Abstract
α7 nicotinic acetylcholine receptors (nAChRs) and N-methyl-D-aspartate receptors (NMDARs) are key mediators of central cholinergic and glutamatergic neurotransmission, respectively. In addition to numerous well-established functional interactions between α7 nAChRs and NMDARs, the two receptors have been proposed to form a multimeric complex, and in the present study we have investigated this putative α7 nAChR/NMDAR assembly in human and murine brain tissues. By α-bungarotoxin (BGT) affinity purification, α7 and NMDAR subunits were co-purified from human and murine cortical and hippocampal homogenates, substantiating the notion that the receptors are parts of a multimeric complex in the human and rodent brain. Interestingly, the ratios between GluN1 and α7 levels in BGT pull-downs from cortical homogenates from Alzheimer’s disease (AD) brains were significantly lower than those in pull-downs from non-AD controls, indicating a reduced degree of α7 nAChR/NMDAR complex formation in the diseased tissue. A similar difference in GluN1/α7 ratios was observed between pull-downs from cortical homogenates from adult 3xTg-AD and age-matched wild type (WT) mice, whereas the GluN1/α7 ratios determined in pull-downs from young 3xTg-AD and age-matched WT mice did not differ significantly. The observation that pretreatment with oligomeric amyloid-β1–42 reduced GluN1/α7 ratios in BGT pull-downs from human cortical homogenate in a concentration-dependent manner provided a plausible molecular mechanism for this observed reduction. In conclusion, while it will be important to further challenge the existence of the putative α7 nAChR/NMDAR complex in future studies applying other methodologies than biochemical assays and to investigate the functional implications of this complex for cholinergic and glutamatergic neurotransmission, this work supports the formation of the complex and presents new insights into its regulation in healthy and diseased brain tissue.
Highlights
Glutamate (Glu) and acetylcholine (ACh) are major neurotransmitters in the central nervous system (CNS), where both are directly involved in or regulate a wide spectrum of physiological processes
We initially set out to challenge these observations and investigated the putative formation of α7 nicotinic ACh receptors (nAChRs)/N-methyl-D-aspartate receptors (NMDARs) complexes in both murine and human brain tissues
We found the GluN2A to be co-purified with α7 in the Pull Down samples (Fig 1D and 1E), but we α7 nAChR/NMDAR complex formation in healthy and Alzheimer brain tissue consistently obtained more robust signals using the antibody for GluN1 than that for GluN2A
Summary
Glutamate (Glu) and acetylcholine (ACh) are major neurotransmitters in the central nervous system (CNS), where both are directly involved in or regulate a wide spectrum of physiological processes. Dysfunctions in glutamatergic and cholinergic neurotransmission have been implicated in numerous pathological states, and modulation of glutamatergic and/or cholinergic mechanisms holds considerable therapeutic potential when it comes to numerous cognitive, psychiatric and neurodegenerative disorders [1,2,3,4]. Both Glu and ACh mediate their physiological effects through highly heterogeneous families of G-protein coupled receptors (GPCRs) and ligand-gated cation channels (LGICs). The fast cholinergic signalling in the CNS is mediated by the neuronal nicotinic ACh receptors (nAChRs) that are homo- and heteropentameric complexes assembled from α2-α10 and β2-β4 subunits, with the homomeric α7 nAChR being one of the two major physiological neuronal nAChR subtypes [3, 4]
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