Ruolo della morfologia e della funzionalità mitocondriale sulla distribuzione intracellulare dei mitocondri in neuroni di Drosophila

ABSTRACT Mitochondria are the energy producing organelles in eukaryotic cells providing ATP through oxidative phosphorylation (OXPHOS). Mitochondria are highly dynamic and undergo fission, fusion and move into the cell along the microtubules to generate the mitochondrial network. Mitochondrial dynamics play a critical role in the control of organelle shape, size, number, function and quality control of mitochondria. It is regulated by several GTPases that play an important role in fusion and fission processes. In mammals, mitochondrial fusion is controlled by Mitofusin 1 (Mfn1), Mitofusin 2 (Mfn2) and Optic atrophy protein 1 (Opa1), while mitochondrial fission is regulated by Dynamin related protein 1 (Drp1). The aim of this study is to understand how mitochondrial distribution in neuronal cells is affected by mitochondria function and/or morphology. We use Drosophila melanogaster , whose genome contains homologs for all mitochondrial fusion and fission proteins, as a modelorganism to study how loss of fusion and fission protein modify the axonal distribution and motility of mitochondria. We demonstrate that loss of Marf (Mitochondrial associated regulatory factor, homologous to human mitofusins) or Opa1 causes an accumulation of mitochondria in the soma, a defect in the axonal distribution of mitochondria, a severe depletion of mitochondria in neuromuscular junctions (NMJs) and reduced mitochondrial motility. Simultaneous loss of Drp1 rescues the Opa1 phenotype very robustly while loss of Marf essentially does not. Viability data however show the opposite trend. The expression of Marf RNAi or Opa1 RNAi cause lethality, and so does the double down regulation of Opa1 and Drp1. Conversely individuals expressing Marf RNAi and Drp1 RNAi simultaneously survive and are comparable to the controls. We then examined possible alterations of mitochondrial function by analyzing the mitochondrial respiratory capacity, the activity of the respiratory chain complexes and ATP production capacity. The data show that individuals where Marf, Opa1 or simultaneously Opa1 and Drp1 are down-regulated display severe alterations in mitochondrial function, while there are no obvious energy defects in individuals in which the expression of Marf and Drp1 is simultaneously reduced. Collectively our results obtained suggest that mitochondrial morphology is important for a homogeneous distribution of mitochondria along the axon and their transport to synapses and that these mechanisms are independent of mitochondria function.