Basic Science and Pathogenesis.

Background: The aggregation of α-synuclein protein, encoded by the SNCA gene, forms Lewy bodies (LBs) in neurons and is a key pathological feature of Lewy body dementia (LBD). Interestingly, the apolipoprotein E gene (APOE), primarily expressed in glial cells, is the strongest genetic modifier for LBD. The ε4 allele of this gene (APOE4) notably increases the risk of LBD. However, the specific mechanisms through which APOE-regulated glial cells affect α-synuclein aggregation in neurons are not yet fully understood.

Method: To explore this, we conducted spatial transcriptomics using the 10X Genomics Visium CytAssist platform on Formalin-Fixed Paraffin-Embedded tissue slides from the temporal cortex of human LBD brains (N = 4 brains in controls and N = 6 brains in LBD group, with an equal ratio of APOE3/3 and APOE3/4 genotypes). We defined the neuronal layers of the gray matter (Layers 1 to 6, L1-6) and white matter in each brain based on the gene signature of each spot. We also identified LBs from hematoxylin and eosin (H&E) staining on the same slides, and annotated LB-positive (LB+), LB-surrounding and LB-negative (LB-) spots in each brain using Loupe Browser. We evaluated SNCA and APOE gene expression in each layer and analyzed cell-cell communication using the CellChat tool across the layers.

Result: We observed that the SNCA gene was highly expressed in L5, which corresponded to the areas where most LBs were found. Notably, within LB+ spots across all layers, both SNCA and APOE gene expression levels were elevated in APOE3/4 cases compared to APOE3/3 cases. Furthermore, we identified cell-cell communication networks between LB+ and LB-surrounding spots, particularly in L4 and L5. These networks were enriched by microglia-related signaling pathways, including APOE signaling, migration inhibitory factor (MIF) signaling, and colony stimulating factor 1 (CSF1) signaling. Interestingly, APOE3/4 brains exhibited significantly reduced interactions and lower interaction strength in these networks compared to APOE3/3 samples.

Conclusion: Our findings indicate that impaired cell-cell communication, such as microglia-mediated immune responses around LB regions, may be associated with the increased risk presented by the APOE4 allele in LBD brains.