Mitochondrial Complex I and β-Amyloid Peptide Interplay in Alzheimer's Disease: A Critical Review of New and Old Little Regarded Findings

Alzheimer's disease (AD) is the most common neurodegenerative disorder and the main cause of dementia which is characterized by a progressive cognitive decline that severely interferes with daily activities of personal life. At a pathological level, it is characterized by the accumulation of abnormal protein structures in the brain-β-amyloid (Aβ) plaques and Tau tangles-which interfere with communication between neurons and lead to their dysfunction and death. In recent years, research on AD has highlighted the critical involvement of mitochondria-the primary energy suppliers for our cells-in the onset and progression of the disease, since mitochondrial bioenergetic deficits precede the beginning of the disease and mitochondria are very sensitive to Aβ toxicity. On the other hand, if it is true that the accumulation of Aβ in the mitochondria leads to mitochondrial malfunctions, it is otherwise proven that mitochondrial dysfunction, through the generation of reactive oxygen species, causes an increase in Aβ production, by initiating a vicious cycle: there is therefore a bidirectional relationship between Aβ aggregation and mitochondrial dysfunction. Here, we focus on the latest news-but also on neglected evidence from the past-concerning the interplay between dysfunctional mitochondrial complex I, oxidative stress, and Aβ, in order to understand how their interplay is implicated in the pathogenesis of the disease.

The Cleavage-Specific Tau 12A12mAb Exerts an Anti-Amyloidogenic Action by Modulating the Endocytic and Bioenergetic Pathways in Alzheimer's Disease Mouse Model

Beyond deficits in hippocampal-dependent episodic memory, Alzheimer's Disease (AD) features sensory impairment in visual cognition consistent with extensive neuropathology in the retina. 12A12 is a monoclonal cleavage specific antibody (mAb) that in vivo selectively neutralizes the AD-relevant, harmful N-terminal 20-22 kDa tau fragment(s) (i.e., NH₂htau) without affecting the full-length normal protein. When systemically injected into the Tg2576 mouse model overexpressing a mutant form of Amyloid Precursor Protein (APP), APPK670/671L linked to early onset familial AD, this conformation-specific tau mAb successfully reduces the NH₂htau accumulating both in their brain and retina and, thus, markedly alleviates the phenotype-associated signs. By means of a combined biochemical and metabolic experimental approach, we report that 12A12mAb downregulates the steady state expression levels of APP and Beta-Secretase 1 (BACE-1) and, thus, limits the Amyloid beta (Aβ) production both in the hippocampus and retina from this AD animal model. The local, antibody-mediated anti-amyloidogenic action is paralleled in vivo by coordinated modulation of the endocytic (BIN1, RIN3) and bioenergetic (glycolysis and L-Lactate) pathways. These findings indicate for the first time that similar molecular and metabolic retino-cerebral pathways are modulated in a coordinated fashion in response to 12A12mAb treatment to tackle the neurosensorial Aβ accumulation in AD neurodegeneration.

Mitochondria Have Made a Long Evolutionary Path from Ancient Bacteria Immigrants within Eukaryotic Cells to Essential Cellular Hosts and Key Players in Human Health and Disease

Mitochondria have made a long evolutionary path from ancient bacteria immigrants within the eukaryotic cell to become key players for the cell, assuming crucial multitasking skills critical for human health and disease. Traditionally identified as the powerhouses of eukaryotic cells due to their central role in energy metabolism, these chemiosmotic machines that synthesize ATP are known as the only maternally inherited organelles with their own genome, where mutations can cause diseases, opening up the field of mitochondrial medicine. More recently, the omics era has highlighted mitochondria as biosynthetic and signaling organelles influencing the behaviors of cells and organisms, making mitochondria the most studied organelles in the biomedical sciences. In this review, we will especially focus on certain 'novelties' in mitochondrial biology "left in the shadows" because, although they have been discovered for some time, they are still not taken with due consideration. We will focus on certain particularities of these organelles, for example, those relating to their metabolism and energy efficiency. In particular, some of their functions that reflect the type of cell in which they reside will be critically discussed, for example, the role of some carriers that are strictly functional to the typical metabolism of the cell or to the tissue specialization. Furthermore, some diseases in whose pathogenesis, surprisingly, mitochondria are involved will be mentioned.

Therapeutic Potential of Targeting Mitochondria for Alzheimer's Disease Treatment

Alzheimer's disease (AD), a chronic and progressive neurodegenerative disease, is characterized by memory and cognitive impairment and by the accumulation in the brain of abnormal proteins, more precisely beta-amyloid (β-amyloid or Aβ) and Tau proteins. Studies aimed at researching pharmacological treatments against AD have focused precisely on molecules capable, in one way or another, of preventing/eliminating the accumulations of the aforementioned proteins. Unfortunately, more than 100 years after the discovery of the disease, there is still no effective therapy in modifying the biology behind AD and nipping the disease in the bud. This state of affairs has made neuroscientists suspicious, so much so that for several years the idea has gained ground that AD is not a direct neuropathological consequence taking place downstream of the deposition of the two toxic proteins, but rather a multifactorial disease, including mitochondrial dysfunction as an early event in the pathogenesis of AD, occurring even before clinical symptoms. This is the reason why the search for pharmacological agents capable of normalizing the functioning of these subcellular organelles of vital importance for nerve cells is certainly to be considered a promising approach to the design of effective neuroprotective drugs aimed at preserving this organelle to arrest or delay the progression of the disease. Here, our intent is to provide an updated overview of the mitochondrial alterations related to this disorder and of the therapeutic strategies (both natural and synthetic) targeting mitochondrial dysfunction.

Tau Cleavage Contributes to Cognitive Dysfunction in Strepto-Zotocin-Induced Sporadic Alzheimer's Disease (sAD) Mouse Model

Tau cleavage plays a crucial role in the onset and progression of Alzheimer’s Disease (AD), a widespread neurodegenerative disease whose incidence is expected to increase in the next years. While genetic and familial forms of AD (fAD) occurring early in life represent less than 1%, the sporadic and late-onset ones (sAD) are the most common, with ageing being an important risk factor. Intracerebroventricular (ICV) infusion of streptozotocin (STZ)—a compound used in the systemic induction of diabetes due to its ability to damage the pancreatic β cells and to induce insulin resistance—mimics in rodents several behavioral, molecular and histopathological hallmarks of sAD, including memory/learning disturbance, amyloid-β (Aβ) accumulation, tau hyperphosphorylation, oxidative stress and brain glucose hypometabolism. We have demonstrated that pathological truncation of tau at its N-terminal domain occurs into hippocampi from two well-established transgenic lines of fAD animal models, such as Tg2576 and 3xTg mice, and that it’s in vivo neutralization via intravenous (i.v.) administration of the cleavage-specific anti-tau 12A12 monoclonal antibody (mAb) is strongly neuroprotective. Here, we report the therapeutic efficacy of 12A12mAb in STZ-infused mice after 14 days (short-term immunization, STIR) and 21 days (long-term immunization regimen, LTIR) of i.v. delivery. A virtually complete recovery was detected after three weeks of 12A12mAb immunization in both novel object recognition test (NORT) and object place recognition task (OPRT). Consistently, three weeks of this immunization regimen relieved in hippocampi from ICV-STZ mice the AD-like up-regulation of amyloid precursor protein (APP), the tau hyperphosphorylation and neuroinflammation, likely due to modulation of the PI3K/AKT/GSK3-β axis and the AMP-activated protein kinase (AMPK) activities. Cerebral oxidative stress, mitochondrial impairment, synaptic and histological alterations occurring in STZ-infused mice were also strongly attenuated by 12A12mAb delivery. These results further strengthen the causal role of N-terminal tau cleavage in AD pathogenesis and indicate that its specific neutralization by non-invasive administration of 12A12mAb can be a therapeutic option for both fAD and sAD patients, as well as for those showing type 2 diabetes as a comorbidity.

Mitochondria Can Cross Cell Boundaries: An Overview of the Biological Relevance, Pathophysiological Implications and Therapeutic Perspectives of Intercellular Mitochondrial Transfer

Mitochondria are complex intracellular organelles traditionally identified as the powerhouses of eukaryotic cells due to their central role in bioenergetic metabolism. In recent decades, the growing interest in mitochondria research has revealed that these multifunctional organelles are more than just the cell powerhouses, playing many other key roles as signaling platforms that regulate cell metabolism, proliferation, death and immunological response. As key regulators, mitochondria, when dysfunctional, are involved in the pathogenesis of a wide range of metabolic, neurodegenerative, immune and neoplastic disorders. Far more recently, mitochondria attracted renewed attention from the scientific community for their ability of intercellular translocation that can involve whole mitochondria, mitochondrial genome or other mitochondrial components. The intercellular transport of mitochondria, defined as horizontal mitochondrial transfer, can occur in mammalian cells both in vitro and in vivo, and in physiological and pathological conditions. Mitochondrial transfer can provide an exogenous mitochondrial source, replenishing dysfunctional mitochondria, thereby improving mitochondrial faults or, as in in the case of tumor cells, changing their functional skills and response to chemotherapy. In this review, we will provide an overview of the state of the art of the up-to-date knowledge on intercellular trafficking of mitochondria by discussing its biological relevance, mode and mechanisms underlying the process and its involvement in different pathophysiological contexts, highlighting its therapeutic potential for diseases with mitochondrial dysfunction primarily involved in their pathogenesis.

Mitochondrial Bioenergetics in Different Pathophysiological Conditions

Mitochondria are complex intracellular organelles involved in many aspects of cellular life, with a primary role in bioenergy production via oxidative phosphorylation (OXPHOS) [...].

Systemic delivery of a specific antibody targeting the pathological N-terminal truncated tau peptide reduces retinal degeneration in a mouse model of Alzheimer's Disease

Retina and optic nerve are sites of extra-cerebral manifestations of Alzheimer's Disease (AD). Amyloid-β (Aβ) plaques and neurofibrillary tangles of hyperphosphorylated tau protein are detected in eyes from AD patients and transgenic animals in correlation with inflammation, reduction of synapses, visual deficits, loss of retinal cells and nerve fiber. However, neither the pathological relevance of other post-translational tau modifications-such as truncation with generation of toxic fragments-nor the potential neuroprotective action induced by their in vivo clearance have been investigated in the context of AD retinal degeneration. We have recently developed a monoclonal tau antibody (12A12mAb) which selectively targets the neurotoxic 20-22 kDa NH2-derived peptide generated from pathological truncation at the N-terminal domain of tau without cross-reacting with its full-length normal protein. Previous studies have shown that 12A12mAb, when intravenously (i.v.)-injected into 6-month-old Tg2576 animals, markedly improves their AD-like, behavioural and neuropathological syndrome. By taking advantage of this well-established tau-directed immunization regimen, we found that 12A12mAb administration also exerts a beneficial action on biochemical, morphological and metabolic parameters (i.e. APP/Aβ processing, tau hyperphosphorylation, neuroinflammation, synaptic proteins, microtubule stability, mitochondria-based energy production, neuronal death) associated with ocular injury in the AD phenotype. These findings prospect translational implications in the AD field by: (1) showing for the first time that cleavage of tau takes part in several pathological changes occurring in vivo in affected retinas and vitreous bodies and that its deleterious effects are successfully antagonized by administration of the specific 12A12mAb; (2) shedding further insights on the tight connections between neurosensory retina and brain, in particular following tau-based immunotherapy. In our view, the parallel response we detected in this preclinical animal model, both in the eye and in the hippocampus, following i.v. 12A12mAb injection opens novel diagnostic and therapeutic avenues for the clinical management of cerebral and extracerebral AD signs in human beings.

Functional Foods: An Approach to Modulate Molecular Mechanisms of Alzheimer's Disease

A new epoch is emerging with intense research on nutraceuticals, i.e., "food or food product that provides medical or health benefits including the prevention and treatment of diseases", such as Alzheimer's disease. Nutraceuticals act at different biochemical and metabolic levels and much evidence shows their neuroprotective effects; in particular, they are able to provide protection against mitochondrial damage, oxidative stress, toxicity of β-amyloid and Tau and cell death. They have been shown to influence the composition of the intestinal microbiota significantly contributing to the discovery that differential microorganisms composition is associated with the formation and aggregation of cerebral toxic proteins. Further, the routes of interaction between epigenetic mechanisms and the microbiota-gut-brain axis have been elucidated, thus establishing a modulatory role of diet-induced epigenetic changes of gut microbiota in shaping the brain. This review examines recent scientific literature addressing the beneficial effects of some natural products for which mechanistic evidence to prevent or slowdown AD are available. Even if the road is still long, the results are already exceptional.

An Intriguing Involvement of Mitochondria in Cystic Fibrosis

Cystic fibrosis (CF) occurs when the cystic fibrosis transmembrane conductance regulator (CFTR) protein is not synthetized and folded correctly. The CFTR protein helps to maintain the balance of salt and water on many body surfaces, such as the lung surface. When the protein is not working correctly, chloride becomes trapped in cells, then water cannot hydrate the cellular surface and the mucus covering the cells becomes thick and sticky. Furthermore, a defective CFTR appears to produce a redox imbalance in epithelial cells and extracellular fluids and to cause an abnormal generation of reactive oxygen species: as a consequence, oxidative stress has been implicated as a causative factor in the aetiology of the process. Moreover, massive evidences show that defective CFTR gives rise to extracellular GSH level decrease and elevated glucose concentrations in airway surface liquid (ASL), thus encouraging lung infection by pathogens in the CF advancement. Recent research in progress aims to rediscover a possible role of mitochondria in CF. Here the latest new and recent studies on mitochondrial bioenergetics are collected. Surprisingly, they have enabled us to ascertain that mitochondria have a leading role in opposing the high ASL glucose level as well as oxidative stress in CF.

Synthesis and metabolism of methylglyoxal, S-D-lactoylglutathione and D-lactate in cancer and Alzheimer's disease. Exploring the crossroad of eternal youth and premature aging

Both cancer and Alzheimer's disease (AD) are emerging as metabolic diseases in which aberrant/dysregulated glucose metabolism and bioenergetics occur, and play a key role in disease progression. Interestingly, an enhancement of glucose uptake, glycolysis and pentose phosphate pathway occurs in both cancer cells and amyloid-β-resistant neurons in the early phase of AD. However, this metabolic shift has its adverse effects. One of them is the increase in methylglyoxal production, a physiological cytotoxic by-product of glucose catabolism. Methylglyoxal is mainly detoxified via cytosolic glyoxalase route comprising glyoxalase 1 and glyoxalase 2 with the production of S-D-lactoylglutathione and D-lactate as intermediate and end-product, respectively. Due to the existence of mitochondrial carriers and intramitochondrial glyoxalase 2 and D-lactate dehydrogenase, the transport and metabolism of both S-D-lactoylglutathione and D-lactate in mitochondria can contribute to methylglyoxal elimination, cellular antioxidant power and energy production. In this review, it is supposed that the different ability of cancer cells and AD neurons to metabolize methylglyoxal, S-D-lactoylglutathione and D-lactate scores cell fate, therefore being at the very crossroad of the "eternal youth" of cancer and the "premature death" of AD neurons. Understanding of these processes would help to elaborate novel metabolism-based therapies for cancer and AD treatment.

Including the mitochondrial metabolism of L-lactate in cancer metabolic reprogramming

Glucose avidity, high glycolysis and L-lactate production, regardless of oxygen availability, are the main traits of cancer metabolic reprogramming. The idea that mitochondria are dysfunctional in cancer, thus causing a glycolysis increase for ATP production and L-lactate accumulation as a dead-end product of glucose catabolism, has oriented cancer research for many years. However, it was shown that mitochondrial metabolism is essential for cancer cell proliferation and tumorigenesis and that L-lactate is a fundamental energy substrate with tumor growth-promoting and signaling capabilities. Nevertheless, the known ability of mitochondria to take up and oxidize L-lactate has remained ignored by cancer research. Beginning with a brief overview of the metabolic changes occurring in cancer, we review the present knowledge of L-lactate formation, transport, and intracellular oxidation and underline the possible role of L-lactate metabolism as energetic, signaling and anabolic support for cancer cell proliferation. These unexplored aspects of cancer biochemistry might be exploited for therapeutic benefit.

Extracellular truncated tau causes early presynaptic dysfunction associated with Alzheimer's disease and other tauopathies

The largest part of tau secreted from AD nerve terminals and released in cerebral spinal fluid (CSF) is C-terminally truncated, soluble and unaggregated supporting potential extracellular role(s) of NH2 -derived fragments of protein on synaptic dysfunction underlying neurodegenerative tauopathies, including Alzheimer's disease (AD). Here we show that sub-toxic doses of extracellular-applied human NH2 tau 26-44 (aka NH 2 htau) -which is the minimal active moiety of neurotoxic 20-22kDa peptide accumulating in vivo at AD synapses and secreted into parenchyma- acutely provokes presynaptic deficit in K+ -evoked glutamate release on hippocampal synaptosomes along with alteration in local Ca2+ dynamics. Neuritic dystrophy, microtubules breakdown, deregulation in presynaptic proteins and loss of mitochondria located at nerve endings are detected in hippocampal cultures only after prolonged exposure to NH 2 htau. The specificity of these biological effects is supported by the lack of any significant change, either on neuronal activity or on cellular integrity, shown by administration of its reverse sequence counterpart which behaves as an inactive control, likely due to a poor conformational flexibility which makes it unable to dynamically perturb biomembrane-like environments. Our results demonstrate that one of the AD-relevant, soluble and secreted N-terminally truncated tau forms can early contribute to pathology outside of neurons causing alterations in synaptic activity at presynaptic level, independently of overt neurodegeneration.

A disease with a sweet tooth: exploring the Warburg effect in Alzheimer's disease

After more than 80 years from the revolutionary discoveries of Otto Warburg, who observed high glucose dependency, with increased glycolysis and lactate production regardless of oxygen availability in most cancer cells, the 'Warburg effect' returns to the fore in neuronal cells affected by Alzheimer's disease (AD). Indeed, it seems that, in the mild phase of AD, neuronal cells "prefer" to use the energetically inefficient method of burning glucose by glycolysis, as in cancer, proving to become resistant to β-amyloid (Aβ)-dependent apoptosis. However, in the late phase, while most AD brain cells die in response to Aβ toxicity, only small populations of neurons, exhibiting increased glucose uptake and glycolytic flux, are able to survive as they are resistant to Aβ. Here we draw an overview on the metabolic shift for glucose utilization from oxidative phosphorylation to glycolysis, focusing on the hypothesis that, as extreme attempt to oppose the impending death, mitochondria-whose dysfunction and central role in Aβ toxicity is an AD hallmark-are sent into quiescence, this likely contributing to activate mechanisms of resistance to Aβ-dependent apoptosis. Finally, the attempt turns out fruitless since the loss of the adaptive advantage afforded by elevated aerobic glycolysis exacerbates the pathophysiological processes associated with AD, making the brain susceptible to Aβ-induced neurotoxicity and leading to cell death and dementia. The understanding of how certain nerve cells become resistant to Aβ toxicity, while the majority dies, is an attractive challenge toward the identification of novel possible targets for AD therapy.

Characterization of mitochondrial function in cells with impaired cystic fibrosis transmembrane conductance regulator (CFTR) function

Evidence supporting the occurrence of oxidative stress in Cystic Fibrosis (CF) is well established and the literature suggests that oxidative stress is inseparably linked to mitochondrial dysfunction. Here, we have characterized mitochondrial function, in particular as it regards the steps of oxidative phosphorylation and ROS production, in airway cells either homozygous for the F508del-CFTR allele or stably expressing wt-CFTR. We find that oxygen consumption, ΔΨ generation, adenine nucleotide translocator-dependent ADP/ATP exchange and both mitochondrial Complex I and IV activities are impaired in CF cells, while both mitochondrial ROS production and membrane lipid peroxidation increase. Importantly, treatment of CF cells with the small molecules VX-809 and 4,6,4'-trimethylangelicin, which act as "correctors" for F508del CFTR by rescuing the F508del CFTR-dependent chloride secretion, while having no effect per sè on mitochondrial function in wt-CFTR cells, significantly improved all the above mitochondrial parameters towards values found in the airway cells expressing wt-CFTR. This novel study on mitochondrial bioenergetics provides a springboard for future research to further understand the molecular mechanisms responsible for the involvement of mitochondria in CF and identify the proteins primarily responsible for the F508del-CFTR-dependent mitochondrial impairment and thus reveal potential novel targets for CF therapy.

NH2-truncated human tau induces deregulated mitophagy in neurons by aberrant recruitment of Parkin and UCHL-1: implications in Alzheimer's disease

Disarrangement in functions and quality control of mitochondria at synapses are early events in Alzheimer's disease (AD) pathobiology. We reported that a 20-22 kDa NH2-tau fragment mapping between 26 and 230 amino acids of the longest human tau isoform (aka NH2htau): (i) is detectable in cellular and animal AD models, as well in synaptic mitochondria and cerebrospinal fluids (CSF) from human AD subjects; (ii) is neurotoxic in primary hippocampal neurons; (iii) compromises the mitochondrial biology both directly, by inhibiting the ANT-1-dependent ADP/ATP exchange, and indirectly, by impairing their selective autophagic clearance (mitophagy). Here, we show that the extensive Parkin-dependent turnover of mitochondria occurring in NH2htau-expressing post-mitotic neurons plays a pro-death role and that UCHL-1, the cytosolic Ubiquitin-C-terminal hydrolase L1 which directs the physiological remodeling of synapses by controlling ubiquitin homeostasis, critically contributes to mitochondrial and synaptic failure in this in vitro AD model. Pharmacological or genetic suppression of improper mitophagy, either by inhibition of mitochondrial targeting to autophagosomes or by shRNA-mediated silencing of Parkin or UCHL-1 gene expression, restores synaptic and mitochondrial content providing partial but significant protection against the NH2htau-induced neuronal death. Moreover, in mitochondria from human AD synapses, the endogenous NH2htau is stably associated with Parkin and with UCHL-1. Taken together, our studies show a causative link between the excessive mitochondrial turnover and the NH2htau-induced in vitro neuronal death, suggesting that pathogenetic tau truncation may contribute to synaptic deterioration in AD by aberrant recruitment of Parkin and UCHL-1 to mitochondria making them more prone to detrimental autophagic clearance.

Glucose-6-phosphate tips the balance in modulating apoptosis in cerebellar granule cells

A metabolic shift from oxidative phosphorylation to glycolysis (i.e. the Warburg effect) occurs in Alzheimer's disease accompanied by an increase of both activity and level of HK-I. The findings reported here demonstrate that in the early phase of apoptosis VDAC1 activity, but not its protein level, progressively decreases, in concomitance with the physical interaction of HK-I with VDAC1. In the late phase of apoptosis, glucose-6-phosphate accumulation in the cell causes the dissociation of the two proteins, the re-opening of the channel and the recovery of VDAC1 function, resulting in a reawakening of the mitochondrial function, thus inevitably leading to cell death.

Tissue-specific mtDNA abundance from exome data and its correlation with mitochondrial transcription, mass and respiratory activity

Eukaryotic cells contain a population of mitochondria, variable in number and shape, which in turn contain multiple copies of a tiny compact genome (mtDNA) whose expression and function is strictly coordinated with the nuclear one. mtDNA copy number varies between different cell or tissues types, both in response to overall metabolic and bioenergetics demands and as a consequence or cause of specific pathological conditions. Here we present a novel and reliable methodology to assess the effective mtDNA copy number per diploid genome by investigating off-target reads obtained by whole-exome sequencing (WES) experiments. We also investigate whether and how mtDNA copy number correlates with mitochondrial mass, respiratory activity and expression levels. Analyzing six different tissues from three age- and sex-matched human individuals, we found a highly significant linear correlation between mtDNA copy number estimated by qPCR and the frequency of mtDNA off target WES reads. Furthermore, mtDNA copy number showed highly significant correlation with mitochondrial gene expression levels as measured by RNA-Seq as well as with mitochondrial mass and respiratory activity. Our methodology makes thus feasible, at a large scale, the investigation of mtDNA copy number in diverse cell-types, tissues and pathological conditions or in response to specific treatments.

Extracellular ADP prevents neuronal apoptosis via activation of cell antioxidant enzymes and protection of mitochondrial ANT-1

Apoptosis in neuronal tissue is an efficient mechanism which contributes to both normal cell development and pathological cell death. The present study explores the effects of extracellular ADP on low [K(+)]-induced apoptosis in rat cerebellar granule cells. ADP, released into the extracellular space in brain by multiple mechanisms, can interact with its receptor or be converted, through the actions of ectoenzymes, to adenosine. The findings reported in this paper demonstrate that ADP inhibits the proapoptotic stimulus supposedly via: i) inhibition of ROS production during early stages of apoptosis, an effect mediated by its interaction with cell receptor/s. This conclusion is validated by the increase in SOD and catalase activities as well as by the GSSG/GSH ratio value decrease, in conjunction with the drop of ROS level and the prevention of the ADP protective effect by pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS), a novel functionally selective antagonist of purine receptor; ii) safeguard of the functionality of the mitochondrial adenine nucleotide-1 translocator (ANT-1), which is early impaired during apoptosis. This effect is mediated by its plausible internalization into cell occurring as such or after its hydrolysis, by means of plasma membrane nucleotide metabolizing enzymes, and resynthesis into the cell. Moreover, the findings that ADP also protects ANT-1 from the toxic action of the two Alzheimer's disease peptides, i.e. Aβ1-42 and NH2htau, which are known to be produced in apoptotic cerebellar neurons, further corroborate the molecular mechanism of neuroprotection by ADP, herein proposed.

Morphological and bioenergetic demands underlying the mitophagy in post-mitotic neurons: the pink-parkin pathway

Evidence suggests a striking causal relationship between changes in quality control of neuronal mitochondria and numerous devastating human neurodegenerative diseases, including Parkinson’s disease, Alzheimer’s disease, Huntington’s disease, and amyotrophic lateral sclerosis. Contrary to replicating mammalian cells with a metabolism essentially glycolytic, post-mitotic neurons are distinctive owing to (i) their exclusive energetic dependence from mitochondrial metabolism and (ii) their polarized shape, which entails compartmentalized and distinct energetic needs. Here, we review the recent findings on mitochondrial dynamics and mitophagy in differentiated neurons focusing on how the exceptional characteristics of neuronal populations in their morphology and bioenergetics needs make them quite different to other cells in controlling the intracellular turnover of these organelles.

AD-linked, toxic NH2 human tau affects the quality control of mitochondria in neurons

Functional as well as structural alterations in mitochondria size, shape and distribution are precipitating, early events in progression of Alzheimer's Disease (AD). We reported that a 20-22kDa NH2-tau fragment (aka NH2htau), mapping between 26 and 230 amino acids of the longest human tau isoform, is detected in cellular and animal AD models and is neurotoxic in hippocampal neurons. The NH2htau -but not the physiological full-length protein- interacts with Aβ at human AD synapses and cooperates with it in inhibiting the mitochondrial ANT-1-dependent ADP/ATP exchange. Here we show that the NH2htau also adversely affects the interplay between the mitochondria dynamics and their selective autophagic clearance. Fragmentation and perinuclear mislocalization of mitochondria with smaller size and density are early found in dying NH2htau-expressing neurons. The specific effect of NH2htau on quality control of mitochondria is accompanied by (i) net reduction in their mass in correlation with a general Parkin-mediated remodeling of membrane proteome; (ii) their extensive association with LC3 and LAMP1 autophagic markers; (iii) bioenergetic deficits and (iv) in vitro synaptic pathology. These results suggest that NH2htau can compromise the mitochondrial biology thereby contributing to AD synaptic deficits not only by ANT-1 inactivation but also, indirectly, by impairing the quality control mechanism of these organelles.

Dissecting the molecular mechanism by which NH2htau and Aβ1-42 peptides impair mitochondrial ANT-1 in Alzheimer disease

To find out whether and how the adenine nucleotide translocator-1 (ANT-1) inhibition due to NH2htau and Aβ1-42 is due to an interplay between these two Alzheimer's peptides, ROS and ANT-1 thiols, use was made of mersalyl, a reversible alkylating agent of thiol groups that are oriented toward the external hydrophilic phase, to selectively block and protect, in a reversible manner, the -SH groups of ANT-1. The rate of ATP appearance outside mitochondria was measured as the increase in NADPH absorbance which occurs, following external addition of ADP, when ATP is produced by oxidative phosphorylation and exported from mitochondria in the presence of glucose, hexokinase and glucose-6-phosphate dehydrogenase. We found that the mitochondrial superoxide anions, whose production is induced at the level of Complex I by externally added Aβ1-42 and whose release from mitochondria is significantly reduced by the addition of the VDAC inhibitor DIDS, modify the thiol group/s present at the active site of mitochondrial ANT-1, impair ANT-1 in a mersalyl-prevented manner and abrogate the toxic effect of NH2htau on ANT-1 when Aβ1-42 is already present. A molecular mechanism is proposed in which the pathological Aβ-NH2htau interplay on ANT-1 in Alzheimer's neurons involves the thiol redox state of ANT-1 and the Aβ1-42-induced ROS increase. This result represents an important innovation because it suggests the possibility of using various strategies to protect cells at the mitochondrial level, by stabilizing or restoring mitochondrial function or by interfering with the energy metabolism providing a promising tool for treating or preventing AD.

Interaction between NH(2)-tau fragment and Aβ in Alzheimer's disease mitochondria contributes to the synaptic deterioration

Although amyloid beta (Aβ) peptide can promote tau pathology and its toxicity is concurrently tau-dependent, the underlying mechanisms of the in vivo interplay of these proteins remain unsolved. Structural and functional mitochondrial alterations play an early, precipitating role in synaptic failure of Alzheimer's disease (AD) pathogenesis and an aggravated mitochondrial impairment has been described in triple APP/PS/tau transgenic mice carrying both plaques and tangles, if compared with mice overexpressing tau or amyloid precursor protein (APP) alone. Here, we show that a neurotoxic aminoterminal (NH(2))-derived tau fragment mapping between 26 and 230 amino acids of the human tau40 isoform (441 amino acids)-but not the physiological full-length protein-preferentially interacts with Aβ peptide(s) in human AD synapses in association with mitochondrial adenine nucleotide translocator-1 (ANT-1) and cyclophilin D. The two peptides-Aβ 1-42 and the smaller and more potent NH(2)-26-44 peptide of the longest 20-22 kDa NH(2)-tau fragment-inhibit the ANT-1-dependent adenosine diphosphate-adenosine triphosphate (ADP/ATP) exchange in a noncompetitive and competitive manner, respectively, and together further aggravate the mitochondrial dysfunction by exacerbating the ANT-1 impairment. Taken together, these data establish a common, direct and synergistic toxicity of pathological APP and tau products on synaptic mitochondria and suggest potential, new pathway(s) and target(s) for a combined, more efficient therapeutic intervention of early synaptic dysfunction in AD.

Alzheimer's proteins, oxidative stress, and mitochondrial dysfunction interplay in a neuronal model of Alzheimer's disease

In this paper, we discuss the interplay between beta-amyloid (Aβ) peptide, Tau fragments, oxidative stress, and mitochondria in the neuronal model of cerebellar granule neurons (CGNs) in which the molecular events reminiscent of AD are activated. The identification of the death route and the cause/effect relationships between the events leading to death could be helpful to manage the progression of apoptosis in neurodegeneration and to define antiapoptotic treatments acting on precocious steps of the death process. Mitochondrial dysfunction is among the earliest events linked to AD and might play a causative role in disease onset and progression. Recent studies on CGNs have shown that adenine nucleotide translocator (ANT) impairment, due to interaction with toxic N-ter Tau fragment, contributes in a significant manner to bioenergetic failure and mitochondrial dysfunction. These findings open a window for new therapeutic strategies aimed at preserving and/or improving mitochondrial function.

L-lactate generates hydrogen peroxide in purified rat liver mitochondria due to the putative L-lactate oxidase localized in the intermembrane space

In order to ascertain whether and how mitochondria can produce hydrogen peroxide (H(2)O(2)) as a result of L-lactate addition, we monitored H(2)O(2) generation in rat liver mitochondria and in submitochondrial fractions free of peroxisomal and cytosolic contamination. We found that H(2)O(2) is produced independently on the respiratory chain with 1:1 stoichiometry with pyruvate, due to a putative flavine-dependent L-lactate oxidase restricted to the intermembrane space. The L-lactate oxidase reaction shows a hyperbolic dependence on L-lactate concentration and is inhibited by NAD(+) in a competitive manner, being the enzyme different from the L-lactate dehydrogenase isoenzymes as shown by their pH profiles.

Pyruvate kinase in pig liver mitochondria

The existence of the pyruvate kinase (PK) in pig liver mitochondria was shown by monitoring photometrically the PK reaction in solubilised mitochondria with either phosphoenolpyruvate (PEP) or ADP used as a substrate. In distinction with the cytosolic isoenzyme, the mitochondrial PK showed a sigmoidal dependence on either PEP or ADP concentrations. The occurrence of the mitochondrial PK was confirmed by immunological analysis. Titration with digitonin showed that mPK is restricted to the matrix. PEP addition to mitochondria resulted in reduction of the intramitochondrial NAD(P)+ inhibited by either the non-penetrant thiol reagent mersalyl or by arsenite, an inhibitor of the pyruvate dehydrogenase complex. Citrate/oxaloacetate appearance outside mitochondria also occurred as result of PEP addition to PLM. Taken together these findings support a role for PEP itself in triggering fatty acid synthesis via its mitochondrial metabolism.

Genistein and daidzein prevent low potassium-dependent apoptosis of cerebellar granule cells

We have investigated the ability of certain dietary flavonoids, known to exert beneficial effects on the central nervous system, to affect neuronal apoptosis. We used cerebellar granule cells undergoing apoptosis due to potassium deprivation in a serum-free medium in either the absence or presence of the flavonoids genistein and daidzein, which are present in soy, and of catechin and epicatechin, which are present in cocoa. These compounds were used in a blood dietary concentration range. We found that genistein and daidzein, but not catechin and epicatechin, prevented apoptosis, with cell survival measured 24h after the induction of apoptosis being higher than that of the same cells incubated in flavonoid free medium (80% and 40%, respectively); there was no effect in control cells. A detailed investigation of the effect of these compounds on certain mitochondrial events that occur in cells en route to apoptosis showed that genistein and daidzein prevented the impairment of glucose oxidation and mitochondrial coupling, reduced cytochrome c release, and prevented both impairment of the adenine nucleotide translocator and opening of the mitochondrial permeability transition pore. Interestingly, genistein and daidzein were found to reduce the levels of reactive oxygen species, which are elevated in cerebellar granule cell apoptosis. These findings strongly suggest that the prevention of apoptosis depends mainly on the antioxidant properties of genistein and daidzein. This could lead to the development of a flavonoid-based therapy in neuropathies.

A peptide containing residues 26-44 of tau protein impairs mitochondrial oxidative phosphorylation acting at the level of the adenine nucleotide translocator

Having confirmed that adenovirus-mediated overexpression of NH(2)-tau fragment lacking the first 25 aminoacids evokes a potent neurotoxic effect, sustained by protracted stimulation of NMDA receptors, in primary neuronal cultures we investigated whether and how chemically synthesized NH(2)-derived tau peptides, i.e. NH(2)-26-44 and NH(2)-1-25 fragments, affect mitochondrial function. We tested both fragments on each step of the processes leading to ATP synthesis via oxidative phosphorylation: i) electron flow via the respiratory chain from physiological substrates to oxygen with the activity of each individual complex of the respiratory chain investigated in some detail, ii) membrane potential generation arising from externally added succinate and iii) the activity of both the adenine nucleotide translocator and iv) ATP synthase. Oxidative phosphorylation is not affected by NH(2)-1-25 tau fragment, but dramatically impaired by NH(2)-26-44 tau fragment. Both cytochrome c oxidase and the adenine nucleotide translocator are targets of NH(2)-26-44 tau fragment, but adenine nucleotide translocator is the unique mitochondrial target responsible for impairment of oxidative phosphorylation by the NH(2)-26-44 tau fragment, which then exerts deleterious effects on cellular availability of ATP synthesized into mitochondria.

Different sources of reactive oxygen species contribute to low potassium-induced apoptosis in cerebellar granule cells

An early increase in ROS production is characteristic of cerebellar granule cells undergoing apoptosis in the presence of 5 mM KCl. However, the sources of this increase have not been investigated in detail. In particular whether there is a single enzymatic source or the increase in ROS production is the consequence of the involvement of different enzymes has not been studied in depth. Different enzymatic pathways may indeed contribute to the up-regulation of intracellular ROS production either directly or via side-chain reactions and a number of candidate enzymes are known to be involved in the apoptotic process in various cell types. The aim of this study was to identify the cellular sources of the ROS generated by CGCs undergoing apoptosis by low K+. A panel of specific inhibitors against phospholipase, cytochromes P450, cyclooxygenase, lipoxygenase, xanthine oxidase, ribonucleotide reductase and NADPH oxidase were used. We provide evidence that no single source of ROS can be identified in apoptotic CGCs, but the ROS generated through the arachidonic acid (AA) pathways, mainly via lipoxygenase activities, seems to be the most prominent.

Identification of a caspase-derived N-terminal tau fragment in cellular and animal Alzheimer's disease models

Biochemical modifications of tau proteins have been proposed to be among the earliest neurobiological changes in Alzheimer's disease (AD) and correlate better with cognitive symptoms than do beta-amyloid plaques. We have recently reported that adenovirus-mediated overexpression of the NH2 26-230aa tau fragment evokes a potent NMDA-mediated neurotoxic effect in primary neuronal cultures. In order to assess whether such N-terminal tau fragment(s) are indeed produced during apoptosis or neurodegeneration in vivo, we attempted to ascertain their presence in cell and animal models using an anti-tau antibody directed against the N-terminal sequence of human protein located downstream of the caspase(s)-cleavage site DRKD(25)-QGGYTMHQDQ. We provide biochemical evidence that a caspase(s)-cleaved NH2-terminal tau fragment of 20-22 kDa, consistent with the size of the NH2 26-230aa neurotoxic fragment of tau, is generated in vitro in differentiated human SH-SY5Y cells undergoing apoptosis by BDNF withdrawal or following treatment with staurosporine. In addition this NH2-terminally cleaved tau fragment, whose expression correlates with a significant up-regulation of caspase(s) activity, is also specifically detected in vivo in the hippocampus of 15 month-old AD11 transgenic mice, a model in which a progressive AD-like neurodegeneration is induced by the expression of transgenic anti-NGF antibodies. The results support the idea that aberrant activation of caspase(s), following apoptotic stimuli or neurodegeneration insults, may produce one or more toxic NH2 tau fragments, that further contribute to propagate and increase cellular dysfunctions in AD.

Is there a pyruvate kinase in pig liver mitochondria?

In order to ascertain whether mammalian mitochondria possess their own pyruvate kinase, we isolated mitochondria from liver of Large White pig and investigated pyruvate kinase occurrence both via immunological analysis and by assaying photometrically the pyruvate kinase reaction. We show that mitochondria contain pyruvate kinase located in the inner compartments; the pyruvate kinase reaction shows hyperbolic dependence on the substrate concentration, is inhibited by malonate and shows maximum activity at pH between 7-7.6 and Ea equal to 33 kJ/mol.

Transport and metabolism of L-lactate occur in mitochondria from cerebellar granule cells and are modified in cells undergoing low potassium dependent apoptosis

Having confirmed that externally added L-lactate can enter cerebellar granule cells, we investigated whether and how L-lactate is metabolized by mitochondria from these cells under normal or apoptotic conditions. (1) L-lactate enters mitochondria, perhaps via an L-lactate/H+ symporter, and is oxidized in a manner stimulated by ADP. The existence of an L-lactate dehydrogenase, located in the inner mitochondrial compartment, was shown by immunological analysis. Neither the protein level nor the Km and Vmax values changed en route to apoptosis. (2) In both normal and apoptotic cell homogenates, externally added L-lactate caused reduction of the intramitochondrial pyridine cofactors, inhibited by phenylsuccinate. This process mirrored L-lactate uptake by mitochondria and occurred with a hyperbolic dependence on L-lactate concentrations. Pyruvate appeared outside mitochondria as a result of external addition of L-lactate. The rate of the process depended on L-lactate concentration and showed saturation characteristics. This shows the occurrence of an intracellular L-lactate/pyruvate shuttle, whose activity was limited by the putative L-lactate/pyruvate antiporter. Both the carriers were different from the monocarboxylate carrier. (3) L-lactate transport changed en route to apoptosis. Uptake increased in the early phase of apoptosis, but decreased in the late phase with characteristics of a non-competitive like inhibition. In contrast, the putative L-lactate/pyruvate antiport decreased en route to apoptosis with characteristics of a competitive like inhibition in early apoptosis, and a mixed non-competitive like inhibition in late apoptosis.

Nitric oxide has dual opposite roles during early and late phases of apoptosis in cerebellar granule neurons

The involvement and the role of nitric oxide (NO) as a signaling molecule in the course of neuronal apoptosis, whether unique or modulated during the progression of the apoptotic program, has been investigated in a cellular system consisting of cerebellar granule cells (CGCs) where apoptosis can be induced by lowering extracellular potassium. Several parameters involved in NO signaling pathway, such as NO production, neuronal nitric oxide synthase (nNOS) expression, and cyclic GMP (cGMP) production were examined in the presence or absence of different inhibitors. We provide evidence that nitric oxide has dual and opposite effects depending on time after induction of apoptosis. In an early phase, up to 3 h of apoptosis, nitric oxide supports survival of CGCs through a cGMP-dependent mechanism. After 3 h, nNOS expression and activity decreased resulting in shut down of NO and cGMP production. Residual NO then contributes to the apoptotic process by reacting with rising superoxide anions leading to peroxynitrite production and protein inactivation. We conclude that whilst NO over-production protects neurons from death in the early phase of neuronal damage, its subsequent reduction may contribute to neuronal degeneration and ultimate cell death.

Teaching the role of mitochondrial transport in energy metabolism

Studies from our laboratories over recent years have uncovered the existence, and established the properties of a variety of mitochondrial transporters. The properties of these transporters throw light on a variety of biochemical phenomena that were previously poorly understood. In particular the role of mitochondrial transport in energy metabolism has been investigated under a variety of physio-pathological conditions. Consistently we describe the procedure to investigate mitochondrial traffic in isolated mitochondria as a model system for students to learn. Here we report some observations that contribute to novel knowledge of the role of mitochondria in glycolysis, urea and purine nucleotide cycle, and nitrogen metabolism with particular reference to the malate/oxaloacetate shuttle and fumarate, glutamine, and lactate metabolism.

Phosphoenolpyruvate metabolism in Jerusalem artichoke mitochondria

We report here initial studies on phosphoenolpyruvate metabolism in coupled mitochondria isolated from Jerusalem artichoke tubers. It was found that: (1) phosphoenolpyruvate can be metabolized by Jerusalem artichoke mitochondria by virtue of the presence of the mitochondrial pyruvate kinase, shown both immunologically and functionally, located in the inner mitochondrial compartments and distinct from the cytosolic pyruvate kinase as shown by the different pH and inhibition profiles. (2) Jerusalem artichoke mitochondria can take up externally added phosphoenolpyruvate in a proton compensated manner, in a carrier-mediated process which was investigated by measuring fluorimetrically the oxidation of intramitochondrial pyridine nucleotide which occurs as a result of phosphoenolpyruvate uptake and alternative oxidase activation. (3) The addition of phosphoenolpyruvate causes pyruvate and ATP production, as monitored via HPLC, with their efflux into the extramitochondrial phase investigated fluorimetrically. Such an efflux occurs via the putative phosphoenolpyruvate/pyruvate and phosphoenolpyruvate/ATP antiporters, which differ from each other and from the pyruvate and the adenine nucleotide carriers, in the light of the different sensitivity to non-penetrant compounds. These carriers were shown to regulate the rate of efflux of both pyruvate and ATP. The appearance of citrate and oxaloacetate outside mitochondria was also found as a result of phosphoenolpyruvate addition.

Mitochondria from the left heart ventricles of both normotensive and spontaneously hypertensive rats oxidize externally added NADH mostly via a novel malate/oxaloacetate shuttle as reconstructed in vitro

A substantial increase in NADH production, arising from accelerated glycolysis, occurs in cardiac hypertrophy and this raises the question of how the NADH is oxidised. We have addressed this problem by reconstructing appropriate mitochondrial shuttles in vitro, using mitochondria from the left ventricles of both normotensive and spontaneously hypertensive rats at 5 and 24 weeks of age as model systems for left ventricle hypertrophy and hypertrophy/hypertension respectively. We found that most NADH oxidation occurs via a novel malate/oxaloacetate shuttle, the activity of which increases with time and with the progression of hypertrophy and development of hypertension as judged by statistical ANOVA analysis. In contrast, alpha-glycerol-phosphate and the malate/aspartate shuttles were shown to make only a minor contribution to NADH oxidation in a manner essentially independent of age and progression of hypertrophy/hypertension. The rate of malate transport in exchange with oxaloacetate proved to limit the rate of NADH oxidation via this malate/oxaloacetate shuttle.

Caspase-dependent alteration of the ADP/ATP translocator triggers the mitochondrial permeability transition which is not required for the low-potassium-dependent apoptosis of cerebellar granule cells

We investigated ADP/ATP exchange mediated by the adenine nucleotide translocator and opening of the mitochondrial permeability transition pore in homogenates from cerebellar granule cells en route to apoptosis induced by low potassium. We showed that, in the first 3 h of apoptosis, when maximum cytochrome c release had already occurred, adenine nucleotide translocator function was impaired owing to the action of reactive oxygen species, but no permeability transition pore opening occurred. Over 3-8 h of apoptosis, the permeability transition pore progressively opened, owing to caspase action, and further ADP/ATP translocator impairment occurred. The kinetics of transport and permeability transition pore opening were inversely correlated, both in the absence and presence of inhibitors of antioxidant and proteolytic systems. We conclude that, en route to apoptosis, alteration of the adenine nucleotide translocator occurs, resulting in permeability transition pore opening. This process depends on the action of caspase on pore component(s) other than the ADP/ATP translocator, because no change in either amount or molecular weight of the latter protein was noted during apoptosis, as measured by western blotting. Cell death occurs via apoptosis in the presence of cyclosporin A, the permeability transition pore inhibitor, thus showing that permeability transition pore opening, not needed for cytochrome c release, is also unnecessary for apoptosis to occur.

Jerusalem artichoke mitochondria can export reducing equivalents in the form of malate as a result of d-lactate uptake and metabolism

We found that as a result of d-lactate uptake and metabolism by Jerusalem artichoke mitochondria, reducing equivalents were exported from the mitochondrial matrix to the exterior in the form of malate. The rate of malate efflux, as measured photometrically using NADP+ and malic enzyme, depended on the rate of transport across the mitochondrial membrane. It showed saturation characteristics (K(m) = 5 mM; V(max) = 9 nmol/min mg of mitochondrial protein) and was inhibited by non-penetrant compounds. We conclude that reducing equivalent export from mitochondria is due to the occurrence of a putative d-lactate/malate antiporter which differs from other mitochondrial carriers, as shown by the different inhibitor sensitivity.

Transport and metabolism of d-lactate in Jerusalem artichoke mitochondria

We report here initial studies on D-lactate metabolism in Jerusalem artichoke. It was found that: 1) D-lactate can be synthesized by Jerusalem artichoke by virtue of the presence of glyoxalase II, the activity of which was measured photometrically in both isolated Jerusalem artichoke mitochondria and cytosolic fraction after the addition of S-D-lactoyl-glutathione. 2) Externally added D-lactate caused oxygen consumption by mitochondria, mitochondrial membrane potential increase and proton release, in processes that were insensitive to rotenone, but inhibited by both antimycin A and cyanide. 3) D-lactate was metabolized inside mitochondria by a flavoprotein, a putative D-lactate dehydrogenase, the activity of which could be measured photometrically in mitochondria treated with Triton X-100. 4) Jerusalem artichoke mitochondria can take up externally added D-lactate by means of a D-lactate/H(+) symporter investigated by measuring the rate of reduction of endogenous flavins. The action of the d-lactate translocator and of the mitochondrial D-lactate dehydrogenase could be responsible for the subsequent metabolism of d-lactate formed from methylglyoxal in the cytosol of Jerusalem artichoke.

An increase in the ATP levels occurs in cerebellar granule cells en route to apoptosis in which ATP derives from both oxidative phosphorylation and anaerobic glycolysis

Although it is recognized that ATP plays a part in apoptosis, whether and how its level changes en route to apoptosis as well as how ATP is synthesized has not been fully investigated. We have addressed these questions using cultured cerebellar granule cells. In particular, we measured the content of ATP, ADP, AMP, IMP, inosine, adenosine and L-lactate in cells undergoing apoptosis during the commitment phase (0-8 h) in the absence or presence of oligomycin or/and of citrate, which can inhibit totally the mitochondrial oxidative phosphorylation and largely the substrate-level phosphorylation in glycolysis, respectively. In the absence of inhibitors, apoptosis was accompanied by an increase in ATP and a decrease in ADP with 1:1 stoichiometry, with maximum ATP level found at 3 h apoptosis, but with no change in levels of AMP and its breakdown products and with a relatively low level of L-lactate production. Consistently, there was an increase in the cell energy charge and in the ratio ([ATP][AMP])/[ADP](2). When the oxidative phosphorylation was completely blocked by oligomycin, a decrease of the ATP content was found both in control cells and in cells undergoing apoptosis, but nonetheless cells still died by apoptosis, as shown by checking DNA laddering and by death prevention due to actinomycin D. In this case, ATP was provided by anaerobic glycolysis, as suggested by the large increase of L-lactate production. On the other hand, citrate itself caused a small decrease in ATP level together with a huge decrease in L-lactate production, but it had no effect on cell survival. When ATP level was further decreased due to the presence of both oligomycin and citrate, death occurred via necrosis at 8 h, as shown by the lack of DNA laddering and by death prevention found due to the NMDA receptor antagonist MK801. However, at a longer time, when ATP level was further decreased, cells died neither via apoptosis nor via glutamate-dependent necrosis, in a manner similar to something like to energy catastrophe. Our results shows that cellular ATP content increases in cerebellar granule cell apoptosis, that the role of oxidative phosphorylation is facultative, i.e. ATP can also derive from anaerobic glycolysis, and that the type of cell death depends on the ATP availability.

Partial reconstruction of in vitro gluconeogenesis arising from mitochondrial l-lactate uptake/metabolism and oxaloacetate export via novel L-lactate translocators

In the light of the occurrence of L-lactate dehydrogenase inside the mitochondrial matrix, we looked at whether isolated rat liver mitochondria can take up and metabolize L-lactate, and provide oxaloacetate outside mitochondria, thus contributing to a partial reconstruction of gluconeogenesis in vitro. We found that: (1) L-lactate (10 mM), added to mitochondria in the presence of a cocktail of glycolysis/gluconeogenesis enzymes and cofactors, can lead to synthesis of glyceraldehyde-3-phosphate at a rate of about 7 nmol/min per mg mitochondrial protein. (2) Three novel translocators exist to mediate L-lactate traffic across the inner mitochondrial membrane. An L-lactate/H+ symporter was identified by measuring fluorimetrically the rate of endogenous pyridine nucleotide reduction. Consistently, L-lactate oxidation was found to occur with P/O ratio=3 (where P/O ratio is the ratio of mol of ATP synthesized to mol of oxygen atoms reduced to water during oxidative phosphorylation) and with generation of membrane potential. Proton uptake, which occurred as a result of addition of L-lactate to RLM together with electron flow inhibitors, and mitochondrial swelling in ammonium L-lactate solutions were also monitored. L-Lactate/oxaloacetate and L-lactate/pyruvate anti-porters were identified by monitoring photometrically the appearance of L-lactate counter-anions outside mitochondria. These L-lactate translocators, which are distinct from the monocarboxylate carrier, were found to differ from each other in V(max) values and in inhibition and pH profiles, and proved to regulate mitochondrial L-lactate metabolism in vitro. The role of lactate/mitochondria interactions in gluconeogenesis is discussed.