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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. [ABSTRACT FROM AUTHOR]

Abstract 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. Highlights • M1 mAChRs facilitate the acquisition of reversal learning in the Morris water maze. • M1 mAChRs induce LTD and GluA2 endocytosis dependent on PKC in the hippocampus. • The effect of M1 mAChR can be abolished by GluA2 K882A mutation and PKC inhibitor. • Facilitated reversal learning is not associated with memory extinction. • The study prompts targeting M1 mAChRs to prevent cognitive inflexibility. [ABSTRACT FROM AUTHOR]