Reduced LACTB expression in myeloid cells is associated with elevated succinylcarnitine levels and reduced Alzheimers disease risk.

This study demonstrates that reduced LACTB expression in myeloid cells lowers Alzheimer's disease risk by increasing succinylcarnitine levels and enhancing mitochondrial function, thereby improving microglial interaction with amyloid plaques.

The Gist

The Big Picture: A Tiny Enzyme, a Big Brain Problem

Imagine your brain is a bustling city. In this city, there are special sanitation workers called microglia. Their job is to clean up trash (like dead cells and protein clumps) to keep the streets safe. In Alzheimer's disease, the city gets overwhelmed with a specific type of sticky, toxic trash called amyloid plaques. The sanitation workers get tired, confused, or stop cleaning effectively, and the city falls into disrepair.

This paper discovers a new "manager" for these sanitation workers called LACTB. The researchers found that if you tell the sanitation workers to stop listening to this manager (reduce LACTB), the workers actually become more efficient at cleaning up the toxic trash, potentially lowering the risk of Alzheimer's.

Here is how they figured it out, step-by-step:

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1. The Genetic Clue: The "Volume Knob"

The researchers started by looking at human DNA. They found a specific spot in our genetic code (a "volume knob") that controls how much LACTB protein is made in immune cells.

• The Discovery: People who naturally have their LACTB "volume" turned down have a lower risk of getting Alzheimer's.
• The Metaphor: Think of LACTB as a strict foreman who tells the sanitation crew to work slowly and save energy. If you fire the foreman (lower LACTB), the crew actually works better and keeps the city cleaner.

2. The Chemical Connection: The "Clogged Pipe"

The researchers wanted to know why turning down LACTB helps. They found a chemical called succinylcarnitine.

• The Role of LACTB: LACTB acts like a specialized pair of scissors (an enzyme) that cuts up succinylcarnitine.
• The Discovery: When LACTB is low, the scissors are missing. This means succinylcarnitine builds up (it isn't cut up).
• The Twist: Surprisingly, having more of this chemical (succinylcarnitine) is actually good for the brain. It's like having extra fuel in the tank. High levels of this chemical are linked to a lower risk of Alzheimer's.
• The Analogy: Imagine LACTB is a drain that sucks away a helpful fuel. If you plug the drain (remove LACTB), the fuel (succinylcarnitine) stays in the system, giving the brain's cleanup crew more energy to fight the disease.

3. The Cellular Shift: From "Sleeping" to "Super-Active"

What happens to the sanitation workers when they have less LACTB and more fuel?

• Energy Boost: The cells switch to a high-energy mode called Oxidative Phosphorylation. They stop wasting energy on making unnecessary proteins and start focusing their power on cleaning.
• The Metaphor: Normally, the sanitation crew might be spending their day filling out paperwork (protein synthesis) and sitting around. When LACTB is removed, they throw away the paperwork, turn on their high-powered vacuums, and get straight to work.
• The Result: They become better at efferocytosis—a fancy word for "eating dead cells and debris." They are better at clearing the toxic amyloid plaques that cause Alzheimer's.

4. The Stress Test: When the City is on Fire

The researchers also tested what happens when the city is under attack (inflammation).

• The Finding: When the immune system is stressed (by viruses or inflammation), LACTB usually gets turned up. But when the researchers forced LACTB to stay down, the cells handled the stress better.
• The Analogy: If a fire breaks out, a strict foreman might tell the crew to "calm down and save resources." But if you remove the foreman, the crew goes into "emergency mode," working harder and faster to put out the fire (clear the inflammation).

5. The Real-World Test: The Mouse Experiment

To prove this works in a living brain, they took human cells with "low LACTB" and transplanted them into the brains of mice that were genetically programmed to get Alzheimer's.

• The Result: The human cells with low LACTB didn't just survive; they hugged the amyloid plaques more tightly. They clustered around the toxic trash and started cleaning it up more aggressively than the normal cells.
• The Takeaway: Even though the mice still had some plaques, the "low LACTB" cells were much more effective at finding and engaging with the problem areas.

Why This Matters: A New Way to Treat Alzheimer's

This is exciting for two main reasons:

1. It's Druggable: LACTB is an enzyme, which means scientists can design small molecules (drugs) to specifically block it, just like we have drugs to block other enzymes. We don't have to invent a whole new machine; we just need to turn off the "strict foreman."
2. A Built-in Test: Because we know that blocking LACTB causes succinylcarnitine to rise, doctors can measure succinylcarnitine levels in blood or spinal fluid. If the levels go up, they know the drug is working! It's like a dashboard light that tells you the engine is running right.

Summary

This paper tells us that Alzheimer's might be partly a problem of "too much management." The protein LACTB acts as a brake on our brain's immune cells. By releasing that brake (lowering LACTB), we allow the brain's cleanup crew to run on high-octane fuel (succinylcarnitine), work harder, and clear out the toxic plaques that cause dementia. It's a promising new path toward a treatment that could help the brain heal itself.

Technical Summary

1. Problem Statement

Alzheimer's Disease (AD) is a complex neurodegenerative disorder with a strong genetic component. While many risk loci have been identified, the functional mechanisms linking specific genes to AD pathogenesis, particularly within myeloid cells (microglia and macrophages), remain poorly understood.

• The Locus: The gene LACTB (Lactamase $\beta$) is located in a well-established AD risk locus on chromosome 15q22 (the "APH1B locus"). Previous proteomic studies suggested an association between LACTB and AD, but its causal role and mechanism were unknown.
• The Metabolite: Loss-of-function mutations in LACTB have been genetically linked to elevated levels of succinylcarnitine, a metabolite that paradoxically correlates with lower AD risk in cerebrospinal fluid (CSF) and brain tissue.
• The Gap: It was unclear whether LACTB directly cleaves succinylcarnitine, how LACTB loss affects myeloid cell metabolism and function, and whether modulating LACTB activity could influence AD pathology in vivo.

2. Methodology

The authors employed a multi-disciplinary approach combining human genetics, cell biology, metabolomics, and in vivo xenotransplantation models.

• Genetic & Statistical Analysis:
  • Mendelian Randomization (MR): Used cis-eQTLs (from STARNET monocyte-derived macrophages), mQTLs, and AD GWAS data to establish causal relationships between LACTB expression, succinylcarnitine levels, and AD risk.
• Cell Models:
  • Human THP-1 Macrophages: Differentiated from monocytes and treated with siRNA to knock down (KD) LACTB.
  • Human iPSC-derived Microglia (iMGLs): Generated isogenic LACTB Knock-Out (KO) lines (3 independent clones) from the WTC11 iPSC line using CRISPR-Cas9.
  • Mouse Models:
    • Enzymatically Dead (ED) Mice: Generated via a conditional knock-in of the S162I mutation (inactivating the catalytic triad) in the Lactb gene.
    • Xenotransplantation: Human LACTB KO and Wild-Type (WT) iMGL precursors were transplanted into the brains of newborn 5XFAD-hCSF1 mice (immunodeficient mice overexpressing human CSF1 and AD mutations) to assess in vivo interactions with amyloid plaques.
• Omics & Functional Assays:
  • Transcriptomics: Bulk and single-cell RNA sequencing (scRNA-seq) to analyze gene expression changes.
  • Metabolomics & Lipidomics: Ion chromatography-mass spectrometry (IC-MS) and LC-MS/MS to quantify succinylcarnitine, carnitine species, and lipid profiles.
  • Enzymatic Assays: Cell-free assays using recombinant LACTB and $^{13}$C-labeled succinylcarnitine to confirm direct cleavage. Fluorogenic substrates (succinyl-D-Asp) measured DAEP activity.
  • Functional Phenotyping: Seahorse assays (mitochondrial respiration), EdU proliferation assays, protein synthesis assays (Click-iT AHA), efferocytosis (myelin phagocytosis), and cytokine profiling.
  • In Vivo Imaging: Immunohistochemistry (IHC) and confocal microscopy to quantify microglia-plaque association, plaque load, and activation markers (HLA-DR, LGALS3, CD9).

3. Key Contributions

1. Causal Link Establishment: Demonstrated via Mendelian Randomization that lower LACTB expression in myeloid cells causally leads to higher succinylcarnitine levels and reduced AD risk.
2. Enzymatic Discovery: Identified LACTB as the primary mitochondrial enzyme responsible for hydrolyzing succinylcarnitine into carnitine and succinate.
3. Regulatory Mechanism: Discovered that LACTB activity is inhibited by lysine succinylation, creating a negative feedback loop where high succinyl-CoA/succinylcarnitine levels may suppress LACTB, further elevating succinylcarnitine.
4. Metabolic Reprogramming: Defined a novel metabolic axis where LACTB loss shifts myeloid cells toward oxidative phosphorylation (OXPHOS), reduces protein synthesis, and lowers triacylglycerides.
5. In Vivo Validation: Showed that human LACTB KO microglia exhibit enhanced association with amyloid plaques and altered activation states in an AD mouse model, without significantly reducing plaque load but potentially improving plaque surveillance.

4. Key Results

• Genetics & Metabolism:

  • MR analysis confirmed: Lower LACTB $\rightarrow$ Higher Succinylcarnitine $\rightarrow$ Lower AD Risk.
  • LACTB KD/KO in human macrophages and microglia, and Lactb ED in mice, resulted in a significant accumulation of succinylcarnitine in cell lysates and media.
  • Recombinant LACTB directly cleaved $^{13}$C-succinylcarnitine; this activity was abolished by a specific LACTB inhibitor and by the S162I mutation.
  • Succinylation of LACTB (induced by diethyl succinate treatment) inhibited its enzymatic activity, leading to increased intracellular succinylcarnitine.

• Transcriptomics & Cellular Function:

  • Gene Expression: LACTB loss upregulated pathways related to interferon (IFN) and TNF responses, oxidative phosphorylation, and antigen presentation. It downregulated proliferation and protein synthesis pathways.
  • Metabolism:
    • OXPHOS: LACTB depletion increased basal and maximal mitochondrial respiration (Seahorse).
    • Protein Synthesis: Nascent protein synthesis was significantly reduced in LACTB KD/KO cells.
    • Lipids: Reduced levels of cholesterol esters (CE) and triacylglycerides (TG); increased ceramides in macrophages.
  • Efferocytosis: Under inflammatory stimulation (IFN/TNF), LACTB KD/KO cells showed reduced myelin phagocytosis and lysosomal acidification, suggesting a shift away from "exhausting" clearance tasks toward a more surveillance-oriented state.

• In Vivo Xenotransplantation (5XFAD-hCSF1):

  • Engraftment: Human LACTB KO microglia engrafted successfully.
  • Plaque Interaction: LACTB KO microglia showed a statistically significant increase in association with amyloid plaques compared to WT.
  • Plaque Load: No significant reduction in total plaque area or number, but a trend toward fewer large plaques and increased plaque compaction (ThioS/MOAB ratio).
  • Activation: LACTB KO microglia showed increased expression of activation markers (HLA-DR, CD9, LGALS3), indicating a more reactive or surveillant phenotype.

5. Significance and Implications

• Therapeutic Target: LACTB is a druggable serine hydrolase. Small-molecule inhibitors of bacterial $\beta$-lactamases (LACTB's orthologs) are already in clinical use, suggesting a potential pathway for repurposing or developing LACTB inhibitors for AD.
• Biomarker Potential: Succinylcarnitine serves as a proximal, mechanistically-linked biomarker (endophenotype) for LACTB activity and target engagement, measurable in blood, CSF, or urine.
• Mechanistic Insight: The study proposes that reducing LACTB activity in myeloid cells creates a "metabolically efficient" state (high OXPHOS, low protein synthesis) that enhances microglial surveillance and interaction with amyloid plaques without triggering the metabolic exhaustion often seen in AD.
• Feedback Loop: The discovery of the succinylation-mediated inhibition of LACTB provides a novel regulatory mechanism linking cellular metabolic state (succinyl-CoA levels) to immune cell function.

Conclusion: The paper defines a novel axis where reduced LACTB expression/activity in myeloid cells elevates succinylcarnitine, reprograms cellular metabolism toward OXPHOS, and enhances microglial association with amyloid pathology, ultimately conferring protection against Alzheimer's Disease. This positions LACTB inhibition as a promising therapeutic strategy for modulating neuroinflammation in AD.