Evaluating the mTOR Pathway in Physiological and Pharmacological Settings

Torin1 restores proliferation rate in Charcot-Marie-Tooth disease type 2A cells harbouring MFN2 (mitofusin 2) mutation

Objective

Mitofusin 2 (MFN2) is a mitochondrial outer membrane protein that serves primarily as a mitochondrial fusion protein but has additional functions including the tethering of mitochondrial-endoplasmic reticulum membranes, movement of mitochondria along axons, and control of the quality of mitochondria. Intriguingly, MFN2 has been referred to play a role in regulating cell proliferation in several cell types such that it acts as a tumour suppressor role in some forms of cancer. Previously, we found that fibroblasts derived from a Charcot-Marie-Tooth disease type 2A (CMT2A) patient with a mutation in the GTPase domain of MFN2 exhibit increased proliferation and decreased autophagy.

Methods

Primary fibroblasts from a young patient affected by CMT2A harbouring c.650G > T/p.Cys217Phe mutation in the MFN2 gene were evaluated versus a healthy control to measure the proliferation rate by growth curves analysis and to assess the phosphorylation of protein kinase B (AKT) at Ser473 in response to different doses of torin1, a selective catalytic ATP-competitive mammalian target of rapamycin complex (mTOR) inhibitor, by immunoblot analysis.

Results

Herein, we demonstrated that the mammalian target of rapamycin complex 2 (mTORC2) is highly activated in the CMT2A^MFN2 fibroblasts to promote cell growth via the AKT(Ser473) phosphorylation-mediated signalling. We report that torin1 restores CMT2A^MFN2 fibroblasts' growth rate in a dose-dependent manner by decreasing AKT(Ser473) phosphorylation.

Conclusions

Overall, our study provides evidence for mTORC2, as a novel molecular target that lies upstream of AKT to restore the cell proliferation rate in CMT2A fibroblasts.

Glaucocalyxin A as a natural product increases amyloid β clearance and decreases tau phosphorylation involving the mammalian target of rapamycin signaling pathway

Alzheimer's disease (AD) is a progressive neurodegenerative disorder correlated with age, characterized by the accumulation of amyloid β (Aβ) plaques and neurofibrillary tangles. The mammalian target of rapamycin (mTOR) is an important protein that regulates Aβ clearance and tau phosphorylation. Therefore, mTOR has become a pivotal therapeutic target for AD treatment. In this study, we discovered a natural product, glaucocalyxin A (GLA), as a new mTOR inhibitor based on a high-throughput screening platform with α-screen technology against our natural product library. Further study showed that GLA increased Aβ clearance involving the protein kinase B/mTOR/autophagy signaling pathway and inhibited tau phosphorylation involving the mTOR/70-kDa ribosomal protein S6 kinase pathway, which highlighted the therapeutic potential of GLA for the AD treatment.

Osteopontin counters human immunodeficiency virus type 1-induced impairment of neurite growth through mammalian target of rapamycin and beta-integrin signaling pathways

Despite the fact that human immunodeficiency virus type 1 (HIV-1) does not enter or replicate in neurons, its infection of a subset of resident brain glia cells (microglia and astrocytes) induces via disparate mechanisms, dysregulation of glutamate metabolism, neurotoxicity, and inflammation. Antiretroviral therapies suppress viral load, but cellular activation and release of proinflammatory factors, some of which is likely related to viral reservoirs, continue to promote a microenvironment that is injurious to neurons. However, the molecular mechanisms remain to be identified. Osteopontin (OPN) is a proinflammatory cytokine-like, extracellular matrix protein that is elevated within the brain and CSF in several neurodegenerative disorders, including HIV-associated cognitive disorder. However, the impact of elevated OPN on neuronal integrity and function in HIV-infected individuals who exhibit cognitive dysfunction remains unknown. In this study, using a neuronal cell line and primary cultures of cortical rat neurons, we identify the mammalian target of rapamycin pathway involvement in a signaling interaction between OPN-β1-integrins and the HIV-1 envelope glycoprotein, which stimulates neurite growth. These findings link for the first time HIV X4-envelope receptor engagement and osteopontin-mediated signaling through β1-integrin receptors to the mTOR pathway and alterations in the cytoskeleton of cortical neurons.

O-GlcNAc regulation of autophagy and α-synuclein homeostasis; implications for Parkinson's disease

Post-translational modification on protein Ser/Thr residues by O-linked attachment of ß-N-acetyl-glucosamine (O-GlcNAcylation) is a key mechanism integrating redox signaling, metabolism and stress responses. One of the most common neurodegenerative diseases that exhibit aberrant redox signaling, metabolism and stress response is Parkinson’s disease, suggesting a potential role for O-GlcNAcylation in its pathology. To determine whether abnormal O-GlcNAcylation occurs in Parkinson’s disease, we analyzed lysates from the postmortem temporal cortex of Parkinson’s disease patients and compared them to age matched controls and found increased protein O-GlcNAcylation levels. To determine whether increased O-GlcNAcylation affects neuronal function and survival, we exposed rat primary cortical neurons to thiamet G, a highly selective inhibitor of the enzyme which removes the O-GlcNAc modification from target proteins, O-GlcNAcase (OGA). We found that inhibition of OGA by thiamet G at nanomolar concentrations significantly increased protein O-GlcNAcylation, activated MTOR, decreased autophagic flux, and increased α-synuclein accumulation, while sparing proteasomal activities. Inhibition of MTOR by rapamycin decreased basal levels of protein O-GlcNAcylation, decreased AKT activation and partially reversed the effect of thiamet G on α-synuclein monomer accumulation. Taken together we have provided evidence that excessive O-GlcNAcylation is detrimental to neurons by inhibition of autophagy and by increasing α-synuclein accumulation.