Abstract
ABSTRACTThe loss of function of DJ-1 caused by mutations in DJ1 causes a form of familial Parkinson's disease (PD). However, the role of DJ-1 in healthy and in PD cells is poorly understood. Even its subcellular localization in mammalian cells is uncertain, with both cytosolic and mitochondrial locations having been reported. We show here that DJ-1 is normally located in the cytoplasm in healthy Dictyostelium discoideum cells. With its unique life cycle, straightforward genotype-phenotype relationships, experimental accessibility and genetic tractability, D. discoideum offers an attractive model to investigate the roles of PD-associated genes. Furthermore, the study of mitochondrial biology, mitochondrial genome transcription and AMP-activated protein kinase-mediated cytopathologies in mitochondrial dysfunction have been well developed in this organism. Unlike mammalian systems, Dictyostelium mitochondrial dysfunction causes a reproducible and readily assayed array of aberrant phenotypes: defective phototaxis, impaired growth, normal rates of endocytosis and characteristic defects in multicellular morphogenesis. This makes it possible to study whether the underlying cytopathological mechanisms of familial PD involve mitochondrial dysfunction. DJ-1 has a single homologue in the Dictyostelium genome. By regulating the expression level of DJ-1 in D. discoideum, we show here that in unstressed cells, DJ-1 is required for normal rates of endocytic nutrient uptake (phagocytosis and, to a lesser extent, pinocytosis) and thus growth. Reduced expression of DJ-1 had no effect on phototaxis in the multicellular migratory ‘slug’ stage of the life cycle, but resulted in thickened stalks in the final fruiting bodies. This pattern of phenotypes is distinct from that observed in Dictyostelium to result from mitochondrial dyfunction. Direct measurement of mitochondrial respiratory function in intact cells revealed that DJ-1 knockdown stimulates whereas DJ-1 overexpression inhibits mitochondrial activity. Together, our results suggest positive roles for DJ-1 in endocytic pathways and loss-of-function cytopathologies that are not associated with impaired mitochondrial function.
Highlights
Attention was first drawn to the role of DJ-1 in Parkinson’s disease (PD) with the report by Bonifati et al (2003) of the first two DJ-1related PD cases, involving a 14 kb deletion from a Dutch kindred and an L166P mutation from an Italian background
Dictyostelium DJ-1 is localized in the cytoplasm, where its loss causes cytopathological defects in two endocytic pathways, phagocytosis and macropinocytosis, that are unaffected by mitochondrial dysfunction
The results showed that DJ-1 does not have a characteristic mitochondrial targeting signal in the form of a positively charged amphipathic helix at its N-terminus and is unlikely to be localized to the mitochondria, while Predotar predicted that an endoplasmic reticulum (ER) location might be possible (Fig. S1 and Table S1)
Summary
Attention was first drawn to the role of DJ-1 in Parkinson’s disease (PD) with the report by Bonifati et al (2003) of the first two DJ-1related PD cases, involving a 14 kb deletion from a Dutch kindred and an L166P mutation from an Italian background. DJ1 was shown to interact with other PDlinked genes, such as PINK1 and α-synuclein, which have been reported to cause PD through mitochondrial dysfunction (Beilina et al, 2005; Dev et al, 2003). These interactions imply that DJ-1 might function as an indirect regulator in PD. The subcellular localization of DJ-1 has been very controversial, because the cytoplasm, nucleus and mitochondria have all been proposed as its site of residence in the cell (Bonifati et al, 2003; Canet-Avilés et al, 2004; Zhang et al, 2005; Junn et al, 2009)
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