The fate of Notch-deficient nephrogenic progenitor cells during metanephric kidney development

Notch3 deletion regulates HIV-1 gene expression and systemic inflammation to ameliorate chronic kidney disease

Anti-retroviral therapy (ART) has decreased human immunodeficiency virus (HIV)-1-associated morbidity. However, despite ART, immune cells remain latently infected, leading to chronic inflammation and HIV-1-associated comorbidities. New strategies are needed to target viral proteins and inflammation. We found activation of Notch3 in renal cells of the HIV-1 transgenic mouse model (HIV-Tg26) and in patients with HIV-associated nephropathy. We hypothesized that targeting NOTCH3 activation constitutes an effective therapy for HIV-related chronic kidney disease. We generated HIV-Tg26 mice with Notch3 knocked out (Tg-N3KO). Compared to HIV-Tg26 mice at 3 months, Tg-N3KO mice showed a marked reduction in renal injury, skin lesions and mortality rate. They also showed reduced renal infiltrating cells and significantly reduced expression of HIV genes. Moreover, Notch3 activated the HIV long terminal repeat promoter, and induction of HIV-1 increased Notch3 activation, indicating a feedback mechanism. Further, bone marrow-derived macrophages from HIV-Tg26 mice showed activation of Notch3, indicating systemic effects. Consistent with that observation, systemic levels of TNF and MCP-1 were reduced in Tg-N3KO compared to HIV-Tg26 mice. Thus, Notch3 deletion/inhibition has a dual-therapeutic effect in HIV-related chronic kidney disease, which might extend to other HIV-related pathologies.

Stepwise developmental mimicry generates proximal-biased kidney organoids

The kidney maintains body fluid homeostasis by reabsorbing essential compounds and excreting waste. Proximal tubule cells, crucial for renal reabsorption of a range of sugars, ions, and amino acids, are highly susceptible to damage, leading to pathologies necessitating dialysis and kidney transplants. While human pluripotent stem cell-derived kidney organoids are used for modeling renal development, disease, and injury, the formation of proximal nephron cells in these 3D structures is incomplete. Here, we describe how to drive the development of proximal tubule precursors in kidney organoids by following a blueprint of in vivo human nephrogenesis. Transient manipulation of the PI3K signaling pathway activates Notch signaling in the early nephron and drives nephrons toward a proximal precursor state. These "proximal-biased" (PB) organoid nephrons proceed to generate proximal nephron precursor cells. Single-cell transcriptional analyses across the organoid nephron differentiation, comparing control and PB types, confirm the requirement of transient Notch signaling for proximal development. Indicative of functional maturity, PB organoids demonstrate dextran and albumin uptake, akin to in vivo proximal tubules. Moreover, PB organoids are highly sensitive to nephrotoxic agents, display an injury response, and drive expression of HAVCR1 / KIM1 , an early proximal-specific marker of kidney injury. Injured PB organoids show evidence of collapsed tubules, DNA damage, and upregulate the injury-response marker SOX9 . The PB organoid model therefore has functional relevance and potential for modeling mechanisms underpinning nephron injury. These advances improve the use of iPSC-derived kidney organoids as tools to understand developmental nephrology, model disease, test novel therapeutics, and for understanding human renal physiology.

Notch3 deletion regulates HIV-1 gene expression and systemic inflammation to ameliorate chronic kidney disease

Antiretroviral therapy (ART) has decreased HIV-1 associated morbidity. However, despite ART, immune cells remain latently infected and slowly release viral proteins, leading to chronic inflammation and HIV-1 associated comorbidities. New strategies are needed to target viral proteins and inflammation. We found activation of Notch3 in several renal cells of the HIV-1 mouse model (HIV-Tg26) and in patients with HIV associated Nephropathy. We hypothesized that targeting Notch3 activation constitutes an effective therapy for HIV-related chronic kidney diseases (HIV-CKD). We generated HIV-Tg26 mice with Notch3 knocked out (Tg-N3KO). Compared to HIV-Tg26 mice at 3 months, HIV-Tg-N3KO mice showed a marked reduction in renal injury, skin lesions and mortality rate. Bulk RNA sequencing revealed that N3KO not only reduced renal infiltrating cells but significantly reduced the expression of HIV genes. Moreover, Notch3 activated the HIV- promoter and induction of HIV-1 resulted in increased Notch3 activation indicating a feedback mechanism. Further, bone marrow derived macrophages (BMDMs) from HIV-Tg26 mice showed activation of Notch3 indicating systemic effects. Consistent with that, systemic levels of TNF-α, MCP-1 and other inflammatory chemokines and cytokines were reduced in Tg-N3KO mice. Thus, Notch3 inhibition/deletion has a dual therapeutic effect in HIV-CKD and may extend to other HIV-related pathologies.

Variants in tubule epithelial regulatory elements mediate most heritable differences in human kidney function

Kidney disease is highly heritable; however, the causal genetic variants, the cell types in which these variants function, and the molecular mechanisms underlying kidney disease remain largely unknown. To identify genetic loci affecting kidney function, we performed a GWAS using multiple kidney function biomarkers and identified 462 loci. To begin to investigate how these loci affect kidney function, we generated single-cell chromatin accessibility (scATAC-seq) maps of the human kidney and identified candidate cis-regulatory elements (cCREs) for kidney podocytes, tubule epithelial cells, and kidney endothelial, stromal, and immune cells. Kidney tubule epithelial cCREs explained 58% of kidney function SNP-heritability and kidney podocyte cCREs explained an additional 6.5% of SNP-heritability. In contrast, little kidney function heritability was explained by kidney endothelial, stromal, or immune cell-specific cCREs. Through functionally informed fine-mapping, we identified putative causal kidney function variants and their corresponding cCREs. Using kidney scATAC-seq data, we created a deep learning model (which we named ChromKid) to predict kidney cell type-specific chromatin accessibility from sequence. ChromKid and allele specific kidney scATAC-seq revealed that many fine-mapped kidney function variants locally change chromatin accessibility in tubule epithelial cells. Enhancer assays confirmed that fine-mapped kidney function variants alter tubule epithelial regulatory element function. To map the genes which these regulatory elements control, we used CRISPR interference (CRISPRi) to target these regulatory elements in tubule epithelial cells and assessed changes in gene expression. CRISPRi of enhancers harboring kidney function variants regulated NDRG1 and RBPMS expression. Thus, inherited differences in tubule epithelial NDRG1 and RBPMS expression may predispose to kidney disease in humans. We conclude that genetic variants affecting tubule epithelial regulatory element function account for most SNP-heritability of human kidney function. This work provides an experimental approach to identify the variants, regulatory elements, and genes involved in polygenic disease.

miR-140-5p protects cartilage progenitor/stem cells from fate changes in knee osteoarthritis

Cartilage progenitor/stem cells (CPCs) are promising seed cells for cartilage regeneration, but their fate changes and regulatory mechanisms in osteoarthritis (OA) pathogenesis remain unclear. This study aimed to investigate the role and potential mechanism of the microRNA-140-5p (miR-140-5p), whose protective role in knee OA has been confirmed by our previous studies, in OA CPCs fate reprogramming. Firstly, the normal and OA CPCs were isolated, and the fate indicators, miR-140-5p, Jagged1, and Notch signals were detected and analyzed. Then, the effect of miR-140-5p and the Notch pathway on CPCs fate reprogramming and miR-140-5p on Jagged1/Notch signaling was investigated in IL-1β-induced chondrocytes in vitro. Finally, the effect of miR-140-5p on OA CPCs fate reprogramming and the potential mechanisms were validated in OA rats. As a result, CPCs percentage was increased in the mild OA cartilage-derived total chondrocytes while decreased in the advanced OA group. Significant fate changes (including reduced cell viability, migration, chondrogenesis, and increased apoptosis), increased Jagged1 and Notch signals, and reduced miR-140-5p were observed in OA CPCs and associated with OA progression. IL-1β induced OA-like changes in CPCs fate, which could be exacerbated by miR-140-5p inhibitor while alleviated by DAPT (a specific Notch inhibitor) and miR-140-5p mimic. Finally, the in vitro phenomenal and mechanistic findings were validated in OA rats. Overall, miR-140-5p protects CPCs from fate changes via inhibiting Jagged1/Notch signaling in knee OA, providing attractive targets for OA therapeutics.

Glucocorticoids Inhibit EGFR Signaling Activation in Podocytes in Anti-GBM Crescentic Glomerulonephritis

Glucocorticoids are commonly used to treat anti-GBM crescentic glomerulonephritis, however, the mechanism underlying its therapeutic effectiveness is not completely understood. Since podocyte EGFR/STAT3 signaling is known to mediate the development of anti-GBM glomerulonephritis, we investigated the effect of glucocorticoids on EGFR/STAT3 signaling in podocytes. We found that the levels of phosphorylated (activated) EGFR and STAT3 in podocytes were markedly elevated in anti-GBM patients without glucocorticoids treatment, but were normalized in patients with glucocorticoids treatment. In a rat model of anti-GBM glomerulonephritis, glucocorticoids treatment significantly attenuated the proteinuria, crescent formation, parietal epithelial cell (PEC) activation and proliferation, accompanied by elimination of podocyte EGFR/STAT3 signaling activation. In cultured podocytes, glucocorticoids were found to inhibit HB-EGF-induced EGFR and STAT3 activation. The conditioned medium from podocytes treated with HB-EGF in the absence but not presence of glucocorticoids was capable of activating Notch signaling (which is known to be involved in PEC proliferation and crescent formation) and enhancing proliferative activity in primary PECs, suggesting that glucocorticoids prevent podocytes from producing secreted factors that cause PEC proliferation and crescent formation. Furthermore, we found that glucocorticoids can downregulate the expression of EGFR ligands, EGF and HB-EGF, while upregulate the expression of EGFR inhibitor, Gene 33, explaining how glucocorticoids suppress EGFR signaling. Taken together, glucocorticoids exert therapeutic effect on anti-GBM crescentic glomerulonephritis through inhibiting podocyte EGFR/STAT3 signaling and the downstream pathway that leads to PEC proliferation and crescent formation.

Understanding stem cells and its pivotal role in regenerative medicine

Stem cells (SCs) are clonogenic cells that develop into the specialized cells which later responsible for making up various types of tissue in the human body. SCs are not only the appropriate source of information for cell division, molecular and cellular processes, and tissue homeostasis but also one of the major putative biological aids to diagnose and cure various degenerative diseases. This study emphasises on various research outputs that occurred in the past two decades. This will give brief information on classification, differentiation, detection, and various isolation techniques of SCs. Here, the various signalling pathways which includes WNT, Sonic hedgehog, Notch, BMI1 and C-met pathways and how does it effect on the regeneration of various classes of SCs and factors that regulates the potency of the SCs are also been discussed. We also focused on the application of SCs in the area of regenerative medicine along with the cellular markers that are useful as salient diagnostic or curative tools or in both, by the process of reprogramming, which includes diabetes, cancer, cardiovascular disorders and neurological disorders. The biomarkers that are mentioned in various literatures and experiments include PDX1, FOXA2, HNF6, and NKX6-1 (for diabetes); CD33, CD24, CD133 (for cancer); c-Kit, SCA-1, Wilm's tumor 1 (for cardiovascular disorders); and OCT4, SOX2, c-MYC, EN1, DAT and VMAT2 (for neurological disorders). In this review, we come to know the advancements and scopes of potential SC-based therapies, its diverse applications in clinical fields that can be helpful in the near future.

Notch signaling regulates Akap12 expression and primary cilia length during renal tubule morphogenesis

Alagille syndrome patients present with loss of function mutations in either JAG1 or NOTCH2. About 40%-50% of patients have kidney abnormalities, and frequently display multicystic, dysplastic kidneys. Additionally, gain-of-function mutations in NOTCH2 are associated with cystic kidneys in Hajdu-Cheney syndrome patients. How perturbations in Notch signaling cause renal tubular cysts remains unclear. Here, we have determined that reduced Notch signaling mediated transcription by ectopic expression of dominant-negative mastermind-like (dnMaml) peptide in the nephrogenic epithelia from after the s-shaped body formation and in the developing collecting ducts results in proximal tubular and collecting duct cysts, respectively. An acute inhibition of Notch signaling for two days during kidney development is sufficient to disrupt tubule formation, and significantly increases Akap12 expression. Ectopic expression of Akap12 in renal epithelia results in abnormally long primary cilia similar to that observed in Notch-signaling-deficient epithelia. Both loss of Notch signaling and elevated Akap12 expression disrupt the ability of renal epithelial cells to form spherical structures with a single lumen when grown embedded in matrix. Interestingly, Akap12 can inhibit Notch signaling mediated transcription, which likely explains how both loss of Notch signaling and ectopic expression of Akap12 result in similar renal epithelial abnormalities. We conclude that Notch signaling regulates Akap12 expression while also ensuring normal primary cilia length and renal epithelial morphogenesis, and suggest that one aspect of diseases associated with defective Notch signaling, such as Alagille syndrome, maybe mechanistically related to ciliopathies.

Notch Signaling in Kidney Development, Maintenance, and Disease

Kidney development involves formation of nephrons intricately aligned with the vasculature and connected to a branched network of collecting ducts. Notch signaling plays multiple roles during kidney development involving the formation of nephrons composed of diverse epithelial cell types arranged into tubular segments, all the while maintaining a nephron progenitor niche. Here, we review the roles of Notch signaling identified from rodent kidney development and injury studies, while discussing human kidney diseases associated with aberrant Notch signaling. We also review Notch signaling requirement in maintenance of mature kidney epithelial cell states and speculate that Notch activity regulation mediates certain renal physiologic adaptations.

Aberrant Regulation of Notch3 Signaling Pathway in Polycystic Kidney Disease

Polycystic kidney disease (PKD) is a genetic disorder characterized by fluid-filled cysts in the kidney and liver that ultimately leads to end-stage renal disease. Currently there is no globally approved therapy for PKD. The Notch signaling pathway regulates cellular processes such as proliferation and de-differentiation, which are cellular hallmarks of PKD. Thus we hypothesized that the Notch pathway plays a critical role in PKD. Evaluation of protein expression of Notch signaling components in kidneys of Autosomal Recessive PKD (ARPKD) and Autosomal Dominant PKD (ADPKD) mouse models and of ADPKD patients revealed that Notch pathway members, particularly Notch3, were consistently upregulated or activated in cyst-lining epithelial cells. Notch3 expression correlated with rapidly growing cysts and co-localized with the proliferation marker, PCNA. Importantly, Notch inhibition significantly decreased forskolin-induced Notch3 activation and proliferation of primary human ADPKD cells, and significantly reduced cyst formation and growth of human ADPKD cells cultured in collagen gels. Thus our data indicate that Notch3 is aberrantly activated and facilitates epithelial cell proliferation in PKD, and that inhibition of Notch signaling may prevent cyst formation and growth.

Rho-Kinase Blockade Attenuates Podocyte Apoptosis by Inhibiting the Notch Signaling Pathway in Diabetic Nephropathy

Podocyte apoptosis is a key process in the onset of diabetic nephropathy. A significant body of evidence shows that the Notch signaling pathway plays a central role in this process. We found that Rho-kinase mediates transforming growth factor β (TGF-β)-induced Notch ligand Jag1 expression. Importantly, TGF-β-mediated podocyte apoptosis was attenuated by Rho-kinase inhibition. Mechanistically, Rho-kinase regulated Jag1 induction via the extracellular signal-regulated kinase (ERK) 1/2 and c-Jun N-terminal kinase (JNK) but not Smad pathways. Consistently, the Rho-kinase inhibitor fasudil prevented albuminuria and the urinary excretion of nephrin in db/db mice and reduced the prevalence of podocyte apoptosis and Jag1 expression. Finally, the expression of Jag1 and apoptosis markers such as Bax and cyclin-dependent kinase inhibitor 1A (CDKN1A) was decreased in podocytes derived from db/db mice treated with fasudil. The present study provides evidence that Rho-kinase plays a key role in podocyte apoptosis. Rho-kinase is an attractive therapeutic target for diabetic nephropathy.

Growth Factor Regulation in the Nephrogenic Zone of the Developing Kidney

New nephrons are induced by the interaction between mesenchymal progenitor cells and collecting duct tips, both of which are located at the outer edge of the kidney. This leading edge of active nephron induction is known as the nephrogenic zone. Cell populations found within this zone include collecting duct tips, cap mesenchyme cells, pretubular aggregates, nephrogenic zone interstitium, hemoendothelial progenitor cells, and macrophages. The close association of these dynamic progenitor cell compartments enables the intricate and synchronized patterning of the epithelial and the vascular components of the nephron. Understanding signaling interactions between the distinct progenitor cells of the nephrogenic zone are essential to determining the basis for new nephron formation, an important goal in regenerative medicine. A variety of technologies have been applied to define essential signaling pathways, including organ culture, mouse genetics, and primary cell culture. This chapter provides an overview of essential signaling pathways and discusses how these may be integrated.

Differential activation of CD95-mediated apoptosis related proteins in proximal and distal tubules during rat renal development

The CD95-mediated apoptotic pathway is the best characterized of the death receptor-mediated apoptotic pathways. The present study characterized localization and expression of proteins involved in CD95-mediated apoptosis during rat renal development. Kidneys were obtained from embryonic (E) 18 and 20-day-old fetuses and postnatal (P) 1-, 3-, 5-, 7-, 14-, and 21-day-old pups. Immunohistochemical characterization revealed that CD95, FasL and cleaved caspase-3 were strongly expressed in proximal tubules and weakly expressed in distal tubules, but that expression of caspase-8 in distal tubules was stronger than that in proximal tubules. Results from terminal deoxynucleotidyl transferase dUTP nick end labeling assays showed that levels of apoptosis in proximal tubules slowly increased after E18, while those of distal tubules slowly decreased after P5. Western blotting demonstrated that expression of CD95, FasL and FADD was very weak during embryonic development, but rapidly increased at P14. Expression of cleaved caspase-3 was maintained at high levels after P1, while caspase-8 expression gradually reached a peak at P7. Results from this study reveal that the CD95-mediated apoptotic pathway is a key driver of apoptosis in proximal tubules during late postnatal kidney development in rats and suggest that apoptosis in distal tubules is mediated by a different apoptotic pathway.

Partitioning-Defective 1a/b Depletion Impairs Glomerular and Proximal Tubule Development

The kidney is a highly polarized epithelial organ that develops from undifferentiated mesenchyme, although the mechanisms that regulate the development of renal epithelial polarity are incompletely understood. Partitioning-defective 1 (Par1) proteins have been implicated in cell polarity and epithelial morphogenesis; however, the role of these proteins in the developing kidney has not been established. Therefore, we studied the contribution of Par1a/b to renal epithelial development. We examined the renal phenotype of newborn compound mutant mice carrying only one allele of Par1a or Par1b. Loss of three out of four Par1a/b alleles resulted in severe renal hypoplasia, associated with impaired ureteric bud branching. Compared with kidneys of newborn control littermates, kidneys of newborn mutant mice exhibited dilated proximal tubules and immature glomeruli, and the renal proximal tubular epithelia lacked proper localization of adhesion complexes. Furthermore, Par1a/b mutants expressed low levels of renal Notch ligand Jag1, activated Notch2, and Notch effecter Hes1. Together, these data demonstrate that Par1a/b has a key role in glomerular and proximal tubule development, likely via modulation of Notch signaling.

Nephron Patterning: Lessons from Xenopus, Zebrafish, and Mouse Studies

The nephron is the basic structural and functional unit of the vertebrate kidney. To ensure kidney functions, the nephrons possess a highly segmental organization where each segment is specialized for the secretion and reabsorption of particular solutes. During embryogenesis, nephron progenitors undergo a mesenchymal-to-epithelial transition (MET) and acquire different segment-specific cell fates along the proximo-distal axis of the nephron. Even if the morphological changes occurring during nephrogenesis are characterized, the regulatory networks driving nephron segmentation are still poorly understood. Interestingly, several studies have shown that the pronephric nephrons in Xenopus and zebrafish are segmented in a similar fashion as the mouse metanephric nephrons. Here we review functional and molecular aspects of nephron segmentation with a particular interest on the signaling molecules and transcription factors recently implicated in kidney development in these three different vertebrate model organisms. A complete understanding of the mechanisms underlying nephrogenesis in different model organisms will provide novel insights on the etiology of several human renal diseases.

Atlas of Cellular Dynamics during Zebrafish Adult Kidney Regeneration

The zebrafish is a useful animal model to study the signaling pathways that orchestrate kidney regeneration, as its renal nephrons are simple, yet they maintain the biological complexity inherent to that of higher vertebrate organisms including mammals. Recent studies have suggested that administration of the aminoglycoside antibiotic gentamicin in zebrafish mimics human acute kidney injury (AKI) through the induction of nephron damage, but the timing and details of critical phenotypic events associated with the regeneration process, particularly in existing nephrons, have not been characterized. Here, we mapped the temporal progression of cellular and molecular changes that occur during renal epithelial regeneration of the proximal tubule in the adult zebrafish using a platform of histological and expression analysis techniques. This work establishes the timing of renal cell death after gentamicin injury, identifies proliferative compartments within the kidney, and documents gene expression changes associated with the regenerative response of proliferating cells. These data provide an important descriptive atlas that documents the series of events that ensue after damage in the zebrafish kidney, thus availing a valuable resource for the scientific community that can facilitate the implementation of zebrafish research to delineate the mechanisms that control renal regeneration.

New insights into glomerular parietal epithelial cell activation and its signaling pathways in glomerular diseases

The glomerular parietal epithelial cells (PECs) have aroused an increasing attention recently. The proliferation of PECs is the main feature of crescentic glomerulonephritis; besides that, in the past decade, PEC activation has been identified in several types of noninflammatory glomerulonephropathies, such as focal segmental glomerulosclerosis, diabetic glomerulopathy, and membranous nephropathy. The pathogenesis of PEC activation is poorly understood; however, a few studies delicately elucidate the potential mechanisms and signaling pathways implicated in these processes. In this review we will focus on the latest observations and concepts about PEC activation in glomerular diseases and the newest identified signaling pathways in PEC activation.

Glomerular development--shaping the multi-cellular filtration unit

The glomerulus represents a highly structured filtration unit, composed of glomerular endothelial cells, mesangial cells, podocytes and parietal epithelial cells. During glomerulogenesis an intricate network of signaling pathways involving transcription factors, secreted factors and cell-cell communication is required to guarantee accurate evolvement of a functional, complex 3-dimensional glomerular architecture. Here, we want to provide an overview on the critical steps and relevant signaling cascades of glomerular development.

Renal tubular Notch signaling triggers a prosenescent state after acute kidney injury

The aging kidney has a diminished regenerative potential and an increased tendency to develop tubular atrophy and fibrosis after acute injury. In this study, we found that activation of tubular epithelial Notch1 signaling was prolonged in the aging kidney after ischemia/reperfusion (IR) damage. To analyze the consequences of sustained Notch activation, we generated mice with conditional inducible expression of Notch1 intracellular domain (NICD) in proximal tubules. NICD kidneys were analyzed 1 and 4 wk after renal IR. Conditional NICD expression was associated with aggravated tubular damage, a fibrotic phenotype, and the expression of cellular senescence markers p21 and p16 INK4a. In wild-type mice pharmacological inhibition of Notch using the γ-secretase inhibitor N-[ N-(3,5-difluorophenacetyl)-l-alanyl]- S-phenylglycine t-butyl ester (DAPT) improved tubulo-interstitial damage and antagonized the prosenescent pathway activation after IR. In vitro, activation of Notch signaling with delta-like-ligand-4 caused prosenescent changes in tubular cells while inhibition with DAPT attenuated these changes. In conclusion, our data suggest that sustained epithelial Notch activation after IR might contribute to the inferior outcome of old kidneys after injury. Sustained epithelial activation of Notch is associated with a prosenescent phenotype and maladaptive repair.

The integrin β1 subunit regulates paracellular permeability of kidney proximal tubule cells

Epithelial cells lining the gastrointestinal tract and kidney have different abilities to facilitate paracellular and transcellular transport of water and solutes. In the kidney, the proximal tubule allows both transcellular and paracellular transport, while the collecting duct primarily facilitates transcellular transport. The claudins and E-cadherin are major structural and functional components regulating paracellular transport. In this study we present the novel finding that the transmembrane matrix receptors, integrins, play a role in regulating paracellular transport of renal proximal tubule cells. Deleting the integrin β1 subunit in these cells converts them from a "loose" epithelium, characterized by low expression of E-cadherin and claudin-7 and high expression of claudin-2, to a "tight" epithelium with increased E-cadherin and claudin-7 expression and decreased claudin-2 expression. This effect is mediated by the integrin β1 cytoplasmic tail and does not entail β1 heterodimerization with an α-subunit or its localization to the cell surface. In addition, we demonstrate that deleting the β1 subunit in the proximal tubule of the kidney results in a major urine-concentrating defect. Thus, the integrin β1 tail plays a key role in regulating the composition and function of tight and adherens junctions that define paracellular transport properties of terminally differentiated renal proximal tubule epithelial cells.

Renal involvement and the role of Notch signalling in Alagille syndrome

Alagille syndrome is an autosomal dominant disorder with variable multisystem organ involvement that is caused by mutations in one of two genes in the Notch signalling pathway, JAG1 or NOTCH2. Alagille syndrome is characterized by bile duct paucity, along with at least three of the following features: cholestasis, cardiac defects, skeletal abnormalities, ocular abnormalities and characteristic facies. However, the clinical features of Alagille syndrome are highly variable, and children or adults may also present with predominantly renal findings and little or no hepatic involvement. Renal involvement occurs in 40% of JAG1-mutation-positive individuals. Renal insufficiency is common and has been specifically reported in children with Alagille syndrome who have end-stage liver disease. The role of NOTCH2 and JAG1 in formation of proximal nephron structures and podocytes might explain the observed phenotypes of renal dysplasia and proteinuria in patients with Alagille syndrome, and renal tubular acidosis may be the result of JAG1 expression in the collecting ducts. Renal vascular hypertension in patients with Alagille syndrome is explained by the widespread vasculopathy and the role of Notch signalling in vascular development. Increased awareness of Alagille syndrome amongst nephrologists may lead to more diagnoses of Alagille syndrome in patients with apparently isolated renal disease.

HNF1B controls proximal-intermediate nephron segment identity in vertebrates by regulating Notch signalling components and Irx1/2

The nephron is a highly specialised segmented structure that provides essential filtration and resorption renal functions. It arises by formation of a polarised renal vesicle that differentiates into a comma-shaped body and then a regionalised S-shaped body (SSB), with the main prospective segments mapped to discrete domains. The regulatory circuits involved in initial nephron patterning are poorly understood. We report here that HNF1B, a transcription factor known to be involved in ureteric bud branching and initiation of nephrogenesis, has an additional role in segment fate acquisition. Hnf1b conditional inactivation in murine nephron progenitors results in rudimentary nephrons comprising a glomerulus connected to the collecting system by a short tubule displaying distal fates. Renal vesicles develop and polarise normally but fail to progress to correctly patterned SSBs. Major defects are evident at late SSBs, with altered morphology, reduction of a proximo-medial subdomain and increased apoptosis. This is preceded by strong downregulation of the Notch pathway components Lfng, Dll1 and Jag1 and the Irx1/2 factors, which are potential regulators of proximal and Henle's loop segment fates. Moreover, HNF1B is recruited to the regulatory sequences of most of these genes. Overexpression of a HNF1B dominant-negative construct in Xenopus embryos causes downregulation specifically of proximal and intermediate pronephric segment markers. These results show that HNF1B is required for the acquisition of a proximo-intermediate segment fate in vertebrates, thus uncovering a previously unappreciated function of a novel SSB subcompartment in global nephron segmentation and further differentiation.

Inhibition of Notch pathway attenuates the progression of human immunodeficiency virus-associated nephropathy

The Notch pathway is an evolutionarily conserved signaling cascade that is critical in kidney development and has also been shown to play a pathogenetic role in a variety of kidney diseases. We have previously shown that the Notch signaling pathway is activated in human immunodeficiency virus-associated nephropathy (HIVAN) as well as in a rat model of the disease. In this study, we examined Notch signaling in the well established Tg26 mouse model of HIVAN. Notch signaling components were distinctly upregulated in the kidneys of these mice as well as in immortalized podocytes derived from these mice. Notch1 and Notch4 were upregulated in the Tg26 glomeruli, and Notch4 was also expressed in tubules. Notch ligands Jagged1, Jagged2, Delta-like1, and Delta-like 4 were all upregulated in the tubules of Tg26 mice, but glomeruli showed minimal expression of Notch ligands. To examine a potential pathogenetic role for Notch in HIVAN, Tg26 mice were treated with GSIXX, a gamma secretase inhibitor that blocks Notch signaling. Strikingly, GSIXX treatment resulted in significant improvement in both histological kidney injury scores and renal function. GSIXX-treated Tg26 mice also showed diminished podocyte proliferation and dedifferentiation, cellular hallmarks of the disease. Moreover, GSIXX blocked podocyte proliferation in vitro induced by HIV proteins Nef and Tat. These studies suggest that Notch signaling can promote HIVAN progression and that Notch inhibition may be a viable treatment strategy for HIVAN.

Membrane trafficking in podocyte health and disease

Podocytes are highly specialized epithelial cells localized in the kidney glomerulus. The distinct cell signaling events and unique cytoskeletal architecture tailor podocytes to withstand changes in hydrostatic pressure during glomerular filtration. Alteration of glomerular filtration leads to kidney disease and frequently manifests with proteinuria. It has been increasingly recognized that cell signaling and cytoskeletal dynamics are coupled more tightly to membrane trafficking than previously thought. Membrane trafficking coordinates the cross-talk between protein networks and signaling cascades in a spatially and temporally organized fashion and may be viewed as a communication highway between the cell exterior and interior. Membrane trafficking involves transport of cargo from the plasma membrane to the cell interior (i.e., endocytosis) followed by cargo trafficking to lysosomes for degradation or to the plasma membrane for recycling. Yet, recent studies indicate that the conventional classification does not fully reflect the complex and versatile nature of membrane trafficking. While the increasing complexity of elaborate protein scaffolds and signaling cascades is being recognized in podocytes, the role of membrane trafficking is less well understood. This review will focus on the role of membrane trafficking in podocyte health and disease.

Notch signaling in diabetic nephropathy

Notch signaling is an evolutionarily conserved cell-cell signaling system that controls the fate of cells during development. In this review, we will summarize the literature that notch signaling during development controls nephron number and segmentation and therefore could influence kidney disease susceptibility. We will also review the evidence that Notch is reactivated in adult-onset diabetic kidney disease where it promotes the development of nephropathy including glomerulopathy, tubulointerstitial fibrosis and possibly arteriopathy and inflammation. Finally, we will review the evidence that blockade of pathogenic Notch signaling alters the natural history of diabetic nephropathy and thus could represent a novel therapeutic approach to the management of diabetic kidney disease.

Role of mitofusin 2 in the renal stress response

The role of mitofusin 2 (MFN2), a key regulator of mitochondrial morphology and function in the renal stress response is unknown. To assess its role, the MFN2 floxed gene was conditionally deleted in the kidney of mice (MFN2 cKO) by Pax2 promoter driven Cre expression (Pax2Cre). MFN2 cKO caused severe mitochondrial fragmentation in renal epithelial cells that are critical for normal kidney tubular function. However, despite a small (20%) decrease in nephron number, newborn cKO pups had organ or tubular function that did not differ from littermate Cre-negative pups. MFN2 deficiency in proximal tubule epithelial cells in primary culture induced mitochondrial fragmentation but did not significantly alter ATP turnover, maximal mitochondrial oxidative reserve capacity, or the low level of oxygen consumption during cyanide exposure. MFN2 deficiency also did not increase apoptosis of tubule epithelial cells under non-stress conditions. In contrast, metabolic stress caused by ATP depletion exacerbated mitochondrial outer membrane injury and increased apoptosis by 80% in MFN2 deficient vs. control cells. Despite similar stress-induced Bax 6A7 epitope exposure in MFN2 deficient and control cells, MFN2 deficiency significantly increased mitochondrial Bax accumulation and was associated with greater release of both apoptosis inducing factor and cytochrome c. In conclusion, MFN2 deficiency in the kidney causes mitochondrial fragmentation but does not affect kidney or tubular function during development or under non-stress conditions. However, MFN2 deficiency exacerbates renal epithelial cell injury by promoting Bax-mediated mitochondrial outer membrane injury and apoptosis.

Notch in the kidney: development and disease

Notch signalling is a highly conserved cell-cell communication mechanism that regulates development, tissue homeostasis, and repair. Within the kidney, Notch has an important function in orchestrating kidney development. Recent studies indicate that Notch plays a key role in establishing proximal epithelial fate during nephron segmentation as well as the differentiation of principal cells in the renal collecting system. Notch signalling is markedly reduced in the adult kidney; however, increased Notch signalling has been noted in both acute and chronic kidney injury. Increased glomerular epithelial Notch signalling has been associated with albuminuria and glomerulosclerosis, while tubular epithelial Notch activation caused fibrosis development most likely inducing an improper epithelial repair pathway. Recent studies thereby indicate that Notch is a key regulator of kidney development, repair, and injury.