Aims

Several lines of recent evidence indicate that the amyloid precursor protein-derived C-terminal fragments (APP-CTFs) are etiological triggers of Alzheimer’s disease (AD) pathology[1]. Altered mitochondrial homeostasis is also considered as an early event in AD development. Strikingly, we and others demonstrated the localization of APP-derived fragments in mitochondria-associated membranes[2]. However, the specific contribution of APP-CTFs to mitochondrial structure, function, and mitophagy defects remains to be established.

Methods

We used human neuroblastoma SH-SY5Y cells expressing the familial APPswe mutation or C99 fragment. We studied 2xTgAD, 3xTgAD mice and adeno-associated-virus (AAV)-C99 injected mice[3]. To discriminate between APP-CTFs and Abeta, we modulated pharmacologically secretases activity. Finally, we analyzed human post-mortem sporadic AD brains.

Results

We demonstrated in cells, that APP-CTFs fragments induce mitochondrial fragmentation, cristae disorganization, mitochondrial hyperpolarization, and a higher production of mitochondrial ROS, independently of Abeta. Moreover, APP-CTFs trigger mitophagic failure characterized by the activation and accumulation of autophagic markers and mitochondrial proteins. We confirmed the contribution of APP-CTFs accumulation to morphological mitochondria alteration and impaired basal mitophagy in vivo. Importantly, we showed that APP-CTFs accumulation correlates with mitophagy failure in AD brains[4].

Conclusions

This study unravels the toxicity of APP-CTFs, independently of Abeta, towards mitochondria dysfunctions and mitophagy in AD. Potential pharmacological approaches should focus on mitophagy activation to force the elimination and/or renewal of harmful mitochondria.

[1] Lauritzen I, et al. Acta Neuropathol 2016.

[2] Del Prete D. et al. J Alzheimers Dis 2017.

[3] Bourgeois A, et al. Neurobiol Aging 2018.

[4] Vaillant-Beuchot L.^#, Mary A.^# (co-authors), Acta Neuropathol 2020, under revision.

Aims

Besides the two main Alzheimer’s disease (AD) hallmarks (neurofibrillary tangles and senile plaques), the alteration of the structure and function of mitochondria was shown to be linked to the toxicity of Aβ peptides, in particular through the increase of reactive oxygen species (ROS) production. Furthermore, we have recently shown that APP C-terminal fragments (C83 and C99) localize to mitochondria-associated membranes (1) and trigger mitochondrial dysfunctions, overproduction of ROS and blockade of the mitophagy process (2).

Aims : We assessed mitochondrial structure and function and analyzed mitophagy markers in a cohort of fibroblasts isolated from sporadic AD patients.

Methods

We used fibroblasts from control, mild-cognitive impairment (MCI), or AD (AD) patients. Mitochondrial cristae organization as well as area, perimeter and number were quantified by electron microscopy. Measurements of mitochondrial ROS production and mitochondrial potential were analyzed by flow cytometry. The expression of proteins involved in mitophagy and mitochondrial dynamics were evaluated by SDS-PAGE.

Results

Fibroblasts from AD patients exhibit disorganization of mitochondria cristae ultrastructure with an increase of size in comparison of control and MCI cells. Electronic microscopy analyses also reveal an enlargement of endosomes in AD fibroblasts. Furthermore, mitochondrial function is also disrupted with an alteration of the mitochondrial membrane potential and modifications of expression of mitophagy markers.

Conclusions

Altogether, our data highlight mitochondria impairment in peripheral cells of AD patients and emphasize their potential use in AD diagnosis and personalized medicine.

(1) Del Prete D, et al. JAD 2017.

(2) Vaillant-Beuchot L^#, Mary A^#, et al. Under revision, Acta Neuropathologica.

Aims

Mitochondria structure and function alterations are major features of AD (1). The AMP-activated protein kinase (AMPK) promotes mitochondrial health, and multiple AMPK targets are involved in various aspects of mitochondrial homeostasis, mitophagy, and inflammatory response. We investigate the impact of AMPK signaling cascade on mitochondrial dysfunctions, mitophagy and neuroinflammation in AD.

Methods

We used SH-SY5Y cells overexpressing the AD Swedish mutation (APPswe), which accumulate Aβ and APP C-terminal fragments (APP-CTFs), ex vivo in hippocampal organotypic slices transduced with APPswe lentiviruses, and in 3xTgAD mice. We studied mitochondrial functions and mitophagy using biochemical, and imagery approaches. We analyzed neuroinflammation in mice using immunohistochemistry, RT-qPCR, and flux cytometry analyses. APP processing was modulated using γ-secretase inhibitor, or harboring the APPswe expression. We used pharmacological and genetic tools to modulate AMPK expression and activity.

Results

Cells expressing APPswe display a repressed AMPK cascade. Inversely, γ-secretase-mediated blockade of Aβ production and accumulation of APP-CTFs enhance AMPK activity, mitochondrial dysfunctions, and trigger mitophagy blockade. Interestingly, repressing AMPK cascade amplify APP-CTFs accumulation, impair mitochondrial functions, activate mitophagy and trigger dendrite shape alterations. Interestingly, activation of AMPK shows beneficial effects by increasing the number of mature dendritic spines and rescue the learning capacity of 3xTgAD mice. Studies investigating the impact of AMPK modulation on neuroinflammation in 3xTgAD mice are ongoing.

Conclusions

Our data highlight a pathogenic role of AMPK cascade repression in AD and question the potential of AMPK stimulation as a novel track for AD therapy.

(1) Vaillant-Beuchot, L^#., Mary, A^#. et al. ^#Co authors. Acta Neuropathologica (under revision).