Inherited effects of low-dose exposure to methylmercury in neural stem cells.

Methylmercury (MeHg) is an environmental contaminant with recognized neurotoxic effects, particularly to the developing nervous system. In the present study, we show that nanomolar concentrations of MeHg can induce long-lasting effects in neural stem cells (NSCs). We investigated short-term direct and long-term inherited effects of exposure to MeHg (2.5 or 5.0 nM) using primary cultures of rat embryonic cortical NSCs. We found that MeHg had no adverse effect on cell viability but reduced NSC proliferation and altered the expression of cell cycle regulators (p16 and p21) and senescence-associated markers. In addition, we demonstrated a decrease in global DNA methylation in the exposed cells, indicating that epigenetic changes may be involved in the mechanisms underlying the MeHg-induced effects. These changes were observed in cells directly exposed to MeHg (parent cells) and in their daughter cells cultured under MeHg-free conditions. In agreement with our in vitro data, a trend was found for decreased cell proliferation in the subgranular zone in the hippocampi of adult mice exposed to low doses of MeHg during the perinatal period. Interestingly, this impaired proliferation had a measurable impact on the total number of neurons in the hippocampal dentate gyrus. Importantly, this effect could be reversed by chronic antidepressant treatment. Our study provides novel evidence for programming effects induced by MeHg in NSCs and supports the idea that developmental exposure to low levels of MeHg may result in long-term consequences predisposing to neurodevelopmental disorders and/or neurodegeneration.

Restorative effects of platelet derived growth factor-BB in rodent models of Parkinson's disease.

Parkinson's disease is characterized by motor deficits caused by loss of midbrain dopaminergic neurons. Neurotrophic factors and cell transplantation have partially restored function in models of Parkinson's disease, but have had limited effects in humans. Here we show that intracerebroventricular administration of platelet-derived growth factor-BB can offer an alternative strategy to restore function in Parkinson's disease; In animal models of nigrostriatal injury, a two weeks treatment with platelet-derived growth factor-BB resulted in long-lasting restoration of striatal dopamine transporter binding sites and expression of nigral tyrosine hydroxylase. It also normalized amphetamine-induced rotational behavior in 6-hydroxydopamine lesioned rats. Platelet-derived growth factor-BB promoted proliferation of neural progenitor cells in the subventricular zone. The effects on dopaminergic neurons and functional recovery could be blocked by co-infusion with a proliferation inhibitor, indicating a link between the proliferative and anti-parkinsonian effects. Based on the current data, we consider platelet-derived growth factor-BB a clinical candidate drug for treatment of Parkinson's disease.

Bromodeoxyuridine infused into the cerebral ventricle of adult mice labels nigral neurons under physiological conditions--a method to detect newborn nerve cells in regions with a low rate of neurogenesis.

Methodological differences may explain discrepancies in adult mammalian neurogenesis in the brain outside the widely accepted neurogenic regions, i.e. hippocampus and olfactory bulb/subventricular zone. Here, we describe a method to dissolve and administer bromodeoxyuridine (BrdU) at high concentrations (150mg/mL) into the adult mouse brain to demonstrate neuronal incorporation of this thymidine analogue in CNS regions with a low rate of neurogenesis. The dosage and duration of BrdU appear critical, since exposure to low doses did not result in a robust label using this marker for proliferation [Zhao et al., 2003. Proc Natl Acad Sci USA 100; 7925]. Mice (five per cage in an enriched environment) received BrdU in the right cerebral ventricle from a subcutaneous osmotic pump (0.9mg/day, 3 weeks, infused at 0.25microL/h). To avoid labelling of cells with a fast turnover, the mice were allowed to survive 3 weeks after ending the BrdU delivery. Midbrain sections were processed for tyrosine hydroxylase (TH) immunohistochemistry, post-fixed with 4% paraformaldehyde, denaturated with 2N HCl and 0.025% pepsin, followed by immunolabelling of nuclear BrdU in single-stranded DNA. Double-labelled cells were analysed in a confocal laser microscope and showed segmented nuclear BrdU-label surrounded by TH-immunoreactive cytoplasm, never displaying apoptotic morphological features. Nigral neuronal proliferation was confirmed using another marker [(3)H]thymidine, delivered via an intraperitoneal osmotic pump. The protocol was well tolerated by the mice and not found to be toxic for the region studied, i.e. did not alter the total number of nigral neurons identified with TH/cresyl violet immunohistochemistry.