Inositol Phosphorlyceramide Synthase null Leishmania major (Δipcs-) are viable and virulent in animal infections where salvage of host sphingomyelin predominates

Abstract

Many pathogens synthesize inositolphosphorylceramide (IPC) as the major sphingolipid (SL), differing from the mammalian host where sphingomyelin (SM) or more complex SLs predominate, and the divergence between IPCS and mammalian sphingolipid synthases has prompted interest as a potential drug target. However, in the trypanosomatid protozoan Leishmania, cultured insect stage promastigotes lacking de novo sphingolipid synthesis ({Delta}spt2-) and sphingolipids entirely survive and remain virulent, as infective amastigotes salvage host sphingolipids and continue to produce IPC. To further understand the role of IPC, we generated null IPCS mutants in L. major ({Delta}ipcs-). Unexpectedly and unlike fungi where IPCS is essential, {Delta}ipcs- was remarkably normal in culture and highly virulent in mouse infections. Both IPCS activity and IPC were absent in {Delta}ipcs- promastigotes and amastigotes, arguing against an alternative route of IPC synthesis. Notably, salvaged mammalian sphingomyelin (SM) was highly abundant in purified amastigotes from both WT and {Delta}ipcs-, and salvaged SLs could be further metabolized into IPC. SM was about 7-fold more abundant than IPC in WT amastigotes, establishing that SM is the dominant amastigote SL, thereby rendering IPC partially redundant. These data suggest that SM salvage likely plays key roles in the survival and virulence of both WT and {Delta}ipcs- parasites in the infected host, confirmation of which will require the development of methods or mutants deficient in host SL/SM uptake in the future. Our findings call into question the suitability of IPCS as a target for chemotherapy, instead suggesting that approaches targeting SM/SL uptake or catabolism may warrant further emphasis.