Astrocytic LRP4 Mediates Gliovascular Remodeling after Pilocarpine-Induced Status Epilepticus

Technical Summary: Astrocytic LRP4 Mediates Gliovascular Remodeling after Pilocarpine-Induced Status Epilepticus

Problem Statement
Hippocampal sclerosis, characterized by neuronal loss, astrogliosis, and blood-brain barrier (BBB) dysfunction, is a hallmark of temporal lobe epilepsy (TLE). The pilocarpine-induced status epilepticus (SE) model recapitulates these features, including spontaneous recurrent seizures (SRS). While Low-density lipoprotein receptor-related protein 4 (LRP4) has been identified as a regulator of neuron-glia communication and seizure susceptibility, its specific role during the latent phase of epileptogenesis—particularly regarding its interaction with the neurovascular unit and BBB repair—remains unclear. Previous studies manipulating LRP4 prior to SE induction suggest a complex, time-dependent role, potentially confounding the distinction between acute seizure protection and long-term remodeling. This study investigates whether restoring astrocytic LRP4 expression during the latent phase (post-acute) can mitigate hippocampal sclerosis and SRS, and elucidates the underlying mechanisms involving gliovascular communication.

Methodology
The study utilized male C57BL/6J mice subjected to a repeated low-dose pilocarpine protocol to induce SE. To isolate the effects of the latent phase, astrocyte-specific overexpression of human LRP4 (hLRP4) was achieved via stereotaxic injection of an adenovirus (ADV) into the hippocampus 7 days post-SE. Control groups received PBS or a control ADV.

• Behavioral and Histological Assessment: Spontaneous recurrent seizures were monitored via continuous video recording from day 14 to day 28. Hippocampal tissues were analyzed at day 28 using immunofluorescence (NeuN for neuronal loss, GFAP for astrogliosis) and Western blotting.
• Single-Cell RNA Sequencing (scRNA-seq): Hippocampal tissues from four groups (PBS control, Pilocarpine-only, Control ADV, hLRP4-OE ADV) were processed for scRNA-seq using the DNBelab C4 platform. Data were analyzed using Seurat and Harmony for integration, clustering, and cell-type annotation.
• Computational Analysis: Researchers performed differential expression analysis, trajectory inference (pseudotime), and ligand-receptor interaction modeling (CellChat-style analysis) to map cell-cell communication networks, specifically focusing on astrocyte-endothelial-pericyte crosstalk.
• Functional Validation: To test the necessity of the Agrin-LRP4 axis, human recombinant Agrin was infused into the lateral ventricles of hLRP4-overexpressing mice to disrupt the signaling pathway, followed by behavioral and histological assessment.

Key Results

1. Phenotypic Protection: Astrocytic hLRP4 overexpression during the latent phase significantly reduced the frequency of spontaneous recurrent seizures and alleviated hippocampal sclerosis-like pathology. Specifically, it preserved neuronal density (NeuN+) and reduced reactive astrogliosis (GFAP+) in the CA1 region compared to control groups.
2. Astrocyte State Remodeling: scRNA-seq revealed that pilocarpine-induced SE drove astrocytes toward an AP3 state characterized by metabolic and lipid-remodeling signatures (e.g., Aldoc, Clu, oxidative phosphorylation). In contrast, hLRP4 overexpression shifted the astrocyte population away from the AP3 reactive state toward an AP6-like state. The AP6 state was enriched for genes associated with structural remodeling, stress response, and vascular/endfoot support (e.g., Gfap, Vim, Sparc, Hspa1a).
3. Enhanced Gliovascular Communication: Ligand-receptor analysis demonstrated that hLRP4 overexpression strengthened communication between astrocytes and vascular cells (endothelial cells and pericytes). Unlike the pilocarpine-only condition, where vascular cells acted as the primary signaling hubs, the hLRP4-overexpressing condition featured astrocytes as dominant receivers of vascular-derived signals.
4. Agrin-LRP4 Axis: The study identified a specific enrichment of Agrin-LRP4-Dag1 signaling in hLRP4-positive astrocytes. These astrocytes showed elevated expression of endfoot-associated genes (Aqp4, Mlc1, Kcnj10) and preferentially received basement membrane and extracellular matrix (ECM) cues.
5. Mechanistic Validation: Disruption of the Agrin-LRP4 interaction via intracerebroventricular Agrin infusion in hLRP4-overexpressing mice abolished the protective effects. These mice exhibited increased seizure frequency, greater neuronal loss, and enhanced astrogliosis, confirming that the therapeutic benefit relies on functional Agrin-LRP4 signaling.
6. BBB Repair Signatures: Analysis of pericyte and endothelial cells showed that while pilocarpine induced vascular stress and BBB dysfunction, hLRP4 overexpression promoted a remodeling program associated with BBB repair, angiogenesis, and basement membrane stabilization.

Significance and Claims
The authors claim that astrocytic LRP4 overexpression during the latent phase of epileptogenesis exerts a protective effect not merely by suppressing astrocyte reactivity, but by actively promoting a repair-associated astrocyte state. This state is characterized by enhanced communication with the neurovascular unit, specifically through the reception of Agrin-derived signals from the vascular basement membrane.

The study posits that LRP4 functions as a sensor for perivascular cues, coordinating the re-establishment of astrocyte endfoot polarity and BBB integrity. By shifting astrocytes from a metabolic/lipid-remodeling reactive state (AP3) to a vascular-supportive repair state (AP6), LRP4 facilitates gliovascular remodeling that stabilizes the BBB and reduces seizure progression. The authors conclude that the Agrin-LRP4 pathway represents a potential therapeutic target for restoring BBB homeostasis and mitigating hippocampal sclerosis following status epilepticus.