Pyroptosis in Alzheimer's disease: cell type-specific activation in microglia, astrocytes and neurons

The major neuropathological hallmarks of Alzheimer's disease (AD) are amyloid β (Aβ) plaques and neurofibrillary tangles (NFT), accompanied by neuroinflammation and neuronal loss. Increasing evidence is emerging for the activation of the canonical NOD-, LRR- and pyrin domain-containing 3 (NLRP3) inflammasome in AD. However, the mechanisms leading to neuronal loss in AD and the involvement of glial cells in these processes are still not clear. The aim of this study was to investigate the contribution of pyroptosis, a pro-inflammatory mechanism of cell death downstream of the inflammasome, to neurodegeneration in AD. Immunohistochemistry and biochemical analysis of protein levels were performed on human post-mortem brain tissue. We investigated the presence of cleaved gasdermin D (GSDMD), the pyroptosis effector protein, as well as the NLRP3 inflammasome-forming proteins, in the medial temporal lobe of 23 symptomatic AD, 25 pathologically defined preclinical AD (p-preAD) and 21 non-demented control cases. Cleaved GSDMD was detected in microglia, but also in astrocytes and in few pyramidal neurons in the first sector of the cornu ammonis (CA1) of the hippocampus and the temporal cortex of Brodmann area 36. Only microglia expressed all NLRP3 inflammasome-forming proteins (i.e., ASC, NLRP3, caspase-1). Cleaved GSDMD-positive astrocytes and neurons exhibited caspase-8 and non-canonical inflammasome protein caspase-4, respectively, potentially indicating alternative pathways for GSDMD cleavage. Brains of AD patients exhibited increased numbers of cleaved GSDMD-positive cells. Cleaved GSDMD-positive microglia and astrocytes were found in close proximity to Aβ plaques, while cleaved GSDMD-positive neurons were devoid of NFTs. In CA1, NLRP3-positive microglia and cleaved GSDMD-positive neurons were associated with local neuronal loss, indicating a possible contribution of NLRP3 inflammasome and pyroptosis activation to AD-related neurodegeneration. Taken together, our results suggest cell type-specific activation of pyroptosis in AD and extend the current knowledge about the contribution of neuroinflammation to the neurodegenerative process in AD via a direct link to neuron death by pyroptosis.