Gender-Dependent Effects of Thimerosal on Human Progenitor Neurons: A Potential Link to Regressive Autism

Abstract

Regressive Autism [RA] is a subtype of autism spectrum disorder (ASD) that manifests as the loss of previously acquired developmental milestones and skills. Early life dysregulation of neurodevelopment due to exposure to toxic metals has been associated with ASD, but the underlying biological mechanisms by which metals influence neurodevelopment remain unclear. We explored the potential role of thimerosal or ethylmercury on neurite formation and oxytocin receptor (OXTR) modulation in four human-developing neuronal cell lines of male and female origin (N = 2 each). We exposed the cell lines to three levels of thimerosal that closely represented the concentrations an infant (equivalent to 1 L of blood volume), an adolescent (~5 L), and an adult (~10 L) might receive from multiple doses of vaccine containing 100 µg/mL of thimerosal. We found that exposure to vaccine-equivalent concentrations of thimerosal induced significantly greater neurite dysregulation, including central chromatolysis, neurite abnormalities (i.e., axonal length, relative diameter, and pathways in neurons), and syncytia formation in undifferentiated and partially differentiated human developing neurons compared to controls. Exposure of male neurons to thimerosal significantly affected neurite formation and OXTR expression compared to the female neurons. In developed nations, most vaccines are thimerosal-free; nonetheless, some still incorporate ethylmercury as a preservative. In contrast, due to the lack of refrigeration in many developing nations, thimerosal is still used widely in most vaccines. Our study shows that ethylmercury induced profound neurite dysregulation and downregulated OXTR expression. The progenitor neurons from males were significantly more susceptible to thimerosal than those from females. However, internal factors in vaccine recipients may trigger dysfunction in the Blood-Brain Barrier (BBB), and screening potentially vulnerable individuals for conditions that may contribute to a BBB breach before vaccination might be beneficial.