Morphogenesis and molecular mechanisms involved in human kidney development

From ureteric bud to the first glomeruli: genes, mediators, kidney alterations

The development of the mammalian kidney is a complex and in part unknown process which requires interactions between pluripotential/stem cells, undifferentiated mesenchymal cells, epithelial and mesenchymal components, eventually leading to the coordinate development of multiple different specialized epithelial, endothelial and stromal cell types within the kidney architectural complexity. We will describe the embryology and molecular nephrogenetic mechanisms, a fascinating traffic of cells and tissues which takes place in five stages: (1) ureteric bud (UB) development; (2) cap mesenchyme formation; (3) mesenchymal-epithelial transition (MET); (4) glomerulogenesis and tubulogenesis; (5) interstitial cell development. In particular, we will analyze the multiple cell types involved in these dramatic events as characters moving between different worlds, from the mesenchymal to the epithelial world and back, and will start to define the multiple factors that propel these cells during their travels throughout the developing kidney. Moreover, according with the hypothesis of renal perinatal programing, we will present the results reached in the fields of immunohistochemistry and molecular biology, by means of which we can explain how a loss or excess of molecular factors governing nephrogenesis may cause the onset of pathologies of different gravity, in some cases leading to a chronic kidney disease at different times from birth.

Ectopic liver tissue in the kidney: case report and literature review

The presence of ectopic liver tissue has been reported in various abdominal and extra-abdominal sites, most often in the gall bladder. Other rare sites include the retroperitoneum and the adrenal gland, which are anatomically closer to the kidney. However, our literature search did not reveal any reports of ectopic liver tissue within the kidney. We present such a case, detected incidentally during a fetal autopsy histologic examination, and we review the possible developmental aspects causing it. Ectopic liver is usually asymptomatic, although it can carry pathology similar to the orthotopic liver. It can be associated with other congenital anomalies and, rarely, can be the cause for clinical emergencies. The most significant implication, however, is development of hepatocellular carcinoma because of an increased predisposition compared with the native liver. Hence, we suggest that ectopic liver in the kidney should be considered by histopathologists, even in unsuspecting cases.

Glomerular development--shaping the multi-cellular filtration unit

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

Emerging biomarkers and metabolomics for assessing toxic nephropathy and acute kidney injury (AKI) in neonatology

Identification of novel drug-induced toxic nephropathy and acute kidney injury (AKI) biomarkers has been designated as a top priority by the American Society of Nephrology. Increasing knowledge in the science of biology and medicine is leading to the discovery of still more new biomarkers and of their roles in molecular pathways triggered by physiological and pathological conditions. Concomitantly, the development of the so-called "omics" allows the progressive clinical utilization of a multitude of information, from those related to the human genome (genomics) and proteome (proteomics), including the emerging epigenomics, to those related to metabolites (metabolomics). In preterm newborns, one of the most important factors causing the pathogenesis and the progression of AKI is the interaction between the individual genetic code, the environment, the gestational age, and the disease. By analyzing a small urine sample, metabolomics allows to identify instantly any change in phenotype, including changes due to genetic modifications. The role of liquid chromatography-mass spectrometry (LC-MS), proton nuclear magnetic resonance (1H NMR), and other emerging technologies is strategic, contributing basically to the sudden development of new biochemical and molecular tests. Urine neutrophil gelatinase-associated lipocalin (uNGAL) and kidney injury molecule-1 (KIM-1) are closely correlated with the severity of kidney injury, representing noninvasive sensitive surrogate biomarkers for diagnosing, monitoring, and quantifying kidney damage. To become routine tests, uNGAL and KIM-1 should be carefully tested in multicenter clinical trials and should be measured in biological fluids by robust, standardized analytical methods.

Tbx18 is essential for normal development of vasculature network and glomerular mesangium in the mammalian kidney

Tbx18 has been shown to be essential for ureteral development. However, it remains unclear whether it plays a direct role in kidney development. Here we addressed this by focusing on examining the pattern and contribution of Tbx18+ cells in the kidney and its role in kidney vascular development. Expression studies and genetic lineage tracing revealed that Tbx18 is expressed in renal capsule, vascular smooth muscle cells and pericytes and glomerular mesangial cells in the kidney and that Tbx18-expressing progenitors contribute to these cell types. Examination of Tbx18(-/-) kidneys revealed large reduction in vasculature density and dilation of glomerular capillary loops. While SMA+ cells were reduced in the mutant, PDGFRβ+ cells were seen in early capillary loop renal corpuscles in the mutant, but fewer than in the controls, and further development of the mesangium failed. Analysis of kidney explants cultured from E12.5 excluded the possibility that the defects observed in the mutant were caused by ureter obstruction. Reduced proliferation in glomerular tuft and increased apoptosis in perivascular mesenchyme were observed in Tbx18(-/-) kidneys. Thus, our analyses have identified a novel role of Tbx18 in kidney vasculature development.

Differentiation of human pluripotent stem cells into nephron progenitor cells in a serum and feeder free system

Objectives

Kidney disease is emerging as a critical medical problem worldwide. Because of limited treatment options for the damaged kidney, stem cell treatment is becoming an alternative therapeutic approach. Of many possible human stem cell sources, pluripotent stem cells are most attractive due to their self-renewal and pluripotent capacity. However, little is known about the derivation of renal lineage cells from human pluripotent stem cells (hPSCs). In this study, we developed a novel protocol for differentiation of nephron progenitor cells (NPCs) from hPSCs in a serum- and feeder-free system.

Materials and Methods

We designed step-wise protocols for differentiation of human pluripotent stem cells toward primitive streak, intermediate mesoderm and NPCs by recapitulating normal nephrogenesis. Expression of key marker genes was examined by RT-PCR, real time RT-PCR and immunocytochemistry. Each experiment was independently performed three times to confirm its reproducibility.

Results

After modification of culture period and concentration of exogenous factors, hPSCs can differentiate into NPCs that markedly express specific marker genes such as SIX2, GDNF, HOXD11, WT1 and CITED1 in addition to OSR1, PAX2, SALL1 and EYA1. Moreover, NPCs possess the potential of bidirectional differentiation into both renal tubular epithelial cells and glomerular podocytes in defined culture conditions. In particular, approximately 70% of SYN-positive cells were obtained from hPSC-derived NPCs after podocytes induction. NPCs can also form in vitro tubule-like structures in three dimensional culture systems.

Conclusions

Our novel protocol for hPSCs differentiation into NPCs can be useful for producing alternative sources of cell replacement therapy and disease modeling for human kidney diseases.

Single-gene causes of congenital anomalies of the kidney and urinary tract (CAKUT) in humans

Congenital anomalies of the kidney and urinary tract (CAKUT) cover a wide range of structural malformations that result from defects in the morphogenesis of the kidney and/or urinary tract. These anomalies account for about 40-50 % of children with chronic kidney disease worldwide. Knowledge from genetically modified mouse models suggests that single gene mutations in renal developmental genes may lead to CAKUT in humans. However, until recently, only a handful of CAKUT-causing genes were reported, most of them in familial syndromic cases. Recent findings suggest that CAKUT may arise from mutations in a multitude of different single gene causes. We focus here on single-gene causes of CAKUT and their developmental origin. Currently, more than 20 monogenic CAKUT-causing genes have been identified. High-throughput sequencing techniques make it likely that additional CAKUT-causing genes will be identified in the near future.

Clear cell sarcoma of the kidney demonstrates an embryonic signature indicative of a primitive nephrogenic origin

Clear cell sarcoma of the kidney (CCSK) is a tumor affecting children with a median age of 3 years at diagnosis. The cell of origin of CCSK is unknown and data on the molecular changes giving rise to CCSK is scarce. This has hindered the identification of positive diagnostic markers and development of molecularly targeted treatment protocols for CCSK. We have characterized a panel of CCSK to gain information regarding its molecular profile and possible origin. High-resolution genomic analysis with single nucleotide polymorphism array of 37 tumors did not reveal any clues to the mechanisms behind tumor development as remarkably few genetic imbalances were found. Gene expression analysis revealed a highly characteristic gene signature, enriched for pathways involved in embryonic development, including kidney formation. The presence of markers for two different developmental lineages in the embryonic kidney was therefore investigated in the tumor cells. FOXD1 which identifies cells giving rise to stromal elements, and CITED1, a marker for cells primed for nephrogenic epithelial differentiation, were both highly expressed in CCSK. In addition, the early embryonic marker OSR1 was expressed at higher levels in CCSK than in Wilms tumor, normal fetal kidney or adult kidney. As this marker discriminates the intermediate mesoderm from other mesodermal structures, our study could suggest that CCSK arises from a mesodermal cell type that retains the capacity to initiate differentiation towards both nephrons and stroma, but remains locked in a primitive state.

Structural links between the renal stem/progenitor cell niche and the organ capsule

A special feature of the renal stem/progenitor cell niche is its always close neighborhood to the capsule during organ development. To explore this link, neonatal kidney was investigated by histochemistry and transmission electron microscopy. For adequate contrasting, fixation of specimens was performed by glutaraldehyde including tannic acid. The immunohistochemical data illustrate that renal stem/progenitor cells are not distributed randomly but are positioned specially to the capsule. Epithelial stem/progenitor cells are found to be enclosed by the basal lamina at a collecting duct (CD) ampulla tip. Only few layers of mesenchymal cells are detected between epithelial cells and the capsule. Most impressive, numerous microfibers reacting with soybean agglutinin, anti-collagen I and III originate from the basal lamina at a CD ampulla tip and line between mesenchymal stem/progenitor cells to the inner side of the capsule. This specific arrangement holds together both types of stem/progenitor cells in a cage and fastens the niche as a whole at the capsule. Electron microscopy further illustrates that the stem/progenitor cell niche is in contact with a tunnel system widely spreading between atypical smooth muscle cells at the inner side of the capsule. It seems probable that stem/progenitor cells are supplied here by interstitial fluid.

Advanced fixation for transmission electron microscopy unveils special extracellular matrix within the renal stem/progenitor cell niche

As well in light as in transmission electron microscopy can be seen that the renal stem/progenitor cell niche shows a special arrangement of two different kinds of stem/progenitor cells. Epithelial cells are found in the tip of an ureteric bud derived CD ampulla encircled by a special basal lamina. Mesenchymal cells are separated from them by a striking interstitial interface. Specimens fixed by conventional glutaraldehyde solution show that the interface looks bright and unremarkable. In contrast, fixation of specimens with glutaraldehyde in combination with cupromeronic blue, ruthenium red, or tannic acid illustrates that the interface contains a remarkable network of extracellular matrix spanning between epithelial and mesenchymal stem/progenitor cells. After unpacking this particular extracellular matrix for electron microscopy, elaboration of related functions such as structural composition of contained molecules, binding of morphogenetic factors, and influence on parenchyma development is under current experimental work.

Developmental origins of chronic renal disease: an integrative hypothesis

Cardiovascular diseases are one of the leading causes of mortality. Hypertension (HT) is one of the principal risk factors associated with death. Chronic kidney disease (CKD), which is probably underestimated, increases the risk and the severity of adverse cardiovascular events. It is now recognized that low birth weight is a risk factor for these diseases, and this relationship is amplified by a rapid catch-up growth or overfeeding during infancy or childhood. The pathophysiological and molecular mechanisms involved in the “early programming” of CKD are multiple and partially understood. It has been proposed that the developmental programming of arterial hypertension and chronic kidney disease is related to a reduced nephron endowment. However, this mechanism is still discussed. This review discusses the complex relationship between birth weight and nephron endowment and how early growth and nutrition influence long term HT and CKD. We hypothesize that fetal environment reduces moderately the nephron number which appears insufficient by itself to induce long term diseases. Reduced nephron number constitutes a “factor of vulnerability” when additional factors, in particular a rapid postnatal growth or overfeeding, promote the early onset of diseases through a complex combination of various pathophysiological pathways.

CD44 immunoreactivity in the developing human kidney: a marker of renal progenitor stem cells?

CD44 is a transmembrane adhesion glycoprotein, functioning as a hyaluronan receptor and participating in the uptake and degradation of hyaluronan. Recently, CD44 has been proposed in the adult kidney as a marker of activated glomerular parietal epithelial cells, the putative niche stem cells that, in case of damage to podocytes, might migrate inside the glomerular tuft and undergo transition to podocytes. Here, immunoreactivity for CD44 was tested in 18 human fetuses and newborns with a gestational age ranging from 11 to 39 weeks. CD44 immunoreactivity was observed in all but one developing kidneys, being localized in several renal cell types including intraglomerular, capsular, cortical and medullary interstitial cells and nerve cells. In some cases, CD44 marked scattered cells in nephrogenic subcapsular zone. Our data indicate that CD44 is involved in human nephrogenesis, probably marking a subset of progenitor/stem cells involved in early phases of kidney development and, putatively, in podocyte and/or interstitial cell differentiation.

The interstitial interface within the renal stem/progenitor cell niche exhibits an unique microheterogeneous composition

Repair of parenchyma by stem/progenitor cells is seen as a possible alternative to cure acute and chronic renal failure in future. To learn about this therapeutic purpose, the formation of nephrons during organ growth is under focus of present research. This process is triggered by numerous morphogenetic interactions between epithelial and mesenchymal cells within the renal stem/progenitor cell niche. Recent data demonstrate that an astonishingly wide interstitial interface separates both types of stem/progenitor cells probably controlling coordinated cell-to-cell communication. Since conventional fixation by glutaraldehyde (GA) does not declare in transmission electron microscopy the spatial separation, improved contrasting procedures were applied. As a consequence, the embryonic cortex of neonatal rabbit kidneys was fixed in solutions containing glutaraldehyde in combination with cupromeronic blue, ruthenium red or tannic acid. To obtain a comparable view to the renal stem/progenitor cell niche, the specimens had to be orientated along the cortico-medullary axis of lining collecting ducts. Analysis of tissue samples fixed with GA, in combination with cupromeronic blue, demonstrates demasked extracellular matrix. Numerous braces of proteoglycans cover, as well, the basal lamina of epithelial stem/progenitor cells as projections of mesenchymal stem/progenitor cells crossing the interstitial interface. Fixation with GA containing ruthenium red or tannic acid illustrates strands of extracellular matrix that originate from the basal lamina of epithelial stem/progenitor cells and line through the interstitial interface. Thus, for the first time, improved contrasting techniques make it possible to analyze in detail a microheterogeneous composition of the interstitial interface within the renal stem/progenitor cell niche.

Cell projections and extracellular matrix cross the interstitial interface within the renal stem/progenitor cell niche: accidental, structural or functional cues?

BACKGROUND

During nephron induction, morphogenetic molecules are reciprocally exchanged between epithelial and mesenchymal stem/progenitor cells within the renal stem/progenitor cell niche. That these molecules remain concentrated, it is assumed that both cell populations stand in close contact to each other. However, recently published data illustrate that epithelial and mesenchymal cells are separated by an astonishingly wide interstitial interface.

METHODS

To gain deeper morphological insights into the spatial distribution of mesenchymal and epithelial stem/progenitor cells, the embryonic zone of neonatal rabbit kidney was fixed either with glutaraldehyde (GA) or in a combination with cupromeronic blue, ruthenium red or tannic acid. Transmission electron microscopy was then performed on exactly orientated sections.

RESULTS

Conventional fixation with GA illustrates that epithelial and mesenchymal stem/progenitor cells are separated by a bright but inconspicuously looking interstitial interface. In contrast, fixation of specimens in GA containing cupromeronic blue, ruthenium red or tannic acid elucidates that part of the interstitial interface exhibits a special extracellular matrix extending like woven strands between mesenchymal and epithelial stem/progenitor cells. In parallel, filigree projections from mesenchymal stem/progenitor cells cross the interstitial interface to penetrate the basal lamina of epithelial cells. Fusion of the plasma membranes cannot be observed. Instead, touching mesenchymal cell projections form a cone at the contact site with tunneling nanotubes.

CONCLUSIONS

The results demonstrate that the contact between mesenchymal and epithelial stem/progenitor cells does not form accidentally but physiologically and appears to belong to a suspected system involved in the exchange of morphogenetic information.

Lower urinary tract development and disease

Congenital anomalies of the lower urinary tract (CALUT) are a family of birth defects of the ureter, the bladder, and the urethra. CALUT includes ureteral anomaliesc such as congenital abnormalities of the ureteropelvic junction (UPJ) and ureterovesical junction (UVJ), and birth defects of the bladder and the urethra such as bladder-exstrophy-epispadias complex (BEEC), prune belly syndrome (PBS), and posterior urethral valves (PUVs). CALUT is one of the most common birth defects and is often associated with antenatal hydronephrosis, vesicoureteral reflux (VUR), urinary tract obstruction, urinary tract infections (UTI), chronic kidney disease, and renal failure in children. Here, we discuss the current genetic and molecular knowledge about lower urinary tract development and genetic basis of CALUT in both human and mouse models. We provide an overview of the developmental processes leading to the formation of the ureter, the bladder, and the urethra, and different genes and signaling pathways controlling these developmental processes. Human genetic disorders that affect the ureter, the bladder and the urethra and associated gene mutations are also presented. As we are entering the postgenomic era of personalized medicine, information in this article may provide useful interpretation for the genetic and genomic test results collected from patients with lower urinary tract birth defects. With evidence-based interpretations, clinicians may provide more effective personalized therapies to patients and genetic counseling for their families.

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.

The role of immunohistochemistry in the study of the newborn kidney

The identification of the different cell types involved in human nephrogenesis, when solely based on morphology, may lead to errors in its interpretation, given the complexity of the histological picture of the fetal and of the newborn kidney. In this study, the most recent works utilizing immunohistochemistry for the identification of the multiple cell types involved in human nephrogenesis are reviewed. The role of WT1, MUC1, Thymosin beta 10, Thymosin beta 4, CD10 and CD44 in the different phases of glomerulogenesis and of tubulogenesis is here described, with particular emphasis on their expression in the early phases of nephrogenesis. On the basis of our data, immunohistochemistry appears to be a useful tool in the study of human nephrogenesis, giving new data on the different steps of the differentiation of metanephric mesenchyme towards the multiple cell types characterizing the mature human kidney. Moreover, allowing a better knowledge of the protein products involved in the generation of new nephrons, immunohistochemistry could open new perspectives in the field of renal regenerating medicine, evidencing the factors able to prolong nephrogenesis after birth, helping us to reach our goal: allowing newborn kidneys to restore their nephron endowment, escaping susceptibility to hypertension and renal disease in adulthood.

Membrane trafficking in podocyte health and disease

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

Mi-2/NuRD is required in renal progenitor cells during embryonic kidney development

Development of the nephron tubules, the functional units of the kidney, requires the differentiation of a renal progenitor population of mesenchymal cells to epithelial cells. This process requires an intricate balance between self-renewal and differentiation of the renal progenitor pool. Sall1 is a transcription factor necessary for renal development which is expressed in renal progenitor cells (cap mesenchyme). Sall1 recruits the Nucleosome Remodeling and Deacetylase (NuRD) chromatin remodeling complex to regulate gene transcription. We deleted Mi2β, a component of the NuRD complex, in cap mesenchyme (CM) to examine its role in progenitor cells during kidney development. These mutants displayed significant renal hypoplasia with a marked reduction in nephrons. Markers of renal progenitor cells, Six2 and Cited1 were significantly depleted and progenitor cell proliferation was reduced. We also demonstrated that Sall1 and Mi2β exhibited a strong in vivo genetic interaction in the developing kidney. Together these findings indicate that Sall1 and NuRD act cooperatively to maintain CM progenitor cells.

Illustration of extensive extracellular matrix at the epithelial-mesenchymal interface within the renal stem/progenitor cell niche

BACKGROUND

Stem/progenitor cells are promising candidates to treat diseased renal parenchyma. However, implanted stem/progenitor cells are exposed to a harmful atmosphere of degenerating parenchyma. To minimize hampering effects after an implantation investigations are in progress to administer these cells within an artificial polyester interstitum supporting survival. Learning from nature the renal stem/progenitor cell niche appears as a valuable model. At this site epithelial stem/progenitor cells within the collecting duct ampulla face mesenchymal stem/progenitor cells. Both cell types do not have close contact but are separated by a wide interstitium.

METHODS

To analyze extracellular matrix in this particular interstitium, special contrasting for transmission electron microscopy was performed. Kidneys of neonatal rabbits were fixed in solutions containing glutaraldehyde (GA) or in combination with cupromeronic blue, ruthenium red and tannic acid.

RESULTS

GA revealed a basal lamina at the ampulla and a bright but inconspicuously looking interstitial space. In contrast, GA containing cupromeronic blue exhibits numerous proteoglycan braces lining from the ampulla towards the interstitial space. GA containing ruthenium red or tannic acid demonstrates clouds of extracellular matrix protruding from the basal lamina of the ampulla to the surface of mesenchymal stem/progenitor cells.

CONCLUSIONS

The actual data show that the interstitium between epithelial and mesenchymal stem/progenitor cells contains much more and up to date unknown extracellular matrix than earlier observed by classical GA fixation.

Wound healing in development

Wound healing is the inherent ability of an organism to protect itself against injuries. Cumulative evidence indicates that the healing process patterns in part embryonic morphogenesis and may result in either organ regeneration or scarring, phenomena that are developmental stage- or age-dependent. Skin is the largest organ. Its morphogenesis and repair mechanisms have been studied extensively due not only to its anatomical location, which allows easy access and observation, but also to its captivating structure and vital function. Thus, this review will focus on using skin as a model organ to illustrate new insights into the mechanisms of wound healing that are developmentally regulated in mammals, with special emphasis on the role of the Wnt signaling pathway and its crosstalk with TGF-β signaling. Relevant information from studies of other organs is discussed where it applies, and the clinical impact from such knowledge and emerging concepts on regenerative medicine are discussed in perspective.