Regulation of β-amyloid clearance in APP/PS1 astrocytes by AMPK activation and MTORC1 inhibition

Abstract

Alzheimer disease (AD) is the most common neurodegenerative dementia. It is characterized by the progressive accumulation of amyloid beta (Aβ) aggregates and hyperphosphorylated tau. This aberrant accumulation has been associated with autophagy dysfunction, occurring in early stages of the disease. AMPK and AKT-MTORC1 signaling pathways are central nodes in the balance between anabolism and catabolism. It is generally accepted that MTORC1 activation leads to the inhibition of autophagy, whereas AMPK activation is supposed to enhance this process. Some regulatory cross-talk between both pathways has been reported. Accordingly, autophagy inducers such as MTORC1 inhibitors may have beneficial effects in the clearance/prevention of protein aggregates in the brain, as we previously reported in the APP/PS1 AD mouse model. Until recently, most of the mechanisms that mediate protein clearance have been studied in neurons and the contribution of glial cells remains to be elucidated. The aim of this work is to determine the contribution of astrocytes to Aβ clearance through autophagy. In an attempt to analyze the effect of autophagic flux modulation, we performed primary cultures of astrocytes form APP/PS1 mice and their wild type littermates. We treated them with different described drugs that act as autophagy inductors through the modulation of MTORC1 and AMPK pathways and evaluated the activation of these pathways and their effect in the autophagic flux by western blot. We found a slight increase of autophagy with rapamycin, a well-known MTORC1 inhibitor. Surprisingly, AMPK activation with metformin did not enhance autophagy, which could be due, at least in part, to an insufficient inhibition of MTORC1. Finally, we measured the direct effect of this modulation on autophagy-dependent amyloidosis through ELISA. These results suggest that AMPK activation, in contrast to mTORC1 inhibition, is not sufficient to enhance autophagy in primary astrocytes, as we previously described in neurons. Thus, the protective mechanisms of autophagy against neurodegeneration must be further examined, with the final aim of describing mechanisms that allow an effective treatment for this neurodegenerative disorder.