Inhibition of HIV type 1 reverse transcriptase assay by nucleases produced by contaminating mycoplasmas

Role of Mycoplasma penetrans endonuclease P40 as a potential pathogenic determinant

Recently, we reported the purification to homogeneity and characterization of Ca(2+)- and Mg(2+)-dependent endonuclease P40 produced by Mycoplasma penetrans (M. Bendjennat, A. Blanchard, M. Loutfi, L. Montagnier, and E. Bahraoui, J. Bacteriol. 179; 2210-2220, 1997), a mycoplasma which was isolated for the first time from the urine of human immunodeficiency virus-infected patients. To evaluate how this nuclease could interact with host cells, we tested its effect on CEM and Molt-4 lymphocytic cell lines and on peripheral blood mononuclear cells. We observed that 10(-7) to 10(-9) M P40 is able to mediate a cytotoxic effect. We found that 100% of cells were killed after 24 h of incubation with 10(-7) M P40 while only 40% cytotoxicity was obtained after 72 h of incubation with 10(-9) M P40. Phase-contrast microscopy observations of P40-treated cells revealed morphological changes, including pronounced blebbing of the plasma membrane and cytoplasmic shrinkage characteristic of programmed cell death, which is in agreement with the internucleosomal fragmentation of P40-treated cell DNA as shown by agarose gel electrophoresis. We showed that (125)I-radiolabeled or fluorescein isothiocyanate-labeled P40 was able to bind specifically in a dose-dependent manner to the cell membrane of CEM cells, which suggested that the cytotoxicity of P40 endonuclease was mediated by its interaction with the cell surface receptor(s). The concentration of unlabeled P40 required to inhibit by 50% the formation of (125)I-P40-CEM complexes was about 3 x 10(-9) M, indicating a high-affinity interaction. Both P40 interaction and cytotoxicity are Ca(2+) dependent. Our results suggest that the cytotoxicity of M. penetrans observed in vitro is mediated at least partially by secreted P40, which, after interaction with host cells, can induce an apoptosis-like death. These results strongly suggest a major role of mycoplasmal nucleases as potential pathogenic determinants.

Purification and characterization of Mycoplasma penetrans Ca2+/Mg2+-dependent endonuclease

The major nuclease from Mycoplasma penetrans has been purified to homogeneity. The enzyme seems to be present as a membrane-associated precursor of 50 kDa and as a peripheral membrane monomeric polypeptide of 40 kDa that is easily removed by washing of cells with isotonic buffers and in the aqueous phase upon Triton partitioning of Triton X-114-solubilized protein. The 40-kDa nuclease was extracted from M. penetrans cells by Triton X-114 and phase fractionation and was further purified by chromatography on Superdex 75 and chelating Sepharose (Zn2+ form) columns. By gel filtration, the apparent molecular mass was 40 kDa. The purified enzyme exhibits both a nicking activity on superhelical and linear double-stranded DNA and a nuclease activity on RNA and single-stranded DNA. No exonuclease activity was found for this enzyme. This nuclease required both Mg2+ (optimum, 5 mM) and Ca2+ (optimum, 2 mM) for activity and exhibited a pH optimum between pH 7 and 8 for DNase activity. It was inhibited by Zn2+, Mn2+, heparin, sodium dodecyl sulfate, and chelator agents such EDTA and EGTA, but no effect was observed with ATP, 2-mercaptoethanol, N-ethylmaleimide, dithiothreitol, nonionic detergents, phenylmethylsulfonyl fluoride, and iodoacetamide. Nuclease activity was inhibited by diethylpyrocarbonate at both pH 6 and 8 and by pepstatin, suggesting the involvement of a histidine and an aspartate in the active site. When added to human lymphoblast nuclei, the purified M. penetrans endonuclease induced internucleosomal fragmentation of the chomatin into oligonucleosomal fragments. On the basis of this result, and taking into account the fact that M. penetrans has the capacity to invade eucaryotic cells, one can suggest, but not assert, that produced Ca2+/Mg2+-dependent endonuclease may alter the nucleic acid metabolism of host cells by DNA and/or RNA degradation and may act as a potential pathogenic determinant.