Regulatory mechanisms and plasticity of nicotinic receptors in transgenic mice overexpressing human beta-amyloid (APPswe) and acetylcholinesterase : implications for Alzheimer’s disease

Abstract

Alzheimer’s disease (AD) is the most frequent type of dementia and is characterized neuropathologically by deposition of plaques containing beta-amyloid (Abeta) and by neurofibrillary tangles, of which the main component is hyperphosphorylated tau. The degenerative progression seen in AD is accompanied by disturbances in several neurotransmitter systems, especially the cholinergic neurotransmitter system.

Three different transgenic mouse models have been used in this thesis: APPswe transgenic mice harbouring the 670/671 APP Swedish mutation (APPswe), transgenic mice that overexpress human acetylcholinesterase in the brain (hAChE-Tg) and double transgenic mice generated by crossing the two mouse strains (hAChE-Tg//APPswe). The overall aim of these studies was to investigate how pathological processes and disruption of a balanced cholinergic neurotransmission, as created in these transgenic mouse models, influence the plasticity and regulation of the neuronal nicotinic acetylcholine receptors (nAChRs).

Immunostaining of Abeta in the brain of APPswe showed Abeta deposition from 9 months of age. Similar investigation in the brain of hAChE-Tg//APPswe showed deposition of Abeta142 at 7 months of age and deposition consisting of Abeta1-40 and Abeta1-42 at 10 months of age. The nAChR subtypes, measured by receptor binding techniques, and levels of mRNA encoding nicotinic receptor subunits were quantified. A persistent increase of the alpha4 nAChR subtype initiated at transcription level and evident at post-transcription level, was found in the cortex and striatum of hAChE-Tg mice, from early postnatal to adult age, compared to control mice. The increase in the alpha4 nAChR in the hAChE-Tg mice most likely reflects a compensatory action from the excess of AChE that causes lack of neurotransmitter in the synaptic cleft and thereby increases the alpha4 nAChR subtypes both at mRNA and binding site level. No major differences in the alpha4 nAChR subtypes were observed in the APPswe or hAChETg//APPswe transgenic mice in comparison to non-transgenic mice.

The alpha7 nAChR subtypes were significantly increased in most brain regions in the APPswe mice at 4 months and the increase was still detectable at 17-19 months of age compared to control mice. Similarly, an increase in alpha7 nAChRs, initiated at mRNA level at 3 and 7 months and evident at binding site level at 7 months of age, was found in the cortex of hAChETg//APPswe transgenic mice compared to controls. The increase in the alpha7 nAChR subtypes seen in these transgenic mice probably reflect a compensatory mechanisms in response to Abeta burden. The increase in alpha7 nAChRs might also reflect the noncholinergic distribution of the alpha7 nAChRs. No major changes in alpha7 nAChR binding sites were detected in the hAChE-Tg mice.

This thesis also investigated changes in nAChR and muscarinic acetylcholine receptor (mAChR) subtypes in the brain of hAChE-Tg and control mice following treatment with the AChE inhibitor (AChEl) galantamine. In addition to its effect of inhibiting AChE, galantamine is proposed to act as an allosteric ligand, at the nicotinic receptor. No significant change in nAChR binding was observed in the galantamine treated hAChE-Tg compared to saline treated hAChE-Tg mice. The results showed that the alpha4 and the alpha7 nAChR subtypes were upregulated following treatment with galantamine in the hippocampus (alpha4), cortex (alpha7) and thalamus (alpha7) in the control mice. The M1 and M2 mAChRs were found to be nonselectively decreased in the brain following galantamine treatment in both the hAChETg and control mice. The lack of further increase in the nAChR binding sites in the hAChE-Tg mice following galantamine treatment may be a due to the already extensively elevated number of nicotinic receptors in these mice.