Francisco J. Carvajal

Peroxisome Proliferator-Activated Receptor (PPAR) γ and PPARα Agonists Modulate Mitochondrial Fusion-Fission Dynamics: Relevance to Reactive Oxygen Species (ROS)-Related Neurodegenerative Disorders?

Recent studies showed that the activation of the retinoid X receptor, which dimerizes with peroxisome proliferator-activated receptors (PPARs), leads to an enhanced clearance of Aβ from the brain of transgenic mice model of Alzheimer’s disease (AD), because an increased expression of apolipoprotein E and it main transporters. However, the effects observed must involve additional underlying mechanisms that have not been yet explored. Several studies conducted in our laboratory suggest that part of the effects observed for the PPARs agonist might involves mitochondrial function and, particularly, mitochondrial dynamics. In the present study we assessed the effects of oxidative stress challenge on mitochondrial morphology and mitochondrial dynamics-related proteins in hippocampal neurons. Using immunofluorescence, we evaluated the PPARγ co-activator 1α (PGC-1α), dynamin related protein 1 (DRP1), mitochondrial fission protein 1 (FIS1), and mitochondrial length, in order to determine if PPARs agonist pre-treatment is able to protect mitochondrial population from hippocampal neurons through modulation of the mitochondrial fusion-fission events. Our results suggest that both a PPARγ agonist (ciglitazone) and a PPARα agonist (WY 14.643) are able to protect neurons by modulating mitochondrial fusion and fission, leading to a better response of neurons to oxidative stress, suggesting that a PPAR based therapy could acts simultaneously in different cellular components. Additionally, our results suggest that PGC-1α and mitochondrial dynamics should be further studied in future therapy research oriented to ameliorate neurodegenerative disorders, such as AD.

Interactions of AChE with Aβ Aggregates in Alzheimer’s Brain: Therapeutic Relevance of IDN 5706

Acetylcholinesterase (AChE; EC 3.1.1.7) plays a crucial role in the rapid hydrolysis of the neurotransmitter acetylcholine, in the central and peripheral nervous system and might also participate in non-cholinergic mechanism related to neurodegenerative diseases. Alzheimer’s disease (AD) is a neurodegenerative disorder characterized by a progressive deterioration of cognitive abilities, amyloid-β (Aβ) peptide accumulation and synaptic alterations. We have previously shown that AChE is able to accelerate the Aβ peptide assembly into Alzheimer-type aggregates increasing its neurotoxicity. Furthermore, AChE activity is altered in brain and blood of Alzheimer’s patients. The enzyme associated to amyloid plaques changes its enzymatic and pharmacological properties, as well as, increases its resistant to low pH, inhibitors and excess of substrate. Here, we reviewed the effects of IDN 5706, a hyperforin derivative that has potential preventive effects on the development of AD. Our results show that treatment with IDN 5706 for 10 weeks increases brain AChE activity in 7-month-old double transgenic mice (APPSWE–PS1) and decreases the content of AChE associated with different types of amyloid plaques in this Alzheimer’s model. We concluded that early treatment with IDN 5706 decreases AChE–Aβ interaction and this effect might be of therapeutic interest in the treatment of AD.

Tetrahydrohyperforin prevents cognitive deficit, Aβ deposition, tau phosphorylation and synaptotoxicity in the APPswe/PSEN1ΔE9 model of Alzheimer's disease: a possible effect on APP processing

Alzheimers disease (AD) is a neurodegenerative disorder characterized by a progressive deterioration of cognitive abilities, amyloid-β peptide (Aβ) accumulation and synaptic alterations. Previous studies indicated that hyperforin, a component of the St Johns Wort, prevents Aβ neurotoxicity and some behavioral impairments in a rat model of AD. In this study we examined the ability of tetrahydrohyperforin (IDN5607), a stable hyperforin derivative, to prevent the cognitive deficit and synaptic impairment in an in vivo model of AD. In double transgenic APPswe/PSEN1ΔE9 mice, IDN5706 improves memory and prevents the impairment of synaptic plasticity in a dose-dependent manner, inducing a recovery of long-term potentiation. In agreement with these findings, IDN5706 prevented the decrease in synaptic proteins in hippocampus and cortex. In addition, decreased levels of tau hyperphosphorylation, astrogliosis, and total fibrillar and oligomeric forms of Aβ were determined in double transgenic mice treated with IDN5706. In cultured cells, IDN5706 decreased the proteolytic processing of the amyloid precursor protein that leads to Aβ peptide generation. These findings indicate that IDN5706 ameliorates AD neuropathology and could be considered of therapeutic relevance in AD treatment.

Nicotine Prevents Synaptic Impairment Induced by Amyloid-β Oligomers Through α7-Nicotinic Acetylcholine Receptor Activation

An emerging view on Alzheimer disease’s (AD) pathogenesis considers amyloid-β (Aβ) oligomers as a key factor in synaptic impairment and rodent spatial memory decline. Alterations in the α7-nicotinic acetylcholine receptor (α7-nAChR) have been implicated in AD pathology. Herein, we report that nicotine, an unselective α7-nAChR agonist, protects from morphological and synaptic impairments induced by Aβ oligomers. Interestingly, nicotine prevents both early postsynaptic impairment and late presynaptic damage induced by Aβ oligomers through the α7-nAChR/phosphatidylinositol-3-kinase (PI3K) signaling pathway. On the other hand, a cross-talk between α7-nAChR and the Wnt/β-catenin signaling pathway was revealed by the following facts: (1) nicotine stabilizes β-catenin, in a concentration-dependent manner; (2) nicotine prevents Aβ-induced loss of β-catenin through the α7-nAChR; and (3) activation of canonical Wnt/β-catenin signaling induces α7-nAChR expression. Analysis of the α7-nAChR promoter indicates that this receptor is a new Wnt target gene. Taken together, these results demonstrate that nicotine prevents memory deficits and synaptic impairment induced by Aβ oligomers. In addition, nicotine improves memory in young APP/PS1 transgenic mice before extensive amyloid deposition and senile plaque development, and also in old mice where senile plaques have already formed. Activation of the α7-nAChR/PI3K signaling pathway and its cross-talk with the Wnt signaling pathway might well be therapeutic targets for potential AD treatments.

Andrographolide reduces cognitive impairment in young and mature AβPPswe/PS-1 mice

Alzheimer’s disease (AD) is a neurodegenerative disorder in which the amyloid-β (Aβ) oligomers are a key factor in synaptic impairment and in spatial memory decline associated with neuronal dysfunction. This impairment includes synaptic failure associated with the loss of synaptic proteins that contribute to AD progression. Interestingly, the use of natural compounds is an emergent conceptual strategy in the search for drugs with therapeutic potentials for treating neurodegenerative disorders. In the present study, we report that andrographolide (ANDRO), which is a labdane diterpene extracted from Andrographis paniculata, increases slope of field excitatory postsynaptic potentials (fEPSP) in the CA1 region of hippocampal slices and inhibits long-term depression (LTD), protecting the long-term potentiation (LTP) against the damage induced by Aβ oligomers in vitro, most likely by inhibiting glycogen synthase kinase-3β (GSK-3β). Additionally, ANDRO prevents changes in neuropathology in two different age groups (7- and 12-month-old mice) of an AβPPswe/PS-1 Alzheimer’s model. ANDRO reduces the Aβ levels, changing the ontogeny of amyloid plaques in hippocampi and cortices in 7-month-old mice, and reduces tau phosphorylation around the Aβ oligomeric species in both age groups. Additionally, we observed that ANDRO recovers spatial memory functions that correlate with protecting synaptic plasticity and synaptic proteins in two different age groups. Our results suggest that ANDRO could be used in a potential preventive therapy during AD progression.

Peroxisome Proliferators Reduce Spatial Memory Impairment, Synaptic Failure, and Neurodegeneration in Brains of a Double Transgenic Mice Model of Alzheimer's Disease

Alzheimers disease (AD) is a neurodegenerative disorder characterized by a progressive deterioration of cognitive abilities, accumulation of the amyloid-β peptide (Aβ), increase of oxidative stress, and synaptic alterations. The scavenging of reactive oxygen species through their matrix enzyme catalase is one of the most recognized functions of peroxisomes. The induction of peroxisome proliferation is attained through different mechanisms by a set of structurally diverse molecules called peroxisome proliferators. In the present work, a double transgenic mouse model of AD that co-expresses a mutant human amyloid-β protein precursor (AβPPswe) and presenilin 1 without exon 9 (PS1dE9) was utilized in order to assess the effect of peroxisomal proliferation on Aβ neurotoxicity in vivo. Mice were tested for spatial memory and their brains analyzed by cytochemical, electrophysiological, and biochemical methods. We report here that peroxisomal proliferation significantly reduces (i) memory impairment, found in this model of AD; (ii) Aβ burden and plaque-associated acetylcholinesterase activity; (iii) neuroinflammation, measured by the extent of astrogliosis and microgliosis; and (iv) the decrease in postsynaptic proteins, while promoting synaptic plasticity in the form of long-term potentiation. We concluded that peroxisomal proliferation reduces various AD neuropathological markers and peroxisome proliferators may be considered as potential therapeutic agents against the disease.

Age Progression of Neuropathological Markers in the Brain of the Chilean Rodent Octodon degus, a Natural Model of Alzheimer's Disease

Alzheimers disease (AD) is the most common neurodegenerative disorder and the leading cause of age‐related dementia worldwide. Several models for AD have been developed to provide information regarding the initial changes that lead to degeneration. Transgenic mouse models recapitulate many, but not all, of the features of AD, most likely because of the high complexity of the pathology. In this context, the validation of a wild‐type animal model of AD that mimics the neuropathological and behavioral abnormalities is necessary. In previous studies, we have reported that the Chilean rodent Octodon degus could represent a natural model for AD. In the present work, we further describe the age‐related neurodegeneration observed in the O. degus brain. We report some histopathological markers associated with the onset progression of AD, such as glial activation, increase in oxidative stress markers, neuronal apoptosis and the expression of the peroxisome proliferative‐activated receptor γ coactivator‐1α (PGC‐1α). With these results, we suggest that the O. degus could represent a new model for AD research and a powerful tool in the search for therapeutic strategies against AD.

Role of NMDA Receptor-Mediated Glutamatergic Signaling in Chronic and Acute Neuropathologies

N-Methyl-D-aspartate receptors (NMDARs) have two opposing roles in the brain. On the one hand, NMDARs control critical events in the formation and development of synaptic organization and synaptic plasticity. On the other hand, the overactivation of NMDARs can promote neuronal death in neuropathological conditions. Ca2+ influx acts as a primary modulator after NMDAR channel activation. An imbalance in Ca2+ homeostasis is associated with several neurological diseases including schizophrenia, Alzheimers disease, Parkinsons disease, Huntingtons disease, and amyotrophic lateral sclerosis. These chronic conditions have a lengthy progression depending on internal and external factors. External factors such as acute episodes of brain damage are associated with an earlier onset of several of these chronic mental conditions. Here, we will review some of the current evidence of how traumatic brain injury can hasten the onset of several neurological conditions, focusing on the role of NMDAR distribution and the functional consequences in calcium homeostasis associated with synaptic dysfunction and neuronal death present in this group of chronic diseases.

Tetrahydrohyperforin Decreases Cholinergic Markers associated with Amyloid-β Plaques, 4-Hydroxynonenal Formation, and Caspase-3 Activation in AβPP/PS1 Mice

Alzheimers disease (AD) is a neurodegenerative disorder characterized by a progressive deterioration of cognitive abilities, amyloid-β peptide (Aβ) accumulation, neurofibrillary tangle deposition, synaptic alterations, and oxidative injury. In AD patients, acetylcholinesterase (AChE) activity is low in most regions of the brain, but increased within and around amyloid plaques, where it accelerates the Aβ assembly into oligomers and fibrils, increasing its neurotoxicity. Tetrahydrohyperforin (THH), a semi-synthetic derivative of hyperforin, reduces tau phosphorylation and Aβ accumulation in AD mouse models. In the present study, we examined the effects of THH on Aβ-AChE complexes, α7-nicotinic acetylcholine receptors (α7-nAChR), 4-hydroxynonenal (4-HNE) adducts, caspase-3 activation, and spatial memory in young AβPPSwe/PSEN1ΔE9 (AβPP/PS1) transgenic mice, in order to evaluate its potential preventive effects on the development of the disease. We report here that treatment with THH prevents the association of AChE to different types of amyloid plaques; partially restores the brain distribution of AChE molecular forms; increases α7-nAChR levels in the hippocampus of treated mice; decreases the amount of these receptors in amyloid plaques; and reduces the oxidative damage, evidenced by 4-HNE adduct formation and caspase-3 activation on AβPP/PS1 mice brain; demonstrating the neuroprotective properties of THH. Finally, we found that the acute treatment of hippocampal neurons with THH, in the presence of Aβ-AChE complexes, prevents 4-HNE adduct formation and caspase-3 activation. Our data support a therapeutic potential of THH for the treatment of AD.

Quercetin Exerts Differential Neuroprotective Effects Against H2O2 and Aβ Aggregates in Hippocampal Neurons: the Role of Mitochondria

Amyloid-β peptide (Aβ) is one of the major players in the pathogenesis of Alzheimer’s disease (AD). Despite numerous studies, the mechanisms by which Aβ induces neurodegeneration are not completely understood. Oxidative stress is considered a major contributor to the pathogenesis of AD, and accumulating evidence indicates that high levels of reactive oxygen species (ROS) are involved in Aβ-induced neurodegeneration. Moreover, Aβ can induce the deregulation of calcium homeostasis, which also affects mitochondrial function and triggers neuronal cell death. In the present study, we analyzed the effects of quercetin, a plant flavonoid with antioxidant properties, on oxidative stress- and Aβ-induced degeneration. Our results indicate that quercetin efficiently protected against H2O2-induced neuronal toxicity; however, this protection was only partial in rat hippocampal neurons that were treated with Aβ. Treatment with quercetin decreased ROS levels, recovered the normal morphology of mitochondria, and prevented mitochondrial dysfunction in neurons that were treated with H2O2. By contrast, quercetin treatment partially rescued hippocampal neurons from Aβ-induced mitochondrial injury. Most importantly, quercetin treatment prevented the toxic effects that are induced by H2O2 in hippocampal neurons and, to a lesser extent, the Aβ-induced toxicity that is associated with the superoxide anion, which is a precursor of ROS production in mitochondria. Collectively, these results indicate that quercetin exerts differential effects on the prevention of H2O2- and Aβ-induced neurotoxicity in hippocampal neurons and may be a powerful tool for dissecting the molecular mechanisms underlying Aβ neurotoxicity.