Endogenous oxidative stress in sporadic Alzheimer's disease neuronal cybrids reduces viability by increasing apoptosis through pro-death signaling pathways and is mimicked by oxidant exposure of control cybrids

The Alzheimer's Disease Mitochondrial Cascade Hypothesis: A Current Overview

Viable Alzheimer's disease (AD) hypotheses must account for its age-dependence; commonality; association with amyloid precursor protein, tau, and apolipoprotein E biology; connection with vascular, inflammation, and insulin signaling changes; and systemic features. Mitochondria and parameters influenced by mitochondria could link these diverse characteristics. Mitochondrial biology can initiate changes in pathways tied to AD and mediate the dysfunction that produces the clinical phenotype. For these reasons, conceptualizing a mitochondrial cascade hypothesis is a straightforward process and data accumulating over decades argue the validity of its principles. Alternative AD hypotheses may yet account for its mitochondria-related phenomena, but absent this happening a primary mitochondrial cascade hypothesis will continue to evolve and attract interest.

Neuroepigenetic Mechanisms of Action of Ultrashort Peptides in Alzheimer's Disease

Epigenetic regulation of gene expression is necessary for maintaining higher-order cognitive functions (learning and memory). The current understanding of the role of epigenetics in the mechanism of Alzheimer’s disease (AD) is focused on DNA methylation, chromatin remodeling, histone modifications, and regulation of non-coding RNAs. The pathogenetic links of this disease are the misfolding and aggregation of tau protein and amyloid peptides, mitochondrial dysfunction, oxidative stress, impaired energy metabolism, destruction of the blood–brain barrier, and neuroinflammation, all of which lead to impaired synaptic plasticity and memory loss. Ultrashort peptides are promising neuroprotective compounds with a broad spectrum of activity and without reported side effects. The main aim of this review is to analyze the possible epigenetic mechanisms of the neuroprotective action of ultrashort peptides in AD. The review highlights the role of short peptides in the AD pathophysiology. We formulate the hypothesis that peptide regulation of gene expression can be mediated by the interaction of short peptides with histone proteins, cis- and transregulatory DNA elements and effector molecules (DNA/RNA-binding proteins and non-coding RNA). The development of therapeutic agents based on ultrashort peptides may offer a promising addition to the multifunctional treatment of AD.

Neuroprotective Effects of the DPP4 Inhibitor Vildagliptin in In Vivo and In Vitro Models of Parkinson's Disease

Parkinson’s disease (PD) is characterized by loss of dopaminergic neurons in the substantia nigra pars compacta (SNpc) of the midbrain. Restoration of nigrostriatal dopamine neurons has been proposed as a potential therapeutic strategy for PD. Because currently used PD therapeutics only help relieve motor symptoms and do not treat the cause of the disease, highly effective drugs are needed. Vildagliptin, a dipeptidyl peptidase 4 (DPP4) inhibitor, is an anti-diabetic drug with various pharmacological properties including neuroprotective effects. However, the detailed effects of vildagliptin against PD are not fully understood. We investigated the effects of vildagliptin on PD and its underlying molecular mechanisms using a 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced mouse model and a 1-methyl-4-phenylpyridium (MPP⁺)-induced cytotoxicity model. Vildagliptin (50 mg/kg) administration significantly attenuated MPTP-induced motor deficits as evidenced by rotarod, pole, and nest building tests. Immunohistochemistry and Western blot analysis revealed that vildagliptin increased tyrosine hydroxylase-positive cells in the SNpc and striatum, which was reduced by MPTP treatment. Furthermore, vildagliptin activated MPTP-decreased PI3k/Akt and mitigated MPTP-increased ERK and JNK signaling pathways in the striatum. Consistent with signaling transduction in the mouse striatum, vildagliptin reversed MPP⁺-induced dephosphorylation of PI3K/Akt and phosphorylation of ERK and JNK in SH-SY5Y cells. Moreover, vildagliptin attenuated MPP⁺-induced conversion of LC3B-II in SH-SY5Y cells, suggesting its role in autophagy inhibition. Taken together, these findings indicate that vildagliptin has protective effects against MPTP-induced motor dysfunction by inhibiting dopaminergic neuronal apoptosis, which is associated with regulation of PI3k/Akt, ERK, and JNK signaling transduction. Our findings suggest vildagliptin as a promising repurposing drug to treat PD.

The neuroprotective effects of icariin on ageing, various neurological, neuropsychiatric disorders, and brain injury induced by radiation exposure

Epimedium brevicornum Maxim, a Traditional Chinese Medicine, has been used for the treatment of impotence, sinew and bone disorders, “painful impediment caused by wind-dampness,” numbness, spasms, hypertension, coronary heart disease, menopausal syndrome, bronchitis, and neurasthenia for many years in China. Recent animal experimental studies indicate that icariin, a major bioactive component of epimedium may effectively treat Alzheimer’s disease, cerebral ischemia, depression, Parkinson’s disease, multiple sclerosis, as well as delay ageing. Our recent study also suggested that epimedium extract could exhibit radio-neuro-protective effects and prevent ionizing radiation-induced impairment of neurogenesis. This paper reviewed the pharmacodynamics of icariin in treating different neurodegenerative and neuropsychiatric diseases, ageing, and radiation-induced brain damage. The relevant molecular mechanisms and its anti-neuroinflammatory, anti-apoptotic, anti-oxidant, as well as pro-neurogenesis roles were also discussed.

Dysfunction of thioredoxin triggers inflammation through activation of autophagy in chicken cardiomyocytes

Thioredoxin (Txn) is a hydrogen carrier protein and exists widely in organism. Txn deficiency implicates cardiomyocytes injury has been proven. However, the exact mechanism remains unclear. To understand the mechanistic response of cardiomyocytes subsequent to Txn suppression, we established the model of Txn dysfunction by employing gene interference technology (siRNA) and Txn inhibitor (PX-12) in cardiomyocytes. We detected the ROS levels, inflammation factors, and key proteins in the autophagy and apoptosis. In addition, heat map was used for further analysis. Our results revealed that Txn dysfunction increased the release of ROS and induced activation of autophagy via upregulation of Becline-1, LC3-1, 2, which further regulated the inflammatory response, meanwhile, Txn silence inhibited apoptosis in chicken cardiomyocytes through Caspase-3 inhibition. Altogether we concluded that Txn-deficient chicken cardiomyocytes experienced autophagy, which caused severe inflammatory reactions and resulting in damage to cardiomyocytes.

Centella Asiatica Improves Memory and Promotes Antioxidative Signaling in 5XFAD Mice

Centella asiatica (CA) herb is a traditional medicine, long reputed to provide cognitive benefits. We have reported that CA water extract (CAW) treatment improves cognitive function of aged Alzheimer’s disease (AD) model Tg2576 and wild-type (WT) mice, and induces an NRF2-regulated antioxidant response in aged WT mice. Here, CAW was administered to AD model 5XFAD female and male mice and WT littermates (age: 7.6 +/− 0.6 months), and object recall and contextual fear memory were tested after three weeks treatment. CAW’s impact on amyloid-β plaque burden, and markers of neuronal oxidative stress and synaptic density, was assessed after five weeks treatment. CAW antioxidant activity was evaluated via nuclear transcription factor (erythroid-derived 2)-like 2 (NRF2) and NRF2-regulated antioxidant response element gene expression. Memory improvement in both genders and genotypes was associated with dose-dependent CAW treatment without affecting plaque burden, and marginally increased synaptic density markers in the hippocampus and prefrontal cortex. CAW treatment increased Nrf2 in hippocampus and other NRF2 targets (heme oxygenase-1, NAD(P)H quinone dehydrogenase 1, glutamate-cysteine ligase catalytic subunit). Reduced plaque-associated SOD1, an indicator of oxidative stress, was observed in the hippocampi and cortices of CAW-treated 5XFAD mice. We postulate that CAW treatment leads to reduced oxidative stress, contributing to improved neuronal health and cognition.

Mitochondria and Mitochondrial Cascades in Alzheimer's Disease

Decades of research indicate mitochondria from Alzheimer's disease (AD) patients differ from those of non-AD individuals. Initial studies revealed structural differences, and subsequent studies showed functional deficits. Observations of structure and function changes prompted investigators to consider the consequences, significance, and causes of AD-related mitochondrial dysfunction. Currently, extensive research argues mitochondria may mediate, drive, or contribute to a variety of AD pathologies. The perceived significance of these mitochondrial changes continues to grow, and many currently believe AD mitochondrial dysfunction represents a reasonable therapeutic target. Debate continues over the origin of AD mitochondrial changes. Some argue amyloid-β (Aβ) induces AD mitochondrial dysfunction, a view that does not challenge the amyloid cascade hypothesis and that may in fact help explain that hypothesis. Alternatively, data indicate mitochondrial dysfunction exists independent of Aβ, potentially lies upstream of Aβ deposition, and suggest a primary mitochondrial cascade hypothesis that assumes mitochondrial pathology hierarchically supersedes Aβ pathology. Mitochondria, therefore, appear at least to mediate or possibly even initiate pathologic molecular cascades in AD. This review considers studies and data that inform this area of AD research.

Walnut Protein Hydrolysates Play a Protective Role on Neurotoxicity Induced by d-Galactose and Aluminum Chloride in Mice

In recent years, with an increase in the aging population, neurodegenerative diseases have attracted more and more attention. This study aimed to investigate the potential neuroprotective effect of defatted walnut meal protein hydrolysates (DWMPH) on neurotoxicity induced by d-galactose (d-gal) and aluminum chloride (AlCl₃) in mice. The animal models were established by combining treatments with d-gal (200 mg/kg/day, subcutaneously) and AlCl₃ (100 mg/kg in drinking water) for 90 days. During the 90 days, 1 g/kg of DWMPH was administrated orally every day. The results indicated that DWMPH treatment alleviated oxidative stress, reversed cholinergic dysfunction, and suppressed the release of proinflammatory cytokines in the brains of d-gal + AlCl₃-treated mice, and thus improving the learning and memory functions of these mice, which was closely correlated with the strong antioxidant activity of DWMPH. This finding suggests that DWMPH might be a promising dietary supplement in improving neuronal dysfunctions of the brain.

Phytochemicals and PI3K Inhibitors in Cancer-An Insight

In today's world of modern medicine and novel therapies, cancer still remains to be one of the prime contributor to the death of people worldwide. The modern therapies improve condition of cancer patients and are effective in early stages of cancer but the advanced metastasized stage of cancer remains untreatable. Also most of the cancer therapies are expensive and are associated with adverse side effects. Thus, considering the current status of cancer treatment there is scope to search for efficient therapies which are cost-effective and are associated with lesser and milder side effects. Phytochemicals have been utilized for many decades to prevent and cure various ailments and current evidences indicate use of phytochemicals as an effective treatment for cancer. Hyperactivation of phosphoinositide 3-kinase (PI3K) signaling cascades is a common phenomenon in most types of cancers. Thus, natural substances targeting PI3K pathway can be of great therapeutic potential in the treatment of cancer patients. This chapter summarizes the updated research on plant-derived substances targeting PI3K pathway and the current status of their preclinical studies and clinical trials.

Neuron and microglia/macrophage-derived FGF10 activate neuronal FGFR2/PI3K/Akt signaling and inhibit microglia/macrophages TLR4/NF-κB-dependent neuroinflammation to improve functional recovery after spinal cord injury

Therapeutics used to treat central nervous system (CNS) injury were designed to repair neurites and inhibit cell apoptosis. Previous studies have shown that neuron-derived FGF10 exerts potential neuroprotective effects after cerebral ischemia injury. However, little is known about the role of endogenous FGF10 in the recovery process after spinal cord injury (SCI). In this study, we found that FGF10 is mainly produced by neuron and microglia/macrophages, and its expression is increased after SCI. Exogenous treatment of FGF10 improved functional recovery after injury by reducing apoptosis, as well as repairing neurites via FGFR2/PI3K/Akt pathway. On another hand, inhibiting the PI3K/Akt pathway with LY294002 partially reversed the therapeutic effects of FGF10. In addition, small interfering RNA knockdown of FGFR2 suppressed PI3K/Akt pathway activation by FGF10 and abolished its anti-apoptotic and neurite repair effects in vitro. Furthermore, FGF10 treatment inhibited the activation and proliferation of microglia/macrophages through regulation of TLR4/NF-κB pathway, and attenuated the release of pro-inflammatory cytokines after SCI. Thus, the increased expression of FGF10 after acute SCI is an endogenous self-protective response, suggesting that FGF10 could be a potential treatment for CNS injury.

Neuroprotective properties of icariin in MPTP-induced mouse model of Parkinson's disease: Involvement of PI3K/Akt and MEK/ERK signaling pathways

BACKGROUND

Epimedium sagittatum is a traditional Chinese herb normally which is used to treat the osteoporosis, cardiovascular dysfunction, and to improve neurological and sexual function in China, Korea and Japan. Icariin is the major active ingredient in Epimedium sagittatum. In the present research, we examined the neuroprotective effects of icariin on dopaminergic neurons and the possible mechanisms in a mouse model of Parkinson's disease (PD).

METHODS

Ovariectomized PD mice were treated with vehicle or icariin (3 days before MPTP injections) with or without the phosphatidylinositol 3-kinase (PI3K) inhibitor LY294002 or mitogen-activated protein kinase kinase (MEK) inhibitor PD98059. The dopamine (DA) content in the striatum was studied by HPLC. Western blot was used to determine the protein expressions of Bcl-2, Bax and Caspase 3 in the striatum. The numbers of tyrosine hydroxylase-immunoreactive (TH-IR) neurons in the substantial nigra pars compacta (SNpc) were assessed by immunohistochemistry. The activation of Akt and ERK by icariin were detected in doparminergic MES23.5 cells.

RESULTS

Icariin pretreatment could ameliorate the decreased striatum DA content and the loss of TH-IR neurons in the SNpc induced by MPTP. The MPTP-induced changes of Bcl-2, Bax and caspase 3 protein expressions in the striatum could be reversed by icariin pretreatment. Blockade of PI3K/Akt or MEK/ERK signaling pathway by LY294002 or PD98059 could attenuate the increase of DA content in the striatum and TH-IR in the SNpc induced by icariin in PD mice model. Additionally, icariin treatment alone significantly induced the phosphorylation of Akt and ERK in a time dependent pattern in dopaminergic MES 23.5 cells. These effects were abolished by co-treatment with LY294002 or PD98059.

CONCLUSION

These data demonstrated that icariin has neuroprotective effect on dopaminergic neurons in PD mice model and the potential mechanisms might be related to PI3K/Akt and MEK/ERK pathways.

Mitochondria, Cybrids, Aging, and Alzheimer's Disease

Mitochondrial and bioenergetic function change with advancing age and may drive aging phenotypes. Mitochondrial and bioenergetic changes are also documented in various age-related neurodegenerative diseases, including Alzheimer's disease (AD). In some instances AD mitochondrial and bioenergetic changes are reminiscent of those observed with advancing age but are greater in magnitude. Mitochondrial and bioenergetic dysfunction could, therefore, link neurodegeneration to brain aging. Interestingly, mitochondrial defects in AD patients are not brain-limited, and mitochondrial function can be linked to classic AD histologic changes including amyloid precursor protein processing to beta amyloid. Also, transferring mitochondria from AD subjects to cell lines depleted of endogenous mitochondrial DNA (mtDNA) creates cytoplasmic hybrid (cybrid) cell lines that recapitulate specific biochemical, molecular, and histologic AD features. Such findings have led to the formulation of a "mitochondrial cascade hypothesis" that places mitochondrial dysfunction at the apex of the AD pathology pyramid. Data pertinent to this premise are reviewed.

Relationships Between Mitochondria and Neuroinflammation: Implications for Alzheimer's Disease

Mitochondrial dysfunction and neuroinflammation occur in Alzheimer's disease (AD). The causes of these pathologic lesions remain uncertain, but links between these phenomena are increasingly recognized. In this review, we discuss data that indicate mitochondria or mitochondrial components may contribute to neuroinflammation. While mitochondrial dysfunction could cause neuroinflammation, neuroinflammation could also cause mitochondrial dysfunction. However, based on the systemic nature of AD mitochondrial dysfunction as well as data from experiments we discuss, the former possibility is perhaps more likely. If correct, then manipulation of mitochondria, either directly or through manipulations of bioenergetic pathways, could prove effective in reducing metabolic dysfunction and neuroinflammation in AD patients. We also review some potential approaches through which such manipulations may be achieved.

Dose-specific effect of simvastatin on hypoxia-induced HIF-1α and BACE expression in Alzheimer's disease cybrid cells

Background

Alzheimer’s disease (AD) is associated with vascular risk factors; brain ischemia facilitates the pathogenesis of AD. Recent studies have suggested that the reduction of AD risk with statin was achieved by decreased amyloidogenic amyloid precursor protein.

Methods

We used mitochondrial transgenic neuronal cell (cybrid) models to investigate changes in the levels of intracellular hypoxia inducible factor 1α (HIF-1α) and β-site amyloid precursor protein cleaving enzyme (BACE) in the presence of simvastatin. Sporadic AD (SAD) and age-matched control (CTL) cybrids were exposed to 2 % O₂ and incubated with 1 μM or 10 μM simvastatin.

Results

There was no significant difference between cell survival by 1 or 10 μM simvastatin in both SAD and CTL cybrids. In the presence of 1 μM simvastatin, intracellular levels of HIF-1α and BACE decreased by 40–70 % in SAD, but not CTL cybrids. However, 10 μM simvastatin increased HIF-1α and BACE expression in both cybrid models.

Conclusion

Our results suggest demonstrate differential dose-dependent effects of simvastatin on HIF-1α and BACE in cultured Alzheimer’s disease cybrid cells.

Dieckol, isolated from the edible brown algae Ecklonia cava, induces apoptosis of ovarian cancer cells and inhibits tumor xenograft growth

PURPOSE

Ecklonia cava is an abundant brown alga and has been reported to possess various bioactive compounds having anti-inflammatory effect. However, the anticancer effects of dieckol, a major active compound in E. cava, are poorly understood. In the present study, we investigated the anti-tumor activity of dieckol and its molecular mechanism in ovarian cancer cells and in a xenograft mouse model .

METHODS

MTT assay, PI staining, and PI and Annexin double staining were performed to study cell cytotoxicity, cell cycle distribution, and apoptosis. We also investigated reactive oxygen species (ROS) production and protein expression using flow cytometry and Western blot analysis, respectively. Anti-tumor effects of dieckol were evaluated in SKOV3 tumor xenograft model.

RESULTS

We found that the E. cava extract and its phlorotannins have cytotoxic effects on A2780 and SKOV3 ovarian cancer cells. Dieckol induced the apoptosis of SKOV3 cells and suppressed tumor growth without any significant adverse effect in the SKOV3-bearing mouse model. Dieckol triggered the activation of caspase-8, caspase-9, and caspase-3, and pretreatment with caspase inhibitors neutralized the pro-apoptotic activity of dieckol. Furthermore, treatment with dieckol caused mitochondrial dysfunction and suppressed the levels of anti-apoptotic proteins. We further demonstrated that dieckol induced an increase in intracellular ROS, and the antioxidant N-acetyl-L-cysteine (NAC) significantly reversed the caspase activation, cytochrome c release, Bcl-2 downregulation, and apoptosis that were caused by dieckol. Moreover, dieckol inhibited the activity of AKT and p38, and overexpression of AKT and p38, at least in part, reversed dieckol-induced apoptosis in SKOV3 cells.

CONCLUSION

These data suggest that dieckol suppresses ovarian cancer cell growth by inducing caspase-dependent apoptosis via ROS production and the regulation of AKT and p38 signaling.

Bioenergetic dysfunction and inflammation in Alzheimer's disease: a possible connection

Inflammation is observed in Alzheimer’s disease (AD) subject brains. Inflammation-relevant genes are increasingly implicated in AD genetic studies, and inflammatory cytokines to some extent even function as peripheral biomarkers. What underlies AD inflammation is unclear, but no “foreign” agent has been implicated. This suggests that internally produced damage-associated molecular pattern (DAMPs) molecules may drive inflammation in AD. A more complete characterization and understanding of AD-relevant DAMPs could advance our understanding of AD and suggest novel therapeutic strategies. In this review, we consider the possibility that mitochondria, intracellular organelles that resemble bacteria in many ways, trigger and maintain chronic inflammation in AD subjects. Data supporting the possible nexus between AD-associated bioenergetic dysfunction are discussed.

Cytoplasmic hybrid (cybrid) cell lines as a practical model for mitochondriopathies

Cytoplasmic hybrid (cybrid) cell lines can incorporate human subject mitochondria and perpetuate its mitochondrial DNA (mtDNA)-encoded components. Since the nuclear background of different cybrid lines can be kept constant, this technique allows investigators to study the influence of mtDNA on cell function. Prior use of cybrids has elucidated the contribution of mtDNA to a variety of biochemical parameters, including electron transport chain activities, bioenergetic fluxes, and free radical production. While the interpretation of data generated from cybrid cell lines has technical limitations, cybrids have contributed valuable insight into the relationship between mtDNA and phenotype alterations. This review discusses the creation of the cybrid technique and subsequent data obtained from cybrid applications.

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The cytoplasmic hybrid (cybrid) model can be used to determine mitochondrial DNA (mtDNA) contributions to phenotypic alterations.

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Cybrids are used to study mitochondriopathies such as Parkinson’s disease and Alzheimer’s disease.

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mtDNA heteroplasmy threshold and nuclear DNA-mtDNA compatibility can be determined using cybrid models.

Cadmium and its neurotoxic effects

Cadmium (Cd) is a heavy metal that has received considerable concern environmentally and occupationally. Cd has a long biological half-life mainly due to its low rate of excretion from the body. Thus, prolonged exposure to Cd will cause toxic effect due to its accumulation over time in a variety of tissues, including kidneys, liver, central nervous system (CNS), and peripheral neuronal systems. Cd can be uptaken from the nasal mucosa or olfactory pathways into the peripheral and central neurons; for the latter, Cd can increase the blood brain barrier (BBB) permeability. However, mechanisms underlying Cd neurotoxicity remain not completely understood. Effect of Cd neurotransmitter, oxidative damage, interaction with other metals such as cobalt and zinc, estrogen-like, effect and epigenetic modification may all be the underlying mechanisms. Here, we review the in vitro and in vivo evidence of neurotoxic effects of Cd. The available finding indicates the neurotoxic effects of Cd that was associated with both biochemical changes of the cell and functional changes of central nervous system, suggesting that neurotoxic effects may play a role in the systemic toxic effects of the exposure to Cd, particularly the long-term exposure.

Thymoquinone-induced reactive oxygen species causes apoptosis of chondrocytes via PI3K/Akt and p38kinase pathway

Thymoquinone (TQ), a bioactive ingredient of the volatile oil of black seed (Nigella sativa), has been shown to possess anti-neoplastic and anti-inflammatory effects on a variety of tumours. However, the precise mechanism of action is not clear in normal cells such as primary chondrocytes. So, we have investigated the effects of TQ on the apoptosis of chondrocytes with a focus on reactive oxygen species (ROS) production. In in vitro experiments, chondrocytes were cultured with increasing concentrations of TQ for 24 h or with 20 µmol/L TQ for the indicated time periods, and various experiments were performed to detect the apoptotic effects caused by TQ. The results showed that TQ significantly increases apoptosis. Apoptosis was dose- and time-dependently expressed, and the generation of ROS also dramatically increased in a dose-dependent manner. Pretreatment of N-acetyl-L-cysteine (NAC), an inhibitor of ROS, inhibited both TQ-induced apoptosis and ROS generation. Also, TQ up-regulated phosphorylation of phosphatidylinositol 3-kinase/Akt (PI3K/Akt) and mitogen-activated protein kinases ([MAPKs] p38kinase, ERK-1/-2, and JNKinase), and these effects were prevented by pretreatment of NAC. However, pretreatment with inhibitors of PI3K/Akt and MAPKs did not inhibit TQ-caused ROS generation. Among the inhibitors of PI3K/Akt, p38kinase, ERK-1/-2, and JNKinase, pretreatment with LY294002 and SB203580 abolished TQ-induced apoptosis, but PD98059 and SP600125 did not have any effect on TQ-caused apoptosis. These findings suggest that TQ-induced ROS generation regulates apoptosis by modulating PI3K/Akt and p38kinase pathways.

Cadmium-induced apoptosis in primary rat cerebral cortical neurons culture is mediated by a calcium signaling pathway

Cadmium (Cd) is an extremely toxic metal, capable of severely damaging several organs, including the brain. Studies have shown that Cd disrupts intracellular free calcium ([Ca²⁺]_i) homeostasis, leading to apoptosis in a variety of cells including primary murine neurons. Calcium is a ubiquitous intracellular ion which acts as a signaling mediator in numerous cellular processes including cell proliferation, differentiation, and survival/death. However, little is known about the role of calcium signaling in Cd-induced apoptosis in neuronal cells. Thus we investigated the role of calcium signaling in Cd-induced apoptosis in primary rat cerebral cortical neurons. Consistent with known toxic properties of Cd, exposure of cerebral cortical neurons to Cd caused morphological changes indicative of apoptosis and cell death. It also induced elevation of [Ca²⁺]_i and inhibition of Na⁺/K⁺-ATPase and Ca²⁺/Mg²⁺-ATPase activities. This Cd-induced elevation of [Ca²⁺]_i was suppressed by an IP₃R inhibitor, 2-APB, suggesting that ER-regulated Ca²⁺ is involved. In addition, we observed elevation of reactive oxygen species (ROS) levels, dysfunction of cytochrome oxidase subunits (COX-I/II/III), depletion of mitochondrial membrane potential (ΔΨm), and cleavage of caspase-9, caspase-3 and poly (ADP-ribose) polymerase (PARP) during Cd exposure. Z-VAD-fmk, a pan caspase inhibitor, partially prevented Cd-induced apoptosis and cell death. Interestingly, apoptosis, cell death and these cellular events induced by Cd were blocked by BAPTA-AM, a specific intracellular Ca²⁺ chelator. Furthermore, western blot analysis revealed an up-regulated expression of Bcl-2 and down-regulated expression of Bax. However, these were not blocked by BAPTA-AM. Thus Cd toxicity is in part due to its disruption of intracellular Ca²⁺ homeostasis, by compromising ATPases activities and ER-regulated Ca²⁺, and this elevation in Ca²⁺ triggers the activation of the Ca²⁺-mitochondria apoptotic signaling pathway. This study clarifies the signaling events underlying Cd neurotoxicity, and suggests that regulation of Cd-disrupted [Ca²⁺]_i homeostasis may be a new strategy for prevention of Cd-induced neurodegenerative diseases.

Long-term Aβ exposure augments mCa2+-independent mROS-mediated depletion of cardiolipin for the shift of a lethal transient mitochondrial permeability transition to its permanent mode in NARP cybrids: a protective targeting of melatonin

Mitochondrial dysfunction is a hallmark of amyloid β-peptide (Aβ)-induced neurodegeneration of Alzheimer's disease (AD). This study investigated whether mtDNA T8993G mutation-induced complex V inhibition, clinically associated with neurological muscle weakness, ataxia, and retinitis pigmentosa (NARP), is a potential risk factor for AD and the pathological link for long-term exposure of Aβ-induced mitochondrial toxicity and apoptosis in NARP cybrids. Using noninvasive fluorescence probe-coupled laser scanning imaging microscopy and NARP cybrids harboring 98% mutant genes along with its parental 143B osteosarcoma cells, we demonstrated that Aβ-augmented mitochondrial Ca(2+) (mCa(2+))-independent mitochondrial reactive oxygen species (mROS) formation for a cardiolipin (CL, a major mitochondrial protective phospholipid)-dependent lethal modulation of the mitochondrial permeability transition (MPT). Aβ augmented not only the amount but also the propagation rate of mROS-induced mROS formation to significantly depolarize mitochondrial membrane potential (∆Ψ(m)) and reduce mCa(2+) stress. Aβ-augmented mROS oxidized and depleted CL, thereby enhances mitochondrial fission and movement retardation, which promoted the NARP-augmented lethal transient-MPT (t-MPT) to switch to its irreversible mode of permanent-MPT (p-MPT). Interestingly, melatonin, a multiple mitochondrial protector, markedly reduced Aβ-augmented mROS formation and therefore significantly reduced mROS-mediated depolarization of ∆Ψ(m), fission of mitochondria and retardation of mitochondrial movement to stabilize CL and hence the MPT. In the presence of melatonin, Aβ-promoted p-MPT was reversed to a protective t-MPT, which preserved ∆Ψ(m) and lowered elevated mCa(2+) to sublethal levels for an enhanced mCa(2+)-dependent O(2) consumption. Thus, melatonin may potentially rescue AD patients associated with NARP symptoms.

Thinking outside the cleft to understand synaptic activity: contribution of the cystine-glutamate antiporter (System xc-) to normal and pathological glutamatergic signaling

System x(c)(-) represents an intriguing target in attempts to understand the pathological states of the central nervous system. Also called a cystine-glutamate antiporter, system x(c)(-) typically functions by exchanging one molecule of extracellular cystine for one molecule of intracellular glutamate. Nonvesicular glutamate released during cystine-glutamate exchange activates extrasynaptic glutamate receptors in a manner that shapes synaptic activity and plasticity. These findings contribute to the intriguing possibility that extracellular glutamate is regulated by a complex network of release and reuptake mechanisms, many of which are unique to glutamate and rarely depicted in models of excitatory signaling. Because system x(c)(-) is often expressed on non-neuronal cells, the study of cystine-glutamate exchange may advance the emerging viewpoint that glia are active contributors to information processing in the brain. It is noteworthy that system x(c)(-) is at the interface between excitatory signaling and oxidative stress, because the uptake of cystine that results from cystine-glutamate exchange is critical in maintaining the levels of glutathione, a critical antioxidant. As a result of these dual functions, system x(c)(-) has been implicated in a wide array of central nervous system diseases ranging from addiction to neurodegenerative disorders to schizophrenia. In the current review, we briefly discuss the major cellular components that regulate glutamate homeostasis, including glutamate release by system x(c)(-). This is followed by an in-depth discussion of system x(c)(-) as it relates to glutamate release, cystine transport, and glutathione synthesis. Finally, the role of system x(c)(-) is surveyed across a number of psychiatric and neurodegenerative disorders.

Mitochondria and cell bioenergetics: increasingly recognized components and a possible etiologic cause of Alzheimer's disease

SIGNIFICANCE

Mitochondria and brain bioenergetics are increasingly thought to play an important role in Alzheimer's disease (AD).

RECENT ADVANCES

Data that support this view are discussed from the perspective of the amyloid cascade hypothesis, which assumes beta-amyloid perturbs mitochondrial function, and from an opposite perspective that assumes mitochondrial dysfunction promotes brain amyloidosis. A detailed review of cytoplasmic hybrid (cybrid) studies, which argue mitochondrial DNA (mtDNA) contributes to sporadic AD, is provided. Recent AD endophenotype data that further suggest an mtDNA contribution are also summarized.

CRITICAL ISSUES AND FUTURE DIRECTIONS

Biochemical, molecular, cybrid, biomarker, and clinical data pertinent to the mitochondria-bioenergetics-AD nexus are synthesized and the mitochondrial cascade hypothesis, which represents a mitochondria-centric attempt to conceptualize sporadic AD, is discussed.

Insulin in central nervous system: more than just a peripheral hormone

Insulin signaling in central nervous system (CNS) has emerged as a novel field of research since decreased brain insulin levels and/or signaling were associated to impaired learning, memory, and age-related neurodegenerative diseases. Thus, besides its well-known role in longevity, insulin may constitute a promising therapy against diabetes- and age-related neurodegenerative disorders. More interestingly, insulin has been also faced as the potential missing link between diabetes and aging in CNS, with Alzheimer's disease (AD) considered as the "brain-type diabetes." In fact, brain insulin has been shown to regulate both peripheral and central glucose metabolism, neurotransmission, learning, and memory and to be neuroprotective. And a future challenge will be to unravel the complex interactions between aging and diabetes, which, we believe, will allow the development of efficient preventive and therapeutic strategies to overcome age-related diseases and to prolong human "healthy" longevity. Herewith, we aim to integrate the metabolic, neuromodulatory, and neuroprotective roles of insulin in two age-related pathologies: diabetes and AD, both in terms of intracellular signaling and potential therapeutic approach.

May "mitochondrial eve" and mitochondrial haplogroups play a role in neurodegeneration and Alzheimer's disease?

Mitochondria, the powerhouse of the cell, play a critical role in several metabolic processes and apoptotic pathways. Multiple evidences suggest that mitochondria may be crucial in ageing-related neurodegenerative diseases. Moreover, mitochondrial haplogroups have been linked to multiple area of medicine, from normal ageing to diseases, including neurodegeneration. Polymorphisms within the mitochondrial genome might lead to impaired energy generation and to increased amount of reactive oxygen species, having either susceptibility or protective role in several diseases. Here, we highlight the role of the mitochondrial haplogroups in the pathogenetic cascade leading to diseases, with special attention to Alzheimer's disease.

Methionine sulfoxide reductase A (MsrA) protects cultured mouse embryonic stem cells from H2O2-mediated oxidative stress

Methionine sulfoxide reductase A (MsrA), a member of the Msr gene family, can reduce methionine sulfoxide residues in proteins formed by oxidation of methionine by reactive oxygen species (ROS). Msr is an important protein repair system which can also function to scavenge ROS. Our studies have confirmed the expression of MsrA in mouse embryonic stem cells (ESCs) in culture conditions. A cytosol-located and mitochondria-enriched expression pattern has been observed in these cells. To confirm the protective function of MsrA in ESCs against oxidative stress, a siRNA approach has been used to knockdown MsrA expression in ES cells which showed less resistance than control cells to hydrogen peroxide treatment. Overexpression of MsrA gene products in ES cells showed improved survivability of these cells to hydrogen peroxide treatment. Our results indicate that MsrA plays an important role in cellular defenses against oxidative stress in ESCs. Msr genes may provide a new target in stem cells to increase their survivability during the therapeutic applications.

Effects of lead and/or cadmium on the oxidative damage of rat kidney cortex mitochondria

Lead acetate (300 mg/L) and/or cadmium chloride (50 mg/L) were administered as drinking water to Sprague-Dawley rats for 8 weeks to investigate the possible combined effects of these metals on the damage in renal cortex mitochondria. Increased malonaldehyde levels due to exposure to these metals were detected by colorimetric method, which demonstrated the lipid peroxidation in the renal cortex. Ultrastructural observations and real-time quantitative PCR analyses were performed on kidney cortex pieces to identify the mitochondrial damage and quantify the relative expression levels of cytochrome oxidase subunits (COX-I/II/III), respectively. The most striking ultrastructural modifications involved distortion of mitochondrial cristae and an unusual arrangement, which were more evident when the mixture was ingested. There were significant differences in the expression levels of COX-I, II, and III between the control group and the exposed groups, whereas those in the (lead+cadmium) group were all significantly lower than that in the lead or cadmium group. In conclusion, there was an obvious synergistic oxidative damage effect of lead combined with cadmium on rat kidney cortex mitochondria, which increased defects in mitochondrial oxidative metabolism.

Nerve growth factor attenuates oxidant-induced β-amyloid neurotoxicity in sporadic Alzheimer's disease cybrids

Although mitochondrial dysfunction has been linked to Alzheimer's disease (AD), it is not fully understood how this dysfunction may induce neuronal death. In this study, we show that transmitochondrial hybrid cells (cybrids) expressing mitochondrial genes from patients with sporadic AD (SAD) have substantial alterations in basal upstream tyrosine kinase signaling and downstream serine-threonine kinase signaling that are mediated by intracellular free radicals. This is associated with reduced tropomyocin receptor kinase (TrkA) and p75 neurotrophin receptor receptor expression that profoundly alters nerve growth factor signaling, increases generation of Aβ and decreases viability. Many of these observed effects in SAD cybrids would be predicted to increase risk of premature neuronal death and reduce resistance to stressors and add further support for the pathogenic role of mtDNA expression in the pathogenesis of SAD.

Role of reactive oxygen species-elicited apoptosis in the pathophysiology of mitochondrial and neurodegenerative diseases associated with mitochondrial DNA mutations

A wide spectrum of pathogenic mutations of mitochondrial DNA (mtDNA) has been demonstrated to cause mitochondrial dysfunction and overproduction of reactive oxygen species (ROS), in relation to mitochondrial and neurodegenerative diseases. Our previous studies have shown that large-scale deletions of mtDNA not only serve as an indicator of oxidative damage, but also result in greater susceptibility of human cells to apoptosis triggered by UV irradiation and other apoptotic stimuli. In this review, we focus on the involvement of mtDNA-mutation-associated oxidative stress and susceptibility to apoptosis in the pathophysiology of mitochondrial and neurodegenerative diseases. Different lines of research have provided concordant data to suggest that the mtDNA-mutation-elicited energy insufficiency and enhanced oxidative stress and damage lead to cell dysfunction, and increase the susceptibility of affected cells to apoptosis in patients with these diseases. Moreover, accumulating experimental evidence has shown that antioxidant therapy is a good strategy for decreasing intracellular ROS and alleviating oxidative-stress-induced apoptosis in cells of patients that harbor pathogenic mtDNA mutations.

Oxidative stress and transcriptional regulation in Alzheimer disease

Alzheimer disease (AD) is defined by progressive impairments in memory and cognition and by the presence of extracellular neuritic plaques and intracellular neurofibrillary tangles. However, oxidative stress and impaired mitochondrial function always accompany AD. Mitochondria are a major site of production of free radicals [ie, reactive oxygen species (ROS)] and primary targets of ROS. ROS are cytotoxic, and evidence of ROS-induced damage to cell membranes, proteins, and DNA in AD is overwhelming. Nevertheless, therapies based on antioxidants have been disappointing. Thus, alternative strategies are necessary. ROS also act as signaling molecules including for transcription. Thus, chronic exposure to ROS in AD could activate cascades of genes. Although initially protective, prolonged activation may be damaging. Thus, therapeutic approaches based on modulation of these gene cascades may lead to effective therapies. Genes involved in several pathways including antioxidant defense, detoxification, inflammation, etc, are induced in response to oxidative stress and in AD. However, genes that are associated with energy metabolism, which is necessary for normal brain function, are mostly down-regulated. Redox-sensitive transcription factors such as activator protein-1, nuclear factor-kappaB, specificity protein-1, and hypoxia-inducible factor are important in redox-dependent gene regulation. Peroxisome proliferators-activated receptor-gamma coactivator (PGC-1alpha) is a coactivator of several transcription factors and is a potent stimulator of mitochondrial biogenesis and respiration. Down-regulated expression of PGC-1alpha has been implicated in Huntington disease and in several Huntington disease animal models. PGC-1alpha role in regulation of ROS metabolism makes it a potential candidate player between ROS, mitochondria, and neurodegenerative diseases. This review summarizes the current progress on how oxidative stress regulates the expression of genes that might contribute to AD pathophysiology and the implications of the transcriptional modifications for AD. Finally, potential therapeutic strategies based on the updated understandings of redox state-dependent gene regulation in AD are proposed to overcome the lack of efficacy of antioxidant therapies.

Mitochondria in cybrids containing mtDNA from persons with mitochondriopathies

The cytoplasmic hybrid (cybrid) technique allows investigators to express selected mitochondrial DNA (mtDNA) sequences against fixed nuclear DNA (nDNA) backgrounds. Cybrids have been used to study the effects of known mtDNA mutations on mitochondrial biochemistry, mtDNA-nDNA inter-species compatibility, and mtDNA integrity in persons without mtDNA mutations defined previously. This review discusses events leading up to creation of the cybrid technique, as well as data obtained via application of the cybrid strategies listed above. Although interpreting cybrid data requires awareness of technique limitations, valuable insights into mtDNA genotype-functional phenotype relationships are suggested.

Mitochondrial cascade hypothesis of Alzheimer's disease: myth or reality?

Mitochondria are recognized to play a pivotal role in neuronal cell survival or death because they are regulators of both energy metabolism and apoptotic pathways. Morphologic, biochemical, and molecular genetic studies suggest that mitochondria might be a convergence point for neurodegeneration, including Alzheimer's disease (AD). The functions and properties of mitochondria might render subsets of selectively vulnerable neurons intrinsically susceptible to cellular aging and stress. However, the question, "Is mitochondrial dysfunction a necessary step in neurodegeneration?" is still unanswered. This review presents the ways in which malfunctioning mitochondria and oxidative stress might contribute to neuronal death in AD.

Alzheimer's disease: a lesson from mitochondrial dysfunction

Extensive literature exists supporting a role for mitochondrial dysfunction and oxidative damage in the pathogenesis of Alzheimer's disease. Mitochondria are a major source of intracellular reactive oxygen species and are particularly vulnerable to oxidative stress. This review discusses evidence supporting the notion that mitochondrial dysfunction is intimately associated with Alzheimer's disease pathogenesis. Furthermore, the potential connection between mitochondrial dysfunction/oxidative stress and autophagy in Alzheimer's disease is also discussed. As a result of insufficient digestion of oxidatively damaged macromolecules and organelles by autophagy, neurons progressively accumulate lipofuscin (biological garbage) that could exacerbate neuronal dysfunction. The knowledge that mitochondrial dysfunction has a preponderant role in several pathological conditions instigated the development of mitochondrial antioxidant therapies. Mitochondria-targeted antioxidant treatments are briefly discussed in this review.

Chronic exposure to sub-lethal beta-amyloid (Abeta) inhibits the import of nuclear-encoded proteins to mitochondria in differentiated PC12 cells

Studies on amyloid beta (Abeta|), the peptide thought to play a crucial role in the pathogenesis of Alzheimer's disease, have implicated mitochondria in Abeta-mediated neurotoxicity. We used differentiated PC12 cells stably transfected with an inducible green fluorescent protein (GFP) fusion protein containing an N'-terminal mitochondrial targeting sequence (mtGFP), to examine the effects of sub-lethal Abeta on the import of nuclear-encoded proteins to mitochondria. Exposure to sub-lethal Abeta(25-35) (10 mumol/L) for 48 h inhibited mtGFP import to mitochondria; average rates decreased by 20 +/- 4%. Concomitant with the decline in mtGFP, cytoplasmic mtGFP increased significantly while mtGFP expression and intramitochondrial mtGFP turnover were unchanged. Sub-lethal Abeta(1-42) inhibited mtGFP import and increased cytoplasmic mtGFP but only after 96 h. The import of two endogenous nuclear-encoded mitochondrial proteins, mortalin/mtHsp70 and Tom20 also declined. Prior to the decline in import, mitochondrial membrane potential (mmp), and reactive oxygen species levels were unchanged in Abeta-treated cells versus reverse phase controls. Sustained periods of decreased import were associated with decreased mmp, increased reactive oxygen species, increased vulnerability to oxygen-glucose deprivation and altered mitochondrial morphology. These findings suggest that an Abeta-mediated inhibition of mitochondrial protein import, and the consequent mitochondrial impairment, may contribute to Alzheimer's disease.

Mitochondria and neurodegeneration

Many lines of evidence suggest that mitochondria have a central role in ageing-related neurodegenerative diseases. However, despite the evidence of morphological, biochemical and molecular abnormalities in mitochondria in various tissues of patients with neurodegenerative disorders, the question "is mitochondrial dysfunction a necessary step in neurodegeneration?" is still unanswered. In this review, we highlight some of the major neurodegenerative disorders (Alzheimer's disease, Parkinson's disease, Amyotrophic lateral sclerosis and Huntington's disease) and discuss the role of the mitochondria in the pathogenetic cascade leading to neurodegeneration.

Low dietary inorganic phosphate affects the brain by controlling apoptosis, cell cycle and protein translation

Inorganic phosphate (Pi) plays a key role in diverse physiologic functions. In a previous study, we showed that high dietary Pi perturbs brain growth through Akt/ERK signaling in developing mice. However, no study has investigated the response of the brain to low dietary Pi. In this study, we addressed this question by studying the effects of low dietary Pi on the cerebrum of developing mice. Two-week-old weaned mice were fed with a low phosphate diet for 4 weeks. At the end of the study, their cerebrum was dissected and signals important for protein translation, apoptosis and cell cycle were examined. The low phosphate diet did not cause physiologically significant changes; it increased the protein expression of phosphatase and tensin homolog deleted on chromosome 10 but decreased Akt activity. In addition, expression of eukaryotic translation initiation factor binding protein coupled with increased complex formation of eukaryotic translation initiation factor 4E/eukaryotic translation initiation factor binding protein 1 was induced in the cerebrum by low phosphate, leading to reduced cap-dependent protein translation. Finally, low phosphate facilitated apoptosis and suppressed signals important for the cell cycle in the cerebrum of dual-luciferase reporter mice. In summary, our results showed that a low phosphate diet affects the brain by controlling protein translation, apoptosis and cell cycle in developing mice. Our results support the hypothesis that Pi works as a stimulus capable of increasing or decreasing several pivotal genes for normal development and suggest that regulation of Pi consumption is important in maintaining a healthy life.

N-acetyl-L-cysteine improves outcome of advanced cerebral adrenoleukodystrophy

Hematopoietic stem cell transplantation as a treatment for childhood cerebral adrenoleukodystrophy (ALD) has historically only been successful in early disease. As ALD is associated with oxidative damage, we reasoned that adjunctive therapy with an antioxidant agent, N-acetyl-L-cysteine (NAC), may provide protection from rapid neurologic decline in boys with advanced cerebral disease. We report three boys with advanced ALD, whose neurologic status and brain radiographic findings were stabilized by treatment including NAC 8-11 months after hematopoietic stem cell transplantation. These results contrast with previous survival data in cerebral ALD patients who had a similar degree of brain involvement, all of whom died within 1 year of stem cell infusion despite a full donor engraftment. Thus, NAC merits investigation as a therapeutic strategy for patients with advanced ALD as an intervention that could change this lethal disease to a condition amendable to treatment with hematopoietic stem cell transplantation.

In vivo administration of D609 leads to protection of subsequently isolated gerbil brain mitochondria subjected to in vitro oxidative stress induced by amyloid beta-peptide and other oxidative stressors: relevance to Alzheimer's disease and other oxidative stress-related neurodegenerative disorders

Tricyclodecan-9-yl-xanthogenate (D609) has in vivo and in vitro antioxidant properties. D609 mimics glutathione (GSH) and has a free thiol group, which upon oxidation forms a disulfide. The resulting dixanthate is a substrate for glutathione reductase, regenerating D609. Recent studies have also shown that D609 protects brain in vivo and neuronal cultures in vitro against the potential Alzheimer's disease (AD) causative factor, Abeta(1-42)-induced oxidative stress and cytotoxicity. Mitochondria are important organelles with both pro- and antiapoptotic factor proteins. The present study was undertaken to test the hypothesis that intraperitoneal injection of D609 would provide neuroprotection against free radical-induced, mitochondria-mediated apoptosis in vitro. Brain mitochondria were isolated from gerbils 1 h post injection intraperitoneally (ip) with D609 and subsequently treated in vitro with the oxidants Fe(2+)/H(2)O(2) (hydroxyl free radicals), 2,2-azobis-(2-amidinopropane) dihydrochloride (AAPH, alkoxyl and peroxyl free radicals), and AD-relevant amyloid beta-peptide 1-42 [Abeta(1-42)]. Brain mitochondria isolated from the gerbils previously injected ip with D609 and subjected to these oxidative stress inducers, in vitro, showed significant reduction in levels of protein carbonyls, protein-bound hydroxynonenal [a lipid peroxidation product], 3-nitrotyrosine, and cytochrome c release compared to oxidant-treated brain mitochondria isolated from saline-injected gerbils. D609 treatment significantly maintains the GSH/GSSG ratio in oxidant-treated mitochondria. Increased activity of glutathione S-transferase, glutathione peroxidase, and glutathione reductase in brain isolated from D609-injected gerbils is consistent with the notion that D609 acts like GSH. These antiapoptotic findings are discussed with reference to the potential use of this brain-accessible glutathione mimetic in the treatment of oxidative stress-related neurodegenerative disorders, including AD.

Cell and animal models of mtDNA biology: progress and prospects

The past two decades have witnessed an evolving understanding of the mitochondrial genome's (mtDNA) role in basic biology and disease. From the recognition that mutations in mtDNA can be responsible for human disease to recent efforts showing that mtDNA mutations accumulate over time and may be responsible for some phenotypes of aging, the field of mitochondrial genetics has greatly benefited from the creation of cell and animal models of mtDNA mutation. In this review, we critically discuss the past two decades of efforts and insights gained from cell and animal models of mtDNA mutation. We attempt to reconcile the varied and at times contradictory findings by highlighting the various methodologies employed and using human mtDNA disease as a guide to better understanding of cell and animal mtDNA models. We end with a discussion of scientific and therapeutic challenges and prospects for the future of mtDNA transfection and gene therapy.

The role of mitochondria in inherited neurodegenerative diseases

In the past decade, the genetic causes underlying familial forms of many neurodegenerative disorders, such as Huntington's disease, Parkinson's disease, Alzheimer's disease, amyotrophic lateral sclerosis, Friedreich ataxia, hereditary spastic paraplegia, dominant optic atrophy, Charcot-Marie-Tooth type 2A, neuropathy ataxia and retinitis pigmentosa, and Leber's hereditary optic atrophy have been elucidated. However, the common pathogenic mechanisms of neuronal death are still largely unknown. Recently, mitochondrial dysfunction has emerged as a potential 'lowest common denominator' linking these disorders. In this review, we discuss the body of evidence supporting the role of mitochondria in the pathogenesis of hereditary neurodegenerative diseases. We summarize the principal features of genetic diseases caused by abnormalities of mitochondrial proteins encoded by the mitochondrial or the nuclear genomes. We then address genetic diseases where mutant proteins are localized in multiple cell compartments, including mitochondria and where mitochondrial defects are likely to be directly caused by the mutant proteins. Finally, we describe examples of neurodegenerative disorders where mitochondrial dysfunction may be 'secondary' and probably concomitant with degenerative events in other cell organelles, but may still play an important role in the neuronal decay. Understanding the contribution of mitochondrial dysfunction to neurodegeneration and its pathophysiological basis will significantly impact our ability to develop more effective therapies for neurodegenerative diseases.

Melatonin in Alzheimer's disease and other neurodegenerative disorders

Increased oxidative stress and mitochondrial dysfunction have been identified as common pathophysiological phenomena associated with neurodegenerative disorders such as Alzheimer's disease (AD), Parkinson's disease (PD) and Huntington's disease (HD). As the age-related decline in the production of melatonin may contribute to increased levels of oxidative stress in the elderly, the role of this neuroprotective agent is attracting increasing attention. Melatonin has multiple actions as a regulator of antioxidant and prooxidant enzymes, radical scavenger and antagonist of mitochondrial radical formation. The ability of melatonin and its kynuramine metabolites to interact directly with the electron transport chain by increasing the electron flow and reducing electron leakage are unique features by which melatonin is able to increase the survival of neurons under enhanced oxidative stress. Moreover, antifibrillogenic actions have been demonstrated in vitro, also in the presence of profibrillogenic apoE4 or apoE3, and in vivo, in a transgenic mouse model. Amyloid-β toxicity is antagonized by melatonin and one of its kynuramine metabolites. Cytoskeletal disorganization and protein hyperphosphorylation, as induced in several cell-line models, have been attenuated by melatonin, effects comprising stress kinase downregulation and extending to neurotrophin expression. Various experimental models of AD, PD and HD indicate the usefulness of melatonin in antagonizing disease progression and/or mitigating some of the symptoms. Melatonin secretion has been found to be altered in AD and PD. Attempts to compensate for age- and disease-dependent melatonin deficiency have shown that administration of this compound can improve sleep efficiency in AD and PD and, to some extent, cognitive function in AD patients. Exogenous melatonin has also been reported to alleviate behavioral symptoms such as sundowning. Taken together, these findings suggest that melatonin, its analogues and kynuric metabolites may have potential value in prevention and treatment of AD and other neurodegenerative disorders.

A high inorganic phosphate diet perturbs brain growth, alters Akt-ERK signaling, and results in changes in cap-dependent translation

Inorganic phosphate (Pi) plays a key role in diverse physiological functions. Recently, considerable progress has been made in our understanding of the function and regulation of the brain-specific sodium-dependent inorganic phosphate transporter 1 (NPT1), which is found to exist principally in cerebrum and cerebellum. The potential importance of Pi as a novel signaling molecule and the poor prognosis of diverse neurodegenerative diseases that involve brain-specific NPT1 have prompted us to define the pathways by which Pi affects mouse brain growth. A high phosphate diet caused an increase in serum Pi accompanied by a decrease in calcium, and a decrease in body weight coupled with a decreased relative weight of cerebellum. A high phosphate diet caused a significant increase in protein expression of NPT1, both in cerebrum and cerebellum. Additionally, the high phosphate diet increased Homo sapiens v-akt murine thymoma viral oncogene homolog 1 (Akt) phosphorylation at Ser473 in cerebrum and cerebellum, whereas suppression of Akt phosphorylation at Thr308 was observed only in cerebellum. Selective suppression of eukaryotic translation initiation factor-binding protein (eIF4E-BP1) in cerebrum was induced by high levels of Pi, which induced cap-dependent and cap-independent protein translation in cerebrum and cerebellum, respectively. Phosphorylation of extracellular regulated kinase 1 (ERK1) in comparison with that of ERK2 was significantly reduced in both cerebrum and cerebellum. High levels of Pi reduced protein expressions of proliferating cell nuclear antigen (PCNA) and cyclin D1 in cerebrum and cerebellum. In conclusion, the results indicate that high dietary Pi can perturb normal brain growth, possibly through Akt-ERK signaling in developing mice.

Mitochondrial microheteroplasmy and a theory of aging and age-related disease

We implicate a recently described form of mitochondrial mutation, mitochondrial microheteroplasmy, as a candidate for the principal component of aging. Microheteroplasmy is the presence of hundreds of independent mutations in one organism, with each mutation usually found in 1-2% of all mitochondrial genomes. Despite the low abundance of single mutations, the vast majority of mitochondrial genomes in all adults are mutated. This mutational burden includes inherited mutations, de novo germline mutations, as well as somatic mutations acquired either during early embryonic development or later in adult life. We postulate that microheteroplasmy is sufficient to explain the pathomechanism of several age-associated diseases, especially in conditions with known mitochondrial involvement, such as diabetes (DM), cardiovascular disease, Parkinson's disease (PD), and Alzheimer's disease (AD) and cancer. The genetic properties of microheteroplasmy reconcile the results of disease models (cybrids, hypermutable PolG variants and mitochondrial toxins), with the relatively low levels of maternal inheritance in the aforementioned diseases, and provide an explanation of their delayed, progressive course.

Mitochondria take center stage in aging and neurodegeneration

A critical role of mitochondrial dysfunction and oxidative damage has been hypothesized in both aging and neurodegenerative diseases. Much of the evidence has been correlative, but recent evidence has shown that the accumulation of mitochondrial DNA mutations accelerates normal aging, leads to oxidative damage to nuclear DNA, and impairs gene transcription. Furthermore, overexpression of the antioxidant enzyme catalase in mitochondria increases murine life span. There is strong evidence from genetics and transgenic mouse models that mitochondrial dysfunction results in neurodegeneration and may contribute to the pathogenesis of Alzheimer's disease, Parkinson's disease, Huntington's disease, amyotrophic lateral sclerosis, hereditary spastic paraplegia, and cerebellar degenerations. Therapeutic approaches targeting mitochondrial dysfunction and oxidative damage in these diseases therefore have great promise.