Activation of Ca2+ phosphatase Calcineurin regulates Parkin translocation to mitochondria and mitophagy in flies.

Selective removal of dysfunctional mitochondria via autophagy is crucial for the maintenance of cellular homeostasis. This event is initiated by the translocation of the E3 ubiquitin ligase Parkin to damaged mitochondria, and it requires the Serine/Threonine-protein kinase PINK1. In a coordinated set of events, PINK1 operates upstream of Parkin in a linear pathway that leads to the phosphorylation of Parkin, Ubiquitin, and Parkin mitochondrial substrates, to promote ubiquitination of outer mitochondrial membrane proteins. Ubiquitin-decorated mitochondria are selectively recruiting autophagy receptors, which are required to terminate the organelle via autophagy. In this work, we show a previously uncharacterized molecular pathway that correlates the activation of the Ca2+-dependent phosphatase Calcineurin to Parkin translocation and Parkin-dependent mitophagy. Calcineurin downregulation or genetic inhibition prevents Parkin translocation to CCCP-treated mitochondria and impairs stress-induced mitophagy, whereas Calcineurin activation promotes Parkin mitochondrial recruitment and basal mitophagy. Calcineurin interacts with Parkin, and promotes Parkin translocation in the absence of PINK1, but requires PINK1 expression to execute mitophagy in MEF cells. Genetic activation of Calcineurin in vivo boosts basal mitophagy in neurons and corrects locomotor dysfunction and mitochondrial respiratory defects of a Drosophila model of impaired mitochondrial functions. Our study identifies Calcineurin as a novel key player in the regulation of Parkin translocation and mitophagy.

USP8 Down-Regulation Promotes Parkin-Independent Mitophagy in the Drosophila Brain and in Human Neurons.

Stress-induced mitophagy, a tightly regulated process that targets dysfunctional mitochondria for autophagy-dependent degradation, mainly relies on two proteins, PINK1 and Parkin, which genes are mutated in some forms of familiar Parkinson's Disease (PD). Upon mitochondrial damage, the protein kinase PINK1 accumulates on the organelle surface where it controls the recruitment of the E3-ubiquitin ligase Parkin. On mitochondria, Parkin ubiquitinates a subset of mitochondrial-resident proteins located on the outer mitochondrial membrane, leading to the recruitment of downstream cytosolic autophagic adaptors and subsequent autophagosome formation. Importantly, PINK1/Parkin-independent mitophagy pathways also exist that can be counteracted by specific deubiquitinating enzymes (DUBs). Down-regulation of these specific DUBs can presumably enhance basal mitophagy and be beneficial in models in which the accumulation of defective mitochondria is implicated. Among these DUBs, USP8 is an interesting target because of its role in the endosomal pathway and autophagy and its beneficial effects, when inhibited, in models of neurodegeneration. Based on this, we evaluated autophagy and mitophagy levels when USP8 activity is altered. We used genetic approaches in D. melanogaster to measure autophagy and mitophagy in vivo and complementary in vitro approaches to investigate the molecular pathway that regulates mitophagy via USP8. We found an inverse correlation between basal mitophagy and USP8 levels, in that down-regulation of USP8 correlates with increased Parkin-independent mitophagy. These results suggest the existence of a yet uncharacterized mitophagic pathway that is inhibited by USP8.

Mitochondrial autophagy in the sleeping brain.

A significant percentage of the mitochondrial mass is replaced on a daily basis via mechanisms of mitochondrial quality control. Through mitophagy (a selective type of autophagy that promotes mitochondrial proteostasis) cells keep a healthy pool of mitochondria, and prevent oxidative stress and inflammation. Furthermore, mitophagy helps adapting to the metabolic demand of the cells, which changes on a daily basis. Core components of the mitophagy process are PINK1 and Parkin, which mutations are linked to Parkinson's Disease. The crucial role of PINK1/Parkin pathway during stress-induced mitophagy has been extensively studied in vitro in different cell types. However, recent advances in the field allowed discovering that mitophagy seems to be only slightly affected in PINK1 KO mice and flies, putting into question the physiological relevance of this pathway in vivo in the whole organism. Indeed, several cell-specific PINK1/Parkin-independent mitophagy pathways have been recently discovered, which appear to be activated under physiological conditions such as those that promote mitochondrial proteome remodeling during differentiation or in response to specific physiological stimuli. In this Mini Review we want to summarize the recent advances in the field, and add another level of complexity by focusing attention on a potentially important aspect of mitophagy regulation: the implication of the circadian clock. Recent works showed that the circadian clock controls many aspects of mitochondrial physiology, including mitochondrial morphology and dynamic, respiratory activity, and ATP synthesis. Furthermore, one of the essential functions of sleep, which is controlled by the clock, is the clearance of toxic metabolic compounds from the brain, including ROS, via mechanisms of proteostasis. Very little is known about a potential role of the clock in the quality control mechanisms that maintain the mitochondrial repertoire healthy during sleep/wake cycles. More importantly, it remains completely unexplored whether (dys)function of mitochondrial proteostasis feedbacks to the circadian clockwork.

Pathways of flower infection and pollen-mediated dispersion of Pseudomonas syringae pv. actinidiae, the causal agent of kiwifruit bacterial canker.

Flowers can provide a protected and nutrient-rich environment to the epiphytic microflora, thus representing a sensible entry point for pathogens such as Pseudomonas syringae pv. actinidiae (Psa). This bacterium can colonize both male and female Actinidia flowers, causing flower browning and fall, and systemic invasion of the host plant, eventually leading to its death. However, the process of flower colonization and penetration into the host tissues has not yet been fully elucidated. In addition, the presence of Psa in the pollen from infected flowers, and the role of pollination in the spread of Psa requires confirmation. The present study employed a Psa strain constitutively expressing the fluorescent GFPuv protein, to visualize in vivo flower colonization. Microscopy observations were performed by means of confocal laser scanning and wide-field fluorescent microscopy, and were coupled with the study of Psa population dynamics by quantitative PCR (q-PCR). The pathogen was shown to colonize stigmata, move along the stylar furrow, and penetrate the receptacles via the style or nectarhodes. Once the receptacle was invaded, the pathogen migrated along the flower pedicel and became systemic. Psa was also able to colonize the anthers epiphytically and endophytically. Infected male flowers produced contaminated pollen, which could transmit Psa to healthy plants. Finally, pollinators (Apis mellifera and Bombus terrestris) were studied in natural conditions, showing that, although they can be contaminated with Psa, the pathogen's transmission via pollinators is contrasted by its short survival in the hive.

Role of Metcalfa pruinosa as a Vector for Pseudomonas syringae pv. actinidiae.

After 20 years of steady increase, kiwifruit industry faced a severe arrest due to the pandemic spread of the bacterial canker, caused by Pseudomonas syringae pv. actinidiae (Psa). The bacterium penetrates the host plant primarily via natural openings or wounds, and its spread is mainly mediated by atmospheric events and cultural activities. Since the role of sucking insects as vectors of bacterial pathogens is widely documented, we investigated the ability of Metcalfa pruinosa Say (1830), one of the most common kiwifruit pests, to transmit Psa to healthy plants in laboratory conditions. Psa could be isolated both from insects feeding over experimentally inoculated plants, and from insects captured in Psa-infected orchards. Furthermore, insects were able to transmit Psa from experimentally inoculated plants to healthy ones. In conclusion, the control of M. pruinosa is recommended in the framework of protection strategies against Psa.