Contribution of cholinergic innervation to adult hippocampal neurogenesis in dementia

Aberrations in the adult hippocampal neurogenesis in AD patients include dysfunctional maturation indicated by accumulated number of immature neurons. The hippocampus receives cholinergic afferents from the medial septum/diagonal band of Broca (MS/DB). Plasticity of septohippocampal cholinergic innervation in AD was reported by multiple studies including higher hippocampal choline acetyltransferase (ChAT) activity in MCI cases with Braak stage III-IV than control and AD cases. Septohippocampal cholinergic pathway is crucial to adult hippocampal neurogenesis in rodents. While several studies have reported association between these two factors in human cases, a range of markers investigated have been limited and the mechanisms remain unclear. The aim of this first project is to use immunohistochemistry (IHC) to measure the extents of cholinergic innervation with ChAT and vesicular acetylcholine transporter (VAChT) and to evaluate the changes in adult hippocampal neurogenesis with immature neuron markers DCX, HuB/D, and proliferative marker PCNA. Statistical power for the main study was calculated from the pilot study and reported to request for the main cohort cases. The main cohort consisted of 36 cases of all Braak stages with low to negligible vascular conditions from the MRC brain bank. ChAT staining in the dentate gyrus (DG) was significantly higher in the cases with Braak III than cases with Braak IV-VI. The number of HuB/D positive cells without PCNA was significantly correlated with ChAT staining and significantly lower in Braak IV-V cases compared with others. The number of DCX-positive cells, unlike the pilot study and other studies, was not different across the groups. Since the third year of the project, Dr Emily Clarke has started her project on D427V heterozygous GBA1 mouse as a potential model for dementia with Lewy bodies (DLB). Several studies demonstrated AD-like abnormal adult hippocampal neurogenesis in DLB and even more severe degeneration of the septohippocampal cholinergic tracts in DLB than in AD cases. This led to our collaboration and the second part of this thesis on characterising the cholinergic innervation and adult hippocampal neurogenesis in this mouse model. The aim is to use IHC to investigate the distribution of ChAT, VAChT, and synaptophysin; to compare/confirm IHC findings on cholinergic profiles with Western blot; and to evaluate the changes in adult hippocampal neurogenesis with immature neuron markers DCX, HuB/D, and PCNA. Although ChAT staining was slightly higher in the hippocampus and cortex of the 12 month-old GBA1 D427V mice, VAChT staining was significantly reduced. The number of cholinergic neurons in the basal forebrain was not different but upregulation of ChAT level and ectopic fibre sprouting was observed. In the DG, 12 month-old GBA1 D427V mice had higher density of DCX-positive neurons. Furthermore, cells double-positive for DCX and HuB/D are more common in human than in mouse. This study confirmed the potential of HuB/D as a human-specific biomarker for healthy adult hippocampal neurogenesis and also demonstrated differential regulation of VAChT and ChAT in different disease types that may have future diagnostic values.