DRP1 and BIF-1 regulations in mitochondrial dynamics during apoptosis

Recent studies have revealed that mitochondria' fragmentation is a critical event in apoptosis. Mitochondria become fragmented and notably, the fragmentation causes the permeabilization of the mitochondria' outer membrane and consequently contributes to mitochondria! dysfunction and apoptotic cell death. In apoptosis, mitochondrial fragmentation involves the activations of Drpl, a key fission protein, and Bif-1, a protein originally identified to interact with Bax. However, the molecular mechanisms by which Drpl and Bif-1 regulate mitochondrial dynamics during apoptosis remain unclear.;In the first study of my thesis work, I investigated Drpl regulation and its role in apoptosis of rat proximal tubular cell (RPTC) following ATP depletion. During ATP depletion, Drpl was shown to be dephosphorylated at serine-637. The dephosphorylation could be suppressed by cyclosporine A and FK506, two calcineurin inhibitors, which also prevented mitochondrial fragmentation, Bax accumulation, cytochrome c release and apoptosis in RPTC. The results suggest that Drpl is activated by calcineurin-mediated dephosphorylation at serin-637. Upon activation, Drpl stimulates mitochondrial fragmentation and the permeabilization of outer membrane, resulting in the release of apoptogenic factors and apoptosis.;In the second study, I detected Bif-1 translocation to mitochondria during apoptosis of RPTC. Notably, apoptotic events including mitochondrial fragmentation, Bax insertion and oligomerization, and cytochrome c release were all suppressed in Bif-1 deficient cells. Mechanistically, we showed that during apoptosis, Bif-1 bound to prohibitin-2 (PHB2), a mitochondrial protein implicated in mitochondrial inner membrane regulation. Furthermore, PHB2 was shown to form hetero-oligomeric complex with prohibitin-1 (PHB I) in control cells and the complex broke down upon apoptosis, which was accompanied by the proteolysis of optic atrophy 1 (OPAL), the mitochondrial inner membrane fusion protein. In Bif-1 deficient cells, the breakdown of PHB complexes and OPAL proteolysis were both inhibited, supporting a critical role of Bif-1 in mitochondrial inner membrane fragmentation by regulating PHB2 and OPA 1.;Our studies have shed new light on the critical molecular mechanisms responsible for the alteration of mitochondrial dynamics upon cell stress, resulting in mitochondrial fragmentation, injury and apoptosis.;INDEX WORDS: Drpl, Bif-1, OPAL, PHBI/2, Calcineurin, Mitochondria! Dynamics, Cyclosporine A, FK506, Bax, cytochrome c, Mitochondria' Fragmentation, Mitochondrial Outer Membrane Permeabilization (MOMP).