Accumulation of metal-binding peptides in fission yeast requires hmt2+.

The fission yeast Schizosaccharomyces pombe detoxifies cadmium by synthesizing phytochelatins, peptides of the structure (gamma-GluCys)nGly, which bind cadmium and mediate its sequestration into the vacuole. The fission yeast protein HMT2, a mitochondrial enzyme that can oxidize sulphide, appears to be essential for tolerance to multiple forms of stress, including exposure to cadmium. We found that the hmt2- mutant is unable to accumulate normal levels of phytochelatins in response to cadmium, although the cells possess a phytochelatin synthase that is active in vitro. Radioactive pulse-chase experiments demonstrated that the defect lies in two steps: the synthesis of phytochelations and the upregulation of glutathione production. Phytochelatins, once formed, are stable. hmt2- cells accumulate high levels of sulphide and, when exposed to cadmium, display bright fluorescent bodies consistent with cadmium sulphide. We propose that the precipitation of free cadmium blocks phytochelatin synthesis in vivo, by preventing upregulation of glutathione production and formation of the cadmium-glutathione thiolate required as a substrate by phytochelatin synthase. Thus, although sulphide is required for phytochelatin-mediated metal tolerance, aberrantly high sulphide levels can inhibit this pathway. Precise regulation of sulphur metabolism, mediated in part by HMT2, is essential for metal tolerance in fission yeast.

A fission yeast gene for mitochondrial sulfide oxidation.

A cadmium-hypersensitive mutant of the fission yeast Schizosaccharomyces pombe was found to accumulate abnormally high levels of sulfide. The gene required for normal regulation of sulfide levels, hmt2(+), was cloned by complementation of the cadmium-hypersensitive phenotype of the mutant. Cell fractionation and immunocytochemistry indicated that HMT2 protein is localized to mitochondria. Sequence analysis revealed homology between HMT2 and sulfide dehydrogenases from photosynthetic bacteria. HMT2 protein, produced in and purified from Escherichia coli, was soluble, bound FAD, and catalyzed the reduction of quinone (coenzyme Q2) by sulfide. HMT2 activity was also detected in isolated fission yeast mitochondria. We propose that HMT2 functions as a sulfide:quinone oxidoreductase. Homologous enzymes may be widespread in higher organisms, as sulfide-oxidizing activities have been described previously in animal mitochondria, and genes of unknown function, but with similarity to hmt2(+), are present in the genomes of flies, worms, rats, mice, and humans.

Role of determinants of cadmium sensitivity in the tolerance of Schizosaccharomyces pombe to cisplatin.

The genetic mechanisms underlying cisplatin (DDP) resistance in yeast were investigated by examining the cytotoxicity of DDP to Schizosaccharomyces pombe mutants that were either hypersensitive or resistant to Cd. Despite reports that have linked glutathione (GSH) to DDP resistance in human cancer cells, we found that a mutant of S. pombe that was hypersensitive to Cd by virtue of a 15-fold reduction in GSH level and lack of phytochelatin production was as tolerant as the wild-type strain to DDP. A mutant that harbored a mutation in hmt1, the gene encoding an ATP-binding cassette-type transporter for vacuolar sequestration of a phytochelatin/Cd complex, exhibited only mild hypersensitivity to DDP even though it was 100-fold more sensitive to Cd. Overexpression of hmt1 in wild-type or mutant cells conferred tolerance to Cd but failed to do the same for DDP. However, a strain that produced 6-fold more sulfide than wild-type cells was found to be 6-fold more resistant to DDP and twice as resistant to Cd; an association between DDP resistance and sulfide production was observed in three other strains that were examined, and overproduction of sulfide was accompanied by reduced platination of DNA. These results indicate that GSH and the GSH-derived phytochelatin peptides do not play critical roles in determining sensitivity to DDP in S. pombe but rather identify increased production of sulfide as a possible new mechanism of DDP resistance that may also be relevant to human cells.

Dystrophic neurites infiltrate extracellular neurofibrillary tangles in Alzheimer disease.

The neurotrophic activity of beta-amyloid protein (beta-AP) has been suggested to be responsible for the dystrophic neurites that surround beta-AP deposits in senile plaques of Alzheimer disease. The recent finding that neurofibrillary tangles (NFT) that remain as remnants in the extracellular space (E-NFT) after the death of the neuron contain beta-AP, suggested that dystrophic neurites might also be associated with E-NFT. In this study, we use a probe for E-NFT, basic fibroblast growth factor (bFGF)-binding to show that E-NFT do contain dystrophic neurites. Since these neurites contain the amyloid precursor protein whose cleavage can lead to beta-AP, they may also play a role in further beta-AP deposition in the E-NFT.

Severe respiratory infection with Branhamella catarrhalis in an African child.

A pure growth of Branhamella catarrhalis was obtained from a purulent bronchial exudate in a 28-month-old Rwandese girl, hospitalized for acute inspiratory dyspnoea with fever. The outcome was favourable under treatment with ampicillin, although the isolate was shown to produce a beta-lactamase in vitro.