Elsevier

NeuroToxicology

Volume 80, September 2020, Pages 71-75
NeuroToxicology

Full Length Article
Metal-induced neurotoxicity in a RAGE-expressing C. elegans model

https://doi.org/10.1016/j.neuro.2020.06.013Get rights and content

Abstract

Environmental and occupational metal exposure poses serious global concerns. Metal exposure have severally been associated with neurotoxicity and brain damage. Furthermore, receptor for advanced glycation end products (RAGE) is also implicated in neurological disorders, particularly those with altered glucose metabolism. Here, we examine potential compounding effect of metal exposure and RAGE expression on dopamine (DA) and serotonin (SER) neurons in C. elegans. In addition, we evaluate the effect of RAGE expression on DA and SER neurons in hyperglycemic conditions. Newly generated RAGE-expressing C. elegans tagged with green fluorescent proteins (GFP) in DAergic and SERergic neurons were treated with cadmium (Cd) or manganese (Mn). Additionally, the RAGE-expressing worms were also exposed to high glucose conditions. Results showed metals induced neurodegeneration both in the presence and absence of RAGE expression, but the manner of degeneration differed between Cd and Mn treated nematodes. Furthermore, RAGE-expressing worms showed significant neurodegeneration in both DAergic and SERergic neurons. Our results indicate co-occurrence of metal exposure and RAGE expression can induce neurodegeneration. Additionally, we show that RAGE expression can exacerbate hyperglycemic induced neurodegeneration.

Introduction

The receptor for advanced glycation end products (RAGE) is a multi-ligand molecule characterized as a binder for advanced glycation end products (AGEs), one of its numerous ligands (Pinkas et al., 2019). RAGE has been implicated in hyperglycemic- and heavy metal-induced neurotoxicity (Milatovic et al., 2009; Shirdhankar et al., 2017). While AGEs, the end products of glycation exogenously originate from certain foods via Maillard’s reaction, hyperglycemic-induced endogenous formation of AGE has also been observed. They accumulate in the body, and subsequently induce oxidative stress and inflammation (Monnier, 1990; Pinkas and Aschner, 2016). AGEs bind to their multi-ligand receptors RAGE to generate reactive oxygen species (ROS) and induce up-regulation of inflammatory cytokines and transcription factors that have been implicated in a wide range of pathologies, including cardiovascular diseases, nephropathy, and diabetes, as well as Alzheimer’s disease (AD), Parkinson’s disease (PD), and other neuropathies (Juranek et al., 2015; Ott et al., 2014; Ramasamy et al., 2016). As such, RAGE expression can be targeted as both an indicator and therapeutic tool in understanding the neurodegeneration that is associated with AGE-related diseases.

The overproduction and accumulation of AGEs has been implicated as a potential cellular mechanism for neurotoxicity under high glucose conditions (Singh et al., 2001; Wendt et al., 2006). Schlotterer et al. (2009) used a C. elegans model to demonstrate the lifespan-shortening effect of hyperglycemic conditions as a result of AGE-mediated modification of mitochondrial proteins and increased ROS formation (Pinkas et al., 2018b; Schlotterer et al., 2009).

Previous studies have shown that chemical elements, such as heavy metals, can potentially affect RAGE expression, leading to the onset or exacerbation of the aforementioned pathologies (Heimfarth et al., 2018; Niño et al., 2018; Pinkas et al., 2018a). Manganese (Mn) is an essential heavy metal whose nutritional and metabolic function is paralleled by its role as a neurotoxicant upon occupational or nutritional exposure. Excessive Mn exposure is a significant non-genetic risk factor for PD, as it is implicated in DAergic neurodegeneration in the substantia nigra, causing the motor deficits. Cadmium (Cd) is a non-essential heavy metal which is known to induce oxidative stress and impair normal nervous system functioning as a result of occupational exposure (Chen et al., 2016). Both metals have been implicated in RAGE mediated hyperglycemic neurotoxicity (Pinkas and Aschner, 2016; Pinkas et al., 2018a; Zhuang et al., 2012).

Here we performed a study to examine the potential link between RAGE and metal-induced neurotoxicity, specifically focusing on RAGE activation as a potential mechanism of metal-induced neurodegeneration. This study uses a novel transgenic RAGE-expressing C. elegans model to examine the neurotoxic impact of Mn and Cd exposure on DAergic and SERergic systems in the presence of RAGE. We hypothesize that due to the contributions of both RAGE and heavy metal exposure to oxidative stress and inflammation-related neuronal death, the combination of RAGE expression and metal exposure will exacerbate the toxic effects in C. elegans, establishing RAGE activation as a mediator in the pathogenicity caused by heavy metal exposure. Furthermore, we evaluated the involvement of RAGE in hyperglycemia-induced neurodegeneration.

Section snippets

C. elegans strains and maintenance

C. elegans were grown and maintained on OP50-seeded nematode growth media (NGM), at 20 °C. Strains used in this study include the BY200 [dat-1::GFP] and GR1333 [tph-1::GFP] to visualize dopamine (DA) and serotonin (SER) neurons respectively. These were obtained from the Caenorhabditis Genetics Center (CGC), USA. We previously designed pan-neuronal RAGE-expressing C. elegans strains (Pinkas et al., 2019).

Strains crossing

The worm strains tagged with GFP in DA [dat-1::GFP] or SER [tph-1::GFP] neurons were crossed

Results

Two-ANOVA analysis showed no significant interaction [P = 0.1425] between RAGE expression and Cd treatments on DAergic neuron mean GFP fluorescence. Additionally, there was no significant strain effect [P = 0.7473] on DAergic neurons. However, a significant dose-dependent effect [P < 0.001] was observed. Further post-hoc analysis showed significantly reduced GFP signal intensity in DAergic neurons in the absence or presence of RAGE-expression following 50 mM Cd treatment compared with

Discussion

The neurotoxic effect of hyperglycemia and heavy metal exposure has been demonstrated (Bishak et al., 2015; Chen et al., 2016; Pinkas and Aschner, 2016; Pinkas et al., 2018b). In this report, we have used a novel transgenic RAGE-expressing C. elegans model to examine the neurodegenerative impact of Mn and Cd exposure on DAergic and SERergic neurons in the presence of RAGE, concomitantly highlighting the role of hyperglycemia in neuronal loss. We took advantage of the transparent body of C.

CRediT authorship contribution statement

Michael Lawes: Investigation, Validation, Visualization, Writing - original draft, Data curation, Formal analysis. Adi Pinkas: Investigation, Writing - review & editing. Bailey A. Frohlich: Investigation, Writing - review & editing. Joy D. Iroegbu: Investigation, Writing - review & editing. Omamuyovwi M. Ijomone: Conceptualization, Investigation, Writing - review & editing. Michael Aschner: Conceptualization, Project administration, Writing - review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

MA is supported by National Institute of Health (NIH), USA grants; NIEHS R01 10563, NIEHS R01 07331 and NIEHS R01 020852.

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