Leishmania salvage and remodelling of host sphingolipids in amastigote survival and acidocalcisome biogenesis.

Sphingolipids (SLs) play essential roles in most eukaryotes, but in the trypanosomatid protozoan Leishmania major their functions differ significantly. Previously we showed that null mutants defective in de novo sphingoid base synthesis (spt2-) lacked SLs but grew well and retained lipid rafts while replicating as promastigotes in vitro. However, they experienced catastrophic defects in membrane trafficking on entry into stationary phase, and failed to differentiate to the infective metacyclic form. Here we showed this mutant retained the ability to enter macrophages silently and inhibit activation, although as expected most parasites were destroyed. However, in mouse infections, after a delay rapidly progressive lesions appeared, and purified amastigotes were fully virulent to macrophages and mice. Mass spectrometry of spt2- amastigote lipids revealed the presence of high levels of parasite-specific inositol phosphorylceramides (IPCs) not synthesized by the mammalian hosts. Inhibitor studies showed that salvage occurs at the level of complex SLs, suggesting that parasites carry out 'headgroup' remodelling. Additionally, we describe a new defect of the spt2- promastigotes involving 'empty' acidocalcisomes (ACs), which may point to the origin of this organelle from the lysosome-related organelle/multivesicular body biogenesis pathway. However, ACs in spt2- amastigotes appeared quantitatively and morphologically normal. Thus salvage of SLs and other molecules by intracellular amastigotes play key roles in AC biogenesis and parasite survival in the host.