Changes in Drosophila mitochondrial proteins following chaperone-mediated lifespan extension confirm a role of Hsp22 in mitochondrial UPR and reveal a mitochondrial localization for cathepsin D

Mitochondrial unfolded protein response mechanism and its cardiovascular protective effects

The mitochondrial unfolded protein response (UPRmt) is a cytoprotective response in response to cellular stress that is activated in response to mitochondrial stress to maintain intra-protein homeostasis, thereby protecting the cell from a variety of stimuli. The activation of this response has been linked to cardiovascular diseases. Here, we reviewed the current understanding of UPRmt and discussed its specific molecular mechanism, mainly in mammals, as well as addressing its protective role against cardiovascular diseases, so as to provide direction for further research on UPRmt and therapies targeting cardiovascular diseases in the future.

Identification of a mitochondrial targeting sequence in cathepsin D and its localization in mitochondria

Cathepsin D (CTSD) is a major lysosomal protease harboring an N-terminal signal peptide (amino acids 1-20) to enable vesicular transport from endoplasmic reticulum to lysosomes. Here, we report the possibility of a mitochondrial targeting sequence and mitochondrial localization of CTSD in cells. Live-cell imaging analysis with C-terminal enhanced green fluorescent protein-tagged CTSD (EGFP-CTSD) indicated that CTSD localizes to mitochondria. CTSD amino acids 21-35 are responsible for its mitochondrial localization, which exhibit typical features of mitochondrial targeting sequences, and are evolutionarily conserved. A proteinase K protection assay and sucrose gradient analysis showed that a small population of endogenous CTSD molecules exists in mitochondria. These results suggest that CTSD is a dual-targeted protein that may localize in both lysosomes and mitochondria.

Insight into the mitochondrial unfolded protein response and cancer: opportunities and challenges

The mitochondrial unfolded protein response (UPR^mt) is an evolutionarily conserved protective transcriptional response that maintains mitochondrial proteostasis by inducing the expression of mitochondrial chaperones and proteases in response to various stresses. The UPR^mt-mediated transcriptional program requires the participation of various upstream signaling pathways and molecules. The factors regulating the UPR^mt in Caenorhabditis elegans (C. elegans) and mammals are both similar and different. Cancer cells, as malignant cells with uncontrolled proliferation, are exposed to various challenges from endogenous and exogenous stresses. Therefore, in cancer cells, the UPR^mt is hijacked and exploited for the repair of mitochondria and the promotion of tumor growth, invasion and metastasis. In this review, we systematically introduce the inducers of UPR^mt, the biological processes in which UPR^mt participates, the mechanisms regulating the UPR^mt in C. elegans and mammals, cross-tissue signal transduction of the UPR^mt and the roles of the UPR^mt in promoting cancer initiation and progression. Disrupting proteostasis in cancer cells by targeting UPR^mt constitutes a novel anticancer therapeutic strategy.

Combined Transcriptomic and Proteomic Analysis of Perk Toxicity Pathways

In Drosophila, endoplasmic reticulum (ER) stress activates the protein kinase R-like endoplasmic reticulum kinase (dPerk). dPerk can also be activated by defective mitochondria in fly models of Parkinson's disease caused by mutations in pink1 or parkin. The Perk branch of the unfolded protein response (UPR) has emerged as a major toxic process in neurodegenerative disorders causing a chronic reduction in vital proteins and neuronal death. In this study, we combined microarray analysis and quantitative proteomics analysis in adult flies overexpressing dPerk to investigate the relationship between the transcriptional and translational response to dPerk activation. We identified tribbles and Heat shock protein 22 as two novel Drosophila activating transcription factor 4 (dAtf4) regulated transcripts. Using a combined bioinformatics tool kit, we demonstrated that the activation of dPerk leads to translational repression of mitochondrial proteins associated with glutathione and nucleotide metabolism, calcium signalling and iron-sulphur cluster biosynthesis. Further efforts to enhance these translationally repressed dPerk targets might offer protection against Perk toxicity.

Structural and functional properties of proteins interacting with small heat shock proteins

Small heat shock proteins (sHsps) are ubiquitous molecular chaperones found in all domains of life, possessing significant roles in protein quality control in cells and assisting the refolding of non-native proteins. They are efficient chaperones against many in vitro protein substrates. Nevertheless, the in vivo native substrates of sHsps are not known. To better understand the functions of sHsps and the mechanisms by which they enhance heat resistance, sHsp-interacting proteins were identified using affinity purification under heat shock conditions. This paper aims at providing some insights into the characteristics of natural substrate proteins of sHsps. It seems that sHsps of prokaryotes, as well as sHsps of some eukaryotes, can bind to a wide range of substrate proteins with a preference for certain functional classes of proteins. Using Drosophila melanogaster mitochondrial Hsp22 as a model system, we observed that this sHsp interacted with the members of ATP synthase machinery. Mechanistically, Hsp22 interacts with the multi-type substrate proteins under heat shock conditions as well as non-heat shock conditions.

Valosin-Containing Protein, a Calcium-Associated ATPase Protein, in Endoplasmic Reticulum and Mitochondrial Function and Its Implications for Diseases

Endoplasmic reticulum (ER) and mitochondrion are the key organelles in mammal cells and play crucial roles in a variety of biological functions in both physiological and pathological conditions. Valosin-containing protein (VCP), a newly identified calcium-associated ATPase protein, has been found to be involved in both ER and mitochondrial function. Impairment of VCP, caused by structural mutations or alterations of expressions, contributes to the development of various diseases, through an integrating effect on ER, mitochondria and the ubiquitin–proteasome system, by interfering with protein degradation, subcellular translocation and calcium homeostasis. Thus, understanding the role and the molecular mechanisms of VCP in these organelles brings new insights to the pathogenesis of the associated diseases, and leads to the discovery of new therapeutic strategies. In this review, we summarized the progress of studies on VCP, in terms of its regulation of ER and mitochondrial function and its implications for the associated diseases, focusing on the cancers, heart disease, and neurodegenerative disorders.

The Mitochondrial Small Heat Shock Protein HSP22 from Pea is a Thermosoluble Chaperone Prone to Co-Precipitate with Unfolding Client Proteins

The small heat shock proteins (sHSPs) are molecular chaperones that share an alpha-crystallin domain but display a high diversity of sequence, expression, and localization. They are especially prominent in plants, populating most cellular compartments. In pea, mitochondrial HSP22 is induced by heat or oxidative stress in leaves but also strongly accumulates during seed development. The molecular function of HSP22 was addressed by studying the effect of temperature on its structural properties and chaperone effects using a recombinant or native protein. Overexpression of HSP22 significantly increased bacterial thermotolerance. The secondary structure of the recombinant protein was not affected by temperature in contrast with its quaternary structure. The purified protein formed large polydisperse oligomers that dissociated upon heating (42 °C) into smaller species (mainly monomers). The recombinant protein appeared thermosoluble but precipitated with thermosensitive proteins upon heat stress in assays either with single protein clients or within complex extracts. As shown by in vitro protection assays, HSP22 at high molar ratio could partly prevent the heat aggregation of rhodanese but not of malate dehydrogenase. HSP22 appears as a holdase that could possibly prevent the aggregation of some proteins while co-precipitating with others to facilitate their subsequent refolding by disaggregases or clearance by proteases.

Identification of Genes that Control Silk Yield by RNA Sequencing Analysis of Silkworm (Bombyx mori) Strains of Variable Silk Yield

Silk is an important natural fiber of high economic value, and thus genetic study of the silkworm is a major area of research. Transcriptome analysis can provide guidance for genetic studies of silk yield traits. In this study, we performed a transcriptome comparison using multiple silkworms with different silk yields. A total of 22 common differentially expressed genes (DEGs) were identified in multiple strains and were mainly involved in metabolic pathways. Among these, seven significant common DEGs were verified by quantitative reverse transcription polymerase chain reaction, and the results coincided with the findings generated by RNA sequencing. Association analysis showed that BGIBMGA003330 and BGIBMGA005780 are significantly associated with cocoon shell weight and encode uridine nucleosidase and small heat shock protein, respectively. Functional annotation of these genes suggest that these play a role in silkworm silk gland development or silk protein synthesis. In addition, we performed principal component analysis (PCA) in combination with wild silkworm analysis, which indicates that modern breeding has a stronger selection effect on silk yield traits than domestication, and imply that silkworm breeding induces aggregation of genes related to silk yield.

Curcumin supplementation increases survival and lifespan in Drosophila under heat stress conditions

Harsh climate induces physiological stress thus compromising organismal survival. Our previous studies demonstrated that curcumin (CUR) supplementation increased survival of turtle under heat stress (HS). Here, we span this work to investigate the survival and lifespan of HS Drosophila fed a diet supplemented with CUR. For this purpose, female and male flies were fed basal diet (N) and CUR diet (0.2 mg/g), and exposed to three conditions: 25°C and 29°C continuously, and 34 °C for 2 h at days 1, 4, and 7, then kept at 25 °C. Lifespan analysis showed that, compared to N-25 °C flies, the mean lifespans of N-29 °C and N-34 °C flies were decreased significantly by 8.5-15.7% in males, and 3.7-7.9% in females. Conversely, in the CUR-supplemented diet, mean lifespans of C-29 °C and C-34 °C flies were significantly extended by 8.7-16.4% in males, and by 8.9-12.8% in females, compared to that of temperature-matched flies fed basal diets. The MDA levels of C-34 °C flies were significantly lower than those of N-34 °C flies, indicating CUR reduced oxidative stress caused by HS. Furthermore, CUR palliated the increased oxidative stress caused by HS, by increasing the expression of SOD1, CAT, and PHGPx and decreasing the expression of Hsp70 and Hsp83. Our results indicated that CUR supplementation increases the survival rate of Drosophila by enhancing thermal tolerance. © 2018 BioFactors, 44(6):577-587, 2018.

Ras-ERK-ETS inhibition alleviates neuronal mitochondrial dysfunction by reprogramming mitochondrial retrograde signaling

Mitochondrial dysfunction activates the mitochondrial retrograde signaling pathway, resulting in large scale changes in gene expression. Mitochondrial retrograde signaling in neurons is poorly understood and whether retrograde signaling contributes to cellular dysfunction or is protective is unknown. We show that inhibition of Ras-ERK-ETS signaling partially reverses the retrograde transcriptional response to alleviate neuronal mitochondrial dysfunction. We have developed a novel genetic screen to identify genes that modify mitochondrial dysfunction in Drosophila. Knock-down of one of the genes identified in this screen, the Ras-ERK-ETS pathway transcription factor Aop, alleviates the damaging effects of mitochondrial dysfunction in the nervous system. Inhibition of Ras-ERK-ETS signaling also restores function in Drosophila models of human diseases associated with mitochondrial dysfunction. Importantly, Ras-ERK-ETS pathway inhibition partially reverses the mitochondrial retrograde transcriptional response. Therefore, mitochondrial retrograde signaling likely contributes to neuronal dysfunction through mis-regulation of gene expression.

Loss of mitochondrial function activates the mitochondrial retrograde signaling pathway resulting in large scale changes in nuclear gene transcription. Very little is known about retrograde signaling in the nervous system and how the transcriptional changes affect neuronal function. Here we identify Ras-ERK-ETS signaling as a novel mitochondrial retrograde signaling pathway in the Drosophila nervous system. Inhibition of Ras-ERK-ETS signaling improves neuronal function in Drosophila models of mitochondrial disease. Targeting Ras-ERK-ETS signaling may therefore have therapeutic potential in mitochondrial disease patients. Using a transcriptomic approach, we find that inhibition of Ras-ERK-ETS signaling partially reverses the mitochondrial retrograde transcriptional response. Surprisingly therefore, the mitochondrial retrograde transcriptional response contributes to neuronal dysfunction.

Stress tolerance in diapausing embryos of Artemia franciscana is dependent on heat shock factor 1 (Hsf1)

Embryos of the crustacean, Artemia franciscana, may undergo oviparous development, forming encysted embryos (cysts) that are released from females and enter diapause, a state of suppressed metabolism and greatly enhanced stress tolerance. Diapause-destined embryos of A. franciscana synthesize three small heat shock proteins (sHsps), p26, ArHsp21 and ArHsp22, as well as artemin, a ferritin homologue, all lacking in embryos that develop directly into nauplii. Of these diapause-specific molecular chaperones, p26 and artemin are important contributors to the extraordinary stress tolerance of A. franciscana cysts, but how their synthesis is regulated is unknown. To address this issue, a cDNA for heat shock factor 1 (Hsf1), shown to encode a protein similar to Hsf1 from other organisms, was cloned from A. franciscana. Hsf1 was knocked down by RNA interference (RNAi) in nauplii and cysts of A. franciscana. Nauplii lacking Hsf1 died prematurely upon release from females, showing that this transcription factor is essential to the survival of nauplii. Diapause cysts with diminished amounts of Hsf1 were significantly less stress tolerant than cysts containing normal levels of Hsf1. Moreover, cysts deficient in Hsf1 possessed reduced amounts of p26, ArHsp21, ArHsp22 and artemin, revealing dependence on Hsf1 for expression of their genes and maximum stress tolerance. The results demonstrate an important role for Hsf1, likely in concert with other transcription factors, in the survival and growth of A. franciscana and in the developmentally regulated synthesis of proteins responsible for the stress tolerance of diapausing A. franciscana cysts.

TRAP1 regulates autophagy in lung cancer cells

BACKGROUND

Expression of TRAP1, a member of the HSP90 chaperone family, has been implicated in tumour protective effects, based on its differential mitochondrial localization and function.

DESIGN

This work was designed to provide new insights into the pathways involved in TRAP1-provided cytoprotection on NSCLC. For this, TRAP1-depleted A549 human NSCLC cells and MRC-5 normal lung fibroblasts were produced using a siRNA approach and main cellular quality control mechanisms were investigated.

RESULTS

TRAP1-depleted A549 cells displayed decreased cell viability likely due to impaired mitochondrial function including decreased ATP/AMP ratio, oxygen consumption and membrane potential, as well as increased apoptotic indicators. Furthermore, the negative impact of TRAP1 depletion on mitochondrial function was not observed in normal MRC-5 lung cells, which might be due to the differential intracellular localization of the chaperone in tumour versus normal cells. Additionally, A549 TRAP1-depleted cells showed increased autophagic flux. Functionally, autophagy inhibition resulted in decreased cell viability in both TRAP1-expressing and TRAP1-depleted tumour cells with minor effects on MRC-5 cells. Conversely, autophagy stimulation decreased cell viability of both A549 and MRC-5 TRAP1-expressing cells while in A549 TRAP1-depleted cells, increased autophagy augmented viability.

CONCLUSIONS

Our results show that even though TRAP1 depletion affects both normal MRC-5 and tumour A549 cell proliferation, inhibition of autophagy per se led to a decrease in tumour cell mass, while having a reduced effect on the normal cell line. The strategy of targeting TRAP1 in NSCLC shows future potential therapeutic applications.

Identification of proteins interacting with the mitochondrial small heat shock protein Hsp22 of Drosophila melanogaster: Implication in mitochondrial homeostasis

The small heat shock protein (sHsp) Hsp22 from Drosophila melanogaster (DmHsp22) is part of the family of sHsps in this diptera. This sHsp is characterized by its presence in the mitochondrial matrix as well as by its preferential expression during ageing. Although DmHsp22 has been demonstrated to be an efficient in vitro chaperone, its function within mitochondria in vivo remains largely unknown. Thus, determining its protein-interaction network (interactome) in the mitochondrial matrix would help to shed light on its function(s). In the present study we combined immunoaffinity conjugation (IAC) with mass spectroscopy analysis of mitochondria from HeLa cells transfected with DmHsp22 in non-heat shock condition and after heat shock (HS). 60 common DmHsp22-binding mitochondrial partners were detected in two independent IACs. Immunoblotting was used to validate interaction between DmHsp22 and two members of the mitochondrial chaperone machinery; Hsp60 and Hsp70. Among the partners of DmHsp22, several ATP synthase subunits were found. Moreover, we showed that expression of DmHsp22 in transiently transfected HeLa cells increased maximal mitochondrial oxygen consumption capacity and ATP contents, providing a mechanistic link between DmHsp22 and mitochondrial functions.

Loss of the Drosophila m-AAA mitochondrial protease paraplegin results in mitochondrial dysfunction, shortened lifespan, and neuronal and muscular degeneration

The progressive accumulation of dysfunctional mitochondria is implicated in aging and in common diseases of the elderly. To oppose this occurrence, organisms employ a variety of strategies, including the selective degradation of oxidatively damaged and misfolded mitochondrial proteins. Genetic studies in yeast indicate that the ATPase Associated with diverse cellular Activities (AAA⁺) family of mitochondrial proteases account for a substantial fraction of this protein degradation, but their metazoan counterparts have been little studied, despite the fact that mutations in the genes encoding these proteases cause a variety of human diseases. To begin to explore the biological roles of the metazoan mitochondrial AAA⁺ protease family, we have created a CRISPR/Cas9 allele of the Drosophila homolog of SPG7, which encodes an inner membrane-localized AAA⁺ protease known as paraplegin. Drosophila SPG7 mutants exhibited shortened lifespan, progressive locomotor defects, sensitivity to chemical and environmental stress, and muscular and neuronal degeneration. Ultrastructural examination of photoreceptor neurons indicated that the neurodegenerative phenotype of SPG7 mutants initiates at the synaptic terminal. A variety of mitochondrial defects accompanied the degenerative phenotypes of SPG7 mutants, including altered axonal transport of mitochondria, accumulation of electron-dense material in the matrix of flight muscle mitochondria, reduced activities of respiratory chain complexes I and II, and severely swollen and dysmorphic mitochondria in the synaptic terminals of photoreceptors. Drosophila SPG7 mutants recapitulate key features of human diseases caused by mutations in SPG7, and thus provide a foundation for the identification of Drosophila paraplegin substrates and strategies that could be used to ameliorate the symptoms of these diseases.

Mitophagy in maintaining skeletal muscle mitochondrial proteostasis and metabolic health with ageing

Skeletal muscle is important for overall functionality and health. Ageing is associated with an accumulation of damage to mitochondrial DNA and proteins. In particular, damage to mitochondrial proteins in skeletal muscle, which is a loss of mitochondrial proteostasis, contributes to tissue dysfunction and negatively impacts systemic health. Therefore, understanding the mechanisms underlying the regulation of mitochondrial proteostasis and how those mechanisms change with age is important for the development of interventions to promote healthy ageing. Herein, we examine how impairment in the selective degradation of damaged/dysfunctional mitochondria through mitophagy may play a central role in the loss of mitochondrial proteostasis in skeletal muscle ageing, as well as its broader implications for systemic health. Further, we explore how stimulating mitophagy through exercise may promote healthy ageing.

Small heat shock proteins in ageing and age-related diseases

Small heat shock proteins (sHSPs) are gatekeepers of cellular homeostasis across species, preserving proteome integrity under stressful conditions. Nonetheless, recent evidence suggests that sHSPs are more than molecular chaperones with merely auxiliary role. In contrast, sHSPs have emerged as central lifespan determinants, and their malfunction has been associated with the manifestation of neurological disorders, cardiovascular disease and cancer malignancies. In this review, we focus on the role of sHSPs in ageing and age-associated diseases and highlight the most prominent paradigms, where impairment of sHSP function has been implicated in human pathology.