Mitochondrial dynamics, mechanisms and meaning: Investigations into fusion and fission proteins Mitofusin 2 and MAPL

Abstract

It has recently been established that mitochondria function as an interconnected reticulum that is maintained through dynamic fission and fusion events. The research performed throughout the course of this doctoral thesis has profound impacts on cell biology, impacting mitochondrial dynamics, programmed cell death, intracellular signaling, and peroxisome biology. Research into the protein mechanisms of mitochondrial fusion and fission is presented as two distinct research aims. The first objective of this thesis investigated the mechanisms and meaning of mitochondrial fusion; specifically the role of Mitofusin 2. It is presented as a manuscript which describes Mfn2 functioning as a signaling GTPase for mitochondrial fusion, rather then a mechanoenzyme, and that activated Mfn2 protects against both cytosolic and mitochondrial apoptosis as well as inhibits Bax activation. The second objective of this doctoral thesis was the investigation into the mechanisms and meaning of mitochondrial fission; specifically the role of a novel uncharacterized protein FLJ12875 (subsequently named MAPL). The identification of MAPL and a previously unrecognized pathway for mitochondrial vesiculation as a means to segregate selected cargo into distinct transport vesicles which traffic to the peroxisome is presented. MAPL is evolutionarily conserved from plants to bacteria and is an integral mitochondrial outer membrane protein. MAPL has two transmembrane domains and a carboy-terminal RING domain. Additionally it has an uncharacterized highly conserved middle domain positioned between the two membranes, the BAM domain (Beside A Membrane) MAPL overexpression results in a stimulation of Drp1-dependent mitochondrial fragmentation. Unexpectedly, it was found that in cells where mitochondrial fission was blocked, a pool of small (70-100 nm) vesicles enriched for MAPL and or TOM20 was observed. Convincing data are presented documenting that this pool of vesicles generates independently of the canonical mitochondrial fission pathway. This thesis describes the identification of a novel vesicular trafficking pathway for the cell. Both the signaling capabilities of Mfn2 and the identification of Mitochondrial Derived Vesicles (MDV's), provide a new platform for investigations of mitochondrial dynamics and opens up new avenues of research into mitochondrial mechanism and meaning.