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Rictor associates with mTOR to form the mTORC2 complex, which activity regulates neuronal function and survival. Neurodegenerative diseases are characterized by the presence of neuronal dysfunction and cell death in specific brain regions such as for example Huntington’s disease (HD), which is characterized by the loss of striatal projection neurons leading to motor dysfunction. Although HD is caused by the expression of mutant huntingtin, cell death occurs gradually suggesting that neurons have the capability to activate compensatory mechanisms to deal with neuronal dysfunction and later cell death. Here, we analyzed whether mTORC2 activity could be altered by the presence of mutant huntingtin. We observed that Rictor levels are specifically increased in the striatum of HD mouse models and in the putamen of HD patients. Rictor-mTOR interaction and the phosphorylation levels of Akt, one of the targets of the mTORC2 complex, were increased in the striatum of the R6/1 mouse model of HD suggesting increased mTORC2 signaling. Interestingly, acute downregulation of Rictor in striatal cells in vitro reduced mTORC2 activity, as shown by reduced levels of phospho-Akt, and increased mutant huntingtin-induced cell death. Accordingly, overexpression of Rictor increased mTORC2 activity counteracting cell death. Furthermore, normalization of endogenous Rictor levels in the striatum of R6/1 mouse worsened motor symptoms suggesting an induction of neuronal dysfunction. In conclusion, our results suggest that increased Rictor striatal levels could counteract neuronal dysfunction induced by mutant huntingtin. [ABSTRACT FROM AUTHOR]

It has recently been reported that the phosphoinositide 3-kinase (PI3K)/Akt/mammalian target of rapamycin (mTOR) signaling pathway regulates neuronal differentiation of neural stem cells (NSCs) derived from rats or mice and is essential for the self-renewal of human embryonic stem cells (hESCs). However, the roles of PI3K/Akt/mTOR signaling pathways during proliferation and dopaminergic neuronal differentiation of human neural stem cells (hNSCs) are poorly understood. In this study, we examined the effect of regulation of these intracellular signaling pathways in hNSCs on the potential to maintain proliferation and induce dopaminergic neuronal differentiation. Dopaminergic neuronal differentiation depended on the concentration of insulin in our culture system. Inhibition of PI3K/Akt with LY294002 reduced proliferation and inhibited dopaminergic neuronal differentiation of these cells. We also found that rapamycin, a specific inhibitor of mTOR, significantly reduced neuronal differentiation without affecting proliferation. Inhibition of the Akt/mTOR signaling pathway led to inhibition of p70 ribosomal S6 kinase (S6K) signaling, which reduced dopaminergic neuronal differentiation in hNSCs. Inhibition of S6K by a specific chemical inhibitor, PF-4708671 inhibited dopaminergic neuronal differentiation of hNSCs. As expected, transduction with a dominant negative S6K1 (S6K1-DN) construct impaired dopaminergic neuronal differentiation of hNSCs. Conversely, overexpression of constitutively active S6K1 (S6K1-CA) promoted dopaminergic neuronal differentiation of these cells. In a survival study, 4 weeks after transplantation, no or very few donor cells were viable in striata grafted with S6K1-DN-transduced hNSCs. In contrast, S6K1-CA-transduced hNSCs survived, integrated into striata to generate tubular masses of grafts and differentiated toward TH-positive cells. Taken together, these data demonstrated that insulin promotes dopaminergic neuronal differentiation through a PI3K/Akt/mTOR-dependent pathway and that S6K plays a critical role in dopaminergic neuronal differentiation in hNSCs. [ABSTRACT FROM AUTHOR]