Regenerating the brain: Stem cell differentiation and cholinergic dysfunction in Alzheimer disease

Abstract: Hippocampal neurogenesis in the adult brain is important for learning and memory processes, which are heavily affected in Alzheimer disease (AD). Thus, targeting processes that could stimulate neurogenesis is a logical step in the search for strategies offering neuronal renewal and brain repair. However, the pathological lesions in the AD brain start to accumulate decades before the onset of clinical symptoms and neuronal plasticity is likely to be compromised by the pathological burden. The aim of this thesis was to provide a deeper understanding of how pathological mechanisms in AD affect stem cell differentiation and cholinergic signaling mechanisms, with implications for future regenerative therapies. The progressive loss of cholinergic neurons in AD may be a consequence of accumulation of β-amyloid (Aβ) in the brain. In paper I, there were prevailing differences in Aβ assemblies between early onset AD and late onset AD and that reduced cholinergic activity correlated with distinct Aβ oligomers. In paper II, the impact of nerve growth factor (NGF) and Aβ treatment on the development of cholinergic neurons from human embryonic stem (hES) cells was investigated. NGF treatment increased differentiation into functional cholinergic neurons, oligomeric Aβ treatment decreased the number of functional neurons, and fibrillar Aβ promoted glial differentiation. In paper III, fibrillar Aβ treatment altered the secretion of cholinergic enzymes from hES cells, resulting in low levels of acetylcholine. These changes were linked with an altered secretion pattern for cytokines, reduced neuronal differentiation and increased gliogenesis. In paper IV, the effects of hippocampal human neural stem cell transplantation alone, or in combination and modulation of Aβ levels with (+)-phenserine or the partial α7 nicotinic acetylcholine receptor (nAChR) agonist JN403 on neurogenesis, graft survival, astrocytosis and cognitive performance in young Tg2576 mice (representing the early stages of AD) was studied. Neural stem cell transplantation increased endogenous neurogenesis and reduced memory impairment in AD mice not receiving the drugs but not in those receiving the drugs. JN403 decreased the number of α7 nAChR-expressing astrocytes, which correlated with reduced neurogenesis. We thus hypothesize that α7 nAChR-expressing astrocytes are involved in neurogenic processes during the development of neuropathology. It is hoped that the findings presented in this thesis will provide novel targets for further studies, with potential for stimulating neuronal regeneration in AD.