Cytoplasmic SIR2 homologue overexpression promotes survival of Leishmania parasites by preventing programmed cell death.

The Silent Information Regulator (SIR2) family of genes have been cloned from a variety of species ranging from bacteria to man. In previous studies, we reported the characterization of a Leishmania major gene encoding a protein with extensive homology to yeast SIR2p and expressed by different Leishmania species and parasite developmental stages and thus termed LmSIR2. Unlike the yeast SIR2p, LmSIR2p is mainly localized within the cytoplasm. In the present study, sequencing of a homologue encoding gene in another Leishmania species, Leishmania infantum, revealed 93% overall amino acid identity with L. major SIR2 gene. Further, using L. infantum as a recipient for a plasmid vector (pTEX) which allows overexpression of LmSIR2p led to the accumulation of the protein in the parasite cytoplasm of both promastigote and amastigote forms and a striking increase in the survival of amastigotes, the vertebrate stage of the parasite, when maintained under normal axenic culture conditions. This phenotype was also observed when L. infantum parasites were transfected with a cosmid vector (CLHyg), isolated from a L. infantum cosmid library, carrying the L. infantum SIR2 gene (CLHyg-LiSIR2). In contrast, no effect was observed on survival of the promastigote forms (insect stage) under similar culture conditions. However, when the glucose was used as a unique source of energy under starvation conditions, the viability of promastigotes was significantly enhanced. Moreover, we showed that amastigote forms in the stationary phase of culture died with a feature of apoptosis as revealed by the appearance of YOPRO-1 positive cells and that expression of LmSIR2 protein substantially delays this phenomenon. Taken together, these results demonstrate the existence of SIR2-related proteins encoding genes in different Leishmania species and suggest that LmSIR2p could participate among other factors in the control of cell death.