Dottorato in GENETICA E BIOLOGIA MOLECOLARE

Titolo della tesi: Assessing an unprecedented role of Heterochromatin Protein 1a in mitochondrial homeostasis

Heterochromatin Protein 1a (HP1a) is a conserved non histone nuclear protein that predominantly binds heterochromatin and play essential roles in heterochromatin organization, heterochromatin- related gene expression regulation, DNA repair and telomere maintenance. My thesis work highlights an unprecedented localization of HP1a also at the outer membrane of mitochondria of Drosophila melanogaster and mammalian cells, thus indicating that the association of HP1a (and its human ortholog HP1 with these organelles is evolutionarily conserved. My genetic and biochemical data revealed that HP1a downregulation in flies results in an increased mitochondrial mass, altered mitochondria ultrastructure, reduced OXPHOS activities without affecting the expression of mitochondrial biogenesis factors. In addition, RNA interference of HP1a-encoding Drosophila gene Su(var)205 in muscle cells determined locomotor impairment at different developmental stages, very likely as a direct consequence of altered mitochondrial ultrastructure and mitochondrial fragmentation. Analogously, HP1 alpha downregulation in human cell lines induces changes in mitochondrial mass and dynamics. Moreover, I found that silencing of HP1 alpha-encoding gene cbx5 increased LC3 positive vesicles and autophagosome- lysosomes fusion indicating that depletion of HP1 stimulates autophagy. More interestingly, my results also show that cells lacking HP1 alpha exhibit consistent accumulation of Parkin- independent Mitophagy receptor BNIP3L/NIX, leading us to speculate that HP1 alpha could act as a regulator of mitophagy. Collectively, my functional studies indicate that HP1a/HP1 has evolved an evolutionarily conserved function in the regulation of Mitochondria Quality Control. Thus, I can hypothesize that HP1a/HP1 alpha is part of the rapidly increasing number of proteins that are required for maintaining mammalian genome integrity and display a dual mitochondrial and nuclear function.