Felix Distelmaier

Mitochondrial complex I deficiency: from organelle dysfunction to clinical disease

Mitochondria are essential for cellular bioenergetics by way of energy production in the form of ATP through the process of oxidative phosphorylation. This crucial task is executed by five multi-protein complexes of which mitochondrial NADH:ubiquinone oxidoreductase or complex I is the largest and most complicated one. During recent years, mutations in nuclear genes encoding structural subunits of complex I have been identified as a cause of devastating neurodegenerative disorders with onset in early childhood. Here, we present a comprehensive overview of clinical, biochemical and cell physiological information of 15 children with isolated, nuclear-encoded complex I deficiency, which was generated in a joint effort of clinical and fundamental research. Our findings point to a rather homogeneous clinical picture in these children and drastically illustrate the severity of the disease. In extensive live cell studies with patient-derived skin fibroblasts we uncovered important cell physiological aspects of complex I deficiency, which point to a central regulatory role of cellular reactive oxygen species production and altered mitochondrial membrane potential in the pathogenesis of the disorder. Moreover, we critically discuss possible interconnections between clinical signs and cellular pathology. Finally, our results indicate apparent differences to drug therapy on the cellular level, depending on the severity of the catalytic defect and identify modulators of cellular Ca(2+) homeostasis as new candidates in the therapy of complex I deficiency.

OXPHOS mutations and neurodegeneration.

Mitochondrial oxidative phosphorylation (OXPHOS) sustains organelle function and plays a central role in cellular energy metabolism. The OXPHOS system consists of 5 multisubunit complexes (CI–CV) that are built up of 92 different structural proteins encoded by the nuclear (nDNA) and mitochondrial DNA (mtDNA). Biogenesis of a functional OXPHOS system further requires the assistance of nDNA‐encoded OXPHOS assembly factors, of which 35 are currently identified. In humans, mutations in both structural and assembly genes and in genes involved in mtDNA maintenance, replication, transcription, and translation induce ‘primary’ OXPHOS disorders that are associated with neurodegenerative diseases including Leigh syndrome (LS), which is probably the most classical OXPHOS disease during early childhood. Here, we present the current insights regarding function, biogenesis, regulation, and supramolecular architecture of the OXPHOS system, as well as its genetic origin. Next, we provide an inventory of OXPHOS structural and assembly genes which, when mutated, induce human neurodegenerative disorders. Finally, we discuss the consequences of mutations in OXPHOS structural and assembly genes at the single cell level and how this information has advanced our understanding of the role of OXPHOS dysfunction in neurodegeneration.

Computer-assisted live cell analysis of mitochondrial membrane potential, morphology and calcium handling

Mitochondria are crucial for many aspects of cellular homeostasis and a sufficiently negative membrane potential (Deltapsi) across the mitochondrial inner membrane (MIM) is required to sustain most mitochondrial functions including ATP generation, MIM fusion, and calcium uptake and release. Here, we present a microscopy approach for automated quantification of Deltapsi and mitochondrial position, shape and calcium handling in individual living cells. In the base protocol, cells are stained with tetramethyl rhodamine methyl ester (TMRM), a fluorescent cation that accumulates in the mitochondrial matrix according to Deltapsi, and visualized using video-microscopy. Next, the acquired images are processed to generate a mitochondria-specific binary image (mask) allowing simultaneous quantification of mitochondrial TMRM fluorescence intensity, shape and position. In a more advanced version of this protocol a mitochondria-targeted variant of green fluorescent protein (mitoAcGFP1) is expressed to allow mask making in TMRM-stained cells. The latter approach allows quantification of Deltapsi in cells with a substantially depolarized Deltapsi. For automated quantification of mitochondrial calcium handling in space and time mitoAcGFP1-expressing cells are stained with rhod-2, a fluorescent calcium indicator that accumulates in the mitochondrial matrix. In this paper, a detailed step-by-step description of the above approaches and its pitfalls is provided.

Life cell quantification of mitochondrial membrane potential at the single organelle level.

Mitochondrial membrane potential (Δψ) is key to mitochondrial function and cellular survival. Here, we aimed to develop an automated protocol allowing sensitive quantification of Δψ in living cells at the level of individual mitochondria. Human skin fibroblasts were stained with the fluorescent cation tetramethyl rhodamine methyl ester (TMRM), which is sequestered by mitochondria according to their Δψ. Cells were visualized by videomicroscopy and the acquired images were processed to generate a mitochondria‐specific mask. The latter was superimposed on the original image to allow quantification of TMRM fluorescence. Following validation, our approach revealed that mitochondria with different Δψ coexisted within the same cell. Furthermore, our method allowed reproducible detection of small (<10%) reductions in TMRM intensity induced by the complex III inhibitor antimycin A. Mitochondrial uncoupling by p‐trifluoromethoxy carbonyl cyanide phenyl hydrazone (FCCP) greatly reduced mitochondrial TMRM fluorescence. Under these conditions faithful mask calculation and TMRM intensity analysis were still possible using a mitochondria‐targeted green fluorescence protein (mitoAcGFP1), expressed in the cells using baculoviral transfection.

A guide to diagnosis and treatment of Leigh syndrome

Leigh syndrome is a devastating neurodegenerative disease, typically manifesting in infancy or early childhood. However, also late-onset cases have been reported. Since its first description by Denis Archibald Leigh in 1951, it has evolved from a postmortem diagnosis, strictly defined by histopathological observations, to a clinical entity with indicative laboratory and radiological findings. Hallmarks of the disease are symmetrical lesions in the basal ganglia or brain stem on MRI, and a clinical course with rapid deterioration of cognitive and motor functions. Examinations of fresh muscle tissue or cultured fibroblasts are important tools to establish a biochemical and genetic diagnosis. Numerous causative mutations in mitochondrial and nuclear genes, encoding components of the oxidative phosphorylation system have been described in the past years. Moreover, dysfunctions in pyruvate dehydrogenase complex or coenzyme Q10 metabolism may be associated with Leigh syndrome. To date, there is no cure for affected patients, and treatment options are mostly unsatisfactory. Here, we review the most important clinical aspects of Leigh syndrome, and discuss diagnostic steps as well as treatment options.

Pseudotumor cerebri as an important differential diagnosis of papilledema in children

INTRODUCTION Primary pseudotumor cerebri (PTC) in childhood is a rare but important differential diagnosis in children presenting with papilledema. It is defined as elevated cerebrospinal fluid (CSF) pressure of more than 20 cm H(2)O, normal CSF composition, and exclusion of underlying structural or systemic causes. Visual loss is a serious complication, which requires careful monitoring and management. PATIENTS AND METHODS We conducted a retrospective chart review of 12 patients with primary PTC. The mean age at presentation was 8212 years, and there was a male-to-female ratio of 7:5. The aim of this study was to investigate the clinical features of primary PTC in children, and to highlight the different treatment options in normalizing intracranial pressure in these patients. RESULTS In the majority of cases, children presented with headache. Four patients had no obvious symptoms and papilledema was found on routine eye examination. Obesity was uncommon and there was no distinct sex predilection. Acetazolamide was our drug of choice for the initial treatment. Furosemide and prednisone were used as second-line agents. Treatment was gradually decreased after resolution of the papilledema with exception of the two youngest children, who remained symptomatic. One child underwent ventricular-peritoneal shunting. DISCUSSION The treatment goals of PTC are the relief of symptoms, and preservation of visual function. Acetazolamide is an effective first-line method of lowering raised intracranial pressure. In our study group especially the young children were difficult to treat. This might indicate an age-related difference in the etiology of PTC. When medical treatment remains ineffective and visual function deteriorates, surgical treatment should be considered.

Phytanic acid impairs mitochondrial respiration through protonophoric action

Abstract.Refsum disease is a rare, inherited neurodegenerative disorder characterized by accumulation of the dietary branched-chain fatty acid phytanic acid in plasma and tissues caused by a defect in the alphaoxidation pathway. The accumulation of phytanic acid is believed to be the main pathophysiological cause of the disease. However, the exact mechanism(s) by which phytanic acid exerts its toxicity have not been resolved. In this study, the effect of phytanic acid on mitochondrial respiration was investigated. The results show that in digitonin-permeabilized fibroblasts, phytanic acid decreases ATP synthesis, whereas substrate oxidation per se is not affected. Importantly, studies in intact fibroblasts revealed that phytanic acid decreases both the mitochondrial membrane potential and NAD(P)H autofluorescence. Taken together, the results described here show that unesterified phytanic acid exerts its toxic effect mainly through its protonophoric action, at least in human skin fibroblasts.

The antioxidant Trolox restores mitochondrial membrane potential and Ca2+-stimulated ATP production in human complex I deficiency

Malfunction of mitochondrial complex I caused by nuclear gene mutations causes early-onset neurodegenerative diseases. Previous work using cultured fibroblasts of complex-I-deficient patients revealed elevated levels of reactive oxygen species (ROS) and reductions in both total Ca2+ content of the endoplasmic reticulum (ERCa) and bradykinin(Bk)-induced increases in cytosolic and mitochondrial free Ca2+ ([Ca2+]C; [Ca2+]M) and ATP ([ATP]C; [ATP]M) concentration. Here, we determined the mitochondrial membrane potential (Δψ) in patient skin fibroblasts and show significant correlations with cellular ROS levels and ERCa, i.e., the less negative Δψ, the higher these levels and the lower ERCa. Treatment with 6-hydroxy-2,5,7,8-tetramethylchromane-2-carboxylic acid (Trolox) normalized Δψ and Bk-induced increases in [Ca2+]M and [ATP]M. These effects were accompanied by an increase in ERCa and Bk-induced increase in [Ca2+]C. Together, these results provide evidence for an integral role of increased ROS levels in complex I deficiency and point to the potential therapeutic value of antioxidant treatment.

Ovarian small cell carcinoma of the hypercalcemic type in children and adolescents: a prognostically unfavorable but curable disease.

Ovarian small cell carcinoma of the hypercalcemic type is a rare neoplasm that is associated with a poor prognosis. The objective of the current study was to investigate the clinicopathologic features of this tumor and to develop preliminary diagnostic and therapeutic guidelines.

C. elegans ATAD-3 is essential for mitochondrial activity and development.

Background Mammalian ATAD3 is a mitochondrial protein, which is thought to play an important role in nucleoid organization. However, its exact function is still unresolved. Results Here, we characterize the Caenorhabditis elegans (C. elegans) ATAD3 homologue (ATAD-3) and investigate its importance for mitochondrial function and development. We show that ATAD-3 is highly conserved among different species and RNA mediated interference against atad-3 causes severe defects, characterized by early larval arrest, gonadal dysfunction and embryonic lethality. Investigation of mitochondrial physiology revealed a disturbance in organellar structure while biogenesis and function, as indicated by complex I and citrate synthase activities, appeared to be unaltered according to the developmental stage. Nevertheless, we observed very low complex I and citrate synthase activities in L1 larvae populations in comparison to higher larval and adult stages. Our findings indicate that atad-3(RNAi) animals arrest at developmental stages with low mitochondrial activity. In addition, a reduced intestinal fat storage and low lysosomal content after depletion of ATAD-3 suggests a central role of this protein for metabolic activity. Conclusions In summary, our data clearly indicate that ATAD-3 is essential for C. elegans development in vivo. Moreover, our results suggest that the protein is important for the upregulation of mitochondrial activity during the transition to higher larval stages.