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Abstract
Memory and long term potentiation require de novo protein synthesis. A key regulator of this process is mTORC1, a complex comprising the mTOR kinase. Growth factors activate mTORC1 via a pathway involving PI3-kinase, Akt, the TSC complex and the GTPase Rheb. In non-neuronal cells, translocation of mTORC1 to late endocytic compartments (LEs), where Rheb is enriched, is triggered by amino acids. However, the regulation of mTORC1 in neurons remains unclear. In mouse hippocampal neurons, we observed that BDNF and treatments activating NMDA receptors trigger a robust increase in mTORC1 activity. NMDA receptors activation induced a significant recruitment of mTOR onto lysosomes even in the absence of external amino acids, whereas mTORC1 was evenly distributed in neurons under resting conditions. NMDA receptor-induced mTOR translocation to LEs was partly dependent on the BDNF receptor TrkB, suggesting that BDNF contributes to the effect of NMDA receptors on mTORC1 translocation. In addition, the combination of Rheb overexpression and artificial mTORC1 targeting to LEs by means of a modified component of mTORC1 fused with a LE-targeting motif strongly activated mTOR. To gain spatial and temporal control over mTOR localization, we designed an optogenetic module based on light-sensitive dimerizers able to recruit mTOR on LEs. In cells expressing this optogenetic tool, mTOR was translocated to LEs upon photoactivation. In the absence of growth factor, this was not sufficient to activate mTORC1. In contrast, mTORC1 was potently activated by a combination of BDNF and photoactivation. The data demonstrate that two important triggers of synaptic plasticity, BDNF and NMDA receptors, synergistically power the two arms of the mTORC1 activation mechanism, i.e., mTORC1 translocation to LEs and Rheb activation. Moreover, they unmask a functional link between NMDA receptors and mTORC1 that could underlie the changes in the synaptic proteome associated with long-lasting changes in synaptic strength.
Highlights
Long-lasting changes in synaptic strength, in particular those associated with the late phase of long term potentiation (L-LTP), are thought to represent the cellular substrate of memory encoding [1, 2]
NMDA receptors and brain-derived neurotrophic factor (BDNF) trigger mechanistic target of rapamycin complex 1 (mTORC1) activation in hippocampal neurons To assess the effects of different neuronal stimuli on mTORC1 activity in hippocampal cultures, we first performed western blot analyses using the phosphorylation of ribosomal S6 protein on serines 240 and 244 as a specific readout for mTORC1 activity [69], as protein S6 (pS6) is targeted by p70S6 kinase (p70S6K), a major effector of mTORC1
This treatment markedly increased the quantity of phosphorylated pS6 (P-pS6) (Additional file 1: Figure S1B) and was suppressed by the mTOR inhibitor rapamycin (200 nM), indicating an effect on mTORC1 activity
Summary
Long-lasting changes in synaptic strength, in particular those associated with the late phase of long term potentiation (L-LTP), are thought to represent the cellular substrate of memory encoding [1, 2]. Many studies have shown that de novo protein synthesis is essential to the formation of different form of neuronal plasticity including LTP [2,3,4,5]. The whole machinery needed for mRNA translation is present in these compartments and local protein synthesis may be elicited by synaptic inputs [5, 11]. A major pathway that controls this balance is the mechanistic target of rapamycin complex 1 (mTORC1) [14]. This ubiquitous complex, possessing a serine/threonine kinase activity, is a master controller of several anabolic and catabolic processes, that integrates many environmental and intracellular inputs such as nutrients, energy levels and growth factors. Despite the remarkable progress made recently on understanding mTOR signaling in health and diseases in specific tissues, little is known about the regulation of mTORC1 signaling in brain [14, 23]
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