Neurotrophin-3 signalling in neurons derived from human embryonic stem cells

The objective of this Thesis was to analyse TrkC-mediated signalling in human neurons following activation of this receptor tyrosine kinase by its ligand neurotrophin-3 (NT3). Neurons were generated from human embryonic stem cells and found to express the brain-derived neurotrophic factor (BDNF) receptor TrkB at higher levels than TrkC. Whilst this dual expression reflects the situation in the mammalian CNS where most neurons express both receptors, it significantly complicates the analysis of NT3-mediated TrkC signalling as NT3 also readily activates TrkB. As a result, very little is known about TrkC activation by NT3 in neurons. To gain mechanistic insights about the impact of selective TrkC activation, human embryonic stem cells were engineered to eliminate ligand-activated forms of TrkB using the CRISPR/Cas9 system. Neurons generated from these engineered cells were found to express unchanged levels of TrkC and gene expression was analysed by RNAseq following TrkC activation by NT3. The transcriptional changes turned out to closely resemble those previously observed following TrkB activation. They include a number of genes associated with neuronal activity and synaptic plasticity, including a rapid increase in the levels of Arc/Arg3.1, a postsynaptic protein extensively studied in the context of synaptic plasticity. The overall conclusion is that TrkC can be activated in human neurons by picomolar concentrations of NT3, lower than the concentrations of BDNF required to activate TrkB, and that TrkC activation leads to changes relevant to synaptic plasticity. The selectivity of NT3/TrkC signalling in neurons co-expressing TrkB is suggested to be determined by the availability of the ligand, not by the downstream consequences of Trk activation. This notion is in line with previous results indicating that the expression of BDNF and NTF3 in neurons is regulated by different mechanisms