Control of mammalian kidney development by the Hedgehog signaling pathway

Hedgehog-GLI mediated control of renal formation and malformation

CAKUT is the leading cause of end-stage kidney disease in children and comprises a broad spectrum of phenotypic abnormalities in kidney and ureter development. Molecular mechanisms underlying the pathogenesis of CAKUT have been elucidated in genetic models, predominantly in the mouse, a paradigm for human renal development. Hedgehog (Hh) signaling is critical to normal embryogenesis, including kidney development. Hh signaling mediates the physiological development of the ureter and stroma and has adverse pathophysiological effects on the metanephric mesenchyme, ureteric, and nephrogenic lineages. Further, disruption of Hh signaling is causative of numerous human developmental disorders associated with renal malformation; Pallister-Hall Syndrome (PHS) is characterized by a diverse spectrum of malformations including CAKUT and caused by truncating variants in the middle-third of the Hh signaling effector GLI3. Here, we outline the roles of Hh signaling in regulating murine kidney development, and review human variants in Hh signaling genes in patients with renal malformation.

Pallister-Hall syndrome, GLI3, and kidney malformation

Pallister-Hall syndrome (PHS) is a rare autosomal dominant disease diagnosed by the presence of hypothalamic hamartoma, mesoaxial polydactyly and a truncating variant in the middle third of the GLI-Kruppel family member 3 (GLI3) gene. PHS may also include a wide range of clinical phenotypes affecting multiple organ systems including congenital anomalies of the kidney and urinary tract (CAKUT). The observed clinical phenotypes are consistent with the essential role of GLI3, a transcriptional effector in the hedgehog (Hh) signaling pathway, in organogenesis. However, the mechanisms by which truncation of GLI3 in PHS results in such a variety of clinical phenotypes with variable severity, even within the same organ, remain unclear. In this study we focus on presentation of CAKUT in PHS. A systematic analysis of reported PHS patients (n = 78) revealed a prevalence of 26.9% (21/78) of CAKUT. Hypoplasia (± dysplasia) and agenesis were the two main types of CAKUT; bilateral and unilateral CAKUT were reported with equal frequency. Examination of clinical phenotypes with CAKUT revealed a significant association between CAKUT and craniofacial defects, bifid epiglottis and a Disorder of Sex Development, specifically affecting external genitalia. Lastly, we determined that PHS patients with CAKUT predominately had substitution type variants (as opposed to deletion type variants in non-CAKUT PHS patients) in the middle third of the GLI3 gene. These results provide a foundation for future work aimed at uncovering the molecular mechanisms by which variant GLI3 result in the wide range and severity of clinical features observed in PHS.

Nephronophthisis-Pathobiology and Molecular Pathogenesis of a Rare Kidney Genetic Disease

The exponential rise in our understanding of the aetiology and pathophysiology of genetic cystic kidney diseases can be attributed to the identification of cystogenic genes over the last three decades. The foundation of this was laid by positional cloning strategies which gradually shifted towards next-generation sequencing (NGS) based screenings. This shift has enabled the discovery of novel cystogenic genes at an accelerated pace unlike ever before and, most notably, the past decade has seen the largest increase in identification of the genes which cause nephronophthisis (NPHP). NPHP is a monogenic autosomal recessive cystic kidney disease caused by mutations in a diverse clade of over 26 identified genes and is the most common genetic cause of renal failure in children. NPHP gene types present with some common pathophysiological features alongside a diverse range of extra-renal phenotypes associated with specific syndromic presentations. This review provides a timely update on our knowledge of this disease, including epidemiology, pathophysiology, anatomical and molecular features. We delve into the diversity of the NPHP causing genes and discuss known molecular mechanisms and biochemical pathways that may have possible points of intersection with polycystic kidney disease (the most studied renal cystic pathology). We delineate the pathologies arising from extra-renal complications and co-morbidities and their impact on quality of life. Finally, we discuss the current diagnostic and therapeutic modalities available for disease management, outlining possible avenues of research to improve the prognosis for NPHP patients.

Molecular pathways involved in injury-repair and ADPKD progression

The major hallmark of Autosomal Dominant Polycystic Kidney Disease (ADPKD) is the formation of many fluid-filled cysts in the kidneys, which ultimately impairs the normal renal structure and function, leading to end-stage renal disease (ESRD). A large body of evidence suggests that injury-repair mechanisms are part of ADPKD progression. Once cysts have been formed, proliferation and fluid secretion contribute to the cyst size increase, which eventually causes stress on the surrounding tissue resulting in local injury and fibrosis. In addition, renal injury can cause or accelerate cyst formation. In this review, we will describe the various mechanisms activated during renal injury and tissue repair and show how they largely overlap with the molecular mechanisms activated during PKD progression. In particular, we will discuss molecular mechanisms such as proliferation, inflammation, cell differentiation, cytokines and growth factors secretion, which are activated following the renal injury to allow the remodelling of the tissue and a proper organ repair. We will also underline how, in a context of PKD-related gene mutations, aberrant or chronic activation of these developmental pathways and repair/remodelling mechanisms results in exacerbation of the disease.

Generation of infant- and pediatric-derived urinary induced pluripotent stem cells competent to form kidney organoids

BACKGROUND

Human induced pluripotent stem cells (iPSCs) are a promising tool to investigate pathogenic mechanisms underlying human genetic conditions, such as congenital anomalies of the kidney and urinary tract (CAKUT). Currently, iPSC-based research in pediatrics is limited by the invasiveness of cell collection.

METHODS

Urine cells (UCs) were isolated from pediatric urine specimens, including bag collections, and reprogrammed using episomal vectors into urinary iPSCs (UiPSCs). Following iPSC-quality assessment, human kidney organoids were generated.

RESULTS

UCs were isolated from 71% (12/17) of single, remnant urine samples obtained in an outpatient setting (patients 1 month-17 years, volumes 10-75 ml). Three independent UCs were reprogrammed to UiPSCs with early episome loss, confirmed pluripotency and normal karyotyping. Subsequently, these UiPSCs were successfully differentiated into kidney organoids, closely resembling organoids generated from control fibroblast-derived iPSCs. Importantly, under research conditions with immediate sample processing, UC isolation was successful 100% for target pediatric CAKUT patients and controls (11/11) after at most two urine collections.

CONCLUSIONS

Urine in small volumes or collected in bags is a reliable source for reprogrammable somatic cells that can be utilized to generate kidney organoids. This constitutes an attractive approach for patient-specific iPSC research involving infants and children with wide applicability and a low threshold for participation.

Saikosaponin B2 attenuates kidney fibrosis via inhibiting the Hedgehog Pathway

BACKGROUND

Renal interstitial fibrosis is a common pathway through which chronic kidney disease progresses to end-stage renal disease. There are currently no effective drugs available to treat kidney fibrosis, so traditional medicine is likely to be a candidate. The therapeutic potential of saikosaponin B2 (SSB2), a biologically active ingredient of Radix Bupleuri, on renal fibrosis has not been reported.

METHODS

A unilateral ureteral obstruction (UUO) model was conducted to induce renal interstitial fibrosis in mice. SSB2's effect was valuated by histological staining and exploring the changes in expression of relative proteins and mRNAs. A conditional medium containing sonic hedgehog variant protein stimulating normal rat kidney interstitial fibroblast cells (NRK-49F) was used in an in vitro model to determine the possible mechanism. The molecular target of SSB2 was verified using several mutation plasmids.

RESULTS

SSB2 administration reduced kidney injury and alleviated interstitial fibrosis by decreasing excessive accumulation of extracellular matrix components in UUO mice. It could also reduce the expression of α-SMA, fibronectin and Gli1, a crucial molecule of the hedgehog (Hh) signaling pathway both in vivo and in vitro. In NIH-3T3 cells simulated by conditional medium containing sonic hedgehog variant protein, SSB2 showed the ability to decrease the expression of Gli1 and Ptch1 mRNA. Using a dual-luciferase reporter assay, SSB2 suppressed the Gli-luciferase reporter activity in NIH-3T3 cells, and the IC₅₀ was 0.49 μM, but had no effect on the TNF-α/NF-κB and Wnt/β-catenin signaling pathways, indicating the inhibition selectivity on the Hh signaling pathway. Furthermore, SSB2 failed to inhibit the Hh pathway activity evoked by ectopic expression of Gli2ΔN and Smo D473H, suggesting that SSB2 might potentially act on smoothened receptors.

CONCLUSION

SSB2 could attenuate renal fibrosis and decrease fibroblast activation by inhibiting the Hh signaling pathway.

Arsenic trioxide and curcumin attenuate cisplatin-induced renal fibrosis in rats through targeting Hedgehog signaling

Renal fibrosis is a progressive process resulting from a sustained injury that may ultimately cause renal failure. Cisplatin is an antitumor drug that induces renal injury and nephrotoxicity and is widely employed as a model for acute and chronic renal injury. Several signaling pathways are implicated in fibrogenic cell activation among which is Hedgehog (Hh) signaling. We here investigated the effects of arsenic trioxide (Ars) and curcumin in ameliorating cisplatin-induced kidney fibrosis via regulating Hh signaling. Cisplatin (4.5 mg/kg) was administered in Sprague-Dawley rats for two consecutive days and renal fibrosis was induced after 21 days. Once renal fibrosis was confirmed, Ars (3.5 mg/kg/day, orally) and curcumin (200 mg/kg/day, orally) were administered daily for another 21 days. Ars and curcumin corrected kidney function markers as creatinine clearance and urea nitrogen. Both agents ameliorated fibrosis as shown by lowered TGF-β1 mRNA levels, α-SMA protein levels, and hydroxylproline content. Cisplatin-activated Hh signaling which was blocked by both Ars and curcumin as demonstrated by decreased mRNA levels of Shh, Smo, and Ptch and suppressed renal Gli1 and Gli2 protein levels. Our results indicate new therapeutic roles for Ars and curcumin and suggest that blocking Hh signaling may be a promising approach for alleviating renal fibrosis. Symbols indicate α-SMA, alpha-smooth muscle actin; TGF-β, transforming growth factor-beta; Ptch, patched; Smo, smoothened; Shh, sonic hedgehog; Ihh, Indian hedgehog; Dhh, desert hedgehog; and SUFU, suppressor of fused.

How Tubular Epithelial Cell Injury Contributes to Renal Fibrosis

The renal tubules are the major component of the kidney and are vulnerable to a variety of injuries including ischemia, proteinuria, toxins, and metabolic disorders. It has long been believed that tubules are the victim of injury. In this review, we shift this concept to renal tubules as a driving force in the progression of kidney disease. In response to injury, tubular epithelial cells (TECs) can synthesize and secrete varieties of bioactive molecules that drive interstitial inflammation and fibrosis. Innate immune-sensing receptors on the TECs also aggravate immune responses. Necroinflammation, an auto-amplification loop between tubular cell death and interstitial inflammation, leads to the exacerbation of renal injury. Furthermore, TECs also play an active role in progressive renal injury via mechanisms associated with the conversion into collagen-producing fibroblast phenotype, cell cycle arrest at both G1/S and G2/M checkpoints, and metabolic disorder. Thus, a better understanding the mechanisms by which tubular injury drives AKI and CKD is necessary for the development of therapeutics to halt the progression of CKD.

Hedgehog Interacting Protein Promotes Fibrosis and Apoptosis in Glomerular Endothelial Cells in Murine Diabetes

We investigated whether renal hedgehog interacting protein (Hhip) expression contributes to the progression of diabetic nephropathy (DN) and studied its related mechanism(s) in vivo and in vitro. Here, we show that Hhip expression is highly elevated in glomerular endothelial cells of adult type 1 diabetic (T1D) Akita and T2D db/db mouse kidneys as compared to non-diabetic control littermates. Hyperglycemia enhances reactive oxygen species (ROS) generation via NADPH oxidase 4 (Nox4) activation and stimulates renal Hhip gene expression, and that elevated renal Hhip gene expression subsequently activates the TGFβ1- Smad2/3 cascade and promotes endothelial to mesenchymal transition associated with endothelial cell fibrosis/apoptosis in vivo and in vitro. Furthermore, kidneys of low-dose streptozotocin-induced diabetic heterozygous Hhip deficient (Hhip^+/−) mice displayed a normal albumin/creatinine ratio with fewer features of DN (glomerulosclerosis/fibrosis and podocyte apoptosis/loss) and less evidence of renal compensation (glomerular hypertrophy and hyperfiltration) as compared to diabetic wild type controls (Hhip^+/+). Thus, our studies demonstrated that renal Hhip expression is associated with nephropathy development in diabetes and that hyperglycemia-induced renal Hhip expression may mediate glomerular endothelial fibrosis and apoptosis in diabetes, a novel finding.

Genomic study of severe fetal anomalies and discovery of GREB1L mutations in renal agenesis

PURPOSE

Fetal anomalies represent a poorly studied group of developmental disorders. Our objective was to assess the impact of whole-exome sequencing (WES) on the investigation of these anomalies.

METHODS

We performed WES in 101 fetuses or stillborns who presented prenatally with severe anomalies, including renal a/dysgenesis, VACTERL association (vertebral defects, anal atresia, cardiac defects, tracheoesophageal fistula, renal anomalies, and limb abnormalities), brain anomalies, suspected ciliopathies, multiple major malformations, and akinesia.

RESULTS

A molecular diagnosis was obtained in 19 cases (19%). In 13 of these cases, the diagnosis was not initially suspected by the clinicians because the phenotype was nonspecific or atypical, corresponding in some cases to the severe end of the spectrum of a known disease (e.g., MNX1-, RYR1-, or TUBB-related disorders). In addition, we identified likely pathogenic variants in genes (DSTYK, ACTB, and HIVEP2) previously associated with phenotypes that were substantially different from those found in our cases. Finally, we identified variants in novel candidate genes that were associated with perinatal lethality, including de novo mutations in GREB1L in two cases with bilateral renal agenesis, which represents a significant enrichment of such mutations in our cohort.

CONCLUSION

Our study opens a window on the distinctive genetic landscape associated with fetal anomalies and highlights the power-but also the challenges-of WES in prenatal diagnosis.

AT2 R deficiency mediated podocyte loss via activation of ectopic hedgehog interacting protein (Hhip) gene expression

Angiotensin II type 2 receptor (AT₂ R) deficiency in AT₂ R knockout (KO) mice has been linked to congenital abnormalities of the kidney and urinary tract; however, the mechanisms by which this occurs are poorly understood. In this study, we examined whether AT₂ R deficiency impaired glomerulogenesis and mediated podocyte loss/dysfunction in vivo and in vitro. Nephrin-cyan fluorescent protein (CFP)-transgenic (Tg) and Nephrin/AT₂ RKO mice were used to assess glomerulogenesis, while wild-type and AT₂ RKO mice were used to evaluate maturation of podocyte morphology/function. Immortalized mouse podocytes (mPODs) were employed for in vitro studies. AT₂ R deficiency resulted in diminished glomerulogenesis in E15 embryos, but had no impact on actual nephron number in neonates. Pups lacking AT₂ R displayed features of renal dysplasia with lower glomerular tuft volume and podocyte numbers. In vivo and in vitro studies demonstrated that loss of AT₂ R was associated with elevated NADPH oxidase 4 levels, which in turn stimulated ectopic hedgehog interacting protein (Hhip) gene expression in podocytes. Consequently, ectopic Hhip expression activation either triggers caspase-3 and p53-related apoptotic processes resulting in podocyte loss, or activates TGFβ1-Smad2/3 cascades and α-SMA expression to transform differentiated podocytes to undifferentiated podocyte-derived fibrotic cells. We analyzed HHIP expression in the kidney disease database (Nephroseq) and then validated this using HHIP immunohistochemistry staining of human kidney biopsies (controls versus focal segmental glomerulosclerosis). In conclusion, loss of AT₂ R is associated with podocyte loss/dysfunction and is mediated, at least in part, via augmented ectopic Hhip expression in podocytes. Copyright © 2017 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Fluid mechanics as a driver of tissue-scale mechanical signaling in organogenesis

PURPOSE OF REVIEW

Organogenesis is the process during development by which cells self-assemble into complex, multi-scale tissues. Whereas significant focus and research effort has demonstrated the importance of solid mechanics in organogenesis, less attention has been given to the fluid forces that provide mechanical cues over tissue length scales.

RECENT FINDINGS

Fluid motion and pressure is capable of creating spatial gradients of forces acting on cells, thus eliciting distinct and localized signaling patterns essential for proper organ formation. Understanding the multi-scale nature of the mechanics is critically important to decipher how mechanical signals sculpt developing organs.

SUMMARY

This review outlines various mechanisms by which tissues generate, regulate, and sense fluid forces and highlights the impact of these forces and mechanisms in case studies of normal and pathological development.

Hedgehog Gli signalling in kidney fibrosis

Kidney fibrosis is the common final pathway of virtually all progressive injury to the kidney and a promising therapeutic target in chronic kidney disease (CKD). The Hedgehog pathway has been reported to be critical in kidney development, and recent evidence suggests a role in kidney injury and fibrosis. This review provides an overview of recent data suggesting an important role of Gli transcriptional activators in kidney injury and repair. We have reported that the hedgehog transcriptional activator Gli1 specifically marks perivascular mesenchymal stem cells, which are an important source of kidney myofibroblasts. Genetic ablation of these cells ameliorated kidney and heart fibrosis and stabilized organ function after injury. Recent data suggest that Gli2 is an important driver of myofibroblast cell cycle progression and a promising therapeutic target in kidney fibrosis progression and CKD. However, the non-canonical mechanism of Gli activation in kidney fibrosis remains an open question, and further studies are needed to elucidate the role of Hedgehog Gli and Gli1⁺ perivascular cells in human kidney fibrosis.

Developmental signalling pathways in renal fibrosis: the roles of Notch, Wnt and Hedgehog

Kidney fibrosis is a common histological manifestation of functional decline in the kidney. Fibrosis is a reactive process that develops in response to excessive epithelial injury and inflammation, leading to myofibroblast activation and an accumulation of extracellular matrix. Here, we describe how three key developmental signalling pathways - Notch, Wnt and Hedgehog (Hh) - are reactivated in response to kidney injury and contribute to the fibrotic response. Although transient activation of these pathways is needed for repair of injured tissue, their sustained activation is thought to promote fibrosis. Excessive Wnt and Notch expression prohibit epithelial differentiation, whereas increased Wnt and Hh expression induce fibroblast proliferation and myofibroblastic transdifferentiation. Notch, Wnt and Hh are fundamentally different signalling pathways, but their choreographed activation seems to be just as important for fibrosis as it is for embryonic kidney development. Decreasing the activity of Notch, Wnt or Hh signalling could potentially provide a new therapeutic strategy to ameliorate the development of fibrosis in chronic kidney disease.

Kidney Regeneration: Lessons from Development

A number of genes involved in kidney development are reactivated in the adult after acute kidney injury (AKI). This has led to the belief that tissue repair mechanisms recapitulate pathways involved in embryonic development after AKI. We will discuss evidence to support this hypothesis by comparing the mechanisms of development with common pathways known to regulate post-AKI repair, or that we identified as cell-specific candidates based on public datasets from recent AKI translational profiling studies. We will argue that while many of these developmental pathways are reactivated after AKI, this is not associated with general cellular reprogramming to an embryonic state. We will show that reactivation of these developmental genes is often associated with expression in cells that are not normally involved in mediating parallel responses in the embryo, and that depending on the cellular context, these responses can have beneficial or detrimental effects on injury and repair after AKI.

An overview of hedgehog signaling in fibrosis

The Hedgehog (Hh) signaling pathway plays a key role during embryogenesis and tissue regeneration. Recently, studies revealed that overactivated Hh signaling leads to fibrogenesis in many types of tissues. The activation of Hh signaling is involved in the epithelial-mesenchymal transition and excessive extracellular matrix deposition. Blockade of Hh signaling abolishes the induction of the epithelial-mesenchymal transition and ameliorates tissue fibrosis. Therefore, new therapeutic targets to alleviate fibrosis based on the Hh signaling have attracted a great deal of attention. This is a new strategy for treating fibrosis and other related diseases. In this review, we discuss the crucial role of Hh signaling in fibrogenesis to provide a better understanding of their relationship and to encourage the study of novel targeted therapies.

Maternal diabetes modulates kidney formation in murine progeny: the role of hedgehog interacting protein (HHIP)

AIMS/HYPOTHESIS

We hypothesised that maternal diabetes impairs kidney formation in offspring via augmented expression of hedgehog interacting protein (HHIP). Our gene-array results were performed in neonatal kidneys from our murine model of maternal diabetes and indicated that Hhip expression was significantly modulated by maternal diabetes.

METHODS

We systematically examined the functional role of HHIP in kidney formation in our murine maternal diabetes model and elucidated the potential mechanisms related to dysnephrogenesis in vitro.

RESULTS

The kidneys of the offspring of diabetic dams, compared with those of the offspring of control non-diabetic dams, showed retardation of development--small kidneys and less ureteric bud (UB) branching morphogenesis. Augmented HHIP expression was observed in the offspring of diabetic dams, initially localised to differentiated metanephric mesenchyme and UB epithelium and subsequently in maturing glomerular endothelial and tubulointerstitial cells. The heightened HHIP targeting TGF-β1 signalling was associated with dysmorphogenesis. In vitro, HHIP overexpression decreased sonic hedgehog and paired box gene 2 proteins (SHH and PAX2, respectively) and increased transcriptional nuclear factor-kappa B (NFκB, p50/p65), phosphorylation of p53, and TGF-β1 expression. In contrast, overexpression of PAX2 inhibited HHIP and NFκB and activated SHH, N-myc and p27(Kip1) expression. Moreover, high glucose stimulated HHIP expression, and then targeted TGF-β1 signalling. Thus, PAX2, via a negative autocrine feedback mechanism, attenuated the stimulatory effect of high glucose on HHIP expression.

CONCLUSIONS/INTERPRETATION

Maternal diabetes modulates kidney formation in young progeny mediated, at least in part, via augmented HHIP expression.

Developmental signaling: does it bridge the gap between cilia dysfunction and renal cystogenesis?

For more than a decade, evidence has accumulated linking dysfunction of primary cilia to renal cystogenesis, yet molecular mechanisms remain undefined. The pathogenesis of renal cysts is complex, involving multiple cellular aberrations and signaling pathways. Adding to this complexity, primary cilia exhibit multiple roles in a context-dependent manner. On renal epithelial cells, primary cilia act as mechanosensors and trigger extracellular Ca(2+) influx in response to laminar fluid flow. During mammalian development, primary cilia mediate the Hedgehog (Hh), Wnt, and Notch pathways, which control cell proliferation and differentiation, and tissue morphogenesis. Further, experimental evidence suggests the developmental state of the kidney strongly influences renal cystic disease. Thus, we review evidence for regulation of Ca(2+) and cAMP, key molecules in renal cystogenesis, at the primary cilium, the role of Hh, Wnt, and Notch signaling in renal cystic disease, and the interplay between these developmental pathways and Ca(2+) signaling. Indeed if these developmental pathways influence renal cystogenesis, these may represent novel therapeutic targets that can be integrated into a combination therapy for renal cystic disease.

Sonic hedgehog is a novel tubule-derived growth factor for interstitial fibroblasts after kidney injury

Tubular epithelium constitutes the majority of the renal parenchyma and is the primary target of various kidney injuries. However, how the injured tubules drive interstitial fibroblast activation and proliferation remains poorly understood. Here, we investigated the role of sonic hedgehog (Shh), a secreted extracellular signaling protein, in fibroblast proliferation. Shh was induced in renal tubular epithelia in animal models of CKD induced by ischemia/reperfusion injury (IRI), adriamycin, or renal mass ablation, and in renal tubules of kidney biopsy specimens from CKD patients with different etiologies. Using Gli1-CreER(T2) reporter mice, we identified interstitial fibroblasts as the principal targets of renal Shh signaling in vivo. In vitro, incubation with Shh promoted normal rat kidney fibroblast proliferation, which was assessed by cell counting, MTT assay, and BrdU incorporation assay, and stimulated the induction of numerous proliferation-related genes. However, Shh had no effect on the proliferation of renal tubular epithelial cells. In vivo, overexpression of Shh promoted fibroblast expansion and aggravated kidney fibrotic lesions after IRI. Correspondingly, blockade of Shh signaling by cyclopamine, a small molecule inhibitor of Smoothened, inhibited fibroblast proliferation, reduced myofibroblast accumulation, and attenuated renal fibrosis. These studies identify Shh as a novel, specific, and potent tubule-derived growth factor that promotes interstitial fibroblast proliferation and activation. Our data also suggest that blockade of Shh signaling is a plausible strategy for therapeutic intervention of renal fibrosis.

Embryofetal development study of vismodegib, a hedgehog pathway inhibitor, in rats

Vismodegib (Erivedge) is a first-in-class small-molecule hedgehog pathway inhibitor for the treatment of adults with advanced basal-cell carcinoma. Because this pathway is known to play key roles in patterning and growth during vertebrate development, vismodegib was anticipated to be embryotoxic. To support marketing applications, an embryofetal development study was completed in which a limited number of pregnant rats (n = 6/group) was administered vismodegib by oral gavage on gestation days 6 to 17. When vismodegib was administered at ≥60 mg/kg/day, doses associated with evidence of pharmacologic activity in previous rat toxicity studies, all conceptuses were resorbed at an early embryonic stage in the absence of significant maternal toxicity. When administered at 10 mg/kg/day, corresponding to an exposure (AUC0-24h ) approximately 15% of the median in patients at steady state, a variety of malformations were observed, including absent/fused digits in the hindlimb of multiple fetuses, multiple craniofacial abnormalities in one fetus, and an anorectal defect in one fetus. In addition, the incidence of variations, including dilated renal pelvis or ureter and incompletely or unossified skeletal elements, was significantly greater when compared with the controls. These results confirmed that vismodegib is likely to be embryotoxic at clinically relevant maternal exposures, and doses ≥60 mg/kg/day resulted in a 100% incidence of embryolethality that likely resulted from severe defects in early embryonic development. In contrast, craniofacial defects typically associated with hedgehog pathway inhibition were only observed in one fetus at the low dose of 10 mg/kg/day, which likely reflected minimal or intermittent pathway inhibition at low exposures.

Sonic Hedgehog Signaling and VACTERL Association

Hedgehog (Hh) signaling is vital for the patterning and organogenesis of almost every system. The specificity of these developmental processes is achieved through a tight spatio-temporal regulation of Hh signaling. Mice with defective Hh signal exhibit a wide spectrum of anomalies, including Vertebral defects, Anal atresia, Cardiovascular anomalies, Tracheoesophageal fistula, Renal dysplasia, and Limb defects, that resemble strikingly the phenotypes observed in VACTERL association in humans. In this review, we summarize our current understanding of mammalian Hh signaling and highlight the relevance of various mouse models for studying the etiology and pathogenesis of VACTERL association. In addition, recent advances in genetic study for unraveling the complexity of genetic inheritance of VACTERL and the implication of the Sonic hedgehog pathway in disease pathogenesis are also discussed.

Targeting pericyte differentiation as a strategy to modulate kidney fibrosis in diabetic nephropathy

Pericytes are a heterogeneous group of extensively branched cells located in microvessels where they make focal contacts with endothelium. Pericytes stabilize blood vessels, regulate vascular tone, synthesize matrix, participate in repair, and serve as progenitor cells, among other functions. Recent work has highlighted the role of pericytes and pericyte-like cells in fibrosis, in which chronic injury triggers pericyte proliferation and differentiation into collagen-secretory, contractile myofibroblasts with migration away from vessels, causing microvascular rarefaction. In this review the developmental origins of kidney pericytes and perivascular fibroblasts are summarized, pericyte to myofibroblast transition in type I diabetic nephropathy is discussed, and the regulation of pericyte differentiation into myofibroblasts as a therapeutic target for treatment of diabetic nephropathy is described.

Molecular regulation of kidney development

Genetically engineered mice have provided much information about gene function in the field of developmental biology. Recently, conditional gene targeting using the Cre/loxP system has been developed to control the cell type and timing of the target gene expression. The increase in number of kidney-specific Cre mice allows for the analysis of phenotypes that cannot be addressed by conventional gene targeting. The mammalian kidney is a vital organ that plays a critical homeostatic role in the regulation of body fluid composition and excretion of waste products. The interactions between epithelial and mesenchymal cells are very critical events in the field of developmental biology, especially renal development. Kidney development is a complex process, requiring inductive interactions between epithelial and mesenchymal cells that eventually lead to the growth and differentiation of multiple highly specialized stromal, vascular, and epithelial cell types. Through the use of genetically engineered mouse models, the molecular bases for many of the events in the developing kidney have been identified. Defective morphogenesis may result in clinical phenotypes that range from complete renal agenesis to diseases such as hypertension that exist in the setting of grossly normal kidneys. In this review, we focus on the growth and transcription factors that define kidney progenitor cell populations, initiate ureteric bud branching, induce nephron formation within the metanephric mesenchyme, and differentiate stromal and vascular progenitors in the metanephric mesenchyme.

Investigating mechanisms of chronic kidney disease in mouse models

Animal models of chronic kidney disease (CKD) are important experimental tools that are used to investigate novel mechanistic pathways and to validate potential new therapeutic interventions prior to pre-clinical testing in humans. Over the past several years, mouse CKD models have been extensively used for these purposes. Despite significant limitations, the model of unilateral ureteral obstruction (UUO) has essentially become the high-throughput in vivo model, as it recapitulates the fundamental pathogenetic mechanisms that typify all forms of CKD in a relatively short time span. In addition, several alternative mouse models are available that can be used to validate new mechanistic paradigms and/or novel therapies. Here, we review several models-both genetic and experimentally induced-that provide investigators with an opportunity to include renal functional study end-points together with quantitative measures of fibrosis severity, something that is not possible with the UUO model.

The hedgehog signal induced modulation of bone morphogenetic protein signaling: an essential signaling relay for urinary tract morphogenesis

BACKGROUND

Congenital diseases of the urinary tract are frequently observed in infants. Such diseases present a number of developmental anomalies such as hydroureter and hydronephrosis. Although some genetically-modified mouse models of growth factor signaling genes reproduce urinary phenotypes, the pathogenic mechanisms remain obscure. Previous studies suggest that a portion of the cells in the external genitalia and bladder are derived from peri-cloacal mesenchymal cells that receive Hedgehog (Hh) signaling in the early developmental stages. We hypothesized that defects in such progenitor cells, which give rise to urinary tract tissues, may be a cause of such diseases.

METHODOLOGY/PRINCIPAL FINDINGS

To elucidate the pathogenic mechanisms of upper urinary tract malformations, we analyzed a series of Sonic hedgehog (Shh) deficient mice. Shh(-/-) displayed hydroureter and hydronephrosis phenotypes and reduced expression of several developmental markers. In addition, we suggested that Shh modulation at an early embryonic stage is responsible for such phenotypes by analyzing the Shh conditional mutants. Tissue contribution assays of Hh-responsive cells revealed that peri-cloacal mesenchymal cells, which received Hh signal secreted from cloacal epithelium, could contribute to the ureteral mesenchyme. Gain- and loss-of-functional mutants for Hh signaling revealed a correlation between Hh signaling and Bone morphogenetic protein (Bmp) signaling. Finally, a conditional ablation of Bmp receptor type IA (BmprIA) gene was examined in Hh-responsive cell lineages. This system thus made it possible to analyze the primary functions of the growth factor signaling relay. The defective Hh-to-Bmp signaling relay resulted in severe urinary tract phenotypes with a decrease in the number of Hh-responsive cells.

CONCLUSIONS/SIGNIFICANCE

This study identified the essential embryonic stages for the pathogenesis of urinary tract phenotypes. These results suggested that Hh-responsive mesenchymal Bmp signaling maintains the population of peri-cloacal mesenchyme cells, which is essential for the development of the ureter and the upper urinary tract.

Mammalian kidney development: principles, progress, and projections

The mammalian kidney is a vital organ with considerable cellular complexity and functional diversity. Kidney development is notable for requiring distinct but coincident tubulogenic processes involving reciprocal inductive signals between mesenchymal and epithelial progenitor compartments. Key molecular pathways mediating these interactions have been identified. Further, advances in the analysis of gene expression and gene activity, coupled with a detailed knowledge of cell origins, are enhancing our understanding of kidney morphogenesis and unraveling the normal processes of postnatal repair and identifying disease-causing mechanisms. This article focuses on recent insights into central regulatory processes governing organ assembly and renal disease, and predicts future directions for the field.

Hedgehog-Gli pathway activation during kidney fibrosis

The Hedgehog (Hh) signaling pathway regulates tissue patterning during development, including patterning and growth of limbs and face, but whether Hh signaling plays a role in adult kidney remains undefined. In this study, using a panel of hedgehog-reporter mice, we show that the two Hh ligands (Indian hedgehog and sonic hedgehog ligands) are expressed in tubular epithelial cells. We report that the Hh effectors (Gli1 and Gli2) are expressed exclusively in adjacent platelet-derived growth factor receptor-β-positive interstitial pericytes and perivascular fibroblasts, suggesting a paracrine signaling loop. In two models of renal fibrosis, Indian Hh ligand was upregulated with a dramatic activation of downstream Gli effector expression. Hh-responsive Gli1-positive interstitial cells underwent 11-fold proliferative expansion during fibrosis, and both Gli1- and Gli2-positive cells differentiated into α-smooth muscle actin-positive myofibroblasts. In the pericyte-like cell line 10T1/2, hedgehog ligand triggered cell proliferation, suggesting a possible role for this pathway in the regulation of cell cycle progression of myofibroblast progenitors during the development of renal fibrosis. The hedgehog antagonist IPI-926 abolished Gli1 induction in vivo but did not decrease kidney fibrosis. However, the transcriptional induction of Gli2 was unaffected by IPI-926, suggesting the existence of smoothened-independent Gli activation in this model. This study is the first detailed description of paracrine hedgehog signaling in adult kidney, which indicates a possible role for hedgehog-Gli signaling in fibrotic chronic kidney disease.

Sonic hedgehog signaling mediates epithelial-mesenchymal communication and promotes renal fibrosis

Sonic hedgehog (Shh) signaling is a developmental signal cascade that plays an essential role in regulating embryogenesis and tissue homeostasis. Here, we investigated the potential role of Shh signaling in renal interstitial fibrogenesis. Ureteral obstruction induced Shh, predominantly in the renal tubular epithelium of the fibrotic kidneys. Using Gli1(lacZ) knock-in mice, we identified renal interstitial fibroblasts as Shh-responding cells. In cultured renal fibroblasts, recombinant Shh protein activated Gli1 and induced α-smooth muscle actin (α-SMA), desmin, fibronectin, and collagen I expression, suggesting that Shh signaling promotes myofibroblast activation and matrix production. Blockade of Shh signaling with cyclopamine abolished the Shh-mediated induction of Gli1, Snail1, α-SMA, fibronectin, and collagen I. In vivo, the kidneys of Gli1-deficient mice were protected against the development of interstitial fibrosis after obstructive injury. In wild-type mice, cyclopamine did not affect renal Shh expression but did inhibit induction of Gli1, Snail1, and α-SMA. In addition, cyclopamine reduced matrix expression and mitigated fibrotic lesions. These results suggest that tubule-derived Shh mediates epithelial-mesenchymal communication by targeting interstitial fibroblasts after kidney injury. We conclude that Shh/Gli1 signaling plays a critical role in promoting fibroblast activation, production of extracellular matrix, and development of renal interstitial fibrosis.

Hedgehog signaling: networking to nurture a promalignant tumor microenvironment

In addition to its role in embryonic development, the Hedgehog pathway has been shown to be an active participant in cancer development, progression, and metastasis. Although this pathway is activated by autocrine signaling by Hedgehog ligands, it can also initiate paracrine signaling with cells in the microenvironment. This creates a network of Hedgehog signaling that determines the malignant behavior of the tumor cells. As a result of paracrine signal transmission, the effects of Hedgehog signaling most profoundly influence the stromal cells that constitute the tumor microenvironment. The stromal cells in turn produce factors that nurture the tumor. Thus, such a resonating cross-talk can amplify Hedgehog signaling, resulting in molecular chatter that overall promotes tumor progression. Inhibitors of Hedgehog signaling have been the subject of intense research. Several of these inhibitors are currently being evaluated in clinical trials. Here, we review the role of the Hedgehog pathway in the signature characteristics of cancer cells that determine tumor development, progression, and metastasis. This review condenses the latest findings on the signaling pathways that are activated and/or regulated by molecules generated from Hedgehog signaling in cancer and cites promising clinical interventions. Finally, we discuss future directions for identifying the appropriate patients for therapy, developing reliable markers of efficacy of treatment, and combating resistance to Hedgehog pathway inhibitors.