Comprehensive single cell profiling of ageing glial cells reveals impaired Wnt signalling and Jun transcription factors regulating cortical astrocytes

This study utilizes comprehensive single-cell profiling to demonstrate that ageing impairs Wnt signaling and reduces Jun transcription factors in cortical astrocytes, leading to altered cell interactions, while showing that restoring JunD expression can rejuvenate astrocyte proliferation and function in the aged brain.

The Gist

The Big Picture: The Brain's "Maintenance Crew" is Tired

Imagine your brain as a bustling, high-tech city. In this city, neurons are the citizens who think, feel, and move. But citizens can't survive without a support system. Enter the astrocytes: the brain's "maintenance crew" or "glue." They feed the neurons, clean up trash, and keep the streets (synapses) in perfect condition.

As we age, the city gets older. The citizens (neurons) start to slow down, but this study found something surprising: the maintenance crew (astrocytes) is actually the first to get tired and confused. When the crew stops working right, the whole city starts to crumble, leading to memory loss and brain fog.

The Investigation: A Deep Dive into the "Old" City

The researchers took a high-tech microscope (single-cell RNA sequencing) to look at the brains of young mice (adults) and very old mice. They wanted to see exactly what was happening inside the cells.

What they found:

1. The Crew is Overworked and Confused: In old brains, the astrocytes started shouting at the "security guards" (microglia) too much, causing inflammation. At the same time, they stopped talking to the citizens (neurons), leaving them unsupported.
2. The Broken Blueprint: The researchers discovered that the astrocytes had lost the instructions for a specific communication system called Wnt Signaling.
   • The Analogy: Imagine Wnt signaling as the city's "Power Grid." In young brains, the grid is humming, sending energy to keep everything running smoothly. In old brains, the grid is flickering and dying.
3. The Saboteurs: The study found that in old astrocytes, the "off switches" for this power grid were stuck in the "ON" position, while the "on switches" were broken. Specifically, a protein called JunD (a key manager for the crew) had vanished.

The Breakthrough: The "Rejuvenation" Experiment

The team had a bold idea: If we can't fix the whole city, can we just give the maintenance crew a specific tool to wake them up?

They identified JunD as the missing manager. In young brains, JunD is abundant, keeping the astrocytes energetic and ready to repair damage. In old brains, JunD is gone.

The Experiment:
The researchers used a harmless virus (like a delivery truck) to inject a dose of JunD directly into the brains of old mice. They targeted only the astrocytes.

The Result:
It was like hitting the "Reset" button.

• The Crew Woke Up: The astrocytes that received the JunD dose started acting young again. They began to divide and repair themselves (proliferate), which old astrocytes usually refuse to do.
• The Power Grid Returned: Levels of a protein called HMGB1 (a marker of cellular youth and health) went up.
• The Takeaway: By simply adding back this one missing piece (JunD), they partially reversed the aging process in the brain's maintenance crew.

Why This Matters

Think of your brain's aging not just as the citizens (neurons) getting old, but as the maintenance crew (astrocytes) losing its tools.

• Before this study: We thought brain aging was inevitable and that once the maintenance crew got old, they were stuck that way.
• After this study: We now know the crew is just "asleep" because they lost a specific manager (JunD). If we can wake them up and give them their tools back, we might be able to keep the brain city running smoothly for much longer, potentially preventing diseases like Alzheimer's before they even start.

Summary in One Sentence

This paper discovered that as we age, the brain's support cells lose a vital manager called JunD, causing them to stop working; but by injecting this manager back into old brains, scientists were able to "rejuvenate" these cells, making them act young and capable of repair again.

Technical Summary

1. Problem Statement

Brain ageing is a complex, multicellular process leading to cognitive decline and neurodegeneration. While astrocytes are known to be among the first cell types affected by ageing, the specific molecular pathways driving their dysfunction and the potential for reversing these changes remain poorly understood. Previous studies have highlighted inflammatory phenotypes in aged astrocytes, but there is a lack of integrated multi-omics data characterizing cell-type-specific transcriptomic changes, intercellular communication shifts, and the reversibility of these aged states. Specifically, the role of Wnt signalling and its downstream effectors in the ageing of cortical astrocytes had not been comprehensively mapped.

2. Methodology

The authors employed a multi-omics approach combining single-cell RNA sequencing (scRNA-seq), single-nucleus multiome analysis (RNA + ATAC-seq), and spatial validation techniques in both mouse and human datasets.

• Animal Models: Adult (3–4 months) and aged (22–24 months) C57BL/6J mice were used. Specific transgenic lines (Aldh1l1-eGFP, Tcerg1l-CreERT2/tdTomato) were utilized for cell-type-specific isolation and layer-specific dissection.
• Single-Cell/Nuclei Sequencing:
  • scRNA-seq: Cortical grey matter (GM) was dissociated using a mild protocol optimized for aged tissue to capture glial cells. Adult (n=3) and aged (n=2) samples were sequenced (10x Genomics).
  • Multiome (scATAC + scRNA): Single nuclei from adult and aged cortices were processed using 10x Genomics Multiome kits to simultaneously profile gene expression and chromatin accessibility.
  • Layer-Specific Analysis: Astrocytes were manually dissected from specific cortical layers (Layers 2-4, 5, 6) and sorted via MACS for scRNA-seq.
• Cross-Species Validation: Publicly available human snRNA-seq data (young vs. aged prefrontal cortex) was analyzed to validate findings across species.
• Spatial and Molecular Validation:
  • RNAscope: Used to quantify RNA levels of Wnt pathway genes (Maml2, Daam2) in astrocytes (co-stained with Slc1a3 or Aldh1l1-eGFP).
  • Immunohistochemistry (IHC): Antibodies against Jun family transcription factors (Jun, JunD, JunB) and astrocyte markers (S100b, Sox9) were used to validate protein levels and distribution.
• Functional Rescue Experiment:
  • Viral Delivery: Lentiviral vectors (MK-G pseudotyped) expressing JunD (with mScarlet reporter) or control were injected into the cortical GM of aged mice.
  • Proliferation Assay: Mice were administered 5-EdU via drinking water for 7 days to label proliferating cells.
  • Readouts: Quantification of EdU+ and HMGB1+ (a marker of youth/anti-senescence) in RFP+ (transduced) astrocytes.
• Bioinformatics: Analysis included standard pipelines (CellRanger, Scanpy, Harmony) and advanced regulatory network inference using SCENIC+ to link chromatin accessibility to transcription factor (TF) activity.

3. Key Contributions

• Comprehensive Cell-Type Profiling: Generated a high-resolution map of age-induced transcriptomic changes across all major cortical cell types, revealing that glial cells (astrocytes, microglia) show predominant gene upregulation, while neurons show downregulation.
• Discovery of Wnt Dysregulation: Identified a specific, conserved dysregulation of the Wnt signalling pathway in aged cortical astrocytes, characterized by the upregulation of inhibitors and downregulation of effectors.
• Identification of Jun Transcription Factors: pinpointed the downregulation of AP-1 complex members, specifically JunD and JunB, as key downstream consequences of impaired Wnt signalling in ageing.
• Demonstration of Reversibility: Provided functional evidence that overexpressing JunD in aged astrocytes can partially "rejuvenate" them, restoring proliferative capacity and increasing HMGB1 levels.
• Cross-Species Conservation: Validated that the specific astrocyte substate characterized by Wnt dysregulation and reduced Jun expression exists in both mouse and human ageing brains.

4. Key Results

A. Global Transcriptomic Shifts

• Cell-Type Specificity: Ageing induced distinct transcriptional shifts. Microglia and astrocytes showed significant upregulation of genes related to immune response and cell projection organization. Neurons and oligodendrocyte progenitors (OPCs) showed downregulation of synaptic and metabolic genes.
• Intercellular Communication: Aged astrocytes exhibited increased communication with microglia (promoting inflammation/synapse engulfment via Il33-Il1rap) and decreased communication with neurons (loss of synapse organization via Bcan-Nrcam and Spon1-App pairs).

B. Wnt Signalling Dysregulation in Astrocytes

• Upregulation of Inhibitors: Aged astrocytes showed increased expression of Wnt pathway inhibitors and regulators, including Maml2 (Notch co-activator antagonizing Wnt), Daam2, Znrf3, and Wwox.
• Downregulation of Effectors: There was a significant reduction in canonical Wnt targets (Myc, Ctnnb1) and non-canonical effectors.
• Cluster Specificity: Re-clustering revealed a specific "aged" astrocyte cluster (Cluster 2) where Wnt dysregulation was most pronounced. This cluster also showed enrichment for the "SNAP-a" ageing gene program.

C. Downstream Transcriptional Impact (Jun Factors)

• AP-1 Complex Reduction: The study identified a significant downregulation of Jun, JunD, and JunB (components of the AP-1 transcriptional complex) in aged astrocytes.
• Validation: RNAscope and IHC confirmed that Maml2 and Daam2 RNA levels increased in aged astrocytes, while JunD and JunB protein levels significantly decreased.
• Layer Distribution: Jun+ astrocytes were naturally more abundant in upper cortical layers (2-4) in adults, but this distribution pattern was maintained in ageing, with a global reduction in expression across all layers.

D. Cross-Species Validation

• Analysis of human prefrontal cortex snRNA-seq data revealed a conserved "aged" astrocyte cluster with similar dysregulation of Wnt pathway genes (Tcf7l2, Maml2, Wwox) and reduced expression of JUN family members compared to young human astrocytes.

E. Functional Rescue via JunD Overexpression

• Proliferation: Injection of JunD-expressing lentivirus into aged mouse cortex significantly increased the proliferation of astrocytes (measured by EdU incorporation) compared to controls, reversing the age-related decline in astrocyte reactivity.
• Senescence Markers: JunD overexpression increased nuclear HMGB1 levels in transduced astrocytes. Since HMGB1 loss is associated with senescence, this suggests a shift toward a more youthful, protective state.

5. Significance

• Mechanistic Insight: The study establishes a causal link between impaired Wnt signalling, the downregulation of Jun transcription factors, and the loss of astrocyte plasticity (specifically proliferation and neuroprotective capacity) during ageing.
• Therapeutic Potential: By demonstrating that overexpressing a single transcription factor (JunD) can restore youthful phenotypes (proliferation and HMGB1 expression) in aged astrocytes, the paper identifies a promising therapeutic target for combating brain ageing and age-related neurodegeneration.
• Paradigm Shift: It challenges the view that astrocyte ageing is a fixed, irreversible state, suggesting instead that these cells retain plasticity that can be harnessed for "rejuvenation" strategies.
• Translational Relevance: The conservation of these findings in human data strengthens the potential for developing interventions targeting the Wnt-Jun axis to promote healthy brain ageing in humans.