Heart failure is a leading cause of morbidity and mortality in industrialized countries. Although infection with microorganisms is not involved in the development of heart failure in most cases, inflammation has been implicated in the pathogenesis of heart failure. However, the mechanisms responsible for initiating and integrating inflammatory responses within the heart remain poorly defined. Mitochondria are evolutionary endosymbionts derived from bacteria and contain DNA similar to bacterial DNA. Mitochondria damaged by external haemodynamic stress are degraded by the autophagy/lysosome system in cardiomyocytes. Here we show that mitochondrial DNA that escapes from autophagy cell-autonomously leads to Toll-like receptor (TLR) 9-mediated inflammatory responses in cardiomyocytes and is capable of inducing myocarditis and dilated cardiomyopathy. Cardiac-specific deletion of lysosomal deoxyribonuclease (DNase) II showed no cardiac phenotypes under baseline conditions, but increased mortality and caused severe myocarditis and dilated cardiomyopathy 10 days after treatment with pressure overload. Early in the pathogenesis, DNase II-deficient hearts showed infiltration of inflammatory cells and increased messenger RNA expression of inflammatory cytokines, with accumulation of mitochondrial DNA deposits in autolysosomes in the myocardium. Administration of inhibitory oligodeoxynucleotides against TLR9, which is known to be activated by bacterial DNA, or ablation of Tlr9 attenuated the development of cardiomyopathy in DNase II-deficient mice. Furthermore, Tlr9 ablation improved pressure overload-induced cardiac dysfunction and inflammation even in mice with wild-type Dnase2a alleles. These data provide new perspectives on the mechanism of genesis of chronic inflammation in failing hearts.

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Mitochondrial DNA that escapes from autophagy causes inflammation and heart failure

Sites of transcription of ribosomal RNA in HeLa cells were visualized by electron microscopy. Cells were either incubated with Br-uridine, or permeabilized and then incubated with BrUTP, before sites containing Br-RNA were immunolabeled with gold particles. Short incubations ensured that most incorporated analogue remained at synthetic sites. Fibrillar centres were unlabelled except at their periphery; label was concentrated over certain regions of the surrounding dense fibrillar component. These results suggest that the dense fibrillar component is the site of rRNA transcription. After dispersing the granular component and the dense fibrillar component by a hypotonic treatment, removal of most chromatin and preparation of resinless sections, fibrillar centres remained fixed to a nucleoskeleton. These structural and functional features are incorporated into a model for rRNA transcription.

/pdf/site-of-transcription-of-ribosomal-rna-and-intranucleolar-1vua036fab.pdf

Site of transcription of ribosomal RNA and intranucleolar structure in HeLa cells

Stomatin is a major lipid-raft component of platelet α granules

Active nucleolar chromatin of Xenopus laevis oocytes was prepared for electron microscopy by a step gradient method, which separates the chromatin from proteins and other constituents which might unspecifically bind at low ionic strength. Between putative RNA polymerases and within the non-transcribed spacer region, the chromatin appears as smooth, thin filaments, indistinguishable from pure DNA adsorbed to the same specimen. These filaments are found under all conditions tested, even in the presence of 100 mM NaCl. On the other hand, bulk rat liver chromatin, which was co-prepared with the active nucleolar chromatin, shows nucleosomes containing fibers, which condense into supranucleosomal structures with increasing ionic strength. Since the appearance and the behaviour of active nucleolar chromatin at different ionic strength and pH resembles that of pure DNA, but not that of any known type of chromatin, it is suggested that, except for the transcription apparatus, few macromolecular constituents are associated with ribosomal DNA during transcription.

Structure of the active nucleolar chromatin of Xenopus laevis oocytes.

Does the Synthesis of Ribosomal RNA Take Place within Nucleolar Fibrillar Centers or Dense Fibrillar Components? A Critical Appraisal

For better understanding of nucleolar architecture, different techniques have been used to localize DNA within the dense fibrillar component (DF) or within the fibrillar centers (FC) by electron microscopy (EM). Since it still remains controversial which components contain DNA, we investigated the distribution of DNA in human Sertoli cells using various approaches. In situ hybridization (ISH) with human total genomic DNA as probe and the use of anti-DNA antibody were followed by immunogold detection. This allowed statistical evaluation of the signal density over individual components. The Feulgen-like osmium-ammine (OA) technique for the selective visualization of DNA was also applied. The anti-DNA antibodies detected DNA in mitochondria, in chromatin, and in the DF of the nucleolus. ISH using human total genomic DNA showed similar labeling patterns. The OA technique revealed DNA filaments in the FC and focal agglomerates of decondensed DNA within the DF. We conclude that (a) EM staining techniques that utilize colloidal gold appear to be less sensitive for DNA detection than the OA method, (b) the DF consists of different domains with different molecular composition, and (c) decondensed DNA is not necessarily confined to one particular nucleolar component.

Distribution of DNA in human Sertoli cell nucleoli.

Nucleoli were noted by biologists very early (Fontana, 1781) and a lot of data has been accumulated on the structure of nucleoli in the last decades. Due to the work of Perry (1962; Perry et al., 1961) it is known that the nucleolus is the site of ribosome biogenesis. Its visibility both in the light-and the electron microscope is due to the fact that the genes for ribosomal RNA are not only transcribed but also processed and complexed with ribosomal proteins in nucleoli, rendering them morphologically different from the rest of the karyoplasm.

M. Derenzini

Papers

Distribution of DNA in human Sertoli cell nucleoli.

Ribosomal genes and nucleolar morphology