Mitochondrial dysfunction triggered by loss of HtrA2 results in the activation of a brain-specific transcriptional stress response

Current perspective of mitochondrial biology in Parkinson's disease

Parkinson's disease (PD) is one of the most common neurodegenerative movement disorder characterized by preferential loss of dopaminergic neurons of the substantia nigra pars compacta and the presence of Lewy bodies containing α-synuclein. Although the cause of PD remains elusive, remarkable advances have been made in understanding the possible causative mechanisms of PD pathogenesis. An explosion of discoveries during the past two decades has led to the identification of several autosomal dominant and recessive genes that cause familial forms of PD. The investigations of these familial PD gene products have shed considerable insights into the molecular pathogenesis of the more common sporadic PD. A growing body of evidence suggests that the etiology of PD is multifactorial and involves a complex interplay between genetic and environmental factors. Substantial evidence from human tissues, genetic and toxin-induced animal and cellular models indicates that mitochondrial dysfunction plays a central role in the pathophysiology of PD. Deficits in mitochondrial functions due to bioenergetics defects, alterations in the mitochondrial DNA, generation of reactive oxygen species, aberrant calcium homeostasis, and anomalies in mitochondrial dynamics and quality control are implicated in the underlying mechanisms of neuronal cell death in PD. In this review, we discuss how familial PD-linked genes and environmental factors interface the pathways regulating mitochondrial functions and thereby potentially converge both familial and sporadic PD at the level of mitochondrial integrity. We also provide an overview of the status of therapeutic strategies targeting mitochondrial dysfunction in PD. Unraveling potential pathways that influence mitochondrial homeostasis in PD may hold the key to therapeutic intervention for this debilitating neurodegenerative movement disorder.

Mitochondrial Chaperones in the Brain: Safeguarding Brain Health and Metabolism?

The brain orchestrates organ function and regulates whole body metabolism by the concerted action of neurons and glia cells in the central nervous system. To do so, the brain has tremendously high energy consumption and relies mainly on glucose utilization and mitochondrial function in order to exert its function. As a consequence of high rate metabolism, mitochondria in the brain accumulate errors over time, such as mitochondrial DNA (mtDNA) mutations, reactive oxygen species, and misfolded and aggregated proteins. Thus, mitochondria need to employ specific mechanisms to avoid or ameliorate the rise of damaged proteins that contribute to aberrant mitochondrial function and oxidative stress. To maintain mitochondria homeostasis (mitostasis), cells evolved molecular chaperones that shuttle, refold, or in coordination with proteolytic systems, help to maintain a low steady-state level of misfolded/aggregated proteins. Their importance is exemplified by the occurrence of various brain diseases which exhibit reduced action of chaperones. Chaperone loss (expression and/or function) has been observed during aging, metabolic diseases such as type 2 diabetes and in neurodegenerative diseases such as Alzheimer's (AD), Parkinson's (PD) or even Huntington's (HD) diseases, where the accumulation of damage proteins is evidenced. Within this perspective, we propose that proper brain function is maintained by the joint action of mitochondrial chaperones to ensure and maintain mitostasis contributing to brain health, and that upon failure, alter brain function which can cause metabolic diseases.

mPOS is a novel mitochondrial trigger of cell death - implications for neurodegeneration

In addition to its central role in energy metabolism, the mitochondrion has many other functions essential for cell survival. When stressed, the multifunctional mitochondria are expected to engender multifaceted cell stress with complex physiological consequences. Potential extra-mitochondrial proteostatic burdens imposed by inefficient protein import have been largely overlooked. Accumulating evidence suggests that a diverse range of pathogenic mitochondrial stressors, which do not directly target the core protein import machinery, can reduce cell fitness by disrupting the proteostatic network in the cytosol. The resulting stress, named mitochondrial precursor overaccumulation stress (mPOS), is characterized by the toxic accumulation of unimported mitochondrial proteins in the cytosol. Here, we review our current understanding of how mitochondrial dysfunction can impact the cytosolic proteome and proteostatic signaling. We also discuss the intriguing possibility that the mPOS model may help untangle the cause-effect relationship between mitochondrial dysfunction and cytosolic protein aggregation, which are probably the two most prominent molecular hallmarks of neurodegenerative disease.

Mitochondrial retrograde signaling in the nervous system

Mitochondria generate the majority of cellular ATP and are essential for neuronal function. Loss of mitochondrial activity leads to primary mitochondrial diseases and may contribute to neurodegenerative diseases such as Alzheimer's and Parkinson's disease. Mitochondria communicate with the cell through mitochondrial retrograde signaling pathways. These signaling pathways are triggered by mitochondrial dysfunction and allow the organelle to control nuclear gene transcription. Neuronal mitochondrial retrograde signaling pathways have been identified in disease model systems and targeted to restore neuronal function and prevent neurodegeneration. In this review, we describe yeast and mammalian cellular models that have paved the way in the investigation of mitochondrial retrograde mechanisms. We then discuss the evidence for retrograde signaling in neurons and our current knowledge of retrograde signaling mechanisms in neuronal model systems. We argue that targeting mitochondrial retrograde pathways has the potential to lead to novel treatments for neurological diseases.

Metformin reverses TRAP1 mutation-associated alterations in mitochondrial function in Parkinson's disease

The mitochondrial proteins TRAP1 and HTRA2 have previously been shown to be phosphorylated in the presence of the Parkinson's disease kinase PINK1 but the downstream signalling is unknown. HTRA2 and PINK1 loss of function causes parkinsonism in humans and animals. Here, we identified TRAP1 as an interactor of HTRA2 using an unbiased mass spectrometry approach. In our human cell models, TRAP1 overexpression is protective, rescuing HTRA2 and PINK1-associated mitochondrial dysfunction and suggesting that TRAP1 acts downstream of HTRA2 and PINK1. HTRA2 regulates TRAP1 protein levels, but TRAP1 is not a direct target of HTRA2 protease activity. Following genetic screening of Parkinson's disease patients and healthy controls, we also report the first TRAP1 mutation leading to complete loss of functional protein in a patient with late onset Parkinson's disease. Analysis of fibroblasts derived from the patient reveal that oxygen consumption, ATP output and reactive oxygen species are increased compared to healthy individuals. This is coupled with an increased pool of free NADH, increased mitochondrial biogenesis, triggering of the mitochondrial unfolded protein response, loss of mitochondrial membrane potential and sensitivity to mitochondrial removal and apoptosis. These data highlight the role of TRAP1 in the regulation of energy metabolism and mitochondrial quality control. Interestingly, the diabetes drug metformin reverses mutation-associated alterations on energy metabolism, mitochondrial biogenesis and restores mitochondrial membrane potential. In summary, our data show that TRAP1 acts downstream of PINK1 and HTRA2 for mitochondrial fine tuning, whereas TRAP1 loss of function leads to reduced control of energy metabolism, ultimately impacting mitochondrial membrane potential. These findings offer new insight into mitochondrial pathologies in Parkinson's disease and provide new prospects for targeted therapies.

Multi-omics analysis identifies ATF4 as a key regulator of the mitochondrial stress response in mammals

Mitochondrial stress activates a mitonuclear response to safeguard and repair mitochondrial function and to adapt cellular metabolism to stress. Using a multiomics approach in mammalian cells treated with four types of mitochondrial stressors, we identify activating transcription factor 4 (ATF4) as the main regulator of the stress response. Surprisingly, canonical mitochondrial unfolded protein response genes mediated by ATF5 are not activated. Instead, ATF4 activates the expression of cytoprotective genes, which reprogram cellular metabolism through activation of the integrated stress response (ISR). Mitochondrial stress promotes a local proteostatic response by reducing mitochondrial ribosomal proteins, inhibiting mitochondrial translation, and coupling the activation of the ISR with the attenuation of mitochondrial function. Through a trans-expression quantitative trait locus analysis, we provide genetic evidence supporting a role for Fh1 in the control of Atf4 expression in mammals. Using gene expression data from mice and humans with mitochondrial diseases, we show that the ATF4 pathway is activated in vivo upon mitochondrial stress. Our data illustrate the value of a multiomics approach to characterize complex cellular networks and provide a versatile resource to identify new regulators of mitochondrial-related diseases.

Structural insights into the activation mechanisms of human HtrA serine proteases

Human HtrA1-4 proteins belong to the HtrA family of evolutionarily conserved serine proteases and function as important modulators of many physiological processes, including maintenance of mitochondrial homeostasis, cell signaling and apoptosis. Disturbances in their action are linked to severe diseases, including oncogenesis and neurodegeneration. The HtrA1-4 proteins share structural and functional features of other members of the HtrA protein family, however there are several significant differences in structural architecture and mechanisms of action which makes each of them unique. Our goal is to present recent studies regarding human HtrAs. We focus on their physiological functions, structure and regulation, and describe current models of activation mechanisms. Knowledge of molecular basis of the human HtrAs' action is a subject of great interest; it is crucial for understanding their relevance in cellular physiology and pathogenesis as well as for using them as targets in future therapies of diseases such as neurodegenerative disorders and cancer.

Sex specific activation of the ERα axis of the mitochondrial UPR (UPRmt) in the G93A-SOD1 mouse model of familial ALS

The mitochondrial unfolded protein response (UPRmt) is a transcriptional program aimed at restoring proteostasis in mitochondria. Upregulation of mitochondrial matrix proteases and heat shock proteins was initially described. Soon thereafter, a distinct UPRmt induced by misfolded proteins in the mitochondrial intermembrane space (IMS) and mediated by the estrogen receptor alpha (ERα), was found to upregulate the proteasome and the IMS protease OMI. However, the IMS-UPRmt was never studied in a neurodegenerative disease in vivo. Thus, we investigated the IMS-UPRmt in the G93A-SOD1 mouse model of familial ALS, since mutant SOD1 is known to accumulate in the IMS of neural tissue and cause mitochondrial dysfunction. As the ERα is most active in females, we postulated that a differential involvement of the IMS-UPRmt could be linked to the longer lifespan of females in the G93A-SOD1 mouse. We found a significant sex difference in the IMS-UPRmt, because the spinal cords of female, but not male, G93A-SOD1 mice showed elevation of OMI and proteasome activity. Then, using a mouse in which G93A-SOD1 was selectively targeted to the IMS, we demonstrated that the IMS-UPRmt could be specifically initiated by mutant SOD1 localized in the IMS. Furthermore, we showed that, in the absence of ERα, G93A-SOD1 failed to activate OMI and the proteasome, confirming the ERα dependence of the response. Taken together, these results demonstrate the IMS-UPRmt activation in SOD1 familial ALS, and suggest that sex differences in the disease phenotype could be linked to differential activation of the ERα axis of the IMS-UPRmt.

dATF4 regulation of mitochondrial folate-mediated one-carbon metabolism is neuroprotective

Neurons rely on mitochondria as their preferred source of energy. Mutations in PINK1 and PARKIN cause neuronal death in early-onset Parkinson's disease (PD), thought to be due to mitochondrial dysfunction. In Drosophila pink1 and parkin mutants, mitochondrial defects lead to the compensatory upregulation of the mitochondrial one-carbon cycle metabolism genes by an unknown mechanism. Here we uncover that this branch is triggered by the activating transcription factor 4 (ATF4). We show that ATF4 regulates the expression of one-carbon metabolism genes SHMT2 and NMDMC as a protective response to mitochondrial toxicity. Suppressing Shmt2 or Nmdmc caused motor impairment and mitochondrial defects in flies. Epistatic analyses showed that suppressing the upregulation of Shmt2 or Nmdmc deteriorates the phenotype of pink1 or parkin mutants. Conversely, the genetic enhancement of these one-carbon metabolism genes in pink1 or parkin mutants was neuroprotective. We conclude that mitochondrial dysfunction caused by mutations in the Pink1/Parkin pathway engages ATF4-dependent activation of one-carbon metabolism as a protective response. Our findings show a central contribution of ATF4 signalling to PD that may represent a new therapeutic strategy. A video abstract for this article is available at https://youtu.be/cFJJm2YZKKM.

Growth differentiation factor 15 is a myomitokine governing systemic energy homeostasis

Chung et al. show that the myomitokine GDF15 can act to modulate oxidative and lipolytic function in a non–cell-autonomous manner, thereby regulating systemic energy homeostasis in skeletal muscle-specific Crif1-deficient mice. This pathway may be a potential therapeutic target for preventing the onset of obesity and insulin resistance.

Reduced mitochondrial electron transport chain activity promotes longevity and improves energy homeostasis via cell-autonomous and –non-autonomous factors in multiple model systems. This mitohormetic effect is thought to involve the mitochondrial unfolded protein response (UPR^mt), an adaptive stress-response pathway activated by mitochondrial proteotoxic stress. Using mice with skeletal muscle–specific deficiency of Crif1 (muscle-specific knockout [MKO]), an integral protein of the large mitoribosomal subunit (39S), we identified growth differentiation factor 15 (GDF15) as a UPR^mt-associated cell–non-autonomous myomitokine that regulates systemic energy homeostasis. MKO mice were protected against obesity and sensitized to insulin, an effect associated with elevated GDF15 secretion after UPR^mt activation. In ob/ob mice, administration of recombinant GDF15 decreased body weight and improved insulin sensitivity, which was attributed to elevated oxidative metabolism and lipid mobilization in the liver, muscle, and adipose tissue. Thus, GDF15 is a potent mitohormetic signal that safeguards against the onset of obesity and insulin resistance.

Mitochondrial Quality Control Proteases in Neuronal Welfare

The functional integrity of mitochondria is a critical determinant of neuronal health and compromised mitochondrial function is a commonly recognized factor that underlies a plethora of neurological and neurodegenerative diseases. Metabolic demands of neural cells require high bioenergetic outputs that are often associated with enhanced production of reactive oxygen species. Unopposed accumulation of these respiratory byproducts over time leads to oxidative damage and imbalanced protein homeostasis within mitochondrial subcompartments, which in turn may result in cellular demise. The post-mitotic nature of neurons and their vulnerability to these stress factors necessitate strict protein homeostatic control to prevent such scenarios. A series of evolutionarily conserved proteases is one of the central elements of mitochondrial quality control. These versatile proteolytic enzymes conduct a multitude of activities to preserve normal mitochondrial function during organelle biogenesis, metabolic remodeling and stress. In this review we discuss neuroprotective aspects of mitochondrial quality control proteases and neuropathological manifestations arising from defective proteolysis within the mitochondrion.

PINK1 in the limelight: multiple functions of an eclectic protein in human health and disease

The gene PINK1 [phosphatase and tensin homologue (PTEN)-induced putative kinase 1] encodes a serine/threonine kinase which was initially linked to the pathogenesis of a familial form of Parkinson's disease. Research on PINK1 has recently unravelled that its multiple functions extend well beyond neuroprotection, implicating this eclectic protein in a growing number of human pathologies, including cancer, diabetes, cardiopulmonary dysfunctions, and inflammation. Extensive studies have identified PINK1 as a crucial player in the mitochondrial quality control pathway, required to label damaged mitochondria and promote their elimination through an autophagic process (mitophagy). Mounting evidence now indicates that PINK1 activities are not restricted solely to mitophagy, and that different subcellular and even sub-mitochondrial pools of PINK1 are involved in distinct signalling cascades to regulate cell metabolism and survival. In this review, we provide a concise overview on the different functions of PINK1 and their potential role in human diseases. Copyright © 2016 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Sending Out an SOS: Mitochondria as a Signaling Hub

Normal cellular physiology is critically dependent on numerous mitochondrial activities including energy conversion, cofactor and precursor metabolite synthesis, and regulation of ion and redox homeostasis. Advances in mitochondrial research during the last two decades provide solid evidence that these organelles are deeply integrated with the rest of the cell and multiple mechanisms are in place to monitor and communicate functional states of mitochondria. In many cases, however, the exact molecular nature of various mitochondria-to-cell communication pathways is only beginning to emerge. Here, we review various signals emitted by distressed or dysfunctional mitochondria and the stress-responsive pathways activated in response to these signals in order to restore mitochondrial function and promote cellular survival.

Distinct 3D Architecture and Dynamics of the Human HtrA2(Omi) Protease and Its Mutated Variants

HtrA2(Omi) protease controls protein quality in mitochondria and plays a major role in apoptosis. Its HtrA2^S306A mutant (with the catalytic serine routinely disabled for an X-ray study to avoid self-degradation) is a homotrimer whose subunits contain the serine protease domain (PD) and the regulatory PDZ domain. In the inactive state, a tight interdomain interface limits penetration of both PDZ-activating ligands and PD substrates into their respective target sites. We successfully crystalized HtrA2^V226K/S306A, whose active counterpart HtrA2^V226K has had higher proteolytic activity, suggesting higher propensity to opening the PD-PDZ interface than that of the wild type HtrA2. Yet, the crystal structure revealed the HtrA2^V226K/S306A architecture typical of the inactive protein. To get a consistent interpretation of crystallographic data in the light of kinetic results, we employed molecular dynamics (MD). V325D inactivating mutant was used as a reference. Our simulations demonstrated that upon binding of a specific peptide ligand NH₂-GWTMFWV-COOH, the PDZ domains open more dynamically in the wild type protease compared to the V226K mutant, whereas the movement is not observed in the V325D mutant. The movement relies on a PDZ vs. PD rotation which opens the PD-PDZ interface in a lid-like (budding flower-like in trimer) fashion. The noncovalent hinges A and B are provided by two clusters of interfacing residues, harboring V325D and V226K in the C- and N-terminal PD barrels, respectively. The opening of the subunit interfaces progresses in a sequential manner during the 50 ns MD simulation. In the systems without the ligand only minor PDZ shifts relative to PD are observed, but the interface does not open. Further activation-associated events, e.g. PDZ-L3 positional swap seen in any active HtrA protein (vs. HtrA2), were not observed. In summary, this study provides hints on the mechanism of activation of wtHtrA2, the dynamics of the inactive HtrA2^V325D, but does not allow to explain an increased activity of HtrA2^V226K.

PARL: The mitochondrial rhomboid protease

The rhomboid family comprises evolutionary conserved intramembrane proteases involved in a wide spectrum of biologically relevant activities. A mitochondrion-localized rhomboid, called PARL in mammals, and conserved in yeast and Drosophila as RBD1/PCP1 and rho-7, respectively, plays an indispensable role in cell homeostasis as illustrated by the severe phenotypes caused by its genetic ablation in the various investigated species. Although several substrates of PARL have been proposed to explain these phenotypes, there remains a lot of controversy in this important area of research. We review here the putative functions and substrates of PARL and its orthologues in different species, highlighting areas of uncertainty, and discuss its potential involvement in some prevalent diseases such as type II diabetes and Parkinson's disease.

Molecular changes in the postmortem parkinsonian brain

Parkinson disease (PD) is the second most common neurodegenerative disease after Alzheimer disease. Although PD has a relatively narrow clinical phenotype, it has become clear that its etiological basis is broad. Post-mortem brain analysis, despite its limitations, has provided invaluable insights into relevant pathogenic pathways including mitochondrial dysfunction, oxidative stress and protein homeostasis dysregulation. Identification of the genetic causes of PD followed the discovery of these abnormalities, and reinforced the importance of the biochemical defects identified post-mortem. Recent genetic studies have highlighted the mitochondrial and lysosomal areas of cell function as particularly significant in mediating the neurodegeneration of PD. Thus the careful analysis of post-mortem PD brain biochemistry remains a crucial component of research, and one that offers considerable opportunity to pursue etiological factors either by 'reverse biochemistry' i.e. from defective pathway to mutant gene, or by the complex interplay between pathways e.g. mitochondrial turnover by lysosomes. In this review we have documented the spectrum of biochemical defects identified in PD post-mortem brain and explored their relevance to metabolic pathways involved in neurodegeneration. We have highlighted the complex interactions between these pathways and the gene mutations causing or increasing risk for PD. These pathways are becoming a focus for the development of disease modifying therapies for PD. Parkinson's is accompanied by multiple changes in the brain that are responsible for the progression of the disease. We describe here the molecular alterations occurring in postmortem brains and classify them as: Neurotransmitters and neurotrophic factors; Lewy bodies and Parkinson's-linked genes; Transition metals, calcium and calcium-binding proteins; Inflammation; Mitochondrial abnormalities and oxidative stress; Abnormal protein removal and degradation; Apoptosis and transduction pathways. This article is part of a special issue on Parkinson disease.

Mitonuclear communication in homeostasis and stress

Mitochondria participate in crucial cellular processes such as energy harvesting and intermediate metabolism. Although mitochondria possess their own genome--a vestige of their bacterial origins and endosymbiotic evolution--most mitochondrial proteins are encoded in the nucleus. The expression of the mitochondrial proteome hence requires tight coordination between the two genomes to adapt mitochondrial function to the ever-changing cellular milieu. In this Review, we focus on the pathways that coordinate the communication between mitochondria and the nucleus during homeostasis and mitochondrial stress. These pathways include nucleus-to-mitochondria (anterograde) and mitochondria-to-nucleus (retrograde) communication, mitonuclear feedback signalling and proteostasis regulation, the integrated stress response and non-cell-autonomous communication. We discuss how mitonuclear communication safeguards cellular and organismal fitness and regulates lifespan.

Mitochondria Retrograde Signaling and the UPR mt: Where Are We in Mammals?

Mitochondrial unfolded protein response is a form of retrograde signaling that contributes to ensuring the maintenance of quality control of mitochondria, allowing functional integrity of the mitochondrial proteome. When misfolded proteins or unassembled complexes accumulate beyond the folding capacity, it leads to alteration of proteostasis, damages, and organelle/cell dysfunction. Extensively studied for the ER, it was recently reported that this kind of signaling for mitochondrion would also be able to communicate with the nucleus in response to impaired proteostasis. The mitochondrial unfolded protein response (UPR^mt) is activated in response to different types and levels of stress, especially in conditions where unfolded or misfolded mitochondrial proteins accumulate and aggregate. A specific UPR^mt could thus be initiated to boost folding and degradation capacity in response to unfolded and aggregated protein accumulation. Although first described in mammals, the UPR^mt was mainly studied in Caenorhabditis elegans, and accumulating evidence suggests that mechanisms triggered in response to a UPR^mt might be different in C. elegans and mammals. In this review, we discuss and integrate recent data from the literature to address whether the UPR^mt is relevant to mitochondrial homeostasis in mammals and to analyze the putative role of integrated stress response (ISR) activation in response to the inhibition of mtDNA expression and/or accumulation of mitochondrial mis/unfolded proteins.

New roles for mitochondrial proteases in health, ageing and disease

Recent advances in mitochondrial biology have revealed the high diversity and complexity of proteolytic enzymes that regulate mitochondrial function. We have classified mitochondrial proteases, or mitoproteases, on the basis of their function and location, and defined the human mitochondrial degradome as the complete set of mitoproteases that are encoded by the human genome. In addition to their nonspecific degradative functions, mitoproteases perform highly regulated proteolytic reactions that are important in mitochondrial function, integrity and homeostasis. These include protein synthesis, quality control, mitochondrial biogenesis and dynamics, mitophagy and apoptosis. Impaired or dysregulated function of mitoproteases is associated with ageing and with many pathological conditions such as neurodegenerative disorders, metabolic syndromes and cancer. A better understanding of the mitochondrial proteolytic landscape and its modulation may contribute to improving human lifespan and 'healthspan'.

Mitochondrial protein quality control: the mechanisms guarding mitochondrial health

SIGNIFICANCE

Mitochondria are complex dynamic organelles pivotal for cellular physiology and human health. Failure to maintain mitochondrial health leads to numerous maladies that include late-onset neurodegenerative diseases and cardiovascular disorders. Furthermore, a decline in mitochondrial health is prevalent with aging. A set of evolutionary conserved mechanisms known as mitochondrial quality control (MQC) is involved in recognition and correction of the mitochondrial proteome.

RECENT ADVANCES

Here, we review current knowledge and latest developments in MQC. We particularly focus on the proteolytic aspect of MQC and its impact on health and aging.

CRITICAL ISSUES

While our knowledge about MQC is steadily growing, critical gaps remain in the mechanistic understanding of how MQC modules sense damage and preserve mitochondrial welfare, particularly in higher organisms.

FUTURE DIRECTIONS

Delineating how coordinated action of the MQC modules orchestrates physiological responses on both organellar and cellular levels will further elucidate the current picture of MQC's role and function in health, cellular stress, and degenerative diseases.

Ameliorative effects of baicalein in MPTP-induced mouse model of Parkinson's disease: A microarray study

Baicalein, a flavonoid from Scutellaria baicalensis Georgi, has been shown to possess neuroprotective properties. The purpose of this study was to explore the effects of baicalein on motor behavioral deficits and gene expression in N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced mice model of Parkinson's disease (PD). The behavioral results showed that baicalein significantly improves the abnormal behaviors in MPTP-induced mice model of PD, as manifested by shortening the total time for climbing down the pole, prolonging the latent periods of rotarod, and increasing the vertical movements. Using cDNA microarray and subsequent bioinformatic analyses, it was found that baicalein significantly promotes the biological processes including neurogenesis, neuroblast proliferation, neurotrophin signaling pathway, walking and locomotor behaviors, and inhibits dopamine metabolic process through regulation of gene expressions. Based on analysis of gene co-expression networks, the results indicated that the regulation of genes such as LIMK1, SNCA and GLRA1 by baicalein might play central roles in the network. Our results provide experimental evidence for the potential use of baicalein in the treatment of PD, and revealed gene expression profiles, biological processes and pathways influenced by baicalein in MPTP-treated mice.

Mouse models rarely mimic the transcriptome of human neurodegenerative diseases: A systematic bioinformatics-based critique of preclinical models

Translational research for neurodegenerative disease depends intimately upon animal models. Unfortunately, promising therapies developed using mouse models mostly fail in clinical trials, highlighting uncertainty about how well mouse models mimic human neurodegenerative disease at the molecular level. We compared the transcriptional signature of neurodegeneration in mouse models of Alzheimer׳s disease (AD), Parkinson׳s disease (PD), Huntington׳s disease (HD) and amyotrophic lateral sclerosis (ALS) to human disease. In contrast to aging, which demonstrated a conserved transcriptome between humans and mice, only 3 of 19 animal models showed significant enrichment for gene sets comprising the most dysregulated up- and down-regulated human genes. Spearman׳s correlation analysis revealed even healthy human aging to be more closely related to human neurodegeneration than any mouse model of AD, PD, ALS or HD. Remarkably, mouse models frequently upregulated stress response genes that were consistently downregulated in human diseases. Among potential alternate models of neurodegeneration, mouse prion disease outperformed all other disease-specific models. Even among the best available animal models, conserved differences between mouse and human transcriptomes were found across multiple animal model versus human disease comparisons, surprisingly, even including aging. Relative to mouse models, mouse disease signatures demonstrated consistent trends toward preserved mitochondrial function protein catabolism, DNA repair responses, and chromatin maintenance. These findings suggest a more complex and multifactorial pathophysiology in human neurodegeneration than is captured through standard animal models, and suggest that even among conserved physiological processes such as aging, mice are less prone to exhibit neurodegeneration-like changes. This work may help explain the poor track record of mouse-based translational therapies for neurodegeneration and provides a path forward to critically evaluate and improve animal models of human disease.

Inhibition of mitochondrial genome expression triggers the activation of CHOP-10 by a cell signaling dependent on the integrated stress response but not the mitochondrial unfolded protein response

Mitochondria-to-nucleus communication, known as retrograde signaling, is important to adjust the nuclear gene expression in response to organelle dysfunction. Among the transcription factors described to respond to mitochondrial stress, CHOP-10 is activated by respiratory chain inhibition, mitochondrial accumulation of unfolded proteins and mtDNA mutations. In this study, we show that altered/impaired expression of mtDNA induces CHOP-10 expression in a signaling pathway that depends on the eIF2α/ATF4 axis of the integrated stress response rather than on the mitochondrial unfolded protein response.

Neural-specific deletion of Htra2 causes cerebellar neurodegeneration and defective processing of mitochondrial OPA1

HTRA2, a serine protease in the intermembrane space, has important functions in mitochondrial stress signaling while its abnormal activity may contribute to the development of Parkinson’s disease. Mice with a missense or null mutation of Htra2 fail to thrive, suffer striatal neuronal loss, and a parkinsonian phenotype that leads to death at 30–40 days of age. While informative, these mouse models cannot separate neural contributions from systemic effects due to the complex phenotypes of HTRA2 deficiency. Hence, we developed mice carrying a Htra2-floxed allele to query the consequences of tissue-specific HTRA2 deficiency. We found that mice with neural-specific deletion of Htra2 exhibited atrophy of the thymus and spleen, cessation to gain weight past postnatal (P) day 18, neurological symptoms including ataxia and complete penetrance of premature death by P40. Histologically, increased apoptosis was detected in the cerebellum, and to a lesser degree in the striatum and the entorhinal cortex, from P25. Even earlier at P20, mitochondria in the cerebella already exhibited abnormal morphology, including swelling, vesiculation, and fragmentation of the cristae. Furthermore, the onset of these structural anomalies was accompanied by defective processing of OPA1, a key molecule for mitochondrial fusion and cristae remodeling, leading to depletion of the L-isoform. Together, these findings suggest that HTRA2 is essential for maintenance of the mitochondrial integrity in neurons. Without functional HTRA2, a lifespan as short as 40 days accumulates a large quantity of dysfunctional mitochondria that contributes to the demise of mutant mice.

Therapeutic strategies in Parkinson's disease: what we have learned from animal models

Parkinson's disease (PD), the second most common neurodegenerative disorder, is characterized by a loss of dopaminergic neurons in the substantia nigra, as well as in other brain areas. The currently available dopamine replacement therapy provides merely symptomatic benefit and is ineffective because habituation and side effects arise relatively quickly. Studying the genetic forms of PD in animal models provides novel insight that allows targeting of specific aspects of this heterogenic disease more specifically. Among others, two important cellular deficits are associated with PD; these deficits relate to (1) synaptic transmission and vesicle trafficking, and (2) mitochondrial function, relating respectively to the dominant and recessive mutations in PD-causing genes. With increased knowledge of PD, the possibility of identifying an efficient, long-lasting treatment is becoming more conceivable, but this can only be done with an increased knowledge of the specific affected cellular mechanisms. This review discusses how discoveries in animal models of PD have clarified the therapeutic potential of pathways disrupted in PD, with a specific focus on synaptic transmission, vesicle trafficking, and mitochondrial function.

Prolonged fasting identifies heat shock protein 10 as a Sirtuin 3 substrate: elucidating a new mechanism linking mitochondrial protein acetylation to fatty acid oxidation enzyme folding and function

Although Sirtuin 3 (SIRT3), a mitochondrially enriched deacetylase and activator of fat oxidation, is down-regulated in response to high fat feeding, the rate of fatty acid oxidation and mitochondrial protein acetylation are invariably enhanced in this dietary milieu. These paradoxical data implicate that additional acetylation modification-dependent levels of regulation may be operational under nutrient excess conditions. Because the heat shock protein (Hsp) Hsp10-Hsp60 chaperone complex mediates folding of the fatty acid oxidation enzyme medium-chain acyl-CoA dehydrogenase, we tested whether acetylation-dependent mitochondrial protein folding contributes to this regulatory discrepancy. We demonstrate that Hsp10 is a functional SIRT3 substrate and that, in response to prolonged fasting, SIRT3 levels modulate mitochondrial protein folding. Acetyl mutagenesis of Hsp10 lysine 56 alters Hsp10-Hsp60 binding, conformation, and protein folding. Consistent with Hsp10-Hsp60 regulation of fatty acid oxidation enzyme integrity, medium-chain acyl-CoA dehydrogenase activity and fat oxidation are elevated by Hsp10 acetylation. These data identify acetyl modification of Hsp10 as a nutrient-sensing regulatory node controlling mitochondrial protein folding and metabolic function.

Unravelling mitochondrial pathways to Parkinson's disease

Mitochondria are essential for cellular function due to their role in ATP production, calcium homeostasis and apoptotic signalling. Neurons are heavily reliant on mitochondrial integrity for their complex signalling, plasticity and excitability properties, and to ensure cell survival over decades. The maintenance of a pool of healthy mitochondria that can meet the bioenergetic demands of a neuron, is therefore of critical importance; this is achieved by maintaining a careful balance between mitochondrial biogenesis, mitochondrial trafficking, mitochondrial dynamics and mitophagy. The molecular mechanisms that underlie these processes are gradually being elucidated. It is widely recognized that mitochondrial dysfunction occurs in many neurodegenerative diseases, including Parkinson's disease. Mitochondrial dysfunction in the form of reduced bioenergetic capacity, increased oxidative stress and reduced resistance to stress, is observed in several Parkinson's disease models. However, identification of the recessive genes implicated in Parkinson's disease has revealed a common pathway involving mitochondrial dynamics, transport, turnover and mitophagy. This body of work has led to the hypothesis that the homeostatic mechanisms that ensure a healthy mitochondrial pool are key to neuronal function and integrity. In this paradigm, impaired mitochondrial dynamics and clearance result in the accumulation of damaged and dysfunctional mitochondria, which may directly induce neuronal dysfunction and death. In this review, we consider the mechanisms by which mitochondrial dysfunction may lead to neurodegeneration. In particular, we focus on the mechanisms that underlie mitochondrial homeostasis, and discuss their importance in neuronal integrity and neurodegeneration in Parkinson's disease.

Compartmental stress responses correlate with cell survival in bystander effects induced by the DNA damage agent, bleomycin

Physical or chemical stress applied to a cell system trigger a signal cascade that is transmitted to the neighboring cell population in a process known as bystander effect. Despite its wide occurrence in biological systems this phenomenon is mainly documented in cancer treatments. Thus understanding whether the bystander effect acts as an adaptive priming element for the neighboring cells or a sensitization factor is critical in designing treatment strategies. Here we characterize the bystander effects induced by bleomycin, a DNA-damaging agent, and compartmental stress responses associated with this phenomenon. Mouse fibroblasts were treated with increasing concentrations of bleomycin and assessed for DNA damage, cell death and induction of compartmental stress response (endoplasmic reticulum, mitochondrial and cytoplasmic stress). Preconditioned media were used to analyze bystander damage using the same end-points. Bleomycin induced bystander response was reflected primarily in increased DNA damage. This was dependent on the concentration of bleomycin and time of media conditioning. Interestingly, we found that ROS but not NO are involved in the transmission of the bystander effect. Consistent transcriptional down-regulation of the stress response factors tested (i.e. BiP, mtHsp60, Hsp70) occurred in the direct effect indicating that bleomycin might induce an arrest of transcription correlated with decreased survival. We observed the opposite trend in the bystander effect, with specific stress markers appearing increased and correlated with increased survival. These data shed new light on the potential role of stress pathways activation in bystander effects and their putative impact on the pro-survival pro-death balance.

Integrative analysis of independent transcriptome data for rare diseases

High-throughput technologies used to interrogate transcriptomes have been generating a great amount of publicly available gene expression data. For rare diseases that lack of clinical samples and research funding, there is a practical benefit to jointly analyze existing data sets commonly related to a specific rare disease. In this study, we collected a number of independently generated transcriptome data sets from four species: human, fly, mouse and worm. All data sets included samples with both normal and abnormal mitochondrial function. We reprocessed each data set to standardize format, scale and gene annotation and used HomoloGene database to map genes between species. Standardized procedure was also applied to compare gene expression profiles of normal and abnormal mitochondrial function to identify differentially expressed genes. We further used meta-analysis and other integrative analyses to recognize patterns across data sets and species. Novel insights related to mitochondrial dysfunction was revealed via these analyses, such as a group of genes consistently dysregulated by impaired mitochondrial function in multiple species. This study created a template for the study of rare diseases using genomic technologies and advanced statistical methods. All data and results generated by this study are freely available and stored at http://goo.gl/nOGWC2, to support further data mining.

Mitochondrial proteolytic stress induced by loss of mortalin function is rescued by Parkin and PINK1

The mitochondrial chaperone mortalin was implicated in Parkinson's disease (PD) because of its reduced levels in the brains of PD patients and disease-associated rare genetic variants that failed to rescue impaired mitochondrial integrity in cellular knockdown models. To uncover the molecular mechanisms underlying mortalin-related neurodegeneration, we dissected the cellular surveillance mechanisms related to mitochondrial quality control, defined the effects of reduced mortalin function at the molecular and cellular levels and investigated the functional interaction of mortalin with Parkin and PINK1, two PD-related proteins involved in mitochondrial homeostasis. We found that reduced mortalin function leads to: (1) activation of the mitochondrial unfolded protein response (UPR(mt)), (2) increased susceptibility towards intramitochondrial proteolytic stress, (3) increased autophagic degradation of fragmented mitochondria and (4) reduced mitochondrial mass in human cells in vitro and ex vivo. These alterations caused increased vulnerability toward apoptotic cell death. Proteotoxic perturbations induced by either partial loss of mortalin or chemical induction were rescued by complementation with native mortalin, but not disease-associated mortalin variants, and were independent of the integrity of autophagic pathways. However, Parkin and PINK1 rescued loss of mortalin phenotypes via increased lysosomal-mediated mitochondrial clearance and required intact autophagic machinery. Our results on loss of mortalin function reveal a direct link between impaired mitochondrial proteostasis, UPR(mt) and PD and show that effective removal of dysfunctional mitochondria via either genetic (PINK1 and Parkin overexpression) or pharmacological intervention (rapamycin) may compensate mitochondrial phenotypes.

Arabidopsis AtPARK13, which confers thermotolerance, targets misfolded proteins

Mutations in HTRA2/Omi/PARK13 have been implicated in Parkinson disease (PD). PARK13 is a neuroprotective serine protease; however, little is known about how PARK13 confers stress protection and which protein targets are directly affected by PARK13. We have reported that Arabidopsis thaliana represents a complementary PD model, and here we demonstrate that AtPARK13, similar to human PARK13 (hPARK13), is a mitochondrial protease. We show that the expression/accumulation of AtPARK13 transcripts are induced by heat stress but not by other stress conditions, including oxidative stress and metals. Our data show that elevated levels of AtPARK13 confer thermotolerance in A. thaliana. Increased temperatures accelerate protein unfolding, and we demonstrate that although AtPARK13 can act on native protein substrates, unfolded proteins represent better AtPARK13 substrates. The results further show that AtPARK13 and hPARK13 can degrade the PD proteins α-synuclein (SNCA) and DJ-1/PARK7 directly, without autophagy involvement, and that misfolded SNCA and DJ-1 represent better substrates than their native counterparts. Comparative proteomic profiling revealed AtPARK13-mediated proteome changes, and we identified four proteins that show altered abundance in response to AtPARK13 overexpression and elevated temperatures. Our study not only suggests that AtPARK13 confers thermotolerance by degrading misfolded protein targets, but it also provides new insight into possible roles of this protease in neurodegeneration.

Integrated transcriptome analysis across mitochondrial disease etiologies and tissues improves understanding of common cellular adaptations to respiratory chain dysfunction

Mitochondrial diseases are heterogeneous, multi-systemic disorders for which mechanistic understanding is limited. To investigate common downstream effects of primary respiratory chain dysfunction on global gene expression and pathway regulation, we reanalyzed transcriptome datasets from all publicly available studies of respiratory chain dysfunction resulting from genetic disorders, acute pathophysiologic processes, or environmental toxins. A general overview is provided of the bioinformatic processing of transcriptome data to uncover biological insights into in vivo and in vitro adaptations to mitochondrial dysfunction, with specific examples discussed from a variety of independent cell, animal, and human tissue studies. To facilitate future community efforts to cohesively mine these data, all reanalyzed transcriptome datasets were deposited into a publicly accessible central web archive. Our own integrated meta-analysis of these data identified several commonly dysregulated genes across diverse mitochondrial disease etiologies, models, and tissue types. Overall, transcriptome analyses provide a useful means to survey cellular adaptation to mitochondrial diseases.

Essential role of TID1 in maintaining mitochondrial membrane potential homogeneity and mitochondrial DNA integrity

The tumorous imaginal disc 1 (TID1) protein localizes mainly to the mitochondrial compartment, wherein its function remains largely unknown. Here we report that TID1 regulates the steady-state homogeneity of the mitochondrial membrane potential (Δψ) and maintains the integrity of mitochondrial DNA (mtDNA). Silencing of TID1 with RNA interference leads to changes in the distribution of Δψ along the mitochondrial network, characterized by an increase in Δψ in focal regions. This effect can be rescued by ectopic expression of a TID1 construct with an intact J domain. Chronic treatment with a low dose of oligomycin, an inhibitor of F1Fo ATP synthase, decreases the cellular ATP content and phenocopies TID1 loss of function, indicating a connection between the disruption of mitochondrial bioenergetics and hyperpolarization. Prolonged silencing of TID1 or low-dose oligomycin treatment leads to the loss of mtDNA and the consequent inhibition of oxygen consumption. Biochemical and colocalization data indicate that complex I aggregation underlies the focal accumulation of Δψ in TID1-silenced cells. Given that TID1 is proposed to function as a cochaperone, these data show that TID1 prevents complex I aggregation and support the existence of a TID1-mediated stress response to ATP synthase inhibition.

The role of PARL and HtrA2 in striatal neuronal injury after transient global cerebral ischemia

The presenilin-associated rhomboid-like (PARL) protein and high temperature requirement factor A2 (HtrA2) are key regulators of mitochondrial integrity and play pivotal roles in apoptosis. However, their roles after cerebral ischemia have not been thoroughly elucidated. To clarify these roles, mice were subjected to transient global cerebral ischemia, and striatal neuronal injury was assessed. Western blot and coimmunoprecipitation analyses revealed that PARL and processed HtrA2 localized to mitochondria, and that PARL was bound to HtrA2 in sham animals. Expression of PARL and processed HtrA2 in mitochondria significantly decreased 6 to 72 hours after ischemia, and the binding of PARL to HtrA2 disappeared after ischemia. In contrast, expression of processed HtrA2 increased 24 hours after ischemia in the cytosol, where HtrA2 was bound to X chromosome-linked inhibitor-of-apoptosis protein (XIAP). Administration of PARL small interfering RNA inhibited HtrA2 processing and worsened ischemic neuronal injury. Our results show that downregulation of PARL after ischemia is a key step in ischemic neuronal injury, and that it decreases HtrA2 processing and increases neuronal vulnerability. In addition, processed HtrA2 released into the cytosol after ischemia contributes to neuronal injury via inhibition of XIAP.

Loss of PINK1 enhances neurodegeneration in a mouse model of Parkinson's disease triggered by mitochondrial stress

Parkinson's disease (PD) shows a complex etiology, where both genetic and environmental factors contribute to initiation and advance of pathology. Mitochondrial dysfunction and mutation of genes implicated in mitochondria quality control are recognized contributors to etiopathology and progression of PD. Here we report the development and characterization of a genetic mouse model of PD with a combined etiology comprising: 1) induction of mitochondrial stress achieved through the expression of a mitochondrial matrix protein that accumulates in an unfolded state and 2) deletion of PINK1 gene. Using this model we address the role of PINK1 in mitochondrial quality control and disease progression. To induce mitochondrial stress specifically in catecholaminergic neurons we generated transgenic animals where the conditional expression of mitochondrial unfolded ornithine transcarbamylase (dOTC) is achieved under the tyrosine hydroxylase (Th) promoter. The mice were characterized in terms of survival, growth and motor behaviour. The characterization was followed by analysis of cell death induced in dopaminergic neurons and responsiveness to l-dopa. We demonstrate that accumulation of dOTC in dopaminergic neurons causes neurodegeneration and motor behaviour impairment that illustrates a parkinsonian phenotype. This associates with l-dopa responsiveness validating the model as a model of PD. The combined transgenic model where dOTC is overexpressed in PINK1 KO background presents increased neurodegeneration as compared to dOTC transgenic in wild-type background. Moreover, this combined model does not show responsiveness to l-dopa. Our in vivo data show that loss of PINK1 accelerates neurodegenerative phenotypes induced by mitochondrial stress triggered by the expression of an unfolded protein in this organelle.

Insights into mitochondrial quality control pathways and Parkinson's disease

The brain uses more energy than any other human organ, accounting for 20 % of the body's total demand. Mitochondria are energy-converting organelles with a pivotal role in meeting the energetic needs of the human brain. Therefore, the decline of these cellular powerhouses can have a negative impact on the function and plasticity of neurons and is believed to have a prominent role in ageing and in the occurrence of several neurological disorders, such as Parkinson's disease (PD). As a consequence of their physiological roles, mitochondria are subjected to high levels of stress and have therefore developed several stress-protective mitochondrial quality control mechanisms that ensure the optimal activity of their molecular machinery. Here, we review some of the most recent advances in our understanding of the regulation of mitochondrial stress pathways with particular emphasis on how defective mitochondrial quality control might contribute to PD.

Inhibition of the mitochondrial Hsp90 chaperone network: a novel, efficient treatment strategy for cancer?

Research has shown that cancer cells exhibit multiple deregulated pathways, involving proliferation, migration and cell death. Heat-shock-proteins have evolved as "central regulators" and are implicated in the modulation of these pathways and in organelle-specific signaling. In this instance, heat-shock-proteins (Hsps) assist cancer cells in the maturation of proteins. Hsp90 is of particular interest because its enzymatic ATPase activity is elevated in malignant cells as compared to non-neoplastic counterparts. Consistent with its high-activity in cancer cells, Hsp90 stabilizes a considerable number of proteins being instrumental in carcinogenesis and the maintenance and growth of highly malignant cancers. Among its distribution Hsp90 is also localized within mitochondria of neoplastic cells of various origin, interacting with another chaperone, TRAP1 (Tumor necrosis factor type 1 receptor-associated protein or Heat-shock-protein 75) to antagonize the cell death promoting properties of the matrix protein, Cyclophilin-D. Several preclinical studies, including in vivo studies in both orthotopic and genetic animal models, have confirmed that targeting mitochondrial Hsp90 may be a novel efficient treatment method for highly recalcitrant tumors. This review summarizes the most recent findings of mitochondrial Hsp90 signaling and its potential implications for cancer therapy.

Drosophila Trap1 protects against mitochondrial dysfunction in a PINK1/parkin model of Parkinson's disease

Mitochondrial dysfunction caused by protein aggregation has been shown to have an important role in neurological diseases, such as Parkinson's disease (PD). Mitochondria have evolved at least two levels of defence mechanisms that ensure their integrity and the viability of their host cell. First, molecular quality control, through the upregulation of mitochondrial chaperones and proteases, guarantees the clearance of damaged proteins. Second, organellar quality control ensures the clearance of defective mitochondria through their selective autophagy. Studies in Drosophila have highlighted mitochondrial dysfunction linked with the loss of the PTEN-induced putative kinase 1 (PINK1) as a mechanism of PD pathogenesis. The mitochondrial chaperone TNF receptor-associated protein 1 (TRAP1) was recently reported to be a cellular substrate for the PINK1 kinase. Here, we characterise Drosophila Trap1 null mutants and describe the genetic analysis of Trap1 function with Pink1 and parkin. We show that loss of Trap1 results in a decrease in mitochondrial function and increased sensitivity to stress, and that its upregulation in neurons of Pink1 mutant rescues mitochondrial impairment. Additionally, the expression of Trap1 was able to partially rescue mitochondrial impairment in parkin mutant flies; and conversely, expression of parkin rescued mitochondrial impairment in Trap1 mutants. We conclude that Trap1 works downstream of Pink1 and in parallel with parkin in Drosophila, and that enhancing its function may ameliorate mitochondrial dysfunction and rescue neurodegeneration in PD.

Temperature-induced changes of HtrA2(Omi) protease activity and structure

HtrA2(Omi), belonging to the high-temperature requirement A (HtrA) family of stress proteins, is involved in the maintenance of mitochondrial homeostasis and in the stimulation of apoptosis, as well as in cancer and neurodegenerative disorders. The protein comprises a serine protease domain and a postsynaptic density of 95 kDa, disk large, and zonula occludens 1 (PDZ) regulatory domain and functions both as a protease and a chaperone. Based on the crystal structure of the HtrA2 inactive trimer, it has been proposed that PDZ domains restrict substrate access to the protease domain and that during protease activation there is a significant conformational change at the PDZ-protease interface, which removes the inhibitory effect of PDZ from the active site. The crystal structure of the HtrA2 active form is not available yet. HtrA2 activity markedly increases with temperature. To understand the molecular basis of this increase in activity, we monitored the temperature-induced structural changes using a set of single-Trp HtrA2 mutants with Trps located at the PDZ-protease interface. The accessibility of each Trp to aqueous medium was assessed by fluorescence quenching, and these results, in combination with mean fluorescence lifetimes and wavelength emission maxima, indicate that upon an increase in temperature the HtrA2 structure relaxes, the PDZ-protease interface becomes more exposed to the solvent, and significant conformational changes involving both domains occur at and above 30 °C. This conclusion correlates well with temperature-dependent changes of HtrA2 proteolytic activity and the effect of amino acid substitutions (V226K and R432L) located at the domain interface, on HtrA2 activity. Our results experimentally support the model of HtrA2 activation and provide an insight into the mechanism of temperature-induced changes in HtrA2 structure.

Mitochondrial Regulation by PINK1-Parkin Signaling

Two genes responsible for the juvenile Parkinson’s disease (PD), PINK1 and Parkin, have been implicated in mitochondrial quality control. The inactivation of PINK1, which encodes a mitochondrial kinase, leads to age-dependent mitochondrial degeneration in Drosophila. The phenotype is closely associated with the impairment of mitochondrial respiratory chain activity and defects in mitochondrial dynamics. Drosophila genetic studies have further revealed that PINK1 is an upstream regulator of Parkin and is involved in the mitochondrial dynamics and motility. A series of cell biological studies have given rise to a model in which the activation of PINK1 in damaged mitochondria induces the selective elimination of mitochondria in cooperation with Parkin through the ubiquitin-proteasome and autophagy machineries. Although the relevance of this pathway to PD etiology is still unclear, approaches using stem cells from patients and animal models will help to understand the significance of mitochondrial quality control by the PINK1-Parkin pathway in PD and in healthy individuals. Here I will review recent advances in our understanding of the PINK1-Parkin signaling and will discuss the roles of PINK1-Parkin signaling for mitochondrial maintenance and how the failure of this signaling leads to neurodegeneration.

Loss of HtrA2/Omi activity in non-neuronal tissues of adult mice causes premature aging

mnd2 mice die prematurely as a result of neurodegeneration 30-40 days after birth due to loss of the enzymatic activity of the mitochondrial quality control protease HtrA2/Omi. Here, we show that transgenic expression of human HtrA2/Omi in the central nervous system of mnd2 mice rescues them from neurodegeneration and prevents their premature death. Interestingly, adult transgenic mnd2 mice develop accelerated aging phenotypes, such as premature weight loss, hair loss, reduced fertility, curvature of the spine, heart enlargement, increased autophagy, and death by 12-17 months of age. These mice also have elevated levels of clonally expanded mitochondrial DNA (mtDNA) deletions in their tissues. Our results provide direct genetic evidence linking mitochondrial protein quality control to mtDNA deletions and aging in mammals.

Control of tumor bioenergetics and survival stress signaling by mitochondrial HSP90s

Tumors successfully adapt to constantly changing intra- and extracellular environments, but the wirings of this process are still largely elusive. Here, we show that heat-shock-protein-90-directed protein folding in mitochondria, but not cytosol, maintains energy production in tumor cells. Interference with this process activates a signaling network that involves phosphorylation of nutrient-sensing AMP-activated kinase, inhibition of rapamycin-sensitive mTOR complex 1, induction of autophagy, and expression of an endoplasmic reticulum unfolded protein response. This signaling network confers a survival and proliferative advantage to genetically disparate tumors, and correlates with worse outcome in lung cancer patients. Therefore, mitochondrial heat shock protein 90s are adaptive regulators of tumor bioenergetics and tractable targets for cancer therapy.

Induced Mist1 expression promotes remodeling of mouse pancreatic acinar cells

BACKGROUND & AIMS

Early embryogenesis involves cell fate decisions that define the body axes and establish pools of progenitor cells. Development does not stop once lineages are specified; cells continue to undergo specific maturation events, and changes in gene expression patterns lead to their unique physiological functions. Secretory pancreatic acinar cells mature postnatally to synthesize large amounts of protein, polarize, and communicate with other cells. The transcription factor MIST1 is expressed by only secretory cells and regulates maturation events. MIST1-deficient acinar cells in mice do not establish apical-basal polarity, properly position zymogen granules, or communicate with adjacent cells, disrupting pancreatic function. We investigated whether MIST1 directly induces and maintains the mature phenotype of acinar cells.

METHODS

We analyzed the effects of Cre-mediated expression of Mist1 in adult Mist1-deficient (Mist1(KO)) mice. Pancreatic tissues were collected and analyzed by light and electron microscopy, immunohistochemistry, real-time polymerase chain reaction analysis, and chromatin immunoprecipitation. Primary acini were isolated from mice and analyzed in amylase secretion assays.

RESULTS

Induced expression of Mist1 in adult Mist1(KO) mice restored wild-type gene expression patterns in acinar cells. The acinar cells changed phenotypes, establishing apical-basal polarity, increasing the size of zymogen granules, reorganizing the cytoskeletal network, communicating intercellularly (by synthesizing gap junctions), and undergoing exocytosis.

CONCLUSIONS

The exocrine pancreas of adult mice can be remodeled by re-expression of the transcription factor MIST1. MIST1 regulates acinar cell maturation and might be used to repair damaged pancreata in patients with pancreatic disorders.

Utilization of the cellular stress response to sensitize cancer cells to TRAIL-mediated apoptosis

INTRODUCTION

Tumor necrosis factor-related apoptosis inducing ligand (TRAIL) is a promising death ligand who has received significant attention due to its specific anti-cancer activity. Recently, a number of clinical trials involving either recombinant soluble TRAIL or agonistic death receptor (DR) antibodies have even been initiated. One major caveat in TRAIL-based anti-cancer therapies is that a considerable number of cancer cells are notorious resistant to apoptosis induction by TRAIL. Overcoming this primary or secondary evolved resistance is an utmost important goal of present cancer research. The current literature suggests that TRAIL resistance is mediated by a number of endogenous factors.

AREAS COVERED

According to recent research, stress-related transcription factors have acquired a pivotal role in the sensitization of highly resistant cancer cells, for example, pancreatic cancer and glioblastoma cells, to TRAIL-mediated cell death. Out of this transcription factor family, C/EBP-homologous protein (CHOP) is linked to the control of DR-mediated apoptosis by modulation of several apoptotic and anti-apoptotic factors. Stress responses in certain organelles, such as endoplasmic reticulum (ER) and mitochondria, are potent inductors of CHOP expression. This report focuses on the influence of stress responses on endogenous or acquired resistance to extrinsic apoptosis in tumor cells and summarizes recent findings and results. The Medline and ClinicalTrials database with key words were used for this review.

EXPERT OPINION

A potential novel treatment strategy for highly treatment-resistant tumors is the induction of a cellular stress response in cancer cells. The induction of an organelle-related stress response, such as nuclear, ER and mitochondrial stress, leads to a dramatic sensitization of a broad variety of cancer cells of different tumor entities to the apoptotic ligand, TRAIL. Importantly, non-neoplastic cells are not sensitized to TRAIL-mediated cell death through the unfolded protein response in most instances, suggesting that this treatment is not only of high efficacy, but even more less of unwanted toxicity in patients.

HtrA2 deficiency causes mitochondrial uncoupling through the F₁F₀-ATP synthase and consequent ATP depletion

Loss of the mitochondrial protease HtrA2 (Omi) in mice leads to mitochondrial dysfunction, neurodegeneration and premature death, but the mechanism underlying this pathology remains unclear. Using primary cultures from wild-type and HtrA2-knockout mice, we find that HtrA2 deficiency significantly reduces mitochondrial membrane potential in a range of cell types. This depolarisation was found to result from mitochondrial uncoupling, as mitochondrial respiration was increased in HtrA2-deficient cells and respiratory control ratio was dramatically reduced. HtrA2-knockout cells exhibit increased proton translocation through the ATP synthase, in combination with decreased ATP production and truncation of the F1 α-subunit, suggesting the ATP synthase as the source of the proton leak. Uncoupling in the HtrA2-deficient mice is accompanied by altered breathing pattern and, on a cellular level, ATP depletion and vulnerability to chemical ischaemia. We propose that this vulnerability may ultimately cause the neurodegeneration observed in these mice.