Characterisation of new factors regulating mitochondrial dynamics and cellular metabolism
Mitochondria are dynamic organelles that possess an inherent plasticity which governs their shape, size and distribution. This dynamic behaviour depends on continuous cycles of fission or fusion, which is mediated by large GTPase enzymes, facilitating the remodelling of the network to adapt and answer to cellular needs. However, the underlying molecular details of these events have not been fully elucidated. Recently, the uncharacterised protein, mitochondrial fission regulator 1 like protein (MTFR1L), has been identified as a substrate for the metabolic kinase adenosine monophosphate kinase (AMPK). Phylogenetically, MTFR1L has been categorised within the mitochondrial fission regulator 1 (MTFR1) family, which includes paralogues MTFR1 and MTFR2. Whilst these two proteins have been implicated in regulating mitochondrial fission via unknown mechanisms, the function of MTFR1L is unknown. The main questions addressed here, have been to characterise the role of MTFR1L in mitochondrial morphology regulation, including its localisation, binding partners, and behaviour during cellular and mitochondrial stresses. The results showed that MTFR1L is alternatively spliced to generate two different isoforms, which are differentially localised to the outer and inner mitochondrial membranes. Silencing experiments, or CRISPR/CAS9 mediated knockout of MTFR1L, led to a drastic mitochondrial hyperfusion, which was accompanied by an increase of the fusion factor OPA1, and a reduction of phosphorylated mitochondrial fission factor (pMFF), a well characterised fission factor. Furthermore, cells lacking MTFR1L harboured reduced AMPK activation and consequently dampened signalling upon electron transport chain (ETC) dysfunction, associated with a resistance against mitochondrial fragmentation. Alternatively, MTFR1L was phosphorylated under different conditions of AMPK activation. MTFR1L is therefore a new regulator of mitochondrial morphology that protects mitochondria from stress induced ETC dysfunction. Lastly, the underlying mechanisms that terminate mitochondrial division have not been fully elucidated. Here, Golgi-localised Arf1 and its downstream effector PI(4)KIIIß, were responsible for localised generation of the lipid PI(4)P on Trans-Golgi network vesicles, which are involved in mitochondrial division. Indeed, silencing of both genes led to a drastic mitochondrial hyperfusion, accompanied by excessive branching, in different mammalian cell lines. Interestingly, this phenotype was resistant to enforced mitochondrial fragmentation, suggesting that these proteins play a role in the late stage of this process. Lastly, the controversial role of Dynamin 2 regulating fission was investigated across different cell lines.