FRIENDLY REGULATES MITOCHONDRIAL DISTRIBUTION, FUSION, AND QUALITY CONTROL IN ARABIDOPSIS
Mitochondria are vital semi-autonomous organelles, containing their own genome (mtDNA) and protein synthesizing machinery, that provide energy for life and a trigger for death. Known as the powerhouse of the cell due to their roles in energy transduction, plant mitochondria are highly dynamic and their distribution inside a cell can be strikingly heterogeneous as a result of their differentially controlled motility on the cytoskeleton. Plant mitochondrial dysfunction can cause male sterility, defects in carbon metabolism and intracellular signaling. In humans, loss of mitochondrial function is implicated in numerous diseases such as cancer, Alzheimer’s, diabetes, blindness, Parkinson’s and many additional neurological and neuromuscular diseases. Mitochondria have key roles in energy conversion and integrated signaling pathways such as regulation of Ca2+ signals and programmed cell death (PCD) in plants and animals. Despite the highly dynamic nature of plant mitochondria, there is little specific scientific evidence linking mitochondrial morphology, distribution and movement to organelle and cell function. Plant mitochondria are normally distributed throughout the cytosol as single, discrete, spherical, or tubular-shaped organelles. However, the friendly mitochondria (fmt, friendly) mutant of Arabidopsis thaliana (Arabidopsis) contains clusters of tens of mitochondria in contrast to the wild type distribution. The results presented in this thesis show that the mutant has a short-root phenotype, non-viable cells in the roots, increased size and number of acidic compartments (autolysosomes) and displays an increased frequency of transient depolarization events, termed “pulses”, that are indicative of cellular stress and a failure of mitochondrial quality control. Furthermore, transmission electron microscopy (TEM) of ultra-thin sections shows that clustered mitochondria are discrete organelles. Fluorescence recovery after photobleaching (FRAP) studies confirmed the TEM results by showing no obvious connectivity of the matrices within a cluster. A recent study that identified the Drosophila homologue of the FRIENDLY, clueless, demonstrated that it genetically interacts with parkin, the Drosophila orthologue of a human gene responsible for many familial cases of Parkinson’s disease. Despite this genetic evidence linking clueless to a putative mitochondrial quality control pathway, the function of FRIENDLY protein and its orthologues and, therefore, the mechanism(s) underpinning the mutant clustered-mitochondria phenotype, remain unknown. Furthermore, we show that the C.elegans CLU-1 protein is required for normal mitochondrial morphology and clu-1 mutants display an increased number and size of autophagosomes compared to wild type. Moreover, interrelationships between clustered mitochondria, actin, and microtubules were studied, demonstrating a clear dynamic association between clustered mitochondria and actin, with no observed association between mitochondria and microtubules. We hypothesized that friendly mitochondrial clusters develop due to a fusion defect. In wild type cells, mitochondria that meet due to either rearrangement of, or movement on, actin filaments first interact through a “handshake process” involving FRIENDLY. However, in the absence of FRIENDLY, the handshake is prolonged such that clusters of mitochondria develop between slow hand-shaking organelles. Overall, it is proposed that FRIENDLY is the first protein to be identified that is involved in plant mitochondrial fusion and that fusion is necessary to allow mitochondria to minimize damage arising from oxidative stress. Lastly, this research suggests that disruption of mitochondrial association, motility, and chondriome structure in friendly affects mitochondrial quality control, and leads to mitochondrial stress, photosynthetic dysfunction, and cell death.
autophagy, dynamics, cell death, cytoskeleton, fusion, membrane potential, mitochondria, photosynthesis, C.elegans Clu-1,quality control.
Doctor of Philosophy (Ph.D.)