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57
through decreased IRS-2 phosphorylation and decreased Ras activation. These results are consistent with findings that Ras induces premature replicative senescence in primary mammalian cells and that SIRT1-deficient mouse embryonic fibroblasts (MEF) have a dramatically extended replicative life span (Chua et al., 2005; Serrano et al., 1997). As expected if SIRT1 acts upstream of Ras, SIRT1 was not required for Ras-dependent accelerated replicative senescence (Chua et al., 2005). Thus, it is possible that inhibition of IRS-2 upstream of Ras is responsible for this effect of SIRT1 deficiency on replicative senescence. In agreement with this model our immunoblots indicated that SIRT1 is almost exclusively retained in the cytosol of adult brain cells and SIRT1 co-precipitated with IRS-2. Interestingly, the PI3K/Akt pathway, another major effector downstream of IRS-2 was unaffected by SIRT1 inhibition, although over-expression of SIRT1 was reported to increase Akt activation under insulin-resistant conditions but not under normal conditions (Sun et al., 2007). It is interesting that only the ERK1/2 pathway but not the PI3K/Akt pathway appears to be affected by IRS-2 in our system. Although IRS-1 and IRS-2 are distributed similarly in many tissues and their functions often overlap, the relative contributions of IRS-1 and IRS-2 are different. In some organs, IRS-1 seems to mediate the majority effects of insulin and transmit signal to PI3K and Ras/ERK pathway. In other tissues like muscles, IRS-2 appears to selectively transduct to ERK signaling, while IRS-1 preferentially activates Akt1; and Akt2 and p38MAPK lie downstream of both IRS-1 and IRS-2 (Byron et al., 2006; Huang et al., 2005). These imply that the coupling of IRS to the downstream signaling may depend on the cell type
Object Description
| Title | Roles of SIRT1 in neuronal oxidative damage and brain function |
| Author | Li, Ying |
| Author email | lying@usc.edu; yingraceli@yahoo.com |
| Degree | Doctor of Philosophy |
| Document type | Dissertation |
| Degree program | Neuroscience |
| School | College of Letters, Arts and Sciences |
| Date defended/completed | 2008-09-12 |
| Date submitted | 2008 |
| Restricted until | Unrestricted |
| Date published | 2008-10-30 |
| Advisor (committee chair) | Longo, Valter D. |
| Advisor (committee member) |
Baudry, Michel Pike, Christian J. Madigan, Stephen A. |
| Abstract | Aging is a common phenomenon of multiple organisms. In humans aging is frequently accompanied by cognitive decline and occurrence of neurodegenerative diseases which reduce the quality of life and impose financial stress on society. Delaying the aging process, extending life span and decreasing the occurrence of age-related brain function deficit have always been aspirations of human kind. Extensive research has advanced our understanding of the mechanisms underlying aging, among which is the ability of calorie restriction to increase longevity, and the pivotal regulatory roles of insulin/IGF-1 signaling pathway. Some recent studies identified silent information regulator 2 (Sir2; SIRT1 is the mammalian homolog) as a key mediator of the beneficial effects of calorie restriction and this prompted development of SIRT1 activators for human consumption to delay aging and accompanying cognitive decline. However, our laboratory previously showed in yeast that Sir2 can increase stress sensitivity and limit life span extension under certain conditions, calling for more detailed characterization of SIRT1. In the research described in this dissertation I extended this study to the mammalian system and focused on the role of SIRT1 on the health of neurons and brain functions, especially learning and memory.; This dissertation consists of three chapters. In chapter 1 I briefly review some recent progress on aging, oxidative stress, insulin/IGF-1 signaling pathway and learning and memory with emphasis on the involvement of SIRT1 in these processes. In chapter 2 I focused on the role of SIRT1 in oxidative stress in neurons and its mechanisms. I found that SIRT1 inhibition increased resistance to oxidative damage and this effect is partially mediated by a reduction in IGF-I/IRS-2/Ras/ERK1/2 signaling. In chapter 3 I studied the functions of SIRT1 in learning and memory. The experiments showed that deletion of SIRT1 impairs a certain form of synaptic plasticity and reduce performance in several different learning and memory tasks while overexpressing SIRT1 did not substantially affect learning and memory.; Together, my studies reveal that SIRT1 exacerbates neuronal oxidative damage but is essential in learning and memory, indicating that SIRT1 plays multiple roles in aging and brain functions and that caution should be exercised in designing anti-aging or therapeutic approaches that involve targeting SIRT1. |
| Keyword | SIRT1; neurons; brain; oxidative damage; learning and memory |
| Language | English |
| Part of collection | University of Southern California dissertations and theses |
| Publisher (of the original version) | University of Southern California |
| Place of publication (of the original version) | Los Angeles, California |
| Publisher (of the digital version) | University of Southern California. Libraries |
| Provenance | Electronically uploaded by the author |
| Type | texts |
| Legacy record ID | usctheses-m1723 |
| Contributing entity | University of Southern California |
| Rights | Li, Ying |
| Repository name | Libraries, University of Southern California |
| Repository address | Los Angeles, California |
| Repository email | cisadmin@lib.usc.edu |
| Filename | etd-LI-2405 |
| Archival file | uscthesesreloadpub_Volume44/etd-LI-2405.pdf |
Description
| Title | Page 67 |
| Contributing entity | University of Southern California |
| Repository email | cisadmin@lib.usc.edu |
| Full text | 57 through decreased IRS-2 phosphorylation and decreased Ras activation. These results are consistent with findings that Ras induces premature replicative senescence in primary mammalian cells and that SIRT1-deficient mouse embryonic fibroblasts (MEF) have a dramatically extended replicative life span (Chua et al., 2005; Serrano et al., 1997). As expected if SIRT1 acts upstream of Ras, SIRT1 was not required for Ras-dependent accelerated replicative senescence (Chua et al., 2005). Thus, it is possible that inhibition of IRS-2 upstream of Ras is responsible for this effect of SIRT1 deficiency on replicative senescence. In agreement with this model our immunoblots indicated that SIRT1 is almost exclusively retained in the cytosol of adult brain cells and SIRT1 co-precipitated with IRS-2. Interestingly, the PI3K/Akt pathway, another major effector downstream of IRS-2 was unaffected by SIRT1 inhibition, although over-expression of SIRT1 was reported to increase Akt activation under insulin-resistant conditions but not under normal conditions (Sun et al., 2007). It is interesting that only the ERK1/2 pathway but not the PI3K/Akt pathway appears to be affected by IRS-2 in our system. Although IRS-1 and IRS-2 are distributed similarly in many tissues and their functions often overlap, the relative contributions of IRS-1 and IRS-2 are different. In some organs, IRS-1 seems to mediate the majority effects of insulin and transmit signal to PI3K and Ras/ERK pathway. In other tissues like muscles, IRS-2 appears to selectively transduct to ERK signaling, while IRS-1 preferentially activates Akt1; and Akt2 and p38MAPK lie downstream of both IRS-1 and IRS-2 (Byron et al., 2006; Huang et al., 2005). These imply that the coupling of IRS to the downstream signaling may depend on the cell type |
