Ribosomal Protein S6 Kinase Is a Critical Downstream Effector of the Target of Rapamycin Complex 1 for Long-term Facilitation in Aplysia

Abstract

Long-term facilitation (LTF) inAplysia is a leading cellular model for elucidating the biochemical mechanisms of synaptic plasticity underlying learning. In Aplysia, LTF requires translational control downstream of the target of rapamycin (TOR) complex 1 (TORC1). The major known downstream targets of TORC1 are 4E binding protein (4E-BP) and S6 kinase (S6K). By removing the site within these regulators required for their interaction with TORC1, we have generated dominant negative proteins that disrupt specific pathways downstream of TORC1. Expression of dominant negative S6K, but not dominant negative 4E-BP, in Aplysia sensory neurons (SNs) blocked 24-h LTF. TORC1 is directly activated by the small GTP-binding protein, Ras homologue enriched in brain (Rheb). To determine the effects of TORC1 activation on translation in Aplysia neurons, we have examined the effects of expressing a constitutively active form of the Aplysia orthologue of Rheb, ApRheb (ApRheb(Q63L)). Expression of ApRheb(Q63L) increased 4E-BP phosphorylation and the level of general, cap-dependent translation within the SN cell soma in a rapamycin-sensitive manner. This increase in cap-dependent translation was blocked neither by dominant negative 4E-BP nor dominant negative S6K. Thus, we demonstrate that S6K is an important downstream target of TORC1 in Aplysia and that it is necessary for 24-h LTF, but not for TORC1-mediated increases in somatic cap-dependent translation.Memories, changes in synaptic strength within a neural network, are composed of mechanistically distinct phases. Whereas short-term memories rely on post-translational modification of pre-existing synaptic proteins, more persistent memories require the production of new proteins and likely involve synaptic growth (1). This de novo protein synthesis within neurons, underlying long-term synaptic plasticity, involves, not only transcriptional regulation, but translational regulation as well (2, 3). Moreover, the protein kinase complex, target of rapamycin (TOR) 3 complex 1 (TORC1), a major regulator of translation and growth in eukaryotic cells (4), has been shown to play an essential role in this process (5). In many cell types, TORC1 is activated when conditions are permissive for cell growth through the integration of signaling pathways that sense the presence of these permissive cues (growth factors, amino acids, and energy (ATP)) (4). In neurons, TORC1 is activated during both in vitro models of synaptic plasticity and in vivo models of memory formation and, similar to other cells, acts as a gatekeeper regulating neuronal growth and plasticity (6).The facilitation of neurotransmitter release at the sensoryto-motor neuron (SN-MN) synapse, in the mollusk, Aplysia californica, is a leading model system for the characterization of the biochemical basis of memory formation, because an increase in the strength of this synapse has been shown to contribute to behavioral sensitization of the reflex (7). In particular, whereas short-term facilitation (STF) is independent of protein synthe...

Translational Control Mechanisms in Long-lasting Synaptic Plasticity and Memory

“…In contrast, in Aplysia neurons, S6K, but not 4E-BP, appears to be the major effector of mTORC1 for long-term facilitation (44). In mammals, S6K appears to be involved mainly in the regulation of long-lasting decreases in synaptic strength because metabotropic glutamate receptor-induced LTD is enhanced in slices from S6K2 Ϫ/Ϫ mice (but not in those from S6K1 Ϫ/Ϫ mice) (42).…”

BDNF-induced local protein synthesis and synaptic plasticity

“…Differential mechanisms of regulation of translation activity were also reported in Aplysia sensory neurons, where the target of rapamycin complex 1 (TORC1)-mediated phosphorylation of S6 kinase contributes to long-term facilitation of sensory-motor neuron synapses, which requires protein synthesis. In contrast, S6 kinase does not play a role in TORC1-mediated increase in somatic cap-dependent translation in cultured Aplysia sensory neurons (Weatherill et al, 2010).…”

A recollection of mTOR signaling in learning and memory

“…This seems exceedingly unlikely given the current data. Rapamycin only blocks a small component of overall protein synthesis (Choo et al 2008), including in the nervous system (Yanow et al 1998) Cold Spring Harbor Laboratory Press on April 4, 2019 -Published by learnmem.cshlp.org Downloaded from dependent increase in overall translation has been dissociated from the requirement of mTORC1 for plasticity (Weatherill et al 2010). In most cases, however, the protein synthesis under control of mTORC1 is not sufficient to induce plasticity.…”