Regulation of amyloid beta generation and its involvement in synaptic function: studies in human iPSC-derived cortical neurons

Abstract

Alzheimer’s disease (AD) is a neurodegenerative disease that affects millions of individuals worldwide and exerts a profound societal and economic impact. Clinically characterized by a gradual loss of memory, cognitive and functional abilities, AD begins decades before the onset of symptoms with the accumulation of an endogenously produced peptide, amyloid beta (Aβ). Aβ is produced through the enzymatic cleavage of amyloid precursor protein (APP) by β- and γ-secretases and its functions include regulating synaptic plasticity and activity, although excessive accumulation can disrupt neuronal function. Inhibition of Aβ generation could enable early disease prevention, however greater insights into the mechanisms of Aβ production and its functions at the synapse are needed to avoid side-effects. Furthermore, a deeper understanding of Aβ’s toxic effects on synapses would improve our ability to detect Aβ-induced synaptic dysfunction and degeneration in patients, allowing to better monitor effective treatments. Therefore, this thesis aims to deepen our understanding of Aβ generation and its pathophysiological effects on synapses in human neurons. In paper I, using a cellular model of human iPSC-derived neurons we found that increased Aβ secretion correlated with increased APP/β-secretase colocalization in early endosomes, and a possible inhibitory function of APP-CTFβ, the intermediate product of β-cleavage, on β-secretase. In paper II, we investigated the secretion of ten potential biomarkers of synaptic dysfunction in AD, from human iPSC-derived neurons. We found that synapse formation, neuronal activity and exposure to exogenous toxic oligomeric Aβ affected secretion of the synaptic proteins differently. In paper III, we explored the consequences of high-dose β-secretase inhibition on synaptic function in human iPSC-derived neurons. We found that acute synaptic dysfunction following β-secretase inhibition seems to involve mechanisms other than reduction of Aβ secretion or APP accumulation at synapses. Finally, in paper IV, we developed a protocol to differentiate human stem cells into mature, synaptically active neurons without the need for glial support. Collectively, our insights into the intricate mechanisms of APP trafficking and cleavage, Aβ generation and its impact on synaptic function and dysfunction will advance the field of AD research and will hopefully provide directions to enhance the success rate of clinical trials targeting AD.