TAZ/Wnt-ß-catenin/c-MYC axis regulates cystogenesis in polycystic kidney disease.

Autosomal-dominant polycystic kidney disease (ADPKD) is the most common genetic renal disease, primarily caused by germline mutation of PKD1 or PKD2, leading to end-stage renal disease. The Hippo signaling pathway regulates organ growth and cell proliferation. Herein, we demonstrate the regulatory mechanism of cystogenesis in ADPKD by transcriptional coactivator with PDZ-binding motif (TAZ), a Hippo signaling effector. TAZ was highly expressed around the renal cyst-lining epithelial cells of Pkd1-deficient mice. Loss of Taz in Pkd1-deficient mice reduced cyst formation. In wild type, TAZ interacted with PKD1, which inactivated ß-catenin. In contrast, in PKD1-deficient cells, TAZ interacted with AXIN1, thus increasing ß-catenin activity. Interaction of TAZ with AXIN1 in PKD1-deficient cells resulted in nuclear accumulation of TAZ together with ß-catenin, which up-regulated c-MYC expression. Our findings suggest that the PKD1-TAZ-Wnt-ß-catenin-c-MYC signaling axis plays a critical role in cystogenesis and might be a potential therapeutic target against ADPKD.

Autophagy in FOXD1 stroma-derived cells regulates renal fibrosis through TGF-ß and NLRP3 inflammasome pathway.

Renal fibrosis is the final common pathway of various renal injuries and it leads to chronic kidney disease. Recent studies reported that FOXD1-lineage pericyte plays a critical role in tubulointerstitial fibrosis (TIF). However the regulatory mechanisms remain unclear. Autophagy is a cellular process of degradation of damaged cytoplasmic components that regulates cell death and proliferation. To investigate the role of autophagy in FOXD1-lineage pericytes on renal TIF, we generated the FOXD1-lineage stromal cell-specific Atg7 deletion (Atg7△FOXD1) mice. FOXD1-lineage stromal cell-specific Atg7 deletion enhanced renal TIF through Smad-dependent transforming growth factor (TGF)-ß signaling after unilateral ureteral obstruction (UUO). FOXD1-lineage stromal cell-specific Atg7 deletion increased the accumulation of interstitial myofibroblasts and enhanced the differentiation of pericytes into myofibroblasts after UUO. Peritubular capillary rarefaction was accelerated in Atg7△FOXD1 mice after UUO. Atg7△FOXD1 mice increased the accumulation of SQSTM1/p62-positive aggregates in the obstructed kidney and resulted in increased expression of NLRP3 inflammasome, interleukin (IL) 1-ß and caspase-1 signaling pathway, which enhanced apoptosis of interstitial cells after UUO. In summary, our data showed that autophagy in FOXD1-lineage stromal cells plays a protective role in renal TIF through regulating the Smad4 dependent TGF-ß an NLRP3 inflammasome signaling pathway.

Aquaporin 2-labeled cells differentiate to intercalated cells in response to potassium depletion.

The mammalian renal collecting duct consists of principal cells (PCs) and intercalated cells (ICs). Both PCs and ICs are involved in potassium (K(+)) homeostasis, PCs through their role in K(+) secretion and ICs through their ability to facilitate K(+) resorption. We previously hypothesized that PCs may differentiate into ICs upon K(+) depletion. However, no direct evidence has yet been obtained to conclusively demonstrate that PCs differentiate into ICs in response to K(+) depletion. Here, we present direct evidence for the differentiation of PCs into ICs by cell lineage tracing using aquaporin 2 (AQP2)-Cre mice and R26R-EYFP transgenic mice. In control mice, AQP2-EYFP(+) cells exhibited mainly a PC phenotype (AQP2-positive/H(+)-ATPase-negative). Interestingly, some AQP2-EYFP(+) cells exhibited an IC phenotype (H(+)-ATPase-positive/AQP2-negative); these cells accounted for 1.7 %. After K(+) depletion, the proportion of AQP2-EYFP(+) cells with an IC phenotype was increased to 4.1 %. Furthermore, some AQP2-EYFP(+) cells exhibited a "null cell" phenotype (AQP2-negative/H(+)-ATPase-negative) after K(+) depletion. Collectively, our data demonstrate that AQP2-labeled cells can differentiate into ICs, as well as null cells, in response to K(+) depletion. This finding indicates that some of AQP2-labeled cells possess properties of progenitor cells and that they can differentiate into ICs in the adult mouse kidney.

Descending thin limb of the intermediate loop expresses both aquaporin 1 and urea transporter A2 in the mouse kidney.

A new intermediate type of Henle's loop has been reported that it extends into the inner medulla and turns within the first millimeter beyond the outer medulla. This study aimed to identify the descending thin limb (DTL) of the intermediate loop in the adult C57Bl/6 mouse kidney using aquaporin 1 (AQP1) and urea transporter A2 (UT-A2) antibodies. In the upper part of the inner stripe of the outer medulla (ISOM), AQP1 was expressed strongly in the DTL with type II epithelium of the long loop, but not in type I epithelium of the short loop. The DTL of the intermediate loop exhibited weak AQP1 immunoreactivity. UT-A2 immunoreactivity was not observed in the upper part of any DTL type. AQP1 expression was similar in the upper and middle parts of the ISOM. UT-A2 expression was variable, being expressed strongly in the DTL with type I epithelium of the short loop, but not in type II epithelium of the long loop. In the innermost part of the ISOM, AQP1 was expressed only in type III epithelium of the long loop. UT-A2-positive and UT-A2-negative cells were intermingled in type I epithelium of the intermediate loop, but were not observed in type III epithelium of the long loop. UT-A2-positive DTLs of the intermediate loop extended into the UT-A2/AQP1-negative type I epithelium in the initial part of the inner medulla. These results demonstrate that the DTL of the intermediate loop is composed of type I epithelium and expresses both AQP1 and UT-A2. The functional role of the DTL of the intermediate loop may be distinct from the short or long loops.

Deep learning predicts the differentiation of kidney organoids derived from human induced pluripotent stem cells.

BACKGROUND: Kidney organoids derived from human pluripotent stem cells (hPSCs) contain multilineage nephrogenic progenitor cells and can recapitulate the development of the kidney. Kidney organoids derived from hPSCs have the potential to be applied in regenerative medicine as well as renal disease modeling, drug screening, and nephrotoxicity testing. Despite biotechnological advances, individual differences in morphological and growth characteristics among kidney organoids need to be addressed before clinical and commercial application. In this study, we hypothesized that an automated noninvasive method based on deep learning of bright-field images of kidney organoids can predict their differentiation status. METHODS: Bright-field images of kidney organoids were collected on day 18 after differentiation. To train convolutional neural networks (CNNs), we utilized a transfer learning approach. CNNs were trained to predict the differentiation of kidney organoids on bright-field images based on the messenger RNA expression of renal tubular epithelial cells as well as podocytes. RESULTS: The best prediction model was DenseNet121 with a total Pearson correlation coefficient score of 0.783 on a test dataset. W classified the kidney organoids into two categories: organoids with above-average gene expression (Positive) and those with below-average gene expression (Negative). Comparing the best-performing CNN with human-based classifiers, the CNN algorithm had a receiver operating characteristic-area under the curve (AUC) score of 0.85, while the experts had an AUC score of 0.48. CONCLUSION: These results confirmed our original hypothesis and demonstrated that our artificial intelligence algorithm can successfully recognize the differentiation status of kidney organoids.

The role of autophagy in unilateral ureteral obstruction rat model.

AIM: Autophagy is a cellular process of degradation of damaged cytoplasmic components and regulates cell death or proliferation. Unilateral ureteral obstruction (UUO) is a model of progressive renal fibrosis in the obstructed kidney. And UUO is followed by compensatory cellular proliferation in the contralateral kidney. We investigate the role of autophagy in the obstructed kidney and contralateral kidney after UUO. METHODS: To obtain the evidence and the patterns of autophagy during UUO, the rats were sacrificed 3, 7 and 14 days after UUO. To examine the efficacy of the autophagy inhibitors, 3-methyladenine (3-MA), the rats were treated daily with intraperitoneal injection of 3-MA (30 mg/kg per day) for 7 days. RESULTS: After UUO, autophagy was induced in the obstructed kidney in a time-dependent manner. Inhibition of autophagy by 3-MA enhanced tubular cell apoptosis and tubulointerstitial fibrosis in the obstructed kidney after UUO. In the contralateral kidney, autophagy was also induced and prolonged during UUO. Inhibition of autophagy by 3-MA increased the protein expression of proliferating cell nuclear antigen significantly in the contralateral kidney after UUO. The Akt-mammalian target of rapamycin (mTOR) signalling pathway was involved in the induction of autophagy after UUO in both kidneys. CONCLUSION: Our present results support that autophagy induced by UUO has a renoprotective role in the obstructed kidney and regulatory role of compensatory cellular proliferation in the contralateral kidney through Akt-mTOR signalling pathway.

In Situ Detection of Kidney Organoid Generation From Stem Cells Using a Simple Electrochemical Method.

Organoids that mimic the structural and cellular characteristics of kidneys in vitro have recently emerged as a promising source for biomedical research. However, uncontrollable cellular heterogeneity after differentiation often results in the generation of off-target cells, one of the most challenging issues in organoid research. This study proposes a new method that enables the real-time assessment of kidney organoids derived from stem cells. When placed on a conductive surface, these organoids generate unique electrochemical signals at ≈0.3 V with intensities proportional to the amount of kidney-specific cell types. Off-target cells (i.e., non-kidney cells) produce an electrical signature at 0 V that is distinguishable from other surrounding cell types, enabling non-destructive assessment of both the differentiation, and maturation levels of kidney organoids. The developed platform can be applied to other types of organoids and is thus highly promising as a tool for organoid-based drug screening, toxicity assessment, and therapeutics.

Kidney Decellularized Extracellular Matrix Enhanced the Vascularization and Maturation of Human Kidney Organoids.

Kidney organoids derived from human pluripotent stem cells (hPSCs) have extensive potential for disease modelling and regenerative medicine. However, the limited vascularization and immaturity of kidney organoids have been still remained to overcome. Extracellular matrix (ECM) can provide mechanical support and a biochemical microenvironment for cell growth and differentiation. Here in vitro methods using a kidney decellularized extracellular matrix (dECM) hydrogel to culture hPSC-derived kidney organoids, which have extensive vascular network and their own endothelial cells, are reported. Single-cell transcriptomics reveal that the vascularized kidney organoids cultured using the kidney dECM have more mature patterns of glomerular development and higher similarity to human kidney than those cultured without the kidney dECM. Differentiation of α-galactosidase A (GLA)-knock-out hPSCs generated using CRISPR/Cas9 into kidney organoids by the culture method using kidney dECM efficiently recapitulate Fabry nephropathy with vasculopathy. Transplantation of kidney organoids with kidney dECM into kidney of mouse accelerates the recruitment of endothelial cells from the host mouse kidney and maintains vascular integrity with the more organized slit diaphragm-like structures than those without kidney dECM. The kidney dECM methodology for inducing extensive vascularization and maturation of kidney organoids can be applied to studies for kidney development, disease modeling, and regenerative medicine.

Human kidney organoids reveal the role of glutathione in Fabry disease.

Fabry disease is an X-linked lysosomal storage disease caused by a mutation in the galactosidase alpha (GLA) gene. Despite advances in therapeutic technologies, the lack of humanized experimental models of Fabry disease has limited the development of new therapies to cure the disease. Herein, we modeled Fabry disease using human inducible pluripotent stem cell (iPSC)-derived kidney organoids and the CRISPR-Cas9 genome-editing system. GLA-mutant human kidney organoids revealed deformed podocytes and tubular cells with accumulation of globotriaosylceramide (Gb3). Ultrastructural analysis showed abundant electron-dense granular deposits and electron-dense lamellate lipid-like deposits that formed concentric bodies (zebra bodies) in the cytoplasm of podocytes and tubules. The oxidative stress level was increased in GLA-mutant kidney organoids, and the increase was accompanied by apoptosis. Enzyme replacement treatment (ERT) with recombinant human α-Gal A decreased the Gb3 accumulation and oxidative stress, which resulted in amelioration of the deformed cellular structure of the GLA-mutant kidney organoids. Transcription profile analyses showed decreased glutathione (GSH) metabolism in GLA-mutant kidney organoids. GSH replacement treatment decreased oxidative stress and attenuated the structural deformity of the GLA-mutant kidney organoids. GSH treatment also increased the expression of podocyte and tubular markers and decreased apoptosis. In conclusion, GLA-mutant kidney organoids derived from human iPSCs are valuable tools for studying the mechanisms and developing novel therapeutic alternatives for Fabry disease.

Human kidney organoids model the tacrolimus nephrotoxicity and elucidate the role of autophagy.

BACKGROUND/AIMS: Tacrolimus has been used as an immunosuppressive agent in organ transplantation. Despite the therapeutic benefits, tacrolimus's use is limited due to its nephrotoxicity. To reduce tacrolimus nephrotoxicity, effective humanized experimental models may be helpful. Here, we modeled tacrolimus nephrotoxicity using kidney organoids derived from human inducible pluripotent stem cells (iPSCs) in vitro. METHODS: Kidney organoids were differentiated from the CMC11 iPSC cell line, re-seeded in 96-well plates, and treated with tacrolimus at doses of 0, 30, or 60 µM for 24 hours. This in vitro model was compared to a mouse model of tacrolimus nephrotoxicity and the associated mechanisms were investigated. RESULTS: The size of the kidney organoids and cell viability decreased in dose-dependent manners after treatment with tacrolimus. The number of tubular cells decreased with a loss of polarity, similar to the effects seen in mouse tacrolimus nephrotoxicity. Ultrastructural analysis showed numerous vacuoles in the proximal tubular cells of the kidney organoids treated with tacrolimus. Tacrolimus treatment induced oxidative stress and mitochondrial dysfunction, and autophagic activity was enhanced in the kidney organoids. Rapamycin, an autophagy inducer, accelerated cell death in the kidney organoid model of tacrolimus nephrotoxicity, which was attenuated by treatment with 3-methyladenine, an autophagy inhibitor. These findings indicate that the augmentation of autophagy by rapamycin treatment accelerated tacrolimus nephrotoxicity. CONCLUSION: Our data suggest that human kidney organoids are an effective in vitro model of tacrolimus nephrotoxicity and that autophagy plays a critical role in tacrolimus nephrotoxicity.

Three-dimensional cell-printing of advanced renal tubular tissue analogue.

Despite significant progress in the development of renal tissue, recapitulation of perfusable complex renal tubular tissue with clinically relevant cellular heterogeneity is still remaining a challenge. In this study, using coaxial 3D cell-printing technique, we present microfluidic hollow tubes to realize tubular/vascular renal parenchyma composed of renal tubular epithelial and endothelial cells, respectively. We developed a functional hybrid bioink that inherits microenvironments for vascularized native kidney tissue with rapidly crosslinkable character to optimize cell functionality and retain the predefined hollow tubular structure. In addition, the novel bioink and 3D coaxial cell-printing technique provided a complex tube with tunable feature of monolayer and bilayer structure across the length of printed tube. Through prototyping a vascularized renal proximal tubule-on-a-chip, we showed its applicability to novel microfluidic renal tissue models. The renal subcapsular transplantation of the hollow tubes showed a long-term graft survival with the therapeutic capability of the tubular constructs in in vivo model of renal disease, which serves their applicability in regenerative medicine.

Correction to: Graft immaturity and safety concerns in transplanted human kidney organoids.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Autophagy attenuates tubulointerstital fibrosis through regulating transforming growth factor-ß and NLRP3 inflammasome signaling pathway.

Renal fibrosis is the final common pathway of various renal injuries and it leads to chronic kidney disease. Autophagy is a cellular process of degradation of damaged cytoplasmic components and regulates cell death and proliferation. Cellular response during unilateral ureteral obstruction (UUO) is tubular segment specific. Thus the role of autophagy on renal tubulointerstitial fibrosis (TIF) after UUO may be different according to segment of nephron. The role of autophagy during UUO remains unclear especially in distal tubules. In this study, we investigated the role of autophagy in distal tubules on renal TIF using conditional knockout mice in which Atg7 was genetically ablated specifically in distal tubular epithelial cell (TEC). In green fluorescent protein (GFP)-LC3 transgenic mice, GFP-LC3 puncta was highly expressed in distal tubular cells of the obstructed kidneys after UUO. Genetic deletion of Atg7 specifically in distal TEC increased renal tubulointerstial fibrosis and epithelial-mesenchymal transition-like phenotype change after UUO through Smad4-dependent transforming growth factor (TGF)-ß pathway. Distal tubule-specific autophagy-deficient mice increased the accumulation of damaged mitochondria and SQSTM1/p62-positive aggregates in the obstructed kidney and resulted in increased expression of NLRP3 inflammasome, interleukin (IL) 1-ß and caspase-1. Distal TEC-specific Atg7 deletion enhanced apoptosis of TECs after UUO. In summary, our data showed that autophagy in distal TEC plays a protective role in development of renal tubulointerstial fibrosis through regulating the expression of TGF-ß an IL1-ß after UUO.

Atg7-dependent canonical autophagy regulates the degradation of aquaporin 2 in prolonged hypokalemia.

Prolonged hypokalemia induces a decrease of urinary concentrating ability via down-regulation of aquaporin 2 (AQP2); however, the precise mechanisms remain unknown. To investigate the role of autophagy in the degradation of AQP2, we generated the principal cell-specific Atg7 deletion (Atg7Δpc) mice. In hypokalemic Atg7-floxed (Atg7f/f) mice, huge irregular shaped LC3-positive autophagic vacuoles accumulated mainly in inner medullary collecting duct (IMCD) cells. Total- and pS261-AQP2 were redistributed from apical and subapical domains into these vacuoles, which were not co-localized with RAB9. However, in the IMCD cells of hypokalemic Atg7Δpc mice, these canonical autophagic vacuoles were markedly reduced, whereas numerous small regular shaped LC3-negative/RAB9-positive non-canonical autophagic vacuoles were observed along with diffusely distributed total- and pS261-AQP2 in the cytoplasm. The immunoreactivity of pS256-AQP2 in the apical membrane of IMCD cells was markedly decreased, and no redistribution was observed in both hypokalemic Atg7f/f and Atg7Δpc mice. These findings suggest that AQP2 down regulation in hypokalemia was induced by reduced phosphorylation of AQP2, resulting in a reduction of apical plasma labeling of pS256-AQP2 and degradation of total- and pS261-AQP2 via an LC3/ATG7-dependent canonical autophagy pathway.

Graft immaturity and safety concerns in transplanted human kidney organoids.

For chronic kidney disease, regeneration of lost nephrons with human kidney organoids derived from induced pluripotent stem (iPS) cells is proposed to be an attractive potential therapeutic option. It remains unclear, however, whether organoids transplanted into kidneys in vivo would be safe or functional. Here, we purified kidney organoids and transplanted them beneath the kidney capsules of immunodeficient mice to test their safety and maturity. Kidney organoid grafts survived for months after transplantation and became vascularized from host mouse endothelial cells. Nephron-like structures in grafts appeared more mature than kidney organoids in vitro, but remained immature compared with the neighboring mouse kidney tissue. Ultrastructural analysis revealed filtration barrier-like structures, capillary lumens, and tubules with brush border in the transplanted kidney organoids, which were more mature than those of the kidney organoids in vitro but not as organized as adult mammalian kidneys. Immaturity was a common feature of three separate differentiation protocols by immunofluorescence analysis and single cell RNA sequencing. Stroma of transplanted kidney organoid grafts were filled with vimentin-positive mesenchymal cells, and chondrogenesis, cystogenesis, and stromal expansion were observed in the long term. Transcription profiles showed that long-term maintenance after kidney organoid transplantation induced transcriptomic reprogramming with prominent suppression of cell-cycle-related genes and upregulation of extracellular matrix organization. Our data suggest that kidney organoids derived from iPS cells may be transplantable but strategies to improve nephron differentiation and purity are required before they can be applied in humans as a therapeutic option.

Applications of kidney organoids derived from human pluripotent stem cells.

The establishment of protocols to differentiate kidney organoids from human pluripotent stem cells provides potential applications of kidney organoids in regenerative medicine. Modeling of renal diseases, drug screening, nephrotoxicity testing of compounds, and regenerative therapy are attractive applications. Although much progress still remains to be made in the development of kidney organoids, recent advances in clustered regularly interspaced short palindromic repeat (CRISPR)-CRISPR-associated system 9 (Cas9) genome editing and three-dimensional bioprinting technologies have contributed to the application of kidney organoids in clinical fields. In this section, we review recent advances in the applications of kidney organoids to kidney disease modelling, drug screening, nephrotoxicity testing, and regenerative therapy.

Notch signaling in the collecting duct regulates renal tubulointerstitial fibrosis induced by unilateral ureteral obstruction in mice.

Background/Aims: Mind bomb-1 (Mib1) encodes an E3 ubiquitin ligase, which is required for the initiation of Notch signaling. Recently, it was demonstrated that the renal collecting duct plays an important role in renal fibrosis. Here, we investigated the role of Notch signaling in renal fibrosis using conditional knockout mice with the specific ablation of Mib1 in renal collecting duct principal cells. METHODS: Mib1-floxed mice (Mib1f/f) were crossed with aquaporin 2 (AQP2)-Cre mice in order to generate principal cell-specific Mib1 knockout mice (Mib1f/f :AQP2-Cre+). Unilateral ureteral obstruction (UUO) was performed, and mice were sacrificed 7 days after UUO. RESULTS: After performing the UUO, renal tubulointerstitial fibrosis and the expression of transforming growth factor ß were markedly enhanced in the obstructed kidneys of Mib1f/f mice compared with the sham-operated kidney of Mib1f/f mice. These changes were shown to be even more pronounced in the obstructed kidneys of Mib1f/f :AQP2-Cre+ mice than in those of the Mib1f/f mice . Furthermore, the number of TUNNEL-positive cells in renal collecting duct was higher in the obstructed kidneys of Mib1f/f :AQP2-Cre+ mice than in the kidneys of Mib1f/f mice. Conclusions: Notch signaling in the renal collecting duct plays an important role in the regulation of renal tubulointerstitial fibrosis and apoptosis after UUO.

The Hippo-Salvador signaling pathway regulates renal tubulointerstitial fibrosis.

Renal tubulointerstitial fibrosis (TIF) is the final pathway of various renal injuries that result in chronic kidney disease. The mammalian Hippo-Salvador signaling pathway has been implicated in the regulation of cell proliferation, cell death, tissue regeneration, and tumorigenesis. Here, we report that the Hippo-Salvador pathway plays a role in disease development in patients with TIF and in a mouse model of TIF. Mice with tubular epithelial cell (TEC)-specific deletions of Sav1 (Salvador homolog 1) exhibited aggravated renal TIF, enhanced epithelial-mesenchymal transition-like phenotypic changes, apoptosis, and proliferation after unilateral ureteral obstruction (UUO). Moreover, Sav1 depletion in TECs increased transforming growth factor (TGF)-ß and activated ß-catenin expression after UUO, which likely accounts for the abovementioned enhanced TEC fibrotic phenotype. In addition, TAZ (transcriptional coactivator with PDZ-binding motif), a major downstream effector of the Hippo pathway, was significantly activated in Sav1-knockout mice in vivo. An in vitro study showed that TAZ directly regulates TGF-ß and TGF-ß receptor II expression. Collectively, our data indicate that the Hippo-Salvador pathway plays a role in the pathogenesis of TIF and that regulating this pathway may be a therapeutic strategy for reducing TIF.

Role of Prox1 in the Transforming Ascending Thin Limb of Henle's Loop during Mouse Kidney Development.

The homeobox transcription factor Prox1 is critical to the development of many embryonic organs and tissues, although current understanding of its expression in the developing renal medulla is limited. We examined the functional role of Prox1 during mouse kidney development with particular emphasis on the developing loop of Henle. Our data show that Prox1 is expressed in the transdifferentiating region from the NKCC2-positive thick ascending limb, into the CLC-K1-positive ascending thin limb of Henle's loop beginning at embryonic day 18. From 1 to 14 days of age, Prox1-positive cells gradually disappeared from the papillary tip, and remained in the initial part of inner medulla after 21 days. In this transforming area, no Prox1 was observed in cells undergoing apoptosis but was expressed strongly in the remaining cells, which differentiated into ascending thin limb epithelial cells. In vitro and in vivo approaches showed that Prox1 expression increases where the osmolality is near optimal range, but decreases at below- or above-optimal ranges. Renal hypoosmolality induced by furosemide (NKCC2 inhibitor) inhibited Prox1 expression and delayed maturation of the ascending limb of Henle's loop. Together, these studies suggest that Prox1 appears to be a critical stage specific regulator of specifying ascending thin limb cell fate and that its expression is regulated by osmolality.

Altered response of pendrin-positive intercalated cells in the kidney of Hoxb7-Cre;Mib1f/f mice.

The anion exchanger pendrin is exclusively expressed by non-type A intercalated cells (ICs), type B ICs and non A-non B ICs. Pendrin-positive ICs are mainly localized in the cortical collecting duct (CCD) and connecting tubule (CNT) rather than the outer medullary collecting duct (OMCD). Our previous study reported that Notch signaling is required for the specification of ureteric bud cells to the principal cells (PCs) and ICs in the medullary collecting duct. The purpose of this study was to determine whether the deletion of Mind bomb-1 (Mib1), an E3 ubiquitin ligase required for the initiation of Notch signaling, would affect the differentiation of pendrin-positive type B and non A-non B ICs in Hoxb7-Cre;Mib1f/f mice. In Hoxb7-Cre;Mib1f/f mice, there was a significant increase in the fraction of pendrin-negative/AE1-positive type A ICs not only in the OMCD (67.02±2.04% vs. 33.78±0.71%; Hoxb7-Cre;Mib1f/f vs. Mib1f/f) but also in the CCD (23.70±2.68% vs. 19.71±0.43%; Hoxb7-Cre;Mib1f/f vs. Mib1f/f) and CNT (23.70±2.68% vs. 19.71±0.43%; Hoxb7-Cre;Mib1f/f vs. Mib1f/f) as compared with Mib1f/f. In contrast, there were no significant differences in the fraction of pendrin-positive type B ICs (7.11±3.84% vs. 7.61±4.45%; Hoxb7-Cre;Mib1f/f vs. Mib1f/f) between the two groups in the cortex including CCD and CNT. Furthermore, there was a significant decrease in the fraction of non A-non B ICs (8.95±2.28% vs. 13.06±4.81%; Hoxb7-Cre;Mib1f/f vs. Mib1f/f) in these tubules in the Hoxb7-Cre;Mib1f/f mice. These results suggest that the degree of differentiation of subtypes of ICs may vary depending on the Notch signaling pathway.