Inhibition of mitochondrial nucleic acid synthesis by methyl mercury

Evidence for interactions between intracellular calcium stores during methylmercury-induced intracellular calcium dysregulation in rat cerebellar granule neurons

Acute exposure to methylmercury (MeHg) causes severe disruption of intracellular Ca(2+) ([Ca(2+)](i)) regulation, which apparently contributes to neuronal death. Activation of the mitochondrial permeability transition pore (MTP) evidently contributes to this effect. We examined in more detail the contribution of mitochondrial Ca(2+) ([Ca(2+)](m)) to elevations of [Ca(2+)](i) caused by acute exposure to a low concentration of MeHg in primary cultures of rat cerebellar granule neurons. In particular, we sought to determine whether interactions occurred between Ca(2+)(i) pools in response to MeHg. Prior depletion of Ca(2+)(m) using carbonyl cyanide m-chlorophenylhydrazone (CCCP) and oligomycin significantly decreased the amplitude of [Ca(2+)](i) release from intracellular stores, and delayed the onset of whole-cell [Ca(2+)](i) elevations, caused by 0.5 microM MeHg. CCCP alone hastened the MeHg-induced release of Ca(2+) within the cell, whereas oligomycin alone delayed the MeHg-induced influx of extracellular Ca(2+). In granule cells loaded with rhod-2 acetoxymethylester to measure changes in [Ca(2+)](m), MeHg exposure caused a biphasic increase in fluorescence. The initial increase in fluorescence occurred in the absence of extracellular Ca(2+) and was abolished by mitochondrial depolarization. The secondary increase was associated with spreading of the dye from punctate staining to whole-cell distribution, and was delayed significantly by the MTP inhibitor cyclosporin A and the smooth endoplasmic reticulum Ca(2+) ATPase inhibitor thapsigargin. We conclude that MeHg causes release of Ca(2+) from the mitochondria through opening of the MTP, which contributes the bulk of the elevated [Ca(2+)](i) observed during MeHg neurotoxicity. Additionally, the Ca(2+) that enters the mitochondria seems to originate in the smooth endoplasmic reticulum, providing a mechanism for the observed mitochondrial Ca(2+) overload.

Mitochondrial transduction of ocular teratogenesis during methylmercury exposure

BACKGROUND

The purpose of the present study was to investigate the correlation between MeHg developmental toxicity and mitochondrial 16S ribosomal RNA (16S rRNA) expression in the embryonic forebrain and pharmacological intervention with PK11195, a ligand for the mitochondrial peripheral-type benzodiazepine receptor (Bzrp).

METHODS

Pregnant CD-1 mice were dosed with methylmercury (II) chloride (MeHg) with or without 4 mg/kg PK11195 on Day 9 of gestation. Fetuses were examined on Day 9 (RT-PCR), Day 15 (histology), and Day 17 (teratology).

RESULTS

MeHg (10 mg/kg) induced microcephaly, microphthalmia and cleft palate. The mean incidences of malformed fetuses were 47.7% with MeHg (P < 0.001) and 19.2% with PK11195 co-treatment (P < 0.01 for rescue). Cleft palates were 12.8% and 1.5%, respectively. An estimate of neurocranial circumference revealed a small (5%) but highly significant (P < 0.001) reduction that was rescued in a subset of co-treated fetuses (P < 0.05). RT-PCR analysis of the Day 9 forebrain revealed inhibition of 16S rRNA expression 3.0 hr after 5 mg/kg MeHg exposure (P < 0.001). This effect was rescued with PK11195 (P < 0.001). Preliminary findings revealed a similar response-rescue in cultured embryos exposed to 1 microM Hg(II) when exogenous 5-aminolevulinic acid (ALA) was added. Protoporphyrin-IX (PP9), the penultimate precursor to heme and an endogenous ligand of the Bzrp, increased in a manner that was ALA-dependent and PK11195-sensitive.

CONCLUSION

At least some teratological effects of Hg appear linked with late steps in the heme biosynthesis pathway through the Bzrp. PK11195, a ligand for these mitochondrial receptors, significantly lessens the risk of microphthalmia, microcephaly, and cleft palate in Hg-poisoned embryos.

Mechanism of methylmercury cytotoxicity

Although a large number of epidemiological, clinical, and pathological studies on methylmercury intoxication have been published, these investigations have not been able to elucidate the detailed mechanisms by which the metal alkyl causes a wide variety of biological dysfunctions. Thus, the cultured cells which are free from the influence of whole body complexities, such as absorption, distribution, metabolism, etc., which complicate the interpretation of in vivo experimental results, attract the attention of many scientists who are interested in clarifying the mode of toxic action of methylmercury. The aim of this article is to review the recent studies on the toxicity of methylmercury at the cellular level and to outline the mechanisms which have been proposed to be responsible for cell injuries.

In vitro methyl mercury inhibition of protein synthesis in neonatal cerebellar perikarya

In an effort to characterize the principal mechanism(s) underlying the methyl mercury-induced defect in protein synthesis in vivo and in vitro, we examined the dose and time related effects of methyl mercury chloride (MeHg) on a variety of cellular functions using bulk-isolated neonatal cerebellar perikarya. This cell preparation demonstrated a high specific activity for protein synthesis which was optimal between 6 and 12 days of age and declined rapidly thereafter. In vitro MeHg inhibited synthesis with an ID50 of approximately 14 microM without causing significant release of lactate dehydrogenase. The inhibition of protein synthesis occurred independently of mercurial effects on RNA synthesis, mitochondrial function, ATP content or intracellular levels of Na+ and K+. Uptake of [3H]-phenylalanine was not appreciably affected by MeHg. The accumulated evidence suggests that in this in vitro cell model, MeHg inhibition of protein synthesis occurs via a direct interaction with the protein synthetic machinery.