Cobalt toxicity and iron metabolism in Neurospora crassa

Efficaciousness of Low Affinity Compared to High Affinity TSPO Ligands in the Inhibition of Hypoxic Mitochondrial Cellular Damage Induced by Cobalt Chloride in Human Lung H1299 Cells

The 18 kDa translocator protein (TSPO) plays an important role in apoptotic cell death, including apoptosis induced by the hypoxia mimicking agent cobalt chloride (CoCl₂). In this study, the protective effects of a high (CB86; Ki = 1.6 nM) and a low (CB204; Ki = 117.7 nM) affinity TSPO ligands were investigated in H1299 lung cancer cell line exposed to CoCl₂. The lung cell line H1299 was chosen in the present study since they express TSPO and able to undergo programmed cell death. The examined cell death markers included: ATP synthase reversal, reactive oxygen species (ROS) generation, mitochondrial membrane potential (Δψm) depolarization, cellular toxicity, and cellular viability. Pretreatment of the cells with the low affinity ligand CB204 at a concentration of 100 µM suppressed significantly (p < 0.05 for all) CoCl₂-induced cellular cytotoxicity (100%), ATP synthase reversal (67%), ROS generation (82%), Δψm depolarization (100%), reduction in cellular density (97%), and also increased cell viability (85%). Furthermore, the low affinity TSPO ligand CB204, was harmless when given by itself at 100 µM. In contrast, the high affinity ligand (CB86) was significantly effective only in the prevention of CoCl₂–induced ROS generation (39%, p < 0.001), and showed significant cytotoxic effects when given alone at 100 µM, as reflected in alterations in ADP/ATP ratio, oxidative stress, mitochondrial membrane potential depolarization and cell death. It appears that similar to previous studies on brain-derived cells, the relatively low affinity for the TSPO target enhances the potency of TSPO ligands in the protection from hypoxic cell death. Moreover, the high affinity TSPO ligand CB86, but not the low affinity ligand CB204, was lethal to the lung cells at high concentration (100 µM). The low affinity TSPO ligand CB204 may be a candidate for the treatment of pulmonary diseases related to hypoxia, such as pulmonary ischemia and chronic obstructive pulmonary disease COPD.

Use of Co speciation and soil properties to explain variation in Co toxicity to root growth of barley (Hordeum vulgare L.) in different soils

The influence of soil properties on the bioavailability and toxicity of Co to barley (Hordeum vulgare L.) root elongation was investigated. Ten soils varying widely in soil properties were amended with seven doses of CoCl2. Soil properties greatly influenced the expression of Co toxicity. The effective concentration of added Co causing 50% inhibition (EC50) ranged from 45 to 863 mg kg(-1), representing almost 20-fold variation among soils. Furthermore, we investigated Co toxicity in relation to Co concentrations and free Co2+ activity in soil solution. The EC50 values showed variation among soils of 17- and 29-fold, based on the Co concentration in soil solution and free Co2+ activity, respectively. Single regressions were carried out between Co toxicity threshold values and selected soil properties. Models obtained showed that soil effective cation exchange capacity (eCEC) and exchangeable calcium were the most consistent single predictors of the EC50 values based on soil added Co.

Importance of mitochondria in survival of Cryptococcus neoformans under low oxygen conditions and tolerance to cobalt chloride

Cryptococcus neoformans is an environmental fungal pathogen that requires atmospheric levels of oxygen for optimal growth. For the fungus to be able to establish an infection, it must adapt to the low oxygen concentrations in the host environment compared to its natural habitat. In order to investigate the oxygen sensing mechanism in C. neoformans, we screened T-DNA insertional mutants for hypoxia-mimetic cobalt chloride (CoCl₂)-sensitive mutants. All the CoCl₂-sensitive mutants had a growth defect under low oxygen conditions at 37°C. The majority of mutants are compromised in their mitochondrial function, which is reflected by their reduced rate of respiration. Some of the mutants are also defective in mitochondrial membrane permeability, suggesting the importance of an intact respiratory system for survival under both high concentrations of CoCl₂ as well as low oxygen conditions. In addition, the mutants tend to accumulate intracellular reactive oxygen species (ROS), and all mutants show sensitivity to various ROS generating chemicals. Gene expression analysis revealed the involvement of several pathways in response to cobalt chloride. Our findings indicate cobalt chloride sensitivity and/or sensitivity to low oxygen conditions are linked to mitochondrial function, sterol and iron homeostasis, ubiquitination, and the ability of cells to respond to ROS. These findings imply that multiple pathways are involved in oxygen sensing in C. neoformans.

Cryptococcus neoformans is an obligate aerobic fungus that requires atmospheric levels of oxygen (21%) for optimal growth. However, the fungus is able to cause life-threatening brain infections in humans, where the oxygen tension is significantly lower than 21%. To understand the pathobiology of Cryptococcus neoformans, it is important to explore the molecular mechanisms adopted by the fungus to survive under low oxygen conditions. By using cobalt chloride, a hypoxia-mimicking agent, we isolated a number of mutants that are unable to grow in the presence of 0.7 mM CoCl₂ as well as at low oxygen conditions. In this study, we show that mitochondria play an important role for C. neoformans to survive in low oxygen conditions. We demonstrate that mutants harboring mutations in the genes related to mitochondrial functions have mitochondrial membrane permeability defect and lowered respiration rate and are more sensitive to stress generating chemicals, in addition to their inability to survive at low oxygen conditions. Finally, we also show that when wild-type cells are exposed to hypoxia-mimicking cobalt chloride, expression of genes involved in respiration and iron and sterol homeostasis, as well as ubiquitination, changes significantly.

Interrelationships in trace-element metabolism in metal toxicities in a cobalt-resistant strain of Neurospora crassa

A strain of Neurospora crassa was isolated by training the mould to grow on media containing high concentrations of Co(2+). This strain, the Co(R) strain, exhibited approximately tenfold the resistance of the parent strain to Co(2+) and Ni(2+) but not to Zn(2+) or Cu(2+). Co(2+) toxicity in the Co(R) strain was reversed by Mg(2+) but not by Fe(3+). Also, Co(2+) did not affect iron metabolism in this strain. It is suggested that the mechanism of resistance in the Co(R) strain involves an alteration in the pattern of iron metabolism such that the latter is no longer adversely affected by toxic concentrations of Co(2+). The Co(R) strain is genetically stable and is most probably a result of a resistance mutation in N. crassa induced by Co(2+).

The mechanism of uptake of cobalt ions by Neurospora crassa

Uptake of Co(2+) by 3-day-old mycelia of Neurospora crassa involves cell-surface binding as well as transport into the intracellular space. The surface binding is rapid and accounts for 30-40% of the total Co(2+) uptake. Transport of Co(2+) occurs at a rate of 40mug/h per 100mg dry wt. Surface binding and overall uptake show different temperature dependence. Metabolic inhibitors such as azide, dinitrophenol and fluoride depress transport of Co(2+). The overall uptake of Co(2+) exhibits a high K(m) value and hence the concentration mechanism is one of low ;affinity' for the metal. The uptake of Co(2+) varies linearly with pH in the range pH3 to pH6. Mg(2+) inhibits both surface binding and transport of Co(2+). It is suggested that the system that transports Mg(2+) is also involved in Co(2+) uptake by N. crassa.