Trichloroethylene and its metabolite TaClo lead to degeneration of substantia nigra dopaminergic neurones: Effects in wild type and human A30P mutant α-synuclein mice

Dry-Cleaning Chemicals and a Cluster of Parkinson's Disease and Cancer: A Retrospective Investigation

BACKGROUND

Environmental exposure to trichloroethylene (TCE), a carcinogenic dry-cleaning chemical, may be linked to Parkinson's disease (PD).

OBJECTIVE

The objective of this study was to determine whether PD and cancer were elevated among attorneys who worked near a contaminated site.

METHODS

We surveyed and evaluated attorneys with possible exposure and assessed a comparison group.

RESULTS

Seventy-nine of 82 attorneys (96.3%; mean [SD] age: 69.5 [11.4] years; 89.9% men) completed at least one phase of the study. For comparison, 75 lawyers (64.9 [10.2] years; 65.3% men) underwent clinical evaluations. Four (5.1%) of them who worked near the polluted site reported PD, more than expected based on age and sex (1.7%; P = 0.01) but not significantly higher than the comparison group (n = 1 [1.3%]; P = 0.37). Fifteen (19.0%), compared to four in the comparison group (5.3%; P = 0.049), had a TCE-related cancer.

CONCLUSIONS

In a retrospective study, diagnoses of PD and TCE-related cancers appeared to be elevated among attorneys who worked next to a contaminated dry-cleaning site. © 2024 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Parkinson's Disease Is Predominantly an Environmental Disease

Parkinson's disease is the world's fastest growing brain disorder, and exposure to environmental toxicants is the principal reason. In this paper, we consider alternative, but unsatisfactory, explanations for its rise, including improved diagnostic skills, aging populations, and genetic causes. We then detail three environmental toxicants that are likely among the main causes of Parkinson's disease- certain pesticides, the solvent trichloroethylene, and air pollution. All three environmental toxicants are ubiquitous, many affect mitochondrial functioning, and all can access humans via various routes, including inhalation and ingestion. We reach the hopeful conclusion that most of Parkinson's disease is thus preventable and that we can help to create a world where Parkinson's disease is increasingly rare.

Low-dose inhalation exposure to trichloroethylene induces dopaminergic neurodegeneration in rodents

Trichloroethylene (TCE) is one of the most pervasive environmental contaminants in the world and is associated with Parkinson disease (PD) risk. Experimental models in rodents show that TCE is selectively toxic to dopaminergic neurons at high doses of ingestion, however, TCE is a highly volatile toxicant, and the primary pathway of human exposure is inhalation. As TCE is a highly lipophilic, volatile organic compound (VOC), inhalation exposure results in rapid diffusion throughout the brain, avoiding first-pass hepatic metabolism that necessitated high doses to recapitulate exposure conditions observed in human populations. We hypothesized that inhalation of TCE would induce significantly more potent neurodegeneration than ingestion and better recapitulate environmental conditions of vapor intrusion or off gassing from liquid TCE. To this end, we developed a novel, whole-body passive exposure inhalation chamber in which we exposed 10-month-old male and female Lewis rats to 50 ppm TCE (time weighted average, TWA) or filtered room air (control) over 8 weeks. In addition, we exposed 12-month-old male and female C57Bl/6 mice to 100 ppm TCE (TWA) or control over 12 weeks. Both rats and mice exposed to chronic TCE inhalation showed significant degeneration of nigrostriatal dopaminergic neurons as well as motor and gait impairments. TCE exposure also induced accumulation of pSer129-αSyn in dopaminergic neurons as well as microglial activation within the substantia nigra of rats. Collectively, these data indicate that TCE inhalation causes highly potent dopaminergic neurodegeneration and recapitulates some of the observed neuropathology associated with PD, providing a future platform for insight into the mechanisms and environmental conditions that influence PD risk from TCE exposure.

Low-dose inhalation exposure to trichloroethylene induces dopaminergic neurodegeneration in rodents

Trichloroethylene (TCE) is one of the most pervasive environmental contaminants in the world and is associated with Parkinson disease (PD) risk. Experimental models in rodents show that TCE is selectively toxic to dopaminergic neurons at high doses of ingestion, however, TCE is a highly volatile toxicant, and the primary pathway of human exposure is inhalation. As TCE is a highly lipophilic, volatile organic contaminant (VOC), inhalation exposure results in rapid diffusion throughout the brain, avoiding first-pass hepatic metabolism that necessitated high doses to recapitulate exposure conditions observed in human populations. We hypothesized that inhalation of TCE would induce significantly more potent neurodegeneration than ingestion and better recapitulate environmental conditions of vapor intrusion or off gassing from liquid TCE. To this end, we developed a novel, whole-body passive exposure inhalation chamber in which we exposed 10-month-old male and female Lewis rats to 50 ppm TCE (time weighted average, TWA) or filtered room air (control) over 8 weeks. In addition, we exposed 12-month-old male and female C57Bl/6 mice to 100 ppm TCE (TWA) or control over 12 weeks. Both rats and mice exposed to chronic TCE inhalation showed significant degeneration of nigrostriatal dopaminergic neurons as well as motor and gait impairments. TCE exposure also induced accumulation of pSer129-αSyn in dopaminergic neurons as well as microglial activation within the substantia nigra of rats. Collectively, these data indicate that TCE inhalation causes highly potent dopaminergic neurodegeneration and recapitulates some of the observed neuropathology associated with PD, providing a future platform for insight into the mechanisms and environmental conditions that influence PD risk from TCE exposure.

Risk of Parkinson Disease Among Service Members at Marine Corps Base Camp Lejeune

Importance

An increased risk of Parkinson disease (PD) has been associated with exposure to the solvent trichloroethylene (TCE), but data are limited. Millions of people in the US and worldwide are exposed to TCE in air, food, and water.

Objective

To test whether the risk of PD is higher in veterans who served at Marine Corps Base Camp Lejeune, whose water supply was contaminated with TCE and other volatile organic compounds (VOCs), compared with veterans who did not serve on that base.

Design, Setting, and Participants

This population-based cohort study examined the risk for PD among all Marines and Navy personnel who resided at Camp Lejeune, North Carolina (contaminated water) (n = 172 128), or Camp Pendleton, California (uncontaminated water) (n = 168 361), for at least 3 months between 1975 and 1985, with follow-up from January 1, 1997, until February 17, 2021. Veterans Health Administration and Medicare databases were searched for International Classification of Diseases diagnostic codes for PD or other forms of parkinsonism and related medications and for diagnostic codes indicative of prodromal disease. Parkinson disease diagnoses were confirmed by medical record review.

Exposures

Water supplies at Camp Lejeune were contaminated with several VOCs. Levels were highest for TCE, with monthly median values greater than 70-fold the permissible amount.

Main Outcome and Measures

Risk of PD in former residents of Camp Lejeune relative to residents of Camp Pendleton. In those without PD or another form of parkinsonism, the risk of being diagnosed with features of prodromal PD were assessed individually and cumulatively using likelihood ratio tests.

Results

Health data were available for 158 122 veterans (46.4%). Demographic characteristics were similar between Camp Lejeune (5.3% women, 94.7% men; mean [SD] attained age of 59.64 [4.43] years; 29.7% Black, 6.0% Hispanic, 67.6% White; and 2.7% other race and ethnicity) and Camp Pendleton (3.8% women, 96.2% men; mean [SD] age, 59.80 [4.62] years; 23.4% Black, 9.4% Hispanic, 71.1% White, and 5.5% other race and ethnicity). A total of 430 veterans had PD, with 279 from Camp Lejeune (prevalence, 0.33%) and 151 from Camp Pendleton (prevalence, 0.21%). In multivariable models, Camp Lejeune veterans had a 70% higher risk of PD (odds ratio, 1.70; 95% CI, 1.39-2.07; P < .001). No excess risk was found for other forms of neurodegenerative parkinsonism. Camp Lejeune veterans also had a significantly increased risk of prodromal PD diagnoses, including tremor, anxiety, and erectile dysfunction, and higher cumulative prodromal risk scores.

Conclusions and Relevance

The study's findings suggest that the risk of PD is higher in persons exposed to TCE and other VOCs in water 4 decades ago. Millions worldwide have been and continue to be exposed to this ubiquitous environmental contaminant.

Cognitive heterogeneity in Parkinson's disease: A mechanistic view

Cognitive impairment occurs in most individuals with Parkinson's disease (PD), exacting a high toll on patients, their caregivers, and the healthcare system. In this review, we begin by summarizing the current clinical landscape surrounding cognition in PD. We then discuss how cognitive impairment and dementia may develop in PD based on the spread of the pathological protein alpha-synuclein (aSyn) from neurons in brainstem regions to those in the cortical regions of the brain responsible for higher cognitive functions, as first proposed in the Braak hypothesis. We appraise the Braak hypothesis from molecular (conformations of aSyn), cell biological (cell-to-cell spread of pathological aSyn), and organ-level (region-to-region spread of aSyn pathology at the whole brain level) viewpoints. Finally, we argue that individual host factors may be the most poorly understood aspect of this pathological process, accounting for substantial heterogeneity in the pattern and pace of cognitive decline in PD.

Mitochondrial Toxicant-Induced Neuronal Apoptosis in Parkinson's Disease: What We Know so Far

Parkinson's disease (PD) is one of the most common progressive neurodegenerative diseases caused by the loss of dopamine-producing neuronal cells in the region of substantia nigra pars compacta of the brain. During biological aging, neuronal cells slowly undergo degeneration, but the rate of cell death increases tremendously under some pathological conditions, leading to irreversible neurodegenerative diseases. By the time symptoms of PD usually appear, more than 50 to 60% of neuronal cells have already been destroyed. PD symptoms often start with tremors, followed by slow movement, stiffness, and postural imbalance. The etiology of PD is still unknown; however, besides genetics, several factors contribute to neurodegenerative disease, including exposure to pesticides, environmental chemicals, solvents, and heavy metals. Postmortem brain tissues of patients with PD show mitochondrial abnormalities, including dysfunction of the electron transport chain. Most chemicals present in our environment have been shown to target the mitochondria; remarkably, patients with PD show a mild deficiency in NADH dehydrogenase activity, signifying a possible link between PD and mitochondrial dysfunction. Inhibition of electron transport complexes generates free radicals that further attack the macromolecules leading to neuropathological conditions. Apart from that, oxidative stress also causes neuroinflammation-mediated neurodegeneration due to the activation of microglial cells. However, the mechanism that causes mitochondrial dysfunction, especially the electron transport chain, in the pathogenesis of PD remains unclear. This review discusses the recent updates and explains the possible mechanisms of mitochondrial toxicant-induced neuroinflammation and neurodegeneration in PD.

Trichloroethylene: An Invisible Cause of Parkinson's Disease?

The etiologies of Parkinson's disease (PD) remain unclear. Some, such as certain genetic mutations and head trauma, are widely known or easily identified. However, these causes or risk factors do not account for the majority of cases. Other, less visible factors must be at play. Among these is a widely used industrial solvent and common environmental contaminant little recognized for its likely role in PD: trichloroethylene (TCE). TCE is a simple, six-atom molecule that can decaffeinate coffee, degrease metal parts, and dry clean clothes. The colorless chemical was first linked to parkinsonism in 1969. Since then, four case studies involving eight individuals have linked occupational exposure to TCE to PD. In addition, a small epidemiological study found that occupational or hobby exposure to the solvent was associated with a 500% increased risk of developing PD. In multiple animal studies, the chemical reproduces the pathological features of PD.Exposure is not confined to those who work with the chemical. TCE pollutes outdoor air, taints groundwater, and contaminates indoor air. The molecule, like radon, evaporates from underlying soil and groundwater and enters homes, workplaces, or schools, often undetected. Despite widespread contamination and increasing industrial, commercial, and military use, clinical investigations of TCE and PD have been limited. Here, through a literature review and seven illustrative cases, we postulate that this ubiquitous chemical is contributing to the global rise of PD and that TCE is one of its invisible and highly preventable causes. Further research is now necessary to examine this hypothesis.

How Well Do Rodent Models of Parkinson's Disease Recapitulate Early Non-Motor Phenotypes? A Systematic Review

The prodromal phase of Parkinson's disease (PD) is characterised by many non-motor symptoms, and these have recently been posited to be predictive of later diagnosis. Genetic rodent models can develop non-motor phenotypes, providing tools to identify mechanisms underlying the early development of PD. However, it is not yet clear how reproducible non-motor phenotypes are amongst genetic PD rodent models, whether phenotypes are age-dependent, and the translatability of these phenotypes has yet to be explored. A systematic literature search was conducted on studies using genetic PD rodent models to investigate non-motor phenotypes; cognition, anxiety/depressive-like behaviour, gastrointestinal (GI) function, olfaction, circadian rhythm, cardiovascular and urinary function. In total, 51 genetic models of PD across 150 studies were identified. We found outcomes of most phenotypes were inconclusive due to inadequate studies, assessment at different ages, or variation in experimental and environmental factors. GI dysfunction was the most reproducible phenotype across all genetic rodent models. The mouse model harbouring mutant A53T, and the wild-type hα-syn overexpression (OE) model recapitulated the majority of phenotypes, albeit did not reliably produce concurrent motor deficits and nigral cell loss. Furthermore, animal models displayed different phenotypic profiles, reflecting the distinct genetic risk factors and heterogeneity of disease mechanisms. Currently, the inconsistent phenotypes within rodent models pose a challenge in the translatability and usefulness for further biomechanistic investigations. This review highlights opportunities to improve phenotype reproducibility with an emphasis on phenotypic assay choice and robust experimental design.

A Forgotten Principle in Immunocytochemistry: Optimal Dilution

Immunocytochemical (ICC) techniques are frequently used in basic and clinical research. Here, we focus on the importance of using antisera/antibodies at optimal dilutions to achieve specificity and reduce costs. Unfortunately, the basic principle, the necessity to test method specificity of the staining by a series of increasing dilutions of primary antiserum/antibodies, is only occasionally seen in papers using ICC. Many researchers rely on the company's information or others' published data. In this study, we show examples with monoclonal antibodies used in the peroxidase-based ICC technique in mouse and guinea pig brain sections. We show images of ICC staining of phospho-S129 alpha-synuclein in A53T mice and NeuN in guinea pig brains and demonstrate that optimal staining with them can be achieved at least at two to three orders of magnitude higher dilutions than generally used in the literature. We strongly recommend that when antisera/antibodies are used for the first time in any laboratory, independent of what the manufacturer or vendor recommends or are found in the literature, a dilution curve should be set up to identify the optimal dilution. This practice provides not only the highest specificity but is also an economic approach.

Environmental triggers of Parkinson's disease - Implications of the Braak and dual-hit hypotheses

Idiopathic Parkinson's disease (PD) may take decades to develop, during which many risk or protective factors may come into play to initiate the pathogenesis or modify its progression to clinical PD. The lack of understanding of this prodromal phase of PD and the factors involved has been a major hurdle in the study of PD etiology and preventive strategies. Although still controversial, the Braak and dual-hit hypotheses that PD may start peripherally in the olfactory structures and/or the gut provides a theoretical platform to identify the triggers and modifiers of PD prodromal development and progression. This is particularly true for the search of environmental causes of PD as the olfactory structures and gut are the major human mucosal interfaces with the environment. In this review, we lay out our personal views about how the Braak and dual-hit hypotheses may help us search for the environmental triggers and modifiers for PD, summarize available experimental and epidemiological evidence, and discuss research gaps and strategies.

Neurotoxicity and Underlying Mechanisms of Endogenous Neurotoxins

Endogenous and exogenous neurotoxins are important factors leading to neurodegenerative diseases. In the 1980s, the discovery that 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) contributes to Parkinson's disease (PD) symptoms led to new research investigations on neurotoxins. An abnormal metabolism of endogenous substances, such as condensation of bioamines with endogenous aldehydes, dopamine (DA) oxidation, and kynurenine pathway, can produce endogenous neurotoxins. Neurotoxins may damage the nervous system by inhibiting mitochondrial activity, increasing oxidative stress, increasing neuroinflammation, and up-regulating proteins related to cell death. This paper reviews the biological synthesis of various known endogenous neurotoxins and their toxic mechanisms.

Stem Cells for Aging-Related Disorders

This review captures recent advances in biological and translational research on stem cells. In particular, we discuss new discoveries and concepts regarding stem cell treatment of aging-related disorders. A myriad of stem cell sources exists, from hematopoietic to mesenchymal and neural cell lineages. We examine current applications of exogenous adult bone marrow-derived stem cells as an effective and safe transplantable cell source, as well as the use of electrical stimulation to promote endogenous neurogenesis for Parkinson's disease. We also explore the potential of transplanting exogenous umbilical cord blood cells and mobilizing host resident stem cells in vascular dementia and aging. In addition, we assess the ability of small molecules to recruit resident stem cells in Alzheimer's disease. Finally, we evaluate mechanisms of action recently implicated in stem cell therapy, such as the role of long non-coding RNAs, G-protein coupled receptor 5, and NeuroD1. Our goal is to provide a synopsis of recent milestones regarding the application of stem cells in aging.

Visualization of early oligomeric α-synuclein pathology and its impact on the dopaminergic system in the (Thy-1)-h[A30P]α-syn transgenic mouse model

Aggregation of alpha-synuclein (α-syn) into Lewy bodies and Lewy neurites is a pathological hallmark in the Parkinson´s disease (PD) brain. The formation of α-syn oligomers is believed to be an early pathogenic event and the A30P mutation in the gene encoding α-syn, causing familial PD, has been shown to cause an accelerated oligomerization. Due to the problem of preserving antigen conformation on tissue surfaces, α-syn oligomers are difficult to detect ex vivo using conventional immunohistochemistry with oligomer-selective antibodies. Herein, we have instead employed the previously reported α-syn oligomer proximity ligation assay (ASO-PLA), along with a wide variety of biochemical assays, to discern the pathological progression of α-syn oligomers and their impact on the dopaminergic system in male and female (Thy-1)-h[A30P]α-syn transgenic (A30P-tg) mice. Our results reveal a previously undetected abundance of α-syn oligomers in midbrain of young mice, whereas phosphorylated (pS129) and proteinase k-resistant α-syn species were observed to a larger extent in aged mice. Although we did not detect loss of dopaminergic neurons in A30P-tg mice, a dysregulation in the monoaminergic system was recorded in older mice. Taken together, ASO-PLA should be a useful method for the detection of early changes in α-syn aggregation on brain tissue, from experimental mouse models in addition to post mortem PD cases.

Hippocampal network hyperexcitability in young transgenic mice expressing human mutant alpha-synuclein

Abnormal excitability in cortical networks has been reported in patients and animal models of Alzheimer's disease (AD), and other neurodegenerative conditions. Whether hyperexcitability is a core feature of alpha(α)-synucleinopathies, including dementia with Lewy bodies (DLB) is unclear. To assess this, we used two murine models of DLB that express either human mutant α-synuclein (α-syn) the hA30P, or human wild-type α-syn (hWT-α-syn) mice. We observed network hyperexcitability in vitro in young (2–5 months), pre-symptomatic transgenic α-syn mice. Interictal discharges (IIDs) were seen in the extracellular local field potential (LFP) in the hippocampus in hA30P and hWT-α-syn mice following kainate application, while only gamma frequency oscillations occurred in control mice. In addition, the concentration of the GABA_A receptor antagonist (gabazine) needed to evoke IIDs was lower in slices from hA30P mice compared to control mice. hA30P mice also showed increased locomotor activity in the open field test compared to control mice. Intracellular recordings from CA3 pyramidal cells showed a more depolarised resting membrane potential in hA30P mice. Quadruple immunohistochemistry for human α-syn, and the mitochondrial markers, porin and the complex IV enzyme cytochrome c oxidase subunit 1 (COX1) in parvalbumin (PV+)-expressing interneurons showed that 25% of PV+ cells contained human α-syn in hA30P mice. While there was no change in PV expression, COX1 expression was significantly increased in PV+ cells in hA30P mice, perhaps reflecting a compensatory change to support PV+ interneuron activity. Our findings suggest that hippocampal network hyperexcitability may be an important early consequence of α-syn-mediated impairment of neuronal/synaptic function, which occurs without any overt loss of PV interneurons. The therapeutic benefit of targeting network excitability early in the disease stage should be explored with respect to α-synucleinopathies such as DLB.

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Young transgenic α-syn mice exhibit network hyperexcitability in the hippocampus in vitro.

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Young transgenic α-syn mice have increased locomotor activity in an open field test.

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Hippocampal pyramidal cells are more depolarised in young transgenic α-syn mice.

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Increased mitochondrial cytochrome c oxidase (complex IV) function in PV+ interneurons in young transgenic a-syn mice.

Early Disruption of Cortical Sleep-Related Oscillations in a Mouse Model of Dementia With Lewy Bodies (DLB) Expressing Human Mutant (A30P) Alpha-Synuclein

Changes in sleep behavior and sleep-related cortical activity have been reported in conditions associated with abnormal alpha-synuclein (α-syn) expression, in particular Parkinson’s disease (PD) and dementia with Lewy bodies (DLB). Notably, changes can occur in patients years before the onset of cognitive decline. Sleep-related network oscillations play a key role in memory function, but how abnormal α-syn impacts the generation of such activity is currently unclear. To determine whether early changes in sleep-related network activity could also be observed, prior to any previously reported cognitive dysfunction, we used mice that over-express human mutant α-syn (A30P). Recordings in vivo were performed under urethane anesthesia in the medial prefrontal cortex (mPFC) and CA1 region of the hippocampus in young male (2.5 – 4 months old) A30P and age-matched wild type (WT) mice. We found that the slow oscillation (SO) < 1 Hz frequency was significantly faster in both the mPFC and hippocampus in A30P mice, and Up-state-associated fast oscillations at beta (20 – 30 Hz) and gamma (30 – 80 Hz) frequencies were delayed relative to the onset of the Up-state. Spindle (8 – 15 Hz) activity in the mPFC was also altered in A30P mice, as spindles were shorter in duration and had reduced density compared to WT. These changes demonstrate that dysregulation of sleep-related oscillations occurs in young A30P mice long before the onset of cognitive dysfunction. Our data suggest that, as seen in patients, changes in sleep-related oscillations are an early consequence of abnormal α-syn aggregation in A30P mice.

Trichloroethylene, a ubiquitous environmental contaminant in the risk for Parkinson's disease

Organic solvents are common chemicals used in industry throughout the world, however, there is evidence for adverse health effects from exposure to these compounds. Trichloroethylene (TCE) is a halogenated solvent that has been used as a degreasing agent since the early 20th century. Due to its widespread use, TCE remains one of the most significant environmental contaminants in the US, and extensive research suggests TCE is a causative factor in a number of diseases, including cancer, fetal cardiac development, and neurotoxicity. TCE has also been implicated as a possible risk factor in the development of the most common neurodegenerative movement disorder, Parkinson's disease (PD). However, there is variable concordance across multiple occupational epidemiological studies assessing TCE (or solvent) exposure and risk for PD. In addition, there remains a degree of uncertainty about how TCE elicits toxicity to the dopaminergic system. To this end, we review the specific neurotoxic mechanisms of TCE in the context of selective vulnerability of dopaminergic neurons. In addition, we consider the complexity of combined risk factors that ultimately contribute to neurodegeneration and discuss the limitations of single-factor exposure assessments.