Mitochondria are critical organelles in the process of survival at the cellular level. Mitochondria accumulate damage over time to become dysfunctional and contribute to the process of ageing and eventually to the death of an organism [1]. Mitochondria are known to be the powerhouse of the cell, but its role goes beyond the production of ATP (oxidative phosphorylation); it is responsible for various aspects of energy homeostasis, oxidative stress, calcium handling, cell signalling, and cell survival [2]. The central role of mitochondria in energy regulation and signalling implies that its dysfunction would have devastating effects upon cellular functions. Such is especially true for the nervous system because subtle changes in signalling can have catastrophic consequences leading to neurodegenerative disease.
The shape and size of mitochondria are not fixed properties; mitochondrial morphology depends upon a number of factors, including stage of the cell cycle and cell type, and can change quite quickly in response to external stimuli or metabolic cues. Changes to the mitochondrial network seem to differentially influence a number of signalling pathways [3]. Therefore, mitochondria undergo fission and fusion frequently in order to change their structure in response to the specific requirements of the cell, under a wide range of circumstances. Mitochondrial fission allows segregation of damaged mitochondria components while the process of fusion facilitates the exchange of mitochondrial material vital to maintain homeostasis within the mitochondrial network. Mitochondrial fusion helps compromised mitochondria, with highly damaged DNA and protein, to actively exchange components with other more healthy mitochondria to decrease the severity of heteroplasmy, and help with functional complementation [4, 5]. Mitochondrial fission allows for the segregation of irreversibly damaged portions of the mitochondrial network and subsequent degradation. Mitochondrial fission necessarily requires dynamin-related protein 1 (Drp1) and FIS1 [6]. Nevertheless, an explicit understanding of the factors promoting fission and fusion remains limited.
The Drp1 gene encodes a dynamin family GTPase protein comprised of a characteristic Dynamin like protein family domain (Dynamin and Mx protein domains), a dynamin central domain, and a dynamin GTPase effector domain. While predominantly located in the cytoplasm, a small fraction is located upon the cytoplasmic surface of the mitochondrial tubules. Overexpression of Drp1 causes mitochondrial fragmentation, whereas inhibition results in the elongation of the mitochondrial network. Drp1 protein function is regulated by post-translational modification via phosphorylation, where a well-documented phosphorylation site, S616, promotes fission through an increase in activity. In contrast, phosphorylation of another site, S637, acts to lessen fission through reduced activity [7, 8, 9]. Drp1 polymerizes to form a spiral structure around the mitochondrial tubule, and then utilizes its GTPase activity to constrict the tubule and eventually cause fragmentation of the mitochondria [10, 11, 12 13]. Presumably, a low membrane potential promotes fission, while a high membrane potential obstructs fission [14]. Through its role in mitochondrial fission, Drp1 controls mitochondrial morphology and function.
The role of Drp1 protein is not limited to mitochondrial fission. In addition, it participates in peroxisomal fragmentation [15] and maintains the morphology and function of the Endoplasmic Reticulum (ER) [16, 17]. Also, Drp1 is required for a standard rate of cytochrome c release and caspase activation during programmed cell death [18]. The role of Drp1 in apoptosis is not clear, but the product of the Bax gene, a pro-apoptotic Bcl-2 family protein, has been found to co-localize with Drp1 at the site of mitochondrial fission [19]. Drp1 plays role in Bcl-2 regulated apoptosis, peroxisomal and ER fragmentation.
The Drp1 protein interacts with other proteins involved in a number of mitochondrial processes, such as the products of Pink1 (PTEN-induced putative kinase 1) and park [20]. Mutation of the Pink1 and park genes are among the most prominent causes of early onset PD. The roles of Pink1 and park are vital to ubiquitin-dependent mitophagy. The protein product of park is involved in the process of ubiquitinylation that plays a crucial role in the proteasomal directed degradation of proteins, such as Drp1 [21]. Activated Akt can increase Drp1 phosphorylation and its localization to the mitochondria, to promote “mitofission” with an accompanying increase in the generation of reactive oxygen species [22]. Drp1 knockouts increase nuclear translocation of transcription factor foxo and enhance the expression of its downstream targets [23]. Akt protein inhibits the nuclear translocation of foxo [24, 25] and regulates Pink1/park dependent mitophagy [26]. Foxo transcriptionally upregulates the Pink1/park pathway in mammals and Pink1 appears to function upstream of foxo in D. melanogaster [27, 28]. Such a feedback mechanism implies a complex relationship between Drp1 and park, Akt1 and foxo.
Here, we propose that gain-of-function of Drp1 results in a Parkinsonian-like phenotype. We use drosophila to model Parkinson disease because it is an excellent model system in which to study interactions between genes, including PD genes [29, 30]. The inhibition of Drp1 in the dopaminergic neurons of a mouse MPTP model of PD gives protection against mitochondrial translocation of p53 and the loss of dopaminergic neurons [31]. In our experiment, we used the UAS-Gal4 system to direct and to inhibit the expression of the Drp1 gene in Drosophila. The GMR-GAL4 transgene directs expression in the developing eye [32] and the Ddc-Gal4 transgene directs expression in dopaminergic and serotonergic neurons [33, 34]. Overexpression of Drp1 has toxic effects; although, its inhibition slightly improves the lifespan, the climbing ability over time is compromised. In an established park-RNAi model of PD [35], we directed and inhibited the expression of the Drp1 gene. The PD -like phenotypes of Ddc-Gal4 park RNAi were rescued by the expression of Drp1-RNAi transgenes. The strategy is to identify the basic mechanism in simple model organism and then further validate the finding in mammalian model organism.