p85S6K sustains synaptic GluA1 to ameliorate cognitive deficits in Alzheimer's disease.

BACKGROUND: Ribosomal protein S6 kinase 1 (S6K1) is a serine-threonine kinase that has two main isoforms: p70S6K (70-kDa isoform) and p85S6K (85-kDa isoform). p70S6K, with its upstream mammalian target of rapamycin (mTOR), has been shown to be involved in learning and memory and participate in the pathophysiology of Alzheimer's disease (AD). However, the function of p85S6K has long been neglected due to its high similarity to p70S6k. The role of p85S6K in learning and memory is still largely unknown. METHODS: We fractionated the postsynaptic densities to illustrate the differential distribution of p85S6K and p70S6K. Coimmunoprecipitation was performed to unveil interactions between p85S6K and the GluA1 subunit of AMPA receptor. The roles of p85S6K in synaptic targeting of GluA1 and learning and memory were evaluated by specific knockdown or overexpression of p85S6K followed by a broad range of methodologies including immunofluorescence, Western blot, in situ proximity ligation assay, morphological staining and behavioral examination. Further, the expression level of p85S6K was measured in brains from AD patients and AD model mice. RESULTS: p85S6K, but not p70S6K, was enriched in the postsynaptic densities. Moreover, knockdown of p85S6K resulted in defective spatial and recognition memory. In addition, p85S6K could interact with the GluA1 subunit of AMPA receptor through synapse-associated protein 97 and A-kinase anchoring protein 79/150. Mechanistic studies demonstrated that p85S6K could directly phosphorylate GluA1 at Ser845 and increase the amount of GluA1 in synapses, thus sustaining synaptic function and spine densities. Moreover, p85S6K was found to be specifically decreased in the synaptosomal compartment in the brains of AD patients and AD mice. Overexpression of p85S6K ameliorated the synaptic deficits and cognitive impairment in transgenic AD model mice. CONCLUSIONS: These results strongly imply a significant role for p85S6K in maintaining synaptic and cognitive function by interacting with GluA1. The findings provide an insight into the rational targeting of p85S6K as a therapeutic potential for AD.

PKC and Ras are Involved in M1 Muscarinic Receptor-Mediated Modulation of AMPA Receptor GluA1 Subunit.

M1 muscarinic acetylcholine receptors (M1 mAChRs) have long been an attractive target for the treatment of Alzheimer's disease (AD), the most common cause of dementia in the elderly. M1 mAChR agonists show desirably preclinical activities; however, most have not gone further into late clinical trials due to ineffectiveness or side effects. Thus, to understand the signaling pathways involved in M1 mAChR-mediated memory improvement may be important for design of biased agonists with on-target therapeutic effects. M1 mAChRs are classically coupled to Gαq or ectopically to Gαs to activate multiple kinases such as protein kinase C (PKC), Ras and protein kinase A (PKA). Our previous studies have found that M1 mAChRs could improve learning and memory through modulating AMPA receptor GluA1 subunit via PKA-PI3K-Akt signaling. Here, we further investigated whether PKC and Ras were involved in M1 mAChR-mediated modulation of GluA1. We demonstrated the role of PKC and Ras in the signaling pathway, as both PKC inhibitors Ro-31-8425 or Gö6983 and Ras inhibitor salirasib abolished the membrane insertion of GluA1 and enhancement of its phosphorylation at Ser845 induced by M1 mAChRs in the primary cultured neurons and hippocampus in vivo. We further showed that PKC and Ras modulated PKA-PI3K-Akt signaling since the increases of PKA, Akt and mTOR activities by M1 mAChR activation were blocked by PKC and Ras inhibitors. These data demonstrated the detailed mechanism underlying M1 mAChR-mediated modulation of GluA1 through Gαq/11 coupling, broadening the knowledge of the downstream signaling after M1 mAChR-Gαq/11 coupling.

M1 muscarinic receptors regulate the phosphorylation of AMPA receptor subunit GluA1 via a signaling pathway linking cAMP-PKA and PI3K-Akt.

M1 muscarinic acetylcholine receptors are highly expressed in key areas that control cognition, such as the cortex and hippocampus, representing one potential therapeutic target for cognitive dysfunctions of Alzheimer's disease and schizophrenia. We have reported that M1 receptors facilitate cognition by promoting membrane insertion of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor AMPA receptor subunit 1 (GluA1) through phosphorylation at Ser845. However, the signaling pathway is still unclear. Here we showed that adenylyl cyclase inhibitor 2',5'-dideoxyadenosine and PKA inhibitor KT5720 inhibited enhancement of phosphorylation of Ser845 and membrane insertion of GluA1 induced by M1 receptor activation. Furthermore, PI3K inhibitor LY294002 and protein kinase B (Akt) inhibitor IV blocked the effects of M1 receptors as well. Remarkably, the increase of the activity of PI3K-Akt signaling induced by M1 receptor activation could be abolished by cAMP-PKA inhibitors. Moreover, inhibiting the mammalian target of rapamycin (mTOR) complex 1, an important downstream effector of PI3K-Akt, by short-term application of rapamycin attenuated the effects of M1 receptors on GluA1. Furthermore, such effect was unrelated to possible protein synthesis promoted by mTOR. Taken together, these data demonstrate that M1 receptor activation induces membrane insertion of GluA1 via a signaling linking cAMP-PKA and PI3K-Akt-mTOR pathways but is irrelevant to protein synthesis.-Zhao, L.-X., Ge, Y.-H., Li, J.-B., Xiong, C.-H., Law, P.-Y., Xu, J.-R., Qiu, Y., Chen, H.-Z. M1 muscarinic receptors regulate the phosphorylation of AMPA receptor subunit GluA1 via a signaling pathway linking cAMP-PKA and PI3K-Akt.

M1 muscarinic receptors facilitate hippocampus-dependent cognitive flexibility via modulating GluA2 subunit of AMPA receptors.

Cognitive flexibility is an important aspect of executive function. The cholinergic system, an important component of cognition, has been shown to modulate cognitive flexibility mainly through the striatum and prefrontal cortex. The role of M1 muscarinic receptors (M1 mAChRs), an important therapeutic target in the cholinergic system, in hippocampus-dependent cognitive flexibility is unclarified. In the present study, we demonstrated that selective activation of M1 mAChRs promoted extinction of initial learned response and facilitated acquisition of reversal learning in the Morris water maze, a behavior test that is mainly dependent on the hippocampus. However, these effects were abolished in GluA2 mutant mice with deficiency in phosphorylation of Ser880 by protein kinase C (PKC). Further long-term depression (LTD) in the hippocampal CA1 area induced by M1 mAChR activation was shown to be dependent on AMPA receptor subunit GluA2 but not GluA1. M1 mAChRs increased GluA2 endocytosis through phosphorylation of Ser880 by PKC. Inhibition of PKC blocked M1 mAChR-mediated LTD, memory switching and reversal learning facilitation. Moreover, the slow memory extinction observed in GluA2 mutant mice and PKC inhibitor-treated mice appeared to affect the consolidation and retrieval of reversal learning. Thus, these results demonstrate that M1 mAChRs mainly facilitate acquisition in spatial reversal learning and further elucidate that such an effect is dependent on the phosphorylation of GluA2 by PKC. The study helps clarify the role of M1 mAChRs in cognitive flexibility and may prompt the earlier prevention of cognitive inflexibility.

M1 muscarinic receptor facilitates cognitive function by interplay with AMPA receptor GluA1 subunit.

M1 muscarinic acetylcholine receptors (M1 mAChRs) are the most abundant muscarinic receptors in the hippocampus and have been shown to have procognitive effects. AMPA receptors (AMPARs), an important subtype of ionotropic glutamate receptors, are key components in neurocognitive networks. However, the role of AMPARs in procognitive effects of M1 mAChRs and how M1 mAChRs affect the function of AMPARs remain poorly understood. Here, we found that basal expression of GluA1, a subunit of AMPARs, and its phosphorylation at Ser845 were maintained by M1 mAChR activity. Activation of M1 mAChRs promoted membrane insertion of GluA1, especially to postsynaptic densities. Impairment of hippocampus-dependent learning and memory by antagonism of M1 mAChRs paralleled the reduction of GluA1 expression, and improvement of learning and memory by activation of M1 mAChRs was accompanied by the synaptic insertion of GluA1 and its increased phosphorylation at Ser845. Furthermore, abrogation of phosphorylation of Ser845 residue of GluA1 ablated M1 mAChR-mediated improvement of learning and memory. Taken together, these results show a functional correlation of M1 mAChRs and GluA1 and the essential role of GluA1 in M1 mAChR-mediated cognitive improvement.-Zhao, L.-X., Ge, Y.-H., Xiong, C.-H., Tang, L., Yan, Y.-H., Law, P.-Y., Qiu, Y., Chen, H.-Z. M1 muscarinic receptor facilitates cognitive function by interplay with AMPA receptor GluA1 subunit.