Methylmercury elicits rapid inhibition of cell proliferation in the developing brain and decreases cell cycle regulator, cyclin E

Abstract

The developing brain is highly sensitive to methylmercury (MeHg). Still, the initial changes in cell proliferation that may contribute to long-term MeHg effects are largely undefined. Our previous studies with growth factors indicate that acute alterations of G1/S phase transition can permanently affect cell numbers and organ size. Therefore, we determined whether an environmental toxicant could also impact brain development with rapid (6-7h) effects on DNA synthesis and cell cycle machinery in neuronal precursors. In vivo studies in newborn rat hippocampus and cerebellum, two regions of postnatal neurogenesis, were followed by in vitro analysis of two precursor models, cortical and cerebellar cells, focusing on the proteins that regulate G1/S transition. In postnatal day 7 (P7) pups, a single subcutaneous injection of MeHg (3μg/g) acutely (7h) decreased DNA synthesis in the hippocampus by 40% and produced long-term (2 weeks) reductions in total cell number, estimated by DNA quantification. Surprisingly, cerebellar granule cells were resistant to MeHg effects in vivo at comparable tissue concentrations, suggesting region-specific differences in precursor populations. In vitro, MeHg altered proliferation and cell viability, with DNA synthesis selectively inhibited at an early timepoint (6h) corresponding to our in vivo observations. Considering that G1/ S regulators are targets of exogenous signals, we used a well-defined cortical cell model to examine MeHg effects on relevant cyclin dependent kinases (CDK) and CDK inhibitors. At 6h, MeHg decreased by 75% levels of cyclin E, a cell cycle regulator with roles in proliferation and apoptosis, without altering p57, p27, or CDK2 nor levels of activated caspase 3. In aggregate, our observations identify the G1/S transition as an early target of MeHg toxicity and raise the possibility that cyclin E degradation contributes to both decreased proliferation and eventual cell death.

Proliferation potential of human amniotic fluid stem cells differently responds to mercury and lead exposure

“…5). Neuronal precursor cells show unchanged p27 as well as unchanged or decreased cyclin D1 levels upon MeHg exposure (Burke et al 2006;Falluel-Morel et al 2007;Xu et al 2010). The here observed pronounced effect of MeHg on cyclin A expression could be explained by MeHg-induced cell cycle arrest and stop of proliferation (further confirming effects described in proliferation experiments above) or by functional interaction between MeHg and cyclin A.…”

“…Interestingly, MeHg also reduces the levels of cyclin E, which is a critical promoter of G1/S progression regulated by p21. The decrease of a cell cycle promoter suggests that MeHg is able to target cell machinery also by controlling the G1/S transition (Burke et al 2006;Xu et al 2010).…”

Literature review on in vitro and alternative Developmental Neurotoxicity (DNT) testing methods

“…The 12 citations revealing 'True Positives' for DNT utilized Lead acetate, Manganese(II)chloride, MeHgCl, PCB126 and Toluene. Of those, only MeHgCl is a true inhibitor of 'Neural Proliferation' in vivo , Burke et al, 2006, whereas the other compounds affect neurodevelopment through different target processes (e.g. Toluene affects astroglial differentiation (Burry et al, 2003) or Manganese alters the dopaminergic system (Tran et al, 2002)).…”

Mechanisms of neurotoxicity induced in the developing brain of mice and rats by DNA-damaging chemicals

“…Since a large number of processes such as proliferation, migration, and differentiation of neural cells occur during an extended period of development, the fetal central nervous system (CNS) is sensitive to diverse environmental factors such as alcohol (Young et al, 2003;Lee et al, 2005), metals (Burke et al, 2006), irradiation (Kimler, 1998;Ferrer et al, 1996;Bolaris et al, 2001;D'Sa-Eipper et al, 2001;Fukuda et al, 2004), mycotoxins (Sehata et al, 2004;Doi and Uetsuka, 2011), endocrine disruptors (Kudo et al, 2004) and DNA-damaging chemicals (Fujimori et al, 1983;Klocke et al, 2002;Nitta et al, 2004;Xiaro et al, 2007;Yamaguchi et al, 2009). Such environmental stresses and stimuli can affect the completion of normal CNS development (Rodier, 1995;Mendola et al, 2002;Costa et al, 2004).…”