Localized signaling of neurotrophins in axons versus somas of basal forebrain cholinergic neurons
Creators: name: Dasgupta, Srestha
Publisher: name: No Publisher Supplied
Year: 2023

description: ABSTRACT OF THE DISSERTATIONLocalized signaling of Neurotrophins in axons versus somas ofBasal Forebrain Cholinergic NeuronsBy Srestha DasguptaDissertation Director: Dr. Wilma J. FriedmanNeurons are highly polarized cells and can be differentially stimulated at the axon terminal or the soma. Basal forebrain cholinergic neurons (BFCNs) extend long projections to multiple targets in the brain, including the neocortex, hippocampus, and amygdala to regulate cognitive functions and are compromised in numerous neurodegenerative disorders, one of which is Alzheimer's Disease (AD). Basal forebrain neuron survival, growth, synaptic maintenance, and apoptosis is governed primarily by neurotrophins (NT), a group of 4 secreted growth factors: Nerve growth factor (NGF), Brain derived neurotrophic factor (BDNF), Neurotrophin-3 (NT3) and Neurotrophin-4 (NT4). NTs signal both in their mature, cleaved (mature NT) and precursor (proNT) forms, primarily through the Tropomyosin receptor kinase (Trk) and p75 Neurotrophin Receptor (p75NTR) receptor, respectively. The basal forebrain neurons express p75NTR, as well as all three Trk receptors: TrkA, TrkB, and TrkC, throughout life. Previous studies showed that treatment of BFCNs with mature NTs promotes survival while pro-neurotrophins (pro-NT) trigger apoptosis. Moreover, combined pro and mature NT treatment results in cell death. Owing to their elaborate axonal projections BFCNs may receive proNT or mature NT from their targets, or locally from glia in the basal forebrain. We investigated whether treatment of BFCNs with NT or proNT at the axon or soma has different consequences for neuronal survival and axonal growth. Microfluidic and filter chamber cultures were used to segregate BFCN soma and axons in vitro, allowing for compartmentalized treatment with pro or mature neurotrophins.To assess whether NTs have specific and differential effects when provided at the soma or axon terminal, BFCNs in microfluidic cultures were treated compartmentally with pro/mature NTs to study the effects on axon morphology and cellular survival. Clear differences in the effect on survival as well as on axon morphology were noticed in BFCNs treated with pro or mature NTs in compartmentalized microfluidic chambers in comparison to BFCNs treated non-compartmentally. Biochemical studies of filter cultures treated compartmentally with either pro or mature NTs or a combination of the two, show differences in signaling in the axons compared to the soma. In particular, our results showed that in vitro compartmentalized treatments with proNGF in the axons of BFCNs lead to p75NTR mediated retrograde axon degeneration and cell death. Moreover, neurons lacking p75NTR were resistant to proNT-induced degeneration. Our studies also show that retrograde transport and nascent protein synthesis in the axons are necessary for axonal proNT induced BFCN axon degeneration and cell death. Interestingly, an induction of proneurotrophins and p75NTR signaling has been shown to be required for injury induced neurodegeneration at the cortex. However, the effect of a change in the BFCN axonal neurotrophic environment on their survival has not been investigated. Using an in vivo Fluid percussion injury (FPI) model, we investigated whether injury at a BFCN target region such as the cortex had an effect on the survival of projecting BFCNs. Our studies showed a significant induction of proBDNF and proNGF in the injured cortex and hippocampus, but not in the basal forebrain, 3 days post injury. However, significantly fewer BFCNs were observed ipsilateral to the injury compared to the contralateral side of the brain 7 and 14 days after the injury, an effect which was absent in p75 knockout mice. These results suggest a retrograde degenerative effect of the cortical injury on the projecting BFCNs mediated through p75NTR, demonstrating that brain injury can impact neurons at distal locations from the injury site via their axonal projections.Overall, this investigation demonstrates that the signaling mechanisms and sub-cellular responses in BFCNs differ between the axon and the soma following pro or mature NT stimulation. In this study I show that the difference in location of neurotrophin stimulation in the basal forebrain cholinergic neurons determines their survival or death. Also, the dominance of proNT mediated apoptotic signaling over the mature NT mediated survival signaling in basal forebrain cholinergic neurons depends on the localization of the stimuli in the somas versus the axons. Further, I provide evidence of a novel role for p75NTR in mediating retrograde degeneration of BFCNs as a response to axon sourced proNTs, both in vivo under conditions of brain injury and in vitro.