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Single-cell discovery of mTOR-associated microglial clusters in human mTORopathies

This study utilizes single-cell RNA sequencing to identify and validate three distinct mTOR-associated microglial populations in human mTORopathies, revealing disease-specific alterations in cell–cell communication with inhibitory neuronal circuits and shared activation states across neurodevelopmental and neurodegenerative disorders.

Original authors: Rozemarijn S. Kalf, Mirte Scheper, Esmée E. Ter Horst, Jasper J. Anink, Floor E. Jansen, Wim van Hecke, Angelika Mühlebner, Erwin A. van Vliet, James D. Mills, Eleonora Aronica

Published 2026-07-24

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Rozemarijn S. Kalf, Mirte Scheper, Esmée E. Ter Horst, Jasper J. Anink, Floor E. Jansen, Wim van Hecke, Angelika Mühlebner, Erwin A. van Vliet, James D. Mills, Eleonora Aronica

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Brain's Tiny Janitors and the Glitchy Circuit

Imagine your brain is a bustling, high-tech city. In this city, there are millions of neurons acting as the citizens, sending electrical messages back and forth to keep you thinking, moving, and feeling. But a city needs more than just citizens; it needs maintenance crews. Enter microglia. Think of these as the brain's tiny, super-smart janitors and security guards. Under normal conditions, they quietly sweep up cellular trash, prune away unnecessary connections (like trimming a hedge), and make sure the neighborhood stays safe and organized. They are essential for the brain to develop correctly and function smoothly.

However, sometimes the city's power grid gets stuck in a "high voltage" mode. In certain rare brain disorders, a master switch called mTOR gets stuck in the "ON" position. This switch controls how cells grow and use energy. When it's stuck on, it causes the brain's construction sites to go haywire, leading to malformed brain tissue and severe, drug-resistant epilepsy. Scientists have long known that this "stuck switch" messes up the neurons, but they didn't fully understand what it was doing to the janitors (the microglia). Do they just get confused, or do they change their entire job description? Understanding this is crucial because if the janitors start acting strangely, they might accidentally tear down the wrong parts of the city or fail to fix the broken wires, making the seizures and other problems worse.

The Discovery: When Janitors Change Jobs

In this study, researchers took a deep dive into the brains of people with two specific conditions caused by that "stuck switch": Tuberous Sclerosis Complex (TSC) and a type of brain malformation called Focal Cortical Dysplasia (FCD) type II. They used a powerful tool called single-cell RNA sequencing, which is like taking a snapshot of the genetic "instruction manual" inside thousands of individual cells at once. This allowed them to see exactly what the microglia were doing in these diseased brains compared to healthy ones.

The researchers found that the microglia in these patients weren't just a single, confused group. Instead, the "stuck switch" had caused the janitors to split into three distinct, unusual teams that you don't usually see in healthy brains:

The "Grease" Team (Lipid-associated): These microglia were packed with genes related to eating fat and cleaning up cellular debris. They looked a lot like the "Disease-Associated Microglia" (DAM) seen in aging brains, but here they were driven by the mTOR glitch.
The "Angry" Team (Pro-inflammatory): These cells were revved up and shouting. They were pumping out inflammatory signals, essentially sounding the alarm and trying to rally the immune system, which can sometimes cause more chaos than help in a developing brain.
The "Virus Alarm" Team (Interferon-responsive): These microglia were acting as if they were fighting a viral infection, even though there was no virus. They were turned on by a specific type of immune signal called interferon, putting the brain on high alert.

The Communication Breakdown: A Broken Walkie-Talkie

Perhaps the most fascinating part of the discovery wasn't just what the microglia looked like, but how they talked to the neurons. The researchers used a digital map to trace the conversations between cells. They found that in healthy brains, the microglia and the "inhibitory" neurons (the ones that act as the brain's brakes to stop things from getting too excited) had a balanced, two-way conversation.

But in the brains with the stuck mTOR switch, this conversation was broken. Specifically, the "brake" neurons stopped talking to the microglia as much as they should. In response, the microglia started talking back to the neurons more aggressively. It's like a walkie-talkie where one side goes silent, and the other side starts shouting, creating a feedback loop of confusion. This breakdown in communication happened even though the microglia still looked like normal microglia on the surface. The researchers suggest this "functional reprogramming" might be a key reason why the brain circuits become unstable and lead to seizures.

What This Means (and What It Doesn't)

The study confirms that the mTOR glitch doesn't just break the neurons; it fundamentally reshapes the brain's immune system. It creates these three new, specialized microglial teams and disrupts the delicate conversation between the immune cells and the neurons. The researchers also found that these strange microglial teams share genetic similarities with those found in neurodegenerative diseases like Alzheimer's, suggesting that the "stuck switch" might be a common culprit behind different types of brain trouble.

However, the paper is careful to note that this is a snapshot of what is happening in the tissue, not a cure. The researchers suggest that these findings open up new doors for understanding why the brain becomes unstable, but they haven't yet proven that fixing the microglia will stop the seizures. They propose that because these microglia are so active, they might be a new target for future therapies—perhaps drugs that calm down the "Angry" or "Virus Alarm" teams could help restore the balance. For now, this study gives us a clearer picture of the chaotic city inside the brain, showing us that when the power grid glitches, the janitors don't just get tired; they change their uniforms and start shouting at the wrong people.

Technical Summary: Single-cell discovery of mTOR-associated microglial clusters in human mTORopathies

Problem Statement
Microglia are critical regulators of central nervous system (CNS) homeostasis, synaptic pruning, and circuit refinement during brain development. While microglial heterogeneity has been extensively characterized in neurodegenerative disorders (e.g., Alzheimer's and Parkinson's disease), it remains poorly understood in developmental brain disorders. Tuberous sclerosis complex (TSC) and focal cortical dysplasia (FCD) type II are mTORopathies characterized by mTORC1 hyperactivation, leading to cortical malformations, drug-resistant epilepsy, and neurodevelopmental comorbidities. The authors hypothesized that mTOR dysregulation in these developmental disorders reshapes the microglial landscape, promoting distinct activation states and altering neuron–microglia communication, potentially contributing to network instability.

Methodology
The study utilized a multi-modal approach integrating single-cell RNA sequencing (scRNA-seq) with immunofluorescence (IF) validation and computational cell-cell communication analysis.

Cohort: Cortical tissue was obtained from patients with TSC ( $n=5$ ), FCD with MTOR mutations ( $n=5$ ), and FCD with DEPDC5 mutations ( $n=3$ ). Age- and region-matched control cortical tissue ( $n=3$ ) from autopsy donors without neurological history served as the baseline.
scRNA-seq: Frontal cortical tissue was dissociated and processed using the 10x Genomics Chromium Single Cell Gene Expression Flex protocol. Approximately 8,000 cells per sample were sequenced.
Bioinformatics: Data were processed using Cell Ranger and Seurat (v5.0.1). After quality control and integration via reciprocal principal component analysis (RPCA), microglial clusters were identified based on canonical markers (e.g., P2RY12, CX3CR1, TMEM119). Peripheral macrophages and T-cells were excluded based on specific marker expression.
Analysis: Differential expression analysis (DESeq2) was performed using a pseudo-bulk strategy. Functional enrichment (Gene Ontology) and Disease Ontology enrichment (DisGeNET) were conducted to characterize cluster functions. Cell-cell communication networks were inferred using the CellChat R package to analyze ligand-receptor interactions between microglia and other cortical cell types.
Validation: Immunofluorescence staining was performed on FFPE tissue sections from TSC, FCD_mTOR, and FCD_DEPDC5 cases to validate protein expression of markers identified in scRNA-seq clusters (specifically HLA-DR, CCL2, GPNMB, and ISG15).

Key Results
The study identified a complex microglial landscape comprising ten transcriptionally distinct clusters. After excluding peripheral immune cells and stress-associated clusters (enriched in controls), the analysis focused on eight microglial states.

Shared Microglial States: Three microglial populations were shared between control and mTORopathy samples, retaining core transcriptional identities:
- Cluster 1 (Homeostatic): Characterized by high expression of canonical markers (P2RY12, CX3CR1, TMEM119) and immune-sensing pathways. Remodeling in this cluster was largely TSC-specific.
- Cluster 2 (Synapse-associated): Enriched for neuronal and synaptic transcripts (e.g., NPTX1, VGF), suggesting active microglia-neuron interaction.
- Cluster 3 (Hypo-responsive): Characterized by a general downregulation of gene programs related to immune signaling and motility, suggesting a functionally attenuated phenotype.
mTOR-Associated Microglial Clusters: Three distinct microglial populations were significantly expanded in mTORopathy tissues compared to controls:
- Cluster 0 (Lipid-associated DAM-like): Defined by upregulation of lipid-handling genes (TREM2, APOE, GPNMB, SPP1) and antigen presentation pathways. Disease ontology linked this cluster to neurodegenerative disorders.
- Cluster 4 (Pro-inflammatory): Characterized by the coordinated upregulation of inflammatory mediators (IL1B, TNF, CCL2, CCL3) and NF-κB signaling components. This cluster also showed enrichment for synaptic regulation pathways.
- Cluster 7 (Interferon-responsive DAM-like): Defined by a robust interferon-driven transcriptional program (ISG15, MX2, IFIT2, OAS2), indicating activation of innate immune sensing and antiviral defense mechanisms.
Altered Cell-Cell Communication:
- Global Remodeling: Network remodeling was most pronounced in TSC, followed by FCD_mTOR, with FCD_DEPDC5 showing more modest changes.
- Inhibitory Circuit Disruption: A striking alteration was observed in communication between GABAergic neurons and the synapse-associated microglial cluster (Cluster 2). In TSC and FCD_mTOR, there was a selective reduction in signaling from GABAergic neurons to microglia, accompanied by increased signaling from microglia to GABAergic neurons. This suggests a functional reprogramming of inhibitory circuits. In contrast, FCD_DEPDC5 exhibited a different polarity with increased GABAergic input to microglia.
- Intra-microglial Signaling: Homeostatic and hypo-responsive clusters showed reduced intra-microglial signaling strength in mTORopathies.
Validation: Immunofluorescence confirmed the presence of HLA-DR+ cells co-expressing CCL2 (pro-inflammatory), GPNMB (lipid-associated), and ISG15 (interferon-responsive) in human mTORopathy tissues, validating the transcriptomic findings.

Significance and Claims
The authors claim that this study provides the first single-cell resolution map of the microglial landscape in human mTORopathies. The key contributions are:

Identification of Novel States: The discovery of three specific mTOR-associated microglial clusters (lipid-associated, pro-inflammatory, and interferon-responsive) that extend the known spectrum of microglial activation states beyond classical neurodegenerative contexts.
Functional Reprogramming: The finding that while core transcriptional identities of shared microglial populations are preserved, their intercellular communication networks are significantly altered, particularly within inhibitory neuronal circuits. This suggests that microglial dysfunction in mTORopathies involves a reprogramming of signaling dynamics rather than a complete loss of identity.
Shared Mechanisms: The disease ontology analysis links these mTOR-associated microglial states to both neurodevelopmental and neurodegenerative disorders, suggesting that mTOR signaling may act as a convergent regulatory axis driving shared microglial activation states across different disease etiologies.
Therapeutic Implications: The identification of an interferon-responsive microglial state suggests that modulating interferon signaling pathways (e.g., via JAK-STAT inhibition) could be a novel therapeutic strategy to dampen maladaptive microglial activation and restore network stability in mTORopathies.

The paper concludes that mTOR hyperactivation drives a heterogeneous and context-dependent microglial landscape, where altered communication with inhibitory neurons may contribute to the network hyperexcitability and epilepsy observed in these disorders.

The Brain's Tiny Janitors and the Glitchy Circuit

The Discovery: When Janitors Change Jobs

The Communication Breakdown: A Broken Walkie-Talkie

What This Means (and What It Doesn't)

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