Blackburn, Emily Victoria; (2025) Investigating BDNF/TrkB signalling in neuronal models of Alzheimer's disease in Down syndrome. Doctoral thesis (Ph.D), UCL(University College London).

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Abstract

Brain derived neurotrophic factor (BDNF) and its receptor tropomyosin-related kinase B (TrkB) play a crucial role in neuronal growth and development, synaptic transmission, and neuroplasticity. Deficits in BDNF/TrkB signalling and impairments in endolysosomal trafficking have been identified in several neurodegenerative diseases, including Alzheimer’s disease (AD). Individuals with Down syndrome (DS) are at a massively increased risk of developing AD. In this Thesis, I characterise the endolysosomal pathway in basal forebrain cholinergic neurons (BFCNs), a highly vulnerable neuronal population in AD and DS, isolated from the Dp1Tyb mouse, a novel animal model of DS. This mouse has an extra copy of 63% of human chromosome 21 orthologous genes in the mouse genome, including the AD associated genes APP, SYNJ1 and DYRK1A. I found that Dp1Tyb BFCNs display enlarged early endosomes and increased levels of active Rab5. Furthermore, the retrograde axonal transport of BDNF/TrkB within endolysosomal organelles was analysed by both live-cell imaging and somatic accumulation assays in microfluidic chambers. In the presence of BDNF, Dp1Tyb BFCNs display slower retrograde velocities and increased pausing of signalling endosomes compared to WT. Additionally, when I delivered BDNF selectively to axons, I found a significant decrease in the somatic accumulation of activated TrkB in Dp1Tyb BFCNs, compatible with a retrograde transport deficit. When investigating the signalling pathways activated by BDNF, I found reduced activation of ERK1/2 in both the axon and the soma of Dp1Tyb BFCNs. Furthermore, direct inhibition of ERK1/2 in WT neurons resulted in a reduction of signalling endosome transport speeds, suggesting that the BDNF-induced enhancement of signalling endosome transport occurs via ERK1/2 activation. Interestingly, when I selectively treated Dp1Tyb axons with Leucettinib21, a DYRK1A inhibitor, signalling endosome transport speeds were partially restored, suggesting a role for DYRK1A in axonal transport. Overall, I have identified deficits in the retrograde axonal transport of neurotrophin/Trk complexes in AD-DS, highlighting possible therapeutic targets. Finally, endosome morphology and BDNF/TrkB transport were characterised in iPSC-derived glutamatergic neurons generated from an individual with DS. I found no significant differences in the size of early endosomes compared to the isogenic control. Furthermore, control iPSC-derived neurons did not respond to BDNF with regards to signalling endosome transport speeds, questioning the use of this iPSC line and/or cortical neurons in these analyses.

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