Date of Award
Doctor of Philosophy (PhD)
David M Feliciano
Neuron development involves intricate morphological changes reliant on the serine/threonine kinase mammalian target of rapamycin complex 1 (mTORC1), a cell growth regulator that supports anabolic processes such as mRNA translation. Upregulated mTORC1 activity during neuron development causes altered neuron morphology, which is a hallmark of neurodevelopmental disorders such as Tuberous Sclerosis Complex (TSC). While the role of mTORC1 in neuron development is well documented, there is insufficient information on what precise upstream and downstream components of the mTOR pathway are involved. Two underexplored regulators of neuron development are the unc-51 like autophagy activating kinase 1 (Ulk1) and solute carrier family 7 member 5 (Slc7a5). Ulk1 is a pro-autophagic kinase and direct substrate of mTORC1. Autophagy is important to general cell maintenance and is linked to synaptic refinement via dendritic spine pruning. Ulk1 non-canonical functions involve Golgi-Endoplasmic Reticulum trafficking, which is also vital for neuron development and function. Slc7a5 is an amino acid transporter and upstream driver of mTORC1 activity by signaling the mTOR complex to the lysosomal surface for activation when amino acids are present. Herein, the role of mTORC1, Slc7a5, and Ulk1 is explored in mouse neuron development, and we apply this insight to develop and propagate a neural stem cell conditional Tsc2 knockout mouse colony. We demonstrate through genetic manipulation that Ulk1 and Slc7a5 play a role in neuron development in vivo. Overexpression of Ulk1 in developing cortical neurons resulted in decreased dendrite complexity. Increased Ulk1 Serine 757 phosphorylation in Ulk1 overexpressing neurons was associated with dendrite complexity closer to control levels. Thus, mTORC1 mediated Ulk1 Serine 757 phosphorylation is likely required during neuron development to control Ulk1 activity. Slc7a5 knockdown was detrimental to olfactory bulb granule cell development, specifically at later stages between Postnatal day 14-30. This is a time of synaptic integration and continued growth of dendrites in granule cells. Slc7a5 also regulates mTORC1 activity in granule cells during development. Knockdown of Slc7a5 decreased mTORC1 activity as indicated by Ribosomal S6 phosphorylation. Decreased dendrite complexity following Slc7a5 knockdown was partially rescued by Ras homolog enriched in brain (Rheb) overexpression, which drives mTORC1 activity. Finally, we documented defects in a Tsc2 transgenic mouse model and documented developmental defects in Tsc2 null granule cells including dendrite hypertrophy and increased soma size. Taken together, the presented research demonstrates how neuron development is tightly regulated, with implications for potential alternative therapeutic avenues in patients with altered mTOR activity in the brain.
Sokolov, Aidan Mikkel, "Regulation of Neuron Development by Slc7a5-mTOR-Ulk1" (2021). All Dissertations. 2776.