Semaphorin3a inhibits ureteric bud branching morphogenesis

Adriamycin-induced podocyte injury via the Sema3A/TRPC5/Rac1 pathway

Podocytopathies encompass kidney diseases where direct or indirect podocyte injury leads to proteinuria or nephrotic syndrome. Although Semaphorin3A (Sema3A) is expressed in podocytes and tubular cells in adult mammalian kidneys and has a common effect on the progression of podocyte injury, its mechanism remains unclear. Previous studies have shown increased Sema3A expression in various glomerulopathies, indicating a gap in understanding its role. In this study, analysis of human data revealed a positive correlation between the levels of urinary Sema3A and Podocalyxin (PCX), suggesting a close relationship between Sema3A and podocyte loss. Furthermore, the impact of Adriamycin on podocytes was investigated. Adriamycin induced podocyte migration and apoptosis, along with an increase in Sema3A expression, all of which were ameliorated by the inhibition of Sema3A. Importantly, TRPC5 was found to increase the overexpression of Sema3A in podocytes. A TRPC5 inhibitor, AC1903, alleviated podocyte migration and apoptosis, inhibiting the formation of lamellar pseudopodia in the podocyte cytoskeleton by lowering the expression of Rac1. Furthermore, AC1903 relieved massive albuminuria and foot process effacement in the kidneys of Adriamycin-treated mice in vivo. In conclusion, our findings suggest that Sema3A may impact the cytoskeletal stability of podocytes through TRPC5 ion channels, mediated by Rac1, ultimately leading to foot process effacement. Notably, AC1903 demonstrates the potential to reverse Adriamycin-induced foot process fusion and urine protein. These results contribute to a deeper understanding of the mechanisms involved in podocytopathies and highlight the therapeutic potential of targeting the Sema3A-TRPC5 pathway.

Central role of podocytes in mediating cellular cross talk in glomerular health and disease

Podocytes are highly specialized epithelial cells that surround the capillaries of the glomeruli in the kidney. Together with the glomerular endothelial cells, these postmitotic cells are responsible for regulating filtrate from the circulating blood with their organized network of interdigitating foot processes that wrap around the glomerular basement membrane. Although podocyte injury and subsequent loss is the hallmark of many glomerular diseases, recent evidence suggests that the cell-cell communication between podocytes and other glomerular and nonglomerular cells is critical for the development and progression of kidney disease. In this review, we highlight these key cellular pathways of communication and how they might be a potential target for therapy in glomerular disease. We also postulate that podocytes might serve as a central hub for communication in the kidney under basal conditions and in response to cellular stress, which may have implications for the development and progression of glomerular diseases.

Role of Semaphorin 3A in Kidney Development and Diseases

Kidney diseases are worldwide public health problems affecting millions of people. However, there are still limited therapeutic options against kidney diseases. Semaphorin 3A (SEMA3A) is a secreted and membrane-associated protein, which regulates diverse functions, including immune regulation, cell survival, migration and angiogenesis, thus involving in the several pathogeneses of diseases, including eyes and neurons, as well as kidneys. SEMA3A is expressed in podocytes and tubular cells in the normal adult kidney, and recent evidence has revealed that excess SEMA3A expression and the subsequent signaling pathway aggravate kidney injury in a variety of kidney diseases, including nephrotic syndrome, diabetic nephropathy, acute kidney injury, and chronic kidney disease. In addition, several reports have demonstrated that the inhibition of SEMA3A ameliorated kidney injury via a reduction in cell apoptosis, fibrosis and inflammation; thus, SEMA3A may be a potential therapeutic target for kidney diseases. In this review article, we summarized the current knowledge regarding the role of SEMA3A in kidney pathophysiology and their potential use in kidney diseases.

β-Catenin in the kidney stroma modulates pathways and genes to regulate kidney development

BACKGROUND

Kidney development is regulated by cellular interactions between the ureteric epithelium, mesenchyme, and stroma. Previous studies demonstrate essential roles for stromal β-catenin in kidney development. However, how stromal β-catenin regulates kidney development is not known. We hypothesize that stromal β-catenin modulates pathways and genes that facilitate communications with neighboring cell populations to regulate kidney development.

RESULTS

We isolated purified stromal cells with wild type, deficient, and overexpressed β-catenin by fluorescence-activated cell sorting and conducted RNA Sequencing. A Gene Ontology network analysis demonstrated that stromal β-catenin modulates key kidney developmental processes, including branching morphogenesis, nephrogenesis and vascular formation. Specific stromal β-catenin candidate target genes that may mediate these effects included secreted, cell-surface and transcriptional factors that regulate branching morphogenesis and nephrogenesis (Wnts, Bmp, Fgfr, Tcf/Lef) and secreted vascular guidance cues (Angpt1, VEGF, Sema3a). We validated established β-catenin targets including Lef1 and novel candidate β-catenin targets including Sema3e which have unknown roles in kidney development.

CONCLUSIONS

These studies advance our understanding of gene and biological pathway dysregulation in the context of stromal β-catenin misexpression during kidney development. Our findings suggest that during normal kidney development, stromal β-catenin may regulate secreted and cell-surface proteins to communicate with adjacent cell populations.

The Potential Biotherapeutic Targets of Contrast-Induced Acute Kidney Injury

Contrast-induced acute kidney injury (CI-AKI) is manifested by an abrupt decline in kidney function as a consequence of intravascular exposure to contrast media. With the increased applicability of medical imaging and interventional procedures that utilize contrast media for clinical diagnosis, CI-AKI is becoming the leading cause of renal dysfunction. The pathophysiological mechanism associated with CI-AKI involves renal medullary hypoxia, the direct toxicity of contrast agents, oxidative stress, apoptosis, inflammation, and epigenetic regulation. To date, there is no effective therapy for CI-AKI, except for the development of strategies that could reduce the toxicity profiles of contrast media. While most of these strategies have failed, evidence has shown that the proper use of personalized hydration, contrast medium, and high-dose statins may reduce the occurrence of CI-AKI. However, adequate risk predication and attempts to develop preventive strategies can be considered as the key determinants that can help eliminate CI-AKI. Additionally, a deeper understanding of the pathophysiological mechanism of CI-AKI is crucial to uncover molecular targets for the prevention of CI-AKI. This review has taken a step further to solidify the current known molecular mechanisms of CI-AKI and elaborate the biomarkers that are used to detect early-stage CI-AKI. On this foundation, this review will analyze the molecular targets relating to apoptosis, inflammation, oxidative stress, and epigenetics, and, thus, provide a strong rationale for therapeutic intervention in the prevention of CI-AKI.

Sox11 gene disruption causes congenital anomalies of the kidney and urinary tract (CAKUT)

Congenital abnormalities of the kidney and the urinary tract (CAKUT) belong to the most common birth defects in human, but the molecular basis for the majority of CAKUT patients remains unknown. Here we show that the transcription factor SOX11 is a crucial regulator of kidney development. SOX11 is expressed in both mesenchymal and epithelial components of the early kidney anlagen. Deletion of Sox11 in mice causes an extension of the domain expressing Gdnf within rostral regions of the nephrogenic cord and results in duplex kidney formation. On the molecular level SOX11 directly binds and regulates a locus control region of the protocadherin B cluster. At later stages of kidney development, SOX11 becomes restricted to the intermediate segment of the developing nephron where it is required for the elongation of Henle's loop. Finally, mutation analysis in a cohort of patients suffering from CAKUT identified a series of rare SOX11 variants, one of which interferes with the transactivation capacity of the SOX11 protein. Taken together these data demonstrate a key role for SOX11 in normal kidney development and may suggest that variants in this gene predispose to CAKUT in humans.

Gpr97 Exacerbates AKI by Mediating Sema3A Signaling

Background G protein-coupled receptors (GPCRs) participate in a variety of physiologic functions, and several GPCRs have critical physiologic and pathophysiologic roles in the regulation of renal function. We investigated the role of Gpr97, a newly identified member of the adhesion GPCR family, in AKI.Methods AKI was induced by ischemia-reperfusion or cisplatin treatment in Gpr97-deficient mice. We assessed renal injury in these models and in patients with acute tubular necrosis by histologic examination, and we conducted microarray analysis and in vitro assays to determine the molecular mechanisms of Gpr97 function.Results Gpr97 was upregulated in the kidneys from mice with AKI and patients with biopsy-proven acute tubular necrosis compared with healthy controls. In AKI models, Gpr97-deficient mice had significantly less renal injury and inflammation than wild-type mice. Gpr97 deficiency also attenuated the AKI-induced expression of semaphorin 3A (Sema3A), a potential early diagnostic biomarker of renal injury. In NRK-52E cells subjected to oxygen-glucose deprivation, siRNA-mediated knockdown of Gpr97 further increased the expression of survivin and phosphorylated STAT3 and reduced toll-like receptor 4 expression. Cotreatment with recombinant murine Sema3A protein counteracted these effects. Finally, additional in vivo and in vitro studies, including electrophoretic mobility shift assays and luciferase reporter assays, showed that Gpr97 deficiency attenuates ischemia-reperfusion-induced expression of the RNA-binding protein human antigen R, which post-transcriptionally regulates Sema3A expression.Conclusions Gpr97 is an important mediator of AKI, and pharmacologic targeting of Gpr97-mediated Sema3A signaling at multiple levels may provide a novel approach for the treatment of AKI.

Urinary semaphorin 3A as an early biomarker to predict contrast-induced acute kidney injury in patients undergoing percutaneous coronary intervention

Contrast-induced acute kidney injury (CI-AKI) is a serious complication of diagnostic coronary angiograph and percutaneous coronary intervention (PCI). However, the exact pathophysiological mechanisms underlying CI-AKI development are largely unknown. The present study examined whether urinary semaphorin 3A levels predict the development of CI-AKI in patients undergoing PCI. This study enrolled 168 patients with stable angina undergoing elective PCI. Serial urine samples, obtained at baseline and 2, 6, 12, 24, 36, and 48 h post-PCI were analyzed by semaphorin 3A and neutrophil gelatinase-associated lipocalin (NGAL) ELISA kit. AKI was defined as an increase in serum creatinine beyond 50% according to the RIFLE classification system. Receiver operator characteristic (ROC) curve analyses identified optimal semaphorin 3A and NGAL values for diagnosing CI-AKI. CI-AKI occurred in 20 of 168 patients. There were no significant differences in the baseline clinical characteristics and angiographic findings between non-AKI patients group and AKI patients group. Both urinary semaphorin 3A and NGAL levels significantly increased at 2 and 6 h post-PCI. ROC analysis showed that the cut-off value of 389.5 pg/mg semaphorin 3A at 2 h post-PCI corresponds to 94% sensitivity and 75% specificity and the cut-off value of 94.4 ng/mg NGAL at 2 h post-PCI corresponds to 74% sensitivity and 82% specificity. Logistic regression showed that semaphorin 3A levels at 2 and 6 h post-PCI were the significant predictors of AKI in our cohort. Urinary semaphorin 3A may be a promising early biomarker for predicting CI-AKI in patients undergoing PCI.

Neuropilin-1 is a glial cell line-derived neurotrophic factor receptor in glioblastoma

The aim of this study was to identify the receptor for glial cell line-derived neurotrophic factor (GDNF) in glioblastoma multiforme (GBM). After GST pull-down assays, membrane proteins purified from C6 rat glioma cells were subjected to liquid chromatography-tandem mass spectrometry (LC-MS/MS). The differentially expressed proteins were annotated using Gene Ontology, and neuropilin-1 (NRP1) was identified as the putative GDNF receptor in glioma. NRP1 was more highly expressed in human GBM brains and C6 rat glioma cells than in normal human brains or primary rat astrocytes. Immunofluorescence staining showed that NRP1 was recruited to the membrane by GDNF, and NRP1 co-immunoprecipitated with GDNF. Using the NRP1 and GDNF protein structures to assess molecular docking in the ZDOCK server and visualization with the PyMOL Molecular Graphics System revealed 8 H-bonds and stable positive and negative electrostatic interactions between NRP1 and GDNF. RNAi knockdown of NRP1 reduced proliferation of C6 glioma cells when stimulated with GDNF. NRP1 was an independent risk factor for both survival and recurrence in GBM patients. High NRP1 mRNA expression correlated with shorter OS and DFS (OS: χ²=4.6720, P=0.0307; DFS: χ²=11.013, P=0.0009). NRP1 is thus a GDNF receptor in glioma cells and a potential therapeutic target.

The level of urinary semaphorin3A is associated with disease activity in patients with minimal change nephrotic syndrome

Semaphorin3A is a secreted protein known to be involved in organogenesis, immune responses and cancer. In the kidney, semaphorin3A is expressed in the glomerular podocytes, distal tubules and collecting tubules, and believed to play a role in the regulation of the kidney development and function. We examined the serum and urinary semaphorin3A levels in 72 patients with renal disease and 5 healthy volunteers. The patients had been diagnosed with thin basement membrane disease (n=4), minimal change nephrotic syndrome (MCNS; n=22), IgA nephritis (n=21), membranous nephropathy (n=16) and focal segmental glomerular sclerosis (n=9). The level of urinary semaphorin3A in MCNS patients tended to be relatively high among all disease groups. We also investigated the urinary semaphorin3A level in 7 patients with MCNS from disease onset to remission during the drug therapy. MCNS patients in pre-remission states had higher urinary semaphorin3A levels than those in post-remission states receiving immunosuppressive therapies. These results suggested that the urinary semaphorin3A level correlates with the MCNS activity. Semaphorin3A has the potential as a biomarker for MCNS to clarify the reactivity for therapy and may be useful in examining other glomerular diseases with proteinuria as well.

Podocyte Shape Regulation by Semaphorin 3A and MICAL-1

Podocytes are complex epithelial cells with foot processes that are essential for the integrity and function of the kidney glomerular filters. Podocyte foot processes linked by slit diaphragms constitute signaling platforms that tightly regulate the cell shape and the function of the filtration barrier. Semaphorin (Sema) 3A is a class 3 semaphorin secreted by podocytes that has autocrine and paracrine functions in the kidney. We have shown that Sema3A regulates podocyte shape and that excess Sema3A signaling induces glomerular disease and aggravates diabetic nephropathy. MICAL-1 is an actin-binding protein that mediates Sema3A signals in podocytes. This chapter describes the methods used to examine how Sema3A signaling regulates podocyte shape.

Blood vessel crosstalk during organogenesis-focus on pancreas and endothelial cells

Blood vessels form a highly branched, interconnected, and largely stereotyped network of tubes that sustains every organ and tissue in vertebrates. How vessels come to take on their particular architecture, or how they are 'patterned,' and in turn, how they influence surrounding tissues are fundamental questions of organogenesis. Decades of work have begun to elucidate how endothelial progenitors arise and home to precise locations within tissues, integrating attractive and repulsive cues to build vessels where they are needed. Conversely, more recent findings have revealed an exciting facet of blood vessel interaction with tissues, where vascular cells provide signals to developing organs and progenitors therein. Here, we discuss the exchange of reciprocal signals between endothelial cells and neighboring tissues during embryogenesis, with a special focus on the developing pancreas. Understanding the mechanisms driving both sides of these interactions will be crucial to the development of therapies, from improving organ regeneration to efficient production of cell based therapies. Specifically, elucidating the interface of the vasculature with pancreatic lineages, including endocrine cells, will instruct approaches such as generation of replacement beta cells for Type I diabetes. WIREs Dev Biol 2016, 5:598-617. doi: 10.1002/wdev.240 For further resources related to this article, please visit the WIREs website.

Role of Neuropilin-1 in Diabetic Nephropathy

Diabetic nephropathy (DN) often develops in patients suffering from type 1 or type 2 diabetes mellitus. DN is characterized by renal injury resulting in proteinuria. Neuropilin-1 (NRP-1) is a single-pass transmembrane receptor protein devoid of enzymatic activity. Its large extracellular tail is structured in several domains, thereby allowing the molecule to interact with multiple ligands linking NRP-1 to different pathways through its signaling co-receptors. NRP-1’s role in nervous system development, immunity, and more recently in cancer, has been extensively investigated. Although its relation to regulation of apoptosis and cytoskeleton organization of glomerular vascular endothelial cells was reported, its function in diabetes mellitus and the development of DN is less clear. Several lines of evidence demonstrate a reduced NRP-1 expression in glycated-BSA cultured differentiated podocytes as well as in glomeruli from db/db mice (a model of type 2 Diabetes) and in diabetic patients diagnosed with DN. In vitro studies of podocytes implicated NRP-1 in the regulation of podocytes’ adhesion to extracellular matrix proteins, cytoskeleton reorganization, and apoptosis via not completely understood mechanisms. However, the exact role of NRP-1 during the onset of DN is not yet understood. This review intends to shed more light on NRP-1 and to present a link between NRP-1 and its signaling complexes in the development of DN.

Unwrapping the origins and roles of the renal endothelium

The renal vasculature, like all vessels, is lined by a thin layer of simple squamous epithelial cells called an endothelium. These endothelial-lined vessels can be subdivided into four major compartments: arteries, veins, capillaries and lymphatics. The renal vasculature is a highly integrated network that forms through the active processes of angiogenesis and vasculogenesis. Determination of the precise contribution of these two processes and of the molecular signaling that governs the differentiation, specification and maturation of these critical cell populations is the focus of an actively evolving field of research. Although much of the focus has concentrated on the origin of the glomerular capillaries, in this review we extend the investigation to the origins of the endothelial cells throughout the entire kidney and the signaling events that cause their distinct functional and molecular profiles. A thorough understanding of endothelial cell biology may play a critical role in a better understanding of renal vascular diseases.

Organ In Vitro Culture: What Have We Learned about Early Kidney Development?

When Clifford Grobstein set out to study the inductive interaction between tissues in the developing embryo, he developed a method that remained important for the study of renal development until now. From the late 1950s on, in vitro cultivation of the metanephric kidney became a standard method. It provided an artificial environment that served as an open platform to study organogenesis. This review provides an introduction to the technique of organ culture, describes how the Grobstein assay and its variants have been used to study aspects of mesenchymal induction, and describes the search for natural and chemical inducers of the metanephric mesenchyme. The review also focuses on renal development, starting with ectopic budding of the ureteric bud, ureteric bud branching, and the generation of the nephron and presents the search for stem cells and renal progenitor cells that contribute to specific structures and tissues during renal development. It also presents the current use of Grobstein assay and its modifications in regenerative medicine and tissue engineering today. Together, this review highlights the importance of ex vivo kidney studies as a way to acquire new knowledge, which in the future can and will be implemented for developmental biology and regenerative medicine applications.

Urinary semaphorin 3A correlates with diabetic proteinuria and mediates diabetic nephropathy and associated inflammation in mice

Semaphorin 3A (sema3A) was recently identified as an early diagnostic biomarker of acute kidney injury. However, its role as a biomarker and/or mediator of chronic kidney disease (CKD) related to diabetic nephropathy is unknown. We examined the expression of sema3A in diabetic animal models and in humans and tested whether sema3A plays a pathogenic role in the development of diabetic nephropathy. The expression of sema3A was localized to podocytes and epithelial cells in distal tubules and collecting ducts in control animals, and its expression was increased following the induction of diabetes. Quantification of sema3A urinary excretion in three different diabetic mouse models showed that excretion was increased as early as 2 weeks after the induction of diabetes and increased over time, in conjunction with the development of nephropathy. Consistent with the mouse data, increased sema3A urinary excretion was detected in diabetic patients with albuminuria, particularly in those with macroalbuminuria. Genetic ablation of sema3A or pharmacological inhibition with a novel sema3A inhibitory peptide was protected against diabetes-induced albuminuria, kidney fibrosis, inflammation, oxidative stress, and renal dysfunction. We conclude that sema3A is both a biomarker and a mediator of diabetic kidney disease and could be a promising therapeutic target in diabetic nephropathy. Key messages Diabetes induced sema3A excretion in urine. Increased semaphorin 3A was associated with severity of albuminuria. Seme3A-mediated diabetes induced glomerulosclerosis. Peptide-based inhibition of semaphorin3A suppressed diabetic nephropathy.

Semaphorin 3A inactivation suppresses ischemia-reperfusion-induced inflammation and acute kidney injury

Recent studies show that guidance molecules that are known to regulate cell migration during development may also play an important role in adult pathophysiologic states. One such molecule, semaphorin3A (sema3A), is highly expressed after acute kidney injury (AKI) in mice and humans, but its pathophysiological role is unknown. Genetic inactivation of sema3A protected mice from ischemia-reperfusion-induced AKI, improved tissue histology, reduced neutrophil infiltration, prevented epithelial cell apoptosis, and increased cytokine and chemokine excretion in urine. Pharmacological-based inhibition of sema3A receptor binding likewise protected against ischemia-reperfusion-induced AKI. In vitro, sema3A enhanced toll-like receptor 4-mediated inflammation in epithelial cells, macrophages, and dendritic cells. Moreover, administration of sema3A-treated, bone marrow-derived dendritic cells exacerbated kidney injury. Finally, sema3A augmented cisplatin-induced apoptosis in kidney epithelial cells in vitro via expression of DFFA-like effector a (cidea). Our data suggest that the guidance molecule sema3A exacerbates AKI via promoting inflammation and epithelial cell apoptosis.

Increased urine semaphorin-3A is associated with renal damage in hypertensive patients with chronic kidney disease: a nested case-control study

BACKGROUND

Semaphorins are guidance proteins implicated in several processes such as angiogenesis, organogenesis, cell migration, and cytokine release. Experimental studies showed that semaphorin-3a (SEMA3A) administration induces transient massive proteinuria, podocyte foot process effacement and endothelial cell damage in healthy animals. While SEMA3A signaling has been demonstrated to be mechanistically involved in experimental diabetic glomerulopathy and in acute kidney injury, to date its role in human chronic kidney disease (CKD) has not been investigated.

METHODS

To test the hypothesis that SEMA3A may play a role in human CKD, we performed a cross-sectional, nested, case-control study on 151 matched hypertensive patients with and without CKD. SEMA3A was quantified in the urine (USEMA) by ELISA. Glomerular filtration rate was estimated (eGFR) by the CKD-EPI formula and albuminuria was measured as albumin-to-creatinine ratio (ACR).

RESULTS

USEMA levels were positively correlated with urine ACR (p = 0.001) and serum creatinine (p < 0.001). USEMA was higher in patients with both components of renal damage as compared to those with only one and those with normal renal function (p < 0.007 and <0.001, respectively). The presence of increased USEMA levels (i.e. top quartile) entailed a fourfold higher risk of combined renal damage (p < 0.001) and an almost twofold higher risk of macroalbuminuria (p = 0.005) or of reduced eGFR, even adjusting for confounding factors (p = 0.002).

CONCLUSIONS

USEMA is independently associated with CKD in both diabetic and non diabetic hypertensive patients. Further studies may help clarify the mechanisms underlying this association and possibly the pathogenic changes leading to the development of CKD.

Semaphorin3a signaling, podocyte shape, and glomerular disease

Semaphorin3a (sema3a), a member of class 3 semaphorins, is a guidance protein that regulates angiogenesis, branching morphogenesis, axon growth, and cell migration, and has pleiotropic roles on organogenesis, immune response, and cancer. Sema3a is secreted by podocytes and is required for normal kidney patterning and glomerular filtration barrier development. We recently discovered that after completion of kidney development, Sema3a gain-of-function in podocytes leads to proteinuric glomerular disease in mice. Excess sema3a causes foot process effacement, glomerular basement lamination, and endothelial damage in vivo, and disrupts cell autonomously podocyte shape by down-regulating nephrin and inhibiting αvβ3 integrin. We identified a novel direct interaction between nephrin and plexinA1, the sema3a signaling receptor. Nephrin-plexinA1 interaction links the slit-diaphragm signaling complex to extracellular sema3a signals. Hence, sema3a functions as an extracellular negative regulator of the structure and function of the glomerular filtration barrier.

Gene regulatory network of renal primordium development

Animal development progresses through the stepwise deployment of gene regulatory networks (GRN) encoded in the genome. Comparative analyses in different species and organ systems have revealed that GRN blueprints are composed of subcircuits with stereotypical architectures that are often reused as modular units. In this review, we report the evidence for the GRN underlying renal primordium development. In vertebrates, renal development is initiated by the induction of a field of intermediate mesoderm cells competent to undergo lineage specification and nephric (Wolffian) duct formation. Definition of the renal field leads to the activation of a core regulatory subcircuit composed of the transcription factors Pax2/8, Gata3 and Lim1. These transcription factors turn on a second layer of transcriptional regulators while also activating effectors of tissue morphogenesis and cellular specialization. Elongation and connection of the nephric duct to the cloaca (bladder/urethra primordium) is followed by metanephric kidney induction through signals emanating from the metanephric mesenchyme. Central to this process is the activation and positioning of the glial cell line-derived neurotrophic factor (Gdnf)-Ret signaling pathway by network subcircuits located in the mesenchyme and epithelial tissues of the caudal trunk. Evidence shows that each step of the renal primordium developmental program is regulated by structured GRN subunits organized in a hierarchical manner. Understanding the structure and dynamics of the renal GRN will help us understand the intrinsic phenotypical variability of congenital anomalies of the kidney and urinary tract and guide our approaches to regenerative medicine.

Genetic controls and cellular behaviors in branching morphogenesis of the renal collecting system

The mammalian kidney, which at maturity contains thousands of nephrons joined to a highly branched collecting duct (CD) system, is an important model system for studying the development of a complex organ. Furthermore, congenital anomalies of the kidney and urinary tract, often resulting from defects in ureteric bud branching morphogenesis, are relatively common human birth defects. Kidney development is initiated by interactions between the nephric duct and the metanephric mesenchyme, leading to the outgrowth and repeated branching of the ureteric bud epithelium, which gives rise to the entire renal CD system. Meanwhile, signals from the ureteric bud induce the mesenchyme cells to form the nephron epithelia. This review focuses on development of the CD system, with emphasis on the mouse as an experimental system. The major topics covered include the origin and development of the nephric duct, formation of the ureteric bud, branching morphogenesis of the ureteric bud, and elongation of the CDs. The signals, receptors, transcription factors, and other regulatory molecules implicated in these processes are discussed. In addition, our current knowledge of cellular behaviors that are controlled by these genes and underlie development of the collecting system is reviewed.

Protein/peptide transduction in metanephric explant culture

While gene targeting methods have largely supplanted cell/explant culture models for studying developmental processes, they have not eliminated the need for or value of such approaches in the investigator's technical arsenal. Explant culture models, such as those devised for the metanephric kidney and its progenitors, remain invaluable as tools for screening regulatory factors involved in tissue induction or in the inhibition of progenitor specification. Thus, some factors capable of inducing tissue condensations or nephronic tubule formation in explants of metanephric mesenchyme have been identified through direct treatment of cultures rather than lengthy genetic engineering in animals. Unfortunately, renal progenitors are largely refractory to most contemporary methods for gene manipulation, including transfection and viral transduction, so the applications of explant culture have been rather limited. However, methods for protein or peptide transduction offer greatly improved efficiencies for uptake and expression/regulation of proteins within cells and tissues. Biologically active TAT- or penetratin-fusion proteins/peptides are readily taken up by most cells in metanephric explants or monolayer cultured cells (Plisov et al., J Am Soc Nephrol 16:1632-1644, 2005; Osafune et al., Development 133:151-161, 2006; Wang et al., Cell Signal 22:1717-1726, 2010; Tanigawa, Dev Biol 352:58-69, 2011), allowing a direct functional evaluation of theoretically any protein, including biologically active enzymes and transcription factors, or any targeted interactive domain within a protein.

Excess podocyte semaphorin-3A leads to glomerular disease involving plexinA1-nephrin interaction

Semaphorin-3A (Sema3a), a guidance protein secreted by podocytes, is essential for normal kidney patterning and glomerular filtration barrier development. Here, we report that podocyte-specific Sema3a gain-of-function in adult mice leads to proteinuric glomerular disease involving the three layers of the glomerular filtration barrier. Reversibility of the glomerular phenotype upon removal of the transgene induction provided proof-of-principle of the cause-and-effect relationship between podocyte Sema3a excess and glomerular disease. Mechanistically, excess Sema3a induces dysregulation of nephrin, matrix metalloproteinase 9, and αvβ3 integrin in vivo. Sema3a cell-autonomously disrupts podocyte shape. We identified a novel direct interaction between the Sema3a signaling receptor plexinA1 and nephrin, linking extracellular Sema3a signals to the slit-diaphragm signaling complex. We conclude that Sema3a functions as an extracellular negative regulator of the structure and function of the glomerular filtration barrier in the adult kidney. Our findings demonstrate a crosstalk between Sema3a and nephrin signaling pathways that is functionally relevant both in vivo and in vitro.

Semaphorin 3A is a new early diagnostic biomarker of experimental and pediatric acute kidney injury

BACKGROUND

Semaphorin 3A is a secreted protein that regulates cell motility and attachment in axon guidance, vascular growth, immune cell regulation and tumor progression. However, nothing is known about its role in kidney pathophysiology. Here, we determined whether semaphorin3A is induced after acute kidney injury (AKI) and whether urinary semaphorin 3A can predict AKI in humans undergoing cardiopulmonary bypass (CPB).

METHODS AND PRINCIPAL FINDINGS

In animals, semaphorin 3A is localized in distal tubules of the kidney and excretion increased within 3 hr after reperfusion of the kidney whereas serum creatinine was significantly raised at 24 hr. In humans, using serum creatinine, AKI was detected on average only 48 hours after CPB. In contrast, urine semaphorin increased at 2 hours after CPB, peaked at 6 hours (2596±591 pg/mg creatinine), and was no longer significantly elevated 12 hours after CPB. The predictive power of semaphorin 3A as demonstrated by area under the receiver-operating characteristic curve for diagnosis of AKI at 2, 6, and 12 hours after CPB was 0.88, 0.81, and 0.74, respectively. The 2-hour urine semaphorin measurement strongly correlated with duration and severity of AKI, as well as length of hospital stay. Adjusting for CPB time and gender, the 2-hour semaphorin remained an independent predictor of AKI, with an odds ratio of 2.19.

CONCLUSION

Our results suggest that semaphorin 3A is an early, predictive biomarker in experimental and pediatric AKI, and may allow for the reliable early diagnosis and prognosis of AKI after CPB, much before the rise in serum creatinine.

Morpholino-mediated gene knockdown in mammalian organ culture

We examined the role of semaphorin3a in ureteric bud (UB) branching morphogenesis using mouse metanephric organ culture [Tufro et al. (Mech Dev 125:558-568, 2008)]. In vitro UB injection of Sema3a antisense morpholino resulted in increased branching morphogenesis. Cellular and tissue uptake of oligonucleotides was facilitated by a peptide-mediated method. Our findings were validated by in vitro translation and in Sema3a null mice. This chapter describes a method to perfuse the UB lumen with fluorescein-labeled oligonucleotides bound to a peptide carrier.

Semaphorin-1 and netrin signal in parallel and permissively to position the male ray 1 sensillum in Caenorhabditis elegans

Netrin and semaphorin axon guidance cues have been found to function in the genesis of several mammalian organs; however, little is known about the underlying molecular mechanisms involved. A genetic approach could help to reveal the underpinnings of these mechanisms. The most anterior ray sensillum (ray 1) in the Caenorhabditis elegans male tail is frequently displaced anterior to its normal position in smp-1/semaphorin-1a and plexin-1/plx-1 mutants. Here we report that UNC-6/netrin and its UNC-40/DCC receptor signal in parallel to SMP-1/semaphorin-1a and its PLX-1/plexin-1 receptor to prevent the anterior displacement of ray 1 and that UNC-6 plus SMP-1 signaling can account entirely for this function. We also report that mab-20/semaphorin-2a mutations, which prevent the separation of neighboring rays and cause ray fusions, suppress the anterior displacements of ray 1 caused by deficiencies in SMP-1 and UNC-6 signaling and this is independent of the ray fusion phenotype, whereas overexpression of UNC-40 and PLX-1 cause ray fusions. This suggests that for ray 1 positioning, a balance is struck between a tendency of SMP-1 and UNC-6 signaling to prevent ray 1 from moving away from ray 2 and a tendency of MAB-20/semaphorin-2a signaling to separate all rays from each other. Additional evidence suggests this balance involves the relative adhesion of the ray 1 structural cell to neighboring SET and hyp 7 hypodermal cells. This finding raises the possibility that changes in ray 1 positioning depend on passive movements caused by attachment to the elongating SET cell in opposition to the morphologically more stable hyp 7 cell. Several lines of evidence indicate that SMP-1 and UNC-6 function permissively in the context of ray 1 positioning.

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.

Neuropilins: expression and roles in the epithelium

Initially found expressed in neuronal and then later in endothelial cells, it is well established that the transmembrane glycoproteins neuropilin-1 (NRP1) and neuropilin-2 (NRP2) play essential roles in axonal growth and guidance and in physiological and pathological angiogenesis. Neuropilin expression and function in epithelial cells has received little attention when compared with neuronal and endothelial cells. Overexpression of NRPs is shown to enhance growth, correlate with invasion and is associated with poor prognosis in various tumour types, especially those of epithelial origin. The contribution of NRP and its ligands to tumour growth and metastasis has spurred a strong interest in NRPs as novel chemotherapy drug targets. Given NRP's role as a multifunctional co-receptor with an ability to bind with disparate ligand families, this has sparked new areas of research implicating NRPs in diverse biological functions. Here, we review the growing body of research demonstrating NRP expression and role in the normal and neoplastic epithelium.

PI3K signaling in the regulation of branching morphogenesis

Branching morphogenesis drives the formation of epithelial organs including the mammary gland, lung, kidney, salivary gland and prostate. Branching at the cellular level also drives development of the nervous and vascular systems. A variety of signaling pathways are orchestrated together to establish the pattern of these branched organs. The phosphoinositide 3-kinase (PI3K) signaling network is of particular interest because of the diverse outcomes it generates, including proliferation, motility, growth, survival and cell death. Here, we focus on the role of the PI3K pathway in the development of branched tissues. Cultured cells, explants and transgenic mice have revealed that the PI3K pathway is critical for the regulation of cell proliferation, apoptosis and motility during branching of tissues.

Semaphorin 3A contributes to distal pulmonary epithelial cell differentiation and lung morphogenesis

Rationale

Semaphorin 3A (Sema3A) is a neural guidance cue that also mediates cell migration, proliferation and apoptosis, and inhibits branching morphogenesis. Because we have shown that genetic deletion of neuropilin-1, which encodes an obligatory Sema3A co-receptor, influences airspace remodeling in the smoke-exposed adult lung, we sought to determine whether genetic deletion of Sema3A altered distal lung structure.

Methods

To determine whether loss of Sema3A signaling influenced distal lung morphology, we compared pulmonary histology, distal epithelial cell morphology and maturation, and the balance between lung cell proliferation and death, in lungs from mice with a targeted genetic deletion of Sema3A (Sema3A^-/-) and wild-type (Sema3A^+/+) littermate controls.

Results

Genetic deletion of Sema3A resulted in significant perinatal lethality. At E17.5, lungs from Sema3A^-/- mice had thickened septae and reduced airspace size. Distal lung epithelial cells had increased intracellular glycogen pools and small multivesicular and lamellar bodies with atypical ultrastructure, as well as reduced expression of type I alveolar epithelial cell markers. Alveolarization was markedly attenuated in lungs from the rare Sema3A^-/- mice that survived the immediate perinatal period. Furthermore, Sema3A deletion was linked with enhanced postnatal alveolar septal cell death.

Conclusions

These data suggest that Sema3A modulates distal pulmonary epithelial cell development and alveolar septation. Defining how Sema3A influences structural plasticity of the developing lung is a critical first step for determining if this pathway can be exploited to develop innovative strategies for repair after acute or chronic lung injury.

Semaphorins in kidney development and disease: modulators of ureteric bud branching, vascular morphogenesis, and podocyte-endothelial crosstalk

Semaphorins are guidance proteins that play important roles in organogenesis and disease. Expression of class 3 semaphorins and their receptors is regulated during kidney development. Gain- and loss-of-function experiments demonstrated that tight semaphorin3a gene dosage is required for podocyte differentiation, and for the establishment of a normal glomerular filtration barrier. Sema3a modulates kidney vascular patterning acting as a negative regulator of endothelial cell migration and survival. Excess podocyte semaphorin3a expression causes glomerular disease in mice. In addition, Sema3a is a negative regulator of ureteric bud branching, whereas Sema3c is a positive regulator of ureteric bud and endothelial cell branching morphogenesis. In summary, secreted semaphorins modulate ureteric bud branching, vascular patterning, and podocyte-endothelial crosstalk, suggesting that they play a role in renal disease. Understanding the signaling pathways downstream from semaphorin receptors will provide insight into the mechanism of action of semaphorins in renal pathology.

Microarray analysis of gene expression patterns in human proximal tubule cells over a short and long time course of cadmium exposure

Numerous studies showed that renal proximal tubules cells are the cell type critically affected by chronic exposure to cadmium (Cd(2+)). The aim of the present study was to apply global gene expression technology and a human renal epithelial cell culture model (HPT) to determine whether time of exposure to Cd(2+) exerts a major influence on the resulting pattern of global gene expression. HPT cells were exposed to Cd(2+) for a short, 1-d, period of exposure (9, 27, and 45 μM) versus a longer, 13-d, period (4.5, 9, and 27 μM), with the hypothesis being that the stress response of the cells would be more active during the short time of exposure. The results showed that the differential expression of genes was very extensive for HPT cells exposed to Cd(2+) for 1 d, with more than 1848 genes displaying alterations compared to control and with the major categories of genes being involved in stress responses; cell death; checkpoint arrest, DNA repair, and the cell cycle; inflammatory responses; and cell adhesion, motion and differentiation. In contrast, HPT cells exposed to Cd(2+) for 13 d showed 923 genes to be differentially expressed, with a marked reduction in the number of differentially expressed stress response genes and a significant increase in the number of genes involved in development and differentiation. There were 387 differentially expressed genes common to both times of exposure. Data suggest that unless one is actively seeking to study the acute stress response, global gene expression technology should not be applied within an early time course of toxicant exposure.

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.

Semaphorin3a regulates endothelial cell number and podocyte differentiation during glomerular development

Semaphorin3a (Sema3a), a chemorepellant guidance protein, plays crucial roles in neural, cardiac and peripheral vascular patterning. Sema3a is expressed in the developing nephron, mature podocytes and collecting tubules. Sema3a acts as a negative regulator of ureteric bud branching, but its function in glomerular development has not been examined. Here we tested the hypothesis that Sema3a regulates glomerular vascular development using loss- and gain-of-function mouse models. Sema3a deletion resulted in defects in renal vascular patterning, excess endothelial cells within glomerular capillaries, effaced podocytes with extremely wide foot processes and albuminuria. Podocyte Sema3a overexpression during organogenesis resulted in glomerular hypoplasia, characterized by glomerular endothelial cell apoptosis, delayed and abnormal podocyte foot process development, a complete absence of slit diaphragms and congenital proteinuria. Nephrin, WT1 and VEGFR2 were downregulated in Sema3a-overexpressing kidneys. We conclude that Sema3a is an essential negative regulator of endothelial cell survival in developing glomeruli and plays a crucial role in podocyte differentiation in vivo. Hence, a tight regulation of Sema3a dosage is required for the establishment of a normal glomerular filtration barrier.

Trps1 functions downstream of Bmp7 in kidney development

During embryonic development, the mesenchyme of the lungs, gut, kidneys, and other tissues expresses Trps1, an atypical member of the GATA-type family of transcription factors. Our previous work suggested the possibility that Trps1 acts downstream of bone morphogenic protein 7 (Bmp7), which is essential for normal renal development. To examine the role of Trps1 during early renal development, we generated Trps1-deficient mice and examined their renal histology. Compared with wild-type mice, Trps1-deficient newborn mice had fewer tubules and glomeruli, an expanded renal interstitium, and numerous uninduced metanephric mesenchymal cells, which resulted in fewer nephrons. In wild-type kidneys, Trps1 expression was present in ureteric buds, cap mesenchyme, and renal vesicles, whereas Trps1 was virtually absent in Bmp7-deficient kidneys. Furthermore, Trps1-deficient kidneys had low levels of Pax2 and Wt1, which are markers of condensed mesenchymal cells, suggesting that a lack of Trps1 affects the differentiation of cap mesenchyme to renal vesicles. In cultured metanephric mesenchymal cells, Bmp7 induced Trps1 and E-cadherin and downregulated vimentin. Knockdown of Trps1 with small interference RNA inhibited this Bmp7-induced mesenchymal-to-epithelial transition. Last, whole-mount in situ hybridization of Wnt9b and Wnt4 demonstrated prolonged branching of ureteric buds and sparse cap mesenchyme in the kidneys of Trps1-deficient mice. Taken together, these findings suggest that normal formation of nephrons requires Trps1, which mediates mesenchymal-to-epithelial transition and ureteric bud branching during early renal development.

Cell and molecular biology of kidney development

Abnormalities of kidney and urinary tract development are the most common cause of end-stage kidney failure in childhood in the United States. Over the past 20 years, the advent of mutant and transgenic mice and the manipulation of gene expression in other animal models has resulted in major advances in identification of the cellular and molecular mechanisms that direct kidney morphogenesis, providing insights into the pathophysiology of renal and urologic anomalies. This review focuses on the molecular mechanisms that define kidney progenitor cell populations, induce nephron formation within the metanephric mesenchyme, initiate and organize ureteric bud branching, and participate in terminal differentiation of the nephron. Highlighted are common signaling pathways that function at multiple stages during kidney development, including signaling via Wnts, bone morphogenic proteins, fibroblast growth factor, sonic hedgehog, RET/glial cell-derived neurotrophic factor, and notch pathways. Also emphasized are the roles of transcription factors Odd1, Eya1, Pax2, Lim1, and WT-1 in directing renal development. Areas requiring future investigation include the factors that modulate signaling pathways to provide temporal and site-specific effects. The evolution of our understanding of the cellular and molecular mechanisms of kidney development may provide methods for improved diagnosis of renal anomalies and, hopefully, targets for intervention for this common cause of childhood end-stage kidney disease.

Met and the epidermal growth factor receptor act cooperatively to regulate final nephron number and maintain collecting duct morphology

Ureteric bud (UB) branching during kidney development determines the final number of nephrons. Although hepatocyte growth factor and its receptor Met have been shown to stimulate branching morphogenesis in explanted embryonic kidneys, loss of Met expression is lethal during early embryogenesis without obvious kidney abnormalities. Met(fl/fl);HoxB7-Cre mice, which lack Met expression selectively in the UB, were generated and found to have a reduction in final nephron number. These mice have increased Egf receptor expression in both the embryonic and adult kidney, and exogenous Egf can partially rescue the branching defect seen in kidney explants. Met(fl/fl);HoxB7-Cre;wa-2/wa-2 mice, which lack normal Egfr and Met signaling, exhibit small kidneys with a marked decrease in UB branching at E14.5 as well as a reduction in final glomerular number. These mice developed progressive interstitial fibrosis surrounding collecting ducts with kidney failure and death by 3-4 weeks of age. Thus, in support of previous in vitro findings, Met and the Egf receptor can act cooperatively to regulate UB branching and mediate maintenance of the normal adult collecting duct.

Branching morphogenesis

Tubular structures are a fundamental anatomical theme recurring in a wide range of animal species. In mammals, tubulogenesis underscores the development of several systems and organs, including the vascular system, the lungs, and the kidneys. All tubular systems are hierarchical, branching into segments of gradually diminishing diameter. There are only two cell types that form the lumen of tubular systems – either endothelial cells in the vascular system, or epithelial cells in all other organs. The most important feature in determining the morphology of the tubular systems is the frequency and geometry of branching. Hence, deciphering the molecular mechanisms underlying the sprouting of new branches from pre-existing ones is the key to understanding the formation of tubular systems. The morphological similarity between the various tubular systems is underscored by similarities between the signaling pathways which control their branching. A prominent feature common to these pathways is their duality – an agonist counterbalanced by an inhibitor. The formation of the tracheal system in Drosophila melanogaster is driven by fibroblast growth factor (FGF) and inhibited by Sprouty/Notch. In vertebrates, the analogous pathways are FGF and transforming growth factor β in epithelial tubular systems, or vascular endothelial growth factor and Notch in the vascular system.

Vegf as an epithelial cell morphogen modulates branching morphogenesis of embryonic kidney by directly acting on the ureteric bud

There is growing evidence that vascular endothelial growth factor (Vegf), a well-recognized angiogenic factor, plays a regulatory role in non-endothelial tissues such as neurons and epithelial cells. In the kidney Vegf receptors have been detected in proximal tubule cells of the adult kidney and Vegf has been show to stimulate branching morphogenesis of the developing kidney. In this study, using laser-microdissection as well as manual separation of the UB, we demonstrate that Vegf receptors are present in the ureteric bud (UB). Furthermore, we determine that Vegf stimulates UB branching in whole kidney explant that is mediated directly by signaling through Vegfr2. In addition, Vegf also induced branching response in isolated UBs that are free of the surrounding mesenchyme. These responses seem to be strictly dependent on the dose of Vegf such that higher doses are inhibitory while lower dose are stimulatory. These data place Vegf in a unique position of being able to modulate vascular as well as epithelial development in the embryonic kidney.

Patterning mechanisms of branched organs

Branching morphogenesis is one of the earliest events essential for the success of metazoans. By branching out and forming cellular or tissue extensions, cells can maximize their surface area and overcome space constraints posed by organ size. Over the past decade, tremendous progress has been made toward understanding the branching mechanisms of various invertebrate and vertebrate organ systems. Despite their distinct origins, morphologies and functions, different cell and tissue types use a remarkably conserved set of tools to undergo branching morphogenesis. Recent studies have shed important light on the basis of molecular conservation in the formation of branched structures in diverse organs.

Branching morphogenesis

Tubular structures are a fundamental anatomic theme recurring in a wide range of animal species. In mammals, tubulogenesis underscores the development of several systems and organs, including the vascular system, the lungs, and the kidneys. All tubular systems are hierarchical, branching into segments of gradually diminishing diameter. There are only 2 cell types that form the lumen of tubular systems: either endothelial cells in the vascular system or epithelial cells in all other organs. The most important feature in determining the morphology of the tubular systems is the frequency and geometry of branching. Hence, deciphering the molecular mechanisms underlying the sprouting of new branches from preexisting ones is the key to understanding the formation of tubular systems. The morphological similarity between the various tubular systems is underscored by similarities between the signaling pathways which control their branching. A prominent feature common to these pathways is their duality--an agonist counterbalanced by an inhibitor. The formation of the tracheal system in Drosophila melanogaster is driven by fibroblast growth factor and inhibited by Sprouty/Notch. In vertebrates, the analogous pathways are fibroblast growth factor and transforming growth factor-beta in epithelial tubular systems or vascular endothelial growth factor and Notch in the vascular system.

A functional role for semaphorin 4D/plexin B1 interactions in epithelial branching morphogenesis during organogenesis

Semaphorins and their receptors, plexins, carry out important functions during development and disease. In contrast to the well-characterized plexin A family, however, very little is known about the functional relevance of B-type plexins in organogenesis, particularly outside the nervous system. Here, we demonstrate that plexin B1 and its ligand Sema4d are selectively expressed in epithelial and mesenchymal compartments during key steps in the genesis of some organs. This selective expression suggests a role in epithelial-mesenchymal interactions. Importantly, using the developing metanephros as a model system, we have observed that endogenously expressed and exogenously supplemented Sema4d inhibits branching morphogenesis during early stages of development of the ureteric collecting duct system. Our results further suggest that the RhoA-ROCK pathway, which is activated downstream of plexin B1, mediates these inhibitory morphogenetic effects of Sema4d and suppresses branch-promoting signalling effectors of the plexin B1 signalling complex. Finally, mice that lack plexin B1 show early anomalies in kidney development in vivo. These results identify a novel function for plexin B1 as a negative regulator of branching morphogenesis during kidney development, and suggest that the Sema4d-plexin B1 ligand-receptor pair contributes to epithelial-mesenchymal interactions during organogenesis via modulation of RhoA signalling.