Alzheimer’s disease (AD) is the most common neurodegenerative disease, characterized by progressive neuronal loss triggered by the presence of amyloid plaques and neurofibrillary tangles. Cell signaling cascades are currently considered a source of promising therapeutical targets for the design of new drugs blocking the progression of AD. Among them, PI3K/PDK1/Akt signaling pathway is hyperactivated in AD brains. This pathway plays an important role in restraining TACE α-secretase activity, that leads to decreased APPα processing and TNFR1 shedding, resulting in increased Aβ release and TNF-α hypersensitivity. Strikingly, inhibition of PDK1 reduced brain pathology and memory impairment in AD mice models, but at the same time exhibited deleterious effects, since the animals died after 5 months of treatment. Results in the laboratory allow to postulate that inhibition of the PDK1 effector Akt would protect mice against AD, while inhibition of the other PDK1 effectors (S6K, SGK, RSK and PKC) would be responsible for the toxicity. Supporting this hypothesis, PDK1K465E/K465E PH-domain knock-in mice with deficient Akt activation show increased TACE activity and elevated TNFR1 and APPα processing, which protect neurons against TNF-α and Aβ toxicity. Pharmacological Akt inhibition in immune cell lines accentuates the phosphorylation of NFκB, a transcription factor that plays an important role in regulating gene expression of inflammatory cytokines; however, TNF-α, IL-6 or IL-1β gene transcription is not affected by this posttranslational modification. Another deleterious cellular consequence of AD is the stress of endoplasmic reticulum (ER), leading to an induction of the Unfolded Protein Response (UPR) in temporal cortex and hippocampus. The UPR is a protective cellular response induced upon ER stress caused by perturbations in protein folding, such as Aβ accumulation. It represents a mechanism of adaptation and survival, which leads to the reduction of the unfolded protein load and to the reestablishment of protein-folding homeostasis. If these mechanisms are not enough to recover the homeostasis, the UPR will dictate cell death. ER stress may interfere with normal non-amyloidogenic trafficking of APP, leading to higher Aβ production. The increased production of Aβ peptides induces an overactivation of the PI3K/Akt signaling pathway, that could also contribute to activate the UPR, thereby exacerbating the Aβ production and the deposition of amyloid plaques. Reducing Akt activity levels by PDK1K465E/K465E knock-in mutation also protects mice neurons against ER stress mediated toxicity, suggesting that Akt could contribute to AD pathology by inducing the UPR. This is supported by the hyperactivation of Akt and UPR observed in APP/Tau mutant mice models of AD. Tunicamycin, an inhibitor of the first step in the biosynthesis of N-linked glycans in proteins, causes ER stress and reduces cell viability by inducing the UPR in both HEK293T and Neuro2A cells. Tunicamycin-induced loss of cell viability decreases cell density and alters cell morphology without inducing cell death, by means of a reversible cell cycle arrest. The inhibition of Akt with the MK2206 compound restores the phenotype of tunicamycin-treated cells by attenuating the UPR and antagonizing the cell cycle arrest. Moreover, the inhibition of other effectors of the PI3K/PDK1 signaling pathway (S6K, SGK or RSK) is not protective against tunicamycin, since it does not attenuate the UPR. Chemical inhibition of Akt is not protective but rather toxic in tunicamycin-treated postmitotic cells and neurons with lower Akt activity levels. In addition, inhibition of the major UPR kinase, PERK, can also mitigate the consequences of ER stress by attenuating the UPR, thereby protecting cells against tunicamycin toxicity without affecting Akt activation levels. Altogether, these results suggests that Akt contributes to the disruption of protein homeostasis, thereby partial inhibition of Akt could be a therapeutic approach to attenuate the neuronal loss occurring in AD brains.
Contribution of the PI3K/PDK1/Akt signaling pathway to Alzheimer’s disease.
Martínez Arenas, L. (Author). 8 Nov 2024
Student thesis: Doctoral thesis
Student thesis: Doctoral thesis