Markus Napirei

Features of systemic lupus erythematosus in Dnase1-deficient mice.

Systemic lupus erythematosus (SLE) is a multifactorial autoimmune disease that affects over one million people in the United States. SLE is characterized by the presence of anti-nuclear antibodies (ANA) directed against naked DNA and entire nucleosomes. It is thought that the resulting immune complexes accumulate in vessel walls, glomeruli and joints and cause a hypersensitivity reaction type III, which manifests as glomerulonephritis, arthritis and general vasculitis. The aetiology of SLE is unknown, but several studies suggest that increased liberation or disturbed clearance of nuclear DNA-protein complexes after cell death may initiate and propagate the disease. Consequently, Dnase1, which is the major nuclease present in serum, urine and secreta, may be responsible for the removal of DNA from nuclear antigens at sites of high cell turnover and thus for the prevention of SLE (refs 7–11). To test this hypothesis, we have generated Dnase1-deficient mice by gene targeting. We report here that these animals show the classical symptoms of SLE, namely the presence of ANA, the deposition of immune complexes in glomeruli and full-blown glomerulonephritis in a Dnase1-dose-dependent manner. Moreover, in agreement with earlier reports, we found Dnase1 activities in serum to be lower in SLE patients than in normal subjects. Our findings suggest that lack or reduction of Dnase1 is a critical factor in the initiation of human SLE.

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.

Synchronized integrin engagement and chemokine activation is crucial in neutrophil extracellular trap–mediated sterile inflammation

There is emerging evidence that neutrophil extracellular traps (NETs) play important roles in inflammatory processes. Here we report that neutrophils have to be simultaneously activated by integrin-mediated outside-in- and G-protein-coupled receptor (GPCR) signaling to induce NET formation in acute lung injury (ALI), which is associated with a high mortality rate in critically ill patients. NETs consist of decondensed chromatin decorated with granular and cytosolic proteins and they can trap extracellular pathogens. The prerequisite for NET formation is the activation of neutrophils and the release of their DNA. In a neutrophil- and platelet-dependent mouse model of ventilator-induced lung injury (VILI), NETs were found in the lung microvasculature, and circulating NET components increased in the plasma. In this model, blocking integrin-mediated outside-in or either GPCR-signaling or heteromerization of platelet chemokines decreased NET formation and lung injury. Targeting NET components by DNAse1 application or neutrophil elastase-deficient mice protected mice from ALI, whereas DNase1(-/-)/Trap1(m/m) mice had an aggravated ALI, suggesting that NETs directly influence the severity of ALI. These data suggest that NETs form in the lungs during VILI, contribute to the disease process, and thus may be a promising new direction for the treatment of ALI.

Expression pattern of the deoxyribonuclease 1 gene: lessons from the Dnase1 knockout mouse.

The tissue distribution of deoxyribonuclease 1 (DNASE1, DNase I), a Ca2+ and Mg2+/Mn2+-dependent secretory endonuclease, has previously been investigated. However, most of these studies did not account for the existence of different members of the DNASE1 gene family, did not differentiate between endogenous DNASE1 protein synthesis and its extracellular occurrence or were not performed with methods allowing both a sensitive and a specific detection. Now we re-examined the DNASE1 gene expression pattern by taking advantage of the Dnase1 knockout mouse model. Direct comparison of samples derived from wild-type (Dnase1+/+) and knockout (Dnase1-/-) mice allowed an unambiguous detection of Dnase1 gene expression at the mRNA and protein level. For the detection of Dnase1 activity, we developed a highly sensitive nuclease zymogram method. We observed high Dnase1 gene expression in the parotid and submandibular gland as well as in the kidney and duodenum, intermediate expression in the ileum, mesenterial lymph nodes, liver, ventral prostate, epididymis, ovary and stomach, and low expression in the sublingual, preputial, coagulation and pituitary gland. We report for the first time the lachrymal and thyroid glands, the urinary bladder and the eye to be Dnase1-expressing organs as well. Since Dnase1 knockout mice with the 129xC57Bl/6 mixed genetic background have indicated the protection against an anti-DNA autoimmune response as a new physiological function of Dnase1, knowledge of the physiological sites of its synthesis might prove helpful to find new therapeutic strategies.

Deoxyribonuclease 1 aggravates acetaminophen-induced liver necrosis in male CD-1 mice

An overdose of acetaminophen (APAP) (N‐acetyl‐p‐aminophenol) leads to hepatocellular necrosis induced by its metabolite N‐acetyl‐p‐benzoquinone‐imine, which is generated during the metabolic phase of liver intoxication. It has been reported that DNA damage occurs during the toxic phase; however, the nucleases responsible for this effect are unknown. In this study, we analyzed the participation of the hepatic endonuclease deoxyribonuclease 1 (DNASE1) during APAP‐induced hepatotoxicity by employing a Dnase1 knockout (KO) mouse model. Male CD‐1 Dnase1 wild‐type (WT) (Dnase1+/+) and KO (Dnase1−/−) mice were treated with 2 different doses of APAP. Hepatic histopathology was performed, and biochemical parameters for APAP metabolism and necrosis were investigated, including depletion of glutathione/glutathione‐disulfide (GSH+GSSG), β‐nicotinamide adenine dinucleotide (NADH+NAD+), and adenosine triphosphate (ATP); release of aminotransferases and Dnase1; and occurrence of DNA fragmentation. As expected, an APAP overdose in WT mice led to massive hepatocellular necrosis characterized by the release of aminotransferases and depletion of hepatocellular GSH+GSSG, NADH+NAD+, and ATP. These metabolic events were accompanied by extensive DNA degradation. In contrast, Dnase1 KO mice were considerably less affected. In conclusion, whereas the innermost pericentral hepatocytes of both mouse strains underwent necrosis to the same extent independent of DNA damage, the progression of necrosis to more outwardly located cells was dependent on DNA damage and only occurred in WT mice. Dnase1 aggravates APAP‐induced liver necrosis. (HEPATOLOGY 2006;43:297–305.)

Loss of connexin36 increases retinal cell vulnerability to secondary cell loss

Accruing evidence indicates that gap junctions are involved in neuronal survival after brain injury. The present study was aimed at clarifying the contribution of the neuronal gap‐junction protein connexin36 (Cx36) to secondary cell loss after injury in the mouse retina. A focal retinal lesion was induced by infrared laser photocoagulation. Remarkably, this model allowed spatial and temporal definition of the lesion with high reproducibility. Moreover, Cx36 is abundantly expressed in the retina and plays an essential role in the visual transmission process. Taking advantage of these features, cell death was assessed using TUNEL assay and light and electron microscopy, and the extent of Cx36 expression was studied by immunohistochemistry, Western blot, in situ hybridization and real‐time RT‐PCR. Secondary cell loss was most prominent between 24 and 48 h after lesioning. This peak was accompanied by an increase in Cx36 expression. When cultured explanted retinas were subjected to gap‐junction blockers a significant increase in the extent of secondary cell loss after laser photocoagulation became evident. Using the same experimental paradigm we compared the incidence of cell death in wild‐type and Cx36–/– mice. A significant increase in total number of TUNEL‐positive cells occurred in the Cx36–/– mice compared to controls. From these data we conclude that Cx36 contributes to the survival and resistance against damage of retinal cells and thus constitutes a protective factor after traumatic injury of the retina.

Murine serum nucleases – contrasting effects of plasmin and heparin on the activities of DNase1 and DNase1-like 3 (DNase1l3)

DNase1 is regarded as the major serum nuclease; however, a systematic investigation into the presence of additional serum nuclease activities is lacking. We have demonstrated directly that serum contains DNase1‐like 3 (DNase1l3) in addition to DNase1 by an improved denaturing SDS‐PAGE zymography method and anti‐murine DNase1l3 immunoblotting. Using DNA degradation assays, we compared the activities of recombinant murine DNase1 and DNase1l3 (rmDNase1, rmDNase1l3) with the serum of wild‐type and DNase1 knockout mice. Serum and rmDNase1 degrade chromatin effectively only in cooperation with serine proteases, such as plasmin or thrombin, which remove DNA‐bound proteins. This can be mimicked by the addition of heparin, which displaces histones from chromatin. In contrast, serum and rmDNase1l3 degrade chromatin without proteolytic help and are directly inhibited by heparin and proteolysis by plasmin. In previous studies, serum DNase1l3 escaped detection because of its sensitivity to proteolysis by plasmin after activation of the plasminogen system in the DNA degradation assays. In contrast, DNase1 is resistant to plasmin, probably as a result of its di‐N‐glycosylation, which is lacking in DNase1l3. Our data demonstrate that secreted rmDNase1 and murine parotid DNase1 are mixtures of three different di‐N‐glycosylated molecules containing two high‐mannose, two complex N‐glycans or one high‐mannose and one complex N‐glycan moiety. In summary, serum contains two nucleases, DNase1 and DNase1l3, which may substitute or cooperate with each other during DNA degradation, providing effective clearance after exposure or release from dying cells.

Host DNases prevent vascular occlusion by neutrophil extracellular traps

Blood DNases hack the NET Neutrophil extracellular traps (NETs) are lattices of processed chromatin decorated with select secreted and cytoplasmic proteins that trap and neutralize microbes. However, their inappropriate release may do more harm than good by promoting inflammation and thrombosis. Jiménez-Alcázar et al. report that two deoxyribonucleases (DNases), DNASE1 and DNASE1L3, have partially redundant roles in degrading NETs in the circulation (see the Perspective by Gunzer). Knockout mice lacking these enzymes were unable to tolerate chronic neutrophilia, quickly dying after blood vessels were occluded by NET clots. Furthermore, the damage unleashed by clots during septicemia was enhanced when these DNases were absent. Science, this issue p. 1202; see also p. 1126 Deoxyribonucleases work together to control vascular occlusion by neutrophil-induced blood clots. Platelet and fibrin clots occlude blood vessels in hemostasis and thrombosis. Here we report a noncanonical mechanism for vascular occlusion based on neutrophil extracellular traps (NETs), DNA fibers released by neutrophils during inflammation. We investigated which host factors control NETs in vivo and found that two deoxyribonucleases (DNases), DNase1 and DNase1-like 3, degraded NETs in circulation during sterile neutrophilia and septicemia. In the absence of both DNases, intravascular NETs formed clots that obstructed blood vessels and caused organ damage. Vascular occlusions in patients with severe bacterial infections were associated with a defect to degrade NETs ex vivo and the formation of intravascular NET clots. DNase1 and DNase1-like 3 are independently expressed and thus provide dual host protection against deleterious effects of intravascular NETs.

Histopathology of lupus-like nephritis in Dnase1-deficient mice in comparison to NZB/W F1 mice:

Deoxyribonuclease 1 (Dnase1)-de”cient mice develop symptoms of Systemic lupus erythematosus (SLE). Here we analysed the renal histopathology of these animals in comparison to F1 hybrids of New Zealand black and white mice (NZB/W F1), an established model of SLE. Animals were divided into three groups according to the presence of anti-nuclear antibodies (ANA) and renal lesions. Groups 1a–1c were healthy, whereas group 2 and 3 were classified as lupus-prone and affected. Subendothelial and/or mesangial immune complex deposits, mesangial and endocapillary proliferation,haematoxylin bodies and platelet aggregation were detected in both mouse strains but were more severe in the NZB/W F1 mice. The lupus nephritis was classi”ed as a proliferating (WHO type III or IV), which appeared to be preceded by a mesangial form (WHO type II). Subclassification of the ANA revealed a high prevalence of anti-nucleosome antibodies in Dnase1-deficient mice, whereas NZB/W F1 mice developed autoantibodies against a broad range of chromatin constituents. Mapping of the murine Dnase1 gene locus to chromosome 16A1-3 did not coincide with one of the reported susceptibility loci in the NZB/W F1 model, although a reduced Dnase1 serum and urine activity has been described previously in these mice.

Holocrine secretion of sebum is a unique DNase2-dependent mode of programmed cell death.

Sebaceous glands produce sebum via holocrine secretion, a largely uncharacterized mode of programmed cell death that contributes to the homeostasis and barrier function of the skin. To determine the mechanism of DNA degradation during sebocyte cell death, we have inactivated candidate DNA-degrading enzymes by targeted gene deletions in mice. DNase1 and DNase1-like 2 were dispensable for nuclear DNA degradation in sebocytes. By contrast, epithelial cell-specific deletion of lysosomal DNase2 blocked DNA degradation in these cells. DNA breakdown during sebocyte differentiation coincided with the loss of LAMP1 and was accelerated by the abrogation of autophagy, the central cellular program of lysosome-dependent catabolism. Suppression of DNA degradation by the deletion of DNase2 resulted in aberrantly increased concentrations of residual DNA and decreased amounts of the DNA metabolite uric acid in secreted sebum. These results define holocrine secretion as a DNase2-mediated form of programmed cell death and suggest that autophagy-dependent metabolism, DNA degradation, and the molecular composition of sebum are mechanistically linked.