Imaging Biomarkers of Neurotoxicity in Welders

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Abstract Manganese (Mn) is an established neurotoxicant that affects the same motor and cognitive brain pathways affected in Parkinson disease (PD), notably the nigrostriatal system. A large body of research, largely from our collaborative team, demonstrates that Mn-exposed workers have a clinical phenotype which overlaps substantially with PD, including Mn-dose-dependent progressive parkinsonism and nigrostriatal dysfunction on positron emission tomography (PET) brain imaging. In the first five years of the present PET imaging project, we used PET biomarkers of the dopaminergic system in the brain to establish that Mn-exposed workers had lower caudate binding of the PET radioligand 6-[18F]fluoro-L-dopa (FDOPA), as compared to non-exposed workers. These differences remained over five years of follow-up. We also found Mn-dose-dependent upregulation of dopamine type 2 receptor (D2R) binding, as measured by the radioligand [11C](N- methyl)benperidol (NMB), in the substantia nigra, the same region of the brain most dramatically affected in PD. In addition, we found an inverse Mn-dose-response association with thalamic (i.e., extrastriatal) [11C]dihydrotetrabenazine (DTBZ) binding, as well as a decline in DTBZ binding in caudate, putamen, and substantia nigra over time. More recently, our lab found evidence of Mn-induced neuroinflammation (microglial activation) in the brains of deceased Mn miners providing a potential mechanism for the in vivo findings in our welder cohort. This renewal builds on our previous studies by exploring the role of neuroinflammation in Mn- induced dopaminergic neurotoxicity. Our hypothesis is that Mn-induced progressive parkinsonism is a neurodegenerative disorder and is due to Mn-dose-dependent dopaminergic degeneration. We will perform repeat DTBZ and NMB PET imaging in research participants from our longitudinal Mn-exposed worker cohort who have already undergone baseline DTBZ and NMB scans to investigate the association between lifetime cumulative Mn exposure, informed by state-of-the-art neutron activated bone Mn quantication, and annual rate of change in DTBZ and NMB binding. We will also perform N-acetyl-N-(2-[(11)C]methoxybenzyl)-2-phenoxy-5- pyridinamine [(11)C]PBR28 (PBR28) PET imaging in these workers to characterize patterns of microglial activation in the same brain regions, and investigate the relationship between lifetime cumulative Mn exposure and PBR28 binding as a marker of neuroinflammation. We then will explore the role of neuroinflammation as a mediator of Mn-induced striatal degeneration. Finally, we enlarge our cohort to enrich for active workers and will perform longitudinal PET imaging to characterize the relation between lifetime cumulative Mn exposure and the annual rate of change in both striatal and extrastriatal binding/uptake of these radioligands between active and retired Mn-exposed workers. Successful completion of these aims will provide a unique opportunity to understand the mechanism of an important environmental nigrostriatal neurotoxicant and potentially inform the understanding of the environmental pathogenesis of PD.