Date of Award

Spring 2017

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Biology - (Ph.D.)

Department

Federated Department of Biological Sciences

First Advisor

Radek Dobrowolski

Second Advisor

Wilma Friedman

Third Advisor

Edward Michael Bonder

Fourth Advisor

Nan Gao

Fifth Advisor

Eliseo A. Eugenin

Abstract

The molecular mechanisms leading to and responsible for age-related, sporadic Alzheimer’s disease (AD) remain largely unknown. It is well documented that aging patients with elevated levels of the amino acid metabolite homocysteine (Hcy) are at high risk of developing AD. The impact of Hcy on molecular clearance pathways in mammalian cells, including in-vitro cultured induced pluripotent stem cell-derived forebrain neurons and in-vivo neurons in mouse brains is investigated in this research project. Exposure to high Hcy levels results in up-regulation of the mechanistic target of rapamycin complex 1 (mTORC1) activity, one of the major kinases in cells that is tightly linked to anabolic and catabolic pathways. Moreover, Hcy-mediated mTORC1 activity is only specific to Hcy and not to Hcy metabolites such as Hcy-thiolactone and cysteine. Homocysteine is sensed by a constitutive protein complex composed of leucyl-tRNA-synthetase (LeuRS) and folliculin (Flcn), which regulates mTOR tethering to lysosomal membranes. In hyper-homocysteinemic human cells and cystathionine β-synthase-deficient mouse brains, an acute and chronic inhibition of autophagy, the molecular clearance pathway is detected. In Hcy-treated cells, mTORC1 mediates phosphorylation of ULK1 which consequently reduces LC3 lipidation. mTORC1 also dampens TFEB-driven transcription of autophagy-related genes resulting in a buildup of abnormal proteins, including β-amyloid and phospho-Tau. Formation of these protein aggregates leads to AD-like neurodegeneration. This pathology can be prevented by inhibition of mTORC1 or by induction of autophagy, through TAT-Beclin1 treatments. Here it is discussed that an increase of intracellular Hcy levels predisposes neurons to develop abnormal protein aggregates, which are hallmarks of AD and its associated onset and pathophysiology with age.

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