Activated extracellular signal-regulated kinases are necessary and sufficient to initiate tubulogenesis in renal tubular MDCK strain I cell cysts

Prevalence and associated risk factors of simple renal cysts in primary hyperparathyroidism

PURPOSE

Previous studies have pointed to the role of parathormone (PTH) in the development of simple renal cysts. However, there is insufficient evidence related to simple renal cysts in primary hyperparathyroidism (PHPT). Therefore, this study aimed to evaluate the prevalence and risk factors associated with renal cysts in PHPT patients.

METHODS

In this retrospective study, we included 235 patients treated surgically for PHPT and 147 control subjects. The clinical and physical examination findings, laboratory test results, and neck and urinary system ultrasonography examination findings were evaluated. Postoperative parathyroid adenoma volume and weight were also recorded.

RESULTS

Simple renal cysts were found in 53 PHPT patients (22.6%) and in 15 control subjects (10.2%) (p = 0.002). Kidney stones were found in 33 PHPT patients (14%) and in only seven control subjects (4.8%) (p = 0.004). The presence of PHPT and high PTH levels independently supported the development of a simple renal cyst. Advanced age was determined as a risk factor for the development of simple renal cysts, and the presence of kidney stones was an independent predictor of simple renal cysts in PHPT patients. There was no relationship between the presence of simple renal cysts and the volume and weight of parathyroid adenoma.

CONCLUSION

A diagnosis of PHPT, high PTH levels, advanced age, and kidney stones are independent risk factors for simple renal cysts in PHPT patients. These patients should be evaluated regularly for complications of renal cysts and nephrolithiasis.

Hypoxia inhibits nephrogenesis through paracrine Vegfa despite the ability to enhance tubulogenesis

Reduced nephron number predisposes to hypertension and kidney disease. Interaction of the branching ureteric bud and surrounding mesenchymal cells determines nephron number. Since oxygen supply may be critical for intrauterine development, we tested whether hypoxia and hypoxia-inducible factor-1α (HIF-1α) influence nephrogenesis. We found that HIF-1α is required for branching of MDCK cells. In addition, culture of metanephric mouse kidneys with ureteric bud cell-specific stabilization or knockout of HIF-1α revealed a positive impact of HIF-1α on nephrogenesis. In contrast, widespread stabilization of HIF-1α in metanephric kidneys through hypoxia or HIF stabilizers impaired nephrogenesis, and pharmacological HIF inhibition enhanced nephrogenesis. Several lines of evidence suggest an inhibitory effect through the hypoxia response of mesenchymal cells. HIF-1α was expressed in mesenchymal cells during nephrogenesis. Expression of the anti-branching factors Bmp4 and Vegfa, secreted by mesenchymal cells, was increased upon HIF stabilization. The conditioned medium from hypoxic metanephric kidneys inhibited MDCK branching, which was partially rescued by Vegfa antibodies. Thus, the effect of HIF-1α on nephrogenesis appears context dependent. While HIF-1α in the ureteric bud is of importance for proper branching morphogenesis, the net effect of hypoxia-induced HIF activation in the embryonic kidney appears to be mesenchymal cell-dependent inhibition of ureter branching.

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.

Epithelial morphogenesis of MDCK cells in three-dimensional collagen culture is modulated by interleukin-8

Epithelial morphogenesis is dependent upon a variety of factors, many of which involve complex interactions between cells and their surrounding environments. We analyzed the patterns of differential gene expression associated with Madin-Darby canine kidney (MDCK) renal epithelial cells grown within a collagen gel in three-dimensional (3D) culture compared with those grown atop a collagen gel in two-dimensional (2D) culture. Under these conditions, MDCK cells spontaneously formed either hollow spherical cysts or flat monolayer sheets, respectively. Microarray analysis of gene expression revealed a twofold or greater expression difference in 732 gene sets from MDCK cysts compared with monolayers (false discovery rate or FDR-adjusted P values <0.05). Interleukin-8 (IL-8) was reproducibly found to be among the genes whose expression was most dramatically upregulated, and this behavior was verified through real-time PCR analysis. The level of IL-8 protein expression was significantly increased in 3D MDCK cultures compared with that detected in cells in 2D culture. Hepatocyte growth factor (HGF) induces MDCK cells in 3D culture to form linear tubule-like structures. We found that HGF stimulation caused MDCK cells in 3D culture to decrease the expression of IL-8 at both the mRNA and protein levels. Furthermore, the addition of recombinant IL-8 to HGF-stimulated 3D MDCK cultures was sufficient to partially reverse the tubulogenic effects of HGF, resulting in the formation of cystic structures. These data suggest that IL-8 participates in the formation of cystic structures by MDCK cells in 3D culture and that HGF may stimulate tubulogenesis through the suppression of IL-8.

Polarity in mammalian epithelial morphogenesis

Cell polarity is fundamental for the architecture and function of epithelial tissues. Epithelial polarization requires the intervention of several fundamental cell processes, whose integration in space and time is only starting to be elucidated. To understand what governs the building of epithelial tissues during development, it is essential to consider the polarization process in the context of the whole tissue. To this end, the development of three-dimensional organotypic cell culture models has brought new insights into the mechanisms underlying the establishment and maintenance of higher-order epithelial tissue architecture, and in the dynamic remodeling of cell polarity that often occurs during development of epithelial organs. Here we discuss some important aspects of mammalian epithelial morphogenesis, from the establishment of cell polarity to epithelial tissue generation.

Measuring collective cell movement and extracellular matrix interactions using magnetic resonance imaging

Collective cell behaviors in migration and force generation were studied at the mesoscopic-level using cells grown in a 3D extracellular matrix (ECM) simulating tissues. Magnetic resonance imaging (MRI) was applied to investigate dynamic cell mechanics at this level. MDCK, NBT2, and MEF cells were embedded in 3D ECM, forming clusters that then migrated and generated forces affecting the ECM. The cells demonstrated MRI contrast due to iron accumulation in the clusters. Timelapse-MRI enabled the measurement of dynamic stress fields generated by the cells, as well as simultaneous monitoring of the cell distribution and ECM deformation/remodeling. We found cell clusters embedded in the 3D ECM can exert translational forces to pull and push, as well as torque, their surroundings. We also observed that the sum of forces generated by multiple cell clusters may result in macroscopic deformation. In summary, MRI can be used to image cell-ECM interactions mesoscopically.

Protein kinase D-mediated phosphorylation of polycystin-2 (TRPP2) is essential for its effects on cell growth and calcium channel activity

PKD2 is mutated in 15% of patients with autosomal dominant polycystic kidney disease. The PKD2 protein, polycystin-2 or TRPP2, is a nonselective Ca2+-permeable cation channel that has been shown to function at several locations, including primary cilia, basolateral membrane, and at the endoplasmic reticulum (ER). Nevertheless, the factors that regulate the channel activity of polycystin-2 are not well understood. Polycystin-2 has been shown previously to be regulated by phosphorylation at two serine residues (Ser812 and Ser76) with distinct functional consequences. Here, we report the identification of a previously unrecognized phosphorylation site within the polycystin-2 C terminus (Ser801), and we demonstrate that it is phosphorylated by protein kinase D. Phosphorylation at this site was significantly increased in response to serum and epidermal growth factor stimulation. In nonciliated Madin-Darby canine kidney I cells, inducible expression of polycystin-2 inhibited cell proliferation compared with wild-type cells. Mutagenesis at Ser801 abolished these effects and reduced ATP-stimulated Ca2+ release from ER stores. Finally, we show that a pathogenic mutation (S804N) within the consensus kinase recognition sequence abolished Ser801 phosphorylation. These results suggest that growth factor-stimulated, protein kinase D-mediated phosphorylation of polycystin-2 is essential for its ER channel function and links extracellular stimuli to its effects on cell growth and intracellular calcium regulation.

Exocyst Sec10 protects epithelial barrier integrity and enhances recovery following oxidative stress, by activation of the MAPK pathway

Cell-cell contacts are essential for epithelial cell function, and disruption is associated with pathological conditions including ischemic kidney injury. We hypothesize that the exocyst, a highly-conserved eight-protein complex that targets secretory vesicles carrying membrane proteins, is involved in maintaining renal epithelial barrier integrity. Accordingly, increasing exocyst expression in renal tubule cells may protect barrier function from oxidative stress resulting from ischemia and reperfusion (I/R) injury. When cultured on plastic, Madin-Darby canine kidney (MDCK) cells overexpressing Sec10, a central exocyst component, formed domes showing increased resistance to hydrogen peroxide (H2O2). Transepithelial electric resistance (TER) of Sec10-overexpressing MDCK cells grown on Transwell filters was higher than in control MDCK cells, and the rate of TER decrease following H2O2 treatment was less in Sec10-overexpressing MDCK cells compared with control MDCK cells. After removal of H2O2, TER returned to normal more rapidly in Sec10-overexpressing compared with control MDCK cells. In collagen culture MDCK cells form cysts, and H2O2 treatment damaged Sec10-overexpressing MDCK cell cysts less than control MDCK cell cysts. The MAPK pathway has been shown to protect animals from I/R injury. Levels of active ERK, the final MAPK pathway step, were higher in Sec10-overexpressing compared with control MDCK cells. U0126 inhibited ERK activation, exacerbated the H2O2-induced decrease in TER and cyst disruption, and delayed recovery of TER following H2O2 removal. Finally, in mice with renal I/R injury, exocyst expression decreased early and returned to normal concomitant with functional recovery, suggesting that the exocyst may be involved in the recovery following I/R injury.

High prevalence of simple kidney cysts in patients with primary hyperparathyroidism

PTH has been demonstrated to promote renal epithelial cell proliferation and cysts development. The study aimed to evaluate the prevalence of kidney cysts in patients with primary hyperparathyroidism (PHPT). The prevalence of renal cysts diagnosed at abdominal ultrasound examinations in 172 PHPT patients (59.4+/-15.1 yr, mean age+/-SD; female/male 2.8) with preserved renal function was compared with that observed in 210 age- and sex-matched healthy controls. All patients underwent clinical, serum, and urine evaluations, and bone mineralization assessment by dual X-ray absorptiometry. Simple kidney cysts occurred with a higher prevalence in both male and female PHPT patients in comparison with healthy controls (34.9% vs 16.2% p<0.001). Kidney cysts were absent in patients younger than 39 yr, whereas they were present in one third of PHPT patients in their 4th, 5th, and 6th decades, increasing up to 45% after the age of 70. Multiple renal cysts were larger and more frequent than single cysts. PHPT patients with renal cysts were affected by a more active PTH secretion than patients without renal cysts as indicated by significant higher hypercalcemia and lower tubular maximal phosphate (TmP) reabsorption, while renal function, the occurrence of kidney stones, and osteoporosis were similar in both groups. Reduced TmP values were associated with about 3-fold increase in the risk of kidney cysts. In conclusion, simple renal cysts might be considered as a benign kidney complication of PHPT and might be related to the action of the chronic elevated PTH levels on tubular epithelial cells.

Ultrastructural insights in the interface between generated renal tubules and a polyester interstitium

In regenerative medicine, stem/progenitor cells are emerging as potential candidates for the treatment of renal failure. However, the mechanism of regeneration of renal tubules from stem/progenitor cells is not well-elucidated. In this study, a new method was developed for the generation of tubules replacing coating by extracellular matrix proteins. Renal stem/progenitor cells are mounted between layers of polyester fleece. This artificial interstitium supports spatial development of tubules within 13 days of perfusion culture in chemically defined Iscove's modified Dulbecco's medium (IMDM) containing aldosterone as the tubulogenic factor. Whole mount label by soybean agglutinin (SBA) showed that generated tubules exhibited a lumen and a continuously developed basal lamina. Immuno-labeling for cytokeratin Endo-A demonstrated the presence of isoprismatic epithelial cells, and laminin gamma1, occludin, and Na/K-ATPase alpha5 labeling revealed typical features of a polarized epithelium. To get first insight in the interface between tubules and polyester interstitium, transmission electron microscopy (TEM) was performed. The results showed that the generated tubules exhibited polar differentiation with a continuously developed basal lamina consisting of a lamina rara interna, lamina densa, and lamina rara externa. Collagen type III was found to be the linking molecule between the basal lamina and the surrounding polyester fibers by immuno labeling studies. Thus, the findings demonstrate that the spatial development involves the interface between the tubular basal lamina and the polyester interstitium of tubules and is not restricted to the epithelial portion.

The small molecule Mek1/2 inhibitor U0126 disrupts the chordamesoderm to notochord transition in zebrafish

Background

Key molecules involved in notochord differentiation and function have been identified through genetic analysis in zebrafish and mice, but MEK1 and 2 have so far not been implicated in this process due to early lethality (Mek1-/-) and functional redundancy (Mek2-/-) in the knockout animals.

Results

Here, we reveal a potential role for Mek1/2 during notochord development by using the small molecule Mek1/2 inhibitor U0126 which blocks phosphorylation of the Mek1/2 target gene Erk1/2 in vivo. Applying the inhibitor from early gastrulation until the 18-somite stage produces a specific and consistent phenotype with lack of dark pigmentation, shorter tail and an abnormal, undulated notochord. Using morphological analysis, in situ hybridization, immunhistochemistry, TUNEL staining and electron microscopy, we demonstrate that in treated embryos the chordamesoderm to notochord transition is disrupted and identify disorganization in the medial layer of the perinotochordal basement mebrane as the probable cause of the undulations and bulges in the notochord. We also examined and excluded FGF as the upstream signal during this process.

Conclusion

Using the small chemical U0126, we have established a novel link between MAPK-signaling and notochord differentiation. Our phenotypic analysis suggests a potential connection between the MAPK-pathway, the COPI-mediated intracellular transport and/or the copper-dependent posttranslational regulatory processes during notochord differentiation.

The ADAMTS12 metalloproteinase exhibits anti-tumorigenic properties through modulation of the Ras-dependent ERK signalling pathway

Members of the ADAMTS (a disintegrin and metalloproteinase with thrombospondin motifs) family of proteolytic enzymes are implicated in a variety of physiological processes, such as collagen maturation, organogenesis, angiogenesis, reproduction and inflammation. Moreover, deficiency or overexpression of certain ADAMTS proteins is directly involved in serious human diseases, including cancer. However, the functional roles of other family members, such as ADAMTS12, remain unknown. Here, by using different in vitro and in vivo approaches, we have evaluated the possible role of ADAMTS12 in the development and progression of cancer. First, we show that expression of ADAMTS12 in Madin-Darby canine kidney (MDCK) cells prevents the tumorigenic effects of hepatocyte growth factor (HGF) by blocking the activation of the Ras-MAPK signalling pathway and that this regulation involves the thrombospondin domains of the metalloproteinase. We also show that addition of recombinant human ADAMTS12 to bovine aortic endothelial cells (BAE-1 cells) abolishes their ability to form tubules upon stimulation with vascular endothelial growth factor (VEGF). Additionally, tumours induced in immunodeficient SCID mice injected with A549 cells overexpressing ADAMTS12 show a remarkable growth deficiency in comparison with tumours formed in animals injected with parental A549 cells. Overall, our data suggest that ADAMTS12 confers tumour-protective functions upon cells that produce this proteolytic enzyme.

The tubulogenic effect of aldosterone is attributed to intact binding and intracellular response of the mineralocorticoid receptor

Little is known about the extra- and intracellular stimuli inducing renal stem/progenitor cells to develop into three-dimensionally structured tubules. To study this specific development in a controlled environment, we used an advanced culture technique. Embryonic tissue derived from neonatal rabbit kidney was placed in a perfusion culture container at the interface of an artificial interstitium made of a polyester fleece. Culture was carried out in chemically defined Iscove’s Modified Dulbecco’s Medium (IMDM) for 13 days. Development of tubules was histochemically detected on cryosections labeled with Soybean Agglutinin (SBA). The experiments showed that aldosterone exerts a specific tubulogenic effect. Application of aldosterone (1 × 10−7 M) raised numerous SBA-labeled tubules, while in the absence of the steroid hormone the development of tubules was lacking. Specificity of hormone action was analyzed by the use of aldosterone antagonists. Administration of spironolactone (1 × 10−4 M) and canrenoate (1 × 10−5 M) completely inhibited the development of tubules. Finally, disrupting the intracellular molecular complex of the mineralocorticoid receptor (MR) and heat shock proteins by geldanamycin (2 μg/ml) prevented the development of tubules. Our results suggest that the tubulogenic effect induced by aldosterone is attributed to both hormone binding and an undisturbed intracellular response of the MR.

Intracellular signaling via ERK/MAPK completes the pathway for tubulogenic fibronectin in MDCK cells

A classic in vitro model of branching morphogenesis utilizes the Madin-Darby canine kidney (MDCK) cell line. MDCK Strain II cells form hollow monoclonal cysts in a three-dimensional collagen matrix over the course of 10 days and tubulate in response to hepatocyte growth factor (HGF). We and our colleagues previously showed that activation of the extracellular-signal regulated kinase (ERK, aka MAPK) pathway is necessary and sufficient to induce tubulogenesis in MDCK cells. We also showed in a microarray study that one of the genes upregulated by HGF was the known tubulogene fibronectin. Given that HGF activates a multitude of signaling pathways, including ERK/MAPK, to test the intracellular regulatory pathway, we used two distinct inhibitors of ERK activation (U0126 and PD098059). Following induction of MDCK Type II cells with HGF, tubulogenic fibronectin mRNA was upregulated fourfold by real-time PCR, and minimal or no change in fibronectin expression was seen when HGF was added with either U0126 or PD098059. We confirmed these results using an MDCK cell line inducible for Raf, which is upstream of ERK. Following activation of Raf, fibronectin mRNA and protein expression were increased to a similar degree as was seen following HGF induction. Furthermore, MDCK Strain I cells, which originate from collecting ducts and have constitutively active ERK, spontaneously initiate tubulogenesis. We show here that MDCK Strain I cells have high levels of fibronectin mRNA and protein compared to MDCK Strain II cells. When U0126 and PD098059 were added to MDCK Strain I cells, fibronectin mRNA, and protein levels were decreased to levels seen in MDCK Strain II cells. These data allow us to complete what we believe is the first description of a tubulogenic pathway from receptor/ligand (HGF/CMET), through an intracellular signaling pathway (ERK/MAPK), to transcription and, finally, secretion of a critical tubuloprotein (fibronectin).

Cell confluence regulates hepatocyte growth factor-stimulated cell morphogenesis in a beta-catenin-dependent manner

Following organ injury, morphogenic epithelial responses can vary depending on local cell density. In the present study, the role of cell confluence in determining the responsiveness of renal epithelial cells to the dedifferentiating morphogenic signals of hepatocyte growth factor (HGF) was examined. Increasing confluence resulted in a greater tendency of cells to organize into epithelial tubes and a significant decrease in migratory responsiveness to HGF. Analysis of downstream signaling revealed that the HGF receptor c-Met was equally activated in confluent and nonconfluent cells following HGF stimulation but that phosphoinositide 3-kinase-dependent activation of Akt and Rac were selectively diminished in confluent cells. In nonconfluent cells treated with HGF, the high level of Akt activation resulted in inhibitory phosphorylation of glycogen synthase kinase 3beta (GSK-3beta) and increased beta-catenin nuclear signaling. In contrast, confluent cells, in which HGF-stimulated Akt activation was diminished, displayed less inhibitory phosphorylation of GSK-3beta and less nuclear signaling by beta-catenin. Overexpression of beta-catenin (SA), which cannot be phosphorylated by GSK-3beta and targeted for ubiquitination, significantly increased migration in fully confluent cells. Thus, cells maintained at high confluence selectively downregulate signaling events such as Rac activation and beta-catenin-dependent transcription that would otherwise promote cell dedifferentiation and migration.

H-Ras, R-Ras, and TC21 differentially regulate ureteric bud cell branching morphogenesis

The collecting system of the kidney, derived from the ureteric bud (UB), undergoes repetitive bifid branching events during early development followed by a phase of tubular growth and elongation. Although members of the Ras GTPase family control cell growth, differentiation, proliferation, and migration, their role in development of the collecting system of the kidney is unexplored. In this study, we demonstrate that members of the R-Ras family of proteins, R-Ras and TC21, are expressed in the murine collecting system at E13.5, whereas H-Ras is only detected at day E17.5. Using murine UB cells expressing activated H-Ras, R-Ras, and TC21, we demonstrate that R-Ras-expressing cells show increased branching morphogenesis and cell growth, TC21-expressing cells branch excessively but lose their ability to migrate, whereas H-Ras-expressing cells migrated the most and formed long unbranched tubules. These differences in branching morphogenesis are mediated by differential regulation/activation of the Rho family of GTPases and mitogen-activated protein kinases. Because most branching of the UB occurs early in development, it is conceivable that R-Ras and TC-21 play a role in facilitating branching and growth in early UB development, whereas H-Ras might favor cell migration and elongation of tubules, events that occur later in development.