Identification and expression of deoxyribonuclease (DNase) I alternative transcripts in the rat

Interplay of immune and kidney resident cells in the formation of tertiary lymphoid structures in lupus nephritis

Kidney involvement confers significant morbidity and mortality in patients with systemic lupus erythematosus (SLE). The pathogenesis of lupus nephritis (LN) involves diverse mechanisms instigated by elements of the autoimmune response which alter the biology of kidney resident cells. Processes in the glomeruli and in the interstitium may proceed independently albeit crosstalk between the two is inevitable. Podocytes, mesangial cells, tubular epithelial cells, kidney resident macrophages and stromal cells with input from cytokines and autoantibodies present in the circulation alter the expression of enzymes, produce cytokines and chemokines which lead to their injury and damage of the kidney. Several of these molecules can be targeted independently to prevent and reverse kidney failure. Tertiary lymphoid structures with true germinal centers are present in the kidneys of patients with lupus nephritis and have been increasingly recognized to associate with poorer renal outcomes. Stromal cells, tubular epithelial cells, high endothelial vessel and lymphatic venule cells produce chemokines which enable the formation of structures composed of a T-cell-rich zone with mature dendritic cells next to a B-cell follicle with the characteristics of a germinal center surrounded by plasma cells. Following an overview on the interaction of the immune cells with kidney resident cells, we discuss the cellular and molecular events which lead to the formation of tertiary lymphoid structures in the interstitium of the kidneys of mice and patients with lupus nephritis. In parallel, molecules and processes that can be targeted therapeutically are presented.

DNase I Induces Other Endonucleases in Kidney Tubular Epithelial Cells by Its DNA-Degrading Activity

Endonuclease-mediated DNA fragmentation is both an immediate cause and a result of apoptosis and of all other types of irreversible cell death after injury. It is produced by nine enzymes including DNase I, DNase 2, their homologs, caspase-activated DNase (CAD) and endonuclease G (EndoG). The endonucleases act simultaneously during cell death; however, regulatory links between these enzymes have not been established. We hypothesized that DNase I, the most abundant of endonucleases, may regulate other endonucleases. To test this hypothesis, rat kidney tubular epithelial NRK-52E cells were transfected with the DNase I gene or its inactive mutant in a pECFP expression vector, while control cells were transfected with the empty vector. mRNA expression of all nine endonucleases was studied using real-time RT-PCR; DNA strand breaks in endonuclease genes were determined by PCR and protein expression of the enzymes was measured by Western blotting and quantitative immunocytochemistry. Our data showed that DNase I, but not its inactive mutant, induces all other endonucleases at varying time periods after transfection, causes DNA breaks in endonuclease genes, and elevates protein expression of several endonucleases. This is the first evidence that endonucleases seem to be induced by the DNA-degrading activity of DNase I.

Plasma Concentrations of Extracellular DNA in Acute Kidney Injury

Current diagnostic methods of acute kidney injury (AKI) have limited sensitivity and specificity. Tissue injury has been linked to an increase in the concentrations of extracellular DNA (ecDNA) in plasma. A rapid turnover of ecDNA in the circulation makes it a potential marker with high sensitivity. This study aimed to analyze the concentration of ecDNA in plasma in animal models of AKI. Three different fractions of ecDNA were measured—total ecDNA was assessed fluorometrically, while nuclear ecDNA (ncDNA) and mitochondrial DNA (mtDNA) were analyzed using quantitative real-time PCR. AKI was induced using four different murine models of AKI-bilateral ureteral obstruction (BUO), glycerol-induced AKI (GLY), ischemia–reperfusion injury (IRI) and bilateral nephrectomy (BNx). Total ecDNA was significantly higher in BUO (p < 0.05) and GLY (p < 0.05) compared to the respective control groups. ncDNA was significantly higher in BUO (p < 0.05) compared to SHAM. No significant differences in the concentrations of mtDNA were found between the groups. The plasma concentrations of different fractions of ecDNA are dependent on the mechanism of induction of AKI and warrant further investigation as potential surrogate markers of AKI.

Endonuclease G modulates the alternative splicing of deoxyribonuclease 1 mRNA in human CD4+ T lymphocytes and prevents the progression of apoptosis

Apoptotic endonucleases act cooperatively to fragment DNA and ensure the irreversibility of apoptosis. However, very little is known regarding the potential regulatory links between endonucleases. Deoxyribonuclease 1 (DNase I) inactivation is caused by alternative splicing (AS) of DNase I pre-mRNA skipping exon 4, which occurs in response to EndoG overexpression in cells. The current study aimed to determine the role of EndoG in the regulation of DNase I mRNA AS and the modulation of its enzymatic activity. A strong correlation was identified between the EndoG expression levels and DNase I splice variants in human lymphocytes. EndoG overexpression in CD4⁺ T cells down-regulated the mRNA levels of the active full-length DNase I variant and up-regulated the levels of the non-active spliced variant, which acts in a dominant-negative fashion. DNase I AS was induced by the translocation of EndoG from mitochondria into nuclei during the development of apoptosis. The DNase I spliced variant was induced by recombinant EndoG or by incubation with EndoG-digested cellular RNA in an in vitro system with isolated cell nuclei. Using antisense DNA oligonucleotides, we identified a 72-base segment that spans the adjacent segments of exon 4 and intron 4 and appears to be responsible for the AS. DNase I-positive CD4⁺ T cells overexpressing EndoG demonstrated decreased progression towards bleomycin-induced apoptosis. Therefore, EndoG is an endonuclease with the unique ability to inactivate another endonuclease, DNase I, and to modulate the development of apoptosis.

Hypoxia induces upregulation of the deoxyribonuclease I gene in the human pancreatic cancer cell line QGP-1

We have previously demonstrated that ischemia caused by acute myocardial infarction induces an abrupt increase of serum deoxyribonuclease I (DNase I) activity. In this study, we examined whether hypoxia can affect the levels of DNase I activity and/or its transcripts in vitro. We first exposed the human pancreatic cancer cell line QGP-1, which is the first documented DNase-I-producing cell line, to hypoxia (2% O2), and found that this induced a significant increase in both the activity and transcripts of DNase I. This response was mediated by increased transcription only from exon 1a of the two alternative transcription-initiating exons utilized simultaneously in the human DNase I gene (DNASE1); exposure of QGP-1 cells to hypoxia for 24 h resulted in a 15-fold increase of DNASE1 transcripts starting from exon 1a compared with the expression level under normoxic conditions. Promoter, electrophoretic mobility shift, and chromatin immunoprecipitation assays with QGP-1 cells exposed to hypoxia or normoxia showed that the region just upstream from exon 1a was involved in this response in a hypoxia-induced factor-1-independent, but at least in a Sp1 transcription factor-dependent manner possibly through enhanced binding of Sp1 protein to the promoter. These results indicate that DNASE1 expression is upregulated by hypoxia in the cells.

Induction of renal endonuclease G by cisplatin is reduced in DNase I-deficient mice

Nephrotoxicity from the chemotherapeutic drug cisplatin is associated with DNA fragmentation and cell death. We have recently demonstrated that DNase I knockout mice are significantly protected against cisplatin nephrotoxicity, but it is unknown whether the DNA fragmentation that occurs is produced by DNase I or another endonuclease. In this study we assessed the expression of several endonucleases involved in cell death after injection of cisplatin and found that the expression of endonuclease G (EndoG) increased whereas the expression of DNase I decreased almost to zero. Immunostaining showed that some nuclei contained both fragmented DNA and EndoG, suggesting that EndoG may cause DNA fragmentation induced by cisplatin. The increase in expression of EndoG was greater in wild-type mice than in DNase I knockout mice, indicating a potential link between the two endonucleases. In support of such a link, overexpression of DNase I in cultured mouse tubular epithelial cells also induced EndoG. Furthermore, gene silencing of EndoG in vitro provided significant protection against cell death. Taken together, our data suggest that both DNase I and EndoG mediate cisplatin injury to tubular epithelial cells.

Characterization of human deoxyribonuclease I gene (DNASE1) promoters reveals the utilization of two transcription-starting exons and the involvement of Sp1 in its transcriptional regulation

Levels of deoxyribonuclease I (DNase I) activity in vivo have been shown to be altered by physiological and/or pathological processes. However, no information is available on the regulation of DNase I gene (DNASE1) expression in vivo or in vitro. We first mapped the transcription start sites of DNASE1 in human pancreas and in the DNase I-producing human pancreatic cancer cell line QGP-1, and revealed a novel site approximately 12 kb upstream of exon 1, which was previously believed to be the single transcription-starting exon. This initiation site marks an alternative starting exon, designated 1a. Exons 1 and 1a were used simultaneously as transcription-starting exons in pancreas and QGP-1 cells. Promoter assay, EMSA and chromatin immunoprecipitation analysis with QGP-1 cells showed the promoter region of exon 1a in which the Sp1 transcription factor is specifically involved in promoter activity. This is the first to be identified as a transcription factor responsible for gene expression of vertebrate DNase I genes. Furthermore, RT-PCR analysis indicated alternative splicing of human DNASE1 pre-mRNA in pancreas and QGP-1 cells. Only two transcripts among eight alternative splicing products identified can be translated to produce intact DNase I protein. These results suggest that human DNASE1 expression is regulated through the use of alternative promoter and alternative splicing.

Cisplatin Nephrotoxicity Is Mediated by Deoxyribonuclease I

Cisplatin is commonly used for chemotherapy in a wide variety of tumors; however, its use is limited by kidney toxicity. Although the exact mechanism of cisplatin-induced nephrotoxicity is not understood, several studies showed that it is associated with DNA fragmentation induced by an unknown endonuclease. It was demonstrated previously that deoxyribonuclease I (DNase I) is a highly active renal endonuclease, and its silencing by antisense is cytoprotective against the in vitro hypoxia injury of kidney tubular epithelial cells. This study used recently developed DNase1 knockout (KO) mice to determine the role of this endonuclease in cisplatin-induced nephrotoxicity. The data showed that DNase I represents approximately 80% of the total endonuclease activity in the kidney and cultured primary renal tubular epithelial cells. In vitro, primary renal tubular epithelial cells isolated from KO animals were resistant to cisplatin (8 microM) injury. DNase I KO mice were also markedly protected against the toxic injury induced by a single injection of cisplatin (20 mg/kg), by both functional (blood urea nitrogen and serum creatinine) and histologic criteria (tubular necrosis and in situ DNA fragmentation assessed by the terminal deoxynucleotidyl transferase nick end-labeling). These data provide direct evidence that DNase I is essential for kidney injury induced by cisplatin.

Bovine DNase I: gene organization, mRNA expression, and changes in the topological distribution of the protein during apoptosis in lens epithelial cells

Genomic DNA sequencing and alignment with the known DNase I mRNA showed that the bovine gene consists of 9 exons and that only the last 8 encode the protein, since initial ATG was found at exon II. RT-PCR was used to identify DNase I mRNA in lens epithelium in vivo and in cultured epithelial cells. We found DNase I transcripts having the same nucleotide sequence as the pancreas form and others lacking almost all exon V. The lens protein presented a slightly higher relative molecular weight than the pancreatic enzyme. Lens DNase I was located in secretory pathway organelles and excluded from the nucleus. Nevertheless, in apoptotic lens epithelial cells in vitro, DNase I translocated to the nucleus and co-localized with TUNEL positive chromatin aggregates. These results indicate that cells in the lens epithelium constitutively express DNase I, and suggest a direct involvement of this nuclease in the final phases of chromatin degradation.