Division of Alzheimer Neurobiology Department of Neurobiology, Care Sciences and Society Karolinska Institutet, Stockholm, Sweden β-Amyloid Processing and α7 Neuronal Nicotinic Acetylcholine Receptors in Alzheimer’s Disease-related Mouse Models - Interactive Mechanisms with Focus on New Drug Targets

Accumulation of beta-amyloid (Aβ), neuronal loss and changes in neurotransmitter systems, in particular the cholinergic system, are consistent features of Alzheimer’s disease (AD). Aβ is thought to play a critical role in the pathogenesis of AD and it has therefore become a target of interest as regards a therapeutic approach. The aim of this work was to investigate how different Aβ peptides influence neurotransmission in the brains of AD-related mouse models, as well as to evaluate how drugs, acting via the cholinergic and glutamatergic neurotransmitter systems, affect these processes. Three different transgenic mouse models were utilized: APPswe, hAChETg//APPswe and 3xTg-AD mice, which revealed diverse brain Aβ pathologies. The hAChE-Tg//APPswe mice exhibited accelerated plaque pathology and showed, even at early ages, increased levels of insoluble Aβ in their brains compared with agematched APPswe mice, whereas the latter expressed increased soluble Aβ. The 3xTgAD mice showed no extracellular Aβ plaques, while intraneuronal APP/Aβ immunostaining was evident in the cortex and the hippocampus. Whereas APPswe mice showed a more pronounced Aβ pathology, with high Aβ levels both in the cortex and the hippocampus, the 3xTg-AD mice showed detectable Aβ peptides solely in the hippocampus. A biphasic effect was found on cortical α7 neuronal nicotinic acetylcholine receptors (nAChRs) in APPswe mice, with an initial decrease at early ages, followed by an increase at later ages, while the N-methyl-D-aspartate (NMDA) receptors showed a persistent increase from a young age. The increased receptor levels probably reflect compensatory mechanisms in response to a high Aβ burden. No changes were found in α7 nAChRs in hAChE-Tg//APPswe mice or 3xTg-AD mice. At a very early age, the APPswe mice expressed increased cortical synaptophysin levels, followed by a persistent decrease. In 3xTg-AD mice, the observed reduction in cortical synaptophysin might be ascribed to the presence of intraneuronal Aβ. The choice of transgenic mouse model, as well as the stage of Aβ pathology, was shown to strongly affect the outcome of drug treatment. While decreasing cortical insoluble Aβ1−40 and Aβ1−42 in APPswe mice, L(-)-nicotine increased cortical insoluble Aβ1–40 and soluble Aβ1–42 (both enantiomers) in hAChE-Tg//APPswe mice. However, in both mouse models nicotine reduced glial fibrillary acidic protein (GFAP) immunoreactive astrocytes. In APPswe mice, L(-)-nicotine increased hippocampal and cortical α7 nAChRs, while D(+)-nicotine treatment resulted in an increase in these receptors in hAChE-Tg//APPswe mice. Huprine X decreased hippocampal insoluble Aβ1−40, the levels of α7 nAChRs in the caudate nucleus, and increased cortical synaptophysin levels in 3xTg-AD mice, while only increasing the levels of α7 nAChRs in the hippocampi of APPswe mice. Treatment with the current AD drug galantamine increased cortical synaptophysin levels and affected the NMDA receptors, indicating plastic changes in the brain, while memantine reduced cortical levels of membranebound amyloid precursor protein (APP), which eventually may decrease Aβ levels. In conclusion, the diverse forms of Aβ displayed in the mouse models studied gave rise to differences in brain neuropathology, with diverging results regarding synapses and neuronal receptors as well as the outcome of drug treatment. The results in this work clearly highlight the fact that the choice of transgenic mouse model, as well as the stage of Aβ pathology, contribute importantly to the outcome of drug treatment. This emphasizes the importance of using different transgenic mouse models for evaluating the effects of new drug candidates for this devastating disease. LIST OF PUBLICATIONS This thesis is based on the following papers, which are referred to in the text by their roman numerals. I. Unger C, Hedberg MM, Mustafiz T, Svedberg MM and Nordberg A Early changes in Aβ levels in the brain of APPswe transgenic mice – Implication on synaptic density, α 7 neuronal nicotinic acetylcholineand N-methylD-aspartate receptor levels Molecular and Cellular Neuroscience, 2005, 30(2), 218-227 II. Unger C, Svedberg MM, Yu W-F, Hedberg MM and Nordberg A Effect of subchronic treatment of memantine, galantamine and nicotine in the brain of APPswe transgenic mice Journal of Pharmacology and Experimental Therapeutics, 2006, 317(1):30-36 III. Hedberg MM, Svedberg MM, Mustafiz T, Yu W-F, Mousavi M, Guan Z-Z, Unger C and Nordberg A Transgenic mice overexpressing human acetylcholinesterase and the Swedish amyloid precursor protein mutation: effect of nicotine treatment Neuroscience, 2008, 152(1):223-233 IV. Hedberg MM, Clos MV, Ratia M, Gonzalez D, Unger Lithner C, Camps P, Muñoz-Torrero D, Badia A, Giménez-Llort L and Nordberg A Effect of huprine X on Aβ, synaptophysin, and α7 neuronal nicotinic acetylcholine receptors in the brain of 3xTg-AD and APPswe transgenic mice Manuscript