with adult-onset focal segmental glomerular sclerosis. Using a clinically relevant model, our present study demonstrated that after podocyte injury, parietal epithelial cells expressing PAX2 are deployed into the glomerular tuft to assist in repair in wild-type mice, but this mechanism is impaired in Pax2A220G/+ mice.

Podocyte number and glomerulosclerosis indices are associated with the response to therapy for primary focal segmental glomerulosclerosis

Corticosteroid therapy, often in combination with inhibition of the renin-angiotensin system, is first-line therapy for primary focal and segmental glomerulosclerosis (FSGS) with nephrotic-range proteinuria. However, the response to treatment is variable, and therefore new approaches to indicate the response to therapy are required. Podocyte depletion is a hallmark of early FSGS, and here we investigated whether podocyte number, density and/or size in diagnostic biopsies and/or the degree of glomerulosclerosis could indicate the clinical response to first-line therapy. In this retrospective single center cohort study, 19 participants (13 responders, 6 non-responders) were included. Biopsies obtained at diagnosis were prepared for analysis of podocyte number, density and size using design-based stereology. Renal function and proteinuria were assessed 6 months after therapy commenced. Responders and non-responders had similar levels of proteinuria at the time of biopsy and similar kidney function. Patients who did not respond to treatment at 6 months had a significantly higher percentage of glomeruli with global sclerosis than responders (p < 0.05) and glomerulosclerotic index (p < 0.05). Podocyte number per glomerulus in responders was 279 (203-507; median, IQR), 50% greater than that of non-responders (186, 118-310; p < 0.05). These findings suggest that primary FSGS patients with higher podocyte number per glomerulus and less advanced glomerulosclerosis are more likely to respond to first-line therapy at 6 months. A podocyte number less than approximately 216 per glomerulus, a GSI greater than 1 and percentage global sclerosis greater than approximately 20% are associated with a lack of response to therapy. Larger, prospective studies are warranted to confirm whether these parameters may help inform therapeutic decision making at the time of diagnosis of primary FSGS.

Dock5 Deficiency Promotes Proteinuric Kidney Diseases via Modulating Podocyte Lipid Metabolism

Podocytes are particularly sensitive to lipid accumulation, which has recently emerged as a crucial pathological process in the progression of proteinuric kidney diseases like diabetic kidney disease and focal segmental glomerulosclerosis. However, the underlying mechanism remains unclear. Here, podocytes predominantly expressed protein dedicator of cytokinesis 5 (Dock5) is screened to be critically related to podocyte lipid lipotoxicity. Its expression is reduced in both proteinuric kidney disease patients and mouse models. Podocyte-specific deficiency of Dock5 exacerbated podocyte injury and glomeruli pathology in proteinuric kidney disease, which is mainly through modulating fatty acid uptake by the liver X receptor α  (LXRα)/scavenger receptor class B (CD36) signaling pathway. Specifically, Dock5 deficiency enhanced CD36-mediated fatty acid uptake of podocytes via upregulating LXRα in an m6 A-dependent way. Moreover, the rescue of Dock5 expression ameliorated podocyte injury and proteinuric kidney disease. Thus, the findings suggest that Dock5 deficiency is a critical contributor to podocyte lipotoxicity and may serve as a promising therapeutic target in proteinuric kidney diseases.

What the BTBR/J mouse has taught us about diabetes and diabetic complications

Human and mouse genetics have delivered numerous diabetogenic loci, but it is mainly through the use of animal models that the pathophysiological basis for their contribution to diabetes has been investigated. More than 20 years ago, we serendipidously identified a mouse strain that could serve as a model of obesity-prone type 2 diabetes, the BTBR (Black and Tan Brachyury) mouse (BTBR T+ Itpr3tf/J, 2018) carrying the Lepob mutation. We went on to discover that the BTBR-Lepob mouse is an excellent model of diabetic nephropathy and is now widely used by nephrologists in academia and the pharmaceutical industry. In this review, we describe the motivation for developing this animal model, the many genes identified and the insights about diabetes and diabetes complications derived from >100 studies conducted in this remarkable animal model.

Diabetic nephropathy with marked extra-capillary cell proliferation: a case report

BACKGROUND

Extra-capillary hypercellularity is a common finding in crescentic glomerulonephritis (GN) and focal segmental glomerulosclerosis (FSGS). In diabetic nephropathy (DN), extra-capillary hypercellularity is often observed as a finding of complications such as IgA nephropathy or microscopic polyangiitis superimposed on DN. However, in rare cases, epithelial cell proliferation may accompany DN. We experienced a case of nodular diabetic glomerulosclerosis with marked extra-capillary hypercellularity and revealed the origin of this atypical lesion using immunostainings.

CASE PRESENTATION

A man in his 50 s was admitted to the hospital with nephrotic syndrome, and a renal biopsy was performed. Diffuse nodular lesions and extra-capillary hypercellularity were observed, but the results of serological examination or immunofluorescent assays did not implicate any other crescentic GN. Immunostaining for claudin-1 and nephrin was performed to identify the origin of the extra-capillary lesions. Given the clinical course and pathological findings, a diagnosis of DN-associated extra-capillary cell proliferation was made.

CONCLUSIONS

Extra-capillary hypercellularity, which resembles FSGS or crescentic GN, is a rare finding in DN and should therefore be treated with caution. In such cases, co-staining for claudin-1 and nephrin may facilitate the diagnosis of DN.

Sestrin2 Signaling Pathway Regulates Podocyte Biology and Protects against Diabetic Nephropathy

Sestrin2 regulates cell homeostasis and is an upstream signaling molecule for several signaling pathways. Sestrin2 leads to AMP-activated protein kinase- (AMPK-) and GTPase-activating protein activity toward Rags (GATOR) 1-mediated inhibition of mammalian target of rapamycin complex 1 (mTORC1), thereby enhancing autophagy. Sestrin2 also improves mitochondrial biogenesis via AMPK/Sirt1/peroxisome proliferator-activated receptor-gamma coactivator 1 alpha (PGC-1α) signaling pathway. Blockade of ribosomal protein synthesis and augmentation of autophagy by Sestrin2 can prevent misfolded protein accumulation and attenuate endoplasmic reticulum (ER) stress. In addition, Sestrin2 enhances P62-mediated autophagic degradation of Keap1 to release nuclear factor erythroid 2-related factor 2 (Nrf2). Nrf2 release by Sestrin2 vigorously potentiates antioxidant defense in diabetic nephropathy. Impaired autophagy and mitochondrial biogenesis, severe oxidative stress, and ER stress are all deeply involved in the development and progression of diabetic nephropathy. It has been shown that Sestrin2 expression is lower in the kidney of animals and patients with diabetic nephropathy. Sestrin2 knockdown aggravated diabetic nephropathy in animal models. In contrast, upregulation of Sestrin2 enhanced autophagy, mitophagy, and mitochondrial biogenesis and suppressed oxidative stress, ER stress, and apoptosis in diabetic nephropathy. Consistently, overexpression of Sestrin2 ameliorated podocyte injury, mesangial proliferation, proteinuria, and renal fibrosis in animal models of diabetic nephropathy. By suppressing transforming growth factor beta (TGF-β)/Smad and Yes-associated protein (YAP)/transcription enhancer factor 1 (TEF1) signaling pathways in experimental models, Sestrin2 hindered epithelial-mesenchymal transition and extracellular matrix accumulation in diabetic kidneys. Moreover, modulation of the downstream molecules of Sestrin2, for instance, augmentation of AMPK or Nrf2 signaling and inhibition of mTORC1, has been protective in diabetic nephropathy. Regarding the beneficial effects of Sestrin2 on diabetic nephropathy and its interaction with several signaling molecules, it is worth targeting Sestrin2 in diabetic nephropathy.

Cellular phenotypic transitions in diabetic nephropathy: An update

Diabetic nephropathy (DN) is a major cause of morbidity and mortality in diabetes and is the most common cause of end stage renal disease (ESRD). Renal fibrosis is the final pathological change in DN. It is widely believed that cellular phenotypic switching is the cause of renal fibrosis in diabetic nephropathy. Several types of kidney cells undergo activation and differentiation and become reprogrammed to express markers of mesenchymal cells or podocyte-like cells. However, the development of targeted therapy for DN has not yet been identified. Here, we discussed the pathophysiologic changes of DN and delineated the possible origins that contribute to myofibroblasts and podocytes through phenotypic transitions. We also highlight the molecular signaling pathways involved in the phenotypic transition, which would provide valuable information for the activation of phenotypic switching and designing effective therapies for DN.

Mechanisms of podocyte injury and implications for diabetic nephropathy

Albuminuria is the hallmark of both primary and secondary proteinuric glomerulopathies, including focal segmental glomerulosclerosis (FSGS), obesity-related nephropathy, and diabetic nephropathy (DN). Moreover, albuminuria is an important feature of all chronic kidney diseases (CKDs). Podocytes play a key role in maintaining the permselectivity of the glomerular filtration barrier (GFB) and injury of the podocyte, leading to foot process (FP) effacement and podocyte loss, the unifying underlying mechanism of proteinuric glomerulopathies. The metabolic insult of hyperglycemia is of paramount importance in the pathogenesis of DN, while insults leading to podocyte damage are poorly defined in other proteinuric glomerulopathies. However, shared mechanisms of podocyte damage have been identified. Herein, we will review the role of haemodynamic and oxidative stress, inflammation, lipotoxicity, endocannabinoid (EC) hypertone, and both mitochondrial and autophagic dysfunction in the pathogenesis of the podocyte damage, focussing particularly on their role in the pathogenesis of DN. Gaining a better insight into the mechanisms of podocyte injury may provide novel targets for treatment. Moreover, novel strategies for boosting podocyte repair may open the way to podocyte regenerative medicine.

The Role of Parietal Epithelial Cells in the Pathogenesis of Podocytopathy

Podocytopathy is the most common feature of glomerular disorder characterized by podocyte injury- or dysfunction-induced excessive proteinuria, which ultimately develops into glomerulosclerosis and results in persistent loss of renal function. Due to the lack of self-renewal ability of podocytes, mild podocyte depletion triggers replacement and repair processes mostly driven by stem cells or resident parietal epithelial cells (PECs). In contrast, when podocyte recovery fails, activated PECs contribute to the establishment of glomerular lesions. Increasing evidence suggests that PECs, more than just bystanders, have a crucial role in various podocytopathies, including minimal change disease, focal segmental glomerulosclerosis, membranous nephropathy, diabetic nephropathy, IgA nephropathy, and lupus podocytopathy. In this review, we attempt to dissect the diverse role of PECs in the pathogenesis of podocytopathy based on currently available information.

Molecular Mechanisms of Kidney Injury and Repair

Chronic kidney disease (CKD) will become the fifth global cause of death by 2040, thus emphasizing the need to better understand the molecular mechanisms of damage and regeneration in the kidney. CKD predisposes to acute kidney injury (AKI) which, in turn, promotes CKD progression. This implies that CKD or the AKI-to-CKD transition are associated with dysfunctional kidney repair mechanisms. Current therapeutic options slow CKD progression but fail to treat or accelerate recovery from AKI and are unable to promote kidney regeneration. Unraveling the cellular and molecular mechanisms involved in kidney injury and repair, including the failure of this process, may provide novel biomarkers and therapeutic tools. We now review the contribution of different molecular and cellular events to the AKI-to-CKD transition, focusing on the role of macrophages in kidney injury, the different forms of regulated cell death and necroinflammation, cellular senescence and the senescence-associated secretory phenotype (SAPS), polyploidization, and podocyte injury and activation of parietal epithelial cells. Next, we discuss key contributors to repair of kidney injury and opportunities for their therapeutic manipulation, with a focus on resident renal progenitor cells, stem cells and their reparative secretome, certain macrophage subphenotypes within the M2 phenotype and senescent cell clearance.

Punicalagin alleviates renal injury via the gut-kidney axis in high-fat diet-induced diabetic mice

Diabetic renal injury was associated with dysbiosis of the gut microbiota and intestinal barrier. Punicalagin (PU) from pomegranates potentially impacts the microbial ecosystem, intestinal barrier, and renal function. Therefore, we hypothesized that PU may improve diabetic renal injury by modulating the gut-kidney axis. The present study evaluated the effect of PU on the gut-kidney axis and kidney function in a diabetic renal injury mouse model induced by a high-fat diet (HFD). Mice were fed a HFD without PU or with at doses of 50 and 100 mg kg^-1 d^-1 for 8 weeks. Targeted metabolomics by GC-MS and 16S rRNA sequencing were implemented to determine short-chain fatty acids (SCFAs) and microbes. Further RNA sequencing analyses were performed to determine which differentially expressed genes were changed by PU. Compared with the DM model group, PU supplementation improved diabetic renal injury, ameliorated kidney architecture and function, and reshaped gut microbial ecology. Additionally, PU reversed HFD-induced gut barrier dysfunction, promoted cecal SCFA concentrations and inhibited serum lipopolysaccharide (LPS) and diamine oxidase (DAO) levels. Moreover, correlation analysis found that cecal SCFAs were significantly negatively correlated with inflammation-related genes in the kidney. The present results indicated that PU, a promising bioactive polyphenol, successfully improved diabetic renal injury, most likely through the gut-kidney axis.

Regression of diabetic nephropathy by treatment with empagliflozin in BTBR ob/ob mice

BACKGROUND

The SGLT2 inhibitor empagliflozin lowers blood glucose via reduced tubular reabsorption of filtered glucose and is an important new therapy for diabetic nephropathy (DN). This study tested whether treatment with empagliflozin would ameliorate proteinuria and the pathologic alterations of DN including podocyte number and integrity in the leptin deficient BTBR ob/ob mouse model of DN.

METHODS

Study cohorts included wild type BTBR mice, untreated diabetic BTBR ob/ob mice, and mice treated with empagliflozin for six weeks after development of established DN at 18 weeks of age.

RESULTS

Hyperglycemia, proteinuria, serum creatinine, accumulation of mesangial matrix and the extent of mesangiolysis were reversed with empagliflozin treatment. Treatment with empagliflozin resulted in increased podocyte number and podocyte density, improvement in the degree of podocyte foot process effacement and parietal epithelial cell activation. SGLT2 inhibition reduced renal oxidative stress, measured by urinary excretion of markers of RNA/DNA damage and in situ demonstration of decreased carbonyl oxidation. There was no discernable difference in accumulations of advanced glycation endproducts by immunohistochemistry.

CONCLUSION

The structural improvements seen in BTBR ob/ob mice treated with empagliflozin provide insight into potential long term benefits for humans with DN, for whom there is no comparable biopsy information to identify structural changes effected by SGLT2 inhibition. The findings suggest SGLT2 inhibition may ameliorate diabetic nephropathy through glucose lowering-dependent and -independent mechanisms that lead to podocyte restoration and delay or reversal of the disease progress.

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.

The recruitment mechanisms and potential therapeutic targets of podocytes from parietal epithelial cells

Podocytes are differentiated postmitotic cells which cannot be replaced after podocyte injury. The mechanism of podocyte repopulation after injury has aroused wide concern. Parietal epithelial cells (PECs) are heterogeneous and only a specific subpopulation of PECs has the capacity to replace podocytes. Major progress has been achieved in recent years regarding the role and function of a subset of PECs which could transdifferentiate toward podocytes. Additionally, several factors, such as Notch, Wnt/ß-catenin, Wilms’ tumor-1, miR-193a and growth arrest-specific protein 1, have been shown to be involved in these processes. Finally, PECs serve as a potential therapeutic target in the conditions of podocyte loss. In this review, we discuss the latest observations and concepts about the recruitment of podocytes from PECs in glomerular diseases as well as newly identified mechanisms and the most recent treatments for this process.

Parietal epithelial cell dysfunction in crescentic glomerulonephritis

Crescentic glomerulonephritis represents a group of kidney diseases characterized by rapid loss of kidney function and the formation of glomerular crescents. While the role of the immune system has been extensively studied in relation to the development of crescents, recent findings show that parietal epithelial cells play a key role in the pathophysiology of crescent formation, even in the absence of immune modulation. This review highlights our current understanding of parietal epithelial cell biology and the reported physiological and pathological roles that these cells play in glomerular lesion formation, especially in the context of crescentic glomerulonephritis.

Establishment and characterization of a novel conditionally immortalized human parietal epithelial cell line

Parietal epithelial cells (PECs) are epithelial cells in the kidney, surrounding Bowman's space. When activated, PECs increase in cell volume, proliferate, migrate to the glomerular tuft and excrete extracellular matrix. Activated PECs are crucially involved in the formation of sclerotic lesions, seen in focal segmental glomerulosclerosis (FSGS). In FSGS, a number of glomeruli show segmental sclerotic lesions. Further disease progression will lead to increasing number of involved glomeruli and gradual destruction of the affected glomeruli. Although the involvement of PECs in FSGS has been acknowledged, little is known about the molecular processes driving PEC activation. To get more insights in this process, accurate in vivo and in vitro models are needed. Here, we describe the development and characterization of a novel conditionally immortalized human PEC (ciPEC) line. We demonstrated that ciPECs are differentiated when grown under growth-restrictive conditions and express important PEC-specific markers, while lacking podocyte and endothelial markers. In addition, ciPECs showed PEC-like morphology and responded to IL-1β treatment. We therefore conclude that we have successfully generated a novel PEC line, which can be used for future studies on the role of PECs in FSGS.

Combined Melatonin and Extracorporeal Shock Wave Therapy Enhances Podocyte Protection and Ameliorates Kidney Function in a Diabetic Nephropathy Rat Model

(1) Background: Diabetic nephropathy (DN) is common complication of diabetes. Current therapy for DN does not include promotion of podocyte protection. Therefore, we investigated the therapeutic effect of melatonin (Mel) combined extracorporeal shock wave (SW) therapy on a DN rat model. (2) Methods: The DN rats were treated with Mel (5 mg/kg) twice a week for 6 weeks and SW treatment once a week (0.13 mJ/mm²) for 6 weeks. We assessed urine microalbumin, albumin to creatinine ratio (ACR), glomerular hypertrophy, glomerular fibrosis, podocyte markers (Wilm’s tumor protein-1, synaptopodin and nephrin), cell proliferation, cell survival, cell apoptosis, renal inflammation and renal oxidative stress. (3) Results: The Mel combined SW therapy regimen significantly reduced urine microalbumin excretion (3.3 ± 0.5 mg/dL, p < 0.001), ACR (65.2 ± 8.3 mg/g, p < 0.001), glomerular hypertrophy (3.1 ± 0.1 × 10⁶ μm³, p < 0.01) and glomerular fibrosis (0.9 ± 0.4 relative mRNA fold, p < 0.05). Moreover, the Mel combined SW therapy regimen significantly increased podocyte number (44.1 ± 5.0% area of synaptopodin, p < 0.001) in the Mel combined SW group. This is likely primarily because Mel combined with SW therapy significantly reduced renal inflammation (753 ± 46 pg/mg, p < 0.01), renal oxidative stress (0.6 ± 0.04 relative density, p < 0.05), and apoptosis (0.3 ± 0.03 relative density, p < 0.001), and also significantly increased cell proliferation (2.0 ± 0.2% area proliferating cell nuclear antigen (PCNA), p < 0.01), cell survival, and nephrin level (4.2 ± 0.4 ng/mL, p < 0.001). (4) Conclusions: Mel combined SW therapy enhances podocyte protection and ameliorates kidney function in a DN rat model. Mel combined SW therapy may serve as a novel noninvasive and effective treatment of DN.

Growth hormone induces mitotic catastrophe of glomerular podocytes and contributes to proteinuria

Glomerular podocytes are integral members of the glomerular filtration barrier in the kidney and are crucial for glomerular permselectivity. These highly differentiated cells are vulnerable to an array of noxious stimuli that prevail in several glomerular diseases. Elevated circulating growth hormone (GH) levels are associated with podocyte injury and proteinuria in diabetes. However, the precise mechanism(s) by which excess GH elicits podocytopathy remains to be elucidated. Previous studies have shown that podocytes express GH receptor (GHR) and induce Notch signaling when exposed to GH. In the present study, we demonstrated that GH induces TGF-β1 signaling and provokes cell cycle reentry of otherwise quiescent podocytes. Though differentiated podocytes reenter the cell cycle in response to GH and TGF-β1, they cannot accomplish cytokinesis, despite karyokinesis. Owing to this aberrant cell cycle event, GH- or TGF-β1-treated cells remain binucleated and undergo mitotic catastrophe. Importantly, inhibition of JAK2, TGFBR1 (TGF-β receptor 1), or Notch prevented cell cycle reentry of podocytes and protected them from mitotic catastrophe associated with cell death. Inhibition of Notch activation prevents GH-dependent podocyte injury and proteinuria. Similarly, attenuation of GHR expression abated Notch activation in podocytes. Kidney biopsy sections from patients with diabetic nephropathy (DN) show activation of Notch signaling and binucleated podocytes. These data indicate that excess GH induced TGF-β1-dependent Notch1 signaling contributes to the mitotic catastrophe of podocytes. This study highlights the role of aberrant GH signaling in podocytopathy and the potential application of TGF-β1 or Notch inhibitors, as a therapeutic agent for DN.

Diabetic condition induces hypertrophy and vacuolization in glomerular parietal epithelial cells

Diabetic nephropathy (DN) is accompanied by characteristic changes in the glomerulus, but little is known about the effect of diabetes on parietal epithelial cells (PECs). In this study, a descriptive analysis of PECs was undertaken in diabetic db/db mice and in diabetic patients. PEC hypertrophy was significantly more prominent in diabetic mice than in nondiabetic mice, and this was evident even at the early stage. Additionally, the number of vacuoles in PECs was markedly increased in diabetic mice, suggesting the presence of cellular injury in PECs in DN. Although rare, binuclear cells were observed in mice with early diabetes. In cultured PECs, a high glucose condition, compared with normal glucose condition, induced cellular hypertrophy and apoptosis. Flow cytometry showed that some PECs in the G0 phase reentered the cell cycle but got arrested in the S phase. Finally, in human diabetic subjects, hypertrophy and vacuolization were observed in the PECs. Our data showed that PECs undergo substantial changes in DN and may participate in rearrangement for differentiation into podocytes.

miR-379 deletion ameliorates features of diabetic kidney disease by enhancing adaptive mitophagy via FIS1

Diabetic kidney disease (DKD) is a major complication of diabetes. Expression of members of the microRNA (miRNA) miR-379 cluster is increased in DKD. miR-379, the most upstream 5'-miRNA in the cluster, functions in endoplasmic reticulum (ER) stress by targeting EDEM3. However, the in vivo functions of miR-379 remain unclear. We created miR-379 knockout (KO) mice using CRISPR-Cas9 nickase and dual guide RNA technique and characterized their phenotype in diabetes. We screened for miR-379 targets in renal mesangial cells from WT vs. miR-379KO mice using AGO2-immunopreciptation and CLASH (cross-linking, ligation, sequencing hybrids) and identified the redox protein thioredoxin and mitochondrial fission-1 protein. miR-379KO mice were protected from features of DKD as well as body weight loss associated with mitochondrial dysfunction, ER- and oxidative stress. These results reveal a role for miR-379 in DKD and metabolic processes via reducing adaptive mitophagy. Strategies targeting miR-379 could offer therapeutic options for DKD.

Renal tubular Bim mediates the tubule-podocyte crosstalk via NFAT2 to induce podocyte cytoskeletal dysfunction

Diabetic nephropathy (DN) is mainly regarded as diabetic glomerulopathy, and its progression is tightly correlated with tubular epithelial lesions. However, the underlying molecular mechanisms linking tubular damage and glomerulopathy are poorly understood.

Methods: We previously reported that the upregulation of Bim mediated proximal tubular epithelial cell (PTEC) apoptosis and was crucial in the early stages of DN. Herein we modulated Bim expression in PTECs and subsequently determined podocyte (PC) cytoskeletal arrangement by building a Transwell co-culture system in high glucose (HG).

Results: Compared to normal glucose, exposure to 40 mM of HG for 48 h induced significant expression of Bim in PTECs and disorganization in the PC cytoskeleton. When cocultured with PTECs in HG, exacerbated filamentous actin (F-actin) rearrangement and reduced synaptopodin levels were detected in PCs. In contrast, gene knockdown of Bim in PTECs was correlated with the absence of PC cytoskeletal disorganization. NFAT2 level and its nuclear translocation in PTECs were decreased by suppressing Bim expression. Upregulating NFAT2 disrupted the beneficial effects on F-actin organization in PCs obtained by inhibiting Bim. LncRNA microarray analysis identified NONHSAT179542.1, which was implicated in Bim-mediated PC cytoskeletal disorder.

Conclusion: Our study clarified the functional role of Bim, a pro-apoptotic factor, which is involved in the crosstalk between PTECs and PCs. Bim promotes NFAT2 activation in PTECs, inducing the downregulation of lncRNA NONHSAT179542.1 in PCs, contributing to the cytoskeletal damage. Identification of the role of the Bim/NFAT2 pathway may represent a promising research direction for a better understanding of DN development.

Podocyte and endothelial-specific elimination of BAMBI identifies differential transforming growth factor-β pathways contributing to diabetic glomerulopathy

Transforming growth factor-β (TGF-β) is a central mediator of diabetic nephropathy. The effect of TGF-β, mediated by the type I TGF-β receptor, ALK5, and subsequent Smad2/3 activation results in podocyte apoptosis and loss. Previously, we demonstrated that the genetic deletion of the BMP and Activin Membrane-Bound Inhibitor (BAMBI), a negative modulator TGF-β signaling, accelerates diabetic nephropathy in mice. This was associated with heightened ALK1-mediated activation of Smad1/5 in the glomerular endothelial cells (ECs). Therefore, to evaluate the glomerular cell-specific effects of TGF-β in diabetic nephropathy we examined the effects of the podocyte- or EC-specific loss of Bambi (Pod-Bambi-/- or EC-Bambi-/-) in streptozotocin-induced diabetic mice with endothelial nitric oxide synthase deficiency. Interestingly, although hyperglycemia and body weight loss were similar in all groups of diabetic mice, significant hypertension was present only in the diabetic EC-Bambi-/- mice. While the podocyte or EC-specific loss of BAMBI both accelerated the progression of diabetic nephropathy, the worsened podocyte injury and loss observed in the diabetic Pod-Bambi-/- mice were associated with enhanced Smad3 activation. Increased Smad1/5 activation and EC proliferation were apparent only in the glomeruli of diabetic EC-Bambi-/- mice. The enhanced Smad1/5 activation in diabetic EC-Bambi-/- mice was associated with increased glomerular expression of plasmalemma vesicle-associated protein, pointing to the involvement of immature or dedifferentiated glomerular ECs in diabetic nephropathy. Notably, diabetic EC-Bambi-/- mice displayed podocyte injury and loss that were comparable to diabetic Pod-Bambi-/- mice. Thus, our results highlight the glomerular cell-specific contribution of TGF-β signaling and the intricate cross-talk between injured glomerular cells in the progression of diabetic nephropathy.

Beneficial effect on podocyte number in experimental diabetic nephropathy resulting from combined atrasentan and RAAS inhibition therapy

Podocyte loss and proteinuria are both key features of human diabetic nephropathy (DN). The leptin-deficient BTBR mouse strain with the ob/ob mutation develops progressive weight gain, type 2 diabetes, and diabetic nephropathy that has many features of advanced human DN, including increased mesangial matrix, mesangiolysis, podocyte loss, and proteinuria. Selective antagonism of the endothelin-1 type A receptor (ET_AR) by atrasentan treatment in combination with renin-angiotensin-aldosterone system inhibition with losartan has been shown to have the therapeutic benefit of lowering proteinuria in patients with DN, but the underlying mechanism for this benefit is not well understood. Using a similar therapeutic approach in diabetic BTBR ob/ob mice, this treatment regimen significantly increased glomerular podocyte number compared with diabetic BTBR ob/ob controls and suggested that parietal epithelial cells were a source for podocyte restoration. Atrasentan treatment alone also increased podocyte number but to a lesser degree. Mice treated with atrasentan demonstrated a reduction in proteinuria, matching the functional improvement reported in humans. This is a first demonstration that treatment with the highly selective ET_AR antagonist atrasentan can lead to restoration of the diminished podocyte number characteristic of DN in humans and thereby underlies the reduction in proteinuria in patients with diabetes undergoing similar treatment. The benefit of ET_AR antagonism in DN extended to a decrease in mesangial matrix as measured by a reduction in accumulations of collagen type IV in both the atrasentan and atrasentan + losartan-treated groups compared with untreated controls.

Renal progenitor cells modulated by angiotensin II receptor blocker (ARB) medication and differentiation towards podocytes in anti-thy1.1 nephritis

Background

Mesangial proliferative glomerulonephritis (MsPGN) is an epidemic disease with increasing occurrence. As important as mesangial cells, podocytes are key innate cells for MsPGN prognosis and recovery. Renal progenitor cells, located at the urinary pole (UP) of Bowman’s capsule (BC), could alleviate kidney injury through their capacity to differentiate into podocytes.

Methods

Seventy-two male rats were categorized randomly into the sham (n=24), untreated Thy-1 (n=24) and losartan-treated (n=24) groups. We administered vehicle or losartan (50 mg/kg by gavage) daily to treat rats with anti-thy1.1 nephritis, an ideal model to simulate human MsPGN. Two weeks after the intravenous injection of antibody, urinary protein and blood samples were analyzed, pathological changes were examined, the number of podocytes was determined, and renal progenitor cells were studied.

Results

Anti-thy1.1 nephritis was significantly alleviated after losartan treatment, as reported previously and as expected. Compared with the untreated Thy-1 group, the number of podocytes in the losartan group increased, and the area of renal progenitor cells significantly increased. The protein expression of components of the p-ERK pathway was determined during the development of renal progenitor cells differentiating into podocytes.

Conclusions

The data in this paper show the direct glomerular cell action of angiotensin II receptor blocker (ARB) treatment in improving outcomes in anti-thy1.1 nephritis. The positive effects of ARB medication on anti-thy1.1 nephritis were due to an increase in the number of renal epithelial progenitor cells (defined as PECs that expressed only stem cell markers without podocyte proteins).

Panax Notoginseng Ameliorates Podocyte EMT by Targeting the Wnt/β-Catenin Signaling Pathway in STZ-Induced Diabetic Rats

Introduction

Epithelial–mesenchymal transition (EMT) may contribute to podocyte dysfunction in diabetic nephropathy (DN). Aiming to identify novel therapeutic options, we investigated the protective effects of Panax notoginseng (PN) on podocyte EMT in diabetic rats and explored its mechanisms.

Methods

Diabetes was induced in rats with streptozotocin (STZ) by intraperitoneal injection at 55 mg/kg. Diabetic rats were randomly divided into three groups, namely, diabetic rats, diabetic rats treated with beraprost sodium (BPS) at 0.6 mg/kg/d or PN at 0.4 g/kg/d p.o., for 12 weeks. Urinary albumin/creatinine ratio (ACR), biochemical parameters, renal histopathology, and podocyte morphological changes were evaluated. Protein expression of EMT markers (desmin, α-SMA, and nephrin) as well as components of the Wnt/β-catenin pathway (wnt1, β-catenin, and snail) was detected by immunohistochemistry and Western blot, respectively.

Results

In diabetic rats, severe hyperglycemia and albuminuria were detected. Moreover, mesangial expansion and podocyte foot process effacement were found markedly increased in diabetic kidneys. Increased protein expression of wnt1, β-catenin, snail, desmin, and α-SMA, as well as decreased protein expression of nephrin was detected in diabetic kidneys. All these abnormalities found in DN rats were partially restored by PN treatment.

Conclusion

PN ameliorated albuminuria and podocyte EMT in diabetic rats partly through inhibiting Wnt/β-catenin signaling pathway. These findings provide experimental arguments for a novel therapeutic option in DN.

The Presence and Location of Podocytes in Glomeruli as Affected by Diabetes Mellitus

The primary purpose of the kidney, specifically the glomerulus, is filtration. Filtration is accomplished through the glomerular filtration barrier, which consists of the fenestrated endothelium, glomerular basement membrane, and specialized epithelial cells called podocytes. In pathologic states, such as Diabetes Mellitus (DM) and diabetic kidney disease (DKD), variable glomerular conditions result in podocyte injury and depletion, followed by progressive glomerular injury and DKD progression. In this work we quantified glomerulus and podocyte structural changes in histopathology image data derived from a murine model of DM. Using a variety of image processing techniques, we studied changes in podocyte morphology and intra-glomerular distribution across healthy, mild DM, and DM glomeruli. Our feature analysis provided feature trends which we believe are reflective of DKD pathology; while glomerular area peaked in mild DM, average podocyte number and distance from the urinary pole continued to decrease and increase, respectively, throughout DM. Ultimately, this study aims to augment the set of quantifiable image biomarkers used for evaluation of DKD progression in digital pathology, as well as underscore the importance of engineering biologically-inspired image features.

Differential expression of parietal epithelial cell and podocyte extracellular matrix proteins in focal segmental glomerulosclerosis and diabetic nephropathy

In healthy glomeruli, parietal epithelial cell (PEC)-derived extracellular matrix (ECM) proteins include laminin-β1, perlecan, and collagen type IV-α2 and podocyte-specific ECM proteins include laminin-β2, agrin, and collagen type IV-α4. This study aimed to define individual ECM protein isoform expression by PECs in both experimental and human focal segmental glomerulosclerosis (FSGS) and diabetic nephropathy (DN) and to determine if changes were CD44 dependent. In experimental FSGS induced with a cytotoxic podocyte antibody and in the BTBR ob/ob mouse model of DN, PEC-derived protein staining was significantly increased in PECs. Dual staining also showed de novo expression of the podocyte-specific ECM proteins laminin-β2 and agrin in PECs. Similar findings were observed in biopsies from patients with FSGS and DN. Increases in individual ECM proteins colocalized with CD44 in PECs in disease. To determine the role of CD44, FSGS was induced in CD44-/- and CD44+/+ mice. PEC staining for perlecan, collagen type IV-α2, laminin-β2, and agrin were significantly lower in diseased CD44-/- mice compared with diseased CD44+/+ mice. These results show that in experimental and human FSGS and DN, PECs typically in an activated state, produce both PEC-derived and podocyte-specific ECM protein isoforms, and that the majority of these changes were dependent on CD44.

Histological Examination of the Diabetic Kidney

The increasing prevalence of diabetes worldwide has led to a concomitant rise in diabetic kidney disease (DKD) as a major cause of end-stage renal disease. Glomerular lesions constitute the most striking and consistent features identified in biopsies from patients with DKD, although tubulointerstitial injury has an important and often under-recognized role in the progression to overt nephropathy. In advanced stages of the disease, podocyte detachment is a pivotal event in the loss of glomerular filtration barrier integrity and may explain, at least in part, the inability of current therapies to halt renal function decline. This chapter details the systematic method that can be used to study renal tissue samples from diabetic patients, and the specific role of different imaging techniques, such as light microscopy, immunofluorescence microscopy, and transmission and scanning electron microscopy in detecting histologic lesions specific to DKD.

Mechanisms of Scarring in Focal Segmental Glomerulosclerosis

Focal segmental glomerulosclerosis (FSGS) presents with scar in parts of some glomeruli and often progresses to global and diffuse glomerulosclerosis. Podocyte injury is the initial target in primary FSGS, induced by a circulating factor. Several gene variants, for example, APOL1, are associated with increased susceptibility to FSGS. Primary FSGS may be due to genetic mutation in key podocyte genes. Increased work stress after loss of nephrons, epigenetic mechanisms, and various profibrotic pathways can contribute to progressive sclerosis, regardless of the initial injury. The progression of FSGS lesions also involves crosstalk between podocytes and other kidney cells, such as parietal epithelial cells, glomerular endothelial cells, and even tubular epithelial cells. New insights related to these mechanisms could potentially lead to new therapeutic strategies to prevent progression of FSGS.

Low-Energy Extracorporeal Shock Wave Therapy Ameliorates Kidney Function in Diabetic Nephropathy

Background

Diabetic nephropathy is the most common cause of end-stage renal disease. Traditional therapy for diabetic nephropathy has focused on supportive treatment, and there is no significant effective therapy. We investigated the effect of low-energy extracorporeal shock wave therapy on a diabetic nephropathy rat model.

Methods

Streptozotocin-induced diabetic nephropathy rats were treated with six sessions of low-energy extracorporeal shock wave therapy (weekly for six consecutive weeks) or left untreated. We assessed urinary creatinine and albumin, glomerular volume, renal fibrosis, podocyte number, renal inflammation, oxidative stress, and tissue repair markers (SDF-1 and VEGF) six weeks after the completion of treatment.

Results

The six-week low-energy extracorporeal shock wave therapy regimen decreased urinary albumin excretion as well as reduced glomerular hypertrophy and renal fibrosis in the rat model of diabetic nephropathy. Moreover, low-energy extracorporeal shock wave therapy increased podocyte number in diabetic nephropathy rats. This was likely primarily attributed to the fact that low-energy extracorporeal shock wave therapy reduced renal inflammation and oxidative stress as well as increased tissue repair potency and cell proliferation.

Conclusions

Low-energy extracorporeal shock wave therapy preserved kidney function in diabetic nephropathy. Low-energy extracorporeal shock wave therapy may serve as a novel noninvasive and effective treatment of diabetic nephropathy.

FGF1ΔHBS ameliorates chronic kidney disease via PI3K/AKT mediated suppression of oxidative stress and inflammation

Currently, there is a lack of effective therapeutic approaches to the treatment of chronic kidney disease (CKD) with irreversible deterioration of renal function. This study aimed to investigate the ability of mutant FGF1 (FGF1^ΔHBS, which has reduced mitogenic activity) to alleviate CKD and to study its associated mechanisms. We found that FGF1^ΔHBS exhibited much weaker mitogenic activity than wild-type FGF1 (FGF1^WT) in renal tissues. RNA-seq analysis revealed that FGF1^ΔHBS inhibited oxidative stress and inflammatory signals in mouse podocytes challenged with high glucose. These antioxidative stress and anti-inflammatory activities of FGF1^ΔHBS prevented CKD in two mouse models: a diabetic nephropathy model and an adriamycin-induced nephropathy model. Further mechanistic analyses suggested that the inhibitory effects of FGF1^ΔHBS on oxidative stress and inflammation were mediated by activation of the GSK-3β/Nrf2 pathway and inhibition of the ASK1/JNK signaling pathway, respectively. An in-depth study demonstrated that both pathways are under control of PI3K/AKT signaling activated by FGF1^ΔHBS. This finding expands the potential uses of FGF1^ΔHBS for the treatment of various kinds of CKD associated with oxidative stress and inflammation.

FoxO3 activation in hypoxic tubules prevents chronic kidney disease

Acute kidney injury (AKI) can lead to chronic kidney disease (CKD) if injury is severe and/or repair is incomplete. However, the pathogenesis of CKD following renal ischemic injury is not fully understood. Capillary rarefaction and tubular hypoxia are common findings during the AKI to CKD transition. We investigated the tubular stress response to hypoxia and demonstrated that a stress responsive transcription factor, FoxO3, was regulated by prolyl hydroxylase. Hypoxia inhibited FoxO3 prolyl hydroxylation and FoxO3 degradation, thus leading to FoxO3 accumulation and activation in tubular cells. Hypoxia-activated Hif-1α contributed to FoxO3 activation and functioned to protect kidneys, as tubular deletion of Hif-1α decreased hypoxia-induced FoxO3 activation, and resulted in more severe tubular injury and interstitial fibrosis following ischemic injury. Strikingly, tubular deletion of FoxO3 during the AKI to CKD transition aggravated renal structural and functional damage leading to a more profound CKD phenotype. We showed that tubular deletion of FoxO3 resulted in decreased autophagic response and increased oxidative injury, which may explain renal protection by FoxO3. Our study indicates that in the hypoxic kidney, stress responsive transcription factors can be activated for adaptions to counteract hypoxic insults, thus attenuating CKD development.

LRG1 Promotes Diabetic Kidney Disease Progression by Enhancing TGF-β-Induced Angiogenesis

BACKGROUND

Glomerular endothelial dysfunction and neoangiogenesis have long been implicated in the pathogenesis of diabetic kidney disease (DKD). However, the specific molecular pathways contributing to these processes in the early stages of DKD are not well understood. Our recent transcriptomic profiling of glomerular endothelial cells identified a number of proangiogenic genes that were upregulated in diabetic mice, including leucine-rich α-2-glycoprotein 1 (LRG1). LRG1 was previously shown to promote neovascularization in mouse models of ocular disease by potentiating endothelial TGF-β/activin receptor-like kinase 1 (ALK1) signaling. However, LRG1's role in the kidney, particularly in the setting of DKD, has been unclear.

METHODS

We analyzed expression of LRG1 mRNA in glomeruli of diabetic kidneys and assessed its localization by RNA in situ hybridization. We examined the effects of genetic ablation of Lrg1 on DKD progression in unilaterally nephrectomized, streptozotocin-induced diabetic mice at 12 and 20 weeks after diabetes induction. We also assessed whether plasma LRG1 was associated with renal outcome in patients with type 2 diabetes.

RESULTS

LRG1 localized predominantly to glomerular endothelial cells, and its expression was elevated in the diabetic kidneys. LRG1 ablation markedly attenuated diabetes-induced glomerular angiogenesis, podocyte loss, and the development of diabetic glomerulopathy. These improvements were associated with reduced ALK1-Smad1/5/8 activation in glomeruli of diabetic mice. Moreover, increased plasma LRG1 was associated with worse renal outcome in patients with type 2 diabetes.

CONCLUSIONS

These findings identify LRG1 as a potential novel pathogenic mediator of diabetic glomerular neoangiogenesis and a risk factor in DKD progression.

Integrated Functional Genomic Analysis Enables Annotation of Kidney Genome-Wide Association Study Loci

BACKGROUND

Linking genetic risk loci identified by genome-wide association studies (GWAS) to their causal genes remains a major challenge. Disease-associated genetic variants are concentrated in regions containing regulatory DNA elements, such as promoters and enhancers. Although researchers have previously published DNA maps of these regulatory regions for kidney tubule cells and glomerular endothelial cells, maps for podocytes and mesangial cells have not been available.

METHODS

We generated regulatory DNA maps (DNase-seq) and paired gene expression profiles (RNA-seq) from primary outgrowth cultures of human glomeruli that were composed mainly of podocytes and mesangial cells. We generated similar datasets from renal cortex cultures, to compare with those of the glomerular cultures. Because regulatory DNA elements can act on target genes across large genomic distances, we also generated a chromatin conformation map from freshly isolated human glomeruli.

RESULTS

We identified thousands of unique regulatory DNA elements, many located close to transcription factor genes, which the glomerular and cortex samples expressed at different levels. We found that genetic variants associated with kidney diseases (GWAS) and kidney expression quantitative trait loci were enriched in regulatory DNA regions. By combining GWAS, epigenomic, and chromatin conformation data, we functionally annotated 46 kidney disease genes.

CONCLUSIONS

We demonstrate a powerful approach to functionally connect kidney disease-/trait-associated loci to their target genes by leveraging unique regulatory DNA maps and integrated epigenomic and genetic analysis. This process can be applied to other kidney cell types and will enhance our understanding of genome regulation and its effects on gene expression in kidney disease.

The Presence of Urinary Renal Progenitor Cells in Stable Kidney Transplant Recipients Anticipates Allograft Deterioration

Long-term kidney transplant outcomes have reached mild improvements recently. Parietal epithelial cells (PECs) are progenitor cells located along the Bowman’s capsule that can be isolated in urine, and display the capability to replace podocytes, but in certain situations cause glomerulosclerosis. In this study, a cohort of stable kidney transplant recipients with 6 months protocol biopsy was divided in two groups depending on the presence (uPEC+; n = 41) or absence (uPEC-; n = 25) of PECs in urine and followed for 2 years. No differences were found between groups at 6 months after transplantation considering clinical variables, alloimmune response, renal function, albuminuria and graft pathology. However, uPEC+ group showed increased podocyturia and a higher rate of proliferating PECs along the Bowman’s capsule, without concomitant enhancement of the CD44 pro-sclerotic activation marker. Accordingly, 2 years follow up evidenced poorer outcomes in the uPEC+ group with worse renal function, increased albuminuria, wider mesangial expansion and more severe IFTA. In summary, chronic allograft damage can progress in certain stable-supposed grafts by podocyte detachment and reactive PECs proliferation, being the uPEC presence a biomarker of this process. This damage-response regenerative process, if sustained in time, might fail in preserve the allograft function and histology. Our study raises new prospects to overcome current limits on long-term allograft results.

Pathology identifies glomerular treatment targets in diabetic nephropathy

The development of the glomerular injury in diabetic nephropathy involves interactions between podocytes, endothelium, and the mesangium. Loss of podocytes is an early and critical step in the development of diabetic nephropathy, and analysis of structural lesions within the mesangium such as mesangiolysis implicate the loss of podocytes as a key mediating event. The BTBR ob/ob mouse has proved a useful tool to demonstrate that restoration of podocyte density, once thought to be an absolute barrier to glomerular repair, can be achieved with replacement of the hormone leptin that is constitutively absent in these mice. Restoration of podocyte density is associated with reversal of the structural lesions of morphologically advanced diabetic glomerular injury in this model. This finding, in conjunction with the demonstration in human diabetic patients with morphologically advanced diabetic nephropathy and with long-standing functioning pancreatic transplants of ten years duration that their diabetic nephropathy can be reversed, suggests that restoration of podocyte number and density is an appropriate target for the development of new therapeutics for diabetic nephropathy.

Modulation of apolipoprotein L1-microRNA-193a axis prevents podocyte dedifferentiation in high-glucose milieu

The loss of podocyte (PD) molecular phenotype is an important feature of diabetic podocytopathy. We hypothesized that high glucose (HG) induces dedifferentiation in differentiated podocytes (DPDs) through alterations in the apolipoprotein (APO) L1-microRNA (miR) 193a axis. HG-induced DPD dedifferentiation manifested in the form of downregulation of Wilms' tumor 1 (WT1) and upregulation of paired box 2 (PAX2) expression. WT1-silenced DPDs displayed enhanced expression of PAX2. Immunoprecipitation of DPD cellular lysates with anti-WT1 antibody revealed formation of WT1 repressor complexes containing Polycomb group proteins, enhancer of zeste homolog 2, menin, and DNA methyltransferase (DNMT1), whereas silencing of either WT1 or DNMT1 disrupted this complex with enhanced expression of PAX2. HG-induced DPD dedifferentiation was associated with a higher expression of miR193a, whereas inhibition of miR193a prevented DPD dedifferentiation in HG milieu. HG downregulated DPD expression of APOL1. miR193a-overexpressing DPDs displayed downregulation of APOL1 and enhanced expression of dedifferentiating markers; conversely, silencing of miR193a enhanced the expression of APOL1 and preserved DPD phenotype. Moreover, stably APOL1G0-overexpressing DPDs displayed the enhanced expression of WT1 but attenuated expression of miR193a; nonetheless, silencing of APOL1 reversed these effects. Since silencing of APOL1 enhanced miR193a expression as well as dedifferentiation in DPDs, it appears that downregulation of APOL1 contributed to dedifferentiation of DPDs through enhanced miR193a expression in HG milieu. Vitamin D receptor agonist downregulated miR193a, upregulated APOL1 expression, and prevented dedifferentiation of DPDs in HG milieu. These findings suggest that modulation of the APOL1-miR193a axis carries a potential to preserve DPD molecular phenotype in HG milieu.

SIRT3-KLF15 signaling ameliorates kidney injury induced by hypertension

Renal fibrosis participates in the progression of hypertension-induced kidney injury. The effect of SIRT3, a member of the NAD⁺-dependent deacetylase family, in hypertensive nephropathy remains unclear. In this study, we found that SIRT3 was reduced after angiotensin II (AngII) treatment both in vivo and in vitro. Furthermore, SIRT3-knockout mice aggravated hypertension-induced renal dysfunction and renal fibrosis via chronic AngII infusion (2000 ng/kg per minute for 42 days). On the contrary, SIRT3-overexpression mice attenuated AngII-induced kidney injury compared with wild-type mice. Remarkably, a co-localization of SIRT3 and KLF15, a kidney-enriched nuclear transcription factor, led to SIRT3 directly deacetylating KLF15, followed by decreased expression of fibronectin and collagen type IV in cultured MPC-5 podocytes. In addition, honokiol (HKL), a major bioactive compound isolated from Magnolia officinalis (Houpo), suppressed AngII-induced renal fibrosis through activating SIRT3-KLF15 signaling. Taken together, our findings implicate that a novel SIRT3-KLF15 signaling may prevent kidney injury from hypertension and HKL can act as a SIRT3-KLF15 signaling activator to protect against hypertensive nephropathy.

Early and late scanning electron microscopy findings in diabetic kidney disease

Diabetic nephropathy (DN), the single strongest predictor of mortality in patients with type 2 diabetes, is characterized by initial glomerular hyperfiltration with subsequent progressive renal function loss with or without albuminuria, greatly accelerated with the onset of overt proteinuria. Experimental and clinical studies have convincingly shown that early interventions retard disease progression, while treatment if started late in the disease course seldom modifies the slope of GFR decline. Here we assessed whether the negligible renoprotection afforded by drugs in patients with proteinuric DN could be due to loss of glomerular structural integrity, explored by scanning electron microscopy (SEM). In diabetic patients with early renal disease, glomerular structural integrity was largely preserved. At variance SEM documented that in the late stage of proteinuric DN, glomerular structure was subverted with nearly complete loss of podocytes and lobular transformation of the glomerular basement membrane. In these circumstances one can reasonably imply that any form of treatment, albeit personalized, is unlikely to reach a given cellular or molecular target. These findings should persuade physicians to start the putative renoprotective therapy soon after the diagnosis of diabetes or in an early phase of the disease before structural integrity of the glomerular filter is irreversibly compromised.

High glucose induces podocyte epithelial‑to‑mesenchymal transition by demethylation‑mediated enhancement of MMP9 expression

Abnormal expression of matrix metalloproteinase 9 (MMP9) is correlated with podocyte epithelial-to-mesenchymal transition (EMT) in diabetic nephropathy (DN). However, the mechanisms underlying this process are not well defined. Site-specific demethylation may sustain high expression levels of target genes. In the present study, in order to investigate the association between DNA demethylation of MMP9 promoter and podocyte EMT in DN, human podocytes were cultured in high-glucose (HG) medium and a rat model of DN was established by intraperitoneal injection of streptozotocin (STZ) to determine whether site-specific demethylation of the MMP9 promoter was involved in regulating podocyte EMT in DN. The MTT assay was used to assess the effects of HG culture on the growth of podocytes, and the demethylation status of the MMP9 promoter was assessed by bisulfite sequencing polymerase chain reaction. mRNA and protein expression levels of MMP9, α-smooth muscle actin (α-SMA), podocalyxin and fibronectin-1 in podocytes were assessed by reverse transcription-quantitative PCR (RT-qPCR) and western blot analyses. The results demonstrated that HG treatment up regulated the expression of MMP9, α-SMA and fibronectin-1, but down regulated the expression of podocalyxin in podocytes. The MMP9 promoter region was revealed to contain a variety of demethylated CpG sites, and HG treatment reduced the rate of MMP9 promotermethylation, which, in turn, enhanced its promoter activity. In summary, these data suggested that demethylation of the MMP9 promoter may serve an important role in podocyte EMT in DN. The demethylation status of the MMP9 promoter maybe used as an important prognostic marker of DN in clinic.

Application of Muse Cell Therapy for Kidney Diseases

The kidney plays an essential role in the maintenance of homeostasis in healthy individuals, e.g., by regulating the amount of water and concentration of electrolyte in the body. Owing to the structural complexity, renal dysfunction is caused by a myriad of diseases and conditions, and in severe cases, it progresses to end-stage renal disease in which patients require renal replacement therapy, i.e., maintenance dialysis or kidney transplantation. The currently available therapeutic modalities, with the exception of renal transplantation, cannot recover severely deteriorated renal function. Thus, regenerative medicine holds considerable promise as a potential means for developing next-generation renal therapeutics. Mesenchymal stem cell (MSC) transplantation has been investigated in acute kidney injury and chronic kidney disease models, and clinical studies have already been started for some kinds of kidney diseases. However, most of these studies concluded that the main underlying mechanism of therapeutic effect of MSC transplantation was paracrine. Recently, we reported that Muse cell therapy in a murine model of chronic kidney disease resulted in differentiation of intravenously injected Muse cells into glomerular cells after preferential homing to damaged glomerulus and improvement in renal function. The result suggested the potentiality of Muse cell therapy for glomerular regeneration. Muse cells are a promising cell source for regenerative therapy for kidney diseases.

A Novel Type 2 Diabetes Mouse Model of Combined Diabetic Kidney Disease and Atherosclerosis

Diabetic kidney disease and atherosclerotic disease are major causes of morbidity and mortality associated with type 2 diabetes (T2D), and diabetic kidney disease is a major cardiovascular risk factor. The black and tan, brachyury (BTBR) mouse strain with leptin deficiency (Lep^ob) has emerged as one of the best models of human diabetic kidney disease. However, no T2D mouse model of combined diabetic kidney disease and atherosclerosis exists. Our goal was to generate such a model. To this end, the low-density lipoprotein (LDL) receptor was targeted for degradation via inducible degrader of the LDL receptor (IDOL) overexpression, using liver-targeted adenoassociated virus serotype DJ/8 (AAV-DJ/8) in BTBR wild-type and BTBR Lep^ob mice. Liver-targeted IDOL-AAV-DJ/8 increased plasma LDL cholesterol compared with the control enhanced green fluorescent protein AAV-DJ/8. IDOL-induced dyslipidemia caused formation of atherosclerotic lesions of an intermediate stage, which contained both macrophages and smooth muscle cells. BTBR Lep^ob mice exhibited diabetic kidney disease. IDOL-induced dyslipidemia worsened albuminuria and glomerular macrophage accumulation but had no effect on mesangial expansion or podocyte numbers. Thus, by inducing hepatic degradation of the LDL receptor, we generated a T2D model of combined kidney disease and atherosclerosis. This model provides a new tool to study mechanisms, interactions, and treatment strategies of kidney disease and atherosclerosis in T2D.

Segmental Sclerosis and Extracapillary Hypercellularity Predict Diabetic ESRD

Pathogenetic markers of diabetic kidney disease (DKD) progression to ESRD are lacking. We characterized the prognostic value of histologic findings in DKD for time to ESRD in native kidney specimens from biopsies performed from 1995 to 2011 with diabetic glomerulosclerosis as the only glomerular disease diagnosis (n=109). Biopsy specimens were analyzed according to standard methods, including determination of diabetic nephropathy class, as defined by the Renal Pathology Society. Clinical data were extracted from electronic medical records. We used competing risk models, with death as the competing risk, to estimate subdistribution hazard ratios (HRs) for ESRD. All multivariable models included age, sex, black race, baseline eGFR, and baseline proteinuria. Pathologic characteristics achieving P<0.1 were added into successively complex models. ESRD occurred in 56% of patients, and 26% of patients died before reaching ESRD. In univariate analyses, diabetic nephropathy class was not statistically significant in predicting time to ESRD. The final multivariable model (n=106) showed a borderline association between mild mesangial expansion and decreased risk for ESRD (subdistribution HR, 0.64; 95% confidence interval, 0.40 to 1.00). Poor prognostic factors in the final model included segmental sclerosis and extracapillary hypercellularity (subdistribution HR, 2.04; 95% confidence interval, 1.36 to 3.05; and subdistribution HR, 2.21; 95% confidence interval, 1.19 to 4.11, respectively). In conclusion, we identified segmental sclerosis and extracapillary hypercellularity as novel, poor prognostic indicators of time from DKD to ESRD. Whether these indicators represent a distinct pathogenetic phenotype of DKD will require a large study with a broad spectrum of disease severity.

High resolution molecular and histological analysis of renal disease progression in ZSF1 fa/faCP rats, a model of type 2 diabetic nephropathy

ZSF1 rats exhibit spontaneous nephropathy secondary to obesity, hypertension, and diabetes, and have gained interest as a model system with potentially high translational value to progressive human disease. To thoroughly characterize this model, and to better understand how closely it recapitulates human disease, we performed a high resolution longitudinal analysis of renal disease progression in ZSF1 rats spanning from early disease to end stage renal disease. Analyses included metabolic endpoints, renal histology and ultrastructure, evaluation of a urinary biomarker of fibrosis, and transcriptome analysis of glomerular-enriched tissue over the course of disease. Our findings support the translational value of the ZSF1 rat model, and are provided here to assist researchers in the determination of the model’s suitability for testing a particular mechanism of interest, the design of therapeutic intervention studies, and the identification of new targets and biomarkers for type 2 diabetic nephropathy.

Single-cell RNA-sequence analysis of mouse glomerular mesangial cells uncovers mesangial cell essential genes

Mesangial cells are essential for the structure and function of glomeruli, but the mechanisms underlying these roles are not well understood. Here, we performed a single-cell RNA-sequence (RNA-seq) analysis of mouse mesangial cells using the Fluidigm C1 platform. We found that gene expression in individual mesangial cells was tremendously heterogeneous, with mean correlation coefficients of 0.20, and most mesangial genes were actually expressed in only a portion of mesangial cells and are therefore presumably dispensable. In contrast, 1,045 genes were expressed in every single mesangial cell and were considered mesangial cell essential genes. A gene ontology analysis revealed a significant enrichment of genes associated with the endothelium, supporting the inference that mesangial cells function as pericytes. Among 58 endothelium-associated genes, 18 encode proteins that are secreted and may be directly involved in endothelial homeostasis. Importantly, 11 (Angpt2, Anxa5, Axl, Ecm1, Eng, Fn1, Mfge8, Msn, Nrp1, Serpine2, and Sparc) were upregulated, while 2 (Apoe and Fgf1) were downregulated in various glomerulopathies. The enrichment of genes associated with other reported functions of mesangial cells was also found. Furthermore, we identified 173 genes specifically expressed in every mesangial cell in glomeruli from the mesangial cell essential gene list. Finally, based on single mesangial cell RNA-seq results, we found that commonly used glomerular cell type markers, including Fhl2, Igfbp5, Wt1, Tek/Tie2, Kdr/Flk1, Flt1/Vegfr1, and Cd34, are actually not specific. Thus, single mesangial cell RNA-seq analysis has provided insights into the functions and underlying mechanisms of mesangial cells.