Exacerbation of excitotoxic neuronal death induced during mitochondrial inhibition in vivo: relation to energy imbalance or ATP depletion?

AMPK and its Activator Berberine in the Treatment of Neurodegenerative Diseases

Neurodegenerative disorders are heterogeneous diseases associated with either acute or progressive neurodegeneration, causing the loss of neurons and axons in the central nervous system (CNS), showing high morbidity and mortality, and there are only a few effective therapies. Here, we summarized that the energy sensor adenosine 5'-monophosphate (AMP)-activated protein kinase (AMPK), and its agonist berberine can combat the common underlying pathological events of neurodegeneration, including oxidative stress, neuroinflammation, mitochondrial disorder, glutamate excitotoxicity, apoptosis, autophagy disorder, and disruption of neurovascular units. The abovementioned effects of berberine may primarily depend on activating AMPK and its downstream targets, such as the mammalian target of rapamycin (mTOR), sirtuin1 (SIRT1), nuclear factor erythroid-2 related factor-2 (Nrf2), nuclear factor-κB (NF-κB), phosphoinositide 3-kinase/protein kinase B (PI3K/Akt), nicotinamide adenine dinucleotide (NAD+), and p38 mitogen-activated protein kinase (p38 MAPK). It is hoped that this review will provide a strong basis for further scientific exploration and development of berberine's therapeutic potential against neurodegeneration.

Delayed Onset and Reduced Cognitive Deficits through Pre-Conditioning with 3-Nitropropionic Acid is Dependent on Sex and CAG Repeat Length in the R6/2 Mouse Model of Huntington's Disease

BACKGROUND

Impairments in energy metabolism are implicated in Huntington's disease (HD) pathogenesis. Reduced levels of the mitochondrial enzyme succinate dehydrogenase (SDH), the main element of complex II, are observed post mortem in the brains of HD patients, and energy metabolism defects have been identified in both presymptomatic and symptomatic HD patients.

OBJECTIVE

Chemical preconditioning with 3-nitropropionic acid (3-NP), an irreversible inhibitor of SDH, has been shown to increase tolerance against experimental hypoxia in both heart and brain. Here we studied the effect of chronic preconditioning in the R6/2 mouse model of HD using mice carrying CAG repeat lengths of either 250 or 400 repeats. Both are transgenic fragment models, with 250CAG mice having a more rapid disease progression than 400CAG mice.

METHODS

Low doses of 3-NP (24 mg/kg) were administered via the drinking water and the effect on phenotype progression and cognition function assessed.

RESULTS

After 3-NP treatment there were significant improvements in all aspects of the behavioural phenotype, apart from body weight, with timing and magnitude of improvements dependent on both CAG repeat length and sex. Specifically, a delay in the deterioration of general health (as shown by delayed onset of glycosuria and increased survival) was seen in both male and female 400CAG mice and in female 250CAG mice and was consistent with improved appearance of 3-NP treated R6/2 mice. Male 250CAG mice showed improvements but these were short term, and 3-NP treatment eventually had deleterious effects on their survival rate. When cognitive performance of 250CAG mice was assessed using a two-choice discrimination touchscreen task, we found that female mice showed significant improvements.

DISCUSSION

Together, our results support the idea that energy metabolism contributes to the pathogenesis of HD, and suggest that improving energy deficits might be a therapeutically useful target.

Conditioned medium of human adipose-derived mesenchymal stem cells mediates protection in neurons following glutamate excitotoxicity by regulating energy metabolism and GAP-43 expression

Glutamate excitotoxicity has been implicated as one of the pathological mechanisms contributing to neuronal cell death and is involved in many neurological disorders. Stem cell transplantation is a promising approach for the treatment of nervous system damage or diseases. Previous studies have shown that mesenchymal stem cells (MSCs) have important therapeutic effects in experimental animal and preclinical disease model of central nervous system pathology. However, it is not well understood whether neurogenesis of MSCs or MSC conditioned-medium (CM) containing microparticles mediates therapeutic effects. Here, we investigated the neuroprotective effects of human adipose-derived MSCs (AMSCs) on cortical neurons using models of glutamate excitotoxicity. Following exposure to glutamate (100 μM, 15 min), cortical neurons were co-cultured with either AMSCs separated by a semiporous membrane (prohibiting direct cell-cell contact) or with AMSC-CM for 18 h. Compared to untreated control groups, AMSCs and AMSC-CM partially and similarly reduced neuronal cell damages, as indicated by reduced LDH release, a decreased number of trypan-positive cells and a decline in the number of apoptotic nuclei. Protection by CM was associated with increased GAP-43 expression and an elevated number of GAP-43-positive neurites. Furthermore, CM increased levels of ATP, NAD(+) and NADH and the ratio of NAD(+)/NADH, while preventing a glutamate-induced decline in mitochondrial membrane potential. These results demonstrate that AMSC-CM mediates direct neuroprotection by inhibiting neuronal cell damage/apoptosis, promoting nerve regeneration and repair, and restoring bioenergy following energy depletion caused by glutamate excitotoxicity.

Synaptic activity and bioenergy homeostasis: implications in brain trauma and neurodegenerative diseases

Powered by glucose metabolism, the brain is the most energy-demanding organ in our body. Adequate ATP production and regulation of the metabolic processes are essential for the maintenance of synaptic transmission and neuronal function. Glutamatergic synaptic activity utilizes the largest portion of bioenergy for synaptic events including neurotransmitter synthesis, vesicle recycling, and most importantly, the postsynaptic activities leading to channel activation and rebalancing of ionic gradients. Bioenergy homeostasis is coupled with synaptic function via activities of the sodium pumps, glutamate transporters, glucose transport, and mitochondria translocation. Energy insufficiency is sensed by the AMP-activated protein kinase (AMPK), a master metabolic regulator that stimulates the catalytic process to enhance energy production. A decline in energy supply and a disruption in bioenergy homeostasis play a critical role in multiple neuropathological conditions including ischemia, stroke, and neurodegenerative diseases including Alzheimer’s disease and traumatic brain injuries.

Regulation of neuronal bioenergy homeostasis by glutamate

Bioenergy homeostasis is crucial in maintaining normal cell function and survival and it is thus important to understand cellular mechanisms underlying its regulation. Neurons use a large amount of ATP to maintain membrane potential and synaptic communication, making the brain the most energy consuming organ in the body. Glutamate mediates a large majority of synaptic transmission which is responsible for the expression of neural plasticity and higher brain functions. Most of the energy cost is attributable to the glutamatergic system; under pathological conditions such as stroke and brain ischemia, neural energy depletion is accompanied by a massive release of glutamate. However, the specific cellular processes implicated in glutamate-dependent bioenergy dynamics are not well understood. We find that glutamate induces a rapid and dramatic reduction of ATP levels in neurons, through reduced ATP genesis and elevated consumption. ATP reduction depends on NMDA receptor activity, but is not a result of neuronal firing, gap junction-mediated leaking or intracellular signaling. Similar changes in ATP levels are also induced by synaptic glutamate accumulation following suppression of glutamate transporter activity. Furthermore, the glutamate-induced ATP down-regulation is blocked by the sodium pump inhibitor ouabain, suggesting the sodium pump as the primary energy consumer during glutamate stimulation. These data suggest the important role of glutamate in the control of cellular ATP homeostasis.

Protein targets for carbonylation by 4-hydroxy-2-nonenal in rat liver mitochondria

Protein carbonylation has been associated with various pathophysiological processes. A representative reactive carbonyl species (RCS), 4-hydroxy-2-nonenal (HNE), has been implicated specifically as a causative factor for the initiation and/or progression of various diseases. To date, however, little is known about the proteins and their modification sites susceptible to "carbonyl stress" by this RCS, especially in the liver. Using chemoprecipitation based on a solid-phase hydrazine chemistry coupled with LC-MS/MS bottom-up approach and database searching, we identified several protein-HNE adducts in isolated rat liver mitochondria upon HNE exposure. The identification of selected major protein targets, such as the ATP synthase β-subunit, was further confirmed by immunoblotting and a gel-based approach in combination with LC-MS/MS. A network was also created based on the identified protein targets, which showed that the main protein interactions were associated with cell death, tumor morphology and drug metabolism, implicating the toxic nature of HNE in the liver mitoproteome. The functional consequence of carbonylation was illustrated by its detrimental impact on the activity of ATP synthase, a representative major mitochondrial protein target for HNE modifications.

3-Nitropropionic acid as a tool to study the mechanisms involved in Huntington's disease: past, present and future

Huntington's disease (HD) is an inheritable autosomal-dominant disorder whose causal mechanisms remain unknown. Experimental models have begun to uncover these pathways, thus helping to understand the mechanisms implicated and allowing for the characterization of potential targets for new therapeutic strategies. 3-Nitropropionic acid is known to produce in animals behavioural, biochemical and morphologic changes similar to those occurring in HD. For this reason, this phenotypic model is gaining attention as a valuable tool to mimick this disorder and further developing new therapies. In this review, we will focus on the past and present research of this molecule, to finally bring a perspective on what will be next in this promising field of study.

Redox proteomics identification of 4-hydroxynonenal-modified brain proteins in Alzheimer's disease: Role of lipid peroxidation in Alzheimer's disease pathogenesis

Numerous studies have shown that neuronal lipids are highly susceptible to oxidative stress including in those brain areas directly involved in the neurodegenerative process of Alzheimer's disease (AD). Lipid peroxidation directly damages membranes and also generates a number of secondary biologically active products (toxic aldehydes)that are capable of easily attacking lipids, proteins, and DNA. Accumulating evidence has demonstrated regionally increased brain lipid peroxidation in patients with AD; however, extensive studies on specific targets of lipid peroxidation-induced damage are still missing. The present study represents a further step in understanding the relationship between oxidative modification of protein and neuronal death associated with AD. We used a proteomics approach to determine specific targets of lipid peroxidation in AD brain, both in hippocampus and inferior parietal lobule, by coupling immunochemical detection of 4-hydroxynonenal-bound proteins with 2-D polyacrylamide gel electrophoresis and MS analysis. We identified 4-hydroxynonenal-bound proteins in the hippocampus and inferior parietal lobule brain regions of subjects with AD. The identified proteins play different biological functions including energy metabolism, antioxidant system, and structural proteins, thus impairing multiple molecular pathways. Our results provide further evidence for the role of lipid peroxidation in the pathogenesis of AD.

Effects of mitochondrial poisons on glutathione redox potential and carotid body chemoreceptor activity

Low oxygen sensing in chemoreceptor cells involves the inhibition of specific plasma membrane K(+) channels, suggesting that mitochondria-derived reactive oxygen species (ROS) link hypoxia to K(+) channel inhibition, subsequent cell depolarization and activation of neurotransmitter release. We have used several mitochondrial poisons, alone and in combination with the antioxidant N-acetylcysteine (NAC), and quantify their capacity to alter GSH/GSSG levels and glutathione redox potential (E(GSH)) in rat diaphragm. Selected concentrations of mitochondrial poisons with or without NAC were tested for their capacity to activate neurotransmitter release in chemoreceptor cells and to alter ATP levels in intact rat carotid body (CB). We found that rotenone (1 microM), antimycin A (0.2 microg/ml) and sodium azide (5mM) decreased E(GSH); NAC restored E(GSH) to control values. At those concentrations mitochondrial poisons activated neurotransmitter release from CB chemoreceptor cells and decreased CB ATP levels, NAC being ineffective to modify these responses. Additional experiments with 3-nitroprionate (5mM), lower concentrations of rotenone and dinitrophenol revealed variable relationships between E(GSH) and chemoreceptor cell neurotransmitter release responses and ATP levels. These findings indicate a lack of correlation between mitochondrial-generated modifications of E(GSH) and chemoreceptor cells activity. This lack of correlation renders unlikely that alteration of mitochondrial production of ROS is the physiological pathway chemoreceptor cells use to signal hypoxia.

Mild mitochondrial inhibition in vivo enhances glutamate-induced neuronal damage through calpain but not caspase activation: role of ionotropic glutamate receptors

Glutamate neurotoxicity is exacerbated when energy metabolism is impaired. In vitro studies show that neuronal death in these conditions is related to mitochondrial dysfunction, ATP depletion, and the loss of calcium homeostasis. We have recently observed that, in vivo, enhancement of glutamate toxicity elicited by previous mitochondrial inhibition does not involve severe ATP depletion, suggesting the involvement of other processes. Factors such as the activation of different proteases may determine the extent and type of cell death. Protease activation might be triggered by internal or external factors, such as mitochondrial damage or the activation of a particular glutamate receptor subtype. In the present study we aimed to investigate whether moderate inhibition of mitochondrial metabolism facilitates glutamate toxicity through caspase-3 or calpain activation, as well as the contribution of NMDA and non-NMDA glutamate ionotropic receptors to this activation. Rats were pre-treated with a subtoxic dose of 3-NP and 4 h later intrastriatally injected with glutamate. Results show that neither of these treatments alone (3-NP or Glu) or in combination (3-NP+Glu) activated caspase-3. Conversely, calpain activity is induced after glutamate injection both in intact and 3-NP pre-treated rats. Inhibition of calpain activity by MDL-28170 significantly prevented striatal damage. NMDA and non-NMDA receptors contributed equally to calpain activation and to the induction of neuronal death. Results suggest that enhancement of glutamate toxicity due to inhibition of mitochondrial metabolism in vivo, does not recruit caspase-dependent apoptosis but favors calpain activation through the stimulation of both subtypes of glutamate ionotropic receptors.

Excitotoxic neuronal death and the pathogenesis of Huntington's disease

Huntington's disease (HD) is a neurodegenerative hereditary illness originated by the mutation of the gene encoding the huntingtin-protein (htt). Mutated htt (mhtt) is characterized by an increased number of glutamine repeats in the N-terminal end; when 40 or more glutamine residues are present, the disease is manifested. Expression of mhtt leads to the selective death of the medium spiny neurons (MSN) in the neostriatum, resulting in the appearance of generalized involuntary movements, the main phenotypic alteration of HD. The relationship between the expression of mhtt and the death of the MSN is not fully understood. Nonetheless, according to experimental evidence indicating that MSN are selectively vulnerable to the toxicity of glutamate (excitotoxicity) or its analogues, excitotoxic neuronal death is suggested to be involved in neurodegeneration associated with HD. Support for this hypothesis comes from studies in HD postmortem tissue and transgenic mice models, suggesting a correlation between mhtt expression and altered glutamatergic neurotransmission, mainly altered conductance of the N-methyl-D-aspartate (NMDA) glutamate receptor subtype and decreased levels of glutamate transporters. On the other hand, alterations in energy metabolism are well documented in HD patients, which might facilitate excitotoxicity. Throughout this review we will discuss relevant evidence suggesting that altered glutamatergic neurotransmission plays a role in neurodegeneration associated with HD, as well as the possible contribution of deficient energy metabolism to the development of an excitotoxic cell death cascade in MSN. We show data supporting protection by energy substrates against neuronal damage in a rat model combining energy deficit and glutamate toxicity.

Excitotoxic damage, disrupted energy metabolism, and oxidative stress in the rat brain: antioxidant and neuroprotective effects of L-carnitine

Excitotoxicity and disrupted energy metabolism are major events leading to nerve cell death in neurodegenerative disorders. These cooperative pathways share one common aspect: triggering of oxidative stress by free radical formation. In this work, we evaluated the effects of the antioxidant and energy precursor, levocarnitine (L-CAR), on the oxidative damage and the behavioral, morphological, and neurochemical alterations produced in nerve tissue by the excitotoxin and free radical precursor, quinolinic acid (2,3-pyrindin dicarboxylic acid; QUIN), and the mitochondrial toxin, 3-nitropropionic acid (3-NP). Oxidative damage was assessed by the estimation of reactive oxygen species formation, lipid peroxidation, and mitochondrial dysfunction in synaptosomal fractions. Behavioral, morphological, and neurochemical alterations were evaluated as markers of neurotoxicity in animals systemically administered with L-CAR, chronically injected with 3-NP and/or intrastriatally infused with QUIN. At micromolar concentrations, L-CAR reduced the three markers of oxidative stress stimulated by both toxins alone or in combination. L-CAR also prevented the rotation behavior evoked by QUIN and the hypokinetic pattern induced by 3-NP in rats. Morphological alterations produced by both toxins (increased striatal glial fibrillary acidic protein-immunoreactivity for QUIN and enhanced neuronal damage in different brain regions for 3-NP) were reduced by L-CAR. In addition, L-CAR prevented the synergistic action of 3-NP and QUIN to increase motor asymmetry and depleted striatal GABA levels. Our results suggest that the protective properties of L-CAR in the neurotoxic models tested are mostly mediated by its characteristics as an antioxidant agent.