Salmonella Typhi asparaginase-dependent activation of GCN2 promotes bacterial killing in murine macrophages

This study demonstrates that *Salmonella* Typhi asparaginase depletes host asparagine to activate the mTOR-GCN2 integrated stress response in murine macrophages, a pathway essential for bacterial clearance and cytokine production that highlights key nutrient-sensing differences between restrictive murine and susceptible human hosts.

The Gist

The Big Picture: A Bacterial Heist and the Cell's Alarm System

Imagine your body's immune system as a highly secure castle. The macrophages (a type of white blood cell) are the castle guards. Their job is to spot invaders, swallow them whole, and destroy them.

Usually, when a bacteria like Salmonella gets inside a guard, the guard sounds a general alarm. But this paper focuses on a very specific, tricky burglar: Salmonella Typhi (the bacteria that causes Typhoid fever). This burglar is a master thief that usually only works on humans. However, the researchers used mouse guards as a "training simulation" because mouse guards are very good at killing this specific burglar, unlike human guards who often let it survive.

The researchers wanted to know: How do the mouse guards know exactly how to fight this specific burglar?

The Discovery: The "Amino Acid" Shortage

Inside every cell, there is a "stress response system" called the Integrated Stress Response (ISR). Think of this as the cell's "Check Engine" light. When the cell runs out of fuel (specifically, amino acids, which are the building blocks of proteins), this light turns on, and the cell changes its behavior to survive or fight back.

The researchers found that when Salmonella Typhi gets inside a mouse guard, it doesn't just sit there. It actively steals the cell's supply of a specific amino acid called Asparagine.

• The Analogy: Imagine the burglar (Salmonella) sneaks into the guard's kitchen (the cell) and eats all the flour (Asparagine). The guard realizes, "Hey, my flour is gone! Someone is stealing from me!" This realization triggers the alarm.

The Key Player: GCN2 (The Smoke Detector)

The cell has a specific sensor protein called GCN2. You can think of GCN2 as a super-sensitive smoke detector that only goes off when it smells the specific scent of "missing flour" (lack of asparagine).

• What happened in the experiment:
  • When the mouse guards had a working GCN2 sensor, they smelled the missing flour, sounded the alarm, and successfully killed the bacteria.
  • When the researchers removed the GCN2 sensor (like taking the batteries out of the smoke detector), the guards didn't realize the flour was missing. They stayed calm, didn't sound the alarm, and the bacteria survived much better.

The Culprit: The Bacterial "Flour-Eater" (AnsB)

Why was the flour missing in the first place? The bacteria has a special tool called an enzyme named AnsB.

• The Analogy: The bacteria is like a thief who brings a vacuum cleaner (AnsB) into the kitchen. This vacuum sucks up all the asparagine and turns it into something else.
• The Proof: When the researchers created a version of the bacteria without this vacuum cleaner (a mutant without the ansB gene), the bacteria couldn't steal the flour. The guards didn't smell the shortage, the alarm didn't go off, and the bacteria actually did worse at surviving in the mouse liver. This proves the bacteria needs to steal the flour to trigger the alarm that helps it survive (a bit of a twist: the alarm helps the host, but the bacteria tries to manipulate it).

The Twist: The "Manager" (mTOR)

Here is where it gets interesting. The researchers found that the GCN2 smoke detector doesn't work on its own. It needs a Manager called mTOR to be "licensed" or turned on first.

• The Analogy: Think of mTOR as the building superintendent. Even if the smoke detector (GCN2) smells smoke, it won't ring the fire alarm unless the superintendent (mTOR) gives the okay.
• The bacteria triggers the superintendent (mTOR) to get ready. Then, when the bacteria steals the flour, the superintendent tells the smoke detector, "Okay, now you can ring the alarm!"
• If the researchers blocked the superintendent (using a drug), the smoke detector never rang, even if the flour was stolen.

Why This Matters: Humans vs. Mice

The most surprising part of the story is the difference between mice and humans.

• Mouse Guards: They have a working GCN2 sensor that detects the stolen flour and fights back effectively.
• Human Guards: The researchers tested human cells, and they did not trigger this specific alarm when the bacteria stole the flour. The human "smoke detector" seems broken or disconnected in this specific scenario.

The Takeaway:
This explains why Salmonella Typhi is so dangerous to humans but not to mice. The mouse immune system has a clever "flour-theft" detection system (GCN2 + mTOR) that helps it kill the bacteria early. Humans seem to lack this specific detection link, allowing the bacteria to hide and cause Typhoid fever.

Summary in One Sentence

Salmonella Typhi tries to survive by stealing a specific nutrient (asparagine) from immune cells; in mice, this theft triggers a specific alarm system (GCN2) that kills the bacteria, but humans lack this specific alarm, which is why the bacteria can make us sick.

Technical Summary

1. Problem Statement

• Host Restriction: Salmonella enterica serovar Typhi (S. Typhi) is a human-restricted pathogen causing typhoid fever. It does not productively infect mice, making murine models "restrictive" hosts where the bacteria are rapidly cleared. Understanding the mechanisms behind this restriction in mice can reveal innate immune defenses that S. Typhi evades in humans.
• Nutrient Sensing in Immunity: Intracellular pathogens often manipulate host nutrient pools. Depletion of amino acids is a known danger signal that triggers the Integrated Stress Response (ISR). However, the specific mechanisms by which S. Typhi triggers ISR in macrophages, and the specific amino acid sensors involved, remain unclear.
• Knowledge Gap: While the ISR kinase GCN2 is known to sense amino acid starvation, its role in S. Typhi infection and the specific bacterial factors driving this sensing in a restrictive host model had not been defined.

2. Methodology

The study employed a combination of in vitro cell culture, genetic manipulation, pharmacological inhibition, and in vivo murine infection models:

• Cell Models: Primary murine Bone Marrow-Derived Macrophages (BMDMs) from Wild-Type (WT), Gcn2^-/-, and Pkr^-/-Gcn2^-/- double knockout mice. Human monocyte-derived macrophages (U-937 cells) were used for comparative analysis.
• Bacterial Strains: S. Typhi (Ty2) and S. Typhimurium (SL1344). Mutants generated included:
  • ansB deletion (ΔansB): Lacks asparaginase II.
  • ansB T111A: Catalytic dead point mutant.
  • ansB complementation (ΔansB+ansB-F): Restored expression with a FLAG tag.
  • ansP deletion (ΔansP): Lacks the asparagine transporter.
• Assays:
  • ISR Activation: Measured via ATF4 protein levels (immunoblot) and nuclear translocation (quantitative immunofluorescence microscopy).
  • Kinase Activity: Phosphorylation of GCN2, mTOR, p70 S6K, and AKT via Western blot.
  • Metabolic Manipulation: Supplementation of Essential Amino Acids (EAAs) vs. Non-Essential Amino Acids (NEAAs), and specific individual amino acids.
  • Pharmacology: Use of Torin 1 (pan-mTOR inhibitor) and Rapamycin (mTORC1 inhibitor) to dissect signaling pathways.
  • In Vivo Models: Intraperitoneal infection of C57BL/6 and knockout mice to measure bacterial burden in spleen, liver, and gallbladder, and cytokine levels in plasma.

3. Key Contributions

• Identification of GCN2 as the Primary Sensor: The study establishes that GCN2 (EIF2AK4) is the specific eIF2α kinase responsible for ISR activation during S. Typhi infection in murine macrophages.
• Discovery of Asparagine Depletion as the Trigger: The authors identified that S. Typhi specifically depletes the non-essential amino acid asparagine to trigger the ISR, contrasting with previous findings in other systems where essential amino acids (leucine/isoleucine) were the triggers.
• Role of Bacterial Asparaginase (AnsB): The study proves that the bacterial enzyme AnsB (L-asparaginase II) is the direct effector causing asparagine depletion and subsequent ISR activation.
• mTOR-GCN2 Axis: The research reveals a novel signaling hierarchy where mTOR activation is required upstream to "license" or sensitize GCN2 to asparagine depletion, rather than GCN2 acting solely as a direct starvation sensor.

4. Detailed Results

A. S. Typhi Induces ISR via GCN2 in Murine Macrophages

• Live S. Typhi, but not heat-killed bacteria or S. Typhimurium, induced robust ISR activation (ATF4 upregulation) in murine BMDMs.
• In Gcn2^-/- BMDMs, S. Typhi failed to induce ATF4, confirming GCN2 is essential for this response.
• Gcn2^-/- macrophages showed impaired bacterial clearance (higher bacterial survival at 24h) and reduced secretion of cytokines IL-6 and IL-10 compared to WT.

B. Asparagine Depletion is the Specific Signal

• Supplementation of infected cultures with NEAAs (but not EAAs) suppressed ISR activation.
• Among individual NEAAs, only L-asparagine supplementation prevented ISR induction.
• This suggests S. Typhi actively depletes intracellular asparagine pools.

C. AnsB is the Bacterial Effector

• S. Typhi mutants lacking ansB (ΔansB) or possessing a catalytic dead mutation (T111A) failed to induce ISR (no ATF4 increase) in murine macrophages.
• Complementation with ansB restored ISR induction.
• Interestingly, while ΔansB failed to trigger ISR, it did not show increased survival in vitro compared to WT, suggesting other factors contribute to killing. However, in vivo, ΔansB showed reduced bacterial burden in the liver compared to WT, indicating AnsB contributes to early survival.

D. mTOR is Upstream of GCN2

• Treatment with Torin 1 (pan-mTOR inhibitor) or Rapamycin (mTORC1 inhibitor) prevented GCN2 phosphorylation and ISR induction during S. Typhi infection.
• This indicates mTOR activity is required to sensitize GCN2 to the asparagine depletion caused by AnsB.
• Notably, deleting ansB did not alter mTOR phosphorylation levels, suggesting AnsB acts downstream of mTOR activation but requires mTOR to function effectively.

E. Species-Specific Differences (Mouse vs. Human)

• Human U-937 macrophages did not activate GCN2 during S. Typhi infection, even though they could activate the ISR via other stressors (e.g., tunicamycin).
• Artificially activating GCN2 in human cells with halofuginone did not reduce bacterial burden.
• This highlights a critical difference in nutrient-sensing pathways between the restrictive murine host and the permissive human host.

5. Significance

• Mechanism of Host Restriction: The paper elucidates a specific mechanism (AnsB-mediated asparagine depletion $\rightarrow$ mTOR-licensed GCN2 activation $\rightarrow$ ISR) that allows murine macrophages to detect and kill S. Typhi.
• Therapeutic Insight: The finding that asparagine depletion triggers a protective immune response suggests that manipulating amino acid availability or targeting the AnsB-mTOR-GCN2 axis could be strategies to enhance host defense against typhoid fever.
• Evolutionary Context: The study highlights how S. Typhi may have evolved to evade this specific sensing mechanism in humans (where GCN2 is not activated by S. Typhi), allowing it to establish chronic infection, whereas the murine host utilizes this pathway for restriction.
• Complex Signaling: It challenges the linear view of amino acid sensing, demonstrating a complex, context-dependent interplay where mTOR acts as a prerequisite "licensing" step for GCN2 activation during infection.