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Mitochondrial dysfunction and aberrant mitochondrial homeostasis are key aspects of Parkinson’s disease (PD) pathophysiology. Mutations in PINK1 and Parkin proteins lead to autosomal recessive PD, suggesting that defective mitochondrial clearance via mitophagy is key in PD etiology. Accelerating the identification and/or removal of dysfunctional mitochondria could therefore provide a disease-modifying approach to treatment. To that end, we performed a high-content phenotypic screen (HCS) of ~125,000 small molecules to identify compounds that positively modulate mitochondrial accumulation of the PINK1-Parkin-dependent mitophagy initiation marker p-Ser65-Ub in Parkin haploinsufficiency (Parkin +/R275W) human fibroblasts. Following confirmatory counter-screening and orthogonal assays, we selected compounds of interest that enhance mitophagy-related biochemical and functional endpoints in patient-derived fibroblasts. Identification of inhibitors of the ubiquitin-specific peptidase and negative regulator of mitophagy USP30 within our hits further validated our approach. The compounds identified provide a novel starting point for further investigation and optimisation.

Protein misfolding has a key role in several neurological disorders including Parkinson's disease. Although a clear mechanism for such proteinopathic diseases is well established when aggregated proteins accumulate in the cytosol, cell nucleus, endoplasmic reticulum and extracellular space, little is known about the role of protein aggregation in the mitochondria. Here we show that mutations in both human and fly PINK1 result in higher levels of misfolded components of respiratory complexes and increase in markers of the mitochondrial unfolded protein response. Through the development of a genetic model of mitochondrial protein misfolding employing Drosophila melanogaster, we show that the in vivo accumulation of an unfolded protein in mitochondria results in the activation of AMP-activated protein kinase-dependent autophagy and phenocopies of pink1 and parkin mutants. Parkin expression acts to clear mitochondria with enhanced levels of misfolded proteins by promoting their autophagic degradation in vivo, and refractory to Sigma P (ref(2)P), the Drosophila orthologue of mammalian p62, is a critical downstream effector of this quality control pathway. We show that in flies, a pathway involving pink1, parkin and ref(2)P has a role in the maintenance of a viable pool of cellular mitochondria by promoting organellar quality control.

The conventional treatment of cancer relies upon radiotherapy and chemotherapy. Such treatments supposedly mediate their effects via the direct elimination of tumor cells. Nonetheless, there are circumstances in which conventional anti-cancer therapy can induce a modality of cellular demise that elicits innate and cognate immune responses, which in turn mediate part of the anti-tumor effect. Although different chemotherapeutic agents may kill tumor cells through an apparently homogeneous apoptotic pathway, they differ in their capacity to stimulate immunogenic cell death. We discovered that the pre-apoptotic translocation of intracellular calreticulin (endo-CRT) to the plasma membrane surface (ecto-CRT) is critical for the recognition and engulfment of dying tumor cells by dendritic cells. Thus, anthracyclines and gamma-irradiation that induce ecto-CRT cause immunogenic cell death, while other pro-apoptotic agents (such as mitomycin C and etoposide) induce neither ecto-CRT nor immunogenic cell death. Depletion of CRT abolishes the immunogenicity of cell death elicited by anthracyclines, while exogenous supply of CRT or enforcement of CRT exposure by pharmacological agents that favor CRT translocation can enhance the immunogenicity of cell death. For optimal anti-tumor vaccination and immunogenic chemotherapy, the same cells have to expose ecto-CRT and to succumb to apoptosis; if these events affect different cells, no anti-tumor immune response is elicited. These results may have far reaching implications for tumor immunology because (i) ecto-CRT exposure by tumor cells allows for the prediction of therapeutic outcome and because (ii) the re-establishment of ecto-CRT may ameliorate the efficacy of chemotherapy.

In contrast to prior belief, tumor cell apoptosis is not necessarily silent but can be immunogenic. By tracing how anthracyclines and γ-irradiation trigger immunogenic cell deaths, we found that they were causally connected to the exposure of calreticulin on the tumor cell surface, before apoptosis in the tumor cell itself occurred. Furthermore, we showed that calreticulin exposure was necessary and sufficient to increase proimmunogenic killing by other chemotherapies. Our findings suggest that calreticulin could serve as a biomarker to predict therapy-associated immune responses, and that tactics to expose calreticulin might improve the clinical efficacy of many cancer therapies. [Cancer Res 2007;67(17):7941–4]

Damage or stress in many organelles may trigger apoptosis by several not yet fully elucidated mechanisms. A cell death pathway is induced by endoplasmic reticulum (ER) stress elicited by the unfolded protein response and/or by aberrant Ca(2+) signalling. Reticulon-1C (RTN-1C) belongs to the reticulon family, neuroendocrine-specific proteins localized primarily on the ER membrane. In the present study, we demonstrate that RTN-1C is able to modulate, in a mutually exclusive way, the cellular sensitivity to different apoptosis pathways in human neuroblastoma cells. In fact, the increase of RTN-1C protein levels per se results in ER stress-induced cell death, mediated by an increase of cytosolic Ca(2+), and significantly sensitizes cells to different ER stress inducers. In line with these findings, the reduction of RTN-1C, by antisense DNA expression, reduced the sensitivity to ER-stressors. In the presence of high RTN-1C levels, genotoxic drugs become ineffective as a consequence of the cytoplasm translocation of p53 protein, while the silencing of endogenous RTN-1C results in the potentiation of the genotoxic drugs action. These data indicate that RTN-1C is able to modulate the cellular sensitivity to different apoptotic pathways representing a promising molecular target for new drug development.

Apoptotic and autophagic cell death have been implicated, on the basis of morphological and biochemical criteria, in neuronal loss occurring in neurodegenerative diseases and it has been shown that they may overlap. We have studied the relationship between apoptosis and autophagic cell death in cerebellar granule cells (CGCs) undergoing apoptosis following serum and potassium deprivation. We found that apoptosis is accompanied by an early and marked proliferation of autophagosomal-lysosomal compartments as detected by electron microscopy and immunofluorescence analysis. Autophagy is blocked by hrIGF-1 and forskolin, two well-known inhibitors of CGC apoptosis, as well as by adenovirus-mediated overexpression of Bcl-2. 3-Methyladenine (3-MA) an inhibitor of autophagy, not only arrests this event but it also blocks apoptosis. The neuroprotective effect of 3-MA is accompanied by block of cytochrome c (cyt c) release in the cytosol and by inhibition of caspase-3 activation which, in turn, appears to be mediated by cathepsin B, as CA074-Me, a selective inhibitor of this enzyme, fully blocks the processing of pro-caspase-3. Immunofluorescence analysis demonstrated that cathepsin B, normally confined inside the lysosomal-endosomal compartment, is released during apoptosis into the cytosol where this enzyme may act as an execution protease. Collectively, these observations indicate that autophagy precedes and is causally connected with the subsequent onset of programmed death.

Mutations in PINK1 cause early-onset Parkinson's disease (PD). Studies in Drosophila melanogaster have highlighted mitochondrial dysfunction on loss of Pink1 as a central mechanism of PD pathogenesis. Here we show that global analysis of transcriptional changes in Drosophila pink1 mutants reveals an upregulation of genes involved in nucleotide metabolism, critical for neuronal mitochondrial DNA synthesis. These key transcriptional changes were also detected in brains of PD patients harbouring PINK1 mutations. We demonstrate that genetic enhancement of the nucleotide salvage pathway in neurons of pink1 mutant flies rescues mitochondrial impairment. In addition, pharmacological approaches enhancing nucleotide pools reduce mitochondrial dysfunction caused by Pink1 deficiency. We conclude that loss of Pink1 evokes the activation of a previously unidentified metabolic reprogramming pathway to increase nucleotide pools and promote mitochondrial biogenesis. We propose that targeting strategies enhancing nucleotide synthesis pathways may reverse mitochondrial dysfunction and rescue neurodegeneration in PD and, potentially, other diseases linked to mitochondrial impairment.

Mutations in PINK1, a mitochondrially targeted serine/threonine kinase, cause autosomal recessive Parkinson's disease (PD). Substantial evidence indicates that PINK1 acts with another PD gene, parkin, to regulate mitochondrial morphology and mitophagy. However, loss of PINK1 also causes complex I (CI) deficiency, and has recently been suggested to regulate CI through phosphorylation of NDUFA10/ND42 subunit. To further explore the mechanisms by which PINK1 and Parkin influence mitochondrial integrity, we conducted a screen in Drosophila cells for genes that either phenocopy or suppress mitochondrial hyperfusion caused by pink1 RNAi. Among the genes recovered from this screen was ND42. In Drosophila pink1 mutants, transgenic overexpression of ND42 or its co-chaperone sicily was sufficient to restore CI activity and partially rescue several phenotypes including flight and climbing deficits and mitochondrial disruption in flight muscles. Here, the restoration of CI activity and partial rescue of locomotion does not appear to have a specific requirement for phosphorylation of ND42 at Ser-250. In contrast to pink1 mutants, overexpression of ND42 or sicily failed to rescue any Drosophila parkin mutant phenotypes. We also find that knockdown of the human homologue, NDUFA10, only minimally affecting CCCP-induced mitophagy, and overexpression of NDUFA10 fails to restore Parkin mitochondrial-translocation upon PINK1 loss. These results indicate that the in vivo rescue is due to restoring CI activity rather than promoting mitophagy. Our findings support the emerging view that PINK1 plays a role in regulating CI activity separate from its role with Parkin in mitophagy., Author Summary Two genes linked to heritable forms of the neurodegenerative movement disorder Parkinson's disease (PD), PINK1 and parkin, play important roles in mitochondrial homeostasis through mechanisms which include the degradation of dysfunctional mitochondria, termed mitophagy, and the maintenance of complex I (CI) activity. Here we report the findings of an RNAi based screen in Drosophila cells for genes that may regulate the PINK1-Parkin pathway which identified NDUFA10 (ND42 in Drosophila), a subunit of CI. Using a well-established cellular system and in vivo Drosophila genetics, we demonstrate that while NDUFA10/ND42 only plays a minimal role in mitophagy, restoration of CI activity through overexpression of either ND42 or its co-chaperone sicily is able to substantially rescue behavioral deficits in pink1 mutants but not parkin mutants. Moreover, while parkin overexpression is known to rescue pink1 mutants, it apparently achieves this without restoring CI activity. These results suggest that increasing CI activity or promoting mitophagy can be beneficial in pink1 mutants, and further highlights separable functions of PINK1 and Parkin.

International audience; Cristae are mitochondrial inner-membrane structures that concentrate respiratory chain complexes and hence regulate ATP production. Mechanisms controlling crista morphogenesis are poorly understood and few crista determinants have been identified. Among them are the Mitofilins that are required to establish crista junctions and ATP-synthase subunits that bend the membrane at the tips of the cristae. We report here the phenotypic consequences associated with the in vivo inactivation of the inner-membrane protein Pantagruelian Mitochondrion I (PMI) both at the scale of the whole organism, and at the level of mitochondrial ultrastructure and function. We show that flies in which PMI is genetically inactivated experience synaptic defects and have a reduced life span. Electron microscopy analysis of the inner-membrane morphology demonstrates that loss of PMI function increases the average length of mitochondrial cristae in embryonic cells. This phenotype is exacerbated in adult neurons in which cristae form a dense tangle of elongated membranes. Conversely, we show that PMI overexpression is sufficient to reduce crista length in vivo. Finally, these crista defects are associated with impaired respiratory chain activity and increases in the level of reactive oxygen species. Since PMI and its human orthologue TMEM11 are regulators of mitochondrial morphology, our data suggest that, by controlling crista length, PMI influences mitochondrial diameter and tubular shape.

The human brain is a highly complex organ with remarkable energy demands. Although it represents only 2% of the total body weight, it accounts for 20% of all oxygen consumption, reflecting its high rate of metabolic activity. Mitochondria have a crucial role in the supply of energy to the brain. Consequently, their deterioration can have important detrimental consequences on the function and plasticity of neurons, and is thought to have a pivotal role in ageing and in the pathogenesis of several neurological disorders. Owing to their inherent physiological functions, mitochondria are subjected to particularly high levels of stress and have evolved specific molecular quality-control mechanisms to maintain the mitochondrial components. Here, we review some of the most recent advances in the understanding of mitochondrial stress-control pathways, with a particular focus on how defects in such pathways might contribute to neurodegenerative disease.

Some chemotherapeutic agents can elicit apoptotic cancer cell death, thereby activating an anticancer immune response that influences therapeutic outcome. We previously reported that anthracyclins are particularly efficient in inducing immunogenic cell death, correlating with the pre-apoptotic exposure of calreticulin (CRT) on the plasma membrane surface of anthracyclin-treated tumor cells. Here, we investigated the role of cellular Ca(2+) homeostasis on CRT exposure. A neuroblastoma cell line (SH-SY5Y) failed to expose CRT in response to anthracyclin treatment. This defect in CRT exposure could be overcome by the overexpression of Reticulon-1C, a manipulation that led to a decrease in the Ca(2+) concentration within the endoplasmic reticulum lumen. The combination of Reticulon-1C expression and anthracyclin treatment yielded more pronounced endoplasmic reticulum Ca(2+) depletion than either of the two manipulations alone. Chelation of intracellular (and endoplasmic reticulum) Ca(2+), targeted expression of the ligand-binding domain of the IP(3) receptor and inhibition of the sarco-endoplasmic reticulum Ca(2+)-ATPase pump reduced endoplasmic reticulum Ca(2+) load and promoted pre-apoptotic CRT exposure on the cell surface, in SH-SY5Y and HeLa cells. These results provide evidence that endoplasmic reticulum Ca(2+) levels control the exposure of CRT.

The endoplasmic reticulum (ER) is the cellular site of polypeptide folding and modification. When these processes are hampered, an unfolded protein response (UPR) is activated. If the damage is too broad, the mammalian UPR launches the apoptotic program. As a consequence, mobilization of ER calcium stores sensitizes mitochondria to direct proapoptotic stimuli. We make use of a mouse Apaf1-deficient cell system of proneural origin to understand the roles played in this context by the apoptosome, the most studied apoptotic machinery along the mitochondrial pathway of death. We show here that in the absence of the apoptosome ER stress induces cytochrome c release from the mitochondria but that apoptosis cannot occur. Under these circumstances, Grp78/BiP and GADD153/CHOP, both hallmarks of UPR, are canonically up-regulated, and calcium is properly released from ER stores. We also demonstrate that caspase 12, a protease until now believed to play a central role in the initiation of ER stress-induced cell death in the mouse system, is dispensable for the mitochondrial pathway of death to take place.

Autophagy is a critical regulator of organellar homeostasis, particularly of mitochondria. Upon the loss of membrane potential, dysfunctional mitochondria are selectively removed by autophagy through recruitment of the E3 ligase Parkin by the PTEN-induced kinase 1 (PINK1) and subsequent ubiquitination of mitochondrial membrane proteins. Mammalian sequestrome-1 (p62/SQSTM1) is an autophagy adaptor, which has been proposed to shuttle ubiquitinated cargo for autophagic degradation downstream of Parkin. Here, we show that loss of ref(2)P, the Drosophila orthologue of mammalian P62, results in abnormalities, including mitochondrial defects and an accumulation of mitochondrial DNA with heteroplasmic mutations, correlated with locomotor defects. Furthermore, we show that expression of Ref(2)P is able to ameliorate the defects caused by loss of Pink1 and that this depends on the presence of functional Parkin. Finally, we show that both the PB1 and UBA domains of Ref(2)P are crucial for mitochondrial clustering. We conclude that Ref(2)P is a crucial downstream effector of a pathway involving Pink1 and Parkin and is responsible for the maintenance of a viable pool of cellular mitochondria by promoting their aggregation and autophagic clearance.