Renal epithelial development in organotypic culture

From shape-shifting embryonic cells to oncology: The fascinating history of epithelial mesenchymal transition

Epithelial-to-mesenchymal transition or transformation (EMT) is a cell shape-changing process that is utilized repeatedly throughout embryogenesis and is critical to the attainment of a precise body plan. In the adult, EMT is observed under both normal and pathological conditions, such as during normal wounding healing, during development of certain fibrotic states and vascular anomalies, as well as in some cancers when malignant cells progress to become more aggressive, invasive, and metastatic. Epithelia derived from any of the three embryonic germ layers can undergo EMT, including those derived from mesoderm, such as endothelial cells (sometimes termed Endo-MT) and those derived from endoderm such as fetal liver stroma. At the cellular level, EMT is defined as the transformation of epithelial cells towards a mesenchymal phenotype and is marked by attenuation of expression of epithelial markers and de novo expression of mesenchymal markers. This process is induced by extracellular factors and can be reversible, resulting in mesenchymal-to-epithelial transformation (MET). It is now clear that a cell can simultaneously express properties of both epithelia and mesenchyme, and that such transitional cell-types drive tumor cell heterogeneity, an important aspect of cancer progression, development of a stem-like cell state, and drug resistance. Here we review some of the earliest studies demonstrating the existence of EMT during embryogenesis and discuss the discovery of the extracellular factors and intracellular signaling pathways that contribute to this process, with components of the TGFβ signaling superfamily playing a prominent role. We mention early controversies surrounding in vivo EMT during embryonic development and in adult diseased states, and the maturation of the field to a stage wherein targeting EMT to control disease states is an aspirational goal.

Organoid technology and applications in cancer research

During the past decade, the three-dimensional organoid technology has sprung up and become more and more popular among researchers. Organoids are the miniatures of in vivo tissues and organs, and faithfully recapitulate the architectures and distinctive functions of a specific organ.

These amazing three-dimensional constructs represent a promising, near-physiological model for human cancers, and tremendously support diverse potential applications in cancer research. Up to now, highly efficient establishment of organoids can be achieved from both normal and malignant tissues of patients. Using this bioengineered platform, the links of infection-cancer progression and mutation-carcinogenesis are feasible to be modeled. Another potential application is that organoid technology facilitates drug testing and guides personalized therapy. Although organoids still fail to model immune system accurately, co-cultures of organoids and lymphocytes have been reported in several studies, bringing hope for further application of this technology in immunotherapy. In addition, the potential value in regeneration medicine might be another paramount branch of organoid technology, which might refine current transplantation therapy through the replacement of irreversibly progressively diseased organs with isogenic healthy organoids.

In conclusion, organoids represent an excellent preclinical model for human tumors, promoting the translation from basic cancer research to clinical practice. In this review, we outline organoid technology and summarize its applications in cancer research.

CD146(+) cells are essential for kidney vasculature development

The kidney vasculature is critical for renal function, but its developmental assembly mechanisms remain poorly understood and models for studying its assembly dynamics are limited. Here, we tested whether the embryonic kidney contains endothelial cells (ECs) that are heterogeneous with respect to VEGFR2/Flk1/KDR, CD31/PECAM, and CD146/MCAM markers. Tie1Cre;R26R(YFP)-based fate mapping with a time-lapse in embryonic kidney organ culture successfully depicted the dynamics of kidney vasculature development and the correlation of the process with the CD31(+) EC network. Depletion of Tie1(+) or CD31(+) ECs from embryonic kidneys, with either Tie1Cre-induced diphtheria toxin susceptibility or cell surface marker-based sorting in a novel dissociation and reaggregation technology, illustrated substantial EC network regeneration. Depletion of the CD146(+) cells abolished this EC regeneration. Fate mapping of green fluorescent protein (GFP)-marked CD146(+)/CD31(-) cells indicated that they became CD31(+) cells, which took part in EC structures with CD31(+) wild-type ECs. EC network development depends on VEGF signaling, and VEGF and erythropoietin are expressed in the embryonic kidney even in the absence of any external hypoxic stimulus. Thus, the ex vivo embryonic kidney culture models adopted here provided novel ways for targeting renal EC development and demonstrated that CD146(+) cells are critical for kidney vasculature development.

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.

Mouse embryonic stem cell-derived embryoid bodies generate progenitors that integrate long term into renal proximal tubules in vivo

The metanephric kidney is a mesodermal organ that develops as a result of reciprocal interactions between the ureteric bud and the blastema. The generation of embryonic stem (ES) cell-derived progenitors offers potential for regenerative therapies but is often limited by development of tumor formation. Because brachyury (T) denotes mesoderm specification, a mouse ES cell line with green fluorescence protein (GFP) knocked into the functional T locus as well as lacZ in the ROSA26 locus (LacZ/T/GFP) was used in cell selection and lineage tracing. In the absence of leukemia inhibitory factor, mouse ES cells give rise to embryoid bodies that can differentiate into mesoderm. Culture conditions were optimized (4 d, 10 ng/ml Activin-A) to generate maximal numbers of renal progenitor populations identified by expression of the specific combination of renal markers cadherin-11, WT-1, Pax-2, and Wnt-4. LacZ/T/GFP+ cells were further enriched by FACS selection. Five days after injection of LacZ/T/GFP+ cells into embryonic kidney explants in organ culture, beta-galactosidase immunohistochemistry showed incorporation into blastemal cells of the nephrogenic zone. After a single injection into developing live newborn mouse kidneys, co-localization studies showed that the LacZ/T/GFP+ cells were stably integrated into proximal tubules with normal morphology and normal polarization of alkaline phosphatase and aquaporin-1 for 7 mo, without teratoma formation. It is concluded that defined differentiation of ES cells into embryoid bodies with Activin-A and selection for T expression provides a means to isolate and purify renal proximal tubular progenitor cells with the potential for safe use in regenerative therapies.

Early embryonic renal tubules of wild-type and polycystic kidney disease kidneys respond to cAMP stimulation with cystic fibrosis transmembrane conductance regulator/Na(+),K(+),2Cl(-) Co-transporter-dependent cystic dilation

Metanephric organ culture has been used to determine whether embryonic kidney tubules can be stimulated by cAMP to form cysts. Under basal culture conditions, wild-type kidneys from embryonic day 13.5 to 15.5 mice grow in size and continue ureteric bud branching and tubule formation over a 4- to 5-d period. Treatment of these kidneys with 8-Br-cAMP or the cAMP agonist forskolin induced the formation of dilated tubules within 1 h, which enlarged over several days and resulted in dramatically expanded cyst-like structures of proximal tubule and collecting duct origin. Tubule dilation was reversible upon withdrawal of 8-Br-cAMP and was inhibited by the cAMP-dependent protein kinase inhibitor H89 and the cystic fibrosis transmembrane conductance regulator (CFTR) inhibitor CFTR(inh)172. For further testing of the role of CFTR, metanephric cultures were prepared from mice with a targeted mutation of the Cftr gene. In contrast to kidneys from wild-type mice, those from Cftr -/- mice showed no evidence of tubular dilation in response to 8-Br-cAMP, indicating that CFTR Cl(-) channels are functional in embryonic kidneys and are required for cAMP-driven tubule expansion. A requirement for transepithelial Cl(-) transport was demonstrated by inhibiting the basolateral Na(+),K(+),2Cl(-) co-transporter with bumetanide, which effectively blocked all cAMP-stimulated tubular dilation. For determination of whether cystic dilation occurs to a greater extent in PKD kidneys in response to cAMP, Pkd1(m1Bei) -/- embryonic kidneys were treated with 8-Br-cAMP and were found to form rapidly CFTR- and Na(+),K(+),2Cl(-) co-transporter-dependent cysts that were three- to six-fold larger than those of wild-type kidneys. These results suggest that cAMP can stimulate fluid secretion early in renal tubule development during the time when renal cysts first appear in PKD kidneys and that PKD-deficient renal tubules are predisposed to abnormally increased cyst expansion in response to elevated levels of cAMP.

Developmental Changes in the Expression of Tight Junction Protein Claudins in Murine Metanephroi and Embryonic Kidneys

Claudins are the major constituents of tight junction (TJ) strands and participate in the cell-cell adhesion and permeability barrier in epithelial cell layers. To investigate the suitability of metanephroi for analysis of the function of the TJ protein claudins in renal tubular formation, mouse metanephroi from embryos at day 12 of gestation were cultured and expression of claudins was compared with that in embryonic kidneys. During in vitro culture for 8 days, the metanephroi showed expression patterns very similar to those observed in embryonic kidneys in reverse transcription-polymerase chain reaction for the claudins examined, including claudins 1-4, 8, 10, 11, and 16, and the TJ proteins occludin and ZO-1. Immunofluorescence microscopy for claudins 1-4, 8, 10, and 16 showed localization of these claudins at the TJ with occludin and ZO-1 in some restricted tubular segments. These findings indicate that the metanephroi show developmental changes in the expression of the TJ protein claudins, representing those in embryonic kidneys, and thus suggest that the mouse metanephros is suitable to examine the functions of specific claudins in the kidney.

EGF-related growth factors in the pathogenesis of murine ARPKD11See Editorial by Wilson, p. 2441

BACKGROUND

Epidermal growth factor (EGF), transforming growth factor-alpha (TGF-alpha) and their receptor, EGFR, play key roles in polycystic kidney disease (PKD) pathogenesis. Renal expression of two related growth factors, amphiregulin and heparin-binding EGF, has not been examined previously in PKD. The aims of this study of murine autosomal-recessive polycystic kidney disease (ARPKD) were (1) to characterize amphiregulin and heparin-binding EGF expression in cystic versus normal kidneys and cells; and (2) to identify the functional effects of abnormal EGF-related growth factor expression.

METHODS

Amphiregulin and heparin-binding-EGF expression were examined by immunohistology and Western blot of kidneys and conditionally-immortalized collecting tubule cells obtained from cystic bpk mice (a murine model of ARPKD) and normal littermates. EGF, TGF-alpha, amphiregulin, and heparin-binding EGF in vitro effects on cystic and control collecting tubule cells were assessed by cell proliferation, cyst fluid mitogenicity, and EGFR activation.

RESULTS

By immunohistology, amphiregulin and heparin-binding EGF localized to apical and basolateral surfaces of proximal tubule cysts > normal proximal tubules. In cystic collecting tubules, heparin-binding EGF (but not amphiregulin) localized to both apical and basolateral surfaces; whereas in normal collecting tubules, amphiregulin and heparin-binding EGF localized to the basolateral surface only. Increased amphiregulin and heparin-binding EGF expression by Western blot was seen in cystic vs. normal kidneys and increased heparin-binding EGF (but not amphiregulin) expression was present in cystic collecting tubule cell lines vs. controls. EGF, TGF-alpha, amphiregulin, and heparin-binding EGF were all mitogenic to cystic > control collecting tubule cells. Immunoprecipitation of EGF and TGF-alpha reduced cyst fluid mitogenicity by almost 80%, whereas heparin-binding EGF and amphiregulin immunoprecipitations had minimal effects. Differential receptor activation was also seen: Heparin-binding EGF markedly activated EGFR (>EGF = TGF-alpha > amphiregulin), with a greater effect seen in cystic vs. control collecting tubule cells.

CONCLUSION

Multiple EGF-related growth factors are abnormally expressed in murine ARPKD and may have differential roles in disease pathogenesis. In particular, newly identified abnormalities in heparin-binding EGF expression in cystic kidneys and cells may have important implications for disease pathogenesis.

The role of hepatocyte growth factor (HGF) at progressive stages of metanephric development

Several lines of evidence suggest that hepatocyte growth factor (HGF), a soluble protein secreted by mesenchymal cells, may elicit a morphogenic response in the developing metanephros. We investigated the role of HGF at three different stages of murine metanephric development utilizing serum-free organ culture. Cultures were initiated at E-13, E-15, and E-17; treated with exogenous HGF or antibodies to HGF (to block endogenous HGF) for 120 h in vitro; and evaluated for growth and differentiation in comparison to control explants cultured for 120 h in basal medium. HGF treatment of E-13 explants resulted in a reduction of growth and differentiation compared to control explants. Treatment of E-13 explants with antibodies to HGF produced explant growth and differentiation indistinguishable from control explants. In contrast to the results of E-13 cultures, explants initiated at E-15 and E-17 demonstrated an increased growth and differentiation profile when treated with HGF compared to controls. Treatment of E-15 and E-17 explants with antibodies to HGF resulted in a decrease growth and differentiation profile compared to control or HGF-treated explants. These data demonstrate that HGF has differential effects on renal morphogenesis at progressive developmental stages of metanephric development.

Antisense oligonucleotides to Cux-1, a Cut-related homeobox gene, cause increased apoptosis in mouse embryonic kidney cultures

Cux-1 is a murine homeobox gene that is highly and transiently expressed in the developing kidney. To further evaluate the role of Cux-1 in mammalian kidney development, organotypic cultures of embryonic mouse kidney were incubated with phosphorothioate-coupled antisense Cux-1 oligonucleotides (ODNs) in the presence of cationic liposomes. Inhibition of Cux-1 expression by antisense ODNs was verified by reverse transcription-PCR. Metanephroi that were incubated with antisense Cux-1 ODNs were 23% smaller than metanephroi that were incubated with sense Cux-1 ODNs. Morphologic analysis of metanephroi that were treated with antisense Cux-1 ODNs revealed that ureteric buds and induced epithelial structures were present. However, extensive areas of cell death containing shrunken cells with pyknotic nuclei were also evident. The presence of increased apoptosis was verified by ultrastructural and terminal transferase-mediated dUTP nick end labeling analyses. Two different antisense Cux-1 ODNs targeting either the translation start codon or the homeobox produced increased apoptosis. In contrast, metanephroi incubated with sense ODNs exhibited only occasional apoptotic cells. We conclude that the presence of antisense Cux-1 ODNs does not block nephron induction, but results instead in increased apoptosis. Proper regulation of Cux-1 expression may be necessary for normal kidney development.

Rat metanephric organ culture in terato-embryology

The development of the permanent mammalian kidney, or metanephros, depends on mesenchymal-epithelial interactions, leading to branching morphogenesis of the ureteric bud that forms the collecting ducts and to conversion of the metanephric mesenchyme into epithelium that forms the nephrons. Rat metanephric organ culture in which these interactions are maintained is a valuable in vitro model system for investigating normal and abnormal renal organogenesis. Methods were designed to evaluate either the capacity of the ureteric bud to branch or that of the mesenchyme to form nephrons. Both are based on specific staining of the ureteric bud and the glomeruli with lectins. Using this approach, we have shown that retinoids are potent stimulating factors of nephrogenesis, acting through an increase in the branching capacity of the ureteric bud. On the other hand, several drugs such as gentamicin and cyclosporin A were found to reduce the number of nephrons formed in vitro. While gentamicin affects the early branching pattern of the ureteric bud, cyclosporin may affect the capacity of the mesenchyme to convert into epithelium. This methodology therefore appears a potentially useful tool for toxicological studies of new drugs.

Tyrosine kinase activity of the EGF receptor in murine metanephric organ culture

Epidermal growth factor (EGF) and its fetal form, transforming growth factor alpha (TGF-alpha) are renal mitogens which induce epithelial hyperplasia, proximal tubular cyst formation (TC), and accelerated distal nephron differentiation in metanephric organ culture. To delineate the intracellular mechanisms mediating these growth factor effects, we studied the specific role of the epidermal growth factor receptor (EGF-R), the common receptor for both ligands, as an activated tyrosine kinase in TC formation and nephrogenesis. Fetal murine metanephric explants were incubated for 120 hours in control, and EGF (15 ng/ml)/TGF-alpha (10 ng/ml) supplemented medium with and without EGF-R blocking monoclonal antibody (50 mg/ml), or tyrosine kinase inhibitor. EGF-R tyrosine kinase inhibition was achieved by incubation with a synthetic tyrphostin (TP B42) (0.1 microM) or genestein (5.5 micrograms/ml). The following parameters were assessed: (a) segment-specific nephron development using morphometry and immunohistology; (b) tubular epithelial hyperplasia by protein content and BrdU uptake; and (c) TC formation by morphometric cystic index. Both growth factors produced hyperplastic proximal TC, significantly increased explant growth, and significantly increased distal nephron differentiation. Inhibiting the ligand-EGF-R interaction with EGF-R blocking monoclonal antibody abolished all growth factor-induced effects and resulted in increased amounts of undifferentiated mesenchyme and decreased distal nephron differentiation. Inhibition of EGF-R tyrosine kinase activity with either Tyrphostin B42 or genestein blocked TC formation and produced nodular blastemal hyperplasia and decreased distal nephron differentiation.

Normal and abnormal nephrogenesis

During the past decade, exciting advances in the fields of cell and molecular biology have provided new insight into the processes of normal and abnormal nephron induction and renal morphogenesis. Although the specific molecular signals that control renal mesenchymal-epithelium inductive interaction remain unknown, recent data suggest that postinductive nephrogenesis may be regulated by the overall balance of a number of local autocrine and/or paracrine growth factor systems. Alterations in the critical balance of regulatory factors might produce a variety of hypoplastic and dysplastic nephropathies or hyperplastic lesions such as tubular cysts. Additional studies demonstrate that extracellular matrix components and cell surface integrins have important regulatory roles in ureteric bud development and branching. Perturbations in matrix or integrin expression due to altered gene activity or toxin exposure would be expected to produce a variety of renal abnormalities ranging from failure of nephron induction (aplasia) to focal disruptions of differentiation (segmental dysplasia). Finally, several groups of genes encoding transcriptional regulatory proteins have been identified that appear to regulate aspects of cell proliferation, pattern formation, and segment-specific differentiation during normal and abnormal nephrogenesis. Future studies will elucidate the roles that specific genes and proteins play in renal development and will ultimately reveal the manner in which their dysregulation or dysfunction causes a variety of developmental renal disorders.

Gastrointestinal complications of gastrocystoplasty

The cases are reported of five children with chronic renal failure who underwent gastrocystoplasty for a variety of urological disorders. Gastrocystoplasty comprises the transplantation of a vascularised segment of stomach to the bladder to form an augmented neobladder. The patients had gastrointestinal complications after the operation, including considerable weight loss in all five patients, accompanied by marked failure to thrive in four of the five patients, and food aversion, feeding intolerance, dumping syndrome, delayed gastric emptying, and oesophagitis in two patients. Three of the five patients developed severe abdominal pain and haemorrhagic cystitis secondary to gastric acid secretion in the neobladder from the transplanted gastric pedicle. Nutritional and pharmacological interventions were used to manage the gastrointestinal problems. Explanations are offered for the pathophysiology of the observed complications of gastrocystoplasty. It is believed that the use of this procedure in infants and children, particularly those with chronic renal failure and uraemia, warrants caution until successful long term follow up and experience with this procedure have been reported.

Development and growth of mouse embryonic kidney in organ culture and modulation of development by soluble growth factor

Differentiation of the metanephrogenic mesenchyme is triggered by an inductive tissue interaction between an inducer tissue and the mesenchyme. It is generally believed that the epithelial ureter bud acts as an inducer during in vivo development. In response to the inductive stimulus most of the mesenchymal cells convert into epithelial cells, while a small fraction differentiates into stromal cells. In vitro, differentiation of isolated mesenchyme to epithelium can be induced by a variety of embryonic tissues, but nothing is known about the molecular nature of the inducing stimulus. In recent years, large numbers of polypeptide growth factors have been described, which in addition to proliferative effects were shown to exert effects on a variety of biological phenomena such as chemotaxis, inflammation, tissue repair, or induction of embryonic development. We therefore analyzed whether growth factors in the absence of inducer tissue can induce isolated kidney mesenchyme to differentiate into epithelium or interstitium. As expected, both growth and differentiation into epithelium were stimulated by an inducer tissue, the spinal cord. We found that none of the various growth factors tested (including epidermal growth factor, transforming growth factors alpha and beta, insulin-like growth factors I and II, fibroblast growth factor, platelet-derived growth factor, and retinoic acid) could mimick the effect of an inducer tissue, although we tested the factors over a wide concentration range. One of the tested factors, epidermal growth factor (EGF) stimulated the mesenchymal cells to become stromal cells, although it could not stimulate development into epithelium. EGF could stimulate stromal development both when the mesenchyme was cultured in isolation and when the mesenchyme was stimulated by an inducer tissue to become epithelium. The expansion of the stromal compartment in response to EGF treatment occurred at the expense of the epithelial cells, but EGF could not completely suppress the formation of epithelium. These data suggest the presence of EGF receptors in the developing kidney, but since application of soluble EGF leads to abnormal development, soluble EGF cannot be the natural ligand. We suggest that locally produced mitogens with an EGF-like structure may regulate the relative amounts of stroma (interstitium) and epithelium in the developing kidney.

Polypeptide growth factors in metanephric growth and segmental nephron differentiation

Although the developing nephron expresses receptors for various polypeptide growth factors, the specific roles of such factors in renal organogenesis are unknown. Therefore, the effects of epidermal growth factor (EGF) (8.2 x 10(-11) M-1.6 x 10(-8) M), multiplication stimulating activity (MSA) (6.6 x 10(-10) M-1.3 x 10(-8) M) and transforming growth factor beta (TGF-beta) (1 x 10(-12) M-1 x 10(-9) M) on organotypic renal growth and segmental nephron differentiation were studied in a serum-free hormone-supplemented, murine metanephric organ culture system. Following culture in control or growth-factor-supplemented medium, explant growth was assessed, and explant growth and differentiation were determined morphometrically in four defined neprhon segments which were identified morphologically or immunohistologically with segment-specific antibodies and/or lectins: glomeruli, proximal tubules, thick ascending limb-early distal tubules, and collecting tubules. Results showed that EGF increased overall renal growth and specific differentiation of distal elements, but retarded differentiation of glomeruli and proximal tubules. EGF also induced hyperplastic cystic malformation in proximal tubules. MSA stimulated explant growth and promoted segmental differentiation of all tubular segments. TGF-beta globally retarded in vitro nephrogenesis. Such data demonstrate that polypeptide growth factors have multiple and often disparate effects on overall renal growth in relation to differentiation of discrete nephron segments and provide insight into the factors which may regulate normal and abnormal renal embryogenesis.