Loss Of Basal Forebrain Cholinergic Neurons Research Articles

Increased Levels of Phosphorylated-P38α Induce WNT/β-Catenin and NGF/P75NTR/TrkA Pathways Disruption and SN56 Cell Death following Single and Repeated Chlorpyrifos Treatment.

Chlorpyrifos (CPF) biocide, exposure to which is mainly produced in the human population through diet, induces several neurotoxic effects. CPF single and repeated exposure induces memory and learning disorders, although the mechanisms that produce these outcomes are complex and not well understood. CPF treatment (single and repeated) of cholinergic septal SN56 cells induced an increase in phosphorylated-P38α levels that led to WNT/β-Catenin and NGF/P75NTR/TrkA pathways disruption and cell death. These results provide new knowledge on the mechanisms that mediate CPF basal forebrain cholinergic neuronal loss induced by CPF single and repeated exposure and can help unravel the way through which this compound produces cognitive decline and develop efficient treatments against these effects.

Bisphenol-A Neurotoxic Effects on Basal Forebrain Cholinergic Neurons In Vitro and In Vivo.

The widely used plasticizer bisphenol-A (BPA) is well-known for producing neurodegeneration and cognitive disorders, following acute and long-term exposure. Although some of the BPA actions involved in these effects have been unraveled, they are still incompletely known. Basal forebrain cholinergic neurons (BFCN) regulate memory and learning processes and their selective loss, as observed in Alzheimer's disease and other neurodegenerative diseases, leads to cognitive decline. In order to study the BPA neurotoxic effects on BFCN and the mechanisms through which they are induced, 60-day old Wistar rats were used, and a neuroblastoma cholinergic cell line from the basal forebrain (SN56) was used as a basal forebrain cholinergic neuron model. Acute treatment of rats with BPA (40 µg/kg) induced a more pronounced basal forebrain cholinergic neuronal loss. Exposure to BPA, following 1- or 14-days, produced postsynaptic-density-protein-95 (PSD95), synaptophysin, spinophilin, and N-methyl-D-aspartate-receptor-subunit-1 (NMDAR1) synaptic proteins downregulation, an increase in glutamate content through an increase in glutaminase activity, a downregulation in the vesicular-glutamate-transporter-2 (VGLUT2) and in the WNT/β-Catenin pathway, and cell death in SN56 cells. These toxic effects observed in SN56 cells were mediated by overexpression of histone-deacetylase-2 (HDAC2). These results may help to explain the synaptic plasticity, cognitive dysfunction, and neurodegeneration induced by the plasticizer BPA, which could contribute to their prevention.

Cortical cholinergic denervation in primary progressive aphasia with Alzheimer pathology.

To investigate the status of the basal forebrain cholinergic system in primary progressive aphasia (PPA) as justification for cholinergic therapy. A cohort of 36 brains from PPA participants with the neuropathology of Alzheimer disease (PPA-AD, n = 14) or frontotemporal lobar degeneration (PPA-tau, n = 12; PPA-TDP, n = 10) were used for semiquantitative rating of degeneration and gliosis of basal forebrain cholinergic neurons (BFCN). A subpopulation of 5 PPA-AD and 7 control brains underwent detailed analysis of BFCN pathology and cortical cholinergic axonal loss employing immunohistochemical and histochemical methods and stereologic analysis. Semiquantitatively, 11 (∼80%) PPA-AD participants were rated as having moderate/severe BFCN loss and gliosis, whereas none of the PPA-tau and only 1 (10%) PPA-TDP participant received such a rating. Detailed analysis in the subpopulation of PPA-AD participants revealed substantial tangle formation, loss of BFCN, and degeneration of cortical cholinergic axons. Compared to controls, loss of p75 low affinity neurotrophin receptor-positive BFCN was detected in the PPA-AD participants (p < 0.01). Acetylcholinesterase-positive cholinergic axons in all cortical areas studied displayed loss in PPA-AD (p < 0.005-0.0001). The loss was more severe in the language-dominant left hemisphere and, within the left hemisphere, in language-affiliated cortical areas. Our results demonstrate prominent depletion of BFCN and cortical cholinergic axons in PPA-AD when compared with normal control or other neuropathologic variants of PPA. The demonstration of cholinergic denervation with an anatomy that fits the clinical picture suggests that cholinergic treatment is justified in patients with PPA who have positive AD biomarkers.

Manganese induced ROS and AChE variants alteration leads to SN56 basal forebrain cholinergic neuronal loss after acute and long-term treatment

Manganese (Mn) induces cognitive disorders and basal forebrain (BF) cholinergic neuronal loss, involved on learning and memory regulation, which could be the cause of such cognitive disorders. However, the mechanisms through which it induces these effects are unknown. We hypothesized that Mn could induce BF cholinergic neuronal loss through oxidative stress generation, cholinergic transmission and AChE variants alteration that could explain Mn cognitive disorders. This study shows that Mn impaired cholinergic transmission in SN56 cholinergic neurons from BF through alteration of AChE and ChAT activity and CHT expression. Moreover, Mn induces, after acute and long-term exposure, AChE variants alteration and oxidative stress generation that leaded to lipid peroxidation and protein oxidation. Finally, Mn induces cell death on SN56 cholinergic neurons and this effect is independent of cholinergic transmission alteration, but was mediated partially by oxidative stress generation and AChE variants alteration. Our results provide new understanding of the mechanisms contributing to the harmful effects of Mn on cholinergic neurons and their possible involvement in cognitive disorders induced by Mn.

Percellome toxicogenomics project: newly designed repeated dose study

Chlorpyrifos (CPF) is an organophosphate insecticide reported to induce, both after acute and repeated exposure, learning and memory dysfunctions, although the mechanism is not completely known. CPF produces basal forebrain cholinergic neuronal loss, involved on learning and memory regulation, which could be the cause of such cognitive disorders. This effect was reported to be induced through apoptotic process, partially mediated by AChE overexpression, although neuronal necrosis was also described after CPF exposure. Accordingly, we hypothesized that CPF induces apoptotic and necrotic basal forebrain cholinergic cell death. We evaluated, in septal SN56 basal forebrain cholinergic neurons, the CPF effect after 24 h and 14 days exposure on apoptosis and necrosis induction and the apoptotic and necrotic gene expression pathways. This study shows that CPF induces, after acute and long-term exposure, apoptosis and necrosis, partially mediated through AChE overexpression. Evaluation of cell death pathways supports the necrosis and apoptosis data and revealed that some genes are altered at lower concentrations than those at which the effects observed are produce and below the No Observed Adverse Effect Level (NOAEL). The present finding suggests that the use of gene expression profile could be a more sensitive and accurate way to determine the CPF's NOAEL.

Cholinergic Basal Forebrain Lesion Decreases Neurotrophin Signaling without Affecting Tau Hyperphosphorylation in Genetically Susceptible Mice.

Alzheimer's disease (AD) is aprogressive, irreversible neurodegenerative disease that destroys memory and cognitive function. Aggregates of hyperphosphorylated tau protein are aprominent feature in the brain of patients with AD, and are amajor contributor to neuronal toxicity and disease progression. However, the factors that initiate the toxic cascade that results in tau hyperphosphorylation in sporadic AD are unknown. Here we investigated whether degeneration of basal forebrain cholinergic neurons (BFCNs) and/or aresultant decrease in neurotrophin signaling cause aberrant tau hyperphosphorylation. Our results reveal that the loss of BFCNs in pre-symptomatic pR5 (P301L) tau transgenic mice results in adecrease in hippocampal brain-derived neurotrophic factor levels and reduced TrkB receptor activation. However, there was no exacerbation of the levels of phosphorylated tau or its aggregation in the hippocampus of susceptible mice. Furthermore the animals' performance in ahippocampal-dependent learning and memory task was unaltered, and no changes in hippocampal synaptic markers were observed. This suggests that tau pathology is likely to be regulated independently of BFCN degeneration and the corresponding decrease in hippocampal neurotrophin levels, although these features may still contribute to disease etiology.

Toxicogenomic profile of apoptotic and necrotic SN56 basal forebrain cholinergic neuronal loss after acute and long-term chlorpyrifos exposure

Chlorpyrifos (CPF) is an organophosphate insecticide reported to induce, both after acute and repeated exposure, learning and memory dysfunctions, although the mechanism is not completely known. CPF produces basal forebrain cholinergic neuronal loss, involved on learning and memory regulation, which could be the cause of such cognitive disorders. This effect was reported to be induced through apoptotic process, partially mediated by AChE overexpression, although neuronal necrosis was also described after CPF exposure. Accordingly, we hypothesized that CPF induces apoptotic and necrotic basal forebrain cholinergic cell death. We evaluated, in septal SN56 basal forebrain cholinergic neurons, the CPF effect after 24h and 14days exposure on apoptosis and necrosis induction and the apoptotic and necrotic gene expression pathways. This study shows that CPF induces, after acute and long-term exposure, apoptosis and necrosis, partially mediated through AChE overexpression. Evaluation of cell death pathways supports the necrosis and apoptosis data and revealed that some genes are altered at lower concentrations than those at which the effects observed are produce and below the No Observed Adverse Effect Level (NOAEL). The present finding suggests that the use of gene expression profile could be a more sensitive and accurate way to determine the CPF's NOAEL.

SN56 basal forebrain cholinergic neuronal loss after acute and long-term chlorpyrifos exposure through oxidative stress generation; P75(NTR) and α7-nAChRs alterations mediated partially by AChE variants disruption.

Chlorpyrifos (CPF) is an organophosphates insecticide reported to induce, both after acute and repeated exposure, cognitive disorders and basal forebrain cholinergic neuronal loss, involved on learning and memory regulation, which could be the cause of such cognitive disorders. This neuronal loss was mediated partially by AChE variants alteration, suggesting other mechanisms are involved. In this regard, CPF induces oxidative stress that is implicated in the induction of cognitive deficits, changes in AChE variants expression and neuronal loss. Otherwise, it has been shown that P75(NTR) and the α7-nAChRs expression is altered in basal forebrain of rats after CPF long-term exposure; this alteration has been related with oxidative stress induction, cholinergic cell loss, and disruption of learning and memory processes. According to these data, we hypothesized that CPF induces basal forebrain cholinergic neuronal loss through induction of oxidative stress produced by P75(NTR) and α7-nAChRs altered expression, which could mediate this action in part through AChE variants disruption. We evaluated this hypothesis in septal SN56 basal forebrain cholinergic neurons, after 24h and 14days CPF exposure in vitro. This study shows that CPF upregulated P75(NTR) and downregulated α7-nAChRs expression, which increased H2O2 and malondialdehyde content and reduced cell viability partially through AChE variants induction. Alpha7-nAChRs repression induced oxidative stress and cell death partially through this mechanism, but P75(NTR) overexpression did not produce these effects, although it increased oxidative stress and cell death after CPF treatment, showing that its overexpression increases cell vulnerability. Our present results provide new understanding of the mechanisms contributing to the harmful effects of CPF.

Acute and long-term exposure to chlorpyrifos induces cell death of basal forebrain cholinergic neurons through AChE variants alteration.

Chlorpyrifos (CPF) is one of the most widely used organophosphates insecticides that has been reported to induce cognitive disorders both after acute and repeated administration similar to those induced in Alzheimer's disease (AD). However, the mechanisms through which it induces these effects are unknown. On the other hand, the cholinergic system, mainly basal forebrain cholinergic neurons, is involved in learning and memory regulation, and an alteration of cholinergic transmission or/and cholinergic cell loss could induce these effects. In this regard, it has been reported that CPF can affect cholinergic transmission, and alter AChE variants, which have been shown to be related with basal forebrain cholinergic neuronal loss. According to these data, we hypothesized that CPF could induce basal forebrain cholinergic neuronal loss through cholinergic transmission and AChE variants alteration. To prove this hypothesis, we evaluated in septal SN56 basal forebrain cholinergic neurons, the CPF toxic effects after 24h and 14 days exposure on neuronal viability and the cholinergic mechanisms related to it. This study shows that CPF impaired cholinergic transmission, induced AChE inhibition and, only after long-term exposure, increased CHT expression, which suggests that acetylcholine levels alteration could be mediated by these actions. Moreover, CPF induces, after acute and long-term exposure, cell death in cholinergic neurons in the basal forebrain and this effect is independent of AChE inhibition and acetylcholine alteration, but was mediated partially by AChE variants alteration. Our present results provide a new understanding of the mechanisms contributing to the harmful effects of CPF on neuronal function and viability, and the possible relevance of CPF in the pathogenesis of neurodegenerative diseases.

Interactions between Aβ oligomers and presynaptic cholinergic signaling: Age-dependent effects on attentional capacities

Substantial evidence suggests that cerebral deposition of the neurotoxic fibrillar form of amyloid precursor protein, β-amyloid (Aβ), plays a critical role in the pathogenesis of Alzheimer's disease (AD). Yet, many aspects of AD pathology including the cognitive symptoms and selective vulnerability of cortically projecting basal forebrain (BF) cholinergic neurons are not well explained by this hypothesis. Specifically, it is not clear why cognitive decline appears early when the loss of BF cholinergic neurons and plaque deposition are manifested late in AD. Soluble oligomeric forms of Aβ are proposed to appear early in the pathology and to be better predictors of synaptic loss and cognitive deficits. The present study was designed to examine the impact of Aβ oligomers on attentional functions and presynaptic cholinergic transmission in young and aged rats. Chronic intracranial infusions of Aβ oligomers produced subtle decrements in the ability of rats to sustain attentional performance with time on task, irrespective of the age of the animals. However, Aβ oligomers produced robust detrimental effects on performance under conditions of enhanced attentional load in aged animals. In vivo electrochemical recordings show reduced depolarization-evoked cholinergic signals in Aβ-infused aged rats. Moreover, soluble Aβ disrupted the capacity of cholinergic synapses to clear exogenous choline from the extracellular space in both young and aged rats, reflecting impairments in the choline transport process that is critical for acetylcholine (ACh) synthesis and release. Although aging per se reduced the cross-sectional area of BF cholinergic neurons and presynaptic cholinergic proteins in the cortex, attentional performance and ACh release remained unaffected in aged rats infused with the control peptide. Taken together, these data suggest that soluble Aβ may marginally influence attentional functions at young ages primarily by interfering with the choline uptake processes. However, age-related weakening of the cholinergic system may synergistically interact with these disruptive presynaptic mechanisms to make this neurotransmitter system vulnerable to the toxic effects of oligomeric Aβ in robustly impeding attentional capacities.

Mitochondrial Dysfunction and Alzheimer’s Disease

Alzheimer’s disease (AD) is a neurodegenerative disorder that is characterized by progressive loss of basal forebrain cholinergic neurons, leading to reduction in transmission through cholinergic fibers involved in processes of attention, learning, and memory. Mitochondria provide and regulate cellular energy and are crucial for proper neuronal activity and survival. Mitochondrial dysfunction is evident in early stages of AD and is involved in AD pathogenesis. This review focuses on the evidence supporting a clear association between amyloid-β toxicity, mitochondrial dysfunction, oxidative stress and neuronal damage/death in Alzheimer’s disease. To date, the beta amyloid (Aβ) cascade hypothesis still remains the main pathogenetic model of Alzheimer’s disease (AD), but its role in the majority of sporadic AD cases is uncertain. Furthermore, the “mitochondrial cascade hypothesis” could explain many of the biochemical, genetic, and pathological features of sporadic AD. This hypothesis promotes mutations in mitochondrial DNA (mtDNA) as the basis for Alzheimer’s disease. The mutations could lead to energy failure, increased oxidative stress, and accumulation of Aβ, which in a vicious cycle reinforces the mtDNA damage and oxidative stress.

Effects of complete immunotoxin lesions of the cholinergic basal forebrain on fear conditioning and spatial learning.

Administration of muscarinic cholinergic antagonists such as scopolamine impairs the acquisition of contextual fear conditioning, but the role of the basal forebrain (BF) cholinergic system in consolidation is unclear. To test the hypothesis that BF cholinergic neurons are critical for acquisition and consolidation of fear conditioning, male Sprague-Dawley rats with 192 IgG-saporin lesions of the entire cholinergic BF made either before or after fear conditioning were tested for conditioned fear to context and tone by assessing freezing and 22 kHz ultrasonic vocalization (USV) responses. Spatial learning in a 1-day water maze task provided a comparison for effects of the BF lesions on fear conditioning. In the test phase, neither pre-training nor posttraining BF lesions affected freezing to the context or tone. During both training and testing, pre-lesioned rats were impaired in production of USVs associated with fear. Postlesioned rats emitted fewer USVs only during testing. Acquisition of a spatial water maze task was mildly impaired in lesioned rats, although probe trial and cued performance was unimpaired. Nevertheless, these data suggest that conditioned fear-induced USVs are more sensitive to the loss of BF cholinergic neurons than is conditioned fear-induced freezing. The failure of BF cholinergic lesions to impair contextual fear conditioning indicates that scopolamine-induced impairments in fear conditioning may not be mediated by affecting cholinergic input to the hippocampus and neocortex.