Funding tool: Basic research projects
Project-ID: LS10-026
Project start: 01. Oktober 2011
Project end: will follow
Runtime: 36 months / finished
Funding amount: € 270.000,00

Brief summary:

The ubiquitous heavy metal mercury is a significant neurodevelopmental toxicant. Prenatal exposure can cause neurodevelopmental disorders and subclinical brain dysfunction. The immature brain is extremely sensitive to neurotoxicity of methyl mercury (MeHg), an organic mercury compound. Humans are exposed to MeHg through consumption of contaminated fish and seafood. There is increasing evidence that also low MeHg exposure causes neurobehavioral deficits in children, and that viability of neuronal cells is impaired even by very low MeHg concentrations. Mercury easily traverses the placenta and the blood-brain-barrier and subsequently accumulates in fetal liver, kidney, and brain. Umbilical cord blood concentrations are almost twice those in maternal blood. This finding has been explained by active mercury transport to the fetal side. In addition, the variability of the ratio cord blood/maternal blood is very high between individuals, which cannot be explained by fish consumption habits. This indicates substantial inter-individual differences in placental mercury transfer. Because of the high neurotoxic potential of mercury it is fundamental to know the genetic background which modulates the placental transfer of mercury. The net transfer and the retention capacity of the placental layers define the amount of mercury that reaches the fetal circulation and finally the blood-brain-barrier. In this way, the placenta is limiting or expanding the potential of mercury to target the fetal brain.
Even though we know since decades that mercury is neurotoxic and is easily transferred to the fetus, surprisingly little is known on the placental mechanisms involved in mercury toxicokinetics. From animal experiments and in vitro studies we know that mercury uptake involves amino acid transporters and that mercury is bound to and detoxified by metallothioneins and glutathione and effluxed via transporters of the ABC family. Several other candidate transporters and enzymes are assumed to play a role in toxicokinetics (i.e., uptake, biotransformation, distribution, and efflux) of mercury. Overall 25 candidate proteins may be involved in cellular mercury toxicokinetics. However, it is not known whether these proteins are actually involved in the placental mercury toxicokinetics. We also do not know whether genetic variation (polymorphisms) in candidate genes, which are known to alter protein activity and function, may alter mercury accumulation in placenta. Finally, also our knowledge on (sub)cellular localization of the involved transporters and enzymes in placenta layers is more than fragmentary. At present, we can only speculate to which site (fetal and/or maternal) the involved transporters are transferring mercury.
To confine a set of candidate proteins/genes, for which involvement in placental mercury toxicokinetics is truly indicated, genoptype-phenotype associations will be studied in vivo and in vitro. We will examine the individual maternal/infant genetic background (functional polymorphisms) in relation to mercury contents in maternal blood, placenta, and cord blood. The expected data will yield new insights into how genotypes influence perinatal mercury exposure. This is crucial because at date we have very poor knowledge on the genetic background, which renders humans susceptible to prenatal mercury exposures and associated neurological impairment. In parallel, experiments in isolated and cultured primary human trophoblasts will be performed. We will knock-down candidate genes as single-knockdown or double-knockdown. Wildtype (WT) and knockdown cells are then exposed to MeHg to test whether mercury accumulation and cell viability are affected by elimination of the gene(s). To obtain a conclusive model for mercury transport across the placental barrier, the exact localization of candidate proteins remains to be demonstrated.
The prevention of toxic mercury insults during intrauterine life is of outstanding interest in human toxicology. Our approach to examine human placenta tissues and to knock down such high number of candidate genes in primary human trophoblasts cells is unique. The expected results will substantially increase our knowledge on the candidate proteins involved in maternal-fetal transfer of mercury. This enables us to study placental mercury transport with a confined set of candidate proteins in the future. In general, the expected data will contribute to a better understanding of cellular mercury toxicity and toxicokinetics. These results can be highly relevant for understanding placental transfer of other toxicants and for mercury membrane transport in liver, kidneys and blood-brain-barrier. Finally, the findings may open up new ways in therapy of mercury intoxications and will allow defining the susceptible risk groups, for which prevention of (prenatal) mercury exposures is crucial.

Keywords:
prenatal exposure, methyl mercury, placenta barrier, syncytiotrophoblast, fetal endothel cells, siRNA knockdown, protein localization