Associations of Environmental Exposure to Arsenic, Manganese, Lead and Cadmium on Alzheimer's Disease: A Review of Recent Evidence from Mechanistic Studies

1. Introduction

Alzheimer’s disease (AD) is one of the most prevalent neurodegenerative diseases across the world (Islam et al., 2022; Rahman et al., 2021). About 50 million individuals worldwide suffer from AD or AD-related dementia, which both serious global health concern and one of the most common types of dementia (Alzheimer's Association, 2016; Guo et al., 2020). It has been projected that the prevalence of AD will dramatically increase due to rising life expectancy worldwide. As the degeneration of neuron takes place, AD gradually impairs cognitive abilities, including memory, attention, language, judgment and thinking (DeTure and Dickson, 2019). In the advanced stages of AD, patients experience significant memory loss, confusion, hallucinations, and a lack of self-sufficiency. Eventually, AD patients die from respiratory syndrome, infection, or starvation (Guo et al., 2020). Additionally, AD is the most common form of dementia among older adults (Tyczyńska et al., 2024). The increase of AD and Alzheimer’s disease related dementia (ADRD) to an estimated 152 million people by 2050 will put remarkable pressure on healthcare systems (GBD 2019 Dementia Forecasting Collaborators, 2022; Li et al., 2022). The initial stage of AD is characterized by the aggregation of amyloid β-protein (Aβ) in senile plaques, the formation of neurofibrillary tangles composed of abnormally phosphorylated tau protein, and a reduction in neuronal cell number, all contributing to declining cognition (Rostagno, 2022; Tyczyńska et al., 2024). Currently, four genes are known to contribute to the development of AD: (i) the amyloid precursor protein (APP), (ii) presenilin 1 (PSEN1), (iii) presenilin 2 (PSEN2) and (iv) the apolipoprotein E (APOE) gene (Blacker, and Tanzi, 1998; Lanoiselée et al., 2017).

The typical onset of sporadic or late-onset AD may be associated with several environmental, lifestyle, and genetic risk factors and thus considered as a multifactorial disorder (Arora et al., 2023; Bellenguez et al., 2022; Bird, 2008; Lau et al., 2023; Rahman et al., 2020; Xia et al., 2018). The most studied mechanisms of the AD pathogenesis are Aβ protein and tau-associated mechanisms (Murphy and LeVine, 2010; Medeiros et al., 2011), glial dysfunction (Yu et al., 2024), mitochondrial dysfunction (Ashleigh et al., 2023), oxidative stress (Dhapola et al., 2024; Gella and Durany, 2009), neuroinflammation (Heneka et al., 2015), calcium dysregulation (Joshi et al., 2023; Yu et al., 2009), as well as various lifestyle and environmental factors also associated with AD pathobiology (Arora et al., 2023; Dosunmu et al., 2007; Guo et al., 2020). Although age, genetics, and lifestyle factors have been established as primary risk factors to the onset of AD, growing evidence indicates that environmental exposures, such as toxic metals, may have adverse effects on AD/ADRD pathogenesis (Rahman et al., 2020; Huat et al., 2019; Suresh et al., 2023; Yegambaram et al., 2015).

Among the environmental risk factors for AD and dementia, air pollution including exposure to ozone, heavy metals, pesticides, Bisphenol A, and microplastics are the most prominent risk factors (Suresh et al., 2023). These factors may be associated with oxidative stress or inflammation as well as numerous genetic and environmental risk factors intersect with the etiology of AD and therefore, illustrating the complexity of the illness (Sheppard and Coleman, 2020). Experts have proposed multiple mechanisms to elucidate the complexity of AD. For instance, Henderson et al. (1988) proposed two hypotheses based on epidemiological data. The first hypothesis suggests that toxic or pathogenic particles aggregate in brain and may induce AD over time. The second hypothesis suggests that environmental exposure, genetic susceptibility, and/or age-related changes of a biological process are essential for cerebral/neuronal dysfunction contributing to AD (Henderson et al., 1988; Suresh et al., 2023). Environmental exposure to metals may induce AD or dementia through one of the two mechanisms highlighted in the above-mentioned hypotheses.

Environmental toxicants, especially heavy metals including arsenic (As), manganese (Mn), lead (Pb), and cadmium (Cd), are widespread pollutants that accumulate in various ecosystems due to human activities such as industrialization, mining, agricultural practices, and urbanization (Angon et al., 2024; Tchounwou et al., 2012). Studies have found that environmental exposure to heavy metals may induce neurotoxicity and possibly influencing the onset of neurodegenerative disorders such as AD and Parkinson's disease (PD) (Bakulski et al., 2020; Chin-Chan et al., 2015; Ijomone et al., 2020; Islam et al., 2022). Prospective associations between neurodegenerative disease (i.e., AD) and elevated exposure to specific metals (such as As, Mn, Pb and Cd) have been found through epidemiological and experimental investigations (Bakulski et al., 2020; Horton et al., 2019; Min and Min, 2016; Spitznagel et al., 2023; Tripathi et al., 2022; Wang et al., 2021; Zhang et al., 2021; Zhou et al., 2018). For example, a population-based cross-sectional study demonstrated that prolonged As exposure may adversely affect adults' cognitive performance in a dose-dependent manner (Wang et al., 2021). Likewise, Mn, a necessary trace element, has shown neurotoxic effects at high concentrations that may cause damage to dopaminergic neurons and cognitive impairment leading to AD development (Peres et al., 2016; Martin et al., 2008; Martins et al., 2019). A study was conducted among Chinese elders by Tong et al. and revealed that Mn levels were shown to be markedly elevated in the brains of AD patients with dementia in comparison to healthy individuals and suggested that high Mn levels may have a role in the progression of AD as a critical pathogenic factor (Tong et al., 2014). Prolong Pb exposure, historically linked to cognitive deficit in children (Canfield et al., 2005; Neuwirth et al., 2020), and exposure to Pb during childhood has also been associated with an increased risk of developing neurodegenerative disorders, including AD in adults (Reuben, 2018; Sanders et al., 2009). In the context of neurodegeneration, Cd exposure is involved in calcium signaling disruption (Jiang et al., 2015), inducing oxidative stress (Branca et al., 2020), and causing mitochondrial dysfunction (Li et al., 2024) which are intricately associated with AD pathology (Ali et al., 2021; Arruebarrena et al., 2023). A population-based study conducted by Peng and colleagues demonstrated that there is a positive relationship between Cd exposure and mortality from AD (Peng et al., 2017). Similarly, another epidemiological study showed a significant association between blood Cd levels and AD mortality among older adults in the US (Min and Min, 2016).

Despite over century since AD was first identified by German psychiatrist and neuropathologist Dr. Alois Alzheimer in 1906, intricate molecular pathways underlying the pathophysiology of AD remain inadequately understood (Hippius and Neundörfer, 2003; Rostagno, 2022). In particular, the development of AD induced by metal exposures primarily due to As, Mn, Pb and Cd and the underlying mechanisms are yet to be fully understood. Notably, most published reviews have focused on single metal exposure rather than providing comprehensive insights and thorough comparisons of the mechanisms that illustrate the pathways of the development of AD/ADRD (Brown et al., 2019; Chib and Singh, 2022; Chin-Chan et al., 2019; Gong and OʼBryant, 2010; Rahman et al., 2021). Therefore, the aim of this review is to summarize contemporary mechanistic research exploring the links between multiple metals such as As, Mn, Pb, and Cd, exposures and their contribution in AD development using data from the most recent mechanistic studies, i.e., studies conducted in the past 10 years. Through the review of the recent data, we seek to pinpoint common routes, processes and molecular mechanisms by which these metals could affect and/or contribute to the development and onset of AD.

2. Methods

2.1. Search Strategy

Relevant primary research articles, published from 2014 through 2024, were identified using the PubMed Database. The literature search was conducted during August-September 2024 to collect information on the associations of As, Mn, Pb and Cd with Alzheimer’s Disease (AD). We restricted our searches to studies published in peer-reviewed English-language journals, within the last 10 years. The following keywords were used: Arsenic AND Alzheimer’s Disease; Cadmium AND Alzheimer’s Disease; Manganese AND Alzheimer’s Disease; and ("Lead"[Mesh]) AND "Alzheimer Disease"[Mesh]. A total of 391 articles were retrieved and screened using the following inclusion and exclusion criteria (Figure 1 and Figure 2). Selected articles were then retrieved for full-text screening and data extraction.

2.2. Inclusion Criteria

Studies were included if (1) focused on metal exposure and Alzheimer’s Disease (AD) mechanisms, (2) were published in English language within the last 10 years, (3) investigated single metal exposure (As or Cd or Mn or Pb) and AD mechanisms, (4) were only mechanistic study (i.e., only cell and animal models) (Figure 2).

2.3. Exclusion Criterion

Studies were excluded if they (1) did not focus on AD, (2) were non-original articles (e.g., reviews) or abstracts, (3) investigated mix metal exposure and AD mechanisms, (4) were epidemiological studies, in silico studies, (5) were duplicated, editorials, and other non-scientific publications, (6) were older than 10 years or non-mechanistic (Figure 2).

2.4. Data Extraction

The following predetermined variables were extracted from the selected articles (n=46): (1) Bibliographical data (authors name, year of publication), (2) Title, (3) Objective of the study, (4) Design of the study, (5) Exposure, (6) Outcomes, (6) Key findings of the study. A total of 125 unique studies were identified. After reviewing abstracts, full-text articles and their reference sections, 46 articles were finally selected for further analysis for this review (Figure 1).

3. Results

After an extensive literature search and final screening, a total of 46 original research articles were selected for this review. Among the 46 articles, 6 are related to As exposure, 9 to Mn exposure, 21 to Pb exposure and 10 to Cd exposure and AD pathobiology. The results of the selected studies are presented in Table 1, Table 2, Table 3 and Table 4.

3.1. Mechanisms and Pathobiology of AD Development Related to Arsenic Exposure

Arsenic (As) is one of the well-known naturally occurring toxic metalloid prevalent in the Earth's crust. Drinking water and As-contaminated foods are the primary sources of human exposure to environmental As (Rahaman et al., 2021; 2022). Although the molecular mechanism of As-associated pathobiology in AD remain unclear, experimental research and epidemiological data indicate that As produces neurotoxicity and affects memory and cognition, which are the symptoms of the AD patients (Rahman et al., 2021; Tyler and Allan, 2014). Exposure to environmental As and its metabolites triggers several pathogenic events, including oxidative damage, inflammation, mitochondrial dysfunction, ER stress, apoptosis, compromised protein homeostasis, and aberrant calcium signaling. Research has shown that these changes align with the majority of AD's pathological, biochemical, and clinical manifestations (Rahman et al., 2021).

The review provides updated mechanistic insights into the impact of arsenic exposure on Alzheimer's disease (AD) development and pathology, based on six recent studies. Of these, three were conducted using mice model (Tripathi et al., 2022; Pakzad et al., 2021; Niño et al., 2018a), two were conducted on Rats (Niño et al., 2021; Niño et al., 2018b) and one study was carried out using SH-SY5Y Cells (Wisessaowapak et al, 2021). The key findings from these mechanistic studies are grouped into three thematic categories, which are discussed below and summarized in Table 1.

3.1.1. Insights from Mouse Models of Arsenic Exposure

Arsenic and Bioenergetic Dysfunction in Alzheimer’s Disease

Bioenergetic dysfunction is a hallmark in Alzhemer’s disease (AD) and play crucial role in its pathogenesis. Employing the 3xTg-AD mouse, a well-established and widely used model for AD, Nino et al., demonstrated that chronic arsenic exposure (3 ppm) from gestation to 6 months disrupts mitochondrial bioenergetics. Mitochondria isolated from the hippocampus of these mice exhibit reduced ATP production, impaired respiratory chain activity, and increased reactive oxygen species (ROS) generation. Immunohistochemical analysis further showed increased amyloid plaques and tau phosphorylation, two characteristic features of AD pathology. Behavioral studies revealed spatial memory deficits and disruption of circadian rhythms, indicating that arsenic accelerates neurodegenerative processes through mitochondrial dysfunction and exacerbating oxidative stress, both of which are central to AD pathology (Niño et al., 2018a).

Arsenic and Tau Phosphorylation: A Key Marker of AD

The increase level of hyperphosphorylated tau is another common feature of Alzheimer's disease. Using male mice, Pakzad et al. explored the effects of different dosages (1 and 10 ppm) of arsenic trioxide exposure on tau protein phosphorylation, specifically at the Ser262 site, which plays a critical role in tauopathies. The researchers found that after three months of exposure to arsenic at 10 ppm, significant increases in tau phosphorylation at Ser262 were observed. Interestingly, low-dose exposure (1 ppm) did not cause significant changes, suggesting a dose-dependent effect of arsenic on tau hyperphosphorylation. The increased tau phosphorylation observed in the high-exposure group parallels key features of AD and tauopathies, reinforcing the idea that arsenic exposure could contribute to the risk of developing neurodegenerative disorders through tau-related mechanisms (Pakzad et al., 2021).

Arsenic-Induced Nitric Oxide Dysregulation and Neurotoxicity

Beyond mitochondrial dysfunction and tau pathology, another critical signaling pathway affected by arsenic is nitric oxide (NO) signaling. Nitric oxide is involved in a variety of cellular processes, including the regulation of mitochondrial function and apoptosis. In an elegant study, Tripathi et al. revealed that arsenic-exposed mice exhibited significant alterations in S-nitrosylation (SNO) signaling pathways, which regulate mitochondrial respiration, antioxidant defense, and apoptosis. These mice also displayed cognitive dysfunctions similar to those observed in autism spectrum disorder (ASD) and AD models. Furthermore, SNO-enrichment analysis revealed disrupted processes involving mitochondrial function, cytoskeleton maintenance, and transcriptional regulation, all of which are critical for neuronal health and survival. These findings underscore the potential role of NO and SNO signaling in arsenic-mediated neurotoxicity and highlight the convergence of arsenic exposure effects with the genetic mutations associated with ASD and AD (Tripathi et al., 2022).

3.1.2. Findings from Rat Models of Arsenic Exposure

Neurotoxicity and Amyloid-β Production, Synaptic and Cortical Changes

Chronic arsenic exposure is increasingly recognized as a potential environmental risk factor for neurodegenerative diseases such as AD. Several studies have investigated its impact on various cellular and molecular processes associated with neurodegeneration. One study by Niño et al. (2018) investigated the effects of arsenic exposure on amyloid-β (Aβ) production and clearance in Wistar rats. Rats were exposed to 3 ppm of arsenic from gestation to four months of age, resulting in behavioral deficits and a marked increase in cleaved Aβ(1–42) levels in brain lysates, a key pathological feature of AD. This was accompanied by elevated receptor for advanced glycation end products (RAGE) and increased β-secretase (BACE1) activity, suggesting that arsenic exposure may disrupt Aβ processing and contribute to amyloid accumulation in the brain (Niño et al., 2018b).

Building on these findings, a separate study by Niño et al. (2021) examined the effects of chronic arsenic exposure on cortical synaptic connectivity and structure in both Wistar rats and a triple-transgenic AD mouse model. Chronic exposure to 3 ppm sodium arsenite from gestation through postnatal development led to changes in cortical microstructure, as indicated by diffusion-weighted imaging showing altered apparent diffusion coefficient (ADC) and fractional anisotropy (FA) values. Furthermore, the analysis of synaptic markers revealed decreased synaptic connectivity, with abnormal dendritic spine morphology and increased spine density observed at younger ages. These structural changes in the cortex and synaptic regions may underlie the cognitive impairments seen with arsenic exposure and could potentially progress to neurodegeneration in older animals (Niño et al., 2021).

3.1.3. Mechanistic Insights from Cell Line: Tau Hyperphosphorylation and Aggregation

In a recent study, Wisessaowapak et al. (2021) explored the effects of long arsenic exposure on tau phosphorylation in differentiated SH-SY5Y neuroblastoma cells, which may contribute to Alzheimer's disease (AD) pathogenesis. They showed that low micromolar concentrations of sodium arsenite increased tau phosphorylation at specific residues (pS202), while decreasing dephosphorylated tau (Tau 1). Arsenic exposure activated two key kinases: GSK3β and ERK1/2. Inhibition of these kinases using specific inhibitors (BIO, SB216763, lithium, and U0126) reversed the effects of arsenic on tau phosphorylation. Additionally, arsenic exposure increased tau aggregation and promoted the redistribution of tau from the membrane fraction to the cytosol, where neurofibrillary tangles may form. This study suggests that arsenic-induced tau hyperphosphorylation, mediated by GSK3β and ERK1/2 activation, could contribute to the development of sporadic AD (Wisessaowapak et al., 2021).

Collectively, the recent mechanistic studies mentioned above revealed the deleterious impact of arsenic exposure on the biological processes of the nervous system, leading to cognitive impairments resembling those observed in AD.

3.2. Mechanisms and Pathobiology of AD Development Related to Manganese Exposure

Manganese (Mn) is one of the essential trace metals primarily obtained through dietary sources. It plays a vital role in enzyme function, bone formation, carbohydrate metabolism, and antioxidant activity. While Mn is necessary in trace amounts, inhaling high concentrations of the metal can cause Mn buildup in the brain and manganism, an illness that resembles parkinsonian disease. Mn accumulation can be occurred in the brain through dietary intake as it crosses the Blood Brain Barrier (BBB) as well as through inhaled manganese, absorbed by the olfactory transport system. Animal studies have established that excessive Mn exposure can cause oxidative stress, impaired MnSOD activity, and contribute to AD pathology including Aβ accumulation and tau phosphorylation (Bakulski et al., 2020; Rizor et al., 2021). Manganism is a unique neurological disease linked to extremely high manganese exposure. The effect of Mn exposure on the nervous system are influenced by the levels of other essential metals.

In this review, we summarized findings from nine mechanistic studies that highlight the pathobiology and disease development of AD related to Mn exposure. Five of these studies examined the pathobiology of AD using cell lines (Xu et al., 2021; Rizor et al., 2021; Pajarillo et al., 2020; Lu et al., 2018; Kirkley et al., 2017), while the remaining four studies investigated the disease mechanism using both mouse models and cell lines exposed to Mn (Fan et al. 2024; Spitznagel et al., 2023; Pajarillo et al., 2022; Yang et al., 2021). The key findings and details of the nine mechanistic studies related to Mn exposure in AD development mechanisms are discussed below and summarized in Table 2.

3.2.1. Cell line and Manganese Exposure

Drp1 Role in Neuroprotection and Mn-Induced Toxicity

Dynamin-related protein 1 (Drp1) is crucial for mitochondrial dynamics and fission. Partial inhibition of Drp1 is protective in neurodegenerative disorders like Parkinson’s (PD) and Alzheimer’s (AD), primarily by improving mitochondrial function. However, studies reveal that Drp1 inhibition also reduces protein aggregation, implicating autophagy. Dose-response studies in HeLa and N27 rat immortalized dopamine neurons showed that Mn impairs autophagy flux without affecting mitochondrial function. Autophagy dysregulation was observed in dopamine neurons, but not in GABA neurons of low Mn-treated mouse. Drp1 knockdown or partial inhibition mitigated Mn-induced autophagy impairment and reduced α-synuclein aggregation, suggesting Drp1’s protective effects extend beyond mitochondrial fission. Drp1 inhibition improves autophagy flux, independent of its mitochondrial role, offering potential therapeutic insights for neurodegenerative diseases linked to autophagy impairment (Fan et al, 2024).

Impact of Mn Exposure on Autophagy

Using the mouse N2a blastoma cell line, Xu et al. revealed that Mn levels above 500 μmol/L caused cell damage and oxidative stress by downregulating PP2Ac methylation, a key regulator of autophagy. This led to mTORC1 activation and autophagy dysfunction. Interventions with the PPME-1 inhibitor ABL-127 and LCMT1 overexpression restored PP2Ac methylation, ameliorating autophagy dysfunction, oxidative stress, and cytotoxicity, suggesting that enhancing PP2Ac methylation offers a protective strategy against Mn-induced neurotoxicity (Xu et al., 2021).

Mn exposure Impairs Astrocytic Glutamate Transporter, EAAT2

Chronic manganese (Mn) exposure impairs the astrocytic glutamate transporter EAAT2, contributing to neurodegenerative diseases like Parkinson's and Alzheimer's. Rizor et al. showed that when the H4 human astrocyte cell line is exposed to pathologically relevant concentrations of Mn (250 μM), it induces oxidative stress and TNF-α production, leading to the activation of the IKK-β kinase. This triggers NF-κB p65 translocation, increasing transcription factor Ying Yang (YY1) levels and repressing EAAT2 (Rizor et al., 2021).

REST Protects Dopaminergic Neurons Against Manganese-Induced Neurotoxicity

Chronic Mn exposure causes dopaminergic dysfunction and neurodegeneration that result in disorders like manganism and AD. In a recent study, Pajarillo et al., demonstrated that the transcription factor REST represses Mn-induced toxicity in dopaminergic neurons through a concurrent induction of the expression of tyrosine hydroxylase, the enzyme that catalyzes dopamine synthesis. REST binds to the promoter of TH, recruits the epigenetic modifiers CBP/p300, and thereby increases TH transcription, mRNA, and protein. Moreover, REST reduced Mn-induced oxidative stress, inflammation, and apoptosis by reducing TNF-α, IL-1β, IL-6, IFN-γ, the proapoptotic proteins Bax and Daxx, while promoting antioxidant proteins such as catalase, Nrf2, and HO-1. These results pointed to the potential therapeutic role of REST in treatment against Mn-induced dopaminergic neurodegeneration (Pajarillo et al., 2020).

Glutamate Excitotoxicity in Mn-Induced Neurotoxicity

Excessive glutamate stress is implicated in conditions like cerebral ischemia, brain trauma, and neurodegenerative diseases such as PD and AD. Disruption of glutamate homeostasis is central to manganese (Mn) neurotoxicity. Astrocytes, which maintain glutamate balance and constitute ~50% of CNS neuronal cells, are highly sensitive to Mn-induced toxicity. Their gap junctions (GJ), primarily formed by connexin43 (Cx43), mediate intercellular communication of small molecules like glutamate. In experiments with cultured astrocytes exposed to Mn (0–1000 μM), Lu et al., recently revealed that Mn exposure reduces astrocyte viability, increases apoptosis, and elevates intracellular and extracellular glutamate levels in a dose-dependent manner. Additionally, Mn impairs gap junction intercellular communication (GJIC) by reducing connexin43 (Cx43) expression and inhibiting gap junction function, disrupting the transfer of signaling molecules like glutamate. These findings suggest that Mn-induced glutamate excitotoxicity is driven by GJ dysfunction and reduced Cx43 expression (Lu et al., 2018).

Role of Mn Exposure and NF-κB in Neuroinflammation

Microglia are the immune cells of the central nervous system (CNS). They become activated in response to injury, infection, or neurological diseases, including Alzheimer's disease (AD), often serving as an early indicator of neurological distress—a process known as microgliosis. Microgliosis is often followed by astrogliosis—reactive responses of astrocytes (another type of glial cell)—both of which contribute to neuroinflammation and exacerbate neuronal damage. While both microgliosis and astrogliosis occur in Alzheimer's disease and manganese-induced neurotoxicity, how these glial populations interact with each other is still poorly understood. Kirkly et al. showed that Mn exposure caused dose-dependent increases in pro-inflammatory gene expression and morphological changes in microglia, leading to the release of cytokines and chemokines. Conditioned media from Mn-activated microglia amplified inflammatory responses in astrocytes by increasing mRNA levels of Tnf, IL-1β, IL-6, Ccl2, and Ccl5. Blocking NF-κB inhibited astrocyte activation, while TNF knockdown partially reduced inflammation, highlighting TNF’s role in microglia-astrocyte crosstalk. The findings underscore NF-κB as a key regulator of neuroinflammation and a potential target for reducing astrocytic inflammatory responses and neurodegeneration (Kirkly et al., 2017).

3.2.2. Mouse Model and/or cell Line Manganese Treatment

Manganese-Induced Toxicity Impairs Glutamatergic Signaling

Manganese (Mn) induced neurotoxicity and Alzheimer’s disease (AD) exhibit dysregulated glutamatergic signaling. Mn toxicity may impair glutamate clearance, disrupt glutamatergic homeostasis, and increase susceptibility to seizures. Indeed, in a recent study, using APP/PSEN1 and wild-type mice, researchers combined in vitro and in vivo approaches to assess astrocytic glutamate clearance and behavioral outcomes following acute Mn exposure. Results show that even limited Mn exposure accumulates in brain tissue, disrupts glutamate clearance in cortical astrocytes, and heightens seizure susceptibility without affecting hippocampal long-term potentiation (LTP). These effects occur in young adult mice before significant AD-related pathology, suggesting glutamate dysregulation as an early AD feature and Mn exposure as a potential amplifier of genetic risk (Spitznagel et al., 2023).

Astrocytic REST (Repressor Element-1 Silencing Transcription Factor) Role in Mn-Induced Neurotoxicity

Manganese (Mn) accumulates in the brain, especially in brain regions such as the striatum and globus pallidus, through disturbing mitochondrial function, oxidative stress, inflammation, and excitotoxicity—common neurodegenerative pathways observed in Alzheimer's disease and Parkinson's disease. REST, a zinc-finger transcription factor, regulates neuronal and astrocytic gene expression, including glutamate transporters like EAAT2, which protect neurons from excitotoxicity. While neuronal REST’s protective role in AD and PD is well-documented, the role of astrocytic REST remains underexplored. Studies show that astrocytic REST deletion exacerbates PD-related dopaminergic neuronal loss and increases inflammation. In Mn-exposed mice, REST deletion in the striatum led to worsened dopaminergic dysfunction, motor deficits, cognitive impairments, inflammation, and reduced glutamate transporter GLT-1. These findings highlight REST’s critical role in mitigating Mn-induced neurotoxicity and glutamate dysregulation, emphasizing astrocytic REST as a potential therapeutic target for Mn-related neurodegeneration and broader neurodegenerative disorders. The striatum was chosen for this study due to its vulnerability to Mn accumulation, excitotoxic lesions, and dopaminergic degeneration, providing insights into region-specific astrocytic REST functions (Pajarillo et al., 2022).

Mn-Induced Dysregulation Amyloid Precursor Protein (APP) Processing and Cognitive Impairment

Excessive manganese (Mn) exposure causes cognitive deficits similar to Alzheimer’s disease (AD) by affecting amyloid precursor protein (APP) and its processing. APP, critical in AD pathology, undergoes cleavage by α- and β-secretases, influencing synaptic function. In Mn-exposed mice and Neuro-2a cells, APP, α-secretase (ADAM10), and soluble APP alpha (sAPPα) levels decreased, along with synaptic protein expression and α-secretase activity. However, β-secretase, Aβ peptides, and β-secretase activity were unaffected. Mn exposure alters non-amyloidogenic APP processing, impairing cognition and synaptic function, linking APP dysregulation to cognitive decline (Yang et al. 2021).

3.3. Mechanisms and Pathobiology of AD Development Related to Lead Exposure

Lead (Pb) exposure-related toxicity was mentioned surprisingly as early as in 370 BC with major sources including old plumbing, paints in the old house, industrial applications, Pb smelters, waste incinerators, automobile Pb-acid batteries (Bakulski et al., 2020). Approximately 1% of global disease burden is associated with Pd exposure, which also has lasting effects on behavioral issues and children's IQ (WHO, 2010; Grandjean, and Bellanger, 2017). Although Pb exposure is particularly more devastating on child health and development, it also contributes to various disease development in older adults including amyotrophic lateral sclerosis (ALS), Parkinson’s disease (PD), hearing loss, age-related cataracts, glaucoma, and other chronic conditions (Bakulski et al., 2020). Environmental Pb exposure can occur through ingestion, inhalation and endogenous sources, with the majority of exposure happening via inhalation or ingestion. Once in the body, Pb is absorbed into cells and tissues, circulating in the bloodstream. Pb can cross both the placental and blood-brain-barrier (BBB), accumulating in the brain due to low excretion rate. Pb, a well-established neurotoxicant, causes oxidative stress by depleting thiols and disrupting the antioxidant defense system. The primary neurotoxic mechanisms of the Pb exposure include excessive oxidative stress, triggering the endoplasmic reticulum stress, mitochondrial disfunction and neuronal cell death it also creates neuroinflammations, excitotoxicity and disruption of the essential metal homeostasis in the brain (Bakulski et al., 2020).

Animal models (i.e., mouse, rats, and zebrafish) treated with lead exhibit the mechanisms and symptoms of AD. While consistent AD-related deficits are documented, the effects of Pb exposure differ by species, timing, dose, and length of exposure. The current review discusses twenty-one mechanistic studies on Pb exposure and AD pathobiology. Among these, seven studies employed different cell lines, ten used mice as models and/or different cell lines, two used zebrafish, and two used rats as a model animal to investigate the link between environmental Pb exposure and AD pathobiology. Table 3 summarizes the main findings of these twenty-one mechanistic investigations.

3.3.1. Mechanism of Pb-Induced AD Pathology in Mice/Cell Line

Mice serve as the predominant animal model for investigating the effects of Pb exposure on the brain, owing to the availability of transgenic mice predisposed to AD. A study by Wang et al. (2022) demonstrated that miR-124-3p/BACE1 pathway modulation is critically involved in Pb-induced AD-like amyloidogenic processing in Pb-exposed mice. Pb exposure contribute to the development of AD through several mechanism in experimental mouse model, including the elevation of tau protein and mRNA levels in aged mice (Bihaqi et al., 2014), reprogramming the expression of epigenetic intermediates involved in DNA methylation or histone modification that in turn regulate latent AD-related gene expression (Eid et al., 2016), early-life microglial damage and Amyloid accumulation (vonderEmbse et al., 2017), changes in miRNA expression targeting AD-related proteins (Masoud et al., 2016), elevated level of phosphorylated tau protein (p-tau), abnormal changes in BBB junction proteins, imbalanced the production and clearance abilities of Aβ (Wu et al., 2020). Pb exposure also increases permeability surface area product, affects brain perfusion and damage the blood brain barrier system (Gu et al., 2020), induces histone modification and DNA methylation (Eid et al., 2018), increases α-Synuclein, GSK-3β, Caspase-3 and tau hyperphosphorylation (Bihaqi et al., 2018), causes mitochondrial copper overload due to COX17-mediated translocation, mitochondrial dysfunction, excessive mtROS accumulation, microglia activation (Huang et al., 2024).

3.3.2. Mechanistic Insights from cell Line Studies

Researchers use several specific cell lines (i.e., SH-SY5Y, iPSCs, BV-2 microglial cells) in laboratory studies to model the Alzheimer's disease pathology, test potential treatments, and investigate the role of amyloid beta plaques, tau protein abnormalities, and other key features of AD in a controlled environment. These cell experiments are essential to understand the molecular mechanisms at the cellular level. In this review, we highlight seven studies that used different cell lines to investigate the AD pathobiology and molecular mechanism due to environmental Pb exposure. The reported molecular mechanisms of Pb exposure-related AD are inhibited APP translation and disruption of iron homeostasis (Rogers et al., 2016), PINK1/Parkin mediated mitophagy and dysfunctional mitochondria mediated apoptosis (Bandaru et al., 2022), altered calcium homeostasis, synaptic plasticity, elevated AD-like pathogenesis markers, including phosphorylated tau, tau aggregates, and Aβ42/40 (Xie et al., 2023), mitochondrial dysfunction and oxidative stress (Bandaru et al., 2023), mitochondrial dysfunction and excessive mtROS accumulation (Huang et al., 2024), oxidative stress and apoptosis (Ayyalasomayajula et al., 2019), oxidative stress mediated DNA damage, decrease in total antioxidant capacity and involvement of CDK5-p25 signaling (Lokesh et al., 2024).

3.3.3. Insights from Rat Models of Lead Exposure

The laboratory rat has been valuable model for brain research in disease model over the years. This review identifies two studies that examined the pathobiology of AD in relation to environmental Pb exposure using rat model. Zhou et al. (2018) demonstrated that Pb exposure contributed to early AD-like pathology in young growing rats by disturbing brain cholesterol metabolism, increasing Aβ accumulation and amyloid plaque deposition. Pb exposure impaired cholesterol homeostasis, decreased brain cholesterol levels, activated the SREBP2-BACE1 pathway, decreased HMG-CR and LDL-R expression and promoted the expression of LXR-α and ABCA1 (Zhou et al., 2018). In contrast, Liu et al. (2023) showed that chronic Pb exposure increased the affinity of Aβ40 to cerebral vasculature, intensified Aβ40 buildup and impaired LRP1 expression in both the brain parenchyma and vasculature (Liu et al., 2023).

3.3.4. Findings from Zebrafish Models of Lead Exposure

Zebrafish model shows potential for studying neurodegenerative disease mechanisms like AD. Embryonic exposure to Pb at levels as low as 10 µg/L disturbed global gene expression patterns in a sex-specific manner, leading to neurological alterations later in life. Pb exposure during embryogenesis in zebrafish led to neurodegenerative gene expression changes (Lee and Freeman, 2016). In a study, examining the 12-month-old adult female and male zebrafish brain, exposed to either a control (0 μg/L) or 10 μg/L Pb only during embryogenesis (1-72 hours post-fertilization), gene ontology and pathway analysis demonstrated that both sexes had similar upper disease and functional categories, but female zebrafish exhibited 4.3 times more genetic alterations. Genes linked to the development and function of the nervous system were particularly more affected; adult females were found to have altered versions of 89 genes linked to AD, including sortlin-related receptor precursor (SORL1), apolipoprotein (APOE), and amyloid precursor protein (APP) (Lee and Freeman, 2016). Another study showed that environmental Pb exposure These CNAs are linked to AD outcomes with most genes are interconnected within a molecular network with amyloid precursor protein (APP), a crucial molecular target linked to the etiology of AD (Lee and Freeman, 2020).

3.4. Mechanisms and Pathobiology of AD Development Related to Cadmium Exposure

Cadmium (Cd) is a naturally occurring heavy metal that is bluish white in color and persistent in nature. The earth’s crust serves as one of the natural sources of Cd, while anthropogenic sources include mining, refining, fossil fuel combustion, waste incineration, and other industrial activities such as manufacturing phosphate fertilizers (ATSDR, 2012). Cd is not an essential element; therefore, it has no physiological function in humans. However, according to the International Agency for Research on Cancer (IARC), Cd is classified as a Group-I carcinogen (cancer causing agent) and has numerous detrimental health effects upon exposure (Straif et al., 2009). Most cases of human exposure to environmental Cd through contaminated food, making diet the primary source of Cd exposure (Satarug et al., 2010). Additionally, People are exposed to Cd through inhalation, particularly by inhaling cigarette smoke which is another significant route of exposure. Chronic environmental Cd exposure increases the risks of various human diseases including kidney damage, high blood pressure, diabetes, decreased pulmonary function and osteoporosis (Satarug et al., 2010). Recently, Cd also has emerged as a neurotoxicant. Inhaled Cd can enter the brain through the olfactory bulb, and the blood cerebrospinal fluid barrier. Mechanistic studies have demonstrated that Cd exposure triggers oxidative stress, alters the permeability of the BBB, causes Aβ aggregation, produces tau neurofibrillary tangles, induces neuroinflammation and leads to apoptotic neuronal cell death (Bakulski et al., 2020).

This review delineates ten mechanistic laboratory studies concerning environmental Cd exposure and AD pathobiology. Among these ten studies, two utilized cell lines (SN56 and Neuro-2a cells), six employed mouse models, one was an experimental study, and one used a rat model to examine the association between environmental Cd exposure and AD pathobiology. Table 4 presents the primary findings and details of these ten mechanistic studies regarding the influence of Cd exposure on the pathways of AD development. Mouse models and cell culture treated with Cd exhibit mechanisms and symptoms of AD. However, the degree of influence of Cd exposure depends on several factors, including species, timing, dose, and length of exposure.

3.4.1. Insights from Mouse Models of Cadmium Exposure

Mouse models have shown promise in the neurological research field and providing insights into the molecular mechanisms underlying environmental Cd exposure and AD pathobiology. While oxidative stress, neuroinflammation, and apoptotic neuronal cell death are well-established molecular pathways involved in cadmium (Cd) exposure and AD pathobiology, emerging mechanisms continue to be identified over time. This current review identifies six mouse model studies related to environmental Cd exposure and its contribution in AD pathways. Among these studies, one explored the effects of low-dose environmental Cd exposure on AD progression and underlying mechanism using wild type C57BL/6J and APP/PS1 double transgenic mice. The findings are noteworthy as both genotypic (C57BL/6J and APP/PS1) exhibited similar Cd levels in the blood after exposure to the same Cd dose, yet the toxic effects differed in genotypes. Environmental Cd exposure exerts more severe damage in APP/PS1 mice compared to C57BL/6J mice, leading to increased anxiety-like behavior, chaotic movement, spatial reference memory damage, Aβ plaque deposition and microglial activation in the brain, as well as increased expression of IL-6 in the cortex and serum. These results indicate that low-dose environmental Cd exposure contributes to AD progression through mechanisms, including BBB disruption, increased Aβ production, reduced Aβ clearance and increased inflammatory responses (Liu, et al., 2023). Other potential molecular mechanisms of Cd exposure-induced AD progression include genotype- and gender-specific gut dysbiosis, increased microbial AD biomarkers, reduced energy supply-related pathways in gut and blood, and enhanced hepatic pathways involved in inflammation and xenobiotic biotransformation (Zhang et al., 2021).

In another study, Wang et al. (2022) demonstrated that Cd dysregulated brain and liver drug-processing genes in a sex- and ApoE-genotype specific manner, contributing to variations in susceptibility to Cd neurotoxicity. Proinflammatory genes were enriched in Cd-exposed ApoE4 males' livers while Cd up-regulated Cyp2j isoforms in the brains of ApoE3 mice. Dysregulation of cation transporters was male-specific in the brains. Matsushita and colleagues investigated the genetic and conditional stimulation of adult neurogenesis and its rescue role in Cd-induced cognitive impairment in ApoE4-KI mice (Matsushita et al., 2023). They showed that specific and conditional stimulation of adult neurogenesis rescued Cd-induced impairments in hippocampus-dependent short-term spatial memory in a gene-environment interactions (GxE) model of ApoE4 and Cd exposure, demonstrating a direct link between memory impairment and adult neurogenesis in the GxE mice model (Matsushita et al., 2023).

Similarly, Zhang et al. (2020) revealed that a GxE between ApoE4 and Cd exposure accelerates cognitive impairment with impaired adult hippocampal neurogenesis identified as potential mechanism. Furthermore, male mice were more susceptible than female mice to the GxE effect during youth (Zhang et al., 2020). Qian et al. (2024) reported that Cd promotes neural senescence by activating the p53/p21/Rb pathway. Their findings revealed that SigmaR1 depletion contributed to neural senescence through Ca2+ dyshomeostasis and mitochondrial dysfunction. This mouse model-combined with SHSY5Y cell culture studies revealed a novel senescence-associated regulatory route for the SEL1L/HRD1/SigmaR1 axis that affects the pathological progression of Cd exposure-associated AD. The study concluded that SigmaR1 functions as a neuroprotective biomarker of neuronal senescence, and pharmacological activation of SigmaR1 could serve as a promising therapeutic strategy for AD.

3.4.2. Mechanistic Insights from Cell Line of Cadmium Exposure

Cell culture studies are crucial for elucidating the molecular mechanism and cellular pathways associated with environmental risk factors and AD pathobiology. Several cell lines, including AD Pathophysiology Model (i.e., induced pluripotent stem cells (iPSCs), SH-SY5Y, PC12, and primary neuronal cultures) are commonly used as models of AD pathophysiology to study the molecular mechanism of AD progression and the contribution of the environmental factors. In this review, two in vitro studies using SN56 and Neuro-2a cells were identified for review and thoroughly discussed. Deng et al. (2024) carried out an in vitro study using mouse neuroblastoma cells (Neuro-2a cells) to investigate the effect of autophagy on environmental Cd-induced AD progression and the underlying molecular mechanisms. Their findings revealed that Cd exposure disrupted autophagosome-lysosome fusion and impaired lysosomal function, leading to defects in autophagic clearance followed by APP accumulation and nerve cell death (Deng et al., 2024). Their study also demonstrated that SIRT5 is an essential molecular target in Cd-impaired autophagic flux. Mechanistically, Cd exposure reduced SIRT5 expression, increasing the succinylation of RAB7A at lysine 31 and inhibiting RAB7A activity, which contributed to autophagic flux blockade. This indicates that SIRT5-catalysed RAB7A desuccinylation is an essential adaptive mechanism for the amelioration of Cd-induced autophagic flux blockade and AD-like pathogenesis (Deng et al., 2024). Del Pino et al. (2016) investigated the mechanism of Cd exposure-induced basal forebrain cholinergic neurons cell death. They showed that Cd induces cell death on cholinergic neurons through the blockade of M1 receptor, overexpression of AChE-S and GSK-3β, down-regulation of AChE-R and increase in Aβ and total and phosphorylated tau protein levels (Del Pino et al., 2016).

3.4.3. Findings from Rat Models of Cadmium Exposure

Arab et al. (2023) investigated the Cd-induced neurotoxicity mechanisms and examined the potential neuroprotective role of topiramate against Cd-induced cognitive deficits in rats, with an emphasis on hippocampal oxidative insult, apoptosis, and autophagy. They found that Cd exposure triggered spatial learning/retention memory impairments, deterioration of the recognition memory, increased hippocampal neurodegeneration signals (elevated hippocampal levels of Aβ42 and p-tau), augmented hippocampal glutamate content, and altered the SIRT1/Nrf2/HO-1 axis and AMPK/mTOR signaling pathway (Arab et al., 2023).

4. Discussion

The growing body of research into environmental toxins has highlighted the significant role that metals such as arsenic (As), manganese (Mn), lead (Pb), and cadmium (Cd) play in the development and progression of neurodegenerative diseases, particularly Alzheimer's disease (AD). These toxicants induce a range of pathological processes that intersect with well-known mechanisms of AD, such as oxidative stress, mitochondrial dysfunction, protein aggregation, and neuroinflammation. The studies reviewed here provide a deeper understanding of how these elements contribute to AD and suggest potential pathways for therapeutic intervention.

Arsenic exposure has long been recognized for its carcinogenic properties, but emerging evidence has also linked it to neurodegenerative diseases like AD. Chronic exposure to arsenic in animal models results in mitochondrial dysfunction, tau hyperphosphorylation, and amyloid-beta (Aβ) accumulation—key features of AD pathology. These effects are coupled with disruptions in memory, circadian rhythms, and neuroinflammation, all of which are hallmarks of neurodegenerative conditions. Mechanistically, arsenic has been shown to interact with critical protein kinases such as GSK3β and ERK1/2, leading to tau aggregation and the exacerbation of Aβ accumulation. These insights position arsenic as an environmental risk factor that could significantly contribute to the development of AD in exposed populations.

Similarly, manganese toxicity has been implicated in the pathogenesis of neurodegenerative diseases, particularly through its effects on glutamate regulation and neuroinflammation. Manganese exposure leads to the suppression of the glutamate transporter EAAT2, resulting in excitotoxicity and neuroinflammation. This dysregulation contributes to synaptic dysfunction, cognitive deficits, and abnormal amyloid precursor protein (APP) processing. The disruption of autophagy, an essential cellular process for clearing damaged proteins, further exacerbates the pathological effects of manganese. Research also suggests that transcription factors like REST may play a protective role against Mn-induced neurotoxicity by modulating oxidative stress and inflammation, offering potential therapeutic strategies (Figure 3).

Lead and cadmium, while historically associated with other toxicological concerns, have more recently emerged as significant neurotoxicants linked to AD. Lead exposure disrupts calcium homeostasis, synaptic plasticity, and mitochondrial function, processes that are integral to the development of neurodegenerative diseases. Lead’s ability to induce oxidative stress and epigenetic reprogramming further exacerbates its neurotoxic effects. On the other hand, cadmium has been shown to activate the p53/p21/Rb pathway, inducing neural senescence, while also disrupting gut-brain interactions and aggravating neuroinflammation. Both lead and cadmium impair autophagic flux and mitochondrial function, leading to cellular dysfunction and contributing to AD pathology (Figure 3).

These studies underscore the importance of considering both genetic predispositions and environmental exposures in understanding AD. Research using a variety of models, including mouse, rat, and zebrafish models, has significantly advanced our understanding of how environmental toxins contribute to AD. These models offer insights into sex-specific differences, genotype-environment interactions, and the molecular pathways through which these toxins exert their effects. However, several gaps remain in our understanding of the chronic, low-level exposure scenarios that are more typical of real-world environments. Moreover, the variability in genetic and environmental contexts across studies complicates efforts to generalize findings.

5. Conclusions and Future Directions

The evidence presented here clearly points to arsenic, manganese, lead, and cadmium as critical environmental risk factors for the development and progression of Alzheimer's disease. These metals disrupt essential brain processes such as oxidative stress regulation, mitochondrial function, and autophagy, all of which contribute to neurodegeneration. Arsenic’s impact on tau phosphorylation and amyloid-beta accumulation, manganese’s role in glutamate dysregulation and excitotoxicity, and the effects of lead and cadmium on mitochondrial bioenergetics and cellular senescence highlight the intricate molecular mechanisms through which these toxicants contribute to AD.

While animal and cell line studies have provided invaluable insights into the molecular and cellular underpinnings of metal-induced neurotoxicity, further research is needed to address existing gaps. A more comprehensive understanding of the cumulative effects of chronic, low-level exposure to these toxicants, particularly in the context of genetic susceptibility, is essential for developing targeted therapeutic strategies. The complexity of Alzheimer’s disease necessitates a holistic approach that integrates environmental, genetic, and epigenetic factors.

5.1. Future Directions

Future research should prioritize several key areas to enhance our understanding of the environmental contributions to Alzheimer’s disease and identify potential interventions.

Longitudinal and Epidemiological Studies: Large-scale studies that assess the cumulative effects of arsenic, manganese, lead, and cadmium exposure on neurodegeneration are essential. These studies should be designed to track exposure levels over time and examine their correlation with cognitive decline and AD incidence. Epidemiological data will provide critical insights into the dose-dependent effects of these metals and their role in the progression of AD in human populations.

Mechanistic Studies: Further research is needed to explore the interaction between environmental exposures and genetic susceptibility to AD. For example, studies could investigate how genetic risk factors such as APOE4 interact with metal-induced oxidative stress, inflammation, and mitochondrial dysfunction. Understanding these interactions could reveal novel biomarkers for early diagnosis and targeted therapeutic interventions.

Therapeutic Development: Targeting the molecular pathways affected by arsenic, manganese, lead, and cadmium exposure could lead to the development of new therapeutic strategies for AD. Inhibitors of kinases like GSK3β and ERK1/2, which are involved in tau phosphorylation and aggregation, could be explored as potential treatments. Additionally, enhancing the function of transcription factors like REST, which may protect against metal-induced neurotoxicity, could offer a novel therapeutic approach.

Intervention Studies: Investigating the protective effects of antioxidants, chelating agents, and epigenetic modulators may provide strategies to counteract the neurotoxic effects of these metals. Compounds that restore glutamate transporter function or enhance mitochondrial resilience could mitigate excitotoxicity and bioenergetic deficits, potentially providing therapeutic benefits.

Environmental Policy and Public Health: Translating research findings into actionable policies is crucial for mitigating the impact of environmental toxins on public health. Efforts to regulate arsenic, manganese, lead, and cadmium levels in drinking water and food sources, coupled with public education campaigns, could reduce exposure and the associated risks of neurodegenerative diseases. Stronger environmental regulations and improved waste management practices are key to preventing further contamination.

Advanced Models: The use of organoid models and humanized systems will improve the translational relevance of findings. These models can better replicate human-specific aspects of AD pathology and allow for high-throughput screening of potential interventions. By advancing animal models that more closely mimic human AD, we can enhance the ability to translate preclinical findings into clinical applications.

In conclusion, addressing the environmental contributions to Alzheimer’s disease, particularly from arsenic, manganese, lead, and cadmium exposure, is essential for mitigating the growing public health burden of neurodegenerative diseases. By integrating mechanistic studies, longitudinal research, and public health initiatives, we can improve our understanding of these environmental risk factors and develop more effective strategies for prevention and treatment.