The Notch pathway in podocytes plays a role in the development of glomerular disease

Glomerular proteinuria: a complex interplay between unique players

Protein leak in the urine is a harbinger of disruption of the glomerular filtration barrier. It also correlates with disease progression and development of ESRD. At present, therapies are aimed at decreasing proteinuria to decrease further damage to the filter and as a marker of remission. Understanding the mechanism of molecular events that lead to protein leak is vital to developing new therapeutic interventions. There has been tremendous progress over the last decade in identifying gene defects which result in hereditary proteinuric defects. This has led to identifying pathways by which these genes regulate the structure and function of the components of the filtration barrier, namely the podocytes, mesangial cells, endothelial cells, and the basement membrane. Using gene knockout mouse models, a role of tubular cells in regulating proteinuria is also emerging. In this review, we have attempted to present some of the prevailing understanding of the underlying mechanisms and physiology of proteinuria.

Podocyte biology for the bedside

The explosion of podocyte biology during the last decade has radically altered our views on the pathophysiologic process of proteinuria, glomerular disease, and progressive kidney disease. In this review, we highlight some of these landmark findings, but focus on recent advances in the field and implications for translating this biology into therapy for podocyte diseases.

Role of mTOR in podocyte function and diabetic nephropathy in humans and mice

Chronic glomerular diseases, associated with renal failure and cardiovascular morbidity, represent a major health issue. However, they remain poorly understood. Here we have reported that tightly controlled mTOR activity was crucial to maintaining glomerular podocyte function, while dysregulation of mTOR facilitated glomerular diseases. Genetic deletion of mTOR complex 1 (mTORC1) in mouse podocytes induced proteinuria and progressive glomerulosclerosis. Furthermore, simultaneous deletion of both mTORC1 and mTORC2 from mouse podocytes aggravated the glomerular lesions, revealing the importance of both mTOR complexes for podocyte homeostasis. In contrast, increased mTOR activity accompanied human diabetic nephropathy, characterized by early glomerular hypertrophy and hyperfiltration. Curtailing mTORC1 signaling in mice by genetically reducing mTORC1 copy number in podocytes prevented glomerulosclerosis and significantly ameliorated the progression of glomerular disease in diabetic nephropathy. These results demonstrate the requirement for tightly balanced mTOR activity in podocyte homeostasis and suggest that mTOR inhibition can protect podocytes and prevent progressive diabetic nephropathy.

mTORC1 activation in podocytes is a critical step in the development of diabetic nephropathy in mice

Diabetic nephropathy (DN) is among the most lethal complications that occur in type 1 and type 2 diabetics. Podocyte dysfunction is postulated to be a critical event associated with proteinuria and glomerulosclerosis in glomerular diseases including DN. However, molecular mechanisms of podocyte dysfunction in the development of DN are not well understood. Here we have shown that activity of mTOR complex 1 (mTORC1), a kinase that senses nutrient availability, was enhanced in the podocytes of diabetic animals. Further, podocyte-specific mTORC1 activation induced by ablation of an upstream negative regulator (PcKOTsc1) recapitulated many DN features, including podocyte loss, glomerular basement membrane thickening, mesangial expansion, and proteinuria in nondiabetic young and adult mice. Abnormal mTORC1 activation caused mislocalization of slit diaphragm proteins and induced an epithelial-mesenchymal transition-like phenotypic switch with enhanced ER stress in podocytes. Conversely, reduction of ER stress with a chemical chaperone significantly protected against both the podocyte phenotypic switch and podocyte loss in PcKOTsc1 mice. Finally, genetic reduction of podocyte-specific mTORC1 in diabetic animals suppressed the development of DN. These results indicate that mTORC1 activation in podocytes is a critical event in inducing DN and suggest that reduction of podocyte mTORC1 activity is a potential therapeutic strategy to prevent DN.

A phase 1, single-dose study of fresolimumab, an anti-TGF-β antibody, in treatment-resistant primary focal segmental glomerulosclerosis

Primary focal segmental glomerulosclerosis (FSGS) is a disease with poor prognosis and high unmet therapeutic need. Here, we evaluated the safety and pharmacokinetics of single-dose infusions of fresolimumab, a human monoclonal antibody that inactivates all forms of transforming growth factor-β (TGF-β), in a phase I open-label, dose-ranging study. Patients with biopsy-confirmed, treatment-resistant, primary FSGS with a minimum estimated glomerular filtration rate (eGFR) of 25  ml/min per 1.73  m(2), and a urine protein to creatinine ratio over 1.8  mg/mg were eligible. All 16 patients completed the study in which each received one of four single-dose levels of fresolimumab (up to 4  mg/kg) and was followed for 112 days. Fresolimumab was well tolerated with pustular rash the only adverse event in two patients. One patient was diagnosed with a histologically confirmed primitive neuroectodermal tumor 2 years after fresolimumab treatment. Consistent with treatment-resistant FSGS, there was a slight decline in eGFR (median decline baseline to final of 5.85 ml/min per 1.73  m(2)). Proteinuria fluctuated during the study with the median decline from baseline to final in urine protein to creatinine ratio of 1.2  mg/mg with all three Black patients having a mean decline of 3.6  mg/mg. The half-life of fresolimumab was ∼14 days, and the mean dose-normalized Cmax and area under the curve were independent of dose. Thus, single-dose fresolimumab was well tolerated in patients with primary resistant FSGS. Additional evaluation in a larger dose-ranging study is necessary.

Wnt-signaling pathways in progressive renal fibrosis

INTRODUCTION

The prevention and potential reversal of interstitial fibrosis is a central strategy for the treatment of progressive renal disease. This strategy requires a better understanding of the underlying pathophysiologic processes involved in progressive renal fibrosis.

AREAS COVERED

The developmental processes in which Wnt (combination of 'wingless' and 'INT')/frizzled signaling is involved is discussed in this review, including cell fate determination, cell polarity, tissue patterning and control of cell proliferation. These pathways are also active in the adult where they play key roles in the maintenance of tissue homeostasis, wound repair and chronic tissue damage.

EXPERT OPINION

Wnt biology helps to control cell polarity, moderates cell proliferation and underlies other processes linked to renal homeostasis. Reactivation and dysregulation of the Wnt pathways underlie chronic fibrosis and progressive renal failure. Wnt signaling is, however, context-dependent: the pathways are complex and undergo many levels of cross-talk with other regulatory systems and regulatory pathways. On one hand, this may help to explain the positive effects of Wnt-signaling blockades seen in some animal models of chronic renal damage and, on the other, this suggests that it may be difficult to predict how modifications of the Wnt pathway may influence a process.

Inhibition of the protein kinase MK-2 protects podocytes from nephrotic syndrome-related injury

While mitogen-activated protein kinase (MAPK) activation has been implicated in the pathogenesis of various glomerular diseases, including nephrotic syndrome (NS), its specific role in podocyte injury is not known. We hypothesized that MK-2, a downstream substrate of p38 MAPK, mediates the adverse effects of this pathway and that inhibition of MK-2 would protect podocytes from NS-related injury. Using cultured podocytes, we analyzed 1) the roles of MK-2 and p38 MAPK in puromycin aminonucleoside (PAN)-induced podocyte injury; 2) the ability of specific MK-2 and p38 MAPK inhibitors to protect podocytes against injury; 3) the role of serum albumin, known to induce podocyte injury, in activating p38 MAPK/MK-2 signaling; and 4) the role of p38 MAPK/MK-2 signaling in the expression of Cox-2, an enzyme associated with podocyte injury. Treatment with protein kinase inhibitors specific for both MK-2 (C23, a pyrrolopyridine-type compound) or p38 MAPK (SB203580) reduced PAN-induced podocyte injury and actin cytoskeletal disruption. Both inhibitors reduced baseline podocyte p38 MAPK/MK-2 signaling, as measured by the degree of phosphorylation of HSPB1, a downstream substrate of MK-2, but exhibited disparate effects on upstream signaling. Serum albumin activated p38 MAPK/MK-2 signaling and induced Cox-2 expression, and these responses were blocked by both inhibitors. Given the critical importance of podocyte injury to both NS and other progressive glomerular diseases, these data suggest an important role for p38 MAPK/MK-2 signaling in podocyte injury and identify MK-2 inhibition as a promising potential therapeutic strategy to protect podocytes in various glomerular diseases.

Wnt/β-catenin pathway in podocytes integrates cell adhesion, differentiation, and survival

Diabetic kidney disease (DKD) is the single most common cause of albuminuria and end-stage kidney disease in the United States. We found increased expression of Wnt/β-catenin (Ctnnb1) pathway transcripts and proteins in glomeruli and podocytes of patients and mouse models of DKD. Mice with podocyte-specific expression of stabilized Ctnnb1 exhibited basement membrane abnormalities, albuminuria, and increased susceptibility to glomerular injury. Mice with podocyte-specific deletion of Ctnnb1 or podocyte-specific expression of the canonical Wnt inhibitor Dickkopf-related protein 1 (Dkk1) also showed increased susceptibility to DKD. Podocytes with stabilized Ctnnb1 were less motile and less adhesive to different matrices. Deletion of Ctnnb1 in cultured podocytes increased the expression of podocyte differentiation markers and enhanced cell motility; however, these cells were more susceptible to apoptosis. These results indicate that Wnt/Ctnnb1 signaling in podocytes plays a critical role in integrating cell adhesion, motility, cell death, and differentiation. Balanced Ctnnb1 expression is critical for glomerular filtration barrier maintenance.

Target inhibition in antiangiogenic therapy a wide spectrum of selectivity and specificity

Recent studies have revealed a previously unsuspected degree of vascular specialization within the host tissue and a tumor's microenvironment. The "vascular zip code" has been used to describe the unique expression of cell-surface molecules found in each vascular bed. Characterization of tumor blood vessels includes selective overexpression of a heterogenous group of proteins such as proteases, integrins, growth factor receptors, and proteoglycans. The process of angiogenesis consists of a "true cytokine storm," requiring many molecular events and biological steps. Antiangiogenic drugs may target a single critical kinase pathway or may interact with several nonspecific molecular targets via a process termed extended spectrum kinase inhibition. The latter strategy may lead to an absence of selectivity and specificity and may result in enhanced toxicities. In this review, we discuss recent developments in the pathogenesis of commonly observed adverse events and summarize new strategies that may ultimately improve efficacy and limit toxicity.

The role of metabolic and haemodynamic factors in podocyte injury in diabetes

Podocyte loss is a common feature in human diabetes as well as in experimental diabetes in rodents. Almost all components of the diabetic milieu lead to serious podocyte stress, driving the cells towards cell cycle arrest and hypertrophy, detachment and apoptosis. Common pathway components induced by high glucose and advanced glycation end-products are reactive oxygen species, cyclin-dependent kinases (p27(Kip1)) and transforming growth factor-beta. In addition, mechanical stresses by stretch or shear forces, insulin deficiency or insulin resistance are independent components resulting in podocyte apoptosis and detachment. In this review, we discuss the common pathways leading to podocyte death as well as novel pathways and concepts of podocyte dedifferentiation and detachment that influence the progression of diabetic glomerulopathy.

Effects of chlorpyrifos exposure on kidney Notch2-Jagged1 pathway of early prenatal embryo

AIMS

To evaluate the effects of this insecticide on the embryonic development of kidney and to assess the important role of Notch2-Jagged1 pathway in this duration.

METHODS AND RESULTS

Chlorpyrifos (CPF) 5 mg/kg/d were administrated on gestation 7.5-11.5 day by subcutaneous injection. On gestation 16.5 day, the normal embryo kidney developed through S shape duration to the original kidney, which had the nephrons and could start to secret the urine. But for the CPF-treated mice, the embryo kidney developed much more slowly, they did not show the S shape and the nephrons. The Notch2-Jagged1 pathway should be expressed stronger in the normal embryo kidney on gestation 16.5 day, but for the CPF-treated mice we found the obvious weak pathway staining.

CONCLUSIONS

CPF broke the Notch2-Jagged1 pathway during the embryo kidney development, and the Notch2-Jagged1 pathway plays an important role in the S shape to original kidney formation duration.

Tumor-derived JAGGED1 promotes osteolytic bone metastasis of breast cancer by engaging notch signaling in bone cells

Despite evidence supporting an oncogenic role in breast cancer, the Notch pathway's contribution to metastasis remains unknown. Here, we report that the Notch ligand Jagged1 is a clinically and functionally important mediator of bone metastasis by activating the Notch pathway in bone cells. Jagged1 promotes tumor growth by stimulating IL-6 release from osteoblasts and directly activates osteoclast differentiation. Furthermore, Jagged1 is a potent downstream mediator of the bone metastasis cytokine TGFβ that is released during bone destruction. Importantly, γ-secretase inhibitor treatment reduces Jagged1-mediated bone metastasis by disrupting the Notch pathway in stromal bone cells. These findings elucidate a stroma-dependent mechanism for Notch signaling in breast cancer and provide rationale for using γ-secretase inhibitors for the treatment of bone metastasis.

Association analysis of Notch pathway signalling genes in diabetic nephropathy

AIMS/HYPOTHESIS

Several studies have provided compelling evidence implicating the Notch signalling pathway in diabetic nephropathy. Co-regulation of Notch signalling pathway genes with GREM1 has recently been demonstrated and several genes involved in the Notch pathway are differentially expressed in kidney biopsies from individuals with diabetic nephropathy. We assessed single-nucleotide polymorphisms (SNPs; n = 42) in four of these key genes (JAG1, HES1, NOTCH3 and ADAM10) for association with diabetic nephropathy using a case-control design.

METHODS

Tag SNPs and potentially functional SNPs were genotyped using Sequenom or Taqman technologies in a total of 1371 individuals with type 1 diabetes (668 patients with nephropathy and 703 controls without nephropathy). Patients and controls were white and recruited from the UK and Ireland. Association analyses were performed using PLINK (http://pngu.mgh.harvard.edu/∼purcell/plink/) and haplotype frequencies in patients and controls were compared. Adjustment for multiple testing was performed by permutation testing.

RESULTS

In analyses stratified by centre, we identified six SNPs, rs8708 and rs11699674 (JAG1), rs10423702 and rs1548555 (NOTCH3), rs2054096 and rs8027998 (ADAM10) as being associated with diabetic nephropathy before, but not after, adjustment for multiple testing. Haplotype and subgroup analysis according to duration of diabetes also failed to find an association with diabetic nephropathy.

CONCLUSIONS/INTERPRETATION

Our results suggest that common variants in JAG1, HES1, NOTCH3 and ADAM10 are not strongly associated with diabetic nephropathy in type 1 diabetes among white individuals. Our findings, however, cannot entirely exclude these genes from involvement in the pathogenesis of diabetic nephropathy.

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

To determine which nephron segments require Notch signals for development, we conditionally deleted Rbpj, a transcription factor required for canonical Notch signaling, in nephrogenic progenitors (NPs) of the metanephric mesenchyme. The retinoic acid receptor-β2 (Rarb2) promoter efficiently directed Cre-recombinase (Cre) activity to these progenitors. Conditional knockout of Rbpj in mice (Rarb2Cre(+)/Rbpj (f/-)) caused severe renal hypoplasia, as indicated by a 70-95% reduction in nephron number and the development of tubular cysts. To track the fate of NPs following Rarb2Cre expression, we labeled them with membrane-associated enhanced green fluorescent protein (GFP). In TomatoGFP(+)/Rarb2Cre(+) control mice, NPs differentiated into epithelia of all nephron segments, except into collecting ducts. In TomatoGFP(+)/Rarb2Cre(+)/Rbpj (f/-) conditional knockout mice, NPs developed into podocytes or distal tubular epithelia, indicating that canonical Notch signals were not required for mesenchymal-to-epithelial transition or for the specification of these nephron segments. Conversely, the few proximal tubules and associated cysts that developed in these mice were derived from the 5-10% of NPs that had failed to express Cre and, therefore, had intact Notch signaling. Thus, our fate mapping studies establish that the profound effect of Notch signaling on nephrogenesis is due to the specification of proximal but not distal tubules or podocytes.

The story of Notch and chronic kidney disease

PURPOSE OF REVIEW

The Notch pathway is an evolutionary conserved cell-cell communication mechanism that plays a key role in kidney development. Here, we will discuss a number of recently published papers describing the role of Notch signaling in kidney development, homeostasis, injury and repair.

RECENT FINDINGS

Recent gene expression studies identified regulation of the Notch pathway in patients with chronic kidney disease (CKD). Mechanistic experiments performed using transgenic and knock-out mouse models indicate that Notch plays an important functional role in the development of proteinuria and renal fibrosis. Inhibition of the Notch pathway ameliorated diabetic kidney disease, nephrotic syndrome and fibrosis in different rodent models.

SUMMARY

An increasing amount of evidence suggests that Notch plays a role in CKD development. Understanding the role of Notch signaling in the kidney can aid in the development of new therapeutics for CKD.

Notch activation differentially regulates renal progenitors proliferation and differentiation toward the podocyte lineage in glomerular disorders

Glomerular diseases account for 90% of end-stage kidney disease. Podocyte loss is a common determining factor for the progression toward glomerulosclerosis. Mature podocytes cannot proliferate, but recent evidence suggests that they can be replaced by renal progenitors localized within the Bowman's capsule. Here, we demonstrate that Notch activation in human renal progenitors stimulates entry into the S-phase of the cell cycle and cell division, whereas its downregulation is required for differentiation toward the podocyte lineage. Indeed, a persistent activation of the Notch pathway induced podocytes to cross the G(2)/M checkpoint, resulting in cytoskeleton disruption and death by mitotic catastrophe. Notch expression was virtually absent in the glomeruli of healthy adult kidneys, while a strong upregulation was observed in renal progenitors and podocytes in patients affected by glomerular disorders. Accordingly, inhibition of the Notch pathway in mouse models of focal segmental glomerulosclerosis ameliorated proteinuria and reduced podocyte loss during the initial phases of glomerular injury, while inducing reduction of progenitor proliferation during the regenerative phases of glomerular injury with worsening of proteinuria and glomerulosclerosis. Taken altogether, these results suggest that the severity of glomerular disorders depends on the Notch-regulated balance between podocyte death and regeneration provided by renal progenitors.

Diabetic nephropathy is accelerated by farnesoid X receptor deficiency and inhibited by farnesoid X receptor activation in a type 1 diabetes model

OBJECTIVE

The pathogenesis of diabetic nephropathy is complex and involves activation of multiple pathways leading to kidney damage. An important role for altered lipid metabolism via sterol regulatory element binding proteins (SREBPs) has been recently recognized in diabetic kidney disease. Our previous studies have shown that the farnesoid X receptor (FXR), a bile acid-activated nuclear hormone receptor, modulates renal SREBP-1 expression. The purpose of the present study was then to determine if FXR deficiency accelerates type 1 diabetic nephropathy in part by further stimulation of SREBPs and related pathways, and conversely, if a selective FXR agonist can prevent the development of type 1 diabetic nephropathy.

RESEARCH DESIGN AND METHODS

Insulin deficiency and hyperglycemia were induced with streptozotocin (STZ) in C57BL/6 FXR KO mice. Progress of renal injury was compared with nephropathy-resistant wild-type C57BL/6 mice given STZ. DBA/2J mice with STZ-induced hyperglycemia were treated with the selective FXR agonist INT-747 for 12 weeks. To accelerate disease progression, all mice were placed on the Western diet after hyperglycemia development.

RESULTS

The present study demonstrates accelerated renal injury in diabetic FXR KO mice. In contrast, treatment with the FXR agonist INT-747 improves renal injury by decreasing proteinuria, glomerulosclerosis, and tubulointerstitial fibrosis, and modulating renal lipid metabolism, macrophage infiltration, and renal expression of SREBPs, profibrotic growth factors, and oxidative stress enzymes in the diabetic DBA/2J strain.

CONCLUSIONS

Our findings indicate a critical role for FXR in the development of diabetic nephropathy and show that FXR activation prevents nephropathy in type 1 diabetes.

Epithelial Notch signaling regulates interstitial fibrosis development in the kidneys of mice and humans

Chronic kidney disease is a leading cause of death in the United States. Tubulointerstitial fibrosis (TIF) is considered the final common pathway leading to end-stage renal disease (ESRD). Here, we used pharmacologic, genetic, in vivo, and in vitro experiments to show that activation of the Notch pathway in tubular epithelial cells (TECs) in patients and in mouse models of TIF plays a role in TIF development. Expression of Notch in renal TECs was found to be both necessary and sufficient for TIF development. Genetic deletion of the Notch pathway in TECs reduced renal fibrosis. Consistent with this, TEC-specific expression of active Notch1 caused rapid development of TIF. Pharmacologic inhibition of Notch activation using a γ-secretase inhibitor ameliorated TIF. In summary, our experiments establish that epithelial injury and Notch signaling play key roles in fibrosis development and indicate that Notch blockade may be a therapeutic strategy to reduce fibrosis and ESRD development.

Podocyte injury and albuminuria in mice with podocyte-specific overexpression of the Ste20-like kinase, SLK

SLK expression and activity are increased during kidney development and recovery from renal ischemia-reperfusion injury. In cultured cells, SLK promotes F-actin destabilization as well as apoptosis, partially via the p38 kinase pathway. To better understand the effects of SLK in vivo, a transgenic mouse model was developed where SLK was expressed in a podocyte-specific manner using the mouse nephrin promoter. Offspring of four founder mice carried the SLK transgene. Among male transgenic mice, 66% developed albuminuria at approximately 3 months of age, and the albuminuric mice originated from three of four founders. Overall, the male transgenic mice demonstrated about fivefold greater urinary albumin/creatinine compared with male non-transgenic mice. Transgenic and non-transgenic female mice did not develop albuminuria, suggesting that females were less susceptible to glomerular filtration barrier damage than their male counterparts. In transgenic mice, electron microscopy revealed striking podocyte injury, including poorly formed or effaced foot processes, and edematous and vacuolated cell bodies. By immunoblotting, nephrin expression was decreased in glomeruli of the albuminuric transgenic mice. Activation-specific phosphorylation of p38 was increased in transgenic mice compared with non-transgenic animals. Glomeruli of SLK transgenic mice showed around 30% fewer podocytes, and a reduction in F-actin compared with control glomeruli. Thus, podocyte SLK overexpression in vivo results in injury and podocyte loss, consistent with the effects of SLK in cultured cells.

Jagged/Notch signalling is required for a subset of TGFβ1 responses in human kidney epithelial cells

The Jagged/Notch pathway has been implicated in TGFβ1 responses in epithelial cells in diabetic nephropathy and other fibrotic conditions in vivo. Here, we identify that Jagged/Notch signalling is required for a subset of TGFβ1-stimulated gene responses in human kidney epithelial cells in vitro. TGFβ1 treatment of HK-2 and RPTEC cells for 24h increased Jagged1 (a Notch ligand) and Hes1 (a Notch target) mRNA. This response was inhibited by co-incubation with Compound E, an inhibitor of γ-secretase (GSI), an enzyme required for Notch receptor cleavage and transcription regulation. In both cell types, TGFβ1-responsive genes associated with epithelial-mesenchymal transition such as E-cadherin and vimentin were also affected by γ-secretase inhibition, but other TGFβ1 targets such as connective tissue growth factor (CTGF) and thrombospondin-1 (THBS1) were not. TGFβ1-induced changes in Jagged1 expression preceded EMT-associated gene changes, and co-incubation with GSI altered TGFβ1-induced changes in cell shape and cytoskeleton. Transfection of cells with the activated, cleaved form of Notch (NICD) triggered decreased expression of E-cadherin in the absence of TGFβ1, but did not affect α-smooth muscle actin expression, suggesting differential requirements for Notch signalling within the TGFβ1-responsive gene subset. Increased Jagged1 expression upon TGFβ1 exposure required Smad3 signalling, and was also regulated by PI3K and ERK. These data suggest that Jagged/Notch signalling is required for a subset of TGFβ1-responsive genes, and that complex signalling pathways are involved in the crosstalk between TGFβ1 and Notch cascades in kidney epithelia.

Activation of Notch signaling pathway in HIV-associated nephropathy

OBJECTIVE

HIV-associated nephropathy (HIVAN) is characterized by the development of glomerulosclerosis and is associated with glomerular epithelial cell proliferation. It has recently been shown that activation of the Notch signaling pathway in podocytes results in glomerulosclerosis and podocyte proliferation. To determine whether Notch signaling is involved in renal disorder associated with HIVAN, we evaluated the expression of Notch receptors in HIVAN.

DESIGN

We evaluated the expression of the Notch signaling pathway using an HIV-transgenic (HIV-Tg) rat model of HIVAN, and biopsy samples from HIVAN and normal controls.

METHODS

Paraffin sections and kidney lysates were used for immunohistochemistry, immunofluorescence and western blot analysis.

RESULTS

A collapsing variant of glomerulosclerosis and focal segmental sclerosis was observed in HIV-Tg rats. Glomeruli of HIV-Tg rats demonstrated activation of Notch1 and Notch4, as determined by the presence of the intracellular domains. In addition, we observed increased expression of the Notch target protein, hairy enhancer of split homolog-1 in glomeruli of these animals. The expression of the Groucho homolog transducin-like enhancer protein 4, a Notch effector protein, and the homeodomain protein cut homeobox 1 were also significantly increased in glomeruli of HIV-Tg rats, and this was associated with decreased expression of the cyclin kinase inhibitor p27. Intriguingly, renal biopsy samples from HIVAN patients also showed upregulation of cleaved Notch1 and Notch4 in the glomeruli compared with the expression in normal kidneys.

CONCLUSION

Our results demonstrate activation of Notch signaling pathway in HIVAN, thereby underscoring its role in disease pathogenesis.

A Notch ligand, Delta-like 1 functions as an adhesion molecule for mast cells

Mast cells (MCs) accumulate in chronic inflammatory sites; however, it is not clear which adhesion molecules are involved in this process. Recently, the expression of Notch ligands was reported to be upregulated in inflammatory sites. Although Notch receptors are known as signaling molecules that can activate integrins, their contributions to the adhesion of MCs have not been studied. In this study, we demonstrated that mouse MCs efficiently adhered to stromal cells forced to express a Notch ligand, Delta-like 1 (Dll1). Surprisingly, the adhesion was a consequence of direct cell-cell interaction between MCs and Dll1-expressing stromal cells rather than activation of downstream effectors of Notch receptor(s)-Dll1. The adhesion of MCs to Dll1-expressing stromal cells remained even when the cell metabolism was arrested. The recognition was blocked only by inhibition of Notch receptor(s)-Dll1 interaction by addition of soluble DLL1, or mAbs against Dll1 or Notch2. Taken together, these results indicate that Notch receptor(s) and Dll1 directly promote the adhesion of MCs to stromal cells by acting as adhesion molecules. This appreciation that Notch receptor-ligand interactions have an adhesion function will provide an important clue to molecular basis of accumulation of MCs to inflammatory sites.

Fine tuning gene expression: the epigenome

An epigenetic trait is a stably inherited phenotype resulting from changes in a chromosome without alterations in the DNA sequence. Epigenetic modifications such as DNA methylation, together with covalent modification of histones, are thought to alter chromatin density and accessibility of the DNA to cellular machinery, thereby modulating the transcriptional potential of the underlying DNA sequence. As environmental changes influence epigenetic marks, epigenetics provides an added layer of variation that might mediate the relationship between genotype and internal and external environmental factors. Integration of our knowledge in genetics, epigenomics, and genomics with the use of systems biology tools may present investigators with new, powerful tools to study many complex human diseases such as kidney disease.

Modulation of notch-1 signaling alleviates vascular endothelial growth factor-mediated diabetic nephropathy

OBJECTIVE

Disturbances in podocytes are typically associated with marked proteinuria, a hallmark of diabetic nephropathy. This study was conducted to investigate modulation of Notch-1 signaling in high glucose (HG)-stressed human podocytes and in a diabetic animal model.

RESEARCH DESIGN AND METHODS

Expression of the Notch signaling components was examined in HG-treated podocytes, human embryonic kidney cells (HEK293), and kidneys from diabetic animals by RT-qPCR, Western blot analysis, and immunohistochemical staining. The association between the Notch signaling, VEGF expression, and podocyte integrity was evaluated.

RESULTS

Notch-1 signaling was significantly activated in HG-cultured human podocytes and HEK293 cells and kidneys from diabetic animals. HG also augmented VEGF expression, decreasing nephrin expression and podocyte number-a critical event for the development of proteinuria in diabetic nephropathy. After use of pharmacological modulators or specific shRNA knockdown strategies, inhibition of Notch-1 signaling significantly abrogated VEGF activation and nephrin repression in HG-stressed cells and ameliorated proteinuria in the diabetic kidney.

CONCLUSIONS

Our findings suggest that upregulation of Notch-1 signaling in HG-treated renal podocytes induces VEGF expression and subsequent nephrin repression and apoptosis. Modulation of Notch-1 signaling may hold promise as a novel therapeutic strategy for the treatment of diabetic nephropathy.

HIV-1 Rev-binding protein accelerates cellular uptake of iron to drive Notch-induced T cell leukemogenesis in mice

Somatic activating mutations in Notch1 contribute to the pathogenesis of T cell acute lymphoblastic lymphoma (T-ALL), but how activated Notch1 signaling exerts this oncogenic effect is not completely understood. Here we identify HIV-1 Rev-binding protein (Hrb), a component of the clathrin-mediated endocytosis machinery, as a critical mediator of Notch-induced T-ALL development in mice. Hrb was found to be a direct transcriptional target of Notch1, and Hrb loss reduced the incidence or delayed the onset of T-ALL in mouse models in which activated Notch1 signaling either contributes to or drives leukemogenesis. Consistent with this observation, Hrb supported survival and proliferation of hematopoietic and T cell precursor cells in vitro. We demonstrated that Hrb accelerated the uptake of transferrin, which was required for upregulation of the T cell protooncogene p21. Indeed, iron-deficient mice developed Notch1-induced T-ALL substantially more slowly than control mice, further supporting a critical role for iron uptake during leukemogenesis. Taken together, these results reveal that Hrb is a critical Notch target gene that mediates lymphoblast transformation and disease progression via its ability to satisfy the enhanced demands of transformed lymphoblasts for iron. Further, our data suggest that Hrb may be targeted to improve current treatment or design novel therapies for human T-ALL patients.

Patterning a complex organ: branching morphogenesis and nephron segmentation in kidney development

The two major components of the kidney, the collecting system and the nephron, have different developmental histories. The collecting system arises by the reiterated branching of a simple epithelial tube, while the nephron forms from a cloud of mesenchymal cells that coalesce into epithelial vesicles. Each develops into a morphologically complex and highly differentiated structure, and together they provide essential filtration and resorption functions. In this review, we will consider their embryological origin and the genes controlling their morphogenesis, patterning, and differentiation, with a focus on recent advances in several areas.

Up-regulation of the homophilic adhesion molecule sidekick-1 in podocytes contributes to glomerulosclerosis

Focal segmental glomerulosclerosis (FSGS) is a leading cause of nephrotic syndrome and end-stage renal disease worldwide. Although the mechanisms underlying this important disease are poorly understood, the glomerular podocyte clearly plays a central role in disease pathogenesis. In the current work, we demonstrate that the homophilic adhesion molecule sidekick-1 (sdk-1) is up-regulated in podocytes in FSGS both in rodent models and in human kidney biopsy samples. Transgenic mice that have podocyte-specific overexpression of sdk-1 develop gradually progressive heavy proteinuria and severe FSGS. We also show that sdk-1 associates with the slit diaphragm linker protein MAGI-1, which is already known to interact with several critical podocyte proteins including synaptopodin, alpha-actinin-4, nephrin, JAM4, and beta-catenin. This interaction is mediated through a direct interaction between the carboxyl terminus of sdk-1 and specific PDZ domains of MAGI-1. In vitro expression of sdk-1 enables a dramatic recruitment of MAGI-1 to the cell membrane. Furthermore, a truncated version of sdk-1 that is unable to bind to MAGI-1 does not induce podocyte dysfunction when overexpressed. We conclude that the up-regulation of sdk-1 in podocytes is an important pathogenic factor in FSGS and that the mechanism involves disruption of the actin cytoskeleton possibly via alterations in MAGI-1 function.

Expression of Notch pathway proteins correlates with albuminuria, glomerulosclerosis, and renal function

Recent studies indicate that the Notch signaling pathway plays an important role in the development of diabetic kidney disease and focal segmental glomerulosclerosis (FSGS). Here we analyzed the degree of expression and localization of Notch ligands (Jagged1 and Delta1) and activated (cleaved) receptors (Notch1 and Notch2) in healthy human kidneys and in renal biopsies from a wide variety of kidney diseases. These included patients with minimal change disease, membranous nephropathy, lupus nephritis ISN/RPS classes III/IV/V, hypertensive nephrosclerosis, crescentic glomerulonephritis, tubulointerstitial fibrosis, IgA nephropathy, diabetic kidney disease, and FSGS. We found that cleaved Notch1, Notch2, and Jagged1 are expressed on podocytes in proteinuric nephropathies and their level of expression correlated with the amount of proteinuria across all disease groups. The degree of glomerulosclerosis correlated with podocyte expression of cleaved Notch1, while the severity of tubulointerstitial fibrosis and the estimated glomerular filtration rate correlated with expression of cleaved Notch1 in the tubulointerstitium. Hence, our results raise the possibility that Notch pathway activation is a common mechanism in the pathophysiology of a wide range of acquired renal diseases.

A novel method for isolating podocytes using magnetic activated cell sorting

BACKGROUND

A large body of accumulated data has now revealed that podocytes play a major role in the development of proteinuria. However, the mechanisms of podocyte injury, leading to foot process effacement and proteinuria, are still unclear partly due to the current lack of an appropriate strategy for preparing podocytes. In this study, we have developed a novel method of rapid isolation of podocytes from mice using magnetic activated cell sorting with an anti-nephrin antibody.

METHODS

After endothelial cell depletion using anti-CD31 antibody, nephrin-positive cells were prepared from mouse kidneys using magnetic activated cell sorting with polyclonal rabbit anti-nephrin antibody. Purity of the positively sorted cells was determined by confocal microscopy and fluorescence-activated cell sorting (FACS) analysis. Expression profiles of podocyte-specific molecules in the sorted fractions were characterized by qualitative PCR and immunoblot analysis.

RESULTS

Nephrin-positive cells, isolated from mouse kidneys within 6 h, showed dual positivity for synaptopodin and rabbit IgG on confocal microscopy. FACS analysis revealed that the purity of the positively sorted fractions was ∼75%. The nephrin-positive cells sorted by this approach showed a significantly higher expression of podocyte-specific molecules compared with nephrin-negative fractions.

CONCLUSIONS

These data strongly suggest that our novel method for isolating podocytes has great utility for various downstream applications such as genomic analysis, proteomics and transcriptomics to elucidate molecular profiling of podocyte biology in vivo compared with conventional methods as our approach requires only several hours to complete and no tissue culture.

Reduced Notch signaling leads to renal cysts and papillary microadenomas

The formation of proximal nephron segments requires canonical Notch2 signaling, but other functions of Notch signaling during renal development are incompletely understood. Here, we report that proximal tubules forming with reduced Notch signaling, resulting from delayed conditional inactivation of Notch1 and/or Notch2, are prone to cyst formation and tubular epithelial stratification. Conditional inactivation of the DNA binding factor RBP-J, which mediates Notch signaling, also resulted in multiple congenital cysts arising from the proximal tubule. Moreover, a few stratified foci/microadenomas containing hyperproliferative cells, resembling precursors of papillary renal cell carcinoma, formed in these proximal tubules. Epithelial stratification correlated neither with reduced expression of the transcriptional regulator of ciliary proteins TCF2/HNF1beta nor with loss of apical-basal polarity. Instead, Notch signaling helped to restrict the orientation of epithelial mitotic spindles to a plane parallel to the basement membrane during nephron elongation. In the absence of Notch, random spindle orientation may explain the epithelial stratification and cyst formation. Furthermore, post hoc analysis of human class 1 papillary renal cell carcinoma revealed reduced Notch activity in these tumors, resulting from abundant expression of a potent inhibitor of canonical Notch signaling, KyoT3/FHL1B. In summary, these data suggest that canonical Notch signaling maintains the alignment of cell division in the proximal tubules during nephrogenesis and that perturbations in Notch signaling may lead to cystic renal disease and tumorigenesis.

Notch2 activation in the embryonic kidney depletes nephron progenitors

Successive activation of Wnt4 and Notch2 generates nephrons from the metanephric mesenchyme. Mesenchymal-to-epithelial transition requires Wnt4, and normal development of the proximal nephron (epithelia of glomeruli and proximal tubules) requires Notch2. It is unknown, however, whether Notch2 dictates the fate of the proximal nephron directly. Here, we generated a mutant strain of mice with activated Notch2 in Six2-containing nephron progenitor cells of the metanephric mesenchyme. Notch2 activation did not skew the cell fate toward the proximal nephron but resulted in severe kidney dysgenesis and depletion of Six2-positive progenitors. We observed ectopic expression of Wnt4 and premature tubule formation, similar to the phenotype of Six2-deficient mice. Activation of Notch2 in the progenitor cells suppressed Pax2, an upstream regulator of Six2, possibly through Hesr genes. Taken together, these data suggest that a positive feedback loop exists between Notch2 and Wnt4, and that Notch2 stabilizes, rather than dictates, nephron fate by shutting down the maintenance of undifferentiated progenitor cells, thereby depleting this population.

Stress-induced switch in Numb isoforms enhances Notch-dependent expression of subtype-specific transient receptor potential channel

The Notch signaling pathway plays an essential role in the regulation of cell specification by controlling differentiation, proliferation, and apoptosis. Numb is an intrinsic regulator of the Notch pathway and exists in four alternative splice variants that differ in the length of their phosphotyrosine-binding domain (PTB) and proline-rich region domains. The physiological relevance of the existence of the Numb splice variants and their exact regulation are still poorly understood. We previously reported that Numb switches from isoforms containing the insertion in PTB to isoforms lacking this insertion in neuronal cells subjected to trophic factor withdrawal (TFW). The functional relevance of the TFW-induced switch in Numb isoforms is not known. Here we provide evidence that the TFW-induced switch in Numb isoforms regulates Notch signaling strength and Notch target gene expression. PC12 cells stably overexpressing Numb isoforms lacking the PTB insertion exhibited higher basal Notch activity and Notch-dependent transcription of the transient receptor potential channel 6 (TRPC6) when compared with those overexpressing Numb isoforms with the PTB insertion. The differential regulation of TRPC6 expression is correlated with perturbed calcium signaling and increased neuronal vulnerability to TFW-induced death. Pharmacological inhibition of the Notch pathway or knockdown of TRPC6 function ameliorates the adverse effects caused by the TFW-induced switch in Numb isoforms. Taken together, our results indicate that Notch and Numb interaction may influence the sensitivity of neuronal cells to injurious stimuli by modulating calcium-dependent apoptotic signaling cascades.

Extracellular BMP-antagonist regulation in development and disease: tied up in knots

Developmental processes are regulated by the bone morphogenetic protein (BMP) family of secreted molecules. BMPs bind to serine/threonine kinase receptors and signal through the canonical Smad pathway and other intracellular effectors. Integral to the control of BMPs is a diverse group of secreted BMP antagonists that bind to BMPs and prevent engagement with their cognate receptors. Tight temporospatial regulation of both BMP and BMP-antagonist expression provides an exquisite control system for developing tissues. Additional facets of BMP-antagonist biology, such as crosstalk with Wnt and Sonic hedgehog signaling during development, have been revealed in recent years. In addition, previously unappreciated roles for the BMP antagonists in kidney fibrosis and cancer have been elucidated. This review provides a description of BMP-antagonist biology, together with highlights of recent novel insights into the role of these antagonists in development, signal transduction and human disease.

A GREM1 gene variant associates with diabetic nephropathy

Gremlin, a cell growth and differentiation factor, promotes the development of diabetic nephropathy in animal models, but whether GREM1 gene variants associate with diabetic nephropathy is unknown. We comprehensively screened the 5' upstream region (including the predicted promoter), all exons, intron-exon boundaries, complete untranslated regions, and the 3' region downstream of the GREM1 gene. We identified 31 unique variants, including 24 with a minor allele frequency exceeding 5%, and 9 haplotype-tagging single nucleotide polymorphisms (htSNPs). We selected one additional variant that we predicted to alter transcription factor binding. We genotyped 709 individuals with type 1 diabetes of whom 267 had nephropathy (cases) and 442 had no evidence of kidney disease (controls). Three individual SNPs significantly associated with nephropathy at the 5% level, and two remained significant after adjustment for multiple testing. Subsequently, we genotyped a replicate population comprising 597 cases and 502 controls: this population supported an association with one of the SNPs (rs1129456; P = 0.0003). Combined analysis, adjusted for recruitment center (n = 8), suggested that the T allele conferred greater odds of nephropathy (OR 1.69; 95% CI 1.36 to 2.11). In summary, the GREM1 variant rs1129456 associates with diabetic nephropathy, perhaps explaining some of the genetic susceptibility to this condition.

Toward the development of podocyte-specific drugs

Most kidney diseases that ultimately lead to end-stage renal failure originate within the glomerulus and are associated with proteinuria. Treatment options are unspecific and offer partial cures at best because available therapies do not primarily treat glomerular cells but rather act systemically and thus cause many side effects. Most glomerulopathies directly stem from injury to podocytes, cells that have a key role in the maintenance of the glomerular filter. Thus, these cells constitute an obvious and promising target for the development of novel kidney-protective drugs. During the last decade, enormous advances have been made in the understanding of podocyte structure and function. A number of pathways that are altered during glomerular diseases may be targeted by novel small- and large-molecule drugs as well as biologicals that have been identified in nephrology and other areas of drug development. Cultured podocytes provide a valuable model for high-throughput drug screening assays. Furthermore, podocytes have been shown to possess many features that make them particularly good target cells for renal protection. This mini-review discusses some of the most recent promising data related to potential drug therapy for proteinuria and kidney disease through direct podocyte targeting.

ADAM10 is expressed in human podocytes and found in urinary vesicles of patients with glomerular kidney diseases

Background

The importance of the Notch signaling in the development of glomerular diseases has been recently described. Therefore we analyzed in podocytes the expression and activity of ADAM10, one important component of the Notch signaling complex.

Methods

By Western blot, immunofluorescence and immunohistochemistry analysis we characterized the expression of ADAM10 in human podocytes, human urine and human renal tissue.

Results

We present evidence, that differentiated human podocytes possessed increased amounts of mature ADAM10 and released elevated levels of L1 adhesion molecule, one well known substrate of ADAM10. By using specific siRNA and metalloproteinase inhibitors we demonstrate that ADAM10 is involved in the cleavage of L1 in human podocytes. Injury of podocytes enhanced the ADAM10 mediated cleavage of L1. In addition, we detected ADAM10 in urinary podocytes from patients with kidney diseases and in tissue sections of normal human kidney. Finally, we found elevated levels of ADAM10 in urinary vesicles of patients with glomerular kidney diseases.

Conclusions

The activity of ADAM10 in human podocytes may play an important role in the development of glomerular kidney diseases.

Preventive effect of Notch signaling inhibition by a gamma-secretase inhibitor on peritoneal dialysis fluid-induced peritoneal fibrosis in rats

Peritoneal fibrosis, a major complication of peritoneal dialysis, limits the effectiveness of peritoneal dialysis as a treatment of end-stage renal disease. Preventing this complication by identifying targets for therapy has recently received much attention. In the present study, we showed that Notch signaling was highly activated in rats in peritoneal dialysis fluid-induced fibrotic peritoneum, as indicated by increased expression of Jagged-1, Notch-1, and HES-1. Blocking Notch signaling activation by intraperitoneal injection of a gamma-secretase inhibitor, DAPT, significantly attenuated peritoneal fibrosis as indicated by the decreased expression of alpha-smooth muscle actin, collagen I, and vascular endothelial growth factor as well as increased expression of E-cadherin. Moreover, compared with control rats, DAPT-treated rats had a thinner peritoneum with less extracellular matrix accumulation, a lower mass transfer of glucose, and a higher ultrafiltration rate. In addition, transforming growth factor (TGF)-beta1 induced Notch signaling activation in primary rat peritoneal mesothelial cells. DAPT blocked this TGF-beta1-induced Notch signaling activation and therefore significantly inhibited TGF-beta1-induced expression of alpha-smooth muscle actin, collagen I, and vascular endothelial growth factor. Thus, a gamma-secretase inhibitor that interferes with Notch signaling prevents biochemical, histological, and functional consequences of peritoneal fibrosis through inhibiting epithelial to mesenchymal transition of rat peritoneal mesothelial cells. These results support the use of gamma-secretase inhibitors as a novel therapeutic approach for peritoneal fibrosis.

Abnormalities in signaling pathways in diabetic nephropathy

Diabetic nephropathy (DN) is characterized by a plethora of signaling abnormalities that together ultimately result in the clinical and pathologic hallmarks of DN, namely progressive albuminuria followed by a gradual decline in glomerular filtration rate leading to kidney failure, and accompanied by podocyte loss, progressive glomerular sclerosis and, ultimately, progressive tubulointerstitial fibrosis. Over the past few years, the general understanding of the abnormalities in signaling pathways that lead to DN has expanded considerably. In this review, some of the important pathways that appear to be involved in driving this process are discussed, with special emphasis on newer findings and insights. Newer concepts regarding signaling changes in bradykinin, mTOR, JAK/STAT, MCP-1, VEGF, endothelial nitric oxide synthase, activated protein C and other pathways are discussed.

New insights into epithelial-mesenchymal transition in kidney fibrosis

Epithelial-mesenchymal transition (EMT), a process by which differentiated epithelial cells undergo a phenotypic conversion that gives rise to the matrix-producing fibroblasts and myofibroblasts, is increasingly recognized as an integral part of tissue fibrogenesis after injury. However, the degree to which this process contributes to kidney fibrosis remains a matter of intense debate and is likely to be context-dependent. EMT is often preceded by and closely associated with chronic interstitial inflammation and could be an adaptive response of epithelial cells to a hostile or changing microenvironment. In addition to tubular epithelial cells, recent studies indicate that endothelial cells and glomerular podocytes may also undergo transition after injury. Phenotypic alteration of podocytes sets them in motion to functional impairment, resulting in proteinuria and glomerulosclerosis. Several intracellular signal transduction pathways such as TGFbeta/Smad, integrin-linked kinase (ILK) and Wnt/beta-catenin signaling are essential in controlling the process of EMT and presently are potential targets of antifibrotic therapy. This review highlights the current understanding of EMT and its underlying mechanisms to stimulate further discussion on its role, not only in the pathogenesis of renal interstitial fibrosis but also in the onset of podocyte dysfunction, proteinuria, and glomerulosclerosis.

Proteinuria: an enzymatic disease of the podocyte?

Proteinuria is a major health-care problem that affects several hundred million people worldwide. Proteinuria is a cardinal sign and a prognostic marker of kidney disease, and also an independent risk factor for cardiovascular morbidity and mortality. Microalbuminuria is the earliest cue of renal complications of diabetes, obesity, and the metabolic syndrome. It can often progress to overt proteinuria that in 10-50% of patients is associated with the development of chronic kidney disease, ultimately requiring dialysis or transplantation. Therefore, reduction or prevention of proteinuria is highly desirable. Here we review recent novel insights into the pathogenesis and treatment of proteinuria, with a special emphasis on the emerging concept that proteinuria can result from enzymatic cleavage of essential regulators of podocyte actin dynamics by cytosolic cathepsin L (CatL), resulting in a motile podocyte phenotype. Finally, we describe signaling pathways controlling the podocyte actin cytoskeleton and motility and how these pathways can be manipulated for therapeutic benefit.

Molecular genetic analysis of podocyte genes in focal segmental glomerulosclerosis--a review

This review deals with podocyte proteins that play a significant role in the structure and function of the glomerular filter. Genetic linkage studies has identified several genes involved in the development of nephrotic syndrome and contributed to the understanding of the pathophysiology of glomerular proteinuria and/or focal segmental glomerulosclerosis. Here, we describe already well-characterized genetic diseases due to mutations in nephrin, podocin, CD2AP, alpha-actinin-4, WT1, and laminin beta2 chain, as well as more recently identified genetic abnormalities in TRPC6, phospholipase C epsilon, and the proteins encoded by the mitochondrial genome. In addition, the role of the proteins which have shown to be important for the structure and functions by gene knockout studies in mice, are also discussed. Furthermore, some rare syndromes with glomerular involvement, in which molecular defects have been recently identified, are briefly described. In summary, this review updates the current knowledge of genetic causes of congenital and childhood nephrotic syndrome and provides new insights into mechanisms of glomerular dysfunction.

Notch activates Wnt-4 signalling to control medio-lateral patterning of the pronephros

Previous studies have highlighted a role for the Notch signalling pathway during pronephrogenesis in the amphibian Xenopus laevis, and in nephron development in the mammalian metanephros, yet a mechanism for this function remains elusive. Here, we further the understanding of how Notch signalling patterns the early X. laevis pronephros anlagen, a function that might be conserved in mammalian nephron segmentation. Our results indicate that early phase pronephric Notch signalling patterns the medio-lateral axis of the dorso-anterior pronephros anlagen, permitting the glomus and tubules to develop in isolation. We show that this novel function acts through the Notch effector gene hrt1 by upregulating expression of wnt4. Wnt-4 then patterns the proximal pronephric anlagen to establish the specific compartments that span the medio-lateral axis. We also identified pronephric expression of lunatic fringe and radical fringe that is temporally and spatially appropriate for a role in regulating Notch signalling in the dorso-anterior region of the pronephros anlagen. On the basis of these results, along with data from previous publications, we propose a mechanism by which the Notch signalling pathway regulates a Wnt-4 function that patterns the proximal pronephric anlagen.

Endothelial Notch signaling is upregulated in human brain arteriovenous malformations and a mouse model of the disease

Brain arteriovenous malformations (BAVMs) can cause lethal hemorrhagic stroke and have no effective treatment. The cellular and molecular basis for this disease is largely unknown. We have previously shown that expression of constitutively-active Notch4 receptor in the endothelium elicits and maintains the hallmarks of BAVM in mice, thus establishing a mouse model of the disease. Our work suggested that Notch pathway could be a critical molecular mediator of BAVM pathogenesis. Here, we investigated the hypothesis that upregulated Notch activation contributes to the pathogenesis of human BAVM. We examined the expression of the canonical Notch downstream target Hes1 in the endothelium of human BAVMs by immunofluorescence, and showed increased levels relative to either autopsy or surgical biopsy controls. We then analyzed receptor activity using an antibody to the activated form of the Notch1 receptor, and found increased levels of activity. These findings suggest that Notch activation may promote the development and even maintenance of BAVM. We also detected increases in Hes1 and activated Notch1 expression in our mouse model of BAVM induced by constitutively active Notch4, demonstrating molecular similarity between the mouse model and the human disease. Our work suggests that activation of Notch signaling is an important molecular candidate in BAVM pathogenesis and further validates that our animal model provides a platform to study the progression as well as the regression of the disease.