Functional analysis of mammalian m-AAA proteases

Abstract

The m-AAA protease, an ATP-dependent proteolytic complex in the inner mitochondrial membrane, controls mitochondrial protein quality and, acting as a processing enzyme, regulates mitochondrial protein synthesis in yeast. Mammalian m-AAA proteases assemble into several isoenzymes with variable subunit composition. In mice, the three different subunits paraplegin, Afg3l1 and Afg3l2, are expressed in a tissue-specific manner. Loss-of-function mutations in the m-AAA protease subunit paraplegin cause the neurodegenerative disease hereditary spastic paraplegia (HSP) which is mainly characterized by a cell-specific axonal degeneration. Similarly, Afg3l2 mutant mice exhibit neuropathological features with a severe defect in axonal development. The molecular basis of these neuron-specific phenotypes as well as cellular functions of mammalian m-AAA proteases in general remain unclear and are studied within this thesis. Using an HSP mouse model lacking paraplegin, a liver-specific mitochondrial translation defect was observed which is consistent with a functional conservation of the m-AAA protease-dependent control of mitochondrial protein synthesis in mammals. However, a significant impairment of mitochondrial protein synthesis and mitochondrial respiration in brain and spinal cord was not observed suggesting that axonal degeneration in HSP due to a loss of paraplegin occurs in the absence of a general respiratory dysfunction. The analysis of m-AAA isoenzymes on a cellular level using RNA interference revealed redundant functions of the subunits Afg3l1 and Afg3l2 and identified paraplegin as a new substrate which is processed by Afg3l1 and Afg3l2. Depletion of the m-AAA protease in MEFs resulted in mitochondrial fragmentation accompanied by an impaired biogenesis of OPA1, an essential component of the mitochondrial fusion machinery. Long OPA1 isoforms were destabilized and cleaved in an accelerated manner. The overexpression of a non-cleavable long OPA1 isoform in m-AAA protease-depleted cells restored the tubular mitochondrial network identifying impaired OPA1 processing as the primary cause for the mitochondrial morphology defect. Furthermore, the m-AAA protease was found to be essential for inducing mitochondrial hyperfusion, a cellular stress response which results in a highly interconnected mitochondrial network. The findings of this study indicate that the m-AAA protease has crucial functions in the regulation of mitochondrial morphology by controlling the biogenesis of OPA1 isoforms which may provide new insights into the molecular mechanisms of axonopathies caused by the loss of m-AAA protease subunits in mammals