Metabolic Cooperation of Glucose and Glutamine Is Essential for the Lytic Cycle of Obligate Intracellular Parasite Toxoplasma gondii

Abstract

Toxoplasma gondii is a widespread protozoan parasite infecting nearly all warm-blooded organisms. Asexual reproduction of the parasite within its host cells is achieved by consecutive lytic cycles, which necessitates biogenesis of significant energy and biomass. Here we show that glucose and glutamine are the two major physiologically important nutrients used for the synthesis of macromolecules (ATP, nucleic acid, proteins, and lipids) in T. gondii, and either of them is sufficient to ensure the parasite survival. The parasite can counteract genetic ablation of its glucose transporter by increasing the flux of glutamine-derived carbon through the tricarboxylic acid cycle and by concurrently activating gluconeogenesis, which guarantee a continued biogenesis of ATP and biomass for host-cell invasion and parasite replication, respectively. In accord, a pharmacological inhibition of glutaminolysis or oxidative phosphorylation arrests the lytic cycle of the glycolysis-deficient mutant, which is primarily a consequence of impaired invasion due to depletion of ATP. Unexpectedly, however, intracellular parasites continue to proliferate, albeit slower, notwithstanding a simultaneous deprivation of glucose and glutamine. A growth defect in the glycolysis-impaired mutant is caused by a compromised synthesis of lipids, which cannot be counterbalanced by glutamine but can be restored by acetate. Consistently, supplementation of parasite cultures with exogenous acetate can amend the lytic cycle of the glucose transport mutant. Such plasticity in the parasite's carbon flux enables a growth-and-survival trade-off in assorted nutrient milieus, which may underlie the promiscuous survival of T. gondii tachyzoites in diverse host cells. Our results also indicate a convergence of parasite metabolism with cancer cells.Toxoplasma gondii is an opportunistic intracellular pathogen of the phylum Apicomplexa that inflicts life-threatening acute and chronic infections in humans and animals (1). The parasite causes ocular and cerebral toxoplasmosis and sporadic abortion/stillbirth as a consequence of severe tissue necrosis in individuals with a poor or weakened immunity, e.g. neonates, AIDS, and organ-transplantation patients. The lytic cycle of T. gondii starts with the active energy-dependent invasion of host cells by the tachyzoite stage that is followed by rapid intracellular replication, egress by host-cell lysis, and reinvasion of adjacent cells. Upon infection of host cells in vitro, one tachyzoite can usually produce 32-64 daughter cells within a nonfusogenic vacuole, which provides a safe niche for the parasite replication and an interface for acquisition of host resources (1). Such an efficient asexual reproduction and concomitant expansion of the parasite vacuole requires a significant nutritional import and biomass synthesis within the parasite. Moreover, tachyzoites likely need to adjust metabolism according to variable bioenergetic demands of extracellular and intracellular states as well as to the nutritional cues in distinct...

The intracellular parasite Toxoplasma gondii harbors three druggable FNT-type formate and l-lactate transporters in the plasma membrane

“…S4). An apparent redundancy in L-lactate transport is also in accordance with the exceptional metabolic plasticity observed in the central carbon metabolism of tachyzoites (16,18,19). FNTs discriminate between different monocarboxylate substrates by size selection in the ⌽/K filter region (7).…”

“…Besides mere redundancy, it is plausible that they serve different parasite stages during the natural life cycle. Glucose import is critical for Plasmodium species (17,18), whereas T. gondii tachyzoites can survive without it, albeit with a modest growth defect (13,16). In accord with PfFNT, inhibition of the singular hexose transporter in Plasmodium (PfHT) kills the parasite (17,29), validating both PfHT and PfFNT as antimalarial targets.…”

“…Earlier work has demonstrated the expression of high-affinity sugar permeases in both parasites (12,13), which allow import of glucose (besides other hexoses) and its subsequent catabolism through glycolysis (14 -16). Surprisingly, glucose import is essential for asexual growth of the acute (termed merozoite) stage in Plasmodium (17,18) but dispensable for the acute (termed tachyzoite) stage of T. gondii (16,18,19). Previous works by several groups have also characterized lactate dehydrogenases in the two parasites (20,21), which are meant to enable recycling of NADH ϩ to NAD ϩ and thus ensure a continued glycolysis.…”

Using metabolomics to dissect host–parasite interactions

“…While initially thought to have a streamlined metabolism and to be primarily dependent on glycolysis for ATP synthesis [28,29], recent metabolomic profiling and 13 Cglucose/glutamine labeling studies on T. gondii infected human foreskin fibroblasts have shown that intracellular tachyzoites co-utilize host glucose and glutamine and have an active mitochondrial metabolism in which pyruvate/oxaloacetate (derived from glucose) or a-ketoglutarate/g-aminobutyric acid (derived from glutamine) are catabolized in a canonical tricarboxylic acid (TCA) cycle ( Figure 1a). Remarkably, disruption of glucose uptake [30 ] or glutamine deprivation [31] has little effect on tachyzoite proliferation in host cells, suggesting that they are able to readily switch between glycolytic and gluconeogenic metabolic states. Even under conditions where both glucose and glutamine uptake/availability is limited, tachyzoites continue to grow slowly, suggesting that they can use other amino acids or alternative carbon sources, such as acetate/fatty acids [31,32].…”

“…Remarkably, disruption of glucose uptake [30 ] or glutamine deprivation [31] has little effect on tachyzoite proliferation in host cells, suggesting that they are able to readily switch between glycolytic and gluconeogenic metabolic states. Even under conditions where both glucose and glutamine uptake/availability is limited, tachyzoites continue to grow slowly, suggesting that they can use other amino acids or alternative carbon sources, such as acetate/fatty acids [31,32]. The metabolic flexibility of these stages is likely to be an important factor underlying the capacity of tachyzoites to proliferate in many different cell types.…”