A minimal Ksp-cadherin promoter linked to a green fluorescent protein reporter gene exhibits tissue-specific expression in the developing kidney and genitourinary tract

YME1L-mediated mitophagy protects renal tubular cells against cellular senescence under diabetic conditions

BACKGROUND

The senescence of renal tubular epithelial cells (RTECs) is crucial in the progression of diabetic kidney disease (DKD). Accumulating evidence suggests a close association between insufficient mitophagy and RTEC senescence. Yeast mitochondrial escape 1-like 1 (YME1L), an inner mitochondrial membrane metalloprotease, maintains mitochondrial integrity. Its functions in DKD remain unclear. Here, we investigated whether YME1L can prevent the progression of DKD by regulating mitophagy and cellular senescence.

METHODS

We analyzed YME1L expression in renal tubules of DKD patients and mice, explored transcriptomic changes associated with YME1L overexpression in RTECs, and assessed its impact on RTEC senescence and renal dysfunction using an HFD/STZ-induced DKD mouse model. Tubule-specific overexpression of YME1L was achieved through the use of recombinant adeno-associated virus 2/9 (rAAV 2/9). We conducted both in vivo and in vitro experiments to evaluate the effects of YME1L overexpression on mitophagy and mitochondrial function. Furthermore, we performed LC-MS/MS analysis to identify potential protein interactions involving YME1L and elucidate the underlying mechanisms.

RESULTS

Our findings revealed a significant decrease in YME1L expression in the renal tubules of DKD patients and mice. However, tubule-specific overexpression of YME1L significantly alleviated RTEC senescence and renal dysfunction in the HFD/STZ-induced DKD mouse model. Moreover, YME1L overexpression exhibited positive effects on enhancing mitophagy and improving mitochondrial function both in vivo and in vitro. Mechanistically, our LC-MS/MS analysis uncovered a crucial mitophagy receptor, BCL2-like 13 (BCL2L13), as an interacting partner of YME1L. Furthermore, YME1L was found to promote the phosphorylation of BCL2L13, highlighting its role in regulating mitophagy.

CONCLUSIONS

This study provides compelling evidence that YME1L plays a critical role in protecting RTECs from cellular senescence and impeding the progression of DKD. Overexpression of YME1L demonstrated significant therapeutic potential by ameliorating both RTEC senescence and renal dysfunction in the DKD mice. Moreover, our findings indicate that YME1L enhances mitophagy and improves mitochondrial function, potentially through its interaction with BCL2L13 and subsequent phosphorylation. These novel insights into the protective mechanisms of YME1L offer a promising strategy for developing therapies targeting DKD.

Collecting duct NCOR1 controls blood pressure by regulating mineralocorticoid receptor

INTRODUCTION

Nuclear receptor corepressor 1(NCOR1) is reported to play crucial roles in cardiovascular diseases, but its function in the kidney has remained obscure.

OBJECTIVE

We aim to elucidate the role of collecting duct NCOR1 in blood pressure (BP) regulation.

METHODS AND RESULTS

Collecting duct NCOR1 knockout (KO) mice manifested increased BP and aggravated vascular and renal injury in an angiotensin II (Ang II)-induced hypertensive model. KO mice also showed significantly higher BP than littermate control (LC) mice in deoxycorticosterone acetate (DOCA)-salt model. Further study showed that collecting duct NCOR1 deficiency aggravated volume and sodium retention after saline challenge. Among the sodium transporter in the collecting duct, the expression of the three epithelial sodium channel (ENaC) subunits was markedly increased in the renal medulla of KO mice. Consistently, BP in Ang II-infused KO mice decreased significantly to the similar level as those in LC mice after amiloride treatment. ChIP analysis revealed that NCOR1 deficiency increased the enrichment of mineralocorticoid receptor (MR) on the promoters of the three ENaC genes in primary inner medulla collecting duct (IMCD) cells. Co-IP results showed interaction between NCOR1 and MR, and luciferase reporter results demonstrated that NCOR1 inhibited the transcriptional activity of MR. Knockdown of MR eliminated the increased ENaC expression in primary IMCD cells isolated from KO mice. Finally, BP was significantly decreased in Ang II-infused KO mice after treatment of MR antagonist spironolactone and the difference between LC and KO mice was abolished.

CONCLUSIONS

NCOR1 interacts with MR to control ENaC activity in the collecting duct and to regulate sodium reabsorption and ultimately BP. Targeting NCOR1 might be a promising tactic to interrupt the volume and sodium retention of the collecting duct in hypertension.

TFEB and TFE3 drive kidney cystogenesis and tumorigenesis

Birt-Hogg-Dubé (BHD) syndrome is an inherited familial cancer syndrome characterized by the development of cutaneous lesions, pulmonary cysts, renal tumors and cysts and caused by loss-of-function pathogenic variants in the gene encoding the tumor-suppressor protein folliculin (FLCN). FLCN acts as a negative regulator of TFEB and TFE3 transcription factors, master controllers of lysosomal biogenesis and autophagy, by enabling their phosphorylation by the mechanistic Target Of Rapamycin Complex 1 (mTORC1). We have previously shown that deletion of Tfeb rescued the renal cystic phenotype of kidney-specific Flcn KO mice. Using Flcn/Tfeb/Tfe3 double and triple KO mice, we now show that both Tfeb and Tfe3 contribute, in a differential and cooperative manner, to kidney cystogenesis. Remarkably, the analysis of BHD patient-derived tumor samples revealed increased activation of TFEB/TFE3-mediated transcriptional program and silencing either of the two genes rescued tumorigenesis in human BHD renal tumor cell line-derived xenografts (CDXs). Our findings demonstrate in disease-relevant models that both TFEB and TFE3 are key drivers of renal tumorigenesis and suggest novel therapeutic strategies based on the inhibition of these transcription factors.

Rac1 deficiency impairs postnatal development of the renal papilla

Development of the renal medulla continues after birth to form mature renal papilla and obtain urine-concentrating ability. Here, we found that a small GTPase, Rac1, plays a critical role in the postnatal development of renal papilla. Mice with distal tubule-specific deletion of Rac1 reached adulthood but showed polydipsia and polyuria with an impaired ability to concentrate urine. The elongation of renal papilla that occurs in the first weeks after birth was impaired in the Rac1-deficient infants, resulting in shortening and damage of the renal papilla. Moreover, the osmoprotective signaling mediated by nuclear factor of activated T cells 5, which is a key molecule of osmotic response to osmotic stress in renal medulla, was significantly impaired in the kidneys of the Rac1-deficient infants. These results demonstrate that Rac1 plays an important role in the development of renal papilla in the postnatal period, and suggested a potential link between Rac1 and osmotic response.

Preparation and Use of Decellularized Extracellular Matrix for Tissue Engineering

The multidisciplinary fields of tissue engineering and regenerative medicine have the potential to revolutionize the practise of medicine through the abilities to repair, regenerate, or replace tissues and organs with functional engineered constructs. To this end, tissue engineering combines scaffolding materials with cells and biologically active molecules into constructs with the appropriate structures and properties for tissue/organ regeneration, where scaffolding materials and biomolecules are the keys to mimic the native extracellular matrix (ECM). For this, one emerging way is to decellularize the native ECM into the materials suitable for, directly or in combination with other materials, creating functional constructs. Over the past decade, decellularized ECM (or dECM) has greatly facilitated the advance of tissue engineering and regenerative medicine, while being challenged in many ways. This article reviews the recent development of dECM for tissue engineering and regenerative medicine, with a focus on the preparation of dECM along with its influence on cell culture, the modification of dECM for use as a scaffolding material, and the novel techniques and emerging trends in processing dECM into functional constructs. We highlight the success of dECM and constructs in the in vitro, in vivo, and clinical applications and further identify the key issues and challenges involved, along with a discussion of future research directions.

Tubular-specific CDK12 knockout causes a defect in urine concentration due to premature cleavage of the slc12a1 gene

Cyclin-dependent kinase 12 (CDK12) plays a critical role in regulating gene transcription. CDK12 inhibition is a potential anticancer therapeutic strategy. However, several clinical trials have shown that CDK inhibitors might cause renal dysfunction and electrolyte disorders. CDK12 is abundant in renal tubular epithelial cells (RTECs), but the exact role of CDK12 in renal physiology remains unclear. Genetic knockout of CDK12 in mouse RTECs causes polydipsia, polyuria, and hydronephrosis. This phenotype is caused by defects in water reabsorption that are the result of reduced Na-K-2Cl cotransporter 2 (NKCC2) levels in the kidney. In addition, CKD12 knockout causes an increase in Slc12a1 (which encodes NKCC2) intronic polyadenylation events, which results in Slc12a1 truncated transcript production and NKCC2 downregulation. These findings provide novel insight into CDK12 being necessary for maintaining renal homeostasis by regulating NKCC2 transcription, which explains the critical water and electrolyte disturbance that occurs during the application of CDK12 inhibitors for cancer treatment. Therefore, there are safety concerns about the clinical use of these new anticancer drugs.

Channel Function of Polycystin-2 in the Endoplasmic Reticulum Protects against Autosomal Dominant Polycystic Kidney Disease

BACKGROUND

Mutations of PKD2, which encodes polycystin-2, cause autosomal dominant polycystic kidney disease (ADPKD). The prevailing view is that defects in polycystin-2-mediated calcium ion influx in the primary cilia play a central role in the pathogenesis of cyst growth. However, polycystin-2 is predominantly expressed in the endoplasmic reticulum (ER) and more permeable to potassium ions than to calcium ions.

METHODS

The trimeric intracellular cation (TRIC) channel TRIC-B is an ER-resident potassium channel that mediates potassium-calcium counterion exchange for inositol trisphosphate-mediated calcium ion release. Using TRIC-B as a tool, we examined the function of ER-localized polycystin-2 and its role in ADPKD pathogenesis in cultured cells, zebrafish, and mouse models.

RESULTS

Agonist-induced ER calcium ion release was defective in cells lacking polycystin-2 and reversed by exogenous expression of TRIC-B. Vice versa, exogenous polycystin-2 reversed an ER calcium-release defect in cells lacking TRIC-B. In a zebrafish model, expression of wild-type but not nonfunctional TRIC-B suppressed polycystin-2-deficient phenotypes. Similarly, these phenotypes were suppressed by targeting the ROMK potassium channel (normally expressed on the cell surface) to the ER. In cultured cells and polycystin-2-deficient zebrafish phenotypes, polycystin-2 remained capable of reversing the ER calcium release defect even when it was not present in the cilia. Transgenic expression of Tric-b ameliorated cystogenesis in the kidneys of conditional Pkd2-inactivated mice, whereas Tric-b deletion enhanced cystogenesis in Pkd2-heterozygous kidneys.

CONCLUSIONS

Polycystin-2 in the ER appears to be critical for anticystogenesis and likely functions as a potassium ion channel to facilitate potassium-calcium counterion exchange for inositol trisphosphate-mediated calcium release. The results advance the understanding of ADPKD pathogenesis and provides proof of principle for pharmacotherapy by TRIC-B activators.

Pharmacological and Genetic Inhibition of HDAC4 Alleviates Renal Injury and Fibrosis in Mice

Histone deacetylase 4 (HDAC4) has been shown to be involved in cell proliferation, differentiation, and migration and is associated with a variety of cancers. However, the role of HDAC4 in renal fibrogenesis and its mechanisms are unclear. We assessed the role of HDAC4 and possible mechanisms of fibrosis in a murine model of kidney injury induced by unilateral ureteral obstruction (UUO) using tasquinimod, a highly selective HDAC4 inhibitor, and knockout mice with depletion of HDAC4 in renal tubular cells. UUO injury resulted in increased expression of HDAC4 and fibrotic proteins fibronectin and α-smooth muscle actin, while treatment with tasquinimod or knockout of HDAC4 significantly reduced their expression. Pharmacological and genetic inhibition of HDAC4 also decreased tubular epithelial cell arrest in the G2/M phase of the cell cycle, expression of transforming growth factor-β1 and phosphorylation of Smad3, signal transducer and activator of transcription 3, and extracellular signal-regulated kinase 1/2 in the injured kidney. Moreover, tasquinimod treatment or HDAC4 deletion inhibited UUO-induced renal tubular cell injury and apoptosis as indicated by reduced expression of neutrophil gelatinase-associated lipocalin, Bax, and inhibition of caspase-3. Finally, administration of tasquinimod or knockdown of HDAC4 prevented injury-related repression of Klotho, a renoprotective protein. Our results indicate that HDAC4 is critically involved in renal tubular injury and fibrosis and suggest that HDAC4 is a potential therapeutic target for treatment of chronic fibrotic kidney disease.

Genetic Kidney Diseases (GKDs) Modeling Using Genome Editing Technologies

Genetic kidney diseases (GKDs) are a group of rare diseases, affecting approximately about 60 to 80 per 100,000 individuals, for which there is currently no treatment that can cure them (in many cases). GKDs usually leads to early-onset chronic kidney disease, which results in patients having to undergo dialysis or kidney transplant. Here, we briefly describe genetic causes and phenotypic effects of six GKDs representative of different ranges of prevalence and renal involvement (ciliopathy, glomerulopathy, and tubulopathy). One of the shared characteristics of GKDs is that most of them are monogenic. This characteristic makes it possible to use site-specific nuclease systems to edit the genes that cause GKDs and generate in vitro and in vivo models that reflect the genetic abnormalities of GKDs. We describe and compare these site-specific nuclease systems (zinc finger nucleases (ZFNs), transcription activator-like effect nucleases (TALENs) and regularly clustered short palindromic repeat-associated protein (CRISPR-Cas9)) and review how these systems have allowed the generation of cellular and animal GKDs models and how they have contributed to shed light on many still unknown fields in GKDs. We also indicate the main obstacles limiting the application of these systems in a more efficient way. The information provided here will be useful to gain an accurate understanding of the technological advances in the field of genome editing for GKDs, as well as to serve as a guide for the selection of both the genome editing tool and the gene delivery method most suitable for the successful development of GKDs models.

Chronic activation of AMP-activated protein kinase leads to early-onset polycystic kidney phenotype

AMP-activated protein kinase (AMPK) plays a key role in the cellular response to low energy stress and has emerged as an attractive therapeutic target for tackling metabolic diseases. Whilst significant progress has been made regarding the physiological role of AMPK, its function in the kidney remains only partially understood. We use a mouse model expressing a constitutively active mutant of AMPK to investigate the effect of AMPK activation on kidney function in vivo. Kidney morphology and changes in gene and protein expression were monitored and serum and urine markers were measured to assess kidney function in vivo. Global AMPK activation resulted in an early-onset polycystic kidney phenotype, featuring collecting duct cysts and compromised renal function in adult mice. Mechanistically, the cystic kidneys had increased cAMP levels and ERK activation, increased hexokinase I (Hk I) expression, glycogen accumulation and altered expression of proteins associated with autophagy. Kidney tubule-specific activation of AMPK also resulted in a polycystic phenotype, demonstrating that renal tubular AMPK activation caused the cystogenesis. Importantly, human autosomal dominant polycystic kidney disease (ADPKD) kidney sections revealed similar protein localisation patterns to that observed in the murine cystic kidneys. Our findings show that early-onset chronic AMPK activation leads to a polycystic kidney phenotype, suggesting dysregulated AMPK signalling is a contributing factor in cystogenesis.

Cystin genetic variants cause autosomal recessive polycystic kidney disease associated with altered Myc expression

Mutation of the Cys1 gene underlies the renal cystic disease in the Cys1cpk/cpk (cpk) mouse that phenocopies human autosomal recessive polycystic kidney disease (ARPKD). Cystin, the protein product of Cys1, is expressed in the primary apical cilia of renal ductal epithelial cells. In previous studies, we showed that cystin regulates Myc expression via interaction with the tumor suppressor, necdin. Here, we demonstrate rescue of the cpk renal phenotype by kidney-specific expression of a cystin-GFP fusion protein encoded by a transgene integrated into the Rosa26 locus. In addition, we show that expression of the cystin-GFP fusion protein in collecting duct cells down-regulates expression of Myc in cpk kidneys. Finally, we report the first human patient with an ARPKD phenotype due to homozygosity for a deleterious splicing variant in CYS1. These findings suggest that mutations in Cys1/CYS1 cause an ARPKD phenotype in mouse and human, respectively, and that the renal cystic phenotype in the mouse is driven by overexpression of the Myc proto-oncogene.

Pdcd10-Stk24/25 complex controls kidney water reabsorption by regulating Aqp2 membrane targeting

PDCD10, also known as CCM3, is a gene found to be associated with the human disease cerebral cavernous malformations (CCMs). PDCD10 forms a complex with GCKIII kinases including STK24, STK25, and MST4. Studies in C. elegans and Drosophila have shown a pivotal role of the PDCD10-GCKIII complex in maintaining epithelial integrity. Here, we found that mice deficient of Pdcd10 or Stk24/25 in the kidney tubules developed polyuria and displayed increased water consumption. Although the expression levels of aquaporin genes were not decreased, the levels of total and phosphorylated aquaporin 2 (Aqp2) protein in the apical membrane of tubular epithelial cells were decreased in Pdcd10- and Stk24/25-deficient mice. This loss of Aqp2 was associated with increased expression and membrane targeting of Ezrin and phosphorylated Ezrin, Radixin, Moesin (p-ERM) proteins and impaired intracellular vesicle trafficking. Treatment with Erlotinib, a tyrosine kinase inhibitor promoting exocytosis and inhibiting endocytosis, normalized the expression level and membrane abundance of Aqp2 protein, and partially rescued the water reabsorption defect observed in the Pdcd10-deficient mice. Our current study identified the PDCD10-STK-ERM signaling pathway as a potentially novel pathway required for water balance control by regulating vesicle trafficking and protein abundance of AQP2 in the kidneys.

Transcriptome-wide association analysis identifies DACH1 as a kidney disease risk gene that contributes to fibrosis

Genome-wide association studies (GWAS) for kidney function identified hundreds of risk regions; however, the causal variants, target genes, cell types, and disease mechanisms remain poorly understood. Here, we performed transcriptome-wide association studies (TWAS), summary Mendelian randomization, and MetaXcan to identify genes whose expression mediates the genotype effect on the phenotype. Our analyses identified Dachshund homolog 1 (DACH1), a cell-fate determination factor. GWAS risk variant was associated with lower DACH1 expression in human kidney tubules. Human and mouse kidney single-cell open chromatin data (snATAC-Seq) prioritized estimated glomerular filtration rate (eGFR) GWAS variants located on an intronic regulatory region in distal convoluted tubule cells. CRISPR-Cas9-mediated gene editing confirmed the role of risk variants in regulating DACH1 expression. Mice with tubule-specific Dach1 deletion developed more severe renal fibrosis both in folic acid and diabetic kidney injury models. Mice with tubule-specific Dach1 overexpression were protected from folic acid nephropathy. Single-cell RNA sequencing, chromatin immunoprecipitation, and functional analysis indicated that DACH1 controls the expression of cell cycle and myeloid chemotactic factors, contributing to macrophage infiltration and fibrosis development. In summary, integration of GWAS, TWAS, single-cell epigenome, expression analyses, gene editing, and functional validation in different mouse kidney disease models identified DACH1 as a kidney disease risk gene.

Targeting fibroblast growth factor 23-responsive pathways uncovers controlling genes in kidney mineral metabolism

Fibroblast Growth Factor 23 (FGF23) is a bone-derived hormone that reduces kidney phosphate reabsorption and 1,25(OH)2 vitamin D synthesis via its required co-receptor alpha-Klotho. To identify novel genes that could serve as targets to control FGF23-mediated mineral metabolism, gene array and single-cell RNA sequencing were performed in wild type mouse kidneys. Gene array demonstrated that heparin-binding EGF-like growth factor (HBEGF) was significantly up-regulated following one-hour FGF23 treatment of wild type mice. Mice injected with HBEGF had phenotypes consistent with partial FGF23-mimetic activity including robust induction of Egr1, and increased Cyp24a1 mRNAs. Single cell RNA sequencing showed overlapping HBEGF and EGF-receptor expression mostly in the proximal tubule, and alpha-Klotho expression in proximal and distal tubule segments. In alpha-Klotho-null mice devoid of canonical FGF23 signaling, HBEGF injections significantly increased Egr1 and Cyp24a1 with correction of basally elevated Cyp27b1. Additionally, mice placed on a phosphate deficient diet to suppress FGF23 had endogenously increased Cyp27b1 mRNA, which was rescued in mice receiving HBEGF. In HEK293 cells with stable alpha-Klotho expression, FGF23 and HBEGF increased CYP24A1 mRNA expression. HBEGF, but not FGF23 bioactivity was blocked with EGF-receptor inhibition. Thus, our findings support that the paracrine/autocrine factor HBEGF could play novel roles in controlling genes downstream of FGF23 via targeting common signaling pathways.

A substrate-specific mTORC1 pathway underlies Birt-Hogg-Dubé syndrome

The mechanistic target of rapamycin complex 1 (mTORC1) is a key metabolic hub that controls the cellular response to environmental cues by exerting its kinase activity on multiple substrates^1–3. However, whether mTORC1 responds to diverse stimuli by differentially phosphorylating specific substrates is poorly understood. Here we show that Transcription Factor EB (TFEB), a master regulator of lysosomal biogenesis and autophagy^4,5, is phosphorylated by mTORC1 via a substrate-specific mechanism mediated by RagGTPases. Thus, TFEB phosphorylation is strictly dependent on amino acid-mediated activation of RagC/D GTPase but, unlike other mTORC1 substrates such as S6K and 4E-BP1, insensitive to growth factor-induced Rheb activity. This mechanism plays a crucial role in Birt-Hogg-Dubé (BHD) syndrome, a disorder caused by mutations of the RagC/D activator folliculin (FLCN) and characterized by benign skin tumors, lung and kidney cysts and renal cell carcinoma^6,7. We found that constitutive activation of TFEB is the main driver of the kidney abnormalities and paradoxical mTORC1 hyperactivity observed in BHD syndrome. Remarkably, depletion of TFEB in a kidney-specific mouse model of BHD syndrome fully rescued the disease phenotype and associated lethality and normalized mTORC1 activity. Together, these findings identify a substrate-specific control mechanism of mTORC1, whose dysregulation leads to kidney cysts and cancer.

Inhibition of oxygen-sensing prolyl hydroxylases increases lipid accumulation in human primary tubular epithelial cells without inducing ER stress

The role of the hypoxia-inducible transcription factor (HIF) pathway in renal lipid metabolism is largely unknown. As HIF stabilizing prolyl hydroxylase (PHD) inhibitors are currently investigated in clinical trials for the treatment of renal anemia, we studied the effects of genetic deletion and pharmacological inhibition of PHDs on renal lipid metabolism in transgenic mice and human primary tubular epithelial cells (hPTEC). Tubular cell-specific deletion of HIF prolyl hydroxylase 2 (Phd2) increased the size of Oil Red-stained lipid droplets in mice. In hPTEC, the PHD inhibitors (PHDi) DMOG and ICA augmented lipid accumulation, which was visualized by Oil Red staining and assessed by microscopy and an infrared imaging system. PHDi-induced lipid accumulation required the exogenous availability of fatty acids and was observed in both proximal and distal hPTEC. PHDi treatment was not associated with structural features of cytotoxicity in contrast to treatment with the immunosuppressant cyclosporine A (CsA). PHDi and CsA differentially upregulated the expression of the lipid droplet-associated genes PLIN2, PLIN4 and HILPDA. Both PHDi and CsA activated AMP-activated protein kinase (AMPK) indicating the initiation of a metabolic stress response. However, only CsA triggered endoplasmic reticulum (ER) stress as determined by the increased mRNA expression of multiple ER stress markers but CsA-induced ER stress was not linked to lipid accumulation. Our data raise the possibility that PHD inhibition may protect tubular cells from toxic free fatty acids by trapping them as triacylglycerides in lipid droplets. This mechanism might contribute to the renoprotective effects of PHDi in experimental kidney diseases.

Comparison of Gene Delivery to the Kidney by Adenovirus, Adeno-Associated Virus, and Lentiviral Vectors After Intravenous and Direct Kidney Injections

There are many kidney diseases that might be addressed by gene therapy. However, gene delivery to kidney cells is inefficient. This is due, in part, to the fact that the kidney excludes molecules above 50 kDa and that most gene delivery vectors are megaDaltons in mass. We compared the ability of adeno-associated virus (AAV), adenovirus (Ad), and lentiviral (LV) vectors to deliver genes to renal cells. When vectors were delivered by the intravenous (IV) route in mice, weak luciferase activity was observed in the kidney with substantially more in the liver. When gene delivery was observed in the kidney, expression was primarily in the glomerulus. To avoid these limitations, vectors were injected directly into the kidney by retrograde ureteral (RU) and subcapsular (SC) injections in mice. Small AAV vectors transduced the kidney, but also leaked from the organ and mediated higher levels of transduction in off-target tissues. Comparison of AAV2, 6.2, 8, and rh10 vectors by direct kidney injection demonstrated highest delivery by AAV6.2 and 8. Larger Ad and LV vectors transduced kidney cells and mediated less off-target tissue transduction. These data demonstrate the utility of direct kidney injections to circumvent the kidney size exclusion barrier. They also identify the effects of vector size on on-target and off-target transduction. This lays the foundation for the use of different vector platforms for gene therapy of diverse kidney diseases.

Rrm2b deletion causes mitochondrial metabolic defects in renal tubules

Renal diseases impose considerable health and economic burdens on health systems worldwide, and there is a lack of efficient methods for the prevention and treatment due to their complexity and heterogeneity. Kidneys are organs with a high demand for energy produced by mitochondria, in which Rrm2b has critical functions as reported. The Rrm2b kidney-specific knockout mice we generated exhibited age-dependent exacerbated features, including mitochondrial dysfunction and increased oxidative stress; additionally, resulted in severe disruption of mitochondria-related metabolism. Rrm2b is vital not only to supply dNTPs for DNA replication and repair, but also to maintain structural integrity and metabolic homeostasis in mitochondria. Thence, Rrm2b deletion might induce chronic kidney defects in mice. This model can facilitate exploration of novel mechanisms and targeted therapies in the kidney diseases and has important translational and clinical implications.

Induction of human pluripotent stem cells into kidney tissues by synthetic mRNAs encoding transcription factors

The derivation of kidney tissues from human pluripotent stem cells (hPSCs) and its application for replacement therapy in end-stage renal disease have been widely discussed. Here we report that consecutive transfections of two sets of synthetic mRNAs encoding transcription factors can induce rapid and efficient differentiation of hPSCs into kidney tissues, termed induced nephron-like organoids (iNephLOs). The first set - FIGLA, PITX2, ASCL1 and TFAP2C, differentiated hPSCs into SIX2+SALL1+ nephron progenitor cells with 92% efficiency within 2 days. Subsequently, the second set - HNF1A, GATA3, GATA1 and EMX2, differentiated these cells into PAX8+LHX1+ pretubular aggregates in another 2 days. Further culture in both 2-dimensional and 3-dimensional conditions produced iNephLOs containing cells characterized as podocytes, proximal tubules, and distal tubules in an additional 10 days. Global gene expression profiles showed similarities between iNephLOs and the human adult kidney, suggesting possible uses of iNephLOs as in vitro models for kidneys.

Epithelial innate immunity mediates tubular cell senescence after kidney injury

Acute kidney injury (AKI) is a common clinical condition of growing incidence. Patients who suffer severe AKI have a higher risk of developing interstitial fibrosis, chronic kidney disease, and end-stage renal disease later in life. Cellular senescence is a persistent cell cycle arrest and altered gene expression pattern evoked by multiple stressors. The number of senescent cells increases with age and even in small numbers these cells can induce chronic inflammation and fibrosis; indeed, in multiple organs including kidneys, the accumulation of such cells is a hallmark of aging. We hypothesized that cellular senescence might be induced in the kidney after injury and that this might contribute to progressive organ fibrosis. Testing this hypothesis, we found that tubular epithelial cells (TECs) in mice senesce within a few days of kidney injury and that this response is mediated by epithelial Toll-like and interleukin 1 receptors (TLR/IL-1R) of the innate immune system. Epithelial cell-specific inhibition of innate immune signaling in mice by knockout of myeloid differentiation 88 (Myd88) reduced fibrosis as well as damage to kidney tubules, and also prevented the accumulation of senescent TECs. Importantly, although inactivation of Myd88 after injury ameliorated fibrosis, it did not reduce damage to the tubules. Selectively induced apoptosis of senescent cells by two different approaches only partially reduced kidney fibrosis, without ameliorating damage to the tubules. Our data reveal a cell-autonomous role for epithelial innate immunity in controlling TEC senescence after kidney injury, and additionally suggest that early therapeutic intervention is required for effective reduction of long-term sequelae of AKI.

β-catenin ablation exacerbates polycystic kidney disease progression

Polycystic kidney disease (PKD) results from excessive renal epithelial cell proliferation, leading to the formation of large fluid filled cysts which impair renal function and frequently lead to renal failure. Hyperactivation of numerous signaling pathways is hypothesized to promote renal epithelial cell hyperproliferation including mTORC1, extracellular signal-regulated kinase (ERK) and WNT signaling. β-catenin and its target genes are overexpressed in some PKD models and expression of activated β-catenin induces cysts in mice; however, β-catenin murine knockout studies indicate it may also inhibit cystogenesis. Therefore, it remains unclear whether β-catenin is pro- or anti-cystogenic and whether its role is canonical WNT signaling-dependent. Here, we investigate whether β-catenin deletion in a PKD model with hyperactived β-catenin signaling affects disease progression to address whether increased β-catenin drives PKD. We used renal epithelial cell specific Inpp5e-null PKD mice which we report exhibit increased β-catenin and target gene expression in the cystic kidneys. Surprisingly, co-deletion of β-catenin with Inpp5e in renal epithelial cells exacerbated polycystic kidney disease and renal failure compared to Inpp5e deletion alone, but did not normalize β-catenin target gene expression. β-catenin/Inpp5e double-knockout kidneys exhibited increased cyst initiation, cell proliferation and MEK/ERK signaling compared to Inpp5e-null, associated with increased fibrosis, which may collectively contribute to accelerated disease. Therefore, increased β-catenin and WNT target gene expression are not necessarily cyst promoting. Rather β-catenin may play a dual and context-dependent role in PKD and in the presence of other cyst-inducing mutations (Inpp5e-deletion); β-catenin loss may exacerbate disease in a WNT target gene-independent manner.

Insulin receptor signaling regulates renal collecting duct and intercalated cell antibacterial defenses

People with diabetes mellitus have increased infection risk. With diabetes, urinary tract infection (UTI) is more common and has worse outcomes. Here, we investigate how diabetes and insulin resistance impact the kidney's innate defenses and urine sterility. We report that type 2 diabetic mice have increased UTI risk. Moreover, insulin-resistant prediabetic mice have increased UTI susceptibility, independent of hyperglycemia or glucosuria. To identify how insulin resistance affects renal antimicrobial defenses, we genetically deleted the insulin receptor in the kidney's collecting tubules and intercalated cells. Intercalated cells, located within collecting tubules, contribute to epithelial defenses by acidifying the urine and secreting antimicrobial peptides (AMPs) into the urinary stream. Collecting duct and intercalated cell-specific insulin receptor deletion did not impact urine acidification, suppressed downstream insulin-mediated targets and AMP expression, and increased UTI susceptibility. Specifically, insulin receptor-mediated signaling regulates AMPs, including lipocalin 2 and ribonuclease 4, via phosphatidylinositol-3-kinase signaling. These data suggest that insulin signaling plays a critical role in renal antibacterial defenses.

Potassium conservation is impaired in mice with reduced renal expression of Kir4.1

To better understand the role of the inward-rectifying K channel Kir4.1 (KCNJ10) in the distal nephron, we initially studied a global Kir4.1 knockout mouse (gKO), which demonstrated the hypokalemia and hypomagnesemia seen in SeSAME/EAST syndrome and was associated with reduced Na/Cl cotransporter (NCC) expression. Lethality by ~3 wk, however, limits the usefulness of this model, so we developed a kidney-specific Kir4.1 "knockdown" mouse (ksKD) using a cadherin 16 promoter and Cre-loxP methodology. These mice appeared normal and survived to adulthood. Kir4.1 protein expression was decreased ~50% vs. wild-type (WT) mice by immunoblotting, and immunofluorescence showed moderately reduced Kir4.1 staining in distal convoluted tubule that was minimal or absent in connecting tubule and cortical collecting duct. Under control conditions, the ksKD mice showed metabolic alkalosis and relative hypercalcemia but were normokalemic and mildly hypermagnesemic despite decreased NCC expression. In addition, the mice had a severe urinary concentrating defect associated with hypernatremia, enlarged kidneys with tubulocystic dilations, and reduced aquaporin-3 expression. On a K/Mg-free diet for 1 wk, however, ksKD mice showed marked hypokalemia (serum K: 1.5 ± 0.1 vs. 3.0 ± 0.1 mEq/l for WT), which was associated with renal K wasting (transtubular K gradient: 11.4 ± 0.8 vs. 1.6 ± 0.4 in WT). Phosphorylated-NCC expression increased in WT but not ksKD mice on the K/Mg-free diet, suggesting that loss of NCC adaptation underlies the hypokalemia. In conclusion, even modest reduction in Kir4.1 expression results in impaired K conservation, which appears to be mediated by reduced expression of activated NCC.

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.

Nephron segment-specific gene expression using AAV vectors

AAV9 vector provides efficient gene transfer in all segments of the renal nephron, with minimum expression in non-renal cells, when administered retrogradely via the ureter. It is important to restrict the transgene expression to the desired cell type within the kidney, so that the physiological endpoints represent the function of the transgene expressed in that specific cell type within kidney. We hypothesized that segment-specific gene expression within the kidney can be accomplished using the highly efficient AAV9 vectors carrying the promoters of genes that are expressed exclusively in the desired segment of the nephron in combination with administration by retrograde infusion into the kidney via the ureter. We constructed AAV vectors carrying eGFP under the control of: kidney-specific cadherin (KSPC) gene promoter for expression in the entire nephron; Na⁺/glucose co-transporter (SGLT2) gene promoter for expression in the S1 and S2 segments of the proximal tubule; sodium, potassium, 2 chloride co-transporter (NKCC2) gene promoter for expression in the thick ascending limb of Henle's loop (TALH); E-cadherin (ECAD) gene promoter for expression in the collecting duct (CD); and cytomegalovirus (CMV) early promoter that provides expression in most of the mammalian cells, as control. We tested the specificity of the promoter constructs in vitro for cell type-specific expression in mouse kidney cells in primary culture, followed by retrograde infusion of the AAV vectors via the ureter in the mouse. Our data show that AAV9 vector, in combination with the segment-specific promoters administered by retrograde infusion via the ureter, provides renal nephron segment-specific gene expression.

MITF - A controls branching morphogenesis and nephron endowment

Congenital nephron number varies widely in the human population and individuals with low nephron number are at risk of developing hypertension and chronic kidney disease. The development of the kidney occurs via an orchestrated morphogenetic process where metanephric mesenchyme and ureteric bud reciprocally interact to induce nephron formation. The genetic networks that modulate the extent of this process and set the final nephron number are mostly unknown. Here, we identified a specific isoform of MITF (MITF-A), a bHLH-Zip transcription factor, as a novel regulator of the final nephron number. We showed that overexpression of MITF-A leads to a substantial increase of nephron number and bigger kidneys, whereas Mitfa deficiency results in reduced nephron number. Furthermore, we demonstrated that MITF-A triggers ureteric bud branching, a phenotype that is associated with increased ureteric bud cell proliferation. Molecular studies associated with an in silico analyses revealed that amongst the putative MITF-A targets, Ret was significantly modulated by MITF-A. Consistent with the key role of this network in kidney morphogenesis, Ret heterozygosis prevented the increase of nephron number in mice overexpressing MITF-A. Collectively, these results uncover a novel transcriptional network that controls branching morphogenesis during kidney development and identifies one of the first modifier genes of nephron endowment.

The number of nephrons, the functional unit of kidney, varies widely among humans. Indeed, it has been shown that kidneys may contain from 0.3 to more than 2 million of nephrons. Nephrons are formed during development via a coordinated morphogenetic program in which the metanephric mesenchyme reciprocally and recursively interacts with the ureteric bud. The fine-tuning of this cross-talk determines the final number of nephrons. Strong evidence indicates that suboptimal nephron endowment is associated with an increased risk of hypertension and chronic kidney disease, a major healthcare burden. Indeed, chronic kidney disease is characterized by the progressive decline of renal function towards end stage renal disease, which occurs once a critical number of nephrons has been lost. Elucidating the molecular mechanisms that control nephron endowment is, therefore, a critical issue for public health. However, little is known about the factors that determine the final number of nephrons in the healthy population. Our data showed that nephron endowment is genetically predetermined and identified Mitfa, a bHLH transcription factor, as one of the first modifiers of nephron formation during kidney development. By generating an allelic series of transgenic mice expressing different levels of MITF-A, we discovered that MITF-A promotes final nephron endowment. In addition, we elucidated the molecular mechanisms by which MITF-A promotes nephron formation and identified RET as one of the critical effectors.

Smad7 protects against acute kidney injury by rescuing tubular epithelial cells from the G1 cell cycle arrest

Smad7 plays a protective role in chronic kidney disease; however, its role in acute kidney injury (AKI) remains unexplored. Here, we report that Smad7 protects against AKI by rescuing the G1 cell cycle arrest of tubular epithelial cells (TECs) in ischemia/reperfusion-induced AKI in mice in which Smad7 gene is disrupted or restored locally into the kidney. In Smad7 gene knockout (KO) mice, more severe renal impairment including higher levels of serum creatinine and massive tubular necrosis was developed at 48 h after AKI. In contrast, restored renal Smad7 gene locally into the kidney of Smad7 KO mice protected against AKI by promoting TEC proliferation identified by PCNA+ and BrdU+ cells. Mechanistic studies revealed that worsen AKI in Smad7 KO mice was associated with a marked activation of TGF-β/Smad3-p21/p27 signaling and a loss of CDK2/cyclin E activities, thereby impairing TEC regeneration at the G1 cell cycle arrest. In contrast, restored Smad7 locally into the kidneys of Smad7 KO mice protected TECs from the G1 cell cycle arrest and promoted TEC G1/S transition via a CDK2/cyclin E-dependent mechanism. In conclusion, Smad7 plays a protective role in AKI. Blockade of TGF-β/Smad3-p21/p27-induced G1 cell cycle arrest may be a key mechanism by which Smad7 treatment inhibits AKI. Thus, Smad7 may be a novel therapeutic agent for AKI.

Generation of kidney tubular organoids from human pluripotent stem cells

Recent advances in stem cell research have resulted in methods to generate kidney organoids from human pluripotent stem cells (hPSCs), which contain cells of multiple lineages including nephron epithelial cells. Methods to purify specific types of cells from differentiated hPSCs, however, have not been established well. For bioengineering, cell transplantation, and disease modeling, it would be useful to establish those methods to obtain pure populations of specific types of kidney cells. Here, we report a simple two-step differentiation protocol to generate kidney tubular organoids from hPSCs with direct purification of KSP (kidney specific protein)-positive cells using anti-KSP antibody. We first differentiated hPSCs into mesoderm cells using a glycogen synthase kinase-3β inhibitor for 3 days, then cultured cells in renal epithelial growth medium to induce KSP+ cells. We purified KSP+ cells using flow cytometry with anti-KSP antibody, which exhibited characteristics of all segments of kidney tubular cells and cultured KSP+ cells in 3D Matrigel, which formed tubular organoids in vitro. The formation of tubular organoids by KSP+ cells induced the acquisition of functional kidney tubules. KSP+ cells also allowed for the generation of chimeric kidney cultures in which human cells self-assembled into 3D tubular structures in combination with mouse embryonic kidney cells.

Renal tubular angiotensin converting enzyme is responsible for nitro-L-arginine methyl ester (L-NAME)-induced salt sensitivity

Renal parenchymal injury predisposes to salt-sensitive hypertension, but how this occurs is not known. Here we tested whether renal tubular angiotensin converting enzyme (ACE), the main site of kidney ACE expression, is central to the development of salt sensitivity in this setting. Two mouse models were used: it-ACE mice in which ACE expression is selectively eliminated from renal tubular epithelial cells; and ACE 3/9 mice, a compound heterozygous mouse model that makes ACE only in renal tubular epithelium from the ACE 9 allele, and in liver hepatocytes from the ACE 3 allele. Salt sensitivity was induced using a post L-NAME salt challenge. While both wild-type and ACE 3/9 mice developed arterial hypertension following three weeks of high salt administration, it-ACE mice remained normotensive with low levels of renal angiotensin II. These mice displayed increased sodium excretion, lower sodium accumulation, and an exaggerated reduction in distal sodium transporters. Thus, in mice with renal injury induced by L-NAME pretreatment, renal tubular epithelial ACE, and not ACE expression by renal endothelium, lung, brain, or plasma, is essential for renal angiotensin II accumulation and salt-sensitive hypertension.

Kidney-specific genetic deletion of both AMPK α-subunits causes salt and water wasting

AMP-activated kinase (AMPK) controls cell energy homeostasis by modulating ATP synthesis and expenditure. In vitro studies have suggested AMPK may also control key elements of renal epithelial electrolyte transport but in vivo physiological confirmation is still insufficient. We studied sodium renal handling and extracellular volume regulation in mice with genetic deletion of AMPK catalytic subunits. AMPKα1 knockout (KO) mice exhibit normal renal sodium handling and a moderate antidiuretic state. This is accompanied by higher urinary aldosterone excretion rates and reduced blood pressure. Plasma volume, however, was found to be increased compared with wild-type mice. Thus blood volume is preserved despite a significantly lower hematocrit. The lack of a defect in renal function in AMPKα1 KO mice could be explained by a compensatory upregulation in AMPK α2-subunit. Therefore, we used the Cre-loxP system to knock down AMPKα2 expression in renal epithelial cells. Combining this approach with the systemic deletion of AMPKα1 we achieved reduced renal AMPK activity, accompanied by a shift to a moderate water- and salt-wasting phenotype. Thus we confirm the physiologically relevant role of AMPK in the kidney. Furthermore, our results indicate that in vivo AMPK activity stimulates renal sodium and water reabsorption.

Modelling TFE renal cell carcinoma in mice reveals a critical role of WNT signaling

TFE-fusion renal cell carcinomas (TFE-fusion RCCs) are caused by chromosomal translocations that lead to overexpression of the TFEB and TFE3 genes (Kauffman et al., 2014). The mechanisms leading to kidney tumor development remain uncharacterized and effective therapies are yet to be identified. Hence, the need to model these diseases in an experimental animal system (Kauffman et al., 2014). Here, we show that kidney-specific TFEB overexpression in transgenic mice, resulted in renal clear cells, multi-layered basement membranes, severe cystic pathology, and ultimately papillary carcinomas with hepatic metastases. These features closely recapitulate those observed in both TFEB- and TFE3-mediated human kidney tumors. Analysis of kidney samples revealed transcriptional induction and enhanced signaling of the WNT β-catenin pathway. WNT signaling inhibitors normalized the proliferation rate of primary kidney cells and significantly rescued the disease phenotype in vivo. These data shed new light on the mechanisms underlying TFE-fusion RCCs and suggest a possible therapeutic strategy based on the inhibition of the WNT pathway.

Fibroproliferative response to urothelial failure obliterates the ureter lumen in a mouse model of prenatal congenital obstructive nephropathy

Congenital obstructive nephropathy (CON) is the most prevalent cause of pediatric chronic kidney disease and end-stage renal disease. The ureteropelvic junction (UPJ) region, where the renal pelvis transitions to the ureter, is the most commonly obstructed site in CON. The underlying causes of congenital UPJ obstructions remain poorly understood, especially when they occur in utero, in part due to the lack of genetic animal models. We previously showed that conditional inactivation of Sec10, a central subunit of the exocyst complex, in the epithelial cells of the ureter and renal collecting system resulted in late gestational bilateral UPJ obstructions with neonatal anuria and death. In this study, we show that without Sec10, the urothelial progenitor cells that line the ureter fail to differentiate into superficial cells, which are responsible for producing uroplakin plaques on the luminal surface. These Sec10-knockout urothelial cells undergo cell death by E17.5 and the urothelial barrier becomes leaky to luminal fluid. Also at E17.5, we measured increased expression of TGFβ1 and genes associated with myofibroblast activation, with evidence of stromal remodeling. Our findings support the model that a defective urothelial barrier allows urine to induce a fibrotic wound healing mechanism, which may contribute to human prenatal UPJ obstructions.

STIM1fl/fl Ksp-Cre Mouse has Impaired Renal Water Balance

BACKGROUND/AIM

STIM1 is as an essential component in store operated Ca2+ entry. However give the paucity of information on the role of STIM1 in kidney, the aim was to study the function of STIM1 in the medulla of the kidney.

METHODS

we crossed a Ksp-cre mouse with another mouse containing two loxP sites flanking Exon 6 of STIM1. The Ksp-cre mouse is based upon the Ksp-cadherin gene promoter which expresses cre recombinase in developing nephrons, collecting ducts (SD) and thick ascending limbs (TAL) of the loop of Henle.

RESULTS

The offspring of these mice are viable without gross morphological changes, however, we noticed that the STIM1 Ksp-cre knockout mice produced more urine compared to control. To examine this more carefully, we fed mice low (LP) and high protein (HP) diets respectively. When mice were fed HP diet STIM1 ko mice had significantly increased urinary volume and lower specific gravity compared to wt mice. In STIM1 ko mice fed HP diet urine creatinine and urea were significantly lower compared to wt mice fed HP diet, however the fractional excretion was the same.

CONCLUSION

These data support the idea that STIM1 ko mice have impaired urinary concentrating ability when challenged with HP diet is most likely caused by impaired Ca2+-dependent signal transduction through the vasopressin receptor cascade.

Inpp5e suppresses polycystic kidney disease via inhibition of PI3K/Akt-dependent mTORC1 signaling

Polycystic kidney disease (PKD) is a common cause of renal failure with few effective treatments. INPP5E is an inositol polyphosphate 5-phosphatase that dephosphorylates phosphoinositide 3-kinase (PI3K)-generated PI(3,4,5)P₃ and is mutated in ciliopathy syndromes. Germline Inpp5e deletion is embryonically lethal, attributed to cilia stability defects, and is associated with polycystic kidneys. However, the molecular mechanisms responsible for PKD development upon Inpp5e loss remain unknown. Here, we show conditional inactivation of Inpp5e in mouse kidney epithelium results in severe PKD and renal failure, associated with a partial reduction in cilia number and hyperactivation of PI3K/Akt and downstream mammalian target of rapamycin complex 1 (mTORC1) signaling. Treatment with an mTORC1 inhibitor improved kidney morphology and function, but did not affect cilia number or length. Therefore, we identify Inpp5e as an essential inhibitor of the PI3K/Akt/mTORC1 signaling axis in renal epithelial cells, and demonstrate a critical role for Inpp5e-dependent mTORC1 regulation in PKD suppression.

Comparing the Bbs10 complete knockout phenotype with a specific renal epithelial knockout one highlights the link between renal defects and systemic inactivation in mice

BACKGROUND

Bardet-Biedl Syndrome (BBS) is a genetically heterogeneous ciliopathy with clinical cardinal features including retinal degeneration, obesity and renal dysfunction. To date, 20 BBS genes have been identified with BBS10 being a major BBS gene found to be mutated in almost 20 percent of all BBS patients worldwide. It codes for the BBS10 protein which forms part of a chaperone complex localized at the basal body of the primary cilium. Renal dysfunction in BBS patients is one of the major causes of morbidity in human patients and is associated initially with urinary concentration defects related to water reabsorption impairment in renal epithelial cells. The aim of this study was to study and compare the impact of a total Bbs10 inactivation (Bbs10 (-/-)) with that of a specific renal epithelial cells inactivation (Bbs10  (fl/fl) ; Cdh16-Cre (+/-)).

RESULTS

We generated the Bbs10 (-/-) and Bbs10  (fl/fl) ; Cadh16-Cre (+/-) mouse model and characterized them. Bbs10 (-/-) mice developed obesity, retinal degeneration, structural defects in the glomeruli, polyuria associated with high circulating arginine vasopressin (AVP) concentrations, and vacuolated, yet ciliated, renal epithelial cells. On the other hand, the Bbs10  (fl/fl) ; Cadh16-Cre (+/-)mice displayed no detectable impairment.

CONCLUSIONS

These data highlight the importance of a systemic Bbs10 inactivation to trigger averted renal dysfunction whereas a targeted absence of BBS10 in the renal epithelium is seemingly non-deleterious.

Urothelial Defects from Targeted Inactivation of Exocyst Sec10 in Mice Cause Ureteropelvic Junction Obstructions

Most cases of congenital obstructive nephropathy are the result of ureteropelvic junction obstructions, and despite their high prevalence, we have a poor understanding of their etiology and scarcity of genetic models. The eight-protein exocyst complex regulates polarized exocytosis of intracellular vesicles in a large variety of cell types. Here we report generation of a conditional knockout mouse for Sec10, a central component of the exocyst, which is the first conditional allele for any exocyst gene. Inactivation of Sec10 in ureteric bud-derived cells using Ksp1.3-Cre mice resulted in severe bilateral hydronephrosis and complete anuria in newborns, with death occurring 6-14 hours after birth. Sec10 FL/FL;Ksp-Cre embryos developed ureteropelvic junction obstructions between E17.5 and E18.5 as a result of degeneration of the urothelium and subsequent overgrowth by surrounding mesenchymal cells. The urothelial cell layer that lines the urinary tract must maintain a hydrophobic luminal barrier again urine while remaining highly stretchable. This barrier is largely established by production of uroplakin proteins that are transported to the apical surface to establish large plaques. By E16.5, Sec10 FL/FL;Ksp-Cre ureter and pelvic urothelium showed decreased uroplakin-3 protein at the luminal surface, and complete absence of uroplakin-3 by E17.5. Affected urothelium at the UPJ showed irregular barriers that exposed the smooth muscle layer to urine, suggesting this may trigger the surrounding mesenchymal cells to overgrow the lumen. Findings from this novel mouse model show Sec10 is critical for the development of the urothelium in ureters, and provides experimental evidence that failure of this urothelial barrier may contribute to human congenital urinary tract obstructions.

The exocyst gene Sec10 regulates renal epithelial monolayer homeostasis and apoptotic sensitivity

The highly conserved exocyst protein complex regulates polarized exocytosis of subsets of secretory vesicles. A previous study reported that shRNA knockdown of an exocyst central subunit, Sec10 (Sec10-KD) in Madin-Darby canine kidney (MDCK) cells disrupted primary cilia assembly and 3D cyst formation. We used three-dimensional collagen cultures of MDCK cells to further investigate the mechanisms by which Sec10 and the exocyst regulate epithelial polarity, morphogenesis, and homeostasis. Sec10-KD cysts initially demonstrated undisturbed lumen formation although later displayed significantly fewer and shorter primary cilia than controls. Later in cystogenesis, control cells maintained normal homeostasis, while Sec10-KD cysts displayed numerous apoptotic cells extruded basally into the collagen matrix. Sec10-KD MDCK cells were also more sensitive to apoptotic triggers than controls. These phenotypes were reversed by restoring Sec10 expression with shRNA-resistant human Sec10. Apico-basal polarity appeared normal in Sec10-KD cysts, whereas mitotic spindle angles differed significantly from controls, suggesting a planar cell polarity defect. In addition, analysis of renal tubules in a newly generated kidney-specific Sec10-knockout mouse model revealed significant defects in primary cilia assembly and in the targeted renal tubules; abnormal epithelial cell extrusion was also observed, supporting our in vitro results. We hypothesize that, in Sec10-KD cells, the disrupted exocyst activity results in increased apoptotic sensitivity through defective primary cilia signaling and that, in combination with an increased basal cell extrusion rate, it affects epithelial barrier integrity and homeostasis.

Cell-cell interactions driving kidney morphogenesis

The mammalian kidney forms via cell-cell interactions between an epithelial outgrowth of the nephric duct and the surrounding nephrogenic mesenchyme. Initial morphogenetic events include ureteric bud branching to form the collecting duct (CD) tree and mesenchymal-to-epithelial transitions to form the nephrons, requiring reciprocal induction between adjacent mesenchyme and epithelial cells. Within the tips of the branching ureteric epithelium, cells respond to mesenchyme-derived trophic factors by proliferation, migration, and mitosis-associated cell dispersal. Self-inhibition signals from one tip to another play a role in branch patterning. The position, survival, and fate of the nephrogenic mesenchyme are regulated by ECM and secreted signals from adjacent tip and stroma. Signals from the ureteric tip promote mesenchyme self-renewal and trigger nephron formation. Subsequent fusion to the CDs, nephron segmentation and maturation, and formation of a patent glomerular basement membrane also require specialized cell-cell interactions. Differential cadherin, laminin, nectin, and integrin expression, as well as intracellular kinesin and actin-mediated regulation of cell shape and adhesion, underlies these cell-cell interactions. Indeed, the capacity for the kidney to form via self-organization has now been established both via the recapitulation of expected morphogenetic interactions after complete dissociation and reassociation of cellular components during development as well as the in vitro formation of 3D kidney organoids from human pluripotent stem cells. As we understand more about how the many cell-cell interactions required for kidney formation operate, this enables the prospect of bioengineering replacement structures based on these self-organizing properties.

Regeneration and bioengineering of the kidney: current status and future challenges

The prevalence of chronic kidney disease continues to outpace the development of effective treatment strategies. For patients with advanced disease, renal replacement therapies approximate the filtration functions of the kidney at considerable cost and inconvenience, while failing to restore the resorptive and endocrine functions. Allogeneic transplantation remains the only restorative treatment, but donor shortage, surgical morbidity and the need for lifelong immunosuppression significantly limit clinical application. Emerging technologies in the fields of regenerative medicine and tissue engineering strive to address these limitations. We review recent advances in cell-based therapies, primordial allografts, bio-artificial organs and whole-organ bioengineering as they apply to renal regeneration. Collaborative efforts across these fields aim to produce a bioengineered kidney capable of restoring renal function in patients with end-stage disease.

Tissue-specific regulation of the mouse Pkhd1 (ARPKD) gene promoter

Autosomal recessive polycystic kidney disease, an inherited disorder characterized by the formation of cysts in renal collecting ducts and biliary dysgenesis, is caused by mutations of the polycystic kidney and hepatic disease 1 (PKHD1) gene. Expression of PKHD1 is tissue specific and developmentally regulated. Here, we show that a 2.0-kb genomic fragment containing the proximal promoter of mouse Pkhd1 directs tissue-specific expression of a lacZ reporter gene in transgenic mice. LacZ is expressed in renal collecting ducts beginning during embryonic development but is not expressed in extrarenal tissues. The Pkhd1 promoter contains a binding site for the transcription factor hepatocyte nuclear factor (HNF)-1β, which is required for activity in transfected cells. Mutation of the HNF-1β-binding site abolishes the expression of the lacZ reporter gene in renal collecting ducts. Transgenes containing the 2.0-kb promoter and 2.7 kb of additional genomic sequence extending downstream to the second exon are expressed in the kidney, intrahepatic bile ducts, and male reproductive tract. This pattern overlaps with the endogenous expression of Pkhd1 and coincides with sites of expression of HNF-1β. We conclude that the proximal 2.0-kb promoter is sufficient for tissue-specific expression of Pkhd1 in renal collecting ducts in vivo and that HNF-1β is required for Pkhd1 promoter activity in collecting ducts. Additional genomic sequences located from exons 1-2 or elsewhere in the gene locus are required for expression in extrarenal tissues.

Tubular p53 regulates multiple genes to mediate AKI

A pathogenic role of p53 in AKI was suggested a decade ago but remains controversial. Indeed, recent work indicates that inhibition of p53 protects against ischemic AKI in rats but exacerbates AKI in mice. One intriguing possibility is that p53 has cell type-specific roles in AKI. To determine the role of tubular p53, we generated two conditional gene knockout mouse models, in which p53 is specifically ablated from proximal tubules or other tubular segments, including distal tubules, loops of Henle, and medullary collecting ducts. Proximal tubule p53 knockout (PT-p53-KO) mice were resistant to ischemic and cisplatin nephrotoxic AKI, which was indicated by the analysis of renal function, histology, apoptosis, and inflammation. However, other tubular p53 knockout (OT-p53-KO) mice were sensitive to AKI. Mechanistically, AKI associated with the upregulation of several known p53 target genes, including Bax, p53-upregulated modulator of apoptosis-α, p21, and Siva, and this association was attenuated in PT-p53-KO mice. In global expression analysis, ischemic AKI induced 371 genes in wild-type kidney cortical tissues, but the induction of 31 of these genes was abrogated in PT-p53-KO tissues. These 31 genes included regulators of cell death, metabolism, signal transduction, oxidative stress, and mitochondria. These results suggest that p53 in proximal tubular cells promotes AKI, whereas p53 in other tubular cells does not.

Renal cells from spermatogonial germline stem cells protect against kidney injury

Spermatogonial stem cells reside in specific niches within seminiferous tubules and continuously generate differentiating daughter cells for production of spermatozoa. Although spermatogonial stem cells are unipotent, these cells are able to spontaneously convert to germline cell-derived pluripotent stem cells (GPSCs) in vitro. GPSCs have many properties of embryonic stem cells and are highly plastic, but their therapeutic potential in tissue regeneration has not been fully explored. Using a novel renal epithelial differentiation protocol, we obtained GPSC-derived tubular-like cells (GTCs) that were functional in vitro, as demonstrated through transepithelial electrical resistance analysis. In mice, GTCs injected after ischemic renal injury homed to the renal parenchyma, and GTC-treated mice showed reduced renal oxidative stress, tubular apoptosis, and cortical damage and upregulated tubular expression of the antioxidant enzyme hemeoxygenase-1. Six weeks after ischemic injury, kidneys of GTC-treated mice had less fibrosis and inflammatory infiltrate than kidneys of vehicle-treated mice. In conclusion, we show that GPSCs can be differentiated into functionally active renal tubular-like cells that therapeutically prevent chronic ischemic damage in vivo, introducing the potential utility of GPSCs in regenerative cell therapy.

Loss of cilia suppresses cyst growth in genetic models of autosomal dominant polycystic kidney disease

Kidney cysts occur following inactivation of polycystins in otherwise intact cilia or following complete removal of cilia by inactivation of intraflagellar transport-related proteins. We investigated the mechanisms of cyst formation in these two distinct processes by combining conditional inactivation of polycystins with concomitant ablation of cilia in developing and adult kidney and liver. We found that loss of intact cilia suppresses cyst growth following inactivation of polycystins and that the severity of cystic disease was directly related to the length of time between the initial loss of the polycystin proteins and the subsequent involution of cilia. This cilia-dependent cyst growth was not explained by activation of the MAPK/ERK, mTOR or cAMP pathways and is likely to be distinct from the mechanism of cyst growth following complete loss of cilia. The data establish the existence of a novel pathway defined by polycystin-dependent inhibition and cilia-dependent activation that promotes rapid cyst growth.

Kidney specific protein-positive cells derived from embryonic stem cells reproduce tubular structures in vitro and differentiate into renal tubular cells

Embryonic stem cells and induced pluripotent stem cells have the ability to differentiate into various organs and tissues, and are regarded as new tools for the elucidation of disease mechanisms as well as sources for regenerative therapies. However, a method of inducing organ-specific cells from pluripotent stem cells is urgently needed. Although many scientists have been developing methods to induce various organ-specific cells from pluripotent stem cells, renal lineage cells have yet to be induced in vitro because of the complexity of kidney structures and the diversity of kidney-component cells. Here, we describe a method of inducing renal tubular cells from mouse embryonic stem cells via the cell purification of kidney specific protein (KSP)-positive cells using an anti-KSP antibody. The global gene expression profiles of KSP-positive cells derived from ES cells exhibited characteristics similar to those of cells in the developing kidney, and KSP-positive cells had the capacity to form tubular structures resembling renal tubular cells when grown in a 3D culture in Matrigel. Moreover, our results indicated that KSP-positive cells acquired the characteristics of each segment of renal tubular cells through tubular formation when stimulated with Wnt4. This method is an important step toward kidney disease research using pluripotent stem cells, and the development of kidney regeneration therapies.

Inducible expression of kallikrein in renal tubular cells protects mice against spontaneous lupus nephritis

OBJECTIVE

To ascertain whether engineered expression of kallikreins within the kidneys, using an inducible Cre/loxP system, can ameliorate murine lupus nephritis.

METHODS

In mice with a lupus-prone genetic background, we engineered the expression of tamoxifen-inducible Cre recombinase under the control of a kidney-specific promoter whose activation initiates murine kallikrein-1 expression within the kidneys. These transgenic mice were injected with either tamoxifen or vehicle at age 2 months and then were monitored for 8 months for kallikrein expression and disease.

RESULTS

Elevated expression of kallikrein was detected in the kidney and urine of tamoxifen-injected mice but not in controls. At age 10 months, all vehicle-injected mice developed severe lupus nephritis, as evidenced by increased proteinuria (mean ± SD 13.43 ± 5.65 mg/24 hours), increased blood urea nitrogen (BUN) and serum creatinine levels (39.86 ± 13.45 mg/dl and 15.23 ± 6.89 mg/dl, respectively), and severe renal pathology. In contrast, the tamoxifen-injected mice showed significantly reduced proteinuria (6.6 ± 4.12 mg/24 hours), decreased BUN and serum creatinine levels (15.71 ± 8.17 mg/dl and 6.64 ± 3.39 mg/dl, respectively), and milder renal pathology. Tamoxifen-induced up-regulation of renal kallikrein expression increased nitric oxide production and dampened renal superoxide production and inflammatory cell infiltration, alluding to some of the pathways through which kallikreins may be operating within the kidneys.

CONCLUSION

Local expression of kallikreins within the kidney has the capacity to dampen lupus nephritis, possibly by modulating inflammation and oxidative stress.

Generation of c-Myc transgenic pigs for autosomal dominant polycystic kidney disease

After several decades of research, autosomal dominant polycystic kidney disease (ADPKD) is still incurable and imposes enormous physical, psychological, and economic burdens on patients and their families. Murine models of ADPKD represent invaluable tools for studying this disease. These murine forms of ADPKD can arise spontaneously, or they can be induced via chemical or genetic manipulations. Although these models have improved our understanding of the etiology and pathogenesis of ADPKD, they have not led to effective treatment strategies. The mini-pig represents an effective biomedical model for studying human diseases, as the pig's human-like physiological processes help to understand disease mechanisms and to develop novel therapies. Here, we tried to generate a transgenic model of ADPKD in pigs by overexpressing c-Myc in kidney tissue. Western-blot analysis showed that c-Myc was overexpressed in the kidney, brain, heart, and liver of transgenic pigs. Immunohistochemical staining of kidney tissue showed that exogenous c-Myc predominantly localized to renal tubules. Slightly elevated blood urea nitrogen levels were observed in transgenic pigs 1 month after birth, but no obvious abnormalities were detected after that time. In the future, we plan to subject this model to renal injury in an effort to promote ADPKD progression.