Heavy metals toxicity: effect of cadmium ions on amyloid beta protein 1-42. Possible implications for Alzheimer's disease

Reevaluating the role of amyloid β-peptides in Alzheimer's disease: from pathogenic agents to protective chelation mechanisms

Alzheimer's disease (AD) is a multifaceted neurodegenerative disorder with complex etiology, often associated with histological markers of oxidative stress, inflammation, and disturbances in calcium homeostasis. Traditionally, amyloid β-peptides (Aβ) have been considered key contributors to these pathological processes. However, emerging evidence suggests a protective role for Aβ and the enzymes involved in its production. This article further explores the hypothesis published by us a decade before that posits amyloid β-peptides and the β-secretase enzyme (BACE1) are part of an intentionally designed cellular defense mechanism against metal toxicity. This challenges the conventional understanding of their roles in AD pathogenesis. It is not until this BACE1 system, primarily the associated amyloid plaque deposit sites, are saturated with heavy and other metals and the exposure to these cations continues to influx oxidative ions into the brain, do the indications of neurodegeneration begin to become symptomatic. Until this metal oversaturation takes place, the system - Aβ and the enzymes involved in its production and conveyance - keeps the oxidative potential of the metal toxins sequestered extracellularly and out of the way of the neuron's intracellular activities.

Heavy metals exposure and Alzheimer's disease: Underlying mechanisms and advancing therapeutic approaches

Heavy metals such as lead, cadmium, mercury, and arsenic are prevalent in the environment due to both natural and anthropogenic sources, leading to significant public health concerns. These heavy metals are known to cause damage to the nervous system, potentially leading to a range of neurological conditions including Alzheimer's disease (AD), Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), multiple sclerosis (MS), and attention-deficit hyperactivity disorder (ADHD). The present study examines the complex relationship between heavy metal exposure and AD, focusing on the underlying mechanisms of toxicity and potential therapeutic approaches. This review article highlights how these metals can impair brain function through mechanisms such as oxidative stress, inflammation, and neurotransmitter disruption, ultimately contributing to neurodegenerative diseases like AD. It also addresses the challenges in diagnosing heavy metal-induced cognitive impairments and emphasizes the need for further research to explore effective treatment strategies and preventive measures against heavy metal exposure.

SIRT5-Mediated Desuccinylation of RAB7A Protects Against Cadmium-Induced Alzheimer's Disease-Like Pathology by Restoring Autophagic Flux

Cadmium (Cd) is a neurotoxic contaminant that induces cognitive decline similar to that observed in Alzheimer's disease (AD). Autophagic flux dysfunction is attributed to the pathogenesis of AD, and this study aimed to investigate the effect of autophagy on environmental Cd-induced AD progression and the underlying mechanism. Here, Cd exposure inhibited autophagosome-lysosome fusion and impaired lysosomal function, leading to defects in autophagic clearance and then to APP accumulation and nerve cell death. Proteomic analysis coupled with Ingenuity Pathway Analysis (IPA) identified SIRT5 as an essential molecular target in Cd-impaired autophagic flux. Mechanistically, Cd exposure hampered the expression of SIRT5, thus increasing the succinylation of RAB7A at lysine 31 and inhibiting RAB7A activity, which contributed to autophagic flux blockade. Importantly, SIRT5 overexpression led to the restoration of autophagic flux blockade, the alleviation of Aβ deposition and memory deficits, and the desuccinylation of RAB7A in Cd-exposed FAD4T mice. Additionally, SIRT5 levels decrease mainly in neurons but not in other cell clusters in the brains of AD patients according to single-nucleus RNA sequencing data from the public dataset GSE188545. This study reveals that SIRT5-catalysed RAB7A desuccinylation is an essential adaptive mechanism for the amelioration of Cd-induced autophagic flux blockade and AD-like pathogenesis.

Heavy metals toxicity on epigenetic modifications in the pathogenesis of Alzheimer's disease (AD)

Alzheimer's disease (AD) is a progressive decline in cognitive ability and behavior which eventually disrupts daily activities. AD has no cure and the progression rate varies unlikely. Among various causative factors, heavy metals are reported to be a significant hazard in AD pathogenesis. Metal-induced neurodegeneration has been focused globally with thorough research to unravel the mechanistic insights in AD. Recently, heavy metals suggested to play an important role in epigenetic alterations which might provide evidential results on AD pathology. Epigenetic modifications are known to play towards novel therapeutic approaches in treating AD. Though many studies focus on epigenetics and heavy metal implications in AD, there is a lack of research on heavy metal influence on epigenetic toxicity in neurological disorders. The current review aims to elucidate the plausible role of cadmium (Cd), iron (Fe), arsenic (As), copper (Cu), and lithium (Li) metals on epigenetic factors and the increase in amyloid beta and tau phosphorylation in AD. Also, the review discusses the common methods of heavy metal detection to implicate in AD pathogenesis. Hence, from this review, we can extend the need for future research on identifying the mechanistic behavior of heavy metals on epigenetic toxicity and to develop diagnostic and therapeutic markers in AD.

A Combination of Heavy Metals and Intracellular Pathway Modulators Induces Alzheimer Disease-like Pathologies in Organotypic Brain Slices

Alzheimer's disease (AD) is a progressive neurodegenerative disorder that is characterized by amyloid-beta (Aβ) plaques and tau neurofibrillary tangles (NFT). Modelling aspects of AD is challenging due to its complex multifactorial etiology and pathology. The present study aims to establish a cost-effective and rapid method to model the two primary pathologies in organotypic brain slices. Coronal hippocampal brain slices (150 µm) were generated from postnatal (day 8-10) C57BL6 wild-type mice and cultured for 9 weeks. Collagen hydrogels containing either an empty load or a mixture of human Aβ42 and P301S aggregated tau were applied to the slices. The media was further supplemented with various intracellular pathway modulators or heavy metals to augment the appearance of Aβ plaques and tau NFTs, as assessed by immunohistochemistry. Immunoreactivity for Aβ and tau was significantly increased in the ventral areas in slices with a mixture of human Aβ42 and P301S aggregated tau compared to slices with empty hydrogels. Aβ plaque- and tau NFT-like pathologies could be induced independently in slices. Heavy metals (aluminum, lead, cadmium) potently augmented Aβ plaque-like pathology, which developed intracellularly prior to cell death. Intracellular pathway modulators (scopolamine, wortmannin, MHY1485) significantly boosted tau NFT-like pathologies. A combination of nanomolar concentrations of scopolamine, wortmannin, MHY1485, lead, and cadmium in the media strongly increased Aβ plaque- and tau NFT-like immunoreactivity in ventral areas compared to the slices with non-supplemented media. The results highlight that we could harness the potential of the collagen hydrogel-based spreading of human Aβ42 and P301S aggregated tau, along with pharmacological manipulation, to produce pathologies relevant to AD. The results offer a novel ex vivo organotypic slice model to investigate AD pathologies with potential applications for screening drugs or therapies in the future.

Alzheimer's Disease: Models and Molecular Mechanisms Informing Disease and Treatments

Alzheimer's Disease (AD) is a complex neurodegenerative disease resulting in progressive loss of memory, language and motor abilities caused by cortical and hippocampal degeneration. This review captures the landscape of understanding of AD pathology, diagnostics, and current therapies. Two major mechanisms direct AD pathology: (1) accumulation of amyloid β (Aβ) plaque and (2) tau-derived neurofibrillary tangles (NFT). The most common variants in the Aβ pathway in APP, PSEN1, and PSEN2 are largely responsible for early-onset AD (EOAD), while MAPT, APOE, TREM2 and ABCA7 have a modifying effect on late-onset AD (LOAD). More recent studies implicate chaperone proteins and Aβ degrading proteins in AD. Several tests, such as cognitive function, brain imaging, and cerebral spinal fluid (CSF) and blood tests, are used for AD diagnosis. Additionally, several biomarkers seem to have a unique AD specific combination of expression and could potentially be used in improved, less invasive diagnostics. In addition to genetic perturbations, environmental influences, such as altered gut microbiome signatures, affect AD. Effective AD treatments have been challenging to develop. Currently, there are several FDA approved drugs (cholinesterase inhibitors, Aß-targeting antibodies and an NMDA antagonist) that could mitigate AD rate of decline and symptoms of distress.

Alzheimer's disease: the role of extrinsic factors in its development, an investigation of the environmental enigma

In the realm of Alzheimer's disease, the most prevalent form of dementia, the impact of environmental factors has ignited intense curiosity due to its substantial burden on global health. Recent investigations have unveiled these environmental factors as key contributors, shedding new light on their profound influence. Notably, emerging evidence highlights the detrimental role of various environmental contaminants in the incidence and progression of Alzheimer's disease. These contaminants encompass a broad spectrum, including air pollutants laden with ozone, neurotoxic metals like lead, aluminum, manganese, and cadmium, pesticides with their insidious effects, and the ubiquitous presence of plastics and microplastics. By meticulously delving into the intricate web connecting environmental pollutants and this devastating neurological disorder, this comprehensive chapter takes a deep dive into their involvement as significant risk factors for Alzheimer's disease. Furthermore, it explores the underlying molecular mechanisms through which these contaminants exert their influence, aiming to unravel the complex interactions that drive the pathogenesis of the disease. Additionally, this chapter proposes potential strategies to mitigate the detrimental effects of these environmental contaminants on brain health, with the ultimate goal of restoring and preserving typical cognitive function. Through this comprehensive exploration, we aim to enhance our understanding of the multifaceted relationship between neurotoxins and Alzheimer's disease, providing a solid foundation for developing innovative in-vivo models and advancing our knowledge of the intricate pathological processes underlying this debilitating condition.

Common and Trace Metals in Alzheimer's and Parkinson's Diseases

Trace elements and metals play critical roles in the normal functioning of the central nervous system (CNS), and their dysregulation has been implicated in neurodegenerative disorders such as Alzheimer's disease (AD) and Parkinson's disease (PD). In a healthy CNS, zinc, copper, iron, and manganese play vital roles as enzyme cofactors, supporting neurotransmission, cellular metabolism, and antioxidant defense. Imbalances in these trace elements can lead to oxidative stress, protein aggregation, and mitochondrial dysfunction, thereby contributing to neurodegeneration. In AD, copper and zinc imbalances are associated with amyloid-beta and tau pathology, impacting cognitive function. PD involves the disruption of iron and manganese levels, leading to oxidative damage and neuronal loss. Toxic metals, like lead and cadmium, impair synaptic transmission and exacerbate neuroinflammation, impacting CNS health. The role of aluminum in AD neurofibrillary tangle formation has also been noted. Understanding the roles of these elements in CNS health and disease might offer potential therapeutic targets for neurodegenerative disorders. The Codex Alimentarius standards concerning the mentioned metals in foods may be one of the key legal contributions to safeguarding public health. Further research is needed to fully comprehend these complex mechanisms and develop effective interventions.

Delineating the protective action of cordycepin against cadmium induced oxidative stress and gut inflammation through downregulation of NF-κB pathway

Cadmium (Cd) exposure is known to cause gut inflammation. In this study, we investigated the protective effects of cordycepin, a natural compound with pharmacological properties, against gut inflammation induced by Cd exposure. Using zebrafish larvae and colon cell line models, we examined the impact of cordycepin on Cd-induced toxicity and inflammation. Zebrafish larvae were exposed to Cd (2 µg/mL) and treated with different concentrations of cordycepin (12.5, 25 and 50 µg/mL). Cordycepin treatment significantly reduced Cd-induced embryotoxicity in zebrafish larvae. It also alleviated Cd-induced oxidative stress by reducing reactive oxygen species (ROS), lipid peroxidation and apoptosis. Furthermore, cordycepin treatment normalized the levels of liver-related biomarkers affected due to Cd exposure. Additionally, cordycepin (50 µg/mL) demonstrated a significant reduction in Cd bioaccumulation and downregulated the expression of inflammatory genes in both zebrafish larval gut and colon cell lines. These findings suggest that cordycepin could be an effective agent against Cd-induced gut inflammation.

Characterization of Uranyl (UO22+) Ion Binding to Amyloid Beta (Aβ) Peptides: Effects on Aβ Structure and Aggregation

Uranium (U) is naturally present in ambient air, water, and soil, and depleted uranium (DU) is released into the environment via industrial and military activities. While the radiological damage from U is rather well understood, less is known about the chemical damage mechanisms, which dominate in DU. Heavy metal exposure is associated with numerous health conditions, including Alzheimer's disease (AD), the most prevalent age-related cause of dementia. The pathological hallmark of AD is the deposition of amyloid plaques, consisting mainly of amyloid-β (Aβ) peptides aggregated into amyloid fibrils in the brain. However, the toxic species in AD are likely oligomeric Aβ aggregates. Exposure to heavy metals such as Cd, Hg, Mn, and Pb is known to increase Aβ production, and these metals bind to Aβ peptides and modulate their aggregation. The possible effects of U in AD pathology have been sparsely studied. Here, we use biophysical techniques to study in vitro interactions between Aβ peptides and uranyl ions, UO22+, of DU. We show for the first time that uranyl ions bind to Aβ peptides with affinities in the micromolar range, induce structural changes in Aβ monomers and oligomers, and inhibit Aβ fibrillization. This suggests a possible link between AD and U exposure, which could be further explored by cell, animal, and epidemiological studies. General toxic mechanisms of uranyl ions could be modulation of protein folding, misfolding, and aggregation.

A Bio-Indicator Pilot Study Screening Selected Heavy Metals in Female Hair, Nails, and Serum from Lifestyle Cosmetic, Canned Food, and Manufactured Drink Choices

Lifestyles, genetic predispositions, environmental factors, and geographical regions are considered key factors of heavy metals initiatives related to health issues. Heavy metals enter the body via the environment, daily lifestyle, foods, beverages, cosmetics, and other products. The accumulation of heavy metals in the human body leads to neurological issues, carcinogenesis, failure of multiple organs in the body, and a reduction in sensitivity to treatment. We screened for Cr, Al, Pb, and Cd in selected foods, beverages, and cosmetics products depending on questionnaire outcomes from female volunteers. We also screened for Cr, Al, Pb, and Cd on hair, nails, and serum samples using inductively coupled plasma mass spectrometry (ICP-MS) from the same volunteers, and we analyzed the serum cholinesterase and complete blood picture (CBC). We performed an AutoDock study on Cr, Al, Pb, and Cd as potential ligands. Our results indicate that the most elevated heavy metal in the cosmetic sample was Al. In addition, in the food and beverages samples, it was Pb and Al, respectively. The results of the questionnaire showed that 71 percent of the female volunteers used the studied cosmetics, food, and beverages, which were contaminated with Cr, Al, Pb, and Cd, reflecting the high concentration of Cr, Al, Cd, and Pb in the three different types of biological samples of sera, nails, and hair of the same females, with 29 percent of the female volunteers not using the products in the studied samples. Our results also show an elevated level of cholinesterase in the serum of group 1 that was greater than group 2, and this result was confirmed by AutoDock. Moreover, the negative variation in the CBC result was compared with the reference ranges. Future studies should concentrate on the actions of these heavy metal contaminations and their potential health consequences for various human organs individually.

Metals in Alzheimer's Disease

The role of metals in the pathogenesis of Alzheimer's disease (AD) is still debated. Although previous research has linked changes in essential metal homeostasis and exposure to environmental heavy metals to the pathogenesis of AD, more research is needed to determine the relationship between metals and AD. In this review, we included human studies that (1) compared the metal concentrations between AD patients and healthy controls, (2) correlated concentrations of AD cerebrospinal fluid (CSF) biomarkers with metal concentrations, and (3) used Mendelian randomization (MR) to assess the potential metal contributions to AD risk. Although many studies have examined various metals in dementia patients, understanding the dynamics of metals in these patients remains difficult due to considerable inconsistencies among the results of individual studies. The most consistent findings were for Zn and Cu, with most studies observing a decrease in Zn levels and an increase in Cu levels in AD patients. However, several studies found no such relation. Because few studies have compared metal levels with biomarker levels in the CSF of AD patients, more research of this type is required. Given that MR is revolutionizing epidemiologic research, additional MR studies that include participants from diverse ethnic backgrounds to assess the causal relationship between metals and AD risk are critical.

Metal-organic frameworks: A promising option for the diagnosis and treatment of Alzheimer's disease

Beta-amyloid (Aβ) peptide is one of the main characteristic biomarkers of Alzheimer's disease (AD). Previous clinical investigations have proposed that unusual concentrations of this biomarker in cerebrospinal fluid, blood, and brain tissue are closely associated with the AD progression. Therefore, the critical point of early diagnosis, prevention, and treatment of AD is to monitor the levels of Aβ. In view of the potential of metal-organic frameworks (MOFs) for diagnosing and treating the AD, much attention has been focused in recent years. This review discusses the latest advances in the applications of MOFs for the early diagnosis of AD via fluorescence and electrochemiluminescence (ECL) detection of AD biomarkers, fluorescence detection of the main metal ions in the brain (Zn2+, Cu2+, Mn2+, Fe3+, and Al3+) in addition to magnetic resonance imaging (MRI) of the Aβ plaques. The current challenges and future strategies for translating the in vitro applications of MOFs into in vivo diagnosis of the AD are discussed.

miRNAs and Stem Cells as Promising Diagnostic and Therapeutic Targets for Alzheimer's Disease

Alzheimer's disease (AD) is a cumulative progressive neurodegenerative disease characterized mainly by impairment in cognitive functions accompanied by memory loss, disturbance in behavior and personality, and difficulties in learning. Although the main causes of AD pathogenesis are not fully understood yet, amyloid-β peptides and tau proteins are supposed to be responsible for AD onset and pathogenesis. Various demographic, genetic, and environmental risk factors are involved in AD onset and pathogenesis such as age, gender, several genes, lipids, malnutrition, and poor diet. Significant changes were observed in microRNA (miRNA) levels between normal and AD cases giving hope for a diagnostic procedure for AD through a simple blood test. As yet, only two classes of AD therapeutic drugs are approved by FDA. They are classified as acetylcholinesterase inhibitors and N-methyl-D-aspartate antagonists (NMDA). Unfortunately, they can only treat the symptoms but cannot cure AD or stop its progression. New therapeutic approaches were developed for AD treatment including acitretin due to its ability to cross blood-brain barrier in the brain of rats and mice and induce the expression of ADAM 10 gene, the α-secretase of human amyloid-β protein precursor, stimulating the non-amyloidogenic pathway for amyloid-β protein precursor processing resulting in amyloid-β reduction. Also stem cells may have a crucial role in AD treatment as they can improve cognitive functions and memory in AD rats through regeneration of damaged neurons. This review spotlights on promising diagnostic techniques such as miRNAs and therapeutic approaches such as acitretin and/or stem cells keeping in consideration AD pathogenesis, stages, symptoms, and risk factors.

Exposure of metal toxicity in Alzheimer's disease: An extensive review

Metals serve important roles in the human body, including the maintenance of cell structure and the regulation of gene expression, the antioxidant response, and neurotransmission. High metal uptake in the nervous system is harmful because it can cause oxidative stress, disrupt mitochondrial function, and impair the activity of various enzymes. Metal accumulation can cause lifelong deterioration, including severe neurological problems. There is a strong association between accidental metal exposure and various neurodegenerative disorders, including Alzheimer's disease (AD), the most common form of dementia that causes degeneration in the aged. Chronic exposure to various metals is a well-known environmental risk factor that has become more widespread due to the rapid pace at which human activities are releasing large amounts of metals into the environment. Consequently, humans are exposed to both biometals and heavy metals, affecting metal homeostasis at molecular and biological levels. This review highlights how these metals affect brain physiology and immunity and their roles in creating harmful proteins such as β-amyloid and tau in AD. In addition, we address findings that confirm the disruption of immune-related pathways as a significant toxicity mechanism through which metals may contribute to AD.

Development of lab-on-chip biosensor for the detection of toxic heavy metals: A review

Recently, a decrease in water availability and quality has been raised due to rapid industrialization, unsustainable agricultural activities and anthropogenic activities. Heavy metals are considered significant pollutants in the water environment, cause environmental hazards and health effects to humans. For monitoring water contaminants utilized different conventional techniques. Still, they have some drawbacks, such as cost expensive, ecological issues, and processing time, requiring technicians and researchers to operate them effectively. Biosensors have become reasonable devices for screening and identifying environmental contaminants because of their different benefits contrasted with other detecting techniques. This review summarizes the toxic effect of heavy metal and their source, occurrence. A detailed discussion is provided on the heavy metal recognition materials for detecting heavy metals in wastewater. Lab on chip (LOC) is an emerging micro-electrical mechanical system (MEMS) device that intakes liquid and makes it move through the micro-channels, to accomplish fast, cost-effective and profoundly sensitive analysis with significant yield. LOC also provided a discussion on numerous laboratory functions on a single platform. This article attempts to discuss the detection of heavy metals using lab on a chip by suitable recognition materials. Further, the design and fabrication mechanism and their recognition abilities of LOC were also reviewed. The review mainly focuses on the application of LOC biosensors, pros, and cons, and suggests a roadmap towards future development to enhance the practical use in pollutant monitoring.

Alzheimer's Disease Association with Metals and Metalloids Concentration in Blood and Urine

As there is some evidence that the risk for Alzheimer’s disease (AD) is partially attributable to environmental exposure to some metals and metalloids, we examined an association between AD and arsenic, chromium, and selenium in 53 AD patients and 217 controls. Urinary arsenic, blood chromium, and selenium were determined by inductively coupled plasma mass spectrometry. Logistic regression models calculating odds ratios (ORs) and 95% confidence intervals (CI) were used to estimate AD association with arsenic, chromium, and selenium. In AD patients, urinary arsenic and blood chromium were significantly higher, while blood selenium was significantly lower compared to controls. Increased blood selenium was related to a significant decrease in the odds of AD after adjustment for risk factors. Blood selenium per 1 kg × 10⁻⁹/m³ × 10⁻⁴ increment was associated with 1.4 times lower risk of AD (OR = 0.71; 95% CI 0.58–0.87). A significant increase in the odds of AD associated with increased blood chromium was also seen in the adjusted model: the OR per 1 kg × 10⁻⁹/m³ × 10⁻³ chromium increment was 2.39 (95% CI 1.32–4.31). The association of urinary arsenic with the risk of AD was not significant. The data obtained provide evidence that selenium reduces the risk of Alzheimer’s disease, while chromium increases it.

Developmental exposure to environmental levels of cadmium induces neurotoxicity and activates microglia in zebrafish larvae: From the perspectives of neurobehavior and neuroimaging

Cadmium (Cd) is a worldwide environmental pollutant that postures serious threats to humans and ecosystems. Over the years, its adverse effects on the central nervous system (CNS) have been concerned, whereas the underlying cellular/molecular mechanisms remain unclear. In this study, taking advantages of zebrafish model in high-throughput imaging and behavioral tests, we have explored the potential developmental neurotoxicity of Cd at environmentally relevant levels, from the perspectives of neurobehavior and neuroimaging. Briefly, Cd²⁺ exposure resulted in a general impairment of zebrafish early development. Zebrafish neurobehavioral patterns including locomotion and reactivity to environmental signals were significantly perturbed upon Cd²⁺ exposure. Importantly, a combination of in vivo two-photon neuroimaging, flow cytometry and gene expression analyses revealed notable neurodevelopmental disorders as well as neuroimmune responses induced by Cd²⁺ exposure. Both cell-cycle arrest and apoptosis contributed jointly to a significant decrease of neuronal density in zebrafish larvae exposed to Cd²⁺. The dramatic morphological alterations of microglia from multi-branched to amoeboid, the microgliosis, as well as the modulation of gene expression profiles demonstrated a strong activation of microglia and neuroinflammation triggered by environmental levels of Cd²⁺. Together, our study points to the developmental toxicity of Cd in inducing CNS impairment and neuroinflammation thereby providing visualized etiological evidence of this heavy metal induced neurodevelopmental disorders. It's tempting to speculate that this research model might represent a promising tool not only for understanding the molecular mechanisms of Cd-induced neurotoxicity, but also for developing pharmacotherapies to mitigate the neurological damage resulting from exposure to Cd, and other neurotoxicants.

Optical chemosensors for environmental monitoring of toxic metals related to Alzheimer's disease

Alzheimer's disease (AD) is the most common type of dementia and progresses from mild memory loss to severe decline in thinking, behavioral and social skills, which dramatically impairs a person's ability to function independently. Genetics, some health disorders and lifestyle have all been connected to AD. Also, environmental factors are reported as contributors to this illness. The presence of heavy metals in air, water, food, soil and commercial products has increased tremendously. Accumulation of heavy metals in the body leads to serious malfunctioning of bodily organs, specifically the brain. For AD, a wide range of heavy metals have been reported to contribute to its onset and progression and the manifestation of its hallmarks. In this review, we focus on detection of highly toxic heavy metals such as mercury, cadmium, lead and arsenic in water. The presence of heavy metals in water is very troubling and regular monitoring is warranted. Optical chemosensors were designed and fabricated for determination of ultra-trace quantities of heavy metals in water. They have shown advantages when compared to other sensors, such as selectivity, low-detection limit, fast response time, and wide-range determination under optimal sensing conditions. Therefore, implementing optical chemosensors for monitoring levels of toxic metals in water represents an important contribution in fighting AD.

This review briefly summarizes evidence that links toxic metals to onset and progression of Alzheimer's disease. It discusses the structure and fabrication of optical chemosensors, and their use for monitoring toxic metals in water.

Cadmium, an Environmental Contaminant, Exacerbates Alzheimer's Pathology in the Aged Mice's Brain

Cadmium (Cd) is an environmental contaminant, which is a potential risk factor in the progression of aging-associated neurodegenerative diseases. Herein, we have assessed the effects of chronic administration of Cd on cellular oxidative stress and its associated Alzheimer's disease (AD) pathologies in animal models. Two groups of mice were used, one group administered with saline and the other with Cd (1 mg/kg/day; intraperitoneally) for 3 months. After behavioral studies, molecular/biochemical (Immunoblotting, ELISAs, ROS, LPO, and GSH assays) and morphological analyses were performed. We observed an exacerbation of memory and synaptic deficits in chronic Cd-injected mice. Subacute and chronic Cd escalated reactive oxygen species (ROS), suppressed the master antioxidant enzymes, e.g., nuclear factor-erythroid 2-related factor 2 and heme oxygenase-1, and evoked the stress kinase phospho-c-Jun N-terminal kinase 1 signaling pathways, which may escalate AD pathologies possibly associated with amyloidogenic processes. These findings suggest the regulation of oxidative stress/ROS and its associated amyloid beta pathologies for targeting the Cd-exacerbated AD pathogenesis. In addition, these preclinical animal studies represent a paradigm for epidemiological studies of the human population exposed to chronic and subacute administration of Cd, suggesting avoiding environmental contaminants.

Mechanisms of Metal-Induced Mitochondrial Dysfunction in Neurological Disorders

Metals are actively involved in multiple catalytic physiological activities. However, metal overload may result in neurotoxicity as it increases formation of reactive oxygen species (ROS) and elevates oxidative stress in the nervous system. Mitochondria are a key target of metal-induced toxicity, given their role in energy production. As the brain consumes a large amount of energy, mitochondrial dysfunction and the subsequent decrease in levels of ATP may significantly disrupt brain function, resulting in neuronal cell death and ensuing neurological disorders. Here, we address contemporary studies on metal-induced mitochondrial dysfunction and its impact on the nervous system.

Cadmium-induced dysfunction of the blood-brain barrier depends on ROS-mediated inhibition of PTPase activity in zebrafish

Increasing evidence has demonstrated that cadmium accumulation in the blood increases the risk of neurological diseases. However, how cadmium breaks through the blood-brain barrier (BBB) and is transferred from the blood circulation into the central nervous system is still unclear. In this study, we examined the toxic effect of cadmium chloride (CdCl₂) on the development and function of BBB in zebrafish. CdCl₂ exposure induced cerebral hemorrhage, increased BBB permeability and promoted abnormal vascular formation by promoting VEGF production in zebrafish brain. Furthermore, in vivo and in vitro experiments showed that CdCl₂ altered cell-cell junctional morphology by disrupting the proper localization of VE-cadherin and ZO-1. The potential mechanism involved in the inhibition of protein tyrosine phosphatase (PTPase) mediated by cadmium-induced ROS was confirmed with diphenylene iodonium (DPI), a ROS production inhibitor. Together, these data indicate that BBB is a critical target of cadmium toxicity and provide in vivo etiological evidence of cadmium-induced neurovascular disease in a zebrafish BBB model.

Current Quest in Natural Bioactive Compounds for Alzheimer's Disease: Multi-Targeted-Designed-Ligand Based Approach with Preclinical and Clinical Based Evidence

Alzheimer's disease is a common and most chronic neurological disorder (NDs) associated with cognitive dysfunction. Pathologically, Alzheimer's disease (AD) is characterized by the presence of β-amyloid (Aβ) plaques, hyper-phosphorylated tau proteins, and neurofibrillary tangles, however, persistence oxidative-nitrative stress, endoplasmic reticulum stress, mitochondrial dysfunction, inflammatory cytokines, pro-apoptotic proteins along with altered neurotransmitters level are common etiological attributes in its pathogenesis. Rivastigmine, memantine, galantamine, and donepezil are FDA approved drugs for symptomatic management of AD, whereas tacrine has been withdrawn because of hepatotoxic profile. These approved drugs only exert symptomatic relief and exhibit poor patient compliance. In the current scenario, the number of published evidence shows the neuroprotective potential of naturally occurring bioactive molecules via their antioxidant, anti-inflammatory, antiapoptotic and neurotransmitter modulatory properties. Despite their potent therapeutic implications, concerns have arisen in context to their efficacy and probable clinical outcome. Thus, to overcome these glitches, many heterocyclic and cyclic hydrocarbon compounds inspired by natural sources have been synthesized and showed improved therapeutic activity. Computational studies (molecular docking) have been used to predict the binding affinity of these natural bioactive as well as synthetic compounds derived from natural sources for the acetylcholine esterase, α/β secretase Nuclear Factor kappa- light-chain-enhancer of activated B cells (NF-kB), Nuclear factor erythroid 2-related factor 2(Nrf2) and other neurological targets. Thus, in this review, we have discussed the molecular etiology of AD, focused on the pharmacotherapeutics of natural products, chemical and pharmacological aspects and multi-targeted designed ligands (MTDLs) of synthetic and semisynthetic molecules derived from the natural sources along with some important on-going clinical trials.

Amyloid-β: A double agent in Alzheimer's disease?

Amyloid-β (Aβ) accumulation is one of the cardinal pathological hallmarks of Alzheimer's disease and plays an important role in its pathogenesis. Although the neurotoxic effects of Aβ has been extensively studied, recent studies have revealed that it may also have protective effects. Here, we review novel findings that have shifted our understanding of the role of Aβ in the pathogenesis of Alzheimer's disease. An in-depth and comprehensive understanding of Aβ will provide us with a broader perspective on the treatment of Alzheimer's disease.

The nickel-chelator dimethylglyoxime inhibits human amyloid beta peptide in vitro aggregation

One of the hallmarks of the most common neurodegenerative disease, Alzheimer's disease (AD), is the extracellular deposition and aggregation of Amyloid Beta (Aβ)-peptides in the brain. Previous studies have shown that select metal ions, most specifically copper (Cu) and zinc (Zn) ions, have a synergistic effect on the aggregation of Aβ-peptides. In the present study, inductively coupled plasma mass spectrometry (ICP-MS) was used to determine the metal content of a commercial recombinant human Aβ40 peptide. Cu and Zn were among the metals detected; unexpectedly, nickel (Ni) was one of the most abundant elements. Using a fluorescence-based assay, we found that Aβ40 peptide in vitro aggregation was enhanced by addition of Zn2+ and Ni2+, and Ni2+-induced aggregation was facilitated by acidic conditions. Nickel binding to Aβ40 peptide was confirmed by isothermal titration calorimetry. Addition of the Ni-specific chelator dimethylglyoxime (DMG) inhibited Aβ40 aggregation in absence of added metal, as well as in presence of Cu2+ and Ni2+, but not in presence of Zn2+. Finally, mass spectrometry analysis revealed that DMG can coordinate Cu or Ni, but not Fe, Se or Zn. Taken together, our results indicate that Ni2+ ions enhance, whereas nickel chelation inhibits, Aβ peptide in vitro aggregation. Hence, DMG-mediated Ni-chelation constitutes a promising approach towards inhibiting or slowing down Aβ40 aggregation.

Blood Transcriptome Response to Environmental Metal Exposure Reveals Potential Biological Processes Related to Alzheimer's Disease

Alzheimer's disease (AD) is a neurodegenerative disease which is manifested by a progressive and irreversible decline of cognition, memory loss, a shortened attention span, and changes in personality. Aging and genetic pre-dispositions, particularly the presence of a specific form of apolipoprotein E (APOE), are main risk factors of sporadic AD; however, a large body of evidence has shown that multiple environmental factors, including exposure to toxic metals, increase the risk for late onset AD. Lead (Pb) and cadmium (Cd) are ubiquitous toxic metals with a wide range of applications resulting in global distribution in the environment and exposure of all living organisms on earth. In addition to being classified as carcinogenic (Cd) and possibly carcinogenic (Pb) to humans by the International Agency for Research on Cancer, both compounds disrupt metal homeostasis and can cause toxic responses at the cellular and organismal levels. Pb toxicity targets the central nervous system and evidence for that has emerged also for Cd. Recent epidemiological studies show that both metals possibly are etiological factors of multiple neurodegenerative diseases, including Alzheimer's disease (AD). To further explore the association between metal exposure and AD risk we applied whole transcriptome gene expression analysis in peripheral blood leukocytes (PBLs) from 632 subjects of the general population, taken from the EnviroGenomarkers project. We used linear mixed effect models to associate metal exposure to gene expression after adjustment for gender, age, BMI, smoking, and alcohol consumption. For Pb exposure only few associations were identified, including a downregulation of the human eukaryotic translation initiation factor 5 (eIF5). In contrast, Cd exposure, particularly in males, revealed a much stronger transcriptomic response, featuring multiple pathways related to pathomolecular mechanisms of AD, such as endocytosis, neutrophil degranulation, and Interleukin-7 signaling. A gender stratified analysis revealed that the Cd responses were male-specific and included a downregulation of the APOE gene in men. This exploratory study revealed novel hypothetical findings which might contribute to the understanding of the neurotoxic effects of chronic Pb and Cd exposure and possibly improve our knowledge on the molecular mechanisms linking metal exposure to AD risk.

The sustaining effects of e-waste-related metal exposure on hypothalamus-pituitary-adrenal axis reactivity and oxidative stress

Taizhou is one of the three largest electronic waste (e-waste) recycling locations in China. At present, to prevent the environmental problems stem from e-waste dismantling, the local government shut down all the industries in 2015. However, little is known to the sustaining effects of e-waste exposure on human health after the e-waste recycling factories were closed. In this study, we collected blood samples in the residents living near e-waste dismantling factories (exposed group) and a chosen reference area (reference group) for comparison in Taizhou in December 2017.17 metals were quantitatively determined in all blood samples. Among them, the concentrations of altogether 9 metals, including chromium (Cr), arsenic (As), cobalt (Co), nickel (Ni), silver (Ag), stannum (Sn), mercury (Hg), lanthanum (La) and cerium (Ce) were significant higher in the individuals in exposed group than in reference group. Hypothalamus-pituitary-adrenal axis (HPA axis) is critical endocrine system in regulating many physical functions. We found that the concentrations of hormones in HPA axis, including corticotropin releasing hormone (CRH), adrenocorticotropic hormone (ACTH) and cortisol, were higher in exposed group than in reference group. Moreover, we also observed significantly higher concentrations of biomarkers of oxidative stress (OS), including malondialdehyde (MDA) and 8-isoprostane (8-I), in exposed group compared with reference group even though the e-waste industries have been shut down for over 2 years. Meanwhile, the hormones in the HPA axis and the biomarkers of OS showed sinificantly positive correlation. The results of Pearson correlation and multiple linear regression showed that blood Cr and Ni positively correlated with the 3 hormones and 2 biomarkers of OS. Considering the relationship between Ni and Cr, HPA axis, OS, we speculated that high concentrations of Cr and Ni exposure could induce oxidative damage in e-waste exposure persons, and the regulations of HPA axis could play the important role during the process.

Risk assessment and evaluation of heavy metals concentrations in blood samples of plastic industry workers in Dhaka, Bangladesh

To assess the potential health risk caused by heavy metals twenty-six blood samples were collected from plastic industry workers based on ages and smoking status in Dhaka, Bangladesh. Heavy metals were analyzed with an atomic absorption spectrometer. The mean concentrations of Lead (Pb), Cadmium (Cd), Nickel (Ni), and Zinc (Zn) found in blood samples of the exposed workers were 32.78 ± 9.47, 1.08 ± 0.47, 1.42 ± 1.01, and 9.08 ± 1.95 μgL-1, respectively. The average heavy metal concentrations in blood samples of smoking workers show a narrow range of fluctuation than that of non-smoking workers. A review of different age groups of industry workers shows the workers between the ages of 26 and 40 are more likely to contaminated with Pb (35.90 ± 8.06 μgL-1) and Ni (1.61 ± 1.31 μgL-1). The higher level of Cd (1.26 ± 0.46 μgL-1) and Zn (9.91 ± 2.80 μgL-1) was found in >40 years old workers. The mean concentration in indoor dust samples of different industrial subsections reported as 40.27 ± 10.33, 3.24 ± 0.83, 18.08 ± 3.61, and 103.64 ± 8.16 mg kg-1 for Pb, Cd, Ni, and Zn, respectively. Exposed workers have relatively less critical health implications concluded from the average daily intake (ADI), hazard quotient (HQs), and hazard index (HI) values. The HI values of Pb, Cd, Ni, and Zn were reported as 2.0 × 10-2, 4.64 × 10-4, 1.62 × 10-3, and 5.49 × 10-4, respectively, which have imparted minimal risks (as HI < 1) to the health of the workers. The cancer risks of Pb, Cd, and Ni were reported as 1.46 × 10-10, 1.77 × 10-9, and 1.31 × 10-9, respectively lower than the threshold values. Therefore, the result divulged a potentially lower cancer risk compared to EPA limit value of 1 × 10-6 to 1 × 10-4 for exposed industrial workers.

Environmental toxins and Alzheimer's disease progression

Alzheimer's disease (AD) is the most common form of dementia, which causes progressive memory loss and cognitive decline. Effective strategies to treat or prevent remains one of the most challenging undertakings in the medical field. AD is a complex and multifactorial disease that involves several risk factors. Aging and genetic factors both play important roles in the onset of the AD, however; certain environmental factors have been reported to increase the risk of AD. Chronic exposure to toxins has been seen as an environmental factor that may increase the risk of developing a neurodegenerative disease such as AD. Exposure to metals and biotoxins produced by bacteria, molds, and viruses may contribute to the cognitive decline and pathophysiology associated with AD. Toxins may contribute to the pathology of the disease through various mechanisms such as deposition of amyloid-beta (Aβ) plaques and tangles in the brain, induction of apoptosis, inflammation, or oxidative damage. Here, we will review how toxins affect brain physiology with a focus on mechanisms by which toxins may contribute to the development and progression of AD. A better understanding of these mechanisms may help contribute towards the development of an effective strategy to slow the progression of AD.

Caffeic acid phenethyl ester reversed cadmium-induced cell death in hippocampus and cortex and subsequent cognitive disorders in mice: Involvements of AMPK/SIRT1 pathway and amyloid-tau-neuroinflammation axis

Exposure to nonbiodegradable cadmium (Cd) causes many health problems including the damage to the nervous system. This study aimed to increase knowledge about its neurotoxic effects and the neuroprotective potential of caffeic acid phenethyl ester (CAPE, a polyphenol abundant in honeybee propolis). In mice, CAPE (10 μmol/kg/day body weight) attenuated significantly learning and memory deficits induced by CdCl₂ (1.5 mg/kg/day body weight). For the CdCl₂-treated mice, CAPE increased crossing number in open field test, decreased the alternation in Y-maze test, and increased the latency time and error number in step down test. CAPE also inhibited CdCl₂-initiated Aβ accumulation and activation of pro-inflammatory factors and microglia in the brains. Therefore, CAPE could be a food-derived neuroprotective agent against Cd-induced neurotoxicity and neurodegenerative disorders, through attenuating neuronal apoptosis and neuroinflammation via the AMPK/SIRT1 pathway and amyloid-tau-neuroinflammation axis.

Associations between metal exposure and global DNA methylation in potentially affected people in E-Waste recycling sites in Taizhou City, China

Electronic waste (e-waste) has been an emerging environmental health issue, and it has already provoked all aspects of attention. Taizhou is one of the three largest e-waste recycling locations in China. Atpresent, to prevent the environmental problems stem from e-waste dismantling, the local government has shut down all the industries in 2015. In this study, we collected blood samples of residents living near e-waste dismantling factories, and in matched reference areas in Taizhou, in December 2017, after the factories have been shut down for two years. Twenty-five metals were quantified in all blood samples. Among them, the concentrations of As, Ni, Ag, La, and Ce were statistically significant higher in individuals in e-waste recycling locations than those in reference location. Global DNA methylation was measured in blood as a marker of human health. Pearson correlation and multiple linear regression analysis between the changed metals and global DNA methylation in blood were performed. The result showed that only blood Ce was negatively correlated with global DNA methylation level significantly in pre-workers exposed e-waste workers (r = -0.51, p = 0.01). Our findings indicated that high concentrations of exposure to Ce in e-waste dismantling site could have sustained effects on the DNA methylation in blood although the e-waste industry had been closed for 2 years.

Heavy Metals Exposure and Alzheimer's Disease and Related Dementias

Alzheimer's disease and related dementias lack effective treatment or cures and are major public health challenges. Risk for Alzheimer's disease and related dementias is partially attributable to environmental factors. The heavy metals lead, cadmium, and manganese are widespread and persistent in our environments. Once persons are exposed to these metals, they are adept at entering cells and reaching the brain. Lead and cadmium are associated with numerous health outcomes even at low levels of exposure. Although manganese is an essential metal, deficiency or environmental exposure or high levels of the metal can be toxic. In cell and animal model systems, lead, cadmium, and manganese are well documented neurotoxicants that contribute to canonical Alzheimer's disease pathologies. Adult human epidemiologic studies have consistently shown lead, cadmium, and manganese are associated with impaired cognitive function and cognitive decline. No longitudinal human epidemiology study has assessed lead or manganese exposure on Alzheimer's disease specifically though two studies have reported a link between cadmium and Alzheimer's disease mortality. More longitudinal epidemiologic studies with high-quality time course exposure data and incident cases of Alzheimer's disease and related dementias are warranted to confirm and estimate the proportion of risk attributable to these exposures. Given the widespread and global exposure to lead, cadmium, and manganese, even small increases in the risks of Alzheimer's disease and related dementias would have a major population impact on the burden on disease. This article reviews the experimental and epidemiologic literature of the associations between lead, cadmium, and manganese on Alzheimer's disease and related dementias and makes recommendations of critical areas of future investment.

Effects of aluminium and cadmium on hatching and swimming ability in developing zebrafish

Aluminium and cadmium are biologically non-essential metals with a role in neurodegenerative and neuromuscular diseases. As an attractive model for neurobehavioural studies, zebrafish at 6 h post fertilization were exposed to 9, 18, 36 and 72 μM CdCl₂ and 50, 100 and 200 μM AlCl₃, respectively, for 72 h, and motility such as distance moved, mean velocity, cumulative movement, meander and heading were measured by DanioVision equipment. The hatching time was also analysed. A delay in the exit from the chorion was observed in all treated larvae with respect to the controls. CdCl₂ acted on the exit from the chorion of larvae with a dose-dependent delay. By contrast, the delay caused by AlCl₃ was greater at low concentrations. A dose-dependent reduction in swimming performance was observed in the larvae exposed to CdCl₂. Instead, for those exposed to AlCl₃, swimming performance improved at higher concentrations although values were in general lower than those of control. All the parameters had a similar trend except the meander parameter which showed a dose-dependent reduction. These data show that cadmium and aluminium can delay hatching and alter swimming ability in the early developmental stages of zebrafish, albeit with different effects, suggesting that exposure to sublethal concentrations of both metals can change behavioural parameters.

The Administration of Cadmium for 2, 3 and 4 Months Causes a Loss of Recognition Memory, Promotes Neuronal Hypotrophy and Apoptosis in the Hippocampus of Rats

Cadmium (Cd) is a toxic metal and classified as a carcinogen whose exposure could affect the function of the central nervous system. There are studies that suggest that Cd promotes neurodegeneration in different regions of the brain, particularly in the hippocampus. It is proposed that its mechanism of toxicity maybe by an oxidative stress pathway, which modifies neuronal morphology and causes the death of neurons and consequently affecting cognitive tasks. However, this mechanism is not yet clear. The aim of the present work was to study the effect of Cd administration on recognition memory for 2, 3 and 4 months, neuronal morphology and immunoreactivity for caspase-3 and 9 in rat hippocampi. The results show that the administration of Cd decreased recognition memory. Likewise, it caused the dendritic morphology of the CA1, CA3 and dentate gyrus regions of the hippocampus to decrease with respect to the time of administration of this heavy metal. In addition, we observed a reduction in the density of dendritic spines as well as an increase in the immunoreactivity of caspase-3 and 9 in the same hippocampal regions of the animals treated with Cd. These results suggest that Cd affects the structure and function of the neurons of the hippocampus, which contribute to the deterioration of recognition memory. Our results suggest that the exposure to Cd represents a critical health problem, which if not addressed quickly, could cause much more serious problems in the quality of life of the human population, as well as in the environment in which they develop.

Metal Toxicity Links to Alzheimer's Disease and Neuroinflammation

As the median age of the population increases, the number of individuals with Alzheimer's disease (AD) and the associated socio-economic burden are predicted to worsen. While aging and inherent genetic predisposition play major roles in the onset of AD, lifestyle, physical fitness, medical condition, and social environment have emerged as relevant disease modifiers. These environmental risk factors can play a key role in accelerating or decelerating disease onset and progression. Among known environmental risk factors, chronic exposure to various metals has become more common among the public as the aggressive pace of anthropogenic activities releases excess amount of metals into the environment. As a result, we are exposed not only to essential metals, such as iron, copper, zinc and manganese, but also to toxic metals including lead, aluminum, and cadmium, which perturb metal homeostasis at the cellular and organismal levels. Herein, we review how these metals affect brain physiology and immunity, as well as their roles in the accumulation of toxic AD proteinaceous species (i.e., β-amyloid and tau). We also discuss studies that validate the disruption of immune-related pathways as an important mechanism of toxicity by which metals can contribute to AD. Our goal is to increase the awareness of metals as players in the onset and progression of AD.

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.

Concentration-dependent effects of mercury and lead on Aβ42: possible implications for Alzheimer's disease

Mercury (Hg) and lead (Pb) are known to be toxic non-radioactive elements, with well-described neurotoxicology. Much evidence supports the implication of metals as potential risk cofactors in Alzheimer's disease (AD). Although the action mechanism of the two metals remains unclear, Hg and Pb toxicity in AD could depend on their ability to favour misfolding and aggregation of amyloid beta proteins (Aβs) that seem to have toxic properties, particularly in their aggregated state. In our study, we evaluated the effect of Hg and Pb both on the Aβ42 ion channel incorporated in a planar lipid membrane made up of phosphatidylcholine containing 30% cholesterol and on the secondary structure of Aβ42 in an aqueous environment. The effects of Hg and Pb on the Aβ42 peptide were observed for its channel incorporated into a membrane as well as for the peptide in solution. A decreasing Aβ42 channel frequency and the formation of large and amorphous aggregates in solution that are prone to precipitate were both dependent on metal concentration. These experimental data suggest that Hg and Pb interact directly with Aβs, strengthening the hypothesis that the two metals may be a risk factor in AD.

'Evolutionary medicine' perspectives on Alzheimer's Disease: Review and new directions

Evolution by natural selection eliminates maladaptive traits from a species, and yet Alzheimer's Disease (AD) persists with rapidly increasing prevalence globally. This apparent paradox begs an explanation within the framework of evolutionary sciences. Here, I summarize and critique previously proposed theories to explain human susceptibility to AD, grouped into 8 distinct hypotheses based on the concepts of novel extension of the lifespan; lack of selective pressure during the post-reproductive phase; antagonistic pleiotropy; rapid brain evolution; delayed neuropathy by selection for grandmothering; novel alleles selected to delay neuropathy; by-product of selection against cardiovascular disease; and thrifty genotype. Subsequently, I describe a new hypothesis inspired by the concept of mismatched environments. Many of the factors that enhance AD risk today may have been absent or functioned differently before the modern era, potentially making AD a less common affliction for age-matched individuals before industrialization and for the majority of human history. Future research is needed to further explore whether changes in environments and lifestyles across human history moderate risk factors and susceptibility to AD.

Biometal Dyshomeostasis and Toxic Metal Accumulations in the Development of Alzheimer's Disease

Biometal dyshomeostasis and toxic metal accumulation are common features in many neurodegenerative disorders, including Alzheimer’s disease (AD), Parkinson’s disease, and Huntington’s disease. The neurotoxic effects of metal imbalance are generally associated with reduced enzymatic activities, elevated protein aggregation and oxidative stress in the central nervous system, in which a cascade of events lead to cell death and neurodegeneration. Although the links between biometal imbalance and neurodegenerative disorders remain elusive, a major class of endogenous proteins involved in metal transport has been receiving increasing attention over recent decades. The abnormal expression of these proteins has been linked to biometal imbalance and to the pathogenesis of AD. Here, we present a brief overview of the physiological roles of biometals including iron, zinc, copper, manganese, magnesium and calcium, and provide a detailed description of their transporters and their synergistic involvement in the development of AD. In addition, we also review the published data relating to neurotoxic metals in AD, including aluminum, lead, cadmium, and mercury.

Potential health risk of heavy metals in the leather manufacturing industries in Sialkot, Pakistan

This is a systematical report on the potential health risk of heavy metals from the leather industries in Pakistan based on multiple biological matrices of the exposed workers and indoor dust samples. The adverse impacts of heavy metals on the oxidative enzyme and their risks to workers' health were also explored. Our results indicated that the level of Cr in indoor industrial dust was more than twice, compared to the background household dust. Blood, urine and hair samples of exposed workers showed significantly high concentrations of heavy metals, compared to those in the control group. Superoxide dismutase (SOD) level in the blood samples expressed significant positive correlation with Cr and Ni. Total hazard quotients (HQs)/hazard index (HI) were >1, and Cr (VI) exhibited higher cancer risks than that of Cd in the exposed workers. In addition, the PCA-MLR analysis confirmed that the industrial sections; cutting, shivering/crusting, and stitching were the principal contributors of heavy metals in the biological entities of the workers. Taken together, our results highlighted the occupationally exposed groups would likely to experience the potential health risks due to excessive exposure to the heavy metals from the leather industries.

Cadmium and Alzheimer's disease mortality in U.S. adults: Updated evidence with a urinary biomarker and extended follow-up time

Cadmium has been linked to impaired cognitive function in adults and may cause behavioral, physiological and molecular abnormalities characteristic of Alzheimer's disease (AD) in animals. Evidence linking cadmium and AD in humans is limited, but supportive. In the most recent epidemiologic study, blood cadmium in U.S. adults was positively associated with elevated AD mortality 7-13 years later. The association between urinary cadmium - an arguably more appropriate biomarker for studying chronic diseases - and AD mortality has not yet been explored. Further study of cadmium and AD mortality in an independent population, with longer follow-up, and stratified by sex is also needed. We sought to answer these questions using the U.S. National Health and Nutrition Examination Survey (NHANES) (1999-2006 cycles) and NHANES III (interviews in 1988-1994) datasets, separately linked to AD mortality as of 2011. We used survey-weighted Cox regression models predicting age at AD death and adjusted for race/ethnicity, sex, smoking status, education and urinary creatinine. An interquartile range (IQR; IQR=0.51ng/mL) increase in urinary cadmium was associated with 58% higher rate of AD mortality (hazard ratio (HR)=1.58, 95% CI: 1.20, 2.09. p-value=0.0009, mean follow-up: 7.5 years) in NHANES 1999-2006 participants. In contrast, in NHANES III participants, an IQR (IQR=0.78ng/mL) increase in urinary cadmium was not associated with AD mortality (HR=0.85, 95% CI: 0.63, 1.17, p-value=0.31, mean follow-up: 13 years). Also in the NHANES III sample however, when the maximum follow-up time was restricted to 12.7 years (i.e. the same as NHANES 1999-2006 participants) and urinary creatinine adjustments were not made, urinary cadmium was associated with elevated AD mortality (HR=1.11, 95% CI: 1.02, 1.20, p-value=0.0086). Our study partially supported an association between cadmium and AD mortality, but the sensitivity of results to follow-up time and creatinine adjustments necessitate cautious interpretation of the association. Further studies, particularly those on toxicological mechanisms, are required to fully understand the nature of the "cadmium-AD mortality" association.

Editor's Highlight: Exposure to CrVI during Early Pregnancy Increases Oxidative Stress and Disrupts the Expression of Antioxidant Proteins in Placental Compartments

Epidemiologic studies document relationships between chromium VI (CrVI) exposure and increased risk of spontaneous abortion, stillbirth, preterm birth, and neonatal death in pregnant women. Environmental contamination with CrVI is a growing problem both in the United States and developing countries. CrVI is widely used in numerous industries. This study was designed to understand the mechanism of CrVI toxicity on placental oxidative stress and antioxidant (AOX) machinery. Pregnant mother rats were treated with or without CrVI (50 ppm K2Cr2O7) through drinking water from gestational day (GD) 9.5-14.5, and placentas were analyzed on GD 18.5. Results indicated that CrVI reduced the trophoblast cell population. CrVI increased reactive oxygen species (ROS) and decreased the expression of AOX proteins. CrVI disrupts the trophoblast proliferation of the placenta. This study provides insight into the critical role of AOXs in placental function.