A recent study published in the journal Nature Neuroscience revealed that Plexin-B1, encoded by PLXNB1regulates the activation of glial nets around the plaque in Alzheimer’s disease (AD).
AD is a major medical challenge as no effective treatment is available. While amyloid plaques are a hallmark of AD pathology, the factors that drive amyloid-beta plaque deposition, clearance, and compaction are poorly understood. Recent studies have shed light on microglial Aβ uptake and processing.
Study: Regulation of cell spacing in periplaque glial networks by Plexin-B1 affects glial activation and amyloid compaction in Alzheimer’s disease. Image credit: Juan Gaertner / Shutterstock
Microglial phagocytic activity promotes plaque compaction, limiting exposure of the healthy neuron to the plaque. In addition, the microglial covering of plaques serves as a barrier that limits the exposure of neurites to neurotoxic protofibrillar Aβ hotspots. Amyloid plaques are also surrounded by reactive astrocytes. Therefore, microglia and reactive astrocytes closely interact with Aβ and each other, forming glial networks around the plaque.
Plexin-B1 is an axon guidance receptor. Plexins and their cognate ligands mediate cell-cell communication during development, cancer, and adult physiology. Previously, the authors identified plexin-B1 as a hub gene in a subnetwork underlying late-onset AD. Furthermore, other studies have independently implicated plexin-B1 in the pathophysiology of AD. functional though in vivo missing data.
The study and findings
In the present study, the researchers investigated the functional significance of plexin-B1 in AD. They searched a gene expression database and found that plexin-B1 was predominantly expressed in astrocytes in the human and mouse central nervous system (CNS). RNAscope on the spot hybridization and immunofluorescence were performed on brain sections of a six-month-old amyloidogenic AD mouse model (APP/PS1 mice).
A high density of Plxnb1 mRNA was observed in peri-plaque glial fibrillary acidic protein (GFAP+) reactive astrocytes. Activated microglia around the plaque were minimal Plxnb1 mRNA. Further, the expression of plexin-B1 was assessed using a Plxnb1 knockout (PB1-KO) allele containing a LacZ cassette before Plxnb1promoter and initial exons.
The PB1-KO allele was bred APP/PS1 mice. LacZ/X-Gal staining from six months of age Plxnb1+/- mice exhibited distinct X-gal signals near plaques. This confirmed the upregulation of plexin-B1 in peri-plaque regions. Furthermore, immunohistochemical analysis of human AD samples showed high plexin-B1 signals around amyloid plaques.
Next, the cytoarchitecture of the periplaque glial networks was compared APP/PS1 mice with and without Plxnb1. Plexin-B1 deletion resulted in a much smaller footprint of glial nets around the plaque. In addition, the cell-cell distance of plaque-associated astrocytes was reduced in mice without Plxnb1. Plexin-B1 removal resulted in smaller, more compact glial nets with increased plaque coverage.
In addition, deletion of plexin-B1 reduced the number of plaque-associated microglia and the distance of plaque-associated microglia. Next, volume RNA sequencing of the prefrontal cortex of mice with and without plexin-B1 was performed. This revealed over 2,700 differentially expressed genes (DEGs). Pathway analyzes showed that DEGs upregulated in plexin-B1 null mice were enriched for gene ontology terms related to synaptic function and nervous system development.
Conversely, downregulated genes were related to inflammation and tissue damage, mechanical stimulus sensing, and protein deacetylase activity. Overall, plexin-B1 ablation had a protective effect, increased synaptic/neural function, and attenuated neuroinflammation and cell death. Next, monocyte RNA sequencing was performed.
Among the 10 distinct clusters defined by marker gene expression, microglia formed the largest cluster, followed by endothelial cells and astrocytes. Astrocytes formed nine transcriptionally distinct subclusters (sc-0 – sc-8). Sc-8 astrocytes were enriched for disease-associated astrocyte gene signatures (DAAs) observed in a mouse model of AD.
The microglia cluster formed 11 subclusters, with the sc-9 subcluster closely associated with disease-associated microglia (DAM) in the AD mouse model. Furthermore, plexin-B1 deletion increased signaling communication from astrocytes to microglia, with chemoattraction, lipid metabolism, and neuroprotection. Moreover, microglia-astrocyte communication was affected by plexin-B1 deletion, although again it was mainly enhanced.
Histological examination of amyloid plaques showed a significant reduction in plaque burden in PB1-KO mice with much smaller plaque sizes. Finally, Barnes Maze behavioral analyzes were performed to compare his memory performance APP/PS1 mice with and without PB1-KO. They observed a significant improvement in working memory performance in PB1-KO mice in acquisition training tests. In contrast, memory recall was similar on probe trials between PB1-KO and non-PB1-KO mice.
conclusions
The study demonstrated the upregulation of plexin-B1 in astrocytes in glial networks around the plaque and its involvement in glial cell mobilization and cell spacing around the plaques. In the absence of plexin-B1, cell-cell distance was reduced in peri-plaque networks and plaque coverage by glial processes was enhanced. Furthermore, plexin-B1 deletion induced transcriptional changes associated with enhanced DAM and DAA activation and improved signaling communication. Collectively, the results suggest inhibition of plexin-B1 as an alternative therapeutic strategy in AD.
