Novel contribution of cell surface and intracellular M1‐muscarinic acetylcholine receptors to synaptic plasticity in hippocampus

Abstract

Muscarinic acetylcholine receptors (mAChRs) are well known to transmit extracellular cholinergic signals into the cytoplasm from their position on the cell surface. However, we show here that M1-mAChRs are also highly expressed on intracellular membranes in neurons of the telencephalon and activate signaling cascades distinct from those of cell surface receptors, contributing uniquely to synaptic plasticity. Radioligandbinding experiments with cell-permeable and -impermeable ligands and immunohistochemical observations revealed intracellular and surface distributions of M1-mAChRs in the hippocampus and cortex of rats, mice, and humans, in contrast to the selective occurrence on the cell surface in other tissues. All intracellular muscarinic-binding sites were abolished in M1-mAChR-gene-knockout mice. Activation of cell surface M1-mAChRs in rat hippocampal neurons evoked phosphatidylinositol hydrolysis and network oscillations at theta rhythm, and transiently enhanced long-term potentiation. On the other hand, activation of intracellular M1-mAChRs phosphorylated extracellular-regulated kinase 1/2 and gradually enhanced long-term potentiation. Our data thus demonstrate that M1-mAChRs function at both surface and intracellular sites in telencephalon neurons including the hippocampus, suggesting a new mode of cholinergic transmission in the central nervous system. Keywords: cell surface and intracellular GPCR, ERK1/2, LTP, M1-muscarinic receptor, synaptic plasticity. Among the five muscarinic acetylcholine receptor (mAChR) subtypes, M1-mAChRs predominantly exist in the CNS and are involved in cognitive enhancement (Wess 2004;

Muscarinic acetylcholine receptors act in synergy to facilitate learning and memory

“…This is supported by work showing that both M 3 knockout mice and mice with M 3 phosphorylation deficiency have deficits in contextual fear conditioning (Poulin et al 2010). Interestingly, whereas M 1 rather than M 3 is the predominant mediator of muscarinic potentiation of hippocampal LTP (Anagnostaras et al 2003;Shinoe et al 2005;Anisuzzaman et al 2013;Dennis et al 2016), a putative physical substrate for learning, M 3 modulates the inhibition of excitatory synaptic transmission in CA1 (de Vin et al 2015). One route by which M 3 in DH could support learning is by increasing the excitability and intrinsic oscillatory activity of CCK+ interneurons.…”

Intracellular localization of M1 muscarinic acetylcholine receptor through clathrin-dependent constitutive internalization via a C-terminal tryptophan-based motif

“…Such intracellular localization has also been found for endogenous M1-mAChR in N1E-115 cells (Uwada et al, 2011), and in the cortex and hippocampal neurons of rats, mice and humans (Anisuzzaman et al, 2013;Yamasaki et al, 2010). More recently, we observed constitutive internalization of M1-mAChR that was dependent on a WxxW motif in exogenously transfected HEK293 and HeLa cells (our unpublished observations).…”

“…However, the density of endogenous mAChRs in N1E-115 cells is extremely low in contrast to the brain (,100 versus 3000 fmol/mg protein) (Anisuzzaman et al, 2013;Uwada et al, 2011). Therefore, we transfected exogenous mAChRs in order to more readily analyze receptor dynamics.…”

“…Trafficking of mAChRs is regulated in a subtypeand/or cell-type-specific manner (Nathanson, 2008;Reiner and Nathanson, 2012). Interestingly, recent immunohistochemical and pharmacological studies have shown stable intracellular localization of M1-mAChR, especially in neuronal cells (Anisuzzaman et al, 2013;Wang et al, 1994;Yamasaki et al, 2010). In contrast, M2 and M4 subtypes exist predominantly on the cell surface (Liste et al, 2002).…”

“…Thus, it has been suggested that there is an M1-subtype-specific mechanism for its intracellular localization. Additionally, we recently reported that intracellular M1-mAChRs are physiologically functional receptors whose activation leads to upregulation of mitogen-activated protein kinase (MAPK) activity and contributes to regulation of synaptic plasticity in hippocampal neurons (Anisuzzaman et al, 2013;Uwada et al, 2011). Therefore, the intracellular localization of M1-mAChR is important for not only the downregulation of signals from cell surface receptors, but also for the upregulation of functional intracellular receptors.…”