Date of Graduation
12-2013
Document Type
Dissertation (PhD)
Program Affiliation
Neuroscience
Degree Name
Doctor of Philosophy (PhD)
Advisor/Committee Chair
Pramod K. Dash, Ph.D.
Committee Member
Anne Sereno, Ph.D.
Committee Member
Roger Janz, Ph.D.
Committee Member
Andrew Bean, Ph.D.
Committee Member
Michael J. Gambello, MD, Ph.D.
Abstract
The mechanistic Target of Rapamycin Complex 1 (mTORC1) pathway integrates cellular availability of growth factors, energy and amino acids to regulate protein synthesis and autophagy. The mTORC1 pathway has also been shown to be required for memory consolidation, and its dysregulation is associated with many neurological disorders. MTORC1 is negatively regulated by the tuberous sclerosis complex (TSC1/2). When ATP and growth factors are available, TSC1/2 is inhibited and mTORC1 activity can be restored. In a complementary regulatory pathway, amino acids signal to mTORC1 through the Rag GTPases and Ragulator complex, which modulate the translocation of mTORC1 from the cytoplasm to the lysosomal surface where activation occurs. This thesis investigates the role of mTORC1 in memory formation and dysfunction.
It has been shown that in response to traumatic brain injury (TBI), mTORC1 activity is acutely increased, and treatment with rapamycin, a specific inhibitor of mTORC1, has been shown to improve outcome. On the other hand, chronic elevated levels of mTORC1 are required for axonal regeneration of injured peripheral nerves and increased synapse growth after TBI. These results suggest that this mTORC1 could be a potential therapeutic target for TBI patients, but it remains unclear whether inhibition or activation of mTORC1 would improve outcome. Using a TSC2 knock-out mouse model we investigated the effects of elevated mTORC1 activity on outcome using hippocampal-dependent learning and memory tasks. My results suggest that acute overactivation of mTORC1 impairs long-term memory after brain injury and its return to baseline is associated with recovery. These results could aid in the design of treatments for brain-injured patients that differentially target the mTORC1 pathway in a temporally specific manner following brain trauma.
In vitro studies have shown that the amino acid glutamine decreases, while leucine increases, mTORC1 signaling in amino acid deprived cells. However, it remains unclear if glutamine and leucine would be effective in vivo at modulating mTORC1 signaling in the brain in order to influence cognition. My results show that in vivo intrahippocampal infusions of glutamine inhibited mTORC1 signaling and impaired memory formation, effects that were reversed by the co-administration of leucine. Furthermore, oral administration of glutamine to conditional knock-out mice lacking the Tsc2 gene prolonged their survival. Taken together, these findings indicate that glutamine can decrease brain mTORC1 activity, and may have utility in the treatment of neurological problems associated with elevated mTORC1 signaling.
Keywords
mTORC1, glutamine, leucine, TSC2, Traumatic Brain Injury, Rapamycin, memory