Inositolphosphoceramide metabolism in Trypanosoma cruzi as compared with other trypanosomatids

Abstract

Chagas disease is caused by Trypanosoma cruzi and is endemic to North, Central and South American countries. Current therapy against this disease is only partially effective and produces adverse side effects. Studies on the metabolic pathways of T. cruzi, in particular those with no equivalent in mammalian cells, might identify targets for the development of new drugs. Ceramide is metabolized to inositolphosphoceramide (IPC) in T. cruzi and other kinetoplastid protists whereas in mammals it is mainly incorporated into sphingomyelin. In T. cruzi, in contrast to Trypanosoma brucei and Leishmania spp., IPC functions as lipid anchor constituent of glycoproteins and free glycosylinositolphospholipids (GIPLs). Inhibition of IPC and GIPLs biosynthesis impairs differentiation of trypomastigotes into the intracellular amastigote forms. The gene encoding IPC synthase in T. cruzi has been identified and the enzyme has been expressed in a cell-free system. The enzyme involved in IPC degradation and the remodelases responsible for the incorporation of ceramide into free GIPLs or into the glycosylphosphatidylinositols anchoring glycoproteins, and in fatty acid modifications of these molecules of T. cruzi have been understudied. Inositolphosphoceramide metabolism and remodeling could be exploited as targets for Chagas disease chemotherapy.

Fig. 1

Structure of the lipid in the free GIPLs and GPI anchors of the major glycoproteins of Trypanosoma cruzi. The glycoproteins of T. cruzi may be anchored by a glycerolipid (PI) or by a ceramide lipid (IPC). The PI is constituted by an alkylglycerol, either acylated (AAG) or not (AG). The alkylglycerol is always hexadecylglycerol (C_16:0) and may be esterified with fatty acid, mainly palmitic acid (C_{16: 0}) or a C₁₈ fatty acid, which may be unsaturated. No diacylglycerol was found in the mature GPIs. In the IPC anchors only the C₁₈ long chain bases were found, either saturated such as dihydrosphingosine (DHS) or unsaturated in the case of sphingosine (S). The amide fatty acid is mainly palmitic acid, lignoceric acid (C_24:0) or stearic acid (C_18:0). Abbreviations: ethanolaminephosphate (EtNP), glycan (G), inositol (I), phosphate (P), alkyl substituent (R₁), acyl substituent (R₂), amide substituent (R₃), phosphatidylinositol (PI), inositolphosphoceramide (IPC). Epi and trypo mean epimastigote and trypomastigote forms of the parasite. The (−) symbol indicates that no substituent is present in the 2-position of glycerol.

Fig. 2

Structures of IPL, GPI-precursor and mature GPI-anchored proteins (GPI-AP) in mammals, yeast and T. cruzi. A. Core structure of the GPI anchor showing substitutions (R₁–R₇). B. Structure of IPLs and substitutions in the core glycan of GPI precursors and mature GPI-AP. IPL, inositolphospholipid; DAG, diacylglycerol; AAG, alkylacylglycerol; EtNP, ethanolaminephosphate, AEP, aminoethylphosphonic acid; Galf, galactofuranose

Fig. 3

Predicted pathway for IPC biosynthesis and degradation in T. cruzi. Abbreviations: SPL, serine palmitoyl transferase; 3KSR, 3 ketodihydrosphingosine reductase; DHCS, dihydroceramide synthase; DHCD, dihydroceramide desaturase; IPCS, inositolphosphoceramide synthase; ISC, inositol sphingolipid phospholipase C; SK, sphingosine kinase; SPL, sphingosine-1-phosphate lyase; TcPI-PLC, T. cruzi phosphoinositide phospholipase C.

Fig. 4

Proposed biosynthesis of GIPLs in T. cruzi. Inositolphosphoceramide is not an acceptor for the first sugar in the biosynthesis of GIPLs. The glycerolipid in the PI acceptor has AAG or DAG. DAG is not present in mature GIPLs that may contain ceramide. Thus, a remodeling step must introduce a ceramide. The remodeling enzymes were not characterized in T. cruzi. Abbreviations: phosphatidylinositol (PI), alkylacylglycerol (AAG), diacylglycerol (DAG), inositolphosphoceramide (IPC), ceramide (Cer).