Nicotinic acetylcholine receptor (nAChR) activation is neuroprotective and nicotine is a cognitive enhancer. Loss of nAChRs, deposition of tau neurofibrillary tangles, cleavage of amyloid precursor protein (APP) and inflammation are well documented in the pathogenesis of Alzheimer’s disease (AD). Sequential cleavage of APP by β- and γ-secretase enzymes generates soluble Aβ peptides, with oligomeric forms of Aβ implicated in both the control of synaptic excitability and dysregulation of synaptic transmission and induction of neuronal death in AD. Aβ production is inhibited by calcium-dependent recruitment of α-secretase, as exemplified by activation of N-methyl-D-aspartate receptors (NMDAR). All neurodegenerative diseases are associated with inflammation, arising from altered homeostasis of the innate immune system, resulting in heightened activation of immune cells and induction of a pro-inflammatory environment. Stimulation of the α7 subtype of nAChR is anti-inflammatory and also enhances cognition and promotes neuronal survival. This work addressed the hypotheses that stimulation of highly calcium-permeable α7nAChR inhibits Aβ production by promoting α-secretase-mediated processing of APP and also modulates inflammatory cellular behaviour of microglia. Thus, this study assessed the role of α7nAChR at glutamatergic synapses, through probing effects on APP processing and phagocytosis in primary cortical neurons and microglia, respectively. Primary cortical neurons expressed functional α7nAChR and glutamate receptors, and through a number of experimental approaches, including immunoblotting and a cleavage reporter assay, results indicated α7nAChR activation with the α7nAChR-selective agonist PNU-282987 and positive allosteric modulator PNU-120596 had no effect on APP and Tau, in contrast to NMDAR activation that significantly modulated these proteins. Data suggest low expression of α7nAChR, coupled with distinct localisation of presynaptic α7nAChR and postsynaptic APP could explain the lack of effect. In addition, primary microglia were highly responsive to lipopolysaccharide and possessed functional α7nAChR that coupled to ERK phosphorylation. Microglial α7nAChR activation promoted neuroprotective phagocytic behaviour, in agreement with the ‘cholinergic anti-inflammatory pathway’. This study supports the hypothesis that α7nAChR are modulators of anti-inflammatory behaviour, thus α7nAChR-selective ligands are viable candidates for the treatment of AD and promoting cognitive enhancement.
|Date of Award||4 Nov 2014|
|Supervisor||Robert Williams (Supervisor) & Sue Wonnacott (Supervisor)|
- nicotinic acetylcholine receptors
- amyloid precursor protein