PALS1 is a key regulator of the lateral distribution of tight junction proteins in renal epithelial cells

The evolutionarily conserved apical Crumbs (CRB) complex, consisting of the core components CRB3a (an isoform of CRB3), PALS1 and PATJ, plays a key role in epithelial cell-cell contact formation and cell polarization. Recently, we observed that deletion of one Pals1 allele in mice results in functional haploinsufficiency characterized by renal cysts. Here, to address the role of PALS1 at the cellular level, we generated CRISPR/Cas9-mediated PALS1-knockout MDCKII cell lines. The loss of PALS1 resulted in increased paracellular permeability, indicating an epithelial barrier defect. This defect was associated with a redistribution of several tight junction-associated proteins from bicellular to tricellular contacts. PALS1-dependent localization of tight junction proteins at bicellular junctions required its interaction with PATJ. Importantly, reestablishment of the tight junction belt upon transient F-actin depolymerization or upon Ca2+ removal was strongly delayed in PALS1-deficient cells. Additionally, the cytoskeleton regulator RhoA was redistributed from junctions into the cytosol under PALS1 knockout. Together, our data uncover a critical role of PALS1 in the coupling of tight junction proteins to the F-actin cytoskeleton, which ensures their correct distribution along bicellular junctions and the formation of tight epithelial barrier.

PATJ inhibits histone deacetylase 7 to control tight junction formation and cell polarity

The conserved multiple PDZ-domain containing protein PATJ stabilizes the Crumbs-Pals1 complex to regulate apical-basal polarity and tight junction formation in epithelial cells. However, the molecular mechanism of PATJ's function in these processes is still unclear. In this study, we demonstrate that knockout of PATJ in epithelial cells results in tight junction defects as well as in a disturbed apical-basal polarity and impaired lumen formation in three-dimensional cyst assays. Mechanistically, we found PATJ to associate with and inhibit histone deacetylase 7 (HDAC7). Inhibition or downregulation of HDAC7 restores polarity and lumen formation. Gene expression analysis of PATJ-deficient cells revealed an impaired expression of genes involved in cell junction assembly and membrane organization, which is rescued by the downregulation of HDAC7. Notably, the function of PATJ regulating HDAC7-dependent cilia formation does not depend on its canonical interaction partner, Pals1, indicating a new role of PATJ, which is distinct from its function in the Crumbs complex. By contrast, polarity and lumen phenotypes observed in Pals1- and PATJ-deficient epithelial cells can be rescued by inhibition of HDAC7, suggesting that the main function of this polarity complex in this process is to modulate the transcriptional profile of epithelial cells by inhibiting HDAC7.

Development and Characterization of Hybrid, Temperature Sensing and Heating Yarns with Color Change

A person's body temperature is an important indicator of their health status. A deviation of that temperature by just 2 °C already has or can lead to serious consequences, such as fever or hypothermia. Hence, the development of a temperature-sensing and heatable yarn is an important step toward enabling and improving the monitoring and regulation of a person's body temperature. This technology offers benefits to several industries, such as health care and sports. This paper focuses on the characterization and development of a hybrid yarn, which can measure and visualize temperature changes through a thermoresistive and thermochromic effect. Moreover, the yarn is able to serve as a flexible heating element by connecting to a power source. The structure of the yarn is designed in three layers. Each layer and component ensures the functionality and flexibility of the yarn and additional compatibility with further processing steps. A flexible stainless steel core was used as the heat-sensitive and heat-conducting material. The layer of polyester wrapped around the stainless steel yarn improves the wearing comfort and serves as substrate material for the thermochromic coating. The resulting hybrid yarn has a reproducible sensory function and changes its resistance by 0.15 Ω between 20 and 60 °C for a length of 30 cm. In addition, the yarn has a uniform and reproducible heating power, so that temperature steps can be achieved at a defined length by selecting certain voltages. The thermochromic color change is clearly visible between 28 and 29 °C. Due to its textile structure, the hybrid sensory and actuating yarn can easily be incorporated into a woven fabric or into a textile by means of joining technology sewing.

Hypoxia hits APOL1 in the kidney

Individuals of African ancestry carrying two pathogenic variants of apolipoprotein 1 (APOL1) have a substantially increased risk for developing chronic kidney disease. The course of APOL1 nephropathy is extremely heterogeneous and shaped by systemic factors such as a response to interferon. However, additional environmental factors operating in this second-hit model have been less well defined. Here, we reveal that stabilization of hypoxia-inducible transcription factors (HIF) by hypoxia or HIF prolyl hydroxylase inhibitors activates transcription of APOL1 in podocytes and tubular cells. An active regulatory DNA-element upstream of APOL1 that interacted with HIF was identified. This enhancer was accessible preferentially in kidney cells. Importantly, upregulation of APOL1 by HIF was additive to the effects of interferon. Furthermore, HIF stimulated expression of APOL1 in tubular cells derived from the urine of an individual carrying a risk variant for kidney disease. Thus, hypoxic insults may serve as important modulators of APOL1 nephropathy.

Disruption of the Rab7-Dependent Final Common Pathway of Endosomal and Autophagic Processing Results in a Severe Podocytopathy

SIGNIFICANCE STATEMENT

Endocytosis, recycling, and degradation of proteins are essential functions of mammalian cells, especially for terminally differentiated cells with limited regeneration rates and complex morphology, such as podocytes. To improve our understanding on how disturbances of these trafficking pathways are linked to podocyte depletion and slit diaphragm (SD) injury, the authors explored the role of the small GTPase Rab7, which is linked to endosomal, lysosomal, and autophagic pathways, using as model systems mice and Drosophila with podocyte-specific or nephrocyte-specific loss of Rab7, and a human podocyte cell line depleted for Rab7. Their findings point to maturation and fusion events during endolysosomal and autophagic maturation as key processes for podocyte homeostasis and function and identify altered lysosomal pH values as a putative novel mechanism for podocytopathies.

BACKGROUND

Endocytosis, recycling, and degradation of proteins are essential functions of mammalian cells, especially for terminally differentiated cells with limited regeneration rates, such as podocytes. How disturbances within these trafficking pathways may act as factors in proteinuric glomerular diseases is poorly understood.

METHODS

To explore how disturbances in trafficking pathways may act as factors in proteinuric glomerular diseases, we focused on Rab7, a highly conserved GTPase that controls the homeostasis of late endolysosomal and autophagic processes. We generated mouse and Drosophila in vivo models lacking Rab7 exclusively in podocytes or nephrocytes, and performed histologic and ultrastructural analyses. To further investigate Rab7 function on lysosomal and autophagic structures, we used immortalized human cell lines depleted for Rab7.

RESULTS

Depletion of Rab7 in mice, Drosophila , and immortalized human cell lines resulted in an accumulation of diverse vesicular structures resembling multivesicular bodies, autophagosomes, and autoendolysosomes. Mice lacking Rab7 developed a severe and lethal renal phenotype with early-onset proteinuria and global or focal segmental glomerulosclerosis, accompanied by an altered distribution of slit diaphragm proteins. Remarkably, structures resembling multivesicular bodies began forming within 2 weeks after birth, prior to the glomerular injuries. In Drosophila nephrocytes, Rab7 knockdown resulted in the accumulation of vesicles and reduced slit diaphragms. In vitro , Rab7 knockout led to similar enlarged vesicles and altered lysosomal pH values, accompanied by an accumulation of lysosomal marker proteins.

CONCLUSIONS

Disruption within the final common pathway of endocytic and autophagic processes may be a novel and insufficiently understood mechanism regulating podocyte health and disease.

Loss of surface transport is a main cellular pathomechanism of CRB2 variants causing podocytopathies

Crumbs2 (CRB2) is a central component of the renal filtration barrier and part of the slit diaphragm, a unique cell contact formed by glomerular podocytes. Some CRB2 variants cause recessive inherited forms of steroid-resistant nephrotic syndrome. However, the disease-causing potential of numerous CRB2 variants remains unknown. Here, we report the establishment of a live-cell imaging-based assay, allowing a quantitative evaluation of the pathogenic potential of so far non-categorized CRB2 variants. Based on in silico data analysis and protein prediction software, putative disease-associated CRB2 missense variants were selected, expressed as CRB2-GFP fusion proteins, and analyzed in reporter cell lines with BFP-labeled plasma membrane. We found that in comparison with PM-localized WT, disease-associated CRB2 variants remained predominantly at the ER. Accumulation at the ER was also present for several non-characterized CRB2 variants and variants in which putative disulfide bridge-forming cysteines were replaced. Strikingly, WT CRB2 retained inside the ER in cells lacking protein disulfide isomerase A3, indicating that posttranslational modification, especially the formation of disulfide bridges, is a crucial step for the CRB2 PM transport.

The transcription factor ATF4 mediates endoplasmic reticulum stress-related podocyte injury and slit diaphragm defects

Mutations in OSGEP and four other genes that encode subunits of the KEOPS complex cause Galloway-Mowat syndrome, a severe, inherited kidney-neurological disease. The complex catalyzes an essential posttranscriptional modification of tRNA and its loss of function induces endoplasmic reticulum (ER) stress. Here, using Drosophila melanogaster garland nephrocytes and cultured human podocytes, we aimed to elucidate the molecular pathogenic mechanisms of KEOPS-related glomerular disease and to test pharmacological inhibition of ER stress-related signaling as a therapeutic principle. We found that ATF4, an ER stress-mediating transcription factor, or its fly orthologue Crc, were upregulated in both fly nephrocytes and human podocytes. Knockdown of Tcs3, a fly orthologue of OSGEP, caused slit diaphragm defects, recapitulating the human kidney phenotype. OSGEP cDNA with mutations found in patients lacked the capacity for rescue. Genetic interaction studies in Tcs3-deficient nephrocytes revealed that Crc mediates not only cell injury, but surprisingly also slit diaphragm defects, and that genetic or pharmacological inhibition of Crc activation attenuates both phenotypes. These findings are conserved in human podocytes where ATF4 inhibition improved the viability of podocytes with OSGEP knockdown, with chemically induced ER stress, and where ATF4 target genes and pro-apoptotic gene clusters are upregulated upon OSGEP knockdown. Thus, our data identify ATF4-mediated signaling as a molecular link among ER stress, slit diaphragm defects, and podocyte injury, and our data suggest that modulation of ATF4 signaling may be a potential therapeutic target for certain podocyte diseases.

Impact of Pals1 on Expression and Localization of Transporters Belonging to the Solute Carrier Family

Pals1 is part of the evolutionary conserved Crumbs polarity complex and plays a key role in two processes, the formation of apicobasal polarity and the establishment of cell-cell contacts. In the human kidney, up to 1.5 million nephrons control blood filtration, as well as resorption and recycling of inorganic and organic ions, sugars, amino acids, peptides, vitamins, water and further metabolites of endogenous and exogenous origin. All nephron segments consist of polarized cells and express high levels of Pals1. Mice that are functionally haploid for Pals1 develop a lethal phenotype, accompanied by heavy proteinuria and the formation of renal cysts. However, on a cellular level, it is still unclear if reduced cell polarization, incomplete cell-cell contact formation, or an altered Pals1-dependent gene expression accounts for the renal phenotype. To address this, we analyzed the transcriptomes of Pals1-haploinsufficient kidneys and the littermate controls by gene set enrichment analysis. Our data elucidated a direct correlation between TGFβ pathway activation and the downregulation of more than 100 members of the solute carrier (SLC) gene family. Surprisingly, Pals1-depleted nephrons keep the SLC’s segment-specific expression and subcellular distribution, demonstrating that the phenotype is not mainly due to dysfunctional apicobasal cell polarization of renal epithelia. Our data may provide first hints that SLCs may act as modulating factors for renal cyst formation.

α-Galactosidase a Deficiency in Fabry Disease Leads to Extensive Dysregulated Cellular Signaling Pathways in Human Podocytes

Fabry disease (FD) is caused by mutations in the α-galactosidase A (GLA) gene encoding the lysosomal AGAL enzyme. Loss of enzymatic AGAL activity and cellular accumulation of sphingolipids (mainly globotriaosylcermide) may lead to podocyturia and renal loss of function with increased cardiovascular morbidity and mortality in affected patients. To identify dysregulated cellular pathways in FD, we established a stable AGAL-deficient podocyte cell line to perform a comprehensive proteome analysis. Imbalanced protein expression and function were analyzed in additional FD cell lines including endothelial, epithelial kidney, patient-derived urinary cells and kidney biopsies. AGAL-deficient podocytes showed dysregulated proteins involved in thermogenesis, lysosomal trafficking and function, metabolic activity, cell-cell interactions and cell cycle. Proteins associated with neurological diseases were upregulated in AGAL-deficient podocytes. Rescues with inducible AGAL expression only partially normalized protein expression. A disturbed protein expression was confirmed in endothelial, epithelial and patient-specific cells, pointing toward fundamental pathway disturbances rather than to cell type-specific alterations in FD. We conclude that a loss of AGAL function results in profound changes of cellular pathways, which are ubiquitously in different cell types. Due to these profound alterations, current approved FD-specific therapies may not be sufficient to completely reverse all dysregulated pathways.

Evolution of renal-disease factor APOL1 results in cis and trans orientations at the endoplasmic reticulum that both show cytotoxic effects

The recent and exclusively in humans and a few other higher primates expressed APOL1 (apolipoprotein L1) gene is linked to African human trypanosomiasis (also known as African sleeping sickness) as well as to different forms of kidney diseases. Whereas APOL1’s role as a trypanolytic factor is well established, pathobiological mechanisms explaining its cytotoxicity in renal cells remain unclear. In this study, we compared the APOL family members using a combination of evolutionary studies and cell biological experiments to detect unique features causal for APOL1 nephrotoxic effects.

We investigated available primate and mouse genome and transcriptome data to apply comparative phylogenetic and maximum likelihood selection analyses. We suggest that the APOL gene family evolved early in vertebrates and initial splitting occurred in ancestral mammals. Diversification and differentiation of functional domains continued in primates, including developing the two members APOL1 and APOL2. Their close relationship could be diagnosed by sequence similarity and a shared ancestral insertion of an AluY transposable element. Live-cell imaging analyses showed that both expressed proteins show a strong preference to localize at the endoplasmic reticulum (ER). However, glycosylation and secretion assays revealed that—unlike APOL2—APOL1 membrane insertion or association occurs in different orientations at the ER, with the disease-associated mutants facing either the luminal (cis) or cytoplasmic (trans) side of the ER. The various pools of APOL1 at the ER offer a novel perspective in explaining the broad spectrum of its observed toxic effects.

Crumbs2 Is an Essential Slit Diaphragm Protein of the Renal Filtration Barrier

BACKGROUND

Crumbs2 is expressed at embryonic stages as well as in the retina, brain, and glomerular podocytes. Recent studies identified CRB2 mutations as a novel cause of steroid-resistant nephrotic syndrome (SRNS).

METHODS

To study the function of Crb2 at the renal filtration barrier, mice lacking Crb2 exclusively in podocytes were generated. Gene expression and histologic studies as well as transmission and scanning electron microscopy were used to analyze these Crb2^podKO knockout mice and their littermate controls. Furthermore, high-resolution expansion microscopy was used to investigate Crb2 distribution in murine glomeruli. For pull-down experiments, live cell imaging, and transcriptome analyses, cell lines were applied that inducibly express fluorescent protein-tagged CRB2 wild type and mutants.

RESULTS

Crb2^podKO mice developed proteinuria directly after birth that preceded a prominent development of disordered and effaced foot processes, upregulation of renal injury and inflammatory markers, and glomerulosclerosis. Pull-down assays revealed an interaction of CRB2 with Nephrin, mediated by their extracellular domains. Expansion microscopy showed that in mice glomeruli, Crb2 and Nephrin are organized in adjacent clusters. SRNS-associated CRB2 protein variants and a mutant that lacks a putative conserved O-glycosylation site were not transported to the cell surface. Instead, mutants accumulated in the ER, showed altered glycosylation pattern, and triggered an ER stress response.

CONCLUSIONS

Crb2 is an essential component of the podocyte's slit diaphragm, interacting with Nephrin. Loss of slit diaphragm targeting and increasing ER stress are pivotal factors for onset and progression of CRB2-related SRNS.

TrkC Is Essential for Nephron Function and Trans-Activates Igf1R Signaling

BACKGROUND

Injury to kidney podocytes often results in chronic glomerular disease and consecutive nephron malfunction. For most glomerular diseases, targeted therapies are lacking. Thus, it is important to identify novel signaling pathways contributing to glomerular disease. Neurotrophic tyrosine kinase receptor 3 (TrkC) is expressed in podocytes and the protein transmits signals to the podocyte actin cytoskeleton.

METHODS

Nephron-specific TrkC knockout (TrkC-KO) and nephron-specific TrkC-overexpressing (TrkC-OE) mice were generated to dissect the role of TrkC in nephron development and maintenance.

RESULTS

Both TrkC-KO and TrkC-OE mice exhibited enlarged glomeruli, mesangial proliferation, basement membrane thickening, albuminuria, podocyte loss, and aspects of FSGS during aging. Igf1 receptor (Igf1R)-associated gene expression was dysregulated in TrkC-KO mouse glomeruli. Phosphoproteins associated with insulin, erb-b2 receptor tyrosine kinase (Erbb), and Toll-like receptor signaling were enriched in lysates of podocytes treated with the TrkC ligand neurotrophin-3 (Nt-3). Activation of TrkC by Nt-3 resulted in phosphorylation of the Igf1R on activating tyrosine residues in podocytes. Igf1R phosphorylation was increased in TrkC-OE mouse kidneys while it was decreased in TrkC-KO kidneys. Furthermore, TrkC expression was elevated in glomerular tissue of patients with diabetic kidney disease compared with control glomerular tissue.

CONCLUSIONS

Our results show that TrkC is essential for maintaining glomerular integrity. Furthermore, TrkC modulates Igf-related signaling in podocytes.

LIM and SH3 protein 1 (LASP-1): A novel link between the slit membrane and actin cytoskeleton dynamics in podocytes

The foot processes of podocytes exhibit a dynamic actin cytoskeleton, which maintains their complex cell structure and antagonizes the elastic forces of the glomerular capillary. Interdigitating secondary foot processes form a highly selective filter for proteins in the kidney, the slit membrane. Knockdown of slit membrane components such as Nephrin or Neph1 and cytoskeletal adaptor proteins such as CD2AP in mice leads to breakdown of the filtration barrier with foot process effacement, proteinuria, and early death of the mice. Less is known about the crosstalk between the slit membrane-associated proteins and cytoskeletal components inside the podocyte foot processes. Our study shows that LASP-1, an actin-binding protein, is highly expressed in podocytes. Electron microscopy studies demonstrate that LASP-1 is found at the slit membrane suggesting a role in anchoring slit membrane components to the actin cytoskeleton. Live cell imaging experiments with transfected podocytes reveal that LASP-1 is either part of a highly dynamic granular complex or a static, actin cytoskeleton-bound protein. We identify CD2AP as a novel LASP-1 binding partner that regulates its association with the actin cytoskeleton. Activation of the renin-angiotensin-aldosterone system, which is crucial for podocyte function, leads to phosphorylation and altered localization of LASP-1. In vivo studies using the Drosophila nephrocyte model indicate that Lasp is necessary for the slit membrane integrity and functional filtration.

Nephrin Signaling Results in Integrin β1 Activation

BACKGROUND

Patients with certain mutations in the gene encoding the slit diaphragm protein Nephrin fail to develop functional slit diaphragms and display severe proteinuria. Many adult-onset glomerulopathies also feature alterations in Nephrin expression and function. Nephrin signals from the podocyte slit diaphragm to the Actin cytoskeleton by recruiting proteins that can interact with C3G, a guanine nucleotide exchange factor of the small GTPase Rap1. Because Rap activity affects formation of focal adhesions, we hypothesized that Nephrin transmits signals to the Integrin receptor complex, which mediates podocyte adhesion to the extracellular matrix.

METHODS

To investigate Nephrin's role in transmitting signals to the Integrin receptor complex, we conducted genetic studies in Drosophila nephrocytes and validated findings from Drosophila in a cultured human podocyte model.

RESULTS

Drosophila nephrocytes form a slit diaphragm-like filtration barrier and express the Nephrin ortholog Sticks and stones (Sns). A genetic screen identified c3g as necessary for nephrocyte function. In vivo, nephrocyte-specific gene silencing of sns or c3g compromised nephrocyte filtration and caused nephrocyte diaphragm defects. Nephrocytes with impaired Sns or C3G expression displayed an altered localization of Integrin and the Integrin-associated protein Talin. Furthermore, gene silencing of c3g partly rescued nephrocyte diaphragm defects of an sns overexpression phenotype, pointing to genetic interaction of sns and c3g in nephrocytes. We also found that activated Nephrin recruited phosphorylated C3G and resulted in activation of Integrin β1 in cultured podocytes.

CONCLUSIONS

Our findings suggest that Nephrin can mediate a signaling pathway that results in activation of Integrin β1 at focal adhesions, which may affect podocyte attachment to the extracellular matrix.

Nephron-specific knockin of the PIKfyve-binding-deficient Vac14L156R mutant results in albuminuria and mesangial expansion

Intracellular trafficking processes play a key role for the establishment and maintenance of membrane surfaces in renal epithelia. Therefore, dysfunctions of these trafficking processes could be key events and important determinants in the onset and progression of diseases. The presence of cellular vacuoles-observed in many histologic analyses of renal diseases-is a macroscopic hint for disturbed intracellular trafficking processes. However, how vacuoles develop and which intracellular pathways are directly affected remain largely unknown. Previous studies showed that in some cases, vacuolization is linked to malfunction of the Vac14 complex. This complex, including the scaffold protein Vac14, the lipid kinase PIKfyve, and its counteracting lipid phosphatase Fig4, regulates intracellular phosphatidylinositol phosphate levels, which in turn, control the maturation of early-into-late endosomes, as well as the processing of autophagosomes into autophagolysosomes. Here, we analyzed the role of Vac14 in mice and observed that the nephron-specific knockin of the PIKfyve-binding-deficient Vac14^L156R mutant led to albuminuria, accompanied by mesangial expansion, increased glomerular size, and an elevated expression of several kidney injury markers. Overexpression of this Vac14 variant in podocytes did not reveal a strong in vivo phenotype, indicating that Vac14-dependent trafficking processes are more important for tubular than for glomerular processes in the kidney. In vitro overexpression of Vac14^L156R in Madin-Darby canine kidney cells had no impact on apico-basal polarity defects but resulted in a faster reassembly of junctional structures after Ca²⁺ depletion and delayed endo- and transcytosis rates. Taken together, our data suggest that increased albuminuria of Vac14^L156R-overexpressing mice is a consequence of a lowered endo- and transcytosis of albumin in renal tubules.

Nuclear YAP localization as a key regulator of podocyte function

Podocytes are crucial for the establishment of the blood-urine filtration barrier in the glomeruli of the kidney. These cells are mainly affected during glomerulopathies causing proteinuria and kidney function impairment. Ongoing podocyte injury leads to podocyte loss, finally followed by end-stage kidney disease. Podocytes display a predominant nuclear localization of YAP (Yes-associated protein), one effector protein of the Hippo pathway, which regulates the balance between proliferation, differentiation, and apoptosis in cells. Nuclear active YAP seems to be critical for podocyte survival in vivo and in vitro. We can show here that different treatments leading to sequestration of YAP into the cytoplasm in podocytes, like decreased rigidity of the substrate, incubation with dasatinib, or overexpression of Hippo pathway members result in the induction of apoptosis. A RNA sequencing analysis of large tumor suppressor kinase 2 (LATS2) overexpressing podocytes confirmed a significant upregulation of apoptotic genes. The downregulation of Hippo pathway components suggests a feedback mechanism in podocytes. Noteworthy was the regulation of genes involved in cell–cell junction, the composition of the extracellular space, and cell migration. This suggests an influence of Hippo pathway activity on podocyte integrity. As focal segmental glomerulopathy (FSGS) goes along with an activation of the Hippo pathway in podocytes, a comparison of our data with two independent studies of transcriptional regulation in human FSGS glomeruli obtained from the Nephroseq database was performed. This comparison affirmed a multitude of consistent transcriptional changes concerning the regulation of genes influencing apoptosis and the Hippo signaling pathway as well as cell junction formation and cell migration. The link between Hippo pathway activation in podocytes and the regulation of junction and migration processes in vivo might be a fundamental mechanism of glomerular sclerosis and loss of renal function.

Getting a Notch closer to renal dysfunction: activated Notch suppresses expression of the adaptor protein Disabled-2 in tubular epithelial cells

Reactivation of Notch signaling in kidneys of animal models and patients with chronic kidney disease (CKD) has been shown to contribute to epithelial injury and fibrosis development. Here, we investigated the mechanisms of Notch-induced injury in renal epithelial cells. We performed genome-wide transcriptome analysis to identify Notch target genes using an in vitro system of cultured tubular epithelial cells expressing the intracellular domain of Notch1. One of the top downregulated genes was Disabled-2 ( Dab2). With the use of Drosophila nephrocytes as a model system, we found that Dab (the Drosophila homolog of Dab2) knockdown resulted in a significant filtration defect, indicating that loss of Dab2 plays a functional role in kidney disease development. We showed that Dab2 expression in cultured tubular epithelial cells is involved in endocytic regulation and that it also protects cells from TGF-β-induced epithelial-to-mesenchymal transition. In vivo correlation studies indicated its additional role in renal ischemia-induced injury. Together, these data suggest that Dab2 plays a versatile role in the kidney and may impact on acute and CKDs.-Schütte-Nütgen, K., Edeling, M., Mendl, G., Krahn, M. P., Edemir, B., Weide, T., Kremerskothen, J., Michgehl, U., Pavenstädt, H. Getting a Notch closer to renal dysfunction: activated Notch suppresses expression of the adaptor protein Disabled-2 in tubular epithelial cells.

WW and C2 domain-containing proteins regulate hepatic cell differentiation and tumorigenesis through the hippo signaling pathway

The Hippo pathway regulates cell differentiation, proliferation, and apoptosis. Upon activation, it inhibits the import of the transcriptional coactivator yes-associated protein (YAP) into the nucleus, thus suppressing transcription of pro-proliferative genes. Hence, dynamic and precise control of the Hippo pathway is crucial for organ size control and the prevention of tumor formation. Hippo signaling is controlled by a growing number of upstream regulators, including WW and C2 domain-containing (WWC) proteins, which trigger a serine/threonine kinase pathway. One component of this is the large tumor suppressor (LATS) kinase, which phosphorylates YAP, trapping it in the cytoplasm. WWC proteins have been shown to interact with LATS in vitro and stimulate its kinase activity, thus directly promoting cytoplasmic accumulation of phosphorylated YAP. However, the function of the WWC proteins in the regulation of cell proliferation, organ size control, and tumor prevention in vivo has not yet been determined. Here, we show that loss of hepatic WWC expression in mice leads to tissue overgrowth, inflammation, fibrosis, and formation of liver carcinoma. WWC-deficient mouse livers display reduced LATS activity, increased YAP-mediated gene transcription, and enhanced proliferation of hepatic progenitor cells. In addition, loss of WWC expression in the liver accelerates the turnover of angiomotin proteins, which act as negative regulators of YAP activity.

CONCLUSION

Our data define an essential in vivo function for WWC proteins as regulators of canonical and noncanonical Hippo signaling in hepatic cell growth and liver tumorigenesis. Thus, expression of WWC proteins may serve as novel prognostic factors in human liver carcinoma. (Hepatology 2018;67:1546-1559).

Tropomyosin-related kinase C (TrkC) enhances podocyte migration by ERK-mediated WAVE2 activation

Podocyte malfunction is central to glomerular diseases and is marked by defective podocyte intercellular junctions and actin cytoskeletal dynamics. Podocytes share many morphologic features with neurons, so that similar sets of proteins appear to regulate cell process formation. One such protein is the tropomyosin-related kinase C (TrkC). TrkC deficiency in mice leads to proteinuria as a surrogate of defective kidney filter function. Activation of endogenous TrkC by its ligand neurotrophin-3 resulted in increased podocyte migration-a surrogate of podocyte actin dynamics in vivo. Employing a mutagenesis approach, we found that the Src homologous and collagen-like (Shc) binding site Tyr⁵¹⁶ within the TrkC cytoplasmic domain was necessary for TrkC-induced migration of podocytes. TrkC activation led to a mobility shift of Wiskott-Aldrich syndrome family verprolin-homologous protein (WAVE)-2 which is known to orchestrate Arp2/3 activation and actin polymerization. Chemical inactivation of Erk or mutagenesis of 2 of 4 known Erk target sites within WAVE2, Thr³⁴⁶ and Ser³⁵¹, abolished the TrkC-induced WAVE2 mobility shift. Knockdown of WAVE2 by shRNA abolished TrkC-induced podocyte migration. In summary, TrkC signals to the podocyte actin cytoskeleton to induce migration by phosphorylating WAVE2 Erk dependently. This signaling mechanism may be important for TrkC-mediated cytoskeletal dynamics in podocyte disease.-Gromnitza, S., Lepa, C., Weide, T., Schwab, A., Pavenstädt, H., George, B. Tropomyosin-related kinase C (TrkC) enhances podocyte migration by ERK-mediated WAVE2 activation.

Angiotensin II regulates phosphorylation of actin-associated proteins in human podocytes

Within the kidney, angiotensin II (AngII) targets different cell types in the vasculature, tubuli, and glomeruli. An important part of the renal filtration barrier is composed of podocytes with their actin-rich foot processes. In this study, we used stable isotope labeling with amino acids in cell culture coupled to mass spectrometry to characterize relative changes in the phosphoproteome of human podocytes in response to short-term treatment with AngII. In 4 replicates, we identified a total of 17,956 peptides that were traceable to 2081 distinct proteins. Bioinformatic analyses revealed that among the increasingly phosphorylated peptides are predominantly peptides that are related to actin filaments, cytoskeleton, lamellipodia, mammalian target of rapamycin, and MAPK signaling. Among others, this screening approach highlighted the increased phosphorylation of actin-bundling protein, l-plastin (LCP1). AngII-dependent phosphorylation of LCP1 in cultured podocytes was mediated by the kinases ERK, p90 ribosomal S6 kinase, PKA, or PKC. LCP1 phosphorylation increased filopodia formation. In addition, treatment with AngII led to LCP1 redistribution to the cell margins, membrane ruffling, and formation of lamellipodia. Our data highlight the importance of AngII-triggered actin cytoskeleton-associated signal transduction in podocytes.-Schenk, L. K., Möller-Kerutt, A., Klosowski, R., Wolters, D., Schaffner-Reckinger, E., Weide, T., Pavenstädt, H., Vollenbröker, B. Angiotensin II regulates phosphorylation of actin-associated proteins in human podocytes.

Intracellular APOL1 Risk Variants Cause Cytotoxicity Accompanied by Energy Depletion

Population genetic approaches have uncovered a strong association between kidney diseases and two sequence variants of the APOL1 gene, called APOL1 risk variant G1 and variant G2, compared with the nonrisk G0 allele. However, the mechanism whereby these variants lead to disease manifestation and, in particular, whether this involves an intracellular or extracellular pool of APOL1 remains unclear. Herein, we show a predominantly intracellular localization of APOL1 G0 and the renal risk variants, which localized to membranes of the endoplasmic reticulum in podocyte cell lines. This localization did not depend on the N-terminal signal peptide that mediates APOL1 secretion into the circulation. Additionally, a fraction of these proteins localized to structures surrounding mitochondria. In vitro overexpression of G1 or G2 lacking the signal peptide inhibited cell viability, triggered phosphorylation of stress-induced kinases, increased the phosphorylation of AMP-activated protein kinase, reduced intracellular potassium levels, and reduced mitochondrial respiration rates. These findings indicate that functions at intracellular membranes, specifically those of the endoplasmic reticulum and mitochondria, are crucial factors in APOL1 renal risk variant-mediated cell injury.

Pals1 Haploinsufficiency Results in Proteinuria and Cyst Formation

The nephron is the basic physiologic subunit of the mammalian kidney and is made up of several apicobasally polarized epithelial cell types. The process of apicobasal polarization in animal cells is controlled by the evolutionarily conserved Crumbs (CRB), Partitioning-defective, and Scribble protein complexes. Here, we investigated the role of protein associated with LIN-7 1 (Pals1, also known as Mpp5), a core component of the apical membrane-determining CRB complex in the nephron. Pals1 interacting proteins, including Crb3 and Wwtr1/Taz, have been linked to renal cyst formation in mice before. Immunohistologic analysis revealed Pals1 expression in renal tubular cells and podocytes of human kidneys. Mice lacking one Pals1 allele (functionally haploid for Pals1) in nephrons developed a fully penetrant phenotype, characterized by cyst formation and severe defects in renal barrier function, which led to death within 6-8 weeks. In Drosophila nephrocytes, deficiency of the Pals1 ortholog caused alterations in slit-diaphragm-like structures. Additional studies in epithelial cell culture models revealed that Pals1 functions as a dose-dependent upstream regulator of the crosstalk between Hippo- and TGF-β-mediated signaling. Furthermore, Pals1 haploinsufficiency in mouse kidneys associated with the upregulation of Hippo pathway target genes and marker genes of TGF-β signaling, including biomarkers of renal diseases. These findings support a link between apical polarity proteins and renal diseases, especially renal cyst diseases. Further investigation of the Pals1-linked networks is required to decipher the mechanisms underlying the pathogenesis of these diseases.

VE-cadherin interacts with cell polarity protein Pals1 to regulate vascular lumen formation

Blood vessel tubulogenesis requires the establishment of apicobasal polarity of endothelial cells. A novel interaction is described of the cell adhesion molecule VE-cadherin with the cell polarity protein Pals1. The activity of VE-cadherin in regulation of endothelial lumen formation depends on its interaction with both Pals1 and Par3.

Blood vessel tubulogenesis requires the formation of stable cell-to-cell contacts and the establishment of apicobasal polarity of vascular endothelial cells. Cell polarity is regulated by highly conserved cell polarity protein complexes such as the Par3-aPKC-Par6 complex and the CRB3-Pals1-PATJ complex, which are expressed by many different cell types and regulate various aspects of cell polarity. Here we describe a functional interaction of VE-cadherin with the cell polarity protein Pals1. Pals1 directly interacts with VE-cadherin through a membrane-proximal motif in the cytoplasmic domain of VE-cadherin. VE-cadherin clusters Pals1 at cell–cell junctions. Mutating the Pals1-binding motif in VE-cadherin abrogates the ability of VE-cadherin to regulate apicobasal polarity and vascular lumen formation. In a similar way, deletion of the Par3-binding motif at the C-terminus of VE-cadherin impairs apicobasal polarity and vascular lumen formation. Our findings indicate that the biological activity of VE-cadherin in regulating endothelial polarity and vascular lumen formation is mediated through its interaction with the two cell polarity proteins Pals1 and Par3.

TMEM16F Regulates Baseline Phosphatidylserine Exposure and Cell Viability in Human Embryonic Kidney Cells

Background / Aims: TMEM16F is a transmembrane protein from a conserved family of Ca2+-activated proteins, which is highly expressed in several tissues. TMEM16F confers phospholipid scramblase activity and Ca2+-activated electrolyte channel activity. Potentially thereby, TMEM16F is involved in cell cycle control and apoptotic signaling. The present study evaluated the role of TMEM16F on cell proliferation and viability in Human Embryonic Kidney cells.

METHODS

An inducible knockdown of TMEM16F was generated and markers of apoptosis and proliferation were assessed via flow cytometry, western blotting and MTT uptake assay under different conditions.

RESULTS

TMEM16F knockdown resulted in attenuated growth of HEK293 cells. This observation correlated with an increased phosphatidylserine exposure and a decreased fraction of viable cells. Interestingly, the cells were not sensitized to Staurosporine- induced cell death. Western blot analyses displayed a parallel activation of pro- and antiapoptotic signaling pathways: Caspase 3 cleavage and Cyclin D1 abundance were simultaneously increased. Furthermore, knockdown of TMEM16F led to activation of AKT signaling.

CONCLUSION

TMEM16F modifies viability of Human Embryonic Kidney cells via its function as a phospholipid scramblase and activation of AKT signaling pathways.

Trajectories and single-particle tracking data of intracellular vesicles loaded with either SNAP-Crb3A or SNAP-Crb3B

Using a combined approach of pulse chase labeling and single-particle tracking of Crb3A or 3B loaded vesicles we collected trajectories of different vesicle population in living podocyte cells and evaluated statistically their different mobility patterns. Differences in their intracellular mobility and in their directed transport correspond well to the role of Crb3A and 3B in renal plasma membrane sorting (Djuric et al., 2016) [1].

CC chemokine receptor 10 cell surface presentation in melanocytes is regulated by the novel interaction partner S100A10

The superfamily of G-protein-coupled receptors (GPCR) conveys signals in response to various endogenous and exogenous stimuli. Consequently, GPCRs are the most important drug targets. CCR10, the receptor for the chemokines CCL27/CTACK and CCL28/MEC, belongs to the chemokine receptor subfamily of GPCRs and is thought to function in immune responses and tumour progression. However, there is only limited information on the intracellular regulation of CCR10. We find that S100A10, a member of the S100 family of Ca(2+) binding proteins, binds directly to the C-terminal cytoplasmic tail of CCR10 and that this interaction regulates the CCR10 cell surface presentation. This identifies S100A10 as a novel interaction partner and regulator of CCR10 that might serve as a target for therapeutic intervention.

AmotL2 disrupts apical-basal cell polarity and promotes tumour invasion

The establishment and maintenance of apical-basal cell polarity is essential for the functionality of glandular epithelia. Cell polarity is often lost in advanced tumours correlating with acquisition of invasive and malignant properties. Despite extensive knowledge regarding the formation and maintenance of polarity, the mechanisms that deregulate polarity in metastasizing cells remain to be fully characterized. Here we show that AmotL2 expression correlates with loss of tissue architecture in tumours from human breast and colon cancer patients. We further show that hypoxic stress results in activation of c-Fos-dependent expression of AmotL2 leading to loss of polarity. c-Fos/hypoxia-induced p60 AmotL2 interacts with the Crb3 and Par3 polarity complexes retaining them in large vesicles and preventing them from reaching the apical membrane. The resulting loss of polarity potentiates the response to invasive cues in vitro and in vivo in mice. These data provide a molecular mechanism how hypoxic stress deregulates cell polarity during tumour progression.

The Vac14-interaction network is linked to regulators of the endolysosomal and autophagic pathway

The scaffold protein Vac14 acts in a complex with the lipid kinase PIKfyve and its counteracting phosphatase FIG4, regulating the interconversion of phosphatidylinositol-3-phosphate to phosphatidylinositol-3,5-bisphosphate. Dysfunctional Vac14 mutants, a deficiency of one of the Vac14 complex components, or inhibition of PIKfyve enzymatic activity results in the formation of large vacuoles in cells. How these vacuoles are generated and which processes are involved are only poorly understood. Here we show that ectopic overexpression of wild-type Vac14 as well as of the PIKfyve-binding deficient Vac14 L156R mutant causes vacuoles. Vac14-dependent vacuoles and PIKfyve inhibitor-dependent vacuoles resulted in elevated levels of late endosomal, lysosomal, and autophagy-associated proteins. However, only late endosomal marker proteins were bound to the membranes of these enlarged vacuoles. In order to decipher the linkage between the Vac14 complex and regulators of the endolysosomal pathway, a protein affinity approach combined with multidimensional protein identification technology was conducted, and novel molecular links were unraveled. We found and verified the interaction of Rab9 and the Rab7 GAP TBC1D15 with Vac14. The identified Rab-related interaction partners support the theory that the regulation of vesicular transport processes and phosphatidylinositol-modifying enzymes are tightly interconnected.

KIBRA (KIdney/BRAin protein) regulates learning and memory and stabilizes Protein kinase Mζ

The WWC1 gene has been genetically associated with human episodic memory performance, and its product KIdney/BRAin protein (KIBRA) has been shown to interact with the atypical protein kinase protein kinase M ζ (PKMζ). Although recently challenged, PKMζ remains a candidate postsynaptic regulator of memory maintenance. Here, we show that PKMζ is subject to rapid proteasomal degradation and that KIBRA is both necessary and sufficient to counteract this process, thus stabilizing the kinase and maintaining its function for a prolonged time. We define the binding sequence on KIBRA, a short amino acid motif near the C-terminus. Both hippocampal knock-down of KIBRA in rats and KIBRA knock-out in mice result in decreased learning and memory performance in spatial memory tasks supporting the notion that KIBRA is a player in episodic memory. Interestingly, decreased memory performance is accompanied by decreased PKMζ protein levels. We speculate that the stabilization of synaptic PKMζ protein levels by KIBRA may be one mechanism by which KIBRA acts in memory maintenance. KIBRA/WWC1 has been genetically associated with human episodic memory. KIBRA has been shown to be post-synaptically localized, but its function remained obscure. Here, we show that KIBRA shields PKMζ, a kinase previously linked to memory maintenance, from proteasomal degradation via direct interaction. KIBRA levels in the rodent hippocampus correlate closely both to spatial memory performance in rodents and to PKMζ levels. Our findings support a role for KIBRA in memory, and unveil a novel function for this protein.

Common exonic missense variants in the C2 domain of the human KIBRA protein modify lipid binding and cognitive performance

The human KIBRA gene has been linked to human cognition through a lead intronic single-nucleotide polymorphism (SNP; rs17070145) that is associated with episodic memory performance and the risk to develop Alzheimer's disease. However, it remains unknown how this relates to the function of the KIBRA protein. Here, we identified two common missense SNPs (rs3822660G/T [M734I], rs3822659T/G [S735A]) in exon 15 of the human KIBRA gene to affect cognitive performance, and to be in almost complete linkage disequilibrium with rs17070145. The identified SNPs encode variants of the KIBRA C2 domain with distinct Ca(2+) dependent binding preferences for monophosphorylated phosphatidylinositols likely due to differences in the dynamics and folding of the lipid-binding pocket. Our results further implicate the KIBRA protein in higher brain function and provide direction to the cellular pathways involved.

Mechanisms of cell polarity and aquaporin sorting in the nephron

The kidneys participate in whole-body homeostasis, regulating acid-base balance, electrolyte concentrations, extracellular fluid volume, and regulation of blood pressure. Many of the kidney's functions are accomplished by relatively simple mechanisms of filtration, reabsorption, and secretion, which take place in the nephron. The kidneys generate 140-180 l of primary urine per day, while reabsorbing a large percentage, allowing for only the excretion of approximately 2 l of urine. Within the nephron, the majority of the filtered water and solutes are reabsorbed. This is mainly facilitated by specialized transporters and channels which are localized at different segments of the nephron and asymmetrically localized within the polarized epithelial cells. The asymmetric localization of these transporters and channels is essential for the physiological tasks of the renal tissues. One family of these proteins are the water-permeable aquaporins which are selectively expressed in cells along the nephron and localized at different compartments. Here, we discuss potential molecular links between mechanisms involved in the establishment of cell polarity and the members of the aquaporin family. In the first part of this review, we will focus on aspects of apical cell polarity. In the second part, we will review the motifs identified so far that are involved in aquaporin sorting and point out potential molecular links.

Implications of autophagy for glomerular aging and disease

Glomerular diseases lead to a progressive decline in renal function and account for the vast majority of end-stage kidney diseases. Injury and loss of glomerular podocytes are common determining factors of glomerular disease progression and renal failure. Podocytes are a primary glomerular target of toxic, immune, metabolic, and oxidant stress, but little is known of the factors that counteract cellular stress signaling pathways. This review focuses on recent findings that identify autophagy as a critical homeostatic and quality control mechanism maintaining glomerular homeostasis.

Expression, secretion and surface display of a human alkaline phosphatase by the ciliate Tetrahymena thermophila

Background

Tetrahymena thermophila possesses many attributes that render it an attractive host for the expression of recombinant proteins. Surface proteins from the parasites Ichthyophthirius multifiliis and Plasmodium falciparum and avian influenza virus antigen H5N1 were displayed on the cell membrane of this ciliate. Furthermore, it has been demonstrated that T. thermophila is also able to produce a functional human DNase I. The present study investigates the heterologous expression of the functional human intestinal alkaline phosphatase (hiAP) using T. thermophila and thereby presents a powerful tool for the optimization of the ciliate-based expression system.

Results

Functional and full length human intestinal alkaline phosphatase was expressed by T. thermophila using a codon-adapted gene containing the native signal-peptide and GPI (Glycosylphosphatidylinositol) anchor attachment signal. HiAP activity in the cell extract of transformants suggested that the hiAP gene was successfully expressed. Furthermore, it was demonstrated that the enzyme was modified with N-glycosylation and localized on the surface membrane by the C-terminal GPI anchor. A C-terminally truncated version of hiAP lacking the GPI anchor signal peptide was secreted into the medium as an active enzyme. In a first approach to establish a high level expression system up to 14,000 U/liter were produced in a time frame of two days, which exceeds the production rate of other published expression systems for this enzyme.

Conclusions

With the expression of hiAP, not only a protein of commercial interest could be produced, but also a reporter enzyme that offers the possibility to analyze T. thermophila genes that play a role in the regulation of protein secretion. Additionally, the fact that ciliates do not secrete an endogenous alkaline phosphatase provides the possibility to use the truncated hiAP as a reporter enzyme, allowing the quantification of measures that will be necessary for further optimization of the host strains and the fermentation processes.

GSK3β inactivation in podocytes results in decreased phosphorylation of p70S6K accompanied by cytoskeletal rearrangements and inhibited motility

The inhibition of mTOR kinase after renal transplantation has been associated with podocyte injury and proteinuria; however, the signaling pathways regulating these effects are not well understood. We found that prolonged rapamycin treatment in podocytes leads to an increase in glycogen synthase kinase 3β (GSK3β) phosphorylation, resulting in inactivation of total GSK3β kinase activity. To investigate the cellular consequences of the inactivation of GSK3β, we used two inhibitors reducing kinase activity and studied the cross talk between GSK3 function and the Akt/mammalian target of rapamycin (mTOR) pathway. Both GSK3 inhibitors reduced the phosphorylation of the mTOR downstream target, p70(S6K), indicating that GSK3 inhibition in podocytes is able to cause similar effects as treatment with rapamycin. Moreover, GSK3 inhibition was accompanied by the reduced expression of slit diaphragm-associated proteins and resulted in an altered cytoskeletal structure and reduced motility of podocytes, suggesting that GSK3 kinase can modulate Akt/mTOR-dependent signaling in podocytes.

Polycystin-2 activity is controlled by transcriptional coactivator with PDZ binding motif and PALS1-associated tight junction protein

Autosomal dominant polycystic kidney disease (ADPKD) is the most frequent monogenic cause of kidney failure, characterized by the development of renal cysts. ADPKD is caused by mutations of the polycystin-1 (PC1) or polycystin-2 (PC2) genes. PC2 encodes a Ca(2+)-permeable cation channel, and its dysfunction has been implicated in cyst development. The transcriptional coactivator with PDZ binding motif (TAZ) is required for the integrity of renal cilia. Its absence results in the development of renal cysts in a knock-out mouse model. TAZ directly interacts with PC2, and it has been suggested that another yet unidentified PDZ domain protein may be involved in the TAZ/PC2 interaction. Here we describe a novel interaction of TAZ with the multi-PDZ-containing PALS1-associated tight junction protein (PATJ). TAZ interacts with both the N-terminal PDZ domains 1-3 and the C-terminal PDZ domains 8-10 of PATJ, suggesting two distinct TAZ binding domains. We also show that the C terminus of PC2 strongly interacts with PDZ domains 8-10 and to a weaker extent with PDZ domains 1-3 of PATJ. Finally, we demonstrate that both TAZ and PATJ impair PC2 channel activity when co-expressed with PC2 in oocytes of Xenopus laevis. These results implicate TAZ and PATJ as novel regulatory elements of the PC2 channel and might thus be involved in ADPKD pathology.

ADAM10 is expressed in human podocytes and found in urinary vesicles of patients with glomerular kidney diseases

Background

The importance of the Notch signaling in the development of glomerular diseases has been recently described. Therefore we analyzed in podocytes the expression and activity of ADAM10, one important component of the Notch signaling complex.

Methods

By Western blot, immunofluorescence and immunohistochemistry analysis we characterized the expression of ADAM10 in human podocytes, human urine and human renal tissue.

Results

We present evidence, that differentiated human podocytes possessed increased amounts of mature ADAM10 and released elevated levels of L1 adhesion molecule, one well known substrate of ADAM10. By using specific siRNA and metalloproteinase inhibitors we demonstrate that ADAM10 is involved in the cleavage of L1 in human podocytes. Injury of podocytes enhanced the ADAM10 mediated cleavage of L1. In addition, we detected ADAM10 in urinary podocytes from patients with kidney diseases and in tissue sections of normal human kidney. Finally, we found elevated levels of ADAM10 in urinary vesicles of patients with glomerular kidney diseases.

Conclusions

The activity of ADAM10 in human podocytes may play an important role in the development of glomerular kidney diseases.

Hypertension in mice lacking the CXCR3 chemokine receptor

The CXC chemokine receptor 3 (CXCR3) has been linked to autoimmune and inflammatory disease, allograft rejection, and ischemic nephropathy. CXCR3 is expressed on endothelial and smooth muscle cells. Although a recent study posited that antagonizing of CXCR3 function may reduce atherosclerosis, the role of CXCR3 in controlling physiological vascular functions remains unclear. This study demonstrates that disruption of CXCR3 leads to elevated mean arterial pressures in anesthetized and conscious mice, respectively. Stimulation of isolated resistance vessels with various vasoconstrictors showed increased contractibility in CXCR3-/- mice in response to angiotensin II (ANG II) and a decreased vasodilatation in response to acetylcholine (ACh). The increased contractibility was related to higher ANG II type 1 receptor (AT1R) expression, whereas the decreased vasodilatation was related to lower M3-ACh receptor expression in the mesenteric arteries of CXCR3-/- mice compared with wild-type mice. The vasodilatatory response to ACh could be antagonized by the nonselective ACh receptor antagonist atropine and the selective M3 receptor antagonist 4-DAMP, but not by M1, M2, and M4 receptor antagonists. Additionally, EMSA studies revealed that transcription factors SP-1 and EGR-1 interact as a complex with the murine AT1R promoter region. Furthermore, we could show increased expression of SP-1 in CXCR3-/- mice indicating an imbalanced SP-1 and EGR-1 complex formation which causes increased AT1R expression and hypertension. The data indicate that CXCR3 receptor is important in vascular contractility and hypertension, possibly through upregulated AT1R expression.

mTOR regulates expression of slit diaphragm proteins and cytoskeleton structure in podocytes

The immunosuppressive mammalian target of rapamycin (mTOR) inhibitors can cause proteinuria, especially in kidney and heart transplanted patients. Podocytes play a major role in establishing the selective permeability of the blood-urine filtration barrier. Damage of these cells leads to proteinuria, a hallmark of most glomerular diseases. Interestingly, podocyte damage and focal segmental glomerulosclerosis can occur after treatment with an mTOR inhibitor in some transplant patients. To investigate the mechanisms of mTOR inhibitor-induced podocyte damage, we analyzed the effect of rapamycin on mTOR signaling and cellular function in human podocytes. We found that prolonged rapamycin treatment reduced the expression of total mTOR, which correlates with diminished levels of mTOR phosphorylation at Ser(2448) and Ser(2481). In addition, treatment with rapamycin reduced rictor expression and mTORC2 formation, resulting in a reduced phosphorylation of protein kinase B at Ser(473). The expression level of the slit-diaphragm proteins nephrin and transient receptor potential cation channel 6 as well as the cytoskeletal adaptor protein Nck significantly decreased. Moreover, rapamycin reduced cell adhesion and cell motility, which was accompanied by an enhanced formation of dot-like actin-rich structures. Our data provide new molecular insights explaining which pathways and molecules are affected in podocytes by an imbalanced mTOR function because of rapamycin treatment.

Mechanisms of angiotensin II signaling on cytoskeleton of podocytes

Podocytes are significant in establishing the glomerular filtration barrier. Sustained rennin-angiotensin system (RAS) activation is crucial in the pathogenesis of podocyte injury and causes proteinuria. This study demonstrates that angiotensin II (Ang II) caused a reactive oxygen species (ROS)-dependent rearrangement of cortical F-actin and a migratory phenotype switch in cultured mouse podocytes with stable Ang II type 1 receptor (AT1R) expression. Activated small GTPase Rac-1 and phosphorylated ezrin/radixin/moesin (ERM) proteins provoked Ang II-induced F-actin cytoskeletal remodeling. This work also shows increased expression of Rac-1 and phosphorylated ERM proteins in cultured podocytes, and in glomeruli of podocyte-specific AT1R transgenic rats (Neph-hAT1 TGRs). The free radical scavenger DMTU eliminated Ang II-induced cell migration, ERM protein phosphorylation and cortical F-actin remodeling, indicating that ROS mediates the influence of Rac-1 on podocyte AT1R signaling. Heparin, a potent G-coupled protein kinase 2 inhibitor, was found to abolish ERM protein phosphorylation and cortical F-actin ring formation in Ang II-treated podocytes, indicating that phosphorylated ERM proteins are the cytoskeletal effector in AT1R signaling. Moreover, Ang II stimulation triggered down-regulation of alpha actinin-4 and reduced focal adhesion expression in podocytes. Signaling inhibitor assay of Ang II-treated podocytes reveals that Rac-1, RhoA, and F-actin reorganization were involved in expressional regulation of alpha actinin-4 in AT1R signaling. With persistent RAS activation, the Ang II-induced phenotype shifts from being dynamically stable to adaptively migratory, which may eventually exhaust podocytes with a high actin cytoskeletal turnover, causing podocyte depletion and focal segmental glomerulosclerosis.

KIBRA modulates directional migration of podocytes

Asymmetric delivery and distribution of macromolecules are essential for cell polarity and for cellular functions such as differentiation, division, and signaling. Injury of podocytes, which are polarized epithelial cells, changes the dynamics of the actin meshwork, resulting in foot process retraction and proteinuria. Although the spatiotemporal control of specific protein-protein interactions is crucial for the establishment of cell polarity, the mechanisms controlling polarity-dependent differentiation and division are incompletely understood. In this study, yeast two-hybrid screens were performed using a podocyte cDNA library and the polarity protein PATJ as bait. The protein KIBRA was identified as an interaction partner of PATJ and was localized to podocytes, tubular structures, and collecting ducts. The last four amino acids of KIBRA mediated binding to the eighth PDZ domain of PATJ. In addition, KIBRA directly bound to synaptopodin, an essential organizer of the podocyte cytoskeleton. Stable knockdown of KIBRA in immortalized podocytes impaired directed cell migration, suggesting that KIBRA modulates the motility of podocytes by linking polarity proteins and cytoskeleton-associated protein complexes.

A recombinase system facilitates cloning of expression cassettes in the ciliate Tetrahymena thermophila

BACKGROUND

Tetrahymena thermophila is one of the best characterized unicellular eukaryotes and its genome is sequenced in its entirety. However, the AT-richness of the genome and an unusual codon usage cause problems in cloning and expression of the ciliate DNA. To overcome these technical hiatuses we developed a Cre-dependent recombinase system.

RESULTS

We created novel donor and acceptor vectors that facilitate the transfer of expression cassettes from the donor into novel acceptor plasmid. Expression vectors were used that encode the 19 kDa C-terminus of the MSP1 protein of Plasmodium falciparum and a blasticidin S (bsdR) resistance gene, respectively. The functional expression of these genes was demonstrated by western blot analysis with MSP1 specific antibodies and by a blasticidin growing assay.

CONCLUSION

The Cre dependent recombinase system in combination with the modular structure of the donor vectors ease cloning and expression of foreign genes in the ciliate system, providing a powerful tool for protistology research in future.

The bifunctional dihydrofolate reductase thymidylate synthase of Tetrahymena thermophila provides a tool for molecular and biotechnology applications

Background

Dihydrofolate reductase (DHFR) and thymidylate synthase (TS) are crucial enzymes in DNA synthesis. In alveolata both enzymes are expressed as one bifunctional enzyme.

Results

Loss of this essential enzyme activities after successful allelic assortment of knock out alleles yields an auxotrophic marker in ciliates. Here the cloning, characterisation and functional analysis of Tetrahymena thermophila's DHFR-TS is presented. A first aspect of the presented work relates to destruction of DHFR-TS enzyme function in an alveolate thereby causing an auxotrophy for thymidine. A second aspect is to knock in an expression cassette encoding for a foreign gene with subsequent expression of the target protein.

Conclusion

This system avoids the use of antibiotics or other drugs and therefore is of high interest for biotechnological applications.