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Insulin resistance in skeletal muscle is a characteristic feature of diabetes mellitus type 2 (DM2). Several lines of circumstantial evidence suggest that reduced mitochondrial oxidative phosphorylation capacity in skeletal muscle is a primary defect causing insulin resistance and subsequent development of DM2. We have now experimentally tested this hypothesis by characterizing glucose homeostasis in tissue-specific knockout mice with progressive respiratory chain dysfunction selectively in skeletal muscle. Surprisingly, these knockout mice are not diabetic and have an increased peripheral glucose disposal when subjected to a glucose tolerance test. Studies of isolated skeletal muscle from knockout animals show an increased basal glucose uptake and a normal increase of glucose uptake in response to insulin. In summary, our findings indicate that mitochondrial dysfunction in skeletal muscle is not a primary etiological event in DM2.

Early erythroblasts from patients with refractory anemia (RA) and RA with ringed sideroblasts (RARS) show constitutive mitochondrial release of cytochrome c. Moreover, mature erythroblasts in RARS, but not in RA, display aberrant accumulation of mitochondrial ferritin (MtF). We analyzed cytochrome c release, MtF expression, and gene expression during erythroid differentiation in bone marrow cells from myelodysplastic syndrome (MDS) patients and healthy controls. Whereas none or few cultured erythroid cells from healthy individuals and RA patients expressed MtF, those from RARS patients showed MtF expression at an early stage, when cells were CD34+ and without morphologic signs of erythroid differentiation. The proportion of RARS erythroblasts that were MtF+ increased further upon in vitro maturation. Moreover, a significant overexpression of mRNA encoding cytochrome c, and proapoptotic Bid and Bax, was seen in freshly isolated cells from MDS patients. Genes involved in erythroid differentiation were also dysregulated in MDS cells. Importantly, GATA-1 expression increased during normal erythroid maturation, but remained low in MDS cultures, indicating a block of erythroid maturation at the transcriptional level. In conclusion, aberrant MtF expression in RARS erythroblasts occurs at a very early stage of erythroid differentiation and is paralleled by an up-regulation of genes involved in this process.

A set of methods suitable for assessment of respiratory chain function in mitochondria isolated from 25mg of muscle is described. This set of methods includes determination of the mitochondrial ATP production rate (MAPR) and the activities of the respiratory chain complexes I, I+III, II+III, and IV and citrate synthase. MAPR is determined with an optimized version of a luminometric method previously described. The optimized method measures 50-220% higher activities than the original method. The highest MAPRs are recorded using the substrate combinations glutamate+succinate and N,N,N(1),N(1)-tetramethyl-1,4-phenyldiamine+ascorbate. The respiratory chain complex activities are determined with standard spectrophotometric methods, adapted to an automated photometer. The sensitivity in the determination of complex I, I+III, and II+III activities was increased considerably by pretreating the samples with saponin. The set of methods was evaluated on double biopsy samples from five healthy volunteers and showed coefficients of variation between 7 and 14% when citrate synthase was used as reference base. All of the various measures of mitochondrial function showed high correlation coefficients to each other (r=0.84-0.98; p<0.01). It is concluded that the set of methods is suitable for diagnosis of mitochondrial disorders in adults and small children.

Since the mid-1980s, a 'mysterious' wasting disease has been afflicting the moose (Alces alces L.) population of south-western Sweden. In 1994, molybdenosis combined with copper deficiency was suggested as the cause of this complex syndrome of clinical signs, diversity of necropsy findings and changes in trace element concentrations. These findings were corroborated by scientists in many countries by similar observations in other ruminants, particularly cattle and sheep, and also by changes in trace element concentrations and clinical chemical findings in our model experiments with goats. The biochemistry of copper is dependent on a number of copper-dependent enzymes in the animal organism. An important example is cytochrome c oxidase (COX), responsible for oxidative phosphorylation and energy production within the cell. In the present study, COX activity and trace element concentrations were determined in myocardium from affected and healthy moose. Citrate synthase (CS) activity was also measured for use as a mitochondrial marker. COX activity had decreased by 45% and the COX/CS ratio by 37%, while Mo and Na were found to have increased by 140% and 25%, respectively. The increase in Na was indicative of the frequently reported oedematous changes in 'flabby' moose heart. The concentrations of the elements Cu, Mg, Mn, P and Zn had decreased by 20%, 20%, 35%, 7% and 19%, respectively. The simultaneous decrease in COX activity and Cu concentration and the increase in Mo further support the hypothesis that molybdenosis is the cause of the moose disease.

The autosomal dominant cerebellar ataxias (ADCA) are a group of neurodegenerative disorders with ataxia and dysarthria as early and dominant signs. In ADCA type II, retinal degeneration causes severe visual impairment. ADCA type II has recently been mapped to chromosome 3p by three independent groups. In the family with ADCA type II studied here, the disease has been mapped to chromosome 3p12-p21.1. Histochemical examination of muscle biopsies in 5 cases showed slight neurogenic atrophy and irregular lobulated appearance or focal decreases of enzyme activity when staining for NADH dehydrogenase, succinic dehydrogenase and cytochrome oxidase. Ragged-red fibres were scarce. Electron microscopic examination showed uneven distribution of mitochondria with large fibre areas devoid of mitochondria and/or large subsarcolemmal accumulations of small rounded mitochondria, and frequent autophagic vacuoles. These vacuoles contained remnants of multiple small rounded organelles, possibly mitochondria, and had a remarkably consistent ultrastructural appearance. Biochemical investigation of mitochondrial function showed reduced activity of complex IV and slightly reduced activity of complex I in the respiratory chain in a severely affected child while no abnormalities were found in his affected uncle.

A new autosomal dominant syndrome in a Swedish pedigree is described. Five patients were affected with cerebellar ataxia and sensorineural deafness. Four of these patients had symptoms of narcolepsy. Optic atrophy, other neurological abnormalities and psychiatric symptoms developed with increasing disease duration. Three patients had non-neurological disease in addition, including diabetes mellitus in two and hypertrophic cardiomyopathy in one. Autopsy with neuropathological examination was performed in one case. Molecular studies focused on the short arm of chromosome 6, including the HLA DR2 locus associated with narcolepsy and the (CAG)n repeat at the spinocerebellar ataxia type 1 (SCA1) locus. Biochemical investigation of muscle biopsy of one case indicated mitochondrial dysfunction with selective decrease in ATP production for substrates that normally give the highest rates. The activity of glutamate dehydrogenase was reduced, indicating a low mitochondrial density. We postulate an autosomal dominant genetic factor responsible for this syndrome. Linkage was excluded to HLA DR2, and a normal sized SCA1 repeat was observed. We conclude that a locus predisposing to ataxia, deafness and narcolepsy exists outside this region of chromosome 6.