The cis-regulatory element Gsl5 is indispensable for proximal straight tubule cell-specific transcription of core 2 beta-1,6-N-acetylglucosaminyltransferase in the mouse kidney

Diabetes Induces Aberrant DNA Methylation in the Proximal Tubules of the Kidney

Epigenetic mechanisms may underlie the progression of diabetic kidney disease. Because the kidney is a heterogeneous organ with different cell types, we investigated DNA methylation status of the kidney in a cell type-specific manner. We first identified genes specifically demethylated in the normal proximal tubules obtained from control db/m mice, and next delineated the candidate disease-modifying genes bearing aberrant DNA methylation induced by diabetes using db/db mice. Genes involved in glucose metabolism, including Sglt2, Pck1, and G6pc, were selectively hypomethylated in the proximal tubules in control mice. Hnf4a, a transcription factor regulating transporters for reabsorption, was also selectively demethylated. In diabetic mice, aberrant hypomethylation of Agt, Abcc4, Cyp4a10, Glut5, and Met and hypermethylation of Kif20b, Cldn18, and Slco1a1 were observed. Time-dependent demethylation of Agt, a marker of diabetic kidney disease, was accompanied by histone modification changes. Furthermore, inhibition of DNA methyltransferase or histone deacetylase increased Agt mRNA in cultured human proximal tubular cells. Aberrant DNA methylation and concomitant changes in histone modifications and mRNA expression in the diabetic kidney were resistant to antidiabetic treatment with pioglitazone. These results suggest that an epigenetic switch involving aberrant DNA methylation causes persistent mRNA expression of select genes that may lead to phenotype changes of the proximal tubules in diabetic kidney disease.

Kidney specific protein-positive cells derived from embryonic stem cells reproduce tubular structures in vitro and differentiate into renal tubular cells

Embryonic stem cells and induced pluripotent stem cells have the ability to differentiate into various organs and tissues, and are regarded as new tools for the elucidation of disease mechanisms as well as sources for regenerative therapies. However, a method of inducing organ-specific cells from pluripotent stem cells is urgently needed. Although many scientists have been developing methods to induce various organ-specific cells from pluripotent stem cells, renal lineage cells have yet to be induced in vitro because of the complexity of kidney structures and the diversity of kidney-component cells. Here, we describe a method of inducing renal tubular cells from mouse embryonic stem cells via the cell purification of kidney specific protein (KSP)-positive cells using an anti-KSP antibody. The global gene expression profiles of KSP-positive cells derived from ES cells exhibited characteristics similar to those of cells in the developing kidney, and KSP-positive cells had the capacity to form tubular structures resembling renal tubular cells when grown in a 3D culture in Matrigel. Moreover, our results indicated that KSP-positive cells acquired the characteristics of each segment of renal tubular cells through tubular formation when stimulated with Wnt4. This method is an important step toward kidney disease research using pluripotent stem cells, and the development of kidney regeneration therapies.

Selective depletion of mouse kidney proximal straight tubule cells causes acute kidney injury

The proximal straight tubule (S3 segment) of the kidney is highly susceptible to ischemia and toxic insults but has a remarkable capacity to repair its structure and function. In response to such injuries, complex processes take place to regenerate the epithelial cells of the S3 segment; however, the precise molecular mechanisms of this regeneration are still being investigated. By applying the “toxin receptor mediated cell knockout” method under the control of the S3 segment-specific promoter/enhancer, Gsl5, which drives core 2 β-1,6-N-acetylglucosaminyltransferase gene expression, we established a transgenic mouse line expressing the human diphtheria toxin (DT) receptor only in the S3 segment. The administration of DT to these transgenic mice caused the selective ablation of S3 segment cells in a dose-dependent manner, and transgenic mice exhibited polyuria containing serum albumin and subsequently developed oliguria. An increase in the concentration of blood urea nitrogen was also observed, and the peak BUN levels occurred 3–7 days after DT administration. Histological analysis revealed that the most severe injury occurred in the S3 segments of the proximal tubule, in which tubular cells were exfoliated into the tubular lumen. In addition, aquaporin 7, which is localized exclusively to the S3 segment, was diminished. These results indicate that this transgenic mouse can suffer acute kidney injury (AKI) caused by S3 segment-specific damage after DT administration. This transgenic line offers an excellent model to uncover the mechanisms of AKI and its rapid recovery.

Core 2 GlcNAc modification and megalin ligand-binding activity

Megalin, a receptor-like transporter glycoprotein, is expressed on kidney proximal tubular cells and reabsorbs small-molecular-weight proteins from the glomerular filtrate. Here, we report that mouse megalins differently modified with core 2 beta6GlcNAc transferase had different kinetic properties to a fluorescence-labeled ligand, retinol-binding protein (RBP). BALB/c mice, a wild-type strain in terms of the expression of kidney-specific core 2 beta6GlcNAc transferase, express megalin carrying the core 2 extended Le(x) epitope, while DBA/2 mice, a mutant-strain of the core 2 beta6GlcNAc transferase, express megalin lacking the epitope. We purified these two types of megalin using lentil lectin chromatography and measured the ligand-binding activities of the megalins using Cy5-labeled RBP by applying gel permeation chromatography (GPC) and fluorescence correlation spectroscopy (FCS). The analysis by GPC indicated that the apparent V(max) of the interaction between Cy5-labeled RBP and the megalins of BALB/c and DBA/2 mice was 60 microM and 30 microM, respectively, and the apparent K(m) was 11 microM and 17 microM, respectively. Scatchard analysis demonstrated the presence of two binding sites. Linear regression analysis resulted in a two-binding-site model characterized by a high-affinity site (K(dBALB)=12.0 microM; K(dDBA)=20.9 microM) and a low-affinity site (K(dBALB)=36.2 microM; K(dDBA)=58.8 microM). FCS analysis exhibited quite different K(m) and V(max) values from those obtained by GPC, but similar K(m) values for the two types of megalin, and a lower V(max) value for DBA/2 megalin than BALB/c megalin. These results suggest that the core 2 GlcNAc extended glycan chains on megalin can change the ligand-binding affinity and capacity.

Mucin biosynthesis: identification of the cis-regulatory elements of human C2GnT-M gene

Mucin glycan is the primary determinant of mucin functions. These functions are expanded by three branch structures, including core 2, core 4, and blood group I, which are synthesized by core 2 beta1,6 N-acetylglucosaminyltransferase-M (C2GnT-M). Alteration of C2GnT-M gene expression is expected to have a profound effect on mucin functions, which prompted us to study the regulation of this gene. Quantitative real-time PCR analysis of the expression of this gene in 24 human tissues and airway epithelial cells showed that this gene was expressed primarily in mucus-secretory tissues. 5' Rapid amplification of cDNA ends analysis, coupled with sequence alignment with human genome database, revealed that this gene was comprised of three exons and two introns. Northern blotting using exon 1 probe showed the presence of this exon in all transcripts, suggesting the presence of cis-regulatory elements in the proximal region upstream of and/or near the transcription initiation site (+1). Analysis of this DNA region (-417/+187) by a promoter-reporter transient transfection assay, coupled with serial deletion and linker scanning mutagenesis, revealed two positive regulatory regions, including -291/-282, and -62/-43. Further, the promoter activity was enhanced by all-trans retinoic acid (ATRA) and IL-13. Thus, the promoter region is specific to hC2GnT-M gene and subject to regulation by ATRA and IL-13. These cis-regulatory elements may be useful for construction of a mucus cell-specific vector for therapy of mucus hypersecretory diseases.

Green fluorescent protein as a tracer in chimeric tissues: the power of vapor fixation

Green fluorescent protein (GFP) and its variants, small, highly soluble proteins, are routinely used as reporters for patterns of gene expression and the origin of cells in transplantation experiments. When not linked as fusion proteins to other polypeptides, they distribute rapidly in the cytoplasm of a given cell, thus allowing real-time observations on living material. For histological analysis, previous bath fixation of whole organs or tissues seemed obligatory, because, during drop fixation of sections, GFP rapidly leaks from cells whose membrane has been damaged by freezing and/or sectioning. The fluorescence of GFP and its derivatives is retained upon fixation, but most enzyme and antigenic activities of interest will be lost in the whole sample as a consequence of formaldehyde (FA) fixation. We have therefore developed an alternative method to fix GFP in frozen tissue sections by FA vapor. This method prevents leakage and redistribution of GFP and allows any cytochemical method to be applied to unfixed adjacent serial sections.

Species-, sex-, and age-dependent urinary excretion of cauxin, a mammalian carboxylesterase

Domestic cats exhibit physiological proteinuria due to the excretion of cauxin, a carboxylesterase, into the urine. In the present report, we demonstrate that cauxin is excreted in a species-, sex-, and age-dependent manner. Although the cauxin gene is conserved in mammals, including human, mouse, and dog, urinary cauxin was found only in member of the genus Felis and lynx (bobcat, and lynx) and not in other Felidae (genus: Panthera and puma) tested. In mature cats, cauxin excretion was higher in intact males than in castrated males or in intact or spayed females. Daily cauxin excretion decreased immediately after castration. Immunohistochemistry confirmed that cauxin expression in the kidney proximal straight tubules was higher in intact males than in castrated males. Urinary cauxin was detectable by Western blotting in cats older than about 3 months, and its excretion increased with age. In a zymographic esterase assay, urine contained a major cauxin band; by contrast, kidney homogenates contained three major bands, comprising two carboxylesterases and an unidentified esterase, and one minor cauxin band. These results suggest that 1. cauxin excretion is regulated by sex hormones, such as testosterone, 2. cauxin functions as an esterase in the urine rather than in kidney cells, and 3. the decomposition products by cauxin are excreted in a species-, sex-, and age-dependent manner, as is cauxin itself.