Reduced glycerol incorporation into phospholipids contributes to impaired intra-erythrocytic growth of glycerol kinase knockout Plasmodium falciparum parasites

Highlights

• The glycerol kinase gene in P. falciparum was disrupted.

• Glycerol kinase knockout parasites grew slower compared to wild type parasites.

• Glycerol incorporation into phospholipids of knockout parasites was compromised.

• Glycerol metabolism is required for optimal P. falciparum blood stage development.

Abstract

Background

Malaria is a devastating disease and Plasmodium falciparum is the most lethal parasite infecting humans. Understanding the biology of this parasite is vital in identifying potential novel drug targets. During every 48-hour intra-erythrocytic asexual replication cycle, a single parasite can produce up to 32 progeny. This extensive proliferation implies that parasites require substantial amounts of lipid precursors for membrane biogenesis. Glycerol kinase is a highly conserved enzyme that functions at the interface of lipid synthesis and carbohydrate metabolism. P. falciparum glycerol kinase catalyzes the ATP-dependent phosphorylation of glycerol to glycerol-3-phosphate, a major phospholipid precursor.

Methods

The P. falciparum glycerol kinase gene was disrupted using double crossover homologous DNA recombination to generate a knockout parasite line. Southern hybridization and mRNA analysis were used to verify gene disruption. Parasite growth rates were monitored by flow cytometry. Radiolabelling studies were used to assess incorporation of glycerol into parasite phospholipids.

Results

Disruption of the P. falciparum glycerol kinase gene produced viable parasites, but their growth was significantly reduced to 56.5 ± 1.8% when compared to wild type parasites. ¹⁴C-glycerol incorporation into the major phospholipids of the parasite membrane, phosphatidylcholine and phosphatidylethanolamine, was 48.4 ± 10.8% and 53.1 ± 5.7% relative to an equivalent number of wild type parasites.

Conclusions

P. falciparum glycerol kinase is required for optimal intra-erythrocytic asexual parasite development. Exogenous glycerol may be used as an alternative carbon source for P. falciparum phospholipid biogenesis, despite the lack of glycerol kinase to generate glycerol-3-phosphate.

General significance

These studies provide new insight into glycerolipid metabolism in P. falciparum.

Introduction

Malaria is the most deadly vector-borne disease in the world. The World Health Organization estimated that in 2010, Plasmodium parasites were responsible for 216 million cases of infections and approximately 665,000 deaths in 106 malaria-endemic countries [1]. The main cause of morbidity and mortality is Plasmodium falciparum, which is the most lethal of the five parasite species infecting humans. The negative socio-economic impact of the disease, the lack of an effective malaria vaccine, parasite resistance to current anti-malaria drugs and vector resistance to insecticides emphasize the need to identify novel drug targets and develop alternative treatment strategies to combat the disease.

The life cycle of P. falciparum parasites is complex and involves an asexual developmental stage in the human host and a sexual developmental stage in the mosquito vector. Malaria is transmitted through injection of sporozoites from an infected female Anopheles mosquito into the bloodstream of the human host during a blood meal. Sporozoites rapidly migrate to and invade hepatocytes, where they develop to produce tens of thousands of merozoites. These are released into the bloodstream where they invade erythrocytes and during the 48-hour intra-erythrocytic development and replication cycle, a single parasite can produce 16–32 daughter merozoites that subsequently invade other erythrocytes. It is this stage that is responsible for the pathogenesis of malaria [2]. A small proportion of parasites differentiate into male and female gametocytes. These sexual parasites are transmitted to the mosquito during feeding and are essential in ensuring the continued survival of the parasite. Within the mosquito midgut, gamete formation and fertilization take place, followed by the development of thousands of sporozoites within an oocyst and the migration of these sporozoites to the salivary glands completing the Plasmodium life cycle [3].

The extensive proliferation throughout the parasite lifecycle requires a significant increase in lipids, particularly phospholipids (PLs), for successful growth and development of the parasite. Plasmodium-infected red cells contain significantly increased amounts of lipids compared to uninfected erythrocytes. In uninfected red cells, PLs and cholesterol are the major components of red cell membranes, co-existing at a molar ratio of approximately 1.0 [4]. Phosphatidylcholine (30–40%), phosphatidylethanolamine (30–35%), sphingomyelin (15%) and phosphatidylserine (10–20%) are the main PLs. Other phospholipids, such as phosphatidylinositol, phosphatidic acid, cardiolipin and lyso-PLs account for less than 5% of the total PLs, and neutral lipids and fatty acids are scarcely detectable [4]. Upon P. falciparum infection, a 5–6-fold increase [5] in erythrocyte PL content, specifically phosphatidylcholine, phosphatidylethanolamine and phosphatidylinositol, is observed, although the PL composition of the host erythrocyte membrane is not changed significantly [6]. Parasite membranes contain predominantly phosphatidylcholine (40–50%), phosphatidylethanolamine (35–45%) and phosphatidylinositol (4–11%), with sphingomyelin and phosphatidylserine accounting for < 5% [5]. In marked contrast to erythrocytes, there is hardly any cholesterol in parasite membranes due to the parasite's inability to synthesize sterols [5], [7].

P. falciparum meets the high demand for PL species through synthesis from metabolites produced within the parasite and from host precursors [7]. The polar head groups of PLs, as well as fatty acids, are taken up from the host serum, although parasites have the capacity for de novo type II fatty acid synthesis [8].

The parasite's requirement for PLs during growth and development implies that the enzymes, proteins and transporters involved in lipid metabolism may be attractive drug target candidates [9]. Enzymes involved in the Kennedy pathways, for example, are refractory to genetic disruption in Plasmodium berghei [10] and may be essential in blood-stage development of this rodent parasite. Disruption of the P. falciparum phosphoethanolamine methyl transferase in the serine decarboxylation phosphoethanolamine methylation pathway reduced parasite growth rates and lead to increased cell death [11]. Choline analogs have also been shown to inhibit P. falciparum and Plasmodium vivax parasite development in vitro and clear P. falciparum and Plasmodium cynomolgi parasites from infected monkeys [12].

All PLs have a glycerol backbone and glycerophospholipid biosynthesis is initiated when two fatty acid molecules are transferred from a donor to glycerol-3-phosphate (G3P), an important metabolite at the interface of carbohydrate and lipid metabolism. In P. falciparum, G3P is thought to be produced primarily from the glycolytic intermediate, dihydroxyacetone phosphate [13], since glucose consumption is increased up to 100-fold in parasitized red blood cells (RBCs) [14]. However, an alternative route exists whereby glycerol is taken up from the host erythrocyte [15] and incorporated into PLs as shown by metabolic labeling studies in Plasmodium knowlesi [16]. Phosphorylation of glycerol to produce G3P is catalyzed by glycerol kinase (GK), a highly conserved enzyme. The P. falciparum GK orthologue (PfGK, PlasmoDB gene ID PF3D7_1351600) has approximately 50% amino acid similarity to both human and Escherichia coli enzymes [17]. It has been expressed in vitro [17] and crystallized [18]. Interestingly, despite mRNA expression throughout parasite development [19], PfGK gene disruption showed that the enzyme appeared to be non-essential in blood stage development [18]. However, the characterization of a P. falciparum aquaglyceroporin (PfAQP) glycerol facilitator [15], [20], the presence of an erythrocyte aquaglyceroporin (hAQP3) [21] and the ability of the parasite to utilize serum glycerol as a precursor for PL synthesis [16] indicate that the role of glycerol during the intra-erythrocytic developmental stage of the parasite should be further investigated.