Mitochondrial DNA replication and disease

A variety of human diseases, ranging from devastating conditions of infancy through to degenerative disorders of old age, are associated with genetic lesions affecting mitochondrial DNA. To understand the underlying processes we study the mechanisms and machinery of mitochondrial DNA replication in model organisms and humans.

• Data from our lab and many others has documented several distinct DNA replication mechanisms active within mitochondria. A major unresolved issue is whether these are genuinely redundant mechanisms, or manifestations of a single unified mechanism that may show minor differences between species and tissues.

• Our recent focus has been on the molecular machinery of DNA replication in Drosophila mitochondria, based on a genome-wide screen we conducted earlier. Arising from this, we have mainly studied the involvement of the intracellular vesicle trafficking machinery, of the members of the mitochondrial transcription terminator family, and of RNase H1.

Nuclear-mitochondrial interactions in Drosophila

We use various Drosophila mutants with defects in mitochondrial function to analyse the contributions of nuclear and mitochondrial genotype, as well as environmental factors, to organismal phenotype. Mostly we have studied the tko25t mutant strain, which carries a point mutation in the gene for mitoribosomal protein S12, and is a useful model for human mitochondrial disease. We also investigate the roles in animal development of global regulators of mitochondrial function, and the physiological effects of respiratory chain dysfunction in the nervous system, using flies as a model system.

• By creating cybrid and back-crossed strains with different permutations of nuclear and mitochondrial DNA, we have found both nuclear and mitochondrial suppressors/modifiers of the canonical tko25t phenotype (delayed development, sensitivity to mechanical stress). Some modifiers lead to clear phenotypic improvement, whilst others convert a relatively mild phenotype into lethality.

• Altered gene expression and metabolite profiles in tko25t flies indicate the activation of a signaling pathway that slows down development in response to mitochondrial stress. We aim to elucidate the machinery and mechanisms of this stress response.

• Specific groups of glutamatergic and cholinergic (but not dopaminergic) neurons are vulnerable to deficiency of the key mitochondrial respiratory enzyme cytochrome oxidase, resulting in locomotor defects in the adult fly.

Alternative respiratory chain enzymes: a possible therapy for mitochondrial disorders?

The genomes of plants, fungi, many microbes and some primitive animals contain genes for alternative mitochondrial respiratory chain enzymes, which buffer redox and bioenergetic stresses caused by defects in the standard respiratory chain and OXPHOS system. The genes for these alternative enzymes are absent from humans and other complex animals. However, we reasoned that their introduction may alleviate many of the physiological defects associated with mitochondrial disease. We have transferred the relevant genes from the tunicate Ciona, as well as from fungi, into human cells and model organisms, creating proof of concept for such protection.

Our current research focuses on ascertaining the potential of these enzymes in therapy, both by exploring what specific pathologies they can alleviate, as well as testing out ways to safely introduce them in a therapeutically effective form. In addition we are studying their natural biology.

• The alternative oxidase (AOX) and NADH dehydrogenase (NDX) from Ciona can be expressed in model organisms without significant deleterious effects

• AOX (and/or NDX) protects cells, tissues and model organisms from the acute effects of acute respiratory poisons or bacterial toxins, as well as from chronic exposure to toxins such as found in cigarette smoke

• The alternative enzymes can protect model organisms from lethality and other severe phenotypes caused by partial genetic ablation of respiratory chain complexes I, III and IV, but can also exacerbate some phenotypes by abrogating stress responses that should activate repair pathways

Mitochondria are hot

By staining cells with a temperature-dependent mitochondrial dye we were able to determine that mitochondria are 10-12 degrees Celsius hotter that the surrounding cell. We are currently exploring how this affects a variety of cellular processes, and how it may underlie a number of physiological and pathological phenomena.

Working in Howylab

Prospective postdocs are expected to raise their own salary and research expenses from grants and/or fellowships. If interested, please contact howard.jacobs(at)tuni.fi. At this time, the lab is not taking on new PhD students.