Caenorhabditis elegans as a Model to Study Manganese-Induced Neurotoxicity

Abstract

Caenorhabditis elegans (C. elegans) is a nematode present worldwide. The worm shows homology to mammalian systems and expresses approximately 40% of human disease-related genes. Since Dr. Sydney Brenner first proposed C. elegans as an advantageous experimental worm-model system for genetic approaches, increasing numbers of studies using C. elegans as a tool to investigate topics in several fields of biochemistry, neuroscience, pharmacology, and toxicology have been performed. In this regard, C. elegans has been used to characterize the molecular mechanisms and affected pathways caused by metals that lead to neurotoxicity, as well as the pathophysiological interrelationship between metal exposure and ongoing neurodegenerative disorders. Several toxic metals, such as lead, cadmium, and mercury, are recognized as important environmental contaminants, and their exposure is associated with toxic effects on the human body. Essential elements that are required to maintain cellular homeostasis and normal physiological functions may also be toxic when accumulated at higher concentrations. For instance, manganese (Mn) is a trace essential element that participates in numerous biological processes, such as enzymatic activities, energy metabolism, and maintenance of cell functions. However, Mn overexposure is associated with behavioral changes in C. elegans, which are consistent with the dopaminergic system being the primary target of Mn neurotoxicity. Caenorhabditis elegans has been shown to be an important tool that allows for studies on neuron morphology using fluorescent transgenic worms. Moreover, behavioral tests may be conducted using worms, and neurotransmitter determination and related gene expression are likely to change after Mn exposure. Likewise, mutant worms may be used to study molecular mechanisms in Mn toxicity, as well as the expression of proteins responsible for the biosynthesis, transport, storage, and uptake of dopamine. Furthermore, this review highlights some advantages and limitations of using the experimental model of C. elegans and provides guidance for potential future applications of this model in studies directed toward assessing for Mn neurotoxicity and related mechanisms.

Keywords: C. elegans; alternative animal models; manganese; neurodegeneration; neurotoxicology; trace elements.

Figure 1

Worms at the same larval stage (L1 to L4, or even adult worms) can be exposed for minutes or hours in liquid medium (96-wells or microtubes) or NGM dishes. Afterwards, parameters, such as behavioral evaluations, survival/lifespan, Mn determination, and neuronal viability, can be evaluated [33]. The Mn neurotoxicity is observed in fluorescent strains (BZ555 (egIs1 [dat-1p::GFP]), BY200 (vtIs1 (Pdat-1::GFP, pRF4(rol-6(su1006)), and BY250 [vtIs7; Pdat-1::GFP] [102]). Images are acquired in Z-stack, summed, and analyzed in ImageJ (NIH). The presence of puncta, neuronal absence or shrinkage, neuronal gaps, absence of cell bodies, and reduction in the fluorescence intensity are evidence of neurotoxicity [18,103].

Figure 2

dAergic conservancy in C. elegans.

Figure 3

Key targets of Mn on its neurotoxic effects in C. elegans.