Sphingomyelin and ceramide metabolism

Sphingolipids are a class of lipids which possesses sphingoid bases such as sphingosine as backbone. Like other membrane phospholipids, sphingolipids are ampipathic possessing hydrophilic heads and hydrophobic tails. Sphingolipids are important in various cell signalling processes as first and second messengers including cell proliferation, differentiation and apoptosis in higher eukaryotes. They are also important constituents of lipid rafts of cell membranes. The simplest sphingolipids is ceramide which has very limited hydrophilicity as it only has 2 hydroxyl groups and no other polar groups are present. In addition to its role in signalling processes, it is also the precursor for the biosynthesis of other complex sphingolipids [1]. Animals possess two different types of these complex sphingolipids, which are sphiongophospholipids and glycosphingolipids respectively. The main sphingophospholipid in mammals is sphingomyelin. Sphingomyelin possesses a phosphocholine group attached to the ceramide backbone. The glycosphingolipids are molecules which have sugar residues attached to the ceramide backbone. The three different types of glycosphingolipids are cerebrosides (1 sugar moiety attached to hydroxyl-group in ceramide), globosides (more than 1 sugar groups attached to ceramide) and gangliosides (at least 1  N-acetylneuraminic acid residue is attached to the sugar chain). In contrast to animals, inositol phosphorylceramide is generated from ceramide in plants, fungi and kinetoplastids such as Trypanosoma and Leishmania [2].

There are biochemical evidence available in both Plasmodium falciparum and Toxoplasma gondii to suggest the presence of sphingomyelin and ceramide metabolism pathway. The study conducted with P. falciparum sphingomyelin synthase inhibitors showed that it inhibits blood-stage parasite growth and alters the tubovesicular structure [3]. Further evidence demonstrated that the tubovesicular structure development does not require de novo ceramide biosynthesis [4]. The sphingomyelin synthase utilises ceramide derived from host sphingolipids via the action of the enzyme sphingomyelinase (sphingomyelin phosphodiesterase). The inhibition of this enzyme has resulted in the inhibition of intra-erythrocyte stage P. falciparum growth [5]. In addition, inhibition with fumonisin B1, inhibitor of sphingosine N-acyltransferase has not affected production of glycosphingolipids, suggesting it can also occur independent of de novo ceramide biosynthesis. The use of myriocin and cycloserine, inhibitors of serine C-palmitoyltransferase has led to reduction in levels of glycosphingolipid generation and also led to reduction in parasite multiplication levels to certain extent. This suggested the importance of de novo synthesis pathway in parasite cultures [4]. The inhibition of ceramide glucosyltransferase (glucosylceramide synthase) with threo-PPMP has resulted in the depletion of glucosylceramides and resulted in inhibition of intra-erythrocyte parasite growth. This is also suggested to be a good drug-target in P. falciparum [6]. The enzymes sphingomyelinase and glucosylceramide synthase are also included in the MPMP sphingomyelin and ceramide metabolism pathway reconstruction.

Although understanding of sphingolipid biosynthesis in T. gondii is not as advanced as in the case of P. falciparum, the presence of de novo ceramide biosynthesis pathway was demonstrated. This study also confirmed that the glycosphingolipids produced are of globo-type [7]. The incorporation of radio-labelled UDP-galactose was also observed in T. gondii and P. falciparum [8], although the genes for enzyme catalysing this reaction, ceramide galactosyltransferase was not identified in either genomes. The inhibition of in vitro replication of T. gondii with aureobasidin A, an inhibitor of inositol phosphorylceramide synthase (catalyses conversion of ceramide to inositol phosphorylceramide in plants and fungi) is an interesting finding [9] and may suggest the presence of both animal and plant/fungi forms of sphingolipids. The absence of any cytotoxic effects in host cells suggests that this inhibition as an effective drug target. The gene for IPC synthase is not identified in T. gondii genome. As it is not possible to confirm the presence of enzymatic activities of ceramide galactosyltransfearse and IPC synthase in the parasite with the evidence available, these enzymes are not incorporated into the metabolic pathway map below.

Open in a new window

Sources and fates of metabolites