Manganese toxicity in a catecholaminergic cell line

Manganese (Mn) is an essential heavy metal that can be neurotoxic at excessive levels. The fact that, in humans, Mn toxicity presents with Parkinsonian-like symptoms suggests that Mn toxicity involves an interaction with dopamine (DA), although the exact mechanisms are unknown. The following studies were designed to illuminate the mechanisms of Mn toxicity in a cell line that produces DA (CATH.a cells).; Initial studies were performed to observe the effect of antioxidants and DA depletion on Mn toxicity. While the antioxidants glutathione and N-acetyl-cysteine provided protection against Mn-induced toxicity, the depletion of DA offered no protection. This suggests that Mn toxicity, in this cell line, is due to a generation of reactive oxygen species (ROS), rather than a direct involvement of DA. Next, experiments were performed to determine if Mn induced programmed cell death (PCD). One form of cell death, apoptosis was not observed in the CATH.a cells (as evidenced by DNA laddering and caspase-3 activation). Finally, hybridization array analyses showed no significant induction or reduction of any genes, including a variety of apoptosis genes, at two distinct time points and two Mn concentrations.; Clinically, Mn toxicity involves a depletion of DA with preservation of the nigrostriatal pathway. The final set of studies, therefore, explored possible mechanisms for the depletion of DA (without loss of neurons) that initially occurs with Mn toxicity. Tyrosine hydroxylase (TH) is the initial and rate limiting enzyme in catecholamine biosynthesis. TH activity was decreased as Mn concentrations were increased. After cells were exposed for 12 hours to Mn (a time point when there is little cell death), TH mRNA and protein remained unchanged. Kinetic experiments showed that while the Michaelis Menten constant (K_m) was unchanged, the V_max for TH activity was decreased. These results provide evidence that Mn can reduce in TH activity without altering protein or mRNA. The findings are consistent with previous evidence that DA quinone (an oxidation product of DA formed in the presence of Mn) can serve as a suicide inhibitor of TH.; The studies presented here have provided valuable insight into mechanisms of Mn toxicity. The studies suggest that Mn acts to decrease DA through an interaction with the TH enzyme and that cell death eventually occurs due to the formation of ROS.