An in vitro tubulogenesis system using cell lines derived from the embryonic kidney shows dependence on multiple soluble growth factors

Role of extracellular matrix components and structure in new renal models in vitro

The extracellular matrix (ECM), a complex set of fibrillar proteins and proteoglycans, supports the renal parenchyma and provides biomechanical and biochemical cues critical for spatial-temporal patterning of cell development and acquisition of specialized functions. As in vitro models progress towards biomimicry, more attention is paid to reproducing ECM-mediated stimuli. ECM's role in in vitro models of renal function and disease used to investigate kidney injury and regeneration is discussed. Availability, affordability, and lot-to-lot consistency are the main factors determining the selection of materials to recreate ECM in vitro. While simpler components can be synthesized in vitro, others must be isolated from animal or human tissues, either as single isolated components or as complex mixtures, such as Matrigel or decellularized formulations. Synthetic polymeric materials with dynamic and instructive capacities are also being explored for cell mechanical support to overcome the issues with natural products. ECM components can be used as simple 2D coatings or complex 3D scaffolds combining natural and synthetic materials. The goal is to recreate the biochemical signals provided by glycosaminoglycans and other signaling molecules, together with the stiffness, elasticity, segmentation, and dimensionality of the original kidney tissue, to support the specialized functions of glomerular, tubular, and vascular compartments. ECM mimicking also plays a central role in recent developments aiming to reproduce renal tissue in vitro or even in therapeutical strategies to regenerate renal function. Bioprinting of renal tubules, recellularization of kidney ECM scaffolds, and development of kidney organoids are examples. Future solutions will probably combine these technologies.

Toward the renal vesicle: Ultrastructural investigation of the cap mesenchyme splitting process in the developing kidney

Background

A complex sequence of morphogenetic events leads to the development of the adult mouse kidney. In the present study, we investigated the morphological events that characterize the early stages of the mesenchymal-to-epithelial transition of cap mesenchymal cells, analyzing in depth the relationship between cap mesenchymal induction and ureteric bud (UB) branching.

Design and methods

Normal kidneys of newborn non-obese diabetic (NOD) mice were excised and prepared for light and electron microscopic examination.

Results

Nephrogenesis was evident in the outer portion of the renal cortex of all examined samples. This process was mainly due to the interaction of two primordial derivatives, the ureteric bud and the metanephric mesenchyme. Early renal developmental stages were initially characterized by the formation of a continuous layer of condensed mesenchymal cells around the tips of the ureteric buds. These caps of mesenchymal cells affected the epithelial cells of the underlying ureteric bud, possibly inducing their growth and branching.

Conclusions

The present study provides morphological evidence of the reciprocal induction between the ureteric bud and the metanephric mesenchyme showing that the ureteric buds convert mesenchyme to epithelium that in turn stimulates the growth and the branching of the ureteric bud.

Protein expression in vesicoureteral reflux: What about children?

PURPOSE

Pathogenesis of vesicoureteral reflux (VUR) which concerns improper embryonal ureteric bud development still remains controversial, despite current studies have revealed several candidate genes. In this study, we aimed to determine the protein expression of certain genes which might play role in the pathogenesis of VUR, in the resected ureterovesical junction segments.

METHODS

The study group consisted of 19 children; 12(63%) girls, 7(37%) boys who had ureteroneocystostomy (UNC) operation; 3(15.7%) right sided, 7(36.8%) left sided, 9(47.3%) bilateral due to VUR. As a total, 28 ureterovesical junction segments were available for analysis of protein expressions of GDNF/RET, PAX2 and FGFR2 genes by their Western Blot analysis.

RESULTS

Protein based expressions of FGFR2, PAX2 and RET were significantly lower than β-Actin (p = 0.001, for all proteins). Correlation analyses between grade of reflux and protein expressions revealed no significant relations (p>0.05, for all proteins). When we grouped the patients into 2 groups as high grade (grade 4-5) and low grade reflux (grade 1-3) for convenient analyses, no statistically significant difference was found between groups (p>0.05, for all proteins). Renal units were also grouped according to differential functions (≥40% and <40%) obtained by renal scintigraphy and compared in terms of proteins' expressions. There was also no significant difference between two groups regarding FGFR2, PAX2 and RET band areas (p>0.05, for all proteins).

CONCLUSION

Our study revealed decreased protein expressions of GDNF/RET, PAX2 and FGFR2 genes in the patients with VUR. Relation between clinical parameters and expression levels were statistically uncorrelated. Prospective studies of larger sample size are necessary in order to delineate the impact of certain proteins in the etiopathogenesis of VUR.

Guiding Cell Network Assembly using Shape-Morphing Hydrogels

Forces and relative movement between cells and extracellular matrix (ECM) are crucial to the self-organization of tissues during development. However, the spatial range over which these dynamics can be controlled in engineering approaches is limited, impeding progress toward the construction of large, structurally mature tissues. Herein, shape-morphing materials called "kinomorphs" that rationally control the shape and size of multicellular networks are described. Kinomorphs are sheets of ECM that change their shape, size, and density depending on patterns of cell contractility within them. It is shown that these changes can manipulate structure-forming behaviors of epithelial cells in many spatial locations at once. Kinomorphs are built using a new photolithographic technology to pattern single cells into ECM sheets that are >10× larger than previously described. These patterns are designed to partially mimic the branch geometry of the embryonic kidney epithelial network. Origami-inspired simulations are then used to predict changes in kinomorph shapes. Last, kinomorph dynamics are shown to provide a centimeter-scale program that sets specific spatial locations in which ≈50 µm-diameter epithelial tubules form by cell coalescence and structural maturation. The kinomorphs may significantly advance organ-scale tissue construction by extending the spatial range of cell self-organization in emerging model systems such as organoids.

Developing a self-organized tubulogenesis model of human renal proximal tubular epithelial cells in vitro

Three-dimensional tissue culture models which recapitulate the phenotype and function of human renal tissue have attracted significant interest as valuable tools for studying kidney development, disease pathophysiology, and nephrotoxicity. Here, a layer-by-layered three-dimensional (3D) co-culture technique was employed to bioengineer an improved human proximal tubule tissue model through incorporating human renal proximal tubule epithelial cells (RPTECs) with two types of interstitial cells on the layered extracellular matrix-like culture matrix. The resulting cultures were characterized by their growth profile, metabolic and proliferative activity, morphological characteristics as well as their functional gene expression. Our results found that the cultures were able to enable the self-organization of RPTECs and promote the tubule-like structure formation in vitro. A well-defined lumen structure and polarized expression of some key protein markers including actin, P-gp, Na⁺ -K⁺ -ATPase, and SGLT2 were also observed in the 3D co-cultures. Moreover, compared to the 3D monocultures, the tubule-like structures formed within the 3D co-cultures displayed more significant polarity and enhanced functional gene expression. This suggested the important role played by the renal stromal cells in supporting the tubulogenesis and differentiation of RPTECs. Thus, the 3D co-culture model reported here would benefit bioengineering approaches toward more physiologically relevant proximal tubule tissue in vitro, providing more robust tool not only for better understanding kidney development and pathophysiology but also for drug screening for nephrotoxicity.

Identification of matrix physicochemical properties required for renal epithelial cell tubulogenesis by using synthetic hydrogels

Synthetic hydrogels with controlled physicochemical matrix properties serve as powerful in vitro tools to dissect cell-extracellular matrix (ECM) interactions that regulate epithelial morphogenesis in 3D microenvironments. In addition, these fully defined matrices overcome the lot-to-lot variability of naturally derived materials and have provided insights into the formation of rudimentary epithelial organs. Therefore, we engineered a fully defined synthetic hydrogel with independent control over proteolytic degradation, mechanical properties, and adhesive ligand type and density to study the impact of ECM properties on epithelial tubulogenesis for inner medullary collecting duct (IMCD) cells. Protease sensitivity of the synthetic material for membrane-type matrix metalloproteinase-1 (MT1-MMP, also known as MMP14) was required for tubulogenesis. Additionally, a defined range of matrix elasticity and presentation of RGD adhesive peptide at a threshold level of 2 mM ligand density were required for epithelial tubulogenesis. Finally, we demonstrated that the engineered hydrogel supported organization of epithelial tubules with a lumen and secreted laminin. This synthetic hydrogel serves as a platform that supports epithelial tubular morphogenetic programs and can be tuned to identify ECM biophysical and biochemical properties required for epithelial tubulogenesis.

Enhancement of HGF-induced tubulogenesis by endothelial cell-derived GDNF

Tubulogenesis, the organization of epithelial cells into tubular structures, is an essential step during renal organogenesis as well as during the regeneration process of renal tubules after injury. In the present study, endothelial cell-derived factors that modulate tubule formation were examined using an in vitro human tubulogenesis system. When human renal proximal tubular epithelial cells (RPTECs) were cultured in gels, tubular structures with lumens were induced in the presence of hepatocyte growth factor (HGF). Aquaporin 1 was localized in the apical membrane of these tubular structures, suggesting that these structures are morphologically equivalent to renal tubules in vivo. HGF-induced tubule formation was significantly enhanced when co-cultured with human umbilical vein endothelial cells (HUVECs) or in the presence of HUVEC-conditioned medium (HUVEC-CM). Co-culture with HUVECs did not induce tubular structures in the absence of HGF. A phospho-receptor tyrosine kinase array revealed that HUVEC-CM markedly enhanced phosphorylation of Ret, glial cell-derived neurotrophic factor (GDNF) receptor, in HGF-induced tubular structures compared to those without HUVEC-CM. HUVECs produced GDNF, and RPTECs expressed both Ret and GDNF family receptor alpha1 (co-receptor). HGF-induced tubule formation was significantly enhanced by addition of GDNF. Interestingly, not only HGF but also GDNF significantly induced phosphorylation of the HGF receptor, Met. These data indicate that endothelial cell-derived GDNF potentiates the tubulogenic properties of HGF and may play a critical role in the epithelial-endothelial crosstalk during renal tubulogenesis as well as tubular regeneration after injury.

Engineered Kidney Tubules for Modeling Patient-Specific Diseases and Drug Discovery

The lack of engineering systems able to faithfully reproduce complex kidney structures in vitro has made it difficult to efficiently model kidney diseases and development. Using polydimethylsiloxane (PDMS) scaffolds and a kidney-derived cell line we developed a system to rapidly engineer custom-made 3D tubules with typical renal epithelial properties. This system was successfully employed to engineer patient-specific tubules, to model polycystic kidney disease (PKD) and test drug efficacy, and to identify a potential new pharmacological treatment. By optimizing our system we constructed functional ureteric bud (UB)-like tubules from human induced pluripotent stem cells (iPSCs), and identified a combination of growth factors that induces budding morphogenesis like embryonic kidneys do. Finally, we applied this assay to investigate budding defects in UB-like tubules derived from a patient with a PAX2 mutation. Our system enables the modeling of human kidney disease and development, drug testing and discovery, and lays the groundwork for engineering anatomically correct kidney tissues in vitro and developing personalized medicine applications.

Development of new method to enrich human iPSC-derived renal progenitors using cell surface markers

Cell therapy using renal progenitors differentiated from human embryonic stem cells (hESCs) or induced pluripotent stem cells (hiPSCs) has the potential to significantly reduce the number of patients receiving dialysis therapy. However, the differentiation cultures may contain undifferentiated or undesired cell types that cause unwanted side effects, such as neoplastic formation, when transplanted into a body. Moreover, the hESCs/iPSCs are often genetically modified in order to isolate the derived renal progenitors, hampering clinical applications. To establish an isolation method for renal progenitors induced from hESCs/iPSCs without genetic modifications, we screened antibodies against cell surface markers. We identified the combination of four markers, CD9⁻CD140a⁺CD140b⁺CD271⁺, which could enrich OSR1⁺SIX2⁺ renal progenitors. Furthermore, these isolated cells ameliorated renal injury in an acute kidney injury (AKI) mouse model when used for cell therapy. These cells could contribute to the development of hiPSC-based cell therapy and disease modeling against kidney diseases.

Symmetry-breaking in branching epithelia: cells on micro-patterns under flow challenge the hypothesis of positive feedback by a secreted autocrine inhibitor of motility

Branching morphogenesis of epithelia involves division of cells into leader (tip) and follower (stalk) cells. Published work on cell lines in culture has suggested that symmetry-breaking takes place via a secreted autocrine inhibitor of motility, the inhibitor accumulating more in concave regions of the culture boundary, slowing advance of cells there, and less in convex areas, allowing advance and a further exaggeration of the concave/convex difference. Here we test this hypothesis using a two-dimensional culture system that includes strong flow conditions to remove accumulating diffusible secretions. We find that, while motility does indeed follow boundary curvature in this system, flow makes no difference: this challenges the hypothesis of control by a diffusible secreted autocrine inhibitor.

Arl4c is a key regulator of tubulogenesis and tumourigenesis as a target gene of Wnt-β-catenin and growth factor-Ras signalling

Epithelial tubular morphogenesis (tubulogenesis) is a fundamental morphogenetic process of many epithelial organs. In this developmental process, epithelial cells migrate, proliferate, polarize and differentiate towards surrounding mesenchymal tissue to form tubule structures. Although epithelial tissue structures are basically stable in the postnatal period, epithelial cells regain highly proliferative and invasive potentials within mesenchymal tissue during tumour formation (tumourigenesis). Therefore, there must be a common molecular basis orchestrating the cellular behaviours involved in both tubulogenesis and tumourigenesis. ADP-ribosylation factor (Arf)-like protein 4c (Arl4c), which belongs to the small GTP-binding protein family, is expressed by the simultaneous activation of Wnt-β-catenin and growth factor-Ras-mitogen-activated protein kinase signalling, was identified as an essential regulator of tubulogenesis. Arl4c expression was also involved in the tumour formation of colorectal and lung cancers. In this review, we focus on Arl4c as a novel Wnt signal target molecule that links epithelial tubulogenesis to tumourigenesis.

Cell Therapy Using Human Induced Pluripotent Stem Cell-Derived Renal Progenitors Ameliorates Acute Kidney Injury in Mice

Acute kidney injury (AKI) is defined as a rapid loss of renal function resulting from various etiologies, with a mortality rate exceeding 60% among intensive care patients. Because conventional treatments have failed to alleviate this condition, the development of regenerative therapies using human induced pluripotent stem cells (hiPSCs) presents a promising new therapeutic option for AKI. We describe our methodology for generating renal progenitors from hiPSCs that show potential in ameliorating AKI. We established a multistep differentiation protocol for inducing hiPSCs into OSR1+SIX2+ renal progenitors capable of reconstituting three-dimensional proximal renal tubule-like structures in vitro and in vivo. Moreover, we found that renal subcapsular transplantation of hiPSC-derived renal progenitors ameliorated the AKI in mice induced by ischemia/reperfusion injury, significantly suppressing the elevation of blood urea nitrogen and serum creatinine levels and attenuating histopathological changes, such as tubular necrosis, tubule dilatation with casts, and interstitial fibrosis. To our knowledge, few reports demonstrating the therapeutic efficacy of cell therapy with renal lineage cells generated from hiPSCs have been published. Our results suggest that regenerative medicine strategies for kidney diseases could be developed using hiPSC-derived renal cells.

SIGNIFICANCE

This report is the first to demonstrate that the transplantation of renal progenitor cells differentiated from human induced pluripotent stem (iPS) cells has therapeutic effectiveness in mouse models of acute kidney injury induced by ischemia/reperfusion injury. In addition, this report clearly demonstrates that the therapeutic benefits come from trophic effects by the renal progenitor cells, and it identifies the renoprotective factors secreted by the progenitors. The results of this study indicate the feasibility of developing regenerative medicine strategy using iPS cells against renal diseases.

Renal developmental defects resulting from in utero hypoxia are associated with suppression of ureteric β-catenin signaling

Gestational stressors, including glucocorticoids and protein restriction, can affect kidney development and hence final nephron number. Since hypoxia is a common insult during pregnancy, we studied the influence of oxygen tension on kidney development in models designed to represent a pathological hypoxic insult. In vivo mouse models of moderate, transient, midgestational (12% O₂, 48 h, 12.5 dpc) or severe, acute, early-gestational (5.5-7.5% O₂, 8 h, 9.5-10.5 dpc) hypoxia were developed. The embryo itself is known to mature under hypoxic conditions with embryonic tissue levels of oxygen estimated to be 5%-8% (physiological hypoxia) when the mother is exposed to ambient normoxia. Both in vivo models generated phenotypes seen in patients with congenital anomalies of the kidney and urinary tract (CAKUT). Severe, acute, early hypoxia resulted in duplex kidney, while moderate, transient, midgestational hypoxia permanently reduced ureteric branching and nephron formation. Both models displayed hypoxia-induced reductions in β-catenin signaling within the ureteric tree and suppression of the downstream target gene, Ccnd1. Thus, we show a link between gestational hypoxia and CAKUT, the phenotype of which varies with timing, duration, and severity of the hypoxic insult.

Spatially restricted hyaluronan production by Has2 drives epithelial tubulogenesis in vitro

Generation of branched tubes from an epithelial bud is a fundamental process in development. We hypothesized that induction of hyaluronan synthase (Has) and production of hyaluronan (HA) drives tubulogenesis in response to morphogenetic cytokines. Treatment of J3B1A mammary cells with transforming growth factor-β1 or renal MDCK and mCCD-N21 cells with hepatocyte growth factor induced strong and specific expression of Has2. Immunostaining revealed that HA was preferentially produced at the tips of growing tubules. Inhibition of HA production, either by 4-methylumbelliferone (4-MU) or by Has2 mRNA silencing, abrogated tubule formation. HA production by J3B1A and mCCD-N21 cells was associated with sustained activation of ERK and S6 phosphorylation. However, silencing of either CD44 or RHAMM (receptor for HA-mediated motility), the major HA receptors, by RNA interference, did not alter tubulogenesis, suggesting that this process is not receptor-mediated.

A self-avoidance mechanism in patterning of the urinary collecting duct tree

Background

Glandular organs require the development of a correctly patterned epithelial tree. These arise by iterative branching: early branches have a stereotyped anatomy, while subsequent branching is more flexible, branches spacing out to avoid entanglement. Previous studies have suggested different genetic programs are responsible for these two classes of branches.

Results

Here, working with the urinary collecting duct tree of mouse kidneys, we show that the transition from the initial, stereotyped, wide branching to narrower later branching is independent from previous branching events but depends instead on the proximity of other branch tips. A simple computer model suggests that a repelling molecule secreted by branches can in principle generate a well-spaced tree that switches automatically from wide initial branch angles to narrower subsequent ones, and that co-cultured trees would distort their normal shapes rather than colliding. We confirm this collision-avoidance experimentally using organ cultures, and identify BMP7 as the repelling molecule.

Conclusions

We propose that self-avoidance, an intrinsically error-correcting mechanism, may be an important patterning mechanism in collecting duct branching, operating along with already-known mesenchyme-derived paracrine factors.

C-kit(+) cells isolated from developing kidneys are a novel population of stem cells with regenerative potential

The presence of tissue specific precursor cells is an emerging concept in organ formation and tissue homeostasis. Several progenitors are described in the kidneys. However, their identity as a true stem cell remains elusive. Here, we identify a neonatal kidney-derived c-kit(+) cell population that fulfills all of the criteria as a stem cell. These cells were found in the thick ascending limb of Henle's loop and exhibited clonogenicity, self-renewal, and multipotentiality with differentiation capacity into mesoderm and ectoderm progeny. Additionally, c-kit(+) cells formed spheres in nonadherent conditions when plated at clonal density and expressed markers of stem cells, progenitors, and differentiated cells. Ex vivo expanded c-kit(+) cells integrated into several compartments of the kidney, including tubules, vessels, and glomeruli, and contributed to functional and morphological improvement of the kidney following acute ischemia-reperfusion injury in rats. Together, these findings document a novel neonatal rat kidney c-kit(+) stem cell population that can be isolated, expanded, cloned, differentiated, and used for kidney repair following acute kidney injury. These cells have important biological and therapeutic implications.

Cellular and developmental strategies aimed at kidney tissue engineering

BACKGROUND

With the rate of kidney disease on the rise, and a serious imbalance between the number of patients requiring a kidney transplant and the number of available donor kidneys, it is becoming increasingly important to develop alternative strategies to restore organ function to diminish the need for human donors.

SUMMARY

We review the current progress and future directions of a subset of these strategies which are ultimately aimed towards bioengineering a functional, implantable, kidney-like tissue construct or organoid that might be genetically matched to the patient.

KEY MESSAGES

By combining the knowledge about normal kidney development with the rapidly growing knowledge in the field of cell differentiation and transdifferentiation, there is hope that partial or complete kidney function can be restored in patients with kidney disease - including genetic disorders, acute kidney injury, or chronic kidney disease - with tissue-engineered construct(s).

Reforming the kidney starting from a single-cell suspension

BACKGROUND

Chronic kidney disease affects 5-7% of people worldwide. The increasing number of patients and the shortage of transplantable organs create an imperative need to develop new methods for generating kidney tissue.

SUMMARY

Recent advances in our understanding of the developmental biology of the kidney, along with the establishment of novel methodologies in the field of regenerative medicine, have created significant potential for kidney regeneration. These advances incorporate both transplantation of metanephric primordia into adult recipients and construction of 'fetal' kidney tissue from suspensions of single cells of metanephric origin. This paper examines these approaches in the context of organ regeneration.

KEY MESSAGES

The use of transplants of metanephric origin has the advantage over undifferentiated stem cells of already being committed to a renal developmental program. Although several technical difficulties remain to be overcome, the validation of these systems in preclinical models of renal disease will be of decisive importance in the coming years.

A combination of Wnt and growth factor signaling induces Arl4c expression to form epithelial tubular structures

Growth factor-dependent epithelial morphological changes and proliferation are essential for the formation of tubular structures, but the underlying molecular mechanisms are poorly understood. Co-stimulation with Wnt3a and epidermal growth factor (Wnt3a/EGF) induced development of tubes consisting of intestinal epithelial cells by inducing expression of Arl4c, an Arf-like small GTP-binding protein, in three-dimensional culture, while stimulation with Wnt3a or EGF alone did not. Arl4c expression resulted in rearrangement of the cytoskeleton through activation of Rac and inactivation of Rho properly, which promoted cell growth by inducing nuclear translocation of Yes-associated protein and transcriptional co-activator with PDZ-binding motif (YAP/TAZ) in leading cells. Arl4c was expressed in ureteric bud tips and pretubular structures in the embryonic kidney. In an organoid culture assay, Wnt and fibroblast growth factor signaling simultaneously induced elongation and budding of kidney ureteric buds through Arl4c expression. YAP/TAZ was observed in the nucleus of extending ureteric bud tips. Thus, Arl4c expression induced by a combination of growth factor signaling mechanisms is involved in tube formation.

Growth factor-heparan sulfate "switches" regulating stages of branching morphogenesis

The development of branched epithelial organs, such as the kidney, mammary gland, lung, pancreas, and salivary gland, is dependent upon the involvement and interaction of multiple regulatory/modulatory molecules, including soluble growth factors, extracellular matrix components, and their receptors. How the function of these molecules is coordinated to bring about the morphogenetic events that regulate iterative tip-stalk generation (ITSG) during organ development remains to be fully elucidated. A common link to many growth factor-dependent morphogenetic pathways is the involvement of variably sulfated heparan sulfates (HS), the glycosaminoglycan backbone of heparan sulfate proteoglycans (HSPG) on extracellular surfaces. Genetic deletions of HS biosynthetic enzymes (e.g., C5-epimerase, Hs2st), as well as considerable in vitro data, indicate that variably sulfated HS are essential for kidney development, particularly in Wolffian duct budding and early ureteric bud (UB) branching. A role for selective HS modifications by enzymes (e.g., Ext, Ndst, Hs2st) in stages of branching morphogenesis is also strongly supported for mammary gland ductal branching, which is dependent upon a set of growth factors similar to those involved in UB branching. Taken together, these studies provide support for the notion that the specific spatio-temporal HS binding of growth factors during the development of branched epithelial organs (such as the kidney, mammary gland, lung and salivary gland) regulates these complex processes by potentially acting as "morphogenetic switches" during the various stages of budding, branching, and other developmental events central to epithelial organogenesis. It may be that two or more growth factor-selective HS interactions constitute a functionally equivalent morphogenetic switch; this may help to explain the paucity of severe branching phenotypes with individual growth factor knockouts.

EGFR controls IQGAP basolateral membrane localization and mitotic spindle orientation during epithelial morphogenesis

Establishing the correct orientation of the mitotic spindle is an essential step in epithelial cell division in order to ensure that epithelial tubules form correctly during organ development and regeneration. While recent findings have identified some of the molecular mechanisms that underlie spindle orientation, many aspects of this process remain poorly understood. Here, we have used the 3D-MDCK model system to demonstrate a key role for a newly identified protein complex formed by IQGAP1 and the epithelial growth factor receptor (EGFR) in controlling the orientation of the mitotic spindle. IQGAP1 is a scaffolding protein that regulates many cellular pathways, from cell-cell adhesion to microtubule organization, and its localization in the basolateral membrane ensures correct spindle orientation. Through its IQ motifs, IQGAP1 binds to EGFR, which is responsible for maintaining IQGAP1 in the basolateral membrane domain. Silencing IQGAP1, or disrupting the basolateral localization of either IQGAP1 or EGFR, results in a non-polarized distribution of NuMA, mitotic spindle misorientation and defects in single lumen formation.

Novel MAPK-dependent and -independent tubulogenes identified via microarray analysis of 3D-cultured Madin-Darby canine kidney cells

Cystogenesis and tubulogenesis are basic building blocks for many epithelial organs, including the kidney. Most researchers have used two-dimensional (2D) cell culture to investigate signaling pathways downstream of hepatocyte growth factor (HGF). We hypothesize that three-dimensional (3D) collagen-grown Madin-Darby canine kidney (MDCK) cells, which form cysts and then tubulate in response to HGF, are a much more in vivo-like system for the identification of novel tubulogenes. With the use of a canine microarray containing over 20,000 genes, 2,417 genes were identified as potential tubulogenes that were differentially regulated, exclusively in 3D-grown MDCK cells. Among these, 840 were dependent on MAPK signaling. Importantly, this work shows that many putative tubulogenes, previously identified via microarray analysis of 2D cultures, including by us, do not change in 3D culture and vice versa. The use of a 3D-culture system allowed for the identification of novel MAPK-dependent and -independent genes that regulate early renal tubulogenesis in vitro, e.g., matrix metalloproteinase 1 (MMP1). Knockdown of MMP1 led to defects in cystogenesis and tubulogenesis in 3D-grown MDCK cells, most likely due to problems establishing normal polarity. We suggest that data obtained from 2D cultures, even those using MDCK cells treated with HGF, should not be automatically extrapolated to factors important for cystogenesis and tubulogenesis. Instead, 3D culture, which more closely replicates the biological environment and is therefore a more accurate model for identifying tubulogenes, is preferred. Results from the present analysis will be used to build a more accurate model of the signaling pathways that control cystogenesis and tubulogenesis.

Concise review: can the intrinsic power of branching morphogenesis be used for engineering epithelial tissues and organs?

Branching morphogenesis is critical to the development of organs such as kidney, lung, mammary gland, prostate, pancreas, and salivary gland. Essentially, an epithelial bud becomes an iterative tip-stalk generator (ITSG) able to form a tree of branching ducts and/or tubules. In different organs, branching morphogenesis is governed by similar sets of genes. Epithelial branching has been recapitulated in vitro (or ex vivo) using three-dimensional cell culture and partial organ culture systems, and several such systems relevant to kidney tissue engineering are discussed here. By adapting systems like these it may be possible to harness the power inherent in the ITSG program to propagate and engineer epithelial tissues and organs. It is also possible to conceive of a universal ITSG capable of propagation that may, by recombination with organ-specific mesenchymal cells, be used for engineering many organ-like tissues similar to the organ from which the mesenchyme cells were derived, or toward which they are differentiated (from stem cells). The three-dimensional (3D) branched epithelial structure could act as a dynamic branching cellular scaffold to establish the architecture for the rest of the tissue. Another strategy-that of recombining propagated organ-specific ITSGs in 3D culture with undifferentiated mesenchymal stem cells-is also worth exploring. If feasible, such engineered tissues may be useful for the ex vivo study of drug toxicity, developmental biology, and physiology in the laboratory. Over the long term, they have potential clinical applications in the general fields of transplantation, regenerative medicine, and bioartificial medical devices to aid in the treatment of chronic kidney disease, diabetes, and other diseases.

A novel model of surgical injury in adult rat kidney: a "pouch model"

Regenerative mechanisms after surgical injury have been studied in many organs but not in the kidney. Studying surgical injury may provide new insights into mechanisms of kidney regeneration. In rodent models, extrarenal tissues adhere to surgical kidney wound and interfere with healing. We hypothesized that this can be prevented by wrapping injured kidney in a plastic pouch. Adult rats tolerated 5/6 nephrectomy with pouch application well. Histological analysis demonstrates that application of the pouch effectively prevented formation of adhesions and induced characteristic wound healing manifested by formation of granulation tissue. Additionally, selected tubules of the wounded kidney extended into the granulation tissue forming branching tubular epithelial outgrowths (TEOs) without terminal differentiation. Tubular regeneration outside of renal parenchyma was not previously observed, and suggests previously unrecognized capacity for regeneration. Our model provides a novel approach to study kidney wound healing.

Low ambient o(2) enhances ureteric bud branching in vitro

Hypoxia exists widely in developing embryos where it may regulate blood vessel formation. VEGF and FGF2 produced in developing renal primordia (metanephroi) stimulate microvessel formation from embryonic thoracic aorta cultured under hypoxic conditions (HC) relative to room air (RA). The aim of the present study was to provide insight into the participation of hypoxia in a process that occurs concomitant with metanephros vascularization in vivo, ureteric bud (UB) branching. To this end, the arborization of the UB and growth of metanephroi were measured in metanephroi grown in serum-free organ culture for two days under RA or HC. When metanephroi were cultured under HC the arborization of UB was stimulated relative to RA. In the presence of anti-VEGF neutralizing antibody (alphamVEGF), or anti-FGF2 neutralizing antibody (alphahFGF2) UB branching was inhibited under both RA and HC. When both alphamVEGF and alphahFGF2 were added, the inhibition was enhanced. Addition of exogenous VEGF or FGF2 to cultures stimulated UB branching under RA and HC and addition of both stimulated it further. These findings provide evidence for roles of hypoxia and metanephric VEGF and FGF2, as regulators not only for vascularization but also for UB bud branching during renal organogenesis.

Organogenesis forum lecture: In vitro kidney development, tissue engineering and systems biology

Renal replacement therapy (i.e., kidney transplantation) represents the optimal treatment for end-stage renal disease (a condition which is expected to increase in prevalence). However, the demand for transplantable kidneys currently outpaces the availability of donor kidneys, a situation not expected to improve in the foreseeable future. An alternative route to cadaveric or living-related donors would be to engineer kidneys for allograft transplantation from cells based on concepts derived from current understanding of normal kidney development. Although the use of cells for this purpose remains hypothetical, recent research from our laboratory has provided strong evidence that implantation of kidney-like tissue bioengineered from the recombination of in vitro culture systems which model discrete aspects of kidney development (i.e., cell culture, isolated WD, isolated UB and isolated MM) is possible. These recent findings are discussed here. Pathway based system biology approaches to understanding the mechanism(s) of kidney development are also discussed, particularly in the setting of this novel and seemingly powerful xeno-based tissue engineering strategy.

TIMP-1 induces an EMT-like phenotypic conversion in MDCK cells independent of its MMP-inhibitory domain

Matrix metalloproteinases (MMPs) and their endogenous inhibitors (TIMPs) regulate epithelial-mesenchymal transition (EMT) critical for the development of epithelial organs as well as cancer cell invasion. TIMP-1 is frequently overexpressed in several types of human cancers and serves as a prognostic marker. The present study investigates the roles of TIMP-1 on the EMT process and formation of the lumen-like structure in a 3D Matrigel culture of MDCK cells. We show that TIMP-1 overexpression effectively prevents cell polarization and acinar-like structure formation. TIMP-1 induces expression of the developmental EMT transcription factors such as SLUG, TWIST, ZEB1 and ZEB2, leading to downregulation of epithelial marker and upregulation of mesenchymal markers. Importantly, TIMP-1's ability to induce the EMT-like process is independent of its MMP-inhibitory domain. To our surprise, TIMP-1 induces migratory and invasive properties in MDCK cells. Here, we present a novel finding that TIMP-1 signaling upregulates MT1-MMP and MMP-2 expression, and potentiates MT1-MMP activation of pro-MMP-2, contributing to tumor cell invasion. In spite of the fact that TIMP-1, as opposed to TIMP-2, does not interact with and inhibit MT1-MMP, TIMP-1 may act as a key regulator of MT1-MMP/MMP-2 axis. Collectively, our findings suggest a model in which TIMP-1 functions as a signaling molecule and also as an endogenous inhibitor of MMPs. This concept represents a paradigm shift in the current view of TIMP-1/MT1-MMP interactions and functions during cancer development/progression.

Glomerular parietal epithelial cells of adult murine kidney undergo EMT to generate cells with traits of renal progenitors

Glomerular parietal epithelial cells (GPECs) are known to revert to embryonic phenotype in response to renal injury. However, the mechanism of de-differentiation in GPECs and the underlying cellular processes are not fully understood. In the present study, we show that cultured GPECs of adult murine kidney undergo epithelial-mesenchymal transition (EMT) to generate cells, which express CD24, CD44 and CD29 surface antigens. Characterization by qRT-PCR and immunostaining of these clonogenic cells demonstrate that they exhibit metastable phenotype with co-expression of both epithelial (cytokeratin-18) and mesenchymal (vimentin) markers. Transcript analysis by qRT-PCR revealed high expression of metanephric mesenchymal (Pax-2, WT-1, Six-1, Eya-1, GDNF) and uteric bud (Hoxb-7, C-Ret) genes in these cells, indicating their bipotent progenitor status. Incubation of GPECs with EMT blocker Prostaglandin E2, resulted in low expression of renal progenitor markers reflecting the correlation between EMT and acquired stemness in these cells. Additional in vitro renal commitment assays confirmed their functional staminality. When injected into E13.5 kidney rudiments, the cells incorporated into the developing kidney primordia and co-culture with E13.5 spinal cord resulted in branching and tubulogenesis in these cells. When implanted under renal capsule of unilaterally nephrectomized mice, these cells differentiated into immature glomeruli and vascular ducts. Our study demonstrates that EMT plays a major role in imparting plasticity to terminally differentiated GPECs by producing metastable cells with traits of kidney progenitors. The present study would improve our understanding on epithelial cell plasticity, furthering our knowledge of its role in renal repair and regeneration.

A rapid in vivo assay system for analyzing the organogenetic capacity of human kidney cells

Transplantation of human kidney-derived cells is a potential therapeutic modality for promoting regeneration of diseased renal tissue. However, assays that determine the ability of candidate populations for renal cell therapy to undergo appropriate differentiation and morphogenesis are limited. We report here a rapid and humane assay for characterizing tubulogenic potency utilizing the well-established chorioallantoic membrane CAM) of the chick embryo. Adult human kidney-derived cells expanded in monolayer were suspended in Matrigel and grafted onto the CAM. After a week, grafts were assessed histologically. Strikingly, many of the renal cells self-organized into tubular structures. Host blood vessels penetrated and presumably fed the grafts. Immuno- and histochemical staining revealed that tubular structures were epithelial, but not blood vessels. Some of the cells both within and outside the tubules were dividing. Analysis for markers of proximal and distal renal tubules revealed that grafts contained individual cells of a proximal tubular phenotype and many tubules of distal tubule character. Our results demonstrate that the chick CAM is a useful xenograft system for screening for differentiation and morphogenesis in cells with potential use in renal regenerative medicine.

Thiazolidinediones inhibit MDCK cyst growth through disrupting oriented cell division and apicobasal polarity

Thiazolidinediones have been reported to retard cystic disease in rodent models by uncertain mechanisms. We hypothesized that their major effect in retarding cystogenesis was through inhibiting cell proliferation or stimulating apoptosis. In the Madin-Darby canine kidney cell (MDCK) model, rosiglitazone inhibited cyst growth in a time- and dose-dependent manner and this was accompanied by a reduction in basal proliferation and an increase in apoptosis. Unexpectedly, we also observed a striking abnormality in lumen formation resulting in a characteristic multiple lumen or loss of lumen phenotype in treated cells at doses which did not inhibit cell proliferation. These changes were preceded by mislocalization of gp135 and Cdc42, misorientation of the mitotic spindle, and retardation in centrosome reorientation with later changes in primary cilia length and mislocalization of E-cadherin. Cdc42 activation was unaffected by rosiglitazone in monolayer culture but was profoundly inhibited in three-dimensional culture. MDCK cells stably expressing mutant Cdc42 showed a similar mislocalization of gp135 expression and multilumen phenotype in the absence of rosiglitazone. We conclude that rosiglitazone influences MDCK cyst growth by multiple mechanisms involving dosage-dependent effects on proliferation, spindle orientation, centrosome migration, and lumen formation. Correct spatial Cdc42 activation is critical for lumen formation, but the effect of rosiglitazone is likely to involve both Cdc42 and non-Cdc42 pathways.

Receptor tyrosine kinases in kidney development

The kidney plays a fundamental role in the regulation of arterial blood pressure and fluid/electrolyte homeostasis. As congenital anomalies of the kidney and urinary tract (CAKUT) constitute one of the most common human birth defects, improved understanding of the cellular and molecular mechanisms that lead to CAKUT is critical. Accumulating evidence indicates that aberrant signaling via receptor tyrosine kinases (RTKs) is causally linked to CAKUT. Upon activation by their ligands, RTKs dimerize, undergo autophosphorylation on specific tyrosine residues, and interact with adaptor proteins to activate intracellular signal transduction pathways that regulate diverse cell behaviours such as cell proliferation, survival, and movement. Here, we review the current understanding of role of RTKs and their downstream signaling pathways in the pathogenesis of CAKUT.

Constructing kidney-like tissues from cells based on programs for organ development: toward a method of in vitro tissue engineering of the kidney

The plausibility of constructing vascularized three-dimensional (3D) kidney tissue from cells was investigated. The kidney develops from mutual inductive interactions between cells of the ureteric bud (UB), derived from the Wolffian duct (WD), and the metanephric mesenchyme (MM). We found that isolated MMs were capable of inducing branching morphogenesis of the WD (an epithelial tube) in recombination cultures; suggesting that the isolated MM retains inductive capacity for WD-derived epithelial tubule cells other than those from the UB. Hanging drop aggregates of embryonic and adult renal epithelial cells from UB and mouse inner medullary collecting duct cell (IMCD) lines, which are ultimately of WD origin, were capable of inducing MM epithelialization and tubulogenesis with apparent connections (UB cells) and collecting duct-like tubules with lumens (IMCD). This supports the view that the collecting system can be constructed from certain epithelial cells (those ultimately of WD origin) when stimulated by MM. Although the functions of the MM could not be replaced by cultured mesenchymal cells, primary MM cells and one MM-derived cell line (BSN) produced factors that stimulate UB branching morphogenesis, whereas another, rat inducible metanephric mesenchyme (RIMM-18), supported WD budding as a feeder layer. This indicates that some MM functions can be recapitulated by cells. Although engineering of a kidney-like tissue from cultured cells alone remains to be achieved, these results suggest the feasibility of such an approach following the normal developmental progression of the UB and MM. Consistent with this notion, implants of kidney-like tissues constructed in vitro from recombinations of the UB and MM survived for over 5 weeks and achieved an apparently host-derived glomerular vasculature. Lastly, we addressed the issue of optimal macro- and micro-patterning of kidney-like tissue, which might be necessary for function of an organ assembled using a tissue engineering approach. To identify suitable conditions, 3D reconstructions of HoxB7-green fluorescent protein mouse rudiments (E12) cultured on a filter or suspended in a collagen gel (type I or type IV) revealed that type IV collagen 3D culture supports the deepest tissue growth (600 +/- 8 microm) and the largest kidney volume (0.22 +/- 0.02 mm(3)), and enabled the development of an umbrella-shaped collecting system such as occurs in vivo. Taken together with prior work (Rosines et al., 2007; Steer et al., 2002), these results support the plausibility of a developmental strategy for constructing and propagating vascularized 3D kidney-like tissues from recombinations of cultured renal progenitor cells and/or primordial tissue.

Developmental tumourigenesis: NCAM as a putative marker for the malignant renal stem/progenitor cell population

During development, renal stem cells reside in the nephrogenic blastema. Wilms' tumour (WT), a common childhood malignancy, is suggested to arise from the nephrogenic blastema that undergoes partial differentiation and as such is an attractive model to study renal stem cells leading to cancer initiation and maintenance. Previously we have made use of blastema-enriched WT stem-like xenografts propagated in vivo to define a 'WT-stem' signature set, which includes cell surface markers convenient for cell isolation (frizzled homolog 2 [Drosophila] - FZD2, FZD7, G-protein coupled receptor 39, activin receptor type 2B, neural cell adhesion molecule - NCAM). We show by fluorescenceactivated cell sorting analysis of sphere-forming heterogeneous primary WT cultures that most of these markers and other stem cell surface antigens (haematopoietic, CD133, CD34, c-Kit; mesenchymal, CD105, CD90, CD44; cancer, CD133, MDR1; hESC, CD24 and putative renal, cadherin 11), are expressed in WT cell sub-populations in varying levels. Of all markers, NCAM, CD133 and FZD7 were constantly detected in low-to-moderate portions likely to contain the stem cell fraction. Sorting according to FZD7 resulted in extensive cell death, while sorted NCAM and CD133 cell fractions were subjected to clonogenicity assays and quantitative RT-PCR analysis, exclusively demonstrating the NCAM fraction as highly clonogenic, overexpressing the WT 'stemness' genes and topoisomerase2A (TOP2A), a bad prognostic marker for WT. Moreover, treatment of WT cells with the topoisomerase inhibitors, Etoposide and Irinotecan resulted in down-regulation of TOP2A along with NCAM and WT1. Thus, we suggest NCAM as a marker for the WT progenitor cell population. These findings provide novel insights into the cellular hierarchy of WT, having possible implications for future therapeutic options.

Generation of easily accessible human kidney tubules on two-dimensional surfaces in vitro

The generation of tissue-like structures in vitro is of major interest for various fields of research including in vitro toxicology, regenerative therapies and tissue engineering. Usually 3D matrices are used to engineer tissue-like structures in vitro, and for the generation of kidney tubules, 3D gels are employed. Kidney tubules embedded within 3D gels are difficult to access for manipulations and imaging. Here we show how large and functional human kidney tubules can be generated in vitro on 2D surfaces, without the use of 3D matrices. The mechanism used by human primary renal proximal tubule cells for tubulogenesis on 2D surfaces appears to be distinct from the mechanism employed in 3D gels, and tubulogenesis on 2D surfaces involves interactions between epithelial and mesenchymal cells. The process is induced by transforming growth factor-β₁, and enhanced by a 3D substrate architecture. However, after triggering the process, the formation of renal tubules occurs with remarkable independence from the substrate architecture. Human proximal tubules generated on 2D surfaces typically have a length of several millimetres, and are easily accessible for manipulations and imaging, which makes them attractive for basic research and in vitro nephrotoxicology. The experimental system described also allows for in vitro studies on how primary human kidney cells regenerate renal structures after organ disruption. The finding that human kidney cells organize tissue-like structures independently from the substrate architecture has important consequences for kidney tissue engineering, and it will be important, for instance, to inhibit the process of tubulogenesis on 2D surfaces in bioartificial kidneys.

Fibroblast hepatocyte growth factor promotes invasion of human mammary ductal carcinoma in situ

Stromal-derived hepatocyte growth factor (HGF) acting through its specific proto-oncogene receptor c-Met has been suggested to play a paracrine role in the regulation of tumor cell migration and invasion. The transition from preinvasive ductal carcinoma in situ (DCIS) to invasive breast carcinoma is marked by infiltration of stromal fibroblasts and the loss of basement membrane. We hypothesized that HGF produced by the infiltrating fibroblasts may alter proteolytic pathways in DCIS cells, and, to study this hypothesis, established three-dimensional reconstituted basement membrane overlay cocultures with two human DCIS cell lines, MCF10.DCIS and SUM102. Both cell lines formed large dysplastic structures in three-dimensional cultures that resembled DCIS in vivo and occasionally developed invasive outgrowths. In coculture with HGF-secreting mammary fibroblasts, the percentage of DCIS structures with invasive outgrowths was increased. Activation of c-Met with conditioned medium from HGF-secreting fibroblasts or with recombinant HGF increased the percentage of DCIS structures with invasive outgrowths, their degradation of collagen IV, and their secretion of urokinase-type plasminogen activator and its receptor. In agreement with the in vitro findings, coinjection with HGF-secreting fibroblasts increased invasiveness of MCF10.DCIS xenografts in severe combined immunodeficient mice. Our study shows that paracrine HGF/c-Met signaling between fibroblasts and preinvasive DCIS cells enhances the transition to invasive carcinomas and suggests that three-dimensional cocultures are appropriate models for testing therapeutics that target tumor microenvironment-enhanced invasiveness.

Renin-angiotensin system-growth factor cross-talk: a novel mechanism for ureteric bud morphogenesis

The renin-angiotensin system (RAS) plays a critical role in kidney development. Mutations in the genes encoding components of the RAS cause a spectrum of congenital abnormalities of the kidney and renal collecting system, ranging from hypoplasia of the renal medulla and hydronephrosis in mice to renal tubular dysgenesis in humans. However, the mechanisms by which an intact RAS controls proper renal system development and how aberrations in the RAS result in abnormal kidney and renal collecting system development are poorly understood. The renal collecting system originates from the ureteric bud (UB). A number of transcription and growth factors regulate UB branching morphogenesis to ultimately form the ureter, pelvis, calyces, medullary, and cortical collecting ducts. Importantly, UB morphogenesis is a key developmental process that controls organogenesis of the entire metanephros. This review emphasizes emerging insights into the role for the RAS in UB morphogenesis and explores the mechanisms whereby RAS regulates this important process. A conceptual framework derived from recent work indicates that cooperation between the angiotensin II AT(1) receptor and receptor tyrosine kinase signaling performs essential functions during renal collecting system development via control of UB branching morphogenesis.

Beta1-integrin is required for kidney collecting duct morphogenesis and maintenance of renal function

Deletion of integrin-beta1 (Itgb1) in the kidney collecting system led to progressive renal dysfunction and polyuria. The defect in the concentrating ability of the kidney was concomitant with decreased medullary collecting duct expression of aquaporin-2 and arginine vasopressin receptor 2, while histological examination revealed hypoplastic renal medullary collecting ducts characterized by increased apoptosis, ectasia and cyst formation. In addition, a range of defects from small kidneys with cysts and dilated tubules to bilateral renal agenesis was observed. This was likely due to altered growth and branching morphogenesis of the ureteric bud (the progenitor tissue of the renal collecting system), despite the apparent ability of the ureteric bud-derived cells to induce differentiation of the metanephric mesenchyme. These data not only support a role for Itgb1 in the development of the renal collecting system but also raise the possibility that Itgb1 links morphogenesis to terminal differentiation and ultimately collecting duct function and/or maintenance.

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.

The role of the EGF family of ligands and receptors in renal development, physiology and pathophysiology

Mammalian kidney expresses all of the members of the ErbB family of receptors and their respective ligands. Studies support a role for ErbB family receptor activation in kidney development and differentiation. Under physiologic conditions, EGFR activation appears to play an important role in the regulation of renal hemodynamics and electrolyte handling by the kidney, while in different pathophysiologic states, EGFR activation may mediate either beneficial or detrimental effects to the kidney. This article provides an overview of the expression profile of the ErbB family of ligands and receptors in the mammalian kidney and summarizes known physiological and pathophysiological roles of EGFR activation in the organ.

cAMP-dependent chloride secretion mediates tubule enlargement and cyst formation by cultured mammalian collecting duct cells

Polycystic kidney diseases result from disruption of the genetically defined program that controls the size and geometry of renal tubules. Cysts which frequently arise from the collecting duct (CD) result from cell proliferation and fluid secretion. From mCCD(cl1) cells, a differentiated mouse CD cell line, we isolated a clonal subpopulation (mCCD-N21) that retains morphogenetic capacity. When grown in three-dimensional gels, mCCD-N21 cells formed highly organized tubular structures consisting of a palisade of polarized epithelial cells surrounding a cylindrical lumen. Subsequent addition of cAMP-elevating agents (forskolin or cholera toxin) or of membrane-permeable cAMP analogs (CPT-cAMP) resulted in rapid and progressive dilatation of existing tubules, leading to the formation of cystlike structures. When grown on filters, mCCD-N21 cells exhibited a high transepithelial resistance as well as aldosterone- and/or vasopressin-induced amiloride-sensitive and -insensitive current. The latter was in part inhibited by Na(+)-K(+)-2Cl(-) cotransporter (bumetanide) and chloride channel (NPPB) inhibitors. Real-time PCR analysis confirmed the expression of NKCC1, the ubiquitous Na(+)-K(+)-2Cl(-) cotransporter and cystic fibrosis transmembrane regulator (CFTR) in mCCD-N21 cells. Tubule enlargement and cyst formation were prevented by inhibitors of Na(+)-K(+)-2Cl(-) cotransporters (bumetanide or ethacrynic acid) or CFTR (NPPB or CFTR inhibitor-172). These results further support the notion that cAMP signaling plays a key role in renal cyst formation, at least in part by promoting chloride-driven fluid secretion. This new in vitro model of tubule-to-cyst conversion affords a unique opportunity for investigating the molecular mechanisms that govern the architecture of epithelial tubes, as well as for dissecting the pathophysiological processes underlying cystic kidney diseases.

How does the ureteric bud branch?

Many genes that modulate kidney development have been identified; however, the molecular interactions that direct arborization of the ureteric bud (UB) remain incompletely understood. This article discusses how "systems" approaches may shed light on the structure of the gene network during UB branching morphogenesis and the mechanisms involved in the formation of a branched collecting system from a straight epithelial tube in the context of a stage model. In vitro and genetic studies suggest that the stages seem to be governed by a conserved network of genes that establish a "tip-stalk generator"; these genes sustain iterative UB branching tubulogenesis through minimal alterations in the network architecture as a budding system shifts to one that autocatalytically branches through budding. The differential expression of stage-specific positive and inhibitory factors in the mesenchyme, likely presented in the context of heparan sulfate proteoglycans, and effector molecules in the epithelium seems to regulate advancement between stages; similar principles may apply to other branching epithelia such as the lung, salivary gland, pancreas, mammary gland, and prostate. Active mesenchymal interactions with the UB seem to govern vectorial arborization and tapering of the collecting system and its terminal differentiation. Cessation of branching correlates with induction of mesenchyme as well as local extracellular matrix changes. Perturbations of these mechanisms and/or single-nucleotide polymorphisms in genes regulating UB branching may predispose to a variety of renal diseases (e.g., hypertension and chronic kidney disease) by altering nephron number. Decentralization of the gene-protein interaction network may explain the relative paucity of branching phenotypes in mutant mice and in human disease.

Cross-talk between integrins alpha1beta1 and alpha2beta1 in renal epithelial cells

The collagen-binding integrins alpha1beta1 and alpha2beta1 have profoundly different functions, yet they are often co-expressed in epithelial cells. When both integrins are expressed in the same cell, it has been suggested that alpha1beta1 negatively regulates integrin alpha2beta1-dependent functions. In this study we utilized murine ureteric bud (UB) epithelial cells, which express no functionally detectable levels of endogenous integrins alpha1beta1 and alpha2beta1, to determine the mechanism whereby this regulation occurs. We demonstrate that UB cells expressing integrin alpha2beta1, but not alpha1beta1 adhere, migrate and proliferate on collagen I as well as form cellular cords in 3D collagen I gels. Substitution of the transmembrane domain of the integrin alpha2 subunit with that of alpha1 results in decreased cell adhesion, migration and cord formation. In contrast, substitution of the integrin alpha2 cytoplasmic tail with that of alpha1, decreases cell migration and cord formation, but increases proliferation. When integrin alpha1 and alpha2 subunits are co-expressed in UB cells, the alpha1 subunit negatively regulates integrin alpha2beta1-dependent cord formation, adhesion and migration and this inhibition requires expression of both alpha1 and alpha2 tails. Thus, we provide evidence that the transmembrane and cytoplasmic domains of the alpha2 integrin subunit, as well as the alpha1 integrin subunit, regulate integrin alpha2beta1 cell function.

Inhibition of bFGF-receptor type 2 increases kidney damage and suppresses nephrogenic protein expression after ischemic acute renal failure

Recovery from acute renal failure (ARF) requires the replacement of injured cells by new cells that are able to restore tubule epithelial integrity. We have recently described the expression of nephrogenic proteins [Vimentin, neural cell adhesion molecule, basic fibroblast growth factor (bFGF), Pax-2, bone morphogen protein-7, Noggin, Smad 1-5-8, p-Smad, hypoxia-inducible factor-1alpha, vascular endothelial growth factor], in a time frame similar to that observed in kidney development, after ischemic ARF induced in an ischemia-reperfusion (I/R) model. Furthermore, we show that bFGF, a morphogen involved in mesenchyme/epithelial transition in kidney development, induces a reexpression of morphogenic proteins in an earlier time frame and accelerates the recovery process after renal damage. Herein, we confirm that renal morphogenes are modulated by bFGF and hypothesized that a decrease in bFGF receptor 2 (bFGFR2) levels by the use of antisense oligonucleotides diminishes the expression of morphogenes. Male Sprague-Dawley rats submitted to ischemic injury were injected with 112 microg/kg bFGFR2 antisense oligonucleotide (bFGFR2-ASO) followed by reperfusion. Rats were killed, and the expression of nephrogenic proteins and renal marker damage was analyzed by immunohistochemistry and immunoblot. Animals subjected to I/R treated with bFGFR2-ASO showed a significant reduction in morphogen levels (P < 0.05). In addition, we observed an increase in markers of renal damage: macrophages (ED-1) and interstitial alpha-smooth muscle actin. These results confirm that bFGF participates in the recovery process and that treatment with bFGFR2-ASO induces an altered expression of morphogen proteins.

Staged in vitro reconstitution and implantation of engineered rat kidney tissue

A major hurdle for current xenogenic-based and other approaches aimed at engineering kidney tissues is reproducing the complex three-dimensional structure of the kidney. Here, a stepwise, in vitro method of engineering rat kidney-like tissue capable of being implanted is described. Based on the fact that the stages of kidney development are separable into in vitro modules, an approach was devised that sequentially induces an epithelial tubule (the Wolffian duct) to undergo in vitro budding, followed by branching of a single isolated bud and its recombination with metanephric mesenchyme. Implantation of the recombined tissue results in apparent early vascularization. Thus, in principle, an unbranched epithelial tubular structure (potentially constructed from cultured cells) can be induced to form kidney tissue such that this in vitro engineered tissue is capable of being implanted in host rats and developing glomeruli with evidence of early vascularization. Optimization studies (of growth factor and matrix) indicate multiple suitable combinations and suggest both a most robust and a minimal system. A whole-genome microarray analysis suggested that recombined tissue recapitulated gene expression changes that occur in vivo during later stages of kidney development, and a functional assay demonstrated that the recombined tissue was capable of transport characteristic of the differentiating nephron. The approach includes several points where tissue can be propagated. The data also show how functional, 3D kidney tissue can assemble by means of interactions of independent modules separable in vitro, potentially facilitating systems-level analyses of kidney development.

Expression of intermediate filaments, EGF and TGF-alpha in early human kidney development

The spatial and temporal expression patterns of cytokeratins, vimentin, epithelial growth factor (EGF) and transforming growth factor alpha (TGF-alpha), were investigated in the 5-9-week old human mesonephros and metanephros. Vimentin was found in all mesonephric structures, while cytokeratins were seen only in the mesonephric tubules. EGF and TGF-alpha were detected early in all mesonephric structures, and immunoreactivity to both factors decreased in later stages. In the 5-6-week metanephros, vimentin immunoreactivity was found in all structures and later increased in the collecting system and interstitium. In the 5th week, cytokeratins 8 and 19 appeared in the ureteric bud and ampullae, and later showed increasing immunoreactivity in the collecting system and nephrons. The coexpression of intermediate filament proteins in metanephric development is a temporary feature and might be associated with mesenchymal to epithelial transformation of developing nephrons. In adult kidneys, such coexpression is associated with fibrosis or carcinomatous changes. At early stages, immunoreactivity to EGF and TGF-alpha was detected in all metanephric structures and from the 7th week onward, it decreased in differentiating nephrons. EGF and TGF-alpha patterns of appearance indicate their role in induction, proliferation and growth of metanephric structures. Disturbances in that pattern might cause reduction in kidney growth.

Mutations of HNF-1beta inhibit epithelial morphogenesis through dysregulation of SOCS-3

Hepatocyte nuclear factor-1beta (HNF-1beta) is a Pit-1, Oct-1/2, Unc-86 (POU) homeodomain-containing transcription factor expressed in the kidney, liver, pancreas, and other epithelial organs. Mutations of HNF-1beta cause maturity-onset diabetes of the young, type 5 (MODY5), which is characterized by early-onset diabetes mellitus and congenital malformations of the kidney, pancreas, and genital tract. Knockout of HNF-1beta in the mouse kidney results in cyst formation. However, the signaling pathways and transcriptional programs controlled by HNF-1beta are poorly understood. Using genome-wide chromatin immunoprecipitation and DNA microarray (ChIP-chip) and microarray analysis of mRNA expression, we identified SOCS3 (suppressor of cytokine signaling-3) as a previously unrecognized target gene of HNF-1beta in the kidney. HNF-1beta binds to the SOCS3 promoter and represses SOCS3 transcription. The expression of SOCS3 is increased in HNF-1beta knockout mice and in renal epithelial cells expressing dominant-negative mutant HNF-1beta. Increased levels of SOCS-3 inhibit HGF-induced tubulogenesis by decreasing phosphorylation of Erk and STAT-3. Conversely, knockdown of SOCS-3 in renal epithelial cells expressing dominant-negative mutant HNF-1beta rescues the defect in HGF-induced tubulogenesis by restoring phosphorylation of Erk and STAT-3. Thus, HNF-1beta regulates tubulogenesis by controlling the levels of SOCS-3 expression. Manipulating the levels of SOCS-3 may be a useful therapeutic approach for human diseases induced by HNF-1beta mutations.