A mouse model for inherited renal fibrosis associated with endoplasmic reticulum stress

Allelic effects on uromodulin aggregates drive autosomal dominant tubulointerstitial kidney disease

Missense mutations in the uromodulin (UMOD) gene cause autosomal dominant tubulointerstitial kidney disease (ADTKD), one of the most common monogenic kidney diseases. The unknown impact of the allelic and gene dosage effects and fate of mutant uromodulin leaves open the gap between postulated gain-of-function mutations, end-organ damage and disease progression in ADTKD. Based on two prevalent missense UMOD mutations with divergent disease progression, we generated UmodC171Y and UmodR186S knock-in mice that showed strong allelic and gene dosage effects on uromodulin aggregates and activation of ER stress and unfolded protein and immune responses, leading to variable kidney damage. Deletion of the wild-type Umod allele in heterozygous UmodR186S mice increased the formation of uromodulin aggregates and ER stress. Studies in kidney tubular cells confirmed differences in uromodulin aggregates, with activation of mutation-specific quality control and clearance mechanisms. Enhancement of autophagy by starvation and mTORC1 inhibition decreased uromodulin aggregates. These studies substantiate the role of toxic aggregates as driving progression of ADTKD-UMOD, relevant for therapeutic strategies to improve clearance of mutant uromodulin.

UMOD Mutations in Chronic Kidney Disease in Taiwan

UMOD is the first identified and the most commonly mutated gene that causes autosomal dominant tubulointerstitial kidney disease (ADTKD). Recent studies have shown that ADTKD-UMOD is a relatively common cause of chronic kidney disease (CKD). However, the status of ADTKD-UMOD in Taiwan remains unknown. In this study, we identified three heterozygous UMOD missense variants, c.121T > C (p.Cys41Arg), c.179G > A (p.Gly60Asp), and c.817G > T (p.Val273Phe), in a total of 221 selected CKD families (1.36%). Two of these missense variants, p.Cys41Arg and p.Gly60Asp, have not been reported previously. In vitro studies showed that both uromodulin variants have defects in cell membrane trafficking and excretion to the culture medium. The structure model predicted altered disulfide bond formation in both variants, but only p.Gly60Asp was predicted to cause protein destabilization. Our findings extend the mutation spectrum and indicate that the ADTKD-UMOD contributed to a small but significant cause of CKD in the Taiwanese population.

Meta-GWAS Reveals Novel Genetic Variants Associated with Urinary Excretion of Uromodulin

BACKGROUND

Uromodulin, the most abundant protein excreted in normal urine, plays major roles in kidney physiology and disease. The mechanisms regulating the urinary excretion of uromodulin remain essentially unknown.

METHODS

We conducted a meta-analysis of genome-wide association studies for raw (uUMOD) and indexed to creatinine (uUCR) urinary levels of uromodulin in 29,315 individuals of European ancestry from 13 cohorts. We tested the distribution of candidate genes in kidney segments and investigated the effects of keratin-40 (KRT40) on uromodulin processing.

RESULTS

Two genome-wide significant signals were identified for uUMOD: a novel locus (P 1.24E-08) over the KRT40 gene coding for KRT40, a type 1 keratin expressed in the kidney, and the UMOD-PDILT locus (P 2.17E-88), with two independent sets of single nucleotide polymorphisms spread over UMOD and PDILT. Two genome-wide significant signals for uUCR were identified at the UMOD-PDILT locus and at the novel WDR72 locus previously associated with kidney function. The effect sizes for rs8067385, the index single nucleotide polymorphism in the KRT40 locus, were similar for both uUMOD and uUCR. KRT40 colocalized with uromodulin and modulating its expression in thick ascending limb (TAL) cells affected uromodulin processing and excretion.

CONCLUSIONS

Common variants in KRT40, WDR72, UMOD, and PDILT associate with the levels of uromodulin in urine. The expression of KRT40 affects uromodulin processing in TAL cells. These results, although limited by lack of replication, provide insights into the biology of uromodulin, the role of keratins in the kidney, and the influence of the UMOD-PDILT locus on kidney function.

Upregulation of C/EBP Homologous Protein induced by ER Stress Mediates Epithelial to Myofibroblast Transformation in ADTKD-UMOD

Autosomal dominant tubulointerstitial kidney disease due to UMOD mutations (ADTKD-UMOD) results in chronic interstitial nephritis, which gradually develops into end-stage renal disease. It is believed that the accumulation of mutant uromodulin causes the endoplasmic reticulum (ER) stress, then leads to the kidney damage. But the underlying mechanism remains unclear. To find the ADTKD-UMOD patients, UMOD gene screening was performed in 26 patients with unexplained chronic interstitial nephritis, during the past 10 years in our department, and among them three ADTKD-UMOD cases were discovered. Routine pathological staining and electron microscopy sections were reviewed again to confirm their kidney lesions. Immunostaining of UMOD and ER stress marker GRP78, as well as CHOP have all been done. The strong colocalization of UMOD with GRP78 and CHOP in ADTKD-UMOD patients but not in other chronic interstitial nephritis patients had been found. Moreover in vitro experiments, ER stress induced by tunicamycin (TM) not only significantly increased the expression of GRP78 and CHOP, but also caused the epithelial to myofibroblast transformation (EMT) of renal tubular epithelial cells, evidenced by decreased expression of E-cadherin and increased expression of vimentin, and extracellular matrix (ECM) deposition, evidenced by increased expression of fibronectin (FN). CHOP knockdown could restore the upregulation of vimentin and FN induced by TM. Thus, specific activation of CHOP in renal tubular epithelial cells induced by UMOD protein might be the key reason of renal interstitial fibrosis in ADTKD-UMOD patients.

TDAG51 induces renal interstitial fibrosis through modulation of TGF-β receptor 1 in chronic kidney disease

Chronic kidney disease (CKD) is characterized by the gradual loss of renal function and is a major public health concern. Risk factors for CKD include hypertension and proteinuria, both of which are associated with endoplasmic reticulum (ER) stress. ER stress-induced TDAG51 protein expression is increased at an early time point in mice with CKD. Based on these findings, wild-type and TDAG51 knock-out (TDKO) mice were used in an angiotensin II/deoxycorticosterone acetate/salt model of CKD. Both wild-type and TDKO mice developed hypertension, increased proteinuria and albuminuria, glomerular injury, and tubular damage. However, TDKO mice were protected from apoptosis and renal interstitial fibrosis. Human proximal tubular cells were used to demonstrate that TDAG51 expression induces apoptosis through a CHOP-dependent mechanism. Further, a mouse model of intrinsic acute kidney injury demonstrated that CHOP is required for ER stress-mediated apoptosis. Renal fibroblasts were used to demonstrate that TGF-β induces collagen production through an IRE1-dependent mechanism; cells treated with a TGF-β receptor 1 inhibitor prevented XBP1 splicing, a downstream consequence of IRE1 activation. Interestingly, TDKO mice express significantly less TGF-β receptor 1, thus, preventing TGF-β-mediated XBP1 splicing. In conclusion, TDAG51 induces apoptosis in the kidney through a CHOP-dependent mechanism, while contributing to renal interstitial fibrosis through a TGF-β-IRE1-XBP1 pathway.

Uromodulin: Roles in Health and Disease

Uromodulin, a protein exclusively produced by the kidney, is the most abundant urinary protein in physiological conditions. Already described several decades ago, uromodulin has gained the spotlight in recent years, since the discovery that mutations in its encoding gene UMOD cause a renal Mendelian disease (autosomal dominant tubulointerstitial kidney disease) and that common polymorphisms are associated with multifactorial disorders, such as chronic kidney disease, hypertension, and cardiovascular diseases. Moreover, variations in uromodulin levels in urine and/or blood reflect kidney functioning mass and are of prognostic value for renal function, cardiovascular events, and overall mortality. The clinical relevance of uromodulin reflects its multifunctional nature, playing a role in renal ion transport and immunomodulation, in protection against urinary tract infections and renal stones, and possibly as a systemic antioxidant. Here, we discuss the multifaceted roles of this protein in kidney physiology and its translational relevance.

A Uromodulin Mutation Drives Autoimmunity and Kidney Mononuclear Phagocyte Endoplasmic Reticulum Stress

We identified a family with a UMOD gene mutation (C106F) resulting in glomerular inflammation and complement deposition. To determine if the observed phenotype is due to immune system activation by mutant uromodulin, a mouse strain with a homologous cysteine to phenylalanine mutation (C105F) in the UMOD gene was generated using CRISPR-Cas9 gene editing and the effect of this mutation on mononuclear phagocytic cells was examined. Mutant mice developed high levels of intracellular and secreted aggregated uromodulin, resulting in anti-uromodulin antibodies and circulating uromodulin containing immune complexes with glomerular deposition and kidney fibrosis with aging. F4/80⁺ and CD11c⁺ kidney cells phagocytize uromodulin. Differential gene expression analysis by RNA sequencing of F4/80⁺ phagocytic cells revealed activation of the activating transcription factor 5 (ATF5)-mediated stress response pathway in mutant mice. Phagocytosis of mutant uromodulin by cultured dendritic cells resulted in activation of the endoplasmic reticulum stress response pathway and markers of cell inactivation, an effect not seen with wild-type protein. Mutant mice demonstrate a twofold increase in T-regulatory cells, consistent with induction of immune tolerance, resulting in decreased inflammatory response and improved tissue repair following ischemia-reperfusion injury. The C105F mutation results in autoantibodies against aggregated misfolded protein with immune complex formation and kidney fibrosis. Aggregated uromodulin may induce dendritic cell tolerance following phagocytosis through an unfolded protein/endoplasmic reticulum stress response pathway, resulting in decreased inflammation following tissue injury.

Berberine (BBR) Attenuated Palmitic Acid (PA)-Induced Lipotoxicity in Human HK-2 Cells by Promoting Peroxisome Proliferator-Activated Receptor α (PPAR-α)

BACKGROUND Berberine (BBR), a natural alkaloid isolated from Coptis chinensis, has frequently been reported as an antidiabetic reagent, partly due to its lipid-lowering activity. Evidence suggests that BBR ameliorates palmitate-induced lipid deposition and apoptosis in renal tubular epithelial cells (TECs), which tracks in tandem with the enhancement of peroxisome proliferator-activated receptor alpha (PPAR-alpha). The study aim was to investigate the roles of BBR in renal lipotoxicity in vitro, and investigate whether PPAR-alpha was the underlying mechanism. MATERIAL AND METHODS Human TECs (HK-2 cells) were injured with palmitic acid (PA), and then treated with BBR, BBR+PPAR-alpha inhibitor (GW6471), and PA+PPAR-alpha agonist (fenofibrate). Endoplasmic reticulum (ER) stress was assessed by measuring the expression of prospective evaluation of radial keratotomy (PERK), C/EBP-homologous protein (CHOP), and 78 kDa glucose-regulated protein (GRP78). Lipid metabolism was assessed by determining lipid anabolism-associated genes, including fatty acid synthase (FAS), acetyl-CoA carboxylase (ACC), and lipoprotein lipase (LPL), as well as lipid catabolism-associated gene, including carnitine palmitoyl transferase 1 (CPT1). Inflammatory response of HK-2 cells was evaluated by measuring interleukin (IL)-6 and tumor necrosis factor (TNF)-alpha. Cell apoptosis and protein levels of cleaved-caspase-3 were evaluated. RESULTS PA downregulated PPAR-alpha and induced server lipotoxicity in HK-2 cells by ER stress, increasing lipid deposition, and elevating inflammatory response of HK-2 cells accompanied with inducting cell apoptosis and cleaved-caspase-3, which were obviously reversed by additional treatment of BBR or PPAR-alpha agonist. However, the protective effect of BBR in PA-induced lipotoxicity in HK-2 cells was significantly ameliorated by PPAR-alpha inhibitor. CONCLUSIONS BBR attenuated PA-induced lipotoxicity via the PPAR-alpha pathway.

Emodin promotes fibroblast apoptosis and prevents epidural fibrosis through PERK pathway in rats

Background

Laminectomy is usually classed as a common orthopedic surgery, but postoperative epidural fibrosis often leads to less-than-desirable clinical outcomes. As demonstrated by prior studies, emodin (EMO) exerts an anti-fibrotic effect. Here, we carried out investigation into the inhibitory effect created by EMO application on epidural fibrosis after laminectomy in rats.

Methods

The paper conducts a series of experiment. In vitro, we observed the effect of EMO on fibroblasts by Cell Counting Kit-8 (CCK-8) assay. Apoptosis of fibroblasts induced by EMO was detected by western blot, TUNEL assay, and flow cytometry. The results revealed that EMO was capable of inducing fibroblast apoptosis, and the proteins of PERK pathway also changed accordingly. In vivo, the effect of EMO on epidural fibrosis in 12 male Sprague-Dawley rats was observed by histological staining.

Results

CCK-8 assay indicated that EMO was effective in reducing fibroblast viability in a time- and a dose-dependent manner. TUNEL assay and flow cytometry analysis have demonstrated that the apoptotic rate of fibroblasts increased as the EMO concentration rose. Western blot analysis proved that EMO promoted the relative expression of p-perk and p-eIF2α and that the expression of its downstream proteins CHOP and GRP78 was also enhanced. The expression of apoptotic protein Bax and cleaved PARP was upregulated, whereas the expression of anti-apoptotic protein Bcl-2 was downregulated. In addition, histological and immunohistochemical analysis demonstrated that EMO functioned to inhibit epidural fibrosis and increase GRP78 expression in fibrous tissue by promoting apoptosis of fibroblasts.

Conclusions

EMO could have inhibitory effect on epidural fibrosis in a concentration-dependent manner. The potential mechanism might be through PERK signaling pathway to promote fibroblast apoptosis. It has a possibility to be taken as a novel method for the treatment of epidural fibrosis.

Autosomal dominant tubulointerstitial kidney disease

Autosomal dominant tubulointerstitial kidney disease (ADTKD) is a recently defined entity that includes rare kidney diseases characterized by tubular damage and interstitial fibrosis in the absence of glomerular lesions, with inescapable progression to end-stage renal disease. These diseases have long been neglected and under-recognized, in part due to confusing and inconsistent terminology. The introduction of a gene-based, unifying terminology led to the identification of an increasing number of cases, with recent data suggesting that ADTKD is one of the more common monogenic kidney diseases after autosomal dominant polycystic kidney disease, accounting for ~5% of monogenic disorders causing chronic kidney disease. ADTKD is caused by mutations in at least five different genes, including UMOD, MUC1, REN, HNF1B and, more rarely, SEC61A1. These genes encode various proteins with renal and extra-renal functions. The mundane clinical characteristics and lack of appreciation of family history often result in a failure to diagnose ADTKD. This Primer highlights the different types of ADTKD and discusses the distinct genetic and clinical features as well as the underlying mechanisms.

Construction and analysis of a diabetic nephropathy related protein-protein interaction network reveals nine critical and functionally associated genes

Diabetic nephropathy (DN) is one of the common diabetic complications, but the mechanisms are still largely unknown. In this study, we constructed a DN related protein-protein interaction network (DNPPIN) on the basis of RNA-seq analysis of renal cortices of DN and normal mice, and the STRING database. We analyzed DNPPIN in detail revealing nine critical proteins which are central in DNPPIN, and contained in one network module which is functionally enriched in ribosome, nucleic acid binding and metabolic process. Overall, this study identified nine critical and functionally associated protein-coding genes concerning DN. These genes could be a starting point of future research towards the goal of elucidating the mechanisms of DN pathogenesis and progression.

Hepsin-mediated Processing of Uromodulin is Crucial for Salt-sensitivity and Thick Ascending Limb Homeostasis

Uromodulin is a zona pellucida-type protein essentially produced in the thick ascending limb (TAL) of the mammalian kidney. It is the most abundant protein in normal urine. Defective uromodulin processing is associated with various kidney disorders. The luminal release and subsequent polymerization of uromodulin depend on its cleavage mediated by the serine protease hepsin. The biological relevance of a proper cleavage of uromodulin remains unknown. Here we combined in vivo testing on hepsin-deficient mice, ex vivo analyses on isolated tubules and in vitro studies on TAL cells to demonstrate that hepsin influence on uromodulin processing is an important modulator of salt transport via the sodium cotransporter NKCC2 in the TAL. At baseline, hepsin-deficient mice accumulate uromodulin, along with hyperactivated NKCC2, resulting in a positive sodium balance and a better adaptation to water deprivation. In conditions of high salt intake, defective uromodulin processing predisposes hepsin-deficient mice to a salt-wasting phenotype, with a decreased salt sensitivity. These modifications are associated with intracellular accumulation of uromodulin, endoplasmic reticulum-stress and signs of tubular damage. These studies expand the physiological role of hepsin and uromodulin and highlight the importance of hepsin-mediated processing of uromodulin for kidney tubule homeostasis and salt sensitivity.

Autosomal Dominant Tubulointerstitial Kidney Disease with Adult Onset due to a Novel Renin Mutation Mapping in the Mature Protein

Autosomal dominant tubulointerstitial kidney disease (ADTKD) is a genetically heterogeneous renal disorder leading to progressive loss of renal function. ADTKD-REN is due to rare mutations in renin, all localized in the protein leader peptide and affecting its co-translational insertion in the endoplasmic reticulum (ER). Through exome sequencing in an adult-onset ADTKD family we identified a new renin variant, p.L381P, mapping in the mature protein. To assess its pathogenicity, we combined genetic data, computational and predictive analysis and functional studies. The L381P substitution affects an evolutionary conserved residue, co-segregates with renal disease, is not found in population databases and is predicted to be deleterious by in silico tools and by structural modelling. Expression of the L381P variant leads to its ER retention and induction of the Unfolded Protein Response in cell models and to defective pronephros development in zebrafish. Our work shows that REN mutations outside of renin leader peptide can cause ADTKD and delineates an adult form of ADTKD-REN, a condition which has usually its onset in childhood. This has implications for the molecular diagnosis and the estimated prevalence of the disease and points at ER homeostasis as a common pathway affected in ADTKD-REN, and possibly more generally in ADTKD.

Mice with a Brd4 Mutation Represent a New Model of Nephrocalcinosis

Nephrolithiasis (NL) and nephrocalcinosis (NC), which comprise renal calcification of the collecting system and parenchyma, respectively, have a multifactorial etiology with environmental and genetic determinants and affect ∼10% of adults by age 70 years. Studies of families with hereditary NL and NC have identified >30 causative genes that have increased our understanding of extracellular calcium homeostasis and renal tubular transport of calcium. However, these account for <20% of the likely genes that are involved, and to identify novel genes for renal calcification disorders, we investigated 1745 12-month-old progeny from a male mouse that had been treated with the chemical mutagen N-ethyl-N-nitrosourea (ENU) for radiological renal opacities. This identified a male mouse with renal calcification that was inherited as an autosomal dominant trait with >80% penetrance in 152 progeny. The calcification consisted of calcium phosphate deposits in the renal papillae and was associated with the presence of the urinary macromolecules osteopontin and Tamm-Horsfall protein, which are features found in Randall's plaques of patients with NC. Genome-wide mapping located the disease locus to a ∼30 Mbp region on chromosome 17A3.3-B3 and whole-exome sequence analysis identified a heterozygous mutation, resulting in a missense substitution (Met149Thr, M149T), in the bromodomain-containing protein 4 (BRD4). The mutant heterozygous (Brd4^+/M149T ) mice, when compared with wild-type (Brd4^+/+ ) mice, were normocalcemic and normophosphatemic, with normal urinary excretions of calcium and phosphate, and had normal bone turnover markers. BRD4 plays a critical role in histone modification and gene transcription, and cDNA expression profiling, using kidneys from Brd4^+/M149T and Brd4^+/+ mice, revealed differential expression of genes involved in vitamin D metabolism, cell differentiation, and apoptosis. Kidneys from Brd4^+/M149T mice also had increased apoptosis at sites of calcification within the renal papillae. Thus, our studies have established a mouse model, due to a Brd4 Met149Thr mutation, for inherited NC. © 2019 American Society for Bone and Mineral Research.

c‑Jun N‑terminal kinase/transforming growth factor‑β/Smad3 pathway: Is it associated with endoplasmic reticulum stress‑mediated renal interstitial fibrosis?

The present study investigated the role of the c‑Jun N‑terminal kinase (JNK)/transforming growth factor‑β (TGF‑β)/Smad3 pathway in endoplasmic reticulum stress (ERS)‑mediated renal interstitial fibrosis, which would be beneficial for chronic kidney disease (CKD) therapy. In human renal biopsy tissue, the expression levels of glucose‑regulated protein 78 (GRP78) and phosphorylated (p)‑JNK were examined by immunohistochemical analysis. In renal tubular HK‑2 cells, tunicamycin (TM) was used to induce ERS, and the cells were then treated with the chemical ERS inhibitor 4‑phenylbutyrate (4‑PBA) or the chemical JNK pathway inhibitor SP600125, respectively. Western blotting was then performed in the cells to determine the expression levels of GRP78 and p‑JNK proteins, as well as TGF‑β/Smad3 pathway‑associated proteins, including TGF‑β1, p‑Smad3, connective tissue growth factor and α‑smooth muscle actin. The results revealed that GRP78 and p‑JNK were evidently expressed in the renal tissues of patients with CKD, and these expression levels were significantly higher in renal tissues with severe interstitial fibrosis compared with glomerular minor lesion tissues (P<0.01 and P<0.05, respectively). Furthermore, ERS and JNK pathway inhibition decreased the expression levels of TGF‑β/Smad3 pathway signals in cells incubated with TM. ERS pathway inhibition also attenuated the expression levels of p‑JNK in HK‑2 cells. In conclusion, ERS was observed to serve an important role in the pathogenesis of CKD and may induce renal interstitial fibrosis via the JNK/TGF‑β/Smad3 pathway.

Disruption of CUL3-mediated ubiquitination causes proximal tubule injury and kidney fibrosis

Cullin 3 (CUL3) is part of the ubiquitin proteasomal system and controls several cellular processes critical for normal organ function including the cell cycle, and Keap1/Nrf2 signaling. Kidney tubule-specific Cul3 disruption causes tubulointerstitial fibrosis, but little is known about the mechanisms. Therefore, we tested the hypothesis that dysregulation of the cell cycle and Keap1/Nrf2 pathway play a role in initiating the kidney injury upon Cul3 disruption. Cul3 deletion increased expression of cyclin E and p21, associated with uncontrolled proliferation, DNA damage, and apoptosis, all of which preceded proximal tubule injury. The cdk2-cyclin E inhibitor roscovitine did not prevent the effects of Cul3 deletion, but instead exacerbated the kidney injury. Injury occurred despite accumulation and activation of CUL3 substrate Keap1/Nrf2, proposed to be protective in kidney injury. Cul3 disruption led to progressive interstitial inflammation, functionally relevant renal fibrosis and death. Finally, we observed reduced CUL3 expression in several AKI and CKD mouse models and in fibrotic human kidney tissue. These data establish CUL3 knockout mice as a novel genetic CKD model in which dysregulation of the cell cycle may play a primary role in initiating tubule injury, and that CUL3 dysregulation could contribute to acute and fibrotic kidney disease.

Autosomal dominant tubulointerstitial kidney disease-UMOD is the most frequent non polycystic genetic kidney disease

Background

Autosomal dominant tubulointerstitial kidney disease (ADTKD) caused by mutations in the UMOD gene (ADTKD-UMOD) is considered rare and often remains unrecognised. We aimed to establish the prevalence of genetic kidney diseases, ADTKD and ADTKD-UMOD in adult chronic kidney disease (CKD) patients, and to investigate characteristic features.

Methods

We sent questionnaires on family history to all patients with CKD stages 3–5 in our tertiary renal centre to identify patients with inherited renal disease. Details on clinical and family history were obtained from patient interviews and clinical records. Sanger sequencing of the UMOD gene was performed from blood or saliva samples.

Results

2027 of 3770 sent questionnaires were returned. 459 patients reported a family history, which was consistent with inherited kidney disease in 217 patients. 182 non-responders with inherited kidney diseases were identified through a database search. Of these 399 individuals, 252 had autosomal dominant polycystic kidney disease (ADPKD), 28 had ADTKD, 25 had Alports, and 44 were unknown, resulting in 11% of CKD 3–5 patients and 19% of end-stage renal disease patients with genetic kidney diseases. Of the unknown, 40 were genotyped, of whom 31 had findings consistent with ADTKD. 30% of unknowns and 39% of unknowns with ADTKD had UMOD mutations. Altogether, 35 individuals from 18 families were found to have ten distinct UMOD mutations (three novel), making up 1% of patients with CKD 3–5, 2% of patients with end-stage renal disease, 9% of inherited kidney diseases and 56% with ADTKD. ADTKD-UMOD was the most common genetic kidney disease after ADPKD with a population prevalence of 9 per million. Less proteinuria and haematuria, but not hyperuricaemia or gout were predictive of ADTKD-UMOD. The main limitations of the study are the single-centre design and a predominantly Caucasian population.

Conclusions

The prevalence of genetic kidney diseases and ADTKD-UMOD is significantly higher than previously described. Clinical features poorly predicted ADTKD-UMOD, highlighting the need for genetic testing guided by family history alone.

Electronic supplementary material

The online version of this article (10.1186/s12882-018-1107-y) contains supplementary material, which is available to authorized users.

Mechanism of Fibrosis in HNF1B-Related Autosomal Dominant Tubulointerstitial Kidney Disease

BACKGROUND

Mutation of HNF1B, the gene encoding transcription factor HNF-1β, is one cause of autosomal dominant tubulointerstitial kidney disease, a syndrome characterized by tubular cysts, renal fibrosis, and progressive decline in renal function. HNF-1β has also been implicated in epithelial-mesenchymal transition (EMT) pathways, and sustained EMT is associated with tissue fibrosis. The mechanism whereby mutated HNF1B leads to tubulointerstitial fibrosis is not known.

METHODS

To explore the mechanism of fibrosis, we created HNF-1β-deficient mIMCD3 renal epithelial cells, used RNA-sequencing analysis to reveal differentially expressed genes in wild-type and HNF-1β-deficient mIMCD3 cells, and performed cell lineage analysis in HNF-1β mutant mice.

RESULTS

The HNF-1β-deficient cells exhibited properties characteristic of mesenchymal cells such as fibroblasts, including spindle-shaped morphology, loss of contact inhibition, and increased cell migration. These cells also showed upregulation of fibrosis and EMT pathways, including upregulation of Twist2, Snail1, Snail2, and Zeb2, which are key EMT transcription factors. Mechanistically, HNF-1β directly represses Twist2, and ablation of Twist2 partially rescued the fibroblastic phenotype of HNF-1β mutant cells. Kidneys from HNF-1β mutant mice showed increased expression of Twist2 and its downstream target Snai2. Cell lineage analysis indicated that HNF-1β mutant epithelial cells do not transdifferentiate into kidney myofibroblasts. Rather, HNF-1β mutant epithelial cells secrete high levels of TGF-β ligands that activate downstream Smad transcription factors in renal interstitial cells.

CONCLUSIONS

Ablation of HNF-1β in renal epithelial cells leads to the activation of a Twist2-dependent transcriptional network that induces EMT and aberrant TGF-β signaling, resulting in renal fibrosis through a cell-nonautonomous mechanism.

Endoplasmic reticulum stress in the pathogenesis of fibrotic disease

Eukaryotic cells contain an elegant protein quality control system that is crucial in maintaining cellular homeostasis; however, dysfunction of this system results in endoplasmic reticulum (ER) stress and activation of the unfolded protein response (UPR). Severe or prolonged ER stress is associated with the development of degenerative and fibrotic disorders in multiple organs, as evidenced by the identification of disease-causing mutations in epithelial-restricted genes that lead to protein misfolding or mistrafficking in familial fibrotic diseases. Emerging evidence implicates ER stress and UPR signaling in a variety of profibrotic mechanisms in individual cell types. In epithelial cells, ER stress can induce apoptosis, inflammatory signaling, and epithelial-mesenchymal transition. In other cell types, ER stress is linked to myofibroblast activation, macrophage polarization, and T cell differentiation. ER stress-targeted therapies have begun to emerge using approaches that range from global enhancement of chaperone function to selective targeting of activated ER stress sensors and other downstream mediators. As the complex regulatory mechanisms of this system are further clarified, there are opportunities to develop new disease-modifying therapeutic strategies in a wide range of chronic fibrotic diseases.

Vaccarin alleviates hypertension and nephropathy in renovascular hypertensive rats

The kidney is an important organ in the regulation of blood pressure, and it is also one of the primary target organs of hypertension. Kidney damage in response to hypertension eventually leads to renal insufficiency. The authors previously demonstrated that vaccarin exhibits a protective role in endothelial injury. However, the effects of vaccarin on the two-kidney, one clip (2K1C) renovascular hypertension model and subsequent kidney injury have yet to be fully elucidated. The present study was designed to investigate the roles and mechanisms of vaccarin in attenuating hypertension and whether vaccarin had beneficial effects on kidney injury. The 2K1C rats had greater fibrosis, apoptosis, reactive oxygen species production, inflammation, angiotensin II (Ang II) and angiotensin type 1 (AT1) receptors in the right kidney compared with normotensive rats, which were alleviated by a high dose of vaccarin and captopril. Vaccarin treatment attenuated hypertension, reduced fibrosis markers, NADPH oxidase (NOX)-2, NOX-4, 3-nitrotyrosine, tumor necrosis factor-α, interleukin 1β (IL-1β), and IL-6 protein levels and altered pro-apoptotic protein levels including caspase-3, anti-apoptosis protein B cell lymphoma (Bcl)-2 and Bcl-2 associated X, apoptosis regulator in the right kidney of 2K1C rats. These findings suggest that the protective effects of vaccarin on the right kidney in renovascular hypertension are possibly due to downregulation of fibrosis, inflammatory molecules, oxidative stress, Ang II, and AT1 receptor levels.

Uromodulin p.Cys147Trp mutation drives kidney disease by activating ER stress and apoptosis

Uromodulin-associated kidney disease (UAKD) is caused by mutations in the uromodulin (UMOD) gene that result in a misfolded form of UMOD protein, which is normally secreted by nephrons. In UAKD patients, mutant UMOD is poorly secreted and accumulates in the ER of distal kidney epithelium, but its role in disease progression is largely unknown. Here, we modeled UMOD accumulation in mice by expressing the murine equivalent of the human UMOD p.Cys148Trp point mutation (UmodC147W/+ mice). Like affected humans, these UmodC147W/+ mice developed spontaneous and progressive kidney disease with organ failure over 24 weeks. Analysis of diseased kidneys and purified UMOD-producing cells revealed early activation of the PKR-like ER kinase/activating transcription factor 4 (PERK/ATF4) ER stress pathway, innate immune mediators, and increased apoptotic signaling, including caspase-3 activation. Unexpectedly, we also detected autophagy deficiency. Human cells expressing UMOD p.Cys147Trp recapitulated the findings in UmodC147W/+ mice, and autophagy activation with mTOR inhibitors stimulated the intracellular removal of aggregated mutant UMOD. Human cells producing mutant UMOD were susceptible to TNF-α- and TRAIL-mediated apoptosis due to increased expression of the ER stress mediator tribbles-3. Blocking TNF-α in vivo with the soluble recombinant fusion protein TNFR:Fc slowed disease progression in UmodC147W/+ mice by reducing active caspase-3, thereby preventing tubule cell death and loss of epithelial function. These findings reveal a targetable mechanism for disease processes involved in UAKD.

Podocyturia: Potential applications and current limitations

Chronic kidney disease is a prevalent condition that affects millions of people worldwide and is a major risk factor of cardiovascular morbidity and mortality. The main diseases that lead to chronic kidney disease are frequent entities as diabetes mellitus, hypertension and glomerulopathies. One of the clinical markers of kidney disease progression is proteinuria. Moreover, the histological hallmark of kidney disease is sclerosis, located both in the glomerular and in the interstitial compartments. Glomerulosclerosis underscores an irreversible lesion that is clinically accompanied by proteinuria. In this regard, proteinuria and glomerular sclerosis are linked by the cell that has been conserved phylogenetically not only to prevent the loss of proteins in the urine, but also to maintain the health of the glomerular filtration barrier: The podocyte. It can then be concluded that the link between proteinuria, kidney disease progression and chronic kidney disease is mainly related to the podocyte. What is this situation due to? The podocyte is unable to proliferate under normal conditions, and a complex molecular machinery exists to avoid its detachment and eventual loss. When the loss of podocytes in the urine, or podocyturia, is taking place and its glomerular absolute number decreased, glomerulosclerosis is the predominant histological feature in a kidney biopsy. Therefore, tissular podocyte shortage is the cause of proteinuria and chronic kidney disease. In this regard, podocyturia has been demonstrated to precede proteinuria, showing that the clinical management of proteinuria cannot be considered an early intervention. The identification of urinary podocytes could be an additional tool to be considered by nephrologists to assess the activity of glomerulopathies, for follow-up purposes and also to unravel the pathophysiology of podocyte detachment in order to tailor the therapy of glomerular diseases more appropriately.

Uromodulin: from physiology to rare and complex kidney disorders

Uromodulin (also known as Tamm-Horsfall protein) is exclusively produced in the kidney and is the most abundant protein in normal urine. The function of uromodulin remains elusive, but the available data suggest that this protein might regulate salt transport, protect against urinary tract infection and kidney stones, and have roles in kidney injury and innate immunity. Interest in uromodulin was boosted by genetic studies that reported involvement of the UMOD gene, which encodes uromodulin, in a spectrum of rare and common kidney diseases. Rare mutations in UMOD cause autosomal dominant tubulointerstitial kidney disease (ADTKD), which leads to chronic kidney disease (CKD). Moreover, genome-wide association studies have identified common variants in UMOD that are strongly associated with risk of CKD and also with hypertension and kidney stones in the general population. These findings have opened up a new field of kidney research. In this Review we summarize biochemical, physiological, genetic and pathological insights into the roles of uromodulin; the mechanisms by which UMOD mutations cause ADTKD, and the association of common UMOD variants with complex disorders.

Early involvement of cellular stress and inflammatory signals in the pathogenesis of tubulointerstitial kidney disease due to UMOD mutations

Autosomal dominant tubulointerstitial kidney disease (ADTKD) is an inherited disorder that causes progressive kidney damage and renal failure. Mutations in the UMOD gene, encoding uromodulin, lead to ADTKD-UMOD related. Uromodulin is a GPI-anchored protein exclusively produced by epithelial cells of the thick ascending limb of Henle's loop. It is released in the tubular lumen after proteolytic cleavage and represents the most abundant protein in human urine in physiological condition. We previously generated and characterized a transgenic mouse model expressing mutant uromodulin (Tg ^UmodC147W) that recapitulates the main features of ATDKD-UMOD. While several studies clearly demonstrated that mutated uromodulin accumulates in endoplasmic reticulum, the mechanisms that lead to renal damage are not fully understood. In our work, we used kidney transcriptional profiling to identify early events of pathogenesis in the kidneys of Tg ^UmodC147W mice. Our results demonstrate up-regulation of inflammation and fibrosis and down-regulation of lipid metabolism in young Tg ^UmodC147W mice, before any functional or histological evidence of kidney damage. We also show that pro-inflammatory signals precede fibrosis onset and are already present in the first week after birth. Early induction of inflammation is likely relevant for ADTKD-UMOD pathogenesis and related pathways can be envisaged as possible novel targets for therapeutic intervention.