Effects of lead exposure on the expression of amyloid β and phosphorylated tau proteins in the C57BL/6 mouse hippocampus at different life stages

Alzheimer's Disease: Pathogenesis and Therapeutic Interventions

BACKGROUND

Alzheimer's Disease (AD) is the most common cause of dementia. Genetics, excessive exposure to environmental pollutants, as well as unhealthy lifestyle practices are often linked to the development of AD. No therapeutic approach has achieved complete success in treating AD; however, early detection and management with appropriate drugs are key to improving prognosis.

INTERVENTIONS

The pathogenesis of AD was extensively discussed in order to understand the reasons for the interventions suggested. The interventions reviewed include the use of different therapeutic agents and approaches, gene therapy, adherence to healthy dietary plans (Mediterranean diet, Okinawan diet and MIND diet), as well as the use of medicinal plants. The potential of nanotechnology as a multidisciplinary and interdisciplinary approach in the design of nano-formulations of AD drugs and the use of Superparamagnetic Iron Oxide Nanoparticles (SPIONs) as theranostic tools for early detection of Alzheimer's disease were also discussed.

Time-course miRNA alterations and SIRT1 inhibition triggered by adolescent lead exposure in mice

Sirtuin 1 (SIRT1), the NAD-dependent histone deacetylase, has been extensively investigated due to its cognitive protective effect. Studies suggest microRNAs (miRNAs) and histone modifications are key epigenetic regulators of gene expression and play important role in brain development. We previously showed that cognitive impairment by lead (Pb) was associated with downregulation of SIRT1, but the epigenetic role of this is unclear. Thus, we exposed 4-week-old male mice to 0.2% lead acetate solution for three months, and subsequently extracted brain homogenate from mice cortex and hippocampus at the age of 1, 4, and 16 months, respectively. In this study, we found that the protein level of SIRT1 was inhibited in the hippocampus and cortex of 16-month-old aged mice exposed to Pb. Moreover, changes in the levels of miR-138-5p and miR-141-3p, which were considered to the mechanistic target of SIRT1 by bioinformatic analysis, were negative correlations SIRT1 protein expression. We also found miR-34c-3p expression was increased in the cortex of mice at the age of 16 months. Collectively, our results showed the expression of neural SIRT1 and three selected microRNAs at different age nodes of mice for the first time of following Pb exposure. Our results suggest that additional efforts should focus on the consequences of early Pb exposure from an epigenetic perspective.

[The role of aluminum and lead in the development of Alzheimer's and Parkinson's diseases]

The article summarizes the data available in the literature on the toxic effects of aluminum and lead on the human brain and assesses the relationship of these effects to the etiopathogenesis of the most common neurodegenerative diseases, such as Alzheimer's and Parkinson's diseases. The accumulation of ions of these metals in the brain structures leads to chronic intoxication that is manifested by the morphological signs that are typical for Alzheimer's disease, such as deposits of β-amyloid and τ-protein mainly in the frontal and temporal regions of the cortex, and for Parkinson's disease, such as degeneration of dopamine neurons in the substantia nigra and their accumulation of α-synuclein. The most likely forms of participation of aluminum and lead ions in the mechanisms of neurodegeneration are the replacement of bivalent metal ions necessary for brain functioning, oxidative stress initiation, epigenetic modifications of histones, and increased expression of noncoding ribonucleic acids.

Hydrogen sulfide protects hippocampal CA1 neurons against lead mediated neuronal damage via reduction oxidative stress in male rats

H2S plays vital roles in modulation brain function. It is associated with antioxidant and anti-inflammatory properties. We assessed the H2S impact on spatial learning and memory deficit and cell death due to lead exposure, and probable mechanisms of action. The 36 male Wistar rats that (200-220 g), were in random assigned to 3 groups, control group (12 rats), lead acetate group (12 rats), and lead acetate +H2S groups (NaHS as a H2S donor; 5/6 mg/kg; 12 rats). Administration of lead to rats was performed through acute lead poisoning (25 mg/kg of lead acetate, IP through 3 days). Using male Morris water maze, their spatial learning and memory function were measured. We carried out ELISA method to calculate TNF-α and antioxidant enzymes level. Immunohistochemical staining was applied for evaluating the caspase-3 expression levels. Treatment with H2S improved learning and memory impairment in Pb-exposed rats (P<0.05). H2S treatment suppressed Pb-related apoptosis in the hippocampal CA1 subfield (P<0.01). Also, the TNF-α over-expression in the CA1 region of hippocampus due to lead exposure showed a significant reduction (P<0.05) after administrating H2S. Simultaneously, H2S treatment reduced the MDA levels, enhanced SOD, GSH level than the Pb-exposed group in hippocampus (P<0.05). H2S was able to significantly improve Pb-related spatial learning and memory deficit, and neuronal cell death in the CA1 region of hippocampus in the male rats at least partly by reducing oxidative stress and TNF.

Carvacrol reduces hippocampal cell death and improves learning and memory deficits following lead-induced neurotoxicity via antioxidant activity

Carvacrol is a monoterpene with neuroprotective effects in several animal models of neurodegeneration, including epilepsy, ischemia, and traumatic neuronal events. In this study, we aimed to examine the effects of carvacrol on neurodegeneration induced by lead acetate in rats. A total of 50 male Wistar rats were divided into five equal groups. The control group received drinking water, while the neurotoxic group was exposed to 500 ppm of lead acetate in drinking water for 40 days. The three remaining groups, which were also exposed to 500 ppm of lead acetate, received carvacrol at doses of 25, 50, and 100 mg/kg orally for 40 days. The Morris water maze test was employed to examine spatial learning and memory. Pathological damage to the hippocampus was determined by Nissl staining. The level of malondialdehyde (MDA), superoxide dismutase (SOD), and catalase (CAT) were detected using biochemical analysis and the free radical scavenging activity as evaluated by the DPPH test. Administration of carvacrol significantly restored learning and memory impairment induced by lead acetate. Moreover, carvacrol ameliorated neurodegeneration, antioxidative capacity, and lipid peroxidation in the hippocampus of rats exposed to lead. The present results provide a rationale for the inhibitory role of carvacrol in the attenuation of lead-induced neurotoxicity.

Disruption of synaptic expression pattern and age-related DNA oxidation in a neuronal model of lead-induced toxicity

Lead (Pb) is recognized as a potent inducer of synaptic toxicity generally associated with reduced synaptic transmission and increased neuronal fiber excitability, becoming an environmental risk for neurodegenerative processes. Despite numerous toxicological studies on Pb have been directed to the developing brain, attention concerning long-term consequences of pubertal chronic Pb exposure on neuronal activity is still lacking. Thus, we exposed 4-week-old male mice to 0.2 % lead acetate solution for one month, then, conducted behavioral tests or extracted brain homogenate from mice prefrontal cortex (PFC) and hippocampus at the age of 4, 13 and 16-month-old respectively. Our results showed that treated mice exhibited an evident increase in latency to reach platform following pubertal Pb exposure and aging. The increase of 8-OHdG revealed evident neural DNA oxidative damage across time upon pubertal Pb exposure. In the hippocampus of lead exposed mice at three age nodes, the expression of brain-derived neurotrophic factor precursor (proBDNF) increased, while that of mature BDNF (mBDNF), cAMP-response element binding protein (CREB) and phosphorylated CREB (pCREB) decreased compared with the control group. Furthermore, the expression of BACE1 protein and tau phosphorylation level in PFC and hippocampus increased, APP mRNAs in PFC and prolonged induction of BACE1 in hippocampus. Our results show that chronic Pb exposure from pubertal stage onward can either initiate divergent synaptic-related gene expression patterns in adulthood or trigger time-course of neurodegenerative profile within the PFC or hippocampus, which can contribute consistent deficits of cognition across subsequent age-nodes.

Epigenetic and Neurological Impairments Associated with Early Life Exposure to Persistent Organic Pollutants

The incidence of neurodevelopmental and neurodegenerative diseases worldwide has dramatically increased over the last decades. Although the aetiology remains uncertain, evidence is now growing that exposure to persistent organic pollutants during sensitive neurodevelopmental periods such as early life may be a strong risk factor, predisposing the individual to disease development later in life. Epidemiological studies have associated environmentally persistent organic pollutant exposure to brain disorders including neuropathies, cognitive, motor, and sensory impairments; neurodevelopmental disorders such as autism spectrum disorder (ASD) and attention-deficit hyperactivity disorder (ADHD); and neurodegenerative diseases including Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis (ALS). In many ways, this expands the classical “Developmental Origins of Health and Disease” paradigm to include exposure to pollutants. This model has been refined over the years to give the current “three-hit” model that considers the individual's genetic factors as a first “hit.” It has an immediate interaction with the early-life exposome (including persistent organic pollutants) that can be considered to be a second “hit.” Together, these first two “hits” produce a quiescent or latent phenotype, most probably encoded in the epigenome, which has become susceptible to a third environmental “hit” in later life. It is only after the third “hit” that the increased risk of disease symptoms is crystallised. However, if the individual is exposed to a different environment in later life, they would be expected to remain healthy. In this review, we examine the effect of exposure to persistent organic pollutants and particulate matters in early life and the relationship to subsequent neurodevelopmental and neurodegenerative disorders. The roles of those environmental factors which may affect epigenetic DNA methylation and therefore influence normal neurodevelopment are then evaluated.

Metal Ion Effects on Aβ and Tau Aggregation

Amyloid and tau aggregation are implicated in manifold neurodegenerative diseases and serve as two signature pathological hallmarks in Alzheimer's disease (AD). Though aging is considered as a prominent risk factor for AD pathogenesis, substantial evidence suggests that an imbalance of essential biometal ions in the body and exposure to certain metal ions in the environment can potentially induce alterations to AD pathology. Despite their physiological importance in various intracellular processes, biometal ions, when present in excessive or deficient amounts, can serve as a mediating factor for neurotoxicity. Recent studies have also demonstrated the contribution of metal ions found in the environment on mediating AD pathogenesis. In this regard, the neuropathological features associated with biometal ion dyshomeostasis and environmental metal ion exposure have prompted widespread interest by multiple research groups. In this review, we discuss and elaborate on findings from previous studies detailing the possible role of both endogenous and exogenous metal ions specifically on amyloid and tau pathology in AD.

Developmental lead (Pb)-induced deficits in hippocampal protein translation at the synapses are ameliorated by ascorbate supplementation

BACKGROUND

Lead (Pb) is a persistent environmental neurotoxin and its exposure even in minute quantities has been known to induce neuronal defects. The immature brain is singularly sensitive to Pb neurotoxicity, and its exposure during development has permanent detrimental effects on the brain developmental trajectory and neuronal signaling and plasticity, culminating into compromises in the cognitive and behavioral attributes which persists even later in adulthood. Several molecular pathways have been implicated in the Pb-mediated disruption of neuronal signaling, including elevated oxidative stress, alterations in neurotransmitter biology, and mitochondrial dysfunction. Nevertheless, the neuronal targets and biochemical pathways underlying these Pb-mediated alterations in synaptic development and function have not been completely deduced. In this respect, recent studies have shown that synaptic signaling and its maintenance and plasticity are critically dependent on localized de novo protein translation at the synaptic terminals.

MATERIALS AND METHODS

The present study hence aimed to assess the alterations in the synapse-specific translation induced by developmental Pb exposure. To this end, in vitro protein translation rate was analyzed in the hippocampal synaptoneurosomal fractions of rat pups pre- and postnatally exposed to Pb using a puromycin incorporation assay. Moreover, we evaluated the therapeutic effects of ascorbic acid supplementation against Pb-induced deficits in synapse-localized protein translation.

RESULTS

We observed a significant loss in the rates of de novo protein translation in synaptoneurosomes of Pb-exposed pups compared to age-matched control pups. Interestingly, ascorbate supplementation lead to an appreciable recovery in Pb-induced translational deficits. Moreover, the deficit in activity-dependent synaptic protein translation was found to correlate significantly with the increase in the blood Pb levels.

CONCLUSION

Dysregulation of synapse-localized de novo protein translation is a potentially critical determinant of Pb-induced synaptic dysfunction and the consequent deficits in behavioral, social, and psychological attributes of the organisms. In addition, our study establishes ascorbate supplementation as a key ameliorative agent against Pb-induced neurotoxicity.

Effects of sulforaphane and vitamin E on cognitive disorder and oxidative damage in lead-exposed mice hippocampus at lactation

OBJECT

To investigate the effects of sulforaphane (SFN) and vitamin E (VE) on spatial learning and memory ability and oxidative damage of hippocampus in lead-exposed mice at lactation.

METHODS

A total of 18 adult Kunming mice, all 12 female mice were divided into two groups by body weight randomly, 10 mice drank water containing 0.2% lead acetate at lactation, the other 2 mice drank lead free deionized water named as the normal group. Then, they were mated at a 1:2 ratio of male to female. After weaning, the pups were divided into 5 groups by weight randomly (10 each group): normal saline (NS) group, corn oil (CO) group, SFN group, VE group and SFN+VE group. They were subject to gavage daily for four weeks. Gavage doses of SFN and VE were 25mg/kg and 30 IU/kg respectively. Meanwhile, 10 pups of the normal group were selected randomly as the control (C) group. The C group was normally raised for 4 weeks. The spatial learning and memory ability of them were evaluated by the Morris water maze test, and the lead level in the blood was determined by polarography. Superoxide dismutase (SOD) activity and malondialdehyde (MDA) level in hippocampus were measured by the kits.

RESULTS

Compared with the NS and CO groups, the lead level in the blood of SFN and SFN+VE group had a significant decrease. In water maze test, the mice treated with SFN or/and VE performed better than mice of the NS and CO groups. In addition, a remarkable decrease in MDA level was found in mice treated with SFN or/and VE than those in NS and CO groups. What's more, there was no statistical distinction of SOD activity in SFN group than that of NS group. SOD activity significantly increased was observed in VE and SFN+VE groups than that of CO group.

CONCLUSION

Sulforaphane and vitamin E could ameliorate cognitive decline and oxidative damage in pups with lead exposure at lactation from maternal milk.

Early-Life Exposure to Lead Induces Cognitive Impairment in Elder Mice Targeting SIRT1 Phosphorylation and Oxidative Alterations

Pb is a potential risk factor for cognition, mainly mediated by enhanced oxidative stress. Resveratrol, a natural polyphenol with crucial anti-oxidative property, is recently implicated in preventing cognitive deficits in normal aging and neurodegenerative disorders. Its beneficial effects have been linked to sirtuin 1(SIRT1) activation. The aim of this work is to investigate the possible linkage between alterations in Pb-induced oxidative damage and cognitive impairment by prolonged treatment of resveratrol. Male C57BL/6 mice were given Pb(Ac)₂ treatment or deionized H₂O for 12 weeks, and subjected to resveratrol gavage at the dose of 50 mg/kgBw•d or vehicle after Pb exposure. Results from biochemical analysis and immunohistofluorescence showed that Pb induced oxidative DNA damage and decreased cortical antioxidant biomarker. As expected, these abnormalities were improved by resveratrol treatment. Morris water maze test, Western blotting, immunohistofluorescence staining and RT-qPCR indicated that resveratrol ameliorated spatial learning and memory deficits with alterations in hippocampal BDNF-TrkB signaling, promoted nuclear localization and phosphorylation of hippocampal SIRT1, partly increased protein levels of AMPK and PGC-1α involving in modulation of antioxidant response in Pb-exposed mice. Our results support the hypothesis that resveratrol could attenuate Pb-induced cognitive impairment which was associated with activating SIRT1 via modulation of oxidative stress. Additionally, resveratrol also repressed the Pb-induce amyloidogenic processing with resultant decline in cortical Aβ₁₋₋₄₀. Noteworthy, such effects were not mediated by resveratrol treatment alone. These findings emphasize the potential of SIRT1 activator as an efficacious dietary intervention to downgrade the Pb-induced neurotoxic lesion.

Gene-environment interaction between lead and Apolipoprotein E4 causes cognitive behavior deficits in mice

Background

Alzheimer’s disease (AD) is characterized by progressive cognitive decline and memory loss. Environmental factors and gene-environment interactions (GXE) may increase AD risk, accelerate cognitive decline, and impair learning and memory. However, there is currently little direct evidence supporting this hypothesis.

Methods

In this study, we assessed for a GXE between lead and ApoE4 on cognitive behavior using transgenic knock-in (KI) mice that express the human Apolipoprotein E4 allele (ApoE4-KI) or Apolipoprotein E3 allele (ApoE3-KI). We exposed 8-week-old male and female ApoE3-KI and ApoE4-KI mice to 0.2% lead acetate via drinking water for 12 weeks and assessed for cognitive behavior deficits during and after the lead exposure. In addition, we exposed a second (cellular) cohort of animals to lead and assessed for changes in adult hippocampal neurogenesis as a potential underlying mechanism for lead-induced learning and memory deficits.

Results

In the behavior cohort, we found that lead reduced contextual fear memory in all animals; however, this decrease was greatest and statistically significant only in lead-treated ApoE4-KI females. Similarly, only lead-treated ApoE4-KI females exhibited a significant decrease in spontaneous alternation in the T-maze. Furthermore, all lead-treated animals developed persistent spatial working memory deficits in the novel object location test, and this deficit manifested earlier in ApoE4-KI mice, with female ApoE4-KI mice exhibiting the earliest deficit onset. In the cellular cohort, we observed that the maturation, differentiation, and dendritic development of adult-born neurons in the hippocampus was selectively impaired in lead-treated female ApoE4-KI mice.

Conclusions

These data suggest that GXE between ApoE4 and lead exposure may contribute to cognitive impairment and that impaired adult hippocampal neurogenesis may contribute to these deficits in cognitive behavior. Together, these data suggest a role for GXE and sex differences in AD risk.

Perinatal exposure to lead (Pb) promotes Tau phosphorylation in the rat brain in a GSK-3β and CDK5 dependent manner: Relevance to neurological disorders

Hyperphosphorylation of Tau is involved in the pathomechanism of neurological disorders such as Alzheimer's, Parkinson's diseases as well as Autism. Epidemiological data suggest the significance of early life exposure to lead (Pb) in etiology of disorders affecting brain function. However, the precise mechanisms by which Pb exerts neurotoxic effects are not fully elucidated. The purpose of this study was to evaluate the effect of perinatal exposure to low dose of Pb on the Tau pathology in the developing rat brain. Furthermore, the involvement of two major Tau-kinases: glycogen synthase kinase-3 beta (GSK-3β) and cyclin-dependent kinase 5 (CDK5) in Pb-induced Tau modification was evaluated. Pregnant female rats were divided into control and Pb-treated group. The control animals were maintained on drinking water while females from the Pb-treated group received 0.1% lead acetate (PbAc) in drinking water, starting from the first day of gestation until weaning of the offspring. During the feeding of pups, mothers from the Pb-treated group were still receiving PbAc. Pups of both groups were weaned at postnatal day 21 and then until postnatal day 28 received only drinking water. 28-day old pups were sacrificed and Tau mRNA and protein level as well as Tau phosphorylation were analyzed in forebrain cortex (FC), cerebellum (C) and hippocampus (H). Concomitantly, we examined the effect of Pb exposure on GSK-3β and CDK5 activation. Our data revealed that pre- and neonatal exposure to Pb (concentration of Pb in whole blood below 10μg/dL, considered safe for humans) caused significant increase in the phosphorylation of Tau at Ser396 and Ser199/202 with parallel rise in the level of total Tau protein in FC and C. Tau hyperphosphorylation in Pb-treated animals was accompanied by elevated activity of GSK-3β and CDK5. Western blot analysis revealed activation of GSK-3β in FC and C as well as CDK5 in C, via increased phosphorylation of Tyr-216 and calpain-dependent p25 formation, respectively. In conclusion, perinatal exposure to Pb up-regulates Tau protein level and induces Tau hyperphosphorylation in the rat brain cortex and cerebellum. We suggest that neurotoxic effect of Pb might be mediated, at least in part, by GSK-3β and CDK5-dependent Tau hyperphosphorylation, which may lead to the impairment of cytoskeleton stability and neuronal dysfunction.

Lactational exposure of mice to low levels of non-dioxin-like polychlorinated biphenyls increases susceptibility to neuronal stress at a mature age

Lactational exposure to low levels of the sum of the six indicator polychlorinated biphenyls (Σ6 NDL-PCBs, 10ng/kg/day) is known to lead to persistent anxious behavior in young and adult offspring mice at postnatal days 40 and 160, respectively. At more advanced life stages, we evaluated the effects on the mouse brain of neuronal stress induced by the synaptotoxic amyloid-beta (Aβ) peptide. Perinatal exposure of lactating mice to Σ6 NDL-PCBs did not affect short-term memory performances of their offspring male mice aged 14 months as compared to control PCB-naive mice. However, following intracerebroventricular injection of soluble Aβ oligomers, significant impairments in long-term memory were detected in the mice that had been lactationally treated with Σ6 NDL-PCBs. In addition, immunoblot analyses of the synaptosomal fraction of hippocampal tissues from treated mice revealed a lower expression of the synaptic proteins synaptophysin and PSD-95. Though preliminary, our findings suggest for the first time that early exposure to low levels of NDL-PCBs induce late neuronal vulnerability to amyloid stress. Additional experiments are needed to confirm whether early environmental influences are involved in the etiology of brain aging and cognitive decline.

Environmental pollutants as risk factors for neurodegenerative disorders: Alzheimer and Parkinson diseases

Neurodegenerative diseases including Alzheimer (AD) and Parkinson (PD) have attracted attention in last decades due to their high incidence worldwide. The etiology of these diseases is still unclear; however the role of the environment as a putative risk factor has gained importance. More worryingly is the evidence that pre- and post-natal exposures to environmental factors predispose to the onset of neurodegenerative diseases in later life. Neurotoxic metals such as lead, mercury, aluminum, cadmium and arsenic, as well as some pesticides and metal-based nanoparticles have been involved in AD due to their ability to increase beta-amyloid (Aβ) peptide and the phosphorylation of Tau protein (P-Tau), causing senile/amyloid plaques and neurofibrillary tangles (NFTs) characteristic of AD. The exposure to lead, manganese, solvents and some pesticides has been related to hallmarks of PD such as mitochondrial dysfunction, alterations in metal homeostasis and aggregation of proteins such as α-synuclein (α-syn), which is a key constituent of Lewy bodies (LB), a crucial factor in PD pathogenesis. Common mechanisms of environmental pollutants to increase Aβ, P-Tau, α-syn and neuronal death have been reported, including the oxidative stress mainly involved in the increase of Aβ and α-syn, and the reduced activity/protein levels of Aβ degrading enzyme (IDE)s such as neprilysin or insulin IDE. In addition, epigenetic mechanisms by maternal nutrient supplementation and exposure to heavy metals and pesticides have been proposed to lead phenotypic diversity and susceptibility to neurodegenerative diseases. This review discusses data from epidemiological and experimental studies about the role of environmental factors in the development of idiopathic AD and PD, and their mechanisms of action.

Mercury Reduces the Enzymatic Activity of Neprilysin in Differentiated SH-SY5Y Cells

Levels of amyloid beta (Aβ) in the central nervous system are regulated by the balance between its synthesis and degradation. Neprilysin (NEP) is associated with Alzheimer's disease (AD) by its ability to degrade Aβ. Some studies have involved the exposure to mercury (Hg) in AD pathogenesis; therefore, our aim was to investigate the effects on the anabolism and catabolism of Aβ in differentiated SH-SY5Y cells incubated with 1-20 μM of Hg. Exposure to 20 µM of Hg induced an increase in Aβ-42 secretion, but did not increase the expression of the amyloid precursor protein (APP). Hg incubation (10 and 20 µM) increased NEP protein levels; however, it did not change NEP mRNA levels nor the levels of the amyloid intracellular domain peptide, a protein fragment with transcriptional activity. Interestingly, Hg reduced NEP activity at 10 and 20 µM, and circular dichroism analysis using human recombinant NEP showed conformational changes after incubation with molar equivalents of Hg. This suggests that the Hg-induced inhibition of NEP activity may be mediated by a conformational change resulting in reduced Aβ-42 degradation. Finally, the comparative effects of lead (Pb, 50 μM) were evaluated. We found a significant increase in Aβ-42 levels and a dramatic increase in APP protein levels; however, no alteration in NEP levels was observed nor in the enzymatic activity of this metalloprotease, despite the fact that Pb slightly modified the rhNEP conformation. Overall, our data suggest that Hg and Pb increase Aβ levels by different mechanisms.