Mechanistic Investigation of Environmentally Relevant Manganese Neurotoxicity

Neurological and neuropsychological dysfunctions resulting from manganese (Mn) accumulation in the human brain are well acknowledged; however, the underlying mechanisms are not yet fully understood. Amongst currently proposed Mn neurotoxicity mechanisms, some were only detectable at concentrations that can lead to over 50% viability loss in acute insult which cannot represent human brain exposure scenarios. Meanwhile, epidemiological reports suggest that exposures over a timeframe of years to decades at Mn levels near or even lower than the regulatory workspace threshold can still lead to adverse outcomes in the central nervous system. Therefore, how to model environmentally relevant chronic Mn exposures at near-threshold levels in in vitro experimental settings and how neurotoxicity is developed under this exposure paradigm are central questions awaiting to be answered.

Considering the essentiality of Mn as a critical metallic co-factor of multiple enzymes, this study aims to test the hypothesis that interrupted homeostasis of cellular functions that utilize Mn under physiological conditions are the most sensitive respondents to Mn overload. To test this hypothesis, a wide range of Mn concentrations were exposed in cell-based neuronal models across multiple durations. The responses of Mn-dependent biological processes were evaluated by protein phosphorylation quantifications, transcriptomic analyses, and functional measurements. Findings from these assessments highlighted the sensitivity of insulin/PI3K/AKT/mTOR signaling, cAMP/PKA/CREB signaling, cell adhesion, axonal guidance, and homeostatic regulation of divalent metals to near-physiological-threshold Mn overload. Alterations of these pathways illustrate a network of cellular functions that relies on optimal intracellular Mn content and vitally contributes to the neurodegeneration risks induced by chronic Mn exposures.

In conclusion, this work contributes to a more nuanced understanding of Mn neurotoxicity mechanisms, emphasizing the importance of both concentration and duration of exposure in the context of neurodegenerative risk, and paving the way for future research into constructing adverse outcome pathways of Mn via advanced in vitro modeling.