HSC71 acetylation confers protection against Spiroplasma eriocheiris infection by inhibiting apoptosis and promoting ROS production in arthropods

This study reveals that Crat-mediated acetylation of HSC71 at lysine 579 enhances arthropod immunity against *Spiroplasma eriocheiris* by stabilizing HSC71 to suppress apoptosis and weakening its interaction with SOD to promote microbicidal ROS accumulation, a mechanism that can be therapeutically targeted via SIRT1 inhibition.

The Gist

Imagine your body is a bustling city, and its immune system is the police force trying to keep the streets safe from invaders. In this story, the invaders are Spiroplasma eriocheiris, a sneaky bacteria that causes "tremor disease" in crabs. The paper you're reading reveals how the crab's immune system fights back using a special "bodyguard" protein and a clever chemical switch.

Here is the story of how this defense works, broken down into simple concepts:

1. The Bodyguard: HSC71

Think of HSC71 as a highly skilled bodyguard for the crab's cells.

• Its Job: Normally, this bodyguard helps fix broken proteins and keeps the cell from falling apart (preventing "apoptosis," or cell suicide).
• The Problem: When the bacteria attacks, it tries to trick the bodyguard. It forces the bodyguard to lose a specific "badge" (a chemical tag called acetylation). Without this badge, the bodyguard becomes weak, gets fired (degraded), and the cell dies, letting the bacteria take over.

2. The Badge Maker: Crat

The researchers discovered a specific enzyme called Crat (Carnitine O-acetyltransferase).

• The Analogy: If HSC71 is the bodyguard, Crat is the badge maker.
• The Action: Crat attaches the "acetylation badge" to a specific spot on the bodyguard (at position 579).
• The Result: When the badge is attached, two amazing things happen:
  1. The Bodyguard is Protected: A "firing squad" (an enzyme called CHIP) tries to destroy the bodyguard, but the badge acts like a shield. The firing squad can't grab the bodyguard, so the bodyguard stays strong and alive.
  2. The Alarm is Sounded: The badge changes the bodyguard's shape so it lets go of a "fire extinguisher" (an enzyme called SOD).

3. The Fire Extinguisher vs. The Fire (ROS)

This is the most interesting part.

• The Fire Extinguisher (SOD): Normally, the bodyguard (HSC71) holds onto the fire extinguisher (SOD). SOD's job is to put out "chemical fires" called ROS (Reactive Oxygen Species).
• The Twist: The crab wants these chemical fires to burn the bacteria! ROS are like toxic fumes that kill the invading bacteria.
• The Strategy: When the badge maker (Crat) puts the badge on the bodyguard, the bodyguard drops the fire extinguisher. Without the extinguisher, the "chemical fire" (ROS) builds up. This toxic environment is deadly to the bacteria but the crab's cells can handle it.

4. The Saboteur: SIRT1

The bacteria tries to win by using a "saboteur" enzyme called SIRT1.

• The Sabotage: SIRT1 is a "badge remover." It strips the acetylation badge off the bodyguard, causing the bodyguard to be destroyed and the fire extinguisher to be picked back up (stopping the ROS).
• The Counter-Attack: The researchers found a drug called EX-527. This drug is like a "badge remover blocker." It stops SIRT1 from doing its job.
• The Victory: When they gave crabs this drug, the bodyguards kept their badges, the fire extinguishers were dropped, the chemical fires (ROS) roared, and the bacteria were wiped out.

The Big Picture: A Two-Pronged Defense

This study shows that the crab uses a brilliant double-strategy to survive:

1. Keep the Bodyguard Alive: By keeping the acetylation badge on, the cell doesn't commit suicide.
2. Create a Toxic Zone: By dropping the fire extinguisher, the cell fills with toxic ROS that kills the bacteria.

Why Does This Matter?

This isn't just about crabs. The researchers tested this in fruit flies (Drosophila) too, and it worked exactly the same way. This suggests that this "badge system" is an ancient, evolutionary trick used by many animals to fight infections.

In short: The paper discovered that by simply "tagging" a specific protein with a chemical badge, the immune system can simultaneously protect its own cells and create a toxic environment that kills the invader. It's like turning a cell's own defense mechanism into a super-weapon.

Technical Summary

1. Problem Statement

• Context: Heat shock proteins (HSP70), specifically HSC71, are critical molecular chaperones involved in protein homeostasis and innate immunity. However, the specific mechanisms by which post-translational modifications (PTMs), particularly acetylation, regulate HSP70 function during pathogenic infection in invertebrates remain poorly understood.
• Specific Challenge: The intracellular pathogen Spiroplasma eriocheiris causes "tremor disease" in Chinese mitten crabs (Eriocheir sinensis), leading to significant economic losses. The host's immune response involves a dynamic regulation of Reactive Oxygen Species (ROS) and apoptosis, but the molecular switch controlling these processes via HSC71 is unknown.
• Knowledge Gap: While acetylation is known to regulate HSP70 stability and function in vertebrates, the specific acetyltransferases, deacetylases, and downstream effects of HSC71 acetylation in crustaceans and other arthropods had not been elucidated.

2. Methodology

The study employed a multidisciplinary approach combining proteomics, molecular biology, cell biology, and pharmacology in both in vivo (crab) and in vitro (Drosophila S2 cells) models.

• Acetylome Profiling: Mass spectrometry (LC-MS/MS) was used to compare acetylated peptides in hemocytes of healthy vs. S. eriocheiris-infected crabs to identify differentially acetylated proteins.
• Genetic Manipulation:
  • Knockdown: siRNA and dsRNA were used to silence HSC71, Crat (carnitine O-acetyltransferase), and SOD (superoxide dismutase) in crabs.
  • Overexpression/Mutagenesis: Plasmids expressing wild-type HSC71 and mutants (K579Q to mimic hyperacetylation; K579R to mimic hypoacetylation) were transfected into Drosophila S2 cells.
• Interaction Studies: Co-immunoprecipitation (Co-IP) and mass spectrometry were used to identify HSC71-interacting partners. Subsequent Co-IP and Western blotting validated interactions with Crat, CHIP (E3 ubiquitin ligase), and SOD.
• Pharmacological Intervention: The SIRT1 inhibitor (EX-527) and SIRT2 inhibitor (SirReal2) were administered to crabs and S2 cells to determine the role of deacetylases.
• Functional Assays:
  • Apoptosis: Flow cytometry using Annexin V/PI and Rhodamine 123 (mitochondrial membrane potential).
  • ROS Detection: Flow cytometry using DCFH-DA and DHE probes.
  • Pathogen Load: Absolute quantitative PCR to measure S. eriocheiris copy numbers.
  • Survival Analysis: Kaplan-Meier survival curves for infected crabs.

3. Key Contributions

This study establishes a novel regulatory axis where Crat-mediated acetylation of HSC71 at Lysine 579 (K579) orchestrates a dual-defense mechanism against intracellular bacterial infection. The key contributions include:

1. Identification of K579 as a Critical Acetylation Site: Discovery that K579 is strictly conserved in crustaceans and is the primary site of acetylation regulating HSC71 stability and immune function.
2. Elucidation of the Acetyltransferase: Identification of Crat (carnitine O-acetyltransferase) as the specific acetyltransferase for HSC71, a non-canonical role for this metabolic enzyme in immunity.
3. Mechanism of Protein Stabilization: Demonstration that HSC71 acetylation at K579 prevents its ubiquitination and degradation by the E3 ligase CHIP, thereby stabilizing HSC71 levels.
4. Dual-Function Immune Regulation: Uncovering a "switch" mechanism where HSC71 acetylation simultaneously:
   • Suppresses Apoptosis: By maintaining stable HSC71 levels.
   • Promotes ROS Accumulation: By disrupting the HSC71-SOD interaction, thereby reducing SOD activity and allowing microbicidal ROS to accumulate.
5. Evolutionary Conservation: Validation that this mechanism is conserved in Drosophila (via the homolog HSPA8), suggesting a broad arthropod defense strategy.

4. Key Results

• Infection Dynamics: S. eriocheiris infection triggers a rapid deacetylation of HSC71 in crab hemocytes.
• HSC71 Function: Silencing HSC71 increased hemocyte apoptosis and susceptibility to infection, confirming its protective role.
• Role of K579 Acetylation:
  • The hyperacetylation-mimetic mutant HSC71(K579Q) significantly reduced apoptosis and pathogen load in S2 cells compared to wild-type or the hypoacetylated mutant (K579R).
  • K579 acetylation inhibited the interaction between HSC71 and CHIP, reducing HSC71 ubiquitination and increasing its protein stability.
• Role of Crat: Crat was identified as the acetyltransferase. Crat knockdown reduced HSC71 acetylation and stability, increased apoptosis, and lowered ROS levels, leading to higher mortality in crabs.
• ROS and SOD Interaction:
  • Acetylated HSC71 (K579Q) showed weakened binding to SOD, leading to reduced SOD enzymatic activity.
  • Reduced SOD activity resulted in accumulation of intracellular ROS, which is toxic to S. eriocheiris.
  • Conversely, Crat deficiency increased HSC71-SOD binding, enhanced SOD activity, cleared ROS, and allowed pathogen proliferation.
• Pharmacological Validation: Treatment with the SIRT1 inhibitor EX-527 increased HSC71 acetylation in both crabs and Drosophila. This led to:
  • Disruption of the HSC71-SOD complex.
  • Elevated ROS levels.
  • Reduced S. eriocheiris load and improved host survival.

5. Significance

• Novel Immune Mechanism: The study reveals a unique "acetylation switch" that allows the host to balance protein stability (anti-apoptosis) with oxidative stress (antimicrobial activity). By acetylating HSC71, the host sacrifices the chaperone's ability to assist SOD in clearing ROS, intentionally allowing ROS to accumulate to kill the pathogen.
• Therapeutic Potential: The findings suggest that targeting HSC71 acetylation (e.g., using SIRT1 inhibitors like EX-527) could be a viable strategy to enhance innate immunity in aquaculture, offering a potential treatment for S. eriocheiris tremor disease in crabs.
• Evolutionary Insight: The conservation of this mechanism from crabs to Drosophila highlights a fundamental, ancient strategy in arthropod innate immunity involving the crosstalk between chaperone stability, ubiquitination, and redox homeostasis.
• Metabolic-Immune Crosstalk: It identifies Crat, traditionally viewed as a metabolic enzyme, as a critical regulator of innate immunity, expanding the understanding of how metabolic enzymes influence host defense.