Molecular mechanisms of metal toxicity in neuronal cells

Abstract

Modification of signal transduction by toxic agents can affect cell metabolism and physiological activity, impair cell capacity to adequately respond to hormones and growth stimuli and consequently compromise cell survival. This thesis describes studies on the interactions between toxic metals, at concentrations comparable to environmental exposure, and Ca2+ signalling in neuronal cells. Four tri-substituted organotin compounds (triethyltin (TET), trimethyltin (TMT), tributyltin (TBT), triphenyltin (TPT)), as well as organic (methylmercury, MeHg) and inorganic (Hg2+) forms of mercury were studied.

The role of the altered Ca2+ signalling in neurotoxicity of organotin compounds was investigated in PC12 cells. Micromolar concentrations of TBT and TPT caused intracellular Ca2+ overload, due to both enhanced Ca2+ influx and release from intracellular stores. Conditions that caused a transient increase were non-cytotoxic, whereas a Ca2+ elevation sustained for at least 30 min. induced apoptosis. In our system, TMT did not induce any measurable effect. TET induced a modest increase in intracellular Ca2+ and interfered with the Ca2+ signals induced by ATP, bradykinin (Bk) and K+. Although TET alone did not elicit norepinephrine (NE) release, it enhanced the release of NE induced by Bk and ATP. These results suggest that neurotoxic effects of TET are related to its ability to modulate Ca2+ signalling and eventually neurosecretion.

Mercury has been shown to be highly toxic to the CNS, particularly during development. We have found that nanomolar concentrations of Hg2+ enhanced NGF-induced differentiation in PC12 cells. This effect was apparently related to a modification of the activated state of the L-type Ca2+ channel, which resulted in an increased Ca2+ influx during depolarization and agonist stimulation. Conversely, marginally higher Hg2+ concentrations (1-2 ~lM) inhibited depolarization and agonist-induced Ca2+ response and caused cell death. The cytotoxic effects of Hg2+ were further investigated in primary cultures of cerebellar granule cells (CGCs). Similar to observations in PC12 cells, 100 nM Hg2+ enhanced the Ca2+ response to chemical depolarization In this system, however, potentiation of Ca2+ signalling was followed by apoptotic cell death. In addition, Hg2+ potentiated NMDA-induced toxicity. Since depolarization and NMDA signals have been shown to be essential during CNS development, a Hg2+-induced alteration of these signals may lead to permanent CNS damage.

Interestingly, a potentiation of the NMDA-induced toxicity has been observed in in vitro ex vivo experiments in which pregnant rats were exposed to MeHg and CGCs were dissociated from the offspring. The NMDA Ca2+ signal plays an important role during the processes involved in learning and memory. Rats prenatally exposed to MeHg did not show any effect on the ability of learning and memory at six months of age, indicating that the animals could have compensated for possible damages in pathways important for these functions. However, in locomotor behaviour we observed a MeHg effect, which was only detectable in male rats and not in the females, indicating a gender-dependent susceptibility to MeHg neurotoxicity.