Glycosylation defects and virulence phenotypes of Leishmania mexicana phosphomannomutase and dolicholphosphate-mannose synthase gene deletion mutants

Leishmania parasites synthesize an abundance of mannose (Man) -containing g lycoconjugates thought to be essential for virulence to the mammalian host and for viability. These glycoconjugates include lipophosphoglycan (LPG), p roteophosphoglycans (PPGs), glycosylphosphatidylinositol (GPI)-anchored pro teins, glyco-inositolphospholipids (GIPLs), and N-glycans. A prerequisite f or their biosynthesis is an ample supply of the Man donors GDP-Man and doli cholphosphate-Man. We have cloned from Leishmania mexicana the gene encodin g the enzyme phosphomannomutase (PMM) and the previously described dolichol phosphate-Man synthase gene (DPMS) that are involved in Man activation. Sur prisingly, gene deletion experiments resulted in viable parasite lines lack ing the respective open reading frames (Delta PMM and Delta DPMS), a result against expectation and in contrast to the lethal phenotype observed in ge ne deletion experiments with fungi. L. mexicana Delta DPMS exhibits a selec tive defect in LPG, protein GPI anchor, and GIPL biosynthesis, but despite the absence of these structures, which have been implicated in parasite vir ulence and viability, the mutant remains infectious to macrophages and mice . By contrast, L. mexicana Delta PMM are largely devoid of all known Man-co ntaining glycoconjugates and are unable to establish an infection in mouse macrophages or the living animal. Our results define Man activation leading to GDP-Man as a virulence pathway in Leishmania.