Expression and function of the developmental gene Wnt-4 during experimental acute renal failure in rats

The involvement of the Wnt/β-catenin signaling cascade in fibrosis progression and its therapeutic targeting by relaxin

Organ scarring, referred to as fibrosis, results from a failed wound-healing response to chronic tissue injury and is characterised by the aberrant accumulation of various extracellular matrix (ECM) components. Once established, fibrosis is recognised as a hallmark of stiffened and dysfunctional tissues, hence, various fibrosis-related diseases collectively contribute to high morbidity and mortality in developed countries. Despite this, these diseases are ineffectively treated by currently-available medications. The pro-fibrotic cytokine, transforming growth factor (TGF)-β1, has emerged as the master regulator of fibrosis progression, owing to its ability to promote various factors and processes that facilitate rapid ECM synthesis and deposition, whilst negating ECM degradation. TGF-β1 signal transduction is tightly controlled by canonical (Smad-dependent) and non-canonical (MAP kinase- and Rho-associated protein kinase-dependent) intracellular protein activity, whereas its pro-fibrotic actions can also be facilitated by the Wnt/β-catenin pathway. This review outlines the pathological sequence of events and contributing roles of TGF-β1 in the progression of fibrosis, and how the Wnt/β-catenin pathway contributes to tissue repair in acute disease settings, but to fibrosis and related tissue dysfunction in synergy with TGF-β1 in chronic diseases. It also outlines the anti-fibrotic and related signal transduction mechanisms of the hormone, relaxin, that are mediated via its negative modulation of TGF-β1 and Wnt/β-catenin signaling, but through the promotion of Wnt/β-catenin activity in acute disease settings. Collectively, this highlights that the crosstalk between TGF-β1 signal transduction and the Wnt/β-catenin cascade may provide a therapeutic target that can be exploited to broadly treat and reverse established fibrosis.

Dynamics of Wnt/β-catenin reporter activity throughout whole life in a naturally short-lived vertebrate

Wnt/β-catenin signaling plays a major role in regulation of embryogenesis, organogenesis, and adult tissue homeostasis and regeneration. However, the roles played by Wnt/β-catenin and the spatiotemporal regulation of its activity throughout life, including during aging, are not fully understood. To address these issues, we introduced a Wnt/β-catenin signaling sensitive reporter into African turquoise killifish (Nothobranchius furzeri), a naturally ultra-short-lived fish that allows for the analysis of its whole life within a short period of time. Using this reporter killifish, we unraveled the previously unidentified dynamics of Wnt/β-catenin signaling during development and aging. Using the reporter strain, we detected Wnt/β-catenin activity in actively developing tissues as reported in previous reports, but also observed activation and attenuation of Wnt/β-catenin activity during embryonic reaggregation and diapause, respectively. During the aging process, the reporter was activated in the choroidal layer and liver, but its expression decreased in the kidneys. In addition, the reporter also revealed that aging disrupts the spatial regulation and intensity control of Wnt/β-catenin activity seen during fin regeneration, which interferes with precise regeneration. Thus, the employed reporter killifish is a highly useful model for investigating the dynamics of Wnt/β-catenin signaling during both the developmental and aging process.

Ischemia-reperfusion injury: molecular mechanisms and therapeutic targets

Ischemia-reperfusion (I/R) injury paradoxically occurs during reperfusion following ischemia, exacerbating the initial tissue damage. The limited understanding of the intricate mechanisms underlying I/R injury hinders the development of effective therapeutic interventions. The Wnt signaling pathway exhibits extensive crosstalk with various other pathways, forming a network system of signaling pathways involved in I/R injury. This review article elucidates the underlying mechanisms involved in Wnt signaling, as well as the complex interplay between Wnt and other pathways, including Notch, phosphatidylinositol 3-kinase/protein kinase B, transforming growth factor-β, nuclear factor kappa, bone morphogenetic protein, N-methyl-D-aspartic acid receptor-Ca2+-Activin A, Hippo-Yes-associated protein, toll-like receptor 4/toll-interleukine-1 receptor domain-containing adapter-inducing interferon-β, and hepatocyte growth factor/mesenchymal-epithelial transition factor. In particular, we delve into their respective contributions to key pathological processes, including apoptosis, the inflammatory response, oxidative stress, extracellular matrix remodeling, angiogenesis, cell hypertrophy, fibrosis, ferroptosis, neurogenesis, and blood-brain barrier damage during I/R injury. Our comprehensive analysis of the mechanisms involved in Wnt signaling during I/R reveals that activation of the canonical Wnt pathway promotes organ recovery, while activation of the non-canonical Wnt pathways exacerbates injury. Moreover, we explore novel therapeutic approaches based on these mechanistic findings, incorporating evidence from animal experiments, current standards, and clinical trials. The objective of this review is to provide deeper insights into the roles of Wnt and its crosstalk signaling pathways in I/R-mediated processes and organ dysfunction, to facilitate the development of innovative therapeutic agents for I/R injury.

N6-methyladenosine (m6A) methylation in kidney diseases: Mechanisms and therapeutic potential

The N6-methyladenosine (m6A) modification is regulated by methylases, commonly referred to as "writers," and demethylases, known as "erasers," leading to a dynamic and reversible process. Changes in m6A levels have been implicated in a wide range of cellular processes, including nuclear RNA export, mRNA metabolism, protein translation, and RNA splicing, establishing a strong correlation with various diseases. Both physiologically and pathologically, m6A methylation plays a critical role in the initiation and progression of kidney disease. The methylation of m6A may also facilitate the early diagnosis and treatment of kidney diseases, according to accumulating research. This review aims to provide a comprehensive overview of the potential role and mechanism of m6A methylation in kidney diseases, as well as its potential application in the treatment of such diseases. There will be a thorough examination of m6A methylation mechanisms, paying particular attention to the interplay between m6A writers, m6A erasers, and m6A readers. Furthermore, this paper will elucidate the interplay between various kidney diseases and m6A methylation, summarize the expression patterns of m6A in pathological kidney tissues, and discuss the potential therapeutic benefits of targeting m6A in the context of kidney diseases.

Loss of Dact2 alleviates cisplatin-induced nephrotoxicity through regulation of the Igfl-MAPK pathway axis

Wnt signaling is a principal pathway regulating the essential activities of cell proliferation. Here, we investigated the effect of Wnt/β-catenin signaling on in vivo drug-induced renal injury through the deletion of Dact2, a Wnt antagonist, and deciphered the underlying mechanism. Wild-type (WT) and Dact2 knockout (KO) mice were administered a single intraperitoneal injection of cisplatin to induce renal injury. The injury was alleviated in Dact2 KO mice, which showed lower levels of blood urea nitrogen and creatinine. RNA sequencing revealed 194 differentially expressed genes (DEGs) between WT and Dact2 KO mouse kidney before cisplatin treatment. Among them, higher levels of Igf1, one of the Wnt target genes responsible for "Positive regulation of cell proliferation" in KO mice, were confirmed along with the induction of Ki67 expression. In RNA-seq analysis comparing WT and Dact2 KO mice after cisplatin treatment, genes related to "Apoptosis" and "Activation of mitogen-activated protein kinase (MAPK) activity" were among the downregulated DEGs in KO mice. These results were corroborated in western blotting of proteins related to apoptosis and proapoptotic MAPK pathway; the expression of which was found to be lower in cisplatin-treated KO mice. Importantly, β-catenin was found to directly bind to and regulate the transcription of Igf1, leading to the alleviation of cisplatin-induced cytotoxicity by the Wnt agonist, CHIR-99021. In addition, Igf1 knockdown accelerated cisplatin-induced cytotoxicity, accompanied by the MAPK upregulation. Our findings suggest that Dact2 knockout could protect cisplatin-induced nephrotoxicity by inhibiting apoptosis, possibly through the regulation of the Igf1-MAPK axis associated with Wnt/β-catenin signaling.

Further insights into the impact of rebamipide on gentamicin-induced nephrotoxicity in rats: modulation of SIRT1 and β-catenin/cyclin D1 pathways

Gentamicin (GM) is an effective antibiotic administered to treat acute Gram-negative infections. Nevertheless, its clinical application is limited due to nephrotoxicity. Therefore, our research aimed to investigate the potential renoprotective impact of rebamipide (RBM), a gastroprotective drug, on GM-induced kidney damage in rats, as well as putative nephroprotective pathways. RBM was orally administered (100 mg/kg/d for 14 d) commencing 7 d before the administration of GM (100 mg/kg/d, intraperitoneally). Nephrotoxicity was elucidated, and the silent information regulator 1 (SIRT1) and β-catenin/cyclin D1 pathways were assessed. GM induced a significant elevation in the serum levels of creatinine, blood urea nitrogen (BUN), and kidney injury molecule-1 (KIM-1), as well as the relative kidney index. In addition, GM increased lipid peroxidation and lowered total antioxidant capacity (TAC) level and superoxide dismutase (SOD) activity. GM administration also demonstrated a significant amplification in tumor necrosis factor-alpha (TNF-α), interleukin-1 beta (IL-1β), nuclear factor-κappa B p65 (NF-κB p65), p38 mitogen-activated protein kinase (p38 MAPK), and caspase-3 kidney levels, as well as B-cell lymphoma 2 (Bcl-2)-associated X protein (Bax)/Bcl-2 ratio. Notably, RBM treatment amended all these changes induced by GM. Furthermore, the potential role of SIRT1 and β-catenin-dependent signaling pathways in GM-induced renal injury was assessed. Our findings showed that GM-treated rats demonstrated a substantial decrease in SIRT1, nuclear factor E2-related factor 2 (Nrf2), and heme oxygenase-1 (HO-1) along with an increase in β-catenin, forkhead box O-3a (FOXO-3a), and cyclin D1 protein expressions. RMB treatment markedly attenuated the deterioration caused by GM on these pathways. Additionally, RBM alleviated the GM-induced deleterious kidney tissue histopathology. In conclusion, our findings have verified that RBM can halt GM-induced renal injury by partly modulating SIRT1 and β-catenin pathways.

Suppression of Indoxyl Sulfate Accumulation Reduces Renal Fibrosis in Sulfotransferase 1a1-Deficient Mice

Renal fibrosis is the final manifestation of chronic kidney disease (CKD); its prevention is vital for controlling CKD progression. Indoxyl sulfate (IS), a typical sulfate-conjugated uremic solute, is produced in the liver via the enzyme sulfotransferase (SULT) 1A1 and accumulates significantly during CKD. We investigated the toxicopathological role of IS in renal fibrosis using Sult1a1-KO mice and the underlying mechanisms. The unilateral ureteral obstruction (UUO) model was created; kidney IS concentrations, inflammation, and renal fibrosis were assessed on day 14. After UUO treatment, inflammation and renal fibrosis were exacerbated in WT mice, with an accumulation of IS in the kidney. However, they were significantly suppressed in Sult1a1-KO mice. CD206⁺ expression was upregulated, and β-catenin expression was downregulated in Sult1a1-KO mice. To confirm the impact of erythropoietin (EPO) on renal fibrosis, we evaluated the time-dependent expression of EPO. In Sult1a1-KO mice, EPO mRNA expression was improved considerably; UUO-induced renal fibrosis was further attenuated by recombinant human erythropoietin (rhEPO). Thus, UUO-induced renal fibrosis was alleviated in Sult1a1-KO mice with a decreased accumulation of IS. Our findings confirmed the pathological role of IS in renal fibrosis and identified SULT1A1 as a new therapeutic target enzyme for preventing and attenuating renal fibrosis.

Biochemical interaction of pyrvinium in gentamicin-induced acute kidney injury by modulating calcium dyshomeostasis and mitochondrial dysfunction

Acute kidney injury (AKI) has a poor clinical prognosis and increases the risk of chronic kidney failure (CKD). It is a common complication of organ failure in hospitalised patients (10-15% of all hospitalizations) and in intensive care unit (ICU) patients, with an incidence of up to 50%. Concerning ICU, AKI has a mortality rate ranging from 27% to 35%, rising to 60%-65% when dialysis is needed, with roughly 5%-20% of survivors requiring dialysis on discharge. AKI is believed to cause over 7 million deaths per year worldwide. Currently, there is no treatment for AKI or its progression to CKD. When activated by AKI, numerous pathways have been suggested as possible contributors to CKD progression. Wnt/β-catenin is a crucial regulator of kidney development that increases following the injury. Despite the overwhelming evidence that Wnt/β-catenin promotes AKI, tubulointerstitial fibrosis, a hallmark of CKD progression, is also promoted by this pathway. The therapeutic potential of Wnt/β-catenin in the treatment of AKI and the progression from AKI to CKD is being studied. This hypothesis aims to determine whether the Wnt/β-catenin inhibitor pyrvinium has a beneficial effect on the renal dysfunction and damage caused by Gentamicin.

Temporal and sex-dependent gene expression patterns in a renal ischemia-reperfusion injury and recovery pig model

Men are more prone to acute kidney injury (AKI) and chronic kidney disease (CKD), progressing to end-stage renal disease (ESRD) than women. Severity and capacity to regenerate after AKI are important determinants of CKD progression, and of patient morbidity and mortality in the hospital setting. To determine sex differences during injury and recovery we have generated a female and male renal ischemia/reperfusion injury (IRI) pig model, which represents a major cause of AKI. Although no differences were found in blood urea nitrogen (BUN) and serum creatinine (SCr) levels between both sexes, females exhibited higher mononuclear infiltrates at basal and recovery, while males showed more tubular damage at injury. Global transcriptomic analyses of kidney biopsies from our IRI pig model revealed a sexual dimorphism in the temporal regulation of genes and pathways relevant for kidney injury and repair, which was also detected in human samples. Enrichment analysis of gene sets revealed five temporal and four sexual patterns governing renal IRI and recovery. Overall, this study constitutes an extensive characterization of the time and sex differences occurring during renal IRI and recovery at gene expression level and offers a template of translational value for further study of sexual dimorphism in kidney diseases.

The Wnt Signaling Pathway in Diabetic Nephropathy

Diabetic nephropathy (DN) is a serious kidney-related complication of both type 1 and type 2 diabetes mellitus (T1DM, T2DM) and the second major cause of end-stage kidney disease. DN can lead to hypertension, edema, and proteinuria. In some cases, DN can even progress to kidney failure, a life-threatening condition. The precise etiology and pathogenesis of DN remain unknown, although multiple factors are believed to be involved. The main pathological manifestations of DN include mesangial expansion, thickening of the glomerular basement membrane, and podocyte injury. Eventually, these pathological manifestations will lead to glomerulosclerosis, thus affecting renal function. There is an urgent need to develop new strategies for the prevention and treatment of DN. Existing evidence shows that the Wnt signaling cascade plays a key role in regulating the development of DN. Previous studies focused on the role of the Wnt canonical signaling pathway in DN. Subsequently, accumulated evidence on the mechanism of the Wnt non-canonical signaling indicated that Wnt/Ca²⁺ and Wnt/PCP also have essential roles in the progression of DN. In this review, we summarize the specific mechanisms of Wnt signaling in the occurrence and development of DN in podocyte injury, mesangial cell injury, and renal fibrosis. Also, to elucidate the significance of the Wnt canonical pathway in the process of DN, we uncovered evidence supporting that both Wnt/PCP and Wnt/Ca²⁺ signaling are critical for DN development.

Geraniol protects against cyclosporine A-induced renal injury in rats: Role of Wnt/β-catenin and PPARγ signaling pathways

AIMS

The nephrotoxicity of cyclosporine A (CsA) limits its use as an immunosuppressant. Wnt/β-catenin signaling is involved in the pathogenesis of both acute and chronic kidney disease, and it is inhibited by peroxisome proliferator-activated receptor gamma (PPARγ). We aimed to evaluate if geraniol, which can modulate both PPARγ and Wnt signaling, could protect against CsA-induced nephrotoxicity.

MATERIALS AND METHODS

Rats (6 groups) received the vehicle or a combination of CsA (30 mg/kg) with the vehicle, geraniol (50, 100, or 200 mg/kg), or the PPARγ agonist pioglitazone for 4 weeks. Blood pressure (BP), markers of renal injury (serum urea, serum creatinine, blood urea nitrogen, and urinary NAG), oxidative stress (glutathione peroxidase), inflammation (ICAM-1, IL-18, and NF-κB), apoptosis (caspase-3), extracellular matrix remodeling [matrix metalloproteinase-9 (MMP-9)], and fibrosis (TGF-β1, Smad3, and Smad7) were assessed. Renal histological analysis, Wnt signaling components (Wnt-4/β-catenin and E-cadherin), and PPARγ expression were evaluated.

KEY FINDINGS

CsA group had renal injury, as well as increased BP, renal oxidative stress, inflammation, and fibrosis. The latter changes were associated with altered renal architecture, active Wnt signaling (higher Wnt-4 and β-catenin expression and E-cadherin down-regulation), and lower PPARγ levels. Geraniol protected against kidney damage and the associated biochemical and histomorphological changes in a dose-dependent manner. The latter effects were comparable or superior to those of pioglitazone.

SIGNIFICANCE

The down-regulation of Wnt/β-catenin and the increase in PPARγ by geraniol suggest that both pathways are involved in its renoprotective potential. The study highlights geraniol as a valuable protective asset against chemically induced nephrotoxicity.

Knockout of Zeb2 ameliorates progression of renal tubulointerstitial fibrosis in a mouse model of renal ischemia-reperfusion injury

BACKGROUND

Zeb2, a zinc finger E-box-binding homeobox transcription factor, regulates transforming growth factor (TGF)-β signaling pathway. However, its role in the pathogenesis of acute kidney injury (AKI) and AKI to chronic kidney disease (CKD) transition is unclear.

METHODS

We evaluated Zeb2 function in a bilateral renal ischemia-reperfusion injury (IRI)-induced AKI model using proximal tubule-specific Zeb2 conditional knockout (Zeb2-cKO) and wild-type (WT) mice, and in renal biopsy samples.

RESULTS

In Zeb2-cKO mice, the levels of plasma creatinine and blood urea nitrogen post-IRI were significantly lower than that in WT mice. Immunohistological analysis revealed mild tubular injury, reduced neutrophil infiltration, less fibrotic changes, and reduced expression of fibrotic proteins (collagen type IV, α-smooth muscle actin [α-SMA], fibronectin, and connective tissue growth factor [CTGF]), at 3-14 days post-IRI. Zeb2 expression was upregulated in proximal tubular cells post-IRI in WT mice. Zeb2 siRNA transfection reduced TGF-β stimulated mRNA and protein expression of collagen type IV, α-SMA, fibronectin, and CTGF in cultured renal tubular cells. Patients with AKI to CKD transition exhibited high Zeb2 expression in renal tubules, as revealed by renal biopsy. Hypoxia and CoCl2-treatment upregulated Zeb2 promoter activity and mRNA and protein expression in cultured renal tubular epithelial cells, suggesting a regulatory role for hypoxia.

CONCLUSIONS

Zeb2 was upregulated in renal tissues in both mice and humans with AKI. Zeb2 regulates fibrotic pathways in the pathogenesis of AKI and AKI to CKD transition. Therefore, inhibition of Zeb2 could be a potential therapeutic strategy for AKI.

Canonical Wnt signaling in the kidney in different hypertension models

There is a close relationship between the kidney and blood pressure. On the one hand, kidney dysfunction causes an increase in blood pressure; on the other hand, high blood pressure causes kidney dysfunction. Wnt/β-catenin signaling is a key pathway that regulates various cellular processes and tissue homeostasis and is also involved in damage and repair processes. In healthy organs, Wnt/β-catenin signaling is muted, but it is activated in pathological states. The purpose of the present study was to immunohistochemically evaluate and compare the expression of WNT4, WNT10A, Fzd8, β-catenin, and GSK-3ß (glycogen synthase kinase 3β) in the kidneys of rats with essential arterial hypertension (SHR), renal-renal hypertension (2K1C), and DOCA-salt-induced hypertension. The study was performed on five male WKY rats, seven SHRs, and twenty-four (n = 24) young male Wistar rats. The main results showed that during hypertension, there are changes in Wnt/β-catenin signaling in the kidneys of rats, and the severity of these changes depends on the type of hypertension. This study is the first to assess the levels of some elements of the canonical Wnt/β-catenin signal transduction pathway in various types of arterial hypertension by immunohistochemistry and may form the basis for further molecular and functional studies of this pathway in hypertension.

Selective Wnt/β-Catenin Pathway Activation Concomitant With Sustained Overexpression of miR-21 is Responsible for Aristolochic Acid-Induced AKI-to-CKD Transition

Acute kidney injury (AKI) is increasingly recognized as a cumulative risk factor for chronic kidney disease (CKD) progression. However, the underlying mechanisms remain unclear. Using an aristolochic acid (AA)-induced mouse model of AKI-to-CKD transition, we found that the development of tubulointerstitial fibrosis following AKI was accompanied with a strong activation of miR-21 and canonical Wnt signaling, whereas inhibition of miR-21 or selective silencing of Wnt ligands partially attenuated AKI-to-CKD transition. To explore the interaction between miR-21 and Wnt/β-catenin signaling, we examined the effects of genetic absence or pharmacologic inhibition of miR-21 on Wnt/β-catenin pathway expression. In miR-21^-/- mice and in wild-type mice treated with anti-miR21 oligos, Wnt1 and Wnt4 canonical signaling in the renal tissue was significantly reduced, with partial reversal of renal interstitial fibrosis. Although the renal abundance of miR-21 remained unchanged after inhibition or activation of Wnt/β-catenin signaling, early intervention with ICG-001, a β-catenin inhibitor, significantly attenuated renal interstitial fibrosis. Moreover, early (within 24 h), but not late β-catenin inhibition after AA administration attenuated AA-induced apoptosis and inflammation. In conclusion, inhibition of miR-21 or β-catenin signaling may be an effective approach to prevent AKI-to-CKD progression.

Wnt/β-Catenin in Acute Kidney Injury and Progression to Chronic Kidney Disease

Acute kidney injury (AKI) portends a poor clinical prognosis and increases the risk for the development of chronic kidney disease (CKD). Currently, there are no therapies to treat AKI or prevent its progression to CKD. Wnt/β-catenin is a critical regulator of kidney development that is up-regulated after injury. Most of the literature support a beneficial role for Wnt/β-catenin in AKI, but suggest that this pathway promotes the progression of tubulointerstitial fibrosis, the hallmark of CKD progression. We review the role of Wnt/β-catenin in renal injury with a focus on its potential as a therapeutic target in AKI and in AKI to CKD transition.

Protective role of kallistatin in renal fibrosis via modulation of Wnt/β-catenin signaling

Kallistatin is a multiple functional serine protease inhibitor that protects against vascular injury, organ damage and tumor progression. Kallistatin treatment reduces inflammation and fibrosis in the progression of chronic kidney disease (CKD), but the molecular mechanisms underlying this protective process and whether kallistatin plays an endogenous role are incompletely understood. In the present study, we observed that renal kallistatin levels were significantly lower in patients with CKD. It was also positively correlated with estimated glomerular filtration rate (eGFR) and negatively correlated with serum creatinine level. Unilateral ureteral obstruction (UUO) in animals also led to down-regulation of kallistatin protein in the kidney, and depletion of endogenous kallistatin by antibody injection resulted in aggravated renal fibrosis, which was accompanied by enhanced Wnt/β-catenin activation. Conversely, overexpression of kallistatin attenuated renal inflammation, interstitial fibroblast activation and tubular injury in UUO mice. The protective effect of kallistatin was due to the suppression of TGF-β and β-catenin signaling pathways and subsequent inhibition of epithelial-to-mesenchymal transition (EMT) in cultured tubular cells. In addition, kallistatin could inhibit TGF-β-mediated fibroblast activation via modulation of Wnt4/β-catenin signaling pathway. Therefore, endogenous kallistatin protects against renal fibrosis by modulating Wnt/β-catenin-mediated EMT and fibroblast activation. Down-regulation of kallistatin in the progression of renal fibrosis underlies its potential as a valuable clinical biomarker and therapeutic target in CKD.

Expression and Function of WNT6: From Development to Disease

WNT family member 6 (WNT6) is a member of the highly conserved WNT protein family. It plays an essential role in the normal development process, not only in embryonic morphogenesis, but also in post-natal homeostasis. WNT6 functions in mice and humans. This review summarizes the current findings on the biological functions of WNT6, describing its involvement in regulating embryogenesis, decidualization, and organ development. Aberrant WNT6 signaling is related to various pathologies, such as promoting cancer development, lung tuberculosis, and kidney fibrosis and improving the symptoms of Rett syndrome (RTT). Thus, due to its various functions, WNT6 has great potential for in-depth research. This work not only describes the signaling mechanism and function of WNT6 under physiological and pathological conditions, but also provides a theoretical basis for targeted therapy.

Wnt signaling in kidney: the initiator or terminator?

The kidney is a key organ in the human body that excretes toxins and sustains the water-electrolyte balance. During embryonic development and disease progression, the kidney undergoes enormous changes in macrostructure, accompanied by a variety of microstructural histological changes, such as glomerular formation and sclerosis, tubule elongation and atrophy, interstitial establishment, and fibrosis progression. All of these rely on the frequent occurrence of cell death and growth. Notably, to overcome disease, some cells regenerate through self-repair or progenitor cell differentiation. However, the signaling mechanisms underlying kidney development and regeneration have not been elucidated. Recently, Wnt signaling has been noted to play an important role. Although it is a well-known developmental signal, the role of Wnt signaling in kidney development and regeneration is not well recognized. In this review, we review the role of Wnt signaling in kidney embryonic development, tissue repair, cell division, and progenitor cell differentiation after injury. Moreover, we briefly highlight advances in our understanding of the pathogenic mechanisms of Wnt signaling in mediating cellular senescence in kidney parenchymal and stem cells, an irreversible arrest of cell proliferation blocking tissue repair and regeneration. We also highlight the therapeutic targets of Wnt signaling in kidney diseases and provide important clues for clinical strategies.

Wnt/β-catenin agonist BIO alleviates cisplatin-induced nephrotoxicity without compromising its efficacy of anti-proliferation in ovarian cancer

AIMS

Cisplatin is an anticancer agent marred by nephrotoxicity. Limiting this adverse effect may allow the use of higher doses to improve its efficacy. The Wnt/β-catenin signaling pathway plays a critical role in nephrogenesis and repair of renal diseases. BIO, a small molecule agonist of this pathway, exerted a protective effect in adriamycin nephropathy and promoted nephrogenesis. The aim of this study, therefore, was to investigate whether Wnt/β-catenin agonist BIO could protect against cisplatin-induced nephrotoxicity in vivo and in vitro, as well as its possible mechanism.

MAIN METHODS

Male mice and human renal proximal tubular cells (HK-2) were subjected to cisplatin to study reno-protective effect of BIO. Renal function, cell viability, tubular apoptosis, production of reactive oxygen species (ROS) and proliferative level were analyzed respectively. Additionally, xenograft model was induced to investigate if BIO would impair the antitumor effect of cisplatin.

KEY FINDINGS

Cisplatin increased serum creatinine levels and promoted histological renal injury as well as oxidative stress levels. Besides, renal apoptotic level and the expression of pro-apoptotic proteins, Bax/bcl-2 and cleaved-caspase3 included, in the kidney were increased. All these features were decreased by BIO, which also activated Wnt/β-catenin pathway in cisplatin-induced nephrotoxicity. Similarly, accompanied by the motivation of Wnt/β-catenin pathway, BIO exerted a positively protective effect on HK-2 challenged cisplatin. Last, the chemotherapeutic effects of cisplatin in xenograft mice of ovary tumor models and in lung cancer cells weren't compromised by BIO.

SIGNIFICANCE

Wnt/β-catenin agonist BIO has the potential to prevent cisplatin nephrotoxicity without compromising its anti-proliferation efficacy.

WNT-β-catenin signalling - a versatile player in kidney injury and repair

The WNT-β-catenin system is an evolutionary conserved signalling pathway that is of particular importance for morphogenesis and cell organization during embryogenesis. The system is usually suppressed in adulthood; however, it can be re-activated in organ injury and regeneration. WNT-deficient mice display severe kidney defects at birth. Transient WNT-β-catenin activation stimulates tissue regeneration after acute kidney injury, whereas sustained (uncontrolled) WNT-β-catenin signalling promotes kidney fibrosis in chronic kidney disease (CKD), podocyte injury and proteinuria, persistent tissue damage during acute kidney injury and cystic kidney diseases. Additionally, WNT-β-catenin signalling is involved in CKD-associated vascular calcification and mineral bone disease. The WNT-β-catenin pathway is tightly regulated, for example, by proteins of the Dickkopf (DKK) family. In particular, DKK3 is released by 'stressed' tubular epithelial cells; DKK3 drives kidney fibrosis and is associated with short-term risk of CKD progression and acute kidney injury. Thus, targeting the WNT-β-catenin pathway might represent a promising therapeutic strategy in kidney injury and associated complications.

Tubular β-catenin and FoxO3 interactions protect in chronic kidney disease

The Wnt/β-catenin signaling pathway plays an important role in renal development and is reexpressed in the injured kidney and other organs. β-Catenin signaling is protective in acute kidney injury (AKI) through actions on the proximal tubule, but the current dogma is that Wnt/β-catenin signaling promotes fibrosis and development of chronic kidney disease (CKD). As the role of proximal tubular β-catenin signaling in CKD remains unclear, we genetically stabilized (i.e., activated) β-catenin specifically in murine proximal tubules. Mice with increased tubular β-catenin signaling were protected in 2 murine models of AKI to CKD progression. Oxidative stress, a common feature of CKD, reduced the conventional T cell factor/lymphoid enhancer factor-dependent β-catenin signaling and augmented FoxO3-dependent activity in proximal tubule cells in vitro and in vivo. The protective effect of proximal tubular β-catenin in renal injury required the presence of FoxO3 in vivo. Furthermore, we identified cystathionine γ-lyase as a potentially novel transcriptional target of β-catenin/FoxO3 interactions in the proximal tubule. Thus, our studies overturned the conventional dogma about β-catenin signaling and CKD by showing a protective effect of proximal tubule β-catenin in CKD and identified a potentially new transcriptional target of β-catenin/FoxO3 signaling that has therapeutic potential for CKD.

Current understanding of the administration of mesenchymal stem cells in acute kidney injury to chronic kidney disease transition: a review with a focus on preclinical models

Incomplete recovery from acute kidney injury (AKI) can result in long-term functional deficits and has been recognized as a major contributor to chronic kidney disease (CKD), which is termed the AKI-CKD transition. Currently, an effective intervention for this disorder is still lacking. Principally, therapeutic strategies targeting the AKI-CKD transition can be divided into those reducing the severity of AKI or promoting the regenerative process towards beneficially adaptive repair pathways. Considering the fact that mesenchymal stem cells (MSCs) have the potential to address both aspects, therapeutic regimens based on MSCs have a promising future. In light of this information, we focus on the currently available evidence associated with MSC therapy involved in the treatment of the AKI-CKD transition and the underlying mechanisms. All of these discussions will contribute to the establishment of a reliable therapeutic strategy for patients with this problem, who can be easily ignored by physicians, and will lead to a better clinical outcome for them.

Oral delivery of nanoparticle urolithin A normalizes cellular stress and improves survival in mouse model of cisplatin-induced AKI

The popular anticancer drug cisplatin causes many adverse side effects, the most serious of which is acute kidney injury (AKI). Emerging evidence from laboratory and clinical studies suggests that the AKI pathogenesis involves oxidative stress pathways; therefore, regulating such pathways may offer protection. Urolithin A (UA), a gut metabolite of the dietary tannin ellagic acid, possesses antioxidant properties and has shown promise in mouse models of AKI. However, therapeutic potential of UA is constrained by poor bioavailability. We aimed to improve oral bioavailability of UA by formulating it into biodegradable nanoparticles that use a surface-conjugated ligand targeting the gut-expressed transferrin receptor. Nanoparticle encapsulation of UA led to a sevenfold enhancement in oral bioavailability compared with native UA. Treatment with nanoparticle UA also significantly attenuated the histopathological hallmarks of cisplatin-induced AKI and reduced mortality by 63% in the mouse model. Expression analyses indicated that nanoparticle UA therapy coincided with oxidative stress mitigation and downregulation of nuclear factor erythroid 2-related factor 2- and P53-inducible genes. Additionally, normalization of miRNA (miR-192-5p and miR-140-5p) implicated in AKI, poly(ADP-ribose) polymerase 1 levels, antiapoptotic signaling, intracellular NAD+, and mitochondrial oxidative phosphorylation were observed in the treatment group. Our findings suggest that nanoparticles greatly increase the oral bioavailability of UA, leading to improved survival rates in AKI mice, in part by reducing renal oxidative and apoptotic stress.

Oral delivery of nanoparticle urolithin A normalizes cellular stress and improves survival in mouse model of cisplatin-induced AKI

The popular anticancer drug cisplatin causes many adverse side effects, the most serious of which is acute kidney injury (AKI). Emerging evidence from laboratory and clinical studies suggests that the AKI pathogenesis involves oxidative stress pathways; therefore, regulating such pathways may offer protection. Urolithin A (UA), a gut metabolite of the dietary tannin ellagic acid, possesses antioxidant properties and has shown promise in mouse models of AKI. However, therapeutic potential of UA is constrained by poor bioavailability. We aimed to improve oral bioavailability of UA by formulating it into biodegradable nanoparticles that use a surface-conjugated ligand targeting the gut-expressed transferrin receptor. Nanoparticle encapsulation of UA led to a sevenfold enhancement in oral bioavailability compared with native UA. Treatment with nanoparticle UA also significantly attenuated the histopathological hallmarks of cisplatin-induced AKI and reduced mortality by 63% in the mouse model. Expression analyses indicated that nanoparticle UA therapy coincided with oxidative stress mitigation and downregulation of nuclear factor erythroid 2-related factor 2- and P53-inducible genes. Additionally, normalization of miRNA (miR-192-5p and miR-140-5p) implicated in AKI, poly(ADP-ribose) polymerase 1 levels, antiapoptotic signaling, intracellular NAD+, and mitochondrial oxidative phosphorylation were observed in the treatment group. Our findings suggest that nanoparticles greatly increase the oral bioavailability of UA, leading to improved survival rates in AKI mice, in part by reducing renal oxidative and apoptotic stress.

WNT Signaling in Disease

Developmental signaling pathways control a vast array of biological processes during embryogenesis and in adult life. The WNT pathway was discovered simultaneously in cancer and development. Recent advances have expanded the role of WNT to a wide range of pathologies in humans. Here, we discuss the WNT pathway and its role in human disease and some of the advances in WNT-related treatments.

WNT4 Expression in Primary and Secondary Kidney Diseases: Dependence on Staging

BACKGROUND/AIMS

WNT4 protein is important for kidney development. Its expression was found to be altered in experimental models of chronic kidney disease (CKD). However, the expression of the WNT4 gene has yet not been studied in human renal biopsy samples from patients with broad spectrum of glomerular disease and at different stages of CKD. Thus, the aim of the study was to assess the WNT4 gene expression in renal biopsies of 98 patients using the real-time PCR technique.

MATERIALS

In order to assess the relative amounts of mRNA, in samples of patients with manifestation of different renal diseases and separately at different stages of CKD, by QPCR, total RNA was isolated from human kidney tissues collected during renal biopsies. Results of blood and urine samples assessment were used to calculate the correlations of biochemical parameters with WNT4 gene expression in both studied groups.

RESULTS

After pathomorphological evaluation, 49 patients were selected as presenting the most common cases in the studied group. Among the patients who developed focal segmental glomerulosclerosis (FSGS; n = 13), IgA nephropathy (IgAN; n = 10), IgAN with morphological presentation of focal segmental glomerulosclerosis (IgAN/FSGS; n = 8), membranous nephropathy (MN; n = 12), and lupus nephritis (LN; n = 6) were included in the analysis. We found that the level of WNT4 mRNA was higher in kidney specimens obtained from patients with MN as compared to those diagnosed with LN or IgAN. A correlation between WNT4 gene expression and serum albumin and cholesterol levels was observed in patients with FSGS, while WNT4 mRNA levels correlated with plasma sodium in patients diagnosed with LN. After consideration of 98 patients, based on the KDIGO classification of CKD, 20 patients were classified as CKD1 stage, 23 as stage 2, 13 as stage 3a, 11 as stage 3b, 13 as stage 4, and 18 as stage 5. WNT4 gene expression was lower in the CKD patients in stage 2 as compared to CKD 3a. Correlations of WNT4 mRNA level at different stages of CKD with indices of kidney function and lipid metabolism such as serum levels of HDL and LDL cholesterol, TG, urea, creatinine, sodium, and potassium were also found.

CONCLUSIONS

Our results suggest that altered WNT4 gene expression in patients with different types of glomerular diseases and patients at different stages of CKD may play a role in kidney tissue disorganization as well as disease development and progression.

Wnt signaling mediates new nephron formation during zebrafish kidney regeneration

Zebrafish kidneys use resident kidney stem cells to replace damaged tubules with new nephrons: the filtration units of the kidney. What stimulates kidney progenitor cells to form new nephrons is not known. Here, we show that wnt9a and wnt9b are induced in the injured kidney at sites where frizzled9b- and lef1-expressing progenitor cells form new nephrons. New nephron aggregates are patterned by Wnt signaling, with high canonical Wnt-signaling cells forming a single cell thick rosette that demarcates: domains of cell proliferation in the elongating nephron; and tubule fusion where the new nephron plumbs into the distal tubule and establishes blood filtrate drainage. Pharmacological blockade of canonical Wnt signaling inhibited new nephron formation after injury by inhibiting cell proliferation, and resulted in loss of polarized rosette structures in the aggregates. Mutation in frizzled9b reduced total kidney nephron number, caused defects in tubule morphology and reduced regeneration of new nephrons after injury. Our results demonstrate an essential role for Wnt/frizzled signaling in adult zebrafish kidney development and regeneration, highlighting conserved mechanisms underlying both mammalian kidney development and kidney stem cell-directed neonephrogenesis in zebrafish.

Summary: Adult zebrafish kidneys induce Wnt signaling to generate new nephrons from resident kidney progenitor cells, highlighting how embryonic morphogens are reactivated in adult organs to drive regeneration.

Enhancing regeneration after acute kidney injury by promoting cellular dedifferentiation in zebrafish

Acute kidney injury (AKI) is a serious disorder for which there are limited treatment options. Following injury, native nephrons display limited regenerative capabilities, relying on the dedifferentiation and proliferation of renal tubular epithelial cells (RTECs) that survive the insult. Previously, we identified 4-(phenylthio)butanoic acid (PTBA), a histone deacetylase inhibitor (HDI), as an enhancer of renal recovery, and showed that PTBA treatment increased RTEC proliferation and reduced renal fibrosis. Here, we investigated the regenerative mechanisms of PTBA in zebrafish models of larval renal injury and adult cardiac injury. With respect to renal injury, we showed that delivery of PTBA using an esterified prodrug (UPHD25) increases the reactivation of the renal progenitor gene Pax2a, enhances dedifferentiation of RTECs, reduces Kidney injury molecule-1 (Kim-1) expression, and lowers the number of infiltrating macrophages. Further, we found that the effects of PTBA on RTEC proliferation depend upon retinoic acid signaling and demonstrate that the therapeutic properties of PTBA are not restricted to the kidney but also increase cardiomyocyte proliferation and decrease fibrosis following cardiac injury in adult zebrafish. These studies provide key mechanistic insights into how PTBA enhances tissue repair in models of acute injury and lay the groundwork for translating this novel HDI into the clinic.

This article has an associated First Person interview with the joint first authors of the paper.

Summary: Mortality associated with AKI is in part due to limited treatments available to ameliorate injury. The authors identify a compound that accelerates AKI recovery and promotes cellular dedifferentiation.

How Tubular Epithelial Cell Injury Contributes to Renal Fibrosis

The renal tubules are the major component of the kidney and are vulnerable to a variety of injuries including ischemia, proteinuria, toxins, and metabolic disorders. It has long been believed that tubules are the victim of injury. In this review, we shift this concept to renal tubules as a driving force in the progression of kidney disease. In response to injury, tubular epithelial cells (TECs) can synthesize and secrete varieties of bioactive molecules that drive interstitial inflammation and fibrosis. Innate immune-sensing receptors on the TECs also aggravate immune responses. Necroinflammation, an auto-amplification loop between tubular cell death and interstitial inflammation, leads to the exacerbation of renal injury. Furthermore, TECs also play an active role in progressive renal injury via mechanisms associated with the conversion into collagen-producing fibroblast phenotype, cell cycle arrest at both G1/S and G2/M checkpoints, and metabolic disorder. Thus, a better understanding the mechanisms by which tubular injury drives AKI and CKD is necessary for the development of therapeutics to halt the progression of CKD.

Wnt4 is significantly upregulated during the early phases of cisplatin-induced acute kidney injury

Wnt4 is a secreted growth factor associated with renal tubulogenesis. Our previous studies identified that renal and urinary Wnt4 are upregulated following ischemia-reperfusion injury in mice, but the roles of Wnt4 in other forms of acute kidney injury (AKI) remain unclear. Here, we investigated the changes in Wnt4 expression using a cisplatin-induced AKI model. We found that renal and urinary Wnt4 expression increased as early as 12 hours, peaked at day 4 following cisplatin-induced AKI and was closely correlated with histopathological alterations. By contrast, the serum creatinine level was significantly elevated until day 3, indicating that Wnt4 is more sensitive to early tubular injury than serum creatinine. In addition, renal Wnt4 was co-stained with aquaporin-1 and thiazide-sensitive NaCl cotransporter, suggesting that Wnt4 can detect both proximal and distal tubular injuries. These data were further confirmed in a clinical study. Increased urinary Wnt4 expression was detected earlier than serum creatinine and eGFR in patients with contrast-induced AKI after vascular intervention. This study is the first to demonstrate that increased expression of renal and urinary Wnt4 can be detected earlier than serum creatinine after drug-induced AKI. In particular, urinary Wnt4 can potentially serve as a noninvasive biomarker for monitoring patients with tubular injury.

Role of CD133 Molecule in Wnt Response and Renal Repair

Renal repair after injury is dependent on clonal expansion of proliferation-competent cells. In the human kidney, the expression of CD133 characterizes a population of resident scattered cells with resistance to damage and ability to proliferate. However, the biological function of the CD133 molecule is unknown. By RNA sequencing, we found that cells undergoing cisplatin damage lost the CD133 signature and acquired metanephric mesenchymal and regenerative genes such as SNAIL1, KLF4, SOX9, and WNT3. CD133 was reacquired in the recovery phase. In CD133-Kd cells, lack of CD133 limited cell proliferation after injury and was specifically correlated with deregulation of Wnt signaling and E-cadherin pathway. By immunoprecipitation, CD133 appeared to form a complex with E-cadherin and β-catenin. In parallel, CD133-Kd cells showed lower β-catenin levels in basal condition and after Wnt pathway activation and reduced TCF/LEF promoter activation in respect to CD133⁺ cells. Finally, the lack of CD133 impaired generation of nephrospheres while favoring senescence. These data indicate that CD133 may act as a permissive factor for β-catenin signaling, preventing its degradation in the cytoplasm. Therefore, CD133 itself appears to play a functional role in renal tubular repair through maintenance of proliferative response and control of senescence. Stem Cells Translational Medicine 2018;7:283-294.

Renal Tubule Repair: Is Wnt/β-Catenin a Friend or Foe?

Wnt/β-catenin signaling is extremely important for proper kidney development. This pathway is also upregulated in injured renal tubular epithelia, both in acute kidney injury and chronic kidney disease. The renal tubular epithelium is an important target of kidney injury, and its response (repair versus persistent injury) is critical for determining whether tubulointerstitial fibrosis, the hallmark of chronic kidney disease, develops. This review discusses how Wnt/β-catenin signaling in the injured tubular epithelia promotes either repair or fibrosis after kidney injury. There is data suggesting that epithelial Wnt/β-catenin signaling is beneficial in acute kidney injury and important in tubular progenitors responsible for epithelial repair. The role of Wnt/β-catenin signaling in chronically injured epithelia is less clear. There is convincing data that Wnt/β-catenin signaling in interstitial fibroblasts and pericytes contributes to the extracellular matrix accumulation that defines fibrosis. However, some recent studies question whether Wnt/β-catenin signaling in chronically injured epithelia actually promotes fibrosis or repair.

Cellular and molecular pathways of renal repair after acute kidney injury

The acutely injured mammalian kidney mounts a cellular and molecular response to repair itself. However, in patchy regions such intrinsic processes are impaired and dysregulated leading to chronic kidney disease. Currently, no therapy exists to treat established acute kidney injury per se. Strategies to augment human endogenous repair processes and retard associated profibrotic responses are urgently required. Recent studies have identified injury-induced activation of the intrinsic molecular driver of epithelial regeneration and induction of partial epithelial to the mesenchymal state, respectively. Activation of key developmental transcription factors drive such processes; however, whether these recruit comparable gene regulatory networks with target genes similar to those in nephrogenesis is unclear. Extensive complex molecular cross-talk between the nephron epithelia and immune, interstitial, and endothelial cells regulate renal recovery. In vitro-based M1/M2 macrophage subtypes have been increasingly linked to renal repair; however, the precise contribution of in vivo macrophage plasticity to repair responses is poorly understood. Endothelial cell-pericyte intimacy, balance of the angiocrine/antiangiocrine system, and endothelial cell-regulated inflammatory processes have an impact on renal recovery and fibrosis. Close scrutiny of cellular and molecular pathways in repairing human kidneys is imperative for the identification of promising therapeutic targets and biomarker of human renal repair processes.

Wnt Signaling in Kidney Development and Disease

Wnt signal cascade is an evolutionarily conserved, developmental pathway that regulates embryogenesis, injury repair, and pathogenesis of human diseases. It is well established that Wnt ligands transmit their signal via canonical, β-catenin-dependent and noncanonical, β-catenin-independent mechanisms. Mounting evidence has revealed that Wnt signaling plays a key role in controlling early nephrogenesis and is implicated in the development of various kidney disorders. Dysregulations of Wnt expression cause a variety of developmental abnormalities and human diseases, such as congenital anomalies of the kidney and urinary tract, cystic kidney, and renal carcinoma. Multiple Wnt ligands, their receptors, and transcriptional targets are upregulated during nephron formation, which is crucial for mediating the reciprocal interaction between primordial tissues of ureteric bud and metanephric mesenchyme. Renal cysts are also associated with disrupted Wnt signaling. In addition, Wnt components are important players in renal tumorigenesis. Activation of Wnt/β-catenin is instrumental for tubular repair and regeneration after acute kidney injury. However, sustained activation of this signal cascade is linked to chronic kidney diseases and renal fibrosis in patients and experimental animal models. Mechanistically, Wnt signaling controls a diverse array of biologic processes, such as cell cycle progression, cell polarity and migration, cilia biology, and activation of renin-angiotensin system. In this chapter, we have reviewed recent findings that implicate Wnt signaling in kidney development and diseases. Targeting this signaling may hold promise for future treatment of kidney disorders in patients.

AKI on CKD: heightened injury, suppressed repair, and the underlying mechanisms

Acute kidney injury (AKI) and chronic kidney disease (CKD) are interconnected. Although AKI-to-CKD transition has been intensively studied, the information of AKI on CKD is very limited. Nonetheless, AKI, when occurring in patients with CKD, is known to be more severe and difficult to recover. CKD is associated with significant changes in cell signaling in kidney tissues, including the activation of transforming growth factor-β, p53, hypoxia-inducible factor, and major developmental pathways. At the cellular level, CKD is characterized by mitochondrial dysfunction, oxidative stress, and aberrant autophagy. At the tissue level, CKD is characterized by chronic inflammation and vascular dysfunction. These pathologic changes may contribute to the heightened sensitivity of, and nonrecovery from, AKI in patients with CKD.

The hormetic functions of Wnt pathways in tubular injury

Chronic tubulointerstitial damage with tubular epithelial atrophy and interstitial fibrosis is the hallmark of chronic kidney disease (CKD) and a predictor for progression of CKD.Several experiments have now provided evidence that the Wnt signaling pathways are significantly contributing to atrophy and fibrosis; in contrast, it also has been shown that the Wnt system fosters regenerative processes in acute tubular injury.We now have demonstrated that Dickkopf 3 (DKK3) is an agonist for canonical Wnt signaling in CKD and fosters chronic fibrosing inflammation of the tubulointerstitial compartment. Genetic- and antibody-mediated inhibition of DKK3 leads to a pronounced improvement of tubular differentiation and a reduction in fibrosis.In addition, the secreted glycoprotein DKK3 can be used as a non-invasive urinary marker for the extent of CKD in man.

The chronic kidney disease - Mineral bone disorder (CKD-MBD): Advances in pathophysiology

The causes of excess cardiovascular mortality associated with chronic kidney disease (CKD) have been attributed in part to the CKD-mineral bone disorder syndrome (CKD-MBD), wherein, novel cardiovascular risk factors have been identified. New advances in the causes of the CKD-MBD are discussed in this review. They demonstrate that repair and disease processes in the kidneys release factors to the circulation that cause the systemic complications of CKD. The discovery of WNT inhibitors, especially Dickkopf 1 (Dkk1), produced during renal repair as participating in the pathogenesis of the vascular and skeletal components of the CKD-MBD implied that additional pathogenic factors are critical. This lead to the discovery that activin A is a second renal repair factor circulating in increased levels during CKD. Activin A derives from peritubular myofibroblasts of diseased kidneys, wherein it stimulates fibrosis, and decreases tubular klotho expression. Activin A binds to the type 2 activin A receptor, ActRIIA, which is variably affected by CKD in the vasculature. In diabetic/atherosclerotic aortas, specifically in vascular smooth muscle cells (VSMC), ActRIIA signaling is inhibited and contributes to CKD induced VSMC dedifferentiation, osteogenic transition and neointimal atherosclerotic calcification. In nondiabetic/nonatherosclerotic aortas, CKD increases VSMC ActRIIA signaling, and vascular fibroblast signaling causing the latter to undergo osteogenic transition and stimulate vascular calcification. In both vascular situations, a ligand trap for ActRIIA prevented vascular calcification. In the skeleton, activin A is responsible for CKD stimulation of osteoclastogenesis and bone remodeling increasing bone turnover. These studies demonstrate that circulating renal repair and injury factors are causal of the CKD-MBD and CKD associated cardiovascular disease.

Multiple Mechanisms are Involved in Salt-Sensitive Hypertension-Induced Renal Injury and Interstitial Fibrosis

Salt-sensitive hypertension (SSHT) leads to kidney interstitial fibrosis. However, the potential mechanisms leading to renal fibrosis have not been well investigated. In present study, Dahl salt-sensitive (DS) rats were divided into three groups: normal salt diet (DSN), high salt diet (DSH) and high salt diet treated with hydrochlorothiazide (HCTZ) (DSH + HCTZ). A significant increase in systolic blood pressure (SBP) was observed 3 weeks after initiating the high salt diet, and marked histological alterations were observed in DSH rats. DSH rats showed obvious podocyte injury, peritubular capillary (PTC) loss, macrophage infiltration, and changes in apoptosis and cell proliferation. Moreover, Wnt/β-catenin signaling was significantly activated in DSH rats. However, HCTZ administration attenuated these changes with decreased SBP. In addition, increased renal and urinary Wnt4 expression was detected with time in DSH rats and was closely correlated with histopathological alterations. Furthermore, these alterations were also confirmed by clinical study. In conclusion, the present study provides novel insight into the mechanisms related to PTC loss, macrophage infiltration and Wnt/β-catenin signaling in SSHT-induced renal injury and fibrosis. Therefore, multi-target therapeutic strategies may be the most effective in preventing these pathological processes. Moreover, urinary Wnt4 may be a noninvasive biomarker for monitoring renal injury after hypertension.

Dact1, a Wnt-Pathway Inhibitor, Mediates Human Mesangial Cell TGF-β1-Induced Apoptosis

Chronic kidney disease (CKD) is a worldwide public health problem that affects millions of men and women of all ages and racial groups. Loss of mesangial cells (MC) represents an early common feature in the pathogenesis of CKD. Transforming growth factor-β1 (TGF-β1) is a key inducer of kidney damage and triggers several pathological changes in renal cells, notably MC apoptosis. However, the mechanism of MC apoptosis induced by TGF-β1 remains elusive. Here, we demonstrate for the first time a novel regulatory pathway in which the disheveled-binding antagonist of β-catenin 1 (Dact1) gene is upregulated by TGF-β1, inducing MC apoptosis. We also show that the inhibitory effect of Dact1 and TGF-β1 on the transcriptional activation of the pro-survival Wnt pathway is the mechanism of death induction. In addition, Dact1 mRNA/protein levels are increased in kidney remnants from 5/6 nephrectomized rats and strongly correlate with TGF-β1 expression. Together, our results point to Dact1 as a novel element controlling MC survival that is causally related to CKD progression. J. Cell. Physiol. 232: 2104-2111, 2017. © 2016 Wiley Periodicals, Inc.

Cellular plasticity in kidney injury and repair

Terminally differentiated cells can be reprogrammed to pluripotency or directly to another differentiated cell type in vitro, a capacity termed cellular plasticity. Plasticity is not limited to in vitro manipulations but rather represents an important aspect of the regenerative response to injury in organs. Differentiated adult cells retain the capacity to dedifferentiate, adopting a progenitor-like phenotype after injury or, alternatively, to transdifferentiate, directly converting to a different mature cell type. Emerging concepts on cellular plasticity have relevance to our understanding of repair after kidney injury, including epithelial regeneration. Here we discuss work published in the past 5 years on the cellular hierarchies and mechanisms underlying kidney injury and repair, with a particular focus on potential roles for cellular plasticity in this response.

Does Renal Repair Recapitulate Kidney Development?

Over a decade ago, it was proposed that the regulation of tubular repair in the kidney might involve the recapitulation of developmental pathways. Although the kidney cannot generate new nephrons after birth, suggesting a low level of regenerative competence, the tubular epithelial cells of the nephrons can proliferate to repair the damage after AKI. However, the debate continues over whether this repair involves a persistent progenitor population or any mature epithelial cell remaining after injury. Recent reports have highlighted the expression of Sox9, a transcription factor critical for normal kidney development, during postnatal epithelial repair in the kidney. Indeed, the proliferative response of the epithelium involves expression of several pathways previously described as being involved in kidney development. In some instances, these pathways are also apparently involved in the maladaptive responses observed after repeated injury. Whether development and repair in the kidney are the same processes or we are misinterpreting the similar expression of genes under different circumstances remains unknown. Here, we review the evidence for this link, concluding that such parallels in expression may more correctly represent the use of the same pathways in a distinct context, likely triggered by similar stressors.

Wnt4 is a novel biomarker for the early detection of kidney tubular injury after ischemia/reperfusion injury

Earlier intervention after acute kidney injury would promote better outcomes. Our previous study found that Wnt proteins are promptly upregulated after ischemic kidney injury. Thus, we assessed whether Wnt4 could be an early and sensitive biomarker of tubular injury. We subjected mice to bilateral ischemia/reperfusion injury (IRI). Kidney and urinary Wnt4 expression showed an early increase at 3 hours and increased further at 24 hours post-IRI and was closely correlated with histopathological alterations. Serum creatinine slightly increased at 6 hours, indicating that it was less sensitive than Wnt4 expression. These data were further confirmed by clinical study. Both kidney and urinary Wnt4 expression were significantly increased in patients diagnosed with biopsy-proven minimal change disease (MCD) with tubular injury, all of whom nevertheless had normal estimated glomerular filtration rate (eGFR) and serum creatinine. The increased Wnt4 expression also strongly correlated with histopathological alterations in these MCD patients. In conclusion, this is the first demonstration that increases in both kidney and urinary Wnt4 expression can be detected more sensitively and earlier than serum creatinine after kidney injury. In particular, urinary Wnt4 could be a potential noninvasive biomarker for the early detection of tubular injury.

Signaling Crosstalk between Tubular Epithelial Cells and Interstitial Fibroblasts after Kidney Injury

BACKGROUND

A wide variety of kidney diseases ultimately lead to tubulointerstitial damage. The initial site of injury is usually the renal tubules, with activation of fibroblasts occurring later. Self-limited disease is characterized by transient cellular activation with timed deactivation and ultimately a return to normal functioning, whereas sustained responses characterize chronic disease and the development of irreversible fibrosis. The underlying molecular and cellular mechanisms of this cascade of events remain an area of active research. Current data overwhelmingly support a role for crosstalk between the tubular epithelium and the interstitial fibroblast that mediates both repair/regeneration and progressive disease. This epithelial-mesenchymal communication (EMC) is regulated by a variety of soluble ligands binding to cell surface receptors to induce intracellular signaling events.

SUMMARY

EMC is an important mechanism whereby tubular epithelium and fibroblasts/mesenchymal cells crosstalk to affect renal physiology and pathology. Numerous soluble factors such as sonic hedgehog, Wnt ligands, transforming growth factor-β, hepatocyte growth factor, connective tissue growth factor, and angiotensin II all participate in bidirectional EMC. Recent studies have also identified exosomes as a vehicle to mediate EMC during kidney injury. In general, while the short-term activity of EMC factors is renoprotective, prolonged activation of these factors leads to chronic disease and fibrosis.

KEY MESSAGES

The discovery of a complex and intricate system of communication between tubular cells and fibroblasts is a new paradigm in our understanding of renal fibrosis. An appreciation of both their regenerative and pathologic functions will inform the development and use of targeted therapeutic interventions.

Developmental signalling pathways in renal fibrosis: the roles of Notch, Wnt and Hedgehog

Kidney fibrosis is a common histological manifestation of functional decline in the kidney. Fibrosis is a reactive process that develops in response to excessive epithelial injury and inflammation, leading to myofibroblast activation and an accumulation of extracellular matrix. Here, we describe how three key developmental signalling pathways - Notch, Wnt and Hedgehog (Hh) - are reactivated in response to kidney injury and contribute to the fibrotic response. Although transient activation of these pathways is needed for repair of injured tissue, their sustained activation is thought to promote fibrosis. Excessive Wnt and Notch expression prohibit epithelial differentiation, whereas increased Wnt and Hh expression induce fibroblast proliferation and myofibroblastic transdifferentiation. Notch, Wnt and Hh are fundamentally different signalling pathways, but their choreographed activation seems to be just as important for fibrosis as it is for embryonic kidney development. Decreasing the activity of Notch, Wnt or Hh signalling could potentially provide a new therapeutic strategy to ameliorate the development of fibrosis in chronic kidney disease.

Wnt6 regulates epithelial cell differentiation and is dysregulated in renal fibrosis

Diabetic nephropathy is the most common microvascular complication of diabetes mellitus, manifesting as mesangial expansion, glomerular basement membrane thickening, glomerular sclerosis, and progressive tubulointerstitial fibrosis leading to end-stage renal disease. Here we describe the functional characterization of Wnt6, whose expression is progressively lost in diabetic nephropathy and animal models of acute tubular injury and renal fibrosis. We have shown prominent Wnt6 and frizzled 7 (FzD7) expression in the mesonephros of the developing mouse kidney, suggesting a role for Wnt6 in epithelialization. Importantly, TCF/Lef reporter activity is also prominent in the mesonephros. Analysis of Wnt family members in human renal biopsies identified differential expression of Wnt6, correlating with severity of the disease. In animal models of tubular injury and fibrosis, loss of Wnt6 was evident. Wnt6 signals through the canonical pathway in renal epithelial cells as evidenced by increased phosphorylation of GSK3β (Ser9), nuclear accumulation of β-catenin and increased TCF/Lef transcriptional activity. FzD7 was identified as a putative receptor of Wnt6. In vitro Wnt6 expression leads to de novo tubulogenesis in renal epithelial cells grown in three-dimensional culture. Importantly, Wnt6 rescued epithelial cell dedifferentiation in response to transforming growth factor-β (TGF-β); Wnt6 reversed TGF-β-mediated increases in vimentin and loss of epithelial phenotype. Wnt6 inhibited TGF-β-mediated p65-NF-κB nuclear translocation, highlighting cross talk between the two pathways. The critical role of NF-κB in the regulation of vimentin expression was confirmed in both p65(-/-) and IKKα/β(-/-) embryonic fibroblasts. We propose that Wnt6 is involved in epithelialization and loss of Wnt6 expression contributes to the pathogenesis of renal fibrosis.