Insulin Stimulates Postsynaptic Density-95 Protein Translation via the Phosphoinositide 3-Kinase-Akt-Mammalian Target of Rapamycin Signaling Pathway

Abstract

Insulin receptors are highly enriched at neuronal synapses, but whose function remains unclear. Here we present evidence that brief incubations of rat hippocampal slices with insulin resulted in an increased protein expression of dendritic scaffolding protein postsynaptic density-95 (PSD-95) in area CA1. This insulin-induced increase in the PSD-95 protein expression was inhibited by the tyrosine kinase inhibitor, AG1024, phosphatidylinositol 3-kinase (PI3K) inhibitors, LY294002 and wortmannin, translational inhibitors, anisomycin and rapamycin, but not by LY303511 (an inactive analogue of LY294002), and transcriptional inhibitor, actinomycin D, suggesting that insulin regulates the translation of PSD-95 by activating the receptor tyrosine kinase-PI3K-mammalian target of rapamycin (mTOR) signaling pathway. A similar insulin-induced increase in the PSD-95 protein expression was detected after stimulation of the synaptic fractions isolated from the hippocampal neurons. Furthermore, insulin treatment did not affect the PSD-95 mRNA levels. In agreement, insulin rapidly induced the phosphorylation of 3-phosphoinositidedependent protein kinase-1 (PDK1), protein kinase B (Akt), and mTOR, effects that were prevented by the AG1024 and LY294002. We also show that insulin stimulated the phosphorylation of 4E-binding protein 1 (4E-BP1) and p70S6 kinase (p70S6K) in a mTOR-dependent manner. Finally, we demonstrate the constitutive expression of PSD-95 mRNA in the synaptic fractions isolated from hippocampal neurons. Taken together, these findings suggest that activation of the PI3K-Akt-mTOR signaling pathway is essential for the insulin-induced up-regulation of local PSD-95 protein synthesis in neuronal dendrites and indicate a new molecular mechanism that may contribute to the modulation of synaptic function by insulin in hippocampal area CA1.Insulin and its receptor are widely dispersed throughout the brain with the highest density located in the olfactory bulb, cerebral cortex, hypothalamus, and hippocampus, where they are thought to subserve a number of functions including regulation of glucose metabolism, food intake and body weight, fertility and reproduction, learning, memory, and attention (1-4). Brain insulin receptors are present in particularly high concentrations in neurons, and in much lower levels in glia (5). Although the mRNA of insulin receptors is largely localized in neuronal somata, abundant insulin receptors are found in both cell bodies and synapses (5-7). However, very little is known about the functional significance of synaptic insulin receptors in the neurons. Recently, several studies have drawn links between insulin signaling and intracellular trafficking and plasma membrane expression of ion channels and neurotransmitter receptors at the central nervous system synapses. For example, it has been shown that insulin rapidly recruits functional GABA A receptors to postsynaptic domains in hippocampal neurons, resulting in a long-lasting enhancement of GABA A receptor-mediated synaptic transmission (8). ...

Mechanism for long-term memory formation when synaptic strengthening is impaired

“…PSD95 protein expression is regulated by mammalian target of rapamycin (mTOR) and is sensitive to inhibitors of protein synthesis, but not block of transcription (22,23). Thus, if training-induced PSD95 up-regulation were important for LTM formation in T286A mutants, then blockers of protein synthesis and mTOR signaling should impair LTM formation, whereas blockers of mRNA synthesis would have no effect.…”

“…Psd95 mRNA is dendritically localized and locally translated via activation of the mTOR pathway (22,23,30,31). In hippocampal slice cultures, overexpression of PSD95 induces MIS formation (21).…”

Role for mammalian target of rapamycin complex 1 signaling in neuroadaptations underlying alcohol-related disorders

“…Together, these findings raise the possibility that the mTORC1-mediated translation of proteins such as GluR1 and Homer that regulate synaptic functions contribute to the molecular mechanisms that underlie the development, maintenance, and/or expression of excessive alcohol consumption. Interestingly, the level of several other synaptic proteins, including the scaffolding protein postsynaptic density protein 95 (PSD-95) and the activity-regulated cytoskeleton-associated protein (Arc), have been linked to the mTORC1 pathway (36,37). The possibility that the activation of the mTORC1 pathway in response to alcohol exposure results in the translation of these and other synaptic proteins is intriguing and merits further investigation.…”

Ceramide inhibition of chondrocyte proliferation and bone growth is IGF-I independent

“…In further studies, the cells were treated with C2-ceramide (25 mM) in the presence of IGF-I or AG1024, an IGF-I and insulin receptor blocker, (10 mM; Sigma) for 24 h (Lee et al 2005, Sutter et al 2006. The diluent for AG1024 was dimethylsulfoxide (final concentration 0 .…”