Inducers of Chemical Hypoxia Act in a Gender- and Brain Region-Specific Manner on Primary Astrocyte Viability and Cytochrome c Oxidase

Resilience to capsaicin-induced mitochondrial damage in trigeminal ganglion neurons

Capsaicin is an agonist of transient receptor potential cation channel subfamily V member 1 (TRPV1). Strong TRPV1 stimulation with capsaicin causes mitochondrial damage in primary sensory neurons. However, the effect of repetitive and moderate exposure to capsaicin on the integrity of neuronal mitochondria remains largely unknown. Our electron microscopic analysis revealed that repetitive stimulation of the facial skin of mice with 10 mM capsaicin induced short-term damage to the mitochondria in small-sized trigeminal ganglion neurons. Further, capsaicin-treated mice exhibited decreased sensitivity to noxious heat stimulation, indicating TRPV1 dysfunction, in parallel with the mitochondrial damage in the trigeminal ganglion neurons. To analyze the capsaicin-induced mitochondrial damage and its relevant cellular events in detail, we performed cell-based assays using TRPV1-expressing PC12 cells. Dose-dependent capsaicin-mediated mitochondrial toxicity was observed. High doses of capsaicin caused rapid destruction of mitochondrial internal structure, while low doses induced mitochondrial swelling. Further, capsaicin induced a dose-dependent loss of mitochondria and autophagy-mediated degradation of mitochondria (mitophagy). Concomitantly, transcriptional upregulation of mitochondrial proteins, cytochrome c oxidase subunit IV, Mic60/Mitofilin, and voltage-dependent anion channel 1 was observed, which implied induction of mitochondrial biogenesis to compensate for the loss of mitochondria. Collectively, although trigeminal ganglion neurons transiently exhibit mitochondrial damage and TRPV1 dysfunction following moderate capsaicin exposure, they appear to be resilient to such a challenge. Our in vitro data show a dose-response relationship in capsaicin-mediated mitochondrial toxicity. We postulate that induction of mitophagy and mitochondrial biogenesis in response to capsaicin stimulation play important roles in repairing the damaged mitochondrial system.

Cox4i2 Triggers an Increase in Reactive Oxygen Species, Leading to Ferroptosis and Apoptosis in HHV7 Infected Schwann Cells

Emerging evidence suggests that reactive oxygen species (ROS) play a significant role in the pathogenesis of peripheral nerve damage. Our previous study indicated that human herpesvirus 7 (HHV7) induces Bell’s palsy. However, the specific mechanism underlying the effects of ROS in HHV7 infection-induced facial nerve damage is unknown. In this study, we established a rat FN model by inoculating an HHV7 virus solution. The facial grading score and LuxolFastBlue (LFB) staining were used to assess the success of the model. Using mRNA-sequencing analysis, we found that the expression of Complex IV Subunit 4 Isoform 2 (Cox4i2) increased in infected Schwann cells (SCs). Cox4i2 was suggested to increase COX activity, thereby promoting ROS production. The changes in the endogenous oxidant and antioxidant system were assessed, and the results showed that oxidative stress increased after HHV7 infection in vivo and in vitro. However, we found that oxidative injury was relieved after the transfection of shCox4i2 in HHV7-treated SCs by evaluating cell death, cell proliferation, and the ROS level as well as the levels of malondialdehyde (MDA), superoxide dismutase (SOD), and glutathione (GSH). Furthermore, we hypothesised that Cox4i2 loss would attenuate HHV7-induced ferroptosis and apoptosis, which are closely related to ROS in SCs. Our research illustrated that the knockdown of Cox4i2 suppresses HHV7-induced RSC96 cell ferroptosis as well as apoptosis via the ERK signalling pathway. Overall, several in vitro and in vivo methods were adopted in this study to reveal the new mechanism of ROS-induced and Cox4i2-mediated apoptosis and ferroptosis in HHV7 infected SCs.

Amyloid-β1-40 differentially stimulates proliferation, activation of oxidative stress and inflammatory responses in male and female hippocampal astrocyte cultures

Alzheimer's disease (AD) is the most common form of dementia and has a higher incidence in women. The main component of the senile plaques characteristic of AD is amyloid-beta (Aβ), with surrounding astrocytes contributing to the degenerative process. We hypothesized that the sex difference in the incidence of AD could be partially due to differential astrocytic responses to Aβ. Thus, the effect of Aβ_1-40 on cell viability, the inflammatory response, and oxidative status was studied in cultures of hippocampal astrocytes from male and female rats. Aβ_1-40 increased astrocyte viability in both female and male cultures by activating proliferation and survival pathways. Pro-inflammatory and anti-inflammatory responses were induced in astrocytes from both sexes. Aβ_1-40 did not affect endoplasmic reticulum stress although it induced oxidative stress in male and female astrocytes. Interestingly, male astrocytes had an increase in cell number and significantly lower cell death in response to Aβ_1-40. Conversely, astrocytes from females displayed a greater inflammatory response after the Aβ_1-40 challenge. These results suggest that the inflammatory and oxidative environment induced by Aβ_1-40 in female astrocytes may contribute to enhance the vulnerability to AD and warrants further studies to unveil the mechanisms underlying sex differences in astrocytic responses.

Sex differences in the mitochondrial bioenergetics of astrocytes but not microglia at a physiologically relevant brain oxygen tension

Biological sex is thought to influence mitochondrial bioenergetic function. Previous respiration measurements examining brain mitochondrial sex differences were made at atmospheric oxygen using isolated brain mitochondria. Oxygen is 160 mm Hg (21%) in the atmosphere, while the oxygen tension in the brain generally ranges from ∼5 to 45 mm Hg (∼1-6% O2). This study tested the hypothesis that sex and/or brain physiological oxygen tension influence the mitochondrial bioenergetic properties of primary rat cortical astrocytes and microglia. Oxygen consumption was measured with a Seahorse XF24 cell respirometer in an oxygen-controlled environmental chamber. Strikingly, male astrocytes had a higher maximal respiration than female astrocytes when cultured and assayed at 3% O2. Three percent O2 yielded a low physiological dissolved O2 level of ∼1.2% (9.1 mm Hg) at the cell monolayer during culture and 1.2-3.0% O2 during assays. No differences in bioenergetic parameters were observed between male and female astrocytes at 21% O2 (dissolved O2 of ∼19.7%, 150 mm Hg during culture) or between either of these cell populations and female astrocytes at 3% O2. In contrast to astrocytes, microglia showed no sex differences in mitochondrial bioenergetic parameters at either oxygen level, regardless of whether they were non-stimulated or activated to a proinflammatory state. There were also no O2- or sex-dependent differences in proinflammatory TNF-α or IL-1β cytokine secretion measured at 18 h activation. Overall, results reveal an intriguing sex variance in astrocytic maximal respiration that requires additional investigation. Findings also demonstrate that sex differences can be masked by conducting experiments at non-physiological O2.

Effects of seawater temperature increase on economically relevant native and introduced clam species

As a consequence of climate change, global warming is expected to increase during the 21^st century. Taking this into account, the impact of rising temperatures on the native Ruditapes decussatus and introduced R. philippinarum bivalve species was assessed, through biochemical and mRNA transcription analyses. Our findings showed that at 21 °C the electron transport system and antioxidant enzyme activity, as well as the expression of Hsp70 gene were induced in R. decussatus when compared with 17 °C. On the other hand, at 25 °C results suggested that R. decussatus closed their valves during short periods, as a behavioral strategy, down-regulating the expression of genes associated with mitochondrial metabolism (Cox-1 and 16S) and chaperone function (Hsp70) compared with organisms at 17 °C. In addition, the introduced species (R. philippinarum) increased the electron transport system and antioxidant activities, as well as gene expression of antioxidant enzymes and molecular chaperone (Hsp70) at 21 °C. However, antioxidant mechanisms were not enough to prevent lipid membrane damages at 21 °C. At 25 °C R. philippinarum presented increased electron transport system and antioxidant activity, as well as the expression of genes associated with apoptosis regulation and molecular chaperone. Overall, the present findings indicate that in a global warming scenario both species are able to induce different mechanisms to mitigate the impacts of temperature increase.

Control of human energy expenditure by cytochrome c oxidase subunit IV-2

Resting metabolic rate (RMR) in humans shows pronounced individual variations, but the underlying molecular mechanism remains elusive. Cytochrome c oxidase (COX) plays a key role in control of metabolic rate, and recent studies of the subunit 4 isoform 2 (COX IV-2) indicate involvement in the cellular response to hypoxia and oxidative stress. We evaluated whether the COX subunit IV isoform composition may explain the pronounced individual variations in resting metabolic rate (RMR). RMR was determined in healthy humans by indirect calorimetry and correlated to levels of COX IV-2 and COX IV-1 in vastus lateralis. Overexpression and knock down of the COX IV isoforms were performed in primary myotubes followed by evaluation of the cell respiration and production of reactive oxygen species. Here we show that COX IV-2 protein is constitutively expressed in human skeletal muscle and strongly correlated to RMR. Primary human myotubes overexpressing COX IV-2 displayed markedly (>60%) lower respiration, reduced (>50%) cellular H2O2 production, higher resistance toward both oxidative stress, and severe hypoxia compared with control cells. These results suggest an important role of isoform COX IV-2 in the control of energy expenditure, hypoxic tolerance, and mitochondrial ROS homeostasis in humans.

Chemical Conditioning as an Approach to Ischemic Stroke Tolerance: Mitochondria as the Target

It is well established that the brain can be prepared to resist or tolerate ischemic stroke injury, and mitochondrion is a major target for this tolerance. The preparation of ischemic stroke tolerance can be achieved by three major approaches: ischemic conditioning, hypoxic conditioning and chemical conditioning. In each conditioning approach, there are often two strategies that can be used to achieve the conditioning effects, namely preconditioning (Pre-C) and postconditioning (Post-C). In this review, we focus on chemical conditioning of mitochondrial proteins as targets for neuroprotection against ischemic stroke injury. Mitochondrial targets covered include complexes I, II, IV, the ATP-sensitive potassium channel (mitoKATP), adenine dinucleotide translocase (ANT) and the mitochondrial permeability transition pore (mPTP). While numerous mitochondrial proteins have not been evaluated in the context of chemical conditioning and ischemic stroke tolerance, the paradigms and approaches reviewed in this article should provide general guidelines on testing those mitochondrial components that have not been investigated. A deep understanding of mitochondria as the target of chemical conditioning for ischemic stroke tolerance should provide valuable insights into strategies for fighting ischemic stroke, a leading cause of death in the world.

Increased incidence of intermittent hypoxemia in the Ts65Dn mouse model of Down syndrome

In addition to nervous system, cardiovascular and respiratory systems are primarily affected in Down syndrome (DS). The Ts65Dn mouse model is widely used to recapitulate cognitive dysfunction in DS. While these mice consistently show failure in learning and memory along with functional and structural abnormalities in the hippocampal region, the underlying mechanisms behind cognitive dysfunction remain to be fully elucidated. Convergent evidence implicates chronic episodes of hypoxemia in cognitive dysfunction in people with DS. Using an infra-red detection system to assess oxygen saturation in free-moving mice, we assessed arterial blood oxygenation in both adolescent and adult Ts65Dn mice and found a significant increase in the incidence of hypoxemia in both groups. Notably, the severity of hypoxemia increased during the dark cycle, suggesting a link between hypoxemia and increased motor activity. Postmortem analysis showed significant increase in the expression of mitochondrial Cox4i2, the terminal enzyme of the mitochondrial respiratory chain and oxygen response element. Altogether these data suggest early and chronic occurrence of hypoxemia in the Ts65Dn mouse model of DS, which can contribute to cognitive dysfunction in these mice.

Disparate roles of zinc in chemical hypoxia-induced neuronal death

Accumulating evidence has provided a causative role of zinc (Zn²⁺) in neuronal death following ischemic brain injury. Using a hypoxia model of primary cultured cortical neurons with hypoxia-inducing chemicals, cobalt chloride (1 mM CoCl₂), deferoxamine (3 mM DFX), and sodium azide (2 mM NaN₃), we evaluated whether Zn²⁺ is involved in hypoxic neuronal death. The hypoxic chemicals rapidly elicited intracellular Zn²⁺ release/accumulation in viable neurons. The immediate addition of the Zn²⁺ chelator, CaEDTA or N,N,N’N’-tetrakis-(2-pyridylmethyl) ethylenediamine (TPEN), prevented the intracellular Zn²⁺ load and CoCl₂-induced neuronal death, but neither 3 hour later Zn²⁺ chelation nor a non-Zn²⁺ chelator ZnEDTA (1 mM) demonstrated any effects. However, neither CaEDTA nor TPEN rescued neurons from cell death following DFX- or NaN₃-induced hypoxia, whereas ZnEDTA rendered them resistant to the hypoxic injury. Instead, the immediate supplementation of Zn²⁺ rescued DFX- and NaN₃-induced neuronal death. The iron supplementation also afforded neuroprotection against DFX-induced hypoxic injury. Thus, although intracellular Zn²⁺ release/accumulation is common during chemical hypoxia, Zn²⁺ might differently influence the subsequent fate of neurons; it appears to play a neurotoxic or neuroprotective role depending on the hypoxic chemical used. These results also suggest that different hypoxic chemicals may induce neuronal death via distinct mechanisms.

Computational deconvolution of genome wide expression data from Parkinson's and Huntington's disease brain tissues using population-specific expression analysis

The characterization of molecular changes in diseased tissues gives insight into pathophysiological mechanisms and is important for therapeutic development. Genome-wide gene expression analysis has proven valuable for identifying biological processes in neurodegenerative diseases using post mortem human brain tissue and numerous datasets are publically available. However, many studies utilize heterogeneous tissue samples consisting of multiple cell types, all of which contribute to global gene expression values, confounding biological interpretation of the data. In particular, changes in numbers of neuronal and glial cells occurring in neurodegeneration confound transcriptomic analyses, particularly in human brain tissues where sample availability and controls are limited. To identify cell specific gene expression changes in neurodegenerative disease, we have applied our recently published computational deconvolution method, population specific expression analysis (PSEA). PSEA estimates cell-type-specific expression values using reference expression measures, which in the case of brain tissue comprises mRNAs with cell-type-specific expression in neurons, astrocytes, oligodendrocytes and microglia. As an exercise in PSEA implementation and hypothesis development regarding neurodegenerative diseases, we applied PSEA to Parkinson's and Huntington's disease (PD, HD) datasets. Genes identified as differentially expressed in substantia nigra pars compacta neurons by PSEA were validated using external laser capture microdissection data. Network analysis and Annotation Clustering (DAVID) identified molecular processes implicated by differential gene expression in specific cell types. The results of these analyses provided new insights into the implementation of PSEA in brain tissues and additional refinement of molecular signatures in human HD and PD.

Astrocyte reactivity and reactive astrogliosis: costs and benefits

Astrocytes are the most abundant cells in the central nervous system (CNS) that provide nutrients, recycle neurotransmitters, as well as fulfill a wide range of other homeostasis maintaining functions. During the past two decades, astrocytes emerged also as increasingly important regulators of neuronal functions including the generation of new nerve cells and structural as well as functional synapse remodeling. Reactive gliosis or reactive astrogliosis is a term coined for the morphological and functional changes seen in astroglial cells/astrocytes responding to CNS injury and other neurological diseases. Whereas this defensive reaction of astrocytes is conceivably aimed at handling the acute stress, limiting tissue damage, and restoring homeostasis, it may also inhibit adaptive neural plasticity mechanisms underlying recovery of function. Understanding the multifaceted roles of astrocytes in the healthy and diseased CNS will undoubtedly contribute to the development of treatment strategies that will, in a context-dependent manner and at appropriate time points, modulate reactive astrogliosis to promote brain repair and reduce the neurological impairment.

Permeability transition pore-dependent and PARP-mediated depletion of neuronal pyridine nucleotides during anoxia and glucose deprivation

Exposure of rat cortical neurons to combined oxygen and glucose deprivation results in loss of NAD(P)H autofluorescence that is only partially reversible following restoration of oxygen and glucose, suggesting catabolism of pyridine nucleotides. This study tested the hypothesis that metabolic inhibition caused by cyanide-induced chemical anoxia plus glucose deprivation promotes both release of mitochondrial NAD(H) in response to opening of the permeability transition pore (PTP) and NAD(P)(H) degradation through activation of poly (ADP-ribose) polymerase (PARP). The NAD(P)H autofluorescence of rat neonatal cortical neurons was monitored during and following acute (10-30 min) exposure to the respiratory inhibitor, cyanide, in the absence and presence of glucose. Because nitric oxide-derived peroxynitrite is a known activator of PARP, we additionally assessed the effect of a nitric oxide generating agent on the NAD(P)H autofluorescence response to chemical anoxia plus glucose deprivation. Cyanide induced a rapid increase in autofluorescence, followed by a steady decline promoted by the presence of nitric oxide. This decline was primarily due to NAD(H) catabolism, as verified by measurements of total NAD(H) present in cellular extracts. Catabolism was partially blocked by an inhibitor of PARP, by a PTP inhibitor, and by either glucose or pyruvate as a source of reducing power. Overall, data suggest that metabolic, oxidative, and nitrosative stress during in vitro neuronal anoxia and glucose deprivation result in release of mitochondrial pyridine nucleotides in response to PTP opening and rapid, extensive NAD(H) degradation mediated by PARP activation. These events may contribute to the metabolic dysfunction that occurs in vivo during cerebral ischemia and reperfusion and therefore represent prime targets for neuroprotection.

The dual role of astrocyte activation and reactive gliosis

Astrocyte activation and reactive gliosis accompany most of the pathologies in the brain, spinal cord, and retina. Reactive gliosis has been described as constitutive, graded, multi-stage, and evolutionary conserved defensive astroglial reaction [Verkhratsky and Butt (2013) In: Glial Physiology and Pathophysiology]. A well- known feature of astrocyte activation and reactive gliosis are the increased production of intermediate filament proteins (also known as nanofilament proteins) and remodeling of the intermediate filament system of astrocytes. Activation of astrocytes is associated with changes in the expression of many genes and characteristic morphological hallmarks, and has important functional consequences in situations such as stroke, trauma, epilepsy, Alzheimer's disease (AD), and other neurodegenerative diseases. The impact of astrocyte activation and reactive gliosis on the pathogenesis of different neurological disorders is not yet fully understood but the available experimental evidence points to many beneficial aspects of astrocyte activation and reactive gliosis that range from isolation and sequestration of the affected region of the central nervous system (CNS) from the neighboring tissue that limits the lesion size to active neuroprotection and regulation of the CNS homeostasis in times of acute ischemic, osmotic, or other kinds of stress. The available experimental data from selected CNS pathologies suggest that if not resolved in time, reactive gliosis can exert inhibitory effects on several aspects of neuroplasticity and CNS regeneration and thus might become a target for future therapeutic interventions.

Expressions of oxidative stress-related genes and antioxidant enzyme activities in Mytilus galloprovincialis (Bivalvia, Mollusca) exposed to hypoxia

Background

In this study, we investigated transcription and enzyme level responses of mussels Mytilus galloprovincialis exposed to hypoxic conditions. Genes for catalase (CAT), cytochrome P450, glutathione S-transferase (GST), metallothionein, superoxide dismutase (SOD), cytochrome c oxidase subunit 1 (COX-1), and NADH dehydrogenase subunit 2 were selected for study. Transcriptional changes were investigated in mussels exposed to hypoxia for 24 and 48 h and were compared to changes in control mussels maintained at normal oxygen levels. Activities of CAT, GST, and SOD enzymes, and lipid peroxidation (LPO) were also investigated in mussels following exposure to hypoxia for 24, 48, and 72 h.

Results

Relative to the control group, the CAT activity decreased in all hypoxia treatments, while the activity of GST significantly increased in mussels exposed to hypoxia for 24 and 48 h, but decreased in those exposed for 72 h. The LPO levels were significantly higher in mussels in the 24- and 48-h hypoxia treatments than those in the control mussels, but there was no significant change in the SOD activities among all hypoxia treatments. Messenger RNA levels for the CAT, cytochrome P450, GST, metallothionein, and SOD genes were not significantly affected by hypoxic conditions for 48 h, but the expressions of the COX-1 and NADH dehydrogenase subunit 2 genes were significantly repressed in mussels in both the 24- and 48-h exposure treatments.

Conclusions

These results demonstrate the transcriptional stability and changes among several genes related to oxidative stress under oxygen-depletion conditions in M. galloprovincialis and provide useful information about the modulation of antioxidant enzyme activities induced by hypoxia in a marine animal.

Cytochrome c oxidase and its role in neurodegeneration and neuroprotection

A hallmark of neurodegenerative diseases, such as Alzheimer's and Parkinson's diseases, and stroke is a malfunction of mitochondria including cytochrome c oxidase (COX), the terminal enzyme complex of the respiratory chain. COX is ascribed a key role based on mainly two regulatory mechanisms. These are the expression of isoforms and the binding of specific allosteric factors to nucleus--encoded subunits. These characteristics represent a unique feature of COX compared with the other respiratory chain complexes. Additional regulatory mechanisms, such as posttranslational modification, substrate availability, and allosteric feedback inhibition by products of the COX reaction, control the enzyme activity in a complex way. In many tissues and cell types, COX represents the rate-limiting enzyme of the respiratory chain which further emphasizes the impact of the regulation of COX as a central site for regulating energy metabolism and oxidative stress. Two of the best-analyzed regulatory mechanisms of COX to date are the allosteric feedback inhibition of the enzyme by its indirect product ATP and the expression of COX subunit IV isoforms. This ATP feedback inhibition of COX requires the expression of COX isoform IV-1. At high ATP/ADP ratios, ADP is exchanged for ATP at the matrix side of COX IV-1 leading to an inhibition of COX activity, thus enabling COX to sense the energy level and to adjust ATP synthesis to energy demand. However, under hypoxic, toxic, and degenerative conditions, COX isoform IV-2 expression is up-regulated and exchanged for COX IV-1 in the enzyme complex. This COX IV isoform switch causes an abolition of the allosteric ATP feedback inhibition of COX and consequently the loss of sensing the energy level. Thus, COX activity is increased leading to higher levels of ATP in neural cells independently of the cellular energy level. Concomitantly, ROS production is increased. Thus, under pathological conditions, neural cells are provided with ATP to meet the energy demand, but at the expense of elevated oxidative stress. This mechanism explains the functional relevance of COX subunit IV isoform expression for cellular energy sensing, ATP production, and oxidative stress levels. This, in turn, affects neural cell function, signaling, and -survival. Thus, COX is a crucial factor in etiology, progression, and prevalence of numerous human neurodegenerative diseases and represents an important target for developing diagnostic and therapeutic tools against those diseases.

Cytochrome c oxidase subunit 4 isoform 2-knockout mice show reduced enzyme activity, airway hyporeactivity, and lung pathology

Cytochrome c oxidase (COX) is the terminal enzyme of the mitochondrial electron transport chain. The purpose of this study was to analyze the function of lung-specific cytochrome c oxidase subunit 4 isoform 2 (COX4i2) in vitro and in COX4i2-knockout mice in vivo. COX was isolated from cow lung and liver as control and functionally analyzed. COX4i2-knockout mice were generated and the effect of the gene knockout was determined, including COX activity, tissue energy levels, noninvasive and invasive lung function, and lung pathology. These studies were complemented by a comprehensive functional screen performed at the German Mouse Clinic (Neuherberg, Germany). We show that isolated cow lung COX containing COX4i2 is about twice as active (88 and 102% increased activity in the presence of allosteric activator ADP and inhibitor ATP, respectively) as liver COX, which lacks COX4i2. In COX4i2-knockout mice, lung COX activity and cellular ATP levels were significantly reduced (-50 and -29%, respectively). Knockout mice showed decreased airway responsiveness (60% reduced P(enh) and 58% reduced airway resistance upon challenge with 25 and 100 mg methacholine, respectively), and they developed a lung pathology deteriorating with age that included the appearance of Charcot-Leyden crystals. In addition, there was an interesting sex-specific phenotype, in which the knockout females showed reduced lean mass (-12%), reduced total oxygen consumption rate (-8%), improved glucose tolerance, and reduced grip force (-14%) compared to wild-type females. Our data suggest that high activity lung COX is a central determinant of airway function and is required for maximal airway responsiveness and healthy lung function. Since airway constriction requires energy, we propose a model in which reduced tissue ATP levels explain protection from airway hyperresponsiveness, i.e., absence of COX4i2 leads to reduced lung COX activity and ATP levels, which results in impaired airway constriction and thus reduced airway responsiveness; long-term lung pathology develops in the knockout mice due to impairment of energy-costly lung maintenance processes; and therefore, we propose mitochondrial oxidative phosphorylation as a novel target for the treatment of respiratory diseases, such as asthma.

The power of life--cytochrome c oxidase takes center stage in metabolic control, cell signalling and survival

Mitochondrial dysfunction is increasingly recognized as a major factor in the etiology and progression of numerous human diseases, such as (neuro-)degeneration, ischemia reperfusion injury, cancer, and diabetes. Cytochrome c oxidase (COX) represents the rate-limiting enzyme of the mitochondrial respiratory chain and is thus predestined for being a central site of regulation of oxidative phosphorylation, proton pumping efficiency, ATP and reactive oxygen species production, which in turn affect cell signaling and survival. A unique feature of COX is its regulation by various factors and mechanisms interacting with the nucleus-encoded subunits, whose actual functions we are only beginning to understand.