mTOR hyperactivation in Down Syndrome underlies deficits in autophagy induction, autophagosome formation, and mitophagy

Abstract

Down syndrome (DS), a complex genetic disorder caused by chromosome 21 trisomy, is associated with mitochondrial dysfunction leading to the accumulation of damaged mitochondria. Here we report that mitophagy, a form of selective autophagy activated to clear damaged mitochondria is deficient in primary human fibroblasts derived from individuals with DS leading to accumulation of damaged mitochondria with consequent increases in oxidative stress. We identified two molecular bases for this mitophagy deficiency: PINK1/PARKIN impairment and abnormal suppression of macroautophagy. First, strongly downregulated PARKIN and the mitophagic adaptor protein SQSTM1/p62 delays PINK1 activation to impair mitophagy induction after mitochondrial depolarization by CCCP or antimycin A plus oligomycin. Secondly, mTOR is strongly hyper-activated, which globally suppresses macroautophagy induction and the transcriptional expression of proteins critical for autophagosome formation such as ATG7, ATG3 and FOXO1. Notably, inhibition of mTOR complex 1 (mTORC1) and complex 2 (mTORC2) using AZD8055 (AZD) restores autophagy flux, PARKIN/PINK initiation of mitophagy, and the clearance of damaged mitochondria by mitophagy. These results recommend mTORC1-mTORC2 inhibition as a promising candidate therapeutic strategy for Down Syndrome.

Mitophagy: An Emerging Role in Aging and Age-Associated Diseases

“…Consistent with these findings, PINK1 and Parkin-dependent mitophagy is impaired and mTOR is hyperactivated in primary human fibroblasts derived from individuals with Down syndrome. In this context, inhibition of mTOR using AZD8055 restores autophagic flux, as well as mitophagy initiated by PINK1 and Parkin (Bordi et al, 2019).…”

Mammalian Mitophagosome Formation: A Focus on the Early Signals and Steps

“…Studies with CCCP and FCCP in a non-mitophagy context, have reported that these proton uncouplers cause a reduction in mTORC1 activity, suggesting that PINK1/Parkin mitophagy might require mTOR inactivation as bulk autophagy does (Inoki et al, 2003;Kim et al, 2013;Bartolome et al, 2017). Furthermore, recent reports suggest that mTORC1 hyperactivation can have an inhibitory effect on PINK1/Parkin mediated mitophagy as well as general autophagy (Bartolome et al, 2014(Bartolome et al, , 2017Bordi et al, 2019). Importantly, hypoxia leads to a reduction in mTORC1 activity as well, meaning that mTORC1 might play a role in hypoxiainduced mitophagy (Vadysirisack and Ellisen, 2012).…”

Prenatal treatment with rapamycin restores enhanced hippocampal mGluR-LTD and mushroom spine size in a Down’s syndrome mouse model

“…The role of mTORC1 as regulator of both general autophagy and mitophagy induction after oxidative phosphorylation uncoupling is well established (Bartolome et al, 2017). Accordingly, pharmacological inhibition of mTOR using AZD8055, which inhibits both mTORC1 and mTORC2 (Chresta et al, 2010), restored autophagy and mitophagy in DS fibroblasts (Bordi et al, 2019). As mentioned before, we previously demonstrated hyperactivation of mTOR in the Ts1Cje hippocampus (Troca-Marín et al, 2011).…”

“…In fact, altered mitochondrial function has long been associated with DS (reviewed by Valenti et al, 2014;Izzo et al, 2018). It has been recently found damaged mitochondria linked to increased oxidative stress, reduced mitophagy and reduced autophagy, together with mTOR hyperactivation in fibroblasts from DS patients, (Bordi et al, 2019). The role of mTORC1 as regulator of both general autophagy and mitophagy induction after oxidative phosphorylation uncoupling is well established (Bartolome et al, 2017).…”

“…Accordingly, mitochondrial alterations have been described in DS and DS mouse models (reviewed by Valenti et al, 2014;Izzo et al, 2018). In fact, it has been recently shown that reduced autophagy/mitophagy due to mTOR hyperactivation produces damaged mitochondria accumulation in DS fibroblasts (Bordi et al, 2019). In agreement with these data, we have found reduced levels of the B-II isoform of Microtubule-associated protein 1A/1B-light chain 3 (LC3B-II), a marker of autophagy, in Ts1Cje hippocampus, compared to WT (Fig.…”