Chromatin modifiers and recombination factors promote a telomere fold-back structure, that is lost during replicative senescence

Stability of Rad51 recombinase and persistence of Rad51 DNA repair foci depends on post-translational modifiers, ubiquitin and SUMO

The DNA double-strand breaks are particularly deleterious, especially when an error-free repair pathway is unavailable, enforcing the error-prone recombination pathways to repair the lesion. Cells can resume the cell cycle but at the expense of decreased viability due to genome rearrangements. One of the major players involved in recombinational repair of DNA damage is Rad51 recombinase, a protein responsible for presynaptic complex formation. We previously showed that an increased level of this protein promotes the usage of illegitimate recombination. Here we show that the level of Rad51 is regulated via the ubiquitin-dependent proteolytic pathway. The ubiquitination of Rad51 depends on multiple E3 enzymes, including SUMO-targeted ubiquitin ligases. We also demonstrate that Rad51 can be modified by both ubiquitin and SUMO. Moreover, its modification with ubiquitin may lead to opposite effects: degradation dependent on Rad6, Rad18, Slx8, Dia2, and the anaphase-promoting complex, or stabilization dependent on Rsp5. We also show that post-translational modifications with SUMO and ubiquitin affect Rad51's ability to form and disassemble DNA repair foci, respectively, influencing cell cycle progression and cell viability in genotoxic stress conditions. Our data suggest the existence of a complex E3 ligases network that regulates Rad51 recombinase's turnover, its molecular activity, and access to DNA, limiting it to the proportions optimal for the actual cell cycle stage and growth conditions, e.g., stress. Dysregulation of this network would result in a drop in cell viability due to uncontrolled genome rearrangement in the yeast cells. In mammals would promote the development of genetic diseases and cancer.

Telomeric 8-oxo-guanine drives rapid premature senescence in the absence of telomere shortening

Oxidative stress is a primary cause of cellular senescence and contributes to the etiology of numerous human diseases. Oxidative damage to telomeric DNA has been proposed to cause premature senescence by accelerating telomere shortening. Here, we tested this model directly using a precision chemoptogenetic tool to produce the common lesion 8-oxo-guanine (8oxoG) exclusively at telomeres in human fibroblasts and epithelial cells. A single induction of telomeric 8oxoG is sufficient to trigger multiple hallmarks of p53-dependent senescence. Telomeric 8oxoG activates ATM and ATR signaling, and enriches for markers of telomere dysfunction in replicating, but not quiescent cells. Acute 8oxoG production fails to shorten telomeres, but rather generates fragile sites and mitotic DNA synthesis at telomeres, indicative of impaired replication. Based on our results, we propose that oxidative stress promotes rapid senescence by producing oxidative base lesions that drive replication-dependent telomere fragility and dysfunction in the absence of shortening and shelterin loss.

This study uncovers a new mechanism linking oxidative stress to telomere-driven senescence. A common oxidative lesion at telomeres causes rapid premature cellular aging by inducing telomere fragility, rather than telomere shortening.

SCA® Slows the Decline of Functional Parameters Associated with Senescence in Skin Cells

The identification of compounds and natural ingredients that can counteract tissue stress and dysfunction induced by aging in skin cells is warranted. Here, we investigated the activity of the secretion from the snail Cryptomphalus aspersa (SCA^®), an active compound with well-established beneficial effects on skin integrity and aging. To determinate its senescence-regulation mechanisms, we used a model where damage was induced by hydrogen peroxide (H₂O₂). The results showed that SCA^® positively modulated factors involved in cell senescence such as β-galactosidase and cell morphology, secretory efficiency markers (SIRT1/6 and carboxymethyl-lysine), and metabolic and redox homeostasis (mTOR and ROS). This study demonstrated a novel compound that is activity-modulating, reduces cell senescence, and increases longevity to maintain skin homeostasis and functionality.

Telomere Interacting Proteins and TERRA Regulation

Telomere shortening rates inversely correlate with life expectancy and hence it is critical to understand how telomere shortening is regulated. Recently, the telomeric non-coding RNA, TERRA has been implicated in the regulation of replicative senescence. To better understand how TERRA is regulated we employed a proteomics approach to look for potential RNA regulators that associate with telomeric sequences. Based on the results, we have identified proteins that may regulate TERRA in both a positive and negative manner, depending on the state of the telomere. In this mini-review, we discuss and speculate about these data to expand our understanding of TERRA and telomere interactors with respect to telomere shortening dynamics.

The regulation of the DNA damage response at telomeres: focus on kinases

The natural ends of linear chromosomes resemble those of accidental double-strand breaks (DSBs). DSBs induce a multifaceted cellular response that promotes the repair of lesions and slows down cell cycle progression. This response is not elicited at chromosome ends, which are organized in nucleoprotein structures called telomeres. Besides counteracting DSB response through specialized telomere-binding proteins, telomeres also prevent chromosome shortening. Despite of the different fate of telomeres and DSBs, many proteins involved in the DSB response also localize at telomeres and participate in telomere homeostasis. In particular, the DSB master regulators Tel1/ATM and Mec1/ATR contribute to telomere length maintenance and arrest cell cycle progression when chromosome ends shorten, thus promoting a tumor-suppressive process known as replicative senescence. During senescence, the actions of both these apical kinases and telomere-binding proteins allow checkpoint activation while bulk DNA repair activities at telomeres are still inhibited. Checkpoint-mediated cell cycle arrest also prevents further telomere erosion and deprotection that would favor chromosome rearrangements, which are known to increase cancer-associated genome instability. This review summarizes recent insights into functions and regulation of Tel1/ATM and Mec1/ATR at telomeres both in the presence and in the absence of telomerase, focusing mainly on discoveries in budding yeast.