Heteroplasmic mitochondrial disease in Dictyostelium discoideum

The bewildering complexity of the relationship between genotype and phenotype in human mitochondrial diseases has delayed an understanding of the related cytopathological mechanisms. To explore the relationship between mitochondrial dysfunction in Dictyostelium discoideum and the related cytopathologies, we determined whether the phenotypic outcomes were similar regardless of which D. discoideum mitochondrial gene was targeted for disruption. The disruption of the mitochondrial genes resulted in a similar pattern of phenotypes to those caused by other mitochondrial defects. These include impairment of phototaxis, multicellular development and growth on plates and in liquid medium. As the reduced growth rates could have been due to defective phagocytic or macropinocytic nutrient uptake, these processes were tested but found to be unaffected. Since mitochondria have been associated with Legionella pathogenesis of human macrophages, it was also determined if mitochondrially diseased Dictyostelium strains were better or worse than healthy cells at supporting the growth of Legionella pneumophila. The results revealed that the mitochondrially diseased strains supported greater L. pneumophila growth than the wild type Dictyostelium strain (AX2). Quantitative Northern blotting showed a significant reduction in the level of expression of the entire mitochondrial genome, regardless of which mitochondrial gene was targeted for disruption, suggesting a generalized deficiency in mitochondrial gene expression and function. The phenotypic outcomes were the same as those shown previously to result from chronic hyperactivity of the energy-sensing protein kinase, AMPK, after knockdown of mitochondrial chaperonin 60.