Table 1:

Evidence for different mechanisms of neuronal death in AD

Cell Death	Authors	Model System	Finding
Apoptosis	[13] Loo et al. 1993	Primary mouse neuron	In vitro exposure of hippocampal neurons to Aβ peptides led to ultrastructural features of apoptosis in electron microscopy imaging studies.
	[17] Stadelmann et al. 1998	Human tissue	Tissue samples from patients with AD and ponstosubicular neuron necrosis (PSNN), included as a positive control for neuronal apoptosis, were compared to evaluate if neurons in the AD brain exhibit microstructural features of apoptosis. Many of the neurons in the AD samples showed DNA fragmentation but very few showed associated hallmarks of apoptosis.
	[14] Ivins, et al. 1999	Primary neurons	Caspase-8 inhibitor prevented Aβ-induced cell death in neurons; induction of dominant negative FADD also protected neurons. This indicates a possible role for extrinsic apoptosis in AD.
	[12] Yao, Nguyen, and Pike 2005	Primary rat neurons	Aβ significantly reduced the expression of the antiapoptotic molecules Bcl-w and Bcl-xL in primary rat neurons. Overexpression of Bcl-w rescued neuronal cell death upon exposure to Aβ.
	[76] Rohn et al. 2008	3xTG	Overexpression of Bcl-2 decreased caspase-9 cleavage, reduced NFT and plaque formation, and improved place recognition memory in 3xTG mice.
	[16] Kudo et al. 2012	C57B6; hippocampal slice culture	Aβ increased Bim but decreased Bcl-2 levels and led to the activation of Bax and neuronal cell death in both hippocampal slice culture and in vivo. Inhibition of Bax through Bax-inhibiting peptide or Bax gene knockout prevented Aβ-induced neuronal cell death.
	[15] Robbins et al. 2018	hiPSC neuron culture	Aβ peptide induced Caspase-3 cleavage in hiPSC neurons. Clusterin KO prevented reductions in neurite length upon Aβ exposure.
	[77] Zhang et al. 2021	5xFAD	Increased BAD protein levels in AD tissue samples and in 5xFAD mice. Disruption of Bad alleles rescued spatial and leaning deficits in 5xFAD.
Pyroptosis	[34] Halle et al. 2008	Primary mouse neuron	Showed that primary microglia activate the NLRP3 inflammasome in response to fibrillar Aβ. This process is dependent on microglial phagocytosis, lysosomal stress, and cathepsin B release, which lead to subsequent NLRP3 activation.
	[35] Heneka et al. 2013	APP/PS1	Nlrp3−/−APP/PS1 and Casp1−/−APP/PS1 mice were both protected from loss of spatial memory and other sequelae associated with AD. These mice demonstrated reduced brain Caspase-1 and interleukin-1β activation as well as enhanced amyloid-β clearance. Increased cleaved Caspase-1 was found in AD/MCI patient tissue.
	[37] Tan et al. 2014	APP/PS1 / primary culture	Aβ1–42 increased NLRP1 expression in primary cortical neurons, and siRNA-mediated knockdown of Nrlp1 increased neuronal viability after treatment with Aβ. siRNA targeted ablation of Nlrp1 in APP/PS1 mice increased neuronal survival in the hippocampus and cortex.
	[36] Ising et al. 2019	APP/PS1	Loss of NLRP3 inflammasome function reduced tau hyperphosphorylation and aggregation. Tau activated the NLRP3 inflammasome and intracerebral injection of fibrillar amyloid-beta-containing brain homogenates induced tau pathology in an NLRP3-dependent manner.
Necrosis	[21] Behl et al. 1994	Rat primary cortical neurons, PC12 cells	Early investigations using electron microscopy provided evidence that primary cortical neurons undergo necrosis when exposed to Aβ peptides; no apoptotic bodies or nuclear fragmentation was seen, while organellar damage and vacuolization was apparent.
	[22] Tanaka et al. 2020	Human tissue/hiPSC	The necrosis-associated protein HMGB1 was increased in the CSF of MCI, but not AD patients. Post-mortem tissue demonstrated increased staining of the necrosis marker pSer46-MARCKS in MCI but not AD cases compared to healthy controls. In vivo imaging revealed instability of endoplasmic reticulum (ER). Genome-edited human AD iPS cell-derived neurons exhibited decreased nuclear Yes-associated protein (YAP) due to the sequestration into cytoplasmic Aβ-aggregates, supporting the feature of YAP-dependent necrosis.
Necroptosis	[24] Caccamo et al. 2017	Human tissue	Elevated RIPK1 and MLKL protein expression, as well as increased colocalization of RIPK3 and MLKL were found in post-mortem tissue of AD patients indicating increased necroptosis. Necrosome formation was shown to be inversely correlated with brain weight and cognitive scores. Genes regulated by RIPK1 overlapped significantly with multiple independent AD transcriptomic signature. Treatment of 5xFAD mice with the RIPK1 inhibitor Nec-1 reduced attenuated neuronal degeneration.
	[23] Yang et al. 2019	HT22 cells/ APP/PS1	Nec-1 was shown to directly disaggregate Aβ fibrils and oligomers, suggesting a cell-extrinsic function of the small-molecule. Nec-1 also prevented Aβ oligomer-induced cell death in both BV2 and HT22 cells. Finally, i.v. administration of Nec-1 (bi-weekly), for 4 weeks in 8 month old APP/PS1 mice reduced cortical plaque number.
	[26] Koper et al. 2020	Human tissue	Activated necrosome components (RIPK1, RIPK3, MLKL) were detected in neurons proximal to GVD lesions and necrosome markers colocalized with GVD biomarkers (pTDP-43 and CK1δ). GVD neurons with activated necroptosis components inversely correlated with neuronal density in the early affected CA1 region of the hippocampus and in the late affected frontal cortex layer III.
	[25] Jayaraman et al. 2021	Human tissue	CA1 pyramidal neurons from AD patients displayed increased levels of activated necroptotic proteins (e.g. pRIPK3 and pMLKL). The density of pRIPK3+ and pMLKL+ neurons also correlated inversely with total neuron density and showed significant sexual dimorphism within the AD cohort. Exposure of human iPSC-derived glutamatergic neurons to TNF increased necroptotic cell death when apoptosis was inhibited.
Ferroptosis	[51] Chen et al. 2022	5xFAD	5xFAD exhibited elevated levels of the lipid peroxidation end-product 4-HNE. Global overexpression of GPX4 rescued spatial learning deficits and prevented the loss of neurons in layer 5 of the cortex.

Acronym Key: Aβ, amyloid beta; AD, Alzheimer’s disease; ER, endoplasmic reticulum; FADD, Fas-Associated Death Domain; GPX4, glutathione peroxidase-4; HMGB1, High Mobility Group Box-1; MCI, mild cognitive impairment; MLKL, Mixed lineage kinase domain-like protein; MMSE, mini-mental status examination; Nec-1, necrostatin-1; PSNN, ponstosubicular neuron necrosis; RIPK1/3, Receptor-interacting serine/threonine-protein kinase 1/3; YAP, Yes-associated protein.