Disruption of postnatal progenitor migration and consequent abnormal pattern of glial distribution in the cerebrum following administration of methylmercury

Methylmercury exposure causes a persistent inhibition of myogenin expression and C2C12 myoblast differentiation

Methylmercury (MeHg) is a ubiquitous environmental toxicant, best known for its selective targeting of the developing nervous system. MeHg exposure has been shown to cause motor deficits such as impaired gait and coordination, muscle weakness, and muscle atrophy, which have been associated with disruption of motor neurons. However, recent studies have suggested that muscle may also be a target of MeHg toxicity, both in the context of developmental myogenic events and of low-level chronic exposures affecting muscle wasting in aging. We therefore investigated the effects of MeHg on myotube formation, using the C2C12 mouse myoblast model. We found that MeHg inhibits both differentiation and fusion, in a concentration-dependent manner. Furthermore, MeHg specifically and persistently inhibits myogenin (MyoG), a transcription factor involved in myocyte differentiation, within the first six hours of exposure. MeHg-induced reduction in MyoG expression is contemporaneous with a reduction of a number of factors involved in mitochondrial biogenesis and mtDNA transcription and translation, which may implicate a role for mitochondria in mediating MeHg-induced change in the differentiation program. Unexpectedly, inhibition of myoblast differentiation with MeHg parallels inhibition of Notch receptor signaling. Our research establishes muscle cell differentiation as a target for MeHg toxicity, which may contribute to the underlying etiology of motor deficits with MeHg toxicity.

Windows of sensitivity to toxic chemicals in the motor effects development

Many chemicals currently used are known to elicit nervous system effects. In addition, approximately 2000 new chemicals introduced annually have not yet undergone neurotoxicity testing. This review concentrated on motor development effects associated with exposure to environmental neurotoxicants to help identify critical windows of exposure and begin to assess data needs based on a subset of chemicals thoroughly reviewed by the Agency for Toxic Substances and Disease Registry (ATSDR) in Toxicological Profiles and Addenda. Multiple windows of sensitivity were identified that differed based on the maturity level of the neurological system at the time of exposure, as well as dose and exposure duration. Similar but distinct windows were found for both motor activity (GD 8-17 [rats], GD 12-14 and PND 3-10 [mice]) and motor function performance (insufficient data for rats, GD 12-17 [mice]). Identifying specific windows of sensitivity in animal studies was hampered by study designs oriented towards detection of neurotoxicity that occurred at any time throughout the developmental process. In conclusion, while this investigation identified some critical exposure windows for motor development effects, it demonstrates a need for more acute duration exposure studies based on neurodevelopmental windows, particularly during the exposure periods identified in this review.

Low level methylmercury enhances CNTF-evoked STAT3 signaling and glial differentiation in cultured cortical progenitor cells

Although many previous investigations have studied how mercury compounds cause cell death, sub-cytotoxic levels may affect mechanisms essential for the proper development of the nervous system. The present study investigates whether low doses of methylmercury (MeHg) and mercury chloride (HgCl2) can modulate the activity of JAK/STAT signaling, a pathway that promotes gliogenesis. We report that sub-cytotoxic doses of MeHg enhance ciliary neurotrophic factor (CNTF) evoked STAT3 phosphorylation in human SH-SY5Y neuroblastoma and mouse cortical neural progenitor cells (NPCs). This effect is specific for MeHg, since HgCl2 fails to enhance JAK/STAT signaling. Exposing NPCs to these low doses of MeHg (30-300nM) enhances CNTF-induced expression of STAT3-target genes such as glial fibrillary acidic protein (GFAP) and suppressors of cytokine signaling 3 (SOCS3), and increases the proportion of cells expressing GFAP following 2 days of differentiation. Higher, near-cytotoxic concentrations of MeHg and HgCl2 inhibit STAT3 phosphorylation and lead to increased production of superoxide. Lower concentrations of MeHg effective in enhancing JAK/STAT signaling (30nM) do not result in a detectable increase in superoxide nor increased expression of the oxidant-responsive genes, heme oxygenase 1, heat shock protein A5 and sirtuin 1. These findings suggest that low concentrations of MeHg inappropriately enhance STAT3 phosphorylation and glial differentiation, and that the mechanism causing this enhancement is distinct from the reactive oxygen species-associated cell death observed at higher concentrations of MeHg and HgCl2.

Prenatal carbofuran exposure inhibits hippocampal neurogenesis and causes learning and memory deficits in offspring

Neurogenesis is a process of generation of new neurons in the hippocampus and associated with learning and memory. Carbofuran, a carbamate pesticide, elicits several neurochemical, neurophysiological, and neurobehavioral deficits. We evaluated whether chronic prenatal oral exposure of carbofuran during gestational days 7-21 alters postnatal hippocampal neurogenesis at postnatal day 21. We found carbofuran treatment significantly decreased bromodeoxyuridine (BrdU) positive cell proliferation and long-term survival in the hippocampus only but not in the cerebellum. We observed a reduced number of transcription factor SOX-2 and glial fibrillary acidic protein (GFAP) colabeled cells, decreased nestin messenger RNA (mRNA) expression, and decreased histone-H3 phosphorylation following carbofuran treatment, suggesting a decreased pool of neural progenitor cells (NPC). Colocalization of BrdU with doublecortin (DCX), neuronal nuclei (NeuN), and GFAP suggested decreased neuronal differentiation and increased glial differentiation by carbofuran. The number of DCX(+) and NeuN(+) neurons, NeuN protein levels, and fibers length of DCX(+) neurons were decreased by carbofuran. Carbofuran caused a significant downregulation of mRNA expression of the neurogenic genes/transcription factors such as neuregulin, neurogenin, and neuroD1 and upregulation of the gliogenic gene Stat3. Carbofuran exposure led to increased BrdU/caspase 3 colabeled cells, an increased number of degenerative neurons and profound deficits in learning and memory processes. The number and size of primary neurospheres derived from the hippocampus of carbofuran-treated rats were decreased. These results suggest that early gestational carbofuran exposure diminishes neurogenesis, reduces the NPC pool, produces neurodegeneration in the hippocampus, and causes cognitive impairments in rat offspring.

Emergence of delayed methylmercury toxicity after perinatal exposure in metallothionein-null and wild-type C57BL mice

BACKGROUND

Although a long latency period of toxicity after exposure to methylmercury (MeHg) is known to exist in humans, few animal studies have addressed this issue. Substantiation of delayed MeHg toxicity in animals would affect the risk evaluation of MeHg.

OBJECTIVES

Our goal in this study was to demonstrate the existence of a latency period in a rodent model in which the toxicity of perinatal MeHg exposure becomes apparent only later in life. Our study included metallothionein (MT) knockout mice because studies have suggested the potential susceptibility of this strain to the neurodevelopmental toxicity of MeHg.

METHODS

Pregnant MT-null and wild-type C57Bl/6J mice were exposed to MeHg through their diet containing 5 mug Hg/g during gestation and early lactation. We examined behavioral functions of the offspring using frequently used paradigms, including open field behavior (OPF), passive avoidance (PA), and the Morris water maze (MM), at ages of 12-13 and 52-53 weeks.

RESULTS

At 12 weeks of age, behavioral effects of MeHg were not detected, except for OPF performance in MeHg-exposed MT-null females. At 52 weeks of age, the MeHg-exposed groups showed poorer performance both in PA and MM, and their OPF activity differed from controls. These effects of MeHg appeared exaggerated in the MT-null strain. The brain Hg concentration had leveled off by 13 weeks of age.

CONCLUSIONS

The results suggest the existence of a long latency period after perinatal exposure to low-level MeHg, in which the behavioral effects emerged long after the leveling-off of brain Hg levels. Hence, the initial toxicologic event responsible for the late effects should have occurred before this leveling-off of brain Hg.

Methylmercury induces activation of Notch signaling

Methylmercury (MeHg) toxicity in humans manifests deficits in neurological function. Cases of prenatal exposure to mercury have established that the developing nervous system is most highly susceptible to perturbation by MeHg. At a cellular level, MeHg-induced defects result from altered neuronal proliferation, migration and pathfinding. However, the molecular targets of MeHg that give rise to these outcomes are not fully understood. In an overall effort to identify the fundamental molecular targets of MeHg in neural development, we have explored the effects of MeHg on cell surface receptor function using the simplified Drosophila model. In this study, we investigated the potential role of MeHg to alter activity of the Notch receptor pathway, a highly conserved cell-cell signaling mechanism that controls cell fate decisions, proliferation, migration and neurite outgrowth in neural development. Notch receptor activation requires proteolysis by a cell surface ADAM metalloprotease. ADAM proteases are required for normal neural development and are activated by organomercurials, thus presenting a possible mechanism for MeHg neurotoxicity. Here, we demonstrate a concentration- and time-dependent increase in Notch receptor activity with MeHg exposure in three distinct Drosophila cell lines. Ten micromolar MeHg results in a 4-5.5-fold increase in Notch signaling as measured by the upregulation of two enhancer of split (E(spl)) target genes. MeHg-induced Notch activity also correlates with receptor proteolysis. Targeted knockdown of Notch protein expression demonstrates that MeHg induced E(spl) activation specifically requires the Notch receptor. Furthermore, MeHg-induced Notch activity is partially attenuated by the metalloprotease inhibitor, GM6001, consistent with a model in which MeHg promotes activation of ADAM metalloproteases. Finally, we demonstrate that inorganic HgCl(2) is significantly less active in inducing Notch activity, suggesting a mechanism specific to organic species of mercury. Overall, these data identify Notch as a potential target for MeHg toxicity in the developing nervous system.

The magnitude of methylmercury-induced cytotoxicity and cell cycle arrest is p53-dependent

BACKGROUND

Methylmercury (MeHg), a ubiquitous environmental contaminant, is a known potent teratogen selectively affecting the developing central nervous system. While a definitive mechanism for MeHg-induced developmental neurotoxicity remains elusive, in utero exposure has been associated with reduced brain weight and reduction in cell number. This suggests early toxicant interference with critical molecular signaling events controlling cell behavior, i.e., proliferation.

METHODS

To examine the role of p53, a major regulator of the G(1)/S and G(2)/M cell cycle checkpoints, in MeHg toxicity, we isolated GD 14 primary embryonal fibroblasts from homozygous wild-type p53 (p53+/+) and homozygous null p53 (p53-/-) mice. Cells were treated at passages 4-7 for 24 or 48 hr with 0, 1.0, or 2.5 microM MeHg and analyzed for effects on viability, cell cycle progression (using BrdU-Hoechst flow cytometric analysis), and apoptosis via annexin V-FITC and propidium iodide (PI) staining.

RESULTS

The p53+/+ cells are more sensitive than p53-/- cells to MeHg-induced cytotoxicity, cell cycle inhibition, and induction of apoptosis: at 24 hr, 2.5 microM MeHg reduced p53+/+ cell viability to 72.6% +/- 3.2%, while p53-/- viability was 94.6% +/- 0.4%. The p53-/- cells underwent less necrosis and less apoptosis following MeHg treatment. MeHg (2.5 microM) also halted all cycling in the p53+/+ cells, while 42.6% +/- 7.2% of p53-/- cells were able to reach a new G(0)/G(1) in 48 hr. Time- and dose-dependent accumulation of cells in G(2)/M phase (1.0 and 2.5 microM MeHg) was observed independent of the p53 genotype; however, the magnitude of change was p53-dependent.

CONCLUSIONS

These studies suggest that MeHg-induced cell cycle arrest occurs via both p53-dependent and -independent pathways in our model system; however, cell death resulting from MeHg exposure is highly dependent on p53.