| Human cellular mortality is exquisitely regulated in order to prevent both premature loss of cellular replicative potential, which can lead to complications of aging, and the aberrant immortalization of somatic cells, which is associated with tumorigenesis. Human somatic cells experience a finite term of replication, measured in part by telomere attrition. As human somatic cells divide, their telomeres erode due to the end replication problem. When telomeres become critically short, cells enter an irreversible growth arrest called senescence, marked by accumulation of inflammatory mediators, which ultimately cause cell death. Occasionally, cells bypass senescence and continue dividing despite having critically short telomeres. These cells will encounter a second growth arrest check point called crisis, characterized by robust inflammation and profuse cell death. Rarely, cells evade the impetus to stop dividing imposed by senescence and crisis by activating telomerase and becoming immortalized.;Telomerase is a ribonucleoprotein reverse transcriptase, minimally comprised of an RNA subunit, TR, and a catalytic protein subunit, TERT. Cells expressing high levels of telomerase (such as germline and embryonic stem cells) are immortal. In addition, telomerase is activated in and conveys immortality to about 90% of all cancer cells. The most well understood contribution of telomerase to determining cellular mortality is its role in maintaining/extending telomeres, which offsets induction of replicative senescence.;Despite significant advances in senescence and telomerase biology, a complete understanding of the mechanisms regulating senescence and the mechanisms by which telomerase influences cellular mortality is still lacking. Work presented in this dissertation will provide the first evidence confirming a dramatic conformational change within Tetrahymena telomerase RNA (tTR) upon assembly into the telomerase complex that is essential to facilitating telomerase activity. In addition, work described in Chapter 3 provides the first full microRNA profile for replicatively senescent human foreskin fibroblasts. Finally, experiments described in Chapter 4 demonstrate the ability of telomerase to influence expression of miRNAs that undergo regulated expression during senescence and thereby influence a cell's ability to proliferate. A thorough understanding of these miRNA-regulated senescence pathways, and the mechanisms by which telomerase influences these pathways, will facilitate new approaches to treat aging-related disorders and cancer. |
