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 ß (Aß) 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 Aß 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.

Seeding, maturation and propagation of amyloid ß-peptide aggregates in Alzheimer's disease.

Alzheimer's disease is neuropathologically characterized by the deposition of the amyloid ß-peptide (Aß) as amyloid plaques. Aß plaque pathology starts in the neocortex before it propagates into further brain regions. Moreover, Aß aggregates undergo maturation indicated by the occurrence of post-translational modifications. Here, we show that propagation of Aß plaques is led by presumably non-modified Aß followed by Aß aggregate maturation. This sequence was seen neuropathologically in human brains and in amyloid precursor protein transgenic mice receiving intracerebral injections of human brain homogenates from cases varying in Aß phase, Aß load and Aß maturation stage. The speed of propagation after seeding in mice was best related to the Aß phase of the donor, the progression speed of maturation to the stage of Aß aggregate maturation. Thus, different forms of Aß can trigger propagation/maturation of Aß aggregates, which may explain the lack of success when therapeutically targeting only specific forms of Aß.

Sequence of proteome profiles in preclinical and symptomatic Alzheimer's disease.

Proteome profile changes in Alzheimer's disease (AD) brains have been reported. However, it is unclear whether they represent a continuous process, or whether there is a sequential involvement of distinct proteins. To address this question, we used mass spectrometry. We analyzed soluble, dispersible, sodium dodecyl sulfate, and formic acid fractions of neocortex homogenates (mainly Brodmann area 17-19) from 18 pathologically diagnosed preclinical AD, 17 symptomatic AD, and 18 cases without signs of neurodegeneration. By doing so, we identified four groups of AD-related proteins being changed in levels in preclinical and symptomatic AD cases: early-responding, late-responding, gradually-changing, and fraction-shifting proteins. Gene ontology analysis of these proteins and all known AD-risk/causative genes identified vesicle endocytosis and the secretory pathway-related processes as an early-involved AD component. In conclusion, our findings suggest that subtle changes involving the secretory pathway and endocytosis precede severe proteome changes in symptomatic AD as part of the preclinical phase of AD. The respective early-responding proteins may also contribute to synaptic vesicle cycle alterations in symptomatic AD.

Necrosome complex detected in granulovacuolar degeneration is associated with neuronal loss in Alzheimer's disease.

Alzheimer's disease (AD) is characterized by a specific pattern of neuropathological changes, including extracellular amyloid ß (Aß) deposits, intracellular neurofibrillary tangles (NFTs), granulovacuolar degeneration (GVD) representing cytoplasmic vacuolar lesions, synapse dysfunction and neuronal loss. Necroptosis, a programmed form of necrosis characterized by the assembly of the necrosome complex composed of phosphorylated proteins, i.e. receptor-interacting serine/threonine-protein kinase 1 and 3 (pRIPK1 and pRIPK3) and mixed lineage kinase domain-like protein (pMLKL), has recently been shown to be involved in AD. However, it is not yet clear whether necrosome assembly takes place in brain regions showing AD-related neuronal loss and whether it is associated with AD-related neuropathological changes. Here, we analyzed brains of AD, pathologically defined preclinical AD (p-preAD) and non-AD control cases to determine the neuropathological characteristics and distribution pattern of the necrosome components. We demonstrated that all three activated necrosome components can be detected in GVD lesions (GVDn+, i.e. GVD with activated necrosome) in neurons, that they colocalize with classical GVD markers, such as pTDP-43 and CK1δ, and similarly to these markers detect GVD lesions. GVDn + neurons inversely correlated with neuronal density in the early affected CA1 region of the hippocampus and in the late affected frontal cortex layer III. Additionally, AD-related GVD lesions were associated with AD-defining parameters, showing the strongest correlation and partial colocalization with NFT pathology. Therefore, we conclude that the presence of the necrosome in GVD plays a role in AD, possibly by representing an AD-specific form of necroptosis-related neuron death. Hence, necroptosis-related neuron loss could be an interesting therapeutic target for treating AD.

Aß-induced acceleration of Alzheimer-related τ-pathology spreading and its association with prion protein.

Extracellular deposition of amyloid ß-protein (Aß) in amyloid plaques and intracellular accumulation of abnormally phosphorylated τ-protein (p-τ) in neurofibrillary tangles (NFTs) represent pathological hallmark lesions of Alzheimer's disease (AD). Both lesions develop in parallel in the human brain throughout the preclinical and clinical course of AD. Nevertheless, it is not yet clear whether there is a direct link between Aß and τ pathology or whether other proteins are involved in this process. To address this question, we crossed amyloid precursor protein (APP) transgenic mice overexpressing human APP with the Swedish mutation (670/671 KM → NL) (APP23), human wild-type APP (APP51/16), or a proenkephalin signal peptide linked to human Aß42 (APP48) with τ-transgenic mice overexpressing human mutant 4-repeat τ-protein with the P301S mutation (TAU58). In 6-month-old APP23xTAU58 and APP51/16xTAU58 mice, soluble Aß was associated with the aggravation of p-τ pathology propagation into the CA1/subiculum region, whereas 6-month-old TAU58 and APP48xTAU58 mice neither exhibited significant amounts of p-τ pathology in the CA1/subiculum region nor displayed significant levels of soluble Aß in the forebrain. In APP23xTAU58 and APP51/16xTAU58 mice showing an acceleration of p-τ propagation, Aß and p-τ were co-immunoprecipitated with cellular prion protein (PrPC). A similar interaction between PrPC, p-τ and Aß was observed in human AD brains. This association was particularly noticed in 60% of the symptomatic AD cases in our sample, suggesting that PrPC may play a role in the progression of AD pathology. An in vitro pull-down assay confirmed that PrPC is capable of interacting with Aß and p-τ. Using a proximity ligation assay, we could demonstrate proximity (less than ~ 30-40 nm distance) between PrPC and Aß and between PrPC and p-τ in APP23xTAU58 mouse brain as well as in human AD brain. Proximity between PrPC and p-τ was also seen in APP51/16xTAU58, APP48xTAU58, and TAU58 mice. Based on these findings, it is tempting to speculate that PrPC is a critical player in the interplay between Aß and p-τ propagation at least in a large group of AD cases. Preexisting p-τ pathology interacting with PrPC, thereby, appears to be a prerequisite for Aß to function as a p-τ pathology accelerator via PrPC.

Inhibition of an Alzheimer's disease-associated form of necroptosis rescues neuronal death in mouse models.

Necroptosis is a regulated form of cell death that has been observed in Alzheimer's disease (AD) along with the classical pathological hallmark lesions of amyloid plaques and Tau neurofibrillary tangles. To understand the neurodegenerative process in AD, we studied the role of necroptosis in mouse models and primary mouse neurons. Using immunohistochemistry, we demonstrated activated necroptosis-related proteins in transgenic mice developing Tau pathology and in primary neurons from amyloid precursor protein (APP)-Tau double transgenic mice treated with phosphorylated Tau seeds derived from a patient with AD but not in APP transgenic mice that only exhibited ß-amyloid deposits. Necroptosis proteins in granulovacuolar degeneration (GVD) bodies were associated with neuronal loss in mouse brain regions also known to be vulnerable to GVD in the human AD brain. Necroptosis inhibitors lowered the percentage of neurons showing GVD and reduced neuronal loss, both in transgenic mice and in primary mouse neurons. This suggests that a GVD-associated form of necroptosis that we refer to as "GVD-necroptosis" may represent a delayed form of necroptosis in AD. We propose that inhibition of necroptosis could rescue this type of neuronal death in AD.

Senescence-related impairment of autophagy induces toxic intraneuronal amyloid-ß accumulation in a mouse model of amyloid pathology.

Aging is the main risk factor for Alzheimer's disease (AD) and other neurodegenerative pathologies, but the molecular and cellular changes underlying pathological aging of the nervous system are poorly understood. AD pathology seems to correlate with the appearance of cells that become senescent due to the progressive accumulation of cellular insults causing DNA damage. Senescence has also been shown to reduce the autophagic flux, a mechanism involved in clearing damaged proteins from the cell, and such impairment has been linked to AD pathogenesis. In this study, we investigated the role of cellular senescence on AD pathology by crossing a mouse model of AD-like amyloid-ß (Aß) pathology (5xFAD) with a mouse model of senescence that is genetically deficient for the RNA component of the telomerase (Terc-/-). We studied changes in amyloid pathology, neurodegeneration, and the autophagy process in brain tissue samples and primary cultures derived from these mice by complementary biochemical and immunostaining approaches. Postmortem human brain samples were also processed to evaluate autophagy defects in AD patients. Our results show that accelerated senescence produces an early accumulation of intraneuronal Aß in the subiculum and cortical layer V of 5xFAD mice. This correlates with a reduction in amyloid plaques and Aß levels in connecting brain regions at a later disease stage. Neuronal loss was specifically observed in brain regions presenting intraneuronal Aß and was linked to telomere attrition. Our results indicate that senescence affects intraneuronal Aß accumulation by impairing autophagy function and that early autophagy defects can be found in the brains of AD patients. Together, these findings demonstrate the instrumental role of senescence in intraneuronal Aß accumulation, which represents a key event in AD pathophysiology, and emphasize the correlation between the initial stages of amyloid pathology and defects in the autophagy flux.

LATE-NC aggravates GVD-mediated necroptosis in Alzheimer's disease.

It has become evident that Alzheimer's Disease (AD) is not only linked to its hallmark lesions-amyloid plaques and neurofibrillary tangles (NFTs)-but also to other co-occurring pathologies. This may lead to synergistic effects of the respective cellular and molecular players, resulting in neuronal death. One of these co-pathologies is the accumulation of phosphorylated transactive-response DNA binding protein 43 (pTDP-43) as neuronal cytoplasmic inclusions, currently considered to represent limbic-predominant age-related TDP-43 encephalopathy neuropathological changes (LATE-NC), in up to 70% of symptomatic AD cases. Granulovacuolar degeneration (GVD) is another AD co-pathology, which also contains TDP-43 and other AD-related proteins. Recently, we found that all proteins required for necroptosis execution, a previously defined programmed form of neuronal cell death, are present in GVD, such as the phosphorylated necroptosis executioner mixed-lineage kinase domain-like protein (pMLKL). Accordingly, this protein is a reliable marker for GVD lesions, similar to other known GVD proteins. Importantly, it is not yet known whether the presence of LATE-NC in symptomatic AD cases is associated with necroptosis pathway activation, presumably contributing to neuron loss by cell death execution. In this study, we investigated the impact of LATE-NC on the severity of necroptosis-associated GVD lesions, phosphorylated tau (pTau) pathology and neuronal density. First, we used 230 human post-mortem cases, including 82 controls without AD neuropathological changes (non-ADNC), 81 non-demented cases with ADNC, i.e.: pathologically-defined preclinical AD (p-preAD) and 67 demented cases with ADNC. We found that Braak NFT stage and LATE-NC stage were good predictors for GVD expansion and neuronal loss in the hippocampal CA1 region. Further, we compared the impact of TDP-43 accumulation on hippocampal expression of pMLKL-positive GVD, pTau as well as on neuronal density in a subset of nine non-ADNC controls, ten symptomatic AD cases with (ADTDP+) and eight without LATE-NC (ADTDP-). Here, we observed increased levels of pMLKL-positive, GVD-exhibiting neurons in ADTDP+ cases, compared to ADTDP- and controls, which was accompanied by augmented pTau pathology. Neuronal loss in the CA1 region was increased in ADTDP+ compared to ADTDP- cases. These data suggest that co-morbid LATE-NC in AD impacts not only pTau pathology but also GVD-mediated necroptosis pathway activation, which results in an accelerated neuronal demise. This further highlights the cumulative and synergistic effects of comorbid pathologies leading to neuronal loss in AD. Accordingly, protection against necroptotic neuronal death appears to be a promising therapeutic option for AD and LATE.