Summary Successive cell divisions during embryonic cleavage create increasingly smaller cells, so intracellular structures must adapt accordingly. Mitotic spindle size correlates with cell size, but the mechanisms for this scaling remain unclear. Using live cell imaging, we analyzed spindle scaling during embryo cleavage in the nematode Caenorhabditis elegans and sea urchin Paracentrotus lividus. We reveal a common scaling mechanism, where the growth rate of spindle microtubules scales with cell volume, which explains spindle shortening. Spindle assembly timing is however constant throughout successive divisions. Analyses in silico suggest that controlling the microtubule growth rate is sufficient to scale spindle length and maintain a constant assembly timing. We tested our in silico predictions to demonstrate that modulating cell volume or microtubule growth rate in vivo induces a proportional spindle size change. Our results suggest that scalability of the microtubule growth rate when cell size varies adapts spindle length to cell volume.

Mitochondrial diseases are severe and largely untreatable. Owing to the many essential processes carried out by mitochondria and the complex cellular systems that support these processes, these diseases are diverse, pleiotropic, and challenging to study. Much of our current understanding of mitochondrial function and dysfunction comes from studies in the baker's yeast Saccharomyces cerevisiae. Because of its good fermenting capacity, S. cerevisiae can survive mutations that inactivate oxidative phosphorylation, has the ability to tolerate the complete loss of mitochondrial DNA (a property referred to as ‘petite-positivity’), and is amenable to mitochondrial and nuclear genome manipulation. These attributes make it an excellent model system for studying and resolving the molecular basis of numerous mitochondrial diseases. Here, we review the invaluable insights this model organism has yielded about diseases caused by mitochondrial dysfunction, which ranges from primary defects in oxidative phosphorylation to metabolic disorders, as well as dysfunctions in maintaining the genome or in the dynamics of mitochondria. Owing to the high level of functional conservation between yeast and human mitochondrial genes, several yeast species have been instrumental in revealing the molecular mechanisms of pathogenic human mitochondrial gene mutations. Importantly, such insights have pointed to potential therapeutic targets, as have genetic and chemical screens using yeast.

Mitochondria are organelles that have their own DNA (mitochondrial DNA, mtDNA) whose maintenance is necessary for the majority of ATP production in eukaryotic cells. Defects in mtDNA maintenance or integrity are responsible for numerous diseases. The DNA polymerase γ (POLG) ensures proper mtDNA replication and repair. Mutations in POLG are a major cause of mitochondrial disorders including hepatic insufficiency, Alpers syndrome, progressive external ophthalmoplegia, sensory neuropathy and ataxia. Mutations in POLG are also associated with parkinsonism. To date, no effective therapy is available. Based on the conservation of mitochondrial function from yeast to human, we used Saccharomyces cerevisiae and Caenorhabditis elegans as first pass filters to identify a chemical that suppresses mtDNA instability in cultured fibroblasts of a POLG-deficient patient. We showed that this unsuspected compound, clofilium tosylate (CLO), belonging to a class of anti-arrhythmic agents, prevents mtDNA loss of all yeast mitochondrial polymerase mutants tested, improves behavior and mtDNA content of polg-1-deficient worms and increases mtDNA content of quiescent POLG-deficient fibroblasts. Furthermore, the mode of action of the drug seems conserved as CLO increases POLG steady-state level in yeast and human cells. Two other anti-arrhythmic agents (FDA-approved) sharing common pharmacological properties and chemical structure also show potential benefit for POLG deficiency in C. elegans. Our findings provide evidence of the first mtDNA-stabilizing compound that may be an effective pharmacological alternative for the treatment of POLG-related diseases.

Isolated complex I deficiencies are one of the most commonly observed biochemical features in patients suffering from mitochondrial disorders. In the majority of these clinical cases the molecular bases of the diseases remain unknown suggesting the involvement of unidentified factors that are critical for complex I function. The Saccharomyces cerevisiae NDI1 gene, encoding the mitochondrial internal NADH dehydrogenase was previously shown to complement a complex I deficient strain in Caenorhabditis elegans with notable improvements in reproduction and whole organism respiration. These features indicate that Ndi1p can functionally integrate the respiratory chain, allowing complex I deficiency complementation. Taking into account the Ndi1p ability to bypass complex I, we evaluate the possibility to extend the range of defects/mutations causing complex I deficiencies that can be alleviated by NDI1 expression. We report here that NDI1 expressing animals unexpectedly exhibit a slightly shortened lifespan, a reduction in the progeny, and a depletion of the mitochondrial genome. However, Ndi1p is expressed and targeted to the mitochondria as a functional protein that confers rotenone resistance to those animals without affecting their respiration rate and ATP content. We show that the severe embryonic lethality level caused by the RNAi knockdowns of complex I structural subunit encoding genes (e.g., NDUFV1, NDUFS1, NDUFS6, NDUFS8, or GRIM-19 human orthologs) in wild type animals is significantly reduced in the Ndi1p expressing worm. All together these results open up the perspective to identify new genes involved in complex I function, assembly, or regulation by screening an RNAi library of genes leading to embryonic lethality that should be rescued by NDI1 expression.