Dynamic Mechanism For The Initiation Of Cellular Replicative Senescence

Cellular replicative senescence refers to that cells halt division after a limited numbers of cell replication, accompanied by a prominent change in cell morphology and metabolism. It is experimentally demonstrated that the shortening of telomeres promotes cellular senescence. Telomeres are specialized chromatin structures that cap the ends of linear chromosomes and protect them from deterioration, end-to-end fusion, and being recognized as damaged DNA. Telomeres shorten with DNA replication. On the other hand, the tumor suppressor p53 is recognized as a main trigger of senescence. Nevertheless, the molecular mechanism for senescence still remains elusive. In contrast to previous theoretical models, the present thesis is the first to characterize the dynamics of both telomere shortening and the p53-centered signaling network in human fibroblasts; the two subsystems are coupled via the TRF2 protein. By theoretical analysis and numerical simulation, we find that seriously eroded telomeres activate p53 through the TRF2-ATM-p53-Siahl positive feedback loop and p53 can induce target genes to trigger and maintain cellular senescence. Specifically, we propose a switch-like dynamic mechanism underlying senescence initiation, which is sufficiently robust. Our results are in good agreement with experimental observations, and testable predictions are also made. The main results are presented in the following.First, we characterize the shortening of 92 telomeres during cell replication in human fibroblast. The function of a telomere depends on its length, specific DNA structure and engagement of telomere proteins. Either end of a chromosome has repetitive nucleotide sequences (5’-TTAGGG-3’). Telomere DNA forms a lariat structure (called t-loop) by invading of the 3’-overhang into the duplex telomeric repeat arrays. Here, we take into accounts three mechanisms responsible for telomere shortening. Because conventional DNA polymerases are unable to replicate the very ends of chromosomes, telomeres in normal human somatic cells shorten with each round of DNA replication. The two others refer to processing of C-rich strands by exonuclease and unrepaired single-strand breaks caused by oxidative stress. Notably, whether all three mechanisms play a role at each telomere depends on whether the telomere originates from the leading strand or lagging strand of the mother telomere. Moreover, the telomeres should be considered in pairs for each chromosome, i.e., whether a telomere at one end of the chromosome is shortened after cell division depends on what occurs on the telomere at the other end. Based on Gillespie’s method and considering the stochasticity in telomere shortening, we develop a stochastic algorithm to simulate the temporal evolution of the length of every telomere in a cell. We analyze in detail the contribution of each mechanism to telomere shortening, and our results are well consistent with experimental observations.Second, we explore the underlying mechanism for triggering senescence by coupling telomere shortening with p53 activation. We construct a mathematic model for the p53-centered signaling network, including the TRF2-ATM-p53-Siahl and p53-Mdm2 feedback loops. TRF2 is mainly localized in the nucleus. TRF2 proteins form dimers once they are produced. TRF2 dimers bind to telomeric repeats, mediating T-loop formation and preventing the ATM kinase from initiating the DNA damage signaling at functional telomeres. On the other hand, free, unbound TRF2 dimers can be targeted for degradation by the E3-liagase Siahl, which is induced by p53. We express the concentration of telomere-bound TRF2 as a function of telomere length and the level of TRF2 in the nucleus. Through deterministic analyses and stochastic simulations, we show that the ATM-p53-Siahl loop is activated when telomeres are badly eroded and reduce to a critical length. Whereas the p53 level rises to a high plateau, the TRF2 level drops quickly to a low level. As a transcription factor, p53 can modulate expression of hundreds of targets genes to trigger and maintain senescence. Such a switch-like mechanism for triggering senescence represents a robust one:only when telomeres are sufficiently short can senescence be induced; and senescence is irreversible once it is evoked in the absence of active telomerase. We also characterize the telomere dynamics at the population level.The organization of this thesis is as follows:In Chapter 1, some essential background knowledge of cellular senescence is introduced, including the types of senescence, the relationship between senescence, aging and cancer, and the current research status.In Chapter 2, we construct a model to characterize the shortening of 92 telomeres during DNA replication in human fibroblast.In Chapter 3, we build a model for the p53-centered signaling network, and explore the dynamic mechanism for senescence initiation.Finally, we present a summary of main conclusions and prospects for further studies in Chapter 4.