Evidence for a role of protein phosphatases 1 and 2A during early nephrogenesis

Role of protein phosphatase 2A in kidney disease (Review)

Kidney disease affects millions of people worldwide and is a financial burden on the healthcare system. Protein phosphatase 2A (PP2A), which is involved in renal development and the function of ion-transport proteins, aquaporin-2 and podocytes, is likely to serve an important role in renal processes. PP2A is associated with the pathogenesis of a variety of different kidney diseases including podocyte injury, inflammation, tumors and chronic kidney disease. The current review aimed to discuss the structure and function of PP2A subunits in the context of kidney diseases. How dysregulation of PP2A in the kidneys causes podocyte death and the inactivation of PP2A in renal carcinoma tissues is discussed. Inhibition of PP2A activity prevents epithelial-mesenchymal transition and attenuates renal fibrosis, creating a favorable inflammatory microenvironment and promoting the initiation and progression of tumor pathogenesis. The current review also indicates that PP2A serves an important role in protection against renal inflammation. Understanding the detailed mechanisms of PP2A provides information that can be utilized in the design and application of novel therapeutics for the treatment and prevention of renal diseases.

Protein phosphatase 2A modulates podocyte maturation and glomerular functional integrity in mice

Background

Protein phosphorylation & dephosphorylation are ubiquitous cellular processes that allow for the nuanced and reversible regulation of protein activity. Protein phosphatase 2A (PP2A) is a multifunction phosphatase that is well expressed in all cell types of kidney during early renal development, though its functions in kidney remains to be elucidated.

Methods

PP2A conditional knock-out mice was generated with PP2A fl/fl mice that were crossed with Podocin-Cre mice. The phenotype of Pod-PP2A–KO mice (homozygous for the floxed PP2A allele with Podocin-Cre) and littermate PP2A fl/fl controls (homozygous for the PP2A allele but lacking Podocin-Cre) were further studied. Primary podocytes isolated from the Pod-PP2A-KO mice were cultured and they were then employed with sing label-free nano-LC − MS/MS technology on a Q-exactive followed by SIEVE processing to identify possible target molecular entities for the dephosphorylation effect of PP2A, in which Western blot and immunofluorescent staining were used to analyze further.

Results

Pod-PP2A–KO mice were developed with weight loss, growth retardation, proteinuria, glomerulopathy and foot process effacement, together with reduced expression of some slit diaphragm molecules and cytoskeleton rearrangement of podocytes. Y box protein 1 (YB-1) was identified to be the target molecule for dephosphorylation effect of PP2A. Furthermore, YB-1 phosphorylation was up-regulated in the Pod-PP2A–KO mice in contrast to the wild type controls, while total and un-phosphorylated YB-1 both was moderately down-regulated in podocytes from the Pod-PP2A-KO mice.

Conclusion

Our study revealed the important role of PP2A in regulating the development of foot processes and fully differentiated podocytes whereas fine-tuning of YB-1 via a post-translational modification by PP2A regulating its activity might be crucial for the functional integrity of podocytes and glomerular filtration barrier.

Graphic abstract

Electronic supplementary material

The online version of this article (10.1186/s12964-019-0402-y) contains supplementary material, which is available to authorized users.

A genome search for primary vesicoureteral reflux shows further evidence for genetic heterogeneity

Vesicoureteral reflux (VUR) is the most common disease of the urinary tract in children. In order to identify gene(s) involved in this complex disorder, we performed a genome-wide search in a selected sample of 31 patients with primary VUR from eight families originating from southern Italy. Sixteen additional families with 41 patients were included in a second stage. Nonparametric, affected-only linkage analysis identified four genomic areas on chromosomes 1, 3, and 4 (p < 0.05); the best result corresponded to the D3S3681-D3S1569 interval on chromosome 3 (nonparametric linkage score, NPL = 2.75, p = 0.008). This region was then saturated with 26 additional markers, tested in the complete group of 72 patients from 24 families (NPL = 2.01, p = 0.01). We identified a genomic area on 3q22.2-23, where 26 patients from six multiplex families shared overlapping haplotypes. However, we did not find evidence for a common ancestral haplotype. The region on chromosome 1 was delimited to 1p36.2-34.3 (D1S228-D1S255, max. NPL = 1.70, p = 0.03), after additional fine typing. Furthermore, on chromosome 22q11.22-12.3, patients from a single family showed excess allele sharing (NPL = 3.35, p = 0.015). Only the chromosome 3q region has been previously reported in the single genome-wide screening available for primary VUR. Our results suggest the presence of several novel loci for primary VUR, giving further evidence for the genetic heterogeneity of this disorder.

Absence of PPP2R1A mutations in Wilms tumor

Evidence from genetic linkage analysis indicates that a gene located at 19q13.4, FWT2, is responsible for predisposition to Wilms tumor in many Wilms tumor families. This region has also been implicated in the etiology of sporadic Wilms tumor through loss of heterozygosity analyses. The PPP2R1A gene, encoding the alpha isoform of the heterotrimeric serine/threonine protein phosphatase 2A (PP2A), is located within the FWT2 candidate region and is altered in breast and lung carcinomas. PPP2R1B, encoding the beta isoform, is mutated in lung, colon, and breast cancers. These findings suggested that both PPP2R1A and PPP2R1B may be tumor suppressor genes. Additionally, PP2A is important in fetal kidney growth and differentiation and has an expression pattern similar to that of the Wilms tumor suppressor gene WT1. Since PPP2R1A was therefore a compelling candidate for the FWT2 gene, we analysed the coding region of PPP2R1A in DNA and RNA samples from affected members of four Wilms tumor families and 30 sporadic tumors and identified no mutations in PPP2R1A in any of these 34 samples. We conclude that PPP2R1A is not the 19q familial Wilms tumor gene and that mutation of PPP2R1A is not a common event in the etiology of sporadic Wilms tumor.

Expression and genomic characterization of protein phosphatase inhibitor-1: a novel marker for mesothelium in the mouse

Protein phosphatase inhibitor-1 (inhibitor-1 or I-1) is involved in signal transduction and is an endogenous inhibitor of protein phosphatase-1. The mouse I-1 protein sequence has been deduced from cDNA and is strongly homologous to the published rat sequence. A mouse genomic library was screened, and the I-1 gene was characterized and localized by fluorescent in situ hybridization (FISH) to chromosome 15F. Protein expression in a range of embryonic and adult tissue was analysed using confocal microscopy. Inhibitor-1 is expressed by: the coelomic epithelium; the epithelial bounding layer of cells of the kidney, lung, liver, heart, intestine and gonad; and the surface ectoderm. The blast cells of the kidney do not express I-1. We conclude that I-1 is a marker for mesothelium.

Protein phosphatase inhibitors arrest cell cycle and reduce branching morphogenesis in fetal rat lung cultures

Protein phosphatase 2A (PP2A) is a key signal transduction intermediate in the regulation of cellular proliferation and differentiation in vitro. However, the role of PP2A in the context of a developing organ is unknown. To explore the role of PP2A in the regulation of lung development, we studied the effect of PP2A inhibition on new airway branching, induction of apoptosis, DNA synthesis, and expression of epithelial marker genes in whole organ explant cultures of embryonic (E14) rat lung. Microdissected lung primordia were cultured in medium containing one of either two PP2A inhibitors, okadaic acid (OA, 0-9 nM) or cantharidin (Can, 0-3,600 nM), or with the PP2B inhibitor deltamethrin (Del, 0-10 microM) as a control for a PP2A-specific effect for 48 h. PP2A inhibition with OA and Can significantly inhibited airway branching and overall lung growth. PP2B inhibition with Del did not affect lung growth or new airway development. Histologically, both PP2A- and PP2B-inhibited explants were similar to controls. Increased apoptosis was not the mechanism of decreased lung growth and new airway branching inasmuch as OA-treated explant sections subjected to the terminal deoxynucleotidyltransferase dUTP nick end labeling reaction demonstrated a decrease in apoptosis. However, PP2A inhibition with OA increased DNA content and 5-bromo-2'-deoxyuridine uptake that correlated with a G(2)/M cell cycle arrest. PP2A inhibition also resulted in altered differentiation of the respiratory epithelium as evidenced by decreased mRNA levels of the early epithelial marker surfactant protein C. These findings suggest that inhibition of protein phosphatases with OA and Can halted mesenchymal cell cycle progression and reduced branching morphogenesis in fetal rat lung explant culture.

Developmental expression of protein phosphatase 2A in the kidney

Although a number of growth and transcription factors are known to regulate renal growth and development, the signal transduction molecules necessary to mediate these developmental signals are relatively unknown. Therefore, the activity and mRNA and protein expression of the signal transduction molecule protein phosphatase 2A (PP2A) were examined during rat kidney development. Northern analysis of total kidney RNA or Western analysis of kidney protein homogenates from embryonic day 15 to 90-d-old adults demonstrated developmental regulation of the catalytic, major 55-kD B regulatory subunit and A structural subunit with the highest levels of expression in late embryonic and newborn kidneys. Similarly, okadaic acid-inhibitable phosphatase enzyme activity was highest in the embryonic and newborn kidney. To map cell-specific expression of PP2A in the developing kidney, in situ hybridization with a catalytic subunit digoxigenin-labeled cRNA was performed on embryonic day 20 and newborn kidneys. PP2A was found predominately in the nephrogenic cortex and particularly in the developing glomeruli and non-brush border tubules in the embryonic day 20 and newborn kidneys. Similarly, immunocytochemistry with a specific PP2A catalytic subunit polyclonal anti-peptide antibody demonstrated catalytic subunit protein particularly concentrated in the podocytes of glomeruli in the newborn kidney. In the adult kidney, PP2A protein was no longer detectable except in the nuclei of distal tubular cells. Therefore, the developmental regulation of PP2A activity and protein during kidney development and its mapping to the nephrogenic cortex, developing glomeruli, and tubules suggests a role for PP2A in the regulation of nephron growth and differentiation.

Apoptosis in the kidney

Apoptosis is a highly regulated mechanism of cell death. Although apoptosis has a functional role in normal development and tissue homeostasis, aberrant triggering of the process by toxicants may lead to abnormal function or disease. Low level exposures to toxicants that induce apoptosis in kidney may therefore create a critical disturbance in kidney homeostasis, contributing to renal neoplasia or renal disease. In this report, we review the involvement of apoptosis in normal kidney development and in renal disease and discuss some of the toxicants and molecular factors involved in regulation of the process in renal cells.

Transitional stages in the development of the rabbit renal collecting duct

The collecting duct (CD) epithelium of the mammalian kidney is an extraordinary structure with respect to its functional changes during development and its heterogeneous composition when matured. All of the different nephron epithelia of the mammalian kidney consist of one single cell type. In contrast, the differentiated CD is composed of at least three distinct cell types [principal, alpha intercalated-, and beta intercalated cells] that are responsible for the multiple physiological functions of this kidney compartment. During development the function of the CD changes: initially, the CD ampulla serves as an embryonic inducer, while the matured epithelium plays a key role in maintaining the homeostasis of body fluids. At present the process of CD maturation is not well understood. Neither the time course of development nor the morphogenic factors leading to the heterogeneously composed epithelium are known. In the present study the differentiation of the CD epithelium was investigated using newly developed monoclonal antibodies and well-characterized antisera. The morphological changes induced during differentiation were monitored by immunohistochemistry and scanning electron microscopy. The experiments were performed on neonatal and adult rabbit kidneys. Results obtained by light microscopical techniques and scanning electron microscopy revealed that the ampullary tip can be distinguished from the ampullary neck, as well as from the maturing CD. A number of proteins that were not detectable in the ampulla were detected in the neonatal CD and were found at even higher concentrations in the adult CD (PCD8, chloride/bicarbonate exchanger). Other proteins (PCD9) were downregulated during differentiation. For the first time the transient character of the differentiation stage of the neonatal CD could be demonstrated unequivocally. Furthermore, considerable heterogeneity in protein expression patterns (PCD6 and PCD9) was demonstrated within the beta IC cell population of the mature CD.

The development of the kidney

This chapter describes the earlier stages of development of the vertebrate metanephric kidney. It focuses on the mouse and descriptive morphology is used for considering both molecular mechanisms, underpinning kidney morphogenesis and differentiation, and the ways in which these processes can go awry and lead to congenital kidney disorders—particularly in humans. The mature kidney is a fairly complex organ attached to an arterial input vessel and two output vessels, the vein and the ureter. Inside, the artery and vein are connected by a complex network of capillaries that invade a large number of glomeruli, the proximal entrance to nephrons, which are filtration units that link to an arborized collecting-duct system that drains into the ureter. The ability of the kidney and isolated metanephrogenic mesenchyme, to develop in culture means that the developing tissues can be subjected to a wide variety of experimental procedures designed to investigate their molecular and cellular properties and to test hypotheses about developmental mechanisms.

Developmental expression and localization of the catalytic subunit of protein phosphatase 2A in rat lung

Protein phosphatase type-2A (PP2A) is a highly conserved serine/threonine phosphatase known to play a key role in cell proliferation and differentiation in vitro, but the role of PP2A in mammalian embryogenesis remains unexplored. No particular information exists as to the tissue or cell specific expression of PP2A or the relevance of PP2A expression to mammalian development in vivo. To examine expression of PP2A during mammalian lung development, we studied fetal rats from day 14 of gestation (the lung bud is formed on day 12 of gestation) to parturition. Western analysis with a specific PP2A catalytic subunit antibody identified a single 36 kDa protein, with protein levels two-fold higher in the 17 and 19 day embryonic lung as compared to the adult. With in situ hybridization and immunohistochemistry, both mRNA and protein for PP2A were localized equally to the epithelial lining of the embryonic lung airway and the surrounding mesenchyme in the 14 day embryonic lung. With maturation of the lung, PP2A becomes highly expressed in respiratory epithelium. The highest level of expression was in the earliest developing airways with columnar epithelium (the pseudoglandular stage, 15-18 days of gestation). There was a decrease in expression with the transformation to cuboidal epithelium by day 20 of gestation. This was most noticeable in the developing bronchial epithelium of the 19 and 20 day gestation lungs where only an occasional cell continues to express PP2A. Mesenchymal hybridization was most obvious in early endothelial cells of forming vascular channels at 17-19 days of gestation. PP2A respiratory epithelial expression mimicked the centrifugal development of the respiratory tree where the highest expression was in the peripheral columnar epithelium (15-18 days gestation) with only an occasional central bronchiolar cell continuing to express PP2A at 19 and 20 days gestation. Endothelial hybridization decreased with muscularization of large pulmonary arteries with low levels of expression detected in bronchial or vascular smooth muscle. In the newborn lung PP2A expression was decreased, but detectable in alveolar epithelium and vascular endothelium. In summary; 1) PP2A mRNA and protein exhibit cell specific expression during rat lung development; 2) PP2A is highly expressed in the respiratory epithelium of the fetal rat lung and is temporally related to the maturation of the bronchial epithelium; 3) and the PP2A subunit is highly expressed in early vascular endothelium, but not smooth muscle of the rat lung.

Protein kinase C in the developing kidney: isoform expression and effects of ceramide and PKC inhibitors

Protein kinase C (PKC) is a serine/threonine kinase recognized as a key enzyme in signal transduction mechanisms in various biological processes. During development, PKC is involved in the regulation of growth and differentiation. In mature tissue PKC is important for homeostatic functions. We studied PKC with regard to expression and effects on differentiation, growth and apoptosis in the developing kidney. Using in situ hybridization, we demonstrate age-dependent expression of PKC alpha, PKC delta, PKC zeta and PKC lambda during fetal and postnatal kidney development. The endogenous sphingolipid product ceramide, as well as specific PKC inhibitors, disturbed nephron formation and induced apoptosis in organ cultures of E13 kidneys. In primary cell cultures of proximal tubule cells, ceramide and the specific PKC inhibitors induced apoptosis. In conclusion, PKC alpha, PKC delta, PKC zeta and PKC lambda are expressed in an age-dependent pattern during kidney development. Inhibition of PKC disturbs nephron formation, inhibits growth and induces apoptosis in the developing kidney. The findings suggest that PKC plays an important role in regulating normal kidney growth and differentiation.

Early defect in branching morphogenesis of the ureteric bud in induced nephron deficit

Development of the metanephric kidney during embryogenesis can be altered both in vivo and in vitro by exposure to gentamicin, which may lead to oligonephronia. To study the role of the ureteric bud in nephron deficit genesis, we used metanephros organ cultures exposed to gentamicin as a model of impaired nephrogenesis. Ultrastructural localization of the antibiotic showed that by eight hours it was already present within the epithelial cells of the ureteric bud and in its growing ends, and also trapped in the adjacent blastema. Using confocal microscopy and image analysis, we devised a quantitative approach to analyze the branching pattern of the ureteric bud, and showed that by 24 hours of culture, despite no change of explants growth, gentamicin had significantly decreased the number of branching points. This effect involved the early branching events and was limited to end buds that had no nephron anlagen nearby. Our findings indicate that impaired branching morphogenesis of the ureteric bud is the likely event of gentamicin-induced nephron deficit.