Toll-like receptor-3-induced mitochondrial dysfunction in cultured human hepatocytes

Several studies have shown the presence of liver mitochondrial dysfunction during sepsis. TLR3 recognizes viral double-stranded RNA and host endogenous cellular mRNA released from damaged cells. TLR3 ligand amplifies the systemic hyperinflammatory response observed during sepsis and in sepsis RNA escaping from damaged tissues/cells may serve as an endogenous ligand for TLR3 thereby modulating immune responses. This study addressed the hypothesis that TLR3 might regulate mitochondrial function in cultured human hepatocytes. HepG2 cells were exposed to TLR-3 ligand (dsRNA--polyinosine-polycytidylic acid; Poly I:C) and mitochondrial respiration was measured. Poly I:C induced a reduction in maximal mitochondrial respiration of human hepatocytes which was prevented partially by preincubation with cyclosporine A (a mitochondrial permeability transition pore-opening inhibitor). Poly-I:C induced activation of NF-κB, and the mitochondrial dysfunction was accompanied by caspase-8 but not caspase-3 activation and by no major alterations in cellular or mitochondrial ultrastructure.

Characterization of the Effect of the Mitochondrial Protein Hint2 on Intracellular Ca2+ dynamics

Hint2, one of the five members of the superfamily of the histidine triad AMP-lysine hydrolase proteins, is expressed in mitochondria of various cell types. In human adrenocarcinoma cells, Hint2 modulates Ca2+ handling by mitochondria. As Hint2 is highly expressed in hepatocytes, we investigated if this protein affects Ca2+ dynamics in this cell type. We found that in hepatocytes isolated from Hint2−/− mice, the frequency of Ca2+ oscillations induced by 1 μM noradrenaline was 150% higher than in the wild-type. Using spectrophotometry, we analyzed the rates of Ca2+ pumping in suspensions of mitochondria prepared from hepatocytes of either wild-type or Hint2−/− mice; we found that Hint2 accelerates Ca2+ pumping into mitochondria. We then resorted to computational modeling to elucidate the possible molecular target of Hint2 that could explain both observations. On the basis of a detailed model for mitochondrial metabolism proposed in another study, we identified the respiratory chain as the most probable target of Hint2. We then used the model to predict that the absence of Hint2 leads to a premature opening of the mitochondrial permeability transition pore in response to repetitive additions of Ca2+ in suspensions of mitochondria. This prediction was then confirmed experimentally.

Impairment of mitochondrial tRNAIle processing by a novel mutation associated with chronic progressive external ophthalmoplegia

We report a sporadic case of chronic progressive external ophthalmoplegia associated with ragged red fibers. The patient presented with enlarged mitochondria with deranged internal architecture and crystalline inclusions. Biochemical studies showed reduced activities of complex I, III and IV in skeletal muscle. Molecular genetic analysis of all mitochondrial tRNAs revealed a G to A transition at nt 4308; the G is a highly conserved nucleotide that participates in a GC base-pair in the T-stem of mammalian mitochondrial tRNA(Ile). The mutation was detected at a high level (approx. 50%) in muscle but not in blood. The mutation co-segregated with the phenotype, as the mutation was absent from blood and muscle in the patient's healthy mother. Functional characterization of the mutation revealed a six-fold reduced rate of tRNA(Ile) precursor 3' end maturation in vitro by tRNAse Z. Furthermore, the mutated tRNA(Ile) displays local structural differences from wild-type. These results suggest that structural perturbations reduce efficiency of tRNA(Ile) precursor 3' end processing and contribute to the molecular pathomechanism of this mutation.

Hypoxia inducible factor-1 alpha induction by tumour necrosis factor-alpha, but not by toll-like receptor agonists, modulates cellular respiration in cultured human hepatocytes

BACKGROUND/AIMS: Genes encoding for some of the mitochondrial proteins are under the control of the transcriptional factor hypoxia inducible factor-1 alpha (HIF-1 alpha), which can accumulate under normoxic conditions in inflammatory states. The aim of this study was to evaluate the effects of cobalt chloride (CoCl(2), a hypoxia mimicking agent), tumour necrosis factor-alpha (TNF-alpha) and toll-like receptor (TLR) -2, -3 and -4 agonists on HIF-1 alpha accumulation, and further on HIF-1 alpha-mediated modulation of mitochondrial respiration in cultured human hepatocytes. METHODS: The human hepatoma cell line HepG2 was used in this study. Cells were treated with CoCl(2), TNF-alpha and TLR-2, -3 and -4 agonists. HIF-1 alpha was determined by Western blotting and mitochondrial respiration in stimulated cells by high-resolution respirometry. RESULTS: CoCl(2), TNF-alpha and TLR agonists induced the expression of HIF-1 alpha in a time-dependent fashion. TNF-alpha and CoCl(2), but not TLR agonists, induced a reduction in complex I-, II- and IV-dependent mitochondrial oxygen consumption. TNF-alpha-associated reduction of cellular oxygen consumption was abolished through inhibition of HIF-1 alpha activity by chetomin (CTM). Pretreatment with cyclosporine A prevented CoCl(2)-induced reduction of complex I- and II-dependent mitochondrial oxygen consumption and TNF-alpha-induced reduction of complex-I-dependent respiration, implicating the involvement of the mitochondrial permeability transition pore openings. TNF-alpha and TLR-2, -3 and -4 agonists induced the expression of vascular endothelial growth factor, which was partially abolished by the blockage of HIF-1 alpha with CTM. CONCLUSIONS: The data suggest that HIF-1 alpha modulates mitochondrial respiration during CoCl(2) and TNF-alpha stimulation, whereas it has no effect when induced with TLR-2, -3 and -4 agonists.

Novel mitochondrial tRNA(Ile) m.4282A>G gene mutation leads to chronic progressive external ophthalmoplegia plus phenotype

BACKGROUND/AIM To investigate the underlying pathomechanism in a 33-year-old female Caucasian patient presenting with chronic progressive external ophthalmoplegia (CPEO) plus symptoms. METHODS Histochemical analysis of skeletal muscle and biochemical measurements of individual oxidative phosphorylation (OXPHOS) complexes. Genetic analysis of mitochondrial DNA in various tissues with subsequent investigation of single muscle fibres for correlation of mutational load. RESULTS The patient's skeletal muscle showed 20% of cytochrome c oxidase-negative fibres and 8% ragged-red fibres. Genetic analysis of the mitochondrial DNA revealed a novel point mutation in the mitochondrial tRNA(Ile) (MTTI) gene at position m.4282G>A. The heteroplasmy was determined in blood, buccal cells and muscle by restriction fragment length polymorphism (RFLP) combined with a last fluorescent cycle. The total mutational load was 38% in skeletal muscle, but was not detectable in blood or buccal cells of the patient. The phenotype segregated with the mutational load as determined by analysis of single cytochrome c oxidase-negative/positive fibres by laser capture microdissection and subsequent LFC-RFLP. CONCLUSIONS We describe a novel MTTI transition mutation at nucleotide position m.4282G>A associated with a CPEO plus phenotype. The novel variant at position m.4282G>A disrupts the middle bond of the D-stem of the tRNA(Ile) and is highly conserved. The conservation and phenotype-genotype segregation strongly suggest pathogenicity and is in good agreement with the MTTI gene being frequently associated with CPEO. This novel variant broadens the spectrum of MTTI mutations causing CPEO.

The TIM translocase in T. brucei contains Rhomboid-like proteins that are essential for mitochondrial protein import

The parasitic protozoon Trypanosoma brucei is often considered as one of the earliest branching eukaryotes that have mitochondria capable of oxidative phosphorylation. Its protein import systems are therefore of great interest. Recently, it was shown that the outer mitochondrial membrane protein translocase is of similar complexity yet different composition than in other eukaryotes (1). In the inner membrane however, only a single orthologue of the pore forming Tim17/22/23 protein family was identified and termed TbTim17. Based on this finding it has been suggested that, instead of separate TIM22 and TIM23 complexes as in other eukaryotes, trypanosomes may have a single multifunctional translocase of the inner mitochondrial membrane (TIM) of reduced complexity. To elucidate the composition of the trypanosomal TIM complex we performed co-immunoprecipitations (CoIP) of epitope-tagged TbTim17 in combination with SILAC-based quantitative mass spectrometry. This led to the identification of 22 highly enriched TbTim17-interacting proteins. We tagged two of the top-scoring proteins for reciprocal CoIP analyses and recovered a set of ten proteins that are highly enriched in all three CoIPs. These proteins are excellent candidates for core subunits of the trypanosomal TIM complex. Eight of them were present in the previously determined inner membrane proteome and four show homology to small Tim chaperones. Three candidates, a novel trypanosomatid-specific 42 kDa protein, termed Tim42, and two putative orthologues of probably inactive rhomboid proteases were chosen for further analysis. All three proteins are essential in both life cycle stages and in a cell line that can grow in the absence of mitochondrial DNA. Additionally, their ablation by RNAi results in a strong protein import defect both in vivo and in vitro. Blue native PAGE reveals that Tim42, like TbTim17 is present in a high molecular weight complex. Moreover, ablation of either Tim42 or TbTim17 leads to a destabilization of the complex containing the other protein, suggesting a tight interaction of the two proteins. In summary our study shows that unlike anticipated trypanosomes have a highly complex TIM translocase that has extensively been redesigned. We have characterized three novel TIM subunits that have never been associated with mitochondrial protein import before. Two of them belong to the rhomboid protease family, a member of which recently has been implicated in the ERAD translocation system. Our study provides insight into mitochondrial evolution over large phylogenetic distances and suggests an exciting analogy between protein translocation systems of mitochondria and the ER.