Invadolysin, a conserved lipid droplet-associated protease interacts with mitochondrial ATP Synthase and regulates mitochondrial metabolism in Drosophila
Invadolysin (inv) is a member of the M8 class of zinc-metalloproteases and is conserved throughout metazoans. It is essential for development and invadolysin homozygous Drosophila mutants are third instar larval lethal. These larvae exhibit a reduced larval brain size and an absence of imaginal discs. Detailed analysis showed that inv mutants exhibit pleiotropic effects, including defects with chromosome architecture, cell cycle progression, spindle assembly, nuclear envelope dynamics, protein turnover and problems with germ cell migration. These findings indicated that Invadolysin must have a critical role in Drosophila. In order to better understand these roles, I set out to identify genetic interactors of invadolysin. I performed a genetic screen scoring for enhancer/suppressor modification of a ‘rough eye’ phenotype induced by invadolysin overexpression. Screening against the Drosdel ‘deficiency kit’ identified numerous genetic interactors including genes linked to energy regulation, glucose and fatty acid pathways. Immunofluorescence experiments in cultured cells showed that H. sapiens Invadolysin localises to the surface of lipid droplets (LD), and subcellular fractionation confirmed its enrichment to these structures. Lipid droplets are highly dynamic organelles involved not only in energy storage but also in protein sequestration, protein and membrane trafficking, and cell signaling. Drosophila fat bodies are enriched in LDs and therefore important energy stores. In addition, they are nutritional sensors and regulators, which are proposed to be the ortholog of vertebrate liver and adipose tissue. Mutant inv fat bodies appeared smaller and thinner than wild type fat body, and accumulated lower levels of triacylgylcerides. This indicated that the loss of invadolysin might be affecting lipid metabolism and storage, confirming the genetic data. However, it was not clear whether these effects were due to the direct action of Invadolysin. Hence, transgenic fly lines expressing either HA, RFP or FLAG tagged forms of Invadolysin were generated to identify physical interactors of Invadolysin. Subsequent mass spectrometry analysis detected ATP synthase-α, -β and -d as interactors. This result suggested that Invadolysin might play a role in regulating mitochondrial function, which might then be manifest in the fat body as the defects previously observed. Energy levels are known to affect the cell cycle, cell growth, lipid metabolism and inevitably development. Further in vivo and in vitro experiments confirmed this hypothesis. Genetic crosses confirmed the interaction of invadolysin with ATP-synthase subunit-α, whilst staining of mitochondria in mutant third instar larval fat bodies suggested decreased mitochondrial activity. Mutants also showed lower ATP levels and an accumulation of reactive oxygen species, hence indicating the possibility of a dysfunctional electron transport chain. Lipid droplets are known to interact with mitochondria, whilst ATP synthase has been found on lipid droplets by proteomic studies in Drosophila. Therefore, based on these data, we propose that Invadolysin is found, with ATP synthase, on lipid droplets, where Invadolysin (likely acting as a protease) could be aiding the normal processing or assembly of ATP synthase. This interaction is vital for the proper functioning of ATP synthase, and hence mitochondria. In this scenario, cellular ATP needs are not met, energy levels drop which results in an inhibition of fatty acid synthesis, cell and organismal growth defects.
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