Perturbations in fumarate levels in Plasmodium berghei leads to cysteine succination and impairs ookinete formation

This study demonstrates that perturbing fumarate metabolism in *Plasmodium berghei* leads to cysteine succination and oxidative stress, which specifically impairs ookinete formation while sparing erythrocytic stages, thereby highlighting parasite-specific fumarate hydratase as a promising target for transmission-blocking antimalarial drugs.

The Gist

The Big Picture: A Parasite's Two-Part Life

Imagine the malaria parasite (Plasmodium) as a traveler with a very specific itinerary. It starts its journey inside a human (the "blood stage"), where it multiplies rapidly. To survive and spread, it must eventually switch gears, turn into a special form called a gametocyte, and get picked up by a mosquito. Once inside the mosquito, it must transform again into a ookinete to complete its life cycle and infect the next human.

This study asks: What happens if we break the parasite's internal "engine" while it's trying to make that switch?

The Engine: The TCA Cycle

Inside the parasite's cells, there is a metabolic engine called the TCA cycle (or Krebs cycle). Think of this engine as a factory assembly line that processes fuel (sugar and amino acids) to create energy and building blocks.

The researchers focused on a specific part of this assembly line involving two chemicals: Fumarate and Malate. They also looked at the "delivery trucks" (transporters) that move these chemicals in and out of the cell's power plant (the mitochondria).

They created "knockout" parasites—essentially parasites with specific genes deleted, meaning they lack the enzymes or trucks needed to handle these chemicals.

The Experiment: Breaking the Trucks and the Factory

The team tested several types of broken parasites:

1. Missing the Factory Worker (FH): The enzyme that turns Fumarate into Malate.
2. Missing the Other Worker (MQO): An enzyme that processes Malate.
3. Missing the Delivery Trucks (DTC & OGC): The transporters that move these chemicals.

The Result:

• In the Human (Blood Stage): Surprisingly, the broken parasites didn't care. They multiplied just fine. The blood stage is like a "lazy" mode where the parasite doesn't need this specific part of the engine running at full speed.
• In the Mosquito (Ookinete Stage): Disaster struck. When the parasites tried to turn into the mosquito-infecting form, they completely failed. The "broken" parasites could not make the transformation. They were stuck in limbo and died.

The Villain: The "Sticky" Glue (Succination)

Why did they fail? The researchers found a toxic side effect.

Normally, the enzyme FH acts like a drain, keeping the level of Fumarate low. When FH is missing (or the trucks are broken), Fumarate builds up like a backed-up traffic jam.

Fumarate is chemically "sticky." It acts like super-glue.

• The parasite has a vital antioxidant called Glutathione (GSH), which acts like a "fire extinguisher" to put out oxidative stress (chemical fires) in the cell.
• The sticky Fumarate glues itself to the Glutathione. This process is called succination.
• Once Glutathione is "glued" (succinated), it can't work anymore. The fire extinguisher is broken.
• Without a working fire extinguisher, the cell gets overwhelmed by oxidative stress (chemical fires), and the parasite dies before it can become an ookinete.

The researchers even found that this "glue" was sticking to important proteins inside the parasite, messing up their functions, much like gum stuck in a car engine.

The Parasite's Desperate Attempt to Fix It

The parasite tried to fight back. It realized it was under chemical stress and tried to ramp up its own fire-fighting system.

• It turned on a backup generator called the Pentose Phosphate Pathway (PPP).
• This pathway produces NADPH, a chemical that helps recharge the fire extinguishers.
• The researchers saw that the parasite was working overtime to make this fuel, but it wasn't enough to overcome the massive amount of "glue" (succination) caused by the Fumarate backup.

The Twist: Who Drives Which Truck?

The paper also solved a mystery about the delivery trucks (transporters):

• DTC Truck: Previously thought to carry only one type of cargo, the researchers found it primarily carries Malate (and some Fumarate).
• OGC Truck: This one is specialized for Fumarate.
• When these trucks were removed, the chemicals they usually carried piled up outside the power plant, causing the toxic backup.

The Takeaway: A New Way to Stop Malaria

This is exciting for malaria control because:

1. It's a Weak Spot: The parasite needs this specific pathway to survive in the mosquito, but not in humans.
2. It's Unique: The parasite's version of the enzyme (FH) is structurally different from the human version.
3. The Strategy: If we can develop a drug that specifically blocks the parasite's FH enzyme (or its trucks), we can cause a "traffic jam" of Fumarate inside the parasite. This will glue up its fire extinguishers, kill it in the mosquito, and stop the malaria from spreading to the next person.

In short: The researchers found that clogging the parasite's chemical drain causes a toxic glue to form, destroying the parasite just as it tries to jump from human to mosquito. This gives scientists a new, highly specific target to block malaria transmission.

Technical Summary

1. Problem Statement

Malaria remains a significant global health burden, and the emergence of drug resistance necessitates new intervention strategies. While the Tricarboxylic Acid (TCA) cycle is known to be non-essential for the asexual blood stages of Plasmodium parasites, its role in the mosquito-stage development (specifically the formation of ookinetes, which are crucial for transmission) is less understood.

• The Gap: Previous studies suggested that TCA cycle enzymes like Fumarate Hydratase (FH) and Malate Quinone Oxidoreductase (MQO) are non-essential in asexual stages. However, the metabolic consequences of disrupting fumarate and malate metabolism during the transition from gametocytes to ookinetes were unclear.
• The Hypothesis: The authors hypothesized that perturbations in fumarate metabolism (via gene knockouts) would lead to specific metabolic toxicities (such as protein succination) that impair the parasite's ability to develop into transmission-competent ookinetes.

2. Methodology

The study utilized Plasmodium berghei (Pb) as the model organism, employing a combination of genetic, metabolomic, and isotopic tracing techniques.

• Genetic Manipulation: The researchers generated single and double knockout (KO) lines for genes involved in fumarate/malate metabolism and transport:
  • Enzymes: fh (fumarate hydratase), mqo (malate quinone oxidoreductase), mdh (malate dehydrogenase).
  • Transporters: dtc (dicarboxylate-tricarboxylate carrier), ogc (citrate-oxoglutarate carrier).
  • Double KOs: mqo/mdh (mm) and dtc/ogc (od).
• Phenotypic Assays:
  • Assessed asexual erythrocytic growth rates and mouse mortality.
  • Measured gametocyte formation (male/female ratios).
  • Quantified in vitro ookinete conversion efficiency.
• Metabolomics:
  • Semi-targeted LC-MS: Performed on gametocytes to quantify 23 metabolites, focusing on TCA intermediates, glutathione species, and nucleotides.
  • Proteomics: Western blotting and Mass Spectrometry (MS/MS) were used to detect 2-(succino)cysteine (2SC) modifications on proteins.
• Isotope Tracing:
  • $^{13}$C$_6$-Glucose: Traced carbon flux through glycolysis and the TCA cycle to determine substrate entry and pathway activity.
  • $^{13}$C$_5$$^{15}$N$_2$-Glutamine: Traced anaplerotic entry of glutamine-derived carbon into the TCA cycle.
• Controls: Uninfected red blood cells (uRBCs) were analyzed to rule out host contributions to metabolite levels.

3. Key Contributions & Results

A. Phenotypic Impact: Specific Block in Transmission

• Asexual Stages: All knockout lines (Δfh, Δmqo, Δmdh, Δmm, Δdtc, Δogc, Δod) showed no significant defect in asexual blood-stage growth or gametocyte formation (with the exception of a slight reduction in gametocytes for Δdtc).
• Mosquito Stage: There was a dramatic impairment in ookinete formation in all lines except Δmdh.
  • Δfh, Δmqo, and Δmm showed a complete absence of mature ookinetes.
  • Δdtc, Δogc, and the double knockout Δod showed severe defects, with Δod being almost completely defective.
  • Conclusion: The TCA cycle and associated transporters are non-essential for blood stages but critical for the mosquito-stage transition.

B. Metabolic Mechanism: Fumarate Accumulation and Succination

• Fumarate Toxicity: All knockout lines exhibited significantly elevated fumarate levels in gametocytes.
• Cysteine Succination: The accumulation of fumarate led to the non-enzymatic Michael addition of fumarate to cysteine thiols, resulting in:
  • Increased levels of 2-(succino)cysteine (2SC) (free cysteine adduct).
  • Increased levels of succinyl-glutathione (succ-GSH).
  • Protein Succination: Western blots and MS/MS identified seven proteins in Δfh gametocytes containing 2SC modifications, including RNA helicase, tRNA import protein, and chromatin assembly factor.
• Oxidative Stress: The conversion of reduced glutathione (GSH) to succ-GSH (which cannot be recycled by glutathione reductase) depletes the antioxidant pool, leading to oxidative stress. This stress is identified as the primary cause of the ookinete formation block.

C. Transporter Specificity (DTC vs. OGC)

The study redefined the substrate specificity of mitochondrial transporters in P. berghei, contradicting previous findings in P. falciparum:

• DTC (Dicarboxylate-Tricarboxylate Carrier): Primarily transports malate, with lower affinity for fumarate. (Evidence: Δdtc showed massive malate accumulation but only minor fumarate/succination increases).
• OGC (Citrate-Oxoglutarate Carrier): Primarily transports fumarate. (Evidence: Δogc showed massive fumarate accumulation and high succination, despite normal malate levels).
• Double Knockout (Δod): Showed the most severe accumulation of both metabolites and the most severe phenotype.

D. Compensatory Mechanisms: The Pentose Phosphate Pathway (PPP)

• IMP and NADPH Upregulation: Isotope tracing revealed a drastic increase in the M+5 isotopologue of Inosine Monophosphate (IMP) in knockouts.
• Mechanism: This suggests an upregulation of the Pentose Phosphate Pathway (PPP) to generate Ribose-5-Phosphate (for IMP synthesis) and, crucially, NADPH.
• Purpose: The increased NADPH is likely a compensatory response to mitigate the oxidative stress caused by GSH succination, attempting to maintain redox homeostasis.

4. Significance and Implications

• Novel Mechanism of Parasite Death: The study identifies cysteine succination driven by fumarate accumulation as a specific metabolic vulnerability that blocks malaria transmission. Unlike general metabolic starvation, this is a chemical toxicity mechanism.
• Transmission-Blocking Strategy: The findings highlight Fumarate Hydratase (FH) as a prime target for transmission-blocking drugs.
  • Selectivity: Parasite FH is a Class I enzyme containing a 4Fe-4S cluster, whereas human FH is a Class II enzyme. This structural difference allows for the development of selective inhibitors (e.g., thiomalate derivatives) that would kill the parasite without harming the host.
• Broader Metabolic Insight: The research clarifies the distinct roles of DTC and OGC in P. berghei (Malate vs. Fumarate transport), correcting previous assumptions based on P. falciparum asexual stages.
• Therapeutic Potential: Since the accumulation of fumarate (and subsequent succination) is lethal to the mosquito stage, drugs that inhibit FH, MQO, or these transporters could effectively stop malaria transmission without necessarily curing the acute blood-stage infection, thereby reducing the spread of the disease.

Conclusion

This paper demonstrates that while the TCA cycle is dispensable for the asexual blood stages of Plasmodium, the precise regulation of fumarate levels is critical for the development of the mosquito-stage ookinete. Disruption of fumarate metabolism leads to toxic succination of cysteine residues in glutathione and proteins, causing oxidative stress that halts transmission. This vulnerability offers a promising new avenue for developing transmission-blocking antimalarials.